ELEMENTS 
 
 OF 
 
 PHYSIOLOGICAL PSYCHOLOGY 
 
ELEMENTS 
 
 OP 
 
 PHYSIOLOGICAL PSYCHOLOGY 
 
 A TREATISE OF THE ACTIVITIES AND NATURE 
 OF THE MIND 
 
 FROM THE PHYSICAL AND EXPERIMENTAL POINTS OF VIEW 
 
 (THOROUGHLY REVISED AND RE-WRITTEN) 
 
 BY 
 
 GEORGE TRUMBULL LADD, LL.D. 
 
 IERITUS PROFESSOR OF MORAL PHILOSOPHY AND METAPHYSICS IN YALE UNIVERSITY 
 
 AND 
 
 ROBERT SESSIONS WOODWORTH, PH.D. 
 
 PROFESSOR OF PSYCHOLOGY IN COLUMBIA UNIVERSITY 
 
 ILLUSTRATED 
 
 NEW YORK 
 
 CHARLES SCRIBNER'S SONS 
 1911 
 
L2- 
 
 BIOLOGY 
 
 LIBRARY 
 
 G 
 
 COPYRIGHT 1887, 1911, BY 
 CHARLES SCRIBNER'S SONS 
 
 Published April, 1887 
 
 Reprinted July 1888 ; July 1889 ; Jan. 1890; Jan., Sept. 1892; 
 Nov. 1894; Nov. 1896; Dec. 1897; Mar. 1900; Feb. 1907 
 
 New Edition Published May, 1911 
 
PREFACE 
 
 IT is now nearly a quarter of a century since the appearance of 
 the first edition of this treatise, which attempted to gather within the 
 limits of a single volume the main results of the then modern study 
 of psychology from the physiological and experimental points of 
 view. It was at that time (1887) the only work of its kind in 
 English; and so far as the author was aware, it was the only work 
 in any language, with the sole exception of Professor Wundt's 
 Grundzuge der physiologischen Psychologic, which was then in its 
 second edition. The task involved in any such attempt was even 
 so long ago exceedingly large and difficult. But the prompt favora- 
 ble response given to its accomplishment, not only in this country, 
 but also from abroad, was on this account all the more unexpectedly 
 gratifying. 
 
 During the interval since the first edition, many trained and 
 skilled workmen have assiduously devoted themselves to the solu- 
 tion of the various subordinate problems involved in the study of 
 mental life and mental development with the carefully guarded 
 methods prescribed by modern science. Much light has been 
 thrown on these problems, their nature and their probable solu- 
 tion; while many new ones or new aspects of old problems have 
 been uncovered and are still awaiting future investigation. And 
 there is no prospect that this process will come to an end. But it is 
 just this fact which evinces in an unmistakable way the vitality 
 of the science of psychology, as it does the vitality of all the other 
 forms of human knowledge. On the other hand, it is entirely 
 safe to say that neither the extravagant hopes nor the extravagant 
 fears of twenty-five years ago, with reference to the results of the 
 so-called " new psychology," have been verified. The fundamental 
 problems with regard to the nature of man's mind and its relations 
 to the organism, its place in the scale of development and its destiny, 
 remain essentially unchanged. 
 
 281804 
 
vi PREFACE 
 
 Those acquainted with the first edition will notice several impor- 
 tant changes on comparing with it the present revised edition. Of 
 these changes perhaps the most important may be summed up in 
 the fact that, of the two editions, this is much the most distinctly 
 physiological. The reason for this change is chiefly the following: 
 While many excellent volumes and articles have meantime appeared 
 to give the results of experimental psychology, more strictly so-called, 
 comparatively little has been published, of a worthy character, 
 which could be brought under the head of physiological psychology, 
 with any strict application of this latter term. With the same end in 
 view, two entire chapters one on THE PLACE OF THE NERVOUS 
 MECHANISM IN THE ANIMAL KINGDOM, and the other on THE 
 DEVELOPMENT OF THE NERVOUS SYSTEM IN THE INDIVIDUAL have 
 been added to Part I, and all the other chapters of this part have 
 been carefully rewritten and in most cases considerably expanded. 
 The new chapters on THE LOCALIZATION OF CEREBRAL FUNCTIONS 
 have been transferred to it. In this way the aim has been to give 
 a compact and complete summary of the nervous mechanism in its 
 relations to mental life. The hope has also been indulged that 
 the book might thus be made more useful to students of physiology 
 and medicine, who, in the opinion of the authors, ought to know 
 far more than they now do about the science of psychology as 
 approached from the physiological point of view. 
 
 The subjects more definitely falling under the head of experi- 
 mental psychology have, however, by no means been neglected. 
 Indeed, so far as these are connected with the functions of the 
 organs of sense, and with the measurement of all sorts of reactions 
 to all kinds of stimuli, they are themselves distinctly physiological 
 in their character and in their conclusions. And even when the 
 students of experimental psychology attempt the more difficult 
 problems connected with the mental functions involved in memory, 
 association, thinking, and learning, they can not possibly quite sep- 
 arate themselves from the closely allied physiological problems. 
 On the other hand, the problems which are attacked and more or 
 less successfully handled by the most purely experimental methods 
 are available and important data for the student of the same phe- 
 nomena from the physiological point of view. While, then, Part II 
 
PREFACE vii 
 
 has not been correspondingly extended, it, too, has been revised with 
 equal thoroughness, with the intention of making it a fairly complete 
 but succinct treatment of similar data as regarded from a somewhat 
 changed point of view. In all this, however, the controlling pur- 
 pose of the book has never been lost out of sight, which is to sum- 
 marize what modern science knows, or reasonably conjectures, about 
 the correlations existing between the nervous mechanism and the 
 mental life of man. 
 
 The other principal change which will be noted is the great re- 
 duction made in the Third Part of the book. This has been partly 
 on account of economy of space. But it has been chiefly due to 
 the fact that the author of the first edition has, since its appear- 
 ance, expressed his views on the more distinctly speculative ques- 
 tions involved, in a series of monographs. 1 The two chapters to 
 which this part has been reduced to summarize them in a single 
 word simply maintain the contention that the study of ultimate 
 psychological problems, from the physiological and experimental 
 points of view, finally leaves them in essentially the same condition 
 as that in which it finds them; and this position is that of a naive 
 and so-called " common-sense " dualism, as distinguished from the 
 metaphysical theories of parallelism, materialism, or subjective 
 idealism. To this dualism, neither psychology nor physiology 
 and physics have anything to oppose on scientific grounds. The 
 settlement of these questions, then, if they are to be settled at all, 
 must be relegated to philosophy. 
 
 While, as has already been stated, the entire subject has been 
 studied afresh and the book rewritten throughout, it has been all 
 the more gratifying to find that as candid and objective a reconsider- 
 ation of his own views as it is easily practicable for any student to 
 give, did not seem to require essential modifications in any of the 
 psychological theories advanced by the first edition on alleged 
 scientific grounds. But if the same candidly critical reception 
 is accorded to this edition which was so happily experienced by its 
 predecessor, it will be all that can be justly asked perhaps more 
 than, under existing circumstances, can be reasonably expected. 
 
 The authors of this edition wish to make general acknowledg- 
 
 > Especially " Philosophy of Mind" (1895), and "A Theory of Reality" (1899). 
 
viii PREFACE 
 
 ment of obligations to the many authorities whom they have con- 
 sulted and without whose assistance, of course, the book could 
 never have been written. So far as was practicable, they have 
 endeavored to mention them by name, either in the text or in the 
 foot-notes. But they desire to make special mention in this place 
 of their obligations to the writers and publishers of the neurological 
 journals and treatises from which so many of the figures illustrat- 
 ing the anatomy of the nervous system have been borrowed. 
 
 During the progress in preparation of this edition, the author 
 of the first edition found that his obligations to Professor R. S. 
 Woodworth could be justly discharged only by placing his name 
 upon the title-page as its joint author. And this act of justice 
 was cheerfully rendered. 
 
 GEORGE TRUMBULL LADD. 
 
 NEW HAVEN, May, 1911. 
 
PREFACE TO THE FIRST EDITION 
 
 THERE can be no doubt that an important movement in psychol- 
 ogy has ar^en in recent times through the effort to approach the 
 phenomena of mind from the experimental and physiological point 
 of view. Different students of psychological science will estimate 
 differently both the net result already reached by this effort and 
 the promise of further additions to the sum of our knowledge from 
 continued investigation of the same kind. Some writers have cer- 
 tainly indulged in extravagant claims as to the past triumphs of so- 
 called Physiological Psychology, and in equally extravagant expec- 
 tations as to its future discoveries. On the other hand, a larger 
 number, perhaps, have been inclined either to fear or to depreciate 
 every attempt to mingle the methods, laws, and speculations of the 
 physical sciences with the study of the human soul. These latter 
 apparently anticipate that some discovery in the localization of 
 cerebral function, or in psychometry, may jeopard the birthright 
 of man as a spiritual and rational being. Or possibly they wish 
 to regard the soul as separated, by nature and with respect to its 
 modes of action, from the material body in such a way as to render 
 it impossible to understand more of the one by learning more about 
 the other. 
 
 As a result of some years of study of the general subject, I 
 express with considerable confidence the opinion that there is no 
 ground for extravagant claims or expectations, and still less ground 
 for any fear of consequences. In all cases of new and somewhat 
 rankly growing scientific enterprises, it is much the better way to 
 waive the discussion of ac ual or possible achievements, as well as 
 of welcomed or dreaded revelations of new truth, and proceed at 
 once to the business on hand. It is proposed in this book to follow 
 this better way. It will be the task of the book itself to set forth 
 the assured or alleged results of Physiological Psychology; and 
 this will be done at every step with such degree of assurance as 
 
 ix 
 
x PREFACE TO THE FIRST EDITION 
 
 belongs to the evidence hitherto attainable upon the particular 
 subject discussed. With declamation, either in attack or defence 
 of the "old psychology," of the "introspective method," etc., one 
 may dispense without serious loss. 
 
 The study of the phenomena of consciousness by the method here 
 proposed necessarily requires some acquaintance with a considera- 
 ble circuit of sciences which are not usually all alike closely allied. 
 The number of scholars who can form opinions with equal freedom 
 and confidence in all of these sciences is very small. Moreover, 
 since all psycho-physical laws are supposed as the very term indi- 
 cates to govern the correlations of phenomena of consciousness 
 with phenomena of the nervous system, a peculiar mystery belongs 
 to much of the domain within which psycho-physical science is 
 compelled to move. These facts may fitly, on the one hand, excite 
 caution in the writer; and, on the other hand, excuse him for 
 many inevitable failures to set forth with perfect defmiteness and 
 confidence the conclusions he has to propose. Much will be said 
 that must be accepted as provisional, as only probably true. Much 
 room must also be made for conjecture and speculation. What is 
 most important, however, is that conjecture should not be put 
 forth as ascertained fact, or speculation as unquestioned law. 
 
 It would have been a great assistance to me if I had had more 
 predecessors in the path which I am to take. But with the excep- 
 tion of Wundt's masterly work (Grundzuge der physiologischen 
 Psychologic, second edition in 1880), no one book has attempted to 
 cover, even in a summary way, the entire ground. The number 
 of monographs, however, which have dealt with individual ques- 
 tions subordinate to, or part of, the main inquiry is very great. 
 These two facts also render the attempt at a general survey of 
 Physiological Psychology for readers of English both peculiarly 
 attractive and peculiarly difficult. I can only indulge the hope 
 that I have done something toward breaking this path and render- 
 ing it easier and more secure, both for myself and for others, in the 
 future. 
 
 The investigators and authors to whom I am under obligations 
 for material upon the various questions discussed, or statements 
 made, in this book are by no means all mentioned by name. Of 
 
PREFACE TO THE FIRST EDITION xi 
 
 course, much of what is said on the structure of the nervous system, 
 and on the phenomena of sensation and perception, has already 
 become part of that general fund of facts and laws which belongs 
 alike to all students of the subject. But by quoting certain author- 
 ities in the text, and by a few (in comparison with the number 
 which might have been cited) references in foot-notes, I have con- 
 nected some of the discoveries and views of modern psycho-physical 
 science with their authors. These may serve somewhat as guide 
 to those persons who wish to pursue such studies still further. 
 
 I am under particular obligations to Dr. James K. Thacher, 
 Professor of Physiology in the Yale Medical School, for valuable 
 assistance in that description of the Nervous Mechanism, its struct- 
 ure and functions, which the First Part of the book contains. If I 
 have escaped the mistake of assuming to teach more than is really 
 known upon this subject, it has been in large measure due to his 
 friendly and skilful guidance. Valuable assistance has also been 
 received from Russell H. Chittenden, Professor of Physiological 
 Chemistry, and Charles S. Hastings, Professor of Physics both of 
 the Sheffield Scientific School. 
 
 The method and arrangement of the book have been chosen so 
 as to fit it for use, both as a text-book by special students of the 
 subjects of which it treats, and also by the general reader who is 
 interested in knowing what results have been reached by the more 
 modern and even the latest psycho-physical researches. 
 
 GEORGE T. LADD. 
 
 YALE UNIVERSITY, NEW HAVEN, February, 1887. 
 
TABLE OF CONTENTS 
 
 PAGE 
 
 INTRODUCTION . . 1-10 
 
 PART FIRST 
 THE NERVOUS MECHANISM 
 
 CHAPTER I 
 
 THE PLACE OP THE NERVOUS SYSTEM IN THE ANIMAL KINGDOM 13-35 
 
 1, Necessity of the Comparative Method. 2, Primary Division of 
 Animals. 3-4, Description of the Amoeba. 5, Specialization of Re- 
 ceptors and Effectors. 6-8, Nerve-Net Type of Nervous System. 
 10-11, Segmented Type of Nervous System. 12, Centralized Type 
 of Nervous System. 13-15, Nerve-Centres of Vertebrates. 16, 
 Structure of the Cerebellum. 17, The Mid-Brain. 18, Inter-Brain 
 and End-Brain. 19, Development of the Cortex. 20-21, Relation of 
 Size to Intelligence. 22, Essential Function of Nervous Tissue. 
 
 CHAPTER II 
 
 THE DEVELOPMENT OP THE NERVOUS SYSTEM IN THE INDIVIDUAL 36-62 
 
 1-2, Reproduction in Unicellular Organisms. 3, Formation of 
 the Neural Groove. 4, Constitution of the Neural Tube. 5-6, 
 Growth of Nerve Axons and Fibres. 7, Formation of the Spinal Cord. 
 8-9, Growth of the Five Brain Vesicles. 10-11, Development of 
 the Cortex. 13-14, The Twelve Pairs of Cranial Nerves. 15-16, 
 Factors in Growth of the Nervous System. 17, Function of the Myelin 
 Sheath. 18-20, Variations in Weight of Human Brain. 
 
 CHAPTER III 
 
 GROSS STRUCTURE OP THE NERVOUS SYSTEM 63-96 
 
 1, Importance of Relation between Nervous Elements. 2-3, Dif- 
 ferentiation of Nervous Function. 4, Distinction of Sympathetic and 
 Cerebro-Spinal. 5, Membranes of Brain and Spinal Cord. 6-7, Col- 
 umns and Commissures of the Cord. 8-9, White and Gray Matter of the 
 Cord. 10-11, Nerve-Fibres and Nerve-Cells in the Cord. 12, Different 
 Aspects of the Encephalon. 13, Twelve Pairs of Cranial Nerves. 14- 
 
 xiii 
 
xiv TABLE OF CONTENTS 
 
 15, Internal Structure of Brain-Stem. 16-18, Continuations of the 
 Brain-Stem. 19-21, Structure and Relations of Inter-Brain. 22, The 
 Cerebellum. 23-24, Tracing and Naming of Nerve-Tracts. 25-27, 
 Nerve-Tracts and Their Connections. 28, Principal Motor Pathway. 
 29-35, Systems of Nerve-Centres and Connections. 
 
 CHAPTER IV 
 
 PAGE 
 
 ELEMENTS OF THE NERVOUS STRUCTURE 97-116 
 
 1-2, Development of the Nervous Elements. 3, Structure of the 
 Nerve-Fibre. 4, Size of Different Nerve-Fibres. 5, Constitution of the 
 Gray Matter. 6, Size and Shape of Nerve-Cells. 7-8, Internal Struct- 
 ure of Nerve-Cells. 9, Arrangement of Nerve-Cells. 10, Structure 
 and Office of the Dendrites. 11, Nerve-Cells of the Cerebellum.! 12, 
 Structure and Function of the Axon. 13, The Neurone Theory. 14, 
 The Neuroglia Cells. 
 
 CHAPTER V 
 
 CHEMISTRY OF THE NERVOUS SYSTEM 117-126 
 
 1-2, General Chemistry of Brain and Nerves. 3, Chief Chemical 
 Elements of Brain. 4-5, The Lipoids of the Nervous Substance. 6, 
 Non-Fatty Substances in Brain. 7, Cholesterin. 8-11, The Phosphor- 
 ized Lipoids in the Nervous Substance. 12, Specific Chemistry of Nervous 
 Elements. 
 
 CHAPTER VI 
 
 THE NERVES AS CONDUCTORS 127-144 
 
 1-2, Most General Function of the Nerves. 3, Methods of Experi- 
 mentation. 4, Excitability of the Nerve. 5, The Laws of Du Bois- 
 Reymond. 6, The Refractory Period. 7, Speed of the Nervous 
 Current. 8, Alterations in Conductivity of Nerves. 9, Double Con- 
 duction of the Nerves. 10, Nature of the Process Conducted.! 11, 
 v Phenomena of Fatigue. 12, The Current of Action. 13-14, Electro- 
 tonic Changes in Nerves. 15, The Core Model or Core Conductor. 
 16-17, The Current of Rest. 18, Theories of Nervous Function. 
 
 CHAPTER VII 
 
 REFLEX FUNCTIONS OF THE NERVOUS SYSTEM 145-174 
 
 1-2, Nature of Reflex Arc. 3, Instinctive and Simple Reflexes. 
 4-5, Automatic Action of Nervous Centres. 6-7, The Ganglionic 
 Reflexes. 8, Reflexes of the Skeletal Musculature. 9-12, Local Re- 
 flexes of the Spinal Cord. 13-14, Reflexes of the Cerebellum. 15, 
 Reflexes of the Medulla. 16, Influence of Cerebrum on Lower Centres. 
 17-18, Nervous Mechanism of Reflexes. 19-20, Phenomena of 
 Inhibition. 21-22, The Refractory Period. 23, Diphasic Impulses. 
 24, Latent Time of Nervous Reflexes. 25-26, Intensity and Ex- 
 
TABLE OF CONTENTS xv 
 
 tent of Reflexes. 27-28, Facilitation of Reflexes. 29-30, Inter- 
 ference of Reflexes. 31, General Characteristics of Reflex Action. 
 
 CHAPTER VIII 
 
 PAGE 
 
 END-ORGANS, OR RECEPTORS, OF THE NERVOUS SYSTEM 175-212 
 
 1-3, Function of End-Organs in the Nervous System. 4-5, End- 
 Organs of Smell. 6-7, End-Organs of Taste. 8-9, End-Organs of 
 Touch. 10, The Pacinian Corpuscles. 11-12, Structure of the Eye- 
 Ball. 13, Refracting Media of the Eye. 14, Muscles of the Eye-Ball. 
 15, The Problem Before the Organ of Vision. 16, Indices of Refraction. 
 17. Mechanism of Accommodation. 18, The Stimulus of Vision. 
 19-20, Nervous Elements of the Retina. 21. The Yellow Spot and the 
 Blind-Spot. 22, Office of the Rods and Cones. 23, Chemical Changes 
 in the Retina. 24, General Description of the Eye. 25, Three Princi- 
 pal Parts of the Ear. 26, Structure of the Middle Ear. 27, Office 
 of the Tympanum. 28, The Eustachian Tube. 29, Structure of the 
 Internal Ear. 30, Distribution of the Auditory Nerve. 31-32, The 
 Organ of Corti. 33, Theory of Sympathetic Vibration. 34-36, Func- 
 tion of the Semicircular Canals. 37, End-Organs of Motion. 
 
 CHAPTER IX 
 
 THE CEREBRAL HEMISPHERES AND THEIR FUNCTIONS 213-234 
 
 1-4, The Problem of Cerebral Functions. 5, Evidence from Com- 
 parative Anatomy. 6-7, Evidence from Removal or Injury. 8, The 
 Problem Stated. 9, General Description of Cerebral Hemispheres. 10, 
 Principal Divisions of the Cortex. 11, Projection and Association Fibres. 
 12-13, Nervous Elements of the Cortex. 14, History of the Investiga- 
 tion. 15, The Three Lines of Evidence. 16, The Method of Extirpa- 
 tion. 17, Evidence of Human Pathology. 18, Evidence from His- 
 tology. 19, Summary of the Situation. 
 
 CHAPTER X 
 
 THE CEREBRAL HEMISPHERES AND THEIR FUNCTIONS [Continued] .... 235-274 
 
 1, The Work of Fritsch and Hitzig. 2-3, Localization in the Brains 
 of Monkeys and Anthropoid Apes. 4, Paralyses of the Motor Area. 
 5-7, Definition and Function of the Motor Area. 8, Restitution of 
 Motor Functions. 9-11, The Somesthetic Area. 12, The Visual Area. 
 13-14, Phenomena of Psychical Blindness. 15, The Auditory Centre. 
 16-17, Centres of Smell and Taste. 18, The So-called "Silent Areas." 
 20, Results of Injuries to the Occipital Lobe. 21, To the Temporal 
 Lobe. 22-23, To the Parietal Lobe. 24-25, Disturbances of Speech 
 Functions. 26-29, Discussion of Broca's Speech Centre. 30, Localiza- 
 tion of a Writing Centre. 31-33, Functions of the Frontal Lobe. 34, 
 Summary of the Results. 35-38, Cell-Layers in the Cortex. 39-40, 
 Histological Mapping of the Cortex. 41-43, Grouping of Functions in 
 the Cortex. 
 
xvi TABLE OF CONTENTS 
 
 CHAPTER XI 
 
 PAGE 
 
 MECHANICAL THEORY OF THE NERVOUS SYSTEM 275-293 
 
 1-3, Machine-like Nature of the Organism. 4-5, Significance of 
 Chemical Constitution. 6-7, Arrangement of the Nervous Elements. 
 8, Equilibrating of Different Parts. 9-10, Imperfect Character of 
 Present Theory. 11-12, Theory of Specific Energies. 13-15, Mech- 
 anism of the Nerve-Centres. 16-17, Function of the Nerve-Cells. 
 18-19, Significance of the Synapse. 20, Rival Metabolic Theories. 
 21-23, Review of the Evidence. 
 
 PART SECOND 
 
 CORRELATIONS OF THE NERVOUS MECHANISM 
 AND MENTAL PHENOMENA 
 
 CHAPTER I 
 THE QUALITY OF SENSATIONS 297-323 
 
 1-3, Changes in Points of View. 4-5, Classification of the Sensa- 
 tions. 6-7, Need of Further Analysis. 8, The So-called Simple 
 Sensation a Fiction. 9, Questions Requiring an Answer. 10, Bear- 
 ing of the Specific Energy of the Nerves. 11-12, Stimuli of Olfactory 
 Sensations. 13-15, Analysis of Olfactory Sensations. 16, Stimuli 
 of Gustatory Sensations. 17-18 Analysis of Gustatory Sensations. 19, 
 Chemical Stimulus of Sensations of Taste. 20-21, General Nature of 
 Auditory Sensations. 22, Characteristics of Musical Sounds. 23, The 
 Pitch of Musical Sounds. 24, Sensitiveness of the Ear to Differences of 
 Pitch. 25, And Purity of Interval. 26, Means of Judging Musical 
 Sounds. 27, Nature of the "Clang." 28, Sounds Used in Music. 
 29-30, Theory of Consonance and Dissonance. 31, The Difference- 
 Tone. 
 
 CHAPTER II 
 
 THE QUALITY OF SENSATIONS [Continued] 324-352 
 
 1, Intricacy of Visual Sensations. 2, Stimulus of Visual Sensations. 
 3, Local Values of the Retina. 4, The General Problem of Visual 
 Sensations. 5, The Spectral Color-Tones. 6, Relative Brightness of 
 Color-Tones. 7, Composite Nature of Ordinary Colors. 8, Number 
 of Colors Distinguishable. 9-12, Theory of Complementary Colors. 
 13-14, Phenomena of Color-Blindness. 15-16, Negative and Positive 
 After-images. 17, Phenomena of Contrast. 18-21, Theories of 
 Color Vision. 22, Symbolic Representation of Visual Sensations. 
 23-24, Sensations of the Skin. 25, Nature of Temperature Stimuli. 
 26-27, Sensations of Pain. 28, The So-called Muscular Sense. 29, 
 Labyrinthic Sensations. 30, Visceral or Organic Sensations. 31, Sum- 
 mary of Evidence for the Specific Energy of the Nerves. 
 
TABLE OF CONTENTS xvii 
 
 CHAPTER III 
 
 PAGE 
 
 THE QUANTITY OF SENSATIONS 353-379 * 
 
 1-2, Distinction of Quantity from Quality. 3-4, Unscientific 
 Form of Ordinary Usage. 5-6, Character of the Quantitative Problems. 
 7, Method of Determining the Limits. 8, Methods of Determining 
 the Least Perceptible Difference. 9-11, Weber's Law and Fechner's 
 Formulas. 12-13, The Perception of Weight. 14, Sensitiveness to 
 Light Pressure. 15, Discriminations of Temperature. 16-17, Sensi- 
 tiveness of Acoustic Perception. 18, Least Perceptible Difference in In- 
 tensity of Tones. 19-20, Quantitative Discriminations of Sight. 21, 
 Weber's Law Applied to Visual Sensations. 22, Perceptible Minimum of 
 Sensations of Light. 23, Extensive Sensations of Light. 24-25, In- 
 tensity of Gustatory Sensations. 26, Intensity of Sensations of Smell, 
 27, Review of Weber's Law. 28-29, Explanations of Weber's Law, 
 30, Fechner's Intepretation of the Phenomena. 31, Summary of Re- 
 sults. 
 
 CHAPTER IV 
 
 PRESENTATIONS OF SENSE, OR SENSE-PERCEPTIONS 380-412 */ 
 
 1, Artificial Character of Simple Sensations. 2, Necessity of Analy- 
 sis. 3-4, Nature and Stages of Sense-Perception. 5, Necessity of 
 Mental Synthesis or Fusion. 6-7, Nativistic and Empiristic Schools. 
 8, Nature of a Spatial Series. 9, Nature of the Local Signs. 10, Stages 
 of Sense-Perception. 11, Activity of Higher Faculties. 12, Perceptions 
 of Smell. 13, Perceptions of Taste. 14, Perceptions of Hearing. 
 15-18, Localization of Sounds. 19, Construction of the Field of 
 Touch. 20-22, Explanation of Weber's "Sensation-Circles." 23-25, 
 The "Two-Point Threshold." 26, Sensuous Basis of Discrimination. 
 27, Mixed and Tangled Skin-Sensations. 28-29, Localization of Tem- 
 perature. 30, Nature of the Muscular Sense. 31-32, Judgments of 
 Bodily Movements. 33, Co-operation of Eye with Hand. 34, Feelings 
 of Double Contact. 
 
 CHAPTER V 
 
 PRESENTATIONS OF SENSE, OR SENSE-PERCEPTIONS [Continued] 413-469 *^ 
 
 1, Special Difficulties of Visual Perceptions. 2-3, Data of Visual 
 Perceptions. 4-5, Formation of the Retinal Field. 6, Statement of 
 the Problem. 7, Values of Different Retinal Sensations. 8, Muscles 
 of the Eye-Ball and Its Movements. 9, The Law of Listing. 10, Effects 
 of Accommodation. 11-13, Conditions of Binocular Vision. 14, Vis- 
 ual Perception of Depth. 15-16, Visual Perception of Distance and 
 Size. 17, Visual Perceptions of Motion. 18-19, Judgment in Errors 
 of Sense. 20, Geometrical Optical Illusions. 21, Central Factor in 
 Illusions. 22, Illusions of Angles. 23, Illusions of Areas. 24, 
 Theories of Visual Illusions. 25, The Central Theories. 26, The 
 Dynamic Theory. 27, The Confusion Theory. 28, Binocular Mixing 
 and Contrast of Colors. 29-30, Upright and Inverted Vision. 31, 
 Inferences from Errors of Sense. 32, General Conclusions as to Theory 
 of Vision. 33, Influence of Eye-Movements on Space Perception. 
 34, Graphic Records of Eye-Movements. 35-36, Speed of Eye-Move- 
 
xviii TABLE OF CONTENTS 
 
 ments. 37, Value of Objective Measurements. 38-39, Development 
 of Visual Perception. 40, Relations of Visual and Muscular Perception. 
 41, Perception as an Achievement of Mind. 
 
 CHAPTER VI 
 
 PAGE 
 
 TIME-RELATIONS OF MENTAL PHENOMENA 470-499 
 
 1-2, Experiments in Reaction-Time. 3, Simple Reaction-Time. 
 4, Inertia of the End-Organs. 5, Measurement of Smallest Intervals. 
 6, The Point of Starting. 7-8, Variations in Reaction-Time. 9, 
 Effect of Increasing Intensity. 10, Character of Reacting Movement. 
 11, Influence of Central Conditions.! 12, The Three Periods.! 13, 
 Wundt's Analysis. 14, So-called Sensorial and Muscular Reactions. 
 15, Complex Reactions. 16-18, Reaction with Discrimination. 19, 
 Discernment of Intensities. 20-22, Associative Reaction-Time. 23- 
 24, Amount of Conscious Process Involved. 25-26, General Character of 
 Reaction-Time. 
 
 CHAPTER VII 
 FEELING, EMOTION, AND EXPRESSIVE MOVEMENTS 500-541 
 
 1-2, Variable Conceptions of the Nature of Feeling. 3-4, Difficulties 
 of Analysis. 5, Methods of Investigation. 6, Physiological Theories 
 of Feeling. 7, Kinds of Feeling. 8, Pleasantness and Unpleasantness. 
 9, Induced Electrical Changes. 10-11, Other Theories of Feeling. 
 12, General Classification of the Feelings. 13, Changeable Charac- 
 teristics of all Feeling. 14-15, Pleasureable and Painful Tone of Feeling. 
 16, Forms of Common Feeling. 17-18, Feeling-Tone of Sensations. 
 19, Mixtures of Feeling. 20-22, Nature of the Emotions.! 23, 
 Peripheral Theory of the Emotions. ! 24, Teleological Value of the Emo- 
 tions. ! 25, The Intellectual Feelings. 26-27, The ^Esthetic Feelings. 
 28, Nature of the Sentiments. 29-30, Dynamogenic Effects of 
 the Emotions. ! 31, Automatic and Ideo-Motor Movements. !! 32-33, 
 Phenomena of Fatigue.!! 34-35, Causes of Fatigue. 36, The Kinds 
 of Fatigue. 37, The Refractory Period. 
 
 CHAPTER VIII 
 MEMORY AND THE PROCESS OF LEARNING 542-592 
 
 1, Memory as Recognitive, Inexplicable. 2-3, Processes Involved 
 in Memory. ! 4, Learning Among Invertebrates. 5, Learning Among 
 Vertebrates. ! 6, Process of Selection in Learning. ! 7, Adjustment of 
 Psycho-physical Mechanism. 8, Learning by Trial and Error. ! 9-10, 
 Learning by Ideas. ! 11, Solution of Mechanical Puzzles. ! 12, Acquire- 
 ment of Skill. 13-14, Analysis of Feats of Skill.! 15, The Rate of 
 Improvement by Practice. ! 16, Relation of Consciousness to Learning. 
 !! 17-19, Transference of Learning !! 20-21, Processes of Memorizing. 
 ! 22, Methods of Studying Memory. 23, The Curve of Forgetting. 
 ! 24-25, Forming of Association. ! 26, The Psycho-physical Mechanism. 
 27-28, The Culture of Memory. !! 29-31, Phenomena of Reproduction. 
 ! 32, Inhibition of Reproductive Processes. ! 33, Phenomena of Per- 
 severation. 34-35, Varieties of Association. 36-37, Recall with 
 Recognition. 38, The Guarantee of Memory. 
 
TABLE OF CONTENTS xlx 
 
 CHAPTER IX 
 
 PAGE 
 
 THE MECHANISM OF THOUGHT 593-625 
 
 1-2, Logical Terms Inadequate.! 3, Perception as a Form of Re- 
 action. 4, The Process of Abstraction. 5, The Span of Attention. 
 6-7, Shifting and Fluctuation of Attention. 8, The Direction of Atten- 
 tion. 9, Development of Abstract Concepts. 10, The Nature of 
 Comparison. 11, The Psychology of Reasoning. 12, Reasoning as 
 Search for Premises. 13, Immediate and Mediate Reasoning. 14-15, 
 Correlations of Mechanism and Mental Life. 16-18, Cerebral Conditions 
 of Consciousness. 19, The Mechanism of Attention. 20, The Power 
 of Varied Reaction. 21-22, Physiology of Local Signs. 23-25, 
 Cerebral Action in Memory. 26-28, The Mechanism of Associations. 
 29-30, Physiology of the Higher Units. 31-33, Collecting and Dis- 
 tributing Mechanisms. 34, Limitations of all Explanation. 
 
 PART THIRD 
 THE NATURE OF THE MIND 
 
 CHAPTER I 
 GENERAL RELATIONS OF BODY AND MIND 629-667 
 
 1-2, The Metaphysical Problem Proposed. 3, Nature of Uncriti- 
 cal Dualism. 4, Reality of Correlations. 5-7, The Brain as "Seat" 
 of the Mind. 8-10, The Brain as "Organ" of the Mind. 11, The 
 Conception of a Bond Between Brain and Mind. 12, The Body as the 
 Tenement of Mind. 13-14, The Conception of Product as Applied to 
 the Case. 15-16, Reciprocal Influence of Body and Mind. 17-18, 
 General Objections to Causal Theory. 19-20, Analysis of Conceptions. 
 21-23, Conservation of Cerebral Energy. 24, Some Kind of Causal 
 Relation Necessary. 25, Necessity of a Further Hypothesis. 26, The 
 Three Processes Involved. 27-29, The Conception of Development 
 Necessary. 30, Mental Condition of the Embryo. 31, Earliest Develop- 
 ment of the Child. 32, Development of the Adult. 33, The Changes of 
 Old Age. 34, Argument from Phenomena of Decay. 35, Peculiarly 
 Mental Elements. 36, Peculiarly Mental Operations. 37, The Con- 
 clusion Drawn. 
 
 CHAPTER II 
 REALITY AND UNITY OF THE MIND 668-687 
 
 1-2, Nature of the Claims Discussed. 3, Conceptions of Reality 
 and Unity Expounded. 4, Unity as Involving Plan. 5-6, Reality 
 and Unity of Mind, not Merely Phenomenal. 7-8, The Activity of 
 Apperception. 9-11, Mental Unity not Atomic. 12-13, The Dis- 
 tinction of Ego and Non-Ego. 14-15, Mind as Unifying Actus. 16, 
 Mind not a Static Unity. 17-18, The Spirituality of Mind. 19-20, 
 Mind as a Unity of Growth. 21-22, Limits of Physiological Psychology. 
 
 INDEX OF AUTHORS 689 
 
 INDEX OF SUBJECTS . . . 694 
 
PHYSIOLOGICAL PSYCHOLOGY 
 
 INTRODUCTION 
 
 1. A clear conception of Physiological Psychology requires some 
 knowledge of the nature and methods of those two sciences, the 
 results of whose investigations it endeavors to combine. These 
 sciences are Psychology and Physiology the latter being under- 
 stood so as to include also various applications of the general 
 theory of physics to the functions of the animal organism. But as 
 the form taken by this compound term would itself seem to indicate, 
 the two do not stand upon precisely the same level in effecting this 
 combination, whether we consider the end that science desires to 
 reach, or the means that it employs to reach the end. For the noun 
 (" psychology") in the compound term may be said more particularly 
 to define this end; while the adjective ("physiological") defines the 
 character of the means which it is proposed especially to employ. 
 Hence "Physiological Psychology" can scarcely claim to be an in- 
 dependent science; it is rather to be regarded simply as psychology 
 approached and studied from a certain the so-called "physiologi- 
 cal " side or point of view. It is necessary, then, in the first place, 
 to define what we understand by the science of psychology. 
 
 2. Perhaps the most common definition of psychology, until 
 recent times, has regarded it as " the science of the human soul." 
 If this definition had always been taken only in a provisional way, 
 and with the implied confession that it is the business of psychol- 
 ogy itself to demonstrate the existence of " the soul, " and to show 
 how such an entity is needed to explain the phenomena of con- 
 sciousness, then little valid objection could have been made to it. 
 But such has by no means been the case. Objections have, there- 
 fore, been more or less fitly and forcefully urged against this defi- 
 nition as ordinarily employed. It has been said that clearly we 
 have no right to assume any such entity as the soul ; and it has even 
 been claimed, especially of late, that there may be a "psychology 
 without a soul," and, indeed, that this kind of psychology is alone 
 worthy of being considered truly scientific. Further objection to 
 the same definition has been made in other quarters, because it 
 
2 ^INTRODUCTION 
 
 seeins^ito regard the question as settled, whether there may not be 
 more than one subject (or "ground") of the manifold phenomena 
 called psychical. Recent researches into so-called " subconscious- 
 ness" as involving mental processes which go on "below the 
 threshold," and theories of double and triple selves, have served 
 further to confuse or discredit the time-honored concept of a 
 soul, or mind, as a permanent and quasi-independent entity. It 
 would be aside from the course of our inquiries to consider these 
 objections in detail at this time. They may all be, for the present, 
 excluded by stating the course of procedure which the study of psy- 
 chology from the physiological point of view seems to us plainly to 
 recommend. 
 
 Accordingly, it will serve our purpose best to define our science, 
 in at least a preliminary way, by ascribing to it a certain more or 
 less definite sphere of phenomena. We shall, therefore, consider 
 psychology as that science which has for its primary subject of 
 investigation all the phenomena of human consciousness, or of the 
 sentient life of man. If the term "sentience" be employed as pref- 
 erable to consciousness, it must be understood as equivalent to 
 consciousness in the broader sense of the latter word. This defini- 
 tion, or rather description, plainly implies an acquaintance experi- 
 mentally with certain phenomena that cannot, strictly speaking, 
 be defined. These are the phenomena of consciousness; and one 
 result of all our subsequent investigations will be to show us that 
 consciousness and its primary phenomena can never be defined. 
 
 Nevertheless it would be very inconvenient, not to say impos- 
 sible, to begin and continue the investigation of psychical phenom- 
 ena, using only roundabout phrases through fear of implying the 
 real existence of some spiritual entity called the Soul or the Mind. 
 In some sort there cannot be any description, much less any scien- 
 tific study, of the phenomena of consciousness without employing 
 some word like these. In all languages, and in the constant every- 
 day use of them all, men in stating and describing the phenomena of 
 their own sentient life make the distinctions involved in such terms 
 as "I" and "me," and place in a kind of contrast with them such 
 other terms as "thou" and "he" or "it." In all the earlier part 
 of this treatise the word "mind" will, therefore, be employed simply 
 as the equivalent of the subject of the phenomena of consciousness. 
 In other words, whatever all men mean by the word "I" (the em- 
 pirical ego of philosophy), whenever they say I think, or feel, or 
 intend this or that; and whatever they understand others to mean by 
 using similar language thus much, and no more, we propose at 
 first to include under the term " mind." This term is preferred to 
 the word " soul," in part out of concession to the prejudices to which 
 
INTRODUCTION 3 
 
 reference has already been made, and in part because it seems to 
 admit of the handling which it is proposed to give to it subsequently, 
 with more freedom from entangling alliances with ethical, social, 
 and religious ideas. In brief, we wish to begin and continue our 
 investigation, as far as'possible, upon purely scientific grounds. 
 
 3. In accordance with what has already been said concerning 
 the nature of psychology, we may define Physiological Psychology 
 as the science which investigates the phenomena of human con- 
 sciousness with the "physiological" point of view and method of 
 approach; or, remembering the cautions which have already been 
 expressed, we may say that it is the science of the human mind 
 as investigated by means of its relations to the human physical or- 
 ganism. A more accurate definition, however, requires that some- 
 thing further should be said concerning the nature and method of 
 that science which furnishes the adjective to our compound term. 
 Human Physiology is the science of the functions (or modes of the 
 behavior) of the human physical organism. As studied at present 
 it implies an acquaintance with the fields of gross and special micro- 
 scopic anatomy (histology), of embryology and the general doctrine 
 of development, of biology, including the allied phenomena of 
 plant life, of molecular physics and chemistry as related to the 
 structure and action of the bodily tissues; and of other forms of 
 kindred knowledge. It is only a relatively small part of this vast 
 domain, however, with which Physiological Psychology has directly 
 to deal; for it is only a part of the human organism which has any 
 direct relation to the phenomena of consciousness. As will appear 
 subsequently, it is with the nervous system alone that our science 
 has its chief immediate concern. Indeed it might be described 
 though in a still somewhat indefinite, but more full and complete 
 way as the science which investigates the correlations that exist 
 between the structure and functions of the human nervous mechan- 
 ism and the phenomena of consciousness; and which derives there- 
 from conclusions as to the laws and nature of the so-called mind, 
 or subject of these phenomena. 
 
 4. Physiology is compelled, from its very nature as a physical 
 science, to regard the nervous system as a mechanism. Physiologi- 
 cal Psychology, inasmuch as it relies so largely upon physiology for 
 its data and method and points of view, is also required to consider 
 this system in the same way. Those unique relations in which the 
 structure and functions of the nervous substance of the body stand 
 to the phenomena of mental life cannot deter the investigator from 
 assuming toward it the so-called mechanical point of view. Physi- 
 ology presejUs^psychology with a description of this nervous sub- 
 stance as a vast and complex system of materialjnolecules, which 
 
4 INTRODUCTION 
 
 are acted upon by different forms of the energy of nature outside 
 (external stimuli), and by intimate changes in the contiguous mole- 
 cules of the other substances of the body (internal stimuli); and 
 which behave as they do on account of the influences thus received, 
 as well as on account of their own molecular constitution and ar- 
 rangement. But all this is the description of a material mechanism. 
 Indeed, it is only as falling under this general conception that these 
 molecules admit of any scientific treatment at all. 
 
 Whatever is to be said further upon the conception of the nervous 
 system as a mechanism must appear in its proper place in the order 
 adopted for the discussion of the general subject. Physiological 
 Psychology, however, can scarcely establish itself at all unless it 
 is willing to receive from the proper one of the two sciences which 
 enter into it the conclusions at which this science has arrived as the 
 result of the most successful modern researches. As far as the ner- 
 vous system admits of being subjected at all to scientific treatment, 
 for the purpose of attaining a more complete knowledge of the nature 
 of its functions, it is necessarily considered as a complex molecular 
 mechanism. We shall, then, receive, in a grateful and docile man- 
 ner, all that the noble science of human physiology has to teach us, 
 under the guidance of the conception of a mechanism, both directly 
 concerning the manner in which the nervous matter of the human 
 body performs its wonderful functions, and more indirectly concern- 
 ing the relations in which these functions stand to the facts and the 
 development of man's mental life. 
 
 5. The remark just made introduces the truth that there are 
 many indirect relations, which need investigation, between the phe- 
 nomena of human consciousness and the constitution and functions 
 of the human nervous organism. Indeed, a large portion of this 
 treatise will make little use of explanations derived directly from 
 the facts and laws of anatomy, histology, and physiology. The 
 entire Second Part falls more properly under the head of Psycho- 
 physics or Experimental Psychology, than under Physiological 
 Psychology, strictly so called. Still all investigators are convinced 
 that the phenomena of sensation, reproduction in the form of mem- 
 ory or imagination, association of ideas, and thought-processes, so 
 far as they have any basis or correlate in the physical world, are 
 either immediately or indirectly dependent upon the structure and 
 functions of that mechanism which human physiology investigates. 
 Only, in the great majority of cases, we have as yet no knowledge 
 of precisely how this mechanism behaves, so interior, hidden, and 
 subtle are its relations to mental phenomena. After making this 
 confession, we are only complying with a fairly well-established usage 
 in giving the ti tie "Physiological Psychology" to the entire treatise. 
 
INTRODUCTION 5 
 
 6. Physiological Psychology it is by this time apparent par- 
 takes of the nature and methods of two sciences that differ widely 
 from each other. One is a science which involves introspection; 
 for it is only by the method of introspection that the actual and pres- 
 ent facts of human consciousness can be reached. The other is a 
 physical science, and involves external observation to determine the 
 external facts of the structure, development, and functions of a 
 physical mechanism. Two sets of phenomena must then be exam- 
 ined in their relations to each other, and, so far as possible, the 
 laws (or permanent modes) of these relations pointed out. It is 
 due to this fact, in part, that both the peculiar difficulties and the 
 peculiar interest and value of psycho-physical researches are so 
 great. 
 
 In every science a beginning is first made by ascertaining and 
 comparing together all the important phenomena; the laws, or 
 regular modes of the occurrence of the phenomena in relation to 
 each other, are then investigated; and finally, certain conclusions 
 are drawn concerning the nature and significance of those real be- 
 ings which reason compels us to assume as permanent subjects of 
 the different classes of phenomena. In its effort to establish itself 
 upon a scientific basis, Physiological Psychology has no choice but 
 to follow essentially the same method of procedure. In its case, 
 however, as has already been remarked, the phenomena which are 
 to be ascertained and compared belong to two orders that obviously 
 differ greatly from each other; and the laws which it is sought to 
 discover are laws which maintain themselves between these two or- ^ 
 ders of phenomena. It has already been said that the phenomena 
 of the nervous system, like all physical phenomena, consist in changes 
 in the constitution and mutual relation of material masses and mole- 
 cules; and that the psychical phenomena are states of consciousness, 
 constantly shifting modes of the behavior of that subject which we 
 have agreed as much as possible without involving any premature 
 assumptions to call the Mind. Still the above-mentioned two 
 orders of phenomena are obviously to a large extent related to each 
 other; they may, in fact, be said to be correlated in a unique manner. 
 The constant forms of this correlation constitute the laws for the 
 discovery of which Physiological Psychology undertakes its special 
 researches. It endeavors to bring the two orders of phenomena 
 face to face, to look at them as they stand thus related to each other, 
 and, as far as possible, to unite them in terms of a uniform character, ^ 
 under law. 
 
 7. It might seem that simply to attempt the accomplishment 
 of so difficult and complicated a task as that just described should 
 satisfy all legitimate demands. And, again, we remind ourselves 
 
6 INTRODUCTION 
 
 that no little protest has of late been made against any introduc- 
 tion of metaphysics, whether in the form of assumptions or con- 
 clusions, into the science of psychology; especially when this science 
 is studied from the physiological and experimental points of view. 
 And has it not just been agreed that metaphysical assumptions 
 shall prejudice as little as possible our statement of psychological 
 facts and laws ? But our science, like every other science, has the 
 right to form and announce conclusions as to the real nature of 
 the subject-matter which it investigates, if these conclusions seem 
 to follow legitimately from its discussions of phenomena and laws. 
 It has even a right to indulge in well-founded and reasonable spec- 
 ulation. Such things are not necessarily objectionable when in- 
 dulged in by any of the more purely physical sciences. Indeed, 
 there is not one of these sciences which would not look compara- 
 tively bare and unattractive if wholly stripped of its more or less 
 questionable inferences, its metaphysical assumptions, its guessings 
 and speculations. 
 
 8. The remarks immediately foregoing serve to indicate what 
 are the principal divisions of this work. The First Part will con- 
 sist of a description of the structure and functions of the Nervous 
 System. In order that this system in man's case may be the better 
 understood, the first two chapters give a brief description of its place 
 in the developmental series. As has already been said, this system 
 will be considered under the conception of a mechanism, and as far as 
 possible without much direct or indirect reference to the phenomena 
 of consciousness as determined by introspection. An important 
 exception may seem to be made in the case of the chapters which 
 treat of the "Localization of Cerebral Function"; and which 
 thus bring forward the relations that have recently been estab- 
 lished in fact between the conditions and activities of the supreme 
 nervous centres and the phenomena of conscious sensation and 
 volition. 
 
 Again, reference is constantly made throughout this Part to the 
 phenomena of automatic and reflex action of the nervous system 
 as a whole; and so of necessity to certain correlated mental phe- 
 nomena. But such topics seem indissolubly connected with a satis- 
 factory description of the nervous mechanism; and to introduce 
 them here avoids much otherwise necessary repetition. 
 
 The Second Part discusses more particularly the relations which 
 exist between the quality, quantity, combination, and order of suc- 
 cession in time, of the various stimuli which act upon the nervous 
 system, and .the kind, magnitude, composite result, and time-re- 
 lations of the mental phenomena. Hence the significance of the 
 term psycho-physics. As Physiological Psychology is ordinarily 
 
INTRODUCTION 7 
 
 and legitimately treated, it includes these more specially psycho- 
 physical researches. 
 
 Besides the foregoing groups, or classes, certain observations 
 which have more or less of scientific confirmation and value, may 
 be made regarding the physical basis of the feelings and volitions 
 controlling the bodily members, and of the higher faculties of mem- 
 ory, association of ideas, etc. 
 
 The Third Part will fitly introduce, at the close of the psycho- 
 physical researches, the briefest possible presentation of such con- 
 clusions as may be legitimately gathered, or more speculatively in- 
 ferred, concerning the nature (considered as a real being) of the 
 human mind. The justification of the order and extent of the en- 
 tire discussion, and especially of the Third Part as a whole, has 
 already been given to some extent; the rest must be left to the prog- 
 ress and result of the discussion itself. 
 
 9. It has already been said that the peculiarity of Physiological 
 Psychology, considered as a branch of the general science of mind, 
 consists largely in the method of its approach to its subject. At- 
 tention must now be more specifically called to this method as 
 necessarily partaking of the methods of the two sciences whose 
 researches it undertakes to combine. The method of physiology, 
 which is in general that of external observation as employed in all 
 the physical sciences, should be applied only when supplemented by 
 the many delicate and accurate instruments of observation now at 
 command, and guarded and checked by that accumulation of expe- 
 rience concerning the best ways of studying nature and concern- 
 ing her ways of working which the whole body of such sciences 
 has made. On the other hand, the method of psychology has or- 
 dinarily been defined as solely the method of introspection or self- 
 consciousness. These two methods are obviously very different. 
 It would not be strange, then, if the science which finds it neces- 
 sary to combine the two should experience some special difficulty. 
 This difficulty has, however, more often been exaggerated than ex- 
 plained and (what is quite possible) for the most part removed. 
 
 Our present purpose does not require that we should examine at 
 length the question whether the introspective method is the only 
 one possible in psychology. Scarcely more is necessary than the 
 statement of the bearing of this question upon the inquiries it 
 is proposed to make. There should in general be no mystery or 
 arrogant assumption about the use of such words as "science" and 
 " scientific method." Science is nothing but knowledge real, veri- 
 fiable, and systematic. Scientific method is nothing but the way 
 of arriving at such knowledge. Now, although Physiological Psy- 
 chology brings the investigator face to face with some of the most 
 
8 INTRODUCTION 
 
 interesting and distinctive mysteries, it is not, as a science, to be 
 regarded as especially mysterious. Inasmuch as its specific busi- 
 ness is to ascertain and combine, under definite laws, two widely 
 differing classes of facts (facts of the human nervous mechanism and 
 facts of human consciousness), it is, of course, compelled, first of all, 
 to ascertain both kinds of facts. The phenomena of consciousness, 
 as primary facts, can be ascertained in no other way than in and by 
 consciousness itself. There is no way of directly examining con- 
 sciousness but the way of being conscious one's self. On the other 
 hand, it is perfectly obvious to students of psychology and of its 
 history (on grounds which need not be stated here) that the scientific 
 treatment of the facts of consciousness can never be, to any satis- 
 factory extent, accomplished by introspection alone. For psy- 
 chology, in order to make valid its claim to be a science, must not 
 merely display the alleged facts of individual mental experience; 
 it must treat these facts analytically, must resolve them into their 
 ultimate factors, and trace the stages of their development from what 
 is simpler to what is more complex; it must also show on all sides 
 their connections and causes, thus placing the phenomena of 
 the mind as much as possible in interaction with the rest of the 
 world. 
 
 The following statements will, accordingly, be found to hold 
 good concerning the method of Physiological Psychology. It must 
 employ faithfully the methods distinctive of both the two sciences 
 which it endeavors to combine. Facts as to the structure and 
 functions of the nervous mechanism, and as to the effect upon it of 
 various kinds of physical energy acting as stimuli, must be ascer- 
 tained by external observation. The primary facts of conscious- 
 ness must be ascertained from consciousness itself; or, since they have 
 already been for a long time subjected to this form of observation, 
 and tabulated, compared, and classified, they may be accepted 
 from the science of introspective psychology. Care must be taken, 
 however, to make sure that all alleged psychical facts are really 
 facts; but upon this point, again, there is no other way of making 
 sure than in and through consciousness. The final result of re- 
 search will doubtless be, not only to supplement and explain, but 
 also to modify and correct, the previous statements of psychologi- 
 cal science as to its laws and inferences. But here, as in other sci- 
 entific research, we shall be obliged to work our way through many 
 mistakes, obscurities, and other obstacles, progressively nearer the 
 complete and verifiable knowledge of the truth. 
 
 10. What has already been indicated will become more evi- 
 dent in the course of the following investigations namely, that 
 we are seldom or never able to proceed directly with the work of 
 
INTRODUCTION 9 
 
 comparing the immediate physical antecedents or consequents of 
 the mental phenomena with these phenomena themselves, and so 
 of drawing conclusions at once as to the laws by which the two 
 classes of facts are connected. Such immediate antecedents and 
 consequents are hid in the inexplorable recesses of the living and 
 molecularly active brain. It is seldom, indeed, that our direct ob- 
 servation can approach within the tenth, or it may be within the 
 hundredth, remove of what goes on in these recesses. We are 
 obliged to examine the physical phenomena from a greater distance 
 and in a more indirect way. For example, physics can inform us 
 what combinations of what wave-lengths of the vibration of ether 
 fall on the eye when a certain form of conscious sensation, which 
 we call "yellow" or "red" or "blue," arises; physiology can locate 
 the nervous elements of the retina upon which the waves fall, can 
 conjecture something as to the chemical changes there produced, 
 and trace doubtfully the paths along which the resulting nervous 
 impulses rise to the brain and diffuse themselves over certain of 
 its areas; psycho-physics can tell approximately the relations in 
 which the varying quantities of the stimulus stand to the resulting 
 degrees of the sensations. But in all this we are still at a great dis- 
 tance from the enjoyment of those opportunities which would seem 
 necessary to make the science of Physiological Psychology as com- 
 prehensive and exact as could readily be wished. As a rule, certain 
 kinds and amounts of physical energy, more or less definitely meas- 
 urable, are known to be acting on the peripheral parts of the body, 
 and the next series of observed facts is the emergence in conscious- 
 ness of a psychical experience quite unlike all kinds of physical en- 
 ergy. To be sure, Fechner's l conception of psycho-physics is that 
 it treats those " physical activities which are the bearers ( Trager) or 
 conditions of the psychical, and accordingly stand in direct func- 
 tional relation with them"; or again, "psycho-physics is an exact 
 doctrine of the relations of function or dependence between body 
 and soul of the universals that lie between the bodily and spirit- 
 ual, the physical and psychical world." But the course of investi- 
 gation will make clear the fact that of such physical activities we 
 have little or no assured knowledge; although we have the best of 
 grounds for believing that such activities exist, and that they stand 
 in important relations under law with the facts of the conscious 
 psychical life. 
 
 11. If the correctness of the remarks last made be admitted, the 
 inquiry may be raised: What justification has this so-called sci- 
 ence of Physiological Psychology for the large claims which it has 
 made of late; and, indeed, what right has it to exist as a special 
 l Elemente d. Psychophysik, pp. 8 and 10 (Leipzig, 1860). 
 
10 INTRODUCTION 
 
 discipline at all? The full answer to the call for self-justification 
 must be made by the actual achievements of the science itself. 
 
 The history of modern investigation, and the conclusions of the 
 modern science of man, both physical and psychological, emphasize 
 the necessity of studying his nature and development as that of a 
 living unity. Such science shows man to be at the head of a series 
 of physical and psychical existences; he cannot be understood as 
 he is, in his whole nature and in his place within nature at large, 
 without taking both sides of this living unity into account. For 
 man is known to himself as body and mind and not as bodiless 
 spirit or a mindless congeries of moving molecules. That the struct- 
 ure and functions of the body, especially of the nervous mechan- 
 ism, and the activities of the mind, are extensively and intimately 
 correlated, is a fact beyond all doubt. It is the particular task of 
 Physiological Psychology to show in what manner, and to what ex- 
 tent, such correlation exists. .Moreover, there are few questions 
 more interesting, from a philosophical and an ethical point of view, 
 than such as the following: What is the nature of mind, considered 
 in the light of its correlations with the body? and, Do the so-called 
 physiological and the so-called psychical phenomena belong to one 
 subject, or to more than one ? But these and similar questions can 
 be scientifically answered only by giving a speculative treatment 
 to the conclusions of psycho-physical investigation. 
 
 In brief, it may be said that introspective psychology, important 
 as its results have been, and indispensable as its method is, has 
 shown its incompetency to deal with many of the most interesting 
 inquiries which it has itself raised. On the other hand, psychology 
 as pursued by the experimental and physiological method has al- 
 ready thrown a flood of fresh light upon many of these inquiries. 
 We may affirm with Wundt, 1 without fear of successful contradic- 
 tion: "Psychology is compelled to make use of objective changes 
 in order, by means of the influences which they exert on our con- 
 sciousness, to establish the subjective properties and laws of that 
 consciousness/' On this fact and on the real achievements of the 
 method we confidently rest its claims to serious and permanent con- 
 sideration. 
 
 'Art., "Ueber psychophysischen Methoden," Philosophische Studien (1881), 
 Heft 1, p. 4. 
 
PART FIRST 
 THE NERVOUS MECHANISM 
 
CHAPTER I 
 
 THE PLACE OF THE NERVOUS SYSTEM IN THE ANIMAL KINGDOM 
 
 1. It is essential to the development of a physiological psy- 
 chology that the part played by the nervous system in animal life 
 and the manner of its working should be understood, as far as pres- 
 ent knowledge permits. As has already been explained in the In- 
 troduction, we wish to study man's mental life in its relation to the 
 nervous system and, in general, to the life of the organism; we wish 
 to understand the functions of the nervous system in their relations 
 to mental phenomena. In such a study, the comparative method 
 should prove of much assistance. For both the nervous structures 
 and the mental activities of man are so enormously complex that 
 the discovery of their relations is of necessity a very difficult task. 
 If, however, we turn to animals of a lower grade, with much less 
 complexity of nervous organization and functions, we may hope to 
 glean some facts, of a fundamental sort, regarding the most gen- 
 eral relation of structure to behavior. We should, at the outset of 
 such a study, lay aside for the time our psychological preoccupa- 
 tion with such complex mental performances as imagination, rea- 
 soning, and will, and limit ourselves to an objective examination of 
 the simpler behavior of lower types of animal which present little 
 indication of these complex activities. We should, at the same time, 
 banish any preconceived notion, founded on human experience, 
 that the nervous system is essentially the servant, or the organ, of 
 mental life, and look for its primary function in animals that possess 
 the most rudimentary nervous structures, and for its increase in 
 function with increase in the complication of its structure. We 
 shall, of course, be unable to present in this book a full account of 
 the comparative anatomy of the nervous system and of the behavior 
 of different classes of animals both of which studies have been 
 zealously prosecuted of late years but must limit ourselves to a 
 brief sketch. 
 
 2. The primary division of animals distinguishes two classes: 
 the protozoa and the metozoa. The protozoa are described as 
 "unicellular"; while the metozoa are composed of a number, usu- 
 ally a great multitude, of cells. A "cell," in the biological sense, is 
 a microscopic bit of semi-fluid matter, bounded either by a definite 
 
 13 
 
14 THE NERVOUS SYSTEM IN THE ANIMAL KINGDOM 
 
 membrane or at least by a "surface of separation," which has the 
 important property of resisting free diffusion of dissolved sub- 
 stances. Though cells will live either when immersed in the water 
 or else, as in higher animals, when bathed in the fluids of the body, 
 substances dissolved within the cell do not freely pass out; nor do 
 dissolved substances from without freely pass in, through the sur- 
 face of the cell. There is, of course, much interchange of materi- 
 als between the inside and the outside of the cell; such interchange, 
 however, is not free, but selective. Certain substances can pass in, 
 others out. No property of the cell is more necessary to its life 
 than this; for, if the cell boundary interposed no barrier to free dif- 
 fusion, the cell would promptly lose its individuality and be merged 
 in the surrounding medium. The semi-fluid matter of the cell, 
 called "protoplasm," consists of a large proportion of water, in 
 which are dissolved salts of sodium, potassium, and calcium, as well 
 as the highly complex compounds of carbon, nitrogen, hydrogen, 
 oxygen, and sulphur, which are called proteins. Other organic com- 
 pounds, and other salts, may also be present. Within this mass of 
 protoplasm is a smaller mass called the nucleus, which appears to 
 be more nearly solid than the protoplasm, and which has a some- 
 what different chemical composition, inasmuch as its protein con- 
 tains phosphorus, in addition to the other elements mentioned. 
 Some cells, indeed, have no nucleus, but these do not possess the 
 power of reproducing their kind, and it is doubtful if they should 
 be regarded as true cells, in the full sense of the word. The 
 nucleus, the protoplasm, and the surface of separation, may there- 
 fore be regarded as the essential parts of every living cell. 
 
 3. As an example of a unicellular animal we may take the 
 amoeba, a minute spherical creature, which lives in stagnant water, 
 feeding on the organic matter contained in the water. It maintains 
 its own chemical composition, which is different from that of the 
 water; it takes in organic matter from the water, changes or "as- 
 similates" it, and so grows; and it excretes waste products. Besides 
 these distinctly chemical processes, it shows the phenomenon of re- 
 production. When it has grown to a sufficient size, it divides into 
 two "daughter cells," each of which is an amoeba. It has further 
 the power of motility; its movements being of two opposed sorts, 
 one consisting in the bulging outward of some part of its surface 
 into a temporary arm or branch, while the other consists in the draw- 
 ing in of these temporary arms and the resumption of the spher- 
 ical shape. Certain stimuli cause it to exhibit one of these move- 
 ments, and other stimuli cause it to exhibit the other. A bit of 
 food in its neighborhood, that sends out particles through the water, 
 acts as a stimulus to the putting forth of a branch; the amoeba 
 
DESCRIPTION OF THE AMOEBA 15 
 
 bulges out on the side toward the food, and then slowly flows into 
 the protruding arm, thus advancing toward the food. When the 
 bit of food is reached, two or more arms bulge out around it, and 
 unite on the further side, enclosing the food within the amoeba's 
 body. On the other hand, a sudden contact with a solid body, or a 
 jar to the water, acts as a stimulus to the drawing in of the tempo- 
 rary arms (see Fig. 1). 
 
 Such is animal behavior when observed and described in its low- 
 est terms. If we leave aside the chemical and reproductive ac- 
 tivities of the amoeba, we may say that its behavior exhibits the 
 powers of motility, irritability or sensitivity, and conductivity. " Ir- 
 
 Fio. 1. The Amoeba. (Verworn.) Four stages in the ingestion of a food particle. 
 
 ritability" refers to the fact that the animal moves in response to 
 forces acting on it; these forces do not actually move the animal, as 
 the current of the water moves it, but they arouse its inherent power 
 of motion. A force acting to arouse an inherent power of an animal 
 is called a stimulus; and the movement so aroused is called a re- 
 sponse or "reaction" to stimulus. The stimulus may be said to 
 "discharge" the movement, as the blow of a trigger discharges a 
 cartridge. "Conductivity" refers to the fact that the part of the 
 animal's body which moves need not be the same as the part which 
 receives the stimulus; there must, therefore, be a conduction or 
 transmission of the excitement through the body. The stimulus 
 acts directly on one side of the body, which responds by bulging 
 outward; but the response spreads to remoter parts, till all of the 
 protoplasm is in motion. It is not the external stimulus which is 
 transmitted, but some sort of activity lying within the body itself. 
 
 4. In the amceba there are no organs to take care of the differ- 
 ent functions; no part of the protoplasm, more than any other, is 
 specially concerned with digestion, or with motion, or with receiving 
 stimuli, or with conduction. There is a complete absence of di- 
 vision of labor, of specialization or differentiation of parts. Every 
 part of the surface can receive stimuli and be thrown into activity 
 by the stimuli; every part of the protoplasm can conduct the exci- 
 tation to adjoining parts; and every part can respond by motion. 
 
16 THE NERVOUS SYSTEM IN THE ANIMAL KINGDOM 
 
 On the other hand, the progress from lower to higher types of 
 animal is marked, not only by increase in size, but by increasing 
 specialization of the parts. Every multicellular animal begins its 
 individual life as a single cell. This divides into two; and the daugh- 
 ter cells divide in their turn, and so on, as will be more fully de- 
 scribed in the following chapter. The many cells generated in 
 this manner remain adherent to one another; and as their multi- 
 plication proceeds, it is seen that they begin to manifest character- 
 istic differences one from another. They differ both in* visible 
 appearance and also in their powers. Comparatively few retain 
 full reproductive power, so as to be capable of giving rise to a new 
 individual. Some groups of cells become specialized in their chem- 
 ical powers, and produce powerful digestive fluids. Others become 
 specialized in the power of motion and develop this power to a much 
 higher degree than that possessed by the amoeba. Some groups of 
 cells, lying for the most part on the surface of the animal's body, be- 
 come specialized in the line of irritability; that is to say, they become 
 highly sensitive to specific stimuli, such as contact and jarring, or 
 light, or certain chemical substances. Still other groups of cells 
 develop to a high degree the power of conduction. The cells which 
 have specially developed powers of receiving stimuli are called " re- 
 ceptors"; those with special powers of conduction may be called 
 "conductors"; and those with special powers of movement, along 
 with others which respond to stimuli with chemical or (in certain 
 species of animals) electrical effects, are called "effectors." The 
 receptors form the essential part of the sense-organs ; the conductors 
 of the nerves; and the effectors of the muscles and other organs 
 which produce effects of importance to the animal. 1 
 
 5. In a general way, the specialization both of receptors and also 
 of effectors, in different orders of animals, keeps pace one with the 
 other. Where, on the one hand, the receptors are adapted to a 
 great variety of stimuli, the effectors are, on the other, numerous 
 and capable of a great variety of effects. Perhaps, on the whole, in 
 the lower forms of animal life, the development of effectors keeps 
 
 1 The use of the term "receptors" in place of the more familiar "sense-organs" 
 is justified by the fact that a sense-organ, such as the eye or ear, contains many 
 accessory structures in addition to the sensitive cells which receive the stimuli; 
 and by the further consideration that "sensation" properly implies conscious- 
 ness; and we do not know, or wish to imply, that all animals provided with 
 receptors are conscious; while on the other hand we do know, in the case of 
 human beings, that many stimuli acting on receptors in the internal organs, 
 and producing reactions, do not give rise to distinguishable sensations. The 
 use of the term "effectors" is justified by the frequent need of including under a 
 single term glands and other organs which, as well as muscles, often execute the 
 animal's response to a stimulus. 
 
SPECIALIZATION OF RECEPTORS AND EFFECTORS 17 
 
 somewhat ahead of the development of receptors. Every effector 
 cell is a receptor as well, to this extent, at least, that it can be aroused 
 by the direct application to it of what are called the " general stim- 
 uli," such as mecnanical jars, sudden changes of temperature, electric 
 shocks, and certain chemical agents. In sponges, which are re- 
 garded as the lowest, or least differentiated, among the metazoa, there 
 are two sorts of motor cell, one 
 of which, lying along the sides of 
 the pores that run in from the 
 exterior of the sponge, is pro- 
 vided with vibratile hairs or 
 "cilia"; these, continually lash- 
 ing inward, produce currents of 
 the sea water in through the 
 pores to the interior cavity of the 
 sponge. The other motor cells 
 form a ring or "sphincter" 
 around the 'mouth of the cavity 
 at the top of the sponge, and by 
 contracting close the mouth and 
 check the circulation of water 
 through the sponge. This oc- 
 curs in response to stimuli ap- 
 plied directly to the sphincter. 
 But the cilia in the pores are not 
 checked in their activity by this 
 stimulus when thus applied; they 
 still continue to lash the water, 
 though without effect, since the 
 outlet is closed. The sponge, 
 then, show,s some specialization 
 of effectors, but little if any of 
 receptors, and none of a con- 
 ducting mechanism which would 
 
 bring the cilia into harmonious action with the sphincter. In other 
 words, the sponges possess no nervous tissue, and show no con- 
 duction from one cell or group of cells to another; in general, 
 they exhibit an almost complete lack of co-ordination (see Fig. 2). 
 6. A nervous system is met in its most elementary form in the 
 ccelenterates, of which the jelly-fish may be taken as an example. 
 The body of the jelly-fish may be roughly described as consisting of 
 the "umbrella," which lies uppermost in the usual position of the 
 jelly-fish, and is a gelatinous mass, with no power of sensitivity or 
 motility. The motor organs lie partly in a circular band which is 
 
 FIG. 2. Diagram of a Sponge. (Parker.) 
 The narrow pores on the side are lined 
 with cilia, lashing the water inward; the 
 large opening at the top is surrounded by 
 the sphincter. 
 
18 THE NERVOUS SYSTEM IN THE ANIMAL KINGDOM 
 
 attached around the rim of the umbrella, and partly in the tenta- 
 cles which hang down at intervals from the rim, and in the mouth 
 stalk suspended from the centre of the animal. As to sense-organs, 
 the tentacles are sensitive, and so is the circular band; and special- 
 ized receptors of several kinds are arranged at intervals around the 
 rim. There is also a simple form of nervous system, lying princi- 
 pally in the circular band, and composed of a special form of cells 
 
 with long slender branches. The 
 uniting or "anastomosing" of 
 each cell with the branches of 
 neighboring cells forms a network 
 which extends all through the 
 circular band around the rim, 
 and also into the tentacles. 
 Branches of this "nerve-net" 
 pass to the specialized receptors 
 along the rim, and to all parts of 
 the sensitive surface and to all 
 parts of the muscular tissue (see 
 Fig. 3). Thus every receptor and 
 every muscle-cell is connected 
 with the nerve-net, and all parts 
 of the net are mutually connected, 
 so that the net forms a universal 
 medium of connection between 
 receptors and effectors. The in- 
 ternal structure of the cells and 
 branches forming the nerve-net 
 
 is highly specialized, the principal feature being the presence of 
 fine fibrils " neurofibrils " extending along the branches from one 
 cell to another or from one branch into another. 
 
 7. To understand the function of the nerve-net in these ani- 
 mals, their behavior under natural and experimental conditions 
 should be briefly considered. The swimming action of the jelly- 
 fish consists of rhythmical contractions of the musculature in the 
 circular band, all portions of which act in unison. This rhythmical 
 movement is aroused by influences proceeding from the specialized 
 receptors in the rim, for if these are all cut off the movements cease. 
 They do not cease, if all but one are cut off, but remain the same, 
 no matter which one has been retained. This fact shows an absence 
 of specialization in the reactions to different receptors. Any small 
 portion of the body, containing a receptor along with some of the 
 muscle and nerve-net, will execute the same rhythmic movements, 
 showing that any part of the nerve-net can do the same sort of work 
 
 FIQ. 3. The Jelly-Fish (schematic). (Bethe.) 
 V, umbrella; B, circular band, containing 
 N, the ring of nervous tissue; R, recep- 
 tor; T, tentacle; M, mouth stalk. 
 
NERVE-NET TYPE OF NERVOUS SYSTEM 
 
 19 
 
 as the whole net. If all but one of the special receptors are removed, 
 and then a further cut is made through the circular band on one 
 side of the remaining receptor, the whole band will even then exe- 
 cute its rhythmic movements; and it makes no difference on which 
 side of the receptor the division is made. This shows that the con- 
 duction is equally good in both directions. Moreover, even if many 
 cuts are made part-way through the circular band, in such a manner 
 as to leave the band of a zig-zag shape, the rhythmic contractions 
 will still spread from the remaining receptor throughout the band, 
 provided only that the 
 divisions are not com- 
 plete. This shows that 
 the conduction can go 
 around corners and in 
 every direction through 
 the circular band. That 
 it is the nerve-net which 
 supplies the means of 
 conduction is shown by 
 experiments in which a 
 complete separation is 
 made between two parts 
 of the muscular tissue, 
 without division of the 
 
 nerve-strands connecting the two parts; the conduction is not inter- 
 fered with so long as the nervous connections are left (see Fig. 4). 
 
 One further fact should be brought forward to complete the de- 
 scription of the conductive process. If a tentacle is gently stimu- 
 lated by lightly touching it with a glass rod, it responds by a slight 
 muscular contraction confined to the part touched. If the touch 
 is a little heavier, the muscular reaction involves a larger part of 
 the tentacle. If the touch reaches the intensity of a slight blow, 
 the whole tentacle responds; and if the blow is made stronger and 
 stronger, the reaction spreads to other tentacles, to the foot, and 
 finally to the swimming muscles. 1 
 
 8. The facts have now been presented with sufficient detail to 
 permit of a proper conception of the nervous system of the jelly- 
 fish and other coelenterates. Anatomically, it is a diffuse network, 
 continuous throughout, and connected with all receptors and with 
 all motor organs. Experiment shows it to have no functional cen- 
 tre, all parts of it being equivalent. It conducts alike in all direc- 
 tions, and serves to bring about a general contraction of the mus- 
 
 1 See A. Bethe, Allgemeine Anatomic und Physiologic des Nervensy stems, 1903, 
 p. 110. 
 
 FIG. 4. The Nerve -Net (somewhat schematic). 
 (Bethe.) R, receptor; M F, part of circular band 
 free from muscle. 
 
20 THE NERVOUS SYSTEM IN THE ANIMAL KINGDOM 
 
 culature when any receptor is excited. The observations on the 
 tentacles, however, show a stronger conduction to neighboring than 
 to more distant parts; inasmuch as the spread of the reaction oc- 
 curs only within narrow limits when the stimulus is feeble, and ex- 
 tends more and more widely as the strength of the stimulus is in- 
 creased. Universal and indiscriminate conduction, limited only by 
 a dying out of the influence conducted with the distance traversed, is 
 the characteristic of this simplest type of nervous system. 
 
 9. This ccelenterate type of nervous system may be called the 
 "nerve-net type," because of the branches connecting the nerve- 
 cells; it is also called the "diffuse type," because of its being spread 
 widely through the epithelium, and also because of its diffuse con- 
 duction. The first distinct sign of centralization is met in the ner- 
 vous system of the flatworms. In them a large proportion of the 
 nerve-cells is located in two bunches or ganglia, situated at the for- 
 ward or mouth end of the animal. The centralization here is far 
 from complete; for many nerve-cells lie diffusely at the surface, and 
 these cells are united by their branches into a network, as in the 
 jelly-fish. In mollusks the condition is much the same as in the flat- 
 worms; although rather more concentration into ganglia is visible. 
 The nerve-net is still complete enough to make possible co-ordi- 
 nated contractions of the muscles of the whole body, even after the 
 removal of the "centres" or ganglia. The significance of the cen- 
 tral system is seen, however, from the fact that reactions of parts dis- 
 tant from the point of stimulation are much more prompt when the 
 ganglia are intact than when they are removed. The essential 
 point of advantage in the ganglionic system of these creatures ap- 
 pears to be that the ganglia can be connected with the muscles and 
 receptors by long strands of nerve, and not simply by the short 
 branches which connect the cells of the nerve-net. These long 
 strands thus afford more rapid conduction between distant parts 
 than is provided by the net. 
 
 10. In contrast with the diffuse nerve-net of the coelenterates, 
 and the partially modified net of the flatworms and mollusks, the 
 nervous system of the higher, segmented worms or annelids, and of 
 the arthropods (crabs, insects, etc.) and vertebrates, may be called 
 a centralized system. Nerve-nets are not indeed entirely absent 
 from these higher forms of animal life; they persist in certain se- 
 cluded and protected situations, such as the walls of the blood-ves- 
 sels, heart, stomach, and intestines of vertebrates, including man. 
 But the receptors of the external surface of the body, by which it 
 is brought into relation with its environment, and the muscles that 
 move the body or its parts and so produce external effects, are no 
 longer, in these higher forms, connected with each other by a diffuse 
 
SEGMENTED TYPE OF NERVOUS SYSTEM 
 
 21 
 
 net of anastomosing nerve-cells. Instead, the receptors are con- 
 nected by long fibres with masses of nerve-cells lying along the 
 middle line of the body (these animals all being bilaterally sym- 
 metrical), and the muscles are likewise connected by similar fibres 
 with the central masses of cells. The fibres which connect the cen- 
 tres with the receptors and effectors are characterized by fibrils 
 running lengthwise within them, and are true nerve-fibres, as in the 
 nerve-net. Such fibres, however, do not anastomose with each other 
 
 CG 
 
 FIG. 5. Diagram of the Nervous Systems of (4) the Flatworm, of (B) the Earth- 
 worm, and (C) the Mollusk. (Bethe.) CG, cerebral ganglion. 
 
 outside of the centres, but, where they are gathered into the 
 bundles which are called nerves, each maintains its individuality, 
 somewhat as the wires bound together in a telephone cable 
 remain separated from each other. Connections between the 
 fibres from receptors and those to effectors are made only in the 
 centres. 
 
 Further: the manner of communication within the centres them- 
 selves differs from that observed in the nerve-net. In the nerve-net, 
 the branches of different cells unite; in the nerve-centres, there seems 
 to be no union of the branches of different cells ; at least, this is true 
 of all the stouter branches. If there is any real continuity between 
 different cells in the nerve-centres, it must be due to passage from 
 one to another of extremely fine strands extremely fine even as 
 tested by the highest powers of the microscope. But the very fine 
 
22 THE NERVOUS SYSTEM IN THE ANIMAL KINGDOM 
 
 fibrils, which course through the nerve-cells and along their branches, 
 would seem, from the observations of some good authorities, to 
 have a certain independence, and to pass freely from one nerve-cell 
 to another, forming between the cells an exceedingly tenuous and 
 intricate network, a network of much more delicate strands than 
 those which form the nerve-net. The existence of such an extra- 
 cellular network of fibrils has been made out, with some probability, 
 in worms and perhaps in crabs; in vertebrates, however, it has not 
 been demonstrated to the satisfaction of those best qualified to 
 judge; and many authorities are disposed to believe that no complete 
 continuity exists between the different cells, but only a close con- 
 tiguity of their fine branches. The evidence on this point will be 
 presented in a later chapter on the nerve-cell. Whatever may be 
 the truth of this difficult question, there is no doubt that the path 
 of communication existing in the centres of the higher animal forms 
 including the worms is less diffuse and indiscriminate than that 
 which obtains in the primitive nerve-net. There is less of wide- 
 spread distribution of the influence coming from a receptor to all 
 the musculature. In other words, the influence is confined within 
 narrower limits, and accordingly more specialization and greater 
 variety of movements result. This is one cardinal feature of the 
 centralized form of a nervous system. Another is the existence of 
 long fibres, which afford quicker connection between distant parts 
 of the body than occurs through the primitive nerve-net. 
 
 11. From these general considerations we may turn to a brief 
 sketch of the nervous system and behavior of the annelids, taking 
 as a simple example the earthworm (see Fig. 5). Like other anne- 
 lids, and like the arthropods and vertebrates as well, the body of the 
 earthworm is built on a segmental plan, the segments succeeding 
 one another along the length of the animal, and being visible, in the 
 earthworm, in the ringed appearance of the animal's surface. Each 
 segment, except at the front and back ends of the worm, is much like 
 every other, and each is fairly complete in itself. Each has a ring 
 of muscle, little protruding bristles which are of use in locomotion, 
 and a set of simple sense-organs. From the sense-organs and from 
 the muscles nerve-fibres run to pairs of ganglia within the segment, 
 one of each pair lying on the right and one on the left. The nerve- 
 fibres connecting with the receptors are branches of the receptive 
 cells; those connected with the muscles are branches of cells lying 
 within the ganglia. Connections within the ganglia are formed, as 
 above described, between the fibres from the receptors and those 
 issuing to the muscles; and thus a local reflex path is provided, by 
 means of which stimuli affecting the receptors of the segment arouse 
 activity in the muscles of that segment (see Fig. 6). 
 
SEGMENTED TYPE OF NERVOUS SYSTEM 
 
 23 
 
 Local reflexes can be obtained from single segments cut out of a 
 worm of this species. But the segments are not entirely independent 
 
 FIG. 6. Nerve-Cells and Fibres in Ganglia of the Earthworm. (Retzius.) 
 Only a few out of many cells and fibres are shown. A motor fibre 
 is indicated by an outward-pointing arrow, and a sensory fibre by 
 an inward-pointing arrow. 
 
 of one another, nor are their ganglia without mutual connections. 
 Between each ganglion and that of the segment next in front or be- 
 
24 THE NERVOUS SYSTEM IN THE ANIMAL KINGDOM 
 
 hind is a strand of nerve-fibres; so that the ganglia form a connected 
 chain extending along the worm near its ventral or under surface. 
 The nerve-fibres which compose the longitudinal strands connect- 
 ing the ganglia are, for the most part, derived neither from the re- 
 ceptor cells nor from the nerve-cells which send out branches to the 
 muscles, but from another class of nerve-cells, which lie in the 
 ganglia and send their branches in a longitudinal direction from one 
 ganglion to the next or to the next but one or two. These central 
 fibres connect neighboring ganglia with each other, and by the series 
 of them in successive ganglia the whole chain is made a continuous 
 conducting medium. There are also a few much longer fibres, which 
 connect distant ganglia directly with each other. At the forward 
 end of the worm there is somewhat of a departure from the regular 
 segmental scheme; a ring of nervous tissue extends from the fore- 
 most pair of ganglia around the gullet and joins these ganglia to a 
 single ganglion lying on the dorsal side, which is called the cerebral 
 ganglion or " brain." In the earthworm this so-called brain is not 
 highly developed. 
 
 Experiment shows that the chain of ganglia is the only path of 
 communication between the receptors and effectors of the worm. 
 If the ganglia of a segment are cut out, that segment is paralyzed: 
 its muscles no longer respond to stimuli applied to any receptor, 
 and its own receptors, on being stimulated, give rise to no reac- 
 tion in any part of the body. If the longitudinal cord connecting 
 the ganglia is severed at any point, no stimulus applied to the part 
 of the worm forward of the break arouses any response in the part 
 lying behind the break; and vice versa; the waves of muscular con- 
 traction that sweep backward over the animal in its creeping, stop 
 at the point where the cord is severed, and all co-ordination be- 
 tween the front and rear parts is abolished. We find in the earth- 
 worm three main classes of movement: (1) local reflexes, which 
 can be carried out by the receptors, effectors, and connecting nerve- 
 fibres of a single segment; (2) slowly moving waves of muscular con- 
 traction, the impulse to which is probably transmitted from segment 
 to segment by the short fibres of the longitudinal cord, and (3) 
 sudden jerks of the whole worm, which are probably due to con- 
 duction along the few long central fibres. 
 
 12. As compared with the lowly earthworm, the higher forms 
 of annelid, and crustaceans and insects, while preserving the same 
 general type of segmented nervous system, show a superior nervous 
 development in many respects. There is an increase in the number 
 of cells in the ganglia, which is partly to be explained by the greater 
 complexity of the sensory and motor apparatus. There is a still 
 greater increase in the amount of branching of the nerve-cells within 
 
CENTRALIZED TYPE OF NERVOUS SYSTEM 25 
 
 the ganglia, which probably means a higher organization of the con- 
 nections between fibres. There is a great increase in the number 
 of long fibres within the ganglion chain, and a high development of 
 the cerebral ganglion. Both of the last-mentioned changes are to 
 be interpreted in the light of the development of the head, which in 
 the earthworm is not much of an affair, but which becomes richly 
 equipped with various organs in crabs and insects. The head is 
 first of all the mouth end of an animal, and as such it is natural that 
 it should go first in locomotion, and that it should have a certain 
 predominance over the rest of the body. Around the mouth are de- 
 veloped motor organs to serve it, and sense-organs concerned with 
 the finding and testing of food; and thus the importance of the head 
 is progressively increased. 
 
 The nervous connections must keep pace with the development 
 of receptors and effectors, and thus it is that the cerebral ganglion 
 grows in size, and becomes more and more dominant over the ganglia 
 lying behind. It is, further, to be expected that such highly special- 
 ized receptors as the eye should appear in this same region. The eye 
 is the type of a "distance receptor"; its importance lies in the fact 
 that it is excited by objects at a greater or less distance from the 
 surface of the body. With the appearance of distance receptors 
 comes a great increase in the range of the environment that can be 
 reacted to; and thus the life of the animal comes to be largely dom- 
 inated by these organs. Since the distance receptors are located in 
 the head, the importance of the cerebral ganglion and its connections 
 is still further increased; and to this is to be ascribed, not only the 
 increase in the size of the ganglion, but the increase in the number 
 of long fibres leading backward from it, and bringing distant mus- 
 cles into quick communication with the sense-organs of the head. 
 
 13. The vertebrates fishes, amphibia, reptiles, birds, and mam- 
 mals are, like annelids, crustaceans, and insects, built on a seg- 
 mental plan, the segmentation appearing clearly in the vertebral col- 
 umn or backbone. The nervous system in vertebrates shows some 
 of the segmental origin, in the regular succession of the nerves which 
 issue, a pair at each vertebra, along the length of the trunk. The 
 central organs of vertebrates are located inside the vertebrae and 
 skull, i. e., in the spinal cord and brain, and these show very little 
 sign of segmentation. In fact, one of the characteristics of the verte- 
 brate nerve-centres is their visible continuity and absence of apparent 
 segmentation. It is much as if the ganglia of the annelid's nerve- 
 chain had grown together into a continuous cord. A second differ- 
 ence between the vertebrate nervous system and that of the annelids, 
 etc., is seen in the position of the spinal cord. The nerve-chain of 
 invertebrates lies near the ventral surface, the digestive tract lying 
 
26 THE NERVOUS SYSTEM IN THE ANIMAL KINGDOM 
 
 above it, while in the vertebrates the cord lies near the dorsal sur- 
 face and above the digestive tract. The cerebral ganglion of the 
 earthworm, it will however be remembered, lies dorsal to the mouth, 
 and this corresponds to the brain of vertebrates. 
 
 A third point of difference between the vertebrate and inverte- 
 brate systems concerns the location of the cells, the branches of 
 which connect the receptors with the centres. In invertebrates, it 
 is the receptive cells, lying at the periphery, the branches of which 
 pass into the centres; but in vertebrates the receptor cells do not 
 themselves provide the fibres for their connections, but are sup- 
 plied by fibres which grow out from special ganglia that lie close 
 to the spinal cord and brain, and are enclosed in the same bony cov- 
 ering. The receptive cells of the sense of smell form an exception, 
 and send branches of their own into the brain, preserving the in- 
 vertebrate condition. The nerve-fibres in vertebrates very com- 
 monly show a more complicated structure than in invertebrates, in 
 that the branch of the nerve-cell becomes enveloped by a sheath of 
 " myelin," a fat-like substance the significance of which is not quite 
 clear, but will be discussed in a later chapter. On the whole, and 
 with exceptions, the nerve-centres of vertebrates may be considered 
 as more advanced in development than those of invertebrates; and 
 this is seen especially in the richer provision of long fibres connect- 
 ing distant parts of the system. 1 
 
 14. The nerve-centres of vertebrates may be considered as con- 
 sisting of (1) a fundamental system, comprising the cord and the 
 brain-stem, and (2) accessory organs developing as outgrowths of 
 the brain-stem, the chief of these being the cerebellum and the cere- 
 brum. The development of the accessory structures is very unequal 
 in different forms of vertebrate animals ; the size of the cerebellum 
 being closely related to the animal's powers of locomotion, and the 
 size of the cerebrum with his powers of learning new and specific 
 adaptations. The fundamental system is, on the other hand, fairly 
 constant throughout the vertebrate series. This is especially true 
 of the spinal cord, the size of which seems to depend almost wholly 
 on the size of the animal, i. e., on the bulk of the receptor and ef- 
 fector organs with which it is connected. Thus, though the brain 
 of an ox is much smaller than that of a man, the spinal cord of the 
 former is both longer and thicker. 
 
 The fundamental system (compare Fig. 7), as most clearly seen in 
 the spinal cord, consists of the above-mentioned sensory ganglia, the 
 cells of which provide fibres connecting the receptors of the trunk 
 
 1 The following treatment of the vertebrate nervous system is based mostly on 
 L. Edinger, " Vorlesungen iiber den Bau der nervosen Zentralorgane," Band 2, 
 Vergleichende Anatomie des Gehirns (Leipzig, 1908). 
 
NERVE-CENTRES OF VERTEBRATES 
 
 27 
 
 N. VIII 
 
 and limbs with the spinal cord; of nerve-cells lying within the cord, 
 branches of which pass out to the muscles and other effector organs ; 
 and of numerous central units cells whose branches do not extend 
 beyond the centres, but run up or down or across from one side of 
 the cord to another, and so bring its different parts into connection. 
 Most of these central fibres 
 are short; but there are some 
 sets of long ones, which are 
 present in all vertebrates, and 
 which connect the cord di- 
 rectly with the brain stem. 
 
 15. The brain-stem 
 shows the same basal char- 
 acteristics as the cord, but 
 its structure is greatly com- 
 plicated by the presence of 
 the important receptors 
 which are located in the 
 head, and connected with 
 this part of the central sys- 
 tem. Where a given set of 
 receptors is highly developed 
 and much used, the part of N. ix, x, xi 
 the brain-stem which receives 
 fibres from it is itself highly 
 developed. The receptors 
 of the vertebrate head can 
 be, though only roughly, in- 
 dicated by mentioning the FIG. 7. Nerve-centres 
 human nose, eye, ear, 
 mouth, and facial skin. Of 
 these, the development of the eye is perhaps the most equal through- 
 out the vertebrates. The sense of smell is much more important 
 in some forms than in others, and the associated parts of the brain 
 are correspondingly unequal. The skin of the face, especially of 
 the snout, and of the front of the mouth and nose, is much more 
 important as a receptive surface in some species of animals than in 
 others; and the "trigeminus" nerve, which supplies the cutaneous 
 receptors of the face, varies correspondingly. The ear of land- 
 living forms is the representative of a much more extensive system 
 of receptors in fishes, which is excited by vibrations and currents 
 in the water; this constitutes the so-called "lateral line" system, 
 the incoming fibres from which give rise to a considerable expansion 
 of the brain-stem at their place of entrance. Again, the sense of 
 
 Olf. nerve 
 
 Olf. lobe 
 
 > Cerebrum 
 
 Interbrain 
 
 Optic lobe 
 Cerebellum 
 
 Bulb 
 
 Spinal cord 
 
 of the Frog. (Ecker- 
 Wiedersheim-Gaupp.) The brain and upper part 
 of the cord are shown from the dorsal side. 
 
28 THE NERVOUS SYSTEM IN THE ANIMAL KINGDOM 
 
 taste in man is provided with a comparatively slight outfit; but in 
 fishes it has an extensive system of chemo-receptors, which lie not 
 only within the mouth and throat, but on the external surface of 
 the head and even of the trunk; and these receptors also, being con- 
 nected with a certain region of the brain-stem, are the occasion of a 
 complex local development in that region. Thus it comes about 
 that the brain-stem in vertebrates is not only larger, but less uni- 
 form in size than the spinal cord. 
 
 Another difference between these two parts of the fundamental 
 nervous system lies in the more extensive development, within the 
 brain-stem, of the short central fibres, which connect the neighboring 
 parts of the system. The impressions received by the various sense- 
 organs of the head are so important in regulating the vital functions 
 of the animal that it is but natural to find extensive paths of com- 
 munication leading from the nerves of these senses in various di- 
 rections. The impressions received by the internal ear (and by the 
 lateral line in fishes) are important in governing locomotion and 
 preserving equilibrium, and in maintaining the "tone" and readi- 
 ness for action of the muscles. The sense of taste is important in 
 regulating digestion. The impressions made on receptors in the 
 lungs the nerve-fibres of which also enter the brain-stem are im- 
 portant in breathing. The "centres" for the digestive organs, and 
 for respiration and the closely allied function of circulation, are lo- 
 cated in the brain-stem; and one of the important centres for loco- 
 motion is located there as well. (By a so-called "centre" is to be 
 understood, anatomically, a set of connections between incoming 
 and outgoing fibres, by which receptors can influence the right 
 effectors. Such connections are largely provided by the central 
 fibres, which accordingly abound in the region concerned with the 
 vital functions.) 
 
 The brain-stem, like the cord, does not belong wholly to the 
 fundamental system, but is invaded by fibres belonging to the acces- 
 sory systems of the cerebrum and cerebellum. In the higher mam- 
 mals, the fundamental system is quite overgrown by fibres belong- 
 ing to these accessory systems. These are long fibres, in contrast 
 with the short interconnections of the fundamental system. The 
 higher the development of the accessory organs, the greater is the 
 proportion of long fibres in the brain-stem and the cord; and the 
 more the fundamental system retires into the background. No 
 doubt, however, it always remains fundamental; it is played upon 
 by the accessory organs, between which and the effectors it always 
 intervenes (compare Fig. 8). 
 
 16. As already stated, the cerebellum varies in size according 
 to the locomotor powers of the animal, so that within closely related 
 
STRUCTURE OF THE CEREBELLUM 
 
 29 
 
 groups, such as the lizards, the tortoises, or the eels, those species 
 which are active swimmers have a much larger development of this 
 organ than those which crawl. Birds, with their high powers of 
 locomotion in three dimensions, have a large cerebellum; in mammals 
 also this organ is large in man specially large, perhaps on account 
 of his upright position and the special demands which this entails 
 on the muscles for locomotion and the maintenance of equilibrium. 
 
 BIRO 
 
 F- MAMMAU 
 
 Fia. 8. Longitudinal Sections through the Brains of Different Classes of Vertebrates. 
 (Edinger.) The cerebellum is represented in black. 
 
 The cerebellum receives fibres, directly or indirectly, from a large 
 share of the receptors of the whole body, including the head. The 
 fibres which conduct outward from the cerebellum run mostly to 
 groups of cells in the brain-stem; and the fibres from these groups 
 which belong to the fundamental connecting or co-ordinating system, 
 undoubtedly spread the influence of the cerebellum to the cells which 
 directly control the muscles. The development of the cerebellum 
 is not, however, entirely independent of that of the cerebrum, since 
 in mammals there is a mass of fibres coming from the cerebrum 
 and making connections with the cerebellum; and a large addition 
 to the cerebellum, on each side, seems to result from this connection 
 with the cerebrum. 
 
30 THE NERVOUS SYSTEM IN THE ANIMAL KINGDOM 
 
 17. Before we pass to the other principal accessory organ, the 
 cerebrum, we should pause to recognize the existence of the in- 
 creased development, which occurs in vertebrates near the forward 
 end of the brain-stem, in the part called the mid-brain. This part 
 reaches a great size in many vertebrates, and is in fact the best-de- 
 veloped part of the brain in fishes and amphibia; in reptiles and 
 birds as well it is very prominent. The most elementary fact re- 
 garding this part is that it receives fibres from one of the most im- 
 portant of all receptors, the eye; and that it, in turn, sends out 
 motor fibres to the muscles of the eye. The swollen portions of the 
 brain-stem which lie on the back of the mid-brain are accordingly 
 called, except in mammals, the "optic" lobes. The fibres of the 
 optic nerve here terminate in close connection with large collec- 
 tions of variously formed cells, some of which send forth only short 
 branches, and serve to afford rich connections within this mass of 
 cells, while others send out long fibres that run in large measure 
 downward to the lower parts of the brain-stem, and to the cord, 
 and thus carry the influence of the eye to effectors throughout the 
 body. But the optic lobes are not exclusively concerned with the 
 eye; for fibres can be traced to them also from the cord and brain- 
 stem; by this means they receive impressions from the other sense- 
 organs of the body, and especially from the ears. Thus the optic 
 lobes come to be a general receiving and distributing centre; and 
 in animals which have not developed the cerebrum to any great ex- 
 tent, the optic lobes are probably the dominant part of the whole 
 nervous system. 
 
 18. The part of the brain-stem which lies next forward of the 
 mid-brain is called the inter-brain or thalamus; it is constantly pres- 
 ent throughout the vertebrate series, but varies greatly in extent; 
 and in mammals it is quite definitely an adjunct of the cerebrum, 
 since it contains masses of cells which are relay stations in the path- 
 ways of impressions from all the receptors to the cerebrum. Thus, 
 the principal connections of the eye, which in other vertebrates are 
 almost exclusively with the optic lobes, become in mammals mostly 
 transferred to the thalamus, from which fibres run to the cortex of 
 the cerebrum. But besides this cerebral part of the thalamus, there 
 is in all forms of vertebrates a more primitive system of cells and 
 fibres, connecting largely with the portion of the brain still further 
 forward (the olfactory region). The thalamus is not, however, 
 the direct or immediate centre of smell; and in fact its significance 
 is difficult to discover. The thalamus has several gland-like off- 
 shoots, and in some of the lower vertebrates, some of these appear 
 to act as sense-organs. The " parietal organs," which are offshoots 
 of the thalamus, appear in some forms to serve as eyes, which are 
 
INTER-BRAIN AND END-BRAIN 31 
 
 fairly well developed, though not by any means as highly developed 
 as the eyes customarily so called. It is possible that this part of 
 the brain-stem is primitively, like the mid-brain, the local centre for 
 certain sense-organs which in most vertebrates have gone out of 
 function; and that later, the thalamus was taken possession of, so 
 to speak, by the adjoining cerebrum. 
 
 What lies forward of the thalamus is called the fore-brain or end- 
 brain. It consists of two parts: (1) the basal portion, which be- 
 longs to the fundamental system of the brain-stem, and (2) the ex- 
 pansions to the side and back, which form the greatest part of the 
 whole brain in mammals, but in many lower vertebrates are of very 
 small dimensions. The basal portion of the fore-brain is again read- 
 ily divisible into two parts: the "corpus striatum," and the olfactory 
 lobe and bulb. The striatum, which lies very near the thalamus, is, 
 probably, closely related to it in function, though we must admit that 
 on this point our knowledge is far from complete. The function 
 of the olfactory bulb, which lies at the very front of the whole organ, 
 is clear from its direct connection with the incoming fibres from the 
 olfactory receptors in the nose. And the olfactory lobe, further, 
 is directly connected with the bulb, and must also be regarded as 
 part of the olfactory system. A portion of it, however, receives 
 fibres from the skin of the face and mouth, and is perhaps, as sug- 
 gested by Edinger, 1 a centre for the " oral sense," which in animals 
 that use the snout or tongue as an exploratory organ, must be of 
 much service in directing their behavior. There is no doubt that, 
 as the mid-brain is primarily the central mechanism of the eye, the 
 fore-brain is primarily the local centre for the sense of smell. In the 
 fishes, it seems to be little else; only the basal portion develops, the 
 dorsal remaining (except for one small portion) a mere membrane, 
 without nerve-cells or fibres in it. In amphibia, however, the dorsal 
 portion, called the "pallium" or mantle, begins to show nervous 
 structures, and in reptiles, this is still more markedly the case; in 
 birds more yet; and in mammals the pallium completely overshad- 
 ows the primitive basal structures of the fore-brain. In amphibia the 
 nerve-cells of the pallium are irregularly arranged, but, beginning 
 with the reptiles, there is a true bark, or " cortex," on the outer sur- 
 face of the pallium, consisting of cells arranged in a definite fashion, 
 and with an extraordinary richness of fine branches. 
 
 19. At its first appearance, in reptiles, the cortex is, to judge 
 from its fibre connections, an adjunct of the primary olfactory and 
 oral sense-centres which lie in the adjacent basal portion of the fore- 
 brain. This oldest part of the pallium is called the " archi-pallium," 
 in distinction from the " neo-pallium," the connections of which are 
 1 L. Edinger, op. cit., p. 261. 
 
32 THE NERVOUS SYSTEM IN THE ANIMAL KINGDOM 
 
 with more distant parts of the brain-stem and cord (see Fig. 9). 
 In birds, the connections with the optic lobes become prominent; 
 and in mammals, through the intermediary of the thalamus, the 
 cortex comes into connection with all the receptors of the body, 
 and also sends outgoing fibres further and further back along the 
 
 FIG. 9. Archipallium and Neopallium in the Brain of the Calf. 
 (Edinger.) In the right half of the figure, the archipallium (un- 
 shaded) is separated from the partially shaded neopallium by the 
 fissura limbica. 
 
 brain-stem and cord, thus assuming control over more and more of 
 the fundamental system. With the increase in the neopallium, the 
 older, olfactory portion of the cortex is left behind near the base 
 and median line of the cerebrum, while the neopallium spreads to 
 the sides, and upward, forward, and backward. The archi- 
 pallium still forms a distinguishable portion of the brain, even of 
 man, and is probably still concerned, as in reptiles, with smell and 
 related senses. 
 
RELATION OF SIZE TO INTELLIGENCE 33 
 
 Among the mammals, the advance of the brain from the lower 
 to the higher forms consists in the development of the neopallium. 
 This advance is marked by the increase of the extent of the cortex, 
 by the growing richness of cells and their fine branches within the 
 cortex, and by the increase of long fibres connecting different parts 
 of the cortex with each other and with the cord, the brain-stem, 
 and the cerebellum. The increase in total extent of the cortex is 
 provided for partly by expansion of the skull and partly by the fold- 
 ing of the surface of the brain into "fissures," which are few in the 
 lower mammals, but numerous in many of the higher forms. The 
 increase of long fibres in the higher forms is as marked as the. in- 
 crease in the extent of the cortex. It will be recalled that, even 
 among the worms, an advance in organization was marked by an 
 increase in the long central fibres, bringing distant parts of the sys- 
 tem into direct connection. This same principle appears in compar- 
 ing the brains of different vertebrates. Man, of all vertebrates, 
 possesses the greatest proportion of long fibres, both those leading 
 into or out of the cerebrum, and those connecting its own non-adja- 
 cent parts. In particular, the supply of fibres connecting the right 
 with the left hemispheres of the cerebrum shows a great advance 
 from lower mammals to higher, and in man the cross connection 
 between the hemispheres, called the "corpus callosum," is a prom- 
 inent feature of the brain. 
 
 20. In a general way, the intelligence of different vertebrates 
 is somewhat proportional to the size of their brains, and especially 
 to their development of cerebral cortex. We have, indeed, as yet 
 no accurate measures of the intelligence of different animals, with 
 the exception of a few which have been subjected to experimental 
 test. The monkey is more intelligent than the cat or dog, as tested 
 by the speed of learning or the number and variety of associations 
 formed. 1 The chimpanzee and other man-like apes seem to be 
 superior to the smaller monkeys; and the primates, in general, seem 
 to surpass most other animals. Some mammals which possess 
 markedly large brains, and high cortical development such as the 
 seal, porpoise, walrus, and whale are little known as respects 
 their behavior and intelligence. 
 
 One thing is reasonably certain: the brain-weight of a species, 
 or of a breed, is partly dependent on the size of the body. Larger 
 breeds have larger brains, but the relative size of the brain is greater 
 in the smaller breeds. There is apparently a purely somatic factor 
 in the determination of brain development, and this factor must be 
 allowed for before the true relation between brain-size and intelli- 
 
 1 Thorndike, "The Mental Life of the Monkeys," Psychol. Rev., Monogr. Suppl. 
 15, 1901. 
 
34 THE NERVOUS SYSTEM IN THE ANIMAL KINGDOM 
 
 gence can be seen. Suggestions have been made toward the de- 
 termination and elimination of this somatic factor, but there are 
 still many elements of uncertainty in the calculation. 
 
 The following table of approximate brain-weights and body- 
 weights of mammals is selected from the much more extensive tables 
 of Warnecke. 1 
 
 
 Brain-Weight 
 in Grams 
 
 Body-Weight 
 in Grams 
 
 
 Mouse . .... 
 
 4 
 
 20 
 
 
 Squirrel 
 
 6 
 
 400 
 
 
 Cat 
 
 30 
 
 3,500 
 
 
 Beaver 
 
 35 
 
 20000 
 
 
 Kangaroo 
 
 65 
 
 45,000 
 
 #T 
 
 Monkey (macaque) . 
 Dog (very large) . . . 
 Sheep 
 
 100 
 120 
 130 
 
 5,000 
 46,000 
 50,000 
 
 y$* 
 
 . >3P 
 
 **t 
 
 Lion 
 
 220 
 
 120,000 
 
 
 Seal 
 
 300 
 
 26000 
 
 
 Bear 
 
 400 
 
 200 000 
 
 o 
 
 Gorilla 
 
 400 
 
 90,000 
 
 
 Moose 
 
 435 
 
 200,000 
 
 3Zn 
 
 Cow 
 
 450 
 
 175 000 
 
 f 
 
 Porpoise 
 
 500 
 
 55,000 
 
 X 
 
 Hippopotamus .... 
 Horse 
 
 580 
 600 
 
 1,750,000 
 300,000 
 
 >4 V 
 
 Giraffe 
 
 680 
 
 530 000 
 
 
 Man . ... 
 
 1 400 
 
 70,000 
 
 ] y^ytf 
 
 Elephant 
 
 5,000 
 
 2,500,000 
 
 
 Whale 
 
 7000 
 
 70 000 000 
 
 
 
 
 
 
 21. Within the order of primates, we find a large range of brain- 
 size, the cerebrum of the common tailed monkeys (e. g., macacus 
 rhesus) weighing about 80-100 grams, that of the anthropoid apes 
 (chimpanzee, gorilla, orang-outang) running to about 400 grams, 
 while that of man averages about 1,400 grams. Remains of extinct 
 forms of primates, showing a cerebral development intermediate 
 between that of the gorilla and that of man, have been unearthed 
 in only a few cases; and there has been considerable difference of 
 judgment regarding the interpretation to be made of these inter- 
 vening forms. On the whole, it seems allowable to recognize at 
 least one intermediate form between the anthropoid apes and man 
 as he exists to-day. This intermediate form (Pithecanthropus 
 
 1 Journal f. Psychol. u. Neurol., 1908, XIII, 355. A collection of brain-weights 
 of birds, by the same author, is to be found in the same journal, 1907, IX, 93. 
 
ESSENTIAL FUNCTION OF NERVOUS TISSUE 35 
 
 erectus), as represented by a single specimen found in Java, 1 is 
 reckoned with the apes rather than with man, but has a skull capac- 
 ity, as well as other anatomical peculiarities, that separate him from 
 existing apes, and indicate a brain-weight of perhaps 600-700 grams. 
 The paucity of the evidence, however, must continue to throw doubt 
 on the conclusion. 
 
 A considerable number of specimens of great antiquity have been 
 unearthed, which are undoubtedly the relics of the human species. 
 In regard to skull capacity, these oldest surviving specimens of 
 inhabitants of Europe do not differ much, if any, from the present 
 inhabitants. In countries like Egypt, also, in which skulls are found 
 representing several thousand years of history, there is no sign of 
 an increase in brain-size. Among existing races, there are racial 
 differences in the average brain- weight; but these differences are 
 small compared with the wide range of variation between indi- 
 viduals of the same race. For example, the average brain-weight 
 of negroes is perhaps two ounces less than that of Europeans; but 
 the difference between individuals of either race may amount to as 
 much as twenty-five ounces. It should be added that the differ- 
 ences in mental capacity of different races appear much smaller to 
 the ethnologist, who knows different races, than they appear to 
 those members of any race who, not having studied other races 
 scientifically, judge in accordance with race prejudice and pride 
 rather than in an objective manner. Even this difference may be 
 chiefly due to the fact that, in the more highly civilized races, the 
 large amount and variety of the educative processes develops 
 (that is, occasions a growth in size of) many of the nerve elements 
 which continue relatively undeveloped in the less civilized races. 
 
 22. A review of the foregoing sketch of the varieties of the 
 nervous system in the animal kingdom leaves no doubt that the 
 most elementary and essential function of nervous tissue is to provide 
 lines of conduction between receptors and effectors. No other possi- 
 ble function of the nervous system would have any biological utility 
 without this. The progress from the diffuse nerve-net of the jelly- 
 fish to the highly organized system of mammals is marked, first by 
 centralization; second, by the appearance of numbers of central or 
 co-ordinating cells; third, by the dominance of the system by the 
 receptors of the head; and fourth, by an increase in the plasticity or 
 modifiability of the system, or of certain parts of it. Conduction, 
 co-ordination, integration, and "learning" (this word in a figurative 
 sense) may be assigned as the functions of the nervous system; and 
 it is probable that, from the stand-point of inner mechanics, all of 
 these are specializations of the primary function of conduction. 
 1 Discovered and described by E. Dubois. 
 
CHAPTER II 
 
 THE DEVELOPMENT OF THE NERVOUS SYSTEM IN THE 
 INDIVIDUAL 1 
 
 1. The study of the growth of the nervous system in embry- 
 onic life and childhood is useful in two ways: it throws some light 
 on the functions of the various parts, and it is of great aid in gain- 
 ing clear conceptions of the highly complicated structure of the 
 central organs. These organs first appear in relatively simple 
 forms and relations, and develop gradually in complexity; an under- 
 standing of the simpler facts of the embryonic brain is thus a good 
 point of starting for the study of its fully developed condition. 
 
 In unicellular organisms reproduction consists simply in division. 
 The adult, full-grown cell divides into two cells of half its size, which 
 may be called young cells, or "daughter cells"; and these in their 
 turn grow and subsequently divide. In all but the simplest forms 
 of living beings, there exists a differentiation among the numerous 
 cells that compose the body; most of them form organs which, 
 from the stand-point of reproduction and the perpetuation of the 
 species, are accessory, and simply serve to provide favorable con- 
 ditions of life for the relatively few cells which preserve the repro- 
 ductive power. These reproductive cells alone are capable of 
 producing daughter cells which can grow into new individuals. In 
 plants and animals which show the distinction of sex, the repro- 
 ductive cells are of two sorts, usually (in animals) located in sepa- 
 rate individuals, male and female. The reproductive cells for the 
 female animal give rise to cells called eggs or ova; while those of 
 the male give rise to spermatozoa. In some low forms the ova are 
 capable of developing into new individuals without aid from the 
 spermatozoa; but in most cases, and always in vertebrates, the 
 ovum does hot develop far unless it has fused with a spermato- 
 zoon. The fusion of these two cells, the ovum from the female 
 individual and the spermatozoon from the male, is called fertiliza- 
 tion; and the ovum which has fused with a spermatozoon is called 
 a fertilized ovum. Leaving aside as unessential for our purpose 
 
 1 This chapter is based principally on the work of His, Die Entwickelung des 
 menschlichen Gehirns, 1904; and on the treatment by O. Strong, in Bailey and 
 Miller's "Textbook of Embryology," 1909, pp. 454-572. 
 
 36 
 
REPRODUCTION IN UNICELLULAR ORGANISMS 37 
 
 the very curious changes which occur in these two cells in prepara- 
 tion for their fusion, we will take our start with the fertilized ovum, 
 which may properly be called a new individual at the very earliest 
 stage of its life, and which promptly begins to develop itself. 
 
 2. As has already been said, the fertilized ovum, itself a 
 single cell, proceeds to divide into two, 1 which, however, do not sep- 
 arate, but remain in contact with each other. Each of these daugh- 
 ter cells, after growing in size, again divides into two; and this proc- 
 ess is repeated time after time, and for a while with considerable 
 regularity, so that the young individual consists successively of one 
 cell, two cells, four, eight, sixteen, and thirty-two. The details 
 of this process differ, however, in different orders of animals, and 
 in all the regular doubling of the number of cells is broken up after 
 a few divisions by the beginning of the differentiation among the 
 daughter cells and their unequal rates of growth and division. 
 
 A process preliminary to this propagation of cells by division 
 may be noticed briefly at this point. When a cell is on the point of 
 dividing into two, the nucleus can be seen to separate into smaller 
 bodies, the number of which is constant for any given sort of cell. 
 In preparation for their fusion, both ovum and spermatozoon ex- 
 trude part of their nuclear matter, and at the time of fertilization, 
 each has only half of the number of so-called "chromosomes" ap- 
 propriate to the animal in question. Thus the fertilized ovum, 
 which consists of both ovum and spermatozoon, contains exactly 
 the full number of chromosomes appropriate to the species. When 
 the fertilized ovum divides into two cells, half of its chromosomes go 
 to each cell, and half of each half is from the original ovum, half 
 from the spermatozoon. The same thing is true of the succeeding 
 generations of "daughter cells." Thus each parent is equally 
 represented, not only in the fertilized ovum, but in each cell of the 
 successive generations of cells that result from repeated division. 
 
 3. At first the cells of the mass resulting from repeated cleavage 
 of the ovum adhere compactly together, presenting somewhat the 
 appearance of a mulberry. Later, spaces appear between some of 
 the cells, and a cavity is formed inside the mass. Then the cells 
 
 1 The very early stages in the development of the human embryo have not 
 been actually observed, but as those mammals which have been studied resemble 
 each other in essential points at this stage, it is inferred that the beginnings of 
 human development do not differ much from the general mammalian condition. 
 The earliest human embryos so far available seem to be of about two weeks' 
 growth. Within this period they have increased in size from 0.2 millimetre 
 (or lis" of an inch, the diameter of the ovum) to a length of some two millime- 
 tres. They have become elongated and cylindrical, and show the beginnings of 
 head and trunk, and distinct rudiments of several" organs, among which are the 
 brain and spinal cord. 
 
38 
 
 THE NERVOUS SYSTEM IN THE INDIVIDUAL 
 
 Am 
 
 lining the cavity become differentiated from those of the exterior 
 surface, and a third group of cells sprouts in between the two layers, 
 so that there are formed, in all, no fewer than three " germ layers." 
 Of these, the exterior layer is called the ectoderm, the inner layer the 
 entoderm, and the intervening layer the mesoderm. From this very 
 early stage of development on, these three layers retain their indi- 
 viduality. The entoderm forms the epithelial wall of the throat, 
 
 gullet, stomach, and intes- 
 tines, and also of the windpipe 
 and lungs ; it gives rise also to 
 the glands which connect with 
 these organs, including the 
 liver and pancreas. The meso- 
 derm gives rise to the bones 
 and connective tissues, to the 
 muscles, the heart and blood- 
 vessels, and to the kidneys and 
 the sex glands. The ectoderm 
 gives rise to the outer layer of 
 the skin and of the mucous 
 membrane of the mouth and 
 nose, to the appendages of the 
 skin, such as hair and nails, to 
 the essential portions of the 
 sense-organs, and to the ner- 
 vous system. It appears 
 strange, at first thought, that 
 the nerve-centres, which lie 
 so far to the interior and are 
 
 enclosed in bone, should develop from the superficial layer of the 
 embryo, but the early stages in the development of the nervous 
 system make this clear. 
 
 Early in the development, when the embryo is less than two weeks 
 old age being reckoned from time of fertilization there appears 
 a shallow groove (see Fig. 10) extending lengthwise along what may 
 be called the back of the embryo. This groove is formed by an in- 
 bending of the ectoderm, and is called the neural groove. The 
 groove increases in depth, its edges arch together over the top, and, 
 where they meet, grow together, thus transforming the groove into 
 a tube a hollow tube of ectoderm lying beneath the surface of the 
 embryo, and later separated widely from this surface by the in- 
 growth of mesoderm between it and the superficial ectoderm. This 
 is the so-called neural tube, and it is the rudiment of the nervous 
 system, which, in fact, always retains the fundamental character- 
 
 Fio. 10. Dorsal View of a Two Weeks' Human 
 Embryo. (Von Spec.) The future head end 
 is above in the figure. The "neurenteric 
 canal" is an early communication between 
 the neural tube and the rudimentary intes- 
 tine. Magnified 20 diameters. 
 
CONSTITUTION OF THE NEURAL TUBE 
 
 39 
 
 Brain 
 (groove still 
 unclosed) 
 
 Amnion 
 
 istic of a tube. The lower portion of the tube gives rise to the spinal 
 cord, the walls of which become so thickened that the small central 
 cavity remaining in adult life and called the central canal is very in- 
 conspicuous. The upper portion of the tube develops into the 
 brain ; and here the form of a tube is still more concealed, by fold- 
 ings, swellings, or out- 
 growths of the tube, and 
 great but irregular thick- 
 enings of the wall. The 
 central cavity persists, how- 
 ever, and appears in the 
 adult brain in the form of 
 spaces of various shape, 
 called the ventricles. It is 
 of great assistance in gain- 
 ing a conception of the 
 complicated structure to 
 bear in mind the fact that 
 the brain is primarily a 
 tube (compare Fig. 11). 
 
 4. The nerves which 
 connect the brain and 
 cord with the muscles and 
 sense-organs, arise partly 
 as outgrowths of the neu- 
 ral tube, and partly from 
 that portion of the ecto- 
 derm which formed the 
 lips of the neural groove 
 
 and which Was left OUt- Neur enterte 
 
 side when the groove 
 closed to form the tube. 
 
 FIG. 11. Dorsal View of a Human Embryo, Some- 
 what More Advanced than the Preceding. (Eter- 
 nod.) About 5 r. The neural groove has closed to 
 
 Vertebra 
 
 canal 
 
 Neural tube 
 
 Neural groove 
 (still unclosed) 
 
 a tube in the middle of its extent, but is still open 
 at the cerebral and lumbar ends. 
 
 This portion of the ecto- 
 derm is called the neural 
 crest, and it is quickly 
 overgrown by the adja- 
 cent ectoderm, and so comes to lie beneath the surface of the em- 
 bryo, and close to the neural tube, one strip at the right side of the 
 tube and one at the left. This part of the ectoderm gives rise to 
 the sensory nerves and to part, at least, of the sympathetic nervous 
 system; while the neural tube itself gives rise to the motor nerves 
 and to the brain and cord. 
 
 The neural tube, when first formed, does not as yet constitute a 
 nervous tissue; its cells are not nerve-cells, but are still in an un- 
 
40 THE NERVOUS SYSTEM IN THE INDIVIDUAL 
 
 specialized condition. The process by which this tube of undiffer- 
 entiated cells becomes a cord of nervous tissue is as follows. In 
 the wall of the tube next to the cavity or "lumen" some cells be- 
 come reproductively active, and divide, giving rise to daughter cells, 
 which move away toward the outside of the wall, but stop midway, 
 and proceed to change their form, sending out branches that 
 
 Germinal layer 
 
 Nerve-cell 
 layer 
 
 Marginal 
 layer 
 
 Fia. 12. Cross Section of the Neural Tube and Crests of a Human Embryo of about Five 
 Weeks. (His.) The dorsal side is uppermost in the figure. Many facts which are men- 
 tioned in the text can be observed in this figure. The three layers of the tube are readily 
 distinguished: the germinal layer, next to the lumen of the tube, is dark with its closely 
 packed cells. The nerve-cell layer, outside of this, is more loosely packed, while the mar- 
 ginal' layer is free from cells. The fact that the nerve-cell layer develops to the sides, 
 and not directly in the mid-ventral and mid-dorsal lines, is evident. Commissural axons, 
 crossing at the mid-ventral line, can be observed ; also axons passing into and out of the 
 tube in the dorsal and ventral roots. The neural crest is clearly distinguished from the 
 surrounding tissues in the left side of the figure. A streaky appearance in the crest indi- 
 cates the direction of the growing axons toward the dorsal root. Within the tube, close 
 to the entrance of the dorsal root, is seen the rudiment of the dorsal column which is 
 composed of the axons which enter by the root. 
 
 unite with branches of other similar cells. In this way a network of 
 minute fibres is formed. These cells, again, are not nerve-cells, 
 nor is this tissue of fibres true nervous tissue; it is a framework or 
 scaffolding; it is the rudiment of the supporting tissue of the nerve- 
 centres, which in its fully developed form is called neuroglia. Al- 
 ways the growth of this supporting framework precedes the appear- 
 ance of nerve-cells in any region of the centres. The neuroglia 
 strengthens the texture of the nerve-centres. In later life, it "pro- 
 liferates " or multiplies wherever injury or degeneration of the ner- 
 vous tissue tends to produce vacant spaces. The neuroglia, grow- 
 
CONSTITUTION OF THE NEURAL TUBE 
 
 41 
 
 ing into the spaces, fills them. When the true nerve-cells begin to 
 grow, the network of neuroglia serves to guide them into their 
 places. 
 
 The nerve-cells originate in the same manner as the neuroglia 
 cells. At the same portion of the tube, namely the layer next to 
 the lumen, cells divide, giving rise to daughter cells which migrate 
 outward to the middle of the wall. Here they stop, perhaps be- 
 cause checked by a dense layer of neuroglia fibres, and here they 
 proceed to develop peculiarities which mark them as nerve-cells. 
 
 FIG. 13. The Growing Axon. (Cajal.) From the embryo 
 of a duck. A is the neural tube, the letter being placed 
 about at the junction of the nerve-cell and the marginal 
 layers; B, the space immediately surrounding the tube. 
 
 The wall of the neural tube thus comes to consist of three layers, 
 the inner or germinal layer, where the process of cell-division is 
 active, and where the young nerve-cells originate; the outermost or 
 marginal layer, which is free from nerve-cells ; and a middle or nerve- 
 cell layer, in which the nerve-cells are developing (see Fig. 12). 
 
 5. The first visible step in the specialization of the nerve-cell 
 is the appearance of a few fine fibrils, the beginning of the " neuro- 
 fibrils," which later develop into an extensive system within the 
 cell and its outgrowths. The next step of importance is the ap- 
 pearance of a branch, which grows out from the cell toward the 
 marginal layer. The growing end of this branch then becomes en- 
 larged into a cone which seems to press its way between other cells 
 
42 THE NERVOUS SYSTEM IN THE INDIVIDUAL 
 
 and into the meshes of the neuroglia net, and so the branch elongates. 
 The name given to this branch of the cell is the "axis cylinder 
 process," or, more briefly, the axon. The axon itself branches, 
 but not very richly; its branches are usually short and at right angles 
 to its own course. The branches of the axon are known as collat- 
 erals. While the axon is pushing its way farther and farther from 
 the cell body, the latter is developing its internal system of fibrils; 
 and it presently puts forth, on the side opposite to the axon, other 
 branches, quite different from the axon, more richly branched, 
 branched in many cases much like a tree, and hence called den- 
 drites (compare Fig. 13). 
 
 Leaving further description of the cell body and dendrites to a 
 later page, let us return to the axon, and endeavor to follow its de- 
 velopment. In most parts of the neural tube, the axon emerges 
 from the layer of nerve-cells into the marginal layer, which thus 
 becomes a layer of axons. Its further course differs much in differ- 
 ent cases. In the case of a very important group of cells, which 
 lie on the ventral 1 side of the neural tube, the axon passes straight 
 out through the marginal layer and into the surrounding tissues; 
 there it grows into a developing muscle, and, as with the general 
 growth of the body the muscle comes to lie further and further from 
 the neural tube, the axon continues to elongate. In this manner 
 it comes to connect some small portion of the muscle with the spinal 
 cord. This axon is a motor nerve-fibre, and its cell, which lies 
 back in the ventral portion of the neural tube, is a motor nerve-cell. 
 Later this cell will send "nerve impulses" down along the axon, 
 arousing a portion of the muscle to action. As the motor nerve-cells 
 are numerous and near together, their axons emerge from the tube 
 in bundles or "rootlets," which come together to form "roots." 
 These roots, emerging from the ventral side of the tube, are the 
 ventral or motor roots of the spinal nerves (see Fig. 14). 
 
 There are also dorsal or sensory roots. These are formed, not 
 by outgrowth of axons from cells in the tube, but by the growing into 
 the tube of axons from outside cells that lie in the two strips which 
 are formed from the neural crest, on the right and the left of the tube 
 and toward its dorsal side. Cells within these strips develop in 
 much the same way as the nerve-cells within the tube; except that 
 each of the former sends out two axons, one of which grows toward 
 the periphery of the body and connects with some sense-organ, while 
 the other grows toward and penetrates the neural tube. The axon 
 which grows toward the periphery, with others from near by, forms 
 a dorsal root; this dorsal root soon joins the ventral root from the 
 
 1 "Ventral" means on the side toward the animal's ventrum or belly; and 
 "dorsal," the corresponding word, means on the side toward the dorsum or back. 
 
GROWTH OF NERVE AXONS AND FIBRES 
 
 43 
 
 adjacent part of the tube, and the two together form one of the spinal 
 or cranial nerves. 
 
 The nerves come off in pairs, on the right and left sides of the 
 tube. The two strips which gave rise to the sensory cells and to 
 their axons become compacted into two rows of "ganglia," or 
 bunches of nerve-cells, which lie on the dorsal roots, and are called 
 
 FIG. 14. Axons Growing in Different Directions. (Cajal.) From 
 the spinal cord of an embryonic chick. A, ventral root; B, 
 spinal ganglion and dorsal root; R, bifurcation of dorsal root 
 axons on entering the cord ; d, cell giving rise to ventral root 
 axon; c, c, commissural axons. 
 
 spinal ganglia. The cells within these ganglia, which at first sent 
 out an axon in each direction, later take a place to one side of their 
 axons, and then withdraw to some distance from them, retaining 
 connection by a single strand; in their fully developed form they ap- 
 pear to have but a single axon, which, however, speedily divides 
 into two. These ganglion cells put out no dendrites; but that one 
 of their axons which penetrates the neural tube splits in the marginal 
 
44 
 
 THE NERVOUS SYSTEM IN THE INDIVIDUAL 
 
 layer into an ascending and a descending branch; it also sends off 
 collaterals to the neighboring motor nerve-cells. 
 
 Not all of the nerve-cells in the tube send axons out to form motor 
 
 fibres. The axons of many of 
 them, on reaching the marginal 
 layer, simply turn upward or 
 downward within it, while still 
 others pass horizontally around 
 through the marginal zone, and 
 cross in its ventral portion to 
 the other side of the tube. By 
 this contrivance the right and 
 left halves of the tube are con- 
 nected. These connections are, 
 therefore, called "commissural 
 fibres," and their crossing-place 
 at the mid-ventral line is called 
 the "ventral commissure." 
 
 6. The axons thus far de- 
 scribed may now be grouped 
 into three classes: (1) the "ven- 
 tral root axons," which arise 
 from cells in the ventral part of 
 the tube and pass out, as mo- 
 tor fibres, connecting the tube 
 with muscles; (2) the "dorsal 
 root axons," which arise from 
 cells in the spinal ganglia, and 
 by their two axons, connect the 
 tube with sense-organs; and (3) 
 the "central axons, 
 wholly within the 
 connect one part of it with 
 another (compare Fig. 15). 
 There is still a fourth class of 
 axons to be added. Some of the 
 cells in the neural crest, instead 
 of remaining near the dorsal part 
 of the tube and sending axons 
 into it, migrate in a ventral di- 
 rection and develop into nerve-cells; but these send out only a single 
 axon, which passes to some of the organs of the interior of the body 
 the stomach, intestines, pancreas, salivary glands, heart, and blood- 
 vessels and enters into connection with muscular or glandular tissue 
 
 FIG. 15. Development of the Nerve-Cells of 
 the Spinal Ganglia. (Cajal.) The drawing 
 is from an embryo chick. A is the spinal 
 cord, containing d, a motor nerve-cell, from 
 which issues an axon; this with other simi- 
 lar axons emerges from the cord into B, the 
 ventral root (the connection between this 
 root and the cord has been accidentally 
 broken in this section). C is the dorsal root, 
 consisting of axons which issue from the 
 spinal ganglion D. Axons issuing from this 
 ganglion in the opposite direction join those 
 of the ventral root, to form E, a spinal nerve. 
 Within the spinal ganglion are seen cells 
 which are still bipolar, h; others, i, which are 
 becoming transformed into unipolar cells; 
 and others, as ;, which are already distinctly 
 unipolar. F is a sympathetic ganglion, 
 showing cells with axons, a, which join the 
 spinal nerve, and others with axons, e, which 
 do not join this nerve, but which, as a mat- 
 ter of fact, pass up or down in the " sym- 
 pathetic chain." 
 
 " which lie 
 tube, and 
 
FORMATION OF THE SPINAL CORD 45 
 
 there. Such are the visceral or "sympathetic" nerve-fibres; and 
 their cell bodies are gathered in the ganglia of the "sympathetic 
 system," some of which lie in two rows close to the backbone on its 
 ventral side, while others are scattered among the internal organs. 
 These axons and their cell bodies, it will be noted, are entirely out- 
 side of the neural tube. They do not, however, remain uncon- 
 nected with it; for certain cells exist in the lateral portions of the 
 tube, which send out axons through the ventral roots to the sympa- 
 thetic ganglia. 
 
 7. We are now in a position to understand the process by which 
 the lower part of the neural tube becomes the spinal cord. We have 
 first the three layers of the wall, the germinal layer next the lumen, 
 the nerve-cell layer in the middle, and on the outside the marginal 
 layer, which becomes the axon layer. Later, the axons receive 
 sheaths of a white "medullary" substance called "myelin"; and 
 thus the axon layer becomes "white matter"; the white matter of 
 the cord lies in the marginal layer. The nerve-cell layer, on the 
 contrary, does not acquire much of this medullary substance, but 
 retains its light-gray or watery color, and is therefore called the 
 "gray matter." Thus the gray matter of the cord is surrounded 
 by white matter. Meanwhile the germinal layer decreases in thick- 
 ness, since the cells in it divide, one after another, giving rise to 
 neuroglia and nerve-cells, which migrate outward; in this way the 
 germinal layer finally becomes reduced to a single thickness of cells 
 lining the cavity of the tube. The other two layers on the con- 
 trary increase greatly in thickness the cell layer by receiving 
 new cells from the germinal layer, by growth in size of the cells al- 
 ready present in it, by growth of the dendrites of these cells, and by 
 the incoming of the terminations of axons into the cell layer. The 
 axon layer increases by the constant growth of new axons into it 
 from the adjoining part of the cell layer, and by the elongation of 
 axons from the marginal layer above and below. The thickening 
 of the cell and axon layers is not, however, uniform all around the 
 tube ; in particular, the mid-ventral and mid-dorsal portions do not 
 develop cell layers of much thickness; since their cells migrate 
 mostly to the right and to the left. At the mid-ventral line, there- 
 fore, we find in the developed cord, first, a thin layer of gray matter 
 next to the central canal, then a thin layer of white, consisting of 
 fibres crossing from one side of the cord to the other (the ventral 
 commissure), and then a deep groove or fissure, the "ventral 
 fissure," which runs lengthwise of the cord between the swollen 
 walls of the adjacent parts of the tube. In the dorsal mid-line, there 
 is a thin layer of gray matter, and then a thin sheet of neuroglia, 
 the "dorsal septum," which has resulted from the narrowing of 
 
46 THE NERVOUS SYSTEM IN THE INDIVIDUAL 
 
 the cavity to a slit, and the subsequent growing together of its 
 side walls. 
 
 The gray matter is most abundant in the ventral half of the cord, 
 which contains the cell bodies of the motor axons. These bodies 
 are especially numerous at the levels of the shoulders and loins, 
 where the nerves to the arms and legs originate. The dorsal half 
 of the gray matter is thinner in these parts, for the reason that the 
 cell-bodies of the sensory axons, instead of lying within the cord, are 
 in the spinal ganglia outside of the cord. From the ventral half of 
 
 FIG. 16. Cross Section of a Child's Spinal Cord. (Marburg.) Magnified 8 diameters. 
 The stain employed to bring out the features of the cord has darkened the nerve-fibres 
 and therefore the white matter, and left the gray matter light. The ventral side of the 
 cord lies below in the figure. The cracks, showing white in the figure, are produced in 
 the process of preparing the section, but doubtless represent natural lines of cleavage, 
 determined by the course of neuroglia fibres. 
 
 the gray matter emerge the motor roots; and into the dorsal half 
 enter the sensory roots. Thus at these two places, the gray matter 
 approaches nearest the surface of the cord, forming the ventral 
 and dorsal "horns" of the gray matter. Between these horns, and 
 continuous with them, lies a middle or lateral portion of the gray 
 matter. The cells in the lateral portion and in the dorsal horn give 
 rise to central axons, which do not emerge from the cord, but turn 
 upward or downward in the marginal layer. Many central axons 
 come also from the ventral horn (see Fig. 16). 
 
 The main principle to be borne in mind in studying the marginal 
 or axon layer later the white matter of the cord is that an axon 
 entering it from the gray matter usually turns up or down very 
 promptly, and thus starts its longitudinal course near to the gray 
 matter. As it elongates itself, whether upward or downward, it 
 
FORMATION OF THE SPINAL CORD 
 
 47 
 
 is crowded outward by axons emerging at the level which it has 
 reached. The further it goes, the more is it crowded toward the 
 outer surface of the cord. There is, however, one striking excep- 
 tion to this general rule. Most of 
 the axons from the " motor area " 
 of the brain, which grow down into 
 the cord late in its development 
 (during the fifth month of foetal 
 life), instead of lying at the very 
 outside of the cord, bore their way 
 through the midst of the lateral 
 strand of white matter. With this 
 and a few other exceptions, the 
 axons which lie near the gray mat- 
 ter are near their 
 cell bodies, while 
 those which lie to- 
 ward the outside 
 of the cord are far 
 from their place of 
 origin. For this 
 reason, that por- 
 tion of the white 
 matter which lies 
 next the gray is 
 called the " ground 
 bundle," while the 
 portions further 
 out are composed 
 in large measure 
 of definite groups or "tracts" of 
 axons connecting distant portions 
 of the cord and brain. Many of 
 the axons in the ground bundle 
 never extend far from their cells, 
 but turn back into the gray matter 
 and so connect neighboring levels 
 of the cord. From comparative 
 study of the nervous systems of 
 various orders of animals, it is 
 believed that these short, axons of the ground bundle represent 
 a primitive and universal system of connections within the nerve- 
 centres; while the long axons of the tracts represent a later and 
 higher development. 
 
 FIG. 17. Bifurcation of the Sensory 
 Root Fibres, at Their Entrance to the 
 Cord. (Cajal.) 
 
48 THE NERVOUS SYSTEM IN THE INDIVIDUAL 
 
 The sensory axons entering by the dorsal roots bifurcate into 
 ascending and descending branches, and these, like other axons, 
 entering the marginal layer, lie at first near to their point of entry 
 (compare Fig. 17). As they grow upward (the downward branches 
 remain short) they are subsequently crowded away from the region of 
 the dorsal roots by other similar axons. They therefore turn into the 
 marginal layer on the dorsal side of their roots, i. e., toward the mid- 
 dorsal line; and they become still further crowded toward this line 
 by the fresh fibres entering higher up the cord. In this manner a 
 thick column of sensory axons is formed between the dorsal roots and 
 the mi'd-dorsal line; and at each level of the cord, those axons which 
 lie nearest the mid-dorsal line have come from farthest down the cord, 
 while those which lie nearest the roots have come from near by. 
 
 The dorsal column of white matter is clearly marked off from the 
 rest of the white matter by the intervention of the dorsal roots and 
 the dorsal horn of the gray matter. The remainder of the white 
 matter is less completely divided by the ventral horn and roots into 
 a lateral and a ventral column. 
 
 8. The brain is formed from the forward or headward end of 
 the neural tube. In the human embryo of two weeks' growth the 
 tube shows at this end a considerable degree of complication. A 
 sharp bend appears in it, the front end being bent in the ventral 
 direction. In front of this "cephalic flexure" the tube expands 
 somewhat into a pouch or vesicle, and behind into another vesicle 
 which tapers downward into the spinal part of the tube. Just be- 
 fore and behind the flexure are two shallow transverse grooves which 
 mark off a third vesicle between the other two. These three primary 
 brain vesicles, appearing thus early in the individual's life, are con- 
 stant throughout all the vertebrates, and represent the fundamental 
 divisions of the brain. Already, however, a division of the fore- 
 most vesicle by another shallow transverse groove is visible in the 
 human embryo; and a little later a backward bend within the hind- 
 most vesicle makes it convenient to regard this vesicle, also, as sub- 
 divided. Thus there are five vesicles out of which the various parts 
 of the brain are developed. They are called, from before backward, 
 the end-brain, the inter-brain, the mid-brain, the hind-brain, and the 
 after-brain or medulla; or, in Greek derivatives, the telencephalon 
 ("encephalon" meaning brain), the diencephalon, the mesenceph- 
 alon, the metencephalon, and the myelencephalon. 1 Of these five 
 
 1 "Myel" is the Greek derivative equivalent to the Latin "medulla," which 
 means marrow. The spinal cord was called the "spinal marrow," or medulla 
 spinalis; and the enlarged extension of the cord at the base of the cranial cavity 
 was known as the medulla oblongata. Recently, the name "medulla spinalis" 
 has been little used, and "medulla" has come to mean the medulla oblongata. 
 The latter is also, most briefly, designated as the "bulb." 
 
GROWTH OF THE FIVE BRAIN VESICLES 
 
 the after-brain develops into the medulla; the hind-brain develops 
 into the pons and cerebellum; the mid-brain consists largely of the 
 corpora quadrigemina, and the inter-brain of the optic thalamus; 
 while the end-brain develops into the cerebrum. The eye, which is 
 embryologically a part of the brain, first appears as a pouch bulging 
 outw.ard at about the junction of the inter-brain and end-brain; 
 its permanent attachment is to the thalamus (compare Fig. 18). 
 
 In the lowest part of the medulla the conditions which affect the 
 external form of the parts do not differ essentially from those present 
 in the cord; the walls of the 
 neural tube thicken greatly, leav- 
 ing a small cavity in the centre. 
 A little further up, however, the 
 dorsal wall, without growing in 
 thickness, increases greatly in 
 width, and lies as a mere skin, 
 containing no nerve-cells, over 
 the expanded cavity, which here 
 is called the " fourth ventricle of 
 the brain." 
 
 Forward of this, the dorsal 
 wall becomes enormously thick- 
 ened, forming the cerebellum. 
 Still further forward, in the 
 mid-brain, the dorsal wall main- 
 tains a moderate thickness, and 
 swells into two hills on each side, 
 the corpora quadrigemina. The 
 cavity of the neural tube here 
 
 remains small, and forms the "aqueduct" connecting the fourth 
 with the third ventricle. It is in the inter-brain that the aqueduct 
 expands into the third ventricle; and here again the dorsal wall of 
 the tube becomes a broad, thin membrane, folded into the ventricle 
 and carrying blood-vessels with it. As the cerebral hemispheres 
 undergo their enormous growth, the cavity expands with them, and 
 remains connected with the third ventricle by two small openings, 
 the foramina of Munro. Within the hemispheres, the cavities are 
 known as the lateral ventricles; in the numbering they count as the 
 first and second. Thus, in spite of all its bendings and thicken- 
 ings, the neural tube remains a tube, and its lumen is continuous 
 from the lateral ventricles, through the foramina of Munro, the 
 third ventricle, the aqueduct, and the fourth ventricle, with the 
 central canal of the cord. This cavity is filled with a lymph known 
 as the cerebro-spinal fluid. 
 
 FIG. 18. Brain of a Developing Chick. 
 (Mihalkovics.) The numbers indicate: 
 I, the end-brain; II, the inter-brain; ///, 
 the mid-brain; IV, the hind-brain; V, the 
 after-brain or bulb, tapering downward 
 into the cord. 
 
50 
 
 THE NERVOUS SYSTEM IN THE INDIVIDUAL 
 
 9. In the end-brain of the two-weeks-old embryo, certain parts 
 can be distinguished which retain their identity throughout the later 
 development (compare Fig. 19). The ventral wall, on each side of 
 the middle line, shows a slight swelling, which becomes pronounced 
 at the age of four weeks, and later grows forward and is entered 
 
 Ganglion of fifth nerve Ear 
 
 Liver 
 
 Arm 
 
 Leg 
 
 FIG. 19. Human Embryo of About Four Weeks. (Streeter.) 1, 2, 3, 4, 5, the brain 
 vesicles; continuous with 5, the spinal cord can be followed down, close beneath the 
 ectoderm, to the posterior extremity. The spinal ganglia show as round, dark masses, 
 connected by (light) rootlets with the cord, and on the other side connected with the 
 nerves, which also show light. The rudiments of arm and leg show as transparent masses, 
 and a group of nerves can be seen entering each. 
 
 by axons of cells lying in the upper part of the nasal cavity. This 
 is the olfactory lobe. Its forward end swells into the olfactory bulb, 
 which receives the axons from the nose and contains nerve-cells 
 whose axons in turn pass back through the hinder part of the lobe 
 (the olfactory tract) to other parts of the brain. Just lateral to the 
 ventral portion of the end-brain out of which the olfactory lobe is 
 formed, there appears, at four weeks, a shallow furrow, which indi- 
 
GROWTH OF THE FIVE BRAIN VESICLES 51 
 
 cates the future position of the mass of gray matter in the base of 
 the hemispheres known as the corpus striatum. The dorsal half 
 of the end-brain shows, even at two weeks, a slight swelling, which 
 grows rapidly. This is called the "brain-mantle," or "pallium," 
 and it gives rise to the great bulk of the cerebral hemispheres. The 
 pallium is the part of the nervous system whose complex develop- 
 ment characterizes the higher mammals, and especially man. 
 
 In comprehending the general shape of the cerebral hemispheres, 
 there is one fact of prime importance. The great increase in size 
 of this part of the neural tube does not proceed by further extension 
 of its length, from its end forward. The middle of the forward 
 end does not itself grow, but remains as a fixed limit, or so-called 
 "lamina terminalis," which is a membrane without nervous func- 
 tions. The growth of the end-brain" may, therefore, be described 
 as primarily lateral; and then, within each lateral half, forward, 
 backward, and upward, as well as to the side. One result of 
 this method of growth is the division of the swelling pallium into 
 right and left hemispheres a division which appears very early 
 in the growth of the embryo. Accompanying this is the enlarge- 
 ment of the cavity which forms the lateral ventricles, and which 
 extends forward and backward within each hemisphere. The back- 
 ward growth of the hemispheres causes them to overlap the thala- 
 mus and mid-brain, and finally, in the human species, the cere- 
 bellum as well. The corpora striata grow forward and backward 
 with the pallium, and overlap the thalamus so as to appear, in the 
 adult brain, rather to the side of the thalamus than in front of it. 
 A vertical or "frontal" section through the midst of the brain shows, 
 at the exterior surface, the cortex; next this the white matter of the 
 hemispheres; then the corpus striatum; and in the centre the thal- 
 amus. 
 
 10. As the hemispheres grow forward, backward, and upward 
 from their place of attachment to the inter-brain, each of them comes 
 to present a two-lobed appearance, somewhat like that of a bean, 
 the notch corresponding to the point of attachment. The front and 
 rear lobes grow at first downward, and then toward each other, 
 making the notch deeper; the wall above the notch grows out and 
 overhangs it. Thus is formed a fissure, the "fissure of Sylvius," 
 the first and principal landmark on the lateral surface of the hemi- 
 sphere. The part in front of this fissure is the frontal lobe, and that 
 behind and below it is the temporal lobe. The surface within the 
 fissure, which is considerable, is clearly visible at the fourth month 
 of foetal life, but becomes partly concealed at the seventh month, 
 by the overgrowth of the frontal and temporal lobes; in the fully 
 developed brain it is wholly concealed, and from its secluded posi- 
 
52 THE NERVOUS SYSTEM IN THE INDIVIDUAL 
 
 tion is called the " island." Both the Sylvian fissure and the longi- 
 tudinal fissure which separates the hemispheres are visible during 
 the second month; in the succeeding months many other fissures 
 appear, till the surface becomes thickly seamed with them. 
 
 Lateral 
 ventricle 
 
 Thalamus 
 
 Lenticular. 
 
 FIG. 20. Frontal Section of the Fore-brain of an Eleven Weeks' Embryo. (His.) The skull 
 as well as the brain is shown, and, on the left side of the figure, the eye. Gray matter 
 appears dark, and white matter light; the skull also appears light, and the ventricular 
 spaces, or cavity of the neural tube, appear white. There is also a clear space between 
 the skull and the brain. The two largest and nearly equal spaces are the lateral ven- 
 tricles; each is surrounded by the pallium, except below, where it is bounded by the 
 dark rounded mass of the caudate nucleus, a part of the striatum. On the left side of 
 the figure, the caudate nucleus is wholly separated by a white mass of fibres from another, 
 less dark nucleus, the lenticular, which also is part of the striatum. The mass of fibres 
 separating these two parts of the striatum is the internal capsule, and the fibres connect 
 the pallium with the thalamus. The thalamus appears as two nearly semicircular masses 
 of gray matter, separated by a median cleft, which is the third ventricle, part of the cavity 
 of the tube. On the right side, thalamus and striatum are not yet joined by the crossing 
 fibres. The pallium is thickest at its junction with the striatum; it shows three layers, 
 the germinal layer (dark) next the lateral ventricle, a light fibre-layer next, and on the 
 outside another dark layer of cells, the cortex. The lowermost part of the brain, in the 
 figure, is olfactory. Into each lateral ventricle extends a membrane, which is a part of 
 the wall of the tube doubled inward and carrying blood-vessels. 
 
 The development of neuroglia, nerve-cells, and axons in the brain 
 proceeds in essentially the same manner as in the cord. The germi- 
 nal cells, which by division give birth to cells destined to grow into 
 neuroglia and nerve-cells, in the brain as in the cord, lie next to the 
 cavity of the neural tube. The primitive neuroglia framework is 
 first formed; after which the cells that migrate from the germinal 
 
DEVELOPMENT OF THE CORTEX 
 
 53 
 
 layer into the middle or cell layer begin to take on the character- 
 istics of nerve-cells. Thus there appear in the developing cortex of 
 the cerebrum the same three layers as in the cord: the germinal 
 layer inside, then the nerve-cell layer, and, outermost, the marginal 
 layer. But there is one marked difference between the cord and the 
 cortex. The axons of the nerve-cells, instead of issuing from the 
 cell layer into the marginal layer, go from their cells in the opposite di- 
 rection, toward the 
 germinal layer. 
 Before reaching this, 
 they turn to the side; 
 and so form a layer 
 of axons between the 
 cell layer and the in- 
 ner cavity or ventri- 
 cle. Thus it is that 
 the white matter of 
 the hemispheres lies 
 inside the gray, in- 
 stead of outside as 
 in the cord. From 
 its position on the 
 outside, the gray mat- 
 ter of the hemispheres 
 has been named the 
 "bark "or "cortex." 
 The first nerve-fibres 
 to appear in the 
 white matter of the 
 cerebrum do not 
 arise from the cor- 
 tex, but come into 
 the hemispheres 
 from below. They 
 arise from cells in 
 the thalamus, and break through the corpus striatum, dividing its 
 gray matter into two principal masses, and forming the "internal 
 capsule" between them. From here they pass into the adjoining 
 wall of the hemisphere, and run along in it, between the germinal 
 and nerve-cell layers. The axons from the thalamus are sensory: 
 they come from groups of nerve-cells, which are connected with the 
 nervous apparatus of sight, hearing, and touch; and they termi- 
 nate in areas of the cortex which come thus to be chiefly concerned 
 with the corresponding senses. 
 
 FIG. 21. Brain of an Embryo of About Three Months. 
 (His.) Inter-brain and mid-brain are already concealed 
 by the pallium; the cerebellum and bulb are shown; also, 
 through the fissure of Sylvius, some of the olfactory re- 
 gion can be seen. 
 
54 THE NERVOUS SYSTEM IN THE INDIVIDUAL 
 
 11. The cortex develops late in comparison with other parts of 
 the nervous system. Nerve-cells begin to appear here at about the 
 beginning of the third month of foetal life, or more than a month later 
 than in the cord (compare Fig. 20). The appearance of the axons 
 and dendrites of the cortical cells is also late, and indeed the cor- 
 tex is by no means fully developed at birth. The specialization of 
 the pallial wall into a nervous structure, with gray cortex and under- 
 lying white matter, begins at a definite part of the wall; namely, 
 where it joins the corpus striatum, or at the junction of the pallial 
 
 FIG. 22. Frontal Section of the Brain at the End of the Fourth 
 Month. (His.) The callosum appears clearly. At each side of 
 the figure a depression in the wall of the hemisphere indicates 
 the fissure of Sylvius. 
 
 outgrowth with the main stem of the neural tube. From here, the 
 development of a cortex spreads up over the lateral surface, and 
 finally over the top down into the median fissure between the hemi- 
 spheres. Near the bottom of this fissure the cortex ends, leaving a 
 narrow strip of the wall free from the usual covering of gray matter. 
 The exposed strip of white matter of the right hemisphere is in close 
 proximity to that of the left, and at a later stage the two strips grow 
 together in the central portion of their extent, and thus the white mat- 
 ter of the two hemispheres becomes continuous. Axons, in vast 
 numbers, here cross over from one hemisphere to the other, bring- 
 ing the two into functional connection. This bridge between the 
 hemispheres is the corpus callosum (see Fig. 22). 
 
 12. In the cerebellum, the process of inner growth is essen- 
 tially the same as in the cerebrum. Here, too, the axons of the cells 
 which have reached the nerve-cell layer pass inward, and not 
 toward the marginal layer, so that the gray matter lies on the sur- 
 
THE TWELVE PAIRS OF CRANIAL NERVES 55 
 
 face and the white matter inside. Possibly the axons take the inner 
 course because, in outgrowths of the tube such as the cerebrum and 
 cerebellum, this is the shortest path to the fundamental centres of 
 the tube proper. The external location of the gray matter, further, 
 favors the spreading out of its great mass into a thin layer readily 
 accessible at all points to axons coming from without. In the cere- 
 bellum also, as in the cerebrum, there are, besides the cortex, other 
 collections of nerve-cells lying in the basal part near to its attach- 
 ment. The cerebellum resembles the cerebrum also in its late de- 
 velopment; in man its nervous structures are far from complete at 
 birth. 
 
 13. The brain-stem cannot be properly understood without 
 reference to the nerves which issue from it. These are called the 
 cranial nerves, and there are twelve of them on each side, desig- 
 nated by numbers and also by names more or less expressive of 
 their distribution and function. The list comprises: 
 
 I. The olfactory. 
 
 II. The optic. 
 
 III. The oculomotor. This is the principal motor nerve to the 
 eye; it innervates the muscle of accommodation, and also four of the 
 six muscles which move the eyeball. 
 
 IV. The trochlear. This is the motor nerve to the "trochlear" 
 or superior oblique muscle of the eyeball. 
 
 V. The trigeminal, so called because it promptly splits into three 
 branches, which, between them, supply the skin of the face, and the 
 sensitive surfaces of the eyeball, nose, and front of the mouth, with 
 sensory axons. The fifth pair is thus the nerve of "touch" for the 
 whole face. It also contains motor axons in smaller quantity; these 
 innervate the muscles of mastication. 
 
 VI. The abducens, the motor nerve to the external rectus muscle 
 of the eyeball, which rolls the eye to the side. 
 
 VII. The facial. This is the main motor nerve of the face, and 
 controls the muscles of facial expression. It also includes a small 
 and distinct bundle of sensory fibres, which come from the taste- 
 buds of the front portion of the tongue. 
 
 VIII. The auditory. This again consists of two distinct por- 
 tions, both sensory, one of which is connected with the cochlea of 
 the inner ear, the organ of hearing, and the other with the remainder 
 of the inner ear, which apparently has to do, not with hearing, but 
 with sensations of movement and posture. 
 
 IX. The glosso-pharyngeal, or sensory nerve to the back of the 
 tongue and to the throat, including the taste-buds in this region. 
 
 X. The so-called vagus, in allusion to its wide or wandering dis- 
 tribution. It contains both sensory and motor fibres between the 
 
56 THE NERVOUS SYSTEM IN THE INDIVIDUAL 
 
 central organs and the larynx, windpipe, lungs, gullet, stomach, 
 pancreas, upper part of intestine, and heart. The outgoing fibres 
 to the heart can be called "motor" only in a loose sense, since their 
 action is confined to the slowing or stoppage of the heart-beat. 
 
 XL The spinal accessory, a motor nerve to some of the muscles 
 at the back of the neck which participate in head movements. 
 
 XII. The hypoglossal, the motor nerve of the tongue. 
 
 Of these twelve pairs of nerves the first consists of axons arising 
 from cells in the olfactory mucous membrane of the upper part of 
 the nose, these cells being the peripheral organs of smell, and being 
 derived from the ectoderm. They thus represent a primitive con- 
 dition of the sensory nerves (compare p. 26), in which the sensitive 
 cells located in the periphery have sent axons to the centre, instead 
 of being supplied by axons growing out from the centre or from 
 ganglia located near the centre. The axons of the olfactory nerve 
 enter the brain at the olfactory bulb, as previously described. 
 
 14. The optic nerve also is peculiar in its origin, but in a differ- 
 ent way. Both this nerve and the retina of the eye arise as an out- 
 growth of the neural tube; they are, therefore, properly to be reck- 
 oned as a part of the brain. Besides the rods and cones, which are 
 specialized nerve-cells sensitive to light, the retina contains many 
 nerve-cells similar to those found in the nervous centres; and some 
 of these send their axons back through the optic nerve. The nerves 
 from the right and left eyes meet at the base of the brain in the optic 
 "chiasm" or crossing. Here many of the axons cross to the other 
 half of the body; and after crossing, the nerves proceed and enter 
 the thalamus. 
 
 The nerve of the inner ear is somewhat peculiar in its mode of 
 development, in that its ganglion lies within the ear itself; and 
 besides, the cells in this ganglion retain their "bipolar" form. 
 
 The remaining cranial nerves develop in the same manner as the 
 spinal nerves. Their sensory axons arise from ganglia, analogous 
 to the spinal ganglia, which lie on the roots of the nerves; and their 
 motor axons grow out from cells in the tube 
 
 15. The process of development, as well as the final structure, 
 of the brain-stem is highly complicated, and it will be sufficient for 
 our purposes to mention one general principle which is of consid- 
 erable aid in understanding the whole matter. We make a distinc- 
 tion between the more and the less primitive structures contained 
 in any part. The more primitive include (1) the nerves entering 
 or leaving the part, (2) the groups of cells connected with these 
 nerves, and (3) the fibres which link together these groups of cells. 
 These, taken together, form the local mechanism of the part. Less 
 primitive, in relation to any particular region, are fibres which grow 
 
FACTORS IN GROWTH OF THE NERVOUS SYSTEM 57 
 
 into it from other parts of the neural tube, and also any cells de- 
 veloped locally but having connection with fibres from other parts. 
 Now since the local mechanisms of any part develop in advance of 
 the less primitive structures, these accordingly grow over the primi- 
 tive structures, here as in the spinal cord. Thus the order of de- 
 velopment helps toward an understanding of the position of the 
 numerous structures which are found within the brain-stem. All 
 along the neural tube, it is the ventral side which develops first; 
 and accordingly we find, in the adult condition of the brain-stem, 
 that the most primitive structures lie on the ventral side of the ventri- 
 cles, and close to them. In the bulb, as was previously said, the 
 dorsal side of the neural tube merely expands into a membrane. 
 The lateral wall of the tube, however, gives rise to many nerve-cells, 
 and these, developing later than the ventral portion, are less primi- 
 tive and more related to distant parts of the system. Many of these 
 lateral cells migrate in a ventral direction, and settle in positions 
 ventral to the original ventral cells; and the fibres which in immense 
 numbers grow into the bulb from above and below, pass on either 
 the ventral or the lateral side of the primitive structures. Thus it 
 finally comes about that the bulb contains, close beneath the ven- 
 tricle, a rather small central mass devoted to the local mechanisms, 
 and giving rise to the local nerves; while enclosing this mass on the 
 sides and below is the large mass of cells and fibres which are less 
 local in character. The development of the pons and mid-brain 
 proceeds along the same lines as that of the bulb, except that in the 
 mid-brain there is an important development of the dorsal wall of 
 the tube, which gives rise to the corpora quadrigemina. 
 
 16. The factors in the growth of the nervous system, as thus 
 far mentioned and applied, are (1) the production of young nerve- 
 cells from the germinal cells at the lumen of the tube; (2) the migra- 
 tion of these young nerve-cells away from the lumen; and (3) the 
 outgrowth and prolongation of the axon. The first of these factors 
 is only temporary in its action; the reproductive activity in the ger- 
 minal layer comes to an end at different times in different parts of 
 the brain and cord, but in the cortex of the cerebrum, where it 
 starts latest and is longest continued, it ceases during the fourth 
 month of fcetal life. It is probable that all the nerve-cells which will 
 finally belong to the individual adult are formed at this early period 
 in embryonic development. The migration of young nerve-cells 
 is also in the main a temporary phenomenon. The lengthening 
 of the axon, on the contrary, is continued for a long time, and the 
 growth of the dendrites of the cell is also a slow and long-continued 
 process. Many of the young nerve-cells remain throughout foetal 
 life and even much longer in an undeveloped state. Many of them 
 
58 THE NERVOUS SYSTEM IN THE INDIVIDUAL 
 
 never develop. It appears that undeveloped nerve-cells in the cor- 
 tex can be made to grow by stimuli affecting the region where they 
 lie and arousing neighboring cells to activity in other words, by 
 experience and education. For example, Donaldson, 1 on examin- 
 ing the brain of Laura Bridgman, who became blind and deaf when 
 a baby of less than two years, found those parts of the cortex which 
 are normally connected with the eye and ear to be under-developed, 
 and to contain an unusual number of undeveloped nerve-cells. The 
 growth of the gray matter, after the production of new nerve-cells 
 has ceased, results from the growth of the cells and especially of 
 their dendrites; from the coming-in of axons from the white matter 
 to branch and terminate in the gray matter; and from the continued 
 growth of the neuroglia framework. 
 
 The increase of the white matter in the nervous system is due in 
 part to the lengthening of axons and the advent of new axons from 
 the developing cells in the gray. It is also partly due to a growth 
 in thickness of the individual axons. But it is very largely due to 
 another factor which has barely been mentioned thus far. The 
 axon becomes invested with a sheath of " myelin," a mixture of fat- 
 like substances; and the volume of this sheath, at first small, in- 
 creases gradually till it often becomes much greater than the vol- 
 ume of the axon which it invests. Whether this myelin is secreted 
 by the axon, or by neuroglia cells, is not yet determined. The 
 myelinization of an axon does not occur as soon as the axon has 
 made its appearance, but there is often a considerable interval 
 during which the axon remains without any sheath. In general, 
 those bundles of axons which develop early also acquire myelin 
 early, and those bundles which appear late become myelinated late. 
 Thus, for example, the ventral and dorsal root axons and the short 
 axons of the ground bundle become myelinated early; while the latest 
 of all, in the spinal cord, are the long descending axons which come 
 from cells in the cerebral cortex. Within the cerebrum, the first 
 axons to appear are those from the thalamus, and these also, after 
 an interval of several months, are the first to become myelinated. 
 The inward-growing or sensory axons of the cerebrum develop 
 before the outward-growing or motor; and these in turn before the 
 numerous "association" axons which extend from one part of the 
 cortex to another. 
 
 17. The function of the myelin sheath, to which we shall re- 
 turn in a later chapter, is not definitely known; but it seems rea- 
 sonable to suppose that it has some function, and that therefore the 
 axon itself does not become fully functional till it has become envel- 
 
 1 American Journal of Psychology, 1890, III, 293. Growth of the Brain, 1898, 
 p. 240. 
 
FACTORS IN GROWTH OF THE NERVOUS SYSTEM 59 
 
 oped by its sheath. It was an easy inference that a given bundle of 
 axons, and the connected portions of gray matter, did not become 
 functional at all till these axons were myelinated; and on this basis 
 it seemed possible to assert (so Flechsig 1 ) that, within the cortex, 
 the sensory portions first became functional, then the motor, and 
 later still the remaining and presumably more intellectual parts. 
 It has, however, been shown by Watson 2 that in the white rat, co- 
 ordinated reflex action, requiring the use of both sensory and motor 
 axons and also their central connections, occurs promptly at birth, 
 though no myelinization whatever has so far taken place; and further 
 that the rat is capable of forming and retaining definite associations 
 at an age when its cortex, which is in all probability concerned in 
 such learning, is entirely unprovided with myelinated connections. 
 The case of the rat, therefore, seems conclusive against the view 
 that orderly function is impossible before the advent of myeliniza- 
 tion. On the other hand, von Bechterew reports 3 that sharply 
 localized response to electrical stimulation of small portions of the 
 nerve-centres occurs only after myelinization has set in ; and Held 4 
 found that opening one eye of a new-born animal whose eyes do 
 not naturally open for some time after birth causes the central 
 connections of this eye to become myelinated earlier than those of 
 the other eye. W r e may, perhaps, conclude that, in some cases at 
 least, function is possible without myelin; but that actual function- 
 ing stimulates the deposition of the myelin, which in turn in some 
 way assists function. There is also evidence to show, in the case of 
 the human species, that control of the muscular system which we 
 call "voluntary" requires to make use of myelinated nerve- 
 tracts. 
 
 18. The weight of the human brain at birth averages about 
 380 grams, or 13 ounces. This is about one-eighth of the weight 
 of the entire body. During childhood and youth, other organs 
 rapidly outstrip the brain, so that at maturity the brain weighs but 
 one-fiftieth of the entire body. The spinal cord at birth weighs 
 one one-hundredth as much as the brain, and at maturity one- 
 fiftieth. In other words, the growth of the brain is carried further in 
 foetal life than the growth of the other organs. This is true as re- 
 gards size, but not, perhaps, as regards the minute details of struct- 
 ure which count for so much in the functions of the nerve-centres. 
 
 1 Flechsig, Gehirn und Seele, 1896. 
 
 2 J. B. Watson, Animal Education, 1903, pp. 115 ff. 
 
 3 W. v. Bechterew, Die Leitungsbahnen im Gehirn und Riickenmark, 1899, 
 p. 104. 
 
 4 Cited from Edinger, Vorlesungen uber den Ban der nervosen Zentralorgane, 
 1904, p. 32. 
 
60 THE NERVOUS SYSTEM IN THE INDIVIDUAL 
 
 The increase of the brain's weight occurs mostly in childhood, as 
 is seen from the following figures: 
 
 At birth, the weight of the brain averages . . . 380 grams 
 
 At one year, " " " " " " ... 945 " 
 
 At two years, " " " " " " ... 1,025 " 
 
 At three years, " " " " " " ... 1,100 " 
 
 At four years, " " " " " " ... 1,300 " 
 
 After the age of four, the human brain continues to increase more 
 slowly, reaching 1,400 grams at from eight to ten years; after which 
 it gains in weight scarcely at all. According to some series of meas- 
 urements, however, the maximum weight is reached at fifteen, after 
 which age a very gradual decrease sets in. There is no doubt that 
 there is uniformly some decrease after the age of fifty-five ; and in old 
 age, there is often a marked shrinking in its weight. The above 
 figures apply to the brain of males. In the female, the weight at 
 birth is nearly the same as in the male; the increase during child- 
 hood is somewhat less, and the adult weight averages about 150 
 grams less than for the male. In proportion to the total weight 
 of the body, however, the female brain is as heavy as that of the male; 
 or it may be slightly heavier. Brain-weight also depends to some 
 extent on stature; distinctly tall individuals averaging about 2 per 
 cent, more than distinctly short individuals. 
 
 It should be noted, however, that the figures quoted above are 
 i gross averages, and that there is much variation. The extreme 
 range, if idiots are excluded", is from 1,000 to perhaps 1,900 grams; 
 90 per cent, of males lying between the limits 1,200 and 1,600 
 grams, and 60 per cent, between 1,300 and 1,500. Most of the 
 individuals whose brains have been weighed on autopsy, and have 
 thus formed the basis for scientific study, 1 have been inmates of 
 workhouses and similar public institutions; they therefore repre- 
 sent the poorer and less successful portion of the population. There 
 is some evidence that the more prosperous and successful portion 
 have, on the average, somewhat heavier brains, and that the growth 
 of their brains is longer continued. This evidence is derived from 
 the post-mortem weight of the brains, sometimes of men who were 
 so eminent for mental ability that the examination was desired in 
 the interests of science, and sometimes of men who realized the 
 
 1 The figures given have been taken from Donaldson, who, in his work on The 
 Growth of the Brain, 1898, has brought together the results of the most reliable 
 series of measurements by various authorities, and subjected them to careful 
 analysis. The brain-weights given for the ages from six to sixteen are derived 
 from the measurement of so few individuals that it has not seemed worth while 
 to attempt to trace the full curve of growth. The main outlines, however, as 
 given in the text seem fairly reliable. 
 
VARIATIONS IN WEIGHT OF HUMAN BRAIN 61 
 
 value to science of such an examination, and therefore directed 
 that their brains should be made available for study. As a result, 
 there are now on record the brain-weights of about one hundred 
 men of more or less eminence. The average of these is about 
 1,470 grams, which is two to four per cent, above the average of 
 workhouse inmates. They range from 1,200 to 2,000 grams, 
 overlapping the range of ordinary men to such an extent that it 
 would clearly be impossible to draw any conclusion, from the mental 
 achievements of an individual, as to his brain-weight, or vice versa. 
 A few well-known names may be cited from the list: 1 
 
 Cuvier, naturalist 1830 grams 
 
 Thackeray, novelist 1658 
 
 Siemens, physicist 1600 
 
 Daniel Webster, statesman 1518 
 
 Chalmers, theologian 1503 
 
 Agassiz, naturalist 1495 
 
 De Morgan, mathematician 1494 
 
 Gauss, mathematician 1492 
 
 Broca, anthropologist 1484 
 
 Grote, historian 1410 
 
 Bertillon, anthropologist 1398 " 
 
 Liebig, chemist V. . . ... . .' . . 1352 " 
 
 The average of this group of certainly eminent minds is above 
 that of the whole number of more or less eminent men. But the 
 correlation between eminence and brain-weight is, at the best, far 
 from close. 
 
 19. The question of decrease of brain-weight with advanced age 
 can be examined in the light of the evidence from the lists of " emi- 
 nent" men, who may be taken as at least representing the more 
 intellectual part of the population. As few young men are included 
 in the lists, it will be necessary to group all not older than fifty-five 
 together. The averages for the different ages come out as follows: 
 
 No. of 
 Individuals 
 
 Ages 
 
 Av. Brain- 
 Weight 
 
 32 
 33 
 24 
 15 
 
 25-55 
 56-65 
 66-75 
 
 76-89 
 
 1482 
 1492 
 1448 
 1389 
 
 In view of the wide variation of the individual cases, so small a 
 difference as that between the first and second age-groups must not 
 
 1 These data are quoted from E. A. Spitzka, "A Study of the Brains of Six 
 Eminent Scientists," etc., in Transactions of the American Philosophical Society, 
 Philadelphia, 1907. In this paper he has brought together all previous records 
 of the brain- weight of eminent men. 
 
62 THE NERVOUS SYSTEM IN THE INDIVIDUAL 
 
 be taken to mean that the brain-weight of this class of men increases 
 beyond the age of fifty-five; it is conceivable that it should do so, 
 but the data are not sufficient to show it. There is, however, some 
 indication that the brain-weight does not start to decrease much 
 before sixty-five. The measurements of the less favored and in- 
 tellectual classes show 1 a loss of about 50 grams from the age of 
 fifty-five to that of sixty-five, and an equal loss in the succeeding 
 decade of life. It is probable, therefore, that the brain reaches a 
 greater size in the more intellectual classes, and maintains its weight 
 longer. The differences in size are, however, too small to serve as a 
 measure of the differences between men in intellectual ability. We 
 must conceive the growth of the brain which takes place after the 
 age of eight to consist for the most part in the development of very 
 minute structures, such as the dendrites and the fine terminations 
 of axons. These structures are individually so small that a vast 
 increase in their number would make but a small impression on the 
 total weight of the brain. 
 
 20. It is the cerebrum which we should expect to show most de- 
 velopment after early childhood, because it is probably this part of 
 the nervous system which is chiefly concerned with learning, educa- 
 tion, and all individual acquisitions. More definitely, the develop- 
 ment to be expected would occur in the cortex, and in the axons 
 connecting its various parts. That such development does take 
 place is not merely to be expected; but it seems to be demonstrated 
 by microscopical study. Already mentioned are the observations 
 tending to show an increase in the number of developed nerve-cells 
 (with their dendrites) as the result of use. There are other obser- 
 vations showing an increase of the myelinated axons and branches 
 of axons traversing the cortex. The axons which enter the cortex 
 from the white matter below are found to penetrate further and 
 further into the cortex, or at least to become myelinated further and 
 further, as the individual advances toward maturity. Axons and 
 their collaterals which traverse the cortex in directions parallel to 
 its surface, both in the outer or marginal layer and in two or more 
 layers in the midst of the nerve-cells, also increase greatly in myeliniz- 
 ation as late as the thirtieth year of life, and probably later still. This 
 growth in complexity of organization of the cortex appears sufficient, 
 so far as the evidence goes, to be paralleled with the increasing com- 
 plexity of mental capabilities which occurs during early life. 2 
 
 1 See Donaldson, The Growth of the Brain, 1898, p. 325. 
 
 2 See Watson, op. cit., p. 103; Kaes, Archiv fur Psychiatric und Nervenkrank- 
 heiten, 1894, XXV, p. 695; Donaldson, Growth of the Brain, 1898, p. 241; Edinger, 
 Vorlesungen uber den Ban der nervosen Zentralorgane, 7th ed., 1904, vol. I, p. 335. 
 
CHAPTER III 
 
 GROSS STRUCTURE OF THE NERVOUS SYSTEM 1 
 
 1. Regarded as isolated, and as possessed only of those proper- 
 ties which belong to all living matter of the peculiar chemical con- 
 stitution and structural form which are described by the word 
 "nervous," the nerve-fibres and nerve-cells are of great interest to 
 physiological and psycho-physical researches. But in their normal 
 position and activity they are always combined into organs, which 
 are then arranged in a symmetrical whole. Thus combined they 
 are dependent upon each other for the parts which they play in the 
 entire system. The condition and function of each element are 
 thus determined by the condition and function of the rest. One 
 part of this system excites another, or modifies the excitation re- 
 ceived from another. We are, therefore, unable to isolate perfectly 
 any one of these elements, and so study its normal functions apart. 
 It is, indeed, possible to dissect out a nerve with a muscle attached, 
 to keep it alive for a time, and thus to inquire what an isolated nerve 
 will do. In this way many of the most important discoveries in the 
 general physiology of the nerves have been made. But every nerve 
 is itself a compound of nervous elements which have been placed 
 for purposes of experiment under abnormal conditions. The action 
 of the nerve-cells, even when gathered into small masses called 
 ganglia, is not easily open to direct inspection. Moreover, when 
 different tracts of nerves, or different regions in the central organs 
 where ganglion-cells abound, are partially isolated by being laid 
 bare for the direct application of stimulus, just so far as they are 
 separated from the system they are in abnormal condition and show 
 abnormal results; and just so far as they are normal in condition and 
 function they are still connected with the system. It is the mutual 
 condition and reciprocal action of the elements, when combined 
 into this totality, which constitute the nervous mechanism. To 
 describe in brief outline the gross structure of this mechanism is 
 the purpose of this chapter. 
 
 1 Among the very numerous and excellent treatments of the anatomy of the 
 nervous centres, reference may be made to Edinger, Vorlesungen uber den Ban 
 der nervosen Zentralorgane, 7th ed., Leipzig, 1904; Schafer and Symington, 
 " Neurology," in Quain's Anatomy, llth ed., London, 1909; O. S. Strong, in 
 Bailey's Textbook of Histology, New York, 1910. 
 
 63 
 
64 GROSS STRUCTURE OF THE NERVOUS SYSTEM 
 
 2. It will be of great service toward understanding such a de- 
 scription if it is begun under the guidance of some appropriate idea. 
 Nerve-fibres and nerve-cells exist in enormous numbers within the 
 human nervous system, and are combined in different proportions 
 to make the different organs of this system. The significance of 
 the combination appears only in the light of reflection upon the 
 amount and kind of work which is to be done. The most general 
 office of the nervous mechanism may be said to be that of " concate- 
 nating" all the functions of the living body in accordance with the 
 complex internal and external conditions to which it is subject. 
 But in the case of any of the higher animals, and especially in the 
 case of man, this one office requires the doing of a quantity and 
 variety of work that are proportionate to the complexity of these 
 conditions. How shall such a quantity and variety of work be done ? 
 The actual arrangement of the elements of this system, in the exer- 
 cise of their reciprocally conditioned activities, is the solution of 
 the problem. As in all very complex questions of this sort, so this 
 particular problem is solved by a wise division of labor. 
 I The manner in which the human nervous mechanism is developed 
 as a response to the before-mentioned problem has already been 
 made clear in its general outlines, especially as applied to the lower 
 forms of animal life. Even the simple protoplasmic speck called 
 an amceba may be considered as a living molecular mechanism. 
 Minute and almost structureless as it appears, the amoeba is really 
 composed of a great number of molecules that are undergoing con- 
 stant change; and, as we have seen, it is capable of exercising several 
 wonderful functions that do not belong to any non-living collection 
 of molecules. Most important for our present purpose is the fact 
 that the amceba is irritable and automatic. With these properties 
 the molecular mechanism of this small bit of protoplasm, under the 
 stimulus of changes in the pressure and temperature of its medium, 
 and in accordance with the unknown laws of its internal self-orig- 
 inating changes, solves the problem of constant readjustment which 
 its environment presents to it. 
 
 3. Let it be supposed that the same problem becomes more 
 complicated, and the animal structure which is to solve it corre- 
 spondingly complex. The metabolic function of the animal may 
 then be assigned to a separate system of structures; and the closely 
 related secretory and excretory functions as well. The repro- 
 ductive function may then also acquire its own peculiar organs. 
 The muscles perform movements in masses because they retain in 
 an eminent degree the "amoeboid" contractility. But the property 
 of being irritable and automatic becomes the special endowment of 
 the nervous system. All these different systems, in order that they 
 
DIFFERENTIATION OF NERVOUS FUNCTION 65 
 
 may be moved in united masses, are then adjusted to a mechanical 
 framework (of indifferent value so far as really vital changes are 
 concerned) of cartilage, bone, etc. 
 
 But the eminently irritable and automatic system of molecules 
 called nervous must also undergo a further differentiation of func- 
 tion. In the structureless protoplasm of the amoeba, the external 
 molecules are, of course, the ones primarily to be affected by the 
 external stimuli. It is with the internal molecules, on the other 
 hand, that the changes called "automatic" begin. But the con- 
 tinual flux of its protoplasmic substance indicates that, in its sim- 
 plest form, any of the molecules of the animalcule may in turn act 
 either as irritable or as automatic. The primary differentiation of 
 this substance by the so-called "surface of separation" (see p. 14) 
 points, however, to a division of labor. 
 
 We have already seen that, in the somewhat higher forms of ani- 
 mal life, we come upon an increased differentiation of parts into 
 "receptors," "conductors," and "effectors" (see p. 16), and a cor- 
 responding further division of labor. When a somewhat compli- 
 cated nervous mechanism appears in the ascending scale of animal 
 development, the idea which lies at the base of this rudimentary 
 differentiation of the system calls for these three kinds of nervous 
 substance: (1) superficial cells susceptible to external stimuli; (2) 
 central and eminently automatic cells, also susceptible to internal 
 stimuli; (3) a strand of irritable protoplasm connecting the two. 
 In order that the more highly organized animal may exercise "a 
 will of its own," certain of its muscle-fibres must be placed under 
 the control of the central and automatic cells. In order, also, that 
 the entire muscular system may feel the reflex influence of external 
 stimuli, and so, by co-ordinated contractions, adapt the organs of 
 the body to the changes of its environment, the muscle-fibres must 
 be indirectly connected with such superficial cells as are sensitive 
 to these stimuli. The nervous system, therefore, in its most funda- 
 mental form consists of these three sets of contrivances with their 
 respective functions: (^L) sensitive cells upon the surface of the body; 
 (B) central cells that are both automatic and modifiers and dis- 
 tributers of sensory impulses; (0) connecting tracts, or strands, that 
 can convey the nervous impulses either centripetally from A to B, 
 or centrifugally from B to the contractile muscular tissues of the 
 body. 
 
 Higher developments of this triple-formed fundamental type of 
 a nervous system are reached by further differentiations of A, B, 
 and C. If various kinds of stimuli are to act upon this system, then 
 the sensitive cells upon the surface (^4) must be modified into vari- 
 ous external organs of sense. The terminations of the centrifugal 
 
FIG. 23. View of the Cerebro-spinal Axis. 
 (After Bourgery.) . The right half of the 
 cranium and trunk has been removed, and the 
 roots of the spinal nerves dissected out and 
 laid on their several vertebrae. F, T, O, cere- 
 brum; C, cerebellum; P, pons^ Varolii; mo, 
 medulla oblongata; ms,ms, upper and lower 
 extremities of the spinal marrow. Cl to 
 CVIII are cervical nerves; DI to DXII, 
 dorsal; LI to LV, lumbar; SI to SV, sacral; 
 Col, coccygeal. 
 
SYMPATHETIC AND CEREBROSPINAL 67 
 
 or motor nervous strands may also be variously modified so as to 
 connect with and control the contractile tissue of many sets of mus- 
 cles. The central cells may be variously grouped and arranged, 
 with functions more or less localized, so as to receive, modify, and 
 distribute, in manifold ways, the different sensory impulses. Other 
 such central cells may become more particularly related to the phe- 
 nomena of conscious sensation and volition. 
 
 Such a highly developed nervous system will then consist of the 
 following parts: (^4) End-organs of Sense, like the skin, the eye, and 
 the ear; (A 1 ) End-organs of Motion, like the so-called motor end- 
 plates; (B) Central Organs, like the various peripheral and sporadic 
 ganglia, the spinal cord, and brain, in which may come to exist 
 (6) certain portions more distinctively automatic, (6 1 ) certain others 
 more concerned in receiving and distributing reflexly the sensory 
 impulses, and (fr 11 ) still others more particularly connected with the 
 phenomena of consciousness; and (C) Conducting Nerves, which will 
 be either (c) centripetal, afferent, and sensory, or (c 1 ) centrifugal, 
 efferent, and motor, designed to connect the central organs and the 
 end-organs. 
 
 4. The nerves and ganglionic masses of nervous matter in the 
 human body are arranged in two great systems, the Sympathetic 
 and the Cerebro-spinal. The Sympathetic Nervous System con- 
 sists of a pair of nervous cords, situated one on each side of the spinal 
 column; of three main plexuses, situated in the cavities of the 
 thorax and abdomen; of a great number of smaller ganglia, lying 
 in relation to the viscera of the same cavities, and widely distrib- 
 uted over the body, especially in connection with the vascular sys- 
 tem; and of a great multitude of fine distributory nerves. Each 
 of the two cords comprises a number of ganglia united by interme- 
 diate nerves. In the other regions of the spinal column the num- 
 ber of these ganglia equals that of the vertebrae (sacral 5, lumbar 
 5, thoracic or dorsal 12), but in the neck (cervical) there are only 
 3. From this gangliated cord a communicating and a distribu- 
 tory series of nerve-branches are derived. By the communicating 
 branches the two systems are brought into close anatomical and 
 physiological relation, and a kind of double interchange takes place 
 between them. The distributory branches of nerves in the sympa- 
 thetic system bring the gangliated cord into connection with the 
 blood-vessels and viscera of the body. The involuntary muscles 
 in the coats of these vessels and in the walls of the viscera are thus 
 related, and through the sympathetic fibres brought into connec- 
 tion with the cerebro-spinal axis. The three main plexuses re- 
 ferred to are collections of nerve-cells and a dense plexiform ar- 
 rangement of nerve-fibres. One of them is situated at the base of 
 
68 GROSS STRUCTURE OF THE NERVOUS SYSTEM 
 
 the heart, to which it gives off branches that wind around that 
 organ and penetrate its muscular substance; another is placed at 
 the upper part of the abdominal cavity, and gives origin to numer- 
 ous plexiform branches that supply the viscera of the abdomen; 
 the third is in front of the last lumbar vertebra, and supplies the 
 vaso-motor nerves and nerves of the muscular coats and mucous 
 membranes of the various organs in that region of the body. Fur- 
 ther details in the anatomy of the sympathetic nervous system are 
 of little interest to psycho-physical studies. To such studies it is 
 
 Fia. 24. The Cranium Opened to Show the Falx Cerebri and Tentorium Cerebelli, and the 
 Places of Exit for the Cranial Blood-vessels. (Schwalbe.) a, a, Falx; 6, &, the 
 tentorium; 3, 3, Sinus transversus, and 2 to 3, Sinus rectus, receiving from in front the 
 Vena magna Galena; 4, internal jugular vein; 5, superficial temporal vein; and 6, mid- 
 dle temporal vein. 
 
 of great interest, however, to know that this system forms a bond 
 between the sensations, emotions, and ideas which have their 
 physical basis in the molecular condition of the cerebro-spinal 
 centres, and those various organs in the thoracic and abdominal 
 regions whose condition is so closely related to such psychical 
 states. 
 
 5. The Brain and Spinal Cord are the great centres of the cere- 
 bro-spinal system. These bodies are situated in the bony cavity of 
 the skull and spinal column. They have three Coverings or Mem- 
 branes, the innermost one of which is directly united with the sur- 
 face of the nervous substance, and sends numerous processes into 
 
B 
 
 FIG. 25. A, Ventral, and B, Dorsal, View of the 
 Spinal Cord and Medulla Oblongata. B 1 the Filum 
 terminate, which has been cut off from A and B. 
 1, Pyramids of the medulla, and I 1 , their decussa- 
 tion; 2, olives; 3, lateral strands of the medulla; 
 4 1 , calamus scriptorius; 5, the funiculus gracilis; 
 and 6, the funiculus cuneatus; 7, the ventral and 
 9, the dorsal, fissures; 8, the ventro-lateral im- 
 pression; 10, dorso-lateral groove. C, the cervical, 
 and L, the lumbar, enlargements of the cord. 
 
 JO- 
 
 7 
 
 Pb 
 
70 GROSS STRUCTURE OF THE NERVOUS SYSTEM 
 
 its interior. (1) The Dura Mater, which is the membrane lying 
 next to the wall of the bony cavity, is tough, white, fibrous, and of 
 structure somewhat different in the cranial from the spinal cavity. 
 Three processes of the dura mater divide only incompletely 
 the cavity of the skull into two symmetrical halves and into an 
 upper and lower space : (a) the falx cerebri, a sickle-shaped process 
 between the two hemispheres of the large brain ; (6) the falx cere- 
 belli, a similar process between the two lateral lobes of the cerebel- 
 lum, or small brain; and (c) the tentorium cerebelli, an arched process 
 over the cerebellum separating it from the back portions of the large 
 brain. The membrane lying next inward is called (2) Arachnoid; 
 this membrane is transparent and of delicate connective tissue. 
 The space below this surface is called subarachnoid; the subarach- 
 noid or cerebro-spinal fluid, which fills the intercommunicating 
 compartments into which this space is divided by bundles of deli- 
 cate areolar tissue, is alkaline and poor in albumen. (3) The Pia 
 Mater is a vascular membrane, a minute network of fine branches 
 of arteries and veins held together by delicate connective tissue. 
 These ramifications of the blood-vessels in the pia mater are on their 
 way to or from the nervous substance of the spinal cord and brain. 
 The membrane, therefore, closely invests this substance, being, 
 however, more intimately attached to the cord than to the brain. 
 The pia mater is well supplied with nerves. 
 
 By these three membranes the nervous masses of the cerebro- 
 spinal system are protected, held together and in place with a soft 
 and yielding but sufficiently firm pressure, and nourished by the 
 
 6. The Spinal Cord, or Medulla Spinalis, extends in the verte- 
 bral canal from the aperture in the cranial cavity (foramen magnum), 
 above which it is continuous with the medulla oblongata, down- 
 ward to opposite the body of the first lumbar vertebra, where, after 
 tapering off, it is spun out into a slender thread of gray nervous 
 substance (filum terminate) that lies in the axis of the sacral canal. 
 Its length is from fifteen to eighteen inches; its weight, when di- 
 vested of membranes and nerves, about an ounce, or not far from 
 one-fiftieth of that of the brain. It is nearly cylindrical in shape, 
 its front and back surfaces being somewhat flattened; it has two 
 considerable enlargements of its girth an upper (cervical), from 
 which arise the nerves that supply the upper limbs; and a lower 
 (lumbar), which supplies the lower limbs with nerves. 
 
 7. The external structure of the spinal cord requires us to 
 notice the fissures which almost completely divide it for its whole 
 length into right and left (lateral) halves, and are, therefore, fitly 
 called "median"; of these fissures (a) the one in front (anterior 
 
COLUMNS AND COMMISSURES OF THE CORD 71 
 
 or ventral 1 median) is somewhat broader than (b) the one behind 
 (posterior or dorsal median). The ventral fissure is penetrated by 
 the pia mater, carrying blood-vessels; the dorsal fissure is not a 
 genuine fissure, but a wall or septum of neuroglia. 
 
 Each of these symmetrical and nearly half-cylindrical halves of 
 the cord is subdivided by the lines of entrance of the dorsal and 
 ventral nerve-roots into three columns: (a) the ventral, which lies 
 between the ventral fis- 
 sure and the ventral root; 
 (6) the dorsal, which lies 
 between the dorsal fissure 
 and the dorsal root; and 
 (c) the lateral column, 
 which lies at the side of 
 the cord between the other 
 two columns. The line 
 of division between the 
 lateral and ventral col- 
 umns is not perfectly 
 sharp, because the fibres 
 of the ventral roots emerge 
 over a considerable width 
 of the surface. 
 
 The Commissures of 
 the spinal cord are two 
 bands of nervous matter 
 which unite its halves, 
 thus preventing it from 
 being completely sepa- 
 rated by the fissures. The 
 one in front, at the bot- 
 tom of the ventral median 
 
 fissure, is composed of transverse nerve-fibres and is called (a) 
 the ventral white commissure; the one behind, at the bottom of the 
 dorsal fissure, is (6) the dorsal gray commissure. The gray commis- 
 sure is nearly twice as large as the white, except at the cervical 
 and lumbar enlargements of the cord, where the white is larger. 2 
 Along its whole length the gray commissure encloses *a circular or 
 
 1 There is a certain advantage in the use of " ventral " and " dorsal " in place 
 of the common words "anterior" and "posterior." The advantage lies in the 
 fact that ventral and dorsal apply with equal fitness to all animals, whether they 
 have the erect position or not. It is convenient also, in some parts of the brain, 
 to speak of anterior in the sense of "rostral" or "toward the mouth." 
 
 2 See Henle, Anaiomie des Menschen, text, p. 309. 
 
 FIG. 26. A, Ventral, and B, Lateral, View of a 
 Portion of the Cord from the Cervical Region, f . 
 (Schwalbe.) 1, ventral median, and 2, dorsal medi- 
 an, fissures. At 3 is the ventro-lateral impression, 
 over which spread the ventral roots (5). The dorsal 
 roots (6), with their ganglion (6 1 ), arise from the 
 dorso-lateral groove, and uniting with the ventral 
 roots form the compound nerve (7). 
 
72 GROSS STRUCTURE OF THE NERVOUS SYSTEM 
 
 elliptical canal (central canal), the vestige of the cavity of the 
 embryonic neural tube. The gray commissure consists for the 
 most part of extremely fine nerve-fibres devoid of medullary sheath; 
 while the white commissure is composed of medullated fibres. 
 The thickness of the commissures is, as a rule, proportional to the 
 
 FIG. 27. Transverse Section Through the Spinal Cord in the Upper Thoracic Region. 
 (From Starr's Atlas of Nerve Cells, by permission of the Columbia University Press.) 
 The section was stained with the "Weigert stain," which leaves the gray matter light, 
 while darkening the white matter. The dorsal columns and horns are above in the fig- 
 ure, the ventral columns and horns below. Magnified 10 diameters. 
 
 size of the corresponding nerve-roots; their form, as they pass 
 into the lateral parts of the cord, varies in different sections of its 
 length. 
 
 8. Transverse sections of the spinal cord show us that, as its 
 external appearance would indicate, the substance of which it is 
 composed is arranged in two symmetrical halves, almost, but not 
 quite, separated by the median fissures. This substance, like that 
 of all the nervous centres, consists of both white and gray nervous 
 matter. The former is external and composes the columns of the 
 cord; while the latter is internal and is surrounded by the white. 
 The relative amount of the two kinds of nervous matter varies in 
 the different parts of the cord. At its beginning from the filum 
 terminale scarcely any white matter appears; the amount of such 
 
WHITE AND GRAY MATTER OF THE CORD 73 
 
 matter, however, increases from below up- 
 ward, and is largest in the cervical part of the 
 cord. The amount of gray matter is greatest 
 in the upper and lower enlargements of the 
 cord. 
 
 The gray columns on either side of the 
 cord, together with the commissures which 
 unite them, form a figure somewhat like a 
 large Roman H, with diverging sides; but 
 the lateral masses of these crescent-shaped 
 bodies are narrower in the thoracic (or dorsal) 
 region, and broader in the cervical and lum- 
 bar enlargements. Sometimes the figure is 
 rather like that of a pair of butterflies' wings. 
 The two limbs of each side of the figure into 
 which the gray columns are thu ci formed are 
 called Horns ; (a) the ventral horn is rounded, 
 (b) the dorsal long and narrow (compare 
 Fig. 27). 
 
 9. The gray matter of the cord contains 
 nerve-cells and their dendrites, and short 
 lengths of axons which pass from the white 
 columns into the gray to terminate there in a 
 tuft of fine branches; collaterals of axons 
 (compare p. 42) similarly enter the gray 
 matter and split up into fine branches; in 
 addition, the gray matter contains neuroglia. 
 From cells in the ventral horns of gray matter 
 issue axons which make their way through the 
 white matter to the surface of the cord, and 
 pass outside, to form the ventral roots (com- 
 pare p. 42). These are the fibres which then 
 pass to muscles and other effectors; and, 
 therefore, the cells from which they arise are 
 called motor nerve-cells. Accordingly, the 
 ventral horn is largely motor in function. The 
 dorsal horns, on the contrary, do not contain 
 the cells of origin for the fibres in the dorsal 
 or sensory roots; but these fibres, as has al- 
 ready been stated (p. 42), arise from cells in 
 the ganglia situated on the dorsal roots (the 
 spinal ganglia) and grow into the cord by 
 the dorsal roots. Since the cells of the sen- 
 sory root fibres lie outside the cord, while 
 
 FIG. 28. Transverse Sec- 
 tions of the Cord at Dif- 
 ferent Levels. (Erb.) The 
 ventral side of the cord 
 lies above in each cross- 
 section. The shaded areas 
 are the pyramidal or cor- 
 tico-spinal tracts, which 
 have degenerated in this 
 individual because of in- 
 jury to the motor area of 
 the left hemisphere. The 
 shaded area in the lateral 
 column is the "crossed" 
 pyramidal tract, and that 
 in the ventral column the 
 "direct" pyramidal. 
 
74 GROSS STRUCTURE OF THE NERVOUS SYSTEM 
 
 those of the motor root fibres lie in the ventral horns, it is natural 
 that the latter should be thicker than the dorsal horns. The thick- 
 ness of the ventral horns, however, varies greatly in different 
 levels of the cord (see Fig. 28). 
 
 While in the cervical and lumbar enlargements of the cord, from 
 which issue the nerves to the extremities, the ventral horns are 
 large, in the mid-thoracic region, the nerves from which supply the 
 less mobile trunk, the motor fibres are relatively few, and the ventral 
 horns are correspondingly slender. More careful study of the shape 
 of the ventral horns in the enlargements shows that they increase 
 by the addition of gray matter at their sides; and the evidence is 
 that these lateral portions of the ventral horns are the seat of the 
 cells whose fibres supply the muscles of the extremities. The cells 
 of origin of the fibres which pass to the sympathetic, seem to lie in 
 the dorsal part of the ventral horn. Thus a certain amount of 
 localization of function can be made out in the ventral horn of 
 the cord. 
 
 Well-defined groups of cells appear elsewhere in the gray matter 
 of the cord. For example, one column of large cells at the base of 
 the dorsal horn ("Clarke's column") gives rise to fibres which pass 
 up to the cerebellum. 
 
 The spinal cord shows no clear division into segments, such 
 as appear, for instance, in the ganglion chain of the earthworm 
 (p. 22); and the spinal "centres," or groups of nerve-cells which 
 control particular muscles, do not exist as compact nuclei, but 
 rather as slender columns of cells, within the ventral horn, extend- 
 ing for a distance of two or three vertebrae up and down the cord. 
 The fibres destined for a particular muscle issue from the cord by 
 two or three adjacent ventral roots; and each root carries fibres 
 destined for several muscles. The spinal centres of neighboring 
 muscles overlap. 
 
 The spinal ganglia, which supply the sensory fibres for the skin 
 and other tissues, are clearly segmental; and the fibres from each 
 ganglion have a definite field of distribution. But the distribu- 
 tions of adjacent ganglia overlap in the skin, so that destruction 
 of a single ganglion or dorsal root does not entirely abolish sensa- 
 tion in any cutaneous area. 
 
 The White Substance of the spinal cord, besides connective tis- 
 sue and lymph- and blood-vessels, is composed of nerve-fibres of 
 comparatively large or of medium size. The essential constituent 
 of these fibres is the axon, the diameter of which is generally one- 
 third or one-fourth of their breadth. When fully developed, they 
 are rarely or never without a medullary sheath, but probably have no 
 neurilemma. Their diameter is not constant; the thickest fibres 
 
NERVE-FIBRES AND NERVE-CELLS IN THE CORD 75 
 
 Cbl 
 
 (TWO to TOTO- of an inch) are found in the outer portions of the 
 ventral columns, where their size is tolerably uniform. In the 
 lateral columns the nerve-fibres vary greatly in size, the finer ones 
 lying inward near the gray matter. In the dorsal columns they in- 
 crease in thickness as they approach the posterior gray commissure. 
 In the upper thoracic, and through the whole of the cervical, region, 
 there is found a wedge-shaped bundle of fine fibres that is separated 
 off from the dorsal columns toward the middle line of the cord by a 
 strong septum; this is 
 called fasciculus gra- 
 cilis, or "column of 
 Goll." 
 
 10. Some idea of 
 the complexity of the 
 cord may perhaps be 
 gained from the 
 counts of fibres which 
 have been made by 
 several authors. In so 
 small an animal as 
 the frog, Hardesty 1 
 found, in general 
 agreement with the 
 
 parliW work nf Riro-P FlG ' 2 9--View of the Brain in Profile. *. (Henle.) Cb, 
 r WOrK 01 Dirge, cerebrum ; Cbl, cerebellum; Mo, medulla oblongata; P. 
 that the total number Pns Varolii; *, nssure of Sylvius. 
 
 of fibres in all the 
 
 dorsal and ventral spinal roots combined would be some such 
 number as 20,000 to 30,000. The number varied with the size 
 of the frog, increasing with the body-weight. The fibres of the 
 dorsal roots were more numerous than those of the ventral roots; 
 in one specimen there were 8,572 dorsal and 6,211 ventral in the 
 roots of one side. In man, Ingbert 2 determined the number of 
 fibres in the dorsal roots entering one side of the cord; the total 
 count was about 650,000. The ventral roots must contain at least 
 half as many more, so that the total number of root fibres entering 
 or leaving the human cord cannot be less than two million. 
 
 11. The same elements of nerve-fibres and nerve-cells, in con- 
 junction with connective tissue and neuroglia, and enveloped in the 
 three enclosing membranes (dura mater, arachnoid, and pia mater) 
 
 1 Irving Hardesty, "The Number and Arrangement of the Fibres Forming the 
 Spinal Nerves of the Frog," Journal of Comparative Neurology, IX, 64-112, 1899. 
 
 2 Chas. Ingbert, "An Enumeration of the Medullated Nerve Fibres in the 
 Dorsal Roots of the Spinal Nerves of Man," Journal of Comparative Neurology, 
 XIII, 53-120, 1903. 
 
76 GROSS STRUCTURE OF THE NERVOUS SYSTEM 
 
 already described, are combined with an increased variety and com- 
 plexity of arrangement to form those intercranial central organs 
 with which the upper end of the spinal cord is continuous. Uni- 
 formity of elementary parts, together with the greatest intricacy of 
 
 FIG. 30. Under Surface of the Brain. (Van Gehuchten.) The Roman numerals at the 
 left margin of the figure indicate the 12 cranial nerves; hyp, hypophysis; ch, optic 
 chiasm; c. mam, mammillary body; pc, peduncle of the cerebrum; pr, pons; o, olive; 
 PV, pyramids; CI, first spinal nerve. 
 
 arrangement, prevails, above all other regions of the body, in the 
 structure of the brain. The significance of the elements and ele- 
 mentary parts can, therefore, only be understood when they are 
 considered in the localities and relations to other parts which are 
 assigned them by this so intricate arrangement. 
 
DIFFERENT ASPECTS OF THE ENCEPHALON 77 
 
 12. The Encephalon, or Brain, in the most extended sense of 
 the word, includes all that portion of the central nervous system 
 which is contained within the cavity of the skull. Its division into 
 five principal parts, and the subdivisions of some of these, have been 
 mentioned in the chapter on the development of the individual 
 nervous system (see p. 48). On removing the entire human brain 
 from the skull, and viewing it from above, one sees only the cere- 
 bral hemispheres, which have grown back and covered the other 
 parts. A view from the side shows the cerebellum lying beneath 
 
 Cb 
 
 FIG. 31. Mesial Section of the Brain. ^. (After a photograph by Retzius.) Cb, the 
 mesial surface of the right cerebral hemisphere; Cbl, the middle lobe of the cerebellum 
 in section; Te, roof of the mid-brain; Pe, peduncle of the cerebrum; v4, fourth ventricle; 
 Aq, aqueduct; Th, thalamus; Mam, mammillary body; //, optic nerve. 
 
 the back part of the cerebrum, and the bulb lying beneath the cere- 
 bellum (compare Fig. 29); the transverse bundles of the pons can 
 also be seen. From the under side can be seen, in addition, the 
 peduncles or crura of the cerebrum, which form the ventral part of 
 the mid-brain; the mammillary bodies and the hypophysis, which 
 are parts of the inter-brain; and also the cranial nerves. In the 
 bulb can be distinguished on this surface, two central hillocks 
 called the "pyramids"; and two somewhat similar at the sides, 
 called the " olives " (see Fig. 30). If a section is made in the median 
 plane, separating the cerebrum into its hemispheres, and dividing 
 the rest of the brain into right and left halves, the cerebrum, cere- 
 bellum, and brain stem are easily distinguished; and the pons is 
 known by the swelling on the ventral side made by its transverse 
 fibres. Next forward of the pons and cerebellum lies the mid- 
 
78 GROSS STRUCTURE OF THE NERVOUS SYSTEM 
 
 brain; the dorsal part of this, close to the cerebellum, consists of 
 the quadrigemina, four rounded eminences, of which two, the right 
 anterior and posterior, show in the section. The broad cavity show- 
 ing in this section beneath the cerebellum is the fourth ventricle, 
 and the slender prolongation of this under the quadrigemina is the 
 "aqueduct." Forward of this the cavity swells into the third ven- 
 tricle, the walls of which constitute the inter-brain. To the inter- 
 brain belong the pineal gland above and the hypophysis and 
 mammillary body beneath. The main portion of the inter-brain, 
 
 Callosum Fornix 
 
 / / / Caudate nucleus 
 
 Habenul 
 Pineal gla 
 
 Anterior quadrigeminu 
 
 Posterior quadrigeminum' 
 VI 
 
 Cerebellu 
 
 Third ventricle 
 Thalamus 
 
 External geniculate body 
 Internal geniculate body 
 
 Superior cerebellar 
 peduncle 
 
 Fourth ventricle 
 Bulb 
 
 FIG. 32. Dorsal Surface of the Brain-Stem. (Sobotta and McMurrich.) 
 
 the thalamus, lies to the side of the third ventricle, and its inner 
 surface can be seen in the figure. Of the cerebrum, the median 
 surface of the frontal, parietal, and occipital lobes, and of the tip 
 of the temporal lobe, is shown ; and the callosum and the f ornix are 
 seen in section (compare Fig. 31). 
 
 To obtain a view of the dorsal surface of the brain-stem, it is 
 necessary to trim away the cerebellum and most of the cerebrum. 
 We then see the bulb swelling out from the cord; and the fourth 
 ventricle beginning below in an acute angle. The quadrigemina 
 are fully exposed; slightly further forward is the pineal gland; and, 
 to its side, the habenula. The large masses to the front and side of 
 the quadrigemina are the thalami (one on each side); and still 
 further in the same direction is seen the caudate nucleus, a part 
 of the striatum. Appended to the thalamus are two small eminences, 
 
TWELVE PAIRS OF CRANIAL NERVES 
 
 79 
 
 called the geniculata, median and lateral, which are of interest as 
 portions respectively of the auditory and visual apparatus (see 
 Fig. 32). 
 
 13. It is important again to note in this connection the nerves 
 which belong with each part of the brain, and which issue from the 
 brain-stem. A list of the twelve pairs of cranial nerves has already 
 been given (see p. 55). To emphasize their relations to the brain 
 more clearly, we note that the first or olfactory nerve enters the 
 olfactory bulb, the end-station or terminal nucleus of this nerve; 
 and that fibres passing back from this through the olfactory tracts 
 
 ec 
 
 FIG. 33. Section of the Bulb at the Decussation of the Pyramids. 6/1. (Schwalbe.) 
 f.l.a., ventral fissure; s.l.p., dorsal fissure; py, py, bundles of pyramidal fibres, crossing 
 at d; V, ventral column; S, lateral column; C.a, ventral horn with groups of ganglion 
 cells, a and &; cc. central canal ; f.r., reticular formation; ce and g, dorsal horns; H l and 
 H 2 , dorsal columns; n.c. and n.g., their nuclei; x, central gray matter. 
 
 serve to connect this sense-organ with other parts of the system. 
 The second, or optic, pair of nerves are seen to enter the inter-brain. 
 The third and fourth nerves, both motor to the eye, issue from the 
 mid-brain. The fifth or trigeminal, the great sensory nerve of the 
 face, including also a motor root to the muscles of mastication, 
 issues from the pons. The remaining seven are connected with the 
 bulb; the sixth, seventh, and eighth with its forward end, the others 
 further back. 
 
 14. The brain-stem may properly be considered as a continua- 
 tion of the cord. As compared with the cord, it is in the first place 
 
80 GROSS STRUCTURE OF THE NERVOUS SYSTEM 
 
 thicker, owing (1) to the large body of nerves which it receives; (2) 
 to the passage through it of all the fibres which connect the cere- 
 brum and cerebellum with the cord; and (3) to the appearance of 
 special groups of cells within it. In shape, the difference of the 
 brain stem from the cord is largely due to the circumstance that the 
 
 central cavity of the neural tube 
 sometimes opens out into ventri- 
 cles, and at all times lies near 
 the dorsal side. Passing upward 
 from the cord, we find the central 
 canal, in the lower region of the 
 bulb, first verging toward the 
 dorsal surface, and then opening 
 out into the fourth ventricle. 
 The dorsal wall here becomes a 
 wide membrane, overlying the 
 ventricle, and itself overlain by 
 the cerebellum. The fourth ven- 
 tricle continues upward through 
 the pons, narrows in the mid- 
 brain to the aqueduct, which 
 broadens again in the inter-brain 
 to the third ventricle. This last 
 is continuous through a narrow 
 opening with the " lateral ventri- 
 cle" of each hemisphere. 
 
 15. A series of sections across 
 the brain-stem will give some idea 
 of its internal structure (compare 
 Fig. 33). 
 
 If we make our first section 
 through the bulb, shortly above 
 the imaginary line which sepa- 
 rates it from the spinal cord, we 
 find an arrangement of parts 
 much like that of the cord. The 
 ventral, lateral, and dorsal columns of white matter can be identified; 
 also the ventral and dorsal horns of gray matter. The most striking 
 feature of this section is the appearance of large numbers of fibres, 
 crossing from the ventral column of each side to the lateral column 
 of the other. TWs is the crossing or " decussation of the pyramids " ; 
 here motor fibres from the left hemisphere cross to the right side of 
 the cord, and vice versa, so that the left hemisphere of the brain con- 
 trols the right half of the body. This section shows another pe- 
 
 fla. 
 
 FIG. 34. Section of the Bulb at the Level 
 of the Sensory Decussation. (Schwalbe.) 
 s.l.p., dorsal, and f.l.a., ventral fissures; 
 cc., central canal, surrounded by n. XI 
 and n. XII, the nuclei of the llth and 
 12th nerves; H and H z , the dorsal col- 
 umns, with n.g. and n.c. (also n.c 1 .), their 
 nuclei; a. V., spinal root of the fifth nerve, 
 the fibres of which terminate successively 
 in the adjoining gray matter, g, which may 
 be regarded as a continuation of the dor- 
 sal horn of the cord; F.r., the "reticular 
 formation," containing fibres which issue 
 from the nuclei of the dorsal columns and 
 of the fifth nerve, and which cross the 
 middle line in the sensory decussation, 
 d.a.; /.a., /.a. 1 , f.a 2 ., arciform fibres, passing 
 toward the cerebellum; n.L, o l , o, n.ar., 
 different nuclei of gray matter, of which 
 o is the lower end of the olivary nucleus; 
 py, pyramid. 
 
INTERNAL STRUCTURE OF BRAIN-STEM 
 
 81 
 
 culiarity, as compared with the cord; there is a small mass of gray 
 matter in the midst of the dorsal column. 
 
 A little higher up the bulb, as the next section shows, the dorsal 
 columns are chiefly filled with gray matter, and their white matter 
 has nearly disappeared; for the fibres which have ascended the cord 
 in the dorsal columns end here, in this gray matter, and their places 
 are taken by new fibres arising from this gray matter. These new 
 fibres, instead of continuing upward in the dorsal region, promptly 
 
 Solitary bundle 
 Vestibule-spinal tract Nucleus X 
 
 Vestibular nucleus 
 Nucleus IX 
 
 Dorsal spino- 
 cerebellar tract 
 
 Quinto- spinal 
 tract 
 
 Ventral spino- 
 cerebellar tract 
 
 Thalamo-olivary 
 
 Accessory olivary 
 nucleus 
 
 Olivary nucleus 
 Accessory olivary 
 nucleus 
 
 Ext.-arcuate fibres 
 
 Pyramid 
 
 FIG. 35. Section of the Bulb Through the Olive. (Magnified 4 diameters.) 
 Kopsch, Lehrbuch der Anatomie.) 
 
 (Rauber- 
 
 cross to the other half of the medulla, and pass upward near the 
 middle line. The gray matter in the dorsal columns is named the 
 "nuclei of the dorsal columns"; the crossing of the sensory fibres 
 is the "sensory decussation," and the bundle of these fibres which 
 runs up near the middle line, and which can be traced to the thala- 
 mus, is the "fillet," or " bulbo-thalamic tract." This is one of 
 the chief sensory pathways toward the cerebral cortex. 
 
 16. A section near the upper limit of the bulb shows that the 
 central canal has now opened out into the fourth ventricle. On 
 the floor of the ventricle are seen the nuclei of the local nerves. 
 Near the nucleus of the tenth or vagus nerve is a spot which is es- 
 sential for life, for if it is punctured, breathing ceases. This is the 
 "respiratory centre." The most striking feature of the bulb at this 
 
82 GROSS STRUCTURE OF THE NERVOUS SYSTEM 
 
 level is the olivary nucleus showing in section as a wavy line (see 
 Fig. 35). Of the numerous crossing fibres which appear in this 
 section, some are from the olives, and some belong to the sensory 
 decussation, which is not yet complete. At the side is seen the in- 
 ferior peduncle of the cerebellum, not as yet quite separate from the 
 bulb; a little higher up the separation is complete. 
 
 At the upper limit of the bulb, where it passes over into the pons, 
 there enters the eighth nerve (see Fig. 36), its two branches, the 
 
 it JET* 
 
 FIG. 36. Section of the Bulb at the Entrance of the Eighth Nerve. (Schwalbe.) Note 
 particularly the division of the eighth nerve, VIII, into a, its cochlear portion, and 6, its 
 vestibular portion. The portion left clear in the drawing is to be understood as filled 
 largely with decussating fibres. Py, pyramid; o, olive; V, fibres from the fifth nerve; 
 c.r., inferior peduncle of the cerebellum. 
 
 cochlear and the vestibular, separating as they enter, and having 
 entirely different central terminations. At this level are large masses 
 of gray matter connected with these two nerves. The central con- 
 tinuation of the cochlear nerve is called the "lateral fillet." 
 
 17. A section through the pons, at the level of entrance of the 
 fifth nerve (compare Fig. 37), shows the peculiar arrangement which 
 gives the name of "bridge" to this part of the brain-stem. Numer- 
 ous transverse fibres extend across the middle line, and appear, 
 indeed, to run from one side of the cerebellum to the other; but this 
 appearance is illusory, as these fibres all originate from cells in the 
 pons itself; they cross from one side to the other, and enter the cere- 
 
INTERNAL STRUCTURE OF THE BRAIN-STEM 83 
 
 bellum, forming its middle peduncle. Interwoven with these 
 transverse pontine fibres are the longitudinal fibres of the pyramids. 
 Small collections of nerve-cells (the "pontine nuclei") lie imbedded 
 in the meshes of this network of fibres, and give origin to the pontine 
 fibres. Above this bridge-like portion of the pons is another re- 
 
 FIG. 37. Cross Section of the Pons at the Level of the Fifth Nerve. (Marburg.) V (at the 
 left) is the fifth nerve; parts of the sixth, seventh, and eighth nerves also are indicated by 
 Roman numerals; FPo., bundles of pontine fibres, which pass to BPo, the middle cere- 
 bellar peduncle. The cerebellum is cut away, but is to be thought of as lying to each 
 side and on top, above the fourth ventricle which shows at the top of the figure. Py 
 is the pyramidal tract; Lm, the fillet; Flm, close beneath the ventricle, is the median 
 longitudinal bundle; ND is the nucleus of Deiters; Crst, a remnant of the inferior cere- 
 bellar peduncle; Vs, the descending bundle of fibres from the fifth nerve, which has been 
 shown in sections lower down. The "hood" includes everything between the ventricle 
 and the uppermost pontine fibres. 
 
 sembling the bulb, the bundles of which are indeed continued up- 
 ward here. This dorsal part of the pons is called the "hood" or 
 tegmentum; and the ventral part is called the foot. 
 
 18. A section somewhat farther forward cuts the mid-brain at 
 the level of the posterior quadrigemina (compare Fig. 38). The 
 fillets, medial and lateral, are visible above the foot; and the lateral 
 fillet is seen to have moved dorsally, its fibres passing to the posterior 
 quadrigeminum, which is a centre for hearing. The abundant 
 decussating fibres above the fillet are from the superior peduncles 
 of the cerebellum. The third ventricle has now narrowed to the 
 aqueduct, which is surrounded by gray matter, containing the 
 nuclei of the third and fourth nerves, which are motor to the eye 
 muscles. 
 
 19. A section through the anterior quadrigemina (Fig. 39) may 
 strike also the hind part of the inter-brain, which overlaps the mid- 
 
84 GROSS STRUCTURE OF THE NERVOUS SYSTEM 
 
 FIG. 38. Cross-section of the Mid-Brain at the Level of the Posterior Quadrigemina. (Mar- 
 burg.) In place of the fourth ventricle we have here the narrow Aqueduct, Aq. Above 
 this, on each side, lie the posterior quadrigemina, Qp. The aqueduct is immediately 
 surrounded by gray matter, below which is Flm, the median longitudinal bundle. Below 
 this is a large mass of decussating fibres, D, from the superior cerebellar peduncles; below 
 and to the side of this is the fillet, Lm; and above this, near the quadrigeminum, the 
 lateral fillet, LI. So much for the tegmentum; the foot still shows the interweaving of 
 pontine and pyramidal fibres. 
 
 Gray matter of the Aqueduct 
 Aqueduct 
 
 Red nucleus 
 
 FIG. 39. Section Through the Anterior Quadrigemina. Magnified about It diameters. 
 (Dejerine.) II, 7/7,the second and third nerves; Th, thalamus; Ge and Gi, the external 
 and internal geniculate bodies; P, peduncle of the cerebrum, containing the pyramidal 
 tracts; F, fillet; Flm, median longitudinal bundle; N iil , nucleus of the third nerve. 
 
STRUCTURE AND RELATIONS OF INTER-BRAIN 85 
 
 brain. In the mid-brain can still be recognized the foot and the 
 hood, but the foot has bifurcated into the peduncles of the right and 
 left cerebral hemispheres. The tegmentum contains the red 
 nucleus, in which terminate most of those fibres from the cerebel- 
 lum, that were seen decussating in the preceding section. At the 
 
 FIG. 40. Horizontal Section Through the Brain. (Edinger.) $. White matter shows white, 
 gray matter gray, and ventricular spaces black. 
 
 side appear the median and lateral geniculate bodies. The cut-off 
 stump of the second or optic nerve is seen entering the lateral genicu- 
 late body. 
 
 20. The relations of the inter-brain can perhaps be better seen 
 in a section (see Fig. 97, p. 223), which is made vertically through 
 the cerebrum, and further to the front than the preceding section; 
 but which includes the thalamus, because of that overlapping of 
 
86 GROSS STRUCTURE OF THE NERVOUS SYSTEM 
 
 some of the end-brain and inter-brain of which mention has al- 
 ready been made (see p. 51). 
 
 Of the pallium, the temporal lobe is seen below, and the frontal 
 lobe above, separated by the fissure of Sylvius, which spreads out 
 at its bottom into the " Island." Passing inward from the island, 
 we encounter first a strip of white matter, then a strip of gray 
 ^(the "claustrum"), next the lenticular nucleus, showing three parts; 
 after that, the white matter of the internal capsule, and finally the 
 thalamus toward the centre and the caudate nucleus above. The 
 caudate and lenticular nuclei are parts of the striatum, and were 
 separated, in the early development of the brain (p. 53), by fibres 
 growing through from the thalamus and from the cortex. 
 
 21. A horizontal section through the cerebrum gives another 
 view of these same structures. The full extent of the internal 
 capsule is better seen here than in the vertical section. It will be 
 noted that the lateral ventricles, the caudate nucleus, and the cal- 
 losum are each cut twice in the section. This results from the curved 
 growth of the cerebrum about a fixed point (compare p. 51), which 
 gives to each of these structures an arched shape. The arch of the 
 callosum can be seen in Fig. 31, which also shows another arched 
 band, the fornix, lying beneath the callosum. The fornix appears 
 in section in Fig. 40. It consists of fibres which arise in the temporal 
 lobe, and which arch forward over the thalamus, and then descend 
 to the mammillary body in the base of the inter-brain. It belongs 
 to the archipallium, and some of its fibres decussate, forming the 
 " psalterium," which is the commissure of the archipallium, as the 
 callosum is the commissure of the neopallium. 
 
 22. The cerebellum consists of a middle lobe called the vermis, 
 and two lateral hemispheres. As in the cerebrum, the greater part 
 of the gray matter lies outside, in a cortex, whose surface is folded 
 into numerous fissures. Other nuclei lie in the base of the vermis. 
 The cerebellum is attached to the brain-stem by three pairs of pe- 
 duncles (see Fig. 41). 
 
 The inferior peduncle consists of afferent fibres from the spinal 
 cord, from the nuclei of the sensory cranial nerves, and from the 
 olive. The superior contains a bundle of fibres from the cord, but 
 is largely composed of fibres which originate in the cerebellum and 
 pass forward to the mid-brain and thalamus. The middle peduncle 
 consists of the pontine fibres, which arise from the pontine nuclei, 
 and are connected with fibres descending from the cerebrum. This 
 is the path of communication from the cerebrum to the cerebellum. 
 The pontine fibres, then, are afferent to the cerebellum; they pass 
 to the cortex of the cerebellar hemispheres; whereas the rest of the 
 afferent fibres go to the vermis. The development of the cere- 
 
TRACING AND NAMING OF NERVE-TRACTS 87 
 
 bellar hemisphere varies with that of the cerebrum, being largest 
 in man. 
 
 (Further study of the inner structure of the cortices and other 
 nuclei of the brain is deferred to a chapter on the nerve-cell and its 
 connections; and, in case of the cerebral cortex, to a special chapter 
 on that cortex.) 
 
 23. As can readily be imagined from the large number of tracts 
 and nuclei which have been mentioned even in the above hasty 
 review, the internal structure of the brain presents a bewildering 
 complexity of collections of nerve-cells and of bundles of fibres 
 
 FIG. 41. Lower Surface of Cerebellum. #. (After Sappey.) 1, vermis; 3, hemisphere. 
 The pons, bulb, and various pairs of nerves are also seen, thus: 12, 13, the fifth nerve; 
 14, the abducens; 15, the facial; 16, the "intermediate"; 17, the auditory; 18, the 
 glosso-pharyngeal; 19, the vagus; 20, the accessory; 21, the hypoglossal. 
 
 coursing in various directions. In attempting to unravel this com- 
 plexity, the most important task is that of tracing fibres from the 
 cells which give them off to the cells which receive them. If we 
 knew the fibres which connect the various "centres" or collections 
 of cells, we should have taken a long step toward discovering the 
 functions of the centres thus connected. 
 
 Many ingenious methods for tracing the nerve-tracts have been 
 employed, the chief of which are the embryological and the patho- 
 logical or degeneration method. The principle of the former the 
 myelinization of different tracts at different times in the individu- 
 al's development has already been mentioned in the chapter on 
 embryology (see p. 58). The principle of the pathological method 
 is the general fact that any part of a living cell, when cut off from the 
 cell nucleus, dies. Thus, a nerve-fibre separated from its cell of 
 origin speedily begins to degenerate, and is then stained by several 
 reagents (especially by osmic acid) to a different degree from normal 
 
88 GROSS STRUCTURE OF THE NERVOUS SYSTEM 
 
 nerve-fibres (compare Fig. 42). If, therefore, a bundle of fibres has 
 been severed by disease or injury, or by the knife of an experimenter, 
 the fibres of this bundle degenerate, and can thus be traced in their 
 course amidst other fibres. This method has proved remarkably 
 serviceable. Many other methods have been employed. Von 
 Bechterew mentions 1 no fewer than eleven, of which we may cite the 
 comparative study of different vertebrates, especially of lower forms 
 with simpler fibre systems; and the physiological method which deals 
 with a living animal by exciting to activity some limited region a 
 
 FIG. 42. Degeneration of Tracts after Injury to the Cord. (Strtimpell.) Five sections of 
 the cord are arranged in order, the uppermost being at the left. The middle section, 
 lying close to an injury which has completely severed the cord, shows degeneration in 
 every part. The amount of degeneration decreases gradually in the sections further from 
 the injury. The upper sections show "ascending" degeneration in the dorsal and the 
 outside of the lateral columns. The lower sections show descending degeneration in a 
 certain part of the lateral column. 
 
 sense-organ, a tract of fibres, or a portion of gray matter and then 
 observing either the locus and character of the end-effect, or the 
 electrical change which accompanies the conduction of the nervous 
 impulse in the nerve-fibres. 
 
 The object of such studies is attained when we know: (1) the 
 origin of a tract, i. e., the group of nerve-cells from which the fibres 
 proceed; (2) the course of the tract to (3) its termination in some 
 other group of nerve-cells; and (4) the connections of the tract with 
 other tracts at its origin and termination. The fact is also to be 
 considered that nerve-fibres often send out collateral branches at 
 some part of their course, and thus a tract may have more than one 
 terminus. It is part of our object to discover the function subserved 
 by each tract. If the connections of the tract were thoroughly 
 known, this alone might be a sufficient indication of its function; 
 but in the present state of knowledge, the physiological method is 
 often the only one which gives a knowledge of the function; and in 
 many cases even this fails us. 
 
 24. A system of naming the nerve-tracts, now coming into 
 favor, consists in the use of compound terms, in which the origin 
 and terminus of the tract are indicated. For example, the " cortico- 
 spinal tract" arises in the cortex and terminates in the cord; it is 
 
 1 Die Leitungsbahnen im Gehirn und Ruckenmark, 1899, pp. 2-9. 
 
NERVE-TRACTS AND THEIR CONNECTIONS 89 
 
 what we have previously called the pyramidal tract. A list of the 
 analytical names of the tracts will convey some notion of the prog- 
 ress thus far made in the unravelling of the white matter; but it 
 should be added that very many fibres still remain to be traced. 1 
 
 25. The most interesting of these tracts to the student of 
 mental life are probably those which convey sensory impulses to 
 the cerebral cortex and those which convey motor impulses away 
 from the cortex; and these may now be examined with some care. 
 
 The sensory pathway from the trunk and limbs enters the cord 
 by the dorsal roots. On reaching the cord, the sensory fibres turn 
 in different directions. Some pass directly to the ventral horn of 
 the gray matter, where they form connections with the motor cells, 
 and so influence the motor fibres and provide a direct reflex pathway 
 back to the muscles. Others of the sensory fibres turn up the dorsal 
 columns; and some of them continue in these columns up through 
 the cord to the bulb, where they terminate in the nuclei of the 
 dorsal columns. The cells in these nuclei send out axons which 
 continue the sensory pathway toward the cerebrum, by first crossing 
 to the other side of the bulb, and then proceeding upward in the 
 
 1 In understanding this list it should be remembered that the names of the 
 different nerve-tracts are designed to combine the place of origin and the place 
 of termination; and that tracts having the same origin are grouped together. 
 Those which have hitherto been fairly well made out are the following: 
 
 Radiculo-bulbar. Tecto-bulbar; Tecto-spinal, lateral and 
 Spino-cerebellar; Spino-olivary; Spino- medial. 
 
 tectal; Spino-thalamic. Thalamo-spinal; Thalamo-olivary; Tha- 
 Cervico-lumbar. lamo-habenular; Thalamo-parietal. 
 
 Nucleo-cerebellar. Thalamo-temporal; Thalamo-occipital. 
 
 Bulbo-tectal; Bulbo-thalamic; Bulbo- Habenulo-peduncular. 
 
 mammillary. Mammillo-thalamic; Mammillo - teg- 
 Acustico-tectal; Acustico-thalamic. mental. 
 
 Vestibulo-spinal. Striato-peduncular; Striato-thalamic. 
 
 Nono-spinal (or " solitary bundle ") Fronto - spinal (cortico - spinal), direct 
 Quinto-spinal; Quinto-thalamic. and crossed; Fronto-bulbar; Fronto- 
 
 Retino - thalamic and Retino - tectal pontine. 
 
 (optic). Occipito - temporo - pontine; Occipito- 
 Olfacto-cortical (or ammonic) ; Olfacto- thalamic. 
 
 habenular; Olfacto-mammillary. Cortico-tectal; Cortico-habenular. 
 
 Olivo-cerebellar. Ammono-mammillary. 
 
 Ponti-cerebellar. Fronto-thalamic. 
 
 Rubro-spinal; Rubro-thalamic. Temporo -frontal; Temporo - parietal; 
 Cerebello-rubral; Cerebello-thalamic. Temporo-occipital. 
 
 To these should be added the median longitudinal bundle and the numerous 
 commissures. 
 
 This list will probably need some revision from future discoveries; and it will 
 certainly require numerous additions. But, as it now stands, it is a striking 
 monument to the industry of neurologists. 
 
90 GROSS STRUCTURE OF THE NERVOUS SYSTEM 
 
 "fillet" to the thalamus, where they terminate; but the path is fur- 
 ther continued by fibres arising from the cells of the thalamus and 
 passing up through the internal capsule to the cortex. Three sets 
 of fibres, placed end to end, are therefore required to convey sensory 
 impulses from the limbs and trunks to the cortex; three tracts are 
 linked to form this sensory pathway tracts which may be named 
 the radiculo-bulbar, the bulbo-thalamic, and the thalamo-cortical. 
 This is the most clearly traced of the sensory pathways from the 
 limbs and trunk; but apparently it is not the pathway for the sense 
 of touch, for interruption of this pathway by injury does not abolish 
 conscious sensations of touch, temperature, and pain. Injury to 
 the dorsal columns interferes with the " muscle-sense," whereas the 
 cutaneous senses are affected by injury to the lateral columns of 
 the cord. 
 
 Now some of the sensory fibres, on entering the cord, terminate 
 in the dorsal horn of gray matter; and cells located there send out 
 fibres which, after crossing to the other side of the cord, ascend in 
 the lateral columns, and can be traced up to the thalamus. This 
 spino-thalamic tract is apparently employed by the cutaneous senses, 
 though it appears rather too slender to constitute the sole path from 
 the skin to the brain. 
 
 Some of the dorsal root fibres pass to that part of the gray matter 
 of the cord which is called Clarke's column, and here connect with 
 the cells which give rise to a tract to the cerebellum. Still other con- 
 nections are formed in the spinal cord between the incoming sen- 
 sory fibres and various tracts running to different parts of the brain. 
 Probably, however, these tracts, as well as the cerebellar, are not 
 concerned in conscious sensation. 
 
 Cutaneous sensation from the face is provided for by the fifth 
 pair of cranial nerves. From the terminal nuclei of these nerves, 
 in the pons and bulb, arise fibres which pass to the thalamus 
 the quinto-thalamic tract and end there, as do the sensory tracts 
 from the cord. 
 
 CThe sense of taste is served by fibres of the seventh and ninth 
 pairs of nerves; these fibres end in a common terminal nucleus, but 
 the further course of the gustatory pathway toward the cortex can 
 not yet be stated. 
 
 The fibres from the cochlear branch of the eighth nerve, the nerve 
 of hearing, end in nuclei close to the entrance of the nerve. The 
 secondary fibres, issuing from here, cross the middle line of the bulb 
 and end in another mass of gray matter, called the superior olive. 
 From these, tertiary fibres arise which conduct the auditory impres- 
 sions forward to the mid-brain and thalamus more precisely, to 
 that part of the mid-brain known as the posterior quadrigeminal 
 
NERVE-TRACTS AND THEIR CONNECTIONS 
 
 91 
 
 body, and to that part or appendage of the thalamus known as the 
 internal geniculate body. 
 
 26. The central connections of the second or optic nerve the 
 nerve of sight are particularly worth noting. The right and left 
 optic nerves, as they pass backward, approach each other, meet, 
 and appear to cross in much the shape of an X or of the Greek letter 
 
 FIELD OF VIEW 
 
 Left 
 
 Right 
 
 Left hemisphere 
 
 Right hemisphere 
 
 BRAIN 
 
 FIG. 43. Diagram of the Semi-Decussation of Optic Fibres in the Chiasm. 
 
 X, from which resemblance the crossing is called the " optic chiasm." 
 The nerves which lead back from the crossing and into the brain 
 are called the "optic tracts." Just how much crossing of fibres 
 occurs in the chiasm depends on the species of animal, and on the 
 position of the eyes in the head. Those animals which have eyes 
 on the sides of their heads, and directed to right and left so as to 
 give almost totally different fields of view, show at the chiasm a 
 nearly complete crossing of fibres. In such animals, therefore, the 
 right eye is connected with the left half of the brain, and the left 
 
92 GROSS STRUCTURE OF THE NERVOUS SYSTEM 
 
 eye with the right half of the brain. This crossed relation holds 
 for the other sense-organs. But in animals whose eyes are placed 
 somewhat forward, with fields of view more or less overlapping, 
 the decussation at the chiasm is less complete; and in animals such 
 as man, whose eyes are directed straight forward and have almost 
 identical fields of view, the crossing becomes a semi-decussation 
 (see Fig. 43). The nerve-fibres from the nasal half of each retina 
 cross, whereas those from the temporal half do not cross, but bend 
 into the optic tract of the same side, and pass back to the same side 
 of the brain. The result is that the right half of the brain receives 
 the fibres from the right half of each eye, and the left half similarly. 
 Now since the rays of light cross within the eyeball, the right half 
 of each retina receives light from the left side, and therefore the 
 right half of the brain receives the impressions that come from the 
 left side. The net result is accordingly the same in animals with 
 eyes directed forward as in animals with eyes directed to the side: 
 in each case the impressions originating at one side of the middle 
 line are conveyed to the opposite half of the brain. The brain has, 
 therefore, the same crossed relation with the outer world in the 
 case of vision as in the case of all other receptors. 
 
 The fibres of the optic tracts end in the inter-brain and in the ad- 
 joining mid-brain. In the former, the end-stations are the "pul- 
 vinar" and more particularly the "external geniculate body"; in 
 the latter, their ending is the anterior quadrigeminum. The quad- 
 rigeminum is the principal ending in fishes, reptiles, and birds. But 
 in mammals the inter-brain endings, especially the geniculatum, 
 receive most of the optic fibres; and in man the mid-brain receives 
 only a few fibres, which are concerned mostly with the pupillary 
 reflex. The principal connections of the optio nerve, in man, are, 
 directly with the thalamus and external geniculate body, and in- 
 directly, through fibres arising from these bodies, with the cortex 
 of the occipital lobe. 
 
 27. The olfactory path begins with the fibres of the olfactory 
 nerve, which terminate in the olfactory bulb. A secondary and then 
 a tertiary tract leads to the cortex of the " archipallium," in the 
 pyriform lobe and the hippocampus. 
 
 The previous survey of the sensory pathways shows that, with 
 the exception of the olfactory and possibly of the gustatory, all of 
 these pathways lead to the inter-brain. The thalamus, with its ac- 
 cessory bodies, is an intermediate station in the sensory paths 
 toward the cortex. It is curious that the thalamus should inter- 
 vene in this manner, and no well-grounded explanation presents 
 itself. Why should not the sensory fibres run right up to the cortex, 
 without interruptions, first in the terminal nuclei and then in the 
 
PRINCIPAL MOTOR PATHWAY 93 
 
 thalamus ? In general, we can see that where a fibre ends by split- 
 ting into fine branches which are mingled with the fine branches of 
 other fibres, opportunities are afforded for something analogous to 
 the switching that goes on at a railway junction. Sensory impulses 
 from various receptors may here be collected; and those from any 
 one receptor may be widely distributed. The terminal nuclei, quite 
 surely, provide for the distribution of sensory impulses to the motor 
 nerves, to the cerebellum and to the thalamus. The thalamus, since 
 it possesses many short fibres connecting its own parts, is probably 
 something more than a mere way-station. Apparently, the sen- 
 sory impulses from different receptors come together here and join 
 in such a way that the impulses which pass from here to the cortex 
 are already organized or synthesized to a certain extent. For ex- 
 ample, the adult man possesses a really remarkable power of lo- 
 cating visual objects in reference to the body, when the position of 
 the head differs greatly; and although a given visual appearance of 
 the object means quite different locations in space according to the 
 position in which the head happens to be at the moment. There 
 must clearly be some means of bringing together (or "synthesizing") 
 the visual impulses with those other sensory impulses which indi- 
 cate the position of the head. The conjecture that this means is 
 furnished by the thalamus serves, at least, to direct attention to the 
 kind of work which is apparently done in the switching stations 
 along the route of sensory impulses to the cortex. 
 
 28. The most interesting motor pathway, from the psycholo- 
 gist's point of view, is that which leads from the "motor area" of 
 the cortex, and which may be called the pyramidal, or the cortico- 
 spinal; or, taking more precise account of its origin, the fronto- 
 spinal. This is the longest of all the tracts, and is visible in every 
 cross section of the cord or brain-stem. After passing down through 
 the white matter underlying the cortex, its fibres come together in- 
 to a compact bundle in the internal capsule (where they are fre- 
 quently injured by hemorrhage, in apoplexy), then emerge upon the 
 ventral surface of the mid-brain. They retain this ventral position 
 down through the pons and bulb; but in the lowest part of the bulb 
 each pyramidal tract (the right and the left) splits into two, the 
 smaller of which continues down the ventral column of the cord, 
 while the larger part passes over to the opposite side of the cord and 
 descends in the lateral column. Those fibres which do not cross in 
 the bulb do so, one by one, at different levels of the cord. The 
 cortico-spinal fibres terminate in the gray matter of the cord, and the 
 motor pathway is continued by axons from the motor cells of the 
 cord, which pass out by the ventral roots, and finally reach the 
 muscles. 
 
94 GROSS STRUCTURE OF THE NERVOUS SYSTEM 
 
 29. Though the connections of the cerebrum with the sense- 
 organs and muscles together with the paths of association within 
 the cerebrum itself, to which we shall return in another chapter 
 constitute the most interesting system of nervous connections, yet 
 a false impression would be created if these only were described; 
 for not all the nuclei and fibre-bundles which exist in the brain and 
 cord are directly subservient to the cerebrum. We may recognize 
 the existence of several systems of tracts and nuclei which are more 
 or less independent in their growth and function, though connected 
 at various points. We may, therefore, recognize the following 
 systems of centres and connections: the fundamental, the mesen- 
 cephalic, the cerebellar, the archipallial, and the neopallial. A 
 brief description of each follows. 
 
 30. (1) The fundamental system consists of the sensory and 
 motor nerves, with the cells of origin of the motor fibres and the 
 groups of cells, called terminal nuclei, into which the sensory fibres 
 lead and in which they terminate. In addition, the fundamental 
 system includes the central fibres which directly connect the motor 
 and sensory nuclei. In the cord, this system includes the gray mat- 
 ter and the ground bundles; in the brain-stem, it includes the motor 
 and terminal nuclei of the cranial nerves, and many short connect- 
 ing fibres. An important part of the fibre connections is represented 
 by the "median longitudinal bundle" of the brain-stem, a continu- 
 ation of the ground bundles of the cord. It extends the length of 
 the brain-stem, and consists mostly of short fibres connecting neigh- 
 boring nuclei; it affords direct connection, for example, between the 
 nuclei of the several motor nerves of the eye, and so contributes to 
 the co-ordination of the eye muscles. 
 
 The fundamental system is the oldest in the race, and the earliest 
 to develop in the individual. In accordance with a principle set 
 forth in the chapter on embryology (see p. 57), the structures be- 
 longing to this system lie close to the cavity of the neural tube; in 
 the bulb, they lie close beneath the ventricles, and in the mid-brain, 
 close about the aqueduct. They are overlain by the more volumi- 
 nous structures of the later-developing systems. 
 
 The function of the fundamental system is to provide for local 
 reflexes, and also, through its connecting fibres, for reflexes that are 
 more wide-spread. Since it includes the origins of the motor nerves, 
 it has direct control of the muscles; and other systems probably act 
 first on it, and only through it on the muscles. 
 
 31. (2) The mesencephalic or, more precisely, the "tectal" 
 system 1 is apparently the dominant system in fishes, amphibia, 
 
 1 From "tectum," the "roof" of the mid-brain, comprising the corpora quad- 
 rigemina. 
 
SYSTEMS OF CENTRES AND CONNECTIONS 95 
 
 reptiles, and birds; but in mammals and especially in man, its im- 
 portance has decreased. Its nuclei are those of the quadrigemina; its 
 incoming fibres are partly from the optic nerve, and partly from the 
 terminal nuclei of the cochlear nerve; it also receives slender tracts 
 from the cord. Its outgoing fibres lead to the nuclei of the motor 
 nerves of the eye, and also, by slender tracts, to the bulb and cord. 
 
 32. (3) The cerebellar system is, in man, very extensive. To 
 its gray matter must be reckoned the cortex of the cerebellum, the 
 nuclei which lie imbedded in the base of the cerebellum, and four 
 other nuclei, which lie outside the cerebellum, but near it in the 
 brain-stem, and closely connected with it. These are the olivary 
 nucleus, the nucleus of Deiters, the pontine nuclei, and the red 
 nucleus of the mid-brain. The olivary and pontine nuclei send 
 numerous fibres into the cerebellum; the red nucleus receives many 
 fibres from the cerebellum; and the nucleus of Deiters both sends 
 and receives. The cerebellum also receives many fibres directly 
 from the terminal nuclei of the sensory nerves; the "direct cerebel- 
 lar" tract from the cord is an example, and there are other bundles 
 from the cranial nerves. The connection of the vestibular nerve 
 with the cerebellum seems especially close. To the cerebellar sys- 
 tem should also be reckoned those tracts which lead into the olives 
 (from the cord below, and from the thalamus above), and into the 
 pontine nuclei (from the cerebral cortex). This connection be- 
 tween the cerebrum and the cerebellum, by way of the pontine 
 nuclei, may be reckoned either with the cerebral or with the cere- 
 bellar system, and is interesting as showing a broad path of communi- 
 cation between the two great organs. The path conducts, appar- 
 ently, from the cerebrum to the cerebellum. 
 
 All the tracts of the cerebellar system, thus far mentioned, lead 
 toward the cerebellum. The principal outgoing tract passes out 
 of the cerebellum by the superior peduncle into the mid-brain, 
 where its fibres terminate mostly in the red nucleus. From this 
 nucleus arise several tracts, one of which passes down into the cord, 
 and must afford a means by which the influence of the cerebellum 
 is exerted on the cord and so on the muscles. From Deiters' nu- 
 cleus, also, a tract passes down into the cord. This account of the 
 cerebellar system, though still far from exhaustive, is enough to 
 awaken respect for this organ and its probable importance. 
 
 33. (4) The " archipallial" system, closely related in function 
 to the sense of smell, includes in its gray matter certain portions of 
 the cortex ("pyriform lobe," "hippocampus"), and certain nuclei 
 in the inter-brain ("mammillary body," "habenula"). Its fibre- 
 bundles include the olfactory tracts, the "fornix" (connecting the 
 archipallial cortex with the inter-brain), and several other tracts. 
 
96 GROSS STRUCTURE OF THE NERVOUS SYSTEM 
 
 34. (5) The neopallial system includes much the greatest part 
 of the human cortex, and so the greatest part of all the gray mat- 
 ter of the nervous system. Besides this, it includes other nuclei, 
 the chief of which lie in the inter-brain. Most of the thalamus be- 
 longs here, and also the external and internal geniculate bodies. 
 The interposition of these nuclei in the path of sensory impulses to 
 the cortex has already been discussed. The tracts of the neopallial 
 system include the various sensory tracts which lead into the inter- 
 brain, the fibres leading from the inter-brain to the cortex, the out- 
 going fibres from the cortex (cortico-spinal, cortico-bulbar, cortico- 
 pontine, cortico-thalamic, etc.), and, most numerous of all, the fibres 
 joining one part of the cortex with another. 
 
 35. Regarding the relations of these several systems, in some 
 instances (as from the neopallium to the cerebellum) broad paths 
 of communication are anatomically visible; in other cases, such con- 
 nections have yet to be discovered. Since all systems make use of 
 the same muscles, they must all converge upon them, and therefore, 
 as was said before, on the fundamental system which alone has a 
 direct connection with the muscles. In this sense, the fundamental 
 system might be called the centre of the whole mechanism. On 
 the other hand it is clear that, in intelligent human behavior, the 
 neopallial is the dominant system. Such a survey as has preceded 
 serves to correct the tendency to formulate a too simple and dia- 
 grammatic scheme of the inner relations of the nervous system. 
 To regard the whole system as a connected mechanism is eminently 
 proper, but it is distinctly a biological mechanism, and its plan shows 
 the marks of growth and adaptation, and of possible effects from use 
 and from learning, being quite different from such a design as is 
 drawn up by an engineer for a machine or an electric system. 
 
 The previous description makes the significant fact quite clear 
 that the white matter of the brain and cord, instead of being, as it 
 appears, a homogeneous mass, is a vast and intricate network of 
 fibres and bundles of fibres, which have the office of forming me- 
 chanical connections between definite parts of the gray matter. 
 These connections are manifold, but are not indiscriminate. Ap- 
 parently, there is no one centre to which all paths lead; all parts of 
 the nervous mechanism are interconnected, but some much more 
 directly than others. In this way, both long and short paths, both 
 converging and radiating paths, between both the nearer and the 
 more remote parts, seem to be characteristic features of the entire 
 system. 
 
CHAPTER IV 
 ELEMENTS OF THE NERVOUS STRUCTURE 
 
 1. In considering the nervous systems of invertebrates, and 
 the embryology of the human system, we have already acquired 
 some knowledge of the elementary structures of which the nerves 
 and nerve-centres are composed. In the development of the nervous 
 system, the germinal cells lining the cavity of the neural tube gen- 
 erate daughter cells, the earlier of which become neuroglia cells, 
 and the later form the nerve-cells. From the latter the so-called 
 axons are branches, which often run to great distances; some of 
 them pass out of the neural tube, and others come from the cells of 
 the spinal ganglia, to contribute to the formation of the nerves. 
 Besides the axons, the cells in the brain and cord give out other 
 branches, which are shorter and less cylindrical. Because they 
 often branch like the limbs of a tree, they are called "dendrites." 
 Nerve-cells, axons, and dendrites, along with neuroglia cells, are, 
 therefore, the best-known elements of the nervous system. 
 
 The different kinds of branches which the nerve-cells give out 
 were first fully described by Deiters. 1 He based his work on that of 
 R. Wagner 2 and Remak. 3 The term "dendrites" was suggested 
 by His. In general, as has already been indicated, the dendrites 
 are relatively thick and short, and have a knotted appearance. 
 That these knots, or knobs, are not the result of treatment, but are 
 the natural characteristic of these elements, would seem to be proved 
 by the facts that all kinds of stainings show them, and that their 
 distribution is characteristic of different kinds of cells. And since 
 they are more marked in the embryo, they may be looked upon as 
 points of growth. The axons are more uniform in calibre, smoother; 
 and their diameter differs in some correspondence to their lengths. 
 In some cells of the so-called " Golgi type," the branches of the axon 
 soon end in a fine network. In certain cases where a branch leads 
 off from the main axon, before the latter emerges from the gray 
 
 1 Untersuchungen uber Gehirn u. Rilckenmark d. Menschen u. d. Saugetiere, 
 pp. 55 ff. (Braunschweig, 1865). 
 
 2 Handworterb., Ill, i, pp. 377 ff. 
 
 3 "Ueber multipolar Ganglienzellen," in the Berichte uber d. Verhandl. d. Kgl. 
 preuss. Academie, pp. 29 ff. (Berlin, 1854). 
 
 -97 
 
98 
 
 ELEMENTS OF THE NERVOUS STRUCTURE 
 
 FIG. 44. Part of the Cross Section of a 
 Nerve. (Schafer). The myelin sheath is 
 
 matter, it may be regarded as of importance enough to be called 
 a " paraxon." Sometimes, also, the axon divides into two branches, 
 each with its own medullary sheath. In the spinal cord, some axons 
 even have three branches. 
 
 In general, the axon is developed before the dendrites appear. 
 The first dendrite usually appears just opposite the axon and travels 
 
 toward the centre. The axon 
 itself starts from the neuroblast 
 and, " guided by some mysterious 
 power," grows through the em- 
 bryonic body to its appropriate 
 muscle. This, and similar facts, 
 have led some observers to affirm 
 that " all nerve-fibres of the body 
 are extraordinarily long out- 
 growths from either central or 
 peripheral ganglion cells." Every 
 nerve-fibre is thus " to be recog- 
 nized as being, from beginning to 
 end, a product or, more correctly, 
 
 iNerve. tcnarer;. Tne myenn sneatn is . n i 11 l 
 
 stained black while the axon remains white. a P art > ot a Single nerve cell. 
 
 2. The peripheral nerves, 
 
 and the white matter of the centres, contain no nerve-cells or 
 dendrites; and the nerves contain no neuroglia. A cross section 
 of a nerve (see Fig. 44) shows great numbers of nerve-fibres, 
 which then appear as little circles. Teasing of a nerve divides 
 
 1 Lenhosse'k, Der feinere Ban des Nervensy stems (Berlin, 1895), p. 89. There 
 are few more obscure and uncertain problems than that proposed by the question: 
 "What controls the direction of the fibres in their outward growth?" But this 
 problem may be considered as only a special case of the general biological prob- 
 lem as to the causes which control the direction of all growths in living bodies. 
 And biology is not yet able satisfactorily to solve this problem, even when, as is 
 highly probable, influences from a variety of controlling forces must be admitted 
 to share in the result. In the special case of the nerve-fibre, some have held 
 that the growth of its protoplasmic node as it moves forward through the tissues, 
 taking up nutrition from the surroundings, has its direction controlled by the 
 mechanical ease of passing through certain tissues rather than others (so His: 
 "Die Entwickelung d. ersten Nervenbahnen beim menschlichen Embryo," 
 Archiv f. Anat. u. Phys., Anat. Abt., 1887, pp. 376 ff.; and "Die Entwickelung 
 d. Nervensystems bei Wirbelthieren," Abhandlungen d. math-phys. Klasse d. kgl. 
 sacks. Ges. d. Wiss., Bd. XVIII, 189). Others account for the phenomenon as 
 due to the chemical attractions of secretions given off from the muscles (so 
 Cajal). Still others consider that the cells are all the time functioning, and 
 this primitive functioning accounts for their growth; and also, perhaps, that their 
 growth is dependent on electrical stimuli. All these explanations, however inter- 
 esting and helpful they may be, leave the matter still an unexplained mystery, 
 of a character similar to that of all the performances of living tissues. 
 
STRUCTURE OF THE NERVE-FIBRE 
 
 99 
 
 it into fibres which run along parallel to each other. They are 
 bound together by sheets of connective tissue, which give the 
 necessary tensile strength to the nerve. The whole nerve is en- 
 closed by connective tissue (called the epineurium), and within it 
 can be distinguished larger and smaller bundles 
 of fibres each bundle being enclosed by con- 
 nective tissue (the perineurium); while more 
 delicate sheets of the same tissue penetrate the 
 bundles between the individual fibres (the 
 endoneuriurri). The branching of a nerve 
 often consists simply in the separation of one 
 of its bundles from the rest; in the same way 
 the minuter subdivision of the branches con- 
 sists of the separation of the fibres from one 
 another. The fibres, however, maintain their 
 individuality throughout the peripheral nerve 
 and its branches. Only at the very end of 
 the fibre, in the receptor or effector organ, does 
 the single nerve-fibre split up into fine branches. 
 3. The nerve-fibre is thus the element or 
 unit of which the nerve is essentially composed. 
 The fibre itself consists of the axon, surrounded 
 by one or more sheaths (compare Fig. 45). 
 Most of the axons in the peripheral nerves 
 have two sheaths, the "primitive sheath" on 
 the outside, and the " medullary " or "myelin 
 sheath," inside of the primitive sheath and next 
 to the axon. The primitive sheath is a thin 
 membrane, while the medullary sheath is often 
 comparatively thick, and is composed of a 
 white, fatlike substance called "myelin." A 
 considerable proportion of the fibres in the 
 peripheral nerves do not possess a myelin 
 sheath, and are called non-medullated fibres; 
 in general, the fibres originating in the sympa- 
 thetic ganglia belong to this class. On the other 
 hand, the fibres in the white matter of the brain 
 and cord possess the myelin sheath but not 
 the primitive sheath, and there is a short length 
 of the axon, just after it emerges from its cell, which is not provided 
 with either of the sheaths. It seems clear, accordingly, that the 
 axon is the most essential part of the nerve-fibre, since it is the 
 only part which is present in all fibres and in all parts of the 
 nervous system. 
 
 FIG. 45. Short Pieces 
 of Two Nerve-Fibres. 
 (Schafer.) The axon ap- 
 pears gray, and shows 
 its fibrils; the myelin 
 sheath is stained black; 
 the primitive sheath 
 appears white. R, node 
 of Ranvier; c, the nu- 
 cleus of one of the 
 sheath-cells. 
 
100 
 
 ELEMENTS OF THE NERVOUS STRUCTURE 
 
 FIG. 46. Motor Cell of the Ventral Horn of the 
 Cord, with Scheme of the Course of its Axon. 
 (Barker.) n, the nucleus, with n l , nucleolus; 
 d, dendrites, only the stumps of which are 
 shown; a.h., hillock from which the axon 
 arises; at ra, the axon becomes invested with 
 the myelin sheath; n.R., a node, with branch- 
 ing of the axon; m l , a muscle, in which the 
 axon terminates in td., the motor end-plate. 
 
 A nerve-fibre has the gen- 
 eral form of a thread or long 
 narrow cylinder. It is not, 
 however, a perfect cylinder, 
 but shows at short intervals 
 slight constrictions or " nodes." 
 The space between two nodes 
 may be called a segment; it 
 is primarily a segment of the 
 primitive sheath, correspond- 
 ing to one cell of the sheath- 
 forming substance. The mye- 
 lin sheath is also interrupted 
 at each node, and thus ap- 
 pears in segments. The axon, 
 however, is continuous through 
 the node, though it is nar- 
 rowed at this point. 
 
 The inner structure of the 
 axon is so minute that even 
 the higher pjowers of the mi- 
 croscope afford barely enough 
 magnification to enable the 
 histologist to see it. 
 
 4. The size of the differ- 
 ent nerve-fibres in the human f 
 body varies greatly, according J 
 to their kind, position, and, 
 perhaps, function. As a rule 
 the non-medullated fibres are 
 smaller than the medullated, 
 the former being from iniW to 
 jrAnr f an mcn m diameter, 
 and the latter (in the trunk 
 and branches of the nerve) 
 from i-gVir to ^TT of an inch. 
 But this rule is not always 
 followed. In the white mat- 
 ter of the cord the medullated 
 fibres range in size from -rhrv 
 to -a 0*0 o of an inch, in parts 
 of the anterior columns, and 
 about TffVfr of an inch in those 
 regions of the lateral and pos- 
 
CONSTITUTION OF THE GRAY MATTER 
 
 101 
 
 terior columns which are nearest the gray matter of the cord. In the 
 gray matter of the cord and brain the fibres are much finer being 
 from TroiF to 14000 of an inch in diameter, or even of an almost 
 
 immeasurable fineness; they are finest of all in the superficial layers 
 of the brain and in the nerves of special sense. In some instances the 
 axis-cylinder may be not more than 1 0*0 o o of an inch in diameter. 
 
 The number of fibres which enter into the composition of indi- 
 vidual nerves also varies greatly. In the common motor nerve of 
 the tongue it has been estimated at about five thousand, in that of 
 the eyes at fifteen thou- 
 sand, in the optic nerve 
 at one hundred thou- 
 sand at least. 
 
 Success in analysis of 
 the nerve depends, in 
 large measure, on the 
 discovery of stains 
 which color some por- 
 tions more deeply than 
 others, and so bring out 
 their differences. The 
 most important fact re- 
 garding the inner struct- 
 ure of the axon is that 
 it consists of plasma in 
 which run very minute 
 
 fibrils. These fibrils ex- FlG - 47. Purkinje Cell. (Starr, Strong and Learning.) 
 
 tend lengthwise of the 
 
 axon, and are continuous for long distances; it is inferred by many 
 authorities that they are the conducting part of the axon. In 
 short, just as a moderate magnification of a nerve shows it to be 
 a bundle of nerve-fibres, so higher magnification applied to the single 
 fibre seems to show that it too is but a bundle, and that the real unit 
 of the nerve is the minute fibril. It should be mentioned, however, 
 that, in our lack of exact knowledge of the physical or chemical 
 process which is conducted along the nerve we cannot make an 
 inference, with any assurance, from the structure of the axon to 
 the function of its separate parts. 
 
 5. The white matter of the brain and cord is made up much 
 as are the nerves, except that the sheets of connective tissue are 
 absent, and also the primitive sheath of the fibre. The nervous 
 substance is protected by the enveloping bone and membranes, 
 and strengthened by the fibres of the neuroglia which pass between 
 the nerve-fibres. 
 
102 
 
 ELEMENTS OF THE NERVOUS STRUCTURE 
 
 The gray matter contains neuroglia, nerve-cells and their den- 
 drites, the terminations of axons which enter from the adjoining 
 white matter, and blood-vessels, which are present also, though in 
 less abundance, in the white matter and in the nerves. That this 
 list exhausts the contents of the gray matter is regarded by some 
 authorities as improbable, in view of the small bulk of the nerve- 
 
 \ 
 
 Fio. 48. Pyramidal Cells from the Cerebral Cortex. (Kolliker.) n, axon; p, apical 
 
 dendrite. 
 
 cells. Donaldson 1 estimates that the nerve-cells occupy but 1.3 
 per cent, of the bulk of the gray matter; while the dendrites, as he 
 believes, cannot occupy more than half the space of the cells. 
 This estimate seems to leave much space unaccounted for. Nissl 2 
 has advanced similar indirect evidence in favor of yet unknown com- 
 ponents of the gray matter. It should be said, however, that the 
 
 1 Journal of Comparative Neurology, 1899, IX, 141. 
 
 2 Die Neuronenlehre und ihre Anhdnger, 1903, p. 75. 
 
SIZE AND SHAPE OF NERVE-CELLS 
 
 103 
 
 best stains, such as that of Cajal, show 
 the gray matter to be very largely filled 
 with fine fibres, which appear to be the 
 branches of dendrites and of axons. 1 
 
 6. The true nerve-cells vary in size 
 as much as in shape; the limits may, 
 perhaps, be given as from about yj^ to 
 TrsW of an inch. The general shape 
 of a nerve-cell is chiefly dependent on 
 the dendrites and the manner or place 
 of their leaving the cell-body. The ac- 
 companying figures (Nos. 46, 47, 48, 49) 
 show several different forms. The cells 
 of the ventral horn of the spinal cord 
 which through their axons directly con- 
 trol the muscles send out dendrites in 
 every direction, and are accordingly 
 called " multipolar." The Purkinje cell 
 from the cerebellar cortex sends out one 
 dendritic stem, which branches into a 
 beautiful tree, though this branching is 
 
 1 Progress in tracing the branches of the nerve- 
 cells within the gray matter has depended largely 
 on the invention of methods of staining gray 
 matter in ways which bring out one or another 
 of its features. For showing the external form 
 of the cells and their branches, a method which 
 has done remarkable service is the silver method 
 of Golgi, according to which pieces of gray mat- 
 ter that have first been soaked in potassium 
 bichromate are treated with nitrate of silver, 
 with the result that a deposit of dark silver 
 chromate is precipitated on some of the cells. 
 The peculiarity of this method is that com- 
 paratively few cells are thus blackened, but 
 these few are blackened throughout, even to 
 their fine branches; in this manner an individual 
 cell, which would otherwise be lost in its intri- 
 cate interlacing with other cells, is made to stand 
 out clearly. The internal structure of the cell is 
 not, however, brought out by the Golgi method, 
 which simply encrusts the surface with a black 
 deposit. Other methods have been found which 
 permit of study of the internal structure. The 
 method of Nissl consists in first staining densely 
 
 with a basic dye such as methylene blue, and then dissolving out some of the 
 dye with alcohol; this leaves some parts of the cell still deeply stained. 
 Apdthy and Bethe have also introduced methods for selectively staining the 
 fibrils within the cells. 
 
 G. 49. Cells of the Mid-brain 
 (Optic Lobe of Chick). (Van 
 Gehuchten.) pr. cyl., the axon, 
 which here emerges from a den- 
 drite; /. opt, axon entering from 
 a distance, and connecting with 
 the dendrite. 
 
104 ELEMENTS OF THE NERVOUS STRUCTURE 
 
 peculiar in that it is confined to one plane. The pyramidal cells 
 of the cerebral cortex send out one long dendritic stalk toward 
 the surface of the cortex, and other stalks to the sides. Many 
 , small cells have only a 
 
 small development of den- 
 drites. The cells of the 
 spinal ganglia, and of the 
 similar ganglia of the sen- 
 sory cranial nerves, have 
 no dendrites (see Fig. 15, 
 p. 44). 
 
 The nerve-cell is usually, 
 at first, a round or some- 
 what angular body without 
 branches. From this a band 
 
 ^ } ^M / AX^^>^r<-^7S/M k of tissue issues > like a sort 
 
 f P^o-podium and then 
 breaks up into branches, 
 thus forming the dendrites. 
 Some cells in the brain (as 
 the Pur kin je and pyramidal 
 cells) develop dendrites after 
 birth. Most central cells 
 have only one axon (mon- 
 axion); but in the external 
 layer of the cerebrum poly- 
 axion cells are said to be 
 found. Bipolar cells are 
 found in abundance in the 
 dorsal root ganglia; and the 
 peripheral sympathetic cells 
 are polyaxion. Wholly an- 
 axion cells are rare; but are 
 said to be found in the olfac- 
 tory bulb, the ear, and the 
 se of some animals. 
 o special function, either 
 sensory or motor, has been 
 proved to be assignable to any of these different shapes. 
 
 Dendrites never extend far from their cell-body, but bifurcate 
 repeatedly in the immediate neighborhood of the cell. In both 
 these respects they differ from the typical axon, which is long, 
 narrow, cylindrical, and in general branches but little. Where it 
 does divide, the branch comes off at right angles to the trunk of 
 
 FIG. 50. Ceil with Short and Much Branched 
 Axon. (Van Gehuchten.) pr. cyL, the axon. 
 
INTERNAL STRUCTURE OF NERVE-CELLS 
 
 105 
 
 FIG. 51. A Nerve-Cell, Stained 
 by the Nissl Method, and 
 Highly Magnified. (Ewing.) 
 The blue of the stain is here 
 represented by black. 
 
 the axon, and is called a collateral. The 
 axon is peculiar also in that it passes into 
 the white matter, and acquires a myelin 
 sheath. It often extends, in the white 
 matter and in the peripheral nerves, to a 
 length of several feet. But there is another 
 type of axon (compare Fig. 50), which is 
 short, and branches abundantly near the 
 cell-body; it can still be distinguished from 
 the dendrites by its uniform slenderness, 
 and by the rectangular character of its 
 branching. 
 
 7. The inner structure of the cell-body 
 shows a nucleus with a minute nucleolus 
 within the nucleus. When the Nissl stain 
 is employed (compare Fig. 51), there are 
 seen many large granules, often of spin- 
 dle shape, with spaces between them. 
 These "Nissl bodies" are present in the dendrites as well as in 
 the cell-body, but are absent from the axon and from the conical 
 projection of the cell from which the axon arises. It is not easy 
 to make sure that such appearances as these represent structures 
 which exist in the living cell; for the chemical treatment through 
 which the cell must pass in order to show details may cause coagu- 
 lations and other changes of the substances within the cell. 
 What is certain is that the cells contain a particular substance 
 w T hich has an affinity for basic dyes; and that the same sort of 
 nerve-cell always gives the same sort of picture 
 when it is treated by the Nissl method, if it has 
 come from an animal in normal condition. But 
 there are many conditions, more or less abnor- 
 mal, which cause the cells to present quite a dif- 
 ferent picture after staining. Thus poisoning 
 with lead, mercury, arsenic, alcohol, strychnine,, 
 and many other poisons, asphyxia, or excessive 
 activity and fatigue, causes a diffusion of the 
 stainable substance throughout the cell-body 
 a condition known as chromatolysis (see Fig. 52). 
 Further action of the poison may cause a com- 
 plete disappearance of the stainable substance. 
 The results suggest, but do not fully prove, that 
 FIG 52 Chromatoiy- * m * s substance is of the nature of stored food or 
 sis. (Ewing.) TO fuel, which the cell utilizes in its activity, and 
 
 be compared with 
 
 Fig. 51. 
 
 uses up in excessive activity. 
 
106 
 
 ELEMENTS OF THE NERVOUS STRUCTURE 
 
 8. Methods of staining nerve-cells have also been introduced 1 
 which show numerous fibrils in the cell-body and dendrites, as well 
 as in the axon (see Fig. 53). Small bundles of fibrils from each 
 dendrite enter and pass through the cell-body into other dendrites 
 or into the axon. Fibrils thus pass from each branch of the cell 
 into nearly or quite every other branch. In the cells of the verte- 
 brate brain and cord, the fibrils 
 seem to maintain their individu- 
 ality through the cell, not anas- 
 tomosing with each other. 2 
 
 In the spinal ganglion cells, 
 however, the fibrils unite with 
 each other into a network or 
 latticework; and the same is 
 true of the cells in the nerve- 
 centres of invertebrates, as de- 
 monstrated by Apathy. 
 
 Occasionally a deposit of pig- 
 ment is found in nerve-cells ; the 
 amount of it increases with the 
 age of the individual. The sig- 
 nificance of the pigment is other- 
 wise unknown. 
 
 9. The arrangement of cells, 
 dendrites, and terminations of 
 axons in the gray matter is of 
 no less importance than the 
 structure of the single cell. In 
 general, it may be said that the 
 interweaving of branches from 
 different cells is very dense and 
 intricate. The number of cells 
 is great, even in a small ganglion 
 or nucleus; the number of axons 
 entering and terminating is also great; and the relations of the 
 axons to the cells near which they terminate is not by any means 
 easy to make out. It is fairly certain that axons, on entering a 
 mass of gray matter, come into definite functional relations with 
 the cells located there, or with their dendrites; but the exact 
 mode of connection is often obscure. In certain localities, how- 
 ever, the relations are clear as to certain facts. 
 
 10. The connections of the fibres of the olfactory nerve, for 
 example, are specially clear. These fibres arise from sensory cells 
 1 By Bethe, Bielchowsky, Cajal. 2 Bethe, op. cit., pp. 56-60. 
 
 Fro. 53. Nerve-Cell Stained for Fibrils. 
 (Bethe.) a, b, c, d, the stumps of several 
 dendrites; Ax, stump of the axon. 
 
STRUCTURE AND OFFICE OF THE DENDRITES 107 
 
 in the mucous membrane of the nose, and, passing through the bone 
 into the brain cavity, enter the olfactory bulb, where each axon 
 breaks up into a little bush of branches. Interlacing with these 
 are the branches of a dendrite which belongs to a cell of the olfac- 
 tory bulb. This dendrite, extending outward from its cell-body, 
 meets the axon coming in from the nose, and the two break up 
 
 FIG. 54. Diagram to Show the Connection of Axons and Dendrites in the Olfactory Bulb. 
 (Schafer, from Quain's Anatomy, by permission of Longmans, Green & Co.) olf.c., olfac- 
 tory cells in the nasal mucous membrane; olf.n., the olfactory nerve, consisting of axons 
 from the cells just mentioned; gl., "glomeruli," in which the terminal branches of these 
 axons are interwoven with dendrites of cells of the olfactory bulb; m.c., these cells; a, 
 their axons, passing further into the brain. 
 
 together into a mass of interlacing branches (see Fig. 54). On the 
 other side, the axon of the cell in the olfactory bulb runs back to 
 other parts of the brain. 
 
 Now it is certain that the line of communication must, in this 
 case, lead from the nose to the brain. It enters the brain by the 
 fibres of the olfactory nerve; it must therefore pass from these 
 fibres to the structures into which they enter into relation; and 
 these structures are the dendrites of the cells of the olfactory bulb. 
 The case thus shows communication from the axon of one cell 
 to the dendrites of another, and from these dendrites to their cell- 
 body and its axon. The dendrites must, therefore, be the receptive 
 part of the cell, i. e., the part which receives the nervous influences 
 or impulses from other more peripheral parts of the nervous 
 
108 ELEMENTS OF THE NERVOUS STRUCTURE 
 
 system. What is certain in this case, is probable enough in many 
 other parts of the gray matter, in which sensory axons terminate; 
 for example, in the case of the optic path. There are always 
 dendrites present which may be the recipients of the sensory im- 
 pulses. Moreover, there are no anatomical peculiarities of any 
 part of the gray matter which would discredit the general concep- 
 tion of the dendrites as receptive organs; or which would make 
 
 Fio. 55. Baskets of Axon-branches Around Nerve-cells. (Veratti, Edinger.) The spheri- 
 cal bodies are nerve-cells, the branches of which do not show in the figure; but the cell- 
 bodies are seen to be closely enveloped by "baskets" of fibres which result from the 
 splitting up of axons from other, distant cells. 
 
 more probable any other function for them. That they have this 
 receptive function may, therefore, be taken as a highly probable 
 and generally accepted view. 
 
 There are, however, indications that impulses are sometimes 
 received directly at the surface of the cell-body, as well as through 
 the dendrites. In certain cases the terminal branchings of an axon 
 are closely applied to the body of another cell. The cells of the 
 " trapezium," a portion of the bulb and pons which is closely con- 
 nected with the auditory nerve, show examples of this (compare 
 Fig. 55). Here the structure strongly suggests that the axon in- 
 fluences the cell-body directly. 
 
 11. The cerebellum affords interesting examples of various 
 forms of communication between one cell and another. The cor- 
 tex of this organ contains several varieties of nerve-cells, such as 
 
NERVE-CELLS OF THE CEREBELLUM 
 
 109 
 
 the Purkinje cells, with their richly branched dendrites, granule 
 cells, " basket cells," and cells with short and much branched 
 axons. Of all these, the Purkinje cells are those which send axons 
 away from the cortex of the cerebellum; apparently, therefore, the 
 influence of the cerebellum on the other parts of the nervous system 
 is exerted through these axons of the Purkinje cells. Accordingly, 
 the different nerve impulses 
 which may be present in 
 the cerebellar cortex must 
 be concentrated on the 
 Purkinje cells. 
 
 There are fibres enter- 
 ing this cortex from other 
 parts of the nervous system, 
 and some of these, called 
 " climbing fibres," grow up 
 the dendrites of the cells 
 of Purkinje, like a vine 
 up a tree; other incoming 
 fibres, however, do not 
 come into direct relation 
 with the Purkinje cells at 
 all, but end in peculiar, 
 mossy terminations in the 
 neighborhood of the little 
 granule cells. The axons 
 of these latter cells then 
 pass upward to the level of the Purkinje dendrites, and extend 
 in great numbers through, or between, the branches of these 
 dendrites. The "basket cells" lie in the same region as the 
 Purkinje dendrites; but their axons divide into several branches 
 each of which splits up into a basket-like arrangement of fine 
 branches around the cell-body of a cell of Purkinje. Thus one 
 basket cell appears " to hold the reins on " several cells of Purkinje. 
 The cells with much branched axons seem to spread influences, 
 similarly, over many granule cells. We certainly are far from a 
 full comprehension of these intricate relations of cells; though the 
 main fact that the cells of Purkinje are subject to a combination of 
 influences from other cells is clear; and it is also highly probable that 
 both the dendrites and the cell-body of the Purkinje cell are recep- 
 tive of influences from the axons of other cells (see Figs. 56 and 57). 
 
 12. If the dendrites are receptive in function, the terminations 
 of the axon must be transmissive ; i. e., they must pass on the nerve- 
 impulse to the dendrites and cell-bodies of other cells. There is 
 
 FIG. 56. Fibre-baskets Around Purkinje Cells. 
 (Cajal.) 
 
110 ELEMENTS OF THE NERVOUS STRUCTURE 
 
 much evidence to favor this view, and little opposed to it. The 
 nerve-cell may therefore be said to be " polarized " ; since one end of 
 it is capable of taking up stimuli, and the other is capable of giving 
 
 FIG. 57. Diagram of the Cells of the Cerebellar Cortex. (Kolliker.) gl, neuroglia 
 cell; gr, granule cell; p, axon of a Purkinje cell; /, " moss-fibre" ; k, climbing fibre, and 
 fc 1 , its termination; ra, small nerve-cell; n, cell with short and much branched axon; m 1 , 
 basket cell, the axon of which branches at zk, about the body of a Purkinje cell. 
 
 off stimuli. In case of the sensory fibres which have no dendrites 
 such as those of the dorsal roots of the cord, whose cells lie in the 
 spinal ganglia the peripheral termination of the axon in the sense- 
 organ becomes the functional equivalent of the dendrites; i. e., it 
 is the receptive part. In the usual type of cell in the centres, the 
 impulse seems to enter at the dendrites, to pass thence to the cell- 
 
STRUCTURE AND FUNCTION OF THE AXON 111 
 
 body and thence into the axon, and out at the terminal arboriza- 
 tions of the axon and its collaterals. 
 
 The axon, therefore, usually conducts toward its own branched 
 ends. Experiment proves, indeed, that the axon is inherently 
 capable of conducting in either direction. But experiment also 
 seems to prove that conduction from an axon to the dendrites of 
 another cell can occur only in this one, and not in the reverse, di- 
 rection. Thus, in the spinal cord, an impulse passing in by the 
 
 vr 
 
 FIG. 58. Diagram of a Synapse in the Cord, dr, fibre of the dorsal root, passing into the 
 ventral horn, and connecting, at the synapse, with a motor cell from which arises the 
 fibre of the ventral root, vr. Stimulating dr arouses vr to activity, but stimulation of vr 
 does not arouse dr. 
 
 fibres of the dorsal roots is transmitted over to the motor fibres of 
 the ventral roots; but an impulse artificially generated in these 
 motor fibres, and conducted back into the cord, does not make its 
 appearance in the dorsal roots. Now we have already seen (p. 89) 
 that the connection between the dorsal and the ventral fibres lies in 
 the gray matter of the ventral horn; it appears, therefore, to be 
 a connection between the terminations of the incoming sensory 
 axons and the dendrites of the large cells of the ventral horn. If 
 this appearance is correct, the above experimental result may be 
 restated in the following terms: Nerve-impulses are conducted from 
 the terminations of the sensory axons to the dendrites of the motor 
 cells, but will not pass in the reverse direction. Thus the connection 
 between axonic terminations and dendrites acts as a sort of valve, 
 allowing nerve-impulses to pass in only one direction. 
 
 The above-mentioned and similar facts such as that conduction 
 is slower in the gray matter than along the axons of the nerves, 
 and more liable to interruption by the action of drugs, etc. have 
 
112 ELEMENTS OF THE NERVOUS STRUCTURE 
 
 led physiologists to the conception of a certain looseness of connec- 
 tion within the gray matter, or lack of such complete continuity as 
 obtains between the parts of the axon. They have accordingly given 
 a special name to the connection between the terminations of an 
 axon and the dendrites of another cell. This name, synapse (com- 
 pare Fig. 58), signifies a "fitting" together, as distinguished from 
 a "growing" together. In this way, axon and dendrites are con- 
 ceived of as dovetailed together; perhaps very snugly, but with- 
 out such complete continuity as obtains between an axon and its 
 cell-body, or between the cell-body and its dendrites. A cell-body, 
 with the axon and dendrites especially belonging to it, is thus thought 
 of as a continuous whole, within which conduction is easy and can 
 occur in any direction; whereas a certain degree of discontinuity 
 or separation is conceived to exist between one cell and another. 
 When regarded in this way, each cell with its branches is called a 
 neurone; and the doctrine that some degree of discontinuity exists 
 between the neurones is called the neurone theory. 
 
 13. The history of the neurone theory is interesting and sug- 
 gestive as to the difficulties and the nature of the conclusions in 
 this entire field of inquiry. The first clearly defined theory re- 
 garding the connections existing within the gray matter was that 
 of Gerlach, who, in 1870, concluded from the knowledge then in 
 hand that the dendrites and other branches of the nerve-cells 
 united with one another into a dense and continuous network 
 throughout the gray matter. This "nerve-net" conception held 
 the field for about twenty years. Meanwhile the use of the Golgi 
 method of staining, especially in the hands of the Spanish investi- 
 gator, S. Ram6n y Cajal, gave pictures of individual cells with 
 numerous branches, but with no indication of anastomoses between 
 cells, or of a continuous network of branches. On the contrary, 
 each cell, with its branches, appeared separate from every other 
 cell. This anatomical, or histological, result was quite in accord 
 with the embryological observations of His, namely (see above, 
 p. 41), that the gray matter starts as a collection of separate cells, 
 out of which the branches grow. Summing up the anatomical and 
 embryological evidence, Waldeyer, in 1891, formulated the theory 
 that nerve-cells, however much they might branch, remained, as 
 they had begun, separate units which he named " neurones." The 
 rapidity with which this new conception won its way among all 
 classes of students is notable in scientific history. To the physiolo- 
 gists it was welcome as affording an explanation of the slow con- 
 duction and valve-like action of the connections within the gray 
 substance of the nervous system. To the pathologist, it was also 
 welcome as affording an explanation of the peculiar fact that de- 
 
THE NEURONE THEORY 113 
 
 generation of nervous tissue resulting from injury extends only to 
 the terminations of the axons injured; and does not spread freely 
 over to other cells. Even the psychologist found the neurone theory 
 useful, since learning and association could now be understood as 
 dependent on the formation of new synapses; while sleep and un- 
 consciousness might very well be due to slight influences obstruct- 
 ing the synapse. Many other mental facts received help from this 
 theory, in respect of their psycho-physical interpretation. From 
 about 1891, accordingly, the neurone theory attained great vogue 
 and came into well-nigh universal acceptation. 
 
 There was always, however, a minority of able neurologists who 
 did not fully accept the neurone theory, and who were stimulated 
 by it to look more carefully for direct connections between the cells 
 of the gray matter. Protoplasmic bridges were found to exist in 
 the nervous systems of very low orders of invertebrates (compare 
 p. 18); but in the higher invertebrates and in vertebrates, such 
 bridges were not found; and all now admit that they do not exist. 
 The centre of discussion was, accordingly, shifted with the new dis- 
 coveries regarding the inner structure of the nerve-cell and its 
 branches (see p. 106). Since the fibrils, from their appearance, 
 are probably the real conductors of nerve-impulses, continuity 
 between two cells might be established if simply the fibrils, rather 
 than larger protoplasmic branches such as the dendrites, could be 
 traced as emerging from one cell and entering into another cell. 
 In some invertebrates, a passage of fibrils from cell to cell was an- 
 nounced by Apathy; but this alleged discovery has not been con- 
 firmed by some of the competent authorities who have followed 
 Apathy's methods. 
 
 In the exceedingly intricate gray matter of vertebrates, the task 
 of tracing such minute structures as the nerve-fibrils from one cell to 
 another would at best be extremely difficult; and for this reason, 
 no strong disproof can be based on failure to trace them. In point 
 of fact, however, the evidence for such passage of fibrils from one 
 cell to another in the gray matter is, up to the present, slight and 
 dubious. There is, indeed, an appearance observed on, or near, 
 the surface of many cell-bodies, which has been called, from its 
 discoverer, the "Golgi net," and which consists of fine fibrils; and 
 some observers, as Bethe, 1 have found some evidence of fibrils 
 issuing from within the cell and joining the Golgi net. On the 
 other hand, this net has seemed to be in continuity with axons ap- 
 proaching the cell from elsewhere. These observations seem to 
 show a possible path of fibrillar connection between the nerve-cells. 
 
 1 Allgemeine Anatomic und Physiologic des Nervensy stems, pp. 65-78 (Leipzig, 
 1903). 
 
114 ELEMENTS OF THE NERVOUS STRUCTURE 
 
 The full meaning of the fibrillar theory may be summed up in 
 this: that the fibrils are the real units of the nervous system. The 
 cells, which are the units according to the neurone theory, are to the 
 fibrillar theory merely the medium in which the fibrils grow and 
 by which they are nourished. The fibrils issue freely from the cells 
 and perhaps form a fibril network in the spaces between them. 
 This network, according to the theory, would therefore be the es- 
 sential structure of the gray matter; it would contain the connections 
 between incoming and outgoing axons. Fibrils would enter the 
 network from axons coming into the gray matter; and fibrils from 
 the network would enter the dendrites and cell-bodies, and then 
 pass at least some of them into the axons of these cell-bodies 
 and so away to other parts. 
 
 As between the neurone theory and the fibrillar theory, it is im- 
 possible at present to decide with certainty; but there is no doubt 
 that the neurone theory still commands the support of the majority 
 of authorities; and that it serves, for the present, the useful purpose 
 of summing up a large proportion of the known facts that have a 
 bearing on the connections within the gray matter of the nervous 
 system. 
 
 14. It has already been said (p. 40) that a considerable part 
 of the central nervous system consists of a substance which is char- 
 acteristically different from either of the definitely recognized nerve 
 elements, and to which the name of "neuroglia" has been given. 
 This name was originally designed to embody the opinion that it 
 acted as a kind of " nerve-cement " (nerven-kiti); and neuroglia has 
 frequently been classified with the connective tissue. But as long 
 ago as 1880, Henle said of this substance: "It is at all events to 
 be distinguished from connective tissue on account of its chemical 
 properties." l Microscopic examination shows that the neuroglia 
 is by no means a homogeneous mass, but is composed of innumer- 
 able minute cells which differ in their physical characteristics from 
 the typical nerve-cells. The cell-body is small; 2 the branches are, 
 in general, of tolerably uniform size, and at the very edge the fibres 
 end in minute balls or knobs. The method of the branching of 
 the neuroglia cells, when any branching occurs, enables the ob- 
 server to differentiate them from the dendrites of the true nerve- 
 cell. Thus their appearance has caused some of them to be called 
 "spider cells" (see Fig. 59). 
 
 Neuroglia cells differ both in size and in distribution, in the differ- 
 ent parts of the central nervous system. They are most numerous 
 
 1 Anatomie d. Menschen, text, p. 306. 
 
 2 Some of these cells are, indeed, exceedingly minute, being scarcely more than 
 fuVu or ssVff of an inch in diameter. 
 
THE NEUROGLIA CELLS 
 
 115 
 
 where there are fewest nervous elements as though their office were 
 to fill in the interstices, and to constitute the background or soil for 
 holding the nerve-cells and nerve-fibres. They sometimes arrange 
 themselves in regular order, as, for example, around the central 
 commissural region of the cord. The distribution of their branches 
 is fairly uniform in the middle of the cord, but changes at the edge, 
 where the massing of the fibres constitutes the outer wall of the cord 
 and the posterior fissure. 
 
 As respects their origin, these spider cells seem to be ectodermal 
 and not connective tissue cells. They therefore arise from the same 
 
 FIG. 59. A Spider Cell from the Spinal Cord. (V. LenhossSk.) 
 
 embryological elements as the nerve-cells. This, however, does 
 not prove that they are, functionally considered, true nerve-cells; 
 for cells from the same source may have altogether different func- 
 tions in their developed form. In general, the supporting cells of 
 the nervous substance are older, ontogenetically, than the true 
 nervous elements. They begin in primitive cells, which are ar- 
 ranged radially around the cord centre. But these primitive cells 
 gradually undergo modifications of shape, before their place ap- 
 pears to be taken by the spider cells. On the other hand, some of 
 the spider cells seem to come directly from the ectodermal cells, with- 
 out passing through the intermediate cell period. 
 
116 ELEMENTS OF THE NERVOUS STRUCTURE 
 
 It thus appears that we cannot as yet be perfectly certain as to 
 what is the whole functional relation of the neuroglia to the acknowl- 
 edged elements of the nervous system. And, indeed, although we 
 now know much more than when these words were written, it still 
 remains true, as Eckhard 1 said years ago: "If we start the inquiry, 
 what formal elements of the brain and cord take part in the activi- 
 ties of these organs, and in what way they do take part, we are 
 able to give to it only a very unsatisfactory answer." 2 
 
 1 Hermann, Handbuch d. Physiologic, II, ii, p. 15. 
 
 2 The modern view of the nature of neuroglia was first hinted at by Kolliker 
 (Handb. d. Gewebelehre, 4th Aufl., 1862, pp. 304 ff.); it was worked out more fully 
 by Golgi in contributions to the histology of the central nervous system, published 
 in the Rivista clinica di Bologna, during 1871-1872. Further important contri- 
 butions to our knowledge were made by F. Boll (" Die Histologie u. Histiogenese 
 d. nervosen Centralorgane," Archiv. /. Psychiatric u. Nervenkrankh., Bd. IV, p. 1); 
 by H. Gieke ("Die Stiitzsubstanz d. Centralsystems," Archiv. f. mikr. Anatomie, 
 Bd. XXV, pp. 441 ff., and Bd. XXVI, pp. 129 ff.); and by W. Vigual ("Sur le 
 deVelloppement des elements de la moelle des mammiferes," Archives de PhysioL 
 normale et pathol., I, 1884, pp. 230 ff.). The conclusions of these investigators, 
 which were at first rather vague, have subsequently been on the whole con- 
 firmed by His, Cajal, and other later workers. 
 
CHAPTER V 
 CHEMISTRY OF THE NERVOUS SYSTEM 
 
 1. It is conceivable that, in the progress of the sciences concerned, 
 the action of the nervous substance should be brought into such re- 
 lations with its chemistry and physics, that nervous functions might 
 be stated very largely in terms of physical and chemical processes. 
 At present so engaging a prospect seems indefinitely remote; partly 
 because of our meagre knowledge of the chemistry of the nervous 
 tissues, and partly because of insufficient knowledge regarding the 
 intimate functions of the nervous elements. Much the same thing 
 is true, indeed, of the muscles and the glands : the known functions 
 of these organs, too, cannot as yet be explained in terms of physics 
 and chemistry. In regard to nervous tissue, we know fairly well 
 the chemical elements which enter into its composition; and we 
 know some of the compounds which exist there. In general, the 
 same chemical elements are represented in nervous as in other 
 tissues, though in somewhat different proportions. Also, the same 
 general classes of compounds occur in nervous and in other tissues. 
 In respect neither of their elements nor of their compounds, there- 
 fore, do the different tissues present such striking differences as 
 might be expected from their various functions. 
 
 2. The general chemistry of the brain and nerves is still in an 
 undeveloped condition. Apparently, a great number of somewhat 
 similar compounds exist in the brain, the isolation and analysis of 
 which is attended with great difficulty. These difficulties are not 
 due simply to the complex constitution of most of the substances 
 with which we have to deal. They are also very largely due to the 
 fact that these substances are products of life; and living tissue can- 
 not be at the same time kept in normal condition and subjected to 
 the handling necessary for chemical analysis. As soon as it is no 
 longer alive, or at any rate long before any chemical analysis can be 
 completed, the constitution of such tissue is changed. However 
 carefully the chemical elements, the constituents, which enter into 
 the nervous substance may be preserved, their constitution, their 
 chemical arrangement and behavior, cannot be preserved. It is 
 impossible for example for the chemist even to determine the 
 specific gravity of uncoagulated blood, "except by operating with 
 extreme expedition and at temperatures below C." 
 
 117 
 
118 CHEMISTRY OF THE NERVOUS SYSTEM 
 
 The specific gravity of the white matter is somewhat greater than 
 that of the gray. Danilewski found the specific gravity of the gray 
 matter in the human brain to vary from 1.029 to 1.039; that of the 
 white matter from 1.039 to 1.043. 1 Other authors have assigned 
 slightly lower values for both. This difference in the weight of the 
 white and the gray matter is chiefly due to difference in the relative 
 amounts of water and of solids which they respectively contain. 
 Determinations by several authors 2 agree in assigning about 86 
 per cent, of water and 14 per cent, of solids to the gray matter (cere- 
 bral cortex); whereas the white matter shows only from 70 to 71 
 per cent, of water. The water-content of the peripheral nerves is 
 still less, ranging from 40 to 70 per cent. 
 
 3. The chemical elements chiefly present in the brain, as in all 
 living cells and tissues, are carbon, hydrogen, oxygen, nitrogen and 
 phosphorus, chlorine, sodium, and potassium. 
 
 Of chemical compounds, the brain, like all living cells, contains 
 water, salts (such as sodium chloride and potassium phosphate), 
 proteids, and lipoids. 
 
 The proteids are highly complex organic compounds of carbon, 
 hydrogen, oxygen, and nitrogen; they form a large proportion of 
 the substance of all living cells, whether animal or vegetable, and ap- 
 pear to play a very important part in the life of the cells. They 
 have, indeed, been regarded as the essentially living parts of the 
 cells; but it is probable that the role of the salts held in solution, 
 and of the lipoids, is no less essential to life. 
 
 Of the solids contained in the nervous centres, more than one-half 
 in the gray, and about one-third in the white, consist of these pro- 
 teid substances. Four such bodies are now generally recognized; 
 of which the most abundant is a nucleo-proteid containing phos- 
 phorus, which is soluble in water; it is believed to come in large 
 measure from the nuclei of the cells, and is analogous to the nucleo- 
 proteids derived from all other cellular tissues. Two other pro- 
 teids found in the brain substance are globulins, differing in their 
 solubilities and in the temperatures at which they coagulate (47 
 C. and 70 C. respectively); they are also quite similar to the globu- 
 lins derived from all other cellular tissues, such as muscle, liver, and 
 kidney. 3 It will be noticed that all these three proteids have been 
 found to be soluble and coagulable at varying temperatures. 
 
 1 Med. CentralbL, xviii, p. 241. 
 
 2 Cited by S. Frankel, in his "Gehirn-Chemie," in Asher and Spiro's Ergeb- 
 nisse der Physiologic, 1909, VIII, p. 217. This work has been followed largely in 
 the statements made in the following pages with regard to the chemistry of the 
 nervous substance of the brain. 
 
 3 Bernhardt; also Halliburton, Journal of Physiology, 1893, XV, 90; Chemistry 
 of Muscle and Nerve, 1904, p. 61. 
 
LIPOIDS OF THE NERVOUS SUBSTANCE 119 
 
 Besides these three soluble proteids, the brain yields a small pro- 
 portion of an insoluble proteid, called neurokeratin, 1 which is 
 similar in its properties to the keratin that forms so large a pro- 
 portion of the hair, horn, nails, and other like appendages of the skin. 
 This substance is found both in the medullated nerves and in the 
 central organs according to Kuhne in both gray and white matter, 
 but according to W. Koch 2 only in the white. As the researches 
 of Kiihne and his pupils showed some years ago, neurokeratin oc- 
 curs also in the retina. It contains carbon, hydrogen, nitrogen, and 
 sulphur, besides inorganic constituents. 
 
 4. It is the lipoids which are likely to be of most interest in 
 their relation to the physiology and psycho-physics of the brain. 
 Indeed, the brain is chiefly characterized, chemically considered, 
 by its exceptionally large content of these substances. It is the 
 white matter, however, rather than the gray, which is peculiarly 
 rich in them; and this is the same thing as saying that the white 
 matter is chemically more specialized than is the gray; while the 
 latter seems to differ but little from the substance of other living 
 cells. Since the myelin sheath of the nerve-fibres is the distin- 
 guishing feature of the white nervous matter, it appears probable 
 that the lipoids are constituents of the myelin sheath; this, indeed, 
 may be regarded as certain; and it is likely also that a large share of 
 the lipoid content of gray matter is due to the fine myelinated 
 fibres which penetrate it. In a word, the chemistry of the brain, 
 so far as it presents anything peculiar, appears for the most part 
 to be the chemistry of the myelin sheath. And the chemistry of 
 the myelin sheath is for the most part the study of its " lipoids." 
 
 5. The term "lipoid" means, etymologically, a "fat-like body"; 
 it covers a considerable number of organic compounds which are 
 not exactly fat, but which resemble fat in some of its physical and 
 chemical properties. To understand the use of the term, it is neces- 
 sary to consider briefly the chemistry of living cells in general. 
 
 As we have already seen (p. 14), one of the most essential prop- 
 erties of a living cell is its power of taking in and letting out dis- 
 solved substances through its surface; while, nevertheless, not al- 
 lowing the passage of such substances to be perfectly free. This 
 we have called "the selective," or "preferential," power of the living 
 cell. Such a product could not live without exchange of substances 
 with the medium in which it lives; but, at the same time, it could 
 not live if the diffusion of these substances were perfectly free, for 
 then it would soon differ in no respect from its surrounding medium. 
 
 1 Ewald and Kiihne, Verhandlungen der nat.-hist.-med. Vereins, N. F. I, 357 
 (Heidelberg, 1877). 
 
 2 Zeitschrift fur physiologische Chemie, 1902, XXXVI, p. 134. 
 
120 CHEMISTRY OF THE NERVOUS SYSTEM 
 
 The individuality of the cell depends on its power of resisting free 
 osmosis through its surface. Its surface is not freely permeable, 
 but "semi-permeable." In many cells, accordingly, the outer sur- 
 face consists of a more or less definite membrane, and the necessary 
 "semi-permeability" is very likely a property of this membrane. 
 Certain chemical agents dissolve or otherwise alter the membrane; 
 among these are the cell-narcotics, ether, chloroform, benzol, and 
 others. From this it is inferred that substances which are dis- 
 solved out of the cell by the application of ether, etc., have special 
 importance in the membrane of the living cell, and probably have 
 much to do with the resistance offered by the cell to free osmosis. 1 
 
 Now since substances extracted by ether, etc., include, besides 
 the true fats, the class of bodies called lipoids, the definition of the 
 term ("lipoid") may be given in terms of this property of being 
 extracted by ether and other cell-narcotics; or, more biologically 
 but less definitely, in terms of the probable function of these bodies 
 in the cell membrane. This function may be shared by the fats 
 proper. 
 
 6. The brain contains a large amount of substances which can 
 be extracted by ether, etc., but which are not fats. It is probable, 
 as was said before, that these substances come chiefly from the myelin 
 sheath, which may perhaps be regarded as a highly developed cell 
 membrane. Thudicum, 2 whose work on the chemistry of the 
 brain is very elaborate and fundamental, recognizes three well- 
 defined classes of brain lipoids, besides others less well understood. 
 The compounds of the first class contain both nitrogen and phos- 
 phorus; those of the second contain nitrogen but no phosphorus; 
 and those of the third class contain neither of these elements, but 
 only carbon, hydrogen, and oxygen. He is further able to subdi- 
 vide these classes, and to point out several members existing within 
 some of the groups; but inasmuch as most of these substances are 
 as yet imperfectly defined, and as great difference of opinion re- 
 garding some of them exists among chemical authorities, it will 
 not be worth our while to give a full list of them here; we may con- 
 fine our attention to a few of the best-established. 
 
 7. Cholesterin is the best established of all the brain lipoids ; 
 it belongs to the third of the classes just mentioned, containing 
 neither phosphorus nor nitrogen. Its composition corresponds 
 
 1 See Ivar Bang, "Biochemie der Zell-lipoide," in Asher and Spiro's Ergebnisse 
 der Physiologic, 1907, VIII, 1 and 2 Abth., pp. 134 ff. 
 
 a "Researches on the Chemical Constitution of the Brain," in Reports of the 
 Medical Officer of the Privy Council and Local Government Board, 1874; and 
 Die chemische Konstitution des Gehirns des Menschen und der Tiere (Tubingen, 
 1901). 
 
THE PHOSPHORIZED LIPOIDS 121 
 
 closely to the formula C 27 H44O, which may also be written, as 
 a slight indication of its structure, C 27 H 48 OH. It belongs to 
 the alcohols, being the hydroxide of a hypothetical hydrocarbon, 
 C 2 7H 44 , which has not yet been prepared. It is a "monatomic" 
 alcohol, having one OH group by which it combines with acids, 
 as with the fatty acids, and thus forms compounds which are prob- 
 ably present in the brain. Cholesterin is also widely present in 
 other tissues, and indeed in all tissues, both animal and vegetable; 
 but is especially abundant in nervous tissue. Cholesterin is a solid 
 at body temperature, its melting-point being 147 C. Its specific 
 gravity is 1.046. It is insoluble in water, though a mixture of it 
 and other lipoids is soluble. It is soluble in hot alcohol, in ether, 
 chloroform, etc., and crystallizes out of these solutions in the form 
 of fine needles (out of ether) or of rhombic tables (out of alcohol). 
 It is electrically a non-conductor. 
 
 To the class of lipoids which contain nitrogen but no phospho- 
 rus are to be reckoned a considerable number of substances, such 
 as cerebrin, cerebron, phrenosin, and kerasin. Some of these 
 differ but little among themselves, according to the analyses, and 
 may even be identical, if allowance be made for accidental impuri- 
 ties. 1 
 
 8. Most interest, however, attaches itself to the class of phos- 
 phorized lipoids. The fact that the brain is rich in phosphorus 
 has long been deemed significant, and at one time the rather crude 
 dictum, "No thought without phosphorus," attained considerable 
 vogue. But on the one hand, there are many other elements which 
 are essential to the brain's activity; and, on the other hand, phos- 
 phorus is present in all living cells. Phosphorus as a chemical 
 element of the nervous substance has, therefore, no monopoly; 
 and the statement, " No life without phosphorus," would more ade- 
 quately represent the facts of the case. The phosphorus of the 
 brain is for the most part contained in its lipoids, that is to say, in 
 the myelin sheaths of its nerve fibres; and, instead of hoping to 
 discover some direct connection between phosphorus and con- 
 sciousness, we should, if we desire to follow a scientific and promis- 
 ing course of investigation, seek first for the part played by phos- 
 phorus-containing lipoids in the myelin sheath. 
 
 While a large share of the lipoid substances which have been ex- 
 tracted from the brain do, without doubt, contain phosphorus, and 
 while a considerable number of different compounds, belonging 
 to this class, have been obtained, analyzed, and named by chemists, 
 the difficulties of satisfactory determination are here so great that 
 
 1 See Posner and Gies, Journal of Biological Chemistry, I, 59; and S. Frankel, 
 in Asher and Spiro's Ergebnisse der Physiologic, 1909, VIII, 249. 
 
122 CHEMISTRY OF THE NERVOUS SYSTEM 
 
 there is no one of these bodies which has been generally accepted 
 as a preformed constituent of the brain. There is no doubt con- 
 cerning some of the decomposition products of these lipoids, and 
 none, therefore, regarding some essential facts of their structure; 
 but the agreement ceases when the exact composition and formula 
 of the brain constituents is in question. Controversy between au- 
 thorities has been especially bitter in this field, and the substances 
 around which the controversy has principally centered are these two: 
 lecithin and protagon. 
 
 9. Lecithin is a well-recognized and fairly well-understood 
 substance; but its occurrence preformed in the living brain is not 
 established beyond dispute. It occurs abundantly in the yolk of 
 eggs, and is known also to occur in muscle tissue; it probably also 
 occurs quite widely in cells of different kinds. The formulae as- 
 signed for it by different authors differ somewhat, possibly because 
 there is more than one lecithin. Thudicum's formula for brain 
 lecithin is C4 3 H 8 4NPO 9 . It is described as a phosphorized fat, 
 and its relation to the ordinary fats may be roughly expressed by 
 saying that, whereas these consist of glycerin combined with fatty 
 acids, lecithin contains, in addition, phosphoric acid and the alka- 
 loid cholin. 1 
 
 Lecithin is an unstable compound, easily breaking up into simpler 
 compounds, while on the other hand it apparently enters into still 
 more complex compounds with proteids and with cholesterin. 
 Physically, lecithin is a yellowish-white, waxy, hygroscopic solid, 
 which in thin layers shines with a silky lustre. It is soluble in 
 alcohol, ether, etc., but in water it swells to a sort of paste, much like 
 starch. Electrically it is a non-conductor; and bound up with this 
 is its property of resisting free osmosis through any membrane into 
 which it enters. It may also be called a highly sensitive substance, 
 since its physical condition, while in solution, is readily changed (as 
 by the action of ions). 2 All of these properties are interesting and 
 suggestive of the probable importance of lecithin in the life of the 
 cells. As was stated above, the existence of lecithin preformed in 
 the brain is not completely made out; but if it does not occur there, 
 then other similar substances, such as Thudicum's kephalin, or else 
 more complex compounds into which lecithin and kephalin enter, 
 
 1 The complex structure of lecithin is more precisely stated by saying that 
 glycerin contains three hydroxyl groups; by one of which it takes up phosphoric 
 acid, and by the others, two molecules of fatty acid; meanwhile, the phosphoric 
 acid also combines with the base, cholin, which has itself a rather complex struct- 
 ure, as expressed by its analytic name, trimethyl-oxyethyl-ammonium hydroxide. 
 
 2 W. Koch, "The Lecithans," Decennial Publications of the University of Chicago, 
 1902, vol. X; American Journal of Physiology, 1904, XI, 303. 
 
THE PHOSPHORIZED LIPOIDS 123 
 
 must exist in the brain. In other words, the phosphorus of the brain 
 is partly contained in compounds presenting the general character- 
 istics which have been described as those of lecithin. 
 
 10. Protagon was discovered, as a new proximate principle 
 that can be separated from the brain, in 1864, by Dr. Oscar Lieb- 
 reich; his discovery was announced in a paper 1 published in 1865. 
 This investigator gave to this substance the name which it still bears, 
 as in his opinion the first to be definitely ascertained among the 
 specific constituents of the brain (TrpwTayds, leading the van). He 
 assigned to it the formula C 116 H 2 4iN 4 O22P. More recent work 
 has changed this formula to some extent especially in assigning 
 five atoms of N to each one of P, and in adding a small proportion 
 of sulphur to the molecule. 
 
 The process by which he obtained it from the brain may be thus 
 briefly described (the description will serve to illustrate in general 
 the processes of physiological chemistry): Perfectly fresh ox's 
 brains are freed from the blood and membranes, and are then di- 
 gested for about a day in eighty-five per cent, alcohol; from this 
 fluid, when filtered, a quantity of white flocculent precipitate is 
 obtained, and the cholesterin and other bodies soluble in ether are 
 dissolved out; from the substance left undissolved, when dried and 
 reduced to powder and digested for many hours with alcohol, and 
 then filtered and cooled, microscopic crystals separate themselves, 
 arranged for the most part in rosettes. The substance thus crystal- 
 lized is protagon. Repetition of the alternate processes of solution 
 and crystallization is resorted to for purifying the protagon. 
 
 11. The history of the protagon problem is interesting and curi- 
 ous in several ways, even apart from the importance which has been 
 assigned this substance as the most characteristic chemical sub- 
 stance of the brain, and the one which would therefore have espe- 
 cially to be considered in any attempt to correlate the composition 
 of the brain with its functions. A substance apparently identical 
 with Liebreich's protagon had been obtained by several previous 
 observers (1834-1850), with whose work Liebreich was unac- 
 quainted. The consensus of expert opinion on the chemical unity 
 of the substance has swung back and forth several times in the last 
 half century. At first, on Liebreich's discovery, it was accepted, 
 and lecithin was regarded as probably non-existent in the fresh 
 brain; then the work of Diaconow (1868) and of Thudicum (1874) 
 discredited protagon, by tending to show that it was a mixture of 
 no constant composition; and for years the belief in its unity was 
 relegated to the limbo of exploded views. Next, Gamgee (1879) re- 
 
 die chemische Beschaffenheit der Gehirnsubstanz," Annalen der 
 Chemie und Pharmacie, CXXXIV, pp. 29-44. 
 
124 CHEMISTRY OF THE NERVOUS SYSTEM 
 
 suscitated it, and it enjoyed for two decades general acceptance. 
 Beginning about 1900, results supporting Thudicum have been ob- 
 tained by several writers, who do not hesitate to say that the chemi- 
 cal unity of protagon has been fully disproved. Gies and his col- 
 laborators 1 have shown that if the method employed for purify- 
 ing protagon is repeated time after time, instead of approximating 
 to a constant composition, the protagon changes at each repetition 
 of the process. They infer that pure protagon, if it exists, has never 
 been prepared, and that probably it does not exist as a definite com- 
 pound. Their results have been confirmed and accepted by other 
 observers. 2 
 
 On the other hand, Cramer has found it possible to prepare from 
 the brain, by an entirely different method, a substance which has the 
 same quantitative composition as Liebreich's protagon. This con- 
 stancy of composition under different methods of preparation, taken 
 in connection with the crystalline form of the substance, leads au- 
 thorities such as Hammersten 3 and Halliburton 4 to conclude that its 
 chemical identity has not been disproved. The former of these two 
 authorities regards protagon as a crystalline substance which is 
 extremely difficult to separate from other substances that are in 
 part its decomposition products. But FrankePs position 5 is alone 
 tenable; for he regards the whole state of the subject as at present 
 thoroughly unsatisfactory. To illustrate and enforce this truth 
 may be regarded as the principal justification for the attention 
 which we have ourselves bestowed upon so-called protagon. 
 
 12. The specific chemistry of the histological elements of the 
 nervous system, or of the various parts of such elements, is yet more 
 meagre and doubtful than its general chemistry. The differences 
 between the composition of gray and of white matter indicate that 
 the nerve-cell body is rich in proteids, whereas the myelin sheath 
 consists very largely of lipoids. The internal structure of the cells 
 and axons, as revealed by selective stains, is an indication of chemi- 
 cal differences between the parts differentiated by the stains; but 
 the interpretation of the reactions on staining is by no means easy, 
 and little can at present be said regarding the chemistry of parts of 
 the cells. 
 
 13. It need scarcely be said, in conclusion, that we have little 
 knowledge respecting the relation which exists between the chemical 
 constitution and chemical processes of the nervous system, on the 
 one hand, and, on the other, the phenomena of so-called mind. 
 
 1 See Posner and Gies, Journal of Biological Chemistry, 1905, vol. I, p. 98. 
 a Rosenheim and Tebb, 1907, Journal of Physiology, XXXVI, pp. 1 ff. 
 
 3 Lehrbuch der physiologischen Chemie, 1907, p. 483. 
 
 4 Biochemistry of Muscle and Nerve, 1904, p. 65. 5 Op. cit., p. 251. 
 
SPECIFIC CHEMISTRY OF NERVOUS ELEMENTS 125 
 
 Nevertheless, certain important general relations may be point- 
 ed out between the chemical nature of the nervous mechanism and 
 its psycho-physical functions. The extremely high organization 
 and chemically sensitive constitution of this mechanism are beyond 
 doubt related to all its distinctive activities. Like every other nat- 
 ural material structure, the nervous system is obviously adapted 
 to its peculiar kind of work. Chemically considered, it appears as 
 composed of a number of extremely complex and highly unstable 
 compounds. It therefore holds in its chemical constitution a large 
 amount of disposable energy; this energy it yields readily when the 
 equilibrium of its molecules is in any way disturbed. Within, cer- 
 tain limits, it explodes with increasing surrender of its disposable 
 energy as the number and intensity of the demands upon it are in- 
 creased very much as would a gun which should be arranged so 
 as to go off with greater energy as the pressure of the finger on its 
 trigger is repeated or increased. 
 
 It is probable that the substance of the nerves is the seat of a 
 chemical synthesis, as the result of which still more complex bodies 
 are constructed from the already complex alimentary material fur- 
 nished by the blood; such bodies have a high value as combus- 
 tibles, and thus, as has been said, possess a significant amount of 
 disposable energy. The relation of a supply of oxygen to the 
 nerve-centres is also important to notice. The nerve-fibres require 
 comparatively a small amount of oxygen. It may be conjectured 
 that in their case, as in the case of muscle-fibre, intra-molecular 
 oxygen is of some use in preparing explosive materials. But at 
 present we must be satisfied with conjecture on this point. On the 
 contrary, the vascular nature of the central organs creates a pre- 
 sumption that the chemical processes which have their seat in them 
 require an abundance of oxygen. Experience confirms this pre- 
 sumption. The respiratory centre in the medulla oblongata is 
 chiefly controlled in its action by the amount of oxygen which 
 reaches it in the blood. The phenomena of consciousness vanish 
 when the supply of oxygenated blood is cut off from the brain. 
 
 Although we are still in the dark as to the precise significance of 
 the visual purple, the phenomena which the study of it has brought 
 to light are suggestive of unseen chemical processes that are set up 
 in the retina, and so serve as stimulus for the fibrils of the optic 
 nerve. In general we know that certain sensations are dependent 
 upon the chemical constitution and activity of the various end- 
 organs of sense. 
 
 Further researches can scarcely fail to enlarge our knowledge 
 of those facts of relation which exist between the chemical constitu- 
 tion and changes of the nervous mechanism and the phenomena of 
 
126 CHEMISTRY OF THE NERVOUS SYSTEM 
 
 psychical life. Perhaps the more particular statements of fact may 
 ultimately be gathered into those more general statements of fact, 
 more or less verifiable by experiment, which we consider sufficient 
 to constitute scientifically verifiable laws. But why certain chemi- 
 cal constituents, when combined and changed in definite fashion, 
 should be specifically connected with certain conscious experiences, 
 will probably always remain a quite unanswerable inquiry. 
 
CHAPTER VI 
 
 THE NERVES AS CONDUCTORS 
 
 1. In that threefold economy of organs which characterizes the 
 developed nervous mechanism, the office of propagating the neural 
 process between the central organs and the end-organs has been 
 assigned to the nerves. The power to originate this process under 
 the action of external stimuli, although experiment shows that it 
 belongs to the nerves, is not exercised by them while in their nor- 
 mal place within the mechanism. It is the office of the end-organs 
 to transmute the physical molecular processes, which are their stim- 
 uli, into the physiological and neural process, and hand it over, as it 
 were, to these conducting cords. But the office of the nerves as 
 conductors is, of course, not like that of a tube which conducts along 
 its channel some kind of fluid, nor is it like that of the wire or bell- 
 metal which is thrown into vibration throughout. It is a molecular 
 commotion which, when started at any point in the nerves, moves 
 in both directions from point to point along its course. The in- 
 timate connection between the two functions of excitation and con- 
 duction becomes, then, at once apparent. Indeed, excitation may 
 be considered as the setting-up of the process of conduction; con- 
 duction as the uninterrupted continuance, or propagation from 
 point to point, successively, of the process of excitation. Each 
 minute subdivision of the nerve, then, must be regarded as consti- 
 tuting, in some sort, a source or centre of stimulation with respect 
 to its neighboring subdivisions. If the nerve-commotion is to 
 move along the nerve N, between two distant portions of its struct- 
 ure, a and z, then a must act upon its neighbor b as a stimulus, b 
 upon c, and so on successively until y is found stimulating 2, and 
 the process of progressive excitation or conduction is complete. 
 
 2. It follows from what has just been said that, in considering 
 the nerves as conductors, the conditions and laws of the origination 
 of that process of excitation which they conduct must be taken into 
 account. It is neither necessary nor convenient, however, to carry 
 throughout a distinction between the two functions the excitabil- 
 ity and the conductivity of the nerves; it is better to regard them 
 as one process, looked at from somewhat different points of view. 
 
 127 
 
128 THE NERVES AS CONDUCTORS 
 
 Clearly, if the "nerve impulse" is to be propagated, it must first be 
 aroused at some point in the nerve; and under normal conditions, 
 by the action on it of other organs. In the case of sensory nerve- 
 fibres, the stimulus is furnished by the action of modified receptive 
 cells; as, for example, the optic nerve is normally excited by the rods 
 and cones, which themselves have been aroused by the action of 
 light. In the case of motor nerve-fibres, the normal source of their 
 activity lies in the spinal cord or brain-stem; here they are aroused 
 by the action on them of other nerve-fibres such action occurring 
 at the synapse, where the fine terminations of an axon come into 
 relation with the dendrites of another cell and so, indirectly, with 
 the axon of that cell. 
 
 3. In studying the physiology of the nerves, it is often neces- 
 sary to separate them from their normal connections in the body, 
 and arouse them by artificial means. At first sight, this artifici- 
 ality in the experimental procedure would seem to vitiate the results; 
 but control experiments show that the nerve impulse behaves in 
 the same way so far as can be observed whether it is aroused 
 in the natural or in an artificial manner. Apparently a nerve-im- 
 pulse is a nerve impulse, however aroused. The objection that the 
 experiments are made on the nerves of animals, from which an in- 
 ference to human nerves may be doubtful, is also met by the state- 
 ment that a nerve is a nerve, from whatever species of animal. Such 
 observations as have been possible on human nerves show no differ- 
 ence between them and the nerves of other mammals; and there are 
 only minor differences between the latter and the nerves of the frog, 
 which have most often been used in experiments. While this is 
 true of the medullated nerves, unmedullated nerves form, in many 
 respects, a separate class. 
 
 In experimenting with a nerve, some sort of indicator is needed 
 to show when it is active; for an active nerve does not differ visibly 
 from an inactive one. For a motor nerve, a good indicator is af- 
 forded by the attached muscle, which contracts when the nerve 
 impulse reaches it, and so indicates the activity of the nerve. For 
 a sensory nerve a reflex movement or a sensation (in the human 
 subject) may be used as the indicator. Since, however, reflexes and 
 sensations involve the complex and variable activity of nerve cen- 
 tres, the physiologist prefers the motor nerves, and makes a large 
 share of his experiments on what he calls a " nerve-muscle prepara- 
 tion." 
 
 This preparation consists of a muscle freshly taken from the liv- 
 ing animal with its attached nerve dissected out; for example, the 
 gastrocnemius muscle of the frog with the attached sciatic nerve. 
 Such a preparation may be kept alive for some time in a moist 
 
METHODS OF EXPERIMENTATION 129 
 
 chamber. By the simple contrivance of connecting the end of the 
 muscle with a lever, arming the lever with some means of making 
 a mark either pen, or bristle, or needle and bringing its point 
 thus armed to bear on a rapidly travelling surface (plain paper, 
 or smoked paper or glass), the time and amount of the contrac- 
 tions of the muscle may be recorded. The most refined means 
 for noting the exact instant when the stimulus is applied, and also 
 the state of the effects produced at every succeeding instant of their 
 duration, are of first importance. The nerve may be stimulated 
 with different kinds, degrees, and directions of the electrical cur- 
 rent (or with other forms of stimuli) at any points preferred in its 
 stretch, and under a great variety of conditions with respect to tem- 
 perature, moisture, mechanical pressure or stricture, integrity and 
 vitality of its structure, etc.; and the effects of such stimulations 
 upon the contractions of the muscle may be noted and compared 
 as they have been recorded. Means for testing the most delicate 
 and rapid changes in the electrical or thermometric conditions of 
 the nerve may be applied to it at any point of its stretch. Varia- 
 tions and refinements of experiments essentially the same may be 
 almost indefinitely multiplied; the experiments may be repeated, 
 and verified or corrected, by the same observer or by others. In- 
 asmuch as the preparation is both muscle and nerve, an acquaint- 
 ance with the behavior of the muscle, and with the laws of its con- 
 traction, is necessary in order that it may be known how much of 
 the complex phenomena is to be ascribed to the functional activity 
 of muscle, how much to that of nerve. But into a statement of the 
 general laws of contractile tissues, and of the nature and explana- 
 tion of the behavior of muscle when irritated, we cannot enter in 
 great detail. 1 
 
 Being thus provided with a fresh nerve and a muscle to serve as 
 indicator of the nerve's activity, the physiologist is able to observe 
 and draw inferences with regard to the excitability and conduc- 
 tivity of the nerve. 
 
 4. As respects its excitability, the nerve is found to be easily 
 aroused by stimuli applied directly to it at any point. Pinch the 
 nerve, and the muscle instantly contracts; drop a light weight on 
 the nerve, and again the muscle contracts. These are examples 
 of "mechanical stimuli," which are thus proved to be capable of 
 arousing the nerve to activity. A variety of chemical substances 
 also excite the nerve; acids are especially effective, bases less so; 
 
 1 For a description of the method and results of experimenting with the nerve- 
 muscle preparation, more accessible to the general reader than the books to which 
 reference will chiefly be made, see Howell's Text-book of Physiology, chap. I, 
 p. 5. 
 
130 THE NERVES AS CONDUCTORS 
 
 of salts, some, like sodium chloride, act as stimuli, while others, 
 such as most of the salts of the heavy metals, kill the nerve with- 
 out exciting it. The same effect follows use of too strong a solu- 
 tion of any of these chemical agents. Sudden heating of the nerve 
 acts as a stimulus. Electric shocks, especially in the form of in- 
 duced currents, are highly effective. These four classes of agents 
 mechanical, chemical, thermal, and electrical are called "gen- 
 eral stimuli," because they are capable of arousing, not only nerves, 
 but a great variety of forms of living matter, including the muscles 
 and glands of the higher animals and the less specialized protoplasm 
 of protozoa. 
 
 5. The effectiveness of any of these stimuli depends in part 
 on its suddenness. Slow changes of temperature, for example, do 
 not excite the nerve; and very gradually increasing pressure may 
 crush the nerve without exciting it. The electric current, too, 
 must have a degree of suddenness in its changes, if it is to act as a 
 stimulus to nerve. 
 
 In regard to this last point, Du Bois-Reymond, one of the pio- 
 neers in " electrophysiology," announced in 1845 l the discovery of 
 two closely related laws of the electrical excitation of nerve. The 
 first law is that the mere passage of a current through the nerve 
 does not arouse it; only changes in the electric current act as stimuli; 
 but these changes may consist either in increasing the current or in 
 decreasing it. This law is the expression of a fact which is easily 
 verified on the nerve-muscle preparation. When the current from 
 a battery is passed through a nerve, the muscle gives a twitch at 
 the starting of the current, and then, usually, remains at rest dur- 
 ing the passage of the current, only to twitch again at the stopping 
 of the current. Or, if the current is not entirely stopped, but is 
 suddenly cut down in strength, still the muscle twitches, as it does 
 also if the current is suddenly increased. The second law of Du 
 Bois-Reymond is that the exciting effect of any change of current 
 is greater, the more sudden the change. If the current is gradually 
 increased by a certain amount, the muscle does not twitch, whereas 
 if the same amount of increase is effected quickly, the muscle 
 twitches. 
 
 The validity of the first of these laws has not been much affected 
 by later investigations, at least so far as concerns the nerves. Cer- 
 tain other tissues are excited by the passage of a steady current, but 
 the nerve is usually not excited in such a way as to arouse the muscle 
 which is attached to it. The nerve is not, however, by any means 
 in its normal quiescent state during the passage of a current, but is 
 
 1 In a paper communicated to the Physiological Society in Berlin; see also 
 his Untersuchungen uber thierische Electricitat, I, 258. 
 
THE LAWS OF DU BOIS-REYMOND 131 
 
 in quite an unusual condition as concerns its excitability and con- 
 ductivity. This fact will be brought out later in the chapter. 
 
 The second law of Du Bois-Reymond needs qualification in the 
 light of further experience. With the introduction of very rapidly 
 oscillating currents ("Tesla" or "d'Arsonval currents"), it has 
 been found that this effectiveness as stimuli by no means increases 
 indefinitely with the rapidity of the oscillations. Oscillations up 
 to 1,000,000 per second will indeed excite the nerve, but only if the 
 current is very intense. 1 Beyond a certain point, the effectiveness 
 of the oscillations decreases as their rate increases. It seems prob- 
 able, therefore, that in order to have the greatest exciting effect on 
 the nerve, the change of current must have a certain high speed, 
 on either side of which its effectiveness decreases. Nerves would 
 be in this respect like other irritable tissues, some of which are best 
 stimulated by rather slow changes in the current, while others are 
 best excited by more rapid changes. Each kind of tissue is best 
 excited by a certain speed of change of the current; but the nerve 
 has the distinction of being that tissue which is adapted to the 
 most rapid change. 
 
 6. Another similar distinction of the nerve is its very short 
 "refractory period." An organ which is excited by a stimulus is 
 apt to be inexcitable by another stimulus which follows immedi- 
 ately; it is said to be refractory during a certain period after commenc- 
 ing its response to the first stimulus. The length of this refractory 
 period differs in different organs. In the heart, in which the phe- 
 nomenon is most striking, the refractoriness lasts undiminished 
 until the full strength of the muscular contraction is reached (or 
 about 0.4 second), after which it gradually passes away. In the 
 spinal cord and brain, the refractory period of some groups of nerve- 
 cells may be as long, or even longer, than that of the heart. On the 
 contrary, skeletal muscle has a much shorter period, not exceeding 
 .01 second; while nerve-fibres have a refractory period of not over 
 .002 second. 2 The nerve is, then, in all respects a quick-acting 
 organ : it is best excited by a sudden stimulus ; its response is prompt 
 and brief, and it is very soon ready for a fresh stimulus. 
 
 7. In still another and very important respect the nerve is dis- 
 tinguished for its quick action; and that is the speed of its con- 
 duction. In measuring the speed of conduction in a motor nerve, 
 the nerve-muscle preparation is again employed. The determina- 
 tion of the rate of transmission in nerves was an achievement of 
 
 1 Einthoven. Cited by Biedermann, Ergebnisse der Physiologie, 1903, II, 
 part 2, 114. 
 
 a See Gotch and Burch, Journal of Physiol., 1899, XXIII, p. xxii. 
 
132 THE NERVES AS CONDUCTORS 
 
 Helmholtz, 1 and it was the more noteworthy because it followed 
 close on the prediction by Johannes Mu'ller, 2 another eminent 
 physiologist, that the speed of nerve-conduction would never be de- 
 termined, because, as he was led to suppose, its speed was compar- 
 able with the speed of light. The method of Helmholtz was, how- 
 ever, simple enough. He measured the time elapsing between the 
 moment of stimulation and the beginning of the muscular contrac- 
 tion, when the stimulus was applied as near the muscle as possible ; 
 and he then compared this with the corresponding time, when the 
 stimulus was applied as far from the muscle as possible. The 
 difference in time and the length of nerve between the two points 
 of stimulation enabled him to calculate the speed of transmission 
 between the same two points. Helmholtz's work has often been 
 repeated and confirmed; and in this way, the rate of transmission 
 in frog's nerve is known to be approximately 28 metres per second. 
 In the human subject, similar experiments, in which a motor nerve 
 is stimulated through the skin, at two distant points, have led to 
 slightly higher values, or about 33 metres per second. For a con- 
 venient approximation, in English measures, we may take 100 feet 
 per second as the rate in the motor nerve. In sensory nerves, the 
 rate is much harder to measure, because the nerve leads into the 
 centre, and the activity of the centre, which consumes a considerable 
 and variable time, intervenes before any movement, reflex or vol- 
 untary, can be made to indicate that the nerve-impulse has arrived. 3 
 When stimulation is confined to single "touch spots," and spots of 
 equal sensitivity are chosen, fairly precise results have been ob- 
 tained, 4 indicating a speed of about 30-33 metres per second in the 
 sensory nerves of the arms and legs. 
 
 This speed of about 100 feet per second is certainly modest in 
 comparison with the speed of light. Yet, in this respect, as well 
 as in the others mentioned in preceding paragraphs, nerves occupy 
 a place of distinction among living tissues. This is specially true 
 of medullated nerves. The non-medullated nerves have a much 
 slower rate of transmission, which varies greatly in different ani- 
 mals. Mammalian unmedullated nerve conducts at about the speed 
 of 8 metres per second; in the lobster, figures have been obtained 
 ranging from 8 to 12 metres; but in some other invertebrates, 
 speeds as low as a centimetre or even a millimetre per second have 
 been observed. In human "striped " muscle, the rate of trans- 
 mission of activity is 10-13 metres per second; in the cold-blooded 
 
 1 Archiv f. Anat. u. PhysioL, 1850, pp. 276-364. 
 8 See his Handbuch der Physiologic, I, pp. 581 f. (Coblenz, 1844). 
 3 Cf. Dolley and Cattell, Psychological Review, 1894, I, 159. 
 *Kiesow, Zeitschr. /. Psychol, 1903, XXXIII, 444. 
 
ALTERATIONS IN CONDUCTIVITY 133 
 
 frog, the speed is less than one half of this; and in the "unstriped" 
 muscles of the stomach and other internal organs it is much slower 
 still. 1 
 
 8. The conductivity of a nerve is altered by the physical and 
 chemical agents applied to it. For example, cooling a nerve slows 
 its rate of conduction, and warming it, within narrower limits, 
 hastens the conduction. The anaesthetics, alcohol, ether, and 
 chloroform, when applied in the form of vapor to a length of nerve 
 enclosed in a small gas-tight chamber, lower the conductivity, and 
 may entirely abolish it. The passage of an electric current through 
 a length of nerve also interferes with its conductivity, as will be more 
 fully explained later. 
 
 It is a curious fact that some of these agents act differently on 
 the conductivity and on the excitability of the nerve. Thus, al- 
 cohol vapor, though lowering the conductivity, may even, at a cer- 
 tain stage in its action, raise the excitability of the portion of nerve 
 to which it is applied, so that weaker stimuli will take effect there 
 than in the portions under normal conditions. Carbon dioxide 
 gas has the opposite effect: if applied to a short length of nerve, 
 it lowers or abolishes the excitability of that portion without, for 
 a time, interfering with the conduction through it of nerve impulses 
 generated elsewhere. Cold, also, though lowering the speed and ef- 
 fectiveness of conduction, may even raise the excitability to certain 
 classes of stimuli; especially, to those which consist of relatively 
 slow changes. Cold, that is to say, makes the nerve a slower-act- 
 ing mechanism, and so adapts it to take up slower changes in the 
 agents which are applied to it. 
 
 It appears from these results that the excitability of a nerve is to 
 some degree independent of its conductivity, and a separation of 
 the two functions is indicated. Perhaps a better generalization 
 of the results would be to say that the taking up of stimuli arti- 
 ficially applied to the surface of the nerve is subject to different 
 conditions from those which determine its normal activity in taking 
 up stimuli from adjoining parts of itself. 2 
 
 9. Another important fact regarding the conductivity of nerve- 
 fibres is that they conduct in both directions. Under the normal 
 conditions of stimulation a nerve-fibre has occasion to conduct only 
 in one direction. Sensory fibres receive stimulation only from the 
 receptive cells with which their peripheral ends are in connection; 
 and they conduct only toward the nervous centres. Motor nerves, 
 
 1 The figures are from several authorities; some of them have been brought 
 together by Biedermann, Ergebnisse der Physiologie, 1903, II, part 2, 147, and 
 by Gotch in Schdfer's Textbook of Physiology, 1900, II, p. 482. 
 
 2 For these results, and a discussion of them, see Gotch, op. cit., pp. 484 ff. 
 
134 THE NERVES AS CONDUCTORS 
 
 in normal conditions, receive stimuli only at their central ends in 
 the cord or brain stem; and they conduct only outward to the 
 muscles. But when, in experimental work, a nerve-fibre is excited 
 somewhere along its course, and not at its natural end, it is found 
 to conduct in both directions; this it does, apparently, as perfectly 
 in one direction as in the other. The evidence for these statements 
 is not quite so easy to obtain as would at first appear, because most 
 nerves are composed partly of sensory and partly of motor fibres, 
 which, normally, conduct in opposite directions. The dorsal roots 
 of the spinal nerves are, indeed, composed almost or quite entirely 
 of sensory fibres, and the ventral roots of motor fibres. But in at- 
 tempting to use these roots for experimental purposes, the difficulty 
 arises of finding some sure indicator. If the fibres in the sensory 
 root did conduct outward to the sense-organ, they could produce 
 no movement or other change in that organ to show their outward 
 conduction. And if the motor roots did, when excited, conduct in- 
 ward to the cord, there is nothing to show this; because these fibres 
 end in synapses in the gray matter of the cord, and no conduction 
 occurs across these synapses, except in the outward direction. 1 
 
 There is, however, an indicator in the wave of electrical change, 
 which attends the impulse in its passage along a nerve, and which 
 can be detected by a galvanometer. When this electrical " current 
 of action" is used as an indicator, excitation of a sensory root is 
 found to cause an outward-travelling impulse, and excitation of a 
 motor nerve is found to cause an inward-travelling impulse which 
 betrays itself by an electrical change in the motor roots. 
 
 Additional evidence of double conduction in nerves is afforded 
 by experiments in which the muscle is utilized as the indicator. 
 When, as happens in certain muscles, the motor axons split on ap- 
 proaching the muscle, one branch of each axon going to one part 
 of the muscle, and the other branch to another part, it may be possi- 
 ble to cut the muscle in two without injuring the nerve, and then 
 to excite one branch of the nerve and obtain contraction of both 
 parts of the muscle. The excitation has been conveyed by the 
 branched axons, which have conducted up along one branch to 
 the point of bifurcation, and then down along the other branch. 
 The motor axons have, therefore, conducted in both directions. 
 To make this result convincing, the nerve must be severed from the 
 spinal cord; otherwise the farther part of the muscle might be ex- 
 cited by reflex action the upward conduction being afforded by 
 sensory axons, which are sure to be interspersed with the motor 
 fibres in the nerve. But sensory axons can transmit their excita- 
 
 1 In regard to the irreversibility of conduction at the synapse, we have 
 spoken in a preceding chapter. 
 
NATURE OF PROCESS CONDUCTED 135 
 
 tion to motor axons only through the nerve-centres, and severing 
 the nerve from the cord destroys this possibility. When these con- 
 ditions are complied with, the positive result of the experiment, 
 combined with the positive result obtained on using the current of 
 action as an indicator, leaves no room for doubting the double con- 
 ductivity of nerve-fibres. 
 
 10. What is it that is conducted along the nerve with the speed 
 of 100 feet per second, but which is blocked by a cold stretch in 
 the nerve, and by ether? Any satisfactory answer to this question 
 is extraordinarily difficult. There is no visible motion of the nerve ; 
 no pull or push is exerted through it; no "animal spirits" circulate 
 rapidly through it, to constitute the nerve-impulse. What is con- 
 ducted must, therefore, be of a much more subtle nature. 
 
 By analogy with other living tissues, the activity of a nerve is 
 readily thought of as a chemical process. It might be likened, 
 in a measure, to the setting-off of a train of gunpowder or of a fuse, 
 in which the oxidation of one part acts to ignite the next. The simile 
 would halt, since the fuse is burnt out in its activity, while the nerve 
 remains in good condition and ready for further activity. A com- 
 parison with other living tissues, such as the muscle, is more appro- 
 priate. The muscle, after one contraction, remains in good con- 
 dition, and yet oxidation has gone on in the muscle; explosive 
 material has been consumed; in a word, catabolism has occurred in 
 the active tissue. Since living cells are subject to the law of the con- 
 servation of energy, and cannot perform work without the utiliza- 
 tion of stored energy, the conclusion seems unavoidable that there 
 must be catabolism also in an active nerve. 
 
 If the activity of the nerve is like other chemical and catabolic 
 processes, it should give rise to the waste products of catabolism, 
 even as the active muscle gives rise to carbon dioxide, lactic acid, 
 etc. With this in view physiologists have looked for the carbon 
 dioxide produced by nerve action, but the most delicate tests have 
 been unable to detect any. They have looked for the acid reaction 
 which should betray catabolism, but have got no clear evidence of 
 the production of acid products. They have looked for the heat 
 which is evolved in all known instances of oxidation, and have ap- 
 plied instruments capable of detecting a rise in temperature of 
 x-fos of a degree centigrade, but have not been able to detect the 
 slightest production of heat. These negative results impress differ- 
 ent physiologists differently. Some conclude that the activity of the 
 nerve is, after all, not a chemical process, or at least not a process 
 involving catabolism. Others are still confident that there is catabol- 
 ism there, only that it is too slight for our present means of de- 
 tection. 
 
136 THE NERVES AS CONDUCTORS 
 
 11. There is another line of evidence which is of special in- 
 terest. A universal result of catabolic activity in any organ is, ap- 
 parently, fatigue. The using-up of the store of available energy 
 weakens a muscle or gland; and the accumulation of the waste 
 products of activity also depresses the activity of the organ. 1 If, 
 therefore, nerve action is catabolic, the nerve should be fatigued 
 by any long-continued activity. 
 
 In examining the question of fatigue in nerves, great pains must 
 be taken to find a suitable indicator. We cannot simply take the 
 nerve-muscle preparation, and, after repeatedly exciting the nerve, 
 look for evidence of its fatigue in a weakening of the muscular con- 
 tractions; for the muscle is itself subject to fatigue, and its fatigue 
 is quite sufficient to mask the fatigue of the nerve. To use the mus- 
 cle as an indicator, it must itself be protected from excitation, while 
 the nerve is continually excited. This result has been reached in 
 several different ways. Bowditch 2 used the drug curare, which 
 blocks the passage of excitation from the motor nerve to its muscle, 
 without paralyzing either the nerve or the muscle. He kept the 
 animal alive by artificial respiration for four or five hours, exciting 
 the nerve many times a second during all this time, at the end of 
 which, as the effect of the drug began to pass off, the muscle began 
 to contract. Thus the nerve had not been exhausted by several 
 hours of continued activity many times as much as would have 
 reduced the muscle to an extreme condition of exhaustion. The 
 same experiment has recently been repeated with an improvement, 
 which consists in giving an antidote to the curare, which acts with 
 great promptness. In this way the block between the active nerve 
 and the protected muscle can be instantly removed, and it is then 
 found that four to ten hours of continued activity have left the nerve 
 still with unimpaired powers of functioning. 3 The non-medullated 
 nerves of mammals seem to have the same absence of fatigue: 4 
 it has been claimed, however, that the olfactory nerve of the pike, 
 a non-medullated nerve, gives signs of fatigue as tested by the " cur- 
 rent of action." 5 The action current has also been used in medul- 
 lated nerves as an indicator, and has been found not to weaken in 
 the course of long-continued excitation of the nerve at the rate of 
 many shocks per second. 8 
 
 1 A fuller discussion of the phenomena of fatigue will be given later on. 
 
 8 Journal of Physiology, 1885, VI, 133. 
 
 8 This form of the classic experiment is due to Durig, and the antidote is 
 the salicylate of physostigmin. See Zentralblatt f. Physiol, 1902, XV, 751. 
 
 4 Brodie and Halliburton, Journal of Physiology, 1902, XXVIII, 181. 
 
 6 Sowton, Proceedings of the Royal Society of London, 1900, LXVI, 379; 
 Garten, Pfliiger's Archiv f. d. gesammte Physiol., 1899, LXXVII. 
 
 6 Edes, Journal of Physiology, 1892, XIII, 431. 
 
PHENOMENA OF FATIGUE 137 
 
 The evidence, therefore, all goes to demonstrate the practical in- 
 defatigability of nerve-fibres. It may be, of course, that this re- 
 markable trait is after all only relative ; that, in truth, the catabolism 
 of the nerve is excessively slight, and it therefore recovers with ex- 
 traordinary promptness from the effects of each phase of activity, 
 just as the heart beats at a moderate rate for many hours without 
 fatigue, recovering from the effects of each beat in the pause that 
 intervenes before the next. And this is the view of Biedermann, 1 
 who also brings forward some evidence, not wholly convincing, 
 that a perfectly continuous electric stimulation, as distinguished 
 from a rapid series of brief shocks, produces fatigue of nerve. 
 
 There are one or two facts of a different order which bear on the 
 same problem. The blood supply of the nerves is very slight; and 
 their need of oxygen is also very slight. It has been shown, 2 in- 
 deed, that several hours' confinement in an atmosphere perfectly 
 free from oxygen destroys the excitability of a nerve, and that there 
 is prompt recovery as soon as oxygen is admitted; and this is taken 
 to mean that the nerve consumes oxygen and therefore undergoes 
 catabolism; but some doubt still remains as to whether it is the 
 activity of the nerve which demands oxygen, since even an inactive 
 nerve gets out of condition in the absence of oxygen. 
 
 Another line of evidence in favor of assuming catabolism in the 
 active nerve is adduced by Waller. 3 The action current of a nerve 
 is increased by activity of the nerve, as induced by exciting it with 
 a rapid series of induction shocks. If, then, the character of the 
 activity of the nerve is changed by its own action, this is prima 
 facie evidence that the nerve has itself changed; and the change is 
 probably chemical. Now the same increase in the action current 
 is caused by exposing the nerve to a very slight dose of carbon 
 dioxide; and Waller is therefore inclined to infer that the increased 
 action current after activity is due to the production within the nerve 
 of very small amounts of this gas. The evidence, however, is at 
 best indirect. 
 
 12. In the absence of mechanical, thermal, or chemical mani- 
 festations of the nerve-impulse, we should be wholly at a loss re- 
 garding its physical nature, were it not for the fact that there is 
 an electrical manifestation, in the "current of action," to which 
 reference has already several times been made. If a nerve is ex- 
 cited by any means at one point, and if the poles of a galvanometer or 
 capillary electrometer are laid on a distant part of the nerve, every 
 excitation of the nerve is followed by a swing of the galvanometer 
 
 1 In Ergebnisse der Physiologie, 1903, II, part 2, 131 f. 
 
 2 A. v. Bayer, Zeitschrift /. allgem. Physiologie, 1902, II, 169. 
 
 * Philosophical Transactions of the Royal Society of London, 1896. 
 
138 THE NERVES AS CONDUCTORS 
 
 needle, or by a movement of the mercury in the electrometer 
 thus indicating the existence of an electric current in the nerve 
 itself. The rate at which this electrical disturbance travels along 
 the nerve is the same as the speed of the nerve-impulse. Such a 
 current of action is, indeed, not peculiar to the nerve; it occurs in 
 muscles, glands, the retina, and apparently in all active living tis- 
 sues. In most of these other cases, there are other manifestations 
 of activity, and the current of action is usually regarded as a mere 
 by-product. But since the nerve shows no other manifestation, 
 the suggestion has been made by several physiologists that there is 
 no other process; but that the nerve-impulse is essentially an elec- 
 trical disturbance moving along the nerve. 
 
 Without committing ourselves to this view of the nerve-impulse 
 further than to recognize it as a respectable hypothesis with a con- 
 siderable number of facts in its favor, we may employ it to give 
 point to our further consideration of the electrical phenomena of 
 the nerve in action. The study of these phenomena has engaged 
 the attention of many able investigators and has proved a fruitful 
 field. To present the subject fully, we should need to enter much 
 more deeply into the science of electricity and the closely related 
 science of physical chemistry than can be expected within the lim- 
 its of a brief discussion. A rough sketch is all that can be here 
 attempted. 
 
 13. The facts bearing on this subject may be conveniently 
 grouped under two heads: (1) the effects of the electrical current 
 on the nerve; and (2) the electrical phenomena developed by the 
 nerve itself. To the latter class belongs the current of action. 
 
 Let us suppose that we have at our disposal, for examining its 
 effects on the nerve, a current from a zinc-copper battery, and let 
 us follow the common usage, and speak of the copper pole as posi- 
 tive and the zinc as negative, and of the current as "flowing," out- 
 side of the battery, along any conductor which is provided, in the 
 direction from copper to zinc. For applying the current to the nerve, 
 we shall need to employ " non-polarizable electrodes." On con- 
 necting the positive or copper pole with one point of the nerve, and 
 the negative pole with another point, the current may be conceived 
 of as entering the nerve at the positive pole, or anode, and leaving 
 at the negative pole or cathode. A key is provided in the circuit, 
 which enables us to start and stop the passage of the current, at 
 will. As has already been stated, the nerve is excited by the start- 
 ing or stopping of the current, but not, ordinarily, by its steady flow. 
 The excitation is found to occur at the cathode when the current 
 is started, and at the anode when the current is stopped. In other 
 words, the nerve-impulse starts at the cathode when the circuit is 
 
ELECTROTONIC CHANGES IN NERVES 139 
 
 closed and at the anode when the circuit is broken. Further, it 
 was shown by Pfliiger 1 that the regions of the anode and of the 
 cathode are each in a condition of altered excitability during the 
 passage of the current. There is an increase of excitability at and 
 near the cathode, and a decrease at and near the anode. On the 
 stopping of the current, a back swing occurs, making the anode 
 for a time over-excitable and the cathode under-excitable. The 
 changes in excitability are greatest at the poles and decrease gradu- 
 ally as we move into the extrapolar regions and into the region be- 
 tween the poles; in the latter region there is an indifference point 
 where the cathodic effect merges into the opposite anodic. On 
 increasing the strength of the current, the extent of the nerve af- 
 fected increases, and, between the poles, the anodic depression en- 
 croaches more and more on the cathodic exaltation. This depress- 
 ing effect also occurs when the current is prolonged; the depression 
 then finally invades even the region of the cathode. The lowering 
 of excitability at the anode during the passage of the current is 
 attended by a lowering of the conductivity of the nerve for nervous 
 impulses; the conductivity is so far lost at the anode during the pass- 
 age of strong currents as to form a complete block to the passage 
 of the impulses generated elsewhere. These changes in the nerve 
 during the passage of an electric current are grouped under the 
 name " electrotonus " ; and the anodic depression is sometimes 
 known as anelectrotonus, while the cathodic exaltation is called 
 catelectrotonus. 
 
 14. Electrotonus may then be defined as an altered excita- 
 bility and conductivity of the nerve produced by the passage of a 
 current through some part of its extent. But there are also certain 
 purely electrical results from the passage of the current, in the form 
 of additional currents which appear in the nerve, or on its surface; 
 and these are called the electrotonic currents. Such currents appear 
 in the extrapolar region, and their direction is the same as that of the 
 current that is led into the nerve. They cannot, therefore, be mere 
 leakages of the exciting current; they are new currents generated in 
 the nerve by the action of this external current. 
 
 15. These electrotonic currents have formed the starting-point 
 for an interesting line of experiments, the object of which is to de- 
 termine whether the phenomena of nerve action can be imitated in 
 a physical model. Since the model may be regarded as an arti- 
 ficial nerve, just as an arrangement of tubes and syringes gives a 
 working model of the circulatory system, it is hoped in this way to 
 throw some light on the physical structure of the nerve. Such an 
 instrument of investigation is called a "core model" (see Fig. 60), 
 1 Untersuchungen iiber Electrotonus, Berlin, 1859. 
 
140 THE NERVES AS CONDUCTORS 
 
 or a "core conductor." It consists of a central core of good elec- 
 trical conductivity, surrounded by a sheath of lower conductivity. 
 It may be constructed of various materials; that of Hermann 1 con- 
 sisted of a platinum wire (the core) stretched the length of a glass 
 tube filled with a solution of some salt. The glass tube was em- 
 ployed simply for the purpose of holding the "sheath" of solution in 
 place, and was provided with small side tubes through which the 
 exciting current could be led in, and the electrotonic currents (if any) 
 led out to a galvanometer. Hermann found, in fact, that currents 
 
 B 
 
 FIG. 60. Hermann's Core Model. AB, glass tube; ab, platinum wire; 
 c, d, e, f, g, h, side tubes. 
 
 equivalent to those of electrotonus were produced in the extrapolar 
 regions of the core model as in the nerve; he, accordingly, con- 
 cluded that the physical structure of a nerve resembles that of the 
 core conductor, and that the electrotonic currents were purely 
 physical phenomena. 
 
 The same line of investigation has been carried further by Bo- 
 ruttau, 2 who has found that, not only the electrotonic currents, but 
 all the electrical phenomena occurring in the nerve from the action 
 of a current on it can be imitated in the core model. In this way, 
 even the current of action can be imitated; and as this is closely 
 associated, in a genuine nerve, with the transmission of the nerve 
 impulse, Boruttau advances the view that the core conductor, like 
 the nerve, can really be "excited"; and that the activity of a nerve 
 is imitated, in all essential respects, by the action of the core con- 
 ductor. According to this view, the nerve owes its functional power 
 to its physical structure, which makes of it an electrical conductor 
 of a certain type namely, a so-called "core conductor." Nerve 
 action would, then, not be a catabolic affair, but a physical rather 
 than a chemical action, consisting in the propagation of a certain 
 electrical state. The electrical state may be generated in the nerve 
 
 1 Handbuch der Physiologic, vol. II, i, 174. 
 
 2 In several articles in Pfliiger's Archiv fur die gesammte Physiologie, from 
 1894 on. 
 
CURRENT OF REST OR INJURY 141 
 
 by various sorts of stimuli, as was mentioned in a preceding page. 
 In the core model also, the electric wave can be initiated by a me- 
 chanical "stimulus" as well as by an electrical; and this fact rather 
 strengthens the analogy between the nerve and the core conductor. 
 The rate of propagation of the electric wave, in a core conductor, 
 varies according to the materials of which it is constructed; but it 
 may be as low as 100 metres per second, and thus it is not incom- 
 parable with the speed of transmission in nerve. The fact that 
 electricity travels along a wire between the poles of a battery or 
 dynamo at an infinitely faster rate is therefore no disproof, as was 
 formerly held, of the view that the nerve impulse is essentially elec- 
 trical. 
 
 The actual microscopic structure of a nerve-fibre affords some 
 reason for believing that it may act as a core conductor. There is 
 the axon, containing its fibrils; and, if the fibrils are better electrical 
 conductors than the protoplasm which surrounds them, the axon 
 corresponds in structure to that of the core model. Or, again, 
 and less hypothetically, if the axon as a whole is a better conductor 
 than the myelin sheath, then the axon represents the core, and the 
 myelin sheath the less conductive envelope. 
 
 16. A further set of facts bearing on the theory of the nerve 
 has not yet been touched upon. When a nerve is cut, and the cut 
 end connected with one pole of a galvanometer, while the other pole 
 is connected with the uninjured convex surface of the nerve, a cur- 
 rent flows through the galvanometer from the convex surface to 
 the cut end. The cut end is negative with respect to the convex 
 surface. This current was discovered by Du Bois-Reymond, who 
 named it the " current of rest," thus distinguishing it from the " cur- 
 rent of action," and indicating also his view that the current so 
 demonstrated existed in the resting and uninjured nerve. Her- 
 mann showed good reason for doubting this interpretation, and for 
 believing that the negative potential of the cut end was a conse- 
 quence of the injury done it in cutting. A controversy waged for 
 years regarding this matter, one of the most bitter controversies 
 in the history of science; but the evidence adduced by Hermann 
 gradually won the assent of physiologists, and the name "current 
 of injury" was substituted for Du Bois-Reymond's "current of 
 rest." Hermann's conception was that chemical changes occurred 
 in the injured and therefore dying part of the nerve, and that these 
 changes were the cause of its negative potential. The phenomenon 
 is not peculiar to the nerve, but appears in all dying tissues. Her- 
 mann brought this negativity of dying tissues into relation with the 
 negativity displayed by active tissues toward tissues when resting. 
 The negative potential of active tissue is revealed by the current of 
 
142 THE NERVES AS CONDUCTORS 
 
 action; for the active portion of a nerve becomes negative to the 
 resting portion. Hermann's views, which may be said to be the 
 views now generally accepted, are summed up in the statements 
 that dying tissue is electro-negative with respect to living; and act- 
 ing tissue is also electro-negative with respect to resting tissue. 
 
 17. In spite of the wide-spread acceptance of Hermann's views, 
 very strong evidence has recently been presented in favor of a radi- 
 cally different conception of the current of rest, a conception which 
 has something in common with that of Du Bois-Reymond, but which 
 is based on discoveries in physical chemistry which have been made 
 since his time. 
 
 The considerations on which this new conception is based are the 
 following: 1 We have in the nerve-fibre a core containing a solu- 
 tion of salts (the axon), surrounded by the myelin and primitive 
 sheaths, which are known to be electrically poor conductors; while 
 outside of these sheaths is also a solution of salts, the lymph which 
 bathes the nerve as it bathes the outside of all the cells of the body. 
 The sheaths, therefore, like the bounding membrane of all living 
 cells, interpose resistance to the free diffusion of salts between the 
 solutions that are within and those without. If, now, the internal 
 and external solutions were different in strength or in concentra- 
 tion, we should have here the making of a "concentration cell"; 
 and, further, if the separating membrane were ruptured at any 
 point, so that diffusion occurred between the external and internal 
 solutions, the diffusion would be attended by electric currents. 
 It is then only necessary to suppose that the internal solution is 
 more concentrated than the external, and the former would be 
 negative with respect to the latter: thus we should have the exact 
 conditions necessary to give rise to the current of rest or of injury, 
 as this current is actually observed. 
 
 Macdonald 2 has given definite evidence in favor of this theory 
 by showing that the current due to injury of a nerve is increased by 
 weakening the external solution, and diminished or even reversed by 
 making the external solution sufficiently concentrated. The re- 
 sults come out as they would on the supposition that the nerve, in 
 case of the current of injury, acts as a concentration cell, with the 
 more concentrated solution normally inside the sheaths. Such a 
 current can be imitated by a core model constructed of two solu- 
 tions of different concentration separated by a membrane (Borut- 
 tau). The evidence of these experiments is therefore favorable 
 
 1 See J. S. Macdonald, "The Injury Current of Nerve," in reports of the 
 Thompson Yates Laboratories, Liverpool, 1902, III, 213-347; Proceedings of 
 the Royal Society of London, 1900, LXVII, 310. 
 
 3 Op. cit., pp. 273, 288. 
 
THEORIES OF NERVOUS FUNCTION 143 
 
 to the conception of the nerve as a core conductor of the construc- 
 tion already described. The non-conducting character of the 
 myelin sheath is indicated by direct observation. 1 
 
 18. According to this view, which is thought to explain the 
 electrical phenomena as well as the absence of signs of catabolism 
 in the active nerve, the nerve-impulse is a special sort of electrical 
 wave propagated from point to point along the nerve-fibre, and capa- 
 ble of exciting a muscle-fibre where the terminations of the axon 
 come into close contact with the muscular substance. The ob- 
 jection to this theory which is derived from the fact that a dead 
 nerve will not conduct, may be met by maintaining that a physical 
 structure which is capable of transmitting the electrical wave de- 
 pends upon the life of the fibre. 
 
 While, then, certain competent authorities still oppose this theory 
 and it would be out of place in a book like this to take sides in a 
 controversial matter, it must be admitted that it accounts for more 
 of the very puzzling phenomena than do any other of the present 
 theories of conduction in the nerves. It may, then, properly serve as 
 a hypothesis about which to gather the principal facts of the physi- 
 ology of nerves. At any rate, the only rival view, at the moment, 
 seems to be a chemical theory, which must either controvert the 
 doctrine of the conservation of energy, or else admit such an amaz- 
 ing ability for promptness and completeness of metabolism as it 
 is difficult to imagine. To suppose the nerve-fibre capable of in- 
 stantaneously recombining, without any detectable loss of energy, 
 the elements which have been separated by the work done through 
 hours of continuous functioning, is to convert it into a wonderful 
 kind of laboratory. But it can scarcely be denied that nature may 
 impart such a capacity to such a kind of living tissue. In a word, 
 then, the physico-chemical theory of the process in which consists 
 the impulse passing along an excited nerve, is still in need of pro- 
 longed and careful investigation. 2 But whatever, more precisely, 
 
 1 Gothlin, Upsala Lakareforenings Forhandlingar, 1902, VIII, 156 ff. 
 
 3 It seems to us that there is no incompatibility between the two theories 
 which have been presented in this chapter and which are customarily looked 
 upon as rival ways of explaining the phenomena. On the contrary, both the 
 classes of phenomena to be explained, and also the theories that are set forth 
 in their explanation, are supplementary and necessary to be combined in 
 order to account for all the facts. It will have been noticed that in the model 
 core conductor, as in the nerve itself, one of the materials used in its con- 
 struction is a saline solution. If this were not so, the phenomena could not 
 be obtained. But saline solutions are electrolytic conductors; and electrol- 
 ysis is the chemical change which is analogous to catabolism in living tissues. 
 It is perfectly possible, however, to construct an electrolytic conductor which, 
 on passing a weak current through it, and yet a current far within the limits 
 of detection by a galvanometer, will give no detectable evidence of any chemi- 
 
144 THE NERVES AS CONDUCTORS 
 
 this impulse is found to be, there is no doubt that conduction is 
 the fundamental function of nervous tissue; and all its other functions 
 are secondary and derived. 
 
 cal or thermal change along its course except at the poles. Even here, the 
 chemical changes may be so small that it would take thousands of years for it 
 to disengage from one pole, and transfer to the other, a single cubic centimetre 
 of copper; but all the while chemical changes, too small to be detected, would 
 be taking place along the entire stretch of the conducting medium. 
 
 Still further, it is not proper to speak of what does take place in an electro- 
 lytic conductor as the passage of a mere wave of electricity. It is doubtful 
 whether there can be any such "mere wave" in any material structure. 
 
 Now, we know that catabolism does take place in the muscle-fibre; and 
 there is sufficient evidence to show that it also takes place in the nerve-cells 
 of the central organs. These are the terminals, between which the nerve- 
 fibres are conductors. The analogy, we repeat, seems to suggest a combination of 
 both theories. 
 
CHAPTER VII 
 REFLEX FUNCTIONS OF THE NERVOUS SYSTEM 
 
 1. When a stimulus acting on a sense-organ arouses to activity 
 a muscle, by the medium of a sensory nerve, a nervous centre, and 
 a motor nerve, the entire transaction is said to be reflex; and its 
 paHTls" called a " reflex arc." In the case of the lower forms of 
 animal life we have already seen how the simplest reactions of an 
 organism which depend on nervous action are of the reflex type. 
 The reflex is, therefore, the unit or element of nervous function. 
 (Compare Fig. 61). 
 
 2. In the case of the higher animals, or of man, there are many 
 prompt reactions to stimuli which, because they involve either voli- 
 tion, or previous learning, or both, are not usually classed as re- 
 flexes. In reading aloud, for example, the reaction conforms to 
 the neurological type of a reflex; since a stimulus to the eye arouses 
 promptly a response of the vocal muscles; but the reaction has been 
 learned. A true reflex should be not learned, but innate. A good 
 example of the true reflex is afforded by the contraction of the pupil 
 in response to bright light entering the eye. This is not an ac- 
 quired reaction, nor is it dependent on the will. Of some of these 
 true reflexes we are wholly unconscious; in the case of others, such 
 as flushing, shivering, starting, the secretion of saliva, we are aware 
 of their occurrence, but have no voluntary control over them; in 
 the case of still others, such as coughing, sneezing, winking, we have 
 some degree of control, and yet there can be no doubt that they were 
 never learned by the individual. There are yet other reactions, of 
 which a good example is afforded by the turning of the eyes toward 
 any "attractive" object, which appear at a very early age in the 
 infant, and without any evidence that they are learned; but which 
 are very closely interwoven with our conscious life, and which are 
 controllable, to a large extent, by the adult animal. There is thus 
 a graded series of reactions to stimuli, which are alike in being native 
 to the individual, but which vary in their relations to consciousness. 
 The most satisfactory basis for classifying reflexes would seem, 
 then, to be found in the history of the reaction in the individual 
 according as it is innate or has to be learned. 
 
 145 
 
146 REFLEX FUNCTIONS OF THE NERVOUS SYSTEM 
 
 Another graded series of reflexes can be arranged on the basis 
 of their complexity. The pupillary reflex, for example, is about as 
 simple and strictly local as any; only one or two muscles are called 
 into action. Much the same is true of winking and of rotations of 
 the eyes. Coughing,, by comparison, is highly complicated; in 
 this reflex, a strong inspiration is followed by forced expiration, at 
 first against the closed glottis, which then is suddenly opened, al- 
 lowing the air to escape with a rush. Some reflexes call into action 
 only one of the limbs, whereas others involve all four limbs, the 
 
 FIG. 61. Diagram of a Reflex Arc. 
 
 trunk, and perhaps the head besides. Since it is impossible to 
 draw any sharp line between the most simple and the more com- 
 plex, the degree of complexity cannot be used as the basis for sepa- 
 rating reflexes from other reactions. 
 
 3. Instinct, also, is a word which cannot be sharply distin- 
 guished from the type of reactions called reflex. Both have in com- 
 mon the notion of innate, as distinguished from learned, reaction. 
 "Instinct," however, is seldom applied to such simple reactions as 
 that of the pupil to light; it is most frequently applied to complex 
 series of reactions, like those of birds in nest-building, or of frogs in 
 hibernating. It is also probable that such typical instincts are at- 
 tended by much of consciousness, and even of desire. But, as we 
 have seen, neither in terms of the attendant consciousness nor in 
 terms of complexity, is it anywhere possible to draw a sharp line; 
 and instinct and reflex may best be regarded as synonymous terms, 
 definable as innate reactions to stimuli. 
 
 The conception of a "simple reflex," or better, perhaps, of an 
 "isolated reflex," should also be considered in this connection. 
 The point here is not that the movement evoked shall be as lim- 
 ited and free from complexity as possible, but that it shall run its 
 course uncomplicated with other reactions that may be occurring 
 at the same time. In any complex nervous mechanism, there is 
 much going on simultaneously; many receptors are undergoing 
 
INSTINCTIVE AND SIMPLE REFLEXES 147 
 
 / 
 
 stimulation, and many effectors are acting at once. A quite simple 
 reflex would, therefore, be one which went through without inter- 
 fering with other reactions, or being interfered with by them. This 
 requires that a certain reflex centre, starting from a condition of 
 rest, shall be acted on by one group of ^enspry fibre.s, and shall dis- 
 charge into a group of motor fibres, the centre meanwhile remaining 
 unaffected by influences from any other parts of the system. But 
 such simplicity is seldom, if ever, realized. Every centre is con- 
 tinually subjected to some degree of excitation or other influence, 
 from the periphery; and the centres are so richly connected by fibres 
 with one another, that mutual influences are probably always in 
 operation. Hence Sherrington 1 has called the "simple reflex" a 
 convenient but artificial abstraction, and has emphasized the fact 
 that "the nervous system functions as a whole"; while Dewey 2 di- 
 rects attention to the psychological errors which result from treating 
 this abstraction as an actuality and building a psychology on it. 
 
 In distinguishing purely reflex from learned and from voluntary 
 reactions, we need to notice that many, or perhaps all, reactions 
 of the higher class are based on reflexes, and include reflex elements. 
 It is within the power of an adult to direct his eyes on an object 
 at will; but in so doing he makes use of the same movement which 
 appears reflexly in the infant. A similar combination of volitional, 
 habitual, and instinctive factors is seen in speaking, chewing, or 
 walking. Reflex elements are therefore more common in human 
 action than would at first appear; and a knowledge of the reflexes 
 is important in the psychology of human behavior, as well as in 
 understanding the inner mechanism of the nervous system. 
 
 4. The reflex, it has been stated above, is the unit of ner- 
 vous function. There is, indeed, another possible unit another 
 sort of functioning of the nerve-centres, which is known by the name 
 of automatic action. In this, the centre discharges into its motor 
 nerves, without itself receiving any stimulus, either from a sense- 
 organ or from any other part of the nerve-centres. Such activity 
 is supposed to originate within the centre. Its cause may be sought 
 either in the inner metabolism of the centre; or in stimuli acting 
 directly on the centre, as for example from the chemical action of 
 the blood. In these cases, the centre itself would seem to play the 
 part of a receptor. For a time, the concept of "automatic" action 
 seemed to fall into discredit; since it was found that certain reactions 
 which had been considered such were really reflex. For example, 
 the convulsions which result from strychnine poisoning, and which 
 
 1 The Integrative Action of the Nervous System, 1906, p. 114. 
 
 2 "The Reflex Arc Concept in Psychology," Psychological Review, 1896, III, 
 357. 
 
148 REFLEX FUNCTIONS OF THE NERVOUS SYSTEM 
 
 were formerly attributed to automatic discharge of the motor cells 
 of the spinal cord, do not take place, if all the sensory nerves lead- 
 ing to the spinal cord are severed. 1 
 
 One of the most probable instances of automatic action occurs 
 in the respiratory centre in the bulb. It is certain that venous 
 blood i. e., blood poor in oxygen and rich in carbon dioxide acts 
 as a stimulant to this centre, raising its irritability and thus increas- 
 ing respiration. When the blood circulating through the bulb is 
 abnormally rich in carbon dioxide, the rate of breathing is increased; 
 and when the blood is poor in carbon dioxide, the rate of breathing is 
 slackened. These facts have led to the view that the normal stimulus 
 to respiration is venous blood, and especially the carbon dioxide in 
 such blood, which, by circulating through the medulla, directly 
 excites the respiratory centre. More precisely, the centre is con- 
 ceived as automatic, at least so far as concerns inspiration; expi- 
 ration, in quiet breathing, is mere passive relaxation; and the cause 
 of the relaxation has been traced to sensory impulses reaching the 
 centre through the tenth or vagus nerve. The fibres concerned in 
 expiration come from the lungs, and are excited by the distension 
 of the lungs which occurs in inspiration. The effect of these sen- 
 sory impulses, themselves resulting from inspiration, is to "inhibit'* 
 the centre, and check inspiration, thus giving rise to passive ex- 
 piration. According to this view, inspiration would be an auto- 
 matic movement, but expiration a reflex. But even inspiration 
 can be shown to be sometimes the result of sensory stimuli, origi- 
 nating in the lungs or elsewhere; so that the possibility remains 
 that inspiration may be, in part at least, a reflex function. 2 
 
 5. Very recently, a strong support to the theory of automatic 
 functions has come from the discoveries of physiological chemistry. 
 It is found that certain chemical compounds, called "hormones" 
 especially those formed by the internally secreting glands, and some 
 which can even be made synthetically when introduced into the 
 blood, have the power to select definite tissues in the animal organ- 
 ism, and produce directly in them specific reactions of a type neces- 
 sary (some of them, absolutely) to the maintenance of the physio- 
 logical system. If this is true of other than the nerve tissues, it 
 
 1 H. E. Hering, in Archiv fur experimentelle Pathologic und Pharmakologie, 
 1896, XXXVIII, 276. 
 
 2 For a fuller discussion of this question, the reader may be referred to the 
 text-books of physiology, and to the authorities, especially to Rosenthal (Die 
 Athembewegungen, 1865), who supported the automatic conception by isolating 
 the bulb though not quite completely from the sensory nerves, -and finding 
 that a slow and imperfect respiration persisted in spite of this operation; and 
 to Head (Journal of Physiology, 1889, X, 1-70, 279-290), whose work went 
 far toward making the reflex conception seem probable. 
 
THE GANGLIONIC REFLEXES 149 
 
 would seem remarkable that the capacity for such special automatic 
 reactions should be denied to the tissues of the central nervous 
 system. All our theory of the specialized functions of this system, 
 especially in its higher forms of development and of activity, seems, 
 therefore, to favor the opinion that it is, among all the organic 
 structures, pre-eminently automatic. It is, therefore, highly proba- 
 ble that the reflex and the automatic forms of its functioning are 
 most frequently, if not uniformly, combined in ever-varying pro- 
 portions. 
 
 6. From these preliminary considerations, we may turn, first, 
 to a brief survey of the reflexes observed in mammals, and, after 
 that, to a discussion of the laws of reflex action in general. 
 
 In beginning a survey or inventory of reflexes, attention should 
 first be given to a class of actions which may be even simpler than 
 reflexes. An example is the beating of the heart. Since the heart 
 is supplied with nerves, the early assumption was that the heart- 
 beat is a reflex. Later, it was discovered that all of these nerves 
 could be cut, or the heart taken entirely out of the body, without 
 destroying the power of the heart to beat. An excised heart, if 
 supplied with suitable blood, will continue to beat, of itself, for 
 hours. Since the walls of the heart contain nerve-fibres and ganglia 
 of nerve-cells, the next view adopted was that the reflex was strictly 
 local, the ganglia acting as reflex centres. Another theory was pro- 
 posed by Gaskell and by Engelmann. 1 According to this theory, 
 the contractions of the heart muscle are essentially independent 
 of any nervous influence; or, in other words, the heart muscle is 
 automatic, depending only on chemical or other stimuli acting di- 
 rectly on the muscle itself. This view was supported by the ob- 
 servation that pieces of the heart muscle, so cut as to contain no 
 nerve-cells, showed the rhythmical beat. The evidence is perhaps 
 less convincing now than formerly, since the advance of histological 
 study has shown the existence of minute nerve ganglia in parts of 
 the muscle which were formerly thought to be free from them. 
 There is, however, no inherent improbability in the view that the heart 
 muscle has the power of reacting directly and rhythmically to 
 chemical stimuli affecting it; and it is said that the heart commences 
 to beat, in the embryo, before the growth of nervous tissue into it. 
 Essentially the same things may be said in regard to the movements 
 of other internal organs, such as the stomach and intestines, ureter, 
 etc. This view would accord, too, with the facts mentioned in the 
 preceding article. 
 
 1 Gaskell, Philosophical Transactions, 1882, p. 993; Engelmann, Pfluger's 
 Archiv fur die gesammte Physiologic, 1882, XXIX, 425; and many other papers 
 by each of these authors. 
 
150 REFLEX FUNCTIONS OF THE NERVOUS SYSTEM 
 
 7. The appearance of the chain of sympathetic ganglia is such 
 as to suggest that it is a series of connected reflex centres, much like 
 the ganglionic chain of a worm. It was long accepted, as a matter 
 of course, that this appearance furnished a correct view of the func- 
 tions of the entire sympathetic system. The fibres of this system 
 are ultimately distributed to the heart, stomach, intestines, and other 
 internal organs; and to the walls of the blood-vessels, the iris of the 
 eye, the sweat-glands, and the cutaneous muscles which erect the 
 hairs. Associated with the sympathetic fibres in the control of 
 these organs are other nerve-fibres which issue from the brain and 
 cord, but which do not pass to the sympathetic chain, though 
 they do make connections with other more scattered ganglia. 
 This whole system of nerves has been named the "autonomic 
 system." 1 It is characteristic of the organs supplied by the fibres 
 of this system that they have a large measure of local automatism, 
 as was noted above in the case of the heart. Yet they are all also 
 subject to reflex effects. The problem is, therefore, to determine 
 the reflex centres concerned. Are they located in the sympathetic 
 ganglia themselves? Goltz and Ewald, 2 in some remarkable ex- 
 periments in which they removed the spinal cord leaving the upper 
 part, or that necessary for breathing from dogs, which were kept 
 alive for a long period after the operation, found indeed that the 
 actions of many of these organs for example, the evacuation of 
 the bladder and rectum, and the bearing and nursing of young 
 were retained. But they found also that the action of these organs 
 was no longer influenced by stimuli acting on distant parts of the 
 body. In short, there was evidence of the automatic action of 
 these organs, but not of reflex action on them exerted through the 
 sympathetic ganglia. More minute experiments 3 have shown that 
 it is impossible to secure long-distance reflexes through the medium 
 of the sympathetic chain. For such reflexes, the connections of 
 the sympathetic with the cord are essential. It is evident, accord- 
 ingly, that the older conception of the sympathetic as a relatively 
 independent reflex centre, charged with the co-ordination of the 
 more "vegetative" functions of the organism, must be abandoned. 
 The centre for such co-ordination is to be sought in the central 
 nervous system, of which the sympathetic is but an adjunct. It 
 is even doubtful whether limited local reflexes occur by way of the 
 sympathetic ganglia. The latter are, more probably, not reflex 
 
 1 See the article by Langley in Schafer's Textbook of Physiology, 1900, vol. 
 II, pp. 616-696. 
 
 2 Pfliiger's Archiv fur die gesammte Physiologic, 1896, LXIII, 362. 
 
 3 See Langley, op. cit. 
 
REFLEXES OF THE SKELETAL MUSCULATURE 151 
 
 centres at all, but simply relay stations in the path of the outgoing 
 fibres to the organs mentioned. 1 
 
 8. In contrast to the musculature of the internal organs, what 
 is called the skeletal or "voluntary" musculature does not have 
 local automatism and is not controlled by the fibres of the sympa- 
 thetic system. Its nerve supply comes directly from the cord and 
 brain-stem, on which it is dependent for all normal stimuli to ac- 
 tivity. If severed from the central nervous system, the skeletal 
 musculature becomes incapable of functioning, and even degener- 
 ates and atrophies, thus losing entirely the characteristics peculiar 
 to it. The muscles of the limbs are therefore dependent on spinal 
 reflex action for their very existence. They are not, however, de- 
 pendent in the same way on the brain; but only on the ventral horn 
 of the cord, from which the motor nerve-fibres for these muscles 
 issue. 
 
 For an exact study of the reflexes of this order, it is necessary 
 to exclude the action of the brain on the muscles; and for studying 
 the reflex powers of any particular portion of the brain-stem or 
 cord, it is necessary to isolate this portion from the rest of the cen- 
 tral system. Physiologists who investigate the laws of reflex action 
 begin, therefore, by making a reflex or, usually, a spinal prepara- 
 
 1 The nervous system, by the connections which it establishes between dif- 
 ferent parts of the body, is the supreme factor in the co-ordination or integra- 
 tion that is necessary if the body is to maintain life and efficiency. But not 
 all co-ordination is effected through the nervous system. The chemical inte- 
 gration of the body is very largely accomplished by transmission of chemical 
 substances from one organ to another, through the circulation (see p. 148). 
 Thus, the fuel taken in at a meal is, to a large extent, stored in the liver, and 
 doled out into the blood as muscular activity makes demand for it. The 
 mechanism by which the demands of the muscles are transmitted to the liver 
 is supplied by the circulation. An active muscle draws fuel from the blood, 
 thus lowering the proportion of sugar in the circulation; and when the blood 
 circulating through the liver has less than a certain proportion of sugar, the 
 liver gives out enough to bring back the proportion to the normal. There 
 are many "internal secretions," produced by certain organs, as the adrenal 
 bodies, the pancreas, and the pituitary body, which pass, by the circulation, 
 to other organs, and affect their activity. One of the best examples of chemi- 
 cal co-ordination is that discovered by Bayliss and Starling (Journal of 
 Physiology, 1902, XXVIII, 325. See also Starling, Recent Advances in the 
 Physiology of Digestion, 1906), through which the secretion of the pancreas 
 is excited. The hydrochloric acid which forms part of the gastric juice, when 
 it passes with the partly digested food from the stomach into the intestine, 
 excites cells in the wall of the intestine to secrete into the blood a substance, 
 called secretin, which, being carried by the blood to the pancreas, excites that 
 gland to pour its secretion into the intestine. Thus a reaction which formerly 
 was taken to be a nervous reflex is seen to be of quite a different nature. It 
 is probable that this sort of chemical action of one organ on another is of great 
 importance in the processes of growth and metabolism. 
 
152 REFLEX FUNCTIONS OF THE NERVOUS SYSTEM 
 
 tion. For this purpose, the brain need not actually be destroyed; 
 but it must be disconnected from the part of the cord to be exam- 
 ined. Since the cord and brain-stem afford the only path of nervous 
 connection between the brain and the cord, or between the brain 
 and the muscles, a transection of the brain-stem or cord, at any 
 level, isolates the part below the cut from the brain, and makes of 
 the part thus severed from the brain a "reflex preparation." Sev- 
 eral varieties of such preparations have been employed. The " spi- 
 nal frog" is constituted by simply decapitating the animal. Since the 
 frog, like other cold-blooded animals, can maintain life without 
 breathing, such a spinal preparation can be kept alive for days and 
 even for months. 1 In mammals, however, decapitation is promptly 
 fatal, unless artificial respiration be provided; the animal must also 
 be kept warm and much loss of blood prevented, in order to secure 
 the best results. With these precautions, a decapitated cat can be 
 kept alive for several hours, and forms one of the most useful of 
 preparations for the study of reflex action. 2 
 
 Another very useful preparation is the " decerebrate " animal, 
 which differs from the spinal animal in that the transection is made 
 through, or just above, the mid-brain thus leaving the cerebellum, 
 pons, bulb, and cord in continuity, but excluding the action of the 
 cerebrum, thalamus, and (usually) part of the mid-brain. Goltz 
 successfully performed decerebration of a dog, by removing the 
 cerebrum in three operations, allowing the animal to recover, as 
 far as possible, from each operation before resorting to the next. 
 In this way he kept one animal alive for 18 months. 3 
 
 Sherrington has found that, if care is taken to prevent loss of 
 blood and of bodily heat, a decerebrate dog or cat can be easily pre- 
 pared and kept alive for several hours, during which time it shows 
 many reflex phenomena. 4 
 
 9. The different separate parts of the cord can be isolated by 
 similar methods. In the frog, for example, a transection of the cord 
 in the middle of the back, secures a reflex preparation consisting 
 principally of the hind-limbs and their nerves, together with the 
 part of the cord which receives these nerves. Similarly, two cuts 
 across the frog's cord, just above and just below the exit of the 
 brachial nerves, isolate a fore-limb preparation; and both of these 
 fractions of the animal show reflex activity. In mammals, it is 
 common to sever the cord below the exit of the phrenic nerve, a pro- 
 
 1 See especially the work of Schrader, Pfliiger's ArcMv fur die gesammte 
 Physiologic, 1887, XLI, 82, and 1888, XLIV, 175. 
 
 2 Sherrington, Journal of Physiology, 1909, XXXVIII, 375. 
 
 3 Goltz, Pfliiger's Archiv fur die gesammte Physiologic, 1892, LI, 570. 
 
 4 Sherrington, Proceedings of the Royal Society, 1896, LX, 411. 
 
LOCAL REFLEXES OF THE SPINAL CORD 153 
 
 cedure which allows respiration to persist and the animal to be kept 
 alive for months, while at the same time isolating the lower two- 
 thirds of the cord as a reflex preparation. Such "spinal dogs" show 
 a variety of reflexes. A second transection can be made at some 
 distance below the first; the animal then, while remaining a unit 
 so far as concerns digestion and circulation, is divided into three, 
 so far as concerns the nervous system. The "fore-dog" includes 
 the head and the fore limbs, and retains brain action; the "mid- 
 dog" consists of the trunk (or a large part of it) with its nerves and 
 the central portion of the cord; while the "hind-dog" consists of the 
 hind-limbs, tail, and pelvic region, with its nerves and the lower por- 
 tion of the cord. In the case of such a preparation, the "hind- 
 dog" shows many reflexes; the "mid-dog" in accordance with 
 the small variety of trunk movements shows comparatively few 
 reflexes; while the "fore-dog," retaining as it does the brain intact, 
 is a "normal" as opposed to a "reflex" animal, and behaves about 
 as dogs usually do, except that it receives no sensations from the 
 middle and rear portions of its body, and has no control over the 
 movements of those parts. 1 
 
 In the human subject, accidents to the spine sometimes sever 
 the cord, leaving the lower portion isolated from the brain, as in 
 the animal preparations; and hemorrhage more frequently inter- 
 rupts the motor pathway from the cerebrum to the brain-stem, leav- 
 ing a condition of the cord which corresponds, in some respects, 
 to that of the decerebrate animal. The discoveries with regard 
 to the reflex functions of the nervous system have, however, been 
 chiefly obtained from the animal preparations. 
 
 10. One of the most striking results of experiment is the fact 
 that reflexes are elicited from very short lengths of the cord, pro- 
 vided the sensory and motor nerves attached to the part in question 
 are intact. In the frog, for example, the "flexion reflex" of the 
 hind leg, which occurs when one foot is gently pinched, consists of a 
 drawing-up of the whole leg, with flexion at hip, knee, and ankle. 
 This is obviously a highly co-ordinated movement, since it brings 
 several muscles into play; it has also the character of "purpos- 
 iveness" or utility, being in fact a protective reflex. Another ex- 
 ample obtained from the frog preparation is the so-called "clasp 
 reflex" which involves both of the fore-limbs. This reflex is ob- 
 tained from a fore-limb preparation of the male frog, by touching 
 the skin of the chest, and consists of a clasping action. In the 
 normal frog, this action is elicited only by the presence of the female 
 frog at the breeding season; but in the reflex preparation it can be 
 
 1 Goltz and Ewald, Pftiiger's Archiv fur die gesammte Physiologie, 1896, LXIII, 
 362. 
 
154 REFLEX FUNCTIONS OF THE NERVOUS SYSTEM 
 
 elicited by so indifferent a stimulus as a mere touch by the finger. 
 Like the "flexion reflex/' it is clearly co-ordinated and "purposive." 
 The characteristic of purposiveness is in fact true of reflexes in 
 general. 
 
 The best interpretation of all such reflexes assumes that, the 
 sensory and motor nerves to a part being intact, and likewise the 
 central connections between them, the reflex is to be expected 
 as a matter of course. The results show that the necessary connec- 
 tion between the sensory and motor nerves to a given part is estab- 
 lished at, or very near, their place of entrance into the cord. Such 
 a connection is what would be anticipated from the minute anatomy 
 of the sensory and motor fibres of the cord (see p. 89); since some 
 branches of the sensory fibres bend immediately into the ventral 
 horn of the gray matter, from which, in turn, issue motor fibres that 
 pass directly out of the cord into the motor roots. These short re- 
 flex pathways may be called local arcs, and their reflexes local re- 
 flexes. The most local of all reflex arcs are those which start in 
 a given muscle and lead back to the same muscle. The receptors 
 concerned in such cases are sense-organs situated in the muscle and 
 its tendon, and excited by movements, active or passive, of the mus- 
 cle itself; they may be called proprioceptors. This is to say that 
 movements and tensions within a muscle act reflexly on the muscle 
 itself, causing it, according to the exact nature of the stimulus, to 
 contract, or to relax, or to maintain the degree of contraction which 
 it already has. 1 Similar reflexes are important in maintaining 
 posture against the action of gravity; and also in giving steadiness 
 and persistence to muscular action in general. 
 
 11. Among other local reflexes may be mentioned the "wiping" 
 reflex of the frog, which is excited by placing a bit of paper moistened 
 in weak acid on the skin of the flank; the leg of the same side is 
 then brought up and brushed across the place stimulated; and this 
 action is repeated a number of times. If the leg of the same side 
 is held, and the stimulus continued, the leg of the other side is 
 brought up and across, to perform the wiping movement. In the 
 spinal dog may be mentioned the "stepping" or "marking-time" 
 reflex, which is aroused by supporting the animal under the arms 
 and letting the hind legs hang down. In this case, the weight of 
 the legs seems to constitute the stimulus; and the reaction is an al- 
 ternate raising and lowering of the two hind legs as in normal walk- 
 ing. The " extensor thrust " (Sherrington) is a brief but strong push- 
 ing down of the foot when pressure is exerted upward on the sole. 
 
 Sherrington, "On Plastic Tonus and Proprioceptive Reflexes," Quarterly 
 Journal of Experimental Physiology, 1909, II, 108; and Integrative Action of 
 the Nervous System, 1906, pp. 129 ff. 
 
LOCAL REFLEXES OF THE SPINAL CORD 155 
 
 Other reflex arcs pass from receptors in one hind leg to muscles in 
 the other, or from the tail to the hind legs. There are also vaso- 
 motor and visceral reflexes, obtainable from the " hind-dog"; some 
 of them, such as micturition and defecation, are accompanied by 
 suitable movements or postures of the limbs. 
 
 From a " mid-dog" preparation can be obtained such reflexes 
 as shivering, curving of the trunk toward the side stimulated, and 
 shaking of the trunk. From an isolated fore-limb preparation can 
 be obtained reflexes similar to those of the hind-limbs. In case of 
 the monkey, even more varied and detailed reflexes can be ob- 
 tained. In man, reflexes are obtained which are on the whole 
 similar in character to those seen in the lower animals; but the 
 reflex activity of severed portions of the cord is usually com- 
 paratively small in man; perhaps, in part, because of the greater 
 roughness and severity of the accidental, as compared with the ex- 
 perimental severing of the cord; but more probably, because of the 
 greater importance, in man's case, of the dependence of the cord 
 on the brain and the receptors of the head. 
 
 12. If longer portions of the cord, or if the whole cord, be left 
 in continuity, there are added to the local reflexes others involving 
 longer arcs, and the co-ordination of more muscles. Thus, if 
 the cord is transected at about the level of the shoulders, the 
 "scratch reflex" of the hind leg can be aroused by tickling or prick- 
 ing the skin as far forward as nearly to the fore-limbs. If the whole 
 cord is left in continuity, pinching the pinna of the ear may evoke 
 a combination of movements of the ear, neck, all four limbs, trunk, 
 and tail. These larger movements are, like the simpler local re- 
 flexes, co-ordinated; they often have either a protective or a loco- 
 motor character. The movements of the four limbs are frequently 
 combined as in the trotting common to quadrupeds. Such loco- 
 motor movements are not very efficient, however, when the cord is 
 isolated from the brain; one important difference is that balance is 
 not maintained. In the decerebrate animal some of this deficiency 
 is supplied; and a decerebrate frog can jump and swim in almost a 
 normal manner. The decerebrate dog or cat, soon after the opera- 
 tion, usually does little more than crawl; but the animal which Goltz 
 kept alive came to walk normally. The contribution of the brain- 
 stem to the function of locomotion is in large measure to be explained 
 by reference to the vestibular branch of the eighth nerve, with its 
 receptors in the inner ear, and its central connections in the pons 
 and bulb. These receptors are located in the vestibule and semi- 
 circular canals; they are excited by movements and positions of 
 the head, and their reflexes consist of compensatory or corrective 
 movements and postures, and also result in maintaining the tonus of 
 
156 REFLEX FUNCTIONS OF THE NERVOUS SYSTEM 
 
 many muscles. Such reflexes are essential for the maintenance of 
 the erect posture and for well-directed locomotion. 
 
 13. The cerebellum is closely related to the reflex functions 
 just mentioned. 1 It is located, as an outgrowth of the brain-stem, 
 near the level of the eighth nerve, and receives many fibres from its 
 vestibular branch. The disturbances which result from injuries 
 to the cerebellum resemble those which result from injury to this 
 nerve or to its receptors. The cerebellum seems, therefore, to be 
 fundamentally an expansion of the local centre of the vestibular 
 branch of the eighth nerve. But it receives also numerous fibres 
 from other sense-organs, especially, as appears probable, from those 
 situated in and about the muscles, and belonging to that proprio- 
 ceptive system which was mentioned a few paragraphs back in 
 connection with local reflexes (see p. 154). We there took note of 
 the action reflexly exerted on a muscle by stimuli arising in the mus- 
 cle itself from its own contractions and from pulls and pushes ex- 
 erted on it. Such local reflexes were found to be important in 
 efficient muscular action. But it is likely that these same stimuli 
 act on the cerebellum through the cerebellar tracts (compare p. 95) ; 
 and this portion of the brain thus becomes an organ where are 
 gathered together sensory impulses from all the muscles of the body. 
 In this way, the cerebellum receives information, as it were, regard- 
 ing the condition of every muscle; in it is formed a sort of representa- 
 tion, or reproduction, detailed and yet comprehensive though, as 
 far as known, unattended with consciousness of the dynamic con- 
 dition of the entire musculature. To this is added the very im- 
 portant function, provided for by the receptors in the inner ear, 
 which responds to the position and movements of the head in space. 
 Thus the posture, movements, muscular tensions, and external 
 strains exerted on the body at every moment, act on the cerebellum, 
 and through it, reflexly, react on the muscles. 
 
 We need to recall, further, that in mammals the cerebellum re- 
 ceives a large mass of fibres which apparently bring impulses from 
 the cerebrum. The exact function of this connection is altogether 
 unknown; but it is reasonable enough to suppose that, if the cere- 
 bellum is to preside over the dynamic condition of the muscles, it 
 should receive advance information from the cerebrum regarding 
 what movements are next to be made; and in the higher mammals, 
 it is the cerebrum which very largely determines the nature of the 
 postures assumed and the general course of bodily movements. 
 
 In accordance with this rather vague conception of the function 
 
 1 See Luciani, II cerveletto, 1891; Sherrington, in Schafer's Textbook of 
 Physiology, 1900, vol. II, pp. 903-910; and Integrative Action of the Nervous 
 System, 1906, p. 347. 
 
REFLEXES OF THE CEREBELLUM 157 
 
 of the cerebellum, we find that the results of injury to it are seen 
 principally in defects of balancing power or of co-ordinated move- 
 ment. The muscles that maintain the posture of the animal, in 
 which this organ has been injured, do not maintain their "tone," or 
 mild steady state of contraction, so well as do the muscles of a nor- 
 mal animal. The movements are also lacking in force and are 
 affected with a tendency to tremor. There is uncertainty in main- 
 taining equilibrium against gravity, and in keeping to a straight 
 line in locomotion. If the injury to the cerebellum is one-sided, the 
 symptoms are even more striking; because there is a lack of sym- 
 metry or balance between the movements of the two sides of the 
 body: "forced movements" occur, such as an uncontrollable turning, 
 or rolling, to one side (the "circus movement"). It is indeed re- 
 markable to what an extent the early symptoms of cerebellar 
 injury disappear with time. Sometimes, in the case of man, very 
 extensive destructions of cerebellar substance, if they come on 
 gradually, betray themselves scarcely at all to ordinary observa- 
 tion. It should be said, however, that such cases have not yet been 
 studied with the minuteness which the subject demands. Part of 
 the difficulty of the study, and part of the vagueness to our minds 
 of the resulting conception of the reflex functions of the cerebellum, 
 may be due to the fact that the matters over which this organ pre- 
 sides are not customarily dependent on conscious control, and so 
 do not arouse our attention. 
 
 14. The completeness with which locomotion occurs in de- 
 cerebrate animals makes it probable that the cerebrum is not 
 fundamentally concerned in this form of co-ordination. That loco- 
 motion is reflex, in the sense of not needing to be learned, is clear 
 in the case of those animals which walk, run, crawl, swim, or fly, 
 at birth or on emerging from the egg. In animals which pass 
 through a period of helpless infancy, the case is not so clear; but it 
 was proved by Spalding 1 that birds, at the proper age, fly perfectly, 
 even when they have been prevented from seeing old birds fly and 
 from exercising their own wings. In the human infant, walking 
 seems, from such observations as have been collected, 2 to occur at 
 the proper age without training or unsuccessful efforts. 
 
 15. The bulb, or medulla, from its being the place of entry of 
 the vagus nerve, is the local reflex centre for the receptors, supplied 
 by that nerve, and located in the lungs, heart, stomach, etc. (compare 
 p. 148). It contains the chief centres for respiration, for regulating 
 the rate and force of the heart-beat, for controlling the diameter of 
 the blood-vessels and so the distribution of the blood, for swallow- 
 
 1 Nature, 1875, XII, 507. 
 
 2 See Woodworth, Le Mouvement, 1903, p. 315. 
 
158 REFLEX FUNCTIONS OF THE NERVOUS SYSTEM 
 
 ing and for regulating the movements of the stomach. These, and 
 other related functions, are not fundamentally interfered with by 
 severing the brain-stem close above the bulb. Other visceral re- 
 flexes, as previously mentioned, have centres in the cord, especially 
 in its lower portion, where also are the centres of the fundamental 
 sexual functions, these also being reflex. 
 
 It appears, accordingly, that visceral and locomotor reactions 
 are fundamentally reflex; and to these should be added the protective 
 reactions aroused by "painful" or injurious stimuli; of the latter 
 class of reflexes, the flexion and scratch reflexes are examples; and 
 so also is the pupillary reflex, whose centre is in the mid-brain. 
 With the protective reflexes may perhaps be classed cries of pain 
 and anger, and some movements of facial expression, which appear 
 in the decerebrate mammal. All in all, it would seem that the funda- 
 mental co-ordinations of movement are, generally speaking, of a 
 reflex nature. It is as provided with such materials that the proc- 
 esses of learning movements and of gaining voluntary control take 
 their start. 
 
 16. As to the influence of the cerebrum on the reflex activity 
 of the lower centres, it was formerly held 1 that reflexes to present 
 stimuli are more regularly and easily elicited, but that anything like 
 spontaneous movement is absent, in the decerebrate animal. This 
 statement of the case is, however, too simple to cover all the facts. 
 It is indeed true that many reflexes are more certainly evoked in a 
 spinal or a decerebrate animal than in a normal animal; among such 
 are especially the protective reflexes. But in the higher mammals, 
 separation from the brain seems on the whole to depress the reflex 
 activity. Much depends, too, on the level at which the transection 
 occurs. If special care is taken not to injure the mid-brain, thai- 
 ami, and optic nerves, removal of the cerebrum is followed by much 
 less of apparent loss of "spontaneity." 2 The difference in such 
 cases must, therefore, be largely due to the retention of the connec- 
 tions with the organ for vision, and so of visual stimuli. Cutting 
 off so important a class of stimuli necessarily reduces the animal's 
 activity. It is unavoidable, in all removals of the cerebrum, that 
 the olfactory lobe, and the central connections of the sense of smell, 
 should be destroyed; the loss of this class of stimuli, also, lowers the 
 activity of the animal particularly in the case of those animals 
 which depend greatly on the sense of smell. In mammals, most of 
 the central connections of sight and hearing, as well as of smell, 
 run through the cerebrum; loss of the hemispheres therefore renders 
 
 1 See Ferrier, Functions of the Brain, 1886, p. 109. 
 
 * Schrader, Pfluger's Archiv filr die gesammte Physiologie, 1887, XLI, 75; 
 and 1888, XLIV, 175. 
 
NERVOUS MECHANISM OF REFLEXES 159 
 
 the mammal practically blind and deaf. Accordingly, the loss of 
 all these senses needs to be considered, whenever the behavior of 
 a decerebrate mammal is examined. There is no doubt that loss 
 of the cerebrum means loss of learned movements; and it also in- 
 volves the loss of both the inhibitory and the tonic influences which 
 are exerted normally by the cerebrum on spinal and brain-stem re- 
 flexes. 
 
 Regarding the general characteristics of the reflex functions of the 
 nervous system, much may be gleaned from the incomplete inventory 
 of reflexes which has been given in the preceding paragraphs. 
 Much, however, still demands more special and detailed considera- 
 tion. Probably the greatest authority on reflex action in general 
 is Sherrington; and in what follows reliance will be placed chiefly 
 on his numerous special studies, and especially on his philosophical 
 presentation of the whole matter in his book with the title "Inte- 
 grative Action of the Nervous System." 1 
 
 17. Returning to the conception of the reflex arc, we see that 
 the particular muscular movement which is to follow any stimulus 
 is dependent on the nervous paths that lead from the receptor which 
 has been stimulated. Since a most general characteristic of re- 
 flexes is to bring into play a considerable amount of musculature 
 in response to the excitation of even a very small group of receptors, 
 it follows that the reflex arc must undergo more or less of branching, 
 so as to distribute the excitation sufficiently widely. Such distri- 
 bution (compare Fig. 62) is provided for, to a limited extent, by the 
 branching of motor nerve-fibres, each of which may innervate sev- 
 eral muscle-fibres. But the required distribution is much more the 
 result of the branching which takes place within the nervous cen- 
 tres themselves. As has already been shown (see p. 47), the sen- 
 sory fibres, when they enter the cord, branch widely, by means of 
 collaterals; and it is highly probable that central neurones, or what 
 von Monakow has called "interpolated cells/ 52 intervene between 
 the sensory and the motor cells, and act as still further distributing 
 agents. Such central cells are shown by histology to exist in abun- 
 dance, and part of their function is, probably, the distribution of 
 impulses. The character of any reflex is dependent, then, first of 
 all, on a large amount of branching in the pathway which extends 
 from any given receptor to many effector units. How wide this 
 distribution can become, is seen most clearly when the stimulus 
 is very intense; for then the reflex may even spread over a large 
 share of the muscles of the entire body. In certain abnormal con- 
 ditions of the nerve-centres, such as, especially, the condition brought 
 
 1 New York, 1906. 
 
 2 Ergebnisse der Physiologic, 1902, I, part ii, 563. 
 
160 REFLEX FUNCTIONS OF THE NERVOUS SYSTEM 
 
 about by strychnine poisoning, a stimulus to any receptor calls into 
 action almost the whole musculature; and this result shows that, po- 
 tentially at least, paths exist from each receptor to every effector unit. 
 But the distribution of nervous impulses over large areas of the 
 musculature is by no means at random; for, in normal conditions 
 
 Muscle 
 
 Motor 
 neurones 
 
 Central 
 neurones 
 
 A 
 
 B 
 
 FIG. 62. The Distribution of Sensory Impulses in the Cord. In A, the distribution is 
 accomplished by the branching of the sensory axon; in B, by the branching of an 
 interpolated central axon. 
 
 of the nervous centres, the reflex is not a general convulsion, but a 
 co-ordinated movement. The distribution, in other words, is 
 highly selective: the excitation is carried to muscles which work in 
 harmony; while other muscles, which would work against those 
 employed, are passed over in the distribution of excitation. The 
 distribution is different, also, according as it comes from one par- 
 ticular receptor or from some other: one receptor calls into play a 
 
NERVOUS MECHANISM OF REFLEXES 161 
 
 certain combination of muscles; another receptor, a very different 
 combination. 
 
 18. Distribution of excitation, by means of branching path- 
 ways, is therefore a primary characteristic of reflex action. An- 
 other equally salient feature of this class of nervous functions is 
 almost the reverse of this. There is a convergence of pathways, 
 so that the same effector organ can be excited from any one of 
 many receptors. This is made evident, first of all, by the fact that 
 the same reflex can be excited from any one of many different points. 
 The scratch reflex, for example, can be evoked by suitable stimula- 
 tion applied anywhere within a large area of the skin of the back 
 and sides. The flexion reflex of a limb can be evoked by a stimulus 
 applied almost anywhere on the same limb. The pupillary re- 
 flex can be aroused by a beam of light falling anywhere on the ret- 
 ina. In general, the "receptive field" of a reflex is often wide; and 
 from anywhere within this field the same muscles are thrown into 
 action, though not always to the same degree, or in the same pro- 
 portion. It is made obvious, from such facts as these, that the 
 paths from the numerous receptors within any receptive field must 
 converge upon the same muscles. And, plainly, such convergence 
 does not occur within the sensory or the motor nerves themselves; 
 for the fibres in these nerves run their courses parallel and inde- 
 pendent, with no chance for communication from one to another. 
 The convergence must, therefore, occur within the nervous centres; 
 and it seems to be provided for, in part, by the spreading dendrites, 
 which are capable of receiving excitation from many axons, and which 
 converge upon their own cell-body and axon. It is probable, also, 
 that central or interpolated cells have a share in the convergence 
 of excitation, as well as in its distribution. 
 
 The convergence of reflex paths from all parts of the receptive 
 field of a single reflex is only one case of convergence; and it is the 
 simplest case. But the same muscle may be employed in different 
 reflexes. For example, the muscles which bend and which extend 
 the knee are active in the scratch reflex, and also in the stepping 
 reflex. The receptive field for the latter lies within the muscles 
 themselves, far distant from the receptive field of the scratch re- 
 flex. Moreover, though the two reflexes employ the same muscles, 
 they employ them differently; the action of the knee muscles is dif- 
 ferently combined with the action of other muscles, and besides, 
 the rhythm of the scratch reflex is faster than that of the stepping 
 reflex. Since the same muscles act differently in different reflexes, 
 the difference cannot be attributed to the muscles themselves; they 
 are merely obedient to the different excitations which they receive. 
 Nor can the difference lie in the peculiarities of the receptors, as 
 
162 REFLEX FUNCTIONS OF THE NERVOUS SYSTEM 
 
 can be proved by suitable experiments. 1 Such differences, on the 
 contrary, must be located in the nerve-centres; and the converging 
 paths that lead within the centres toward the motor cells govern- 
 ing a given muscle must, therefore, not simply converge, but dis- 
 charge their impulses with differing force and rhythm. Among 
 the central cells of the cord there must apparently be, not only col- 
 lectors of sensory impulses, but different collectors for the several 
 reflexes which employ the same muscle; and these different collec- 
 tors must have differing rates of discharge, etc. The collectors them- 
 selves have also differing distributions to the motor cells of the cord. 
 
 If these inferences make the connections within the cord seem high- 
 ly complicated, we must remember that the complexity of connec- 
 tions revealed by histology is as ample as could be desired by the 
 physiologist. 
 
 19. There is a yet further complication to be noted. The ef- 
 fect exerted reflexly on a muscle is not always that of arousing it 
 to activity; it may be the exact opposite, that of suppressing what- 
 ever activity is going on in a muscle. This process of checking ac- 
 tivity is called inhibition, and the process of inhibition seems to be 
 scarcely less important than that of excitation, in securing harmoni- 
 ous action by the muscles. It is seldom, indeed, that a stimulus 
 to a sense-organ finds the system in a completely resting condition; 
 almost always, some action is already going on within the system. 
 In particular, the muscles which maintain the posture of the animal 
 are usually in activity; such are, especially, the extensor muscles 
 of the limbs and neck, and the muscle which supports the jaw against 
 gravity. 
 
 Now, whatever movement is called for by any stimulus is pretty 
 sure to require the temporary abandonment of the existing posture; 
 and if the muscles which maintain this posture were left in their 
 active state, they would hinder the quick and powerful execution 
 of the newly required reflex movement. It is found, as a matter of 
 fact, that the contraction existing in such muscles is inhibited by 
 the reflex action of the new stimulus, simultaneously with the excita- 
 tion of the muscles which execute the reflex movement. 2 This has 
 been demonstrated by Sherrington in the case of the extensor mus- 
 cles of the knee, which lose their tonic contraction simultaneously 
 with any reflex action of the flexors of the knee; he has also demon- 
 
 1 Sherrington (op. cit., pp. 55-61) finds that the rhythm of the scratch 
 reflex is not interfered with by simultaneously or alternately exciting two 
 points on the dog's back, and argues that, if the rhythm were determined in 
 the receptors, the rhythmic impulses sent in from one point of stimulation 
 would combine with those sent in from the other point, and produce a rhythm 
 of twice the frequency, or at least change the rhythm, as it fails to do. 
 
 2 Sherrington, op. cit., 83-101. 
 
PHENOMENA OF INHIBITION 163 
 
 strated the same phenomena in the muscles of the eye and in other 
 pairs of antagonistic muscles. When one muscle of a pair of an- 
 tagonists is excited reflexly, the opposing muscle is simultaneously 
 deprived of whatever contraction it may have. 1 There are no doubt 
 exceptions, of a sort, to this rule, since in voluntary action it is possi- 
 ble to fix any joint in a rigid position by contracting at the same 
 time the antagonistic muscles that act on the joint. But in move- 
 ments, the inhibition of antagonists seems to be a very general 
 principle. 
 
 Inhibition is a frequent phenomenon in some of the internal 
 organs. The movements of the stomach and intestines, which, 
 as noted above (p. 149), go on without the action of the central 
 nervous system, can yet be checked by outgoing impulses from the 
 nervous centres. The muscles of the arteries, similarly, are made to 
 relax by the action of the "vaso-dilator" nerves. But the most in- 
 teresting case is that of the heart. The vagus nerve, which sends a 
 branch to the heart, acts to check the heart-beat. When this nerve 
 is stimulated, the heart-beat is slowed down or even stopped for a 
 time. Since this result is obtained by stimulating the outgoing 
 nerve to the heart, it is obvious that the inhibition operates within 
 the heart itself; and strong evidence has been offered to show that 
 it operates within the heart muscle. But such inhibition as we are 
 here considering namely, the inhibition of the skeletal muscles 
 does not operate within those muscles; for no good evidence ex- 
 ists that there are any specifically inhibitory fibres running to the 
 muscles, as there are running to the heart by the vagus. The in- 
 hibition of the skeletal muscles operates within the spinal cord. 
 The postural activity of these muscles is itself of a reflex nature, 
 being maintained through reflex centres; and the inhibition works 
 on those centres, stopping them from sending out excitatory im- 
 pulses to the muscles. 
 
 When inhibition as well as excitation is taken into account, the 
 breadth of a reflex action, or the extent of its distribution, is seen to 
 be twice as great as at first appears. For the influence extends not 
 only to the muscles which become active, but also to those muscles 
 which are inhibited. A small group of sensory fibres may thus exert 
 a wide influence on the motor cells of the cord; but they may excite 
 some of these cells and others they may inhibit or depress. 
 
 20. It is not easy to form a complete conception of the mechan- 
 ism of inhibition, 2 but there is one significant fact about it which 
 
 1 In the case of movements which amount to changes of posture, the in- 
 hibited muscle often loses only a part of its contraction (Sherrington, Quar- 
 terly Journal of Experimental Physiology, 1909, II, pp. 109 ff.). 
 
 3 See, however, pp. 289 ff. 
 
164 REFLEX FUNCTIONS OF THE NERVOUS SYSTEM 
 
 is of no small assistance to this end. Inhibition itself is not merely 
 an interruption of activity; for it has an after-effect which is the 
 opposite of depression. When a muscle has undergone inhibi- 
 tion, it becomes at once readier for a new phase of activity. It is 
 more easily aroused than it was before, and it is likely to show more 
 force in its next contraction. That is, the phase of inhibition is 
 followed by a rebound to greater activity; 1 and the rebound, like 
 the inhibition, is primarily a central, and not a muscular affair. 
 This after-effect of inhibition is probably important in the numer- 
 ous alternating movements which occur in locomotion, breathing, 
 chewing, pounding, etc.; the muscles (or their controlling nerve- 
 cells) which are inhibited in one phase of the movement are thereby 
 made ready for the succeeding, opposite phase. 
 
 21. Closely related to inhibition is the phenomenon of the "re- 
 fractory period," already mentioned (p. 131) in the case of nerve- 
 fibres. Immediately after acting, or starting to act, any excitable 
 organ loses its excitability and becomes refractory or unrespon- 
 sive for a brief period. The duration of the refractory period differs 
 in different organs; it is shortest in the case of nerve-fibres, where it 
 does not exceed .002 sec. It is much longer than this in some of 
 the reflexes, but varies greatly from one reflex to another. The 
 "extensor thrust," for example, has a long refractory period, which 
 may reach a full second; this means that repeating the stimulus 
 within a second after a thrust has been evoked does not evoke a 
 second thrust. More concretely stated and illustrated: If a gentle 
 upward pressure is exerted on the "pads" of a spinal-dog's hind 
 foot, the leg responds by a vigorous downward thrust (as if in 
 jumping). Now, though this thrust lasts for only a fifth of a sec- 
 ond, yet repeating the upward pressure on the pads does not evoke 
 another thrust, unless an interval of a full second is allowed to 
 elapse between the two stimuli. The duration of the refractory 
 period in this, as in other reflexes, is, however, somewhat vari- 
 able. 
 
 The length of the refractory period in the winking reflex is about 
 the same as that in the extensor thrust; and in the swal.lowing re- 
 flex it is half a second or longer. In the "stepping reflex" it is about 
 two-fifths of a second, and in the "scratch reflex," about one-fifth. 
 In other movements, which are not obviously rhythmical, and which 
 show a prolonged contraction of the muscles (such as the flexion 
 reflex), it is found, on careful examination, that the apparently 
 steady contraction of the muscle includes a series of waves, follow- 
 ing one another at the rate of eight to twelve per second. These 
 waves represent discharges from the cord, which are, in effect, 
 1 Sherrington, op. cit., p. 206. 
 
THE REFRACTORY PERIOD 165 
 
 fused into the steady prolonged contraction. The refractory phase, 
 in such cases, varies from an eighth to a twelfth of a second; as we 
 know by the fact that the waves are not increased in frequency by 
 stimulating the receptor at a faster rate, as, for example, by elec- 
 tric shocks at an interval of from twenty to fifty per second. 
 
 22. Like those other peculiarities of reflex action that have al- 
 ready been mentioned, the refractory phase is not a peripheral 
 phenomenon, the cause 'of which resides either in the muscles or 
 in the receptors. That it does not reside in the muscles is evident 
 from the fact that the same muscle shows refractory periods of 
 different duration according to the reflex combinations into which 
 it enters. The extensor muscle of the knee, for example, takes 
 part in both the scratch reflex and the extensor thrust; but in the 
 first reflex, its refractory period is only a fifth of a second, while in 
 the extensor thrust this period lasts for a full second. That the 
 refractory phase cannot be attributed to the receptors is evident 
 from the fact that a reflex evoked by stimulating one receptor is 
 refractory, for the usual time, to stimuli applied to any other re- 
 ceptor which normally evokes the same reflex. Accordingly, the 
 refractory period must be considered as pre-eminently a central 
 
 Ehenomenon; and it probably belongs to those central or interpo- 
 ited neurones which have been referred to in previous paragraphs, 
 as taking an important part in the reflex functions of the nervous 
 system. Different central mechanisms have refractory periods of 
 different duration; and the duration is in each case adapted to se- 
 cure the final purpose, or greatest utility, of the particular reflex. 
 The general utility of a refractory phase is clearest in the case of 
 rhythmical or alternating movements, such as scratching, walking, 
 etc. The stimulus is here continuous; but a single prolonged con- 
 traction of the muscles would not be an efficient response. The 
 refractory phase, on the contrary, secures a rhythmical response to 
 a continuous stimulus. 
 
 23. It seems obvious, accordingly, that the "impulses" which 
 pass along the fibres in the nerve-centres are, very often at least, 
 diphasic. An excitatory phase is followed at once by an inhibi- 
 tory phase. In other cases the inhibitory phase precedes, and is 
 followed by a phase of rebound to a condition of heightened ex- 
 citability. In general, the duration of the phases varies in differ- 
 ent reflexes i. e., in different fibres or neurones, or at different 
 synapses. 
 
 We have, therefore, not exhausted our knowledge of reflex action, 
 and of the central mechanisms which control it, until we have taken 
 note of all the facts regarding the time, extent, and intensity of the 
 different reactions. 
 
166 REFLEX FUNCTIONS OF THE NERVOUS SYSTEM 
 
 24. As to the time, the rhythmic character of reflex action has 
 already been sufficiently considered. The duration of the reflex 
 contraction depends to some extent on the duration of the stimulus; 
 but such dependence is fa? less close in the case of reflex action than 
 when a muscle is directly excited by an artificial stimulus, or when 
 it is excited by stimulating artificially its motor nerve. In these 
 cases of direct stimulation, the response lasts as long as the stimu- 
 lus, and stops when the stimulus stops or nearly so subject of 
 course to modification by fatigue. But in reflex action, the move- 
 ment in response has in many cases a fixed duration that is more or 
 less independent of the duration of the stimulus. This is true, for 
 example, of the winking reflex and of the extensor thrust. In the 
 scratch reflex, also, the alternating movement of the leg is likely to 
 stop during the continuation of the stimulus. On the other hand, 
 if the stimulus is brief, but sufficiently intense, the scratching con- 
 siderably outlasts it. Other reflexes, too, outlast a brief stimulus; 
 and, in general, the duration of this "after-discharge" 1 is greater 
 for a strong stimulus than for a weak. 
 
 Between the beginning of the stimulus and the beginning of the 
 muscular response there is always an interval, which is called the 
 latent time of the reflex, or, more briefly, the re-flex time. This is 
 analogous, on the one hand, to the "reaction time" of voluntary 
 movements, and, on the other, to the "latent time" of muscular 
 contraction. When a muscle is directly excited by an artificial 
 stimulus, a brief interval elapses between the stimulus and the com- 
 mencement of visible movement in the muscle. This latent time 
 usually appears as about .01 sec. It is probable that each recep- 
 tor, likewise, has a latent period, i. e., an interval between the ap- 
 plication of the stimulus and the starting of the nerve impulse along 
 the sensory nerve. The latent time is probably different for the 
 different kinds of receptors; but, in general, it is fully as brief as 
 that given above for the muscle. The total Beflex time includes 
 the latent periods of both receptors and muscles; in addition, it 
 includes the time consumed in nerve transmission to and from the 
 centres; and, finally, it includes whatever time is consumed within 
 the gray matter of the centres. It is, therefore, a highly composite 
 affair; but its most interesting and important factor, as bearing 
 upon our knowledge of the reflex functions of the nervous system, 
 concerns the problem of how much of this time is lost, or absorbed, 
 in the nerve-centres. This problem may be approximately solved, 
 in the following way: If we take the speed of nerve conduction 
 to be 30 metres per second, and assume the receptor and muscular 
 latent periods to be each .01 second, then, knowing the length of 
 1 Sherrington, Integrative Action of the Nervous System, p. 26 ff. 
 
LATENT TIME OF NERVOUS REFLEXES 167 
 
 nerve traversed in the reflex arc, we can subtract the time con- 
 sumed in the nerves, and in the receptors and muscles, and have 
 left the so-called "reduced reflex time." 1 Some of the uncertain- 
 ties of this reduction can be avoided by stimulating the sensory 
 nerve close to the spinal cord. The shortest reduced reflex times 
 seem to amount to about .01 second. 
 
 We have already seen, however, that the reflex time is far from 
 constant. It varies, first, with the intensity of the stimulus, becom- 
 ing shorter with increasing stimulus. Thus, the latent time for 
 the flexion reflex of the dog may rise to .20 second with weak stim- 
 uli, and sink to .02 second with strong stimuli; and the time for the 
 scratch reflex varies, similarly, between .14 and .50 second. 2 These 
 figures show that the reflex time varies, not only with the intensity 
 of the stimulus, but also with the character of the particular reflex. 
 The flexion reflex is about the quickest of all; and here the final 
 purpose of protecting the organism applies; for this reflex needs to 
 be prompt, since its normal stimulus is a harmful agent from which 
 the muscular reaction must snatch the limb away. The scratch 
 reflex is much less prompt. The winking reflex is also relatively 
 slow, giving times of about .05 of a second. These differences can 
 not be due to the muscles; and not entirely, at least, to the receptors; 
 they must be attributed, principally, to differences in the character 
 of the different central neurones and of the central connections in- 
 volved. 3 
 
 25. The intensity, or muscular force of a reflex varies in about 
 the same ways as the latent time. In many reflexes, the response 
 increases in force with increasing intensity of the stimulus. But 
 
 1 Exner, Pfluger's Archiv filr die gesammte Physiologic, 1874, VIII, 526. 
 
 2 Sherrington, op. cit., p. 21. 
 
 3 The " knee jerk," or kick of the lower leg aroused by a blow on the patellar 
 tendon just below the knee-cap, has a very brief latent time as short, in some 
 measurements, as .02 second. This extreme shortness leads to a doubt 
 whether this movement is properly a reflex at all; though it is often called 
 the "patellar reflex" and is regarded as a true reflex by many authorities. 
 The difficulty with this view is that the length of nerve which must be trav- 
 ersed, from the knee to the spinal cord, and back to the quadriceps muscle 
 of the thigh, which performs the movement, is so great that, at the accepted 
 rate of nerve transmission, a time of .03 would be consumed in the nerve; and, 
 of course, some time must also be allowed for the latent period of the muscle 
 and of the receptors. These times add up to considerably more than the 
 total latent time of the knee jerk; and this fact has led to the conclusion that 
 this jerk is a direct response of the muscle to the mechanical stimulus. On 
 the contrary, it is found that the knee jerk cannot be got unless the sensory 
 and motor nerves of the muscle are both intact; it is dependent on the exist- 
 ence of reflex "tone" in the muscle; therefore, whether it is itself a reflex or 
 not, it serves admirably as a test of the reflex condition of the muscle, and so 
 of the condition of the cord. 
 
168 REFLEX FUNCTIONS OF THE NERVOUS SYSTEM 
 
 there are exceptions to this rule, in which the reflex shows about 
 the same force whatever the intensity of the stimulus, provided 
 only that the latter is strong enough to arouse any reflex at all. 
 The extensor thrust, for example, is found by Sherrington to show 
 this peculiarity. 
 
 The reflex movement in response to stimulation also differs in 
 strength according to the receptor stimulated. Within the re- 
 ceptive field of a reflex, some points yield a stronger effect than 
 others. To give an example: the pupillary reflex is stronger, i. e., 
 the pupil is more narrowed, when a beam of light falls on the centre 
 of the retina than when it falls toward the side of the retina. A 
 comparison of different reflexes shows that there is little general 
 correspondence between the energy of the stimulus and that of the 
 response. Some reflexes show a powerful response to a weak 
 stimulus, while others give a comparatively feeble response to a 
 strong stimulus. A clear example of the former class is seen in the 
 violent reaction which is made to a tickling stimulus; in this case, 
 slight brushing of the skin evokes a much stronger reaction than 
 does a firm pressure. Or, again, a feeble force exerted on a point 
 of the skin, so as to prick it, gives a strong reaction; while a much 
 greater force exerted on a large area of the skin results in a compara- 
 tively feeble reaction. The same muscle, too, may contract strongly 
 in one reflex and much less strongly in another. From such phe- 
 nomena as these it is safe to conclude. that the central mechanisms 
 of the different reflexes differ in the intensity with which they ex- 
 cite the motor cells of the cord and brain-stem. 
 
 26. The extent of the reflex movement, or the amount of the 
 musculature which it brings into play, differs greatly in different 
 reflexes; since some of them, as, for example, winking or the con- 
 traction of the pupil of the eye, involve only a small mass of muscle, 
 whereas others, as locomotion, bring into play a large share of 
 the whole musculature. In other words, the pathways of some re- 
 flex arcs are much branched and widely distributory the pathways 
 of others much less so. But to speak of this difference tells only 
 half the story. The distribution is not fixed in extent for each re- 
 flex, but, as we have seen, increases with the strength of the stimulus. 
 Even in case of the pupillary reflex, any considerable increase of 
 the intensity of the light entering the eye causes an extension of 
 this reflex beyond the little muscle of the iris, to which it may be 
 confined when the light is of ordinary intensities. A very intense 
 light even causes the eyelids to close, the head to be turned away, 
 the hand to be brought up before the eyes, or a general movement 
 of flight to be begun. Other similar examples can be obtained 
 from the spinal or the decerebrate animal. Pinching the pinna 
 
INTENSITY AND EXTENT OF REFLEXES 169 
 
 of a spinal-cat's ear will, if the stimulus is weak, arouse nothing 
 more than a twitch of the pinna. If the stimulus is increased, the 
 head may be turned to the other side, and the fore limb of the same 
 side brought forward toward the ear. Still stronger stimulation 
 arouses movements of all four limbs. Again, pinching gently the 
 forefoot of a decerebrate cat evokes only the flexion reflex in that 
 limb; but a firmer or more persistent pinch causes the head to turn 
 toward the point stimulated, and, perhaps, the jaw to snap; loco- 
 motion may also be evoked in this way, and strong stimuli may 
 bring out a snarl or whine. Sometimes, as the stimulus is increased, 
 the original local response is abruptly abandoned, and another re- 
 action of a different, more efficient nature is substituted. But, in 
 many cases, the original local reflex is maintained, and other move- 
 ments are added to it. 
 
 The wider distribution of the response with increasing stimulus 
 is often called by such names as "irradiation" or "spread" of re- 
 flexes. Neither of these names, however, is entirely suitable, since 
 both seem to present a picture of a general diffusion of the same 
 effect. The fact is, on the contrary, that the effect is far from be- 
 ing diffused indiscriminately to neighboring regions of the cord, 
 or of the body. It is indeed true that the reflex often spreads to 
 neighboring regions as from the pinna to the neck before it 
 reaches more distant members. But it always spreads only to 
 muscles which give a harmonious total result. Or, more precisely, 
 the stimulus is "selective," and excites only muscles which combine 
 harmoniously; when it spreads to their antagonists, also, its effect is 
 to serve the same purpose of the organism by inhibiting them. Still 
 more precise would probably be the statement that the reflex, in 
 spreading beyond its local area, excites the central mechanisms of 
 other reflexes, which belong more intimately to other stimuli oc- 
 curring in their own locality, but which are allied to the reflex which 
 in the given case is primary. Other reflexes, however, which are 
 antagonistic to the primary reflex of the moment, are not excited 
 in this spread. This "alliance" is, beyond all doubt, pre-eminently 
 a central and not a peripheral phenomenon; it must be an "alli- 
 ance" amongst the central mechanisms of different reflexes; and it 
 therefore depends on the particular distribution of the neurones in 
 the centres. 
 
 These facts, regarding the distribution, convergence, time, and 
 intensity relations of reflex action, show the extreme nicety with 
 which the reflexes are, in general, adapted to their several uses. 
 When taken neurologically, they convey some conception of the 
 complex, co-operative workings of the spinal cord and the other re- 
 flex centres. 
 
170 REFLEX FUNCTIONS OF THE NERVOUS SYSTEM 
 
 27. It remains to take due account of the fact stated near the 
 beginning of this chapter; the fact, namely, that the "simple" 
 or isolated reflex, which has virtually been the theme of the last 
 few pages, is an abstraction. This appears true for the following 
 reasons: first, that more than one reflex, and indeed several reflexes, 
 are simultaneously in progress, interacting as a rule with each other; 
 and, second, that any new stimulus breaks in upon a condition which 
 is not one of rest and neutrality, but upon an existing condition of 
 reflex activity already established. We have, therefore, always to 
 consider how the reflex effect of a given stimulus is modified by 
 the action of other stimuli, both simultaneous and preceding. 
 
 Light has been thrown on this rather complicated problem by 
 the studies of physiologists, among whom Exner 1 and Sherrington 2 
 deserve special mention. The two notions of " facilitation" and 
 " inhibition/' first clearly formulated by Exner, seem to give the key 
 to the true explanation of the phenomena. Exner used the German 
 word, Bahnung; which cannot easily be rendered into English. 
 Its meaning is, however, that one stimulation of any part of the ner- 
 vous mechanism may prepare a path, or break open a way, and so 
 increase the effect of other and subsequent stimulations. The same 
 meaning is fairly well expressed by saying that one stimulus "fa- 
 cilitates" the action of another. "Reinforcement" is a fact closely 
 related to facilitation; and "inhibition" is the opposite of facilita- 
 tion. The application of these conceptions to reflex action will 
 now be illustrated by a few examples. 
 
 Suppose that two stimuli act simultaneously on a spinal animal, 
 and we observe carefully their combined effect. The simplest case is 
 that in which both stimuli are of the same nature, and applied within 
 the receptive field of the same reflex. In this case, each of them, 
 taken by itself, would tend to evoke a merely local response; and 
 this response would spread if the stimulus were made more intense. 
 But taken together, they facilitate or reinforce each other's effect. 
 If then each of them, taken alone, is just too weak to arouse any re- 
 flex whatever, the two acting simultaneously, or nearly so, will call 
 out the customary, normal reflex. And if each of them, taken alone, 
 is of such an intensity as only to evoke a feeble or moderate response, 
 the two acting together reinforce each other's effect, so that the re- 
 sponse becomes relatively strong. This phenomenon can be easi- 
 ly obtained in the case of the scratch reflex. Feeble irritation of 
 two points of the spinal-dog's back may evoke the scratching, though 
 neither of the two irritations does so by itself; and the strength of 
 the scratching movement evoked by moderate irritation at one point 
 
 1 Pfliiger's Archiv fur die gesammte Physiologie, 1882, XXVIII, 487. 
 
 2 Integrative Action of the Nervous System, pp. 114-234. 
 
FACILITATION OF REFLEXES 
 
 171 
 
 40- 
 30- 
 20- 
 10- 
 
 10- 
 80- 
 
 30- 
 
 mnt 
 
 NORMAL 
 
 TIME 
 
 is increased by moderately irritating another point. The nearer 
 together the two points are on the skin, the more powerfully do 
 they act in this co-operative way. 
 
 The more interesting cases of facilitation are those in which the 
 two points of stimulation do not lie in the same receptive field; and, 
 therefore, do not tend to call out the same local reflex. It was 
 noticed above that a reflex, in spreading, takes up local reflexes 
 from other parts and, as it were, incorporates them into itself. For 
 example, pinching 
 either hind leg 
 causes, as its local 
 response, a pulling- 
 up of the same leg; 
 but if this reflex 
 spreads, it results 
 in a similar move- 
 ment in the oppo- 
 site fore limb. This 
 movement of the 
 fore limb is the 
 local flexion reflex 
 to pinching the 
 forefoot; and it 
 can, accordingly, 
 be more readily 
 aroused in this lat- 
 ter way than by 
 stimulating the 
 hind limb. Now 
 if both of these 
 
 points for example, the left forefoot and the right hindfoot are 
 stimulated at the same time, but so feebly that neither stimulus, 
 taken alone, would arouse the movement of the fore limb, this limb 
 may, nevertheless, be made to move through the combined effect of 
 the two stimuli. In general, distant stimuli may facilitate each other's 
 action. The impulses started at two widely separated receptors 
 may converge upon the same central cells, and so produce move- 
 ment of the same muscles. Even an auditory stimulus may facili- 
 tate a reflex in the hind limb. In man, the knee jerk is strengthened 
 by a sudden noise or other stimulus which has the effect of startling 
 the subject of experiment. 1 
 
 Mental states may exert a similar reinforcement on various re- 
 flex functions of the nervous system; and artificial stimuli applied 
 1 See .Lombard, American Journal of Physiology, 1887, I, 1. 
 
 QT02* 0.4* 
 
 10" 
 
 IT' 
 
 FIG. 63. Reinforcement of the Knee Jerk Giving Way to 
 Inhibition. ( Bo wditch and Warren.) Distances along the 
 horizontal line represent the time elapsing between the 
 clenching of the fist and the tap on the tendon which 
 evoked the jerk. Distances above the "normal" line rep- 
 resent the amount by which clenching the fist increased 
 the movement of the foot, and distances below the line 
 represent the amount of decrease. The increase gives 
 way to decrease at about 0.4 sec. 
 
172 REFLEX FUNCTIONS OF THE NERVOUS SYSTEM 
 
 to the cortex of the brain may facilitate the reflex effect of stimula- 
 tion to the skin. 
 
 28. This facilitating effect is, however, customarily one of 
 brief duration; and it probably is subject to the diphasic law which 
 was noted above (compare p. 165 and see Fig. 63). That is to say, 
 the reinforcement soon gives way to its opposite, inhibition, which 
 latter then fades away more gradually. In some experiments on the 
 knee jerk, when reinforced by a vigorous voluntary clenching of the 
 fist, it appeared that the phase of reinforcement yielded to the phase 
 of inhibition, after about half a second, and that no effect of any 
 sort could be discovered after a period much exceeding two seconds. 1 
 
 29. Not every two stimuli, however, facilitate each other's 
 action; but only two stimuli which, when taken singly, lead to the 
 same reaction or to "allied" reactions. If the two stimuli, singly, 
 lead to opposed or inconsistent reactions, the one does not facili- 
 tate the other, but rather tends to inhibit it. For example, the ex- 
 ecution of the ^scratch reflex with one hindleg of the animal is 
 inconsistent with the attempt to execute the same reflex with the 
 other hindleg; because, when one limb is engaged in scratching, 
 the other must be used to support and brace the body. Accord- 
 ingly, if the skin of the back is irritated in two spots at the same 
 time, one on each side of the mid-line, two stimuli are acting simul- 
 taneously, which would tend to call out inconsistent results. One 
 of these stimulations must inhibit the other. 
 
 But it is important to inquire into the exact character of such 
 cases of inhibition. The law of the results obtained from apply- 
 ing two stimulations simultaneously does not follow the parallelo- 
 gram of forces, nor any sort of algebraic addition; neither does it 
 give an average or compromise of the two reflexes. The actual re- 
 sult is that either one leg or the other scratches; that is to say, one 
 stimulus gains the right of way, and the other is excluded. By 
 nicely balancing the stimuli, it may be possible to obtain a mutual 
 inhibition of both reflexes, and it is often possible to delay the onset 
 of either; but, finally, one or the other is pretty sure to break through. 
 In a normal state of the nervous centres, it is probable that the mus- 
 cles are never simultaneously excited for antagonistic reflexes; and 
 this is true, although the sensory stimuli for both are acting at the 
 same time. However mixed the stimuli at any moment and they 
 are often much mixed the reaction itself is never a mixture of 
 opposing movements, but is always a harmonious whole, consisting 
 of allied reflexes, with their antagonists shut out. Nothing could 
 be more essential to well-directed and efficient reaction than this 
 principle. And no more conclusive proof can be demanded of the 
 1 Bowditch and Warren t Journal of Physiology, 1890, XI, 25. 
 
INTERFERENCE OF REFLEXES 173 
 
 selective and purposeful character of the functions of the nervous 
 mechanism at least so far as its normal reflex activities are con- 
 cerned. 
 
 The suppression of any given reflex by an opposing reflex is 
 not, however, the end of the matter. Inhibition, we saw (p. 164), 
 is followed by a rebound effect; and a reflex which has been inhib- 
 ited is, the next moment, specially sensitive to a new stimulus. 
 The scratch reflex, in the experiment mentioned in the last para- 
 graph, begins on one side; but, if the stimuli continue to act, one 
 on each side of the animal, the scratching may shift from one leg to 
 the other. Each of the opposed reflexes has its turn. This is 
 true, at least, when the stimuli are about evenly balanced. And, 
 in general, a reflex which has just been inhibited is for some time 
 afterward much more readily excited. 
 
 30. The situation in the spinal cord and other reflex centres 
 at any time is accordingly one of competition between different 
 stimuli for the control of the muscles. Some of the stimuli are al- 
 lied, in the sense that their action on the muscles would be the same ; 
 but other stimuli are antagonistic. There are not simply two par- 
 ties, since the same muscle may be used in several different ways 
 by as many different reflexes. The scratch reflex of one leg is not 
 only inconsistent with that of the other leg; but it is also incompati- 
 ble with the flexion reflex in either leg, with the extensor thrust, and 
 with the tonic postural reflex which would maintain the standing 
 position. However keen and many-sided the competition for the 
 control of a muscle may be, the control goes, at any moment, to 
 one reflex; and this same reflex and its allies have control of all 
 the muscles, either to excite them or to inhibit their action. "All 
 the muscles" makes the preceding statement pretty strong; it is 
 doubtless true in the case of intense stimuli and widely spreading 
 reflex effects; but in the case of weak stimuli, some reflexes may be 
 so neutral to others that each may go on without interfering with 
 the other. 
 
 In the competition of reflexes, certain kinds usually have more or 
 less advantage over others. As a rule, protective reflexes have the 
 advantage over all others. Postural reflexes, on the other hand, are 
 usually the most at a disadvantage, and are liable to be thrust aside 
 by almost any stimulus which calls for a movement. 
 
 The fatality and predictability of reflex action have sometimes 
 been overstated. In the case of protective reflexes, which have 
 the right of way over anything else in the cord, prediction is easy; 
 but in the case of many others, prediction is by no means sure, be- 
 cause of the diversity of the stimuli which are likely to be acting 
 at the same time. In general, reflex action is much less predictable 
 
174 REFLEX FUNCTIONS OF THE NERVOUS SYSTEM 
 
 than the response of a muscle to direct excitation; and the reflexes 
 obtained from abbreviated nervous centres, such as the "mid- 
 dog" and "hind-dog" mentioned earlier in the chapter, are more 
 predictable than those obtained from an entire and uninjured spinal 
 cord. The rule seems to be that the greater the number of influ- 
 ences to which any organ is exposed, the greater will be the varia- 
 bility of its action, and the less of regularity and fatality will appear 
 in its responses to stimuli. 
 
 31. Many of the characteristics of reflex action such as dis- 
 tribution and convergence, facilitation and inhibition are to be 
 regarded as fundamental properties of the action of nerve-centres, 
 and as applicable to the brain, in all probability, as well as to the 
 cord. There are psychological facts (to be brought forward later) 
 which are closely analogous to these facts of reflex action; and the 
 analogy is of great importance in any attempt to comprehend the 
 action of the brain in its relation to these psychological phenom- 
 ena. The cord offers a simpler field for the unravelling of nervous 
 function, and the results obtained are also of great importance in a 
 physiological psychology. 
 
CHAPTER VIII 
 
 END-ORGANS, OR RECEPTORS, OF THE NERVOUS SYSTEM 
 
 1. In order to understand the end-organs, or receptors, it is 
 necessary to refer again to the place which they hold in the threefold 
 arrangement of the nervous mechanism. In the general division 
 of labor, the function of certain cells situated at the surface of the 
 body becomes that of receiving the action of the stimuli, of modify- 
 ing this action, and thus of setting up in the conducting nerves the 
 neural process which is propagated to the central organs. It is 
 obvious, then, that the structure and grouping of such superficial 
 cells must bear some definite relation both to the external stimulus 
 and also to the nerve-fibres which convey inward the nervous im- 
 pulse occasioned by it. The end-organs of sense may then all be 
 described as special adaptations of the superficial cells to different 
 kinds of stimuli. Even undifferentiated living matter is chiefly 
 sensitive to certain stimuli, such as mechanical jar, heat, chemical 
 agents, and electricity. But in the process of differentiation, cer- 
 tain cells become further specialized in the direction of their sensi- 
 tivity, so that very slight stimuli of a particular sort are capable 
 of arousing them. This increase of sensitivity is "specific" in 
 the sense that each receptor is thus made highly sensitive to one 
 kind of physical agent; whereas it loses rather than gains in its re- 
 ceptiveness toward other agents. The eye, for example or more 
 precisely the retina combines with its very delicate sensitivity 
 to ether vibrations of certain frequencies relative insensitivity to 
 mechanical jar and even to other vibrations of the low frequency 
 which affect the sense of temperature. 
 
 It is such specialization of receptors that gives precision and detail 
 to the deliverances of the senses. If the eye were as sensitive to 
 sound as it is to light, we should see so much that we should get 
 little definite information regarding surrounding objects. That is, 
 there could be no " apperceptive " vision. The particular agent to 
 which a receptor is adapted is called its "adequate stimulus." 
 
 2. In the end-organs of the special senses the fibrils of the 
 sensory nerves, as a rule, terminate in cellular structures which have 
 the morphological significance of metamorphosed epithelial cells. 
 The end-organs of smell show this characteristic development 
 
 175 
 
END-ORGANS OF SMELL 
 
 177 
 
 Most physiologists follow Schultze in holding that the two kinds 
 of cells are distinct both in form and in function, and that only the 
 "olfactory" cells are connected with the end-fibrils of the nerve of 
 smell, the others serving the purpose of supporting the olfactory 
 cells. The internal or proximal process of the olfactory cell is a 
 nerve-fibre, which passes back through the cribriform or sieve-like 
 plate of the ethmoid bone into the brain cavity; and these fibres 
 from the numerous olfactory cells are made up into small bundles 
 which, taken together, are called the olfactory nerve. Arrived 
 within the brain cavity, the fibres penetrate the 
 olfactory bulb, and there end, forming synapses 
 with other fibres which conduct further back 
 into the brain (compare p. 107). 
 
 5. The contrivance for applying the stimulus 
 to the end-organs of smell is very simple; in 
 general it is only necessary that a current of air, 
 in which the stimulating particles float, shall be 
 drawn through the nasal passages over the 
 mucous membrane of the regio olfactoria. Even 
 ammonia and camphor, when placed under the 
 nostrils, have no smell so long as the breath is 
 held or drawn through the mouth. In quiet inr 
 spiration much the greater part of the current 
 of air is conducted to the pharynx directly, and 
 comparatively little reaches the ridge situated 
 above the nasal dam at the back of the nose, 
 where the end-organs of smell are placed. In 
 full inspiration, and still more when short and 
 deep draughts are drawn through the nasal 
 passages, a considerable amount of the air is 
 forced over the sensory parts. By snuffing we 
 increase the amount of air drawn into the region by first creating 
 a partial vacuum in its cavity, and also by creating eddies in the 
 air current, which carry the odoriferous substance out of the main 
 stream and into the olfactory recess. In expiration the breathing 
 passage is so located as to carry nearly all the air past the sensory 
 parts without striking them. For this reason smelling is almost 
 exclusively confined to inspiration; it has been disputed whether the 
 current of expiration can be smelled at all. But Debrou showed 
 that the odor of orange blossoms, when water tinctured with them 
 has been drunk, can be detected in the expired air. The current 
 which passes through the anterior part of the nasal passages seems to 
 be the more important. This is probably the reason why the loss 
 of the nose is so frequently attended with loss of the sense of smell. 
 
 FIG. 64. Olfactory 
 Cells and Epithelial 
 Cells from the Mucous 
 Membrane of the 
 Nose. >%. (After 
 Schultze.) 
 
178 END-ORGANS OF THE NERVOUS SYSTEM 
 
 6. The end-organs of taste, called gustatory bulbs or flasks, 
 or more commonly taste-buds, lie, for the most part, on the upper 
 surface and edges of the tongue; though some occur on the soft 
 palate, and even on the epiglottis and in the larynx. More precisely, 
 they are situated in the papillae or projections which give a rough 
 surface to the tongue. The roughness is, however, chiefly due to 
 the filiform papillae, which do not contain taste-buds; the latter 
 being found in the circumvallate papillae (Fig. 65) at the back of 
 the tongue, and in the fungiform, scattered over its upper surface. 
 The circumvallate, few in number but large in size, are circular 
 
 and surrounded by a trench, in 
 the walls of which are embedded 
 the taste-buds. The fungiform 
 papillae are comparatively small 
 structures; the taste-buds lie in 
 their tops and sides. 
 
 The extent of distribution of 
 the taste-buds, and of the sense 
 
 FIG. 65. Transverse Section through a of taste, On the upper Surface of 
 Papilla Circumvallata of a Calf. Show- ,1 , u* 
 
 ing the arrangement and distribution of the tongue Varies Somewhat in 
 
 the gustatory bulbs. *. (Engeimann). different individuals, and con- 
 siderably as between children 
 
 and adults. In young children all of the upper surface of the 
 tongue has taste-buds, but in adults the front third is free from 
 them, except at the edges; and there is no sense of taste in this 
 part of the tongue. 
 
 7. Taste-buds resemble minute bulbs (less than one-tenth of 
 a millimetre in length) growing in the mucous membrane. In 
 shape they are flask-like, narrowing to a neck and opening to the 
 surface by a little pore (see Fig. 65). They are composed of cells 
 which are arranged like the leaves of a bud in closely compressed 
 rows around the axis. As in the olfactory end-organ, there are here 
 two sorts of cells (Figs. 66 and 67), the supporting and the gustatory. 
 The latter, also called taste-cells, are slender with a central enlarge- 
 ment containing the nucleus, and two processes. One process ex- 
 tends toward the mouth or pore of the taste-bud, and terminates 
 in a short hair-like projection, which enters into the pore. The other 
 process extends away from the pore, and is often branched. 
 
 Sensory nerve-fibres penetrate the taste-bud, and branch among 
 and around the taste-cells. These fibres are derived from three of 
 the cranial nerves. The glosso-pharyngeal supplies the rear of 
 the tongue, and the lingual branch of the trigeminus the front of 
 the tongue; while the vagus supplies the few taste-buds in the phar- 
 ynx and larynx. The course of the fibres from the front of the 
 
END-ORGANS OF TOUCH 
 
 179 
 
 tongue is curiously intricate and apparently varies in different 
 individuals. From the lingual nerve they pass to the chorda 
 tympani, which crosses the cavity of the middle ear; in some cases 
 they enter the medulla by way of the intermediate nerve. 
 
 8. An interesting example of the far-reaching results of that 
 more discriminating and thorough analysis in which modern science 
 delights, is afforded by the case of the so-called Sense of Touch. 
 It was formerly customary to lump together all forms of sensation 
 caused by irritating any area of skin, and to classify them all as 
 one of the five senses with which the human animal was endowed. 
 
 FIG. 66. Isolated Gus- FIG. 67. a, Isolated Gustatory Cells, from the Lateral 
 tatory Bulb, from the Organ of the Rabbit; 6, an Investing and Two Gus- 
 tatory Cells, isolated but still in connection. ^^. 
 (Engelmann.) 
 
 Lateral Gustatory Or- 
 gan of the Rabbit. 
 (Engelmann.) 
 
 It is now recognized, however, that the conscious states which re- 
 sult from applying different stimuli to the superficial area of the 
 body resemble one another scarcely more closely than do sight and 
 hearing, and no more closely than do smell and taste. 
 
 In considering the sensory end-organs in the skin, we must there- 
 fore anticipate what will be said in the chapter on qualities of sen- 
 sation regarding the division of the old "sense of touch" into several 
 senses, namely, those for contact, for temperature, whether warm 
 or cold, and for pain. The problem which psychology proposes 
 to physiology is, accordingly, that of discovering, if possible, the 
 minute structures in the skin which serve as receptors for these 
 different forms of sensation. It may be said at once, however, 
 that this is possible only to a limited extent from present knowl- 
 edge. 
 
 9. The short hairs on hairy surfaces, which comprise ninety- 
 five per cent, of the area of the skin, are to be regarded as specific 
 organs of the tactile sense. About the root of the hair is coiled 
 the termination of a sensory nerve-fibre. The nerve-ending is 
 thus excited by touching the hair, or by touching the skin on the 
 "windward" side of the hair. 
 
180 
 
 END-ORGANS OF THE NERVOUS SYSTEM 
 
 FIG. 68. Corpuscle of Touch 
 from the Palm of the Human 
 
 On hairless surfaces there occurs in 
 great numbers a form of nerve-ending 
 which apparently takes the place of the 
 hair-receptor. This form is called the 
 "touch-corpuscle" (see Fig. 68). These 
 corpuscles occur in some of the papillae 
 of the skin of the palm and sole. They 
 are composed of a capsule of connective 
 tissue and a core of cells among which 
 winds the branched termination of a 
 nerve-fibre. 
 
 These two forms of end-organ may 
 with probability be assigned to the sense 
 of touch proper; but it is not certain 
 that none of the other forms of nerve- 
 ending met with in the skin are con- 
 nected with this sense. 
 
 "End-bulbs" of various shapes also 
 Forefinger. (Ranvier.) n, sen- occur j n the skin, some cylindrical, some 
 
 sory axon ; a, its branching , , . , ' . / . , 
 
 termination within the corpuscle, nearly spherical. 1 heir structure is much 
 
 like that of the touch-corpuscles: a cap- 
 
 sule encloses a core, into which penetrates a sensory nerve-fibre. 
 The nerve-fibre loses its sheaths, and coils and ramifies as a 
 naked axon in the interior of the bulb (see Fig. 69). 
 
 The most highly developed in structure of the cutaneous end- 
 organs are the Pacinian corpuscles (compare 
 Fig. 70). They are larger than the other forms, 
 and their capsule is composed of concentric 
 plates of connective tissue like the layers of an 
 onion. Each Pacinian corpuscle is entered at 
 one end by a nerve-fibre, which loses its myelin 
 sheath in the interior of the corpuscle, and, pass- 
 ing along the axis, terminates near the other end. 
 The Pacinian corpuscles lie just beneath the 
 skin, or deeper in; and also in other situations, 
 as in the muscles, in the neighborhood of 
 tendons, ligaments, and bones, and in the 
 mesentery. 
 
 There is still another mode of nerve-ending on 
 the skin, quite different from the forms already Fig 
 described, in that the nerve-fibre branches freely 
 among the cells of the skin, being unprovided 
 with a capsule (Fig. 71). This form may be its branching termi- 
 
 ,11 i ( ( e jj. rf nation within c, the 
 
 called a "free nerve-ending. capsule. 
 
 _ End . Bulb 
 from the Human 
 
THE PACINIAN CORPUSCLES 
 
 181 
 
 10. As to the specific function of the end-bulbs, Pacinian 
 corpuscles, and free nerve-ends, there is nothing of a conclusive 
 nature to bring forward. The distribution of the various forms 
 over the skin is unequal, and, since the distribution of the four 
 cutaneous senses is also unequal, attempts have been made 1 to 
 determine the function of the end-organs by correlating their dis- 
 tribution with that of the senses. Thus, spherical end-bulbs occur 
 in the conjunctiva of the eye, which, to many observers, lacks tac- 
 tile sensation, though possessing that of 
 temperature and especially that of cold; 
 this form of end-organ may very well, 
 then, be the receptor for cold. On ac- 
 count of the longer reaction-time to 
 warmth than to cold, and of the less 
 sharp localization of the "warmth- 
 spots," it is believed that the end- 
 organs for this sense probably lie in 
 the deeper layers of the skin, and this 
 fact leads to the supposition that a 
 cylindrical form of end-bulb, which 
 occurs deep in the skin, may be the 
 warmth-receptor. 
 
 The pain sense is perhaps served by 
 the free nerve-ends. A fact supporting 
 this view is the absence of other sense 
 qualities from the cornea, the nerve- FIG. 70. Corpuscle of Pacini (or 
 ends in which are of the free-branching 
 
 type. The Pacinian Corpuscles, from 
 
 their location, must serve a subcutane- 
 ous form of sensibility perhaps what is thYnerve-fibre ends. 
 known as "deep sensibility" to pressure. 
 
 Sensory endings of complicated structure are found in muscles 
 and their tendons, and are of interest in connection with the "mus- 
 cle sense." The "muscle spindle" (Fig. 72) occurs embedded in 
 the substance of the muscle, and is composed of several modified 
 muscle fibres bound together by a capsule of connective tissue, and 
 supplied by one or more sensory nerve-fibres, which, losing their 
 myelin sheath, break up into fine branches that coil around the 
 muscle-fibres within the spindle. That these are sense-organs has 
 
 1 Especially by von Frey, Beitrdge zur Sinnesphysiologie der Haut, in Be- 
 richte d. k. sacks. Gesellsch. d. Wissensch. zu Leipzig, math.-phys. Klasse, 1894, 
 1895, 1897; see also later discussions by Sherrington, in Schafer's Textbook of 
 Physiology, 1900, II, 920 ff., and by Thunberg in Nagel's Handbuch der Physi- 
 ologic, 1905, III, 654. 
 
 its sheaths; 6, system of tunics 
 constituting the capsule of the 
 corpuscle; c, axial canal, in which 
 
182 
 
 END-ORGANS OF THE NERVOUS SYSTEM 
 
 been proved by showing that the nerve-fibres which enter them do 
 not degenerate after section of the ventral spinal roots supplying 
 the muscle in question; 1 these nerve-fibres, therefore, come from the 
 dorsal roots and are sensory. Tendon spindles occur in the part 
 of a tendon near to its muscle, and are of very similar structure to 
 the muscle spindles. Other simpler sensory end-organs Pacinian 
 corpuscles and end-bulbs are found in muscles, tendons, the cap- 
 sules of joints, the periosteum and interior of bones; and these all 
 may be connected with that complex of sensory apparatus which 
 goes by the inexact name of "muscle-sense." 
 
 11. With the exception perhaps of the ear, the eye is by far 
 the most elaborate and complicated of the end-organs of sense. 
 This is true of those portions of it which are designed merely to 
 
 \~? 
 
 Fig. 71. Free Branching Sensory Axons from the Larynx. (Retzius.) 
 n, axons. 
 
 bring the external stimulus to bear upon the nervous structure, as 
 well as of this structure itself. Considering it as a whole, we 
 may say that the peripheral organ of sensations of light and color 
 is an optical instrument constructed on the plan of a water camera 
 obscura, with a self-adjusting lens, and a concave, sensitive, nervous 
 membrane, as a screen on which the image is formed. 
 
 12. The eyeball consists of three coats or tunics enclosing three 
 translucent refracting media. Since, however, the front part of 
 the outer one of these coats is itself translucent and refracting, 
 the number of refracting media in the eye is really four. (l)The 
 first or external coat consists of two parts: (a) the Sclerotic or 
 posterior five-sixths part ("white of the eye"), which is a firm, 
 fibrous membrane formed of connective tissue intermingled with 
 elastic fibres; and (6) the Cornea, or translucent anterior one-sixth 
 part, which is circular and convex in form, and covered with con- 
 
 1 Sherrington, Journal of Physiology, 1894, XVII, 211 ff. Sensory endings 
 are found also in the mucous membrane and quite widely distributed through 
 the viscera. 
 
STRUCTURE OF THE EYEBALL 
 
 183 
 
 J'unctival epithelium. The. cornea rises and bulges in the middle 
 ike a watch-glass. (2) The second coat, or tunic of the eye, also 
 consists of two parts: these are (a) the Choroid coat, which com- 
 prises much its larger portion, is of a dark brown color, due to its 
 pigment cells (except in the case of 
 albinos), and is abundantly provided 
 with nerves and blood-vessels; and 
 (6) the Iris, a circular, flattened, 
 disk-shaped diaphragm in front of 
 the lens (the colored part of the 
 visible eyeball), bathed with aqueous 
 humor, and having in its centre a 
 circular aperture called the " pupil" 
 of the eye. The anterior border 
 (corpus ciliare) around the iris con- 
 sists of the ciliary muscle and the 
 ciliary processes. (3) The Retina is 
 the third or inner coat of the eye. 
 It is a delicate membrane of exqui- 
 site transparency and almost perfect 
 optical homogeneity; it has a highly 
 complex structure, consisting of nine 
 or ten layers, the truly nervous por- 
 tions of which contain nerve-fibres, 
 nerve-cells, and special end-organs, , 
 together with supporting tissue and 
 blood-vessels. The inner surface of 
 the retina is moulded on the vitreous 
 body, and it extends from the en- 
 trance of the optic nerve nearly as 
 far forward as the ciliary processes. 
 13. The eyeball has four trans- 
 lucent refracting media. The first of 
 these enumerating inward from the 
 outside front is (1) the Cornea, Fig. 72. A Muscle Spindle. (Ciaccio.) 
 already spoken of as the anterior 
 
 one-sixth of the outer coat of the eye. (2) The Aqueous Humor 
 fills the space between the cornea and the lens, and is divided by 
 the iris into two chambers, of which the front one is much the 
 larger. It is limpid and watery; it holds in solution the salts of 
 the blood-serum, with traces of organic substances. (3) The Crys- 
 talline Lens is situated between the iris and the vitreous body. It is 
 a transparent biconvex lens, with its antero-posterior diameter about 
 one-third less than the transverse diameter. It consists of a capsule 
 
 rfnc 
 
184 
 
 END-ORGANS OF THE NERVOUS SYSTEM 
 
 and enclosed body. It is of "buttery consistency," composed, like 
 an onion, of a number of easily separable layers. Each layer consists 
 of fibres, which within the layer are, as a rule, radial. Between 
 the entire ciliary part of the retina and the corresponding part of 
 the vitreous humor is interposed a structureless membranous body, 
 
 FIG. 73. Horizontal Section through the Left Eye. 
 from Gegenbaur.) 
 
 (Schematic, 
 
 to which the edge of the lens is attached, and which radiates out- 
 ward and maintains the lens in tension. It is called the suspen- 
 sory ligament (or Zonida of Zinn), and its office is very important 
 in the accommodating of the eye to different distances. (4) The 
 Vitreous Humor consists of a number of firm sheets or layers 
 (lamellae), between which fluid is contained, built into a body that 
 is, optically considered, transparent and homogeneous. It occupies 
 
MUSCLES OF THE EYEBALL 
 
 185 
 
 most of the space enclosed by the tunics of the eye. It is thought 
 to be a gelatinous form of connective tissue, and is composed most- 
 ly of water with salts in solution, of proteids and mucin, fats and 
 extractive matters especially urea. Its peculiar structure is of 
 little significance for the physiology of the eye. 
 
 14. Of the appendages or accessory parts of the eye such as 
 the eyebrows, the eyelids, lachrymal apparatus, muscles of the 
 eyeball only the mechanism by which the eye is moved in its 
 
 fr 
 
 o.r 
 
 Fia. 74. Muscles of the Left Hu- 
 man Eye, seen from above, rs, 
 rectus superior; re, rectus ex- 
 ternus; and rit, rectus internus; 
 os, superior oblique, with its 
 tendon, t, which runs through the 
 membranous pulley, u, at the 
 inner wall of the cavity of the 
 eyeball. 
 
 FIG. 75. Muscles of the Left Human Eye, 
 seen from the outside. Ir, levator of the up- 
 per eyelid, which covers the rectus superior, 
 rs, re, os, as in the preceding figure; rif, 
 rectus inferior; oi, inferior oblique. 
 
 orbit has any special significance for physiological psychology. 
 The building-up of a world of visible objects, and even the forma- 
 tion of a so-called "field of vision," is dependent upon the great 
 mobility of the eye. The eyeball is moved in its bony socket, 
 where it is embedded in a mass of fat as in a socket-joint, by six 
 muscles, which are attached to it somewhat like the bridle to the 
 horse's head. Four of these muscles spring from the bony wall 
 near the point where the optic nerve enters, extend through the 
 length of the socket, and pass directly to the eyeball, where they 
 are attached to it, one above, one below, one on the outer, and one 
 on the inner side (the recti internus and ext-ernus, superior and infe- 
 rior). In moving both eyes up or down, the same muscles in both 
 contract simultaneously; in moving the eyes to the right, the outer 
 muscle of the right eye and the inner of the left contract simul- 
 taneously (and vice versa) ; in turning both eyes inward to converge 
 them upon a near object, the two inner muscles contract together. 
 
tK, 
 
 ** 
 
 186 END-ORGANS OF THE NERVOUS SYSTEM 
 
 The other two of the six muscles of the eye are called oblique. Of 
 these one is superior and internal; it does not pass directly forward 
 from its place of origin, at the posterior aperture through which 
 the optic nerve enters to the eye, but first runs through a ring, then 
 turns around, and is attached obliquely to the upper surface of the 
 eyeball. The other oblique muscle begins at the inner wall in the 
 socket, passes under the eyeball, and is attached to it opposite to 
 the superior oblique muscle. The two oblique muscles combine 
 with the four recti to move the eyes in various directions which 
 would be impossible for the latter alone. 
 
 15. The problem which is to be solved by the end-organ of 
 vision may be stated in a general form as follows: A mosaic of 
 localized sensations must be so constructed that changes in the 
 quantity, quality, local relation, and sequence of these sensations 
 shall be quickly interpreted as indicative of the color, shade, size, 
 shape, locality, and motion of external visible objects. The most 
 important part of the solution of this problem falls upon the nervous 
 structure of the retina. It is itself a mosaic of nervous elements, the 
 excitation of which may vary in quality, quantity, local coloring, 
 and sequence of the different elements excited. But in order that 
 the retina may exercise its function with the precision and delicacy 
 of detail for which its structure fits it, the rays of light reflected 
 from a single point of the surface of the visible object must excite a 
 single one, or at most a small and definite group, of the retinal 
 nervous elements. The sensations thus occasioned can then un- 
 
 rgo a systematic arrangement by the mind. It is the work of 
 the translucent refracting media of the eye to apply the stimulus to 
 retinal elements exactly discriminated, and in an order correspond- 
 ing to the object; that is to say, the cornea, the humors of the eye, 
 and the lens must form an image on the retina. 
 
 16. Light entering the eye passes successively through the cor- 
 nea, the aqueous humor, the lens, and the vitreous humor. At 
 each of the surfaces between these media the light suffers refrac- 
 tion. Since the surfaces at which the refraction occurs are approxi- 
 mately spherical, and the centres of the spherical surfaces all lie in 
 one straight line the so-called "optic axis" the eye is, optically 
 considered, a centred system. An important result of this arrange- 
 ment is that it avoids distortion in the image. For example, the pen- 
 cils of rays issuing from the various points of any plane perpendicular 
 to the axis will be finally focussed in points which lie in a plane hav- 
 ing the same direction;. and the image formed of these latter points 
 will, therefore, be in approximately true proportion to the seen object. 
 
 To determine the refractive power of the eye, we need to know 
 (1) the index of refraction of each of the media through which the 
 
INDICES OF REFRACTION 187 
 
 light passes, (2) the radius of curvature of each of the surfaces at 
 which refraction occurs, and (3) the distance apart of these sur- 
 faces. 
 
 The indices of refraction which have been worked out by vari- 
 ous ingenious methods are in round numbers the following: for 
 aqueous and vitreous humors, 1.34, or very nearly the same as for 
 water; for the cornea, 1.38; for the lens, increasing from 1.39 in the 
 outer layers to 1.41 at the centre. Since the curvature of the layers 
 of the lens becomes sharper from the outside inward, the refract- 
 ive power of the lens is increased; taken as a whole, it is equiva- 
 lent to a lens of the same size and shape with an index of about 
 1.44. But since the cornea is thin and differs but little, in its 
 index of refraction, from the adjacent aqueous humor, it may be 
 considered as a part of the latter. The only surfaces at which in- 
 coming rays are effectively bent are, therefore, the outer surface of 
 the cornea, and the front and rear surfaces of the lens. The radii 
 of these surfaces are as follows: of the cornea, about 8 millimetres; 
 front of lens, about 10 millimetres; back of lens, about 6 milli- 
 metres. It should be understood, however, that these measure- 
 ments differ considerably in different individuals. 
 
 The distance from the corneal surface to the front of the lens 
 is about 3.6 millimetres, and the thickness of the lens about the 
 same. The data are thus provided for calculating the strength of 
 the eye as an optical instrument, and the calculation gives it a 
 strength of about 67 diopters. Its power is such that parallel 
 rays rays from a distant object entering the eye are brought to a 
 focus at a distance of about 20 millimetres behind the cornea 
 a distance which in the normal eye corresponds to the actual lo- 
 cation of the retina. 1 If, however, the axis of the eye, from the 
 cornea to the retina, is too long for the refractive power, the image 
 of distant objects is formed in front of the retina, and only near 
 objects can be clearly seen (near-sightedness or myopia); whereas 
 if the axis of the eyeball is too short, the image of distant objects 
 will be formed behind the retina, and the refractive power of the 
 eye must be increased to permit of clear vision (hypermetropia). If, 
 as is common, the surface of the cornea is not truly spherical, the 
 rays of light are brought to a line rather than a point on the retina 
 (astigmatism). 
 
 17. The preceding remarks apply to what is called the "rest- 
 ing eye." The eye, when its internal mechanism is at rest, is fo- 
 cussed on distance, and does not form clear images of near objects. 
 
 1 These measurements and calculations are from several investigators, and 
 are here cited after Schenck, in NagePs Handbuch der Physiologie, 1905, III, 
 38 ff. 
 
188 END-ORGANS OF THE NERVOUS SYSTEM 
 
 To obtain clear vision for near objects, the optical power of the eye 
 must be increased. The process by which this is accomplished is 
 called "accommodation." The mechanism of accommodation 
 differs in different orders of animals; 1 in amphibia and reptiles 
 the lens is moved forward as in the camera. In mammals this is 
 not the case, but the power of the eye is increased by increasing 
 the convexity of the lens; its front surface bulges forward, becoming 
 more curved, while its rear surface remains in position. 
 
 There are several methods of experiment which demonstrate 
 that in accommodation for near distances the front of the lens be- 
 comes more strongly arched. When accommodation is taking 
 place, the pupil may be seen not only to contract, but also to draw 
 its edge forward. Helmholtz calculated the amount of this forward 
 movement for two cases at about -^ and -fa of an inch, respectively. 
 Moreover, by an ingenious contrivance the image reflected from the 
 anterior surface of the lens may be watched as it becomes smaller 
 and more distinct on adjustment for near distances, thus showing 
 that the surface from which it is reflected has increased its curva- 
 ture. 
 
 It is obvious that the mechanism for adjusting the eye must be 
 under the brain's control, since adjustment is voluntary; and that 
 it must consist of muscles which lie within the eyeball. The ac- 
 cepted hypothesis concerning the nature and action of this mechan- 
 ism was first proposed by Helmholtz. This investigator assumes 
 that the lens, when the eye is at rest, does not have the form which 
 corresponds to a condition of equilibrium in its own elastic power. 
 If it were not held in by its surroundings, it would be more arched 
 than it is both before and behind. But it is kept flattened by the 
 radial tension of the suspensory ligament; when this tension is with- 
 drawn the lens becomes curved by the action of its own elasticity. 
 The withdrawal of the tension is accomplished by the action of the 
 ciliary muscle, the fibres of which have their point of fixation at the 
 edge of the cornea, and run from here in the direction of a merid- 
 ian toward the equator of the eye. When the ciliary muscle con- 
 tracts, the free ends of its fibres are drawn toward its fixed ends 
 on the edge of the cornea; the radial tension of the suspensory lig- 
 ament is thus relaxed, and the lens is allowed to assume its natural 
 form under the equipoise of its own elastic forces (Fig. 76). 
 
 The iris of the eye corresponds to the diaphragm of the camera. 
 It contains two sets of contractile fibres, one circular, the effect of 
 which is to diminish the size of the pupil; the other radial and en- 
 larging the pupil. Contraction of the pupil occurs under two con- 
 ditions: When bright light (or, more exactly, light brighter than that 
 1 Beer, Wiener klin. Wochenschrift, 1898, XI, 942. 
 
MECHANISM OF ACCOMMODATION 
 
 189 
 
 to which the retina is at the time adapted) enters the eye; and when 
 the eye is accommodated for a near object. In both cases the effect 
 of narrowing the pupil is to increase the clearness of vision. A sud- 
 den bright light has a dazzling effect, and this is lessened by di- 
 minishing the quantity of light entering the eye. The divergent 
 rays from a near object are not so well focussed as are the more 
 nearly parallel rays, on account of spherical aberration; but this bad 
 effect is diminished by cutting off the more oblique rays which 
 enter through the periphery of the cornea and lens. 
 
 The nerves which supply the muscles of the eye are the third, 
 fourth, and sixth of the cranial nerves, and the sympathetic. Of 
 these, the third or oculomotor is the largest; in it are contained the 
 fibres which supply 
 the ciliary muscle, 
 and those which 
 supply the circular 
 muscle of the iris. 
 The fibres which 
 supply the radial 
 muscle of the iris 
 are from the sym- 
 pathetic, and arise Fig. 76. The Change of the Lens in Accommodation, 
 from a ppntrp in thp (Helmholtz.) The left half of the figure shows the lens 
 focussed on a distant object, the right half on a very near 
 Cervical COrd. The object, c, the ciliary muscle; s, the suspensory ligament. 
 
 fibres of the oculo- 
 motor nerve originate in the mid-brain, in the floor of the third 
 ventricle. Convergence, accommodation, and constriction of the 
 pupil can all and severally be excited by stimulating special por- 
 tions of this region. The pupillary reflex to light has as its sensory 
 nerve the optic; the fibres concerned are those which pass to the 
 anterior quadrigemina, whence, apparently, other fibres pass to the 
 nucleus of the oculomotor nerve and so excite the motor fibres to 
 the iris. 
 
 18. Given the formation of the image upon the retina, it is fur- 
 ther required in order to vision that this physical process shall 
 be changed into a physiological process. We now examine briefly 
 the mechanism by which such a change is accomplished. The 
 retina, or inner tunic of the eye, contains the nervous elements by 
 whose action the system of refracted rays is changed into a mosaic 
 of nerve commotions. But light does not act as a stimulus to the 
 nervous substance, either fibres or cells, unless it have an intensity 
 which is nearly deadly to that substance. Since we are able to 
 see the feeblest rays of the moon as reflected from white paper, the 
 nervous excitation which is the condition of vision cannot be pro- 
 
190 
 
 END-ORGANS OF THE NERVOUS SYSTEM 
 
 duced by the direct action of light on the nerve-fibres or nerve-cells 
 of the eye. A photo-chemical substance and process, as well as 
 a special end-apparatus, seems therefore to be necessarily involved 
 in the problem which is given to the retina to solve. 
 
 19. The nervous and other elements of the retina are arranged 
 
 OUTER SURFACE. 
 
 10 .... Layer of pigment cells. 
 
 Layer of rods and cones. 
 
 , Mernbrana limitans externa. 
 
 7 .... Outer nuclear layer. 
 
 .... Outer molecular layer. 
 
 .... Inner nuclear layer. 
 
 .... Inner molecular layer. 
 
 .... Layer of nerve-cells. 
 
 .... Layer of nerve-fibres. 
 
 . . Mernbrana limitans interna. 
 
 INNER SURFACE. 
 
 FIG. 77. Diagrammatic Section of the Human Retina. 
 
 (Schultze.) 
 
 (see Figs. 77 and 78) in the following ten layers, counting from within 
 outward and backward: (1) the membrana limitans interna, which is 
 the retinal border toward the vitreous body; (2) the layer of optic 
 nerve-fibres distributed from the papilla where this nerve breaks in 
 through the tunics of the eye; (3) the ganglion cell-layer; (4) the 
 inner molecular layer; (5) the inner nuclear layer; (6) the outer 
 molecular layer; (7) the outer nuclear layer; (8) the membrana limi- 
 
NERVOUS ELEMENTS OF THE RETINA 
 
 191 
 
 tans externa; (9) the bacillary layer, or 10 
 
 layer of rods and cones; (10) the pigment- 
 epithelium layer. The membranes (Nos. 
 (1) and (8) ) are not really uninterrupted 
 layers, but an extremely fine network. 
 
 By no means all the retinal substance 
 is nervous. Indeed, the numerous radial 
 fibres (fibres of Midler) which seem to 
 penetrate its entire thickness are now held 
 to be in great part elements of the support- 
 ing tissue; moreover, the whole connective 
 substance is a kind of sponge-like tissue, 
 in the gaps of which the true nervous ele- 
 ments lie embedded. The gaps thus filled 
 are especially large in the second, third, 
 fifth, and seventh layers. 
 
 The principal nervous elements of the 
 retina can be arranged in three parallel 
 sets, and may, therefore, be spoken of as 
 lying in three layers, each of which in- 
 cludes more than one of the layers just 
 mentioned. These three sets of nerve-cells 
 f are (1) the ganglion or optic nerve-cells, (2) 
 the bipolar cells, and (3) the rod and cone 
 cells. The fibres of the optic nerve arise 
 in the main from the ganglion cells; they 
 are, indeed, the axons of these cells, and 
 have no medullary sheaths in their course 
 over the retina to the point of exit of the 
 nerve. They converge from all parts of 
 the retina to the nerve, thus forming the 
 second of the ten layers mentioned above. 
 Their arrangement is special at the yellow 
 spot, so as to surround and not cover it. 
 The dendrites of the ganglion cells branch 
 in the fourth layer, the "molecular" ap- 
 pearance of which, in cross section, is due 
 to the cut ends of numerous fine dendritic 
 branches. This molecular layer may also 
 be called a synapse layer, since it contains 
 the synapses between the ganglion cells and 
 
 the bipolar Cells. The Cell-bodies of the FIG. 78. Diagrammatic Repre- 
 
 latter lie in the fifth or inner nuclear layer; ^N^|^^ 
 their short axons branch and terminate Retina. (Schuitze.) The 
 among the dendrites of the ganglion cells. SS^ta'Sfi*^ ref ' 
 
 B 
 
192 
 
 END-ORGANS OF THE NERVOUS SYSTEM 
 
 The dendrites of the bipolar cells extend toward the layer of rods 
 and cones, branching in the outer molecular layer, which is, again, 
 a synapse layer between the bipolar cells and the rods and cones. 
 The cell-bodies of the rod-and-cone-cells lie in the outer nuclear 
 layer; their axons run inward to form synapses with the dendrites 
 
 FIG. 79. Rods and Cones of the Human Retina. 
 (Schultze.) A, showing inner segments of the 
 rods, s s s, and of the cones, z z\ the latter in 
 connection with the cone-nuclei and fibres as far 
 as the outer molecular layer. & -\ Q -. B, inner 
 segment of a cone with a cone-nucleus. 
 C, isolated interior portion of a cone. 
 
 FIG. 80. Rod and Cone 
 from the Human Reti- 
 na, preserved in peros- 
 mic acid, showing the 
 fine fibres of the sur- 
 face and the different 
 lengths of the inter- 
 nal segment. JL ^-^. 
 (Schultze.) The outer 
 segment of the cone is 
 broken into disks which 
 are still adherent. 
 
 of the bipolar cells, while what correspond to their dendrites are 
 the specialized branches called rods and cones. These branches 
 are the real receptors of the retina; it is they which are sensi- 
 tive to light; and the nerve impulses set up in them by the action 
 of light pass from them to the bipolar cells; from the bipolar to the 
 ganglion cells, and so, by the axons of the latter cells to the optic 
 nerve and its terminations in the brain (compare p. 91). 
 
NERVOUS ELEMENTS OF THE RETINA 193 
 
 The researches of Cajal and other recent histologists have un- 
 earthed a wealth of further facts regarding the internal structure 
 of the retina, to only a few of which brief reference can now be 
 made. It is found that some of the fibres of the optic nerve orig- 
 inate within the brain, and terminate in the retina; these probably 
 conduct outward from the brain, but their exact function is not 
 clearly made out. There are also cells in the retina which do not 
 conduct in the direction of the rod-and-cone-bipolar-ganglion cells, 
 but spread out horizontally, as if to associate cells of the same layer. 
 Among the bipolar cells, two classes are recognized, linked respec- 
 tively to the rods and to the cones. Several rods are linked to the 
 same bipolar cell; whereas it appears that each cone may have an 
 individual bipolar cell connected with it. 
 
 20. To this description of the minute nervous elements of the 
 retina, a brief notice of some of the more distinctively physical 
 characteristics of certain of its parts may now be added. As its 
 very name suggests: The layer of rods and cones (No. 9) consists of 
 a multitude of elongated bodies arranged side by side, like rows of 
 palisades, with their largest extension in the radial direction. These 
 bodies are of two kinds one cylindrical, and called "rods of the 
 retina," the other rather flask-shaped, and called "cones of the 
 retina" (compare Figs. 79 and 80). The^rods extend the entire 
 thickness of the layer, and are about -^ inch in length, but the cones 
 are" shorter; the roctslire about 1 4 1 o inch in diameter, the smallest 
 cones of the central depression Triinr inch. Each rod or cone is 
 composed of an inner and an outer segment or limb; the latter is 
 highly refractile, the former only feebly so. The inner limbs ap- 
 pear under the microscope like a mass of protoplasm. 
 
 In general, the rods are more numerous than the cones. The 
 distribution of the two~elements~is different for different parts of 
 the retina. In the yellow spot only cones appear, but these are of 
 more slender form, and of increased length, so that not less than one 
 million are supposed to be set in a square T V inch; while not far from 
 this spot each cone is surrounded by a crown-shaped border of 
 rods. Toward the ora serrata the cones become continually rarer. 
 In close connection with the rods and cones stand the cells of the 
 pigment-epithelium. These cells form a regular mosaic of flat, 
 six-sided cells, which send out pigmented processes between the 
 outer limbs of the rods and cones (see Figs. 81 and 82). 
 
 21. Two minute portions of the inner surface of the retina re- 
 quire to be distinguished from the rest of its area: the yellow spot 
 (macula lutea) and the "blind spot" (papilla opticd). The yellow 
 spot is of oval shape, about ^ of an inch in its long diameter, and 
 has in the centre a depression called the fovea centralis. It is the 
 
194 END-ORGANS OF THE NERVOUS SYSTEM 
 
 place of clearest vision, and the physiological centre of the eye. 
 About J of an inch inside the eye, or 15, from the middle of the 
 yellow spot, is the middle of the papilla, or place where the optic 
 nerve breaks into the retina (compare Fig. 83). The blind spot, 
 
 or portion of the retina which 
 can be experimentally shown 
 to be inoperative in vision, 
 has been proved by Helm- 
 holtz to correspond in both 
 size and shape to that covered 
 by this papilla. Its diameter 
 is about ^ or T V of an inch, 
 
 FIGS. 81 and 82. Superficial Aspect of the Ar- . .1 U1 ,.- 
 
 rangement of the Rods and Cones in the Ret- Varying considerably for dlf- 
 
 ina. *. (Schultze.) The former is from ferent eyes. It is Wanting in 
 
 the region of the macula lutea; the latter 11,1 i 
 
 from the peripheral region. all the nervous elements, ex- 
 
 cept, of course, thenerve-fibres. 
 
 22. In answer to the question, What elements of the retina are 
 directly affected by the light ? both anatomy and physiology refer to 
 the layer of rods and cones. This layer alone possesses that mosaic 
 nervous structure which appears to correspond to the demands 
 made upon the end-apparatus of 
 vision. It can be demonstrated that 
 the waves of light pass through the 
 structure of the retina, and that 
 the nervous process must begin in 
 the back part of this structure. In- 
 deed, it is possible, by an experi- 
 ment (devised by Purkinje), to per- 
 ceive with one's own retina the 
 figure formed by the shadow of the 
 blood-vessels expanded upon its 
 front part. 
 
 23. Certain changes which oc- 
 cur in the retina under the stimulus 
 
 of light are of interest from their FIG. 83. Equatorial Section of the Right 
 
 S?i i ,1 IP Eye, showing the Papilla of the optic 
 
 possible bearing On the mode of ne j rvet the Blood-vessels radiating 
 
 this organ's action. Electrical cur- f" 1 / 1 ' a d the Macula lutea. |. 
 
 & . , 1111 (Henle.) S, sclerotic; Ch, choroid; 
 
 rents can be detected, both when and R, retina. 
 
 the eye is kept in the dark and 
 
 when it is suddenly exposed to light; these currents are analogous, 
 
 in a general way, to the currents which appear in nerves and 
 
 in muscles; they show certain peculiarities, but not much use can 
 
 as yet be made of them for an explanation of the function of the 
 
 retina. 
 
CHEMICAL CHANGES IN THE RETINA 195 
 
 Of the chemical changes in the retina, the most striking is con- 
 nected with the famous visual purple. When an eye has been kept 
 for some minutes in the dark, the outer limbs of the rods become 
 tinged with purple or rose-color. This color is quickly bleached 
 by exposure to light, and reappears again in the dark. The bleach- 
 ing effect of different parts of the spectrum is of very different 
 strength, being greatest for a yellowish green. Along with the 
 changes in the visual purple go movements of the cones and of the 
 pigment cells of the tenth layer. The reaction to light consists in 
 a shortening of the cones by drawing back toward the membrana 
 limitans externa; and, on the part of the pigment cells, by a pushing 
 forward of their processes between the rods. When exposed to the 
 dark, the processes of the pigment cells are withdrawn, and the 
 cones extend backward. 
 
 It has long appeared probable that the first effect of light on the 
 retina must be of a chemical character, and therefore the discovery 
 of the visual purple, and of its reaction to light, was hailed as open- 
 ing the way to a more complete understanding of retinal function. 1 
 It soon appeared, however, that the visual purple was not essential 
 to sight. Kiihne showed that a frog, with the purple bleached by 
 exposure to light, still saw. It is now clear that the changes pro- 
 duced by light in the visual purple are not the photo-chemical proc- 
 esses which were looked for as the intermediary between light and 
 nervous activity. At the same time, it is not impossible to see a 
 relation between the visual purple and retinal function. One of 
 the most remarkable facts in the physiology of the retina is its power 
 of adaptation to different degrees of illumination. Every one is 
 familiar with the fact, that on passing from light to dark the eye 
 seems at first nearly blind, but soon becomes "used to the dark," or 
 dark-adapted. Also, in passing from dark to light, the first effect 
 is that of being dazzled; but soon the eye grows used to the light, 
 or light-adapted. Adaptation to the dark goes on rapidly within 
 the first ten minutes after passing into the dark, and thereafter more 
 slowly for half an hour or more. Adaptation to the light occurs 
 much more swiftly. These times of adaptation correspond rather 
 closely with the times for bleaching and regeneration of the visual 
 purple. It is probable, from this and other correspondences, that 
 the changes in the color of the rods have something to do with the 
 processes of adaptation to light and dark. 
 
 The fact that the centre of clear vision, corresponding to the fovea, 
 is blind in dim light, and does not become adapted to it, combined 
 with the fact that the fovea contains only cones, in connection with 
 
 1 The important names in connection with this discovery are those of Boll 
 and Kuhne. 
 
196 END-ORGANS OF THE NERVOUS SYSTEM 
 
 other similar facts, has led to the view that the cones are insensitive 
 to very faint light, and have less power of dark-adaptation than the 
 rods. The further fact that clear vision, and vision for colors, 
 diminish from the fovea toward the periphery of the retina along 
 with the diminution of cones has led to the view that color vision, 
 and distinctness of vision, are properties of the cones. There is 
 thus some evidence of a difference in function between the rods and 
 the cones. 1 
 
 In very dim light, all colored objects appear gray, and the rela- 
 tive brightness of different colors is much different from what it is 
 under good illumination. Thus, a red and a green which appear 
 equally bright in good light are seen in very dim light as if the green 
 were much brighter than the red (the Purkinje phenomenon). 
 The point of greatest brightness in the spectrum, which is in the 
 yellow under good illumination, moves into the green in very dim 
 light. These changes do not occur in central or foveal vision. 
 
 24. In a word to repeat our summary: The human eye is a 
 camera, which if one were advertising it might be described as 
 "a wonderfully compact little instrument, capable of being focussed 
 on any distance from five inches upward, provided with the only 
 original iris diaphragm, and having the special feature of a self- 
 renewing plate, which automatically alters its sensitivity to suit 
 the illumination, and also gives colored photographs. The camera 
 cannot, however, be guaranteed, as some specimens are defective, 
 and even the best are liable to be injured by hard usage; none will 
 be replaced, though some of the defects can be partially corrected." 
 
 25. The end-organ of hearing is the Ear. But in this case, as 
 in that of the eye, a very large part of the apparatus of sense is sig- 
 nificant simply as a contrivance for applying the stimulus to the 
 true end-organ, to the differentiations of epithelial cells and nervous 
 cells connected with the terminal fibrils of the sensory nerve. The 
 entire human ear consists of three parts, or ears: namely, the ex- 
 ternal ear, the middle ear, or tympanum, and the inner ear, which 
 is also called the "labyrinth/' from its complex construction. 
 
 I. The External Ear exclusive of the cartilaginous plate which 
 is extended from the side of the head consists of (a) the concha, a 
 deep hollow, and (b) the external meatus, or passage leading from 
 the bottom of this hollow to the drum of the ear. The concha is 
 probably of only slight service in sharpening and defining our per- 
 ceptions of sound. Its position, however, favors the reception of 
 sound-waves which come from in front rather than behind; it may, 
 
 1 The principal champion of this "duplex theory" of vision is von Kries. 
 See his papers in Zeitschrift f. PsychoL, IX, 82; XII, 1; XIII, 242; and his 
 article in Nagel's Handbuch der Physiologic, 1905, III, 184. 
 
THE EXTERNAL EAR 
 
 197 
 
 therefore, be of some service in enabling us to determine more read- 
 ily and accurately the direction from which the sound-waves come. 
 The most patent office of the external meatus is the protection 
 of the ear-drum; the passage is so curved that the drum cannot be 
 reached from the outside in a straight line. Helmholtz called at- 
 tention to the fact that certain tones of a high pitch resound strong- 
 ly in the ear when the meatus is of normal length, but cease so to 
 resound when its length is increased artificially. The meatus 
 
 Liy. mallei 
 
 8Uf. 
 
 Processtis 
 
 /of. 
 
 Insert io muse, 
 tens. 
 
 FIG. 84. Drum of the Right Ear with 
 the Hammer, seen from the inside, 
 f . (Henle.) 1, chorda tympani; 2, 
 Eustachian tube; *, tendon of the 
 tensor tympani muscle cut off close 
 to its insertion; ma, anterior ligament 
 of the malleus; Mcp, its head; and 
 Ml, its long process. Stp, Spina 
 tympanica posterior. 
 
 Chorda 
 
 FIG. 85. Side Wall of the Cavity of 
 the Tympanum, with the Hammer 
 (M) and the Anvil (J). The former 
 shows the connection of its handle 
 with the drum. T, Eustachian tube, 
 f. (Gegenbaur.) 
 
 probably, therefore, modifies certain tones by its own resonant 
 action strengthening the high ones, and deadening the low, in 
 some degree. 
 
 Simple experiments such as placing a resounding body in con- 
 tact with the teeth prove that the surrounding cranial bones con- 
 duct sound to the ear. Various paths for this conduction, both 
 direct by way of the cranial and petrous bones to the inner ear, 
 and indirect by way of the ear-drum and bones of the middle ear 
 to the fenestra ovalis, are theoretically possible. But the amount of 
 conduction to be assigned to each is difficult of precise determina- 
 tion. 
 
 26. II. The Middle Ear, or Tympanum (Figs. 84 and 85), is a 
 chamber irregularly cuboidal in form, and situated in the temporal 
 bone, between the bottom of the meatus and the inner ear. Its 
 outer wall is (a) the membrana tympani, which consists of three lay- 
 ers an external tegumentary, an internal mucous, and the inter- 
 
* 
 
 198 
 
 END-ORGANS OF THE NERVOUS SYSTEM 
 
 Hep 
 
 mediate membrana propria, composed of unyielding fibres arranged 
 both radially and circularly. In the inner wall, which separates 
 the tympanum from the labyrinth, are two openings or windows 
 the fenestra ovalis, which corresponds to the vestibule of the laby- 
 rinth, and the fenestra rotunda, which corresponds to the tympanic 
 passage in the cochlea. Near its anterior part the tympanum opens 
 
 into (b) the Eustachian tube, a canal 
 which communicates with the nasal 
 compartment of the pharynx. 
 
 (c) The auditory bones are three 
 in number, called Malleus, Incus, and 
 Stapes, and arranged so as to form 
 an irregular chain stretched across 
 the cavity from the outer to the 
 inner wall of the tympanum (see Fig. 
 86). The malleus has a head, sepa- 
 rated by a constricted neck from an 
 elongated handle; its handle is con- 
 nected with the centre of the mem- 
 brana tympani; its head articulates 
 with the incus. The incus has a 
 body and two processes. On the 
 front surface of the body is a saddle- 
 
 film 
 
 FIQ. 86. Bones of the Ear, as seen 
 in their connection from in front. 
 f. (Henle.) /, Incus (anvil), of 
 which 76 is the short, and II the 
 long, process; c, its body, and pi, 
 the process for articulation with the 
 
 stapes (processus orbicuiaris) M, shaped hollow, in which the head of 
 
 Malleus (hammer), of which Me is . r ,, n * i 
 
 the malleus fits; the short process is 
 bound by a ligament to the posterior 
 wall of the 
 
 the neck, Mcp the head, Ml the long 
 process, and Mm the manubrium; 
 S, stapes (stirrup), with its capitu- 
 lum, cp. 
 
 ie tympanum; the long proc- 
 ess ends in a rounded projection (os 
 
 orbiculare) through which it articulates with the stapes. The 
 stapes, or stirrup-shaped bone, has a head and neck, a base and 
 two crura. The head articulates with the incus; from the con- 
 stricted neck the two crura curve inward to the base, which is 
 attached to the fenestra ovalis. These bones are moved on each 
 other at their joints by (d) two small muscles the tensor tympani 
 and the stapedius. The first of these is inserted into the malleus, 
 near the root, and serves to tighten the tympanic membrane by 
 drawing the handle of the malleus inward; the stapedius is inserted 
 into the neck of the stapes, and draws the stapes from the fenestra 
 ovalis, thus diminishing the pressure of the chain of bones and 
 lessening the tension of the tympanic membrane; it therefore acts 
 as the antagonist of the tensor tympani. 
 
 27. The general office of the tympanum may be described as 
 that of transmitting the acoustic waves to the inner ear, while at the 
 same time modifying their character. Some modification is neces- 
 
OFFICE OF THE TYMPANUM 199 
 
 sary in order that these waves may occasion such vibrations in the 
 elements of the inner ear as shall be adapted for the excitation of 
 its end-organs. The acoustic motion of the molecules of air, in the 
 form in which it reaches the ear-drum, has a large amplitude, but 
 a small degree of intensity. This motion must be changed into one 
 of smaller amplitude and greater intensity; and it must be trans- 
 mitted, with as little loss as possible, to the fluids of the labyrinth. 
 The transmitting vibrating media must also have the power of an- 
 swering to the different tones of any pitch perceptible by the ear. 
 The description of the manner in which this apparatus of membrane 
 and bones solves so complicated a mechanical problem belongs to 
 the physics of anatomy; it has been worked out with great detail 
 by Helmholtz and others, although certain questions still remain un- 
 solved. We can here only indicate one or two particulars. 
 
 A flat membrane, evenly stretched, whose mass is small in pro- 
 portion to the size of its superficies, is easily thrown into vibration 
 by the impact of acoustic waves upon one of its sides. Such a 
 membrane responds readily to tones which approach its own funda- 
 mental tone; but if divergent tones are sounded the membrane is 
 unaffected. A motion which consists of a series of harmonious 
 partial tones cannot then be repeated by such a membrane in the 
 form in which the air brings it. If, then, the membrane of the 
 tympanum were not so arranged and connected as to have no pre- 
 ponderating tone of its own, it could not be the medium of our 
 hearing a great variety of tones. The property of taking up the 
 vibrations of a large scale of tones is secured for the tympanum by 
 its funnel-shaped form and by its being loaded. It is contracted 
 inward into a depression of the right shape by means of the handle 
 of the hammer; it is therefore unequally and only slightly stretched, 
 and has no fundamental tone. It is also loaded with the audi- 
 tory bones, which deprive it of every trace of such a tone and act as 
 dampers to prevent long-continued vibrating. Moreover, since the 
 apex of its funnel bulges inward, the force of the vibrations from all 
 sides is concentrated in vibrations of greater intensity in the centre, 
 where it is spent in setting the chain of ear-bones in motion. 
 
 The acoustic vibrations of the auditory bones, which are occa- 
 sioned by the movements of the ear-drum, are not longitudinal, 
 but transverse; they do not, however, resemble the vibrations of a 
 stretched cord or a fixed pin. They do not vibrate by reason of their 
 elasticity, but like very light small levers vibrating as a system, 
 with a simultaneous motion around a common axis. Direct obser- 
 vation of these bones in motion shows that their sympathetic vibra- 
 tions vary greatly for tones of different pitch and similar intensity, 
 from a scarcely observable motion to a surprisingly great elongation. 
 
200 END-ORGANS OF THE NERVOUS SYSTEM 
 
 The effect of the muscles of the tympanum upon the transmis- 
 sion of tones of different pitch is not quite clearly 'demonstrated. 
 In general, the stretching of the tensor muscle, within the limits 
 which have thus far been investigated, seems to weaken the higher 
 much less than the lower tones. But the tension of the drum un- 
 der the influence of this muscle does not indicate the slightest 
 change on passing from low to high tones. The tensor tympani 
 can, therefore, scarcely be regarded as the mechanism which has 
 complete control of accommodation to pitch. On the other hand, 
 its reflex contractions are most easily excited by tones of high pitch. 
 The resulting favoring of high tones, and corresponding dampening of 
 simultaneously sounding low tones, would seem thus to be of assist- 
 ance in picking out a high tone from a mixture of sounds of differing 
 pitch. The stretching of the tendon of the stapedius muscle has 
 no observable influence on the acoustic vibrations of the tympanum. 
 
 28. The Eustachian Tube, when in its normal position, is 
 neither closely shut nor wide open. Its office is to effect a renewal of 
 the air in the tympanum, to maintain the equilibrium of atmos- 
 pheric pressure on both sides of the tympanic membrane, and to 
 convey away the fluids which collect in the tympanic cavity. If it 
 remained open, so as to permit the acoustic waves of the air from 
 the mouth to enter, our own voices would be heard as a roaring 
 sound, and the passage of air inward and outward during respira- 
 tion would affect the position and tension of the tympanic mem- 
 brane. That it is opened, however, on swallowing, Valsalva proved 
 two centuries ago. For if we keep the nose and mouth closed and 
 then swallow, with the cheeks blown violently out, a feeling of 
 pressure is felt in the ears and the hearing is weakened. These ef- 
 fects are due to the forcing of the air through the Eustachian tube 
 into the tympanic cavity. The tube is thus of indirect service in 
 respect to the physiological functions of the middle ear. 
 
 29. III. The Internal Ear, or Labyrinth, is the complex organ 
 in which the terminal fibrils of the auditory nerve are distributed 
 and the end-organs of hearing situated. The so-called "bony 
 labyrinth" is a series of cavities channelled out of the petrous bone. 
 It consists of three parts the Vestibule, the Semicircular Canals, 
 and the Cochlea. In each osseous part a membranous part is sus- 
 pended, corresponding to it in shape, but filling only a small portion 
 of the bony cavity which contains it. It is in the labyrinth that the 
 acoustic waves transmitted by the tympanum are analyzed and 
 changed from a physical molecular process to a nerve-commotion, 
 by the special end-apparatus of hearing (see Fig. 87). 
 
 (A) The Vestibule is the central cavity of the internal ear; it is 
 the part of the labyrinth which appears first in animals and is most 
 
STRUCTURE OF THE INTERNAL EAR 201 
 
 constant. The membranous vestibule is composed of two sac-like 
 dilatations the upper and larger of which is named utriculus, the 
 lower sacculus. In the outer wall of the vestibule is the fenestra 
 ovalis; its anterior wall communicates with the scala vestibuli of 
 the cochlea, and at its posterior wall the fine orifices of (B) the Semi- 
 circular Canals open into the utriculus. These canals are three in 
 number, are bent so as to form nearly two-thirds of a circle, and are 
 about an inch in length and -fa of an inch in diameter. They are 
 called the superior, the posterior or vertical, and the external or 
 horizontal canals. The contiguous ends of the superior and pos- 
 
 FIG. 87. No. 1, Cast of the Osseous Labyrinth of the Left Ear, from below; No. 2, of 
 the Right Ear, from the inside; No. 3, of the Left Ear, from above. (Henle.) Av, 
 aqueduct of vestibule; Fc, fossa of the cochlea; Fee, its fenestra (rotunda); Fv, fe- 
 nestra of the vestibule (ovalis); ha, external ampulla; h, external semicircular canal; 
 Tsf, tractus spiralis foram^nos^ls; vaa, ampulla of the superior semicircular canal; vc, 
 posterior semicircular canal; and vpa, its ampulla. 
 
 terior canals blend together and have a common orifice into the 
 vestibule. They all have a regular relative position, their planes 
 being nearly at right angles to each other. Near the vestibule they 
 dilate to about twice their average diameter and form the so-called 
 ampulla. Both the osseous vestibule and the osseous canals con- 
 tain a fluid (the perilymph) in which the membranous vestibule 
 and canals are suspended; the membranous labyrinth is also dis- 
 tended with a similar fluid (the endolymph). 
 
 (C) The Cochlea is by far the most complex part of the laby- 
 rinth, and, according to present evidence, the only part directly con- 
 cerned with auditory sensations and perceptions (compare Figs. 
 88, 89, and 90). It is about of an inch long, and is shaped like 
 the shell of a common snail. It, too, consists of a membranous sac 
 embedded in an osseous cavity. The whole passage of the cochlea is 
 imperfectly divided into two canals by a partition-wall of bone, which 
 is wound 2-J- times around an axis (the modiolus), from the base to the 
 apex, somewhat like a spiral stair-case. It is called the osseous lamina 
 spiralis. Of the two canals or passages thus formed, the one which 
 faces the base of the cochlea is called the scala tympani; since it has 
 
202 
 
 END-ORGANS OF THE NERVOUS SYSTEM 
 
 its origin in the circular aperture (fenestra rotunda) which leads to 
 the tympanic cavity. The other, which faces toward the apex, 
 opens into the vestibule, and is called the scala vestibuli. At the 
 apex of the cochlea these two scalse communicate with each other 
 through a small hole (helicotrema). The division of the membranous 
 cochlea is completed by a membrane (the basilar membrane, or 
 membranous spiral lamina), which bridges the interval between the 
 free edge of the osseous spiral lamina and the outer wall of the pas- 
 
 FIG. 88. Osseous Cochlea of the Right 
 Ear, exposed from in front, -f. 
 (Henle.) t, section of the division- 
 wall of the cochlea; ft, upper end of 
 the same. Fee, fenestra; H, ham- 
 ulus; Md, modiolus; Ls, lamina 
 spiral is. 
 
 FIG. 89. Cross Section through the 
 Acoustic Nerve and the Cochlea, f . 
 (Henle.) Nc, nerve of the cochlea; 
 Nv, nerve of the vestibule; St, scala 
 tympani; Sv, scala vestibuli; and 
 between them the ductus cochlearis, 
 DC. Ls and Md, as in preceding 
 figure. 
 
 sage; it is attached to this wall by the spiral ligament. Another mem- 
 brane (the membrane of Reissner) arises from a spiral crest (lim- 
 bus, or crista spiralis) attached to the free edge of the osseous 
 lamina, and extends to the spiral ligament, so as to form a small 
 aqueduct between it and the basilar membrane (the scala inter- 
 media, or ductus cochlearis, or canal of the cochlea). It is in the 
 vestibule, in the ampullae of the canals, and in the scala intermedia 
 that the nervous end-organs of hearing are to be found. 
 
 30. The auditory nerve, on approaching the labyrinth, divides 
 into a vestibular and a cochlear branch (compare Fig. 89). As 
 stated in another chapter (see p. 82), these two parts of the eighth 
 nerve separate also at their entrance to the medulla and pass to 
 different portions of the central organs. They serve two senses, 
 
DISTRIBUTION OF AUDITORY NERVE 
 
 203 
 
 Liyamentum 
 spirale 
 
 the end-organs of both of which are located in the inner ear. The 
 vestibular branch or nerve enters the vestibule, and divides into five 
 branches, one for the utriculus, one for the sacculus, and one for 
 each of the three ampullae. In each of these dilatations the mem- 
 branous wall forms a thickened projection, which is called the crista 
 in the ampulla and the macula in the utricle and saccule. The 
 characteristic feature of each of these is the presence of epithelial 
 cells provided with tufts of fine hairs. The hairs, instead of pro- 
 jecting directly into the endolymph, are embedded (as discovered by 
 Retzius) in a soft gelatin- 
 ous or mucous-like mass. 
 In the ampullae this mass 
 is dome-shaped, and the 
 hairs are of considerable 
 (microscopic) length. In 
 the utricle and saccule, the 
 hairs are shorter, and the 
 soft mass in which they 
 are embedded is flatter, 
 but is principally remark- 
 able for containing little 
 particles of carbonate of 
 lime (limestone), which are 
 called the otoliths, or "ear- 
 stones." The base of the 
 hair-cells, in the ampullae, utricle, and saccule, are embraced by the 
 terminal ramifications of the fibres of the vestibular nerve; there is 
 no doubt, accordingly, that the hair-cells are sensory cells (compare 
 Fig. 91). 
 
 31. The terminal nerve-apparatus which constitutes the spe- 
 cial end-organ of hearing is noteworthy for its exceedingly compli- 
 cated structure and striking appearance. The cochlear branch of 
 the auditory nerve pierces the axis of the cochlea and gives off 
 laterals which enter the canals of the osseous spiral membrane. 
 Here they radiate to the membranous spiral lamina, and are con- 
 nected with a ganglion which contains the cell-bodies of the fibres 
 of the cochlear nerve. Beyond this ganglion, they form a plexiform 
 expansion, from which the delicate fibrils losing their medullary 
 sheath and becoming extremely fine axis-cylinders pass through a 
 gap in the edge of the lamina into the organ of Corti, where they 
 terminate around the base of certain hair-cells, soon to be described 
 (compare Fig. 92). 
 
 It is the so-called "organ of Corti," however, in which the in- 
 genuity of the natural processes engaged in constructing the mech- 
 
 Lamina 
 
 basilaris 
 
 FIG. 90. Section through one of the Coils of the 
 Cochlea. . (Schematic, from Gegenbaur.) 
 
204 
 
 END-ORGANS OF THE NERVOUS SYSTEM 
 
 anisms of the human body would seem to have reached the 
 ultimatum of endeavor to create an elaborate and effective, but 
 mystifying, structure for the conversion of physical stimuli into 
 nervous impulses. A study of the accompanying figures, together 
 with the following description of some of its more obvious features, 
 will suffice the purposes of our treatise. 
 
 It will be noticed that the organ of Corti is a wonderful arrange- 
 ment of -cells, some of which are elongated and curved, and are 
 gathered into two groups that may be designated, respectively, 
 as an inner and an outer. Since the cells of the inner group pro- 
 
 Fig. 91. Nerve Endings in an Ampulla. (Retzius.) n, sensory 
 axons; h, hair-cells. 
 
 ject forward and outward, while those of the outer group incline 
 forward and inward, the two form a sort of bow which arches over 
 an exceedingly minute channel (the canal of Corti) between them 
 and the membrane on which they both rest. This membrane 
 itself is composed of fibres arranged in a transverse direction, and 
 in such manner that a single rod-like cell rests upon one or two of 
 these fibres. 
 
 Internal, and almost parallel to the inner one of the groups just 
 described, is a row of columnar cells with short and stiff hair-like 
 processes (inner hair-cells). External and almost parallel to the 
 outer group are four or five rows of hair-cells (outer hair-cells) 
 which are attached to the basilar membrane, while their other ex- 
 tremity projects as a brush of hairs through the reticular membrane 
 (membrane of Kolliker). This latter membrane is a very delicate 
 
THE ORGAN OF CORTI 
 
 205 
 
 framework, perforated with holes, through which the hairs of the 
 outer hair-cells project, and which extends from the inner rods to 
 the external row of hair-cells. It acts as a support for the ends of 
 these cells. The interval between the outer hair-cells and the spiral 
 ligament is occupied by cells of a columnar form (the supporting 
 cells of Hensen). The organ of Cord is covered over and separated 
 from the endolymph of the ductus cochlearis by the so-caljed mem- 
 brana tectoria. 
 
 32. The problem before the labyrinth of the ear is in part the 
 same as that solved by the tympanum, namely, the problem of con- 
 veying the acoustic waves to the true end-apparatus of hearing. 
 
 Tectorial Inner Outer 
 
 membrane hair-cells hair-cells 
 
 Nerve-fibres Inner rod Outer rod Basilar membrane 
 
 Fig. 92. The Organ of Corti. (Retzius.) 
 
 The repeated shocks of the stirrup at the fenestra ovalis -and per- 
 haps, in far less degree, the pulsations of air at the fenestra ro- 
 tunda produce waves in the fluid of the labyrinth. Any molec- 
 ular oscillations of this fluid, thus occasioned, cannot, however, 
 act directly as the appropriate stimulus of the sensations of sound. 
 Since the dimensions of the whole mass thrown into vibration are 
 so small in comparison with the length of the acoustic waves that 
 the extension of the shock from the stirrup would be practically 
 instantaneous throughout, and since the surrounding walls may be 
 regarded as absolutely immovable by any such impact, the laby- 
 rinth-water would act as an incompressible fluid. It would, there- 
 fore, be unsuitable for the transmission of various kinds of acoustic 
 waves. But different parts of the labyrinth are capable of yielding 
 to the waves in the fluid caused by the repeated shocks of the 
 stirrup. This is especially true of the membrane of the fenestra 
 rotunda which is left free to bulge out into the tympanic cavity. 
 Waves started by impact at the fenestra ovalis would pass into the 
 scala vestibuli of the cochlea; and from there it is probable 
 through the membrane of Reissner, the fluid in the cochlear due- 
 
206 END-ORGANS OF THE NERVOUS SYSTEM 
 
 tus, and the basilar membrane, into the air of the middle ear. In 
 their passage through the basilar membrane, they would, of course, 
 cause it to vibrate and so excite the sensory hair-cells which rest 
 on this membrane. 
 
 33. We may be fairly certain that the process by which vi- 
 brations excite the end-organ of hearing is as just described 
 namely, in brief, that vibrations of the fluid in the cochlea excite 
 the hairs of the hair-cells. But a more difficult problem is that of 
 explaining the great variety of responses which the ear makes to 
 different stimuli, and which we know by means of the many dis- 
 tinguishable noises, tones, and tone-combinations. Since we dis- 
 tinguish a great number of pitches, there must be some special re- 
 action of the ear corresponding to each of these tones. One of 
 the most important facts to bear in mind in framing a theory of the 
 action of the cochlea is the analytic power of the ear. The vi- 
 brations which reach the ear from the air, when a chord of several 
 tones, or even when a single tone, is sounded with its accompanying 
 overtones, are highly complex; but a "trained ear" can analyze the 
 complex into the separate tones of which it is composed. The 
 "training" occurs, no doubt, in the brain and not in the ear; but 
 the brain could not distinguish the components unless the ear had 
 first broken up the complex vibration into its elements, and sent 
 to the brain an impulse corresponding to each. 
 
 A theory which accounts in an elegant manner for this analytic 
 power of the ear, and for its power to respond differently to tones 
 of differing pitch, is the resonance or sympathetic vibration theory 
 of Helmholtz. 1 What is meant by "sympathetic" vibration is 
 illustrated when a piano string, for instance, which is tuned to 
 vibrate at a certain rate, takes up vibrations of this rate from the 
 air, and is itself set in vibration by them. If the dampers are lifted 
 from the strings of a piano, and a particular tone be sung into it, 
 it answers with the same tone; if two or more tones are simulta- 
 neously sung into it, it answers with the same combination of tones, 
 the strings tuned to these tones having been set into sympathetic 
 vibration. Helmholtz conceived that the transverse fibres of the 
 basilar membrane might be likened to the strings of a piano, and 
 since they are of different length, might be tuned to tones of dif- 
 ferent pitch, and vibrate sympathetically, each to its own vibration 
 rate. The vibration of any fibre of the basilar membrane would 
 naturally excite the hair-cells in its immediate neighborhood; 
 different hair-cells would thus be set in vibration for different pitches, 
 and -different combinations of them for different combinations of 
 
 1 See his Sensations of Tone, 1895, pp. 145 ff. 
 
THEORY OF SYMPATHETIC VIBRATION 207 
 
 tones; and thus the sense for pitch and the analytic power of the 
 organ would find their explanation. 
 
 The theory of sympathetic vibration has been worked out with 
 great thoroughness to cover the various peculiarities of auditory 
 sensation, and in general has been found to be adequate. It is not, 
 however, altogether free from internal difficulties, the chief of which 
 lies in the extreme minuteness and small range in length of the 
 fibres which are supposed to be tuned to the various audible pitches. 
 The longest of them is but half a millimetre in length, and it is 
 difficult to conceive that a fibre of such length, even though loaded 
 with its segment of the organ of Corti, and even though suspended, 
 not in air, but in a liquid, could be tuned so low as the lowest string 
 of the piano. Moreover, though the number of fibres in the basilar 
 membrane about 24,000 is adequate for the range of audible 
 pitch and the number of discriminable pitches which a trained ear 
 can detect, the difference in length of these fibres is only about as 
 one to twelve, much less than would be required for tuning to the 
 range of audible pitch unless, indeed, we can conceive of these 
 fibres as being under different tension or differently loaded. M. 
 Meyer 1 has called attention to a physiological difficulty in the way 
 of accepting the suggestion that the fibres are under different ten- 
 sion: when a living tissue is subjected to a continued tension, it 
 yields or accommodates itself, so that the tension is relieved; and 
 there is no evidence that this rule is broken in the case of the 
 basilar membrane. Hardesty, 2 on minute anatomical examination, 
 finds that the basilar membrane contains, not only the radial fibres 
 which the Helmholtz theory likens to the strings of a harp, but also 
 numerous other fibres running athwart these and binding them 
 so tightly together that it seems a physical impossibility that they 
 could vibrate singly or in small groups, as required by the theory. 
 
 Such difficulties have caused dissatisfaction, in the minds of 
 many students of acoustics, with the Helmholtz theory, and several 
 other theories have been put forward, but the working of them out 
 in detail, to explain, as well as the Helmholtz theory does, the vari- 
 ous facts of hearing, has not yet been accomplished. The most 
 prominent of the opposing theories is that of Ewald. 3 By experi- 
 menting with a little model of the basilar membrane, he finds that 
 even so minute a membrane as this is set into vibration of a fixed 
 form, giving a fixed vibration figure, by the action of vibrations of 
 any given rate; and that the figure varies with the rate; and, further, 
 
 1 University of Missouri Studies, 1907, II, 1. 
 
 2 American Journal of Anatomy. 1908, VIII, 109. 
 
 3 Pftiiger's Archiv /. d. gesammte Physiologic, 1899, LXXVI, 147, and 1903, 
 XCIII, 485. 
 
208 END-ORGANS OF THE NERVOUS SYSTEM 
 
 that the combined action of two vibration rates is such as to in- 
 duce in the membrane a compound vibration-figure, in which, how- 
 ever, the component figures are not obliterated. As applied to the 
 cochlea, these % facts would indicate that vibrations of any given 
 rate, acting on the basilar membrane, would cause it to vibrate in 
 certain places not simply in one place as according to the Helm- 
 holtz theory and that therefore corresponding parts of the sensory 
 apparatus would be excited by each vibration rate. Apparently a 
 combination of nerve-fibres, from different parts of the cochlea, 
 would be excited by even a single tone; and thus the nervous result 
 would be less simple and easy to conceive than according to the 
 sympathetic vibration theory. But Ewald has made it probable 
 that the basilar membrane does vibrate in the manner supposed by 
 his theory. 
 
 With regard to these and all similar physiological theories, how- 
 ever, it must be remembered that we are not in search of a nervous 
 apparatus which can be listened to by the brain, or by the soul in 
 the brain, as we listen, so to say, to the tones produced by the 
 vibrations of the strings of a piano. What we are seeking, the 
 rather, is some sufficient account for a series of more or less 
 complex nervous changes which can be adequately correlated 
 with the varieties of elements into which we can analyze our sense 
 experience. 
 
 34. From the fact that the utricle, saccule, and semicircular 
 canals are parts of the inner ear, it was natural to suppose for them 
 an auditory function; and they have often been regarded as connected 
 with the reception of noises and with the perception of the direction 
 from which sound comes. There is, however, no positive evidence 
 of an auditory function for these structures, while there is positive 
 evidence of a function of another sort. Experimental knowledge 
 of this, their true function, began with Flourens, who applied, about 
 1825, the method of extirpation to the canals of pigeons, and found 
 the result to consist in certain disturbances of movement. Since 
 then numerous results of the same tenor have accumulated, and the 
 technique of stimulating and extirpating these minute structures 
 has been perfected, till, in the hands of Ewald, 1 very delicate opera- 
 tions with precise results have been achieved. 
 
 The position of the canals should first be noted. As has already 
 been said, the three canals of one labyrinth lie in three planes 
 nearly at right angles to each other; and the planes of the canals of 
 the two labyrinths are related to each other, so that the two hori- 
 
 1 Physiologische Untersuchungen uber das Endorgan des Nervus Octavus 
 (Wiesbaden, 1892). See also Kreidl, in Asher and Spiro's Ergebnisse der Physi- 
 ologic, 1905, V, 572, and Nagel, Handbuch der Physiologic, 1905, III, 778. 
 
OFFICE OF SEMICIRCULAR CANALS 
 
 209 
 
 zontal canals are parallel, and the superior of either side is parallel 
 with the posterior of the other side. The planes of the canals are 
 not, however, the three primary planes of the head, but lie at angles 
 of about 45 to these (compare Fig. 93). 
 
 The three planes at right angles remind one of the planes of 
 reference in co-ordinate geometry; and this peculiarity of arrange- 
 ment has suggested to some 
 authors the theory that the 
 canals are concerned with 
 the perception of the direc- 
 tion of sound; and to one 
 author the still vaguer 
 theory that they y furnish 
 sensations of three-dimen- 
 sional space. 1 To under- 
 stand the real function of 
 the canals, it is necessary 
 first to have in mind the 
 class of reflexes which are 
 called compensatory move- 
 ments. If a frog is placed 
 on a board, and the board 
 is so tilted as to lower the 
 frog's head, he responds by 
 raising his head; and simi- 
 larly, he lowers his head if 
 the board is so tilted as to 
 raise it, and turns his head 
 to the right if the board is 
 so rotated as to turn it to 
 the left, etc. These reac- 
 tions "compensate for," or 
 
 correct, the movement impressed on him. Similar movements 
 are found through a wide range of animals, including fishes, 
 birds, and mammals. Along with compensatory movements of 
 the head go compensatory rotations of the eyes, and movements 
 of the body as a whole. These reflexes are of great importance in 
 maintaining the position and equilibrium of the body, and in keeping 
 it to a straight line in locomotion. Now one marked result of the 
 extirpation of the semicircular canals is the loss of compensatory 
 movements; and another closely related result is the loss of the 
 "tonus" of many of the muscles, and the consequent inefficiency 
 
 1 Von Cyon, Pfliiger's Archiv /. d. gesammte Physiologie, 1900, LXIX, 211. 
 
 Fig. 93. The Position of the Semicircular Canals in 
 the Head. (Ewald.) 
 
210 END-ORGANS OF THE NERVOUS SYSTEM 
 
 of many movements. Locomotion becomes uncertain, and, if the 
 destruction of the canals is unilateral or otherwise partial, "forced 
 movements," such as running in a circle, or rolling to one side, 
 appear. These forced movements can be explained as the result 
 of the loss of some of the compensatory movements, along with 
 retention of the rest. Thus, if every incipient movement to the right 
 calls up a compensatory movement, while movements to the left 
 call up none, the result must be a constant turning to the left. On 
 stimulation of a canal, as with a current of electricity, movements 
 occur which are apparently identical with the compensatory move- 
 ments. Stimulation of the ampulla of any single canal arouses a 
 movement in the plane of that canal. Stimulation, at once, of 
 two canals which lie at right angles to each other arouses a move- 
 ment in a plane between those of the stimulated canals. 
 
 Such facts as these indicate the use of the canals, but do not also 
 show how they are normally excited. The now generally accepted 
 theory of these organs dates from about 1873-75, when it was put 
 forth independently by Mach, Breuer, and Crum Brown. It is 
 based partly on physical considerations. Each canal opens at 
 both ends into the utricle, and may be considered as a circular pipe, 
 at one point of which (the ampulla) a sensory end-organ projects 
 into the pipe. Rotation of the head in the plane of any canal must 
 therefore cause, by inertia of the contained endolymph, a back- 
 flow in the canal. On account of the very small calibre of the pipe, 
 and the consequent friction against the wall, the back-flow of en- 
 dolymph would certainly be very slight, but inertia must have some 
 such effect. A back-flow through the ampulla would act on the 
 dome-like mass in which the hairs of the sensory cells are embedded, 
 and thus bend the hairs and no doubt excite the cells. Even a ro- 
 tation not in the exact plane of a canal would cause some back- 
 flow, unless the rotation were in a plane at right angles to that of 
 the canal; and the more nearly the plane of rotation approached 
 that of the canal the greater would be the effect within that canal. 
 Given, therefore, three canals at right angles to one another, and 
 no rotation of the head can occur without exciting currents in at 
 least one canal; and no two rotations, in different directions, can 
 excite exactly the same direction and proportion of back-flow in 
 the three canals. Thus this physical theory explains the facts made 
 evident by compensatory movements: first, that the canals are some- 
 how stimulated by head rotations, and second, that they are differ- 
 ently stimulated by rotations in different directions. 
 
 More direct evidence is at hand in support of the theory; for 
 Ewald has been able to produce artificial currents of the endolymph 
 in a canal, and the canal is thereby excited, as is shown by the oc- 
 
OFFICE OF SEMICIRCULAR CANALS 211 
 
 currence of a reflex movement. Moreover, this movement is in 
 the same direction as the current in the canal ; and this is as it should 
 be to agree with the theory; for both the back-flow and the com- 
 pensatory movement are opposed to the impressed rotation, and 
 therefore act in the same direction, one with the other. 
 
 35. Regarding the utricle and saccule, physical considerations 
 again suggest a similar theory (Breuer). Since the gelatinous mass 
 in which the hairs of the sensory cells are here embedded is weighted 
 with the otoliths, it would tend to sag downwards, and would pull 
 differently on the hairs according to the position of the head with 
 reference to gravity. Probably we have here a sense-organ for 
 indicating the position of the head; this would explain certain 
 compensatory positions, which disappear on destruction of the laby- 
 rinth. Not only gravity, but any rectilinear acceleration would be 
 expected to act on the otoliths; and there is some evidence that this 
 is the case. 
 
 36. It should be noted that it is acceleration and retardation, 
 or change in movement rather than movement as such, which would 
 be expected to excite the canals. The inertia current would gradu- 
 ally cease, as the fluid became carried along by friction with the 
 walls of the canal, till finally the fluid would move with the head. 
 If then the rotation of the head should cease, inertia would tend to 
 cause a continued movement of the fluid, and the effect would be 
 the same as if rotation were begun, from rest, in the opposite 
 direction. This is what actually happens in dizziness, and the facts 
 of dizziness lend further support to the theory. Many deaf per- 
 sons those, probably, whose disease extends to the semicircular 
 canals are not made dizzy by rotation. 1 The swimming sensa- 
 tion of the head which accompanies dizziness is probably to be 
 ascribed to the semicircular canals, and the somewhat similar sen- 
 sations which occur on starting up or down in an elevator are prob- 
 ably connected with the utricle and saccule. With milder stimu- 
 lation of these organs, the sensations are not obtrusive, but the 
 power of perceiving changes in the speed or direction of rotation 
 is very keen. 
 
 We apparently possess, then, in the labyrinth, the end-organ of 
 a sense for the positions and movements of the head, which provides 
 for perceptions of rotation, etc., and which reflexly excites compen- 
 satory movements and muscular tonus. The nerve of this sense 
 is the vestibular branch of the eighth. As the central connections 
 of this nerve with the cerebellum are close, and as the results of 
 extirpating the labyrinth resemble those of injuring the cerebellum 
 (compare p. 156), it is probable that the reflex functions connected 
 1 James, American Journal of Otology, 1882, IV, 239. 
 
212 END-ORGANS OF THE NERVOUS SYSTEM 
 
 with this sense-organ have their centre, in large measure, in the 
 cerebellum. 
 
 37. A brief description of the End-Organs of Motion, or motor 
 end-plates, will suffice for our purposes. In general, the termina- 
 tions of the efferent nerves are connected either with electrical 
 organs (as, for example, in the torpedo), or with secretory glands, 
 or with the muscular fibre. We consider only the last of these 
 three cases. 
 
 After an efferent nerve has entered the substance of the so-called 
 voluntary or striated muscle, it subdivides among the individual 
 muscular fibres, separating these fibres from each other. Such 
 nerve-twigs usually lose their medullary sheath, and their axis- 
 cylinder splits up into fibrils, whose exact mode of termination has 
 been much debated. It appears now to be demonstrated (by 
 Kiihne, Margo, Rouget, and others) that the axis-cylinder itself 
 pierces the sarcolemma or sheath of the muscular fibre; that the 
 neurilemma becomes continuous with the sarcolemma; and that 
 the fibrils, into which the axis-cylinder divides, form a flat, branch- 
 ing mass within certain peculiar, disk-shaped bodies situated inside 
 the sarcolemma, and called "motor end-plates." In the non-striated 
 (or non-voluntary) muscles, the nerves divide and subdivide to form 
 more and more minute plexuses of nerve-fibres, which are distrib- 
 uted in the connective tissue that separates the muscular fibres from 
 each other, and finally applied to the surfaces of the muscular 
 fibres. The exact manner of this application is of no particular 
 interest to psychology, even when approached from the physi- 
 ological point of view. 
 
CHAPTER IX 
 
 THE CEREBRAL HEMISPHERES AND THEIR FUNCTIONS 
 
 1 . Ordinary observation recognizes the fact that the phenomena 
 of consciousness are more or less definitely correlated with the con- 
 dition of the bodily organs. Certain alterations in our mental 
 states, on account of the injury of any of its masses, as well as a con- 
 stant dependence of those states upon the way some of the masses 
 stand related to each other and to the outside world, impress the 
 fact upon our daily experience. It is by no means so obvious that 
 the nervous substance has any particular relation to the thoughts 
 and feelings of the mind. For the functions of the nervous system 
 are not exercised in giving information as to itself, its own condi- 
 tion and changes. By aid of these functions, however, we have pre- 
 sented in consciousness a more or less clear picture of the condition 
 and changes of the superficial parts of the body. In the same way a 
 knowledge is gained of the successive states of tension belonging 
 to the muscles in movement, and even though rather obscurely 
 of the place and condition of the internal organs. But as long as 
 they are healthy and excited with only a moderate intensity of 
 their stimuli, the nerves do not even reveal their own existence; 
 and when they are injured or unduly excited, the notice they fur- 
 nish of the fact comes in the form of painful feeling which we have 
 learned to localize, not in the nervous substance itself, but in the 
 adjacent parts of muscle and skin. Attention may be called, how- 
 ever, to the peripheral nerves by the accident or the dissecting- 
 knife which exposes them to sight. But in the case of the central 
 nervous organs, and especially in the case of the brain, there is little 
 in ordinary experience which leads to a suspicion of their signifi- 
 cance or even of their existence. 
 
 It is not very strange, then, that no general recognition of the 
 supreme importance of the brain, in relation to the phenomena of 
 consciousness, is to be found in early history. It is true that 
 Plutarch 1 and Theophrastus 2 inform us of the opinion of the 
 physician Alcmseon, who is said to have been a younger contem- 
 porary of Pythagoras, and who regarded the brain as the common 
 meeting-place of the senses. The same view is also ascribed to the 
 
 1 De Placitis Philosophorum, IV, 17, 1. *De Sensu, 25 f. 
 
 213 
 
214 THE CEREBRAL HEMISPHERES 
 
 celebrated Hippocrates. Later on Plato accepted it. But Aris- 
 totle, 1 the greatest of all thinkers in antiquity, the son of a phy- 
 sician, especially educated in physical science, and well acquainted 
 for the time in the dissection of animals, regarded the brain as a 
 lump of cold substance, quite unfit to be the seat and organ of the 
 sensus communis. This important office he ascribed rather to the 
 heart. The brain he considered to be chiefly useful as the source 
 of fluid for lubricating the eyes, etc. 
 
 2. The opinion of Exner, 2 however, who supposes that feeling 
 in no way immediately informs us that we think with the head, 
 still less with the brain or the cortex of the cerebrum, seems some- 
 what extreme. For we certainly localize in the head certain phe- 
 nomena of consciousness that are inextricably interwoven with the 
 processes of thought. The act of attention, for example, results 
 in feelings which indicate that the muscles of the eye are being in- 
 nervated; or in the more indefinite and diffused sense of strain 
 produced by contracting the skin of the forehead and adjacent 
 parts of the face. The special sensations of hearing, smelling, and 
 tasting, which impress so strongly our conscious life, are frequently 
 referred to the head. The same thing is true of many of the sen- 
 sations of sight particularly of such as appear when the eyes are 
 closed, in the form of after-images, or spectra, or indefinite and 
 changing color-spots, seated in the upper front part of the face. 
 Moreover, that inchoate and sometimes half-articulated language, 
 with which we support our trains of thought, even when we are not 
 conscious of resorting to the expedient of "talking to ourselves," is 
 felt to be going on within the head. When one has been engaged 
 for some time in intense thought, or in eager and concentrated 
 observation, one is suddenly made aware of more or less painful 
 feelings which are somewhat indefinitely ascribed to the same 
 cerebral region. Men commonly lean the head upon the hand 
 in supporting meditation; or rub it vigorously to awaken the pow- 
 ers of memory and reasoning; or stroke it to relieve the disagreea- 
 ble sensations which follow severe mental excitement. Headache, 
 of more or less intensity, thus becomes associated with active ex- 
 ercise of the intellect. The head is wearied with thought; and 
 not only so, but also with intense physical exercise. The dis- 
 comfort which bodily strain produces in the hinder regions of the 
 head are an indication, although of only a very general kind, that 
 processes have gone on in that locality which are of great impor- 
 tance to the succeeding states of consciousness. All this apparent 
 
 1 See De Partibus Animalium, 652, b. 5; (II, 7); 656, b. 22 (II, 10); De 
 Juvent., 467, b. 28; and De Anima, III, 1 and 2. 
 
 2 See Hermann's Handb. d. PhysioL, II, ii, p. 192. 
 
THE PROBLEM OF CEREBRAL FUNCTIONS 215 
 
 testimony of immediate feeling is, doubtless, somewhat exaggerated 
 in an age so distinctively "nervous" as our own; but it cannot well 
 be doubted that a certain amount of testimony from immediate 
 feeling as to the important relation which exists between the state 
 of mind and the contents of the cranial cavity, belongs to all human 
 experience. 
 
 However uncertain the witness of immediate feeling upon the 
 point in question may be, very little observation of others is needed 
 to amplify and confirm its witness. We are not infrequently led to 
 notice how quickly and profoundly the states of consciousness are 
 changed by injuries to the brain. The effect of a blow upon the 
 head in suspending consciousness is decisive of this question. It 
 is but a step from this conclusion to a recognition of the truth that 
 the physiological significance of the contents of the cranial cavity 
 consists in their affording a field upon which all the impressions of 
 sense can meet together, and so furnish the basis and material of 
 comparative thought. Indeed, it was this line of inquiry which 
 probably led certain ancient anatomists, like Herophilus and Galen, 
 to locate the soul, or psychical principle, in the brain. 1 
 
 3. A great multitude of physical considerations, advanced by 
 modern science, place beyond doubt the supreme importance of 
 the brain in its influence upon the phenomena of consciousness. 
 The free circulation of arterial blood, with its supply of oxygen, 
 is a necessary condition for the fulfilment of the functions of all 
 the central organs ; but this necessity is especially marked in the case 
 of the brain. The stoppage of one of the great arteries leading to 
 this organ, either by compression in the neck, or by embolism at 
 some point along its course, at once produces profound distur- 
 bances and even complete cessation of consciousness. 
 
 Certain other arguments of a similar nature, which have been ad- 
 vanced from time to time, have either been rendered doubtful or 
 quite discredited by recent investigations. Such are the claims 
 made by Lombard and Schiff that a rise of temperature either in 
 the entire cerebral area or in particular circumscribed regions of 
 the cortex, results from all kinds of psychical activities. Indeed, 
 the amount of such variation (less than yi^ C.) would seem to fall 
 below the limits of accurate observation. So, too, the claim of 
 Byasson and others to measure with any reasonable approach to 
 accuracy the amount of thought accomplished, by the increase of 
 waste in the cerebral tissues, and by the resulting quantity of sul- 
 phates and phosphates excreted, may be said to have involved orig- 
 
 1 In the subsequent discussions, such terms as "seat," "localize," "local- 
 ization," " resides in," and similar terms, must be understood only as involv- 
 ing a convenient figure of speech. 
 
216 THE CEREBRAL HEMISPHERES 
 
 inally far too many uncertain factors; and besides, it is not confirmed 
 by more recent work. For the present, then, all such arguments 
 for the special connection of the cerebral hemispheres with the phe- 
 nomena of consciousness must be left in abeyance. 1 
 
 4. In the case of man, the cerebral hemispheres are, appar- 
 ently, the only portions of the nervous system, between the size, 
 condition, and molecular activity of which and the phenomena of 
 consciousness there is a direct correlation. If, then, we are to speak 
 of mental activities as "localized" at all, the locality must be in 
 the cortex of the cerebrum. The position that, in the case of man, 
 the spinal cord and all the intercranial organs below the cerebral 
 hemispheres, are incapable of acting as the immediate physical 
 basis of mental states, is confirmed even by those experiments upon 
 other animals, which seem at first sight to discredit it. The hypoth- 
 esis that consciousness has a seat in the spinal cord of the frog; 
 that, in fact, we may properly speak of the decapitated animal as 
 having a soul has been urged by eminent physiologists (Pfliiger, 
 for example). That the cord alone is capable of various purpose- 
 ful activities, such as serve, lender certain circumstances, as signs 
 of a psychical experience, may be demonstrated by experiment. 
 But unless one is prepared to maintain that all purposeful activity, 
 as resulting from excited nervous substance, must be correlated 
 with phenomena of conscious sensation and volition, one can scarce- 
 ly assume with confidence that such phenomena accompany the 
 movements of the decapitated frog. 
 
 5. The evidence from comparative anatomy in favor of re- 
 garding the cerebrum as, in some peculiar manner, the "seat" of 
 mental life has been so fully set forth in preceding chapters (pp. 
 33, 61), that it needs here but a brief note. In general, the size of 
 the cerebrum bears a fairly close relation, as between different orders 
 of vertebrates, with the apparent intelligence of the animal. The 
 size of the cerebrum is indeed correlated also with the size of the 
 animal; and there are some apparent exceptions, in which animals, 
 such as the ruminants, though possessing large hemispheres do 
 not give the impression of special intelligence. It must be admitted, 
 however, that comparative psychology has not yet progressed to 
 such a stage that we can definitely assign the grade of intelligence 
 of each of these animals ; and our casual observations may have de- 
 ceived us in regard to them. In a general way, it certainly seems 
 that the intelligence of animals is correlated with the size of their 
 cerebra, and not, on the other hand, to the same degree with the 
 size of any other part of the nervous system. The power of learn- 
 
 1 This paragraph is in correction of the views expressed in the earlier edi- 
 tion of this work. See Elements of Physiological Psychology, p. 242. 
 
EVIDENCE FROM REMOVAL OR INJURY 217 
 
 ing also seems to depend on the development of the cerebrum. 
 It cannot, indeed, be said that nothing can be learned without de- 
 veloping this particular part of the brain; for even a fish can learn 
 to some extent; and the behavior of invertebrate animals has been 
 found to be modifiable. 1 It would be rash to assert that absolutely 
 no plasticity, no modifiability by experience, is retained by the spinal 
 cord; and it would be impossible to demonstrate in logical form that 
 no dim and rudimentary consciousness resides in the cord; but com- 
 parative anatomy leaves no serious doubt that the highly developed 
 consciousness of man, and his great power of learning by experi- 
 ence, are associated with the functions of his cerebral hemispheres. 
 
 6. Reference should also be made to our previous discussion 
 (compare p. 158) of the functions destroyed and spared, in ani- 
 mals, by removal of the cerebrum. We found that the functions 
 remaining after such an operation are the vegetative, locomotor, 
 and protective reflexes along with some of the simpler expressive 
 movements comprising, all in all, the elementary movements of 
 the members, but involving a few of the most essential combina- 
 tions of such movements. What was destroyed by the operation 
 was, most clearly, the learned reactions, together with the power 
 of new learning, and the "anticipatory reactions," or reactions of 
 only indirect utility. From these facts, the conclusion seemed 
 justified that the cerebrum is the organ for learned reactions and for 
 reactions to the wider environment, that is, for the reactions made 
 possible by the existence of the "distance receptors." Now it is 
 especially with reference to this "wider environment," as revealed 
 by these receptors, that learned reactions have their chief importance 
 in securing physical well-being and mental development. 
 
 7. Perhaps the most conclusive evidence that the cerebral 
 hemispheres are the seat of consciousness and of the intellectual 
 operations, as well as, chiefly, of learned movements and reactions, 
 is afforded by the results of partial injuries to the brain. When the 
 injury is total or general, as in the case of a blow on the head, an 
 objector might possibly reply that breaking of the neck, or piercing 
 the heart, also promptly abolishes consciousness and all mental 
 function. The argument, in this bald form, cannot be taken seri- 
 ously at the present day; yet it has this much of force, that the loss 
 of consciousness following brain shock is, in strict logic, evidence 
 only that the brain is necessary to consciousness even as the 
 heart is. That is to say, as a vital organ, it is part of the bodily 
 mechanism with which consciousness is associated. It might 
 still be true that the whole nervous system cerebrum, cerebellum, 
 brain-stem, cord and nerves are the seat and organ of the mental 
 1 See Jennings, Behavior of the Lower Organisms (New York, 1906). 
 
218 THE CEREBRAL HEMISPHERES 
 
 life. But partial injuries, which do not abolish all consciousness, 
 but simply make impossible some of its phenomena, cannot be 
 considered in the same light. Life persists; the organ destroyed 
 is not vital; and yet some functions of the mental life are rendered 
 impossible. 
 
 To introspection, it certainly seems as if the consciousness of 
 the hand, for example, had its seat in the hand itself; and as if the 
 skilled movements which the hand learns are learned by the hand 
 itself; and common forms of speech agree with this naive view. 
 This seeming consciousness may even be projected into the tool 
 which the hand is using. But if the nerves going to the hand are 
 severed, at any point between the hand and the spinal cord, stimuli 
 affecting the hand are no longer perceived, and no power of skilled 
 or even of reflex movement is retained. The hand has become as 
 though non-existent for consciousness. Yet the power to think 
 of the hand has not been lost; nor the power to will movements of 
 the hand, which are, however, no longer carried out. In cases of 
 amputation of the hand, stimulation of the stump and, thus, of 
 the remnants of the nerves which formerly ran to the hand often 
 arouses sensations located in the hand as if it were still there. Such 
 observations show that the consciousness of the hand resides not 
 in the hand, but somewhere in the central nervous system. 
 
 The same argument can be carried further by noting the results 
 of injuries to the cord. If the cord has suffered such an injury 
 that the lower part of it is severed from the upper, the whole lower 
 part of the body is cut off from consciousness and all mental in- 
 fluences, just as the hand was in the preceding case. The con- 
 sciousness of the legs does not then reside in their reflex centres in 
 the cord, but somewhere higher up, in the brain. In the same way, 
 injuries may occur to the brain-stem, which, without severing it 
 entirely for this would be a mortal injury, and would therefore 
 destroy the value of the observation from our present point of view 
 may yet interrupt the sensory or motor tracts connecting the cere- 
 bral cortex with the cord; and in such cases, the same general re- 
 sult, as regards consciousness and learned movements, is to be 
 noted. The power to think of the parts of the body concerned is 
 retained, and the power to will movements; but the execution of 
 the movements is impossible, and sensations of the members no 
 longer arise in consciousness. It seems possible, in this way, to 
 push back the seat of consciousness from the periphery to the cere- 
 bral cortex, and to conclude that the latter is the essential seat of 
 consciousness. 
 
 The argument is further strengthened by observing the effect of 
 partial injuries to the cortex itself. Injuries to certain portions of 
 
THE PROBLEM STATED 219 
 
 it as will be set forth in more detail in the next chapter cause a 
 loss of certain of the more elementary functions of normal life. 
 A person may lose, as the result of such injury, the sense of sight, 
 or that of hearing, or the sensations from any part of the periphery. 
 The hand and its nerves, it may be, as well as the cord, brain- 
 stem, and cerebellum, are uninjured, but the destruction of a small 
 part of the cortex has destroyed all sensation of the hand. Indeed, 
 the person thus afflicted seems unable even to imagine the sensation 
 as localized in his hand, in any very realistic way. As far as con- 
 sciousness is concerned, therefore, he has suffered a more severe 
 loss than would have been suffered if the injury had affected 
 simply the nerves of the hand. So again, destruction of certain 
 portions of the cortex deprives a person of visual sensations, and 
 sometimes of visual images as well. Much the same sort of evi- 
 dence is available in regard to other than strictly sensory functions. 
 Learned movements, memories of any particular sort, mental oper- 
 ations in general, may be thrown out of power to function, even 
 though the injury is not wide-spread or severe enough to abolish 
 all consciousness. 
 
 8. Having been convinced by this evidence that the cerebral 
 hemispheres are, in some special sense, the seat and organ of mental 
 functions, the psychologist next appeals to anatomy and to physi- 
 ology for an answer to these questions : (1) What plan is followed in 
 the apportionment of the various mental functions over the whole 
 extent of the cortex ? and (2) What is the precise character of the 
 functions of this organ in its relations to the phenomena of con- 
 sciousness ? Both problems that of the "localization of cerebral 
 functions/' and that of the intimate physiology of the cortex are 
 fraught with such difficulty that the answers given are still very 
 fragmentary and often tentative. The results of investigation, 
 however, are well worthy the serious attention of the psychologist. 
 
 9. The cerebral hemispheres consist, as has been stated in 
 an earlier chapter (see pp. 50 f.), of the olfactory lobes, the corpora 
 striata, and the pallium. It is the latter with which we are now con- 
 cerned. The pallium itself can be divided, on the basis of compara- 
 tive anatomy (compare pp. 31 f.) into the archipallium and the neo- 
 pallium; of these the former is older in the animal series, and is con- 
 cerned especially with the sense of smell and with the "snout sense." 
 In mammals the archipallium is eclipsed in size by the neopallium, 
 which, by expanding laterally, forward and backward, leaves the 
 archipallium in a central location and for the most part conceals it. 
 It cannot be seen on the upper, lower, or lateral surfaces of the brain, 
 and only a little of it is visible on the mesial surface of a hemisphere. 
 It includes the fornix and the dentate gyre, but it is best seen, as 
 
220 THE CEREBRAL HEMISPHERES 
 
 the "horn of Ammon," in a section through the brain (see Figs. 
 9 and 97). 
 
 10. The cortex is divided, for convenience of reference, into 
 lobes. The great Sylvian fissure makes a clear division between the 
 frontal lobe above it, and the temporal lobe beneath. The fissure 
 of Rolando, or central fissure (compare Fig. 94), constitutes the 
 
 Frontal lobe 
 
 Precentral gyre 
 
 Central fissure 
 
 Postcentral gyre 
 
 Parietal lobe 
 
 Occipital lobe 
 
 FIG. 94. The Upper Surface of the Cerebral Hemispheres. 
 (Sobotta-McMurrich.) 
 
 boundary between the frontal and the parietal lobes. The pari- 
 etal lobe is separated from the temporal by the posterior portion 
 of the fissure of Sylvius. The rearmost portion of the cortex is 
 designated as the occipital lobe. A partial boundary between the 
 occipital and parietal lobes is afforded by the parieto-occipital 
 fissure. There is no clear boundary between the occipital and the 
 temporal lobes. In fact, the whole division into lobes is somewhat 
 artificial. There is no break in the cortex between one lobe and 
 the next; for the cortex extends down the sides of the boundary 
 fissures, and continuously around the bottom of the fissures into 
 the adjoining lobes. Within the Sylvian fissure, the cortex ex- 
 pands into a considerable area, cut up by smaller fissures; and this 
 
PRINCIPAL DIVISIONS OF THE CORTEX 221 
 
 concealed part of the cortex is known as the "island of Reil," or, 
 more briefly, as the Island. 
 
 Besides the fissures which have been chosen as the grand bound- 
 aries between the lobes, many other fissures in the superficial man- 
 ner above indicated divide the lobes into smaller parts known as 
 convolutions or gyres. Some of these smaller fissures are incon- 
 stant, when one individual brain is compared with another; but 
 certain of the more important are fairly constant, and so form ap- 
 
 Central fissure 
 
 Supramarginal gyre 
 
 Angular gyre 
 
 Superior frontal 
 gyre 
 
 Middle frontal 
 Precentral gyre 
 
 Inferior 
 temporal 
 gyre 
 
 Cerebellum 
 
 Fia. 95. Lateral Surface of the Left Cerebral Hemisphere. (Edinger.) 
 
 propriate landmarks on the surfaces of the cortex. Those of most 
 interest are the following: In the "central region" (see Fig. 95), 
 or that immediately adjacent to the central fissure, we may dis- 
 tinguish two gyres, one on each side of the central fissure, and ex- 
 tending along it; they are separated from the rest of the frontal 
 and parietal lobes, respectively, by the precentral and postcentral 
 fissures. These gyres are named the precentral and the postcentral. 
 To the front of the precentral fissure lies the great extent of the frontal 
 lobe, which is partially divided into superior, middle, and inferior 
 gyres by two horizontal fissures, called the superior and the infe- 
 rior frontal. In the temporal lobe, also, there is a series of hori- 
 zontal fissures, which divide the lobe into the superior, middle, 
 and inferior temporal gyres. The subdivision of the parietal and 
 occipital lobes is not quite so simply made. The intraparietal 
 fissure separates the superior parietal gyre from the rest of the lobe ; 
 below this, two parts are easily distinguished namely, the supra- 
 marginal gyre, which curves around the end of the fissure of Syl- 
 
222 
 
 THE CEREBRAL HEMISPHERES 
 
 vius, and the angular gyre, which curves around the end of the supe- 
 rior temporal fissure. On the lateral surface of the occipital lobe 
 the divisions are not specially clear or constant; but superior, mid- 
 dle, and inferior gyres are customarily recognized. 
 
 On the mesial surface (Fig. 96), a prominent fissure is the 
 cingulate, extending parallel to the callosum, and separating the 
 limbic lobe from the frontal and parietal. The central fissure 
 often shows itself on the mesial surface; and the gyre which curves 
 
 Cingulate fissure. 
 
 Central fissure 
 
 Limbic lobe 
 
 Paracentral lobule 
 
 Precuneus 
 
 Parieto-occipital 
 Assure 
 
 Optic chiasm 
 
 FIG. 96. Mesial Surface of the Right Cerebral Hemisphere. (Edinger.) 
 
 around its end is called the paracentral lobule. The parie to- 
 occipital fissure is strongly marked on this surface, and forms a 
 clear division between the parietal and occipital lobes. Within 
 the occipital lobe, there appears on the mesial surface a prominent 
 fissure, called the calcarine, which is of great importance in dis- 
 cussions of localization. The calcarine fissure joins the parieto- 
 occipital, and the triangular gyre which lies between them is called 
 the cuneus, while the gyre which lies immediately beneath the cal- 
 carine fissure is the lingual; beneath this, again, is the fusiform. 
 
 These, then, are the chief landmarks on the surface of the brain, 
 which are of use in our studies for the localization of cerebral func- 
 tions. 
 
 11. A section through the cerebral hemispheres (Fig. 97) 
 shows the gray matter, or cortex, on the surface, and extending 
 to and around the bottom of the various fissures; but beneath the 
 
PROJECTION AND ASSOCIATION FIBRES 
 
 223 
 
 cortex appears a large mass of white matter. This white matter 
 is composed of medullated (or myelinated) nerve-fibres, which are 
 of varied origin. Some, as was stated in considering the develop- 
 ment of the brain (see p. 53), grow into the cerebrum from the 
 interbrain, and pass to various regions of the cortex. Others orig- 
 inate in the cortex, and grow down, converging into the internal cap- 
 sule, whence they pass to the peduncle and so on down to the pons. 
 These two classes of fibres form paths of communication between the 
 cortex and the lower parts of the nervous system; they are known 
 as "projection fibres/' in distinc- 
 tion from the "association fibres," 
 which pass from one part of the cor- 
 tex to another. Much the great- 
 est part of the cerebral mass of 
 white matter, however, is composed 
 of association fibres, some of which 
 are short, and connect neighboring 
 gyres, while others are long and 
 connect distant parts of the cor- 
 tex with each other. Of these long 
 association fibres, several distinct 
 bundles can be distinguished: a 
 massive bundle of fibres running 
 between the temporal and occipital 
 lobes; a bundle running between 
 the temporal and frontal lobes; 
 another between the temporal and 
 parietal lobes; one between the frontal and occipital lobes; and one 
 extending from one end to the other of the curved limbic lobe. Not 
 all the fibres in a bundle, however, extend the whole length of its 
 course, but some enter or leave at various points (compare Fig. 98). 
 
 Though projection fibres have been traced to or from nearly 
 all parts of the cortex, they are by far most numerous in certain 
 definite portions of the cortex; such are, especially, the central 
 region, on both sides of the central fissure; the calcarine region; 
 and the superior temporal gyre. This fact in itself suggests, at 
 least, a certain amount of localization of function ; since the regions 
 directly connected by fibres with lower parts of the system would 
 probably be the regions most directly connected with the senses 
 and with movement. This suggestion is borne out, as we shall see, 
 by the results of other methods of localization. 
 
 As was stated in considering the development of the brain (see 
 pp. 58 f.), different bundles of fibres become myelinated at different 
 times. At birth the only myelinated fibres passing to the cortex 
 
 Subtt. nigra 
 
 FIG. 97. Frontal Section through the 
 Cerebral Hemispheres. (Gegenbaur.) 
 A portion of the archipallium is seen 
 at the part marked "Hippocampus." 
 
224 THE CEREBRAL HEMISPHERES 
 
 of the human infant go to the central zone; but soon other regions 
 receive myelinated fibres. Flechsig, on the basis of extended 
 studies of the date of myelinization of different regions, is able to 
 divide the cortex into areas (Fig. 99), some of which show mye- 
 linated fibres early, others late, and others at intermediate dates. 
 He believes that the map of the cortex so obtained is also to be 
 regarded as a map of the distribution of functions; and in particular 
 he supposes that the regions whose fibres receive their myelin 
 sheath early are the centres of the lower functions of sensation 
 
 Fia. 98. Association Fibres of the Cerebrum. (Starr's Atlas of Nerve 
 Cells, by permission of the Columbia University Press.) 
 
 and movement, while the late-myelinating regions are the seat of 
 the highest intellectual functions. As far as concerns the early- 
 myelinating areas, this theory is confirmed by the results of other 
 methods of study; as far as concerns the localization of the intel- 
 lectual functions, the theory should be entertained with considerable 
 reserve. There is no improbability, however, in the view that areas 
 distinguished on the basis of their differing dates of myelinization 
 should also have different functions. 
 
 12. Microscopic examination of the cortex reveals a wealth 
 of nerve-cells, embedded in an intricate net-work of fine fibres. 
 The study of these cells and fibres, and of the differences which they 
 present in different parts of the cortex, has been prosecuted with 
 great vigor and success during recent years. The most logical order 
 for presentation of this subject would demand at once some account 
 of the results of these histological studies. But they will mean more 
 to the reader if he is first made acquainted with the results of other 
 
NERVOUS ELEMENTS OF THE CORTEX 
 
 225 
 
 work in the localization of functions in the cortex. It will, there- 
 fore, be better to abandon the strict logical order of presentation for 
 a historical order. The physiological and pathological results have, 
 in fact, preceded the histological, and have served as the chief im- 
 petus to the minute examination of all parts of the cortex with a 
 view to determine whether regions which have special functions 
 are also characterized by a specialized structure. 
 
 To superficial examination, indeed, the cerebrum appears to be 
 a fairly homogeneous mass, of soft consistency an appearance which 
 
 FIG. 99. Division of the Cortex on the Basis of Date of Myelinization. (Flechsig.) 
 The frontal lobe lies to the left. The numbers indicate the approximate order of 
 myelinization. The early-myelinating areas are heavily shaded, the intermediate areas 
 lightly shaded, and the late-myelinating areas left clear. 
 
 easily gave rise to the notion that it functions as a whole, much as 
 a gland functions. Such an epigram as "The brain secretes 
 thought, as the liver secretes bile," has even at times gained currency. 
 But when the microscope, aided by differential stains, revealed 
 the inner structure of this mass, and when it was seen to consist, 
 like the nerves, of multitudes of fibres running in various direc- 
 tions in the white matter, and entering and leaving the gray, the 
 glandular simile lost its force, and the opinion that each fibre or 
 set of fibres like each nerve had something specific to do, became 
 almost a matter of compulsion. 
 
 13. We begin, then, our search for more definite knowledge as to 
 the localization of cerebral functions with strong presumptions in 
 its favor. The cerebral cortex is itself a very complex organ, or 
 
226 
 
 THE CEREBRAL HEMISPHERES 
 
 system of organs. Its different regions are marked by compara- 
 tively slight, and yet not insignificant, differences of structure; they 
 stand in different local relations and nervous connections with one 
 another and with the ganglia lying below. This outlying rind of 
 gray nervous matter is, of course, not a homogeneous mass. It is 
 made up of innumerable nervous elements combined in various 
 ways and multiform connections. It may be regarded, then, as a 
 complex of organs. 
 
 Most of our definite knowledge concerning the functions of the 
 other parts of the nervous mechanism also creates a presumption in 
 
 Fig. 99a. The Same as Fig. 99, Mesial Surface. 
 
 favor of some localization of cerebral functions. All the different 
 parts of this mechanism are, indeed, constructed by combining vari- 
 ously a few elements of essentially the same structure; all of them 
 likewise are capable of exercising essentially the same neural func- 
 tions. But each part of this mechanism has also its special func- 
 tions. Thus we found that the different nerves become classified 
 functionally; some are motor, voluntary or involuntary, some in- 
 hibitory, some secretory, some sensory, etc. Hints of a certain 
 kind of classification may be discovered for the smaller ganglia or 
 collections of nerve-cells. In making transverse sections of the 
 cord, different regions with different functions appear. Consid- 
 ered longitudinally, the cord is capable of being more or less defi- 
 nitely divided into several so-called centres, with specifically different 
 functions. Localized centres, where specific kinds of reflex-motor 
 
HISTORY OF THE INVESTIGATION 227 
 
 activity have their particular seats, are fairly crowded together in 
 the medulla oblongata. All the lower parts of the encephalon 
 appear subject, in a measure, to the principle of localization. Shall 
 we, then, stop short in our attempts at differencing the functions of 
 the locally separate parts of the nervous system just at the point 
 where we reach the most complex and extended organ, or rather 
 collection of organs, which this system contains? 
 
 14. Notwithstanding the strong presumption in favor of the 
 localization of cerebral function, the beginnings of a successful 
 attempt to establish this theory are comparatively recent. The 
 doctrines of Gall, Spurzheim, and others in the older school of 
 phrenologists, proved so inconclusive as to bring contempt upon 
 subsequent attempts to divide the hemispheres of the brain into 
 different functional areas. Moreover, certain indisputable facts 
 seemed to render impossible the assured beginnings of a theory of 
 cerebral localization. Considerable portions of the human brain, 
 it was found, might be lost without destroying any one sensory or 
 motor function. Moreover, the gray matter of the cerebral hemi- 
 spheres, it was then thought, could not be directly excited by elec- 
 tricity or by other forms of stimuli. The greatest experimenters 
 in physiology, such as Longet, Magendie, Flourens, Matteucci, Van 
 Deen, Budge, and Schiff, declared against the localizing of cerebral 
 function. In 1842 Longet 1 affirmed that he had experimented upon 
 the cortical substance of dogs, rabbits, and kids, had irritated it me- 
 chanically, cauterized it with potash, nitric acid, etc., and had passed 
 galvanic currents through it in different directions, without obtain- 
 ing any sign whatever of resulting muscular contraction. In the 
 same year Flourens 2 asserted, on the basis of numerous experiments 
 in extirpation, that the lobes of the cerebrum perform their func- 
 tions with their whole mass; that there is no special seat for any of 
 the cerebral activities; and that even a small remnant of the hemi- 
 spheres can serve all the uses of their collective functions. 
 
 So great was the authority of the distinguished names just men- 
 tioned, that their confident opinions gained general credence. The 
 evidence brought forward by Broca and others seemed, however, 
 to show some special connection between a single convolution of 
 the frontal lobe and the complex activities of articulate speech; 
 and the anatomist, Meynert, held the opinion that the structure 
 and connections of the cerebrum show its anterior portion to be 
 in general used for motor, its posterior for sensory, functions. In 
 1867 Eckhard repeated the significant observation which had been 
 
 1 Anatomic et physiologic du systeme nerveux, etc., Paris, 1842, I., pp. 644 f. 
 
 2 Recherches experimentales sur Us proprieties et les fonctions du systeme 
 nerveux, etc., pp. 99 f. 
 
228 THE CEREBRAL HEMISPHERES 
 
 made by Haller and Zinn more than a century before: namely, 
 that, on removing parts of the cortical substance of an animal's 
 brain, convulsive movements occur in its extremities. 
 
 It was not until 1870 that the "epoch-making" experiments 
 of Fritsch and Hitzig 1 began the modern era of investigation into 
 this subject. These observers announced the fact that the cere- 
 bral cortex of dogs is, at least in certain minute areas of it, excita- 
 ble by electricity. They pointed out the further fact that, while 
 some parts of the convexity of the cerebrum are capable of motor 
 excitation and others not, the motor parts lie in general to the 
 front, the non-motor to the rear of this convexity. By stimulating 
 with an electrical current the so-called motor parts, co-ordinated 
 contractions of the muscles in the opposite half of the body were 
 obtained. Of such so-called "motor-centres" they indicated, in 
 their first announcement, the following five: One for the muscles 
 of the neck, another for the extension and abduction of the fore 
 limb, another for the bending and rotation of the same limb, an- 
 other for the hind limb, and lastly one for the face. From such 
 facts they drew the conclusion that the principle announced by 
 Flourens is demonstrably false. We must rather admit, say they, 
 that "certainly several psychical functions, and probably all, are 
 shown to have their point of entrance into matter or of origin from 
 it at circumscribed centres of the cerebral cortex." The same 
 principle was subsequently defended at length by Hitzig, and the 
 number of so-called cerebral centres increased. The most note- 
 worthy facts which these experimenters first made clear and de- 
 monstrable have since been verified by many investigators. Among 
 the physiologists who have amplified the results of Fritsch and Hit- 
 zig, the following deserve special mention: Ferrier, for his work 
 on the monkey's brain; and, more recently, Sherrington and Grtin- 
 baum, for their work on the anthropoid apes, the brains of which 
 are anatomically much closer to the human brain than are those of 
 the lower monkeys. The testimony of human pathology, and the 
 evidence of comparative anatomy and of histology, have also been 
 largely drawn upon either to confirm or to confute the conclusions 
 originally based on experiments with animals. Before considering 
 the conclusions themselves, it is necessary to understand the true 
 nature and extent of the various kinds of evidence. 
 
 15. Three great lines of evidence, leading from three great 
 groups of facts, must be considered. These are the evidence from 
 
 1 See the article by G. Fritsch and E. Hitzig in the Archiv f. Anat., Phys- 
 iol., etc., 1870, pp. 300-332; and subsequent articles by Hitzig in the same 
 Archiv, 1871, 1873, 1874, 1875, 1876; also his collected works Physiologische 
 und klinische Untersuchungen uber das Gehirn (Berlin, 1904). 
 
THE THREE LINES OF EVIDENCE 229 
 
 experimentation, the evidence from pathology, and the evidence 
 from histology and comparative anatomy. Each of the three has 
 its peculiar advantages and value; each also its peculiar difficul- 
 ties and dangers. It is only by regarding the combined testimony 
 of the three that the highest probability at present possible can be 
 attained. . 
 
 Experimentation with a view to discover the localized functions 
 of the cerebral cortex is of two kinds, stimulation and extirpation. 
 In stimulation experiments, the procedure is as follows: having 
 first, under anaesthesia, removed the bony and membranous cover- 
 ings of a portion of the brain, a weak current of electricity is applied 
 to a minute portion of the exposed cortex, and a watch is kept for 
 resulting movements in any part of the body. It was this experi- 
 ment which, in the hands of Flourens and other early observers, 
 gave negative results, and which first succeeded in the hands of 
 Fritsch and Hitzig. The failure of the older experimenters is not 
 difficult to explain; for only a small proportion of the entire cortex, 
 on stimulation by currents of low or moderate strength, responds 
 with any bodily movements whatever. The rest of the cortex is 
 said to be "silent" under stimulation. In this fact lies a limitation 
 of the method; it supplies information only regarding the fraction of 
 the cortex which gives motor responses. 
 
 A further difficulty with the method of stimulation is its depend- 
 ence on the use of the electric current. This is an admirable stimu- 
 lus in most respects, but it is subject to one or two limitations, which 
 need to be guarded against by the experimenter; and sometimes by 
 those who would accept his results. One difficulty is the "spread- 
 ing" of the current, which is thus likely to excite parts not imme- 
 diately in contact with the electrodes, and so deceive the observer. 
 In the most recent work, the method of "unipolar stimulation" 
 in which one pole of the battery is broad, and applied to a distant 
 part of the body, while the other is a needle point applied to the sur- 
 face of the brain has been employed with success in finer localiza- 
 tions. At one time, it was feared that the spreading of the electrical 
 current seriously jeopardized all the results of cortical stimulation; 
 for it was argued the movements might be due not to arousal of 
 the cortex, after all, but to the arousal of some deeper-lying struct- 
 ure. A variety of checks have shown, however, that the cortex 
 is actually the part aroused. Perhaps the best worth citing of 
 these checks is the fact that, in the large brain of the chimpanzee, 
 with its thick cortex, and relatively long distances between parts, weak 
 unipolar stimulation easily arouses movements, but only when the 
 stimulus is applied to certain limited regions; and the same move- 
 ments are obtained with great constancy from the same spots. 
 
230 THE CEREBRAL HEMISPHERES 
 
 Such regular results could hardly be got by diffusion of current to 
 subcortical parts; and, in fact, the same responses cannot be got 
 by stimulation of any remote subcortical parts. On the contrary, the 
 same responses, or nearly the same, can be got by stimulating the 
 white matter immediately below a given region of the cortex as are 
 got from the region itself; but this is to be expected, since any area 
 must, of course, exert its influence through the fibres issuing from it. 
 
 Some use has been made of a similar, but reverse use of the 
 method of stimulation; namely, that of observing the electrical 
 changes in the cortex on stimulation of certain peripheral nerves. 
 Just as a nerve, excited at one end, is traversed by an electric wave, 
 so it has been found by several physiologists (Caton, Danielewski, 
 and others) that on flashing a light into the eye of an animal, elec- 
 tric currents were produced in a certain definite region of the cor- 
 tex, which it is then concluded must be closely connected with 
 the retina. The value of this method is diminished by the fact 
 that the currents seem excessively weak except in certain regions; 
 and that these same regions happen to have their functions fairly 
 well located by other methods. 
 
 16. In the method of extirpation, as practised by physiologists, 
 a well-defined area of the cortex is cut out; the animal is allowed to 
 recover from the general effects of the operation, and is watched and 
 tested with a view to determine precisely what loss of function has 
 attended the loss of brain substance. The assumption that the 
 two correspond is antecedently probable; but he would be an un- 
 wary experimenter who should, at the present day, assert unquali- 
 fiedly that the injured function had its peculiar, not to say its only, 
 seat in the injured part. Indeed, later on the injured animal often 
 shows a partial or nearly complete recovery of the lost functions. 
 This species of restitution is a puzzling side of the results of the 
 method of extirpation; it will be referred to again. Certain other 
 difficulties of this method can be largely avoided by good procedure. 
 For example, the inflammatory after-effects of an operation, which 
 often impaired the value of the early work, can now be avoided by 
 operating with aseptic precautions. The danger of cutting too 
 deep, and so of not simply extirpating the desired area of the cortex, 
 but also of interfering with other areas by incidentally severing 
 their projection and association fibres, cannot be wholly avoided; 
 but examination of the brain after the death of the animal may reveal 
 the truth in this respect. 
 
 One difficulty with all the methods of the physiologist is that he 
 deals with animals, and can learn of their mental processes only in- 
 directly and imperfectly. To a limited extent, there exist observa- 
 tions on man which are of the same nature as those of the physiolo- 
 
EVIDENCE OF HUMAN PATHOLOGY 231 
 
 gists on animals. In some surgical operations, it is necessary to re- 
 move definite parts of the brain; and in order to locate these parts 
 precisely, the electric current is applied, when the part is in the 
 "motor region." Thus some opportunity is afforded for examining 
 the effects of stimulation and of extirpation in the human subject. 
 
 17. The chief source of direct evidence regarding localization 
 in the human brain is pathology. Injuries occur to parts of the 
 brain, by gunshot wounds, or by fractures of the skull necessitating 
 the removal of splinters of bone and with them some of the brain 
 substance. More common are destructive lesions due to tumors, 
 hemorrhages, or local softening from impaired circulation in one 
 or more branches of the cerebral arteries. It will be observed that 
 accident and disease do much the same thing here to the human brain 
 that the physiologist's knife does to the brains of the animals which 
 he is studying. The pathological method is essentially the same "as 
 the method of extirpation. It has the advantage of giving results 
 on the human subject, regarding whose mental operations we have 
 much better sources of information than are available in the case 
 of animals. The pathological method has, on the other hand, the 
 disadvantage that the seat and limits of the lesion are not prede- 
 termined by the observer. Too much of the brain substance is 
 usually affected to afford a good subject for precise localization 
 of function. Indeed, in many cases a large portion of the entire 
 cortex is more or less affected, either by pressure exerted by a grow- 
 ing tumor, or by general disturbances of the blood supply, or by 
 spreading of the chemical influences of a diseased spot. The result 
 is that few pathological cases afford perfectly clean experimental 
 evidence. Another difficulty is that the individual whose brain is 
 to be injured by these natural causes is not known beforehand; he, 
 therefore, cannot be examined beforehand as to his mental character- 
 istics, as the animal subject can be. For these reasons, and also be- 
 cause of the intricate interweaving of mental functions, the progress 
 of localization in the human brain has been slow; a large accumula- 
 tion of material, and good judgment in interpreting the material, 
 are still needed in order to reach sound conclusions. In spite of 
 these difficulties, however, it may be said that by using as guides the 
 best-established localizations in animals, the study of human path- 
 ology has established the truth, that the fundamental facts of localiza- 
 tion are the same in the brains of both the lower animals and of man. 
 
 18. In addition to the physiological and the allied pathological 
 methods, there is a group of methods which belong under the science 
 of anatomy in a broad sense of the latter word. 
 
 The comparative anatomy of the brain affords some scientific 
 information regarding the functions of its different parts. The best 
 
232 THE CEREBRAL HEMISPHERES 
 
 instance of the use of this method for purposes of localization is 
 the case of the archipallium, and its probable connection with the 
 sense of smell and other related functions. This matter has al- 
 ready been sufficiently discussed. The logic of the method, in all 
 such cases, is as follows: When the behavior of a species of animals 
 shows that a certain function is highly develope'd (or the opposite), 
 peculiar developments of the brain in these species may, with some 
 probability, be regarded as related to this function. This argu- 
 ment is, indeed, similar to that of Gall and the phrenologists; but 
 they endeavored to apply the method in the comparison of human 
 individuals, and in the first instance, they sought to infer the de- 
 velopment of the brain from the external appearance of the skull. 
 The latter inference is now known to be very insecure. There seems 
 no antecedent reason to conclude, however, that a comparison of 
 the brains of different human individuals, whose mental peculiari- 
 ties were well known, might not assist in the localization of mental 
 functions. The method is still in use; but so far its results are not 
 very trustworthy partly because of insufficient psychological analy- 
 sis of the individuals concerned, and partly because of insufficient 
 microscopic study of the brains. 
 
 Comparative anatomy is useful in another way, which has al- 
 ready been noticed in our study of the nerve-tracts (compare pp. 
 31, 88). Brains of simpler construction afford a better opportunity 
 for the tracing of nervous connections than is afforded by the ex- 
 tremely intricate mass of fibres in the white matter of the human 
 brain. It is a valid assumption, that the connections of any part 
 of the cortex are of great, and even decisive importance, in assign- 
 ing the function of that part. If, for example, it is possible as it 
 is to trace the fibres of the optic nerve back to certain parts of 
 the interbrain, and other fibres thence to a certain region of the cor- 
 tex, this fact is the best possible indication that the region of the 
 cortex to which these fibres run is, somehow, specially concerned 
 with the sense of sight. Indeed, the most decisive localization of 
 the sensory functions of the cortex has been, to a large extent, ob- 
 tained by this method. To take another example: If the fibres of 
 the cortico-spinal or pyramidal tract, the principal motor tract pass- 
 ing from the cerebrum to the cord, can be traced back to a certain 
 area of the cortex, there is good reason for calling this the "motor 
 area." Such tracing of fibres, as previously explained (compare 
 pp. 87 f.), has been accomplished by several methods. 
 
 In the case, too, of parts of the cortex which are directly connected 
 with lower ganglia (either sensory or motor), the tracing of fibre-con- 
 nections has decisive weight. In general, increased knowledge of the 
 connections of other parts of the cortex with its sensory and motor 
 
EVIDENCE FROM HISTOLOGY . 233 
 
 areas, and with one another, would conduce greatly to an under- 
 standing of the functions of all these areas. If, for example, a cer- 
 tain limited region can be shown to be the origin of the fibres con- 
 necting the cortex with the muscles of the tongue, and if some other 
 area can be shown to be connected with this motor tongue area by 
 especially numerous fibres, then there is evidence that this second 
 area, too, is concerned in movements of the tongue. The probability 
 arises in this way, that all these areas are concerned with the com- 
 plex functions of speech. In other words, a truly neurological con- 
 ception of the working of the brain, as distinguished from a rough 
 assignment of this or that gross function or "faculty" to this or that 
 part of the cortex, would be advanced by nothing so much as by 
 an unravelling of the paths of the association fibres. Unfortunately, 
 such a task is extraordinarily difficult. Since it is in this respect that 
 man differs most from the animals, the work must be largely done 
 on the human brain; it must, therefore, be subject to the difficulties 
 that attend other work with pathological material. Still, progress 
 is being made even here, and at an accelerated rate. 
 
 Another method of an anatomical character which has recently 
 begun to show great promise is the histological mapping of the 
 cortex. Some of the results of this method will be referred to, later 
 on, in their relation to the more successful of the efforts to solve 
 the problems of the localization of cerebral functions, and of the 
 more precise nature, in themselves considered, of these functions. 
 
 19. We close this critical survey of the evidence for the localiza- 
 tion of cerebral functions, with these observations as to the nature 
 of the evidence itself and as to the trustworthiness of its results. 
 The evidence is always extremely complex and often very conflict- 
 ing; but it is in general cumulative, and in certain cases it is entitled 
 to be pronounced quite convincing. It must always be remembered, 
 however, that the possibility of differentiations and even of idiosyn- 
 crasies is as great in the human brain as it is in any of nature's 
 most complicated products. On the psychological side, too, we 
 find ourselves always faced with great, and often insuperable, 
 difficulties in our attempts to perfect the necessary analyses. The 
 most conclusive results are obtained in those cases where, on the 
 side of the nervous mechanism, we find all the lines followed by 
 the different methods converging on the same result; and where, 
 on the side of the mental life and development, we are dealing with 
 those forms of conscious activity such as the co-ordination and 
 control of bodily movements and the experiences of the more simple 
 and fundamental of the activities of sensation and association 
 which are shared by man with the lower animals, and which are, 
 whether considered from the biological or the psychological point 
 
234 THE CEREBRAL HEMISPHERES 
 
 of view, most essential to his existence and to the capacity for learn- 
 ing to adapt himself to a varied environment. 
 
 More precisely: A hundred years of the use of these different 
 methods of investigation, advanced in the most recent times to a 
 high degree of refinement in the hands of many diligent and skil- 
 ful workmen, have brought about a general agreement and a rea- 
 sonable certainty as respect the following of the simpler functions: 
 The "motor area" is definitely located; the "visual area" likewise; 
 and the location of the areas for hearing and smell is only a little 
 less definite. 
 
CHAPTER X 
 
 THE CEREBRAL HEMISPHERES AND THEIR FUNCTIONS 
 (CONTINUED) 
 
 1. The first functional area to be localized was the motor area. 
 The pioneer work of Fritsch and Hitzig 1 located it in the case of 
 the dog (Fig. 100). Its position is far forward on the surface of the 
 dog's cortex, in the "sigmoid gyre." Here a small area was de- 
 tected, on stimulating which with a weak current of electricity, the 
 muscles of the neck were thrown into 
 contraction; another similar area 
 was found for extension of the fore 
 limb; another for flexion of the same 
 limb; and another for facial move- 
 ments. Movements of the back, 
 abdomen, and tail were also ob- 
 tained, though their precise area was 
 not, at that time, sharply localized. 
 Many later observers have confirmed 
 and refined these results. As it is 
 not our purpose here to give a com- 
 parative description of cerebral local- 
 ization in different animals, we may 
 simply note that motor areas have 
 been located in other carnivora, in 
 rodents, and indeed in various orders 
 of animals. We turn at once to the 
 primates. Ferrier rendered service 2 
 to the growing science of localiza- 
 tion by extending the study to the 
 monkeys an important extension, 
 because of the similarity of shape and fissuration between the brains 
 of these animals and the human brain; and also because a much 
 more detailed localization was found possible in the primate brain. 
 Ferrier located the motor area in the central region of the monkey's 
 brain, and, indeed, in both of the central gyres, the precentral and 
 the postcentral. In other words, his motor area extended along the 
 
 1 Archiv f. Anat. und PhysioL, 1870, pp. 312 f. 
 
 2 Functions of the Brain (London, 1876); 2d edition, 1886. 
 
 235 
 
 Fia. 100. Motor Area of the Dog. 
 (Fritsch and Hitzig.) The two hem- 
 ispheres are drawn from different 
 animals, a, the sulcus, around 
 which the sigmoid gyre bends; A, 
 area for muscles of the neck; +, fore 
 limb; *, hind limb, 000 , facial mus- 
 cles. 
 
236 THE CEREBRAL HEMISPHERES 
 
 central fissure, or fissure of Rolando, and on both sides of it. With- 
 in this area, he distinguished smaller areas for movements of dif- 
 ferent groups of muscles; movements of the hind limbs were ob- 
 tained from the upper part of the region, near the middle line, 
 while movements of the fore limbs were obtained about half-way 
 down, and movements of the face at the bottom of the region, near 
 the fissure of Sylvius. 
 
 2. Ferrier's results have been abundantly confirmed, with 
 one principal exception, which will be stated directly . We can 
 pass quickly over this work on the lower monkeys and consider the 
 case of the anthropoid apes, whose brain resembles the human 
 brain still more closely. Owing to the difficulty of obtaining ani- 
 mals for experimentation, these highest and most interesting forms 
 were not early examined by physiologists. Beevor and Horsley 
 published in 1890 1 a physiological study of the brain of a single 
 orang; and, more recently, Grlinbaum and Sherrington 2 have ex- 
 perimented on all three species of anthropoid apes, including a con- 
 siderable number of individuals. Their work, done in the light of 
 all the experience of previous investigators, and with improved 
 methods, assigns much the same position to the motor area in the 
 anthropoid brain as that found by Ferrier for the smaller monkeys. 
 Griinbaum and Sherrington, however, employed unipolar stimula- 
 tion (compare p. 229), by which means the electrical stimulus can 
 be more sharply limited in its application than by older methods; 
 for this reason, among others, they were able to show that the ex- 
 fcntable region did not extend to the rear of the central fissure, as 
 Terrier had found in the monkeys, but was limited to the precentral 
 gyre. This led to renewed examination 3 of the brain of the smaller 
 monkeys, with the result that here, too, the motor area was con- 
 fined to the anterior side of the central fissure. In this respect, 
 then, the results of Ferrier have received an important correction. 
 The excitable region extends down into the central fissure, and even 
 to its very bottom, but does not reach the free surface of the post- 
 central gyre. The limits of the motor region, in the forward di- 
 rection, are less sharp than on the rear. The area is broad at the 
 top, where it extends also over upon the mesial surface of the hemis- 
 phere; lower down the precentral gyre, it becomes narrower. 
 
 1 Philosophical Transactions of the Roy. Soc. of London, 1890, B. p. 129. 
 
 2 Proceedings of the Royal Society of London, 1901, LX1X, 206; and 1903, 
 LXXII, 152; Transactions of the Pathological Society of London, 1902, 53, 
 part I, pp. 127-136; Sherrington, Integrative Action of the Nervous System, 
 New York, 1906. 
 
 3 See the very extensive investigations of C. and O. Vogt, Journal /. Psy- 
 chol und NeuroL, 1907, VIII, 277. 
 
BRAINS OF MONKEYS AND ANTHROPOID APES 237 
 
 These investigators found (compare Figs. 101 and 102) the move- 
 ments of the different parts of the body represented within the motor 
 region about as Ferrier had said: the hind limbs were excitable 
 from the upper part, near the middle line; the area for the trunk lay 
 below these, opposite the upper of two well-marked curves in the 
 central fissure, which serve as valuable landmarks. Below this 
 same curve is the area for the arms; further below, and opposite 
 
 Uits cenfrdlis. 
 
 cords. 
 
 FIG. 101. Lateral Surface of the Brain of a Chimpanzee. (Grunbaum and Sherrington.) 
 The left hemisphere is shown, with the frontal lobe to the left. The extent of the 
 motor area is indicated by the darkened portion, though it should be understood that 
 a large part of the area lies in the central fissure. 
 
 the lower of the two bends of the fissure, is a region for the neck; 
 and below this, again, near the bottom of the fissure, lies the area 
 for the head and face. Still more minute localizations were found 
 to be possible: thus, within the arm area, the sequence, from above 
 downward, is shoulder, elbow, wrist, hand; and in the region cor- 
 responding to the lower extremity the sequence is pelvic region, 
 toes, ankle, knee, and hip. 
 
 Movements of the eyeballs were not obtained in the anthropoid 
 ape from stimulation of the precentral gyre; there was, however, a 
 considerable area further forward, in the middle and inferior frontal 
 gyres, by excitation of which conjugate movement of both eyes to 
 
238 
 
 THE CEREBRAL HEMISPHERES 
 
 the other side was obtained. This agrees with the results of experi- 
 ments with the monkey. 
 
 /$ 3. The method of extirpation, applied to the motor area of the 
 anthropoid brain, results in paralysis of the entire opposite side, 
 or of parts of this side. For example, extirpation of the hand area 
 of the right hemisphere causes an immediate and severe paralysis 
 of the left hand, but without any sign of paralysis in either the face 
 
 Sulc.Central. A ** 
 
 Sulccattoso X^r-,/ Sulc.srecencrmar0 
 
 mar$ 
 Sulc.parieto 
 
 C.S.S.4& 
 
 FIG. 102. Mesial Surface of the Brain of a Chimpanzee. (Grttnbaum and Sherring- 
 ton.) The left hemisphere is shown, with the frontal lobe to the right. 
 
 or the leg; and extirpation in the leg area causes paralysis of part 
 of the opposite leg. These symptoms are, however, as is usual 
 in such extirpations, recovered from, in large measure, in the course 
 of a few weeks. All these extirpations are in the precentral gyre; 
 when, on the contrary, part of the postcentral is cut out, no paraly- 
 sis results. 
 
 4. An uncritical transfer from the ape's brain to the human 
 brain, of the localizations obtained in these ways, would not be en- 
 tirely justified, in spite of the great similarity of the two, both as re- 
 spects their external form, and also the course of fibre-tracts with- 
 in them. But, as previously stated, it sometimes happens that sur- 
 geons, in operating on the brain, have occasion to use the electric 
 stimulus for the purpose of orientation. Previous to these results 
 of Griinbaum and Sherrington, observations of this sort had not 
 
PARALYSES OF THE MOTOR AREA 
 
 239 
 
 definitely contradicted the older localization, which placed the motor 
 area on both sides of the central fissure. But since the problem 
 has been more sharply defined, and their attention more definitely 
 directed, by the recent work on the 
 brains of anthropoid apes, oper- 
 ating surgeons have found that the 
 excitable region in the human cor- 
 tex also lies in the precentral, and 
 not the postcentral, gyre. The ar- 
 rangement of special centres within 
 this general region of the human 
 brain is substantially the same as 
 
 ^.^that in the anthropoid brain name- 
 ly, the leg at the top, the arm half- 
 way down, and the face at the bottom 
 
 - of the precentral gyre. 
 
 The evidence from paralyses re- 
 sulting from disease or injury of 
 this region in man is also in general 
 agreement with the same system of 
 localization. The anatomical meth- 
 od lends further support to the same 
 conclusion; for it is after lesion of 
 the precentral gyre that the pyram- 
 idal or cortico-spinal tracts de- 
 generate. And since there is no 
 doubt that these tracts are the prin- 
 cipal motor path (Fig. 103) of con- 
 nection between the cortex and the 
 cord, their origin from the precen- 
 tral gyre is strong evidence that this 
 gyre is the true "motor area." A 
 specially favorable opportunity for 
 examining this question is afforded 
 by cases of the disease known as 
 "amyotrophic lateral sclerosis." 
 
 This disease produces at the same Flo . 103 ._ Diagrara of the Motor 
 time a gradual atrophy or the mus- way from the cortex. 
 
 cles, and also progressive degenera- 
 tion of those portions of the nervous system which are connected 
 with the muscles ; namely, of the cells of the ventral horn of the cord, 
 and of the cortico-spinal tract throughout its whole extent. Ac- 
 cordingly the origin of these motor tracts in the cortex ought to be 
 similarly affected; and recent careful examination by several au- 
 
240 THE CEREBRAL HEMISPHERES 
 
 thorities 1 has shown that, in fact, the cortex of the precentral gyre 
 shows profound pathological changes in cases of this disease. The 
 "giant cells/ 7 which are characteristic of this gyre, are very much 
 / reduced in number, and the other cells are also affected. The large 
 fibres which issue from the cortex and which pass many of them 
 at least downward to form the pyramidal tracts have largely 
 disappeared; but no corresponding changes are found in the post- 
 central gyre. The bottom of the central fissure is, by this method, 
 indicated as being, in the human brain, the hinder boundary of 
 the motor region; and although the forward boundary is less sharply 
 defined, the changes are for the most part confined to the precentral 
 gyre. Still more convincing is the experimental study by Holmes 
 and May, 2 who, after severing the pyramidal tract, in different ani- 
 mals, located the exact origin of this tract by aid of the chroma- 
 tolysis which occurs in cells after their axons have been cut. This 
 symptom was limited to the precentral gyre (sometimes extending 
 slightly into the superior and middle frontal gyres), and it seemed 
 also to be limited to the giant pyramidal cells in the inner pyram- 
 idal layer. The authors conclude that " The cortico-spinal fibres 
 arise only from the giant pyramidal cells . . . and these cells 
 probably give origin only to cortico-spinal fibres." 
 
 5. To these lines of evidence may be added the fact that the 
 area thus marked out as motor is characterized by a peculiar struct- 
 ure; and that the limits of this structure are nearly the same as 
 those indicated by the other methods of localization. All in all, 
 then, there can be little further hesitation in accepting the locali- 
 zation of the motor area in the anthropoid brain, as valid also for 
 the human brain. 
 
 6. What, exactly, is the function of this so-called "motor" 
 area ? This question is not easy to answer. From the fact that 
 it is the origin of the principal path of conduction from the cortex 
 to the motor nuclei in the cord and brain-stem, the inference is 
 clearly valid, that the control of the cortex over the bodily move- 
 ments is largely exerted through this area. It may, therefore, be 
 regarded as a collecting centre for impulses from various parts of 
 the cortex the impulses thus collected giving rise to discharges 
 down the cortico-spinal tracts and so to muscular contractions 
 and relaxations. Since, however, there are no direct fibres from 
 the motor area, or from any part of the cortex, to the muscles, but 
 only fibres running to the lower motor nuclei, it would be more proper 
 
 1 Among others, see Campbell, Histological Studies on the Localization of Cere- 
 bral Function, pp. 38 ff. (Cambridge, 1905); Schroder, Journal fur Psycholo- 
 gie und Neurologic, 1910, XVI, 60-78; Janssens, ibid., 1910, XV, 245-256. 
 
 2 Brain, 1909, XXXII, 1. 
 
DEFINITION OF THE MOTOR AREA 241 
 
 to speak of the motor area as controlling the lower nuclei, than to 
 speak of it as controlling the muscles. The movements obtained 
 by stimulating the motor area are co-ordinated movements, in 
 much the same way as reflexes are co-ordinated. That is to say: 
 
 ; Neither isolated contractions of single muscles, nor general contrac- 
 tions of all the muscles in a limb, are usually obtained on exciting 
 the motor area. The movements are such as, for example, flex- 
 ions and extensions of the limbs, clenching or opening the fist, 
 pricking up the ear, mastication, turning both eyes to the side, 
 etc. In the higher apes, movements of separate fingers can be ob- 
 tained ; but even these are to be regarded as co-ordinated movements. 
 A clear indication of the co-ordinated nature of these movements 
 is the fact, first discovered by H. E. Hering and Sherrington, 1 that 
 the contraction of a group of muscles, when aroused by excitation 
 of its appropriate cortical area, is attended by relaxation of the an- 
 tagonistic muscles. This is the same result as appears in reflex 
 action (see p. 162). It is quite likely, then, that the same "central 
 cells " of the cord, which were conceived of as the mechanisms con- 
 trolling the co-ordination of reflexes, are excited by the fibres of the 
 cortico-spinal tract. In this case, the function of the motor area 
 may be said to be, the control of spinal co-ordinating mechanisms, 
 in accordance with impulses reaching the motor area from various 
 parts of the cortex. Accordingly, we can neither regard the motor 
 area as standing in direct relation with the muscles, nor as the orig- 
 inal starting-point of cortical - influence on the muscles. But it is 
 more properly thought of as an intermediary between the cortex 
 in general and the co-ordinating mechanisms of the cord and brain- 
 stem (compare p. 94). 
 
 7. The foregoing may be taken as a fairly safe and even ob- 
 vious induction. But there is still considerable difficulty in reach- 
 ing a satisfactory conception of the precise function of the motor 
 area. The difficulty arises chiefly from the facts of that restitu- 
 tion of function, which so often follows removal or disease of this 
 region, or of parts of it. In connection with the results of extir- 
 pating limited portions of the motor area of the anthropoid brain, 
 it was mentioned above that the paralysis of the arm or leg, which is 
 the immediate result, usually disappears in the course of a few weeks. 
 The same result has often been noted in man ; for, in certain cases 
 of irritation of small portions of the motor area, resulting in epilep- 
 tiform convulsions, it has long been found practicable to cure the 
 convulsions by cutting out the irritated portion; and, though this 
 gives rise to temporary paralysis of some group of muscles, the 
 
 l Pfluger's Archiv f. d. gesammte PhysioL, 1897, LXVIII, 222; Journ. of PhysioL, 
 1899, 23 Suppl. 
 
242 THE CEREBRAL HEMISPHERES 
 
 paralysis is recovered from, at least by young persons. When the 
 part destroyed by experiment on the animals, or by disease or acci- 
 dent in man, is more extensive, the recovery of function is less com- 
 plete. Locomotion is restored, at least to a considerable degree; 
 but in man's case, after extensive destruction of the motor area in 
 both hemispheres, the gait remains insecure and subject to spasm. 
 The more specialized movements are less completely restored. A 
 dog, deprived of the motor region on both sides, is at first pretty 
 completely paralyzed, except for reflexes and such vital movements 
 as breathing. Soon, however, he recovers locomotion; but he is 
 stated (Munk, Monakow) never to recover the use of the forepaw 
 as a hand for holding a bone, etc. 
 
 Now since restitution of function is not the result of restitution 
 of the injured parts of the brain by new growth, it is a puzzling 
 phenomenon. It has sometimes been explained as due to the taking 
 up of the function of the destroyed part by other parts ("vicarious 
 function"). Such an explanation seems improbable, since it would" 
 call for the growth of new projection fibres, a growth which prob- 
 ably does not occur; and since, moreover, it calls for too much fresh 
 learning of complicated functions. Another explanation is to the 
 effect that other parts of the cortex, besides the part which gives 
 rise on excitation to a given movement, are also connected with that 
 movement, though in a minor degree. They may, therefore, be 
 considered as auxiliary centres, unused to taking full charge of a 
 movement, yet always employed in conjunction with the principal 
 motor centres. After destruction of the principal centre, these 
 auxiliary centres, continuing to act, might in time come to exercise 
 an efficient control. Such auxiliary centres should be looked for 
 in the immediate neighborhood of the principal focus; or in the cor- 
 responding area of the other hemisphere. Thus the representation 
 of a given movement in the cortex might be centred at a given point 
 or small area, but still spread somewhat over neighboring areas. 
 There is indeed some evidence, anatomical as well as physiological, 
 that the motor area of the left hemisphere, though principally con- 
 nected with the right side of the cord and of the body, is connected 
 to a much slighter degree with its own side. It may even be, as 
 Von Monakow is inclined to believe, 1 that there are "motor" spots,, 
 scattered generally over the cortex, and connected by projection j 
 fibres, perhaps with the thalamus and mid-brain, and so, through \ 
 the thalamo-spinal, tecto-spinal, and rubro-spinal tracts, with the v 
 spinal cord. There do appear to be projection fibres issuing from 
 various parts of the cortex; but hitherto the existence of scattered 
 motor spots has not been demonstrated. 
 
 1 Ergebnisse der Physiologie, 1902, I, part 2, p. 611. 
 
FUNCTION OF THE MOTOR AREA 243 
 
 Another conception of Von Monakow 1 seems more worthy of at- 
 tention. It is a mistake, he urges, to consider the functions which 
 are lost immediately after the destruction of a given area of the cor- 
 tex as exclusively appertaining to that area. The area in question 
 was connected by nerve-fibres with other parts of the cortex and with 
 subcortical centres; and the destruction of the area cuts all these 
 fibres, abolishes their functions, and induces pathological changes 
 in them. It thus interferes with the function of the various parts 
 to which, or from which, they lead. These connected centres are 
 thus left in a subnormal condition; but although they might not dis- 
 charge their functions at once, they might come in time, since they 
 are not destroyed, to function again in a normal manner, or at least 
 in a manner approaching the normal. 
 
 Still another possible explanation of restitution of function fol- 
 lowing destruction of the motor area holds that the lower centres, 
 in the brain-stem and cord, are responsible for the movements after- 
 ward executed. This is likely to be correct to a certain extent 
 just as the subcortical centres are responsible, in normal condi- 
 tions, for much of the co-ordination and efficiency of voluntary 
 movements. But it can hardly be held that the cortex has noth- 
 ing to do with movements after restitution; because the movements 
 occur with some regard to volition and to other mental influences. 
 
 Some of the most suggestive and puzzling cases of the restitu- 
 tion of function in a manner to suggest a certain amount of substi- 
 tution of one cortical area for another, have recently occurred in 
 connection with the surgical practice of nerve-anastomosis. It has 
 been found, for example, that where the facial nerve had been com- 
 pletely severed and all its motor and sensory functions quite de- 
 stroyed, a partial, and in extremely favorable instances, an almost 
 entire recovery of functions could be obtained by uniting the pe- 
 ripheral end of the injured facial nerve with that portion of the ac- 
 cessory nerve which supplies the trapezius muscle. Now the cere- 
 bral areas which control these two nerves are not far distant in 
 space, but in their normal functions are largely different. It would 
 seem, therefore, that such cases must depend for their physiological 
 explanation on the spread of the impulse originating in the cortical 
 areas over their customary limits, when the demand for this is made 
 by abnormal conditions. And on the other hand, there can be no 
 doubt about their emphasizing the cortical factors in all cases of 
 recovered functions, since in these cases, imagination and will were 
 shown to be about the most influential forms of excitement, in the 
 interests of a restoration of function. 2 
 
 1 Op. cit., p. 569, and passim. 
 
 2 See a discussion of some of these cases: Ladd, "A Suggestive Case of Nerve- 
 Anastomosis," The Popular Science Monthly, August, 1905. 
 
244 THE CEREBRAL HEMISPHERES 
 
 
 8. Restitution of function has to be reckoned with in consid- 
 ering the localization of other functions besides the motor, and even 
 to a greater degree; for the localization of the sensory functions is 
 dependent more largely on the method of extirpation and on the 
 corresponding pathological method. The method of excitation 
 is not applicable for purposes of localization in most parts of the 
 cortex, since, with the exception of the motor area and of a few 
 other regions which will be mentioned in due course, the rest of the 
 cortex is " silent" under excitation; i. e., does not give rise to move- 
 
 Central fissure Su-pramargindl gyre 
 
 Angular gyre 
 
 Fissure of Sylvius 
 
 FIG. 104. The Somesthetic Area. 
 
 ments. It has only recently been found that the weak electric 
 current can be safely applied to an exposed surface of the human 
 cortex, in surgical cases, without the use of an anaesthetic; pain does 
 not result. It will therefore be possible, in the future, as it has not 
 been in the past, to explore, little by little, as occasion offers, the 
 surface of the brain with the exciting current, in conscious human 
 beings. In this way, direct and convincing testimony can be ob- 
 tained as to the sensations or ideas, if any, aroused by the excitation 
 of definite cortical areas. 
 
 9. Besides the motor area, there have been found areas which 
 are closely related to several of the senses. We pass, then, to the 
 consideration of the "sensory areas " (Fig. 104). 
 
 The history of the localization of the so-called "somesthetic 
 area," or area for the sense of "touch" which, in the older mean- 
 ing of the term, included the cutaneous senses and the muscular 
 sense is of interest. It was often observed that injuries to the 
 motor area, in both animals and man, were accompanied by loss of 
 
THE SOMESTHETIC AREA 245 
 
 sensation as well as by motor paralysis. If, for example, the injury 
 affected the upper part of the central gyres, on the left side, paraly- 
 sis, and also, in many cases, loss of sensation in the right leg, would 
 result. This led to the doctrine that the sensory and motor centres 
 are the same; and many authorities preferred to call the cortical 
 area involved sensory rather than motor. But there were, also, 
 many negative cases i. e., cases in which paralysis occurred without 
 loss of sensation, or in which loss of sensation occurred without paral- 
 ysis. Such facts as these seemed to disprove the doctrine that the 
 sensory and motor centres are coincident. The discovery that the 
 motor area, instead of occupying both sides of the central fissure, is * 
 confined to the frontal side, made necessary a complete revision of 
 all the older observations. It was no longer possible simply to 
 describe an injury as lying in the central gyres, without more minute 
 specification. Critical review of the older cases, and especially the 
 more attentive observation of recent cases, have seemed to show that 
 injury, when strictly confined to the precentral gyre, is not attended 
 by loss of sensation; the precentral gyre, or motor area, can there- 
 fore be excluded from the somesthetic area. But destruction of 
 the region immediately behind the central fissure is attended by 
 loss of sensation. Accordingly, the central fissure seems to be the 
 anterior boundary of the somesthetic area; although its posterior 
 limits cannot yet be distinctly assigned. Some authorities 1 are in- 
 clined to include within it, in addition to the postcentral gyre, the 
 front part of the superior and inferior parietal; while others 2 would 
 limit it to the postcentral, or even to the anterior part of that gyre. 
 Within this somesthetic area, the representation of the different 
 parts of the body seems to have about the same arrangement as in 
 the adjoining motor area: namely, the leg area at the top; the arm 
 area half-way down; and the head area at the bottom. In this 
 manner the sensory and the motor areas for a given member are 
 brought into close proximity to each other, but on opposite sides 
 of the central fissure. 
 
 10. The losses of sensation which attend destruction of the 
 somesthetic area do not affect all varieties of sensation equally. The 
 pain sense is little or not at all affected, except temporarily; the sense 
 of pressure and contact is considerably more diminished; the tem- 
 perature sense is so much reduced that only extremes of heat and 
 cold are perceived; the muscular sense is almost entirely destroyed; 
 and the perception of form, size, location, etc., by use of the hand 
 is usually abolished. These are the permanent results of extensive 
 
 1 Von Monakow, Ergebnisse der Physiologic, 1902, I, part 2, pp. 621-641. 
 
 2 Campbell, Histological Studies in the Localization of Cerebral Function, 1905, 
 pp. 97 ff. 
 
246 THE CEREBRAL HEMISPHERES 
 
 destruction involving the postcentral and front part of the parietal 
 regions. 1 This would seem to indicate that the ability to make 
 fine distinctions in sensations and to recognize their "intellectual" 
 .qualities is most easily lost. 
 
 11. To the pathological and experimental evidence just men- 
 tioned may be added the evidence of an anatomical nature in favor 
 of locating the somesthetic area just behind the central fissure. 
 /"The primary receiving station for the body senses must be that 
 I region to which the sensory tracts from the cord and brain-stem 
 > NeaiL In a previous chapter (compare pp. 81, 89 f.), these tracts were 
 ^traced through the "fillet" to the thalamus; they run to a particular 
 part of the thalamus, namely, the ventral nucleus, where the fibres 
 from the cord and bulb terminate. Prom this part of the thalamus 
 issue fresh fibres, which pass to the cortex; and these have been 
 traced 2 by the degeneration method to the region behind the cen- 
 tral fissure. The myelinization method has led to much the same 
 result. 3 This part, therefore, may with some probability be ac- 
 cepted as the cortical termination of the sensory path from the skin 
 and muscles. On the whole, it seems a highly reasonable conject- 
 ure that the function of this somestheJJQ. area is that of receiving 
 sensory impulses, and of distributing them, by association fibres, 
 to neighboring and distant parts of the cortex. Accordingly, if 
 the postcentral gyre, close to the fissure, is the receiving station, the 
 posterior part of this gyre, and the adjoining margin of the parietal 
 convolutions, may be held to be concerned with the combination, 
 "elaboration," perception, interpretation, etc., of somesthetic data. 
 12. The visual area was first located with some exactness by 
 I Munk. 4 He found that removal of portions of the occipital lobe] 
 I was followed, in dogs and monkeys, by disturbances of vision; and 
 insisted that destructions of other portions of the cortex were not 
 necessarily followed by visual loss. Though this view was for a 
 time subject to much doubt and contradiction, the accumulation 
 of experimental and pathological data, and especially of anatomi- 
 cal facts, has shown that Munk's localization is correct. Ferrier, 
 indeed, located the visual area further forward, in the angular gyre; 
 but this is now agreed to have been an error, due to the fact that the 
 "optic radiation" i. e., the fibres leading from the external genicu- 
 
 1 Von Monakow, op. cit., pp. 629-630. 
 
 2 Von Monakow, op. cit., p. 636. 
 
 3 The myelinization method has not yet excluded the precentral gyre from be- 
 ing part of the receiving station; and the inner structure of the precentral is in 
 some important respects like that of known sensory centres. There is still a 
 chance that some sensory fibres lead directly to the motor area. 
 
 4 Ueber d. Functionen d. Grosshirnrinde (Berlin, 1881; 2d ed., 1890). 
 
THE VISUAL AREA 247 
 
 latum to the occipital lobe, and constituting the continuation of 
 the optic nerves lies close beneath the cortex of the angular gyre, 
 and thus is likely to be unintentionally affected by either stimulation 
 or extirpation of this region. 
 
 The particular sort of visual disturbance resulting from injury 
 or disease within the occipital lobe varies greatly in different cases. 
 In some cases it is described as "psychic blindness," under which 
 head may be included several varieties, such as inability to recog- 
 nize objects, or to read, or to perceive colors, or to utilize vision for 
 
 Parielo-occipital 
 fissure 
 
 Cuneus 
 
 Calcarine 
 fissure 
 
 Lingual 
 gyre 
 
 FIG. 105. The Visual Area in the Occipital Lobe. 
 
 purposes of orientation. All of these symptoms may occur with- 
 out blindness in the strict sense; in such cases, the patient can still 
 see, but has lost the uses of sight, or some of them. In another class 
 of cases, there is complete blindness for a certain part of the field 
 of view. If the occipital lobe is destroyed in the right hemisphere, 
 for instance, the patient cannot see what lies to his left, or, more 
 precisely, what lies to the left of the vertical meridian of his eyes. 
 It is as if the right half of each retina were blinded. This form of 
 blindness is called "hemianopsia." The peculiar distribution of 
 such blindness is well explained by the semi-decussation of the optic 
 nerves at the chiasm (compare pp. 91 f.). In hemianopsia, however, 
 central or foveal vision is not destroyed in either eye, which leads to 
 the supposition that the fibres from the fovea of each eye are dis- 
 tributed to the occipital lobes of both hemispheres. In hemian- 
 
248 THE CEREBRAL HEMISPHERES 
 
 opsia, blindness for the affected half of the field of vision is com- 
 plete; all that is left, in man's case, is the pupillary reflex. 
 
 13. Clearly, hemianopsia is a more radical form of blindness 
 than is "psychical blindness." It must, therefore, be considered 
 as the sign of destruction of the primary receiving station for nerve 
 impulses from the eye, or of the fibres leading from the eye to that 
 receiving station. The localization of this primary visual area is 
 therefore the first step required toward a more precise localization of 
 visual functions within the wider limits of the occipital lobe. To 
 Henschen 1 is due the credit for a sharper localization of this pri- 
 mary visual area. On the basis of clinical cases, he located it in a 
 relatively small portion of the occipital lobe, a portion which lies, 
 in man, almost entirely on the mesial surface, and in the immediate 
 neighborhood of the calcarine fissure. Later observers have on 
 the whole confirmed Henschen's conclusion that hemianopsia, 
 when it is due strictly to injury of the cortex, is dependent on lesion 
 of the calcarine region (Fig. 105). 
 
 Anatomical evidence of convincing character is available in con- 
 firmation of this localization It will be recalled (see p. 92) that 
 the optic nerves, after their semi-decussation, terminate in thein- 
 terbrain, and for the most part in the external geniculate bodies. 
 The fibres which arise from the geniculatum have been traced by 
 Flechsig, Vogt, and others, by the method of myelinization; they 
 are found to pass back, underneath the angular gyre and the lateral 
 portion of the occipital lobe, and to terminate in the lips and im- 
 mediate neighborhood of the calcarine fissure. Other methods of 
 tracing fibres have led to concordant results. The facts, however, 
 that some few fibres pass from the lower optic centres to other parts 
 of the occipital lobe, and that central vision is often preserved after 
 the destruction of the calcarine region, would seem to favor the view 
 of Von Monakow. 2 He holds that central or foveal vision has no 
 sharply defined cortical centre. 
 
 14. Within the calcarine region, some degree of more minute 
 localization is indicated by the observations of Henschen and others. 
 Apparently the posterior part of the primary visual area is connected 
 with the lower part of the retinas, and the anterior part of the vis- 
 ual area with the upper part of the retinas. It would appear likely 
 that the retinas are projected, point for point, though perhaps not 
 quite so minutely as this, upon the visual cortex. The projection 
 of corresponding points of the two retinas must be superimposed 
 in the visual areas, the right half of each retina being projected on 
 
 1 Klinische und anatomische Beitrdge zur Pathologic des Gehirns (Upsala, 1890- 
 1892); "On the Visual Path and Centre," Brain, 1893, p. 170. 
 3 Ergebnisse der Physiologic, 1902, I, part 2, pp. 653, 661. 
 
PHENOMENA OF PSYCHICAL BLINDNESS 249 
 
 the right visual cortex, and the left half of each retina on the left 
 visual area. 
 
 ^^plxcitation of the occipital lobe gives rise to movements of the 
 eyes, and Schafer 1 found that different movements were obtained 
 by exciting different portions of this lobe; for example, excitation 
 of the posterior part gave an upward movement of both eyes. This 
 result is harmonious with that of Henschen just stated, since an 
 upward movement of the eyes is what occurs when a bright light 
 from above suddenly shines into the eye, striking the lower part 
 
 Central fissure 
 
 Superior 
 temporal gyre 
 
 Middle 
 temporal gyre 
 
 Inferior 
 temporal gyre 
 
 Fissure of Sylvius 
 
 FIG. 106. The Auditory Area. 
 
 of the retina. It is probable that our ordinary movements of the 
 eyes in looking at an object, i. e., in directing the centre of clear 
 vision upon it, are reactions through the visual area, and not through 
 the motor area. And motor fibres leading downward from the 
 visual area toward the mid-brain have been demonstrated by von 
 Monakow. 
 
 If the calcarine region is the receiving station for optic impulses, 
 it is probable that neighboring portions of the occipital lobes are 
 concerned with those more complicated visual functions, the loss 
 of which is betrayed by the different sorts of "psychical blindness." 
 
 15. The evidence in favor of an auditory centre is of the same 
 kind as that in favor of the visual ce'ntre; and the history of its local- 
 ization is much the same. There has been general agreement, from 
 an early period of the study, that the auditory functions were spe- 
 
 1 Brain, 1888, XI, 1; Textbook of Physiology, 1900, vol. II, p. 755. 
 
250- THE CEREBRAL HEMISPHERES 
 
 cially connected with the temporal lobe. The symptoms which re- 
 sult from lesions here cover much the same range as the visual 
 symptoms which attend lesions of the occipital lobe. They are in 
 some cases of the "psychic" variety, such as "word-deafness" 
 (inability to understand spoken words) or as "amusia" (inability 
 to apprehend melodies); or such as loss of memory for words or 
 for melodies. In other cases, there is genuine deafness in the oppo- 
 site ear; but the findings here seem not to be perfectly clear, per- 
 haps because of an incomplete decussation of the incoming fibres 
 from the organ of hearing. Complete deafness is not caused ex- 
 cept by destruction of both temporal regions. 
 
 Given this general localization of auditory (Fig. 106) functions in 
 the 'temporal lobe, the efforts of investigators were devoted to subdi- 
 viding the area, and especially to finding the primary receiving sta- 
 tion for impulses from the ear. Clinical evidence tends to limit the 
 primary auditory area to the upper temporal gyre, and to the rear 
 two-thirds of this gyre. Here again, as with the visual centre, the 
 tracing of the incoming path is of decisive importance. We have 
 already seen (pp. 90 f .) that the path from the cochlea, the organ of 
 hearing, is traced up to the inter-brain, and to that part of it which 
 is called the internal geniculatum. By the myelinization method 
 (Flechsig, Vogt), the fibres arising thence are seen to pass to the 
 first temporal gyre; and, indeed, to a limited portion of it, which lies, 
 in man, mostly within the fissure of Sylvius. This is, then, the 
 primary auditory area, the receiving and distributing station for 
 impulses coming in from the ear. Neighboring parts of the temporal 
 lobe, and perhaps also of the parietal and of the island, are "elab- 
 orative," combinational, or perceptive with respect to the audi- 
 tory impulses. Excitation of the temporal lobe, in animals, gives 
 rise to movements of the ears, and to turning of the eyes and head 
 to the opposite side. These are, in appearance, "listening" move- 
 ments, and their occurrence indicates that the primary motor ad- 
 justment to sound occurs through the auditory area rather than 
 through the motor area. 
 
 16. Regarding the cortical representation of the sense of smell, 
 we have already mentioned the evidence afforded by comparative 
 anatomy that the archipallium is concerned with this and allied 
 senses. The fibres from the olfactory part of the nose pass to the 
 olfactory lobe; and fibres can be traced back from this lobe to the 
 pyriform lobule, which, accordingly, is regarded as the receiving 
 station for olfactory impulses. The pyriform lobule usually shows 
 a high development in animals which make great use of the sense 
 of smell. In man, it is comparatively small. There is not much 
 pathological evidence regarding the localization of smell in the 
 
 C 
 
THE SO-CALLED "SILENT AREAS" 251 
 
 human cortex, and the fibre connections just mentioned, on which 
 the localization of the olfactory area is based, have been traced in 
 animals not closely related to man. But what pathological evi- 
 dence exists points, for the most part, to the pyriform lobule or to 
 its neighborhood. Excitation of this general region, by Ferrier, 
 gave movements of the nostrils an adjustment of the sense-organ 
 analogous to those which are got by exciting the visual and auditory 
 areas. 
 
 17. Little of a definite nature can be stated regarding the corti- 
 cal representation of the sense of taste; but it is believed to lie some- 
 where within the archipallium or its immediate neighborhood. 
 
 18. On reviewing the localizations which have been established, 
 we find that the retina is connected most directly with the calcarine 
 region; the organ of hearing with that portion of the first temporal 
 which lies in the side of the Sylvian fissure; the organ of smell with 
 the pyriform lobule; and the cutaneous and muscular senses with 
 the postcentral gyre; while the principal motor pathway arises from 
 the precentral gyre. Other special motor pathways, controlling the 
 adjustment of sense-organs, arise from the visual, auditory, and 
 olfactory areas. In addition, there is an area in the frontal lobe, 
 the connections of which are not as yet well made out, but which, 
 on excitation, gives movements of the eyes. With the exception of 
 this last area, the receiving and departing stations are of limited 
 extent; they occupy a comparatively small proportion of the entire 
 surface of the cortex, especially in the higher animals. They are 
 relatively smallest in the human brain. But this relative diminu- 
 tion, as we ascend the scale of animals, is owing to increase in the 
 intervening areas, which are not so directly connected with the sense- 
 organs or with the muscles. 
 
 What, now, are the functions of these "silent" areas? It is easy 
 to call them "intellectual centres"; but if such a term is used, the 
 implication of the word "intellectual" must be made very broad. 
 It must be made broad enough, for example, to cover the mental 
 performances of the dog, since even in the dog there is much "silent" 
 space left between the primary sensory and the motor areas. Flech- 
 sig proposed to call such areas "association centres"; but here 
 again if this term is used, it should, for the present, be under- 
 stood in an anatomical rather than in a psychological sense. It 
 should have reference, namely, to the association fibres; and the 
 idea conveyed should be that the association centres are those areas 
 whose connections are provided by these fibres; that is to say, their 
 connection with the organs of sense and of movement is indirect. 
 Even this cannot be stated as an absolutely valid distinction; for, 
 though the principal bundles of projection fibres are traceable to 
 
252 THE CEREBRAL HEMISPHERES 
 
 the sensory and motor areas, there seems to be a scattered distri- 
 bution of similar fibres over a large portion of the whole cortex. 
 
 19. With regard to the specific functions of other parts of the 
 cortex, not thus far discussed, we have little definite information; 
 and yet we are not wholly without grounds of conjecture. It js 
 obvious, for example, that the areas surrounding the calcarine 
 region are necessary for the proper utilization of visual data; that 
 portions of the temporal lobe, near the primary auditory station, 
 are necessary for the utilization of auditory data; and that parts of 
 the parietal lobe, adjoining the somesthetic area, are necessary for 
 the utilization of cutaneous and muscular sensation. These out- 
 lying regions are richly connected by short association fibres with 
 their respective sensory distributing stations. Most of the evi- 
 dence for these statements has been obtained from observation of 
 human patients suffering from brain diseases, but some evidence 
 ,_js also to be got from the results of extirpation in animals. Munk 
 / found that excising small portions of the temporal lobe, in dogs, 
 resulted not in deafness, for the animals would still prick up the 
 ears at a sound, but in what he called "psychic deafness," as shown 
 by the loss of habitual reactions to words of command, etc. Re- 
 movals of small parts of the occipital region were followed, similarly, 
 not by blindness, but by failure to recognize familiar objects, such 
 as food, or the master's hand extended as if to "shake hands." 
 
 Losses of function which follow destructions of limited portions 
 of the human cortex have been grouped under a considerable num- 
 ber of descriptive terms. These defects seldom occur singly; usu- 
 ally a number of them occur together; yet cases of isolated loss are 
 not unknown, and are properly regarded as specially important, 
 because they afford the best opportunity for purposes of localiza- 
 tion, and for analysis of complicated mental functions into compo- 
 nent functions of less complexity. 
 
 20. A number of defects have been distinguished, which result 
 from injury of the occipital lobe or of the adjacent part of the pari- 
 etal lobe. The principal ones are given below. 
 
 Alexia is inability to read, occurring, of course, in a person who 
 previously could read, and in one who has not become blind. In 
 such cases, the printed characters are seen clearly enough, but have 
 lost their significance. In "pure" cases, the individual can under- 
 stand what is spoken, and may even be able to write, though unable 
 to read what he has just written. 
 
 Asymbolia. The use of this term varies, at present, between a 
 strict definition, covering cases in which numerals, or other con- 
 ventional signs, are no longer grasped, and a wider definition, ac- 
 cording to which it includes cases of "object blindness." In the 
 
RESULTS OF INJURIES TO THE LOBES 253 
 
 latter cases, the patient is unable to recognize familiar objects; 
 the visual impression is no longer for him a sign of the object, 
 though he may know the object at once on handling it. 
 
 Achromatopsia. Occasionally there occurs, without blindness, 
 a loss of color vision in half of the field of view. 
 
 In some cases, there is, without blindness, a loss of the power to 
 perceive depth, or of the power to orientate oneself in familiar sur- 
 roundings. In other cases, there is deficiency in the clearness of 
 central vision; and in still others, a loss of visual imagination. Two 
 or more of these various defects are likely to be combined in the 
 same individual; but there is no necessary connection between any 
 two of them, nor between any one of them and hemianopsia. Nor 
 is it possible as yet to connect any of them with destruction of any 
 particular limited area; but, when taken together, they give some 
 notion of the function of the cortex in the neighborhood of the visual 
 area. 
 
 21. Analogous defects are met in certain cases of lesion of the 
 temporal lobe and neighboring parts of the island and parietal 
 lobe: 
 
 Amusia is a loss of ability to apprehend melodies and other 
 musical complexes; it has been observed in musicians afflicted with 
 brain disease. 
 
 Word-deafness is a similar inability to recognize the meaning of 
 spoken words. 
 
 Verbal amnesia and paraphasia belong to the aphasias. In the 
 former, there is abnormal difficulty in finding the right word to 
 express one's meaning. Resort must be had to round-about modes 
 of expression, or absurdly general terms, such as "the what-d' 
 you-call-it." In paraphasia, there is less hesitation, but much sub- 
 stitution of wrong words, or of jargon. 
 
 22. Injuries to the parietal lobe, in the neighborhood of the 
 postcentral gyre, are often attended by 
 
 Aster eognosis. This is a loss of the " stereognostic sense," or, 
 better, of the ability to recognize the size, shape, and consistency of 
 objects by handling them. Such judgments must depend on util- 
 izing a combination of impressions from both the cutaneous and 
 the muscular senses. 
 
 From a survey of these defects, and of the location of the lesions 
 which cause them, we may properly reach the general conclusion 
 that the immediate neighborhood of the receiving station for each 
 sense is concerned with functions which are closely related to that 
 sense. 
 
 23. Similar defects occur on the side of movement, and pass 
 by the general name of apraxia, of which several forms are recog- 
 
254 THE CEREBRAL HEMISPHERES 
 
 nized. In general, "apraxia" may be defined as a loss of ability 
 to perform learned or skilled acts, in the absence of paralysis, or 
 ataxia, or pronounced sensory or perceptional defect. The pa- 
 tient knows what he wishes to do, but cannot make his hands do 
 it; yet his hands are not paralyzed, and may be able to perform very 
 simple acts, and to take part in instinctive movements, such as 
 locomotion, or as pointing toward an object at which he is looking. 
 Often, indeed, there is paralysis of the right hand, with apraxia 
 of the left hand, which is not paralyzed. Sometimes the apraxia 
 is associated, not with motor paralysis of one hand, but with object- 
 blindness; but in these cases, the difficulty consists rather in not 
 knowing what is to be done, than in inability to carry out an in- 
 tention. Sometimes, also, the difficulty lies in the combination of 
 the single movements which, in proper sequence, make up a skilled 
 act. In this connection, it is worth noting that even very familiar 
 and apparently simple acts, such as lighting a candle or picking a 
 flower, are really composed of an orderly sequence of movements, 
 and therefore need to be learned, in childhood, by a long course of 
 training. The results of this training are lost, in some forms of 
 apraxia, in consequence of localized disease of the brain. There 
 are other cases in which apraxia is associated with a "general" de- 
 fect of intellect. 
 
 The close study of apraxia is only of recent date, and has been 
 prosecuted with special vigor by Liepmann 1 and Pick. 2 
 
 One case, narrated by Liepmann, is of special interest; because 
 the patient, while free from paralysis or ataxia on either side, was 
 apractic on the right side only. He was also free from "general 
 intellectual" defect, as was shown by his ability to describe the act 
 which he wished to perform, and by his performing it himself with 
 his left hand. With his right hand, however, he could not success- 
 fully perform even simple learned acts. The post-mortem examina- 
 tion of the brain in this case revealed destructions of the white mat- 
 ter in such situations as to separate the two central gyres of the left 
 hemisphere from the rest of that hemisphere, and also, through in- 
 jury of the callosum, from the right hemisphere. Being thus iso- 
 lated, the motor area for the right hand was beyond the influence of 
 other parts of the cortex concerned with the intention and the proper 
 ordering of separate movements into a skilled act. 
 
 Agraphia, or inability to write, may be regarded as a special 
 form of apraxia. It seldom occurs alone, but usually in associa- 
 
 1 H. Liepmann, Das Krankheitsbild bei Apraxie (Berlin, 1900); Der weitere 
 KrarikheitsverlauJ bei dem einseitig Apraktischen (Berlin, 1906); Ueber Storungen 
 des Handelns bei Gehirnkranken (Berlin, 1905). 
 
 3 A. Pick, Studien -fiber motorische Apraxie (Leipzig and Vienna, 1905). 
 
DISTURBANCES OF SPEECH FUNCTIONS 255 
 
 tion with some form of aphasia. In the few recorded cases where it 
 has occurred alone, examination of the brain has revealed destruc- 
 tion of the cortex in the middle frontal gyre, just forward of the motor 
 centre for the arm and hand. This localization is disputed by 
 many authorities, and is to be accepted only tentatively. 
 
 24. No class of learned and skilled acts is more highly special- 
 ized than the various linguistic performances, such as using articu- 
 late language in speaking, understanding what is spoken, reading 
 and writing. No losses of learned acts are, therefore, easier to de- 
 tect than are those of language. For this reason, the study of 
 aphasia, or loss of speech, has a much longer history than has the 
 study of apraxia or of asymbolia. And since aphasia is a result of 
 localized brain lesions, it has been, and is still, one of the chief con- 
 cerns of the student of cerebral localization. 
 
 For about a decade, however, previous to the discoveries of 
 Fritsch and Hitzig, in 1870, the facts which seemed definitely to 
 connect the loss of speech with a certain region of the left cerebral 
 hemisphere were nearly all to which any advocate of the localiza- 
 tion of the cerebral function could confidently appeal in behalf of 
 his theory. As long ago as 1825, Bouillaud located the articulation 
 of words in the frontal lobes. Subsequently (1836) M. Dax main- 
 tained the proposition that " lesions of the left half of the enceph- 
 alon are coincident with forgetfulness of the symbols of thought." 
 
 In treatises of the years 1861-1865, Broca first announced the 
 substantially true discovery that the gyrus frontalis inferior on the 
 left side of the cerebrum is especially concerned in using the pow- 
 er of speech. This circumstance he connected with the fact that 
 men generally use the left hemisphere more than the right for the 
 expression of thought with the right hand and arm, whether in 
 writing or in the mechanical arts. The literal meaning of the 
 statements made by Broca such as that this part of the brain is 
 "the seat of the faculty of articulate language" 1 is, however, not 
 simply inappropriate to the facts; it is even absurd. There is no 
 one "faculty" of language which can, in any possible meaning of 
 the word, be regarded as having its "seat" or locality confined ^o 
 some particular region of the brain. Speech involves, in a very: 
 complicated and large way, all the faculties; strictly speaking, then,\ 
 it cannot be located, with all its attendant operations of self-con- \ 
 scious, rational mind, in any one cerebral area. But that the J 
 phenomena of aphasia show some special connection of certain 
 cerebral centres with the complex process of apprehending and ex- 
 
 1 "Sur le siege de la faculte* du langage articule*," etc., Bull de la Soc. anat., 
 August, 1861; "Du siege de la faculte" du langage articule dans rhe"misphere 
 gauche du cerveau," Bull, de la Soc. d'anthropol, June, 1865. 
 
256 THE CEREBRAL HEMISPHERES 
 
 pressing articulate language, seems entitled to credit as an induc- 
 tion based upon a wide range of facts. Of course, in this particu- 
 lar attempt at localization of function, no real help can be derived 
 from experiments upon the lower animals. 
 
 25. The phenomena of various classes, among which the truly 
 aphasic cases must be discriminated, vary all the way from those 
 resembling the results of momentary inattention such as that of 
 the German professor who certified in writing, "A. B. has attended 
 my remarkable lectures in chemistry with inorganic assiduity" to 
 the impairment and utter loss of speech in progressive paralysis 
 with dementia. A few of the more curious and instructive in- 
 stances furnish facts like the following: The aphasic patient may 
 be entirely speechless, and yet understand what is said to him, and 
 be able to write his wishes down on paper. Some thus afflicted re- 
 tain the power to pronounce words of one syllable, but are obliged 
 to resort to writing in order to communicate anything further. Oth- 
 ers possess a small stock of words, which they make more servicea- 
 ble with expressive gestures. Others, still, are simply able to speak 
 "a few senseless, and often very extraordinary, syllables and words." 
 
 Among the surprising phenomena of the disease of aphasia, 
 none are perhaps more so than those occasioned by the ability 
 to utter certain syllables or words, when accompanied by an utter 
 inability to put the same letters into slightly different combination. 
 One patient, who could say "Bonjour, monsieur," tolerably well, 
 could not pronounce the word "bonbon" at all. Another, whose 
 vocabulary was almost entirely limited to the meaningless sylla- 
 bles, "cousisi," was quite unable to utter either "coucon" or "sisi." 
 The celebrated case of the aphasic Le Long, reported by Broca, 
 was that of a man confined to five words for his entire vocabulary. 
 These words were, "oui, non, tois, instead of trois, toujours, and 
 Le Lo instead of Le Long." The first two and the last were used 
 with their appropriate meaning; "tois" indicated all ideas of num- 
 ber whatever; and "toujours" was the word used when the patient 
 could not express his meaning by gestures and the other four 
 words. It appears, then, that Le Long could pronounce the 
 r in "toujours," but not in "trois," and the nasal sound in "non," 
 but not in his own name. In another class of cases, the aphasic 
 person can utter only a few or no words spontaneously and cor- 
 rectly, but can repeat and write without difficulty words that are 
 spoken before him. Such inability is somtimes called "simple 
 aphasia of recollection." Different classes of words, as a rule, slip 
 from the memory in succession, as it were. Proper names are 
 "most frequently forgotten; then substantives generally, and some- 
 times verbs, adjectives, pronouns, and all other parts of speech. 
 
DISTURBANCES OF SPEECH FUNCTIONS 
 
 257 
 
 "The more concrete the idea/' says Kussmaul, 1 "the more readily 
 the word to designate it is forgotten, when the memory fails." 
 Many cases of disease occur where the patient has lost the power 
 mentally to find the appropriate words, although his power of ar- 
 ticulation is unimpaired. Such disturbances of speech may, or may 
 not, be accompanied by a corresponding impairment of general in- 
 telligence. This complication increases the difficulty of studying 
 the phases of this disease. 
 
 Aphasia may also be accompanied by so-called "word-deafness" 
 and "word-blindness." Persons thus afflicted hear words as con- 
 
 Central fissure 
 
 Middle 
 
 frontal gyre 
 Precentral gyre 
 
 Inferior 
 
 frontal gyre 
 
 Fissure of Sylvius 
 
 FIG. 107. Areas of the Cortex Supposed to be Specially Connected with Speech. 
 
 fused murmurings, or see them as blurred images. The individ- 
 ual letters may be intelligently heard or read, but their combina- 
 tion has become unintelligible. The same thing sometimes happens 
 with figures; as in the case of the accountant who could read the 
 sum 766, figure for figure, but did not know what the figure 7 
 meant as placed before the two 6's. At other times the disturb- 
 ance of speech takes the form of grammatical ataxy, as it were, or 
 of verbal delirium a medley of words, partly in themselves signifi- 
 cant and partly unmeaning. 
 
 26. In the analysis of these varied defects of speech, an im- 
 portant step was taken by Wernicke, 2 who, in 1874, made a dis- 
 tinction between motor aphasia and sensory aphasia (compare 
 Fig. 107). Accepting Broca's localization as correct for the motor 
 
 1 Ziemssen's Cyclopaedia, XIV, p. 759. 
 
 3 Der aphasische Symptomencomplex (Breslau, 1874). 
 
258 THE CEREBRAL HEMISPHERES 
 
 form, he assigned for sensory aphasia quite a different seat namely, 
 in the temporal lobe of the left hemisphere. The distinguishing 
 symptom of motor aphasia is inability to combine articulatory move- 
 ments into spoken words, in the absence, however, of paralysis of 
 the muscles of articulation. The distinguishing symptom of the 
 sensory form, according to Wernicke, is a loss of the auditory images 
 of words. Now, since speech is before all an auditory affair, loss 
 of the auditory images of words is equivalent to a loss of memory for 
 words, and so of the use of speech. So reasoned Wernicke. Wheth- 
 er the psychological terms in which he expressed his theory are cor- 
 rect, or not, the distinction between motor and sensory aphasia has 
 proved to be valid; and also the separate localization of the two 
 forms. Even in milder cases, in which both the sensory and the 
 motor aphasic patient have retained some use of speech, there is 
 still a difference between the two. The motor aphasic shows much 
 hesitation and effort in enunciation, and speaks disjointedly, having 
 lost his hold on grammatical form; whereas the sensory aphasic 
 may speak fluently enough, and with preservation of grammatical 
 form, but with the wrong use of words, and in some cases in a per- 
 fect jargon. Wernicke also introduced the conceptions of "corti- 
 cal," "subcortical," and " transcortical " aphasia a subdivision 
 which can be applied to both main types. In the cortical vari- 
 eties, the centre itself is supposed to be destroyed, and therefore 
 the loss of function is complete. In the subcortical variety, the 
 centre, remaining intact, allows of an internal speech, which, how- 
 ever is disconnected, by subcortical lesion, from the ear, in the one 
 case, and from the muscles of articulation, in the other; so that 
 the patient becomes either word-deaf or else incapable of actually 
 enunciating. In the transcortical or associative variety, the centre, 
 by severance of the association fibres, is thought to be isolated from 
 other parts of the cortex concerned in the use of language. Though 
 it is not difficult to find cases presenting these varieties of symptoms, 
 the anatomical findings do not, as yet, clearly substantiate the in- 
 terpretations indicated. 
 
 27. In the recent history of the aphasia problem, great impor- 
 tance attaches to the work of P. Marie. 1 This authority concludes, 
 from extensive material of his own, and from a review of cases pre- 
 
 1 Marie's original articles appeared in the Semaine medicate, in 1906. His 
 point of view, and the evidence for it, are well presented in the extensive mono- 
 graph of his pupil Moutier, L'Aphasie de Broca, Paris, 1908. Marie's conten- 
 tions have provoked wide-spread discussion among authorities. Reference may 
 be made to the reviews and discussions by C. von Monakow, Ergebnisse der Physi- 
 ologic, 1907, VI, 334-605; and by A. Meyer, Psychological Bulletin, 1907, IV, 
 181-193; 1908, V, 275-282. 
 
DISCUSSION OF BROCAS SPEECH CENTRE 259 
 
 viously published, comprising the original cases of Broca, that 
 actual injury always extends beyond the limits of Broca's "speech 
 centre," and that the assignment of special speech functions to this 
 region is entirely unjustified. He even goes so far as to deny any 
 speech function to the third left frontal convolution. He holds 
 that the motor symptoms in aphasia are due, not to any cortical 
 lesion, but to destruction of parts lying beneath the Broca region, 
 especially the lenticular nucleus and the neighboring white matter. 
 The motor symptoms, taken alone, would not, in his view, con- 
 stitute a true aphasia, but simply a defect in the co-ordination of 
 the speech muscles. True aphasia, he believes, is always of the 
 Wernicke or "sensory" type, and is due to injury of "Wernicke's 
 region," namely the rear of the temporal lobe and adjoining parts 
 of the parietal. He objects, however, to calling this aphasia "sen- 
 sory," insisting that it is, rather, to be called intellectual. In sup- 
 port of this last contention, he points out that practically all cases 
 of true aphasia show other intellectual defects. 
 
 Without doubt it is time to inaugurate a much needed reform 
 in the study of aphasia, especially on the anatomical side. Above 
 all, do the negative cases, of which there are many with regard to 
 the Broca region, need to be taken much fuller account of than has 
 previously been done. The fact of a negative case is, however, 
 always hard to establish; since defects of speech may be recovered 
 from after a brain injury, and may never form part of the record 
 of the patient's clinical history. 1 The anatomical examination of 
 the brain of aphasic patients has usually been quite superficial, 
 from a modern stand-point, which demands that the underlying 
 portions, as well as the cortex, be microscopically examined in 
 "serial sections." This laborious method is only beginning to be 
 applied to cases of aphasia, and for this reason, most of the older 
 attempts to localize its various symptoms are subject to suspicion. 
 
 Marie's view of aphasia as essentially a loss of general intellectual 
 power corresponds indeed to the fact that patients who suffer from 
 disturbances of speech usually show also some degree of apraxia 
 or asymbolia, and are often mentally inefficient. In most cases, 
 they are persons of advanced years, and often with a more or less 
 diffused disturbance of cerebral circulation, or with other diseased 
 
 1 Compare, in this connection, the case recorded by Liepmann, in Journal /. 
 Psychologic u. Neurologic, 1907, IX, 284, in which almost complete destruction 
 of Broca 's region appeared on post-mortem examination of the brain of a man 
 whose hospital record showed no evidence of aphasia, but whose previous 
 history revealed a "shock," ten years before, which had been followed by 
 temporary aphasia. Cases like this are likely to be frequent among those which 
 are reported as completely negative. 
 
260 THE CEREBRAL HEMISPHERES 
 
 conditions which affect a considerable area of the cortex. Intel- 
 lectual deficiency in other directions, however, may be slight in 
 comparison with the disturbance of speech; and in surgical cases, 
 in which local destruction has been brought about in healthy brains, 
 it is not uncommon to find aphasia unattended by any marked 
 .intellectual defect. 1 
 
 28. The conclusion seems warranted, then, that there is a 
 group of functions, which are indispensable for language, but not 
 equally essential for all kinds of intellectual performance; and that 
 these functions may be disturbed or destroyed by partial destruc- 
 tion of the cerebrum. This is to express the conclusion very cau- 
 tiously. There can be scarcely any doubt, further, that the part 
 of the cortex which is most vulnerable as concerns the linguistic 
 functions is " Wernicke's region," in close proximity to the receiving 
 station for auditory impulses. Adolf Meyer, 2 who has studied 
 the subject, both clinically and anatomically, with great thorough- 
 ness, concludes that the special symptom, word-deafness, or, as 
 he prefers to call it, " word-imperception," is the result of destruc- 
 tion of the primary auditory receiving station in the left hemi- 
 sphere; whereas paraphasia "jargon-aphasia," and verbal amnesia 
 are associated with destructions within the rest of Wernicke's 
 region. This region contains nervous mechanisms which are es- 
 sential to orderly speech, and which are more closely related to 
 the understanding of heard speech than to co-ordination of the 
 muscles of articulation. We may. then properly speak of a speech 
 area in the temporal lobe, with the understanding that the area is 
 too broad to be a unit in function; although special localizations 
 within it are not yet clearly made out. 
 
 29. To return now to the historic speech area of Broca, which 
 has been assailed with such energy: Marie's attempt to show that 
 the injury which causes the motor symptoms of aphasia is located 
 in the neighborhood of the lenticular nucleus has not met with 
 much favor; and clear cases have been brought forward 3 in which 
 the destruction involved Broca's region, without affecting deeper- 
 lying parts. Such negative cases are, indeed, not fully convincing 
 (see p. 259), because of the possibility of recovery later. But the 
 fact of restitution of motor speech, after extensive or even complete 
 destruction of the third left frontal gyre, is an undoubted fact; and 
 it is not exceptional. Restitution of function is as puzzling a fact 
 here as in the case of injury to the motor area (compare p. 241). 
 It cannot be interpreted as due to the assumption of linguistic func- 
 
 1 C. v. Monakow, op. cit., p. 395. 
 
 2 Journal /. Psychologic u. Neurologic, 1908, XIII, pp. 203 ff. 
 
 8 See Liepmann, Journal fur Psychologic u. Neurologic, 1907, X, 280. 
 
DISCUSSION OF BROCAS SPEECH CENTRE 261 
 
 tions by some part of the cortex which previously had nothing to 
 do with them; for the recovery is often very sudden; and besides this 
 objection, such an elaborate system of reactions cannot be easily 
 learned at the time of life of most sufferers from aphasia. Usually, 
 after restitution in case of injury to the Broca region, there are some 
 residual defects, such as difficulty in pronouncing long words, slow 
 and hesitating speech, with perhaps stuttering, spasm, easy fa- 
 tigue; also difficulty in managing the grammatical constructions. 1 
 The amount of restitution depends, in part, on the general condi- 
 tion of the brain and of the bodily health. In some cases, with poor 
 general condition, loss of the Broca region alone has brought about 
 complete motor aphasia, without later recovery. Also, there is 
 good evidence that the Broca region is the most vulnerable part of 
 the cortex, as regards the motor co-ordination of speech. 
 
 There seems, therefore, on the whole, to be good ground for still 
 retaining Broca's speech centre; while relieving it of part of its 
 supposed duties. It is probable that the rearmost part of the third 
 frontal gyre, in the left hemisphere, is intimately connected with the 
 adjoining part of the precentral gyre, from which issues the motor 
 pathway to the muscles of speech. It is further probable that this 
 part of the third frontal has something to do with the combination 
 of elementary movements of the organs of speech into those highly 
 practised sequences which constitute words. Psychological anal- 
 ysis of the exceedingly complex mental activities involved in all the 
 uses of human spoken and written language tends to confirm the 
 conjecture of such writers as Campbell and Von Monakow, that 
 speech functions cover a somewhat wider area in the frontal lobe. 
 Among such functions might be those combinations which are of 
 a higher order than that of elementary vocal movements into spoken 
 words, such as phrasing, inflection of the voice, syntactical forms, 
 etc., etc. Indeed, the entire cerebrum would seem to be, of neces- 
 sity, involved in man's linguistic attainments and uses. 
 
 30. A writing centre in the second frontal gyre, in close prox- 
 imity to the motor area for the hand, has been asserted as the teach- 
 ing of a few strikingly positive cases; but the negative and mixed 
 cases have sufficient weight to prevent a general acceptance of this 
 localization. Since the neighborhood of each sensory receiving 
 station is concerned with the impressions received at that station 
 from its sense-organ, it is a tempting conjecture that the writing area 
 lies close to the motor area of the hand. This conjecture is fortified 
 by the analogous case of the localization of the organs of speech. And 
 there is some, but not as yet conclusive, evidence that the frontal 
 
 1 Von Monakow, op. cit., p. 528. 
 
262 THE CEREBRAL HEMISPHERES 
 
 region, just forward of the motor area, in the left hemisphere, is 
 of special importance in all learned and skilled movements. 
 
 31. Even after adding to the primary sensory areas those neigh- 
 boring areas that are probably concerned in the utilization of sen- 
 sory data, and allowing a similar bordering zone along the motor 
 area for the combination of elementary movements into learned 
 actions, there remains a considerable part of the cortex of the human 
 
 in without known relationship to any special function. Con- 
 siderable portions of the parietal and temporal lobes are still un- 
 accounted for; also the island; and, especially, a large part of the 
 frontal lobes. The frontal lobes, because of their superior develop- 
 ment in man, have long been regarded with special interest. They 
 have been by many suspected of being the great intellectual cen- 
 tres. Statistics of the symptoms accompanying brain tumors 
 in different regions 1 have shown intellectual disturbances in 80 
 per cent, of the cases, when the tumor was in the frontal lobes, as 
 compared with 54 to 66 per cent, when the tumor was located 
 elsewhere on the cortex. Bolton, 2 after examining many cases of 
 dementia and idiocy, found that the cortex was usually thinned out, 
 especially in the frontal lobes; and from this fact he concluded that 
 the frontal region is the organ of attention and general "orderly 
 co-ordination of psychic processes." Loss of power of attention 
 and of inhibition has been claimed by several observers to be the 
 result of injuries to this region. " Witzelsucht," or an addiction to 
 practical jokes of a weak order, with lack of respect for propriety 
 or the rights of others, has been frequently observed. Some au- 
 thors use terms as broad and vague as "loss of character" or "of 
 personality," in attempting to formulate the symptoms of frontal 
 injury. Indeed, all of the clinical observations are rather vaguely 
 formulated as regards the mental symptoms. On the contrary, in 
 some remarkable cases of destruction of large parts of the frontal 
 lobes, no marked symptoms whatever have appeared. Injury of 
 the right frontal lobe has more frequently been attended by no 
 marked mental symptoms than has injury of the corresponding 
 area in the left hemisphere. 
 
 32. Of experimental investigations of the frontal lobe in ani- 
 mals, we may recall the existence of an area, which on excitation 
 responded with movements of eye, ear, and head. Changes in the 
 rate of breathing and of the heart beat, were also sometimes ob- 
 served. 3 This excitable frontal area is not yet fully understood. 
 Of work done by the method of extirpation, the most promising is 
 
 1 Compare Schuster, Psychische Storungen bei Hirntumoren (Stuttgart, 1902). 
 * Brain, 1903, XXVI, 215-241; Journal of Mental Science, 1905, 1906. 
 3 Langelaan and Beyerman, Brain, 1903, XXVI, 81-93. 
 
FUNCTIONS OF THE FRONTAL LOBE 263 
 
 that of Franz, performed by a combination of physiological and psy- 
 chological methods. Having first taught a cat or monkey certain 
 specialized acts, such as getting into a cage by turning a certain but- 
 ton or pulling a certain string, Franz 1 removed parts of the frontal 
 lobes, and later tested the animal to see if it retained the recently 
 learned act. In general the act was not retained after the operation. 
 The objection that the shock of operation, or the mere removal 
 of a certain quantity of brain substance, no matter from what locality, 
 was sufficient to explain the loss, was met by operations in other 
 regions, which were not followed by loss of the learned act. Some- 
 times removals of parts of the frontal lobe itself were not followed 
 by such loss; and the act could be relearned after the frontal opera- 
 tion, the new learning taking about the same time as if the previous 
 learning had not occurred. Habits of long standing, on the other 
 hand, were not disturbed by such extirpations. Injury to only one 
 of the frontal lobes caused, indeed, a slowing of the act, but not a 
 loss of it. The author concludes that the frontal lobes are con- 
 cerned in the acquisition of new performances of the sort used; but 
 that no one spot is indispensable for the acquisition of a particular 
 act; and that long-continued practice in a performance reduces it 
 to an automatic or semi-reflex condition, in which the frontal lobes 
 are no longer necessary. 
 
 33. Attention should be called to the predominance of the left 
 hemisphere in right-handed persons. The various defects which 
 have been described as resulting from injury to different portions 
 of the cortex usually result from injury to the left hemisphere. 
 Simple paralysis or loss of sensation may result, indeed, from in- 
 V jury to either hemisphere. But object-blindness, word-blindness 
 or word-deafness, the various aphasias and apraxias, usually result 
 from injury to the left hemisphere. From a study of ninety cases 
 of hemiplegia (one-sided paralysis due to injury to the motor area 
 or the white matter beneath it), Liepmann 2 found that injuries to 
 the left hemisphere which caused paralysis of the right hand caused 
 also awkwardness and disturbance of skilled movements in the left 
 hand. Injury of the callosum by severing the connection between 
 the two hemispheres resulted in disturbance of skilled movements 
 of the left hand, but not of the right. 
 
 It would almost seem, from the evidence obtained, that the left 
 hemisphere so completely takes charge of acts of skill, and of the in- 
 tellectual processes concerned in them, as to leave nothing for the 
 great bulk of the right hemisphere to do. Such a conclusion is, 
 
 1 "The Frontal Lobes," Archives of Psychology, 1907, No. 2. 
 
 2 Munchner medicinische Wochenschrift, 1905, reviewed in Journal f. Psychol. 
 u. Neurol, 1906, VII, 190. 
 
264 THE CEREBRAL HEMISPHERES 
 
 of course, in itself extremely improbable, especially in view of the 
 nearly equal size and inner development of the two hemispheres; 
 but it must be admitted that the role of the right hemisphere, aside 
 from the simplest sensory and motor functions, is not at all clearly 
 made out. In general, it may be that of assistance, or in case of 
 need, of substitution, for the more constant and important functions 
 of the left hemisphere: and for this view there is a considerable 
 accumulation of evidence. This special culture of the left hemi- 
 sphere if we may so express the fact may well enough be con- 
 nected, both as cause and effect, with the prevalent right-handedness 
 of the human species. 
 
 34. We have now passed in review all of the localizations of 
 function in the cortex which have much claim to attention. With 
 the exception of the sensory and motor areas, the localizations so 
 far established are somewhat vague, as respects both their extent 
 on the cortex, and also the exact function which is performed by 
 any given area. What is needed, before a more satisfactory ap- 
 portionment of functions over the cortex can be attained, is, on the 
 physiological side, a more detailed knowledge of the structure of 
 the cortex as a whole, and in its different parts; and, on the psycho- 
 logical side, a thorough analysis of such vague and gross so-called 
 functions as "speech," or "skilled movement," or "perception of 
 objects," or "orientation in space," into their elementary func- 
 tional factors. It is highly probable that any concrete mental per- 
 formance involves, physiologically, a complex of activities of various 
 parts of the brain; the performance as a whole, therefore, cannot be 
 localized, although the elementary functions may and, without 
 doubt, do each depend on certain particular nervous connections 
 that have a definite location. It is certain, psychologically, that 
 all these mental activities involve a vast and tangled complex of 
 simpler factors, which have either never come before us for con- 
 scious inspection, or have already been quite lost out of conscious- 
 ness, for purposes of such inspection. But to this view of the whole 
 subject we shall return again. 
 
 35. On the histological side, the mapping of the cortex into 
 areas of different structure has recently made great advances. 
 
 The picture seen on examining a cross section of the cortex under 
 the microscope differs greatly according to the dye with which the 
 tissue has been stained. Some dyes stain the cells, some the 
 fibres. Prominent, for example, among the cell-stains is Nissl's 
 methylene blue stain; prominent among the fibre-stains is the hema- 
 toxylin of Weigert. The Golgi stain shows cells with their den- 
 drites and axons, but usually stains only a few of the whole number 
 of cells present, thus, however, making it easier to trace out those 
 
CELL-LAYERS IN THE CORTEX 
 
 265 
 
 that do show themselves. The silver-reduction methods of Cajal 
 and of Bielschowsky show the fibrils within the cells, and altogether 
 give the most complete pictures of the intricacy of the cortex. Each 
 
 of these methods has its peculiar ad- __ _. 
 
 vantages, and a combination of the 
 results of all is necessary in order to 
 get an adequate notion of the minuter 
 intricacies of cortical structure. 
 
 36. To begin with the cells, let 
 us consider a picture like that given 
 in Fig. 108. The cells here are seen 
 to be of several shapes, and of a 
 considerable range of sizes. Prom- 
 inent in the cerebral cortex are the 
 "pyramidal cells," or "pyramids," 
 cells of generally triangular appear- 
 ance, with a long apical dendrite 
 extending upward toward the outer 
 surface of the cortex and several 
 basal dendrites, extending horizon- 
 tally and obliquely downward. The 
 axon of a pyramidal cell emerges 
 from the base and usually passes 
 downward into the white matter, 
 giving off a few collaterals as it goes. 
 The pyramids differ in size from 
 small to large. 1 
 
 Besides these radially arranged 
 cells, there are many others of dif- 
 ferent orientation. Some send their 
 dendrites downward and their axon 
 upward into the outer layer; some 
 have short axons which split into 
 fine branches close to their cell; 
 
 Some present a general horizontal FIG. 108. Section through the Cerebral 
 
 arrangement of dendrites and axons. 
 Examples of these varieties are 
 shown in Fig. 109. 
 
 37. For purposes of histological examination and description, 
 it is convenient to divide the cortex into layers. The boundaries 
 between some of the layers are not perfectly definite, and authori- 
 ties have disagreed both as to the number to be recognized, and also 
 
 1 Cajal, Studien iiber die Hirnrinde des Menschen, Heft 2, pp. 27-28 (Leip- 
 zig, 1900). 
 
 Cortex, Stained by the Golgi Method. 
 (Kolliker.) The numbers refer to 
 the layers mentioned on p. 266. 
 
266 
 
 THE CEREBRAL HEMISPHERES 
 
 as to their names. A division into six layers has very high author- 
 ity, and is defended by Brodmann 1 as being the fundamental 
 type throughout the neopallium of all orders of mammals. The 
 
 six layers are, he states, 
 clearly visible everywhere at 
 an early stage of develop- 
 ment; but he admits that 
 they become obscured in 
 the adult condition. These 
 six layers, arranged in their 
 order from the external sur- 
 face to the white matter, 
 may be named: 
 
 (1) The zonal or plexi- 
 form layer. 
 
 (2) The outer granule or 
 small-celled layer. 
 
 (3) The outer pyramidal 
 layer. 
 
 (4) The inner granule 
 layer. 
 
 (5) The inner pyramidal 
 layer. 
 
 (6) The spindle or multi- 
 form layer. 
 
 The plexiform layer, 
 though it contains cells, es- 
 pecially the so-called "hori- 
 zontal cells," consists mostly 
 of fibres. According to Ca- 
 jal, 2 some of these, the " tan- 
 gential fibres," arise from 
 the horizontal cells of this 
 layer, and may be regarded 
 as association fibres, and 
 others are axons from cells 
 lying in the deeper layers, or 
 
 dendrites from the pyramidal cells. The plexiform layer is therefore 
 a place in which numerous synaptic connections are formed be- 
 tween the various cells of the cortex. But the plexiform layer is 
 not the only plexus of nerve-fibres in the cortex. Intricate networks 
 
 1 Vergleichende Lokalisationslehre der Grosshirnrinde in ihren Principien dar- 
 gestellt auf Grund des Zellenbaus, pp. 14-42 (Leipzig, 1909). 
 
 2 Op. cit., p. 23. 
 
 FIG. 109. Different Types of Cells Found in the 
 Cortex. (Starr, Strong, and Learning.) 
 
CELL-LAYERS IN THE CORTEX 
 
 267 
 
 of fibres, medullated and unmedullated, exist in the various layers. 
 When the cortex is stained for medullated fibres (Fig. 110), some 
 layers are seen to contain many such fibres, and some only a few. 
 In the deepest layers, near the white matter, the network of hori- 
 zontal and oblique fibres is specially dense. Those layers which 
 show but few medullated fibres are not, however, free from a net- 
 
 FIQ. 110. Layers of the Cortical Cells and Fibres. (Vogt.) 
 
 work of fibres; for on treating the cortex with the Golgi stain, dense 
 plexuses are seen. Cajal 1 describes what he calls the sensory 
 plexus, which is very dense in the sensory and motor areas, and ex- 
 ists also, with less density, in the rest of the cortex. His tracing 
 of the fibres which give rise to this plexus leads him to believe that 
 they form the path of incoming impulses to the cortex (see Fig. 
 111). The outgoing path is, he thinks, provided by the axons of the 
 pyramidal cells; and the inner connections within any minute portion 
 of the cortex are provided partly, but not exclusively, through 
 
 1 Op. cit., p. 83. 
 
268 
 
 THE CEREBRAL HEMISPHERES 
 
 the plexiform layer. Numerous interweavings of fine dendrites and 
 terminations of axons and their collaterals occur at all levels. 
 38. Certain areas of the cortex especially the precentral gyre, 
 
 \ 
 
 FIG. 111. The Sensory Plexus of the Cortex. (Cajal.) A is the zonal layer. 
 
 the calcarine region, the pyriform lobe, and parts of the limbic 
 lobe are strikingly peculiar in structure (see Figs. 112 and 113). 
 The precentral gyre is characterized by the large size of its pyra- 
 mids, some of which are real giants, and are so named. These giant 
 
CELL-LAYERS IN THE CORTEX 
 
 269 
 
 pyramids are the chief origin of the long fibres of the cortico-spinal 
 tract. The calcarine cortex is marked by a very prominent stripe, 
 which is here called the stripe of Gennari, from the observer who 
 first noticed it in this area. 1 The 
 
 1 calcarine cortex also has a few 
 very large pyramids. Another of 
 
 2 its peculiarities is a layer of large 
 stellate cells between the two 
 layers of small cells into which 
 
 the internal granule layer is ' ffiSjpM 
 split up. Cajal 2 has attempted, -^'<'*^ 
 
 3 in Golgi preparations, to trace 
 the interrelations of the cells 
 and fibres in this part of the cor- 
 tex. His results lead him to con- 
 sider it probable that the sensory 
 
 4 impulses from the retina impinge 
 first on the large stellate cells, 
 which would thus constitute the 
 primary receiving station. The 
 axons of these cells he regards 
 as association fibres by which 
 
 J the effect of the visual impres- 
 sions is communicated to other 
 parts of the cortex; and the 
 motor fibres which are known to 
 originate from the calcarine re- 
 gion and which give rise to move- 
 ments of the eyes, he believes to 
 arise from the large pyramidal 
 cells. These cells and fibres 
 
 4a 
 
 4b 
 
 4c 
 
 ( 
 
 f Ni ryr 
 
 m- 
 
 Fl the n Motor rt A X rea f would thus account f or the partic- 
 (Lewis, from Fer- u j ar functions of the visual area. 
 
 Tier's Functions of or . T , , , , 
 
 the Brain.) 39. It has long been known 
 
 that the internal structure of the 
 
 FIG. 113. Cortex of the 
 Calcarine Area. (Lew- 
 is, from Ferrier's 
 Functions of the 
 Brain.) The splitting 
 of layer 4 into tbree 
 layers is characteristic 
 of this area. The 
 fibre-stripe of Gennari 
 comes in layer 4b. 
 
 cortex differs in different parts; but not till quite 
 
 recently has the laborious work of examining 
 
 systematically the whole cortex, and mapping 
 
 out the limits of each variety of its structure, been attempted. 
 
 This has now been done, independently, by different observers, 
 
 with results that are in close agreement; and this work is one 
 
 1 This stripe is visible, on close inspection, in most areas of the cortex, and 
 bears in general the name of Baillarger. 
 
 2 Studien uber die Hirnrinde des Menschen, Heft 1, pp. 62-70 (Leipzig, 1900). 
 
270 
 
 THE CEREBRAL HEMISPHERES 
 
 of the greatest recent ad- 
 ditions to our knowledge 
 of the brain. Campbell 
 made a complete examina- 
 tion of several human 
 brains, and also of the 
 brains of the anthropoid 
 ape and of the dog, cat, 
 and pig. His examination 
 was directed both to the 
 cells and to the medullated 
 fibres. 1 A yet more exten- 
 sive program is now in 
 process of execution in 
 Professor Vogt's Neuro- 
 biological Institute at Ber- 
 lin, the work being divided 
 among several collabora- 
 tors, and extending to all 
 orders of mammals. Brod- 
 mann 2 has published his 
 studies on the cells of the 
 cortex in various mammals. 
 Mauss 3 has issued a study 
 of the medullated fibres of 
 the cortex in monkeys ; and 
 Vogt 4 has given a map of 
 the frontal lobes in man as 
 divided on the basis of the 
 medullated fibres. The 
 maps of the cortex afforded 
 by these different studies 
 agree in the main, although 
 Campbell did not subdi- 
 
 1 A. W. Campbell, Histological 
 Studies on the Localization of 
 Cerebral Function (Cambridge, 
 1905). 
 
 2 Vergleichende Localisations- 
 lehre der Grosshirnrinde (Leip- 
 zig, 1908), and several papers in 
 
 FIG. 114. Medullated Fibres in Two Regions of the the Journal f. Psvchol. 
 Cortex. (Campbell.) A is from the precentral or 3 j m , rr , n i t PoL/,^7 ,, 
 motor area, which is extraordinarily rich in fibres; , fonc VTTT o?o oo^ 
 
 B is from the pref rental area, which is compara- Tol -> iyus > Alii, Zb6-625 
 tively poor in medullated fibres. * Ibid., 1910, XV, 221-232. 
 
HISTOLOGICAL MAPPING OF CORTEX 
 
 271 
 
 vide so minutely as did the Berlin observers; and among the lat- 
 ter, Vogt has been able, by studying the fibres, to make more sub- 
 divisions than were recognizable from the study of the cells alone. 
 40. The principles that have guided all these investigators in 
 marking the boundaries between the cerebral areas may be illus- 
 trated by a few examples. The precentral area is characterized 
 by the giant pyramids, and these stop abruptly at the bottom of the 
 central fissure, thus establishing the posterior border of this area; 
 its anterior border is less abrupt, but can be determined within a 
 
 FIG. 115. Histological Map of the Cortex. Lateral Surface. (Campbell.) The areas ex- 
 tend down into the fissures: thus, the precentral and postcentral meet at the bottom of 
 the central fissure; and the audito-sensory lies almost entirely within the fissure of Sylvius. 
 
 millimetre or two. Another characteristic of the precentral area 
 is its lack of a well-defined inner granular layer, and the consequent 
 fusion of the outer and inner pyramidal layers. The granule layer 
 appears promptly behind the central fissure. The thickness of 
 the cortex in the precentral area is very great, and medullated fibres 
 exist in extraordinary richness; the wealth of fibres is one of the dis- 
 tinctive marks of this area. 
 
 The calcarine area is distinguished by a splitting of the inner 
 granular layer into two, with a layer of large stellate cells between 
 (see p. 269); and in fibre preparations, by the distinctness of the 
 stripe of Gennari, which can be seen with the naked eye; hence 
 this area is named also the "area striata." The boundaries of 
 this area are perfectly sharp; the stripe appears suddenly, and the 
 subdivision of the granular layer is also very sudden. 
 
272 
 
 THE CEREBRAL HEMISPHERES 
 
 There are other instances of sudden transitions between one 
 type of structure and the adjoining types; but as a rule, the 
 transitions are less abrupt. The distinctions between neighbour- 
 ing areas are based partly on the fusion or subdivision of layers, 
 and partly on quantitative grounds, such as the thickness of the 
 whole cortex and of the separate layers, the size and frequency 
 of the cells, the calibre of the single fibres, and the density of the 
 fibre network. 
 
 41. Some of the areas thus marked out in terms of structure 
 by the accompanying histological maps (Figs. 115 and 116) are 
 
 FIG. 116. The Same, Mesial Surface. (Campbell.) Brodmann's map differs from this of 
 Campbell in that it subdivides many of the areas into two or more. 
 
 already familiar in the localization of function. The precentral 
 area, for example, coincides almost exactly with the motor area as 
 delimited by Grunbaum and Sherrington; the only difference is 
 that the excitable area extends a millimetre or two further forward 
 than the area of giant pyramids. The area striata coincides with 
 the visual area as marked out by the termination of the optic radia- 
 tion, and hence is named by Campbell the " visuo-sensory " area; 
 and there is an area in the temporal lobe corresponding to the end- 
 station of the auditory fibres. This correspondence of differences 
 of structure with the known differences in function lends force to 
 the view that the other differences in structure, over parts of the 
 
GROUPING OF FUNCTIONS IN CORTEX 273 
 
 cortex whose function is not known, may also be taken as indica- 
 tions of differences of function. 
 
 42. It is a reasonable conjecture, then, that these structural maps 
 of the cortex are functional maps as well. We cannot, indeed, as yet 
 infer the function from the structure; but it is certainly significant 
 that the motor area and the sensory areas present the appearance 
 of fuller development than do most of the other areas. These 
 other areas are also later in becoming myelinated in the individual, 
 and later in making their appearance in the race. They are, there- 
 fore, probably concerned with functions which are less used and 
 less essential than the elementary functions of sensation and move- 
 ment. 
 
 Another important suggestion is afforded by the fact that it is 
 possible to mark out areas which possess a nearly or quite uniform 
 structure. The fact that a uniform structure exists over any con- 
 siderable area of the cortex, giving place at its borders to areas of 
 other structure, would seem plainly to indicate that within each area 
 the elements have something in common in the manner of their 
 functioning. Another fact may be brought forward, which points 
 in the same direction. Rich provision seems to be made, both in 
 the plexiform layer and by the dense plexuses in deeper layers, for 
 causing neighboring cells to act together. It seems impossible, 
 then, that the cells in these areas should be thrown into action 
 singly; they must act in groups or neighborhoods. Some of these 
 neighborhoods may, indeed, be microscopic in size, much smaller 
 than the structural areas which are mapped out; also, it is probable 
 that these neighborhoods are not sharply limited in extent, but that 
 the co-operating cells, in any given function, are most active in a 
 certain part of each neighborhood, and less and less active the fur- 
 ther we go from this centre out. In this case, the neighborhoods 
 concerned in different allied functions would certainly overlap. 
 
 If such a conception as the foregoing is even half-way true, con- 
 tiguous spots of the cortex must possess related, and even over- 
 lapping functions; so that the transition from one function to an- 
 other would be gradual over a certain area; and this area would 
 accordingly be the seat of a certain group or range of related junc- 
 tions. Such considerations make it seem highly probable that the 
 structural areas which the histologists have mapped out are, indeed, 
 areas for different groups of functions. 
 
 43. But what are the "groups of functions" which we have a 
 right to expect to be thus localizable? As to this, little that is 
 definite and satisfactory can be said at present. It is a question 
 to the answer of which psychology must make the preliminary, 
 directing if not determining, contribution. And the analysis of 
 
274 THE CEREBRAL HEMISPHERES 
 
 mental functions into their elements, in a manner suitable for physi- 
 ological use, has scarcely been begun. The so-called "facul- 
 ties" of memory, imagination, attention, and reasoning, are scarcely 
 to be thought of in this connection. They are gross, unanalyzed 
 terms of a popular psychology, having no claim to represent ele- 
 mentary functions of the mental life. The results of local injury 
 to the cortex show that memory for visual objects is dependent 
 on the integrity of the occipital lobes; for auditory objects, on the 
 temporal lobes, etc.; and the same thing is true with respect to the 
 imagining of these objects. As for attention, which some l have 
 sought to localize in the frontal lobes, experience shows that any 
 sort of thing may, on occasion, claim the attention and drive out, 
 or inhibit, other claimants to control. In studying spinal inhibi- 
 tion (see pp. 172 f.) the fact was noted that different reflex functions 
 exert a mutually inhibitory influence; any one can, on occasion, in- 
 hibit the others. But this is apparently the case with mental 
 functions, when studied from the point of view of introspection, 
 as well; any one can, on occasion, be dominant and inhibit others 
 which are antagonistic to it. No one centre of attention, or of in- 
 hibition, can therefore be expected to be discovered. In general, 
 instead of seeking to localize such so-called " faculties," it is neces- 
 sary to start with complex and concrete acts and processes, and by 
 a study of analysis discover the more simple and elementary acts 
 and processes which have a value for the ends of physiological 
 psychology. Such analysis, however, is extraordinarily difficult; 
 and it is probable that in the future, as in the past, the efforts of 
 normal psychology will need to be supplemented by the study of 
 mutilated functions, as they are offered for observation by disease 
 and injury. 
 
 In spite of the fact that it has, of late, been much decried and even 
 ridiculed in certain quarters, we shall follow this method of careful 
 and minute analysis in the part of the work to which we now pro- 
 ceed. In this part it is our purpose to point out certain more spe- 
 cific forms of correlation which exist between our psychical ex- 
 periences and the functions of the nervous mechanism, when under 
 different kinds and degrees of external stimulation and variously 
 influenced by acquired habits and associations. But, first, it is in 
 place to summarize our conclusions as to the mechanical nature of 
 this system, taken as a whole. 
 
 1 Most prominently Wundt, Physiol. Psychol, 6th ed., 1908, I, 378. 
 
CHAPTER XI 
 MECHANICAL THEORY OF THE NERVOUS SYSTEM 
 
 1. The nature of the process by which the nervous system is 
 developed, as well as the nature of the developed structure and its 
 functions, so far as physical science can go at all, points in the di- 
 rection of a mechanical theory. But in respect to both, such a 
 theory is at present in an exceedingly fragmentary and uncertain 
 condition. Further investigations may largely remove the present 
 limitations. But the most complete theory possible can scarcely 
 be more than a statement of the order and extent of physical changes, 
 the real causes and meaning of which it lies beyond the power of a 
 mechanical theory to give. 
 
 The impregnated ovum does, indeed, become converted into the 
 developed organism by an evolution that, at every step in its course, 
 appears as an alteration in the arrangement of material molecules, 
 under conditions derived from the original nature of the molecules 
 themselves, from their necessary relations to each other, and from 
 the action of their total environment. By division of that which 
 was single into several parts, by bending of that which was straight, 
 by stretching in one direction and compressing elsewhere, by swell- 
 ing and dilating in the various outlines under the influence of press- 
 ure, by folding and tucking in so as to close up an opening here 
 and form another there, by laying down cells of the same kind in 
 right lines or grouping them in masses, etc. in brief, by motion 
 of particles of matter in such a way that the motion of each is con- 
 ditioned upon that of the others, the nervous mechanism is built 
 up. What it can accomplish in the way of further molecular mo- 
 tion, after it is thus built up, depends of course in large measure 
 upon what it is made to be by the very process of building. How 
 far it is possible even to propound a mechanical theory of the build- 
 ing process belongs to the speculations of embryologists to con- 
 sider. It is our problem, at present, to consider as a whole the few 
 data upon which it has been thought possible to base a mechanical 
 theory of the behavior of the nervous system after it has once been 
 constructed as a result of the embryonic process. 
 
 2. The machine-like nature of much of the structure and move- 
 ment of the human body does not escape the most ordinary obser- 
 
 275 
 

 276 THE NERVOUS MECHANISM 
 
 vation. When this body, either as a whole or with respect to some 
 of its parts, changes its position in space, its various masses sup- 
 port and act upon each other in essentially the same manner as 
 the masses of matter which compose the parts of any machine con- 
 structed by human skill. Such movement is possible for it, because 
 its framework of bones has a rigidity sufficient to sustain the other 
 less rigid organs; and because the bones are so divided, and yet 
 articulated, that they can assume different relations toward one an- 
 other in accordance with the simplest principles of mechanics. 
 The laws of the lever, of the pulley, the ball-and-socket joint, etc., 
 need no modification when applied to this particular machine of the 
 human body. 
 
 The action of certain other of its parts, which do not belong 
 to the bony framework but which are of muscular or epithelial 
 structure, is also plainly of the same machine-like character. The 
 movement of the heart, for example, is in part to be explained as 
 that of a pump with chambers and valves; and the flow of the 
 blood through the arteries as that of a fluid pumped through con- 
 duits, of unlike and changeable sizes. So, too, the lungs may be, 
 with considerable propriety, compared to bellows which alternately 
 suck in and expel the surrounding atmosphere. The optics of the 
 eye and the acoustics of the ear are special only so far as the struct- 
 ure of the organs makes necessary a special application of the gen- 
 eral laws of those sciences. Moreover, the distribution of the fluids 
 among the tissues of the body takes place in part, at least, if in part 
 only under the laws which govern the distribution of fluids generally 
 when separated by membranes which they can permeate. Nor is 
 the chemistry of the same tissues and fluids by any means wholly 
 unlike that with which the experiments of the laboratory make us 
 familiar. When, however, we begin to speak of those changes of 
 relative position which take place at extremely minute distances 
 among the molecular elements of which the larger masses of the 
 body are composed, we seem compelled to drop the conception of 
 a machine and to seek both another conception and another title. 
 
 The very attempt, then, to explain the motion of the more purely 
 machine-like parts of the human body, leads us to consider certain 
 activities of other parts for which the word "mechanism" is more 
 appropriate. The movement of none of the more or less rigid or- 
 gans of the body originates within these organs themselves. The 
 changes of relative position in the parts, with which the ordinary 
 laws of mechanics deal, imply antecedent molecular changes in 
 other parts with which these laws cannot deal. The motion which 
 finds its final expression in the swing of the arm, or of the leg, in 
 the lifting of a weight, and even in the contraction of the heart, or 
 
MACHINE-LIKE NATURE OF ORGANISM 277 
 
 in the rising and falling of the chest, does not begin in arm, or 
 leg, or ribs, or diaphragm, or cardiac muscles. The change of posi- 
 tion of so considerable masses of matter is but the summing-up of 
 innumerable minute molecular changes which began within the 
 body, but outside of the masses themselves. If, for example, we 
 inquire as to what causes the bones to move however strictly their 
 motion, once begun, may follow the laws of mechanics the answer 
 is to be found in the pull of the tendons, or cord-like structures, 
 which are attached to them. And if we then inquire, What causes 
 the tendons to pull upon the bones by means of their attachment ? 
 the answer must be, That it is the contraction of the muscles which 
 pulls upon the tendons. 
 
 The next step in following this chain of causes, however, intro- 
 duces us to a different class of considerations from any of the fore- 
 going. For we cannot say that the contraction of the muscles is 
 caused by the pull of the nerves upon them. The movement of 
 muscular fibre in contraction is an altogether different affair from 
 the movement of the bones as they are pulled by the muscles; nor 
 do the nerves act upon the muscles as the muscles act upon the 
 tendons. The elasticity of the muscles is, indeed, a mechanical 
 quality, like that of which we avail ourselves in the construction of 
 machines. But the quality of elasticity does not fully explain the 
 behavior of the so-called muscle-nerve machine when its muscular 
 tissue is contracting or relaxing. Yet the living muscle, in itself 
 considered, may certainly be looked upon as a molecular mechan- 
 ism. It is a system of minute particles of matter which act upon 
 each other at indefinitely small distances; and which, when any 
 motion is set up at one part of it, propagates such motion accord- 
 ing to laws that are given in the very constitution and arrangement 
 of the particles themselves. This is precisely what we understand 
 by a physical molecular mechanism. The office of the nerve with 
 respect to the muscle is simply, as we know, to start that molecular 
 activity which it is the function of the irritated muscle itself to ex- 
 ercise. The nerve, however, cannot perform its office of irritating 
 the muscle without being in a state of molecular commotion called 
 the "excitement" of the nerve. And, further, this excited condi- 
 tion of that part of the nerve which is in immediate contact with the 
 muscle is itself a state of the nerve which has been propagated from 
 a distant point of the nervous matter. All the machine-like move- 
 ments of the masses of the body require us, therefore, to look for 
 their origin in minute molecular changes that originate in its ner- 
 vous elements. And for the further account of these neural molec- 
 ular changes we are to look to a mechanical theory of the nervous 
 system. 
 
278 THE NERVOUS MECHANISM 
 
 3. The basis for a general view of the nervous system as a 
 mechanism has been laid in all the preceding examination; and 
 it cannot be denied that the results of this examination are such 
 as to dispose us favorably toward the attempt to develop such a 
 view into a complete mechanical theory. Physical science, as a 
 matter of course, strives to establish such a theory. It knows no 
 other way of considering any group of phenomena when brought 
 before it for examination. To deny totally the application of the 
 conception of a mechanism to the action of the nervous system 
 would be to refuse to apply to its phenomena the same scientific 
 treatment which we apply to all other physical phenomena. To 
 limit, a priori, such application would be to restrict improperly, on 
 merely theoretical grounds, the area of the phenomena with which 
 such science is entitled to deal. The fact that molecular changes 
 here are correlated with another class of phenomena which we call 
 "mental," in no wise destroys the propriety of pushing our physical 
 science of the nervous system to its furthest possible limits. The 
 movements of all material bodies, whether in the elemental shape 
 of the molecules, or in the shape of the same molecules when aggre- 
 gated into masses, as well as the laws under which such bodies in 
 movement act and react upon each other, constitute the legitimate 
 sphere of physical science. But it is to a system of interacting 
 molecules that the conception of mechanism especially applies. 
 The aim of physical research with regard to any given system of 
 this kind is, therefore, not accomplished until all the movements 
 of its different parts are explained in the light of a consistent me- 
 chanical theory. This general principle of all physical science 
 neither needs nor permits a special exception in the' case of the 
 human nerves, organs of sense, and brain. 
 
 On the other hand, the very unsatisfactory condition of the data 
 for a mechanical theory of the human nervous system has been im- 
 plied in each of the preceding chapters. It will appear all the 
 more plainly now as we present briefly a statement of two or three 
 such theories in the form in which it has been found possible for 
 different investigators to state and to defend them. Nor can we 
 express much confidence that physics and physiology combined 
 will ever be able to point to a complete theory of so intricate and 
 delicate a mechanism as this nervous system. Moreover, we do not 
 by any means affirm that a purely mechanical treatment, however 
 complete, would of itself suffice to furnish a satisfactory understand- 
 ing of all the phenomena; or even that the phenomena in general 
 could by any possibility be brought solely under the terms of such 
 treatment. We only affirm the unrestricted right of physical sci- 
 ence to attempt, in the light of the conception of mechanism, an ex- 
 
SIGNIFICANCE OF CHEMICAL CONSTITUTION 279 
 
 planation of the nervous system as well as of all other physical 
 objects; and also its right to its persistent faith that So far as 
 physical science can explain any object, all the special difficul- 
 ties of the nervous system can be fitly considered only in this way. 
 
 4. The chemical constitution and structural form of the ele- 
 ments of nervous matter require that the system which they com- 
 pose should be regarded in the light of the conception of mechan- 
 ism. It is true that physical science cannot give an accurate descrip- 
 tion of the chemical processes which take place in the formation of 
 the nerve-fibres and nerve-cells, or during their functional activity; 
 it cannot do so much as this for the living tissues generally. But 
 it finds here the same chemical elements which exist elsewhere 
 in nature, especially the four elements, oxygen, hydrogen, nitrogen, 
 and carbon. It nowhere finds these elements behaving differently 
 in the nervous system from the way in which it is their nature to 
 behave elsewhere, under similar circumstances. And the fact that 
 precisely similar circumstances do not occur to induce the same 
 combination and interaction of these elements outside of the ner- 
 vous system, is traced back to its causes in a succession of occur- 
 rences that all have the character belonging to the chemistry of 
 living tissues. We know of no sap which is suitable for forming 
 organisms in general, but which is itself a perfectly homogeneous 
 fluid. Nucleated granules in the very chemical constituents which 
 give conditions to all the subsequent activity of the molecules, are 
 revealed by microscopic examination of those cells from which the 
 whole body springs. This fact, together with the character of the 
 subsequent process, may lead some to insist that a certain special 
 form of energy (called "vital force," or by some less obnoxious 
 title) is marshalling the minute particles under its superior con- 
 trol. But such way of considering the phenomena whether ad- 
 missible or inadmissible does not at all help us to dispense with the 
 purely mechanical point of view. In the original living germ with 
 which the organism began, and in all its subsequent development, 
 every chemical change in nervous matter is nothing more than a 
 movement of physical molecules, strictly under the conditions fur- 
 nished by their constitution and previous arrangement. 
 
 5. The general significance of the form and chemical consti- 
 tution of the nervous tissue is by no means wholly obscure. Es- 
 pecially is this true in regard to the form of the minute structures 
 which compose the system. The long wire-like form of the nerve- 
 fibre is understood by reference to its function of conduction. Since 
 the nerve-fibre does nothing which is directly beneficial to the body, 
 but simply serves to transmit "impulses" which shall arouse ac- 
 tivity in the muscles and other effective organs, the smaller the size 
 
280 THE NERVOUS MECHANISM 
 
 of the nerve-fibre, the better. Within the central organs, the rami- 
 fying of the fine branches of the fibre and of the cell seems fitted for 
 the function which the centres are known to exert, namely that of 
 providing for co-ordinated action, and also for action which can 
 vary according to circumstances. 1 
 
 In regard to chemical composition, less is perhaps known that 
 affords an insight into the mechanism of nervous activity. The 
 protoplasmic portions of the cell-body, dendrites and axon, seem to 
 differ only in minor respects from the protoplasm of other tissues. 
 The lipoids of the sheath seem to play a part as insulators or non- 
 conductors. 
 
 6. There can be no doubt that the arrangement of the nervous 
 elements into a system corresponds to the conception of mechanism. 
 A certain work of concatenating the different physical systems of 
 the body, and of adjusting its relations to the changes in its en- 
 vironment, requires to be accomplished. This problem demands a 
 three-fold exercise of function; it is a problem in the construction 
 of a mechanism. The nervous system actually is of threefold con- 
 struction; its threefold construction is the answer which it prac- 
 tically makes to the above-mentioned problem. One part of the 
 complex problem consists in the conversion of certain of those 
 molecular motions which take place in nature outside of the living 
 organism into molecular motion within the tissues of such organism. 
 The solution of this part of the problem is furnished by the end- 
 organs of the nervous system. The end-organs are those -special 
 mechanisms which are adapted to convert the molecular motions 
 called stimuli into the molecular motions called neural excitation. 
 That by far the larger portion of the eye and ear, for example, acts 
 in a purely mechanical way, there is no doubt. It is the office of 
 the great mass of the eye to transmit and refract the rays of light; 
 of the ear to transmit and condense the acoustic waves. But when 
 the nervous elements of the retina and of the organ of Corti re- 
 ceive the physical processes transmitted to them, they transmute 
 these physical processes into physiological neural processes; in 
 doing this they act as special molecular mechanisms. 
 
 The second part of the complex problem before the nervous sys- 
 tem consists in the conduction in all necessary directions of these 
 neural processes; only on this condition can distant parts of the 
 nervous system act, as it were, in view of each other, and thus the 
 
 1 It is true that we cannot describe the peculiar place in the mechanism, or the 
 specific chemistry if such there be which belong to the different locations and 
 shapes of nerve-cells, bipolar, multipolar, stellate, etc. But all that we are learn- 
 ing about the minute structure and detailed functions of the nervous elements 
 seems to indicate that these more obvious mechanical characteristics are of 
 value in the entire complex working of the nervous system. 
 
ARRANGEMENT OF THE NERVOUS ELEMENTS 281 
 
 whole body be bound into a living unity under the influence of 
 changes in its environment, and in the ideas and impulses of the 
 mind. The nerve-fibres solve this part of the problem. This they 
 do by acting as mechanisms, which have such a molecular constitu- 
 tion and function that a commotion, started at any point in the 
 physical elements of the system, spreads from molecule to molecule, 
 in accordance with the laws of the system. 
 
 The third part of the same complex problem requires for its solu- 
 tion structures and functions still more intricate and inexplicable. 
 Incoming molecular disturbances must be modified and redistrib- 
 uted so as to give rise to outgoing molecular disturbances along 
 definite tracts, in order that definite groups of muscles may be made 
 to contract. Only in this way can the whole physical organism, by 
 a so-called reflex activity, adjust its condition, in view of the pres- 
 ence of given kinds and degrees of stimuli. Moreover, the vital 
 functions the movements that control respiration, digestion, cir- 
 culation of the blood and of other fluids, etc. must be united so as 
 to work to a common end, and with the modified forms and degrees 
 of their respective energies, which the changing circumstances re- 
 quire. Still further, not only must the neural processes set up by 
 the end-organs and conducted inward by the afferent nerves have 
 a place of meeting in proximity with the centres of origin for the 
 corresponding efferent impulses; but all the neural processes in this 
 place of meeting must also be so modified and made mutually de- 
 pendent that they can be correlated, under psycho-physical laws, 
 with the processes of mind. It is the central organs which alone 
 possess the molecular construction and functions necessary for such 
 wonderful reflex and automatic activities. In their highest form 
 the hemispheres of the human brain they solve the problem of 
 providing a system of molecules, whose constitution and changes 
 may be immediately related with the phenomena of mind. These 
 central organs are extremely intricate physical structures. It can- 
 not be pretended that even a beginning has been made toward a 
 satisfactory theory of their functional activity considered as a special 
 case in molecular physics. But this fact does not affect the con- 
 fidence which is based upon what is known of physical structures 
 in general, that in these organs, the changes which take place are 
 essentially of the same order as are those with which the science of 
 molecular physics has elsewhere to deal. They are modes of motion 
 in which the behavior of each molecule, regarded as a constituent 
 element of the system, is conditioned upon the constitution and 
 behavior of the other members of the same system. That is to 
 say, the central organs must be regarded in the light of the con- 
 ception of mechanisin. 
 
282 THE NERVOUS MECHANISM 
 
 7. The general office of the nervous system may, then, be de- 
 scribed in somewhat the following manner. The development of a 
 rich and varied life, both animal and intellectual, requires a great 
 store of sensations and of motions. The sensations are primarily 
 designed to serve as signs of changes in the environment of the 
 animal to which his condition must be adapted by movement of 
 his bodily parts; but they are also to serve as a basis for intel- 
 lectual attainment and development. The forces of external nature 
 continually storm the peripheral parts of the animal's body. In 
 order that any of these forces may act as the stimuli of sensations, 
 they must be converted into molecular motions within the tissues 
 of this body. In order, further, that the masses of the body may 
 constantly be readjusted to the external changes of which the sen- 
 sations are signs, the molecular motions must, in turn, be converted 
 into movements of these masses. In other words, a process of con- 
 stant interchange must take place between the animal organism 
 and external nature. 
 
 Disturbances in one part of the body, by the play upon it of nat- 
 ure's energy, instead of becoming injurious or destructive, are 
 thus made serviceable through inducing the needed disturbances 
 of other parts of the same body. The equilibrium on which life 
 depends is maintained. Moreover, the material necessary for self- 
 conscious development, for a growing knowledge of the so-called 
 outside world, is furnished through the conduction of these dis- 
 turbances to their common meeting-places in the central organs. 
 The nervous system, especially in its supreme central organs, must 
 therefore be the kind of mechanism that can so to say be 
 " trained " into co-ordinated forms of action, so as to serve as a basis 
 for the mental activities involved in processes of learning. 
 
 8. To accomplish the general work of equilibrating the inter- 
 action of the different parts of the body, of readjusting its condition to 
 the changing condition of its surroundings, some special construc- 
 tion and arrangement of material molecules is necessary. If the 
 work is to be done in a highly elaborate way, a very intricate ar- 
 rangement of an indefinitely great number of chemically complex 
 molecules is necessary. Such an arrangement is the human ner- 
 vous system. But just because its arrangement and function are 
 of this kind, it is a "mechanism." As a highly complex molecular 
 mechanism it utilizes the disturbances which arise from the en- 
 vironment. It binds together all the other systems of the body in 
 living reciprocity of energies and functions. Its superficial parts 
 are so constructed that they can be set in motion by various forms 
 of physical energy by light, heat, sound, chemical change, etc.; 
 they are also adapted fitly to modify the impressions thus received. 
 
EQUILIBRATING OF DIFFERENT PARTS 283 
 
 The molecules of its conducting nerves are so constituted and ar- 
 ranged that they can indicate the path along which the disturbance 
 thus occasioned must pass; they can dictate the conditions and 
 laws under which its course must be completed. The molecules of 
 its central organs are capable of assuming inconceivably varied re- 
 lations to each other, of thus transmuting and redistributing the 
 nerve-commotions which reach them along the incoming tracts, and 
 even (it would seem) of starting automatically outgoing disturb- 
 ances in response to self-conscious sensations and ideas. 
 
 But all the foregoing offices of the nervous system are nothing 
 but the movements of physical elements, in constant reciprocal de- 
 pendence upon each other, though in response to excitations lying 
 outside of the system itself. To move thus is the function of a 
 molecular mechanism. So far as science can control the different 
 parts of the nervous system for experimental purposes, it finds 
 them behaving in such a manner as to make a plain demand for a 
 physical and mechanical theory in explanation of their behavior. 
 
 9. The foregoing description of the nervous system as a mech- 
 anism, like all similar descriptions, undoubtedly lacks scientific 
 quality. It is neither exact nor in such form as to admit of ex- 
 perimental verification. It is largely based upon conjectures, full 
 of gaps and assumptions; and were it pressed at every point for 
 proof, it would be obliged to rely much upon general principles in 
 mechanics (the special applications of which to the case in hand are 
 by no means certain or obvious), and even to indulge in hopes and 
 promises with reference to the future rather than present demon- 
 stration. May we not know more precisely the nature of the mo- 
 lecular changes which constitute the functions of nerve-fibres and 
 nerve-cells ? Cannot physical science help us to complete these be- 
 ginnings of a theory? 
 
 10. In answering the question just raised, we have to consider 
 separately the case of the nerve-fibre and that of the nerve-centre. 
 The former is much the simpler problem, and there is more of ex- 
 perimental knowledge to form the basis of a theory for its solution. 
 In a preceding chapter (pp. 135 ff.) we noted the disagreement be- 
 tween physiologists as to whether the activity of a nerve is to be 
 conceived of as a chemical process, which involves the consumption 
 of fuel, and the setting free of potential energy; or whether, on the 
 contrary, the nerve impulse is a purely physical process, like the 
 transmission of a wave over the surface of water, or of electricity 
 along a wire. It appears that in the former case, the amount of 
 catabolism in the nerve must be exceedingly slight, and the recovery 
 of the fibre exceedingly prompt. This absence of signs of chemical 
 change in the active nerve, we then saw, has led a group of physi- 
 
284 THE NERVOUS MECHANISM 
 
 ologists to regard the nerve-impulse as an electrical affair, similar 
 to a movement of ions along a "core conductor." In the interest 
 of reconciling these views it was suggested (foot-note, p. 143) that a 
 mechanical theory of the nerves and their functions, which shall ac- 
 count for all the phenomena, is entirely feasible and consistent with 
 what is known of chemical change, as related to electrical conduction, 
 in other cases. It is a reasonable hope, then, that in the not distant 
 future the facts of "summation/' "interference," "facilitation," etc., 
 may be statable in terms of such a combination theory. It is sig- 
 nificant that these very terms are applicable only to phenomena 
 which admit of a mechanical explanation. 
 
 11. Bound up with this is the further question, whether the 
 nerve-impulse in all nerve-fibres is of the same quality; or whether 
 different nerves have different "specific energies," to use a term 
 introduced by the celebrated physiologist, Johannes Miiller. Miil- 
 ler called attention 1 to the fact that the optic nerve, whether excited 
 normally through the action of light on the retina, or artificially by 
 the passage of an electric current through the forehead, or by a blow 
 on the head, always gives rise to a sensation of light. Similarly, 
 each of the sensory nerves, however excited, gives rise to a specific 
 sensation. He accordingly interpreted these facts to mean that 
 "Either the nerves themselves may communicate impressions 
 different in quality to the sensorium" or brain "which in every 
 instance remains the same; or else the vibrations of the nervous 
 principle may in every nerve be the same and yet give rise to the 
 perception of different sensations in the sensorium, owing to the 
 parts of the latter with which the nerves are connected having dif- 
 ferent properties. The proof of either of these propositions I re- 
 gard as at present impossible." Miiller, however, inclined to the 
 former opinion namely, that each sensory nerve has an action pe- 
 culiar to it, a "specific energy," differing from the energy of all 
 other nerves. With the progress of the study of the nerves, the ap- 
 parent similarity in the action of all of them has led most physiolo- 
 gists to the second member of the alternative, according to which 
 the specific quality is no longer believed to inhere in the action of 
 nerve-fibres, but rather in the organs, central or peripheral, with 
 which they are connected. Thus, in the case of motor nerves, the 
 nerve-fibres which run to the biceps muscle seem not to differ in 
 their mode of action from the fibres running to the triceps, or to 
 any other muscle. These nerves are, accordingly, regarded as in- 
 different conductors; and the difference in the motor results of their 
 activity is believed to inhere simply in the muscles to which they 
 
 1 In 1844, in his Physiology. See the English translation, 1848, pp. 1059 ff. 
 
THEORY OF SPECIFIC ENERGIES 285 
 
 lead, and in the attachments of these muscles to the bones. This 
 may be affirmed to be true, in general, of motor nerve-fibres. 
 
 12. Strong additional evidence in favor of this view is obtained 
 by "crossing" two nerves. If, for example, the nerve A leads to 
 the muscle a, and nerve B to muscle 6, and if both nerves are cut, 
 and the central end of A sutured to the peripheral end of B, and 
 vice versa, then, after time has been allowed for the regeneration 
 of the peripheral ends, nerve A excites muscle b, and nerve B mus- 
 cle a, so that muscle a contracts when muscle b should contract, and 
 vice versa. That is to say, the impulses issue from the nerve-centres 
 in the normal way, but, the nerves being crossed, the impulses are 
 conveyed to the wrong muscle. It appears, however, that, with 
 practice, a fair degree of co-ordination can be newly learned. But 
 the conclusion from the experiment, from our present point of view, 
 is that nerves are capable of exciting other muscles than those to 
 which they normally lead. The motor nerves are thus, to a large 
 degree at least, interchangeable, and have no specific energies of 
 their own, but differ, as was said, only in their connections. 
 
 In regard to the sensory nerves, equally conclusive evidence does 
 not seem to be at hand; but the prevalent view is, probably, that 
 the action of the fibres of the optic nerve differs in no essential 
 respect from the action of the fibres of the auditory nerve, or of 
 other sensory nerves; the same principle, therefore, holds here as 
 in the case of the motor nerves namely, that the difference be- 
 tween them lies in their connections. In the case of the sensory 
 nerves, the connections to be considered are central the connec- 
 tions of the incoming sensory fibres with motor fibres or with cen- 
 tral cells. The differences in conscious sensation between the sen- 
 sory nerves would, accordingly, be associated with differences, not 
 in these nerves themselves, but with differences in the cortical 
 centres to which these nerves lead. Visual sensations, that is, 
 would result from the activity of the visual area, no matter by what 
 means it is excited; and the same for the other sensory areas. It 
 has been objected by Hering 1 that transferring the specific quality 
 from the nerves to the sensory areas does not make the differences 
 between sensations any more intelligible, and that our methods of 
 studying nerve-fibres are much too crude to detect fine differences 
 in the character of their activity, should such exist. The objec- 
 tion does not, however, counterbalance the evidence derived from 
 more recent cases of anastomosis, where mixed nerves are involved. 
 For it has been found that, when the juncture between the cut ends 
 of the two nerves is made perfect, and time is allowed for healing 
 
 1 Zur Theorie der Nerventdtigkeit (Leipzig, 1899). 
 
286 THE NERVOUS MECHANISM 
 
 and for the effects of practice, both the sensory and the motor func- 
 tions of such a nerve can be performed by another nerve, whose 
 normal connections commit it to quite a different and even distant 
 area of the body (compare p. 243). 
 
 13. In regard to the mechanism of the nerve-centres, a con- 
 siderable change of opinion seems to be taking place in the minds 
 of students on the subject. In the early days after the microscope 
 had revealed the cell elements in the gray matter, and after the 
 fact was established that the gray matter constituted the real cen- 
 tral organs of the nervous system, it seemed almost self-evident that 
 the nerve-cells were the essential structures of the nerve-centres; 
 and that, therefore, the mechanics of the centres was the mechanics 
 of the nerve-cell. These cells were regarded as exercising control 
 over the motor nerves and so over the muscles, much as a general, 
 seated in his tent, exercises control over an army. Co-ordination 
 was supposed to be the special function of the nerve-cells. They 
 were also believed to contain large stores of potential energy which 
 they, at the time of their activity, discharged along the nerve-fibres 
 leading from them. The cells in the sensory areas of the brain 
 were regarded as the essential termini of the sensory nerves, and the 
 activity of these and other cells in the brain was supposed to be cor- 
 related with consciousness. Memories were spoken of, metaphori- 
 cally, as being "stored" in the cell; the meaning being that modifi- 
 cation of the cells by any experience is the physical condition of 
 the later revival of the experience. In all respects, the nerve-cells 
 were regarded as the organs of reflex and mental activities. 
 
 14. With the progress of histology, it became evident, however, 
 that the nerve-cells are only a part of the gray matter, and, in bulk, 
 a small part. The branches of the cells were seen to be fully as 
 characteristic a feature of the gray matter, and to fill much more of 
 the cranial space. As the conception of the nerve-fibres and their 
 branches as conductors gained precision, the view came into prom- 
 inence, that the connections established by the fine branches of 
 cells in the gray matter are the important fact. This view assigns 
 more importance to the branches by which the connections are made 
 than to the cell-bodies themselves. The neurone conception, too 
 (see pp. 110 ff.), added force to the same view; for it holds that the 
 branches of the different cells simply come into contact or close 
 proximity with each other, so that the surfaces of contact would be, 
 in all probability, the most critical points in the system of nervous 
 communication. Within any one neurone, all parts cell-body, 
 axon, and dendrites are in continuity, and apparently conduction 
 is free throughout. Between one neurone and another, conduction 
 is less free, because of the lack of perfect continuity. Nerve-im- 
 
MECHANISM OF THE NERVE-CENTRES 287 
 
 pulses must indeed pass from one neurone to another; but the pas- 
 sage would probably be more difficult than from one part to another 
 of a single neurone. Therefore the delay which occurs in the trans- 
 mission of a nerve-impulse through a nerve-centre is localized in 
 the passage from one neurone to another. That is, the delay oc- 
 curs at the "synapse." 
 
 15. Now, delay in transmission is a type of several of the pe- 
 culiarities of central nervous action. One of these is the "blocking" 
 of nerve-impulses. In general, this seems due to the fact that the 
 impulse can pass from the terminal branches of an axon over to the 
 dendrites of another cell, but not in the reverse direction. This 
 fact means that the synapse is the place at which such a peculiarity 
 of central conduction occurs. Oftentimes, also, an impulse is 
 blocked in the forward direction. This is the case when anaes- 
 thetics act on the centres; and since nerve-centres are more suscepti- 
 ble than nerves to the action of ether, chloroform, and alcohol, the 
 probability is that the loss of function brought on by anaesthetics 
 is due to the blocking of the synapse. Still other instances of 
 blocking a nerve impulse on its way through a nerve-centre are seen 
 in the phenomena of inhibition; and here, again, the continuity of 
 structure between the various parts of a neurone makes the view 
 probable that all these blocks occur at the place of separation be- 
 tween one neurone and another. In fine, it seems possible to con- 
 ceive of the action of the nerve-centres as a process of the trans- 
 mission of nerve-impulses that is subject to the peculiarities of cen- 
 tral conduction. 1 Most of these peculiarities can be stated in terms 
 of resistance resistance in general high, but variable with many con- 
 ditions. If the highest resistance in the path of a nerve-impulse 
 through the centre lies at the synapse, then this would be the crit- 
 ical and typical part of the centre. Thus, for example, memory 
 would not consist so much in a modification of the cell-bodies as 
 in the improvement of synaptic connections between different cells. 
 
 16. Our reasoning on this subject, it is admitted, has about it 
 much that is vague and uncertain. It is founded partly on the ap- 
 parent structure of the gray matter, and partly on the conception 
 of nerve-action as being essentially that of conduction. There 
 are, however, a few experiments which throw additional light on 
 the mechanism of the nerve-cells. If nerve function, even in the 
 gray matter, is essentially conduction, then the cell-bodies might 
 be dispensed with, were it not for the fact that they are usually in- 
 terposed between dendrites and axon, and so form a link in the chain 
 of conduction; and were it not for the further fact, that they are 
 
 1 For a fuller account of these peculiarities, see Sherrington, Integrative Action 
 of the Nervous System, p. 11. 
 
288 THE NERVOUS MECHANISM 
 
 certainly necessary for the nutrition and continued life of all parts 
 of the neurone. If, however, the cell-bodies could be cut out of a 
 nerve-centre without interrupting the continuity between the den- 
 drites of each cell and the axon, such an operation would not neces- 
 sarily at once destroy the function of the nerve-centre. Even the 
 conception of such an operation seems wild enough when only the 
 vertebrate cord and brain are considered; but in some invertebrates 
 the motor neurones are unipolar dendrites and axon being di- 
 rectly continuous while the cell-body, with its nucleus, lies off to 
 the side, connected with the rest by a slender strand. In the crab, 
 for example, the cell-bodies are bunched together at the outside 
 of each ganglion, and can be destroyed without seriously injuring 
 the axons and dendrites with their fine branches and connections. 
 The experiment has been tried by Bethe, 1 with the very striking 
 result that the reflex functions of a ganglion are retained after its 
 cell-bodies have been removed. To be sure, the function is not 
 retained permanently in the absence of the cells, but it is retained 
 for a few days; and even if it were only retained for a few hours, the 
 evidence would be conclusive to show that normal activity of the 
 ganglion can occur without the presence of nerve-cells. Accordingly 
 the activity of a nerve-centre is not essentially the activity of the cell- 
 bodies in it. The evidence applies in the first instance only to the 
 crab, but we have nothing of a contrary teaching in regard to verte- 
 brates. 
 
 17. If, then, the query should be raised anew as to what is the 
 function of the nerve-cells, the answer may be found by recalling 
 that the condition of the centre slowly deteriorates after the removal 
 of the cell-bodies. A nerve-fibre, too, when cut off from its cell of 
 origin, undergoes degeneration. Similar facts are true of other 
 than nervous tissue. The nucleus of a cell is necessary to the nu- 
 trition and good condition of the whole cell, and any part severed 
 from the nucleus suffers and usually degenerates. Now the cell- 
 body of a neurone the "cell" as we usually call it is principally 
 distinguished as the part of the neurone containing the nucleus. 
 We may then be sure that the cell-body has the highly important 
 function of serving the nutrition of the whole neurone; it is neces- 
 sary for maintaining the axon and dendrites in proper condition 
 for work, even though it may take no peculiar part in the actual 
 doing of the work. 
 
 18. If we accept the view that the synapse is the locus of the 
 most important peculiarities of central function, our inquiry be- 
 comes: Is it possible to form a reasonable conception of the mechanics 
 
 1 "Das Zentralnervensystem von Carcinus Maenas," in Archiv. f. mikroscop. 
 Anatomic, 1897, L, 629 ff. 
 
SIGNIFICANCE OF THE SYNAPSE 289 
 
 of the synapse ? In answer to this question, a theory which has the 
 merit of simplicity and tangibility has been put forward by Duval 1 
 and others. Duval supposes that the fine branches of axons and 
 dendrites, which by their close proximity to each other form the 
 synapse, have the power of motility, in much the same way as that 
 shown by the amoeba (compare pp. 14 f.). As, then, the amoeba 
 puts out temporary branches, but retracts them under certain con- 
 ditions, so, according to this theory, the fine branches of axons 
 and dendrites can be thrust out under certain conditions and re- 
 tracted under others. When they are thrust out they come into 
 closer contact; when they are retracted they separate. As the con- 
 duction across a synapse would naturally be better the closer the 
 contact between the branches forming it, the protrusion of the 
 branches would mean better conduction through the centre, and the 
 retraction of them poorer conduction, or even a blocking of the path. 
 Special application of this theory has been made to the case of un- 
 consciousness from anaesthetics or from fatigue. It has been 
 claimed that the retraction of the dendrites would block the path of 
 impulses, and thus reactions to stimuli would be prevented. The 
 promoters of this theory also believed they had evidence that the 
 fine branches of the dendrites were shorter in animals subjected to 
 ether, than in animals killed suddenly without the use of anaes- 
 thetics. In. general, however, the evidence is against any power 
 of motility mtKe nerve-cell or its branches; and the theory does 
 not command the assent of the best authorities. It may, however, 
 serve a useful purpose as giving a sort of rough diagram of what 
 goes on in the synapse. There need be no actual motion of the 
 branches as wholes; but there may be molecular motions within 
 them, or chemical changes within them, which would have the 
 same effect of increasing or decreasing the conductivity of the 
 synapse. 
 
 19. In view of the attractiveness of the electrical theory, as ap- 
 plied to the nerve-fibre, it will be of much interest to see whether 
 the same theory can be extended to give a reasonable account of the 
 action of the synapse. This has been attempted with considerable 
 success by Sherrington 2 . There is a certain degree of discontinu- 
 ity at the synapse; it is a boundary between cells, as is seen in this 
 fact, among others, that the degeneration which occurs in a nerve- 
 fibre on being separated from its cell-body extends to the fine 
 branches of that fibre, but does not pass over the synapse into an- 
 other neurone. The synapse is a cell boundary, or surface of sep- 
 aration between cells. It should have, therefore, the physical 
 
 1 Comptes rendus de la Societe de Biologie, 1895, p. 74. 
 
 2 Integrative Action of the Nervous System, 1906, pp. 15-18. 
 
290 THE NERVOUS MECHANISM 
 
 properties of other cell boundaries, one of the most important of 
 which (see above, p. 14) is the resistance interposed by it to free 
 diffusion and to the passage of electricity. The position of the 
 synapse as such a boundary would therefore account for the funda- 
 mental fact that conduction through a nerve-centre is less free than 
 along the nerve-fibre. The allied facts mentioned above regarding 
 the peculiarities of conduction through the nerve-centres are sus- 
 ceptible of possible explanation in the same terms. The strange 
 fact of the irreversibility of conduction through a nerve-centre be- 
 comes a little less mysterious when it is recalled that some degree 
 of irreversibility of diffusion is characteristic of cell boundaries. 
 The fact that ether, chloroform, etc., have a powerful effect on con- 
 duction through nerve-centres is also partly cleared up by the well- 
 known influence of these chemical substances on cell membranes. 
 Cell boundaries can have their permeability altered by many causes, 
 and their variability in this respect may well be brought forward 
 in explanation of the variability of conductivity through a nerve- 
 centre. 
 
 The synapse or cell-boundary theory of the action of nerve- 
 centres seems, therefore, when worked out in detail, to be more 
 capable of giving an expression in physico-chemical terms to most 
 of the known peculiarities of central function than any other theory 
 which has been put forward. 
 
 20. Of other theories, 1 the most prominent have conceived the 
 action of nerve-centres as essentially a chemical and metabolic 
 process, and have laid stress on assimilation and dissimilation, or 
 anabolism and catabolism, as the fundamental processes involved. 
 The most thoroughly worked out of these metabolic theories are 
 those of Wundt 2 and of Verworn. 3 It does not seem possible, 
 with present knowledge, to fit these theories to the details of the 
 function of nerve-centres so nicely as has been done for the cell 
 membrane theory. As affecting all these theories, the fundamental 
 question concerns the evidence of catabolism in central activity. 
 This is the same question which was raised before in regard to the 
 activity of the nerves (pp. 135 ff.); but the answer here would seem, 
 at first, to be very different from that reached in the case of the nerves. 
 There, the evidence showed unmistakably that the catabolism of 
 the nerve-fibres is, at most, very small in amount. In the case of 
 
 1 See a critical discussion of the theories of central function by Bethe, in Er- 
 gebnisse der Physiologie, 1906, V, 250-288. 
 
 2 Untersuchungen zur Mechanik der Nerven und Nervenzentren, 1876; and in 
 the successive editions of his Physiologische Psychologic. 
 
 3 See the chapter, "Vom Mechanismus des Lebens," in the various editions 
 of his Allgemeine Physiologie, and also many special papers. 
 
RIVAL METABOLIC THEORIES 291 
 
 the centres, however, the prevailing view has been that much ca- 
 tabolism occurs during their activity. Perhaps the chief fact in 
 favor of this view is the rich blood supply of the brain and cord, 
 and the great dependence of their functions on their blood supply. 
 Stoppage of the arteries to the brain results in speedy unconscious- 
 ness. The brain must have blood, must have plenty of oxygen. 
 And it uses up this oxygen, for the blood returning from the brain 
 by the veins has been deprived of its oxygen. The conclusion seems 
 almost self-evident that this oxygen was used by the brain in proc- 
 esses of combustion; and that much combustion was needed to 
 supply the energy consumed in brain activity. But the matter is 
 not so simple. For much blood circulates through the brain even 
 in conditions of mental quiescence, sleep, in anaesthesia; and even 
 in these conditions the blood returning from the brain has lost much 
 of its oxygen and become venous. Respiration experiments on 
 men performing hard mental work have failed to detect any in- 
 crease in the consumption of oxygen, or in the production of car- 
 bon dioxide, over the amounts consumed and produced, respec- 
 tively, in conditions of rest. 1 Other tests for the consumption of 
 oxygen by brain work have yielded no conclusive evidence. On 
 the whole, it seems that brain work is not a strongly catabolic 
 process, and that the mechanics of brain activity, and of nerve- 
 centre activity in general, is of the same general sort as the mechanics 
 of the nerves. All this is, however, no conclusive argument against 
 our recognizing the evidence for the view that chemical processes 
 in the form of catabolism, are an essential part of the mechanics of 
 the nervous system. And modern discoveries are constantly show- 
 ing more clearly what enormous amounts of promptly available 
 energy may be stored in very small amounts of material substance. 
 Among such kinds of substance, the structure of the nerve-fibres and 
 cells seems to have a distinguished, if not a pre-eminent position. 
 
 21. Our main contention namely, that science must view 
 the structure of the nervous system in the light of a mechanism, 
 and its functions as a species of mechanics, in the most vague and 
 general meaning of these terms would not be in the least impaired 
 or altered, if it should continue forever impossible to explain the 
 phenomena in terms of pure chemistry and physics. Let it be 
 found necessary to revive the conception of "vital force," or of a 
 considerable group of so-called "vital forces." This would not 
 at all essentially change the conditions of the problem. Structural 
 changes in material substances, and forces assumed to account for 
 the performances of such substances, can be described and explained 
 
 1 Atwater, Ergebnisse der Physiologie, 1904, III, part 1, pp. 609 ff. 
 
292 THE NERVOUS MECHANISM 
 
 only in terms of a mechanical theory. Considered as a mechanism, 
 the human brain, with its marvellous outfit of fibres and cells, is 
 no more worthy to be dubbed spiritual, or have applied to it terms 
 that are derived from the phenomena of consciousness, than are the 
 most obvious and grossest forms of matter. 
 
 22. Our review of the various molecular theories proposed to 
 account for the nervous mechanism, either as a whole or in any of 
 its parts, has made plain the important fact that such theories are 
 all obliged to assume the origin and continuance of a peculiar mo- 
 lecular structure for this mechanism. In other words, no attempt 
 to explain how the nervous system acts can avoid the conclusion 
 that the determining factor in the explanation must be found in 
 what the nervous system is. The physiological functions of the 
 nerve depart when the nerve dies. The nerve dies when it is sev- 
 ered from the ganglion-cell. Both cell and nerve must, therefore, 
 constitute a living molecular unity, in order that their normal phys- 
 iological functions may be performed. The explanation of these 
 functions assumes the molecular constitution of the organs them- 
 selves. But how shall we explain, in accordance with the known 
 laws of molecular physics, the origin and preservation of such a mo- 
 lecular constitution? It is the business of biology rather than of 
 physiology to attempt an answer to this question. But the question 
 itself asks from science the performance of a task no smaller than 
 that of framing a mechanical theory of life. Biological science can, 
 as yet, do little toward framing such a theory. Throughout our en- 
 tire discussion of the nervous mechanism we have carefully avoided 
 raising any inquiry as to the nature of life, as to the source and con- 
 ditions of that very molecular constitution which determines the 
 nature and working of this mechanism. We have simply assumed 
 and argued that, taking the nervous system for what it really is 
 and really does, its structure and functions admit of scientific ex- 
 planation, so far as such explanation is possible at all, only when 
 they are regarded as belonging to a molecular mechanism. The 
 question of a mechanical theory for the origin and constitution of 
 living organisms in general lies outside of the inquiries of Physio- 
 logical Psychology. 
 
 23. One other important question has also thus far been 
 avoided. What is the relation of the mind to the working of the 
 nervous mechanism ? Can the mind set this molecular mechanism 
 at work, or can it in any way determine the character of its func- 
 tions ? As far as our consideration of the nervous system has gone 
 hitherto, all might very well have been the same without the exist- 
 ence of a single act of conscious thought or feeling occurring in 
 any relation whatever to this system. Given the molecular mechan- 
 
REVIEW OF THE EVIDENCE 293 
 
 ism as it is constituted and conserved by the forces which control 
 as long as life continues; and given the necessary impact of out- 
 side forces upon the end-organs, and the proper changes of blood 
 within the central organs; and it has been assumed that this mechan- 
 ism would exercise its functions in ways thus far described. But 
 the consideration of another class of phenomena is now to be in- 
 troduced; these are the phenomena of human consciousness, the 
 phenomena of Mind. The question whether such phenomena can 
 be true causes of any of the changes in the molecular mechanism 
 is a part of the general question as to the correlations that exist 
 between two classes of facts. The answer to such general question 
 belongs to the following divisions of our work. 
 
PART SECOND 
 
 CORRELATIONS OF THE NERVOUS 
 
 MECHANISM AND MENTAL 
 
 PHENOMENA 
 
CHAPTER I 
 THE QUALITY OF SENSATIONS 
 
 1. A considerable change in the point of view, and a corre- 
 sponding change in the methods of investigation, will be found 
 necessary for effective treatment of the subjects which are to occupy 
 our attention from this time onward. Thus far we have endeav- 
 ored, as much as the nature of the subject made possible, to look 
 at the nervous system from a purely objective point of view, and 
 to arrive at an understanding of its structure and functions by em- 
 ploying the methods of the physico-chemical and biological sciences. 
 In a word, this system has been examined as a material mechanism, 
 which like everything known to human minds, must be known in 
 terms of the human consciousness, but which may be known, as 
 other, material structures are known, without any preconceived 
 opinions, or preconceived theories, as to its special relations to this 
 consciousness. Viewed in this way, the nervous mechanism is 
 an object to have its constitution determined by the dissecting 
 knife, the microscope, and the various means for physical and chem- 
 ical analysis; while its functions consist of molecular changes and 
 chemical processes, that are in all important respects assumed to 
 be like those with which science is familiar in other living bodies. 
 
 It can scarcely have escaped observation, however, that this prom- 
 ise to keep clear of all the more strictly psychological implications 
 and complications has not been completely fulfilled. And, indeed, 
 no amount of painstaking, or even of distinct aversion to all that 
 has a remote connection with psychological topics, could possibly 
 have resulted in its complete fulfilment. No treatise of the human 
 nervous mechanism is possible, that does not admit somewhat freely 
 implications derived from the science which has for its subject- 
 matter the mind's conscious states; or that does not adopt some pro- 
 visional attitude toward a number of complicated psychological 
 problems. And in giving the results of modern researches as to 
 the functions of certain parts of the nervous system, the language 
 which it is found necessary to employ is, strictly considered, often 
 much more psychological than physiological. 
 
 Examples illustrating our contention will readily occur to any 
 one who passes before his mind in review the descriptions given, 
 
 297 
 
298 THE QUALITY OF SENSATIONS 
 
 theories favored, and laws demonstrated, in Part First of this book. 
 For instance, in tracing the development of the nervous system in 
 the animal series, and the phylogenetic peculiarities of different 
 species, it was found that a certain at least rough and indefinite, 
 but no less real, correlation must be assumed between this develop- 
 ment and the development of what, from the psychologist's point 
 of view, we speak of as the "mind." In studying the development 
 of the nervous system in the individual man, we found it necessary 
 to assume a yet more strict, if not more complicated network of 
 correlations between the two kinds of development. The differ- 
 ent main parts of this system in man were seen to be plainly adapted 
 to the performance of functions which, if not employed in the im- 
 mediate present, would before long be needed to serve as the 
 physical correlates of the main classes of mental activities that are 
 involved in the more elaborate processes of learning. Provision 
 must be made for the individual's becoming consciously aware of 
 the nature of his environment, and for his reacting on that environ- 
 ment with different kinds and degrees of consciously directed activity. 
 Moreover, it was found impossible to avoid the conclusion that the 
 complexity and size of the nervous system, and, in a very special 
 way, of the cerebral hemispheres, are correlated with the complexity 
 and the extent of a possible mental development. 
 
 2. It was, however, when we came to consider the so-called 
 "localization of cerebral function," that we found ourselves com- 
 pelled to receive the terms, the analyses, the conclusions, and even 
 the conjectures, of psychology, as derived from a study of conscious- 
 ness, into our fullest confidence. Indeed, the functions described 
 as cerebral, and located in different parts of the cerebrum, are, 
 properly speaking, not cerebral at all. They are different factors, or 
 phases, or aspects, of conscious acts. They can only be described 
 as such, in a relatively satisfactory manner. Modern science, by a 
 skilful combination of the methods of histology, pathology, and ex- 
 perimentation, has established in some cases, what parts of the cere- 
 bral areas are in some manner concerned in furnishing the condi- 
 tions or accompaniments of these factors, phases, or aspects, of 
 the mental performances; and it has a few rather uncertain con- 
 jectures as to the physical and chemical changes in which these 
 conditions and concomitants consist. But it is these latter alone, 
 that are, strictly speaking, localized in the cerebral areas. With- 
 out psychology that is, without a study of the states of conscious- 
 ness we should not even know where to look for any of those 
 physical and chemical reactions which are the special performances 
 of the nervous system. And did we not see that a more complete 
 and satisfactory analysis, from the psychological point of view, is 
 
CHANGES IN POINTS OF VIEW 299 
 
 at present a prime requisite for clearing up the very puzzling counter 
 evidences and contradictions which still cling to the doctrine of 
 cerebral localization? 
 
 The same thing is even more obviously true of all our treatment 
 of those portions of the nervous mechanism which serve the pur- 
 poses of end-organs of sense. It might almost be said that there is 
 no physiology proper of the end-organs of sense. There is increas- 
 ing knowledge as to their histology; and there is some growth to 
 our knowledge of the physical and chemical changes which they 
 undergo when subjected to the various forms of stimuli to which 
 they, specifically, respond. But as "end-organs of sense" we 
 know them only through our conscious sensations in dependence 
 upon their integrity of structure and normal ways of functioning. 
 This fact results in making the physiology of the special senses, 
 too, very largely psychological rather than distinctly physiological 
 a description and analysis of how we feel, rather than of how a 
 piece of mechanism looks and acts, when examined in a purely ob- 
 jective fashion. 
 
 3. It will be seen, then, that after all, the change which is to 
 take place in our point of view, and the corresponding change in 
 the methods of investigation, are by no means absolute and com- 
 plete. We have all along been getting some very decided and fixed, 
 if not always definite and mathematically accurate, impressions 
 as to the correlations which exist in fact, between the structure, func- 
 tions, and development of the nervous system in man, and the nature, 
 activities, and development of man's mental life. But having 
 obtained a sufficiently full and clear notion as to what sort of con- 
 trivance this nervous mechanism is, in fact, and of how it can oper- 
 ate, with its three-fold outfit of receptors, conductors, and central 
 organs, we wish now to set it agoing, so to say. In this way we can 
 study the results, as they appear in consciousness, of the action 
 upon it of various kinds and degrees of stimulus, and the laws of 
 mental life, as it develops in dependence, more or less remote, 
 upon these results. It is thus much of change which has seemed 
 to us to justify the beginning here of a "Part Second" of our treat- 
 ise on the one subject of Physiological Psychology. 
 
 4. A study of the correlations which exist between the ner- 
 vous mechanism and the mental life in man, requires a certain 
 amount of preliminary analysis of experience. The first topic 
 to be approached in this way is our experience in the use of our 
 senses. 
 
 The world as known to us by our senses consists of a great num- 
 ber of so-called "things" that are believed to be separate existences, 
 
300 THE QUALITY OF SENSATIONS 
 
 but possess certain common characteristics, and stand in certain 
 relations to each other, of space, time, and action. It is with the 
 things, their common qualities and mutual relations, that unreflect- 
 ing practical life is chiefly concerned. But even without special re- 
 flection, every one learns that his knowledge of such external objects 
 depends upon the kind and degree of the effect they exercise upon 
 his consciousness through the senses. Attention is thus turned from 
 the things themselves to the sensations produced in us by their ac- 
 tion. The variety of such sensations, at first bewilderingly great, 
 is soon reduced to some order by a classification which refers them 
 to the different organs through which they come. Thus, certain 
 sensations are received through the nose, others through the mouth, 
 the ear, the eye, or the skin especially as covering that part of the 
 body (the hand) which is most active in touch. Smell, taste, hear- 
 ing, sight, and touch are the five classes of sensation, as the group- 
 ing is made by the unprejudiced judgment of all; and until recently 
 this was considered sufficient for scientific purposes. 
 
 A further rough and scientifically inadequate classification takes 
 place among the sensations of the same sense. Those of smell, in- 
 deed, defy classification, whether popular or scientific. Among 
 tastes, the most familiar are easily distinguished; such are the 
 sweet, the sour, and the bitter. The two principal classes of sensa- 
 tions of sound are easily discriminated, as either noises or musical 
 tones; the former are further classified as respects the character of 
 the feeling which accompanies them, and the latter as high or low 
 in pitch. The different more prominent colors including black 
 and white are recognized by all persons of normal vision as modes 
 of the sensations of sight; hence the colors commonly named, and 
 the various so-called "shades" of these colors. That more than 
 one class of sensations arise through the skin is shown by the popu- 
 ular use of the word to "feel." Things feel hard and soft, smooth 
 and rough, as well as warm and cold. But things are also said 
 to feel heavy or light. The feeling by which their weight is esti- 
 mated, however, is only ascribed in a very indefinite way to the 
 parts of the body that are chiefly concerned in passively supporting, 
 or actively lifting, or pushing against their weight. The particular 
 use of tactual feeling, as well as the general use of the muscular 
 sense, in gaining this class of sensations is little noticed by ordinary 
 reflection. 
 
 5. All the sensations are also regarded as having some place 
 in a scale of degrees of sensation; they are either strong or faint, 
 or else lie somewhere between the two extremes. They are also 
 habitually thought of as related to time, and as being connected 
 with the motion in space of the objects that occasion them. Of 
 
NEED OF FURTHER ANALYSIS 301 
 
 the molecular action of their stimuli upon the end-organs of special 
 sense; of the hidden chemical, electrical, or other processes con- 
 nected with the activity of the peripheral and central nervous sys- 
 tem; of the physiological, psycho-physical, and psychological laws 
 under which the mind reacts in the form of simple sensations, and 
 combines these sensations into the composite objects of sense; of 
 all these and other similar matters, the unreflecting conception of 
 sensation takes no account. 
 
 6. It is obvious that the analysis of sense-percepts which suf- 
 fices for working-day life will in no respect answer the demands of 
 science. Its " common-sense" character is a distinct mark of its 
 inadequacy. An adequate scientific treatment of this branch of 
 Physiological Psychology requires at least four things: (1) to dis- 
 tinguish the simple sensations from those complex objects of experi- 
 ence with which alone our adult consciousness is familiar; (2) to 
 point out the varieties of quality and degrees of quantity which be- 
 long to these sensations, and to discover the laws which relate them 
 to changes in the form and intensity of their stimuli; (3) to show 
 how the simple sensations are constructed by the mind into the so- 
 called "presentations of sense" under mental laws of time-form 
 and space-form; and (4) to indicate how far, if at all, the higher 
 mental activities of association, memory, will, and judgment may 
 be brought under laws similar to those upon which the formation 
 of these presentations of sense depends. It is upon these four 
 heads of inquiry that modern psychology, as studied from the 
 psycho-physical point of view, has expended most of its painstak- 
 ing researches. Its success has been by no means complete. All 
 these fields of inquiry still include many unanswered questions; all 
 of them present the results of researches that seem in various re- 
 spects conflicting. Yet it is precisely in these fields that modern 
 psychology has achieved its most brilliant successes. 
 
 7. The distinctions with which scientific analysis begins are to 
 a large extent received from ordinary experience. Some of the 
 most essential of the distinctions are confirmed by the results of 
 this analysis. They all, however, require to be carried farther and 
 to be fixed with much more of accuracy than belongs to the im- 
 pressions of common life. New distinctions also have to be intro- 
 duced. For example, scientific investigation maintains the differ- 
 ence between sensations of smell and sensations of taste; but it 
 points out what is not ordinarily apparent namely, that certain 
 results commonly referred to the latter sense really belong to the 
 former. It also adds the sensations of the muscular sense to the 
 classes popularly described; and it discriminates more clearly be- 
 tween the several kinds of sensations that have the skin for their 
 
302 THE QUALITY OF SENSATIONS 
 
 organ. As we have already seen, it assigns two very different 
 classes of sensations to the ear. 
 
 Psycho-physical science, moreover, accepts the common distinc- 
 tion between the quality and the quantity of the different sensa- 
 tions. But it describes with all possible accuracy the limits within 
 which alone this distinction can be carried out. It shows that the 
 quality and quantity of sensation are inseparably connected; that, 
 as Lotze held (a view confirmed by von Kries and others), changes 
 in quality can be distinguished from changes in intensity, with 
 perfect confidence, only in the case of sensations of hearing. It is 
 possible that even here the distinction is largely made on the basis 
 of complex experience. Very intense sensations of heat and cold so 
 far change their specific character as to tend to pass into each other, 
 or, perhaps, to become submerged in a common tone of painful 
 feeling. Minimum sensations of heat and pressure are difficult to 
 distinguish from each other; maximum sensations of pressure are 
 likely to lose the characteristic quality of touch and be displaced 
 by sensations of pain. To treat scientifically of the quality of 
 sensations requires, then, a large amount of the most careful an- 
 alysis. 
 
 8. It is essential, in the first place, to distinguish "simple 
 sensations" from "presentations of sense," or those complex ob- 
 jects of consciousness which result from an act of mental synthesis 
 on the basis of several simultaneous affections of sense. As respects 
 developed experience, the simple sensation is a necessary fiction of 
 psycho-physical science. 1 Consciousness is scarcely more able di- 
 rectly to analyze a presentation of sense into those factors out of 
 which it originated than it is to analyze a drop of water into its 
 component oxygen and hydrogen gases. Simple sensations, there- 
 fore, are not objects which can be examined in the direct light of 
 introspection. Yet they are factors which, as scientific analysis 
 shows, actually enter into all such objects as can properly be spoken 
 of under the term "presentations of sense." Any sensation which 
 is absolutely unanalyzable with respect to distinctions of quality, 
 and which, therefore, cannot be considered as consisting of com- 
 ponent parts, is called simple. It is distinguished 'as a sensation 
 from all other elementary forms of feeling or knowledge, by the 
 relation which it sustains to the presentations of sense. A sensa- 
 tion, unlike the feeling of grief, of desire, or of weariness, etc., is a 
 
 1 All our subsequent work will be completely misunderstood unless this state- 
 ment is constantly borne in mind. On the one hand, the scientific study of 
 sense-experience is impossible without the analysis which employs this fiction; 
 on the other hand, to give reality to the fiction, as though it were an experienced 
 element in consciousness, is to favor an atomistic theory of mental life. 
 
QUESTIONS REQUIRING AN ANSWER 303 
 
 potential factor of a material object. Through the senses we know 
 "things"; not, indeed, as though they appeared before the mind 
 by immediate apprehension in the form of exact copies of extra- 
 mental realities. But every sensation is an affection of the mind 
 recognized as connected with an extra-mental reality, through the 
 activity of the senses. Simple sensations are those elementary 
 factors, themselves indecomposable, out of which the presentations 
 of sense are composed. The objects of sense, however, do not have 
 the character of mere compounds of simple sensations. Sensations 
 must not only be associated and compounded, but also localized 
 and projected without (that is, set in systematic relations of space- 
 form), in order to constitute the objects of sense. 
 
 9. The foregoing remarks suffice to indicate, in a preliminary 
 way, what is the nature and value of the psycho-physical investi- 
 gation of sensation. We inquire, in the next two chapters, as to 
 the Quality of Sensations. The inquiry, when conducted from the 
 psycho-physical point of view, involves an answer to three questions : 
 (1) What is the precise locality in the organism where the specific 
 excitation which occasions each kind of sensation originates; and 
 what is the nature of the action of the stimulus in producing such 
 excitation ? (2) What are the kinds of sensations which appear in 
 consciousness as the result of the various excitations ? (3) What are 
 the laws by which the quality of the sensations is related to the 
 kinds of excitation? Neither of these three questions can be 
 answered completely. The investigation of the first is much re- 
 stricted by our almost complete ignorance of those processes in 
 the central organs that are in all cases the proximate internal 
 stimuli or immediate antecedents of the sensations. uX Moreover, as 
 has already been made apparent, our knowledge of the intimate 
 structure of the end-organs of sense, and of the nature of the physi- 
 cal processes which excite them, is still very incomplete. The de- 
 tection of obscure but important differences in the qualities of con- 
 scious states of sensation is by no means easy; it requires great skill, 
 strict and trained attention, and unwearied repetition of experiment. 
 But these conditions of success have a great effect in altering the 
 quality of the sensations themselves. Besides all this, remarkable 
 idiosyncrasies not infrequently appear; and language can only im- 
 perfectly describe even the most common factors of the varied and 
 living experiences with which science tries to deal. 
 
 In investigating the laws that define the relations between our 
 subjective experience, called sensation, and objective phenomena 
 in the shape of physical energy acting upon the nervous mechanism, 
 there is often the greatest doubt as to what manner of laws are be- 
 ing investigated. They may be considered as purely physiological, 
 
304 THE QUALITY OF SENSATIONS 
 
 or as psycho-physical, or as purely psychological. It is not strange, 
 therefore, that different theories exist for accounting for all the 
 more important groups of facts, depending upon the emphasis laid 
 by different investigators upon the value of each of the three possi- 
 ble modes of explanation. The truth is, that each sensation is sepa- 
 rated by a series of intricate physiological and psychical processes 
 from the application of the stimulus in the gross, as it were, to the 
 end-organ of sense. 
 
 10. What has already been said regarding the "specific energy 
 of the nerves" (compare pp. 284 f.) must be assumed in discussing the 
 quality of sensations. The possession of common functions can- 
 not, indeed, be denied to the nerve-fibres in general. Conduction 
 has been seen to be the one universal property of all the nervous 
 elements; and even the end-organs, or receptors, of sense-impres- 
 sions are made up of elaborate and ingenious combinations of these 
 elements. But the phenomena of sensation require a further exten- 
 sion of the conception of specific functions, on whatever physical or 
 physiological basis this differentiation is founded. Consciously 
 made distinctions in the quality of sensations depend upon the ex- 
 citation of specific corresponding elements of the nervous mechanism. 
 Sensations of light and color depend upon different species of the ex- 
 citation of the optic nerve; and similar specific quality cannot be 
 denied to the functional activity of the nerves of smell, taste, hear- 
 ing, and touch. But the nature of the evidence and the conclu- 
 sions which must be drawn from it will be much better appreciated 
 at a later period in the discussion. 
 
 11. Little of a scientific character is known concerning Sensa- 
 tions of Smell t considered as respects their specific quality. The 
 physical and nervous structure of the apparatus employed in ex- 
 citing this species of sensations, and the way in which the stimulus 
 is customarily applied, have already been described (see p. 176). 
 Under ordinary conditions the stimulus must act in gaseous form, 
 or else be vaporizable, with the existing degree of temperature. 
 The degree of temperature at which different substances become 
 vapori/able, and therefore odorous, varies greatly according to their 
 physical characteristics. Arsenic, for example, which at ordinary 
 temperatures is inodorous, when raised to a dark-red heat excites 
 intense sensations of smell by the vapor it gives off. 
 
 Whether an odorous substance must actually reach the olfactory 
 mucous membrane in a vaporous form or whether a solution will 
 in any case excite sensations of smell, cannot easily be ascertained; 
 since neither the positive nor the negative results which have been 
 obtained by different experimenters can withstand all criticism. In 
 the case of negative results, such as those of \Veber, who found that 
 
STIMULI OF OLFACTORY SENSATIONS 305 
 
 when the head was tilted back and the nostrils filled with a ten per 
 cent, solution of eau de cologne, the sense of smell was not excited, 
 attention must be directed to the fact that treatment of the membrane 
 by soaking it in a liquid disturbs the function of the olfactory ap- 
 paratus for a considerable time. In the case of the positive re- 
 sults, the difficulty is that of being sure that the narrow chink in 
 which the end-organs are situated is actually filled with the liquid; 
 if it were not, but some air remained, then the odorous particles 
 might really reach the olfactory surface in the form of a vapor. 
 On the other side, it may be argued that, since at least a thin layer 
 of liquid must cover the olfactory membrane, the odorous particles 
 must finally be dissolved or suspended in this liquid, and so excite 
 the sensations of smell. 
 
 12. Whatever decision may be reached as to the possibility 
 under highly abnormal conditions, the ordinary and "adequate" 
 stimulus of smell is rightly assumed to consist in certain exceedingly 
 minute particles contained in the odorous gas or vapor which is 
 drawn in with the current of air over the mucous membrane of 
 the regio olfactoria. The question is as yet scarcely decided, 
 whether other forms of stimulus, besides these odorous particles 
 mechanical, electrical, thermic, or so-called subjective can ex- 
 cite the sensation of smell. The older experimenters (Volta, 
 Pfaff, Fowler, and Humboldt) failed to obtain any certain proof 
 that the electrical current is an excitant of this sense. In one place, 
 however, Pfaff speaks of a sensation resembling the smell of sul- 
 phur as caused by the application of electricity to the sensory pas- 
 sages of the nose. Ritter (in 1798) experimented by using bits of 
 graphite and zinc thrust into these passages, and also by holding 
 one pole of a battery in the hand and placing the other in the nos- 
 tril. In the latter way he thought that he excited a genuine spe- 
 cific sensation of this sense. He describes the positive pole in the 
 nostril as producing an inclination to sneeze and a trace of a smell 
 like that of ammonia; the negative pole placed there does away 
 with this inclination and produces a kind of "sour" smell. Such 
 phenomena are probably, however, all to be assigned to the nerves 
 of taste, touch, and common feeling. More recent investigations 
 have done little to remove the reasons for doubt. 1 The smell of 
 phosphorus which is developed by the action of the electrical ma- 
 chine is probably due to the ozone set free; it is not a case, then, 
 of the direct excitation by electricity of the sensation of smell. 
 Some physiologists (notably Valentin) have observed that this sen- 
 sation may be awakened by mechanical stimulation, such as strong 
 vibration of the nostrils, violent sneezing, etc. ; others have failed 
 1 See W. Nagel, Handbuch d. Physiologic des Menschen, 1905, III, 602. 
 
306 THE QUALITY OF SENSATIONS 
 
 to produce this specific sensory effect in such ways. It does not 
 appear that thermic stimulation will excite the sensation of 
 smell. 
 
 Experiments to prove that subjective sensations of smell may be 
 produced by injecting odorous substances into the veins of animals 
 are very uncertain. Human pathological cases, in spite of the cus- 
 tomary indefiniteness of the patient's testimony as to the nature of 
 his sensory affection, tend to show that compression of the olfactory 
 nerve by tumors, etc., may produce sensations of smell. Dis- 
 turbances of the central organs, such as occur in cases of disease, 
 may doubtless have the same result. The powerful effect which 
 some odors have upon the brains of some persons, so that nausea, 
 giddiness, and other disturbances of feeling result, scarcely needs 
 mention; but it cannot easily all be resolved into mental associations 
 connected with the sense impressions. Hallucinations of smell are 
 among the prominent and persistent symptoms of certain forms of 
 insanity. 
 
 13. As concerns the varieties of odors, from the psychological 
 point of view, the important scientific problem is that of attempting 
 to reduce this manifold to order. In other senses, particularly in 
 sight and taste, such attempts have already met with a considerable 
 degree of success. The great variety of colors can be ordered by 
 reference to a few primary colors; and the tastes can be reduced to 
 mixtures of a few elementary tastes. There are some indications 
 that a similar "component theory" is moving in the right direction 
 in the case of smell also. This theory would hold that there is 
 a relatively small number of primary odors, each aroused by a spe- 
 cific physico-chemical stimulus, while the great variety of other 
 odors have their complex nature due to their being aroused by 
 mixed stimuli. But owing to the uncertain and vague character 
 of the evidence, a designation of the primary sensations of smell, 
 or of their specific stimuli, is still very far from an accomplished 
 fact. 
 
 Introspectively, odors by no means separate themselves into ele- 
 mentary and compound, but all seem simple. It is, however, possi- 
 ble to draw a line between smell proper and tactile sensations of 
 the nasal organ. The interior of the nose is supplied not only by 
 the olfactory nerve but also by a branch of the trigeminus; and many 
 sharp "odors" are in part stinging sensations from the end-organs 
 of the latter nerve, and persist in individuals who are, through in- 
 jury or congenital absence of the olfactory nerve or similar causes, 
 completely anosmic, or lacking in the sense of smell. It is also 
 possible to draw a line between smell and the sensations of taste 
 which sometimes arise from vapors, as the sweet taste of inhaled 
 
ANALYSIS OF OLFACTORY SENSATIONS 307 
 
 chloroform, which is properly due 1 to excitation of taste-buds on 
 the soft palate and in the larynx. Even these distinctions are not 
 easy for introspection; and in common life we ascribe the mixture 
 of smell, taste, and touch sensations which may result from sniffing 
 a vaporous substance, all to the sense of smell. 
 
 14. If all these extraneous sensations are excluded, the intro- 
 spective analysis of the sensations of smell proper seems impossible. 
 About all that can be done is to notice resemblances and differences 
 between odors, and so to arrive at some sort of classification. None 
 of the classifications of odors which have been offered are thoroughly 
 satisfactory, however; nor do any of them afford much promise 
 of leading to the discovery of the elementary smell-stimuli. The 
 only one which has any claim to attention has come down to us 
 from Linnaeus, the great naturalist of the eighteenth century; it 
 has been adopted and somewhat developed by Zwaardemaker, who 
 ranks as the leader among contemporary students of the sense of 
 smell. Zwaardemaker's classification is as follows: 
 
 (1) Ethereal odors, including the odors of fruits. 
 
 (2) Aromatic or spicy odors. 
 
 (3) Fragrant odors, including the scents 6f flowers, and also 
 vanilla, tea, balsam, etc. 
 
 (4) Ambrosial odors, of which musk is the most familiar example. 
 
 (5) Alliaceous odors, including onion, india rubber, chlorin, and 
 iodin. 
 
 (6) Empyreumatic or burnt odors, including burnt foods, and 
 also tar, gasoline, etc. 
 
 (7) Hircine or goaty odors, including cheese, rancid butter, etc. 
 
 (8) Repulsive odors, as of certain insects and plants. 
 
 (9) Nauseous odors, as of decaying flesh. 
 
 Some of these classes seem broader than others; especially do 
 we notice that numbers 8 and 9 include, for many individuals, 
 smells that might be selected from all the other classes; and all are 
 capable of subdivision into subordinate classes. More important 
 still is the fact that some odors do not readily find a place in the 
 scheme at all. If it were possible to discover any chemical com- 
 munity between the members of any class or sub-class, which should 
 set that class off, as stimuli, from the other classes, a long step would 
 be taken toward a scientific understanding of these sensations; but, 
 so far, there is little sign of a common chemical character among 
 resembling odors. Perhaps more hope of advance is afforded by 
 
 1 W. Nagel, Handbuch der Physiologic, 1905, III, 611; Zwaardemaker, Physi- 
 ologie des Geruchs, 1895. 
 
308 THE QUALITY OF SENSATIONS 
 
 the results of examining the odors of closely related groups of chem- 
 ical substances. It is found, in the first place, that odorous sub- 
 stances, with few exceptions, contain elements belonging to only 
 three of Mendelejeff's groups, namely the fifth (of which nitrogen, 
 phosphorus, and antimony are members), the sixth (including oxy- 
 gen, sulphur, and chromium), and the seventh (including chlorin, 
 bromin, and iodin). When similar compounds of the elements of 
 one of these groups are arranged in the order of their atomic weight, 
 the odors are found to run along a scale, shading off from one to 
 another. Thus, the odors of chlorin, bromin, and iodin can be re- 
 garded as arranged along a linear scale, and the same is true of like 
 compounds of these three. Similar scales are found on arranging 
 the fatty acids in order, or the monatomic alcohols, or other like 
 series of organic compounds. The members at each end of such a 
 series are often odorless; and the intensity as well as the quality 
 of the odor is likely to change in a gradual manner along the 
 series. 1 
 
 15. On the psychological or physiological side, more light can 
 be expected from studying the results of mixture of odors, of fa- 
 tigue of the organs, etc., than from attempts to classify odors in- 
 trospectively. Partial anosmia a condition in which the nose is 
 insensitive to certain odors but not to all is not infrequent; and 
 the distinction draws a physiological line between some odors and 
 others. It is well known that prolonged action of an odorous sub- 
 stance results in a condition of fatigue (or adaptation), in which 
 the odor can no longer be perceived. In this condition, it is found 
 that the nose becomes insensitive to certain other odors besides the 
 particular one which has been acting, while it remains sensitive 
 to still other odors of a different character. The odors which are 
 fatigued together would seem to belong together physiologically, 
 and they are also likely, in fact, to be subjectively similar. 
 
 Somewhat analogous results have been obtained by studying 
 mixtures of different stimuli which excite sensations of smell. Care 
 must be taken, in such experiments, to avoid the possibility of a 
 chemical union between the two vapors, and this is perhaps best ac- 
 complished by Zwaardemaker's device of conducting one vapor 
 to the right nostril and the other to the left, each through a separate 
 tube. The effect of such mixed stimulation is, in some cases, the 
 production of an intermediate odor, but in others the masking of 
 one odor by the other, or even the complete neutralization of each 
 by the other so that no odor is perceived from the mixture. Similar 
 
 1 See Haycraft, Brain, 1889, II, 166, and in Schafer's Textbook of Physiology, 
 1900, II, 1254. The results appear to be an entering wedge into the tough prob- 
 lem of the classification of the stimuli of sensations of smell. 
 
STIMULI OF GUSTATORY SENSATIONS 309 
 
 effects of the blending, or contrast, or opposition of sensations are 
 obtained in other classes of our sensory experience. It is along 
 these lines that the unravelling of the intricacies of the sense of 
 smell may be expected to make most progress. 
 
 16. The condition of scientific attainment as to sensations of 
 taste and their stimuli is somewhat better than that as to the allied 
 sense of smell. The adequate specific stimulus for the nerves of 
 this sense consists in certain tastable substances; such substances, 
 however, do not excite the end-apparatus unless they act upon it 
 under definite conditions. Only fluid bodies, or such as are at 
 least to some small degree soluble in a fluid or menstruum, excite 
 sensations of taste; absolutely insoluble bodies are, without excep- 
 tion, tasteless. This fact may be due to the concealed position of the 
 inner cells of the gustatory flasks, which is such that they cannot be 
 reached by substances undissolved. By no means all soluble sub- 
 stances, however, have a taste. No known law regulates the rela- 
 tion between the solubility of bodies and their power to excite sen- 
 sations of this class. The adequate stimulus of taste is thus chemi- 
 cal; and taste-buds are "chemo-ceptors." Whether they can be 
 aroused by other classes of stimuli mechanical, thermal, electrical 
 is not yet ascertained with certainty. But thermal stimuli, at 
 least, are probably without any effect on the taste end-organs. 
 As to mechanical stimuli, some good observers have reported sen- 
 sations of taste resulting from pressing or tapping the tongue; but 
 others have been unable to obtain the same results. Sensations 
 of this class are certainly aroused by the passage of a current of 
 electricity through the tongue. If, for example, the positive pole 
 or anode is applied to the tongue, the sensation is sour; if the nega- 
 tive pole or cathode is applied, the taste is not so easy to describe, 
 but is called sharp, alkaline, or bitter, by different observers. Wheth- 
 er the sensations produced in this way are the direct result of the 
 action of the current on the end-organs of taste is a question which, 
 after a hundred and fifty years of study, is not yet settled. The in- 
 sufficiency of the evidence is due to the fact that the passage of a 
 current sets up electrolysis; and though this may be prevented from 
 taking place in the saliva of the mouth, there is no way to make sure 
 that electrolysis is not going on within the taste-buds, or at some 
 polarizable surface through which the current has to pass. Now 
 electrolysis of the salts dissolved in the saliva or body fluids would 
 set free acid ions at the anode and alkaline at the cathode; and thus 
 the tastes actually observed could be accounted for as due to chemi- 
 cal stimuli generated by electrolysis, rather than to electrical stimuli. 
 This view has considerable probability as an explanation of the 
 "electrical tastes"; it is at least not excluded by any results yet ob- 
 
310 THE QUALITY OF SENSATIONS 
 
 tained. 1 Apparently, therefore, we have in the taste end-organs a 
 striking example of the general characteristic of special receptors 
 namely, that, though highly sensitive to one special stimulus, they 
 are rather obtuse to all other sorts of stimuli. 
 
 17. In attempting to delimit, analyze, and order the sensations 
 of taste, we are met with the same difficulty as that which confronted 
 us in the case of smell namely, that tastes usually occur in close 
 combination with other sensations. The tongue and mouth possess 
 the tactile, temperature, pain, and muscular senses as well as taste; 
 and the "feeling" of food contributes largely to its apparent "taste." 
 For example, astringent and oily "tastes" are really tactile sensa- 
 tions. It is difficult to distinguish between taste and smell, except 
 by artificially excluding smell. A simple experiment for accomplish- 
 ing this is due to Chevreul (1824), and consists in holding the nose 
 while taking into the mouth substances of various flavor, but reduced 
 to like consistency so as to avoid help in discrimination from the 
 side of the tactile sense. Under these circumstances, the surpris- 
 ing fact comes to light that it is impossible to distinguish, by taste 
 alone, between coffee and quinine, or between apple and onion. 
 The conclusion from this experiment, when fully carried out, is 
 that most so-called tastes are really odors. 
 
 Excluding the sense of smell does not, however, interfere at all 
 in the perception of the following four tastes: sweet, sour, bitter, 
 saline. These four are universally recognized as simple tastes, 
 and it is very doubtful whether any others should be added to this 
 list. Metallic and alkaline are added by some authorities, and there 
 is still some question regarding them; but it is clear that they are 
 at least partly composed of mixtures of sweet, sour, bitter, and saline, 
 along with sensations of touch and smell. 
 
 18. The following question now arises: Does each of the four 
 elementary tastes have a special end-organ? Histologically, no 
 difference appears between the various taste-buds which can be 
 taken as an indication of functional differentiation. Yet it is clear 
 from experiment that the taste-buds are not all equivalent. The 
 fact is familiar that bitter is best tasted at the back of the tongue, 
 and sweet best at the tip; while the edges of the tongue are the most 
 sensitive to sour. Experiment has made these results more pre- 
 cise by sharply localizing the stimulation. If weak solutions are 
 applied to individual fungiform papillse by means of a camers-hair 
 brush, it is found that the sensitivity of the papillse varies consider- 
 ably; although it is impossible to localize the stimulus to single 
 taste-buds. A few papillae are found which respond only to sweet, 
 
 1 See the summary of evidence by Nagel, in his Handbuch der Physiologic, 
 1905, III, 630. 
 
ANALYSIS OF GUSTATORY SENSATIONS 311 
 
 or only to sour, etc. Others respond to two, and others to three, 
 some to all four, classes of the sensation of taste; while some do not 
 respond at all. 1 These facts, so far as they go, are favorable to 
 the hypothesis that individual taste-buds are specialized, each for 
 one taste. 
 
 The physiological relations maintained between the four elemen- 
 tary tastes are not yet clearly made out. A condition of partial 
 ageusia, or lack of taste, can be produced by chewing gymnema 
 leaves; the sense for sweet and for bitter is thus temporarily de- 
 stroyed, while that for sour and saline substances remains. The 
 mixture of two taste-stimuli does not give rise to a series of clear 
 intervening qualities, as does, for example, the mixture of red and 
 yellow, but the two tastes in some cases remain side by side and dis- 
 tinct; in other cases they alternate; and in still others, the two neu- 
 tralize each other. Kiesow has, however, found that a mixture of 
 weak solutions of sugar and common salt gives a flat alkaline taste, 
 weaker than that of either of the components of the mixture, and 
 not like either of them. Contrast effects, such as the extreme sour- 
 ness of an acid fruit after a sweet substance has been eaten, are well 
 known; but it seems at present impossible to utilize this experience 
 in explaining the dynamic relations of the four simple sensations 
 of taste. Thare is no clear indication that the tastes can be ar- 
 ranged in a linear scale, as the primary colors are, nor that any taste 
 stands to any other definitely in the relation of opposite or comple- 
 mentary. On the whole, it appears as if the four tastes were rather 
 isolated from each other, each representing almost an independent 
 sense. There is much blending, to be sure; but the amount is ap- 
 parently no greater between one taste and another than between 
 tastes and odors. 
 
 19. Little can be said regarding the nature of the chemical 
 stimulus which arouses any of the sensations of taste. It is true 
 that bodies of similar chemical composition usually evoke similar 
 tastes; though a curious exception to this rule is found in the fact 
 that sweet and bitter are often evoked by bodies of very similar 
 composition, or even by the same body. On the other hand, sub- 
 stances of very different chemical composition such as, for ex- 
 ample, cane sugar and sugar of lead sometimes excite the same 
 sensation of taste. Since it is in solution that substances act 
 on the end-organs of taste, the theory of solutions needs to be 
 taken into account. It may be that dissociated ions are the 
 essential stimulus; and there is some positive evidence that acid 
 
 1 Ohrwall, Skandinav. Archivf. Physiol, 1890, II, 1; Kiesow, Wundt's Philosoph. 
 Studien, 1898, XIV, 591. 
 
312 THE QUALITY OF SENSATIONS 
 
 taste is due to the action of hydrogen ions, and alkaline taste to 
 hydroxyl ions. 1 
 
 20. On passing to the consideration of sensations of sound much 
 more help is received from the science of physics. But modern in- 
 vestigations, in the form in which they concern us, do not go back 
 of the great work of Helmholtz, 2 who made the entire field pecul- 
 iarly his own. Since the first appearance of this work, the subject 
 has also been greatly enriched by the original researches of Oetting- 
 en, Mach, Preyer, Hensen, Stumpf, and other still later writers. In 
 speaking of the stimuli of these sensations, we are compelled to re- 
 fer chiefly to the vibrations of air, which are only remote excitants 
 of the end-organs of this sense. Neither physics nor physiology 
 has yet been able to fix the precise locality in the organism (the ner- 
 vous structure of the cochlea) where the immediate stimulation of 
 the end-apparatus takes place; or to tell what is the exact nature 
 of its action. We are obliged, then, to confine ourselves in the 
 main to considering a relation between the vibratory energy of the 
 air and certain states of consciousness, without attempting to ex- 
 plain the many intermediate links. 
 
 All sensations which arise in the mind by means of the irritation 
 of the auditory nerve are called sensations of sound. The word 
 "stnmd" is thus used by psychology for a wholly subjective affair, 
 which has no more resemblance to those vibrations which physics 
 designates by the same word than has the taste sweet to the un- 
 known physical properties that produce it. The trained mind, or 
 "trained ear," as we say, has indeed the power directly to analyze 
 a compound musical sound into its constituent elements. But each 
 of these elements is purely a sensation, a subjective affair. It car- 
 ries in itself no token that it has been produced by vibrations of 
 any kind; or that it sustains any numerical relation whatever to 
 the vibrations of which some other sensation of sound is composed. 
 We know nothing directly, through sensations, either of the struct- 
 ure of the ear or of vibrating strings and particles of air, or of the 
 mathematics and physics of music. 
 
 21. Sounds are of two classes tones, or musical sounds, and 
 noises. The former are due to periodic motions of sonorous bodies; 
 the latter to non-periodic. Noises are those sounds which, ob- 
 jectively considered, are wanting in the periodic regularity of stimu- 
 lation which characterizes all musical sounds, and, subjectively 
 considered, in the peculiar, pleasant modification of consciousness 
 
 1 Richards, Amer. Chem. Journal, 1898, XX; Kastle, ibid.} Hober and Kiesow, 
 Zeitschr. f. physicalische Chemie, 1898, XXVII. 
 
 2 Die Lehre von d. Tonempfundungen als physiolog. Grundlage filr d. Theorie 
 d. Musik (Braunschweig, 1st ed., 1862; and several subsequent editions). 
 
CHARACTERISTICS OF MUSICAL SOUNDS 313 
 
 which the latter produce. But noises accompany almost all tones; 
 and, conversely, tones may be detected by the trained ear as mingled 
 with the noises of every-day life. No player of the violin avoids all 
 noise of scraping from the bow; no stroke of a workman's hammer, 
 or slamming of a door, that does not start and catch up into itself 
 some trace of musical tone. The interest of science has hitherto 
 been almost wholly concentrated upon musical sounds, and little 
 has been done by either physics or physiology toward the analysis 
 of noises. It is characteristic of a noise, according to Helmholtz, 
 that there is a quick and irregular alternation of different kinds of 
 sensation of sound. This distinctive character can generally be 
 detected "by attentive aural observation without artificial assist- 
 ance." We can compound noises out of musical tones; as, for ex- 
 ample, by striking together all the keys of an octave on the piano. 
 Hensen 1 distinguished three "categories of unmixed noises" the 
 "beats" or pulsations which disturb the purity of musical tones; 
 the crackle, crack, or crash; and hissing sounds. These three shade 
 into each other, and, when mixed with different kinds and quantities 
 of musical sounds, make up the noises which we hear on every hand. 
 
 It is possible, therefore, to analyze tonal elements out of most, 
 if not all, kinds of noises. And although no one has succeeded in 
 completely analyzing a noise into such elements, success has been 
 great enough to encourage the opinion that a complete analysis of 
 all noises might break them up into combinations of various in- 
 harmonic tones. 
 
 22. Musical sounds differ, not only in quality, but also in 
 quantity or intensity of sensation as dependent upon the ampli- 
 tude of the vibrations which produce them. With respect to their 
 quality they are distinguished as either simple or complex, accord- 
 ing as they result from one set of regularly recurrent (periodic) vi- 
 brations of a given number in a given unit of time, or result from 
 a combination of two or more sets of such vibrations. The musi- 
 cal sounds of ordinary experience are complex. The blending of 
 the simple tones into the complex tone is not so complete, however, 
 that it cannot be at least partially analyzed directly by a trained 
 ear. The complex sound, which results from this compounding 
 of the contrasts or coincidences of several simple musical sounds, 
 may be called by the term "clang" in this meaning borrowed 
 from the usage of the German. The quality of tones considered as 
 simple sensations is their pitch, which varies according to a scale of 
 states of consciousness that are immediately apprehended and com- 
 pared with each other, and that are discovered by objective meth- 
 
 1 In Hermann's Handb. d. PhysioL, III, ii, pp. 3-142, and works by the same 
 author there referred to. 
 
314 THE QUALITY OF SENSATIONS 
 
 ods to correspond to a scale of changes in the number of the vi- 
 brations of the waves which occasion them. The pitch of tones is 
 therefore spoken of as "high" or "low," according to the place 
 which we assign to the resulting sensations in this' scale. Such 
 place in the scale may be considered either with respect to the re- 
 lation of any particular tone to the upper or lower limits of the 
 scale, or with respect to the relation of the different tones to one 
 another. "Clangs," or complex tones the musical sounds with 
 which we are made acquainted by all ordinary experience have 
 also a variable quality called timbre, or "tone-color"; the timbre 
 of the clang is dependent upon the pitch, number, and relative in- 
 tensity of the simple tones which compose it. Thus a note having 
 the same place in the musical scale (for example, a of the once- 
 marked octave 440 vibrations) sounds differently, as we say, on 
 the piano, violin, cornet, or when sung by the human voice. The 
 pitch of the tone as produced by all these different methods is the 
 same; but its tone-color is determined by the character of the over- 
 tones which are blended with the fundamental tone. 
 
 23. The pitch of tones depends upon the rapidity of the peri- 
 odic vibrations (the number in a given unit of time usually one 
 second) which occasion them, or what is the same thing upon 
 the length of the sound-waves. This class of sensations, however, 
 has both an upper and a lower limit; that is to say, vibrations either 
 below or above a certain number per second, or what is the 
 same thing wave-lengths that are either shorter or longer than a 
 given limit, produce no sensations of musical sound. The difficulty 
 of determining these limits is great, because the intensity of ex- 
 tremely low or high tones has to be enormously increased in order 
 that they may be heard at all; because the perceptions of the 
 acoustic sense are so very blunt near the limits that the different 
 sensations are almost certain to be confused; because distracting 
 sensations of common feeling mingle in these ranges of tone with 
 the sensations of sound, and because near the lower limits the 
 over- tones especially the octave above become so strong as to be 
 mistaken for the fundamental tones. On account of these diffi- 
 culties the older investigators made numerous mistakes. 
 
 Individual peculiarities are also very important in determining 
 sensations of pitch. Some persons can hear tones below or above 
 t those audible to most others; and, in general, the range of audible 
 pitch decreases somewhat with age beginning in some cases with 
 the twentieth year. This difference in individuals is illustrated 
 by the judgment of Helmholtz, who thought that sensations of 
 tone begin to cease when the vibrations fall below 34 per second; 
 some tuning-forks of great size, which vibrated only 28 times per 
 
THE PITCH OF MUSICAL SOUNDS 315 
 
 second, seemed to him, however, to have a trace of tone in the form 
 of a "weak drone." But Preyer 1 found that while 14 vibrations 
 produced no tone that he could hear, at 16 vibrations he was able 
 to hear a tone; others could distinguish a musical sound only at 19 
 or 23 vibrations. The same observer experienced as a sensation of 
 musical sound more than 40,000 vibrations per second; Turnbull 
 found that the majority of those with whom he experimented 
 could not hear more than about 20,000 to 22,500 vibrations per sec- 
 ond, and only one a musician heard 30,000; Despretz succeeded 
 in producing with tuning-forks audible tones that had 32,000 vi- 
 brations. Of late years, the Galton whistle (a little organ pipe, of 
 adjustable length), especially in the improved form introduced by 
 Edelmann, has been much used in determinations of the highest 
 audible tone. Edelmann reports that tones of 50,000 vibrations 
 are sometimes heard, while Bruner, 2 who tested a large number of 
 individuals, finds an average of 32,000, with individuals of appar- 
 ently normal hearing varying from 22,000 to 43,000. 
 
 Setting aside the evidence from more or less exceptional cases, it 
 appears that vibrations slower than 28 to 30 per second produce in 
 most ears only a buzzing or groaning sound; the more acute tones 
 are unpleasant, or even painful, and finally inaudible to all ears. 
 These results cannot be considered as very concordant or precise. 
 They show, however, that the range of the average human ear is 
 not far from ten octaves, reaching from about A 8 of the sub-contra 
 octave (27J vibrations per second) to about c 8 of the eight-times- 
 marked octave (33,792 vibrations per second). 
 
 The table 3 on next page gives the pitch of all the musical tones au- 
 dible to the human ear, in the key of C major, on a scale in which 
 a 1 is fixed at 440 vibrations. Only about seven of the rather more 
 than eleven octaves of the table are, however, usable in music; 
 these seven reach upward from C^ of the contra, or from A 2 of the 
 subcontra octave, to b 4 namely, the seven or seven and a half 
 octaves of the modern piano. 
 
 24. The sensitiveness of the ear to differences of pitch also 
 varies greatly with different individuals, and for the different oc- 
 taves of the musical scale. Preyer found that unpractised persons, 
 within the octaves from c to c 3 (132-1,056 vibrations by the table, 
 but 128-1,024 by the scale adopted for his experiments), distinguish 
 
 1 Grenzen d. Tonwahrnehmung, p. 23 f. 
 
 3 Archives of PsychoL, 1908, No. 11, p. 11; this work contains references to the 
 older authorities. 
 
 3 Taken from Stumpf, Tonpsychologie, I, p. xiv, and giving the German scale; 
 for the purposes of physics and psychology, C, 1 or "middle C," is usually fixed 
 at 256 vibrations. 
 
316 
 
 THE QUALITY OF SENSATIONS 
 
 a difference of from 8 to 16 vibrations as producing a distinct dif- 
 ference in the sensation of pitch. Extreme cases of deafness to 
 differences in pitch are recorded; as, for example, that of the man 
 who, in the middle part of the scale, could not distinguish an in- 
 terval of less than a third, and, in the higher and lower parts, of 
 less than a seventh. Persons insensitive to differences of a tone or 
 half-tone, who are sometimes said "not to know one note from 
 
 
 C 
 
 D 
 
 E 
 
 F 
 
 G 
 
 A 
 
 B 
 
 son 
 
 611 
 
 123f 
 247^ 
 495 
 990 
 1,980 
 3,960 
 7,920 
 15,840 
 31,680 
 
 
 Subcontra octave 
 
 16* 
 33 
 66 
 132 
 264 
 528 
 1,056 
 2,112 
 4,224 
 8,448 
 16,896 
 33,792 
 
 18i 9 ff 
 37J 
 74i 
 148J 
 297 
 594 
 1,188 
 2,376 
 4,752 
 9,504 
 19,008 
 38,016 
 
 20| 
 41i 
 82i 
 165 
 330 
 660 
 1,320 
 2,640 
 5,280 
 10,560 
 21,120 
 42,240 
 
 22 
 44 
 88 
 176 
 352 
 704 
 1,408 
 2,816 
 5,632 
 11,264 
 22,528 
 
 24| 
 49i 
 99 
 198 
 396 
 792 
 1,584 
 3,168 
 6,336 
 12,672 
 25,344 
 
 27i 
 55 
 110 
 220 
 440 
 880 
 1,760 
 3,520 
 7,040 
 14,080 
 28,160 
 
 C z , D a , etc. 
 C lt D^etc. 
 C, D, etc. 
 c, d, etc. 
 c 1 , d 1 , etc. 
 c 2 , d 2 , etc. 
 c 3 , d, etc. 
 c 4 , d 4 , etc. 
 c 5 , d 5 , etc. 
 c 6 , d 6 , etc. 
 tf, d?, etc. 
 
 Contra octave 
 
 Great octave 
 
 Small octave 
 
 Once-marked octave 
 
 Twice-marked octave .... 
 Thrice-marked octave. . . . 
 Four-times-marked octave 
 Five-times-marked octave 
 Six -times-marked octave . 
 Seven-times-marked octave 
 Eight-times-marked octave 
 
 
 
 
 
 
 another," are by no means infrequently met. Sensitiveness to 
 pitch is, however, generally capable of rapid cultivation, and may 
 reach a high degree of perfection in persons who have what is called 
 "a good natural ear" for musical tones, if the ear be also highly 
 trained. Such persons may become able to discriminate differ- 
 ences in the sensations caused by changing the number of vibra- 
 tions not more than a third of a single vibration per second, in the 
 region of the scale between c and g 2 . In the octave from a 1 to a 2 
 more than 1,200 tones are distinguishable. But above and below 
 this region the distinctions possible are less fine; above c 5 even well- 
 trained ears commit errors in identifying two notes that differ by 
 100 or even by 1,000 vibrations. Not only the musical quality of 
 tones, but also the power of distinguishing differences in them, 
 diminishes rapidly as we approach the upper limits of the scale. 
 
 It appears, therefore, that sensitiveness to pitch is, in general, 
 greatest in the most used part of the musical scale; or, in other 
 words, in that part which falls within the common range of human 
 voices. Within this range, the least perceptible difference is a con- 
 stant quantity, when expressed in number of vibrations. 1 
 
 25. The fineness of the possible distinctions of purity of in- 
 terval also differs for different individuals and for different inter- 
 
 1 On this whole subject compare the lengthy and interesting discussion on 
 " Individuality des Sinnes und Gediichtnisses fur Tonqualitaten, " in Stumpf, 
 Tonpsychologie, I, pp. 262 ff. 
 
THE PURITY OF MUSICAL INTERVALS 317 
 
 vals. The results of experiment on this point have been tabulated 
 by Hensen as derived from data drawn from Preyer's investiga- 
 tions. This investigator found the degree of sensitiveness to be 
 greatest in the case of the octave, next of the fifth, next the whole 
 tone, and so on to the minor third, where it is least of all. The 
 figures which give the number of vibrations off from the pure in- 
 terval which is distinguishable, for the above intervals, are respect- 
 ively, 0.13; 0.23; 0.85, and 1.90. Other investigators have ob- 
 tained somewhat different results. 
 
 Immediate judgment of absolute tone (as the a 1 carried in mind 
 by musicians) is possible; judgment between two tones as to which 
 is higher or lower in pitch is also immediate, and may be exer- 
 cised independently of everything except the two sensations them- 
 selves. The latter judgment is the common power of mind be- 
 longing to this sense; the former is, as a rule, exercised only by 
 skilled persons, and by them only very imperfectly. Experiments 
 of Stumpf, 1 upon himself and three other musicians, showed that 
 the mistakes in judgment of absolute tone amounted, in the lower 
 region of the scale (from C : to B^), to 15%-100% of the trials; in 
 the middle region (from a-g l , or from g-e 2 ), to 0%-70%; in the 
 upper region (from ^-/ 4 , or from f-a 4 ), to 7%-80%. Only one of 
 the four persons experimented upon seemed to approach the point 
 of infallibility. Judgment of absolute tone is, therefore, a different 
 matter from that which makes distinctions in intervals or in the 
 least observable differences of pitch, and is much more precarious. 
 
 26. Those psychologists appear to be in the right who claim 
 that some power of the mind immediately to judge differences of 
 quality in pitch, purely as such, must be assumed in order to ac- 
 count for the foregoing phenomena. 2 Such judgment, however, 
 may be, and ordinarily is, much assisted by auxiliary discrimina- 
 tions of other sensations which blend with those of musical tone. 
 Among such secondary helps the most important are the muscular 
 sensations which accompany the innervation of the larynx and other 
 organs used in producing musical tones. For we ordinarily inner- 
 vate these organs (at least in an inchoate and partial way) that is, 
 we sound the tone to ourselves when trying carefully to judge of 
 its pitch. But the niceness of these muscular sensations is not 
 great enough, even when most highly trained, to account for the 
 discriminations of the "good ear." The trained musician can de- 
 
 1 Tonpsychologie, I, pp. 305 ff. 
 
 2 On this subject compare Lotze, Medicin. Psychologic, pp. 265 ff., 480 f.; 
 Strieker, Studieniiber d. Association d. Vorstellungen, 1883, pp. 2 f.; G. E. Miiller, 
 Zur Grundlegung d. Psychophysik, pp. 276 ff., Berlin, 1878; and Stumpf, Ton- 
 psychologie, I, pp. 134 ff. 
 
318 THE QUALITY OF SENSATIONS 
 
 tect by ear a difference in quality between two tones of 400 and 
 400-J vibrations per second; but the most skilful singer Jenny 
 Lind, for example scarcely succeeds in singing in quarter-tones. 
 Fairly good singers often err by 5-10 vibrations in striking a note, 
 and cannot prevent the voice from varying by as much as this in 
 attempting to sustain the same tone for a second or two. 1 More- 
 over, the relative powers of larynx and ear by no means keep 
 pace with each other in the same person. It should also be re- 
 membered that all our ordinary discriminations of musical sound 
 apply to composite tones, or "clangs"; in discriminating these we 
 are aided by the tone-color, or tone-feeling, which belongs to each 
 note as sounded by some sonorous body with whose peculiarities 
 we are previously more or less acquainted. 
 
 It follows, then, that the judgment is supplied, by the varying 
 qualities of musical tones, with the means for arranging them in a 
 continuous series which may be symbolized by different positions 
 assigned along an uninterrupted straight line. Of any three un- 
 like tones, one must be, and only one can be, arranged as respects 
 pitch between the other two. And whenever any two tones, as m 
 and n, are given, another sliding tone, which begins with m and 
 ends with n, is possible. Moreover, within the bounds of our ex- 
 perience of tones, as we advance along the scale toward either the 
 upper or the lower limit, we see no tendency in the qualities of 
 the sejisations to approach each other. In this respect the scale 
 of sound-tones is wholly different from that of color-tones. There 
 are not two ways, for example, of getting from a 1 to c 3 (one through 
 6 1 , c 2 , etc., and the other through g l , f l , etc., around to 3 , d 3 , and 
 then c 3 ), as there are two ways of going from yellow to blue (i. e., 
 through green and blue-green, or through orange, red, and violet). 
 We speak, then, of the series of tones as a constant and infinite 
 series; although, of course, no series of states of consciousness is 
 really infinite, and although the upper and lower limits of the 
 musical scale, as well as the limits of the least observable differ- 
 ences between two tones, are not constant but variable for differ- 
 ent individuals. 
 
 The symbolism taken from relations of space, which we employ 
 when we speak of certain acoustic sensations as "high" and of 
 others as "low" in pitch, or when we distinguish so-called "in- 
 tervals" between the tones as large and small, is strictly applicable 
 only to the complex tactual, visual, and muscular sensations that 
 accompany the acoustic. In sounding the lower tones with the 
 voice the organs are depressed; in sounding the higher, they are 
 elevated. Low tones have a certain breadth and gravity which 
 1 Cameron, Psychol Rev. Monogr. Suppl. No. 34, 1907, p. 227. 
 
NATURE OF THE "CLANG" 319 
 
 correspond to the foundations of a spatial structure; as sensations 
 they require more time to come into and depart from conscious- 
 ness, as it were. A great intensity and slower tempo belong to the 
 bass-viol than to the violin. We read up for the notes of highest 
 pitch, and down for those of lowest pitch, in the written musical 
 scale. 
 
 27. We have seen that tones, like rays of light, come to us as 
 compounded into "clangs"; these really composite tones being 
 esteemed as single tones in ordinary experience. The nature of 
 such composition determines the so-called "timbre," or "tone- 
 color," of the compound tone. In a word, each sensation of a clang 
 is a summing-up in consciousness of several absolute qualities of 
 musical sound; the stimulus which occasions this complex subjec- 
 tive state is a complex sound-wave made up of the contrasts and co- 
 incidences of several single waves that have the character of simple 
 pendular vibrations. The quality of each clang depends upon the 
 combination of simple waves, of different lengths, within this com- 
 plex sound-wave. 
 
 In the interests of psycho-physical science, or the study of the 
 various qualities and combinations of sensations of sound from 
 the point of view held by physiological psychology, it is not necessary 
 to consider with any detail the mathematics and physics of the 
 waves, whether simple or in combination, which act upon the organ 
 of hearing and so become the stimuli of these sensations. To af- 
 ford some explanation of those acoustic sensations which appear 
 in consciousness as having an sesthetical value, and which, when 
 combined and arranged in certain ways, give rise to the art of music, 
 it is enough to recall the physical fact that a sounding body, such 
 as a piano string, gives forth when struck not only the "funda- 
 mental" pitch to which it is tuned, but also a series of "overtones," 
 the vibration rates of which are 2, 3, 4, 5, 6, 7, 8, 9, 10, etc., times 
 the vibration rate of the fundamental. The string may be said to 
 be vibrating, not only as a whole, but simultaneously in halves, 
 thirds, fourths, etc. The vibrations impressed by the string on 
 the air and by the air on the drum of the ear, are therefore highly 
 compound; and a fundamental fact in the physiology of hearing, 
 as noted in an earlier chapter (see p. 206), is that overtones can, 
 with training and attention, be heard out of a clang, and that there- 
 fore the ear must act as an analytic organ. Ordinarily, however, 
 we do not consciously analyze a clang, but experience it as a whole. 
 
 Of the whole series of overtones, some are favored by one instru- 
 ment, and others by another instrument, and thus the tones of the 
 instruments differ. It was an achievement of Helmholtz to analyze 
 the clangs of different instruments, and to show that it is the differ- 
 
320 
 
 THE QUALITY OF SENSATIONS 
 
 ence in overtones which gives the peculiar quality to the tone of 
 each instrument, each human voice, each vowel, etc. Instruments 
 which give off few and weak overtones have a soft but rather dull 
 tone; those which favor the lower overtones have a rich but mellow 
 tone; those which favor the higher overtones tend toward shrill- 
 ness. Besides the overtones, other elements contribute to the pe- 
 culiar quality of an instrument; e. g., the noises made in operating 
 it, the suddenness with which its sound begins and ends, etc. 
 
 28. Of the very numerous pitches which are distinguishable, 
 music makes use of only a small selection. It is not so much the 
 absolute pitches, as the intervals between the notes of the scale, that 
 determine the qualitative effects of the music. The scales in use 
 among the different peoples differ greatly: Chinese music, for ex- 
 ample, uses quite a different scale from European music; and the 
 scale used in the native Japanese music may be called "indefinitely 
 penta tonic." The scale of European music has come down from 
 the Greeks, and perhaps from the Egyptians; it is said to have been 
 reduced by the Pythagoreans to a mathematically exact form, ac- 
 cording to which the relative vibration rates of the tones of the 
 diatonic scale are as follows: 
 
 1st 
 
 2d 
 
 3d 
 
 4th 
 
 5th 
 
 6th 
 
 7th 
 
 Octave 
 
 C 
 
 D 
 
 E 
 
 F 
 
 G 
 
 A 
 
 B 
 
 C 1 
 
 1 
 
 ! 
 
 1 
 
 f 
 
 f 
 
 1 
 
 
 
 2 
 
 8 
 
 9 
 
 10 
 
 101 
 
 12 
 
 m 
 
 15 
 
 16 
 
 That is to say, while the tone C makes one vibration, D makes 
 nine-eighths, and E makes five-fourths, etc.; or while C makes 
 8 vibrations D makes 9, E makes 10, etc. Of these relations in 
 the number of vibrations, the simplest is, of course, that of the 
 octave, 1:2; and the octave seems to be present in almost, or quite, 
 all musical scales; whereas the other intervals are less universal. 
 Even in European music, the practical necessities of instruments 
 with keyboards (the piano, etc.) have led to the abandonment of 
 the strict mathematical ratios, and the substitution of the "equally 
 tempered" scale, in which the octave is first divided into twelve 
 equal semitones, and the different intervals of the scale then built 
 up as well as possible out of these semitones. The result is that 
 all the intervals except the octave are slightly mistuned by compari- 
 son with the Pythagorean relations; yet the effect is satisfactory 
 to even musical ears which have become accustomed to this scale. 
 
THEORY OF CONSONANCE AND DISSONANCE 321 
 
 Indeed, European ears, to which Oriental music seems at first de- 
 void of musical quality, are able, after practice with the Oriental 
 scale, to find beauty in it; and this fact, as well as the fact of the 
 equally tempered scale, suggests that the use of a particular scale 
 is partly a matter of custom. It does not seem possible, in view of 
 all the facts, to lay so much stress on "simple ratios" as was done 
 by the Pythagoreans and many later theorists; yet, on the other 
 hand, it is not possible, at present, to substitute any other theory 
 of the psychology of scale formation which would meet with the 
 general approval of authorities. 
 
 29. Modern European music makes great use of the simul- 
 taneous sounding of two or more tones, forming chords and dis- 
 cords. A chord or consonance may be described as a combina- 
 tion of tones which gives a smooth and agreeable impression; a 
 discord or dissonance as a combination which gives a rough and dis- 
 agreeable impression. Among the combinations of two tones, the 
 most perfect consonance is no doubt presented by the octave, with 
 vibration rates of 2:1; other recognized consonant pairs are the 
 Fifth (theoretical ratio 3 : 2), the Fourth (4 : 3), the Major Third 
 (5 : 4), Minor Third (6 : 5), Major Sixth (5 : 3), and Minor Sixth 
 (8 : 5). That custom or habituation has much to do with the agree- 
 ableness or disagreeableness of many of these combinations is 
 evidenced by the history of European music; for the older music 
 rejected Thirds and Sixths as dissonant, whereas they are extremely 
 satisfactory to modern ears; while recent music makes great use 
 of Seconds, Sevenths, and Ninths, in combination with other notes 
 of the scale. It seems more proper, however, to speak of these 
 dissonances as "tolerated," and to regard the sesthetical pleasure 
 as consisting in an appreciation of this contrast with preceding or 
 following consonance. 
 
 Such facts as the foregoing make it difficult to formulate a theory 
 of consonance and dissonance and yet consonance and dissonance 
 certainly are phenomena which call for explanation, in view of 
 their importance in music, which is, psychologically considered, the 
 most highly developed system of sensory qualities, treated as such, 
 rather than for their associations or meanings. A partial ex- 
 planation of some of the phenomena of consonance and dis- 
 sonance is found in a fact brought forward by Stumpf, 1 that 
 if a tone and its octave are sounded together, they not only 
 produce a smooth and agreeable impression, but fuse or blend to 
 such an extent that even a musically trained ear may be in doubt 
 whether one or two tones are being sounded. If, on the contrary, 
 
 1 C. Stumpf, "Konsonanz und Dissonanz," in his B&itrage zur Akustik und 
 Musikurissentschaft, 1898, I, 1. 
 
322 THE QUALITY OF SENSATIONS 
 
 a tone and its Second or Seventh are sounded, the fact that two 
 tones are being sounded is evident, usually, even to an untrained 
 ear. In general, the degree of fusion corresponds to the perfec- 
 tion of the consonance; Stumpf, therefore, suggests that the fusion 
 is the source of the consonance, or, in other words, that the unity 
 of the impression is the measure of its harmoniousness. The 
 physiological basis for the blending of certain tones and the com- 
 parative refusal of others to blend, remains to be worked out. 
 
 30. It is important in this connection to bear in mind the com- 
 plexity of the stimulus when two or more tones are sounded to- 
 gether. Each single tone is a clang with overtones. The over- 
 tones of one of two complex tones may coincide, to a greater or 
 less extent, with the overtones of the other. In the case of a funda- 
 mental tone and its octave, all the overtones of the upper tone co- 
 incide with overtones of the lower; but in case of the smaller in- 
 tervals, the correspondence of overtones is less complete. Thus the 
 degree of consonance runs parallel, to a considerable extent, with 
 the amount of coincidence of overtones; and Helmholtz and others 
 have laid stress on this point in their theories of consonance, and 
 of the relatedness which is felt between the various tones of the 
 scale. 
 
 Now we know that when two sounding bodies, of different vi- 
 bration rates, act on the same body of air, the waves from the one 
 periodically strengthen and weaken the waves from the other; and 
 the number of such "interferences" per second is equal to the dif- 
 ference between the two vibration rates. Thus, if tones of 200 and 
 201 vibrations are sounded together, the compound tonal effect 
 will wax and wane once a second. This change in intensity can be 
 distinguished, and is called a beat. As the difference between the 
 vibration rates becomes greater, the beats become more frequent, 
 and also rougher and more unpleasant, reaching the maximum of 
 unpleasantness at about 30 per second. Beyond this point, the 
 effect of beats becomes less marked, and disappears at a difference in 
 the vibration rate of about 50-60. Rough, unpleasant beats are 
 inconsistent with consonance; this fact, therefore, forms the basis 
 of a negative theory of consonance, which defines dissonance as the 
 presence of beats, and consonance as the absence of dissonance. 
 Helmholtz, who bases his theory of consonance in this way, calls 
 attention to the disagreeableness of other intermittent sensations, 
 such as a flickering light, for example. It is, however, not only the 
 beats of the fundamental tones which have to be considered; for, even 
 if the fundamentals do not beat, their overtones may do so; and in 
 general, the consonances which appear the most perfect are those 
 in which the overtones do not beat observably, whereas the Minor 
 
THE DIFFERENCE-TONE 323 
 
 Third and Minor Sixth, for example, have overtones which produce 
 audible beats. 
 
 31. Another element of the complex mass produced by the 
 simultaneous sounding of two tones is the difference-tone. If two 
 preferably high tones are sounded together, attentive observation 
 reveals a low tone sounding with them; this low tone is not heard 
 when either of the high tones is sounded alone, but only when the 
 two sound together. The pitch of this additional tone is equal to 
 the difference in vibration rate of the two inducing tones. This in- 
 duced tone is called the first difference-tone, to distinguish it from 
 others of like character but of lower or higher pitch. The external 
 stimulus which gives rise to the difference-tone is the same as that 
 which gives rise to beats namely, the periodic interference between 
 two sets of waves of different vibration rates; but the starting-point 
 of the difference-tone, as a separate series of vibrations, is apparently 
 in the ear, either in the tympanic membrane or in the fenestra ro- 
 tunda or ovalis. Difference-tones are, therefore, of importance in 
 connection with theories of hearing; and also in connection with 
 theories of consonance, especially in the explanation of the discordant 
 effect of slightly mistuned intervals. In such intervals, the differ- 
 ence-tones of different order are likely to beat with each other. 1 
 
 When we consider the great complexity of the acoustic effect 
 of a chord of even two tones, with its overtones, beats, difference- 
 tones, and the degree of fusion of all these elements, it is not to be 
 wondered at that students of this branch of psychology have not 
 yet been able to come to a generally acceptable theory of the cause 
 of the harmonious or inharmonious effect of the chord. 
 
 'See F. Krueger, " Beobachtungen iiber Zweiklangen," in Wundt's Philo- 
 sophische Studien, 1900, XVI, 307, 568. 
 
CHAPTER II 
 THE QUALITY OF SENSATIONS (CONTINUED) 
 
 1. The analysis of the qualities of different Sensations of Sight 
 is much more intricate than that of any of the other senses. They 
 may all be described as sensations of color and light; but an in- 
 definite number of colors is known to experience, and many de- 
 grees of the sensation of light. Moreover the quantity of the white 
 light which acts as stimulus upon the eye has an important effect 
 upon the quality of the resulting color-sensation; in other words, 
 the tone of the color is dependent upon the amount of white light 
 which is mixed with the "saturated" spectral color. The size of 
 the colored object and the resulting breadth of the sensation, as 
 well as the intensity of the stimulus and the time during which it 
 acts, also affect the quality of the sensation. Still further, the same 
 stimulus produces different sensations as it falls upon different por- 
 tions of a normal retina; while a considerable class of persons are 
 color-blind, or incapable of certain kinds of color-sensations. The 
 previous condition of the retina, and the relations between the con- 
 tiguous portions when any considerable area of it is under stimu- 
 lation, must also be taken into account. The fundamental laws 
 governing sensations of sight can, therefore, be discovered only by 
 excluding for the time many of those variable elements which, in 
 fact, always enter into the determination of the exact character of 
 our experiences in the use of our eyes. Thus defining the first prob- 
 lem before us, we find that it may be stated in the following terms. 
 What sensations result from the stimulation of a sufficiently small, 
 but not too small, area of the most central part of the normal retina, 
 for a given time, when it is not fatigued and the eye is at rest, and 
 with neither too great nor too small intensity of a given kind of 
 light? Such sensations may be called (though somewhat ineptly) 
 normal sensations of color. When the foregoing question is an- 
 swered we may go on to consider the most important variations 
 possible on account of various forms of departure from the so-called 
 normal conditions of sensation. 
 
 2. The ordinary stimulus, the application of which to the eye 
 gives rise to the sensations of sight, is light or certain exceedingly 
 rapid oscillations of luminiferous ether. Some forms of mechani- 
 
 324 
 
STIMULUS OF VISUAL SENSATIONS 325 
 
 cal and electrical stimuli also produce the same sensations. Any 
 violent shock to the eye, such as a blow upon the back of the head, 
 may fill the whole field of vision with an intense light. The action 
 of mechanical pressure of moderate intensity upon a limited part 
 of the retinal elements may be studied by rolling the eyeball in- 
 ward and using the fingernail, or a small, blunted stick, upon the 
 outer surface of the closed lids. By such stimulation disks of 
 light (called phosphenes), with darkly colored edges, are produced 
 in the field of vision of the closed eye. Some observers have 
 claimed that very strenuous exertion of the apparatus for accom- 
 modation occasioned in their eyes similar phenomena ("phos- 
 phenes of accommodation"). On making or breaking a weak elec- 
 trical current sent through the eye, the entire field of vision is 
 lighted up; the constant current also seems to excite the optic 
 nerve. The quality of the sensations thus excited is found to de- 
 pend upon the direction of the current through the nerve. When 
 the current is ascending, the place where the nerve enters the ret- 
 ina appears as a dark disk upon a field of vision that is brighter 
 than it, and of pale violet color; when it is descending, as a bright 
 bluish disk on a field of dark or reddish-yellow color. The retina 
 has also a "light of its own" (Eigenlicht)', for its nervous elements 
 are rarely or never inactive, but have a continuous tonic excitation. 
 Hence some persons see the most gorgeous and varied coloring, 
 when the eyes are closed in a darkened room. This normal light 
 of the retina is not constant either in degree or in quality; both 
 the form and the color of the different minute parts of the field 
 of vision, as lighted by it, are very changeable. It may be said 
 to have the rhythmic movement of all tonic excitation. Such ex- 
 citation is supposed to be due to chemical effects, wrought by the 
 changing supply of blood, upon the nervous elements of the retina 
 and (perhaps, also) of the central organs of the brain. The pe- 
 culiar action of the ascending and descending electrical current 
 has been thought by some 1 to be due to its catelectrotonic or an- 
 electrotonic effect upon the central organs by way of the optic 
 nerve. Aubert has estimated the retina's own light to be about 
 equal (in his case) to half the brightness of a sheet of white paper 
 when seen in the full light of the planet Venus. 
 
 3. The place where the light acts (and here, as is supposed, only 
 indirectly through photo-chemical and perhaps electro-motive 
 changes in the pigments of the eye) upon the end-organs of vision 
 must be located at the back of the retina in the rods and cones. 
 The argument by which we have already (see p. 194) connected 
 
 ^ee Fick, "Physiolog. Optik," in Hermann's Handb. d. Physiol, III, i, p. 
 230. 
 
326 
 
 THE QUALITY OF SENSATIONS 
 
 the analytic power of vision with the structure of this nervous 
 layer may be carried yet further into details. It appears likely 
 that each element of the structure at least in some parts of the 
 retina should be regarded as an isolated sensitive spot, which 
 corresponds on the one side to definite excitations from the appro- 
 priate stimuli, and on the other side to the smallest localized sen- 
 sations of color and light. Accordingly, in order that two visual 
 sensations may be seen as separate, yet side by side, in an object, 
 two neighboring retinal elements must be excited by the stimulus. 
 This implies that the breadth of retinal surface stimulated must be, 
 at least, about that of the distance between two such elements. 
 
 With this hypothesis the facts of his- 
 tology and experimental physiology 
 agree fairly well. 
 
 The degree of accuracy which sight 
 can attain seems to be related to the 
 size of the retinal elements directly 
 affected by the light. Certain indi- 
 vidual differences, either native or 
 due to training, in the fineness of vis- 
 ual discriminations must, apparently, 
 be allowed. Hooke observed that no 
 one can distinguish two stars as two, 
 unless they are apart at least 30" ; 
 few, indeed, can distinguish them 
 
 when less distant from each other than 60". E. H. Weber could 
 not perceive as separate two lines whose distance did not cover 
 at least 73" of the angle of vision; Helmholtz puts the limit of his 
 sharpness of vision at 64". The numbers 60", 64", and 73", in 
 the angle of vision, correspond to a size of the retinal elements 
 varying from 0.00438 mm. to 0.00526 mm.; and this agrees very 
 closely with the calculated breadth (by Kolliker) of the thickness 
 of the cones in the yellow-spot namely, 0.0045 mm. to 0.0055 
 mm. (0.000177 in. to 0.0002165 in.). If white lines be drawn on 
 a dark ground so closely together as to approximate this limit of 
 vision, they will appear, not straight, but knotted and nicked. This 
 fact is due to the action of the stimulus on the mosaic of rods and 
 cones, as seen by the accompanying figure (No. 117). The di- 
 minishing sharpness of vision as we move away on the surface of 
 the retina from its most central area corresponds to the compar- 
 ative paucity of the nervous elements, especially of the cones, which 
 enter into the structure of the peripheral parts. 
 
 4. Excluding consideration of those changes in the quantity, 
 as such, of visual sensations which are produced by changes in in- 
 
 B 
 
 11 
 ** 
 
 FIG. 117. A shows the appearance of 
 lines drawn very closely together, 
 which is supposed to be due to their 
 falling upon the nervous elements 
 of the retina in the manner shown 
 by B. 
 
THE SPECTRAL COLOR-TONES 327 
 
 tensity of the light, and confining our attention to what has already 
 been defined as the normal action of the eye, we treat scientifically 
 all the different sensations of sight when we describe (1) the wave- 
 lengths of the different kinds of colored light, or pure color-tones, 
 and (2) the relations in which the different colors stand with re- 
 spect to the amounts of white (or colorless light) and saturated 
 light (or light of pure color-tone) which enter into them. The fore- 
 going distinctions in the quality of our color-sensations may be con- 
 firmed by an appeal to experience. Red is unlike yellow in "color- 
 tone," and both are unlike blue; but orange is more like either red 
 or yellow than it is like blue, while violet is more like blue than it 
 is like either yellow or red. Yet we distinguish colors of the same 
 class (red, green, or violet) as being like or unlike with respect 
 to their "brightness"; and in respect of brightness, a certain shade 
 of red may differ more from another shade of red than it differs 
 from some shade of yellow, green, or blue. The brightness of 
 a color is, scientifically speaking, dependent both upon the degree 
 of saturation which the color possesses and upon the total inten- 
 sity of the light. 
 
 5. A color-tone is said to be "pure" or "saturated" when it is 
 free from all admixture of other color-tones. Pure or saturated 
 color-tones can be obtained only by the use of the spectrum, which, 
 on account of the different refrangibility of the different colored 
 rays that compose it, analyzes the compound ray of white light into 
 its constituent color-tones. By stimulating with different simple 
 rays those nervous elements which have the same local situation 
 at, or very near, the pole of the eye, we test the question whether 
 each special color-sensation corresponds to a special physical con- 
 struction of the stimulus. It is thus discovered that the compound 
 ray of sunlight, so far as it stimulates the human eye, is made up 
 of components formed by oscillations varying all the way between 
 about three hundred and seventy billions and about nine hundred 
 billions per second; and that the color-tone of the sensation changes' 
 as the number of these oscillations changes. The table 1 on next 
 page exhibits these facts on the scale of Fraunhofer's lines, which 
 mark those portions of the spectrum where its principal colors ap- 
 pear most obvious to the normal eye. 
 
 Rays of light which have a number of oscillations less than four 
 hundred and seventy billions per second, so far as they affect the 
 retina at all, occasion the sensation of Red; and this sensation does 
 not vary essentially in quality when the oscillations are four hundred 
 and forty to four hundred and sixty billions. But when their number 
 
 1 Taken from Pick, " Physiolog. Optik," in Hermann's Handb. d. Physiolog., 
 Ill, i, p. 173. 
 
328 
 
 THE QUALITY OF SENSATIONS 
 
 increases beyond four hundred and seventy billions (0) the quality 
 of the sensation changes rapidly, takes on a yellow tone (Orange- 
 yellow), and finally, at about five hundred and twenty-six billions 
 (D), corresponds to what we definitely call Yellow. This yellow 
 becomes greenish as the oscillations increase in number, until they 
 reach about five hundred and eighty-nine billions (E), when Green 
 appears. (Changes from yellow to green occupy only a small zone 
 
 Name of the line 
 
 
 
 Number of vibra- 
 tions per second 
 
 Wave-length in the air 
 
 B 
 
 Billions. 
 450 
 
 Millimetres. 
 0006878 
 
 C 
 
 472 
 
 0006564 
 
 D 
 
 526 
 
 0005888 
 
 E 
 
 589 
 
 0005260 
 
 F 
 
 640 
 
 0004843 
 
 G .. . . . . 
 
 722 
 
 0004291 
 
 H 
 
 790 
 
 0.0003928 
 
 
 
 
 in the spectrum.) The green in turn becomes bluish; at six hun- 
 dred and forty billions (F) Blue begins to appear. From this point 
 to seven hundred and twenty-two billions (F-G) the color-tones 
 that lie between blue and violet are run through; beyond the latter 
 number Violet comes to view. 
 
 The color-tones of the spectrum are, therefore, not sharply sepa- 
 rated, but pass gradually into each other. The nearer together two 
 colors are situated in the spectrum, the more nearly do they corre- 
 spond in the quality of their sensations. Nor has the spectrum any 
 sharply defined limit at either end, but passes gradually into black 
 more gradually at the violet than at the red end. The energy 
 of the ultra-red rays, as measured by their physical and chemical 
 action, is greater than that of the more highly refrangible rays. 
 The fact that these rays do not excite visual sensations must, then, 
 be due to the structure of the retina. The ^ra-violet end of the 
 spectrum has been made visible for a certain extent by experiment; 1 
 it produces the sensation of a glimmer of lavender-gray color. Our 
 inability to perceive these ultra-red and ultra-violet rays is not to 
 be considered an imperfection of the eye, as Tyndall thought. It 
 is rather purposeful, and of the greatest importance for vision; 
 since, if these ultra rays were visible, the clearness of objects would 
 be much disturbed by the chromatic aberration of the refracting 
 apparatus of the eye. 
 
 1 See Helmholtz, Physiolog. Optik, pp. 232 f. 
 
RELATIVE BRIGHTNESS OF COLOR-TONES 
 
 329 
 
 6. Besides the foregoing distinctions of color-tones, the im- 
 pression made by the green-yellow of the spectrum (D-E, and im- 
 mediately about D) is by far the strongest; or, as we should say, 
 this color is naturally the "brightest" of the spectral colors. From 
 
 . 
 
 FIG. 118. (From Fick.) The letters on the horizontal line stand for Fraunhofer's lines. 
 The ordinates of the interrupted curved line show the brightness of rays as seen; the ordi- 
 nates of the dark curved line, the intensity of the rays as measured by calorific effect. 
 
 the region immediately around D, the brightness of the color-tones 
 diminishes toward both the red and the violet ends of the spectrum 
 at first slowly, then more quickly, and then more slowly again. 
 Such a relation cannot be due to the spectrum as an objective 
 affair; for if we measure by other physical means the amount of 
 energy belonging to its different regions, we find that of the red 
 rays (which are by no means brightest) to be strongest. We must, 
 then, seek an explanation in the structure of the retina, and conclude 
 that it is peculiarly sensitive to stimulations by oscillations of about 
 five hundred and fifty billions per second (compare Fig. 118). The 
 sensitiveness of the retina to slight variations in color-tone, as de- 
 pendent upon differences in the wave-lengths of the stimulus, is also 
 different at different portions of the spectrum. It is greatest in the 
 green and blue-green regions (D and F). 
 
 The following table represents both the foregoing laws. The 
 numbers of the second and third columns show the relative bright- 
 
 
 Fraunhofer 
 
 Vierordt 
 
 Mandelstamm and Dobn 
 
 >wolsky 
 
 Red B ... 
 
 32 
 
 22 
 
 B 
 
 lilT 
 
 Orange, C 
 
 94 
 
 128 
 
 C 
 
 T^T 
 
 Reddish-yellow D 
 
 640 
 
 780 
 
 C-D 
 
 tir 
 
 Yellow, D-E 
 
 1,000 
 
 1,000 
 
 D.. ... 
 
 jfa 
 
 Green, E 
 
 480 
 
 370 
 
 D-E 
 
 sis 
 
 Blue-green, F 
 
 170 
 
 128 
 
 E 
 
 5^ 
 
 Blue, G 
 
 31 
 
 8 
 
 E-F 
 
 eh 
 
 Violet H 
 
 56 
 
 07 
 
 P 
 
 -W 
 
 
 
 
 G . 
 
 
 
 
 
 H 
 
 yiir 
 
 
 
 
 
 
 ness with which the different colors of the spectrum appear to the 
 eye, as calculated by different methods and by two observers. It 
 
330 THE QUALITY OF SENSATIONS 
 
 will be seen that the results agree substantially, though by no 
 means perfectly. In the last two columns the letters stand for 
 Fraunhofer's lines, and the figures give the fractional variation in 
 the wave-lengths which produces an observable variation in the 
 color-tone for different regions of the spectrum. 
 
 7. The colors of every-day experience, like its musical tones, are 
 not simple and pure color-tones, such as are obtained by spectral 
 analysis; they are composite. Inquiry must therefore be raised as 
 to the effect produced in sensation from the co-working of two 
 homogeneous rays of light upon the same elements of the retina 
 under all the normal conditions to which reference was previously 
 made. In pursuing this inquiry no direct assistance can be ob- 
 tained from the discriminations of consciousness; for sensations of 
 color, unlike those of musical clang, cannot be mentally analyzed 
 
 ' into their constituent elements. The science of optics makes us 
 acquainted, however, with the following facts: When the wave- 
 
 1 lengths of the two colors mixed vary but slightly (a few billions of 
 oscillations in a second) from each other, the color resulting from 
 the mixture lies between, and may be recognized as a "shade" of, 
 the colors mixed. By selecting for mixture color-tones that lie 
 apart at the various distances possible along the spectrum, an in- 
 definite number of impressions of color may be obtained, which all 
 differ from those obtained by the homogeneous colors. These 
 mixed color-impressions classify in such a way, however, that the 
 number of the qualities of resulting sensations is far less than that 
 .of the compound physical processes which stimulate the retina. 
 J The character of the colors making up our ordinary visual ex- 
 perience depends both upon the _place in the spectrum from which 
 
 . the simple color-tones are selected for mixture, and also upon the 
 relative intensity of the ones selected. For example, if a ray of 
 four hundred and fifty billions of oscillations per second (red) be 
 mixed with one of seven hundred and ninety billions (violet), a new 
 series of impressions of color (the purples) is attained by varying 
 the intensities of the two. These impressions are more or less like 
 red or like violet, according to the relative amounts of the rays of 
 four hundred and fifty billions and of seven hundred and ninety 
 billions which enter into the mixture. Moreover, there are found 
 to be two ways of advancing by this process of mixing color-tones 
 toward any one of the composite colors. Thus, we may pass from 
 yellow to blue either through green-yellow, green, and blue-green, 
 or through orange, red, purple, and violet. The table 1 on next page 
 is of interest in this connection. Where two colors are given as 
 
 *Made according to investigations by J. J. Miiller, and taken from Fick, in 
 Hermann's Handb. d. PhysioL, III, i., p. 190. 
 
NUMBER OF COLORS DISTINGUISHABLE 
 
 331 
 
 resulting from the mixture, the variation is to be understood as de- 
 pendent upon the prevailing intensity of one of the two compo- 
 nents. 
 
 8. The number of colors distinguishable by the human eye is 
 not easily stated with accuracy; like the number of musical tones, 
 it varies with different individuals. The usual number of seven 
 
 Components 
 
 Tone of the color obtained 
 by mixture 
 
 Degree of 
 saturation 
 
 Red and Yellow 
 
 Orange 
 
 Spectral. 
 
 Orange and Yellow-green. 
 
 Yellow 
 
 Spectral. 
 
 Yellow and Green 
 
 Yellow-green 
 
 Whitish 
 
 Yellow-green and Blue-green 
 
 Green 
 
 Very whitish. 
 
 Green and Cyanic Blue 
 
 Blue-green 
 
 Whitish. 
 
 Blue-green and Indigo 
 
 Cyanic Blue 
 
 Spectral. 
 
 Cyanic Blue and Violet 
 
 Indigo . . ... 
 
 Spectral. 
 
 Red and Yellow-green 
 
 Orange or Yellow 
 
 Spectral. 
 
 Red and Green 
 
 Orange or Yellow or Yellow- 
 
 
 
 green 
 
 Whitish. 
 
 Violet and Blue-green 
 
 Indigo or Cyanic Blue 
 
 Spectral. 
 
 Violet and Green 
 
 Indigo or Cyanic Blue or Blue- 
 
 
 
 ereen 
 
 Whitish. 
 
 Violet and Orange 
 
 Red 
 
 Whitish. 
 
 Red and Cyanic Blue 
 
 Indigo or Violet 
 
 Whitish. 
 
 Red and Indigo 
 
 Violet 
 
 Slightly whitish. 
 
 
 
 
 fundamental colors, as fixed by Newton, with the intent of forming 
 an octave in the scale of color-tones, has no sufficient claim to 
 acceptance. Six of the seven namely, red, orange, yellow, green, 
 blue, violet are indeed names in common use. But indigo, as an 
 intermediate tone, or kind of semi-tone, between blue and violet, 
 has perhaps no more real right to recognition than various other 
 intermediate color-tones. Bonders put the number of color-tones 
 distinguishable in oil-colors at one hundred; Von Kries the rec- 
 ognizable number of spectral tints at about two hundred and thirty. 
 But, as has already been said, colors also differ according to the 
 degree of their saturation or purity, due to freedom from admix- 
 ture of white light. Another series of variations of sensation must 
 be allowed for, which are due to differences in "brightness" or 
 intensity. Introducing these two variable elements, Von Kries 
 calculates the number of distinctions of color-sensations, possible 
 for all degrees of purity of tone and intensity of light, at about five 
 hundred thousand to six hundred thousand. This number stands 
 midway between the "many millions" of which Aubert speaks and 
 the five thousand allowed by Bonders. Herschel thought that the 
 
332 
 
 THE QUALITY OF SENSATIONS 
 
 workers on the mosaics of the Vatican must have distinguished at 
 least thirty thousand different colors. 
 
 9. Experiment also shows that if certain color-tones with a 
 given intensity are united on the retina, the result is a sensation 
 unlike that of any other of the colors, whether pure or mixed. 
 This sensation we call " white," and the two colors which by their 
 admixture produce it are called "complementary." Complementary 
 colors may be mixed upon the retina in various ways; either by al- 
 lowing two spectral rays properly selected to be superimposed at 
 the same spot, or by blending the reflected images of two colored 
 wafers, or by blending the direct visual impressions of colored 
 surfaces on a swiftly revolving top or wheel, etc. But however 
 mixed, the resultant sensation is that of a so-called "white" color, 
 in which all trace of the constituent elements is lost. Following is 
 a table of complementary colors 1 : the wave-lengths are given in 
 millionths of a millimetre. 
 
 Color 
 
 Wave-length 
 
 Complementary 
 color 
 
 Wave-length 
 
 Relation of 
 wave-lengths 
 
 Red 
 
 656 2 
 
 Green-blue 
 
 492 1 
 
 1 334 
 
 Orange 
 
 607 7 
 
 Blue 
 
 489 7 
 
 240 
 
 Gold-yellow 
 
 585.3 
 
 Blue 
 
 485 4 
 
 206 
 
 Gold-yellow 
 
 573.9 
 
 Blue 
 
 482 1 
 
 190 
 
 Yellow 
 
 567 1 
 
 Indisro-blue 
 
 464 5 
 
 221 
 
 Yellow 
 
 564 4 
 
 Indigo-blue 
 
 461 8 
 
 222 
 
 Green-yellow 
 
 563.6 
 
 Violet 
 
 433 l and 
 
 301 
 
 
 
 
 ' ( less 
 
 
 It will be noted that no complementaries are assigned for the 
 colors between green-yellow and green-blue, with wave-lengths 
 from five hundred and sixty-four to four hundred and ninety-two. 
 In other words, the central greens have no complementary colors 
 within the limits of the spectrum. In the series of color sensations, 
 however, green has a complementary, namely purple, the stimulus 
 for which is a mixture of long and short wave-lengths. 
 
 10. If the foregoing facts and laws are held to be true of the 
 "normal" connection between light and visual sensations, then 
 various classes of circumstances must be taken account of as "ab- 
 normal," which, nevertheless, enter into all our daily experience 
 with this sense. Indeed, the connection between stimulus and 
 sensation is not the same for different individuals who possess sub- 
 stantially the same color-sensations; frequently the complementary 
 colors for two different individuals are not precisely the same. 
 1 From Helmholtz, op. cit., p. 277. 
 
THEORY OF COMPLEMENTARY COLORS 333 
 
 Even the two eyes of the same individual often differ perceptibly 
 in this regard. Important changes in the quality of the sensations, 
 other than those directly ascribable to changes in the wave-lengths 
 of light, take place when the intensity of the light approaches either 
 a maximum or a minimum. At the maximum intensities of the 
 stimulus all sensations of color-tone cease, and even homogeneous 
 rays appear white. Previous to reaching this maximum, red and 
 yellowish green pass over into yellow, and bluish green and violet 
 into blue. At the minimum intensities of light every color-tone 
 except the pure red of spectral saturation appears colorless when seen 
 alone on a perfectly black ground. 
 
 In other words, light of any wave-length can be weakened till 
 it gives no sensation of color, while still giving a sensation of light. 
 There is, accordingly, a "photo-chromatic interval" between the 
 threshold for the perception of light and the threshold for the per- 
 ception of the color of the light; but this interval is extremely small 
 or absent in the case of the longest waves which give the sensation 
 of red. This phenomenon is closely connected with the process of 
 dark-adaptation, which has been described in an earlier chapter 
 (see p. 195) and may be restated as follows: With decreasing illu- 
 mination, the retina becomes "adapted," or gets into a condition 
 in which it is sensitive to very faint lights, but does not respond with 
 sensations of color to these very faint lights. Moreover, this adap- 
 tation, or increase in sensitiveness, varies greatly with the different 
 wave-lengths; and, indeed, scarcely exists at all with those which 
 produce sensations of red. 
 
 11. Changes of color also take place when the time of the ac- 
 tion of the light is reduced to a minimum. Sensations of satu- 
 rated color can be produced by instantaneous illumination of the 
 spectrum with the electrical spark. More time is needed, however, 
 to produce these sensations with smaller intensities of the light. 
 The different colors, even when of the same brightness, appear to re- 
 quire different amounts of time in order to reach the maximum of 
 their effect red, 0.0573; blue, 0.0913; green, 0.133 of a second. 
 The tone of the color varies with the duration of the impression as 
 well as with the intensity of the light. Very minute objects, too, 
 appear of a different color on account of their size. In general, the 
 larger the surface, the less the intensity of the light necessary to 
 produce the sensation of any particular color-tone; the greater the 
 intensity of the light, the smaller the surface which will suffice for 
 such sensation. Fick 1 showed that the color-sensations derived 
 from small distinct points support each other, as it were, in the same 
 way as the contiguous points of a colored surface. For if we make 
 1 Pfiuger's Archiv, XVII, p. 152. 
 
334 THE QUALITY OF SENSATIONS 
 
 with a fine needle a single hole (of about 0.6 mm. in diameter) in 
 a sheet of paper and look through it at colored paper distant some 
 six and a half metres, the color of the paper cannot be distinguished. 
 But if the number of holes be as many as sixteen, the color can be 
 distinguished at the same distance, even when the holes through 
 which we look are smaller. Subsequent experiment has shown that 
 the smaller the distance between the single perforations, the greater 
 the distance at which the eye can recognize colors through them. 
 In general, then, two weak sensations, both of which belong to one 
 eye, may fuse together into one stronger sensation. 
 
 12. Very important changes in the visual sensations occur as 
 dependent on the part of the retina which is stimulated. In this 
 respect a great difference exists between the central and the pe- 
 ripheral parts. The entire field of this organ may be somewhat 
 indefinitely divided into three zones a central or polar, a middle, 
 and an outer or peripheral. It is probably true that the periph- 
 eral parts of the retina produce no sensations which cannot be 
 produced by stimulating the central zone. But it is equally true 
 that, under the same circumstances, the same stimulus produces 
 a markedly different effect upon sensation when applied to differ- 
 ent localities of the retina. Rays which, falling on the polar zone, 
 produce the impression of red, yellow, or green, all make an im- 
 pression of yellow when they fall on the surrounding zone (a few 
 millimetres from the fovea centralis), and this yellow is so much 
 the paler, the greener the impression on the polar zone. Rays 
 which make on the polar zone the impression of blue or violet 
 make on the outer zone the impression of blue; and this blue is 
 so much the paler, the nearer the impression on the polar zone is to 
 green. 
 
 Any conclusions which we might be tempted to draw, as to physi- 
 ological differences in the different parts of the retina, in dependence 
 upon histological differences, are somewhat complicated by the fact 
 that a certain red, a certain yellow, a certain green, and a certain 
 blue can be found, which do not undergo this change of color-tone 
 in passing from the polar to the intermediate zone; but the yellow 
 and the blue remain unchanged, while the red and the green change 
 directly into gray, without first changing to yellow or blue. These 
 four special color-tones are, therefore, called the stable colors: the 
 stable yellow and blue are about what we should ordinarily call a 
 typical yellow or blue; but the stable red is a somewhat purplish 
 red, lying outside of the spectrum; and the stable green is a some- 
 what bluish green, as described in customary terms. The stable 
 blue and yellow are complementary, and this is true also of the 
 stable red and green. 
 
PHENOMENA OF COLOR-BLINDNESS 335 
 
 The so-called stable colors are clearly of great theoretical im- 
 portance; and the more so because of the following curious fact: 
 If the stable yellow and blue are made of such relative intensity 
 that, when mixed, they give a white or gray, the limit at which they 
 lose their color and become gray, on being moved toward the periph- 
 ery, is the same for both; and, similarly, if the stable red and green 
 are of such relative intensity as to give gray when mixed, both 
 change to gray at the same limit, on passing from the polar region 
 outward. 1 
 
 13. A certain proportion of persons (from three to five per cent, 
 of males, and apparently a much smaller proportion of females) 
 have a defect of vision which is known as "color-blindness." There 
 are several forms of color-blindness, in one of which (very rare) no 
 distinctions of color are possible, but only of brightness; and the 
 spectrum appears to such persons as differing, from part to part, 
 only in intensity. Light of any color appears to them identical 
 with light of any other color, provided the intensities be rightly pro- 
 portioned. In dim light, the vision of these individuals is the same 
 as that of normal persons, who also, it will be recalled (see p. 333), 
 lose color-vision under similar conditions. With the increase of 
 light, the normal eye begins to distinguish colors, i. e., to observe 
 qualitative differences between different parts of the spectrum; but 
 this the totally color-blind eye fails to do. The further fact that, 
 often at least, the yellow spot of the totally color-blind eye yields no 
 visual sensations whatever, suggests that such an eye possesses only 
 rod-vision and no cone-vision (compare pp. 195 f.). 
 
 Much more common, however, are the cases of blindness to cer- 
 tain colors, or, more correctly expressed, of inability to distinguish 
 certain colors which are readily distinguished by the normal eye. 
 It should be said that, though we speak of the normal eye as dis- 
 tinguished from the color-blind, there is no diseased condition pres- 
 ent in the color-blind eye. The defect is inherited, and is perfectly 
 consistent with normal vision in all other respects except that of 
 distinguishing certain colors. In the form which is called "red- 
 green blindness," the colors which are most frequently confused 
 are shades of red and green, or of purple and greenish blue. By 
 properly adjusting the intensity and saturation, it is possible to 
 produce confusion, in these persons, of any of the color-tones that 
 lie between red and green, and also of any lying between purple and 
 greenish blue. More precisely studied, the color-vision of these 
 individuals is found to reduce apart from matters of intensity and 
 
 1 See a review of the somewhat conflicting literature on this point, and an ex- 
 perimental research establishing the above results, in Baird's The Color Sensi- 
 tivity of the Peripheral Retina (Washington, 1905). 
 
336 THE QUALITY OF SENSATIONS 
 
 saturation to a single qualitative distinction; and this is the dis- 
 tinction between two colors that are excited, respectively, by the 
 long waves and by the short waves of light. From the red end of 
 the spectrum to a point in the green (wave-length about 490-500), 
 all visual sensations are with such persons shadings of a single color; 
 and any part of the scale may be matched with any other provided 
 the intensities are rightly selected. In similar manner, from this 
 point in the green to the violet (and purple), all is a shading of a 
 single color. Colors taken from opposite sides of this limiting 
 point in the green are, however, clearly distinguished. This bound- 
 ary itself is indistinguishable from gray; and the particular mixture 
 of long and short wave-lengths, which produces in the normal eye 
 the impression of a purplish red complementary to this green, to 
 the color-blind eye is also indistinguishable from gray. 
 
 What precisely are the color-sensations of the red-green blind 
 individual whether his white and gray look the same to him as 
 white and gray look to the normal individual, and whether the red 
 half of the spectrum seems to him shaded in red, or in yellow, the 
 blue half in blue or in violet is a difficult question to approach; 
 since evidently normal and color-blind individuals cannot "com- 
 pare notes" on such a matter. There have been examined one or 
 two cases of color-blindness confined to one eye, while the other saw 
 normally; and the testimony of these individuals seems to indicate 
 that, to the color-blind eye, gray and white appear the same as to 
 the normal eye, but that the red end of the spectrum is shaded in 
 yellow, and the blue end in blue. 
 
 Under the head of red-green blindness are recognized two sub- 
 classes, sometimes called the red-blind and the green-blind, the dif- 
 ference being that the red-blind are comparatively insensitive to 
 reds near the end of the spectrum. A few (pathological) cases 
 have also been observed of a different form of color-blindness, called 
 blue or yellow-blue blindness. This form of color-vision, like that 
 in red-green blindness, is dichromatic, and apparently the two col- 
 ors retained are red and green. In these cases, the neutral point 
 in the spectrum falls in the yellow. 
 
 14. Color-blindness is evidently of great importance for an 
 understanding of normal color-vision ; we may even say that dichro- 
 matic vision, from its greater simplicity, is better understood than 
 the normal, polychromatic fofm. The evidence, so far as it goes, 
 shows that the color-vision of the red-green blind agrees exactly 
 with that of the intermediate yellow-blue zone of the normal ret- 
 ina; the two agree, at any rate, in being dichromatic. 
 
 15. Important modifications of the normal action of the retina 
 I are also produced by previous excitation. The most important of 
 
. NEGATIVE AND POSITIVE AFTER-IMAGES 337 
 
 these changes is the adaptation to dark or to light which has already 
 been described. Let us assume that the eye has been exposed for 
 some time to light of a moderate intensity, and that it is then turned 
 upon a much brighter light, with prolonged fixation, and the latter 
 will appear to grow less bright; but if, after this, the eye is turned 
 back on its previous moderate light, this appears darker than before. 
 If, however, the eye is turned from moderate light to a dark field, 
 the latter seems to grow brighter with continued fixation; and if 
 then the eye is returned to the moderate field, this appears brighter 
 than before. These effects can be seen more strikingly, if only a 
 part of the field of view is bright or dark. After gazing steadily, 
 for example, at a bright patch on<a medium background, and then 
 turning to a uniform medium surface, a dark patch appears on 
 this, corresponding to the light patch previously fixated; and this 
 dark spot moves about on the background, as the eyes move. If, 
 similarly, a black spot on a gray ground is gazed at, and then the 
 eye turned to a plain gray ground, the place of the black spot is 
 now taken by a bright spot. If a figure containing both bright and 
 dark parts is fixated, and the eyes are then turned to the gray field, 
 an image of the figure appears, bright where the figure was dark and 
 dark where it was bright. This residual effect of excitation is 
 called the negative after-image. 
 
 Negative after-images of colors are also seen, the colors of the 
 image being complementary to those of the stimulus. If, for ex- 
 ample, a green spot is steadily fixated, it appears to lose some of 
 its color, becoming less saturated; and if then the eye is turned to a 
 plain gray field, there appears, in place of the green, its complemen- 
 tary purple. If the eye is turned from green to a purple ground, 
 the after-image of the green is still seen, in the form of a spot of 
 still deeper purple; it is in this way that the most saturated color 
 effects possible can be obtained. These after-images can be ex- 
 plained in terms of adaptation, or in terms of fatigue. A full expla- 
 nation, however, would need to take account of many curious de- 
 tails, one or two of which are worth mentioning. When a patch of 
 color is being steadily fixated, it not only grows less saturated, but 
 also changes its color.-tone; under such conditions, it seems to ap- 
 proach either yellow or blue the colors which are nearer to yel- 
 low approaching yellow, and those which are nearer to blue ap- 
 proaching blue. Yellow and blue, themselves, however, do not 
 change their color-tone on prolonged fixation; nor do two other 
 colors namely, a bluish green and a purplish red. These four col- 
 ors are stable here as they were in passing from central to periph- 
 eral vision. 1 A further detail is that the color of the after-image, 
 1 Voeste, Zeitschrift /. Psychol, 1898, XVIII, 257. 
 
338 THE QUALITY OF SENSATIONS 
 
 after strong stimulation, changes from moment to moment, and 
 passes through a series of colors. 
 
 16. Another difficulty in the way of a simple conception of the 
 after-image is the existence of positive after-images, between which 
 and the negative it is not easy to draw any valid distinction. The 
 names positive and negative are indeed justified by the fact that the 
 positive after-image presents both color and light-and-dark sen- 
 sations as they appear during the direct action of the stimulus; 
 whereas the negative after-image reverses everything. In order to 
 study the positive after-image we need only to look for a second or 
 two at some luminous object, and then close the eyes or turn them 
 toward a dark ground. A bright image of the luminous object 
 will be seen for a few seconds. Longer fixation favors the negative 
 image; when both are obtained, the positive precedes, though, in 
 case of the prolonged images which follow looking at a very bright 
 object (as the sun), changes occur from negative to positive and back 
 again as the background is changed from light to dark. Thus the 
 whole series of facts becomes rather complicated, and this is still 
 more the case when minute examination is directed to the immedi- 
 ate after-effects of the stimulus. These effects can be best exam- 
 ined when the stimulus is moved at a moderate rate over a dark 
 ground, while the eye remains fixed in position; the after-effects 
 then appear strung out behind the moving light. Three images 
 appear, separated by dark spaces; of these, the first image corre- 
 sponds to the sight of the light itself; the second is fairly sharp, and 
 often called the "ghost"; the third is less sharp and passes over 
 gradually into black. Translating the spatial relations of this ex- 
 periment into terms of time, we conclude that, when a light acts 
 on the retina for an instant, the first, wholly positive effect outlasts 
 the stimulus for a small fraction of a second, and is succeeded by 
 a brief interval of no effect, then by a recurrence of light effect, then 
 by another interval and another recurrence of light which gradu- 
 ally fades out and gives place to a negative effect (black). The three 
 images may all be called positive, except that the " ghost" is nega- 
 tive as regards color. 1 Many other details have been observed, but 
 these are sufficient to show something of the intricacy of the reaction 
 of the retina to light, and the difficulty of forming a clear concep- 
 tion of what takes place. Individual peculiarities appear to count 
 for much in these, as in all other details, of our visual experience. 
 
 17. The different parts of the retina are interdependent in the 
 production of sensation; or to employ the statement of Wundt 2 
 "The sensation which arises through the stimulation of any given 
 
 1 See W. McDougall, British Journal of Psychology, 1904, I, 78. 
 
 2 Quoted from the Grundzuge d. physiolog. Psychologic (2d ed., I, p. 439). 
 
PHENOMENA OF CONTRAST 339 
 
 point of the retina is also a function of the state of other immedi- 
 ately contiguous points." Hence arise, in part at least, the phe- 
 nomena of contrast, which are of two kinds contrast of bright- 
 ness and contrast of color-tone. The fundamental fact in the first 
 class of contrasts is this: every bright object appears brighter with 
 surroundings darker than itself, and darker with surroundings 
 brighter than itself. These phenomena are explained by Helm- 
 holtz 1 as deceptions of judgment, such as we are accustomed to in 
 our estimates of distances. To this explanation, however, Fick, 2 
 Hering 3 , and others oppose strong and apparently conclusive ob- 
 jections. They would explain the same phenomena by the modify- 
 ing influence of the excitation of one part of the retina upon the 
 excitation of contiguous parts. 
 
 When colored instead of white light is used in experimenting 
 under the law of contrast, phenomena similar to those of comple- 
 mentary colors are obtained A small square of white on a sur- 
 face of green, when covered with a transparent sheet of tissue-paper, 
 appears as red on a surrounding surface of a whitish hue; on a red 
 ground it appears as green, on a blue ground as yellow, and vice 
 versa. More complicated illustrations of this principle of inter- 
 dependence may be obtained in various ways. For example, if 
 on a pale-green background, in size 36 mm. by 44 mm., which is 
 divided into squares of 1.8 mm. by lines of white 0.4 mm. in width, 
 a red letter E, 21 mm. by 34 mm. in size, be constructed out of 
 similar squares, on observing the figure for a few seconds with a 
 fixed gaze, some of the red squares will disappear and be replaced 
 by green squares similar to the background. 4 Or if on sheets of 
 different colored paper, 20 cm. by 30 cm. in size, small strips, 
 1 cm. by 20 cm., of various colors are laid, as on a background, and 
 the whole then observed at a distance of about 3 m. with a fixed gaze, 
 exceedingly varied illusions of disappearance and substitution may 
 be obtained. 5 
 
 These, and similar phenomena, of which a great variety might 
 be mentioned, certainly cannot be explained as deceptions of "judg- 
 ments" in any justifiable meaning of the term. They do, however, 
 all seem to imply some physiological explanation which ascribes 
 to each part of the retina an influence on contiguous parts. Still 
 less in accordance with the facts of experience would the view of 
 Helmholtz appear to be, in the cases of those individuals who can 
 
 1 Physiolog. Optik, pp. 388 ff. 
 
 2 Hermann's Handb. d. Physiologic, III, i, 231 f. 
 
 3 Sitzgsber. d. Wiener Acad., June, 1872; Dec., 1873. 
 
 4 Fick's Lehrb. d. Augenheilkunde, 1897, p. 50. 
 6 Ladd, "A Color Illusion," Yale Studies, VI, 1898. 
 
340 THE QUALITY OF SENSATIONS 
 
 by an act of will produce, with eyes closed, simple colored shapes 
 in the retinal field, which upon opening the eyes will cast their 
 complementary images upon a white background. 1 
 
 18. It will by this time readily be seen that a theory which shall 
 satisfactorily account for all the complicated phenomena of visual 
 sensations is difficult to establish. Though ingenuity of the highest 
 order has been employed in the development of theories of color- 
 vision, not all the phenomena are well explained by any one theory. 
 Among the more significant facts which must find a place in any 
 adequate explanation are the following: (1) The colorless vision 
 which occurs in dim light, in the outer zone of the retina, and over 
 the whole retina in cases of total color-blindness. The conception 
 of two kinds of vision, one making no distinction of colors, and pro- 
 vided for, probably, by the rods, the other distinguishing colors 
 and provided for by the cones, serves admirably to explain these 
 facts, and a number of other details. This theory is associated 
 principally with the name of Von Kries (compare pp. 195 f.). With 
 colorless vision thus explained, as the function of the rods of the 
 retina, there remain to be explained only the facts of color-vision. 
 
 (2) The principal facts of color mixture are, first, the discovery 
 of Newton that all the colors of the rainbow, when mixed in proper 
 proportion, produce the sensation of white; then the fact of comple- 
 mentary colors, i. e., that pairs of colors, suitably chosen, mix to 
 produce the sensation of white; and further the fact that all color- 
 tones can be produced by the mixture of three (or more, but not 
 less than three) properly chosen colors. 
 
 (3) To these two classes of facts must be added the facts of after- 
 images and of contrast; and (4) The existence of dichromatic vision, 
 in the intermediate zone of the retina, in red-green blindness, in 
 intense illumination, and (partially at least) in the prolonged ex- 
 posure of the eye to light of a fixed color. In explanation of the 
 effect of intense light, and of the dichromatic zone of the retina, we 
 know that the two colors to which vision is reduced are yellow and 
 blue; in the case of color-blindness we are not so sure, but the indi- 
 cations point to the same two colors; in the case of prolonged fix- 
 ating of one color, again, the color tends to change toward yellow or 
 blue, i. e., there is a tendency to dichromatic vision with these two 
 colors remaining. In all these cases, moreover, special interest at- 
 taches to a certain bluish green, and its complementary, a purplish 
 red, since these are "stable," not being reduced completely to, 
 or in the direction of, yellow and blue, but changing directly to gray 
 or white. 
 
 1 See Ladd, " On the Direct Control of the Retinal Field," Psychol Rev., July, 
 1894; Mar., 1903. 
 
THEORIES OF COLOR VISION 341 
 
 (5) The psychological simplicity of white, i. e., the fact that it 
 cannot be introspectively analyzed into colors, though its stimulus 
 is always a complex of color stimuli, is also of prime importance. 
 Black, too, seems a positive and elementary sensation. It should 
 be noted that the occasion on which this sensation arises is not, pre- 
 cisely (as was formerly held), the absence of all light; for prolonged 
 absence of light gives rather the sensation of gray, and deep black 
 comes always as an effect of contrast or as an after-image. All of 
 the colors, to many psychological analysts, appear equally elemen- 
 tary; though others assert that orange, for example, seems to them, 
 introspectively analyzed, to consist of red and yellow components. 
 
 19. Of the theories which attempt to account for the facts of 
 our experience with color sensations, it will be sufficient to refer to 
 the two most prominent; but this must be done with the understand- 
 ing that, as neither of these has yet proved itself perfectly adequate, 
 constant attempts are still required to supplement or modify them 
 both, when taken singly and together. One of these theories is 
 called the Young-Helmholtz theory, having been propounded by 
 Thomas Young, and elaborated and defended by Helmholtz; the 
 other bears the name of Ewald Hering. The Young-Helmholtz 
 theory approaches the matter from the side of physics, i. e., pri- 
 marily from the facts of color mixture; the Hering theory takes its 
 start rather from physiological and psychological facts, such as the 
 apparent simplicity and positiveness of white and black, the facts 
 of contrast, etc. 
 
 Since all color-tones can be produced by mixing no fewer than 
 three of the number, there must apparently exist at least three sorts 
 of receptors or sensitive substances in the retina, each of them at- 
 tuned to ether vibrations of different wave-lengths. The Young- 
 Helmholtz theory supposes that there are, in fact, three such sub- 
 stances, one of them attuned to the long waves of the red end of 
 the spectrum, one to the short waves of the blue end, and one 
 to waves of intermediate length. Since, however, color-sensations 
 arise from stimulation by waves of any length, within the limits 
 of the visible spectrum, each of the three substances must be ex- 
 citable, not simply by waves of one length, but also, in diminishing 
 degree, by waves differing from this length. The relative sensi- 
 tivity of the three substances to light-waves of different lengths 
 would be somewhat as indicated by the curves in the accompanying 
 diagram, in which the horizontal distances denote distances in 
 the spectrum, the lettered lines indicating the prominent lines of 
 the spectrum. The height of the curve G over any part of the spec- 
 trum, indicates the sensitivity of the "green-receiving substance" to 
 light from that part of the spectrum; and the same for the curves 
 
342 
 
 THE QUALITY OF SENSATIONS 
 
 R and B, which show the varying sensitivity of the red and of the 
 blue (or violet) receptors. Light of any wave-length, according 
 to this diagram, would stimulate all three receptors, but in varying 
 degrees according to its length. White light would excite all of 
 them equally, we may suppose. 
 
 20. The theory of Hering is even more ingenious. It supposes, 
 first, that white light is not, physiologically, a mixture of colors, 
 but is due to the excitation of a special "brightness-receptor," 
 which responds, though in unequal degrees, to light of any wave- 
 
 FIG. 119. Diagram from Fick, illustrating the Young- Helmholtz Theory. 
 (For explanation, see the text.) 
 
 length. The whole white-black series, including all the shades of 
 neutral gray, is therefore due to the activity of this receptor, which 
 is accordingly named the "white-black substance." But the sen- 
 sation of black, according to Hering, is not due to the total inac- 
 tivity of the white-black substance, but to a form of activity op- 
 posed to that which occurs in response to bright light. This one 
 receptor is supposed to have two opposite forms of activity, which 
 give the sensations of the opposites, white and black. What the 
 two forms of activity may be, is not of so great consequence; Hering 
 supposed them to be anabolism or assimilation, and catabolism 
 or dissimilation the latter resulting from the action of bright light, 
 and the former from the action of lights more dim than that to 
 which the retina is, at any particular time, adapted. Other op- 
 posed chemical or electrical processes would, however, do as well. 
 Now if white and gray are due to the activity of a special recep- 
 tor, and not to the mixture of color-impressions, it follows that color 
 stimuli, when mixed, as they are in sunlight or in complementary 
 colors, must have the power of neutralizing each other; and so al- 
 lowing free play to the white-black process. The so-called pri- 
 
THEORIES OF COLOR VISION 343 
 
 mary colors must, therefore, exist in pairs, the members of each pair 
 being antagonistic. Hering therefore assumed, in addition to his 
 white-black substance, two other receptors, one for yellow and blue, 
 the other for red and green. The red must be slightly purplish and 
 the green slightly bluish, in order that they may be exactly com- 
 plementary and thus antagonistic. Light of long wave-length ex- 
 cites in the yellow-blue receptor a process which gives us the sen- 
 sation of yellow; but the short wave-lengths arouse in it an opposite 
 process, which gives us the sensation of blue. If both long and short 
 waves reach the receptor at once, the one neutralizes the other, and 
 the receptor remains at rest, leaving the white-black receptor to 
 act. Most colored lights excite simultaneously all three receptors, 
 and the resulting sensation is compounded of so much light (or dark), 
 so much yellow (or blue), and so much red (or green). 
 
 21. Without attempting to evaluate these opposing theories 
 by applying them in detail to the many facts which have to be ex- 
 plained, we may say that both are equally good as applied to the 
 facts of color mixture; while neither seems inconsistent with the 
 main facts of contrast and after-images. The psychological an- 
 alysis of colors as simple or as positive is perhaps of no conclusive 
 weight; but of all the facts, the most crucial seem to be those of 
 dichromatic vision. The most promising way of approaching an 
 explanation of these facts, with either theory, is therefore to sup- 
 pose that color-blindness is due to the absence, or lack of functional 
 power, of one set of color receptors ; since, in either theory, the sub- 
 traction of one set of receptors would leave dichromatic vision. 
 This same explanation could also be applied to the intermediate 
 dichromatic zone of the retina. The Young-Helmholtz theory has 
 a certain advantage in dealing with the two classes of the red-green 
 blind, which it supposes to be due to the absence, respectively, of 
 the red-receptors and of the green-receptors, and accordingly names 
 red-blind and green-blind. But other facts strongly favor the Her- 
 ing theory. For in the intermediate zone of the retina it is certain, 
 and in red-green blindness rather probable, that dichromatic vision 
 is ,yellow-blue vision. Now if yellow and blue are two of the pri- 
 mary colors, since they are complementary, no other single primary 
 would suffice; for white light could not be secured by the mixture of 
 two complementaries with any other single color; nor could all the 
 colors be produced by the mixture of three such colors. In fact, 
 only half the range of color-tones could be produced by such mix- 
 tures. The facts of dichromatic vision, therefore, apparently make 
 impossible any three-color theory for normal vision. There must, 
 accordingly, be at least four primary colors; and if four, the two 
 besides yellow and blue must themselves be complementary, in 
 
344 THE QUALITY OF SENSATIONS 
 
 order that the mixture of all four may give white. This turns 
 our minds back to the two stable colors colors which, each singly, 
 become gray in dichromatic vision the bluish green and its com- 
 plementary purplish red. These are the red and green which 
 Hering chose for primaries, and there can be no doubt that the pe- 
 culiar behavior of these two particular colors, as revealed in the 
 studies of recent years, has done much to strengthen the Hering 
 theory, at least as far as concerns the selection of the primary colors. 
 
 22. Much ingenuity and painstaking have been expended in de- 
 vising some form of symbolism which should represent to the eye in 
 geometrical relations the laws of the sensations of light and color. 
 Obviously the sensations of this sense cannot, like those of hearing, 
 be symbolized by the relations of points along a straight line. 
 Color-tones, unlike musical tones, form a series of qualitatively dif- 
 ferent sensations that, at certain places in the scale, separate from 
 each other with varying degrees of rapidity, and then toward the 
 broken ends, as it were, of this scale, tend to approach each other 
 again. Such relations are, perhaps, most successfully set forth by a 
 triangle in which the different color-tones may be regarded as lying 
 together along a curved line, from red to violet, and the difference 
 in any two color-tones as measured by the angle which two lines 
 make when drawn from a point within the triangle through the points 
 
 upied on the curve by the two color-tones. 
 
 23. Of sensations arising from excitation of the Skin, intro- 
 n enables us readily to distinguish those of temperature as 
 different in quality from those of contact and pressure; and to sepa- 
 rate sensations of warmth as different in quality from those of cold. 
 Further, the sensation of pain which comes from pricking the skin 
 can scarcely be regarded as anything but a distinct quality of sensa- 
 tion (i. e., in the narrower meaning of both words, "pain" and 
 "sensation"). These introspective distinctions are corroborated 
 in a most interesting way by experiments with minutely localized 
 stimuli applied to the skin. Such experimentation 1 reveals the ex- 
 istence of what are- called "touch-spots," "warmth-spots," "cold- 
 spots," and "pain-spots." If a small area of the skin, free or freed 
 from hairs, is explored, first with a bristle or hair which requires 
 very little force to bend it, no sensation is felt at most points on 
 pressing the end of the hair against the skin, but at certain points, 
 
 1 Since it was inaugurated by Blix in 1882, by Goldscheider in 1884, and by 
 Donaldson in 1885, this work has been continued by Von Frey, Kiesow, Alrutz, 
 and many others. See Goldscheider's Gesammelte Abhandlungen, Physiologic 
 der Hautsinnesnerven (Leipzig, 1908), also Sherrington in Schafer's Textbook of 
 Physiology, 1900, II, 920-1001, and Thunberg in Nagel's Handbuch der Physiol- 
 ogic, 1905, III, 647-733. 
 
SENSATIONS OF THE SKIN 345 
 
 a clear sensation of touch emerges. These are the touch-spots 
 (Fig. 120); and if they are marked for identification and retested 
 on another day, careful exploration finds their arrangement un- 
 changed. If, next, the skin is explored with a brass cone, cooled 
 a few degrees below the temperature of the skin, and applied with 
 its (somewhat rounded) point against the skin, no temperature sen- 
 sation will be felt at most points; but at certain points a clear sensa- 
 tion of cold appears. These cold-spots are not identical with the 
 
 ODE 
 
 t 
 
 
 Fia. 120. Arrangement of Pressure-spots (Goldscheider). A, dorsal and radial surface of 
 the first phalanx of the index finger; B, membrane between thumb and index finger; C, 
 dorsal surface of forearm; D, back; E, inner surface of forearm; F, back of hand. 
 
 touch-spots (see Fig. 121). Exploration with a warm cone brings 
 out, though with more difficulty, the existence of another series of 
 spots specifically sensitive to warmth; and exploration with a fine- 
 pointed needle or stiff bristle reveals numerous spots which give a 
 sharp, minute, pricking sensation. The pressure needed to excite 
 these pain-spots is much greater than that needed for the touch- 
 spots. 
 
 The skin thus resembles a mosaic of differently sensitized spots; 
 it differs from a mosaic in this, however, that there are insensitive 
 spots between those which are sensitive. No great care is required 
 to convince oneself of the general truth of these observations; but, 
 on the other hand, great care is necessary to reach precise results, 
 largely because of the difficulty of properly confining the stimulus 
 to the point which it is desired, at any moment, to examine; if the 
 pressure, the warmth, or the cold, is too great, it spreads quickly 
 to neighboring regions and the observations become uncertain and 
 confused. 
 
346 
 
 THE QUALITY OF SENSATIONS 
 
 24. The number of spots of the four varieties already mentioned 
 is, in general, very unequal, the warmth-spots being the fewest, 
 and the pain-spots the most numerous. The relative numbers may 
 be roughly stated as 1 warmth-spot to 10 cold, 10 touch, and 40 
 pain-spots. This proportion varies, however, in different parts of 
 the skin. From the cornea, for example, it seems that only sensations 
 of pain arise; while from certain areas on the inner surface of the 
 cheek, over against the gums, no sensations of pain can be elicited. 1 
 Hairs are organs of touch, and the corresponding sensation can 
 be aroused either by bending or pulling the hair, or by pressure on 
 
 B 
 
 FIG. 121. Arrangement of Temperature-spots. A, cold-spots; and B, warmth-spots 
 from the palm of the left hand (Goldscheider). 
 
 the skin over the hair follicle, on the "windward" side of the point 
 of exit of the hair. In general, the arrangement of the spots seems 
 to be irregular within any small area. 
 
 Touch-spots are excited by either depressing or by pulling out 
 the skin, or, in general terms, by any deformation of the skin; they 
 can also be excited by electricity. Pain-spots can be excited me- 
 chanically, electrically, chemically, or thermally; they seem to be 
 adapted to receive any stimulus which is intense enough, or almost 
 intense enough, to injure the skin. Cold-spots can be aroused, not 
 only by cold objects, but also by warm objects ("the paradoxical 
 sensation of cold"); warmth-spots cannot, perhaps, be aroused by 
 anything but warmth. 
 
 25. Concerning the exact nature of temperature stimuli, two 
 theories are in the field. The one (that of Weber) holds that the 
 warmth organs are excited whenever the temperature of the skin 
 is rising, and the cold receptors whenever the temperature of the 
 skin is falling. The other theory, by Hering, holds that any portion 
 of the skin is, at any moment, adapted to a certain temperature, 
 which may be called its zero or indifference point; 2 and that tempera- 
 tures above this zero point excite the sensation of warmth, and 
 temperatures below the sensation of cold or cool. The difference 
 between the two theories will become clearer when applied to the fol- 
 
 1 Kiesow, in Wundt's Philosophische Studien, 1898, XIV, 567. 
 2 More properly speaking, not a "point," but a zone extending for about the 
 space of one degree (Fahrenheit). 
 
NATURE OF TEMPERATURE STIMULI 347 
 
 lowing experiment. Suppose that the temperature of the skin of 
 both hands is 80 Fahrenheit, and feels neither warm nor cool. 
 Immerse one hand in water at 100, the other in water at 60, and, 
 after leaving them there for a minute, plunge both into water at 80. 
 This will now feel warm to the hand which has been in the cold 
 water, and cool to the hand which has been in hot water. The ex- 
 planation, according to the first theory, is that the temperature of 
 the hand which has been heated is now lowered, that of the other 
 hand raised by the same water. The other theory says that immer- 
 sion of one hand in hot water has adapted it to a temperature above 
 80, i. e., that its zero point has been raised, so that 80 is now below 
 its zero and feels cool; and the opposite for the other hand. Such 
 an experiment does not, however, enable us to decide between the 
 two theories. A more crucial instance is afforded by prolonged ex- 
 posure to warmth or (moderate) cold. If one is in a hot bath, or 
 sitting before a fire, or has a fever, the sensation of warmth persists, 
 though perhaps in diminished degree, for a very long time. The 
 first theory would have us suppose that the temperature of the skin 
 is constantly rising during all this time which is hardly possible. 
 The other theory has simply to argue that the process of adaptation 
 to temperature has limits i. e., that the zero point, though it can 
 be raised or lowered, cannot be raised or lowered very far. The 
 latter, or "adaptation theory" has the better of it in this case; as 
 it has also in the case of a sensation of warmth which lasts for a time 
 after emerging from a hot bath, in spite of the fact that the skin 
 at this time is cooling off. However, such every-day observations 
 have not the precision needed for a convincing test of either theory; 
 and experimental observations, 1 in which care has been taken to 
 excite a limited portion of the skin continuously and without allow- 
 ing neighboring parts to be affected, have been more favorable to 
 Weber's theory; inasmuch as prolongation of the stimulus did not pro- 
 long the sensation beyond a minute or at most three or four minutes. 
 26. Other sensations from the skin are apparently compounds 
 or modifications of the four classes already mentioned. Hard and 
 soft, rough and smooth, moist and dry, are judgments regarding 
 objects, founded on a combination of cutaneous sensations and of 
 these with muscular sensations. Itch seems closely akin to pain; 
 tickle, at least one form of it, seems closely related to touch proper, 
 since it can be aroused by brushing the hairs, which are organs of 
 touch. The sensation of heat or of burning has clearly an element 
 of pain, and probably arises from the excitation of pain-spots along 
 with warmth-spots. It is also probable, from what was said above 
 
 1 By Holm, Skandinavisches Archiv /. Physiol., 1903, XIV, 242; cited by Thun- 
 berg in Nagel's Handbuch. 
 
348 THE QUALITY OF SENSATIONS 
 
 regarding the paradoxical sensation of cold, that a hot object ex- 
 cites the cold-spots as well, so that a burning sensation would be a 
 blend of three elementary sensations. 
 
 Regarding pain, the view has long been held and is still enter- 
 tained by some authorities, that this sensation is the result of ex- 
 cessive stimulation of any sensory nerve; and the painfulness of 
 very loud sounds, or very bright lights, and of high temperatures, 
 was regarded as good evidence in favor of this view. The evi- 
 dence is not, however, conclusive, since it is impossible to prevent 
 excessive stimulation from involving other end-organs besides those 
 of the special sense supposed to be under stimulation. Very bright 
 light affects not only the retina, but causes a strong reaction in the 
 iris and eyelids. Loud sounds cause strong reactions in the mid- 
 dle ear; and high temperatures, as we have just seen, affect the pain- 
 rts as well as the warmth-spots. If we distinguish, as we certainly 
 uld, between mere unpleasantness and pain, we can hardly 
 doubt that there is a specific cutaneous pain, which arises from the 
 excitation of certain definite points; and accordingly it has become 
 customary to speak of a pain sense, as of a warmth sense, a cold 
 sense, and a touch sense. There are, however, several different 
 qualities of sensory pain and ache, as from the bones, teeth, fa- 
 tigued muscles, etc. 
 
 27. Quite recently a new line of study of the cutaneous senses 
 has been opened through the work of Head and his collaborators. 1 
 When a nerve supplying the sense-organs in any area of the skin 
 is severed by accident, a certain region of the skin is entirely de- 
 prived of sensibility; but, bordering this region is a zone in which 
 sensibility is impaired in a definite way, without being entirely de- 
 stroyed. This border region may be insensitive to light touch or 
 to minor degrees of warmth or cold, and be deprived of spatial 
 discrimination; a prick may be felt, but not be localizable, since 
 it gives rise to only a diffuse, tingling sensation. In the area in 
 which the skin is totally insensitive, a deep or subcutaneous sensi- 
 bility is still retained. Pressure through the skin is felt here; but, 
 if the skin is lifted from the underlying tissue into a fold, no stimulus 
 applied to this fold of skin is felt. The pressure which is appreci- 
 ated by the subcutaneous sense can itself be localized, and yet 
 spatial discrimination, as between the point and the head of a pin, 
 or between two points and one point of the compasses, is impossible 
 with the deep sensibility alone. Dull pain is produced by strong 
 pressure on the subcutaneous organs. 
 
 1 Head and Sherren, "The Consequences of Injury to the Peripheral Nerves of 
 Man," Brain, 1905, XXVIII, 241; Rivers and Head, " A Human Experiment in 
 Nerve Division," Brain, 1908, XXXI, 323. 
 
THE SO-CALLED MUSCULAR SENSE 349 
 
 Such phenomena show that the severance of the cutaneous nerve 
 has served to isolate the subcutaneous sense, and thus they lead to 
 a distinction between the tactile sense of the skin and the pressure 
 sense of the subcutaneous tissue; and also to a distinction between 
 the pain sense of the skin (pricking) and the duller pain of the sub- 
 cutaneous sense. Besides this isolation of the subcutaneous sense, 
 Head believes that the partial sensibility of the intermediate zone 
 of the skin represents the isolation of a rudimentary form of cu- 
 taneous sensibility, which is sensitive to pricking and to extremes 
 of heat and cold, from the perfected form of the normal skin, which 
 is sensitive to warmth and coolness and to light touch, and which has 
 the power of spatial discrimination. The more rudimentary form 
 he calls " protopathic " sensibility, and the more perfect form, "epi- 
 critic" sensibility. This separation of two distinct forms of cu- 
 taneous sensibility has been called in question; since it has been 
 shown by Franz 1 and by Trotter and Davies 2 that the transition 
 from one to the other is really gradual. But the sufficiently sharp 
 delimitation of the subcutaneous sense appears to be correct, and a 
 certain region in which the transition appears gradual is precisely 
 what we should expect. The discovery, therefore, represents a con- 
 siderable advance in our .knowledge of these classes of sensation. 
 
 28. The subcutaneous sense merges with the so-called muscular 
 sense. Sensory end-organs are found in muscles and tendons, 
 about the joints, and in the bones (see p. 181). Goldscheider 3 and 
 others have shown that cutaneous anaesthesia alone does not destroy 
 nor much impair the power of perceiving the movements and posi- 
 tions of the different members of the body, nor the power of accu- 
 rately co-ordinated movement, which latter, as well as, of course, the 
 former, is destroyed by complete anaesthesia of a member. At one 
 time, there was much disposition to believe in a definite joint-sense, 
 with end-organs in the articular surfaces, excited by the rubbing 
 of these surfaces over each other; but evidence is lacking of sense- 
 organs in these surfaces. Yet the region of the joints must be the 
 location of end-organs important in the perception of movement, 
 for Goldscheider found that anaesthetizing the joint, by passing a 
 current through it, greatly impaired the perception of motion. The 
 observations of Pillsbury 4 indicate that the sensibility of the joint 
 region is closely connected with the tendons which pass over the 
 joint. It is apparently the movement of these tendons in their 
 
 1 Journ. of Comp. Neurol. and Psychol, 1909, XIV, 107, 215. 
 
 2 Journ. of PhysioL, 1909, XXXVIII, 134. 
 
 3 See Goldscheider's GesammeUe Abhandlungen, Physiologic des Muskelsinnes 
 (Leipzig, 1909). 
 
 4 American Journ. of Psychol., 1901, XII, 346. 
 
350 THE QUALITY OF SENSATIONS 
 
 grooves, rather than the movement of the articular surfaces over 
 each other, which gives rise to sensations of movement. Of the 
 qualities of sensation belonging to this "muscle" sense it is less easy 
 to speak than of the perceptions of motion, position, and resistance 
 which are founded on them. We may, however, distinguish with 
 certainty between the unpleasant sensations of muscular fatigue 
 and soreness and the more matter-of-fact sensations of movement 
 and resistance. Of these latter, several varieties can be observed; 1 
 but nothing like a complete analysis or system of such sensory quali- 
 ties has yet been attempted. 
 
 29. From the labyrinth of the inner ear the semicircular 
 canals and vestibule arise a class of sensations which may be 
 named " labyrinthic " (see above, p. 206). Here again, though it 
 is easy to mention the perceptions and the reflex movements which 
 result from the stimulation of these organs, it is not possible as yet 
 to say much regarding the sensory qualities belonging to the laby- 
 rinthic sense. The swimming sensations in the head in dizziness, 
 and the milder degrees of the same which can be observed in rota- 
 tion, and in going up or down in an elevator, belong here without 
 doubt. 
 
 30. Under the head of visceral or organic sensations may be 
 grouped hunger, thirst, nausea, suffocation, and probably a host 
 of other sensory compounds which contribute much to our feeling 
 of well-being or illness and in general to the "coensesthesia" or 
 "common sensation" of the organism. Analysis here is difficult, 
 for it is never easy to analyze a sensory complex into its elements 
 unless we can control and isolate the stimuli. Little can be done 
 at present toward a physiological explanation of these sensations, 
 except to call attention to their existence. 2 
 
 31. In closing the subject treated in the last two chapters, 
 attention is again called to the large amount and cumulative char- 
 acter of the evidence afforded by the special sensations, considered 
 as respects their quality, for the law of the Specific Energy of the 
 Nerves. It is impossible to account for the above-mentioned phe- 
 nomena without carrying this law to a great length in its applica- 
 tion to the special senses. We may not be able at present to affirm 
 that two sensations are distinguishable as respects quality only in 
 case they are occasioned by two individually different elements of 
 the nervous system. For, as we advance in our investigations we 
 shall see even more clearly that the quality of sensations depends 
 upon their quantity, upon their relation to preceding and con- 
 
 1 See Woodworth, Le Mouvement, pp. 25 ff. (Paris, 1903). 
 8 For a more extended treatment of the entire subject of qualities of sensation, 
 reference may be made to Titchener's Textbook of Psychology (New York, 1909). 
 
SPECIFIC ENERGY OF THE NERVES 351 
 
 temporaneous sensations, and upon innumerable considerations 
 other than merely the one of what particular nerve-fibre or element 
 of the end-apparatus was acted upon by the stimulus. Moreover, 
 there is no warrant for saying that identically the same nervous 
 apparatus cannot be excited variously according to the nature of 
 the stimulus which acts upon it, or according to the combination 
 with other parts of the system into which it enters for the time. It 
 is obvious, however, that the differentiation of function, and the as- 
 signment to specifically distinct apparatus of particular nervous im- 
 pressions corresponding to, particular mental states, is carried to a 
 great length in the special senses. In this differentiation of func- 
 tion it is not wholly or chiefly the nerve-fibres, as such, which should 
 be taken into account; it is also the minute subdivisions of the end- 
 organs of sense, and the connections set up within the correspond- 
 ing regions of the central organs. In accounting for those complex 
 sensations which appear in ordinary consciousness, the law of per- 
 mutations and combinations has, of course, to be considered. A 
 vast variety of such ^sensations may be made up by changing the 
 relations to each other of comparatively few simple elements. But 
 in each of the senses our analysis, when carried to its utmost limit, 
 leaves a number in some of the senses very large of simple sensa- 
 tions, which apparently must have their normal physical basis in 
 the excitation of specifically distinct elements of the nervous mech- 
 anism. 
 
 The sense of smell apparently requires that the law of the specific 
 energy of the nerves should be carried to such a length as almost 
 to reduce it to an absurdity. Histology has discovered only one 
 essential kind of olfactory end-organ, and that of comparatively 
 simple structure; and yet experience gives, as the result of its ex- 
 citation, a bewildering variety of sensations so specifically different 
 as to baffle all our attempts to classify them. From the case of 
 this sense an argument may then be derived which leads in either 
 direction. It may be objected to the law that it is absurd to sup- 
 pose a complexity of the end-organs of smell such as to correspond 
 to each specific kind of olfactory stimulus with a specific sensation 
 for example, the smell of musk, or of sulphuretted hydrogen. It 
 may be replied to the objection that, in the case of the ear, there 
 are at least 16,000 or 20,000 distinct units of auditory end-appa- 
 ratus corresponding to the different musical tones; and it is there- 
 fore by no means impossible that the entire regio olfactoria may 
 contain enough specifically different forms of its own peculiar end- 
 apparatus to suffice for all the simple sensations of smell. 
 
 The sense of taste does not occasion so many difficulties in rela- 
 tion to the law of the specific energy of the nerves. We have seen 
 
352 THE QUALITY OF SENSATIONS 
 
 that physiologists incline to reduce all the sensations of taste to four, 
 or at most six, different species. It is easy to suppose as many spe- 
 cifically different forms of the nervous apparatus corresponding 
 to the different classes of sensations sweet and sour, salt and bit- 
 ter, alkaline and metallic. On combining these with sensations 
 of smell and of touch, it is assumed to find an explanation for all 
 the varieties of the tastes of our daily experience. But even if we 
 have greatly to increase the number of primary sensations of the 
 gustatory species, the theory of specific functions for different ner- 
 vous elements and combinations of such elements would seem able 
 to meet the demand. 
 
 The strongest defence of the most extreme form of the theory of 
 the specific energy of the nerves has hitherto been found in sensa- 
 tions of musical sound. Here we undoubtedly have a wide range 
 of qualitatively distinct states of consciousness which are appar- 
 ently dependent upon the excitation of a correspondingly large 
 number of distinct nervous elements. 
 
 The recent discoveries as to the existence of pressure-spots, 
 warmth-spots, and cold-spots in the skin add important evidence to 
 that already existing in favor of the law under discussion. 
 
 It is, undoubtedly, still difficult to make any thorough-going appli- 
 cation of the law of specific energies to the case of the color sensa- 
 tions. The principal reasons for this are twofold: first, histology 
 has revealed no such differentiation of the cones as would enable 
 us to divide them into organs for the several primary colors; and, 
 second, the peculiar, antagonistic relations of white and black, 
 yellow and blue, and red and green, are hard to explain by any 
 theory of specific energies. The Hering theory, as we have seen, 
 interprets these relations to mean that the same end-organ is capa- 
 ble of two antagonistic reactions. Accordingly, it would seem that 
 the nerve-fibres connected with these end-organs must transmit two 
 kinds of impulses to the brain. The theory of their specific energy, 
 therefore, needs further testing before it can be satisfactorily fitted 
 to the facts of our vision of color. 
 
 It will further appear, when we consider the process of localiza- 
 tion in the so-called "geometrical senses" of the eye and the skin, 
 that the very possibility of such a process demands a somewhat 
 strict and far-reaching application of the law of the specific energy 
 of the nerves. Precisely how we are to state and limit this law, 
 neither its opponents nor its advocates have as yet been able satis- 
 factorily to show. The exact expression of the theory waits for 
 further evidence from experiment, although there can be little doubt 
 that in its main features it is already secure. 
 
CHAPTER III 
 THE QUANTITY OF SENSATIONS 
 
 1. By an act of mental analysis, which all men readily perform, 
 changes in the amount of sensation are distinguished from changes 
 in its quality. This distinction, in itself considered, obviously re- 
 quires for its performance nothing beyond what is immediately 
 given in consciousness. The simple fact of experience is, that all 
 sensations appear to differ among themselves, not only with re- 
 spect to the nature of the impression which serves to classify them 
 into groups (as sensations of sight, sound, etc.), but also with re- 
 spect to the degree in which each particular impression possesses 
 the sphere of conscious attention and feeling. The best illustra- 
 tion of an alteration in the intensity of sensation, while its charac- 
 teristic quality remains unaltered, may be derived from musical 
 tones. The dying-out of a single tone when the bow is drawn 
 with decreasing force across the string of a violin, or a single 
 key of the piano is struck and the pedal held, may be considered 
 as a change in the quantity of sensation, while its quality is un- 
 changed. A more complex case is the experience we have when 
 approaching or receding from a bell that is sounding or a steam- 
 whistle that is blowing. Noises of a certain complex quality 
 such as slamming, hissing, grating, etc. are continually described 
 as very loud, moderately loud, or of weak intensity. So, too, 
 when approaching a white or colored light, with our attention 
 fixed upon it, we generally disregard almost wholly the changes 
 in its color-tone which take place, and consider chiefly the changes 
 in its intensity and apparent size. The pressure of different weights 
 upon different parts of our skin is ordinarily regarded as the same 
 in quality and as varying only in amount and locality. The same 
 thing is true, in almost precisely the same way, with sensations 
 of temperature. The thing we touch is called slightly cold or very 
 cold, somewhat warm or very hot, our attention being directed 
 chiefly to our judgment of the quantum of sense experience which 
 it calls forth. In other words, it is generally the same kind of 
 pressure and temperature, with a varying degree of intensity, of 
 which we believe ourselves to be conscious. 
 
 It is more difficult, however, even in the most indefinite way, to 
 discriminate between the quantities of our sensations of smell and 
 
 353 
 
354 THE QUANTITY OF SENSATIONS 
 
 taste and the changes in specific quality of the same sensations. A 
 concentrated sweet or acid so strongly excites a variety of forms of 
 feeling which mingle indistinguishably with the sensations of taste 
 that we are compelled to attend to the very decided qualitative 
 changes which are taking place. The increased intensity of the 
 sweet or sour we may indeed speak of as "very" much of the same 
 sensation which was excited in less degree by the diluted form of 
 the stimulus; but we are' more likely to regard it as constituting a 
 complete change in the kind of taste. In the same manner, at- 
 tention is forcibly directed toward the kind of sensation which re- 
 sults from increasing the quantity of any specific sensation of smell. 
 
 It is further obvious that the distinction which we make between 
 changes in the quantity and changes in the quality of our sensa- 
 tions is to some extent applicable for comparing the sensations of 
 different senses. And here the distinction, when applied to sub- 
 species under certain specific forms of sensation, affords us a means 
 of transition for such comparisons. Some yellows are bright and 
 others dull; and the same thing is true of the reds and the blues. 
 The sours, the sweets, the bitters, may be compared with each 
 other as respects the degree of intensity which they possess. We 
 may next, in a very indefinite way, compare the quantities of the 
 sensations of the different senses as they appear side by side, or 
 successively, in consciousness. We are ordinarily satisfied, how- 
 ever, with simply describing the varying degrees of intensity pos- 
 sessed by our different sensations as "weak" or "strong" (with 
 or without the emphatic "very") or as only "moderate." Thus 
 we may judge that both the light which we see and the tone which 
 we hear (either simultaneously or one immediately after the other) 
 are, or are not, to be classed together under the same one of these 
 three popular forms of discriminating degrees of intensity. 
 
 2. That changes in the intensity of our sensations are not, in 
 fact, wholly independent of changes in their specific nature has al- 
 ready been stated. Only in the case of musical tones are we able 
 at the same time to attend carefully to both the quantity and quality 
 of our sensations, and so discover with perfect confidence that the 
 former is changing while the latter remains unchanged. Even in 
 this case, since the tones which we ordinarily hear are composite, 
 any considerable alteration of their intensity changes also their tone- 
 coloring, through the alteration which it produces in the comparative 
 intensities of the overtones. Any increase in the brightness of a par- 
 ticular color invariably changes its characteristic color-tone. A 
 white of less intensity is not merely less white, but it has become a 
 gray; and by constantly diminishing its intensity white can be shaded 
 through the different grays toward black, which is certainly not a 
 
UNSCIENTIFIC FORM OF ORDINARY USAGE 355 
 
 feebler degree of the sensation of white. The same dependence of 
 quality on quantity is true in all sensations of smell, taste, pressure, 
 and temperature. It is no less than an inexcusable psychological 
 blunder, however, on this account to consider "quantity" of sen- 
 sations as only another name for shades of quality; or to deny that 
 we can apply terms of measurement to these reactions of the mind 
 upon the excitation of the nervous apparatus of sense. Scientific 
 analysis confirms the distinction made by ordinary experience be- 
 tween "the way" we feel and " how much" we feel in any particular 
 way. 
 
 3. All descriptions of the changing intensities of sensations, 
 when made on the basis of ordinary experience solely, leave the 
 subject in a very indefinite and unscientific form. That a certain 
 noise is louder or weaker than another of precisely the same kind, 
 one may be quite ready to affirm; one may even be ready to say 
 that one judges this noise to be about twice or three times as loud 
 as the other. But when more precise estimates are demanded, one 
 is obliged to hesitate before giving them. Is this musical tone ten 
 (or a hundred) times as loud as the other; or is it only nine and 
 nine-tenths (or ninety-nine and nine-tenths) as loud? Few would 
 venture so nice an estimate with any confidence. Yet the case of 
 sound is much more favorable than that of most of the other senses 
 for forming an exact judgment as to its intensity. It would be 
 difficult under the most favorable circumstances to affirm that the 
 sensation of the light a is twice or three times as bright as that of 
 the light b; or that of the shadow x one-half or one-third as bright 
 as y. The comparative intensities of different color-tones are yet 
 more difficult to fix subjectively even in the most indefinite way. 
 This particular yellow may seem about as bright a color, of its kind, 
 as does the red near it, of its kind. But the precise moment could 
 not readily be told when the blue of the sky appears exactly twice 
 as intense as the green of the grass. Still further, all estimates 
 of the quantity of sensation approach the point at which they lose 
 their meaning and tend to become absurd, when we compare, for 
 example, sensations of smell or taste with those of pressure, tempera- 
 ture, or sight. We never say: The rose smells as sweet as it looks 
 red; or the lemon is twice as sour as the sky is blue. And yet each 
 qualitatively different sensation is assumed to have its place some- 
 where in that scale of intensities through which the different quali- 
 ties may run; each may, therefore, be compared with every other, 
 with respect to the general position which it occupies in its char- 
 acteristic scale. 
 
 4. The variation of sensations in intensity, though it is an ob- 
 vious fact of the commonest experience, leads, when we attempt to 
 
356 THE QUANTITY OF SENSATIONS 
 
 give it exact scientific form, to a host of difficulties, which have been 
 the occasion of an immense amount of discussion among psychol- 
 ogists. All the resources of the higher mathematics have been em- 
 ployed in order to discover and demonstrate some law of universal 
 applicability, which shall regulate satisfactorily the quantitative 
 relations existing between the functions of the nervous mechanism, 
 on the one hand, and on the other, the states of consciousness to 
 which psychology gives the name of sensations, or "sensation-com- 
 plexes." In some cases, as notably that of Fechner, the attempt 
 has even been made to raise this alleged psycho-physical law to the 
 dignity of a universal metaphysical principle. Meantime, the fact 
 has become more and more apparent that what we have, on the 
 one side, is certain largely unknown but always highly complex 
 chemico-physical and nervous reactions, and on the other side, 
 even more complex, and often unanalyzable, conscious experiences, 
 in which the faculty of discrimination always plays the leading part, 
 and which are quite uniformly influenced by individual conditions 
 of mental habit, fatigue, specific sensitiveness, either congenital 
 or acquired, and even individual idiosyncrasies. We do not ex- 
 pect, then, to reach anything like an exact and universal formula, 
 statable in terms of mathematics, after the fashion of the results 
 obtainable by a successful research in physics or chemistry. As 
 students of psychology from the physiological point of view, it is 
 our duty (so we have always held) first to discover the data, and then 
 as far as possible give them that kind of interpretation of which 
 alone the science of psychology admits. 
 
 5. The fundamental characteristic of the intensity of a given 
 sensation, that which fits it for scientific study and measurement, is 
 this: its various degrees can be arranged in an ordered series, from 
 less intense to more intense; and thus every intensity can be judged, 
 with reference to any other, by comparing it with a standard, or 
 some one simple category of magnitude. Such well-ordered series 
 can be found, not only in reference to intensity, but also in refer- 
 ence to extent and duration; and, in some instances, in reference 
 to quality. Tones, for example, can be arranged in an ordered series 
 as regards their pitch; lights, as regards their color-tone. The scien- 
 tific problems which arise in connection with such series are (1) to 
 assign the limits of the series; and (2) to discover, if possible, the 
 customary, or fixed, relations between the different terms of the 
 series. As applied to the pitch series, these problems lead to the 
 determination of the lowest and highest pitch, the measurement of 
 the least noticeable difference of pitch, and the examination of the 
 " intervals." As applied to color, however, the problems of measure- 
 ment are reducible to the determination of the limits of the colored 
 
CHARACTER OF QUANTITATIVE PROBLEMS 357 
 
 spectrum, the least noticeable difference in color, and such relations 
 as that of complementary colors. 
 
 As applied to the intensity series, these same problems call for 
 (1) the determination of the weakest and strongest sensations, of 
 any given quality, of which we are capable; (2) the relation between 
 neighboring members of the series which is indicated by the least 
 noticeable difference in intensity; and (3) the relations (if any of 
 interest can be found) between distant members of the series. In 
 all this kind of work, it is clearly impossible to deal with sensations 
 apart from the stimuli which give rise to them. The stimuli are 
 needed to arouse the sensations; control and identification of the 
 stimulus is necessary in order to control and (approximately) iden- 
 tify the sensation; and furthermore measurement can be applied 
 directly to the stimulus, but not to the sensation. It is clear, then, 
 that quantitative statements regarding the intensity of sensation 
 must always be based on quantitatively determined stimuli. 
 
 6. The quantitative problems which arise in connection with 
 the intensity of sensation may then be somewhat crudely generalized 
 as follows: (1) To determine how little and how much of each kind 
 of stimulus will produce respectively the least and the greatest quan- 
 tity of each kind of sensation of which the mind is capable, or to 
 find the quantitative limits within which sensations of each sense 
 are possible; and (2) to determine the law of the relation under which 
 changes in the intensity of sensations, as estimated in conscious- 
 ness, are dependent upon changes in the intensity of the stimuli. 
 
 7. Two methods of determining the lower limit, or minimum 
 of stimulus producing a sensation, are possible. In the use of one 
 method, a weak stimulus, but somewhat above the amount needed 
 to produce a sensation, is applied; its intensity is then diminished 
 by minute gradations until the exact point is reached and noted at 
 which it ceases to produce any sensation at all. In the use of the 
 other method a stimulus too weak to produce any sensation is first 
 applied; its intensity is then very gradually increased until it be- 
 gins to produce the smallest observable sensation. Both ways may 
 be combined, and thus the "sensitiveness" of each organ of sense, 
 and of each part of each organ, may be determined. Such sensi- 
 tiveness increases, of course, in inverse ratio to the amount of stim- 
 ulus necessary for producing any sensation at all, or for producing 
 a sensation estimated as having a definite degree of energy. The 
 effort to determine the lower limit of sensations of sight and of 
 sound is embarrassed by the facts that the retina is always under 
 excitation from the chemical changes going on in its pigments, and 
 therefore has a certain quantum of so-called "light of its own," 
 and that such a thing as "absolute stillness" cannot probably be 
 
358 THE QUANTITY OF SENSATIONS 
 
 secured for the ear. Total absence of sensation in the ear, could 
 it be secured, would not be comparable to the black which we see 
 with the eyes closed. 
 
 The upper limit of the intensity series would be determined if 
 we should find that increasing the stimulus beyond a certain limit 
 gave no further increase in the sensation. Practically, however, 
 such a determination is scarcely ever possible, both because of the 
 danger to the sense-organs, and also because very intense stimuli 
 affect other organs besides that whose sensations we wish to examine 
 and so lead to confused results. 
 
 8. The invention of methods for determining the least percepti- 
 ble difference in intensity, and other relations between members 
 of the intensity series, has been one of the main tasks of experimental 
 psychology, and has engaged the attention of many of the ablest 
 students of this specialty. The greatest contributions, in this mat- 
 ter of method, were made by Fechner in his Psychophysik (I860). 1 
 
 We cannot enter here into the technical details which must be 
 attended to in any actual use of these methods, but will simply 
 attempt to characterize them briefly. The following are the prin- 
 cipal ones: (1) The "method of just noticeable differences" de- 
 termines that difference in the intensity of two stimuli which is just 
 large enough to be recognized as such. Of two lights, for example, 
 which are at first equally bright, one is made brighter by slow de- 
 grees, till it is first judged brighter; then a second determination 
 is made, starting with one light obviously the brighter, and diminish- 
 ing this one till it no longer seems brighter than the other. The 
 mean of these two determinations gives a measure of the least 
 noticeable difference, or, it may also be called, the threshold of 
 difference. It is not true, however, as the name might imply, that 
 this threshold is a perfectly fixed quantity, below which all dif- 
 ferences are unperceived, while above it all are perceived; for per- 
 ception is a variable process, and a difference which at one moment 
 may be clearly detected will pass unnoticed at another moment. 
 It is necessary, therefore, to repeat the determinations a number 
 of times, and to take an average, according to the well-known rules 
 regulating such experimentation. 
 
 (2) In the "method of average error," as in the preceding method, 
 there are two stimuli, one of which is fixed in intensity, and the other 
 
 1 Next to Fechner, the development of these " psychophysical methods " owes 
 most to G. E. Mullen Zur Grundlegung der Psychophysik (Berlin, 1878); Die 
 Gesichtspunkte und die Tatsachen der psychophysischen Methodik (Wiesbaden, 
 1904). For both an elementary account of these methods and a full history and 
 discussion of the subject, see Titchener, Experimental Psychology, Quantitative 
 (New York, 1905). 
 
THE LEAST PERCEPTIBLE DIFFERENCE 359 
 
 adjustable. In the use of this method, however, the effort is made 
 to adjust the second stimulus so that it shall appear equal to the 
 first. Some slight error is sure to be committed; the error will 
 vary from trial to trial, and the average error is determined, and also 
 any constant errors which may appear; and, as well, the varia- 
 bility of the performance. In some respects the variability is the 
 most important of all of these determinations, since it gives a measure 
 of the variation which a stimulus can undergo without ceasing to 
 appear the same. 
 
 (3) In the "method of right and wrong cases," neither of the two 
 stimuli is adjustable, but they are fixed at a difference too small to 
 be perceived with certainty and regularity, but not too small to be 
 detected most of the time. With this fixed difference, a long series 
 of judgments is made, and the per cent, of right judgments gives a 
 measure of the perceptibility of the chosen difference between the 
 stimuli. If, for example, a weight of 105 grams is judged heavier 
 than a weight of 100 grams 70 per cent, of the time, while 205 is 
 judged heavier than 200 only 60 per cent, of the time, it is clear that 
 the difference 200-205 is less perceptible than the difference 100- 
 105; and if 200210 is, like 100-105, judged correctly 70 per cent, 
 of the time, then these two differences are equally perceptible. 
 Mathematical analysis, based on the theory of probability, enables 
 us to go further than this, and compute, from the observed per cent, 
 of right cases with a given difference, the difference which should 
 give any desired per cent, of right cases. 1 
 
 While the three preceding methods deal with the perception of 
 small differences differences at or near the threshold the two 
 which remain to be noticed deal with larger, supraliminal differences. 
 
 (4) The " method of mean gradations" uses three stimuli, two 
 of which are fixed, while the third is to be so adjusted as to seem 
 midway between the other two. The point is to see whether the 
 stimulus which gives this "middle sensation" is the arithmetical 
 mean between the extreme stimuli, or the geometrical mean, or 
 some other proportion. Various similar problems, involving the 
 relations of distant members of the intensity series, can be ap- 
 proached in the same way. 
 
 1 It would seem that the " method of right and wrong cases " can scarcely be 
 freed from certain inherent difficulties; the chief of which concern, first, the proper 
 treatment of those cases in which the judgment is undecided; and second, the 
 relation of the values found by this method to those found by the method of 
 the least noticeable difference. Neither of the solutions hitherto proposed for 
 these difficulties seerns altogether satisfactory. For a discussion of the relations 
 of the two methods of measurement from the point of view of their comparative 
 value, the student of the subject may refer to Fullerton and Cattell, On the 
 Perception of Small Differences, pp. 10, 35 (Philadelphia, 1892). 
 
360 THE QUANTITY OF SENSATIONS 
 
 (5) What may be called the "discrimination time method" is 
 of recent introduction. 1 It consists in determining the time re- 
 quired to judge of a difference between two stimuli. If, starting 
 at the threshold, we gradually increase the difference between the 
 stimuli, requiring the observer to make his judgments as promptly 
 as possible, we find the judgment rather slow near the threshold, 
 and more and more prompt as the difference is increased. The 
 impulse to judge, or the "steering force" exerted by the pair of 
 stimuli, goes on increasing far beyond the threshold; and this is a 
 fact of some theoretic moment, as showing the artificiality of the 
 threshold. Now differences that are judged rightly and in equal 
 times may be called subjectively equal; they are equal in their ef- 
 fect on perception, or equally perceptible this is the basis of the 
 method. 
 
 """ 9. The problem to which these methods have chiefly been ap- 
 ' plied is that of determining equally perceptible, or equal-appearing 
 differences, at all parts of the scale of intensities. The commence- 
 ment of this study, which soon took on the proportions of one of 
 the most important topics of experimental psychology, dates from 
 about 1830, and is due to the physiologist E. H. Weber. 2 Desiring 
 to determine the power of the skin and of the muscle-sense to dis- 
 criminate weights, Weber devised the method of least noticeable 
 differences, and found that, when poised in the hand, a weight of 
 30 ounces could just be distinguished from one of 29 ounces; but 
 that if he started with 30 drams instead of ounces, the least noticea- 
 ble difference was one dram. It seemed that the least noticeable 
 difference was not itself a fixed quantity, but varied with the stimu- 
 lus, being always a certain fixed fraction of the stimulus. Weber 
 found, however, that the fraction which gave the least noticeable 
 difference was not the same for all the senses : it was larger, for ex- 
 ample, in case of pressure on the skin than in case of the lifting 
 of weights; but in the case of judging the length of lines it was much 
 smaller, or only about ifo i. e., a line of 100 units could barely 
 be distinguished from a line of 101 units. Weber therefore con- 
 cluded, on generalizing from his results, that the least noticeable, 
 difference, in each kind of sense-perception, is a constant fraction 
 of the stimulus. 
 
 Some years later, Fechner became acquainted with this general- 
 
 1 Henmon's The Time of Perception as a Measure of Differences in Sensations 
 (New York, 1906) is the most important study made by the use of this method, 
 and contains a history of the method. 
 
 2 An account of Weber's experiments can be found reprinted in his collected 
 works (1851) and also in his article in Wagner's Handworterbuch der Physiologic, 
 1846, III, ii, 481. 
 
WEBER'S LAW AND FECHNER'S FORMULAS 361 
 
 ization, and being greatly impressed with its importance, he named 
 it Weber's law, and made it the corner-stone of his Psychophysics^f 
 Fechner tested this law by extensive series of experiments, whicTThe 
 conducted according to several of the above-mentioned methods. 
 And he further sought to utilize it as a means for attaining to a scien- 
 tific measure of all our sensory experience. To accomplish this, 
 he had to make one or two assumptions. He must, of course, as- 
 sume that every sensation is a measurable quantity, and this im- 
 plies that sensations can be subdivided, added and subtracted; and 
 he further assumed that the least noticeable difference corresponds 
 to a genuine unit of sensation, and that all least noticeable differ- 
 ences, within the same kind of sense-perception, represent equal 
 steps or units of sensation. With these assumptions, Fechner could 
 express Weber's law in either of the two following ways: (1) The 
 addition of equal units of sensation is accomplished by the successive 
 multiplication of the stimulus by a constant fraction; or (2) to make I 
 sensation increase in arithmetical progression, the stimulus must \ 
 increase in geometrical progression. Now when two quantities are 
 related, in this way, the one increasing in arithmetical progression 
 while the other increases in geometrical progression, the former is 
 proportional to the logarithm of the latter; and thus Fechner reached 
 the most compact form of his modification of Weber's law, which 
 reads that sensation is proportional to the logarithm of the stimulus. 1 \ 
 This modification is called Fechner's law; and it is to be noted that, 
 as it involves certain assumptions from which Weber's law is free, 
 the latter is the more empirical expression of the facts, so far as, 
 indeed, the facts are found to agree with it. 
 
 10. That Weber's law is at least an approximation to the truth 
 is borne out by our most common experiences. These evince the^ 
 simple fact that an amount of difference which is easily percepti- ' 
 
 1 For the detailed mathematical discussion and expression of Weber's law the 
 reader is referred to the technical works, especially of Fechner and G. E. Miiller. 
 A simple statement of Fechner 's principle may be given as follows: Let T=the 
 ^intensity of the light of one-half of a white field; jf^the smallest fraction of 
 stimulus added to H that will produce an observable increase in this intensity; 
 and H + f f = the intensity of the other half of the same field. Then let S = 
 the sensation produced by H; + s = the sensation produced by # + T |^; and s 
 will, of course, represent the so-called least observable difference at this point in 
 the scale. We have, then, H produces S; # + T f , or Hi H, produces S+s; 
 Hi ff + ijf J?, or Hi-Hi H, produces + s + s; and so on. That is to say, if 
 s is to be kept of the same magnitude, then H must be multiplied by the same 
 magnitude (Hi). 
 
 The three fundamental formulas which Fechner has employed to state and 
 demonstrate the law are the following: Let S be the magnitude of the sensa- 
 tion caused by the stimulus S, and AS a just observable increase in this sen- 
 
362 THE QUANTITY OF SENSATIONS 
 
 'ble when the stimuli are weak becomes imperceptible when the 
 stimuli are strong. If, for example, when a single electric bulb 
 is burning, one more is turned on, the increase in illumination is 
 very marked; but when a hundred are burning, the addition of one 
 more is barely perceptible; and when a thousand are burning the 
 addition of one more is quite unnoticed. To hear the pin drop, 
 the room must be very quiet. But such e very-day observations are 
 not a sufficient test of the law, which states something definite and 
 exact namely, that the least perceptible difference is a constant 
 fraction of the stimulus; or, when somewhat generalized, that equally 
 perceptible differences require the stimuli to be in a constant ratio. 
 Moreover, while the empirical data upon which the advocates of 
 Weber's law rely are very numerous, their value and trustworthi- 
 ness are often much diminished by the fact that many of these ex- 
 perimenters have failed to isolate sufficiently the exact problem which 
 it was desired to solve. Nevertheless, the data show that the law 
 summarizes many facts reasonably well within a certain range of 
 sensations lying near the middle of the scale of quantity. The same 
 general fact of experience, which this law attempts to summarize, 
 holds, roughly, of our perception of magnitudes of space and time, 
 as well as of our estimate of intensities of sensation. Near both the 
 upper and the lower limits the law fails to prove applicable; even in 
 the regions and under the circumstances which are most favorable 
 it is only approximately true. Many fluctuations of unknown sig- 
 nificance and origin occur in all the senses. 
 
 11. In the following brief summary of the empirical results 
 of a study of the intensity of sensation, we shall group together 
 under each sense the data on the two chief problems: that of the 
 least observable stimulus; and that of the least observable difference 
 between two stimuli. Weber's law is concerned only with the latter; 
 
 sation which is caused by an increase of the stimulus = AS. Let C be a con- 
 
 CA2 
 
 stant dependent on the units chosen for S and S. Then AS=^-. Let it be 
 further assumed that A5 remains constant whatever values for S and AS are as- 
 sumed; then dS = C^, and by integration S = C log. S, which is Fechner's "funda- 
 mental formula." But if the stimulus is just below the least observable amount, 
 and be =S, then substituting in the above formula we have = C log S; 
 from which Fechner derives formula No. 2 (the formula of measurement) 
 
 namely, S = C log. J, which means that the magnitude of the sensation is 
 " negative, " in case the stimulus sinks below the least observable. If two sensations 
 (S and S') are observably different, then SS' =C (log. S log. S') = (7 log /; 
 this is called the "formula of difference," and means that the difference in the 
 intensity of two sensations is proportional to the logarithm of the quotient of the 
 magnitudes of their stimuli. 
 
THE PERCEPTION OF WEIGHT 
 
 363 
 
 and wherever this law is empirically true, it is possible to express the 
 result, in regard to the least observable difference, or the discrimi- 
 native sensibility of the sense in question, in the form of a single 
 fraction. It should be noted, however, that a single sense may 
 furnish more than one intensity series, and that Weber's law must 
 accordingly be tested in each such series separately. 
 
 12. We begin where Weber began, with the perception of weight. 
 Weight is appreciated partly by pressure on the skin and subcu- 
 taneous pressure organs, and partly by the muscular sense, as in 
 lifting and poising the weight. One of Weber's results was to the 
 effect that a smaller difference between weights can be detected 
 when they are lifted than when they are simply allowed to press on 
 the skin, and this result is now generally accepted. Weber also 
 decided that the discriminative sensitivity, for lifted weights, was 
 expressible as a constant fraction, about -fa, of the total weight. 
 His experiments were, however, confined to a narrow range of weight. 
 
 Biedermann and Lowit, by the method of just observable differ- 
 ences, obtained results departing widely from Weber's law. 1 By 
 experimenting with weights varying from 10 to 500 grams they 
 found that the sensitiveness to pressure rose with the increase of 
 the weights from 10 to 400 grams, and then fell off rapidly, as the 
 following table will show: 
 
 Absolute weight 
 Grams 
 
 Least observable difference 
 Grams 
 
 Quotient of sensitiveness 
 
 10 
 
 0.7 
 
 iV 
 
 50 
 
 1.7 
 
 *v 
 
 100 
 
 2.4 
 
 & 
 
 200 
 
 3.6 
 
 A 
 
 300 
 
 4.6 
 
 * 
 
 400 
 
 5.2 
 
 lv 
 
 450 
 
 6.5 
 
 A 
 
 500 
 
 25.5 
 
 & 
 
 The trustworthiness of these results is impaired, however, by the 
 fact that no method, except the doubtful one of directing "atten- 
 tion" exclusively to the sensations of pressure, was employed to 
 exclude the disturbing effect of the muscular sensations. The 
 same observers concluded, also, that the fineness of the muscular 
 sense, when isolated, does not vary according to Weber's law. They 
 fixed it at -fa for weights of 250 grams, YTT f r weights of 2,500 
 grams, -gV for weights of 2,750 grams. 
 
 1 See Hering, Sitzgsber. d. Wiener Acad., LXXII, iii, 342 f. 
 
364 THE QUANTITY OF SENSATIONS 
 
 The absolute sensitiveness of the complex sensations dealt with 
 in the above experiments differs greatly for different localities on the 
 surface of the body. This kind of sensitiveness was at one time 
 thought to be chiefly dependent upon the number of the nervous 
 elements present in the skin, its thickness, the character of its 
 tension over the underlying parts, etc.; but its variations are by 
 no means parallel with those of the sharpness of the sense of lo- 
 cality. The foregoing and similar conclusions all need to be re- 
 vised in the light of Goldscheider's determinations of the pressure- 
 spots. 
 
 Several other authors have tested Weber's law in the perception 
 of lifted weights, and have regularly found the least perceptible 
 fraction of the stimulus to decrease as the total weight increased. 
 The least perceptible difference of weight does, indeed, increase in 
 absolute magnitude as the total weight increases, but it increases 
 more slowly than the weight, contrary to Weber's law. The same 
 is true of other forms of perception depending on the muscle-sense, 
 such as the extent or force of a movement. In all these varieties 
 of muscle-sense perception, the least noticeable difference increases 
 more slowly than the total magnitude; the fraction which should 
 remain constant grows smaller as the magnitude increases. It is 
 rather disappointing to find that the sphere of perception which 
 first suggested the formulation of Weber's law should prove to be 
 the sphere, of all others, to which it is least applicable. 
 
 13. Let us now turn to the determinations of the least observa- 
 ble stimulus in the realm of the muscle-sense. Such determina- 
 tions have been chiefly carried out by Goldscheider, 1 who has meas- 
 ured the least perceptible extent of movement at various joints. 
 This varies greatly for different joints. The most sensitive joints, 
 i. e., those at which the least angular rotation is perceived, are the 
 wrist, shoulder, and the joint between the hand and the finger; at 
 these joints a rotation of .30 to .40 can be perceived. Less sensi- 
 tive are the elbow, hip, and knee, with thresholds of .40 to .80; still 
 less sensitive are the ankle and the two joints within each finger 
 (.75-1.50); and least sensitive of all are the joints within the foot 
 (2-4). The measure of sensitivity varies with the speed of the 
 impressed movement, the joints being more sensitive for rapid than 
 for slow movements. For active movements (that is, movements 
 made voluntarily by the observer's own muscular action), the sen- 
 sitivity is perhaps slightly greater than for passive or impressed 
 movements; but the difference, if any, is very small. 
 
 14. If we next turn to the skin and subcutaneous pressure 
 organs, we easily observe that the absolute sensitiveness of different 
 1 Gesammelte Abhandlungen, II. 
 
SENSITIVENESS TO LIGHT PRESSURE 365 
 
 parts differs greatly. Aubert and Kammler found the lightest 
 weight which produced a sensation of touch to be 0.002 gram on 
 the forehead, temples, and dorsal side of the forearm and hands; 
 0.003 gram for the volar side of the forearm; 0.005 gram for the 
 nose, lips, chin, eyelids, and skin of abdomen; 0.005-0.015 gram 
 for the volar side of the fingers; and 1 gram for the finger-nails and 
 skin of the heel. Later observations have to some extent varied 
 these results; and it has come to be recognized that the most sig- 
 nificant experiments are those which determine the least percepti- 
 ble pressure on single touch-spots. Von Frey 1 concludes that in- 
 dividual touch-spots differ comparatively little from each other in 
 sensitiveness, the threshold ranging from .5 to 4 grams pressure 
 per square millimetre of the surface pressed on. Kiesow 2 has de- 
 termined the sensitivity of touch-spots in various regions of the trunk 
 and limbs, and finds it to vary little within the limbs, but to be com- 
 paratively blunt on the chest and back. The presence of hairs 
 or hairlets greatly increases this kind of sensitivity. 
 
 The sensitivity to differences of pressure also varies from one 
 area of skin to another, being greatest on the face, where the least 
 noticeable difference may be as low as ^ of the total pressure, 
 whereas on the leg the fraction rises above yV 3 Probably the 
 most thorough test of Weber's law in this field has been made by 
 Stratton, 4 who found that the fraction expressing the least observa- 
 ble increase in an existing pressure was large for such small press- 
 ures as 10 grams, and decreased considerably up to 75 grams, but 
 from here on remained fairly constant up to 200 grams ; accordingly, 
 Weber's law held good for a medium range of stimuli but not for 
 weak stimuli. The skin, as well as the joint sense, is more sensi- 
 tive to rapid than to slow changes. 
 
 15. Extraordinary difficulties accompany the attempt to apply 
 Weber's law to sensations of temperature. As has already been seen 
 (compare p. 346), we do not know exactly what to measure whether 
 the rising and falling of the thermic apparatus, or its actual tempera- 
 ture in relation to its own zero-point as constituting the quanti- 
 tative changes in the stimuli. Even Fechner admits that Weber's 
 law does not apply to the sensitiveness of the hand to changes in 
 temperature when it is itself cooling off; but he thinks the law 
 holds good approximately for degrees of warmth varying between 
 25 and 37.5 C. (77-99.5 Fahr.), if 18.71 C. (65.66 Fahr.) be 
 
 1 Pfliiger's Archiv f. d. gesammte PhysioL, 1900, LXXXII, 399. 
 
 2 Wundt's Philosophised Studien, 1902, XIX, 307. 
 
 3 Eulenberg, Zeitschrift /. rationelle Medecin, 1861, X, cited after Sherrington 
 in Schafer's Textbook of Physiology, 1900, II, 929. 
 
 4 Wimdt's Philosophische Studien, 1896, XII, 525, 
 
366 THE QUANTITY OF SENSATIONS 
 
 taken as -the zero-point. The assumption of this zero-point is, 
 however, arbitrary. No general rule for the quantity of sensations 
 of temperature can well be given except this: the skin is most sensi- 
 tive to changes which lie near its own zero-point. In comparing 
 two temperatures it is most favorable to nice discrimination that 
 one should lie slightly above, the other slightly below, this point. 
 The degrees of the thermometer between which the maximum of 
 sensitiveness is attainable are given differently by different ob- 
 servers: By Nothnagel, 27-33 C. (80.6-91.4 Fahr.); by Linde- 
 mann, 26-39 C., by Alsberg, 35-39 C.; by Fechner, 12-25 
 C. where it is so great as not to be easily measurable by a good 
 quicksilver thermometer (about -fa Fahr.). Cold and heat alike, 
 when applied for some time, reduce greatly the sensitiveness of 
 the skin to minute changes of temperature; by heat it can be so 
 dulled as not to distinguish alterations of less than 4 or J Fahr.; 
 by cold it can be rendered insensible to changes measuring from 
 2 to 5J. 
 
 Another complicating circumstance is that the sense of tempera- 
 ture depends for its fineness upon the extent and locality of the sur- 
 face excited. Weber found that water at 29^ R., in which the 
 whole hand was immersed, seemed warmer than that at 32 R., to a 
 single finger. Nothnagel placed the following values upon the fine- 
 ness of discrimination, for minute variations in temperature, of dif- 
 ferent parts of the body: Middle breast, 0.6 C.; sides of the same, 
 0.4; middle of the back, 1.2; sides of the same, 0.9; hollow of the 
 hand, 0.5-0.4; back of the same, 0.3; parts of upper and lower 
 arm, 0.2; cheeks, 0.4-0.2; temples, 0.4-0.3. 
 
 More recent investigations have shown that the table of sensi- 
 tiveness for the different parts of the body must take account of 
 the division of the temperature-sense into two species, and of the 
 locality of the warmth-spots and cold-spots in all such different parts. 
 On the basis of experiment with areas of the skin whose topography, 
 with respect to the temperature-sense had previously been investi- 
 gated, Goldscheider has given a lengthy statement 1 of the sensi- 
 tiveness of different parts of the body. Thus he finds that the skin 
 of the head is, in general, little developed for the sense of cold, and 
 only in a few places for the sense of heat. The sensitiveness of the 
 forehead to cold is intense, but to heat only moderate; that of the 
 breast to cold moderate along the sternum, and elsewhere very in- 
 tense, while to heat it is only moderate except near the nipples; 
 that of the back everywhere very intense to cold, and only moderate 
 to heat; while in all parts of the hand the intensity of sensitiveness 
 
 1 See the Archiv /. Anat. u. PhysioL, Pysiolog. Abth., 1885, Supplement-Band, 
 pp. 60 ff. 
 
SENSITIVENESS OF ACOUSTIC PERCEPTION 367 
 
 to both cold and heat is alike. In general, the skin in the median 
 line of the body seems much less sensitive to changes in temperature 
 than at its sides; and the number of thermic elements (according 
 to Goldscheider, the distributory fibrils of the temperature-nerves), 
 the thickness of the skin, etc., are determining factors. 
 
 16. The possibility of executing or appreciating a musical 
 passage in which the intensity of the successive tones is brought 
 to a certain standard of memory, or in which these tones are nicely 
 shaded so as to constitute a crescendo or a diminuendo, appears to 
 depend upon applying to sensations of sound some law resembling 
 that of Weber. It is partly by comparing such sensations with 
 their images in memory that the singer or player reproduces certain 
 notes previously executed, with about the same stress of tone. 
 Moreover, in order to shade the relative intensities of successive 
 tones, our appreciation of their differences needs to be much greater 
 for those that have a low degree of intensity. Many obstacles, 
 however, stand in the way of determining either the lower limit 
 or the least observable difference for sensations of sound. The 
 greatest difficulty is of a physical nature namely, to obtain a source 
 of sound which shall be free from disturbing variations in pitch 
 and clang, both of which greatly affect the apparent intensity of 
 sound; and especially to secure a physical measure of the intensity 
 of the stimulus. If sound is generated, as has frequently been the 
 case in tests of Weber's law, by dropping a metal or hard rubber ball 
 upon a block of wood or slate, it is easy to calculate the energy of 
 impact from the weight of the ball and the height of its fall. But 
 the real stimulus is not this impact, but the vibrations of air which 
 strike the tympanic membrane; for a full determination of the stimu- 
 lus, we should therefore need to know how much of the energy of 
 impact went into the production of vibrations of the air, and how the 
 amplitude of these vibrations decreased in its passage to the ear. 
 In regard to the latter point, the law of inverse squares would prob- 
 ably hold in a space that was perfectly open, and so free from all 
 reflection; but in a closed room reflection and resonance make it 
 impossible to deduce the intensity at the ear from the distance be- 
 tween the ear and the sounding body. Comparative tests may, how- 
 ever, be made by keeping both the apparatus and the observer at 
 the same places in the same room throughout. In regard to the 
 question as to how the intensity of the sound varies with the height 
 of the fall, there has been much difference of opinion, but perhaps it 
 is safe to assume that, for a given piece of apparatus, the intensity 
 varies as the height of the fall (and therefore as the energy of im- 
 pact). 1 If this is granted, the least perceptible difference in inten- 
 1 See Kampfe, Philosophische Studien, 1893, VIII, 526. 
 
368 THE QUANTITY OF SENSATIONS 
 
 sity of sound (noise) as produced by this apparatus, is about ; 
 i. e., the two noises can just be distinguished when the heights of 
 fall are as 3 to 4. This ratio remains fairly constant within a mod- / 
 erate range of intensity, in confirmation of Weber's law. 
 
 17. Such an experiment as the above, though it does not meas- 
 ure the real stimulus at the tympanic membrane, is satisfactory for 
 the testing of Weber's law; but not for determining the least stimu- 
 lus which can be perceived. Through the work of several authors, 
 among whom may be mentioned Lord Rayleigh and Max Wien, 
 an approximation to the desired measure can now be given. The 
 sound given out by a telephone receiver can be calculated if the ex- 
 cursions of the telephone plate are known ; and these excursions can 
 be measured by direct microscopic examination. If now a tele- 
 phone receiver be held to the ear, and actuated by an electric current 
 which causes the plate to vibrate with a known amplitude, as well 
 as a known frequency, the amplitude of the vibrations entering the 
 ear is known, and it can be assumed that this does not change much 
 in the passage to the tympanic membrane. By such means, Wien 1 
 has found that a tone is barely audible when the energy of the at- 
 mospheric vibrations is from .00032 to .000,000,000,002,5 ergs (per 
 second, over an area of one square centimetre). Now the erg is 
 itself a small unit; and since the air vibrations need not act for a whole 
 second, a small fraction of a second being long enough, the above 
 figures would need to be considerably reduced in order to express 
 the minimum stimulus which will produce the sensation of sound. 
 Other authorities, it should be said, have not assigned values as 
 small as the smallest of Wien, but they all lie within the range of 
 those cited from this author. 
 
 Individuals differ greatly in absolute sensitivity; Bruner 2 found 
 the energy of the least noticeable stimulus to vary in the ratio of 
 1 to 400, among normally hearing individuals of the white race. 
 The sensitivity to sound varies also, and greatly, with the pitch. 
 The largest of the numbers, quoted above from Wien, is the energy 
 required for a low tone (50 vibrations per second); the smallest is 
 for tones of 1,600 and 3,200 vibrations. The ear is, accordingly, 
 more than 100,000,000 times as sensitive to the high tones as to 
 the low ones; but the threshold is nearly constant in the interval 
 from 800 to 6,400 vibrations. It increases rapidly above and below 
 this range. It will be seen that the ear is most sensitive to decidedly 
 high tones (from "high G," or g 2 , up for about three octaves). This 
 may appear strange in view of the fact that these high notes lie 
 beyond the ordinary use of the human voice; but the importance of 
 
 1 Pfliiger's Archiv f. d. gesammte Physiol, 1903, XCVII, 1. 
 a The Hearing of Primitive Peoples, p. 95 (New York, 1908). 
 
QUANTITATIVE DISCRIMINATIONS OF SIGHT 369 
 
 overtones in the appreciation of the clang, or complex character, of 
 a sound, should be borne in mind in this connection. The ability 
 to distinguish different noises, voices, vocal sounds, etc., is as im- 
 portant as anything in hearing, and requires that the ear shall be 
 sensitive to high and faint overtones. 
 
 18. Wien also employed the telephone method for determining 
 the least noticeable difference of intensity of tones. He found the 
 fraction expressing the least noticeable difference to vary with the 
 pitch of the tone (-J- to -J- from a to a 1 ); but for each tone it pre- 
 serves a fair equality over a considerable middle range of intensity. 
 There is, therefore, good ground for concluding that Weber's law 
 is fairly applicable to intensity of sound. 
 
 19. Attention was early called to the law of judgment in esti- 
 mating the quantitative relations of sensations of sight, on account 
 of its connection with astronomical observation. In the eighteenth 
 century French physicists had already begun to investigate the 
 sensitiveness of the eye to varying intensities of light. Bouguer, in 
 answer to the question, What force must a light have in order to 
 make a more feeble one disappear? placed the fraction of least 
 observable difference in the intensities of two shadows at ^j-. That 
 the magnitudes of the stars are not to be classified according to 
 their absolute brightness as determined by photometric observa- 
 tions, was, of course, assumed by Sir John Herschel when he made 
 the latter vary in the series 1 : J : i : tV, while the former vary in 
 the series 1:2:3:4. That the least observable difference in the 
 intensity of two sensations of sight is absolutely much smaller for 
 those of the lowest grade of intensity is a truth needed to explain 
 many every-day experiences. For example, the finer gradations of 
 shade in a lithograph or photograph are not lost when we take it 
 from the open sunlight into a rather dimly lighted room; we can 
 also observe them through smoked glass, if it be not too black. 
 Through the same media we can measure rather delicate shades 
 of brightness on the clouds. We observe, however, that in all 
 such cases either too great or too weak intensity of the light 
 destroys our power to distinguish the finest gradations of its 
 intensity. 
 
 20. It has already been shown (p. 325) that the retina is never 
 free from light of its own which has a varying intensity; this fact 
 greatly increases the difficulty of fixing accurately either the lower 
 limit or the least observable difference of visual sensations. In the 
 effort to apply Weber's law to sensations of color, the laws of change 
 in the quality operate to obscure the laws of change in the quantity 
 of the sensations. Experiments with shadows for the sake of test- 
 ing Weber's law were first conducted by A. W. Volkmann and others, 
 
370 THE QUANTITY OF SENSATIONS 
 
 under the direction of Fechner. 1 By measuring the distance to 
 which a candle must be removed from an object in order that the 
 shadow produced by its light might disappear in that of another 
 candle of like intensity situated at a fixed near distance from the ob- 
 ject, the quotient for the least observable difference was found to be 
 rJ-fr. This quotient was also found to remain nearly constant for 
 absolute intensities varying from 1 to 38.79. If, however, the light 
 of the background diminished to 0.36 in intensity, marked varia- 
 tions in the law occurred; the difference in the brightness of the two 
 shadows had then to be greater than Y^ to be observable. Later 
 experiments of the same observer yielded results less favorable to 
 Weber's law. 2 The quotient was found to vary from -g-J.^ for weak 
 intensities of light to T ^ for stronger intensities. 
 
 By using rotating disks and comparing the grayish circles made 
 upon them when revolving rapidly, through the admixture of small 
 black stripes with the white of their surfaces ("Masson's Disks"), 
 Helmholtz 3 found the medium value of the quotient of least observ- 
 able difference to be T J-j; this quotient is not constant, however, and 
 increases, especially for sensations near the upper or the lower limit. 
 By changing the method somewhat, Aubert obtained a variation 
 f TST to T-Jhr in the degree of sensitiveness to differences in the 
 brightness of lights, even when not going above the middle of the 
 scale of intensity. Experiments with such intensities as lie nearest 
 the limits showed much greater departures from Weber's law. Just 
 above the lower limit, an addition of even J to J to the stimulus 
 might be necessary in order to produce an observable difference 
 in the resulting sensation. Similar results have been obtained by 
 Delboeuf, but, on the whole, more favorable to Weber's law than 
 the results of Aubert. 
 
 21. Of the many experiments which have been undertaken 
 with the object of testing Weber's law in the comparison of intensi- 
 ties of light, those of Konig and Brudhun 4 apparently cover the 
 greatest range of intensities and so afford the most complete test. 
 These experimenters used intensities varying in the proportion of 
 1 to 50,000,000, and found the least perceptible difference to be a 
 nearly constant fraction (.017 to .018 of the total intensity) over a 
 considerable range of medium intensities. But the fraction became 
 considerably larger for very feeble, or for very intense, light (.036 
 for the most intense employed, and .695 for the weakest). We 
 may accordingly regard Weber's law as verified for the middle range 
 
 1 See Elemente d. Psychophysik, pp. 148 f. 
 
 2 A. W. Volkmann, Physiolog. Untersuchungen, I, pp. 56 f . 
 
 3 Physiologische Optik, pp. 315 f. 
 
 4 Sitzungsberichte der Berliner Akad. d. Wissensch., 1888 and 1889. 
 
MINIMUM SENSATIONS OF LIGHT 371 
 
 of intensity of light. The fraction which expresses the least per- 
 ceptible difference varies greatly, however, with the special condi- 
 tions of observation ; it is smallest when the two lights to be compared 
 appear side by side with no intervening space, so that the effect 
 of contrast helps discrimination. When the lights are separated, 
 comparison is much less precise, and if one light is seen a second 
 after the other has been removed 1 the fraction may rise as high as J. 
 
 Older experiments seemed to show that the sensitivity of the 
 eye to differences in intensity varies according to the color of the 
 light; but the authors just quoted found that this was not the case, 
 within the same range of brightness. The comparison of the bright- 
 ness of one color with that of a different color is, however, difficult 
 and uncertain. 
 
 22. The minimum of the intensity of light appreciable by the 
 eye is dependent on many circumstances, of which the most im- 
 portant is the condition of adaptation to light or dark (see p. 195). 
 The color or wave-length also makes a great difference, as also the 
 part of the retina stimulated, and the extent and duration of the 
 stimulus. The eye is much more sensitive to rays from the middle 
 of the spectrum, especially yellow and green, than to rays from near 
 the red or the violet end. The central area of clear vision of the 
 retina is less sensitive to very dim light than is the region lying just 
 about it; this difference appears during dark adaptation. Lights 
 of very small area or duration will make no impression, although 
 the same intensity, with greater extent or duration, would suffice 
 to give a sensation. S. P. Langley 2 sought to determine the energy 
 of the least quantity of light which could, under the most favorable 
 conditions, produce a perceptible impression, and reached a figure 
 of .00000003 ergs. Though this is a very small quantity of energy, 
 it is considerably greater than the minimum determined by Wien as 
 required to excite an auditory sensation. 
 
 23. Weber applied his own law to so-called extensive sensa- 
 tions of light. He showed that in judging of the comparative 
 length of lines the least observable difference is, for each person, a 
 tolerably constant fraction of the absolute length of the line with 
 which the comparison is made. This fraction is different for differ- 
 ent persons; and has a range from ^ to T^TF* Fechner 3 defends 
 the validity of the law for lines of lengths varying between 10 and 
 240 mm. (f to 9-fc in.), with the eye removed from 300 to 800 mm. 
 (12-32 in.). The lower limit for such cases has been fixed by A. W. 
 Volkmann at lines of length from 0.2 to 3.6 mm. It is obvious, 
 
 1 See Fullerton and Cattell, The Perception of Small Differences, 1892, p. 140. 
 
 2 Philosophical Magazine, 1889, XXVII, 1. 
 1 Elements d. Psychophysik, I, 211 f. 
 
372 THE QUANTITY OF SENSATIONS 
 
 however, that we are here not dealing with pure quantity of visual 
 sensations, but with judgments of local relation which, in case the 
 eyes are moved, have their basis, at least partly, in our power to dis- 
 criminate minute differences in the sensations of the muscular sense 
 connected with such movements. 
 
 24. The law of Weber can, of course, derive little or no sup- 
 port from sensations of taste and smell. In the case of these two 
 senses our knowledge of both series of quantities of the intensity 
 of the stimulus and of the amount of specific sensation which re- 
 sults from its application is altogether too inadequate to admit 
 of trustworthy comparison. We cannot measure forms of energy 
 like those by which smellable particles and tas table solutions act 
 on the end-organs of sense, until we have a unit of measurement and 
 some information as to what the object is to which the standard 
 should be applied. Nor can we compare amounts of sensations 
 that are so largely matters of individual origin and capricious 
 change, and that are so overlaid with other forms of feeling, as are 
 the sensations of these senses. Moreover, the element of time both 
 as respects the interval elapsing between the two sensations com- 
 pared and also the order in which the sensations follow each other 
 is here a very important influence. 
 
 The intensity of taste depends upon a variety of circumstances 
 besides the objective quantity of the stimulus. Among these cir- 
 cumstances is the extent of surface excited. Camerer 1 found by 
 experimenting with common salt in solutions of different degrees 
 of concentration that the number of correct guesses increased almost 
 in exact proportion to the number of gustatory papilla? upon which 
 the solutions were placed. Certain mechanical and thermic con- 
 ditions also have a great influence. Substances even in fluid form, 
 when quickly swallowed, have little taste; pressing and rubbing 
 against the gustatory organs, movement of the tastable matter in 
 the mouth, increase the excitatory effect of the stimuli. It is 
 doubtful whether this effect is due solely to the mechanical result 
 of spreading the stimulus over the surface and urging it into the 
 pores against the end-organs of the sense, or in part also to some 
 direct physiological cause. The influence of temperature on the 
 intensity of sensations of taste is well known. Weber showed that 
 if the tongue is held for J to 1 minute in very cold water, or in wa- 
 ter of about 125 Fahr., the sweet taste of sugar can no longer be 
 perceived. Cold also destroys for a time the susceptibility to bit- 
 ter tastes. Keppler 2 endeavored to test Weber's law by determining 
 the sensitiveness to minute changes in the four principal kinds of 
 
 1 See Zeitschr. /. Biologie, 1870, VI, pp. 440 f. 
 
 2 Pfliiger's Archiv, 1869, II, 449 f. 
 
INTENSITY OF GUSTATORY SENSATIONS 373 
 
 taste; and arrived at a negative result. Fechner, however, con- 
 siders that Keppler's experiments with common salt confirm We- 
 ber's law, and that his other experiments were not adapted to yield 
 any assured result. We can only repeat the statement that other 
 causes than mere increase in the quantity of the stimulus so largely 
 determine the intensity of the resulting sensations as to discredit 
 any arguments from the experiments either for or against applying 
 Weber's law to sensations of taste. 
 
 25. The experiments of Valentin and others, to determine how 
 weak solutions of various substances will excite the end-organs of 
 taste, are chiefly valuable as gratifying our curiosity. The figures 
 are not to be accepted as exact, but as showing in general the ex- 
 treme fineness of this sense, and the great difference of different 
 substances in their power to excite it. Valentin found, for exam- 
 ple, that 0.24 gram of a solution containing 1.2 per cent, of cane- 
 sugar excited the sensation of sweet; a solution containing -%fa 
 part of common salt was scarcely detectable; of sulphuric acid 
 TToWfr could be discerned, loooTnnr not; extract of aloes contain- 
 ing woVoir could be distinguished from distilled water; ssooir 
 of sulphate of quinine was plainly observable, and the observer 
 thought he could detect a slight trace of bitter when the solution 
 was diluted to 1000000 of this substance. In general, a smaller 
 absolute quantity of stimulus, when in a relatively concentrated 
 solution, will suffice to excite the end-organs of taste. It will 
 readily be seen that the minimum of some of these substances 
 which will give rise to a sensation under the most favorable cir- 
 cumstances is exceedingly small. 
 
 26. The intensity of sensations of smell is also largely depend- 
 ent on other causes than changes in the quantity of the stimuli. 
 The amount of sensation appears to be largely governed by the 
 extent of surface excited; since it is greater when we smell with 
 both nostrils, and with the current of inspiration which carries the 
 exciting particles over more of the sensitive membrane. No as- 
 sured results on this point, however, have yet been reached. Val- 
 entin supposes that a smaller number of odorous particles will 
 excite sensation if presented in a concentrated rather than a dilute 
 form. When the intensity of the stimulus increases beyond a cer- 
 tain point, the character of the resulting sensation changes often- 
 times from a pleasant to an unpleasant tone of feeling. All are 
 familiar with the fact that a large increase of some smells for 
 example, musk does not give the same kind of sensation. 
 
 This sense has a great degree of "sharpness," or power to be ex- 
 cited by small quantities of stimulus, as distinguished from "fine- 
 ness," or power to distinguish minute variations in the sensations. 
 
374 THE QUANTITY OF SENSATIONS 
 
 It is undoubtedly different in different species of animals, as de- 
 pendent upon unknown differences in their psycho-physical con- 
 stitution; but it is tolerably uniform among men where there is the 
 same cultivation of it, and the same concentration of attention. It 
 is well known that certain animals have an astonishing fineness of 
 smell, and are able by it even to detect the individual variations 
 that are quite imperceptible to man. Little value can be attached 
 to the results reached by experiments to fix the least quantity of 
 smellable substances which can excite the human end-organs of 
 this sense. In general, we can say that incredibly small quanti- 
 ties of some substances will suffice. Valentin found that a current 
 of air containing -3-0 0*000 of vapor of bromine excited a strong un- 
 pleasant sensation. Atmosphere polluted with even 1700000 of 
 sulphuretted hydrogen could be detected. It was calculated by 
 this observer that -a o o o o o o f a milligram of alcoholic extract of 
 musk is about as little as can be perceived; of mercaptan, a still 
 smaller quantity is perceptible, since the odor can be detected when 
 a litre of air contains only T8iroVo"oTr of a milligram of this substance ; 
 and it is probable that only a small part of a litre would be inhaled 
 in snuffing the air. 
 
 Zwaardemaker 1 has devised an olfactometer, which, though not 
 measuring the stimulus in absolute units, permits of relative de- 
 terminations, and has applied his instrument to the testing of 
 Weber's law. The results were necessarily inexact, but the law was 
 believed to be approximately verified. 
 
 27. A review of the facts bearing on the validity of Weber's law 
 shows, first, that the law is regularly departed from at both the lower 
 and the upper extremes of the scale of intensities; next, that it holds 
 very well for an extensive middle range of intensities of light; further, 
 that it seems to hold approximately for the middle range of inten- 
 sities of sound, smell, and pressure on the skin, though in none of 
 these cases has anything like the full range of intensities been 
 brought under examination. In the perception of lifted weights, 
 and generally in perceptions dependent on the muscular sense, 
 the departures from the law, even within the middle range, are great 
 and all in the same direction, so that it is impossible to regard the 
 law as holding for these kinds of perception. 
 
 28. The value of Weber's law is so restricted, even as stating 
 a general fact of experience, that it would seem scarcely necessary 
 to discuss at length its higher significance. Three possible modes 
 of explanation have all had their defenders; these are the physio- 
 logical, the psycho-physical, and the psychological. The first of 
 the three assumes that the physical construction of the nervous 
 1 Physiologic des Geruchs, 1895. 
 
EXPLANATIONS OF WEBER'S LAW 375 
 
 system, including chiefly the end-organs of sense and their central 
 representatives and connections, is such as to supply the reason 
 for this relation between the intensity of sensations and that of 
 their stimuli. And certainly, if we were to adopt off-hand the sim- 
 plest assumption, it would be that the quantitative relation between 
 the last antecedent molecular changes in the brain and the mental 
 changes to which they give rise, is one of simple proportion; the more 
 work done by means of the excitation in the appropriate cerebral 
 centres, the more of physical basis laid, as it were, for a resulting 
 quantity of psychical movement. 
 
 If, then, the sensations vary in quantity in an arithmetical pro- 
 portion, while their external stimuli vary in a geometrical propor- 
 tion, the explanation of the fact must be found somewhere in the 
 chain of events between the external stimuli and the nerve-commo- 
 tions set up as a result in the appropriate centres of the brain. And 
 without doubt, in all of the senses, the end-organs profoundly mod- 
 ify the intensity of the stimulus they receive. In the so-called chem- 
 ical senses (smell, taste, sight) a profound quantitative modifica- 
 tion takes place, even before the stimulus reaches the fibrils of the 
 sensory nerve. In the case of the mechanical sense of hearing we 
 cannot say how much of the effect stated in Weber's law may not 
 have been gained even before the acoustic waves set agoing the 
 nervous elements of the organ of Corti. As to profounder modifi- 
 cations in the same direction by reason of the interaction of different 
 nerve-elements in the brain we are yet more ignorant. 
 
 The psycho-physical explanation of Weber's law is that adopted 
 by Fechner. This explanation insists upon making the law one of 
 the utmost generality and of the highest import as stating the re- 
 lations between organic and spiritual activities. Although Fech- 
 ner's view confessedly grew out of his speculation that body and 
 mind are only two phenomenal aspects, as it were, of one and the 
 same underlying reality, it has been defended by him with a great 
 amount of mathematical science and experimental research. No 
 other form of explanation, however, takes us so much into the 
 regions of utter obscurity. Why the quantitative relations of body 
 and mind should be such, and such only, that a geometrical series 
 of changes in the one should invariably be represented by an arith- 
 metical series of changes in the other, must indeed remain an ulti- 
 mate mystery. And the experimental proof of Weber's law is as 
 yet much too incomplete to make us ready to accept it as an ulti- 
 mate psycho-physical principle. 
 
 The psychological explanation of Weber's law resolves it into a 
 special case under the greater law of the relativity of our inner \ 
 states. It is not so much, then, a law of the absolute quantity of 
 
376 THE QUANTITY OF SENSATIONS 
 
 sensations as dependent on stimuli, but rather a law of our ap- 
 prehension in consciousness of the relation of our own feelings. In 
 general, it may be said that every mental state has its value de- 
 termined, both as respects its quality and its so-called quantity, by 
 its relation to other states. It is the amount of change rather than 
 the absolute amount of feeling which the mental apperception esti- 
 mates. That some psychological explanation is needed to account 
 for the facts there can be no doubt when we consider how impor- 
 tant are the elements of attention, mental habit, power of acute 
 discrimination, etc., in determining our estimates of the quanti- 
 tative relations of our sensations. Estimates that is, acts of the 
 / comparing judgment are involved in the experience upon which 
 reliance is placed for a demonstration of Weber's law. 
 
 29. It does not seem possible, however, to make a strict de- 
 duction of Weber's law from the principle of relativity alone ; for the 
 proportionality required by Weber's law is not the only form of 
 relation that can hold between quantities. Wundt has laid some 
 stress on the "method of mean gradations" as being specially 
 suited to test the view that, in comparing different intensities, we 
 judge by ratios rather than by additions and subtractions. If we 
 judge by ratios, the subjective mean between two intensities should 
 be given by the geometrical mean between the stimuli. The re- 
 sults, on application of the method, have been uneven : in some cases 
 the subjective mean has corresponded rather closely to the geo- 
 metrical mean of the stimuli, and in others to the arithmetical mean. 
 This has led Wundt to offer the suggestion that two kinds of com- 
 parison are possible by ratios and by absolute differences ac- 
 cording to the psychological attitude induced in the observer by 
 the conditions of the experiment. This importance of the psycho- 
 logical attitude he regards as good evidence in favor of the psycho- 
 logical interpretation of Weber's law. 1 The prevailing opinion at 
 the present day probably favors some form of physiological inter- 
 pretation for Weber's law. In this connection a suggestion of Fuller- 
 ton and Cattell 2 is of interest. They would interpret the least notice- 
 able difference, like the average error, as belonging to the class of 
 "errors of observation," and as due, fundamentally, to the varia- 
 bility which the process of discrimination shares with all other or- 
 ganic processes. In general, the variability of a large phenomenon is 
 greater than that of a small; and the perception of a strong stimu- 
 lus may probably be regarded as involving a larger organic response 
 than the perception of a weak stimulus, and thus as involving more 
 sources of variability. Accordingly, the error of observation, and 
 
 1 Physiologische Psychologic, 1902, I, 544; 6th ed., 1908, I, 635. 
 
 2 Op. cit. 
 
FECHNER'S INTERPRETATION 377 
 
 the least noticeable difference, would increase with the stimulus. 
 It need not, however, increase as fast as the stimulus; and indeed 
 these authors suggest that it should typically increase according to 
 the general law of composition of independent variations namely, 
 according as the square root of the stimulus. This formula fits 
 the facts better than Weber's law, in the case of perception of weight 
 and bodily movement, and the deviations from Weber's law in 
 the other departments of sense-perception are usually toward the 
 law of the square root. 
 
 This ingenious hypothesis may be helped out by ascribing the 
 departures from Weber's law to the peculiarities of structure in the 
 end-organs and to the complexity of the physiological process in- 
 volved in different sense-experiences. It is to be noted, however, 
 that it does not do away with the required psychological explana- 
 tion, but, the rather, involves it as its necessary correlate. For 
 conscious discrimination is involved in all the experiments, of every 
 kind and by whatever method tested, that are cited in the interests 
 of Weber 's law. And conscious discrimination, as viewed from the 
 introspective point of view, is an exceedingly complex and variable 
 process. 
 
 30. As an experimental problem, our confidence in the scientific 
 accuracy of Weber's, or indeed of any other law looked upon as 
 an invariable rule of our sense-experience, quantitatively considered, 
 would be much increased, if there could be any agreement as to ex- 
 act standards of measurement, and as to the values of the different 
 ways of treating the experimental data. But, as yet there is no 
 such agreement. 
 
 Fechner interpreted Weber's law as affording a measure of 
 sensations, and so modified the more empirical expression of 
 Weber as to express the quantity of sensation in terms of the loga- 
 rithm of the stimulus. This conception implies the possibility of 
 adding one sensation to another, and of treating the least notice- 
 able difference as a certain quantum of sensation. Against this 
 it has been argued, with much force, that the least noticeable differ- 
 ence is not a sensation, but simply a fact about perception; that 
 every sensation, be it of great or small intensity, is, for conscious- 
 ness, a unit and not made up of smaller sensations; and that, in 
 short, a sensation is not a measurable quantity, but that what we 
 call a difference in intensity is merely a difference in one of the many 
 directions along which sensations differ a certain kind of quali- 
 tative difference. Probably this general point of view has been 
 rather generally accepted, in opposition to Fechner. But it has 
 been found possible to retain Fechner's logarithmic formula, while 
 
378 THE QUANTITY OF SENSATIONS 
 
 revising the interpretation of it. 1 Granted that it is no longer con- 
 sidered as giving a measure of sensation; it may be retained as indi- 
 cating the position of a sensation in the scale of intensities. Any 
 convenient intensity can be taken as the zero or centre of reference, 
 and any convenient unit can be chosen, and then the location of 
 any intensity can be indicated in multiples of the unit above or 
 below the chosen zero. This view does not require us, as Fech- 
 ner's did, to conceive of a sensation as made up of smaller sensa- 
 tions, but only to conceive the scale of intensities of any kind of 
 sensation as marked off in equal units. Fechner's formula then 
 shows the relation of points in the intensity series to the corres- 
 ponding points in the series of stimuli which produce the sen- 
 sations. 
 
 It is doubtful, however, if even this improved interpretation of 
 Fechner's law can be regarded as a justifiable way of stating the 
 facts discovered in measuring least noticeable differences. One 
 difficulty may be expressed as follows: If a least noticeable differ- 
 ence is taken as a unit, the difference in intensity between any two 
 sensations must be equal to a certain number of these units. The 
 units must therefore be additive; they must fit together, one after 
 another, without loss. But this, in general, they will not do, be- 
 cause of the adaptation of the sense-organ to different intensities 
 of the stimulus. In the case of sensations of sight, for example, 
 on account of the adaptation of the eye, it may even happen that 
 the same intensity of sensation is aroused by stimuli of widely dif- 
 ferent strength, in spite of the fact that, in passing gradually from 
 one to the other, we have perceived many successive increases of 
 intensity. Our units have telescoped; we have been like a man 
 on a ship, carefully pacing off the distance between two points 
 on the neighboring shore, unmindful of the fact that the ship is 
 moving in the opposite direction. The same difficulty would 
 be met in other senses besides sight, since all have some power of 
 adaptation. 
 
 31. It seems better, then, to drop Fechner's logarithmic law, 
 and abide by the more empirical expression of Weber. The facts 
 which are discovered in experiments on the least noticeable differ- 
 ence are not, directly at least, facts regarding the intensity of sensa- 
 tion, but facts regarding the discrimination of intensities of the stim- 
 
 1 Those who have contributed to this revision are principally Delboeuf, Ele- 
 ments de psychophysique, 1883; Stumpf, Tonpsychologie, 1883; and Ebbinghaus, 
 Sitzungsberichte d. Berl. Akad. d. Wissensch., 1887. Convenient references are 
 Ebbinghaus, Grundzuge der Psychologie, 1905, p. 529, and Titchener, Experimental 
 Psychology, Quantitative, Instructor's Manual, 1905, p. cxvi. 
 
SUMMARY OF RESULTS 379 
 
 ulus j 1 and Weber's law, where it holds good, expresses an important 
 truth regarding the perception of intensities, the interpretation of 
 which, as already explained, probably must involve both physi- 
 ological and psychological factors. Indeed, we shall have to return 
 to the discussion of the whole subject of our sense-experience, from 
 higher points of view. 
 
 1 That all the studies which have been considered in this chapter are, properly 
 speaking, studies in the process and limitations of sense-perception, and not sim- 
 ply in the intensity of sensation, is a growing conviction among psychologists, 
 and has been presented with special force by Miiller, in Die Gesichtspunkte und 
 Tatsachen der psychophysichen Methodik, p. 234 (Wiesbaden, 1904), and by 
 Aliotta, in La Misura in Psicologia Sperimentale, p. 103 (Firenze, 1905). 
 
CHAPTER IV 
 PRESENTATIONS OF SENSE, OR SENSE-PERCEPTIONS 
 
 1. It has already been made clear that those hypothetical ele- 
 ments which we have called "simple sensations," and whose vari- 
 ations in quality and quantity have been found to be dependent 
 on the locality, structural peculiarities, and characteristic physical 
 and chemical processes, of the nervous system, have themselves no 
 real existence in our sensuous experience. We are never con- 
 sciously aware of simple, unlocalized sensations, as such. Even our 
 most carefully prepared means of scientific analysis do not succeed 
 in disentangling them from the intricate complex of experience in 
 which we find them to be actually engaged. For this experience 
 of feeling, smelling, tasting, hearing, and seeing things, is of some- 
 thing infinitely more complex. And to it, when viewed from the 
 psychological point of view, such titles as the following have been 
 given: "Sense-perceptions," or "apperceptions," "presentations 
 of sense," "sense experience," or when designated in a manner 
 seeming to imply less of introspective theory, "objects of sense." 
 
 It is the ambition of physiological psychology, as it is of course 
 the aim of psychological science from whatever point of view its 
 problems may be approached, to explain experience. And expla- 
 nation here, as elsewhere, would involve an analysis of these com- 
 pounds (if we may be pardoned a somewhat inappropriate figure 
 of speech) into their simpler elements; or better, a descriptive his- 
 tory of the genesis and growth of the more complex from the simpler 
 and more nearly primitive: a study of the causes which have given 
 rise to the process of development, and of the relations which have 
 been called into existence between its different products; and, finally, 
 as far as possible, the establishment of those most general facts of 
 relationship, or so-called laws, which have presided over the whole 
 course of this development. 
 
 2. But while our study shares to the full in the desire to accom- 
 plish this aim of all psychological science, it has its own somewhat 
 special stand-point to maintain, its special obligations to fulfil, 
 and its somewhat special problems to investigate. All these have 
 been sufficiently many times over defined. We are studying the 
 whole subject from the point of view of one who lays emphasis 
 
 380 
 
NATURE AND STAGES OF SENSE-PERCEPTION 381 
 
 on the relations existing between the development of mental life 
 and the growth and increasingly complicated functionings of the 
 nervous mechanism. Our special ambition, accordingly, is to dis- 
 cover, or to conjecture, the most plausible explanation of the devel- 
 opment of the mental life of the human individual by correlating 
 it with the genesis and growth of the human nervous mechanism. 
 
 It is plain, however, that from this time onward we must be more 
 than ever dependent for our problems, for suggestions as to the best 
 manner of approaching the study of them, and even for suggestions 
 as to the directions in which to look for their answer, upon intro- 
 spective psychology. It is conscious experience which we wish to 
 explain. And since the particular kind of experience, which we ob- 
 tain through the use of our senses, by no means consists in the 
 mere having of simple, or of indefinitely complex, sensations, we 
 must constantly bear in mind the fact that as everybody knows 
 attention, association, memory, discrimination, judgment, are all 
 involved in gaining a knowledge of things. All these so-called 
 faculties, however, have a genesis and development which, while 
 it involves a constant and ever more complicated system of actions 
 and reactions, may in many respects be made the subjects, each 
 one, of a special investigation from the physiological point of view. 
 
 3. We shall now briefly summarize some of those conclu- 
 sions, or suggestions from introspective psychology, which have 
 most bearing on every attempt to build up a theory of the presenta- 
 tions of sense, or so-called sense-perception. 
 
 (1) Sensations are, when considered from the introspective point 
 of view, modes of our being affected. There is a wide interval 
 between our consciousness of being ourselves affected and the per- 
 ception of "things" as objective and having qualities of their own. 
 This interval is filled, in nature, by the development of mental 
 life as conditioned upon its environment of sense-stimuli; it must be 
 filled in psychological theory, by a description of the process of de- 
 velopment. But how shall such a description be obtained? The 
 psychologist does not remember by what stages he first learned to 
 see or feel the extended and external objects of sense. The child 
 cannot describe the process to the psychologist. The infant's 
 first sense-experience is already exceedingly complex; and in all 
 stages of human growth the analyzable contents of consciousness 
 represent only very imperfectly the nature of the basis upon which 
 they rest. 
 
 (2) The forms of being and happening in the world, outside of the 
 body, furnish in themselves no adequate explanation whatever of 
 the presentations of sense. This is as true of the colored or smooth 
 extension of an object as it is of its sweet taste or disagreeable smell. 
 
382 PRESENTATIONS OF SENSE 
 
 Whatever exists eotfra-mentally, so far as its pure existence goes, is 
 of no account to the mind. It is only as so-called " things" act upon 
 us, or in other words get themselves expressed by causing changes 
 in our mental states, that any theory of knowledge by the senses can 
 make use of them. What is true of all that exists and happens out- 
 side of the body is just as true of all the bodily conditions and proc- 
 esses. Strictly speaking, they can in themselves furnish no ade- 
 quate explanation for the rise and development of the presentations 
 of sense. Only psychical factors can be built into mental products. 
 The simple sensations have no theoretical value except as they may 
 be considered as psychical elements. The image on the retina, for 
 example, is a necessary physical condition of the clear vision of 
 outside objects; it may also become an object for the inspection 
 of another observer. But the retinal image never becomes a kind 
 of inner object for one's own brain or mind. The mind is never to 
 be conceived of as contemplating a spatial picture of its object 
 formed somewhere within the cerebral substance. 
 
 Even more obvious is the worthlessness, for purposes of strictly 
 psychological analysis, of all theory as to the precise spatial ar- 
 rangement of the fibrils of sensory nerves within the skin or muscular 
 fibre. We have nothing approaching an immediate cognition of 
 the extended network of sensory fibrils in the skin or muscles; much 
 less of the extended muscle or area of the skin. No copy in space- 
 form of the various simultaneous or successive rubbings and stretch- 
 ings of these peripheral fibrils is propagated to the brain; and if it 
 were, the mind could not be regarded as taking account of any of 
 these neural processes. 
 
 (3) A further negative statement may be made with entire confi- 
 dence. The place at which each organ of sense is found in the pe- 
 riphery of the body, or the place at which any such organ is acted 
 on by the stimulus, cannot of itself furnish a reason for the spatial 
 perception of such place or for distinguishing it from other places 
 near or remote. The locality where a stimulus is applied, except 
 as this locality affects the mental coloring or qualitative shading 
 of the sensations which result, is a matter of complete indifference 
 to the mind. 
 
 4. In contrast to all theories like those just rejected, the fol- 
 lowing positive affirmations are to be held firmly. (1) Sensations, 
 as the elements of so-called "presentations of sense," are psychical 
 states whose place so far as they can be said to have one and to 
 speak figuratively is in consciousness. The transference of these 
 sensations from mere mental states to an interpretation of physical 
 processes located in the periphery of the body, or of qualities of 
 things projected in space external to the body, is a mental act. It 
 
NECESSITY OF MENTAL SYNTHESIS 383 
 
 may rather be said to be a mental achievement; for it is an act which 
 in its perfection results from a long and intricate process of psychical 
 as well as physical development. 
 
 (2) The presentation of sense, or rather, the thing perceived, 
 has "space-form"; it is extended, and consists of an indefinite num- 
 ber of visible or tangible parts that are systematically arranged be- 
 side each other into a continuous whole; it is related with respect to 
 position, magnitude, etc., to other similar objects of sense. And, 
 indeed, the one most important characteristic which the presenta- 
 tions of sense possess is space-form. "Space-form" (whatever 
 metaphysics may decide to be the nature, origin, and validity of 
 our idea of space) must be regarded by psychology simply as the 
 mental form of the presentations of sense. The problem which 
 physiological psychology has to solve in this direction may then be 
 stated as follows: On the basis of what combinations of physical and 
 nervous processes do the different resulting sensations come to be com- 
 bined into presentations of sense under the characteristic of space- 
 form f 
 
 5. The most complete answer possible to the question just 
 raised is obliged to recognize the following particular truths: 
 
 (1) A combination (or "fusion," or "synthesis," or "association") 
 of two or more qualitatively different series of sensations is ordinarily 
 if not absolutely necessary in order that presentations of sense in 
 space-form may be constructed. A series of sensations of one kind 
 only, like the pure differences in pitch of musical tone, or of degrees 
 of brightness and saturation of color-tone, or of pressure, tempera- 
 ture, or muscular innervation, is not adapted to form the material 
 for constructing extended objects of sense. 
 
 (2) The characteristic differences in quality of the sensations of 
 some of the senses, and so their adaptability to form graded series, 
 are such as to fit these sensations for combination with other simi- 
 lar sensations into the presentations of sense under space-form; the 
 sensations of other senses have not, at least, to the same extent, 
 these characteristic differences and this adaptability. We may then 
 speak of peculiarly " spatial series" of sensations, and of other series 
 of sensations as at least, relatively non-spatial. The sensations 
 of smell, for example, are manifestly not fitted to form a so-called 
 spatial series; indeed, they are incapable of being arranged in any 
 series at all. On the contrary, the various series of complex sensa- 
 tions that come through the eye and skin (including those of the 
 muscular sense) are qualitatively adapted to enter into such rela- 
 tions to each other as shall give a ground in their combined ex- 
 istence for a perception of things. Accordingly the eye and skin 
 are the so-called "geometrical senses." 
 
384 PRESENTATIONS OF SENSE 
 
 (3) The locally different parts of the organ of sense if this 
 organ is itself to become known (as in the case of the skin), or if 
 through its being stimulated an extended object outside of the body 
 is to be perceived (as in the case of both skin and eye) must have 
 some mental representative in the sensations which stimulation 
 of each calls forth. It is therefore assumed that every complex 
 sensuous experience, besides its general characteristic quality as 
 belonging to this or that particular sense, must have a peculiar 
 "local stamp," or shade, or mixture of quality, dependent upon the 
 place of the organism at which the stimuli are applied; otherwise 
 such experience cannot serve as a factor in the construction of an 
 extended object of sense. This peculiar local stamp, or shade, or 
 mixture of quality has been called a "local sign." It is to Lotze 
 that we owe the first elaborate theory of "local signs," and of their 
 relation to the formation of the presentations of sense. 
 
 (4) Various stages in the process of elaborating the objects of 
 perception must be recognized. Thus the knowledge of the things 
 we handle the fork, the tool, the pen stands at a farther remove 
 from the simplest perceptions of touch than does the discrimina- 
 tion of one area at the surface of the body as warmer or under more 
 pressure than the surrounding spots. Two noteworthy stages, or 
 "epoch-making" achievements, in the process of elaborating the 
 presentations of sense would seem to require a special consideration. 
 These have been spoken of as "localization" or the assigning of 
 the sensuous experience to more or less definitely fixed points or 
 areas of the body; and "eccentric projection" (sometimes called 
 "eccentric perception"), or the recognition by the senses of the 
 qualities of objects as situated within a field of space and either in 
 contact with, or more or less remotely distant from, the body. 
 
 (5) The entire process of elaborating the presentations of sense, 
 or objects of perception by the senses, presupposes for its explana- 
 tion a constant activity of the mind in reacting upon the stimuli 
 which produce various forms of molecular disturbance in the ner- 
 vous system; and, furthermore, its activity in combining the sensa- 
 tions into ever more complex forms. This combining activity 
 is best called "synthetic" or constructive. 1 It may, indeed, always 
 have a physical basis in some central organic combination of the 
 
 'The word "synthesis" for this mental activity is employed and defended 
 by Wundt (Physiolog. Psychologic, 2d ed., ii, pp. 28 f., 164 f., 177), who justly 
 objects to the word "association" and the theories which have used the word, 
 because of their concealment of the truth that the process imparts new proper- 
 ties to its product. He also calls attention (p. 175) to the fact that John Stuart 
 Mill, a chief defender of the "association hypothesis," virtually admits the 
 theory of a mental synthesis by using the term "psychical chemistry." 
 
NATIVISTIC AND EMPIRISTIC SCHOOLS 385 
 
 neural processes which result from stimulating, simultaneously or 
 in the right succession, the different end-organs and areas of the 
 end-organs of sense. And, indeed, our science assumes that this is 
 so; although about this, as a matter of accurate knowledge, we are 
 almost wholly in the dark. 
 
 6. It follows, then, that an analysis of the presentations of sense 
 leads us to find our explanation of certain primary facts and results 
 in the nature of the Mind itself. It is in vain to object that to do 
 this leaves the subject, ultimately, still shrouded in mystery. As 
 a matter of fact, the analysis of psycho-physical science does end 
 in the recognition of ultimate mystery. This is no reproach to it; 
 nor is it a failure or fault peculiar to it alone. All physical science, 
 even, is obliged to accept the same result from its keenest analyses 
 when most vigorously pushed. 
 
 The foregoing remarks indicate what is the correct attitude of 
 the science of physiological psychology so far as it is necessary for 
 it to take any position whatever toward the two rival theories 
 as to the nature and origin of presentations of sense. These theo- 
 ries have been named the "nativistic" (or intuitional) and the "em- 
 piristic." Properly speaking, they are not two fundamentally 
 different theories, but rather two tendencies which appear in the at- 
 titude assumed by two classes of observers toward the admission 
 of certain alleged facts, or in the manner of explaining such facts 
 as are admitted by all. These different tendencies are largely due 
 to differences of position on certain fundamental philosophical ques- 
 tions. Thus influenced, the advocates of the so-called "Nativistic 
 School" prefer to emphasize the intuitional and underived activ- 
 ities of the mind. 
 
 The so-called "Empiristic School," on the other hand, is inclined 
 to give little or no place to the mind's native intuition; it prefers 
 to fill the gaps in the explanation as based on experiment, with 
 probable conjecture and hypothesis. It often aims to show how 
 what we call "mind" is itself rather the result of a genesis induced 
 by the activity of things through the nervous system. The one 
 school is inclined to look upon the space-form, which presentations 
 of sense possess, as the mind's form, in some large sense native to 
 it and not to be explained as the result of a development. The 
 other is inclined to look upon space-form as wholly a form which 
 "things" have come to acquire, and which will be fully explained 
 when science has described the empirical process by which solely 
 this acquisition is gained for them. 
 
 7. Certain principles adopted both by the empiristic and by the 
 nativistic school have their undoubted rights; and no satisfactory 
 theory of sense-perception can be framed without admitting them. 
 
386 PRESENTATIONS OF SENSE 
 
 There can be no doubt that the presentations of sense which so 
 largely constitute our every-day adult experience are not direct re- 
 sults of untrained organic and mental activities; they are not sim- 
 ple intuitions dependent solely on the native and inherent powers 
 of the mind. With whatever speed and certainty they are formed, 
 and however the impression they make is characterized by a per- 
 fect "immediateness," they are really extremely complex products, 
 involving not only the organic habit of the species and individual 
 peculiarities of mind and body, but also the acquisitions of experi- 
 ence through memory, attention, association, and so-called "instinc- 
 tive inference." All this is as true of the unhesitating localization 
 of a burning or cutting pain in some area of the skin as it is of the 
 most deliberate judgment about the distance of a mountain. 
 
 On the other hand, however far the "empiricist" may succeed 
 in resolving these "intuitions" of sense into more nearly prim- 
 itive elements, and however minutely he may describe the processes 
 and laws of their development, he will never succeed in withholding 
 from the mind itself the ascription of all its so-called native powers. 
 The elements reached by his most complete analysis must always 
 be considered as reactions of the mind upon the stimulation of the 
 nervous centres through the end-organs of sense; they all imply a 
 native disposition and ability of the subject of the sensations. And 
 both theories must alike admit that the nature of the elements and 
 of the synthetic process is conditioned at every step upon the action 
 of the central nervous mechanism as sensitive and excited through 
 stimulation of the end-organs of sense. Nevertheless, the triumphs 
 of scientific research all lie along the line of the construction of a 
 more and more perfect genetic theory. 
 
 8. Before proceeding to illustrate and confirm in detail the 
 principles already laid down, several questions raised by the mere 
 statement of these principles require an answer. And first: What 
 are those characteristic differences in quality which the sensations be- 
 longing to some of the senses possess, and which adapt them to com- 
 bine into presentations of sense under space-form ? In other words, 
 what kinds of sensations are fitted to constitute a so-called "spatial 
 series" ? Plainly, it is not necessary that those elements of the com- 
 plex objects of sense, which make the objects appear to be composed 
 of parts set together side by side, should themselves be immediately 
 known as side by side. What is really necessary is that both series 
 of sensations, if they are to be combined into one presentation of 
 sense, shall be capable of clearly and reciprocally determining each 
 other as series of sensations. They must both have, that is to say, 
 the common qualities and mutual relations of a so-called "spatial 
 
NATURE OF A SPATIAL SERIES 387 
 
 (1) Of the qualities which characterize spatial series the following 
 are the most important: Series of sensations of like quality, which 
 are adapted to combine into extended objects of sense, must admit of 
 easy, rapid, and frequent repetition in varying order of arrangement. 
 If a portion of the body be moved, as, for example, a finger, an arm, 
 a leg, or the bending of the back a graded series of sensations, due 
 to the varying quality and quantity of strain upon the different 
 muscles, joints, etc., is the result. This series is composed of indi- 
 vidual compound sensations that shade into each other with no 
 apparent interruption, each of them having a certain value and 
 temporal position in consciousness. In adult experience the series 
 is rapidly concluded, and instantaneously interpreted as a whole. 
 But they may be reproduced in a measure by slowly moving a limb 
 in any direction, and endeavoring to pay strict and exclusive atten- 
 tion to the succession of feelings which results. Every motion of 
 each limb, from about the same position a to about the same posi- 
 tion m, relative to the whole body, with similar energy, speed, and 
 other concomitant circumstances, yields a nearly identical series 
 of sensations (a, /3, 7, . . . /*). Other motions of different limbs, 
 or differing otherwise (in energy, speed, point of starting or of con- 
 clusion, etc.), yield series differing in the value and ordering of 
 their individual members. What is true of the muscular sensations 
 that result from the movement of the limbs is also true of the ac- 
 companying sensations of the skin, such as arise from changes in 
 its tension, etc. These sensations, however, largely blend with the 
 series of muscular sensations so as to be nearly or quite inseparable 
 in consciousness. The same thing also holds good of the series 
 of tactual sensations (sensations of light pressure or touch proper) 
 developed by moving an object over the skin, or by moving a tactile 
 organ (especially the hand) over an object at rest. The muscular 
 and tactual sensations which result from motion of the eye also 
 have the qualities of a graded spatial series. 
 
 Accordingly, senses like those of the eye and hand, which have 
 organs capable of rapid and precise motion, are equipped with a 
 peripheral mechanism adapted to the production of so-called spa- 
 tial series of sensations. The succession of sensations of light and 
 color which accompany the movement of an object in the field of 
 vision, or of the glance from one object to another, are of the kind 
 favorable to forming a spatial series. In all these cases the rate of 
 the sensations is important. Either too slow or too rapid movement 
 of the organ will not yield a spatial series of sensations. Moreover, 
 such series are capable of repetition, not only forward, as a, ft, 7, 8, 
 ... /*, or in inverse order, as p, \, K, . . . ft, a, but also in an 
 endless variety as an intersecting network of sensations. 
 
388 PRESENTATIONS OF SENSE 
 
 (2) The second class of qualifications which must be possessed 
 by a spatial series of sensations secures their habitual combination 
 with other series, also of a spatial kind. They must be in nature 
 comparable and associable with each other, and, in fact, simultane- 
 ously experienced by the mind. In singing a musical scale a series 
 of sounds is accompanied by another series of muscular and tactual 
 sensations occasioned by the use of the vocal organs; both series 
 may be produced in inverse order by singing the same scale back- 
 ward. Thus we know not only that we are singing the scale with 
 the vocal organs, but also that we are at the same time hearing it 
 with the ear. We know both these facts, however, through sensa- 
 tions of muscle and skin that have already become inseparably 
 associated and localized in our own body. 
 
 On the contrary, from the dawn of consciousness onward through 
 all the development of experience, series of sensations of light and 
 color are constantly accompanied by, and combined with, other se- 
 ries of tactual and muscular sensations of the eye. So, too, the 
 different series of sensations that arise from the irritation of the 
 nerves in muscle and skin are, of necessity, habitually combined. 
 In forming the field of touch, the fact that certain parts of the pe- 
 riphery of the body so frequently come into contact with other parts 
 is of the highest significance. Two series of complex sensations, 
 corresponding to the terms " touching" and "being touched/' are 
 thus brought into juxtaposition, as it were, in consciousness. This 
 "juxtaposition" in consciousness is not itself, of course, a spatial 
 juxtaposition; the former is, however, the necessary precondition 
 of the latter. 
 
 (3) The third characteristic of the spatial series of sensations 
 is the possession of a system of local signs. It may safely be as- 
 sumed that on neither side that of an active nervous mechanism, 
 or that of a conscious, sensuous experience are the means that 
 make an interpretation of the locality of our body as affected by the 
 stimuli, of the spatial relation t>ne to another of its parts, of the whole 
 body to its environment, and of the different objects in this environ- 
 ment one to another, a simple affair. On the contrary, those feel-j 
 ings, on the interpretation of which, as "local signs," our sense-per-j 
 ceptions all depend, are always exceedingly complex. And cor- 
 respondingly complex are the quickly changing and combining 
 functions of the nervous mechanism. For this reason, if no other, 
 it is uniformly difficult to submit them to a complete analysis. 
 Several views, for example, are possible as to the nature of the local 
 signs of the skin. It has been held that they are not qualitative 
 differences at all, but differences in the intensity and time course 
 of the tactual sensations. Again, it has been held that the local 
 
NATURE OF THE LOCAL SIGNS 389 
 
 signs of touch are qualitativedifferences_of sensation dependent 
 upon the modifications whicTTTEe stimulus undergoes on account 
 of the changing character of the skin witn respect to tension, nature 
 of the substance of muscle, tendon, and bone over which it is 
 stretched, etc. Finally, it may also be held that the local signs of 
 the skin are qualitative differences of sensation peculiar to the differ- 
 ent nervous ^laments yiating in different parts of this organ 
 sense. They are the direct result, that is, of the mind's reaction 
 upon the specific energies of the nervous elements as called out by 
 the stimulus. This is, of course, to fall back upon the ultimate 
 mystery involved in the original nature of that reaction which the 
 mind makes as dependent upon the locally individual nervous ele- 
 ments being stimulated. 
 
 What is certain of the feelings, or "sensation-complexes," in 
 dependence on which this class of objects of sense becomes known 
 to us, is yet more certain of those on the interpretation of which all 
 our knowledge of visual objects is dependent. Here, not only is 
 it true that no one theory among the several proposed seems ade- 
 quate to account for all our experience, but something may possibly 
 be taken from them all which will prove of assistance. And more 
 refinements borrowed from the psychology of association, memory, 
 habit, and judgment, must be added in order to approximate, even 
 somewhat remotely, a satisfactory theory of the local signs of the 
 eye. 
 
 9. In view of all the evidence, it would seem that the gene 
 theory of local signs must be constructed in somewhat the follow- 
 ing way: Within certain limits, which it is impossible for science 
 as yet definitely to fix, the irritation of the different nervous ele- 
 ments of certain organs of sense gives rise to sensations which dif- 
 fer in the shading of their quality according to the locality in the 
 
 ^jgE^a*****"* 1 "*" i "^''^P"* 1 "'"* t/ O 9P^MMMK9Vi> 
 
 organ at which the elements are situated. This is probably true of 
 both peripheral and central areas of the total organ. It is true of 
 the latter areas as dependent on the excitation of the former. The 
 simultaneous irritation of several locally related elements of the 
 organ (and the irritation is seldom or never confined to a single 
 element) results, then, in a certain mixture of feeling dependent 
 upon the number and local relation of all the elements thus simul- 
 taneously irritated. For example, the color-tone of the complex 
 sensations aroused by irritating together the retinal elements a, /3, y, 
 5, etc., differs from that aroused by irritating the elements 7, S, e, , 
 etc. The same thing holds true of locally related nervous elements 
 of the skin. Just how much in every case of the local coloring is 
 due, on the physiological side, to differences in structure and how 
 much to differences in processes, how much to peripheral elements 
 
390 PRESENTATIONS OF SENSE 
 
 and how much to central nervous connections, it may be impossible 
 to say. Each of the spatial series of sensations is characterized by 
 this shading of its elements. We must, therefore, hold that every 
 sense which is the medium of space-perceptions has a system of 
 local signs of its own. 
 
 Further: not only each "geometrical sense," but also each of the 
 "spatial series" of sensations arising through the total operation 
 of that sense, consists of members that have a local coloring pe- 
 culiar to the series. Thus the spatial series of tactual impressions 
 produced by moving an object from a to d on the hand differs from 
 that produced by moving it from a to n; the series of muscular sen- 
 sations developed by raising one pound differs, with respect to the 
 color-tone of its members, from that developed by raising two 
 pounds, with the hand. 
 
 But another important consideration remains. The local signs 
 of the different spatial series which frequently combine in the opera- 
 tion of the same organ must necessarily modify each other. Hence 
 there arise ever more complex admixtures of feeling dependent 
 upon the combined specific energies of the nervous elements simul- 
 taneously excited, with a given amount of energy and with given 
 relations to preceding conditions. -We define the local sign, then, 
 as that mixture of feeling which gives to the sense-experience its 
 peculiar coloring, and is dependent upon the combined result of excit- 
 ing the nerves of a given locality of the organ. 
 
 10. The most noteworthy stages, or "epoch-making" achieve- 
 ments, in the process of elaborating the presentations of sense, have 
 been declared to be " localization" and " eccentric projection." The 
 first, primarily, gives us the knowledge of our own body, mainly by 
 passive sensations of touch; the knowledge of our own body which 
 comes through sight is by eccentric projection. We immediately 
 feel the peripheral parts of the body as the places where the sensa- 
 tions are localized; we see some of the same parts as projected in 
 space before our eyes. Objects that are not a part of ourselves are 
 given to us as projected eccentrically, either by touch through their 
 being in contact with the skin and occasioning sensations of mus- 
 cular exertion, or by sight as having distance in its field of vision. 
 
 Localization and projection are not, however, to be regarded as 
 two phases of one and the same process; we do not first have the 
 presentations of sense as parts of the periphery of our bodies, and 
 then, on further experience, push them beyond this periphery, either 
 to an infinitesimal distance or to one remote. Localization and ec- 
 centric projection are rather two processes, largely unlike, which 
 go on contemporaneously and are set up chiefly on the basis of dif- 
 ferent classes of sensations. Where two parts of the sensitive skin 
 
ACTIVITY OF HIGHER FACULTIES 391 
 
 of our own bodies come together, the conditions for both of the above- 
 mentioned processes are fulfilled. Accordingly, one part has local- 
 ized in it those complex sensations which make us aware that this 
 part of our body is touching something; the other has localized in 
 it those sensations which make us aware that this part is being 
 touched by something. Which of the two parts shall be regarded 
 as touching, and which as being touched, depends on various con- 
 siderations. Those members of the body which are most used in 
 active touch are generally known as touching, and the less active 
 parts as being touched. 
 
 11. Two things more must constantly be borne in mind in any 
 attempt to construct even a fairly plausible theory of sense-percep- 
 tion in terms of physiological psychology. First: so far as we can 
 penetrate the mysteries of beginning mental life, there is never at 
 any stage an experience corresponding to "pure sensations," or 
 "simple sensations." Such terms, if employed at all, must always 
 be understood as applying to hypothetical elements of already com- 
 plex psychoses, which are, however, of value, especially to the 
 student of physiological psychology as enabling him to correlate the 
 mental life, in its development, with the increasing complexity of 
 the secondary activities of the brain which result from the combina- 
 tion of its more primary reactions to the elementary sensory impulses. 
 That there is growth in knowledge as to the spatial and temporal 
 relations of the bodily members, and of external objects and the 
 sequence of events among things, there can be no manner of doubt. 
 Our problem is to account for this growth. 
 
 But second: the so-called "higher faculties" of the mind are as 
 truly implied in the very beginnings of sensuous experience as in 
 its latest developments. The infant attends, discriminates, judges, 
 and so learns, as truly and in all probability, in essentially the same 
 way as the adult man of science. Indeed, the most astonishing 
 and antecedently incredible thing about the whole of these earlier 
 stages is the intensity, and the fine quality, of those mental activ- 
 ities which initiate, conduct, and control all the more primitive 
 processes of learning to know, by the senses, the bodily organism 
 and external things. For these secondary and "synthetic" reactions, 
 the highly developed, but as yet unused, cerebral hemispheres of the 
 human infant seem especially adapted. 
 
 The foregoing principles must now be illustrated and confirmed 
 by a brief statement of facts which relate to the formation and de- 
 velopment of presentations of sense by a synthesis of simple sen- 
 sations. Attention will, for obvious reasons, be directed almost 
 exclusively to those presentations of sense which come through the 
 eye and skin, including in both the influence of muscular sensations. 
 
392 PRESENTATIONS OF SENSE 
 
 12. Perceptions of Smell differ only in fineness, duration, and ac- 
 companying tone of feeling; they have no size or shape, no spatial 
 properties of any kind. Considered apart from their accompani- 
 ment of muscular and tactual sensations, they cannot even be said 
 to be localized. Fineness of smell, or power to make minute dis- 
 tinctions in quality, and so infer the presence or direction of an ob- 
 ject previously known to excite such quality of sensations, differs 
 greatly in different species of animals and in different individuals 
 of the same species. The exploits of some animals give ground for 
 the conjecture that every species, and even every individual, has an 
 odor of its own. The direction and nature of the object which 
 causes the sensations are judged by variations of intensity on turn- 
 ing the head, or on approaching or receding from the object. Sen- 
 sations of smell are known to come through the nose, by localizing 
 there the accompanying muscular and tactual sensations with their 
 strong tone of feeling. This is readily done, since we draw the air 
 through the nostrils and feel its double effects in producing the two 
 classes of sensations. As to the simultaneous influence of two smells, 
 little is known beyond the fact that the stronger overwhelms the 
 weaker. The power of discrimination may, of course, be culti- 
 vated in this sense as in every other. 1 
 
 13. Most of the remarks just made as to perceptions of smell 
 apply also to Perceptions of Taste. Sensations of taste, however, are 
 much more closely connected with those of touch; since the tongue 
 is a chief organ of active touch. It is the tactual and muscular 
 sensations, and not the purely qualitative affections of taste, which 
 are localized in the mouth. Concerning contrast and compensation 
 of tastes, little is known which does not belong to ordinary experi- 
 ence. Valentin 2 alleges that when a sour mass is laid on one half, 
 and a bitter mass on the other half, of the root of the tongue, the 
 predominating taste may sometimes be determined by our choice. 
 It is well known that certain tastes compensate each other, as it 
 were, in experience, without any chemical equivalence of their prop- 
 erties. The sugar neutralizes the acid of the lemonade, not in the 
 vessel that contains the mixture, but in the nervous system of him 
 who drinks it. Briicke holds 3 that the neutralizing of one sensa- 
 tion of taste by the other takes place in the brain. The sensation 
 of bitter is especially difficult to cover or neutralize. 
 
 14. Perceptions of Hearing next demand consideration. More 
 difficulty accompanies the effort to establish the proposition that 
 
 1 On the whole subject see von Vintschgau's monograph in Hermann, Handb. 
 d. Physiol, III, pp. 225 ff. 
 
 2 Lehrbuch der Physiol. d. Menschen, etc., Abth. ii, p. 308 (2d ed.). 
 
 3 Vorlesungen uber Physiol. (ed. 1884), ii, p. 262. 
 
LOCALIZATION OF SOUNDS 393 
 
 sensations of sound are not directly localized, but are projected 
 in a space constituted chiefly by the eye and the hand, through 
 complicated indirect inferences. 
 
 The chief facts which must be accounted for by a theory for the 
 localization of sounds are the following: A sound can be recog- 
 nized with certainty as coming from the right or the left side, or as 
 coming from a point either to the right or left of the median plane 
 of the head. Moreover, the angle at which the sound approaches 
 the ear, as measured from the median plane, can be recognized with 
 considerable accuracy. Or, if a sound is produced within this 
 plane (extended into space), this fact can be well recognized. But 
 as to the direction within this plane from which the sound comes 
 whether from above, before, or behind judgment is uncertain and 
 subject to large errors. And the same difficulty is experienced in 
 judging the exact direction of sounds which come from the side. 
 How much their direction differs from the median plane can, as 
 was stated, be told; but whether from before, behind, up or down, 
 is only poorly distinguished. In other words, if a sphere be con- 
 ceived as surrounding the head, with the north and south poles 
 located opposite the ears and the equator coinciding with the median 
 plane of the head, then the latitude of a sound can be detected with 
 considerable accuracy, but the longitude is subject to much error. 
 Judgment of the latitude is, however, most accurate near the equa- 
 tor, and least accurate near the poles. 1 
 
 15. Another set of facts comes to light when different sounds 
 are made to affect the two ears. If the two sounds are of different 
 pitch or timbre, each is apt to be heard and localized separately; 
 but if the two are alike, except that one is stronger than the other, 
 they usually appear as one, which is localized on the side of the 
 stronger stimulus; and if they are alike in intensity as well as in 
 pitch and timbre, usually one sound is heard, which seems to come 
 from the median plane. This last result, which is as instructive 
 as it is curious, can be obtained by sounding two tuning forks of the 
 same pitch, with equal loudness, one opposite each ear; or by con- 
 veying a sound through a branched tube, one branch being inserted 
 in each meatus; or, finally, by employing a branched telephone cir- 
 cuit, with a receiver held to each ear. In the latter two cases, the 
 sound is subjectively localized in the interior of the head. Similar 
 results are obtained by applying the shank of a vibrating tuning 
 fork to the skull at various points; the tone is localized in that ear 
 which is more strongly excited, but if the fork is applied at a point 
 
 1 See D. Starch, University of Iowa Studies in Psychology, 1905, IV, 1; and Psy- 
 chological Review, Monograph Supplement, No. XXXVIII, 1908. 
 
394 PRESENTATIONS OF SENSE 
 
 in the median plane, the sound is localized in the median plane, and 
 often in the interior of the head. When sound is thus conveyed 
 to the ear by bone conduction, closing the meatus of one ear by 
 holding the palm against it increases the effect on that ear, and 
 causes the sound to appear louder, and, therefore, to be localized 
 in the ear that is closed (Weber's experiment). As such conduction 
 is, in large measure, the means by which the sound of our own voices 
 reaches our ears, the same experiment can be tried by humming a 
 low note with closed lips, and observing the effect of closing the ex- 
 ternal meatus with the palm. If one ear is thus closed, the sound 
 is localized in that ear; if both ears are closed at once, the sound 
 appears to come from the interior of the head. 
 
 16. The first step toward an explanation of the power of local- 
 izing sounds is thus made clear: evidently, as the last-mentioned ex- 
 periment shows, each ear has a "local sign" of its own, by which a 
 stimulus affecting chiefly one ear is distinguished from a stimulus 
 affecting the other ear. It should be noted that sound never affects 
 one ear alone, unless the other ear is totally deaf; for conduction by 
 the bones occurs between the two ears to a surprising degree. Nei- 
 ther closing the external meatus of one ear, nor bringing the source 
 of sound close to the other ear, suffices to produce strictly monaural 
 hearing. These devices do, however, ensure a stronger excitation 
 of one ear than of the other. There can be no doubt as a second 
 indication for a correct theory of the localization of sounds that a 
 sound which excites one ear more strongly than the other is local- 
 ized on the more excited side; nor that when both ears are excited 
 equally, the sound is localized in the median plane. It would seem, 
 accordingly, that the decisive factor in localizing sounds to the right 
 or left must be the relative intensities of the stimulation of the two 
 ears. This factor would account for the more accurate element in 
 localization. The much less accurate sense of locality for front 
 and back may be dependent on the position of the auricle, which, 
 in the human being, would seem to favor the entrance of sounds 
 from the front; just as, in animals possessing movable ears, the 
 localization of sounds certainly appears to depend on adjusting 
 the position of the pinna most favorably for the direction of the 
 sound. 
 
 17. The principle of relative intensities can therefore be ap- 
 plied satisfactorily to the case of sounds originating near the ear; 
 but difficulty arises in the case of sounds from distant sources, for 
 here the difference in distance of the ears from the source of sound 
 is too small, in comparison with the total distance, to account 
 for a different intensity of excitation of the two ears. One ear is 
 practically as far from the source as the other; both ears should 
 
LOCALIZATION OF SOUNDS 395 
 
 therefore be equally excited, and the sound be localized in the median 
 plane, instead of being, as it often is, very clearly and correctly as- 
 signed to one side or the other. The ear which is toward the source 
 of sound is indeed exposed to the direct impact of the waves, whereas 
 the other would seem to be shielded from them by the head. But 
 here we meet a physical difficulty, for, as Lord Rayleigh has shown, 1 
 the head is too small a shield to cast a "sound shadow," at least 
 when the sound is of low pitch and, therefore, of great wave-length 
 with respect to the diameter of the head. Sound-waves bend read- 
 ily around the head, and enter the further ear with, it would seem, 
 little possible diminution of their energy. The waves entering the 
 two ears would, however, be at any moment in different phases, 
 and Lord Rayleigh believed that this difference of phase afforded 
 a basis for distinguishing the direction of the sound a view which 
 appears rather improbable. Myers and Wilson, 2 taking account of 
 bone conduction, are, however, able to show that the difference of 
 phase must result in a difference in intensity at the two ears. It 
 should also be noted that, as the orifice of the ear lies nearer to the 
 back than to the front or top of the head, the path of sound-waves 
 around the head is shorter in some directions than in others, and 
 that, accordingly, the waves which reach the further ear will them- 
 selves vary in phase and therefore interfere with each other and have 
 less effective intensity than the sounds which, approaching the 
 nearer ear across clear space, act on it without this mutual inter- 
 ference. High tones, with short wave-length, would evidently be 
 more subject to this interference than low tones. At any rate, there 
 is no doubt that the nearer ear is more intensely excited than the 
 further ear, for this can be proved by direct measurements of the 
 least audible sound, coming from various directions. 3 A weaker 
 sound can be heard when the source is directly opposite the 
 right or left ear, than when it is situated in or near the median 
 plane. 
 
 The quality or timbre of a sound is also affected by the angle 
 from which it approaches the ear, and is different for the nearer 
 and for the farther ear. 
 
 The shorter the waves, the more effectively does the head screen 
 the farther ear; it thus cuts out the higher partial tones of a clang 
 or noise more than the lower components, and changes the quality 
 of the sound. This difference in the quality of sound to the two 
 ears, according to the direction from which the sound comes, may be 
 
 1 Nature, 1876, XIV, 32; Philosophical Magazine, 1907, XIII, 214. 
 
 2 Proceedings of the Royal Society, 1908, A, LXXX, 260; British Journal of Psy- 
 chology, 1908, II, 363. 
 
 3 See Starch, op. cit. 
 
396 PRESENTATIONS OF SENSE 
 
 of importance not only in the binaural localization of sounds to the 
 right or left, but also in the (often considerable) power of localiza- 
 tion as possessed by a one-eared person; 1 and, further, in the as- 
 signment of sounds to front and rear, above and below. 
 
 18. What has thus far been said of localization of sound has re- 
 ferred to the perception of its direction. The judgment of its dis- 
 tance must be dismissed with a few words. If the sound is familiar, 
 the distance of its source can be judged from its apparent intensity. 
 But even when the intensity of the sound at its source is not known, 
 it may still be possible to recognize the distance from which it comes. 
 It is probable that the change in the quality of a sound with dis- 
 tance, due to the dropping out of the weaker overtones, has much to 
 do with the recognition of distance. It is significant that simple 
 tones are more poorly localized, both as to distance and as to direc- 
 tion, than clangs and noises containing an abundance of high par- 
 tial tones. 2 In one-eared individuals pure tones are entirely un- 
 localizable. 
 
 The proposition would seem then to be established that the so- 
 called perceptions of hearing are localized by means of the varying 
 intensities and complex qualities (or "local signs") of the sensations 
 of sound, in a field of space which has been "constructed" so to 
 say out of other forms of sense-experience. 
 
 19. An account of the process by which a Field of Touch is con- 
 structed, and extended objects are known as in contact with the 
 skin at definite points or areas of it, must begin by enumerating the 
 data which the mind has for such activity. The most important 
 of these data are indicated by certain facts as to the fineness of the 
 so-called "sense of locality" belonging to the skin. E. H. Weber 
 first attempted a rule for measuring the degree of this fineness ac- 
 curately; he also mapped out the entire field of the surface of the 
 body into areas differing greatly in their fineness. 3 For a measur- 
 ing instrument he used the two points of a pair of dividers, blunted 
 so as to prevent the sensation of being pricked; the principle of 
 measurement was that the minimum distance apart at which the 
 two points, when touching the skin of any region, are felt as two 
 localized sensations, is the measure of the sensitiveness to local dis- 
 tinction of that region. The following table gives some of the re- 
 sults of Weber's experiments; the figures indicate the number of 
 
 1 See Angell and Fite, Psychological Review, 1901, VIII, 225, 449. 
 
 2 On this point, see J. R. Angell, Psychological Review, 1903, X, 1. For general 
 discussion and literature, see K. L. Schaefer, in Nagel's Handbuch der Physiologic, 
 1905, III, 573, and C. S. Myers, Textbook of Experimental Psychology, 1909, 286. 
 
 3 Annot. Anatom., vii, p. 4 f.; Wagner's Handworterb. d. PhysioL, III, Abth. 
 ii, p. 529 f. 
 
CONSTRUCTION OF THE FIELD OF TOUCH 397 
 
 millimetres 1 apart which the points of the dividers were when the 
 given area of the organ was just able to distinguish them : 
 
 Tip of the tongue 1 
 
 Volar side of the last phalanx of the finger 2 
 
 Red part of the lips " 5 
 
 Volar side of the second and dorsal side of the third phalanx of the finger . 7 
 
 White of the lips, and metacarpus of the thumb 9 
 
 Cheek, and plantar side of the last phalanx of the great-toe 11 
 
 Dorsal side of the first phalanx of the finger 16 
 
 Skin on the back part of cheek-bone, and forehead 23 
 
 Back of the hand 31 
 
 Knee-pan, and surrounding region .36 
 
 Forearm, lower leg, back of the foot near the toes 40 
 
 Skin of the nape, and of the back in the five upper cervical vertebrae ... 54 
 
 Skin of the middle of the back, and of the upper arm and leg 68 
 
 Weber also found that the fineness of the sense of locality is 
 greater in a transverse than in a longitudinal direction, on both arms 
 and legs. On these surfaces of the skin the "sensation-circles," 
 or areas within which the minimum distances of the dividers' points 
 are felt as two points, have an elliptical shape, with their long axes 
 up and down. That the size of the sensation-circles, or the fineness 
 of the sense of locality, largely forms the basis for our judgments 
 of the position, number, and magnitude of the localized sensations 
 in the field of touch may be shown by a simple experiment. If 
 the points of the dividers be separated somewhat less than is neces- 
 sary in order to distinguish them as two on the cheek just in front 
 of the ear, and then (the distance apart of the points remaining un- 
 changed) be slowly moved until one point rests upon the upper and 
 the other upon the lower lip, to a person blindfold, and unpreju- 
 diced by knowing what is to take place, the point first felt as one 
 will appear to become two, and then the two recede from each 
 other continually as the parts with a finer sense of locality are trav- 
 ersed. The same experiment may be tried upon any other part 
 of the body. It appears, therefore, that the mental representation 
 of the magnitude of the distance between two impressions varies in 
 inverse proportion to the real magnitude of the smallest perceiv- 
 able distance, on any given area of the skin. The same principle 
 appears to hold good when all the space between the impressions 
 is filled up, so as to make a continuum of localized sensations. 
 
 20. The explanation of Weber's "sensation-circles" of the skin 
 has been the subject of much debate. It is natural at first to as- 
 
 1 The numbers were given by Weber in Parisian lines; in the table they are 
 taken from W T undt, Physiolog. Psychologic (2d ed.), ii, p. 7, who has reduced them 
 to even millimetres. 
 
398 PRESENTATIONS OF SENSE 
 
 sume that each entire circle is provided with one and only one 
 nerve-fibre, whose terminal expansion covers the circle, and whose 
 excitation is represented in consciousness by a sensation of a spe- 
 cific value. Doubtless certain anatomical differences in the nerve- 
 fibres of the skin, and certain corresponding physiological differ- 
 ences in their function, must be assumed as the basis of every the- 
 ory to account for the skin's sense of locality. But Goldscheider's 
 experiments show that a number of pressure-spots must be recog- 
 nized within each sensation-circle, and each pressure-spot at least 
 should have a sensory fibre. Moreover, every point within each 
 sensation-circle is itself sensitive (however large the circle may be), 
 and the limits of none of the circles are fixed as would be the ex- 
 panse of a single nerve-fibre distributed over them. Still further, 
 different individuals differ greatly in the size of these circles (and 
 we cannot well suppose a corresponding difference in the number 
 of sensory nerves of the skin), and practice suddenly and greatly 
 diminishes the area covered by a single circle. It must at least be 
 admitted that "the smallest perceivable distance is not a direct 
 measure for the diameter of the sensation-circle." Weber himself 
 assumed that sensation-circles always contain a number of isolated 
 nerve-fibres; and that, in order to have the impression of two local- 
 ized sensations, several unexcited fibres must exist between the two 
 excited. The number of these unexcited fibres serves the mind as 
 a kind of means for the approximate measurement of distances on 
 the skin. The highly conjectural, and in general the unpsycho- 
 logical, nature of all these explanations renders them unsatisfactory. 
 21. When we attempt to apply the theory of local signs to this 
 subject, difficulty arises in assigning a conclusive reason why the 
 different areas of the skin should differ so greatly in the fineness of 
 their capacity for making local distinctions. In the view of Lotze, 1 
 this difference is chiefly due to the varying character of the areas 
 of the skin with respect to richness in nerve-fibres, thickness and 
 so sensitiveness, support and tension according as the skin is 
 stretched over underlying soft or hard parts fat, muscle, tendon, 
 bone, etc. Doubtless all such influences enter into the determina- 
 tion of that mixture of feeling which characterizes the local signs 
 of the skin. The theory suggested by Vierordt, 2 on the basis of 
 experiments made by himself and his pupils, should also be men- 
 tioned. This investigator concluded that the fineness of the sense 
 of locality belonging to any area of the skin increases in direct pro- 
 portion with the distance of that area from the axes about which it 
 
 1 See Medicin. Psychologic, p. 405 f. 
 
 2 Pfluger's Archiv, 1869, II, pp. 297 ff.; and Zeitschr. f. Biologic VI, VII, IX, 
 X, XI. 
 
EXPLANATION OF WEBER'S "SENSATION-CIRCLES" 399 
 
 is rotated. The relative fineness of the organ's local sense is a 
 function of its mobility. Thus an uninterrupted increase of the 
 power of localization exists in the arm from the acromion to the 
 tips of the fingers; an increase of its movableness, on the whole, also 
 exists. If a value of 100 be assigned to the power of discrimination 
 exercised at the acromion, 151 will represent that of the upper arm, 
 272 that of the lower arm, 659 of the hand, 2,417 of the thumb, and 
 2,582 of the tips of the fingers. In estimating the relative movable- 
 ness of these different parts, it should be remembered that they 
 not only all move in an enlarging circuit from the shoulder-joint 
 downward, but that each of them from the elbow-joint downward 
 has its special increased circuit and more numerous forms of mo- 
 tion. 
 
 But even if Vierordt's law could be strictly demonstrated for 
 every portion of the body, its meaning would have to be translated 
 into other terms in order to be of any real service to psychology. 
 It is therefore suggested by Funke that the increased power of 
 discrimination which belongs to the more movable areas of the skin 
 is really due to the superior facility which they thus have for exer- 
 cise; it therefore falls under the law of habit. Furthermore as 
 we have occasion to remark concerning many similar functions of 
 the mind in correlation with the nervous mechanism the effect 
 of acquired habit is not limited to the experience of the individual; 
 it belongs also to the race. The superior fineness of local sense in 
 some parts of the body may therefore be regarded as largely native 
 to the individual. 
 
 22. The view which must be taken of Weber's "sensation-cir- 
 cles," and of the entire subject of the localization of areas of press- 
 ure on the skin, has been largely changed by the more recent experi- 
 ments of Goldscheider 1 and others. We have already seen (p. 344) 
 that the finest point, when it touches a "pressure-spot," produces 
 a sensation of pressure, and not one of being pricked; but touching 
 other spots does not produce a sensation of pressure at all. It musL 
 be held, then, that the sensations produced by laying a single 
 blunted dividers' point upon the skin, as in Weber's classical ex- 
 periment, are really very complex, and are composed of the sensa- 
 tions from several pressure-spots blended with other sensations 
 from the rest of the same area not covered by the pressure-spots. 
 The fineness of discrimination possible in any area of the skin de- 
 pends, then, upon how all the points irritated stand related to the 
 specific pressure-spots. Goldscheider found that only when two 
 irritating points touch two pressure-spots are they felt as two. But 
 
 1 Archiv f. Anat. u. Physiol, Physiolog. Abth., 1885, Supplement-Band, pp. 
 1-104; especially, p. 84 f. 
 
400 
 
 PRESENTATIONS OF SENSE 
 
 when one of the points touches a pressure-spot, and the other touches 
 some place in the contiguous area of skin which is free from such 
 spots, the two points are not both felt; in this case only the one rest- 
 ing on the pressure-spot is felt. 
 
 The table of minimum distances at which two points can be felt 
 as two, when the exact nature of the area of the skin on which we 
 are experimenting is known, and everything made as favorable as 
 possible, consists of numbers very much reduced from those of 
 Weber. Following are some citations from Goldscheider's table: 
 
 Part of the body mm. 
 
 Back 4-6 
 
 Breast 0.8 
 
 Forehead 0.5-1.0 
 
 Cheek 0.4-0.6 
 
 Nose and chin 0.3 
 
 Upper and lower arm . . . . 5-1 . 
 
 Part of the body mm. 
 
 Back of hand 0.3-0.6 
 
 I. and II. phalanges (volar) . . 2-0 . 4 
 
 I. and II. phalanges (dorsal) 0.4-0.8 
 
 Upper leg 3.0 
 
 Lower leg 0.8-2.0 
 
 Back, and sole of foot . .0.8-1.0 
 
 23. Yet more recent experiments by Von Frey 1 and his col- 
 laborators, Bruckner and Metzner, differ considerably from those 
 of Goldscheider in the absolute values assigned to the touch-spots. 
 . When two points are applied to neighboring spots, the threshold 
 is indeed very small provided the points are applied successively, 
 but not when both are applied simultaneously, as in the experi- 
 ments of Weber. It had previously been known that a difference 
 of location could be perceived with much less distance between the 
 points stimulated when one was touched after the other than when 
 both were touched at once. 2 Von Frey and Metzner, applying 
 stimuli to previously identified touch-spots, concluded that a dif- 
 ference of location could be appreciated between any two touch- 
 spots, no matter how near together they might be, when one was ex- 
 cited shortly (say J to 1 second) after the other, but not when both 
 were excited simultaneously. In the latter case, the "two-point 
 threshold" was not much less when touch-spots were specifically 
 touched than when the compasses were applied in the usual indiscrimi- 
 nate manner. The perception of simultaneous double touch, there- 
 fore, is evidently a perception made under difficulties, and cannot 
 give an ultimate measure of the fineness of the system of local signs. 
 Other facts tend in the same direction. If two points are simul- 
 taneously applied to the skin at a distance too small to permit of 
 a clear recognition of twoness, still the sensation aroused may dif- 
 fer from that of a single point, in possessing a certain breadth. In 
 pathological cases, notably after injury to the nerves supplying 
 some region of the skin, the power of discriminating two points 
 
 1 Zeitschrift fur Psychologie, 1901, XXVI, 33, and 1902, XXIX, 161. 
 
 2 See Judd, Wundt's Philosophische Studien, 1896, XII, 409. 
 
THE "TWO-POINT THRESHOLD" 401 
 
 from one may be practically abolished, while nevertheless the power 
 of localizing single touches may remain excellent, being provided 
 for by the subcutaneous sense, which possesses good powers of 
 localization but none of spatial discrimination. 1 
 
 24. We conclude, then, that the discrimination of two points 
 applied to the skin is not simply related to the system of local signs, 
 but is, as we should expect, closely bound up with other mental 
 factors. Of these may be mentioned attention, practice, sugges- 
 tion, and association. Under the head of attention we note that 
 persons unfamiliar with the compass test suffer much more than 
 trained observers from the strain to which they are subjected; 
 previous mental or bodily exertion seems to make it difficult to 
 adapt the attention to the rather unusual demands of the test. For 
 this reason this method has been regarded by some as a suitable 
 device for testing mental and muscular fatigue. 2 The direction 
 of attentiorfTs also an important factor in determining the threshold; 
 for if the observer seeks to interpret the impression of breadth that 
 arises from two points which are applied close together, he may 
 translate this into terms of double touch, whereas if he insists 
 a clear impression of doubleness, he needs to have the points much 
 farther separated. 3 Such differences in the direction of attention 
 probably account in part for the enormous individual differences 
 which seem to exist in the fineness of this kind of perception. A. W. 
 Volkmann 4 showed the remarkable effect of exercise upon the culti- 
 vation of the sense of locality. After fixing the value of the least 
 perceivable differences of locality for a number of small areas in 
 the field of touch, Volkmann found that each successive series of 
 experiments with each area increased its fineness of perception, 
 until within a few hours twice the original degree of fineness could 
 be reached. The growth in perceptive skill of the skin was slower 
 at first for areas not ordinarily used for touch; quicker for those 
 accustomed to daily use. The improvement ceased at a certain 
 limit, and was soon lost by disuse, so that a few months out of 
 
 1 Compare Head and Sherren, Brain, 1905, XXIX, 109; Spearman, British 
 Journal of Psychology, 1905, I, 286. 
 
 2 Griesbach, Archiv fur Hygeine, 1895, XXIV, and Internationales Archiv fur 
 Schulhygeine, 1905, I, 317. Griesbach introduced the two-point threshold as an 
 index of mental fatigue, especially in school pupils. He asserts that the two 
 points must be more widely separated after mental work, in order that they 
 may be felt as two. Of others who have tried the method, some regard it as 
 valid, but many have got only negative results. See a summary of this and other 
 points in the recent literature by Spearman in Archiv fur die gesammte Psychol- 
 ogic, 1906. 
 
 3 Binet, Annee Psychologique, 1903, IX, 199. 
 
 * Berichte d. sdchsischen Gesellschaft d. Wissenschaften, 1858, pp. 38 f. 
 
402 PRESENTATIONS OF SENSE 
 
 practice served to reduce the acquired tact of any area to its origi- 
 nal condition. A most surprising discovery of this experimenter 
 was, that the practice exclusively of a member of the body on one 
 side resulted in improving the fineness of touch of the correspond- 
 ing member of the other side. Thus, if the smallest perceivable 
 distance for the tip of a left finger was, to begin with, 0.75 line, 
 and that of the corresponding place on the right finger, 0.85, prac- 
 tice with the left finger exclusively reduced the distance for both 
 fingers for the left to 0.45 line, and for the right to 0.4. 
 
 It is well known that the blind, who have no spatial series of 
 sensations or presentations of extended objects by the eye, attain by 
 exercise a high degree of fineness for certain space-perceptions of 
 the skin. 1 In the case of those who have sight, the most movable 
 and discriminating organs of the skin such as the tips of the fin- 
 gers are capable of being cultivated to great delicacy of touch; 
 but Funke 2 did not succeed, even by an education lasting an en- 
 tire month, in reducing the obtuseness of the skin of the back be- 
 tween the shoulder-blades and in the lumbar region more than by 
 about one-fourth. 
 
 25. These earlier results are subject to some modification in 
 consequence of later work. The rapid improvement with train- 
 ing is ascribed by Tawney 3 to the influence of suggestion, while 
 Judd and Von Frey and Metzner found that the improvement did 
 not occur in the form of the experiment in which the two points 
 are excited successively. Probably the improvement which occurs 
 in the case of simultaneous application is due to the acquisition of 
 skill in interpreting the broad impression produced by exciting two 
 near-by points. 
 
 The influence of suggestion and of chance associations has been 
 brought out clearly by Solomons 4 and by Messenger, 5 who, by syste- 
 matically mistraining an observer, were able to induce a condition 
 in which false judgments were the rule, and in which there might 
 even be a complete reversal of the judgments "one" and "two." 
 The fact is that the impressions derived from two points simul- 
 taneously excited appear in consciousness as a single, blended im- 
 pressionj'which, however, differs slightly from that produced by the 
 excitation of one point; and the more distant are the two points, 
 the more the difference in the two impressions increases. When the 
 two points are quite far apart, it becomes possible to single out 
 
 1 Compare Czermak, Sitzgsber. d. Wiener Acad., XVII, Abth. ii, pp. 563 f. 
 
 2 See Hermann's Handb. d. PhysioL, III, ii, p. 382. 
 
 3 Wundt's Philosophische Studien, 1898, XIII, 163. 
 
 4 Psychol. Rev., 1897, IV, 249. 
 
 8 Psychol. Rev., Monog. Suppl., XXII, 1903. 
 
SENSUOUS BASIS OF DISCRIMINATION 403 
 
 either of them and devote attention to it separately, and thus the 
 perception of two is clear. When the points are too near to permit 
 of either being singled out in this way, it may still be true that the 
 total impression of the two-point stimulation differs perceptibly from 
 that of one-point stimulation, and these impressions may then be 
 associated with the numbers one and two, so that correct judgments 
 of the stimulus will be established. 
 
 In the case of the blind, though their skill in judging the shape 
 of objects by touch is highly developed, it does not appear that their 
 two-point threshold is specially low. 1 Their skill in touch judg- 
 ments must, therefore, be the result of practice in interpreting the 
 total impressions derived from objects of different size and shape. 
 
 26. Since our experience shows that localization of the different 
 minute areas of the skin includes not only the existence of discrim- 
 inable local signs, but also an active process of discrimination, any 
 physiological theory of this class of perceptions must include some 
 attempt to account for the nervous correlates of the process of dis- 
 crimination itself. The fact that discrimination is easier when the 
 stimuli are presented successively than when simultaneously holds 
 good not only in the case here under consideration, but also in many 
 others, such as the discrimination of w r eights or of tones. The 
 transition from one stimulus to another following it may produce 
 a "shock of difference " 2 even though the two stimuli blend when 
 applied simultaneously. Physiologically considered, this blending 
 may be related to the convergence and summation of two compati- 
 ble stimuli which was seen to occur in the case of reflex action (see 
 pp. 161 f.). The confluence of sensory impulses from two excited 
 points must be less complete in the case of perception than in the 
 case of reflex action; otherwise, it would seem, no discrimination 
 would be possible. To account for the facts of the two-point 
 threshold, Bernstein 3 proposed a theory which has much in its 
 favor. He conceived that the sensory impulses reaching, let us say, 
 the somesthetic area of the brain did not impinge simply on a single 
 point, but were distributed over a certain neighborhood most in- 
 tensely, however, to a central point in this neighborhood, and less 
 and less intensely to more and more distant parts of the same. The 
 distribution of impulses from two near-by points in the skin might 
 therefore overlap in the brain to a greater or less degree. Where 
 the overlapping was but slight, there would be two points of maxi- 
 
 1 Haines, Psychological Review, 1905, XII, 207. 
 
 2 See James, Principles of Psychology, 1890, I, 495. 
 
 3 Untersuchungen uber den Erregungsprozess im Muskel- und Nervensystem 
 (Heidelberg, 1870). See also Thunberg in Nagel's Handbuch der Physiologic, 
 1905, III, 720; and Myers, Textbook of Experimental Psychology, 1909, p. 235. 
 
404 PRESENTATIONS OF SENSE 
 
 mum activity within the somesthetic area, and this condition would 
 favor discrimination; where the overlapping was considerable, on 
 the contrary, there would be only one point of maximum activity, 
 and therefore no possibility of a true sense of double stimulation; 
 though, if the combined distribution was broad, the total impression 
 might be recognizably different from that of a single point, with its 
 narrower cerebral distribution. To account for the fact that two 
 points can be discriminated at a much less distance on some por- 
 tions of the skin than on others, we make the probable assumption 
 that the cortical area connected with a highly sensitive region of 
 the skin is broad, and that connected with a less sensitive region 
 narrow, in comparison with the corresponding areas of the skin. 
 Accordingly, the cortical overlapping of impulses need be no greater 
 from closely adjacent points on the finger-tips than from widely 
 separated points of the back. Such a theory does not pretend to 
 explain the process of discrimination, but only one of the nervous 
 correlates or prerequisites of discrimination; other conditions also 
 must be met in order that discrimination may occur, for two points 
 need not be distinguished though separated by a very considerable 
 distance. On the psychological side, the process corresponds to 
 what, when exercised in a more deliberate way, we call the "weigh- 
 ing of data" before "making up the mind" to an act of judgment. 
 
 27. Closely connected with the foregoing is the difference of 
 different parts of the skin in furnishing data for discriminating the 
 fact, the amount, and the direction of motion in contact with the body. 
 Upon this point the experiments of G. Stanley Hall 1 are of special 
 interest. These experiments seem to show that we are more likely, 
 when in doubt, to judge motion on the surface of the limbs to be 
 up rather than down their axis; on the breast, the shoulder-blades, 
 and the back, the tendency is to judge motion to be toward the 
 head. The discriminative sensibility of the skin for motion is 
 much greater than that for separate touch, as determined by We- 
 ber's experiments. Thus, while at least a distance of 25 mm. be- 
 tween the dividers' points was needed on the volar surface of the 
 right arm, in order to perceive them as two points, both the fact 
 and the direction of motion could be discriminated at an average 
 distance of between 6 and 7 mm. In judging the rate and distance 
 of motion over the skin the liability to error is always great; but, 
 as a rule, distances rapidly traversed are judged to be relatively 
 shorter than the same distances more slowly traversed. Inasmuch, 
 however, as the judgment of motion on the left arm was expressed 
 by reproducing the rate and distance with the right hand, we have 
 
 1 Motor Sensations on the Skin, by Professor G. S. Hall and Dr. H. H. Donald- 
 son, in Mind, October, 1885, pp. 557 ff. 
 
MIXED AND TANGLED SKIN SENSATIONS 405 
 
 a double liability to error involved in regulating the muscular move- 
 ment of this hand by means of its series of muscular and tactual 
 sensations. 
 
 Hall found the motor sensibility of different parts of the surface 
 of the skin to be different; but the differences do not appear to 
 correspond to those belonging to Weber's sensation-circles. The 
 average distance, in millimetres, which a metallic point of 12 mm. 
 in diameter could move over the skin at a rate of 2 mm. per second 
 before a judgment of direction could be formed was found, for one 
 subject of experiment, as follows: forehead, 0.20; upper arm, 0.40; 
 forearm, 0.44; shin, 0.60; palm, 0.74; back, 0.85. Motion can be 
 produced so slowly as not to be discriminated at all, even when the 
 body in contact has really moved from 6 to 12 centimetres. It can 
 also be produced so rapidly as to make it impossible to tell when it 
 begins and when ends. Heavy weights seem to move faster than 
 light ones going at the same rate; but here other sensations are 
 called out by the deep pressure, and combined with those of con- 
 tact. Hall concludes that heat-spots and cold-spots traversed by 
 the moving body are of great service in judging motion and its di- 
 rection on the skin; the cold-spots more than heat-spots, "be- 
 cause of the fainter sensation and wider irradiation" of the latter. 
 
 Further experiments with a travelling metallic point that carried 
 the stimulus of an electrical current over the surface of the skin 
 showed an astonishing diversity of sensations developed at different 
 points of the area thus traversed. Points of cutting pain, "thrill- 
 points," "tickle-points," "acceleration-points" (or places where the 
 rate of motion seems suddenly to increase without any real change 
 in the speed of the moving metal), "blind-points" (or spots where 
 all impression of contact is momentarily lost), are all to be differen- 
 tiated. Yet the sharp differentiation of these sensations is ren- 
 dered difficult by the fact that the various kinds are so impacted 
 and run together, in a tangle of sensation. The experimenters also 
 speak as though many dermal sensations may thus be partially dis- 
 entangled, for the description of which language furnishes no ade- 
 quate terms. All these facts agree exceedingly well with the theory 
 of local signs already proposed. These dermal signs are complex 
 "mixtures" of feeling, which give to each discernible locality a 
 characteristic local stamp. The fact that our sensibility to motion 
 is so much greater in each area of the skin than our susceptibility 
 to the distance of stationary points accords with the same theory. 
 Our ability to localize the dermal sensations is dependent upon the 
 degree and rate of the changes in the color-tone of these sensations. 
 Hall is undoubtedly right in holding that, by moving the touch- 
 ing surface over the surface touched, we do not simply multiply, but 
 
406 
 
 PRESENTATIONS OF SENSE 
 
 also diversify, our data for filling up the dermal blind-spots and 
 judging the nature of impressions. 
 
 28. The localizing of sensations of temperature in the skin is, 
 in principle, the same as that of sensations of light pressure or of 
 motion. The former, however, are in all our ordinary experience 
 interwoven with the latter; they therefore have the help of the lat- 
 ter in getting a place assigned to them in the periphery of the body. 
 Goldscheider 1 experimented to determine how far apart the heat- 
 spots and cold-spots must be, respectively, in order that two of 
 them, when stimulated, may be felt as two. Both kinds of sensations 
 are localized, not as points, but as minute warm or cold drops in 
 contact with the skin. By the following table, which gives the 
 minimum distances for different areas of the body, it appears that 
 the sense of locality connected with the cold-spots is about twice 
 as fine, as a rule, .as that connected with the heat-spots. The dis- 
 tances are given in millimetres. 
 
 
 Part of the body 
 
 Cold-spots 
 
 Heat-spots 
 
 Forehead, cheek, 
 
 
 8 
 
 3-5 
 
 Breast 
 
 
 2 
 
 4-5 
 
 Abdomen . 
 
 
 1-2 
 
 4-6 
 
 Back. . . . 
 
 
 1 5-2 
 
 4-6 
 
 
 
 1.5-2.0 
 
 2-3 
 
 Lower arm 
 
 
 2-3 
 
 2-3 
 
 Hollow of the ha] 
 
 id 
 
 0.8 
 
 2.0 
 
 Back of the hand 
 
 , and upper and lower leg . . . 
 
 2-3 
 
 3-4 
 
 29. Some basis seems to be laid in the foregoing facts for a sys- 
 tem of local signs of the skin, that consist in a mixture of color-tones 
 and temperature-sensations. Yet sensations of heat or cold, in them- 
 selves considered, differ chiefly, if not wholly, in intensity. In 
 themselves, therefore, they are not well fitted to constitute a so-called 
 "spatial series" of sensations. If, for example, a certain area of 
 the skin be stimulated simultaneously by both heat and cold, at 
 points too near together to be distinguished by touch, the result is 
 neither a modification of one sensation by the other nor a localizing 
 of the two sensations as lying closely side by side. 2 A wavering of 
 perception rather takes place, similar to the strife of colors in vi- 
 sion; the experience is as though the skin were being touched with 
 
 1 Archiv /. Anat. u. Physiol., Physiolog. Abth., 1885, Supplement-Band, pp. 
 70 ff. 
 
 2 See Czermak, Sitzgsber. d. Wiener Acad., March, 1855, p. 500; confirmed by 
 Klug and others. 
 
NATURE OF THE MUSCULAR SENSE 407 
 
 a single body alternately hot and cold. Klug also found that the 
 least observable distance between two points touching the skin at 
 the same time depends upon their temperature relative to that of 
 the skin. The fineness of our sense of locality, as well as of our 
 sensitiveness to motion, is increased by exciting sensations of tem- 
 perature up to the point where pain intervenes. But the localizing 
 of these sensations is primarily dependent, to a great extent, upon 
 their connection with localized sensations of touch. If we bring 
 two parts of the skin, that differ considerably in temperature, into 
 contact for example, a cool hand and warm forehead, or a cool hand 
 and a warm one it is often difficult by strict attention to the sensa- 
 tions of temperature alone to tell which part is cooler, which warmer. 
 The difficulty is doubtless largely due to the fact that each part 
 which feels the temperature of the other is also changing its own 
 temperature in the direction of the temperature of the other. A 
 confusion of the data for judgment, accordingly, takes place. 
 
 Any localization of the sensations which occurs under such cir- 
 cumstances is largely dependent upon secondary considerations, 
 and especially upon the direction of the attention. We judge of 
 depth by sensations of temperature, indirectly, and through our 
 ability to remove or change the intensity and locality of these sensa- 
 tions by changing the position of the body in space as related to 
 what we know to be hot and cold bodies or surrounding media. 
 
 30. The specific sensations of the muscular sense constitute 
 another spatial series which combines with the foregoing in the 
 localizing of areas at the periphery, and of external objects as 
 projected in space and yet known as in contact with the body. In- 
 deed, it is upon this particular system of local signs that the mind 
 is chiefly dependent for its data other than the visual in the 
 synthetic construction of its presentations of bodies that stand re- 
 lated to each other in three dimensions in objective space. Three 
 principal theories have been held as to the nature of the so-called 
 muscular sensations: (1) They are to be resolved into "central feel- 
 ings of innervation," which differ only in intensity and not in specific 
 quality, and which result from the changes, initiating movement of 
 the bodily organs, that take place in the brain as correlated with 
 impulses of the will; (2) they are not specific sensations, but are due 
 to interpretations of those feelings in the skin which originate on 
 account of its changes of position, tension, etc., as the underlying 
 muscles are moved; (3) they are specific sensations dependent on 
 a nerve-apparatus of sense, which has its end-organs in the muscle- 
 fibre, and which is excited by the contraction of the latter in a man- 
 ner dependent upon the kind, amount, and direction of the mus- 
 cular movement taking place. 
 
408 PRESENTATIONS OF SENSE 
 
 We have already given certain reasons for preferring the last of 
 the foregoing views; other reasons are implied in considering the nat- 
 ure of what has been called the " feeling of innervation " or of ' 'active 
 energy." The muscular sense, like all the other senses which con- 
 tribute to our presentations of objects extended in space, appears 
 to have its own system of local signs. The muscular sensations are 
 qualitatively (and not merely quantitatively) different, according to 
 the combination of the muscles moved, and according to the exten- 
 sion over the muscular area of the stimulus imparted to the sensory 
 nerve-fibres situated in the muscle by the changing condition of the 
 latter as it contracts and relaxes. At each step in the flexing of the 
 leg for example the muscular sensations have a specific quality 
 and value as local signs, in our consciousness, of the position of the 
 member. The same thing is true of the bending arm, back, or 
 single toe or finger. These sensations are indeed intimately, and 
 even inextricably, combined with the spatial series of specifically 
 dermal sensations; but in themselves they have a different quality, 
 and are not localized simply at the surface of the body. As the ex- 
 tent of the circuit of motion gone through by any limb increases, or 
 the intensity of the strain becomes greater, the quality of the mass of 
 resulting muscular sensations is perpetually changing. These 
 sensations are, accordingly, localized over a broader area of the body 
 and deeper in its substance, as it were. Every one knows what new 
 mixtures of sensation are produced in consciousness by calling into 
 vigorous exercise the unused more deeply lying muscles of the body. 
 
 The muscular sensations also assist the more strictly tactual in 
 discriminating locality for all cases where the pressure upon the 
 skin exceeds a certain small degree of intensity. In strong contact 
 or heavy pressure the sensory nerves of the underlying muscle are 
 excited; we have the feeling, not simply of being touched, but also 
 of being pressed. The combination of these two spatial series 
 gives to the mind a doubly constituted system of local signs; hence, 
 as the experiments of G. Stanley Hall 1 show, our judgment of di- 
 rection of motion is quicker as the weight resting on the skin is 
 increased up to the limit where other disturbing sensations inter- 
 vene. The superior discriminating power which any member of 
 the body has when permitted to move that is, to call forth fa- 
 miliar series of muscular sensations is largely due to the help 
 which the local signs of this system render to the mind. When 
 the particular member (the hand) which is capable of the nicest 
 tactual discrimination is also permitted to move over an object 
 freely, and to acquire abundant data from all the sources described 
 above, we have fulfilled the most advantageous conditions for the 
 l Mind, October, 1885, p. 567. 
 
JUDGMENT OF BODILY MOVEMENTS 409 
 
 utmost nicety of knowledge possible to "touch," in the widest 
 meaning of the word. 
 
 31. In point of precision, judgments of extent of bodily move- 
 ment far surpass judgments of extent based on cutaneous sensation 
 alone, but are inferior, in their turn, to judgments of extent by the 
 eye. The perception of extent of movement is one among several 
 judgments which may be passed on the movement of a limb, since 
 the direction, duration, and speed of the movement, the resistance 
 encountered, and the end-positions of the limb can all be judged 
 with considerable accuracy. The perception of any one of these 
 characteristics of a movement is usually confused by introducing 
 irregularities into any other of them. Thus, movements by different 
 limbs, or by the same limb in different positions or directions, can- 
 not be so accurately compared in regard to their extent as when 
 the two movements to be compared are as nearly as possible dupli- 
 cates of each other in all respects. Systematic or "constant" er- 
 rors often creep in when judgments of extent are attempted in com- 
 paring diverse movements. Thus, a slow movement seems longer 
 than a fast movement of equal extent. 1 But, curiously enough, a 
 movement made against resistance seems no longer than a free 
 movement of the same extent. 2 This last result seems to prove 
 that the extent of the movement of a limb is not judged in terms of 
 the muscular work performed in executing it; while the close in- 
 terrelation of the speed of a movement and its apparent extent, 
 along with other facts, has been thought to indicate that extent was 
 judged in terms of the duration of a movement. On this point 
 opinions have differed widely, but there seems to be no cogent 
 reason for singling out the duration of a movement as that on which 
 the judgment of extent is based. It is more probable that the judg- 
 ment of extent, like that of duration and also of speed, is based di- 
 rectly on the entire complex of sensations which is produced by 
 moving the limb. 3 
 
 32. The fineness of the spatial perception connected with the 
 "muscle-sense" is of great significance in forming a just concep- 
 tion of "touch" as an organ of space-perception. Seldom, in the 
 common use of touch for discovering the form and size of ob- 
 jects as in the dark do we attempt to rely on cutaneous im- 
 pressions alone; we handle the object, bringing into play sensations 
 of movement and position as well as sensations from the skin. Such 
 
 Pfliiger's Archiv fur die gesammte Physiologic, 1887, XLVI, 1; Dela- 
 barre, Ueber Bewegungsempfindungen (Freiburg i. B., 1891). 
 
 2 Delabarre, op. cit.; Angler, Zeitschrift fur Psychologic, 1905, XXXIX, 429. 
 3 Compare Hollingworth, The Inaccuracy of Movement, 1909, p. 40. 
 
410 PRESENTATIONS OF SENSE 
 
 perceptions are not tactile alone, but "stereognostic." 1 The per- 
 ception of the shape of objects simply laid or pressed on the skin 
 is much inferior to the judgment based on handling them. Hence 
 there seems to be sufficient evidence for our view, 2 which is also 
 that of Wundt 3 namely, that "touch space'' in so far as it can be 
 conceived as independent of visual space, results from a union, fusion, 
 or synthesis of cutaneous sensation with sensations of bodily move- 
 ment and position. 
 
 33. It is unnecessary to illustrate further the process by which 
 the mind's native activity of discrimination, with the help of quali- 
 tatively different sensation-complexes, or local signs, constructs its 
 field of touch. The localization of certain points in the area of the 
 body which are of marked local characteristics, and frequently re- 
 current in experience, is the first achievement in constructing this 
 field. To these landmarks, as it were, other points or areas, sub- 
 sequently discovered, are referred. One hand learns to know the 
 other; the right hand chiefly explores the left arm and side and the 
 upper right leg; the left hand, the right arm and side and the upper 
 left leg. The finger-tips, especially of the right hand, have an 
 office similar to that performed by the yellow-spot of the retina; 
 they are the centre or hearth of clear perceptions of touch. But in 
 order to bring them to their object they must be moved; through this 
 motion fresh combinations of muscular and tactual sensations re- 
 sult. But long before the entire field of touch has been constructed 
 with any considerable approach to completeness, the eye has al- 
 ready explored those parts of the body which are open to its in- 
 spection. It learns first to know the hand, which nature keeps 
 constantly in motion before it. As objects rest on the hand, it 
 notes the place where they rest; with its perceptions of sight cer- 
 tain combinations of tactual sensations thus become associated. As 
 the hand moves over other objects, or especially over the other parts 
 of the body, the eye marks its successive progress; combined sen- 
 sations of muscular and tactual kind are thus associated with each 
 position of the hand and with each area of the body which it touches. 
 Very early in the development of a normal experience the eye comes 
 to be the leader and critic of the discriminations connected with 
 the muscular and tactual sensations. Its power of rapid movement 
 over its total field, and its delicate judgment on account of the 
 finely shaded complex local signs which it calls forth with a com- 
 prehensive simultaneousness, give a great superiority to the organ 
 of vision as a geometrical sense. The results of such superiority it 
 
 1 Hoffmann, Stereognostische Versitche (Strassburg, 1883). 
 
 2 Compare Ladd, Elements of Physiological Psychology, 1st ed., 1887, 417 f. 
 
 3 Physiologische Psychologic, 6th ed., 1910, II, 517. 
 
FEELINGS OF DOUBLE CONTACT 411 
 
 constantly places at the disposal of the more slowly moving and 
 less delicate sense of touch. For this reason, one born blind can 
 never attain the same quality (of "comprehensive simultaneous- 
 ness") for his spatial intuitions and ideas of spatial relations; even 
 the field of touch, in spite of the greater refinement which the 
 muscular and tactual sensations of such an unfortunate person ac- 
 quire through use, cannot possess this quality as it is imparted by 
 the eye. 
 
 The familiar experiments of trying to estimate the size, shape, 
 and relation of objects, the amount and direction of motion, etc., 
 when blindfold, show our dependence upon the organ of sight. It 
 must not be forgotten, however, that the discriminations possible 
 through the muscular and tactual sensations alone are wonderfully 
 exact; and that in certain circumstances touch has sight at a dis- 
 advantage, as it were. Thus the player on the violin who should 
 adjust his spacing of the strings by the sensations of the eye, with 
 the unaccustomed and unfavorable perspective made necessary by 
 its position in relation to the left hand, would not attain the art of 
 making true and pure tones. 
 
 34. Among the most complex perceptions of which the skin 
 and muscles by their combined action are capable are the so-called 
 "feelings of double contact." It is largely by means of these feel- 
 ings that skill is acquired in the use of tools, weapons, and musi- 
 cal instruments. In these cases the process of projection goes so 
 far that we seem to feel the object with which the implement is in 
 contact, not so much in the hand (the feelings of contact being 
 located there), by the external means of the implement, but rather 
 as ourselves being in the implement and using it as a sentient part 
 of the organism. The carver in wood feels his chisel move through 
 the stuff he is shaping, and guides it as unerringly as he would his 
 finger, so as to lay it with a given degree of pressure upon a given 
 spot. We are all familiar with the experience of feeling the ground 
 we are about to tread, with a cane or other stick. If the fingers be 
 lightly brushed over the hair when it stands out from the head, it 
 will be difficult to localize the sensations of pressure at the scalp 
 rather than in the hair. We feel the touch of our finger at the end 
 of the tooth, where the contact takes place, instead of where the 
 sensory nerves really receive the stimulus and convert it into a 
 nerve-commotion. 
 
 The management of the implement is, of course, really made 
 possible by delicate changes in the shades of feeling called out by 
 its changing pressure upon the nerves terminating in the skin and 
 muscles of the hand, and by the accompanying feelings of strain 
 and of effort that result from the movement of the arm which 
 
412 PRESENTATIONS OF SENSE 
 
 carries the hand. These feelings are aroused by the end of the 
 implement which is in contact with the body, and are primarily 
 localized in that part of the body; but they are felt through a more 
 artificial and elaborate process of localization, as though directly 
 dependent upon the other end of the implement. Upon the aes- 
 thetic and pleasurable uses of these feelings of double contact Lotze 1 
 has remarked at length. 
 
 At this point the further discussion of the development of our 
 presentations of sense in general must be arrested, in order to con- 
 sider more in detail the activities of the other great "geometrical 
 sense." 
 
 1 See Microcosmus, i, pp. 586 ff. (Edinburgh, 1885). 
 
CHAPTER V 
 
 PRESENTATIONS OF SENSE; OR SENSE-PERCEPTIONS (CONTINUED) 
 
 1. The discussions of the last chapter as to the data furnished in 
 the form of complex sensations, and as to the mental activities in- 
 volved in the discrimination, association, and interpretation of these 
 data, for the localization and knowledge of objects of sense through 
 the skin and the muscles, must have convinced us that the problem 
 which nature solves with such apparent ease in a practical way, is 
 exceedingly difficult perhaps impossible of a complete theoretical 
 solution. Moreover, the application of the general principles which 
 control the development of our sense-experience to the particular 
 case of the eye has many peculiar difficulties. The physiological 
 psychology of visual perception is, therefore, a much controverted 
 and very obscure domain. This fact is doubtless in part due to 
 the amount of experimenting and speculating which has been be- 
 stowed upon it; but peculiar difficulties are intrinsic in the case of 
 the eye. These are caused by the great complexity of its native ac- 
 tivities, and by the speed with which it reaches a generous maturity! 
 of development. Nature has equipped this organ with superior 
 means for furnishing to the mind a variety of data, as respects both 
 quantity and quality, for the nicest discriminations; it has also pro- 
 vided it with such constant stimulation as to cause it to acquire an 
 incomparable facility. The character of its structure, functions, 
 and development is, therefore, such as to make it difficult to dis- 
 entangle the simple factors from those complex forms into which 
 the synthetic activity of the mind has constructed them. 
 
 It is affirmed by one authority 1 that no less than eight different 
 data, or motifs, are used even in monocular vision by the adult for 
 perceiving the third dimension of space and of visual objects in 
 space. These are the changes with respect to (1) extent and (2) 
 clearness, of the complex of the sensations of color and light, as de- 
 pendent on distance; (3) the perspective elevation of the bottom 
 
 1 Volkmann von Volkmar, Lehrb. d. Psychologic, II, p. 84. 
 413 
 
414 PRESENTATIONS OF SENSE 
 
 of distant objects above the horizon; (4) the covering of known dis- 
 tant objects by those placed nearer; (5) the alterations of light and 
 shadow on the curved surfaces of the object, according as they are 
 nearer or more remote; (6) the perspective contraction of the retinal 
 image; (7) the change of the visor angle in proportion to the dis- 
 tance of the object; (8) the muscular sensations of the accommoda- 
 tion of the eye. To these eight data, two others at least must be 
 added for binocular vision namely (9), the stereoscopic double 
 images, and (10) the sensations arising from convergence of the 
 axes. These ten sets of variable experiences may be combined, of 
 course, in an almost infinite variety of proportions. 
 
 Moreover, it is not improbable that we shall have to admit still 
 other data as entering into the complex perceptions of sight. The 
 question must be raised: Do not the visual sensations themselves 
 have a certain local coloring directly dependent upon the nervous 
 elements of the retina which are excited by the stimuli? If we 
 answer this question affirmatively, we shall have a system of local 
 retinal signs as constituting one of the most primary of the spatial 
 series of sensations entering into the space-perceptions of this sense. 
 
 2. Several of the data just enumerated, however, are plainly of 
 only secondary rank and value; they do not necessarily enter into 
 every perception of a visual object as such. What does seem neces- 
 sary to the most elementary form of visual perception may be stated 
 as follows: Sensations of light and color, differing in intensity and 
 quality, but simultaneously present in consciousness, must be syste- 
 matically arranged with reference to each other by being localized with 
 the help of retinal signs. For any development of visual percep- 
 tion as the adult human being has experience of it, we must add: 
 These sensations must be associated with other spatial series of mus- 
 cular and tactual sensations that arise from accommodation of the 
 eye and from its position and motion. The complexity of the com- 
 binations arising in the normal use of the organ of vision is, of course, 
 increased by the fact that there are two eyes, and, therefore, two 
 retinas with their systems of retinal signs, two images of each object, 
 and two sets of motions. But the two eyes are (as we shall see sub- 
 sequently) in a certain sense to be regarded as one eye certainly 
 as constituting one organ of vision. So that, even when one eye is 
 closed, the other does not see what it sees without being influenced 
 by the closed and relatively inoperative part of the one organ. The 
 constancy with which the eyes act together explains, in part, why 
 they are one organ as the two hands are not; but the frequency with 
 which we voluntarily suppress the activity of one eye by closing it 
 explains, in part, why they are not one organ as are the two nostrils 
 or the two ears. 
 
DATA OF VISUAL PERCEPTIONS 415 
 
 3. Could we select an adult human being who had never seen, 
 and proceed to develop his visual perceptions, experimentally, in 
 the direct order of their complexity, we might possibly rely upon 
 his description of his experience to solve certain problems that now 
 seem unsolvable. At present, however, it is quite impossible to 
 say what the experience of the subject of such experiments would 
 be. Nothing remains, then, but to employ the data which physi- 
 ological optics has secured, in order to make a theoretic reconstruc- 
 tion (confessedly imperfect and doubtful) of the process that nature 
 is all the while successfully completing. In this effort we naturally 
 follow the order of nature, so far as possible; beginning with the 
 simplest conceivable case (this is substantially the course followed 
 by Wundt), we find three things to be considered in explaining the 
 developed perceptions of sight: (1) The retinal image of the eye 
 at rest, and the motifs which it furnishes; (2) the single eye as 
 moved, and the influence of these movements; (3) the conditions 
 furnished by the existence and relations of the two eyes exercis- 
 ing their functions in common. But, in reality, from the very be- 
 ginning of its activity the eye is in motion, and acts as a double organ. 
 
 Corresponding to the three sets of considerations just mentioned, 
 we may speak of three fields of vision which are to be constructed 
 in the order of their complexity. They may be called, respectively, 
 the retinal field of vision, the field of monocular vision, and the 
 field of binocular vision. In the "retinal field of vision" we mean 
 to include only such a perception or mental spatial arrangement 
 of sensations of color and light as points lying side by side as 
 would be presented through the excited expanse of nervous ele- 
 ments constituting the retina of one motionless eye, in case there 
 had been no previous vision with both eyes in motion. The field 
 of monocular vision, when completely constructed, includes all that 
 can be seen with one eye as the result of its experience, devel- 
 oped, but unaided by the other eye. The field of binocular vision 
 includes all that can be seen by both eyes. The first two so-called 
 "fields of vision" are, strictly speaking, fictitious and theoretically 
 constructed in order to explain the process by which the mind 
 reaches the construction of the third and last. Indeed, the ques- 
 tion may be pressed, whether we can speak of a purely "retinal 
 field of vision," and whether the excited mosaic of nervous elements 
 on which the image is formed, without aid from previous experi- 
 ence of sensations of position and motion, could furnish any true 
 presentations of sight, or visual perceptions of objects as they actu- 
 ally exist in space. 
 
 4. The "retinal field" has no clearly defined limits, or bound- 
 ary-lines; it may be described rather as having its expanse of sensa- 
 
416 PRESENTATIONS OF SENSE 
 
 tions distinguished by a shifting, graded transition into a region of 
 no-sensations. This fact is, of course, due to the constantly chang- 
 ing activity of the nervous elements of the retina. Yet the sensa- 
 tions which are massed in the foregoing experience constitute a 
 true spatial expanse; that is, they are not simply recognized as differ- 
 ing in color-tone, or brightness or intensity of effect, but as having 
 true local distinctions, and as being arranged into a system of points 
 of color and light lying side by side. In other words, the different 
 sensations do not fall together in consciousness so as to resemble 
 the one sensation of smell produced by irritating simultaneously a 
 number of fibres of the olfactory nerve; nor are they simply ana- 
 lyzable into several qualitatively different factors, as is the com- 
 plex sensation of a musical clang. They are presented as spatially 
 systematized. The "retinal field" at least, as it appears in adult 
 vision may, then, be said to be extended in two dimensions; and 
 the minima msibilia which compose it all have local relations to 
 each other. It cannot properly be said, however, to have depth 
 (as Stumpf 1 and Hering 2 hold that it does); for the different col- 
 ored points are not projected as different in distance, nor can we 
 be said to look into the colored space thus presented before the 
 mind. It is true that the expanse of the retinal field is not like 
 that of a darkly colored wall or curtain placed in front of the eye. 
 But the quasi-appearance of depth is due to constant change in 
 color-tone and brightness of the minute portions of the field, which 
 has an effect somewhat like that we get on looking at a very dense 
 mist of particles differently colored and drifting. In other words, 
 the secondary and derived data give to it an appearance which we 
 have learned to associate with the perception of depth. 
 
 On the other hand, the retinal field can scarcely be considered 
 as two-dimensional in a way to distinguish it from the three-dimen- 
 sional field of binocular vision, without implying a suggestion of 
 the third dimension, as though it were itself seen projected in a 
 three-dimensional space. The only safe conclusion from all this 
 is, therefore, that this simplest ( ?) form of adult visual experience 
 is so far removed from the form with which its own development 
 began, as to be no adequate representation of it. In particular, 
 it leaves the question of the influence, and even of the absolute 
 necessity, of sensations of position and motion for any visual per- 
 ception, still unsolved 
 
 1 Ueber d. physiolog. Ursprung d. Raumwrstellung (Leipzig, 1873). Stumpf 
 holds that "Space is just as originally and directly perceived as quality" (p. 
 115). 
 
 2 In Hermann's Handb. d. Physiol, III, i, pp. 572 f. 
 
FORMATION OF THE RETINAL FIELD 417 
 
 5. Further experiment, however, with this so-called "retinal 
 field" serves to show how complicated its apparently simple char- 
 acter really is. In the first place, even this field is the result of 
 the combined action of the two retinas. If, with both eyes closed, 
 a "phosphene" be produced in either eye by pressing upon its 
 ball, the colored circle will be located in the corresponding part of 
 the field; but the character of the entire field, as formed by the ac- 
 tivity of both retinas, will be changed. It is, of course, impossible 
 to suppress the action of one retina, and thus examine a monocular 
 "retinal field," as it were. But it may easily be shown that, even 
 in vision with one eye open and in motion, the character of the whole 
 field of vision is under the influence of the retinal activity of the 
 closed eye. Let one of the eyes both hitherto closed and motion- 
 less now be opened. Immediately a picture of all the objects 
 falling within the field of monocular vision appears before us; each 
 object seen with its position, magnitude, and spatial relations de- 
 termined according to the laws of visual perception. This monocu- 
 lar field seems bounded on one side (the left side if the right eye 
 is opened, the right side if the left) by the rather dim outline of the 
 nose and lower line of the forehead. What has become of the retinal 
 field of the closed eye ? It has been submerged or overwhelmed by 
 the field of the open eye, on account of the latter's stationary and 
 clearly defined images and strong arrest and fixation of attention. 
 But if a character to arrest and fix the attention be given to the field 
 of the closed eye, it may be made in turn to overwhelm that of the 
 open eye. This can be accomplished by producing strong "phos- 
 phenes" in the former. On pressing the closed eye brightly col- 
 ored circles are presented in the corresponding part of the field; and 
 by using sufficient pressure the objects seen as projected in space 
 by the open eye are drowned in a shower of minute, vivid sparks. 
 
 The "retinal field" has its character determined also by as- 
 sociated muscular sensations dependent upon the movement of 
 both eyes. It will be found impossible to make any definite area 
 of this retinal field, which lies much to the right or left, to the upper 
 or lower part, of its centre, a matter of regard without detecting 
 slight movements of the eyes according to the direction in which 
 the attention is to be fixed. The value of muscular movements in 
 this case cannot consist in their enabling a clear image of objects 
 situated in different relations to the eye to be formed on its retina; 
 for with closed eyes no change is occasioned in the retinal images 
 by motion of the eyes. Moreover, it will be found that the extent 
 of this entire field and its prolongation, as it were, in any given di- 
 rection are dependent upon the accommodation and motion of the 
 eyes. 
 
418 PRESENTATIONS OF SENSE 
 
 6. The foregoing facts undeniably afford considerable support 
 to the "empiristic" theory of visual perception; but they do not 
 show that the considerations it brings forward are entirely con- 
 clusive. It would seem that, after excluding all the factors which 
 combine into our ordinary presentations of sight such as double 
 images, accommodation, convergence of the axes of the eyes, and 
 secondary helps by way of shadows, perspective, elevation, etc. 
 a certain spatial quality still remains to the simplest sensations of 
 color and light which we are able to reproduce. It will naturally 
 be objected that these sensations are the reactions of a mind that 
 has had a long previous experience in localizing visual sensations 
 by means of just such helps as the foregoing. The question then 
 recurs: Is the fact that the sensations of light and color, which are 
 produced by the simultaneous excitation of many nervous elements 
 of the retina, appear as locally distinct (even when the eyes are closed 
 and motionless) an otherwise unexplained datum due to an original 
 activity of the mind under the law of the specific energy of these 
 nervous elements; or is it a result of acquired experience, to be 
 explained by the revival of images of previously associated impres- 
 sions obtained when the eyes were both open and moving? To 
 take the former position is to adopt, so far forth, the nativistic 
 theory of visual perception; to take the latter is to espouse the 
 empiristic opinion. Either position has its difficulties. The for- 
 mer seems to us, however, nearer to the ultimate truth. 
 
 7. That the sensations of light and color occasioned by stimu- 
 lating different elements of the retina have a different value in 
 consciousness, and that the recognition of this value, and the pres- 
 entation of the sensations as locally separate and arranged into 
 a spatial system, is native to the mind, may be argued from the 
 following among other reasons: The peculiar mosaic structure of 
 the retina is obviously the fundamental cause for the pre-eminence 
 of the eye as a "geometrical sense." It has already been shown 
 (chap. VIII, 15) that each element of this structure may be regarded 
 as an isolated sensitive spot, which corresponds, on the one side, to 
 individual irritations from the stimuli, and, on the other, to the 
 smallest localized sensations of light and color. But the latter part 
 of this statement could not be true unless each of the elements in 
 this nervous mosaic had a certain peculiar representative value in 
 consciousness. In other words, sensations of light and color are 
 localized in part, at least, by means of the specific local quality 
 which belongs to the result of the different points in the retina 
 being simultaneously irritated. The very construction of this or- 
 gan, as well as the correspondence between its construction and 
 the nicety attained in its use for local distinctions, indicates that 
 
VALUES OF DIFFERENT RETINAL SENSATIONS 419 
 
 the spatial quality of our visual percepts depends upon its specific 
 functions. 
 
 Moreover, unless the series of light- and color-sensations had an 
 original spatial character, it is difficult to see how they could com- 
 bine with the other spatial series of the eye into perceptions of ex- 
 tended colored objects. It is difficult to see what advantage they 
 would then have over the series of musical tones varying in pitch. 
 Still further, it is as impossible to prove experimentally as it is to 
 make seem true to consciousness that the arrangement of the points 
 of light and color which appears before us with closed and motion- 
 less eyes is only the residuum, as it were, of past sensations of a mus- 
 cular kind. Such an appeal to consciousness could not be made, 
 indeed, with any confidence, if scientific analysis were able to show 
 that the color-sensations can be perceived simultaneously, as a sys- 
 tem of points lying side by side, without having the characteristics 
 of a spatial series. But in view of our inability to do this, we only 
 account for the facts of consciousness by admitting what the very 
 structure of the organ suggests, and what general psychological 
 theory seems to confirm, when we hold that spatial perception, at 
 least in germinal form, is native to the mind as a synthesis of the 
 qualitatively different sensations which result from stimulating simul- 
 taneously the retinal mosaic of nervous elements. 
 
 The foregoing view is very different from that which assumes 
 that we have an immediate knowledge of the retinal image; or 
 that a knowledge of the direction from which the light falls upon 
 the retina is an unresolvable intuition of the mind. To such mis- 
 taken statements it is a sufficient reply to show that the subjective 
 image (or mental presentation) of the object does not correspond 
 either to the image on the retina or to the real object as it is other- 
 wise known to exist in space. The mental presentation, for exam- 
 ple, has no blind-spot; it is a different representation of the real 
 object from that offered by the retinal image, with more inaccu- 
 racies than belong to the latter as seen by an observer looking 
 at it from without. To the question, then, whether sensations of 
 light and color would have space-form if they came only from an 
 excited but motionless retina, and were uncombined with other 
 sensations of a spatial series, we can give only a tentative and par- 
 tial answer. Doubtless the "presentations of sense" formed by 
 combining such sensations alone would be indescribably different 
 from those to which we now ascribe visual space-form. An animal 
 with a single immovable expanse of nervous elements susceptible 
 to irritations from light could not be said to have what we call 
 "vision." But, on the one hand, the spatial quality which belongs 
 to the visual sensations of man cannot all be resolved into muscu- 
 
420 PRESENTATIONS OF SENSE 
 
 lar and tactual sensations of eye and hand; and on the other hand, 
 sensations of light and color do have the quality which insures 
 their arrangement in consciousness in spatial order. This fact is 
 due to the working of the law of the specific energy of the nervous 
 retinal elements in connection with the native activity of mind in 
 synthesizing these sensations. The law as applied to the eye is 
 essentially the same as that already demonstrated for the skin; the 
 activity assumed as native to man is not essentially different from 
 that ascribed to the lower animals in the use of their senses. That 
 this tact for the individual has been largely won by the development 
 of the race is a proposition to which our attitude is determined by 
 more general conclusions. But physiological optics cannot account 
 for the phenomena of vision without assuming both the original 
 exercise of this tact and the theory of local retinal signs as data 
 hitherto unresolvable by its analysis. 
 
 8. Whatever may be thought of the foregoing assumptions, it 
 is certain that ordinary adult visual perception involves the motion 
 of the open eye monocular vision, of one eye and binocular of 
 both. The sensations which accompany such motion must be com- 
 bined with sensations of light and color to make the complete pres- 
 entations of sight. The consideration of the simplest case requires 
 that we should recur to the physiology of the eye. Only one small 
 spot in the retina (the so-called "fovea centralist see p. 193), is 
 capable of giving a perfectly clear image of an object. When, then, 
 we desire to see an object clearly, we bring its image upon this 
 spot and fixate it there. That point of the object to which the 
 centre of the retinal area of clearest vision corresponds is called 
 the "point of regard" (or "fixation-point"). In ordinary vision, 
 then, the eye constantly changes its point of regard, and so brings 
 successively upon its most sensitive area the images of the different 
 points of its object. 
 
 The different changes of position in the point of regard are ac- 
 complished by the six muscles of the eyeball. This wandering of 
 the point of regard over an object may be considered as due to 
 rotating the eye upon a pivotal point, or "centre of rotation," by 
 motions that have different axes of rotation. The centre of rota- 
 tion is, however, only theoretically a point, but is really an inter- 
 axial space. It has been variously located for normal eyes at about 
 13.45-13.73 mm. behind the cornea, and 1.24-1.77 mm. behind 
 the middle of the optical axes. Of such axes of rotation, three are 
 especially to be distinguished an antero-posterior, a vertical, and 
 a transverse. A line drawn from the centre of rotation to the point 
 of regard is called the "line of regard"; since each eye has its own 
 centre of rotation, there are, in vision with both eyes, two lines of 
 
MUSCLES AND MOTIONS OF THE EYE-BALL 421 
 
 obl. s sup. 
 
 regard. A plane passing through these two lines is called "the 
 plane of regard" (or "plane of vision"). 1 In the "primary posi- 
 tion" the head is erect and the line of regard directed toward the 
 distant horizon. The plane passing through the lines of regard 
 of both eyes in this position is the "primary plane of vision." In 
 this position for most eyes, however, the line of vision is inclined 
 somewhat below the horizontal plane. 
 
 Starting from the primary position, one set of positions are suc- 
 cessively assumed by moving the eye upon its transverse and ver- 
 tical axes. When the eye rotates 
 round the former, the line of regard 
 is displaced either above or below; 
 it thus makes a varying angle with 
 the line corresponding to its first 
 direction, and this is called the 
 "angle of vertical displacement" 
 (so Helmholtz), or the "ascensional 
 angle." When it moves about the 
 vertical axis, the line of regard is 
 displaced from side to side, and 
 forms with the median plane of 
 the eye a varying angle called "the 
 angle of lateral displacement." In 
 passing from the primary position 
 to the foregoing secondary position 
 no rotation of the axis itself oc- 
 curs. Another order of positions is 
 assumed by an apparent rotation 
 on the anteroposterior axis, com- 
 bined with lateral or vertical dis- 
 placements; this movement results 
 in bringing the eye to an oblique 
 position, and is really a torsion of the eye. The angle which the 
 plane of regard makes with the transverse plane measures the 
 amount of torsion, and is called the "angle of torsion." 
 
 9. The law which seems to govern the eye's movements of 
 torsion or combined movements sideways, and either up or down 
 was conjectured by Listing, whose name it bears, and elaborated 
 by Helmholtz. Listing's law is stated by Helmholtz 2 in the fol- 
 
 1 For the detailed theory of the movements of the eye, see Hering, in Her- 
 mann's Handb. d. Physiol., III, i, chaps. IX-XI; Helmholtz, Physiolog. Optik, 
 27; Wundt, Physiolog. Psychologic (6th ed.), II, 548; and Zoth in NagePs Handb. 
 d. Physiol. (1905), III, 282 ff. 
 
 2 Physiolog. Optik, p. 466. 
 
 FIG. 122. Diagram of the Attachments 
 of the Muscles of the Eye, and Their 
 Axes of Rotation the latter being 
 shown by dotted lines. The axis of 
 rotation of the rectus externus, and 
 internus, being perpendicular to the 
 plane of the paper, cannot be shown. 
 
422 PRESENTATIONS OF SENSE 
 
 lowing terms: "When the line of regard passes from its primary 
 position into any other position, the torsion of the eye (as meas- 
 ured by the angle of torsion) in the second position is the same as 
 if the eye were turned about a fixed axis standing perpendicular to 
 both the first and the second positions of the line of regard." The 
 same principle is stated in different language by Wundt: 1 "All 
 movements of the eye from its primary position take place about 
 fixed axes, each of which at the point of rotation stands at right 
 angles to the plane which is described by revolving the line of re- 
 gard; and all of these axes lie in a single plane, at right angles to 
 the primary position of the line of regard, at its point of rotation." 
 The orientating of the eye, then, for every possible position of the 
 line of regard, may be referred to a constant standard. 
 
 More detailed statement of the laws of the eye's motion in vision 
 is not necessary for the purposes of physiological psychology. It 
 needs only to be noted that the construction of the field of monocular 
 or binocular vision is a synthetic mental achievement dependent upon 
 the varying sensations which result from the wandering of the point of 
 regard over the outline of an object. Starting from its primary posi- 
 tion, the eye may come around, as it were, by a variety of circui- 
 tous paths, to the fixation of any particular point of its object. In 
 the pursuit of these paths it develops various sensations which are 
 fitted to combine into a spatial series of sensations that have the 
 value of "local signs." Thus the field of vision necessarily has the 
 same form as the surface over which the point of regard can be made 
 to wander. Its construction is a progressive synthesis of the mind, 
 stimulated and guided by means which consist in varying states of 
 consciousness. 
 
 10. Certain important consequences follow as to the relation 
 between the lines of the extended and objective "thing" and the 
 lines of the retinal image, as affording the mind data for the spatial 
 ordering of the sensations that arise from stimulating the nervous 
 elements of the eye. Only those objects which are seen by direct 
 vision (their images lying in the line of regard when the eye is in its 
 primary position) appear in their actual place; lines lying outside 
 of the vertical and horizontal meridians of the retina, in order to be 
 seen straight, must be really bent; and all really straight lines in 
 such positions are seen bent. This fact may be proved in various 
 ways. If a sheet of white paper, having a black dot in its centre 
 
 1 Physiolog. Psychologie (2d ed.), II, pp. 79 f. In the sixth edition, the form of 
 statement is somewhat changed, so as to indicate that the various positions which 
 the eye assumes are the same as they would be if reached by movements about 
 the axes so defined. The actual movements of the eye from the primary posi- 
 tion often show considerable irregularity. 
 
EFFECTS OF ACCOMMODATION 423 
 
 to serve as a point of regard, be held at right angles to the line of 
 vision, with the eye in its primary position and constantly fixed 
 upon this point, thin, straight slits of black paper outside of the two 
 meridians will appear bent. Or if the after-images left on these 
 meridians of the retina by light falling through narrow and straight 
 slits be studied when torsion of the eye takes place, these after- 
 images will themselves be found to suffer torsion. 
 
 Besides the help from sensations due to movements of the eye 
 in fixing its point of regard, account must be taken of those which 
 may possibly result from accommodation of the eye (for the mech- 
 anism of accommodation, see pp. 188 f). According to Helmholtz: * 
 "There can be no doubt that any one who has much observed his 
 own changes of accommodation and knows the muscular feeling 
 of the effort belonging to them is in a condition to tell whether, 
 when he fixates an object or an optical image, he is accommodating 
 for a great or small distance." There is scarcely greater doubt that 
 the significance of these changes would not be realized as indicat- 
 ing a third dimension of space, were they not combined with sen- 
 sations belonging to the use of both eyes in conjunction with the 
 organs of touch. Even adult judgment of distance, by accommoda- 
 tion alone, is extremely imperfect. Wundt 2 experimented to de- 
 termine the niceriess of this judgment by regarding a black thread 
 stretched vertically against a white background, with one eye 
 through an aperture in a shield. He found that almost nothing 
 could be told in this way as to the absolute distance of the thread. 
 Its relative position, however, could be discriminated with consider- 
 able accuracy by changes in accommodation; and, as might be ex- 
 pected, with more accuracy when the apparatus was called into 
 more active operation by approach of the object toward the eye. 
 We return to this subject later. 
 
 11. But all the achievements possible to a single eye, when open 
 and in motion, would not avail to produce the presentations of 
 sight as our ordinary experience is familiar with them. Strictly 
 monocular vision is for the most part a fiction of science. What 
 we can see with one eye, after experience in binocular vision, de- 
 pends upon what we have been accustomed to see with both eyes. 
 Indeed, what we see at any instant with one open eye depends, in 
 part, upon the position, motion, and retinal condition of the other and 
 closed eye. A theory of binocular vision, however, requires the con- 
 sideration of two sets of data in addition to those already enumerated. 
 These are the existence and relations of the two retinal images, and 
 the relations and laws of the binocular movements of the two eyes. 
 
 1 Physiologische Optik, 633. 
 
 2 Beitrdge zur Theorie d. Sinneswahrnehmung, pp. 105-118 (1862). 
 
424 
 
 PRESENTATIONS OF SENSE 
 
 The fact that two eyes are ordinarily active, and that there are, 
 therefore, two images of the object, is a fact of the first importance 
 for the theory of visual perception. Each eye is in itself, indeed, 
 a complete optical instrument; each has its own point, line, and 
 plane of regard, and movements of rotation, torsion, and accom- 
 modation. The two eyes, however, act normally as one instrument; 
 and yet they cannot be regarded as mere duplicates. The theory 
 of binocular vision, then, considers the two eyes acting as one. For 
 the purposes of such theory it is not important what shape the two 
 
 retinas are regarded as having; they 
 are usually taken as surfaces with 
 the curvature of the inside of a 
 sphere whose centre lies at a point 
 where all the lines of direction inter- 
 sect. It may be assumed, to begin 
 with, that this point of intersection is 
 the same for accommodation to all 
 distances of the object. If the two 
 retinas were perfectly symmetrical 
 all the nervous elements which com- 
 pose the mosaic of each one might 
 be regarded as situated at points 
 identical with those occupied by the 
 nervous elements of the other. In 
 other words, the surfaces of the two 
 retinas might be regarded as capa- 
 ble of being perfectly superimposed. 
 Upon such retinas, when the eyes 
 were parallel, each single point of an object would have its image 
 formed upon two "identical" points of the two retinas upon 
 points, that is, whose position would be mathematically the same 
 with relation to the centre of each retina. 
 
 But the retinas are not symmetrical, and the physiological centre 
 is not the true mathematical centre; moreover, the eyes, to be of use, 
 must act together in other positions than that called "primary." A 
 distinction must then be made between corresponding points and 
 identical points; the former are such as are found by experiment 
 actually, as a rule, to act together and to combine their images 
 when simultaneously stimulated. But, further, in certain cases the 
 points of the retinas which customarily act together do not so act; 
 points not exactly corresponding sometimes cover each other, and 
 points usually corresponding sometimes fail to cover each other. 
 Hence, a distinction may be made between corresponding points 
 and "covering points"; the latter term being used for those points 
 
 FIG. 123. Diagram to illustrate the 
 theory of corresponding retinal 
 points. The images of objects at 
 a" or b" or c" will fall on corre- 
 sponding points of the retina a 
 and a 1 , 6 and ft 1 , c and c 1 and be 
 seen single. 
 
CONDITIONS OF BINOCULAR VISION 425 
 
 whose impressions, in each individual case of seeing, are actually 
 referred to one and the same point of the object. 1 The two points 
 of regard of the two eyes are in all cases identical, corresponding, 
 and covering. 
 
 Scarcely more than a reference to previous elaborate attempts to 
 determine the corresponding points of the two retinas is necessary 
 for our purpose. Experiment shows that considerable reciprocal 
 substitution takes place among the different points of both retinas. 
 The eyes of most persons, if not of all, are both structurally and 
 functionally incongruous. When the lines of regard lie parallel in 
 the plane of the horizontal meridian of the two retinas, the verti- 
 cal meridians do not correspond. A vertical meridian of the left 
 eye, with its upper end inclined to the left, may be conjoined with 
 a vertical meridian of the right eye that has its upper end inclined 
 at about the same angle to the right. The image of a line which 
 lies on these meridians thus inclined appears in the vertical horizon 
 of the field of vision and divides it into a right and a left half. 
 
 12. That objects are ordinarily seen as single when their images 
 are formed on corresponding points of the retinas, and otherwise as 
 double, may be shown by many familiar experiments. If we hold 
 a finger before the eyes and look, not at it, but at the wall or the 
 sky; or if we point it at some distant object, and keep our eyes 
 steadily fixed on the object two transparent images of the finger, 
 rather than one solid finger, will be seen. Many persons may have 
 difficulty in seeing the two images, but none will fail to notice their 
 transparent character. Under these circumstances the wall, sky, 
 or distant object may readily be seen through the finger. By ex- 
 perimental methods the images of a single object may be dissociated 
 and what is really one be seen as two; on the other hand, images com- 
 ing from two objects may be combined upon corresponding points, 
 and thus what is really two be seen as one. It needs only a little 
 skilful pressure upon one eyeball to create for us the double of each 
 one of a group of friends, and to see one body partially through the 
 transparent image of another. If two objects very similar for 
 example, the two forefingers be held a little way apart at about a foot 
 distant and against a clear sky, three like objects, one solid and two 
 transparent, may be made to appear by combining the two middle 
 images and dissociating the two on the outside. Two systems of 
 regularly recurring similar objects such as a regular small pattern 
 of carpet or wall-paper, or the diamond-shaped spaces of a wire 
 grating may have all their images combined by slipping them, as 
 it were, simultaneously to one side. There is, then, a double-seeing 
 
 1 Wundt, Physiolog. Psychologie (6th ed.), II, 639. 
 
426 
 
 PRESENTATIONS OF SENSE 
 
 of what is really single and a single-seeing of what is really double; 
 but the latter is much rarer than the former, and usually occurs 
 only when brought about for purposes of experiment. 
 
 13. It is obvious that the relations of the two images of an ob- 
 ject cannot remain unchanged when the eyes are moved from their 
 primary position. When the eyes are converged upon an object, 
 the images which are formed on the central spots of the two retinas, 
 
 by rays coming from the point 
 of regard, are exactly identical 
 and corresponding; the object 
 in this case is therefore seen 
 absolutely single. Points of the 
 object lying near to the point 
 of regard in any direction, and 
 thus having their images formed 
 close to the centres of the two 
 retinas, are also seen single. 
 For the points of the retinas 
 on which the images are then 
 formed, although not strictly 
 identical, are corresponding; 
 that is, they have habitually 
 acted together in seeing objects 
 single by binocular vision, and 
 the slight incongruousness of 
 the two sets of images is dis- 
 regarded, as it were, by the 
 
 j T> f 11 ^,'ppfa Ivmo- 
 mma - *>Ut &11 ODJ6CCS lying 
 
 nearer or more remote than 
 
 , i , n , j i , i 
 
 the point fixated by the eyes 
 
 *^re liable to be Seen double* 
 , . , ,, 
 
 for their images do not fall on 
 corresponding points of the ret- 
 
 inas. Objects lying below or above, or to one side or the other, of 
 the point of regard, do not, as a rule, have their images formed 
 on corresponding points; they may, therefore, also be seen double. 
 Some of these points, however, which occupy positions below or 
 above, to the one side or the other, of the point of regard, are seen 
 single. The sum of all the points which are seen single while the 
 point of regard remains the same is called the horopter. 
 
 14. We must now advance to the consideration of the factors 
 which enter into the construction of the field of so-called "three- 
 dimensional space." Here the value, and even the necessity, of 
 combining sensations due to movements of the organism with those 
 
 FIG. 124. (From Hering). f f, the sash of the 
 window, and p, the black spot fixated. On 
 
 the left line of vision i b lies a distant object, 
 
 and on the right line r e another object. The 
 images of 6 and e, as well as the image of p, 
 fall on the place of direct vision and, there- 
 fore, on corresponding points of the two 
 
 retinas. 
 
VISUAL PERCEPTION OF DEPTH 427 
 
 more obscure local signs whose existence is required for the explana- 
 tion of the retinal field, become obvious beyond all doubt. 
 
 The numerous factors which contribute to the perception of 
 depth and relief may be grouped into three classes: (1) sensations 
 of the position of the eyes; (2) parallax between the two eyes as due 
 to movements of the head and body; and (3) associative aids, such 
 as can be utilized to give an appearance of relief to a flat drawing 
 or painting. We consider them briefly in the order just given: 
 (1) Kinesthetic sensations. Since, in accommodating the lens of 
 the eye for different distances, the ciliary muscle assumes various 
 degrees of contraction, the sensory evidence of its contracted or 
 relaxed condition may give an indication of the distance of the ob- 
 ject for which the lens has been accommodated. This indication 
 is hardly available, however, except for distances within a few feet 
 of the eye. Kinesthetic impressions of the degree of convergence 
 of the eyeballs, according to the distance of the object upon which 
 they were converged, should be available for greater distances, per- 
 haps even up to 100 feet. As has been said (p. 423), experiment 
 shows 1 that, when other aids to accurate perception of depth are 
 excluded, the judgments due to this means become very inaccurate. 
 It will not, therefore, account for the accuracy ordinarily attainable. 
 
 (2) In considering the aids to perception of depth grouped under 
 the head of "Parallax," we have to distinguish binocular parallax 
 with unmoved eyes or head, from parallax (essentially monocular) 
 as due to movements of the head or of the whole body. Since the 
 eyes look from slightly different positions, they obtain, as has al- 
 ready been shown (p. 424), different views of the same (tridimen- 
 sional) object. If the visual object is close to the face, the differ- 
 ence between the two views is great; but as the object retreats 
 further and further, the view obtained by both eyes becomes more 
 and more nearly the same. We should expect, then, that binocular 
 parallax would be of importance in perceiving distance and relief 
 when the objects are near at hand; and the actual importance of 
 this factor is proved by means of the binocular stereoscope. If 
 drawings or photographs are made of an object from two points of 
 view that are separated by the distance between the two eyes, and 
 if then the drawing from the more rightward point of view is placed 
 before the right eye, and that from the more leftward before the 
 left eye, and the two views combined by aid of the stereoscope, the 
 appearance of depth is much intensified. For proving the impor- 
 tance of binocular parallax, the most important stereoscopic views 
 
 1 Wundt, Beitrdge z. Theorie d. Sinneswahrnehmung, 1862; Hillebrand, Zeitschr. 
 f. Psychol, 1894, VII, 97 ; Bourdon, La perception visuelle de I'espace, 1902; and 
 others. 
 
428 PRESENTATIONS OF SENSE 
 
 are those which consist of bare outline drawings of prisms, pyra- 
 mids, dodecahedrons, etc., because in such views there is an ab- 
 sence of the associative factors which contribute to the perception 
 of depth. The small amount of line-perspective in these drawings 
 does not alone suffice to make the drawings appear solid, except 
 when they are binocularly combined. 
 
 The "pseudoscopic" effect also is important in this connection: 
 for if the views appropriate to the right and left eyes are interchanged, 
 then, in the absence of associative factors, the relief is reversed; and, 
 even if the associative factors are retained, as when the right and 
 left views of the landscape are interchanged on a stereoscope slide, 
 a certain degree of pseudoscopic effect is received. The appearance 
 of relief in the absence of associative factors, and even, to some de- 
 gree, in opposition to these factors, is good evidence of the real- 
 ity of binocular parallax as an important aid to the perception of 
 depth. On the other hand, the diminution of the pseudoscopic 
 effect when parallax has to contend with associative factors is evi- 
 dence of the value of the latter. 
 
 A more exact conception of the "different views" of the same ob- 
 ject which are obtained by the two eyes may be reached as follows. 
 Suppose two points to lie in the same line extending from the eye, 
 which may be called the line of sight. If the eyes are directed 
 on the nearer point, the farther one will be seen double; if the 
 eyes are fixed on the farther point, the nearer will be seen double. 
 The disparate images of the farther point, when the nearer is fix- 
 ated, are called " homonymous, " because the image which appears 
 to the right is that of the right eye; but when the farther point is 
 fixated, the double images of the nearer point are called "heter- 
 onymous," because the image which appears to the right is that of 
 the left eye. If, now, it can be assumed that the difference between 
 homonymous and heteronymous double images is a difference which 
 affects perception though it is not a difference of which we are 
 consciously aware then this difference would enable the observer 
 to know whether a doubly seen point were nearer or farther from 
 the eye than the point on which the eyes are fixed. 
 
 The accuracy of the perception of depth, by use of binocular 
 parallax, is very great for objects lying near the eye. Helmholtz 1 
 measured the accuracy of this judgment of distance by a simple ex- 
 periment, in which he fixed three pins upright in little blocks, and 
 endeavored to adjust them in a line at right angles to the line of 
 sight screening the base of the pins and the table on which they 
 were manipulated, so as to exclude aids derived from perspective. 
 When the line of the pins was 340 millimetres distant from his eyes, 
 1 Physiologische Optik, p. 644. 
 
VISUAL PERCEPTION OF DEPTH 429 
 
 and the pins 12 millimetres apart, he found that he never committed 
 an error as great as J millimetre. Bourdon 1 repeating Helmholtz's 
 test, with the line of pins placed two metres away from the eyes, 
 found the error (in judging whether the middle pin was at the same 
 distance as the two end pins) to be always less than 2.5 millimetres, 
 and seldom to exceed 1.5 millimetres. These judgments of dis- 
 tance are over 500 times as precise as can be made by use of con- 
 vergence and accommodation alone. 2 
 
 Parallax due to movements of the head has the advantage that 
 the head can move through a greater lateral distance than that 
 which separates the eyes; and thus can receive views of an object 
 which differ more than do those of binocular parallax. There is 
 an offsetting disadvantage, in that the two views are not received 
 simultaneously, and so are not combined in that direct and proba- 
 bly instinctive manner which enables binocular parallax to convey 
 an apparently immediate impression of the third dimension. But 
 there is one way in which the parallax due to head movement is 
 probably utilized a way emphasized by Helmholtz, 3 and tested by 
 Bourdon. 4 It is as follows: If we suppose the head moved to the 
 right, with the eyes remaining fixed in their sockets, then all objects 
 are displaced to the left in the field of view, and the nearer they lie 
 to the eye the greater is their backward displacement. Since, how- 
 ever, the eyes do not remain fixed in their sockets during such a 
 head movement, but instead remain fixated on some visible object, 
 this object must remain at the centre of clear vision; while, on the 
 contrary, objects nearer than it move backward in the field of view, 
 and objects farther off than it move, in the field of view, in the 
 direction of the head's movement. This motion of objects within 
 the field of view is very easily and minutely perceptible, and is 
 capable of giving a perfect indication of the relative distance of 
 objects in the line of sight. The question is, how far this source of 
 information is actually employed in ordinary perception. Bourdon 
 finds that, in attempting to judge of distance with one eye, and with- 
 out the aid of the associative factors, the observer does make in- 
 voluntary sideward movements of the head, and that his judgment 
 of distance is thus much improved. 
 
 (3) We now come to consider the so-called "associative" factors. 
 Under this head belong the space in the field of vision which is oc- 
 cupied by an object of known size; the linear perspective; haze and 
 other atmospheric effects; shadows and the covering of one object 
 
 1 Rev. philos., 1900, XXV, 74. 
 
 2 See O. Zoth, in NagePs Handbuch der Physiologic, 1905, III, 415. 
 
 3 Physiologische Optik, p. 634. 
 
 4 Op. cit., p. 286. 
 
430 PRESENTATIONS OF SENSE 
 
 by another and nearer object; and the general make-up of the field 
 of view. In comparing the distances of objects, all of which are 
 over a mile or two away, we are dependent entirely on these associ- 
 ative factors. One-eyed individuals, also, must be dependent on 
 them, and on parallax due to movements of the head. Since one- 
 eyed individuals do not show a noticeable deficiency in the percep- 
 tion of distance under ordinary conditions, and since a person may 
 even become blind in one eye without being aware of the fact, it 
 is clear that very good indications of the third dimension are pro- 
 vided by the associative factors, in connection with movement- 
 parallax. It is probable enough that the factors here classed as 
 associative are of much influence even in normal binocular vision. 
 The significance and value of these associative factors will be further 
 illustrated especially when we come to consider certain classes of 
 the errors, and illusions, that are so frequent in visual perception. 
 Since these factors bring into prominence the interpretative function 
 of the mind in all perception that is, the presence, in the lowest and 
 earliest stages of the mental life, of the working of the so-called 
 faculties of attention, discrimination, association, etc. such errors 
 and illusions have, hitherto, oftener than not, been laid to the fault 
 of "the judgment." 
 
 15. A fact of profound and far-reaching psychological meaning 
 comes to the surface repeatedly in considering the factors on which 
 judgment of distance and relief is based. The fact is this: We are 
 often unaware of these factors, taken by themselves, and are even 
 unable to become aware of them directly, though their reality can 
 be demonstrated by their effects. We are unconscious, for example, 
 of the existence of two fields of view, one due to each eye; and, in 
 the case of double images, we are unable to tell by introspection 
 which belongs to the right eye and which to the left. It may even 
 happen that an object is visible only to one eye as, for example, 
 when the fingers of both hands are held before the eyes in a sort of 
 lattice-work and yet we cannot tell with which eye it is seen, or 
 whether it is seen with both. Notwithstanding this inability to 
 distinguish between the contributions of the two eyes, the facts of 
 stereoscopic and especially of pseudoscopic ^vision show that bi- 
 nocular perception of depth depends on some sort of (physiological) 
 distinction between the complex nervous impulses coming from the 
 two eyes. To this corresponds the distinction (psychological) of 
 the complexes of resulting sensations. 
 
 Again, perception of the distance of a familiar object and also 
 perception of the size of an unknown object, when its distance is 
 known, are both dependent on using the "visual size" of the ob- 
 ject, or the angle subtended by it, as an indication of its real size 
 
PERCEPTION OF DISTANCE AND SIZE 431 
 
 or of its actual distance; and yet direct judgment of this visual 
 size is much less certain and accurate than judgment of the real 
 size or actual distance. These paradoxes would amount to genuine 
 impossibilities if the whole process of reaching a judgment of size 
 or depth went on within the field of consciousness, and if every part 
 of it were accessible to attentive observation. 
 
 Such experiences as these tend strongly and, may we not say, 
 conclusively ? to confirm our suspicion that innumerable complex 
 "traces" of sensations due to native and acquired motor reactions 
 are so fused with all the local signs of the retina, as to demand recog- 
 nition in every satisfactory theory of the genesis and development of 
 the entire class of visual perceptions. The infant does not even 
 initiate, not to say achieve, the process of objective vision otherwise 
 than through the use of a ceaselessly moving pair of eyes. And 
 that our subsequent analysis, whether introspective or more purely 
 experimental, cannot disentangle and reproduce in consciousness 
 these fused, or synthesized, factors, no more proves that they did 
 not formerly exist than does the similar inability of the accomplished 
 violinist to reproduce all the sensations under the guidance of which 
 he learned correct spacing and bowing for all the different "posi- 
 tions" of his instrument, and its most difficult and delicate work. 
 Indeed, on the one hand, the player's violin is as much a seeming 
 part of his own organism as is the seer's pair of eyes; and on the other 
 hand, the complexity and depth below the threshold of conscious- 
 ness, of the player's former sense-experience bears no resemblance 
 to the complexity of the lost art of learning how to see with the 
 average pair of eyes. 
 
 16. And, indeed, as has already been indicated, the laws which 
 control our estimates of visual magnitudes are psychological, and 
 apply to all the action of the mind in constructing its sense-data into 
 the presentations of sense. Yet more elaborate mental activities, 
 such as take place when the distance, size, and contour of visual ob- 
 jects are deliberately estimated and expressed in terms of an ac- 
 cepted standard, of course imply more of dependence upon skill 
 acquired through experience. 
 
 The degree of fineness with which differences of distance and 
 magnitude can be seen, under the most favorable circumstances, is 
 limited by the least observable differences in the members of the 
 different spatial series of sensations which compose the visual ob- 
 jects. Of such series, those most capable of exceedingly fine differ- 
 entiation are the local retinal signs and the sensations of position 
 and motion accompanying convergence of the eyes for near dis- 
 tances. Different authorities assign different proportions to the 
 different help which these series render in making the finest possi- 
 
432 PRESENTATIONS OF SENSE 
 
 ble distinctions of visual magnitude. Hering 1 denies that any help 
 is obtained from muscular sensations, or "feelings of innervation," 
 in comparing the size of two minute objects near by, and assigns 
 all the work of furnishing such data to the "spatial sense of the 
 retina." Lotze, 2 who admitted the assistance of muscular sensa- 
 tions, nevertheless held that the fineness of the distinctions possible 
 among them is not sufficient to support our ordinary judgments of 
 the size, distance, and direction of objects. Wundt 3 and others 
 claim that it is by gradations in the sensations of eye-movements that 
 we make the most accurate of these estimates; they deny that any 
 "spatial sense" (in Hering's meaning of the words) belongs to the 
 retina. The evidence seems to favor the view that both classes of 
 sensations furnish data for all nice discrimination of visual extension. 
 The particular degree of accuracy with which minute differences 
 in the distance and magnitude of visual objects can be perceived 
 varies greatly, according to different positions of the eyes and of 
 the object, the amount of light, practice, etc. and all these, as con- 
 nected with individual peculiarities of structure and previous func- 
 tion of the organs of sense. That such estimates fall to some extent 
 under Weber's law in other words, that the least observable dif- 
 ference in the length of visual lines and surfaces is relative and 
 not absolute has already been shown (chap. Ill, 23). Chodin 
 found the relative value of the least observable difference, with a 
 variation of the absolute vertical distance from 2.5 to 160 mm., to 
 be as follows when the lines lie in the same direction: 
 
 Absolute distance.. 2.5 5 10 20 40 80 160mm. 
 
 Fraction of observ- 
 able difference . . 
 
 The fineness of ocular judgment is greater for horizontal dis- 
 tances. 
 
 The measuring power of the eye is much less accurate when 
 the distances compared lie in different directions. In particular, 
 points separated by a vertical distance of 20 mm. are estimated as 
 equally far apart with those separated by a horizontal distance of 
 25 mm. 4 Most estimates of direction and distance are compara- 
 tively inaccurate when only one eye is used. A vertical line drawn 
 at right angles to a horizontal appears bent to monocular vision; 
 its apparent inclination is variable, and was found by Bonders 5 to 
 vary between 1 and 3 of the angle within a short time. 
 
 1 Hermann's Handb. d. PhysioL, III, i, pp. 533 f . 
 
 3 Medicin. Psychologic, 384 f . 
 
 8 Physiolog. Psychologic (6th ed.), II, 574. 
 
 4 So Wundt found, op. cit., II, 591. 
 
 6 Archiv f. Ophthalmologie, XXI, iii, pp. 100 f. 
 
VISUAL PERCEPTION OF MOTION 433 
 
 17. The data or motifs already described are the foundation, 
 also, of our perceptions of motion, and of our estimates of its di- 
 rection, speed, and extent. It need scarcely be said that all such 
 perceptions and estimates are relative; they imply the existence of 
 some point which may be regarded as fixed, and the application of 
 a standard of measurement. For perceptions of motion by the eye, 
 the point of regard when the organ is in the primary position 
 furnishes the means of orientating ourselves and of placing the dif- 
 ferent things of vision in their right relations to us and to each 
 other. Suppose the body and head to be erect, and the eyes mo- 
 tionless and looking into the distance with the lines of vision paral- 
 lel; the perception of motion may then arise in either one of two ways. 
 Of these, by far the most frequent is the change of relative position 
 of an object in the field of vision which is occasioned by its move- 
 ment. What is necessary, however, is simply the successive stimu- 
 lation of continuous points or areas of the retina with images that 
 are sufficiently similar to be perceived as one object. The percep- 
 tion of motion may also be produced by the successive stimulation 
 of the same points or areas of the retina with images that are too 
 dissimilar to be regarded as one object. One may thus see motion 
 when neither the eyes nor any external objects are really moved, 
 as is now so familiarly illustrated by the well-known performances 
 of kinetoscope and kinematograph. It is in the latter way that the 
 colored points of the images formed by the retina's own light, when 
 the eyes are closed and motionless, seem to be in constant motion. 
 
 The direction and amount of motion perceived with the eyes is 
 measured off upon the entire field of vision in accordance with pre- 
 vious experience and by means of the data already described. With 
 the eyes at rest, the retinal local signs, or space-values belonging 
 to the retinal elements, furnish the more important data; second- 
 ary helps, and associated sensations and ideas of position and mo- 
 tion, complete the perception. 
 
 It is assumed, in cases like the foregoing, that no sensations in- 
 dicating motion of either the organ of vision, or the head, or the 
 whole body, complicate the problem. But ordinary perceptions of 
 motion are gained with the eyes in motion out of the primary posi- 
 tion. When the eye and the object both move in such a way that 
 the point of regard remains fixed on the object, our perceptions 
 of motion, and estimates of its direction and magnitude, are de- 
 pendent upon muscular and tactual sensations occasioned by the 
 eye's changes of position. We know from experience what kinds 
 and intensities of sensations are produced by keeping the point of 
 regard fixed on an object which is moving about at a given rate in 
 a given direction. If any of the links ordinarily belonging to this 
 
434 PRESENTATIONS OF SENSE 
 
 chain of conscious experiences drop out, our measuring instru- 
 ment fails us either partially or completely. The head, too, is in- 
 variably turned when we are watching an object that is moving in 
 any direction other than straight forward or away from us along the 
 line of regard. The sensations originating in the action of the 
 muscles and skin of the head and neck thus enter into our compu- 
 tation; they must have such a value in consciousness as to inform 
 us about how far the head has gone from the position with which 
 it started, in order to fixate the moving object. According to 
 Helmholtz, 1 the ordinary movements of the head in vision follow 
 the same principle as that followed by the eyes in movement; that 
 is to say, the head turns from its primary position on an axis that 
 is approximately parallel to the axis of the simultaneous rotation 
 of the eyes. But Hering 2 asserts that a difference between the laws 
 of the motion of head and eyes is of essential significance for our 
 perception of space. However this may be, it is certain that the 
 position and motion of the head, as known by its muscular and 
 tactual sensations, must be taken account of in all ordinary visual 
 perception of motion. The same thing is true of the position and 
 motion of the entire body. Many of our errors of sense, or false 
 perceptions of motion its existence, direction, rate, and amount 
 are dependent upon the principles of judgment governing such 
 data of sensations. We are peculiarly liable to error in all cases 
 where the motions of our own bodily organs are passive; in such 
 cases we do not have the ordinary motifs, or data, at our command. 
 Objects are perceived at rest, either when, our organs of vision 
 being themselves at rest, the images of the objects do not change 
 their position in the field of vision, or when sensations of motion 
 occasioned by moving these organs are such and so great as we 
 know by experience correspond to (or compensate for) the changes 
 in the position of their images which are occasioned by their actu- 
 ally remaining at rest. But whenever we look with moving eyes 
 upon a number of objects arranged in fixed position with relation to 
 each other, a conflict between two sets of data really takes place. 
 The result with respect to our perceptions of motion may depend 
 upon which of the two is chiefly effective in arresting attention. 
 When the eyes are brought from the parallel position, which they 
 assume in vision of remote objects, to convergence upon some near 
 object, the two fields of view belonging to the two eyes rotate in 
 opposite directions, while the middle visual line maintains its posi- 
 tion in the median plane. 3 Ordinarily we do not perceive this 
 
 1 Physiolog. Optik, p. 486. 
 
 2 In Hermann's Handb. d. PhysioL, III, i, p. 495. 
 
 3 See Le Conte, Sight, p. 229. 
 
JUDGMENT IN ERRORS OF SENSE 435 
 
 rotary motion of the two fields of vision, but consider the field as 
 one and stationary and ourselves as changing our point of regard in 
 it. By attention, however, we may see that the external objects, 
 although they really continue at rest, appear to move as the rela- 
 tions of their double images are changed. So, also, when the eye 
 or head or body turns in either direction, in order that a new ob- 
 ject may be brought under regard, it is possible either to perceive 
 or not to perceive the entire field of objects sweeping by; and which 
 of the two happens depends upon the direction in which attention 
 is drawn. When strictly attending to the phenomena, we cannot 
 well fail to regard everything as moving in the opposite direction 
 from that in which we know the organ of vision to be turning. 
 
 18. The principles already laid down also suffice to explain 
 most of the ordinary "errors of sense," as well as certain extraor- 
 dinary experiences of a somewhat different kind. The right to 
 speak of errors of sense has been questioned. It has been claimed 
 that such errors belong rather to judgment, and that sense pure 
 and simple cannot err. The claim is based upon a misunderstand- 
 ing of the nature of perception. A very obvious difference exists, 
 indeed, between a mistaken estimate of the distance of a mountain 
 through extraordinary clearness of atmosphere and the seeing of a 
 square of white paper as green on a red ground, or as yellow on 
 a blue ground. But the latter is surely an "error of sense," or 
 sensation, in as pure form as such error is conceivable. That sense 
 cannot err is true only in case we speak of unlocalized and unpro- 
 jected sensation, regarded as not predicating anything beyond itself. 
 In all presentations of sense a certain psychological judgment is 
 involved; for all such presentations imply association of impres- 
 sions discriminated as similar or dissimilar, and a mental synthesis 
 which is dependent upon attention and the interpretation of certain 
 motifs or data according to past experiences. Clear vision is always 
 mental interpretation. 
 
 The attempt to assign the relative amount of blame to sense and 
 to intellect, in cases where our presentations of sense do not rep- 
 resent objective relations of things, assumes an ability to make dis- 
 tinctions which we do not possess. Moreover, the distinction, when 
 made as the objection would have it, will not hold. Innumerable 
 experiences contradict the statement that immediate sense-percep- 
 tion cannot err. When one sees (with no power to see otherwise) 
 a gigantic human form through the fog, or projected against the 
 scenery of a stage, and yet judges that this form is only of usual 
 size, the error is not one of judgment, but just the reverse. Errors 
 of sense are only special instances where the mind makes its syn- 
 thesis unfortunately, as it were, out of incomplete data, instantane- 
 
436 PRESENTATIONS OF SENSE 
 
 ously and inevitably interpreting them in accordance with the laws 
 which have regulated all its experience. As Lotze has remarked, 
 "The whole of our apprehension of the world by the senses is 
 one great and prolonged deception." Objects of sense are in no 
 case exact copies of ready-made things which exist extra-mentally 
 just as they are afterward perceived, and which get themselves 
 copied off in the mind by making so-called impressions upon it; 
 they are mental constructions. In the special case of sight we have 
 seen that, in every particular in its elements, its mode of con- 
 struction, its laws of change the field of vision is a subjective af- 
 fair. The case is in no respect essentially different, whether our 
 presentations of sense are so-called errors or true images of things. 
 In both cases the same data and laws of the use of these data main- 
 tain themselves. Errors of sense, however, are distinguished from 
 hallucinations, because the former result from the activity of an 
 organism which is normal in structure and function, while the lat- 
 ter do not. 
 
 19. The errors of visual perception are almost innumerable; 
 they may be classified in part, however, according as they fall under 
 some one or other of the before-mentioned principles. Such errors 
 may be called "normal," because they are committed in accordance 
 with principles which regulate the ordinary activity of the mind in 
 making its synthesis by the help of the sense-data or motifs fur- 
 nished to it through the excitement of the organism. Deceptions 
 of this class really result, then, from the fidelity of both mind and 
 nervous system. Certain errors of sense, for example, are special 
 examples of the working of the laws which regulate the correspond- 
 ence of the two images in binocular vision. Thus, near objects 
 erroneously appear double when the eye is adjusted for distant 
 vision, distant objects when it is adjusted for near vision; solid 
 things are seen through other solid things; relations in space in 
 general are perceived different from the reality; and all according 
 to the law of the correspondence and non-correspondence of the 
 two retinal images. Accordingly, the inquiry, Why is vision single 
 when it is performed with two eyes ? can demand and receive only 
 one answer. An important condition of the single vision of solid 
 objects is that they shall be seen with two eyes. Whether anything 
 whatever is seen as two or one does not depend, primarily, upon its 
 really being either two or one, or upon the existence of one or two 
 retinal images of it (as though such images were directly perceived) ; 
 it rather depends upon the appropriate data of sensations being 
 furnished to the mind for completing its mental synthesis of the 
 object. The two eyes being simultaneously affected in a certain 
 way, these data are supplied. What is one is seen as one, and 
 
GEOMETRICAL OPTICAL ILLUSIONS 437 
 
 what is two is seen as one, and what is one is seen as two all in 
 essentially the same way. 
 
 20. Under the head of geometrical optical illusions are grouped 
 a large number of curious errors in the perception of lines and angles. 
 Of the numerous figures which have been found to give rise to such 
 illusions, only a small selection can be presented here; but it may be 
 stated as a general principle, that scarcely any geometrical figure 
 is free from illusory effects; or, in other words i '.'that the apparent 
 length and direction of a line is likely to be affected by all its visual 
 environment. 
 
 Among the principal illusions of this general type are such as 
 the following: (1) Specific illusions. Vertical distances are per- 
 ceived as greater than mathematically equal horizontal distances. 1 
 This can be seen in Figs. 125, a and b; but the reader can best 
 
 FIG. 125. 
 
 convince himself of the fact by marking off a vertical distance be- 
 tween two points, and then keeping the original two points stead- 
 ily in the vertical meridian of the field of view locating a third 
 point at what seems to be the same distance to the right or left of 
 one of the original points. The error will then be apparent on 
 measuring the two distances, or on turning the figure so as to inter- 
 change horizontal and vertical. 
 
 By a similar test the upper half of a vertical line can be shown to 
 appear slightly longer than the lower half; 2 and in general, when one 
 of two equal figures lies directly above the other, the upper appears 
 somewhat larger than the lower. The upper and lower halves of 
 a letter "S" or a figure "8" appear of nearly the same size; but when 
 they are inverted ("g" and "g"), the actual difference between 
 
 1 See J. Oppel, Uber geometrisch optische Tauschungen, Jahresbericht d. physi- 
 kal. Vereins zu Frankfurt, 1854-55, p. 37. 
 
 2 Delboeuf, Bulletin de I'acad. roy. de Belgique., 2 Sr., XIX, 2, 195. 
 
438 PRESENTATIONS OF SENSE 
 
 the two halves becomes magnified. There is a somewhat similar 
 illusion in dividing a horizontal line in halves, 1 when only one eye 
 is used, and attention is directed to the middle of the line; the outer 
 
 Fio. 126. 
 
 half needs to be larger than the inner or nasal half in order that the 
 line may appear equally divided. Not all individuals, however, are 
 subject to this illusion, and it is always slight in amount. 
 
 (2) One-dimensional illusions. In distinction from the illusions 
 already mentioned, which seem to be specifically connected with 
 certain parts of the retina or of the field of view, there is a large 
 class of more general errors which are likely to be committed in 
 
 I 
 
 FIG. 127. (From Ebbinghaus.) 
 
 the judgment of lengths. They may be classed as one-dimensional 
 illusions, since they occur within a single straight line, and are not 
 due to the influence of figures in other parts of the field of view. 
 
 The best-known of these one-dimensional illusions is commonly 
 expressed by saying that filled or divided space appears greater 
 than empty or undivided space. This rule, however, is subject 
 to certain qualifications. It is, first, rather the filled or empty ap- 
 
 FIG. 128. 
 
 pearance which counts, than the amount of light received from the 
 spaces in question; for a black line on a white sheet appears longer 
 than an equal distance laid off between two points (Fig. 126), 
 though, objectively, it is the black line which is empty. In the same 
 
 1 Kundt, Poggendorf's Annalen, 1863, CXX, 118. 
 
GEOMETRICAL OPTICAL ILLUSIONS 439 
 
 way, it is not the most finely divided space which seems greatest, 
 but the space of which the division is most obtrusive (Fig. 127). 
 These facts indicate that the illusion is dependent on the figure 
 which we are led to see rather than on the mere distribution of 
 retinal stimulation. There are further qualifications, in detail, 
 to the rule that divided space appears greater than undivided; for 
 example, a single division of a line at its middle, or a division into 
 three parts, of which the middle one is much larger than that at 
 either end, causes, if anything, the opposite illusion (Fig. 128). 
 
 When subdivision of a line causes it to appear longer, the parts 
 into which it is divided (or some of them) themselves appear shorter 
 
 FIG. 129. 
 
 than isolated lines of the same length. 1 If, on the contrary, the 
 division is such as to make the whole line appear shorter, the parts 
 then appear long in comparison with isolated lines of the same length. 
 For example, if a line is divided into a large central piece and two 
 small end-pieces, the whole line appears shortened, and the middle 
 part lengthened ; and this illusion is intensified by leaving the middle 
 part blank, as in Fig. 129, in which, by a combination of these 
 effects, the whole of the shorter line appears shorter than the vacant 
 middle part of the longer line. 2 If, however, the middle part is 
 relatively small, it appears shorter than an equal isolated line, while 
 the whole line appears too long. By cutting out equal distances 
 from the middles of short and of long lines, as in Fig. 130, an 
 effect of contrast is produced. 3 
 
 1 Ebbinghaus, Grundzuge der Psychologic, 1908, II, 59. 
 
 2 Compare the very similar illusions investigated by H. J. Pearce, Psychol. 
 Rev., 1904, XI, 143. 
 
 3 Miiller-Lyer, Archiv /. Physiol, 1889, Suppl., p. 263; Zeitschrift f. Psychol., 
 1896, IX, 1. 
 
440 PRESENTATIONS OF SENSE 
 
 Very similar to these one-dimensional illusions of visual percep- 
 tion are some which occur in the comparison of distances by touch, 
 when lines are applied to the skin, or by touch and the muscle-sense, 
 when the finger is drawn along a line, the divisions being made 
 in such a way as to be appreciated by touch. 1 And much the same 
 illusions occur in the comparison of short intervals of time, when 
 these are marked out by sounds. It appears quite probable, there- 
 fore, that these one-dimensional illusions are not specifically de- 
 pendent on the processes of vision, nor even on the processes of 
 space-perception. One may say that the process of comparison 
 is affected, in all these cases, with the same sort of difficulty and 
 source of confusion; or one may say that the same sort of configura- 
 
 FIG. 130. 
 
 tion is introduced into the percept in all these cases, and that the 
 illusions are incidental to the configuration. 
 
 No fully satisfactory theory has yet been worked out to cover 
 and explain all the facts. The illusions which have been men- 
 tioned in visual perception of lines are rather slight, and even in- 
 constant from one individual to another; but they are made more 
 striking, and harder to escape, by adding to the one-dimensional 
 figure other lines and angles. 
 
 1 Compare Robertson, Psychol. Rev., 1902, IX, 549; Pearce, Archiv /. d. ges 
 Psychol, 1903, I, 31; Psychol. Rev., 1904, XI, 143. 
 
 It should not be understood, however, that the same illusions occur in all cases 
 in touch as in vision. Of the illusions to be mentioned on succeeding pages, that 
 of Miiller-Lyer holds of touch, but that of Poggendorf is reversed (Robertson). 
 In an extended study of the illusion of rilled and empty space in touch (Arch. 
 f.d. ges. Psychol., 1910, XVI, 418), Cook finds (1) that when adjacent areas 
 of the skin are simultaneously excited, a filled space is overestimated by com- 
 parison with an empty, but (2) that when the stimuli are applied successively 
 to the same area, the opposite illusion results; and (3) that when the stimuli 
 are applied successively to different areas, no illusion occurs in the comparison 
 of filled and empty space. Analysis of the total impression, or isolated attention 
 to the particular elements to be compared, is favored, in the case of touch, by 
 successive presentation. In general, it would seem that touch is rather more sub- 
 ject to illusions of this sort than is vision; for analysis is easier in vision. 
 
THE MULLER-LYER ILLUSION 
 
 441 
 
 (3) The Miiller-Lyer illusion. The most striking illusion in 
 length of lines is that first announced by Miiller-Lyer in 1889, and 
 called by him a "confluxion" effect, in opposition to "contrast." 
 
 FIG. 131. The Mtiller-Lyer Illusion. 
 
 Fig. 131 presents the "confluxion" illusion in a common form, 
 but there are numerous variants. A continuous series of transi- 
 tional forms can be constructed, leading from this arrow-head 
 
 FIG. 132. Modified Mdller-Lyer Figure. 
 
 figure to the one-dimensional illusion of Fig. 129: thus, the hori- 
 zontal line may be omitted, and only the arrow-heads left the 
 distances between the points of the arrows being the object of com- 
 
 FIG. 133. Confluxion and Contrast. 
 
 parison; the angle of the arrow-heads may then be diminished to 
 zero, so giving the one-dimensional figure. Another transitional 
 series can be made by substituting for the arrow-heads rectangles 
 (with or without one side open), and then narrowing the rectangle 
 
442 
 
 PRESENTATIONS OF SENSE 
 
 FIG. 134. Reversible MOller-Lyer Illusion. 
 
 to the width of the "line" in the one-dimensional figure (Fig. 132). 
 From the continuity of these series, there can be little doubt that the 
 same principle enters into both extremes of the series; on the other 
 
 hand, the introduction of figures in two 
 dimensions certainly intensifies the illu- 
 / sory effect. 
 
 * The Miiller-Lyer figure, like the one- 
 dimensional figure, and even more, pro- 
 duces a compound or reduplicated 
 illusion. One of the two lines to be 
 compared seems shorter than it is, and 
 the other longer than it is, and therefore 
 the comparison of the two is subject to a 
 double illusion. Further, either of the 
 two principal lines of the Miiller-Lyer 
 figure is influenced by two oblique lines 
 at each end, and any one of these four 
 oblique lines is enough to produce a 
 moderate degree of the illusion. In 
 other words, the sides of an acute angle 
 seem shorter than they are, and the sides 
 
 / of an obtuse angle longer than they are. 
 
 * Since, however, the same sort of illusion 
 
 occurs when rectangular figures, or 
 FIG. 135. The Angle illusion, curves, or even parts of a single straight 
 
 line, are substituted for the arrow-heads, 
 
 a more comprehensive statement of the illusion may take the fol- 
 lowing form: When a point is apprehended as part of a compact 
 figure (as an arrow-head, a rectangle, or a short stretch of line), it 
 becomes attracted, or displaced, toward the centre of that figure. 1 
 
 1 Compare the similar formulations by Judd, Psychol. Rev., 1899, VI, 241; by 
 Pearce, Psychol. Rev., 1904, XI, 143; by Benussi, in Meinong's Untersuchungen 
 zur Gegenstandstheorie und Psychologic, 1904, p. 303; and by Smith and Sowton, 
 Brit. Journ. of Psychol., 1907, II, 196. 
 
CENTRAL FACTOR IN ILLUSIONS 
 
 443 
 
 21. A few words of explanation are needed with reference to the 
 formula just given. Neither " figure" nor "centre" is to be under- 
 stood in a strict geometrical sense. By "figure" is meant any part 
 
 Fro. 136. Modified Angle Illusion. 
 
 of the total presentation (whether this be visual, tactile, or audi- 
 tory) which, usually because of its compactness, is apprehended as 
 possessing a certain degree of unity and isolation. By "centre of 
 the figure" is to be understood something analogous to the centre 
 of gravity, but it is a subjective centre of gravity, depending rather 
 on the figure as apprehended than 
 on the exact geometrical relations 
 within the stimulus. For example, 
 when the inner circle in a, Fig. 
 133, is compared with the outer 
 circle in 6, an illusion occurs which 
 is probably a reduplication of the 
 one-dimensional illusion of Fig. 129; 
 diameters drawn through the cir- 
 cles would give the one-dimensional 
 figure. Both a and b are most 
 readily apprehended as rings, and 
 the centre of attraction lies within 
 the ring and not at the geometrical 
 centre of the circles. In c of the 
 same figure, on the other hand, the 
 ring is not compact, and the inner 
 circle is apprehended as an inde- 
 pendent figure; its centre of attrac- 
 tion lies at its geometrical centre, 
 and the circumference is drawn 
 inward; and thus comparison of FlQ . 137. Double Angle illusion, 
 the inner rings of b and c leads to 
 
 a "contrast" illusion similar to that in the one-dimensional Fig. 
 130. It appears, therefore, that both the "contrast" and the 
 
444 
 
 PRESENTATIONS OF SENSE 
 
 "confluxion" illusions are covered by 
 the comprehensive formula of the pre- 
 ceding paragraph. Which of the two 
 effects shall result depends on the sub- 
 jective grouping of parts. 
 
 The importance of some such mental 
 or "central" factor as apprehension in 
 the causation of these illusions is indi- 
 cated by the fact that opposite illusions 
 can sometimes be got from the same ob- 
 jective figure, according to the way in 
 which its parts are combined. Thus, in 
 Fig. 134, a given objective distance ap- 
 pears now longer and again shorter than 
 another objectively equal distance, ac- 
 cording as attention is fixed on the fig- 
 ures in black or on the figures in white. 
 22. (4) Illusions of angles and the 
 direction of lines. The apparent direc- 
 tion of a line is easily influenced by the 
 FIG. 138. Poggendorf Figure, presence of other lines in its neighbor- 
 hood. The general rule applying to 
 
 these cases is often stated in the following form: Acute angles are 
 over-estimated, and obtuse angles under-estimated. This formula 
 is to be regarded less as 
 an explanation of the large 
 class of illusions to which 
 it is applied than as a con- 
 venient designation of the 
 class itself. These figures 
 seldom require the ob- 
 server to make a direct 
 estimate of the size of an 
 angle, either in degrees 
 or in comparison with an- 
 other angle; he has rather 
 to compare the directions 
 of two lines or to locate 
 the prolongation of a line 
 when it is interrupted 
 The error committed is as 
 
 it would be if the acute FIG. 139. ZSllner Figure. 
 
 angles of the figure were 
 
 over-estimated; but the over-estimation of the angle is not a di- 
 rectly observed fact. Indeed, when a direct comparison of angles 
 
ILLUSIONS OF ANGLES 
 
 445 
 
 is required, under conditions which make comparison difficult 
 such as omission of the vertex, or unequal length of the sides of 
 the angles then quite other sorts of errors appear. 1 This makes it 
 appear doubtful whether the over-estimation of acute angles is, 
 
 FIG. 140a. Hering Figure. 
 
 FIG. 140&. Hering Figure. 
 
 in reality, a universal principle which can be freely invoked for the 
 explanation of these illusions. 
 
 In the "angle illusion" (Fig. 135), the acute angle at the vertex 
 is said to be over-estimated, because the true prolongation of one 
 side falls within the apparent divergence of the sides. The illusion 
 appears to depend in part on the disparity in length of the two lines 
 which form the angle; if both are treated alike, as in Fig. 136, the 
 effect is reduced, made uncertain, and may even be reversed. 
 
 1 Judd, op. cit., A. H. Pierce, Studies in Auditory and Visual Space Perception 
 (New York, 1901), p. 271. 
 
446 
 
 PRESENTATIONS OF SENSE 
 
 Reduplication of the angle illusion gives such effects as are seen 
 in Fig. 137, or, in more pronounced form, in Fig. 138, which 
 last presents the long-famous Poggendorf illusion. Various degrees 
 of this illusion are produced by dissecting the figure and presenting 
 its parts separately; thus the illusion is, if anything, strengthened by 
 leaving only the exterior obtuse angles between the interrupted 
 oblique line and the parallels, while it is much diminished or even 
 reversed by leaving only the acute angles. 1 
 
 By multiplication of the same elements, the figures of Zollner 
 (139) and of Hering (140 a and 6), and other variants, are pro- 
 duced. The same errors in the apparent prolongation of the ob- 
 lique lines can be observed here, but, in addition, the parallels are 
 
 :*:*:*:*.v;v;v; 
 
 FIG. 141. The Twisted Cord Illusion (Fraser). 
 
 affected in their apparent direction, and straight lines may even ap- 
 pear bent. These illusions are interpreted in the following way, as 
 coming under the rule of over-estimation of acute angles: If the 
 angles between the parallels and the intersecting oblique lines are 
 exaggerated, and if the resulting illusion affects chiefly the paral- 
 lels, then these will appear swung around toward right angles with 
 the obliques, and so the Zollner illusion will result. Further, since 
 the over-estimation of the angles is greater, the smaller the angles, 
 the parallels will appear bent when the angle of the intersecting 
 lines varies; and thus the other figures are provided for. 
 
 It is a curious fact that when the oblique lines in the Zollner 
 figure are made exceedingly oblique, while at the same time the 
 principal lines are indistinct, an illusion opposite to that of Zollner 
 is produced. This may be called the " twisted cord illusion," since 
 the figure (No. 141) produced is similar to the drawing of a cord 
 
 1 Jastrow, Amer. Journ. of PsychoL, 1892, IV, 381; A. H. Pierce, Studies in 
 Auditory and Visual Space Perception (New York), 1901, p. 257. 
 
ILLUSIONS OF AREAS 447 
 
 made by twisting together a black and a white strand. 1 The 
 illusion is strengthened by placing the "cord" on a checkered back- 
 ground in such a position that each of the obliques is augmented 
 at each of its ends by parts of the squares of the background. By 
 varied application of this principle, Fraser has produced very strik- 
 ing illusions. Since the deflection of the "cord" is in the direction 
 of the oblique lines, the illusion may be called one of "conflux- 
 ion," and in distinction to it, the Zollner illusion may be called one 
 of contrast. 
 
 23 (5) Illusions of area. The judgment of area, like that of 
 angles and directions, is, introspectively, less direct and confident 
 than the judgment of length. If the areas of the several surfaces 
 in Fig. 142 are compared (without the aid of calculation), much 
 uncertainty is likely to be felt; but the figures will probably appear 
 unequal, though all have very nearly the same area. In general, 
 compactness of form seems to diminish the apparent area of the 
 
 O 
 
 FIG. 142. Comparison of Areas. 
 
 surface. Another cause must be sought for the illusion of Fig. 
 143. Here the judgment of area is perhaps confused by* the ob- 
 trusive inequality of the adjacent sides, or by the evident tendency 
 of the lower figure, if continued upward, to enclose the upper figure. 
 24. The field of geometrical illusions has proved exceedingly 
 fertile in theories, but rather barren of scientific agreement. A 
 twofold division may be made into theories which appeal to pe- 
 ripheral factors and those which appeal to central factors. The 
 chief peripheral factors are those which relate either to the retina 
 or to the eye movements. On the one hand, it is impossible to 
 found any comprehensive theory purely on peculiarities of the retina. 
 On the other hand, the eye-movement theory has been applied 
 to these illusions with much detail and much apparent success. 
 The general conception underlying this theory is that the eye 
 measures lengths, angles, etc., by moving over them, and that move- 
 ments of greater extent or energy, or those which involve more effort, 
 lead to the appearance of greater magnitude. Since, for example, 
 vertical movements of the eyes require more effort than horizontal 
 
 1 Fraser, Brit. Journ. of Psychol., 1908, II, 307. 
 
448 
 
 PRESENTATIONS OF SENSE 
 
 movements, vertical lines seem longer than equal horizontals; di- 
 vided space impedes the movement of the eye over it, and so appears 
 the greater; acute angles call for abrupt changes in movement, and 
 so are felt as large in comparison with obtuse angles, which require 
 but a slight deviation in the eye's direction. In the Muller-Lyer 
 figure, the readiest explanation is that the eye is led to err in its 
 fixations, and so comes to measure, not the distances between the 
 points, as required, but rather the distances between the figures 
 adjacent to the points. 
 
 Actual photographic records of the eye's movements during ex- 
 amination of the Muller-Lyer figure do not, however, as will be 
 
 FIG. 143. An Illusion of Area (Jastrow). 
 
 seen later in the chapter, always substantiate this interpretation. 
 They are more favorable to the statement that the figure with 
 inward-turned obliques impedes the eye's movement, making it 
 "slow and hesitating," whereas the figure with outward-pointing 
 obliques allows the eye free play for "extensive and energetic" 
 movement. 1 Unfortunately, hesitating and impeded movements 
 are required by the theory also for explaining the over-estimation 
 of divided space; so that the two explanations are inconsistent. 
 
 Supporters of the eye-movement theory have often brought for- 
 ward the observation that steady fixation diminishes the illusions, 
 whereas free and rapid roaming of the eyes over the figures favors 
 the illusory effect. On the other hand, opponents of the theory 
 have reported that a momentary exposure of the figures too brief to 
 
 t^ J Wundt, Physiol. Psychol, 6th ed., 1910, II, 583. 
 
CENTRAL THEORIES OF ILLUSIONS 449 
 
 permit the eyes to move from one point to another does not de- 
 stroy the illusions but rather favors them. These two observations, 
 in appearance contradictory, are in reality much the same thing; for, 
 as will be set forth more fully in later paragraphs on this subject, 
 rapid movement of the eyes over a figure amounts to obtaining a 
 series of brief exposures the figure being invisible during the move- 
 ments, and being seen only during the brief fixations of the eye which 
 intervene between the movements. It would seem, then, that the 
 absence of the sensory effects which result from the eyes in motion 
 is one of the factors most directly responsible for such geometrical 
 illusions. Nor is it by any means fair to disregard the sensations 
 of position and tendencies to movement, or memory images of past 
 movements, which may exist in the absence of actual movement. 
 From this point of view, the question of the relation of eye move- 
 ments to these illusions is lost in the general problem of the visual 
 perception of space. 
 
 25. Of "central" theories of the source of these illusions, the 
 most comprehensive are the perspective theory, the dynamic theory, 
 and what may perhaps be called the " confusion" theory. 
 
 The perspective theory starts from the undoubted facts that even 
 simple line drawings readily suggest objects in three dimensions; 
 and that the introduction by suggestion of the third dimension 
 must lead to changes in the apparent length and direction of the 
 lines composing the figures. The application of this principle to 
 some of the figures is readily made. Thus, vertical lines in the 
 field of view very often represent lines extending away from the ob- 
 server and foreshortened by perspective; such lines are interpreted 
 in accordance with their objective length, and therefore as longer 
 than equal horizontal lines of the field of view, which are not sub- 
 ject to foreshortening. In accordance with this tendency, vertical 
 lines in these simple figures appear longer than equal horizontal 
 lines. The application of the principle of perspective to many of 
 the illusions, 1 however, is much less direct, and finds little support 
 in introspection. 
 
 26. The dynamic theory is the work of Lipps, 2 and is, without 
 doubt, the most interesting and even fascinating of all. Lipps 
 urges that account must be taken of the inner activity of the observ- 
 ing subject, even though this activity may not come to conscious- 
 ness as anything separate from the figures, but may rather be pro- 
 
 1 For the application of this theory to many other illusions, see Thie*ry, in 
 Wundt's Philos. Studien, 1895, XI, 307, 603; 1896, XII, 67; and Filehne, 
 Zeitschr. /. PsychoL, 1898, XVII, 15. 
 
 2 Raumesthetik und geometrische Tduschungen, 1897; Zeitschrift /. PsychoL, 
 1898, XVIII, 405; 1905, XXXVIII, 241. 
 
450 
 
 PRESENTATIONS OF SENSE 
 
 jected into the figures, or "felt in" them. Always there is present, 
 in the apprehension of a figure, an "expansion" of the mental grasp 
 to include all of the figure, and an opposed bounding or limiting 
 activity, by which the figure is distinguished from surrounding space. 
 These activities are felt as belonging to the figure, and their balance 
 determines the apparent length of a line. But the presence of 
 other lines, as in the Miiller-Lyer figure, leads to expansions and 
 limitations which are appropriate enough as applied to the whole 
 figure, but not as applied to the particular lines which are to be 
 compared. In many cases, besides these expanding and limiting 
 tendencies, other tendencies, drawn from the dynamics of nature, 
 are felt in the figures. Thus, a vertical line suggests a struggle 
 
 Fia. 144. A Test of the Dynamic Theory. 
 
 against gravity, and the gravity-feeling in it affects its apparent 
 length. The same conceptions are applied by Lipps to the aes- 
 thetics of simple forms, and to architecture. 
 
 A difficulty with the dynamic theory is that it tends to run to 
 fanciful explanations. It explains too easily and too much; it could 
 often be just as well applied if the illusions were the opposite of 
 what they are; for the dynamics of these figures is usually quite am- 
 biguous. For example, a vertical line may be thought of as stand- 
 ing upright or as hanging downward; its relations to gravity are 
 opposite in the two cases; if gravity tends to compress it in the first 
 case, it tends to stretch it in the second, and therefore opposite 
 illusions ought to result from the two ways of looking at it. In 
 Fig. 144, therefore, there should appear quite a strong illusion as 
 between the two vertical lines, and this should be reversed on re- 
 versing the figure. 
 
 27. The "confusion theory" might also be called the attention 
 theory. It takes its start from the evident fact that the illusion 
 figures are so constructed as to make difficult the isolation of the 
 particular feature which is to be judged. It is difficult to fix atten- 
 tion on this one feature, thrusting aside all complicating features. 
 
CONFUSION THEORY OF ILLUSION 451 
 
 If this difficulty is only partially overcome, judgment will be based 
 on a partial confusion of complicating features with the feature 
 which is ostensibly judged. Thus, in attempting to compare the 
 areas of variously shaped surfaces (Figs. 142 and 143), one feels the 
 difficulty of isolating the bare feature of area from the more ob- 
 trusive feature of compactness; and the errors committed go to 
 show that the isolation has not been fully carried out. In the Miiller- 
 Lyer and other "confluxion" illusions, it is much easier to take ar- 
 row-heads, etc., as units, and judge the distance between them, than 
 to isolate the exact points which mark the ends of the distances to 
 be compared; and the errors committed seem to show that the iso- 
 lation is not fully carried out. The nature of the "confusion" in 
 the cases of "contrast" illusions is not so readily pointed out; and 
 in the vertical-horizontal and other specific illusions, special causes 
 would probably have to be invoked. Nor is it entirely easy to ap- 
 ply this conception to the illusions of angle and direction, though 
 the judgment of these matters must be admitted to be rather indi- 
 rect and indefinite, and such, therefore, as might probably be sub- 
 ject to confusion. The confusion theory has at least this advantage, 
 that it almost certainly affords the true explanation of some of the 
 illusions; and this is more than can be said of the other central 
 theories. But reference to confusion takes us only a little way, and, 
 beyond that, leaves various possibilities of explanation open in 
 each particular case. What is needed is to point out the nature of 
 the confusion in each class of illusions; and this cannot yet be done 
 with any certainty. 
 
 Rather in favor of the confusion theory, further, is the fact that 
 practice in making the required judgments, in such cases as the 
 Miiller-Lyer and Poggendorf figures, produces a gradual decrease 
 and ultimate disappearance of the illusion; and this result follows, 
 even though the observer does not know the nature of his errors. 1 
 The probable explanation of this practice effect is that the observer 
 is conscious of the difficulty of isolating the feature to be judged, 
 and therefore devotes himself to this isolation. The skill which he 
 thus acquires in thrusting aside complicating features of the figures 
 is in part a specific aptitude in dealing with a particular figure, and 
 may not be transferred promptly to another figure or even to a 
 changed position of the same figure, yet facility in dealing similarly 
 with another figure is more easily acquired because of the previous 
 practice. That the practice effect truly consists in the acquiring 
 of skin_iDLJsolatiori is indicated by the observation of Benussi 2 
 namely, that thejllusion is mujh,m.pre_oj.ijckly overcome when prac- 
 
 1 Judd, Psychol Rev., 1902, IX, 27; Lewis, Brit. Journ. of PsychoL, 1908, II, 294. 
 3 Op. cit. 
 
452 PRESENTATIONS OF SENSE 
 
 tice is governed from the outset by a clearly formulated intention 
 to isolate a certain feature of the figure; while, on the contrary, 
 practice with the definite intention of always grasping the figure as 
 a whole leads not to the usual decrease, but to an actual increase 
 of the illusion. In concluding the whole matter, we may confidently 
 assert that the geometrical illusions, or most of them, are chiefly 
 due to central rather than peripheral factors; and that these central 
 factors are intimately bound up with the tendency to apprehend 
 figures, and compact parts of figures, as wholes. It is, in most 
 cases, if not in all, as wholes, or units of " apperception," that we 
 have been accustomed to regard them. And this tendency may, 
 therefore, well enough be the chief cause of the illusion. 
 
 28. The necessity of appealing to certain obscure, but effective 
 central factors, as mingled with, or even dominating the peripheral 
 factors, is felt in trying to account for those errors of sense which 
 occur in connection with the strife and prevalence of contours, 
 and the binocular mixing and contrast of colors. If a well-defined 
 image of some contour, such as a sharp-marked limit between two 
 differently colored surfaces, be formed on one retina, and on the 
 corresponding points of the other the image of a uniform-colored 
 background, then only the former will be visible. This is called 
 the "prevalence of contours." But if the contours of the images of 
 two differently colored objects run on the retina so as to cross only 
 in one place, then sometimes one color and sometimes the other will 
 prevail and get itself perceived at that place. This is called "the 
 strife of contours." If two squares of red paper and two of blue, 
 all of equal size and brightness and without any distinguishing 
 marks, be laid side by side at equal distances, and their images 
 then combined, the color of the middle one of the binocular images 
 will at first be sometimes redder and sometimes bluer than that of 
 the two side images, but in no case exactly like either of them. By 
 steadily looking it is said to be possible to mix the colors of the two 
 objects in a binocular image which is reddish blue (or violet). 1 This 
 is called " the binocular mixing of colors." If such a deception can 
 be secured, and made subject to the more or less voluntary direc- 
 tion of attention, it is manifest that the mixing of colors on which it 
 depends must be largely central, and not wholly dependent upon 
 the retinas of the two eyes. If a white stripe be placed upon a black 
 surface and divided into two images, the right one of which is 
 formed by looking at one half through blue glass, the left by look- 
 ing through gray glass, then the right image will be seen blue, but the 
 left will be seen yellow. This is called " binocular contrast of colors." 
 
 1 So Hering asserts, "Physiolog. Optik," in Hermann's Handb. d. Physiol, III, 
 i, p. 592. Binocular mixing of colors has been denied by some authorities. 
 
BINOCULAR MIXING AND CONTRAST 453 
 
 The peculiar perception of lustre is due to a struggle between 
 the two fields of vision which results, not in combining the black 
 images of one field with the white images of the other so as to pro- 
 duce an equal tint of gray, but in a rapid alternation of the two. 
 Very smooth bodies, when they reflect the light perfectly, do not 
 appear lustrous. But when the surface of such bodies as, for 
 example, the surface of a sheet of water becomes ruffled by rip- 
 ples, it becomes lustrous. The perception of lustre may be pro- 
 duced by combining two stereoscopic pictures of an object which 
 are alike in contour, but one of which is black with white lines where 
 the other is white with black lines. Two such pictures not com- 
 bining to produce an equal tint of gray over the whole surface, the 
 images of the separate points on the two retinas enter into a strug- 
 gle with each other; and the rapid alternation of the prevalence, 
 first of one and then of the other, gives rise to the appearance of 
 lustre. 
 
 Similar conclusions seem forced upon us by our experience with 
 certain of the most common and persistent optical illusions of mo- 
 tion. The fact that a steady succession of images (as in the case 
 of watching a fall of water), passing over a particular region of the 
 retina for a long time, sometimes ceases to be perceived as a motion, 
 and that the image of a stationary body on the same retinal region 
 may appear to be moving in the opposite direction, has been ex- 
 plained by "Thomson's law." This law refers the phenomena to 
 the principle of fatigue. Recent investigations, however, seem to 
 show that the explanation is incorrect. They bring out the remark- 
 able result that the same elements of the retina, when stimulated 
 simultaneously, may give rise to impressions both of motion and 
 of rest. For this result some unknown law of cerebral action would 
 seem to afford the only possible explanation. 1 
 
 29. The fact that things are seen upright and in correct rela- 
 tions horizontally, by means of data furnished through inverted reti- 
 nal images, as well as all illusions and errors that are connected with 
 this normal fact, implies yet more maturity of experience. Why do 
 we see the upper part of the object by means of the lower part of the 
 retinal image, and vice versa ? and why do we see the right side of 
 the object by means of the left side of the retinal image, and vice 
 versa? Such questions have often been propounded as psycho- 
 logical puzzles of special difficulty. The only answer possible fol- 
 lows, obviously, from the foregoing principles. Strictly speaking, 
 we neither see the external object nor the retinal image; the field 
 of vision is a subjective affair, and is like neither of these two. The 
 presentation of visual sense is normally dependent upon the retinal 
 1 See Bowditch and Hall, Journal of Physiology, III, pp. 299 f. 
 
454 PRESENTATIONS OF SENSE 
 
 image for the data from which it is constructed; the image is de- 
 pendent upon the external object for its formation by rays of light 
 reflected from the object and converged upon the nervous elements 
 of the retina. The different parts of the object as seen are pri- 
 marily localized simply with reference to each other by means of 
 local retinal signs and of complex sensations produced by motion 
 of the eyes. But as yet the field of vision has no locality in ob- 
 jective space; no part of it can be said to be either up or down, 
 either right or left. The use of such terms of position implies an 
 association of localized sensations of sight with those of touch and 
 of the muscular sense, in giving us a picture of the relation of the 
 different parts of the body to each other, and of the entire body to 
 the ground, the sky, and the various parts of surrounding objects. 
 When the eyes are moved downward, the lower parts of the body 
 and objects situated on the ground successively come into the field 
 of vision; when the eyes are moved upward, the near ground and 
 lower parts of objects successively disappear from the field of vision, 
 and remoter or higher objects come to view. Seeing objects to the 
 right or to the left is accomplished by motion of the eyes in the cor- 
 responding direction. Right is the direction in which the right 
 hand is placed from the middle of the body; left is the direction in 
 which the left hand is found. The massive feelings of touch and 
 muscular sensation keep us informed of the general relation of our 
 bodies to the earth and to objects on its surface. The head is the 
 upper part, or part farthest away from the ground; the feet are the 
 lower part, or members of the body in contact with the ground. 
 Thus we come to use terms for localized sensations of sight which, 
 in this use of them, have no primary reference whatever to the field 
 of vision in itself considered. 
 
 30. The above view of the relation of erect vision to the in- 
 verted retinal image is substantiated by the experience of one look- 
 ing through a microscope. Since the ordinary microscope itself 
 gives an inverted image of the object, the second inversion by the 
 eye gives an image on the retina which is erect with respect to the 
 object, though inverted with respect to the usual retinal image of 
 an object. There is, however, no distortion or internal change 
 (except magnification) in the spatial relations within the image. 
 Nor is there any change in the eye movements necessary to bring 
 any part of the image to the area of clear vision; for these movements 
 all involve a movement of the fovea toward a point of the optical 
 image on the retina, and the latter movement remains the same no 
 matter what inversions, or even distortions, the rays of light may have 
 undergone on their way to the retina. In other words, the eye- 
 movement value, as it may be called, of the various retinal points is 
 
UPRIGHT AND INVERTED VISION 455 
 
 unaltered by changes in the rays of light on their way to the retina. 
 Therefore, the observer, looking through the microscope, obtains 
 an internally correct view of the object, and experiences no difficulty 
 in directing his eyes to any point of it. His difficulty begins, how- 
 ever, when he attempts to move the object under the microscope, 
 so as to bring into view a fresh part of it; for now the long-standing 
 connection between the retina and the movements of the hand plays 
 him false; and he is sure to move the object to the left when he should 
 move it to the right, and up when he should move it down. No long 
 experience with the microscope, however, is needed to overcome this 
 difficulty by establishing a new association between retinal direc- 
 tions and appropriate hand movements. This new association is 
 operative only within the special situation of looking through a 
 microscope; it gives way at once to the usual association on removing 
 the eye from the instrument. Much the same effect of practice 
 is noted in handling objects while viewing them through a mirror; 
 though here the inversion is not complete. 
 
 An interesting experiment, along the line of these observations, 
 was performed by Stratton. 1 While covering one eye, he placed 
 before the other an optical instrument which, like the microscope, 
 gave an inverted image of objects, and so an upright image on the 
 retina. No light was allowed to enter the eye except through this 
 inverting instrument, and this instrument was worn continuously in 
 the daytime, both eyes being covered when the instrument was re- 
 moved for sleep. The experiment was continued for a week, with the 
 result that movements of the hands and body, which on the first day 
 suffered great confusion, became rapidly associated with the novel 
 visual relations, so that before the end of the week all bodily move- 
 ments were correctly and promptly executed under the direction 
 of the eye; and, whereas at first everything had appeared upside 
 down, this appearance also gave place to a normal appearance after 
 a few days. When the instrument was removed at the end of the 
 week, a new period of confusion, false movements, and inverted ap- 
 pearance supervened before the old association was re-established. 
 This experiment, as well as the experiences with the microscope and 
 mirror, are interesting as revealing a remarkable looseness or flexi- 
 bility in the association between the spatial perceptions of different 
 senses. 
 
 31. In concluding this brief survey of the very fruitful field 
 afforded by so-called "errors of sense," as bearing upon any attempt 
 to frame an adequate theory of visual perception, we mention the 
 following inferences as those it seems to us which are most amply 
 
 1 "Vision without Inversion of the Retinal Image," Psychol. Rev., 1897, IV, 
 341, 463. 
 
456 PRESENTATIONS OF SENSE 
 
 supported. It should be said, in a preliminary way, that the great 
 variety of special and, too often, exclusive and partisan theories 
 adopted by different observers, is what, under the circumstances, 
 might be expected. For explanation in general is, for this sort of 
 phenomena, an extremely intricate affair. And this is true, even 
 when the phenomena themselves seem to be of the utmost simplicity 
 in character. A variety of theories, therefore, may well enough ap- 
 peal to certain facts of experience, or rather, to certain aspects of 
 common facts, to sustain their special contentions. Perhaps, too, 
 they are all needed; or at least, they may all be said to contain cer- 
 tain of the elements indispensable to a complete explanation. The 
 difficulty with nearly all of them, however, is that they fail to take 
 sufficient account of the equally important facts on which other 
 rival theories are based. Moreover, it must not be forgotten that 
 essentially the same errors of sense may in differing individual cases, 
 or under slightly differing circumstances, require a somewhat differ- 
 ent set of explanations. The nervous mechanism engaged, and 
 the mental development acquired, in the growth of perception 
 through the use of the eyes, and the associations of this growth with 
 the senses of touch and hearing, are variable to a certain extent 
 which cannot be determined a priori. It is a well-known fact that 
 the mental pictures and conceptions of spatial extensions and of the 
 relations of objects in space, are not by any means the same with 
 all adults. The claim has even been made that they are, in general, 
 somewhat markedly different with the two sexes. And certainly, 
 in this regard, the accomplished geometrician, or microscopist, or 
 astronomer, is at a world-wide remove from the savage or the newly- 
 born infant. 
 
 32. At the same time we repeat, with increased confidence, 
 that the most recent investigations in this line tend to confirm, in 
 its essential features, the theory of visual perception, especially on 
 the side of the apprehension of spatial qualities and spatial relations, 
 which has been advocated in the preceding sections; and which 
 may at least, for most of its more important connections be as- 
 sociated with the great names of Lotze, Helmholtz, and Wundt. 
 Its essential features, as we are inclined to adopt it, while confess- 
 ing the many obscurities and difficulties which still accompany any 
 attempt at completing it, are the following: 
 
 First: The data, or factors, on the basis of which, so to say, the 
 construction of visual space proceeds, have all the characteristics 
 required by the so-called "local signs" (compare p. 386). They 
 have these characteristics in the highest degree. They are series of 
 sensations, which, as series, are of like quality; which admit of 
 easy, rapid, and frequent repetition in a varying order of arrange- 
 
CONCLUSIONS AS TO VISUAL PERCEPTION 457 
 
 ment; and which, by the very nature of the organism and the condi- 
 tions of mental development, are adapted to combine with other 
 series of like characteristics. Of such series the most important are 
 (1) the retinal signs, and (2) the entire complex of tangled and ob- 
 scure sensations which accompany and follow the attaining and 
 holding of any position, by movement of the eyes. 
 
 Second: As to the retinal signs, they may most properly be spoken 
 of as "native" at least, in the sense that their existence and dis- 
 criminable character must be assumed as "given" (datum) in any 
 attempt to explain the initial steps of visual perception. There is 
 scarcely less doubt that the motor reactions which, of necessity, 
 evoke the first complex sensations of position (even of eye-strain) 
 and of motion, are native, in the sense that they are innate powers 
 and are not learned by the infant. Indeed, it is by no means cer- 
 tain that the resulting sensations may not have from the first a cer- 
 tain value as local signs, comparable in a way to that of the associ- 
 ated local signs of the retina. 
 
 In this connection, attention should be called to the fact that the 
 "centre of clear vision" is not a point, but an area within which 
 many points can be distinguished; this makes it difficult to accept 
 any theory which regards visual space as organized about a single 
 point. The analogy of latitude and longitude in geography, or 
 of co-ordinate geometry, is tempting to the psychologist but is rather 
 misleading. The organization is indeed partly of this sort, as is 
 shown by the fact that an object seen in indirect vision attracts the 
 eye to make a movement toward it; and that this movement is 
 about sufficient in extent to bring the object to the centre of clear 
 vision. To regard visual space as a system of "polar co-ordinates," 
 the position of every point in the field of view being determined by 
 its direction from the "origin," or centre of clear vision, and by its 
 distance from that origin, is not, however, an adequate account of 
 visual space. The "origin" is here not really a point; and the sub- 
 division of the field is much finer than can be accounted for in this 
 way. Neighboring and probably also distant points in the field 
 must be related directly to one another, and not simply to the centre 
 of clear vision. The actual organization of visual space is, in gen- 
 eral, from a mathematical point of view, much less simple and clean- 
 cut than a system of polar co-ordinates. 1 
 
 Third: As between nativistic and empiristic theories, the balance 
 of evidence seems to favor the former. Certainly both types of 
 eye movement are innate powers, and not learned by the infant. 
 Their native character does not, perhaps, prove that spatial con- 
 
 1 See Dodge, Psychol Rev., Monograph Suppl. XXXV (1907), p. 64. 
 
458 PRESENTATIONS OF SENSE 
 
 sciousness is innate, but it does show that the spatial organization 
 of the retina and of its nervous connections is partly innate. Bio- 
 logically, it would be difficult to believe that a fixed form of organiza- 
 tion, such as that of visual space, should be newly acquired by every 
 individual. This consideration applies with most force to two- 
 dimensional space, but also with some probability to the third di- 
 mension. In so far as distance from the eye is definitely related to 
 binocular parallax, the reactions of the eyes and limbs to the distance 
 of objects might well be, in a measure, instinctive. They certainly 
 are instinctive in some animals; thus the chick, on emerging from 
 the egg, correctly gauges the distance of a grain at which he pecks; 
 and human infants, in spite of what has sometimes been alleged 
 to the contrary, show little tendency to reach for the moon or for 
 anything that is definitely beyond their reach. 
 
 Fourth: The incalculable, but enormous influence on sense-ex- 
 perience which lies back of all the phenomena obtained for scien- 
 tific treatment, whether from the physiological, the psycho-experi- 
 mental, or the purely introspective point of view, must never be lost 
 out of account. Indeed, these residua of past experiences, if we 
 may so call them, are doubtless in many cases the determining causes 
 of the character of the new experience. They consist of obscure 
 and scarcely recognized sensations, images of previous sensations, 
 motor tendencies and impressions, fusions of unanalyzable elements, 
 flighty and flitting syntheses that have scarcely the quality of even 
 an instinctively formed judgment; and perhaps, above all, 
 workings of the organism which do not result in any effect that rises 
 " above the threshold" of consciousness. But it is just such un- 
 recognized, and largely unanalyzable, factors as these, which 
 chiefly determine, not only our conduct under the direction of sight, 
 but also our seemingly most logical conceptions and deliberate 
 judgments concerning visual objects. 
 
 Fifth: To deny the influence of "central" factors, and the ne- 
 cessity of a theory of perception which gives them their full value, 
 would be to controvert everything which we know about the nature 
 of the nervous mechanism and its relations to the awakening and 
 development of mental life. We have already said that only 
 psychical factors can be fused, or united, or consciously judged to- 
 gether in any sort of relation, in a mental product. Of this declara- 
 tion, the physiological correlate is, of course, the recognition of 
 the influence of the cerebral mechanism in every sort, and every in- 
 dividual experience, of sense-perception. And we only return once 
 more to the psychological point of view when we recognize anew the 
 abundant evidence that attention, active discrimination, varied 
 forms of conscious feeling other than sensation, and innumerable 
 
GRAPHIC RECORDS OF EYE MOVEMENTS 459 
 
 forms of association, all enter into our experience of objects as 
 known by the organism employed in vision. 
 
 33. In all the older discussions there was little exact knowledge 
 of the actual movements made by the eyes in their mastery of the 
 details of spatial perception. Even the laws summarized under 
 the name of Listing were based upon a study of the positions reached 
 by movement rather than of the movements themselves. Recently, 
 several observers have obtained actual registration of these move- 
 ments; and the results must, assuredly, be harmonized with any 
 theory of visual space-perception. 
 
 The simple experiment of comparing the observations of the move- 
 ments of the eyes, as seen by another observer, with those which 
 our consciousness seems to assure us are made, is sufficient to show 
 how uncertain is the latter kind of evidence in support of the facts. 
 In reading a line of print, for example, one may seem to one's self 
 to be moving the eyes smoothly along the line, while objective ob- 
 servation shows that the actual movements are a series of jerks, 
 separated by brief periods of rest. The jerky character of this 
 movement is difficult, or impossible to correct; whereas, in follow- 
 ing a moving object the eyes themselves move in a fairly steady 
 way and at a speed regulated by the speed of the object. 
 
 34. The needed method of obtaining an accurate graphic 
 record of the eye movements was first successfully applied by Dela- 
 barre, 1 and was also employed with success by Huey. 2 These ex- 
 perimenters obtained a record by attaching a light disk to the sur- 
 face of the eyeball; and from the disk a thread ran to a light lever 
 which wrote upon a moving surface. This method had the disad- 
 vantage of weighting the eye and so producing possible distortions 
 in its movements. 
 
 A much improved device for photographing the eye while in 
 motion has more recently been accomplished by several observers. 3 
 This method consists in general of obtaining a photographic record 
 of the movements of a beam of light reflected from the cornea itself, 
 or from some object attached to the cornea. Among the important 
 facts established by this method are one or two relating to the fixa- 
 tion of the eye. When the eye "rests upon an object," the fovea, 
 or centre of clear vision, has a diameter of one or two degrees, and 
 what falls outside of this area is seen with diminishing clearness as 
 
 1 American Journal of Psychology (1898), IX, 572. 
 
 3 Ibid., p. 575; and (1900) XIII, 282. 
 
 3 For a more detailed account of the methods and results of these experiments, 
 see Dodge and Cline, Psychol. Rev. (1901), VIII, 145; Stratton, Phil Stud. (1902), 
 XX, 336, and Psychol Rev. (1906), XIII, 82; and Judd, Psychol. Rev., Mono- 
 graph Supplement XXIX (1905). 
 
460 PRESENTATIONS OF SENSE 
 
 we move toward the periphery of the visual field. In other words, 
 the fovea is not a mathematical point; and, consequently, fixation 
 of the eye is itself not a perfectly precise and rigid affair. Within 
 this area there is a constant slow (exploring ?) movement of the eye ; 
 and if the eye is momentarily turned away from any point in the 
 area, on bringing it back to examine the same point its new posi- 
 tion is not usually the same as the old. 1 
 
 35. In regard to speed, the movements of the eye fall into two 
 main classes. 2 Of the first type are the movements by which an 
 object, first seen in indirect vision, is brought to the centre of clear 
 vision. This is the simple reactive movement of the eye to periph- 
 eral stimulation, with its psychological accompaniment of a trans- 
 ference of attention. Its speed depends upon the extent of the move- 
 ment, being more rapid for longer than for shorter movements; it 
 varies from .029 seconds for movements of 5 degrees to .100 for 
 movements of 40 degrees. 3 In reading, however, the extent of this 
 movement is often less than 5 degrees, and, in time, not more than 
 one-fiftieth of a second. The speed is not readily, or at all, sub- 
 jected to voluntary control. The function of this first type of eye 
 movement is simply that of carrying the eye from one point of 
 fixation to another, with the least possible loss of time. No clear 
 vision results from the stimulation of the retina during these jumps 
 of the eye nothing, in ordinary circumstances, but a brief and 
 featureless blur, which has no value for purposes of visual percep- 
 tion, and which is usually unobserved. 4 
 
 The second main type of movement is the "pursuit" movement, 
 which occurs when a moving object is followed by the eye. This 
 is determined by the speed of the object, but neither very swiftly, 
 nor very slowly, can moving objects be followed by the eye. This 
 
 1 McAllister, Psycholog. Rev., Monograph Supplement XXIX (1905), p. 17. 
 The reader can readily satisfy himself of the truth of these statements by what is 
 called the "after-image method" of studying eye movements a method long 
 in use, and in many respects valuable. An after-image resulting from fixation 
 of a bright object moves with the eye and represents a fixed point of reference 
 in the retina. If a sharp after-image is first secured, and then the attempt is 
 made to look steadily at a point on the wall, the after-image will seem to move 
 about over the wall, indicating the unsteadiness of fixation (see Dodge, Psychol. 
 Rev., Monograph Supplement 35, 1907). A variation of this experiment shows 
 that in repeatedly looking at the same object, there is a certain latitude in the 
 fixations of the eye. If a bright light, such as the rising or setting sun, is looked 
 at repeatedly for a second or two, the eye being turned to some other object be- 
 tween the fixations, quite a group of after-images of the sun will accumulate; 
 and the extent of the group will show the variability of repeated fixations. 
 
 2 Dodge, Amer. Journ. of Physiol., VIII, 301. 
 
 3 See the table given by Dodge and Cline, op. cit., 145. 
 
 4 Dodge, Psychol. Rev., 1900, VII, 454. 
 
SPEED OF EYE MOVEMENTS 461 
 
 second type of movements of the eye differs from the first type, not 
 only in speed, but also in function. Pursuit movements cannot be 
 executed at will; they occur only in the presence of a moving ob- 
 ject. Without them, however, clear vision of moving objects is 
 impossible; as can easily be shown by casting a shadow on a page 
 of print, steadily fixating the shadow, and then either moving the 
 page from side to side before the fixed eye, or causing the shadow to 
 move back and forth over the page and al- 
 lowing the eye to follow it. In either case, 
 the print becomes unreadable through blur- 
 ring. The same effect is produced when the 
 eye passes rapidly from one to another of a 
 few bright spots standing out against a dark 
 background. From these and similar expe- 
 riences, we reach the conclusion that, in read- 
 ing, we obtain fairly clear retinal impressions 
 only during the fixation pauses of the eye, 
 and that the practical valuelessness of the Movement in Tracing the 
 
 j * ui i j Ai_ i j? Outline of a Circle. (Strat- 
 
 periods or blurring leads to the neglect or ton.) 
 them. The same kind of jerky movement 
 
 of the eyes, with periods of fixation between periods of blurring, 
 occurs when we seem to ourselves to be sweeping our eyes slowly 
 over any geometrical figure, or over any scene. This process has 
 been carefully worked out in the case of one reading a printed page. 1 
 The eye moves across the line of a newspaper with a series of 
 jumps, fixating from 3 to 8 points in a line, and with a speed de- 
 pending on the mentality of the reader. 2 The accompanying fig- 
 ure (No. 145), taken from Stratton, 3 shows how the eye "moves 
 around" a circle which has been placed before it. 
 
 36. In general, it must be noted that the power of making 
 spatial distinctions greatly exceeds the accuracy with which dis- 
 tances can be measured by the moving eye. A circle of an inch in 
 diameter, placed five feet from the eye, is practically a unit to fixa- 
 tion, although it contains a multitude of distinguishable points, 
 and a variety of perceptible lines, angles, and figures. Spatial dis- 
 tinctions in indirect vision are finer than the adjustment of eye move- 
 ments to points in indirect vision. The eye can measure the dis- 
 tance between two points much more exactly than it can jump from 
 one to another. 4 It can distinguish forms much more accurately 
 than it can follow them by its movements. 
 
 1 Compare Huey, American Jour, of Psychol. (1900), XIII, 283; and Dearborn, 
 The Psychology of Reading (New York, 1906). 
 
 2 Ruediger, The Field of Clear Vision (New York, 1907). 
 
 3 Philos. Studien (1902), XX, 336, and Psychol. Rev. (1906), XIII, 82. 
 
 4 McAllister, op. cit., p. 17; Dearborn, Psychol. Rev., 1904, XI, 297. 
 
462 PRESENTATIONS OF SENSE 
 
 These experiments also show that we can no longer appeal, as 
 Helmholtz and Wundt formerly did, to the supposed orderly pro- 
 cession of adjacent points of the field of view through the area of 
 clear vision as the eye is moved. There is no such procession; and 
 nothing objective passes in such review. As a specific instance of 
 the difficulty connected with this same view, let us consider a prob- 
 lem which is fundamental in the perception of two-dimensional 
 space. Such a space is characterized, as Helmholtz pointed out, 1 
 by the circumstance that, given any two points in it, A and B, it 
 is possible to enclose A by a circle or any other closed line, so that 
 passage from A to B can only occur by crossing the line. Now 
 Helmholtz endeavors to show that experience could teach us this 
 peculiarity of the visual field, since, whenever we turn the eye from 
 A to B, we always observe the fixation point to cross the separating 
 line. As a matter of fact, no such observation could ordinarily 
 be made, for the separating line would simply disappear, in a brief 
 unobservable blur, when the eye jumped from A to B. If the ex- 
 perience taught anything, it would apparently be that movement 
 from A to B did not take the eye across the line encircling A. So 
 elementary a spatial relation as that of a point or line lying between 
 two other points could not be observed in the course of eye move- 
 ments of the common jump type; for the eye would not show us the 
 middle point in the course of its movement between the extremes. 
 The eye might, of course, fixate first one of the outer points, next 
 the middle point, and finally the other extreme; but it might also 
 pass directly from one extreme to the other, and then to the middle. 
 No fixed ordering of the points in the field of view could result 
 from such experiences. To be sure, when objects move about 
 within the field, the eye may follow them, and during such rela- 
 tively slow movements observe the sequence of stationary objects 
 over which the moving object passes; but the moving object may 
 take any course, and the experience so gained would seem unfitted 
 to give rise to a definite arrangement of points in the visual field, 
 and so to a well-ordered field of space. All such experiences, in 
 conjunction with others of the most varied kind, doubtless con- 
 tribute to our knowledge of the spatial relations of objects; but the 
 point is that the quasi-mathematical correspondence which Helm- 
 holtz conceived to exist between eye movements and visual space 
 breaks down utterly before the fact that the jump of the eye the 
 only type of eye movement which shows any approach to the mathe- 
 matical precision required by the theory is too rapid to admit of 
 perception during its execution, and amounts simply to a sudden 
 shift of clear vision from one object to another. 
 
 1 Physiolog. Optik, pp. 533 ff. 
 
VALUE OF OBJECTIVE MEASUREMENTS 463 
 
 37. Once again, then, in estimating the bearing of the recent 
 experiments to determine more exactly the movements of the eyes, 
 upon our theory of sense-perception, is it necessary to return to the 
 original point of view from which all theories must take their start. 
 Only psychical factors can be built into mental products. Objective 
 measurements of visual movements are, therefore, of little value 
 for theoretical purposes, except as they give sure indication of the 
 present, or past, value of the quantitative and qualitative changes 
 in the psychical factors that are correlated with these movements. 
 A closer examination of these very experiments shows that, on the 
 whole, they do not destroy, or even greatly depreciate, our estimate 
 of this value. We have seen, for example, that when the eyes seem 
 to us to be at complete rest while regarding attentively the field 
 covered by the area of clear vision, they are really going through 
 a succession of exceedingly minute and slow movements, as though 
 ceaselessly " exploring" this field. Something similar may be 
 brought at least vaguely into consciousness, by closing the eyes 
 and watching the behavior of the retinal field as correlated with the 
 changing complex of sensations. Neither the field itself, nor any 
 of the minutest subdivisions of it try as hard as one will to fixate 
 it rigidly can be kept from ceaseless motions; and by directing 
 attention to the psychical accompaniments of these motions, they 
 can themselves be regularly felt. Still further, the jerky and dis- 
 continuous character of the movements of the eyes, with the inter- 
 vening pauses for momentary fixation, when reading print or sur- 
 veying a geometrical figure, or a landscape, corresponds precisely 
 with the correlated mental experience. Neither in reading print 
 (nor in hearing speech), nor in looking at any object whatever, do 
 we become consciously aware of more than a percentage of what, 
 objectively considered, passes through the field of vision. Our 
 mental apprehension is as jerky, as imperfect and full of blurred or 
 blind spots, as are the movements of the eyes. As we have just 
 seen, even the area of clearest vision requires to be diligently ex- 
 plored with slowly moving eyes, in order to become the more thor- 
 oughly apprehended. 
 
 Still further, it needs again to be insisted, that we are examining 
 by relatively simple and coarse methods, the swift and complex 
 processes of nature, where most of what there is to be examined has 
 long ago fallen below the threshold of consciousness, and has be- 
 come quite impossible to revive in anything approaching its orig- 
 inal form. To employ again an illustration already used once before: 
 By aid of delicate tactual and muscular sensations, which he once 
 followed comparatively slowly and on the watch, as it were, to re- 
 produce them, the accomplished violinist has acquired the art of 
 
464 PRESENTATIONS OF SENSE 
 
 spacing and bowing on the violin. But he cannot recall or describe 
 those sensations; and the very effort to do so in any particular in- 
 stance would paralyze his art. Neither can the experimenter de- 
 termine their original quality as local signs, or specific value, by 
 measuring the movements of the ringers and wrist of the left arm, 
 or the sweep and the pressure of the right arm. But let paralysis 
 impair the sensations of the skin and muscles of either of these 
 members, and the art of the violinist instantly disappears. Al- 
 though, however, the accomplished player can determine almost 
 exactly the number of thousands of vibrations of his string, by mov- 
 ing his finger a minute fraction of an inch, his art is simple and 
 coarse compared with that with which Nature endows the common- 
 est pair of eyes. 
 
 38. The nature of the "sense-data" which the mind has at its 
 disposal for constructing its presentations of sense, and the psycho- 
 physical laws which are followed in the process of construction, 
 have been explained in such detail that little need be added con- 
 cerning the development of visual perception. 1 Visual space pre- 
 sents itself to us as a coherent complex of sensations of light and 
 color systematically arranged. The arrangement implies certain 
 native activities of the mind in connection with and dependence 
 upon the action of the nervous organism; but it also implies an 
 immense influence from experience. It is extremely difficult, if not 
 wholly impossible, to distinguish with confidence the limits which 
 must be drawn between what is native and what is learned. The 
 seeing of colors is undoubtedly a far more simple and primary act 
 than the seeing of colored objects as situated in relation to each 
 other in objective space. A colored surface, or a system of color- 
 sensations related to each other as side by side in space-form, re- 
 sults in experience from the weaving together of several spatial 
 series of sensations. Such a surface may theoretically be conceived 
 of as presented to the mind through the activity of the nervous 
 elements belonging to the retina of a single motionless eye. The 
 motifs or data which the mind would have for constructing such a 
 surface must be found in the series of sensations of light and color 
 as varying in intensity and quality according to the locally distinct 
 nervous elements which are simultaneously excited. The evidence 
 seems, on the whole, favorable to the assumption that some indefi- 
 nite picture of visual space might be gained wholly through the 
 excitation of a motionless nervous mosaic (like the retina) sensitive 
 to light. 
 
 But visual space, as experience makes it known to us, requires 
 
 1 On this subject, compare Ladd, Psychology, Descriptive and Explanatory, 
 pp. 321-375; 487-495; and A Theory of Reality, chap. IX. 
 
DEVELOPMENT OF VISUAL PERCEPTION 465 
 
 binocular vision with moving eyes. The firm spatial connection of 
 all the parts requires that a system of lines of direction should be 
 fixed, prescribing the objective points at which the sensations pro- 
 duced by exciting together the different pairs of the covering points 
 of the retina must appear in visual space. To establish such spatial 
 connection, both eyes must move in their conjoined action as a 
 single organ of vision. By this action the field of binocular vision 
 is built up in an order of experience which, on the whole, consists 
 in the successive mastery of more and more complex problems. 
 For the process of learning to localize, the one centre the point 
 of starting and the goal of return is the area of clearest vision of 
 the retina (the yellow-spot), to which the point of regard in the ob- 
 ject corresponds. With the point of regard fixed in the primary 
 position of the eye, the first and most essential means is gained for 
 orientating objects in the field of vision. The meridians, horizontal 
 and vertical, and the locations of different points in the surface of 
 the field of vision thus presented to the ruind, afford the compara- 
 tively simple problems furnished by the primary position. In this 
 way a central area, determining lines, and finally a continuous sur- 
 face are fixed, to which may be referred all the directions and loca- 
 tions of the binocular points and lines of regard in the secondary 
 positions of the eye. 
 
 In constructing the field of binocular vision with moving eyes, the 
 general principle seems to be observed that by motion the relative 
 space-values of the retinal elements are not changed; but their ab- 
 solute values that is, the complex which is formed by combining 
 all these muscular and tactual sensations with the local signs of the 
 retina are changed in equal sense and measure. What moving the 
 eyes does for the retinal images, moving the head and body does 
 for the presentations of sense as constructed in binocular vision; 
 it alters the absolute values of the complex of sensation as related 
 to objective space, while keeping the relative values belonging to 
 the different positions of the eyes unchanged. 
 
 The visual perception of depth involves a later and more complex 
 training from experience than the perception of two-dimensioned 
 extension. To solve at all adequately the problem of depth, bin- 
 ocular vision with moving eyes, and its resulting combination and 
 separation of the double images of objects, seems necessary. The 
 existence and assistance of those secondary helps, which are so 
 important in perceiving the solidity and distance of objects, imply 
 a further development of experience. In all these advances, how- 
 ever, the course of acquisition is not in separate straight lines that 
 run parallel or converge, as it were. More complex experience, 
 when obtained, modifies what is really more simple and primary. 
 
466 PRESENTATIONS OF SENSE 
 
 What we see in monocular vision with an open eye, and even what we 
 see with both eyes closed and motionless, depends upon what we 
 have learned to see with both eyes in varied movement and avail- 
 ing themselves of all possible secondary helps. It also depends 
 upon what we have learned to know of the nature and probable posi- 
 tion and shape of manifold objects of which the eye has already at- 
 tained the mastery. How simple the visual data which the mind 
 may have learned to interpret into terms of complicated visual ob- 
 jects, placed in definite spatial relations, is amply proved by every 
 illustrated lecture, and by the phenomena of visual dreams. 1 Not 
 infrequently indeed, habitually what the eyes present in the form 
 of visual images, strictly so called, are the barest schemata of what 
 the mind sees by way of interpreting these images. 
 
 39. Finally, brief mention must be made of the connections 
 which are constituted, in the development of our perception of ob- 
 jects as having the qualities and relations of space-form, by the 
 joint action and mutual assistance of eye and hand. With the 
 sense-presentations of one of these senses the images of objects as 
 known by the other become most intimately related. It is a misuse 
 of terms, however, and involves the entire subject in confusion, to 
 speak of this joint product as a "sense-perception." It is rather to 
 be spoken of as a mental image or concept. The visual presenta- 
 tion of an object as, for example, a ball, a pen, a table may re- 
 call its tactual presentation. We readily interpret one into terms 
 of the other sight into terms of touch, and touch into terms of 
 sight. But all the perceptions, as such, of spatial properties and 
 relations, whether gained by eye or hand, are kept quite distinct 
 and separable in the mind. No such synthesis takes place between 
 the spatial series of the one sense and the spatial series of the other 
 sense as takes place between the spatial series of the same sense. 
 And all the properties and relations of bodies as known in space- 
 form are given by each of these senses. The view which makes the 
 sense of sight dependent upon the sense of touch and the muscu- 
 lar sense for the construction of its spatial objects is erroneous. 
 While feeling the pen, we can image how it would look; when 
 seeing it, how it would feel. We can image how much exertion 
 would be required to reach a mountain which appears to the eye so 
 far away, or how a mountain would look at a distance of so many 
 miles as measured by the exertion required to walk there. But the 
 true presentations of the visual objects and tactual objects do not 
 mix in one combined perception. They unite only in one image or 
 idea of the object. 
 
 1 On the latter subject compare Ladd, Mind, New Series, I, p. 299. 
 
VISUAL AND MUSCULAR PERCEPTION 467 
 
 40. Interesting experiments have been conducted to determine 
 the degree of accuracy with which perceptions of distance by sight 
 can be translated, as it were, into terms of the tactual and mus- 
 cular sense. Some of these experiments show the amount of har- 
 mony which can be obtained between optical localizing and localiz- 
 ing with the finger. Helmholtz 1 made use of a vertical thread 
 which he tried to locate, as seen in monocular vision, by hitting it 
 with a pencil's point; Donders, 2 of a very small induction-spark, 
 which was to be touched with the index-finger. The result of 50 
 experiments, made for distances along the same line of regard vary- 
 ing between 60 and 610 mm., when only the spark itself was seen in 
 perfectly dark surroundings, showed that the distance was over- 
 estimated 34 times, under-estimated 12, estimated right 4 times. 
 The greatest errors were +35 and 34 mm.; the mean error 
 10.6 mm. When the surroundings were visible and the electrodes 
 seen with open eyes, the eyes then closed, and the finger reached 
 to the estimated distance, the greatest errors were +30 and 12 
 mm., and the mean variable error 9.8 mm., for distances from 80 to 
 630 mm. The exact localizing of the point of regard in terms of 
 touch is more difficult the farther the object is removed and the 
 less assistance is had from secondary helps. Localizing in the 
 same way when the object lies out of the line of regard is still more 
 inaccurate. In 29 experiments, where the spark to be localized 
 was flashed at a distance of 210-600 mm. to one side of this line, 
 the greatest errors were +120 and 68 mm., with a mean error 
 of about 34 mm. 
 
 The problem of comparing the judgments of linear extension 
 made by the eye, the hand, and the arm, and of determining their 
 relative accuracy, has more recently been examined, experimentally, 
 at considerable length by Jastrow. 3 His method was to present a 
 definite length, varying from 5 mm. to 120 mm., to the retina, the 
 skin (by application of a pair of points, or by motion of a single 
 point), to the forefinger and thumb (by being held between the 
 two), or to the arm when in free movement and guiding a pencil to 
 express its estimate. The subject of experiment was required to 
 get a clear perception of the given distance by one of these organs 
 (called, in such case, the " receiving sense"), and then either si- 
 multaneously or successively express this perception through the 
 same or some other one of these organs (the "expressing sense"). 
 In this manner it was discovered that, if the eye is both receiving 
 
 1 Physiolog. Optik, p. 650. 
 
 2 Archiv /. Ophthalmologie, XVII, ii, p. 55. 
 
 3 Article on "The Perception of Space by Disparate Senses," in Mind, October, 
 1886, pp. 539-554. 
 
468 PRESENTATIONS OF SENSE 
 
 and expressing sense, small lengths will be under-estimated and 
 large lengths exaggerated, the point where no error is made being 
 at about 38 mm.; whereas, if the hand is both receiving and ex- 
 pressing, small lengths will be exaggerated and large lengths un- 
 der-estimated, the "indifference-point" being at about 50 mm.; 
 but the arm exaggerates all lengths within the limits of the experi- 
 ments. When, however, the eye expresses and the other organs 
 receive the impression, all lengths are greatly under-estimated; but 
 if the hand is the expressing sense, all lengths are greatly exagger- 
 ated. The arm as expressing sense exaggerates all lengths received 
 by the eye, and under-estimates all received by the hand. 
 
 The relative accuracy of the three senses, whether receiving or 
 expressing, or both, stands in the order of eye, hand, arm the hand 
 being only slightly better than the arm. The degree of confidence 
 felt in the estimate made is naturally greatest where the accuracy 
 is greatest. Inasmuch as "the expressing sense gives the charac- 
 teristic properties to the curve of error," 1 the question arises whether 
 all the phenomena cannot be accounted for by a special applica- 
 tion of the law of habit in connection with the normal action of the 
 sensory apparatus. Each sense, when expressing the estimate, 
 tends to approximate it in size toward those dimensions which it is 
 most accustomed to judge accurately. 
 
 All the foregoing results show plainly that the interpretation of 
 visual distance in terms of the tactual and muscular sense is a mat- 
 ter of complex experience, and is not usually more than very im- 
 perfectly attained. It bears little comparison with the nicety of 
 the spatial perceptions belonging to each one of the two senses 
 concerned when interpreting its own specific data in corresponding 
 terms, as it were. 
 
 41. In closing this subject, the one psychological truth of pre- 
 eminent value which has been most obviously demonstrated should 
 be stated again. Perception is the result of an extremely complex 
 activity of the psychical subject, Mind; it involves the synthesis of 
 a number of sense-data according to laws that are not deducible 
 from the nature of the external objects, or of the physiological ac- 
 tion of the end-organs and central organs of sense. An analysis of 
 these data themselves is not sufficient to explain perception. The 
 descriptions of Physiological Psychology can do no more than 
 enumerate these data, show their dependence on external stimuli, 
 and the value which they have as motifs for the perceiving subject; 
 and then understand the laws of this synthesis as the permanent 
 modes of the behavior of the psychical subject. The object of 
 sense-perception, the presentation of sense, is not an extra-mental 
 
 1 Ibid., p. 549. 
 
PERCEPTION A MENTAL ACHIEVEMENT 469 
 
 entity made up outside of the mind and borne into or impressed 
 upon it through the avenues of sense. It is a mental construction. 
 The field of vision is a subjective affair, and so is the field of touch. 
 The same psychical subject which reacts upon the stimulation of 
 the nervous organs of sense in the form of sensations, by its activity 
 in synthesizing these sensations, constructs the objects of sense. 
 The fundamental fact is the presence and activity of the subject, 
 known as Mind. 
 
CHAPTER VI 
 TIME-RELATIONS OF MENTAL PHENOMENA 
 
 1. So-called "presentations of sense" appear in consciousness, 
 not only as having spatial qualities and relations, but also as oc- 
 curring either simultaneously or successively as respects Time-form. 
 The clearest experience of the manner in which our sensations are 
 located in this framework of time, as it were, is gained by attention 
 to the successive tones of a melody, or to the rhythm of visual 
 or muscular impressions which accompanies a regularly recurrent 
 motion of some member of the body. What is true of the presenta- 
 tions of sense is also true of all mental phenomena, such as the re- 
 produced images of sense, the pure creations of fancy, and the 
 thoughts. All these have that form of occurrence and relation which 
 we call "Time." 
 
 Physiological Psychology, however, can no more give an ultimate 
 explanation of this time-form which belongs to all mental phenom- 
 ena than of the space-form which objects of sense acquire as the 
 result of a mental synthesis. Experimental science cannot explain 
 " time." Nothing is accomplished toward comprehending the ori- 
 gin of the mental representation of time by indicating the speed, 
 number, and order of the various series of conscious experiences. 
 Successive presentations of sense or successive ideas do not of 
 themselves constitute a mental presentation or idea of succession. 
 The idea that a follows or precedes b is not the idea of a nor the 
 idea of b; neither is it the idea of a + b or of a b. Experimental 
 science can explain the order of succession; but in doing this it 
 implies the idea of succession, and this idea is not itself a succes- 
 sion, or an order of succession, or a compound of successive ideas. 1 
 
 Many thousands of experiments have been made (since the work 
 of Bonders in 1868), with the use of the most complicated and deli- 
 cate machinery, in order to fix the amount of time required for the 
 various processes, both nervous and mental, which are the condi- 
 tions of our conscious life. These experiments have succeeded in 
 bringing many interesting facts to light. But the laws thus estab- 
 lished beyond all reasonable question are remarkably few; more- 
 
 1 Compare Volkmann von Volkmar, Lehrb. d. Psychologic (3d ed.), II, pp. 11 f. 
 
 470 
 
EXPERIMENTS IN REACTION-TIME 471 
 
 over, they are nearly all merely restatements in more definite form 
 of already familiar generalizations. That a kind of sluggishness or 
 inertia, which the stimulus must overcome, belongs to all the senses, 
 and that they often continue to act, when once roused, after the ex- 
 citing cause is withdrawn; that different sensations following each 
 other too quickly tend to confuse or destroy each other; that no 
 one can see or think more than about so rapidly, but that this rate 
 varies with different individuals and with the same individual at 
 different times; that it takes more time to perceive or think where 
 the objects are complex, and are either too small or too large or 
 too closely alike; that it takes time to will or choose, less time to 
 act when we know what to expect, and more time to move, in re- 
 sponse to a particular sensation, some part of the body which we 
 are not accustomed to connect with that sensation; that practice 
 increases the speed of our mental and bodily action, and that fatigue 
 and certain drugs diminish it all these statements were matters of 
 common observation long before experimental psychology began its 
 use of scientific methods. 
 
 2. * It is not necessary to describe the construction of the ma- 
 chines 1 which have been used in experimenting upon the time-rela- 
 tions of mental phenomena, or the methods of using them employed 
 and commended by different observers. The general problem 
 is in all cases essentially the same namely, to produce certain 
 definite impressions upon the organs of sense, to secure a definite 
 result in the form of motion of some part of the body as a sign 
 that the impressions have been received (and, perhaps, interpreted 
 and mentally combined), and to measure with extreme accuracy the 
 interval between peripheral stimulation and resulting motion. 
 
 The electrical current is ordinarily used to mark both the in- 
 stant when the external sense-stimulus acts on the organ and that 
 when the resulting motion occurs. The stimulus may consist in 
 the flash or crackle of an electric spark, the appearance of one 
 or more colors or figures, or letters or words, the sounding of a 
 bell or a falling ball, etc.; the motion may be with the finger press- 
 ing a key, or the foot or hand closing or breaking a circuit, or the 
 vocal organs calling into a tube, etc. The one difficult matter 
 which marks the success or the comparative failure of any series 
 of observations is the arrangement of the experiments and their 
 tabulated results so as to analyze the different elements of the com- 
 plex process involved. Such experiments need to be repeated 
 many times upon the same individual, so as to eliminate the vari- 
 
 1 For an excellent account of these machines, see Titchener, Experimental 
 Psychology, Quantitative (1905), chap. Ill; and for the nervous mechanisms in- 
 volved, nearly every chapter of part I of this book may be consulted anew. 
 
472 TIME-RELATIONS OF MENTAL PHENOMENA 
 
 able factors of bodily condition, attention or distraction of mind, 
 practice, etc. ; they need also to be repeated with many individuals, 
 so as to calculate accurately the so-called personal equation. 
 
 3. The interval between the instant when the external stimulus 
 begins to act upon the end-organ of sense and the resulting move- 
 ment of some member of the body has been called "physiological 
 time" by Hirsch and others, and "reaction-time" by Exner. The 
 latter term is preferable. "Reflex time" is a similar conception; 
 but this term refers to the time of organized reflexes, whereas re- 
 action-time refers to reactions which are not instinctively connected 
 with the stimuli used. The reflex time varies greatly, as has al- 
 ready been stated (p. 167), in different reflexes; in some it is shorter 
 than the quickest reaction-time, but in others not. 
 
 Reaction-time is "simple" when all the elements which tend to 
 complicate the processes involved in the reaction, and so to lengthen 
 the time required by it, have been as far as possible eliminated. 
 Reaction obtained in response to a single sensation of known qual- 
 ity, the instant of whose appearance is expected, by executing a 
 single natural and easy motion, best fulfils the conditions of sim- 
 plicity. It is therefore requisite, for all experiments of this sort, 
 that the average simple reaction-time of each individual experimented 
 upon shall be determined; and also the effect of practice, exhaustion, 
 and other influences upon this interval. But even the simplest 
 reaction-time is, of course, a very complex affair. 
 
 Donders 1 distinguished no less than twelve different processes 
 as entering into "physiological time" (or simple reaction-time) 
 and this without interpolating any purely psychical elements, as 
 occupying separate periods, into the entire interval. The analysis 
 of Exner 2 is more pertinent to our purpose. Exner finds seven 
 elements in all reaction-time: (1) An action of the stimulus on the 
 end-organ of sense preparatory to excitation of the sensory nerve; 
 (2) centripetal conduction in this nerve; (3) centripetal conduction 
 in the spinal cord or lower parts of the brain; (4) transformation 
 of the sensory into the motor impulse; (5) centrifugal conduction 
 of the impulse in the spinal cord; (6) centrifugal conduction in 
 the motor nerve; (7) setting-free of the muscular movement. Of 
 these seven factors, however, the fourth is most interesting to psy- 
 chology. It may properly be called "psycho-physical" as distin- 
 guished from more purely physiological time. The other six ele- 
 ments (with the exception of the first, on account of difficulties 
 inherent in the experiments) have been determined with some de- 
 gree of defmiteness. It is, then, theoretically possible to ascertain 
 
 1 Archiv f. Anat., PhysioL, etc., 1868, p. 664. 
 8 See Hermann's- Handb. d. PhysioL, II, ii, p. 271. 
 
INERTIA OF THE END-ORGANS 473 
 
 the amount of these six and subtract them from the entire reaction- 
 time; the remainder would be the interval occupied by the central 
 cerebral processes (that is, by No. 4). Thus Exner assumed 62 
 metres per second as the probable rate of conduction 1 in both sen- 
 sory and motor nerves ; and in the spinal cord, 8 for the sensory and 
 11-12 for the motor process. In this way he calculated that about 
 0.0828 sec. is the " reduced reaction-time," or interval occupied 
 within the cerebral centres in transforming the sensory into motor 
 impulses in the special case of reaction from hand to hand, where 
 the whole reaction- time is 0.1337 sec. The uncertainties of all 
 such calculation, however, occasion the demand for other methods 
 of determining the strictly "psycho-physical" portion of reaction- 
 time. 
 
 4. As bearing on the foregoing problem of analysis, it must be 
 remembered that any psycho-physical theory of the time relations 
 of mental phenomena requires that account be taken of the inertia 
 of the nervous system. As composed of moving molecules, it nec- 
 essarily requires some time to be started by the action of a given 
 stimulus, then reach its maximum of activity in a particular direc- 
 tion, then subside into a negative condition with respect to this 
 direction (called "Anklingen" and "Abklingen" of the nervous ex- 
 citement, by the German investigators). This statement follows as 
 a necessary assumption from the physical nature of the nerve-fibres 
 and nerve-cells, since inertia is a property of every material mechan- 
 ism. It is difficult, however, to justify the assumption experi- 
 mentally, or to fix the exact amount of time consumed by the inertia 
 of different parts of the nervous system. Experiment demonstrates 
 no stadium of latent excitation for the motor nerve, such as is 
 about y^ sec. for the muscle when electricity is used. The case 
 is different, however, with the end-organs of sense. They do ex- 
 hibit a certain sluggishness, and this is one reason why only so many 
 sensations in a given unit of time can be produced by their successive 
 irritation. 
 
 The result of the inertia of the end-organs, as determining the 
 number of separate excitations of which they are capable in a 
 second, varies for the different senses. The nerve-endings of touch 
 probably exceed all others in the promptness with which they re- 
 spond to stimulus and then return to a relative equilibrium. But 
 the number of separate sensations of this sense which can be pro- 
 duced during a given interval depends in a remarkable way upon 
 the quality and intensity of the stimulus, the place where it is ap- 
 plied, etc. The results of different experimenters therefore differ 
 
 Compare p. 132. Recently, by improved methods, Piper (Pfliiger's Archiv, 
 1908, CXXIV, 591) has obtained 120 metres per second in human nerves. 
 
474 TIME-RELATIONS OF MENTAL PHENOMENA 
 
 widely. Preyer thought that 27.6-36.8 stimulations (per second) 
 of the skin fused into one continuous sensation; but Valentin put 
 the limit at 480-640, and von Wittich 1 succeeded in observing 
 a vibrating or discontinuous sensation corresponding to about 
 1,000 separate excitations in this unit of time. Hearing can re- 
 ceive nearly as many separate sensations in a second as can touch. 
 The noise of the electric spark has been heard with one ear only, 
 as separate sensations, at intervals of 0.00205 sec.; but hardly or 
 not at all at intervals of 0.00198 sec. The number of possible sensa- 
 tions of sound may then be placed at about 500 per second. E. H. 
 Weber noticed that we can tell whether two watches are ticking 
 exactly together much better when both are held near the same ear 
 than when one is held at each ear. 
 
 The smallest interval for sensations of sight, when the two stimuli 
 act on the same place of the retina, is still greater. In ordinary 
 daylight, rotating disks whose surface is part white and part black 
 become gray (that is, the sensations fuse) when they attain a motion 
 of about 24 per second. It can be told which of two images of elec- 
 tric sparks that are 0.011 mm. apart on the retina occurs first, if 
 the difference in the time of their occurrence is 0.044 sec. If the 
 two sparks are seen as one with an apparent motion, its direction 
 can be distinguished when the two ends of the line of motion are 
 only 0.014-0.015 sec. apart. But if one stimulus strikes the fovea 
 centralis and the other a point of the retina 6 mm. off, the smallest 
 interval for distinct perception is increased to 0.076 sec. 2 Within 
 certain limits these intervals are independent of the intensity of the 
 light, when it falls on the retina near its centre; but (compare p. 333) 
 the intensity and quality of the sensations are connected with the 
 time during which the stimulus acts. The law for the "time- 
 course" of such retinal excitations has been stated and defended by 
 Fick, 3 as known by the name of "Talbot's principle": If any place 
 of the retina is periodically excited with light of given intensity, for 
 a certain time a, and then left unexcited for a time 6, and if the 
 time a + b is less than about 0.04 sec., 4 then the sensation becomes 
 
 continuous, with a strength corresponding to the excitation 
 
 a + b. 
 
 1 For his remarks on Preyer's experiments, see the article in Pfliiger's Archiv, 
 
 II, pp. 329 ff. 
 
 2 Compare Exner, in Hermann's Handb. d. PhysioL, II, ii, pp. 256 f.; and Sitzgs- 
 ber. d. Wiener Acad., LXXII, pp. 156 f. 
 
 3 Archiv /. Anat., PhysioL, 1863, pp. 739 f.; and Hermann's Handb. d. Physiol, 
 
 III, i, pp. 212 f. 
 
 4 This time, called the "action time of light," is the time during which the 
 stimulus must act in order to produce its maximum effect, in point of apparent 
 intensity. Talbot's principle may be restated by saying that the apparent 
 
MEASUREMENT OF SMALLEST INTERVAL 475 
 
 The measurement of the smallest interval for sensations of smell 
 and taste cannot be made with satisfactory exactness on account of 
 the nature of the stimuli of these senses. Little is known which 
 goes beyond ordinary experience concerning after-tastes analogous 
 to the after-images of the eye. One experimenter (Bidder) thought 
 that the sensation continued after the tongue had been so carefully 
 dried off that no particles of the tastable substance were left remain- 
 ing; but of this we can scarcely be sure. It may be that certain 
 substances leave their after-taste because their tastable particles are 
 dissolved later; or because their effect, being weaker, is at first 
 suppressed by particles of stronger quality. 1 
 
 5. When the successive sensations are of different senses, the 
 " smallest interval" between them, and so the number possible in 
 a second, varies still more. The following table 2 exhibits the re- 
 sults obtained by several different observers: 
 
 Sec. 
 
 Between two sensations of sound (electrical sparks) 0.002 
 
 Between two sensations of light (direct electrical excitation of same 
 
 retinal spot) 0.017 
 
 Between two sensations of touch (impact on finger Mach) .... 0.0277 
 Between two sensations of light (at fovea centralis, by optical images) . . 044 
 Between two sensations of light (at periphery of retina, by optical images) .049 
 Between sensation of sight and sensation of touch (sight following) . . . 05 
 Between sensation of sight and sensation of hearing (sight following) . 0.06 
 Between two sensations of noises (each heard by one ear) . . . . . 064 
 Between sensation of sight and sensation of touch (sight preceding) . 0.071 
 Between two sensations of light, one at the periphery and the other at 
 
 the centre of retina 0.076 
 
 Between sensation of sight and sensation of hearing (sight preceding) . 0.16 
 
 6. The point of starting for determining experimentally all 
 the problems which concern the durations and relations in time of 
 
 intensity of a light which acts on the retina for less than its action time, is di- 
 rectly proportional to the time during which it acts. The action time varies, 
 however, with the intensity of the stimulus, being, according to McDougall 
 (British Journal of Psychology, 1904, I, 151), as long as one-fifth of a second 
 when the stimulus is so weak as to be barely perceptible, and decreasing to 0.03 
 sec. when the intensity of the stimulus is sufficiently increased. According to 
 Kunkel (Pfliiger's Archiv, IX, 206), the action time varies for light of different 
 colors. The action time for sound varies with the intensity, and apparently 
 also with the pitch. With intense stimuli, the maximum subjective intensity was 
 reached, in Sander's work (Psycholog. Studien, 1910, VI, 34), between 0.6 and 
 1.0 sec.; with weak stimuli, Kafka (ibid., 1906, II, 292) found that the maximum 
 effect might not be reached under 1.5 sec. The subjective intensity rises at first 
 rapidly, then more slowly, till the maximum is reached. The rise is more rapid 
 for high than for low tones. 
 
 1 Compare von Vintschgau, in Hermann's Handb. d. Physiol, III, ii, p. 221. 
 
 3 By Exner, in Hermann's Handb. d. Physiol, II, ii, p. 262. 
 
476 TIME-RELATIONS OF MENTAL PHENOMENA 
 
 mental phenomena is, therefore, gained by fixing the so-called 
 " simple reaction-time." In its very simplest form the question 
 may now be stated as follows: How long an interval will elapse, 
 under the most favorable circumstances, between the instant when 
 some end-organ of sense is stimulated and the instant when mo- 
 tion follows as the result of recognizing the fact, in consciousness, 
 that such stimulation has taken place ? As has already been indi- 
 cated: To shorten the reaction-time as much as possible, the sub- 
 ject must know what place of the sensory organism is to be hit by 
 the stimulus, and about when to expect it; he must also be called 
 upon to react, in one and the same easy and natural way, in all 
 cases, as soon as he knows that he is hit at all. 
 
 Under the foregoing conditions the simple reaction- time varies 
 usually within only rather narrow limits. It does vary, however, 
 from one moment to another in the same individual, so that a series 
 of reactions must be taken, and the average and variability com- 
 puted. It varies also from one individual to another. Besides 
 these variations, the reaction-time shows many interesting differences 
 according to the exact conditions surrounding the experiment; such 
 are the character of the stimulus, the nature of the reacting move- 
 ment, the "central" or mental conditions, etc. 
 
 7. Reaction-time varies, first, with the sense-organ which is 
 stimulated. Some of the determinations of the older observers, 
 which have been, in general, confirmed by later work, are brought 
 together in the following table : 1 
 
 Observer. 
 
 Optical stimulus. 
 Sec. 
 
 Acoustic stimulus. 
 Sec. 
 
 Stimulus of touch. 
 Sec. 
 
 Hirsch ..... 
 
 200 
 
 149 
 
 182 (hand). 
 
 Hankel 
 
 225 
 
 0.151 
 
 0.155 
 
 Bonders , . 
 VonWittich . . . 
 Wundt ... . 
 Exner 
 
 0.188 
 0.194 
 0.175 
 . 1506 
 
 0.180 
 0.182 
 0.128 
 . 1360 
 
 0.154 (neck). 
 0.130 (forehead). 
 0.188 
 0.1276 (hand). 
 
 Auerbach .... 
 Von Kries .... 
 
 0.191 
 0.193 
 
 0.122 
 0.120 
 
 0.146 
 0.117 
 
 We conclude, then, that under the most favorable circumstances 
 the reaction-time can scarcely be reduced to ^ of a second, while 
 it rarely rises much above T \ of a second. Reaction to sound and 
 to touch differ, on the average, but little, whereas reaction to light 
 is distinctly slower. 
 
 1 Taken from the article of Kries and Auerbach, Archiv /. Anat. u. PhysioL, 
 Physiol. Abth. (1877), pp. 359 f. 
 
VARIATIONS IN REACTION-TIME 477 
 
 Stimuli to the other senses have been less employed in reaction- 
 time experiments; and, accordingly, the times are less well estab- 
 lished. For warmth and cold, the times, are somewhat longer than 
 for touch, and longer for warmth than for cold. An approximate 
 figure for reaction to. cold is 0.15 sec. and to warmth 0.18 sec. 1 It 
 should be noted that temperature stimuli cannot be so directly ap- 
 plied to the end-organs as can light, sound, and pressure on the 
 skin; the temperature of the skin must first be changed. Hence 
 the reaction-time to warmth and cold varies widely according to 
 the mode of application of the stimulus. The times given above 
 were obtained by bringing a warm or cold piece of metal in contact 
 with the skin. If radiant heat is applied by bringing a candle close 
 to the skin, then, though the final impression may be almost painful, 
 its development is so slow that the reaction-time rises to half a 
 second. 2 The fact that reaction to warmth is slower than to cold 
 is believed to indicate that the warmth-receptors lie deeper in the 
 skin than the cold-receptors (see p. 181). 
 
 Reaction to painful stimuli is especially slow. The fact that the 
 sensation of cutaneous pain is slow in developing can be readily 
 observed introspectively; a painful stimulus, if not applied precisely 
 to a pain spot, is apt to evoke first a sensation of touch, and then, 
 after an appreciable interval, a sensation of pain. If a pain spot 
 is directly excited, by a strong stimulus, two sensations of pain are 
 felt, though at an appreciable interval. The reason for this prob- 
 ably is that the strong stimulus excites the nerve-fibres directly, 
 as well as the slow-acting end-organs of pain. A weak stimulus 
 apparently excites only the end-organs; accordingly Thunberg 
 finds 3 that pain-reaction requires over a second when the stimulus 
 is weak, but drops abruptly, at a certain strength of stimulus, to 
 0.40 sec. 
 
 The reaction to taste is slow, and varies with the part of the tongue 
 to which the stimulus is applied, and with the particular taste 
 aroused. The following figures are given by Kiesow, 4 the stimuli 
 being applied to the tip of the tongue: 
 
 To salt 0.307 sec. 
 
 To sweet . 446 sec. 
 
 To acid . 536 sec. 
 
 To bitter 1.082 sec. 
 
 1 Goldscheider, Archiv f. (Anat. w.) Physiol. (1887), p. 469; Von Vintschgau 
 and Steinach, Pfliiger's Archiv (1888), XLIII, 152. 
 
 2 Tanzi, cited from Sherrington in Schafer's Textbook of Physiology, 1900, II, 
 963. 
 
 3 Nagel's Handbuch der Physiologic, 1905, III, 710. 
 
 4 Zeitschrift /. PsychoL, 1903, XXXIII, 453. 
 
478 TIME-RELATIONS OF MENTAL PHENOMENA 
 
 The reaction to bitter is prompter when the back of the tongue 
 is stimulated. The slowness of reaction to taste may probably be 
 due, in part, to the impossibility of applying the stimulus directly 
 upon the taste-buds. Sensations of taste, as Kiesow notes, are slow 
 and gradual in their development, and the reaction to the first ap- 
 pearance of the specific sensation is a reaction to very faint stimuli, 
 and "sensorial" (see p. 485) in character, and therefore slow. 
 
 The reaction to smell is still slower, though how far this is due 
 to the relative inaccessibility of the end-organs is uncertain. Times 
 of 0.2 to 0.8 sec. have been obtained. 1 
 
 The reaction-time to cutaneous stimuli varies with the part of 
 the skin excited. This would be expected, on account of the un- 
 equal distances of different parts from the brain, and the consequent 
 unequal times which must be consumed in transmission by the nerve. 2 
 However, the observed differences are not all accounted for by this 
 difference in distance. Reaction to a stimulus applied to the fore- 
 head is more delayed than to a stimulus applied to the hand (Ex- 
 ner). Dolley and Cattell 3 measured the time of reaction to stimuli 
 applied to various parts of the skin, and found many differences 
 which could not be accounted for by the lengths of nerve traversed, 
 but only by differences in the closeness of cerebral connection be- 
 tween sensory and motor nerves. Thus, the reaction of the right 
 hand to a stimulus applied to that hand is quicker than to a stimulus 
 applied to the left hand; and the foot, though it always gives slower 
 reactions than the hand, is relatively quick when the stimulus is 
 applied to the foot or leg. In general, it appears that the cerebral 
 path from the skin of a given member to the muscles of that same 
 member is either especially short, or especially open. 
 
 8. It has been argued that the apparent difference in the reac- 
 tion-times of different senses is due to difference in the intensity of 
 the stimuli applied. Increasing the strength of the stimulus de- 
 creases the reaction- time in all the senses; but we have no very 
 good means of measuring stimuli of one sense in terms of another 
 sense. It has been proposed (by Wundt) to reduce them to a com- 
 mon standard by referring the sensations to the point where they 
 barely reach the "threshold of excitation" (Reizschwelle) -, that is, 
 where they are just perceptible in consciousness. In this way the 
 mean result for sound (0.337), light (0.331), and touch (0.327) are 
 found to be almost exactly the same. It has further been argued 
 
 1 Moldenhauer, Wundt's Philosoph. Studien (1883), I, 606; Buccola, Arch, 
 ital. de Biologie, 1884, V, 279; Beaunis, Compt. rend. Acad. de Sciences (Paris, 
 1883), XCVI, 387. 
 
 2 See Kiesow, Zeitschr. /. Psychol (1903), XXXIII, 444. 
 
 3 National Academy of Sciences, 1893, VII, 393; Psychol. Rev., 1894, I, 159. 
 
EFFECT OF INCREASING INTENSITY 479 
 
 that the speed of perception and the duration of psycho-physical 
 time are the same for all the senses. On the contrary, there seems 
 good reason to suppose that the reaction-time of sight is necessarily 
 longer than that of hearing or touch, on account of the photo- 
 chemical nature of its more immediate stimulus. One observer 
 (von Wittich) has even gone so far as to conjecture that the speed 
 of conduction in the optic nerve is less than that of the other 
 nerves of sense; it is rather to be concluded, however, that the la- 
 tent time of the sensory end-apparatus, and of the cerebral proc- 
 esses by which sensory impulses pass over into motor impulses, 
 is different. But if equality of reaction-times in the different 
 senses is to be taken as indicating equally intense stimuli, then a de- 
 cidedly bright light must be regarded as having no more intensity 
 than a very moderate noise or pressure. 
 
 Another suggestion 1 is that the reaction time is very largely con- 
 sumed in the passage of the nerve-impulse across synapses (see p. 
 110); and that the path from the rods and cones to the brain has 
 more synapses than the path from the organ of Corti. On the whole, 
 the suggestion which probably is most generally entertained, and 
 which seems certainly applicable to many of the differences above 
 noted, is that already adopted by us namely, that the inertia or 
 latent time of different sense-organs differs. 
 
 9. The fact that increasing the intensity of the stimulus di- 
 minishes the reaction-time has been incidentally mentioned above, 
 in the cases of pain and temperature. The subject has been quan- 
 titatively examined by several authors; as by Exner in the case of 
 light, by Wundt in the case of sound, by Berger and Cattell in light, 
 sound, and touch, and by a few others. There is nearly universal 
 agreement that the reaction becomes quicker as the intensity of the 
 stimulus increases, though the change is not great, when once the 
 range of very feeble intensities is passed. The most exact determi- 
 nations on this point are those of Froeberg. 2 The stimulus used by 
 this experimenter was light reflected from white, gray, and " black" 
 paper; its relative intensity is stated in the upper line of the follow- 
 ing table, the lower line showing the reaction-time of one individual 
 in seconds: 
 
 100 56 25 16 10 7 3 2 0.7 
 
 .191 .194 .197 .202 .208 .210 .215 .220 .226 
 
 A second individual gave shorter times, but the changes in intensity 
 varied to about the same extent. The result is summarized ap- 
 
 1 Schafer, Textbook of Physiology, 1900, II, 611. 
 
 2 The Relation between the Magnitude of Stimulus and the Time of Reaction 
 (New York, 1907). 
 
480 TIME-RELATIONS OF MENTAL PHENOMENA 
 
 proximately by saying that doubling the intensity of the stimulus 
 decreases the reaction-time by 0.003-0.004 sec.; or, in other words, 
 that the reaction-time decreases in arithmetical progression as the 
 stimulus increases in geometrical progression. The analogy of 
 this result to Fechner's law will at once be noticed. Froeberg ob- 
 tained a similar result for varying intensities of an acoustic stimulus; 
 and for variations in the area and (within the limits of the "action- 
 time") duration of a photic stimulus. 
 
 10. The character of the reacting movement influences the time 
 of reaction. Reactions by the hand or a finger are usually quicker 
 than those of any other member. A member unpracticed in quick 
 reactions gives slower responses in these experiments than a member 
 which is much used in similar performances; this difference de- 
 creases with practice in the unfamiliar movement. 1 When the re- 
 action consists in pressing down a telegraph key, against the force 
 of a spring, the time of reaction increases with the force of the spring. 2 
 This effect is probably purely muscular, as more time is required 
 by the muscles to attain a strong than a weak contraction. The 
 movement more customary in reaction-time experiments, a move- 
 ment in which the key is kept closed by the finger till the stimulus is 
 received, and then released, does not show a similar effect from the 
 force of the spring; and hence is a better device for general use in 
 these experiments. 
 
 The movement executed by the finger in such reactions has been 
 subjected to analysis by several authors 3 and found to be by no means 
 simple in all individuals. In the preliminary period of expectancy, 
 an oscillation between greater and less tension in the finger has been 
 observed in a considerable minority of the individuals tested. At 
 the moment when the stimulus is received, there is in some persons 
 a preliminary increase of the downward pressure on the key, an- 
 tagonistic to the release movement which then follows immediately; 
 but the downward movement consumes time and slows the reaction. 
 When the required movement is itself a downward pressure on the 
 key, the force exerted is, in unpracticed subjects, quite in excess of 
 the amount needed to close the key; but with practice, this excess 
 diminishes, and is accompanied by improvement in the speed of 
 reaction. 4 In general, unpracticed subjects are apt, in these re- 
 
 1 See the summary of these and many other results by Jastrow, in his Time- 
 Relations of Mental Phenomena (New York, 1890). 
 
 2 Breitwieser, Psychol. Rev., 1909, XVI, 352. 
 
 3 W. G. Smith, Mind, 1903, N. S., XII, 47; Judd, McAllister and Steele, Psy- 
 chol. Rev., Monograph Supplement XXIX (1905), p. 141; Titchener, Exper. 
 Psychol., Quantitative, part II (1905), p. 351. 
 
 4 Breitwieser, op. cit. 
 
INFLUENCE OF CENTRAL CONDITIONS 481 
 
 actions as in many other performances (compare p. 549), to make 
 larger and more forceful movements than are required, and improve- 
 ment in speed of reaction is attended by a more precise limitation 
 of the movement. The speed of the reacting movement is, in 
 practiced subjects, nearly uniform and does not follow the varia- 
 tions of the reaction- time; conditions which affect the speed of the 
 reaction do not necessarily affect the speed of the reacting move- 
 ment. 1 
 
 11. The "central" or mental conditions which influence the 
 reaction-time are naturally of special interest to the student of psy- 
 chology; but they are also specially intricate and difficult of exact de- 
 termination. No doubt central factors are interwoven with all 
 the sensory and motor influences which have already been men- 
 tioned. That changes in the general condition of the brain, de- 
 pendent on disease, fatigue, the action of drugs, etc., influence the 
 time employed by all our bodily movements need scarcely be men- 
 tioned. Less obvious are influences dependent on the particular 
 manner in which the experiment is arranged. 
 
 In the usual method of conducting an experiment in reaction- 
 times, the " subject," or reagent, is first made acquainted with a 
 certain stimulus to which he is to react by a prescribed movement 
 (usually a movement of the forefinger). There is no ready-formed 
 connection instinctive or habitual between this particular stim- 
 ulus and this particular movement; and the movement would not, 
 in general, be made in response to the stimulus except for the in- 
 structions of the experimenter. The subject assumes a position of 
 readiness placing his finger on a key by means of which his reac- 
 tion is to be recorded, and perhaps also directing his eyes toward 
 the place where the stimulus (if visual) is to appear. He is usually 
 warned a few seconds in advance to be " ready" for the stimulus. 
 This preliminary signal is an important part of the procedure. If 
 it is omitted, the time of reaction is increased. Wundt 2 found an 
 increase from 0.076 sec. to 0.253 sec. in reacting to a loud noise (a 
 ball falling 25 cm.); and an increase from 0.175 to 0.266 in reacting 
 to a weak sound (a ball falling 5 cm.); these increases being the 
 result of omitting the "Ready!" signal. If the stimulus follows the 
 signal at an irregular interval, the reaction-time is not so short as 
 when the procedure is regular. If, indeed, the procedure is so 
 regular that the moment of the stimulus can be exactly anticipated, 
 the movement may be made to coincide in time with the stimulus, 
 and the whole character of the experiment be thus changed. This 
 
 1 T. V. Moore, Psychol Rev., Monograph Supplement, No. XXIV (1904). 
 
 2 Physiol Psychologic, 3d edition, 1887, II, 287. The increase is not always 
 so great; see Wundt's 5th edition, 1903, III, 434. 
 
482 TIME-RELATIONS OF MENTAL PHENOMENA 
 
 result is avoided by varying the preliminary interval within narrow 
 limits. The most favorable interval between the ready signal and 
 the stimulus is one or two seconds; a shorter time does not allow the 
 subject to prepare himself fully for the stimulus, while a longer period 
 than two seconds allows more time than is needed and so affords a 
 chance for wandering of the attention. 
 
 The preparation for reaction is clearly an essential part of the proc- 
 ess; without some degree of preparation the reaction would not oc- 
 cur at all, and the more perfect the preparation, the more prompt 
 the reaction. The preparation may be said to include an adjust- 
 ment to the stimulus, and an adjustment for the movement to be made 
 the total adjustment including and connecting these two. The 
 more precisely the character of the stimulus is foreknown, the more 
 complete is the adjustment to it, and the more rapid the reaction. 
 Thus, if the intensity of the stimulus varies so that it cannot be ex- 
 actly anticipated, the reaction-time is increased. The increase is 
 the greater if the alternation of intensities is very irregular. This 
 fact is exhibited in the following table i 1 
 
 I. Uniform 
 
 change of intensity 
 
 II. Irregular change of 
 
 intensity 
 
 Loud sound 
 
 Sec. 
 0.116 
 
 Loud sound . 
 
 Sec. 
 . . 0.189 
 
 Feeble sound 
 
 0.127 
 
 Feeble sound 
 
 . . 0.298 
 
 By suddenly intercalating a feeble sound in a series of loud noises 
 the reaction-time may be prolonged to 0.4 or 0.5 sec. Accordingly, 
 the speed of reaction affords a measure of the precision of the pre- 
 liminary adjustment. 
 
 12. In attempting a psychological analysis of the nervous and 
 psychical factors involved in such reaction-time experiments, it 
 is important, then, to recognize that the whole process begins with 
 the warning signal (if not, indeed, still earlier), and that it can prop- 
 erly be divided into three periods 2 : (1) the period of preparation; 
 (2) the period of reaction; and (3) the period immediately following 
 the reaction. These are sometimes named, respectively, the fore- 
 period, the main-period, and the after-period. 
 
 The preparatory period is reckoned as beginning with the 
 "Ready" signal; the reaction period with the stimulus; and the 
 after-period with the reacting movement. These three correspond, 
 in a way, to the cocking of a gun, the discharge, and the smoke. 
 
 1 Wundt, Physiol. Psychologic, 5th edition, 1903, III, 440. 
 
 2 See Ach, Uber die Willenstdtigkeit und das Denken (Gottingen, 1905). 
 
THE THREE PERIODS 483 
 
 During the preparatory period the reacting mechanism is adjusted 
 or set for a certain form of action; then the stimulus comes and 
 causes the predetermined action to take place; and then after-effects, 
 such as feelings of satisfaction or of relief, may occur. When the 
 adjustment is complex, and more than one response has to be made 
 to the stimulus, some parts of the response may occur in the after- 
 period, and are therefore not to be included in the time which is 
 measured, for only those events which occur in the main or reaction 
 period are timed by the experiment. 
 
 The importance of the above division into three periods is made 
 clear by comparing it with the most famous of attempts to analyze 
 the mental or central process entering into a reaction. The analy- 
 sis of Wundt 1 divided the central or psycho-physical part of the re- 
 action (No. 4 of Exner's seven processes, p. 472) into three psycho- 
 physical processes: (1) entrance into the field of consciousness, or 
 simple perception ; (2) entrance into the point of clear consciousness, 
 or apperception (attentive or discerning perception) ; (3) the excita- 
 tion of the will, which sets free in the central organ the motor im- 
 pulse to the reacting muscles. Obviously, the mental processes 
 are here all conceived of after the analogy of sight. Consciousness 
 is regarded as a field of vision ; objects enter it and are at first only 
 obscurely and indefinitely perceived, as are those visual objects 
 whose images enter the field of the eye at the sides of the retina. 
 Time is required for the objects to arrive at the spot of clear 
 vision the fovea centralis of consciousness (Blickpunkt) where 
 discerning attention is bestowed upon them and they are apper- 
 ceived. When they are apperceived, further time is required to 
 get up the corresponding molecular motion in the motor areas of 
 the brain. All three foregoing processes are psycho-physical 
 that is, they comprise physiological processes in the central organs 
 and simultaneous corresponding changes of consciousness occurring 
 in time-form. There is no good reason to suppose that the mind 
 occupies time for its own processes which is separate from and as it 
 were thrown in between the physiological processes. Indeed, all 
 the evidence is contrary to such an hypothesis. 
 
 Wundt has made an elaborate defence of his positions with re- 
 gard to the nature of psycho-physical time. He and his pupils 
 have attempted more definitely to characterize the cerebral changes 
 which correspond to each of the mental elements of (1) perception, 
 (2) apperception, and (3) will; and to determine what part of the 
 total reaction-time must be assigned to each. His figure of speech, 
 which likens all changes of conscious states to those produced by 
 moving an image over the retina to the spot of clear vision, may be 
 \Physiol. Psychologic, 5th edition, 1903, III, 384, 
 
484 TIME-RELATIONS OF MENTAL PHENOMENA 
 
 accepted as helpful to the imagination; it must not be forgotten, 
 however, that it is still a figure of speech. The fact of which it 
 takes account is, that all changes of consciousness require time in 
 order to define themselves with their maximum of clearness and in- 
 tensity. 
 
 13. Let us now suppose that Wundt's analysis be accepted as 
 a true account of the mental process in a voluntary reaction, as dis- 
 tinguished from a reflex or from an habitual and automatic response; 
 and let us also grant that the movement of the forefinger in response 
 to a prearranged stimulus is not a purely reflex or automatic re- 
 sponse; the subject's will, indeed, may be strongly excited to react 
 as quickly as possible. Still it does not follow that all of the 
 processes which Wundt recognizes as included in a voluntary re- 
 action occur during the reaction period of the experiment and so 
 enter separately into the time which is measured. Several of the 
 leading students of reaction-times (Exner, Cattell, Lange, Ach, and 
 others) have observed that no detectable act of will intervenes be- 
 tween the stimulus and the reacting movement, at least in most 
 cases. The will may be exerted in the preparatory period, in set- 
 ting up the adjustment for reaction; but, when once this adjust- 
 ment is established, the stimulus calls out the response without any 
 further act of will. In such cases, accordingly, "will-time" does 
 not usually enter into the time which is measured. Somewhat the 
 same thing is true regarding perception-time. Since the stimulus 
 is anticipated arid adjusted for, it does not need to win its way to 
 the focus of attention, but its way is, as it were, " cleared" before it 
 by the preparatory adjustment. No time is consumed, during the 
 reaction period, in turning the attention upon the stimulus, for the 
 attention is directed toward the stimulus before it arrives. 1 
 
 As to "apperception-time," this is partly covered by the preceding 
 remarks on perception-time; however, the full apperception of a 
 stimulus involves a progressive apprehension of its exact character; 
 it may first be apprehended simply as a change in the situation, and 
 then as a particular color or sound; and so on. The full process of 
 apperception may take considerable time, but it is not ordinarily 
 necessary for this process to be complete before the reactive move- 
 ment is initiated. Unless the reaction is required to occur only when 
 the stimulus has a particular character which must be recognized 
 before reacting, the movement may occur at any stage in the devel- 
 opment of the apperception. 2 Part of the apperceptive process thus 
 occurs in the after-period, and is not included in the measured time. 
 
 1 See Ach, op. cit., p. 116. 
 
 2 See Cattell, Wundt's Philos. Stud., 1886, III, 452; and National Acad. of 
 Sciences, 1893, VII, 393; Ach, op. cit., p. 117. 
 
WUNDT'S ANALYSIS EXAMINED 485 
 
 How much of apperception occurs within the measured time can- 
 not easily be told; the amount is variable, and introspection can 
 scarcely be trusted to indicate at what stage in a rapidly developing 
 process a motor impulse leaves the brain. 
 
 The outcome of this discussion is that Wundt's analysis of a 
 complete reaction cannot be regarded as applying accurately to 
 the process which occurs between the application of the stimulus 
 and the accomplishment of the movement. Experiment does not, 
 therefore, generally afford a measure of the duration (nor the latent 
 period) of the processes of perception, apperception, and will; the 
 amount of conscious process included in the measured time may be 
 very small. What is measured is rather the time needed to set in 
 action a previously prepared adjustment; and the more perfectly 
 the adjustment is prepared, the shorter will be its latent time. If 
 we try to think in neural terms, we shall probably not be far wrong 
 in supposing that the preparatory adjustment consists in some sort 
 of making-ready (perhaps a partial arousal or sub-excitation) of 
 the cortical areas which are to receive the sensory impulse and send 
 out the motor impulse, and of the connections between these two 
 areas; thus the incoming impulse, on reaching the cortex, finds a 
 particular pathway wide open to it, and makes a quick passage to 
 the connected motor nerves. Probably the time consumed in the 
 brain depends partly on the relative directness or circuitousness of 
 the pathway, and partly on the degree to which it is prepared or 
 sub-excited. How much consciousness, therefore, should attend 
 such a nervous impulse in its passage through the brain cannot be 
 judged from the neurological conception. It seems certain, however, 
 that our experiences of apperception and of willing are associated 
 with the activity of longer and more complicated pathways than are 
 traversed by the nerve-impulse in a prepared reaction. When viewed 
 from the more purely psychological point of view, the entire process 
 seems to involve the same substitution of the more speedy automatic 
 elements for the slower but more distinctly conscious operations, 
 with which our theory of the growth of experience by learning 
 makes us abundantly familiar. 
 
 14. The speed of any reaction, as can be judged from the pre- 
 ceding considerations, differs according to the particular adjustment 
 which is prepared ; that is to say, according to the particular require- 
 ments of the experiment. Variations in the reaction-time experi- 
 ment are suited to discover the different speed with which differ- 
 ent adjustments can be set in action. We shall now consider two 
 classes of such variations of the experiment. The first variation 
 falls under the head of the simple reaction-time, and refers to the 
 distinction between "sensorial" and " muscular" (or " sensory" 
 
486 TIME-RELATIONS OF MENTAL PHENOMENA 
 
 and "motor") reactions; while the second class includes the many 
 varieties of "complex reaction- times." 
 
 The distinction of sensory and motor reactions was made by 
 Lange in 1888, 1 and has aroused a great amount of observation 
 and discussion. Lange found that when the subject directed his 
 attention to the expected stimulus, the reaction-time was longer 
 than when he directed it toward the movement which he was about 
 to make. The difference in time between these two modes of re- 
 action was considerable the sensorial type being about 0.100 sec. 
 slower than the muscular. Thus, Lange's figure for the muscular 
 reaction to sound was 0.123, and for sensorial reaction to the same 
 stimulus, 0.227. Most subsequent observers have obtained re- 
 sults which show a much smaller difference, varying all the way 
 down to zero; and some subjects even give quicker reactions when 
 they attempt to take the sensorial attitude. 2 It seems probable, 
 therefore, that some individuals more readily and naturally adopt the 
 sensorial attitude, and others the muscular attitude; but it is also prob- 
 able that the instructions are differently understood by different indi- 
 viduals, so that what has passed for the same attitude, by being called 
 by the same name, has in reality varied much in the different cases. 
 
 From more recent work 3 it appears that the names " sensorial" 
 and "muscular" are not well fitted to indicate the difference in 
 attitude of different subjects; and also that the object to which the 
 attention is directed is not the determining factor in causing the 
 differences in reaction-time. In the sensorial attitude, the subject 
 prepares, not only to react, but also to observe the stimulus; but in 
 the muscular reaction, he does not prepare to observe the movement. 
 Rather, he prepares to make the movement, and to make it as 
 promptly as possible on receiving the stimulus. This preparation 
 for a speedy reaction is always more or less involved in reaction- 
 time experiments; but concentrating the expectant attention on 
 the movement seems to emphasize the matter of speed, by excluding 
 every other consideration besides that of barely reacting. Sensa- 
 tions of tension or of tingling are apt to be felt from the muscles 
 in the preparatory period of a muscular reaction; and premature 
 reactions frequently occur, indicating that the "sub-excitation" 
 of the motor apparatus has gone a little too far. In terms of the 
 neurological conception sketched above we may say that the cere- 
 
 1 Philosoph. Studien, IV, 479. 
 
 2 Miinsterberg, Beitrdge zur exp. PsychoL, I, 74; Baldwin, Psychol. Rev., 1895, 
 II, 259; Angell and Moore, Psychol. Rev,. 1896, III, 245; Flournoy, Observations 
 sur quelques types de reaction simple (Geneva, 1896). 
 
 3 Ach, op. cit.; Breitwieser, Attention and Movement in Reaction Time (New 
 York, 1911). 
 
"SENSORIAL" AND "MUSCULAR" REACTIONS 487 
 
 bral pathway, in a muscular reaction, is as short and uncompli- 
 cated as possible, and as highly prepared as is possible without actual 
 movement. In this way the greater speed of this type of reaction 
 would be explained. 
 
 Moreover, a "pure muscular" reaction is an extreme case, the 
 opposite extreme of which would be an attitude so purely sensorial 
 so exclusively directed toward the apprehension of the stimulus 
 that the preparation to move would not be completed till the 
 stimulus had been fully "apperceived." Such reactions as this 
 seldom occur under experimental conditions; for the mere holding 
 of the finger on the key involves some degree of readiness to make 
 a particular movement. Reactions which seem, introspectively, 
 to conform to the above definition of the pure sensorial type oc- 
 cupy a longer time than the usual sensorial reactions (0.3 to 0.8 
 sec., according to Ach); but also an exceedingly variable time. The 
 usual " sensorial" reaction is, therefore, an intermediate type, in 
 which the adjustment to react is more or less, but not completely, 
 suppressed by the simultaneous preparation to observe the stimulus. 
 The reacting movement usually occurs during the process of obser- 
 vation, but not always at the same stage of this process. According 
 to this conception, therefore, the longer reaction-time with "sen- 
 sorial" preparation might be due either to a relatively long central 
 pathway a pathway including parts corresponding to factors in 
 the process of apperception; or it might also be due to a relatively 
 low degree of sub-excitation of the motor part of the pathway and 
 of its connections with the sensory part. Both of these causes 
 probably operate to produce great variations in the so-called sen- 
 sorial reaction. 
 
 15. "Complex reactions" are those in which several mental 
 factors intervene between the stimulus and the motor response. 
 The sensorial type of "simple reaction" belongs here, in all cases 
 where the reaction is actually held back till a degree of appercep- 
 tion of the stimulus has occurred. Scarcely different from these are 
 reactions in which a variety of stimuli are used, but only a single 
 movement is required the instructions being to react only when the 
 stimulus is recognized. Such instructions uniformly lead to varia- 
 ble reaction- times; because they afford no guarantee against a short- 
 circuit in the brain, through which the reaction may be a direct re- 
 sponse to the mere reception of the stimulus, and the mental process 
 of recognition occur in the after-period. 
 
 A more rigorous procedure requires different movements in re- 
 sponse to different stimuli as, for example, to react with the right 
 hand to one of two stimuli, and with the left hand to the other; or 
 to make no movement except in response to a specified one of the 
 
488 TIME-RELATIONS OF MENTAL PHENOMENA 
 
 stimuli; or to give a vocal response by naming the stimulus. With 
 such procedure, the reaction, to be correct, must be held back till 
 the stimulus is recognized and discriminated from other stimuli. 
 It was formerly assumed that the discrimination of the stimulus 
 must be followed by an act of "choice," by which the proper move- 
 ment is selected; but this natural assumption, like many others 
 which have been made in regard to the mental process in reactions, 
 is not borne out by introspection. 1 Unpracticed subjects, indeed, 
 in their first few reactions, may think of the movement after recog- 
 nizing the stimulus, and before executing the movement; but with 
 a little practice, even this slight trace of choice disappears, and the 
 movement follows directly on the recognition of the stimulus. In 
 other words, each movement becomes so firmly associated with its 
 appropriate stimulus as to follow it immediately. It should be 
 noted that discriminative, or, as Jastrow 2 has called them, "adap- 
 tive reactions," are exceedingly common in ordinary behavior, as 
 in reading and naming objects, in obeying commands, and in mak- 
 ing appropriate responses to the numerous stimuli which affect the 
 senses. When the appropriate responses have been frequently as- 
 sociated with the stimuli, as in the case of reading, no consciousness 
 of a volitional character intervenes between the stimulus and the 
 response; volition has no opportunity to appear except when the 
 reaction is hesitating and uncertain. Choice-time is therefore not 
 included in the measured period of a discriminative reaction; for 
 an act of will occurs only during the phase of preparation. Dis- 
 crimination certainly occurs in the practical sense that the reaction 
 differs according to the stimulus which is presented; but we are not 
 warranted in framing too formal a notion of such discrimination, 
 nor in thinking of it as a well-defined act intervening between the 
 reception of the stimulus and the response. There is usually little 
 trace of a conscious comparison of the actual stimulus with other 
 possible stimuli; but, the rather, there is a simple apprehension of 
 the actual stimulus, followed by movement; and the apprehension, 
 as in the simple reaction, is prepared beforehand, and also need 
 not be completed till after the movement. It must, however, be 
 so far completed as to determine the right movement. 
 
 16. The time occupied by a "reaction with discrimination" 
 varies greatly according to the conditions. Speed is favored by 
 a "natural," i. e., previously well-trained, association between each 
 stimulus and the movement which is assigned to it as its reaction. 
 The speed of reaction increases with practice in associating a given 
 movement with a given stimulus. But the discrimination reaction 
 
 1 See especially Ach, op. cit., pp. 129, 146. 
 
 8 The Time Relations of Mental Phenomena (New York, 1890), p. 26. 
 
REACTION WITH DISCRIMINATION 489 
 
 never becomes so prompt as the muscular form of the simple re- 
 action, though it may become as prompt as the sensorial reaction. 
 This is what we should expect; because the preparation to move can- 
 not be so complete when there is uncertainty, in the period of prep- 
 aration, as to which of two movements is to be made; and besides, 
 the perceptive process must be allowed to go further before the 
 movement is initiated, than is the case in the more purely muscular 
 reaction. The quickest discriminative reactions reported are about 
 .180 sec., when two movements have to be held in readiness and 
 each executed in response to one of two stimuli; when only one 
 movement is held in readiness, and this is to be made in response to 
 only one of two stimuli, the time may sink to .170 or even .160 sec. 
 The preceding remarks suggest, what has been found to be true 
 in fact, that the time of a discriminative reaction increases with the 
 number of movements that are held in readiness, to be executed, each 
 in response to an assigned stimulus. Of the several authors who 
 have investigated this matter, the results of Merkel 1 may be cited. 
 He used as stimuli the numerals 1, 2, 3, 4, 5, and I, II, III, IV, V, 
 by assigning them in a natural way to the ten fingers. He thus ob- 
 tained the following reaction-times, as the average of ten individ- 
 uals: 
 
 One movement (simple reaction) 188 sec. 
 
 Two movements, with two stimuli . . . .. .276 
 Three movements, three stimuli . . . . . . . 330 
 
 Four movements, four stimuli 394 
 
 Five movements, five stimuli 445 
 
 Six movements, six stimuli . . . . . . . . 489 
 
 Seven movements, seven stimuli 526 
 
 Eight movements, eight stimuli 562 " 
 
 Nine movements, nine stimuli 581 " 
 
 Ten movements, ten stimuli 588 " 
 
 The discriminative reaction is lengthened not only when, as above, 
 the preparation of the movements becomes more complex and un- 
 certain, but also when the preliminary adjustment for the stimulus 
 becomes less definite. Thus Cattell found 2 that it required slightly 
 longer to react to one only out of ten possible colors than to react 
 to one out of two possible. 
 
 17. It appears from the preceding paragraphs that discrimina- 
 tive reaction is in part, like the simple reaction, a measure of the 
 perfection of the preparation to react to a stimulus. In part, however, 
 it is also a measure of the difficulty of the discrimination involved. 
 The more difficult the discrimination, i. e., the more nearly alike 
 the stimuli which must be held apart in reaction, the slower is the 
 
 1 Wundt's Philos. Studien, 1885, II, 73. 
 "Wundt's Philos. Stud., 1886, III, 460. 
 
490 TIME-RELATIONS OF MENTAL PHENOMENA 
 
 reaction. On this point we may cite the results of Henmon. 1 The 
 method employed by this experimenter, when the stimuli to be dis- 
 criminated were visual, was the following: Both stimuli were pre- 
 sented simultaneously and side by side, and both hands were made 
 ready to react, each resting on a telegraph key. The subject was 
 directed to react to a prescribed one of the two possible stimuli, 
 reacting with that hand which was on the side of the prescribed 
 stimulus. For example, if red and green were the two stimuli em- 
 ployed in a series of reactions, the red was presented lying sometimes 
 to the right of the green and sometimes to the left, and the subject 
 reacted with the right hand when the red appeared on the right and 
 with the left hand when the red appeared on the left. This form 
 of association between stimulus and movement was found to be es- 
 pecially easy, and so to give short and regular reaction times. 
 
 Experimenting in this manner, Henmon obtained the following 
 times for discriminative reactions between various colors: 
 
 White and black 197 sec. 
 
 Red and green 203 " 
 
 Red and blue 212 " 
 
 Red and yellow 217 " 
 
 Red and orange 246 " 
 
 Red and orange mixed with 25 per cent, red . . . 252 " 
 
 Red and orange mixed with 50 per cent, red . . . 260 " 
 
 Red and orange mixed with 75 per cent, red . . . 271 " 
 
 All of these differences in the stimuli are well above th,e threshold 
 of color discrimination (compare p. 360), and yet they do not all 
 require the same time for discrimination, but the time increases as 
 the difference decreases. 
 
 Similar results were obtained in the times of reaction to differences 
 of pitch, as can be seen in the following table : 
 
 Difference of pitch in vibrations .... 16 12 8 4 
 
 Time of the reaction 290 .299 .311 .334 
 
 Again, when the subject was required to react to the shorter of 
 two lines, exposed side by side, the following times were obtained: 2 
 
 In discriminating lines of 10 and 13 mm. . . . 296 sec. 
 
 H tl 
 
 (f tl 
 
 It II 
 
 It II 
 
 (t 
 
 10 ' 12.5 " . . . . 298 ' 
 
 10 ' 12 " . . . . 305 " 
 
 10 ' 11.5 " . . . . 313 " 
 
 10 ' 11 "... 324 " 
 
 10 ' 10.5 " . . 345 " 
 
 1 The Time of Perception as a Measure of Differences in Sensations (New York, 
 1906). 
 
 2 The times cited are those of the quicker of two subjects; the relations of the 
 times of the slower subject were approximately the same. 
 
REACTION WITH DISCRIMINATION 
 
 491 
 
 In general, then, diminishing the difference between two stimuli 
 increases the discriminative reaction-time; and, further, diminishing 
 the objective difference successively by equal amounts causes a 
 greater and greater increase in the reaction-time, as the threshold 
 of discriminable difference is approached. If we interpret this 
 result in the light of our former discussions, we may probably con- 
 clude that the process of apprehension of the stimulus has to pro- 
 ceed further, before the initiation of the movement, when the differ- 
 ence to be reacted to is small than when the difference is large; and 
 also that the preliminary adjustment of the movement is less com- 
 plete when greater difficulty of discrimination is anticipated. In 
 other words, the smaller the difference between the simuli, the more 
 the discriminative reaction approaches the pnre type of sensorial or 
 apperceptive reaction. 
 
 18. It should now be made clear that the study of discrimina- 
 tive reactions is capable of giving information regarding the com- 
 parative ease or difficulty of different processes of discrimination, 
 provided only that all the conditions of the experiment remain the 
 same, so that the only factor influencing the reaction-time is the 
 difference between the stimuli. In this respect, results like those 
 quoted below from von Kries and Auerbach 1 are of interest. The 
 numbers given in this table are obtained by subtracting the simple 
 reaction-time from the time of the discriminative reaction, and need 
 to be increased by .150-. 200 sec. to make them comparable with 
 the reaction-times previously given. Such comparison cannot, 
 however, be properly made, because of individual differences. The 
 validity of the method of "elimination by subtraction," employed 
 by these authors to determine the pure time of discrimination, will 
 be discussed later; meanwhile, the differences which appear in the 
 table may be taken as indicative of differences in the difficulty of 
 discrimination. 
 
 Discernment of the direction of light 
 
 Discernment between two colors 
 
 Localization of sound (minimum) 
 
 Discernment of tone when higher 
 
 Localization of sensations of touch 
 
 Localization of distance by sight 
 
 Discernment between tone and noise 
 
 Judgment of intensity of sensations of touch (strong) . 
 Discernment of tone when lower ...... 
 
 Judgment of intensity of sensations of touch (weak) . 
 Localization of sound (maximum) 
 
 Auerbach Von Kries 
 
 Sec. 
 0.011 
 0.012 
 0.015 
 0.019 
 0.021 
 0.022 
 0.022 
 0.023 
 0.034 
 0.053 
 0.062 
 
 Sec. 
 0.017 
 0.034 
 0.032 
 0.049 
 0.036 
 0.030 
 0.046 
 0.061 
 0.054 
 0.105 
 0.077 
 
 Archiv /. (Anat. w.) Physiol (1877), p. 298. 
 
492 TIME-RELATIONS OF MENTAL PHENOMENA 
 
 19. Various interesting discoveries were made during the course 
 of the experiments which resulted in preparing the foregoing table. 
 For example, it was found that the simple reaction-time for A. 
 (Auerbach), when stimulus was applied to the middle finger or 
 back of the hand, was 0.146-0.147 sec.; and for K. (Kries), 0.117- 
 0.119 sec. But, as the table shows, when discernment was re- 
 quired of the two observers, the reaction-time of K. was relatively 
 so much increased as to make his discernment time greater than 
 that of A. The result of practice in discernment was found to hold 
 good for other areas of the skin than those in experimenting upon 
 which the practice was gained. For discernment among three 
 places (middle finger, back of hand, and middle of lower arm), the 
 order being unknown and only one to be reacted on the mean in- 
 terval required was for A. 0.028 sec., and for K. 0.050 sec.; fur- 
 ther practice, however, reduced this interval to about the same as 
 that required for two places. 
 
 Discernment between two intensities of the sensation of touch was 
 found to be very uncertain and difficult. Many more false reactions 
 followed the attempt to tell whether the dorsal side of the last of 
 the phalanges of the middle finger was being hit with the weaker 
 or the stronger of two stimuli than occurred in the attempts to 
 localize tactile sensations. The discernment time, when reaction 
 followed the stronger stimulus, was 0.016-0.034 sec. for A., and 
 0.05-0.07 for K.; when reaction followed the weaker stimulus, the 
 discernment time was 0.035-0.069 sec. for A., and 0.089-0.114 for K. 
 The character of our judgments of intensity is, perhaps, dependent 
 on the steepness, as it were, with which the curve rises in con- 
 sciousness; but, however this may be, it appears that we discern 
 how and where we are affected with a sensation more promptly 
 than about how much we are affected. 
 
 When discernment between two simple tones of different pitch is 
 required, the reaction follows the one of higher pitch more promptly. 
 Thus the discernment time, under such circumstances, was for A., 
 0.015-0.044 sec., and for K., 0.043-0.11; but, if reaction followed 
 the tone of lower pitch, the discernment time for A. was 0.03-0.059 
 sec., and for K. 0.045-0.092. To discern tone from noise, when re- 
 action followed the tone, A. required 0.015-0.023 sec., and K. 0.036 
 -0.055; when reaction followed the noise, A.'s discernment time was 
 0.017-0.025 sec., and K.'s, 0.045-0.047. The reaction-time dimin- 
 ishes as the pitch rises; for very high notes it nearly reaches the 
 limit required for hearing the noise of the electric spark. 
 
 The simple reaction-time for sensations of sound remains nearly 
 the same for all changes in the angle by which the locality of the 
 sound diverges from the median plane between the two ears. But 
 
ASSOCIATIVE REACTION-TIME 493 
 
 the time required for discerning the locality of the sound varies 
 greatly for the different sizes of this angle. Thus the discernment 
 time for locality, as to right or left, varied for Auerbach and Kries 
 as follows: 
 
 
 Angle 120-35 
 
 Angle 35-26 
 
 Angle 26-ll 
 
 Auerfo&ch 
 
 . 020 sec. 
 
 0.033 sec. 
 
 . 120 sec. 
 
 Kries 
 
 0.013 " 
 
 0.122 " 
 
 0.153 " 
 
 
 
 
 
 The discernment time required for localizing the direction of a 
 spark by direct vision varied for A. from 0.005 to 0.025 sec., and for 
 K. from 0.006 to 0.029 sec.; by indirect vision, for A. from 0.008 
 to 0.028 sec., and for K. from 0.007 to 0.028 sec. For localizing 
 distance, A. required 0.019 to 0.027 sec. of discernment time, when 
 the object arose in front of the fixation-point, and K. 0.027 to 0.035 
 sec.; but A. required 0.019 to 0.029 sec., and K. 0.021 to 0.036 sec., 
 when the object arose behind this point. 
 
 20. From the discriminative reaction we pass now to the as- 
 sociative. It is true that a certain amount of association is already 
 involved in the discriminative reaction, since definite movements 
 are associated, for the purposes of the experiment, with definite 
 stimuli. But the stimuli and movements employed, and the asso- 
 ciations between them, are few in such cases; and the subject knows, 
 within a narrow range, what stimulus to expect and for what move- 
 ment to prepare. In the typical experiment in association time, 
 also, the subject knows, indeed, the general type of stimulus which 
 he is to receive and the general type of movement which he 
 is to make ; but neither the exact stimulus nor the exact reaction need 
 have been previously mentioned in the course of the experiments. 
 He may be told, for example, that he will be shown some number, 
 and that he is to react by calling out the number next larger; or that 
 he is to be shown a word, and is to react by calling out another 
 word, the first that is suggested by the presented word. The asso- 
 ciation between stimulus and response is here one of long standing, 
 instead of being, as in the discriminative reaction, specially formed 
 for the purposes of the experiment. The long-standing of the asso- 
 ciation favors quick response; but, on the other hand, the immediate 
 preparation for the stimulus and for the movement cannot be so 
 complete here as in the discriminative reaction; so that, on the whole, 
 the associative reaction takes the longer time, and often a very much 
 longer time. There is in fact no upper limit to the time which may 
 be occupied by such a reaction; at its quickest, it occupies about 
 half a second or a little less. 
 
494 TIME-RELATIONS OF MENTAL PHENOMENA 
 
 In the simple and discriminative reactions, we recognized two 
 factors as determining the speed of the performance namely, (1) 
 the perfection of the preliminary adjustment or preparation; and 
 (2) the degree to which the perceptual process must be carried be- 
 fore the impulse to movement could be initiated. In the associative 
 reaction, these two factors are joined by a third namely, the de- 
 gree of closeness of the previously formed association between the 
 stimulus and the response. We may profitably examine the in- 
 fluence of each of these factors on the association time, beginning 
 with the last as the most obvious. 
 
 21. That familiar associations operate more quickly than un- 
 familiar is too trite a fact to require elaboration. It is obvious, for 
 example, that one who is familiar with the multiplication table will 
 call out the product of two presented numbers more promptly than 
 one who is ill trained in multiplication; or that it will take less time 
 to name objects in one's native language than in an unfamiliar 
 language. What is more worthy of note is that the association time 
 reveals differences in the firmness of associations which are not evi- 
 dent to less precise modes of observation. Thus one may have 
 lived for some years in a foreign country and have learned to use 
 its language fluently, and still require more time to name an object 
 in that language than in one's own. 1 Cattell found that it took 
 much less time (0.345 and 0.389 sec., respectively, for two subjects), 
 to name the month following a given month, than it took (0.763 and 
 0.832 sec.) to name the month preceding a given month. Simi- 
 larly, more time was required to name the letter preceding, in the 
 alphabet, a given letter than to name the letter following; or to 
 respond to a given number by the number next smaller than by the 
 number next larger. In all such familiar series, association is 
 much quicker in the direction of the series than in the reverse di- 
 rection. Less obvious is the reason for the fact 2 that it ordinarily 
 takes less time, on the average, to pass from a part to the whole 
 (i. e., to give the name of the whole object when the name of a part 
 is presented as the stimulus), than to pass from the whole to a part; 
 and less to pass from a special class to a more general (e. g., "dog 
 animal") than from the more general to the more special. These 
 differences in speed of reaction are probably to be attributed, in 
 part at least, to differences in the previous training of the associa- 
 tions. Within the same category of associations, some are im- 
 mensely more familiar and quick-acting than others; and even 
 among those which seem, all alike, thoroughly familiar, some are 
 
 1 Cattell, Wundt's PhUos. Stud., 1888, IV, 241; and Mind, XII, 68. 
 
 2 Cattell, op. cit.; Trautscholdt, Wundt's Philos. Studien, I, 213; Watt, Archiv 
 /. d. ges. Psychol, 1905, IV, 289. 
 
ASSOCIATIVE REACTION-TIME 495 
 
 shown by experiment to give quicker reactions than others. Thus, 
 if the requirement is to respond by a word having the opposite 
 meaning to that of the stimulus word, about twice as much time 
 is consumed in reacting to "smooth" or to "broad," as in re- 
 acting to "good" or "long." In this way, the study of associa- 
 tion times reveals facts regarding the experience and training of 
 an individual or of a social group which would otherwise be only 
 vaguely suspected. 
 
 22. The time of an associative reaction is not, however, an 
 unequivocal measure of the firmness with which an association has 
 been established by past experience; the speed of reaction depends 
 in part on the perfection of the momentary preparation and in part 
 on the complexity of the mental process which intervenes between 
 the stimulus and the reaction. Introspective studies 1 have revealed 
 some interesting facts regarding both the period of preparation and 
 the principal or reaction period. The preparation may be more or 
 less precise, according to the character of the experiment. Thus 
 it has become customary to distinguish between "free" and "con- 
 trolled" or "constrained" association: in the former case, the sub- 
 ject is simply directed to respond by the first word suggested by 
 the stimulus word; in the latter, he is required to respond by a word 
 which stands in some assigned relation to the stimulus word as, 
 for example, a word of opposite meaning, or the name of a class 
 of objects within which the object named in the stimulus word is 
 included. The constraint may be more or less complete; since in 
 some cases there is only one right answer, while in others any one 
 of a greater or smaller number of answers is correct. Free associa- 
 tion is intended to be wholly unconstrained; but this ideal cannot 
 be reached in practice, since the subject has at least to make a verbal 
 response, and since, also, in many cases, he involuntarily sets him- 
 self a more definite task. 
 
 One seemingly strange result from these experiments is that free 
 association often requires more time than constrained. The reason 
 probably is that the constraint, by limiting beforehand the field 
 of operations, permits of a more perfect preparation to react. The 
 reality and efficiency of the preparation, in general, is shown by the 
 promptness with which a correct response is given in constrained 
 association; as well as by the fact that changing the preparation 
 for example, from an adjustment to multiply to an adjustment to 
 add two presented numbers leads to an entirely different set of 
 responses to the same stimuli. Watt found that, at the commence- 
 ment of a series of associative reactions of the same type, the task 
 or problem (naming a higher class, etc.) was at first consciously 
 See especially Watt, op. cit. 
 
496 TIME-RELATIONS OF MENTAL PHENOMENA 
 
 represented in the preparation for each single reaction; but that this 
 consciousness of the task decreased as the series of similar reac- 
 tions progressed, till there might be nothing present to indicate 
 the prepared condition except an unspecialized feeling of readiness. 
 The adjustment for the reaction remained in force, however; and 
 it even improved as the consciousness of it waned. 
 
 On changing from one task to another, the subject is likely to 
 become more specifically conscious of the new problem; he is also 
 likely to be slower in his reactions, even though the "new" problem 
 may have been made familiar by previous experiments. This also 
 goes to indicate the reality of some central adjustment by which 
 the pathway of each fresh stimulus through the brain is made ready 
 beforehand. 
 
 23. The amount of conscious process which intervenes be- 
 tween the stimulus and an associative reaction varies enormously 
 in different cases. When the preparation is perfect, and the asso- 
 ciation of response to stimulus is familiar, the time of reaction is 
 brief, and there is little introspective evidence of the process, which 
 seems to have become automatic. This is especially apt to be the 
 case when there is only one correct response. When two or more 
 responses are correct, and about equally familiar, the time is apt 
 to be longer, 1 apparently because of an interference between the 
 tendencies toward the two responses. Introspectively, this inter- 
 ference may be in evidence as an experienced need of selection 
 between two consciously suggested responses. At other times, 
 interference results from the occurrence of a conscious tendency to 
 some false reaction; even if such a tendency is checked in time to 
 prevent an incorrect motor response, it slows the reaction. Time 
 may also be required in searching for a correct response. This 
 searching, in not too difficult eases, is introspectively a waiting for 
 the correct response to be suggested. Sometimes the correct re- 
 sponse seems to come gradually, so that its coming can be observed; 
 at other times it appears suddenly, or jumps into consciousness. 
 
 The interference of one associative tendency with another can 
 also be strengthened by requiring a response other than the one 
 which is likely to be suggested. Thus Ach, 2 after first forming by 
 repetition strong associations between pairs of nonsense syllables, 
 then presented these syllables with the requirement that the response 
 to each should be, contrary to the formed associations, a syllable 
 rhyming with it. Under these conditions, the first response suggested 
 was almost sure to be the familiar but now incorrect syllable. If 
 this wrong response was not checked in time, the false reaction led 
 
 1 Compare Cattell, op. cit. 
 
 2 Uber den Willensakt und das Temperament (Leipzig, 1910). 
 
GENERAL CHARACTER OF REACTION-TIME 497 
 
 to a strong determination to react correctly, and the effectiveness 
 of this strong determination, which formed part of the preparation 
 for the succeeding reactions, was often visible in a successful check- 
 ing of the wrong response in the next trial. When the wrong re- 
 sponse was checked in time, the thought of it was followed by a con- 
 sciousness of the task to be accomplished (such consciousness not 
 ordinarily being present between the stimulus and the response); 
 and, next, by a period of searching for a correct response. 
 
 24. It is clear that the amount of the entire mental process in- 
 tervening between the stimulus and the associative reaction differs 
 greatly in different cases. The time of such a reaction is not al- 
 ways the time of the same processes; because the total associative 
 mechanism which may be called into play is vast and compli- 
 cated, and subject to many varying influences, so that it cannot al- 
 ways be perfectly controlled. The time of an associative reaction, 
 therefore, is a measure partly of the perfection and strength of 
 the preliminary adjustment, partly of the strength of the partic- 
 ular associative tendency which leads to the response, and partly 
 of the amount of interference between different associative ten- 
 dencies. 
 
 25. The object of the study of reaction-times, as it presented 
 itself to the minds of the founders of this study, was that of deter- 
 mining the duration of definite mental operations, such as sensation, 
 perception, apperception, discrimination, association, choice. Cer- 
 tain of these processes they recognized as involved in the simple re- 
 action, and these they labored to isolate by variations of the experi- 
 ment. Others of these processes they supposed to be added when 
 the simple reaction was changed to the discriminative form, or 
 this to the associative. Accordingly, they argued that the time oc- 
 cupied by these added operations could be obtained by subtracting 
 the time of the simple reaction from the discriminative or associ- 
 ative reaction- time; and great ingenuity was displayed in so vary- 
 ing the conditions and requirements of the reaction as to permit of 
 this " elimination by subtraction." More recent studies have shown, 
 as described above, that the total actual process which is timed does 
 not correspond, introspectively, with the analyses drawn up for it; 
 and probably, also, it does not correspond physiologically. These 
 supposed constituent processes perception, apperception, discrim- 
 ination, choice were discovered by logical analysis of the result 
 or outcome, and not by a direct study of the process itself. The 
 process gone through in a discriminative reaction is not that of a 
 simple reaction with a process of discrimination interpolated into 
 it; but the whole performance is different, beginning with the pre- 
 liminary adjustment. It follows, therefore, that the procedure of 
 
498 TIME-RELATIONS OF MENTAL PHENOMENA 
 
 elimination by subtraction is invalid, 1 and that the times occupied 
 by the supposed elementary processes are not revealed by the work 
 on reaction-times. Yet the results obtained are of value, and the 
 method of reaction-times promises to prove of still further value 
 when taken in connection with introspective analysis of the process 
 and with other indications of the nature of the brain's action in 
 elucidating the dynamics of mental operations. 
 
 26. One further remark should be made regarding reaction- 
 times. In these experiments, the aim is to isolate as far as possi- 
 ble a single factor, so as to determine its speed and the process by 
 which it is accomplished. From the very nature of the case, how- 
 ever, it is probable that this aim can never be accomplished per- 
 fectly. In ordinary life, while reactions essentially like the simple 
 discriminative and associative reactions of the experimenters are 
 common enough, they do not occur in isolation, but as parts, usu- 
 ally, of continued performances. Hence the importance of Cat- 
 tell's attempt 2 to study the speed of such reactions under condi- 
 tions approaching those of ordinary application. When a whole 
 page of words or letters was placed before the subject, and he was 
 directed to read or name these consecutively and as rapidly as possi- 
 ble, the time consumed per letter was .188 sec. and per word .200 
 sec. In the isolated reactions, however, the time for a word or 
 letter was .320 to .360 sec. Two important facts appear in these 
 figures. The first is that scarcely more time is required to name a 
 (short) word than to name a single letter; it is clear, therefore, that 
 the reaction to a word does not consist of a sum of reactions to the 
 letters composing it. The reaction is by larger units. The other fact 
 to be noted is that considerably less time is required to react to each 
 one of a series of letters or words when all are visible at once 
 than when it is required to react to a single isolated letter or word. 
 There is evidence here of an overlapping of the successive reactions. 
 While one word is being pronounced, the next is already in process 
 of apprehension. The overlapping, in fact, extends beyond the 
 adjoining unit; by a special experiment, Cattell determined that it 
 extended, in the case of separate letters, over four or five of them. 
 (We note in passing that this corresponds to what we should ex- 
 pect to follow from the facts already brought out in the study of 
 visual perception. See pp. 461 f.) Both overlapping and reaction 
 to large units enter into such skilled performances as those of a 
 typewriter writing from copy or of a musician playing from score. 
 These performances consist of a series of discriminative reactions; 
 
 1 This procedure has been searchingly criticised by several of the authors cited 
 above, and also by Aliotta, La Misura in Psicologia Sperimentale (Firenze, 1905). 
 
 2 Wundt's Philos. Studien, 1885, II, 635. 
 
GENERAL CHARACTER OF REACTION-TIME 499 
 
 and yet their speed may be such that only an eighth or a tenth of 
 a second is occupied by each movement much less than the time of 
 a discriminative reaction. The high speed is due partly to appre- 
 hending and reacting to phrases rather than to single notes, words, 
 or letters; and partly to carrying on processes of apprehension and 
 movement simultaneously. In both these processes, "practice 
 makes perfect" by the expedient of dropping out of consciousness 
 many of its procedures which, although originally necessary, have 
 now become, under the laws of association and habit, quite dis- 
 pensable without impairing, but rather with improving, the speed 
 and certainty with which the desired result is secured. Nor can 
 there be any reasonable doubt that a corresponding "short-circuit- 
 ing" takes place in the nervous mechanism, especially in the cere- 
 bral processes. Thus skill is acquired. To the same question as 
 to how such skill is acquired, we shall have occasion to recur in a 
 later chapter on "learning." We close this discussion by referring 
 again to the truth that both the reactions of the nervous mechanism 
 and the correlated mental activities are immensely complicated; 
 while at the same time, the factors which can be either definitely 
 isolated or reasonably suspected are bound together, in both cases, 
 into a marvellous unity. 
 
CHAPTER VII 
 FEELING, EMOTION, AND EXPRESSIVE MOVEMENTS 
 
 1. We have already had repeated occasion to remark upon the 
 extreme difficulty of recognizing either by purely introspective or 
 by experimental analysis the absolutely elementary factors of our 
 complex mental states. As a matter of course, when such factors 
 are successfully analyzed, they are found to be incapable of iden- 
 tification with one another; or of being substituted for one another, 
 without changing the entire mental complex of which they form a 
 part. All the difficulties connected with, or consequent upon, this 
 fundamental fact of our experience, accompany in an exaggerated 
 manner every attempt to form a satisfactory psychological doctrine 
 of the nature and conditions of the feelings and emotions. And, 
 indeed, as to the essential nature of feeling, no satisfactory definition 
 can be given; since to feel is as simple and fundamental an opera- 
 tion of mind as it is to know. Feeling can never be stated in terms 
 of knowledge. Inasmuch, then, as all definition is only the ex- 
 pression of an elaborate and complex form of knowledge, the nature 
 of feeling is not capable of being defined; it must be felt. When, 
 then, this nature is defined as consisting in some relation to physi- 
 cal sensation or to mental images, it is deprived of the very character- 
 istic which makes it to be feeling rather than sensation or idea. 
 Various theories, however, have succeeded in stating certain con- 
 ditions or antecedents of the reaction of mind in the way of feeling. 
 
 2. To this indefiniteness in the experience, the indefiniteness 
 in the language which attempts to describe the experience bears 
 witness. The term, "feeling," has, in popular speech, a wide and 
 varied usage. Sometimes applied to the sense of touch, it is also 
 used to indicate any and every emotional experience, whether de- 
 pendent chiefly upon certain characteristic sensations, or chiefly 
 upon conditions of an ideal or intellectual sort. An example of 
 this last usage is seen in the expression, "I feel that there is some 
 mistake in the argument, though I cannot tell just where the mis- 
 take lies." Psychologists have abandoned the first of these usages, 
 but are not yet agreed as to the limits to be drawn within the broad 
 field indicated by the second and third usages. Some would pre- 
 fer to retain feeling as a vague term, applicable to any condition of 
 
 500 
 
VARIABLE CONCEPTION OF FEELING 501 
 
 consciousness which is lacking in clear definition. Others would 
 narrowly limit the term to the feelings of pleasantness and unpleas- 
 antness, regarding which no doubt exists that they should be thus 
 named. 
 
 The question is not entirely one of scientific terminology; for, 
 from the point of view of the analysis of consciousness, many of these 
 vague states must be regarded as composite; whereas pleasantness 
 and unpleasantness are apparently not susceptible of further analy- 
 sis, but seem themselves to be of an elementary character. From 
 this point of view, therefore, the question arises as to what elements 
 of consciousness, different from sensation, and analogous to these 
 paradigms, pleasantness and unpleasantness, can be discovered. 
 Even on this question, however, competent opinion is still widely 
 divergent, since some authorities would limit the elementary feel- 
 ings to the two mentioned, while others would increase the number 
 to four or six, and still others hold to a very large and indefinite 
 number. One writer, 1 for example, presents a long list, including 
 feelings of interest, of reality and unreality, of belief and doubt, of 
 clearness, confusion, effort, ease, eagerness, hesitation, pride, hu- 
 mility, admiration, scorn, reasonableness, etc. In the opinion of 
 many psychologists, most of these, as well as the numerous other 
 emotions which might be mentioned, are certainly complexes of 
 many elements. 
 
 The dual conception of feeling, which would reduce all feelings 
 to pleasantness and unpleasantness (in combination with a variety of 
 intellectual elements), is the traditional view, and has usually been 
 in favor with the analytic school of psychologists. Recently, how- 
 ever, Wundt 2 and Royce 3 have made the novel suggestion that there 
 are, in addition to this one dimension of feeling, other dimensions. 
 The latter proposes to regard feeling as a two-dimensional con- 
 tinuum, like a plane; he would, therefore, add to the traditional 
 pleasantness-unpleasantness dimension that of restlessness-quies- 
 cence. This is to say that any state of feeling may differ from any 
 other state in being more or less pleasant and also in being more or 
 less restless. Wundt proposes to regard feeling as a three-dimen- 
 sional continuum, and adds to the dimension of pleasantness and 
 unpleasantness that of excitement and calm or depression (in which 
 connection depression is to be taken as a simple opposite of excitement 
 and is not to include any notion of unpleasantness), and that of 
 tension and relief. Those who, in opposition to these suggestions, 
 
 1 Baldwin: Handbook of Psychology, Feeling and Will, 1894, p. 242. 
 
 2 Grundriss d. Psychologic, 1896, p. 98; Grundzuge d. physiol. Psychol., 6th ed., 
 1910, II, 294. 
 
 3 Outlines o/ Psychology, 1903, 
 
502 FEELING, EMOTION, AND EXPRESSIVE MOVEMENTS 
 
 continue to uphold the one-dimensional theory, 1 believe that the 
 excitement, tension, etc., incident to certain states of feeling, are 
 not elementary, but are compounds of bodily sensations. 
 
 3. In criticism of the foregoing views, we remark that the real- 
 ity of feelings of excitement and depression, of tension and relief, 
 cannot be doubted, but only their elementary character; and this 
 has not yet been established. Among the sensations, some classes 
 are indeed characterized chiefly with a tone of either pleasantness 
 or unpleasantness; here belong tastes, odors, and sensations of 
 temperature. Visual and auditory stimuli do not usually arouse 
 such intense feelings of pleasantness or unpleasantness; unless they 
 have become associated with certain meanings, as in pictures, 
 landscapes, and speech. Visual sensations, more than auditory, 
 are likely to be relatively free from pleasantness and unpleasant- 
 ness. There is much greater unanimity among individuals as to 
 which is the most exciting color than as to which is the most agreea- 
 ble; the latter judgment seems to depend largely on the associa- 
 tions of the various colors, while the exciting or calming effect of 
 a color seems to be more intrinsic. Feelings of tension and relief 
 are especially connected with auditory sensations, perhaps because 
 the temporal sequence of this class of sansations is particularly 
 prominent. In listening to a series of metronome beats to take 
 Wundt's example one can detect a feeling of tension in anticipa- 
 tion of each beat, which gives way to a temporary relaxation or 
 relief on the actual occurrence of the beat. So, also, in listening 
 to, or in reading, a sentence which has a periodic structure, a feeling 
 of tension can be observed which gives way to relief on the comple- 
 tion of the sentence. Tension is thus closely associated with ex- 
 pectancy, and relief with the fulfilment of expectation. Along 
 with the expectancy there may be present, according to the circum- 
 stances, either pleasantness or unpleasantness; and the relief may 
 also be accompanied with either of these feelings, according as the 
 event is agreeable or the opposite. 
 
 As we have already seen, according to the theory of Wundt, the 
 qualities of simple feeling have three dimensions, and no more. 
 But it is very doubtful whether this view gives an adequate account 
 of our feeling experience. Certainly there are many feelings which 
 do not readily fit into these grooves, and which, though we may sus- 
 pect them to be composite, do not as yet admit of being analyzed. 
 Among these may be mentioned a class of feelings connected with 
 memory and judgment. When one is trying to recall a name, the 
 feeling of "being near," or "on the right track," often arises. If an- 
 
 1 See Titchener, Lectures on the Elementary Psychology of Feeling and Atten- 
 tion, 1908; Ebbinghaus, Grundziige der Psychologic, 1905, p. 567. 
 
DIFFICULTIES OF ANALYSIS 503 
 
 other than the right name occurs to the mind, it is usually attended 
 with a feeling of its incorrectness, but when the right name occurs, 
 it is felt to be right. Such affective experiences are accepted as the 
 warrants of correct or incorrect recollection; they cannot be de- 
 fended; they cannot be analyzed; they are distinctly subjective, and 
 not referable to anything beyond the self; in all ways they answer 
 to the conception of feeling rather than of any form of sensation or 
 knowledge. The same can be said of the feeling of familiarity 
 with a face or an odor which is not completely recognized, but is 
 felt to be known as distinguished from " knowing it to be known." 
 Then, too, there is the feeling of conviction which obtains when we 
 pass from the former into the latter state of consciousness. Ex- 
 periments on reaction time and association time have also brought 
 to light feelings of readiness or unreadiness, of clearness or con- 
 fusion, which have resisted analysis. Most of the feelings mentioned 
 in this paragraph may be attended with pleasantness or unpleas- 
 antness, but they are not simply pleasant or unpleasant. On the 
 contrary, they have, each one, a specific character. This character 
 is not identical with the character of the name, for example, which 
 is being recalled, for the same feeling may attach to any name 
 under the circumstances of its being recalled after difficulty. The 
 feeling seems, the rather, to be appropriate to a certain sort of mental 
 performance. To resolve it into a complex of bodily sensations is 
 at least premature; though it might, conceivably, be so composed, 
 it might equally well, for all we can now see, be the specific accom- 
 paniment of a special sort of brain action. As the case stands at 
 present, therefore, psychological analysis does not permit us to 
 limit the number of elementary feelings to two, or to six, or to any 
 specified number. 
 
 4. We recur, then, to our previous statement with added con- 
 fidence. About that aspect of our experience which we call our 
 feelings, or our emotions their nature, origin, relation to a physi- 
 cal basis and to sensations and ideas we know remarkably little. 
 The reason for this fact is not difficult to discover. By their very 
 nature, the phenomena are obscure, indefinite, and yet extremely 
 variable and multiform. They are also connected with our sensa- 
 tions and ideas in such a way as to make all separation in fact quite 
 impossible. The psychology of the feelings, as studied from the 
 introspective point of view, has therefore always been peculiarly un- 
 productive of assured results. The fact that their physiological con- 
 ditions are laid so largely in obscure, rapid, and infinitely varied 
 changes within the central organs, such as cannot be either directly 
 observed or indirectly subjected to experimentation, increases the 
 difficulties of the subject. What is the nature of feeling ? How do 
 the different feelings differ, and what elements have they in com- 
 
504 FEELING, EMOTION, AND EXPRESSIVE MOVEMENTS 
 
 mon? Under what conditions do we have sensuous feelings; and 
 under what conditions are these feelings pleasant or unpleasant? 
 Is feeling ever perfectly indifferent? is there a zero-point of feel- 
 ing? How are the feelings related to the quality and intensity of 
 physical stimuli ? What is the physiological basis (if any exist) of 
 the higher aesthetic, moral, and religious feelings? These and 
 other similar questions may be asked of psycho-physical science 
 with little satisfactory result. The attempt unduly to simplify, 
 instead of increasing our scientific clearness in dealing with the sub- 
 ject, adds to its confusion. 
 
 5. The experimental study of feeling in some of its aspects 
 has developed two methods of investigation, called "the method 
 of impression" and "the method of expression." Both are alike 
 in that they bring to bear on the subject certain known stimuli, in 
 order to determine their effect in exciting the feelings. The stimuli 
 range, in different experiments, from simple colors, tones, odors, or 
 tastes, to complex presentations, such as pictures or witticisms or 
 musical and literary compositions. The two methods differ in the 
 means by which the feeling of the subject is made known. In the 
 method of impression, the person on whom the stimuli are made to 
 act has simply to take note of the resulting changes in his affective 
 tone, and to indicate the result of his observation in words. The 
 method of expression is based on the fact that bodily reactions often 
 appear in connection with changes of feeling such as reactions of 
 the organs of circulation, respiration, secretion, digestion, and 
 involuntary movements and tensions of the muscles. The method 
 of expression, acting on the assumption that these bodily reactions 
 are the signs of inner feeling, seeks to record these signs themselves. 
 The two methods are often joined, in that the subject is asked to 
 report his feelings in words, and the correlation of the feelings, as 
 introspectively observed, with the bodily expressive reactions, is 
 made the main purpose of the experiment. 
 
 In the use of the method of impression, the most approved pro- 
 cedure is to present stimuli in pairs, and then to require a judgment 
 on each pair, as to which of the two is the more agreeable, etc. A 
 variation of this method consists in presenting a whole series of 
 stimuli at once, and requiring that they be arranged in the order 
 of their agreeableness or other value. The use of the method of 
 impression has occurred, largely, in the field of aesthetics, into which 
 Fechner 1 was the first to introduce the experimental method. 
 
 6. Since it is the special purpose of this Treatise to investigate 
 the phenomena of man's mental life in their relations to the func- 
 tions of the nervous mechanism, some attempt at a physiological 
 theory of the feelings and emotions would seem to be our appropri- 
 1 Vorschule der JZsthetik, 1876. 
 
PHYSIOLOGICAL THEORIES OF FEELING 505 
 
 ate task. This method of approach, and the field of conjecture to 
 which it leads, has proved particularly attractive to most theorists. 
 But, unfortunately, far the greater number of such theorists have 
 made no effort to take into consideration, much less to deal ade- 
 quately with, the complex and intricate forms of nervous function- 
 ing, concerned in any comprehensive theory. It is true, as Bain l 
 declares, that "a very considerable number of the facts may be 
 brought under the following principle namely, that states of pleas- t 
 ure are connected with an increase, and states of pain with an abate- 
 ment, of some, or all, of the vital functions." But other facts in no 
 small number cannot be brought under this principle. It is not a 
 difficult task for the physician to abate all the vital functions of 
 the patient, even down to or beyond the line of danger, with the im- 
 mediate result of producing pleasure rather than pain. After ob- 
 jecting to Bain's statement as being "too vague," etc., Grant Allen 2 
 declares the true principle of connection to be the following: "Pleas- 
 ure is the concomitant of the healthy action of any or all of the organs 
 or members supplied with afferent cerebro-spinal nerves, to an ex- 
 tent not exceeding the ordinary powers of reparation possessed by 
 the system." ^Esthetic pleasure he provisionally defines as "the 
 subjective concomitant of the normal amount of activity, not directly 
 connected with life-serving function, in the peripheral end-organs 
 of the cerebro-spinal nervous system." Now, that pleasure is the 
 reflex of healthy and unimpeded activity is an old psychological 
 truism; and that we are dependent upon impulses propagated in 
 the sensory nerves of the cerebro-spinal system for sensations, 
 pleasurable or painful, of muscular, organic, or more special sort, 
 scarcely needs statement as a newly discovered law of "physiologi- 
 cal aesthetics." Nothing, however, could well be more "vague" 
 than the limit fixed by the words "to an extent not exceeding the 
 ordinary powers of reparation possessed by the system." 
 
 Even to undertake the study of the bodily expressions of feeling, 
 it is necessary to have a knowledge of the physiology of circulation, 
 respiration, secretion, reflex action, etc. The movements of breath- 
 ing, for instance, are, indeed, responsive to mental conditions 
 laughing, sobbing, and sighing are striking examples of this re- 
 sponsiveness but they are also responsive to many other influ- 
 ences, such as the need of the muscles for oxygen. Any sensory 
 stimulus is likely to exert an influence on the breathing, and that 
 even in a condition of unconsciousness through anaesthesia. The 
 rate and strength of the heart-beat is likewise subject to many re- 
 flex influences, as well as to influences proceeding downward from 
 the cerebrum; and it is further influenced by the respiration. The 
 
 1 The Senses and the Intellect, pp. 281 f. 
 
 2 Physiological ^Esthetics, 1877, pp. 21-34. 
 
506 FEELING, EMOTION, AND EXPRESSIVE MOVEMENTS 
 
 circulation through any organ is influenced not only by the rate and 
 force of the heart-beat, but also by the constriction or dilatation of 
 the arteries leading to the organ, and to other organs as well. Even 
 the activity of the voluntary muscles is certainly subjected to reflex 
 influences, as well as to the influence of the cerebrum. It thus be- 
 comes a difficult matter to observe the effects of simple states of 
 feeling,. because of the complication of these with other effects. 
 
 The principal kinds of the observation of bodily functions in 
 connection with the feelings have been the following: The move- 
 ments of the chest and perhaps also of the abdomen in breathing 
 have been recorded, by aid of pneumographs; and the relation of 
 different feelings to changes in the rate and depth of respiration has 
 been studied in the graphic records. The pulse has been recorded 
 by sphygmographs applied to various arteries, among which the 
 radial is the most accessible, though the carotid, since it supplies the 
 brain, has appeared to many investigators as the most important 
 for psychological purposes. Changes in the volume of the arm, or 
 of a finger changes which are due to the varying amount of blood 
 contained in the member, and which depend partly on the output of 
 the heart and partly on the vasomotor condition of the member and 
 of other parts have been deemed specially important by many 
 authorities, and have been recorded by means of plethysmographs. 
 Similar instruments can even be applied to the brain itself, in cases 
 where part of it has been exposed through removal of a portion of 
 the skull. One or two other methods of study will be briefly re- 
 ferred to below. 
 
 7. We may begin by bringing forward the most ambitious at- 
 tempt to reduce the results of the method of expression to the form 
 of a law. Wundt, in whose laboratory several experimenters have 
 studied the pulse and respiration during moments of pleasant feel- 
 ing, etc., summarizes the results in the following scheme: 1 
 
 Feeling 
 
 Pulse 
 
 Breathing 
 
 Tension 
 
 Strength 
 
 Speed 
 
 Strength 
 
 Speed 
 
 + 
 
 + 
 + 
 
 + 
 + 
 
 + 
 
 + 
 + 
 
 + 
 + 
 + 
 
 Calm . ... 
 
 Unpleasantness .... 
 Pleasantness 
 Excitation 
 Relief 
 
 
 1 Physiol Psychol, 6th ed., 1910, II, 310. The arrangement of the scheme is 
 here somewhat modified from that given by Wundt. + indicates an increase 
 in strength or rapidity; , a decrease; =, no change. 
 
KINDS OF FEELING 507 
 
 In commenting on this scheme of results, it should be noted, first, 
 that not all the results fit well into the scheme. This is even true 
 of some of the work done in Wundt's laboratory, so that his scheme 
 has undergone certain modifications from its earlier form, in conse- 
 quence of later results which seemed more trustworthy. In the 
 earlier scheme, the positions of excitation and relief are, approx- 
 imately, interchanged; and the same is true of calm and- tension. 
 Moreover, the results obtained by other experimenters are often 
 at variance with those of Wundt's laboratory. In analyzing the 
 results of the table with a view to determine what they probably 
 mean, the manner of conducting the experiments must first be 
 understood. The subject, being taken in as nearly as possible a 
 neutral state of feeling, is presented with certain sensory stimuli; 
 or is required to perform some brief mental operation, such as multi- 
 plying together two numbers. The effects of these interruptions of 
 the neutral state are noted in the records of pulse and respiration, 
 and are then compared with the subject's own testimony as to the 
 change of feeling induced. Interruptions inducing feelings of ten- 
 sion are, particularly, those which arouse attentive and expectant ob- 
 servation on the part of the subject, or those which require him to 
 perform some brief mental operation; while relief is experienced at 
 the successful accomplishment of any such task. That, under these 
 circumstances, the breathing becomes weaker and shallower is a fact 
 observed by practically all experimenters; it may be taken, in con- 
 nection with the correlative fact that the opposite effect occurs at 
 the close of the mental operation, as the best-established result of 
 this entire line of study. At times, indeed, respiration may be en- 
 tirely suspended during a brief attentive act. 
 
 Other movements besides the respiratory show similar changes. 
 When, for example, an audience is restless, the speaker may suc- 
 ceed, for a moment, in checking the slight movements of numerous 
 individuals which go to make up this restlessness, by showing them 
 a picture, pointing to some object, posing a question, or otherwise 
 arousing the mental activity of the audience. When the activity 
 so aroused has reached its goal, there is likely to be a general shift- 
 ing of position throughout the audience, and a return to other un- 
 easy movements. It seems clear, then, that brief and attentive 
 mental acts inhibit bodily movement, and that respiration partici- 
 pates in this inhibition. The subsequent reverse effect is doubtless 
 due, in the case of breathing, to the need of air, i. e., to a slight 
 dyspnoea produced by insufficient respiration during the brief 
 period of mental activity. It would perhaps be better to think of 
 the inhibited respiration during such periods of mental activity as 
 a symptom of the activity (or of "attention") rather than of the feel- 
 
508 FEELING, EMOTION, AND EXPRESSIVE MOVEMENTS 
 
 ing of tension which goes with the activity. It is claimed by those 
 who refuse to admit tension and relief into the category of elementary 
 feelings, that the feeling of tension is itself composed of sensations 
 of shallow breathing and of the immobile and often strained posi- 
 tion of the limbs ; and that the feeling of relief is composed of sensa- 
 tions of deeper breathing (sometimes amounting to a sigh), and of 
 the relaxation of the limbs from their fixed positions. 
 
 It should be noted that the statements made above regarding 
 the effects of mental activity (or of "attention") on pulse and respira- 
 tion hold good mainly of brief periods of activity. When mental 
 work is long continued, the principal effect observed 1 is a quick- 
 ening of the heart-beat. 
 
 As to excitement and its opposite, there is also little doubt that 
 the above table corresponds pretty closely to the facts, and that ex- 
 citement is usually accompanied by an increase in breathing, and 
 calm by a decrease. It is difficult to attain the feeling of calm 
 immediately after vigorous muscular exercise, when pulse and res- 
 piration are strong and rapid. In explaining the fact that the 
 symptoms of tension so closely resemble those of calm, and those 
 of relief so closely resemble those of excitement, it must be remem- 
 bered that tension corresponds to a repression of bodily activity, and 
 that relief brings a compensatory rebound from this repression. 
 
 8. If we now turn to the accompaniments of pleasantness and 
 unpleasantness, we shall find it hard to reach any satisfactory con- 
 clusion. For one thing, it is almost impossible, in an experiment, 
 to keep these feelings free from tension and relief; that is, from the 
 effects of observant attention and its satisfaction. Some stimulus 
 must be used to arouse the pleasant or unpleasant feeling, and this 
 stimulus arouses also the subject's attention. Thus Lehmann, 2 one 
 of those who have most devoted themselves to this line of study, re- 
 ports that colors and tones are unsuited for testing the effects of 
 pleasantness and unpleasantness, since the only effects obtained 
 by their use are those of attention; only olfactory stimuli seemed to 
 him suited for the purpose; .and even with these, the changes in 
 the pulse were not constant, but sometimes consisted in a slowing 
 and sometimes in a hastening. In general, the circulatory and re- 
 spiratory symptoms of these feelings are inconstant, and, according 
 to many observers, there is no clear difference between the accom- 
 paniments of pleasantness and of unpleasantness. 
 
 The same thing is even true of the cerebral circulation, as ob- 
 served in cases where a portion of the brain has been exposed by the 
 
 1 See E. Gley, Etudes de psychologic physiologique et pathologique, 1903, p. 29; 
 Billings and Shepard, Psychol Rev., 1910, XVII, 217. 
 
 8 Die korperliche Ausserungen psychischer Zustdnde, 1905, III, 481-482. 
 
INDUCED ELECTRICAL CHANGES 509 
 
 removal of part of the skull in a surgical operation. The brain 
 can then be observed to expand and contract according to the amount 
 of blood in it; and its volume changes according to the activity of 
 the brain. But whether it expands or contracts, specifically, in 
 conditions of pleasantness and unpleasantness, is a question to 
 which the observations so far reported give divergent answers. 1 
 
 The more intense and, as they are considered, more complex, 
 states of feeling known as emotions, are also far from uniform in 
 their respiratory symptoms. On the whole, therefore, the evidence 
 from this line of study would seem distinctly opposed to analyzing 
 the feelings into combinations of sensations with only a varying feel- 
 ing-tone of pleasantness and unpleasantness. 
 
 9. Of late years, a new symptom of the condition of feeling has 
 come into prominence. The instrument used for the observation 
 of this symptom is the galvanometer, and the bodily change noted 
 is electrical. Fe're' 2 and Tarchanoff, independently, discovered 
 that an electrical current, or a change of potential, could be ob- 
 served by connecting a delicate galvanometer to two points of the 
 skin, and then subjecting the person under observation to stimuli 
 which influenced the state of feeling. Such a stimulus as tickling 
 caused a current to appear in the galvanometer; as did also any 
 emotion or the thought of an emotion. Mental activity of a non- 
 affective sort produces comparatively slight currents. 4 But mus- 
 cular activity produces deflections of the galvanometer. Vera- 
 guth 5 found that if a story were read to the subject while his skin 
 was connected with the galvanometer, deflections occurred when 
 emotional passages were read. Jung 6 turned the experiment to 
 practical account, by showing that ideas having for an individual 
 an emotional import could be detected by reading him a list of 
 words, and observing which words caused an electrical response. 
 The detection of such ideas, or " complexes" of ideas, which have 
 a strong emotional tone for an individual, is often of value in treat- 
 ing hysteria and other neuroses. 
 
 The source of the electricity which is revealed by the galvanom- 
 eter in these experiments is not yet certainly known. We know that 
 activity of any organ is attended by the production of electric cur- 
 
 1 See H. Berger, Uber die korperliche Ausserungen psychischer Zustande, 1904 
 and 1907; E. Weber, Der Einfluss psychischer Vorgange auf den Korper, 1910; 
 and also short articles by these same authors in Zeitschr. /. Psychol., 1910, LVI, 
 299, 305. 
 
 2 C. R. Soc. de Biologie, 1888, p. 217. 
 
 3 Pfliiger's Arch. f. d. ges. Physiol, 1890, XL VI, 46. 
 
 4 Starch, Psychol. Rev., 1910, XVII, 19. 
 
 5 Arch, de Psychol., 1906, VI, 162. 
 
 6 See Peterson and Jung, Brain, 1907, XXX, 153. 
 
510 FEELING, EMOTION, AND EXPRESSIVE MOVEMENTS 
 
 rents. The sweat-glands in the skin, it may be, are excited, through 
 their nerves, by brain activity, and so give rise to the current. 
 Sidis and Nelson, 1 experimenting on animals, used electrodes which 
 penetrated through the skin, and still obtained currents; they there- 
 fore concluded that it is the muscles which produce the electric 
 currents the muscles being involuntarily excited by the activity of 
 the brain. 
 
 Though these electric phenomena are delicate indicators of emo- 
 tional states, they do not, apparently, differ with the particular 
 quality of the emotion. They do not furnish, therefore, a differ- 
 ential symptom of pleasantness and of unpleasantness. It is diffi- 
 cult, indeed, to discover such a symptom in the bodily accompani- 
 ments of feeling. To judge by the method of expression, we should 
 be brought to the unexpected conclusion that pleasant and un- 
 pleasant states of mind resemble each other more than either of 
 them resembles indifference. In other words, they are all alike 
 states of excited feeling. 
 
 10. To recur to a subject to which reference has already been 
 made : It is impossible to make any final statement, or even to offer 
 any weighty evidence, regarding the brain activities which corre- 
 spond to pleasant and unpleasant feelings. The general biologicalN 
 interpretation of the feelings connects pleasure with beneficial 
 stimuli and a good condition of the organism, and unpleasantness 
 v^with injurious stimuli and bad bodily conditions a rule to which, 
 however, there are evident exceptions. But this conception does 
 not yet indicate in what manner the feelings are generated. Is 
 feeling a sensation 2 originating in the excitation of sensory nerve- 
 ends and, if so, are there specific nerves and sense-organs for feel- 
 ing, or can many or all afferent nerves convey to the brain impulses 
 which are capable of arousing these forms of consciousness; or, 
 on the contrary, do pleasantness and unpleasantness arise from 
 conditions of the brain itself ? The latter class of explanations 
 has several representatives. Meynert 3 suggested that feeling is an 
 indication of the nutritive condition of the brain, especially as de- 
 pendent on the blood supply. 
 
 Lehmann's " dynamic theory" of feeling 4 regards pleasantness as 
 
 1 Psychol Rev., 1910, XVII, 98. 
 
 2 Stumpf, ZeUschr. /. Psychol, 1906, XLIV, 1; Titchener, Textbook of Psychology, 
 1909, p. 261. 
 
 3 Psychiatric, p. 171, cited after Ebbinghaus, Grundzuge der Psychologic, 1905, 
 p. 577. 
 
 'Die korperliche Ausserungen psychischer Zustdnde, 1901, II, 291; 1905, III, 
 403; other interesting conceptions of the physiology of feeling have been enter- 
 tained by Marshall in his Pain, Pleasure and ^Esthetics, 1894, and by M. Meyer, 
 Psychol. Rev., 1908, XV, 201. 
 
OTHER THEORIES OF FEELING 511 
 
 the index of a proper balance between the supply and the expendi- 
 ture of energy by the nerve-cells of the brain; while unpleasantness 
 results from an excess of expenditure over supply; and therefore 
 every very intense sensory stimulus is unpleasant, because it over- 
 works some of the brain-cells, causing their expenditure to exceed 
 the supply brought to them by the blood stream. 
 
 The Herbartian doctrine (see just below) of pleasantness as 
 the measure of harmonious co-operation of ideas, and of unpleas- 
 antness as the measure of conflict between ideas, might be trans- 
 lated into the neural terms of facilitation and inhibition, a facili- 
 tated reaction being pleasurable and an inhibited or obstructed 
 response being unpleasurable. Somewhat similar is the theory of 
 Ziehen, 1 that the ready discharge of an excited portion of the cor- 
 tex along paths of projection or of association is pleasurable; where- 
 as the obstruction of such discharge constitutes unpleasantness. 
 Where pertinent facts are so meagre, an attempt at selecting the 
 best of these hypotheses is scarcely worth while. 
 
 11. On account of the nature of our work, only a brief refer- 
 ence is necessary to one of the most elaborate, and in many respects 
 successful, of all the theories which derive the phenomena of the 
 feelings and emotions, in general, from other and different men- 
 tal factors. We refer to that best known as connected with the 
 name of the great German psychologist, Herbart. This theory 
 makes feeling dependent upon the relations of the ideas as further- 
 ing or checking each other. It cannot be admitted, to begin with, 
 that feeling is a secondary or derived form of consciousness. No 
 form of mental activity is more primitive and unanalyzable than 
 feeling; none is earlier in the development of mental life. 2 Be- 
 fore the infant has localized the different sensations, and combined 
 them into percepts of the different parts of its own organism, the 
 consciousness of being affected in a given way, either pleasurable 
 or not, must predominate. Other forms of feeling of desire, un- 
 easiness, comfort, etc. are inseparably connected with its first 
 states of consciousness; they belong to its inherited impulses and 
 instincts, and are only later definitely related to the appropriate 
 ideas. The primary formation of self-consciousness is quite as 
 truly connected with self-feeling , pleasurable or painful, as with the 
 process of ideation in constructing the concept of "me" and "not- 
 me." Volkmann von Volkmar, in his great work, 3 considers feeling 
 
 1 Physiologische Psychologic der Gefuhle und Affekte, 1902. 
 
 2 The same view of the feelings is maintained by Horwicz, and developed at 
 length, polemically, by Lotze (Horwicz, Psychologische Analysen, i, pp. 168 f.; 
 Lotze, Medicin. Psyehologie, pp. 235 f.). 
 
 3 Lehrbuch d. Psychologic, 1884, II, pp. 298 ff. 
 
512 FEELING, EMOTION, AND EXPRESSIVE MOVEMENTS 
 
 as the consciousness of the process of ideation itself as distinguished 
 from consciousness of this or that idea, and it is conditioned upon 
 some resistance being offered to the process. Feeling is, then, no 
 one proper idea, to be placed in conjunction or classed with others. 
 It is rather a becoming conscious of the degree of tension, as it were, 
 which characterizes the process of ideation at each particular mo- 
 ment. The condition of the origin of a feeling is, then, the existence 
 of two simultaneous opposed ideas. Their coexistence occasions 
 a state of tension (" Spannung"), as it were, and this state gives 
 way as one idea triumphs over the other. The type of simple feel- 
 ing may be illustrated by the condition in which the mind finds 
 itself when listening to harmonious or discordant musical sounds. 
 
 As has already been said (p. 511), this theory closely approaches 
 one of the more prominent of the purely physiological theories. 
 
 But, although we are to distinguish sensation from feeling, we 
 must regard the feeling which inseparably accompanies sensation 
 as feeling, strictly speaking, and not as tone of sensation; or, in other 
 words, the tone of every sensation, as either pleasurable or pain- 
 ful, is given to it by the feeling which accompanies and blends with 
 it. The sensation, as having a certain quality, quantity, and lo- 
 cality, is capable of being built into a " Thing" which the mind 
 perceives as not itself. But the feeling, the pleasurable or pain- 
 ful tone of the sensation, is always recognized as purely and simply 
 a way in which the mind is affected. To refuse to speak of sensa- 
 tions and emotions, with all their complicated physical basis, as 
 belonging at all in the realm of " feeling," is to restrict the use of the 
 word unwarrantably. The Herbartian theory commits in this mat- 
 ter the mistake which it is guilty of committing repeatedly; it re- 
 gards the "ideas" as realities that have in some sort a substantial 
 existence, and can do something by way of furthering or hindering 
 each other. But ideas are themselves nothing more than mental 
 products that exist only when and so long as the mind acts with a 
 definite degree and kind of energy. In determining the kind and 
 degree of this ideating energy, the previous action and habit of the 
 mind by way of feeling is quite as influential upon the mode of 
 feeling as the manner of its ideating energy. Finally, this theory 
 wrecks itself upon the denial of all that which the physiological 
 theory maintains and establishes. The two theories, then, sup- 
 plement and correct each other; but even when combined they only 
 tell us in part what are the physical and mental conditions under 
 which feeling arises. 
 
 12. As a logical consequence of the very nature of all kinds 
 and degrees of feeling, a strict classification of our affective and emo- 
 tional states is impossible. The difficulty is even greater than that 
 
CLASSIFICATION OF FEELINGS 513 
 
 which was found to be encountered by every attempt to classify 
 the sensations of smell, as such. In both cases that of certain 
 sensations and that of all feelings the popular language is signifi- 
 cant of this fact of universal experience. Sensations of color are 
 either red, green, blue, etc.; but agreeable sensations of smell are 
 either of a rose, or of a violet; and disagreeable odors are either of 
 asafcetida, or of mercaptan, etc. In somewhat the same way, 
 judgments are fitly classified as either positive or negative, universal 
 or particular or distributive, etc.; but all these classes of judgments 
 may, when considered in their affective or emotional aspect, be 
 distinguished as characterized by feelings of doubt, or of certainty, 
 and accompanied by feelings of sadness, or of joy, or of mixed 
 pleasure and pain. 
 
 It is obvious, then, that any attempt at classifying the feelings 
 is likely to be most practically successful, even if it must surrender 
 the attempt at scientific accuracy, when it takes some one of the 
 points of view from which the subject may be best seen in indirect 
 vision, as it were. Among the classification schemes derived in 
 this way, that is perhaps most convenient which emphasizes those 
 other forms of mental activity on which the various kinds of feel- 
 ing are chiefly dependent. In this way, we may recognize three, 
 or four, great classes of human experiences, when looked upon as 
 characterized by their affective, or emotional, aspect. These would 
 be (1) the sensuous feelings, or those which are dependently re- 
 lated to, and in consciousness blended with, the different qualities 
 and intensities of the sensations of the special senses or the more gen- 
 eral organic functions; (2) the intellectual feelings, or those which 
 precede, accompany, or follow the various activities of discrimina- 
 tion, association, judgment, reasoning, and, indeed, all the forms 
 of functioning of the "mind" in the narrower meaning of the latter 
 word; (3) cesthetic feelings, or those which belong to the perception 
 and appreciation of what we call "the beautiful" (in its various 
 forms), or its opposite; and (4) the moral feelings, or those affective 
 experiences which appertain to the good and the bad, in human con- 
 duct. To these might be added a fifth class, to be called the relig- 
 ious feelings, were it not for the fact that the latter may be satisfac- 
 torily treated as special forms of the combination of the intellectual, 
 aesthetic, and moral feelings. 1 
 
 In adopting this, or any similar classification, it must be remem- 
 bered, as a matter of course, that senses, intellect, and will enter into 
 all the activities connected with every form of complex and highly 
 developed emotion; and such are all the emotional states of our 
 
 1 A similar classification is that proposed by Horwicz: Psychologische Analy- 
 sen, ii, pp. 82 f. 
 
514 FEELING, EMOTION, AND EXPRESSIVE MOVEMENTS 
 
 adult experience. For this reason also, no hard and fixed line can 
 be drawn about the different so-called classes of feelings. The 
 aesthetic feelings cannot be separated from the sensuous; for ex- 
 ample, the feeling which accompanies the sensation of a musical 
 chord, or of the color purple, may be classed under either head. 
 Nor can the intellectual feelings be separated from the aesthetic; the 
 perception of harmony of colors and sound is inseparably connected 
 with aesthetic and sensuous feeling, and the latter is intensified or 
 otherwise modified under the intellectual laws of contrast, change, 
 habit, and higher association. Even the feelings which we call 
 "moral" on account of their connection with will and desire, often 
 have an indefinite part of them so combined with feelings located in the 
 bodily organism, or so dependent on its functions for their quantity 
 and quality, that a strict separation becomes impossible. Love is sel- 
 dom or never so purely ideal as not plainly to involve in itself feel- 
 ing of sensuous and aesthetic sort; hate not mixed with anger, and 
 so supported on some elements of that physical basis which under- 
 lies the latter, is hard to discover in real life. 
 
 13. All feelings are characterized by tone, strength, rhythm, 
 and content. Their content is determined by the ideating activity 
 with which they are directly connected, or to which they are relat- 
 ed; and this content may be comparatively simple, as is the case 
 with the feeling connected with the presentation of a colored surface 
 (for example, purple or green), or obviously complex, as is the case 
 with the sentiments of patriotism, loyalty, and religious devotion. 
 
 Feelings, like all other mental phenomena, occur under time- 
 form; they are, in general, rhythmic in character, and change in re- 
 spect to content, tone, and intensity, with a movement marked more 
 or less distinctly by the quality of periodicity. Their rhythm, 
 with respect to content, is, of course, determined by the recur- 
 rence of changes in the ideating activity as dependent especially 
 upon attention and the laws of association. Feelings of sadness or 
 joy, comfort or discomfort, may come around again in conscious- 
 ness, as it were, according to the rhythmic movement of the sensa- 
 tions which occasion them. Sometimes an alternation of tone takes 
 place, which carries the mind back and forth by the point of indif- 
 ference (or hypothetical zero-point of feeling) between agreeable 
 and disagreeable sensations, or ideas of the same kind. Thus we 
 are sometimes forced to say that we do not know whether a certain 
 combination of colors, or quality of taste or smell, is pleasing to us 
 or not; in such a case feeling seems to move rhythmically back and 
 forth between a slightly pronounced tone of pleasure and a slightly 
 pronounced tone of pain. 
 
 The intensity, too, of feelings rises and falls alternately in de- 
 
CHARACTERISTICS OF ALL FEELING 515 
 
 pendence upon the rhythmic movement of the nervous processes 
 and of the train of ideas. No feeling is kept at a long continuous 
 level with respect to its vigor and pitch of strength. The law of 
 quickly alternating exhaustion and repair of the nervous elements 
 underlies, to a large extent, this rhythmic movement of the inten- 
 sity of the feelings. This is one of many proofs which go to show 
 that, the conditions of the end-organs and of the central organs are 
 determinative of the tone and strength of feeling. Even when we 
 are strictly attending to our painful feeling, the toothache is not a 
 perfectly uniform and steady strain; even when we are doing our 
 best to abstract attention from the pain, we succeed only intermit- 
 tently. But the course of the ideas must also be taken into account 
 as influencing the rhythm of feeling. As our sensations or mental 
 images become more clear and vivid, the feelings attached to them 
 gather strength; as the former become more obscure and feeble, 
 the feelings also die away in consciousness. 
 
 14. It has been said (p. 501) that some psychologists would 
 divide all our affective states into two classes of feelings viz., 
 pleasantness and unpleasantness. But we have seen that this classi- 
 fication is entirely unsatisfactory; primarily because it fails to take 
 account of the almost infinite variety of, not only our complex feel- 
 ings, emotions, and sentiments, but also of those affective experi- 
 ences whose analysis has hitherto resisted all our most ingenious 
 methods, both introspective and experimental. We have, therefore, 
 preferred to regard pleasantness and unpleasantness as the "tone" 
 of feeling. This is to recognize the fact that, not only can we speak 
 appropriately of the feelings of pleasantness and of unpleasantness, 
 but that, with even greater propriety, we speak of nearly, or quite, 
 all our feelings, of every class, as being themselves either pleasant 
 or unpleasant. And, indeed, the feeling of pleasure and pain is 
 probably the most general, most simple, and earliest psychical 
 process. That almost all feelings are characterized by some posi- 
 tive tone or, in other words, are not absolutely indifferent to us 
 there can be no question. Is it agreeable or disagreeable, at least 
 in some slight degree and in some more or less indefinite manner ? 
 is an inquiry which we can pretty readily answer with respect to 
 nearly all our sensations and ideas. The question has for a long 
 time been debated, however, whether this is necessarily true of all 
 our feelings. Is there any such thing as completely "neutral" feel- 
 ing, or feeling that is in no respect or degree either agreeable or 
 disagreeable to us ? Neutral or indifferent feelings were recognized 
 by Reid, but disputed by Hamilton. 1 Bain 2 asserted it as un- 
 
 1 Hamilton's Works of Thomas Reid, p. 311 (Edinburgh, 1854). 
 3 The Emotions and the Will, 3d ed., p. 13. 
 
516 FEELING, EMOTION, AND EXPRESSIVE MOVEMENTS 
 
 doubted that "we may feel, and yet be neither pleased nor pained," 
 and that " almost every pleasurable and painful sensation and emo- 
 tion passes through a stage or moment of indifference." Wundt 1 
 argued, on theoretical grounds, that pleasure and pain, as tones of 
 feeling having a variable intensity, are conditions which may be 
 regarded as on different sides of a zero-point, or point of indif- 
 ference lying between them. It does not follow, however, that, 
 because the mind passes in time from feeling of one positive tone 
 (pleasure) to feeling of the opposite tone (pain), it must, therefore, 
 at some instant be in a state of feeling that has no tone and lies 
 between the two. The curve plotted to represent the rise and fall 
 of feeling is a material line; it cannot be at one time below, and at 
 another above, the abscissa-line, without at some single point (the 
 zero-point) coinciding with it. But it does not follow that, because 
 such a curve is a picture of the phenomena of feeling in one respect, 
 it is so in all other respects. The question whether there is any 
 zero-point to the tone of feeling can only be answered by an appeal 
 to consciousness; and this answer, like all others given to similar 
 appeals, is likely to contain dubious and conflicting elements. It 
 is quite certain that one can pass from a high state of pleasure to 
 one of intense pain without any consciously interpolated neutral 
 feeling. For example, if while one is viewing a beautiful landscape 
 one is stung by hornets, the condition of quiet massive pleasure may 
 be converted into one of great physical suffering without any inter- 
 vening feeling of indifference. 
 
 15. As to the nervous apparatus and physiological processes 
 concerned in the imparting of pleasant or unpleasant tone to the 
 various kinds of feelings, two principal views have been current 
 hitherto. As bearing upon the pleasure-pain tone of the sensations, 
 one view holds that the same apparatus of end-organs, conducting 
 nerve-tracts, and central areas, which on moderate excitement pro- 
 duces the simple sensations of pressure or of temperature, or the 
 more complex sensations of tickling, shuddering, etc., produces the 
 feeling of pain when irritated with increased intensity. Such a 
 view would apparently have also to hold that muscular sensations 
 have the same physical apparatus as do feelings of muscular weari- 
 ness or exhaustion; and, perhaps, that cardialgia and hunger are 
 due to modifications of the action of the same nerves of the stomach. 
 But from the introspective point of view it is as certain that sensations 
 of pressure or mere temperature are unlike the feeling of pleasure 
 produced by gentle rubbing or by comfortable warmth, or the pain 
 that comes from heavy pressure or burning, as it is that sensations 
 of light are unlike those of musical tone. 
 
 1 Physiolog. Psychologic, 2d ed., I, pp. 465 f. 
 
PLEASURE-PAIN TONE OF ALL FEELING 517 
 
 It is now known, however, that there are, for the skin at least, 
 end-organs at the "pain points" (see p. 345.) It becomes a 
 reasonable conjecture that the pain which follows the excessive 
 stimulation of the other end-organs of sense, such as the eye or 
 ear, is due to the excitement of specific "pain nerves" connected 
 with those organs. There are other physiological reasons for doubt- 
 ing the complete identity of the nervous apparatus of pleasurable 
 and painful feeling with that of the sensations with which the feel- 
 ing is allied. The facts upon which Schiff and others supported 
 the view that nervous impulses resulting in pain travel by more or 
 less distinct paths along the spinal cord have already been stated. 
 More recent experiments seem to show that the end-organs of tem- 
 perature, pressure, and pain are locally separable in the different 
 minute areas of the skin. Pathological results indicating the same 
 separation of the nervous elements of feeling also deserve a brief 
 mention. In certain cases the sensibility of the skin to pain is lost, 
 while its sensibility to touch is not weakened or is even increased. 
 The reverse condition also sometimes occurs. "Analgia," as occa- 
 sioned by pathological states of the spinal cord due to lead-poison- 
 ing, was noticed in many cases by Beau. This loss of sensibility to 
 pain can hardly be explained by any change in the activity of cer- 
 tain end-organs common both to touch and to painful feeling. What 
 impairment of function could possibly result in destroying the sen- 
 sitiveness to strong mechanical and thermic excitations, such as 
 ordinarily occasion great pain, while the response by way of sensa- 
 tions of touch to much feebler excitations remains undiminished ? 
 
 The same argument would appear decisive against identifying, 
 locally, the central nervous processes which result in sensation with 
 those which result in feeling. In certain stages of narcosis, pro- 
 duced by ether or chloroform, the patient is able to perceive the 
 slightest contact with the skin, but feels no pain even when the same 
 area is treated severely. Moreover, in some cases of tabes dorsalis, 
 a constant difference seems to exist in the time at which the sensa- 
 tions of pressure and the feelings of pain, simultaneously excited at 
 the end-organ, arise in the mind. If the patient is pricked with a 
 needle, he will instantly feel the contact, and the pain only one to 
 two seconds later. 1 The case of the eye, which responds with sensa- 
 tions of light and color when the optic nerve is moderately excited, 
 and with the painful feeling of being blinded when the stimulus is 
 increased, is not perfectly clear; for cases of amaurosis are on 
 record where the painful feeling persisted after the eye had lost all 
 
 1 See Funke, in Hermann's Handb. d. Physiol, III, ii, pp. 297 f.; such phenomena 
 have been especially discussed by Osthoff, Die Verlangsamung d. Schmerzemp- 
 findung bei Tabes dorsalis, 1874. 
 
518 FEELING, EMOTION, AND EXPRESSIVE MOVEMENTS 
 
 power to distinguish light. It may well be, therefore, that while 
 the specific sensations of light and color are due to the irritation of 
 the optic nerve, the excitement of feeling indicates a simultaneous 
 irritation of part of the trigeminus. 
 
 We are compelled, then, to confess that the localizing of the 
 nervous apparatus, and the nature of the physiological processes 
 which give the tone of painful and pleasurable feeling to our sensa- 
 tions, require further investigation. The tendency of the evidence, 
 however, is toward a theory which assigns to feeling a more or less 
 separate mechanism of end-organs, conducting nerve-tracts, and 
 central areas (or at least of nervous elements in the central areas). 
 
 16. Certain mixtures of vaguely localized sensations, with feel- 
 ings of a more or less pleasant or unpleasant tone, have acquired 
 the name of sensus communis, or "common feeling." Such feel- 
 ing may have more or less of content of one kind or another, ac- 
 cording to the state of perception and ideation with which it is 
 combined. Nervous impulses of indefinite variety and the most 
 manifold peripheral origin are constantly pouring in, as it were, 
 upon the cerebral centres each one contributing some element to 
 the characteristic tone of consciousness. The resulting feelings are 
 modes of our being affected which are not converted into definite 
 presentations of sense, or referred to a particular part of our own 
 bodies. The effect of changes in the minute blood-vessels and 
 other capillaries about the nerve-endings, the presence of impuri- 
 ties in the blood, the condition of the lower cerebral centres, the 
 action of the heart and lungs and other internal organs, and the 
 connection of the sympathetic with the cerebro-spinal nervous sys- 
 tem, are all felt in this way. Moreover, inasmuch as few (if any) 
 sensations are without some tone of feeling, while many sensations 
 are exceedingly heterogeneous in their elements, and not clearly 
 referred to the place of their origin, a melange, as it were, of ob- 
 scure bodily affections is readily formed. 
 
 Sensations in themselves heterogeneous may also be brought 
 into a temporary relation by the partial identity of their source of 
 excitation, and of the nervous connections in the central organs. 
 It is also always a very important question, how the more obscure 
 and mixed bodily feelings stand related to the mind's course of 
 ideation, to attention, association, etc. This relation often de- 
 termines whether such obscure impressions shall be definitely ob- 
 jectified or not; whether they shall not rather run together in the 
 dark stream of common feeling. Let any one suspend for an in- 
 stant a train of interesting thought, which has up to the moment 
 been interrupted only by certain obscure feelings of uneasiness, and 
 such a one will be able instantly to select and localize in the cramped 
 
FORMS OF COMMON FEELING 519 
 
 chest, or oppressed limbs, or tired organs of special sense, most of 
 the sensations whose painful tone has thus colored the stream of 
 common feeling. Separation from localized sensations is, then, the 
 chief negative characteristic of common feeling. Under its dif- 
 ferent principal forms we may distinguish different total results, ac- 
 cording to the general relation in which the being aware merely 
 that we are affected in an agreeable or disagreeable manner stands 
 to the being aware of what affects us in this manner. Thus we 
 sometimes feel well or ill, elevated or depressed, without ability to 
 assign these feelings at all definitely to the physical organism, either 
 as perceived or imaged, or to any reason in the train of ideas. At 
 other times the general impression of being in the body, for some 
 greater or less amount of either weal or woe, is emphatic; we feel 
 ill all over, or seem to enjoy the caursing of the blood through every 
 artery and vein, as though mentally present in the extended tissues. 
 
 This melange of sensations and feelings, the so-called sensus 
 communis, is connected in an important way with the higher forms 
 of self-consciousness, and with our entire sense of personality. 
 Temporary and relatively unimportant disturbances of its more 
 essential characteristics may result in our " feeling queer," or " feel- 
 ing not exactly like ourselves." More important and permanent 
 disturbances take the form of those illusions which are character- 
 istic of certain forms of insanity, such as that some part of the body 
 is made of glass, or that wheels are whirling inside the head, etc. 
 Upon the introspective basis of such experiences of "common 
 feeling" a whole system of perverted judgments may be based, 
 and essential changes in the conduct of life brought about. This 
 is what would be expected as a consequence of the fundamental 
 psychological truth that our feelings are the factors, and the aspects, 
 of our total experience which we attribute most directly and essen- 
 tially to our very "own self," as distinguished from other selves. 
 Functional disturbances of the nervous mechanism, both peripheral 
 and central, chiefly concerned with the feeling aspect of our experi- 
 ence, alter in the most important way our conception of the Self. 
 
 17. The question whether every sensation has some feeling 
 must be distinguished from the question whether every feeling is 
 of either painful or pleasurable tone. The tone of the feeling of 
 sensations is the agreeable or disagreeable affection of conscious- 
 ness which they often carry, as inseparably connected with them. 
 The particular tone belonging to any sensation is, to a large ex- 
 tent, dependent on its intensity. Sensations of moderate inten- 
 sity that is, of intensity below the point at which the minimum of 
 painful feeling begins are usually pleasurable. The feeling of 
 pain rises in intensity, from the point where it begins, as the in- 
 
520 FEELING, EMOTION, AND EXPRESSIVE MOVEMENTS 
 
 tensity of the stimulus increases ; but the curves which represent the 
 increase of feeling and the increase of sensation by no means com- 
 pletely correspond. The amount of pleasurable feeling is also de- 
 pendent on the element of time. It has been thought to reach a 
 maximum at about the point where the strength of sensation is 
 the most favorable for accurate discernment of the objective 
 stimulus. 
 
 As to the dependence of the tone of feeling belonging to a sen- 
 sation upon the quality of the latter, it has been held that no sen- 
 sation is absolutely pleasant or unpleasant irrespective of its in- 
 tensity. Even then, however, it would have to be admitted that 
 qualitatively different sensations differ greatly in the amount which 
 is consistent with an agreeable tone of feeling. It is, of course, 
 with regard to the organic sensations, and the special sensations of 
 touch, smell, and taste, that the relation between tone of feeling 
 and the quality of sensation is most apparent. Doubtless large al- 
 lowance must be made in all cases for individual peculiarities of 
 organism, association, etc. The disagreeable tone of feeling which 
 almost universally attaches itself to certain qualities of sensation, 
 however moderate or unobtrusive their intensity, may be largely ex- 
 plicable on the principle of heredity. But, taking matters as they 
 stand in present experience, it is impossible to maintain that the 
 tone of feeling is not, in certain cases, directly dependent on the 
 quality of sensation. This is a question upon which only conscious- 
 ness can pronounce. All degrees of some tastes and smells are disa- 
 greeable to most persons. Bitter is a distinctive species of the qual- 
 ity of gustatory sensations; but the pleasure which some persons 
 have in greater or less degrees of it is, as a rule, acquired. It is 
 true that some substances, whose odor in large quantity is disagreea- 
 ble, become tolerable, or even pleasant, when the smell from them 
 is faint. But this faint smell is not the same, but a distinctly dif- 
 ferent quality; oftentimes it could not be immediately recognized as 
 coming from the same substance as that which emitted the strong 
 odor. Discordant sounds are, in all degrees of intensity, naturally 
 unpleasant; and so most witnesses would pronounce certain com- 
 plex sensations of the skin (as of creeping, prickling, etc.). 
 
 18. The character of the disagreeable or painful feeling belong- 
 ing to different classes of sensations also differs with respect to the 
 nature of its attachment to a recognized physical basis. Inhar- 
 monious colors produce in us a feeling of mild dissatisfaction, which 
 appears as almost wholly of a spiritual kind. Discordant tones 
 cause more of physical suffering; and disagreeable smells or tastes 
 create a wide-spread sense of organic discomfort. Pains in the 
 skin and interior organs, however, may take a character of intense 
 
MIXTURES OF FEELING 521 
 
 bodily anguish, which is distinctive of no other qualities of sensation, 
 and which is capable of submerging all sensation, as such, in a flood 
 of painful feeling. 
 
 The tone of sensuous feeling is also dependent upon the total 
 condition of consciousness as determined by attention, mental habit, 
 association of the feelings among themselves and with the ideas, 
 control of the will, etc. Such feeling is, therefore, largely a second- 
 ary element of experience, which arises through certain acquired 
 effects of the sensations as connected with previous activities of 
 the mind. But concerning the physical basis of the feelings, in 
 this aspect of them, we have scanty scientific knowledge; and the 
 subject is not as yet one with which physiological psychology can 
 very successfully deal. 
 
 19. Characteristic mixtures of feeling some of them scarcely 
 describable seem to be attached inseparably to different kinds of 
 sensations. This is obvious when we consider the marked differ- 
 ence in the way we are affected by major and minor chords, by suc- 
 cessive tones having different musical intervals (for example, the 
 diminished third, etc.), and by the characteristic clangs of different 
 musical instruments. Writers upon this part of musical theory 
 may disagree as to the precise significance of the violin, clarinet, 
 cornet, or hautboy, with respect to the tone of feeling belonging to 
 each; but they can scarcely deny the fact of a marked difference. 
 Goethe 1 called attention to the change in spiritual tone, as it were, 
 which harmonizes with what the eye sees when looking upon the 
 world through different-colored glasses. Here, again, the precise 
 equivalent, or value, in terms of feeling, which the different color- 
 tones possess, may be a matter of dispute; but the fact that the 
 tones of feeling change with the color-tones is beyond dispute. 
 That feelings of soberness or gloom go with black, of excitement 
 with red, of cheerfulness with light green, of cool quiet with dark 
 blue, of intense sensuous pleasure with saturated purple, would 
 probably be admitted by most persons. Fewer would agree to de- 
 scribing the tone of feeling belonging to dark yellow or spectral 
 orange as one of "suppressed excitement," or to brown as one of 
 "perfectly neutral mood." 
 
 20. Nearly, if not quite, all the various forms of feeling, when 
 they become stronger and the disturbances of their organic basis, 
 so to say, more powerfully show themselves in consciousness, par- 
 take of certain common characteristics which lead to their being 
 classified as "emotions." But, again, they are subdivided in an in- 
 definite way, according to the relation they sustain to their objects, 
 or to the actions which they tend to induce or to influence. In 
 
 1 Farbenlehre, 763. 
 
522 FEELING, EMOTION, AND EXPRESSIVE MOVEMENTS 
 
 this way, such different words as "affection," "passion," "impulse," 
 and "desire" (the former two emphasizing rather the passive, and 
 the latter two, the active aspect of the affective condition) may be 
 applied to essentially the same class of feelings. A sufficient in- 
 crease in their intensity, and a resulting increase of the "somatic 
 resonance," may even convert the higher, and usually milder, in- 
 tellectual, aesthetic, and moral feelings into an emotional experi- 
 ence. In truth, essentially the same mental state, so far as dis- 
 tinctions of affective quality are concerned, may be called simply 
 a feeling, or an emotion, or a passion, or a sentiment. For example, 
 love, whether sexual or so-called Platonic, or that distinctive of any 
 of the various social relations, may on different occasions merit the 
 distinctions involved in any of these terms. Owing to this general 
 fact, the physiology of the various more complex forms of emotion 
 may be studied from the same points of view. 
 
 21. All emotional forms of feeling are accompanied by abrupt 
 and marked changes in the character and time-course of the mental 
 train. Such changes may be regarded as standing in the relation 
 both of cause and of effect to these feelings. Some impression 
 with which strong feeling has become associated is made upon the 
 mind; the result is a transitory interruption of the mental equi- 
 poise. This constitutes in part the justification for the saying that 
 from mere feeling to affection is a "leap." 1 As a rule, the effect of 
 any sudden and surprising impression perception of some object 
 of sense, or remembered image is to start the flow of emotion. 
 Thus anger, fear, desire, avarice, take men "off their guard"; the 
 feelings of such kind that are started by a given mental impression 
 themselves produce a confusion of the mental train. But, on the 
 other hand, this very disturbance of the mental train is itself pro- 
 ductive of a new phase of feeling, such as is associated with the 
 particular ideas that in confused and hurried throngs rush into 
 consciousness, as well as with the general state of consciousness 
 considered as one of haste and confusion. The physical basis of 
 this state is laid in the extraordinary condition of excitation that 
 exists within the central organs the ideo- and sensory-motor cen- 
 tres of the cerebral hemispheres. 
 
 A not unreasonable conjecture as to the central conditions on 
 which the excitement of the more complex forms of emotion are de- 
 pendent, would state the case in somewhat the following way: 
 It is a well-known fact that different individuals differ more widely 
 and incalculably as to the particular feelings evoked, on different 
 particular occasions, than as to the sensations and ideas oc- 
 casioned by changes in the amounts, kinds, and time-rates of the 
 1 Compare Nahlowsky, Das Gefuhlsleben, etc., Einleitung. 
 
NATURE OF THE EMOTIONS 523 
 
 stimuli which act upon the peripheral nervous system. This fact 
 suggests that our feelings are determined by the changeable relations 
 of the neural processes to the constitution, previous habits and 
 temporary mood of the nervous system, and by the relations of 
 each neural process to all the others within the central system, in 
 a more irregular way than are our sensations and our knowledge. 
 Those conditions of the nervous processes which depend immediately 
 upon the quality, intensity, and time-rate of the stimuli that act 
 upon the end-organs of sense, are in general conformable to law; 
 they are regular and as it were to be depended upon. In cor- 
 respondence with them are the regularity and the dependableness 
 of our sensations and of our knowledge by the senses. But over 
 and above the more uniformly recurrent similar elements in all the 
 peripherally originated nervous processes, there is more or less of 
 a " semi-chaotic surplus" of nervous action occasioned in the brain 
 centres. In this semi-chaotic surplus the general character of 
 which depends upon what the whole nervous system was, and is, 
 and has recently been doing, and upon how the various new stimula- 
 tions, running in to the brain centres, fit in with all this and with 
 one another may we find the physiological conditions of the emo- 
 tions. No wonder, then, that these conditions are so indeterminate 
 for different individuals, and so changeable in the same individual. 
 At any particular moment the kind and amount of feeling experi- 
 enced has for its physiological condition the total complex relation 
 in which all the subordinate neural processes, set up by the stimuli 
 of that moment, stand to one another and to the set, or direction, 
 of pre-existing related neural processes. 1 
 
 22. Besides the physiological changes of central origin, which 
 accompany or follow certain perceptions and trains of ideas, the 
 wonderful, characteristic effect which these forms of feeling pro- 
 duce upon certain of the vital organs is the most noteworthy pe- 
 culiarity of all affections, emotions, and passions. Upon this point 
 science has far less than we could wish of information reaching 
 beyond the observations of ordinary experience. Of such informa- 
 tion, perhaps the most important concerns the influence exerted 
 through many groups of muscles, from the central organs, upon the 
 vaso-motor system. The effect of shame, fear, or anger, for ex- 
 ample, upon the circulation of the blood is matter of common re- 
 mark. But some grow pale and others red, when angry. As long 
 ago as 1854, R. Wagner investigated the effect of fear upon the heart 
 of a rabbit. A blow on the table near the animal was found to 
 cause its heart to stand still a short time, and then resume beating 
 
 1 See Ladd, Psychology, Descriptive and Explanatory, 1894, p. 173; and on the 
 whole subject of feeling, ibid., chapters IX, X, XXIII, XXIV, and XXV. 
 
524 FEELING, EMOTION, AND EXPRESSIVE MOVEMENTS 
 
 with accelerated frequency of stroke. Subsequent investigations 
 have made obvious the general effect of emotion upon the curve in- 
 dicating the blood pressure. The effect produced upon the pulse 
 of a dog by hearing the sudden cry of another dog depends for its 
 character upon whether the vagus nerves are cut or not; but even 
 after their severance a marked effect of this kind is still manifest. 1 
 The great influence of these forms of feeling upon all the action of 
 the capillary vessels, upon the secretions, etc., and upon the respira- 
 tion to retard, or accelerate, or make it irregular, is of the same 
 order. That care and anxiety disturb nutrition, that pain and sor- 
 row cause the tears to flow, that fear and love and anger act upon 
 the abdominal organs, is generally recognized. The effect is some- 
 times seen in suddenly innervating, and sometimes in depressing, 
 one or more of the bodily organs; or in both innervating and then 
 depressing them, in certain well-recognized cases. On the basis of 
 such facts, Kant suggested a division of the affections into "sthenic" 
 and "asthenic." But many forms of feeling, as they run their 
 course, become by turns sthenic and asthenic. Strong emotions or 
 passions of all kinds tend to destroy the nervous mechanism; "the 
 sthenic kill by apoplexy, the asthenic by laming the heart." Un- 
 usual tension or relaxation of certain groups of muscles character- 
 izes all these forms of feeling. 
 
 23. The marked effect which certain feelings have upon par- 
 ticular organs of the body is complemented by the fact that such or- 
 ganic effect has in turn a marked effect upon the feelings. The or- 
 ganic disturbances advance step by step to form the physical basis 
 of a rising tide of emotion, and then fall off with equal pace as the 
 tide of emotion subsides. The organic changes are not merely an 
 expression of the mental; they are its material cause and support. 
 Hence, from the physiological point of view every strong emotion 
 must be regarded as involving certain factors of a peripheral origin; 
 and from the introspective point of view, as including factors which 
 should be analyzed into vaguely (or not at all) localized sensations 
 having a somewhat marked tone of pleasant or unpleasant feeling. 
 This general truth is well expressed by Sherrington 2 as follows : 
 "Of points where physiology and psychology touch, the place of 
 one lies at 'emotion/ Built upon sense-feeling much as cognition 
 is built upon sense-perception, emotion may be regarded almost as 
 a feeling a feeling excited, not by a simple little-elaborated sensa- 
 sation, but by a group or train of ideas. To such compound ideas 
 
 lf This subject has been investigated by Conty and Charpentier, by Cyon, 
 Heidenhain, and others; compare Exner, in Hermann's Handb. d. Physiol., II, 
 ii, pp. 289 f. 
 
 2 The Integrative Action of the Nervous System, 1906, p. 256. 
 
PERIPHERAL THEORY OF EMOTIONS 525 
 
 it holds relation much as does feeling to certain species of simple 
 sense-perceptions. It has a special physiological interest in that 
 certain visceral reactions are peculiarly colligate with it. Heart, 
 blood-vessels, respiratory muscles, and secretory glands take special 
 and characteristic part in the various emotions. These viscera, 
 though otherwise remote from the general play of psychical process, 
 are affected vividly by the emotional." Some of these movements and 
 secretions are visible from the outside; others, however, are hidden 
 from view. Under the latter class may be mentioned the inhibition 
 of the wave-like movements of the stomach in anger, as observed by 
 Cannon, 1 by means of the Rontgen rays, in a cat. It is this "so- 
 matic reaction," or "bodily resonance," which largely gives to all 
 the various emotions their peculiar tinge, or characteristic coloring, 
 as they arise in consciousness. 2 
 
 The claim, however, that the emotions are simply the expressions, 
 or outcome, in consciousness of peripherally initiated sensations, 
 or organic reactions of a visceral or vaso-motor character, accom- 
 panied by the reduction of the cerebral and psychical processes to 
 a secondary r61e, as this claim has been put forth, with minor dif- 
 ferences, by Lange, James, and Sergi, contradicts all the evidence 
 which we can at present bring to bear upon the case. It is op- 
 posed to the order of dependent sequence, so far as it can be deter- 
 mined by introspective analysis or followed by experiment; to the gen- 
 eral theory of brain action in its relations to mental life; and it seems 
 to have been distinctly discredited by experimentation upon the 
 lower animals. 3 The latter kind of evidence leads Sherrington to 
 pronounce "untenable" the vaso-motor theory of the production 
 of emotion, and as well the view that visceral sensations or presenta- 
 tions are necessary to emotion. 
 
 On the other hand, there is evidence that some of the expressive 
 movements are as independent of the cerebrum on the one side as 
 they are of the viscera on the other; for, as already mentioned (p. 
 158), movements "expressive" of pain, fear and anger can be elic- 
 ited from a decerebrate animal, and belong therefore to the class of 
 
 1 Amer. Journ. of Medical Sci., 1909, CXXXVII, 480. 
 
 2 For an analysis of the more prominent of the emotions, both on the physi- 
 ological and on the introspective side, and some more specific account of the 
 kinds of this " bodily resonance," see Ladd, Psychology, Descriptive and Explana- 
 tory, pp. 538 ff. 
 
 3 Sherrington, by "appropriate spinal and vagal transection," removed from 
 dogs, completely, all the sensations of the viscera and of the skin and muscles 
 behind the shoulder. In the case of an animal, " selected because of markedly 
 emotional temperament," the almost complete reduction of the field of sensa- 
 tion "produced no obvious diminution of her emotional character." Ibid., 
 pp. 260 f.; Proceedings of the Royal Society of London, 1900, LXVI, 390. 
 
526 FEELING, EMOTION, AND EXPRESSIVE MOVEMENTS 
 
 sub-cortical reflexes. Expressions of pleasure, such as purring and 
 wagging the tail, are not observed in a decerebrate animal. A few 
 cases of visceral anaesthesia, of which the best is that of d'Allonnes, 1 
 show, as we should expect, a loss of the affective tone of certain 
 emotions, in which, in the normal conditions, the visceral sensations 
 play a prominent part. The woman whose case is reported by 
 d'Allonnes, though previously affectionate and emotional, given 
 to worry, etc., became indifferent to her beloved family, and sought 
 admission to the hospital to be cured of this lack of power to feel 
 emotions. She still retained her former habits and principles, and 
 even preferences, but these, she said, no longer had any feeling 
 connected with them. Yet her external expressive movements 
 were retained; in particular, she wept, but without inner grief, 
 and she showed the signs of anger, shame, disgust, etc., while pro- 
 testing that she felt no emotion. Similar indefinite results have, 
 however, been reported as sequent upon injuries to various portions 
 of the cerebrum, where the peripheral organic functions remained 
 practically normal. 
 
 Among the many and often conflicting investigations in this really 
 difficult field may be mentioned a very careful piece of work by 
 Shepard, 2 in which the previous results are reviewed. Shepard 
 made a comparative study of circulatory changes in the brain (in 
 a patient whose brain was exposed) and in the arm, and found that, 
 "in general, all agreeable or disagreeable stimuli, all sensory atten- 
 tion or attention to arithmetical problems, all agreeably exciting 
 light or music," gave a decrease in the volume of the hand 
 and an increase in the volume of the brain. He concludes from 
 these results, and from the studies of circulation and breathing 
 during affective conditions in other cases, that a classification of 
 the feelings cannot be made on the basis of their bodily accompani- 
 ments; that there is no opposition between the accompaniments of 
 pleasantness and unpleasantness, nor any evidence for Wundt's 
 three dimensions of feeling. "In short, all moderate nervous ac- 
 tivity tends to constrict the peripheral vessels and to increase the 
 volume and size of pulse in the brain. All moderate nervous ac- 
 tivity likewise increases the heart-rate." Strong stimuli, such as 
 arouse fear, etc., induce a more complex circulatory reaction, be- 
 ginning, in the brain, with an increase in volume, passing thence 
 to a decrease, and finally rising to a large increase, which gradually 
 passes away. The mechanism of these changes in the blood sup- 
 plied to the brain is probably to be sought, largely, in the vaso-motor 
 control of the great abdominal veins. These investigations, there- 
 
 1 Revue philos., 1905, LX, 592. 
 
 Amer. Journ. of PsychoL, 1906, XVII, 522. 
 
THE INTELLECTUAL FEELINGS 527 
 
 fore, strongly support the central theory of feeling which we have 
 been advocating. 
 
 24. The teleological value of many of the emotions, especially 
 such as man shares with all the higher animals, needs scarcely more 
 than mention in order to be recognized. It is amply illustrated by 
 the observations, both of biology and of psychology, when studied 
 from the comparative and evolutionary points of view. In general 
 these two principles apply to a large class of the emotions: (1) The 
 motor reactions called forth as a part of the bodily resonance are 
 adapted for the defence and preservation of the individual; and (2) 
 by the application of the principles of imitation and sympathy, 
 these same or other reactions operate for the defence and preser- 
 vation of the species. Evolutionary biology is, therefore, justified 
 in considering "the bodily expressions of emotion as instinctive 
 actions reminiscent of ancestral ways of life." 
 
 25. That there is a considerable class of feelings which may be 
 properly classified as " intellectual," no adequate analysis of the 
 extremely complex form of mental activity which we call knowledge, 
 or cognition, allows us for a moment to doubt. Indeed, it might 
 also be said that the influence of feeling is as obvious, and almost if 
 not quite as great, in determining all our acts of knowledge, whether 
 so-called presentations of sense or self-consciousness, as are the ac- 
 tivities of discrimination, comparison, association, etc. The so- 
 called faculties of intellect and feeling blend in all cognition, and the 
 complex result the very object of knowledge is determined by 
 both. 1 
 
 Without attempting the difficult, if not impossible, task of enumer- 
 ating all the intellectual feelings, we may remind ourselves of. the 
 changing affective tone of consciousness when, on comparing two 
 or more objects, as wholes, or qualities of objects, we pass from a con- 
 dition of doubt and uncertainty, through varying shades of the rec- 
 ognition of similarities and differences, into the feeling of certainty 
 and conviction which is both the accompaniment, and the test in a 
 measure, of our arriving at a completed act of cognition. Indeed, 
 this conviction itself involves a sort of "belief in reality," for which 
 we seem compelled to find a place among our most fundamental 
 forms of feeling. 
 
 What can be claimed, or credibly conjectured, as true with regard 
 to the forms of the functioning of the nervous mechanism on which 
 many of these so-called intellectual feelings depend, or with which 
 they are connected, has in large measure been said while treating 
 
 1 For an extended exposition and defence of this doctrine see Ladd, Psychology, 
 Descriptive and Explanatory, chapter XXII; Philosophy of Mind, chapter III; 
 and Philosophy of Knowledge (passim). 
 
528 FEELING, EMOTION, A^D EXPRESSIVE MOVEMENTS 
 
 of the sensations, their combinations into presentations of sense, 
 and the physiological basis of the simpler forms of feeling. Some- 
 thing will be added in the following chapters which deal with 
 the physiological psychology of attention, association, memory, and 
 judgment. In all this field, experimental psychology has not much 
 of clearly ascertained scientific truth, which can be employed to 
 reveal the concomitant, or otherwise connected, processes of the 
 nervous system especially of the cerebral areas. But it is not 
 by any means a wholly unfounded conjecture that hesitancy, in- 
 hibition, confusion, opposition, facilitation, with varying degrees 
 of speed and smoothness, fixed habit in " dynamical associations" 
 within the brain, and a number of similar terms, describe with rea- 
 sonable appropriateness those central nervous processes which are 
 correlated with the mental processes that we are accustomed to 
 describe in similar terms. 
 
 One other thing is worthy of notice in this connection. Some 
 of the intellectual feelings, especially when they take on the emotional 
 character, quite naturally lead to expressive motions, or tendencies 
 to move, or to hold in place, the muscular system, in whole or in 
 part. All our language such as " standing firm," or "pat," "hold- 
 ing on" to one's belief, or opinion, etc., and the whole practice of 
 gesticulating and expressive posturing is significant of the close 
 relations existing between certain intellectual feelings and the mus- 
 cular apparatus. Here, too, we must look for important "somatic 
 reactions." 
 
 26. The (Esthetic feelings arise and develop chiefly in connec- 
 tion with presentations of sense, or with the remembered or created 
 mental images that represent objects of sense. In their elementary 
 form, therefore, they plainly have a physiological side which admits 
 of scientific treatment although they have received such treatment 
 far less than could be wished. Many interesting facts and certain 
 partial generalizations having most application to the lower 
 classes of pleasurable feelings through the organs of smell, taste, 
 and the skin, when viewed in the light of the hypothesis of evolu- 
 tion have been alleged by various observers. But even the most 
 elementary aesthetic feelings cannot be considered as on a par with 
 the sensuous feelings, or as mere aggregates of such feelings. The 
 tone of feeling which characterizes the sensations furnishes a material, 
 as it were, for genuinely aesthetic feeling; but the latter always im- 
 plies also the working of certain intellectual laws, and a union of 
 the simple feelings of sensation under time-form and space-form. 
 ^Esthetic feelings, then, may be said to spring, in a measure, from 
 the manner of the combination of sensuous feelings; time and space 
 furnish the framework in which they are arranged. Hearing is 
 
THE AESTHETIC FEELINGS 529 
 
 the principal sense for combining sensuous feelings so as to produce 
 aesthetic feelings under time-form, and sight under space-form. 
 The development of even the elementary but genuine (Esthetic feel- 
 ings by other senses than the eye and ear is extremely limited. 
 The agreeable and disagreeable feelings which come through sensa- 
 tions of smell, taste, and touch are for the most part sensuous, 
 rather than strictly aesthetic. 
 
 Hearing, as pre-eminently the time-sense, has two forms of aes- 
 thetic feeling harmony and rhythm. The nature of the complex 
 sensations which produce the feeling of consonance and dissonance 
 has already been discussed. Harmony is determined by the co- 
 incidence of certain partial tones belonging to different clangs si- 
 multaneously sounded. The feeling of harmony is colored by the 
 peculiar way in which the combination of the clangs occurs. The 
 principal difference of this sort is that which obtains between major 
 chords and minor chords; in the former the different clangs are 
 perceived as firmly held together by the fundamental clang, while 
 in the latter the coincident overtone performs the same office less 
 obviously. The one is productive of agreeable aesthetic feeling 
 satisfied; the other of such feeling left unsatisfied a feeling of 
 longing. When, then, the one form of feeling becomes very in- 
 tense, it may involve the pain of over-excitement; the other, when 
 intensified, stirs a kind of agreeable pain of unrest. In musical 
 time it is the periodic nature of the excitation, with a change in the 
 individual presentations of sense, which produces the pleasurable 
 aesthetic feeling. 
 
 Two or three regularly recurring impressions, having the same 
 or a different content of musical sound, are combined into a series; 
 certain members among the whole number are then accentuated, 
 in order to form the different series that constitute the various kinds 
 of musical time. All musical time, fundamentally considered as 
 respects its rhythm, is either two-time or three-time. The differ- 
 ence in the feelings which respond to these two classes of musical 
 rhythm is obvious in a pronounced form, in the funeral march, on 
 the one hand, and the waltz, on the other. In general, it is the 
 harmony of music which gives direction to its feeling, and the 
 rhythm which determines the rise and fall of feeling. Thus waves 
 of different kinds of feeling are made by music to pass over the 
 soul. 
 
 27. Even less than is the case with the intellectual feelings, 
 does experimental psychology serve to reveal the exact nature of 
 those physiological processes which are connected with the more 
 complex aesthetic feelings that greet the art of music. Where these 
 feelings rise to an emotional character, the effect of the "somatic 
 
530 FEELING, EMOTION, AND EXPRESSIVE MOVEMENTS 
 
 reactions," and the influence of the associated ideas and tenden- 
 cies to action (as to dance, to fight, to embrace, to march) become 
 more clearly distinctive and powerful. The feelings with which we 
 appreciate whether favorably or unfavorably those peculiar suc- 
 cessions of tones which are required by melodies written in the various 
 keys, or the tone-color of the various instruments in an orchestra, 
 are more subtle and as yet quite indeterminate as to their essen- 
 tial content, not to say their physiological correlates. Those feel- 
 ings of right "relationship" which have come to set the laws of the 
 succession of tones in the affectively best melodies of modern music 
 are of special interest to the investigator. But beyond the mere fact 
 that some successions excite feelings of pleasurable satisfaction, 
 and "finality," or feelings of recognition of the series as a unity, 
 while others do not, little is known at present about the psychol- 
 ogy of musical melody. The cause of this feeling of finality has 
 been described as a "balanced muscular resolution." 1 
 
 The elementary aesthetic feelings which come through sight lead 
 to the consideration of the aesthetic effect of visual form. Such 
 effect can be considered only very imperfectly from the physiologi- 
 cal point of view. In one important particular, however, pleasura- 
 ble aesthetic feeling is directly dependent upon the combination of 
 the sensations, with their accompanying tone of feeling, under the 
 laws of the mechanism of vision with both eyes in motion. Beau- 
 tiful form is determined by the course of the limiting lines; and limit- 
 ing lines, in order to have the effect of arousing agreeable sesthetic 
 feeling, must accommodate themselves to the physiological and psy- 
 cho-physical necessities of the eye when in motion. These neces- 
 sities thus determine both the direction and the extent of the limit- 
 ing lines. Lines of slight curvature, not too far continued in one 
 direction, best comply with such necessities. Lines of very sharp 
 curvature, or lines continued too long in one direction, do not pro- 
 duce a pleasing sesthetic effect. So also must the main lines of a 
 building lie in horizontal or vertical directions, preferably in the 
 former direction. But long oblique lines for example, from a 
 lower right-hand to an upper left-hand corner of a building are 
 scarcely tolerable. The ease with which the eye moves, by jerks 
 (see p. 459), along the lines, in order to make that synthesis of suc- 
 cessive similar presentations of sense in which every perception of 
 a line consists, is plainly a determining factor in all these cases. 
 
 The aesthetic effect of visual form is also determined by the way 
 in which the form is constructed, through repeating similar or un- 
 like simple shapes and combining them into a totality. By this 
 
 1 Studies in Melody, by W. Van Dyke Bingham. Monograph Supplements of 
 the Psych. Rev., 1910, vol. XII, No. 3. 
 
NATURE OF THE SENTIMENTS 531 
 
 means a feeling of pleasure akin to the feeling of musical rhythm 
 is excited by the successive impressions which occur periodically as 
 the eye, with a nearly uniform movement, sweeps the entire field. 
 In horizontal directions, the law for the arrangement of the parts is 
 that of symmetry of the simple parts; in vertical, rather the law 
 of asymmetry. Certain proportions between the connected parts, 
 and between the whole and the parts, are favorable to the develop- 
 ment of aesthetic feeling. Ease of the mental apprehension with 
 which the relations in proportion of the different parts are presented 
 is favorable to agreeable aesthetic feeling. 
 
 28. It appears, then, that the varied aesthetic feelings, pleas- 
 ant and unpleasant, which are dependent upon visual sensations 
 and perceptions, are of all others most closely connected, in respect 
 of their facts and laws, with the so-called "intellectual" side of 
 our objective experience. Conditions which determine our ap- 
 perception of space-relations, as well as the harmony and contrast 
 of colors, varied and often obscure associations, and even heredi- 
 tary factors, are prominent in the causation of the various shades 
 of aesthetic feeling of visual objects. 
 
 In conclusion of this subject it is worth while to notice that the 
 emotions aroused by the markedly different classes of beautiful 
 objects differ in a marked way among themselves, in respect to 
 their " somatic reactions" and to the resulting affective tone of 
 consciousness. Thus, for example, our appreciation of what we 
 consider sublime is distinctly unlike, and in some respects the 
 opposite of, our appreciation of what is beautifully delicate, the 
 handsome, or pretty, so called. So, too, the mingling of sensations 
 and feelings and ideas with which we greet objects that have the 
 beauty of order and proportion (a Greek temple, for example) is 
 very different from that with which we appreciate the beauty of 
 luxuriance and wildness (a tropical forest, for example). Intro- 
 spective analysis of the elements of these complex aesthetic emotions 
 is relatively easy as compared with the experimental analysis of the 
 simpler forms of feeling, under whatever class we may be inclined 
 to place them. 1 
 
 29. Not only do the emotions involve elements derived from 
 the changes initiated in the musculature, but they all tend to ex- 
 press themselves in the muscles of the limbs and trunk, head, eyes, 
 and vocal apparatus. We are probably safe in saying that strained 
 attention is associated with a tense and rigid condition of the mus- 
 cles, and that relief brings relaxation and freer movements. Excite- 
 ment seems to be expressed by much muscular activity, and calm 
 and depression by muscular inactivity. 
 
 1 Compare Ladd, Knowledge, Life, and Reality, 1909, chaps. XVII-XIX. 
 
532 FEELING, EMOTION, AND EXPRESSIVE MOVEMENTS 
 
 In regard to pleasantness and unpleasantness, considerable in- 
 terest has been aroused over the assertion 1 that pleasantness is 
 associated with expansive movements of the organism, and un- 
 pleasantness with contractive movements. The probability is that 
 there is no such clean-cut distinction between the expressive value 
 of these two classes of muscles, in so highly organized and so special- 
 ized a motor apparatus as that of man. Both flexors and exten- 
 sors co-operate in the production of movements which may be, as 
 wholes, reactions to beneficial or harmful stimuli, and so expressive 
 of pleasant or unpleasant feelings. Although, for example, the 
 primary reaction to a painful stimulus is the " flexion reflex" (com- 
 pare p. 153), this changes, when the stimulus is intense or prolonged, 
 to a movement of flight, in which the extensors take the leading part. 
 The rapid alternation, in most active movements, of extensions and 
 flexions, also makes it difficult to believe that either is specifically 
 related to a certain tone of feeling. On the whole, then, there is 
 little evidence of a specific bodily expression for pleasantness or 
 unpleasantness one, that is, which is expressive alike of all con- 
 ditions into which these feelings enter. 
 
 30. There is, however, good evidence of an exciting or depress- 
 ing effect on the bodily activities of many stimuli and of many states 
 of mind. These effects have sometimes been called dynamogenic. 
 They are related to the reinforcement and inhibition which are seen 
 in reflex action. A good index of the excited or depressed condition 
 of the reflex mechanisms of the nerve-centres, and especially of the 
 spinal cord, is afforded by the knee jerk, or smart kick of the foot 
 produced by the extensor muscle of the thigh, when its tendon, 
 passing over the knee, is struck. Though this movement is en- 
 tirely involuntary, it is by no means removed from the influence of 
 mental conditions. For example, anxious attention to the knee and 
 foot inhibits the reflex, so that the physician, to whom the knee jerk 
 is often a symptom of importance, must needs distract the attention 
 of his patient from the knee. This he usually does by requiring the 
 patient to grip or pull vigorously with the hands at the moment when 
 the blow on the tendon is to be struck; under these circumstances the 
 knee jerk is especially strong. The explanation of this result in 
 terms of distraction as if the only reaction of mental activity to 
 the reflex were one of inhibition, which must be put aside by dis- 
 traction is not complete. Lombard 2 found that reinforcement or 
 inhibition of the knee jerk could result from many stimuli and mental 
 influences, even though the subject, from long-continued familiar- 
 ity with the experiment, required no distraction to direct his mind 
 
 1 See Munsterberg, Beitrdge zur exp. PsychoL, 1892, IV, 216. 
 1 Amer. Journ. of PsychoL, 1887, 1, 1. 
 
DYNAMOGENIC EFFECTS OF THE EMOTIONS 533 
 
 away from his knee. Martial music increased the reflex, whereas 
 quiet though interesting music had the opposite effect. An excit- 
 ing noise, such as the cry of an infant, increased it, whereas common- 
 place and insignificant noises, such as the rattle of vehicles in the 
 street, were without effect. In these last examples it is evidently 
 not the mere sensory effect of the stimulus, but rather its meaning 
 for the individual, which exercises the exciting or depressing effect 
 on the spinal cord. 
 
 One salient fact regarding the relation of consciousness to move- 
 ment is, accordingly, that some conscious processes exalt, while 
 others depress, the activity of the reflex centres and through them 
 of the muscles. 
 
 Similar effects were observed by Fere 1 in the case of voluntary 
 movements. He required his subjects to exert their utmost force 
 in squeezing a dynamometer in the hand, and found that this " ut- 
 most " could be increased by sensory stimuli, and that different 
 stimuli possessed different degrees of this dynamogenic influence. 
 Light reinforced the action of the hand muscles, darkness depressed 
 it. The colors were still more powerful and all dynamogenic; but 
 red had the strongest effect. Of tastes, bitter had a strong dyna- 
 mogenic effect, and sweet a weak effect in the same direction; of 
 odors, the sharp and penetrating reinforced the pressure of the hand, 
 and the heavy odors had the opposite effect and this without much 
 regard to the pleasantness or unpleasantness of the odor. It should 
 be said that these effects are not strongly marked except in suggesti- 
 ble subjects; the average, "normal" individual is scarcely influenced 
 at all by colors, for example. But there are certain more complex 
 and, as one may say, more mental influences which do have a strong 
 influence on the motor power of the normal individual. Chief 
 among such influences is perhaps that of competition between in- 
 dividuals who alternately squeeze the dynamometer, or who are 
 running a race. 2 
 
 In fact, the supposedly maximal voluntary effort is, to judge from 
 its muscular effect, a variable quantity, subject to many involuntary 
 influences; and the experiments quoted illustrate the complexity of 
 the mental influences which exalt or depress muscular activity. 
 
 31. The relation of consciousness to movement cannot be 
 fully appraised without considering another class of facts. These 
 relate to the so-called automatic movements, or movements which, 
 in contrast to expressive and voluntary movements, go on without 
 
 1 Sensation et mouvement (Paris, 1887; 2d ed., 1900); Annee psychologique, 
 1900, VII, 69, 82, 143. 
 
 2 Triplett, Amer, Journ, of PsychoL, 1898, IX, 507; Wright, Psychol. Rev., 1906, 
 XIII, 23. 
 
534 FEELING, EMOTION, AND EXPRESSIVE MOVEMENTS 
 
 consciousness, or at least without clear consciousness. A good ex- 
 ample is afforded by any much-practised movement, which con- 
 tinues smoothly while the attention is on something else. Another 
 example is found in instinctive movements, such as breathing, which 
 pursue their way, on the whole, undirected by consciousness. In 
 studying reflex action (p. 173), we noted the solidarity of the nervous 
 system, and the mutual influence which was likely to be exerted 
 by the activity of one part on the activities of any other part; but 
 we also noticed that the activity of one part might, on occasion, be 
 relatively independent of other parts, so that two or more non-inter- 
 fering activities might be going on at the same time in different parts 
 of the nerve-centres. Automatic movement is an illustration of this 
 relative independence of different parts of the system. The regular 
 movements of respiration go on without much reference to what 
 is occurring in the conscious centres of the cerebrum, except in 
 cases of excitement, strained attention, etc. In the same way, the 
 activity of some well-trained part of the cerebrum, which presides 
 over a certain familiar performance, may go on without interfering 
 with the activity of some other part with which the attentive con- 
 sciousness of the moment is connected, and without being interfered 
 with by it. This independence of different parts is, however, only 
 relative; interference is likely to occur, and when it does occur, we 
 have an expressive as opposed to an automatic movement. Those 
 parts of the motor apparatus which are, at any moment, much in- 
 fluenced by conscious processes give rise to expressive movements; 
 those parts which are little influenced give rise to automatic move- 
 ments. Expressive movements, in a word, illustrate the solidarity of 
 the nervous system, while automatic movements reveal a certain 
 degree of dissociation within it. 
 
 In a negative way, automatic movements may themselves be ex- 
 pressive; for they usually cease or are somehow interfered with as 
 soon as the conscious process becomes very intense. Breathing, 
 as has been said, is likely to be inhibited during a brief period of 
 intense mental activity. A man who is automatically walking 
 while immersed in thought may sometimes be observed to stand 
 stock-still when some specially interesting idea strikes him. Lindley 1 
 found two classes of involuntary movements, which might be ob- 
 served, for example in a school-room: one class of movements ac- 
 companied intense mental effort, and consisted in strained positions 
 of the members; the other class appeared when the mind wandered, 
 and consisted in rhythmical movements. The latter, automatic 
 class, gives way to the former, expressive class, on passing from mind- 
 wandering to mental effort. There are, however, considerable 
 1 Amer. Journ. of PsychoL, 1895, VII, 491. 
 
AUTOMATIC AND IDEOMOTOR MOVEMENTS 535 
 
 individual differences in regard to automatisms. Binet 1 observed 
 such differences among children, and Stein 2 in young adults. The 
 last author found some individuals in whom intense mental appli- 
 cation favored instead of inhibited automatic movements. In that 
 peculiarly unstable condition of the nervous system known as hys- 
 teria, the tendency to automatisms and dissociations is at its maxi- 
 mum, and some such individuals may carry on complex acts, such 
 as writing a letter, while the attention is absorbed in something quite 
 different. 3 The relation of consciousness to bodily movement is 
 therefore partly expressed by saying that conscious processes exert 
 general dynamogenic and depressive effects on the motor apparatus, 
 and partly by calling attention to the limitations of these influences. 
 
 But besides these general influences, specific conscious processes 
 have specific motor effects. Certain ideas lead to certain definite 
 movements, with which they have become associated by past ex- 
 perience. They may do so either with or without the full consent 
 of the subject. When an idea leads to its appropriate movement 
 with the full consent of the subject, we call it voluntary movement; 
 but when the idea leads to movement, as it always tends to, while the 
 subject's attention and intention are elsewhere directed, the move- 
 ment is often named ideomotor. Examples of the last are seen in 
 involuntary whispering of what one reads or thinks, in involuntary 
 gestures, and often, in rather an amusing way, in the movements of 
 spectators at an athletic game or an acrobatic show, when they are 
 much absorbed in the movements about to be executed by the per- 
 formers, and unwittingly execute such movements themselves. 
 
 This specific relation between particular ideas and particular 
 movements has often been so conceived as to connect directly only 
 ideas of the movements themselves with those movements. In 
 other words, it has been assumed that any other sort of idea, to issue 
 in movement, must first arouse an idea of the movement, which in 
 turn would arouse the movement. Introspective examination shows 4 
 that in adults, at least, this conception is wide of the mark, and that 
 the most direct conscious antecedent is likely to be an idea of any 
 sort; it is specially likely to be an idea of some end which is to 
 be attained by the movement. There is no reason why the idea of 
 the end should not become directly associated with the making 
 of a movement leading to that end; and there is good reason for 
 thinking that in children, as well as adults, attention is attracted 
 
 1 La suggestibility (Paris, 1900), p. 360. 
 tpsychol. Rev., 1898, V, 295. 
 
 a See especially Janet, L'Automatisme psychologique (Paris, 1889), p. 223. 
 * Woodworth, "The Cause of a Voluntary Movement," in Studies in Philoso- 
 phy and Psychology, pp. 351 f. (Boston, 1906). 
 
536 FEELING, EMOTION, AND EXPRESSIVE MOVEMENTS 
 
 mostly to the end, and that ideas of the movement, in a strict sense, 
 have always but a small place in consciousness. Indeed, of our 
 habitual movements it is, in general, difficult or impossible for us to 
 form any clear idea, strictly so called. 
 
 32. It remains to notice a certain mixture of sensations and 
 feelings, usually attended by a rather strong tone of unpleasantness, 
 which may be grouped under the term, "Fatigue." In ordinary 
 usage, the word fatigue has a somewhat ill-defined meaning, inas- 
 much as it refers partly to a certain feeling or mass of sensations, 
 and partly to a condition of actual inability to perform a certain act, 
 as well before as after the onset of the feeling of fatigue. It is in 
 the latter sense that the term is employed by physiologists, who be- 
 
 FIG. 146. Fatigue Curve of a Frog's Muscle. Each vertical line records a contraction, 
 aroused by an electrical stimulus, and lifting a weight. 
 
 gan their study of fatigue with an isolated frog's muscle. If this 
 muscle was excited by an electric shock once every two seconds, 
 while being protected from drying, the first effect of the repeated 
 stimulation was found to be a slight increase in the force of muscular 
 contraction that is, a slight increase in the height to which the 
 muscle raised the weight with which it was loaded. This " stair- 
 case effect," or period of "warming-up," soon gave way, with 
 further stimulation, to a gradual decline in the force of contraction; 
 and this decline might go on to the zero-point. The course of 
 fatigue, as indicated in such an experiment, is called the "fatigue 
 curve." If the muscle, instead of being entirely isolated, is left 
 with its circulation intact, the period of gradual increase of force 
 is more prolonged, and the subsequent decline is less rapid (Fig. 
 146). If the muscle, instead of being excited by shocks applied 
 directly to its substance, is aroused by exciting its motor nerve, es- 
 sentially the same fatigue curve is obtained. And the result is 
 essentially the same if the muscle is excited reflexly, by stimuli 
 applied to some suitable sensory nerve. 
 
 Mosso 1 devised the ergograph for obtaining a record of the fa- 
 tigue of voluntary movement in man; and many later investigators 
 have labored to add improvements to the technique of this experi- 
 ment. In general, the curve of fatigue obtained from human muscle 
 
 1 Arch. f. (Anat. and} PhysioL, 1890, p. 89; Arch, italiennes de biol., 1890, 
 XIII, 123. 
 
PHENOMENA OF FATIGUE 537 
 
 under excitation by the will is similar to that above described in 
 case of the isolated frog's muscle. The period of warming-up is 
 usually in evidence, and then there is a gradual decline. There is, 
 however, much more variability between the separate contractions 
 than appears in the isolated muscles ; for at times the attention given 
 to the muscular work wanes, and the force of the contraction slightly 
 declines, while at other times there is renewed effort resulting in 
 a " spurt" or rise in the curve. Besides this, there are differences be- 
 tween individuals, some showing a very gradual onset of fatigue, 
 whereas others maintain nearly the original strength for a consider- 
 able time, and then weaken suddenly. Under the best conditions, 
 a well-trained muscle will show but a gradual decline in force, and 
 may be able, after an hour or two of such work, lifting the weight 
 as high as possible once every two seconds, to reach still 60 to 80 
 per cent, of its original performance. 1 In fact, after an initial period 
 of decline, the performance approximates to a level, known as the 
 "level of fatigue." 
 
 Sensory fatigue, apart from " adaptation," of which more will be 
 said later, is known principally in the case of the eye; it consists 
 partly of sensations of soreness and partly of fatigue of the muscles 
 of the eye. An actual loss of the functional power of the eye may 
 result from continued work. 2 
 
 33. Mental fatigue, pure and simple, is difficult of observation, 
 because almost any mental task which can be measured and used 
 as the basis of a test requires the use of the eyes, or of the muscles, 
 and so is likely to involve muscular and sensory fatigue. In case, 
 however, the demand on the eyes and muscles is as moderate as 
 possible, the signs of fatigue i. e., of real inability to perform work 
 are slow in appearing. Reaction-time work has been continued 3 
 for 15 to 20 hours, and memorizing 4 for four or five hours, with 
 little loss of efficiency. On the other hand, continued adding of 
 columns of figures has shown, in some individuals at least, a more 
 pronounced loss. No doubt common experience would make us 
 incline to believe in the reality of mental fatigue, and even in a 
 rather rapid rate in its progress; but there is this to be said, that 
 mental work is not ordinarily done under test conditions, which 
 
 Breves, Arch, ital de biol, 1898,. XXIX, 157, and XXX, 1; also in Pfliiger's 
 Arch. f. d. ges. PhysioL, 1899, LXXVIII, 163; Schenck, Pfliiger's Archiv., 1900, 
 LXXXII, 390; Hough, Amer. Journ. of PhysioL, 1901, V, 240; Woodworth, Le 
 Mouvement (Paris, 1903), p. 371, where a re*sume" of work on motor fatigue is 
 given. 
 
 2 Scripture and von Tobel, Studies from the Yale Psychological Laboratory, 
 1896, IV, 15; Moore, ibid., 1895, III, 87. 
 
 3 Cattell, Wundt's Philos. Studien, 1886, III, 489. 
 * Thorndike, Psychol. Rev., 1900, VII, 571. 
 
538 FEELING, EMOTION, AND EXPRESSIVE MOVEMENTS 
 
 would keep up the incentive to do one's utmost. In ordinary con- 
 ditions, we yield more readily than in an experiment to feelings of 
 ennui and weariness which do not necessarily indicate actual de- 
 crease of power. 1 
 
 34. In considering the causes of fatigue, we may take our start 
 again with the muscle. A priori, three causes might be possible : the 
 structure of the muscle might be partially broken down ; or the fuel 
 from the oxidation of which the muscle derives its energy might be 
 limited; or, finally, the waste products of this oxidation might poison 
 the muscle. There is little sign that the structure of the muscle is 
 impaired in normal fatigue; there is considerable probability that 
 the supply of fuel may run low; or at least, as suggested by Treves, 
 that a store of fuel laid up in the muscle during its previous resting 
 condition may become quickly used up, so that the muscle becomes 
 dependent on the supply brought to it by the blood, and therefore 
 does not obtain its fuel as rapidly as at first. That the muscle pro- 
 duces in its activity "fatigue substances" which partially poison or 
 depress it, is an established fact. The muscle, by its activity, evolves 
 carbon dioxide, lactic acid, and acid potassium phosphate. Lee has 
 shown 2 that two at least of these three substances, when injected 
 into a fresh muscle, cause in it marked signs of fatigue almost from 
 the beginning of its activity. He has further shown 3 that the first 
 effect of a moderate dose of the same substances is to produce the 
 staircase effect; he therefore suggests that this effect may, in the ac- 
 tivity of a muscle, be the result of the beginning accumulation of 
 these substances. 
 
 35. The fatigue of voluntary muscular activity, depending as 
 this activity does on the brain as well as on the muscles, was as- 
 cribed by some of the early experimenters, on what seemed good 
 grounds, to the brain in larger measure than to the muscle. More 
 recently, this view has been subjected to severe criticism. By a 
 method similar to that which was previously used (compare p. 
 136) in examining the fatigue of nerve-fibres, the reflex mechanisms 
 of the spinal cord have beerrthown 4 to be highly resistant to fatigue. 
 When a block is interposed between the spinal cord and a muscle, 
 and a stimulus is applied to a sensory nerve which would, in the 
 absence of the block, arouse a reflex contraction in the muscle 
 
 1 Kraepelin and his pupils have devoted much study to the fatigue curve of 
 mental work, or to the "work curve," and have attempted to analyze out the 
 various factors which contribute to change the working efficiency from moment 
 to moment. See the successive issues of Kraepelin's Psychologische Arbeiten. 
 
 *Amer. Journ. of PhysioL, 1907, XX, 170. 
 
 3 Ibid., 1907, XVIII, 267, 
 
 4 Joteyko, Conies rendues de la Soc. de Biol, 1899, p. 484. 
 
THE KINDS OF FATIGUE 539 
 
 every second or two, the muscle, here serving as an indicator of the 
 condition of the cord, responds well after the removal of the block 
 thus showing that the cord was not much fatigued by a long suc- 
 cession of stimuli which had reached it and caused it to discharge 
 down a motor nerve. 
 
 Tests 1 of the excitability of a muscle which had been employed 
 in voluntary contractions to the point of fatigue showed that the 
 muscle had lost some of its responsiveness, and was therefore actu- 
 ally fatigued. Further, the fact seems important that fatigue is 
 little if any more speedy in its onset in voluntary muscular work 
 than in the work of a muscle excited directly or through its nerve. 
 If the nerve-centres fatigued rapidly, activity involving these centres 
 should show more rapid fatigue than similar activity with the cen- 
 tres left out. The slow rate of fatigue in purely mental work 
 should also be recalled in this connection. 
 
 36. The nature of brain fatigue, in so far as it occurs, is still 
 unexplained. In considering the matter, we are invited to dis- 
 tinguish several sorts of " fatigue" which may be concerned. There 
 is first the metabolic fatigue, such as has been demonstrated in the 
 case of the muscle. Some indications of a metabolic change in 
 the brain after very prolonged and usually excessive activity have 
 been obtained by histologists, 2 but this evidence would, on the whole, 
 tend to show that the metabolism of the brain is slight in amount. 
 
 A second variety of fatigue may be named the " transferred 
 metabolic." This results from the carrying of fatigue products by 
 the blood from the muscle or other organ in which they are produced 
 to some other organ which has been resting. The effect is the 
 same as that shown by the experiment in transfusing fatigue prod- 
 ucts, mentioned above. There is little doubt of the reality of this 
 form of brain fatigue, as is indicated by the drowsiness and inability 
 to perform mental work which immediately follows severe muscu- 
 lar exercise. 
 
 A third variety consists of feelings and sensations of fatigue. This 
 is to be distinguished from true fatigue, as urged above; but at 
 the same time it must be accounted a genuine inhibiting influence 
 acting to cause the cessation of work, especially of the particular 
 work which has been going on. When a small group of muscles 
 is repeatedly innervated, pain may develop in the muscles and their 
 tendons, and the natural reaction to this sensation is, either to stop 
 the work, or at least to substitute the action of some other group of 
 
 1 Storey, Amer. Journ. of PhysioL, 1903, IX, 52. 
 
 2 Hodge, Amer. Journ. of PsychoL, 1889, II, 376; Goldscheider and Flateau, 
 Fortschritte d. Med., 1897, XV, 241; Dejerine, Soc. de Biol, 1897, p. 728; Ewing, 
 Arch, of Neurol. and Psychopathol, 1898, I, 157. 
 
540 FEELING, EMOTION, AND EXPRESSIVE MOVEMENTS 
 
 muscles. This tendency to change may be wellnigh irresistible; 
 its urgency may be compared with that of the impulse to breathe 
 after holding the breath for some time. In mental work, there may 
 occur local fatigue pains, because of the cramped position in which 
 the neck or other parts are sometimes held during strained atten- 
 tion; but besides these, there are more diffuse feelings of restlessness 
 and ennui, the analysis of which is difficult, but the tendency of 
 which is clearly toward a cessation of the activity in progress. 
 
 A fourth effect which is sometimes classed under the head of 
 fatigue is sensory adaptation. In the case of the eye, fatigue was 
 the explanation advanced by Helmholtz for after-images; but since 
 there are histological changes in the retina on exposure to the dark 
 or to light, which are certainly not akin to the forms of fatigue so far 
 mentioned, but constitute, rather, a positively adaptive phenomenon, 
 it is now customary to substitute "adaptation" for " fatigue" in 
 speaking of such changes. Adaptation occurs in the other senses, 
 but its mechanism is unknown; its effect is to cause a stimulus to be 
 less fully sensed after it has been steadily acting for a time than at 
 first; so far, then, the effect resembles that of fatigue. It is probable 
 that adaptation occurs in the central organs as well as in the sense- 
 organs; growing so accustomed to a stimulus as not to notice it 
 (e. g., the ticking of a clock) is often not a purely peripheral phe- 
 nomenon, since the stimulus can be perceived at any moment if 
 occasion arises for noticing it. 
 
 A fifth variety of impairment of efficiency which bears a super- 
 ficial resemblance to fatigue is interference, and is best known in 
 the case of memory. When a series of dissimilar acts must be per- 
 formed in quick succession, one of these is likely to interfere with 
 another. Inasmuch as a short series is relatively free from inner 
 interferences, whereas a long series involves many such chances, 
 a short series is apt to be performed with better success than a long 
 series. The appearance then is that fatigue has influenced the 
 longer performance. But since even this longer performance need 
 be only a few seconds in length, and since recovery from the condi- 
 tion is prompt, such fatigue can scarcely be identified with the pro- 
 gressive and metabolic type. 
 
 37. The "refractory period" (compare p. 131) is also a phe- 
 nomenon related, at least superficially, to fatigue, though in dura- 
 tion it lies at the other extreme from the progressive type. It 
 seems to be of the nature of a back-swing from the condition of 
 activity. There is, apparently, a radical difference between meta- 
 bolic fatigue, which really lowers the power of an organ, and the 
 inhibitions which result from the action of certain stimuli to the 
 organ. It is probable that inhibitions or interferences give the key 
 
THE REFRACTORY PERIOD 541 
 
 to most appearances of intellectual fatigue. Yet the question cannot 
 be said to be wholly cleared up. In certain forms of mental work, 
 such as performing calculations in the head, or certain complex 
 forms of memory work, 1 there is sometimes a sudden drop from effi- 
 ciency to complete inability to continue; in such cases, the key to 
 the situation is lost. Introspectively, this condition appears as a 
 diffusion of attention, or a tendency to relapse into sleep; but the 
 exact cause is not made out. 
 
 Finally, there can be little doubt that such complex intellectual 
 and aesthetic or moral feelings as loss of interest, ennui, the feeling of 
 being bored, desire to do something else, distate, whether of a mild 
 order or amounting to disgust, and moral disapprobation, have 
 much to do with the onset of fatigue, in the psychological meaning 
 of the word. That there is, in such cases, some corresponding 
 "slowing-up" or positive inhibition of the central nervous processes 
 required for doing the work in hand, without the feeling of fatigue, 
 scarcely admits of doubt. There would seem, then, to be no 
 reasonable objection to speaking of this correlated lowering of 
 energy in the cerebral centres as a true case of brain fatigue. 
 Work done under such mental friction or "loading" of the cerebral 
 activities wastes the cerebral stores of energy. 
 
 1 Compare C. S. Yoakum, An Experimental Study of Fatigue, Psychological 
 Monographs, No. XLVI, 1909. 
 
CHAPTER VIII 
 MEMORY AND THE PROCESS OF LEARNING 
 
 1. One of the most distinguishing characteristics of the higher 
 animals, and especially of man, isThe capacity of learning from ex- 
 perience, or of acquiring modes of reaction adapted to the peculiar 
 circumstances of individual life. It is chiefly this which produces 
 the individual modifications of behavior, as distinguished from the 
 so-called "instinctive" behavior, or behavior common to the species. 
 Each individual contracts habits ofjiis own, acquires skill, learns to 
 know things and tojreact to them as known. In this way, above all 
 other animals does man come to have the course of his thoughts as 
 well as of his actions modified by previously formed associations. 
 Physiological psychology, however, can throw no light upon the 
 philosophical significance of recognitive memory; 1 it must content 
 itself with studying the more obvious modifications of behavior and 
 of thought by experience, and with basing upon this study certain 
 conjectures concerning the part played in the process of learning 
 by the nervous mechanism. The method required by this investi- 
 gation will lead us to recite certain facts regarding both animal and 
 human learning (the latter, in cases of acts of skill as well as in 
 cases of memory of facts); and then to seek to discover any general 
 tendencies, or laws of association, which may emerge from our wide 
 though somewhat hasty survey. 
 
 2. All memory involves two events, and a condition persisting 
 between them. The first event is the impression, or formation of an 
 association; the persisting condition is the preservation of the modi- 
 fication, or the retention of the association; the second event is the 
 changed reaction upon the recurrence of the original situation or 
 of something resembling it. In place of the single first event, there 
 may be a number of events, all tending to impress the same modi- 
 fication. In a broad sense, we speak of the series of events which 
 impresses the modification as experience. We speak of the same 
 series as the act of memorizing, when there is a definite intention of 
 recalling later what has been learned; and as practice, or training, 
 
 1 For discussions of the philosophical aspects of memory, see Ladd, Psychology, 
 Descriptive and Explanatory, pp. 397-407; and Philosophy of Knowledge, pp. 
 122 f.; 262 f.; 386 f. 
 
 542 
 
PROCESSES INVOLVED IN MEMORY 543 
 
 when the object in view is the acquisition of some sort of skill. The 
 preservation of the results of experience, or training, through an 
 interval of time, may be called retention. What is retained, how- 
 ever, is not the newly learned fact or action; for the fact is not con- 
 sciously present during the interval, nor is the act continuously 
 performed. If we wish to avoid, for the time being, any physio- 
 logical hypothesis as to the nature of retention, we may, with Stout, 1 
 speak of a disposition left behind by the modifying experience, 
 ready to give rise, on suitable stimulation, to a reaction which shall 
 show the influence of that experience. 
 
 The second event referred to above, or the reaction showing the 
 effect of past experience, is perhaps better covered by the name 
 reproduction than by any other single word. But its character may 
 be so varied, as well as so complex, that no one word is really ade- 
 quate to designate it. In the sphere of conscious memory, it is 
 common to distinguish two component parts of this event, as the 
 recall and the recognition the recovery of something from past ex- 
 perience, and the conscious reference of it to past experience. These 
 two are often indistinguishably blended in the remembering con- 
 sciousness; but the distinction between them is justified by frequent 
 instances in which either may occur without the other. Thus, 
 recognition without recall occurs when, for example, a name re- 
 fuses to come at our bidding, and yet is recognized at once if 
 spoken by some one else. Recall without recognition occurs in 
 the habitual use of familiar words, and in the practice of familiar 
 acts (for these are not attended by a conscious reference to the 
 past), as well as in the interesting cases of "unconscious plagiarism," 
 where the person believes himself to be inventing or composing 
 something newin literature or art, though he is really reproducing 
 something previously read or seen. 
 
 3. Under the four heads of impression, retention, recall, and 
 recognition may be grouped the problems whicfr*memory p>reents 
 for our consideration, and also the many investigations which have 
 been conducted in the hope of solving these problems. Wecjnay 
 stuclyLthe procesg of jorming an association or learning an act of 
 skill, and the conditions which help or hinder this process; or we 
 ffiay study the permanence of acquired dispositions, and the causes 
 which lead to forgetting; and, similarly, the processes of recall and 
 recognition may be chosen as the subjects for examination. The 
 results of these four studies cannot always be so easily disentangled; 
 for neither the formation nor the retention of associations can be ex- 
 amined except through their effects as seen in recall and recognition; 
 and recall and recognition cannot properly be studied without tak- 
 1 Analytic Psychology, 1896, I, 21 ff. 
 
544 MEMORY AND THE PROCESS OF LEARNING 
 
 ing account of the conditions under which the associations were 
 originally formed and have been retained. Neither is it always 
 possible to analyze the results of investigation, so as to assign its 
 part to each of the processes involved in the memory cycle. 
 
 A sort of justification of the above four-fold division of memory 
 is, however, seen in pathological conditions. Among the diseases 
 of memory are some which affect the formation of associations, 
 and others which affect each of the other components. In senility, 
 for example, there is a loss of modifiability or plasticity, with the 
 result that new associations cannot be formed. A similar result ap- 
 pears in the "polyneuritic psychosis" often a,secjuel of alcoholism 
 in which recent events are promptly forgotten, because of their 
 having, apparently, made no impression on memory. As losses of 
 retention may be mentioned the aphasias and similar defects 
 (compare pp. 252 ff.) when these are due to the destruction of some 
 part of the brain substance which was modified in the learning of 
 language, etc. Under the head of loss of recall may properly be 
 counted the amnesias of hysteria; since here what seems to be for- 
 gotten is not wholly lost, but may later be recalled under hypnosis 
 or other conditions. Some of the losses of memory resulting from 
 shock also belong here; since as the shock passes away, memory 
 may return. Finally, under the head of recognition may be classed 
 pathological conditions of two kinds: in one, there is a feeling of 
 strangeness, which deprives things that are really familiar, and that 
 can be dealt with as such, of their air of familiarity; whereas in 
 the condition of " f alsejrecognition," events which are really new 
 are felt to have occurred in exactly the same way before. 
 
 4. Without attempting to trace the phylogenetic development 
 of the power of learning through the ascending scale of animals, 
 we shall first examine modifications of behavior as they occur in 
 the lowest animals; then in the higher animals, and, finally, in man. 1 
 
 Among unicellular animals, Amoe6a^~lParamecium f "and"" Stentor 
 are the forms which have been best studied in regard to their be- 
 havior. 2 Of these, the amoeba is already familiar to us (see p. 
 14); the other two are somewhat more highly organized cells, be- 
 ing provided with special motor organs in the form of hairs or cilia 
 for swimming. Only slight and uncertain evidence of modifia- 
 bility has been attained by observation of Amoeba, but clearer signs 
 appear in the behavior of Stentor. This little animal is of a trumpet 
 
 1 For a presentation of the development of modifiability in the animal series, 
 see M. F. Washburn, The Animal Mind (New York, 1908); E. A. Kirkpatrick, 
 Genetic Psychology (New York, 1909); H. P.ie"ron, L'Evolution de la Memoire 
 (Paris, 1910). The first of these works contains a valuable bibliography. 
 
 2 See especially Jennings, Behavior of the Lower Organisms (New York, 1906). 
 
LEARNING AMONG INVERTEBRATES 545 
 
 shape, and is usually to be found attached by its stalk to some solid 
 object in the water. If, while thus attached, it is affected by a harm- 
 less stimulus, such as a mild jet of water, it reacts at first by bending 
 away from the jet; but if the same stimulus is repeated, the reaction 
 is discontinued. The animal has become "adapted" to the harm- 
 less stimulus. If, however, the stimulus is somewhat harmful, as 
 a jet of some weak chemical solution, the first reaction being the 
 same as before, repetition of the stimulus leads to a more powerful 
 reaction namely, the contraction of the whole animal; and if the 
 stimulus is still repeated, the stentor pulls loose from its attach- 
 ment and moves away. This is an instance of the principle of 
 varied reaction, which Jennings rightly emphasizes as of funda- 
 menfaTimportange in animal behavior. The various possible reac- 
 tions to the same stimulus are not blended and confused, but are 
 tried in turn until they are proved to be not efficacious. 
 
 Similar behavior has been noticed 1 in Paramecium. When one 
 of these boat-shaped creatures was sucked up into a capillary tube, 
 of a diameter less than the length of the animal, it would, as usual, 
 swim till it encountered some obstacle. This it found, under these 
 conditions, in the surface between the water and the outside air, 
 at either end of the capillary tube. On encountering this obstacle, 
 it at first performed its customary reaction, by backing away, mak- 
 ing a slight turn, and then going forward again. This manoeuvre 
 was repeated many times, but finally gave way to a quite different 
 reaction, the creature doubling on itself in the tube, and swimming 
 off toward the other end. Here the same obstacle was encountered, 
 and the same sequence of reactions made. After several such ex- 
 periences, however, the doubling reaction occurred more promptly, 
 till finally, in some individuals, it became the first reaction to each 
 fresh encounter with the surface of air. Such a changeofjaehav- 
 ior can, perhaps, be explained by a heightened excitaollity of the. 
 animal duetto the cons tantlv_recurr ing obstacles to locomotion. 
 HeightenSTexcitability might favor"the more extreme formlTof re- 
 action; just as, in much higher animals, a state of high excitability 
 may lead to violent reactions to feeble and insignificant stimuli. 
 If this is the explanation, the modification of behavior would not be 
 expected to hold over a period of rest; and, in fact, there is little 
 evidence of the retention by protozoa of such modifications ^as have 
 been detected in their behavior. 
 
 In .higher animals, however, a true retention, lasting from one 
 day to an^jiieii^jiasjr^qiieiitly been demonstrated. A spider, on 
 which experiments were made by the Peckhams, 2 made its usual 
 
 1 Stevenson Smith, Journ, of Comp. Neurol and Psychol., 1908, XVIII, 505. 
 
 2 Journal of Morphology, 1887, I, 383. 
 
546 MEMORY AND THE PROCESS OF LEARNING 
 
 defensive reaction of dropping from its web to the ground, when a 
 tuning-fork was sounded near it, and after it had regained its web, 
 it repeated the same reaction on the repetition of the same stimulus; 
 but after several repetitions ceased to do so. On the next day, it 
 reacted as at first; but after many days, it no longer made this reac- 
 tion to this stimulus. It had become permanently accustomed or 
 " adapted" to the sound. 
 
 Other forms of modification may similarly be retained for long 
 periods by the higher animals. Yet^ together with Detention, or rather, 
 as acting against it, can be seen ^tendency tojecoyer from a modifi- 
 cation, if the stimulus which calls it forth is not frequently repeated. 
 Iri~~the protozoa, recovery is prompt and complete, and there are 
 no after-effects; whereas, in many metazoa, after-effects remain 
 when the temporary effects of adaptation, fatigue, and heightened 
 excitability have passed away. But even these after-effects be- 
 come weakened by time, provided the stimulus which produced 
 them is not repeated. This recovery and forgetting are clearly 
 not without utility; for the original, unmodified behavior may be, 
 in general, a better starting-point for meeting any novel situation. 
 
 Still another form of moclification, sometimes dignified by the 
 name of "associative memory," has been observe! in some in- 
 vertebrates. It Consists" in connecting a particular reaction with 
 a form of stimulus which would not in the first instance tend to 
 evoke it. Suppose, to illustrate, that a stimulus A naturally evokes 
 a certain reaction, and that another stimulus B, to which the animal 
 would not react at all, or to which, at least, it would not respond 
 by the reaction in question, is presented time after time along with 
 A: the result may be that B comes to evoke the reaction which was 
 originally appropriate to A. Thus Spaulding 1 found that the hermit 
 crab, which commonly avoids the darker parts of the aquarium, 
 could be attracted thither by the chemical stimulus of food. If a 
 wire screen was placed in the w T ay, the crab learned to go around 
 and behind it, when attracted by this chemical stimulus; and after 
 many such experiences, the crab would go behind the screen when- 
 ever it was placed in the aquarium, even though the stimulus of 
 food was absent. The sight of the screen, from its frequent asso- 
 ciation with the chemical stimulus, had come to evoke the same re- 
 action. 
 
 5. All orders of vertebrates, but especially birols and mammals, 
 give abundant evidence of learning_bvexperience. The sfudy of 
 tKTr subject has recently, and especially since the work of Thorn- 
 dike, 2 taken on a more precise and scientific character than formerly. 
 
 1 Journ. of Comp. Neurol. and PsychoL, 1904, XIV, 49. 
 
 '"Animal Intelligence," PsychoL Rev., Monogr. Suppl. No. VIII, 1898. 
 
LEARNING AMONG VERTEBRATES 547 
 
 Anecdotes and incidental observations have given way to experi- 
 ments, in which the learning process is under observation from the 
 beginning. A comparison of vertebrates with invertebrates, and 
 of higher with lower vertebrates, shows little that is essentially new 
 in kind in the learning of the higher animals; but it does reveal a 
 great increase in fertility and readiness of modification. Reptiles, 
 as represented by the turtle, 1 learn more quickly than amphibia, 
 as represented by the frog, 2 or than fishes. 3 Birds 4 and mammals, 
 on the whole, learn better thanjthe lowerclajses of animals. Among 
 mammals, monkeys 1 * certainly learn more, and more rapidly, than 
 do rats 6 and mice, 7 or dogs and cats, 8 while the raccoon 9 seems to 
 stand, in this respect, intermediate between the cat and the monkey. 
 The various orders, families, and genera differ in the time required 
 for the formation of a habit, in the permanence of the habits ac- 
 quired, in ttee^omplexity of performance, andjn the_ variety and 
 number of the performances which they are capable of acquiring. 
 
 One typical form of experiment, equally applicable, in principle, 
 to a!T"sorts of animals, requires that a path to a desirable object 
 (such as food, the nest, mates, etc.) shall be learned! Only one 
 path is left open, and sometimes this is complicated into a maze. 
 On the first trial, the animal goes at random, but finally reaches the 
 goal; and on repeated trials, if the maze is not too complicated, the 
 number of wrong turns decreases, till finally a fixed and successful 
 path is followed. Fishes, frogs, and turtles have in this way learned 
 to thread very simple mazes, while quite intricate ones have been 
 mastered by rats, birds, and monkeys. The learning process may 
 be described, externally, by saying that aJdapl of natural selection 
 takes place among_the ; djfferent ^ jeactipns, the successful ones being 
 keptlfficl the unsuccessful eliminated. 
 
 A second form of experiment, instead of providing rewards for 
 well-doing, provides only punishment for ill-doing. The training 
 
 1 Yerkes, Pop. Science Monthly, 1901, LVIII, 519. 
 
 2 Yerkes, Harvard Psychological Studies, 1903, I, 579. 
 
 3 Thorndike, Amer. Naturalist, 1899, XXXIII, 923; Triplett, Amer. Journ. 
 of Psychol., 1901, XII, 354. 
 
 4 Thorndike, Psychol. Rev., 1899, VI, 282; Porter, Amer. Journ. of Psychol., 
 1904, XV, 313 and 1906, XVII, 248. 
 
 6 Thorndike, Psychol. Rev., Monogr. Suppl. No. XV, 1901; Kinnaman, Amer. 
 Journ. of Psychol., 1902, XIII, 98, 173; Haggerty, Journ. of Comp. Neurol. and 
 Psychol, 1909, XIX, 337. 
 
 6 Small, Amer. Journ. of Psychol., 1899, XI, 133, and 1900, XII, 206; Watson, 
 Animal Education (Chicago, 1903); and Psychol. Rev., Monogr. Suppl. No. 
 XXXIII, 1907. 
 
 7 Yerkes, The Dancing Mouse (New York, 1907). 
 
 8 Thorndike, Psychol. Rev., Monogr. Suppl. No. VIII, 1898. 
 
 9 Cole, Journ. of Comp. Neurol. and Psychol., 1907, XVII, 211. 
 
548 MEMORY AND THE PROCESS OF LEARNING 
 
 of domestic animals furnishes many rough experiments of this sort, 
 and the results seem to show that punishment is an effective means 
 of selection. More precise experiments have yielded the same re- 
 sult. The young chigk, after once pecking at a bee, avoids bees in 
 the future; 1 mice quickly learn to avoid a spot where they receive 
 an electric shock, 2 and a pike 3 or perch, 4 which instinctively snaps 
 at minnows, learns not to do so if he is separated from them by an 
 
 FIG. 147. Curve of Learning (Thorndike). The gradual descent of the line represents the 
 decrease in time occupied by a cat to escape from a cage. The association here learned 
 (escaping by turning a button) was comparatively difficult for the cat. 
 
 invisible glass partition against which he bumps his nose at every 
 attempt; after some weeks of this experience, the large fish can safely 
 be allowed to swim freely among the little ones. 
 
 The thir^Jorm of experiment which has been frequently em- 
 ployed is like the maze test in offering rewards, but differs from it 
 in requiring not simply locomotion on the part of the animal, but 
 the operation of some simple mechanical device. In Thornolike's 
 experiments, a cat was~placed in a cage^from which escape was pos- 
 sible by pulling a string, or turning a button, or pressing a lever, etc. 
 To ensure an energetic attempt to escape, the cat was taken when 
 hungry, and food was placed outside the cage and in plain sight of 
 the animal. Under these conditions, the animal attacked the sides 
 of the cage vigorously, trying to squeeze between the bars, biting 
 at them, clawing at anything loose, and, in the course of these in- 
 stinctive acts, was pretty sure to turn the button or pull the string, 
 
 1 Lloyd Morgan, Habit and Instinct, p. 53 (London, 1896). 
 8 Yerkes, The Dancing Mouse. 
 
 3 K. Mobius, Die Bewegungen der Tiere und ihre psychischer Horizont, 1873. 
 
 4 Triplett, op. cit. 
 
LEARNING AMONG VERTEBRATES 
 
 549 
 
 
 which opened the door and gave access to the food. On frequent 
 repetition of this experiment, the useless movements gradually de- 
 creased in number, till finally all were eliminated, and only the one 
 successful movement was continued. 
 
 In the experiment just narrated, the time taken by the animal 
 to escape from the cage was noted, on each trial, and the decrease 
 in this time indicated the progress of the animal in learning. An 
 association is established in 
 such cases between the situa- 
 tion of being in a cage while 
 hungry and with food in sight 
 outside, and a particular re- 
 sponse which brings success. 
 This successful resgojise, to 
 use Thorndike's terms, he- 
 comes gradjially " stamped 
 in," while the unsuccessful 
 responses are "staiApSTout." 
 The process of " stamping-in " 
 and "stamping-out" is in 
 some cases rapid, in others 
 very gradual; while in still 
 others, little or no progress 
 is apparent in a long series of 
 trials. The "curve of learn- 
 ing," obtained by plotting the 
 times of the successive trials, 
 gives a graphic view of the 
 progress in formation of the 
 association. The 
 
 FIG. 148. Curve of Learning (Thorndike). The 
 same cat in a performance easier to learn. 
 Here a loop attached to a string and hang- 
 ing in the cage on the side toward the food 
 was to be pulled. The act of pulling the 
 string was, apparently, more a definite unit 
 which would impress itself in connection with 
 the resulting opening of the door; whereas the 
 turning of the button was apt to occur in 
 the midst of random clawing, and so to have 
 little individuality. 
 
 this progress differs with the kind of animal, with the difficulty of 
 the performance, and with the previous experiene of the in3ivio!ual 
 tested. A performance which is learned by a cat only in the course 
 of many trials may be learned by a monkey in few trials; while a 
 performance which requires the monkey many trials may be prac- 
 tically beyond the powers of the cat. The single door-button is 
 slowly learned by a cat, but quickly by a monkey. A combina- 
 tion lock, first used by Kinnaman, in which four fastenings have to 
 be undone in a certain order, was found too hard for cats, but was 
 mastered by monkeys after many trials; an adult man usually mas- 
 ters it in from one to three trials. The influence of the previous 
 experience of the individual animal is brought out by requiring the 
 animal to master one puzzle after another. When the same sort 
 of fastening is employed in two cages, but in different parts of the 
 
550 MEMORY AND THE PROCESS OF LEARNING 
 
 two, learning to operate it in the first cage often hastens the progress 
 of learning to operate it in the second. This means, therefore, that 
 experience adds new(or more highly specialized) mooles of reaction 
 to me^ instjnc^ive store with which the animal begins. 
 
 6. Any attempt at interpreting the process of learning in ani- 
 mals must involve a consideration of some topics which are more 
 germane to the next chapter than to the present. For, as a matter 
 of fact, the formation o associations cannot be understood without 
 reference to processes of discrimination and atterTtTotf. With the 
 quesHon of the limits of animal^TCa*son we are noFhere specially 
 concerned, our object being rather that of understanding the sim- 
 pler mental processes in general. In the experiments already de- 
 scribed, the method by which the animal learns to master a maze or 
 a puzzle-box has been called learning by " trial and error." We pre- 
 fer to call it, "learning by varied reaction through selection of the 
 successful variants." Without variation of reaction, the cat would 
 continue trying to squeeze between the bars toward the food, jus,t 
 as iron filings tend along fixed lines of force toward a magnet from 
 which, perhaps, they are separated by a sheet of paper. On the 
 other hand, without some sort of selection from among the varied 
 reactions, no progressive shortening of the whole time of reaction 
 would occur. 
 
 The principal problems which arise in the interpretation of this 
 sort of learning concern (1) the source of the varied reactions and 
 the manner in which they are called into activity; and (2) the 
 mechanism of selection. In regard to the source of the variations, 
 some are instinctive and others are derived from previous experi- 
 ence. But not all of previous experience is utilized; and sometimes 
 previously learned reactions which would fit the new situation ad- 
 mirably are not called into play at all. The problem here is one 
 which has passed under the name of the "transference" of the effects 
 of training in one situation to another situation. This problem 
 again fereaks up into two : namely, (1) as to how far reactions 
 learned in one situation can be directly applied to another; and (2) 
 as to how reactions which might well be applied are actually re- 
 called by the new situation. In regard to the first point, we ob- 
 serve that there is often something specific about reactions learned 
 in a given situation which unfits them for direct application in an- 
 other situation, even though the two situations may seem, externally 
 regarded, to be essentially the same. The particular combination 
 of movements, for example, which has become stereotyped in oper- 
 ating a fastening in one cage may need some modification to work 
 well in another cage; and therefore the latter may need to be mas- 
 tered afresh. Granted, even, that a mode of reaction has been 
 
PROCESS OF SELECTION IN LEARNING 551 
 
 learned which is directly applicable to the new situation, it by no 
 means follows that it will be called into play; for, since the new situa- 
 tion differs in some respects from the old, the proper reaction de- 
 pends on the chance that the similar features of the two situations 
 shall control the process of its recall. /In every new situation, how- 
 ever, there are many features competing for prominence; so that 
 the familiar feature has only a limited chance to become dominant./ 
 
 The nature of the process of selection, by which the successful 
 variants of reaction are strengthened from trial to trial, and the un- 
 successful eliminated, is indicated to some extent by the curves of 
 learning (see pp.548 f.). Where the descent of these curves is gradual, 
 the process of selection must itself be gradual; but where the curves 
 show a sudden drop from a high to a low level, the process of selec- 
 tion must be correspondingly abrupt. These sudden drops, when 
 (as in human learning) introspection comes to our aid in explain- 
 ing them, are found to be due to a perception of certain facts about 
 the situation, such as the uselessness of some reactions or the value 
 of others. This perception is often a clearly defined event in the 
 individual's experience, and its results are "stamped in" once for 
 all. The absence of sudden drops in many curves of learning, even 
 of those which describe graphically the attainment of a high degree 
 of skill in manipulation, in man's case, is an evidence of the lack 
 of moments of clear perception. In such cases, selection must be 
 possible in some more gradual and mechanical way; but this gradual 
 selection must be highly important and perhaps even fundamental ' 
 in all learning. The following attempt at an interpretation of the 
 process of selection in learning by trial andTerror should be regarded 
 only as tentative, though combined of elements which are fairly 
 sure to be genuine. ^ 
 
 - 7. In the first place, we must assume in the animal an adjustment 
 or determination of the psycho-physical mechanism toward a cer- 
 tain end. The animal desires, as we like to say, to get out and to 
 reach the food. Whatever be his consciousness, his behavior 
 shows that he is, as an organism, set in that direction. This adjust- 
 ment persists till the motor reaction is consummated; it is the driv- 
 ing force in the unremitting efforts of the animal to attain the desired 
 end. His reactions are, therefore, the joint result of the adjustment 
 and of stimuli from various features of the cage. EacL^single re- 
 action tends to become associated with the adjustment. But the 
 unsuccessful reactions are less strongly associated than the success- 
 ful, because each one of the former is at some moment given up 
 or inhibited; and this inhibition, too, being made under the influence 
 of the adjustment, tends to become associated with it, and so to in- 
 terfere with the association between the adjustment and the per- 
 
552 MEMORY AND THE PROCESS OF LEARNING 
 
 formance of this particular reaction. In the case of the successful 
 reaction, however, the phase of inhibition does not occur, and the 
 only association with the adjustment is of the positive sort. /Thus 
 the successful reactions must, in the long run, gain an advantage 
 
 ver the unsuccessful. 
 
 The preceding explanation, although satisfactory as far as it 
 goes, is not fully adequate to account for all the facts. In particular, 
 it does not take full account of the pleasure accompanying success, 
 and of the often strong displeasure that attends baffled effort. Such 
 pleasure and displeasure certainly occurjn humanjgajrning, and_seem 
 present" in thft fofohpr animals. Exactly how these emotions act to 
 strengthen one association and to weaken or counteract another 
 cannot readily be seen ; but it is safe to assume that they correspond 
 to some genuine dynamic process of great efficacy. The displeasure 
 of failure and baffled effort, in particular, must be the indication 
 of a stronger inhibitory process than is provided for in the preceding 
 explanation. ^ 
 
 8. Where the curve of learning, instead of descending slowly, 
 passes after the first few trials from long to short times where, that 
 is to say, the elimination of unsuccessful reactions occurs suddenly, 
 and the right response is quickly associated with the situation 
 there, we may assume, something like a clear perception of the 
 right reaction has intervened. But, in the case of the lower ani- 
 mals and even in man's case, too much meaning must not be 
 read into the word "perception." The required mental act need 
 not involve any insight into the reason why one reaction is successful 
 and the other unsuccessful; there need not be any moment of reflect- 
 ive judgment, such as might be expressed in the words, " This is the 
 way," or " That is not the way." All that isjificessary is_that some 
 feature of the situation should jDrgxail in consciousness, ancj thaTthe 
 reaction to it should have aj^ertain separatenesTrrom_the total series 
 of changing reactions. This distinctness of a certain feature of the 
 situation, and of the reaction to it, probably indicates a high in- 
 tensity of the correlated neural process, and so a favorable condition 
 for a strong association. 
 
 To illustrate this quick learning, and the lack of real insight that 
 may accompany it, we will make brief mention of a hitherto unpub- 
 lished experiment 1 on a chimpanzee a species which, to judge by 
 cerebral development, stands^ considerably higher than the smaller 
 monkeys (compare p. 34). The specimen tested was a young 
 female, about half grown, and corresponding in relative maturity, 
 perhaps, to a child of the human species of ten or twelve years. 
 
 1 The experiment was performed by R. S. Woodworth in 1902-03, in the 
 laboratory of Professor Sherrington at Liverpool. 
 
LEARNING BY TRIAL AND ERROR 553 
 
 A box was prepared, having a slatted front with a door closed by a 
 button, a turn of which through 90 released the door. The chim- 
 panzee, on being placed in front of this box, in which a piece of 
 banana had been placed before her eyes, quickly came to devote 
 most of her efforts to the door (which allowed of some slight mo- 
 tion even with the button closed) pulling it outward, pushing it 
 inward, and shaking it. She soon, also, attacked the button, and 
 alternated, for the most part, between this and the door. In this 
 way, it was not long before she turned the button through 90, 
 then tried the door, and got in, thus securing the food. On a second 
 trial, the chimpanzee worked almost entirely at the door and the 
 button; and from the third trial on, her reason was uniformly 
 prompt and correct. After several more trials, a second button was 
 added a few inches from the first, but much like the first in appear- 
 ance, and operated in the same manner. The chimpanzee at- 
 tacked the box as before, neglecting the second button. After 
 once turning the first button, and pulling the door, which, of course, 
 did not yield, she turned the first button again, so locking the door; 
 then again tried the door, and continued in this way for a long time, 
 before passing to the second button and dealing similarly with it. 
 Entrance was finally secured by a chance placing of both buttons 
 at once in the right position. In the course of several trials, no 
 further progress was made. It seemed to be wholly a matter of 
 chance whether both buttons should be put right at once or not. 
 The experiment showed then a prompt narrowing down of the field 
 of effort to the right feature of the situation; but this important 
 factor in the process of learning seemed to be accompanied by a 
 complete absence of insight into the mechanical principle involved. 
 
 In such experiments, one of the features of a situation which most 
 readily becomes prominent is the place where the successful issue 
 occurs. Often, after a few trials, effort will be concentrated at 
 the right place, though many useless movements are made there. 
 
 9. A contrast is usually drawn between learning by trial and 
 error and learning by ideas, the latter being clearly present in 
 much human learning, but only doubtfully present in the learning 
 of animals. "Ideas," if available, might be of service in several 
 ways. First, an idea called up by some feature of a present situa- 
 tion might suggest some reaction which would not be directly sug- 
 gested by the situation itself, and so might enlarge the range of the 
 varied reactions, and afford greater opportunity for success. There 
 is some evidence, 1 not wholly, conclusive, that ideas may function 
 in this way in some animals. Second, where a reaction has been pre- 
 viously tried and found unsuccessful, it might be mentally rehearsed 
 1 Thorndike, Cole, op. cit. 
 
554 MEMORY AND THE PROCESS OF LEARNING 
 
 without the actual motor performance. Such mental rehearsal of a 
 reaction certainly occurs in human behavior, and possibly in the 
 monkeys, which seem at times to inhibit movement as if in thought; 
 but there is little evidence of it in dogs and cats, whose motor 
 activity is perhaps too prompt and direct to permit of the neces- 
 sary inhibition. 1 A third service of ideas might be the following: 
 the successful reaction and its result might similarly be rehearsed, 
 and thus practice in dealing with a situation might be obtained in 
 the absence of the actual situation. This use of ideas occurs fre- 
 quently in man, but in animals there is no clear evidence of it. 
 Fourth, by a combination of the first with the second or third of 
 the above uses of ideas, some feature of the present situation might 
 suggest a reaction learned in previous experience; the consequences 
 of this reaction might be mentally rehearsed, and its probable suc- 
 cess or failure in the present situation judged without actual trial. 
 This would be equivalent to "thinking out" the solution of a pres- 
 ent difficulty without, or before, actually trying it. So complex a 
 use of ideas, while perfectly within human capacity, occurs seldom 
 even in man, in cases similar to the maze or puzzle-box experiments; 
 for the prolonged suspension of motor reactions which it requires 
 is disagreeable to most men especially if the situation permits of 
 immediate reaction. 2 Fifth, the ideas employed might have the 
 character of general principles, from which the necessities of the 
 present case could be deduced, and so the situation be thoroughly 
 and surely mastered in advance of motor reaction. Such a use of 
 ideas occurs in specially trained human individuals within the range 
 of their specialty; but otherwise is probably rare, at least in any 
 complete form. 
 
 10. The differences between different animals, and between man 
 and the animals, in the power of learning, are not fully accounted 
 for by different degrees of the use of ideas. Greater fertility in 
 association must also be allowed for in the human being. That dis- 
 crimination favors association has been noted above, in saying that 
 any feature of a situation or of the reaction to a situation, which 
 stands out with a degree of separateness, has an especially good 
 chance of becoming associated strongly with the adjustment to 
 deal with the situation. That association favors discrimination has 
 also been suggested by the fact that a previously familiar feature 
 is likely to be singled out for present reaction. Now one of the 
 differences between different species, and also between different 
 
 1 But see Hamilton, "An Experimental Study of an Unusual Type of Reaction 
 in a Dog," Journ, of Comp. N enrol, and Psychol, 1907, XVII, 329. 
 
 2 See Ruger, "The Psychology of Efficiency," Archives of Psychology, No. 
 XV, 1910, p. 9. 
 
LEARNING BY IDEALS 555 
 
 human individuals, lies in their different powers of discrimination. 
 Greater power of analysis or discrimination belongs to man. The 
 terms, as heftrTTsect,"" Imply reacting to^slomenFeature of a situation 
 isolated from the total. Features which an animal cannot isolate 
 are easily isolated by man. For example, in dealing with Kinna- 
 man's combination lock (p. 549), which requires several fastenings 
 to be undone in a fixed order, the cat seems wholly lost, and the 
 monkey succeeds only with difficulty, not because they cannot 
 master the separate fastenings, but because they do not take these 
 in proper sequence; but man quickly observes that a certain order 
 is necessary in dealing with the parts of a similar mechanism. 
 Whether he thinks of an interlocking of hidden parts, or does not 
 clearly formulate his conception in any way, that relation of parts 
 or of movements, which we have designated by the word " order," 
 is a feature which his powers, dependent largely on his past ex- 
 perience, enable him to single out and make the basis of his reac- 
 tions. Man, therefore, reacts to relations which the animal passes 
 by; and much of his superiority in learning to deal with complicated 
 situations is due to this power. 
 
 {/ 11. Human behavior in situations resembling those employed 
 ifi experimenting on animals has been studied by several investi- 
 gators. The solution of mechanical puzzles 1 is a task similar in 
 principle to that presented to the animal in a puzzle box, but of a 
 more difficult order. The behavior of a man on his first attempt 
 to solve such a puzzle often resembles closely that of the animals. 
 Under the influence of an adjustment to solve the puzzle, the man 
 usually begins promptly to manipulate, according as parts of the 
 device catch his eye and lead to instinctive or previously learned 
 reactions. This random procedure may continue till success is 
 accidentally reached; or, continued failure may lead to a period 
 of inhibition of movement, of closer examination of the puzzle and 
 a deliberate attempt to " think it out." This attempt is not always 
 successful, and random manipulation is again resorted to. In 
 the human being, however, the randomness of reaction is likely to 
 be limited by certain conceptions as to the probable line of success- 
 ful effort; and though these conceptions are sometimes wrong and 
 a hindrance to prompt success, at other times they are of great value. 
 The first successful issue comes about, accordingly, in various ways, 
 sometimes by a pure accident which remains a mystery to the per- 
 former; sometimes by an accident which is nevertheless observed 
 and partially understood; sometimes as the outcome of consciously 
 trying whether a certain manipulation will succeed; and sometimes 
 as the result of clear insight into the necessities of the case. The 
 
 1 Ruger, op. cit. 
 
556 MEMORY AND THE PROCESS OF LEARNING 
 
 more there is of conscious prevision of the successful movement, 
 the more certainly will the first success lead to prompt and correct 
 reaction in all subsequent trials. 
 
 Suppose, now, that the solution of the same puzzle is undertaken 
 time after time, with a constant effort to increase the speed. A curve 
 of learning then appears, which may vary in form between extremes 
 such as are represented in the two curves for animals shown above. 
 In general, gradual improvement is visible in the midst of many 
 oscillations. As soon as the main path to success is well known, 
 new difficulties of manipulation emerge, and each of these becomes 
 a problem, which is likely to be solved in any one of the variety 
 of ways which appear in the main solution. Decrease in the total 
 time of manipulation occurs sometimes gradually, from the increas- 
 ing firmness of association between the parts of the performance, 
 and sometimes suddenly, from the adoption of a "short-cut" or 
 better form of manipulation. The steps in the manipulation, 
 which in the early stages of practice are separate acts and require 
 separate attention, come later to be combined into larger units. 
 It thus becomes^Dgssible to keep the attention ahead of the hands, 
 and to prepare ;HeBtally for each step before the time for it arrives. 
 In this way tlmPIs saved in passing from one step to another, and 
 the whole process gains in speed and smoothness. 
 
 12. Much more complex than the solution of a mechanical 
 puzzle are some of the performances which have been made the 
 material for experiments on practice. Bryan and Harter 1 have 
 studied the process of mastering telegraphy, and Book 2 the acqui- 
 sition of skill in typewriting. In contrast with the maze or puzzle, 
 which can sometimes be mastered in a hundred trials though it 
 offers a surprising amount of opportunity for continued improve- 
 ment in matters of detail telegraphy and typewriting require 
 years of practice before the highest expert stage of skill is reached. 
 Progress consists in the formation of numerous specific but inter- 
 related habits, some of which are formed early in the process of 
 training, and may be called habits of the lower order, whereas 
 others are superimposed on these first, and may be called habits of 
 the higher order. All in all, as Bryan and Harter state the case, 
 mastery of such an art consists in the formation of a hierarchy of 
 habits. In learning to telegraph, the first step is to acquire some 
 familiarity with the alphabet of dots and dashes. The beginner, 
 in sending a message, laboriously spells out each word letter by 
 letter. He learns to make a particular combination of dots and 
 
 1 Psycfwl. Rev., 1897, IV, 27, and 1899, VI, 345. 
 
 2 "The Psychology of Skill," University of Montana Publications in Psychol- 
 ogy, 1908. 
 
ACQUIREMENT OF SKILL 
 
 557 
 
 dashes in response to each letter of his "copy"; and the associa- 
 tion so formed between the several letters and the appropriate finger 
 movements constitutes his habits of the lowest order. Soon, how- 
 ever, he comes to run together the letters of familiar words, thinking 
 rather of the rhythm of the whole word than of the patterns of the 
 constituent letters. As his skill increases, he adds more and more 
 word-units to his repertoire; and these word-units constitute his 
 habits of the second order. He does not stop here, however, but 
 
 Fia. 149. Curves Showing the Improvement in Sending and Receiving Telegraphic 
 Messages (Bryan and Harter). 
 
 begins to train his fingers to the rhythm of phrases and other fre- 
 quently recurring combinations of words. The progress of improve- 
 ment in sending telegraphic messages is shown in the upper curve 
 of Fig. 149, in which a rise indicates improvement. The main 
 characteristic of the practice curve, as here illustrated, is the rapid 
 rate of improvement at the beginning, and the gradual slackening 
 of improvement as the practice continues, till the "physiological 
 limit" for this particular performance is reached, and no further 
 improvement is possible. 
 
 The curve of improvement in receiving messages over the wire 
 is more complex, and the whole process of receiving is more inter- 
 esting to study. At first, greater speed is possible in sending than 
 in receiving, and this difference continues for a long period. In 
 sending, the operator has matters in his own hands; he can adjust 
 his rate to the difficulties of the copy, and he can see ahead. In re- 
 
558 MEMORY AND THE PROCESS OF LEARNING 
 
 ceiving, he has none of these advantages. In spite of this early ad- 
 vantage of sending over receiving, in the long run the speed of re- 
 ceiving surpasses that of sending, so that an expert can receive 
 faster than either he or any other man can move his fingers. Habits, 
 or units of perception, of different orders are formed in learning 
 to receive much as in learning to send. The beginner must atten- 
 tively isolate the letters from the series of clicks which he hears; 
 later he becomes able to catch the pattern of frequent combinations 
 of letters and of common words; still later, he apprehends phrases 
 and even short sentences as wholes; while "the real expert has all 
 the details of the language with such automatic perfection that he 
 gives them practically no attention at all. He can, therefore, give 
 his attention freely to the sense of the passage; or, if the message is 
 sent accurately and distinctly, he can transcribe it upon the type- 
 writer while his mind is running on things wholly apart. . . . 
 The feat of the expert receiver for example, of the receiver of 
 press despatches is more remarkable than is generally supposed. 
 . . . To bring the sender's rate up to that of the receiver abbrevi- 
 ated codes have been prepared. The receiver must translate the 
 code into English words, and transcribe these, correctly capital- 
 ized and punctuated, upon the typewriter. He takes, in this way, 
 eighty or eighty-five words a minute. If mistakes are made by the 
 sender, the receiver is expected to correct them as they come, and 
 send a clean copy to press. The work continues for hours without 
 leisure for rereading, the pages being taken away to press as fast 
 as they are finished." 1 
 
 13. For an understanding of the psycho-physical mechanism 
 of learning, and so of the processes of association and discrimina- 
 tion involved in learning, it is important to attempt an analysis of 
 such feats of skill as those described above. An essential feature of 
 the performance of transcribing a telegraphic message, is that the 
 skilled receiver keeps several words behind the message as he hears 
 it. The expert learns not to guess ahead, or anticipate what is 
 coming, for in this way false readings are sure to occur. Yet he does 
 not by any means spell out the letters as they come. He seems 
 rather to let the pattern of the clicks " soak in " for a time, gradually 
 interpreting those that come earlier in the light of those which fol- 
 low. For this purpose, the expert, in receiving a connected de- 
 spatch, "prefers to keep from six to ten or twelve words behind the 
 instrument." This gives an index of the size of the units in which 
 he thinks. 
 
 The process of acquiring skill in typewriting has been even more 
 minutely examined by Book, who employed in his study a combina- 
 1 Bryan and Barter, Psychol. Rev., 1899, VI, 352. 
 
ANALYSIS OF FEATS OF SKILL 559 
 
 tion of objective records and introspective observations. The gen- 
 eral course of practice is the same as in telegraphy, and the same 
 sort of hierarchy of habits appears. Many habits are only tempora- 
 rily formed and used, but are later supplanted by the development 
 of other habits better adapted to a higher degree of skill in writing. 
 Thus, when the beginner is learning by the "touch method," and 
 the keyboard is concealed by a screen, the first associations formed 
 are between the letters and their place on a plan of the keyboard, 
 which may be held in mind as a visual image. Later, this image 
 or plan disappears so completely that a skilful performer may be 
 unable to tell the location of the letters on the keyboard without 
 actually writing and finding out where his fingers automatically 
 place themselves. In such cases the plan has already become un- 
 necessary, because of the direct associations which have become 
 established between the letters and the appropriate movements. 
 When such associations are well learned, and even before they are 
 perfected, frequently occurring sequences of letters begin to be 
 associated with sequences of finger movements, so as to constitute 
 those units of motor reaction to which reference has already been 
 made. Prefixes and suffixes, short words, longer words, and even 
 phrases of some length, become in this way reduced to specific^ 
 motor habits, which stand ready to command. 
 
 Since, however, the possible sequences of words are much too 
 numerous ever to be all reduced to automatic habits, the procedure 
 of the finished expert does not consist simply in writing in large 
 units, but rather in utilizing letter associations, word associations, 
 and phrase associations, according to the exigencies of the copy. 
 This the expert accomplishes while still maintaining a wholly un- 
 broken series of finger movements at the maximum rate; no "units," 
 higher or lower, can be detected in the rhythm of his motor reactions. 
 The means by which this result is accomplished consists in keeping 
 the eyes, on the copy, several words ahead of the fingers. What are 
 the marvellous internal, or psycho-physical processes (mind and 
 brain) which occur during the interval between the reading of the 
 copy and the writing of it? An indication of the answer to this 
 question is afforded by the fact 1 that when some special difficulty 
 appears in the copy, the eyes are slackened in their advance, and the 
 hands partially overtake them. One might have supposed that 
 the hands would slacken, and the eyes get still farther ahead, but 
 the contrary is the case; and, indeed, the hands do not need to 
 slacken, for by the time they have reached the difficult spot, the 
 way has been made ready for them by special attention devoted 
 to the details of the coming movements. There occurs, then, in 
 
 1 Book, op. cit., p. 44. 
 
560 MEMORY AND THE PROCESS OF LEARNING 
 
 this important interval between the reading and the writing of a 
 word, an organization of the coming reaction, in which such of the 
 various ready-formed phrase associations, word associations, and, 
 if need be, letter associations, are utilized as fit the requirements 
 of the case. The entire procedure is an elaborate process of re- 
 call of many associations, and of appropriate combination of these 
 associations. 
 
 14. The foregoing studies of typewriting, telegraphy, and the 
 manipulation of mechanical puzzles serve admirably to throw into 
 relief the importance, for skilled and efficient action, of two princi- 
 ples which were found necessary to a satisfactory theory of percep- 
 tion, and which were mentioned in a previous chapter (p. 498): 
 these are (1) the principle of higher units, 1 and (2) the principle of 
 overlapping. Overlapping of different reactions is clearly in evi- 
 dence when the reading of the copy precedes by several words the 
 writing, or when the hearing of the telegraphic clicks precedes by 
 several words the full interpretation and transcription of the message. 
 Such overlapping is a result of practice; the "span of attention," 
 it may be said, is lengthened by practice. But the increase in the 
 attention span does not mean, mainly, that a large number of dis- 
 tinct items are held at once in consciousness; the meaning is, the 
 rather, that these items are now grasped in larger units, such as 
 words and phrases. In a sense it is true that the expert at the type- 
 writer carries in mind twenty or thirty letters at once, and that the 
 expert telegrapher, in receiving, carries in mind over two hundred 
 clicks; for the total reaction in progress at any moment covers this 
 large number of elements. But the letters or clicks do not remain 
 separate in the performer's mind; they take the form of "units," 
 consisting of words, phrases, etc. The lengthened span of atten- 
 tion, and the extensive overlapping of processes, are the result of 
 the higher units in which the reaction is carried on. 
 
 15. The rate at which improvement progresses with practice is 
 seldom steady. Usually the curve of learning shows temporary 
 descents in the midst of the general rise. Many brief relapses can 
 be explained by poor bodily condition, or by distractions and fail- 
 ures of interest and effort. But in some of these curves there ap- 
 pear longer periods of no appreciable gain, which can scarcely be 
 explained in this way. For weeks, it may be, the learner remains at 
 
 1 A similar demonstration of the change from lower to higher units of reaction 
 has been obtained by Leuba (Psychol. Rev., 1905, VII, 351) in a much simpler 
 performance, which consisted in transliterating German script into English 
 script, and vice versa a key of equivalents being at hand for consulta- 
 tion. At first the German letters were read and transliterated one by one, but 
 later words were reacted to as units. 
 
THE RATE OF IMPROVEMENT 561 
 
 a standstill, and seems to have reached the limit of his capacity. 
 And then, if he perseveres, a sudden advance may carry him to 
 quite a new level of efficiency. He apparently begins a fresh ascend- 
 ing curve on the top of his old one. Such a compound curve is seen 
 in Fig. 149 (the curve for receiving by telegraph). A period of no 
 gain, followed by rapid improvement, has been named the plateau, 
 and the study of the process of learning raises the question, whether 
 this "plateau" is a universal and necessary feature of the practice 
 curve. It can hardly be called universal, for it does not clearly 
 appear in many curves; as, for example, in the sending curve in 
 telegraphy, and in most of the animal curves. In practice with 
 puzzles, however, the plateau is a frequent occurrence, and its ex- 
 ternal explanation is usually to be found in the adoption of some new 
 and improved method, after a prolonged period of practice with in- 
 ferior methods. In the case of the long-protracted plateaus which 
 appear in typewriting and telegraphy, change of method is again the 
 explanation of the rise from the plateau. The plateau occurs where 
 
 ) further improvement is impossible without some change of method. 
 In typewriting and telegraphy, the required change consists in the 
 substitution of words and phrases for letters as the units of perform- 
 ance. In typewriting Book found two plateaus, one at the transi- 
 tion from letter-reactions to word-reactions, and another at the tran- 
 sition from word-reactions to phrase-reactions. But why should 
 not these transitions occur promptly, without the long period of 
 no apparent gain? Bryan and Harter believe that real gain is 
 being made, below the surface, in the further perfection of the more 
 
 Elementary reactions; and, in support of this view, they are able to 
 show, in the case of receiving telegraphic messages, that ability to 
 receive isolated letters and detached words does slowly improve 
 during the plateau. Until, therefore, these simpler forms of reac- 
 tion have reached a high degree of facility and promptness, the higher 
 reactions are impracticable. Swift, 1 who has investigated the proc- 
 ess of learning in a variety of activities, including the learning of a 
 language, subscribes in the main to this view of Bryan and Harter, 
 but shows that the higher forms of reaction do not all wait till the 
 end of the plateau, for some of them are present from an early stage. 
 
 i Book's observations lead him to a somewhat different conclusion. 
 He found that the beginning of a plateau was marked, introspec- 
 tively, by some loss of interest in the work, and by a tendency of 
 the mind to wander. A little later, the learner becomes aware of 
 his lack of progress, and may increase his efforts; but at this par- 
 ticular time his efforts are likely to be ill directed and result in no 
 improvement. What he needs is attention to the details of work- 
 1 Mind in the Making, p. 208 (New York, 1908). 
 
562 MEMORY AND THE PROCESS OF LEARNING 
 
 manship; what he is apt to strive for is speed or higher methods 
 which are at that time not wholly within his grasp and only dimly 
 understood. The loss of interest which initiates the plateau is 
 easily explained by the comparative facility which has been achieved 
 in a certain lower form of reaction. Attention is set free from many 
 of the details which have hitherto held it. What is needed then, 
 from the introspective point of view, is to find new objects of at- 
 tention, and new modes of reaction which shall nevertheless lie 
 w r ithin the learner's powers at the stage which he has reached. 
 Specially ingenious individuals may, as Book found, 1 come quickly 
 upon such modes of reaction, and avoid the plateau altogether. 
 Book concludes that the plateau is actually, as well as apparently, 
 a period of no progress; but this conclusion does not take account 
 of Bryan and Barter's demonstration of progress in the lower habits 
 during this period, nor does it explain how the plateau is ever left 
 behind. The probability is that continued practice does perfect 
 elementary reactions to some extent, and that it thus becomes easier 
 to enter upon the more inclusive reactions. 
 
 It seems to us, however, that all explanations of the process of 
 learning, and, especially in such cases as we are now considering, 
 of the occurrence of the so-called "plateau," which do not concen- 
 trate attention upon what is probably going on in the mechanism 
 of the central nervous system, are surely destined to prove unsatis- 
 factory. Perhaps it would express the truth better to say, that all 
 explanations are satisfactory only in so far as they involve certain 
 facts and laws of the central nervous mechanism. It is plain that 
 as the overlapping of the psycho-physical processes proceeds, and the 
 accompanying higher and higher psycho-physical units take place, 
 more numerous and more locally distant cerebral elements are be- 
 coming involved. This fact implies, of necessity, more numerous 
 chances of inhibitory processes, and also increased difficulty in 
 effecting the required co-ordination of these elements; or to speak 
 figuratively of establishing those coherent "dynamical associa- 
 tions" which are the psycho-physical basis of the higher unities. 
 That cerebral associated activities of this complicated and locally 
 diverse character, as respects the nervous elements taking part in 
 them, should advance by "fits and starts" rather than always at a 
 uniform pace, seems to us, therefore, to accord perfectly with the 
 nature of the central nervous mechanism itself. Indeed, we might 
 well venture to say, that such a way of development accords with 
 the nature of all living substance. The chemico-physical changes 
 in which the life of this substance consists seems to require this 
 irregular manner of behavior. 
 
 1 Op. cit., p. 20. 
 
RELATION OF CONSCIOUSNESS TO LEARNING 563 
 
 16. The relation of consciousness to the process of learning is 
 an important topic on which the conclusions of different investi- 
 gators are still somewhat at variance. Several questions can be 
 raised under this general head as to the participation of conscious- 
 ness, first, in the origination of new methods; second, in the selec- 
 tion of one method and the rejection of others; and last, in the later 
 stages of practice, when automatism has become the condition of 
 the acquired skill. 
 
 In regard to the first question, as to how consciousness is con- 
 cerned in the origination, invention, or " hitting-upon " of new meth- 
 ods, there is little doubt that this process is essentially the same as 
 the "varied reaction" observed in animals. Each new method 
 comes first as a spontaneous variation. But in human learning the 
 variation may occur in the form of a recalled " idea," or in the form 
 of a clear perception of some hitherto unnoticed feature of the situa- 
 tion. In other cases, however, the variation occurs first as a motor 
 reaction. In which of these several forms it shall occur depends on 
 the nature of the situation in relation to the past experience of the 
 learner. Where the difficulties encountered are similar to those 
 which have previously been met, there is likelihood that past ex- 
 periences or ideas derived therefrom will be consciously recalled. 
 But where the situation is very unusual, the contributions of past 
 experience are vague and not suited for definite ideational recall. 
 In such cases, the new variation has to be achieved in the thick of 
 the fight, so to say. Then the individual finds himself acting in 
 some new way, which he did not plan nor foresee, and which as yet 
 he does not fully understand. 
 
 Unusual situations and difficulties are often created by the process 
 of learning itself; for when once the learner has acquired a consid- 
 erable degree of skill, the difficulties which next confront him are 
 such as do not occur in an unskilled performance. Unusual pre- 
 cision of manipulation may be demanded, or overlapping and higher 
 units of reaction may be necessary for further advance. Past ex- 
 perience is not likely to provide modes of reaction, or ideas, which 
 are applicable to the case in hand. The learner may indeed have 
 previously acquired high skill in some other performance, such as 
 the use of language; but each new skilled performance must be pre- 
 cisely adapted to the particular material dealt with, and previous 
 experience can only furnish vague hints for guidance. This vague- 
 ness of relation of the present to the past situation not only makes it 
 difficult to apply past acquisitions, even if they are recalled, but it 
 also renders the recall of them doubtful. Consequently the new 
 variations which lead to the highest skill are likely to make their 
 appearance in the midst of actual performance and without the 
 
564 MEMORY AND THE PROCESS OF LEARNING 
 
 foreknowledge of the learner. This fact has sometimes been rather 
 ineptly expressed by saying that the new variations occur " uncon- 
 sciously." By this, it is by no means intended that they occur in a 
 condition of unconsciousness, or even of inattention; on the con- 
 trary, it is recognized that they are unlikely to occur at all except 
 in a condition of " rapt attention " to the work in hand. Nor are they 
 likely to occur in one part of the performance, when attention is 
 directed to another part; but they occur where attention is keenest, 
 and often where attention is striving to enlarge its span or to per- 
 fect its grasp. They occur, therefore, in the very focus of conscious- 
 ness, but are "unconscious" in the loose sense that they are un- 
 premeditated and not reflectively observed. They are not thought 
 about at the moment when they first occur; consciousness is too much 
 absorbed in doing the thing to classify it or speculate about it. 
 
 As to the second point namely, the participation of consciousness 
 in the selection and rejection of methods of reaction here, too, no 
 one general rule is applicable to all cases. Sometimes new varia* 
 tions of method, without being reflected on or definitely conceived, 
 become habitual and automatic. In such cases, the learner never 
 really knows how he has come to gain the skill. At other times, the 
 variation catches the eye of reflection, is definitely made note of and 
 perhaps formulated in words, and the learner can later tell by what 
 means he has improved. At times this reflective attitude proves to 
 be of value; for a definitely conceived and formulated reaction is 
 more readily recalled under somewhat changed conditions. 
 
 As to the third point, there is no doubt that attention tends to 
 desert a reaction which has become well-drilled and facile, and to \ 
 pass to something else; in illustration of this, the feats of skilled 
 performers on any instrument, while their minds are occupied with 
 something else, are well known. But the very highest skill is never * 
 manifested except when the performance is fully occupying the at- 
 tention. No matter how far the elementary reactions have become 
 automatic, there are always higher units and combinations which 
 demand the best efforts of the performer. Given, then, a constant 
 degree of zeal in the learner, the progress of practice does not lead 
 to a diminution of the consciousness attending the performance, 
 but to a change in the distribution of consciousness. The expert 
 finds within the work new objects of attention which lie entirely 
 beyond the ken of the beginner. 
 
 The changes of feeling which occur in the course of practice are 
 also noteworthy. 1 A new occupation, in which there is some de- 
 gree of immediate success, is interesting and pleasurable; as the 
 near-lying difficulties are overcome, a feeling of monotony and un- 
 
 1 Book, op. cit., p. 71. 
 
TRANSFERENCE OF LEARNING 565 
 
 pleasantness ensues; but as the possibilities of higher forms of re- 
 action open before the learner, the pleasure in the work returns, and 
 the keenest pleasure of all may be felt by the expert, if only there is 
 sufficient incentive to spur him to his best efforts. 
 -^flT? How far skill acquired in one performance can be " trans- 
 ferred" to another performance how far success in dealing with 
 one situation is an equipment for dealing with other situations 
 is a problem of evident importance, regarding which, however, 
 great differences of opinion have appeared. Some remarks on 
 this matter have been included in the preceding discussions (p. 
 550), and need not be repeated here. The history of the question 
 begins with Volkmann 1 and Fechner, 2 who reported, in 1858, ob- 
 servations tending to show that training one function might result 
 in the improvement of other closely related functions. In Volk- 
 mann's case, the function trained was the discrimination of two 
 points applied to the skin (the " two-point threshold," compare p. 
 402) of the left arm, and the other function thus improved was the 
 same discrimination when the points were applied to the right 
 arm. Also, training applied to one finger improved the power of 
 discrimination in the other fingers, but not in the arm. Fechner's 
 observations showed that teaching a boy to write with his right hand 
 might give him power to write with the left hand also. This special 
 form of the transfer of acquired skill, between symmetrical parts of 
 the body, received the name of "cross-education" from Scripture, 3 
 who, with his pupils, 4 demonstrated the reality of it in a variety of 
 cases, such as speed, force, and accuracy of movement. 
 
 The probable explanation of cross-education lies in the fact 
 (compare p. 263) that the same portion of the left hemisphere 
 (in right-handed persons) is concerned in skilled movements of 
 either hand, or in the skilled use of sensory data from either side. 
 If an essential part of the neural mechanism for co-ordinating a 
 skilled movement is the same, whether that movement is executed 
 by the right or by the left hand, then, naturally, training of either 
 hand should improve the other hand also. But since some of the 
 neural connections are different for the two hands, training of one 
 hand would still leave something to be accomplished by special 
 training of the other; and this deduction also corresponds to the 
 facts, for the untrained hand does not at once show all the skill of 
 the trained hand, but only some of it. 
 
 1 Berichte d.k.-sachs. Ges. d. Wissensch., math.-phys. KL, 1858, X, 38. 
 
 2 Ibid., 1858, X, 70. 
 
 3 Scripture, Smith and Brown, Studies from the Yale PsychoL Lab., 1894, 
 II, 115. 
 
 * Davis, ibid., 1898, VI, 6; Scripture, PsychoL Rev., 1899, VI, 165. 
 
566 MEMORY AND THE PROCESS OF LEARNING 
 
 This relatively simple case may contain the germ of an explana- 
 tion of other cases of transferred training. If the brain functioned 
 as a whole, we should expect that training in any special perform- 
 ance would operate equally to produce the improvement of all 
 other performances; if it functioned in compartments, correspond- 
 ing to "memory," "discrimination," or to other so-called "facul- 
 ties" or phrenological organs, then we should expect training of 
 any special performance within the scope of a faculty to benefit alike 
 all other performances included within that faculty. But since the 
 evidence points to a highly detailed localization of cerebral func- 
 tions, and since the neural mechanism employed in any performance 
 cannot be wholly identical with that required for slightly different 
 performances, though it may be partly the same, training in one 
 performance would not be expected to improve another, except 
 in so far as the neural mechanisms involved were in part identical 
 i. e., employed the same cells, fibres, and synapses. As applied 
 to psychology, this would mean that, in order for a transference of 
 skill to occur from one performance to another, there should be, 
 between the two, not simply likeness in the abstract, but some con- 
 crete part-performance in common, as there is between boxing and 
 fighting, or between saying " boot" and saying " book." In general, 
 since the neural process in any reaction undoubtedly has more de- 
 tail than appears either to introspection or to objective observation, 
 it will not always be possible to point out the common features of 
 two reactions; but that there should be features in common, if any 
 transference of training is possible from one to the other, seems 
 necessary from the physiological point of view. 
 
 18. But leaving these speculations for the present, let us return 
 to the investigations of fact. James 1 opened a new line of inquiry 
 by seeking to determine whether training the memory for one kind 
 of material increased the power of remembering other kinds. His 
 method was to practise memorizing the poetry of one author, and 
 to test beforehand and afterward the ability to retain another au- 
 thor. His results showed, on the whole, a lack of improvement of 
 the memory, except for the material on which it was trained. The 
 exceptions he believed could be explained by improved methods of 
 memorizing; he, therefore, concluded that retentiveness, as such, 
 was not susceptible of improvement by training. 
 
 The problem of memory training was taken up again with the 
 same general method, by Ebert and Meumann, 2 who made more ex- 
 tensive experiments, but came to a somewhat different conclusion. 
 These investigators trained their subjects in memorizing lists of 
 
 1 Principles of Psychology, 1890, I, 666. 
 8 Archiv /. d. ges. Psychol, 1905, IV, 1. 
 
TRANSFERENCE OF LEARNING 567 
 
 / 
 
 nonsense syllables, and, before and after this course of training, 
 took a " cross section " of the subjects' powers of memory by testing 
 them with lists of letters, numbers, disconnected words, vocabularies, 
 passages of prose and of poetry, and meaningless visual characters. 
 In all these tests they found great improvement, but much more 
 in some than in others. It seemed to them that the improvement 
 was great in proportion as the new material resembled the nonsense 
 syllables with which the subjects were trained; and they interpreted 
 the results to mean that memory, trained by use of one sort of mate- 
 rial, is thereby trained for other sorts; not however equally for all 
 sorts, but most for those sorts which are closely related to the ma- 
 terial used in the training. They, therefore, conceive of several 
 special "memories" in place of the old faculty of memory, and 
 within the scope of each of these sub-faculties they suppose that skill 
 is readily transferred from one performance to another. This is 
 all rather mysterious, and it is more to the point to note that many 
 improved methods of memorizing were invented by the subjects 
 during their training, and that some of these methods such as 
 giving a rhythmic form to a list were readily applied to different 
 sorts of material. Feelings of distaste, strain, and doubt gave way, 
 with continued experience and success, to pleasure and confidence 
 in the work. Distractions and useless reactions were better re- 
 pressed; the proper direction of attention was better understood; 
 and the subject came to know his own powers and limitations. 
 
 A similar line of experimentation, with similar but better-analyzed 
 results, was undertaken by Fracker. 1 He trained his subjects in 
 an unusual feat of memory, as follows : four sounds, differing in in- 
 tensity, were presented in varying orders, and the subject had to 
 notice the order of each presentation, and reproduce it, designating 
 each sound by a number; but before he was allowed to reproduce 
 one order, he was required to observe a second; and the second 
 tended to interfere seriously with the first and cause confusion. 
 In this novel situation, the subjects tried a variety of means to es- 
 cape from the difficulty, and most of them hit upon some successful 
 device. In some of the subsequent tests with other material, the 
 device so learned was readily applicable and these tests profited 
 by the transference of the device. But to other material, such as 
 poetry, these devices were not applicable; and here the tests showed 
 no improvement as the result of the special training in memorizing 
 the order of sounds. The author's main conclusion is that effective 
 memory-training consists in the development of methods of memo- 
 rizing. 
 
 Most of the studies of transference of acquired skill have suffered 
 1 Psychol. Rev., Monogr. Suppl. No. XXXVIII, 1908, p. 56. 
 
568 MEMORY AND THE PROCESS OF LEARNING 
 
 from an obvious defect of method, which lies in the fact that the 
 performances which are supposed to receive no direct training must 
 themselves nevertheless be tested, and so exercised. But even a 
 little practice in a novel performance may cause considerable im- 
 provement, and therefore the amount of special training received 
 by the "untrained" performances is by no means negligible. In 
 fact, Dearborn, 1 on repeating the preliminary and final tests of 
 Ebert and Meumann, without the intervening special training, 
 found a considerable improvement, and thus showed that the results 
 they obtained need to be considerably discounted. 
 
 19. It was a much-needed addition to the methods of investiga- 
 tion when Bair 2 tried the plan of following up the training of one 
 Eerformance by the training of another, to see whether the curve of 
 earning of the later-trained performance would show any effect from 
 the previous training. In his experiments he made use of a type- 
 writer, using only a part of the keys, which he covered with different 
 colors, while his "copy" consisted of series of color stimuli, to which 
 the subject reacted by striking the corresponding keys. After one 
 particular series of colors had been copied time after time till great 
 speed and accuracy were attained, the order of colors was changed 
 and a new learning curve obtained. The new performance showed 
 from the start the influence of the previous training; for though the 
 new series was not at first written as rapidly as the old had come to 
 be written, it was written more rapidly than the other had been at 
 the start; moreover, it improved rapidly, soon passing the limit of 
 efficiency reached in the first series. This result would indeed be 
 expected, for the single reactions remained the same as before; the 
 same key was struck in response to each color, and only the order 
 of the colors was changed. The fact that there was some loss in pass- 
 ing from one order to another proves that the performance first 
 learned consisted partly in a reaction to the order of the stimuli; 
 and this was an untransferable feature of the performance. In 
 this way, then, both the gain in general facility and the loss in 
 special facility are accounted for. 
 
 Bair now interchanged the colors on the keys, so that a new re- 
 action was required to each single stimulus, though the order of 
 stimuli remained as it had been just previously. This change in 
 the character of the single reactions proved to be more of a dis- 
 turbance than the previous change in order; yet in this case, too, the 
 new performance did not start back where the first had started, but 
 showed from the beginning some of the skill acquired in the previous 
 training. From the point at which it started, the new performance 
 
 l Psychol. Bulletin, 1909, VI, 44. 
 
 3 PsychoL Rev., Monogr. Suppl. No. XIX, 1902. 
 
TRANSFERENCE OF LEARNING 569 
 
 also made more rapid progress than the first had done thus show- 
 ing, beyond doubt, a transference of the previously acquired skill. 
 What was transferred was probably, besides acquaintance with the 
 order of the stimuli, an adjustment to the apparatus and general 
 conditions of the experiment, which remained the same throughout. 
 
 The method of Bair has also been employed in the previously 
 mentioned researches of Leuba, Book, and Ruger. The latter noted 
 several kinds of transference: transference of special methods; trans- 
 ference of more "general" methods, such as transference of the habit 
 or idea of analyzing each new situation, or of trying to induce vari- 
 ations instead of repeating an unsuccessful reaction time after time; 
 transference of a confident and self-reliant attitude toward a new 
 situation; transference of the habit or idea of keeping up active 
 attention during the course of practice, and of looking for improved 
 methods and higher units, instead of settling down to a mediocre 
 performance. Transfer is readiest in the realm of ideas; and the 
 more definitely a method of work, either special or general, has 
 been conceived and formulated, the wider is the field of its prob- 
 able usefulness. It may be remarked in passing that all these ad- 
 mittedly possible forms of so-called "transference," when taken together, 
 amount to a tolerably complete summary of the most essential factors in 
 what is popularly included under the training, or culture, of the mind. 
 
 The conception of a general transference or spreading of the 
 effects of training was called in question by Thorndike and Wood- 
 worth, 1 who experimented, by a method similar to that of James, 
 in performances involving observation, discrimination, estimation 
 of magnitudes, etc., and found, in general, rather a small amount 
 of transference from any specially trained performance to others 
 which, superficially, appear quite similar to it. This led them to 
 conclude that the mind works in great detail, adapting itself, of 
 necessity, to the particular material with which it has to deal; and, 
 therefore, that training in one performance could only help another 
 when the two had elementary factors in common. 2 This is to say 
 that what has been found to be transferred has always been some 
 specific habit, or reaction, or idea, or attitude; although it need not 
 always be a motor reaction. 3 
 
 20. Another line of evidence is available regarding the transfer 
 of training in memory. It is not necessary that the kind of material 
 
 1 Psychol. Rev., 1901, VIII, 247, 384, 553. 
 
 2 Compare also Coover and Angell, Amer. Journ. of Psychol, 1907, XVIII, 328. 
 
 3 For a discussion of the present state of the problem, and of its practical bear- 
 ings, see a symposium between Professors Angell, Pillsbury, and Judd in the 
 Educational Review, 1908, XXXVI, 1, and also the works of Meumann referred 
 to on p. 571. 
 
570 MEMORY AND THE PROCESS OF LEARNING 
 
 should be changed, in order to test for transference. Does learning 
 one list of twelve nonsense syllables make it easier to learn the next 
 list, composed of other nonsense syllables? Of this there is no 
 doubt; the times needed for learning such series decrease rapidly 
 with practice. Now the particular associations formed in learning 
 one series are different from those formed in learning another; and 
 yet the learning of one set increases the power to learn other sets. 
 This, it would seem, is as good evidence as could be desired of 
 training one performance by practising another. But there is an- 
 other curious fact to be considered in connection with the foregoing. 
 The learning of one such list may interfere greatly with the learn- 
 ing of another similar list, especially if little time intervenes be- 
 tween the two. If, immediately after one list has been learned, 
 another is begun, it will be found, later, that neither list is as well 
 retained as if either one had been attempted alone. Therefore it 
 is not the particular associations of the one list which favor the as- 
 sociations of the other. There is no spread of associative power 
 from one association to another unrelated association; but there may 
 be improvement, through practice, in the process of memorizing. - 
 
 Let us consider the meaning of this experience. When a person 
 first takes part in a memory test, there are many unaccustomed 
 features of the situation to which he must react. He must adjust 
 himself to the apparatus and procedure and to the peculiarities 
 of the experimenter; he must master his own distaste for the monot- 
 onous work; he must exclude the distractions which are inherent 
 in the circumstances and in himself; he must become negatively 
 adapted to these, as one becomes adapted to the ticking of a clock. 
 The material to be learned has some special but rather uniform 
 character, and he readily becomes accustomed to that; some ways 
 of trying to memorize it are good and others bad, and he has a 
 chance to learn the most suitable method. In a word, the situation 
 to which he reacts is decidedly complex, and his total reaction is 
 correspondingly complex. What he learns is not merely the list of 
 syllables; but he learns, or begins to learn, how to react to that par- 
 ticular complex situation; and when a similar list is later presented 
 to him, a large share of the situation remains the same and can be 
 reacted to as before. Thus, by degrees, he comes to master that type 
 of situation; and even if the situation is changed somewhat by the 
 introduction of a new sort of material to be learned, many of his old 
 partial reactions are still applicable to changes in the material. 
 
 It would seem, then, that the most practical sort of memory- 
 training, for ordinary purposes, is probably to be obtained by con- 
 necting together things that belong together for some purpose in 
 hand, and so building up a system of valuable associations. The 
 
THE PROCESSES OF MEMORIZING 571 
 
 suggestion of Meumann 1 to the effect that a species of " formal dis- 
 cipline" of the memory might lead to good results, is not without 
 force; since what he means by formal discipline is memory work 
 under conditions resembling those of the experiments which have 
 proved to lead to greatly improved technique in the memorizing 
 of certain kinds of matter. Experimental conditions are stimulat- 
 ing, largely because one has a measure of one's success and progress; 
 and the habit of checking up one's work can scarcely fail to prove 
 of benefit wherever measures of success and failure are practicable. 
 \/' 21. One other most important, and indeed essential, considera- 
 tion must certainly be borne in mind, in every attempt to deal with 
 the problems of memorizing and of the "transference" of acquired 
 skill. In studying the development of sense-perception, we saw 
 that, from the introspective point of view, attention and discrimina- 
 tion are involved in all human knowledge and in all forms of human 
 learning. From the same point of view, these both appear as active 
 forms of consciousness in all memorizing. But it is "I" that at- 
 tend, and "I" that discriminate. What forms of cerebral condi- 
 tions, or of cerebral changes, correspond to these conscious activities, 
 we may be much at a loss to point out. But everything which we 
 do know indicates that these conditions and changes must be of a 
 general character to correspond with the general character of the 
 mental aptitudes involved. It might be, then, quite appropriate 
 to speak of training the "faculty" of attention, and the "faculty" 
 of discrimination, so as to pass over the results of this training fr.om 
 one species of skilled reactions to anothgr. In all such cases, the 
 well-known distinction between the speed of the total motor reac- 
 tion, or of the total act of committing to memory, and the factor 
 consumed by the acts of attending and discriminating, must be taken 
 into the account. For example, experiments conducted in the Yale 
 Laboratory showed that while the reactions, under customary con- 
 ditions, of the master of a fencing club were speedier than those of 
 any of the members of the club, when discrimination was required 
 by unusual conditions, his discrimination-time was slowest of all. 
 And two Yale professors, one of whom had never used foils, while 
 the other had not practised fencing for many years, excelled in the 
 speed of their "discrimination-time" every member of the club 
 with the single exception of a gentleman who was himself both a 
 skilled fencer and highly educated. Indeed, to claim that trained 
 
 1 Vorlesungen zur Einfuhrung in die experimentelle Pddagogik, I, 200 (Leipzig, 
 1907) ; Okonomie und Technik des Geddchtnisses, p. 258 (Leipzig, 1908). 
 
 Improvement of memory with practice under experimental conditions, has 
 also been found by Winch (Brit. Journ. of Psychol, 1904, I, 127; 1906, II, 52; 
 1908, II, 284) to occur in school-children. 
 
572 MEMORY AND THE PROCESS OF LEARNING 
 
 powers of attention and discrimination are not available for trans- 
 ference to unusual situations would contradict the whole round of 
 human experience. At the same time, it must be admitted that a 
 high standard of specialized skill in certain lines may hinder, rather 
 than help, the rapid attainment of skill in other non-cognate lines, 
 through the large number of inhibitory processes which it may in- 
 troduce, if in no other way. Concentration of attention may also 
 be opposed to nimbleness of attention. In a word: It may be possi- 
 ble, by training, to increase the speed and improve the quality of 
 those general cerebral conditions and forms of functioning, to which 
 attention and discrimination correspond from the introspective point 
 of view. 
 
 22. We pass now to a study of that complex form of function- 
 ing which is called "memory," in a more special meaning of the 
 word. Investigations of this subject have been numerous since 
 Ebbinghaus * snowed that it afforded a fruitful field for experiment. 
 Ebbinghaus contributed, first of all, a method for measuring the 
 degrees of memory for all kinds of material; he further introduced 
 a new kind of material namely, so-called " nonsense syllables," 2 
 which possesses the advantage of being comparatively free from 
 ready-formed associations; and he applied both material and method 
 to the study of some fundamental problems of the formation and 
 retention of associations. His method, which passes by the name 
 of the "learning method," consists in presenting a list of nonsense 
 syllables to be memorized, and in determining the time, or the num- 
 ber of readings, necessary before the list can be recited without er- 
 ror. Care must be taken that the list is not "over-learned," i.e., 
 that more study is not given to it than the bare amount necessary 
 to reach the standard of one perfect recitation immediately after the 
 study. 3 
 
 The same method was ingeniously adapted to the study of re- 
 tention: after a given list of nonsense syllables (or, for that matter, 
 any other material) had once been learned up to the above stand- 
 ard, further work on it was suspended for a certain interval, and then 
 it was relearned to the same standard, the time or number of readings 
 needed for relearning being determined as before. Two important 
 
 1 Uber das Geddchtnis (Leipzig, 1885). 
 
 2 A nonsense syllable, as used by Ebbinghaus and later investigators, con- 
 sists of a vowel or diphthong between two consonants. 
 
 3 Some investigators have indeed chosen a higher standard namely, twosuc- 
 cessive recitations without error; and have found that considerable further 
 study is often needed to reach this higher standard. See Radossawljewitsch, 
 Das Behalten und Vergessen bei Kindern und Erwachsenen nach experimentellen 
 Untersuchungen (Leipzig, 1907). 
 
METHODS OF STUDYING MEMORY 573 
 
 facts immediately came to light and served as the foundation for 
 further use of the method. First, when a list was learned barely 
 enough to permit of one perfect recitation immediately at the close 
 of the learning, an interval as brief as twenty minutes, or even five 
 minutes, made another perfect recitation of the list impossible; and 
 second, after a much longer interval, though the list might seem, 
 introspectively, to be altogether forgotten, the time necessary to 
 relearn it was less than the time needed to learn it at first. This 
 showed that the associations formed in the first learning had not been 
 entirely obliterated; there was a partial retention, and it could be 
 measured by the saving (in time or number of readings) apparent 
 in the process of relearning as compared with the first learning. 
 If, for example, the first learning of a list required 10 readings, and 
 relearning after a week required only 8 readings, the saving due 
 to partial retention was 2 readings, or 20 per cent, of the original 
 labor. As thus applied to the study of retention, the Ebbinghaus 
 method is called the "saving method." 
 
 The method of Ebbinghaus was improved in two respects by 
 Miiller and Schumann. 1 They introduced rules for the prepara- 
 tion of lists of nonsense syllables which should be as nearly as pos- 
 sible equal in difficulty; and they provided an apparatus for expos- 
 ing the syllables to the eye at a fixed speed, so making the entire 
 procedure more uniform. 
 
 A second method of experimentally studying memory was in- 
 troduced by Miss Calkins, 2 and more fully formulated by Miiller 
 and Pilzecker, 3 who named it the "Treffermethode." This name has 
 been roughly rendered into English as the "method of hits and 
 misses," or as the "scoring method." The method has also been 
 named, 4 and perhaps most suitably, the "method of paired associ- 
 ates"; since syllables, words, or other materials, are presented in 
 pairs, the effort of the learner being to associate the pairs. Later, 
 one member of each pair is presented and the subject responds, if 
 possible, with its associate. A score of the " hits," or right responses, 
 gives the measure of memory, and the association time for each re- 
 sponse can also be determined. This method has certain advan- 
 tages over the other, in requiring less time, and in permitting a more 
 detailed study of individual associations. 5 
 
 1 Zeitschr. f. Psychol, 1894, VI, 81, 257. 
 
 2 "Association," Psychol. Rev., Monogr. Suppl. No. II, 1896. 
 
 3 " Experimentelle Beitrage zur Lehre vom Gedachtniss," Zeitschr. f. Psychol., 
 Ergdnzungsband 1, 1900; also Jost, Zeitschr. f. Psychol, 1897, XIV, 436. 
 
 4 Thorndike, Psychol. Rev., 1908, XV, 122. 
 
 5 For the latter purpose, Ebbinghaus (Grundzuge der Psychologie, 1905, I, 648) 
 has introduced a modification of the learning method, which may be called the 
 "prompting method," and according to which, after a few preliminary readings, 
 the subject attempts to reproduce the series of syllables, and is immediately 
 
574 MEMORY AND THE PROCESS OF LEARNING 
 
 Two other simpler methods 1 are of use for some purposes; they 
 may be called the "memory span method," and the "method of 
 retained members." The latter, which might also be designated 
 as a method of measuring the accuracy of recall, is the simplest 
 in principle of all, and consists merely in measuring how much of a 
 list of syllables, or of any other material, can be correctly repro- 
 duced. The "memory span" is the largest amount of any given 
 material which can always be correctly reproduced immediately 
 after one presentation. For example, in adults, about 5-7 nonsense 
 syllables, 8-11 one-place numbers, and 15-25 words of easy con- 
 nected prose, can be so reproduced. The span is determined by 
 starting with a short series and passing to longer and longer series 
 till errors begin to appear. The method is susceptible of various 
 modifications. 2 
 
 23. The results of work by all these methods are well worthy 
 of a much more extended analysis than space will here permit. 3 
 We may consider first the decline of retention with the passage of 
 time. Ebbinghaus studied this matter by aid of his saving method, 
 and, as the result of many experiments, in which, however, he alone 
 was the subject, found that the loss of retention was rapid at first, 
 and then slower and slower. If retention is measured by the per- 
 centage of the original time which is saved in relearning, the follow- 
 ing table 4 shows the loss of retention after different intervals: 
 
 Interval since the Per cent, of saving, 
 
 original learning or of retention 
 
 20 min 58 
 
 1 hour 44 
 
 8.8 hours 36 
 
 24 hours 34 
 
 2 days ' ... 28 
 
 6 days 25 
 
 31 days 21 
 
 prompted or corrected when he halts or errs. The number of promptings and 
 corrections gives a measure of the degree to which the series is learned, and also 
 shows which parts of it are learned. The process can be repeated till no more 
 help is needed. See also Ephrussi, Zeitschr. /. Psychol, 1904, XXXVII, 222. 
 
 1 Jacobs, Mind, 1887, XIII, 75; Pohlmann, Experimentelle Beitrage zur Lehre 
 vom Geddchtnis (Berlin, 1906). 
 
 2 Ebert and Meumann, op. cit.; Kirkpatrick, "Studies in Development and 
 Learning," Archives of Psychology, No. XII, 1909. 
 
 3 Probably the best general treatment of the subject is that by Ebbinghaus in 
 his Grundzuge der Psychologic; see also van Biervliet, La memoire (Paris, 1902). 
 A concise account of experimental results is given by Myers in his Textbook of 
 Experimental Psychology, pp. 144-182 (London and New York, 1909). For bib- 
 liographies, see Burnham, Amer. Journ. of Psychol., 1888-89, II, 39, 225, 431, 
 568; Kennedy, Psychol. Rev., 1898, V, 477; Reuther, Wundt's Psychol Studien, 
 1905, I, 93. 
 
 4 Uber das Gedachtnis, 1885, p. 103. 
 
THE CURVE OF FORGETTING 
 
 575 
 
 These results may be plotted into a "curve of forgetting " (or of 
 retention), by making the distances along the horizontal axis pro- 
 portional to the time elapsed, and the vertical distances proportional 
 
 144 
 
 FIG. 150. The Curve of Forgetting (Ebbinghaus). The numbers along the horizontal base 
 line give the hours elapsed since the time of learning; the numbers along the vertical 
 line give the percentage retained. 
 
 to the percentage of material retained. For comparison with this 
 curve of forgetting, a typical curve of learning may be presented; 
 in which case a certain similarity between the two is at once apparent, 
 inasmuch as the rate of change in each is rapid at first and then 
 
 FIG. 151. A Typical Curve of Learning. The curve is not typical in one respect, since it 
 does not show the fluctuations of efficiency which always appear; but it is intended to 
 show the general increase (rise) of efficiency with continued repetition of the performance. 
 Horizontal distances denote the time spent in learning, and vertical distances the meas- 
 ure of efficiency. 
 
576 MEMORY AND THE PROCESS OF LEARNING 
 
 slower and slower. On the other hand, if the two curves are thought 
 of as combined, a certain discontinuity between the fwo is evident, 
 inasmuch as the slow increase of the strength of association toward 
 the end of the process of learning suddenly gives way to a rapid 
 loss on cessation of the learning. But such a combination of the 
 two is unjustified; for the learning w r hich would give rise to such a 
 curve, with slow gain at the end, is carried much further toward 
 perfection than that from which the curve of forgetting takes its 
 start. Ebbinghaus stopped learning as soon as he had reached such 
 a proficiency that he could barely repeat the list of syllables once 
 correctly; while the typical learning curve supposes the learning 
 to be continued almost to the point of automatism. When the learn- 
 ing is thus long continued, subsequent forgetting goes on at a much 
 slower rate. Ebbinghaus himself showed that increasing the num- 
 ber of readings of his nonsense syllables increased markedly the 
 amount retained after twenty-four hours; and Radossawljewitsch, 1 
 who required the lists of nonsense syllables to be studied till two suc- 
 cessive perfect recitations could be made (instead of one, as in 
 Ebbinghaus's experiments), found the loss of retention to proceed 
 still more slow r ly. He also found the retention of poetry to decline 
 more slowly than that of nonsense syllables, as is shown in the fol- 
 lowing table: 
 
 Interval since the Per cent, of retention Per cent, of retention 
 
 original learning in nonsense syllables in poetry 
 
 5 minutes 98 100 
 
 20 minutes 89 96 
 
 1 hour 71 78 
 
 8 hours 47 58 
 
 24 hours 68 . . 79 
 
 2 days 61 ......... 67 
 
 6 days 49 ; .. . 42 
 
 14 days 41 30 
 
 30 days 20 ......... 24 
 
 120 days 3 . . . ? 
 
 The figures given above are the average result from several indi- 
 viduals. The poor retention after eight hours is assigned to fatigue 
 in the latter part of the day. But the principal thing to be noted in 
 the table is the slight loss of retention after five and after twenty min- 
 utes. This shows that after some "over-learning," the onset of 
 forgetting is retarded, so that the transition from the curve of learn- 
 ing to that of forgetting would not be wholly abrupt. When the 
 over-learning has been carried to a very high pitch, as in the experi- 
 ments reported above with the typewriter to so high a pitch that 
 a very large share of the associations involved have been reduced 
 
 1 Op. cit., p. 81. 
 
FORMING OF ASSOCIATIONS 577 
 
 to a condition of automatic efficiency then the onset of forgetting 
 is extremely slow. Book found, 1 after refraining for a full year 
 from all use of the typewriter, that his speed had only decreased 
 by 8 per cent., and this small loss was fully regained in forty minutes 
 of fresh practice. 
 
 Material having "sense" is, as the last preceding table shows, 
 better retained than nonsense syllables. Even after twenty-two years, 
 Ebbinghaus found 2 a perceptible retention of stanzas of poetry 
 learned only once to the point of one perfect recitation, and never 
 since seen. Forgetting can, of course, be stayed by learning anew; 
 and after each new learning the progress of forgetting is slower and 
 slower, till finally recall after an interval is possible with no further 
 study. 3 
 
 24. Regarding the process of memorizing, or the formation 
 of associations, many interesting facts have been established, and 
 some advance has been made toward their explanation. Some of 
 the results of experiment appear rather obvious, from the fact that 
 they are met with in ordinary experience, but they are none the less 
 curious when attentively considered. Among such may be men- 
 tioned the ease with which a short list of words or syllables is memo- 
 rized, and the rapid increase of difficulty which appears as the list 
 is lengthened. A certain length of list, corresponding to the memory 
 span, can be immediately reproduced after one reading; but if the 
 list is lengthened beyond this point, several readings are usually 
 required. Thus Ebbinghaus 4 could recite a list of seven nonsense 
 syllables after one reading; but it took him 13 readings to fix a list 
 of 10 syllables, 17 readings to fix a list of 12, 30 readings to fix a list 
 of 16, 44 readings to fix a list of 24, and 55 readings to fix a list of 
 36. Binet 5 found that a list of 11 one-place numbers could be re- 
 produced after 4 seconds of study, whereas, to learn a list of 13 num- 
 bers, 38 seconds were needed; and 75 seconds for a list of 14. 
 
 When a list is too long to be learned in a single reading, continued 
 reading does not develop the mastery of all parts of it with equal 
 speed ; but some of it will be known long before the rest. Generally 
 the first and last of a list are known long before the middle, which 
 remains like a shapeless mass after the ends have taken shape in 
 the mind. 6 More rapid organization of the middle of a list and 
 therefore more rapid learning of the whole list is accomplished 
 
 1 Op. cit., p. 75. 
 
 3 Grundzuge der Psychologic, 1905, I, 681. 
 
 3 Ebbinghaus, Uber das Gedachtnis, p. 110. 
 
 4 Op. cit., pp. 64, 67. 
 
 5 Psychologic des grands calculateurs, 1894. 
 6 W. G. Smith, Psychol. Rev., 1896, III, 21. 
 
578 MEMORY AND THE PROCESS OF LEARNING 
 
 by following the natural tendency to accent certain members of the 
 list and thus impress on it a rhythmic form. It is not, indeed, an 
 advantage to divide the list into parts and learn these separately, 
 afterward putting them together; for this method is found to take 7 
 more time than learning the whole list together by reading it through | 
 and through. 1 But dividing it into measures or feet, with regular 
 pauses and accents, while still reading the list straight through, is 
 found to be the most economical device for memorizing nonsense 
 lists, 2 and still more for memorizing poetry. If, indeed, the material 
 is not required to be recited as a whole, but in parts, as in the case 
 of vocabularies, or, in general, in experiments by the method of 
 paired associates, then division into parts, and prolonged attention 
 to each part before passing to the next, is apt to give the best results. 
 Again, where the members of a list are individually hard to grasp, 
 it may be economy to repeat or linger on small parts of the list. 3 
 Similar considerations apply to the question as to the best rate of 
 reading a list; a slow rate is more favorable for the formation of 
 single associations between the pairs of which the list is composed, 
 and a rapid rate of reading is relatively favorable for learning the 
 list as a complete list. 4 
 
 25. A partial explanation of the peculiarities of the process of 
 forming associations is afforded by the three following considera- 
 tions. 
 
 (1) Associations are certainly formed, not only between the suc- 
 cessive members of a list, but also between individual members 
 and their positions in the series. The first member becomes 
 strongly associated with the first place, and the last member with 
 the last place; and when the list is rhythmically organized, the ac- 
 cented member of each measure is associated with its place in the 
 rhythmic pattern. This association with position helps to explain 
 not only the advantage of the rhythmic form, but also the disad- 
 vantage of learning a list in disjointed parts; for the latter method 
 generates false associations of position, since the last term of each 
 section becomes associated with a final position and a "full stop" 
 which do not belong to it in the complete list. Any such false asso- 
 ciations must, therefore, be broken up before the list is ready to be 
 handled as a whole. 
 
 (2) "Higher units," similar to those which were assigned such 
 importance in the learning of an act of skill, are present also in the 
 
 1 Lottie Steffens, Zeitschr. f. Psychol, 1900, XXII, 321; Pentschew, Archiv 
 /. d. ges. Psychol, 1903, I, 417. 
 
 2 Miiller and Schumann, Ebert and Meumann, op. cit. 
 
 3 Ephrussi, Zeitschr. f. Psychol, 1904, XXXVII, 161; Pentschew, op. cit. 
 * Ogden, Archiv /. d. ges. Psychol, 1904, II, 93; Ephrussi, op. cit 
 
FORMING OF ASSOCIATIONS 579 
 
 process of memorizing, both when the material to be learned has 
 meaning and when it is, as far as possible, void of meaning. Mean- 
 ing organizes the words of connected speech into phrases and still 
 larger groups, which are grasped as units, and thus it is that prose 
 or verse can be memorized with enormously greater ease than an 
 equal number of unconnected words. But even in learning a list 
 of nonsense syllables, the formation of higher units can be detected. 
 The syllable is itself entitled to rank as a higher unit, for it is found 
 that many more letters can be learned in a given time when they are 
 combined into nonsense syllables than when they are wholly un- 
 combined. And further, the measures or feet, into which a list 
 of nonsense syllables is almost inevitably broken up, and which 
 greatly facilitate the memorizing of the list, also belong in the class 
 of higher units. The dynamic unity, or "dynamical association," 1 
 of the measure is shown by the facts 2 that relearning of a list is made 
 more difficult by breaking up these measures, or even by altering 
 the position of the accent within each measure; while, on the other 
 hand, a list built up out of complete measures taken from different, 
 previously learned lists, is comparatively easy to learn. If the meas- 
 ures are short, they are more readily joined into larger groups which 
 have a certain unity. 
 
 The further fact that associations are formed between the con- 
 stituent syllables and their positions in the list is evidence of some 
 degree of unity in the grasp of the entire list. Associations are also 
 formed between the single measures and their positions in the list. 
 Still another evidence of the unitary grasp of large sections of the 
 list, or even of the whole list, is afforded by the fact that each sylla- 
 ble has a tendency to call up, not only the immediately succeeding 
 syllable, but also the syllable which comes next but one; and, in- 
 deed, the syllable which comes next but two, next but three, etc. 
 But the strength of these "remote associations " decreases with the 
 number of intervening syllables. 3 Associations are even formed in 
 the backward direction, so that the later members have some ten- 
 dency to call up the earlier, at least within the same measure. 
 Enough has been said to show that the learning of a list of nonsense 
 syllables is very far from being simply the formation of a chain of 
 serial associations. The process embodies great multiplicity, and 
 at the same time much unity. 
 
 (3) Inhibition or interference is a factor to be reckoned with, 
 both in memorizing and in recall. A distinction may be drawn be- 
 
 1 For an explanation of this phrase, see Ladd, Psychology, Descriptive and Ex- 
 planatory, pp. 242 f. and 384 f. 
 
 2 Ebbinghaus, Miiller and Schumann, op. cit. 
 
 3 Ebbinghaus, Miiller and Schumann, op. cit. 
 
580 MEMORY AND THE PROCESS OF LEARNING 
 
 tween the two cases, by speaking of "associative inhibition" as that 
 which occurs in the process of learning, and impedes the formation 
 of associations; and "reproductive inhibition," or that which 
 hampers the process of recall or the operation of associations. A 
 clear instance of inhibition appears in the fact that lists of words 
 or syllables, up to a certain length, can be recited after one reading, 
 whereas adding one or two more members to the list makes many 
 readings necessary. Perhaps an individual can recite, after one 
 reading, a list of six syllables, but, on attempting a list of seven, 
 is unable to give more than one or two of them, or even any at all. 
 The seventh syllable has driven away the others, without becoming 
 fixed in their place. 
 
 26. The exact nature of the psycho-physical mechanism in- 
 volved in these inhibitions is by no means easy to "make out. The 
 clearest case is, perhaps, that in which a given stimulus or antecedent, 
 
 A, is already firmly associated with a certain response or consequent, 
 
 B, and now the attempt is made to associate with A a new response, 
 
 C, A leads so promptly to B that C does not have a chance, at least 
 till after B has had its turn; and thus the learning of the sequence 
 AC is impeded, while that of the sequence AB may be further 
 strengthened. Even if the overt reaction B, or the conscious 
 thought of B, should be repressed, A would still exert a tendency 
 toward B, and might sub-excite it, or bring it into "readiness," 
 and thus hinder the formation of the association AC, while being 
 itself somewhat strengthened. If, however, AB is not very strong^ 
 to start with, it is likely to yield to the pressure toward AC, though 
 interfering somewhat with the formation of the latter association. 
 The reality of such interferences has been shown by Miiller and 
 Pilzecker 1 in- the case of learning pairs of nonsense syllables, and 
 by Bergstrom 2 in the case of associating certain movements with 
 certain stimuli. But the relations of interference to such factors 
 as amount and recency of training in the opposing associations are 
 very intricate and by no means fully worked out. It was shown by 
 Miinsterberg 3 and later by Bair, 4 that two opposed responses to the 
 same stimulus might both be learned so well that either could be set 
 into operation at will. They had ceased to interfere with each other 
 to any perceptible degree. 
 
 It is quite possible that interferences between the manifold as- 
 sociations formed in reading over a list of nonsense syllables are the 
 main cause of the difficulty of learning such lists, especially when 
 the length of the lists is increased and the associative tendencies 
 
 1 Op. cit., pp. 138 ff. 
 
 2 Amer. Journ. of Psychol, 1893, V, 356; 1894, VI, 433. 
 
 3 Beitrdge z. exp. Psychol., 1892, IV, 69. * Op. cit. 
 
THE CULTURE OF MEMORY 581 
 
 are thus made more numerous. Repeated reading strengthens the 
 principal or serial associations more than the remote associations, 
 till at last the interferences are unable to prevent the correct reci- 
 tation of the list. 
 
 27. On recurring to the discussion of the problem whether prac- 
 tice in learning, in one rather specialized line of performance, is 
 available for the culture of the faculty of learning in general, we may 
 gain additional light on the reasons for the negative results, from 
 the facts just disclosed by the experiments on the conditions of 
 successful memorizing. It was formerly shown that great skill in 
 making certain motor reactions can be attained only at the risk 
 of establishing increased chances of interferences and inhibitions, 
 when the attempt is made to substitute a certain changed system of 
 such reactions. And now the same thing appears to be true in 
 the cultivation of skill in memorizing. It is a well-known fact, 
 however, that there are certain natural or acquired aptitudes in 
 the retentive and associative factors of conscious memory, as there 
 are aptitudes, both natural and acquired, for performing highly 
 specialized feats of skill in motor reactions. Only a detailed study 
 of each particular instance of failure to pass from one kind of facility 
 to another kind, could eliminate the influence of the inhibitory and 
 interfering processes, which belong to the passage itself. An over- 
 cultivated, and so implastic mind, or brain, in some special kind of 
 learning whether it be in the form of nearly automatic motor re- 
 actions, or of conscious attention, discrimination, and deliberate 
 memorizing might be a temporary hindrance to "transference" 
 of the facility along other lines. But all this affords no satisfactory 
 proof against the almost universal assumption that the power of 
 discriminating, attending, memorizing, and learning in general, 
 can be cultivated. 
 
 28. Another class of interesting facts regarding the process of 
 memorizing may now receive attention. It is found 1 that a list can 
 be learned in fewer readings when these are spread over several 
 days than when all are concentrated into a single learning period. 
 Since a similar fact appears in muscular training, the explanation 
 is probably to be sought in the effects of activity upon the nutrition 
 of the organ exercised. Probably, also, the lapse of time allows the 
 minor interfering associations to die out more than the principal 
 associations, and thus favors the latter in the new impression. A 
 similar dropping out of interferences, and consequent rapid im- 
 provement after an interruption of practice, has been noted by Book 
 in typewriting, and by Ebert and Meumann in memorizing. 
 
 1 Jost, op. cit. A similar result has been obtained in learning other perform- 
 ances by Leuba, op. cit., and by Kirkpatrick, op. cit. 
 
582 MEMORY AND THE PROCESS OF LEARNING 
 
 Another fact, which has been brought out most definitely by 
 Witasek, 1 is the superior value of active recitations of the matter to 
 be memorized, as compared with the more passive reading of the 
 presented lists. Long before a list can be recited entire, it is possi- 
 ble to recite parts of it, and if the learner does this, relying on him- 
 self as far as possible, but being prompted when he hesitates, the 
 list is learned in fewer repetitions than otherwise. In partial ex- 
 planation of this important result, it is a plausible conjecture that 
 the passive or receptive attitude leaves the door open to incidental 
 and interfering associations; while an active reproduction, so far as 
 it succeeds, requires the suppression of interferences and the selec- 
 tion of those associations which contribute to success. From this 
 point of view, there is a certain analogy between learning a list of 
 nonsense syllables, and learning the successful response to a situa- 
 tion by the method of trial and error. This experience also illus- 
 trates the value in all learning of a cultivated power of giving at- 
 tention, in general. 
 
 The same conclusions are further enforced by the fact that to 
 achieve quick learning, it is necessary to arouse the "will to learn." 2 
 If a subject maintains a purely passive attitude toward the lists 
 which are shown him, his learning is indeed slow. He may adopt 
 a decidedly active attitude, but if this is rather that of observation 
 of the members of the series than of associating them so as to be able 
 to recite the list, repeated presentations may leave him without 
 the power to recite it. 3 It is, therefore, clearly possible by an act of 
 will to set up an adjustment specifically favorable for memorizing, 
 though it is by no means clear, introspectively, in what this adjust- 
 ment consists. There is even some evidence that the adjustment 
 may be different, according as the list is to be retained for a long 
 time, or only for a few moments. 4 
 
 29. If now we turn from the "first event," or impression, and 
 pass over the intervening time, during which the disposition left 
 behind by the impression is gradually dying out, we come finally to 
 the "second event," the so-called "reproduction" or "recall" 
 Neither of these terms is perfectly correct; for the original impression 
 is not always, and perhaps never, fully and accurately reproduced. 
 Few people can reinstate an impression in all its sensory fulness 
 
 1 Zeitschr. f. Psychol, 1907, XLIV, 161, 246. 
 
 * Ebert and Meumann, op. cit.; Meumann, Okonomie und Technik des Ge- 
 d&chtnisses, 1908, pp. 24, 232. 
 
 8 Miiller and Schumann, op. cit., p. 291. 
 
 Ohms, Zeitschr. f. Psychol, 1910, LVI, 73; Henderson, "A Study of Mem- 
 ory for Connected Trains of Thought," Psychol. Rev., Monogr. Suppl. No. XXIII, 
 1903, p. 53. 
 
PHENOMENA OF REPRODUCTION 583 
 
 and vividness; many can accomplish this with moderate success; 
 while others are quite incapable of seeing their breakfast table "in 
 their mind's eye," as if it were actually before them, though they 
 are fully capable of recollecting aspects of, or facts about, the orig- 
 inal experience. This difference between individuals is spoken of 
 as a difference in their powers of imagery. 1 Besides this deficiency 
 in fulness and vividness, all reproduction, when tested carefully 
 by comparison with the original experience, is apt to be found in- 
 fected with certain erroneous factors. 
 
 It will be remembered that in considering the topic of "associa- 
 tion times" (see above, p. 493) we found the stimulus A calling up 
 the reaction B, although both stimulus and reaction were internal 
 rather than sensory and motor. This was explained as due to a 
 "disposition" left behind by the previous experience. Recall, 
 then, may at least sometimes be considered as a certain type of 
 reaction. And concretely, the condition would seem to be somewhat 
 as follows: The individual, being in a given situation, and being 
 adjusted or prepared, voluntarily or involuntarily, in a certain di- 
 rection, and having within him a host of dispositions or reproductive 
 tendencies of varying strength and manifold connections, is affected 
 by a certain stimulus, and reacts to it. This reaction is his recall 
 or reproduction, and it is determined by the stimulus, by the indi- 
 vidual's present adjustment, and by his past experience as retained in 
 reproductive tendencies. 
 
 30. If we now ask what determines the reaction to a given 
 stimulus and if, for the present, we leave out of account the im- 
 portant factor of adjustment our previous discussions have suffi- 
 ciently shown that A is likely to recall B when it has been already 
 associated with B; and that the likelihood of its recalling B is greater 
 in some proportion to the frequency and recency of this association. 
 If A has been previously associated with both B and C, then the 
 likelihood of its calling up one rather than the other of these asso- 
 ciates depends on the relative frequency and recency of the two 
 associations, as also on the vividness or intensity of the associative 
 events. 2 
 
 Recall also depends on the degree of success or pleasure which 
 has attended previous reactions to A by B or C (compare pp. 547, 
 552). The tendency of A to call up B is also strengthened, if A has 
 been immediately followed by B, and if attention has been directed 
 to connecting the two; but in a less degree, if A was preceded in- 
 stead of followed by B, or if it was only indirectly followed by B, 
 or if both A and B formed parts of a single group or " higher unit." 
 
 1 Galton, Inquiry into Human Faculty, p. 83 (London, 1883). 
 8 Calkins, op. cit. 
 
584 MEMORY AND THE PROCESS OF LEARNING 
 
 In all these cases, we may speak of A and B as having been con- 
 tiguous in past experience; and we may speak of the present recall 
 as resulting from "association by contiguity" in experience. 1 Prob- 
 ably it is not so much a mere contiguity in " experience" that counts, 
 as some definite contiguity in reaction. Perhaps we may express 
 the general fact best by saying that the contiguity must effect a 
 "dynamic association." When A and B have formed parts of a 
 single unit of reaction such a unit, for example, as one of the 
 rhythmical measures in which a list of syllables is learned, or such 
 as the noting of a relation between A and B they become associated 
 much more strongly than when they have simply been present to con- 
 sciousness at the same time or in immediate succession. 2 
 
 31. The association between A and B may have, at a given 
 time, any strength from zero to a maximum such that A will recall 
 B with certainty and promptness. Below a certain strength, it 
 cannot be utilized for purposes of recall. In other words, a repro- 
 ductive tendency may lie below the "threshold of recall"; 3 or, we 
 may speak of some associations as being "swfr-liminal." The ex- 
 istence of a subliminal association may be shown in several ways: 
 sometimes by a partially correct recall, and sometimes by the feeling 
 of "being near" a name which is vainly sought for in memory. 
 Its activity may also be tested by the following experiment: .Suppose 
 the association AB has proved to be subliminal at a certain time; 
 that is, A, being presented, has not been able to recall B. Now let 
 B itself be presented, but not clearly; let it be shown for too brief 
 an interval to allow of accurate reading, or let it be pronounced 
 through a poor telephone, so that it cannot be distinctly heard; 
 under such conditions, it has been found 4 that B is more likely to 
 be rightly read or heard if it has just been put into a condition of 
 "readiness" by the use of the subliminal association AB. In such 
 a case the subliminal tendency from A toward B facilitates (com- 
 pare p. 170) the arousal of B by another stimulus. It may also 
 inhibit the tendency to some other response. Two associations, 
 
 1 On the possibility of reducing the so-called laws of association to the princi- 
 ple of "contiguity in consciousness," see Ladd, Psychology, Descriptive and 
 Explanatory, pp. 263 ff. 
 
 2 " To be associated it is not enough that two impressions shall occur together 
 or in immediate succession. B may follow upon A as a physical event till 
 doomsday, but it is only as A and B are apperceived as in some sort one and 
 connected," that A will afterward give rise to B. J. Ward, Mind, 1894, N. S., 
 Ill, 509. This statement may perhaps need some qualification; but the experi- 
 mental work on memory shows abundantly that no strong associations are 
 formed by virtue of mere temporal contiguity in consciousness. 
 
 3 Muller and Pilzecker, op. cit., p. 34. 
 
 4 Ohms, Zeitschr. /. Psychol, 1910, LVI, 1. 
 
PHENOMENA OF REPRODUCTION 585 
 
 AB and AC, tending in different directions with slight and about 
 equal strength, may inhibit each other so that neither B nor C is 
 recalled. 1 
 
 Reproductive interference between different associations is 
 probably of great importance in explaining the difficulties of recall. 
 Suppose, for example, that reproductive tendencies AB and AC ex- 
 ist in an individual at a given time, but that AC is the more readily 
 excited; then the stimulus A may lead to the response C, though B 
 may be the response desired. When an association has been very 
 recently exercised, it is left in an excitable condition and is more 
 quickly re-excited to full activity than an old and perhaps well- 
 ingrained association which has not been recently active. In other 
 words, the association time for recently active associations is es- 
 pecially short. 2 A recently active association is, therefore, likely 
 to "get the start of" an older and perhaps better-established asso- 
 ciation, and thus govern the first response. By so doing it acquires 
 even more of the advantage of recency, so that a renewed attempt 
 to arouse the old association may simply lead to the recent one again. 
 This occurs in trying to recall a piece of music which resembles a 
 piece just heard; it is hard to drive one air out of the mind and give 
 the other a chance. The same difficulty occurs in case of a slip in 
 writing or spelling. One sometimes, after making a "slip of the 
 pen," goes back to write the sentence correctly, and commits the 
 same error again. In playing a musical instrument or in a highly 
 skilled athletic performance, an error tends to be repeated and to 
 put the performer "out of form" for the time being, or until the 
 recent bad associative tendencies have time to "cool down." The 
 same sort of trouble occurs at times in trying to recall a familiar 
 name; unless the right response occurs at once, it appears to be short- 
 circuited by the false responses which occur, and the best plan is 
 often to abandon the search for a time, and allow the interferences 
 to go to sleep. It would seem safe to say that, as a rule, an existing 
 reproductive tendency AB must control the reaction to A except 
 for the interference of other instinctive or associative tendencies set 
 into activity by A or by some other stimulus acting at the same time. 
 
 32. A very interesting conception of the interferences operative 
 in preventing the recall of a familiar name, and in causing lapses 
 and slips of various kinds, has been put forward by Freud. 3 Without 
 attempting a lengthy exposition of this theory, we may get a glimpse 
 at its character in the following way: Let us suppose that a name 
 has several associates which tend to be recalled with it. It may be 
 that these associated ideas are unpleasant, or that they have been un- 
 
 1 Miiller and Pilzecker, op. cit. 2 Miiller and Pilzecker, op. cit. 
 
 3 Zur Psychopathologie des Alltagslebens (Berlin, 1904). 
 
586 MEMORY AND THE PROCESS OF LEARNING 
 
 pleasant at some time in our past experience; or it may simply be 
 that they are not welcome at the moment because they would dis- 
 tract attention from the present object of interest. In any of these 
 cases, there is a tendency to repress or inhibit these undesired as- 
 sociates, even before they actually break into consciousness. But 
 in repressing them, we may repress the name as well; or we may only 
 partially suppress it, and so recall a name similar to it. Or, again, 
 some of the associates may not be fully suppressed, but may be 
 operative in recalling a substitute name. In other words, a for- 
 gotten name points to a repressed complex of ideas and emotions, 
 and what is thus suppressed may be discovered by removing the 
 suppression, and allowing a train of perfectly free association to 
 take its start from the forgotten name when, later, it has been found. 
 By this method of "psycho-analysis," Freud, and others following 
 him, have been able to bring to the surface submerged " complexes," 
 which, in nervous persons, are sometimes the source of much mental 
 inefficiency. Freud is inclined to look especially for old emotional 
 suppressions, often of a more or less sexual nature, as lying at the 
 bottom of apparently trivial lapses of memory. It is highly im- 
 probable that all interference with recall possesses this highly 
 elaborate and significant character; interference could hardly act 
 in these complicated ways did it not also act as has been abundantly 
 shown that it does in simple ways; but it may very well be that such 
 inhibitory mechanisms as Freud conceives are sometimes operative. 1 
 33. What was said above of the liability of a recently active 
 association to re-excitation leads to a further important consideration. 
 In certain abnormal conditions of the nervous system, a tendency is 
 visible to repeat an act time after time, when once it has been aroused 
 by some appropriate stimulus. Or, in an association test, which 
 calls for responses to one after another of a series of presented words, 
 there may appear a tendency to give the same response to several 
 words a response which was evidently called up by association in 
 the first instance, but which has no relevance to the later words. 
 The response, once excited, persists or perseveres. This tendency 
 to " perseveration " is not confined to abnormal conditions, but has 
 been demonstrated by Miiller and Pilzecker in normal subjects, 
 
 1 An example may perhaps make this clearer. A certain author has frequently 
 had difficulty in recalling the name "Wadelton." When he applies free associa- 
 tion to this name, the first idea that occurs to him is that of waddling a notion 
 which does not in the least comport with the individual so named. Moreover, 
 this cheap punning on names is a thing to be suppressed, and the author has been 
 at some pains to suppress it. Now it may be to speak in a figurative way 
 that recall, in reaching for the name, got hold also of these unsuitable associates, 
 and, in trying to drop these, dropped the name with them, and so was unable 
 to pull it up into consciousness. 
 
PHENOMENA OF PERSEVERATION 587 
 
 when, in trying to recall nonsense syllables, they are operating with 
 rather weak reproductive tendencies. Perseveration may then ap- 
 pear in the repeated recall of the same syllable, as a response to 
 syllables with which it has not previously been associated. 
 
 Perseveration is most apt to lead to the recall of recent experiences, 
 and is most apt to appear when attention is relaxed, and the mind al- 
 lowed to wander freely. Instances of perseveration are found in 
 the running of a tune in the head, soon after it has been heard; or 
 in the flashing of a scene before the mind's eye soon after it has been 
 actually seen; or in the reminiscences of the day which are apt to 
 come to mind as one is dropping off to sleep. In the case of visual 
 dreams, these may be caused by the persistence of the after-images 
 in the "fundus" of the eye. 
 
 The theoretical importance of such facts lies in the apparent ab- 
 sence of a stimulus to recall. B comes, it seems, not in response to 
 any A, but entirely of itself. It seems to spring up of its own elas- 
 ticity, when the repressive force of other interests is removed. Per- 
 severation seems thus not to belong altogether under the head of 
 association, and has accordingly been assigned an independent 
 standing as a cause of recall. We may conceive, perhaps, that the 
 cerebral mechanism which takes care of B, after being strongly ex- 
 cited, does not at once lapse into quiet, but remains subliminally 
 active; it may thus become the most active part of the brain by vir- 
 tue of a decrease in the activity of other parts. However, it is not 
 necessary to adopt exactly this conception ; for, if a mechanism, be- 
 cause of recent activity, simply is highly excitable, it might be thrown 
 into activity through feeble associative links, of which, as the pre- 
 ceding pages have shown, many are formed between things which 
 seem only remotely connected in consciousness. Such a condition 
 is especially emphasized when, either through the excitement of 
 fever, or the relaxation of control over the train of ideas on account 
 of exhaustion, or in sleep, a perfect hurly-burly of disconnected 
 mental images takes possession of the conscious mind. Whatever 
 be the explanation, the facts indicated by the name perseveration 
 are important in any sketch of the succession of thoughts which pass 
 through the mind. 
 
 34. Perseveration is not the only apparent exception to the rule 
 that recall proceeds along the lines of previously formed associa- 
 tions. The law of association requires that A, in order to recall B', 
 must have previously been associated with it. But often A is new, 
 and so never previously associated with B; or, though A and B may 
 both be old, they may never have been present together in experience. 
 The question is, then, whether, and how, A can recall B without 
 having been previously associated with it. The cases in which 
 
588 MEMORY AND THE PROCESS OF LEARNING 
 
 this seems to occur are chiefly of two kinds: 1 first, A may be sim- 
 ilar to some stimulus which was previously associated with B; 
 and second, A may be similar to B itself. The first case is ex- 
 ceedingly common, for a stimulus seldom recurs exactly as it was 
 formerly received; its quality, intensity, or setting may vary, and yet 
 the same reaction be evoked. The child, having learned the name 
 "dog," for example, applies it readily to dogs of various sizes and 
 colors, seen from various angles and in various attitudes; and even 
 has little hesitation in applying the same name to a variety of other 
 animals. The mastering of a new situation by aid of some act 
 learned in response to a similar situation is an essential factor in 
 learning. This form of recall thus takes us back to our previous 
 discussion of the transference of a learned reaction from one situa- 
 tion to another; and our previous analysis of transference as de- 
 pendent on the existence of something common to the old and new 
 situations and responses is still applicable. The new stimulus need 
 not be an exact copy of the old; neither does the new reaction need 
 to be an exact copy of the old, for it is modified by the exigencies of 
 the present situation; but both stimulus and response are partly 
 old, and, therefore, connected by an old association. 
 
 Less common, but still common enough to be of constant and 
 great importance in mental life, is the case in which A leads to a B 
 which is similar to A itself. This is the famous case of " association 
 by similarity." Although it differs, descriptively, from the pre- 
 ceding case, dynamically it is much the same. A stranger, for ex- 
 ample, reminds us of an acquaintance, because of some resemblance 
 between their faces. In this case, A is the sight of the stranger, and 
 B the thought of the acquaintance, and A and B have never been 
 associated in our past experience. If, now, the resemblance had 
 been so great as to lead us to call the stranger by the name of the ac- 
 quaintance, the response would clearly belong under the preceding 
 case; it would be transference of a reaction rather than association 
 by similarity. But if the resemblance is not strong enough to lead 
 to a transferred motor reaction, it still exerts a tendency in that di- 
 rection, and may give rise to an "idea" of our acquaintance. The 
 two cases differ in the kind of reaction, but not in the mechanism 
 of recall. 
 
 Let us further examine the question in an instance which has been 
 experimentally worked out with some detail. 2 Suppose a collec- 
 tion of two-syllabled "nonsense words" to be made familiar by a 
 moderate amount of study, and a few minutes later let similar non- 
 
 1 Compare Ebbinghaus, Grundzuge der Psychologic, 1905, I, pp. 636, 642. 
 
 2 Peters, "Uber Ahnlichkeitsassociation," Zeitschr. /. PsychoL, 1910, LVI, 161. 
 
VARIETIES OF ASSOCIATION 589 
 
 sense words be presented the similarity consisting in the general 
 construction and in the possession of letters in common; then it is 
 found that a word has considerable power to recall the similar word 
 which has recently been made familiar. For instance, tolaf, be- 
 ing presented, recalls golap, recently studied. Now the common 
 group of letters, ola, was an essential part of the stimulus which led 
 to the percept golaf; and therefore tends to recall golaf, even when 
 presented as part of another complex. If the association formed 
 between A and B were exclusively concerned with A and B as totals, 
 it would be difficult to conceive how anything similar to A could 
 recall B, but since associations are formed connecting parts of A 
 with B, the recall of B by partial recurrence of A presents no real 
 exception to the law of association. 
 
 35. The full importance in intellectual life of association by 
 similarity does not, however, become evident till we consider that, 
 though similarity may be analytically regarded as due to the pos- 
 session of features in common, yet the detection of these common 
 features is not always easy or even possible in concrete cases. It 
 is not usually easy, for instance, to analyze the resemblance be- 
 tween two faces, so as to state exactly what they have in common; 
 and yet one face may remind us of the other. Clearly, therefore, 
 association by similarity does not depend on a conscious analysis 
 of the stimulus, and a conscious discrimination of the features which 
 have power to lead to the recall of some other complex. The recall 
 of similars provides, the rather, an occasion for subsequent analysis. 
 A recalls B, we do not see why, though we are vaguely aware of 
 some resemblance between them. We may, next, become aware of 
 the point of resemblance, and thus be led to an analysis of what would 
 otherwise have remained unanalyzed. So, when a new situation 
 arouses either a thought of some earlier situation, or a reaction 
 which was made to it, the feature common to the two situations 
 may become isolated in thought, and thus a more intelligent grasp 
 of both old and new be promoted. Such seems to be the mechanism 
 at least in part by which new insights and discoveries are often 
 achieved. Thus transferred reactions and associations by similar- 
 ity lead over from memory to productive imagination and thought. 
 
 36. Thinking is, in very large measure, a process of recall, 
 but it involves one factor which has not yet been fully considered. 
 It has been shown that most cases of recall are of the nature of a 
 reaction ; and that, besides the stimulus, and the existing stock of 
 reproductive tendencies, an important factor in the result is the men- 
 tal adjustment present at the moment. Recall is seldom perfectly 
 "free"; usually it is governed or directed by some present inter- 
 est or state of mind. Reproductive tendencies, formed in past 
 
590 MEMORY AND THE PROCESS OF LEARNING 
 
 experience, are numerous, and make possible many reactions to 
 a given stimulus; and which, in particular, of these reactions shall 
 occur is dependent not only on the relative strength of the repro- 
 ductive tendencies, but also on the present adjustment. In reading, 
 for instance, the context so "sets the mind" that the appropriate 
 * meaning of each word is at once recalled, though the word may have 
 many meanings, and each of these may have been previously as- 
 sociated with the word. In arithmetical work, the numbers " three 
 and four" suggest "seven," if the adjustment is to add; but " twelve," 
 if the adjustment is to multiply. Mental work of any kind would be 
 a hopeless tangle of associations, were it ncJTFor the directive power 
 of preparatory adjustments. This Factor is a selective one; without 
 increasing the stock of associations, it gives the preference to some 
 and suppresses others. 
 
 The most important question regarding this "selective mechan- 
 ism" is whether it acts before, or after, the reproductive tendencies 
 have done their work. Does such a word as "fair," for example, 
 when occurring in context, first call up several of its familiar mean- 
 ings, from among which, next, that which suits the context is selected; 
 or does the selection so occur that only the appropriate meaning 
 comes to consciousness? If, in multiplying, we meet the numbers 
 7 and 9, do we recall both their sum and their product, and after- 
 ward select the product ? In such cases, introspection gives a clear 
 answer: usually only the appropriate response appears; and in fact 
 the reaction is objectively too prompt to allow of multiple recall and 
 subsequent selection. The preparatory adjustment here facili- 
 tates one reproductive tendency and Inhibits the others from the 
 very start of the total process. Such, indeed, is the case in all 
 smooth-running mental work. On the other hand, there are not a 
 few cases where the appropriate reproductive tendency is weak, 
 and other reproductive tendencies are relatively strong; and in all 
 cases there are limits to the power of the adjustment. 1 
 
 37. Recall is likely to be attended by feelings of familiarity, of 
 recent occurrence, of correctness or incorrectness, of certainty or 
 doubt; and these feelings are often the chief subjective constituent 
 of what is termed " recognition" A high value is attached to them 
 by the individual, inasmuch as he is usually willing to assert that 
 they indicate the truth. He "feels sure" that he has seen this face 
 before, or that he has recalled this name correctly, or that seven 
 times nine is sixty-three, without further evidence than his immediate 
 impression. Without doubt these feelings have a considerable de- 
 gree of trustworthiness^_especially when they are strong and posi- 
 
 1 Experimental evidenpe in support of these conclusions has been presented 
 in a previous chapter, pi 
 

 THE GUARANTEE OF MEMORY 591 
 
 tive. They are not, however, absolutely trustworthy; for cases are 
 easily found, in experiment, in which a false recall is attended with 
 the feeling of correctnes'" and confidence, as well as cases in which 
 a right recall is attended with a feeling of its incorrectness. 1 Testi- 
 mony, whether in ordinary intercourse or in courts of justice, re- 
 garding events experienced shows similar cases; and in fact, where 
 the testimony can be controlled by a full knowledge of the facts, 
 a considerable proportion of false recalls, attended by a feeling of 
 correctness, is ordinarily found. 2 A witness in court may even be 
 willing to testify under oath, and in perfectly good faith, to the oc- 
 currence of an event or to the identity of a person, when his memory 
 is really at fault. 3 Still, the feelings of recognition are on the whole 
 worthy of confidence. In some persons they are highly trustworthy; 
 in others much less so, according to the temperament; and, again, 
 they are much more reliable as to certain classes of facts than as to 
 other classes. 
 
 38. No subjective guarantee of the correctness of memory is 
 superior to these feelings of recognition. One may seek outside cor- 
 roboration, or one may criticise one's recollections on the basis of 
 probability; but in so far as one must rely on one's own memory, the 
 feelings are ultimate, and there is no way of going behind them to 
 justify or to discredit them. It has indeed been sometimes sug- 
 gested that recognition consists in more recall, i. e., in the recall of 
 the original setting of the particular fact recognized. This is so 
 far true that recall of the setting of a fact often occurs, and permits 
 of a more exact assignment of the fact to its place in past experience; 
 and, besides, recall of the setting reassures us as to the correctness 
 of the central fact. But what guarantees the correctness of the 
 recalled setting? How do we recognize that? If a setting is all 
 that is required, imagination is perfectly capable of conjuring one 
 up about an imagined fact; only, such a setting will not feel genuine 
 and familiar. The recognitive feeling must attach to the setting 
 to give it subjective validity. Try as we will to justify memory by 
 indirect evidence from within ourselves, we have always, in the last 
 analysis, to rely upon a feeling of direct certainty though the 
 certainty may be so categorical as to occasion very little emotional 
 tumult. Such rather neutral states of assurance are the rule where 
 recognition is easy and unhesitating, as when a daily companion 
 is seen, or a well-known date recalled. This feeling of unquestion- 
 ing assurance, as distinguished from feelings of uncertainty, hesita- 
 
 1 Miiller and Pilzecker, op. cit. 
 
 2 Cattell, Science, 1895, N.S. II, 760; Stern, Zur Psychologic der Aussage (Ber- 
 lin, 1902). 
 
 3 Urstein, Zeitschr. f. PsychoL, 1906, XLIII, 423. 
 
592 MEMORY AND THE PROCESS OF LEARNING 
 
 tion, and doubt, is the correlate in consciousness of the uninhibited 
 and smoothly running adjustment and unimpeded dynamic associa- 
 tion of the cerebral mechanism. 
 
 That recognition does not consist essentially in the recall of a set- 
 ting was asserted by Kiilpe, 1 who drew a distinction between the 
 "mediate" or indirect and the "immediate" or direct types of rec- 
 ognition. In the mediate type, the setting is recalled and aids in 
 the recognition; but in the immediate type, the recognition occurs 
 before the setting is recalled, if indeed it is recalled at all. The im- 
 mediate type occurs when recognition is prompt and sure. Experi- 
 ments on the recognition of odors, undertaken by Gamble and 
 Calkins, 2 showed instances in which an odor was recognized without 
 recall of previous experiences of it, and other instances in which 
 past experiences of the odor were recalled only after the odor had 
 been recognized. 
 
 1 Grundriss der Psychologic, 1893, pp. 177. 
 
 2 Zeitschr. /. PsychoL, 1903, XXXII, 177, and XXXIII, 161. 
 
CHAPTER IX 
 THE MECHANISM OF THOUGHT 
 
 1. Logic treats of rational discourse as made up of syllogisms; 
 of these in turn as composed of propositions; and of these, finally, 
 as containing terms and a copula, or sign that the terms belong to- 
 gether. Or, starting from the terms, logic regards the proposition 
 as resulting from comparison of the terms, and the syllogism as 
 resulting from the comparison of propositions. Though this treat- 
 ment is based on the analysis of the linguistic expression of thought 
 rather than on the analysis of concrete mental processes, it may 
 serve to indicate, in a provisional way, the topics suggested for study 
 in the field of thought. 
 
 2. The so-called "terms," or "ideas," of which' a proposition 
 or judgment is supposed to consist, are supplied either by perception 
 or by recall. This fact makes desirable some further consideration 
 of the nature of perception, although the topic has already been 
 discussed (Chapters V and VI of Part II) in detail. It is clear, in 
 the first place, that sense-perception commonly depends on past 
 experience. In general, each thing is perceived as a familiar indi- 
 vidual, or as belonging to a familiar class of things; or as like 
 something already known; or as compounded of known elements. 
 Perception thus involves recognition; and what has been said of recog- 
 nition applies also to perception. In particular, the distinction 
 between mediate and immediate recognition is applicable in the 
 field of thought, and in the study of its mechanism. When an ob- 
 ject appears in a new setting and every setting is likely to differ 
 somewhat from those in which the object previously occurred the 
 old setting may be recalled and apparently aid in recognizing the 
 nature of the object. This is "mediate" perception; but it is far 
 from being the common type, for usually an object of a familiar 
 class is recognized in a new setting promptly and directly; and 
 this is equally true if the present object differs from similar ob- 
 jects previously experienced, even as much as an outline drawing 
 differs from an actual scene. The outline drawing is readily 
 " understood," or recognized as a representation of a certain sort of 
 object, without the recall of the colors or background which an 
 actual object must have. When the same outline drawing can repre- 
 sent more than one object as is the case in the "staircase figure" 
 
 593 
 
594 
 
 THE MECHANISM OF THOUGHT 
 
 and other ambiguous figures the transition from one way of seeing 
 the figure to another is not attended by the recall of a new setting. 1 
 The observer seems to see only the figure before him, but to see it 
 now as this object and again as that. In perception generally, past 
 experience is not overtly present in consciousness; one seems to live 
 
 in and deal with the pres- 
 ent, while all the time one 
 is really utilizing previously 
 learned reactions. 
 
 3. A percept belongs 
 without doubt to the class 
 of reactions to stimuli, and 
 usually to the class of re- 
 actions previously learned. 2 
 Yet introspection can sel- 
 
 FIG. 152. The Staircase Figure (from Wundt). 
 a can be made to appear either nearer or far- 
 ther off than &. 
 
 dom distinguish between the 
 stimulus and the reaction in 
 a perceptual act. It is im- 
 possible to lay aside habitual modes of perception, and to receive 
 the stimulus as a "pure sensation"; it is also usually impossible to 
 detect by introspection an interval between the reception of the 
 stimulus, as such, and the recognition of it as some definite thing. 
 Brain pathology, however (compare p. 252), affords reason to believe 
 that the reception of the stimulus and the perception of it as some 
 known object are two events; and even that an interval occurs 
 between the beginning of the first event and the beginning of the 
 second; but this interval is probably a small fraction of a second, 
 and is not, moreover, an empty interval, but a period of gradual 
 change in consciousness. Introspection, therefore, could not be ex- 
 pected to be aware of the interval, or to distinguish the earlier from 
 the later stages of the total process. 
 
 In cases of difficult perception, however, it is often possible to 
 observe the gradual development of a percept. A sudden and start- 
 ling stimulus may call out an immediate motor response, attended 
 by mental confusion, which then gradually gives way to a clear per- 
 ception of the nature of the stimulus or of the object indicated by it. 
 At other times, an unfamiliar stimulus, especially a curious sound, 
 may give rise to a rapid succession of "trial percepts" which are 
 promptly rejected, each for the next, till one is reached which sat- 
 isfies the mind. A few seconds may see the whole series of tenta- 
 tive interpretations gone through with, and a satisfactory percept 
 reached. In such cases, perception has definitely the character 
 
 1 Woodworth, Journal of Philos., 1907, IV, 169. 
 
 2 Ward, Mind, 1893, N.S., II, 355. 
 
PERCEPTION AS A FORM OF REACTION 595 
 
 of response by "trial and error"; varied reactions, provided by in- 
 stinct and previous training, are tried in succession, till one is 
 reached which gives satisfaction. On the other hand, the percep- 
 tion of a familiar object has the character of a well-learned reaction, 
 which is recalled swiftly and automatically by the stimulus. 
 
 More definite experimental analysis of the process of perception 
 has been obtained in case of complex visual stimuli, such as reading 
 matter or " nonsense drawings." The method used in these studies 
 is that of exposing the object for too brief a time to permit of a com- 
 plete and accurate perception, and then repeating the exposure 
 time after time, noting after each exposure how far the observer is 
 able to reproduce the object. When reading matter is so presented, 1 
 the observer is able, at the first presentation, to read a certain num- 
 ber of words with certainty; to distinguish the general shape of 
 another word or two; and to hazard a guess as to some vague char- 
 acter of the next word or two. On the next exposure, his eyes be- 
 ing directed to words which were only partially grasped the first 
 time, he sees these distinctly and his area of indistinct vision is ex- 
 tended to new words further to the right. It moreover appears cer- 
 tain that the indistinct perception of words is of value in the subse- 
 quent clear perception of them. The indistinct perception renders 
 the clear perception more prompt. In the case of nonsense figures, 
 composed of seven short lines and curves, Judd and Cowling 2 found, 
 with repeated exposures of ten seconds each, a gradual development 
 of mastery of the figure. Some subjects got at first the general out- 
 line and later the details; while others neglected the general out- 
 line at first and mastered a few details, adding to them in later trials. 
 In all cases, there was a shifting of attention as the exposures were 
 repeated, so that now one part, and now another came into promi- 
 nence. Further, in this more deliberate process, as distinguished 
 from the rapid succession of trial percepts spoken of above, there 
 was not a strict following of the method of trial and error, but some 
 individuals adopted a definite plan of operation from the beginning; 
 and formulations in general terms of the way the figure was con- 
 structed were sometimes in evidence before the details were mastered. 
 On the whole, then, perception presents certain features essentially 
 the same as the processes involved in the reaching of a successful 
 motor reaction to a situation, as the latter were described in the pre- 
 ceding chapter. 
 
 4. Logic shows the importance of general and abstract terms, 
 and it will therefore be worth while to consider how abstractions 
 
 1 Hamilton, "The Perceptual Factors in Reading," Archives of Psychol, No. 
 IX, 1907. 
 
 2 Psychol. Rev., Monogr. Suppl. XXXIV, 1907, p. 349. 
 
596 THE MECHANISM OF THOUGHT 
 
 are developed. The prominent characteristic of an abstract con- 
 ception is that it neglects many of the features of a concrete situa- 
 tion or thing, and considers only certain features. A thing cannot 
 exist except in the concrete, and in a concrete setting; but the con- 
 crete setting and many of the concrete details of a particular thing 
 can be neglected for the purposes of thought, and so the thing be 
 treated as abstract. Indeed, if the mind were incapable of this proc- 
 ess of so-called abstraction, all the generalizations of science and 
 of practical life would be impossible. 
 
 Logic usually speaks of ideas or concepts only as being abstract; 
 but psychologically considered, a percept may be, and indeed must 
 be, abstract; since every percept isolates some feature of a thing or 
 situation, and for the moment neglects the other features. A 
 motor reaction does the same, for it is never a reaction equally and 
 indiscriminately to the whole situation, with all its concrete feat- 
 ures, but is predominantly a reaction to some particular feature. 
 Both motor reaction and perception treat a situation, therefore, 
 as the bearer of the feature to which response is made; and thus the 
 situation is generalized, while the feature perceived, or reacted to, 
 is abstracted, though it actually remains concrete. Every percept 
 is, therefore, a partial analysis of the situation, emphasizing one 
 feature to the neglect of others. 
 
 This process of analysis or emphasis is often considered under 
 the name of attention', and some of the results of the study of atten- 
 tion are of importance in our present inquiry. 1 Attention is closely 
 related to analysis and abstraction, since it is always directed to 
 some feature of a situation, others being neglected for the moment; 
 attention is, in other words, selective. Yet attention should not be 
 identified completely with analysis; for analysis occurs without at- 
 tention. A motor response to some particular feature of a situation 
 may occur automatically, while attention is elsewhere directed ; and 
 perception of particular features may also occur without attention 
 being especially directed to them. While attention is directed upon 
 some visible object, sounds which occur may be perceived as voices, 
 footsteps, and the like; or, while attention is directed to some "in- 
 ternal" thought, the eyes, if open, are sure to be accurately di- 
 rected upon some object, and this object is apprehended as an ob- 
 ject (for example, as a word, when the eyes rest inattentively on the 
 page and the thoughts are wandering). What is not especially at- 
 tended to, therefore, by no means remains "pure sensation," or 
 wholly undifferentiated from the total situation. 
 
 Moreover, several motor or perceptual reactions may occur at the 
 
 1 For a fuller treatment, see Pillsbury, Attention (London, 1908), and Wirth, 
 Die experimentelle Analyse der Bewusstseinsphdnomene (Braunschweig, 1908). 
 
THE SPAN OF ATTENTION 597 
 
 same time, but attention is usually concerned, in any special way, 
 with only one of these. These facts make it difficult, and at present 
 impossible, to define attention as a special function, or in any terms 
 except those of degree. The descriptive definition which is usually 
 attempted takes a figurative form. Of all the contents of conscious- 
 ness at any moment, some are said to have the quality of "clear- 
 ness," and it is the clear contents which are spoken of, in other 
 words, as the objects of attention. "Clearness" has here a special 
 meaning, which can only be defined by referring back to attention. 
 A vague or obscure impression may be clear in this sense, since 
 it can be the object of attention. A definition in terms of degrees 
 of consciousness is perhaps admissible. We are, at any moment, 
 more conscious of some objects than of others, and it is the objects 
 of which we are most conscious to which we are said to attend, or 
 which are said to be " clear." Within the field of consciousness there 
 are gradations, and the most conscious part of this broader field 
 is the narrower "field of attention." 1 
 
 5. The attempt to fix the number of objects which can be at- 
 tended to at once, or to measure the so-called " span of attention," 
 has been made by determining how many small and discrete units 
 can be grasped in one presentation. Cattell 2 found that four or five 
 short lines could be apprehended correctly in a brief exposure; be- 
 yond this, perception was not clear and reliable, though approxi- 
 mate judgments were possible with considerably larger numbers. 
 The span was about the same for disconnected letters ; but when the 
 letters were combined into short words, about four words could be 
 perceived. Similar results have been obtained in other senses. It 
 is not so much the span of attention as the span of apprehension that 
 is measured in these experiments ; that is, it is not so much the extent 
 of the field of clear consciousness as the extent and kind of object 
 which can be perceived as a unit, that determines the result. Train- 
 ing increases the " span," not, probably, by increasing the amount 
 of clear consciousness, but by developing "higher units" of percep- 
 tion. After training has done its perfect work, apprehension may 
 proceed by large spans, while attention is elsewhere occupied. The 
 span of attention, therefore, measures, not so much the area of the 
 field of attention, as the number of stimuli which can be taken care 
 of in a brief perceptual reaction (compare p. 498), 
 
 1 For a treatment of attention as one of the " most general forms of all mental 
 life," and the condition and accompaniment of all consciousness, see Ladd, 
 Psychology, Descriptive and Explanatory, chap. III. 
 
 2 Wundt's Philos. Studien, 1886, III, 94, 121. This is one of the oldest ex- 
 periments in psychology, having been performed, in a relatively crude way, by 
 Bonnet and by Sir William Hamilton. 
 
598 
 
 THE MECHANISM OF THOUGHT 
 
 6. The shifting of attention from one object to another is a fact 
 of common observation, of which psychology supplies examples 
 of great precision. The most instructive cases are those in which 
 the objective situation does not change, and yet, from internal 
 causes, now this feature and now that becomes prominent. A good 
 example is afforded by the changes in appearance of the staircase 
 and other similar figures, which have several times been mentioned. 
 Perhaps a still better example is seen in McDougallV " dot figure," 
 
 the particular construc- 
 tion of which can be 
 varied ad libitum. If 
 this figure is steadily ex- 
 amined, ever new groups 
 and arrangements ap- 
 pear within it; but it is 
 practically impossible to 
 hold any one for a long 
 time. " Varied reaction " 
 continually occurs in a 
 pronounced degree. 
 Changes of the fixation 
 point of the eyes are 
 also likely to favor 
 change in the group- 
 ings; but the shifting of 
 appearance is by no 
 means fully explained by these changes of fixation. The main fact 
 is that a complex stimulus, which can be apprehended in several 
 ways, or out of which several features can be isolated all equally 
 easy and of equal significance gives rise to a succession of differ- 
 ent percepts. 
 
 A similar instance of the shifting of attention, or of perception, 
 is found in " binocular rivalry." 2 Ordinarily, both eyes receive 
 only slightly differing views of an object, and these views are com- 
 bined or blended in a single percept (compare p. 425). But if, 
 artificially, and most easily by aid of the stereoscope, very different 
 views are brought before the two eyes, the conditions suitable for 
 single vision are not fulfilled, and there is likely to be an oscilla- 
 tion, in consciousness, between the deliverance of one eye and that 
 of the other-. If a plain red field, for example, is held before one 
 eye, and a plain green field before the other, we see red and green, 
 not usually together, but alternately. When one of the fields is 
 
 1 Mind, 1902, N. S., XI, 316. 
 
 * Compare Helmholtz, Physiologische Optik, p. 766. 
 
 FIG. 153. The Dot Figure (McDougall). 
 
SHIFTING OF ATTENTION 599 
 
 plain, and the other contains a figure, the figure remains seen for 
 most of the time, though the background may oscillate as in the 
 cases of the other figures. Voluntary attention devoted to one field 
 gives it some advantage over the other, by bringing it back more 
 quickly and thus abridging each appearance of the other field. 
 This is true to some extent even if both fields are plain; but volun- 
 tary attention has much more effect in holding a field which pre- 
 sents considerable detail and so allows of a succession of percepts. 1 
 
 It is clear that binocular rivalry belongs in large measure to the 
 level which we are apt to distinguish as " physiological," in contrast 
 with the mental; but it is equally clear that the value of each of the 
 two opposing stimuli is important in giving the advantage to one or 
 the other of them. Binocular rivalry reminds us, on the one hand, 
 of the oscillation of attention between two topics of interest; and, 
 on the other hand, of the alternation of two reflexes when the stimuli 
 for both are continuously acting (p. 173). ' Apparently, then, we 
 may formulate a law of the shiftings of attention in almost the same 
 terms as those of the law for the alternation of reflexes. When two 
 stimuli act simultaneously, either of two results may happen: The 
 two may facilitate each other's action that is, in psychical terms, 
 they may give rise to a combined or blended percept; or they may 
 be incompatible with each other, in which case they will not give 
 a resultant effect, but one or the other will get the "right of way" 
 and inhibit all response to the other for the time being. This in- 
 hibition, however, is followed by a "back swing" which gives the 
 advantage to the response that has just been inhibited; and so an 
 alternation of two responses occurs. Accordingly, it would be better 
 to substitute for "shifting of attention" some such term as "shift- 
 ing of perceptual reaction"; since rivalry between the eyes, and be- 
 tween different appearances of an ambiguous figure as well, oc- 
 curs even if attention is directed to something entirely apart. 
 
 7. What have been called "fluctuations of attention" also oc- 
 cur, if the attempt is made to hold continuously under observation 
 something which is just above the threshold of perceptibility, such 
 as the ticking of a watch when the watch is held at a distance such 
 as to be barely audible; or such as a small and barely perceptible 
 speck of white, or a patch of gray just distinguishable in brightness 
 from its background. In these cases, the watch is heard for a time, 
 then becomes inaudible, and then is heard again; and similar alter- 
 nations occur with the other stimuli. No one interpretation of these 
 experiences is altogether satisfactory; for oscillations in the condition 
 
 1 For these and several other facts regarding this important phenomenon, 
 see Breese, Psychol. Rev., Monogr. Suppl. No. XI, 1899; also Sherrington, 
 The Integrative Action of the Nervous System (New York, 1906). 
 
600 THE MECHANISM OF THOUGHT 
 
 of the sense-organs may as well be the cause as oscillations in atten- 
 tion, or "perceptual reaction"; and the fluctuations in perception 
 do correspond to some extent to oscillations in the blood pressure 
 and in respiration. 1 
 
 The tendency of attention to shift adds another factor to those 
 spoken of in the preceding chapter as determining recall. It is not 
 simply the stimulus, the stock of reproductive tendencies, with their 
 varying strength, and the adjustment of the moment, which are ef- 
 fective in governing recall; but the tendency to change operates 
 counter to perseveration and to the factor of recency, which, with- 
 out it, would probably control the mental train so as to make mental 
 development difficult or impossible. 
 
 8. The direction of attention, or of analytic perception, is de- 
 termined in part by dispositions left behind through past experi- 
 ence, as was said a few paragraphs back. It is not, however, en- 
 tirely so determined; for certain classes of stimuli have a natural 
 or instinctive hold on attention. Very intense stimuli compel at- 
 tention, as do sudden stimuli, sudden changes in a stimulus, or 
 moving objects in the field of view, or over the skin; sharp con- 
 tours in the field of vision, and objects contrasting strongly 
 with their background, have a similar attraction. These causes 
 operate even in young children, independently of previous experi- 
 ence, and lead to some degree of analysis of a presented complex 
 of stimuli. Each individual also possesses native interests, which 
 make their appearance as he grows older, and which lead him to 
 analyze out and react to certain features of situations to which other 
 individuals may remain relatively inattentive. In proportion as 
 these instinctive analyses are exercised and trained, they afford a 
 basis for further analyses of a more varied and more specialized 
 character, so that the adult's attention becomes directed very largely 
 by his acquired tendencies. 
 
 9. Psychology, as studied from the physiological and experi- 
 mental points of view, can make at present only meagre contribu- 
 tions to a theory of the development of abstract concepts. But 
 there is one more fact which should be mentioned in this connection. 
 Though a percept involves abstraction, a recalled impression is 
 likely to surpass its percept in this respect; since the setting is more 
 fully neglected in recall than in perception. Recall of the full setting 
 of an impression is seldom possible (compare p. 582); but recall 
 of a feature which has previously been observed often occurs without 
 recall of the setting. Among the features of a situation, which can 
 
 1 Urbantschitsch, Centralblatt f. d. med. Wissensch., 1877, p. 626; Lange, 
 Wundt's Philos. Stud., 1887, IV, 390; Lehmann, ibid., 1894, IX, 66; Slaughter, 
 Amer. Journ. of Psychol, 1901, XII, 313. 
 
THE NATURE OF COMPARISON 601 
 
 be relatively isolated in perception, are relations, forms, groupings, 
 and meanings of the most varied character. Each of these factors is 
 actually presented in some concrete setting; but it is partially ana- 
 lyzed out of the situation by the act of perception. 1 Now it appears 
 that any such feature which has been observed can be recalled with- 
 out its setting; or, the setting may be only vaguely recalled while the 
 feature in question comes to clear consciousness. 2 Even so "ab- 
 stract" a feature as the meaning of a proverb can be recalled and 
 identified as equivalent to that of another proverb, while the words 
 in which it first occurred are altogether forgotten. 3 This tendency 
 is of importance in the development of abstract concepts. 
 
 10. Comparison, or the judgment of likeness or difference, is, 
 like simple perception, dependent on memory. There are excep- 
 tions to this rule, especially in the case of different colors or degrees 
 of brightness when lying immediately side by side; for then an ele- 
 mentary apprehension of difference involves little more than the per- 
 ception of the line separating the surfaces. But in most cases, the 
 objects to be compared must be examined one after the other, so 
 that memory of the first one examined is necessary. Such judgments 
 have been frequently subjected to psychological experimentation, 
 although attention has usually been directed to the accuracy of the 
 comparison rather than to the process by which the judgment is 
 reached. 
 
 A stock experiment in judgments of this sort consists in the com- 
 parison of two weights, first one and then the other being lifted, and 
 the judgment being announced after the second lift. It would seem 
 theoretically necessary that the experience of lifting the first weight 
 should be recalled for comparison with the experience of lifting the 
 second; but Schumann, 4 who had an intimate knowledge of this ex- 
 periment, asserted that this is not the case, and that no recollection 
 of the first weight need be present in consciousness when the sec- 
 ond is lifted. Kulpe 5 introduced here, as in the case of recognition, 
 the distinction between mediate and immediate judgments, and ex- 
 periment 6 has abundantly justified this distinction, in the compari- 
 son not only of weights but also of other successively presented 
 
 1 Compare Ehrenfels, "Uber Gestaltqualitaten, " Vierteljahrschr. f. wiss. Philos., 
 1890, XIV, 249; Griinbaum, Arch. /. d. ges. PsychoL, 1908, XII, 452. 
 
 2 Compare Wundt, PsychoL Studien, 1907, III, 301; Titchener, The Experi- 
 mental Psychology of the Thought-Processes (New York, 1909). 
 
 3 Biihler, Arch. f. d. ges. PsychoL, 1908, XII, 24. 
 'Zeitschr. f. PsychoL, 1898, XVII, 119. 
 
 5 Grundriss der Psychologic, 1893, p. 212. 
 
 6 Bentley, Amer. Journ. of PsychoL, 1899, XI, 1; Whipple, ibid., 1901, XII, 409, 
 and 1902, XIII, 219. 
 
602 THE MECHANISM OF THOUGHT 
 
 stimuli. Martin and Miiller, 1 who, more than other experimenters, 
 have sought to dissect the process of comparison in the case of 
 weights, also found that no clear recollection of the first weight need 
 be present when the judgment was made. Often introspection 
 revealed nothing like a comparison of the two weights if by " com- 
 parison" be meant some form of holding them together but the 
 judgment seemed to be based simply on the impression produced by 
 the second weight. 
 
 If now we attempt an interpretation of what must seem at first 
 rather a mysterious process, we can make use of conceptions which 
 have become familiar in preceding discussions. It is probable that 
 the first weight arouses an adjustment or preparation for the sec- 
 ond, such that the effect of the second will be different in dependence 
 upon its being either heavier or lighter than the first. The adjust- 
 ment may consist in regulating the muscular force with which the 
 second weight is lifted; if, for instance, the force is so gauged that 
 it would lift the first weight with a moderate speed, and the second 
 weight yields tardily to this force, then the second weight appears 
 heavier than the first; but it appears lighter, if it yields with great 
 promptness. Repeated lifting of the same weight perfects the ad- 
 justment of muscular force to it; so that when this weight is com- 
 pared with other weights a little heavier or lighter, in a long series 
 of trials, the delicacy of adjustment to the central weight may be such 
 that lifting one of the lighter weights will convey an "absolute im- 
 pression" of lightness, etc., and upon this impression the judgment 
 may be based. 2 It is probable that this conception of the adjust- 
 ment set up in comparing weights, which is founded on the theory 
 of Miiller and Schumann, 3 is not fully adequate; and certainly an 
 adjustment of this motor type cannot easily be conceived as available 
 in the comparison of pitch, brightness, and many other kinds of 
 stimuli. Some kind of cerebral adjustment is likely, however, to 
 be set up in anticipation of any stimulus the character of which is 
 approximately foreknown. When the expectation is not fulfilled, 
 a shock of surprise is experienced, but this surprise does not by any 
 means imply that an image of the expected event is present in mind 
 alongside of the actual event, but only that something happens 
 for which we are not ready, i. e., not adjusted. Innumerable ex- 
 amples of this truth may be derived from daily experiences, which 
 show that the way the mind reacts to any new event requiring judg- 
 
 1 Zur Analyse der Unterschiedsempfindlichkeit, 1899. 
 
 2 Martin and Miiller, op. cit. 
 
 3 Arch. f. d. ges. PhysioL, 1889, XLV, 37. Interesting examples of the reality 
 of motor adjustment or " Einstellung " are given in this paper and in a supple- 
 mentary study by Laura Steffens, Zeitschr. /. PsychoL, 1900, XXIII, 241. 
 
THE PSYCHOLOGY OF REASONING 603 
 
 ment depends upon a multiplicity of pre-existing conditions, many 
 of which are barely above the threshold of consciousness or even 
 wholly subliminal. Such experiences vary from the physical 
 shock which we encounter on miscalculating the width or the 
 height of the step of a stair, to the spiritual shock with which an 
 insulting look, or an unpleasant memory, or an immoral thought, 
 is greeted in consciousness. 
 
 Fine comparisons require a much more delicate adjustment 
 than is present in many cases of expectation, but the inner process is 
 probably of the same general character. Since these are generally 
 made with more deliberation, and what may figuratively be called 
 "weighing of evidence," we often find ourselves alternating between 
 mental images that represent the more recent, and the more remote, 
 of the sensuous impressions that are to be compared. But if the 
 attempt is made to tell "just how much" the more recent experi- 
 ence differs from the earlier, a voluntary effort to recall more dis- 
 tinctly the memory image of the earlier is quite sure to follow. In 
 all these cases of so-called "comparison," when considered from 
 the introspective point of view, enough of recollection is involved to 
 account for that recognition of the character, amount, and direction, 
 of the change in the sensation elements of consciousness on which 
 the judgment is based. 
 
 11. The psychology of reasoning is even less understood than 
 that of abstraction and comparison. 1 Little aid can be derived 
 from logic for an understanding of the actual processes which go 
 on, either in consciousness or below the threshold, in an act of 
 reasoning. The formal character of the syllogism is undoubtedly 
 a travesty, rather than a description, of most concrete instances of 
 inference; and, in particular, the major premise, or general principle 
 involved, is almost always implied rather than overtly expressed 
 in the thinking process. But there is a more serious exception to 
 be made. The syllogism treats inference as though it were a 
 straight-ahead process; the premises are supposed to be given, and 
 the conclusion is supposed to follow from them by "laws of thought" 
 which are axiomatic and compelling. If this were a true account, 
 rational thinking would be a much simpler and easier matter than 
 it is. But reasoning is not, in fact, a straight-forward progress, and 
 does not proceed by rule; neither are the premises customarily 
 recognized as given. This is true, at least, in a large proportion of 
 concrete cases of the discovery of causes and the drawing of in- 
 ferences. It is comparatively unusual to ask: "What further use 
 can be made of the knowledge which I possess?" The usual 
 
 1 See Pillsbury, The Psychology of Reasoning (New York, 1910); Dewey, How 
 We Think (Boston, 1910). 
 
604 THE MECHANISM OF THOUGHT 
 
 question is, "What do I know that will help me to solve this par- 
 ticular problem?" What is given is a problem. The solution of 
 the problem corresponds to the conclusion of the syllogism; the 
 minor premise corresponds to an insight into the problem, i. e., 
 to the perception of some feature which is the key to the situation; 
 the major premise consists of previously acquired acquaintance 
 with, and knowledge about, this feature; but this knowledge may 
 or may not have received a definite formulation. 
 
 As a hypothetical example, we may suppose the following thoughts 
 to have passed through the mind of Ebbinghaus, as he considered 
 the problem whether anything once learned was ever forgotten; 
 or to what extent and at what rate it was forgotten. Clearly what 
 was in his mind to start with was something more like the conclusion 
 than like the premises of a chain of reasoning; he could formulate 
 alternative conclusions from the beginning, but would not know 
 which was true. As he considered the problem, it may have first 
 occurred to him that learning produces some effect on the mind, 
 or on the brain, and that a change once induced ought never to be 
 entirely lost; and therefore what has been learned ought never to 
 be entirely forgotten. In other words, he has isolated from the 
 solution the feature, "Change produced in a substance," and has 
 either recalled, or newly formulated, a general principle applica- 
 ble to this feature. But he may also have had his attention directed 
 to another feature namely, that he has now to deal with a living 
 creature, and that atrophy through disuse is characteristic of such 
 creatures; and thus he may have come to doubt the validity of his 
 first conclusion. He may then have been driven to abandon the 
 attempt to solve the problem deductively, and have sought to ap- 
 proach it inductively, or by way of examining concrete instances. 
 He may have thought of the first lines of the "^Eneid," which he 
 learned fifteen years before possibly he can recall some of them 
 now. This failing, he was not satisfied to conclude that they have 
 been all forgotten, for it might be that, once started, he could con- 
 tinue these lines. He hunts up a Vergil, and allows himself to 
 refresh his memory by seeing the first line; but on turning from the 
 book, he is unable to continue. Still unsatisfied, for the first line 
 had a tinge of familiarity which seemed to indicate some degree of 
 memory of it, the thought may have occurred to him: "It might be 
 easier to relearn these lines than to learn an equal number of lines 
 from some other part which had never received special attention." 
 He then tries this supposition, and finds it correct. But now sub- 
 ordinate problems sprout out of the main problem, bearing on the 
 best methods of testing this matter of relearning; till finally the "sav- 
 ing method" and the use of nonsense syllables are decided on as 
 
THE SEARCH FOR PREMISES 605 
 
 suitable means for examining the degree of retention; the experi- 
 ments are tried and the solution reached. Clearly the entire process 
 in this case has been anything but straight-ahead; it has depended on 
 " turning over in the mind " the situation of trying to recall something 
 long forgotten, and being "struck by" this and that aspect of the 
 situation in the light of previous experience. Translated into psy- 
 cho-physical terms, it has been distinctly a process .of " varied reac- 
 tion"; and both the inductive and the deductive parts of the process 
 are alike in this respect. 
 
 12. In a word, reasoning involves hunting for premises, or for 
 features which are significant; and for past knowledge of, or reac- 
 tions to, these features. As in other processes of analysis and recall, 
 the course of thought is determined in part by the present adjust- 
 ment, or direction toward a certain problem; partly by reproductive 
 tendencies of varying strength; partly by the concrete situation with 
 its many incidental and often confusing features; and partly by the 
 fundamental tendency to vary the reaction. 
 
 Though the above example is hypothetical, it follows along the 
 lines of actual instances of rational response to a concrete problem, 
 especially as these are presented in such trivial but easily observed 
 processes as the solution of puzzles. 1 In many such cases, it is 
 difficult to distinguish reasoning sharply from complex cases of 
 "transferred reaction ";[ the reasoning process consists, in fact, of 
 the transfer to a new situation of some mode of response previously 
 acquired -in different situations, and depends on some degree of 
 isolatian-of features which are common to both the new situation and 
 the old^^he transference is regarded as more rational in propor- 
 tion as it is conscious and deliberate; and in proportion, also, as 
 the varied reactions are tried out mentally instead of being impul- 
 sively put to an immediate motor test. Though it is apparently 
 impossible to lay down any straightforward procedure for approach- 
 ing the solution of all problems, a few general maxims emerge 
 from the examination of concrete processes of reasoning. A good 
 supply of major premises in the form of reproductive tendencies 
 ready to be brought into play is a fundamental desideratum. 
 And the more clearly these premises have been thought out and gen- 
 eralized, the more likely they are to be recalled in a variety of situa- 
 tions. Further, the importance of keeping an open mind for feat- 
 ures or possibilities which do not at first appear i. e., the necessity 
 of inventing and adopting varied reaction is so great that it may 
 well be used as a maxim for the practical guidance of reasoning. 
 Finally, as an offset to the preceding maxim, another may be based 
 on the value of dwelling on each feature long enough to afford an 
 1 Compare the previous reference to the work of Ruger, pp. 555 f. 
 
606 THE MECHANISM OF THOUGHT 
 
 opportunity for any reproductive tendencies connected with it to 
 be brought into play. 
 
 13. Reasoning, like recognition, perception, and comparison, 
 may be " immediate" as well as " mediate"; 1 and some of the clever- 
 est instances of reasoning are cases in which a new situation is seen 
 to be an exemplification of some general principle, without conscious 
 recall of the former situations in which the principle was learned, 
 and even without conscious recall of the principle itself as inde- 
 pendent of the case in hand. An abstract general principle, or a 
 principle as exemplified in some former situation, needs always to 
 be applied or adapted to the novel situation; but immediate reason- 
 ing short-circuits this process by seeing the principle, not as "in 
 itself," so to say, but as adapted or exemplified. If we possessed 
 an introspective history of moments of new insights and discoveries, 
 we should probably find that the insight usually started with this 
 immediate or implicit reasoning, which was followed by explicit 
 statements such as are necessary to present an inference in lan- 
 guage. 
 
 On glancing back over the preceding sketch of the psychology of 
 thinking, meagre and disconnected as it must seem, one cannot fail 
 to be impressed by the great similarity of the mental process as it 
 appeared in all those forms of perception, analysis and abstraction, 
 comparison and reasoning, as well as those of learning and recall, 
 presented in the preceding chapter. All of these performances, even 
 the most intellectual, though they differ greatly in content, preserve 
 the method of procedure that was visible in learning by trial and 
 error. In all cases of this procedure there appears the controlling 
 influence of some problem or aim which so sets or adjusts the psy- 
 cho-physical mechanism as to select the relevant associative tenden- 
 cies and make them prevail above others which are irrelevant to the 
 problem in hand. In all cases, too, the process of solution with the 
 exception of persons well drilled in very similar problems is usu- 
 ally far from straightforward; it has rather the form of varied and 
 tentative reaction; and in all, an essential fact is the dealing with 
 parts or features of a situation by emphasizing each in turn to the 
 temporary neglect of other features. Such thinking appears, in 
 other words, as a species of observation with reaction; and the de- 
 grees of its intellectuality and practical success depend largely on 
 what is observed, i. e., on the content or material of the process rather 
 than on its form. It is a question of what features of a situation are 
 isolated or analyzed out, and made the starting-point for recall and 
 motor reaction. To note the fall of an apple is to analyze out a com- 
 
 1 Storriug, Arch. f. d. ges. Psychol, 1908, XI, 1; Pillsbury, The Psychology of 
 Reasoning, pp. 119-120. 
 
IMMEDIATE AND MEDIATE REASONING 607 
 
 paratively obvious feature; to distinguish falling as a feature of the 
 moon's revolution about the earth is still analysis, proceeding in much 
 the same manner, but dealing with more difficult material and work- 
 ing with much more refined and elaborate tools. 
 
 14. During all the discussions of the last five chapters, and, in- 
 deed, throughout the entire Part II of the book, one truth of su- 
 preme importance has been both assumed and illustrated, even 
 where it has not been made perfectly obvious. This truth has been 
 embodied in the language which we have found ourselves forced 
 to employ. The similarity of the terms, and even in many cases 
 their identity, which were used in describing the histological struct- 
 ure and physiological functions of the nervous mechanism, when 
 studied from the points of view afforded by kindred natural and 
 physical sciences, with the terms used in describing the condi- 
 tions and activities of the conscious mental life, when studied from 
 the introspective point of view, can scarcely have escaped notice. 
 This "parallelism" is undoubtedly of the highest significance. It 
 is true that some of these terms are rather highly figurative, and 
 constructed upon a basis of analogy rather than of observed facts; 
 it is also true that many of them only imperfectly suggest what we 
 are led to believe are the actual phenomena, and the laws of their 
 interdependence. On the other hand, it cannot be doubted that they 
 describe, on the whole faithfully, the real character of the facts. 
 But this is to say that, between the nervous mechanism and the phe- 
 nomena of conscious mental life, as respects the constitution and the 
 behavior of both, there is, in man's case also, an extensive and in- 
 tricate system of " correlations" 
 
 Among those correspondences of language which are significant 
 of correlations in fact, the following are some of the more important. 
 In describing the structure and functions of the nervous mechanism 
 and, as well, the nature and development of mental life, it was found 
 necessary to employ terms indicative of a great variety of highly 
 differentiated elements combining in manifold intricate ways, and in 
 different degrees of intensity, to bring about more and more com- 
 plicated and purposeful results. We spoke of "stimuli" and "re- 
 actions" to stimuli, in the case both of mechanism and of mind. 
 But the one is a physical, the other a psychical, sequence. We took 
 note of "transferences" and "consequences"; of "facilitation" 
 and "inhibition," or "interference"; of the acquirement of "habits 
 of reaction" as shown in the recurrence of states or forms of action, 
 essentially similar although modified in various details. There 
 were tokens of the laws of " conservation " and of " perseveration " 
 in both nervous mechanism and mental life. But especially was it 
 
608 THE MECHANISM OF THOUGHT 
 
 significant to see how the development of both mechanism and mind 
 was dependent upon the speedy and, so to say, accurate formation 
 of "higher units" of reaction out of the more elementary factors; 
 and how in the formation of such units the more elementary con- 
 ditions and reactions were absorbed or lost out of sight. Indeed, if 
 now all the facts and laws hitherto treated are passed in review as 
 illumined by this thought of the actuality of the relations between 
 mechanism and mind, as testified to in the very language employed 
 to describe both the one from the objective, and the other from the 
 introspective point of view the significance of the term "physio- 
 logical psychology" will be enforced and illustrated anew. 
 
 15. In closing this Part of our work, therefore, we may fitly 
 attempt to summarize some of the results in the form of established 
 correlations between the nervous mechanism and mental life, while 
 leaving details to the further and renewed study of the same class of 
 facts which have occupied our attention, and been made the basis 
 of our theories, from the beginning of the book. The general 
 problem with which we have been concerned may now be formu- 
 lated anew in somewhat the following way: What conception 
 can be formed of cerebral action, in the combined light of the 
 facts of anatomy, physiology, and pathology, as these were set forth 
 in the first Part of the book, and of the facts of psychology as set 
 forth in its second Part? Without necessarily looking to cerebral 
 action as the explanation of mental activity, one may be convinced 
 that the brain is active in mental operations, and may seek to form 
 scientific conceptions of the character of its activity. It is certainly 
 to psychology that the neurologist must appeal, in large measure, for 
 the facts of behavior on which his conceptions of such activity must 
 be based. It will be admitted at once by the psychologist that the 
 data supplied by him are deficient not only, for the time being, in 
 range and quantity, but also in minuteness and definiteness of de- 
 tail. As was shown in considering the subject of localization of 
 cerebral function, the brain must work in much greater detail than 
 is visible to the observation of the psychologist, whether his observa- 
 tion be of the introspective or of the objective type. From the latter 
 point of view, he determines the reaction of the subject to stimuli, 
 and obtains a rough dynamics of behavior; in introspection, he seems 
 to have some insight into the process intervening between stimulus 
 and reaction, and so to penetrate somewhat to the inside of behavior. 
 But a reaction, as judged by its end-effect, is a mass action; and 
 introspection shows little of the detail which must be present in 
 all brain action. Hence there is no immediate prospect of obtain- 
 ing from psychology a full and minute account of any single men- 
 tal performance, or of reaching by this means anything like a de- 
 
CEREBRAL CONDITIONS OF CONSCIOUSNESS 609 
 
 tailed description of the cerebral process. The most that can be 
 hoped is to gain some indications of the general character of cere- 
 bral action. 
 
 16. Peculiar difficulties encompass the problem of determining 
 what cerebral activities are the indispensable conditions of conscious- 
 ness, in general. For, not only do we find what must appear to us 
 as abundant signs of intelligent and purposeful activity in animals 
 of a simple and low form of nervous organization, or of no strictly 
 nervous mechanism whatever, but also in plant life, and even in 
 the ova and protozoa, and in the individual cells which are aggre- 
 gated to form both vegetable and animal tissues. As to a full ex- 
 planation of consciousness in terms of brain activity, to propose the 
 problem is to expose the absurdity of an attempt at its answer. But 
 to inquire as to what are the cerebral conditions of consciousness in 
 man's case, is a legitimate and interesting problem for physiological 
 psychology. Alas! that the answer to the problem still remains so 
 incomplete. 
 
 Consciousness may reasonably be taken as indicating brain ac- 
 tivity; in other words, when there is consciousness, then the brain 
 is active. And degrees of consciousness may probably be taken as 
 indicative of degrees of brain activity. The field of attention may 
 therefore be taken as an index of the field of greatest brain activity. 
 Many parts of the cerebrum many systems of nervous connec- 
 tions within it are likely to be simultaneously active; it is, therefore, 
 a reasonable supposition that the most conscious part of conscious- 
 ness is an index of the most active part of the brain; so that, when 
 sights and sounds are both simultaneously present in consciousness, 
 but sights occupy the field of attention and sounds are relatively 
 in the background, at such a time the visual area and its immediately 
 connected areas are more active than the auditory area and its con- 
 nections. Even this supposition, however, is not altogether free 
 from difficulties. 
 
 17. The more precise question now arises as to what is meant by 
 the brain activity that is said to be indicated by the existence of 
 consciousness. The mere transmission of nerve-currents along the 
 numerous "association fibres" of the brain is not, probably, what is 
 directly correlated with consciousness. The activity of the nerve- 
 cells in the cortex would be a better guess. But on account of the 
 great functional importance of the synapses, a still more likely sup- 
 position would regard the brain activity indicated by consciousness as 
 occurring at these junctions between neurones; or, perhaps, both 
 in the synapses and in the dendrites and nerve-cells. The activity 
 of the brain resolves itself, accordingly, for the most part into the 
 passage of nervous "currents" or "impulses" across the junctions 
 
610 THE MECHANISM OF THOUGHT 
 
 between the terminations of axons and the dendrites with which they 
 come into contact. 
 
 According to this view, consciousness serves as an index that nerve- 
 currents are traversing synapses in the brain; and incidentally also, 
 of course, passing along the nerve-fibres between one synapse and 
 another. But now a difficulty arises from the fact that well-trained 
 or habitual reactions occur with a minimum of consciousness. It 
 even happens that strong sensory stimuli, if commonplace and of 
 no momentary significance, are neglected by attention for much 
 weaker sensory stimuli; and that fairly energetic motor reactions 
 may be carried on while the centre of consciousness is fixed upon 
 something quite different. " He that runneth may read," or other- 
 wise direct his attention; though no doubt motor quiet would be a 
 more favorable condition for such intellectual activity. On the 
 other hand, there can be no doubt that if reading were originally 
 learned while running, the act of running would facilitate the act 
 of reading. The difficulty is, then, that performances which in- 
 volve the cerebrum, and which seem to require a considerable ac- 
 tivity in the parts of the cerebrum concerned with them, may yet be 
 attended with but a low degree of consciousness. We may note 
 at once, however, that, though intense sensory stimuli and energetic 
 muscular action must require strong activity in the parts of the cere- 
 brum which are concerned with them, this is no proof that other parts 
 of the brain may not be simultaneously in even greater activity. 
 To compete with a strong sensory stimulus, another object of atten- 
 tion must have much momentary attraction or significance; and this 
 probably indicates that its system of cerebral pathways is in a con- 
 dition of very high activity. The focus of activity need not always 
 lie in the sensory or motor areas. 
 
 18. Important, if not decisive, objections maintain themselves to 
 each of two opposed theories : one of which 1 regards clear conscious- 
 ness as indicating an open path for motor discharge; and the other 
 of which 2 considers that the difficulty of free passage stimulates 
 attention and so increases the degree of clear consciousness, or that 
 consciousness itself is the product of a condition of tension at the 
 synapses. 3 The former, the so-called "Action Theory," seems dis- 
 credited by the general experience that well-trained reactions, which 
 imply open motor-pathways, are almost uniformly only dimly repre- 
 
 1 So Miinsterberg, Grundzuge d. Psychologic, 1900, I, 525. 
 
 3 Dewey, Psychol. Rev., 1894, I, 553; 1895, II, 13; Philos. Rev., 1897, VI, 43. 
 
 3 Montague, "Consciousness a Form of Energy," Essays Philosophical and 
 Psychological in Honor of William James, 1908, p. 103. Objections to Wundt'a 
 theory of a special centre for attention or "apperception" in the frontal lobes 
 have already been brought forward (p. 274). 
 
THE MECHANISM OF ATTENTION 611 
 
 sented, if at all, in consciousness. The supposition that such path- 
 ways do not lie in the cerebral hemispheres, but only in the cord and 
 brain-stem, is opposed to the phenomena of aphasia, apraxia, asym- 
 bolia, etc., which follow upon injury to these hemispheres. 
 
 The theory which emphasizes the difficulty of reaction as the 
 cause of increased clearness of consciousness hardly seems to com- 
 port well with all that we know about the preference of objects, both 
 in perception and recall a matter which, except in the most ex- 
 treme cases of selective attention, and largely even then seems to be 
 determined by influences that do not manifest themselves in con- 
 sciousness at all. Nor can it be claimed that the "feeling," or 
 other evidence, of tension is anything like a constant, not to say 
 indispensable, precondition of clear consciousness. 
 
 19. The shifting of attention, and "varied reaction" in general, 
 next claim our notice. This problem has already met us in the 
 sphere of reflex action (pp. 165, 173), in the form of that alterna- 
 tion of responses which sometimes results from a single stimulus. 
 As in reflexes, so here in the shifting of perceptual reactions, the facts 
 point to the existence of branching tracts in the nerve-centres, and 
 so of alternative pathways open to a nerve-current, but unequally 
 open. McDougall, who in his "Physiological Psychology" 1 has 
 made one of the most serious of recent attempts to conceive the 
 neural apparatus involved in mental action, offers a theory of the 
 mode of action of these branching pathways. He makes use of 
 two conceptions: of fatigue, and of the "drainage" of energy from 
 all parts of a neurone into any synapse across which the current 
 actually breaks its way. Suppose to illustrate that a nerve-cur- 
 rent reaches a point where the paths branch; there is synaptic re- 
 sistance to its passage into either of these paths; the nerve-current 
 is, for an instant, dammed up against the entrance to each of them, 
 and gathers tension at each. At whichever entrance the resistance 
 is lower, there the gathering tension will soonest be sufficient to 
 force a passage, and when once an outlet is thus secured, all the 
 energy is drained off into it, so that none takes the alternative path. 
 But now the synapse across which the current is moving suffers fa- 
 tigue from this passage, and its resistance increases till further pass- 
 age is blocked. The result of this blocking of the first outlet may 
 be that the alternative outlet is forced, and so the current takes a 
 new path and gives rise to a different reaction. Such a shifting 
 from one outlet to the other may be repeated time after time. This 
 interpretation, its author thinks, is readily applied to the alternation 
 of percepts in viewing the staircase or dot figures, and probably to 
 all cases of shifting of attention. 
 
 1 London, 1905; see also Mind, 1906, N. S., XV, 329. 
 
612 THE MECHANISM OF THOUGHT 
 
 There are, however, what appear to us decisive objections to 
 this ingenious hypothesis. In the first place, it does not explain 
 the high excitability of a pathway which has just been inhibited. 1 
 Possibly a slight modification will enable the theory to succeed bet- 
 ter at this point. The nerve-current which beats at the door of a 
 pathway without gaining admission is, in fact, a "subliminal stim- 
 ulus" at that point; and the fact of "summation of subliminal 
 stimuli" shows that such a stimulus, though it does not arouse ac- 
 tivity, induces a heightened excitability. Let us suppose, then, that 
 a nerve-current, acting on a branched path, not only breaks through 
 at one point and produces activity there, but raises the irritability 
 of all synapses against which it impinges without being able to 
 break through. It would therefore leave such synapses in a " con- 
 dition of readiness" for any soon-following stimulus. Stated in 
 this modified form, the conception is rather attractive; since it 
 would apply not only to shifts of attention, but equally well to the 
 condition of readiness observed in experiments on memory, when A, 
 previously associated with B, does not now recall B, and yet makes 
 it easier to reach B by some other way. In such a case, we may 
 imagine that the nerve-current started by the presentation of A 
 impinges, at some point in its passage through the brain, upon a 
 synapse leading to the response B, but this synapse has a high re- 
 sistance, and the current does not pass that way; yet it raises the 
 excitability (or lowers the resistance) of this synapse, so that some 
 other nerve-current, reaching the same synapse or nerve-cell by 
 another route, enters and gives rise to the response B. 
 
 A further and more decisive objection to the conception of varied 
 reaction as due to branching paths is this : unicellular animals, like 
 Amoeba and Paramecium, exhibit varied reactions, but, having no 
 nervous systems, can hardly be credited with alternative paths. 
 Their varied reactions are not executed by different effectors; but 
 the whole cell seems to participate in different and often opposed 
 movements. Varied reaction is, therefore, much more primitive and 
 fundamental than branching paths. Indeed, were this not so, the 
 very existence and development of animal life would seem impossi- 
 ble. Besides, fatigue is not admissible as an explanation of the 
 shifting of response in these unicellular forms; since the same 
 structures are active throughout the sequence of movements. Such 
 behavior as that of Stentor, for example (p. 544), in response to a 
 continued stimulus could not be explained in terms of fatigue; for 
 if a cell is fatigued by its first gentle response, how could the same 
 cell shift at once to a more vigorous response to the same stimulus ? 
 Some internal change certainly occurs in the cell, in consequence of 
 1 Compare Sherrington, op. cit., p. 203. 
 
THE POWER OF VARIED REACTION 613 
 
 the first stimulus and response, and by virtue of this change repe- 
 tition of the stimulus gives rise to another response; but this change 
 cannot be of the nature of fatigue. 
 
 20. Although we have no certain knowledge as to its nature, 
 we must at least accept the gross fact that some change occurs in 
 the condition of the cell, such that the same stimulus, on being re- 
 peated, gives rise to a different response. It is not, accordingly, a 
 wholly unreasonable attempt to view the nervous system of man in 
 the light of this behavior of unicellular animals. If we suppose that 
 each neurone has the power of varied reaction, we have a way out 
 of some of our difficulties. The neurone has hitherto been held to 
 have only one form of reaction; but the facts of inhibition, refractory 
 period, etc., seem to indicate that it has at least two opposed modes 
 of reaction, and that these correspond, in a manner, to the two op- 
 posed reactions of the amoeba. Now the amoeba sends out branches 
 in response to some stimuli, and draws them back in response to 
 others. Some neurologists have believed that protraction and re- 
 traction of the dendrites occur, and have based explanations of 
 sleep, etc., on these "amoeboid movements" of the neurone. The 
 balance of evidence is against these movements, but inner changes 
 of a chemical or electrical nature might have the same results. 
 Protraction of the dendrites was supposed to give closer contact 
 at the synapses and so to lower the resistance to passage of a nerve- 
 current, and retraction to have the opposite effect. Similar changes in 
 resistance might, however, be the result of changes of surface tension 
 in the dendrites, or of other changes not involving molar motion. 
 Suppose, then, that each neurone retains so much as this of the prime- 
 val power of varied reaction; suppose that it has two opposite modes 
 of response, positive and negative, corresponding to excitation and 
 inhibition. It may in this case give either response to a stimulus, 
 according, perhaps, to the character of the stimulus, but more par- 
 ticularly according to its own condition as determined by preceding 
 stimuli and responses. Each positive response is followed by a 
 negative phase, and each negative response by a back swing toward 
 a positive phase. Aside from this substitution of negative response 
 for fatigue, the cerebral action in varied reactions could then be 
 conceived as before, in terms of branching paths, drainage, and sum- 
 mation of stimuli. The most recent researches seem to favor such 
 a view of the "selective affinities" of the cells composing different 
 structures of the animal body, and even of individual cells. 
 
 21. On recurring to what was said concerning the known facts 
 and laws of the Presentations of Sense, and the varying qualities 
 and quantities of the sensations composing them, the correspond- 
 ences between nervous mechanism and mental phenomena, as 
 
614 THE MECHANISM OF THOUGHT 
 
 indicated by the language employed to describe each, will be found 
 to pervade the entire treatment of the subject. It is, therefore, 
 necessary to add only a few words of a more general character at 
 this point. In the first place, the distinctive qualities of things as 
 apprehended by the senses are undoubtedly significant of specifically 
 different reactions of the brain to the various kinds of stimuli com- 
 ing into it along the different paths from the different end-organs 
 of sense. Qualitative distinctions, as observed by the conscious 
 mind, imply specific differences of some sort in the correlated brain 
 activities. Something definite as to special localities chiefly con- 
 cerned in the cerebral activities connected with the different senses, 
 and even as to histological differences within those centres, is already 
 scientifically established. And we seem to be on the eve of knowing 
 something, or at least of having some ground for conjecturing some- 
 thing, about the chemico-physical peculiarities of the different 
 states and forms of cerebral functioning concerned with the different 
 senses. 
 
 Further, the whole theory of "local signs," as justified from the 
 introspective point of view, suggests a corresponding theory of the 
 highly complex character of the brain reactions that result from the 
 simultaneous discharge into the cerebral hemispheres of a great vari- 
 ety of weak stimuli from a correspondingly great variety of recep- 
 tors. The quantitative relations between brain activities and the 
 conscious estimate of the intensity of our sensations, and of the mag- 
 nitude of the objects of sense, are too obviously in place here for more 
 than a mere reference to chapter III to be necessary. The same 
 thing is equally true of that sequence in mental events of which 
 we are consciously aware and the objective time occupied by con- 
 current or sequent events in the areas of the cerebral hemispheres. 
 
 In all these cases, the correspondences of quality, quantity, and 
 temporal relations are no simple and easily analyzable affair. 
 Neither do they lend themselves readily to the discovery and con- 
 fident announcement of so-called " laws," whether it is proposed to 
 apply the term to the activities of the brain or to our conscious ex- 
 perience by way of sense-perception. Even less do they, up to the 
 present time, enable us to enunciate a system of fixed and inflexible 
 terms on which the brain and the mind shall, so to say, pay atten- 
 tion to each other. But as to this condition of general correspond- 
 ence, and as to the abstract possibility of extending our knowledge 
 of it more and more into details, the evidence from the study of the 
 development of sense-perception in man's case is, of all the available 
 evidence, quite the most conclusive. 
 
 22. In studying the presentations of sense from this point of 
 view, it must be borne in mind, that they can never be regarded as 
 
CEREBRAL ACTION IN MEMORY 615 
 
 merely fortuitous, or most strictly necessitated, aggregations of 
 sensations, qualitatively and quantitatively different. They all im- 
 ply such activities as selective attention, more or less conscious dis- 
 crimination, and mediate or immediate judgment and voluntary 
 or involuntary recall. They are also creatures of habit, so to say. 
 But attention, discrimination, judgment, and recall, as studied 
 from the introspective point of view, of themselves invite and en- 
 courage the study of the corresponding brain activities. What can 
 be made a matter of reasonable conjecture as to the conditions and 
 functions of the cerebral hemispheres in connection with these con- 
 scious activities belongs, also, to our legitimate attempt to explain, 
 or illustrate, the nature of sense-perception in the light of physiolog- 
 ical psychology. 
 
 23. A certain antagonism between the tendency to change and 
 the tendency to persistence or " perseveration " of a response, has al- 
 ready been remarked. It is certainly a fact that a response just 
 made is more readily evoked a second time. The effects of great 
 "recency" in favoring recall are too clear to be doubted. The con- 
 tradiction between this fact and the fact of shifting of response is 
 only apparent. The refractory period, or negative after-effect, of 
 response is of variable, but usually of very brief duration (compare 
 p. 164), and gives way, in turn, to a condition of heightened excita- 
 bility. This can easily be demonstrated in case of the heart mus- 
 cle, in which both the refractory period and the increased excita- 
 bility after response are present to a marked degree. Skeletal 
 muscle also shows both effects, as do unicellular animals. It is then 
 a legitimate supposition that a cerebral neurone, after recent ac- 
 tivity, is in such condition that it is readily excited again; its synapses 
 have a low resistance, and any nerve-current which impinges on its 
 dendrites from any axon is likely to enter and give rise to a repeti- 
 tion of the recent reaction. 
 
 24. In the last paragraph we have entered upon the attempt 
 to conceive of the cerebral action in Memory and Learning. A re- 
 sponse once made leaves behind a condition favorable to the repeti- 
 tion of the same response. But this after-effect is, in itself, only 
 temporary, and appears in low organisms which show no sign of a 
 permanent modification of behavior. Something must be added to 
 our conceptions to account for true learning and for the formation 
 of habit. The "law of habit" states that a response, once made, 
 is more readily excited again, even after an interval of complete 
 quiescence. Perseveration, "warming up," and the ready recall 
 of very recent responses, are best understood as due to a persistence 
 of activity. The structures concerned have not entirely cooled off, 
 or gone to sleep. The recall of a response after a longer interval 
 
616 THE MECHANISM OF THOUGHT 
 
 corresponds to the awakening of organs which have once been active 
 but have since been asleep. This is quite another thing, and de- 
 mands a quite different interpretation. Some comparatively per- 
 manent change of structure has been induced. Since the structures 
 with which we are here concerned are living, the change can best 
 be thought of as having a metabolic or nutritive character. We 
 know that activity of a muscle enables it, in its subsequent rest, 
 to take up nutriment from >he blood, and so to increase in size. 
 There is evidence (p. 58),^at activity of the brain causes a growth 
 in the fine branphespf^the axons and dendrites, which are the essen- 
 tial structures informing connections. Besides this growth in size, 
 a muscle shows, after exercise, an improvement in its inner condi- 
 tion; it shows this in the fact that its increased strength is often too 
 great to be explained in terms of increased size. The exact nature 
 of this inner change is unknown, but it may reasonably be supposed 
 to be nutritive or metabolic. It is not unlikely that neurones also 
 improve in their inner condition as the result of preceding activity. 
 Both growth of the fine branches and improvement in internal con- 
 dition are, probably, factors in the retention of a response. We may 
 regard the combination of neurones which are concerned in carry- 
 ing out any reaction as an organ, which grows and improves its 
 condition as the result of exercise, and which is thus made more 
 ready for work and more likely to be called into activity. That an 
 organ improves as the result of exercise is the fundamental physi- 
 ological conception in the doctrine of habit and learning. In connec- 
 tion with this must go the explanation of what we have called " dy- 
 namical associations." 
 
 25. Several details of the process of retention, as revealed by 
 psychological experiment, fit easily into the above conception. 
 That retention dies out with the lapse of time seems but an instance 
 of " atrophy through disuse," or to speak in more general terms 
 of the tendency to recovery and "regulation." That repeated ex- 
 ercise gives continued improvement, in motor skill or in the mastery 
 of a series of nonsense syllables or a poem, may be explained as due 
 to the accumulation of the nutritive after-effects of a single period 
 of activity. The advantage of periods of rest between the successive 
 repetitions of an act to be learned, is what would be expected from 
 the behavior of other organs, such as the muscles. No one would 
 hope for great and permanent increase in the strength of a muscle 
 from prolonged over-exercising it at one time; common experience 
 teaches the advantage of spreading out the exercise over many days. 
 A physiological explanation of this fact may be attempted as fol- 
 lows : the nutritive after-effect of exercise occurs largely in the subse- 
 quent period of rest. After a little exercise, rest improves the organ, 
 
 7 
 
MECHANISM OF ASSOCIATIONS 617 
 
 which enters on the next period of activity more capable of deriving 
 benefit from it. It is, perhaps, a larger organ, and so able to absorb 
 more nutriment in its next rest. Massing all the exercise in one 
 period of continued activity prevents, in large measure, the summa- 
 tion of nutritive after-effects. But since, in all learning, it is not iso- 
 lated but co-ordinated, and often widely locally separated activities 
 of the brain which are involved, something of a more dynamic char- 
 acter is needed to account for the phenomenon of "associated" re- 
 tention. 
 
 26. While, then, the physiology of simple retention, as in the 
 case of the muscles, falls readily into accepted biological terms, the 
 physiology of the formation of associations meets with peculiar diffi- 
 culties, such as might be expected from the fact that the capacity 
 for initiating connection between stimuli and responses not hitherto 
 connected with them is peculiar for the most part to the nervous 
 system, and especially to the brain. The problems chiefly suggested 
 are the two following: (1) What is the physiology of learning by 
 " trial and error," and of the selection of the successful response ? 
 (2) What is the physiology of the formation of an association be- 
 tween two presented stimuli? In regard to the mechanism of se- 
 lection from varied reactions, we have already expressed an opinion 
 (pp. 551 fL), though not in strictly physiological terms, and that 
 matter need not be further rehearsed. The second problem is one 
 of so great importance, as to lead one author to declare: " If no solu- 
 tion can be found, physiological psychology is bankrupt." l 
 
 The problem of association by contiguity, as already stated, con- 
 sists in the fact that A and B, between which an association is 
 formed, do not stand from the outset in the relation of stimulus and 
 response. A does not originally give rise to B, but each is inde- 
 pendently supplied. A difficulty therefore enters here which is not 
 present in learning by trial and error. The animal, striving to es- 
 cape from a cage, makes a series of different responses to the situa- 
 tion ; each response is actually called out by the situation acting as 
 stimulus. The nervous connections between the stimulus and re- 
 sponse are traversed by nerve-currents, and therefore strengthened 
 in accordance with the law of exercise; and the only question is to 
 explain the advantage which accrues to the successful reaction. 
 But in the case now before us, as when a person is presented and his 
 name spoken, neither the name nor the face of the person comes as a 
 response to the other. The first sight of the person does not suggest 
 his name, nor the first hearing of a name suggest the appearance of 
 the person. The two associates come independently, one by way 
 of the eye, the other by way of the ear; and there seems no occasion 
 1 McDougall, op. cit., p. 126. 
 
618 THE MECHANISM OF THOUGHT 
 
 for the activity of any neural connection between the parts of the 
 brain thus independently aroused to action. And yet, later, the 
 name recalls the face or the face the name showing, in accordance 
 with our general conceptions of the brain mechanism, that connec- 
 tions have been established between the parts concerned with face 
 and name respectively. When and how have these connections 
 been formed? 
 
 27. Such is the problem of the physiology of association by 
 contiguity. A solution has been attempted by James, 1 and elabo- 
 rated by McDougall. 2 As the latter states the case, a certain part 
 or system of neurones in the brain is thrown into activity by A, the 
 sight of the person, and immediately afterward, anoth'er part or 
 system is thrown into activity by the sound of the name, B. The 
 centre of activity shifts from one part to another, as it does in other 
 shifts of attention, and the second centre of activity attracts, and 
 drains off, the energy of the first centre, so that there is really a trans- 
 mission of nerve-currents from the first to the second. The path 
 from one to the other is thrown into activity, as truly as if the sight 
 of the person had actually called up the name, and thus receives the 
 nutritive after-effect of activity and becomes a favored path, or 
 path of low resistance. 
 
 This explanation probably proceeds in the right direction, so far, 
 at least, as this : Nerve-currents do pass between different centres 
 of activity in the brain; but the theory seems rather too elaborate 
 and highly specialized to explain some cases of association by con- 
 tiguity, while it is not elaborate enough to explain all cases. We 
 have grave doubts about the concept of "drainage," which enters 
 here in even a vaguer form than in the case of the alternation of re- 
 flexes or of percepts of an ambiguous stimulus. To test the matter, 
 let us recall that, in looking at the staircase or the dot figure, the ob- 
 server shifts completely from one appearance to the other; so that 
 the drainage of the first-acting system by the second should be com- 
 plete, as is required by the conception of drainage in the formula 
 of McDougall. But there is, in fact, no such complete shifting in 
 the instance of the face and the name; for the face does not disappear 
 when the name is spoken. The drainage is not, then, complete in 
 this case. In general, the fact that the field of consciousness is 
 broader than the field of attention goes to show how more than one 
 system of neurones may be simultaneously active in the brain; 
 and how no one system exerts complete attraction on the energy of 
 the other systems. Partial drainage, however, is a much less clean- 
 cut conception than complete drainage, and since it has no special 
 
 1 Principles of Psychology, 1890, II, 584. 
 8 Op. cit., p. 126. 
 
MECHANISM OF ASSOCIATIONS 619 
 
 claim to acceptance in regard to the activity of the brain as a whole 
 (though it may be accepted in regard to a single neurone), we may 
 profitably seek some more satisfactory explanation. 
 
 In our search let us, first of all, assume that the neural connections 
 which are formed in learning are not wholly formed at the moment 
 of learning; for it would clearly be impossible to suppose association 
 fibres to grow all the way from the visual to the auditory centre in the 
 few moments needed to connect a face with a name. These fibres 
 already exist in great numbers, and connections are already formed 
 in the rough, and need but a few "finishing touches," so to say, to 
 make them good conductors. Probably the finishing touches are 
 applied to- the synapses. Thus much any theory must assume, 
 and it is entirely in accordance with neurological probability. 1 Now 
 let two centres, thus loosely connected, be thrown into simultaneous 
 or nearly simultaneous excitement. Each centre discharges mainly 
 into some previously trained channel, giving rise to motor reactions, 
 percepts, or associated ideas. But after each has thus discharged 
 itself, its activity does not come to an abrupt end. Each probably 
 continues active to a slight extent, and each is also in a condition of 
 heightened excitability; therefore the conditions are favorable for 
 the passage of currents across the imperfectly formed synapses 
 between them. Each has something to give, and each is ready to 
 take, and so an interchange takes place between them. The fore- 
 going is intended only as a formulation of the minimum conditions 
 of association. Memory experiments have shown the formation 
 of "remote" associations, i. e., associations between non-contiguous 
 members of a series of syllables, etc.; in such cases no shifting of 
 attention occurs directly between the remote terms which become 
 associated; and the conception of drainage does not appear applica- 
 ble. The conception just offered would better suit the multiplicity 
 of associations which experiment has shown to be formed between 
 other than directly adjacent members of a series; it would allow, in 
 fact, for the formation of associative paths between any two cerebral 
 organs which might be thrown into activity at nearly the same time. 
 Taken by itself, therefore, the drainage theory suffers from defect 
 of explaining too much; since it allows for associations which are 
 not demonstrably formed. It is by no means everything present 
 in consciousness at the same or nearly the same time which be- 
 comes associated with everything else so present. But it fails to 
 
 1 McDougall also makes this assumption (p. 127): "We may legitimately 
 assume that all parts of the cerebral hemispheres are connected together in such 
 a way that, under favorable conditions, the excitement of any sensory neurone 
 may spread to any part, just as in the strychnine-poisoned animal the excite- 
 ment of a sensory neurone may spread through all parts of the spinal cord." 
 
620 THE MECHANISM OF THOUGHT 
 
 explain the strong associations formed between successive members 
 of a series which is memorized, and to show why the forward asso- 
 ciations in such a series are so much stronger than those in the 
 backward direction. 
 
 28. It is to these strong serial associations that the drainage 
 theory has been most applied and appears most applicable. But 
 its applicability here is mostly an illusion. More careful analysis 
 shows that this theory rests on two assumptions: (1) that, psycho- 
 logically, serial associations are formed when attention shifts from 
 each member of a series to the next; and (2) that, physiologically, 
 shif tings of attention are always attended by a drainage from the 
 centre first active into the centre next active, with consequent in- 
 hibition of the first centre. Neither of these suppositions is prob- 
 ably true for all cases; and the cases where one is true are the 
 cases where the other is not true. In those cases where psychologi- 
 cal facts point to something like drainage in the cerebral mechan- 
 isms such as the often-mentioned cases of the staircase and dot 
 figures, and numerous others like them one percept vanishes as the 
 other appears; that is, the activity of one centre is accompanied by 
 inhibition of the other. But there is no evidence that such shifts of 
 attention give rise to a serial association between the two percepts; 
 the evidence is rather to the contrary. After experience with an 
 ambiguous figure, one of its appearances does not seem to call up 
 the other; the observer has still to wait for the process of varied 
 reaction to cause the changes. He may obtain a certain degree of 
 control over them, by learning the best fixation point to bring out 
 either appearance; but this control does not establish an association 
 between the two percepts. It is also true that the alternative views 
 are apt to succeed each other more rapidly after experience with 
 the figure; but this is sufficiently accounted for by the greater fa- 
 miliarity of each appearance, taken by itself as a response to the 
 stimulus. The relation between the alternative percepts, however, 
 remains essentially the same after experience of the shifting as be- 
 fore; and the shifting of percepts itself corresponds always to the 
 type of varied reaction, and never to the type of serial association. 
 Essentially the same things can be said of other cases which belong 
 under the type of varied reaction. The result of learning by trial 
 and error is not the formation of serial associations between the suc- 
 cessive reactions; but the successful reactions become associated 
 directly with the situation and with the " adjustment," while the other 
 reactions tend, on the whole, to be dissociated. Once more, the 
 shifting of attention that occurs with a sudden interruption to the 
 course of thought does not result in a strong association between the 
 thought interrupted and the interrupting stimulus. But this ex- 
 
PHYSIOLOGY OF THE HIGHER UNITS 621 
 
 perience is pretty nearly equivalent to proving that cases which do 
 correspond well with the drainage conception, are not cases of 
 strong serial association. 
 
 On the other hand, the cases in which strong serial association 
 does occur do not conform to the type required by the drainage 
 conception. A case in point here is that of the two syllables which 
 form a single measure in a memory series. The movement of at- 
 tention from the first to the second syllable of such a measure differs 
 in two important respects from the movement of attention in exam- 
 ining an ambiguous figure: (1) the first syllable does not pass en- 
 tirely out of consciousness with the coming of the second; and (2) the 
 second is anticipated while the first is being presented. The move- 
 ment of attention is, therefore, continuous in such cases; whereas 
 in viewing an ambiguous figure or in other cases of varied reaction 
 it is discontinuous. The reactions to the two syllables are not 
 opposed and mutually exclusive, but overlap and enter into a " higher 
 unit" of reaction namely, into the measure. Even in memoriz- 
 ing by rote we make use of such higher units; and the strongest 
 associations are those between the parts of such units. But the 
 drainage theory has no place for such higher units as extend over 
 two or more successive acts; for it, each act is antagonistic and in- 
 hibitory to its predecessor. Any attempt to apply the drainage 
 theory to the cases of learning typewriting and telegraphy in 
 which overlapping and higher units of reaction are so much in evi- 
 dence would reveal beyond doubt its utter inadequacy. This 
 theory also has no room for adjustments which last for a while and 
 control a series of acts; but the process of learning can hardly be 
 interpreted without allowing for such adjustments. 
 
 29. The foregoing considerations have increased enormously 
 the complexity of the problem of explaining all kinds of mental 
 association in terms of cerebral activities. Nothing so simple 
 as the shifting of activity from one centre of the brain to another 
 can satisfy the conditions set by experience of the facts. It is best, 
 however, to pause before attacking the physiology of " higher units," 
 in order to see if any case can be made out for purely serial associa- 
 tions, uncomplicated by such higher inclusive units. For it will 
 soon appear that the concept of serial association is needed in ex- 
 planation of more complex associations. Suppose, for instance, 
 that attention moves from A to B, yet not by way of transition be- 
 tween mutually exclusive terms, but with a certain anticipation of 
 what is to come, or condition of expectancy, and with a lingering 
 of what has just gone. Now expectancy means a suspension of 
 reaction till the expected has happened; that is, a reaction is par- 
 tially prepared, but is not "discharged" until the coming stimulus 
 
622 THE MECHANISM OF THOUGHT 
 
 shall have arrived. In physiological terms, expectancy probably 
 indicates a damming-up of nervous energy till the new stimulus 
 comes; then, the accumulated energy is set free, and very likely 
 it is discharged into the centre aroused by the stimulus. If one 
 says, for example, "Let me present Mr. A," the nervous energy 
 aroused by the sight of the man is perhaps held in check till the 
 name is spoken, and then discharged into the centre aroused by the 
 auditory stimulus, which, on account of its being aroused by this stim- 
 ulus, would the more attract other nerve-currents having a partially 
 open path into it. Such a condition would account also for the 
 special clearness with which an expected stimulus is perceived. 
 In other words: It is the movement of expectant attention, rather 
 than the shifts between antagonistic reactions, that gives rise to strong 
 serial reactions. This movement of expectant attention is perfectly 
 consistent with the persistence of adjustments, and with the simul- 
 taneous operation of higher units. 
 
 30. To pass, now, to an attempt at conceiving the neural mechan- 
 ism of "higher units." They may best be conceived of after the 
 analogy of those co-ordinating mechanisms of which convincing evi- 
 dence was found in studying the spinal cord. These mechanisms 
 showed the existence of both collecting and distributing groups of 
 neurones. Nerve-currents from different sources converge by means 
 of collecting mechanisms, and diverge by means of distributing 
 mechanisms. In the case of learned and mental performances, it 
 is easier to conceive of the working of the distributing mechanisms, 
 and of the development of the collecting mechanisms. Let us begin, 
 therefore, with the distributing mechanisms, and take for illustra- 
 tion those which may be supposed to care for learned combinations 
 of movement, or, in other words, for skilled performances. A suc- 
 cession of simple instinctive movements is performed as a single 
 act; it is a veritable unit, in spite of the fact that it appears as a 
 succession of movements which may enter into other combinations. 
 
 A distributing mechanism, in connection with serial associations 
 between the component movements, thus seems adequate to explain 
 the general fact of co-ordination. The distributing group of neu- 
 rones calls into play the groups of cells in the motor area which pre- 
 side directly over the spinal centres for the component movements. 
 Serial association, however, seems necessary to account for these 
 movements occurring in the proper order. The serial connections 
 alone cannot, indeed, account for the co-ordination, since the same 
 component movements enter, in different orders, into several skilled 
 acts, and serial association would be liable to go astray, and lead 
 off into the wrong series; but the distributing mechanism, by holding 
 certain motor groups of cells in readiness, brings about a proper se- 
 
PHYSIOLOGY OF THE HIGHER UNITS 623 
 
 lection among the various possible serial orders. A good example 
 of this is afforded by any word which, as spoken, is a series of 
 articulatory movements. The word is a unit to consciousness and 
 is also a unit in reaction. Yet it is composed of elementary move- 
 ments which, in other words, enter into an enormous number of 
 other combinations. A distributing unit, corresponding to the word 
 and holding in readiness the proper groups of motor-cells, plus the 
 established serial associations between the groups of motor-cells, 
 seems to afford a partially satisfactory conception of the possible 
 neural mechanism. 
 
 The explanation just given is not indeed wholly satisfactory, 
 for it does not explain the differentiation between two words, such 
 as "cat" and "tack/* which are composed of the same movements 
 in different orders. Of the two mechanisms postulated in this ex- 
 planation, the distributing unit is supported by the best evidence, 
 psychological as well as neurological; and if we could attribute to 
 such a unit the power, not only of distributing its nerve-currents 
 to selected groups of motor cells, but also of exciting them in a cer- 
 tain order, our explanation would be made more complete. This 
 last is a difficult conception, however, and serial association seems 
 to be indicated by certain forms of lapses, in which a word, a melody, 
 or a series of manual movements, though rightly begun, runs off the 
 track by switching to some other familiar series of movements. 
 
 31. A conception of collecting mechanisms is specially needed for 
 explaining the physiology of percepts. A number of items, once at- 
 tended to singly (as the clicks in receiving a telegraphic message), 
 come to be apprehended in groups; it would seem, therefore, that the 
 nerve-currents set up by the individual items of stimulus must con- 
 verge upon some unitary mechanism, which can discharge as a unit, 
 and so give rise to unitary motor reactions, and other unitary se- 
 quels. This is not at all a difficult nor improbable conception; but 
 some difficulty arises from the fact that we may pass easily from such 
 wholes of apprehension to the items which arouse them; for such 
 passage implies distribution, in addition to collection. In like man- 
 ner, it is difficult to imagine how a distributing unit could acquire 
 its special form of distribution, except on the supposition that the 
 subsidiary units react upon it which would make it a collector 
 as well as a distributor. We conclude, therefore, that collecting and 
 distributing mechanisms always exist in close relation one to the 
 other. 
 
 32. Can any notion of controlled association be formed in 
 terms of physiological psychology? This would require an instru- 
 ment of selection among the numerous paths leading out from a 
 given starting-point. A word, taken in isolation, may suggest any 
 
624 THE MECHANISM OF THOUGHT 
 
 one of many meanings; but in context it suggests only one. To put 
 the fact in neural terms : many paths lead from the auditory centre for 
 that word to many other groups of cells or systems of neurones; and 
 all of these paths have become developed by previous exercise. Any 
 one of them is a path of low resistance; but, under the given con- 
 ditions, only one of them is traversed by the current starting from 
 the auditory centre. Our question is, What cerebral mechanism 
 corresponds to the context, and selects the proper path ? It is not al- 
 together impossible to reach a rough notion of such a mechanism. 
 It would have the general character of a distributing mechanism, 
 holding certain paths in readiness, and facilitating their action 
 above those not so prepared. Some special cerebral activity is in- 
 dicated by our awareness of the trend of the meaning of a passage; 
 the parts thus active exert their influence, not promiscuously over 
 all the neurones of the cortex, but selectively', that is, they favor cer- 
 tain responses to each new word as it comes. If one says " Add ! " for 
 example, an adjustment is set up which favors the paths developed 
 in learning the addition table; and the nerve-currents started by 
 hearing two numbers in this context take this favored path. 
 
 A certain amount of controlled association would thus reduce, 
 physiologically, to compound or convergent association. It is 
 probable that simple serial association is decidedly the exception, 
 but compound association the rule, in all mental processes; and that, 
 therefore, the cerebral activity indicated by mental processes is 
 by no means a mere succession of currents passing from one system 
 of neurones to another. All our attempts to picture the process 
 in detail must be ludicrously inadequate before its real complexity. 
 But from both the physiological and the psychological points of 
 view, the mechanism involved in the formation of so-called " higher 
 units" is the most important of all psycho-physical mechanisms to 
 conceive of in a clear and definite manner. 
 
 33. In considering the process involved in the isolation of a simple 
 or complex feature from a still more complex situation, our previous 
 studies have shown that such analysis is most apt to occur when the 
 given feature has been previously experienced in other, somewhat 
 different situations. How does it come about that a stimulus which 
 acts now in one combination, and again in another, may come to 
 arouse a reaction on its own account ? We must suppose, here as al- 
 ways, that the cerebral process is much more detailed than con- 
 sciousness would indicate, and that when a combination of stimuli 
 acts on the organism, each component stimulus has some separate 
 cerebral effect, though consciousness reveals but a blended total, 
 and though motor reaction is also but a single joint response to the 
 whole mass of stimuli. Notwithstanding this gross effect, the brain 
 
LIMITATIONS OF ALL EXPLANATION 625 
 
 paths directly reached by the incoming sensory currents have the 
 benefit of exercise, while other paths not so excited do not receive 
 the same benefit. When, therefore, a new combination of stimuli 
 occurs that contains some old components, the brain paths which 
 receive these old components have an advantage over those not pre- 
 viously exercised, and a corresponding especial influence in deter- 
 mining the following reaction. They may even determine a new 
 reaction, such as would not result from the equal balance of all 
 the stimuli entering into the combination. 
 
 34. In closing this part of our work it is scarcely necessary to 
 confess anew the limitations within which the scientific studies of 
 the order required by physiological psychology find themselves con- 
 fined. Everywhere, the complexity and subtlety of nature's proc- 
 esses far surpass all the attempts, however successful, of human 
 science to unravel and depict them. But such are the limitations 
 of every form of science. That they are uncommonly restrictive in 
 this field is, doubtless, in large measure due to the very character of 
 the facts and of the relations which are to be examined. 
 
 Nor can it escape the insight of those accustomed to reflect upon 
 all the phenomena displayed by the human mind in its higher stages 
 of development, that many of the most important and distinctive 
 classes of these phenomena have really not been treated, from the 
 physiological point of view have, indeed, scarcely been mentioned 
 at all. Such are the phenomena of the higher degrees of selective 
 attention, and of deliberative choice, of the more logical forms of 
 judgment and of reasoning, of the thinking involved in the con- 
 struction of scientific systems and philosophical conceptions, of the 
 feelings of the more strictly ethical and sesthetical order, and of the 
 ideals of art, duty, and religion. But all these so-called higher forms 
 of functioning do seem to be involved, as it were, in a sensory- 
 motor basis, to which they are responses of a secondary and derived 
 order, and so require the assumption of the activity and develop- 
 ment of that conscious subject of them all which we call the Soul, 
 or Mind. 
 
PART THIRD 
 
 THE NATURE OF THE MIND 
 
CHAPTER I 
 GENERAL RELATIONS OF BODY AND MIND 
 
 1. Without entering in a definite and detailed way into the field 
 of metaphysics proper, there are certain quasi-metaphysical prob- 
 lems to which psychology, when studied from the physiological and 
 experimental points of view, can scarcely avoid making a constant 
 reference; and, indeed, about the most plausible solution of which 
 it cannot, in fidelity to its own completeness, fail to express some 
 opinion. Such of these problems as we are now proposing briefly 
 to discuss may be conveniently grouped under two heads. We ask, 
 in the first place : In what terms shall we conceive of the most gen- 
 eral relations between the Nervous Mechanism and the Subject of 
 the psychical or mental phenomena ? And in the second place : In 
 what terms shall we conceive of the nature, or most essential and 
 permanent characteristics, of this Subject itself? Or more popu- 
 larly stated: How are Body and Mind essentially related, as re- 
 spects their development and the forms of functioning assigned to 
 each ? and What may be affirmed as to the reality and unity of the 
 so-called Soul or Mind? Both of these questions, however, are 
 to be considered only so far as some indications leading to a cor- 
 rect answer seem to exist on scientific grounds. 
 
 At this point it is necessary to recall what was said at the begin- 
 ning, and what has been amply illustrated and enforced by the facts 
 and the conclusions of the entire treatise. Our problem concerns 
 the very conception of physiological and experimental psychology. 
 The express object of this science is to investigate the relations be- 
 tween the constitution and functions of a certain mechanism, the 
 human Nervous System, at the various stages of its development and 
 under the excitement of various forms of internal and external 
 stimuli, and the changes effected in consciousness, whether of a 
 temporary or more permanent character in a word, the behavior 
 and development of human Mental Life. Since these relations ap- 
 pear, at least, to maintain themselves in both directions, as it were; 
 and since sometimes the physical phenomena are antecedent to, 
 and sometimes consequent upon, the mental phenomena, they may 
 be spoken of as "co-relations." By speaking of co-relations, how- 
 
 629 
 
630 GENERAL RELATIONS OF BODY AND MIND 
 
 ever, we are only adopting a term which summarizes in a convenient 
 way a working scientific hypothesis. 
 
 2. The nature of this hypothesis needs further explanation. 
 For this purpose a brief reference to two particulars is sufficient at 
 present. In the first place, any piece of scientific research, no matter 
 how much it may abjure all definite metaphysical theories, is com- 
 pelled to make certain assumptions and to indulge certain beliefs, 
 which are really of a metaphysical character. It is quite right, how- 
 ever, that these assumptions should be those which have a prima 
 facie evidence for all human thought; and that these beliefs should 
 be such as accord with the so-called "common sense" of mankind. 
 Science, as long as it remains science, does not undertake to furnish 
 a system of critical metaphysics; its metaphysics is naive and pop- 
 ular, rather than the metaphysics of any of the schools of philoso- 
 phy. It is this kind of metaphysical attitude which we purpose to 
 adopt at the beginning of the discussion of both classes of the prob- 
 lems just proposed. And, for the most part, we intend to main- 
 tain this attitude throughout the entire discussion. 
 
 3. Now there can be no doubt as to what " naive metaphysics," 
 which is the commonly and properly adopted attitude of science, has 
 to say as to the general relations of body and mind. Its theory is 
 what the expert in philosophy and its history would call "an un- 
 critical dualism." Of course, everybody, from the most untutored 
 savage to the most accomplished student of psycho-physics, when- 
 ever the subject is not taken as a purely scholastic question, believes, 
 speaks, and acts as though there were two existences, a body and a 
 mind; and as though each one of these two had much to do with the 
 behavior, the welfare, and indeed, the very life of the other. In 
 fact, the body as respects both what it is and what it is doing 
 is always thought of as acting upon and influencing the mind ; and 
 in like manner, the mind is always yielding to or resisting the bod- 
 ily influences. Moreover it habitually uses the bodily organs as 
 though they were its ministers or tools. It is as an embodiment of 
 this dualistic theory, which is commended both by common sense 
 and by science as a working hypothesis, that we have adopted the 
 term "correlations." 
 
 It must be said in the second place, that no one word can serve 
 equally well to define, or even to describe, all those reciprocal in- 
 fluences which furnish conditions to man's complex life and devel- 
 opment as an ensouled body or an embodied soul. It is not, however, 
 at all strange that such should be the fact. For in their very nature, 
 the relations between neural and mental phenomena are of all others 
 about the most abstruse and complex; and in man's case, the beings 
 between which they are assumed to maintain themselves are of all 
 
NATURE OF UNCRITICAL DUALISM 631 
 
 physical and psychical existences, about the most complex and diffi- 
 cult of access. 
 
 Our course would, therefore, seem to be plainly marked out for 
 us, so far as the problems are concerned which fall more properly 
 under the titles of these chapters. However, at once we are met 
 by differences of method; and finally, by two rival and contrary 
 ways of replying to the general inquiry. One of these denies that, 
 in order to account for mental phenomena, we need assume the ex- 
 istence of any reality other than the material substance of the living 
 and active nervous system (especially, or wholly, of the brain). The 
 other, on the contrary, claims that no explanation of mental phe- 
 nomena is possible without referring them to a non-material or spir- 
 itual entity as the real subject of them all. Both of these ways of 
 explanation admit of various modifications. A third view, which 
 regards both the so-called "brain" and the so-called "mind" as 
 merely phenomenal aspects of some one reality that is like neither, 
 but manifests itself in both, requires for its discussion so much of 
 subtle metaphysics, and is so foreign to all the scientific material 
 with which we have thus far been dealing, that it is for the present 
 passed by with a bare reference. 
 
 In the remaining part of our discussion we shall be chiefly occu- 
 pied with considering which one of two theories best accords with 
 all the facts. These facts, which are to test the theory, are facts of 
 the nervous mechanism, and of the correlations between this mechan- 
 ism and the phenomena of consciousness. The question before us 
 may then be stated in the following provisional form: Do the phe- 
 nomena of consciousness require for their explanation nothing more 
 than a statement of those changes in the material mechanism with 
 which they are obviously correlated; or do they also require the as- 
 sumption of one real and non-material being as the subject and 
 ground of them all ? 
 
 4. How, then, we now inquire, are body and mind related, as 
 respects their development and the forms of functioning assigned 
 to each? The two series of phenomena, as has already been 
 said, when looked at superficially, appear correlated reciprocally. 
 What, however, is signified by these apparent or obvious facts as to 
 the real connection of the two? 
 
 Various attempts have been made, from different points of view, 
 to sum up in some single word the relations that maintain themselves 
 between the body and the mind. Thus, the body has frequently 
 been spoken of as the "seat" or "organ" of the mind. Looking at 
 these relations from the more strictly mechanical point of view, men- 
 tal phenomena have been regarded as the "products" of the func- 
 tional activity of the brain. More highly figurative terms even have 
 
632 GENERAL RELATIONS OF BODY AND MIND 
 
 often enough been employed, especially in the supposed interests 
 of morals or religion. The body has then been called the "prison" 
 or "tenement" or "tabernacle" of the soul. Not seldom, also, has 
 the mind been represented as mastering and controlling, and even 
 "moulding" the body somewhat as the rider subdues and guides 
 his horse, or the worker in clay and metal shapes the product of his 
 toil. One form of the doctrine of " animism" has held that the mind 
 is identical with the vital principle, which is busy from the very im- 
 pregnation of the ovum in shaping its increasing molecules accord- 
 ing to an unconscious or dimly conscious plan. Much debate has 
 also been held as to whether the conception of "cause" is applica- 
 ble to any of the relations in which body and mind stand to each 
 other whether, indeed, it must not rather be held that what hap- 
 pens in one is only the "occasion" on which some underlying cause, 
 common to both, operates to produce a change in the other. 
 
 5. The inquiry in what sense, if at all, the brain can be said to 
 be the "seat" of the mind is more easily answered in a negative 
 than a positive way. Nothing but the crudest notions, both of the 
 nervous mechanism and of the mind, would be consistent with any 
 of the more literal and direct interpretations of this word. No one 
 would seriously regard the mind as a special entity, whether con- 
 structed of ordinary material atoms or constituted in ethereal form, 
 that maintains a sitting or other posture amidst the cerebral masses. 
 Nor is it any more correctly conceived of as thinly diffused over the 
 entire mechanism of nerve-cells and nerve-fibres, or as wandering 
 about among the cerebral elements to find its temporary "seat" 
 where occasion seems to require its presence. And, although some 
 of the two classes of phenomena perhaps admit very well of being 
 brought under the conception of an atom, acting and acted upon in 
 varying relations to other atoms of kinds different from itself, no 
 essential gain is made by the attempt to regard the mind as in real- 
 ity anything of the sort. In brief, there is no literal meaning of the 
 words in which we can speak of the mind as seated in the brain. 
 
 The phrase, the brain is the "seat" of the mind, is, however, very 
 well adapted to raise the whole question of the spatial qualities of 
 the mind, and of its alleged spatial relations to different portions of 
 the central nervous system. We shall, then, briefly consider the 
 grounds for the use of this figurative term. There can be no doubt 
 that science justifies ordinary language in speaking of the soul as 
 in the body, in some sense in which this term does not apply to any 
 other collection of material atoms. The human soul is in the human 
 body as it is not in the bird, the tree, the house, the star. Even that 
 way of regarding the mind's nature which does not hesitate to speak 
 as though it were a thinly diffused and half-spiritualized form of 
 
THE BRAIN AS "SEAT" OF THE MIND 633 
 
 matter, assents to the necessity of asserting a special relation in space 
 between it and the body. Hence some old-time philosophies rep- 
 resented the soul in perception as streaming out through the avenues 
 of sense in order to get the sensuous object into its embrace; or else 
 pictured some etherealized copy of this object as streaming into the 
 soul by the same avenues. Modern vagaries, in the form of theories 
 of an "astral body," or of "spiritualistic materializations," are fa- 
 miliarizing us with such representations anew. But even such a 
 view of the nature and activities of the mind is based upon the claim 
 that the body is, in some sort, the peculiar dwelling-place, or " seat," 
 of the mind. 
 
 A correct account of the process by which the world of things be- 
 comes known also shows that all our experience is connected with 
 the establishing and justifying of a similar claim. There are no 
 " things " known to experience except as our sensations, or modes of 
 being affected, are both localized and projected extra-men tally. In- 
 ducements and considerations, such as have already been treated 
 in great detail, irresistibly urge on science to arrange all phenomena 
 under two classes phenomena which are qualities of outside things, 
 and phenomena which are mere states of internal experience. But 
 the same inducements and considerations compel it to look upon 
 certain phenomena of the first class as related to those of the second 
 class in a peculiar way. The world of things outside always (at 
 least in ordinary experience) affects us is perceived by us or modi- 
 fies our consciousness through the body. The mind is, therefore, 
 said to be in the body. 
 
 The conclusion from the foregoing general experience is confirmed 
 by certain experiences of a special order. The feelings of pleasure 
 and pain, which have so immediate and incontestable a value for 
 the life of consciousness, are all connected with sensations more or 
 less definitely localized in the body. So close is the connection be- 
 tween the localized sensations and the painful or pleasurable states 
 of the mind, that the mind actually seems to be suffering in that part 
 of the body where the sensations are localized. When the localiz- 
 ing of sensations connected with feelings of strong "tone" is very 
 indefinite, as it is in cases where the feelings arise from the condi- 
 tion of large areas of the internal organs, the soul seems to be suffer- 
 ing in, and throughout, almost the entire body. 
 
 Furthermore, both ordinary experience and scientific observation 
 require us to regard the mind as standing under certain special re- 
 lations to certain parts of the body. The ancients located the soul 
 in the heart or lower viscera, because of marked connections be- 
 tween conscious states and the condition of these organs. But the 
 obvious connection of the head with the more obtrusive sensations 
 
634 GENERAL RELATIONS OF BODY AND MIND 
 
 of the perceptive order tends to confirm the belief that the mind, as 
 perceptive, has its "seat" in that region of the body. For reasons 
 already given in detail modern scientific researches justify us in nar- 
 rowing more precisely the local domain within which we can affirm 
 the mind to have its seat. The mind is certainly in the nervous 
 system, in a sense in which it is not in any other of the systems of 
 the animal body. More precisely yet, it is pre-eminently in the 
 brain; and, among all the complex groups of encephalic organs, the 
 final and special claim of the cerebral cortex to be the "seat" of the 
 mind is most easily maintained. Here, in this convoluted rind 
 which forms the interlaced "projection-systems" of sensory and 
 voluntary motor-impulses, here if anywhere must it be held that 
 the subject of the states of consciousness has its peculiar dwelling- 
 place and home. And yet, recent experiments in cerebral surgery 
 upon human subjects have demonstrated the fact that the conscious 
 mind, when the sensory areas of the brain are stimulated, itself 
 localizes, or seate, its own sensations and feelings, not in the brain 
 at all, but in the appropriate areas of the periphery. 
 
 6. On the contrary, the results of modern scientific inquiry 
 become unfavorable to the effort yet more precisely to designate a 
 material "seat" for the mind. Is there any one mathematical 
 point, or minute area, in the cerebral cortex that is most especially 
 of all the dwelling-place of mind ? If so, might it not be properly 
 conceived of as ordinarily remaining at this point to receive the 
 messages despatched to it from the various parts of the periphery; 
 and as executing its will over those peripheral parts by sending back 
 to them corresponding messages despatched from the same central 
 point? The pineal gland has undoubtedly lost the significance 
 which Descartes gave to it as the special seat of the soul. But can 
 no substitute be found to take and hold so important a place ? The 
 answer of cerebral histology and physiology to the foregoing ques- 
 tions is, on the whole, a decided negative. 
 
 At this point it is customary to greet with peculiar satisfaction, 
 as though we had found the solution of a puzzling metaphysical 
 problem, the discoveries of cerebral localization, in recent times. 
 Certain areas of the cerebral cortex do, indeed, appear to have a 
 particular connection with the execution of certain functions of the 
 mind. But the very phenomena on which reliance is placed for 
 establishing this connection forbid us to regard the mind, in its 
 special relations to the brain, as limited to any point or small area 
 of the cerebral cortex. Both gross and microscopic anatomy show us 
 that the cortical part of the brain, like all its other parts, is not con- 
 structed on the plan of having its uses for the mind concentrated in 
 any one minute circumscribed spot. In any sense in which the 
 
THE BRAIN AS "SEAT" OF THE MIND 635 
 
 mind can be said to have its "seat" in the brain at all, in that same 
 sense, and with equal propriety, may the entire cerebral cortex, 
 with its vast complexity of nerve-fibres and nerve-cells, be said to 
 be entitled to something of the same distinction. Moreover, the 
 combined testimony of physiology and surgery establishes the truth 
 that the brain never localizes the mental products of perception 
 within its own areas. The simple yet essentially mysterious truth 
 is as has just been pointed out that when certain cerebral areas 
 are properly stimulated, the mind localizes its own sensations, either 
 in the corresponding peripheral parts of the body, or somewhere 
 external to the body. 
 
 7. And now the puzzling question recurs: What that is intelli- 
 gible can be meant by designating the supreme central organs of 
 man's nervous mechanism as the "seat" of his conscious mind? 
 
 The only solution for such a puzzle as the foregoing if solution 
 it can be called ; must always consist in calling attention anew to 
 the essential facts of the case. Certain particles of very highly 
 organized chemical constitution, when grouped into nerve-fibres 
 and nerve-cells, and when further associated into organs, may be 
 acted upon by appropriate stimuli. These material particles are 
 locally in the cranial cavity, and, more precisely, in this or that area 
 or organ of the cranial contents. Moreover, a large and important 
 part of the phenomena of consciousness consists in localized bodily 
 sensations of a painful or pleasurable character. To these facts in- 
 vestigation adds the inference as based upon experiment and ob- 
 servation in the case of others, that the localized sensations are 
 themselves ultimately dependent upon the behavior of the afore- 
 said material molecules in the brain. That is to say, we directly 
 localize many of our mental affections in this or that part of the 
 body; by remote processes of observation and argument we infer 
 that the last material antecedent of them all is the behavior of cer- 
 tain invisible parts of the body within the brain. Therefore we say: 
 The mind is in the brain; or the seat of the mind is in the brain. By 
 this, nothing further can be meant of an assured or intelligible 
 character than the emphatic repetition of the same principal facts: 
 the sensations which we localize at the periphery of the body, or pro- 
 ject from the body in space, all have a sui generis connection with the 
 condition and action of that portion of the same body which is con- 
 tained in the cranial cavity. 
 
 As to the possibility of such a sui generis relation between mate- 
 rial elements which exist in space, and the localizing and other ac- 
 tivities of a being not to be conceived of as, strictly speaking, in 
 space, only experience is entitled to pronounce. Such a relation is 
 an accomplished fact. The fact is, therefore, not to be disputed on 
 
636 GENERAL RELATIONS OF BODY AND MIND 
 
 any so-called a priori grounds. It does not follow, however, that 
 the relation of the mind to the brain is any more essentially myste- 
 rious than that of the molecules of the brain to one another. Nor 
 does it form an insuperable objection to the former relation that 
 it is not, like the latter, a relation of changes of position in space. 
 For who shall undertake to affirm that beings which are not ex- 
 tended and movable in space, because their very nature is of another 
 order, cannot exist in relations of any kind to beings which are thus 
 extended and movable ? It is precisely in this way that the mind is 
 actually related to the brain. To speak of the mind as having its 
 "seat" in the brain is only a figurative way of affirming the reality 
 of such relations. 
 
 8. The term "organ" (or instrument) of the mind, as applied 
 to the body, is particularly calculated to emphasize the relation of 
 the ideas and volitions which arise in consciousness to the control 
 of the muscular apparatus. But the same term may also be used, 
 though with less propriety, to describe the relation of the brain to 
 the mind in sensation and thought. Thus we may be said to feel 
 or think with the brain, in some manner supposed to be analogous 
 to that in which the workman accomplishes his task by availing 
 himself of a particular tool or instrument. It is obvious, however, 
 that the figure of speech suggested by these terms also will not admit 
 of a literal interpretation. We cannot conceive of the mind as a 
 peculiar kind of material entity which, when it desires or wills to 
 move the bodily members in a certain way, lays a clutch as it 
 were upon the nervous substance of the central organs, and so 
 makes the body serve as an " organ" of the desire or volition. Even 
 less are we to conceive of the brain as a complex tool or mechanism 
 which the mind uses in thought and feeling, somewhat as senses and 
 fingers avail themselves of a calculating machine or of a musical 
 instrument. 
 
 In producing changes of shape and position in masses of matter 
 outside of our own bodies, we ordinarily find it convenient to use 
 some material medium between those masses and the various mov- 
 able parts of our own bodies. We can, by means of complicated 
 mechanisms, accomplish a great variety of changes which it would 
 be quite impossible to accomplish without such aid. On the other 
 hand, we sharpen, define, and multiply our sensations and percepts 
 of things in similar manner. The deaf man hears with a trumpet 
 or other acoustic contrivance; and the scientific observer contrives 
 an instrument for observing the absolutely simple tones as analyzed 
 out of the composite clang; and with a prism the optician beholds 
 the colors of the spectrum. 
 
 It is characteristic of all the most skilful use of tools and instru- 
 
THE BRAIN AS ORGAN OF THE MIND 637 
 
 ments that they come to seem to the observer like a part of his own 
 bodily mechanism. By feelings of "double contact" the workman 
 comes to know, with the chisel, the wood or metal which he is carv- 
 ing just as the blind man seems to extend his conscious life to the 
 very end of the stick he is accustomed to carry. In these cases the 
 mental picture before the practised mind is not that of the hand and 
 the way it must be moved, but of the graving tool and the motion to 
 be imparted to it as though the instrument itself were immediately 
 subject to volition. 
 
 The conception of an "organ" or instrument may, with a cer- 
 tain propriety, be extended so as to cover the relation which exists 
 between the nervous system and the muscular, and between the 
 central and peripheral parts of the nervous system. Thus it may 
 be said that the spinal cord and brain move the limbs "with the 
 use" of the afferent nerves, or that the cerebral hemispheres employ 
 the lower ganglia of the brain in effecting certain co-ordinations of 
 sensation and motion; it may even be said that the end-organs of 
 sense communicate with the supreme central organs "by means of" 
 the afferent nerve-tracts and the lower ganglia. All such language 
 expresses, correctly enough for popular usage, the undoubted fact 
 that, in the complicated relations of position and motion which are 
 maintained among the different members of the nervous system, a 
 certain order of action is constantly preserved. Changes originate 
 in one part, and are propagated to other contiguous or more distant 
 parts. In such propagation of the changes a regular tract of the 
 advancing motions is assumed always to exist; and thus to science 
 the parts that lie between the extremes may be looked upon as means 
 or media i. e., as instrumental to the completion of the process. 
 To the uninformed person, however, the result seems to be an " im- 
 mediate" effect of the will. 
 
 9. It is obvious from the foregoing remarks that one part of 
 the nervous mechanism can be said to be the "organ" or instru- 
 ment of another part, in the meaning of the word which cannot prop- 
 erly apply to the relation of the brain and the mind. In a certain 
 justifiable meaning of the word, all the rest of the body may be 
 said to be the organ of the brain. That is to say, those changes in 
 the molecules of the brain's substance which arise there whether 
 because of certain ideas and volitions of the mind, or because of 
 changes in the character of the blood-supply, or of sensory impulses 
 thrown in from the periphery or other lower nervous centres get 
 themselves expressed through the other members of the body. One 
 part serves as an instrument or "organ" for another, because the 
 changes in it effect changes elsewhere, not directly, but through con- 
 tiguous and connected parts. If the necessary contiguous parts are 
 
638 GENERAL RELATIONS OF BODY AND MIND 
 
 wanting or their relations disarranged, if. the connection is inter- 
 rupted or destroyed, then the work cannot be done; the "organ," 
 "instrument," or "means," is lacking. 
 
 But, in truth, only a part of the real relations existing between 
 mind and brain can properly be described under such terms as 
 "organ," "instrument," etc. The brain, with its appropriate func- 
 tions, is an indispensable medium between certain changes in the 
 peripheral parts of the body and corresponding changes in. the states 
 of consciousness. As much as this is true of all the efferent tracts 
 which lead from the cerebral cortex through the lower portions of 
 the encephalon, along the spinal cord, and out to the particular 
 groups of muscles. Something more and special is, however, true 
 of the brain. It is the first of the indispensable physical links in 
 the whole chain; it stands nearest, as it were, to the mind. All 
 the other steps in the execution of the ideas and volitions of the 
 mind depend upon what takes place in the brain. In this sense, 
 at least, the brain is the particular organ of the mind; it is the most 
 intimate and indispensable means for the execution of all its ideas 
 or volitions of motion. 
 
 It does not appear that the foregoing statement by any means 
 exhausts the description of the experience, reflection upon which 
 induces us to regard the brain as the "organ" of the mind. For 
 the brain seems to serve as the special physical basis of the ideas 
 and volitions of motion themselves. Experiments with animals, 
 by extirpating the cortical areas, and observation of human patho- 
 logical cases especially, perhaps, in certain forms of aphasia 
 seem clearly to show that a much more intimate "organic" relation 
 exists between the brain and the mind. With the destruction or 
 derangement of certain cerebral areas, the power even to form cer- 
 tain ideas and volitions, or to have certain feelings, seems to be im- 
 paired or lost. We cannot say, to be sure, that the mind has lost 
 a part of its general faculty to conceive, to feel, and to will. It has, 
 however, suffered in respect of its power to frame a certain set of 
 definite ideas and volitions for the purpose of controlling the mo- 
 tion of the peripheral members. This class of facts is certainly 
 calculated to emphasize strongly our conception of the brain as 
 being, in a special sense, the indispensable means through which the 
 states of consciousness are related to changes in the position of 
 molecules and masses of matter. 
 
 10. There is another most important class of facts which may 
 be partially described under the same terms as the foregoing. The 
 brain is the indispensable means for furnishing the mind with its 
 sensations, and so with its presentations of sense or perceptions of 
 things. This statement is not to be understood as though the brain 
 
A BOND BETWEEN BRAIN AND MIND 639 
 
 could, of itself, construct the sensations and perceptions and hand 
 them over ready-made, as it were, to the mind. Sensations are 
 states of consciousness, not modes of the brain ; and even when they 
 are synthetically united, localized, and projected to the periphery 
 of the body, or into surrounding space, they are brought under no 
 essentially new relations to the nervous mechanism. Sensations 
 are not nerve-commotions, "etherealized" by the optic thalami and 
 cerebral convolutions, and then handed over to consciousness. 
 Therefore the instrumental relation between brain and mind is 
 not that of transmitting a peculiar kind of motion from one phase 
 into another, or from one being to another. Nevertheless, no sen- 
 sations will arise in the mind unless the brain be affected in a cer- 
 tain way. Looking at the chain of sequences as it runs from with- 
 out inward, we might say: The brain is the organ, or instrument, 
 through which the stimuli of the outside world, acting on the end- 
 organs of sense, finally reach the mind. Or, to say the same thing 
 in other terms: The brain is the last and most important physical 
 antecedent to the mind's being affected with the different sensations. 
 11. Still another class of attempts to generalize, and embody 
 in a single term, the various essential relations of the brain to the 
 mind leads to the inquiry after some one special "connection" or 
 "bond" between the two. Here, again, any too literal answer to 
 this inquiry leads at once to manifest absurdity. A material bond 
 designed to unite mind and brain might perhaps be conceived of as 
 connected with the latter, and yet as remaining material; but in 
 order to make it connect with the former (the mind) it would have 
 to become non-material, unless we are ready to concede that the 
 material and the non-material can stand connected without any 
 special bond. In case this concession is once made, however, we 
 cease to feel the need of a special bond between the mind and the 
 brain. But if it be at once admitted that no connection is to be 
 sought, or can be found, between the mind and the brain, beyond 
 the fact that their modes of behavior are mutually dependent, it will 
 not be necessary to appeal to any special mystery. This is simply 
 to admit that general fact of correlations upon which every form of 
 science depends for its conclusions. What bond connects together 
 the planets of the solar system so that each one moves invariably 
 with reference to the position of all the others, and yet in a path pe- 
 culiarly its own ? We can only respond by talking of the force and 
 laws of gravitation. These "laws," however, are only a mathe- 
 matical statement of the uniform modes of the behavior of certain 
 physical beings; this "force" is no entity existing between the indi- 
 viduals, as the rods of the orrery bind its parts to a common centre. 
 Cohesion and chemical affinity are not special bonds; they, too, 
 
640 GENERAL RELATIONS OF BODY AND MIND 
 
 are but expressions for the facts that the elements of material real- 
 ity, under certain conditions and according to the kind to which 
 they belong, behave as though bound. That is, they are actually 
 correlated. The behavior of the so-called atoms, like that of the 
 stars, is, as a simple matter of fact, "relative," each to the others. 
 
 12. It will scarcely be supposed that information of scientific 
 value concerning the nature of the real connection between the 
 body and the soul can be obtained from terms which are yet more 
 purely figurative and poetic than those which have already been 
 examined. The limited and defective nature of our sense-percep- 
 tions, the misery of much of life, the unrealized longings for knowl- 
 edge and happiness, and the work of imagination in framing a pict- 
 ure of some state of existence in which the limitations are removed 
 and the longings realized, have led men in all ages to regard the 
 body as the "prison" of the soul. Because the senses are not more 
 in number than they really are, or more far-reaching and accurate 
 than their construction permits them to be, they are regarded as re- 
 straining the soul, rather than as bringing it information which has 
 the character of satisfying reality. The brevity and uncertainty of 
 life, and the speed with which accident and disease impair or dis- 
 solve the bodily functions, together with the persuasion that the 
 thinking principle will have a continued existence, suggest the re- 
 flection: the body is only the "tenement" or "tabernacle" of the 
 soul. 
 
 13. It appears, then, that all the terms in popular use to convey 
 the impressions of a " dualistic " theory of the relations between the 
 body and the mind are well grounded in facts of experience. They 
 express the truth, although only in an incomplete and figurative 
 way, which we have tried to summarize under the word "correla- 
 tions"; and by this term we understand series of changes occurring 
 in the physical mechanism that are dependently related, either as 
 antecedents or consequents, to series of phenomena occurring in 
 consciousness. So unlike in their most essential characteristics are 
 these two classes of phenomena that we are forced to assign them 
 to different species of beings in this case, minds and bodies; and 
 yet so intimate and regular are at least some of the forms of this re- 
 ciprocal dependence, that we are entitled to speak of the general 
 facts of relation as constituting so-called laws. All the researches 
 of physiological and experimental psychology, as thus far conducted, 
 do not contradict, but rather confirm, this popular and naive dualism. 
 
 The same thing cannot be said, however, of certain other terms 
 which have been proposed in the name of psycho-physical science, 
 and for use by all its various subdivisions and branches, taking the 
 words in their widest possible application. For these terms at- 
 
CONSCIOUSNESS AS A PRODUCT OF BRAIN 641 
 
 tempt in some manner, or to some degree which the popular im- 
 pressions do not warrant, to identify the body and the mind. Now, 
 although this particular word is seldom used, there are theories which 
 have not a few defenders, and which have, indeed, not a few facts 
 of experience in their favor, that would bring the correlations be- 
 tween the mechanism and the mental life under some such concep- 
 tion as that of "product," or other similar term. In their more 
 customary form these theories regard the brain as in some sort the 
 producer of the phenomena of the conscious mental life. 
 
 By the word "product" we ordinarily understand the result of 
 some process of manufacture; or in the case of a living organism 
 like the human body, the results of the secretory or metabolic proc- 
 esses of these organs are spoken of as their "products," after the 
 analogy of the products of the field or of the loom. But to speak 
 of mental states and processes as products of the brain, in any cor- 
 responding meaning of the words, involves us in the grossest ab- 
 surdities. The peculiar secretory product of the brain is the fluid 
 found in certain of its cavities; and its metabolic products are the 
 worn-out materials which it discharges into the venous circulation, 
 or the renewed elements of its own substance. In case it is prepar- 
 ing abnormal or diseased products, these take the form of abscesses 
 or tumors, or of blood-vessels and nerve-cells that have gone wrong. 
 
 A more plausible use of the word product, as applying to certain 
 correlations between the brain and the phenomena of consciousness, 
 might assume the following form: The functional activity of the 
 nervous centres might be regarded as the product of the matter con- 
 stituting these centres. But after admitting the propriety of this 
 manner of speech, the very same problem remains upon our hands; 
 and it is no less difficult and mysterious than it was before. For 
 the problem is: How shall we conceive of the correlations between 
 the different forms of this very nervous activity and the antecedent, 
 concomitant, or sequent changes in the conscious mental life ? If 
 our knowledge of these relations were indefinitely increased, and 
 even if it became perfect, it is still impossible to see how the term 
 "product," or any similar term, would fitly characterize these re- 
 lations. 
 
 14. The term " cause," or necessary precondition, seems of all 
 much most appropriate to describe in general the nature of the cor- 
 relations between the body and the mind. And, indeed, as we shall 
 subsequently show, when properly understood, the essential nature 
 of these relations may be described as causal. But here again, it is 
 customary to give a too strictly mechanical and physical interpreta- 
 tion to the words employed, and to assume that this causal relation 
 works in only one way. 
 
642 GENERAL RELATIONS OF BODY AND MIND 
 
 Thus we are treated to a theory of the correlations between the 
 body and the mind which renders the latter absolutely dependent 
 upon the former, not only for the precise forms of its characteris- 
 tic functioning, but also for its unity and claim to even a temporary 
 existence. This theory assumes that all mental phenomena, what- 
 ever their varied characteristic shading, have their exact equivalents 
 and necessary conditioning causes, only in specific forms of the nerve- 
 commotion of the living human brain. We may give a more definite 
 statement to this mechanical theory in the following way: With 
 changes in the substance of the brain which may be designated 
 A, B, C, D, etc., the mental processes called a, 6, c, d, etc., are uni- 
 formly and necessarily joined ; and with the combination of molecu- 
 lar changes which may be described by A +B + C +D, etc., the mental 
 states a+b+c+d, are as uniformly and necessarily joined. When 
 the same molecular changes recur in a fainter or modified form, as 
 A', B' t C r , D', then there must be a recurrence of the corresponding 
 mental states, only in fainter form, as a', &', c', d'. Finally, it is 
 without exception true so this theory holds that nothing happens 
 in the mental life, by way of conscious sensation, presentation of 
 objects of sense, ideation, reproduction of mental images, and higher 
 aesthetic feeling, or processes of reasoning, or choice, which does 
 not find its only real explanation in the equivalent changing states 
 of the nervous system. How stupendous are the assumptions in- 
 volved in such a theory, and how far it outruns all our knowledge 
 of the facts, has been made obvious by the entire course of our 
 previous investigations. Besides, these investigations have abund- 
 antly shown that no one-sided view of the nature of the corre- 
 lations between body and mind can lay claim to all the facts in its 
 support. 
 
 15. We have now to examine more carefully the propriety of 
 applying terms which imply causation (such as " energy," " action," 
 "force," "impulse," "effective agency," etc.) to the case of mind 
 and brain. Everything which has been said has involved the in- 
 ference that these terms may be so understood as to be really applica- 
 ble. There would be no advantage to the mind in being "seated" 
 in the brain that is, in being under any special relations to a given 
 extent of nervous matter unless it were somehow influenced or 
 acted upon by this nervous matter, and could in turn influence and 
 act upon it. No "organ" or instrument is of any use whatever 
 that is, no thing can become an organ or instrument unless it can 
 be acted upon by that which employs it as an organ, and can in 
 its turn act upon other things. Action of mind on brain is implied 
 in calling the latter the organ of the mind's volitions; action of brain 
 on mind is implied in calling it the organ of the mind's sensations. 
 
INFLUENCE OF BODY ON MIND 643 
 
 In general, to act and to be acted upon is equivalent to standing in 
 the relation of cause and effect. 
 
 It is not at present necessary to point out in detail how much of 
 obscurity and contradiction are involved in all the more popular 
 ways of mentally representing the foregoing relation. The trans- 
 mission of energy (or force) is popularly spoken of as though such 
 energy streamed off from one body and attached itself to another; 
 and as though the quantity of energy thus given off were dependent 
 upon the strength of the blow given by one body to another. Let it 
 be supposed, however, that the application of the law of causation to 
 the case of brain and mind is made in the most approved manner. 
 It is simple matter of fact, as tested by thousands of observations 
 and experiments, that changes in the condition and functional ac- 
 tivity of the nervous centres are followed by changes in states of 
 consciousness, in a regular way; and that, conversely, changes of 
 the latter sort are followed by changes in the relations of the masses 
 of the body, and of the functional activity of nervous centres and 
 end-organs of sense. Now, unless we are ready to be satisfied with 
 simply stating the facts, without making the attempt to find any 
 rational account for them, we are obliged to consider these cor- 
 related changes under the terms of cause and effect; and in fact, 
 were it not for the influence of prejudice derived from speculation 
 upon certain philosophical, ethical, and religious questions, no one 
 would think of hesitating to apply the terms of causation to the case 
 of mind and brain. 
 
 How impossible, indeed, it is to avoid speaking of the connection 
 of mind and brain, in terms of causation, may be illustrated by the 
 relations between the condition of the intercranial blood-supply and 
 the states of consciousness. A slight increase of this circulation, 
 resulting from a small quantity of alcohol or other drugs, or from 
 the hearing of interesting news, produces an increased speed in the 
 mental train. Reaction-time is found to vary with changes in the 
 circulation. In the delirium of fever the wild and quickly moving con- 
 dition of the thoughts, fancies, and sensations is a direct expression 
 of the kind of work which is going on, because of the accelerated 
 heart-beat and the disordered character of the blood, within the 
 cerebral arteries. Schroeder van der Kolk tells of a patient who, 
 when his pulse was reduced by digitalis to 50 or 60 beats per minute, 
 was mentally quiet and depressed; when it was allowed to rise again 
 to 90 beats, his mind was in maniacal confusion. Cox narrates the 
 case of a sick man who, at 40 pulsations in the minute, was " half- 
 dead"; at 50, melancholic; at 70, quite "beside himself"; at 90, 
 raving mad. The character of dreams is determined, to a consid- 
 erable extent, by the position of the head and the way in which 
 
644 GENERAL RELATIONS OF BODY AND MIND 
 
 this position affects the cranial circulation. Hallucinations not in- 
 frequently are immediately made to cease when the person having 
 them assumes the standing posture, or has leeches applied to the 
 head. Indeed, the phenomena which illustrate the causal influ- 
 ence of the nervous mechanism on the mental life constitute a large 
 part of th entire body of the science of psycho-physics and physio- 
 logical psychology. 
 
 16. On the other hand, phenomena which indicate that mind 
 operates as a true cause within the structure of the body are also in- 
 numerable. They are quite as numerous, though perhaps not so 
 obvious and impressive, as those which indicate the reverse rela- 
 tion. The chief reason why these phenomena are relatively little 
 regarded in psycho-physical researches is that the real causes are 
 in this case not readily made the objects of observation and measure- 
 ment. External stimuli constitute those causes of mental changes 
 which we can most easily observe and estimate. Ideas, feelings, 
 and acts of will arising in consciousness, and considered as causes of 
 the resulting bodily changes, cannot be treated by the same methods 
 of experimental science as apply to physical stimuli. But that the 
 mind acts on the body is one of the most familiar of experiences. 
 Such action penetrates and modifies all the life of the body. Hence 
 the material mechanism of the animal structure can never be con- 
 sidered, with a view to explain what is going on within it, as though 
 it were disconnected from the consciousness of the animal. The 
 most purely vegetative of the processes of the human body are 
 dependent for their character upon the states of the human mind. 
 The nutrition of the tissues, the circulation of the blood, the secre- 
 tion of different kinds of fluids, the healthy or diseased nature of 
 the vital processes, are greatly influenced by conscious processes. 
 If abnormal digestion produces melancholy, it is equally true that 
 melancholy causes bad digestion. In the case of the rise of strong 
 emotions, like anger or grief, the increasing affection of the mind 
 builds itself up upon a physical basis of increasing disturbance of 
 the organs; but it is equally obvious that the starting of the emo- 
 tion in consciousness, and the letting of it slip from control, are 
 necessarily followed by gathering momentum to the organic dis- 
 turbance. Irregular action of the heart, caused by organic defect 
 or weakness, occasions a feeling of indescribable alarm in the soul; 
 fear is followed, through the action of the mind upon the nervous 
 centres, by functional incapacity of the heart. The impure condi- 
 tion of the arterial blood which is characteristic of certain diseases 
 brings about a chronic state of mental lassitude or anxiety; care, 
 chagrin, and ennui poison the arterial blood. The lesion of the 
 cortical substance produced by a growing abscess or broken blood- 
 
INFLUENCE OF MIND ON BODY 645 
 
 vessel impairs the mind's powers of sensation and thought; ex- 
 cessive thought and over-excited feeling wear away the brain. 
 
 The entire class of phenomena which we are entitled to call 
 "voluntary," in the widest sense of the word, might be appealed 
 to in proof of the same principle. Whether they show that the 
 mind is "free," in the highest ethical meaning of the word, or not 
 (and upon this question psycho-physical science cannot pronounce), 
 they certainly do show that the condition of the bodily organs is 
 made dependent, through the nervous elements of the brain, upon 
 the states of the mind. And here are, in point, the phenomena of 
 the voluntary innervation of the organ by fixing the attention, of the 
 dependence of reaction-time upon the exercise of the will through 
 attention of the person reacting, of the abstraction of regard from 
 the images of sense when occupied in reflective thought, as well 
 as all the more marvellous instances of self-control in determining 
 the results of disease, etc. 
 
 The elevation of the bodily activities to the most astonishing 
 precision, under the influence of high and strong artistic feeling, 
 or sense of duty, is also a noteworthy fact of the same order. The 
 mind has not the power to constitute, in opposition to fixed chem- 
 ical affinities, a single molecule, or to execute the slightest move- 
 ment of a single muscle, without involving the nervous system in 
 the expenditure of the requisite energy. Moreover, this energy 
 must be started in the appropriate cortical area and descend along 
 the allotted motor tracts. We cannot explain how it is that mole- 
 cules of nervous matter can be acted upon in view of states of con- 
 sciousness. But neither can we explain how one kind of atoms 
 comes to act as it does in view of the presence and action of atoms 
 of another kind. Nevertheless, we can just as little assume to ex- 
 plain away the fact of such obvious causal connection, because we 
 cannot bring the measure of the connection under the same law as 
 that which maintains itself among certain modes of physical mo- 
 tion. 
 
 17. It would scarcely be worth while to consider seriously 
 either the older or the more modern forms of the denial that any 
 real causal relation exists between body and mind, were it not for 
 the fact of their essential agreement upon a false conception of the 
 nature of causal relations in general, and upon a false theory as to 
 the particular case of the correlations of body and mind. Two re- 
 marks bearing upon all such theories are, therefore, necessary. The 
 assumption that matter and mind are separated from each other 
 "by the whole diameter of being," if it be held to mean that the 
 two forms of being are so disparate in nature as to be unable to act 
 on each other, is an unverifiable assumption. It even goes squarely 
 
646 GENERAL RELATIONS OF BODY AND MIND 
 
 in the face of many of the most important psycho-physical facts. 
 We know nothing about what kind of beings can or cannot act on 
 each other, except through our experience of what beings do actually 
 act upon each other. The mystery involved in any one being act- 
 ing on any other is equally deep and unfathomable^ in whatever di- 
 rection we attempt to explore it. Before experience with the facts, 
 we should be quite at a loss to tell whether atoms of oxygen could act 
 on atoms of hydrogen, under the laws of chemical affinity, or not; 
 whether molecules of iron could act on other molecules of iron, 
 under the laws of cohesion, or not, etc. How it is that material masses 
 or molecules can "influence" each other, or what is the real nature 
 of the force which binds them together, physical science is quite 
 unable to say. So that even if we were entitled to regard matter 
 as somewhat, the very essence of which it is to be spread out, and 
 mind as somewhat, the very essence of which it is to be conscious 
 and not to be spread out, we should still be quite without justification 
 in asserting (a priori, as it were) that one cannot act upon the other. 
 But just the contrary if we are to accept, unbiassed, the obvious 
 witness of the facts, we are compelled to affirm: The phenomena of 
 mind and the conditions of the brain are related so constantly and 
 immediately under law, that we are warranted in believing in the 
 action of each upon the other. 
 
 18. But, in the second place, most of the modern objections 
 to speaking of causal relations existing between the nervous mechan- 
 ism and the processes of conscious mental life, are connected with 
 the current theory of the conservation and correlation of physical 
 energy. And here it is interesting to notice certain relations, both 
 of similarity and of difference, between a prominent modern theory 
 as to the mutual action of mind and brain and the now-abandoned 
 views of Occasionalism and Pre-established Harmony. These views 
 had regard to the reality and integrity of the soul, and respect for 
 the ethical character of the Divine relations toward its activities and 
 development. Modern science, on the contrary, raises most of its 
 objections, against regarding the conditions of the central nervous 
 system and the states of consciousness as connected by a real causal 
 tie, out of a profound regard for matter and the laws of physics. 
 The great value and significance of physical phenomena, and the 
 regular modes of their recurrence, if not the independent and eternal 
 existence of material beings, are taken for granted by this theory, 
 whatever difficulties, fears, or hopes to the contrary may arise from 
 the sphere of mind. Elements of material reality (called "atoms") 
 are assumed to exist; the universal form of their relation is held to 
 be the law of the conservation and correlation of energy. By " en- 
 ergy" we are to understand that which moves or tends to move the 
 
ANALYSIS OF CONCEPTIONS 647 
 
 elementary atoms, or their aggregations into molecules and masses. 
 The energy which is regarded as causing actual motion is kinetic; 
 that which is to be regarded as tending to produce motion is stored 
 or potential. But inasmuch as we have no test or suggestion of the 
 presence of energy except motion, we seem compelled to consider 
 the so-called "tendency" to move (potential energy) as motion that 
 is beyond the sphere of the senses, because distributed over so vast 
 a number of minute portions of matter whose amount of motion 
 is too small to be discoverable. All physical elements and masses 
 are, accordingly, always in motion, and the total quantum of this 
 motion is invariable throughout the entire universe. All forms of 
 energy must be classified, as respects quality, by the kind of their 
 motion; and as respects degree, by the amount of their motion. 
 
 19. It requires, however, only a brief and rather superficial 
 examination of the conceptions of physical science, as they are pop- 
 ularly taken, and even as they are correctly and helpfully employed 
 for purposes of scientific research, to discover how full they are of 
 concealed figures of speech and of as yet unverifiable hypotheses. 
 The known facts are of about the following order. Observed events 
 in the elements and masses of the physical world make upon our 
 minds impressions of more or less magnitude and intensity. Our 
 experience, especially with respect to the correlations between our 
 own mental strivings, feelings of effort or so-called " deeds of will," 
 and the sequent changes in the bodily organism, and through this 
 organism in material objects, begets and fosters the conceptions of 
 "power," "force," "energy," etc. This power, force, energy, is 
 conceived of as residing in "these objects," and as "passing over" 
 from one to another of them; and so as being the cause of the inter- 
 dependent, or mutually related, changes. But the causal concep- 
 tion is a distinctly metaphysical conception. For this vague and 
 indeterminate idea of cause, therefore, the physico-chemical sciences 
 very properly strive to substitute mathematical formulas which are 
 designed to express definite relations in the amounts as measured by 
 units of time and space, of these changes. Thus arises the theory 
 of work, actually doing, or that may be expected to be done, by ma- 
 terial masses and their elements. Between certain kinds of changes, 
 either directly observed or capable of an approximately correct cal- 
 culation, science has already established formulas which serve the 
 purposes both of explanation and of prediction. Such are the for- 
 mulas which give the equivalents of the kinetic energy of masses in 
 terms of the energy called heat, or light, or electricity. Further, as 
 a result of their experience, in general, with the quantitative correla- 
 tions existing between the different material objects, the physico- 
 chemical sciences have evolved an hypothesis of a more universal 
 
648 GENERAL RELATIONS OF BODY AND MIND 
 
 character. This is the hypothesis of the conservation of energy. 
 It holds that any system of material elements or masses, so long as 
 it can be kept " closed," or protected from dissipating its energy, and 
 from receiving accessions of energy from the outside, will do only a 
 fixed amount of work; although this gross sum of energy may mani- 
 fest itself in any of the various forms known to physical science, and 
 may be distributed and redistributed among the different elements 
 and masses of the system. Moreover, in the process of distribu- 
 tion and redistribution, the equivalents of the different kinds of 
 energy will remain the same. 
 
 This hypothesis of the conservation of energy is still undergoing 
 a process of experimental testing at the hands of the students of the 
 physico-chemical sciences. Only in the case of certain pairs of the 
 various recognized kinds of so-called energy have any definite, ap- 
 proximately exact formulas for their correlations been as yet dis- 
 covered. In the world of man's experience there is no such thing 
 as a " closed system." And wonderful new forms of energy, and of 
 the doing, and the potentiality of work, on the part of the physical 
 universe, are constantly being discovered. But slowly, and on the 
 whole in a satisfactory way, some hypothesis corresponding in general 
 to that called by the title "The Conservation and Correlation of 
 Energy" seems to be gaining support by the sum-total of human 
 experience with material things. 
 
 20. How far, now, can such an hypothesis as the foregoing be 
 applied to the nervous system of man ? That this system is a mech- 
 anism and like every other mechanism dependent for the amount 
 of work which it can do, upon resources of energy, either stored 
 within itself or derived by accessions from outside itself, there can 
 be no manner of doubt. All the various kinds of automatic and 
 reflex, or sensory-motor activity illustrate and enforce this truth. 
 In these activities the various kinds of energy recognized by physi- 
 cal science as kinetic energy of masses, quantity of heat, electrical 
 energy, and chemical energy, take part. And so far as the laws of 
 their conservation within the nervous system are known they ap- 
 pear to confirm the general hypothesis in question. It is true, how- 
 ever, that the work done by the nervous mechanism cannot as yet 
 be fully explained not to say, even correctly stated in terms of 
 other forms of energy. And thus far, the need of assuming a very 
 special and, indeed, unique form of energy, to be called " nervous," 
 does not seem to be in the least degree diminished. This is only 
 to say: What the nervous mechanism does, considered as having 
 the power to store, to transmit, and to distribute, its own resources 
 for doing its own work, cannot as yet be explained in terms that are 
 mere modifications or combinations of the formulas derived from a 
 
CONSERVATION OF CEREBRAL ENERGY 649 
 
 study of other forms of energy, employed in other kinds of work. 
 But this fact by no means disproves the assumption that the general 
 principle implied in the phrase we are discussing applies somehow 
 to the peculiar functioning of the nervous mechanism. 
 
 21. So, too, if we consider the external relations of man's ner- 
 vous system, we find it dependent on energies outside of itself for 
 the amounts, whether stored or kinetic, of its own peculiar way of 
 doing work. This dependence is of two principal kinds. The 
 nervous system must have nutriment; otherwise it cannot build 
 itself up; cannot store energy; cannot acquire the material to be 
 transmuted into its own form of work. In its metabolic processes, 
 therefore, it is constantly illustrating a species of the law of the con- 
 servation of energy. It is true that we cannot at present give the 
 exact equivalents, in the potentiality or kinetic energy of the nervous 
 mechanism, for the different food products; whether they are con- 
 sidered apart from the whole organism, or as carried to the nervous 
 system in the blood. The problem of food-supply as related to 
 neural energy is immensely complicated not only as a chemical 
 and physiological study, but also as having a large mixture of ob- 
 scure or totally unknown psycho-physical and mental factors. It 
 is conceivable, however, that mathematical formulas may at some 
 time be discovered for stating the quantitative relations between the 
 energies stored in the blood supplied and the energies, stored or 
 kinetic, of the nervous mechanism including its own peculiar 
 form of so-called "nervous energy." 
 
 The second form of the more obvious correlations between ex- 
 ternal forms of energy and the energy of the nervous system is 
 realized every time any appropriate stimulus is applied to the end- 
 organs of this system. How far we know of formulas that state in 
 mathematical terms this kind of correlation has already been suf- 
 ficiently discussed. Indeed, the whole science of psycho-physics 
 involves the attempt to deal with these correlations. 
 
 Let us suppose, then, that all the functions of the nervous mechan- 
 ism, and all their relations to the various forms of energy which act 
 upon it, and upon which it reacts, have become so well known as to 
 form a theory of the conservation of energy completely adapted for 
 application to this mechanism. We should then be able to explain 
 in terms of quantity the successive redistributions of work actually 
 done: between the different parts of the nervous system of man; be- 
 tween it as a whole and the other organs of the body; and between 
 it and the physical world as it affects the body. But the explanation 
 WTmld be couched in terms of quantity only. The mystery of the 
 quality, or kind, of the energizing of the nervous mechanism might 
 remain as great as before. And the mystery of its correlations with 
 
650 GENERAL RELATIONS OF BODY AND MIND 
 
 the phenomena of the mental life would not be changed, or its pro- 
 spective better solution even approximated. For the popular con- 
 ception of energy as a sort of entity that can actually be split up, 
 and distributed, or passed over, is, of course, especially absurd when 
 considered as applied to the relations of the body and the mind. 
 That varying amounts of the activity of the central nervous system 
 are correlated with somewhat comparable changes in the intensity 
 of our consciously felt experiences, is a statement sufficiently war- 
 ranted by a numerous class of observed facts and of reasonable 
 inferences from facts. But all this scarcely touches the point at 
 issue. 
 
 22. The entire conception of the conservation and correlation 
 of energy, as it is made fruitful use of in the physical and chemical 
 sciences, and even as it hopes to be introduced into the biological 
 sciences, is, therefore, essentially and utterly inappropriate to ex- 
 press those peculiar relations which are matters of experience be- 
 tween the body and the mind. This is as true of the brain, with all 
 its complicated structure and subtile forms of functioning, as it is 
 of any of the more massive and grosser forms of the bodily organ- 
 ism. The so-called " mind" is not the kind of an existence in which 
 physical energy can be stored, or to which it can be transferred, or 
 from which it can be derived. The utmost that could possibly be 
 claimed on the valid ground of experience, would be statable only 
 in terms somewhat like the following: The brain, considered as that 
 part of the nervous mechanism which has the most intimate and 
 important relations with the phenomena of conscious mental life, 
 is itself subject to the principle of the conservation of physical energy. 
 The felt intensity of some of the mind's experiences varies in some 
 sort of dependence upon the amounts of the energy expended by the 
 brain ; and, possibly, upon the particular portions of the brain in which 
 these changes of stored into kinetic energy take place. But if this 
 relation is to be looked upon as causal and we hold that it is, in a 
 defensible and rational meaning of the word " causal " it is a quite 
 different kind of causal relation from that which maintains itself 
 between material objects. And, in truth, even for the latter kind of 
 relation, all that the principle of the conservation of energy at its 
 best can do, is to give formulas applicable to the quantitative as- 
 pect of physical changes only. But this kind of quantitative formu- 
 las, and its mathematics, have no valid applications to the phenomena 
 of consciousness, as such. 
 
 23. To go over the ground again in a somewhat different way: 
 On attempting to account for the whole world of phenomena in 
 terms of motion, kinetic or potential, under the law of the conser- 
 vation of energy, we are met with insuperable difficulty as soon as 
 
CONSERVATION OF CEREBRAL ENERGY 651 
 
 we enter the domain of consciousness. States of consciousness 
 are not modes of motion. If they were, the general theory of physics 
 would compel us at once to attempt a strict mathematical correla- 
 tion between physical and mental phenomena. Just as the kinetic 
 energy of masses can be expressed, with a tolerable approximation 
 to exactness, in terms of heat as a mode of motion, so would some 
 formula be conceivable for indicating what amount of chemical 
 changes, or nerve-commotion, in the matter of the brain, is the mathe- 
 matical equivalent of the conception of home, of the sense of obliga- 
 tion, or of the idea of God. In other words, it seems impossible to 
 regard any amount of physical energy as abstracted from the brain, 
 so to speak, and expended or stored up in consciousness. Energy 
 is stored by the process of nutrition in the nervous elements of the 
 brain; it becomes kinetic in connection with the phenomena of con- 
 sciousness. But between the mind, whether regarded as merely 
 the formal subject of consciousness or as a real unit-being whose 
 faculty or power it is to be conscious, and the physical basis of mind 
 in the brain, no correlation, in the sense of a passing back and forth 
 of physical energy, can occur. 
 
 The entire circuit of the transmission and distribution of energy 
 is, therefore, complete within the brain itself. Not a single atom enters 
 its substance that does not come forth unchanged, with all its forces 
 inherent in it. No atom is transferred from brain to mind, as all 
 the atoms are transferred from the blood to the nervous substance 
 of the brain. Not the most infinitesimal amount of energy exists, 
 stored in the constitution of the molecules of this substance, which 
 is not either used up there or returned to external nature in connec- 
 tion with the constitution of the molecules separated from this sub- 
 stance. The stricter we make our application of the law of the 
 conservation of energy within the physical realm, the more impos- 
 sible does it become to apply it at all to the relations of body and 
 mind. 
 
 It is not surprising that, in the estimate of one who is unaccus- 
 tomed to regard with favor any explanation of phenomena which 
 does not come under the most general law of all physics, the case 
 of the mind and the brain should seem to demand the most ex- 
 traordinary treatment. In any event, the facts of consciousness, as 
 facts, cannot be denied. Whether we can explain them or not, they 
 are equally plain and persistent. Men perceive, and imagine, and 
 remember, and reason, and believe in the invisible, and choose, etc. 
 All this they do, as possessed of a body and, particularly, of a ner- 
 vous mechanism, the activities of whose central portion are related 
 in some special and unique way with the doing of all this. And yet 
 sure, beyond doubt it is argued is the existence of the atom, with 
 
652 GENERAL RELATIONS OF BODY AND MIND 
 
 its host of inherent energies; and supreme is the law of the conserva- 
 tion and correlation of these energies regarded as modifications of 
 one fundamental form. But here the whole conception becomes a 
 figure of speech so vague and evanescent that it loses all definable 
 meaning when the attempt is made to render it into terms of actual, 
 matter-of-fact experience. 
 
 24. When freed, then, from a physical hypothesis which, of its 
 very nature, has no valid application to the subject in hand, the con- 
 ception of causal influence, and of reciprocally dependent relations 
 under terms of this influence, between the nervous mechanism and 
 the phenomena of mental life, meets with no objection to itself in 
 a scientific study of the facts. In a word, we may affirm anew that 
 kind of dualism, which holds that some, at least, of our conscious 
 states are caused to be such and no other, by the antecedent or accom- 
 panying functions of the nervous system; and, on the other hand, 
 that some, at least, of the changes which occur in the bodily states are 
 caused through the nervous system, by the antecedent or accompany- 
 ing functions of the mental life. This is as has already been re- 
 peatedly said the popular view. Science has nothing to adduce 
 against it; but much to confirm it. So far may we carry the theory 
 of the natural correlations between body and mind, by a direct ap- 
 peal to the experienced facts. 
 
 25. But the facts also compel us to admit that neither one of these 
 forms of functioning wholly explains the other. What goes on in 
 the nervous mechanism does not completely account for what goes 
 on in consciousness; what goes on in consciousness does not com- 
 pletely account for the activities in the nervous mechanism. In- 
 deed, the very nature of the two series of occurrences is as unlike 
 as it is conceivable that different classes of phenomena should be. 
 The behavior of radium under the influence of radio-active energy, 
 is not so unlike that of the star Sirius under the influence of the force 
 of gravity, as is the simplest conscious sensation from the nerve-com- 
 motion which caused it. Moreover, the brain has always something 
 to say as to what it will do, when under the most determined and 
 strongest influence from the mind. And the mind has always 
 something to -say as to what its own behavior shall be, even when 
 under the strongest of influences from the brain. There is, however, 
 nothing unique about all this, so far as the propriety of any applica- 
 tion of the conception of a causal relation is concerned. For there 
 is no material thing so mean that it does not have a "nature" of its 
 own; and that nature determines, in the most powerful and prac- 
 tical way, what it shall be caused to do by its entering into all sorts 
 of relations with other things. And there is no psychic existence 
 which has not a similar claim to a nature of its own. 
 
THE THREE PROCESSES INVOLVED 653 
 
 In what sense we may assert any reality or unity for the subject 
 of the mental life is not the question at the present time. But all 
 our scientific research, as directed toward this particular kind of 
 correlations, assumes that, in some valid meaning of the word, there 
 is a being to be called the Mind. On the other hand, no one can cross 
 the threshold of the science of physiological psychology, or psycho- 
 physics, without assuming the existence of the body; and of the ner- 
 vous system and brain, as portions of this body. The two existences, 
 body and mind, may not be identified by the science which investi- 
 gates their correlations; for they have markedly different ways of 
 functioning; the phenomena which research both assumes and dis- 
 covers, and so assigns to them, are characteristically unlike. They 
 are, however, dependently connected. Each stands in causal re- 
 lations to the other; although this dependence appears to be by 
 no means complete. Body and Mind, changes in the nervous proc- 
 esses and the changing processes of the conscious mental life, can- 
 not be identified. Neither can be stated with scientific precision, 
 or in any other than a convenient figure of speech, in terms of the 
 other. 
 
 26. In the more particular description of the connection between 
 the mind and the brain, it may be said that all intercourse between 
 material objects and the subject of consciousness involves three 
 processes a physical, a physiological, and a psychical. In these 
 processes the perceived object and the perceiving subject mutually 
 condition each other. This fact, however, does not destroy the ne- 
 cessity, under which all scientific investigation finds itself, of assum- 
 ing that both object and subject exist as real beings. The physical 
 process consists in the action of the appropriate modes of physical 
 energy upon the nervous end-apparatus of sense. The bringing of 
 such modes of energy to bear upon the apparatus is accomplished 
 through mechanical contrivances such as the means for forming 
 an image on the retina in the eye, and for conveying the modified 
 acoustic impulses to the organ of Corti in the ear. 
 
 The second process consists in transmuting the physical ener- 
 gies, in part at least, into a physiological process, a nerve-commo- 
 tion within the nervous system; and in propagating such nerve- 
 commotion along the proper tracts and diffusing it over the various 
 areas of this system. Inasmuch as the physiological process is 
 also a physical process that is, a mode of the motion of material 
 molecules, accompanied by chemical and electrical and other 
 changes it must be conceived of as standing in certain relations 
 of quality and quantity to the first, or more distinctively physical, 
 process. That the law of the conservation of energy, as formulated 
 for much simpler cases of the relations of forces between inorganic 
 
654 GENERAL RELATIONS OF BODY AND MIND 
 
 bodies, applies to the relations of the nervous system and its stimuli 
 or within the different parts of the nervous system itself, we are not 
 yet able to affirm with confidence. But we have valid grounds for 
 the belief that something of the sort is true. 
 
 The third process is psychical; it is a process which is a psychi- 
 cal event, a forth-putting of the energy of mind. It is directly 
 correlated with the physiological process only when the latter has 
 been realized in certain cerebral areas. It is not to be explained 
 as a resultant of the cerebral physiological process, but as an ac- 
 tion of the mind which is conditioned upon that process. So, also, 
 are we entitled to say that, when certain psychical processes, by 
 way of feeling, ideation, and volition, take place, then, and as con- 
 ditioned upon these processes, certain corresponding physiological 
 processes occur in the brain; the physiological processes, being 
 propagated from the central nervous system, end in physical proc- 
 esses returning energy to the world outside of the body. 
 
 When the mental process is a perception of some object, called 
 an "external" object, it is no less truly a psychical process. The 
 mind creates its own objects; presents itself with its own presen- 
 tations of sense; acts to bring forth that which it knows as not 
 itself. But it does all this as dependent upon the processes which 
 take place outside of itself, and with the assumption of extra-men- 
 tal realities as existing, to which it stands in the relation of cause 
 and effect. 
 
 Finally, then, the assumption that the mind is a real being, which 
 can be acted upon by the brain, and which can act on the body through 
 the brain, is the only one compatible with all the facts of experience. 
 There is nothing which we know about the nature of material be- 
 ings and the laws of their relation to each other, or about the nature 
 of spiritual beings and their possible relation to material beings, or 
 about the nature of causal efficiency whether in the form of so-called 
 physical energy or in that of activity in consciousness, which for- 
 bids the aforesaid assumption. And no other assumption, sub- 
 stantially different from this, is compatible with the facts of experi- 
 ence. 
 
 27. A distinctive feature of modern science is its endeavor to 
 satisfy inquiry into the nature of the objects of its investigation by 
 a detailed description of their development. In answer to the in- 
 quiry what a thing is, we are invited to listen to an account of how 
 it became what it is. The history of the egg explains the bird even 
 more than the nature of the bird explains the egg. Indeed the 
 universal process of "Becoming" has been almost personified and 
 deified so as to make it the true ground of all finite and concrete 
 existences. There can be no doubt as to the great fruitfulness and 
 
THE CONCEPTION OF DEVELOPMENT 655 
 
 value of this historical and genetic way of studying everything. Let 
 us, then, now consider the most general relations, or correlations, of 
 body and mind, from the point of view of their dependent develop- 
 ment. 
 
 Undoubtedly, both of the two subjects, with whose correlations 
 Physiological Psychology deals, require for their most satisfactory 
 understanding to be studied by the genetic method. The structure 
 of the nervous system, as we saw in the earliest chapters of this 
 treatise, appears in a new light w r hen regarded as the result of a 
 process of evolution. Beginning with the impregnated ovum, 
 by propagation of cells of living protoplasm, by segmentation of 
 larger sections of these cells, by proliferation of cells and separation 
 into layers, the one portion of the germ from which the mechanism 
 of nerve-fibres and nerve-cells is to unfold itself becomes differenti- 
 ated from the other portions. By vital processes kept up through nu- 
 trition and resulting in the growth of some areas beyond others, and 
 by mechanical influences at work to crowd forward here or push 
 back there, to fold and tuck and cause to dip or curve, etc., this 
 epiblastic portion develops the system of end-organs, central or- 
 gans, and connecting tracts of nerves. 
 
 Psychology, also, has felt strongly the same impulse. It has 
 been forced to confess that its real task is but begun when it has, 
 by introspection, examined and classified the phenomena of adult 
 conscious life. All the mental phenomena undoubtedly have, as 
 respects their genesis and order, a truly vital connection. Those of 
 the present have their roots in those of the past. The so-called 
 faculties of the mind are neither hard and fixed lines drawn to ex- 
 clude from internal relation the various modes of its behavior in 
 consciousness, nor are they kinds of activities that spring up, full- 
 formed at once, at different intervals in its entire history. Percep- 
 tion itself is a result of development; for " things" are not ready-made 
 products existing, as they appear, outside of the mind, but resultants 
 of mental activities that have to be performed anew so often as the 
 things appear. It is in the evolution of the mind that we find our 
 means for understanding its true nature. Moreover, the character- 
 istics which distinguish one mind from another are to be under- 
 stood as largely resulting from the order and relative prominence 
 of different activities in the development of each. 
 
 So far as the connection of mental phenomena with the increasing 
 complexity of the nervous activities, and with the stored energies 
 and hardening habitus of the nervous elements, affords any explana- 
 tion of the development of the mind, we have already said all that 
 is necessary. The growth of the mental life in the acquirement 
 and arrangement of sensations, in the recalling of ideas, in the form- 
 
656 GENERAL RELATIONS OF BODY AND MIND 
 
 ing of judgments about objects of sense, etc., is plainly dependent 
 upon the evolution of the bodily members. But the real nature 
 of the relation which exists between the mental phenomena and 
 the nervous mechanism, so far as this can be learned by studying 
 the development of both, furnishes us with another question. Upon 
 this question, also, the same conflict of view as that to which we have 
 already drawn attention may arise. 
 
 28. There can be no doubt of a general correspondence be- 
 tween the two developments, of the body and of the mind. Nervous 
 system and mental condition are both immature in infancy; both de- 
 velop with great rapidity in early childhood, and then more slowly on 
 into adult life; both it is claimed remain comparatively stationary 
 through the period of man's highest maturity; and as old age ad- 
 vances, both, in some respects at least, customarily keep pace in 
 their decline. Moreover, cases of arrested development of brain are 
 cases of arrested development of mental capacity. Idiots are fre- 
 quently microcephalic; many of them have brains weighing less 
 than thirty ounces. Degeneracy of the tissues of the cerebral hemis- 
 pheres is commonly connected with increasing degeneracy of the 
 mind. As the tides of molecular nerve-commotion rise and fall in 
 the nervous mass, so rise and fall the tides of mental vigor. 
 
 On the other hand, attempts to account for the orderly increase 
 in complexity and comprehensiveness of all the mental phenomena 
 by tracing the physical evolution of the brain are wholly unsatis- 
 factory to many minds. We have no hesitation in classing ourselves 
 among this number. That something more than an absolutely de- 
 pendent and physically conditioned development is implied in the 
 history of each individual mind may be argued on two principal 
 grounds. In the first place, it may be shown that the stages and laws 
 of mental development do not fully correspond to those which are ob- 
 served on tracing the evolution of the nervous system. It may also be 
 shown that certain elements necessarily enter into the development 
 of mind, which have nothing like them, or strictly correlated with 
 them, in the evolution of the material mechanism. Any being may 
 be dependent on other beings for its starting, as it were, and for cer- 
 tain factors that enter into its growth or furnish the indispensable 
 conditions of its growth ; and yet this fact gives us no right whatever 
 to refuse to such a being all title to take rank among other real ex- 
 istences as having a complex nature of its own. No existence loses 
 or impairs its claim to reality by being dependent on other existences 
 for its development. The mind, on the contrary, most indubitably 
 establishes such a claim, because the stages and laws of its unfold- 
 ing, and some of the factors which necessarily enter into this unfold- 
 ing, are peculiar to itself (sui generis). 
 
THE CONCEPTION OF DEVELOPMENT 657 
 
 29. That the words, "development of the mind," stand for some- 
 thing real and verifiable, there can be no reasonable doubt. The 
 sum-total of the conscious experience of each individual is far more 
 than a mere series of states of consciousness. No difference in de- 
 grees under the same kind can be conceived of which is greater 
 than the difference between the most mature and highly developed 
 mental performances and those inconceivably simple activities with 
 which the mental life begins. So far as the character of the phe- 
 nomena of consciousness is concerned, the mind of the adult New- 
 ton or Kant is much farther removed from the mind of the infant 
 Newton or Kant than the latter is from the mind of one of the 
 lower animals. There is no doubt, also, that the incomparable im- 
 provement of the mental processes which distinguishes the adult 
 from the infantile human being is a true development. Each stage 
 of this improvement is dependent upon preceding stages. The 
 changes are all in some sort according to a plan. Thus the life of 
 every individual's mental experiences is capable of being made into 
 a history. A certain tolerably uniform order in the relative devel- 
 opment of the different faculties is discernible. At first the senses 
 are awakened to a lively and varied activity; then memory and imag- 
 ination become more prominent; and, finally, judgment and the 
 reasoning powers assert their sway. Gradually, things become 
 known and conduct shaped under principles which are assumed to 
 have a universal validity as so-called general laws. The history of 
 the mental life of every human being, from the cradle (or even from 
 its embryonic existence) to the grave, has all these marks of un- 
 folding itself in a regular order, in which every characteristic event 
 happens in due sequence and in dependence upon what has pre- 
 ceded. This is the very essence of a true development. 
 
 Can this mental development be explained as merely the result- 
 ant or expression of the physical evolution of the nervous system 
 the latter being regarded as situated in the rest of the bodily en- 
 vironment, and surrounded by the more extended environment of 
 the world of active physical energies outside ? Against an affirma- 
 tive answer to this inquiry stand many facts and laws of all such 
 mental development. In spite of certain striking correspondences 
 between the evolution of the bodily organism and the development 
 of the mental powers, it must be held that there are marked di- 
 vergences as well. At certain epochs of life the evolution of the 
 brain seems to stand far in advance of the mind; at others, the mind 
 appears to have overtaken and passed by the stage reached by its 
 physical substratum. During a long period of life the growth of 
 mental powers is constant and solid, while the growth of the physi- 
 cal basis has nearly ceased, and such changes as are taking place 
 
658 GENERAL RELATIONS OF BODY AND MIND 
 
 in it appear quite inadequate to serve as correlates for the mental 
 growth. Moreover, the most distinctly typical features in the de- 
 velopment of the mind remain the same when malformation or dis- 
 ease or accident have largely changed the physical evolution of the 
 brain. 
 
 30. We have no sufficient means for deciding how far the mental 
 life of the human embryo keeps pace with its organic evolution. 
 We do not even know beyond doubt that the embryo has a mental 
 life, in the only tenable meaning of the words that is, a life of con- 
 scious states. It is probable, however, that its antenatal movements 
 are not all purely reflex, but are accompanied and directed by con- 
 scious sensation, feeling, and volition. But the mental life of the 
 embryo, if it exist at all, can hardly be more than an irregular and 
 fitful succession of the lowest and least complex of conscious proc- 
 esses. Taste, smell, hearing, and sight are, of course, not to be 
 thought of as entering into such a mental life. Touch, as we under- 
 stand the word to express the localized sensations of pressure which 
 arise through the practised organ of the skin, is scarcely more likely 
 to belong to the human embryo. Obscure feelings arising from 
 changes in its relation to the surrounding tissues and fluids of the 
 mother, or from disturbances in its own internal organs, and result- 
 ing equally obscure feelings of position and motion, as its limbs are 
 moved, must constitute the greater part, if not the whole, of its ex- 
 periences. As yet there is no experience, properly so called; no 
 perception of things, no feelings of self, no discrimination of ego and 
 state. Yet long before the child is born it possesses a wonderfully 
 elaborate nervous mechanism, far surpassing in its grade of evolu- 
 tion the nervous system of the most intelligent adult animals. Pre- 
 vious to birth this nervous mechanism must also be constantly in 
 action in a highly complicated way; it is engaged in supervising the 
 processes of nutrition, and in the reflex and automatic activities 
 which are expressed by the changes of the child's position within 
 the womb of the mother. The mind, however, is as yet unawakened; 
 this is not because the nervous mechanism is not complex and active 
 enough to serve as the physical basis of a rich mental development, 
 but because the kinds of sensation visual, tactual, auditory, etc. 
 which start and furnish and direct this development have not yet 
 been supplied. The mental life cannot then be said to have kept 
 pace before birth with the evolution of the brain, or with its dis- 
 tinctive activities. On the contrary, it is far behind the stage al- 
 ready reached by its physical support. It waits to be aroused and 
 set to its own work of combining and interpreting those sensations 
 which are to serve as its chief means of early culture. 
 
 For the first few weeks of infancy the embryonic relation between 
 
MENTAL CONDITION OF THE EMBRYO 659 
 
 the relative developments of the body and soul of the child seems 
 to be maintained. Both are subjects of a rapid growth, but the 
 former is still much in advance of the latter. The newly born infant 
 is, in respect to the condition of its nervous system, much the most 
 highly organized and fully equipped of all young animals; but as 
 judged by the number and quality of its volitions and perceptions, 
 many other young animals are less stupid and insensate. If we 
 may represent its mental condition by anything conceivable through 
 the adult imagination, the human infant is in a dreamless sleep oc- 
 casionally interrupted by instants of unlocalized and unmeaning 
 sensations. 
 
 The cavity of the infant's tympanum is filled with a fluid, the 
 place of which is only gradually taken by the air. Sensations of 
 sound, if they arise at all, must be at first only occasional and faint. 
 Binocular movements of the eyes in the direction of bright objects 
 take place early; and it is through sensations of light and color that 
 the first activities of the mind in perception are chiefly aroused and 
 controlled. But for some weeks there are only sensations and im- 
 pressions, without true perceptions; there is as yet no knowledge of 
 any "Thing." This earliest relation of mind and brain, with re- 
 spect to the degree and rate of their development, is not favorable 
 to any form of the materialistic theory. It rather favors the view 
 that the mental phenomena belong to another principle than any 
 material substratum. The dependence of the mind on the brain 
 is indirect and through the sensations (chiefly of sight and touch) 
 which must somehow be furnished as the primary factors in its de- 
 velopment. The halt in the development of mind at first, and its 
 distinct backwardness with respect to the relative stage it has 
 reached, are due to a lack of such sensations as have the charac- 
 teristics of spatial series, and so are able to stimulate the mind, and 
 to afford it the requisite material for the construction of true pres- 
 entations of sense. 
 
 31. Within a few months after birth the child has undergone 
 an enormous mental development; it has become a mind, in some 
 inchoate way recognizing itself as the subject of states, and per- 
 ceiving a surrounding world of objects of sense. It has also be- 
 gun to attend to the objects presented in consciousness, and to 
 direct its attention by voluntary choice. The mind's relating ac- 
 tivity has been aroused; and acts of memory, discrimination, and 
 judgment, as the basis for those concepts which require articulate 
 language to express them, are repeatedly taking place. And soon, 
 the assumptions of reason, as involved in all human experience of 
 things, and of their action and reaction upon each other, are found 
 to be shaping the growth of the mental powers. 
 
660 GENERAL RELATIONS OF BODY AND MIND 
 
 As accompanying and forming the ground for this sudden blos- 
 soming of the mind in the use of its conscious powers, there is a 
 continuous and yet diminishing monthly increase of the substance 
 of the brain. No new organs are formed within the cranial cavity; 
 but those which have been formed previous to birth are further 
 developed under the changed conditions of nutrition. In respect to 
 the quantity and arrangement of its molecules, the nervous mech- 
 anism certainly undergoes no development during the first year of 
 the child's life which at all corresponds to, or accounts for, the de- 
 velopment of the child's mind. 
 
 It may be claimed, however, that the most important develop- 
 ment of the nervous mechanism has been overlooked in the fore- 
 going description. This development does not consist so much in 
 the increased quantity of the brain's substance, or in the more 
 intricate arrangement of its elements with relation to each other; 
 but, the rather, in the forming of what have already been referred 
 to as "dynamical associations" among the existing elements. The 
 statement that such is the nature of the developing activities of the 
 nervous mechanism, and the assumption that such activities are an 
 indispensable physical condition for the growth of the mind, must 
 be taken for granted. But even then the argument is far from com- 
 plete upon which the development of mind as a real being, with a 
 nature of its own, and with a history controlled by its own laws, 
 can be denied. The formation of so-called "dynamical associa- 
 tions" among the molecules of the nervous mass furnishes no ade- 
 quate account of the development of mind. This development is 
 not in the direction simply of associating together states of feeling, 
 each one of which has an exact physical correlate in a physical as- 
 sociation among the minute parts of the nervous substance. It is 
 rather a development which for its very existence requires something 
 different from such associations. The child might go on forever 
 merely associating together affections of its own mind in correspond- 
 ence to dynamical associations among the nervous molecules, and 
 yet have no growth of experience such as it actually attains. The 
 fact is that within a single year, or within two years, the child has 
 learned to know " Things," to attend to some in preference to others, 
 to refer its states in some crude way to itself, to form concepts and 
 judgments by the mind's relating activity, and to underlay the world 
 of its sensuous experience with another world of assumption re- 
 specting certain non-sensuous realities. To account for this bound- 
 less expansion of the activities of consciousness, with its surprising 
 new factors and mysterious grounds of synthesis and assumption, 
 by proposing an hypothesis of "dynamical associations" among 
 the particles of nervous substance in the brain, is a deification of 
 
DEVELOPMENT OF THE ADULT 661 
 
 impotency. So far as we really know anything about the develop- 
 ment of both brain and mind, we are compelled to say that the latter, 
 when once started by the sensations furnished through excitation 
 of the former, proceeds to unfold its activities with a rapidity and 
 in an order for which no adequate physical causes can be assigned. 
 
 32. During the period of young manhood, or young womanhood, 
 the dependence of the development of the mind on that of the body 
 is most strikingly seen in the influence over the emotions and imag- 
 ination from the sudden unfolding of certain bodily organs and 
 powers. The indirect influence of these acts of feeling and imag- 
 ination upon the more purely intellectual progress of the mind is, 
 of course, correspondingly great. But the dependence of mind on 
 body is by no means such as to favor the view that there is no ground 
 in a real being, other than the brain, for the order and rate of the 
 mental development. 
 
 This same statement is emphatically true of the long period of 
 maturity which constitutes what we call the "middle life" of man. 
 During this time the nervous matter undergoes scarcely any dis- 
 cernible development. Nothing that microscope or electrometer can 
 detect distinguishes the brain characteristic of the man of twenty- 
 five from that of the man of fifty. A few grams of weight have 
 perhaps been added to it during this long period of years. Any one 
 is at liberty to speculate as to the immense development of so-called 
 "dynamical associations" which has taken place during the same 
 period. We are far from denying the possibility of such develop- 
 ment. But the fact that a large development of mind may have 
 taken place during the same period cannot be denied. If it be 
 true that large numbers of mankind remain mentally stationary for 
 most of their adult life, this truth in no way favors a materialistic 
 view of the development of mind. Most observing persons will 
 rightly find the chief account of the failure of mental growth in 
 precisely those kinds of mental activity which least admit of being 
 explained by physical analogies. It is from want of mental curi- 
 osity, attention, careful and comprehensive judgment, sound moral 
 purpose, etc., that most men fail to develop during adult life in 
 their mental powers. And these are mental activities for explain- 
 ing which no one as yet has been able to conjecture any analogous 
 or corresponding class of cerebral changes. 
 
 Many minds, however, not only make vast acquisitions, but also 
 experience a large unfolding of mental capacities during the period 
 of middle life. How mature and wide-reaching do the judgments 
 of some men then become! How profound the insight into the 
 most abstract and difficult speculations comes to be! What cere- 
 bral evolution shall be conceived of as being the only true cause, 
 
662 GENERAL RELATIONS OF BODY AND MIND 
 
 and the exact physical correlate, of the mental development of Kant 
 during the years preceding the appearance of the " Critique of Pure 
 Reason," or of Newton while he was unfolding the calculations and 
 conjectures of the "Principia"? To hold that the changing mole- 
 cules of the brain substance of these thinkers were the sole subjects, 
 really being and acting in the unrolling of these great dramas of 
 human speculation, involves an astonishing credulity. On the 
 contrary, we seem compelled to affirm that no important activity, 
 or law, or fact, in the order of such mental development, fails to 
 demand the assumption of a real and non-material unit-being, un- 
 folding its powers according to its own nature, although in de- 
 pendence upon certain elements and conditions furnished through 
 the brain. 
 
 33. Advancing old age is doubtless, as a rule, characterized by 
 a simultaneous decline both of certain mental and of certain bodily 
 powers. In this period of life, however, the correspondence be- 
 tween the changes in the character of the phenomena of conscious- 
 ness and the altered vigor and quality of the nervous mechanism is 
 not such as to suggest that the two have an altogether common 
 basis. In healthy normal old age the course of the organic life is 
 distinguished chiefly by the dropping out or diminished action of 
 certain factors that are relatively prominent in youth. The circu- 
 lation is slower; the vital energy is declining; the muscles are less 
 promptly and completely under the control of the volitions; the 
 end-organs of sense are less sensitive under impressions; and cer- 
 tain emotions and passions whose physical basis is of the most ob- 
 vious sort become greatly modified or disappear. As to the marked 
 effect of these bodily changes upon the mental development there 
 can be no doubt; and if the previous mental development has been 
 chiefly along lines indicated by organic activities the apparent de- 
 cay of mental vigor when the physical basis begins to fail is, of 
 course, also most plainly marked. 
 
 On the other hand, there are many other cases, where no notable 
 difference can be detected, or even fairly assumed, in the course of 
 the psychical evolution down to the "feebleness" of old age; where 
 the course of mental development continues substantially undis- 
 turbed in all its most important features. The mind of the culti- 
 vated old man, with calm and broad judgment, with refined kind- 
 liness and fixed moral principles, is not to be spoken of as suffering 
 a decline which keeps pace with the failing of his physical powers. 
 It may justly be claimed that the final period of human life, on the 
 whole, favors that theory which regards the mind as by no means 
 wholly conditioned upon the brain for the character, order, and 
 laws of its development. 
 
PHENOMENA OF DECAY 663 
 
 34. The same general view of the development of mind, which 
 is most consistent with the facts of the different stages of life, is 
 also favored by considering those sudden checks or changes in the 
 course of this development that are caused by disturbing or de- 
 stroying considerable portions of the nervous matter. The phe- 
 nomena which follow experimental extirpation of the substance of 
 the brain in the lower animals, and loss of it by serious lesions in 
 the case of man, do not favor a theory which completely identifies 
 the two developments, or one which makes the unfolding of the men- 
 tal life nothing more significant than a dependent expression of the 
 evolution, maturing, and decay of the nervous mechanism. Ex- 
 tensive losses in certain areas of the cerebral hemispheres are often 
 followed by no appreciable disturbance even of any sensory or motor 
 activity. When lesions are followed by such disturbance, their ef- 
 fects may in time wholly or partially disappear. When such dis- 
 turbance is permanent it is not necessarily connected with loss in the 
 power of judgment, in the higher intellectual, aesthetic, and ethical 
 activities of feeling, intellect, or will. Even where aphasia is so 
 severe as to include the loss of all power to utter or understand 
 articulate language, the patient may still show a good degree of 
 mental acuteness by ability to make calculations or play games of 
 skill. 
 
 On the other hand, a much more serious interruption or complete 
 loss of mental development may occur when no adequate explana- 
 tion can be detected in the disturbance or arrest of cerebral develop- 
 ment. It is, of course, natural to conjecture that, in all this latter 
 class of cases, more accurate information would show us some dis- 
 eased condition of the brain as the physical antecedent of the mental 
 defects. We know that subtle changes in the character of the 
 blood-supply, such as we have no physical means whatever for de- 
 tecting, are often the causes of most profound changes either 
 temporary or more permanent in the train of ideas. None the 
 less, however, do both classes of cases above mentioned favor the 
 theory we are advocating, rather than the so-called materialistic 
 theory of mind. 
 
 35. Several references to the second argument for our view of 
 the development of mind have already been made. This argument 
 is based upon the fact that certain indispensable elements enter into 
 the development of mental life which have nothing similar to them, 
 or strictly correlated with them, in the evolution of the material 
 mechanism. The mind can, indeed, undergo no development ex- 
 cept as conditioned upon these elements. But the elements them- 
 selves are of such a nature that they cannot be regarded as the ex- 
 pression in consciousness of merely physical causes, or as flowing 
 
664 GENERAL RELATIONS OF BODY AND MIND 
 
 necessarily from more primitive activities of the mind which may 
 possibly be regarded as the expression of such causes. 
 
 If we accept for the moment the customary classification of modern 
 psychology, we may say: All of those fundamental forms of activity 
 which are recognized in the threefold division of conscious processes 
 namely, acts of feeling, acts of knowledge, and acts of will neces- 
 sarily enter into the development of the mind. Its development 
 consists in increased capacity for these three classes of acts, in their 
 mutual dependence and according to the laws which belong to them. 
 Among each of these three great classes of acts there are certain 
 kinds that defy all attempts whatever to correlate them with changes 
 in the nervous mechanism, or to explain them as necessarily or 
 actually arising out of such physical changes. Such are the feel- 
 ing of moral obligation, the sentiment of justice, the love of truth, 
 and certain of the higher sesthetic feelings. Among the acts of 
 knowledge, such are the mind's relating activity, its use of the prin- 
 ciple of reason and consequent in drawing deductions, its confident 
 assumption that similar phenomena are signs of like realities, and 
 that the world of sensuous individual experience is but the manifes- 
 tation of an invisible world of real beings, with permanent proper- 
 ties and forces, acting and reacting under law. Such, also, are the 
 acts of deliberate choice among courses of conduct, under the in- 
 fluence of moral considerations the so-called acts of "free will" 
 in the highest sense of the term. 
 
 Not one of the higher acts of feeling, knowing, or willing, so far 
 as its sui generis character is concerned, admits of being correlated 
 with, or represented under, any of the conceivable modes of the 
 motion and relation of molecules of nervous substance. Certain 
 sensations and perceptions connected with the rise and growth of 
 the higher forms of feeling have, undoubtedly, a physical basis; 
 but such basis is not assignable to the feelings themselves. Sen- 
 sations and perceptions, which are the resultants (in some meaning 
 of the word) of physical processes, are discriminated by judgment 
 and made the basis of deductions and inductions. But admitting 
 this does not one whit the better enable us to conceive of a physical 
 
 Erocess which can account for the sui generis character of the re- 
 tting activity itself. Acts of "free will," so called, always take 
 place under certain conditions of sensation and perception, as well 
 as of desire; but the physical correlates of these conditions can in 
 no respect be conceived of as being also correlates of the conviction 
 that the choice is responsible and free. 
 
 Now, if such activities as the foregoing do actually constitute in- 
 dispensable elements of mental development and it is obvious that 
 they do this development cannot properly be accounted for by 
 
PECULIARLY MENTAL OPERATIONS 665 
 
 assigning it to a mass of nervous matter undergoing a physical 
 process of evolution, after the manner of the growing human brain. 
 Such development rather implies a being of another than the phys- 
 ical order. And this other being must be thought of as stimulated 
 by the rise and recurrence of sensations and images of past sensa- 
 tions to unfold its own activities as conditioned by its own inherent 
 powers. Like every other real being, the history of its unfolding 
 is dependent upon the relations in which it is placed to other real 
 beings; but it is nevertheless a history determined also by what 
 the being is. 
 
 36. More particularly, it may be said that perceptions are not 
 merely developed forms of sensations which latter are products of 
 the brain; they are rather advanced forms of a mental life developing 
 under the experience of sensation elaborate products of the syn- 
 thetic activity of mind. Moreover, the knowledge of things by per- 
 ception involves the development of mental life in the forms of 
 memory and judgment. But acts of memory and judgment are not 
 developments from perception; they are not merely modified forms 
 of sensations as recurring or combined under the action of physical 
 antecedents. All talk about the "image" of memory as though it 
 were merely a faint or faded-out impression of sense is quite un- 
 availing; it does not hit the real point of inquiry, and consequently 
 does nothing to explain the mystery. The vital element in memory, 
 that which makes it to be memory, is neither a sensation, nor a modi- 
 fied form of sensation, nor a development of sensation. It is the 
 subject Ego's recognition of its own past history as belonging to the 
 life of a mind. 
 
 The same statement is true of judgment. The relating activity 
 of mind, the power to bring two objects together in the unity of con- 
 sciousness, and, while keeping their ideas distinctly separate, to 
 bind them into one under the mental affirmation of their likeness or 
 unlikeness this is a new and startling mode of the activity of mind 
 as contrasted with merely being affected in sensation. Minimize 
 it as we may, we cannot look upon this activity as a mere " re- 
 sultant" of two sensations or images of sensations arising simul- 
 taneously in consciousness. We cannot consider judgment under the 
 principle of the conservation of energy. To treat it as such involves 
 the grossest misapplication of the laws which control the coinci- 
 dence or conflict of physical forces. Nor are the different forms of 
 the relating activity of the mind concept, judgment, deduction, 
 induction to be regarded, strictly speaking, as developments from 
 each other or from any one mental activity simpler than any of them. 
 They may all, indeed, be considered as modes of the relating ac- 
 tivity, because they involve discrimination, the discernment of like- 
 
666 GENERAL RELATIONS OF BODY AND MIND 
 
 nesses and unlikenesses. But each one of them involves somewhat 
 more than simple discrimination; each one involves other elements 
 peculiar to itself. That a sentient being should simply judge, or 
 affirm this or that, is not of itself a sufficient reason why it should 
 also make inferences by syllogistic processes or arrive at general 
 laws by induction. Indeed, the former may belong to many animals 
 which are incapable of the latter. 
 
 We might, properly and almost indefinitely, continue the fore- 
 going line of remarks into the consideration of the mind's most gen- 
 eral activities. Modern psychology, we have seen, is accustomed to 
 distinguish faculties of knowing, feeling, and willing as belonging 
 to the mind. But if we adopt this division, it becomes emphatically 
 true that no one of these three faculties can be regarded as devel- 
 oped from any other one, or from any two combined. That a being 
 feels that is, is affected with a state of consciousness more or less 
 pleasurable or painful, and having a characteristic quality is in 
 itself no ground for explanation of its knowing "Things" through 
 sense-perception and inference. Conversely, a being is conceivable 
 with the knowledge of an archangel, but without experience of de- 
 sire, emotion, or sentiment of attraction or repulsion. Such a being 
 would, indeed, have to attain its knowledge in other ways than those 
 open to us, and we find it difficult or impossible to imagine precisely 
 what such knowledge could be like. But growth in knowledge is 
 a different thing from the unfolding of mere feeling; and the former 
 cannot be explained as arising out of the latter. Acts of will are, 
 indeed, always actually dependent upon knowledge and feeling, 
 and cannot even be conceived of as taking place without this de- 
 pendence. But acts of will are not mere developments of those acts 
 of knowledge and feeling on which they undoubtedly depend. The 
 act of choice involves a new element, an element not to be neces- 
 sarily evolved from the other activities of mind. 
 
 37. The development of mind, therefore, cannot be explained 
 after the analogy of the accretion of molecules within a germ, and 
 the resulting division, multiplication, and advancing arrangement of 
 the living cells into separate organs of the entire system. No real 
 elements of the mind exist which can aggregate to themselves other 
 elements by absorbing them as pabulum, or can grow by arranging 
 the new material thus gained according to the energies inherent in 
 the material already organized. The life of consciousness is a never- 
 ceasing change of states. Yet the result of this change of states is 
 an orderly history, a true development. Such development is not 
 merely the expression of the evolution of the material basis of some 
 of these mental states. For it does not follow precisely the same 
 order or the same laws as govern the material evolution; and some 
 
THE CONCLUSION DRAWN 667 
 
 of its most important factors cannot be regarded as having any 
 physical correlate, or as evolved from other factors which have such 
 a correlate. The development of Mind can only be regarded as the 
 progressive manifestation in consciousness of the life of a real being 
 which, although taking its start and direction from the action of the 
 physical elements of the body, proceeds to unfold powers that are sui 
 generis, according to laws of its own. 
 
CHAPTER II 
 REALITY AND UNITY OF THE MIND 
 
 1. In all the discussions of the previous chapter it was implied 
 that we were dealing with two different existences separable at 
 least in thought, and apparently belonging to widely divergent species 
 of existence. And yet these two existences are in every case related 
 to each other in a peculiarly and even uniquely intimate way: they 
 are so correlated that each furnishes to the other the conditionating 
 antecedent, or cause, of its habitual behavior and characteristic de- 
 velopment. For example: I have, or rather am, a body and also a 
 mind; and you have, or rather are, another and different body and 
 also a mind. My body and my mind are bound together with these 
 peculiar ties of correlation ; and the same thing is true of your body 
 and your mind. The individuality of each of us as human beings 
 is the joint product of the nature and development of the two, in 
 this their naturally appointed form of being correlated. In some 
 such way as this, the popular conceptions relating to the subject 
 would have to be expressed; and as yet we have found nothing in 
 psychological science, as pursued from the physiological and experi- 
 mental points of view, to contradict or substantially to modify the 
 popular conception. 
 
 Now, only an extreme form of scholastic idealism (sometimes 
 called "subjective idealism" or "solipsism"), of which science can 
 take no account, ventures to deny the reality of the human body, 
 in the fullest meaning of that word which can apply to any material 
 entity. A piece of quartz rock and the gold extracted from it, or a 
 ton of steel, is not a bit more real, and truly a part of physical nature, 
 than is the most delicately organized human body, including the 
 wonderfully intricate and tenuous network of nerve-cells in the 
 brain attached to that body. Nor would any one think of denying 
 that the different parts of the body constitute a certain kind of 
 organic whole; or that the brain, considered by itself, shows mani- 
 fest signs of both an architectural and a functional unity. All this, 
 and much more of the same sort, enters into all the sciences of bi- 
 ology, anatomy, histology, and physiology, in their several ways of 
 dealing with the same material substance. All these assumptions, 
 
 668 
 
NATURE OF CLAIMS DISCUSSED 669 
 
 however, are a naive and uncritical but legitimate and scientifically 
 productive form of metaphysics. 
 
 2. When we come to deal with the claims of the subject of the 
 conscious mental life to a reality and a unity of its own, similar as- 
 sumptions are apt to be ruled out, on the ground that they are meta- 
 physical, and that science ought not to have anything to do with 
 metaphysics. But why this prejudice against claiming for the sub- 
 ject of the mental life and mental development a title to some sort 
 of reality, and to some sort of unity; when both these claims are 
 admitted without contest for the bodily organism with which this 
 mental life and mental development are correlated ? In answer to 
 this inquiry, four considerations may be alleged. One of the chief 
 reasons for such discrimination against the claims of the mind is 
 just that lack of the critical analysis of metaphysical conceptions, to 
 which reference has already been made. What, indeed, do we mean 
 by calling things "real," and by speaking of them as having a cer- 
 tain kind of "unity"? Until we have fixed upon some at least 
 provisional answer to this inquiry, how can we say whether any par- 
 ticular thing is entitled to be called real, or to be spoken of as one, 
 two, or any particular number? Another reason is to be found in 
 the peculiar characteristics which affirm the existence in reality of the 
 so-called mind; and which are relied upon to demonstrate its unity. 
 A third reason has operated to produce scepticism ending in a vir- 
 tually materialistic conception, in a yet more powerful way. This 
 is to be found in the vagueness and extravagance of those who, in 
 the supposed interests of morals and religion, have presented what 
 they were pleased to regard as a " spiritual " conception of the human 
 soul. And, finally, a reason is to be found, in many individual cases, 
 in a hardened and highly prejudiced attitude on the part of the ob- 
 server, against admitting anything that does not easily accord with 
 a purely mechanical and materialistic conception of nature and her 
 processes. In the opinion of such minds, nothing can be real that 
 cannot be perceived by the senses, that cannot be weighed and meas- 
 ured in the laboratory or in the field; and nothing can have a real 
 unity that does not admit of having its elements physically or chem- 
 ically combined and separated for future combination with other 
 elements. Each one of these four considerations will now be ex- 
 amined in the order just given. 
 
 3. As to its conceptions of "reality" and "unity," modern psy- 
 chology may properly claim to be following the lead of the chemico- 
 physical sciences. These conceptions are now, more than formerly, 
 of the "dynamical" and less of the statical order. No physical 
 reality not even the ancient atom is to be fitly conceived of as 
 an unchanging mass or unalterable tiny bit of material substance. 
 
670 REALITY AND UNITY OF THE MIND 
 
 To have its claim entered in the world of real beings, every indi- 
 vidual thing must prove this claim by continually doing something 
 and by having something done to it. What kind of a reality any 
 particular thing is to be proved to be, depends upon what it is able 
 to do to other things and to have done to it by other things. In a 
 word, it is action which demonstrates reality both that it is, and what 
 it is. 
 
 In somewhat similar manner, what gives unity to any particular 
 thing is not the fact that it is a solid and indissoluble mass, or an un- 
 analyzable, tiny bit of substance; but its unity is gained for it by the 
 co-operation of its various elements, in their action and in their de- 
 velopment. Or, to say the same thing in other words: Every re- 
 ality becomes "one" through the continuous action and reaction 
 of its elements in definite ways upon one another and upon all the 
 other things which constitute its particular environment. This is true 
 even of so loosely constituted a unity as a heap of sand; the differ- 
 ent grains form one heap, only so long as they are held together in 
 one place by the forces of gravity and cohesion. But there are differ- 
 ent kinds of unity, and different degrees of being united, correspond- 
 ing to these different kinds. In general, the more complex and un- 
 stable the unity, the higher it is in kind. According to modern 
 physics, as respects its elements there is no more rapidly and subtly 
 alterable unity than is the atom; and yet the atom, as the very word 
 signifies, was formerly regarded as the most stable and unalterable 
 of all conceivable unities. In the case of all living forms, the unity 
 which they possess is the oneness that is attained by a process of de- 
 velopment. And here, in general, the original elements are com- 
 paratively few in number and simple in arrangement; but they be- 
 come more and more numerous and highly complex in arrangement, 
 as this process goes forward. Indeed, by certain theories it would 
 seem to be held, that to give a simple descriptive history of this pro- 
 gressive complication of elements is a sufficient explanation of the 
 process itself. Such theories would think to account for the devel- 
 opment by describing it as an increase in "aggregation," "differ- 
 entiation," " integration," etc. ; and would regard the unity resulting 
 from the process as having no more reality than that which belongs 
 to the process; and this is a never-ceasing, but ever restless, activity 
 by way of action and reaction. 
 
 4. If now we insist upon having some further and more satis- 
 factory explanation of a unity which is achieved by a process of 
 development that combines, even into a temporary oneness, a num- 
 ber of elements through the co-operation of a number of kinds of 
 energy, we are obliged to introduce the conception of "plan." No 
 unity of any kind or degree of complexity can be realized without 
 
UNITY AS INVOLVING PLAN 671 
 
 giving evidence of some "indwelling" or "overruling" idea. In 
 the case of such unitary beings as appear to our rninds ready-made, 
 as it were, the plan is thought of as already realized; while in the case 
 of such beings as are found to be still in a process of development, 
 the plan is thought of as in the way of being realized. Whether 
 this manner of explaining the unitary nature of any physical thing 
 is only a sign of the sort of necessity under which the human mind 
 finds itself, and gives us no clue to the Nature of Reality, or whether 
 it is also a trustworthy revelation of the essential character of the 
 Being of the World, is a metaphysical inquiry which need not con- 
 cern us at the present time. 
 
 It would seem, then, that if the phenomena of man's mental life 
 give unmistakable signs of forms of energy, which act upon other 
 real beings and are acted upon by them, and which arrange them- 
 selves according to some plan, then it is certainly permissible to 
 claim reality and unity for the Subject of these phenomena i. e., 
 for the so-called soul or mind. At once, however, we are met by 
 objections based upon the peculiar nature of these phenomena; and 
 especially upon the well-known fact of their dependence even to 
 the point of not appearing at all, or of total disappearance on the 
 constitution and functioning of the bodily organism. 
 
 Now a certain very intimate, and in some respects seemingly ab- 
 solute, dependence of all mental phenomena upon the integrity, con- 
 dition as respects the character of its blood-supply, habitual ways 
 of reacting, established "dynamical associations," "preoccupa- 
 tions," and concurrent sensory and motor nerve-commotions, etc., 
 etc., of the brain, must be admitted. How intimate and far-reach- 
 ing this dependence actually is, has occupied with its statement more 
 than one-half of this entire treatise. It has been shown that the 
 beginnings, the varying phases of intensity, the rise and fall below 
 the so-called " threshold," and even the complete temporary cessa- 
 tion of the conscious states, are involved in this dependence. Here 
 it is not necessary to repeat, however, what has been sufficiently 
 insisted upon in the last chapter, and in numerous other places, 
 namely, that the phenomena which are ascribed to the nervous mech- 
 anism are also dependent for their existence and their character 
 upon the reactions of the conscious mental life. But it is especially 
 necessary at this point to remind ourselves that no kind of physical 
 reality succeeds in escaping a similar condition of dependence. All 
 the phenomena which material substances exhibit as signs of their 
 reality are liable to vary in degree, in characteristic quality, and even 
 to disappear and reappear, or cease entirely, in dependence upon 
 internal changes and upon their relations to other material sub- 
 stances. The higher in the scale of complexity, expecially as a re- 
 
672 REALITY AND UNITY OF THE MIND 
 
 suit of biological development, the living unity has come to be, the 
 more significant, and yet, generally, the more unstable, is its unity 
 apt to become. 
 
 5. If now in the light of even so little criticism of the metaphysi- 
 cal conceptions involved, we examine anew the claims of the mind to 
 reality and unity, we shall find it impossible not to accept them in a 
 certain meaning of the words employed. In some meaning of these 
 words, the mind undoubtedly has an actual existence as a unitary 
 being; it is real and it is one. It has at least what Kant called a 
 "phenomenal" reality; and it develops in time a "phenomenal" 
 unity. To establish so much of a claim we may appeal with con- 
 fidence to every one's indisputable experience. Both the reality 
 and the unity of this sort are, indeed, a matter of degrees and of de- 
 velopment. But why should this fact diminish the trustworthiness 
 or the value of the experience on which the claim is based ? Indeed, 
 this kind of reality and unity is involved in the very conception of a 
 development. As a bare conception not to speak of any corre- 
 sponding facts in reality no development can exclude the neces- 
 sity of a process which is itself characterized by a certain kind of 
 unity, and which has its result in a series of existences possessing 
 a unitary character. All this is conspicuously true of that organism 
 with which the development of the mental life is especially corre- 
 lated. The human nervous mechanism, as was shown in detail 
 in the opening chapters of the book, has resulted from a process 
 which, in the case of each individual man, has followed a somewhat 
 peculiar plan; and at every stage in this complex process, from the 
 beginning of the faintest discernible traces of nerve-fibres and nerve- 
 cells to the maturing of the adult brain, spinal cord, peripheral 
 ganglia, end-organs of sense, etc., etc., this mechanism has exhibited 
 all the signs of a "unitary being." Indeed, were this not true, it 
 could not properly be called either a "system," or a "mechanism/* 
 
 6. But to examine further the claims of the subject of the phe- 
 nomena of the mental life, the so-called mind, to the titles of reality 
 and unity: It was just now said that this claim can scarcely be com- 
 bated, if we qualify the nouns by the adjective "phenomenal." 
 By this it was meant that the phenomena present themselves to our 
 inspection, whether we study them by the method of self-conscious- 
 ness solely or also and chiefly by the methods of physiological and 
 experimental psychology, as matter-of-fact experiences of a uni- 
 tary character. Many ill-advised attempts have recently been made 
 to destroy or to minimize what used to be called " the authority of 
 consciousness," and even to prove that no such experience as self- 
 consciousness, properly so called, can possibly be had. This, to- 
 gether with the loose use of such terms as "unconscious" and "sub- 
 
THE ACTIVITY OF APPERCEPTION 673 
 
 conscious" in application to the phenomena of mental life, has been 
 the source of much obscuration of the entire problem. And when 
 we add to this result the effect of the loose employment of such terms 
 from the mathematics of physics as "double," "triple," etc., to 
 human selfhood or personality, we have a state of affairs in which the 
 plainest testimony of the most indubitable experiences is tolerably 
 sure to be either sophisticated or totally overlooked. Surely, the 
 consistent, tenable, and hopeful course, upon which to enter for the 
 scientific definition of the existence and the nature of any thing, 
 physical or otherwise, is not through the door of a denial of the 
 actual facts of our experience with that particular thing. 
 
 7. Now in the special case of man's mental life, its very es- 
 sence consists in conscious activity, culminating in the achievement 
 of perceptive consciousness (sometimes called "apperception"), 
 or the knowledge of things, on the one hand; and in self-conscious- 
 ness or the knowledge of Self, on the other hand. In this very 
 process of development the mind not only constructs and affirms its 
 own unitary being, but also constructs all the unities which it affirms 
 of other selves and of material things. But when we speak of this 
 reality and this unity as an achievement, we in no wise invalidate or 
 diminish the truthfulness or the value of the claim. In fact nothing 
 that is alive, especially no product of an elaborate process of devel- 
 opment and probably no inorganic physical reality even has 
 come into existence "ready made," as it were. On the contrary, 
 each unity in nature has become one, through a process of self- 
 making. Looked at from the purely objective and scientific point 
 of view, Nature is constantly making and unmaking all those prod- 
 ucts, which appear to our minds to have a certain temporary unity, 
 but which are in fact never wholly delivered from the necessity 
 of ceaseless change. The rather must we say that the reality of 
 their being at all consists in this actual process of change; and that 
 their individual natures are defined to us by the character of the 
 changes which take place in them and which they contribute to 
 produce in other natural objects. But subjectively regarded, as the 
 theory of perception already advanced has sufficiently made clear, 
 all these same natural products are made to be one to us, through 
 no process of merely copying-off, but by the constructive activity of 
 our own minds. 
 
 8. Now one essential thing about the activity of human con- 
 sciousness, as the truth has all along been implied, is to affirm the 
 actuality of the existence of the Subject of all the conscious states. 
 In a word, the reality, or actuality for the time being of this Subject 
 of all the mental life is an indispensable implicate of all the phenomena 
 of the same mental life. The mind must do duty here; and there is 
 
674 REALITY AND UNITY OF THE MIND 
 
 nothing else which can by any possibility be supposed to take its 
 place. Let the case be tried by making a beginning with that sort 
 of testimony with which every one is most familiar. I know that 
 I think, feel, will; that is to say, phenomena take place in my con- 
 sciousness which there is no conceivable way of describing except 
 by attributing them to the subject of all my consciousness to the 
 self-conscious "me" called mind. But because I cannot perceive 
 this subject of all consciousness as an extended and external some- 
 what a "Thing" so large, and shaped and colored in just such a 
 manner, with a definitely hard or soft feel that is to say, because 
 I do not appear to myself in consciousness to be just such a kind of 
 being as are some of the objects of my perception, I begin to raise 
 the question whether this subject (the "I" that thinks, etc.) has any 
 real being at all. May it not in fact be, I ask myself, that some 
 " thing," or collection of things, like those which I have often seen 
 and felt, is the subject to which the thoughts and feelings and acts 
 of will that I have called "mine" should be attributed? Of course, 
 if this question is to be answered in the light of modern physiology 
 with even a provisional affirmative, the particular " thing,"to which 
 such activities as those I am conscious of are to be attributed, is my 
 brain. Nothing, surely, but my brain can think, and feel, and will 
 so to speak for me. 
 
 But the inquiry may now be raised: How do we know that there 
 is actually a brain, which may serve as the real substratum of the 
 phenomena of consciousness? In view of the science of body and 
 mind, it is as fair to ask this question in a sceptical way, as to ask 
 the corresponding question about the mind. It scarcely need be 
 said that no one has any evidence presented directly to the senses 
 that such organ exists within his own cranial cavity. To be con- 
 scious, and at the same time to observe the substratum of one's 
 consciousness, is an unattainable opportunity. 1 It may even be 
 that the particular ego (the " I " of consciousness) which is engaged 
 in the search for its own real being in a material substratum has 
 never seen so much as a single human brain. Since there is such 
 scarcity of direct ocular and tangible demonstration of a special re- 
 lation between the brain and mental phenomena, it is plain that the 
 testimony of experts must be summoned. 
 
 On the other hand, it must be confessed that no expert has any 
 more direct evidence than every self-conscious ego has of the ex- 
 
 1 After all, perhaps this opportunity is not forever in all respects " unattain- 
 able," for the most recent experiences with cerebral surgery without anaesthetics, 
 leave it possible that an arrangement of mirrors might give one the rare sight 
 of seeing his own brain exposed and operated upon to an hitherto inconceivable 
 extent. 
 
THE ACTIVITY OF APPERCEPTION 675 
 
 istence of a real material structure called brain, which may account, 
 by its presence and activities, for his own mental phenomena. Nor 
 can he offer any evidence peculiar to himself for his belief that the 
 particular ego which he calls "himself" is connected with his brain. 
 And how many soever other brains he may have seen, he only knows 
 by a series of very indirect and complicated inferences that any 
 individual whose brain he has not seen really possesses one. But 
 whence these inferences ? and, What are the grounds on which the 
 confidence attached to them is based ? To these questions only 
 one answer is possible. The inferences themselves are acts of 
 knowledge, modes of consciousness, phenomena of mind. The 
 only possible grounds of confidence in them, as valid inferences, 
 must be referred back to our inherent faith in the power of the 
 mind rightly to infer, from its own phenomena, the real existence 
 of beings the phenomena of which it has never perceived. More- 
 over, if the mind had perceived the phenomena of its own brain, 
 there could be nothing in the phenomena themselves to account for 
 the power to make inferences which belong to it as mind. On the 
 ground, then, of an inferred reality called the brain, I am asked 
 to dispense with my confidence in the reality of the being which 
 makes the inference, and which, at the same time, makes a much 
 more irresistible inference as to its own reality as an active infer- 
 ring force. 
 
 9. The case is, however, by no means so favorable as the state- 
 ment just made would imply, for that phase of scientific material- 
 ism which refers the phenomena of consciousness to the brain as 
 their sole cause. For it is not in the brain, as a mere mass of mat- 
 ter whose structure and mechanical functions can be made obvious 
 to any intelligent observer, that the real substratum of mental phe- 
 nomena must be sought. Considered as such a mass, this organ 
 is no better than any other similar soft and pulp-like bulk. It is 
 the wonderful molecular constitution, atomic play, and changing 
 dynamic relations of the invisible particles of this mass, which are 
 responsible for its unique functions. But the very existence of the 
 atoms as real beings, capable of acting on each other and of being 
 acted on how shall this remote and obscure fact be ascertained? 
 and how shall we learn what is the nature of these beings, so as to 
 determine whether or not they are capable of performing the stu- 
 pendous task of bringing forth the various mental phenomena ? 
 
 In attempting to answer the last two questions we are in great 
 danger of losing completely all that we have taken most pains to 
 gain. It is to the all-powerful "atoms," with their potent forces, 
 that we are now looking as the real subjects at once of the molec- 
 ular changes in the brain-mass and of the phenomena of conscious- 
 
676 REALITY AND UNITY OF THE MIND 
 
 ness. From these real beings and their relations there must be 
 derived, not only the activities which physiology ascribes to nervous 
 matter, but also those which psychology is constrained to ascribe 
 to conscious mind. And yet, how do we know that any real beings 
 whatever called atoms exist ? Certainly not by direct evidence of any 
 of the senses. Not even the most pronounced advocate of the reality 
 of physical things, and of the unreality of mind, would venture to 
 affirm that he has seen or touched an atom, or can demonstrate its 
 existence and nature to ordinary observation through the human 
 senses. Atoms are supersensible beings. Moreover, they are hypo- 
 thetical existences, or beings whose existence is inferred in an ex- 
 tremely roundabout way in order that we may be able to give to 
 ourselves a rational account of the grounds on which certain classes 
 of phenomena rest. 
 
 10. Moreover, the best efforts of modern investigation to de- 
 scribe the nature of the atom appear, not only incomplete, but also, 
 in certain particulars, self-contradictory. It is certain that the 
 atom cannot be regarded as an independent reality. What it is can 
 only be described by telling what it does; but in telling what it does 
 we always find ourselves implying certain relations to other atoms. 
 That is to say, we know nothing about the nature of any of the 
 atoms which does not involve also complicated hypotheses concern- 
 ing its mode of behavior as caused by the presence and mode of 
 behavior of other hypothetical beings. In this way the reality of 
 the atoms is made ultimately to depend on the reality of some law, 
 or ideal force, that binds different atoms together, as it were, and 
 makes them work to a unity of plan. But here, again, we are re- 
 minded that we can form no conception of a " plan " which is not a 
 phenomenon of mind, and no conception of a "unity" that does not 
 depend upon the unifying actus of the mind. Moreover, all ideas 
 of "relation" are dependent upon mental activities that are quite 
 without physical analogy. All "Things" are made into the units 
 which they appear to be by the unifying action of the mind. Such 
 action is implied in the most elementary and naive perceiving of 
 things; for the study of perception, from the physiological point of 
 view even, has enabled us to show that no so-called "thing" is a 
 ready-made material product, apprehended by mind in a form which 
 is a copy of some extra-mental existence. 
 
 11. Accordingly, the whole course of argument and the whole 
 weight of conviction appear to be the reverse of what is assumed 
 by the objector to the reality of mind. The material molecules of 
 the brain are not beings about the reality and exact nature of which 
 we have the most indubitable evidence evidence so indubitable 
 that we may venture to press it into the contradiction of the more 
 
DISTINCTION OF EGO AND NON-EGO 677 
 
 immediate data of consciousness. If these elements of all physical 
 being are real, they come to us as inferences and hypotheses; they 
 involve a vast amount of conjecture, indirect inference, and unsolved 
 difficulties, or even contradictions. And if we ask, On what au- 
 thority are these inferences made? Whence comes the demand for 
 any rational explanation whatever? Where do the conjecture, hy- 
 pothesis, and sense of difficulty and seeming contradiction exist? 
 then the only answer to be given to all these questions refers them 
 to the Mind. What atoms and forces and laws can be, or mean, 
 without the being and activity of self-conscious mind, is even harder 
 to conjecture than what a color can be which is not seen, a sound 
 which is not heard, an odor that is not smelled. 
 
 Shall this, then, be the last word of science on the subject? 
 namely, that the one being in whose active energizing all conceptions 
 of all real being arise, feels justified in denying its own reality in the 
 supposed favor of certain of its most remote and doubtful conceptions. 
 
 12. What is meant by affirming the reality of mind may now 
 be made somewhat clearer by pursuing the following train of re- 
 flections: In the development of the mental life its phenomena come 
 inevitably to divide themselves into two great classes. As it ap- 
 pears to adult experience, not only the unfolding, but even the very 
 existence of self-consciousness seems to involve the distinction be- 
 tween the ego and the non-ego between the "I" with its states, 
 and the "Things" which it knows with their manifold properties 
 or attributes. Each of these two classes of phenomena the so- 
 called subjective and the so-called objective is inevitably attributed 
 in consciousness to a different subject; the one to the "I" as its own 
 states, the other to somewhat left undefined, except that it is not 
 the "I," and is called "matter," "material substance," etc. (the un- 
 known X which is not I). It is only as involving all this mental 
 process that any real being whatever, whether Mind or Thing, is 
 known or believed to exist; but the mind in the development of ex- 
 perience inevitably completes the process, which involves the as- 
 sumption that real beings do exist, and that all these real beings are 
 either " things," such as I know, or myself and other conscious be- 
 ings, such as I am. 
 
 13. Peculiar and cogent reasons may be given, however, which 
 further enforce and verify the assumption of a real existence for 
 the Mind. Repeated attention has been called to the fact that there 
 is a class of so-called mental faculties, most important and distinc- 
 tive, for the distinguishing characteristic of which no physical anal- 
 ogies or correspondences whatever can be discovered or imagined. 
 This is true of memory as active reminiscence, of the unity of con- 
 sciousness, of voluntary attention, and of the relating activity. The 
 
678 REALITY AND UNITY OF THE MIND 
 
 existence of these modes of mental behavior requires the assump- 
 tion of a characteristic real being, other than the molecules of the 
 brain, to which they may be referred. Some of these modes of be- 
 havior are conspicuously unintelligible and meaningless without 
 granting such an assumption. For example, an act of recollection 
 involves the presence in consciousness of a state the very essence of 
 which is that it claims to represent (or stand for) an absent past 
 state of consciousness. No way of verifying this claim in any par- 
 ticular instance, which does not involve its acceptance in general, 
 can possibly be devised. For all argument is valid, only if we accept 
 the validity of some memory. But every present act of memory 
 on my part is a process in my consciousness, and that which it 
 claims to represent was also a process in my consciousness. To 
 recollect the past state of another consciousness than my own in- 
 volves an absurdity; to recollect a past state otherwise than as rep- 
 resented in a present state of my own consciousness also involves 
 an absurdity. Of course, such reflection upon the nature of the 
 act of memory affords no demonstration of the claim that the sub- 
 ject of the present state is one and the same real being with the 
 subject of the past state. On the contrary, all demonstration itself 
 rests on this assumption; for without accepting it as valid we could 
 not reach the conclusion of any demonstration. The premises of 
 every syllogism are connected with one another and with their 
 conclusion in a living unity of thought, only on the assumption that 
 one real being is the subject of each of the thoughts which consti- 
 tute the syllogism. To "be really," and to be the one subject of 
 changing states, are, therefore, but different ways of expressing the 
 same truth. In this meaning of the words: the soul exists in reality, 
 above all other kinds of being; because it alone, so far as we know 
 on good evidence, knows itself as the subject of its own states; or, 
 indeed, knows the states of which it is the subject as states belong- 
 ing to itself. 
 
 14. A similar train of argument leads us at once and irresisti- 
 bly to an affirmation of the at least "phenomenal" unity of the de- 
 veloped human mind. In every act of self-consciousness its unity 
 is achieved, as it were, by the very form of its own activity. Every 
 act of memory is a further affirmation of the fact of some sort of 
 unity between the Ego of the present and the Ego of the past. Mem- 
 ory, of the " recognitive " type, recognizes the oneness of the Sub- 
 ject of all the conscious states. All human business and social 
 intercourse, all attribution of conduct and the moral ideas and senti- 
 ments of responsibility, all assignment and distribution of property 
 rights, depend upon the significance and the value of the mind's 
 unity. Even the much overworked and misunderstood phenomena 
 
MIND AS UNIFYING ACTUS 679 
 
 of "double consciousness" confirm rather than confute the same 
 view. Where the phenomena are not proofs of the unsuspected versa- 
 tility, wealth of ideas and resources, and unexplored capacities, 
 of the one mind, they are proofs of a mind disordered, disorganized, 
 or even degenerate. The very conception of the mind at all is de- 
 pendent upon this self-unifying actus of man's conscious mental 
 life. In a word, the experience upon which the conception of oneness 
 is applied to the human mind is that of our actual unification, ac- 
 complished and recognized by the activity of the very subject, whose 
 otherwise disparate states of existence, and phases of development, are, 
 in fact, united. A unifying of energies, that takes place more and 
 more perfectly as the development of mind proceeds toward matu- 
 rity and ascertain characteristic type of the development of each indi- 
 vidual mind all this affords the indisputable basis for affirming 
 the reality of the mind's unity. 
 
 15. Of course, most of the terms applied to the unity of con- 
 sciousness, when carefully examined, turn out to be figurative, and 
 to have no meaning except as interpreted over from entities and re- 
 lations of a material sort into terms of consciousness. By the 
 "unity of consciousness" it cannot be meant that consciousness is 
 some kind of an entity which remains one and unchangeable through- 
 out, like those atoms which physical science formerly supposed to 
 constitute the whole world of material reality. It will be found, 
 however, that no conception can be formed of the unity which is 
 supposed to belong to the atom without involving in it the unity of 
 consciousness. We can, indeed, picture to ourselves a very little 
 bit of extended matter, barely visible under the highest powers of 
 the microscope, which never changes its shape or color, etc., and 
 which always behaves itself in exactly the same way under precisely 
 similar circumstances. But this mental picture would itself have 
 any unity belonging to it only as it existed in the unity of conscious- 
 ness. It is this unifying mental process which makes each "Thing" 
 to be one thing; it is this unification which imparts to all else that 
 is one whatever unity it may have. 
 
 When, then, we speak of the unity of consciousness we mean, 
 first of all and chiefly, to call attention to the following primary 
 fact of experience: All states of consciousness involve a reference 
 of the state to an "I," as the subject of the state; and, in spite of 
 the constant change of states which goes on, so that in reality the 
 same state never recurs, and even the same thing is never twice 
 known, all the states are somehow understood to be states of one 
 and the same subject. This reference and this understanding enter 
 into all our experience; they give conditions to experience and make 
 it possible. Whatever changes experience may be conceived of as 
 
680 REALITY AND UNITY OF THE MIND 
 
 undergoing, they, as conditions of all possible experience, must be 
 conceived of as remaining. To ask us to try to imagine a mental 
 state or act not involving this reference and this understanding, 
 with respect to the unit-subject of consciousness, is to ask us to try 
 to be conscious and unconscious at the same time. The "I" may 
 become unconscious; that is, the phenomena of consciousness in 
 that connected development which characterizes the individual may 
 cease to exist. But phenomena of self-consciousness cannot be con- 
 ceived of without implying the actual and accomplished unifying 
 of the self-conscious Subject the Ego, the Mind. 
 
 16. This kind of reality, and this kind of unity (call it "phe- 
 nomenal," if you will), for the mind and for all things, is all that 
 science can recognize, and all that it can prove. But this is a kind 
 of reality, and a kind of unity, which science cannot refuse to 
 recognize; or of which it cannot fairly deny the sufficient proof. 
 Metaphysics, presuming upon its intimate relations to the "old 
 psychology," has doubtless often made an unwarrantable use of the 
 facts above mentioned. It has often declared that we have an im- 
 mediate and indubitable knowledge of the mind as one and the 
 same real being in all acts of consciousness. The facts have been 
 interpreted as though the case stood as follows: I have the power 
 to loot within myself, and by thus looking I can discern what I 
 really am. I immediately know (that is, know by the introspective 
 act of self-consciousness) that "I" am always, however my states 
 may change, one and the same real being. I am a real, self-identi- 
 cal entity; and if asked how I know that I am all this, my appeal 
 is to the indubitable evidence of the act of self-consciousness. 
 
 The foregoing metaphysical statement of the case is by no means 
 obviously correct; we believe it, on the contrary, to be exaggerated 
 and incorrect. In thus overstating the case, there is liability that 
 the case itself will be lost. Consciousness carries with it no immediate 
 knowledge of any real and self-identical being not even of that real 
 being which we call Mind and, with good reason, assume to exist 
 as the ground or permanent subject of mental phenomena. Meta- 
 physics is the science which treats of those assumptions that under- 
 lie all of our experience with what we call "reality." But it treats 
 of assumptions or beliefs such as we find do actually and inevita- 
 bly enter into all our experience. The real existence of " Things," 
 whether of the masses of matter we daily test by the senses, or of 
 those hypothetical beings called atoms which physical science re- 
 quires in order to account for the phenomena, depends upon such 
 assumptions. If it be admitted that we cannot be immediately 
 conscious of ourselves as real unit-beings, we are no worse off than 
 we are with respect to our belief in the existence of any of the so- 
 
THE SPIRITUALITY OF MIND 681 
 
 called real beings of which all men suppose the world to be com- 
 posed. 
 
 It can also be shown that the case of the mind or soul, with re- 
 spect to its unity as a real being, is made no better by admitting 
 that an immediate consciousness of ourselves as such unit-beings is 
 possible. For let it be supposed that by concentrating all my at- 
 tention upon the present state of consciousness I most clearly and 
 indisputably discern myself as one real being, forming the ground 
 of that state. Let it be supposed that every half-hour in the day I 
 repeat this mental act. It would still have to be assumed, as some- 
 what altogether out of consciousness, that the real being discerned 
 in any one of these acts of introspection is one and the same real 
 being as that discerned in all the rest. A real unit-being that 
 should last only while the difficult act of concentrated introspec- 
 tion was taking place would be of no value to serve as a self-con- 
 scious mind. In fact, such a unit-subject of the individual state 
 would have no claim to be considered as a real being at all. 
 
 17. The question whether the mind is to be spoken of as non- 
 material or " spiritual " scarcely merits at the hands of psychological 
 science the grave and lengthy discussion to which it has often been 
 carried. Materiality, as predicated of any real being, is only a com- 
 plex term including a number of so-called attributes, which are all 
 the subjects of experience only as belonging to individual things. 
 All real things are to be called material which have these attributes, 
 so called. Primarily, as has been frequently shown already, the 
 attributes are simply modes of the affection of the mind which we 
 have learned to localize and objectify as belonging to extra-mental 
 reality. But if we raise the question whether the Mind, too, is 
 known to itself as having those attributes which make up our com- 
 plex, general notion of "materiality," no one would find it easy to 
 think of giving this question an affirmative answer. The mind at- 
 tributes to " things " the qualities of extension, impenetrability, and 
 all the various subordinate modifications of these qualities. It per- 
 ceives these things as colored, cold, hot, rough, smooth, etc. But 
 it does not attribute such qualities to itself; it can find nothing in 
 the modes in which it manifests itself to itself which would warrant 
 the application of similar terms to these modes of its own behavior. 
 
 Indeed, all the terms which do apply to the recognized qualities 
 of mind have to be understood as figurative when, having been 
 borrowed from physical relations, they are made to apply to psychi- 
 cal states. Even in those cases where the analogy seems almost 
 to amount to an identity, closer inspection shows that this seem- 
 ing does not correspond to the actual fact. For example, we do 
 attribute quantity to sensations and feelings. But when the suffer- 
 
682 REALITY AND UNITY OF THE MIND 
 
 ing from pressure becomes more intense, we do not regard the 
 mind as actually passing, like some material thing, under a heav- 
 ier load (sub-jero), against which it must either bear up or break, 
 through the physical strain. Just so, movements of the mind are 
 not to be defined as changes of its position with relation to other 
 things in space. We are, then, surely warranted in affirming that, 
 so far as the mind has any immediate information as to what quali- 
 ties should be assigned to itself and what to "things" which it 
 always looks upon as not-itself it is compelled to regard itself as 
 non-material. 
 
 As has already been pointed out, there is no way of telling what 
 is the real nature of any existence except by enumerating its quali- 
 ties, or those modes of behavior which we attribute to it on account 
 of its affecting our consciousness in certain definite ways. To at- 
 tempt to regard the mind as material, when it manifests itself co 
 itself as non-material, compels us either to use the word " material " 
 in an unwonted and unauthorized way, or else to attribute to matter 
 in general certain occult powers which it never manifests itself to 
 the mind as possessing, and which make it really to be quite differ- 
 ent from what its manifestation of itself would indicate. 
 
 The only way of maintaining the materiality of mind would then 
 appear to be that of denying its real existence at all, and of attrib- 
 uting its phenomena to the material molecules of the brain as their 
 real and material substratum or basis. But the untenable nature 
 of this view has already been sufficiently indicated. Or perhaps a 
 strong temptation may be again felt, at this point, to recur to the 
 hypothesis of a third somewhat, a " two-faced unity," which is the 
 ground of the phenomena of both body and mind. But such hypoth- 
 esis can throw no light whatever on the inquiry whether the mind 
 is material or non-material. The phenomena we call "mental," 
 and attribute to the subject of consciousness, would remain just as 
 radically unlike those which we call "physical," and attribute to 
 matter, after making the hypothesis as before. And to the hypoth- 
 esis itself the same objections would remain opposed. 
 
 The negative conclusion that mind is non-material is quite in- 
 evitable for every one who admits that mind is a real being with 
 any nature whatever. 
 
 18. It is not difficult, also, to show that we must make the cor- 
 responding positive statement, and affirm the spirituality of mind. 
 This we can do with confidence, however, only so long as we mean 
 by the term " spirituality" simply to sum up and express in one word 
 the list of attributes which describe the known activities of mind. 
 To perceive, feel, think, will in brief, to be conscious in some one 
 of the various forms of conscious life this is to be positively spirit- 
 
MIND AS A UNITY OF GROWTH 683 
 
 ual, in the only sense in which science is entitled to affirm spiritual- 
 ity of mind as such. As soon as we conceive of spirituality as some 
 ethereal extension of thinking substance, we enter upon the vain 
 effort to conceive of mind under terms of matter, and at the same 
 time escape the consequences of so conceiving of it. Nor can we 
 hope to vindicate for the mind such spirituality as would be implied 
 in its being freed from all relations to material things, or from de- 
 pendence for the modes of its being upon the material substratum 
 of the brain. How spirit, in the sense of disembodied or unem- 
 bodied mind, would perceive, and feel, and think, and will, is a ques- 
 tion toward the answer to which we can make no beginning. But 
 to control the mental train as distinguished from being a passive 
 member of a psycho-physical mechanism, to reason so as to deduce 
 conclusions and make inductions to general laws, to recognize the 
 call of duty, and to call up and classify in the consciousness the 
 lofty and complex ideas of art and religion these and other similar 
 operations of the mind pre-eminently emphasize its spirituality. 
 
 19. In somewhat the same way must it be admitted that the 
 question of the unity of mind has given rise to much fruitless and 
 by no means altogether pertinent debate. The attempt to conceive 
 of the mind as a unit-being, constituted after the analogy of those 
 physical structures which we are accustomed to regard as unities, 
 inevitably leads to confusion and error. The important psycho- 
 logical fact is, that there is no one of these physical unities which 
 does not derive its unity from the unifying actus of the mind. This 
 statement is true of each such so-called unity, whether it is per- 
 ceived as one or is conceived of as one. The unity which belongs 
 to the percept finds its source in the synthetic activity of the per- 
 ceiving mind; the unity of the conception, in the unifying activity 
 of the mind's relating faculty. It is sometimes supposed, however, 
 that an atom which should have no parts, be perfectly homogeneous 
 throughout, and so incapable of changes of its interior states, would 
 be the highest possible type of a unity of real being. Nothing 
 could ever happen to disturb or destroy such a unity. The tempta- 
 tion has therefore arisen to conceive of the mind after the analogy 
 of a thinking atom. 
 
 20. Now, it must be admitted that such a thinking atom would 
 be in far less danger of suffering from the death of the physical basis 
 of its thought than is the thinking man. But two considerations of 
 great importance are likely to be overlooked in the mere making of 
 the hypothesis of such an atom. Surely such an atom could hardly 
 have any experience corresponding to what we call the unity of 
 our consciousness; and if it had any unity of consciousness what- 
 ever, such unity could no more be explained as arising out of, or 
 
684 REALITY AND UNITY OF THE MIND 
 
 conditioned upon, the simplicity of the physical being of the atom 
 than the unity of our consciousness can be explained as arising out 
 of, or conditioned upon, the complexity of our physical being. 
 
 It is impossible to see how a unity of consciousness at all resem- 
 bling what we understand by the term could find an adequate ma- 
 terial substratum in a single rigid atom. In other words, if a spirit- 
 ual being having a unity of consciousness were brought into special 
 psycho-physical relations with a material being incapable of any 
 interior changes, because possessed of no parts to undergo change, 
 these relations would have to be totally different from any which 
 we can conceive of as holding between the body and mind of man. 
 For the very nature of the mind's unity is dependent upon that 
 variety of experiences which is occasioned in the mind through the 
 changing states of the brain. The physical basis of the human 
 mind is undoubtedly an extremely complex system of interacting 
 molecules. Certain relations can be traced between the character 
 of these physical interactions and the character of the states arising 
 in the mind. These states depend for their character, and even for 
 their very existence, upon the occurrence of the corresponding 
 material changes. A brain that is not in a ceaseless change of ac- 
 tivities of the peculiar sort called "neural" is a dead brain, so far 
 as its influence on the mind is concerned; such a brain could not 
 serve as the substratum or physical cause of mental phenomena. 
 
 Moreover, comparative anatomy shows us that the greater the 
 number of molecules, and the larger the variety and the size of the 
 organs specially related to the mental processes, the richer in vari- 
 ety and nobler in quality the mental processes themselves become. 
 So far as we can ascertain, the highest unity of consciousness be- 
 longs in connection with the greatest complexity of the material 
 substratum. The animals which have the largest cerebral develop- 
 ment appear to have, too, not only the most manifold and extensive 
 mental life, but also, in the highest degree, the capacity for attrib- 
 uting the phenomena of that life to one subject. Those psychical 
 activities which are connected with the physical interaction of the 
 greatest number of material elements are the most numerous and 
 significant; and they are, also, actually most perfectly harmonized 
 into a higher unity of spiritual self-conscious being. 
 
 No information derived from the study of Physiological Psy- 
 chology warrants us in affirming that a highly developed self-con- 
 scious existence must, from the universal necessities of the case, be 
 united with a vastly complex material structure like the human 
 brain. Such study does, however, compel us to affirm that such 
 a unity in variety as is the human mind cannot be conceived of 
 in dependence upon the movements in space of a single perfectly 
 
MIND AS A UNITY OF GROWTH 685 
 
 rigid and unchanging atom. The development of human experi- 
 ence is conditioned upon the arising in consciousness of many 
 sensations of varied quantities, qualities, and orders in time; upon 
 the synthesis of these sensations into presentations of sense; and 
 upon the recall of the presentations in the form of representative 
 ideas. What experience would be, if its basis were not laid in 
 such rise and combination and recurrence of sensations, we cannot 
 even conjecture. In the highest flights of imagination, in the pro- 
 foundest explorations of reflection, we never wholly escape from the 
 influences arising in this basis. The nature of this psychical basis 
 of sensation and perception depends upon the nature of the physi- 
 cal basis of the living and acting brain. In other words, what sen- 
 sations and perceptions constitute, at least in part, the "stuff" of 
 all consciousness, depends upon what the molecules of the central 
 nervous system are doing. We cannot even conceive of any other 
 relations as possible between the mind, on the one hand, and the 
 brain, on the other, than relations between a. system of moving 
 molecules and a corresponding change of conscious states. 
 
 Furthermore, the unity of a single indestructible and eternally 
 unchanging atom would afford no explanation of a mental unity. 
 In the case of man's mind and brain, the variety of the nervous 
 changes in part explains the variety of the mental states; but nothing 
 in the changing relations of the innumerable moving molecules 
 throws any clear light on the origin of the unity of mind in con- 
 sciousness. A material being absolutely without distinction of parts 
 would be, for that fact, no better fitted to become conscious of 
 itself as one. A series of states of consciousness can indeed be 
 attributed by our imagination to such a being. From the purely psy- 
 chological point of view we can conceive of the unit-atom as having 
 an experience resembling our own. We, in our consciousness, can 
 imagine such a being as the subject of states, and as attributing 
 each of these states to one and the same subject namely the "I" 
 of the unit-atom after the fashion of our customary mental be- 
 havior. But this is quite a different thing from explaining the con- 
 sciousness of such an atom as arising, with respect to its unity, out 
 of the material nature of the atom. By the very hypothesis, the 
 material nature of this particular kind of atom can have no states; 
 it never changes; it is always the same. But consciousness is al- 
 ways some particular definite state; and self-consciousness is always 
 the being aware of some particular definite state. There is no con- 
 sciousness in general; there is no consciousness which does not in- 
 volve change of state. Indeed, change is a reality in human con- 
 sciousness, if nowhere else in the universe of being. No particular 
 state of consciousness, whether considered as involving an attribu- 
 
686 REALITY AND UNITY OF THE MIND 
 
 tion of that state to a subject or not, could be explained by reference 
 to the material nature or condition of such a unit-atom. 
 
 21. The foregoing remarks have their value chiefly as a warn- 
 ing against supposing that the unity of the soul suffers any preju- 
 dice because it is not to be regarded or explained from a point of 
 view furnished by physical analogies. To be one, as a rigid mate- 
 rial atom may possibly be regarded as one, would be of no advantage 
 to the soul. Or if it be admitted that, in case it had such unity, it 
 could never cease to exist, it must also be admitted that we are un- 
 able to see how it could ever begin to exist as a self-conscious mind. 
 If the unit-atom could never die, it could also never live as a con- 
 scious psychical existence. And it is the unity which the mind 
 plainly has in self-consciousness that is alone worth contending for. 
 If the mind were really that is, regarded as out of its own con- 
 sciousness one, and yet two or more in consciousness, it would 
 be no better, but rather the worse off. If it were really one, but 
 were obliged not to know itself as one, and could never be aware 
 of its own states, or attribute them to the one "I" which is the 
 subject of them all, it would surely be the worse off. To be one, in 
 the only meaning of the word that is of real value, is to have and 
 to keep the unity of consciousness. If this unity were really a mere 
 seeming a trick of nature to cheat the mind the seeming would 
 forever seem real, would, indeed, be the ground of all reality; the 
 trick would be the kindest of all illusions, and one from which we 
 should crave never to be set free. When, then, we have recognized 
 the fact that all ordering and development of human consciousness 
 implies this kind of unit-being as belonging to the mind, we have 
 gone as far in vindication of the mind's rights as we have any psy- 
 chological interest in going. 
 
 22. It would seem, finally, as though our excursion thus far 
 into the debatable field of metaphysical discussion, might commend 
 itself to any unprejudiced and critical student of those phenomena, 
 which we have called " correlations," existing as matter-of-fact ex- 
 periences, between the human nervous mechanism and the human 
 mental life. The conclusions warranted, in our judgment, by the 
 phenomena, are by no means a final settlement of the metaphysical 
 problems involved. As to the "ground" of these correlations, and 
 as to how it is related to that Being of the World, which science is 
 accustomed to speak of as Nature, and which the different schools 
 of philosophy have conceived of in various ways, as to the first and 
 last things of mind, psychology as studied from the physiological 
 and experimental points of view finds itself unable to pronounce. 
 It cannot, indeed, explain the entire being of the mind as arising 
 out of the development of the physical germ from which the bodily 
 
LIMITS OF PHYSIOLOGICAL PSYCHOLOGY 687 
 
 members unfold themselves. It knows no decisive reason against 
 the belief that such a non-material and real unit-being, as the mind 
 is, should exist in other relations than those which it sustains at 
 present to the structure of the brain. On the contrary, it discloses 
 certain phenomena which at least suggest, and perhaps confirm, 
 the possibility of such existence for the Mind. But, in general, 
 if it remain faithful to its own mission, within its own limits, it en- 
 trusts the full consideration of these questions, after it has cleared 
 the way from barriers of ignorance and prejudice, to Rational 
 Psychology, to Ethics, to Metaphysics, and to Theology. 
 
INDEX 
 
 INDEX OF AUTHORS 
 
 ACH, 482, 484, 486 ff., 496 
 
 Alcmaeon, 213 
 
 Aliotta, 379, 498 
 
 Allen, 505 
 
 d'Allonnes, 526 
 
 Alrutz, 344 
 
 Alsberg, 366 
 
 Angell, F., 569 
 
 Angell, J. R., 396, 486, 569 
 
 Angler, 409 
 
 Apathy, 103, 106, 113 
 
 Aristotle, 214 
 
 d'Arsonval, 131 
 
 Atwater, 291 
 
 Aubert, 325, 331, 365, 370 
 
 Auerbach, 476, 491 ff. 
 
 BAILLARGER, 269 
 
 Bain, 505, 515 
 
 Bair, 568 f., 580 
 
 Baird, 335 
 
 Baldwin, 486, 501 
 
 Bang, 120 
 
 Barker, 100 
 
 von Bayer, 137 
 
 Bayliss, 151 
 
 Beaunis, 478 
 
 von Bechterew, 59, 88 
 
 Beer, 188 
 
 Beevor, 236 
 
 Bentley, 601 
 
 Benussi, 442, 451 
 
 Berger, G. O., 479 
 
 Berger, H., 509 
 
 BergstrOm, 580 
 
 Bernhardt, 118 
 
 Bernstein, 403 
 
 Bethe, 18 f., 21, 103, 106, 113, 288, 290 
 
 Beyerman, 262 
 
 Bidder, 475 
 
 Biedermann, 131, 133, 137, 363 
 
 Bielschowsky, 106, 265 
 
 Billings, 508 
 
 Binet, 401, 535, 577 
 
 Bingham, 530 
 
 Birge, 75 
 
 Blix, 344 
 
 Boll, 116,195 
 
 Bolton, 262 
 
 Bonnet, 597 
 
 Book, 556, 558 f., 561 f., 564, 569, 577 
 
 Boruttau, 140, 142 
 Bouguer, 369 
 Bouillaud, 255 
 Bourdon, 427, 429 
 Bourgery, 66 
 
 Bowditch, 136, 171 f., 453 
 Breese, 599 
 Breitwieser, 480, 486 
 Breuer, 210 f. 
 Broca, 227, 253 ff. 
 Brodhun, 370 
 Brodie, 136 
 
 Brodmann, 266, 270, 272 
 Brown, C., 210 
 Brown, E. M., 565 
 BrQcke, 392 
 BrUckner, 400 
 Bruner, 315, 368 
 Bryan, 556 ff., 561 f. 
 Buccola, 478 
 Budge. 227 
 Buhler, 601 
 Burch, 131 
 Burnham, 574 
 Byasson, 215 
 
 CAJAL, 41, 43 f., 47, 98, 102, 106, 109, 112, 
 
 116, 193, 265 ff. 
 Calkins, 573, 583, 592 
 Camerer, 372 
 Cameron, 318 
 
 Campbell, 240, 245, 261, 270 ff. 
 Cannon, 525 
 Caton, 230 
 Cattell, 132, 359, 371, 376, 478 f., 484, 489. 
 
 494, 496, 498, 537, 591, 597 
 Charpentier, 524 
 Chevreul, 310 
 Chodin, 432 
 Ciaccio, 183 
 Cline, 459 f. 
 Cole, 547 
 Conty, 524 
 Cook, 440 
 Coover, 569 
 Corti, 203 ff. 
 Cowling, 595 
 Cox, 643 
 Cramer, 124 
 von Cyon, 209, 524 
 Czermak, 402, 406 
 
INDEX OF AUTHORS 
 
 D ANILE WSKI, 118, 230 
 
 Davies, 349 
 
 Davis, 565 
 
 Dax, 255 
 
 Dearborn, 461, 568 
 
 Debrou, 177 
 
 Deiters, 83, 95, 97 
 
 Dejerine, 84, 539 
 
 Delabarre, 409, 459 
 
 Delboeuf, 370, 378, 437 
 
 Descartes, 634 
 
 Despretz, 315 
 
 Dewey, 147, 603, 610 
 
 Diaconow, 123 
 
 Dobrowolsky, 329 
 
 Dodge, 457, 459 f. 
 
 Dogiel, 180 
 
 Dolley, 132, 478 
 
 Donaldson, 58, 60, 62, 102, 344, 404 
 
 Donders, 331, 432, 467, 470, 472, 476 
 
 Dubois, 35 
 
 Du Bois-Reymond, 130 f., 141 f. 
 
 Durig, 136 
 
 Duval, 289 
 
 EBBINGHAUS, 378, 438 f., 502, 510, 572 ff. f 
 
 579, 588, 604 
 
 Ebert, 566, 568, 574, 578, 582 
 Ecker, 27, 176 
 Eckhard, 116, 176, 227 
 Edelmann, 315 
 Edes, 136 
 Edinger, 26, 29, 31, 32, 59, 62, 63, 85, 108, 
 
 221 f. 
 
 Ehrenfels, 601 
 Einthoven, 131 
 Engelmann, 149, 178 f. 
 Ephrussi, 574, 578 
 Erb, 73 
 Eternod, 39 
 Eulenberg, 365 
 Ewald, 119, 150, 153, 207 ff. 
 Ewing, 105, 539 
 Exner, 167, 170, 214, 472 ff., 478 f., 483 f., 
 
 524 
 
 FECHNER, 9, 356, 358, 360 ff., 366, 370 f., 373, 
 
 375, 377 f., 504, 565 
 Fe're', 509, 533 
 
 Ferrier, 158, 228, 235 ff., 246, 251 
 Fick, 325, 327, 329 ff., 339, 342, 474 
 Filehne, 449 
 Fite, 396 
 Flateau, 539 
 
 Flechsig, 59, 224, ff., 248, 250 f. 
 Flourens, 208, 227 ff. 
 Flournoy, 486 
 Fowler, 305 
 Fracker, 567 
 Frankel, 118, 121, 124 
 Franz, 263, 349 
 Fraser, 446 f. 
 Fraunhofer, 329 
 Freud, 585 f. 
 
 von Frey, 181, 344, 365, 400, 402 
 Fritsch, 228 f., 235, 255 
 Froeberg, 479 f. 
 Fullerton, 359, 371, 376 
 Funke, 399, 402, 517 
 
 GALEN, 215 
 
 Gall, 227, 232 
 
 Galton, 315, 583 
 
 Gamble, 592 
 
 Gamgee, 123 
 
 Garten, 136 
 
 Gaskell, 149 
 
 Gaupp, 27 
 
 Gegenbaur, 184, 197, 203,*223 
 
 Gennari, 269, 271 
 
 Gerlach, 112 
 
 Gieke, 116 
 
 Gies, 121, 124 
 
 Gley, 508 
 
 Goethe, 521 
 
 Goldscheider, 344 ff., 349, 364, 366 f., 398 ft, 
 
 406, 477, 539 
 Golgi, 97, 103, 116 
 Goltz, 150, 152 f., 155 
 Gotch, 131, 133 
 Gothlin, 143 
 Griesbach, 401 
 Grunbaum, A. A., 601 
 Griinbaum, A. F., 228, 236 ff., 272 
 
 HAGGERTY, 547 
 
 Haines, 403 
 
 Hall, 404 f., 408, 453 
 
 Haller, 228 
 
 Halliburton, 118, 124, 136 
 
 Hamilton, F. M., 595 
 
 Hamilton, G. Van T., 554 
 
 Hamilton, W., 515, 597 
 
 Hammersten, 124 
 
 Hankel, 476 
 
 Hardesty, 75, 207 
 
 Barter, 556 ff., 561 f. 
 
 Haycraft, 308 
 
 Head, 148, 348 f., 401 
 
 Heidenhain, 524 
 
 Held, 59 
 
 Helmholtz, 132, 188 f., 194, 197, 199, 206 ff., 
 
 312 ff., 319, 322, 326, 328, 332, 339, 341, 
 
 370, 421, 423, 428 f., 434, 456, 462, 467, 
 
 540, 598 
 Henderson, 582 
 
 Henle, 71, 75, 114, 194, 197 f., 201 f. 
 Henmon, 360, 490 
 Henschen, 248 f. 
 Hensen, 205, 312 f., 317 
 Herbart, 511 f. 
 Hering, E., 285, 339, 341 ff., 346, 352, 363, 
 
 416, 421, 426, 432, 434, 445 f., 452 
 Hering, H. E., 148, 241 
 Hermann,, 140 ff. 
 Herophilus, 215 
 Herschel, 331, 369 
 Hiilebrand, 427 
 Hippocrates, 214 
 Hirsch, 472, 476 
 
 His, 36, 40, 52 ff., 97 f., 112, 116 
 Hitzig, 228 f., 235, 255 
 Hober, 312 
 Hodge, 539 
 Hoffmann, 410 
 Hollingworth, 409 
 Holm, 347 
 Holmes, 240 
 Hooke, 326 
 
INDEX OF AUTHORS 
 
 691 
 
 Horsley, 236 
 Horwicz, 511, 513 
 Hough, 537 
 Howell, 129 
 Huey, 459, 461 
 Humboldt, 305 
 
 INGBERT, 75 
 
 JACOBS, 574 
 
 James, 211, 403, 525, 566, 569, 618 
 
 Janet, 535 
 
 Janssens, 240 
 
 Jastrow, 446, 448, 467, 480, 488 
 
 Jennings, 217, 544 f. 
 
 Jost, 573, 581 
 
 Joteyko, 538 
 
 Judd, 400, 402, 442, 445, 451,459, 480, 569, 595 
 
 Jung, 509 
 
 KAES, 62 
 
 Kafka, 475 
 
 Kammler, 365 
 
 Kampfe, 367 
 
 Kant, 524, 662, 672 
 
 Kastle, 312 
 
 Kennedy, 574 
 
 Keppler, 372 f. 
 
 Kiesow, 132, 312, 344, 346, 365, 477 f. 
 
 Kinnaman, 547, 549, 555 
 
 Kirkpatrick, 544, 574, 581 
 
 Klug, 406 f. 
 
 Koch, 119, 122 
 
 Kolliker, 102, 110, 116, 204, 265, 326 
 
 Konig, 370 
 
 Kopsch, 81 
 
 Kraepelin, 538 
 
 Kreidl, 208 
 
 von Kries, 196, 302, 331, 340, 476, 491 ff. 
 
 Krueger, 323 
 
 Kuhne, 119, 195, 212 
 
 Kttlpe, 592, 601 
 
 Kundt, 438 
 
 Kunkel, 475 
 
 Kussmaul, 257 
 
 LADD, 243, 339 f., 410, 464, 466, 523, 525, 
 
 527, 531, 542, 579, 584, 597 
 Lange, C., 525 
 Lange, L. f 484, 486 
 Lange, N., 600 
 Langelaan, 262 
 Langley, J. N., 150 
 Langley, S. P., 371 
 Learning, 101, 266 
 Le Conte, 434 
 Lee, 538 
 
 Lehmann, 508, 510, 600 
 von Lenhossgk, 98, 115 
 Leuba, 560, 569, 581 
 Lewis, B., 269 
 Lewis, E. O. 451 
 Liebreich, 123 
 Liepmann, 254, 259 f., 263 
 Lindemann, 366 
 Lindley, 534 
 Linnaeus, 307 
 Lipps, 449 
 Listing, 421 
 
 Loeb, 409 
 
 Lombard, 171, 215, 532 
 
 Longet, 227 
 
 Lotze, 302, 317, 384, 398, 412, 432, 436, 456, 
 
 511 
 
 Lowit, 363 
 Luciani, 156 
 
 MACDONALD, 142 
 
 Mach, 210, 312, 475 
 
 Magendie, 227 
 
 Mandelstamm, 329 
 
 Marburg, 46, 83 f. 
 
 Margo, 212 
 
 Marie, 258 ff. 
 
 Marshall, 510 
 
 Martin, 602 
 
 Matteuci, 227 
 
 Mauss, 270 
 
 May, 240 
 
 McAllister, 460 f., 480 
 
 McDougall, 338, 475, 598, 611, 618 ff. 
 
 McMurrich, 78, 220 
 
 Mendelejeff, 308 
 
 Merkel, 489 
 
 Messenger, 402 
 
 Metzner, 400, 402 
 
 Meumann, 566, 568 f., 571, 574, 578, 582 
 
 Meyer, A., 258, 260 
 
 Meyer, M., 207, 510 
 
 Meynert, 227, 510 
 
 Mihalcovics, 49 
 
 Mill, J. S., 384 
 
 Mobius, 548 
 
 Moldenhauer, 478 
 
 von Monakow, 159, 242 f., 245 f., 248 f., 258, 
 
 260 f. 
 
 Montague, 610 
 Moore, A. W., 486 
 Moore, J. M., 537 
 Moore, T. V., 481 
 Morgan, 548 
 Mosso, 536 
 Moutier, 258 
 Muller, G. E., 317, 358, 361, 379, 573, 578 f., 
 
 582, 584 ff., 591, 602 
 MQller, J., 132, 284 
 Muller, J. J., 330 
 Mttller-Lyer, 439, 441 
 Munk, 242, 246, 252 
 Munsterberg, 486, 532, 580, 610 
 Myers, 395 f., 403, 574 
 
 NAGEL, 208, 305, 307, 310 
 Nahlowsky, 522 
 Nelson, 510 
 Newton, 331, 340, 662 
 Nissl, 102 f. 
 Nothnagel, 366 
 
 OETTINGEN, 312 
 Ogden, 578 
 Ohms, 582, 584 
 Oppel, 437 
 Osthoff, 517 
 
 PARKER, 17 
 Pearce, 439 f., 442 
 Peckham, 545 
 
692 
 
 INDEX OF AUTHORS 
 
 Pentschew, 578 
 
 Peters, 588 
 
 Peterson, 509 
 
 Pfaff, 305 
 
 Pfluger, 139, 216 
 
 Pick, 254 
 
 Pierce, 445 f. 
 
 Pie*ron, 544 
 
 Pillsbury, 349, 569, 596, 603, 606 
 
 Pilzecker, 573, 584, ff., 591 
 
 Piper, 473 
 
 Plato, 214 
 
 Plutarch, 213 
 
 Pohlmann, 574 
 
 Porter, 547 
 
 Posner, 121, 124 
 
 Preyer, 312, 315, 317, 474 
 
 Purkinje, 194 
 
 Pythagoreans, 320 f. 
 
 RADOSSAWLJEWITSCH, 572, 576 
 
 Ranvier, 180 
 
 Rauber, 81 
 
 Rayleigh, 368, 395 
 
 Reid, 515 
 
 Remak, 97 
 
 Retzius, 23, 77, 182, 203 ff. 
 
 Reuther, 574 
 
 Richards, 312 
 
 Ritter, 305 
 
 Rivers, 348 
 
 Robertson, 440 
 
 Rosenheim, 124 
 
 Rosenthal, 148 
 
 Rouget, 212 
 
 Royce, 501 
 
 Ruediger, 461 
 
 Ruger, 554 f., 569, 605 
 
 SANDER, 475 
 
 Sappey, 87 
 
 Schaefer, 396 
 
 Schafer, 63, 98 f., 107, 249, 479 
 
 Schenck, 187, 537 
 
 Schiff, 215, 227, 517 
 
 Schrader, 152, 158 
 
 Schroder, 240 
 
 Schroeder van der Kolk, 643 
 
 Schultze, 176 f., 190 ff., 194 
 
 Schumann, 573, 578 f., 582, 601 f. 
 
 Schuster, 262 
 
 Schwalbe, 68, 71, 79, 80, 82 
 
 Scripture, 537, 565 
 
 Sergi, 525 
 
 Shepard, 508, 526 
 
 Sherren, 348, 401 
 
 Sherrington, 147, 152, 154, 156, 159, 162 ff., 
 166 ff., 170, 181 f., 228, 236 ff., 241, 272, 
 287, 289, 344, 365, 477, 524 f., 552, 599, 612 
 
 Sidis, 510 
 
 Slaughter, 600 
 
 Small, 547 
 
 Smith, S., 545 
 
 Smith, T. L., 565 
 
 Smith, W. G., 442, 480, 677 
 
 Sobotta, 78, 220 
 
 Solomons, 402 
 
 Sowton, 136, 442 
 
 Spalding, 157 
 
 Spaulding, 546 
 
 Spearman, 401 
 
 von Spee, 38 
 
 Spitzka, 61 
 
 Spurzheim, 227 
 
 Starch, 393, 395, 509 
 
 Starling, 151 
 
 Starr, 72, 101, 224, 266 
 
 Steele, 480 
 
 Steffens, Laura, 602 
 
 Steffens, Lottie, 578 
 
 Stein, 535 
 
 Steinach, 477 
 
 Stern, 591 
 
 Storey, 539 
 
 Starring, 606 
 
 Stout, 543 
 
 Stratton, 365, 455, 459, 461 
 
 Streeter, 50 
 
 Strieker, 317 
 
 Strong, 36, 63, 101, 266 
 
 Strttmpell, 88 
 
 Stumpf, 312, 315 ff., 321 f., 378, 416, 510 
 
 Swift, 561 
 
 Symington, 63 
 
 TANZI, 477 
 
 Tarchanoff, 509 
 
 Tawney, 402 
 
 Tebb, 124 
 
 Tesla, 131 
 
 Theophrastus, 213 
 
 Thie-ry, 449 
 
 Thorndike, 33, 637, 546 ff., 569, 573 
 
 Thudicum, 120, 122 ff. 
 
 Thunberg, 181, 344, 347, 403, 479 
 
 Titchener, 350, 358, 378, 471, 480, 502, 510, 
 
 601 
 
 von Tobel, 537 
 Trautscholdt, 494 
 Treves, 537 f. 
 Triplett, 633, 547 f. 
 Trotter, 349 
 Turnbull, 315 
 Tyndall, 328 
 
 URBANTSCHITSCH, 600 
 Urstein, 591 
 
 VALENTIN, 305, 373 f., 392, 474 
 
 Valsava, 200 
 
 Van Biervliet, 574 
 
 Van Deen, 227 
 
 Van Gehuchten, 76, 103 f. 
 
 Veraguth, 509 
 
 Veratti, 108 
 
 Verworn, 15, 290 
 
 Vierordt, 329, 398 f. 
 
 Vigual, 116 
 
 von Vintschgau, 392, 475, 477 
 
 Voeste, 337 
 
 Vogt, C., 236 
 
 Vogt, O., 236, 248, 250, 267, 270 f. 
 
 Volkmann, 369 ff., 401, 565 
 
 Volkmann von Volkmar, 413, 470, 511 
 
 Volta, 305 
 
 WAGNER, 97, 523 
 Waldeyer, 112 
 
INDEX OF AUTHORS 
 
 693 
 
 Waller, 137 
 
 Ward, 584, 594 
 
 Warnecke, 38 
 
 Warren, 171 f. 
 
 Washburn, 544 
 
 Watson, 59, 62, 547 
 
 Watt, 494 f. 
 
 Weber, E., 509 
 
 Weber, E. H., 304, 326, 346 f., 360, 363, 
 
 366, 371 f., 394, 396 ff., 404 f., 474 
 Weigert, 264 
 Wernlcke, 257 ff. 
 Whipple, 601 
 Wien, 368 f., 371 
 Wilson, 395 
 Winch, 571 
 Wirth, 596 
 
 Witasek, 582 
 
 von Wittich, 474, 476, 479 
 
 Woodworth, 157, 350, 535, 537, 552, 569, 594 
 
 Wright, 533 
 
 Wundt, 10, 274, 290; 338, 376, 384, 397, 
 410, 415, 421 ff., 425, 427, 432, 448, 456, 
 462, 476, 478 f., 481 ff., 501 f., 506 f., 516, 
 526, 594, 601, 610 
 
 YERKES, 547 f. 
 Yoakum, 541 
 Young, 341 
 
 ZlEHEN, 511 
 
 Zinn, 228 
 Zoth, 421, 429 
 Zwaardemaker, 307 f., 374 
 
INDEX OF SUBJECTS 
 
 ABSOLUTE PITCH, 317 
 
 Abstraction, 595 ff.; in perception, 596; 
 
 in recall, 600 f. 
 Accommodation of the eye, 188, 325, 414, 
 
 417, 423, 427; to pitch, 200 
 Achromatopsia, 253 
 Action theory of consciousness, 610 f. 
 Action time, 333, 474 f. 
 Acuity, 357; of hearing, 368 f.; of muscle 
 
 sense, 364, 409; of smell, 373 f.; of taste, 
 
 372 f.; of temperature sense, 365 f ., 406 f . ; 
 
 of touch, 365, 397, 408; of vision, 326, 
 
 371 f., 427 ff., 461 
 Adaptation, 545 f., 570; of the retina, 195 f., 
 
 333, 337, 371, 378, 540; of other sense 
 
 organs, 308, 346 f., 378, 540; of central 
 
 organs, 540 
 Adjustment, 482 ff., 489, 491, 494, 551, 555, 
 
 569, 582, 589 f., 602, 605 f., 621 f.; neural 
 
 mechanism of, 624 
 Adolescence, 661 
 ^Esthetics, 504 
 Affection, 500 ff. 
 After-brain, 48 ff. 
 After-discharge, 166 
 After-image, 460, 475, 587; negative, 337, 
 
 343; positive, 338 
 Ageusia, 311 
 Agraphia, 254 f., 261 
 Alexia, 252 
 
 Alliance of reflexes, 169, 172 f. 
 Ambiguous figures, 594, 620 
 Amnesia, 253, 260, 544 
 Amo3ba, behavior of, 14 f., 64, 544, 612 
 Amphibia, brain of, 27, 29 ff.; intelligence 
 
 of, 547 
 
 Ampulla, 201, 203 f., 210 
 Amputation, 218 
 Amusia, 250, 253 
 Analgia, 517 
 Analysis, 186, 302, 307, 310, 312 f., 319, 326, 
 
 330, 341, 343 f., 350 ff., 353, 380, 440, 
 
 450 ff., 458, 500, 551 ff., 569, 605 ff.; de- 
 pendent on association by similarity, 589; 
 
 in perception, 596 ff.; neural mechanism 
 
 of, 624 f. 
 Anastomosis of branches of nerve-cells, 18, 
 
 21, 112 ff.; of nerves, 243, 285 f. 
 Anesthesia, 244 f., 464, 517, 526 
 Anesthetics, 120, 287, 289 ff. 
 Angle illusion, 442 ff. 
 Annelids, nervous system of, 20 ff. 
 Anosmia, 306, 308 
 Anthropoid apes, brain of, 33 ff., 228 f., 
 
 236 ff.; intelligence of, 552 f. 
 Anvil, 197 fit. 
 
 Aphasia, 253, 255 ff., 544, 663; varieties of, 
 256 ff. 
 
 Apoplexy, 93 
 
 Apperception, 380, 452, 483 ff., 487, 497 
 673 
 
 Apprehension, 376, 436, 442, 444, 488, 491, 
 597 
 
 Apraxia, 253 f., 263 
 
 Aqueduct, 49, 78, 83 f. 
 
 Aqueous humor, 183 f., 186 f. 
 
 Arachnoid, 70 
 
 Archipallial system, 95 
 
 Archipallium, 31 f., 86, 219, 223, 250 
 
 Area, judgments and illusions of, 447 f. 
 
 Area of the cortex, functional, 235 ff.; his- 
 tological, 268 ff.; auditory, 249 f., 272; 
 of Broca, 255, 257 ff.; gustatory, 251; in- 
 tellectual, 251, 262; motor, 228, 232, 
 235 ff., 254, 267 ff., 272; olfactory, 250 f.; 
 prefrontal, 270 ff.; sensory, 244 ff., 267 ff; 
 silent, 229, 244, 251; somesthetic, 244 ff., 
 403 f.; of speech, 227, 233, 255 ff.; stri- 
 ate, 271 f.; visual, 246 ff., 271 f.; of 
 Wernicke, 257 ff. 
 
 Association, 402, 427, 429 f., 454 f., 466 ff., 
 493 ff., 502; by contiguity, 584, 617 ff.; 
 by similarity, 588 f.; convergent, 624; 
 controlled, 495, 586, 589, 623 f.; dy- 
 namical, 562, 579, 584, 616, 660 f.; forma- 
 tion of, 542, 544, 546, 549, 551 f.; 554, 570, 
 575 f., 577 ff., 617 ff.; free, 495, 586; 
 neural mechanism of, 617 ff.; operation of, 
 583 ff.; principal and subordinate, 578 f.; 
 remote, 579, 581, 619; serial, 578 f., 581, 
 620 ff.; subliminal, 584, 612 
 
 Association fibres, 58, 96, 223 f., 233, 246, 
 251 f., 269 
 
 Association time, 493 ff., 573, 85 
 
 Astereognosis, 253 
 
 Astigmatism, 187 
 
 Asymbolia, 252 f. 
 
 Atom, concept of, 676; reality of, 680; 
 unity of, 679; the unity of the mind not 
 atomic, 683 ff. \ 
 
 Attention, 401, 4Q7\ 435, 440, 450, 458, 483, 
 532, 541, 550, 556, 561 ff., 571 f., 582 f., 
 587, 596 ff.; centre for, 262, 274; cerebral 
 conditions of, 609 ft, 618 ff.; definition of, 
 596 f.; determination of, 600; expectant, 
 see Expectancy; feelings of, 214, 507; 
 field of, 597, 609; fluctuation of, 599 f.; 
 motor adjustments in, 249 ff., 507 f., 531; 
 shifting of, 598 f., 611, 618, 620 ff., span of, 
 560, 597 
 
 Attraction, 442 f. 
 
 Auditory area, 249 f. 
 
INDEX OF SUBJECTS 
 
 695 
 
 Auditory perception of space, 392 ff. 
 
 Auditory sensations, 312 ff., 367 ff., 475, 
 529 f., 659 
 
 Automatism, 64 f., 147 ff., 158, 485, 496. 
 533 ff., 559, 564, 610 
 
 Autonomic system of nerves, 150 
 
 Average error, 358 f. 
 
 Axis cylinder process, see Axon. 
 
 Axon, 74, 97, 100 ff.; central. 44j classes of, 
 44; conduction in, 111; function of, 109 f.; 
 growth of, 41 ff., 98; motor, 42, 212; sen- 
 sory, 42, 48, 177 ff.; sheaths of, 99, 142; 
 structure of, 100 f.; types of, 104 f., 265; 
 see Nerve-Fibre. 
 
 BAHNUNG, 170 
 
 " Baskets," 108 ff. 
 
 Beats, 322 f. 
 
 Beauty, 529 ff. 
 
 Behavior, of Amoeba, 14 f. ; of earthworm, 
 22; of jelly-fish, 17 ff.; of sponge, 17 
 
 Binocular vision, 414 f., 427 ff., 436, 458, 
 465; contrast, 452; rivalry, 452 f., 598 f.; 
 mixture, 452 
 
 Birds, brain of, 29 ff.; intelligence of, 547 
 
 Blind, tactile sensitivity of the, 402 f., 411 
 
 Blind spot, 193 f. 
 
 Blocking of nerve impulses, 287, 289 f. 
 
 Bodily resonance of emotion, 525 
 
 Body, relations of, to mind, 629 ff. 
 
 Bone-conduction to the ear, 197, 393 f. 
 
 Brain, 66, 75; action of, on mind, 643 f.; 
 chemistry of, 117 ff., 291; circulation in, 
 68, 70, 125, 215, 291, 643, 663; develop- 
 ment of, 48 ff.; functions of, 213 ff.; of 
 invertebrates, 24 ff.; of man, 33 ff.; as 
 organ of mind, 636 ff.; as seat of mind, 
 213 ff., 632 ff.; a tube, 39, 48 f.; of ver- 
 tebrates, 25 ff.; weight of, see Brain- 
 weight 
 
 Brain-stem, 27 f., 77 ff., 155, 241; develop- 
 ment of, 55 ff. 
 
 Brain vesicles, 48 
 
 Brain-weight, of idiots, 656; of mammals, 
 33 f.; of primates, 34 f.; of races of man, 
 35; as related to age, 59 ff., 660 f.; to 
 intelligence, 33, 60 f., 216 f.; to sex, 60; 
 to stature, 33, 60 
 
 Bridgman, Laura, brain of, 58 
 
 Brightness, 327, 329, 331, 333, 354 f., 370 f. 
 
 Bulb or medulla, 48, 66, 69, 75 ff.; develop- 
 ment of, 56 f.; function of, 157 f. 
 
 CALCAKINE region, 223, 248 f., 252, 268 f., 
 
 271 
 
 Callosum, 33, 54, 77 f.; 85 f., 222 ff., 254, 263 
 Calm, 501 f., 506, 508, 531 
 Carbon dioxide in nervous activity, 135, 137, 
 
 291; in respiration, 148; in fatigue, 538 
 Catabolism in nerve, 135 ff.; in brain, 215, 
 
 290 f. 
 Causal relations between brain and mind, 
 
 641 ff. 
 
 Cause, concept of, 647, 650 
 Cell, definition, 13 f.; chemistry of, 119 f.; 
 
 gustatory, 178 f.; nerve, see Nerve-Cell, 
 
 olfactory, 176 f. 
 Cell-body, 286 ff. 
 Cell membrane, 14, 119 f. 
 
 Central canal of the cord, 39, 45, 72 
 
 Central factors in illusions, 444, 447, 449 ff.; 
 in reaction time, 481 f. 
 
 Centralization in the nervous system, 20 ff. 
 
 Cerebellar system, 95 
 
 Cerebellum, 66, 75 ff., 86 f., 108 ff., 221 f.; 
 development of, 49, 54; function of, 156 f., 
 211 f. 
 
 Cerebral ganglion of invertebrates, 24 ff. 
 
 Cerebrin, 121 
 
 Cerebro-spinal fluid, 49 
 
 Cerebrum, 66, 76 ff., 156, 219 ff.; develop- 
 ment of, 49 ff.; in different animals, 26, 
 31 ff.; influence of, on reflexes, 158 f., 
 171 f., 241, 505, 532 ff.; injury of, 215, 
 217 ff., 252 ff., 262, 274, 644, f., 663; 
 localization of functions of, 213 ff., 235 ff.; 
 as related to consciousness, 213 ff., 298, 
 609 ff., 618, 633 ff.; removal of, 158 f., 
 217; as seat of mental functions, 219, 281 ff. ; 
 see Brain 
 
 Character, 262 
 
 Chemical changes in retina, 195, 325 
 
 Chemical elements in brain, 118, 279 
 
 Chemical integration of the body, 151 
 
 Chemical senses, 28, 375, 479 
 
 Chemistry of nervous tissue, 117 ff., 279 f. 
 
 Chinese music, 320 
 
 Choice-time, 484, 488, 497 
 
 Cholesterin, 120 f. 
 
 Chorda tympani, 179 
 
 Choroid, 183 f., 194 
 
 Chromatolysis, 105, 240 
 
 Chromosome, 37 
 
 Ciliary muscle, 183, 188 f., 427 
 
 Circulation, in brain, 68, 70, 125, 215, 291, 
 526, 643 f., 663; during emotion, 505 ff., 
 523 ff.; as a means of co-ordination, 151; 
 in nerves, 137 
 
 Clang, 313 f., 318 f., 369, 395 
 
 Classification of mental functions, 664, 666; 
 of odors, 307; of sensations, 300 ff.; of 
 sounds, 312 f.; of tastes, 310 
 
 Clearness, 597 
 
 " Climbing fibres," 109 f. 
 
 Cochlea, 200 ff., 280 
 
 Cochlear nerve, 82, 90, 203 
 
 Coelenterate, nervous system in, 17 ff. 
 
 Coenaesthesia, 350, 518 f. 
 
 Cold spots, 181, 344 ff., 366, 405 
 
 Collaterals, 42, 73, 88, 111, 265, 268 
 
 Collectors, 162, 240, 622 f. 
 
 Color blindness, 335 f., 343 
 
 Color mixture, 330 ff., 340, 343, 452 
 
 Color-tone, 327, 333, 337 
 
 Color triangle, 344 
 
 Color vision, 196, 324, 327 ff., 352, 475, 490; 
 theories of, 340 ff. 
 
 Columns of the cord, 40, 46 ff., 71, 73 ff., 
 79 ff., 89 f.; of Clarke, 74, 90; of Goll, 75 
 
 Commissures, 40, 43 f., 71 f., 89 
 
 Comparison, 376, 440, 488, 601 ff.; mediate 
 and immediate, 601; process of, 602 f. 
 
 Compensatory movements, 155, 209 ff. 
 
 Competition, between stimuli, 173; dyn- 
 amogenic effect of, 533 
 
 Complementary colors, 332, 334, 339, 342 f. 
 
 "Complexes," 509, 586 
 
 Concatenation, 64, 280 f. 
 
696 
 
 INDEX OF SUBJECTS 
 
 Concept, 466 
 
 Conduction, alterations of, 133, 139; in 
 axon, 111, 133 ff., 279 f.; in gray matter, 
 111, 286 ff.; irreciprocal, 111, 290; in 
 nerves, 127 ff.; in nerve-net, 19 f.; as 
 primary function of nervous tissue, 16, 33, 
 143, 279 f.; rapid, due to long nerve- 
 fibres, 20 f.; reversible, 133 ff.; at syn- 
 apse, 111, 286 f., 290; velocity of, in 
 nerves, 131 ff., 473; in other tissues, 132 
 
 Conductivity, 15 f., 127, 133 
 
 Conductor, 16, 65, 127 ff.; artificial, 139 ff.; 
 indifferent, 284 f. 
 
 Cones of the retina, 190 ff., 325 f., 352 
 
 Confluxion, 441, 444, 447, 451 
 
 Conjunctiva, 180 f., 184 
 
 Consciousness, 651, 666, 685; centre of, 483, 
 564, 597; cerebral conditions of, 609 ff., 
 618, 635; double, 679; elements of, 500 f., 
 511; field of, 431, 483, 597, 618; intensity 
 of, 597, 609 f., 650; limits of, 430 f.; relation 
 of, to learning and habit, 496, 563 f.; to 
 bodily movements, 145 f.; seat of, 213 ff., 
 633 ff.; theories of, 610 f.; unity of, 679, 
 684 ff. 
 
 Conservation of energy, 646 ff.; inappli- 
 cable to relations of mind and body, 650 f., 
 665; within the nervous system, 648 ff. 
 
 Consonance, 321 ff., 529 
 
 Constant errors, 409, 437 ff. 
 
 Contours, prevalence and rivalry of, 452 
 
 Contrast, in perception of space, 439, 441, 
 443, 447, 451; in taste, 311; in tempera- 
 ture, 347; in vision, 339 f., 343, 435, 452 
 
 Convergence, of the eyes, 189, 427; of ner- 
 vous impulses, 93, 161 f., 171, 403, 622 ff. 
 
 Convolution, see Gyre 
 
 Co-ordination, as function of nerve-centres, 
 63 ff., 93, 281 ff., 285 f.; in movements 
 initiated from cortex, 241, 254, 261 f., 
 562, 565, 622 ff.; in reflexes, 153, 155, 
 157 f., 169, 172, 241, 622; in sponge, 17; 
 in worm, 24 
 
 Core conductor, 139 f., 284 
 
 Core model, 139 f. 
 
 Cornea, 181 ff., 186 ff., 346 
 
 Corpuscle, Pacinian, 180 ff.; tactile, 180 
 
 Correlation between the neural and the 
 mental, 3, 6, 9 f., 124 ff., 213 ff., 281, 292 f., 
 298 f., 375, 381, 458, 483, 485, 592, 607 ff., 
 629 ff.; conception of, 629 ff.; limits of, 
 656 ff., 664 ff., 677 f. 
 
 Corresponding points, 248 f., 424 ff. 
 
 Cortex of cerebellum, 54, 86, 103, 108 ff.; 
 of cerebrum, 31 ff., 104, 222 ff., 264 ff.; 
 development of, 52 ff., 62; as seat of 
 mind, 634 
 
 Covering points, 424 
 
 Crab, nervous system of, 24 f., 288; learning 
 by, 546 
 
 Cross-education, 565 
 
 Crossing of nerves, 243, 285 f. 
 
 Cuneus, 222, 247 
 
 Curare, 136 
 
 Current of action, 134, 137 f., 230 
 
 Current of injury, 141 f. 
 
 Current of rest, 141 f. 
 
 Cutaneous senses, 179 ff., 244, 344 ff.,352, 
 363 ff., 473, 475 ff., 517 
 
 DEAF-MUTES, 211 
 
 Deafness, 250, 316 
 
 Decerebrate animal, 152, 155, 158 f., 168 f., 
 525 
 
 Decussations, 69, 79, ff., 91 f., 247, 250 
 
 Deep sensibility, 181 
 
 Degeneration of nerve-fibres, 73, 87 f., 112 f., 
 239, 288 
 
 Dendrites, 42, 97, 101 ff., 265 f., 268; func- 
 tion of, 107 f., 161, 287 ff.; mobility of, 
 289, 613 
 
 Departing station, 251 
 
 Depression, 501 f., 531 
 
 Depth, perception of, 413 f., 416, 423, 426 ff., 
 458, 465 
 
 Determining tendency, 495 ff. 
 
 Development, of the mind, 654 ff.; of the 
 nervous system, in the individual, 36, 
 275, 279, 298, 655, 672; in the race, 13, 
 298; of space-perception, 381, 384 ff., 
 390 f., 410, 431, 458, 463 ff. 
 
 Diaschizis, 243 
 
 Dichromatic vision, J536, 340, 343 f . 
 
 Diencephalon, 48 
 
 Difference tone, 323 
 
 Diffusion, in living cells, 14, 290 
 
 Discrimination, 376 ff., 403 f., 410, 665; 
 auditory, 315 ff., 369; as a factor in learn- 
 ing, 550, 554 f., 571; of intensities, 353 ff., 
 378 f.; olfactory, 374; tactile, 348, 396 ff.; 
 time of, 360, 488 ff., 497, 571; visual, 326, 
 329, 331, 336 
 
 Disposition, 543, 583 
 
 Dissociation, 534 f. 
 
 Distribution of nervous impulses, 93, 159 f., 
 168, 622 ff.; of training, 581, 616 
 
 Divided space, illusion of, 438 ff., 448 
 
 Division of labor in animal economy, 15, 
 64 ff., 175, 351 
 
 Dizziness, 211, 350 
 
 Dorsal column, 40, 48, 80 f. 
 
 Dorsal horn, 46 
 
 Dorsal root, 40, 43 f., 46 ff. 
 
 Dot figure, 598, 618 
 
 Double contact, 411 f., 637 
 
 Double images, 414, 424 ff., 436, 465 
 
 Drainage theory, 611 ff., 618 ff. 
 
 Dreams, 587, 643 
 
 Dualism, 630, 640, 652 
 
 Ductus cochlearis, 202 f., 205 
 
 Duplex theory of vision, 196, 340 
 
 Dura mater, 70 
 
 Dynamogenesis, 532 f. 
 
 EAR, 196 ff.; an analytic organ, 206, 319; 
 
 development of, 56; inertia of, 474, 476, 
 
 479; internal, 28, 155 f. 
 Ear drum, 197 
 
 Eccentric projection, 303, 384, 390, 411 
 Economy in learning, 577 f., 581 f. 
 Ectoderm, 38 ff. 
 Effector, 16, 65, 99 
 Eigenlicht, 325, 357, 369, 433 
 Einfuhlung, 450 
 
 Einstellung, 602; see also Adjustment 
 Electrical excitation of cortex, 228 ff.; of 
 
 nerve, 130 f.; of sense organs, 305, 309, 
 
 325 
 Electrical phenomena in cortex, 230; in 
 
INDEX OF SUBJECTS 
 
 697 
 
 nerves, 137 ff.; in the retina, 138, 194; 
 during emotion, 509 f. 
 
 Electrical theory of nerve impulse, 138, 
 140 ff.; of the synapse, 289 f. 
 
 Electrophysiology, 130 
 
 Electrotonic currents in nerve, 139 f. 
 
 Electrotonus, 138 f. 
 
 Elements of consciousness, 500 f., 511 
 
 Elimination by subtraction, 491, 497 f. 
 
 Embryo, 37 ff.; mental life of, 658 
 
 Embryology, 36 ff. 
 
 Eminent men, brain weights of, 60 f. 
 
 Emotion, 509, 521 ff., 661; bodily symp- 
 toms of, 523 ff.; central factors in, 522, 
 644; sensory factors in, 523, 644; sup- 
 pression of, 586; theories of, 522 f., 525 
 
 Empiristic theory, 385, 418, 458 f. 
 
 End-brain, 31, 48 ff. 
 
 End-bulb, 180 f. 
 
 End-organs, 175 ff., 280, 299, 304, 351 f., 
 375, 473, 479 
 
 End-plate, motor, 100, 136, 212 
 
 Endoneurium, 99 
 
 Enlargements of the cord, 69 f., 74 
 
 Entoderm, 38 
 
 Epicritic sensibility, 349 
 
 Epineurium, 99 
 
 Equally tempered scale, 320 
 
 Equilibrium, 157, 209, 282 
 
 Erect vision, 453 ff. 
 
 Eustachian tube, 197 f., 200 
 
 Excitability, see Irritability 
 
 Excitation, method of, 228 ff., 244, 249 ff. 
 
 Excitement, 501 f., 506, 508, 531 
 
 Exercise, nutritive effects of, 59, 62, 581, 616 
 
 Expectancy, 480, 502, 507, 556, 602; physi- 
 ology of, 621 f. 
 
 Experience, 380, 418, 458. 468, 542, 549 f., 
 563, 593 f., 646, 657 f., 679, 685 
 
 Extensity, 383, 418 f. 
 
 Extent, perception of, 364, 371, 409, 431 f., 
 440, 467 f., 490 
 
 External ear, 196 f., 394 
 
 External meatus, 196 f. 
 
 Extirpation, 229 ff., 238, 241, 262 f., 663 
 
 Eye, axes of, 420 ff.; development of, 49, 
 56; fixation of, 420, 448 f., 459 f.; move- 
 ments of, 185 f., 189, 246 ff., 269, 414 f. 
 417, 420, 427, 447 ff., 454, 457, 459 ff., 
 530, 659; muscles of, 185 f., 420 f.; 
 nerves of, 189, 191; nerve-centres of, 25, 
 30, 56, 83, 92, 158, 189, 237, 246 ff.; per- 
 ceptions of, 413 ff.; positions of, 421 f., 
 465; rotation of, 420 ff.; structure of, 
 182 ff.; torsion of, 421 ff. 
 
 FACILITATION, 170 ff., 511, 584, 590, 599, 612 
 
 Faculties, 255, 274, 391, 566, 571, 581, 655, 
 666; development of, 657 
 
 Fatigue, causes of, 538 ff.; curve of, 536, 
 538; feelings of, 350, 536, 538 f.; level of, 
 537; mental, 401, 537 f., 541; of muscle, 
 536 ff.; of nerve-centres, 538 ff., 611 ff.; 
 of nerve-fibres, 136; products of, 538 f.; 
 of sense organs, 308, 337, 453, 537; varie- 
 ties of, 539 ff. 
 
 Fechner's law, 361, 377 ff., 480 
 
 Feeling, 302, 373, 496, 500 ff., 564 f., 567, 
 658, 666; aesthetic, 505, 513 f., 528 ff., 
 
 664; classification of, 512 ff., 526; com- 
 mon, 518 f.; complex, 518 f., 521 ff.; con- 
 tent of, 514; definition of, 500; dimen- 
 sions of, 501 f., 506 ff., 526; experiments 
 on, 504; expression of, 505 ff.; intel- 
 lectual, 502 f., 513 f., 527 f., 544, 584, 
 590 f.; intensity of. 514; moral, 513 f., 
 664; nature of, 500, 503, 511; neutral, 
 515 f.; number of, 501; physiology of, 
 504 ff.; rhythm of, 514 f.; sensuous, 
 513 f., 518 ff.; sthenic and asthenic, 524; 
 teleology of, 527; theories of, 504 f., 
 510 ff., 522 ff. 
 
 Feeling of familiarity, 503, 544, 590 f.; of 
 strangeness, 544 
 
 Feelings of innervation, 407 f. 
 
 Feeling-tone of sensations, 502, 512, 515 ff.. 
 519 ff., 633, 635 
 
 Fenestra ovalis, 198, 205 
 
 Fenestra rotunda, 198, 205 
 
 Fibrillar theory, 114 
 
 Field of touch, 396 ff.; of vision, 415 ff., 436, 
 438, 456 ff., 461 ff. 
 
 Fillet, lateral, 82 f.; mesial, 81, 83 f., 90, 246 
 
 Filum terminale, 69 f., 72 
 
 Fishes, brain of, 27, 29 ff.; intelligence of, 217 
 
 Fissure, calcarine, 222, 247 ff.; central, 
 220 ff., 236 ff., 244 f., 271; of cerebellum, 
 86; of cerebrum, 33, 51 ff., 220 ff.; cing- 
 ulate, 222; of the cord, 45, 69 ff., 79 f.; 
 intraparietal, 221; longitudinal, 52; par- 
 ieto-occipital, 220, 222, 247; postcentral, 
 221; precentral, 221; of Rolando, 220 ff., 
 236 ff., 244 f., 271; of Sylvius, 51 ff., 75, 
 86, 220 f., 249 f. 
 
 Fixation of the eye. 420, 448 f., 459 f. 
 
 Flatworm, 20 
 
 Fore-brain, 31 
 
 Fore-dog, 153, 155 
 
 Fore-period, 480, 482 ff., 489 
 
 Forgetting, 546, 573 ff. ; curve of, 575 ff. 
 
 Fornix, 77 f., 85 f., 95, 219, 223 
 
 Fovea centralis, 193, 195, 247 f., 334, 420, 
 459 f. 
 
 Fraunhofer's lines, 327 ff. 
 
 Free nerve-ending, 180 f. 
 
 Functions, fundamental of animals, 14; 
 intellectual, 219, 224, 251 ff., 273 f. 
 
 Fundamental system, 26 ff., 47, 56 f., 94, 96 
 
 Fundamental tone, 319 
 
 Fusion, 321 ff., 403 f., 410, 450 f., 599 
 
 GANGLION, cerebral, 24; In Invertebrates, 
 20 ff.; spinal, 26, 42 ff., 50, 56, 73 f., 104, 
 106; spiral, 203 
 
 Genetic theory of space perception, 386 
 
 Geniculatum, 78 f., 84 f., 91 f., 96, 246, 248, 250 
 
 Geometrical illusions, 437 ff.; theories of, 
 447 ff. 
 
 Geometrical senses, 352, 383, 390, 410,418 
 
 Germ layers, 38 
 
 Gestaltqualitaten, 601 
 
 Golgi net, 113; Golgi stain, 103, 112, 264 f., 
 267, 269 
 
 Gray matter, 45, 53 f., 73, 102, 106, 118, 222, 
 226; conduction in, 110, 286 ff. 
 
 Ground bundle, 47, 94 
 
 Growth of the brain, 59 ff.; of the ner- 
 vous system, 57 ff. 
 
INDEX OF SUBJECTS 
 
 Gyre, 221; angular, 221, 246 f.; dentate, 
 219; frontal, 221; fusiform, 222; lin- 
 gual, 222, 247; occipital, 222; parietal, 
 221 f.; postcentral, 220 ff., 235 ff., 244 ff.; 
 precentral, 220 ff., 235 ff., 245 f., 261, 
 268 ff.; supramarginal, 221; temporal, 
 221; sigmoid, 235 
 
 HABENULA, 78, 95 
 
 Habit, 263; formation of, 545 ff.; orders of, 
 556 ff.; law of, 615 ff. 
 
 Hair-receptors, 179 f., 346 
 
 Hallucinations, 306, 436, 644 
 
 Hammer, 197 ff. 
 
 Harmony, 321 ff., 529 
 
 Head, 25, 28; receptors of, 25, 27 f. 
 
 Hearing, acuity of, 368 f.; end-organ of, 
 196 ff., 351 f.; pathway of, 82, 90 f., 203, 
 250; sensations of, 312 ff., 367 ff.; space- 
 perceptions of, 392 ff.; theories of, 205 ff. 
 
 Heart, beat of, 149, 262, 505 ff.; nerves of, 
 157, 163 
 
 Heat, sensation of, 347 f. 
 
 Hemianopsia, 247 f. f 253 
 
 Hemiplegia, 263 
 
 Hemispheres, cerebral, 51 f., 213 ff.; pre- 
 dominance of the left, 263, 565 
 
 Heredity, 37, 520 
 
 Higher units, 498, 559 f., 562, 569, 578 f., 
 583, 597, 621; neural mechanism of, 622 ff. 
 
 Hind-brain, 48 ff. 
 
 Hind-dog, 153, 155, 174 
 
 Hippocampus, 95, 223 
 
 Hood or tegmentum, 83 ff. 
 
 Hormones, 148, 151 
 
 Horn of Ammon, 220 
 
 Horns of the cord, 46, 72 f., 79 f., 89 
 
 Horopter, 426 
 
 Hypermetropia, 187 
 
 Hypophysis, 76 ff. 
 
 Hysteria, 535, 544 
 
 IDEAS, 512, 593; learning by, 553 f., 563; 
 physical basis of, 638 
 
 Identical points, 424, 426 
 
 Idiocy, 656 
 
 Illusion, 339, 409, 430, 434 ff., 519, 544; 
 angle, 442 ff.; of area, 447 f.; confluxion, 
 441, 444, 447, 451; of divided space, 
 438 ff., 448; geometrical, 437 ff.; of 
 Hering, 445 f.; of motion, 453; of Muller- 
 Lyer, 440 ff., 448, 450 f.; one-dimen- 
 sional, 438 ff.; of Poggendorf, 440, 444, 
 446, 451; theories of, 447 ff., 456; of 
 time, 440; of touch, 440; twisted-cord, 
 446 f.; vertical-horizontal, 437, 447 ff.; of 
 Zollner, 444, 446 f. 
 
 Imagery, 583 
 
 Imagination, 219, 243, 253, 274, 589, 661 
 
 Incus, 197 ff. 
 
 Index of refraction, 186 f. 
 
 Indifference point, 346 f., 365 f., 468, 514 ff. 
 
 Indifferent conductor, 284 f. 
 
 Individual differences, 60 ff., 187, 221,232, 
 306, 314 ff., 326, 331 f., 335 f., 338, 368, 
 398, 438, 456, 471 f., 476, 491, 537,542, 
 554 f., 571, 583, 591, 600, 655 
 
 Inertia of the senses, 471 473 ff., 479 
 
 Infancy, mental life in, 658 ff. 
 
 Inhibition, 148, 162 ff., 170 ff., 262, 274, 
 287, 507 f., 511, 532, 539 f., 551, 562, 572, 
 579 f:, 584 ff., 590, 599, 612 f. 
 
 Injury of the brain, 215, 217 ff., 252 ff., 262, 
 274, 644 f., 663; of the cord, 218; of the 
 nerves, 218, 306, 348 f., 400 f. 
 
 Instinct, 146, 542, 550, 600 
 
 Insula, 52 
 
 Intellect, 219, 224, 251, 259 f., 262, 435, 527 
 
 Intelligence of animals, 33, 216 f., 545 ff. 
 
 Intensity of sensations, 300, 302, 353 ff.; 
 of consciousness, 597, 609 f., 650 
 
 Interaction, 640 ff., 654 
 
 Interbrain, 48 ff., 76, 78 f., 83 ff., 91 f., 223 
 248, 250; in different vertebrates, 30 f. 
 
 Interference, 496 f., 534, 540, 551, 570, 
 579 ff., 584 ff.; associative and reproduc- 
 tive, 580, 585; see, also, Inhibition. 
 
 Internal capsule, 52 f., 85 f., 93, 223 
 
 Internal ear, 200 ff., 350 
 
 Interval, 316 f., 320 f., 323 
 
 Introspection, 5, 7 ff., 307, 430, 463, 500, 
 551, 559, 594, 608, 672, 681 
 
 Intuition, 385 
 
 Iris, 183 f., 188 f. 
 
 Irradiation of impulses, 169 
 
 Irritability, 15 f., 64 f., 127, 129 ff., 133, 175, 
 228 f., 539, 545, 585, 587, 612, 615, 619 
 
 Island of Reil, 52, 85 f., 221, 250, 262 
 
 Itch, 347 
 
 JAPANESE music, 320 
 Joint-sense, 182, 349 
 Judgment, 593, 601 ff., 665 f. 
 Jump of the eye, 459 ff. 
 Just noticeable difference, 358, 361 ff., 432; 
 just noticeable interval, 473 ff. 
 
 KEPHALIN, 122 
 
 Kinesthetic sensations, 317, 349 f., 407 ff., 
 
 414, 417, 423, 427, 431 ff., 447 ff., 454 f., 
 
 486 
 Knee jerk, 167, 171 f., 532 f. 
 
 LABYRINTH of the ear, 200 ff., 350 
 
 Language, 252 ff. 
 
 Lapses, 585, 623 
 
 Latent time, 166, 287, 290, 473 ff., 479 
 
 Lateral line in fishes, 27 
 
 Layers, of cortex, 265 ff.; of embryo, 38; 
 of neural tube, 40 ff., 45, 53; of retina 190 f. 
 
 Learning, 542 ff.; as a function of the ner- 
 vous system, 35, 96, 113, 145, 147, 159, 
 216 f., 254, 262 ff., 282, 286 f., 615 ff.; 
 curve of, 548 ff., 556 f., 560 ff., 568, 575; 
 in animals, 544 ff.; in man, 555 ff.; re- 
 lation of consciousness to, 563 f. 
 
 Lecithin, 122 
 
 Le Long, case of, 256 
 
 Lens of the eye, 183 f., 186 ff., 427 
 
 Life, mechanics of, 290, 292 
 
 Limit, physiological, 557; of the scale of 
 intensities, 356 ff., 362, 364 f., 368; of 
 the scale of tones, 314 f.; of the visible 
 spectrum, 328 
 
 Line of regard, 420 
 
 Lipoids of the brain, 118 ff.; function of, 
 119, 280 
 
 Listing's law, 421 f., 459 
 
INDEX OF SUBJECTS 
 
 699 
 
 Lobe, 220 ff.; frontal, 51, 85 f., 220 ff., 227, 
 255, 257, 259 ff., 262 ff., 274; limbic, 222, 
 268; occipital, 85, 92, 220 ff., 246 ff., 
 252 f., 274.; olfactory, 31 f., 50, 158, 219, 
 250; optic, 30, 103; parietal, 220 ff., 
 245, 252 f., 262; pyriform, 95, 250, 268; 
 temporal, 51, 85 f., 220 ff., 249 f., 252 f., 
 257 ff., 262, 272, 274 
 
 Local sign, 384, 388 ff., 394, 398 f., 401, 405 
 ff., 410, 414, 422, 431, 433, 456 f., 464, 614 
 
 Localization of sensations, 384, 390, 467, 
 511, 633, 635; auditory, 392 ff., 492 f.; 
 cutaneous, 348, 396 ff., 406 f., 410; gus- 
 tatory, 392; olfactory, 392; visual, 414 
 
 Localization of functions, 74, 152 ff., 219 ff.; 
 history of, 227 f., 234, 235 f., 255, 257 f., 
 264; in human brain, 238 ff., 298, 614, 
 634; methods of, 87 f., 223, 228 ff., 273 
 
 Locomotion, 155, 157 f., 209, 242, 254 
 
 Logic, 593, 603 
 
 Lustre, 453 
 
 MACULA LTJTEA, 191, 193 f. 
 
 Malleus, 197 ff. 
 
 Mammillary body 76 ff., 86, 95 222 
 
 Mammals, brain of, 29 ff.; intelligence of, 
 547 
 
 Man, nervous system of, 33 ff. ; intelligence 
 of, 549, 553 ff. 
 
 Map of the cortex, 224 ff., 233, 264 
 
 Masson disk, 370 
 
 Materialism, 631, 640 f., 669; criticism of, 
 659, 663, 674 ff. 
 
 Materiality, concept of, 681; inapplicable 
 to mind, 681 
 
 Maturity, 661 f. 
 
 Maze-test, 547 
 
 Mean gradations, 359, 376 
 
 Meaning, as aid to memory, 577, 579; re- 
 call of, 601 
 
 Measurement of sensation, 356 ff., 377 f. 
 
 Mechanism, concept of, as applied to the 
 nervous system, 3, 63 ff., 96, 275 ff., 562; 
 of association, 617 ff.; of attention, 611 ff.; 
 of co-ordination, 159 ff., 622 ff.; of dis- 
 crimination, 403, 624 f.; of habit, 615 ff.; 
 of life, 290, 292; of memory, 615 ff.; of 
 nerve-centres, 159 ff., 286 ff., 660; of per- 
 severation, 587; of thought, 555, 589 f., 
 606, 623 ff.; of trial and error, 551 f.; of 
 varied reaction, 611 ff. 
 
 Median longitudinal bundle, 83 f., 89, 94 
 
 Medulla oblongata, 48, 66, 69, 75, 157 
 
 Medullary sheath, see Myelin 
 
 Membrana tympani, 197 ff. 
 
 Membrane, basilar, 202 ff., 206 ff.; of brain 
 and cord, 68, 70; of Reissner, 202 f., 205; 
 semi-permeable, 14, 119 f.; tectorial, 205; 
 tympanic, 197 ff. 
 
 Memorizing, 542, 566 f., 570, 577 ff.; econ- 
 omy in, 578, 581 f. 
 
 Memory, analysis of, 542 ff. ; associative, in 
 animals, 546; centre for, 274; disturb- 
 ances of, 544; experimental study of, 
 572 ff.; feelings of, 502 f.; neural me- 
 chanics of, 286 f., 615 ff.; span, 574, 577; 
 training of, 566 ff., 570 
 
 Mental work, 291, 507, 509, 537 f., 541, 590; 
 elements, 382; products, 382, 458, 463, 
 
 512, 641, 654; processes, time of, 470 ff., 
 497, 614; life, in embryo, 658; in infancy, 
 658 ff.; in adolescence,/^ 1; in maturity, 
 661; in old age, 662 
 
 Mesencephalic system, 94 f. 
 
 Mesencephalon, 48 
 
 Mesoderm, 38 
 
 Metabolism, in brain, 215, 290 f., 649; in 
 nerve, 135 ff.; in retina, 342 
 
 Metaphysics, its place in psychology, 6, 
 629 f., 669, 686 f. 
 
 Metazoa, 13, 16, 545 ff. 
 
 Metencephalon, 48 
 
 Method, comparative, 13, 88, 216, 219, 231 f.; 
 of expression, 504; of impression, 504; 
 introspective, see Introspection; of lo- 
 calization of functions, 87 f., 223, 228 ff., 
 273; of physical science, 278; of physio- 
 logical psychology, 7, 13, 297 ff.; of psy- 
 chophysics, 357 ff.; of studying animal 
 intelligence, 547 ff.; of studying memory, 
 572 ff. ; of tracing tracts, 87 f . 
 
 Mid-brain, 77 ff., 90, 92, 103, 189; in differ- 
 ent vertebrates, 30; development of, 48 ff. 
 
 Middle ear, 197 ff. 
 
 Mid-dog, 153, 155, 174 
 
 Mind, 2, 10, 468 f., 483, 625; action of, on 
 brain, 642 ff.; dependence of, on brain, 
 215 ff., 643, 671, 685; development of, 
 644 ff.; immateriality of, 681; indepen- 
 dence of, 652; nature of, 385, 632, 652; 
 reality of, 653 f., 668 ff.; relations of, to 
 body, 629 ff.; to brain, 632 ff.; seat of, 
 631 ff.; spatia relations of, 632 ff.; spiritu- 
 ality of, 682 f . ; unity of, 672, 678 ff. 
 
 Mixture, of colors, 330 ff., 340, 343, 452; 
 of cutaneous stimuli, 347 f.; of odors, 308; 
 of tastes, 311 
 
 Modiflability, 35, 217, 286 f., 542 ff., 615 ff. 
 
 Molecules, 275 ff. 
 
 Mollusks, nervous system of, 20 
 
 Monkey, brain of, 33 f., 235 ff.; intelligence 
 of, 547, 549 
 
 Monocular vision, 413 ff., 423, 430 
 
 Motifs of perception, 413 ff., 433, 436, 464 
 
 Motility, 14 ff.; of dendrites, 289 
 
 Motor area, 228, 232, 235 ff., 254, 267 ff. 
 
 Movement, automatic, 485, 496, 533 f., 559, 
 564, 610 f.; compensatory, 155, 209 ff.; 
 expressive, 504 ff., 523 ff., 528 ff., 534, 
 637; forced, 157, 210; ideomotor, 535; 
 involuntary, 534; learned, 145, 147, 159, 
 242, 254 f., 262 ff.; perception of, 211, 
 364, 387 f., 404 f., 433 f., 453; reactive, 
 480 f., 484; reflex, 145 ff.; rhythmical, 
 149, 164 f.; skilled, 242, 254 f., 262 ff.; 
 unconscious, 145; voluntary, 147, 535, 
 538, 645 
 
 Muller-Lyer illusion, 440 ff., 448, 450 f. 
 
 Muscle sense, 317, 349 f., 363 f., 374, 407 ff., 
 440; centre for, 244 f., 253; end-or- 
 gans of, 181 ff., 349; pathway of, 90; 
 theories of, 407 f.; in visual perception, 
 414, 417, 423, 427, 431 ff., 447 ff., 454 f., 
 466 
 
 Muscle spindle, 181 ff. 
 
 Music, feelings aroused by, 521 f., 529 f.; 
 intervals in, 316 f., 320 f., 323, 521; scale 
 of, 316, 320 f. 
 
700 
 
 INDEX OF SUBJECTS 
 
 Myelencephalon, 48 
 
 Myelin, 26, 45, 58, 119 ff.; myelin sheath, 
 
 26, 45, 58, 99 f., 119, 142 f. 
 Myelinization, 58 f., 223 ff., 246 
 Myopia, 187 
 
 NARCOTICS, 120, 287, 289 f. 
 
 Native powers, 146, 386, 419, 457 f., 464, 600 
 
 Nativistic theory, 385, 418 f., 457 f. 
 
 Near-sightedness, 187 
 
 Neopallial system, 96 
 
 Neopallium, 31 ff., 86, 219, 266 
 
 Nerve, 67, 98 ff.; abducens, 55, 87; acces- 
 sory, 56, 87; auditory, 55 f., 82, 87, 90, 
 202; blood supply of, 137; cochlear, 82, 
 202 f.; conduction in, 127 ff., 473; cra- 
 nial, 55 f., 76, 79; development of, 39, 
 42 ff.; excitation of, 129 ff.; facial, 55, 87; 
 function of, 21; glosso-pharyngeal, 55, 87, 
 178; hypoglossal, 56, 87; intermediate, 
 87, 179; oculomotor, 55, 79, 189; ol- 
 factory, 55 f., 79, 106 f., 177, 306; optic, 
 30, 55 f., 79, 191, 193; physiology of, 127 
 ff.; quickness of, 130 ff.; spinal, 42, 66; 
 trigeminus, 27, 55, 79 ff., 87, 90, 178, 306, 
 518; trochlear, 55, 79; vagus, 55, 81, 87, 
 157, 163, 178, 524 f.; vestibular, 82, 95, 
 155 f., 202 f. 
 
 Nerve-cell, 97 ff.; basket, 108 ff.; branches 
 of, 24 f., 41 f., 97. 286; central 23 f., 27 f., 
 33, 65, 67, 159 ff., 165, 167, 241; chem- 
 istry of, 105, 121, 280; in coelenterates, 
 18; of cortex, 265 ff.; development of, 
 41 ff., 57 ff., 97 f., 104; function of, 286 ff.; 
 giant, 240, 268 f., 271; of Golgi type, 97; 
 multipolar, 103; pigment in, 106: Pur- 
 kinje, 101, 103 f., 109 f.; pyramidal, 102, 
 104, 240, 265 ff., 269; of retina, 190 ff.; 
 size of, 103; stellate, 269; unipolar, 43 f.; 
 in worms, 21, 23 
 
 Nerve-centre, 28, 67, 74, 226 ff., 281; aux- 
 iliary, 242; catabolism in, 291; fatigue 
 of, 538 ff.; intellectual, 251, 262; of in- 
 vertebrates, 20; mechanics of, 286 ff.; 
 motor, 235 ff., 253 f.; perceptual, 252 f.; 
 respiratory, 28, 157; sensory, 244 ff.; 
 speech, 255 ff.; of vertebrates, 25 ff. 
 
 Nerve-fibre, 98 ff.; association, 58, 96, 223 f., 
 233, 246, 251 f., 269; catabolism in, 135 ff., 
 283; central, 24, 28, 33, 56; a concentra- 
 tion cell, 142; in cord, 75; a core con- 
 ductor, 140 ff.; in cortex, 266 ff.; elec- 
 trical properties of, 134, 137 ff.; fatigue 
 of, 136; function of, 16, 20, 22, 35, 65, 
 127 ff., 154, 223, 279 ff., 284 ff., 304, 
 473; long and short, 20 f., 25, 27 f., 33 
 56 f., 94; motor, 42, 46, 56, 73, 100, 128, 
 132 f., 151, 154, 212, 279; in nerves, 98 ff. ; 
 nodes of, 99 f.; number of, 75, 101; of 
 olfactory nerve, 26, 56, 107; projection, 
 223, 242, 251; sensory, 22, 26, 73 f., 110, 
 128, 132 f., 176, 178, 182, 191, 203, 285, 
 348 f., 398; size of, 75, 100; structure of, 
 98 ff., 141, 279; unmedullated, 99 f., 128, 
 132, 136 
 
 Nerve impulse, 127 f., 165, 277, 279, 283 ff.; 
 blocking of, 287, 289; in different nerves, 
 284 f.; speed of, 131 f., 473; theories of, 
 135 ff., 143 f., 283 f. 
 
 Nerve-muscle preparation, 128 f. 
 
 Nerve-net, in coelenterates, 18 ff.; in higher 
 animals, 20, 112 
 
 Nervous elements, 97 ff., 279 f. 
 
 Nervous system, of annelids, 22; of coe- 
 lenterates, 17 ff.; of crustaceans and in- 
 sects, 24: development of, 38 ff., 275, 279, 
 298; elements of, 97 ff.; evolution of, 35, 
 298; of flatworms, 20; function of , 13, 16, 
 33, 63 ff., 282 f., 287; fundamental and 
 accessory, 26 ff.; growth of, 57 ff.; a 
 mechanism, 3 f., 63, 96, 275 ff.; of mol- 
 lusks, 20; plan of, 63 ff., 96, 127, 175, 
 280 f.; simplest form of, 17; structure of, 
 63 ff.; types of, 20, 22, 25; of vertebrates, 
 25 ff. 
 
 Neural crest, 39 f., 42 ff. 
 
 Neural groove, 38 
 
 Neural tube, 38 ff., 49 
 
 Neurofibrils, 18, 21 f., 41, 101, 106, 113 f., 265 
 
 Neuroglia, 40 f., 46, 52, 71, 97, 101, 110, 
 114 ff. 
 
 Neurokeratin, 119 
 
 Neurone, 100, 112, 160 
 
 Neurone theory, 98, 112 ff., 286 f. 
 
 Nissl granules, 105 
 
 Nissl stain, 103, 105, 264 
 
 Noises, 312 ff. 
 
 Nonsense syllable, 572, 577, 579 
 
 Nucleus, of the cell, 14, 100, 105, 288; cau- 
 date, 52, 78, 85 f., 223; of cranial nerves, 
 80 ff.; of Deiters, 83, 95; of the dorsal 
 columns, 79 ff.. 89; lenticular, 52, 85 f., 
 223, 259 f.; olivary, 80 ff., 95; pontine, 83, 
 86, 95; red, 84 f., 95, 223; terminal, 93 f. 
 
 Nutrition dependent on exercise, 59, 62, 581, 
 616 f. 
 
 OBJECT of sense, 310 ff., 380 ff., 654 
 
 Object blindness, 252, 263 
 
 Occasionalism, 646 
 
 Odors, classification of, 306 ff. 
 
 Old age, brain- weight in, 61 f.; mental life 
 
 in, 544, 662 
 Olfactometer, 374 
 
 Olfactory bulb, 31, 50, 56, 79, 107, 177 
 Olfactory lobe, 31, 50, 158 
 Olive, 69, 76 f., 79 ff., 86 
 Olive, superior, 90 
 Optic chiasm, 56, 76, 91, 222, 247 
 Optic lobes, 30, 103 
 Organ, concept of, 636 f. 
 Organ of Corti, 203 ff. 
 Orientation, 247, 253 
 Ossicles of the ear, 197 ff. 
 Osmosis in living cells, 14, 119 f. 
 Oral or snout sense, 27, 31 
 Otoliths, 203, 211 
 
 Overlapping, 498 f., 556, 558 ff., 562 
 Over-learning, 572, 576 
 Over-tone, 314, 319 f., 322 f., 369 
 Ovum, development of, 36 ff.; fertilization 
 
 of, 36 f. 
 Oxygen consumption of brain, 125, 215, 291; 
 
 of nerves, 137 
 
 PACINIAN corpuscle, 180 ff. 
 Pain, 179, 181, 245, 302, 344 ff., 348, 477, 
 516 f., 532, 539 f. 
 
INDEX OF SUBJECTS 
 
 701 
 
 Pain spots, 181, 344 ff., 477, 517 
 
 Paired associates, 573, 578 
 
 Pallium, 31, 86, 219; development of, 51 ff. 
 
 Papilla, of skin, 180; of tongue, 178, 310, 372 
 
 Paracentral lobule, 222 
 
 Paradoxical sensation of cold, 346, 348 
 
 Parallax, 427, 429, 458 
 
 Parallelism, 607, 631, 682 
 
 Paralysis, 238 f., 241 f., 245, 254, 263 
 
 Paramecium, behavior of, 545, 612 
 
 Paraphasia, 253, 260 
 
 Paraxon, 98 
 
 Parietal organ, 30 f. 
 
 Path, auditory, 82, 90, 203, 250; branching, 
 159 ff., 168 f., 611 ff.; cerebellar, 95; 
 converging, 161 f.; cutaneous, 89 f.; gus- 
 tatory, 90; kinesthetic, 90; motor, 239, 
 251; olfactory, 92, 107, 250; optic, 91 f., 
 246, 269, 479; reflex, 22, 89, 92; sensory, 
 81, 89 f., 246, 267; of voluntary reactions, 
 478, 485, 487 
 
 Pathological method, 231, 252 ff., 594 
 
 Peduncles, of cerebellum, 78, 82 ff., 86; of 
 cerebrum, 76 ff., 84 f., 223 
 
 Perception. 301 ff., 347, 375 ff., 403, 435, 
 440, 442, 449 ff., 458, 466, 483 f., 593 ff., 
 665, 673; centres for, 245 ff., 249 f., 
 252 ff., 260; of differences, 358 ff., 601 ff.; 
 errors of, 434 ff.; in learning, 551 f.; me- , 
 diate and immediate, 593; of movement/ 
 211, 364, 387 f., 404 f/ 433 f., 467; process 
 of, 594 f.; of relations, 555, 664, f.; selec-/ 
 tion in, 596 ff.; of space, 380 ff., 413 fT; 
 time of, 483 ff., 497 
 
 Perineurium, 99 
 
 Perse verat ion, 586 f., .615 
 
 Personal identity, 678, 680 f. 
 
 Personality, 262, 519, 673 
 
 Perspective, 429 f., 449 
 
 Phenomenal reality, 672, 680 
 
 Phosphene, 325, 417 
 
 Phosphorus of the brain, 121 
 
 Photo-chemical substance, 190 
 
 Photo-chromatic interval, 333 
 
 Phrenology, 227, 232 
 
 Physiological psychology, 1 ff., 13, 297 ff., 
 629; assumptions of, 653; limitations of, 
 469, 625; method of, 7 f., 13, 297 ff., 356, 
 380, 468 
 
 Physiological time, 472 
 
 Physiology, 3 ff.; method of, 7 
 
 Pia mater, 70 f. 
 
 Pineal gland, 78, 634 
 
 Pitch, 206 ff., 313 ff., 490, 492 
 
 Pithecanthropus erectus, 34 f. 
 
 Plane of regard, 421 
 
 Plasticity, 35, 217, 544, 581 
 
 Plateau in the curve of learning, 561 f. 
 
 Pleasantness, 501 ff., 508 ff., 515 ff., 526, 
 532, 552. 564 f., 567, 583 
 
 Plexiform layer, 266, 268, 273 
 
 Plexus in cortex, 267 f., 273 
 
 Poggendorf illusion, 440, 444, 446, 451 
 
 Point of regard, 420, 422 
 
 Pons, 66, 75, 76 ff., 82 ff., 222; development 
 of, 49, 57 
 
 Posture, 157, 162, 173, 209 
 
 Practice, 401 ff., 451 f., 455. 471, 480 f., 488, 
 496, 499, 534, 542, 556 ff., 595 
 
 Practice curve, 548 ff., 556 f . , 660 ff. . 568, 575 f . 
 
 Precuneus, 222 
 
 Pre-established harmony, 646 
 
 Preparation for reaction, 480, 482 ff., 489, 
 491, 493 ff., 590, 602 
 
 Presentations of sense, 301 ff., 380 ff., 413 ff 
 613 f., 659 
 
 Primary position of the eye, 421 f. 
 
 Primates, brain of, 34 f . ; intelligence of 33, 
 547, 549, 551 f. 
 
 Product, concept of, 641 
 
 Projection fibres, 223, 242, 251 
 
 Projection of sensations, 303, 384, 390, 393, 
 411, 414, 633 
 
 Proprioceptors, 154, 156 
 
 Protagon, 123 f. 
 
 Proteid or protein, 14, 118; in brain, 118 f. 
 
 Protopathic sensibility, 349 
 
 Protoplasm, composition, 14 
 
 Protozoa, 13, 544 f. 
 
 Psalterium, 85 f. 
 
 Pseudoscopic vision, 428, 430 
 
 Psychic blindness, 247 ff., 252 f.; deafness, 
 250, 252 f. 
 
 Psycho-analysis, 586 
 
 Psycho-galvanic reaction, 509 
 
 Psychology, 1, 7, 264, 273; comparative, 
 14 ff., 216 f.; experimental, 4, 629, 
 genetic, 381, 655; introspective, 381; phy- 
 siological, see Physiological psychology 
 
 Psycho-physics, 6, 9, 353 ff., 649; methods 
 of, 357 ff. 
 
 Pupil of the eye, 183 f., 188 f. 
 
 Purkinje cell, 101, 103 f., 109 f. 
 
 Purkinje phenomenon, 196, 333 
 
 Purposiveness of reflexes, 153 f., 216, 527 
 
 Pursuit movement of the eye, 460 ff. 
 
 Puzzle test, 555 f., 605 
 
 Puzzle-box test, 548 f., 551, 553 
 
 Pyramids, 69, 76 f., 79^. 
 
 QUADRIGEMINA, 30, 49, 77 f., 83 f., 90, 92, 
 
 95, 189 
 Quality of sensation, 302 ff., 324 ff., 353 ff., 
 
 408, 614 
 Quantity of sensation, 300, 302, 324, 353 ff., 
 
 614 
 
 REACTION, general concept, 15; anticipatory, 
 217; instinctive, 146; learned, 145, 147, 
 545 ff., 569; mental, 583, 594; uncon- 
 scious, 145; varied, 545, 547 f., 550, 555, 
 563, 569, 595, 598, 600, 604 ff.; voluntary, 
 147 
 
 Reaction time, 166, 472 f., 476 ff.; analysis 
 of, 473, 483 ff.; associative, 493 ff.; com- 
 plex, 487; discriminative, 488 ff., 497; 
 experiment in, 481; movement in, 480 f., 
 489; muscular, 485 ff., 489; reduced, 473, 
 483; sensorial, 485 ff., 489, 491; simple, 
 472, 476 ff., 497; to stimuli of different 
 senses, 476 ff.; of different intensities, 
 479 f. 
 
 Readiness, 483, 485 ff., 489, 493, 495 ff., 
 503, 580, 584, 612 
 
 Reading, 252, 257, 461, 498 f., 560, 595 
 
 Reality, conception of, 656, 669 f., 677; of 
 atoms, 675 ff.; of the mind, 653, 656, 662, 
 667, 668 ff. 
 
 \ 
 
702 
 
 INDEX OF SUBJECTS 
 
 Reasoning, 603 ff., 664 ff.; deductive and 
 inductive, 605; mediate and immediate, 
 606 
 
 Rebound after inhibition, 164 f., 173, 599, 
 612 f. 
 
 Recall, 503, 543 f., 550 f., 563, 582 ff.; diffi- 
 culties of, 585 f.; factors determining, 
 583, 600; partial or selective, 582 f., 600 f . ; 
 threshold of, 584 
 
 Receiving station, 246, 248, 250 f., 253, 260, 
 272 
 
 Receptive field, 161, 168 
 
 Receptor, 16, 65, 99, 127, 175 ff., 280, 304, 
 341 ff., 351 f., 375, 477; distance receptor, 
 25, 217; simplest type of, 176 
 
 Receptors, of annelids and arthropods, 25; 
 of jelly-fish, 18; of vertebrates, 27 f. 
 
 Reciprocal innervatipn, 162 
 
 Recitation, value of in memorizing, 582 
 
 Recognition, 543, 590 ff., 665, 678; false, 
 544; feelings of, 590 f.; mediate and im- 
 mediate 592, 593 
 
 Rectus internus, etc., 185 f. 
 
 Reflex, 128, 145 ff.; clasp, 153; compound, - 
 169 ff.; duration of, 166; extensor thrust, 
 
 154, 164, 168, 173; flexion, 153, 161, 164, 
 
 167, 169, 171, 173; force of , 167 f.; local, 
 22 ff., 94, 146, 154; in man, 155; mental 
 influences on, 532 f.; patellar, 167, 171 f., 
 532 f.; postural, 162, 173; protective, 153, 
 
 155, 173, 532; pupillary, 92, 145 f., 161, 
 
 168, 188 f., 248; scratch, 155, 161, 164, 
 167, 172 f.; pinipjs. 146 f., 170; spread of, 
 168 f.; stepping, 154, 164; winking, 164, 
 167; wiping, 154 
 
 Reflex action, 145 ff., 281, 538; cerebral in- 
 fluence on, 158 f., 171 f., 241, 262; char- 
 acteristics of, 159 ff.; fatality of, 173; in- 
 terpretation of, 154; in man, 155; varia- 
 bility of, 174 
 
 Reflex arc or path, 22, 89, 92, 145 f., 154, 
 159 ff. 
 
 Reflex preparation, 151 ff. 
 
 Reflex time, 166 f., 287, 472; reduced, 167 
 
 Refraction in the eye, 183, 186 ff. 
 
 Refractory period, 131, 164 f., 540, 615 
 
 Reinforcement, 170 f., 532 f. 
 
 Relations, perception of, 555, 601 
 
 Relativity, law of, 375 f. 
 
 Relay station, 92 f., 151 
 
 Relief, 501 f., 506 ff., 531 
 
 Relearning, 572, 577 
 
 Reproduction, in protozoa, 14, 36; in met- 
 azoa, 16, 36 f.; of associations, 543, 582 ff. ; 
 see Recall 
 
 Reproductive tendencies, 493 ff., 583 ff., 
 605 f.; selection among, 590 
 
 Reptiles, brain of, 29 ff. ; intelligence of, 547 
 
 Residual defects, 261 
 
 Resistance in gray matter, 287, 289 f. 
 
 Respiration, automatism of, 148; centre for, 
 81, 125, 148, 157, 262; in emotion, 505 ff., 
 525 
 
 Restitution of function, 241 ff., 259 ff., 663 
 
 Restlessness, 501 
 
 Retention, 542 ff., 545 f., 572 ff.; curve of, 
 575; loss of, 546, 574 ff., 585, 616 f.; par- 
 tial, 573; physiology of, 286 f., 615 ff. 
 
 Reticular formation, 79 f. 
 
 Retina, 183 f., 186, 189 ff., 280, 325 f., 438; 
 
 layers of, 190 f.: periphery of, 326, 334; 
 
 projection of, upon cortex, 248 f. 
 Retinal field, 415 ff. 
 Retinal image, 186 f., 189, 382, 415 f., 419, 
 
 453 ff. 
 
 Retinal light, 325, 357, 369, 433, 463 
 Retraction of dendrites, 289, 613 
 Reward and punishment, 547 f. 
 Rhythm as a source of feeling, 529; as an 
 
 aid in memorizing, 578 f., 584 
 Rhythmical movement, 149, 164 f. 
 Right and wrong cases, 359 
 Right-handedness, 263 f. f 565 
 Rivalry, 308, 311, 392 f., 452 f., 598 f. 
 Rods of the retina, 190 ff., 325 f. 
 Roots of the nerves, 40, 42, 44, 47, 50, 56, 
 
 73 f., 134 
 Rotation, sensation of, 210 ff., 350; of the 
 
 eye, 420 ff. 
 
 SACCULE, 201, 203, 208, 211 
 
 Salts of the brain, 118 
 
 Saturation of color, 324, 327, 337 
 
 Saving method, 573 
 
 Scala tympani, 201 ff. 
 
 Scala vestibuli, 201 ff. 
 
 Scale, of colors, 328, 330 f., 344; of ex- 
 tensity, 383; of intensity, 356 f., 362; of 
 tones, 314 ff., 318. 320 f. 
 
 Sclerotic, 182, 184, 194 
 
 Segments of the nervous system, in an- 
 nelids, 22 ff.; in vertebrates, 25, 74 
 
 Selection, neural mechanism of, 623 f.; in 
 the process of learning, 547, 549 ff., 564; 
 in perception, 596; in recall, 590; in 
 thought, 606 
 
 Self, 511, 519, 571, 658 f., 665, 668 ff. 
 
 Semicircular canals, 155, 200 f., 203 f., 208 ff., 
 350 
 
 Sensations, 16, 282, 285, 297 ff., 381 f., 639; 
 classification of, 300; feeling-tone of, 502, 
 512, 515 ff. ; measurement of, 356 ff., 377 f . ; 
 pure, 391, 594, 596, 659; quality of, 
 302 ff., 324 ff., 353 ff., 405, 408; quantity 
 of, 300, 302, 324, 353 ff.; simple, 302 f., 
 327, 341, 351, 380, 391; subjective, 306; 
 unit of, 361, 377 f.; visceral or organic, 
 
 350, 518 f., 525 f. 
 Sensation-circles, 397 ff., 405 
 Sense-organ, 67, 175 ff., 299, 473 ff., 479; 
 
 development, of, 38, 56; see Receptor. 
 Sensitivity, 175, 345, 357, 364 ff., 368 f., 
 
 371 ff.; see also Irritability; differential, 
 
 358 ff., 392, 404, 431 f. 
 Sensorium, 213 f., 284 
 Sensus communis, 214, 350, 518 f. 
 Series of sensations, 356 f., 383, 386 ff. 
 Sheath, medullary or myelin, 26, 45, 58, 99 f.. 
 
 142 f.; primitive, 99 
 Shock of difference, 403 
 Sight, see Vision. 
 Skill, acquisition of, 555 ff., 565 
 Skin, senses of, 179 ff., 244, 344 ff. 
 Sleep, 113, 291 
 Smell, 27, -158, 175 ff., 219, 250 f., 304 ff., 
 
 351, 372 ff., 392, 475, 478 
 Snout sense, 31, 219 
 Solipsism, 668 
 
INDEX OF SUBJECTS 
 
 703 
 
 Somesthetic area, 244 ff., 403 f. 
 
 Soul, 1 f., 10, 215 f., 625, 686; see Mind 
 
 Sounds, 312 ff. 
 
 Space-form, 383, 418 f., 466 
 
 Space-perception, 245, 364, 371, 380 ff., 
 413 ff.; theories of, 385 f., 418 ff. 
 
 Span of attention or apprehension, 560, 
 597; of immediate memory, 574, 577 
 
 Spatial series, 383, 386 ff., 406 f., 419, 422, 
 659 
 
 Spatial quality, 418 f. 
 
 Specialization of organs, 15 f., 18 f., 22, 36, 
 64 f., 175 
 
 Specific energies of nerves, 284 ff., 304, 350 
 ff., 389 f., 418, 420; of parts of the brain, 
 284 f., 614 
 
 Spectrum, 327 ff., 341 
 
 Speech, centres for, 227, 233, 255 ff.; dis- 
 turbances of, 253, 255 ff. 
 
 Spider, learning by, 545 f. 
 
 Spider cell, 114 f. 
 
 Spinal cord, 25 ff., 66, 69 ff., 226, 241; con- 
 sciousness in, 216 f.; development of, 39 
 ff., 59 
 
 Spinal dog, 153 ff. 
 
 Spinal frog, 152 ff. 
 
 Spinal ganglia, 26, 42 ff., 73 f., 103, 106 
 
 Spinal preparation, 151 ff. 
 
 Spirituality, concept of, 682 f. 
 
 Sponge, behavior of, 17 
 
 Spreading of reflexes, 169 
 
 Stable colors, 334 f., 337, 340, 344 
 
 Stains of nervous tissue, 101, 103, 124, 264; 
 fibril, 103, 106, 265; Golgi, 103, 264 f.: 
 Nissl, 103, 105, 264; osmic acid, 87; 
 Weigert, 72, 264 
 
 Staircase effect, 536, 538 
 
 Staircase figure, 594, 618 
 
 Stapedius muscle, 198, 200 
 
 Stapes, 197 ff., 205 
 
 Stentor, behavior of, 544 f., 612 
 
 Stereognosis, 253, 410 
 
 Stereoscope, 427 f., 598 
 
 Stereoscopic vision, 414, 427 f., 430 
 
 Stimulation, method of, 229 f. 
 
 Stimulus, 4, 6, 14 f., 282, 303; adequate, 
 175, 304 f., 307 ff., 311, 324, 327 ff., 332, 
 341, 346 ff., 367; general, 17, 129 f., 175, 
 305, 309 ff., 324 f., 346; mixed, 308, 311, 
 330 ff., 340, 343, 347 f., 389, 452; in re- 
 action time experiments, 471, 476 ff.; 
 and sensation, 357 ff. 
 
 Stirrup, 197 ff., 205 
 
 Striatum, 31, 51, 78 f., 86, 219 
 
 Stripe of Baillarger, 269 
 
 Stripe of Gennari, 269, 271 
 
 Strychnine, 147, 160, 619 
 
 Subconsciousness, 2, 672 f. 
 
 Subcutaneous senses, 181, 348 f., 363 ff. 
 
 Sub-excitation, 485 ff., 580 
 
 Subjective idealism, 668 
 
 Suggestion, 402, 535 
 
 Summation of stimuli, 170 f., 612 
 
 Surface of separation, 14, 65, 119 f., 289 f. 
 
 Surprise, 602 
 
 Suspensory ligament, 184, 188 
 
 Sympathetic system, 66 f., 74, 99; develop- 
 ment of, 44 f.; function of, 150, 189 
 
 Sympathetic vibration, 206 f. 
 
 Synapse, 107, 111 ff., 266, 287 ff., 479, 
 
 609 ff., 619 
 Synthesis, 93, 302 f., 383 f., 391, 407, 410, 
 
 419, 422, 435, 466, 468, 665, 683 
 
 TALBOT'S law, 474 
 
 Taste, 28, 306, 309 ff., 351 f., 372 f., 392, 
 475, 477 f.; centre for, 251; end-organs 
 of, 178 f., 307; pathway of, 00, 178 f. 
 
 Taste-buds, 178 f., 307, 309 ff., 478 
 
 Tectal system, 94 f. 
 
 Tegmentum, 83 ff. 
 
 Telegraphy, learning of, 556 ff., 561, 621 
 
 Telencephalon, 48 
 
 Temperature sense, 179, 181, 245, 344 ff., 
 365 f., 406 f., 477 
 
 Tendon spindle, 182 
 
 Tension, feeling of, 501 f., 506 ff. 
 
 Tension theory of consciousness, 610 f. 
 
 Tensor tympani, 197 f., 200 
 
 Tentorium cerebelli, 68, 70 
 
 Terms of the judgment, 593 
 
 Testimony, reliability of, 589 
 
 Thalamus, 30, 49, 51 ff., 77 f., 84 ff., 90, 
 92 f., 96, 246; in different vertebrates, 30 f. 
 
 Things, 299 f., 303, 381 f., 422, 512, 633, 
 655, 658 ff., 666, 673 f., 676 f., 679 ff. 
 
 Third dimension, perception of, 413 f., 423, 
 426 ff., 449, 458, 465 
 
 Thomson's law, 453 
 
 Thought, 555, 589 f., 593 ff. 
 
 Threshold, of consciousness, 2, 458, 463, 
 671; of difference, 358 ff.; of excitation, 
 357, 478; of recall, 584; of spatial dis- 
 crimination, 326, 396 ff., 565 
 
 Tickle, 347 
 
 Timbre, 314, 318 f., 369, 395 
 
 Time, illusions of, 440; of mental processes, 
 470 ff., 497, 614 
 
 Time-form, 470, 514 
 
 Tone-color, 314, 318 
 
 Tones, 312 ff.; difference, 323; end-organs 
 for, 206 ff.; scale of, 314 ff., 318, 320 f.; 
 simple, 313 
 
 Tonus, 155, 209, 211, 325 
 
 Touch, analysis of, 179, 306, 310, 344 ff.; 
 centre for, 244 ff.; illusions of, 440; in- 
 ertia of, 473 ff., 476; pathway of, 81, 
 89 f.; perceptions of, 391, 396 ff., 466 ff., 
 492 
 
 Touch compasses, 396, 401 
 
 Touch-corpuscle, 180 
 
 Touch-spots, 132, 179 f., 344 ff., 365, 398 ff. 
 
 Tract, 81 ff., cortico-spinal or pyramidal, 47, 
 73, 83 f., 88 f., 93, 232, 239 f., 242, 269; 
 development of, 47; direct cerebellar, 95; 
 list of tracts, 89; methods of tracing, 
 87 f.; naming of, 88 f.; olfactory, 50,79, 
 92; optic, 91. See Path. 
 
 Training, 401 ff., 542, 547 f., 565 ff. 
 
 Transference of training, 402, 550, 563, 
 565 ff., 581, 588, 605 
 
 Treffermethode, 573 
 
 Trial and error, learning by, 550 ff., 582, 
 617, 620; in perception, 594 f. 
 
 Twilight vision, 196, 333, 335, 340 
 
 Twisted cord illusion, 446 f. 
 
 Two-point threshold, 396 ff., 565 
 
 Tympanic membrane, 197 ff., 367 
 
704 
 
 INDEX OF SUBJECTS 
 
 Tympanum, 197 ff. 
 
 Typewriting, learning of, 558 ff., 576 f., 621 
 
 UNCONSCIOUS movements, 145 
 
 Unicellular animal, 13, 14 
 
 Unit, of perception, 442 ff., 597, 623; of 
 reaction, 498, 558 f. t 562, 569, 578 f., 584, 
 621 ff.; of sensation, 361 
 
 Unity, conception of, 670 f., 676; of con- 
 sciousness, 679, 684 ff.; of the mind, 672, 
 678 ff. 
 
 Unpleasantness, 348, 501 ff., 508 ff., 515 ff., 
 526, 532, 552, 564 f., 567, 585 f. 
 
 Utricle, 201, 203, 208, 211 
 
 VARIABILITY, of brain- weight, 60; of per- 
 ception, 359, 376; of reflexes, 174; of 
 reaction time, 476, 481; of voluntary 
 effort, 537 
 
 Varied reaction, 545, 547 f., 550, 555, 563, 
 569, 595, 598, 600, 604 ff., 615; physio- 
 logical theory of, 611 ff., 620 
 
 Ventral horn, 46, 151, 154 
 
 Ventral root, 40 ff. 
 
 Ventricles of the brain, 39, 49, 51, 77 ff., 81, 
 86 
 
 Verbal amnesia, 253, 260 
 
 Vermis, 86 f. 
 
 Vertebrates, nervous system of, 25 ff. 
 
 Vertical-horizontal illusion, 437, 447 ff. 
 
 Vestibular nerve, 82, 95, 155, 203 
 
 Vestibule, 200 ff., 208, 211, 350 
 
 Vicarious function, 242 
 
 Visceral sensation, 350 
 
 Vision, acuity of, 326, 371 f.; aesthetics of, 
 
 530 f.; binocular, 414 f., 423 ff., 436, 458, 
 465; clear, 420, 457, 459 ff.; erect, 453 
 ff.; indirect,- 326, 334, 422, 461; inertia 
 of, 474, 476, 479; monocular, 413 ff., 423, 
 430, 466; peripheral. 326, 334 ff.; sen- 
 sations of, 324 ff.; stereoscopic, 414. 
 See Eye, Color vision. 
 
 Visual area, 246 ff., 285 
 
 Visual perception, 413 ff. 
 
 Visual purple, 125, 195 
 
 Visual sensations, 285, 324 ff., 369 ff. 
 
 Visual size, 430 
 
 Vital force, 279, 291 
 
 Vitreous humor, 183 ff., 187 
 
 WARMING-UP, 536, 538, 545, 615 
 Warmth-spots, 181, 344 ff., 366, 405 
 Weber's law, 361 ff., 432; interpretations of, 
 
 374 ff. 
 
 Weigert stain, 72, 264 
 Weight, perception of, 363 ff., 601 f. 
 Wernicke's region, 257, 259 f. 
 White matter, 45, 53, 74, 101, 118 ff., 223 
 Will, 243, 483 ff., 496, 645, 664, 666; will 
 
 to learn, 582 
 Will-time, 484, 488, 497 
 Word-blindness, 252, 257, 263 
 Word-deafness, 250, 253, 257, 260, 263 
 Writing centre, 254 f., 257, 261 
 
 YELLOW spot of retina, 191, 193 f., 326, 335, 
 371, 465 
 
 ZOLLNER illusion, 444, 446 f. 
 Zones of the retina, 334 ff., 343 
 
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