REESE LIBRARY 
 
 UNIVERSITY OF CALIFORNIA. 
 

 
ELEMENTS 
 
 OF 
 
 PHYSIOLOGICAL PSYCHOLOGY 
 
 A TREATISE OF THE ACTIVITIES AND NATURE 
 OF 'THE MIND 
 
 FROM THE PHYSICAL AND EXPEEIMENTAL POINT OF VIEW 
 
 BY 
 
 GEOEGE T. LADD 
 
 PROFESSOR OF PHILOSOPHY IN YALE UNIVERSITY 
 
 NEW YORK 
 
 CHAKLES SCKIBNEK'S SONS 
 
 1887 ' 
 
QP3, 
 
 BIG 
 
 LIBRARY 
 G 
 
 COPYRIGHT, 1887, BY 
 CHARLES SCRIBNER'S SONS 
 
 TROW'8 
 
 riNG AND BOOKBINDING COMPANY, 
 NEW YORK. 
 
PREFACE. 
 
 THERE can be no doubt that an important movement in psychol- 
 ogy has arisen 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 psychonietry, 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 discus- 
 sion of actual or possible achievements, as well as of welcomed or 
 dreaded revelations of new truth, and proceed at once to the busi- 
 ness 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 al- 
 leged results of Physiological Psychology ; and this will be done at 
 
iv PEEFACE. 
 
 every step with such degree of assurance as 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 seri- 
 ous loss. 
 
 The study of the phenomena of consciousness by the method here 
 proposed necessarily requires some acquaintance with a consider- 
 able 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 com- 
 pelled 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 definiteness and confi- 
 dence 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 ex- 
 ception 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 questions 
 subordinate to, or part of, the main inquiry is very great. These 
 two facts also render the attempt at a general survey of Physiolog- 
 ical 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 rendering 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 
 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- 
 
PREFACE. V 
 
 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, Pro- 
 fessor of Physiology in the Yale Medical School, for valuable as- 
 sistance in that description of the Nervous Mechanism, its structure 
 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. . . 1-14 
 
 PART FIRST. 
 THE NERVOUS MECHANISM. 
 
 CHAPTER I. 
 
 THE ELEMENTS OP THE NERVOUS SYSTEM 17-55 
 
 1-4, General Function of the Nervous System. 5-16, Chem- 
 ical Constitution of the Nervous Elements. 17-30, Structural 
 Form of the Nervous Elements. 31-36, Common Properties of 
 the Nervous Elements. 
 
 CHAPTER II. 
 
 COMBINATION OP THE NERVOUS ELEMENTS INTO A SYSTEM 56-101 
 
 1-3, Threefold Plan of the Nervous System. 4, The Sympa- 
 thetic and Cerebro-spinal Systems. 5, Membranes of Brain and 
 Spinal Cord. 6-12, Structure of the Spinal Cord. 13-14, 
 General Arrangement of the Encephalon. 15, Structure of the 
 Medulla Oblongata. 16, Structure of the Cerebellum. 17, Struct- 
 ure of the Pons Varolii. 18-24, Structure of the Cerebrum. 
 | 25-27, Cortex of the Cerebral Hemispheres. 28-29, Arrange- 
 ment of the Nerve-Tracts. 30, The Cranial and Spinal Nerves. 
 
 CHAPTER III. 
 
 THE NERVES AS CONDUCTORS 102-129 
 
 1-3, General Office of the Nerves. 4, The Nerve-Muscle Ma- 
 chine. 5-8, The Conditions of Neural Action. 9-19, Phenom- 
 ena induced in the Nerves by different Stimuli. 20-23, Electrical 
 and other Processes in the excited Nerve-Stretch. 24-26, Laws 
 
Vlll TABLE OF CONTENTS. 
 
 of the Nerve-Commotion. 27, Speed of the Nerve-Commotion. 
 28, Effect of Section. 29, Nervous Conduction in the Central Or- 
 gans.^ 30-32, Paths of Conduction in the Spinal Cord. 33-35, 
 Paths of Conduction in the Brain. 
 
 CHAPTER IV. 
 
 PAGE 
 
 AUTOMATIC AND REFLEX FUNCTIONS OP THE CENTRAL ORGANS.. 130-162 
 
 1-2, Nature and Kinds of Reflex Action. 3-5, The Spinal 
 Cord as a Central Organ. 6-9, Laws of Spinal Reflexes. 10, 
 Irregular Automatism of the Cord. 11, Centres of the Cord. 12, 
 Excitability of the Cord. 13, Inhibition of the Cord. 14, The 
 Brain as a Central Organ. 15-16, Functions of the Medulla Ob- 
 longata. 17-19, Centres of the Medulla Oblongata. 20, Influ- 
 ence of the Cerebral Lobes. 21, Functions of the Cerebellum. 
 22-27, Functions of the Basal Ganglia. 28, Gray Matter of the 
 Third Ventricle. 
 
 CHAPTER V. 
 
 END-ORGANS OF THE NERVOUS SYSTEM 163-197 
 
 1-2, Characteristics of the End-Organs. 3, The Kinds of End- 
 Organs. ^ 4-5, The End-Organs of Smell. 6-7, The End-Or- 
 gans of Taste. 8-10, The End-Organs of Touch. 11, The End- 
 Organ of Sight. 12-16, Tunics, Media, and Appendages of the 
 Eye. 17, The Mechanism of Accommodation. 18-21, Structure 
 and Functions of the Retina. 22, Photo-Chemistry of Vision. 
 23-26, External and Middle Ear. 27, Structure of the Labyrinth. 
 28, End-Apparatus of the Vestibule. 29, The Organ of Corti. 
 30-31, Problem solved by the Labyrinth. 32, End-Organs of 
 Motion. 
 
 CHAPTER VI. 
 
 THE DEVELOPMENT OF THE NERVOUS MECHANISM 198-213 
 
 1, Nature of Embryonic Life. 2-5, Earliest Development of 
 the Ovum. 6-8, Blastodermic Layers and their Differentia. !)- 
 11, Head-Fold and Brain-Vesicles. 12, Development of Cranial 
 and Spinal Nerves. 13-15, Subsequent Development of the 
 Brain. 16-17, Development of Eye and Ear. 18, Histogenetic 
 Changes in the Embryo. 19, Conclusions. 
 
 CHAPTER VII. 
 
 MECHANICAL THEORY OF THE NERVOUS SYSTEM 214-236 
 
 1, Machine-like Nature of the Body. 2-6, The Nervous Sys- 
 tem as a Mechanism. 7, Summation and Interference in Nerves. 
 8, Evidence from the Electrical Phenomena. 9, Theory of du 
 Bois-Reymond. 10-11, Theory of Hermann. g 12-13, Theory 
 of Wundt. 14-15, General Conclusions as to a Mechanical Theory. 
 
TABLE OF CONTENTS. ix 
 
 PART SECOND. 
 
 CORRELATIONS OF THE NERVOUS MECH- 
 ANISM AND THE MIND. 
 
 CHAPTER I. 
 
 PAGE 
 
 THE LOCALIZATION OF CEREBRAL FUNCTION 239-262 
 
 1-3, Proofs of the Brain's Special Significance. 4-7, The 
 Brain as a Measure of Intelligence. 8, Special Significance of the 
 Cerebral Hemispheres. 9-10, The Question of Localization. 
 11-13, The History of Discovery. 14-16, The Evidence from 
 Experiment. 17, The Evidence from Pathology. 18, The Evi- 
 dence from Anatomy. 19, True Method of Investigation. 
 
 CHAPTER II. 
 
 THE LOCALIZATION OF CEREBRAL FUNCTION [Continued] 263-302 
 
 1-4, Difficulties from Negative Cases. 5-6, Experiments in 
 Stimulation. 7, Experiments in Extirpation. Sj 8-9, Nature of 
 so-called Motor Centres. 10-15, Method and Results of Exner. 
 16, Confirmatory Conclusions from other Sources. 17, The Evi- 
 dence of Histology. 18, Relation of Motion and Sensibility. 19- 
 21, Visual and Auditory Centres of Ferrier and Munk. 22, Ex- 
 ner's Cerehral Field of Vision. 23, Relations between the Retinas 
 and the Cerebrum. 24, Localization of Smell and Taste. 25- 
 27, The Phenomena of Aphasia. 28, Cerebral Lesions in Aphasia. 
 29, Conjectures as to the Frontal Lobes. 30, Negative Conclu- 
 sions of Goltz. 31, Conclusion as to three leading Principles. 
 
 CHAPTER III. 
 
 THE QUALITY OF SENSATIONS 303-324 
 
 1-5, Sensations and Things. 6, The Subjects investigated. 
 7, Specific Energy of the Nerves. 8-11, Sensations of Smell. 
 12-15, Sensations of Taste. 16-17, The Varieties of Sound. 
 18-20, The Pitch of Tones. 21-22, The Composition of 
 Clangs. 23, Analysis of Sounds by the Ear. 
 
 CHAPTER IV. 
 
 THE QUALITY OF SENSATIONS [Continued] 325-355 
 
 1, Analysis of Sensations of Sight. 2-3, The Stimulus of Sight. 
 4, Relation of Quality and Quantity. 5-8, The Different Color- 
 
TABLE OF CONTENTS. 
 
 Tones. 9, The Complementary Colors. 10-13, Conditions of 
 Changes in Color. 14, Phenomena of Contrast. 15-17, Theories 
 of Visual Sensations. 18, Symbolism of Visual Sensations. 19, 
 Sensations of the Skin. 20, The Muscular Sensations. 21/Sen- 
 sations of Pressure. 22-24, Sensations of Temperature. 25, 
 Specific Energy of the Nerves. 
 
 CHAPTER V. 
 
 PAQB 
 
 THE QUANTITY OF SENSATIONS , 356-381 
 
 1-3, Distinction of Variations in Quantity. 4-5, The Meas- 
 urement of Sensations. 6, Nature of the Least Observable Differ- 
 ence. 7, The Determining of the Limits. 8, Methods of Experi- 
 ment. 9, Statement of Weber's Law. 10, Measurement of 
 Sensations of Pressure. 11, Measurement of Sensations of Tem- 
 perature. 12-15, The Intensity of Sounds. 16-18, The In- 
 tensity of Visual Sensations. 19-21, Measurement of Taste and 
 Smell". 22-23, Value of Weber's Law. 
 
 CHAPTER VI. 
 
 THE PRESENTATIONS OF SENSE 382-419 
 
 1-2, Sensations and Things. 3, General Nature of the Pres- 
 entations of Sense. 4-5, Laws of the Synthesis of Sensations. 
 6-7, Nativistic and Empiristic Theories. 8-11, Nature of the 
 Spatial Series. 12-15, The Theory of Local Signs. 16, The 
 Stages of Perception. 17, Perceptions of Smell. 18, Perceptions 
 of Taste. 19-20, Perceptions of Hearing. 21-22, Sense of Lo- 
 cality by the Skin. 23-25, Weber's Sensation-Circles. 26, The 
 Discernment of Motion. 27, Localizing of Temperature-Sensa- 
 tions. 28, Localizing of Muscular Sensations. 29-30, Construc- 
 tion of the Field of Touch. 31, Feelings of Double Contact. 
 
 CHAPTER VII. 
 
 THE PRESENTATIONS OF SENSE [Continued] 420-467 
 
 1, General Principles applied to the Eye. 2, Data or Motifs of 
 Vision. 3-6, Nature of the Primary Retinal Field. 7, Value of 
 the Retinal Elements. 8-9, Motions of the Eye. 10, The Law 
 of Listing. 11, Meridians of the Field of Vision. 12, Effect of 
 Accommodation. 13-16, Single and Double Images. 17, The 
 Fixation of Attention. 18-20, Stereoscopic Vision and Vision of 
 Perspective. Vg 21-23, The Use of Secondary Helps. 24-25, 
 General Office of Experience. 26-28, Judgment of Spatial Exten- 
 sion and Relations. $ 29, Visual Perception of Motion. 30-34, 
 Errors of Sense. 35, Development of Visual Perception. 
 
PHYSIOLOGICAL PSYCHOLOGY. 
 
 ESTTKODUCTIOK 
 
 1. A CLEAB conception of Physiological Psychology requires some 
 special knowledge of the nature and methods of those two sci- 
 ences, the results of whose investigation it endeavors to combine. 
 These sciences are, of course, Psychology and Physiology the latter 
 being understood in a broad way as including also various applica- 
 tions 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 the one science into which both enter 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 particu- 
 larly to define the end desired; the adjective ("physiological") 
 the character of the means which it is proposed especially to em- 
 ploy. Hence "Physiological Psychology " can scarcely claim to be 
 an independent science, or even a definite branch of the science of 
 psychology in general. It is rather to be regarded simply as psychol- 
 ogy approached and studied from a certain the so-called fl physi- 
 ological " side or point of view. It is necessary, then, in the first 
 place, to define what we understand by the science of psychology, 
 and how it is proposed to treat this science as subject to the physi- 
 ological method, and as approached by means of physiological ex- 
 perimentation and researches. 
 
 2. Perhaps the most common definition of psychology, up to the 
 present time, has regarded it as " the science of the human soul." 
 If this definition had always been given, on beginning the pursuit 
 of the science, only in a provisional way, and with the implied or 
 open confession that it is the business of psychology itself to de- 
 monstrate the existence of a particular entity called " the soul," and 
 
2 INTRODUCTION. 
 
 to show how this 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. For example, 
 one writer on the subject (Drbal), at the very commencement of 
 his treatise, asserts that "psychology is the science of the human 
 soul as the real foundation of the spiritual life ; " and another (Erd- 
 mann) declares that " the subject-matter of psychology is the sub- 
 jective spirit," meaning by this term the human soul. Objections 
 have, therefore, been more or less fitly and forcefully urged agninst 
 this definition as ordinarily employed. It has been said that clearly 
 we have no right to assume any such entity as the soul ; and 
 even that a careful study of all the phenomena especially by the 
 experimental and physiological method does not compel or induce 
 us to conclude that such entity exists. It has been claimed, espe- 
 cially of late, that there may be a " psychology without a soul," and, 
 indeed, that this kind of psychology is alone worthy of being con- 
 sidered truly scientific. Further objection to the same definition 
 has been made in other quarters, because it seems to regard the 
 question as settled, whether man has not more than one subject (or 
 "ground") of the manifold phenomena called psychical ; whether, 
 in fact, he may not be the fortunate possessor of both an " animal " 
 and a "rational" soul, etc. It would be aside from the course of 
 our inquiries to consider these objections in detail at this time ; or 
 to state at any length how far we are inclined to agree with them and 
 how far to express dissent. They may all be, for the present, set 
 aside by stating the course of procedure which the study of psy- 
 chology from the physiological point of view seems to us plainly to 
 recommend. 
 
 The satisfactory definition of any science is often one of the latest 
 and most difficult achievements of that science. When such defini- 
 tion is placed at the beginning of an investigation, it must often 
 really include results reached only by going carefully and repeatedly 
 over the entire ground of the science. In all such cases the learner 
 of the science is quite unable fully to comprehend the definition, or 
 to understand the positions upon various disputed questions which 
 it may really involve. In general, then, it is better that the earliest 
 so-called definition should be simply a description of that class of 
 phenomena which it is proposed, as far as possible, to isolate for 
 purposes of inquiry. This remark applies with peculiar force to 
 psychology, both on account of such objections as those mentioned 
 above, and also on account of certain difficulties inherent in the 
 subject itself. Accordingly, it will serve our purpose best to "de- 
 fine " this science simply by ascribing to it a certain more or less 
 
INTRODUCTION. 
 
 definite sphere of phenomena. Thus we shall 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 preferable to con- 
 sciousness, it must be understood as equivalent to consciousness in 
 the broader sense of the latter word. This definition, or rather de- 
 scription, plainly implies an acquaintance experimentally with cer- 
 tain phenomena that cannot, strictly speaking, be defined. These 
 are the phenomena of consciousness ; and one result of all our sub- 
 sequent investigations will be to show us that consciousness and 
 its primary phenomena can never be defined. The definition of 
 psychology need not, however, be understood to imply the real 
 existence of any one entity such as a soul. 
 
 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 implying 
 somewhat which requires us to use a word like these. In all lan- 
 guages, and in the constant everyday use of them all, men in stating 
 and describing the phenomena of their own sentient life employ 
 such terms as "I" and "me," and place in a kind of contrast with 
 them such other terms as "thou" and "he " or "it." Inasmuch 
 as recollection, and the assumption of some kind of continuous 
 personal identity, enter into all their experience, and underlie all 
 their relations with each other and with the physical world which 
 surrounds them, they are compelled to use language implying a 
 permanent subject of the phenomena of consciousness. No one 
 doubts as to his right to ascribe to himself the phenomena of his 
 own consciousness ; and as well to ascribe certain other phenom- 
 ena, which are not attributed to himself as their subject, to other 
 subjects (so-called " persons "), which he supposes to have, each one, 
 a consciousness of his own. No one doubts that this subject is in 
 every case somehow the same with itself from hour to hour and day 
 to day, and even from year to year. In all the earlier part of this 
 treatise the word "mind" will be employed simply as the equiva- 
 lent of the subject (which all language as expressive of universal 
 experience necessarily recognizes) of the phenomena of conscious- 
 ness. In other words, whatever all men inevitably mean by the 
 word "I" (the empirical ego of philosophy), whenever they say / 
 think, or feel, or intend this or that ; and whatever they under- 
 stand others to mean by using similar language thus mueh, and 
 
4 INTRODUCTION. 
 
 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 allusion 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 alli- 
 ances with ethical, social, and religious ideas. In other words, w r e 
 wish to begin and continue as far as possible upon purely scientific 
 grounds. And when, subsequently, these grounds are in part aban- 
 doned for certain fields of rational speculation, we wish to have the 
 connection between the two kept open and unimpeded. 
 
 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 from the " physiological " point of view or method of 
 approach. Remembering the cautions which have already been 
 expressed, we may also say that it is the science of the Immjuynind 
 as investigated by means of its relations ioTthe 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 in its correlated action) of the human physical organism. 
 As studied at present it implies an acquaintance with the fields of 
 gross and special microscopic 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 correla- 
 tions that exist between the structure and functions of the human 
 nervous mechanism and the phenomena of consciousness, and which 
 derives therefrom conclusions as to the laws and nature of the 
 mind. 
 
 4. Physiology is compelled, from its very nature as a physical 
 science, to regard the nervous system as a mechanism. Physiological 
 Psychology, inasmuch as it relies so largely upon physiology for its 
 data and method and points of view, is also required to consider 
 
INTRODUCTION. 5 
 
 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 the mind cannot deter the investigator from as- 
 suming toward it the so-called mechanical point of view. Physiology 
 presents psychology with a description of this nervous substance as 
 a vast and complex system of material molecules, which are acted 
 upon by different forms of the energy of nature outside (external 
 stimuli), and by intimate changes in the contiguous molecules of 
 the other substances of the ftody (internal stimuli) ; and which be- 
 have as they do on account of the influences thus received, as well 
 as on account of their own molecular constitution and arrangement. 
 But all this is the description of a material mechanism. The word 
 "mechanism " is preferable to the word " machine" for describing 
 such a system of interacting molecules as constitute the living ner- 
 vous substance, because we attach to the latter word the mental pict- 
 ure of something which has a certain magnitude and rigidity of 
 parts that act and react upon each other in a palpable way under 
 the ordinary laws of mechanics. A steam-engine is a machine 
 w r hose parts may be seen to push and pull and turn each other after 
 the ordinary fashion of all levers and wheels. But the molecules 
 of the steam, from the activity of which all the motion of the rigid 
 and ponderous parts of this machine is derived, are no less mate- 
 rial and governed by physical law in their changing relations to 
 each other than are the masses of the machine itself. The inter- 
 action of the minute particles of the steam falls more fitly, how- 
 ever, under the conception of mechanism. Indeed, it is only as 
 falling under this general conception that these molecules admit of 
 any scientific treatment at all. Now it is not our purpose to begin 
 the consideration of the human nervous system by debating the 
 question, how completely it falls under the conception of mechan- 
 ism, and whether some other conception be not needed to supple- 
 ment this when the unique relations of this system to the phenomena 
 of the mind are taken into account. Whatever is to be said upon 
 such a question 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 be willing to 
 receive from the proper one of the two sciences which enter into it 
 that conception of the nervous system at which this science has 
 arrived as the result of the most successful modern researches. As 
 far as the nervous 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 com- 
 plex molecular mechanism. We shall, then, receive, in a grateful 
 
6 INTRODUCTION. 
 
 and docile manner, all that the noble science of human physiology 
 has to teach us, under the guidance of the conception of a mech- 
 anism, both directly concerning the manner in which the nervous 
 matter of the human body performs its wonderful functions, and 
 more indirectly concerning the relations in which these functions 
 stand to the phenomena of consciousness. 
 
 5. 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 real 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. The phenomena of the nervous system, like 
 all physical phenomena, consist in changes in the constitution and 
 mutual relation of material masses and molecules. They are, then, 
 of a kind to be related to each other, under the conception of mech- 
 anism, inside of the nervous system and of the entire human body ; 
 and also, outside of the body, to the various forms of physical energy 
 in nature which act upon these masses and molecules. But 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 
 
INTRODUCTION. 7 
 
 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. 
 It might seem that simply to attempt the accomplishment of the 
 task just described should satisfy all legitimate demands. And, 
 indeed, no little protest has of late been made against any attempt 
 on the part of scientific psychology (and how much more when 
 studied from the physiological and experimental point of view) to 
 proceed further than this. All inquirers have been warned, not only 
 against introducing metaphysical assumptions into the beginnings 
 of psychology, but also against allowing any admixture of the two 
 during the investigations pursued by the latter. We have, indeed, 
 just agreed that metaphysical assumptions as to the nature of mind 
 should prejudice as little as possible our statement of psychological 
 facts and laws. But if the warning against so-called " metaphysics " 
 be understood to mean that inquiry must be stopped when the 
 phenomena and their uniform modes of relation have been enume- 
 rated, and that no venture must be made upon any discussions or 
 conclusions regarding the real nature of the subject of them all 
 (the mind), such warning may very well be quietly disregarded. 
 What we are chiefly interested in, on undertaking all psychological 
 investigation, is the real nature the permanent characteristics, the 
 claims to be a substantial existence, a spiritual unity and the ori- 
 gin and destiny of the mind. To assume as little as possible con- 
 cerning all this, at the first, is simply a matter of wise reserve and 
 self-control in the interests of scientific investigation. We feel no 
 hesitancy, however, in announcing our intention, ultimately, to 
 draw whatever conclusions seem to us legitimate and desirable con- 
 cerning many of these so-called "metaphysical" inquiries. Psy- 
 chology no less truly when studied from the physiological and ex- 
 perimental point of view has the undoubted right, and is under 
 the obligation, to contribute as much as possible toward the solu- 
 tion of these inquiries. Nor do observation and wide reading 
 show that the advocates of " psychology without a soul," and freed 
 from all metaphysics, are at all certain to avoid drawing con- 
 clusions, not to say introducing illegitimate assumptions, upon 
 these very same inquiries. In brief, Physiological Psychology has 
 the right, which belongs to it as a science, to introduce whatever 
 conclusions as to the nature of mind follow legitimately from its 
 
8 INTRODUCTION. 
 
 discussions of phenomena and laws. It has even a right to in- 
 dulge in well-founded and reasonable speculation. Such things are 
 not necessarily objectionable when indulged in by any of the more 
 purely physical sciences. Indeed, there is not one of these sciences 
 which would not look comparatively bare and unattractive if wholly 
 stripped of its more or less questionable inferences, its metaphysi- 
 cal assumptions, its guessings, and speculations. 
 
 6. The remarks immediately foregoing serve to indicate what 
 are the principal Divisions of this work. The First Part will 
 consist of a description of the structure and functions of the Ner- 
 vous System. This system will there be considered under the 
 conception of a mechanism, and as far as possible without any 
 direct or indirect reference to the phenomena of consciousness as 
 determined by introspection. The Second Part will describe the 
 various classes of " correlations which exist between the phenomena 
 of the nervous mechanism and mental phenomena. It will also 
 attempt to state what is known of the laws which maintain 
 themselves over these various classes. No attempt will be made, 
 however, to describe and discuss any of the phenomena which may 
 be classed as abnormal, or as consisting (so far as they are psychical) 
 in so-called " disturbances of consciousness," except when reference 
 to such abnormal phenomena is necessary in order to explain those 
 which are called ordinary or normal. The phenomena of insanity, 
 delirium, hypnotism, somnambulism, ecstasy, mind-reading, spir- 
 itualism, and even of sleep and dreaming, will therefore be defi- 
 nitely excluded. The chief reason for such exclusion is to be found 
 in a lack of space, it being difficult even to bring within the limits 
 of a single volume a sufficiently thorough discussion of the more 
 ordinary phenomena with which Physiological Psychology is called 
 upon to deal. 
 
 The various correlations of the mind and the nervous mechanism 
 (of which the Second Part treats) may be conveniently consid- 
 ered under several principal groups or classes. The first of these 
 includes more particularly such relations as can be established be- 
 tween the condition and activity of the supreme nervous centres 
 and the phenomena of conscious sensation and volition. Most of 
 what can be said at present upon this point may be summed up in 
 the discussion of the localization of cerebral function, as taken in 
 connection with the description of the automatic and reflex action of 
 these centres considered as parts of the nervous mechanism. The 
 second class of these correlations covers all the phenomena with 
 which psycho-physics (in the more precise use of the term) attempts 
 to deal. It discusses the relations which exist between the quality, 
 
INTRODUCTION. 9 
 
 quantity, combination, and order of succession in time, of the vari- 
 ous stimuli which act upon the nervous system, and the kind, mag- 
 nitude, composite result, and time-relations of the mental phe- 
 nomena. Hence the significance of the term psycho-physics. As 
 Physiological Psychology is ordinarily and legitimately treated, 
 it includes these more specially psycho-physical researches. An- 
 other class of these correlations covers certain related phenomena 
 of mind and body as dependent upon age, sex, race, etc. 
 
 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 voli- 
 tions controlling the bodily members, and of the higher faculties of 
 memory, association of ideas, etc. The Third Part will fitly intro- 
 duce, at the close of the psycho-physical researches, the presenta- 
 tion of such conclusions as may be legitimately gathered, or more 
 speculatively inferred, concerning the nature (considered as a real 
 being) of the human mind. The justification of the order and ex- 
 tent of the entire discussion, and especially of the Third Part as a 
 whole, has already been given to some extent ; the rest must be 
 left to the progress and result of the discussion itself. 
 
 7. 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 extern al jjbservation 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 
 
10 INTRODUCTION. 
 
 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 
 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. Whatever fault may be found with the so- 
 called introspective method in psychology, on account of its alleged 
 inaccuracy, lack of scientific and progressive quality, etc., from the 
 very nature of the case no other way of ascertaining what the phe- 
 nomena of consciousness in themselves are can ever take the place 
 of the direct examination of consciousness. And there is no way of 
 directly examining consciousness 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 conscious- 
 ness can never be, to any satisfactory extent, accomplished by in- 
 trospection alone. For psychology, in order to make valid its claim 
 to be a science, must not merely display the alleged facts of individ- 
 ual 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 interac- 
 tion with the rest of the world. It is because human physiology can 
 contribute largely to such scientific treatment (as distinguished from 
 the mere observation, grouping, and cataloguing) of the phenomena 
 of the mind that it is entitled to be considered as furnishing one 
 distinctive and fruitful branch of psychological researches. 
 
 8. 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. In general they must be accepted 
 
INTRODUCTION. 11 
 
 by us as contributed from the modern science of human physiology. 
 The primary facts of consciousness must be ascertained from con- 
 sciousness 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 conscious- 
 ness. The principal laws and inferences also of introspective psy- 
 chology may be accepted (at least in a provisional way) on begin- 
 ning the study of Physiological Psychology. The final result of 
 such study will doubtless be, not only to supplement and explain, 
 but also to modify and correct, the statement of these laws and in- 
 ferences. But here, as in other scientific research, we are obliged 
 to work our way through many mistakes, obscurities, and other ob- 
 stacles, progressively nearer the complete and verifiable knowledge 
 of the truth. 
 
 Furthermore, from the nature of the case, Physiological Psychol- 
 ogy takes its point of starting from the facts and laws of physiology 
 as reached by the method of external observation. This follows 
 necessarily from the relation in which the two sciences of physiol- 
 ogy and psychology stand as entering into the proposed combina- 
 tion. The enlargement of our knowledge of the latter is the 
 end to be reached ; but the former is to give us the way by which, 
 and the guidance under which, the approach to this end must be 
 made. 
 
 It will also become evident, in the course of the following inves- 
 tigation, that we are seldom or never able to proceed directly with 
 the work of comparing the immediate physical antecedents or con- 
 sequents of the mental phenomena with these phenomena them- 
 selves, and so of drawing conclusions at once as to the laws by 
 which the two classes of facts are connected. Such immediate an- 
 tecedents and consequents are hid in the inexplorable recesses of 
 the living and moiecularly active brain. It is seldom, indeed, that 
 our direct observation 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 dis- 
 tance 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 lo- 
 cate the nervous elements of the retina upon which the waves fall, 
 can conjecture something as to the chemical changes there produced, 
 
12 INTEODUCTION. 
 
 imd 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 ' conception of psycho-physics is that 
 it treats those "physical activities which are the bearers ( Tr tiger) 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 it will be seen that of 
 such physical activities we have little or no assured knowledge ; al- 
 though 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. 
 
 It follows, then, that Physiological Psychology is, pre-eminently, 
 first experimental and then speculative ; it can never become 
 strictly demonstrative, or even deductive to any considerable ex- 
 tent. That a strictly demonstrative science of the relations be- 
 tween the structure and functions of the nervous mechanism and 
 the phenomena of consciousness is impossible, we might argue from 
 the most ordinary experience. To infer from certain movements 
 of material molecules that certain facts of consciousness must take 
 place, under the most universal laws of all Being, involves a kind 
 and amount of knowledge of which we cannot even clearly conceive. 
 
 In brief, our proper course will be, first, to explain, as completely 
 as possible, the structure and functions of the nervous mechanism ; 
 and then to set forth, as fully as the present means at disposal will 
 permit, the various relations in which its action under stimuli 
 stand to the phenomena of the mind. In attempting the latter 
 problem we shall come upon a few, but only a few, general state- 
 ments of fact which deserve to be spoken of as laws in any strict 
 meaning of the word. 
 
 1 Elements d. Psychophysik, pp. 8 and 10. Leipzig, 1860. 
 
INTRODUCTI01Sr.^H|UFC: -,/ 13 
 
 9. 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 
 discipline at all? The full answer to the call for self-justification 
 must be made by the actual achievements of the science itself. It 
 will be better, then, to leave it to the convictions of the reader when 
 the presentation of these achievements shall have been made. But 
 even at this point an appeal may be taken to certain facts. We 
 have already repeatedly conceded the fact that we are to investigate 
 the phenomena of consciousness (that is, study psychology) by a 
 special method rather than try to establish an independent science 
 or even separate branch of the general science of mind. The de- 
 mand for a justification is then reduced to this Is there valid 
 reason for studying psychology in this particular way ; for approach- 
 ing its domain through the researches and conclusions of physi- 
 ology ? To such a question there can be but one intelligent answer. 
 There is an abundance of valid reason. 
 
 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 treat- 
 ment 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 
 
14 INTRODUCTION. 
 
 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, ' 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. 
 
 1 Art. " Ueber psychopliysiken Metlioden," Pliilosophische Studien, 1881, 
 heft 1, p. 4 
 
PABT FIRST. 
 
 THE NERVOUS MECHANISM 
 
CHAPTER I. 
 THE ELEMENTS OF THE NEEVOUS SYSTEM. 
 
 1. IN all forms of animal life, except the very lowest, the pres- 
 ence and activity of a nervous system constitutes the chief charac- 
 teristic of their difference from all the more nearly corresponding 
 forms of plant life. Both animals and plants are organisms, and 
 their structure regarded as a whole composed of an indefinite 
 number of material masses or particles, which move with reference 
 to each other for the accomplishment of a common piece of work 
 may be considered as a "natural mechanism." Both have mate- 
 rial parts of superior firmness, adapted to divide off and to support 
 their softer parts. Plants, as well as animals, are possessed of liv- 
 ing, and, more especially, of contractile tissue ; they are therefore 
 capable of the functions of nutrition, of propagation, and of that 
 so-called automatic motion which is thought to be a fundamental 
 property of protoplasm. As is well known, science is not yet able 
 always to distinguish between the lowest forms of animal and the 
 lowest forms of plant life. But nervous tissue and its functions 
 do not belong to the vegetable kingdom ; on the contrary, the pos- 
 session and use of at least a rudimentary mechanism of nerve-fibres 
 and nerve-cells are found in most members of the animal kingdom. 
 
 It is true that, even in the case of animals which do possess 
 a nervous system, the structure and functions of the nervous tissue 
 are very closely related to those of the merely contractile tissue. 
 Thus the muscular tissue of the animal might seem to be a connect- 
 ing-link between its own nervous tissue and the contractile tissue 
 of the plant. For the motor nerves, at least, are anatomically con- 
 nected by means of their end-plates with the contractile substance 
 of the muscular fibre, and the result of modern experimentation, 
 with both muscles and nerves, has been to make clear many feat- 
 ures of resemblance between them. On the other hand, even the 
 isolated nervous elements, when subjected to the same experi- 
 mental tests as those which are used to determine the funda- 
 mental properties of contractile tissue, exhibit certain marked 
 differences of behavior ; while the functions of such elements, 
 2 
 
18 FUNCTION OF A NERVOUS MECHANISM. 
 
 when combined into a very simple nervous system, are alto- 
 gether unique. Moreover, as the nervous system of the animal 
 becomes more elaborate and complex, and especially as its central 
 organs spinal cord and brain are relatively developed, other new 
 and wonderful functions are seen to be connected with it. In the 
 case of the superior vertebrate animals, and especially of man, the 
 significance of this particular form of a physical mechanism be- 
 comes, therefore, vastly increased. Thus the minute structure of 
 the nervous mechanism invites the student of chemistry, molecular 
 physics, and histology, to investigations of the greatest interest and 
 yet of extreme difficulty ; while the functions of this mechanism 
 are so curiously and intimately connected with the phenomena, not 
 merely of all higher animal life, but also of human consciousness, 
 that inquiry into them is, among all physical inquiries, the one of 
 unparalleled intellectual interest and importance. 
 
 2. It will be the work of this entire treatise to set forth in some 
 detail the unique functions of the human nervous mechanism, to 
 which allusion has just been made. For the present a very gen- 
 eral and somewhat indefinite statement of these functions must suf- 
 fice. In general, and somewhat indefinitely, it may be said, then, 
 that the one great function of the nervous system is to concatenate (or 
 link together into a whole) the manifold elements, both physical and 
 psycho-physical, which enter into the twofold life of man. Differ- 
 ent and distant parts of the body, whether they belong to the same 
 or to different so-called systems (as, for example, the circulatory, 
 the secretory, the digestive, the muscular), are bound together, and 
 made to exercise their functions in reciprocal dependence and for 
 common ends, by the nervous mechanism. The whole body is also 
 linked to the external world, and kept in either unconscious or con- 
 scious adjustment to the changeful play of its forces, by the same 
 mechanism. And further, the development of the mental life, at 
 least in all its more primitive and fundamental factors, is mediated 
 by the nervous system. For it is certainly in connection with the 
 exercise of nervous functions that sensation takes place ; and, by 
 development and combination of the sensations, all our perceptions 
 of the so-called " Things " of the external world. It is the nervous 
 mechanism which unites the entire body with the physical stimuli 
 of the external world, on the one hand, and, on the other, with the 
 primitive activities of mind. What relation the nervous functions 
 have, and whether they have any direct relation at all, to memory, 
 judgment, and the higher activities of mind in general, we do not 
 now even inquire. 
 
 The significance of the above-mentioned function of " concatena- 
 
EXAMPLES OF EEFLEX ACTION. 19 
 
 tion," so far as it concerns the different and distant parts of the 
 body, might be illustrated in many ways. Inasmuch as the plant 
 is an organism, there is a reciprocal dependence of the structure 
 and action of all its parts. But each part of the plant acts directly 
 and slowly on only contiguous parts in effecting the distribution 
 of the fluids, upon the spread of which the life and growth of the 
 plant depend. In the case of the animal, however, an effect pro- 
 duced in one part of the body may quickly spread to other distant 
 parts by the mediation of the nervous system. The circulation of 
 the blood is made to affect, and to be affected by, the state of the 
 skin and the muscles, the state of the respiratory organs, or the 
 state of the mind's feeling as determined by the ideas before the 
 mind. A draught of cold air, for example, strikes some peripheral 
 portion of the body ; the heart and lungs modify their activities, 
 the muscles contract, and a shudder runs through the physical 
 framework ; the secretions are disturbed, and the mind is, perhaps, 
 seized with a vague feeling of fear. Such a complex effect of the 
 stimulus of cold on some region of the skin has been brought about 
 by the action of the nervous system, with its peripheral end-organs, 
 conducting nerve-fibres, and nervous centres. Or, again, the seeing 
 of some sight or the hearing of some sound is followed by ideas and 
 emotions of shame, or of fear, or of joy. A complex co-ordination 
 of the muscles then takes place, so as to move the limbs in running, 
 to give or ward off a blow, to extend the hand in greeting, to lift 
 up or bow down the head. In this case, also, the action of heart 
 and lungs and secretory organs is greatly modified ; the capillary 
 circulation is altered, and the cheeks are blanched or reddened ; 
 the pupils and lachrymal ducts of the eyes are moved ; the very 
 hair of the head seems to sympathize with the state of the mind. 
 Thus, changes which involve the functions of almost all the tissues 
 and organs of the body are accomplished by the mediation of the 
 nervous mechanism. Unlike the modifications in expression of 
 function which take place in the plant, they are accomplished with 
 what seems a practical instantaneousness. The complexity of the 
 reciprocal changes which characterize the life of the higher animals 
 is due to the general functions of the nervous system ; the speed 
 with which the changes are accomplished is dependent upon the 
 laws of the propagation of nervous impulses within that system. 
 
 Further illustration of this general office of the mechanism of 
 nerve-fibres and nerve-cells in " concatenating " the manifold ele- 
 ments of physical and psycho-physical life may well be left to the 
 progress of our examination. 
 
 3. The application of the term " mechanism " to the nervous sys- 
 
20 FUNCTION OF A NEEVOUS MECHANISM. 
 
 tern of man has already (see p. 4 ff.) been partially explained and 
 justified. We now describe the elementary parts of such a system 
 as considered from the same general point of view which induces 
 us to apply this term to the structure and functions of the entire 
 system. In order to do this, it is necessary to speak, first, of the 
 structure, and, second, of the function of these parts, regarded as the 
 fundamental and distinguishing factors of a complex mechanism. 
 That is to say, two inquiries must be made : What is the composi- 
 tion and form of those ultimate structures called nervous elements, 
 into which microscopic anatomy analyzes the nervous system ? and, 
 What can such structures do which fits them to act as parts of a 
 " mechanism " like that of the nervous system ? It is obvious that 
 the answers to these inquiries lie at the very entrance upon the 
 way toward a complete science of the nervous mechanism. But 
 even if the fullest imaginable answers were already attained, much 
 would remain to be done in order to perfect the science. Histology 
 would still have to inform us precisely how the elements are com- 
 bined into the manifold organs of a system. Physiology would 
 have to discover the laws according to which the functions of the 
 elements are modified, when they act as thus combined. Of course, 
 to know completely the nature, number, and properties of all the 
 individual factors of a mechanical system, and to know also pre- 
 cisely how those factors are combined into the system, as well as 
 how their modes of behavior are affected by such a combination, 
 would be to have a complete science of such system. 
 
 A strictly deductive science of the molecular motion, and con- 
 sequent function of the elements of the nervous mechanism, is, in- 
 deed, a conceivable attainment. But it need scarcely be said that 
 we are indefinitely far from, not only the attainment, but even the 
 reasonable prospect of such a complete physical science of the ner- 
 vous system. None of the questions raised respecting the struct- 
 ure and functions of its elements, whether considered apart or in 
 combination, can be answered with complete satisfaction. More- 
 over, the scientific study and description of the nervous mechanism 
 is compelled from the first to pursue a somewhat different path 
 from that open to many forms of physical science. The direct 
 path to the complete science of the subject is impassable ; it is ren- 
 dered impassable by the most fundamental and universal of our 
 experiences respecting the nature of the phenomena of the nervous 
 system. The immediate effects of the molecular changes which 
 take place in the nervous elements, even when isolated as much as 
 possible, can only with difficulty be made the subject of direct obser- 
 vation. Histology has enormous difficulties to overcome in its effort 
 
STUDY OF NEKVOUS ELEMENTS. 21 
 
 to describe how these elements are combined in the living human 
 body, and physiology has like difficulties in the way of its effort to 
 determine the functions of those organs which are constructed by 
 means of such combination. Only the beginning of a theory which 
 shall correlate that mode of molecular motion which is peculiar to 
 nervous matter with other modes of the motion of matter has yet 
 been made. 
 
 In spite of the foregoing concessions, a careful study of the ele- 
 ments of the nervous system is the indispensable mode of approach 
 to the subject of physiological psychology. It is these elements 
 which, when variously combined, constitute all the organs of the 
 system ; it is they which, when acting in combination, do all the 
 w r ork of the system. 
 
 4. The Elements of the Nervous System of Man, as elements, do 
 not differ in any essential known respect from those of other verte- 
 brate animals. Upon this subject, then, histology with its micro- 
 scope, and physiology with its experimentation, can describe the 
 nerve-fibres and nerve-cells of other animals, and then safely draw 
 certain inferences from them which will apply to the case of man. 
 It is, however, the development of enlarged or of new organs by 
 the combination of these elements, and the development and elab- 
 oration of function as dependent upon such organs, which consti- 
 tute the difference between the nervous system of man and that of 
 the lower animals. It is here that histology meets with its supreme 
 difficulties and its most interesting problems ; it is here that plrysi- 
 ology is most insecure when trying to carry over to the structure and 
 functions of the human nervous mechanism the conclusions which 
 have been reached by experiments upon the lower animals. On 
 the contrary, the nerve-fibres and nerve-cells of these animals are, 
 in most respects, perfectly competent to tell us all we need to know 
 regarding the nerve-fibres and nerve-cells of man. In describing 
 the constitution, structure, and function of the nervous elements, 
 therefore, it will not generally be necessary to pay attention to the 
 specific animal form from which the description is taken. In other 
 words, the discussion of the nervous elements belongs to the most 
 general histology and physiology of the nervous system. 
 
 5. The elements of the nervous mechanism require to be con- 
 sidered in three ways : (1) as respects their chemical constitution ; 
 (2) as respects their formal structure ; (3) as respects their general 
 physiological function. 
 
 6. The Chemistry of the Nervous System is of necessity in an 
 exceedingly unsatisfactory condition. The facts concerning which 
 perfect certainty is attainable are very few in number ; the bearing 
 
22 CHEMISTRY OF NERVOUS ELEMENTS. 
 
 of those facts on our theory of nerve-function is both slight and 
 disputable. Physiological chemistry is in general encompassed 
 with many difficulties. These difficulties are not due simply to the 
 complex constitution of most of the substances with which it has 
 to deal. They are also very largely due to the fact that these sub- 
 stances are products of life ; and living tissue cannot 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 ner- 
 vous 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." 
 
 Moreover, the difficulties which belong to the chemistry of all 
 living tissue are especially great in the case of the nervous tissues. 
 In their natural state the proximate principles which compose these 
 tissues are very complex and unstable compounds. To obtain spe- 
 cific portions or kinds of nervous substance free from foreign ingre- 
 dients as, for example, the axis-cylinder of the nerves, or the rods 
 and cones of the retina is by no means always easy. The analysis 
 of such substance, when once the substance is obtained, is often ex- 
 tremely tedious in respect to process, and doubtful in respect to 
 result. Nevertheless, the principal conclusions, which may be ac- 
 cepted with considerable confidence in their correctness, are as 
 follows : 
 
 7. Nervous Matter is of two kinds, called white or fibrous, and 
 gray or vesicular, which differ not only in color and microscopic 
 structure, but also in specific gravity and chemical constitution. 
 The specific gravity of the white nervous matter is greater than that 
 of the gray. Danilewski l found the specific gravity of the gray 
 matter in man to vary from 1.02927 to 1.03854 ; that of the white 
 matter from 1.03902 to 1.04334. Others (as Bastian, W. Krause, 
 and L. Fischer) calculate the mean specific gravity of the gray mat- 
 ter at about 1.031, of the white at 1.036-1.040. "This difference in 
 the weight of the two is chiefly due to the difference in the relative 
 amount of water and of solids which they contain. Of 100 parts of 
 each, from the brain of the ox, the gray matter was found to be 
 
 1 See Med. Centralbl., xviii., p. 241, as cited by Drechsel, with apparent 
 confidence, in Hermann's Handbuch der Physiologie, V., i., p. 577. Leipzig, 
 1883. 
 
NON-PHOSPHORIZED BODIES IN BRAIN. 
 
 composed of 81.60 parts of water and 18.40 of solids ; the white, 
 of 68.35 of water and 31.65 of solids. The amount of water is also 
 larger in the brain of the young animal than in that of the adult. 
 The brain of the foetus was found by Weisbach to consist of from 
 87.9 to 92.6 parts of water. The amount of water entering into the 
 composition of the different parts of the central nervous system is 
 unequal. The following is a tabulated statement 1 of the facts to 
 which allusion has just been made : 
 
 PROPORTION OF WATER IN ONE HUNDRED PARTS. 
 
 
 Age, 20 to SO. 
 
 Age, 30 to 50. 
 
 Age, 70 to 94. 
 
 White substance of the brain 
 
 69 56 
 
 68 31 
 
 72.61 
 
 Gray substance of the brain . . . 
 
 83 36 
 
 83.60 
 
 84.78 
 
 Cerebellum 
 
 78.83 
 
 77.87 
 
 80.34 
 
 
 73 46 
 
 72.55 
 
 72 74 
 
 
 74.43 
 
 73.25 
 
 73.62 
 
 
 
 
 
 The amount of water varies in the different regions of the spinal 
 cord. Bernhardt found a smaller proportion of water in the cervi- 
 cal (73.05 per cent.) than in the lumbar (76.04) region of the cord. 
 The gray matter also contains more of albumen, lecithin, and lactic 
 acid than the white, and less of cholesterin, fat, and protagon. 
 
 8. Of the solids contained in the matter of the nerve-centres, 
 more than one-half in the gray, and about one-quarter in the white, 
 consist of certain proteid or albuminous substances. Bodies of 
 this general class are the only ones never absent from the active 
 living cells ; they therefore exist in the primordial structures of 
 all vegetable and animal organisms, and occupy a peculiar place 
 among organic proximate principles. Of these proteid substances 
 found in the nerve-centres very little is as yet known. Gamgee a 
 mentions three such substances one soluble in water and probably 
 derived from the gray matter, another a globulin-like body which 
 is dissolved by a ten per cent, solution of common salt, still another 
 a myosin-like body which remains in solution when a ten per cent, 
 salt solution of brain is boiled. 
 
 9. Three other non-phosphorized bodies of different classes 
 from that above mentioned are found in nervous tissues : these are 
 Cholesterin, Neurokeratin, and, more doubtfully, Cerebrin. Cho- 
 
 1 Derived from Weisbach's observations, and found in Gamgee, Physiologi- 
 cal Chemistry of the Animal Body, i. , p. 445. London, 1880. 
 
 2 Physiological Chemistry, i., p. 423 ; see, also, the article of D. Petrowsky, 
 " Zusammensetzung der grauen und der weissen Substanz des Gehirns," Pflii- 
 ger's Archiv, vii., p. 367. 
 
24 CHEMISTEY OF NERVOUS ELEMENTS. 
 
 lesterin is among the most abundant of the constituents of the ner- 
 vous tissues especially of the white matter of the cerebro-spinal 
 axis and of the nerves. It is a "monad alcohol," the only alcohol 
 which occurs in the human body in a free state. On account of its 
 solubility in ether, cold or hot, and in warm alcohol, cholesterin 
 finds its way into both ethereal and alcoholic extracts of the ner- 
 vous tissues. It is a non-nitrogenous body, crystallizing in beau- 
 tiful white crystals, which, when separated pure from its solutions 
 in ether or chloroform, takes the shape of fine needles, and when 
 separated from alcohol takes the shape of rhombic tables. It is sup- 
 posed to exist preformed in the brain. Its formula is C 26 H 44 O + H a O- 
 
 Neurokeratin is most easily derived by treating the medullated 
 nerve-fibres with boiling alcohol and ether, so as to extract the fatty 
 matters of the medullary sheath ; in the place of this sheath there 
 is left, as a kind of irregular framework, a highly refractile sub- 
 stance which resembles the horny matter of epidermis in its power 
 of resistance to chemical agents. This substance is also found in 
 the gray matter of the nerve-centres, and in the retinal epithelial 
 cells and pigment cells of the choroid ; but not in the non-medul- 
 lated nerve-fibres. It contains nitrogen, 2.93 per cent, of sulphur, 
 and leaves 1.6 per cent, of ash. 
 
 Cerebrin was announced by Miiller, in 1858, as a non-phosphorized 
 nitrogenous body, obtained from a precipitate from the brain when 
 pounded up with baryta water to the consistence of thin milk and 
 then boiled. He described it as a loose, white, very light powder, 
 destitute of smell and taste, soluble in boiling alcohol and ether, but 
 insoluble in water. He gave to it the formula C 34 H 33 NO 6 . Thudi- 
 chum believes that brain matter contains a class of nitrogenous bod- 
 ies free from phosphorus, to which he gives the name of "cerebrins." 
 Gamgee, however, thinks it very unlikely that a body produced, 
 like Miiller's cerebrin, "by the prolonged action of a solution of 
 boiling barium hydrate on so complex an organic mixture as brain 
 should be a definite proximate principle of the unaltered organ ;" j 
 but the same authority admits 2 that the precipitate which sepa- 
 rates itself from an alcoholic solution of brain contains, besides 
 cholesterin, protagon, and the so-called lecithins, "a body for 
 which we may retain the name of cerebrin." 
 
 Nuclein was discovered by Miescher in the nuclei of pus-corpus- 
 cles and in the yellow corpuscles of yolk of egg. Other observers 
 subsequently obtained it from various other substances, especially 
 from the nuclei of the red blood-corpuscles of birds and amphibia. 
 
 1 Physiological Chemistry, i., p. "439. 
 'Ibid., i.,p. 433. 
 
PHOSPHORIZED BODIES IN BRAIN. 25 
 
 Von Jaksch l thinks he has discovered miclein in the human brain. 
 His claim seems to be credited by Drechsel. 2 Its formula is given 
 as C 29 H 49 N 9 P 3 O 22 . But the very existence of nuclein, as a definite 
 body, has been denied by chemists like Worm-M filler and Gam- 
 gee ; and the analyses of Von Jaksch do not agree with those ob- 
 tained from other sources than the substance of the human brain. 
 The whole question of nuclein must then be left in doubt. 
 
 10. No other substances found in the nervous system are, how- 
 ever, both so interesting and so difficult to consider, from the mixed 
 chemical and psycho-physical point of view, as certain complex phos- 
 phorized fats. The entire progress of our inquiry will make it obvi- 
 ous how inadequate and misleading is the use often made of such 
 off-hand remarks as the celebrated dictum : " No thought without 
 phosphorus." Yet it is doubtless true that the highly elaborate and 
 unstable compounds containing phosphorus, which enter into the 
 composition of nervous matter, have a significance for physiological 
 and psychological researches such as belongs to no other material 
 bodies. These complex bodies are especially characteristic of the 
 centres of the nervous system. The strife of discovery and of con- 
 firmatory experiment has been chiefly carried on over the following 
 three : Protagon, Lecithin, and Cerebrin. Of these three, however, 
 probably only the two former are phosphorized bodies. The main 
 question involved in controversy concerns the relation in which leci- 
 thin and cerebrin stand to protagon. Is protagon a definite prox- 
 imate principle of the brain, and are lecithin and cerebrin bodies of 
 ill-defined properties and doubtful claim to existence as proximate 
 principles of the brain ? or, are lecithin and cerebrin definite prox- 
 imate principles, and is protagon a mechanical admixture of the 
 two ? The latter view of protagon has been held by Diaconow, 
 Hoppe-Seyler, and Thudichum ; on the contrary, its claims to the 
 position of the " only well-characterized phosphorized proximate 
 principle " of the brain as yet discovered have been defended (and, 
 it may be said, apparently established) by the researches of Gam- 
 gee and others. 
 
 Protagon was discovered, as a new proximate principle that can 
 be separated from the brain, in 1864, by Dr. Oscar Liebreich ; his 
 discovery was announced in a paper 3 published in 1865. This in- 
 vestigator gave to this substance the name which it still bears, as 
 
 1 See article " Ueber das Vorkommen von Nuclein im Menschengehirn," 
 Pfliiger's Archiv, xiii., p. 469. 
 
 2 In Hermann's Handb. d. Physiol., V., i., p. 578. 
 
 3 "Ueber die chemische Beschaffenheit der Gehirnsubstanz." Annalen der 
 Chemie und Pharmacie, cxxxiv., pp. 29-44. 
 
26 CHEMISTEY OF NERVOUS ELEMENTS. 
 
 in his opinion the first to be definitely ascertained among the spe- 
 cific constituents of the brain (Trpwrayos, leading the van). He as- 
 signed to it the formula C U6 H 241 N 4 O 22 P. In spite of subsequent 
 denials and disproofs of its existence, the extremely careful and 
 often-repeated researches of Gamgee l and Blankenhorn have cor- 
 roborated the discovery of Liebreich. The process by which pro- 
 tagon is obtained from the brain may be thus briefly described (the 
 description will serve to illustrate in general the processes of physi- 
 ological chemistry) : Perfectly fresh ox's brains are freed from the 
 blood and membranes, and are then digested for about a day in 
 eighty-five per cent, alcohol ; from this fluid, when filtered, a quan- 
 tity of white flocculent precipitate is obtained, and the cholesterin 
 and other bodies soluble in ether are dissolved out ; from the sub- 
 stance 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 crystallized is protagon. It is con- 
 sidered by some chemists the easiest to obtain, and indeed the only 
 very well-established phosphorized proximate principle of the brain. 
 Such a material substance is indeed a long way removed from the 
 living nervous mass, with its capacity for exercising such marvel- 
 lous physiological and psycho-physical functions. But it is the best 
 representative that chemistry can as yet present of a scientific result 
 upon which to base any attempt to point out definite relations be- 
 tween psychical activities and the chemical constitution of those 
 complex phosphorized fats which exist in the central nervous 
 mechanism. The empirical formula of protagon, as given by Gam- 
 gee, is C 160 H 30S N & PO 35 . It has been made highly probable that pro- 
 tagon cannot be a compound or mixture of cerebrin and lecithin ; it 
 may, then, be announced as a proximate principle of the brain. 
 
 Lecithin is an organic phosphorized compound which exists in 
 large quantities in ova, spermatozoa, etc., as well as in the nervous 
 tissues. It is described as a yellowish-white, w r axy, very hygro- 
 scopic solid, which in thin layers shines with a silky lustre. It is 
 soluble in ether and alcohol ; on being stirred with water it forms a 
 starch-like solution difficult to filter. Diaconow assigns to it the for- 
 mula C 44 H 90 NPO 9 H-H 2 O. Gamgee supposes that the lecithin of Dia- 
 conow is only one of a group of similar bodies which possess a higher 
 percentage of phosphorus than protagon, and the "general smeary 
 characters " of lecithin. We may, then, speak of " the lecithins " as 
 highly phosphorized compounds existing in the matter of the brain. 
 
 1 See his Physiological Chemistry, i., pp. 425-429 ; and article in the Jour- 
 nal of Physiology, ii., pp. 113-131. 
 

 INORGANIC BODIES IN BRAIN. 27 
 
 The various products of the decomposition of protagon and leci- 
 thin it is not necessary to describe. Neurin is the only one of 
 these products which deserves for our purpose even to be named. 
 It may be obtained from either protagon or lecithin. Dr. Thudi- 
 chum's elaborate "Researches on the Chemical Constitution of the 
 Brain " 1 conclude that at least three well-defined groups of phos- 
 phorized bodies may be separated from the brain ; these are dis- 
 tinguished as (1) kephalins, (2) myelins, (3) lecithins. The exist- 
 ence of a group of bodies which may be termed " lecithins " has just 
 above been declared probable. Thudichum thinks that all these 
 bodies contain phosphoric acid combined proximately with glyce- 
 rin, but " differ in the manner in which they contain the nitrogen 
 and the acid radicles which constitute the great bulk of their sub- 
 stance." The researches of Dr. Thudichum still await confirmation. 
 
 11. In addition to the substances already mentioned, the brain 
 contains certain extractive matters which are found also in other 
 tissues, especially in muscle. Among these bodies are creatin, 
 inosite, xanthin, and lactic acids. 
 
 12. The brain also contains an extremely small amount of inor- 
 ganic matters so small, indeed, that few facts can be relied on 
 concerning it. The estimates of this amount vary from 0.1 to 1 
 per cent, of the fresh brain. Among such inorganic matters are 
 alkaline phosphates and sulphates, chalk, magnesia, oxide of iron, 
 etc. It is said that the ash of the gray matter has an alkaline reac- 
 tion, that of the white matter an acid reaction, 2 and that the reac- 
 tion of the former during life is acid, while that of the latter is 
 neutral or weak alkaline. 
 
 13. All quantitative analyses of the brain are exceedingly doubt- 
 ful ; the older results are wholly worthless. The following table of 
 Petrowsky, 3 which gives the chief organic constituents of the brain 
 of the ox, is an object of interest rather than of complete confidence : 
 
 Substance. 
 
 Gray Matter. 
 
 White Matter. 
 
 Albumen and gelatin 
 
 55.37+ per 
 0.53+ 
 
 17.24+ 
 18.68 + 
 6.71 + 
 1.45 + 
 
 cent. 
 
 24. 72+ per 
 9.55- 
 9.90+ 
 51.91- 
 3.34+ 
 0.57+ 
 
 cent. 
 
 Cerebrin 
 
 Lecithin 
 
 Cholesterin and fats 
 
 
 Salts 
 
 
 1 Reports of Medical Officer of the Privy Council and Local Government 
 Board, 1874, pp. 113 ff. 
 
 3 See Gamgee, Physiological Chemistry, i., p. 445; Hermann, Handb. d. 
 Physiol., V., i., p. 577. 
 
 3 u Zusammensetzung der grauen und der weissen Substanz des Gehirns," 
 Pfliiger's Archiv, vii., p. 367. 
 
28 CHEMISTRY OF NERVOUS ELEMENTS. 
 
 14. The specific chemistry of the elements of the nervous sys- 
 tem, or of the various parts of such elements which histological 
 science reveals, is yet more meagre and doubtful than its general 
 chemistry. The micro-chemistry of the nerve-cells tells us simply 
 that they are in the main protoplasmic, and therefore rich in pro- 
 *teid substances ; and since an analysis of the two kinds of nervous 
 matter shows that the gray is much the poorer in complex phos- 
 phorized constituents and in cholesterin, we conclude that the cells 
 which enter so largely into the gray matter are also poor in the 
 same substances. The different structural parts of the nerve-fibres 
 have doubtless a different chemical constitution. This is proved 
 by the difference in their appearance under the microscope, by the 
 different action of reagents upon them, and, to some extent, by 
 chemical analysis. The neurilemrna, or membranous envelope of 
 the nerve-fibres, like the sarcolemma, on prolonged boiling, yields 
 gelatin. The axis-cylinder appears to be a mixture of proteid with 
 complex fat-like bodies. The w r hite substance of Schwann is rich 
 in complex phosphorized fats, in cholesterin, and in the so-called 
 cerebrins. 
 
 The researches of Ktihne l and others for the most part his 
 pupils have developed certain interesting results with respect to 
 the chemical constitution and chemical^ changesof the nervous tis- 
 sue of the eye. Many of the various non-nervous parts of the ear 
 and the eye have been carefully analyzed. The extremely small 
 amount of such material which is obtainable for chemical analysis 
 is one reason why so little can be known concerning the chemical 
 constitution of the substance of the retina. It is said to have an 
 acid reaction. It is a fair surmise, on general grounds, that it con- 
 tains the same bodies as the central nervous system. The two seg- 
 ments into which the rods and cones break up exhibit marked dif- 
 ferences in their chemical as well as optical characters. The inner 
 segments are composed of protoplasm of "a marvellous transpar- 
 ency." The outer limbs of the rods have an external envelope 
 which agrees in its physical characters with neurokeratin. This 
 envelope encloses a mixture of proteid bodies and of other sub- 
 stances similar to those found in the other nervous tissues. 
 
 15. If knowledge of the chemical constitution of the nervous 
 system is so far behind what we could wish, knowledge of the 
 chemical processes and chemical changes which are connected with 
 the physiological functions of this system must be declared to be 
 
 1 For a list of papers by Kuhne and his pupils on this subject, see Gamgee, 
 Physiological Chemistry, i., p. 462 1; and for an account by him of his re- 
 searches and their results, see Hermann, Handb. d. PhysioL, L, L, pp. 235 ff. 
 
IXTRA-MOLECULAE OXTG1 20 
 
 almost wholly wanting. Even the beginnings of scientific general 
 statements, or fates, respecting the relations between the chemical 
 constitution of the nervous system and its various physiological 
 activities have yet to be made. Different investigators will doubt- 
 less differ as to the prospect for such discovery in the future. 
 When chemistry can deduce the molecular behavior of the most 
 highly complex chemical compound from the nature and number 
 of its component chemical elements, and physiology can definitely 
 connect all the physiological functions of nervous matter with the 
 molecular motions of its chemical constituents, we shall have the 
 means for a strictly scientific solution of such problems. 
 
 16. It need scarcely be said that we have little knowledge re- 
 specting the relation which exists between the chemical constitu- 
 tion and chemical processes of the nervous system, on the one hand, 
 and, on the other, the phenomena of so-called mind. 
 
 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 organiza- 
 tion and chemically sensitive constitution of this mechanism are 
 beyond doubt related to all its distinctive f unctions, lake every 
 other natural 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 consti- 
 tution a large amount of disposable energy ; this energy it yields 
 readily when the equilibrium of its molecules is in any way dis- 
 turbed. Within certain limits, it explodes with increasing surren- 
 der of its disposable energy as the number and intensity of the 
 demands upon it are increased very much as would a gun which 
 should be arranged so as to go off with greater energy as the press- 
 ure 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- 
 
30 FOKM OF THE NERVOUS ELEMENTS. 
 
 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 will undoubtedly greatly enlarge our knowl- 
 edge of those facts of relation which exist between the chemical 
 constitution and changes of the nervous mechanism and the phe- 
 nomena of psychical life. Perhaps such particular statements of 
 fact may be gathered into such more general statements of fact, 
 verifiable by experiment, as we consider sufficient to constitute 
 scientifically established laws. But why certain chemical constitu- 
 ents, when combined and changed in definite fashion, should be 
 specifically connected with certain conscious experiences will always 
 remain an unanswerable inquiry. 
 
 17. From the chemical constitution of the elements of the ner- 
 vous system we now pass to their Structural Form. The science 
 which must be our guide is no longer chemistry, but microscopic 
 anatomy, or histology; this science furnishes us with a large amount 
 of trustworthy information mingled with a still larger amount of 
 conjecture and doubt. Even the number of those elements upon 
 which histology is entitled to focus its microscope and declare that 
 such, and no others, are capable of performing distinctively ner- 
 vous functions can scarcely be said, as yet, to be placed beyond all 
 doubt ; neither can it be claimed that the microscope has yet dem- 
 onstrated the ultimate structure of those two species of such ele- 
 ments the reality of whose nervous functions is beyond doubt. 
 
 It is customary to speak of nerve-fibres and ganglion-cells as the 
 only structural elements of the nervous system. If, however, by the 
 term " ganglion -cell " we intend only such bodies as histology usu- 
 ally describes under this type (for example, the so-called motor 
 ganglion-cells of the spinal cord) the limitation is without sufficient 
 warrant. For there are many cells, which almost certainly have 
 some significance as parts of the nervous system, that differ in 
 form very widely from the typical ganglion-cell. Moreover, by such 
 
CORPUSCLES OF DOUBTFUL CHARACTER. 31 
 
 an off-hand twofold division the important question is often silently 
 passed by : What is the significance for the nervous functions of that 
 diffuse, finely granular substance, found in considerable quantity in 
 the great nerve-centres, and called neuroglia, or nerve-cement (Ner- 
 ven-kitt ; Kittsubstanz) ? This substance is most frequently classed 
 with the connective tissue; but, according to Henle, 1 "it is at all 
 events to be distinguished from connective tissue on account of its 
 chemical properties." That certain microscopic forms of so-called 
 "neuroglia" are largely unlike other forms recognized as being 
 nerve-cells beyond doubt cannot be argued in proof of its ina- 
 bility to perform any of the strictly nervous functions, except upon 
 the basis of the assumption that we already know beyond reasonable 
 question what are all the elementary structural forms of true ner- 
 vous matter. But, as says Eckhard, 2 " if we start the inquiry, what 
 formal elements of the brain and spinal cord take part in the activi- 
 ties of these (the nervous) organs, and in what way do they take 
 part, we are able to give to it only a very unsatisfactory answer." 
 We are not in a position, then, either to affirm or to deny abso- 
 lutely the claim sometimes set up for the neuroglia, that it con- 
 tains true nervous elements. 
 
 It is best to recur to the facts which microscopic anatomy dis- 
 closes as a basis for classifying the different structural elements of 
 the nervous system. These may be briefly described as follows : * 
 As to the true nervous character of fibres of various kinds, not only 
 a$ conducting bands between the nervous centres and the peripheral 
 parts of the body, but also within the substance of these centres, 
 there is no dispute. Nerve-fibres undoubtedly constitute one of 
 the structural elements of the nervous mechanism. Besides the 
 nerve-fibres, histology distinguishes in the gray substance of the 
 nervous centres where all the structural elements of the nervous 
 system are to be found in greatest abundance and variety three 
 other species of structural form. Such are (a) certain cells, known 
 more particularly as the " ganglionic nerve-cells." These cells (to 
 be described more minutely hereafter) are irregular masses of finely 
 granular protoplasm, possessed of a clear nucleus and one or more 
 nucleoli, and sending off one or more processes. 
 
 (b) Corpuscles, consisting either of naked nuclei or of nuclei 
 with only a small amount of surrounding protoplasm, and having 
 various shapes sometimes difficult to make out, are also found 
 abundantly in the gray matter of certain nervous centres. Such 
 
 1 Anatomie des Menschen. Text, p. 306. Braunschweig, 1880. 
 
 2 Hermann, Handb. d. Physiol , II., ii., p. 15. 
 
 3 Comp. Henle, Anatomic des Meuschen. Text, p. 306. 
 
32 FORM OF THE NERVOUS ELEMENTS. 
 
 bodies are usually much smaller than the cells of undoubted ner- 
 vous character described above, many of them being scarcely more 
 than -g-gL-^ - -joVo, or even -5-5^0 f an i ncu * n diameter. Some of 
 them, like the typical ganglionic cell, give off processes which are 
 thought to be continuous with nerve-fibres. It is altogether prob- 
 able that these cells of the second class differ only in their dimen- 
 sions from the cells of the first class. In some places (for example, 
 in the cortex of the cerebrum, or large brain) they appear to have 
 the characteristics of transitional forms between the undeveloped 
 granules of the same class and the more highly developed ganglion- 
 cells. In other places (as in the cerebellum) they form independ- 
 ent layers. They may be described as nuclei "invested by only 
 a small quantity of cell-substance." 1 Some are multipolar, some 
 bipolar, some unipolar. Admitting, as we seem compelled to do on 
 account of their resemblance to the typical form of the ganglionic 
 nerve-cell, that some of these cells are true nervous elements, it is 
 impossible for histology to draw the line through the entire class, 
 and so to affirm beyond doubt that any of them are without claim 
 to be counted among such elements. 
 
 (c) The diffuse, finely granular substance, already referred to as 
 so-called "neuroglia," which fills in the gaps between the nerve- 
 fibres and the cells of the two preceding classes, constitutes the other 
 form of matter observed in the nervous" centres. It exists in quan- 
 tity large enough to form an essential constituent of some locali- 
 ties of the brain and spinal cord. It is not always clear, however, 
 to what this appearance of granular or molecular matter, in which 
 the nerve-cells seem embedded, is due. According to some author- 
 ities, it may result from a confused interlacement of fine nerve- 
 fibrils and fine ramifications of nerve-cells ; or from the crushing 
 of such nervous matter in the process of examination. 2 The neu- 
 roglia itself is described as a delicate reticulum, or network, in 
 which certain small cells (neuroglia-cells) supposed to belong to 
 the sustentacular tissue, and other more conspicuous cells, usually 
 stellate in section (" cells of Deiters "), are found. 
 
 18. Of the three foregoing kinds of structural forms found in 
 the gray nervous matter, it is perhaps safest to class the first two 
 together under the term "nerve-cells." We should then have to 
 remember how greatly these vary in size and formation all the 
 way from the naked, or almost naked, nucleus to the large ganglion- 
 cell with its many processes and complex microscopic structure. 
 
 1 See Max Schultze in Strieker, Human and Comparative Histology, i., p. 
 183. London, 1870. 
 
 8 See Quain's Elements of Anatomy, ii., p. 149. London, 1882. 
 
STRUCTURE OF A NERVE. 
 
 33 
 
 The last of the three (neuroglia) may then be regarded as a susten- 
 tacular tissue ; though with the confession that in the brain and 
 spinal cord it is by no means always easy to distinguish susten- 
 tacular from true nervous tissue. 1 
 
 Of the structures known as nerve-fibres and nerve-cells, his- 
 tology enables us to give a further more minute, if not a com- 
 plete, description ; it also excites our interest by making it possi- 
 ble to conjecture what is the regular anatomical relation between 
 the two. When combined with physiological researches, histology 
 also enables us to make considerable progress toward distinguish- 
 ing the separate as well as the combined functions of these ele- 
 ments. We consider, then, with particular detail, the structure and 
 functions of nerve-fibres and ganglionic nerve-cells. 
 
 19. What is ordinarily called a nerve appears to the naked eye, 
 when dissected from an animal, as a cord of a whitish or grayish 
 
 ep 
 
 pn> 
 
 FIG. 1. Cross-section of the Sciatic Nerve of Man. 8 /j. (After Key and Retzius.) The left lower 
 half is schematic, ra, n, Bundles of nerve-fibres, surrounded by pn, pn, the perineurium ; be- 
 tween them appears the connective tissue, epineurium (ep, ep), and adipose substance (ad). 
 
 color, and of uniform structure. The nerve is really, however, one 
 or more bundles, or fascicles, of a larger or smaller size, bound to- 
 gether by connective tissue. Accordingly, on following it toward 
 its peripheral termination we find that it divides and subdivides 
 until its subdivisions consist of a single nervous element called a 
 
 1 Comp. Ranvier, Traite Technique d'Histologie, i., p. 717. 
 3 
 
 Paris, 1875. 
 
34 FORM OF THE NERVOUS ELEMENTS. 
 
 Nerve-fibre. The bundles have a special sheath formed of con- 
 nective tissue (neurilemma, or perineurium), which in the finest 
 branches becomes reduced to a single layer of cells cemented to- 
 gether edge to edge, and is called the "sheath of Henle." On fol- 
 lowing the fibres backward again toward the central organs, it is 
 found that several of them are bound together to form a nerve- 
 fascicle ; a small amount of fibrillar connective tissue appears be- 
 tween the several fibres within the same sheath ; the character of 
 the sheath itself is changed, and it becomes attached to surround- 
 ing structures by a layer of connective tissue. It is the fibres into 
 which the nerves break up on being followed toward their periph- 
 eral terminations, or by junction of which, successively, they are 
 composed on being followed toward their central termination, that 
 are to be considered as the true elements of the nervous system. 
 
 20. Such nerve-fibres as compose the nerves which stretch from 
 the central organs to the peripheral parts of vertebrate animals 
 may readily be divided into two classes : one called medullated 
 fibres or nerve-tubes, and the other non-medullated fibres or fibres 
 of Eeinak. Nerves in which there is a large proportion of medul- 
 lated fibres have a characteristic white or watery appearance ; those 
 in which only non-medullated fibres, or only a few medullated fibres, 
 exist are grayish and slightly translucent. Vertebrates alone have 
 the former. The medullated nerve-tubes belong particularly to 
 the cerebro-spinal system, and are, therefore, of prime interest in 
 psycho-physical researches ; the fibres of Remak are very abundant 
 in all the nerves belonging to the sympathetic system. This two- 
 fold division of nerve-fibres, while admitting of easy application to 
 the constituent elements of the nerve-trunks, becomes more diffi- 
 cult when we attempt to carry it out within the complex nervous 
 matter of the central organs. Here Max Schultze 1 points out sev- 
 eral varieties of nerve-fibres. There are, first, those "very fine 
 threads which lie on the extreme verge of microscopic mensura- 
 tion," and which require an enlargement of from 500 to 800 diame- 
 ters in order to be made visible. In such fibres no internal struct- 
 ure can be detected by the microscope. To these Schultze gives 
 the name of " primitive nerve-fibrils." Second : certain very deli- 
 cate transparent fibres of albuminous composition, and distinguished 
 from the foregoing by their greater size and their manifest fibrillar 
 structure, are found in the central organs. These are the so-called 
 naked axis-cylinders. Both the foregoing, when invested with a 
 medullary sheath, become converted into the third, or medullated, 
 form of nerve-fibre. These fibres in the nerves, while running be- 
 1 See Strieker's Human and Comparative Histology, i. , pp. , 147 ff. 
 
VARIETIES OF NERVE-FIBRES. 35 
 
 tween the central organs and the end-organs, become invested with 
 a delicate membrane, and are thus converted into nerve-tubes of 
 the well-known threefold structure. A fourth form of nerve-fibre 
 occurs in the peripheral nerves, and is distinguished from the fore- 
 going by the absence of the medullary sheath. This is the pe- 
 ripheral non-medullated fibre, or fibre of Remak, already alluded to. 
 As they appear to the microscopist, then, on an examination of all 
 the kinds of nerve-fibres which are found in all the different parts 
 of the nervous system, the following table of varieties is proposed 
 by Schultze : 
 
 T XT -i n i. j ( 1- Primitive fibrils. 
 
 I Non-medullated I ft . v , .... _., 
 fi , -< 2. fasciculi of primitive fibrils. 
 
 ( 3. These last, with a sheath of Schwann. 
 
 !1. Primitive fibrils with medullary sheath. 
 2. Fasciculi of primitive fibrils with such 
 , ,, " 
 sheath. 
 3. These last, with a sheath of Schwann. 
 
 The exposition of Schultze, although of value in setting forth 
 the variety of forms in which the nerve-fibre is actually found by 
 the histologist, does not constitute an objection to the twofold di- 
 vision first proposed. On the contrary, it leads directly to such a 
 division. For it will be noticed that both of the chief classes of 
 fibres are regarded as composed of a number of primitive fibrils ; 
 both are also regarded as becoming invested in their peripheral 
 course with an outside membrane. The two classes, however, are 
 really derived upon the basis of the fact that some of the primitive 
 fibrils, whether they have already become invested with this mem- 
 brane or not, possess a medullary sheath, and others do not. It is 
 the presence or absence of this medullary sheath which constitutes 
 the one important difference between the different classes of nerve- 
 fibres. 
 
 21. Medullated nerve-fibres, or nerve -tubes, have a threefold 
 structure. Such fibres, when separated by teasing with needles 
 from the fascicle of nerve-fibres and examined under the microscope 
 while still fresh, appear pellucid, with a central part and a border 
 on each side, like a translucent liquid in a tube of translucent 
 walls. The lines of this double contour are at first comparatively 
 sharp and regular ; lengthening the focus of the instrument ob- 
 scures slightly the central part, and causes the parts on the border 
 to appear brighter. Little by little the appearance of the fibres 
 changes. The contours become irregular, and the substance (myelin) 
 
36 
 
 FORM OF THE NERVOUS ELEMENTS. 
 
 composing the borders becomes folded, striated, and granulated in 
 appearance. The myelin wells over the sides of the ends of the 
 fibres in irregular globular or contorted masses. Occasionally a 
 pale filament may be seen projecting beyond the torn end of a 
 fibre. Owing to the fact that various solutions have different effects 
 upon the different parts of the nerve-fibres, it is 
 possible to prepare specimens which shall exhibit 
 clearly their threefold structure. Thus, for ex- 
 ample, a solution of picrocarminate of ammonia 
 colors the central part of the fibre, or axis-cylin- 
 der, but not the myelin ; whereas osmic acid 
 stains the axis-cylinder slightly, the myelin thor- 
 oughly, but not the substance of the annular 
 rings. By use, then, of various reagents, to color 
 the nerve-fibres, and by numerous observations 
 of them under various circumstances, their three- 
 fold nature in a living state is thought to be dem- 
 onstrated. We distinguish, then, in the medul- 
 lated fibres : (1) An outer membrane, extremely 
 thin, pellucid, with nuclei in it, and called the 
 primitive sheath or sheath of Schwann ; (2) an in- 
 terior layer of dimly granular, white, and highly 
 FIG. 2. Meduiiated refracting substance, semi-liquid during life, and 
 
 Nerve-fibres, with doub- . -, ,, -, . -. i- 
 
 le and irregular contour atter death undergoing a process resembling co- 
 euowing. (schwaibe.) agu i at i on _ ca n e a the medullary sheath or white 
 substance of Schwann ; and (3) a cylindrical band of albuminous 
 material, transparent, but with marks of fibrillation called the 
 axis-cylinder. Only the last is supposed to constitute the essen- 
 tial nervous structure ; for, as we have already seen, many nerve- 
 fibres are simple naked axis-cylinders, and all fibres for a certain 
 distance from the cells in which they originate are devoid of the 
 medullary sheath. There is considerable evidence that the presence 
 of this sheath depends upon the need of insulation only. 
 
 22. Besides the threefold longitudinal structure of the meclul- 
 lated nerve-fibre, we have to notice certain structural modifications 
 that occur at intervals in its length. The peripheral nerve-tube 
 does not run along as a regular cylinder, but places of constriction 
 appear at certain points situated beneath the outer sheath ; these 
 constrictions are made at the expense of the medullary sheath or 
 myelin. They are called annular constrictions or nodes of Eanvier ; 
 the portion of nerve-fibre included between two of these constric- 
 tions is called an interannular segment. At the constrictions the 
 ends of the segments of the outer sheath are joined together by a 
 
MEDULLATED NERVE-FIBKES. 
 A B 
 
 37 
 
 FlG. 3. A, Medullated Nerve-fibres from the 
 Sciatic of a Rabbit, stained with osmic acid, 
 and dissociated in water. (Ranvier.) 
 
 B, Single Fibre Enlarged 40 % , a, a, An- 
 nular constrictions, or nodes of Ranvier, 
 nearly midway between which is n, the nu- 
 cleus, with protoplasm, p, surrounding it ; 
 ca, axis-cylinder. 
 
 "P 
 71 
 
 FlG. 4. Medullated Nerve-fibres. (Schwalbe.) 
 a, Axis-cylinder ; *, sheath of Schwann ; n, 
 nucleus ; p, p, granular substance at the poles 
 of the nucleus ; r, r, Ranvier's nodes, where 
 the medullary sheath is interrupted and 
 the axis-cylinder appears ; i, i, incisures of 
 Schmidt. 
 
38 
 
 FORM OF THE NERVOUS ELEMENTS. 
 
 thin layer of cementing substance which extends inward toward 
 the axis-cylinder. These interannular segments of the nerve-fibre 
 vary greatly in length. When several nerve-fibres lie parallel with 
 each other, the segments of four or five of them often seem to have 
 about the same length, and then the series appears interrupted by 
 some segment considerably longer or shorter than the rest. 
 
 Each interannular segment of a nerve-fibre has a flattened ellip- 
 tical nucleus, situated nearly equidistant between the two annular 
 constrictions which limit the segment. This nucleus 
 often has a nucleolus ; between the nucleus and the 
 myelin there exists a minute mass of protoplasm which 
 is spread beneath the membrane of Schwann and fixed 
 to it. 
 
 Scattered irregularly along each interannular seg- 
 ment are delicate lines or fissures which seem to run 
 obliquely through the medullary sheath from the mem- 
 brane on the surface of the nerve-fibre to the axis-cyl- 
 inder. Their significance is not yet determined ; they 
 are called the " incisures of Schmidt." (See Fig. 4.) 
 
 23. The complex microscopic structure of the med- 
 ullated nerve-fibre, as described above outer sheath, 
 medullary sheath, axis-cylinder, interannular segments 
 limited at each end by annular constrictions, nucleus 
 and nucleolus, and incisures of Schmidt cannot be 
 considered as "ultimate," even in the restricted sense 
 i n which we use the word as applied to what the eye 
 can see b y the aid of optical instruments. Other still 
 sciatic of an more minute characteristics of its structure must be 
 
 Adult Babbit. 
 
 ' (B ~ Briefly mentioned, although with the understanding 
 
 FIG. 5 Medui- 
 
 an 
 nuiar con- that their interpretation, and even their existence, is 
 
 cy, axis'-cyi- more doubtful than are the characteristics already de- 
 
 i n d e r with . , -, 
 
 double con- Scribed. 
 
 tour showing. The f a(jt ^^ isolated axis-cylinders, when submitted 
 to the action of picrocarminate of ammonia, are stained red along 
 their median line, while an extremely thin homogeneous border is 
 left comparatively uncolored, and the additional fact that minute 
 flakes or scales sometimes seem to appear upon their surface, have 
 led to the conjecture that the axis-cylinder has a double structure. 
 The clear homogeneous border probably corresponds to the so-called 
 " sheath of Mauthner." ' 
 
 The "fibrillated" appearance of the axis-cylinder under the mi- 
 croscope has already been referred to as undoubted ; but the exact 
 1 See Ranvier, Traite Technique d'Histologie, i., pp. 738, 742. 
 
DOUBLE CONTOUR OF AXIS-CYLINDER. 
 
 39 
 
 nature and the interpretation of this appearance are still matters 
 of dispute. On account of the fact that the light must be passed 
 through two or perhaps three cylinders in order to reach the inte- 
 rior structure of the nerve-fibre, its examination under the high 
 powers of the microscope which are necessary to see this fibrillated 
 structure is extremely difficult. In spite of this difficulty, however, 
 Hans Schultze l claims that the fibrils of the axis-cylinder can be 
 
 FIG. 6. Fibrillated Appearance of the Axis-cylinders of Medullated Nerve-fibres. (Hans Schultze.) 
 
 distinctly traced in living fibres, when these are in process of form- 
 ing and are still naked, or where they issue from the cells without 
 a medullary sheath, or where they lose this sheath at the annular 
 constrictions or in the peripheral end-plexuses. Various prepara- 
 tions of dead nerve-fibre, treated with different reagents, seem to 
 demonstrate the same fibrillated structure. Moreover, from the 
 fact that the nervous substance of the fibrils takes a carmine tinge, 
 while the interfibrillary nucleated substance remains stained steel- 
 blue with the nitrate of silver, Schultze argues that the axis-cylinder 
 consists of two chemical substances. The fibrillated appearance 
 can, therefore, scarcely be considered as due to the arrangement of 
 
 1 In the Archiv f. Anat. und Physiol., 1878, Auat. Abtli., pp. 259-285. 
 
40 
 
 FOEM OF THE NEEVOUS ELEMENTS. 
 
 I 
 
 rows of granules in straight lines. 1 According to T. W. Engel- 
 mann, 2 in good preparations these fibrils appear distinct, and are 
 never seen to anastomose or form a plexus of fibrils. By actual count 
 the number of fibrils remains the same at any rate, between any 
 two annular constrictions ; nor are they apparently interrupted in 
 their course by these constrictions. The fibrils, as found in different 
 nerve-fibres, seem not to differ in respect to size or closeness of 
 
 contact, but their number 
 differs in nerve-fibres of 
 different sizes. Engel- 
 mann counted about four 
 hundred in the thickest 
 fibres taken from the mo- 
 tor roots of the spinal 
 
 \ii! HI Illl Mi cord of the frog. The 
 
 closeness of their contact, 
 and the smallness of their 
 number, as compared 
 with that of the fibrils into 
 which the fibre breaks up 
 at its peripheral termina- 
 tions, make it difficult to 
 see how these subdivi- 
 sions of the axis-cylinder 
 can have any separate 
 function as the conduc- 
 tors of nervous impulses. 
 Further information re- 
 l|] HI garding them must be left 
 
 to subsequent researches. 
 (See Fig. 6.) 
 
 The strict continuity of 
 the axis-cylinder through 
 the annular constrictions 
 maybe called in question. 
 Engelmann found that, on being treated with nitrate of silver, the 
 axis-cylinders, as a rule, were broken off at the annular constric- 
 tions or nodes. 3 Out of a hundred cases of broken cylinders only 
 four appeared where they had not parted in the middle of these 
 constrictions. It is not to be inferred from this, however, that nor- 
 mal and living nerve-fibres are interrupted by a space of even mi- 
 
 ' So article of H. D. Schmidt, in the Monthly Micr. Jour. , 1874, vol. xii. 
 a Pfluger's Archiv, xxii., p. 26. 3 Pfliiger's Archiv, xxii., pp. 1-24. 
 
 FIG. 7. Fibrillated Axis-cylinders broken at the Nodes 
 of Ranvier. (Engelmann.) 
 
THE FIBRES OF EEMAK. 
 
 41 
 
 croscopic proportions at these nodes ; no such interruption appears. 
 But it is by no means impossible that these fibres are to be regarded 
 as composed of a number of annular segments cemented together 
 each separate fibril placed exactly end to end with its fellow in the 
 adjoining segments. Such an arrangement 
 would accord with the theory which regards 
 the segments as elongated and developed 
 nerve-cells. 
 
 24. Non-medullated nerve-fibres, or fibres 
 of Remak, differ from those already described 
 in that they do not possess a medullary 
 sheath. They are grayish and translucent, 
 longitudinally striated, with flattened elon- 
 gated nuclei lying at frequent intervals upon 
 their surface. When gathered together within 
 a sheath of neurilemma, they are not placed 
 side by side as are the medullated nerve- 
 tubes ; they are rather formed in the interior 
 of the nerve, where they unite and divide and 
 make an intricate plexus or network of fibres. 
 They are grouped in larger bundles, some- 
 times alone, but more frequently in connec- 
 tion with medullated fibres. Their striated 
 appearance is probably due to the fact that 
 they, like the axis-cylinder of the medullated 
 nerve-fibres, are composed of numerous fibrils. 
 As has already been said, they belong to the 
 sympathetic system. 
 
 25. The size of different nerve-fibres in 
 the human body varies greatly, according to 
 their kind, position, and, perhaps, function. 
 As a rule the non - medullated fibres are 
 smaller than the medullated, the former be- 
 ing from -g-oVir to -g-g-Vo of an inch in diam- 
 eter, and the latter (in the trunk and branches FIG. 8. Fibres of Remak from 
 of the nerve) from y-^Vo to ^^ of an inch. 
 
 But this rule is not always followed. In the K H _ 
 
 white matter of the cord the medullated Bonding to fibrhs. 
 fibres range in size from y^ to ^Vir of an inch, in parts of the 
 anterior columns, and about T ^ of an inch in those regions of 
 the lateral and posterior 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 ^Vo" to iffanr of an inch in diameter, or 
 
 Nucleus with surrounding 
 protoplasm, p ; ft, striae cor- 
 
42 FORM OF THE NERVOUS ELEMENTS. 
 
 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 y^gVo 0- 
 of an inch in diameter. 
 
 26. The number of fibres which enter into the composition of 
 individual 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 thousand, in the optic nerve at one hundred 
 thousand at least. 
 
 27. So-called ganglion-cells, or nerve-cells, are the second of 
 the two structural elements which can be more minutely described 
 as undoubtedly belonging to the nervous system. These bodies 
 vary greatly in size and shape, but they all show, when subjected 
 to microscopic examination, certain well-recognized common charac- 
 teristics. Nerve-cells are irregular masses of protoplasm, finely 
 granular and delicately striated, with a large nucleus which is well- 
 defined and vesicular in appearance, and which usually contains a 
 shining nucleolus ; they send off one or more processes. In the gray 
 matter of the cord and brain they are embedded in the neuroglia 
 or so-called " nerve-cement ; " in the smaller nervous centres, such 
 as the ganglia of the sympathetic and the ganglia on the posterior 
 roots of the spinal cord, they are surrounded by a capsule of con- 
 nective tissue. 
 
 28. Careful microscopic investigation of the nerve-cell with high 
 magnifying powers of the instrument reveals the great complexity 
 of its structure. In describing this complex structure the bipolar 
 ganglion-cell of the fish may be considered as a common type. Such 
 a cell is called by Max Schultze ' a " nucleated swelling of the axis- 
 cylinder." When found in the course of a nerve-fibre it appears 
 at first sight as a complete interruption to the continuity of the 
 fibre. Further examination is thought to show, however, that, 
 when the fibre reaches the cell, the axis-cylinder loses its medul- 
 lary sheath, and the fibrils which constitute the substance of the 
 cylinder become dissociated, and continue their course over the 
 surface of the " ganglionic globe " to its opposite pole ; here they 
 reunite and form a fibre identical with the one that approached 
 the nearer pole of the cell. The "ganglionic globe " itself appears 
 to be composed of granular substance. We may distinguish, then, 
 in such a ganglion-cell these two parts : (1) a fibrillary covering, 
 the fibrils of which are continuous with the fibrils of the axis- 
 cylinder on either side of the cell ; and (2) a granular globe con- 
 taining near its surface a nucleus, within which one or more nucleoli 
 1 In Strieker, Human and Comparative Histology, i. , p. 1 74. 
 
NERVE-CELL FROM SPINAL GANGLION. 
 
 43 
 
 appear. l A repetition of these parts of the structure of the bipolar 
 cell, it is claimed, may be expected and found in ganglionic nerve- 
 cells in general. 
 
 A microscopic structure substantially like that of the bipolar 
 ganglion-cell of the fish, as already described, 
 is found to belong to the multipolar cells of 
 the anterior horns of the spinal cord of man, 
 and of the ox, or of other mammals. Among 
 the many processes given out by such a cell, 
 the researches of Deiters and of others have 
 demonstrated that ordinarily only one be- 
 comes continuous with the axis-cylinder of the 
 peripherally running nerve-fibre. This one, 
 called the "prolongation (or process) of Dei- 
 ters," has sometimes been distinctly seen to 
 be fibrillated ; and it is supposed that its 
 fibrils are, as a rule, continuous with those 
 of the axis-cylinder of the nerve-fibre. Hence 
 it is called the " axis-cylinder process." The 
 other processes from the cell also seem to be 
 fibrillar ; but the quantity of interfibrillar 
 granular substance which they contain is 
 greater than that in the axis-cylinder process. 
 These fibrils ramify, anastomose with each 
 other, and become lost in an intricate net- 
 work of extremely minute nervous filaments. 
 Over the surface and within the interior of the 
 " ganglionic globe " of the multipolar cell the 
 fibrils of all these processes run in every di- 
 rection with bewildering complexity. Their 
 relation to one another, and to the various 
 parts of the substance of the cell, cannot be 
 said to be determined with any degree of cer- 
 tainty. Most of the fibrils appear only to tra- 
 verse the ganglion-cells, but some of them, 
 perhaps, originate within the cells. In the case 
 of any thus originating, it is not as yet possible to say whether 
 or not they arise out of the nuclei and nucleoli, and so, whether we 
 may consider these parts of the cells as the special sources or cen- 
 tres of the nerve-fibrils, as Harless, Meynert, and others have done. 2 
 
 1 See Ranvier, Traite Technique d'Histologie, i. , p. 712. 
 
 a See, on this whole subject, Max Schultze in Strieker, Human and Compara- 
 tive Histology, i., pp. 172-187 ; Ranvier, Traite Technique d'Histologie, i., pp. 
 710 ff. ; and Hans Schultze, Archiv f. Anat. u. Physiol., 1878, pp. 259-285. 
 
 FIG. 9. Nerve-cell from the 
 Spinal Ganglion of the 
 Ray. "5%. (Ranvier.) 
 my, Medullary sheath of 
 nerve-fibre, enclosing ca, 
 the axis-cylinder, the fi- 
 brils of which (/) separate 
 and run over the gangli- 
 onic globe, m; n, nu- 
 cleus. 
 
44 FORM OF THE NERVOUS ELEMENTS. 
 
 29. The variety of shapes taken by the nerve-cells has already 
 "been mentioned, as well as the fact that they may be classified as 
 unipolar, bipolar, and multipolar. Some are nearly round ; others 
 ovoidal, caudate, stellate, or shaped like a flask or the blade of a 
 paddle. Still others appear somewhat like the foot of an animal 
 with claws ; while the branching processes of others give them the 
 appearance of sprawling out irregularly in a half-score of different 
 directions. To a certain extent the shape of the cells is character- 
 istic of that region of the central nervous system where they are 
 
 FIG. 10. Multipolar Ganglion-cell from the Anterior Horn of the Gray Substance of the Spinal 
 Cord of the Ox. (After Deiters.) 1, Nucleus ; 2, axis-cylinder process ; 3, 3, branching 
 
 found, in most abundant numbers, embedded in the neuroglia. 
 For example, large ganglion -cells of irregular shape, with branch- 
 ing processes, which have been called " motor," are found in the ante- 
 rior horns of the gray matter of the spinal cord ; pyramidal cells of 
 various sizes, with processes from both base and apex, are character- 
 istic of the cortex of the cerebrum ; and just at the inner edge of 
 the gray cortical matter in the cerebellum appear irregular globu- 
 lar or ovoidal cells, which send off one or two branching processes 
 toward the surface of the cerebrum. The ganglion-cells of the sym- 
 pathetic also are usually globular or ovoidal, and have one or more 
 processes which pierce their capsule and become non-medullated 
 
THE GANGLION-CELL AS A TYPE. 45 
 
 nerve-fibres. Unipolar cells are found in the spinal ganglia of the 
 higher animals, bipolar in the spinal ganglia of fishes. 
 
 Nerve-cells vary in size as much as in shape ; the limits may per- 
 haps be given as from about ^J^- to ^Vo f an inch- 1 No special 
 physiological significance can in any case be assigned to the shape 
 of the nerve-cell ; we are wholly ignorant of the meaning of such 
 a variety of forms, and of the value of any particlar form in a 
 given position. It is possible, however, that the large size of the 
 so-called "motor-cells" of the anterior horns of the spinal cord is 
 indicative of their special physiological function. We may also 
 fairly incline to interpret the multiplication of ganglion-cells in the 
 central parts of the nervous system as significant of the large 
 amount and high quality of work which must be done by them 
 within these centres. It is possible that the shape of the cells is 
 largely due to the mechanical conditions which control their de- 
 velopment within the embryo ; but upon this subject we have 
 scarcely any trustworthy information. 
 
 30. The structure of the nerve -fibres and nerve-cells, and the 
 nature of the histological relations which apparently exist between 
 the two, have led to a captivating theory intended to reduce all the 
 elements of the nervous mechanism to modifications of a single 
 form. Extremely different in structure as the various parts of the 
 nervous system obviously are, we are told that modern histological 
 science refers them all, for their elements, to " one perfectly defi- 
 nite type ; " 2 this type is the ganglionic nerve-cell. The important 
 common characteristic, that they send out prolongations which 
 become nerve-fibres, is assumed to belong to all such cells. The 
 fibres are, accordingly, considered to be prolongations of the cells, 
 and to be formed of substance like that of the source from which 
 they appear to arise. Nerve-fibres may then be described as nerve- 
 cells drawn out into an extremely long peduncle, which serves to 
 connect them with other similar cells and fibres, or with certain 
 muscular fibres which the nervous matter commands. This mor- 
 phological theory of the nervous elements rests, however, upon a 
 doubtful basis, and certain strong objections may be brought against 
 it. We are probably warranted simply in asserting that both classes 
 of these elements, like the other primary structural forms of the 
 body, may be regarded as differentiations of one type (the cell) 
 under conditions of which we are almost wholly ignorant. 
 
 There is accumulating evidence in favor of the view that nerve- 
 
 1 See an article of J. Hoffmann in the American Journal of Neurology and 
 Psychiatry, August, 1883, pp. 432 ff. 
 
 2 Ranvier, Traite Technique d'Histologie, i. , p. 710. 
 
46 FORM OF THE NERVOUS ELEMENTS. 
 
 fibres are, in general, connected, both histologically and physiologi- 
 cally, with the nerve-cells. One of the processes of each cell may, 
 therefore, as a rule, be regarded as continuous with the axis-cylin- 
 der of a nerve-fibre. It is true that this connection can by no 
 means always be traced by the microscope. A score of years ago 
 one investigator l declared that, after having examined the gray 
 matter of the spinal cord a great number of times, he had demon- 
 strated this alleged connection only very rarely. Eepeated obser- 
 vations since, of the improved modern kind, have not done away 
 with the comparative infrequency of the desired demonstration. 
 But from the very nature of the case a great number of the nerve- 
 fibres must have their connection with the cells broken off by the 
 treatment they receive in preparation for examination. And the 
 positive cases where such connection has been traced may fairly be 
 said to have indicated the rule. Moreover, the facts of physiology 
 (to which reference will be made subsequently) seem to favor such 
 a view of the anatomical relation of these two elements of the ner- 
 vous system. 
 
 Additional evidence upon this subject may perhaps be derived 
 from the recent researches of E. A. Birge. 2 This investigator under- 
 took the gigantic task of counting the nervous elements in the gan- 
 glia and roots of the spinal cord of a large number of frogs. He 
 apparently discovered a general relation indicating some agreement 
 in the number of the so-called motor-cells and the fibres alleged to 
 originate from these cells. In one case (No. 42) an actual count of 
 ten motor-roots gave 5,734 fibres and 5,777 cells on the right, and 
 5,740 cells on the left side of the cord. Other results of counting, 
 however, were by no means so favorable to the statement that the 
 number of the fibres in the roots agrees exactly with the number of 
 cells in the corresponding region of the cord. Nor could more 
 complete results of this kind form any sufficient warrant for the 
 conclusion that everywhere in the nervous system the number of 
 fibres corresponds with the cells, or that the nerve-fibres all spring 
 from the nerve-cells ; much less, that they may be reduced to one 
 form of such cells as to a perfectly definite type. 
 
 31. The discussion of the chemical constitution and structural 
 form of the elements of the nervous system introduces the ques- 
 tion as to the Functions of these Elements. This question must 
 be answered, if at all, by the science of physiology. And in view 
 
 1 Vulpian, see Lemons sur la Physiologie du Systeme Nerveux, p. 318, Lect- 
 ure of July 9, 1864. 
 
 Archiv f. Anat. u. Physiol., 1882, Physiolog. Abtli., pp. 435-479, espe- 
 cially p. 471. 
 
EXCITABILITY AND CONDUCTIVITY. 47 
 
 of our ignorance of the genuine nervous character of all other 
 claimants to a place among the elements of the nervous system, our 
 inquiry is narrowed to the following terms : What can nerve-fibres 
 and ganglionic nerve-cells do? With the activities of these ele- 
 ments, as combined into the complex organs of the human nervous 
 mechanism, the whole of our subsequent examination is designed 
 to deal. We speak here very briefly of certain fundamental prop- 
 erties of the two nerve-elements already described that is ; of the 
 nerve-fibres as gathered into bundles called nerves, and of the cells 
 as collected into ganglia and connected with these nerves. 
 
 Nerves and nerve-cells have certain properties in common ; that 
 is to say, within certain limits both can do the same things. Both 
 are capable of becoming the subjects of a specific kind of molecu* 
 lar motion which we are entitled to consider as distinctively " neu- 
 ral" but about whose nature and mathematical or physical relations 
 to other modes of the molecular motion of matter we are still al- 
 most totally ignorant. Both are also capable of propagating this 
 distinctively " neural commotion " from one portion of their struct- 
 ure to another. In a word, both nerve-fibres and nerve-cells have 
 the properties of Excitability and Conductivity ; and the excitation 
 and conduction of excitation which these nervous elements display 
 are of a kind peculiar to themselves. It is the production, propa- 
 gation, modification, and distribution of this distinctive nerve-com- 
 motion which constitutes the one constant function, or property, 
 of the nervous elements, whether considered as isolated or as com- 
 bined into organs. It is customary with some writers to speak of 
 the production of psychical phenomena as the crowning function 
 of the nervous system. But whatever may be the view we shall 
 find ourselves compelled to take of the relations between the loca- 
 tion, quantity, quality, and combinations of this neural molecular 
 motion and the phenomena of self-conscious life, from our present 
 point of view the utterances of such writers if designed as anything 
 other than figures of speech which need to be explained in detail 
 to be even suggestive of those real facts and relations which they, in 
 truth, only symbolize are of little interest or value. We are speak- 
 ing of a material structure, which, although alive and standing in 
 altogether unique relations to psychical phenomena, is, nevertheless, 
 in itself considered, nothing but a very complex collection of moving 
 molecules. The peculiar form of molecular motion which charac- 
 terizes this structure namely, so-called " nerve-commotion " is 
 its unique function. Inasmuch as such nerve-commotion may be 
 considered as originally set up in a single nervous element or group 
 of elements, and then propagated from this initial point along cer- 
 
48 FUNCTION OF THE NERVOUS ELEMENTS. 
 
 tain more or less definitely marked tracts to other elements or 
 groups of elements, we may divide the one function into two the 
 function of excitation and the function of conduction. 
 
 32. Nerve-commotion, or neural molecular action, is, of course, 
 never an uncaused event. It begins at certain points in the ner- 
 vous elements, where it is set agoing by the application of appropri- 
 ate causes of excitation. The causes of the excitation of the ner- 
 vous elements are called "stimuli." Stimuli are of two general 
 kinds external and internal External stimuli comprise all such 
 modes of the motion of matter as act upon the peripheral parts of the 
 nervous system, and so produce within it a state of excitation or 
 nerve-commotion ; among these are light, heat, chemical changes, 
 etc. Internal stimuli are such as act upon the nerve-cells of the 
 central organs ; they consist, in general, of changes in the blood 
 produced by an increase or decrease of oxygen, the presence of 
 drugs, etc. The susceptibility of a nerve to any form of external 
 stimulus is called its " irritability ; " and when a nerve will no 
 longer respond to such stimulus by being thrown into a condition 
 of excitation, it is said to have lost its irritability. As the word is 
 generally used, then, the irritability of a nerve is its property of 
 excitability under the action of some form of external stimulus. 
 When excited by such stimulus it is said to be irritated. We shall 
 use both sets of words, reserving the words " excitation " and " ex- 
 citability " for the general state and function of all nervous tissue 
 considered as capable of a specific molecular commotion ' a nerve- 
 commotion. 
 
 33. But although all the nervous elements may be said to have 
 the properties of neural excitability and conductivity, important 
 differences arise as to the conditions under which, and as to the 
 modes in which, they exercise their functions when combined into 
 a complex nervous system. In the normal condition of such a s} 7 s- 
 tem it is by no means all of its parts which are directly excitable, 
 whether by external or by internal stimuli ; nor is the effect of the 
 excitation of both the elementary structural forms of such a system 
 exactly the same. A single nerve may, indeed, be separated from 
 the other parts of the nervous system, with a muscle attached, and 
 may then be made to exercise its neural function in moving the 
 muscle by being itself stimulated with different kinds of stimuli at 
 different points along its course. But in their normal place and 
 condition nerves are never excited by the direct application of 
 
 1 It is a pity that we have in English no one word which can be used under 
 all conditions, and compounded ad libitum, in order to designate a property, a 
 process, a state, etc., as can the German word Erregen, Erregung, etc. 
 
GANGLION-CELLS AS DISTRIBUTORY. 49 
 
 stimuli ; they are always excited indirectly by the propagation to 
 them of nerve-commotion which originates in the central organs or 
 in the end-organs. The distinctive office of the nerves is, then, to 
 act as conductors of molecular motion set up in themselves by the 
 direct excitation of the nervous elements in which they either cen- 
 trally or peripherally terminate. Moreover, large portions of the 
 central organs do not respond to the direct application of various 
 kinds of stimuli to their surface. We are obliged, then, to suppose 
 that many of the nerve-cells which compose these organs are excita- 
 ble only by stimulation through the nerve-fibres that run into them. 
 The case of the normal nervous system, with respect to its excita- 
 bility, may, then, be briefly described in the following terms : The 
 end-organs of sense are directly excitable by external stimuli, and 
 each specific kind of end-organ which is characteristic of a particu- 
 lar sense is excitable only by the specific kind of stimuli appropri- 
 ate to that sense. The afferent or centripetal nerves are excited 
 only by the end-organs of sense ; their specific function is to con- 
 duct the nerve-commotion, started by the external stimuli in these 
 end-organs, toward the central organs. The efferent or centrifugal 
 nerves are not directly excited by either internal or external stimuli, 
 but only by the central organs ; their specific function is to conduct 
 the nerve-commotion started in them by the central organs to the 
 muscles, glands, etc. to the peripheral parts of the body which 
 are to be moved through their excitation. The central nerve-cells 
 themselves are excited either through the nerve-commotion brought 
 to them by the afferent nerves or by internal stimuli. Nerve- 
 commotions are also said to arise in them automatically ; but 
 the facts covered by the term " automatic " require further distinc- 
 tions to be made as to the functional activity of the different nerve- 
 elements. 
 
 34. If the distinctive normal function of the nerves is the con- 
 ducting of neural molecular motion between the central organs 
 and the end-organs, the function of the ganglion-cells can by no 
 means be pronounced so simple. These cells are, indeed, also con- 
 ductors of nerve-commotion ; within the central organs they form 
 important parts of the tracts along which such commotion passes. 
 They serve also as points for the division and redistribution of this 
 commotion ; they may be spoken of as switching-places in the sys- 
 tem or network of tracts. In these " shunting-places " of the cell 
 many lines of conduction meet ; and the one of them taken by any 
 impulse entering the cell may depend upon the relative amount of 
 resistance offered by these lines. The work of the cell may then 
 be considered as "re-directive." The office of the cell in distri- 
 4 
 
50 FUNCTION OF THE NERVOUS ELEMENTS. 
 
 bution of the nerve-commotion may also be either to condense or 
 to disperse it ; in the former case the distribution might be spoken 
 spoken of as " associative," in the latter as " dissociative." 1 They 
 may also intensify or diminish the nerve-commotion entering them. 
 But the nerve-cells have also other functions, or forms of the one 
 neural function, which have been classed as either (a) automatic, 
 (6) reflex, or (c) inhibitory. 
 
 (a.) Automatism, or the power of initiating the peculiar form of 
 molecular motion known as "vital impulses," independently of 
 the action of any discoverable stimulus from without, is one of the 
 fundamental properties of protoplasm. An amoeba, for example, is a 
 minute mass of such protoplasm ; it executes movements which can- 
 not be wholly explained by reference to any changes in its environ- 
 ment. The difficulty of distinguishing automatic from reflex action 
 in the spinal cord, and muscular from nervous automatism in the 
 sporadic ganglia, need not concern us at present. According to 
 Eckhard 2 two kinds of this automatic function of the ganglion-cells 
 may be distinguished viz., the automatic-tonic and the automatic- 
 rhythmic. In the former case the control of the cells over the 
 muscular structures by means of the efferent nerves is irregular ; 
 in the latter this control results in the nearly simultaneous contrac- 
 tion of the same set of such structures, repeated at regular intervals ; 
 as is the case in the movements of the heart and luDgs. In neither 
 case, however, can we form any clear conception of the origin within 
 the cells of this neural commotion, of the nature of the forces at 
 work to produce it, or of the changes in material that are involved 
 in it. We can only say that as yet we know no reasons lying out- 
 side of the structure and activities of the living nerve-cells them- 
 selves which will account for the starting of the excitation. In this 
 sense, at least, such neural action is "automatic." 
 
 (b.) The reflex function of the ganglion-cells admits of a some- 
 what more detailed and satisfactory statement ; but the phenomena 
 and laws of reflex nervous action are properly discussed as belong- 
 ing to the central organs of the nervous system. It is enough, at 
 present, to note that the great changes which take place in the 
 character of nervous impulses, when, after entering the central 
 organs by the afferent tracts, they are, as it is said, " reflected " 
 from those organs along the efferent tracts, are indubitable evi- 
 dence of the specific molecular activity of the ganglion-cells. For 
 the afferent impulses are, in fact, not simply reflected in these cells ; 
 
 1 See A. Hill, The Plan of the Central Nervous System, p. 2. Cambridge, 
 1885. 
 
 2 Hermann's Handb. d. Physiol., II., ii., p. 19 f. 
 
GANGLION-CELLS AS INHIBITORY. 51 
 
 they are greatly modified as to their number, intensity, character, 
 and distribution. This modification is proof of profound molecu- 
 lar changes that are instituted in the substance of the cells them- 
 selves. It is one proof, among others, that a large expenditure of 
 energy in the cells accompanies the transmutation of afferent into 
 efferent impulses. 
 
 (c.) The function of inhibition, as ascribed to ganglion-cells, must 
 be pronounced more doubtful in character than either of the two 
 foregoing. It was found by Wundt * that nervous impulses are 
 delayed on passing through the spinal ganglion. Such impulses 
 seem also to consume an amount of time in travelling along or 
 through the cord that cannot readily be accounted for as wholly 
 due to the length of the nervous tracts which they thus traverse. 
 But until our information is more precise as to the microscopic 
 structure of the cord, and as to the tracts within it which the ner- 
 vous impulses follow, we cannot say with confidence how much of 
 this delay is due to molecular changes peculiar to the cells them- 
 selves. That the automatic and reflex functions of the medulla 
 oblongata may be compounded, as it were, in such way as either to 
 inhibit or to accelerate the action of the heart and lungs and mus- 
 cular walls of the arteries, is a well-known fact. It has already 
 been said that nerve-cells may diminish as well as intensify the 
 nerve-commotion entering them. When afferent impulses reach the 
 ganglion-cells of the centres, and find them already at work, such 
 impulses result, according to circumstances, in either inhibiting or 
 augmenting this activity. 2 Moreover, the tone given forth by a 
 muscle, when tetanized by stimulating the nerve to which the mus- 
 cle is attached with repeated induction-shocks, has the same num- 
 ber of vibrations per second as there are of such shocks ; but the 
 tone given forth by muscle tetanized through the spinal cord, or 
 by action of the will, has a constant number of vibrations, namely, 
 about nineteen per second. It would appear from this, also, that 
 the central apparatus of nerve-cells controls the impulses which 
 tetanize the muscle, according to the molecular structure and 
 changes of those cells. In this sense, then, the cells may be said 
 to exercise inhibitory functions under certain conditions. 
 
 35. A consideration of the different effects produced by the 
 conduction of nervous impulses along the different nerves of the 
 system would seem at first to justify the classification of the nerves 
 according to the varieties of their functional activity. In this way 
 
 1 Untersuchungen zur Mechanik der Nerven, 1876, Abth. ii., pp. 45 ff. 
 
 2 Comp. Foster, A Text-book of Physiology, fourth edition, p. 134. New 
 York, 1880. 
 
52 FUNCTION OF THE NERVOUS ELEMENTS. 
 
 we should distinguish the following classes : (a) nerves of motion 
 controlling the muscular apparatus, whether of smooth or of striated 
 muscular fibres ; (6) nerves of inhibition ; (c) nerves of secretion ; 
 (d) trophic nerves, or nerves which have a direct influence upon nu- 
 trition ; (e) centripetal nerves that have no sensory function ; and, 
 finally, (f) sensory nerves, or those the excitation of which may 
 result in conscious sensation. 1 
 
 That the irritation of different nerves may have results so differ- 
 ent as are indicated by the foregoing classes must indeed be ad- 
 mitted ; but it is quite another question whether this difference is 
 not wholly due to the sources of origin for the nerve-commotions 
 sent along them, and to the structures in which it terminates, rather 
 than to any difference in the essential physiological function of the 
 nerves themselves. Just as the same electrical current may pass 
 along the same kind of wire, and write a message, or ring a bell, or 
 move the legs of a frog ; just so the irritation of certain fibres of the 
 pneumogastric nerve results in controlling the motion of the heart ; 
 the irritation of other nerves seems to have an immediate metabolic 
 effect in directing the secretory processes ; that of still others pro- 
 foundly modifies the nutrition of the portions of the body to which 
 they are distributed. All these effects are in appearance greatly 
 unlike the movement of a muscle unde^ stimulation from a nerve. 
 With regard to the influence of the nerves on nutrition (their tro- 
 phic function) it is not necessary, in order to account for it, that 
 some specific action of a particular kind of nerves should be as- 
 sumed. We should suppose, of course, that the chemical processes 
 in which nutrition consists would be changed in character by the 
 molecular changes in the tissue which irritating any of the nerve- 
 fibres running into it would inevitably bring about. 
 
 Further consideration of the six possible classes of nerves given 
 above reveals the fact that they may all be reduced to two, accord- 
 ing to the direction in which their function of conducting nerve- 
 commotion is exercised. The first four conduct it outward from 
 the nervous centres, and are therefore called " efferent ; " the last 
 two conduct it inward toward the nervous centres, and are there- 
 fore called " afferent." Into these two kinds all nerves are custom- 
 arily divided, so far as their physiological function is concerned. 
 
 36. The further question now arises, Whether the general phy- 
 siological function of these two principal classes of nerves differs in 
 kind as well as in direction ; or are afferent and efferent nerves to 
 be identified so far as their specific neural function is concerned ? 
 Inasmuch as every nerve-fibre, in the normal condition of the ner- 
 
 1 Comp. Sigmund Mayer, in Hermann, Handb. d. Physiol., II., i., pp. 200 ff. 
 
AFFERENT AND EFFERENT NERVES. 53 
 
 vous system, is a stretch of nervous matter between two termina- 
 tions a point of origin and a point of issue for the state of excita- 
 tion it might, at first, seem simpler to consider it as intrinsically 
 capable of propagating nerve-commotion in one direction only. It 
 would be concluded, then, that the behavior of afferent and effer- 
 ent nerves, when stimulated, is essentially different with respect 
 to their molecular processes. Certain phenomena are sometimes 
 urged in favor of such a conclusion. 
 
 The application of heat to an efferent (or motor) nerve causes no 
 contraction in the muscle which the nerve supplies ; heat does not 
 appear to be a stimulus of such nerves. On the contrary, Grtitzner * 
 concluded that heating the different kinds of afferent nerves to 
 from about 115 to 125 Fahr. does excite them. The passage of 
 a constant current along an efferent nerve, so long as this cur- 
 rent does not suddenly change in strength, does not stimulate this 
 nerve so that the muscle contracts ; but such a current does excite 
 nervous impulses in a sensory nerve. Moreover, certain chemical 
 substances are said to act as stimuli on efferent nerves which have 
 no such effect upon sensory nerves. 
 
 On the other hand, the rate of conduction in both afferent and 
 efferent fibres, under similar conditions, is about the same. The 
 laws which evince the behavior of nerves under stimulation by elec- 
 tricity, and which are most relied upon as a basis for a mechanical 
 theory of the nervous system, are largely the same for both kinds 
 of fibres. There is a large amount of scientific information, called 
 " general physiology of the nerves," which looks in the direction of 
 identifying the molecular processes in the two classes of nerve- 
 fibres. This is true in particular of the remarkable phenomenon 
 known as the "negative variation "of the nerve-current. More- 
 over, the marked difference (referred to above) in the results ob- 
 tained by stimulating motor nerves on the one hand, and sensory 
 nerves on the other, is plainly, to a great extent, due to the differ- 
 ence in the sources of the stimulation ; the former are excited by 
 the central organs, the latter by the end-organs of sense. The mo- 
 lecular structure of these two sets of organs, and their consequent 
 molecular motion when acted upon by the appropriate stimuli, dif- 
 fer widely ; we do not, then, need to assume a specific difference 
 in the function of the connecting nerve-strands in order to account 
 for a marked difference in the results. Thus it may be assumed 
 that molecular disturbances, which would be quite powerless to stir 
 the sluggish muscle-fibres when transmitted to them by a motor 
 nerve, would occasion profound changes in the more sensitive 
 1 Pfliiger's Archiv, xvii., p. 215. 
 
54 FUNCTION OF THE NERVOUS ELEMENTS. 
 
 structure of the ganglion-cells when transmitted to the latter by a 
 sensory nerve. 
 
 Various attempts have been made to demonstrate, experimen- 
 tally, that motor and sensory nerves can perform each other's func- 
 tions. Such experiments have not yet been altogether successful. 
 They consist, in general, of attempts to unite by healing the cen- 
 tral part of a divided sensory nerve and the peripheral part of a 
 divided motor nerve, and then to show that the nerve thus united 
 discharges certain sensory or motor functions, as the case may 
 be. Philipeaux and Vulpian, 1 after various rather unsuccessful 
 attempts of Flourens, Bidder, Schiff, and others, succeeded in 
 uniting the central portion of the lingual (or sensory gustatory) 
 nerve of young dogs with the peripheral end of the hypoglossal 
 (motor nerve of the tongue) on the same side. Stimulation of the 
 lingual nerve above the point of union then produced contractions 
 in the hypoglossal of the same side, and that even when the lin- 
 gual was divided high up so as to preclude any reflex action. But 
 the action obtained was found to be apparently due to the chorda 
 (motor) fibres present in the lingual. In 1863 Bert succeeded in 
 reversing the course of the nerve-fibres in the tail of a rat, by bend- 
 ing this appendage over and implanting its end in the animal's 
 back. After healing had taken place, the transplanted tail was cut 
 off near its origin, and found to be sensitive of course, in the re- 
 verse direction of the nerve-fibres from the natural one. This 
 experiment would seem, then, to show that sensory nerve-fibres, 
 when reversed, can transmit sensory impulses in the direction 
 which was formerly centrifugal. The experiments of Ktihne * and 
 others upon the intramuscular ramifications of the nerve-fibres in 
 the sartorius muscle of the frog point in the same direction. If 
 the broad end of this muscle be divided by a longitudinal slit into 
 a forked shape, then stimulation of one of the two tines of the fork 
 beyond their division will stimulate the fibrils of the other tine ; 
 that is, the minute twigs of the motor nerve in the tine which is 
 directly stimulated have acted centripetally, and the excitation has 
 then descended the twigs of the other tine. 
 
 For all the foregoing, and for other reasons, we seem warranted 
 in assuming that there is no such specific difference in the func- 
 tion of the two kinds of nerves as is dependent upon the peculiar 
 structure or molecular processes of each kind. Both afferent and 
 
 1 See Vulpian, Lemons sur la Physiologie du Systc^me Nerveux, etc., pp. 
 274 ff. ; and comp. the remarks of Hermann, Handb. d. Physiol., II., i., pp. 
 10 ff., and of Foster, Text-book of Physiology, pp. 503-508. 
 
 2 Archiv f. Anat., Physiol., etc., 1859, pp. 595 ff. 
 
PROPERTIES OF ALL NERVES. 55 
 
 efferent nerves are probably capable of the same kind of molecular 
 commotion called nervous excitation, and of conducting this commo- 
 tion in either direction. The marked difference in the results of the 
 exercise of this function in the two cases is probably due chiefly to 
 the difference in the organs from which the excitation of the nerve 
 starts, and into which it is discharged. With respect to neural mo- 
 lecular disturbances, all nerves are excitable, conductors of excita- 
 tion, and exciters of nerve-cells and muscle-fibres. And if to this 
 description we add the statement that nerve-cells can, acting auto- 
 matically, originate this nerve-commotion, can modify its character 
 profoundly as it passes through them, and distribute it in various 
 directions, we state, in the most general form, what is at present 
 known as to the functions of the nervous elements. 
 
CHAPTEK II. 
 
 COMBINATION OF THE NEEVOUS ELEMENTS INTO A 
 
 SYSTEM. 
 
 1. IN the last chapter the nervous elements were considered, as 
 far as possible, without reference to their combination for the ac- 
 complishment of a common work. Regarded as isolated, and as 
 possessed only of those properties which belong to all living mat- 
 ter of the peculiar chemical constitution and structural form which 
 are described by the word "nervous," these elements are of great 
 interest to physiological and psycho-physical researches. But in 
 their normal position and activity the nerve-fibres and nerve-cells 
 are always combined into certain 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 received from another. We are 
 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 open to direct in- 
 spection. Moreover, when different tracts of nerves, or different 
 regions in the central organs where ganglion-cells abound, are par- 
 tially isolated by being laid bare for the direct application of stimu- 
 lus, 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 
 
MECHANISM OF THE AMCEBA. 57 
 
 the nervous mechanism. A brief description of the manner of this 
 combination is, then, indispensable at this point. 
 
 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 office of the 
 nervous mechanism has been said (p. 18 f.) 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 ? 
 To answer this question may be said speaking figuratively to be 
 the problem before the nervous system. The actual arrangement 
 of the elements of this system, in the exercise 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. 
 
 The manner in which the human nervous mechanism is developed 
 as a response to the before-mentioned problem is made clear by con- 
 sidering, in the first place, a much simpler form of the same prob- 
 lem. The simple protoplasmic speck called an amoeba may be con- 
 sidered as a living molecular mechanism. It appears, even under 
 the higher powers of the microscope, as almost wholly, if not quite, 
 composed of undifferentiated protoplasm, in the midst of which, as 
 a rule, lies a single nucleus. If differentiated at all, it may be ob- 
 served to have a somewhat solid external layer, called an ectosarc, 
 and a more fluid granular interior, called endosarc. But minute 
 and almost structureless as it appears, the amoeba is really com- 
 posed of a great number of molecules that are undergoing constant 
 change ; and it is capable of exercising several wonderful functions 
 that do not belong to any non-living collection of molecules. Its sub- 
 stance is metabolic, respiratory, reproductive. The protoplasm of 
 the amoeba is the subject of constant chemical alterations, by which 
 the old protoplasm is broken up and its products cast off, while 
 new protoplasm is formed. Oxygen is assumed by this substance 
 and carbonic acid excreted. The unit which is constituted by the 
 amoeba may, by fission (or by other means), divide into two parts, 
 each of which becomes a fresh unit. But more important for our 
 
58 PLAN OF A NEKVOUS SYSTEM. 
 
 purpose is the fact that the amoeba is irritable and automatic. It 
 is almost unceasingly in motion. It is living matter ; and when 
 acted on by stimuli, it suffers an explosion of energy which gener- 
 ally results in a change of place and form. Inasmuch as these pe- 
 culiar " amoeboid " movements seem substantially identical with 
 those which occur in a muscle and result in its contraction, the 
 animalcule may be said to be contractile. But inasmuch as some 
 of these movements cannot be ascribed to irritation of the external 
 molecules of the amoeba by the surrounding medium, but seem 
 rather to be due to energy set free in consequence of unknown in- 
 ternal changes, we call it automatic. We say, " it has a will of its 
 own." Thus does 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-originating changes, solve the problem pre- 
 sented to it. 
 
 Let it be supposed that the problem becomes more complicated, 
 and the animal structure which is to solve it correspondingly com- 
 plex. 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 reproductive 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 may be moved in 
 united masses, are then adjusted to a mechanical framework (of in- 
 different value so far as really vital changes are concerned) of carti- 
 lage, bone, etc. 
 
 But the eminently irritable and automatic system of molecules 
 called nervous must undergo a further differentiation of function. 
 In the structureless protoplasm of the amoeba, the external mole- 
 cules are, of course, the ones primarily to be affected by the exter- 
 nal stimuli. It is with the internal molecules, on the other hand, 
 that the changes called " automatic " begin. But the continual 
 flux of its protoplasmic substance indicates that, in its simplest 
 form, any of the molecules of the animalcule may in turn act either 
 as irritable or as automatic. The primary differentiation of this 
 substance into ectosarc and endosarc points, however, to a division 
 of labor. 
 
 By this primary differentiation of the substance of the animal, 
 one cell, or group of cells, becomes more eminently irritable, 
 another automatic. The former has thus been fitted for the spe- 
 
TRIPLE FORM OF THE SYSTEM. 59 
 
 cial work of responding to external stimuli by vital impulses ; 
 the latter for that of initiating so-called automatic impulses. The 
 position of the former in the animal mechanism will then natu- 
 rally be at the surface, where it can be acted upon by the appro- 
 priate external stimuli ; the position of the latter will naturally be 
 withdrawn from the surface, where it can be protected from such 
 stimuli and left undisturbed for action that is either automatic 
 or excited by only internal stimuli. But if the two kinds of sub- 
 stance are to perform one work, although by division of labor, 
 they must be connected ; that is, the eminently irritable protoplasm 
 of the surface must be joined by irritable protoplasmic material 
 with the eminently automatic protoplasm of the interior. Three 
 sets of organs are then called for in this rudimentary differentiation 
 of the nervous substance : (1) superficial cells susceptible to exter- 
 nal stimuli ; (2) central and eminently automatic cells, also suscep- 
 tible to internal stimuli ; (3) a strand of irritable protoplasm con- 
 necting the two. 
 
 Yet one more step in the distribution of functions between the 
 irritable and the automatic protoplasm of the complex animal or- 
 ganism must be taken, in order to reach the fundamental triple ar- 
 rangement of a nervous system. The system of eminently contrac- 
 tile tissue called muscular must be brought into connection with the 
 parts already described. In order that the more highly organized 
 animal may, like the amoeba, both have and 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 indi- 
 rectly connected, through the automatic cells, with such superficial 
 cells as are sensitive to these stimuli. The nervous system, there- 
 fore, in its most fundamental form consists of these three sets of 
 contrivances with their respective functions : (A) sensitive cells 
 upon the surface of the body ; (B) central cells that are both auto- 
 matic and modifiers and distributers of sensory impulses; (C) con- 
 necting cords, or strands, that can convey the nervous impulses 
 either centripetally from A to B, or centrifugally from B to the con- 
 tractile 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 G. If various kinds of stimuli are to act upon this system, 
 then the sensitive cells upon the surface (A) must be modified into 
 various external organs of sense ; and with these organs the ter- 
 
60 PLAN OF A NERVOUS SYSTEM. 
 
 minations of the centripetal or sensory nervous strands must be 
 variously connected. The terminations of the centrifugal or moto* 
 nervous strands may also be variously modified so as to connect 
 with and control the contractile tissue of many sets of muscles. 
 The central cells may be variously grouped and arranged, with 
 functions more or less localized, so as to receive, modify, and dis- 
 tribute, in manifold ways, the different sensory impulses ; and so 
 as to co-ordinate these impulses for definite results in the periph- 
 eral parts of the body. Other such central cells may become more 
 particularly related to the phenomena of conscious sensation and 
 volition. Such a highly developed nervous system will then con- 
 sist of the following parts : (A) End-organs of Sense, like the skin, 
 the eye, and the ear ; (A') End-organs of Motion, like the so-called 
 motor end-plates and terminal nerve-bulbs ; (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, (&') certain others more concerned in re- 
 ceiving and distributing reflexly the sensory impulses, and (6") still 
 others more particularly connected with the phenomena of con- 
 sciousness ; and (C) Conducting Nerves, which will be either (c) 
 centripetal, afferent, and sensory, or (c') centrifugal, efferent, and 
 motor, designed to connect the central organs and the end-organs. 
 We are now to consider the details with which such a highly de- 
 veloped nervous system is actually constructed in the case of man. 
 Our guides will, of course, be anatomy and histology. 
 
 3. In the manner already described (Chapter I., 19) the indi- 
 vidual nerve-fibres are collected and bound together in fascicles or 
 groups of fascicles, called nerves, and in larger bundles or nerve- 
 trunks. The nerve-cells are grouped into minute masses of nervous 
 matter, such as the sporadic ganglia found in the sinus, auricular 
 walls, and auriculo-ventricular groove of the heart ; or they are 
 gathered into larger bodies, intersected with most intricate ramifi- 
 cations of the nerves and interspersed with the finely granular sub- 
 stance called neuroglia, such as constitute the various parts of the 
 brain and spinal cord. 
 
 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 consists 
 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 with the viscera of the same cavities, and widely distrib- 
 uted over the body, especially in connection with the vascular sys- 
 
GANGLIA OF THE SYMPATHETIC. 61 
 
 tern ; and of a great multitude of fine distributory nerves. Each 
 of the two cords consists of 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 each of which contains not only non-medullated nerve- 
 fibres from the sympathetic syste"m to the cerebro-spinal nerves, 
 but also medullated fibres from the cerebro-spinal to the sympa- 
 thetic 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 bound together, and through the sympathetic fibres brought 
 under the control of the cerebro-spinal axis. The three main plex- 
 uses referred to are collections of nerve-cells and a dense plexiform 
 arrangement of nerve-fibres. One of them is situated at the base 
 of 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 
 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. The effect of certain emotions, for example, upon the con- 
 dition of the circulation, digestion, etc., is too well known to re- 
 quire a lengthy statement. 
 
 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 
 its interior. (1) The Dura Mater, which is the membrane lying 
 
FIG. 11. View of the Cerebro-spinal Axis. 
 (After Bourgery.) '/6- The rj g ht h lf 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, 
 cerebrum ; C, cerebellum ; P, pons Varolii ; 
 in o, medulla oblongata ; m s, m s, upper and 
 lower extremities of the spinal marrow. 01. 
 to CVIII. are cervical nerves; 1)1. to DXIL, 
 dorsal ; LI. to LV., lumbar : SI. to SV., sa- 
 cral : Col., coceygeal. 
 
PROCESSES OF THE DURA MA 
 
 63 
 
 next to the wall of the bony cavity, is tough, white, fibrous, and of 
 structure somewhat different in the cranial from the spinal cavity. 
 In the former position it is identical with the inner periosteum of 
 the bones of the skull ; on passing into the spinal column, how- 
 ever, the periosteum divides into two or more lamellae, the inner- 
 most of which is prolonged into the cylindrical tube that includes 
 the spinal cord. 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) ike fate cerebri, a sickle- 
 shaped process between the two hemispheres of the large brain ; 
 
 FIG. 12. 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, 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, middle temporal vein. 
 
 (b) ihefalx cerebelli, a similar process between the two lateral lobes 
 of the cerebellum, 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 fluid necessary to fill up the gaps 
 and smooth over the surfaces of the closed area made by the dura 
 mater is contained in the intercommunicating spaces of the mem- 
 brane lying next inward and called (2) Arachnoid ; this membrane 
 is transparent and of delicate connective tissue. Toward the dura 
 mater it presents a smooth, firm surface, like that of a serous mem- 
 brane, and is covered by a layer of scaly endothelium ; this layer 
 is reflected on to the roots of the spinal and cranial nerves, and 
 becomes continuous with the lining of the dura mater when the 
 
64 THE SPINAL CORD. 
 
 nerves pierce the latter membrane. The space below this surface 
 is called subarachnoid ; the subarachnoid or cerebro- spinal fluid (al- 
 ready referred to as filling the intercommunicating compartments 
 into which this space is divided by bundles of delicate areolar tis- 
 sue) is alkaline and poor in albumen. (3) The Pia Mater is a vas- 
 cular membrane, a minute network of fine branches of arteries 
 and veins held together by delicate connective tissue. These rami- 
 fications 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, how- 
 ever, more intimately attached to the cord than to the brain. Un- 
 like the arachnoid membrane, the pia mater dips into the fissures 
 between the convolutions of the cerebrum. It also sends its pro- 
 longations, not only into the fissures of the cord, but also, as slen- 
 der bands (trabeculae) from its inner surface, into the columns of the 
 cord. These trabeculse branch and anastomose within the white 
 substance of the cord like the midrib of a leaf. 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 
 blood. This great nervous system, as a whole, consists of the cen- 
 tral organs spinal cord and brain and of various roots, divisions, 
 and branches of spinal and cranial, or encephalic nerves. 
 
 6. The Spinal Cord, or Medulla Spinalis, extends in the spinal 
 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 and a half, or not 
 far from one thirty-third of that of the brain. It is nearly cylin- 
 drical in shape, its front and back surfaces being somewhat flat- 
 tened ; it has two considerable enlargements of its girth an up- 
 per (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 no- 
 tice (1) 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 
 median) is somewhat broader than (6) the one behind (posterior 
 
B 
 
 40- 
 
 FlG. 13. A. Anterior, and B, Posterior. View of the 
 Spinal Cord and Medulla Oblongata. B', the Filum 
 terminale, whioh has been cut off from A and B. 1, 
 Pyramids of the medulla, and 1', their decussation. 
 2, olives ; 3, lateral strands of the medulla ; 4', cala- 
 mus scriptorius ; 5, the funiculus gracilis ; and 6, 
 the ftmiculus cuneatus ; 7. the anterior, and 9, the 
 posterior, fissures ; 8, the antero-lateral impression ; 
 10, postero-lateral groove. C, the cervical, and L, 
 the lumbar, enlargements of the cord. 
 
 10 -I 
 
THE SPINAL COED. 
 
 A 
 
 median). Both are filled to their bottom with processes of the pia 
 mater ; and the sides of the posterior fissure are bound closely to- 
 gether by the same membrane. 
 
 Each of these symmetrical and nearly half-cylindrical halves of 
 the cord is subdivided by the lines of the entrance of the posterior 
 and anterior nerve-roots into (2) three Columns : (a) the anterior, 
 which lies between the anterior median fissure and the anterior 
 roots ; (b) the posterior, which lies between the posterior median 
 fissure and the posterior roots ; and (c) the lateral column, which 
 lies at the side of the cord between the other two columns. 
 
 (3) The Commissures of the spinal cord are two bands of ner- 
 vous matter which unite 
 its halves, thus prevent- 
 ing it from being com- 
 pletely separated into 
 two portions by the 
 fissures. The one in 
 front, at the bottom of 
 the anterior median 
 fissure, is composed of 
 transverse nerve-fibres 
 and is called (a) the 
 anterior white commis- 
 sure ; the one behind, 
 at the bottom of the 
 posterior fissure, is (b) 
 the posterior gray com- 
 missure. The gray 
 commissure is nearly 
 
 Fio.14.-A, Anterior, and B, Lateral, View of a Portion of the twice as lar g e a S the 
 
 Cord from the Cervical Region. 2 /i- (Schwalbe.) 1, Anterior w >,itp PTPpnt nt tViP 
 
 median, and 2, posterior median, fissures. At 3 is the an- wmte > except at tne 
 
 tero-lateral impression, over which spread the anterior roots nprviPftl and Inmhnr Pn- 
 
 (5). The posterior roots (6), with their ganglion (6/), arise C 
 
 from the postero-lateral groove, and uniting with the ante- largements of the COl'd 
 rior roots form the compound nerve (7). 
 
 where the white is 
 
 larger. 1 Along its whole length the gray commissure incloses a 
 circular or elliptical canal (central canal), whose diameter is about 
 one-twenty-fifth of an inch and which is lined by ciliated cells. Near 
 the central canal lies a thin layer of gelatinous substance. The rest 
 of the gray commissure consists for the most part of extremely fine 
 nerve-fibres devoid of medullary sheath; while the white com- 
 missure is composed of medullated fibres. The thickness of the 
 
 1 See Henle, Anatomie des Menschen. Text, p. 309. 
 
CRESCENT SHAPE OF THE HORNS. 
 
 67 
 
 commissures is, as a rule, proportional to the size of the corre- 
 sponding 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 termi- 
 nate scarcely any white matter appears ; the amount of such matter, 
 however, increases from below upward, and is largest in the cervi- 
 cal 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 th,e 
 commissures which unite them, form a figure somewhat like a large 
 Roman X, 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 lumbar en- 
 largements. Sometimes the 
 figure is rather like that of a 
 large %, or a pair of butterflies' 
 wings. The two limbs of each 
 side of the figure into which 
 the gray columns are thus 
 formed are called (4) Horns ; 
 (a) the anterior horn is round- 
 ed, (b) the posterior long and 
 narrow. The division into an- 
 terior, posterior, and lateral 
 columns, which is well marked 
 on the external surface of the spinal cord, is gradually lost as we 
 pass inward toward the central gray substance. Of the two horns 
 of each side, the anterior has the appearance of " spongy sub- 
 stance," the posterior of a kernel of such substance surrounded by 
 gelatinous substance. 
 
 9. Careful study of the spinal cord with the higher powers 
 of the microscope has enabled histologists to describe with further 
 
 FIG. 15. Transverse Section through the Spinal 
 Cord. AF, antero-median, and PF, postero-median 
 fissures ; PC, posterior, LC, lateral, and AC, anteri- 
 or columns ; AR, anterior, and PR, posterior nerve- 
 roots ; C, central canal of cord, with its columnar 
 endothclial lining. The pia mater is shown invest- 
 ing the cord, sending processes into the anterior 
 and posterior fissures, as well as delicate prolonga- 
 tions into the columns. The crescentic arrange- 
 ment of the gray matter is shown by the darker 
 shaded portion. 
 
THE SPINAL CORD. 
 
 Ti 
 
 details the manner in which the nervous elements, both fibrillar and 
 granular, are arranged within the connective substance. 
 
 The White Substance of the spinal cord, besides connective tis- 
 sue and lymph- and blood- 
 vessels, is composed of 
 nerve-fibres of compara- 
 tively large or of medium 
 size. The essential constit- 
 uent of these fibres is the 
 axis-cylinder, the diameter 
 of which is generally one- 
 third or one-fourth of their 
 breadth. When fully de- 
 veloped, they are rarely or 
 never without a medullary 
 sheath, but probably have 
 no neurilemma. Their di- 
 ameter is not constant ; 
 the thickest fibres ( rsVo ^ 
 2 oVo f an inch) are found 
 in the outer portions of the 
 anterior columns, where 
 their size is tolerably uni- 
 form. In the lateral col- 
 umns the nerve-fibres vary 
 greatly in size, the finer 
 ones lying inward near the gray matter. In the posterior columns 
 they increase in thickness as they approach the posterior gray com- 
 missure. In the upper thoracic, and through the whole of the cer- 
 vical, region, there is found a wedge-shaped bundle of fine fibres 
 that is separated off from the posterior columns toward the middle 
 line of the cord by a strong septum ; this is called fasciculus gracilis, 
 or " column of Goll." 
 
 The direction of some of the nerve-fibres in the white substance 
 of the cord is vertical, of others, horizontal, of still others, oblique. 
 The vertical fibres are most abundant, are united with a parallel 
 arrangement into fascicles of various sizes, and ascend toward the 
 brain. Horizontal fibres in the white substance of the spinal cord 
 are of two kinds commissural fibres and fibres of the roots. The 
 fibres of the white commissure run horizontally along the median 
 border of the gray matter of the horns, and become interwoven with 
 the vertical bundles of the anterior columns. Most of them pass 
 from the substance of the anterior horn of one side across to the 
 
 FIG. 16. Section of Dorsal Part of the Spinal Cord show- 
 ing the Gray Matter of the Horns. ao /j. (Henle.) 
 Ca, anterior white, and Cg, gray commissure ; Co, cen- 
 tral canal ; v, vesicular column ; s. spongy substance of 
 the posterior horn, surrounded by g, gelatinous sub- 
 stance; Pr, reticular process; Ti, intermedian lateral 
 tract. 
 
COURSE OF FIBRES IN THE COED. 
 
 69 
 
 anterior column of the other side. The fibres of the posterior spi- 
 nal roots run in a nearly horizontal direction inward ; they divide 
 into anastomosing bundles so minute and so intricately interwoven 
 with the vertical fibres of the posterior column that their course 
 is difficult to trace. Part of them (the lateral ones) run directly 
 into the subslantia gelatinosa of the posterior horns, and are, per- 
 haps, continuous with the axis-cylinder processes of the nerve-cells 
 of its spongy kernel ; part of them appear to enter the gray sub- 
 stance of these horns only after curving and running a variable dis- 
 
 FIG. 17. Section of the Spinal Cord at the Level of the Eighth Pair of Dorsal Nerves. 8/ p 
 (Schematic, from Schwalbe.) $.., anterior fissure; s.p., posterior septum (or fissure); c.a., 
 anterior, and c.p., posterior, commissures ; c.c., central canal ; co.a, anterior horn ; co.l., lateral 
 horn ; CO.JD., posterior horn ; #, anterior lateral, and &, anterior median cells ; e, cells of the 
 lateral horn ; d, columns of Clai-ke ; e, solitary cells of the posterior horn ; r.a., the anterior, 
 and r.p., the posterior, roots ; f, bundle of fibres of the posterior horn ; and /', bundle of the 
 posterior column ; _/", longitudinal fibres of the posterior horn ; s.g.R., gelatinous substance of 
 Rolando; /.., anterior,/.;., lateral, and/.p., posterior, columns. 
 
 tance upward, or perhaps downward, in the posterior columns. 
 The fibres of the anterior roots of the spinal cord traverse its white 
 substance obliquely ; some of them enter the gray matter of the 
 anterior horns on the same side, where they probably become con- 
 tinuous with the axis-cylinder processes of its large ganglion-cells ; 
 others of them pass through the anterior commissure to the other 
 side of the cord ; still others pass into the lateral columns and the 
 posterior horns. 
 
 The Gray Substance of the spinal cord, in addition to the same 
 constituents as those of the white substance, has numerous nerve- 
 
70 THE SPINAL CORD. 
 
 cells. Its nerve-fibres, which form the chief part of its mass, and 
 are generally non-medullated, differ from those of the white sub- 
 stance in that they frequently subdivide and thus become attenu- 
 ated into extremely minute plexuses. The ganglion-cells of the 
 spinal cord are multipolar, and give off two kinds of processes ; one 
 an unbranched axis-cylinder process and the others branching pro- 
 cesses, both being of a fibrillated character (comp. Chap. I, 28 
 and 29). The unbranched processes of the ganglion-cells of the 
 anterior horns are probably continuous with the axis-cylinders of 
 the nerve -fibres of the anterior spinal roots. Of most of the simi- 
 lar processes from cells in the posterior horns we cannot yet make 
 the same affirmation. The branching processes of the nerve-cells 
 were traced by Gerlach * until he thought himself able to affirm 
 that their finest ramifications participate in those plexuses of nerve- 
 fibres which he regards as an essential constituent of the gray sub- 
 stance of the cord. Henle 2 and others consider the fate of these 
 processes to be still unknown. 
 
 Characteristic groups of ganglion-cells occur at various places in 
 the sections of the gray matter of the spinal cord. In the anterior 
 horns of the cervical and lumbar regions are three groups of large 
 cells ; one of these is on the side of the horn (lateral), one farther 
 to the front, one on its median border. They all coalesce in the 
 anterior horns of the thoracic region. In the anterior horns also 
 occur isolated nerve-cells of different sizes. The middle part of 
 the gray lateral halves of the spinal cord contains, in parts of the 
 cervical and thoracic regions, isolated groups of cells ; one impor- 
 tant group is situated at the inner angle of the base of the posterior 
 horn, and is called the " columns of Clarke." The other nerve-cells 
 of the posterior horns are small, and are not collected into groups, 
 but are distributed through that part of the substance of the horns 
 which is also traversed by the above-mentioned fine plexuses of 
 nerve-fibres (see Fig. 17). 
 
 10. By careful counting, E. A. Birge 8 ascertained the number 
 of the elements in the spinal cords of several frogs. From his con- 
 clusions something may perhaps be gained toward forming a better 
 conception of this organ. In seven cases Birge found that the num- 
 ber of fibres in the anterior roots varied from 5,984 in the smallest 
 animal to 11,468 in the largest ; the number increasing at the rate of 
 about one thousand four hundred and fifty motor fibres to each added 
 ounce of weight (51.5 to the gram). The diameter of the fibres was 
 
 1 See in Strieker's Human and Comparative Histology, ii., pp. 352 ff. 
 
 2 Anat. des Menschen. Text, pp. 310 ff. 
 
 8 Archiv f. Anat. u. Phvsiol, 1882, Physiolog. Abth., pp. 435-479. 
 
NERVE-TRACTS IN THE CORD. 71 
 
 also found to be much enlarged, according to the size and weight of 
 the animal ; t and the average diameter widely different in the different 
 nerve-roots. For example, it varied from 3,550 fibres, in the sev- 
 enth pair of nerves, to 14,133 in the tenth pair, for a cross-sec- 
 tion one twenty-fifth of an inch square. So, too, were the so- 
 called motor-cells of the anterior gray columns found to vary from 
 4,871 to 11,517, according to the weight of the animal. It was 
 found that the large masses of cells lie in two principal groups, 
 corresponding to the cervical and lumbar enlargements of the 
 cord. 
 
 11. It would be of great interest to our inquiries if it were 
 possible to give a complete description of the tracts of the nerve- 
 fibres in their passage along the spinal cord ; but it is impossible 
 for the microscope to unravel them, and the evidence of physiology 
 is (as we shall see subsequently), somewhat doubtful and even con- 
 flicting. Of late, however, certain of these paths have been traced 
 with considerable certainty by combining the methods of embry- 
 ological and pathological observation. In the development of the 
 spinal cord, the medullary substance of the nerve-fibres along cer- 
 tain tracts of the white columns is formed later, so as to render 
 them distinguishable in cross-sections. Moreover, when the nerve- 
 fibres are separated from their place of origin, degeneration of their 
 elements takes place. The place of the degenerated nervous sub- 
 stance is taken by connective tissue, which behaves differently un- 
 der the influence of staining fluids. By following the course of 
 this degeneration toward their periphery, the paths of conduction 
 in the nerves may be traced. Some time ago, Tiirck * attempted to 
 mark out certain motor tracts in the brain by using this process of 
 degeneration as his guide. Our great authority at present on the 
 paths of the nerve-fibres in the spinal cord and brain, as ascer- 
 tained chiefly by the former of these methods, is the work of 
 Flechsig. 2 
 
 Two tracts in the antero-lateral columns, which extend along the 
 greater part of the spinal cord and into certain parts of the brain, 
 are thus quite certainly made out. From their upper connections 
 they have been named the pyramidal tract (or tracts) and the direct 
 lateral cerebellar tract. The former is directly traceable down from 
 the anterior pyramid of the medulla oblongata. Most of the fibres 
 of this tract cross over in the extreme upper part of the cord, and 
 pass down it in the back part of the lateral column as a compact 
 
 1 Sitzgsb. d. Kaiserl. Acad., vi., pp. 303 ff. 
 
 2 Die Leitungsbalinen im Gehirn u. Ruckenmark d. Menschen. Leipzig, 
 1876. 
 
THE SPINAL CORD. 
 
 bundle. This crossed (or lateral) part of the pyramidal tract can 
 be traced as far as the third or fourth pair of the sacral nerves. 
 But some of the fibres from the pyramids of the medulla do not 
 cross in the upper part of the cord. These 
 form the uncrossed (or anterior) part of the 
 pyramidal tract ; this part gradually dimin- 
 ishes as it passes downward, and ceases in 
 the dorsal region of the cord. The direct 
 lateral cerebellar tract lies between the late- 
 ral pyramidal tract and the outer surface of 
 the cord. It disappears in the lumbar re- 
 gion. It is thought that the rest of the ante- 
 rior column of the cord, besides the anterior 
 pyramidal tract, may be, for the most part, 
 commissural in nature that is, it serves to 
 bind together the two halves of the cord on 
 the same level, or somewhat obliquely those 
 lying slightly below or slightly above. 
 
 In the posterior white column a tract can 
 be traced as far downward as the middle of 
 the dorsal region of the cord ; this is the 
 one already referred to as the " tract (or 
 column) of Goll." 
 
 12. The spinal cord is, therefore, shown 
 to be a mechanism composed by combining 
 the nervous elements so as to serve the 
 great purpose of conducting nerve-commo- 
 tion and acting as a series of reflex and auto- 
 matic centres. In it we find tracts of con- 
 nected nervous elements for the movement 
 of ascending and descending nervous im- 
 pulses. It is also a column or pile of ner- 
 vous centres, each one of which may have a 
 particular value for particular functions ; 
 but which are also all bound together, up 
 and down, right and left, and obliquely, so 
 as to act unitedly under a certain control 
 from each other and from the central organs 
 lying above. It is especially strong in nerve- 
 cells, just where it needs to be so namely, at the enlargements, 
 where it sends off nerves to the upper and lower limbs. Its paths 
 for the passage and diffusion of molecular disturbance are indefi- 
 nitely numerous, and their intricacy extremely great. It has groups 
 
 FIG. 18. Sections through the 
 Spinal Cord at different ele- 
 vations, to show the tracts of 
 White Substance. /., eleva- 
 tion of the sixth cervical 
 nerres. //., of the third; 
 ///., of the sixth ; and TV., 
 of the twelfth, dorsal nerves; 
 and V., of the fourth lumbar 
 nerves ; pv, uncrossed (or 
 anterior) pyramidal tract; 
 jo, crossed (or lateral) pyra- 
 midal tract ; *, direct late- 
 ral cerebellar tract ; fir, tract 
 of Goll. 
 
ELEMENTS OF THE INTEECEANIAL OEGANS. 73 
 
 of nerve-elements, such as belong to the central organs generally, 
 of ganglion-cells embedded in neuroglia ; it has special local mech- 
 anisms within, and yet connected with its general mechanism. It 
 is adapted to do a large amount and variety of work through its 
 pairs of nerves, without calling upon the higher nervous centres ; it 
 is constructed so as to act like a system of relays, not only trans- 
 mitting, but also modifying, inhibiting, enhancing, and distributing 
 the impulses which it receives, both from the more central and from 
 the peripheral portions of the cerebro-spinal system. 
 
 13. The same elements of nerve-fibres and nerve-cells, in con- 
 junction with connective tissue and neuroglia, and enveloped in the 
 three inclosing membranes (dura mater, arachnoid, and pia mater) 
 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. Here, 
 too, these elements are gathered into fascicles of nerve-fibres 
 which converge, or diverge, and run their courses in various direc- 
 tions, and into ganglionic masses, in which, besides the nerve-fibres, 
 nerve-cells and diffused finely granular substance of a doubtful 
 physiological character are found. Uniformity of elementary parts, 
 together with the greatest intricacy of arrangement, prevails, above 
 all other regions of the body, in the structure of the brain. The 
 significance of the elements and elementary 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. 
 
 14. The Encephalon, or Brain, in the most extended sense of 
 the word, includes all that portion of the central nervous axis which 
 is contained within the cavity of the skull. This grand mass of 
 nervous matter may be divided into several parts, somewhat differ- 
 ently marked off according to the point of view from which the 
 division proceeds. The division proposed by Meynert ' to which 
 reference will be made later is based upon the supposed physio- 
 logical significance of the different parts, and upon their arrange- 
 ment so as to discharge the functions of conduction and " suscep- 
 tibility to impressions." For, as this authority rightly claims, " a 
 purely histological description " is of comparatively little service 
 in comprehending the meaning of the architecture of the brain. 
 We shall, first of all, however, describe briefly the contents of the 
 cranial cavity, as it appears both to the unaided eye and under the 
 microscope, without reference to theory. 
 
 1 In Strieker, Human and Comparative Histology, ii. , pp. 367 ff. 
 
74 STRUCTURE OF THE BRAIN. 
 
 On removing the entire brain from the skull, the following four 
 divisions of its mass engage the attention of even the inexperienced 
 observer. Immediately above the section by which it has been sepa- 
 rated from the spinal cord, and appearing as an enlarged prolonga- 
 tion of the cord, is (I.) the Medulla Oblongata. Covering the upper 
 back part of this organ, and extending beyond it on both sides, 
 with its surface divided into small lobes by furrows, is (II.) the 
 Cerebellum, or little or hinder brain. Swelling out in front of and 
 above the medulla is (IH.) the Pons Varolii, or so-called "bridge" 
 of the brain. While in two hemispheres separated by a deep fis- 
 sure, above both pons and cerebellum, and filling the larger part 
 of the cranial cavity, is seen (IV.) the Cerebrum, or large brain, or 
 
 Cbl 
 
 ^^ 
 
 Mo 
 
 Fio. 19. View of the Brain in Profile. %. (Henle.) (76, cerebrum; (757, cerebellum ; J/b, me- 
 dulla oblongata ; / J , pons Varolii, 
 
 brain proper. These divisions are all readily distinguishable on 
 the external surfaces of the Encephalon. 
 
 On laying the encephalic mass open, however, certain bodies of 
 nervous matter are disclosed that have been concealed beneath the 
 cerebellum and the cerebrum, and that^-although ordinarily re- 
 garded as parts of the latter are scarcely to be included in any one 
 of the four main divisions of the brain. We shall describe in order 
 the organs just named. 
 
 15. I. The Medulla Oblongata is somewhat pyramidal in form, 
 about one and one-fourth inch in length, from three-fourths to one 
 inch broad in its widest part, and one-half inch thick ; it extends 
 from the spinal aperture of the cranial cavity (foramen magnum) to 
 the lower border of the pons Varolii. It is continuous with the 
 spinal cord, and somewhat resembles it in the divisions of its ex- 
 ternal surface. Its anterior pyramids appear superficially continu- 
 
EXTEENAL ASPECT OF THE MEDULLA. 
 
 ous with the anterior columns of the cord ; its lateral area shows 
 upon its upper end an oval-shaped elevation called the " olivary 
 body ; " its posterior tracts also appear continuous with the poste- 
 rior columns of the cord. Just outside the upper portion of each 
 posterior tract, and behind the olive, ascends to the cerebellum a 
 strong tract named Cq 
 
 the "restiform body." 
 That portion of the 
 posterior column of 
 the upper cord (al- 
 ready referred to, p. 
 68) which is marked 
 off from the rest by a 
 septum of pia mater, 
 is continued up into 
 the medulla oblonga- 
 ta, and becomes more 
 strongly marked. It 
 is known as the fu- 
 niculus gracilis ; and 
 when traced still far- 
 ther upward is seen to 
 broaden out into an 
 expansion called the 
 clava. A prolongation 
 of the posterior lateral 
 column also gradually 
 expands as it ascends, so that it acquires a "wedge-shape" form, 
 and is accordingly known as the cuneate funiculus. 
 
 The medulla oblongata, like the spinal cord^ is composed of white 
 and gray nervous matter ; it differs from the cord, however, in hav- 
 ing its gray matter not 'confined to the central part, but gath- 
 ered more into special masses or nuclei. A redistribution of the 
 nerve-elements takes place in the medulla, and their arrangement 
 becomes more complex. An important part of this redistribution 
 is accomplished by the divergence of the posterior tracts and resti- 
 form bodies, which opens up tfre central gray mass, and lets it 
 come to the surface between the sides' of the surrounding white 
 matter. Looking at this redistribution as it appears from below, 
 the elements of the cord may be said to be spread out and increased 
 by the addition of new elements ; looking at it as it appears from 
 above, the two great nerve-tracts of the cerebrum (tegmentum and 
 crusta of the crus cerebri), and the tract of the cerebellum, may be 
 
 FIG. 20. Back View of the Medulla Oblongata, the Cerebellum 
 being removed. (Henle.) Cq. corpus quadrigeminum ; Lc, 
 locus cceruleus ; F, flocculus of the cerebellum ; Ac, ala ci- 
 nerea ; and Ac' Stilling's nucleus accessorius ; 01, clava ; Fc, 
 funiculus cuneatus ; Fg, funiculus gracilis. 
 
76 
 
 STRUCTURE OF THE BRAIN. 
 
 said to be gathered up in the medulla, and compressed so as to 
 form in the cord a continuous and symmetrical medullary invest- 
 ment for its central gray matter. 
 
 The intimate structure of this organ is exceedingly complicated ; 
 much of it is doubtful, and as yet impossible to make out satis- 
 factorily. The two important considerations are (1) to trace the 
 nerve-fibres as they ascend through the medulla from the various 
 columns of the cord, and (2) to locate the particular collections 
 of gray matter, whether as continuous with those of the cord or as 
 consisting of independent masses. 
 
 The various tracts of White Matter in the medulla oblongata, 
 although they superficially appear to be prolongations of the col- 
 
 KT 
 
 - cc 
 
 C.a. 
 
 FIG. 21. Section showing the Decussation of the Pyramids at the point where the Spinal Cord 
 passes into the Medulla Oblongata. %. (Schwalbe.) f.l.a., longitudinal anterior fissure, 
 through which the bundles of pyramidal fibres (py, py'), are crossing over at d ; V, anterior, 
 and S. lateral pyramids; <7.a., anterior horn with groups of ganglion-cells, a and b ; cc. cen- 
 tral canal; f.r., formatio reticularis; ce, the neck, and ff, the head, of the posterior horn; 
 M.C., nucleus of the funiculus cuneatus ; and n.gr., of the funiculus gracilis ; II 1 , funiculus gra- 
 cilis ; H *, funiculus cuneatus ; x, group of ganglion cells. 
 
 umns of the spinal cord, are really so to a small extent only. This 
 fact is most clearly made obvious by a comparison of successive 
 transverse sections. A large bundle of fibres, which in the cord 
 lies in the posterior part of the lateral column (see p. 71 f.), pushes 
 its way obliquely through the gray matter of the anterior horn, and 
 passes in front of the central canal to the pyramid of the opposite 
 side. The crossing of this bundle, as seen in the anterior median 
 
DECUSSATION OF THE PYKAMIDS. 77 
 
 fissure at the lower part of the medulla, is called the " decussation 
 of the pyramids." The abrupt passage of so many fibres through it 
 breaks up the anterior horn, separates part of it from the rest, and 
 pushes this separated part over to one side, so that it comes to lie 
 close to a part of the posterior horn. The latter also becomes gradu- 
 ally shifted sidewise by an increase in the size of the posterior 
 tracts, so that it comes to lie almost at right angles to the posterior 
 median fissure ; its head enlarges and approaches close to the sur- 
 face, where it forms a projection (funiculas of Rolando), and, higher 
 up, a distinct swelling (tubercle of Rolando}. Tracing the princi- 
 pal bundles of fibres on their course from the columns of the spinal 
 cord upward through the medulla oblongata, we find (in accordance 
 with what has already been said) that the posterior column forms the 
 substance of the three posterior fuuiculi of the medulla namely, 
 gracilis, cuneatus, and funiculus of Rolando : a considerable part 
 of the lateral column (the lateral pyramidal tract, see p. 72) passes 
 into the opposite pyramid of the medulla, and ascends in it toward 
 the cerebrum in company with a small part of the anterior column 
 of the same side ; while another part of the lateral column (the 
 direct lateral cerebellar tract) passes at about the middle of the 
 medulla obliquely backward to the restiform body, and the rest of 
 it dips under the olives, and is continued toward the corpora quad- 
 rigeminum and optic thalamus. Most of the anterior column dips 
 under the pyramid, and passes upward toward the cerebrum, but 
 part is continued into the pyramid of the same side. 
 
 Curved fibres may also be seen running their course in the plane 
 of the different transverse sections some superficial, some deep 
 (arciform or arcuate fibres). 
 
 As the medulla is a bilateral organ, its halves are bound together 
 by commissural fibres, which run obliquely and decussate in the 
 mesial plane, forming a well-marked band called raph'e. In addi- 
 tion to the fibres of the medulla oblongata which are continuous 
 with those of the spinal cord, others originate within the organ 
 itself. It is a centre of origin for several pairs of encephalic 
 nerves. 
 
 The Gray Matter of the medulla oblongata is, in part, continu- 
 ous with that of the cord, and in part consists of independent masses. 
 The former part is, as we have seen, broken up and rearranged by 
 the decussation of the pyramids. The fate of the posterior horns 
 and of the central gray substance has already been described. The 
 substance of the anterior horns becomes divided into many little 
 masses by the nerve-fibres that traverse it, so as to form a coarse 
 network of nervous matter (formatio reticularis) containing nerve- 
 
78 
 
 STRUCTURE OF THE BRAIN. 
 
 cells, and intersected by bundles of fibres. In the upper part of 
 the organ its interior gray matter appears upon the floor of the 
 fourth ventricle, into which the central canal dilates. Four special 
 
 kernels or nuclei, of gelatinous 
 / '"' ** n.c. appearance and containing few 
 
 n.c. multipolar nerve-cells, are to be 
 noted in each half of the me- 
 dulla. These are (1) the nucle- 
 us arciformis, which is situated 
 just beneath the pia mater, at 
 the front of the anterior pyra- 
 mid ; (2) the nucleus olivaris, or 
 dentate body (corpus dentatum), 
 which is within the inferior 
 olive, a mass of gray matter 
 folded in a zigzag or denticu- 
 lated manner, forming a sort of 
 capsule through the openings 
 of which closely packed masses 
 
 FIG. 22. Section showing Gray Matter of the of fibres run into the SUri'OUnd- 
 Medulla Oblongata, in the region of the upper . / o\ J.T 
 
 crossing of the Pyramids. /i- (Schwalbe.) ing Space | (6) 1116 nucleus Oll- 
 
 il varis accessorius, a smaller gray 
 mass lying on the outside of 
 the dentate body ; and (4) the 
 
 8oryolivary~nucleus; F.r. /formatio reticularis ; nudeUS pyramidallS (sometimes 
 fir, substantia gelatmosa; /.a., /.a. 1 , /.a. 2 , arci- ry 
 
 form fibres. also called " inner accessory nu- 
 
 cleus " of the olive), lying on the inside of the same body. Another 
 kind of collections of gray matter in the medulla consists of those 
 groups of multipolar cells to which the nerves that have here their 
 so-called roots of origin can be traced. These cells resemble those 
 of the gray columns of the cord the larger ones apparently being 
 connected with the roots of the motor nerves, the smaller with 
 those of the sensory. It may be assumed that some of their pro- 
 cesses are continuous with the axis-cylinders of the fibres of the 
 nerve-roots, and that others serve to place the medulla in direct 
 connection with the cerebrum ; positive demonstration of these as- 
 sumptions, however, requires further histological researches. The 
 nerve-nuclei in the medulla receive their name from the nerves 
 whose fibres originate in them. 
 
 16. II. In the Cerebellum, or Little Brain, the general arrange- 
 ment of the two kinds of nervous matter is the reverse of that of 
 the spinal cord and the medulla oblongata : the gray matter is 
 external, the white internal. More precisely, the cerebellum is 
 
 mid in which is n.ar, the nucleus arcifornris; 
 o, beginning of the olivary nucleus : o' acce^ 
 
PEDUNCLES OF THE CEREBELLUM. 79 
 
 a white or medullary mass rising out of three large bundles or 
 stalks of nerve-fibres on each side, and enveloped with a covering 
 of gray nervous matter. Like the other organs of the cerebro- 
 spinal system, it is a bilateral structure. These stalks of nerves 
 connect the cerebellum with three other organs, with parts of 
 which they are continuous. Considered as connections, they are 
 called the " peduncles " or crura of the cerebellum. Of the three 
 peduncles, (1) one (inferior peduncle) on each half of the organ is 
 identical with the restiform fascicle which ascends from the me- 
 dulla to the cerebellum ; (2) another (superior peduncle), similar to 
 the first in size, passes forward over the anterior end of the fourth 
 ventricle, and connects the cerebellum with the tegmentum of the 
 crus ; (3) a third (middle peduncle) passes down on each side into 
 the pons. This middle peduncle forms the larger portion of the 
 white core of the organ. In addition to the fibres from these three 
 sets of peduncles, this core is in part constituted by others which 
 arise in the cerebellum itself ; some of the latter connect together 
 the different regions of the organ lying above or below each other, 
 some unite the opposite and symmetrical regions of its hemi- 
 spheres. 
 
 The interior relations of the fibres from the three peduncles are, 
 on account of the extreme intricacy of their course, not yet fully 
 made out. United in the white core of the cerebellum, they form 
 a rather uniform mass, which is interrupted, however, by certain 
 nuclei of a gelatinous appearance. Within either hemisphere, and 
 to be disclosed by cutting through it a little to the outer side of the 
 median lobe, is a mass of nervous matter arranged like the den- 
 tate body of the medulla oblongata ; it is the corpus dentatum of 
 the cerebellum. Other smaller, round, or oblate masses of gray 
 matter are found toward the middle of the core from the dentate 
 body. 
 
 The arrangement of the gray matter which forms the rind or 
 cortex of the cerebellum is somewhat peculiar ; its characteristics 
 are best seen by examining a cross-section. It is thus found that 
 this cortical gray substance is arranged in thin plates, or lamellae, 
 which are penetrated by prolongations of the white matter of the 
 core ; these prolongations branch off into the interior of the lamel- 
 lae, and give to the cortex the arborescent appearance known by 
 the name of " arbor vitce." The primary branches of this tree-like 
 prolongation of the white matter of the core within the gray mat- 
 ter of the cortex stand either perpendicular or a little inclined to 
 the surface of the core. The smaller branches run from one side to 
 another transversely or forward in concave curves. 
 
80 STRUCTURE OF THE BRAIN. 
 
 The external surface of the cerebellum presents two hemispheres, 
 or lateral lobes, united by a central lobe called the vermiform pro- 
 cess. This central lobe on its upper (or tentorial) surface is a mere 
 elevation, but the " vermiform " character of its lower (or occipital) 
 surface is well defined. The process here lies at the bottom of a 
 deep fossa (vallecula). From the middle peduncle of each hemi- 
 sphere a large horizontal fissure extends backward along its outer 
 border, and divides the hemisphere into its tentorial and occipital 
 
 FIG. 23 Lower Surface of Cerebellum. %. (After Sappey.) 1, inferior vermiform process; 
 2, 2, vallecula : 5, flocculus; 6, pons Varolii ; 8. middle peduncle of the Cerebellum : 9, medulla 
 oblongata. Various pairs of nerves are seen thus : 12 and 13, roots of fifth pair ; 14, sixth 
 pair; 15, facial nerve; 17, auditory; 18, glosso-pharyngeal ; 19, pneurno-gastric ; 20, spinal 
 accessory ; 21, hypoglossal. 
 
 surfaces. Each of these surfaces is divided by fissures into smaller 
 lobes or lobules. 
 
 In the gray matter of the cortex of the cerebellum three distinct 
 layers of nervous substance may be distinguished. Of these the 
 pure gray layer is the most external ; it is sometimes called the 
 " molecular layer." It consists of an extremely delicate framework 
 of connective tissue, in which, together with nuclei of the connec- 
 tive tissue, a few roundish cells and minute fibres of nervous struct- 
 ure appear. The middle layer is cellular and composed of a single 
 irregular row of large ganglion-cells, called " cells or corpuscles of 
 Purkinje." Comparatively large processes from these cells branch 
 into and ramify within the outer layer. According to most observers 
 (Kolliker, Deiters, and others) each of the cells sends a single me- 
 dullated and unbranched process inward, which becomes continu- 
 ous with the axis-cylinder of a fibre of the medullary portion of this 
 organ ; but according to Stilling there are several branches from 
 each, which divide to form a network in the internal layer. This 
 
THE PONS VAROLII. 
 
 81 
 
 layer is rust-colored and merges gradually into the white substance 
 of the core ; it appears to contain multitudes of granules, with a 
 well-defined nucleus surrounded by branching protoplasm. The 
 nature of the granules is not known ; they have been considered 
 by some as elements of sustentacular tissue, by others as lymph- 
 corpuscles, by others as multipolar nerve-corpuscles. 
 
 The cerebellum is thus constituted by a complex arrangement 
 of the nervous elements as a kind of side mechanism of the nervous 
 system, lying out of the course of its direct tracts and yet bound 
 by nervous cords (the peduncles) in all directions to the other 
 organs of the brain. 
 
 17. III. The Pons Varolii, or Bridge of the Brain, has its princi- 
 pal office in the mechanism of the central organs of the cranial 
 cavity as a meeting- and switching-place of nerve-tracts between 
 other organs ; but it is also itself a central organ, as well as a cen- 
 tre of origin for certain nerve-fibres. The pons is really a thicken- 
 
 FIG. 24. Median Section through the Stem of the "Brain. (After Reichert.) M, medulla oblon- 
 gata ; of whichPa are the pyramid?, decussating &tpd ; c, central canal ; pp, restiform body ; 
 Pv, pons Varolii ; F4, fourth ventricle, av, arbor vitse of the cerebellum J p, pyramid ; , 
 uvula ; n, nodule ; as, aqueduct of Sylvius ; Cr, crus cerebri ; Q, corpora quadrigemina ; P, 
 pineal gland ; 7%, optic thalamus. Commissures ; ca, the anterior ; cm, the mollis ; and cp, the 
 posterior. F3, the third ventricle ; A, corpus albicans ; to, tuber cinereum ; i, infundibulum. 
 
 ing of the ventral wall of the fourth ventricle, composed of the mid- 
 dle peduncles of the cerebellum encircling and partly blending with 
 the continuation upward of the medulla oblongata. Its superficial 
 fibres on the ventral surface are transverse in their general direc- 
 tion ; but the middle fibres pass directly across, the lower ascend 
 slightly, and the superior are more curved, and descend obliquely 
 6 
 
82 STRUCTURE OF THE BRAIN. 
 
 to reach the crus cerebelli. On removing these superficial fibres 
 the prolonged fibres of the anterior pyramids are exposed to view. 
 These, as they ascend through the pons, are intersected by the 
 transverse fibres. At the lower part of the organ, behind the fibres 
 from the anterior pyramids, a special set of transverse fibres (tra- 
 pezium) begins at a collection of gray matter (superior olivary 
 nucleus) on one side, and crosses the middle line to ascend to the 
 cerebellum on the other side. 
 
 Nuclei of gray matter with small multipolar nerve-cells are found 
 everywhere between the fibres of the ventral part of the pons. 
 Many of its transverse fibres are probably connected with these 
 cells. The posterior portion of this organ is chiefly constituted by 
 a continuation upward of the formatio reticularis, and of the gray 
 matter of the medulla oblongata. In the reticular formation two 
 or three important collections of nerve-cells lie embedded. One 
 of these is the " superior olivary nucleus," which lies behind the 
 outer part of the trapezium and gives origin to some of its nerve- 
 fibres. Of the other nuclei in this region, one gives origin to the 
 seventh or facial nerve, and others to portions of the fifth nerve. 
 
 18. IV. The Cerebrum, or Large Brain, much exceeds in size all 
 the other contents of the cranial cavity ; but it surpasses them more 
 especially in the variety and complexity of the arrangement here 
 given to the nervous elements ; while its significance for the in- 
 quiries of Physiological Psychology is altogether unique. 
 
 As ordinarily described, this nervous mass includes a consider- 
 able number of organs, which vary in structure, relations, and 
 physiological functions. Besides the hemispheres of the cerebrum, 
 and the great ganglia (corpora striata and optic thalami) which lie 
 at their base, custom includes in this term certain bodies that ap- 
 pear connected with the lower surface of the mass, viz., the corpora 
 quadrigemina, pineal gland, crura cerebri, etc. 
 
 19. The Cerebrum is of ovoid shape and is divided above, in 
 front, and behind into two hemispheres by a deep median longi- 
 tudinal fissure. If these hemispheres are drawn asunder by open- 
 ing this fissure, they are seen to be connected at its bottom by 
 a broad white band of nervous matter, the corpus callosum. The 
 outer surface of each hemisphere is convex and fitted to the con- 
 cave inner side of the bones of the skull ; the inner surface along 
 the median fissure is flat, and separated from the corresponding 
 surface of the other hemisphere by a process of the dura mater 
 (fc&c cerebri) ; its under surface is separated from the cerebellum 
 and the pons by another process of the same membrane (tentoriuni). 
 From the front of the pons the large white nervous cords, called 
 
BASAL ASPECT OF THE CEREBRUM. 
 
 83 
 
 cerebral peduncles, or crura cerebri, pass upward and forward to 
 connect the cerebrum with the organs lying below it. Around 
 each crus winds a flat band, the optic tract ; these tracts come to- 
 gether in front to form the optic commissure from which the two 
 optic nerves arise. The lozenge-shaped space enclosed by the 
 crura cerebri, the optic tracts, and optic commissure, contains a 
 
 Let 
 
 Pec 
 
 Tbo 
 
 Cha 
 
 Tc 
 
 Spa 
 
 In 
 
 'Mo 
 
 FIG. 25. Under Aspect of the Brain. (Henle.) B, basis of the crura cerebri ; Oca, corpora al- 
 bicantia; I 1 , olfactory bulb; II 1 , optic tract; Tc, tuber cinereum ; Lpp, posterior perforated 
 space ; Ccl, corpus callosum ; Let, lamina cinerea terminalis ; Spa, anterior perforated space ; 
 T, tegrmentum ; Tho, thalamus opticus ; P, pons ; Mo, medulla oblongata ; I. to VIII., first to 
 eighth pair of cranial nerves. 
 
 gray layer (posterior perforated space), two small white bodies (cor- 
 pora albicantia), and a gray nodule (tuber cinereum) which is joined 
 to a small reddish-gray oval mass (pituitary body) by a conical Jjro- 
 cess of gray matter (infundibulum). In front of the optic commis- 
 sure is a thin layer of gray substance (lamina cinerea) ; and on each 
 side of .the deep longitudinal fissure stretches the olfactory tract, 
 
84 
 
 STRUCTURE OF THE BRAIN. 
 
 with its bulb. The intercranial part of this "nerve " is now known 
 really to be a projecting portion of the brain. All these structures, 
 together with the cut ends of the several pairs of cranial nerves, 
 ma} r be seen upon the under surface of the cerebrum. 
 
 20. The upper surface of the cerebral hemispheres presents the 
 appearance of gray nervous matter arranged in folds which are 
 called " convolutions " or gyri. These convolutions are separated by 
 " fissures " or sulci of varying depth, some of which are so constant 
 and strongly marked that their presence is employed to divide the 
 surface of the hemispheres into lobes, while others, less strongly 
 marked, separate from each other the convolutions of the same 
 lobe. It is the arrangement of the convolutions, with their sep- 
 arating fissures, which gives the hemispheres of the brain their 
 characteristic appearance, and which fits them for their unique 
 functions in the economy of the nervous mechanism. 
 
 FIG. 26. To show the Right Ventricle and the Left Half of the Corpus Callosum. a, transverse 
 fibres, and 6, longitudinal fibres of corpus callosum ; c, anterior, and d, posterior cornua of lat- 
 eral ventricle ; e, septum lucidum ; /, corpus striatum ; j7, taenia semicircularis ; A, optic thala- 
 mus ; *, choroid plexus ; J, taenia hippocampi ; m, hippocampus major ; , hippocampus 
 minor ; o, eminentia collateral! s. 
 
 21. By cutting off successive slices from the upper part of the 
 hemispheres their general internal structure may be seen. It re- 
 
VENTRICLES OF THE CEREBRUM. 
 
 85 
 
 sembles the plan upon which the cerebellum is constructed. A 
 
 core of white nervous matter is surrounded by a shell or cortex of 
 
 gray ;'the two lateral halves of the core are bound together by a 
 
 strong band of fibres, 
 
 usually described as 
 
 commissural (corpus 
 
 callosum), which is 
 
 itself overlapped by 
 
 one of the most 
 
 marked convolutions 
 
 of the brain (gyrus 
 
 fornicatus). By cut- 
 
 ting still deeper it is 
 
 found that the cor- 
 
 pus callosum forms 
 
 the roof of a space in 
 
 the interior of each 
 
 hemisphere (the late- 
 
 ral ventricles). These 
 
 two cavities or ven- 
 
 tricles are moistened 
 
 by a serous fluid and 
 
 separated by a thin 
 
 transparent wall (sep- 
 
 tum lucidum). The 
 
 roof of another cav- 
 
 ity, the third ventri- 
 
 cle, is formed by an 
 
 expanded fold of the 
 
 pia mater (velum in* 
 
 terpoftitum), the mar- 
 
 gins of which are 
 
 fringed by the so- 
 
 ,, T .. , ., , FIG. 27. Basal Ganglia of the Cerebrum seen from above. (Henle.) 
 
 Called CtlOrOld pleX- Ccl, genu of the corpus callosum ; Cs, corpus striatum ; Vel, 
 ventricle of the septum lucidum ; Of, column of the fornix ; St, 
 stria termmalis ; Tho, optic thalamus ; and Ts, its anterior tu- 
 bercle ; Com, middle commissure between the thalami and over 
 the third ventricle ; Pv, pulvinar ; Cn, conarium or pineal 
 arteries Which SUp- gland ; Cop ' corpus <ladrigeminum. 
 
 ply the nervous structures of this region. Each lateral ventricle is 
 divided into a central space and three curved prolongations or cor- 
 nua ; of the cornua, one (the anterior) extends forward and outward 
 toward the front part of the cerebrum ; one (the posterior) curves 
 backward, outward, and then inward ; and the third (the descending) 
 curves backward, outward, downward, and then forward and inward. 
 
 
 j -I -I , , 
 
 tile latter 
 
STRUCTURE OF THE BRAFN". 
 
 On the floor of each lateral ventricle the exposed portions of the 
 great basal ganglia of the cerebrum are visible. A large pear-shaped 
 body of gray color is here seen with its broad extremity directed 
 forward into the anterior cornu of the ventricle and its narrow 
 end outward and backward. 1 This body, on account of the striped 
 
 FIG. 28. A Deeper Dissection of the Lateral Ventricle, and of the Velum Interpositnm. a, un- 
 der surface of corpus callosum, turned back ; 6, 6, posterior pillars of the fornix, turned back ; 
 c, c, anterior pillars of the fornix ; d, velum interpositum and veins of Galen ; e, fifth ventricle ; 
 /. /, corpus striatum ; ff, ff, tasnia semictrcularis ; A, h, optic thalamus; A-, choroid plexus ; /, 
 tsenia hippocampi ; m, hippocampus major in descending cornu ; n, hippocampus minor ; o, 
 eminentia collateralis. 
 
 appearance which it presents when cut open, is called a " striate 
 body" (corpus striatum). It consists of two masses, the upper one 
 of which (nucleus caudatus) projects into the lateral ventricle ; the 
 lower one is embedded in the white substance of the hemisphere and 
 forms the principal part of the body (nucleus lenticularis). The two 
 are separated by a layer of white matter called the " internal capsule.'' 
 Between the diverging portions of the striate bodies are the oblong 
 
 1 Dalton (Topographical Anatomy of tlie Brain, Philadelphia, 1885, ii. , p. 
 76) and others speak as though the caudate nucleus alone were to be called 
 corpus striatum, the nucleus lenticularis by this name ; and the two considered 
 as separate bodies. 
 
THE CRtJSTA AND THE TEGMENTUM. 87 
 
 or somewhat ovoid masses of the " optic thalami." Each thalamus 
 rests upon and partially embraces one of the crura cerebri ; its me- 
 dian surface forms the side wall of the third ventricle, and upon its 
 outer and back part are two small elevations, one on each side of 
 the optic tract (corpora geniculata, internum and externum). In the 
 depression between each striate body and the optic thalamus is a 
 narrow, whitish, semitransparent band of medullary substance 
 (tcenia semicircular is). Along the entire length of the floor of the 
 descending cornu of the ventricle is a white eminence (hippocampus 
 major or cornu Ammonis) which is the inner surface of the gyrus 
 fornicatus doubled upon itself like a horn. An arch-shaped band 
 of nerve-fibres, consisting of two lateral halves, which, in front, 
 form two pillars that descend to the base of the cerebrum and be- 
 come the corpora albicantia, and which diverge behind into two pil- 
 lars that descend with the descending cornu of the ventricle and 
 connect with a convolution of the brain (gyrus hippocampi), is situ- 
 ated beneath the corpus callosurn ; it is called the/ornia?. Behind 
 and between the optic thalami, and resting on the back surface of 
 the crura cerebri, are four rounded eminences in two pairs, called 
 corpora quadrigemina ; the front pair are the nates, the back pair, 
 testes. 
 
 22. Without mentioning other more minute subdivisions, super- 
 ficial or internal, in the structure of the cerebrum as seen by the 
 unaided eye, we now consider the arrange- 
 ment of the nerve-fibres and nerve-cells in 
 the more important organs already named. 
 
 Of the fascicles of nerve-fibres belonging 
 to the cerebrum, some connect it with the 
 lower organs of the encephalon ; some con- 
 nect together its hemispheres ; some join 
 different structures in the same hemisphere ; FIG. 29. section through the 
 
 * Mid-brain. (Schwalbe.) erg-., 
 
 some are roots of origin for certain nerves. aqueduct of Sylvius ; .n., sub- 
 
 , , n i-i -I-J1 stantianigra ; p, crustaof the 
 
 The fibres 01 the crura cerebri those cms cerebri ; *, tegmentum of 
 strong peduncles of the brain that ascend 
 
 from the pons to the optic thalami and the striate bodies are 
 arranged in two groups (crusta and tegmentum) separated by the 
 gray matter of the substantia nigra. An important part of the 
 fibres of the crusta, or front part (pes) of the crus, is continuous 
 with the longitudinal fibres of the pons which come from the 
 pyramids of the medulla ; it receives some fibres from the gray mat- 
 ter of the substantia nigra. Many of the fibres of the crusta ter- 
 minate in the nuclei of the striate bodies ; but some radiate upward 
 through the internal capsule directly to the gray cortex of the 
 
88 STRUCTURE OF THE BRAIN. 
 
 cerebrum. (Comp. Fig. 24.) Some of the more diffused fibres of 
 the tegmentum, or back and deeper part of the crus, are probably 
 continued from the anterior column of the cord, and may be traced 
 above to the optic thalami. Others of its fibres are collected into 
 more well-defined tracts ; one of the most important of which 
 comes from the superior peduncle of the cerebellum, and has al- 
 ready been traced as it passes forward over the anterior end of the 
 fourth ventricle (see p. 79). The formatio reticularis is continued 
 into the tegmentum ; the latter, therefore, has a considerable 
 amount of gray matter containing nerve-cells. Some of its fibres 
 arise in these cells. The superior peduncle of the cerebellum as- 
 scends, crosses over to the other side beneath the Sylvian aque- 
 duct, and terminates in a collection of large pigmented cells (the 
 nucleus of the tegmentum or red nucleus). 
 
 The intimate structure of the striate bodies is not as yet entirely 
 made out. On its deeper side, which is turned toward the internal 
 capsule, the nucleus caudatus receives from the capsule several 
 bundles of fibres. According to Meynert, some of these bundles 
 serve to connect this nucleus downward with the peduncle of the 
 cerebrum, some upward with its cortex ; but, according to Wer- 
 nicke, it is doubtful if any of them pass to the white matter of the 
 hemispheres, or come directly (that is, without traversing the len- 
 ticular nucleus) from the crusta. All parts of the nucleus lenticu- 
 laris are pervaded with white fibres. Some pass into its inner zone 
 from the adjacent part of the internal capsule ; some connect it with 
 the caudate nucleus ; some pass from it into the corona radiata, 
 and then to the cerebral cortex. These nuclei appear to have a 
 special connection with the frontal and parietal lobes, but also with 
 some convolutions of the temporal lobe and the island of Reil. The 
 gray matter of this organ is composed of delicate connective tissue, 
 with "free nuclei sparingly distributed through it." The nerve- 
 cells of the nucleus caudatus are multipolar and of two sizes ; some 
 are about ^J-g- inch in diameter with many processes, but most are 
 much smaller '(T^TO i ncn ) Between the fibres of the gray matter 
 of the nucleus lenticularis are many ceUs with yellow pigment in 
 them. 
 
 The three collections of gray matter the locus niger, and the cau- 
 date and lenticular nuclei of the striate body with the nerve-fibres 
 which originate in them and bind them together, have been held to 
 constitute a connected chain of nervous organs, to which the name 
 "ganglia of the crusta" has been given by Meynert. Eecently, 
 however, this relation of the corpora striata as "basal ganglia," or 
 "middle-men " between the spinal cord and the cerebral cortex, has 
 
CORPUS GENICTJLATUM AND OPTIC THALAMUS. 
 
 89 
 
 been called in question by Wernicke and A. Hill. The latter argues, 1 
 chiefly on morphological grounds, that the nucleus caudatus should 
 be separated from the optic thalamus, and connected immediately 
 with the cortex. This connection, he thinks, is favored by the na- 
 ture of its development, by its minute structure, which differs from 
 that of the thalamus, and by its resemblance to another nucleus 
 (the amygdaloid) which has an undoubted origin from the cortex. 
 23. Another chain of nervous organs, leading between the pons 
 
 Varolii and the hemispheres of the 
 brain, consists of the tegmentum of 
 the crus and its ganglia the red 
 nucleus (already described), corpus 
 subthalamicon, the corpora genicu- 
 lata, and the optic thalami. 
 
 The arrangement of the nervous 
 elements in the external corpus 
 
 FIG. 31. 
 
 These and the following two Figures show the arrangement of the white and gray substance in 
 the interior of the cerebrum. (All four are from Gegenbaur.) 
 
 FIG. 30. Horizontal Section through the Right Hemisphere. 
 
 FIG. 31. Frontal Section through the Cerebrum in front of the Fornix. Posterior surface of the 
 
 section displayed. 
 
 geniculatum is peculiar : it consists of alternate layers of white and 
 gray matter, as though occasioned by laying a lamina of the gray 
 between two medullary laminae, and then folding them in a zigzag 
 manner. The nerve-cells of this organ are from -g-J-g- to -g-J-g- of an 
 inch in length, and y^-o f an i ncn m breadth ; they are coarsely 
 granular and pigmented. 
 
 The optic thalamus is a mass of gray matter, with multipolar and 
 fusiform cells, traversed by nerve-fibres. This gray matter is par- 
 tially subdivided into two parts, an inner and an outer nucleus. 
 
 1 The Plan of the Central Nervous System, pp. 25 ff. 
 
90 
 
 STRUCTURE OF THE BRAIN. 
 
 Its free surface (inner and upper) is covered by a layer of white 
 fibres. On its* outer surface is the white matter of the internal 
 capsule, formed by fibres diverging from the crusta into the hemi- 
 spheres. All along this surface fibres radiate from the interior of 
 this organ, and mingle with those of the internal capsule on their 
 way to the cerebral hemispheres. Those in front pass to the frontal 
 lobe ; those in the middle pass to the back part of the same lobe 
 and to the parietal lobe ; those behind to the temporo-sphenoidal 
 and occipital lobes. 
 
 "The external and under surfaces of the thalamus are not free, 
 but are united with the other parts of the brain. The under sur- 
 face is united with the tegmental part of the crus cerebri, while 
 the external surface is covered by white substance, that is formed 
 
 FIG. 32. 
 
 Fia. 33. 
 
 Fro. 32. Frontal Section through the Right Hemisphere of the Cerebrum in front of the Com- 
 
 missura Mollis. Posterior surface of section displayed. 
 
 FIG. 33. Frontal Section through the Cerebrum back of the Commissura Mollis. Front surface 
 
 of section displayed. 
 
 of fibres of the crusta, which here diverge into the substance of the 
 hemisphere and pass between the thalamus and the lenticular nu- 
 cleus, forming the so-called 'internal capsule'" 1 (comp. p. 87 f.). 
 The cells of its substance average about -g-J-g- inch in length and 
 2~gVo in breadth ; their long axis is parallel to the course of the 
 nerve-fascicles. According to a recent authority, 2 the thalamus 
 is the primary centre of the optic nerve, and is also connected with 
 the olfactory nerve originally by way of the fornix. 
 
 The nervous substance of the corpora quadrigemina consists 
 mainly of gray matter covered externally with a thin layer of 
 nerve-fibres. In the interior of the upper or front pair the most 
 
 1 Quain's Anatomy, ninth edition, ii., p. 324. 
 
 2 A. Hill, The Plan of the Central Nervous System, pp. 20 ff. 
 
FIBRES OF THE CORONA RADIATA. 91 
 
 characteristic portion of this organ is found ; it is a layer of fine 
 nerve-fibres running longitudinally, between which are small, scat- 
 tered nerve-cells. In the external strata of these bodies multipolar 
 cells are abundant ; in their interior, at the sides of the Sylvian 
 aqueduct, is a collection of gray matter which forms a continuation 
 of the lining of the third ventricle. The nerve-cells in the -corpora 
 quadrigemina vary greatly in size. Most of those in the superficial 
 strata are small ; but in the deeper strata some of them reach the 
 maximum of nearly ^-J-o of an inch. The centres of origin for the 
 third and fourth nerves are in that nervous structure of fine fibrils 
 and fusiform cells which lies along the Sylvian aqueduct. 
 
 24. The arrangement of the nervous elements in all the basal 
 ganglia, as connected with the cerebral peduncles, indicates the na- 
 ture of the mechanism of this region of the brain. It is constructed 
 so as to co-ordinate all the nerve-tracts of motion with those of 
 sense, and thus give to these ganglia important reflex and auto- 
 matic powers over the sensory-motor apparatus, while subordinating 
 them to the control of the nervous centres of the cerebral cortex 
 that lie farther above. 
 
 To the highest and dominating nervous centres in the cerebral 
 hemispheres the paths of nervous impulse are laid from the basal 
 ganglia by that blossoming out, as it were, of the nerve-fibres on 
 their way to the white core of the cerebrum, which is called the 
 "corona radiata." The corona is formed by the fibres that radiate 
 from the striate bodies, from the optic thalami, and the internal 
 capsule, into the convolutions of the lobes of the hemispheres. 
 
 25. The combination of the nervous elements into the pre- 
 eminently complex mechanism of the convolutions of the human 
 cerebrum may be described from two points of view ; the first is 
 that from which their various external surfaces may be regarded by 
 the unaided eye, the second that which histology assumes when it 
 examines under the microscope various sections made from layers 
 of their substance. 
 
 The details of the external aspect of the convolutions vary so much 
 in each individual, and even in the two hemispheres of the same brain, 
 that the only chance of bringing order out of this apparent confusion 
 is to discover what is general, and for the most part constant, in the 
 midst of what is particular and subject to change. In making such 
 discovery the study of embryology is especially important. Certain 
 sulci and their corresponding gyri appear with a marked regular- 
 ity in the earlier and more fundamental stages of the development 
 of the fcetal brain. So, too, does the examination of the surfaces 
 of the hemispheres of the adult brain show certain degrees of 
 
92 THE CEREBEAL COTCTEX. 
 
 strength with which the sulci and gyri are distinguishable, and 
 thus enable the investigator to divide them into so-called pri- 
 mary, secondary, and even tertiary classes. Bischoff, Ecker, and 
 others have aptly compared the primary gyri to the large mountain 
 ranges whose sinuosities give to an entire region its characteristic 
 features ; the secondary gyri are like those subordinate ranges 
 which are brought into existence through the formation of longi- 
 tudinal valleys (secondary sulci) in the main ranges ; while the ter- 
 tiary convolutions may be compared to the small spurs which run 
 out into the valleys between the principal ranges and from their 
 sides. Only the primary gyri are, as a rule, pretty regularly dis- 
 posed. 
 
 It is by means of the primary sulci that the surfaces of the hem- 
 ispheres of the brain have been divided by modern anatomy into 
 five territories or Lobes. 1 The frontier lines of these lobes, how- 
 ever, are clearly laid down only on some of the surfaces, while on 
 other surfaces the lobes encroach on each other without distinct 
 boundaries. The five lobes are called Frontal, Parietal, Temporo- 
 sphenoidal (also Temporal or Sphenoidal), Occipital, and Central, 
 or Insula, or Island of Reil ; the latter does not stand in immediate 
 relation with the walls of the skull. The Frontal Lobe is divided 
 from the parietal on its upper and lateral surface by the Fissure of 
 Rolando (sulcus centralis) ; and on its lower surface from the tem- 
 poral lobe by the horizontal branch of the Fissure of Sylvius. The 
 Parietal Lobe is divided from the temporal for the greater part 
 by the Fissure of Sylvius, and from the occipital on its median 
 surface completely, but on its upper surface only very incompletely 
 by the parieto-occipital fissure. The Temporo-sphenoiclal lobe 
 is distinctly marked off from the frontal and parietal, as already 
 described ; while the boundary line between it and the occipital 
 lobe is ill defined. The Island of Reil lies concealed between the 
 frontal, parietal, and temporo-sphenoidal lobes; its surface, when 
 exposed by drawing aside the margin of the Sylvian Fissure, shows 
 a few short convolutions which radiate forward, upward, and back- 
 ward from a central spot on the lower surface. The occipital, tem- 
 poro-sphenoidal, and frontal lobes, all have three principal convolu- 
 tions arranged in parallel tiers (superior, middle, and inferior) ; in 
 the frontal lobes these three spring from the anterior part of the 
 ascending convolution just in front of the Fissure of Rolando (that 
 is, from the gyrus centralis anterior) and run forward to the front 
 end of the cerebrum. 
 
 1 The convolutions are here described in dependence upon the work of 
 Ecker, The Convolutions of the Human Brain. London, 1873. 
 
ARRANGEMENT OF THE SULCI AN 
 
 %/FOI 
 
 GYRI. 
 
 93 
 
 26. A few of the most important of the sulci and gyri need 
 separate mention ; the accompanying diagrams will make clear 
 
 FIG. 34. 
 
 FIG. 35. 
 
 FIGS. 34 and 35. Profile and Vertex Views of Cerebrum. Fr, the frontal lobe ; Par, parietal ; 
 0c, occipital : Ts, ternporo-sphenoidal lobe ; SS, Sylvian fissure ; ER, fissure of Rolando ; PO, 
 parieto-occipital fissure ; IP, intra-parietal fissure ; PP, Parallel fissure ; SF and IF, supero- 
 and infero-frontal fissures ; 1, 1, 1, inferior, 2, 2, 2, middle, and 3, 3, 3, superior frontal convo- 
 lutions ; 4, 4, ascending frontal convolution ; 5, 5, 5, ascending parietal, 5', postero-parietal, 
 and 6, fi, angular convolutions ; A, supra-marginal, or convolution of the parietal eminence ; 7, 
 7, superior, 8, 8, 8, middle, and 9, 9, 9, inferior temporo-sphenoidal convolutions ; 10, superior, 
 11, middle, and 12, inferior occipital convolutions ; a, , -y, 5, four annectent convolutions. 
 
 further details (see Figs. 34, 35, and 36). Among the sulci which 
 bound the main territories of the cerebral hemispheres the Fissure 
 
94 
 
 THE CEREBRAL CORTEX. 
 
 of Sylvius is much the most important. "It can," says Ecker, 
 " in nowise be considered in the same category as the rest of the 
 sulci on the surface of the brain." The other sulci may be re- 
 garded as mere folds of the cerebral cortex ; the Fissure of Sylvius, 
 on the contrary, is made by folding the entire hemisphere into an 
 arch, with its concave surface downward, about the point of en- 
 trance of the crus cerebri. This fissure exists in the foetal brain at 
 the third month. " It arises," say Foster and Balfour, 1 " at the time 
 when the hemispheres, owing to their growth in front of and be- 
 hind the corpora striata, have assumed somewhat the form of a 
 bean." The Fissure of Rolando is also always present in the human 
 brain. It makes its appearance in the foetus as early as the end of 
 the fifth month. It is rarely, if ever, bridged over by a secondary 
 gyms ; it, therefore, forms a point of departure in the examination 
 of all the convolutions. It is bounded for its entire length by two 
 
 o c 
 
 FIG. 36. Convolutions of the Inner and Tentorial Surfaces of the Left Hemisphere, i. i, i, cal- 
 loso-marginal fissure, I, I, calcarine fissure; m,m, hippocampal fissure ; n, n, collateral fis- 
 sure; PO, parieto-occipital fissure ; 17, 17, marginal convolution ; 18, 18, gyrus fornicatus; 18', 
 quadrilateral lobule ; 19, hippocampal gyrus ; 19', its recurved end ; 25, occipital lobule ; 9, 9, 
 inferior temporo-sphenoidal convolution. 
 
 important convolutions (the anterior and posterior central or as- 
 cending frontal and ascending parietal), which at both of its ends 
 connect together in the form of an arch. The fissure which sepa- 
 rates the parietal from the occipital lobe (parieto-occipital) and the 
 one which runs from before backward through the parietal lobe 
 (intra-parietal) are also to be mentioned among the more important. 
 The intraparietal fissure, on the convexity of the parietal lobe, in- 
 cludes, between it and the median line, the upper parietal convolu- 
 tion, and embraces in its downward and outward bend the angular 
 convolution. The latter convolution, and the marginal convolution 
 form the inferior parietal lobule. 
 
 1 Elements of Embryology, p. 384. London, 1883. 
 
THE FIVE LAMI1SLE OF THE CORTEX. 95 
 
 Besides the superior, middle, and inferior convolutions of the 
 frontal, temporo-sphenoidal, and occipital lobes, and the two cen- 
 tral convolutions on each side of the Fissure of Rolando, the follow- 
 ing, which belong to the median aspect of the hemispheres, are 
 to be noted in particular. The convolution which arches around 
 the corpus callosum, and is separated from the median aspect 
 of the first frontal convolution by a deep and constant fissure 
 (the sulcus calloso-marginalis) is called from its shape, gyrusforni- 
 catus. The back end of this convolution curves downward and then 
 forward, under the name of gyrus hippocampi, to the inner tip of 
 the temporal lobe. The passage of the former convolution, without 
 break, into the latter, Ecker considers one of the most important 
 differences between the hemispheres of the brain of man and those 
 of the ape. 
 
 27. Although the general arrangement of gray nervous matter 
 upon the surface, and of white matter within, is adhered to in all 
 parts of the cerebral cortex, the form and disposition of the cells 
 in the gray matter differ in different regions, and also in different 
 layers of the same regions. But its most common form, which is 
 that seen in the convolutions of the parietal lobe, corresponds to 
 what Meynert 1 has called " the general or five-laminated type of 
 the cortex of the cerebrum." There are, as a rule, that is to say, 
 five layers or laminse to be discovered in the gray matter of the 
 cerebral cortex. The thickness of the entire cortex, thus com- 
 posed, is, in the adult, from -fa to \ of an inch. The first of 
 the layers consists of a matrix, in which delicate nerve- fibrils run 
 parallel to the surface and interlace with a few small globular or 
 elongated branching nerve-cells scattered here and there. The na- 
 ture of this matrix has been the subject of dispute ; by some it is 
 looked upon as connective tissue (Kolliker), by others as neuroglia 
 (Virchow). The second and third layers contain a large number 
 of pyramidal, or spindle-shaped cells ; of these layers the third is 
 the broadest, and contains the largest (but fewest) cells. The cells 
 of the second layer are about ^ T ^ of an inch in diameter, and are 
 closely pressed together to form its substance ; but in the third 
 layer they augment gradually in size until they reach a diameter of 
 T-jjVjr, to perhaps ^ of an inch, with their long axes perpendicu- 
 lar to the cortical surface. The fourth layer contains large num- 
 bers of small, globular, and irregularly-shaped and branching cells ; 
 the fifth, spindle-shaped bodies with long tapering processes, and 
 also a certain number of smaller irregular cells. This innermost 
 layer consists chiefly of a compact accumulation of cells which give 
 1 In Strieker's Human and Comparative Anatomy, ii., p. 381. 
 
96 
 
 THE CEREBRAL CORTEX. 
 
 off lateral processes. Gerlach discovers here, as in the spinal cord 
 (see 9), a very minute network with which these processes are 
 apparently continuous. It is also an assumption, verified by direct 
 observation of some cases, and by the general analogy of the nervous 
 system, that many of the extremely attenuated nerve-fibrils which 
 
 iiiiii ' 
 
 x ' 
 
 FIG. 37. Section through the Cerebral Cortex of Man, prepared with Osmic Acid * 8 /,. 
 (Schwalbe.) 7, principal external, and II, internal, layer ; x, layer lying as a limit between the 
 two ; i, medullary substance sending out bundles of nerve-fibres into //; J, layer poor in cells, 
 but with an external plexus of nerve-fibres (la.) ; 2, layer of small, and 3, of large, pyramidal 
 cells ; 4, inner layer of small nerve-cellB. 
 
 radiate from the white core of the convolutions are continuous with 
 the basal axis-cylinder processes of the cells in the layers of gray 
 substance. Small rounded corpuscles and small stellate cells, so 
 pellucid that they seem to be only free nuclei, are contained in the 
 neuroglia of the gray substance of the cerebral cortex. It is doubt- 
 
WHITE SUBSTANCE OF THE HEMISPHERES. 97 
 
 ful whether these are true nervous elements or not. The number 
 of nerve-cells in the cortical substance is very great. In a portion 
 of this substance, only one millimeter square and y 1 ^ millimeter 
 thick, 100 to 120 have, on an average, been counted. 1 
 
 Modifications of the arrangement which prevails in most of the 
 gray substance of the hemispheres of the brain are found in certain 
 regions. In the cortex of the occipital lobe the number of layers 
 is increased by the intercalation of additional granule layers to 
 seven or eight. In the cortex of the Island of Reil, and of the con- 
 volutions bounding the Fissure of Sylvius, a large proportion of 
 fusiform cells is found. In Jthe fourth layer of the cerebral cortex 
 of the dog, in the region which Hitzig considered to be motor, 
 Betz discovered certain cells lying in scattered groups, with two 
 large and several small processes ; these cells, on acccount of their 
 great size, he called " giant-cells." Similar cells, have been found 
 by him in certain regions of the human cerebral cortex namely, in 
 the entire anterior central, and the upper end of the posterior cen- 
 tral convolutions, and along the lobe which is prolonged backward 
 from the two. 
 
 28. The white substance of the hemispheres of the brain may 
 all be considered as originating in its cortical gray substance ; but 
 the nerve-fibres of which it is composed constitute three classes, 
 according to the destination of the fascicles into which the fibres are 
 gathered. These three are the down-going or peduncular, the com- 
 missural, and the arcuate (or fibrce proprice). It is the business of 
 the peduncular system to connect the cerebrum with the lower parts 
 of the encephalon. This system, called the corona radiata, is nar- 
 rowed into the internal capsule and continued downward to the 
 crura cerebri ; its diminished size shows that a considerable por- 
 tion of its fibres have entered into the optic thalami and striate 
 bodies. But it is also probable that many fibres of the crusta pass 
 directly into the brain's medullary centre, and through this to its 
 gray cortex, without entering these ganglia. Of such tracts the 
 best known is the pyramidal (probably motor). According to 
 Flechsig and others, this is traceable through the internal capsule 
 and corona radiata to certain frontal and parietal convolutions.. 
 Another tract, traceable directly to the convolutions of the cortex, 
 passes from the external part of the crusta into the white matter 
 of the occipital lobe (so-called direct sensory tract). The fibres 
 which come from the tegmentum, and are lost, for the most part, in 
 the thalamus and the subthalamic region, stream outward from the 
 other side of this organ, join the general system of the corona radi 
 1 See Luys, The Brain and its Functions, p. 17. New York, 1882. 
 7 
 
98 THE CEREBRAL CORTEX. 
 
 ata, and diverge to nearly every part of the hemispheres ; but espe- 
 cially to the temporo-sphenoidal and occipital lobes (probably sen- 
 sory). 
 
 The commissural system of fibres has hitherto universally been 
 supposed to connect the two hemispheres of the brain ; but Pro- 
 fessor Hamilton, of Aberdeen, and others, have recently called this 
 statement in question. The principal tract of such fibres is in the 
 corpus callosum. Since this commissure lies in a plane above that 
 of the corona radiata, the two systems of fibres intersect each other 
 on their way to the convolutions of the cerebral hemispheres. A 
 smaller commissure (the anterior) passes below the lenticular nuclei 
 of the striate bodies and connects the convolutions around the Syl- 
 vian fissure binding together the right and left temporo-sphen- 
 oidal lobes ; it also furnishes a root of origin for the olfactory 
 nerve. 
 
 The arcuate fibres extend over more or less territory on the 
 same side, and connect the gray matter of adjacent, or more or 
 less distant, convolutions in the same hemisphere " a garland-like 
 interweaving " of two convolutions around the sulcus between them. 
 In certain localities, where the fascicles into which, these fibres are 
 gathered are strongly marked, they have received special names ; 
 such are the fasciculus uncinatus which crosses the bottom of the 
 Sylvian fissure and connects the convolutions of the frontal with 
 those of the temporo-sphenoidal lobe ; the fillet of the gyrus forni- 
 catus, extending longitudinally in that convolution ; the longitudi- 
 nal inferior fasciculus, connecting the convolutions of the occipital 
 and temporo-sphenoidal lobes. Such fibres are sometimes called 
 longitudinal or collateral fibres. It is by the commissural and arcu- 
 ate fibres that the innumerable ganglion-cells and nerve-granules of 
 the cortex are bound into a unity of form and of function. The pro- 
 cesses of the cells anastomose, and are thus united with immedi- 
 ately adjoining cells by means of a gray fibre-plexus. The axis- 
 cylinder processes become continuous with the medullated fibres, 
 which, gathered into bundles (the fasciculi of the arcuate fibres), 
 line as a continuous layer the inner surface of the cortex. In this 
 manner the nervous elements of that crowning mechanism, which 
 is known as the chief glory of man's nervous system, are made to 
 exhibit a manifoldness, and at the same time a unity of structure, 
 suggestive of a common service joined with diversity of mode in 
 which the service is rendered. 
 
 29. The view of Meynert to which reference has already been 
 made (p. 73) regards the gray masses and converging and diverg- 
 ing tracts of the cerebro-spinal nervous mechanism as a " Projec- 
 
MEYNEKT'S PEOJECTION SYSTEMS. 99 
 
 tion System " (or rather as a series of "projection systems"), which 
 is capped and dominated by the hemispheres of the cerebrum. The 
 sensory nerves may thus be figuratively described as the " feelers," 
 and the motor nerves as the " arms " of its cortical gray matter. 
 This matter is both a "sensory shell," upon which the centripetal 
 nerve-commotions gather and dispose themselves ; and is also the 
 "motor shell" in which certain centrifugal motions originate. It 
 is, therefore, an internal " Projection Field " for the muscular sys- 
 
 Ccl 8 C i SM 
 
 Cop 
 
 Vma 
 
 Ftp 
 
 FIG. 38. Median Section of the Brain. A, aqueduct of Sylvius ; Cba, white commissure ; Cbl, 
 cerebellum ; Oca, corpus albicans ; Ccl, corpus cailosum, of which the different parts are Gel 1 , 
 rostrum, Ccl 2 , the genu, Ccl 3 , the body, and Ccl 4 , splenium ; Cn, conarium, or pineal gland ; 
 Coa, anterior, Com, middle, and Cop, posterior, commissures ; FM, foramen of Monro ; Fta, 
 the anterior, and Ftp, the posterior, transverse fissures; Let, lamina cinerea terrain alis ; and 
 Lq, lamina of the corpora quadrigemina ; Mo, medulla oblongata ; P, pons Vafolii ; SI, septum 
 lucidum ; SM, sulcus of Monro ; Tc, tuber cinereum ; Vma, anterior velum medullare ; V, 
 fourth ventricle; II, optic nerve ; II 1 , chiasm of the optic nerve. 
 
 tern. The gray masses of the brain below its hemispheres (with 
 the exception of the internal tabular mass) may according to 
 Meynert be described as either (a) "Interruption Masses" of the 
 projection system, or as belonging to (6) the " Region of Reduc- 
 tion " of the mass of this system. It is in these lower gray masses 
 that the great bulk of the nerve-tracts (the corona radiata) coming 
 from the cortex of the cerebrum are not only broken and inter- 
 rupted in their course, but are also greatly reduced in size. 1 The 
 
 1 For a clear and concise summary of Meynert' s entire view, see Quain's 
 Anatomy, ninth edition, ii. , pp. 370 ff. 
 
100 
 
 NERVES OF THE CEREBRO-SPINAL SYSTEM. 
 
 functional significance of this relation in which the cerebral cortex 
 stands to all the rest of the nervous mechanism will appear more 
 clearly further on in the discussion. 
 
 30. The cerebro-spinal axis, or central nervous mechanism of 
 the cavity of the spinal column and skull, is connected with the 
 end-organs of motion and of sense by thirty-one pairs of Spinal, 
 and twelve pairs of Cranial or Encephalic Nerves. 
 
 The thirty-one pairs of Spinal Nerves originate in the spinal 
 cord and pass out of the spinal canal through the openings called 
 
 ABC 
 
 FIG. 39. Posterior View of the Spinal Cord with its Nerves. %. (After Sappey. ) I-VIII in A 
 are cervical ; I, II, and III in A, and IV-XII in B, dorsal ; the last in B. and down to V in C, 
 lumbar; I-V in C, sacral ; 10, 10, origin of the posterior roots; 11, 11, posterior median fis- 
 sure ; 12, 12, spinal ganglia ; 13, 13, united nerve ; 15, tapering of the lower end, becoming 16, 
 16, the nlum terminate ; 17, cauda equina. 
 
 " intervertebral foramina." Of the entire number enumerating 
 from above eight pairs are cervical, twelve thoracic or dorsal, 
 five lumbar, five sacral, one coccygeal. Each nerve arises from the 
 side of the cord by two roots, an anterior and a posterior. The 
 posterior root has a swelling or ganglion upon it, the anterior has 
 none ; the former is composed of sensory nerve-fibres, the latter, 
 
THE TWELVE PAIRS OF CRANIAL NERVES. 101 
 
 of motor nerve-fibres. The ganglion of the posterior roots contains 
 unipolar nerve-cells. The roots themselves vary, in the different 
 regions of the cord, both as respects direction and length. Imme- 
 diately outside the ganglion the anterior root joins the posterior, 
 and the united nerve containing a mixture of motor and sensory 
 fibres soon after separates into two divisions, that are formed of 
 elements from each root and that are distributed, one upon the 
 back and the other upon the front and sides, to all parts of the 
 trunk and limbs. 
 
 Of the twelve pairs (adopting the Continental instead of the 
 English division) of Cranial Nerves, which arise from the base of 
 the encephalon and pass through the openings (foramina) in the 
 floor of the cranial cavity, three groups may be distinguished : (a) 
 the sensory nerves, or nerves of special sense ; (6) the motor nerves ; 
 (c) the mixed nerves, which contain both sensory and motor fibres. 
 To the first group belong the olfactory nerve (first pair), the optic 
 (second pair), and the auditory (eighth pair) ; to the second group 
 belong the nerves that supply the principal muscles of the eyeball 
 (oculo-motor, third pair), the superior oblique (trochlear, fourth 
 pair), and external rectus (abducent, sixth pair), muscles of the eye, 
 the muscles of facial expression (seventh pair), the muscles of the 
 tongue (hypoglossal, twelfth pair), and the spinal accessory nerve 
 (eleventh pair) ; to the third group belong the three nerves which 
 are so widely distributed over the mucous membranes and muscles 
 of the face, tongue, pharynx, and internal organs namely, the tri- 
 geminus (fifth pair), the glossopharyngeal (ninth pair), and the pneu- 
 mogastric or vagus (tenth pair). 
 
 31. It is, then, by a process of differentiation of a few compara- 
 tively simple elements, and of infinitely varied arrangement and 
 combination of the elements thus differentiated, that the elaborate 
 mechanism of the human nervous system is constructed, and made 
 fit for the great variety of interconnected functions which it is 
 called upon to perform. Material atoms are chemically united into 
 the complex and unstable molecules of w r hich nervous matter is com- 
 posed. These molecules are arranged into the structural forms of 
 nerve-fibres and nerve-cells ; and the latter, at least, are modified 
 in form according to their location, and perhaps also function. 
 The elements are combined into conducting cords, end-organs, and 
 central organs, according to the threefold plan of a nervous sys- 
 tem ; and the organs are arranged, in the case of man, with an in- 
 tricacy of relations which can be only very inadequately described. 
 
 The description of the mechanism being finished, we consider 
 more in detail what it can do. 
 
CHAPTER III. 
 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 
 intimate connection between the two functions of excitation and 
 conduction becomes, then, at once apparent. Indeed, excitation 
 may be considered as the setting up of the process of conduction ; 
 conduction 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 z, 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 
 
GENERAL PHYSIOLOGY OF NERVES. 103 
 
 as one process from somewhat different points of view. The arising 
 and progressive movement of a unique molecular commotion con- 
 stitutes the distinctively neural action or function of the nerves. 
 And since this so-called nerve-commotion has eluded all the at- 
 tempts hitherto made to discover its more intimate nature, and to 
 bring it under a strict theory, we must be content with describing 
 the following three classes of facts : (1) The Conditions of the pro- 
 cess ; (2) the Phenomena evoked with it, or as part of it, by differ- 
 ent kinds of stimuli ; (3) the Laws of its propagation. 
 
 What is called " the general physiology of nerves " attempts to 
 consider their action while excluding the influence upon it from 
 the central organs and the end-organs. That is, the function of the 
 nerves, as we now consider it, is exercised under abnormal conditions. 
 It has been objected to the view which regards each element of 
 the nerve as the stimulus of neighboring elements, that the ef- 
 fects of direct artificial stimulation must differ in important respects 
 from those obtained by stimulation in the normal way. For exam- 
 ple, Ziemssen and others have shown that the crushed nerve of an 
 animal, or the paralyzed nerve of a man, may be made to set up a 
 nerve-process by reflex stimulation when it will no longer respond 
 to stimulation applied directly to its trunk. And Griinhagen af- 
 firms that after a stretch of nerve has been reduced by the effects 
 of carbonic acid to a lower degree of excitability under direct stim- 
 ulation, it will still propagate through itself the excitation set 
 up elsewhere with undiminished force. Such facts, however, only 
 prove that the application of stimuli to the nerve for purposes of 
 experiment is a very rough and ineffective way compared with nat- 
 ure's method of preparing the stimuli by the modifying influence 
 of the nervous tissues themselves. They do not prove that the 
 neural process is not fundamentally the same in whatever way it is 
 brought about. On the contrary, there is abundant evidence to 
 show that this abnormal activity, when carefully studied, will give 
 us the key to the normal function of the nerves. The advantages 
 of simplifying the problem by experiments upon isolated nerves 
 are too great, and the fund of valid information thus obtained is 
 too important for us to neglect the method proposed. The science 
 called " general physiology of the nerves " is, indeed, very largely 
 built upon experiments with the motor nerves of frogs ; and, of 
 course, it may be said that frogs, with respect to their nervous sys- 
 tems as otherwise, are very unlike men. But with respect to the 
 character of that specific molecular process which is ' set up in the 
 nerve when excited, frogs appear to be essentially the same as men. 
 At any rate, we have no physical means adequate for detecting any 
 
104 THE NERVES AS CONDUCTORS. 
 
 essential differences. In other words, nerves are nerves the world 
 over ; and what they do as nerves simply, is essentially one thing in 
 all cases. What they do in their vastly different arrangements and 
 connections with central organs and end-organs differs vastly in 
 different cases. 
 
 3. The view that each element of every nerve, irrespective of 
 its kind or specific place in the animal mechanism, can only stimu- 
 late its neighbor and be stimulated by its neighbor, suggests an- 
 other interesting inquiry. Is this stimulus of the nerve-elements, 
 this effect in exciting contiguous elements, analogous to any of the 
 so-called external stimuli? Or, in other words, the inquiry may be 
 raised : Is the process of nerve-commotion in the nerves similar to, 
 or identical with, any of those molecular processes which act as in- 
 direct stimuli upon the nerves through the end-organs ? In answer- 
 ing this question it has long been customary to ally nerve-commo- 
 tion with electricity. In a posthumous work by the mathematician 
 Hausen, in 1743, it was first proposed to consider the efficient 
 principle of nervous action as identical with that of the electrical 
 machine. 1 Exactly a century later (1843) du Bois-Keymond an- 
 nounced the discovery of an electrical current in unexcited nerves 
 (the so-called " current of rest "). Since then the phenomena of this 
 current, of the negative variation of the nerve, and of electrotonus 
 all discovered almost simultaneously by the same investigator 
 have been the subject of much painstaking research. This research 
 has resulted in showing that important differences exist between 
 the neural process and that of the electrical current, and in making 
 more and more clear the impossibility of forming a purely electri- 
 cal theory of the nervous functions. On the other hand, it has also 
 revealed many important similarities between the two. It is by 
 experiment with the effect of electrical currents, of different kinds 
 and directions, and under varying conditions, that the science of 
 general physiology of the nerves has been built up. 
 
 4. In order to understand the general results of experiment 
 upon the nerves, the nature and use of the so-called Nerve-muscle 
 Machine must be understood. 
 
 A "nerve-muscle preparation" consists of muscle freshly taken 
 from the living 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 chamber. By the simple contrivance of connecting the 
 
 1 See in du Bois-Reymond's great work, the history of opinions 011 this 
 point : Untersuchungen uber thierische Electricitiit, II. , i. , pp. 209 ff. Berlin, 
 1849. 
 
THE NEKVE-MUSCLE MACHINE. 105 
 
 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 con- 
 tractions 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. l 
 
 Certain terms in constant use to describe the methods and results 
 of experiments with the nerve-muscle machine also require a brief 
 explanation. The line traced by the armed end of the lever, as it 
 rises and falls with the contractions of the muscle, is known as the 
 11 muscle-curve." In so far as it shows changes that are due to the 
 condition of the attached nerve, or to the quality, intensity, and 
 order of the stimulations applied to that nerve, this curve is a 
 measure of the process of neural excitation and conduction. If 
 the electrical current flows with the course of the motor nerve- 
 stretch that is, from the central toward the "neripheral parts it 
 is called " descending," or direct ; if in the opposite direction 
 "ascending," or inverse. The current to be detected in an unex- 
 
 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 Foster's Text-book of Physiology, 
 pp. 43 ff. 
 
106 THE NERVES AS CONDUCTORS. 
 
 cited nerve (a nerve, that is, whose functional activity is not at the 
 time in exercise on account of the application of any kind of stim- 
 lus) is called a " natural current," or a " current of rest." The cur- 
 rent produced by stimulating the nerve, and so calling into exer- 
 cise its physiological function, is a "current of action." When 
 a single induction-shock, or a number of such shocks repeated 
 at sufficient intervals, is sent through a nerve-stretch, the contrac- 
 tile spasm of the muscle in response to each shock shows that 
 a single "nervous impulse " is passing along the nerve. When the 
 single stimulations are repeated with sufficient rapidity, the single 
 spasms fuse into one apparently continuous effort, known as " tet- 
 anus," or "tetanic contraction." The term "tetanus" applies 
 primarily to the muscle only ; but the application of rapidly re- 
 peated shocks to the nerve, such as would produce "tetanic con- 
 traction " of the muscle, may be called the " tetanization of a nerve." 
 The contraction which follows the closing of the current is called 
 the "making contraction," or "closing contraction;" that which 
 follows its opening, the "breaking" or "opening" contraction. 
 
 5. Of the conditions under which alone the nerve is capable of 
 exercising its function of neurility the most important are these 
 three : Vitality, Oxygen, and Becovery from previous exhaustion. 
 
 A nerve cannot act as a conductor of the neural process unless 
 it is vital ; but the death of the nerve is not necessarily simultane- 
 ous with that of the body from which it is taken, or of the muscle 
 to which it is attached. On the contrary, by careful treatment with 
 respect to moisture and temperature, and by guarding it from 
 mechanical or chemical injury, it may be preserved alive for some 
 time after excision. The indirect irritability of the muscle through 
 the excised nerve attached to it frequently continues in warm- 
 blooded animals and in high temperature not longer than about 
 an hour ; in the frog and in a low temperature it may last for sev- 
 eral days. The nerves of the summer frog are much more perish- 
 able than those of the same animal in winter. A nerve is, of course, 
 alive as long as it will excite the muscle to contract. But the nerve 
 is not necessarily dead when the attached muscle no longer responds 
 to its excitation ; the failure may be due to the death of the very 
 sensitive and perishable end-organs which connect the two. Her- 
 mann ' considered that the existence of electrical phenomena in the 
 nerves of rabbits showed the nerves to be alive for several hours 
 after they would no longer stimulate the muscle, and also after the 
 muscle itself could not be irritated directly. Nerves may even be 
 alive after they cease to exhibit electrical phenomena that can be 
 * Handb. d. Physiol., II., i., p. 120. 
 
VITALITY AND DYING OF THE NERVE. 107 
 
 detected by the most delicate tests available. It is possible that 
 the capacity for excitation may linger after the capacity for con- 
 ducting the excitation is lost. Since the nerve, unlike the muscle, 
 has no death-rigor, we cannot say just when it is wholly dead. 
 
 During the stages of dying, nerves exhibit two interesting changes 
 of excitability. Immediately after it is severed from the body 
 the irritability of the nerve increases temporarily, and afterward 
 diminishes by successive degrees until it is wholly lost. The 
 course of these changes in its irritability is found to be different 
 for different parts of the same nerve-stretch. It was discovered by 
 Valli and Hitter 1 that a nerve which has once ceased to stimulate 
 its attached muscle to contract will again excite muscular contrac- 
 tion if the electrodes be applied farther down its stretch ; there- 
 fore the lower portion of the nerve-stretch seems to preserve a 
 given degree of vitality for the longest time. From this fact " Valli's 
 principle " has been derived : Nerves die from the centre to the 
 periphery. The temporary increase of the irritability of the ex- 
 cised nerve belongs indeed to its entire stretch ; but it appears first 
 in the upper part. This fact is connected with the important in- 
 fluence which the cross-section of a nerve has upon its electrical 
 and neural condition. As to the reason for this increase of nervous 
 excitability which accompanies the first stage in the dying of the 
 nerve, we are quite in the dark. 
 
 6. Closely allied to the foregoing changes are those which take 
 place in the structure and functional activity of a nerve that re- 
 mains in the living animal organism after having been separated 
 from the central organs. Such a nerve, after a time, completely 
 loses its irritability. Two investigators, Giinther and Schon, 2 found 
 this time to be, in the case of rabbits or dogs, about three or four 
 days ; in a cold-blooded animal like the frog, the time may be pro- 
 longed to a week, or even more. The law of increased irritability, 
 produced in the entire nerve-stretch, but first manifested in the 
 portion nearest the cross-section, immediately after separation from 
 the central organ, holds good for most observations on nerves cut 
 in situ ; its application is obvious, however, only to the case of the 
 motor nerves. In 1850 Waller announced 3 the discovery that the 
 anatomical changes (a fatty or granular degeneration) which take 
 place in the nerve-fibres after being severed from the central organs 
 proceed from the place of section to the extreme peripheral portion 
 
 1 See in du Bois-Reymond's Untersuchungen, etc., i., pp. 321 ff. 
 
 2 See the Archiv f. Anat. Physiol., etc., 1840, p. 270. 
 
 3 In Philosophical Transactions, 1850, ii., p. 423; and see, also, Archiv f. 
 A.nat. u. Physiol., 1852, p. 392. 
 
108 THE NERVES AS CONDUCTORS. 
 
 of the fibre ; and that the sensory nerves do not degenerate in their 
 peripheral, but in their central portion, when the posterior roots 
 are cut above the ganglion. The central portion of the nerve, when 
 cut at a point lying toward the periphery from the ganglion, may 
 be shown (in the case of the sensory nerves, which alone admit of 
 being experimented upon for this purpose) to retain its irritability 
 for a long time, although it finally loses it through lack of exercise. 
 A cut nerve remaining in situ may be regenerated, and so regain its 
 functional powers. Regeneration takes place by the axis-cylinders 
 growing out from the central portion and running into and between 
 the sheaths of Schwann of the peripheral portion ; it is accom- 
 plished, then, by the influence of the central organs. The irrita- 
 bility of the nerve returns as its structure is regenerated. Accord- 
 ing to some investigators its conductivity is regained earlier than 
 its power of local irritability. Duchenne ' and others claim that 
 the influence of the will is the first form of stimulus to regain con- 
 trol of regenerated motor fibres. 
 
 7. Oxygen, as furnished by the circulation of the arterial blood, 
 is the second condition for the performance by the nerves of their 
 distinctive functions. But nerves, as compared with the central 
 organs or end-organs of the nervous system, or even with the mus- 
 cles, are relatively independent of the presence of oxygen. Indeed, 
 since the muscle is so much more sensitive to changes in the qual- 
 ity of the blood, and is supplied by the same arteries that supply 
 the attached nerves ; and since the irritability of the nerve is tested 
 by the vital contraction of the muscle it is difficult to determine 
 by experiment the exact effect upon the nerves of withdrawing 
 from them the oxygen of the blood. The irritability of the nerves 
 continues about as long in a moist vacuum, or in indifferent gases, 
 as in the air. What little is known of the chemical processes 
 which take place ic the nerves confirms the view that they are rel- 
 atively independent of the presence of oxygen ; and the experi- 
 ments of Severini, who thinks that he has discovered a restorative 
 effect of ozone (if not of ordinary oxygen) upon these organs when 
 dying, are not yet fully confirmed. It may be argued, however, 
 from the marked dependence of the other forms of nervous tissue 
 upon a supply of arterial blood, as well as from the general theory 
 of the nervous system, that the presence of some oxygen is a nec- 
 essary condition of the functional activity of the nerves. 
 
 8. Exhaustion is a condition of the nerves recovery from which 
 is necessary in order that they may exercise their normal functions ; 
 but exhaustion of the nerves is difficult to distinguish experimen- 
 1 Traite de Pelectrisation localisee, second edition. Paris, 1861. 
 
MECHANICAL PROPERTIES OF NERVES. 109 
 
 tally from exhaustion of the central organs or of the end-organs. 
 The experiments of du Bois-Reymond upon the negative varia- 
 tion of the nerve-current under repeated irritation give us the 
 first item of the desired proof. The variation under these circum- 
 stances becomes constantly weaker. By ingeniously separating the 
 proofs of exhaustion in the muscle from those of exhaustion in the 
 nerve, Bernstein 1 has shown that the latter comes on much more 
 slowly than the former ; and that by far the greater amount of 
 the effects attributed to exhaustion in the nerve-muscle machine 
 belong to the muscle-element of this machine. When tired, how- 
 ever, the nerve recovers more slowly than the muscle. Nerve- 
 cells and therefore the central organs and end-organs of the ner- 
 vous mechanism tire much more easily and quickly than nerve- 
 fibres. Indeed, according to Hermann,' 2 it is conceivable that all 
 the phenomena of exhaustion which take place in the normal expe- 
 rience of the nervous system belong really to the organs connected 
 with the nerves rather than to the nerves themselves. When we 
 are tired nervously, it is not ordinarily the nerves that are tired. 
 And yet the law of the exhaustion and recovery of functional ac- 
 tivity doubtless belongs to normal, as it does to excised, nerve-fibres. 
 
 9. The various classes of phenomena which are evoked in con- 
 nection with the starting and propagation of nerve-commotion 
 along a nerve-stretch will be considered from two points of view : 
 First, as regards their dependence upon the character, amount, and 
 method of the application of the stimuli which are used ; and, sec- 
 ond, as indicative of certain processes chemical, thermic, electri- 
 cal, etc. set up in the nerves themselves. We shall thus, as far 
 as possible, avoid repetition. 
 
 10. The mechanical properties of the nerves are of little inter- 
 est to psycho-physical researches ; and comparatively little con- 
 cerning their physiological functions has been learned by the ap- 
 plication to them of mechanical stimuli. The elasticity of nerves 
 in the dead body was found by Wertheim to follow the same laws 
 as that of the muscle their absolute ductility is less than that of 
 muscle ; their cohesion greater. All kinds of mechanical attacks on 
 the nerves excite them, and are followed by pain in the case of 
 sensory nerves, contraction of the muscles in the case of motor 
 nerves. By rapid shocks of this kind for example-, with a toothed 
 wheel or a hammer tetanus may be produced. A certain sudden- 
 ness of influence is, in general, necessary to the effect. Yet Fon- 
 tana succeeded in cutting nerves very quickly with a sharp knife 
 without producing any muscular contraction. Pressure of a nerve 
 
 'In Pfluger's Arcliiv, xv., p. 289 f. 'Handb. d. Physiol., II., i., p. 135. 
 
110 THE NERVES AS CONDUCTORS. 
 
 may be increased very gradually to a high degree without exciting- 
 it ; but its power of conductivity is thus temporarily suspended. 
 Very moderate pressure or slight traction of the nerve has been 
 found by several investigators to increase, at least for a moment, 
 the irritability of the nerve ; and perhaps, also, the speed of con- 
 duction in it. All neural function is, of course, destroyed by any 
 considerable mechanical injury of the nerve, such as often happens 
 by stricture or pressure from a swelling. 
 
 11. Thermic influences upon the phenomena of the neural pro- 
 cess are very marked and important. On the other hand, almost 
 nothing is known as to the specific heat of nerves or as to their 
 power to conduct heat. Hermann thinks it probable that the 
 latter is different in the two main directions of the fibres. The 
 results of experiment differ as to the degree of heat which is neces- 
 sary to act upon the nerves as a stimulus. Valentin, the first ob- 
 server in this line, found that dipping the motor nerves of frogs 
 in water heated to about 100 Fahr. (38 C.) caused contractions ; 
 but Eckhard obtained such results only from temperatures above 
 150.8 to 1544, or below 25 to 22 that is, temperatures that 
 are either deadly or permanently injurious to the nerve. Nor, 
 according to the latter, is the nerve excited by changes in tempera- 
 ture as it is by changes in the electrical current. Slighter changes 
 near the dead-line may have an effect to excite the nerve ; but con- 
 siderable changes in the medium temperatures, as a rule, have no 
 such effect. It is the opinion of some, however, that such thermic 
 changes, when marked and sudden, may act as a stimulus to mo- 
 tor nerves. It was shown by E. H. Weber ' that heat and cold have 
 no effect in producing sensations when applied directly to the sen- 
 sory nerve-trunks of man. 
 
 While there is little evidence, then, to show the direct excitatory 
 effect of heat upon the nerves, there is no doubt whatever as to 
 the importance of thermic influences upon their excitability and 
 conductivity. High degrees of temperature may destroy the pow- 
 er of the nerve to perform its functions, but without killing it. 
 Warmth increases the immediate expenditure of energy in an ex- 
 cised nerve, and so hastens its death ; cold delays this expenditure, 
 and so conserves the nerve. The limit of this increased irritability 
 of the nerve under the influence of heat is reached at about 122 
 Fahr. ; as the degree of heat applied rises from this point toward 
 150, its effect is rapidly felt in causing the death of the nerve. 
 Sudden cooling from about 68 down to 50 may produce a tem- 
 
 ] In Wagner's Handworterb. d. Physio!., III., ii., pp. 496, 578 ; and Archiv 
 f. Anat., Physiol., etc., 1847, p. 342, 1849, p. 273. 
 
EFFECT OF CHEMICAL INFLUENCES. Ill 
 
 porary rise of irritability ; but, in general, cooling below 59 di- 
 minishes the irritability of nerves. The effect of temperature upon 
 the speed of conduction will be referred to elsewhere. 
 
 12. Chemical influences have, for the most part, surprisingly 
 little effect upon the irritability and conductivity of the nerves, 
 especially in view of their great sensitiveness to other external in- 
 fluences. Such indifference is probably due to the protection of 
 the nerve by its membranes. The effect of most chemical agents, 
 when long continued, is to destroy the nerve without irritating it ; 
 but some agents in a concentrated form act upon it as stimuli. 
 The researches of Eckhard, Kolliker, and Ktihne have given us 
 most of the information we have upon this matter. Only two 
 points need mention here. First : Changes of the amount of water 
 in the substance of the nerve affect its functional activity. Drying 
 the nerve produces contractions ending in tetanus ; although, ac- 
 cording to some authorities, these effects do not follow if the dry- 
 ing be very sudden. A. slight amount of drying raises temporarily 
 the irritability of the nerve. The amount of the decrease of water 
 necessary to produce contractions in the attached muscle is given 
 by Birkner at four to eight per cent, of the weight of the nerve ; 
 irritability ceases, although the dried nerve is not dead, with a 
 loss of forty per cent. Others, however, give the latter figure as 
 between eight per cent, and nineteen per cent. Swelling the nerve 
 in water or other indifferent fluids decreases its irritability slowly 
 to the point of entire cessation. 
 
 Second : The effect of certain acid and alkaline solutions upon 
 the nerve is much like that of drying it. Various neutral salt so- 
 lutions, and free alkalies in solution, produce strong muscular con- 
 tractions, ending in tetanus and death. Certain organic sub- 
 stances in concentrated solutions for example, urea, sugar, and 
 glycerine irritate the nerve ; so, according to most observers, does 
 alcohol of from ninety per cent, to eighty per cent. The law seems 
 to be, that all chemical stimulation of the nerves is closely connected 
 with the destruction of the nervous tissue. 
 
 13. The phenomena evoked by applying the stimulus of elec- 
 tricity to the nerve-muscle machine are very numerous and diffi- 
 cult of disentanglement, since they depend upon such a variety of 
 changing conditions. Following is a very brief statement of some 
 of the more important of such phenomena, in as far as they relate 
 to the direct excitatory effect of this stimulus, and also to its effect 
 in modifying the excitability of the nerve. 1 
 
 1 Here, as throughout the subject of the general physiology of the nerves, 
 the chief reliance has been placed upon Hermann, Handb. d. Physiol., II., i. 
 
112 THE NERVES AS CONDUCTORS. 
 
 The resistance which living nerves offer to the electrical current 
 does not differ much from that of living muscle ; it is given by 
 most authorities as somewhat greater. According to Weber's in- 
 vestigationp its resistance is about 50,000,000 times as great as 
 that of copper wire. According to Harless, the conductivity of the 
 nerve is on the average about 14.86 times that of distilled water. 
 Hermann found the conductivity to be much greater in the longi- 
 tudinal than in the transverse direction of the nerve. 
 
 As to the direct excitatory effect upon the nerve of constant 
 currents and of their variations, the main principle is that formu- 
 lated by du Bois-Reymond in 1845. * This principle may be stated 
 as follows : The excitatory effect of the constant current, as judged 
 by the contraction- curve of the muscle, does not correspond to the 
 absolute value of the intensity of the current at each moment, 
 but to the change in this value from one moment to another ; and 
 the effect is greater the less the time in which changes of the same 
 magnitude in the current occur, or the greater their magnitude in 
 the same length of time. The essential fact is that constant cur- 
 rents, while they remain constant, do not irritate the nerve ; vari- 
 ations in such currents do irritate it. The variation may be either 
 from zero or to zero (the making or the breaking of the current), 
 but it must have a certain degree of suddenness to be of any effect. 
 Hence induction-shocks are, relatively to their actual strength, 
 much more effective than the constant current in exciting the 
 nerve. Great difficulties, however, stand in the way of stating 
 definitely the relations that exist between variations in the strength 
 of the constant current and changes in the excitation of the nerve 
 produced by these variations ; Hermann, indeed, pronounces the 
 difficulties "insuperable." 
 
 It is not absolutely certain that the constant current itself, apart 
 from variations in its strength, has any excitatory effect upon the 
 sensory nerves. The sensory effects produced bj such a current, 
 for example, pain in the skin, roaring in the ears, sensations of 
 light and color, electrical taste, giddiness (as when the current is 
 passed transversely through the head at the mastoid processes), 
 etc. are due to the end-organs and the central organs. It is per- 
 haps probable that such a current itself may produce tetanus in 
 certain nerves ; but the effect is very small compared with that 
 produced by variations of this current. Pfluger found tetanus pro- 
 
 1 In a paper communicated to the Physiological Society in Berlin, August 
 8th, of that year ; see, also, his Untersuchungen iiber thierische Electricitat, 
 L, p. 25b. 
 
BISECTION OF THE CURRENT. 
 
 113 
 
 duced by weak currents of about the order of the so-called muscle- 
 current ; but not by strong ones. 
 
 14. The excitatory effect of the constant current is dependent 
 upon its direction. If three grades of strength are assigned to all 
 such currents namely, weak, medium, and strong the results of 
 all the experimenters will be found to agree as to the dependence 
 of the effect of medium and strong currents upon their direction ; 
 as to the case of weak currents, authorities differ. The following 
 table, given by Pfliiger, 1 states the conclusion agreed to by the 
 larger number of observers : 
 
 STRENGTH OF CURRENT. 
 
 Ascending Current. 
 
 Descending Current. 
 
 Making. 
 
 Breaking. 
 
 Making. 
 
 Breaking. 
 
 Weak 
 
 Contraction. 
 Contraction. 
 Rest. 
 
 Rest. 
 Contraction. 
 Contraction. 
 
 Contraction. 
 Contraction. 
 Contraction. 
 
 Rest. 
 
 Contraction. 
 Rest or weak 
 contraction. 
 
 Medium . . 
 
 Strong 
 
 
 The results here tabulated are obtained by experimenting with 
 the excised motor nerves of frogs. In experiments with the sen- 
 sory nerves, or with any of the nerves while remaining in the liv- 
 ing animal, the conditions become so complicated that satisfactory 
 results in confirmation of Pfliiger's conclusions have not yet been 
 reached. 
 
 15. The excitatory effect of the constant current is also depend- 
 ent upon its absolute strength. Du Bois-Reymond, after discovering 
 his law, proceeded to raise the inquiry, whether the height of the 
 current upon which the variation is piled up, as it were, has any 
 influence upon its effect. Various attempts to answer the inquiry 
 have been made ; but the discovery of Pniiger's law of electrotonus 
 has, according to Hermann, 2 changed the form of the question to 
 the following : "What influence upon the excitatory effect of in- 
 creasing catalectrotonus and diminishing anelectrotonus does the 
 absolute amount of existing electrotonus have ? In this form it will 
 be referred to again. 
 
 16. The excitatory effect of the electrical current is influenced 
 by the length of the nerve-stretch through which it flows. From 
 the beginning of electro-physiology the opinion has prevailed that 
 the excitatory effect is increased by the length of the nerve-stretch. 
 This view accords theoretically with deductions from Pfltiger's law 
 of electrotonus. The experimental proof, however, is somewhat 
 vacillating ; in part, doubtless, on account of the admixture of 
 
 1 See Untersuchungen iiber die Physiologie d. Electrotonus, p. 453. Ber- 
 lin, 1859. 2 Handb. d. Pliysiol., II., i., p. 76. 
 
 8 
 
114 THE NERVES AS CONDUCTORS. 
 
 different local conditions where different considerable lengths of a 
 nerve are passed through. Different investigators have found the 
 increase of irritability in the nerve, as dependent upon its length, 
 confined within different limits ; one has fixed the limit at from 
 ^ to ^ inch, another at from J to % inch. Willy found the rule, 
 in general, to hold good only for descending currents. 
 
 17. The excitatory effect of a constant current is influenced by 
 the angle between the axis of the nerve and the direction of the 
 current. After considerable experimentation, with varying results, 
 the more modern researches have, according to careful experiments 
 made by Albrecht and A. Meyer, in the laboratory of Hermann, 1 
 confirmed the opinion of Galvani : The electrical current does not 
 excite the nerve when it flows precisely at right angles to the 
 nerve's axis. 
 
 18. The duration of the current also influences its effect as a 
 stimulus. 
 
 Attention has already been called to the exhausting effects of 
 long-continued stimulation of the nerve, whether by electricity or 
 otherwise. But can a shock be so brief as not to stimulate the 
 nerve at all ? The reason why very brief currents, on breaking the 
 circuit, are not followed by a contraction of the muscle is obvi- 
 ously to be found in the fact that the condition of anelectrotonus, 
 on which the breaking contraction depends, has not had time to 
 develop itself. But J. Konig, working under Helmholtz's direc- 
 tion, found that currents which would produce the making but not 
 the breaking contraction, provided they had sufficient duration, pro- 
 duced no contractions at all if they lasted only 0.001 of a second. 
 On increasing the duration of the current the strength of the con- 
 tractions increased also, until at 0.017-0.018 of a second they 
 reached the same height as that of the contraction produced by the 
 corresponding constant current. It may be said, then, that the 
 electrical current must act upon a nerve for at least about 0.001$ 
 of a second in order to excite it. The nerve on being cooled 
 becomes more sluggish in its response to the stimulus ; at the 
 freezing-point it requires a duration of nearly 0.02 of a second for 
 the stimulus to start it into action. 
 
 19. Besides the direct excitatory effect upon the nerve of elec- 
 trical currents, we have to consider their effect in modifying the 
 action of the nerve under stimuli, whether electrical or of some 
 other kind. If a nerve-stretch is under the influence of a constant 
 current which is being passed through it, the effect of stimuli, 
 when applied to any part of the nerve and judged by sensation or 
 i See his Handb. d. Physiol., II., i., p. 81 1 
 
muscular contraction, is increased. This changed condition of the 
 nerve with respect to its excitability, which the electrical current 
 produces, is called " Electrotonus." The term was introduced into 
 physiology by du Bois-Reymond, who was preceded in his in- 
 vestigations by Hitter, Nobili, and Matteucci, and followed by 
 Valentin, Eckhard, and others. It is Pfluger, however, who is en- 
 titled to have his name permanently attached to the law of elec- 
 trotonus; for it is he who most thoroughly analyzed the facts, 
 separated the variables from the constants, and gave scientific form 
 to the result. It is found that the modified excitability of the 
 electrotonized nerve (that is, of the nerve which has been thrown 
 by the passage of the electrical current into this modified condition 
 of excitability) is not uniform through its entire stretch, but is 
 greatest in the immediate region where the electrodes are applied. 
 Moreover, it differs at the two electrodes the condition at the 
 anode (or positive pole) from that at the cathode (or negative pole). 
 It differs, also, for that part of the stretch which lies between the 
 electrodes as compared with that which is outside of the electrodes. 
 Pfl tiger's law states the whole case as follows : The excitability of a 
 nerve under the action of the constant current is increased in the 
 catelectrotonized region (that is, on both sides of the negative elec- 
 trode), and diminished in the anelectrotonized region (that is, on both 
 sides of the positive electrode). This law is declared by Hermann ' 
 to hold good of all kinds of stimulus, and in all cases with the only 
 apparent exception of the suprapolar region of an ascending current. 
 This electro tonic effect of the constant current, like its direct 
 excitatory effect, is influenced by the strength of the current, by its 
 making and breaking, and by the length of the stretch through 
 which it flows. The change in the excitability of the electrotonized 
 nerve increases with the strength of the current, from the low- 
 est observable point until it soon reaches a maximum ; after this 
 maximum is reached, further increase of electrotonus is to be rec- 
 ognized only by the expanding of this condition over the extra- 
 polar parts of the nerve-stretch. Electrotonus increases also with 
 the length of the nerve-stretch affected; but this relation also 
 finally reaches a maximum. Electrotonic changes in the catelec- 
 trotonic region occur immediately upon making the current ; they 
 then speedily but slightly increase, and more slowly diminish again. 
 The anelectrotonic condition develops and extends itself compar- 
 atively slowly, reaches a maximum, and then gradually falls off 
 again. The immediate consequence of breaking the current is to 
 increase the electrotonic condition of the nerve in the anelectrotonic 
 1 Handb. d. Physiol., II. , i., p. 43. 
 
116 THE NERVES AS CONDUCTORS. 
 
 region, and very briefly to decrease it in the catelectrotonic region ; 
 the former increase gradually vanishes ; the latter decrease is fol- 
 lowed, after a few seconds, by an increase which lasts from one- 
 half a minute to fifteen minutes. 
 
 The so-called " laws of electro tonus " are almost wholly based 
 upon experiments with the motor nerves of frogs. Great and even 
 insuperable difficulties stand in the way of proving experimentally 
 its application to sensory nerves, or to the nerves of living and self- 
 conscious man. The conditions of influence from the central 
 organs and end-organs, from sensation and will upon the nerves 
 in such cases are so complicated as to baffle all attempts to analyze 
 them by means of direct experimentation. 
 
 Further consideration of electrotonus, and of its bearing upon 
 a mechanical theory of the nerves, must be for the present post- 
 poned. 
 
 20. The phenomena evoked in connection with the starting 
 and propagating of nerve-commotion along a nerve-stretch may be 
 presented in the second place (see 9) as indicative of certain 
 Processes set up within the nerves themselves. That the effect 
 of a constant current is not exhausted in direct excitation of the 
 nerve is proved by the changed condition of excitability which it 
 also produces. 
 
 No mechanical process that can be made directly appreciable by 
 the senses or accurately measured by mechapical means, like that 
 which takes place in the contracting muscle, occurs in the nerve 
 when excited to its physiological activity by means of appropriate 
 stimuli. Whatever changes then take place in it are invisible and 
 impalpable. 
 
 21. Nor are we much better able, on the ground of experi- 
 mental tests, to affirm the existence of any thermic process in 
 connection with the excitation of the nerves. If any rise of tem- 
 perature in the nerve is caused by the application of stimulus, it 
 is exceedingly small. Helmholz, 1 in connection with his investiga- 
 tions into the heating of the muscle when in a state of tetanus, 
 could detect no development of heat in the nerve, although his 
 means would have revealed a change of only a few thousandths of 
 a degree. On the other hand, Schiff and Heidenhain both de- 
 tected a rise of temperature in the brain due to nervous excitation. 
 But it is still a question how far this fact indicates anything more 
 than change in the distribution of the arterial blood. Moreover, 
 the former of the two observers failed to obtain any evidence of 
 heating in the cerebellum by sensory excitation. The ease of the 
 1 Archiv f. Anat., Physiol., etc., 1848, p. 158. 
 
THE NATURAL NERVE-CURRENT. 117 
 
 conducting nerve-cords and that of the cellular tissue of the cen- 
 tral organs may very likely be different in this regard. 
 
 22. Nor have any chemical processes been indubitably proved 
 to occur in the nerves as an accompaniment or result of the exer- 
 cise of their physiological function. The only experimental evi- 
 .clence of such a process is the change of reaction which some ob- 
 servers have found. Funke and others have asserted that, not only 
 a certain time after death, but also after exertion as caused by 
 cramping produced with strychnine-poisoning, the nerves show an 
 acid reaction. But Heidenhain and other observers contest this 
 alleged fact. Other assertions of chemical changes set up in the 
 nerves by exciting them are even more uncertain. Ranke's theory 
 of a " respiration of the nerves " is quite without any sufficient ex- 
 perimental proof ; and so is his claim that an absorption of the 
 water of the nervous tissue results from tetanus. If any chemical 
 changes are produced in the nerve by exciting it, they are like the 
 thermic exceedingly small. This fact is proved by the almost 
 complete independence of the nerve with respect to the oxygen 
 of the arterial blood, and by the absence of any observable changes 
 in its temperature when functionally active. But here again we 
 must distinguish between the case of the nerves as conductors and 
 that of the nervous tissue of the central organs. 
 
 23. Evidence of the electrical process in nerves functionally ac- 
 tive is not wanting. It was not, however, until the discovery of du 
 Bois-Keymond, announced in 1843, that any experimental evidence 
 had been obtained to show the existence of electrical currents in 
 the nerves, although it had previously been conjectured that the 
 distinctively neural process is a phase of electricity. This ex- 
 perimenter found that in the case of the nerve, as in that of the 
 muscle, the cross-section artificially made is negative toward the 
 longitudinal surface of the nerve-stretch. Weak longitudinal cur- 
 rents also show themselves between the two cross-sections of a 
 nerve-stretch thus prepared. The current outside the nerve-stretch 
 may be considered as completed by a current in the nerve-stretch 
 from its cut end to the equator. This current (called "natural 
 nerve-current," or "current of rest") is the same in the sensory, 
 the motor, and the mixed nerves of the same animal ; but its elec- 
 tro-motive force is greater the larger and thicker the nerve. Its 
 absolute strength in the sciatic nerve of the frog is given by du 
 Bois-Beymond as 0.022 of a Daniell's cell, but by Engelmann as 
 0.046. It gradually becomes extinct in the nerves of the dead 
 body, but it continues for some time after their irritability is lost. 
 
 The same discoverer, du Bois-Reymond, found that the current 
 
118 THE NERVES AS CONDUCTORS. 
 
 of rest is diminished in energy by tetanizing the nerve-stretch with 
 an electrical current. That is, if when one of the electrodes is placed 
 at the equator, and the other at the cut end of a nerve-stretch, the 
 needle of the galvanometer indicates the passage of a so-called cur- 
 rent of rest, and then the muscle to which the nerve is attached be 
 tetanized by passing an interrupted current through the nerve, the 
 needle will swing back toward zero. This variation is called the 
 "negative variation " of the nerve-current. It may be produced by 
 chemical and mechanical, as well as by electrical, stimulation ; and 
 when the nerve is no longer irritable the negative variation sinks 
 to zero. It shows, therefore, that the electro-motive force of the 
 nerve is diminished by the nerve being excited ; and the degree 
 of the negative variation is a measure of this diminution, although 
 it does not wholly nullify the so-called current of rest. The nega- 
 tive variation of the electrical current in the nerve is closely con- 
 nected with the nerve-commotion which is started and conducted 
 in the nerve. Since the excitation of the nerve is known to be 
 progressive, or of a wave-like character, the nature of this connec- 
 tion, according to Hermann, may be more definitely stated as fol- 
 lows : The electrical condition of each excited place in a nerve- 
 stretch is negative toward all the places of the same nerve-fibre 
 that are unexcited. Hence, between any two points in a nerve- 
 fibre, while the nerve-commotion is passing over the distance, two 
 phases of the current of action occur ; the first phase is in the 
 same direction as the course of the wave of excitation, the second 
 is in the opposite direction. 
 
 24. The Laws which are known to govern the starting and prop- 
 agation of nerve-commotion along the nerves as conductors are 
 few in number ; they deal chiefly with relations between the mag- 
 nitude of the stimulus and the amount of the resulting impulses, 
 and with the conditions for, and speed of, the unbroken propaga- 
 tion of these impulses. 
 
 The relations which exist between the magnitude of the stimulus 
 applied to the nerves in their normal condition and the amount of 
 resulting nervous impulse cannot be given with accuracy. For, in 
 the first place, there is no absolute measure for either of the two 
 values which it is desired to compare. Of the various stimuli 
 which act upon the nerve-fibres, electricity is the only one that ad- 
 mits of even a fairly approximate measurement as an excitant of 
 these fibres ; and the excitatory effect of electricity does not vary 
 in direct proportion to the strength of the current, but in propor- 
 tion to the changes in its strength. With reference to attaining a 
 direct measurement of the amount of the process set up in the 
 
EFFECT OF SEVERAL EXCITATIONS. 119 
 
 nerve by the stimulus, we seem to be in a yet more helpless con- 
 dition. The effect of this process is almost wholly manifested in 
 the organs with which the nerve is connected rather than in the 
 nerve itself. There is evidence in the case of the nerves, however, 
 as in that of the muscles, that their excitation consists in the set- 
 ting free, by the stimulus, of potential energy due to the molecular 
 constitution of the nerve itself. But the exact nature of this en- 
 ergy, and of its mathematical relations, both to the stimulus and to 
 the resulting energy called forth in the organs connected with the 
 nerve, we shall probably never discover. Still further, as Grtitzner ' 
 and others have shown, the same kind and degree of stimulus pro- 
 duces different effects when applied to different nerves. 
 
 25. Allowing for the uncertain factors, however, some approxi- 
 mate statement may be ventured as to the relation between the 
 magnitude of the stimulus and that of the resulting nerve-commo- 
 tion. Measuring the amount of the process in the nerve by the 
 resulting contraction in the muscle, Hermann 2 found that this 
 amount increases, at first rapidly and then more slowly, with the 
 increase of the stimulus. According to Tick, 3 the height to which 
 the lever is raised by the contractions of the muscle varies, within 
 certain limits, in direct proportion -to the amount of the stimulus. 
 The last-mentioned observer also noted two remarkable phenom- 
 ena : (1) On increasing the amount of the stimulus beyond the 
 point necessary to produce the first maximum contraction, another 
 stage is reached in which the effect further increases, in proportion 
 to the stimulus, until a second maximum is gained. (2) In some 
 circumstances, after reaching the first maximum, the effect dimin- 
 ishes with the increase of the stimulus, then rises on further in- 
 crease, until it attains a second maximum. 
 
 The effect of several excitations may be supposed to pass along 
 the nerve as separate waves of nerve-commotion ; but in order to 
 keep the waves separate the interval between them must be more 
 than about -^ of a second (the fraction differing for different 
 nerves, different animals, etc.), otherwise they fuse in the muscle 
 and tetanus results. The combined effects of stimulations having 
 the requisite interval may be piled up, or summed up, in the 
 nerve, and be seen in superimposed contractions of the muscle. 
 Two simultaneous excitations of the same place of the nerve-stretch 
 are thus " summed up " as long as the maximum of excitation is 
 not reached ; the two are, in fact, one. If the cathodes of the two 
 
 1 Pfliiger's Archiv, xvii., pp. 215 ff.; and xxv., pp. 255 if. 
 
 2 See Archiv f. Anat. u. Physiol., 1861, p. 392. 
 
 3 Untersuchungen iiber electrische Nervenreizung. Braunschweig, 1864 
 
120 THE NERVES AS CONDUCTORS. 
 
 exciting currents unite, the same effect takes place. A similar re- 
 sult may be gained by combining the effects of two different kinds 
 of stimuli as, for example, electricity and the drying off of the 
 nerve. 
 
 26. In a rough way the specific excitability also of different 
 nerves, or of different localities in the same nerve, may be dis- 
 covered. Harless found that the excitability of the nerves of the 
 frog is twenty-two times as great in winter as in summer. In the 
 cut nerve it is greater near the artificial cross-section. Many 
 observers have contended that the excitability of the normal nerve 
 ^diminishes toward its peripheral portion. Matteucci investigated 
 ihe subject of local differences of excitability in the sensory nerves ; 
 more recently Rutherford 1 discovered that the reflex effects of 
 ^stimulating a sensory nerve are greater the nearer the central 
 organ the stimulus is applied. Finally, Helmholtz 2 and Hermann 3 
 observed that the lower part of the nerve-stretch is more excitable 
 under the action of an ascending, the upper under that of a de- 
 .scending induction-current. 
 
 27. The Speed with which the process of conduction takes 
 .place in the nerves has been determined with considerable accu- 
 iracy, under a variety of circumstances ; this, notwithstanding the 
 ,fact that the physiologist Joh. Muller * declared it to be forever 
 .impossible no longer ago than 1844. In only 1850, however, 
 Helmholtz B announced that he had succeeded in measuring the 
 .speed of nervous impulses in the motor nerves of the frog. The 
 rate he found to be 26.4 meters, or about 86.6 feet, per second. 
 Another series of investigations, in which the pendulum-myograph 
 was used, gave a result about 3 feet larger (27.25 in.). Subsequent 
 .investigators have substantially confirmed the figures of Helm- 
 .holtz. Bernstein, by a still different method of measurement, found 
 that the speed of conduction in the nerves varies between 25 and 
 .33 meters. In the motor nerves of man the number was still later 
 fixed by Helmholtz and Baxt at 33.9 meters, or about 111 feet, per 
 second. Von Wittich found it to be about 98.5 feet per second. 
 The complexity of the elements which enter into the measurement 
 of the speed of nervous impulses in the sensory nerves makes it near- 
 ly impossible to obtain satisfactory results by experiment. And so 
 far as the calculations take into account changes produced in the 
 
 1 See Journal of Anat. and Physiol., 1871, v., pp. 329 ff. 
 
 2 Archiv f. Anat. u. Physiol., 1850, p. 337. 
 8 Pfluger's Archiv, viii., p. 261; and xvi., p. 262. 
 
 4 See his Handbuch der Physiologic, i. , pp. 581 ff. Coblenz, 1844. 
 *See Archiv f. Anat. u. Physiol., 1850, pp. 276-364. 
 
SPEED OF NERVOUS IMPULSES. 121 
 
 nervous centres with accompanying phenomena of sensation and 
 attention, their discussion belongs elsewhere. Of the four factors 
 that enter into the entire time ("reaction-time") which elapses 
 between the application of stimulus to a sensory nerve and the 
 resulting contraction of the muscle namely, (1) time of conduction 
 in the sensory nerve ; (2) processes in the central organs ; (3) time of 
 conduction in the motor nerve ; and (4) latent period of the muscle 
 it is difficult to disentangle the factor (No. (1) ) required by 
 the attempt at analysis. Hirsch, by experimenting with stimuli ap- 
 plied to the skin at different distances from the brain, found the 
 speed of conduction in the sensory nerves of man to be about 111.5 
 feet per second a result in exceedingly close agreement with the 
 figure obtained by Helmholtz for the motor nerves. Schelske 
 used another method of measurement ; by applying the stimulus 
 to the groin and the foot, and recording the difference of time in 
 the two classes of cases, he obtained results varying between 
 25.294 and 32.608 meters per second. Others have given figures 
 differing more or less widely from those just stated. The general 
 conclusions, however, favor numbers lying between 98 and 131 
 feet per second as giving the speed of conduction in the sensory 
 nerves of man. 
 
 The speed of conduction in all nerves depends upon several va- 
 rying conditions, such as their temperature, the strength of the 
 stimulus, length of the nerve-stretch, and its electrical condition. 
 Experiments in winter give different results from those in summer. 
 In the motor nerves of man the rate can be made, by changes of 
 temperature, to vary from about 98 feet to 295 feet per second. 
 It has been disputed by different observers whether the speed of 
 conduction is dependent in any degree upon the strength of the 
 stimulus ; and even Hermann considers the question undecided. 
 But Vintschgau J has recently shown, as the result of a large number 
 of carefully conducted experiments, that as soon as the stimulus 
 rises above a certain limit of intensity, the speed of the nervous 
 impulses increases with the increase of the intensity of the stimulus. 
 This limit depends, however, upon the direction of the current, upon 
 whether it is a making or a breaking current, upon the animal 
 chosen for experiment, etc. Whether the speed of the nervous im- 
 pulses is directly dependent upon the length of the nerve-stretch 
 is scarcely decided beyond doubt. The effort of the science, gen- 
 eral "nerve-physiology," is directed toward showing how these 
 variations in speed, as experimentally determined, may be explained 
 from the laws of Electrotonus. 
 
 1 In Pfliiger's Archiv, 1883, xxx., pp. 17 ff. 
 
122 THE NERVES AS CONDUCTORS. 
 
 28. Finally, it should be remembered that the fact of any 
 propagation of nervous impulses whatever presupposes the con- 
 tinuity, integrity, and isolation of the nerve tract along which the 
 impulses move. The slightest separation of the substance of the 
 nerve by cross-section, even when the cut ends are left in the 
 closest mechanical contact, destroys the unity of the nerve's 
 physiological function. The ancients knew that tying the nerve 
 prevented its action ; they explained the fact by saying that the 
 flow of nervous fluid was thus hindered. So also does the fineness 
 of the localization which belongs to the organs of motion, but 
 especially to those of sense, as well as the fact that partial section 
 of a nerve only lames part of the field cared for by that nerve, 
 indicate the phytiofagwal isolation of the nerve-fibre during its 
 course between end-organs and central organs. Since the result of 
 stimulating a given nerve is in quality invariably the same, it would 
 seem that the law of the " specific energy " of each nervous element 
 (to which we shall refer elsewhere) is connected with the assump- 
 tions necessary to explain the phenomena attendant upon the 
 starting and propagating of nervous impulses in the conducting 
 nerves. 
 
 29. Inasmuch as the central organs are to a large extent com- 
 posed of nerves, a complete account of the nerves as conductors 
 should include a description of the nature of that nerve-commotion 
 which is propagated from point to point along the nervous ele- 
 ments within these organs, and of the paths or tracts along which 
 it passes. But unfortunately our knowledge upon these matters is 
 exceedingly scanty and uncertain. This is in part due to the fact 
 that the influence of the ganglion-cells, with which the nerve-fibres 
 are mixed to form the central organs, profoundly modifies the 
 neural processes of excitation and conduction. The subject be- 
 longs, then, to a consideration of the functions of the central or- 
 gans rather than of the nerves alone. Certain statements, how- 
 ever, may most fitly be given in this connection. 
 
 When speaking of conduction in the spinal cord or brain we 
 are not to think of a nerve-commotion as always moving along one 
 fixed path, after the analogy of the far simpler case of the nerve in 
 the nerve-muscle machine. It is true that the nerve-fibre in its 
 normal place in the body runs insulated, as it were, between the 
 spinal cord and the end-organ at the periphery. But the spinal 
 cord itself does not act as a perfectly isomorphic medium. The 
 very complex structure of this organ, in which nerve-fibres and 
 nerve-cells are intricately interwoven, has already shown us that it 
 is not adapted to act as such a medium, The case of the brain is 
 
IMPULSES IN THE SPINAL COED. 123 
 
 even clearer. It accords, therefore, with the structure of all central 
 organs, that we should find the speed of conduction slower in them 
 than in the peripheral nerves. Exner ' calculated, from the delay 
 which sensory impulses experience in the cord of man, that their 
 speed there is not more than about 26} feet (8 meters) per sec- 
 ond. The speed of the motor impulses in the cord he gives doubt- 
 fully as varying between 36 feet and 49 feet (11 to 12 and 14 to 15 
 meters). These numbers are substantially confirmed by the con- 
 clusions of Burckhardt (8 to 14 meters). The latter also maintains 
 that the speed of the impulses which occasion sensations of touch is 
 greater than that of those which occasion pain (as 27 to 50 meters 
 compared with 8 to 14). It has also been observed that, in some 
 cases of persons with disease of the posterior strands of the spinal 
 cord, sensations of pain arise in consciousness notably later than 
 those of touch. But the interpretation of all these phenomena is 
 complicated with questions of the cerebral functions ; for sensations 
 of pain are pre-eminently of cerebral origin. Moreover, we can 
 have but meagre confidence in our ability to tell with any pre- 
 cision the length of the paths by which nervous impulses travel in 
 the spinal cord of man. The fact observed by du Bois-Reymond, 
 that the vibrations of the muscle tetanized through the cord are 
 less than would be expected from the number of shocks given by 
 the stimulus, and the fact discovered by Helrnholtz, that muscle 
 when tetanized by an act of will has a uniform tone indicating 
 nineteen vibrations to the second (the rate of vibration into which 
 the muscle is thrown by direct stimulation of the motor nerve, on 
 the contrary, corresponding to the number of shocks), show the 
 profound effect of the central organs over the nervous impulses. 
 Although, then, the experimental evidence is not perfectly conclu- 
 sive, it, on the whole, confirms what we should expect from the 
 anatomical structure of the spinal cord, as to the complexity and 
 relative slowness of conduction in this organ. 
 
 30. Various attempts have been made by experimental physi- 
 ology to demonstrate the paths of conduction in the spinal cord. 
 The evidence from histology on this difficult subject has already 
 been given (p. 71 f.). It is not always easy to make the two lines 
 of evidence coincide. As to one point of experimental physiology } 
 however, no doubt has existed since the "epoch-making discov- 
 ery " of Sir Charles Bell and Magendie. The sensory fibres en- 
 ter the spinal cord by the posterior root, the motor fibres by the 
 anterior. The demonstration of this fact is performed by dividing 
 these roots, respectively, and observing the results. When a pos- 
 1 Pfliiger's Archiv, vii., pp. 632 ff. ; and compare Ibid., viii., pp 532 ff. 
 
124 PATHS OF NERVOUS CONDUCTION. 
 
 terior root is divided all the structures supplied by the same nerve 
 lose their sensibility ; while the muscles supplied by its correspond- 
 ing anterior root continue to be thrown into action by the will 
 and by reflex stimulation. Moreover, stimulation of the central 
 end of the posterior root thus divided produces sensory effects, but 
 stimulation of its peripheral end produces no motion. When an 
 anterior root is divided, on the contrary, the muscles supplied by 
 its nerves cannot be made to act either by volition or by reflex 
 stimulation ; but no sensory paralysis is produced. Moreover, 
 stimulation of the peripheral end of the nerve will now throw the 
 muscles into contraction, but stimulation of the central end will 
 produce no effects. An exception to the exclusively motor effects 
 of the peripheral end occurs in certain cases of so-called "recurrent 
 sensibility; " the sensibility shown in these cases is probably due 
 to the fact that a few sensory fibres from the posterior root, after 
 running a short distance in the mixed nerve, turn back and run 
 upward in the anterior root. The proof is then complete, so far 
 as the direct motor paths to the striated muscles, and the specifi- 
 cally sensory paths which conduct impulses to the cerebral hemi- 
 spheres, are concerned. According to Sigmund Mayer ' it does 
 not necessarily follow, however, that only centripetal impulses are 
 conducted by the posterior, and only centrifugal by the anterior 
 roots. 
 
 31. The general arrangement of the motor paths in that part 
 of the spinal cord, on the same side, where they enter by the an- 
 terior roots of the nerves, and of the sensory paths in the posterior 
 part of the cord, is maintained throughout. In man, that is to 
 say, the impulses pass up or down the cord in that region of it 
 at which they leave, or by which they enter with, the anterior or 
 the posterior roots. But histology shows that the two halves of the 
 cord are anatomically connected by the commissures, and that every 
 part of each half is bound with other parts of the same half, both 
 up and down and to and fro. Physiology, too, indicates that the 
 paths of sensory impulse undergo a partial crossing from right to 
 left, and from left to right. For, after complete section of one lat> 
 eral half of the cord, complete loss of sensibility of either side in 
 that part of the body which is supplied by those nerves of the same 
 side that enter the cord below the place of section does not re- 
 sult. The effects that do result depend upon the animal chosen 
 for experiment, and upon the height at which the section is made. 
 Experiments upon the lower animals seem also to show that in 
 their case a partial crossing of the motor paths takes place in the 
 1 Hermann's Handb. d. Physiol., II., i., p. 217. 
 
PATHS IN THE SPINAL COLUMNS. 125 
 
 spinal cord ; the evidence from pathology makes it doubtful whether 
 in man any crossing from side to side occurs in the voluntary 
 motor paths, at least below a point very high up in the neck. All 
 the evidence shows that in the lateral columns both sensory and 
 motor paths are to be found. 
 
 32. In addition to the general statement just made, experi- 
 mental physiology has little to say confirming or correcting the con- 
 clusions of histology (see p. 71 f.) as to the paths of neural impulses 
 in the spinal cord of man. 1 Experiments which attempt to make 
 a section, either of all the fibres in the anterior columns, leaving all 
 the other fibres intact, or of all the other columns, leaving the fibres 
 in the anterior columns intact, can never, indeed, be quite sure of 
 their success. But, on the whole, their results are confirmatory of 
 the statements made in the last article. Some investigators have 
 endeavored to solve the same problem by directly stimulating the 
 fibres of the different columns in such manner as to confine, as far as 
 possible, the influence of the excitatory current (or other stimulus) 
 to certain definitely selected fibres, and so to exclude all reflex ac- 
 tion. It is found that no reaction, indicative of any sensory im- 
 pulses whatever, follows the stimulation of the central ends of the 
 anterior white columns of the spinal cord ; but stimulation of the 
 peripheral ends of these same columns may be followed by muscu- 
 lar contraction, sometimes (so Longet and Kiirschner found) when 
 mechanical stimuli are used, but oftener with weak electrical cur- 
 rents. Careful cutting of these columns is followed by no signs of 
 pain. 
 
 On the other hand, stimulation of the central cut ends of the 
 posterior columns produces signs of pain, and other sensory effects ; 
 for this purpose Longet has used electrical, and Eigenbrodt and 
 Schiff mechanical stimulation. According to Schiff and others the 
 entire cord can be cut through from before back to the posterior 
 columns, and if these are left the animal will retain the sense of 
 feeling. As to a further differentiation of the sensory function of 
 these columns, different experimenters do not agree. Some would 
 confine their function to impulses that give rise to sensations of 
 touch, on the ground that animals, the substance of whose cord has 
 been entirely cut through with the exception of the posterior col- 
 umns, retain their sensations of touch, but loose their susceptibil- 
 ity to pain from impressions made on the surfaces whose nerves 
 enter the cord below the place of section. Impulses which give 
 
 1 Comp. the generalizations of Eckhard in the chapter on " Verlauf d. mo- 
 torischen u. sensiblen Innervationswege im Riickenmarke," Hermann, Handb. 
 d. Physiol.,H.,ii.,pp. 148 ff. 
 
126 PATHS OF NERVOUS CONDUCTION. 
 
 rise to sensations of pain must therefore pass elsewhere than by 
 the posterior strands ; that is, chiefly by the gray matter of the 
 cord. According to others, however, these strands conduct sen- 
 sory processes only in so far as they serve for the passage through 
 them of the nerves from the sensory roots ; it is, then, the gray sub- 
 stance of the cord which conducts these processes along upward. 
 In addition to the more marked sensory effects of stimulating the 
 posterior columns, some experimenters get effects which they in- 
 terpret as showing the presence of motor, and even of voluntary 
 motor, paths in these columns. Stilling, for example, found that 
 voluntary motions occurred after one entire anterior half of the 
 cord had been cut through. But in the absence of proof that 
 no motor paths in the lateral columns were left intact by his ex- 
 periments, and in view of the fact that a crossing of such paths 
 may take place in some of the animals, the evidence is not conclu- 
 sive. Moreover, Tiirck and others have found that the posterior 
 white columns may be entirely cut through without causing motor 
 disturbances. 
 
 In the lateral columns of the cord, paths of both motor and sen- 
 sory impulses are probably to be found. As to the case of motor 
 paths there is, indeed, no reasonable doubt at least there is no dis- 
 pute. Ludwig and Woroschiloff found that, in the case of the rab- 
 bit, voluntary movements of the hinder extremities took place even 
 after section of the anterior and posterior strands, and of the gray 
 matter of the cord in the cervical region. As to the proofs of sen- 
 sory paths in the lateral columns, the evidence is somewhat con- 
 flicting. Longet and Stilling discovered no proof of their existence ; 
 Schiff pronounces the matter doubtful ; Tiirck found that unmistak- 
 able signs of pain followed the cutting of these portions of the cord. 
 Experiments upon animals and pathological observation, however, 
 on the whole, confirm the view that the sensory are mixed with the 
 motor paths in the lateral columns. As Wundt l expresses the ap- 
 parent truth in the side strands of the cord a part of the system of 
 motor fibres is shoved off toward the limits of the posterior columns 
 and surrounded on all sides by branches of the sensory tract. 
 
 It must be borne in mind that the function of neither the motor 
 nor the sensory tracts is such that a nerve-commotion, when started 
 in one of the columns, must necessarily run its course by the short- 
 est path in that one column, or else not be propagated at all to its 
 destination. Both histology and physiological experiment indi- 
 cate that the interlacing of the nerve-fibres, and the interruption of 
 their course with nerve-cells, provide various secondary paths in 
 'Grundziige d. physiolog. Psychologic, i., p. 101. Leipzig, 1880. 
 
PATHS IN THE BRAIN. 127 
 
 addition to that which may be called the primary or chief. More- 
 over, the gray substance of the cord not only distributes, but also 
 carries forward the nervous impulses. After entire half- section of 
 the cord the sensory tracts of the other half still seem able, in a 
 partially substitutionary way, to accomplish the work normal to 
 both sides. And even in the case of the voluntary motor tracts in 
 man's spinal cord, though such a work of substitution does not take 
 place, we cannot affirm that the paths of voluntary innervation 
 for a definite set of muscles are invariably the same through their 
 entire length. A certain latitude of movement from the straight- 
 forward course of the impulse undoubtedly exists even in such a 
 case. 
 
 33. Difficult as it is for experimental physiology to deal with 
 the tracing of those paths along which the sensory and motor 
 impulses flow in the spinal cord, it is much more so within the 
 nervous mass which fills the cranial cavity. Both the structure and 
 the functions of the cerebrum, as a group of chief central organs, 
 make it nearly impossible experimentally to distinguish between 
 paths of voluntary and paths of merely reflex motion ; or even to 
 conjecture where, within its substance, impulses that have been 
 moving along some more clearly defined tract may not divide and, 
 subdivide indefinitely, or conversely impulses that enter along- 
 several converging paths be concentrated, as it were, into one or two 
 that are more definitely fixed. 
 
 The evidence by which histology has succeeded in tracing cer- 
 tain tracts through the brain, from the medulla oblongata to the 
 convolutions of the cerebral cortex, has been presented at sufficient 
 length in the last chapter (see pp. 76 f., 87 f., and 97 f.). The fuller 
 discussion of the evidence from experimental physiology concerning 
 the same subject will more properly appear in subsequent chapters 
 upon the automatic and reflex functions of the central organs and 
 upon the localization of cerebral function. Certain tracts which pass 
 directly from the crusta through the internal capsule, without en- 
 tering the basal ganglia, into the frontal and parietal convolutions 
 have already been referred to as probably motor. Others which 
 come from the tegmentum, enter the thalamus and subthalamic 
 region, and emerge after being redistributed to find their way es- 
 pecially to the tempero-sphenoidal and occipital lobes, have been 
 declared, in all probability, to be sensory. With this statement, 
 so far as the motor tracts are concerned, we shall see that the con- 
 clusions of experimental physiology accord very well. 
 
 34. But our assured knowledge from experiment concerning the 
 paths by which sensory impulses travel in the brain is exceedingly 
 
128 PATHS OF NERVOUS CONDUCTION. 
 
 meagre. These paths are probably much more numerous and in- 
 tricate than those along which the motor impulses are propagated. 
 Moreover, we can seldom draw conclusions with safety concerning 
 the sensations of the lower animals ; we therefore largely lose our 
 help from experiment upon them to determine these sensory paths. 
 The phenomena connected with all sensory disturbances are exceed- 
 ingly complicated, and the conclusions they seem to warrant are 
 often conflicting. For example, the effect of destroying a sensory 
 nerve-tract in the head does not consist simply in the destruction 
 or laming of some one definite function. On the contrary, if a 
 sensory cranial nerve is severed, the various different functions of 
 feeling pain, of pressure, and temperature, and the power of localiz- 
 ing, in the region supplied by the nerve are all lost. But disease 
 of the cerebro-spinal axis may impair one or more of these func- 
 tions, and leave the others intact, in a given region of the periph- 
 ery. Anaesthetics also may obliterate the sense of pain while leav- 
 ing that of contact relatively unimpaired. 
 
 Still more difficult of comprehension from the point of view fur- 
 nished by the general physiology of the nerves are the degrees of 
 tenacity with which different sensory functions, even when adminis- 
 tered by the same sensory nerve, are combined. Loss of the sense 
 of temperature and of the muscular sense rarely or never occur 
 separately ; but muscular sense not infrequently disappears and 
 the sensitiveness of the skin to pressure is retained. Upon such 
 phenomena we have little clear light to throw. It can simply be said 
 that the distribution of the sensory nerves within the central or- 
 gans must be enormously complicated, and that we have absolute- 
 ly no knowledge as to any differences in the kinds, or velocity, or 
 paths, of the nerve-commotions there, that will help us to account 
 for the facts. Yet such differences in the sensations doubtless rest 
 upon differences in the nerve-commotion that causes them, within 
 that inner projection -system of sensory impressions which is fur- 
 nished by the cortex of the cerebrum. 
 
 It has already been seen that the paths of sensory impulses cross 
 over more or less completely within the spinal cord. They also, 
 like the paths of motor impulses, cross in the region where the 
 nerve-fibres in general decussate namely, in the pons varolii and 
 medulla oblongata. Experiment and pathology both show that 
 the principal paths of sensory impulses from all the peripheral 
 parts of the trunk of the body and from its mucous membrane lie 
 close to those of the motor impulses in the white nervous substance 
 surrounding the basal ganglia. Effusions of blood in this region 
 not only cause hemiplegia, but also produce more or less impair- 
 
PATHS IN THE BASAL GANGLIA. 129 
 
 ment of the different modifications of touch, both in the skin and 
 in the mucous membrane. According to some authorities, lesions 
 in the same region often so impair the muscular sense that the 
 contraction of the muscles which is produced by electrical stimula- 
 tion is no longer felt. Veyssiere and others suppose that injuries 
 to the white fibrous matter of the crura cerebri, the internal cap- 
 sule, and the foot of the corona radiata, invariably produce a loss 
 of sensibility on one side of the body ; while those which are more 
 definitely confined to the striate body have this effect only imper- 
 fectly and for a time the amount of the effect depending upon the 
 amount of the adjoining white substance which is involved in the 
 injury. This view, like many others on the general subject, is 
 doubtful. 
 
 35. Attempts have been made to localize the paths of sensory 
 impulses in the optic thalami and those of motor impulses in the 
 striate bodies ; and in connection with this view it has been held 
 that the former are chiefly concerned in the elaboration of sensory 
 impulses (as sensory ganglionic centres), and the latter in the 
 elaboration of motor impulses (as motor centres). This theory has 
 been wrought out (with much rhetoric and conjecture) by J. 
 Luys. 1 Luys finds in the optic thalami four centres which lying 
 in order, one behind the other in an antero-posterior line conduct 
 and " condense " respectively the olfactory, the visual, the tact- 
 ual, and the auditory impressions ; the corpora striata perform 
 a similar office for the motor impulses. The sensory impressions 
 which come from the periphery, therefore, all run through the op- 
 tic thalami, according to this theory, in order that they may be 
 "intellectualized" (whatever that may mean); the motor through 
 the striate bodies, in order that they may be "materialized." It 
 is enough in this connection to say that no such complete dis- 
 tinction of function in the basal ganglia, whether as conductors 
 or as central organs, has yet been made out. It is true, however, 
 that the paths in the crusta and in and surrounding the striate 
 bodies are probably mainly motor, while those in the tegmentum 
 and in and around the optic thalami are mainly sensory. The 
 tendency of the most recent investigation is toward placing more 
 emphasis upon the fibrous nerve-matter surrounding these organs 
 as furnishing paths for the conduction of both kinds of impulses. 
 
 1 Recherches anatomiques, physiologiques, et pathologiques sur les Centres 
 Nerveux, 1865 ; and The Brain and its Functions, New York, 1882. 
 9 
 
CHAPTER IV. 
 
 AUTOMATIC AND REFLEX FUNCTIONS OF THE CENTRAL 
 
 ORGANS. 
 
 1. When a physiological function is occasioned in a peripheral 
 nerve, independently of a so-called act of will, by the stimulation 
 of some other peripheral nerve, this function is said to be " reflex." 
 Such a reflex function of the nerve is regularly brought about, how- 
 ever, by the mediation of a collection of ganglion-cells and inter- 
 lacing nerve-fibres, known as a central organ. In other words, the 
 second ary stimulation of one peripheral nerve, through a central 
 organ, as a result of a primary stimulation of some other periph- 
 eral nerve, is a reflex action of the nervous elements. The entire 
 cerebro-spinal axis is a pile of nervous centres, increasing, on the 
 whole, in complexity of structure and of function from below up- 
 ward, which, with the nerve-tracts running into and out of it, con- 
 stitutes a complicated mechanism capable of an indefinite variety 
 of such reflex functions. But the spinal cord and the medulla 
 oblongata are the special seat of many such functions. On the 
 other hand, all excitations of the nervous system which originate 
 in the nervous centres themselves' that is as distinguished from 
 being called out there by the nerve-commotion brought to them 
 through the afferent nerves are called "automatic." The word 
 automatic must doubtless often be used to conceal our ignorance 
 of the real origin of a neural process. And doubtless, also, many 
 processes which, on first inspection, appear to be automatic, may 
 be discovered, or suspected, to be in reality reflex. But, as far as 
 our information goes at present, not only movements of the mus- 
 cles through the stimulating of the efferent nerves connected with 
 them, but also the inhibiting of such movements, and the rise of 
 sensations, must be ascribed to the automatic action of the central 
 organs. Changes in the vital conditions to which these organs are 
 subjected by their immediate surroundings, and especially changes 
 in the condition of the blood with which they are supplied, ordina- 
 rily constitute the internals timuli to which they respond by exer- 
 cising their peculiar functions. Automatic activities belong dis- 
 tinctively to the central ganglia of the brain ; it is more difficult to 
 
KINDS OF KEFLEX ACTION. 131 
 
 vindicate their existence in the spinal cord. In general, it is by no 
 means easy confidently to distinguish between the purely reflex 
 and the purely automatic action of particular central organs. The 
 two forms of action are doubtless uniformly blended ; so that what 
 is accomplished by any central organ depends both upon its own 
 internal condition and molecular activity at the moment when the 
 sensory impulse reaches it, and also upon the character of that im- 
 pulse. Inasmuch as it is a vital molecular mechanism connected 
 by an indefinite number of ties with other similar mechanisms, the 
 central organ constantly acts both reflexly and automatically. 
 
 2. It follows, therefore, that several kinds of reflex action are 
 theoretically supposable in the nervous system. When motor 
 nerves are stimulated in a secondary way through a central organ, 
 by applying stimulus to the sensory nerve-endings, the effect may 
 be called reflex-motor. If an excitation of a motor nerve were 
 transferred, without action of the will, to one or more sensory paths, 
 such a conversion of nervous action might be called reflex-sensory. 
 In this way the attempt has been made to explain the feeling of 
 weariness in the muscles when they have been overexerted, or the 
 feeling which we describe as that of a limb being "asleep." It has 
 also been proposed to speak of "co-motor reflexes," in cases where 
 two motor nerves are assumed to be reciprocally combined in their 
 influence, through a central organ; or of "co-sensory," in cases 
 where the same relation is sustained by two sensory nerves. As an 
 example of the latter, attention has been called to the sensation 
 which is felt in the nose when trying to look at the sun. Examples 
 of the three last classes of alleged reflex functions of the nervous 
 system are, however, for the most part very doubtful; or they 
 admit of explanation by recognized causes in another way. 1 It is 
 only concerning the laws of the first class of reflex actions the 
 reflex-motor or sensory-motor that we have assured scientific 
 evidence. The reflex function of a central organ may be defined, 
 then, as being (at least in its simplest form) the " conversion " or 
 "reflexion" of a sensory impulse into a motor excitation. We 
 must guard ourselves carefully, however, against the misconception 
 that lurks in these words : the effect of the central organ is never 
 that of merely converting or reflecting a nerve-commotion from one 
 perfectly definite sensory path to an equally definite motor path. 
 No such simple figure of speech will serve to describe its function. 
 
 3. The spinal cord complex as its structure and functions 
 are is much the simplest and most accessible for experimental 
 purposes of any of the organs of the cerebro- spinal system ; it is 
 1 Comp. Bokhara in Hermann's Handb. d. Physiol., II., ii., pp. 23 ff. 
 
132 THE CORD AS A CENTRAL ORGAN. 
 
 pre-eminently the seat of unconscious reflex-motor functions. It is 
 a column or pile of centres, bound together for the reception of 
 sensory impulses by its posterior roots and for redistributing them, 
 as modified by its own molecular structure and condition, through 
 the efferent fibres of the anterior roots. Such is its office as an 
 organ of reflex action in distinction from its office as an organ for 
 conducting neural impulses. We consider, then, in the first place, 
 the Spinal Cord as a Central Organ. 
 
 4 As the "nerve-muscle machine" is a preparation for testing 
 experimentally the laws of the action of the nerves as conductors, 
 so preparations may be made for testing the laws of the reflex and 
 automatic functions of the spinal cord, by separating that organ 
 from the brain by section below the medulla oblongata. For the 
 purpose of experiment, the "brainless frog " is the most convenient 
 of such preparations and the most fruitful of results. 1 If the flank 
 of such a frog be lightly touched, the resulting reflex motion will 
 be limited to a slight twitching of the muscles that lie immediately 
 beneath the spot on the skin thus stimulated. If its legs be 
 stretched out and one of them pinched, all the segments of the 
 limb thus irritated will be rapidly flexed in the definite purposeful 
 way necessary to withdraw it from the irritation. If the skin of 
 the region near the anal orifice be pinched, a new combination of 
 muscular contractions will take place and a different form of defen- 
 sive movements will result : the feet will be drawn up toward the 
 spot irritated and the legs brusquely extended, as though to push 
 away the irritating agent. If the stimulus applied to the skin of 
 one hind leg be increased by forcibly pinching it, the resulting 
 reflex motions may involve the fore leg of the same side, then the 
 hind leg and fore leg of the opposide side, and finally almost all 
 the muscles of the body. Moreover, changes in the character of 
 these reflex motor activities take place which are plainly adapted to 
 provide for changes in the animal's circumstances. For if the 
 right flank of a brainless frog be irritated with a drop of acid, and 
 at the same time the right leg be held (the member which, if un- 
 hindered, would be, almost without exception, used in the attempt 
 to remove the irritation), or the right foot cut off, the left foot 
 may be used for the same purpose of defence. 
 
 Phenomena, similar to those obtained in the case of the frog, 
 are obtained from other brainless animals. Thus the decapitated 
 salamander, when the skin of one of its sides is pinched, will bend 
 this side into concave shape in order to withdraw it. Not succeed- 
 
 1 For detailed information see Vulpian, Le9ons sur la Physiologic du Sys- 
 teme Nerveux, pp. 311-465. 
 
REFLEXES OF THE SPINAL COED. 133 
 
 in 01 in this way, it will make a movement with its foot as though to 
 push away the cause of the irritation. In the case of the higher 
 animals the reflexes of the spinal cord appear, on first inspection, 
 to be comparatively feeble and lacking in purposeful character. 
 The mammal, for a relatively long time after the division of the 
 cord from the brain, exhibits only very imperfect reactions in parts 
 of the body supplied by nerves which spring from the cord below 
 the point of its section. But if the animal be kept alive for some 
 time, and even without any physiological union of the severed parts, 
 more strong, varied, and complex movements will follow upon the 
 stimulation of the sensory nerves of those parts. Immediately 
 after the spinal cord of a dog is divided low down in the dorsal 
 region, the hind limbs hang limp and motionless ; irritating the 
 skin calls forth only feeble and irregular movements, or none at all. 
 But after some weeks or months have elapsed, reactions resembling 
 those already described in the case of the frog (taking into account, 
 of course, the difference in the structure and normal functions of the 
 two animals) begin to appear. The hind limbs, instead of remain- 
 ing motionless, will, when the animal is held so that they are pen- 
 dent, be drawn up and let down again with a kind of regular rhythm, 
 as a result of the constant stimulation of their motor nerves by the 
 sensory nerves, through the spinal cord. Moreover, it is found that 
 the breed, age, sex, and training of the animal determine the charac- 
 ter of these brainless reflex movements. That is to say, the spinal 
 cord, as a nervous mechanism, embodies in its very structure and 
 functions all the peculiarities due to these causes. And when its 
 activities are elicited through the stimulus which, arising in many 
 various regions, flows in upon it along the sensory nerve-tracts, or 
 through some stronger but limited impulse occasioned by the ap- 
 plication of stimulus to a particular spot 011 the skin with a definite 
 degree of energy, these activities bear the character both of the 
 stimulation and of the mediating central organ. 
 
 5. Little need be added to what has already been said (Chap. 
 II., 9), in description of that mechanism of the cord to which 
 the foregoing remarkable functions are referred. Earlier investi- 
 gators ! assumed the existence of a special system of sensory and 
 motor nerve-fibres, with connecting nerve-cells, designed and appro- 
 priated solely for executing these reflex-motor activities. That the 
 motor tracts for reflex movements are to a certain extent distinct 
 in the spinal cord from those devoted to specifically voluntary ac- 
 tivities, there seems to be good reason for affirming ; but the older 
 
 1 For example, Marshall Hall, in his New Memoir on the Nervous System. 
 London, 1843. 
 
134 THE CORD AS A CENTRAL ORGAN. 
 
 supposition, that there are double tracts, one connected with con- 
 scious and voluntary reaction upon sensation, and one connected 
 with unconscious and involuntary, or merely reflex-motor, reac- 
 tion, between the spinal cord and the end-organs of sensation 
 and motion, is almost certainly incorrect. It seems antecedently 
 very improbable that every spot of the skin should be equipped 
 with such a twofold outfit of both kinds of nerve-fibres. No par- 
 ticular nerves which serve merely for reflex-motor functions, and 
 which have no connection either with conscious sensation or with 
 voluntary motion, can be pointed out. 
 
 What happens with respect to conscious sensation the rise of 
 it or its failure to rise depends rather upon the effect of the 
 stimulus on the end-organ, and upon the condition in which that 
 stimulus finds the central organ on its arrival there. In consider- 
 ing that mechanism of the spinal cord which comes into use when 
 it acts as a central organ in all the reflex-motor activities belonging 
 to it, the office of the ganglion-cells is usually made prominent. 
 And it can be definitely proved that these cells are an important 
 part of the reflex mechanism of the cord. But the extremely del- 
 icate network of interlacing nerve-fibres in which the processes 
 of these cells lose themselves also bears an important part in the 
 same functions. Precisely what elements of the central substance 
 alone act, and precisely how the elements act that do act, it is im- 
 possible to say. 
 
 6. The following laws embody the most important general re- 
 sults of experiment upon the reflex-motor functions of the spinal 
 cord, as applied to a variety of animals under a great number of 
 changing conditions and circumstances. 
 
 The primary stimulation of the sensory nerves must have a cer- 
 tain degree of strength and suddenness in order to produce a sec- 
 ondary excitation of the motor nerves through the centres of the 
 spinal cord. This is true of all the different kinds of stimuli by 
 application of which spinal reflexes can be obtained. Continuous 
 irritation of the skin, if very slowly increased, may be carried to 
 the extent necessary to destroy its sensitive surface, without giving 
 rise to any reflex movements ; but a less degree of stimulus, if 
 suddenly applied, will call forth such movements. Different chem- 
 ical substances, when used as irritants, produce effects dependent 
 upon the strength of the solution. Thus a weak solution ( - -J f] 
 of sulphuric acid is recommended by some experimenters ; and 
 it is asserted that in this way exactly the same reflex move- 
 ments, as respects kind and degree, can be repeatedly got from the 
 same nerve-preparation, with a machine-like regularity. Each chem- 
 
SPEED OF EEFLEX PEOCESSES. 135 
 
 ical stimulus has its lower limit of concentration which will produce 
 any reflex movement, and also its latent period. The time of the 
 latent period for weak solutions of sulphuric acid is said by Baxt ' 
 to increase nearly in geometrical ratio, while the concentration of 
 the acid diminishes in arithmetical ratio. The chemical stimulus, 
 like the mechanical, can be so slowly increased in strength as to 
 produce no effect. The same thing is also true of thermic stimu- 
 lus. A decapitated frog may be placed in water, and the water 
 gradually heated to the point at which heat-rigidity sets in, without 
 showing any reflex activity. This fact, however, may be in part 
 ascribed to the direct effect of the heat, diffused from the skin 
 upon the central organ. The same law which renders stimulus 
 inoperative, when very gradually increasing in strength, applies to 
 the use of the electrical current. Repetition of the shocks is much 
 more effective than a slow increase in the strength of the current. 
 Single induction-currents are relatively powerless, and produce no 
 effect unless they have a high degree of strength. Frequent inter- 
 ruptions greatly increase the efficiency of the constant current in 
 producing reflex movements. It would seem, then, that a kind of 
 summation of afferent impulses may take place in the spinal cord ; 
 that is to say, the repeated excitation of the nervous centre starts 
 a nerve-commotion in its substance, which gathers intensity until 
 it breaks over, as it were, into the adjoining motor tracts. We can 
 scarcely affirm, however, that such summation of many impulses is 
 necessary to start off the nervous centre, as it were, since the sin- 
 gle making of the constant current, or a single strong induction- 
 shock, may be followed by a number of reflex movements. 
 
 7. The speed of reflex processes is apparently increased by 
 increasing the strength of the stimulus. We have already spoken 
 (p. 123) of the delay which the process of conduction suffers in the 
 spinal cord when passing longitudinally. The time of cross-con- 
 duction also in the cord seems to be a function of the strength of 
 the stimulus. Exner 2 calculated by an experiment, which con- 
 sisted in causing one eyelid to move by stimulating the other, that 
 the time consumed in the specifically central operations of the re- 
 flex act can be made to vary between 0.055 and 0.047 of a second 
 by increasing the strength of the stimulus. Rosenthal 3 and others 
 have found that the time for any reflex act diminishes considerably 
 with the increase of the strength of the stimulus ; is greater in trans- 
 verse than in longitudinal conduction ; and is much increased by ex- 
 
 1 Quoted in Hermann, Handb. d. Physiol., II, ii., p. 29. 
 
 2 See Pfliiger's Archiv, viii., p. 530 ff. 
 
 3 Monatsbericht d. Berlin. Acad., 1873, p. 104. 
 
136 THE COED AS A CENTRAL ORGAN. 
 
 haustion of the cord. With very strong stimuli it becomes almost 
 too brief for observation. Wundt, ' however, denies that the time of 
 the reflex act is dependent upon the strength of the stimulus ; on the 
 contrary, he affirms that the time is either very little or none at all 
 affected by changes in strength of the stimulus, or else is even 
 changed in the contrary direction to that required by the alleged 
 law of Exner and Rosenthal. 
 
 8. The condition of the spinal cord, at the time when it re- 
 ceives the impulses of the sensory nerves, undoubtedly determines 
 to a large extent the character of the resulting reflex motions. 
 Lesion increases the excitability of the part below the lesion, and 
 this for example, in the case of reflex movements of the posterior 
 limbs according to the amount of the cord removed from the por- 
 tion of it lying anterior to its nervous connections with these limbs. 2 
 Marked effects are also produced by certain drugs, as strychnine, 
 chloroform, aconite, quinine, etc. Of these drugs, some heighten 
 and some depress its excitability. In an animal slightly poisoned 
 with strychnine, the excitability of the cord is more or less height- 
 ened ; and in cases of strong poisoning with the same drug, the 
 least stimulation may call forth a condition of tetanus or convul- 
 sive cramping extending to the whole body. Two ways of explain- 
 ing this effect upon the mechanism of the central organ are pos- 
 sible : ene, that the excitability of those portions of this organ 
 which mediate between the sensory and motor impulses is so much 
 increased by the poison that, on being stimulated, they explode 
 their molecular energy, as it were, and cause it to be diffused with 
 great strength into unaccustomed paths ; the other, that the effect 
 of the poison is to diminish the resistance along all the network 
 of paths, both habitual and unaccustomed, in the spinal cord. 
 Between these two explanations Eckhard 3 will not decide ; Ilosen- 
 thal seems to prefer the former, Foster 4 and others the latter. 
 Chloroform and various other anaesthetics diminish the reflex ac- 
 tion of the cord. As to the effect of changes in temperature, and 
 in electrical condition, upon the spinal reflexes, the conclusions of 
 different experimenters are somewhat divergent. This power of 
 the nervous mechanism is, as we have already seen, retained longer 
 in low than in high temperatures. According to Cayrade, when 
 the temperature of the whole cord is raised, the reflex movements, 
 however produced, become more energetic and the single con- 
 
 1 Mechanik d. Nerven, abth. ii., pp. 14 ff. Stuttgart, 1876. 
 
 2 Vulpian, Legons, etc., p. 438. 
 
 8 In Hermann, Handb. d. Physiol., II., ii., p. 42. 
 4 Text-book of Physiology, p. 602. 
 
THE EFFECT OF LOCALITY. 137 
 
 tractions last longer. Another observer found a temporary rise of 
 excitability, followed by a depression, on heating sections of the 
 cord between 75 and 158 Fahr. On the other hand, some observ- 
 ers are of the opinion that cold increases the excitability of the 
 cord. In experimenting with the electrical current it is very diffi- 
 cult to distinguish between its effect upon the central organ as the 
 mediating mechanism and the effect of the same stimulus upon 
 the nerve-roots and nerve-paths between which the mediation 
 occurs. 
 
 9. The locality to which the stimulus is applied has a marked 
 influence in determining the extent and character of the resulting 
 reflex movements. The most important difference of all is that 
 found by stimulating some spot of the skin, and then comparing 
 the resulting reflex action with what follows upon the application 
 of the same stimulus to the trunk of the nerve which is distributed 
 to that region of the skin. The simple nervous impulses, which 
 result from stimulating the afferent nerve-fibres directly, call forth 
 irregular spasms in 'a few muscles only ; the complicated nervous 
 impulses, which result from applying the same stimulus to the 
 skin, are followed by extended movement of many muscles directed 
 toward definite ends. Moreover, it is much more easy to produce 
 reflex action by a slight pressure on the skin than by even strong 
 induction-shocks when applied to the nerve-trunk. By separating 
 a small bit of skin from that surrounding it on the back of a brain- 
 less frog, while taking care not to injure the nerves that attach it 
 to the body, the foregoing difference may be made strikingly clear 
 in an experimental way. 1 Whai particular reflex actions will 
 be evoked by the stimulus is, in each case, dependent upon the 
 particular locality of the skin to which the stimulus is applied. 
 Such facts suggest the truth that the entire mechanism of the cord 
 is broken up into centres of activity, which, however, are in close 
 molecular relation with each other, and which are of a somewhat 
 expansive nature. 
 
 In view of the foregoing truths Pfliiger 2 has formulated the fol- 
 lowing laws of relation between the stimulation and the resulting 
 reflex action : (a) In the case of a spinal cord from which the 
 medulla oblongata is wholly severed, all reflex motion confined to 
 one side of the body is due to stimulation of that side, (b) Reflex 
 movements of both sides never occur in a diagonal direction ; that 
 is to say, stimulating one hind limb can never evoke reflex move- 
 
 1 See the article of Fick and Erlenmeyer in Pfliiger's Archiv, iii., p. 326. 
 a In his work, Ueber d. sensorischen Functionen d. Ruckenmarks. Berlin, 
 1853. 
 
138 THE CORD AS A CENTRAL ORGAN. 
 
 merit of that limb and of the fore limb of the opposite side. ' (c) If 
 reflex action is called out in the limbs of both sides, and such action 
 is stronger on one side than on the other, then it is stronger on the 
 side stimulated, (d) If the motor effects of the stimulation show 
 that the excitation has been "irradiated," as it were, from one 
 centre to another, then such movement of irradiation is always 
 downward toward the medulla oblongata in the brain, and upward 
 in the cord toward the same organ. It is by no means certain, 
 however, that these formulas (especially the second No. b) admit 
 of no exceptions which are involved in the peculiar structure and 
 functions of the cords of certain animals. But the general rule 
 appears to be, that the excitation of a sensory nerve with a slight 
 degree of stimulus gives rise to reflex movements which originate 
 in the cord on the same side, at about the same altitude as that at 
 which the sensory impulses enter the cord ; with an increased amount 
 of stimulus, it gives rise to those also that arise in the other half of 
 the cord at the same altitude ; with a still greater amount, to those 
 which arise above and below on both sides of the cord, with the 
 preference given to the same side. That is, the molecular disturb- 
 ance, as it is dispersed or radiated, passes from the cells and net- 
 work of fibres situated near together on the same side of the cord, 
 first to those on the other side of the cord at the same altitude, and 
 then diffuses itself on both sides up and down the cord. 2 Accord- 
 ingly, it is only after allowing for a difference in the obstacles to be 
 overcome along the different paths anatomically open to any nerve- 
 commotion in the spinal cord, that we can adopt the declaration 
 of Luchsinger: 3 When an excitation is started anywhere in the 
 spinal cord, it radiates from this point in all directions, but with 
 diminishing intensity. Hence the title which Flourens and Vul- 
 pian, 4 following him, have given to the spinal cord "the organ for 
 the dispersion of irritations." 
 
 10. Besides such undoubted reflex action as the foregoing, 
 other cases where the spinal cord controls the muscles of the body 
 are less certainly of a purely reflex character. Indeed, for some 
 such cases the title of " automatic " has been employed. The cord 
 is not capable of " irregular automatism " that is, of spontaneous 
 excitation like that which takes place in the higher nervous centres 
 
 1 See the observations of Luchsinger, which seem to show that in some ani- 
 mals as, e.g., the salamander, turtle, and even dogs, when under the influ 
 ence of ether cross reflexes in violation of Pfluger's law do sometimes occur. 
 Pfliiger's Archiv, xxii., pp. 179 ff. 
 
 2 Compare Wundt, Grundziige d. physiol. Psychologic, i., pp. 103 and 109. 
 
 3 Pfluger's Archiv, xxii., p. 178. 
 
 4 Lemons sur la Physiologic, etc. , p. 404. 
 
TONIC ACTION OF THE CORD. 139 
 
 on volition. If a brainless frog, for example, be kept in a condi- 
 tion of perfect equilibrium with respect to stimulus, it will remain 
 wholly motionless. But the cord of such an animal will continue 
 to influence certain muscles of the body through the motor nerves, 
 even in cases where sensory impulses are difficult or impossible to 
 trace. What is called the " tonic action " of the cord upon the 
 skeletal and sphincter muscles, or the smooth muscles of the ar- 
 teries, is a chief illustration of this influence. The fact that such 
 tonic action does not contract all the muscles connected with the 
 cord at the same time, or any one set of them with the same en- 
 ergy as any other, throws some suspicion on its alleged automatic 
 character. A careful sifting of the evidence rather induces us to 
 ascribe this influence to the constant reflex action of stimulus from 
 subtle changes in the external circumstances in which the animal 
 is placed. Moreover, the sensory nerves in the muscles and ten- 
 dons, as well as in the skin and organs of special sense, may occa- 
 sion the rise and continuance of such reflex action. Different in- 
 vestigators, almost without exception, have failed to notice any 
 lengthening of a muscle (or loss of its tone) when the nerve going 
 to it is severed from the cord. That this so-called "tonic" influ- 
 ence is largely reflex-motor is also shown by the fact that the tone of 
 the muscles is lost when the skin covering them is removed, or when 
 the posterior root which furnishes sensory impulses for the motor 
 nerves connected with them is cut. Brondgeest has shown that, 
 when a decapitated frog is hung up after having the sciatic plexus 
 cut on one side, the leg is more flexed (that is, the muscles have 
 more of tone) on the other side. But the same flaccid condition of 
 the muscles can be produced by cutting only the posterior (or sen- 
 sory) roots of this plexus. This observer is satisfied that the con- 
 traction of the muscles in the uninjured limb is due to stimulation 
 from the nerves of the skin ; the tonic action of the cord on the 
 skeletal muscles is, therefore, reflex. The only objection to consid- 
 ering the tone of the sphincter muscles reflex lies in the fact that this 
 tone continues to exist after all other reflex-motor action has been 
 suppressed by narcotics ; but our knowledge of the nervous mechan- 
 ism which controls these muscles is not sufficiently complete to make 
 it certain that we have excluded all possible forms of reflex influence. 
 Of the marked influence of the nervous system upon the cali- 
 bre of the arteries, and through this upon the character of the 
 circulation of the blood, there is abundant evidence. Besides the 
 main vaso-motor centres in the medulla oblongata, certain parts of 
 the -spinal cord are capable of acting as such centres. Circulation 
 may continue with regularity in a beheaded frog ; but the removal 
 
140 THE CORD AS A CENTRAL ORGAN. 
 
 also of any considerable part of the cord affects the circulation 
 through the loss of tone in the blood-vessels which it occasions. 
 The mechanisms for expanding and contracting the arteries are 
 apparently interlaced with those for contracting the skeletal mus- 
 cles, in all portions of the cord. But their chief work undoubt- 
 edly consists in transforming afferent impulses into efferent vaso- 
 motor impulses directed toward the dilatation or constriction of the 
 arteries. Whether they are capable of automatic action in the 
 sense in which the medulla oblongata seems to be thus capable 
 is a question we need not discuss in detail here. 
 
 11. The facts already alluded to, and others similar, form the 
 basis for the assumption of " Centres " in the spinal cord. In general, 
 the application of a given amount of stimulus to a definite group of 
 sensory nerves calls forth reflex-motor activities in definite groups 
 of muscles by means of a certain region of the cord. What groups 
 of muscles are thus moved depends upon the amount of the stimu- 
 lus and the locality of its application. This fact is due to disper- 
 sion of that nerve-commotion which is set up at different points 
 in the course of the cord by the excitation of those points through 
 the sensory nerves. That is to say, the mechanism of this central 
 organ is so constructed as to connect the sensory with the motor 
 tracts, more favorably in some regions than in others. Such re- 
 gions are the so-called reflex centres of the spinal cord. If, how- 
 ever, a more or less constant flow of motor impulses takes place 
 from any region, and this flow is due to molecular activity not 
 occasioned by the sensory nerve-fibres of the region, then such 
 region may also be called an automatic centre. Nothing would 
 seem to prevent the same region from acting as both a reflex and 
 an automatic centre. The general principle may then be formulated 
 as follows : " The spinal cord is the proximate centre, the proximate 
 physiological hearth of excitation, for all the nerves that originate 
 from it." This principle has been defended and illustrated with many 
 researches by Legallois, Volkmann, Pfliiger, Goltz, Luchsinger, and 
 others. In accordance with it, and especially since the "epoch- 
 making " experiments of Goltz upon the spinal cord of dogs, many 
 functions which were formerly ascribed to the brain have been 
 shown to have their proximate centre in the spinal-cord. In ac- 
 cordance with the same principle, it is discovered that different 
 animals have different spinal centres varying in relation to their 
 peripheral structure and their habits. 1 
 
 1 Compare the results of the researches of Langendorff in the Archiv f . 
 Anat. u. Physiol., Physiolog. Abth., 1880, pp. 518 ff., and 1881, pp. 519 ff. ; 
 and of Luchsinger in Pfluger's Archiv, xxii., pp. 158 ff., and xxiii., pp. 308 ff. 
 
CENTRES IN THE SPINAL COED. 141 
 
 Iii illustration of the last point the following facts may be men- 
 tioned : By the sufficiently long-continued and strong stimulation 
 of any portion of the skin of a decapitated frog, reflex movements 
 may be induced in all of its muscles. With rabbits, however, a reflex 
 action of one hind leg can be caused by stimulating the sensory 
 nerves of a fore leg, only in case a portion of the medulla oblon- 
 gata (at least about one-third) be left attached to the cord. With 
 the cord alone, the stimulation of one hind leg fails to excite ac- 
 tion in either of the fore limbs. By using great care and artificial 
 respiration, Luchsinger 1 succeeded in obtaining what he calls a 
 " trotting reflex " from the spinal cord after being completely sev- 
 ered from the medulla oblongata of several young animals with 
 which that form of movement is natural. Thus the diagonal op- 
 posite extremities of goats and cats w T ere moved in response to 
 even such weak stimulation as passive motion of the fore leg, gen- 
 tle pressure, and weak electrical currents. In general, then, it 
 would seem that the spinal cord of every animal is a series of con- 
 nected mechanisms, which are arranged so as to move the muscles 
 of the body, either under the control of the higher nervous centres 
 or in response to stimulation entering it at any point through the 
 sensory peripheral nerves, in accordance with the specific structure 
 and habits of the animal. 
 
 Many of the chief special centres connected with the organic and 
 vital functions are located in the medulla oblongata; those con- 
 nected with the co-ordination of impressions of the special senses 
 and muscular action belong to the still superior portions of the 
 cerebro-spinal system. But the spinal cord also contains mechan- 
 isms which serve as centres of both these kinds. 2 Their location, 
 however, is so much a matter of the special physiology of particular 
 species of animals, and is so indirectly connected with the inquiries 
 of physiological psychology, that it is unnecessary to add anything 
 further upon the subject. 
 
 12. The question whether the spinal cord is excitable as a 
 whole, and in its several parts, by artificial stimulation, has been 
 much debated. Its direct excitability as a whole is denied by 
 
 1 See Pfliiger's Arcliiv, xxviii., pp. 65 ff. 
 
 2 Besides the vaso-motor centres already referred to, those for micturition, 
 defecation, erection, parturition, etc., may also be mentioned. Goltz, in his 
 celebrated researches in 1874 (see Pfliiger's Archiv, viii., pp. 474 ff.), showed 
 that normal micturition may take place in a dog in which the lumbar region 
 has been completely severed from the dorsal region. The influence of the 
 cerebral centres seems, however, to be necessary to cause a steady increase or 
 decrease of the action of the sphincter ani. The cilio-spinal centre, located by 
 Budge at the seventh and eighth cervical roots, is more doubtful. 
 
142 EXCITABILITY OF THE CORD. 
 
 Schiff, ' who declares that the motions obtained by stimulating any 
 part of the cord with electricity comprise only those muscles which 
 are physiologically related, to the exclusion of those which are ana- 
 tomically contiguous through the stimulated part of the cord. A 
 strong local stimulus, he affirms, produces just the same reflex mo- 
 tions as those which are accustomed to arise on occasion of an ex- 
 tended irritation of the skin at the places to which the nerves is- 
 suing from this locality of the cord are distributed. It is inferred, 
 then, that the resulting motions are obtained only reflexly, by in- 
 volving the sensory nerve-roots. Bat that certain longitudinal parts 
 of the cord can be directly stimulated seems capable of demonstra- 
 tion. For Fick and Engelken 2 found that movements of the mus- 
 cles were obtained when the anterior columns were isolated from 
 the rest of the cord for a considerable distance and then stimulated. 
 Luchsinger's 3 experiments, moreover, contradict the conclusions of 
 Schiff; and Mendelssohn 4 found that the reaction-time of the an- 
 terior half, and especially of the anterior columns of the cord, was 
 uniformly less than the reaction-time of its posterior columns. The 
 latter also found that weaker stimuli would suffice to excite motion 
 when applied to the anterior columns. But, according to Schiff 5 
 again, the cord contains no motor elements that are directly exci- 
 table except the central paths of the nerve-roots. He also agrees 
 with van Deen in denying that the gray matter of the cord can be 
 made, by direct stimulation, to originate either motor or sensory 
 impulses. It affords paths, however, for the transmission of both 
 these kinds of impulse when once started by the other nervous ele- 
 ments. Schiff accordingly speaks of the posterior gray columns, 
 and of those parts of the posterior white columns which are not 
 direct prolongations of the nerve-roots, as " cesthesodic." The corre- 
 sponding parts of the anterior cord he calls "kinesodic." The sen- 
 sitiveness of the posterior columns which others discover on experi- 
 ment he regards as only indirect. Vulpian, 6 on the contrary, agrees 
 with BeU, Magendie, Flourens, and Longet, in holding that, while 
 the gray matter is absolutely inexcitable and the posterior columns 
 very excitable, the anterior columns possess only a moderate degree 
 of excitability. 
 
 1 See, especially, articles in Pfliiger's Archiv, xxviii., pp. 537-555, and xxix., 
 pp. 537-555. 
 
 2 Du Bois-Reymond's Archiv, 1867, p. 198 ; and Pfliiger's Archiv, ii., p. 414. 
 "Pfliiger's Archiv, xxii., pp. 169-176. 
 
 4 Archiv f. Anat. u. Physiol., 1883, Physiolog. Abth., pp. 282 ff. 
 
 6 Pfliiger's Archiv, xxix. , p. 598. 
 
 6 Le9ons sur la Physiologic du Systeme nerveux, p. 362. 
 
INFLUENCE OF THE BRAIN ON THE CORD. 143 
 
 By an ingenious arrangement for applying the mechanical stimu- 
 lus of pricks from an extremely fine needle-point to definitely cir- 
 cumscribed spots in the spinal cord of the frog, E. A. Birge ' seems 
 to have demonstrated the susceptibility of the ganglion-cells to di- 
 rect stimulus. Pricking these cells produces movements in defi- 
 nitely located groups of muscles ; and the te tanusis invariably 
 confined to the muscles of the same side as that of the cells stimu- 
 lated, unless (as microscopic examination shows) the effect of the 
 needle has reached certain cells on the other side. Birge also 
 found that different regions of a single cross-section of the cord are 
 excitable in different degrees ; the region from the posterior fissure 
 to the median line of the gray matter being most inactive, and that 
 of the large ganglion-cells in the anterior horn sinvariably being 
 able to produce tetanus. 
 
 In view of such conflict of testimony it can only be said that 
 certain longitudinal parts of the spinal cord are plainly susceptible 
 to direct stimulation, but at present it is difficult to decide which 
 parts, exclusively, are sensitive. 
 
 13. Thus far the spinal cord has been considered as a series of 
 related centres, that act automatically or reflexly when separated 
 from the brain. But in its normal condition the cord always acts, 
 of course, under the influence of the brain. The brain thus exer- 
 cises a profound modifying influence over the automatic and reflex 
 activities of the inferior organ. The cord alone can be depended 
 upon, as it were, to respond with great regularity, in the form of 
 definite reflex movements, to a given amount of stimulus, when 
 applied at a given locality. But the action of the brain, when at- 
 tached to the cord, interferes with this regularity, so that the ex- 
 pected muscular movements may not result when the stimulus is 
 applied. They are then said to be inhibited by the action of the 
 brain. The phenomena of "inhibition," when connected with vo- 
 lition, are familiar enough ; for example, one may voluntarily re- 
 strain those movements of one's legs which the cord, if left to it- 
 self, would produce as the result of tickling the soles of the feet. 
 
 But the brain without conscious volition exercises the same in- 
 hibitory action over the spinal cord. If a frog is suspended by the 
 head, and its legs allowed to dip into a vessel of dilute acid, the in- 
 terval between the contact of the acid and the withdrawal of the 
 legs is considerably lengthened when the spinal cord remains un- 
 divided below the medulla oblongata ; that is to say, the cord alone 
 withdraws the legs quicker than the cord when influenced, or in- 
 hibited, by the brain. The interval between the application of the 
 1 Archiv f. Anat. u. Physiol., 1882, Physiolog. Abth., pp. 481-489. 
 
144 THE BRAIN AS A CENTRAL ORGAN. 
 
 acid and the contraction of the muscles can also be prolonged, when 
 the brain is still connected with the cord, by applying chemical 
 irritation at the same time to the optic lobes ; that is to say, the 
 cord is hindered from performing its reflex-motor function by the 
 stimulation, and consequent influence upon itself, of the higher 
 nervous centre. Moreover, if at the time that one leg of a brain- 
 less frog is dipped into the acid, the sciatic nerve of the other is 
 strongly stimulated with an interrupted current, the same prolon- 
 gation of the period of incubation will be observed ; in some cases, 
 indeed, the reflex act will not take place at all. In discussing the 
 reciprocal relations of the higher centres of the brain, we shall dis- 
 cover many phenomena similar to the foregoing. All these centres 
 may exercise this so-called " inhibitory " action upon other centres, 
 according to their several physiological connections. The phenom- 
 ena of inhibition are not, therefore, confined to the influence of the 
 brain on the spinal cord. 
 
 Elaborate attempts have been made to point out a special mech- 
 anism of inhibition. Thus Setschenow * has advocated the view 
 that localized inhibitory centres exist in the brain, and that the de- 
 pressing effect travels by certain definite tracts in the spinal cord. 
 But on this subject our doubts are entitled to go even beyond the 
 remark of Fender : 3 " The nature of the inhibitory mechanism is 
 exceedingly obscure." We cannot be said to have sufficient grounds 
 for assuming the existence of any such specific mechanism. In 
 general, nerve-commotions modify each other within the central 
 organs ; they either facilitate and increase, or inhibit and diminish, 
 each other's effect, according to the structure and functions of the 
 organs, the amount and kind of stimulus thrown in upon them from 
 without, and the exact condition in which this stimulus finds them. 
 The inhibition of the cord by the brain is, then, only a special case 
 under the general molecular theory of the nervous mechanism. 
 The factors entering into every such case will very likely always 
 prove too varied and complex to be analyzed with complete success. 
 
 14. On passing from the spinal cord into the brain, the diffi- 
 culty of defining the specific functions whether automatic or re- 
 flex of the different central organs becomes greatly increased. 
 The phenomena are vastly more complicated, and the methods of 
 analyzing them experimentally much less readily applied. The 
 
 1 Ueber d. Hemmungsmechanismen 1 d. Reflexthatigkeit im Gehirn d. 
 Frosches, Berlin, 1863 ; and other papers. 
 
 8 Functions of the Brain, London, 1876, p. 18, where he refers to the 
 elaborate paper on Inhibition in the West Riding Reports, vol. iv., by Dr. L. 
 Brunton. 
 
THE METHODS OF RESEARCH. 145 
 
 most complex portions of the nervous substance, in respect both 
 to structure and to function, are most completely withdrawn from 
 the use of strictly scientific methods of research. What is known, 
 however, of the anatomical structure and connections of the dif- 
 ferent organs of the brain, and of the paths along which the ner- 
 vous impulses are propagated between them, prepares the way for 
 the more specific physiology of each organ. The methods of such 
 physiological research are in general these two : Observation of the 
 results which follow the application of stimulus to each of the en- 
 cephalic organs, or to any definite locality in each ; and observation 
 of the results which follow the total extirpation or lesion of these 
 organs, or of any portion of each. Of course, both of these 
 methods are almost wholly applicable only to the lower animals. 
 In using the method of stimulation, the stimulus cannot be ap- 
 plied to the nervous substance of the brain without a certain 
 amount of injury to that substance. To stimulate any of the cranial 
 organs with precision they must be exposed ; those that lie deepest 
 cannot be exposed without injury to other organs and the death of 
 the animal. Moreover, it is difficult precisely to circumscribe the 
 application of the stimulus. Just that form of stimulus which is 
 most convenient, effective, and fruitful in results namely, the 
 electrical current is liable to diffuse its direct effects beyond the 
 region which it is desired to circumscribe. When no result follows 
 the application of the current to a definite locality of the nervous 
 substance, the failure may be due to the weakness of the stimulus, 
 or to the fact that this particular centre is at the moment inhib- 
 ited by its condition or by the activity of some connected centre. 
 When a result does follow, it may be that this particular result is 
 due to the direct or indirect stimulation of some other so-called 
 centre, or to the stimulus hitting, by diffusion or otherwise, some 
 of the contiguous sensory or motor nerve-tracts. 
 
 Objection may also be raised against the nature of the argument 
 by which an inference is drawn from the facts gained by the sec- 
 ond of the above-mentioned methods. Such argument not only as- 
 sumes that the activities which remain, when some of the organs 
 of the brain are partially or wholly destroyed, belong to those 
 organs that remain, but also that those activities which have dis- 
 appeared belong to the organs that have disappeared. Both of 
 these assumptions are, however, doubtful, when we come to apply 
 them to the organs in their normal condition and connections 
 under the action of natural stimuli ; the latter of the two is partic- 
 ularly doubtful. In a word, the different mechanisms of the human 
 brain, in their normal condition and relations, constitute an in- 
 10 
 
146 THE BRAIN AS A CENTKAL OKGAN. 
 
 ter-dependent and intimately related system ; what each so-called 
 organ or centre does, or can do, depends not only upon its own 
 structure and condition at the time, but also upon the condition 
 and behavior of the other organs and centres at the same time. 
 Such interdependence extends not only to those divisions which 
 gross anatomy can mark off and consider under the name "the 
 organs of the brain," nor simply to those minuter subdivisions 
 w r hich histology can distinguish by aid of the microscope ; it doubt- 
 less also extends to the last details of that molecular mechanism 
 which the brain-substance is. These details are different for 
 every individual animal, and for every individual case. Specific 
 differences belonging to the different species of animal life, as well 
 as those idiosyncrasies with which pathology is familiar, must alike 
 be recognized. It is by no means strange, then, that the physi- 
 ology of the brain is able only very slowly and imperfectly to win 
 from nature the truth, and to remove the reproach of apparently 
 conflicting facts. 
 
 In spite of the above-mentioned difficulties certain results may 
 be claimed as resting upon more or less of clear evidence regard- 
 ing the specific automatic and reflex-motor functions of those inter- 
 cranial organs that lie inferior to the cerebral hemispheres. The 
 case of these hemispheres themselves will be subsequently consid- 
 ered in detail. For they are those portions of the nervous mechan- 
 ism about the immediate correlation of which with the phenomena 
 of consciousness there can be no doubt. Since we are now con- 
 sidering the nervous system and its central organs merely as a 
 physical mechanism, we definitely rule out, as far as possible, all 
 allusion to any special relation between it and the phenomena of 
 self-conscious mind. 
 
 15. Besides the spinal cord, the Medulla Oblongata is the cen- 
 tral organ concerning whose automatic and reflex-motor functions 
 the largest amount of precise information exists. The reflex-motor 
 functions of this organ are more intricate and of a higher order 
 than those belonging primarily to the cord. They are especially 
 such as stand related to the vital functions of the heart and blood- 
 vessels ; to respiration and its allied movements of the organs in 
 coughing and sneezing, etc. ; to- the movements of the muscles in 
 swallowing and vomiting ; to the mimetic movements of laughing, 
 weeping, etc. Among the different movements in the execution of 
 which the medulla oblorigata is concerned, some are more purely 
 reflex and some less so. Thus one cannot swallow if the sen- 
 sory tracts from the throat to this central organ are broken ; but 
 the movements of the heart and lungs continue after the reflex- 
 
THE MEDULLA AS AUTOMATIC. 147 
 
 motor paths to them are destroyed. Sensory stimulations of the 
 medulla oblongata, as a rule, occasion reflex movements by second- 
 ary stimulation of a number of motor tracts. Swallowing, sneezing, 
 coughing, shedding of tears, changes in respiration and in the 
 movements of the heart, contortions of the countenance, may all 
 be occasioned, through the mediation of this organ, by one and the 
 same sensory impulse. There is also a marked difference in the 
 extent of the domain over which the motor results of stimulating 
 the different sensory paths connected with the medulla spread 
 themselves. Stimulation of the optic nerve occasions only very 
 limited reflex movements, such as the winking of the eyes, the se- 
 cretion of a few tears, and a slight tendency to sneeze. Stimula- 
 tion of the nerves of taste extends over a wider area of motor 
 tracts ; that of the palate and larynx still wider. 
 
 16. The most important reflex centres of the medulla oblon- 
 gata are also automatic ; of such centres he chief are those con- 
 nected with breathing, the movements of the heart, and the inner- 
 vation of the blood-vessels. The excitation in these cases must be 
 considered as a neural process arising within the central organ 
 itself. The cause of its origin is doubtless to be found in the 
 changes that occur in the supply and character of the blood. Not 
 only all abnormal conditions of respiration, like dyspnoea and 
 apncea, but also the rhythm of normal respiration, are dependent 
 upon the changing condition of the blood with respect to its more 
 or less perfect oxidation. The stimulus to action of the respiratory 
 centre in the medulla, from the condition of the blood, may be in 
 part reflexly applied through the peripheral ends of the afferent 
 nerves in various parts of the body ; but the main effect is doubt- 
 less produced by the direct action of the blood on this centre. Its 
 rhythmic nervous action may then very well be dependent upon the 
 rhythmic action of the lungs, and upon the resulting periodic re- 
 oxidation of the blood. For the nervous substance of the medulla 
 oblongata seems to be peculiarly susceptible to the condition of 
 the blood. 
 
 17. This small central organ into which the spinal cord ex- 
 pands on entering the skull may then be said to be thickly 
 crowded with reflex and automatic centres. To speak of the more 
 important will best serve to exhibit what is known of its mech- 
 anism. 
 
 The respiratory centre was first located by Flourens in that part 
 of the medulla oblongata which serves as the place of origin for 
 the vagus nerve, and then more definitely in the V-shaped apex 
 of the fourth ventricle, or beak of the calamus scriptorius. Since 
 
148 THE BRAIN AS A CENTRAL ORGAN. 
 
 extirpation or injury of this small portion of the nervous sub- 
 stance, when all other parts of the body are left intact, causes 
 immediate and final cessation of respiration, Flourens called it the 
 "vital knot" (noeud vital). Foster 1 locates this centre below the 
 vaso-motor centre, and between it and the calamus scriptonus. 
 Schiff concludes that it is double, and lies on either side in the 
 region of the anterior part of the ala cinerea ; the function of each 
 side, he thinks, is separate. In case of need it may be shifted 
 slightly backward toward the spinal cord. The efforts of Gierke 3 
 to fix it in a definite group of ganglion-cells were not successful. 
 With this same centre all the modifications of respiration in sigh- 
 ing, sobbing, yawning, crying, laughing, coughing, sneezing, and 
 hiccoughing are connected. 
 
 A nervous centre intimately connected with the vaso-motor sys- 
 tem of the different parts of the body exists in the middle part of 
 the medulla oblongata. Since we cannot examine experimentally 
 the effect upon the action of this centre which would be produced 
 by severing all the afferent nerves that lead into it, we cannot 
 demonstrate directly how much of its action is automatic, how 
 much reflex. It is probably both automatic and reflex. But the 
 removal of the parts in front of the medulla, inclusive of the cor- 
 pora quadrigemina, exercises no perceptible influence on the blood- 
 pressure. The principal vaso-motor centres in the brain are then 
 found in this portion of the medulla oblongata. Through it reflex 
 motions are called forth of the most different kinds, and involving 
 muscles widely separated from each other and from the region of the 
 skin where the stimulus is applied. Witness the effect of a draught 
 of air upon the circulation of the blood. The arteries of a rabbit's 
 ear can be made to contract by stimulating any one of more than a 
 half-dozen different sensory nerves, including the sciatic plexus. In 
 this same central organ must be located the so-called cardio-inhib- 
 itory centre. In cases where the heart is stopped by sudden and 
 great emotion, or by severe pain, the stimulus probably reaches the 
 medulla from the hemispheres of the brain. 
 
 The centre of deglutition lies in the medulla higher up than that 
 of respiration. If this part of the organ be destroyed, swallowing 
 is impossible. This centre has been located in the floor of the 
 fourth ventricle. In the floor of the same ventricle, and in the 
 adjoining region, are probably located centres for different secre- 
 tions as, for example, of spittle, or sweat, of tears, and possibly 
 of the pancreatic and other digestive juices. The connection of 
 
 1 Text-Book of Physiology, p. 370. 
 
 2 See Pfluger's Archiv, vii., pp. 583 ff. 
 
INFLUENCE OF MEDULLA ON THE LIMBS. 149 
 
 various sensations and emotions with these secretions is too famil- 
 iar to need description. A central mechanism for winking the 
 eyes Exner would place near the beak of the calamus scriptorium. 
 The central mechanism for the reflex movement of the muscles of 
 the oesophagus and stomach also lies in the medulla oblongata. Of 
 the centre for the production of artificial diabetes, and of other 
 more conjectural centres which are packed within this small bit 
 of nervous matter, scarcely more than an inch in length, we do not 
 need to speak. 
 
 18. The alleged functions of the medulla oblongata in the co- 
 ordination of the movements of the skeletal muscles ally this organ 
 more closely with certain other inferior parts of the brain. The 
 preparation of a frog which has retained this organ, in addition to 
 the spinal cord, although without any of the rest of the brain, will 
 execute movements of the muscles that are not possible for the cord 
 alone. It will not, indeed, move spontaneously ; it still requires 
 external stimulation to start the mechanism of such a preparation. 
 Under such stimulation, however, it will assume a position natural 
 to it in an uninjured state. When laid on its back it will make 
 efforts generally unsuccessful to turn over. The movements of 
 the limbs with which it responds to vaiious sensory impulses are 
 more complicated than those executed by the spinal cord alone ; 
 they even resemble crawling motions or short leaps. Placed in the 
 water, what is left of the animal will swim ; and if its motions are 
 less perfect than those of the perfect frog, they are much more so 
 than those of the cord alone. It is doubtful whether, when placed 
 beneath the water, it will ascend to the surface to breathe, or 
 make efforts to escape from water gradually heated to about 104 
 Fahr., as will the animal that retains its cerebellum and optic 
 lobes. 
 
 Reflex movements of considerable complexity can also be exe- 
 cuted by mammals that have been deprived of all the encephalic 
 centres above the medulla. Vulpian claims that a young rat in 
 this condition will emit a cry, as of pain, when its toes are pinched. 
 Such s a mechanism will swallow and execute certain co-ordinated 
 movements of the limbs. Infants whose nervous centres above the 
 medulla are undeveloped will perform the associated movements 
 of sucking when put to the breast. Moreover, the effects of le- 
 sion of the centres of the medulla are very marked in respect to the 
 co-ordination of motion. Rolando observed that convulsive move- 
 ments followed extensive injury of this central organ. More recent 
 researches seem to show that the seat of these epileptiform move- 
 ments is at the place of union between the medulla and the pons ; 
 
150 THE BRAIN AS A CENTRAL ORGAN. 
 
 it can, therefore, scarcely be located in either alone. 1 One sided 
 lesions are followed by certain so-called " forced " and rotary move- 
 ments of the head, and eyes, and trunk. Such effects are most 
 likely to be produced when the injury affects the region of the 
 tuberculum acuslicum. In the opinion of Bechterew 2 the olivary 
 bodies are in relation with the gray matter of the third ventricle, 
 and with the semicircular canals, as central organs for the co-or- 
 dination of the muscles used in balancing according to impressions 
 of touch. It would then be one chief function of the medulla to 
 secure equipoise through these sensory impressions. On the 
 whole, it appears certain that considerable work in co-ordinating 
 the muscular movements falls upon its mechanisms. Of such work 
 it is probable that the movements concerned in articulate speech 
 are a part. Any indirect relation which it may have to the produc- 
 tion of those sensations and images which are woven into our 
 dreams does not belong in this connection. 
 
 19. The associations among the different centres of the me- 
 dulla oblongata are curious enough ; they involve an extremely 
 intricate physiological apparatus. Some of these centres are in- 
 directly connected with psychical activities. They are not all alike 
 excitable ; they are not all voluntarily so. Thus we can volun- 
 tarily control, within certain limits, the movements of the lungs, 
 but not those of the heart and blood-vessels ; we can cough, but 
 cannot sneeze, at will. Some of their functions are associated 
 together regularly ; some of them seldom ; some never. Swallow- 
 ing is not necessarily connected with the activity of the other cen- 
 tres, unless it be with that for the secretion of saliva ; it takes 
 place, however, during arrest of respiration. The excitation of no 
 other centre necessarily affects this centre. The secretion of saliva 
 is constantly connected with a change in the circulation through 
 the submaxillary glands. 
 
 20. An animal which possesses all, or a considerable part of 
 the other nervous mechanisms of the brain that lie below the cere- 
 bral hemispheres is capable of executing movements which differ 
 greatly from those already described as belonging to the spinal 
 cord and medulla oblongata. Very few of the movements of such 
 a preparation are, indeed, even apparently spontaneous; for al- 
 most all of them a definite form and degree of stimulus acting on 
 the sensory surfaces can be assigned. We are inclined, then, to sus- 
 pect that those movements which are apparently spontaneous are 
 really due to some stimulation from without the central organs 
 
 'See Eckhard, in Hermann, Handb. d. Physiol., II., ii., p. 98. 
 2 Pniiger's Archiv, xxxi., pp. 479 if., and xxix., p. 258 f. 
 
INFLUENCE OF THE CEREBRAL LOBES. 151 
 
 which has escaped our observation. But the range of reflex-motor 
 activities which an animal deprived simply of its cerebral hemi- 
 spheres will execute, in response to appropriate stimuli, is very 
 great ; it may be said to include every form of movement possible 
 for the uninjured animal. The statement is, therefore, warranted 
 by all our knowledge of the facts, that the medulla, pons, crura 
 cerebri, cerebellum, corpora quadrigemina (or optic lobes), and 
 basal ganglia generally are the special mechanism for co-ordi- 
 nating the movements of the muscles with the various impulses of 
 sense. 
 
 A frog from which the cerebral lobes have been removed will 
 respond to appropriate stimuli with all the movements of which a 
 perfect frog is capable. It will swim, leap, and crawl. When 
 placed on its back, it will easily and at once regain its natural posi- 
 tion. When placed on a tilting board, it will constantly adjust 
 the position of its body so as to maintain an equilibrium. It will 
 croak with the regularity of a music-box when its flanks are gently 
 stroked. Thrown into the water it will swim with great regularity 
 of motion until it is exhausted or finds something as a small piece 
 of wood placed in contact with it upon which it can crawl. When 
 submerged in the water, it will rise to the surface for air ; it will 
 not, like a mere spinal cord, remain quietly in water the temper- 
 ature of which is gradually raised, but will make violent efforts to 
 escape. It is guided by the light, for it avoids objects that cast a 
 strong shadow. On the other hand, it appears stupid ; it pays no 
 attention to the flies that are placed near it ; by careful exclusion 
 of all stimuli it may be kept motionless for hours. We cannot 
 argue from this, however, that it is without sensations, for it may 
 not be hungry ; and Heubel 1 asserts that a sound frog may, with 
 careful manipulation, be made to lie still upon its back for a long 
 time. 
 
 Similar phenomena occur in the case of the mammal whose cere- 
 bral hemispheres have been removed. The rabbit or rat thus 
 operated upon will stand and run and leap. Placed on its back, 
 it will regain its feet. It will follow with its head a bright light 
 held in front of it ; it will start and tremble, or run, at a shrill or 
 loud noise. It will utter a prolonged cry when pinched. Its mus- 
 cular motions are obviously co-ordinated in response to sensory 
 impulses from the organs of touch, hearing, and sight. The bird 
 thus operated upon will easily regain its feet when laid upon its 
 side or back, and will stand in a natural and easy posture. It will 
 tuck its head under its wings, clean its feathers, and pick up corn 
 1 Pfliiger's ArcMv, xiv. , pp. 162 ff. 
 
152 THE BRAIN AS A CENTRAL ORGAN. 
 
 or drink water presented to its beak. Thrown into the air, it will 
 fly with considerable precision for some distance, and in its flight 
 will guide itself, though imperfectly, so as to avoid obstacles in its 
 way. It will start at sharp sounds or flashes of light. Such ani- 
 mals have on the whole the appearance of being sleepy and stupid 
 rather than of being deprived of any of their powers for co-ordi- 
 nating sensation and motion. We conclude, then, that the organs 
 which such animals possess are functionally capable of exercising 
 all these powers of co-ordination ; we do not at present raise the 
 question whether this implies the existence of psychical phenomena 
 or not. The phenomena which follow the partial loss of the cere- 
 bral hemispheres in the higher mammals confirm the same conclu- 
 sion. 
 
 It is much more difficult, however, to assign the special place 
 which belongs to each of the organs that lie between the medulla 
 oblongata and the cerebral hemispheres, under their general func- 
 tion as already stated. They are all very intimately related ; act 
 to a large extent dependently ; can, within certain limits, assume 
 each other's functions ; and have largely the same connections with 
 the peripheral organs of sense and of motion, and the same work 
 to do as mediating between the two. 
 
 21. It is impossible to determine the special functions of the 
 Cerebellum, so conflicting is the testimony of different experiment- 
 ers. A high degree of probability, however, attaches itself to the 
 statement that this organ is largely concerned in the co-ordination 
 of motion ; although such statement cannot be held to exhaust its 
 functions. The more specific theory of Wundt 1 "It is the central 
 organ that brings such movements of the animal's body as are ex- 
 cited by impulses from the cerebrum, into accord with its situation 
 as a whole in space" is more doubtful, precisely because it is 
 more specific. Comparative anatomy seems to show that the office 
 of the cerebellum in some animals differs from its office in man ; 
 reasoning from the former to the latter is, therefore, especially pre- 
 carious. Moreover, its functions are so closely connected anatom- 
 ically with those of the pons, the crura cerebri, and the medulla, 
 that it is difficult precisely to separate its work from that done by 
 these organs. 
 
 Testimony as to the result of the extirpation or lesion of the cere- 
 bellum is very conflicting. Apparently almost the entire length 
 and breadth of its surface (in the direction of the posterior bones 
 of the skull), and not only the gray matter, but also the white, as 
 far as near the bifurcation of its strands, may be removed without 
 Grundzuge d. physiologisclie Psychologie, L, p. 201. 
 
LESIONS OF THE CEREBELLUM. 153 
 
 any observable result. J On approaching the middle of its thick- 
 ness and removing the strands connected with the middle peduncles, 
 disturbances of motion begin and increase rapidly in proportion to 
 the amount of substance removed. Most of these disturbances, if 
 the animal recovers well, prove to be only temporary; they are, 
 therefore, probably due largely to traumatic excitation. Permanent 
 disturbances, however, occur when the injuries reach the lower 
 third of the organ, or when they are confined to this third. Vulpian 
 accordingly concludes that the disturbance of gait which results 
 from injury of the cerebellum, is due to the irritation of its more 
 profound white parts or of the adjoining cerebral isthmus. But 
 Schiff believes that the mass of the organ, apart from locality, has 
 a definite influence upon the co-ordination of the bodily movements ; 
 though what that influence is cannot yet be clearly defined. The 
 influence of locality seems to be considerable upon the effect which 
 results from lesions in a given amount of the cerebellar substance ; 
 but since this influence is much more marked near the connections 
 of the cerebellum with other contiguous organs, some observers 
 attribute it largely or wholly to the injury by extension of the 
 lesion, by pressure, or by inflammation of these organs. Thus 
 the place of its union with the medulla oblongata and the regions 
 near the crura cerebri are especially important. But Schiff found, 
 in experimenting upon mammals, that complete vertical section of 
 the cerebellum, in the exact median line of the vermiform process, 
 and removal with the knife or pincers of the entire substance, with 
 the exception of the flocculi and the parts external to the peduncles, 
 produced no appreciable loss of the power of co-ordination. 
 
 The effect of one-sided lesions of the cerebellum in the disturb- 
 ance of motion seems to be, as a rule, much more certain and 
 marked than that of symmetrical lesions of both sides. Schiff, in- 
 deed, asserts that when a bilateral lesion is perfectly symmetrical 
 it produces no impairment whatever of the functions of motion. 
 But the entire evidence from experiment shows that sudden lesion 
 of one hemisphere of this organ is almost uniformly followed by 
 at least temporary impairment of the motor functions. Section of 
 the middle peduncle of the cerebellum of a bird or mammal almost 
 always occasions so-called " forced " movements ; the animal rolls 
 around its own longitudinal axis, generally, though not invariably, 
 toward the injured side. Nystagmus, or the peculiar rolling move- 
 ment of the eyes suggestive of vertigo, and strabismus, take place 
 
 1 Compare Vulpian, Legons sur la Physiologie, etc., pp. 603 ff.; Eckhard, in 
 Hermann, Handb. d. Physiol. IL,ii.,pp. 102 ff.; Schiff, in Pfliiger's Archiv., 
 xxxii., pp. 427 ff.; and Ferrier, Functions of the Brain, pp. 85-123. 
 
154 THE BRAIX AS A CENTRAL ORGAN. 
 
 in such cases. One eye may be moved inward and downward, the 
 other outward and upward. Hitzig 1 and Ferrier a found the same 
 results to follow injury of the lateral lobe. The latter observed that 
 strong stimulation of the cerebellar surface with the interrupted 
 current causes associated movements of the eyes and head and 
 limbs, in cats and dogs and monkeys. But these effects may 
 be largely due to the connection of the cerebellum with the me- 
 dulla oblongata. 
 
 The evidence from pathological cases in man conflicts, to a con- 
 siderable extent, with the conclusions which we might hasten to 
 derive from experiment upon the animals. According to Vulpian 3 
 it is by no means rare to have unilateral lesions of the cerebellum 
 followed by no paralysis of either side. In a great number of such 
 cases no genuine hemiplegia results ; the resulting enfeeblement of 
 motion, moreover, is as often on the same as on the opposite side. 
 M. Andral is said to have made a collection of ninety-three cases 
 of diseases of the cerebellum, in only one of which ataxy was ob- 
 served in any marked way. In most cases w r here crossed hemi- 
 plegia does result, Vulpian thinks it due to the destruction or 
 compression of the adjacent parts, especially the roots of the cere- 
 bellar peduncles. The same authority denies that the superficial 
 parts of this organ are excitable, or that lesion of them is followed 
 by pain or by convulsions of the body, face, or eyes. Such results 
 do, however, follow excitation and lesion of its deeper parts, in 
 proportion to the degree of approach to the peduncles. The dis- 
 crepancy between experiment and pathology may perhaps be re- 
 moved, at least in part, by remembering that the injury is sudden 
 in the one case and not in the other. Moreover, few of the patho- 
 logical cases are clearly enough defined to serve as a sure basis for 
 conclusions. Some of them, however, would seem to warrant cer- 
 tain inferences. More than fifty years since, the well-known case of 
 the girl Alexandrine Labrosse was reported by Combette, 4 and after- 
 ward made known to students of physiology generally by Longet. 5 
 This girl was found, on post mortem, to have no cerebellum ; in its 
 stead was a gelatinous membrane attached to the medulla by two 
 peduncles of like construction. A true pons was also wanting, but 
 no loss of substance seemed to have taken place here. Yet she 
 could co-ordinate all the limbs voluntarily, and had the full use of 
 
 1 Untersuchungen iiber d. Gehirn, pp. 198 ff. 
 
 2 Functions of the Brain, p. 106 f. 
 
 3 Legons sur la Physiologic, etc. , p. 607 f. 
 
 4 Revue medicale, II., p. 57 (1831). 
 
 6 Anatomie et Physiologie du Systeme nerveux, I., p. 764 (1842). 
 
LESIONS OF THE CEREBELLUM. 155 
 
 all the senses. She was, however, subject to falling (se laissait 
 tomber sou vent) and spoke imperfectly. Bouillaud has reported 
 another case of an adult whose entire cerebellum was changed into 
 a brown purulent mass ; this patient could walk, though in a tot- 
 tering and insecure way. Vulpian l also describes a case which 
 came under his own observation. A woman, dying at the age of 
 sixty-nine, after twenty years in the hospital of La Salpetriere, was 
 found to have suffered an entire atrophy of all the cortical gray 
 substance of the cerebellum. This patient preserved great muscu- 
 lar vigor, and could co-orclinate all the muscles ; but her "locomo- 
 tion " was disordered and difficult. 
 
 On the whole, then, it must be admitted that the evidence con- 
 cerning the specific functions of the cerebellum of mammals, and 
 especially of man, is not such as to warrant us in making definite 
 affirmations. Scarcely a single case can be adduced in which it 
 is not possible to maintain that the motor disturbances which fol- 
 lowed lesion or excitation of this organ should be ascribed to an 
 indirect effect upon contiguous organs. Yet the coincidence of 
 evidence from several different lines gives sufficient support to the 
 view that the functions of the cerebellum are in some way con- 
 nected with the balancing, and therefore with the precise and se- 
 cure locomotion of the body in space. More definitely, with refer- 
 ence to the nature of this connection, it is not possible to speak 
 confidently. No disturbance of the senses of hearing, of sight, or 
 of muscular feeling, can be shown to follow injuries of this organ 
 where other parts of the brain are not involved ; on the contrary, 
 all these senses appear to have been perfect in certain cases of the 
 complete absence of this organ. The only disturbance of sensi- 
 bility which frequently follows affections of the cerebellum is ver- 
 tigo ; the same symptom can be produced by passing a current of 
 electricity through the back part of the head, or by the effusions 
 of blood in this region which are sometimes occasioned by alcohol. 
 Vulpian and others are, however, probably right in holding that 
 the result is only indirectly to be ascribed to this organ. Indeed 
 the view of Schiff has much in its favor : this view maintains that 
 the aberration of motion due to lesion of the cerebellum should 
 not be called a loss of co-ordination at all, since all the limbs may 
 be moved in exactly the right relations necessary to carry the body 
 forward or to maintain its equipoise ; but the precision of the mo- 
 tion is impaired, because the nervous impulses from this organ 
 that innervate the neighboring groups of muscles are not rightly 
 adjusted to each other in amount along the different tracts. The 
 1 Le9ons sur la Physiologie, etc., p. 629. 
 
156 THE BRAIN AS A CENTRAL ORGAN. 
 
 balance of the innervating cells is destroyed ; and the result is a 
 loss of nice adjustment of the amount of innervation sent to the 
 particular muscles employed in equipoise and locomotion. 
 
 It scarcely need be added that modern physiology distinctly dis- 
 proves the hypothesis of Gall, who connected the sexual instinct 
 with the cerebellum. There is no good evidence that the hinder 
 brain directly participates in any way in those activities of the 
 nervous system which are immediately correlated with psychical 
 phenomena, whether of emotion, instinct, or intelligence. 
 
 22. The functions of only three other parts of the encephalon 
 require consideration in this connection ; these are the corpora 
 quadrigemina, the optic thalami, and the corpora striata. The 
 crura cerebri and the pons Varolii are, as we have already seen, 
 significant chiefly as organs of conduction. So far as they have 
 also the intermediating functions of central organs, it is not possi- 
 ble to treat of them otherwise than as concerned in that general 
 reflex-motor mechanism which occupies all this region of the brain. 
 
 23. Experiments upon the Corpora Quadrigemina are rendered 
 especially difficult by the small size and deep situation of these 
 organs ; they cannot easily be exposed for stimulation without 
 great effusion of blood, or subjected to lesion without extending the 
 injury to contiguous parts. These difficulties render conclusions 
 from the effect of stimulating or extirpating the corresponding 
 organs (optic lobes) of the frog more than usually precarious. 
 There is no doubt, however, as to some special connection between 
 the corpora quadrigemina and sensory impulses of sight ; such con- 
 nection is, then, of course, to be extended to those motor activities 
 that are dependent upon the sensory impulses of sight. Flourens 
 and many subsequent observers have found that one-sided extirpa- 
 tion of the optic lobes of birds, or of the corpora quadrigemina of 
 mammals, with the cerebral hemispheres intact, produces blindness 
 in the opposite eye. The amount of this blindness is different in 
 different animals, as the decussation of the fibres in the optic chi- 
 asm is more or less complete in different animals. In the rabbit 
 such decussation appears to be complete ; in the cat and dog in- 
 complete. The fact that hemianopsia in both eyes is connected 
 with disease of one side of the brain is an evidence that it is incom- 
 plete in man also. Moreover, when the brain is removed in front 
 of the corpora quadrigemina, and these organs left intact, the ani- 
 mal can still guide and co-ordinate its motions in response to visual 
 impulses. (We do not in this place consider whether we are war- 
 ranted in calling these impulses " sensations " not to say "percep- 
 tions" of sight.) These organs are, then, in some sort, central 
 
LESIONS OF THE CORPORA QUADRIGEMINA. 157 
 
 organs of sight. Since they are connected by nerve-tracts with the 
 cortex of the cerebrum, motor innervation in response to stimulus 
 from the optic nerve may arise either immediately in the corpora 
 quadrigemina themselves or in the gray matter of the cortex. We 
 may therefore suppose, with Wundt, J that destruction of the cere- 
 bral substance abolishes only those movements of the muscles, in 
 response to the stimulus of light, which involve complicated co-or- 
 dinations with other excitations of sense, or with earlier established 
 experience. It is scarcely allowable, however, to locate this special 
 relation to visual impulses definitely in the substance of the corpora 
 quadrigemina considered as isolated from the optic thalami, the 
 optic tracts, and the gray matter at the floor of the third ventricle. 
 There is sound sense in Eckhard's 2 remark that the functions 
 commonly attributed to these bodies should rather be ascribed to 
 the region in which they lie. The nates (or anterior pair) seem to 
 be more especially connected with the sensory, and the testes (or 
 posterior pair) with the motor activities of sight. 
 
 Abnormal movements of a "forced " nature, and impairment of 
 the power of co-ordination, follow the injury or extirpation of the 
 corpora quadrigemina. These phenomena may be due in part to 
 the loss of guidance by visual impressions ; but they are probably 
 due chiefly to the extension of the effects of the injury to the crura 
 cerebri and other surrounding parts. The optic lobes, according 
 to Goltz, are the principal central mechanism for the croaking of 
 the frog deprived of its hemispheres. Vulpian 3 makes a distinction 
 between a merely reflex-motor cry and the plaintive utterance of 
 an animal (e.g., the rabbit) which retains these organs and the pons 
 Varolii. Ferrier, 4 however, was unable to make the distinction so 
 clearly. The latter observer found that very marked phenomena 
 such as dilating the pupils, clenching the jaws, retraction of the 
 ears and angles of the mouth, extending the legs, etc. followed 
 the stimulation of these organs with an electrical current, in the 
 case of cats and dogs. But his experiments do not enable us to 
 say how much of all this belongs to the specific function of the 
 corpora quadrigemina as central organs, and how much to the irri- 
 tation of the nerve-tracts in all the surrounding region. While we 
 seem warranted in connecting these organs with the cerebellum, 
 medulla, and pons, as concerned in the co-ordination of motions 
 necessary for equipoise and locomotion, it is not safe at present to 
 attempt a more precise localization of function. 
 
 1 Physiologische Psyehologie, i. , p. 184. 
 
 2 In Hermann's Handb. d. Physiol., II., ii., p. 131. 
 
 3 Lemons sur la Physiologic, etc. , p. 541 f. 
 
 4 Functions of the Brain, p. 76. 
 
158 THE BRAIN AS A CENTRAL ORGAN. 
 
 24. The office of the so-called basal ganglia Optic Thalami 
 and Corpora Striata in that " projection-system " which connects 
 the cerebral hemispheres with the periphery of the body, has 
 already been spoken of; one chief function of these ganglia has 
 usually been held to be that of acting the part " of middlemen be- 
 tween the cerebral convolutions and the rest of the brain." 1 But 
 they both have further functions as specifically central organs in co- 
 ordinating the movements of the body according to impressions of 
 sense. It is difficult, if not impossible, however, to define precisely 
 what these functions are. Some special relation of the optic 
 thalami to impressions of sight must be admitted. The fact that 
 animals deprived of the cerebral hemispheres are capable of com- 
 plex co-ordination of their muscles as reflex effects of visual im- 
 pressions, seems to indicate that the mechanism of the optic 
 thalami is associated with that of the corpora quadrigemina in 
 performing this function. In mammals complete extirpation of 
 the posterior portion of one thalamus results in permanent ex- 
 pansion of the pupil of the opposite eye ; and Renzi was confident 
 that injury of the upper surface of the anterior portion occasioned 
 blindness. Lussana and Lemoigne found blindness in the opposite 
 eye to be the invariable result of lesion of one thalamus. Cases of 
 the disturbance of vision, or even of complete blindness, have been 
 observed in human patients as the apparent result of disorgani- 
 zation of this organ. It must be admitted, however, that the sig- 
 nificance of the optic thalami for vision may be due simply to the 
 fact that certain fibres of the optic nerve have their origin in it, 
 and are rendered inoperative by injuring it. Experiments and 
 pathological cases connecting the optic thalami with the sensations 
 of smell and taste are more doubtful and conflicting. Ferrier 2 con- 
 cludes that lesions in and around this organ destroy the cutaneous 
 sensation of the opposite side of the body in the monkey ; Veyssiere 
 found the same thing true in dogs. But Nothnagel found that no 
 effect upon sensation followed the destruction of these organs in 
 the rabbit. Not a few cases of disease of the optic thalami in man 
 seem to point to some connection with tactile impressions ; other 
 cases, however, are decidedly unfavorable to this view. On the 
 basis of this rather meagre evidence Wundt 3 is willing to rest the 
 theory that the optic thalami are special centres for the reflex- 
 motions of touch ; by the same theory he also accounts for the dis- 
 turbances of motion which follow injury to these organs. He 
 
 1 See Foster, Text-Book of Physiology, p. 653. 
 
 2 Functions of the Brain, pp. 238 ff. 
 
 3 Physiologische Psychologic, i. , p. 188. 
 
LESIONS OF THE STKIATE BODIES. 159 
 
 thinks it probable, nevertheless, that their function is not exhausted 
 by this description. "Forced " positions and movements, and various 
 other marks of impaired motor activities, follow the experimental 
 lesion of these organs. But such disturbances largely or wholly 
 vanish after a brief time, although they can be again called out by 
 stimulation. They occur, as a rule, only when the lesion affects 
 the posterior part of the thalamus, or the edges of the opening 
 leading from above into the third ventricle. Most of the phenom- 
 ena may be explained as due to the working of a mechanism that 
 has been stimulated to abnormal activity by the mechanical irrita- 
 tion due to the extirpation. 1 We can scarcely, then, be any more ex- 
 plicit than to quote the remark of Vulpian, made some years since : 2 
 "We know nothing of the special functions of the optic thalami." 
 
 25. The special motor significance of the Corpora Striata is 
 undoubted ; although we cannot go to the length of holding that 
 these bodies are concerned only in the elaboration and downward 
 transmission of efferent impulses. Ferrier 3 and others have ob- 
 served that stimulating these bodies with an interrupted current 
 produces strong convulsive movements of the opposite side of the 
 body ; with a very powerful stimulus the whole side is drawn 
 into an arch. No such effect could be produced by stimulating 
 the optic thalami. Ferrier holds 4 that "in man and the monkey 
 there is little, if any, difference perceptible between the complete 
 destruction of the cortical motor-centres and destruction of the 
 corpus striatum." Vivisection of this organ is sometimes followed 
 by hasty forward running motions. Lesions of the striate bodies, in 
 the case of the animals, are usually followed by laming of the limbs 
 of the opposite side ; sometimes, however, no pathological symp- 
 toms result. As a rule, in the case of man, paralysis of the arms 
 and legs of the opposite side follows disease of these organs. Here, 
 as elsewhere in this region of the nervous system, a certain sud- 
 denness of the disturbance appears necessary to secure any marked 
 result. Some experiments seem to point to a difference in the 
 effects of injury to the two main nuclei of the corpora striata. 
 Nothnagel asserted that all mechanical injury to the nucleus len- 
 ticularis of one side results in laming of the opposite side : destruc- 
 tion of this nucleus on both sides brings the animal into nearly 
 the same condition as the removal of the cerebral hemispheres. 
 But voluntary movements persisted after complete destruction of 
 both the nuclei caudal i of the rabbit. 
 
 1 Comp. Eckhard in Hermann, Handb. d. Physiol. , II., ii., p. 125 f. 
 
 2 Lecons sur la Physiologie, etc. , p. 659. 
 
 3 Functions of the Brain, p. 161. 4 Ibid., p. 249. 
 
160 THE BRAIN AS A CENTRAL ORGAN. 
 
 There is much evidence, then, to show that the corpora striata 
 are, as compared with the optic thalami, more especially connected 
 with motor activities. Wundt l considers them to be pre-eminently 
 significant as ganglia for the co-ordination of those motor impulses 
 which are derived from the cerebellum and the cerebrum. The 
 relative importance which they seem to have in the higher, as com- 
 pared with the lower, animals (the monkey and man as compared 
 with the rabbit, etc.) he thinks is like that of all the anterior por- 
 tions of the brain ; such portions are in general, more significant 
 in man than in the other animals. Wundt's view has considerable 
 in its support among other things, the fact that, in case of lesions 
 of the striate bodies, voluntary motions, or those motions whose 
 motor innervation originates above these organs, seem to suffer 
 most. But we positively must not adopt without qualification the 
 statement a that the corpora striata are exclusively motor, and the 
 optic thalami exclusively sensory. In addition to what has already 
 been said (p. 129) to caution one against this view, it may now 
 be added that numerous cases are recorded where injury, appar- 
 ently confined to one corpus striatum, has resulted in loss of feeling 
 on the opposite* side ; and other cases where disease, apparently 
 confined to one optic thalamus, has caused loss of motion as well 
 as of sensation. Moreover, the chief motor effects of injury to the 
 striate bodies (if not all of them) may be due to the fact that the 
 descending motor tracts are necessarily involved in the injury, 
 rather than to any special motor function belonging to these 
 bodies as a central organ. Another theory of the office of the 
 striate bodies rejects entirely the view which regards them as in 
 any true sense basal ganglia, with either specially motor or specially 
 sensory functions ; and regards them as belonging to the cerebral 
 hemispheres, rather than subordinate to the hemispheres in func- 
 tion. 8 But inasmuch as this theory has its principal support, of a 
 physiological kind, from a single case of an idiot's brain, in which 
 these bodies were of nearly normal size, while the cortex was defi- 
 cient in the motor regions and the base of the brain in general 
 small, it can scarcely be regarded as sufficiently confirmed. 
 
 26. The researches of the last few years have tended to show 
 that some special relation exists between the nervous substance of 
 the organs lying at the base of the cerebrum, and the temperature 
 
 1 Physiologische Psychologie, i. , p. 193 f. 
 
 2 As propounded by Carpenter and Todd, and apparently adopted by Fer- 
 rier, Functions of the Brain, 252 f. 
 
 3 See A. Hill, The Plan of the Central Nervous System, p. 276 ; and Jour- 
 nal of Anat. and Physiol. , July, 1885. 
 
GRAY MATTER OF THE THIRD VENTRICLE. 161 
 
 of the body. The earlier observations ' pointed out the limits be- 
 tween the medulla oblongata and the pons as a region, lesion of 
 which was followed by a sudden and large rise of temperature. 
 Still later, other observers ascribed vaso-rnotor functions to the 
 optic thalami, 2 or asserted the existence of vaso-motor fibres in the 
 crura cerebri (so Budge). In 1884, J. Ott pointed out that cutting 
 the corpora striata is speedily followed by a marked rise of tem- 
 perature. Yet more recently two experimenters, 3 working together, 
 have arrived at certain conclusions based upon a large number of 
 experiments, chiefly on rabbits, but also on guinea-pigs and dogs. 
 They discover that, while the cortical substance can be subjected 
 to the most severe and extended lesions without producing a fever- 
 ish rise of temperature, puncturing the brain at the juncture of the 
 sagittal and coronal sutures, down to the level of the striate bodies 
 or deeper, invariably produces a marked rise of temperature. If 
 the lesion only hits the striate bodies (especially the medial side, 
 near Nothnagel's nodus cursorius) the coming-on of the fever is 
 slow and gradual ; but if the needle is carried further toward the 
 base of the brain, the fever springs up at once and reaches a max- 
 imum in two to four hours. In what way these organs act as 
 " fever-centres," or precisely what nervous elements are chiefly in- 
 volved in the action, has not yet been made clear. 
 
 27. Eckhard 4 is inclined to lay down the law that in all verte- 
 brates the mechanisms for a change of place lie rather in the ante- 
 rior part of this general region corpora quadrigemina, etc.; while 
 those for maintaining the upright posture and the equipoise of the 
 body are localized in the region of the pons, cerebellum, and me- 
 dulla oblongata. 
 
 28. It should be added that almost all observers have hitherto 
 failed to attach sufficient importance to the central functions of 
 the gray matter which lines the floor and walls of the third ven- 
 tricle. Bechterew 5 has recently contributed the results of very 
 important experiments to determine the specific function of this 
 central nervous substance. He finds that frogs retain the function 
 of balancing even when the optic lobes are crushed, if no injury is 
 clone to the gray substance of the third ventricle or to the crura 
 
 1 By Tschetschichin, in Archiv. f. Anat. u. Physiol., 1866, pp. 151 ff . ; and 
 Schreiber, Pfluger's Archiv., viii., pp. 576 ff. 
 
 2 Lussana and Christian! (No. 16 of the Verhandlungen d. physiolog. Gesell- 
 schaft zu Berlin, 1883-84). 
 
 3 Ed. Aronsohn and J. Sachs: See Pfliiger's Archiv., xxxvii. (1885), pp. 
 232 ff. 
 
 4 In Hermann's Handb. d. Physiol., II., ii., p. 138. 
 6 Pfluger's Archiv, xxxi. (1883), pp. 479 ff. 
 11 
 
162 THE BRAIN AS A CENTRAL ORGAN. 
 
 cerebri ; they lose this function, however, when a section is made 
 into the third ventricle. Birds (hens and pigeons), also, show the 
 same loss of function when a lesion is produced by running a very 
 fine needle into the cavity of this ventricle. In the case of dogs, 
 Bechterew considers himself able to localize the function of equi- 
 poise precisely, and to point out the special effect of injury done 
 to different definitely fixed localities. For example, bilateral lesion 
 of the lateral or postero-lateral parts of the wall of the third ven- 
 tricle results in the impairment or loss of equipoise and co-ordi- 
 nated motion on both sides of the body : the lost function is re- 
 gained only after a long time, and then but partial!} 7 . In none of 
 these cases were any of the phenomena of motor laming of the 
 extremities apparent, or any very marked disturbance of sensation. 
 This gray matter of the third ventricle operates, Bechterew thinks, 
 in connection with the olivary bodies for the co-ordination of motor 
 impulses in response to sensations of touch, and with the semi- 
 circular canals in response to sensations of sound. It is especially 
 important also in equipoise through visual impulse connected with 
 the changes in the axial direction of the eyes. Thus all the above- 
 mentioned organs operate with the cerebellum as complex and 
 correlated mechanisms for keeping the body balanced in response 
 to changing sensory impulses. 
 
 We stop at this point in our ascending review of the automatic 
 and reflex-motor functions of the central mechanisms. For dis- 
 tinctly psycho-physical and psychological questions the most im- 
 portant of the activities of the nervous mechanism still await our 
 examination ; these are the activities of the cerebral hemispheres. 
 But nothing is known as to the molecular structure of these hemi- 
 spheres, or as to their automatic and reflex-motor centres and activ- 
 ities, which adds anything of importance to the description of the 
 nervous system as a mechanism, or to the mechanical theory of its 
 action. It is with such description and theory that we are now 
 concerned. The correlations which exist between the structural 
 condition, or physiological function of the nervous system, and the 
 phenomena of mind, are chiefly (if not wholly) capable of study as 
 illustrated in the cerebral hemispheres. But the nature of the 
 nervous molecular machinery, and of its working as mere machinery, 
 is understood, as far as our present information will permit, by an 
 examination of the physiology of the spinal cord and of the inter- 
 cranial ganglia lying below the hemispheres. As to the alleged 
 psychical functions of these inferior organs we shall adduce further 
 considerations when we come to consider such functions as belong- 
 ing to the brain proper. 
 
CHAPTER Y. 
 END-ORGANS OF THE NERVOUS SYSTEM. 
 
 1. IN order to understand the end-organs it is necessary to 
 refer again to the place which they hold in the threefold arrange- 
 ment of the nervous mechanism (compare Chapter II., 2). In the 
 general division of labor among its organs, certain cells situated 
 at the surface of the body become especially sensitive to external 
 stimuli. The special function of these cells accordingly becomes 
 that of receiving the action of such stimuli, of modifying this action 
 in accordance with their own peculiar structure, and thus of set- 
 ting up in the conducting nerves the neural process which is prop- 
 agated to the central organs. It is obvious, then, that the struct- 
 ure and grouping of the superficial cells must bear some definite 
 relation both to the external stimulus and also to the nerve-fibres 
 which convey inward the nervous impulse occasioned by it. The 
 end-organs of sense may then all be described as special adapta- 
 tions of the superficial cells to the different kinds of stimuli. With 
 such special adaptations the peripheral terminations of the nerve- 
 fibres must be connected. For the end-organs, as it were, look both 
 outward and inward. They act as mediators between those different 
 modes of external molecular motion which can occasion sensations 
 in us, and the nerves which convey the results of this motion, when 
 it has been changed into a nerve-commotion, onward to the central 
 organs. 
 
 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 and taste show this characteristic development 
 most clearly. These end-organs are, in general, made up of cells 
 which, posteriorly, pass into nerve-threads that are gathered to- 
 gether into the sensory nerve of the special sense ; and which, an- 
 teriorly, pass into conical or fusiform processes. The simplest type 
 of an end-organ may then be described as follows : A hair-like pro- 
 cess extending outward, and connected by a sensitive cell with a 
 nervous filament extending inward. Such processes are probably 
 
164 END-ORGANS OF SENSE. 
 
 extremely sensitive to external stimuli ; and perhaps peculiarly so 
 to the chemical changes which, at least in the case of three of the 
 special senses (smell, taste, and sight), are their immediate excit- 
 ants. 
 
 All the end-organs of sense may be regarded as modifications of 
 the type described above. Only a small part, however, of what are 
 ordinarily called "the organs of the special senses" (e.g., the nose, 
 the mouth, the ear, the eye, the skin) belongs, strictly speaking, 
 to the nervous system. By far the greater part consists of me- 
 chanical contrivances, designed to prepare the external stimuli and 
 conduct to the true nervous apparatus the impulses they occasion. 
 These non-nervous mechanical contrivances, however, modify the 
 nature of the stimulus in so important a manner as to merit some 
 brief description in our consideration of the nervous mechanism. 
 
 3. Besides the end-organs of sense, histology points out another 
 kind of terminal apparatus. The efferent nerves, in order that they 
 may stimulate the muscles, must have some special form of attach- 
 ment to them. Special contrivances for connecting the motor 
 nerves and the muscles are actually discoverable. We distinguish, 
 then, two classes of end-organs : first, End-organs of Sense, and, 
 second, End-organs of Motion. 
 
 4. Among the end-organs of sense, those of Smell have been 
 least successfully investigated. That portion of the mucous mem- 
 brane of the nose which clothes the upper region of the nasal cavity 
 and is marked by a brown-yellow color the region of the expansion 
 of the olfactory nerve is called " regio olfactoria ;" it contains the 
 end-organs of smell. Here Ecker and Eckhardt (in 1855) discovered 
 two different kinds of cells ; but we are indebted to Max Schultze for 
 the first detailed description of them. The epithelial portion of the 
 olfactory organ is supposed to be constructed upon the same type in 
 all the vertebrate animals. Of the two kinds of cells which the last- 
 mentioned investigator described, one is called " epithelial/' the other 
 "olfactory." The epithelial cells are the larger, have an oval nucleus 
 of considerable size, and extend through the whole epithelial layer. 
 Their external half appears more or less cylindrical or columnar (at 
 least in the Triton and Proteus), and is described by some observers ' 
 as striated longitudinally. The form of the inner half of these cells 
 is varied. The olfactory cells are spindle-shaped, with a large, 
 round nucleus, and very long, fine processes. The external process 
 is elongated into a stiff hair, at least in many cases, although 
 Schultze considers that in man the olfactory cells have no cilia. 
 
 1 See Professor Babuchin in Strieker's Human and Comparative Histology, 
 in., p. 207 f. 
 
THE REGIO OLFACTORIA. 
 
 165 
 
 These cells are surrounded by the epithelial cells. Most physiol- 
 ogists follow Schultzein holding that the two kinds of cells are dis- 
 tinct both in form and in function, and that only the " olfactory " 
 cells are connected with the end-fibrils of the nerve of smell ; Ex- 
 ner ' and others, however, believe that the distinction is not a fixed 
 one. In his opinion the structure of one is merged into that of the 
 other, and both are connected, though in a different manner, with 
 the subepithelial net-work in which the fibres of the olfactory nerve 
 are lost. The exact histological relation of the fibrils of the ol- 
 factory nerve to the epithelium of the regio 
 olfactoria is not yet made out. It is probable, 
 however, that the finest of these fibrils, after pen- 
 etrating the epithelial layer, closely embrace the 
 large epithelial cells and enter into connection 
 with the inner extremities of the olfactory cells. 
 According to Exner, the fibres of the nerve do 
 not pass over directly into the processes of the 
 end-organ cells, but are lost in a net-work whose 
 interstices are filled up with granules of nervous 
 matter. The first pair of cranial nerves, the ol- 
 factorius, which, as we have already seen on p. 84, 
 is really a lobe of the brain itself, is the specific 
 nerve of smell. 
 
 5. The contrivance for applying the stimulus 
 to the end-organs of smell is very simple ; in gen- 
 eral it is only necessary that a current of air, in 
 which the stimulating particles float, shall be 
 drawn through the nasal passages over the mu- 
 cous membrane of the regio olfactoria. Even am- 
 monia and camphor, when placed under the nos- 
 trils, have no smell so long as the breath is held or drawn through 
 the mouth. In quiet inspiration much the greater part of the cur- 
 rent 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 inspir- 
 ation, 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 vac- 
 uum in its cavity. 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 
 J Sitzgsber. d. Wiener. Acad. , Ixiii. , p. 44 f . and Ixv. , p, 7 f . 
 
 FIG. 40 Olfactory Cells 
 and Epithelial Cells 
 from the Mucous Mem- 
 brane of the Nose. 60 %. 
 (After Schultze.) 
 
166 END-OKGANS OF SENSE. 
 
 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 impor- 
 tant. This is probably the reason why the loss of the nose is so fre- 
 quently attended with loss of the sense of smell. 
 
 6. The end-organs of Taste are situated in certain papillae, 
 found on the upper surface of the root of the tongue, on the bor- 
 ders and apex of the tongue, and in some cases on the anterior por- 
 tion of the soft palate. These papillae of the tongue, are the papil- 
 Ice circumvallatce and the papillcefungiformes. The lateral portions 
 of the former are pre-eminently the regions of the mucous mem- 
 brane of the tongue where the end-organs of taste are found. The 
 same organs are also found more sparsely distributed in the fungi- 
 form papillae. The circumvallate papillae are composed of connective 
 tissue, which is invested by a pavement epithelium arranged in 
 laminae. The epithelial layer is thinner than elsewhere at the sides of 
 the papillae, in which the end-organs of taste (gustatory flasks or 
 bulbs) form a zone that extends upward to about the level at which 
 the papillae are no longer protected by their lateral wall. In the 
 fungiform papillae the end-organs appear in the epithelium which 
 covers their upper surface, and in the side surfaces. A. Hoffmann 
 also found them in the papillae of the region of the soft palate. It 
 is more doubtful whether they exist, as has been alleged, on the epi- 
 glottis. The papillce jiliformes, which are sometimes classed with 
 the two others, probably have nothing to do with sensations of 
 taste. 1 
 
 Methods of experimenting to discover what surfaces are sensitive 
 to taste are not easily made exact, because the stimulus must be in 
 solution to excite the end-organs, and because the nature of the ex- 
 citatory changes is chemical. There is scarcely a spot from the lips 
 to the stomach which some physiologist has not described as be- 
 longing to the organ of taste. But the regions where the above- 
 described papillae, with their gustatory flasks, are found, are doubt- 
 less the principal and probably they are the only sensitive 
 surfaces. Considerable differences exist, however, among different 
 species of animals, and even among different individual men es- 
 pecially as to the sensitiveness of the tip and edges of the tongue, 
 and of the anterior surface of the palate. All the evidence tends to 
 show that the gustatory flasks are the sole end-organs of taste. 
 
 1 Comp. Brlicke, Vorlesungen iiber Physiologic, ii., p. 257; and von 
 Vintschgauin Hermanns Handb. d. Physiol., III., ii., p. 147. 
 
THE GUSTATORY CELLS. 
 
 167 
 
 FIG. 41. Gustatory Bulbs from the Lateral Gustatory 
 Organ of the Rabbit. 45 %. (Engelmann.) 
 
 7. The microscopic structure of the end-organs of taste is de- 
 scribed in substantially the same way by all investigators, al- 
 though these structures vary 
 considerably, according to 
 their position, and accord- 
 ing to the different species 
 of animals. In general they 
 are like a glass knob with a 
 short neck, and with its 
 length somewhat greater 
 than its greatest width. 
 Hence they are called "gus- 
 tatory knobs" or "bulbs" 
 (so Henle), or, better, " gus- 
 tatory flasks " (so M. Schultze). They occupy flask-shaped cavities 
 of the epithelium, which they completely fill. Their lower or inner 
 
 part rests on the connective 
 tissue of the mucous mem- 
 brane ; their upper and more 
 slender part is surrounded by 
 epithelial cells and has an 
 opening, or pore, of from 4o ^ o6 
 to y^^ of an inch in diameter, 
 at the surface of the epithe- 
 lium. The margin of this pore 
 is usually formed by placing 
 several cells together, but 
 sometimes by a single cell which appears perforated with a round 
 hole. Each of the gustatory flasks consists of from fifteen to thirty 
 long, thin cells, arranged like the leaves of a bud in closely com- 
 pressed rows around the axis. 
 
 All the gustatory flasks are composed of two kinds of cells : 
 some are, essentially, epithelial cells, and have probably no direct 
 connection with the nerves ; the others are highly differentiated 
 structures, are probably directly continuous with the nerve-fibrils 
 and are thought to be true gustatory cells. The epithelial or in- 
 vesting-cells are long, narrow, spindle-shaped, bent, with a nucleus 
 well marked ; the outward end is pointed, the central end branch- 
 ing. The gustatory cells are thin, long, and highly refractive of 
 light, with nearly the whole body of the structure occupied by an 
 elliptical nucleus. The body of the cell is elongated into two pro- 
 cesses, of which the upper or peripheral is tolerably broad and 
 bears a short and fine point like a hair or pencil-point. This point 
 
 FIG. 42. Transverse Section through a Papilla 
 Circumvallata of a Calf. Showing the arrange- 
 ment and distribution of the gustatory buib. 
 26 /j. (Engelmann.) 
 
168 
 
 END-ORGANS OF TASTE. 
 
 lies in a canal, in the epithelial layer, and rarely projects from the 
 pore of the flask. The lower or central process of the cell is much 
 attenuated, and usually divides into two branches. A direct con- 
 
 Fio. 43. Isolated Gustatory 
 Bulb, from the Lateral 
 Gustatory Organ of the 
 Rabbit, ^/j. (Engelmann.) 
 
 Fio. 44. a. Isolated Gustatory Cells, from the Lateral 
 Organ of the Rabbit ; 6, an Investiug and Two Gusta- 
 tory Cells, isolated but still in connection. 6po / lt (En- 
 gelmann.) 
 
 nection of these processes with the fibrils of the gustatory nerve is 
 assumed by all investigators. The manner in which the nerve-fibres 
 terminate within the papillae is different in different animals. 
 
 The glosso-pharyngeal nerve is the principal nerve of taste. It is 
 distributed to the back of the tongue, enters the circum valla te pa- 
 pillae, where it forms a minute plexus, interspersed with nerve-cells, 
 from which both medullated and non-medullated fibres pass to the 
 base of the gustatory flasks. The lingual branch of the trigeminus 
 has also some claims to be, in a minor degree, a nerve of taste. 
 Schiff l considers it as designed for sour taste, with a slight sensi- 
 tiveness to bitter also. 
 
 8. In considering the end-organs of Touch, attention should 
 be directed to the great variety of sensations which are grouped 
 together under the word " touch," in the broadest meaning appli- 
 cable to it. The question is thus raised whether any histological 
 difference is to be detected in the nervous apparatus which may 
 serve as a physical basis for the difference in the sensations. We 
 may set aside for the present all consideration of the feelings of 
 pain, of exertion and fatigue, and the so-called " common feeling " 
 and "muscular sense." The question is thus reduced to this nar- 
 row form : Can histology point out two specifically distinct kinds of 
 end-organs in the skin, one of which serves for sensations of tem- 
 perature, and the other for sensations of pressure ? 
 
 9. Histological examination shows that the sensory nerves dis- 
 tributed to the skin the general organ of touch terminate in two 
 
 1 Molesch. Unters., X., p. 406 f., as referred toby von Vintschgau in Her- 
 mann's Handb. d. Physiol., III., ii., p. 171 f. 
 
THE TACTILE CORPUSCLES. 
 
 
 169 
 
 * 
 
 ways, either in free end-fibrils or in special constructions called 
 "tactile corpuscles" or "end-bulbs" The different varieties, 1 all, 
 however, essentially alike, of these special end-organs of touch have 
 been named after as many different investigators. Their general 
 office is that of modifying and multiplying the effect of the stimu- 
 lus upon the nerve-fibres which terminate in them. The so-called 
 " corpuscles of Pacini " were the first end-apparatus to be discovered 
 in connection with the peripheral termination of the sensory nerves ; 
 they were seen more than one hundred and fifty years ago by Vater. 
 In man they are constantly present in the subcutaneous connective 
 tissue of the palms of the hand and of 
 the soles of the feet ; but are most 
 numerous in the palmar surfaces of 
 the fingers and toes, especially the 
 third phalanges, although they occur 
 in the neck, arms, etc. In some places 
 they are visible to the naked eye as a 
 minute grain of from -^ to J of an 
 
 Z 
 
 medullated nerve-fibres remarkably 
 thickened. 2 Each corpuscle consists 
 of layers of connective tissue, arranged 
 concentrically and more closely packed 
 near the centre ; these surround a cav- 
 ity containing a soft nucleated mate- 
 rial, into the interior of which the 
 nerve penetrates. Here the nerve- 
 fibre, having become a naked axis-cyl- 
 
 * 
 
 inder, appears to terminate in a little 
 
 ., ,. * . .. ... .. . . . . 
 
 bulb. Examination with the highest 
 
 powers of the microscope shows that the axis-cylinder of the fibre 
 
 is fibrillated, and that the terminal bulb consists of finely granular 
 
 substance. 
 
 Closely allied to the foregoing structures are the so-called " end- 
 bulbs of Krause." These are small capsules of connective tissue in 
 which nuclei can be detected. In them the nerve-fibrils of touch 
 terminate either in a coiled mass or in a bulbous extremity. They 
 are from ^ ^ to io * 00 of an inch in diameter, and exist in the con- 
 
 1 On the different kinds of terminal corpuscles, a principal monograph is by 
 Fr. Merkel, Ueber die Endigungen der sensibleii Nerven in der Haut der 
 Wirbeltliiere, Rostok, 1880. 
 
 2 So Biesiadecki in Strieker's Human and Comparative Anatomy, ii. , p. 232. 
 
 Fre y-> . ne r ve with its sheaths ; &, 
 
 system of tunics constituting the cap- 
 
 sule of fch e corpuscle ; c, axial canal, 
 
 in which the nerve-fibre ends. 
 
170 
 
 END-ORGANS OF SENSE. 
 
 junctiva of the eye, in the tongue, the lips, the floor of the buccal 
 cavity, etc. The " corpuscles of Wagner " (or Meissner, who has 
 furnished most of the details) may be described as oval-shaped 
 bodies, made up of superimposed laminae and 
 bearing some resemblance to a miniature fir- 
 cone. The medullated nerve-fibres, like 
 " creeping roots," wind beneath the cutane- 
 ous papillae, and here and there penetrating 
 them, terminate in the corpuscles. Within 
 the corpuscles, 
 according to 
 KoUiker, the 
 
 FIG. 46. End-bulbs from the -.. ., ., 
 
 Conjunctiva of the Human tibl'llS lOl'm LWO 
 Eye. (After Kolliker.) 1, ,, ., 
 
 has two nerve-fibres which Or three COllS, 
 form a coil within the end- -i / _TU- ;; 
 
 bulb; a, has a fatty core, and finally join 
 
 The nerve-fibre of 3 ends fno-p + lipr irt 
 within in the form of a knot. l u b ei 
 
 loops. These 
 
 tactile end-organs are most constant 
 and numerous in the terminal pha- 
 langes of the fingers; they occur in 
 smaller numbers on the palm and 
 back of the hand, on the sole and 
 back of the foot, and sometimes on 
 the nipple, lips, etc. They are seated 
 in the papillae of the skin. Meissner 
 counted four hundred papillae in -fa 
 of an inch square on the third pha- 
 lanx of the index-finger, and found 
 these corpuscles in one hundred and eight of them. Their long 
 diameter lies in the direction of the papillae and extends from -j^ 7 
 to -j-J^ of an inch ; they are about jfa of an inch in thickness. 
 
 10. Since the surface of the skin is in general sensitive to press- 
 ure and to temperature, it follows that the special structures de- 
 scribed above as occurring in parts of this surface, cannot be the 
 sole end-organs of touch. Modern histology has demonstrated 
 the presence of an intricate plexus of non-medullated nerve-fibres 
 which end in free extremities between the cells of the mucous 
 layer. This terminal plexus of nerve-fibres is also the end-organ 
 of so-called general sensibility and of touch. 
 
 None of the attempts hitherto made to establish specific relations 
 between the varieties in the structure of the tactile end-organs and 
 the varieties of the sensations which they administer can be pro- 
 nounced successful. Krause has tried to deduce from the construc- 
 
 FIG. 47. Corpuscles of Touch. (After 
 Frey.) a, from the soft skin of the 
 duck's bill ; 6 and c, from the papillae 
 of the tongue of the same animal. 
 
THE TACTILE CORPUSCLES. 171 
 
 tion of the corpuscles of Pacini their fitness to act as the end- 
 organs of pressure ; but these corpuscles are wanting in many 
 parts of the body that are sensitive to pressure. Wagner con- 
 sidered the corpuscles which bear his name to be special organs of 
 touch. But it has been shown by Merkel that these corpuscles are 
 nothing but aggregates of more elementary forms, the so-called 
 " tactile cells." Some have argued that the end-bulbs of Krause and 
 the corpuscles of Pacini are the organs of general feeling (sensus 
 communis) ; but others, with more probability, assign this function 
 to the free nerve-endings ; while Merkel is of opinion that the latter 
 are specifically concerned in sensations of temperature. Nothing 
 is known on this point beyond the fact that the skin, within which 
 the sensory nerve-fibres terminate, either in free ends or in special 
 tactile corpuscles, is the organ for all the varieties of sensation 
 brought under the most general meaning of the word " touch.' 2 
 
 The more precise manner in which the terminal fibres of the 
 nerves of touch stand related to the individual tactile cells is also 
 still in doubt. Some investigators consider that the fibres enter 
 into the very protoplasm of the cells (Merkel, Frey) ; others that 
 they spread themselves on end-plates superimposed on the cells 
 (Eetzius, Ranvier). 
 
 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 
 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 inclosing 
 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. (1) 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- 
 juuctival epithelium. The cornea rises and bulges in the middle 
 like 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 
 
172 
 
 THE HUMAN EYE. 
 
 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 hav- 
 ing in its centre a circular aperture called the " pupil'' of the eye. 
 The anterior border (corpus ciliare) around the iris consists of the 
 
 JTov. c. \ - 
 
 PI 
 
 Pio. 48. Horizontal Section through the Left Eye. */i- (Schematic, from Gegenbaur.) 
 
 ciliary muscle and the ciliary processes. (3) The Retina is the 
 third or inner coat of the eye. It is a delicate membrane of ex- 
 quisite transparency and almost perfect optical homogeneity ; it 
 has a highly complex structure, consisting of nine or ten layers, the 
 truly nervous portions of which contain nerve-fibres, nerve-cells, 
 and special end-organs, together with connective tissue and blood- 
 vessels. The inner surface of the retina is moulded on the vitreous 
 
THE FOUR REFRACTING MEDIA. 173 
 
 body, and it extends from the entrance of the optic nerve nearly 
 as far forward as the ciliary processes. 
 
 13. The eyeball has four translucent refracting media. The 
 first of these enumerating inward from the outside front is (1) 
 the Cornea, 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 Crystalline Lens 
 is situated between the iris and the vitreous body. It is a transpar- 
 ent biconvex lens, with its antero-posterior diameter about one-third 
 less than the transverse diameter. It consists of a capsule and in- 
 closed 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, 
 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 Zonula 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 
 (lamellse), between which fluid is contained, built into a body that 
 is, optically considered, transparent and homogeneous. It occupies 
 most of the space inclosed 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 eye- 
 ball only the mechanism by which the eye is moved in its or- 
 bit 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 
 
174 
 
 THE HUMAN EYE. 
 
 on the inner side, (the recti ; intemus and. externus, 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 
 
 fr 
 
 FIG. 49. Muscles of the Left Human Eye, 
 seen from above, rs, rectus superior ; 
 re, rectus externus ; and rit, rectus 
 internus ; os, superior oblique, with 
 its tendon, , which runs through the 
 membranous pulley, u, at the inner 
 wall of the cavity of the eyeball. 
 
 FIG. 50. Muscles of the Left Human Eye, seen 
 from the outside. Ir, levator of the upper eye- 
 lid, which covers the rectus superior, rs, re, os, 
 as in the preceding figure ; rif, rectus inferior ; 
 at, inferior oblique. 
 
 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. 
 We cannot move the eyes so that the optical axes do not either 
 meet or remain parallel ; we cannot look with one eye upward and 
 the other downward, nor with one eye to the left and the other to 
 the right ; nor can we voluntarily turn the eyes farther apart than 
 when their axes are parallel. 
 
 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 eye-ball, 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 
 
THE FORMING OF THE IMAGE. 175 
 
 shall be quickly interpreted as indicative of the size, shape, lo- 
 cality, and motion of external visible objects. The most important 
 part of the solution of this problem falls upon the nervous struct- 
 ure 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- 
 dergo 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. To show the pos- 
 sibility of this by calculating how the general laws of optics apply 
 to the special structure of the eye, as anatomy describes it, and to 
 make the calculations accord approximately with the facts, has been 
 the labor of a number of investigators, especially of Helmholtz and 
 his pupils. To the results of this labor only a brief allusion must 
 suffice. 
 
 16. The four media of the eye constitute a system of refracting 
 surfaces, each of which is separated from the one adjoining by a 
 circular cut, as it were, in the whole refraction-substance. Espe- 
 cially is this true of the lens with its concentric layers. The " image " 
 formed upon the first member of this system of surfaces, by its re- 
 fraction of such bundles of rays, from the object, as all lie in a plane 
 at right angles to the axis of the system, thus becomes an " object '' 
 for the second refracting surface of the system ; and the image 
 formed by the second an object for the third ; and so on. The re- 
 sult of any number of such refractions will accordingly always be 
 an image whose points lie in a plane at right angles to the axis of 
 the system of refracting surfaces, and which, as a whole, is in true 
 perspective to the original object. The last image and the object 
 are geometrically similar. 
 
 In tracing the course of the rays of light through the refracting 
 media of the eye, two things must be taken into the account : (1) 
 the indices of refraction of these media, and (2) the geometrical 
 form and position of all the limiting surfaces. (1) The means for 
 attaining a knowledge of the former is by taking the average result 
 of an examination of a number of eyes supposed to be normal. 
 Fortunately for science, death has, for the first twenty-four hours, 
 
176 THE HUMAN EYE. 
 
 little or no effect in changing tbe indices of refraction of the eye. 
 Krause ' found the mean index of refraction of the cornea to be = 
 1.3507, of the aqueous humor = 1.3420, of the vitreous body = 
 
 FIG. 51 Median Section through the Axis of the Lens of the Eye. (Schematic, after Babuchin.) 
 
 1.3485. But Helmholtz (subsequent observers have agreed better 
 with his result than with Krause's) found the two latter indices of 
 refraction to be - 1.3365 and - 1.3382, 
 respectively. The lens of the eye, espec- 
 ially, is not homogeneous throughout as 
 to its index of refraction. Each layer has 
 its own index, and the amount of the 
 index of each layer increases regularly 
 toward the kernel of the lens. The work 
 of refraction done by the lens is, there- 
 fore, greater even than that which could 
 be done by a homogeneous lens with an 
 index of refraction equal to that of the 
 no. 52,-view of the Lens in kernel, or most highly refracting part of 
 
 Profile. /,. (After Arnold.) the lens. 
 
 (2) The position and form of the separating surfaces of the re- 
 fracting media can be only approximately determined in the living 
 eye. Three of these surfaces are of chief importance the anterior 
 surface of the cornea, and the anterior and posterior surfaces of the 
 lens. The convexity of the first of these three is found to depart 
 perceptibly from a sphere ; it is greater toward its edge than at its 
 vertex, where it resembles rather a section of an ellipsoid. The 
 advantage of such a shape is seen in the fact that the images 
 
 1 Krause's experiments refer to rays of the wave-length to which the bright- 
 est place in the solar spectrum corresponds ; that is, to the place at the end of 
 the first third, or quarter between D and E. The refraction-index of water for 
 these rays he assumed at =1.33424. 
 
THE PROCESS OF ACCOMMODATION. 177 
 
 formed when the pupil is expanded are thus made sharper than 
 they could otherwise be. No observable refraction takes place on 
 the posterior surface of the cornea, because the difference between 
 the indices of refraction of the cornea and of the aqueous humor is 
 so slight that the faint images from this surface vanish by proxim- 
 ity to the stronger ones refracted from the front part of the cornea. 
 
 17. The power of altering the refracting conditions of the eye, 
 so as to enable the media to form a single perfect image on the 
 retina, for varying distances of the object, is called its power of 
 " accommodation " or adjustment. Plainly such adjustment of the 
 eye cannot take place, like that of a camera obscura, by changing 
 to any appreciable extent the distance of the lens from the screen 
 on which the image is formed. It must therefore take place, either 
 \)j increasing the indices of refraction of the media of the eye, or 
 by increasing the curvature of one or more of the refracting surfaces. 
 It is now known to be due to changes in the convexity of the lens, 
 principally, if not wholly, of its anterior surface. The posterior 
 apex of the lens remains unmoved. There are several methods of 
 experiment which demonstrate that in accommodation for near dis- 
 tances the front of the lens becomes 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 
 -g^ 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 curvature. 
 
 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 o fthe eye. When the ciliary muscle con- 
 12 
 
178 THE HUMAN EYE. 
 
 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. 
 
 Conjunctiv 
 Corneae 
 
 Proc. ciliarix 
 
 radiarer circularcv 
 
 Ciliarmitslcel 
 
 FIG. 53. Sectional View of the Connections of the Cornea, Ciliary Muscle, Ciliary Processes, etc. 
 
 10 /,. (Gegenbaur.) 
 
 The occulo-motor nerve furnishes the fibres that serve the ciliary 
 muscle ; these fibres run in the posterior strands of its roots. Their 
 central place of origin is in the posterior part of the floor of the 
 third ventricle ; stimulating the front division of this part produces 
 accommodation of the lens ; stimulating the back division of the 
 same part produces contraction of the pupils. Stimulation still 
 further back, where the third ventricle passes into the aqueduct of 
 Sylvius, produces contraction of the internal rectus muscle of the 
 eye ; and the innervation of this muscle is, of course, regularly con- 
 nected with adjustment for near distances. Thus all the mechan- 
 ism of accommodation, both that of the central organs and that of 
 the end-organs, is made to work together for the production of an 
 image upon the retina. 
 
 18. Given the formation of the image upon the retina, it is fur- 
 ther required in order to vision that this physical process should 
 be changed into a physiological process. We now examine briefly 
 the mechanism by which such a change is accomplished. [The 
 reader is referred to the larger specific treatises for the detailed 
 theory of the schematic, the emmetropic, the myopic, and the ^hy- 
 permetropic eye.] The retina, or inner tunic of the eye, contains 
 the nervous elements by whose action the system of refracted rays 
 
THE LAYERS OF THE RETINA. 
 
 "179 
 
 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 sub- 
 stance. Since we are able to see the feeblest rays of the moon as 
 reflected from white paper, the nervous excitation which is the con- 
 dition of vision cannot be produced by the direct action of light on 
 the nerve-fibres or nerve-cells of the eye. A photo-chemical sub- 
 stance and process, as well as a special end-apparatus, seems there- 
 fore 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 
 
 10 Layer of pigment cells. 
 
 9 . . . Layer of rods and cones. 
 & Membrana limitans externa. 
 
 | 7 Outer nuclear layer. 
 
 f .... Outer molecular layer. 
 .... Inner nuclear layer. 
 
 * Inner molecular layer. 
 
 Layer of nerve-cells. 
 
 IQ^F * Layer of nerve-fibres. 
 
 Membrana limitans interna. 
 
 _ , -^ ^ . ' - 
 
 Inner surface. 
 FIG. 54. Diagrammatic Section of the Human Retina. (Schultze.) 
 
 in the following ten layers, counting from within outward and 
 backward : (1) the membrana limitans interna, which is the retinal 
 
10 
 
 Fia. 55. Diagrammatic represen- 
 tation of the Connections of the 
 Nerve - fibres in the Eetina. 
 (Sehultze.) The numbers have 
 the same reference as in Fig. 54. 
 
 THE HUMAN EYE. 
 
 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 nu- 
 clear layer ; (8) the membrana limitans 
 externa ; (9) the bacillary layer, or layer 
 of rods and cones ; (10) the pigment-epithe- 
 lium 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 Mailer) which seem to 
 penetrate its entire thickness are now held 
 to be in great part elements of the sup- 
 porting tissue ; moreover, the whole con- 
 nective substance is a kind of sponge-like 
 tissue, in the gaps of which the true ner- 
 vous elements lie embedded. The gaps 
 thus filled are especially large in the 
 second, third, fifth, and seventh layers. 
 
 A description of the undoubtedly ner- 
 vous elements of the retina includes the 
 following particulars : (a) The retinal 
 fibres of the optic nerve lie parallel to the 
 surface, are non-medullated, and extreme- 
 ly fine ; in general, they are arranged in 
 ray-like bundles, radiating on all sides 
 from the place of the entrance of the 
 nerve. The arrangement is special at the 
 yellow spot, so as to surround, and not 
 cover it. This nerve-fibre layer is thickest 
 at the papilla of the retina, and diminishes 
 continuously from this spot toward the 
 ora serrata ; at about one-third of the 
 distance it becomes single, (b) The gan- 
 glion-cells, which form the principal part 
 of layer No. 3, like the multipolar cells of 
 the rest of the cerebro-spinal system, 
 have one large process of more trans- 
 
THE LAYER OF RODS AND CONES. 
 
 181 
 
 lucent appearance. This process subdivides into fibrils of vanishing 
 fineness, that enter and are lost in the next layer. At the yellow 
 spot these cells are eight or ten deep ; from this centre they dimin- 
 ish toward the ora serrata, where spaces are found between the 
 cells, (c) The nervous elements of the inner molecular layer (No. 4) 
 are not clearly made out. They probably consist of extremely fine 
 filaments, which are connected with the external processes of the 
 ganglion-cells, (d) Most of the nucleus-like bodies of the inner 
 nuclear layer (No. 5) are probably nervous. Each such body has 
 two processes one directed inward, the other outward. The 
 former is thought to be connected with the filaments of the 
 inner (No. 4), and the latter with those of the outer (No. 6) molec- 
 ular layer, (e) In the outer molec- 
 ular layer (No. 6) are nervous fila- 
 ments, like those in No. 4, which are 
 probably connected with the external 
 processes of the inner nuclear layer. 
 Here are also found numerous star- 
 shaped cells probably not nervous. 
 (f) In the outer nuclear layer (No. 
 7) the undoubtedly nervous elements 
 preponderate. Each nucleus-like 
 body in this layer is connected by a 
 radial fibre with one of the nervous 
 elements of the rod-and-cone layer 
 (No. 9). These nuclear bodies are 
 called rod-granules and cone-granules 
 
 respectively, and are to be distin- FIG. 56. Diagrammatic Section of the 
 in i i -, ,-t . . Posterior Part of the Retina of a Pie. 
 
 guisned, not only by their connection *oo/ v - 
 
 7, part of outer nu- 
 
 with these elements, but also by ^I^JffSST HT3C 
 their size and position ; the latter are %S^*i^^;WJ^ 
 larger, and lie on the more external fra , ctile bod y. the function O f which is 
 
 ' unknown. 
 
 side of the layer, (g) 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 ex- 
 tension 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." 
 
 The rods extend the entire thickness of the layer, and are about 
 3-^0 inch in length, but the cones are shorter ; the rods are 
 about T ^ 00 inch in diameter, the smallest cones of the central 
 depression 10 ^ 00 inch. The inner ends of both are continuous 
 with the rod-fibres and cone-fibres of the outer nuclear layer. 
 
182 
 
 THE HUMAN EYE. 
 
 Each rod or cone is composed of an inner and outer segment 
 or limb ; the latter is highly refractile, the former only feebly so. 
 The inner limbs appear under the microscope like a mass of pro- 
 toplasm. The appearance of a most delicate longitudinal line in 
 the inner and outer segments has led to the belief that a nerve- 
 fibril is, as it were, drawn through their axis. The description 
 
 FIG. 57. 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. 80 %. 2?, inner 
 segment of a cone with a cone-nucleus. 120 % . C, isolated 
 interior portion of a cone. 
 
 PIG. 58. Rod and Cone from 
 the Human Retina, preserved 
 in perosmic acid, showing 
 the fine fibres of the surface 
 and the different lengths of 
 the internal segment. 1000 /,. 
 (Schultze.) The outer seg- 
 ment of the cone is broken 
 into disks which are still ad- 
 herent. 
 
 of the two shows that there is no essential anatomical difference 
 between the rods and cones ; nor are we able to distinguish any 
 difference in their physiological significance. 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 ^ inch ; ' while not far from this 
 1 See Le Conte, Sight, p. 58. New York, 1881. 
 
YELLOW SPOT AND BLIND SPOT. 
 
 183 
 
 FIGS. 59 and 60. Superficial Aspectof the Arrangement 
 of the Rods and Cones in the Retina. 50 %. (Schultze.) 
 The former is from the region of the macula lutea ; 
 the latter from the peripheral region. 
 
 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-epithe- 
 lium. These cells form a 
 regular mosaic of flat, six- 
 sided cells, which send out 
 pigmented processes be- 
 tween the outer limbs of the 
 rods and cones. 
 
 The fibres of the optic 
 nerve are supposed to be 
 connected with the rods and cones by means of the ganglion- cells, 
 and of the radial fibres in which the granules of the outer and inner 
 nuclear layers are embedded. 
 
 20. 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 optica). The yellow spot 
 
 is of oval shape, about ^ of an 
 inch in its long diameter, and has 
 in the centre a depression called 
 tbefovea centralis. It is the place 
 of clearest vision, and the physi- 
 ological centre of the eye. About 
 J- of an inch inside the eye from 
 the middle of the yellow spot is 
 the middle of the papilla, or place 
 where the optic nerve breaks into 
 the retina. The blind spot, or 
 portion of the retina which can 
 be experimentally shown to be 
 inoperative in vision, has been 
 proved by Helmholtz to corre- 
 spond in both size and shape to 
 that covered by this papilla. Its diameter is about ^ or -fa of an 
 inch, varying considerably for different eyes. It is wanting in all 
 the nervous elements. 
 
 21. 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 de- 
 mands made upon the end-apparatus of vision. It can be demon- 
 
 FlG. 61. Equatorial Section of the Right Eye, 
 showing the Papilla of the optic nerve, the 
 Blood-vessels radiating from it, and the 
 Macula lutea. 2 /i- (Henle.) S, sclerotic; 
 Ch, choroid ; and R, retina. 
 
184 THE HUMAN EYE. 
 
 strated 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. Indeed, it is possible, by an experiment (devised by 
 Purkinje), to perceive with one's own retina the aborescent figure 
 formed by the shadow of the blood-vessels expanded upon its front 
 part. 
 
 22. We have already seen (Chapter I., 14, 15) that a chemi- 
 cal process may reasonably be conjectured to accompany the action 
 of the nerves in general. Undoubtedly a photo-chemical process 
 is concerned in vision. But after all the careful researches of many 
 observers, especially of Ktihne ! and his pupils it is difficult to 
 point to any results of chemical investigation which serve better to 
 define the exact nature of the physiological action of the end-or- 
 gans of the eye. The relation of the light to any chemical pro- 
 cesses which may take place in the gray substance of the retina can 
 be only indirect. The opto-chemical hypothesis must, therefore, 
 regard the epithelial cells, with which the end-fibrils of the optic 
 nerve are in physiological connection, as the bearers (Tr tiger) of cer- 
 tain photo- chemically decomposable materials or visual substances 
 (Sehstoffe) ; these substances, however, cannot excite chemically 
 the irritable part of the visual cells the protoplasm of the inner 
 limbs of the rods and cones without being themselves decomposed. 
 Visual substance is necessarily some kind of matter easily decom- 
 posable by light, or chemically sensitive to light. The first process, 
 then, in the excitation of the optic nerve, is the decomposition by 
 the light of some substance found in certain epithelial elements of 
 the retina. The second process is the action, as visual excitants 
 (Sehreger), of the decomposition-products of the epithelial cells 
 upon the protoplasm of the end-organs. But in order that such 
 decomposition-products may act as excitants of the end-organs of 
 vision, the visual substance must be rightly placed that is, it must 
 be in local connection with the protoplasm of the outer limbs of 
 the rods and cones. The relation of the two last layers of the 
 retina is such as to secure this necessary connection. We are as 
 yet unable, however, to say what are the visual substances which 
 the successful working of the opto-chemical hypothesis demands. 
 The location of the pigmentum nigrum, and the changes produced 
 in it by light, favor the conjecture that this substance is of the 
 most fundamental and general importance for visual sensations. 
 Visual purple may also be supposed to be a visual substance. The 
 fact that light of different wave-lengths effects changes in this pig- 
 
 1 The few statements here given are taken for the most part from the article 
 of this investigator in Hermann's Handb. d. Physiol., III., i., pp. 235 ff. 
 
PIGMENTS OF THE EYE. 185 
 
 raent with different degrees of speed, suggests the view that it is 
 related to the susceptibility of the eye for different colors. But 
 since invertebrates do not have the visual purple ; since the cones 
 (a thing which no one doubts) see without this purple, and since 
 the rods of some animals, such as hens and doves, and the rods of 
 the ora serrata, perform their functions without it, this pigment 
 can scarcely be said to be the only visual substance. The opto-chem- 
 ical hypothesis, then, seems to require several colored visual sub- 
 stances. Moreover, since animals can see with bleached retinas, 
 and albinos have the power of vision, we are compelled to assume 
 also a colorless visual pigment. As to the nature of the chemical 
 changes necessary to be produced in the protoplasm of the outer 
 limbs of the rods and cones by the action of the decomposition- 
 products of the visual substances, we are quite ignorant. 
 
 23. 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 (6) the external meatus, or passage leading from the 
 bottom of this hollow to the drum of the ear. The concha is prob- 
 ably of little or no use in sharpening our perceptions of sound ; 
 for if a tube be inserted so as to secure a canal for the air to the 
 drum of the ear, the entire concha may be filled with wax, and the 
 result is to increase rather than diminish the sharpness of the 
 sound. It is possible, however, that vibrations of more than one 
 thousand in a second are concentrated by reflection ' from the con- 
 cha. The external ear appears to be of some service in perceiv- 
 ing the direction of sound. Rhine's experiments seem to show 
 that as Harless 2 thought the cartilage of the ear can be thrown 
 into sympathetic vibration with certain acoustic waves, and so re- 
 inforce the sound. At best, however such work done by the con- 
 cha is small. 
 
 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 
 
 1 See Hensen, Physiologie d. Gehors, in Hermann's Handb. d. Physiol., 
 III., ii., p. 23. 
 
 2 Article Horen, in Wagner's Handworterbuch d. Physiol., IV,, 1853. 
 
186 
 
 THE HUMAN EAR. 
 
 reached from the outside in a straight line. Helmholtz called at- 
 tention to the fact that certain tones of a high pitch resound 
 strongly in the ear when the meatus is of normal length, but cease 
 so to resound when its length is increased artificially. The rneatus 
 probably, therefore, modifies certain tones by its own resonant 
 action strengthening the high ones, and deadening the low, in 
 some degree. 
 
 Various simple experiments such as placing a resounding body 
 in contact with the teeth prove that the surrounding cranial bones 
 conduct sound to the ear. It is probable, however, that the path 
 of such conduction is not, for the most part, as was formerly sup- 
 posed, directly to the inner ear by way of the cranial and petrous 
 bones, but indirectly, through the ear-drum and bones of the middle 
 ear to the fenestra ovalis. The amount of direct conduction pos- 
 sible, has not as yet been determined precisely. 
 
 24. II. The Middle Ear, or Tympanum, is a chamber irregu- 
 larly cuboidal in form, and situated in the temporal bone, between 
 the bottom of the meatus and the inner ear. Its outer wall is (a) 
 
 FIG. 62. Drum of the Right Ear with the Ham- 
 mer, seen from the inside. 2 /i- (Henle.) 1, 
 chorda tympani ; 2, Euslachian tube ; *, ten- 
 don of the tensor tympani muscle cut off close 
 to its insertion ; m a, anterior ligament of the 
 malleus ; M c p, its head ; and II 1, its long 
 process. S t p, Spina tympanica posterior. 
 
 Chorda tyin.an.i 
 
 FIG. 63. Side Wall of the Cavity of the Tym- 
 panum, with the Hammer (M) and the Anvil 
 ( J). The former phows the connection of its 
 handle with the drum. T, Eustachian tube. 
 8 /,. (Gegenbaur.) 
 
 the membrana tympani, which consists of three layers an external 
 tegumentary, an internal mucous, and the intermediate 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 labyrinth, and the fenestra 
 rotunda, which corresponds to the tympanic passage in the cochlea. 
 Near its anterior part the tympanum opens into (b) the Eustachian 
 
THE THREE AUDITORY BONES. 187 
 
 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. The malleus has a head, separated by a constricted 
 neck from an elongated handle ; its handle is connected with the 
 centre of the membrana tympani ; its head articulates with the in- 
 cus. The incus has a body and two processes. On the front sur- 
 face of the body is a saddle-shaped hollow, in which the head of 
 the malleus fits ; the short process is bound by a ligament to the 
 posterior wall of the tympanum ; the 
 long process ends in a rounded pro- 
 jection (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 constricted neck 
 the two crura curve inward to the 
 base, which is attached to the fenes- 
 tra ovalis. These bones are moved 
 on each other at their joints by (d) 
 two or three small muscles the ten- 
 
 FIG. 64. Bones of the Ear, as seen in their 
 
 SOT tvmpani, the StapediuS. and, more connection from in front. /i- (Henle.) 
 , , , n ,, T . mv J Incus (anvil), of which Ib is the short, 
 
 doubtfully, the laxator tympani. The and II the long, process ; c, its body, and 
 /> , , . , T . , . , pi, the process for articulation with the 
 
 nrst OI tliese IS inserted into tne stapes (processus orbtcularis.) M, Malleus 
 
 malleus, near the root, and serves to JKBttf J^S^-KS 
 tighten the tympanic membrane by jgSjJj* 8tapes (8tirrup) ' with it3 
 drawing the handle of the malleus 
 
 inward ; the stapedius is inserted into the neck of the stapes, 
 but its function is doubtful apparently it draws the stapes from 
 the fenestra ovalis, and so diminishes the pressure of the chain 
 of bones in that direction. The laxator tympani is inserted into 
 the neck of the same bone, and its action has been supposed by 
 some to be antagonistic to that of the tensor tympani ; but its 
 muscular character is now denied by most observers. 
 
 25. 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- 
 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 
 
188 THE HUMAN EAE. 
 
 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 points 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 with 
 the -vibrations, as it were, 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 load- 
 ed with the auditory 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 sys- 
 tem, with a simultaneous motion around a common axis. Direct 
 observation of these bones in motion shows that their sympathetic 
 vibrations vary greatly for tones of different pitch and similar in- 
 tensity, from a scarcely observable motion to a surprisingly great 
 elongation. 
 
THE EUSTACHIAN TUBE. 189 
 
 The effect of the muscles of the tympanum upon the transmis- 
 sion of tones of different pitch is not as yet 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 stretching of the 
 tendon of the stapedius muscle has no observable influence on the 
 acoustic vibrations of the tympanum. 
 
 26. 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 atmospheric 
 pressure on both sides of the tympanic membrane, and to convey 
 away the fluids which collect in the tympanic cavity. If it re- 
 mained 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 respiration 
 would affect the position and tension of the tympanic membrane. 
 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 press- 
 ure is felt in the ears and the hearing is weakened. These effects 
 are due to the forcing of the air through the Eustachian tube into 
 the tympanic cavity. The tube is thus of indirect service in re- 
 spect to the physiological functions of the middle ear. 
 
 27. 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. It lies in a series of cav- 
 ities 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 suspended, 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 trans- 
 mitted by the tympanum are analysed and changed from a physi- 
 cal molecular process to a nerve-commotion, by the special end- 
 apparatus of hearing. 
 
 (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 
 constant. The membranous vestibule is composed of two sac-like 
 dilatations the upper and larger of which is named utriculus, the 
 lower sacculus. In its outer wall 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 Semicircular Canals open 
 
190 
 
 THE HUMAN EAR. 
 
 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 supe- 
 
 No. 1. 
 
 No. 2. 
 
 No. 3. 
 
 FIG. 65. No. 1, 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. fenestra of the vestibule (ovalis) ; ha, external 
 ampulla ; h, external semicircular canal ; Tsf, tracing spiralia foraminosus ; vaa, ampulla of 
 the superior semicircular canal ; vc, posterior semicircular canal ; and vpa, its ampulla. 
 
 nor, the posterior or vertical, and the external or horizontal canals. 
 The contiguous ends of the superior and posterior canals blend to- 
 gether and have a common orifice into the vestibule. They all 
 
 Ls 
 
 FIG. 66. Osseous Cochlea of the Right Ear, ex- 
 posed from in front. 4 /j. (Henle.) t, section 
 of the division-wall of the cochlea ; ft, upper 
 end of the same. Fee, Fenestra ; H, hamulus ; 
 Md, modiolus ; Ls, lamina spiralis. 
 
 FIG. 67. Cross-section through the Acoustic 
 Nerve and the Cochlea. /,. (Henle.) Nc, 
 nerve of the cochlea ; Nv, nerve of the vesti- 
 bule ; St. scalsi tympani ; Sv, scala vestibuli ; 
 and between them the ductns cochlearis, DC. 
 Ls and Md, as in preceding figure. 
 
 have a regular relative position, their planes being at right angles 
 to each other. Near the vestibule they dilate to about twice their 
 average diameter and form the so-called ampullae. Both the osseous 
 
STRUCTURE OF THE COCHLEA. 
 
 191 
 
 vestibule and the osseous canals contain a fluid (the perilymph), in 
 which the membranous vestibule and canals are suspended ; the 
 membranous labyrinth is also distended with a similar fluid (the 
 endolymph). 
 
 (C) The Cochlea is by far the most complex part of the laby- 
 rinth ; it is about J of an inch long, and is shaped like the shell of 
 a common snail. It, too, consists of a membranous sac embedded in 
 the osseous cavity. The whole passage of the cochlea is imperfectly 
 divided into two canals by a partition-wall of bone, which is wound 
 2J 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 its origin in the cir- 
 cular aperture (fenestra 
 rotunda) w T hich leads to 
 the tympanic cavity. The 
 other, which faces to- 
 ward the apex, opens 
 into the vestibule, and 
 is called the scala vesti- 
 buli. At the apex of the 
 cochlea these two scake 
 communicate with each 
 other through a small 
 hole (helicotrema). The 
 division of the mem- 
 branous cochlea is com- 
 pleted 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 passage ; it is attached to this wall by the spiral 
 ligament. Another membrane (the membrane of Reissner) arises 
 from a spiral crest (limbus, 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 intermedia, 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. 
 
 28. The auditory nerve, on approaching the labyrinth, divides 
 into a vestibular and a cochlear division. The former enters the 
 vestibule and subdivides into five branches one for the utriculus, 
 one for the sacculus, and one for each of the three ampullse. In 
 
 Liffamentuia 
 spiral* 
 
 Stria 
 vasculari* 
 
 Lamina 
 basilaris 
 
 FIG. 68. Section through one of the Coils of the Cochlea. 
 20 /j. (Schematic, from Gegenbaur.) 
 
192 
 
 THE HUMAN EAR. 
 
 each of these dilatations the membranous wall forms a projecting 
 ridge, called the crista acoustica. The endothelial investment of 
 the crista is elongated into columnar cells, intercalated between 
 which are fusiform cells. Each of the latter, according to Max 
 Schultze, and others, has the peripheral and the central process 
 with which we are already familiar in the nerve-cells of other end- 
 organs of sense. The peripheral process projects into the en do- 
 lymph as an auditory 
 hair ; while the central 
 extends into the subendo- 
 thelial tissue where the 
 nerve-plexus of the audi- 
 tory nerve ramifies, with 
 the terminal branches of 
 which it is probably con- 
 tinuous. According to 
 more recent observers 
 (Retzius and others) the 
 auditory hairs are con- 
 nected with the columnar 
 cells, and do not project 
 into the endolymph, but 
 into a soft material of in- 
 distinctly fibrillar struct- 
 ure. The inner surface 
 of the epithelium of the 
 crista is thus clothed with 
 a thick- set " wood " of 
 these hairs. Max Schultze 
 found their length to be 
 inch their ul- 
 timate ends, however, be- 
 ing too fine to discriminate. Calcare as particles, called "ear- 
 stones " (otoliths) appear in both sacc a and utricle, embedded in 
 a soft matrix and lying in contact \v A th the nerve-epithelium. In 
 the vestibule the hair-like prolongations of the epithelial cells are 
 more scanty than in the ampullae. 
 
 29. The terminal nerve-apparatus of the cochlea is even far 
 more complicated and remarkable. The cochlear branch of the 
 auditory nerve pierces the axis of the cochlea (modiolus) and gives 
 off lateral branches which pass into the canals of the osseous spiral 
 membrane. Here they radiate to the membranous spiral lamina, 
 and are connected with a ganglion of nerve-cells ; beyond the gan- 
 
 Fio. 69. Scheme of the Nerve-endings in the Ampullae. 
 (After Rudinger.) 1, membranous wall of the ampullae, 
 with a structureless border. 2 ; through which the nerve- 
 fibre, 3, sends its axis-cylinder, 4 ; 5, plexiform connection 
 of the nerve-fibres ; 6, auditory cells ; 7, supporting cells ; oVjont, -~ 
 8, auditory hairs. 
 
THE OEGA^ OF COETI. 193 
 
 glion 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. The connection of their ultimate fibrils 
 with the cone-cells of this organ may be assumed, but is difficult to 
 demonstrate. 
 
 The organ of Corti is situated on that surface of the basilar 
 membrane which is directed tow r ard the ductus cochlearis. Its 
 structure is a wonderful arrangement of cells. Some of these cells 
 are curved, elongated, and placed in two groups an inner and an 
 outer. They are called the "rods," or "pillars," or "fibres of 
 
 FIG. 70. Organ of Corti in the Dog. 800 / 1 . (Waldeyer.) b c, homogeneous layer of the basilar 
 membrane ; w, its vestibular layer ; , its tympanal layer ; d, blood-vessel ; f, nerves in spiral 
 lamina ; ff, epithelium of spiral groove ; h, nerve-fibres passing toward inner hair-cells, i, k ; /, 
 auditory hairlets on inner hair-cells ; I ^, lamina reticularis ; ra, heads of the rods of Corti 
 jointed together ; the inner rod seen in its whole length ; the outer one broken off ; n, cell at 
 base of inner rod ; p, q, r, outer hair-cells ; s, a cutioular process probably belonging to a cell 
 of Deiters ; , lower ends of hair-cells, two being attached by cuticular processes to the basilar 
 membrane ; w, a nerve-fibril passing into an outer hair-cell ; e, a sustenticular cell of Deiters. 
 
 Corti." The cells of the inner group rest by a broad foot on the 
 inner part of the basilar me'i ''brane, project obliquely forward and 
 outward, and expand into a x - "lated head ; the cells of the outer 
 group rest in the same way, *fifcline forward and inward, and fit 
 into a depression in the head of the cells of the inner group. The 
 two thus make a bow, which arches over an exceedingly minute 
 canal (the canal of Corti) formed between them and the basilar 
 membrane. These rods of Corti increase in length from the base 
 to the apex of the cochlea. The basilar membrane is composed 
 of fibres arranged in a transverse direction, so that each rod rests 
 upon one, or upon a pair of these fibres. Internal and almost 
 parallel to the inner group of rods is a row of compressed conical 
 13 
 
194 THE HUMAN EAR. 
 
 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 extremity projects as a brush 
 of hairs through the reticular membrane (membrane of Kolliker). 
 This latter membrane is a very delicate 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 inter- 
 val 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 Corti is covered over and separated from the endolymph 
 of the ductus cochlearis by the so-called membrana tectoria. 
 
 30. 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. 
 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 mole- 
 cular 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. Four such places, into which, as they yield, the fluid of the 
 labyrinth can retreat (as it were) are designated by Hensen ; ! these 
 are the two openings of the aqueduct of the vestibule, the mem- 
 branes of the aqueduct of the cochlea, the pores of the blood-vessels 
 in the bone, the membrane of the fenestra rotunda by bulging out 
 into the tympanic cavity. Impulses started in the fluid of the 
 labyrinth would thus result in its movement back and forth, so as 
 to produce a friction of the end-apparatus. This friction would 
 be increased by the action of the otoliths, or minute calcareous 
 particles, found in the fluid. Thus the waves started at the fenes- 
 tra ovalis would be diffused over the vestibule and into the scala 
 vestibuli of the cochlea, where they would flow to its head, being 
 1 In Hermann's Handb. d. Physiol., III., ii., p. 106. 
 
THE ANALYSIS OF SOUNDS. 195 
 
 prevented by the separating membrane from entering the scala 
 tympaui. To what extent these waves flow through the helico- 
 trema, or small hole at the apex of the cochlea, into the scala tym- 
 pani, and what are the exact relations between the waves in this 
 latter scala and those in the scala vestibuli cannot be stated con- 
 fidently. Nor can the exact part of the basilar membrane at which 
 the excitation of the end-organs by the oscillations of the structure 
 begins, be indicated with certainty. This membrane is, however, 
 undoubtedly thrown into vibration through the unequal pressure 
 of the moving fluid ; and by its vibration it excites the nervous 
 structures with which it is intimately connected. 
 
 31. A still more difficult problem for the labyrinth to solve 
 may be described in one word as a problem of "analysis." The 
 inner ear is not, indeed, contrived so as to reproduce changes in 
 the form of the acoustic oscillations, as such, after the manner in 
 which these changes can be made apparent to the eye or to touch. 
 But all our analogies for the analysis of composite tones the 
 " clangs " or musical notes of ordinary experience are derived from 
 the process of sympathetic vibrations. "We are led, then, to inquire 
 whether any part of the structure of the ear is capable of enough 
 such sympathetic vibrations to account for the experience which 
 we have in recognizing all the possible degrees of pitch in the scale 
 of musical sounds. The structure must also be such as to receive 
 the impressions produced by a number of simultaneous tones, com- 
 posing a harmony. Moreover, it must be such as to represent 
 tones that follow each other in rapid succession, as do the notes of 
 a melody. The sympathetic vibratory apparatus of the labyrinth 
 must therefore cease its vibrations immediately upon the cessation 
 of the sounds in sympathy with which it vibrates. In other words, 
 it must either have a damper, or be so constructed as to return 
 at once to a state of rest without such a damper. It must be capa- 
 ble of being thus excited, and of returning to a state of rest, no 
 fewer than five hundred times in a second, since the crackling 
 of electric sparks, between which the interval is no more than .002 
 of a second, can be heard as distinct noises. Still further, the end- 
 apparatus of hearing must suffice for all kinds of noise, as distin- 
 guished from musical tones ; and it is extremely difficult to see how 
 the same apparatus which serves for the analysis of the clang can 
 also suffice for all the various sensations of noise. 
 
 The manner is not known in which the auditory hairs and stones 
 and cells of the vestibule and ampullae, and the rods of Corti, the 
 fibres of the basilar membrane, and the conical hair-cells of Dei- 
 ters, in the cochlea, actually discharge the required functions. The 
 
196 THE HUMAN EAR. 
 
 structure of the end-apparatus in the vestibule and semicircular 
 canals is plainly not adapted to the analysis of musical tones. The 
 otoliths found in the vestibule, and the hairs of the ampullae, are 
 not capable of regular sympathetic vibrations ; moreover, they form 
 no scale of structures corresponding to the scale of sensations of tone. 
 This fact has led to the assumption that these organs are designed to 
 act as the end-organs of noise instead of musical sound. The more 
 complicated structures of the ductus cochlearis do seem, on the 
 contrary, to be adapted for the required analytic functions. It was 
 first argued by Helmholtz that the bows formed by the rods or 
 fibres of Corti are enough in number to constitute such a scale of 
 structures that this work of analysis can be assigned to them. 
 Some three thousand of these fibres, arranged in rows upon the 
 basilar membrane like the keys of a piano-forte, if distributed over 
 seven octaves would give about thirty-three for a semitone. They 
 might then be supposed to be elastic ; and since they differ in 
 size, to be tuned for particular sounds, so that the sympathetic 
 vibration of each one of them corresponds to the sensation of a 
 given tone. But the rods of Corti are stiff and not easily vibratory ; 
 and their office is probably simply to constitute a support for the 
 hair-cells. Moreover, birds, which are undoubtedly capable of ap- 
 preciating musical notes, have no rods of Corti. 
 
 Hensen has shown * that the basilar membrane is itself in a good 
 degree graded to pitch ; its continuous structure and expansion in 
 size from the beginning to the end of the ductus cochlearis en- 
 courage the assumption that its individual radii act like stretched 
 strings to respond to the different tones, from the lowest to the 
 highest. The calculations of Helmholtz have tended to confirm the 
 view of Hensen. It is assumed, then, that the parts resting upon 
 this membrane would be moved up and down, and that the excita- 
 tion of the conical hair-cells with which the terminal fibrils of the 
 auditory nerve are supposed to be connected is thus brought 
 about. The number of the acoustic cells is claimed to be about 
 great enough to correspond to the demands made upon the organ 
 which shall be instrumental in the physical analysis required as a 
 basis for the sensations of musical tones. The claim is at best 
 doubtful. As Hensen himself remarks, 2 the possibility is by no 
 means excluded that the working of this complicated and delicate ap- 
 paratus may be altogether different from that conjectured by all such 
 theory. In other words, the physiology of the peripheral mechanism 
 of hearing is as yet in a very incomplete and unsatisfactory state. 
 
 1 Zeitschrift f. wiss. Zool., XIII., p. 481 f. 
 
 a In Hermann's Handb. d. Physiol., III., ii., p. 104 f. 
 
END-OKGANS OF MOTION. 197 
 
 32. 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, Kouget, and others) that the axis-cylinder itself 
 pierces the sarcolemma or sheath of the muscular fibre ; that the 
 neurilemma becomes continuous with the sarcolemma ; 1 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. The exact relation between this extremely minute in- 
 tramuscular network of fibrils and the nuclei of the cells of mus- 
 cular "fibre " is not yet made out. 
 
 The shape and structure of the motor end-plates are different for 
 different animals, and even for different muscles of the same ani- 
 mal. Indeed, the mode of the termination of the motor nerves in 
 the muscle appears to be somewhat distinctive of the different 
 parts of the muscular structure. Sometimes the axis-cylinders are 
 somewhat enlarged, with strongly granular corpuscles attached or 
 adjacent. Sometimes a granular mass with its nuclei forms a kind 
 of base or floor for the terminal nerve-fibres ; and this eminence 
 may be elongated, elliptical, or circular. But the character and 
 variety of these forms are of no particular interest to psychology, 
 even as approached from the physiological point of view. 
 
 1 The question of histology is debated, whether the neurilemma actually 
 becomes continuous with the sarcolemma. Strictly speaking, according to 
 Kiihne, it does not ; but then, strictly speaking, it is not continuous with it- 
 self. It is, as we have seen (p. 36 f), divided by the annular constrictions into 
 members which are separate structures. It is to be considered as fringed 
 out on its edge and cemented to the sarcolemma. [See on this subject the 
 monograph, Die Verbindung d. Nervenscheiden mit dem Sarkolemm, Sepa- 
 ratabdruck aus der Zeitschrift fur Biologie, by Kuhne.] 
 
CHAPTEK VI. 
 THE DEVELOPMENT OF THE NERVOUS MECHANISM. 
 
 1. THE life of the individual man, so far as it can be made an 
 object of immediate observation and scientific description begins 
 as an un differentiated germ, without apparent distinction of bodily 
 organs or of physical and psychical activities. This living germ 
 undergoes a development. Before it can be subjected to ordinary 
 inspection it has unfolded itself into an elaborate organism ; and, 
 in its normal relation to the other systems of this organism (mus- 
 cular, respiratory, metabolic, reproductive, etc.), the nervous system 
 has acquired all its complex mechanism, consisting of an indefinite 
 number of parts. What are the different stages of the development 
 of this nervous system, and what are the laws according to which 
 its different factors and organs become differentiated, it belongs to 
 the science of Embryology to describe. But it belongs to psychology 
 to make such doubtful inferences as suggest themselves concerning 
 the psychical activities that are to be ascribed to the unfolding 
 mind of the embryo. Psychology, indeed, attempts in such a case 
 to form a picture of those earliest and most obscure mental states, 
 the elements of which can no longer be reproduced or recombined 
 in the developed consciousness of the adult. To this fact is due, 
 in part, the doubt which clings to all such inferences. But this 
 doubt is also due to the fact that embryology itself is so incomplete, 
 even in respect to its possession of single facts, and yet more in- 
 complete in respect to its power to set forth any system of general 
 truths and laws. 
 
 Our knowledge of all the earlier states and changes of con- 
 sciousness is wholly a matter of the interpretation of states and 
 movements of the bodily organism, in terms of our own conscious 
 mental experience. If, then, it were found that certain physical states 
 and motions of the human embryo need for their interpretation 
 the assumption of preceding or accompanying mental states, we 
 should have the right to carry our psychological principles back to 
 the life of this embryo even back to its beginning in the undif- 
 ferentiated germ from which the whole development proceeds. 
 
THE TWO-FOLD PROCESS. 199 
 
 But as the case now stands, the proper physical science cannot 
 claim to have furnished us with the requisite description of these 
 antenatal-physical movements and states. Little use for the main 
 purposes of Physiological Psychology, therefore, can be made of 
 facts accessible as to the embryonic development of man. We 
 might even seem warranted in passing by the whole subject with 
 two or three general observations like the following : The two-fold 
 life of man, both nervous mechanism and mind, begins in what is 
 apparent only as a physical unity, in that system of moving mole- 
 cules which constitutes the living germ. Out of this unity, and in 
 indissoluble connection with it, the two-fold human life then pro- 
 gressively develops. The mechanism unfolds itself, increases the 
 complexity of its molecular activities, runs its course of changes, and 
 is broken up again into its material elements. The mind manifests 
 itself in primitive activities, unfolds itself, increases the complexity 
 of its psychical life, and then ceases to make itself known through 
 the physical mechanism, when the mechanism itself is dissolved. 
 And all the while the molecular mechanism and the mind are most 
 closely and mysteriously correlated in their development as a to- 
 talit}', and in their particular activities. 
 
 But in spite of the fact that embryology furnishes psychology 
 with scanty material for any extended and trustworthy conclusions 
 with regard to the earliest activities and development of the mind, 
 at least a sketch of its principal outlines, so far as the nervous sys- 
 tem is concerned, seems desirable. Of knowledge from direct ob- 
 servation concerning the early development of the human embryo 
 there is exceedingly little. Yet the comparatively few facts which 
 are indisputably known, throw considerable light upon the nature 
 and functions of the human nervous mechanism. Moreover, in cer- 
 tain most important particulars there is good reason to believe that 
 the earliest history of the development of the embryos of other 
 animals is substantially like that of the human embryo. The very 
 first things in the life of the chick or better, one of the mammals 
 for example, may be described as probably holding good in all 
 important respects for the life of man. And when those differences 
 which are most strikingly human begin plainly to appear, they 
 show what parts of the nervous system are most worthy of em- 
 phasis as distinctively connected with man's mental life. 1 
 
 2. The immature ovarian ovum of the common fowl like that 
 
 1 The following description is taken to a large extent, and in some places 
 almost verbatim, from Foster and Balfour's Elements of Embryology, London, 
 1883, and F. M. Balfour, Comparative Embryology, vol. ii., pp. 177 ff., Lon- 
 don, 1881. 
 
200 EMBRYONIC LIFE OF MAN. 
 
 of every other animal presents the characters of a simple cell. It 
 is seen to consist of a naked protoplasmic body which contains in 
 its interior a nucleus (the germinal vesicle) and within this a nucle- 
 olus (the germinal spot). It is enclosed in a capsule of epithelium, 
 called the "follicle," or "follicular membrane." As the ovum ma- 
 tures, the body of it grows in size and a number of granules make 
 their appearance in the interior ; while the outermost layer of the 
 protoplasm remains free from them. But as the granules grow 
 larger in the centre, other granules appear also in the periphery 
 of the ovum. The germinal vesicle, during the growth of the ovum, 
 travels toward the periphery where the protoplasm surrounding 
 it remains comparatively free from granules. Accessory germinal 
 spots make their appearance. The cells of the follicular membrane, 
 which were at first arranged in a single row, now become two or 
 more rows deep ; and, whereas the immature ovum is naked, its 
 superficial layer is now converted into a radiately striated mem- 
 brane. Still later, a second membrane appears between this striated 
 membrane and the cells of the follicle ; and the former disappear- 
 ing as the ovum approaches maturity, the second membrane (called 
 the "vitelline") remains alone. Other changes which take place 
 after the ovum has ripened and has been discharged into the ovi- 
 duct, it is not necessary to describe. They result in the formation 
 of the accessory parts of the egg. The only essential constituent 
 of the body of the ovum is an active living protoplasm. 
 
 3. Impregnation takes place in the upper portion of the oviduct, 
 and consists in the entrance of a single spermatozoon into the 
 ovum, followed by the fusion of the two. The spermatozoon itself 
 may be considered as a cell, the nucleus of which is its head. On 
 entering the ovum, the substance of its tail becomes mingled with 
 the protoplasm of the ovum ; while the head enlarges, moves to- 
 ward and fuses with a part of the substance of the ovum, thus 
 constituting the nucleus of the impregnated egg. In this manner 
 the physical and mental peculiarities of both parents are trans- 
 mitted or carried over to the offspring by means of the actual fu- 
 sion of substance derived from the bodies of both. 
 
 4. A process known as segmentation or "yolk-cleavage" follows 
 the fecundation of the ovum. This process consists in a succes- 
 sive division of the ovum into a number of cells, from which all the 
 cells of the full-grown animal are, as it were, the lineal descendants. 
 This process has many variations among the different animals. 
 The chief peculiarity among the mammals is that the whole mass of 
 the yolk is subject to this change. 
 
 By segmentation the germinal disk of the ovum is broken up 
 
THE OVARIAN OVUM. 201 
 
 into a large number of rounded segments of protoplasm, called the 
 blastoderm. Of these segments those that lie uppermost are smaller 
 than those beneath. The beginning of the two layers into which 
 the blastoderm divides is thus made. The behavior of the nucleus 
 formed by the union of substance from the male and the female, 
 during the process of segmentation, has not been so satisfactorily 
 traced ; it appears probable, however, that a process of division goes 
 on in it also. Other nuclei, thought to be derived from the primi- 
 tive nucleus, make their appearance immediately below the blasto- 
 derm. The distinction between the upper and lower layers of the 
 blastoderm now becomes more obvious, for the segments of the 
 former arrange themselves side by side, with their long axes vertical, 
 as a membrane of columnar nucleated cells ; while those of the 
 latter continue granular and round, and form a close, irregular 
 net-work of cells, whose nuclei are not easily seen. 
 
 5. The principal difference between the ovum of a mammal and 
 that of a bird depends upon the amount and distribution of the 
 food-yolk. The ovum of the mammal is small the human ovarian 
 ovum being only from 
 
 T!T ^ liir ^ an i ncn 
 in diameter because 
 it contains so little 
 food-yolk ; but this 
 small supply is dis- 
 tributed uniformly 
 throughout. In con- 
 sequence of the above- 
 mentioned difference, PIG. 71. FIG. 72. 
 
 the OVUm is able to FIGS. 71 and 72. Fructified Human Egg of 12-13 days, seen 
 lf<1r- ir^4-/T. from the surface and the side. In the centre of the former is 
 
 reaK Up into Seg- what Reichert considers the embryonic area. 
 
 ments through the 
 
 whole of its protoplasmic mass. As the process of segmentation 
 goes on, the differences among the ova of different species of ani- 
 mals become more clearly marked. For example, in the rabbit, 
 although the details are differently described by different observers, 
 at the close of the process of segmentation the ovum appears to be 
 composed of "an outer layer of cubical hyaline cells, almost en- 
 tirely surrounding an inner mass of highly granular, rounded, or 
 polygonal cells." In a small circular area, however, the inner mass 
 remains exposed. The outer cells soon close over the exposed spot 
 (called by van Beneden, blastopore), and thus form a superficial 
 layer. A narrow cavity then appears between the two layers, 
 which extends so as to separate them completely, except in the 
 
202 
 
 EMBRYONIC LIFE OF MAN. 
 
 region near to the spot originally exposed. The enlargement of 
 the ovum and of the cavity together, soon give the whole structure 
 the appearance of a vesicle with a thin wall and a large central 
 cavity. This vesicle is called the blastodermic vesicle. The greater 
 part of its walls is composed of a single row of outer flattened 
 cells ; while an inner lens-shaped mass of cells appears attached to 
 
 FIG. 73. Vascular Area and Embryonic Area of the Embryo of a Rabbit, seven days old. 28 /!, 
 (Kolliker.) o o, the vascular or opaque area : ag, embryonic area ; pr, primitive streak and 
 groove ; rf, medullary groove. 
 
 a portion of the inner side of the outer layer. The " blastodermic 
 vesicle " enlarges rapidly ; its inner mass of cells loses its lens-like 
 shape, becomes flattened, and spreads out on the inner side of the 
 outer layer. Its central part remains thicker and forms an opaque 
 circular spot on the blastoderm, which is the beginning of the area 
 where the embryo is to form (the embryonic area). 
 
 6. The immediately subsequent history of the development of 
 the mammalian ovum, until the appearance of the so-called "primi- 
 tive streak," is less perfectly understood : Foster and Balfour ' speak 
 of the following description as " tentative." In the embryonic 
 area the cells of the inner mass become divided into two distinct 
 strata, an upper one of rounded cells which lies close to the 
 flattened outer layer, and a lower one of flattened cells (the " hypo- 
 
 1 Elements of Embryology, p. 316 f. 
 
THE THREE GEEMINAL LAYERS. 203 
 
 blast"). The former becomes fused with the outer layer, and 
 thus gives rise to a layer of columnar cells (the " epiblast "). In 
 this way the embryonic area consists of two layers of cells ; the 
 upper one of which is the epiblast, and the under one the hypoblast. 
 
 The blastoderm at first, then, consists of only two layers, which 
 constitute a double-walled sac (the gastrula) ; but a third layer 
 soon makes its appearance between the other two. These three 
 layers epiblast, mesoblast, and hypoblast are called " germinal 
 layers " and are found in the embryos of all forms of vertebrate, 
 and most forms of invertebrate animals. The middle one, or meso- 
 blast, arises from certain parts of the other two primitive layers, in 
 a manner which need not be described. From these three germi- 
 nal layers, all the different parts of the organism of the animal are 
 developed. The history of the development of every animal in its 
 earlier stages is, therefore, a narrative of the changes which take 
 place in the three layers of the blastoderm. The hypoblast is 
 the secretory layer ; and from it almost all the epithelial lining 
 of the alimentary tract and its glands is derived. The mesoblast 
 is the source of the skeletal, muscular, and vascular systems, and 
 of the connective tissue of all the parts of the body. But it is the 
 epiblast which produces the central and peripheral nervous system, 
 the epidermis, and all the most important parts of the organs of 
 sense. It is to the development of the epiblast exclusively, then, 
 that we now direct our attention. 
 
 7. The process of differentiating the layers of the embryo is 
 intimately connected with another, which results in forming a 
 
 FIG. 74. Primitive Streak of the Embryo of a Rabbit, eight days and nine hours old. 220 /,. 
 (Kolliker.) No medullary groove has yet been formed, ax, primitive streak: pr, primitive 
 groove ; #/, primitive fold ; ect, ectoderm (or epiblast) ; mes, mesoderm (or mesoblast) ; ent, en- 
 toderm (hypoblast). 
 
 structure known as the primitive groove. This process is substan- 
 tially alike in mammals and in birds. A short sickle-like thickening 
 of the blastoderm, which afterward becomes a " narrow strap-like 
 opacity" due to a forward propagation (linear proliferation) of 
 
204 EMBRYONIC LIFE OF MAN. 
 
 epiblast cells in a straight line arises near the junction between 
 the pellucid and the opaque areas of the blastoderm, and stretches 
 inward upon the embryonic area ; it is called the primitive streak. 
 The median line of the primitive streak then shows a shallow fur- 
 row, running along its axis. This furrow is called the primitive 
 groove. (Compare Fig. 73.) 
 
 8. Now occurs the formation of the medullary groove. In that 
 portion of the embryonic area which is in front of the primitive 
 streak, the axial part of the epiblast thickens ; two folds arise along 
 the boundaries of a shallow median groove ; the folds meet in front, 
 diverge behind, and then enclose between them the front part of the 
 primitive streak. These are the medullary folds, and they constitute 
 the first definite features of the embryo. The part bounded between 
 these folds is called the " medullary plate ; " its supreme impor- 
 tance in the embryo appears in the fact that it is the portion of the 
 epiblast which gives rise to the central nervous system. At about 
 the time of the development of the medullary groove (a little earlier) 
 an important change is taking place in the constitution of the 
 hypoblast in front of the primitive streak. An opaque line ap- 
 pears, as seen from the surface, and is continued forward from the 
 front end of the streak, but stops short at a semicircular fold near 
 the front part of the pellucid area. This fold is the future head- 
 fold of the embryo. The opaque line is due to a concentration of 
 cells in the form of a cord ; it is the beginning of what is known as 
 the notochord. It is to subsequent changes in connection with the 
 notochord that we are to look for the development of the distinct- 
 ively vertebral structure of the animal. 
 
 9. From this point onward the shaping of recognizable parts 
 of the embryo proceeds rapidly. The pellucid area, which was at 
 first quite flat or slightly curved, has, in the process of its growth, 
 suffered a " tucking in" as it were of a portion of the blasto- 
 derm, in the form of a crescent. It is this tuck which, when viewed 
 from above, appears as a curved line marking the margin of the 
 medullary groove. Thus the blastoderm is at this spot folded in 
 the form of the reversed letter Z ; the fold is the one already re- 
 ferred to as the "head-fold." Of the two limbs of this 3-fold, 
 the upper is continually growing forward and the lower is contin- 
 ually growing backward. As the head-fold enlarges rapidly, the 
 crescentic groove becomes deeper ; and at the same time, the over- 
 hanging margin of the groove rises up above the level of the blasto- 
 derm. The medullary folds meantime increase in height and lean 
 over from either side toward the middle line. They soon come in 
 contact in the region which will afterward become the brain, and 
 
FIRST CEREBRAL VESICLE. 
 
 205 
 
 thus form a tubular canal (the medullary or neural canal), although 
 they do not for some time coalesce. As the upper limb or head of 
 the embryo becomes more prominent, the medullary folds close 
 rapidly, and, in the region of the head quite coalesce. The open 
 medullary groove is thus converted into a canal or tube, which is 
 closed in front but remains open behind. The front end of this 
 
 V7; 
 
 FIG. 75. Fore-part of an Embryo-chick at the 
 end of the second day, viewed from the Dorsal 
 Side. /,. (Kolliker.) V h. fore-brain ; A b /, 
 occular vesicles; J/A, mid-brain ; H A, hind- 
 brain ; H, part of the heart seen bulging to 
 the right side; Vom, vitelline veins; Mr, 
 medullary canal, spinal part ; M r', medullary 
 wall of the mid-brain ; U u>, proto-vertebral 
 somites. 
 
 FIG. 76 Embryo of a Rabbit, eierht days and 
 fourteen hours old. 22 ' 7 /i (Kalliker.) ap, 
 pellucid area ; , anterior edge of the circuit 
 of the head ; A', fore-brain ; h" ', region of 
 later mid-brain ; h fff , position of the hinder 
 brain ; A, position of the heart ; rf, medullary 
 groove ; rw, medullary ridge ; uw. meso- 
 blastic somite; pz, lateral zone ; stz, vertebral 
 zone. 
 
 neural canal having a more rapid growth than the rest dilates 
 into a small bulb or vesicle, the cavity of which remains continuous 
 with that of the rest of the canal, while its walls are similarly formed 
 of epiblast. This bulb is the so-called first cerebral vesicle ; and the 
 lateral processes which soon grow out from its sides are called optic 
 vesicles. Behind the first vesicle, a second, and afterward behind the 
 second vesicle, a third is soon formed. Thus these three brain-buds, 
 
206 
 
 EMBRYONIC LIFE OF MAN. 
 
 Mid-brain 
 
 Plexus 
 chorioides 
 
 or germinal brains, are made. At the level of the hind end of the 
 head, two shallow pits appear (the auditory pits) which are the rudi- 
 ments of the organ of hearing. Thus the closing-up of the medul- 
 lary canal has converted the original medullary groove into a 
 neural tube ; and three cerebral vesicles have been grown which 
 are to develop into the fore-brain, the mid-brain, and the hind- 
 brain. 
 
 10. The most important changes which now take place in 
 the development of the nervous mechanism, are connected with 
 the growth of the three cerebral vesicles and with the flexure of 
 the medullary canal. The front portion of this canal-^that is, the 
 fore-brain with its vesicles in consequence of inequalities of 
 
 growth in the different 
 parts of the brain, be- 
 comes bent downward ; 
 this is the commence- 
 ment of the cranial flex- 
 ure. As the flexure pro- 
 gresses, the front portion 
 becomes more and more 
 folded down, so that the 
 second vesicle, or mid- 
 brain, comes to project in 
 front of it. From the 
 front part of the fore- 
 brain the vesicles of the 
 cerebral hemispheres 
 grow out and swell lat- 
 erally, so as to make two 
 buds corresponding to 
 the two hemispheres of 
 the brain. Each of these side-buds has a cavity which is continu- 
 ous behind with the cavity of the fore-brain ; each cavity becomes a 
 lateral ventricle of the brain. The original vesicle of the fore-brain, 
 having ceased to occupy its front position, is developed into the 
 parts surrounding the third ventricle. In the hind-brain, or third 
 cerebral vesicle, the part nearest to the mid-brain becomes marked 
 off by a constriction ; the hind-brain is thus separated into two 
 parts the rudimentary cerebellum with the pons in front, the 
 rudimentary medulla oblongata behind. 
 
 11. Various differentiations of the lining of the epiblast, which 
 is involuted along the cerebro-spinal cavity, take place. Through 
 the length of the neural canal this lining is thickened at each 
 
 Communica- 
 tion oflat-. 
 eral ventri- 
 cles 
 
 Plexus 
 chorioidc 
 
 Foramen 
 Monroi 
 
 N. opt. 
 
 Infundi- 
 bulum 
 
 N. trig. 
 
 Fia. 77. 4, Brain of an Embryo of the Rabbit. B, Brain 
 of an Embryo of the Ox. In both cases the side-wall of 
 the left hemisphere is removed. (After Mihalkovics.) 
 
CRANIAL AND SPINAL NERVES. 207 
 
 side, so that the cavity is no longer circular, but resembles a narrow 
 vertical slit. In the region of the cerebral hemispheres, the sides 
 and floor of the canal are much thickened, but in the region of 
 the third and fourth ventricles, its roof becomes excessively thin, 
 so as to form a membrane consisting of scarcely more than a single 
 layer of cells. 
 
 12. Another important event, at about this stage in the 
 development of the embryo, is the formation of the cranial and 
 spinal nerves. The cranial nerves sprout out of a continuous 
 band (the neural band), composed of two plates, which connects 
 the dorsal edges of the neural canal with the external epiblast. 
 This band separates from the epiblast and becomes a crest on the 
 roof of the brain, with its two plates fused together. The crest 
 extends forward as far as the roof of the mid-brain. As the roots 
 of the cranial nerves grow centrifugally and become established, 
 the crest connecting them is partially obliterated. The posterior 
 roots of the spinal nerves are outgrowths of a series of median 
 processes of cells that appear on the dorsal part of the cord. These 
 outgrowths are symmetrically arranged, and attached to the walls 
 of the cord ; but their original attachment is not permanent. Such 
 rudimentary posterior spinal nerves divide subsequently into three 
 portions a rounded portion nearest to the cord, an enlarged 
 middle portion forming the rudiment of a ganglion, and a periphe- 
 ral portion forming the commencement of the nerve. The origin 
 of the anterior roots of the spinal nerves is less satisfactorily made 
 out. 
 
 13. In the further development of the hind-brain the medulla 
 oblongata undergoes changes of a somewhat complicated character. 
 Its roof becomes extended and thinner ; where the two lateral halves 
 of the brain were at first united (at the raphe) a separation takes 
 place, so that the sole union of the two sides is by a single row of 
 cells. The thin roof of the fourth ventricle is thus formed. The 
 floor of the whole hind-brain becomes thickened, and on its outer 
 surface a layer of longitudinal non-medullated nerve-fibres appears. 
 The roof of the anterior part of the hind-brain, which has become 
 thickened instead of thinned out thus forming the rudimentary 
 cerebellum is developed, first, by the formation of the median lobe 
 (or vermiform process) and, afterward, by the swelling of its sides so 
 as to constitute the cerebellar hemispheres. 
 
 14. The changes in the development of the mid-brain (or 
 mesencephalon) are comparatively simple. When the cranial flex- 
 ure has taken place, the mid-brain is left at the front end of the 
 axis of the body, as a single vesicle with a vaulted roof and a curved 
 
208 
 
 EMBRYONIC LIFE OF MAN. 
 
 floor, whose cavity is known as the aqueduct of Sylvius. The cor- 
 pora quadrigemina of the two sides are marked off from each other 
 
 by the appearance of a vertical 
 furrow about the sixth month ; 
 and about a month later a 
 transverse depression sepa- 
 rates the anterior (nates) and 
 posterior (testes) pairs. The 
 thickening of the floor of the 
 mid-brain gives rise to the 
 crura cerebri. 
 
 15. Of the two divisions 
 into which the fore-brain has 
 already become divided, the 
 posterior constitutes the so- 
 called " thalamen-cephalon." 
 
 Fio. 7S.-Head of the Embryo of a Sheep, cut This body IS at first a simple 
 through the middle. 3/j. (Kolliker.) u, under * 
 
 jaw ; *, tongue ; , septum narium ; occipttale vesicle. formed of Spmdle- 
 basilare ; tfi, thalamus options; vt, roof of the 
 
 third ventricle ; cp, posterior commissure ; mh, shaped Cells, With Wails OI 
 mid-brain divided by a fold into two parts ;/, falx . . ., . , T , 
 
 cerebri ; /', terminal plate of the fore-brain. At nearly Uniform thickness. Its 
 the prolongation of the line offm is the foramen 
 of Monro. , tentorium cerebelli ; cl, cerebellum ; 
 pi, plexus of the fourth ventricle. 
 
 floor gives rise to the optic chiasm 
 and the origin of the optic nerves, 
 and to the rudiment of the infundi- 
 bulum ; and its sides become thick- 
 ened to form the optic thalami, while 
 the interval between them enlarges 
 toward the base and constitutes the 
 cavity of the third ventricle. The 
 more complicated changes which its 
 roof undergoes give rise to the pineal 
 gland and other small surrounding 
 structures. It is the anterior and 
 larger portion of the fore-brain which 
 constitutes the rudiment of the cere- 
 bral hemispheres. In this cerebral 
 rudiment, also, a floor and a roof may 
 be distinguished. The former is de- 
 veloped into the principal basal gan- 
 glia, the striate bodies ; the latter into the structures of the cerebral 
 hemispheres proper. The formation of the striate bodies (corpora 
 striata) is in fact due to thickenings of the walls of the floor of this 
 
 Fio. 79 Brain of Human Embryo of five 
 months, with Basal Ganglia laid bare. 
 Natural size. (Kolliker.) st, corpus 
 striatum ; o, optic thalamus ; to, ante- 
 rior lobe (lunatus) of the cerebellum, 
 and Ip, posterior lobe of the same ; , 
 semilunaris superior, and si, inferior ; 
 j, pyramid. 
 
DEVELOPMENT OF THE BRAIN. 
 
 209 
 
 rudiment. The laying of the commissures is the characteristic 
 feature of the development of the mammalian hemispheres. These 
 are the anterior commissure, the fornix, 
 and the corpus callosum. But into the 
 details of this process we do not need 
 to enter. One characteristic of the em- 
 bryonic development of mammals is the 
 early enlargement of the cerebral hemi- 
 spheres ; in the human embryo they are 
 even by the tenth week much larger 
 than all the other parts of the brain. 
 At this time they are hollow bodies 
 with comparatively thin upper walls, the 
 lateral ventricles being dilated and com- 
 municating with each other through a 
 wide opening, and with the third ven- 
 tricle by the foramen of Monro. They 
 grow from before backward, and thus 
 cover up, one after the other, the optic 
 thalami, corpora quadrigemina, and cere- 
 bellum. Their floor keeps on thicken- 
 ing, and thus the striate bodies become 
 greatly enlarged, and project upward into 
 
 the lateral ventricles, giving these cav- rigemina -- (or niesencephaion) ; 
 ities their arched form. 
 
 The following table, exhibits the rela- ments - 
 tions, with respect to their development, in which the different 
 parts of the brain stand to its fundamental rudiments : 
 
 c, cerebellum; mo, medulla ob- 
 longata ; ss, spinal cord with 
 its bracnial and crural enlarge- 
 
 f 1. Prosencephalon, 
 I Fore-brain. 
 
 I. Anterior prima- ' 
 ry vesicle. 
 
 2. Thalamencephalon, 
 Inter-brain. 
 
 II. Middle primary J 3. Mesencephalon, 
 vesicle. Mid-brain. 
 
 III. Posterior prima- 
 ry vesicle. 
 
 4. Epencephalon, 
 
 Hind-brain. 
 
 5. Metencephalon, 
 
 After-brain. 
 
 f Cerebral Hemispheres. Cor- 
 pora Striata, Corpus Callo 
 sum, Fornix, Lateral Ven- 
 tricles, Olfactory bulbs. 
 
 Thalami Optici, Pineal gland, 
 Pituitary body, Third Ven- 
 tricle, Optic nerve (prima- 
 rily). 
 
 Corpora quadrigemina, Crura 
 Cerebri, Aqueduct of Syl- 
 vius, Optic nerve (secon- 
 darily). 
 
 Cerebellum, Pons Varolii, 
 anterior part of the Fourth 
 Ventricle. 
 
 Medulla Oblongata, Fourth 
 Ventricle, Auditory nerve. 
 
 1 Taken from Quain's Anatomy (Ninth Edition), IL, p. 828. 
 
210 
 
 EMBRYONIC LIFE OF MAN. 
 
 ra parieto* c 
 
 FIG. 81. Brain of a Six-months Human Embryo. Natural size. 
 (Kolliker.) ol, olfactory bulb ; /s, fissure of Sylvius ; c, cere- 
 bellum ; p, pons Varolii ; /, flocculus ; o, olive. 
 
 The more important convolutions and sulci of the cerebral hemi- 
 spheres (those called "primitive ") result from the folding of the 
 
 whole substance of 
 the wall of the hemi- 
 sphere ; the less im- 
 portant (the so-called 
 "secondary") consist 
 merely of depressions 
 and elevations of its 
 more superficial por- 
 tion. The former ap- 
 pear earlier the first 
 of the primitive sulci 
 being the fissure of 
 Sylvius, which is visi- 
 ble before the end of 
 the third month. By 
 the end of the seventh 
 month almost all the principal features of the cerebral hemispheres, 
 "both convolutions and sulci, are already fixed. 
 
 16. The nervous parts of the eye are differentiations of certain 
 lateral growths of the germinal brain-buds, called the "optic 
 vesicles." The optic vesicles are outgrowths from the sides of the 
 first cerebral vesicle, and are originally connected with it by short 
 and wide stalks ; at first they stand out at nearly right angles to 
 the axis of the embryo. The stalks soon become narrower and thus 
 form the rudiments of the 
 optic nerves ; 1 at the same 
 time the rudiments of the 
 retina are formed from the 
 vesicles themselves. The 
 bulb of the optic vesicle is * 
 made into a cup with two 
 walls by doubling it upon FlG - 
 itself ; thus a second optic 
 vesicle or "optic cup" is 
 produced, as distinguished 
 from the original one. The 
 lens of the eye is made by 
 thickening some of the superficial epiblast and involuting it in- 
 ward over the front of the optic cup, or secondary optic vesicle. 
 
 1 But His and Kolliker suppose these nerves to be formed by secondary em- 
 anation from the chiasm or nervous centre. 
 
 . Longitudinal Sections of the Eye of an Embryo, 
 in three stages. (From Remak.) 1, commencement 
 of the formation of the lens, /, by depression of a part 
 of A, the corneous layer ; u, r, the primitive ocular vesi- 
 cle is doubled back on itself by the depression of the 
 commencing lens. 2, the depression for the lens is now 
 enclosed, with the lens beginning to be formed on the 
 inner side ; the optic vesicle is more folded back. 3, 
 a third stage, in which the secondary optic vesicle, g I, 
 begins to be formed. 
 
DEVELOPMENT OF EYE AND EAE. 211 
 
 This involution has at first the form of a pit, then of a closed 
 sac with thick walls, then of a solid mass. The cavity between 
 the two walls of the optic cup is closed up by bringing the walls 
 into contact. The subsequent development of the different parts 
 of the eye is conditioned upon the fact that the walls of the 
 optic cup grow more rapidly than does the lens, and that their 
 growth does not take place equally in all portions of the cup. It 
 is by changes in the surrounding mesoblast, which takes on the 
 character of an investment, that the outline of the eyeball is defi- 
 nitely formed (the choroid and sclerotic). The vitreous humor 
 also is a inesoblastic product which is supposed to originate as a 
 kind of transudation through the so-called choroid slit. Of the 
 two walls, the inner or anterior is originally somewhat thicker ; 
 and since, in most parts of the cup it grows more rapidly, it con- 
 stantly increases in relative thickness. But just in front of a line 
 which afterward becomes the ora serrata, both layers soon cease 
 to thicken and then completely coalesce ; thus the hind portion or 
 true retina becomes marked off from the ciliary ridges and the iris, 
 while the wide opening of the optic cup is narrowed into a smaller 
 orifice that constitutes the pupil. By differentiations of the inner 
 or anterior wall of the hind portion of the optic cup its cells mul- 
 tiplying rapidly and undergoing morphological changes while the 
 wall is thickening the different layers of the retina are formed. It 
 is a significant fact that in its early stage this wall resembles the 
 brain in its structure, and may be considered as a part of that organ. 
 It is not necessary to enter into a more detailed description of the 
 development of the different parts of the eye. 
 
 17. The ear originally appears on either side of the hind-brain 
 as an involution of the external epiblast, sunk in a mass of the 
 mesoblast. It is then simply a shallow pit with a wide-open 
 mouth. The mouth closes up and the pit then becomes a closed 
 vesicle (the otic vesicle) which is lined with epiblast and sur- 
 rounded by mesoblast. As the walls of this vesicle thicken, its 
 cavity enlarges. The shape of the vesicle is at first nearly spheri- 
 cal, but it soon becomes triangular, with the apex of the triangle 
 directed inward and downward. It is by elongating this apex that 
 the rudiment of the cochlear canal is formed. Part of the vesicle 
 becomes stretched into a long, narrow, hollow process (the recessus 
 vestibuli), and from the outer wall of the main body two protube- 
 rances grow, which are the rudiments of the vertical semicircular 
 canals. These parts of the auditory labyrinth are soon more clearly 
 defined. The cochlear canal is further elongated and curved ; the 
 recessus is also stretched out more ; and from a new protuberance 
 
212 EMBRYONIC LIFE OF MAN. 
 
 the horizontal canal is developed. Another protuberance, which 
 becomes apparent at the inner commencement of the cochlear 
 canal, is converted into the sacculus by being constricted on either 
 side. The rest of the cavity, into which all the other parts open, 
 may now be called the utriculus. Dilatations of the semicircular 
 canals form the ampullae. When the cochlear canal has reached two 
 and a half coils, the thickened epithelium of its lower surface 
 forms a double ridge, from which the organ of Corti is developed. 
 For the details of the structure of the labyrinth we refer to the 
 previous description of this end-organ of sense. 
 
 18. All the coarser differentiations of structure to which refer- 
 ence has thus far been made are only the expression as it were 
 of certain histogenetic changes which have been secretly taking 
 place. The laying down of delicate threads of nervous tissue, the 
 proliferation of nerve-cells along definite lines of movement, have 
 resulted in combining these elements by a living process into the 
 organs of the nervous mechanism. The white matter of the cord 
 is supposed to result from a differentiation of the outer parts of its 
 superficial cells into longitudinal nerve-fibres ; the latter remain, 
 however, for a considerable time without their medullary sheath. 
 The white matter first appears in four patches at the front and 
 back of either side, in which the individual fibres seem like small 
 dots. The gray matter of the cord is formed by a differentiation 
 of the principal mass of the walls of the medullary canal. The 
 outer cells first lose their epithelial-like arrangement, and then 
 become converted into true nerve-cells, with prolongations that 
 constitute nerve-fibres. The early histological character of the 
 parts of the brain which lie back of the cerebral hemispheres is 
 very similar to that of the spinal cord. In the floor of the hind- 
 brain and mid-brain a superficial layer of delicate nerve-fibres is 
 early formed. The cells internal to the nerve-fibres give rise to 
 the epithelial layer which lines the cavities of the ventricles and to 
 an outer layer of gray matter. In the fore-brain the walls of the 
 hemispheres become divided into two layers, between which the 
 fibres of the crura cerebri interpose themselves. The inner layer 
 unites with these fibres to give rise to most of the white matter of 
 the hemispheres ; the outer layer of rounded cells becomes further 
 differentiated into the outer part of the gray matter, which has 
 comparatively few cells, and a deeper layer with numerous cells, 
 the latter forming the principal mass of the gray matter of the 
 cortex. 
 
 19. The preceding description of the outlines of the develop- 
 ment of the human nervous mechanism is derived, for the most 
 
MECHANICAL THEORY OF THE EMBRYO. 213 
 
 part, from the study of other embryos than those of the human 
 species. It is probably, however, substantially true for the latter 
 also. It is valuable for the purposes of Physiological Psychology, 
 chiefly as emphasizing what has already been said concerning the 
 structure and functions of this mechanism in its developed form. 
 The nature of the process by which the nervous system is devel- 
 oped, as well as the nature of the developed structure and its func- 
 tions, as far as physical science can go at all, leads us in the direc- 
 tion 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 hardly 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 mechani- 
 cal 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 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 next problem 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. 
 
CHAPTEK YIL 
 MECHANICAL THEORY OF THE NERVOUS SYSTEM. 
 
 1. THE machine-like nature of much of the structure and move- 
 ment of the human body does not escape the most ordinary obser- 
 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 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, how- 
 ever, we begin to speak of those changes of relative position which 
 take place at extremely minute distances among the molecular ele- 
 
MACHINE AND MECHANISM. 215 
 
 ments 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 
 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 position 
 of so considerable masses of matter is but the summing-up of in- 
 numerable 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 mo- 
 tion, 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 
 
216 THE NEKVOUS SYSTEM A MECHANISM. 
 
 by a physical molecular mechanism. The office of the nerve with 
 respect to the muscle is simpty, 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 condition 
 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. 
 
 2. 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 
 mechanical 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. 
 
THE CONSTITUTION MECHANICAL. 217 
 
 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- 
 planation of the nervous system as well as of all other physical 
 subjects ; and also its right to its persistent faith that So far as 
 physical science can explain any such subject, all the special difficul- 
 ties of the nervous system can be fitly considered only in this way. 
 
 3. 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 Ies3 obnoxious 
 
218 THE NERVOUS SYSTEM A MECHANISM. 
 
 title), is marshalling the minute particles under its superior control. 
 But such way of considering the phenomena whether admissible 
 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 move- 
 ment of physical molecules, strictly under the conditions furnished 
 by their constitution and previous arrangement. 
 
 The general significance of the chemical constitution of ner- 
 vous matter, with reference to a mechanical theory of the nervous 
 system, is by no means wholly obscure. It is obvious that all the 
 energy expended in the movement of the body as a whole, or of its 
 larger masses, originates in minute molecular changes. The latter 
 changes have, of course, a direct relation to the chemical constitu- 
 tion of the tissues in which they occur. The muscular fibre can 
 contract because its molecules admit of that rearrangement in 
 which the contraction essentially consists ; for doing the amount of 
 work implied in such rearrangement, this fibre is, of course, depen- 
 dent upon its own chemical constitution. But the source of the 
 excitation of the muscle is to be found in antecedent molecular 
 changes within the nervous system ; indeed, all the changes that 
 are to be summed up in the work done by and within the rigid 
 masses of the body have their origin here. It accords, then, with 
 the mechanical conception of the nervous system that its chemistry 
 should be just such as we have seen that it actually is. Nervous 
 matter holds in store a large amount of energy that is easily dis- 
 posable ; of energy that will be yielded freely and rapidly if any- 
 thing occurs to start the process within the system of molecules 
 of which such matter is composed. For the molecules are of such 
 kind as readily break up and recombine their elements in simpler 
 forms ; in doing this they render kinetic a large amount of energy 
 which they have previously held latent. 
 
 No mechanical theory of the nervous system can explain the 
 meaning of all the various structural forms which the elements of 
 this system assume. It cannot be told, for example, what peculiar 
 place in the mechanism belongs to the different shapes of nerve- 
 cells, bipolar, multipolar, stellate, etc. Nor can a complete picture 
 be drawn of the differences in character which the nerve-commotion 
 takes as it passes from the nerve-fibres to the nerve-cells, or from 
 one nerve-cell to another. We can only insist upon the undoubted 
 general fact that all these structural forms have whatever signifi- 
 cance belongs to them, because they are themselves molecular 
 structures, capable of undergoing, in relation to each other, those 
 
THE ARKANGEMENT MECHANICAL. 219 
 
 very changes in which the functional activity of the nervous system 
 consists. 
 
 4. 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 
 environment, 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 
 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 redistributed 
 so as to give rise to outgoing molecular disturbances along definite 
 tracts, in order that definite groups of muscles may be made to con- 
 tract. Only in this way can the whole physical organism, by a so- 
 
220 THE NERVOUS SYSTEM A MECHANISM. 
 
 called reflex activity, adjust its condition, in view of the presence of 
 given kinds and degrees of stimuli. Moreover, the vital functions 
 the movements that control respiration, digestion, circulation 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 re- 
 spective energies, which the changing circumstances require. Still 
 further, not only must the neural processes set up by the end-or- 
 gans and conducted inward by the afferent nerves have a place of 
 meeting in proximity with the centres of origin for the correspond- 
 ing efferent impulses ; but all the neural processes in this place of 
 meeting must also be so modified and made mutually dependent 
 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 won- 
 derful reflex and automatic activities. In their highest form the 
 hemispheres of the human brain they solve the problem of pro- 
 viding 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 cannot be 
 pretended that even a beginning has been made toward a satisfac- 
 tory theory of their functional activity considered as a special case 
 in molecular physics. But this fact does not affect the confidence 
 which is based upon what is known of physical structures in gen- 
 eral, that in these organs, the changes which take place are essen- 
 tially of the same order as are those with which the science of mo- 
 lecular 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 mechanism. 
 
 5. 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 
 
THE EQUILIBRATING MECHANICAL. 221 
 
 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. 
 
 To accomplish the general work of equilibrating the interaction 
 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. 
 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 
 
222 THE NERVOUS SYSTEM A MECHANISM. 
 
 them behaving in such a manner as to make a plain demand for a 
 physical and mechanical theory in explanation of their behavior. 
 
 6. 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 ? 
 
 In answer to the question just raised we have already seen how 
 little satisfaction is afforded on applying to the science of chemistry. 
 On general principles of physical science there can be little doubt 
 that the excitation and conduction of nerve-commotion is dependent 
 upon a chemical change in the nervous tissue itself. Moreover, we 
 know that the process of conduction in the nerve requires each of 
 its molecules to act upon the neighboring elements as the condition 
 of the process continuing. Nor can this process itself be a mere 
 impartation of motion, from molecule to molecule ; on the contrary, 
 the phenomena of electrotonus seem to show that it must also con- 
 sist in the setting free of energy which exists latent within the 
 molecules of the nerve-substance. These molecules contain, then, 
 by virtue of their constitution, stored or potential energy which is 
 converted into kinetic energy in the propagation of the process of 
 excitation, and which is expended, in part, in either inhibiting or 
 increasing the energy of that process. Such potential energy can 
 scarcely be other than chemical. 
 
 Accordingly, we should be tempted to describe the process of 
 progressive excitation of the nerve somewhat as follows : Every 
 element of the nerve, by reason of its highly complex and unstable 
 chemical constitution, contains a large store of energy ; the excite- 
 ment of the nerve consists in the explosive decomposition succes- 
 sively of these elements of the nerve ; and the result of the decom- 
 position is the setting free of the stored energy to be expended in 
 part in the excitation of the next adjoining elements. The process, 
 then, is not altogether unlike the burning of a line of powder 
 grains. Such an hypothesis, however, would at once have to answer 
 several difficult questions. "Why does not the whole of the explosive 
 
INTERFERENCES OF NERVE-COMMOTION. 223 
 
 substance burn up, instead of only an amount of it approximately 
 proportional to the strength of the stimulus which sets the process 
 agoing ? Analogies may indeed be found in the union of chlorine 
 and hydrogen jinder the action of light. What checks the process 
 in the nerve as a whole, and what limits it quantitatively in a differ- 
 ent way at different points in its course, so as to give the phenomena 
 of anelectrotonus and catelectrotonus ? (comp., Chap. III., 19 f.). 
 Moreover, direct observation has as yet discovered no indisputable 
 evidence of functional chemical changes in the nerve-fibres. If 
 such changes exist at all they are exceedingly small. 
 
 7. Allusion has been made (p. 119 f.) to the fact that the effect 
 of several excitations of a nerve-stretch is compounded, as it were, 
 in the action of the attached muscle. That is to say, excitations 
 which are simultaneous, or which follow each other with sufficient 
 and not too great rapidity, are summed up in the nerve, like mo- 
 lecular waves of nerve-commotion piled upon each other. Besides 
 such phenomena of " summation," there exist analagous phe- 
 nomena of so-called " interference ; " and, further, of the facilitat- 
 ing effect which one excitation has upon others following it along 
 the same paths of conduction, especially in the central organs. 
 These and similar phenomena tempt us to consider the activity of 
 the nervous substance in terms of an exceedingly complex sum in 
 the addition and subtraction of molecular disturbances of a wave- 
 like character. Elaborate experiments have been made to deter- 
 mine the laws under which such summation or interference of 
 electrical excitations takes place. Thus G. Valentin 1 assumes that 
 the case of the nerves comes under the general theory of molecular 
 waves that may either be piled upon each other, or may interfere 
 with each other. The interferences he calls " positive " when the 
 currents are moving in the same direction, " negative " when they 
 are moving in opposite directions ; and such currents may, of 
 course, be either ascending or descending. The character of the 
 interferences is to be denned by the way in which the nerve-muscle 
 machine responds to these four kinds of interference. The inter- 
 ference has a heightening effect (is erhohende) when the result 
 indicated by the behavior of the muscle is greater than the sum of 
 two single effects from the partial excitations that are compounded ; 
 a depressing effect when the result is less than this sum. If the 
 effect of the interference is such as to reduce the result to zero, 
 it is called inhibitory. Valentin concludes that, in case of inter- 
 ferences of excitations from one and the same current (with respect 
 
 1 In Pfluger's Archiv, vii. (1873), pp. 458-496, article on the Interfer- 
 ences of Electrical Excitations. 
 
224 THE NERVOUS SYSTEM A MECHANISM. 
 
 to degree, direction, etc.), the character of the effects produced 
 depends upon the original molecular constitution of the nerve. 
 Just as its constitution is decisive with regard to the nature of the 
 muscular contractions that follow a single excitation of the nerve, 
 so is it also decisive with regard to the results of interference. 
 These results, moreover, conform to the same laws after decapita- 
 tion or poisoning as before. And further, the same rules govern 
 in the case of interferences of two currents, if both the currents 
 are of about the same degree of strength. Finally, according to 
 Valentin, the same rules belong to the interferences that occur in 
 cases of reflex action, or of the stimulation of motor nerves through 
 the sensory, as those which apply to the direct stimulation of the 
 motor nerves. It is apparent that the only net gain from the fore- 
 going experiments consists in the information that the molecular 
 constitution of the nerves themselves determines all the variable 
 elements in the results of exciting them. But this would be an 
 assumption fairly made by every attempt at a physical science of the 
 nervous functions. And inasmuch as we can make no such veri- 
 fiable statements concerning the nature of this molecular constitu- 
 tion as will serve the purposes of a precise mechanical theory, it is 
 hard to see what advance has been gained toward the construction 
 of such a theory. 
 
 The phenomena caused by the reciprocal action of different ex- 
 citations within the central nervous system are, of course, much 
 more complex and difficult to bring under a theory of molecular 
 wave-like impulses, than are the phenomena of the comparatively 
 simple nerve-muscle machine. A fortiori, molecular physics is 
 unable to propose a satisfactory theory for the central organs. 
 According to Exner, 1 many of the phenomena are covered by the 
 general principle that one excitation acts to facilitate or, as it were, 
 smooth the path for others passing, after only a brief interval, along 
 the same course. This principle he distinguishes from that of 
 < c summation, " when applied to reflex action. The latter term Exner 
 applies to the accumulation in the central organ of excitations 
 which, taken singly, are too weak to produce any reflex motion, but 
 which by their combined strength do produce such motion. The 
 principle of " facilitation," however, refers to the condition of the 
 central parts after the passage through them of a stimulus which 
 has already called forth some reflex action. Exner's experiments 
 led him to conclude that the motor excitation of some one ex- 
 tremity from the brain (that is, by stimulating, in the brain, the 
 so-called motor area of the extremity) facilitates the subsequent pas- 
 1 Article in Pfluger's Archiv., xxviii., pp. 487 ff. 
 
PHYSICS OF THE CENTRAL ORGANS. 225 
 
 sage of reflex stimulus affecting the same extremity ; and, con- 
 versely, stimulating an extremity reflexly facilitates the passage of 
 a subsequent motor excitation from the area of the brain to the 
 same extremity. Thus, for example, the reflex motions of the fore- 
 leg of a rabbit, produced by stimulating the toes of that leg, were 
 found to be increased in intensity if the so-called cerebral motor 
 centre of the fore-leg was also stimulated. Different reflex excita- 
 tions also may facilitate each other's effect in the same way. For 
 example, the sensory stimulation of the left foot has the effect of 
 facilitating the reflex act which, as it might appear, would relate 
 only to the right foot and its motor area in the central organ ; and 
 such reflex action of the right foot facilitates the contraction set 
 free in the same foot by stimulating the left-foot section of the spi- 
 nal cord. Exner was unable, however, to obtain any inhibitory 
 effect upon the motion of the extremities by stimulating various 
 other places of the cortex of the cerebrum, or by stimulating the 
 cerebellum. He also found that when one side of the cortex of the 
 cerebrum is stimulated by electricity so as to produce a condition 
 of tetanus in one extremity of the animal, the results of two excita- 
 tionsone as a reflex from the foot and one directly from the same 
 side of the brain are compounded in a way which seems incom- 
 patible with any known form of the summation and interference 
 of molecular wave-like disturbances. 
 
 Indeed (to return to the simpler case), Griitzner ' seems justified 
 in saying that, strictly speaking, we cannot without qualification 
 even represent what takes place when two currents of electricity 
 act in combination upon a nerve, as though it were a matter of the 
 addition or subtraction of their separate effects. For it is possible 
 that an electrical current of an intensity equal to the amount of the 
 natural nerve-current (current of rest = a) and the current used as 
 stimulus (current of action = b), taken together (a + b), will not ex- 
 cite a nerve that shows no current at all, although the latter (b) 
 alone will excite the nerve if just previously the former (a) was 
 present in the nerve. The currents already existing in the nerve, 
 when the exciting current is applied, are, therefore, not simply 
 added to or subtracted from the latter ; but they produce molecu- 
 lar changes of an unknown kind which tend to induce the origina- 
 tion of so-called "cathodic" and "anodic" places in the nerve 
 that is, places of exalted and places of depressed excitability. 
 Thus a weaker current will excite the nerve when it is in a condi- 
 tion of exalted excitability ; a stronger current may fail to excite 
 the nerve when in a condition of depressed excitability. 
 
 1 See Pfliiger's Archiv, xxviii., p. 144 f. 
 15 
 
226 THE ELECTRICAL THEORIES. 
 
 How obscure and complicated are the molecular conditions con- 
 nected with the excitation of the nerve is further shown by the 
 effect of giving different treatments to the cross-section of the nerve. 
 If the nerve is simply cut, its behavior under stimulation is differ- 
 ent from that which occurs when it has been bound before the 
 cross-section is made. Binding the nerve produces, for some min- 
 utes after cross-section, a large increase of its excitability in the 
 immediate neighborhood of the injured place ; this is true for all 
 kinds of stimuli, including the electric current in both directions. 
 From five to ten minutes subsequently, however, the making of the 
 current in the opposite direction to the current induced by cross- 
 section has frequently a diminished rather than an increased effect. 
 
 8. On the whole, it would appear, then, that the ability to lay 
 even a basis for a strictly scientific molecular theory of the nervous 
 mechanism depends upon the ability satisfactorily to explain the 
 electrical process in the nerves and their consequent behavior under 
 electrical stimulation. It would by no means follow that a com- 
 plete theory for the comparatively simple phenomena of the nerve- 
 muscle machine would furnish the sure clew, not to say the full 
 explanation, of all the activities of the nervous system. On the 
 contrary, the evidence is overwhelming that the working of the 
 complete nervous mechanism involves other principles than those 
 which may be deemed sufficient for the case of the single nerve 
 and muscle when under electrical stimulation. But, plainly, the 
 more complex case cannot be solved without first solving the far 
 less complex one. And yet the simplest possible case of nervous 
 molecular mechanism the case that can be brought under the 
 most favorable experimental conditions has thus far proved to 
 lie beyond our power to find a satisfactory scientific solution. 
 
 The two most important principles which must enter into any 
 mechanical theory for explaining the behavior of nerves in relation 
 to electricity are, according to Hermann : ' (1) the law of electrical 
 excitation, and (2) the law of the so-called current of action. The 
 phenomena upon which these laws are themselves based are chiefly 
 the phenomena of electrotonus and the phenomena of negative 
 variation. 
 
 It is a fact (see p. 114 f.) that the passage of the electrical current 
 through a nerve-stretch produces in the nerve a changed condi- 
 tion of excitability called electrotonic. This condition is, however, 
 different for different parts of the nerve-stretch. It is dependent 
 upon the nearness of each part to the electrodes, it being greatest 
 in their vicinity. It is dependent on the strength of the polarizing 
 1 Handb. d. Physiol., II., i., p. 193. 
 
THEORY OF DU BOIS-KEYMOND. 227 
 
 current and on the length of the stretch through which it flows. 
 Its intensity is greater on the side of the anode than on the side 
 of the cathode. The condition may be said to be one of increased 
 excitability in the region of the cathode, of diminished excitability 
 in the region of the anode. Helmholtz found that the time of the 
 development of the electrotonic condition is not perceptibly later 
 than that of the electrical current which excites it ; the condition 
 originates at the moment of making, and ceases at the moment of 
 breaking, the polarizing current. Du Bois-Beymond concludes, 
 thereupon, that the electrotonic condition is spread over the nerve 
 with a speed equal to that of the process of excitation. 
 
 It is also a fact (see p. 117 f.) that, in the case of the nerve-stretch 
 as well as in that of the muscle, the galvanometer shows the pas- 
 sage of a current when one of the electrodes is placed at its cut 
 end and the other at its equator. It is a fact that this so-called 
 natural current, or current of rest, is diminished by the stimula- 
 tion of the nerve with an interrupted current, or by other means 
 of exciting it the diminution being shown by the return of the 
 needle of the galvanometer toward the zero-point (the so-called 
 " negative variation "). 
 
 9. The two principal theories which have hitherto attempted 
 to account for the above-mentioned facts are the theories of du 
 Bois-Reymond and of Hermann. Du Bois-Beymond * assumes 
 that in the substance of the nerve there exists an arrangement of 
 electro-motive molecules embedded in an imperfectly conducting 
 medium. Each molecule is like a minute battery with positive and 
 negative poles ; and the molecules present their positive surfaces to 
 the longitudinal surface of the nerve, their negative surfaces to the 
 cut ends or transverse sections of the nerve. The presence of these 
 molecules gives rise to currents in the medium which surrounds 
 them. Owing to the imperfect conductivity of the medium, such 
 currents flow in more or less concentric lines at some distance 
 from each molecule. The current which exists in the nerves 
 (exists, according to du Bois-Beymond, as natural to the nerve and 
 previous to its injury by cross-section), and which is made obvious 
 by the deflection of the needle of the attached galvanometer, may 
 therefore be regarded as the resultant of the numerous unobserv- 
 able currents belonging to the several molecules. In this way the 
 so-called " current of rest " is to be explained. Du Bois-Beymond 
 is forced to account for the fact that such natural currents are 
 
 1 The views of du Bois-Reymond are to be found in his Untersuchungen 
 liber thierische Electricitat, 1848-49, and Gesammelte Abhandlungen, etc., 
 
 1875-77. 
 
228 THE ELECTRICAL THEORIES. 
 
 either exceedingly small or wholly wanting in an uninjured mus- 
 cle by a very artificial hypothesis as to a so-called parelectronomic 
 region at the place where the ends of the muscle come into contact 
 with the tendons. His theory of electrotonus and of the negative 
 variation of the nerve-current is too complicated and doubtful to 
 be even stated here ; it is enough to say that his assumptions as to 
 "peripolar " and "bipolar" molecules, and the effect of the elec- 
 trical current in reversing the molecules, etc., have little to com- 
 mend them to the practical workers in modern physics. 
 
 10. The theory which Hermann, 1 and those who agree with 
 him, would substitute for the theory of du Bois-Reymond takes 
 its point of starting from a discovery made by Matteucci some 
 years ago. In 1863 this truly great investigator noticed phenom- 
 ena similar to those of the electrotonic condition of the nerves in 
 over-spun wires moistened with a conducting fluid. If an electri- 
 cal current is conducted to the moist covering of such a wire, the 
 needle of the galvanometer shows along every part of the wire the 
 presence of a current in the same direction with the primary cur- 
 rent, but with the strength of the former diminishing as the dis- 
 tance increases from the points where the latter is applied to the 
 wire. No such current arises, however, if the wire is made of amal- 
 gamated zinc and its covering is moistened with a solution of sul- 
 phate of zinc. It appears, then, that the electrical condition of the 
 wire, when a current is conducted to it, depends upon the limiting 
 surfaces of its metal centre and of its moistened covering being po- 
 larizable. Very recently 2 Hermann has, as he thinks, still further 
 shown the possibility of explaining all the electrotonic properties 
 of the nerves after the analogy of Matteucci's discovery. A con- 
 ductor consisting of a central and a covering substance, with polar- 
 izable limiting surfaces, as soon as a momentary electric current is 
 conducted through any portion of it begins successively to exhibit 
 a current of the same kind at every other place in it ; the more 
 distant the place from the one to which the current is applied the 
 later its appearance there, so that at the most distant places such 
 current may begin after it has for some time ceased at the primary 
 place. Now, in an analogous manner, every nerve-fibre may be as- 
 sumed to consist of a centre and covering substance, with polarizable 
 limiting surfaces. In the nerve-fibre the limiting surfaces needed 
 for the theory are perhaps actually to be found between the axis- 
 
 1 The views of Hermann may be found in his Untersuchungen zur Physiol- 
 ogie d. Muskeln u. Nerven, 1867-68, and in numerous later papers in Pfliiger's 
 Archiv. 
 
 2 Pfluger's Archiv, 1885, xxxv., p. 23 f. 
 
THE THEORY OF HERMANN. 229 
 
 cylinder and the medullary sheath. Griinhagen, 1 however, affirms 
 that the polarization of the limiting surfaces of the nerve-fibre is a 
 consequence rather than a cause of the current in electrotonus. 
 The first and fundamental cause of this current he considers to be 
 the characteristic difference in the resistance, as conductors, of the 
 kernel and the covering of the nerve-stretch. 
 
 The so-called "natural current," or "current of rest," Hermann 
 does not consider it necessary to explain. What appears to be a 
 natural current Hermann holds to be in all cases the result of in- 
 jury. It is to be considered, then, as due to the peculiar molecu- 
 lar condition into which certain parts of a nerve-stretch are thrown 
 by their mechanical or chemical destruction. In fact, whenever a 
 nerve is cut across, or any of its fibres are injured, the molecules 
 thus disturbed begin at once to die ; they then become negative to- 
 ward the other uninjured parts of the nerve. It is because of this 
 change in the dying molecules that the electrical current is devel- 
 oped. But all the parts of a wholly untouched and unexcited nerve 
 are, according to Hermann, " isoelectric." It is not necessary to 
 give the experimental evidence by which this investigator strives 
 to prove his opinion ; it is enough to say that this evidence is 
 strong, and nearly, if not quite, conclusive. 
 
 Accordingly, Hermann regards the negative variation as not due 
 to the diminution of any current previously existing, but rather as 
 a manifestation of the electro-motive forces which come into opera- 
 tion at the moment, and at the seat, of excitation. This current is, 
 therefore, the only true " current of action." Its rise and flow are 
 explained by the fact that every excited part of a nerve-stretch be- 
 comes negative toward all the other parts. As this wave passes 
 along the nerve-fibre, each minute portion becomes first negative 
 and then positive toward the adjoining minute portions ; and hence 
 the so-called "ad-terminal" and "ab-terminal currents" appear 
 along the nerve-stretch as fast as successive parts of its substance 
 reach their maximum of negativity. The excess of the ab-terminal 
 over the ad-terminal current manifests itself as the so-called " neg- 
 ative variation." 
 
 The phenomena of electrotonus Hermann explains, as has already 
 been said, upon the basis of Matteucci's experiments. An inner 
 polarization, such as takes place between the wire and its moist- 
 ened covering, takes place between the substance which constitutes 
 the core of the nerve and one of its sheaths. The electrotonic cur- 
 rent is, therefore, simply due to an escape of the polarizing cur- 
 rent. It is wanting in the dead nerve, because the inner polariza- 
 1 Pfluger's Archiv, xxxv., p. 534 f. 
 
230 THE ELECTRICAL THEORIES. 
 
 tion belongs only to the nerve in its living state ; it is stopped by 
 ligature or by crushing, because the nervous substance is thus 
 made into dead, indifferent substance, and the functional continu- 
 ity of the nervous core is destroyed. His detailed explanation of 
 " tetanic action -currents " and " phasic action-currents," and of the 
 physiological phenomena of electrotonus and catelectrotonus, need 
 not be repeated. The one principle to which Hermann would re- 
 duce all the electrical phenomena derived from the cut nerve- 
 stretch is this : All excitable protoplasm, when dying or irritated, 
 becomes negative toward its own uninjured and unirritated parts. 
 Such is the nature of its electro-motive reaction. 
 
 11. Objections have been made to the theory of Hermann, but 
 they can scarcely be said to be so formidable as those which he 
 brings against the theory of du Bois-Eeymond. The most forcible of 
 them is, perhaps, the following : If the so-called currents of rest were 
 due solely to the negativity of the dying portion of the substance, 
 we should not expect that the current from the equator to the cross- 
 section would be greater than the current from a point nearer the 
 cross- section, seeing that the resistance is greater in the former case. 
 
 Hermann is himself ready to admit, 1 however, that no simple 
 scheme of polarization will fully satisfy the conditions of the prob- 
 lem offered by the behavior of the nerve-muscle machine under 
 electrical stimulation. " The platinum wire, with its moist sheaths, 
 is no model of the irritable nerve ; it is only a model of its elec- 
 trotonic properties." We must, therefore, after the discussion of 
 all analogies resort again to the unknown molecular constitution 
 and properties of the substance of the nerve, as being sui generis, 
 for our explanation of its peculiar physiological properties. Its 
 functions are a species of molecular change, connected, to be sure, 
 both with chemical changes and with other mechanical changes of 
 a wave-like character, and yet unlike them all ; and these molecular 
 changes, when the nerve is excited, are propagated from point to 
 point along its course with a speed and according to laws which 
 have already been stated (see Chapter III.). But further than this 
 we cannot as yet go with confidence in affirming a mechanical the- 
 ory of even that simplest element of the nervous mechanism for ex- 
 perimental purposes namely, the nerve attached to the muscle 
 and constituting the nerve-muscle machine. 2 
 
 1 See Handb. d. Physiol., II., i., p. 195 f. 
 
 8 Further information upon the two theories of Hermann and du Bois-Rey- 
 mond may be found in Foster's Text-book of Physiology, pp. 101 ff. See, also, 
 a brief statement of Hermann's theory in the Journal of Physiology, I., pp. 
 196-212. 
 
THE THEORY OF WUNDT. 231 
 
 12. A confession of ignorance might fitly close the entire dis- 
 cussion as to the possibility at present of a precise mechanical 
 theory of the nervous system. For on resuming the larger and 
 more complicated inquiry, as to how the physiological functions of 
 all the nervous organs in their mutual relations may be explained 
 according to any known laws of molecular science, we are obliged 
 to approach this inquiry with an acknowledged inability to deal 
 satisfactorily even with the much simpler case of one of the ele- 
 ments of this system. The peculiar forms and laws of the molec- 
 ular activity of the entire nervous mechanism certainly cannot be 
 understood until we are able to describe and explain the molecu- 
 lar activity of a single nerve-muscle machine. A statement of an 
 elaborate theory, framed with a view to meet the whole case, by a 
 distinguished authority, cannot fail, however, to possess a certain 
 interest and value. Accordingly, we shall refer briefly to the 
 theory of Wundt. 1 
 
 Wundt begins his discussion of the mechanics (or molecular 
 physics) of nervous substance by stating two possible ways of ap- 
 proaching the subject. It is conceivable that we might directly 
 investigate the chemical and physical constitution of the nervous 
 elements, and the changes they undergo in the exercise of their 
 physiological functions, with a view to construct a theory of so- 
 called nerve-force by induction from such investigation. But the 
 preferable because the much more promising way of procedure 
 is to assume that the processes which take place in the nervous 
 system are modes of molecular motion connected with each other, 
 and with the forces of external nature, under the general principles 
 of molecular physics ; and then, arguing deductively, to make such 
 a combination and application of these principles as will serve to 
 meet all the demands of the case. It scarcely need be said that 
 Wundt adopts the latter method. 
 
 Assuming, then, the general principles of molecular physics, and 
 especially the law of the conservation of energy, it is possible to 
 show how living beings may be brought under the control of these 
 principles. Such beings, through the regularity with which the 
 making and breaking of chemical combinations goes on within 
 them, take a noteworthy part in the continuous process of inter- 
 changing potential and kinetic (inner and external) energy. It is 
 the nervous system, in all the animals that have one, from which 
 
 1 To be found, in part, in his Untersuchungen zur Mechanik der Nerven, 
 and in later and more complete form in the chapter (vi., Part I.) " Physio- 
 logische Mechanik der Nervensubstauz," in his Grundziige der physiologi- 
 schen Psychologic. Leipzig, 1880. 
 
232 THEORY OF MOLECULAR ENERGIES. 
 
 this process is controlled. The process itself is a species of com- 
 bustion ; the nervous system keeps going those functions which 
 effect the process, regulates the setting free and distributing of 
 the heat, and determines the muscles to movement. The source 
 of the special activities of the nervous system itself lies in the nat- 
 ure of the chemical combinations which compose it. 
 
 The nervous system regarded as unaffected by stimuli that is, 
 as unexcited may be theoretically compared to a fluid in a condi- 
 tion of equilibrium. But, in fact, the nervous system is never hi a 
 condition of perfect equilibrium. For, not only is there a ceaseless 
 play of energy internal to this system, in which the atoms separate 
 from the old combinations as nervous substance to form new com- 
 binations as the same substance ; but a continuous process also 
 goes on by which the molecules of the nervous substance are broken 
 up to form less complex but more stable compounds. Moreover, 
 the building of the nervous substance itself out of the nourishment 
 brought to it is a process the reverse of that last mentioned ; it 
 is a process, that is to say, in which the more stable chemical com- 
 pounds of other substance are broken up and their atoms used to 
 form the more complex but more unstable molecules of the nervous 
 substance. The process of change from the less stable to the more 
 stable combinations represents the setting-free of stored or poten- 
 tial energy ; the reverse process represents the storing of energy 
 and the vanishing of kinetic or actual energy. That energy which 
 is made apparent by the former process "Wundt calls " positive ; " 
 that which is stored up, when the more stable combination disap- 
 pears, he calls "negative." Positive molecular energy of the ner- 
 vous system is recognized as heat set free, as contraction of the 
 muscles, etc. ; its negative molecular energy exists in the form of 
 heat becoming latent, or of inhibitory action upon the course of 
 excitation in the nerves, etc. 
 
 In accordance with the foregoing theory of positive and negative 
 molecular energy, as due to the chemical activity of the nervous 
 substance, Wundt would explain the process of excitation and con- 
 duction in the nerve-fibres. No simple conduction of motion, of 
 course, takes place in the nerve ; but certain molecular processes, 
 peculiar to the constitution of the nerve, are set up at one point by 
 the stimulus, and are then conducted successively to other points 
 along its stretch. In all cases when a nerve is irritated two classes 
 of opposed effects are set up in its substance ; the one is directed 
 toward the production of external energy (secretion, stimulation of 
 the ganglion-cells, movement of the muscles, etc.), the other toward 
 the control of the energy thus set free. The former is positive, 
 
POSITIVE AND NEGATIVE ENERGY. 233 
 
 the latter negative or inhibitory. The general law for all excita- 
 tion of the nerves is, that by the application of stimulus the posi- 
 tive as well as the negative molecular energy of the nervous sub- 
 stance is increased. Stimulating the nerve accelerates both the 
 recombination of the atoms of its highly complex molecules in 
 less complex but more stable forms, and also the escape of the 
 atoms from these forms and their return to the more complex and 
 less stable combinations. The renewal of the nerve depends upon 
 the restitution of the more complex molecules ; but the work 
 which the nerve does external to itself depends upon that process 
 of combustion in which the complex molecules break up and pass 
 into more stable but less complex forms. The latter process 
 involves, of course, the exhaustion of the nerve. External energy 
 (work done outside of the nerve) can then only take place in case 
 the positive molecular energy is more accelerated than the negative, 
 by the application of the stimulus. 
 
 The entire sum of positive molecular energy which is set free 
 when a nerve is irritated may be reckoned as distributed in three 
 directions : a part is spent in the continuous excitation of the 
 nerve ; another part becomes heat ; still another part is converted 
 into negative molecular energy. In this way the peculiar molec- 
 ular condition which the nerve-fibre leading from the peripheral 
 region assumes, when it is irritated, is imparted to the central re- 
 gion of the nerve-cell. 
 
 13. The application of the foregoing theory to the central 
 organs of the nervous mechanism requires us to take account of 
 the fact that a greater intensity of the stimulus is needed to move 
 a muscle through a collection of ganglion-cells than directly by 
 stimulating the nerve-fibre connected with the muscle. We are to 
 conclude, then, that the nervous substance of the central parts 
 offers a far greater resistance to the conduction of the process of 
 excitation than is offered by the nerves themselves. On the other 
 hand, the central organs are in a condition to develop within them- 
 selves a far greater amount of work ; that is, to convert into 
 kinetic form a vast sum of energy stored in their chemical con- 
 stitution. The proofs which Wundt brings forward for this view- 
 are derived from the phenomena of summation of inhibition, and 
 of so-called "reflex-poisons," etc. A detailed discussion of such 
 phenomena leads to the conclusion that, when " summation " (com- 
 pare pp. 223 ff.) takes place, the several excitations along the cen- 
 tripetal tracts have been conducted to different sensory central 
 regions, and have then passed from them, as a result of their being 
 simultaneously excited, over into the same motor elements of the 
 
234 THEORY OF MOLECULAK ENERGIES. 
 
 central organ; but when "inhibition " takes place, such excitations 
 have been conducted so as to come together and counteract each 
 other in the same sensory central region. The external conditions 
 of those relations which obtain among the different senses and 
 sensations are to be found, partly in the constitution of the organs 
 of sense, and partly in the nature of their respective stimuli. 
 
 When speculating as to the molecular changes, with respect both 
 to positive and also to negative energy, which take place in the 
 central organs, our point of starting must be taken from a condition 
 of equilibrium assumed to exist in their ganglion-cells. Excitation 
 of the central organs, like irritation of the nerves, increases both 
 kinds of nervous energy. But the positive molecular energy of the 
 central organs is relatively little increased by a momentary excita- 
 tion. The result of repeated excitation, however, is to make the 
 positive condition largely predominate in the whole central region. 
 An excited ganglion-cell is in a condition analogous to that of the 
 nerve-stretch at the anode when a constant current is passing 
 through it. In the nerve, as a rule, the nervous substance is used 
 up, and the process of storing energy goes on in only a very par- 
 tial manner. In the cells the production of the complex molecules 
 in which energy is stored predominates, as a rule. 
 
 The fundamental properties of nervous matter mechanically 
 considered are (1) to receive external impressions in order by 
 them to be determined in its own molecular condition ; and (2) to 
 transform potential energy into kinetic, partly under the immedi- 
 ate, and partly under the progressive, influence of these impres- 
 sions. 
 
 Wundt also proposes an elaborate and highly speculative view 
 of the molecular constitution and functions of the ganglion-cells. 
 Every such cell possesses, he thinks, two regions (although the 
 word " regions " is not to be interpreted locally). These regions 
 are called " peripheral " and "central," because the former is as- 
 sumed to stand in more intimate relations to the peripheral ner- 
 vous substance, with respect to its own reactions under stimula- 
 tion. Excitations which reach the central region of a ganglion-cell 
 induce a propagation of the processes set up in this region to the 
 other or peripheral region. In the same way do excitations which 
 first touch the peripheral region necessitate the spreading of the 
 form of molecular energy set free here over into the central region. 
 When a process of excitation is frequently conducted in a definite 
 direction through some ganglion-cell, such direction is favorably 
 disposed toward the conduction of future excitations which may 
 reach the same cell. Whether the excitation of any particular 
 
ACTION DEPENDENT ON STRUCTURE. 235 
 
 nerve-fibre connected with a ganglion-cell results in an excitatory or 
 an inhibitory effect depends upon the nature of its connection with 
 the cell. 
 
 But we refrain from further statement of a theory so largely con- 
 jectural. Nothing remains but to repeat a confession of igno- 
 rance and of inability even to suggest a satisfactory solution for so 
 complex a problem in molecular physics as is offered by the human 
 nervous system. 
 
 14. A review of various molecular theories proposed to account 
 for the nervous mechanism, either as a whole or in any of its parts, 
 makes plain the important fact that such theories are all obliged 
 to assume the origin and continuance of a peculiar molecular 
 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 severed 
 from the ganglion-cell. Both cell and nerve must, therefore, con- 
 stitute a living molecular unity, in order that their normal physio- 
 logical 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 an 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. 
 
 15. 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- 
 
236 THEORY OF MOLECULAR ENERGIES. 
 
 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- 
 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 mech- 
 anism would exercise its functions in ways thus far described. 
 But the consideration of another class of phenomena is now to be 
 introduced ; 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 THE MIND. 
 
CHAPTER I. 
 THE LOCALIZATION OF CEEEBEAL FUNCTION. 
 
 1. ORDINARY observation recognizes the fact that the phenomena 
 of consciousness are more or less definitely correlated Avith the 
 condition of the body. Certain alterations in our mental states, on 
 account of the injury of any of its masses, as well as a constant de- 
 pendence 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 ner- 
 vous substance has any peculiar relation to the thoughts and feelings 
 of the mind. For the functions of the nervous system are not ex- 
 ercised in giving information as to itself, its own condition and 
 changes. By aid of these functions we have presented in con- 
 sciousness 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 furnish 
 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. 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 '' and Theophrastus a inform us of the opinion of the 
 
 1 De Placitis Philosophorum, IV. , 17, 1. 
 
 2 De Sensu, 25 f. 
 
240 GENERAL FUNCTION OF THE BRAIN. 
 
 physician Alcmaeon, 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 
 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 Exneiy 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. Concerning the contents of the cranial cavity, indeed, 
 we get no direct information from the feelings connected with 
 the exercise of its functions. But we certainly localize in the head 
 certain phenomena of consciousness that are inextricably inter- 
 woven with the processes of thought. The act of attention 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 powers of memory and reasoning ; or stroke it to relieve the 
 disagreeable sensations which follow severe mental excitement. 
 Headache, of more or less intensity, thus becomes associated with 
 
 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. 
 
NEED OF ARTERIAL BLOOD. 241 
 
 active exercise 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 
 testimony of immediate feeling is, doubtless, somewhat exaggerated 
 in an age so distinctively " nervous" as our own ; and this fact may, 
 perhaps, account in part for the inclination of the ancients to em- 
 phasize the more obvious connection of mental phenomena with the 
 heart, and other lower visceral organs, to the neglect of all connection 
 of these phenomena with the brain. 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. The 
 intimate local connection between the organs of sense and the 
 brain leads naturally to the conclusion that the avenues of sensa- 
 tion and of perception have in the latter a kind of gathering-place, 
 as it were. 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 in- 
 quiry which probably led certain ancient anatomists, like Herophilus 
 and Galen, to locate the soul, or psychical principle, in the brain. 
 
 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. 
 It has already been stated (Part I., Chapter HE., 7) that 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 ; 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 dis- 
 turbances and even complete cessation of consciousness. It has 
 been calculated that, while the weight of the entire encephalon is 
 16 
 
242 GENERAL FUNCTION OF THE BRAIN. 
 
 only about one-forty-fifth of that of the body, the supply of blood 
 used up there is not less than about one-eighth of the whole supply. 
 This expenditure is indicative of the large amount of work done 
 by the intercranial organs. 
 
 More delicate measurements seem to show that the temperature 
 rises and falls in the whole cerebral area, or at particular cir- 
 cumscribed regions of the cortex, in close connection with the 
 psychical activities. Thus Dr. Lombard found, by measurements 
 with exact thermo-electric apparatus, that the temperature of the 
 head during waking hours varies rapidly, though slightly (less 
 than y^j C.) ; and that these variations " appear to be connected 
 with different degrees of cerebral activity. . . . Every cause 
 that attracts the attention a noise, or the sight of some person or 
 other object produces elevation of temperature. An elevation of 
 temperature also occurs under the influence of an emotion, or 
 during an interesting reading aloud." Similar examinations have 
 been carried still further by Schiff, ' who has applied extremely del- 
 icate thermoscopic instruments directly to the cerebral substance 
 of certain animals (corap. Part L, Chapter III, 21). He finds 
 that the arrival of sensorial impressions is followed by a rise of 
 temperature, in certain special areas of the cortical substance, where 
 as he supposes these impressions are diffused ; he also con- 
 cludes that any resulting psychical activity is itself connected with 
 a still further rise of temperature than that which the sensorial 
 impressions alone engender. Schiff 's conclusions, therefore, point 
 not only to the localization in the entire brain of functions connected 
 with the phenomena of conscious psychical life, but also to some 
 distribution of such functions among its various areas. In the 
 same general direction are the conclusions of Byasson 2 and others, 
 as to an increase of waste in the tissues of this organ, which 
 corresponds, to some extent, at least, with the amount of thought 
 accomplished. This investigator found that the quantity of sul- 
 phates and phosphates excreted, in comparison with the quantity 
 estimated as entering into his diet, was notably increased in pro- 
 portion to the amount of his mental work. That is to say, in con- 
 nection with an increase in the number and intensity of the 
 psychical operations stands an increase in the functional activity of 
 the cerebral cells, as shown by the expenditure of their phos- 
 phorized constituents. 3 
 
 1 Archives de Physiologie, 1870, p. 451. 
 
 2 In the Jour. d. Anat. de Robin, 1869, p. 557 f. 
 
 3 See the chapter of Luys on the Physico-chemical Phenomena of Cere- 
 bral Activity ; The Brain and its Functions. 
 
RELATIVE WEIGHT OF BRA^N". 243 
 
 4. Comparative anatomy also indicates the importance of the re- 
 lation between the size, structure, and functions of the intercranial 
 nervous mass and the phenomena of mind. It shows, first of all, a 
 general but indefinite correspondence between the size and weight 
 of the brain of any species of animal, as compared with the weight 
 of its entire mass, and the place of the same species in the scale of 
 intelligence. This fact is roughly exhibited by the following com- 
 parative table : 1 
 
 RELATION OF THE WEIGHT OP THE BRAIN TO THE WEIGHT OF THE BODY. 
 
 Land tortoise 
 
 . .. 1 
 
 2,240 
 
 Eagle . . 
 
 -| 
 
 160 
 
 Shad 
 
 ... 1 
 
 1,837 
 
 Picreon . 
 
 1 
 
 104 
 
 Tadpole 
 
 ... 1 
 
 720 
 
 Rat 
 
 . . . 1 
 
 82 
 
 Elephant 
 
 1 
 
 500 
 
 Gibbon 
 
 1 
 
 48 
 
 Salamander 
 
 .. . 1 
 
 380 
 
 Young cat 
 
 1 
 
 39 
 
 Sheep . . 
 
 . 1 
 
 351 
 
 Sai ape . . 
 
 1 
 
 25 
 
 Doubtless other tables might be compiled which would lead to 
 less satisfactory conclusions than the one given above. Even in 
 this table we note that the elephant stands lower than the sala- 
 mander or the sheep, both of which animals are, in fact, far in- 
 ferior to the elephant in intelligence. Large allowance must also 
 be made in certain cases for peculiarities of physical structure ; for 
 example, the tortoise is rated lower than he would be were it not 
 for his heavy shell. The law itself is confessedly subject to re- 
 markable and unexplained exceptions ; at best it holds good only 
 in a very general way. For example, the relative weight of the 
 brain is not greatly different in the dolphin, in the baboon, and in 
 man. It is much greater in the infancy and youth of the human 
 species than in middle life or old age. In the male child at birth 
 it is about as one to six or seven (according to Tiedemann, 1 to 
 5.85 in the male, and 1 to 6.5 in the female). The brain grows with 
 great rapidity for the first few years the increase during the first 
 year being estimated at about one cubic centimeter daily. But the 
 rest of the body increases so much more rapidly that by the end of 
 the second" year it is about 1:14 ; by the end of the third year, 1:18. 
 It increases in absolute weight until well on into middle life, and 
 then after middle life diminishes at about the average rate of one 
 
 1 Taken from Hermann's Handb. d. Physiol., II., ii., p. 193, as compiled by 
 Exner, on the basis of the works, in part of Cams, and in part of J. Miiller. 
 The figures of comparative weight between the brain and the body are some- 
 what differently given by other authorities. 
 
244 GENERAL FUNCTION OF THE BRAIN. 
 
 ounce in a decade. The average relative weight of the adult brain 
 is one-fortieth or one-fiftieth. Tiederaann found that the relative 
 weight of the brain is dependent upon the absolute weight of the 
 body, and is relatively greatest with light persons. The human 
 brain is, however, absolutely heavier than that of any of the ani- 
 mals except the elephant (8-10 Ibs.) and the whale (5-6 Ibs.). 
 
 Much pains has been taken, by actually weighing different 
 human brains, or by calculating their weight on the basis of careful 
 cranial measurements, to establish a law connecting the amount of 
 the intercranial nervous mass with the comparative intelligence of 
 races and of individuals. 1 The average weight of the brain of the 
 adult European is, for the male, from 4G to 52 ounces ; for the fe- 
 male, from 42 to 46 ounces. Boyd gives the average weight of the 
 brain of the male, at the period of life when it is most developed 
 (twenty- five to forty years of age), as 46.8 ounces (1,321 grams, 91 
 centigrams). This difference between the sexes is not wholly de- 
 pendent on difference in bulk of body, but is an important sexual 
 distinction. The brain of man is on the average ten per cent, above 
 that of woman ; the difference in average stature is about eight per 
 cent. Many human brains rise above the upper average ranges ; 
 others fall below the lower average ranges ; and yet no marked 
 peculiarities of mental development are necessarily connected with 
 these variations. Considerable quantities of the substance of the 
 brain may be lost (at any rate from some areas of the cortical sur- 
 face) without perceptibly changing the mental life. In 278 cases 
 of males, the maximum weight of brain was found to be 65 ounces, 
 the minimum 34 ounces ; in 191 cases of females, the maximum 
 was 56 ounces, the minimum 31 ounces. 2 Numerous instances of 
 large excess in the average weight of brain-mass by individuals 
 eminent for intelligence are on record : for example, Ityron scarcely 
 under 79 ounces ; Cromwell, only 77 grains less, or 78.8 ounces 
 (although Vulpian thinks that the national spirit has exaggerated 
 both these instances) ; Cuvier, 64.5 ounces ; Abercrombie, 63 
 ounces ; Spurzheim, 55 ounces ; Sir J. Y. Simpson, 54 ounces ; Web- 
 ster, 53.5 ounces ; Agassiz, 53.4 ounces ; Chalmers, 53 ounces. 
 Other persons of eminence, however, have had brains of only aver- 
 age, or of under average weight ; thus C. F. Hermann, 46.5 ounces, 
 and J. F. L. Hausmann, 43.3 ounces. Moreover, brains of high 
 weight not infrequently occur without evidence of unusual mental 
 capacity, or even in the case of those mentally inferior. Record is 
 
 1 On the relations of the Brain with respect to weight and mass, see Schwalbe, 
 Lehrb. d. Neurologic, ii., pp. 589 ff. Erlangen, 1881. 
 
 8 Results obtained by Sims, Clendinning, Tiedemaun, and J. Reid. 
 
BKAINS OF DIFFERENT KACES. 245 
 
 made of four male brains, belonging to persons of no repute for in- 
 tellectual ability, which ranged from 62.75 ounces to 61 ounces ; of 
 another such, which weighed 60.75 ounces ; of the brain of a boy 
 of fourteen which weighed 60 ounces. In the West Riding Asy- 
 lum ' for the Insane, out of 375 males examined, the weight of the 
 brain in 30 cases was 55 ounces or upward ; out of 300 females 
 examined, in 26 cases it was 50 ounces or upward. Several persons 
 afflicted with dementia were found to have brains weighing more 
 than 60 ounces. On the contrary, idiots, almost without exception, 
 have brains far below the average in weight ; as a rule, the braiii 
 of such an unfortunate does not weigh so much as 30 ounces. 
 Cases of microcephalous idiots are on record whose brains weighed 
 only 10.5, or even 8.05 ounces. Here, again, however, singular ex- 
 ceptions must be admitted ; for in a few cases the brains of idiots 
 have reached the average weight, and have even, in rare cases, con- 
 siderably surpassed it. 
 
 Although the data adduced to show that the average weight of 
 brain in the more highly civilized races is greater than in the savage 
 races, are by no means abundant or conclusive, yet they are suffi- 
 cient to create a reasonably strong presumption in favor of this 
 view. Calculating from the size of the cranial cavity, as ascertained 
 by measurement of a large number of skulls, it is inferred that the 
 average weight of brain in the African, Australian, and Oceanic 
 races generally, falls from 1 to 4 ounces below that of the more 
 highly civilized European. It is further noted that there is almost 
 a complete absence of cases rising above the higher ranges above 
 54 ounces, for example ; and that there is not the same difference 
 between the two sexes in the uncultivated as in the cultivated 
 peoples. Davis calculated the average weight of brain among the 
 Chinese to be about equal to that of the Caucasian race in Europe ; 
 among the Sandwich Islanders to be some thirty grams less. The 
 surprisingly low weight of the brain of the Hindus is in part a 
 function of their smaller weight and bulk of body. It may fairly 
 be urged in objection, that by the method of measuring skulls 
 taken somewhat at random we should be likely to find a note- 
 worthy absence of such exceptional cases in certain quarters 
 among the European races ; and that the relative increase in size 
 of the female brain among uneducated peoples is probably, in part 
 at least, the result of the response of the nervous system to the 
 demand made upon it for the Hard labor performed by the women 
 among such peoples. 
 
 Any law which refers the intensity and range of the mental 
 3 For these facts see the Encyclopaedia Britannica, ninth ed., I., p. 879 f. 
 
246 GENERAL FUNCTION OF THE BRAIN. 
 
 activities directly to the size and weight of the nervous mass of the 
 brain must, therefore, be held only very loosely. It is to be ex- 
 pected that many unexplained exceptions will meet us, whether we 
 compare men with the other animals, or certain races of men 
 with others, or individual men with one another. No intelligent 
 physiologist would now think of making mere mass the test of 
 mental capacity. 
 
 5. A more intimate relation of dependence exists between the 
 amount of intelligence and the complex structure of the brain as 
 arising to a large extent from the development of the cerebral 
 hemispheres that is, from their relative size and expanse, and 
 from the number and depth of their convolutions. In other 
 words, wealth of expanded and convoluted cerebral hemispheres 
 is, in some general way, a measure of the richness and intensity of 
 mental life. This conviction becomes stronger the more carefully 
 the comparative anatomy of the cerebrum, and the development of 
 the cerebral hemispheres in the human embryo, are examined. 
 The forms of brain found permanently in fishes, amphibians, 
 reptiles, birds, and the lower mammals, are extremely similiar to 
 those shown in succession by the developing brain of the higher 
 mammals, and especially of man. The most distinctive feature of 
 man's superior brain is the marked development in the size, num- 
 ber, and depth of the convolutions of the hemispheres. In fishes 
 generally, both cerebrum and cerebellum are very small ; but the 
 ganglia connected with the organs of sense, especially of vision, are 
 relatively large. In amphibia the cerebral hemispheres are rel- 
 atively enlarged ; are advanced backward still farther in reptiles ; 
 while in birds the vesicles of the mid-brain are partially hidden by 
 the development of the hemispheres. In the lower mammals the 
 enlargement of these same organs by growth backward continues, 
 and their two parts become connected by a commissure ; but they 
 still remain comparatively meagre in size and simple in structure, 
 without much distinction of lobes or division into convolutions. It 
 is only in the most elaborately developed brains of the higher 
 mammals that the occipital lobe enlarges backward so as to cover 
 mid-brain, cerebellum, and medulla oblongata ; and that the frontal 
 lobe spreads forward over the nasal cavities so as to constitute a 
 development of forehead. Meantime the convolutions apparent on 
 the cerebral surface increase in number and depth. 
 
 ^he theory suggested by comparative anatomy is confirmed by 
 the probable view of Meynert, that the whole of this cortical region 
 of the cerebrum is a great " projection-field " on which the sensory 
 impulses are marshalled and systematically ordered (to serve, as it 
 
THE MEASUREMENTS OF WAGNER. 247 
 
 were, for the physical basis of mental phenomena), as they arrive 
 from the peripheral regions and are distributed over the outgoing 
 motor tracts. Certain striking exceptions to the principle of this 
 theory must, however, be acknowledged. Within each great group 
 of animals considerable variations occur in the degree of cerebral 
 convolution, such that it cannot be said accurately to measure the 
 degree of intelligence. For example, among mammals the in- 
 sectivora have brains "poorest" in convolutions, the herbivora 
 are "richest," and the carnivora stand between; the ruminants, 
 although rather dull and incapable of being taught, have numer- 
 ous and deep convolutions enough to rank them much higher than 
 their real intelligence deserves. The marmoset, on the other hand, 
 the relative weight of whose brain is as 1 to 18, shows a compara- 
 tively smooth arid non-convoluted surface, in striking contrast with 
 that of other monkeys. 
 
 Trustworthy data are as yet wanting to place beyond doubt the 
 probable opinion that the brains of less highly civilized races and 
 less highly intellectual individuals are relatively poor in develop- 
 ment of the cerebral hemispheres. The human embryo is, indeed, 
 an illustration in miniature of the truth of this opinion ; the older 
 it becomes the more distinctly marked are the lobes of the cere- 
 bral hemispheres, and the more numerous and deep are their con- 
 volutions. The brains of idiots are said, as a rule, to be poor in 
 convolutions ; this fact is doubtless connected with the embryonic 
 condition in which many of them have remained through arrested 
 development. Hermann Wagner, 1 on the basis of measurements 
 made by his father, undertook to estimate the comparative total 
 surface of the cerebral hemispheres of four brains, viz.: of two 
 males of noteworthy intelligence (Gauss, the mathematician, and 
 Fuchs, the physician), of a male laborer (Krebs), and of a female 
 in middle life. By weighing carefully the amount of gold-foil laid 
 on uniformly, which was required completely and closely to en- 
 velop all the convolutions of these brains, Wagner concluded that 
 the area of concealed surface was, in each case, approximately 
 equal to that exposed. The total surfaces of the four brains were 
 thus found to measure of Gauss, 2,196 square centimeters; of 
 Fuchs, 2,210 ; of the woman, 2,041 ; of Krebs, 1,877. It is a tempt- 
 ing but rather insecure generalization which concludes from so 
 few cases that the richness of the cerebral convolutions (the total 
 surface, both that exposed and that concealed by the sulci), is a 
 general direct measure of the intelligence. 
 
 6. Other interesting attempts have been made to measure the 
 1 Maassbestimmungen d. Oberflache d. grossen Gehirns. Cassel, 1864. 
 
248 GENERAL FUNCTION OF THE BRAIN. 
 
 intelligence of the animal by the relative size and structure of 
 the intercranial nervous mass, and so, definitely, to establish a direct 
 relation between the two ; we notice especially those of J. Miiller, ' 
 and of Meynert. 8 The great physiologist, Miiller, held that the 
 position of an animal in the scale of intelligence may be estimated 
 by comparing the hemispheres of his brain with the corpus quad- 
 rigeminum. According as the latter organ is relatively large, and 
 lies behind the hemispheres, uncovered by them, the animal is low 
 in the scale of intelligence ; according as the hemispheres increase 
 in size, and so envelop and bury beneath them the relatively small 
 corpora quadrigemina, the animal stands high in that scale. This 
 statement, however, scarcely covers anything more explicit than 
 the general fact that relative increase of the cerebral hemispheres 
 is indicative of progressive mental life. Meynert has pointed out 
 other important relations between parts of the brain, by which he 
 proposes to measure the intelligence. In the entire mass of the 
 crura cerebri we may recognize two parts, an upper {tegmenium), 
 which stands in direct connection with the optic thalami and the 
 corpora quadrigemina, and a lower (crusla), which is connected 
 through the lenticular nuclei of the striate bodies with the cere- 
 brum. Now the greater the hemispheres are in comparison with the 
 corpora quadrigemina, the greater must the mass of the crusta be in 
 comparison with that of the tegmentum. The development of the 
 pons Varolii is also essentially dependent on that of the crusta, for 
 the fibres of the latter enter into the former ; the arching of the 
 pons is therefore connected with the development of the hemi- 
 spheres. In general, then, the relative development of the entire 
 tract represented by the crusta, or lower part of the crura cerebri, 
 and the nucleus lenticularis, the fibres of which expand in the cere- 
 brum, is according to Meynert a measure of an animal's intelli- 
 gence. In man the mass of the crusta on the level of the corpora 
 quadrigemina exceeds that of the tegmentum ; in the other mam- 
 mals the reverse is true. 3 
 
 7. The above-mentioned facts of comparative anatomy, with 
 many others similar, show plainly the unique significance which the 
 masses of the brain, and especially the cerebral hemispheres, have, 
 as related to the phenomena of self-conscious mind. They may be 
 supplemented and confirmed through other facts furnished by 
 physiology, especially of the experimental kind. Upon this point, 
 
 1 Handb. d. Physiol. d. Menschen, 1844, I., p. 702 f. 
 'Sitzgsber. d. Wiener Acad., LX., Hi. (1869), pp. 447-462. 
 3 For a brief but judicious discussion of this subject, see Briicke, Vorles- 
 ungen iiber Physiologic, 1884, II., pp. 52 ff. 
 
FUNCTION OF CEREBRAL HEMISPHERES. 249 
 
 for the present, reference is simply made to the results of inves- 
 tigation as already set forth in Part I. (see especially Chapter 
 IV.). Physiology demonstrates that the nervous impulses, so far as 
 they result in sensation, pass along centripetal tracts which con- 
 verge from every portion of the periphery toward the brain ; and 
 that, so far as they result in motion following upon idea and voli- 
 tion, they pass along centrifugal tracts diverging from the same 
 central masses. It thus confirms the same theory which studies of 
 the anatomical structure of the nervous system suggest, namely, 
 that in these masses, and especially upon the cortex of the cere- 
 brum, is the common meeting-place of both kinds of impulses. The 
 section or injury of any nerve-tract, even in the spinal cord, apart 
 from indirect and secondary influences, does not affect the psychical 
 functions. In such an event, the parts of the body lying periphe- 
 rally from the point of interruption are simply withdrawn from all 
 direct connection with sensations or volitions. Sensory impulses, 
 then, no longer occasion sensations ; ideas of motion and volitions 
 to motion, of the parts thus disconnected, become of no effect in 
 producing the customary result. It has also been made obvious 
 that, in proportion as the masses of an animal's brain are removed 
 or incapacitated from performing their functions, the evidences of a 
 varied and complex mental experience are diminished. The simple 
 spinal cord of a frog, acting as a nervous mechanism, will perform 
 a few wonderful feats ; joined with the medulla oblongata, optic 
 lobes, and other lower parts of the brain, it will give largely in- 
 creased signs of psychical phenomena ; it would not be claimed, 
 however, that the cerebral hemispheres of this animal relatively 
 insignificant as they are when compared with those of the higher 
 animals are of no special importance for its highest psychical life. 
 Essentially the same thing, though in more emphatic form, is true 
 of all animals of a higher grade of intelligence. 
 
 8. 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 
 
250 GENERAL FUNCTION OF THE BEAIN. 
 
 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, under certain circumstances, as signs 
 of a psychical experience, may be demonstrated by experiment 
 (comp. Part I, Chapter IV., 4 ff.). But unless one is prepared 
 to maintain that all purposeful activity, as resulting from excited ner- 
 vous substance, must be correlated with phenomena of conscious 
 sensation and volition, one can scarcely assume with confidence 
 that such phenomena accompany the movements of the decapitated 
 frog. 
 
 What the nervous mechanism will do, when set agoing by the 
 appropriate stimuli, depends not only on its original structure, but 
 also on its acquired habits of action. That this law holds good 
 even for the mechanism of the hemispheres of the brain is obvious 
 from various facts. Stimulating those regions of the cerebral cor- 
 tex which are connected with definite groups of muscles, in the case 
 of the adult animal (for example, a dog), does not call out the same 
 responses in the animal newly born (that is, under nine or ten days 
 old). The case of the bird which has lost its cerebral hemispheres, 
 and which executes motions by means of the lower basal ganglia, 
 that seem to indicate a complex psychical life (comp. Part I, 
 Chapter IV., 20) is less easy of solution. Are we to consider 
 such an animal still capable of "sensation," " perception," and "vo- 
 lition ? " If this question means whether any phenomena continue 
 to occur such as correspond to those conscious experiences of our 
 own to which we apply the above-mentioned words, then we must 
 confess our inability to answer it. 
 
 In general, we know extremely little of the conscious mental life 
 of the lower animals. What we conjecture is wholly dependent on 
 the interpretation, given in terms of our human consciousness, to 
 motions of their bodies resembling those which express definite 
 conscious states in ourselves. But a large part of our own 
 bodily activity is ordinarily not definitely correlated with any con- 
 scious mental activity ; for example, breathing, winking, swallow- 
 ing, changing the posture of the body in sleep and in states of 
 profound meditation, and especially the very complex operations 
 involved in walking, singing, playing on musical instruments, or 
 handling a tool, etc. In all these and similar cases, we find that 
 the intricate and purposeful play of the mechanism is by no means 
 necessarily connected with a corresponding series of conscious sen- 
 sations and volitions. But in proportion as the hemispheres of an 
 animal's brain become relatively developed, not only their abso- 
 lute but also their relative significance is increased. The influence 
 
LOSS OF THE CEREBRAL HEMISPHERES. 251 
 
 of the brain proper upon the voluntary movements of an animal 
 is greater, the higher the animal stands in the scale of cerebral 
 development and of intelligence. A frog, or a fowl, deprived of 
 its hemispheres, can do what is quite impossible for a dog or an 
 ape in the same condition. If, then, man's nervous mechanism, 
 especially in case it has been trained to elaborate co-ordinated func- 
 tions, can, without any corresponding accompaniment of mental 
 phenomena, accomplish so much which appears significant of the 
 most elaborate psychical activities; a fortiori, it is likely that we 
 may make this mechanism, working without consciousness, account 
 for most of what is done by the hen or pigeon without its cerebral 
 hemispheres. Moreover, experimental physiology undoubtedly 
 tends toward accounting more and more fully for the most com- 
 plex bodily motions under the terms of physical mechanism. 
 
 The most marked result of an animal's loss of the cerebral 
 hemispheres is the sudden and great, or total departure of its 
 intelligence. This fact is, of course, confirmatory of the impres- 
 sion that the functions of these hemispheres, and of them alone, 
 constitute the physical basis of its intelligence. We confess, 
 however, our inability to affirm that the " psychical life " of every 
 animal is inseparably bound to its continued possession of these 
 organs. There may possibly be a varied psychical life of animals 
 that have no brain. Yet in the case of the higher mammals, and 
 especially in the case of man, we need not hesitate to affirm the 
 probability of such an inseparable connection. The physical basis 
 of the phenomena of human consciousness is pre-eminently, if not 
 exclusively, the convoluted cortex of the cerebrum. 
 
 9. It is impossible, accordingly, tg avoid raising the inquiry 
 whether some more definite scheme of the localization of cerebral 
 functions may not be discovered. The cerebral cortex is itself a 
 very complex organ, or system of organs. Its different regions 
 are marked by comparatively slight, and yet not insignificant, dif- 
 ferences 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. The question 
 therefore arises : Have the different members of this complex of 
 organs different relations to definite motor activities in the pe- 
 ripheral regions, and to the various phenomena of conscious men- 
 tal life ? or, in other words : Have different parts of the cere- 
 bral hemispheres all the same office and value in relation to the life 
 
252 EVIDENCE FOR LOCALIZATION. 
 
 of sensation and voluntary motion ? This is the question generally 
 understood under the term "the localization of cerebral func- 
 tion." 
 
 10. Most of our definite knowledge concerning the functions 
 of the other parts of the nervous mechanism creates a presumption 
 in favor of some localization of cerebral functions. All the different 
 parts of this mechanism are, indeed, constructed by combining 
 variously a few elements of essentially the same structure ; all of 
 them likewise are capable of exercising essentially the same neural 
 functions. But each part of this mechanism has also its special 
 functions. Thus we found that the different nerves become classi- 
 fied functionally ; some are motor, voluntary or involuntary, some 
 inhibitory, some secretory, some sensory, etc. Hints of a certain 
 kind of classification may be discovered for the smaller ganglia or col- 
 lections of nerve-cells. In making transverse sections of the cord, 
 different regions with different functions appear. Considered lon- 
 gitudinally, the cord is capable of being more or less definitely 
 divided into several so-called centres, with specifically different 
 functions. Localized centres, where specific kinds of reflex-motor 
 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 ? 
 
 11. Notwithstanding the strong presumption in favor of the 
 localization of cerebral function, the beginnings of a successful 
 attempt to establish this theory are only about fifteen years old. 
 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 
 
 1 Anatomie et physiologie du systeme nerveux, etc., Paris, 1842, i., p. 644 f. 
 
EXPERIMENTS OF FRITSCH AND HITZIG. 253 
 
 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 obtaining 
 any sign whatever of resulting muscular contraction. In the same 
 year Flourens ' asserted, on the basis of numerous experiments in 
 extirpation, that the lobes of the cerebrum perform their functions 
 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 
 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. 
 
 12. It was not until 1870 that the "epoch-making" experi- 
 ments of Fritsch and Hitzig 2 began the modern era of investiga- 
 tion into this subject. These observers announced the fact that the 
 cerebral cortex of dogs is, at least in certain minute areas of it, ex- 
 citable 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 are 
 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 adduction of the fore- 
 limb, another for the bending and rotation of the same limb, 
 another for the hind-limb, and lastly one for the face. From such 
 facts they drew the conclusion that the principle announced by 
 
 1 Recherches experimentales sur les proprietes et les fonctions du systeme 
 nerveux, etc., p. 99 f. 
 
 2 See the article by G. Fritsch and E. Hitzig in the Archiv f. Anat., Phy- 
 siol., etc., 1870, pp. 300-332 ; and subsequent articles by Hitzig in the same 
 Archiv, 1871, 1873, 1874, 1875, 1876 ; also his work, Untersuchungen iiber 
 das Gehirn, Berlin, 1874. 
 
254 EVIDENCE FOR LOCALIZATION. 
 
 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." J 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. Many 
 of these facts may, with care and skill, be verified by any observer. 
 Dr. Ferrier in particular has used the method of Fritsch and Hitzig 
 to map out the hemispheres of the brain of the monkey into no 
 fewer than fifteen kinds of centres. The testimony of human pa- 
 thology, 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. Be- 
 fore considering the conclusions themselves, it is necessary to 
 understand the true nature and extent of the various kinds of evi- 
 dence. 
 
 13. Exner 2 has well said that " a physiology of the cerebral cortex, 
 in the sense in which there is a physiology of the muscle, etc., scarcely 
 exists at the present time." The reasons for such a deficiency lie 
 partly in the very nature of this organ and the place it holds with- 
 in the animal economy ; as well as partly, perhaps, in certain prej- 
 udices which have hindered the physical theory of a material struct- 
 ure so intimately related to the action of the mind. The cerebral 
 cortex of the animals is experimentally approached only by over- 
 coming immense difficulties. Moreover, those physical and chemi- 
 cal processes of the cerebral substances, to which we must look for 
 any strictly scientific understanding of its physiology, are placed 
 almost utterly beyond reach of investigation. Reasoning must fill 
 up with conjecture the great gaps that lie between a very complex 
 series of physical occurrences, only a part of which are observable, 
 on the one side, and on the other, an equally complex group of 
 psychical occurrences. The latter belong to a different order of 
 phenomena from the former ; and, moreover, in the case of the 
 lower animals which must be selected almost exclusively for ex- 
 perimentwe know nothing of these psychical occurrences except 
 through physical signs that are peculiarly liable to misinterpreta- 
 tion. The result is that our conclusions on the localization of cere- 
 bral function must be reached by considering a great multitude of 
 complicated facts, many of which appear to take sides with contend- 
 
 1 Archiv. f. Anat., Physiol., etc., 1870, p. 332. 
 'See Hermann's Handb. d. Physiol., II., ii., p. 189. 
 
STIMULATION AND EXTIRPATION. 255 
 
 ing champions of different theories who alike appeal to them. It 
 is only by observing the directions in which the different lines of 
 evidence seem to point in common, that we can reach even a prob- 
 able opinion upon a few points. 
 
 14. Three great lines of evidence, leading from three great 
 groups of facts, must be considered. These are the evidence from 
 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 difficulties 
 and dangers. It is only by regarding the combined testimony of 
 the three that the highest probability at present possible can be at- 
 tained. 
 
 Experimentation with a view to discover the localized functions 
 of the cerebral cortex is of two kinds, stimulation and extirpation. 
 Here, too, what has already been said (Parti., chap. IV., 14) con- 
 cerning the difficulties of the same mode of investigation in the 
 sub-cerebral regions of the encephalon must be recalled and made 
 more emphatic. All experiment by stimulation of certain areas of 
 the hemispheres of the brain relies, of course, upon the argument 
 that those areas whose stimulation is followed by the movement of 
 definite groups of muscles are especially connected with such groups 
 of muscles. The further assumption is likely to be made that these 
 areas constitute the special organs which have control, as it were, 
 of the same muscles. Since it seems to be a general principle that 
 the sensory and motor nerve-tracts distributed to any region of the 
 periphery come into tolerably close local relations to each other 
 somewhere within the entire field of the cerebrum, it would seem to 
 follow that some special connection exists between certain classes 
 of sensations and volitions and the circumscribed areas of cortical 
 substance pointed out by experiment. It should not be forgotten, 
 however, that the excitation of any group of muscles, by applying 
 stimulus to some area of the cerebral cortex, proves only that this 
 area is somehow connected with such group of muscles. It still 
 remains to be shown that sensory impulses, on arriving from such a 
 peripheral portion of the body, serve as the physical basis for the 
 psychical phenomena of sensation solely within this circumscribed 
 central area ; or that conscious volitions, in order to be followed 
 by motion in this peripheral portion, must give rise to the mole- 
 cular commotion of the same area. 
 
 15. By far the most efficient and manageable stimulus for ex- 
 perimenting upon the localization of cerebral function is the electrical 
 current. Mechanical or chemical irritation may, however, be em- 
 ployed in certain cases. The use of the electrical current incurs, 
 
256 EVIDENCE FOR LOCALIZATION. 
 
 of course, the danger of its diffusion. Important objections, based 
 upon this fact and upon other grounds connected with the use of 
 electricity, have been raised to the conclusions of Hitzig. 1 To 
 Hitzig's claim that the electrical currents which excite the so-called 
 motor areas are "very weak," and therefore unable, at a very slight 
 distance from the place of the application of the electrodes, to affect 
 the ne*rvous substance, Hermann replies that, on the contrary, con- 
 sidering the effect antecedently to be expected, these currents are 
 " surprisingly strong," and that the brain, in diffusing the currents, 
 must act like any other substance (e.g., a mass of copper) of similar 
 form that is to say, the distribution of such a current in the sub- 
 stance of the brain is a purely geometrical function of the form of 
 this substance and of the position of the electrodes. Moreover, it 
 is found that increasing the strength of the current applied to a 
 so-called "motor area " invariably increases the size of the cortical 
 region thrown into activity. That extra-polar conduction actually 
 takes place in the substance of the brain has been shown by Dupuy, 
 and by Carville and Duret ; contraction of the muscle of the rheo- 
 scopic frog and deflection of the needle of the galvanometer, at re- 
 mote distances from the electrodes, prove that the current passes 
 along the whole extent of the cerebral hemisphere. The excitability 
 of the cortical substance continues for hours after its exposure to the 
 air, or after acids have completely destroyed its external third por- 
 tion. If the cortical area be separated by a circular cut from all 
 connection with the nervous substance below, it is still excitable with 
 only a slight increase in the strength of the stimulus applied. Or 
 if the gray substance of the surface be wholly removed, and the 
 electrodes plunged in the blood of the cavity of one of these so- 
 called motor areas, the customary results follow. Still further, the 
 size of the circle within which the minimum amount of stimulus, 
 when applied to certain gyri, will serve to excite the hind-limb of 
 the animal, remains about the same whether the amount of cortical 
 surface contained in the circle be largely increased by a sulcus 
 crossing it, or not. 
 
 From facts like the foregoing it is argued that, while beyond 
 question the application of a given amount of stimulus to certain 
 gyri of the cortical surface will produce definite motor results, we 
 cannot affirm those gyri to be the true cortical centres of such 
 motion. Such gyri have accordingly been regarded by some as 
 merely connected with the excitation of motion in a mechanical 
 
 1 See especially the article of Hermann describing investigations under- 
 taken by him in company with von Borosnyai, Luchsiuger, and others, 
 Pfliiger's Archiv (1875), x., pp. 77 ff, 
 
EFFECTS OF ELECTRICAL STIMULATION. 257 
 
 way, through their service in conducting the electrical stimulus to 
 other regions of the brain, especially to the basal ganglia. The 
 argument for the theory of localization would need to show, how- 
 ever, that the electrical current stimulates these areas immediately 
 to the exercise of their central nervous functions, and does not 
 simply pass through them to excite other nervous matter lying 
 beneath. 
 
 To the foregoing objections the advocates of the theory of locali- 
 zation make the following among other replies : " The effect of irri- 
 tation of the basal ganglia is capable of exact estimation ; " l and 
 definite localized contraction of single groups of muscles, such as 
 comes from stimulating certain areas of the cortical surface, does 
 not follow from irritating the basal ganglia. Stimulation of other 
 areas of the cortical surface which lie nearer to the basal ganglia 
 for example, of the island of Keil, which immediately overlies the 
 corpus striatum causes no movements. On the contrary, it was 
 found by Carville and Duret that the phenomena evoked by stimu- 
 lating the motor areas persist, even after the destruction of the 
 corpus striatum. Moreover, when the animal is deeply etherized, 
 the excitability of the cortical regions is partially or wholly lost. 2 
 Since the physical conductivity of the gray nervous substance is 
 not impaired by the anaesthesia, the loss of function must be due 
 to the functional condition of this substance. More conclusive do 
 the facts appear to be, which show that the nature of the motor 
 reaction following upon the application of stimulus to the cortical 
 substance is peculiar. Many observers have found that a stronger 
 stimulation is necessary to bring about the same motor results 
 after the cortical surface is removed ; this is what we should expect 
 on the theory of localization, but the reverse of what would be 
 true if the effect of the current was transmitted unchanged through 
 this surface. Then, too, Franck and Pitres 3 have shown that the 
 effect of the electrical current is retarded in the gray matter ; the 
 difference of time, as dependent upon whether the stimulus is 
 applied to the gray matter or to the white lying beneath, being 
 about 0.015 second. This interval must be spent in evolving, 
 under the influence of the stimulus, the distinct neural function 
 which belongs to the gray matter. Finally, the excitation is appar- 
 ently reinforced in strength by the functional activity of the cor- 
 tical substance, since as we have just seen a stronger stimulation 
 is needed to produce the same result after this substance is re- 
 
 1 Ferrier, The Functions of the Brain, London, 1876, p. 133 f. 
 
 2 See Hitzig in Archiv f. Anat., Physiol., etc., 1873, p. 402. 
 
 3 Archives de physiologic, 1875. 
 
 17 
 
258 EVIDENCE FOR LOCALIZATION. 
 
 moved ; such reinforcement is the peculiar property of the central 
 organs. 
 
 It seems obvious, therefore, that experiments with electrical 
 stimulation of the cortical surface demonstrate a special connection 
 between certain more or less definitely circumscribed areas of 
 that surface and definite groups of muscles ; they also create a 
 strong presumption that this connection is not merely anatomical 
 or structural, but also functional. 
 
 16. The second kind of direct experimental evidence is de- 
 rived from observing the effects of extirpation. It is natural to 
 argue that those areas of the brain, the loss of which is followed 
 by the loss or disturbance of motion in definite groups of muscles, 
 or by the loss or disturbance of any class of sensory impressions, 
 are functionally related in a peculiar way to such muscles or organs 
 of sense. But the application of this argument is encompassed 
 with many difficulties. In the first place, it is impossible at 
 each stage of the experiment which often includes several days 
 or months of observation to know precisely what the condition 
 of the brain is. Post-mortem examination of the brain reveals only 
 what was the final effect of the experiment in destroying its 
 tissues. The rise and fall of local or extensive inflammations, the 
 progress of degeneration in the nerve- tracts and of abscesses result- 
 ing from the primary lesions, etc., cannot be followed by the ex- 
 perimenter in detail. Nor can he directly observe the formation 
 and education of the tissue as it is called upon for an increase in 
 the amount of its former functions, or perhaps for the discharge of 
 functions partially new. As a rule, then, it is found that the 
 effects of extirpation change from time to time ; some of them are 
 of first importance and cannot well be overlooked, and others are 
 so delicate and minute as almost wholly to escape observation ; 
 some speedily pass away, others more slowly, still others perhaps 
 not at all. Tha difficulties are, of course, especially great when 
 we try to deal with effects upon the animal's sensory apparatus and 
 his psychical world of sensations and perceptions. To tell whether 
 an animal sees, hears, feels, smells, and tastes, or not ; and to tell 
 precisely in what sense it exercises these functions whether, for 
 example, its deficiency is " soul-blindness " in any of its various 
 degrees are not tasks which it is easy to perform, or about the 
 correct performance of which one can indulge in a boundless 
 confidence. 
 
 The demonstrative value of both kinds of experimental evidence 
 electrization and extirpation is much lessened by the fact that 
 it is almost wholly derived from the lower animals. Ethical con- 
 
FACTS OF HUMAN PATHOLOGY. 259 
 
 siderations, which few investigators dare even occasionally to dis- 
 regard, forbid that the living human brain should be made the 
 subject of similar experiment. In order, then, to draw any safe 
 conclusions from this evidence, it is necessary not only that the 
 application of the principle of localization in general should be as- 
 sumed, but also that some right should be gained to transfer to 
 the human brain from the map of the cortical surface of the ani- 
 mal's brain, the so-called motor and sensory areas which have been 
 determined by experiment. But it is not even in all cases clear, 
 precisely what convolutions or parts of convolutions of the human 
 cerebrum correspond to those previously marked out on the brain 
 of the animal. Moreover, in the effort to make any such transfer- 
 ence of the argument from the animals to man, we meet again with 
 the insuperable difficulty of forming a correct mental picture of 
 the psychical life of the animals. 
 
 17. The evidence from human pathology for the localization of 
 cerebral function has a peculiar value ; but it has also its peculiar 
 puzzles and dangers. Such evidence is free, indeed, from the ob- 
 jections which arise against all attempts to carry the argument over 
 from the cerebral hemispheres of the lower animals directly to 
 those of man. Nature and human intercourse are less kind to this 
 wonderful mass of nerve-cells and nerve-fibres than the electrodes 
 and knife of the physiologist are compelled to be. Accident and 
 disease destroy, either suddenly or progressively, the different 
 areas of the cortical substance of the human brain. They have, in 
 various cases, made such a variety of attacks upon it as to cover all 
 the areas of both hemispheres. If, then, we had a large collection 
 of cases in which the lesions were definitely circumscribed, or the 
 progress made by the destruction of tissue was accurately recorded 
 for every stage ; and if we had also a correspondingly definite and 
 accurate description of the motor and sensory disturbances occa- 
 sioned by these lesions, we might perhaps be able to make a toler- 
 ably conclusive induction. But losses of brain-tissue, when caused 
 by accident and disease, have not the same circumscribed limits 
 which can be observed by the knife or corroding acid of the physi- 
 ologist. Lesions of the cortical areas entirely free from complica- 
 tion with lesions in the sensory and motor tracts below are compar- 
 atively infrequent. Cases of total destruction of any so-called "area " 
 on both hemispheres, and of such area alone, rarely or never occur. 
 
 Furthermore, it is only by careful post-mortem examination that 
 the precise extent of the pathological changes can be known ; this 
 examination, at best, reveals simply the last state of the case. 
 The reports of post-mortem examinations are also, as a rule, lacking 
 
260 EVIDENCE FOR LOCALIZATION. 
 
 in precision. On the other hand, the symptoms of motor or sensory 
 disturbance are rarely described, from beginning to end, with suffi- 
 cient accuracy of detail to be of great service. Many large losses 
 of cerebral substance are followed by no sensory or motor disturb- 
 ances which can be distinctly traced. In large numbers of cases 
 where such disturbances arise, they in time pass almost or quite 
 wholly away. For these and other reasons the best evidence at- 
 tainable from pathological cases, when collected and sifted, appears 
 surprisingly confusing and self-contradictory. Pathology has, there- 
 fore, furnished the common fund of cases from which the most di- 
 verse and even contradictory theories have drawn at sight their stock 
 of so-called proof. It has been used as the careless and false witness 
 upon which either party, and all parties to the suit, could call for 
 precisely the testimony desired. An increase of information and 
 care on the part of those who have opportunity for ante- and post- 
 mortem observation of such cases will doubtless, in time, cause 
 pathology to yield much more assured results. 
 
 18. The third kind of evidence to which the principle of the 
 localization of cerebral function may appeal comes from compara- 
 tive anatomy and histology. Comparative anatomy, however, gives 
 us evidence of only the most general kind. Combined with exper- 
 iment by electrical excitation, it shows that, on the whole, the 
 higher the structure and intelligence of the animal, the more nu- 
 merous and more definitely marked are the " excito-motor areas " 
 which may be discovered on the hemispheres of its brain. Only 
 traces, as it were, of such areas can be found upon the cerebral 
 hemispheres of the frog or the pigeon ; only a few areas can be 
 doubtfully pointed out for the rat or guinea-pig. The indications 
 are clearer and more numerous of localized cerebral function in 
 definite centres of the brains of the rabbit and the sheep. But it 
 is in dealing with the cerebral convolutions of the more highly 
 specialized brains of the dog, and particularly of the monkey or the 
 man-like ape, that the proofs of the theory become most abundant. 
 While, then, the argument from all the other animals to man is' 
 uncertain and should be used only with great caution, the general 
 drift of comparative anatomy encourages us to place the greater 
 confidence in it, the more nearly the brain of the particular animal 
 from whose case we wish to draw the inference resembles the 
 brain of man. At the same time, the rash confidence with which 
 the brain of the monkey has been mapped out in detail, and human 
 pathology thereupon ransacked with the purpose of finding some 
 warrant for copying this map upon the brain of the human species, 
 cannot be too carefully avoided. 
 
THE EVIDENCE OF HISTOL 
 
 Histology supplements and confirms the other evidence by show- 
 ing that the structure and connections of different parts of the 
 cerebrum are such as we should expect them to be, in case the 
 functions of the parts were such as experimentation and pathology 
 seem to have discovered. The modern arts of microscopy and 
 photography have made possible an increasingly accurate knowledge 
 of the intimate structure of the brain. Many great difficulties, 
 however, still remain in the way of such perfection of this knowl- 
 edge as will make it available as a secure foundation for a theory 
 of the localization of cerebral function. At present the histology 
 of the human cerebral hemispheres is not in a condition to take 
 the place of a leader of physiological experiment and pathological 
 observations. Its office is still rather that of rendering supple- 
 mentary evidence in correction or confirmation of the evidence 
 from the other two sources. Thus, for example, if Gliky's belief 
 that he traced the nerve-tracts from the so-called motor centres of 
 the cerebral hemispheres as they bend around the striate bodies 
 and run into the crusta of the crura cerebri should be demon- 
 strated, this fact would constitute an item of confirmatory evidence 
 furnished by histology to experimental physiology and pathology, 
 in favor of their general theory. 
 
 19. According to the foregoing view of the nature of the three 
 kinds of evidence available, it would seem that, in collating and 
 estimating the combined proofs from them all^the following course 
 of inquiry should be pursued. The indications of experiment upon 
 the cerebral hemispheres of the animals especially of those most 
 closely allied to man in their cerebral structure by the two 
 methods of stimulation and extirpation, must first be gathered and 
 carefully weighed. Only those conclusions upon which the two 
 methods are found to yield substantially the same results should be 
 selected for further testing. The instances of localization of cere- 
 bral function thus detected in the other higher mammals must 
 then be allowed to suggest to pathology the questions it should 
 undertake to answer with reference to man. In other words, ex- 
 perimentation with the other animals suggests and strengthens the 
 hypothesis which human pathology must try to satisfy. But in 
 undertaking to test such hypothesis, pathology must be both fair 
 and comprehensive in its observations. All the accessible patho- 
 logical cases must be sifted and those only selected to bring for- 
 ward as evidence which have the definite nature, and have received 
 the careful examination recorded in detail, that are necessary to 
 make them of real value. The corrective or confirmatory evidence 
 of histology must then, so far as possible, be summoned to aid in 
 
262 EVIDENCE FOE LOCALIZATION. 
 
 forming our final conclusions. It is not until all the kinds of proof 
 unite with a large and substantial agreement, if not with an abso- 
 lute uniformity, that we can feel the utmost confidence attainable 
 in our results. If it be found that certain regions of the cerebral 
 hemispheres of the higher animals are the only ones to respond 
 when stimulated with movements in definite peripheral parts of the 
 body, and that the injury of those same central regions alone, or 
 chiefly, causes motor and sensory disturbances in the same periph- 
 eral parts ; if it also be found that lesions of the corresponding 
 regions of the human brain are alone, or chiefly, followed by similar 
 motor and sensory disturbances, and that lesions of other regions 
 alone are rarely or never followed by these same disturbances ; and, 
 finally, if it be found that these same cortical regions have in the 
 human body a special anatomical connection with these same pe- 
 ripheral parts ; then we have reached the most conclusive evidence 
 attainable for a theory that the cerebral functions are localized in 
 the case of man. But precisely what is meant by such " localiza- 
 tion " may still remain more or less a matter of dispute. We con- 
 sider now a summary of the evidence according to the foregoing 
 principles. 
 
CHAPTEK II. 
 THE LOCALIZATION OF CEBEBKAL FUNCTION. [CONTINUED.] 
 
 1. ON attempting to make an induction from all the three kinds 
 of evidence which may be adduced in answer to the question, 
 whether the different functions of the cerebral cortex have special 
 relations to its different localities, no other difficulties are on the 
 whole so great as those which come from so-called "negative cases." 
 These negative cases force the inquirer to undertake a detailed ex- 
 perimental and pathological examination. " That the cortex of the 
 cerebrum, the undoubted material substratum of our mental opera- 
 tions," says Ecker, 1 "is not a single organ, which is brought into 
 play as a whole in the exercise of each and every psychical function, " 
 but consists rather of a multitude of mental organs, each of which 
 is subservient to certain intellectual processes, is a conviction which 
 forces itself upon us almost with the necessity of a claim of reason." 
 But even the proposition that the brain is the "material substratum 
 of our mental operations," is very far indeed from having the char- 
 acter of a rational necessity. The further proposition that the cor- 
 tex of the cerebrum " consists of a multitude of mental organs," is 
 an inadequate statement of a conclusion which, at the very best, 
 we can adopt only as the result of a long series of complex and con- 
 flicting researches. In fact, considerable areas of the cortical sur- 
 face appear, at first, not to have any immediate relation to any psy- 
 chical function whatever. 
 
 The first general principle to be admitted in all attempts at a 
 theory of the localization of cerebral function is, then, of a nega- 
 tive character. This principle is based upon the negative results 
 of physiological inquiry. Considerable areas of the cortical sur- 
 face do not respond with motor activities when stimulated. Con- 
 siderable portions of the cortical substance may be extirpated or 
 lost by disease without the destruction or appreciable disturbance 
 of any motor, sensory, or more purely intellectual functions. To 
 such an astonishing extent is this true as to throw temporary doubt 
 not only over the whole theory of the localization of cerebral func- 
 1 The Convolutions of the Human Brain, p. 1. London, 1873. 
 
264 STIMULATION AND EXTIRPATION. 
 
 tion, but even over the statement that the cerebral cortex, as a -whole, 
 is the only " material substratum " of mental operations. 
 
 2. Attention has already been called (Chap. I., 11) to the fact 
 that Longet, Flourens, and other great physiologists, considered 
 the cerebral hemispheres to be active as a whole in all their func- 
 tions, and this, partly, because they found them not irritable by the 
 electrical current. The discovery of Fritsch and Hitzig in 1870 de- 
 monstrated that a part of the hemispheres of the dog, and a part 
 only, gives signs of being excited by the application of stimulus. 
 This part they called " motor," and located, in general, in the fore 
 part of the hemispheres ; behind lay the region called " non-mo- 
 tor," because it gave no response on being stimulated. 1 Even with- 
 in this so-called " motor " region the early researches of these in- 
 vestigators pointed out only five spots of a small fraction of an inch 
 in diameter (the electrodes were, as a rule, separated not more than 
 2-3 mm.) that could be more definitely related to the movement of 
 certain groups of muscles ; between and around these spots lay the 
 much larger areas of negative result. Subsequent experiments 
 added a few more such irritable areas to the map of the cerebral 
 hemispheres of the dog. A large number of so-called centres, cover- 
 ing an increased amount of the cortical surface, have been pointed 
 out by Ferrier and others on the cerebral hemispheres of the mon- 
 key. Fully half of this number, however, cannot be regarded as 
 having anything like a demonstrable character ; and much fault has 
 justly been found 2 with many operators upon the brains both of 
 monkeys and of dogs, for their lack of precision in experiment, and 
 haste in drawing conclusions. 
 
 Experiments in extirpation also show that considerable areas 
 of the cortical substance may be removed without perceptibly im- 
 pairing any of the motor or sensory functions of the animal. In- 
 deed, even when the loss of the cortical substance, thus artificially 
 produced, extends over almost an entire hemisphere, or over a large 
 portion of both hemispheres, the operation may not result (in the 
 case of the dog, ordinarily does not result), in the permanent and 
 complete loss of any specific function, motor or sensory. So true is 
 this that one eminent observer, Goltz, has maintained, on the basis 
 
 1 Archiv f. Anat., Physiol., etc., 1870, p. 311. 
 
 * See, for example, Munk's strictures of Ferrier, Ueber d. Functionen d. 
 Grosshirnrinde, Berlin, 1881, pp. 14 ff. (also p. 6 f. ; 36 f. " roh war operirt, 
 roll beobachtet, roh gescMossen"). On the other hand, the charge of careless- 
 ness in experiment, and of illogical conclusions, is freely made against Munk 
 himself, both by advocates of rival theories of localization, like Dr. Yeo and 
 others, and also by opponents of all theories of localization, like Goltz, Lob, 
 and others. 
 
EVIDENCE FROM NEGATIVE CASES. 265 
 
 of many experiments in extirpation, that it is chiefly the quantity of 
 the cerebral substance destroyed, in large measure irrespective of 
 the locality, which determines the nature and extent of the result- 
 ing psychical disturbances. The arguments of Goltz (as he him- 
 self admits) do not answer those urged for a certain kind and de- 
 gree of the localization of cerebral function. Bat his experiments 
 furnish a large number of facts which emphasize the negative char- 
 acter of many of the results of experiment. This fact is in itself 
 undeniably unfavorable to any theory which would map out the en- 
 tire cortical surface into so-called centres or areas, to be considered 
 as separate organs of particular psychical processes. 
 
 3. The negative evidence from certain cases in human pa- 
 thology is yet more astonishing and perplexing. At first sight it 
 seems to suggest the conclusion that the mind can dispense, with- 
 out impairment, with a considerable mass of brain-substance, no 
 matter from what region it be subtracted. Many cases of large le- 
 sions of the cerebral hemispheres in man, with no resulting disturb- 
 ance of the psychical functions, are recorded. 1 
 
 Berenger de Carpi tells of a young man who had a foreign body 
 of four fingers' breadth square driven into the substance of his 
 brain until it was buried. Much of this substance was lost when 
 the foreign body was removed, and more yet some thirteen days 
 later. Nevertheless, the patient lived for a long time in the enjoy- 
 ment of all his faculties. 
 
 Longet was acquainted with an army officer who had lost, by a 
 wound in the parietal region, a large quantity of brain-substance ; 
 yet he remained mentally vivacious and showed no other result of 
 the lesion than a tendency to grow tired easily. The same authority 
 communicates 2 the case of an Italian whose skull was crushed in 
 the right parietal region by a stone. So much of the substance of 
 the brain w r as lost on the wound being dressed, and subsequently 
 through a fall from his bed (on the eighteenth day) and through 
 intoxication (on the thirty-fifth day), that the attendant physician 
 calculated the lesion must have reached down nearly to the corpus 
 callosum. The man, however, lived without any apparent impair- 
 ment of psychical functions ; but we note in this case a permanent 
 laming of the limbs of the left side. 
 
 1 See the list of such cases in Ferrier, the Localization of Cerebral Disease, 
 London, 1878, pp. 25 ff.; Hermann, Handb. d. Physiol., II., ii., pp. 333 ff.; 
 Brucke, Vorlesungen iiber Physiol., II., p. 57 f. Wien, 1884; and the works 
 cited by the two former, especially Pitres, Lesions du centre ovale Paris 
 1877. 
 
 2 Recorded, however, by Quesnay. 
 
266 STIMULATION AND EXTIRPATION. 
 
 A remarkable case is narrated by Briicke, 1 on the authority of 
 a certain Dr. Kratter. By a blow from a stone on the parietal 
 region of the skull, one Ivan Mussulin was thrown to the ground ; 
 but within two hours he recovered so that he himself went to 
 the " praetor " and entered complaint against his assailant. For 
 twenty days he lived in apparently full possession of his powers 
 of motion, sensation, and intelligence ; on the twenty-first day he 
 suddenly died. The entire left cerebral hemisphere was found on 
 examination to be a disorganized mass. It is to be noticed, how- 
 ever, that the autopsy did not take place until some eighteen hours 
 after death, and that we have no good means of judging what the 
 condition of the injured hemisphere was during the twenty days 
 preceding his sudden death. 
 
 Eemarkable instances of defective brains are also on record ; for 
 example, the case which Lallemand narrates of a person of normal 
 psychical constitution in whose cerebrum the entire place of the 
 right hemisphere was, after death, unexpectedly found to have 
 been filled with a serous fluid. Here again, however, there had 
 been lameness of the left side of the body from birth. 
 
 Extensive lesions without marked motor or sensory disturbances 
 occur by far most frequently in the frontal lobes of the cerebral 
 hemispheres. Yet similar negative cases are by no means infre- 
 quent also in the occipital and temporo-sphenoidal lobes. Trous- 
 seau narrates the case of an officer who was shot through the head 
 in the middle of the frontal lobes, and who showed until death, 
 which occurred from inflammation, no signs of any kind of paral- 
 ysis. The work of M. Pitres 3 contains a large collection of cases, 
 in which the frontal lobes have been the seat of extensive disease, 
 of softening, or of abscess, without any symptoms of laming what- 
 ever ; in most of which, also, no disturbance of psychical con- 
 dition was observed. That sudden extensive lesions may occur in 
 this region without inducing sensory or motor paralysis, is shown 
 in a marked way by the celebrated "American crowbar case." 3 
 By premature discharge of blasting powder an iron bar, three feet 
 seven inches in length and one and one-fourth inch in diameter, 
 was driven through the brain of a young man. The missile en- 
 tered at the left angle of the jaw, and passed through the top of the 
 head near the sagittal suture in the frontal region ; it was picked 
 up at some distance off, covered with blood and brains. The pa- 
 
 1 Vorlesungen iiber Physiol., II., p. 57. 
 
 2 Lesions du centre ovale. 
 
 a See the paper in the Am. Journal for Med. Sciences, by Dr. Bigelow, July, 
 1850, and the one read before the Mass. Med. So. by Dr. Harlow, June, 1868, 
 
GENEEAL MOTOR EEGION. 267 
 
 tient, although for the moment stunned, recovered in a few min- 
 utes so as to ascend a flight of stairs and give to the surgeon an 
 intelligible account of his injury. He lived twelve and a half years 
 afterward, with no noticeable impairment of his sensory-motor 
 powers. Examination of the skull showed that the substance de- 
 stroyed by the bar must have been confined to the frontal region, 
 with the possible exception of the tip of the temporo-sphenoidal 
 lobe. 
 
 Boyer narrates the case of an epileptic child, that showed, how- 
 ever, no other abnormal nervous phenomena, whose entire temporal 
 lobe on the left side was found to have been destroyed. Instances 
 of extensive lesions in the occipital lobes, without any resulting 
 sensory or motor disturbances, might also be given. 
 
 4. It must be confessed, in the words of Exner, 1 that the 
 understanding of cases of this sort " is made more difficult rather 
 than easier by recent researches." Nevertheless, a large amount 
 of concurrent testimony from all three main sources of evidence 
 proves that some theory may be framed in acknowledgment of a 
 more definite localization of cerebral function. Such theory can 
 be most clearly established with respect to the cerebral region 
 especially concerned in the motor functions. This region is the 
 one l^ing about the great central fissure, or fissure of Kolando ; 
 more precisely still, it embraces the gyrus centralis anterior, the 
 gyrus centralis posterior, and the prolongation of the two on the 
 median surface of the brain in the lobulus paracentralis. (Comp. 
 Figs. 87 and 88). More definite localizations still, of smaller re- 
 gions within the larger one e. g., for the upper limbs, for the lower 
 limbs, for the separate fingers, etc. are more doubtful ; they can 
 by no means appeal to the same amount of evidence as that at com- 
 mand of the more general induction. 
 
 5. The evidence from experiments in stimulation indicates that 
 we are to look for the so-called "motor areas" in the above-mentioned 
 convolutions about the fissure of Kolando. The original experi- 
 ments of Fritsch and Hitzig 2 located the five motor areas as fol- 
 lows : The centre for the muscles of the neck (marked A in the 
 figure) in the middle of the prse-frontal gyrus at the spot where its 
 surface falls off steep ; the centre for the extensor and adductor of 
 the fore-limb, at the outermost end of the post-frontal gyrus in the 
 region near the end of the frontal fissure (-f- in the figure) ; the 
 
 1 In Hermann's Handb. d. Physiol., II., ii., p. 334. 
 
 3 See Archiv f. Anat., Physiol., etc., 1870, p. 312 f.; comp. Taf. IX B. by 
 Hitzig in the same Archiv for 1873, from which the accompanying figure is 
 taken. 
 
268 
 
 STIMULATION AND EXTIKPATION. 
 
 FIG. 83. Hitzig's Motor Areas on the Cortex of the 
 Dog. The left hemisphere belongs to one animal, 
 the right to another; a, the sulcus cruciatus, 
 around which the gyrus Bigmoideus bends ; oooo, 
 area for the face. The other symbols are explained 
 in the text. 
 
 centre for the bending and rotation of the same limb, a little 
 farther back (+ in the figure) ; the centre for the hind-limb, in 
 
 the post-frontal gyrus but to- 
 ward the median line of the 
 hemisphere and back of the 
 preceding two centres (4j= in 
 F the figure) ; the facial centre, 
 * in the middle part of the 
 Q gyrus lying above the fissure 
 of Sylvius ($- O in the figure). 
 These experimenters found 
 4- also that the muscles of the 
 o back, tail, and abdomen, were 
 excited to contraction by 
 stimulating points lying be- 
 * tween those marked as above ; 
 but they could not definitely 
 circumscribe the cortical areas 
 
 w |>:ph wprp tn hp fl<*qi<rnprf fn 
 Wn 
 
 these muscles. 
 
 By reference to the chart 
 
 of the numerous "centres of 
 electrical irritation " which Ferrier ' claims to have discovered on the 
 cerebral hemispheres of the monkey, it will be seen that they are set 
 close together in the two 
 central convolutions 
 (gyri centrales, called 
 by Ferrier the " ascend- 
 ing frontal" and "as- 
 cending parietal ") and 
 in the immediately ad- 
 joining parts of the 
 frontal and temporo- 
 sphenoidal convolu- 
 tions. Thus, the cen- 
 tres (2, in part), (3), (4, 
 
 m part), (5, in part), 
 (6) (7 ) (8 ) (9) and 
 
 (10), are located in the 
 
 anterior central (" ascending frontal ") convolution ; (2, in part), 
 
 (4, in part), (11, in part), and (a), (b), (c), (d), are placed on the poste- 
 
 1 See The Functions of the Brain, pp. 141 ff. , 149 f. , and 305 f. London, 1876. 
 In the second edition (1886), pp. 240 ff. 
 
 FJQ 84 _ Areas on the Left Hemisphere of the Monkey, by 
 stimulating which Ferrier obtains motion in definite groups 
 of muscles 
 
EFFECT OF ELECTRICAL STIMULUS. 269 
 
 rior central (" ascending parietal ") convolution ; the centres (12) and 
 (5, in part) are situated on parts of the superior and middle frontal 
 convolutions adjacent to the anterior central; and (14) on the su- 
 perior temporo-sphenoidal convolution. 
 
 Further and more recent information seems to render the experi- 
 ments in the electrization of the cerebral areas of animals more 
 available for use in confirming the general argument in behalf of 
 some kind of localization of cerebral function in the case of man. 
 Luciani and Tamburini, as well as other experimenters, have agreed 
 with Hitzig and Ferrier in finding small, circumscribed motor cen- 
 tres on the cortex of the dog, monkey, rabbit, and other animals. 
 Some experimenters (Bochefontaine and Vulpian, e.g.) claim to 
 have discovered that the minute areas, at first excitable, after a 
 time cease to be so ; and that other areas, at first not excitable, 
 afterward become excitable that is, a displacement of the excit- 
 able points takes place. 
 
 More recently still, it has apparently been discovered 1 that Ex- 
 ner's view (to be explained subsequently) of the existence of " ab- 
 solute " and " relative " motor fields in the case of man is probably 
 applicable to the animals also. Paneth found that a number of 
 minute areas (or spots) for each one of the several groups of muscles 
 could be detected as lying in the larger " excitable zone " of the 
 cortex. These areas, unlike the immediately surrounding ones, 
 could be excited when cut around, but not when cut beneath ; the 
 fibres whose function it is to bring a definite group of muscles to 
 contraction seem then to proceed directly' from these cortical spots 
 to the lower parts of the brain. A number of such belong to each 
 muscle excitable ; but only two general " fields " are distinguish- 
 able, within which all the isolated motor spots are located ; one 
 field is for the extremities, the other for the orbicularis palpebrarum. 
 The former is situated in the posterior division of the gyrus sig- 
 moideus. The minute areas for the different muscles of the ex- 
 tremities are sharply limited ; they do not wholly cover each other ; 
 and those for any special muscle (the extensor digitorum of the fore- 
 foot, etc.) are of small extent in comparison with the field or zone 
 which may be looked on as common to all the extremities. The 
 excitability of the different muscles is not all alike ; this Paneth ex- 
 plains by assuming that the number of nerve-elements assigned to 
 each is not alike. 
 
 6. Experiments in extirpation confirm, at least in a general way, 
 
 1 See the art. of J. Paneth, "Ueber Lage, Ausdehnung, und Bedeutung der 
 absoluten motorischen Felder," etc., in Pfliiger's Archiv, xxxvii. (1885) pp. 
 520 ff. 
 
270 STIMULATION AND EXTIRPATION. 
 
 the above-mentioned results of experiments in stimulation. The 
 destruction of the substance of those cortical areas which respond 
 to the current of electricity with the co-ordinated movement of def- 
 inite groups of muscles, causes a temporary or permanent impair- 
 ment of the functions connected with the same groups of muscles. 
 In their first report Fritsch and Hitzig called attention to certain 
 experiments of their own in removing from the cerebral hemispheres 
 of two dogs the nervous substance of the centre which had already 
 been fixed upon by them as that for the " right fore extremity " 
 of the animal. These experiments they found confirmatory of the 
 views derived from stimulation. The animals operated upon, 
 when sitting or standing or running, used the right fore-leg un- 
 skilfully ; this part of the body, however, showed no marked dimi- 
 nution of sensibility under Jiard pressure. Other observers have 
 since performed many similar experiments ; especially Ferrier on 
 monkeys, and Goltz and Munk on dogs. Among them all no oth- 
 ers are so carefully refined as are those of Munk. l But the very 
 refinement of these experiments subjects them to more of distrust, 
 in certain particulars. 
 
 7. The earlier experiments of Munk were confined to the convex 
 surfaces of the parietal, occipital, and temporal lobes of dogs ; they 
 consisted in removing clean-cut circular bits of the cerebral sub- 
 stance about three-fifths of an inch in diameter and one-twelfth of an 
 inch thick sometimes simultaneously, from the symmetrical areas 
 of the two hemispheres, and sometimes with an interval between the 
 two operations. Munk's general conclusion is stated as follows : 
 If a line be drawn from the terminal point of the fissure of 
 Sylvius vertically toward the falx cerebri, it will mark, approxi- 
 imately, the limits of two spheres that are sharply distinguished 
 experimentally namely, an anterior motor and a posterior sensory 
 sphere. 3 Extirpations in front of this line always occasion dis- 
 turbances of motion, those back of it never so. More precisely, 
 the cerebral convolutions of the dog may, according to Munk, be 
 mapped out into the several spheres and regions indicated in the ac- 
 companying figure (Fig. 85). It will be noticed that three of these 
 regions (namely, C for the hind leg, D for the fore leg, and E for 
 the head) correspond pretty accurately to the centres of stimulation 
 fixed upon by Fritsch and Hitzig. Extirpations of the cortical sub- 
 stance in these regions, of only a few millimetres broad and not 
 more than two deep, are regularly followed by definitely localized 
 
 1 See his Gesammelte Mittheilungen aus d. Jahren 1877-80, in the book, 
 Ueber d. Functionen d. Grosshirnrinde. Berlin, 1881. 
 2 Munk, ibid., p. 11. 
 
MOTOR AREAS ' OF THE DOG. 
 
 271 
 
 disturbance of motion. For example, let the region (D) be removed 
 from the left cerebral hemisphere of a dog. At the end of from three 
 to five days and after the fever from the operation has subsided, 
 abnormal phenomena connected with the fore-leg of the opposite 
 side will be observed. If any other limb of the animal than the 
 right fore-leg be touched lightly, the dog will look quickly around ; 
 
 FIG. 85. Areas on the Brain of the Dog. (According to Munk.) A, centre of the Eye; S, of 
 the Ear ; C, of the sensations of the hind Leg ; D, of the fore Log ; E< of the Head ; F, of the 
 Apparatus for protecting the Eye ; (?, of the Region of the Ear ; ff, of the Neck ; J, of the 
 Rump. 
 
 and if of a bad temper will try to bite the offending hand. He 
 will also quickly withdraw any other limb when it is subjected to 
 even very slight pressure. But hard pressure and pinching or 
 sticking of the right fore-leg is either followed by no result, or else 
 by mere withdrawal of the limb, as though in reflex motion, without 
 any attention being paid to the attack. Moreover, this particular 
 limb, unlike all the others, can be put into unnatural and uncom- 
 fortable positions can be bent, stretched, set on the ground with 
 
272 STIMULATION AND EXTIRPATION. 
 
 the back of the foot down, etc. without any resistance on the 
 animal's part or any apparent disposition to remove it to the 
 normal and comfortable position. According to Munk, the animal 
 has apparently lost all mental picture of this one limb, and there- 
 fore all power to move it intelligently and voluntarily. If he has 
 been accustomed, on call, to put the right leg into his master's hand, 
 he will now respond with the left instead of the right foot to the 
 same call. The dog no longer handles his food with the right 
 foot. In running he slips on that foot. If he is drawn to the 
 edge of a table and the right leg forcibly stretched out over it, he 
 will allow the leg to hang down thus, although evidently aware of 
 the dangerous position in which this places him. Such an animal 
 can, however, still walk and run, using all four limbs. The " gross 
 mechanism " of motion to borrow Munk's phrase ' still acts as it 
 did before ; but the so-called " cerebral " or intelligent quality in 
 the management of this particular limb has been lost. 
 
 Gradually the phenomena which indicate impairment of cerebral 
 function as related to the movement of the fore-leg diminish in 
 magnitude. Less pressure is then necessary to secure the with- 
 drawal of the injured limb ; the dog is less surprisingly unskilful 
 in its use. At the end of four or five weeks the more marked 
 symptoms of his loss of function have probably disappeared ; at the 
 end of eight or ten weeks it may be difficult or impossible to dis- 
 tinguish his movements from those of a perfectly sound animal. 
 If, however, the size of the pieces of cerebral substance taken from 
 any of the so-called " motor regions " be somewhat larger than that 
 indicated above, recovery is slower and more imperfect. In the 
 opinion of Munk, if the extirpations are considerably enlarged the 
 restitution of function is never complete. 
 
 8. That explanation of the phenomena which regards the va- 
 rious cerebral regions, that seem somehow specially connected with 
 motor activities, as true " motor centres," that is, as areas of the 
 cerebral cortex that have for their peculiar function the initiating 
 of definite motor impulse on occasion of the idea and volition to 
 move definite portions of the periphery of the body, is rejected 
 by Munk. All the regions marked C J, belong rather to what he 
 calls the "feeling-sphere" of the cerebral hemispheres. It is an 
 undoubted fact that the definite co-ordination of the limbs, from the 
 higher cerebral centres, depends upon feelings of contact arid press- 
 ure of the skin, and upon muscular feelings or so-called feelings of 
 innervation. The effects of extirpating centres like (0), (D), and 
 (E), is due, therefore, first to the sudden loss, and subsequently to 
 1 Ueber d. Functionen d. Grosshirurinde, p. 47. 
 
MOTOR AREAS" OF THE MONKEY. 
 
 273 
 
 the gradual restitution of these feelings, and of their correspond- 
 ing mental representations, with respect to given groups of mus- 
 cles. 
 
 Schiff 1 agrees with Munk in the view that the real loss of function 
 due to the extirpation of the above-mentioned cerebral regions is 
 sensory rather than motor ; he considers that the impairment of the 
 power of moving these parts is only an expression of the loss of the 
 sense of touch in the same parts ; in other words, it is tactile an- 
 aesthesia. He calls attention to the significant fact that an animal 
 thus operated upon will freely allow parasites and insects to gather 
 on that surface of the skin whose corresponding cortical area has 
 been removed. Schiff also finds that the use w r hich the higher apes 
 
 FIG. 86. Areas on the Brain of the Monkey. (According to Munk.) The letters have the same 
 reference as in the preceding figure. 
 
 make of their limbs for grasping the rounds of a trellis or ladder 
 is not permanently impaired by removing to considerable depth 
 the convolutions about the central sulcus, unless the trellis or ladder 
 be turned at an angle of 60 to 70, so as to convert the animal's 
 walking into climbing. Apparently the animal cannot climb be- 
 cause he is unable to form a mental picture of the next round so 
 as to reach out and grasp it. Schiff therefore concludes, that "all 
 motions are suppressed (by extirpating the cerebral substance) 
 which, on being excited by the higher senses, receive a special 
 supervision on the side of cerebral sense, in relation to direction, 
 extent, and succession." He also asserts, in opposition to the 
 conclusions of Goltz, that the injured animal never recovers the 
 
 1 See Pfliiger's Archiv, xxx. (1883), pp. 313 ff. 
 18 
 
274 STIMULATION AND EXTIRPATION. 
 
 powers it has once really lost ; in other words, it is not possible to 
 extirpate any of the centres, excitation of which produces a given 
 motion, without effecting some permanent result. 
 
 9. The conclusions of Munk and Schiff undoubtedly have cer- 
 tain facts of experiment in their favor, but they can scarcely be said 
 either to cover all the facts or to be wholly consistent with certain 
 particular ones. Goltz : agrees substantially with Munk in finding 
 that destruction of the cerebral substance of the frontal lobe causes 
 the animal to execute movements of the limbs of the opposite side 
 in a coarse and unskilful manner. He also finds that the tactile 
 sense is temporarily impaired, although by giving increased atten- 
 tion the animal is able to feel the slightest touch on any area of 
 the skin. Indeed, deep and extensive lesions in this region may 
 be followed by hypersesthesia. The muscular sense, on the other 
 hand, seems permanently to suffer. Goltz's conclusions are squarely 
 contradictory of all those which find any permanent laming of any 
 muscle as the result of even the most extensive destruction of the 
 cortical substance in the so-called "motor field." His theory lays 
 more emphasis on the general impairment of intelligence which re- 
 sults from removing any considerable amount of the substance of 
 the brain, from whatever region it may be taken. 
 
 A still more recent investigator 2 calls attention anew to the facts 
 that an animal deprived of the " motor sphere " cannot use the ex- 
 tremities as hands ; cannot hold the foot out on call, or push away 
 the hand by which its chin is grasped, or stretch out the limb so 
 as to grasp the dish containing its food. These phenomena imply, 
 he thinks, some severance between the organ of will and the nerves 
 which execute the will. The motor centres are to be limited, it is 
 claimed, almost exclusively to the gyrus sigmoideus, and those for 
 feelings of the skin and muscles to the region lying above the fis- 
 sure of Sylvius. 
 
 On attempting to reconcile all the results of experiment upon 
 the animals with one another, and with the facts of human pathol- 
 ogy, it must be admitted that great difficulty is experienced ; and 
 even more difficulty when the effort is made to frame a consistent 
 theory which shall cover them all. On the whole, however, it 
 Beems obvious that a certain region or sphere of the cortex of the 
 brains of the higher animals is entitled to be called " motor" in a 
 special sense ; and that this region corresponds in a general way to 
 
 1 See his article in Pfliiger's Archiv, xxxiv. (1884), pp. 450 ff. 
 
 2 Bechterew, art. " Wie siud die Erscheinungen zu verstehen die nach 
 Zerstorung des motorischen Rindenfeldes an Thieren auftreten;" Pfliiger, 
 xxxv. (1885), pp. 137 ff. 
 
FROM ANIMALS TO MAN. 275 
 
 that which (as we shall soon see) pathology indicates as specially 
 motor in the case of man. Stimulation of various minute areas in 
 this region is followed by the movement of definite muscles of the 
 body ; extirpation of this region in its entirety, or in part, is fol- 
 lowed by special disturbances of the motor functions of the animal. 
 These disturbances are not of the kind which indicates so much 
 a laming of any particular muscle, as a loss of cerebral, and so in- 
 telligent, quality in respect to the handling of the extremities. 
 They probably imply more or less of all those various kinds of psychi- 
 cal disturbances and impairments of function, by some one of which 
 exclusively the different investigators are wrongly inclined to ac- 
 count for the phenomena which they observe. Extensive losses of 
 cerebral substance in the motor region result in the loss of those 
 tactile sensations and muscular sensations, by means of which the 
 animal localizes and interprets the meaning of objects, and adapts 
 the finer movements of its limbs accordingly. They also impair 
 the power to express the volition of the animal by motor impulses 
 started, in accordance with the sensations and images of motion, in 
 the appropriate area of the brain. Moreover, such loss of the 
 powers of sensation, sense-perception, and skilful motion, neces- 
 sarily implies more or less of loss of intelligence. 
 
 10. It will always be difficult to designate precisely what fac- 
 tors in the animal's complex sensory-motor activities drop out as the 
 result of the removal of a certain area of cortical substance from the 
 brain of a dog or monkey ; and whether these factors are exclusively 
 sensory or exclusively motor, rather than both sensory and motor. 
 It is doubtful if enough can ever be known, concerning the mental 
 life of the dog or the monkey, to determine confidently in this way 
 the question of the localization of psycho-physical functions. The 
 phenomena of human pathological cases indicate, however, that in 
 man the corresponding general area of the cerebrum that is, the 
 convolutions on both sides of the central fissure and the lobulus 
 paracentralis is especially concerned in both sensory and motor fac- 
 tors for co-ordinated action of the limbs. Without adducing further 
 confirmatory evidence from experiment upon other animals, we 
 pass to the consideration of the evidence from human pathology. 
 The results of experiments in stimulation and extirpation upon the 
 lower animals are not to be transferred in toto, as a matter of 
 course, to the human cerebrum ; they are rather to be consulted 
 as indicating the precise nature of the questions to be proposed to 
 pathology, and of the answers to these questions which are antece- 
 dently probable. 
 
 From this point onward our chief reliance must be placed upon 
 
276 
 
 HUMAN PATHOLOGICAL CASES. 
 
 Exner's ' careful and scientifically classified investigations. The 
 method pursued by this investigator is described at length by him- 
 self. 2 Exner began, with true German thoroughness, by reading 
 several thousand cases of cerebral disease which had been followed 
 by post-mortem examination ; the catalogue of works thus consulted 
 by him occupies more than twenty pages. From all these cases he 
 
 Cft, 
 
 FIG. 87. Lateral View of the Human Brain. (Schematic, Ecker.) F, frontal. P, parietal, O, 
 occipital, and T, temporo-sphenoidal lobes. 8, fissure of Sylvius, with S', the horizontal, and 
 8", the ascending ranms ; C, sulcus centralis ; A, anterior, and B, posterior, central convolu- 
 tions ; Fl, F2, F3, superior, middle, and inferior frontal convolutions ; fl, superior, f2, infe- 
 rior frontal sulci ; f3, sulcus praecentralis ; PI, superior, and P2, inferior parietal lobule; the 
 latter, the gyms supra-marginalis, and P2', the gyrus angularis ; ip, sulcus interparietalis ; cm, 
 end of calloso-marginal fissure ; Ol. O2, O3, occipital convolutions ; po, parieto-occipital fissure ; 
 o, transverse, and o2, inferior longitudinal sulcus ; Tl, T2, T3, tcmporo sphenoidal convolu- 
 tions ; and tl, t2, tempero spnenoidal fissures. 
 
 then made a collection of such only as could safely form the basis 
 of a scientific induction. The conditions of admittance into this 
 collection were as follows : Both the history of the disease and the 
 description of the post-mortem condition must be trustworthy, full, 
 
 1 Untersuchungen iiber d. Localisation d. Functionen in d. Grosshirnrinde 
 d. Menscheii. Wien, 1881. 
 
 2 Ibid, p. 6 f. 
 
THE METHODS OF RECKONING. 
 
 277 
 
 and unambiguous ; and there must have been no other lesion than 
 the one in the cerebral cortex, either elsewhere in the brain or in 
 the spinal cord, to complicate the legitimate inferences. Only two 
 exceptions to the latter rule were, for reasons peculiar to themselves, 
 admitted. Nearly all cases in which symptoms indicative of diffuse 
 meningitis occurred were also excluded. In this cautious way one 
 
 FIG. 88. View of the Human Brain from Above. (Schematic, Ecker.) The letters have the 
 same reference as in the preceding figure. 
 
 hundred and sixty-nine test-cases were secured from the thousands 
 recorded. These test-cases were then tabulated on three sets of 
 maps, according to the following methods of induction : (1) The 
 method of negative cases, (2) the method of "reckoning per cent.," 
 (3) the method of positive cases. 
 
 The method of negative cases (if the number of such cases were 
 large enough) would result in showing what regions of the cerebral 
 hemispheres, if any, are not necessarily connected with motor or 
 
278 
 
 HUMAN PATHOLOGICAL CASES. 
 
 sensory functions both, or either one respectively. The charts 
 constructed by this method would, accordingly, have only those 
 convolutions and parts of convolutions left blank, or unmarked, 
 in which no lesion had occurred that was not followed by some 
 given kind of motor or sensory disturbances. The method of per- 
 centage was designed to show the amount of probability that a 
 given small area of the cerebral cortex will be hit by disease, as 
 it were, in case the lesion has been followed by a given kind of 
 motor or sensory disturbance. For this purpose the entire sur- 
 face of one hemisphere was mapped out into three hundred and 
 
 FIG. 89. Median Aspect of the Right Hemisphere. (Schematic, Ecker.) CC, corpus callosum. 
 Gyri : Gf, fornicatus ; H, hippocampi (with its sulcus, h), and U, uncinatus ; PI', praecuneue: 
 Oz, cuneus ; oc, calcarine fissure, with its two rami, oc' and oc' 7 ; D, gyrus descendens ; T4, the 
 lateral, and T5, the medial, gyrus occipito-temporaliB. 
 
 sixty-seven quadrilateral areas, all small and yet of somewhat dif- 
 ferent sizes. As the different selected cases were recorded by 
 painting the area of the lesions on this set of maps, the intensity of 
 the color used would, of course, deepen in proportion to the cer- 
 tainty of a connection between that particular area and some par- 
 ticular sensory or motor function. Thus a perfect black would 
 indicate one hundred per cent, of cases in which a given quadri- 
 lateral was hit when a given disturbance of function had followed ; 
 pure white, nought per cent, of such cases. The third method (that of 
 positive cases) is the one usually relied upon to prove (?) the theory 
 of localization of cerebral function from pathology ; it is justly re- 
 garded by Exner as the least conclusive, as never of itself forming 
 
FIELD OF LATENT LESIONS. 279 
 
 a basis for anything beyond conjecture. Its principle is the as- 
 sumption that the region where the lesions connected with certain 
 disturbances are most thickly crowded together, is the required 
 cortical area with its specific function. 
 
 11. The result of Exner's comprehensive induction from patho- 
 logical cases, as based on all three of the methods just described, 
 fixes almost beyond doubt the so-called " motor areas " of the hu- 
 man cerebrum. [For understanding Exner's induction, consulta- 
 tion of Ecker's charts, found on p. 2761, figs. 87, 88, and 89, will be 
 found helpful.] The field of wholly "latent lesions" that is, of le- 
 sions which are not necessarily followed by any disturbances of 
 either sense or motion covers a large part of the surfaces of both 
 hemispheres ; it is not, however, precisely the same for them both. 
 While Exner's collection of cases comprised 67 lesions of the right 
 hemisphere, and 101 of the left, the absolute number of latent 
 lesions was the same (namely, 20) for both hemispheres. The 
 chances that a lesion of the right hemisphere will not be followed 
 by any disturbance of function are, therefore, about fifty per cent, 
 greater than the chances that the same thing will occur in the left 
 hemisphere. On the right hemisphere the entire surface, with the 
 exception of the two gyri centrales, the lobulus paracentralis, and 
 certain small portions on the convex and inferior surfaces of the 
 occipital lobe is latent. On the left hemisphere the latent region 
 is of less extent. This result may be regarded as a restatement, 
 on a basis of scientific induction, of the well-known fact that ex- 
 tensive lesions can occur in the frontal, temporal, and occipital 
 lobes, without being followed by any sensory or motor disturb- 
 ances. But it also confirms the impression that the portions of 
 the cerebral cortex lying about the fissure of Rolando are entitled 
 to be called " the exquisitely motor parts of the cortex." 
 
 Yet more precisely, the motor region on either hemisphere may 
 be, according to Exner, marked out by the method of negative 
 cases, and by the method of percentage of cases. The former meth- 
 od shows that, for the upper extremities, the corresponding cortical 
 region on the right hemisphere is the lobulus paracentralis, the gyrus 
 centralis anterior (with the exception of a small part of its lower 
 end) and the upper half of the gyrus centralis posterior. The latter 
 method further confirms the foregoing conclusion. It shows that 
 the "absolute field" for the upper extremities the field, that is, 
 within which lesions are always followed by impaired motion of 
 these extremities covers quite completely the same parts of cere- 
 bral convolutions ; while the " relative field," or portion in which 
 more than fifty per cent, of cases of lesions are followed by similar 
 
280 HUMAN PATHOLOGICAL CASES. 
 
 disturbances, extends over the remaining half of the gyrus centralis 
 posterior, the posterior third or half of the three frontal convolu- 
 tions, the anterior half of the parietal lobe, and more of the neighbor- 
 ing median surface. Corresponding to the better motor education 
 of the right arm is the fact that its motor region on the left hemi- 
 sphere is more extended. Here the absolute field comprises the 
 lobulus paracentralis, the three upper quarters of both gyri centrales, 
 and the greater part of the upper parietal lobe. Portions of the 
 median surface of the occipital lobe may also belong to this field. 
 The relative field for the upper extremity on the left hemisphere 
 includes the posterior half of the gyrus frontalis superior, almost 
 the entire convex surface of the other frontal gyri, the parietal lobe 
 at large, and the upper part of the occipital lobe. 
 
 More specific localization of cerebral areas, corresponding to the 
 different parts of the upper extremities, can as yet be accomplished 
 only with much less confidence and in a conjectural way. The 
 method of positive cases seems to designate the gyrus centralis an- 
 terior as the special cortical area for the hand ; with a probability 
 that the area for the extensors of the hand lies in its middle part, 
 and the area of the thumb somewhat below in the same gyrus. 
 
 12. Exner's collection contained 75 cases of disturbances of 
 motion in the lower extremities ; 26 lesions being on the right, 49 
 on the left hemisphere. The methods both of negative cases and 
 of percentage agree in indicating that the " absolute " cortical field of 
 the left leg comprises the lobulus paracentralis, the uppermost third 
 (as far as the lower end of the sulcus frontalis superior) of the 
 gyrus centralis anterior, portions of the corresponding third of the 
 gyrus centralis posterior, and some small areas behind and below on 
 the lobulus quadratus, all, of course, in the right hemisphere. The 
 " relative field " of the same limb on the same cerebral hemisphere 
 includes both lower thirds of the central convolutions, the back 
 parts of the frontal convolutions, the parietal lobules, and the up- 
 per portion of the occipital lobe. On the median surface of the 
 brain, the posterior part of the gyrus frontalis superior and the 
 anterior half of the lobulus quadratus belong to this field. On the 
 left hemisphere the absolute cortical field for the right leg includes 
 the lobulus paracentralis, the upper half of the gyrus centralis pos- 
 terior, and most of the upper portion of the parietal lobe. A small 
 lateral part of this lobe, and on the median surface the lobulus 
 quadratus, and perhaps the cuneus, must be added to complete the 
 relative field of this lower extremity. Exner does not consider it 
 possible, as yet, to be more precise in designating the cerebral fields 
 for the lower limbs of man. 
 
FIELDS OF FACE, TONGUE, AND NECK. 281 
 
 13. On comparing with each other the foregoing conclusions, 
 it is apparent that the absolute field for the upper extremities en- 
 tirely covers the corresponding field for the lower extremities ; but 
 the gyrus centralis anterior and lower half of the gyrus centralis 
 posterior belong only to this field for the upper extremities. The 
 relative fields, too, for both arms and legs, have a similar relation in 
 extent and intensity. There is considerably greater probability, 
 therefore, that a lesion of a given size in the motor region will af- 
 fect the arms than that it will affect the legs ; indeed, the collection 
 of Exner shows but one case in which the motions of the legs were 
 disturbed and not those of the arms. This greater " sensitiveness " 
 if we may so speak of the cortical region of the upper extrem- 
 ities, corresponds to the fact that their motion is more distinc- 
 tively cerebral and intelligent than that of .the lower extremities. 
 
 14. In this same " exquisitely motor " region of the cerebral 
 cortex, and in the most nearly adjacent regions of the frontal and 
 parietal lobes, certain other cerebral fields corresponding to definite 
 muscles or groups of muscles may be localized, conjecturally. By 
 the method of percentage the cerebral area for those muscles to 
 which the facial nerve is distributed may be rather indefinitely in- 
 dicated as lying in the lower half of the gyrus centralis anterior, and 
 lower third of the gyrus centralis posterior, on the right hemisphere ; 
 while, on the left hemisphere, it appears to be more definitely fixed 
 at a small strip that belongs to the gyrus centralis anterior, and lies 
 between the places where the inferior and superior frontal gyri 
 spring from this central gyrus, but nearer the first of the two. 
 Both methods of induction apparently unite in indicating the 
 cortical region for the tongue as lying where the middle and lower 
 frontal gyri meet with the anterior central gyrus. In the nine cases 
 of the collection in which the muscles of head and neck were af- 
 fected, the lesions were all situated in one of the central convolu- 
 tions ; but a more definite localization within the limits of these 
 convolutions does not appear to be possible. As to the localization 
 also of the cerebral field for the muscles of the eyeball, including 
 that for raising the. upper lid, pathology is able only to say in a 
 general way that this field appears to fall within the general motor 
 area as thus far pointed out. Exner thinks it certain that the 
 rectus interims muscle of one side, and the rectus externus of the 
 other side, are innervated from the same hemisphere of the brain. 
 This we should also argue from their ordinary physiological func- 
 tion. 
 
 15. Positive cases of a nature to strengthen the foregoing in- 
 duction as to the cerebral areas especially connected with the upper 
 
282 HUMAN PATHOLOGICAL CASES. 
 
 and lower extremities might be indefinitely multiplied. Especially 
 interesting are those where disuse, through accident or disease, of 
 one of these extremities has been found, post-mortem, to have re- 
 sulted in atrophy of the corresponding cortical fields. That is to 
 say, the cortical region, being unused on account of the loss of 
 function in the peripheral member, has itself paid the penalty of 
 all failure to exercise the normal functions ; it has lost in size and 
 strength. For example, atrophy of the upper end of the gyrus 
 centralis anterior of the right hemisphere, and of its prolongation 
 in the lobulus paracentralis, was in one case found to have resulted 
 from the amputation of the left leg twenty years before death. 
 
 16. The conclusions of other authorities as to the motor re- 
 gions of the cerebral cortex in man especially of Lepine, ' and of 
 Charcot and Pitres 2 as based on pathology, confirm those of Exner 
 in the main, as well as also in some interesting particulars ; any di- 
 vergences arise almost wholly from the effort to make distinctions 
 more nicely than the present condition of the facts will warrant. 
 The most general conclusions of these investigators may be summed 
 up as follows : 8 " The cortex of the cerebral hemispheres in man 
 may be divided, functionally, into two parts ; motor and non-motor, 
 according as destructive lesions do or do not cause permanent par- 
 alysis of the opposite side of the body." . . " The motor zone in- 
 cludes only the ascending frontal and ascending parietal convolutions 
 and the paracentral lobule." It may be concluded then, as a well-es- 
 tablished induction, that the convolutions on either side of the fissure 
 of Kolando (the gyri centrales anterior and posterior) and the con- 
 nected lobule on the median surface of the brain (lobulus paracen- 
 tralis) are in the highest degree especially connected with the mo- 
 tion of the extremities of the body ; that adjacent parts of the 
 frontal and parietal lobes are thus connected in a less degree ; that 
 the cortical region for the arms lies, on the whole, anterior to that 
 of the legs ; and that, probably, the region for the hand is near the 
 middle part of the front central convolution, and that for the tongue 
 where the middle and lower frontal convolutions meet the front 
 central. More precise localization of the motor functions of man 
 must as yet be made with a. lower degree of confidence. Beyond 
 these general statements lies the undefined field of conjecture. 
 
 17. It cannot be said that histology and comparative anatomy 
 
 1 Localisation dans les maladies cerebrales. Paris, 1875. 
 
 2 Localisation dans les maladies du cerveau, Paris, 1876 ; Revue mensuelle 
 de Med. et de Chir., 1877-1879; tude critique et clinique de la doctrine 
 dans l'6corce des hemispheres ce"rebraux de 1'homme, Paris, 1883. 
 
 8 See Brain, July, 1884, p. 270 f. 
 
TESTIMONY OF HISTOLOGY. 283 
 
 much affect the strength of the argument for the localization of the 
 cerebral motor regions, as derived from experimentation and pa- 
 thology ; whatever evidence they do furnish, however, is confirma- 
 tory of the conclusions reached above. In this connection refer- 
 ence may be made to the conclusion of Meynert,' that the paths 
 of the sensory nerves run more toward the occipital, and those of 
 the motor nerves toward the frontal region of the cerebrum. The 
 existence of nerve-cells of gigantic size, resembling those found in 
 the motor region of the spinal cord, which Betz discovered in the 
 motor regions of the cerebrum of the dog, the monkey, and of man, 
 is an indication in the same direction. It should also be mentioned 
 that the pathological researches of Pitres a into the results of lesions 
 in the medullary substance lying between the cerebral cortex and 
 the basal ganglia, seem to show that such as occur in the fronto- 
 parietal portion of this substance c'ause paralysis of motion and de- 
 generation of the motor tracts. Finally, the general structure of the 
 cerebrum and the courses of its nerve-tracts, as already considered 
 (Part I, Chapters H and DX), are, in the main, accordant with 
 the facts of experimentation and pathology. 
 
 18. The remarkable degree of coincidence in locality which 
 obtains among those circles whose extirpation is followed by dis- 
 turbances of motion (disturbances due, in the opinion of Hitzig, 
 to destruction of the physical basis of the animal's control over its 
 limbs, but, in the opinion of Schiff, rather due to tactile anaesthe- 
 sia) suggests the following question : Is not the cortical field of 
 tactile sensation in the extremities of man coincident, in the main, 
 or even in particular, with the field for the motion of the same ex- 
 tremities? An affirmative answer to this question would seem 
 reasonable, even prior to experimental and pathological evidence. 
 The sensory and motor mechanisms are, of necessity, most inti- 
 mately connected, locally, in all the central organs. This state- 
 ment is certainly true of the spinal cord and of the inferior parts 
 of the brain. Moreover, in consciousness the sensations which 
 guide the volitions in all the finer uses of the peripheral parts of the 
 body are very promptly, and even almost inextricably, interwoven 
 with the volitions. In walking, talking, handling a tool, or plajdng 
 a musical instrument, to be unable to experience certain delicate 
 sensations is to be unable to will the execution of corresponding 
 nicely adjusted motions ; whereas the appearance of the associated 
 sensations may instantaneously call forth the requisite volitions. 
 It would seem, then, that the cerebral mechanism for both the 
 
 1 Sitzgsbr. d. Wiener Acad., LX., heft iii., p. 455 f. 
 
 2 Le'sions du centre ovale. Paris, 1877. 
 
284 HUMAN PATHOLOGICAL CASES. 
 
 sensory and the voluntary motor factors of these complex functions 
 must be composed of elements having the closest local connection. 
 Certain indisputable facts of pathology form, however, a strong 
 objection to such an identification of the cerebral fields of motor 
 function and tactile sensory function. Many cases of motor dis- 
 turbance occur without the disturbance of sensation in the same ex- 
 tremity ; and cases of sensory disturbance without corresponding 
 motor disturbance, although much less frequent, are by no means 
 very rare. How, then, can these facts of pathology be reconciled 
 with any hypothesis which locates in the same cerebral region the 
 so-called " fields " for both classes of function ? 
 
 The answer which Exner gives to the foregoing question is per- 
 tinent, but not wholly conclusive. No absolute cortical field for 
 disturbances of tactile sensation in the extremities of the body can, 
 indeed, be pointed out ; that is to say, there is no portion of the 
 cerebral cortex, lesions of which are invariably and necessarily fol- 
 lowed by tactile anaesthesia, hypersesthesia, etc., in definite parts of 
 the periphery. But that the entire relative field of sensations of touch 
 in the extremities corresponds with that of the motor activities, is 
 made highly probable by the method of positive cases. After ex- 
 cluding doubtful cases in which the patient complained rather in- 
 definitely of a feeling of " heaviness" or " numbness," etc., in some 
 area of muscle or skin Exner's collection was found to contain 
 22 cases where marked disturbances of tactile sensations seemed 
 clearly made out. Of these 22 cases no fewer than 16 were located 
 wholly in the two central convolutions ; and 3 of the remaining 6 
 extended for several millimetres into the same convolutions. Of 
 those still remaining, the one farthest removed from the " exquisitely 
 motor " region was in the gyrus angularis, and therefore in a portion 
 of the relative motor field which has a considerable per cent, of in- 
 tensity. 
 
 On the basis of so complete an agreement of the positive 
 cases, Exner feels warranted in affirming that "the tactile cortical 
 fields for the different divisions of the body coincide in general 
 with their motor cortical fields." It is to be noted, moreover, that 
 the percentage of the cases of disturbance of tactile sensations 
 occurring on the right hemisphere is more than twice as large as 
 that of the left. Sensibility seems, then, to be the predominating 
 function of the right hemisphere, as motion is of the left. This 
 fact, when taken in connection with the greater liability of the left 
 hemisphere to be the seat of cerebral disease, accounts in part for 
 the less frequent occurrence of sensory disturbances following le- 
 sions in this general area. Moreover, we are warranted in assum- 
 
THE CONCLUSIONS OF 
 
 ing that the cortical fields, in which the nervous impulses occasion- 
 ing tactile sensations are projected, are connected with each other, 
 and with the ascending sensory tracts, in a very complicated way. 
 The manner of this connection is doubtless different for the different 
 areas of muscle and skin. Nor does it appear that the sensory areas 
 are so well differentiated as the corresponding motor areas ; although 
 one case, at least, can be pointed out in which loss of sensation in 
 the thumb and index-finger was the definite result of a lesion of 
 the very limited cortical region already conjecturally assigned to 
 these members. Finally, it must be remembered that those descrip- 
 tions of pathological cases on which all our inductions have hitherto 
 been based, are very liable to be faulty with respect to slight dis- 
 turbances of sensibility. 
 
 It is a general conclusion, then, which is entitled to a large de- 
 gree of confidence, that both the gyri centrales, the lobulus para- 
 centralis, and the most nearly adjacent parts of the frontal and 
 parietal convolutions, constitute a cortical region especially related 
 to both the motor and the sensory functions of the extremities of 
 the body. 
 
 The view of Exner concerning the nature of the motor area in 
 man is, on the whole, greatly strengthened by the most recent con- 
 clusions of Luciani. This experimenter finds ' that total or partial 
 extirpation of the " motor zone " in the dog and the monkey is 
 uniformly followed, not only by motor paralysis, but also by cuta- 
 neous and muscular anaesthesia. The " motor " sphere and the 
 " tactile " sphere are largely coincident in these animals ; and " in 
 all experiments upon the tactile sphere there was a manifest and 
 constant crossing of the relations between the peripheral sensory 
 fibres and their respective cortical centres." "What one calls 
 'motor zone ' is the central focus of the large portion of the senso- 
 rial sphere visible on the external aspect of the hemisphere." 
 
 19. The testimony of the facts upon which reliance must be 
 placed in the effort to localize the cerebral field for sensations of sight 
 and hearing in man is by no means so satisfactory as the foregoing. 
 Experiment upon animals by stimulation is of no direct value ; it 
 could at most only discover the cortical regions especially related 
 to some of the motions of the eye or ear and their surrounding 
 parts. Our conclusions from the method of extirpation also must 
 always be somewhat uncertain, since we infer the sensations of the 
 animal only by interpreting his motions into terms of our own self- 
 consciousness. It is not strange, then, that the leading experi- 
 menters differ irreconcilably in certain of their conclusions. There 
 1 See an abstract of his results, in Brain, July, 1884, pp. 145 ff. 
 
286 CEREBRAL FIELDS OF SENSATION. 
 
 is pretty general agreement at present, however, as to the localiza- 
 tion of sight somewhere in the occipital lobe. Hitzig l found that 
 the removal of certain gyri in the posterior lobes of the dog pro- 
 duced blindness of the opposite eye, combined with a paralytic 
 dilatation of its pupil ; stimulation of the same gyri produced con- 
 traction of the pupil. Ferrier 2 claims that destruction (by cauter- 
 ization chiefly) of the gyrus angularis of apes produces blindness of 
 the opposite eye, and this loss of function alone ; stimulation of the 
 same region causes movements of the eye. He therefore considers 
 this convolution as pre-eminently the cortical centre of sight. But 
 Munk, after numerous experiments upon dogs, and some upon 
 monkeys, locates the centre of sight above and behind the place 
 assigned it by Ferrier namely, in the upper and hinder part of the 
 occipital lobe ; the gyrus angularis, on the contrary, he makes the 
 cortical region for the tactile sensations of the eye. Munk's ex- 
 periments are so minute in carefulness, and his conclusions so 
 based upon detailed analysis of the phenomena, that they perhaps 
 deserve to suggest to pathology the exact form in which to put its 
 inquiry. They are, undoubtedly, excessive, however, in the refine- 
 ment to which they would carry the principle of localization. 
 
 20. Munk details the following among other phenomena which 
 result from extirpating the region marked A t (see Fig. 85) from the 
 brain of a dog. The animal thus operated upon is in a condition 
 to which the name of "psychical blindness" (Seelenblindheit) is 
 given ; but it has suffered no other obvious impairment of its sen- 
 sory or motor functions. By " psychical blindness " is meant the 
 inability of the dog to form those visual mental images or ideas 
 which give it the meaning or interpretation, as it were, of its visual 
 impressions. This includes the loss of the use of that portion of 
 the retina which is necessary for distinct vision, and of the immedi- 
 ately surrounding retinal parts. If the region A t be removed from 
 both hemispheres of the brain, when the animal has recovered from 
 the inflammatory reaction, it will still move about freely, guiding 
 itself by sight even under difficult circumstances. But it does not 
 recognize by sight the dish from which it has been accustomed to 
 take food or water, the companions with which it has formerly 
 played, the man who has been its keeper, the threatening hand or 
 
 1 Centralb. f. d. med. Wissenschaft, 1874, p. 548. 
 
 2 The Functions of the Brain, p. 164 f. In the second edition (p. 271 f.) 
 Ferrier acknowledges that he was in error in localizing the visual centres in 
 this gyrus to the exclusion of the occipital lobes. For a very telling criticism 
 of this position of Ferrier, see Munk, Ueber d. Functionen d. Grosshirurinde, 
 p. 14 f. 
 
THE SIGHT- CENTEE OF MUNK. 287 
 
 whip, the burning coal held before its face. It still retains its gen- 
 eral intelligence and makes constant and diligent investigation into 
 the objects by which it is surrounded. As time passes, it gradually 
 learns to recognize again all these visual objects. The more com- 
 plex and infrequent of the objects are the last in the process of re- 
 covery to receive interpretation. At the end of three to five weeks 
 after the operation, the injured animal may be said to have recov- 
 ered ; its restlessness and curiosity have subsided in proportion to 
 the progress made in the knowledge of visual impressions ; it is 
 itself at last, its " soul-blindness " having departed. It may be 
 shown, moreover, that this recovery consists in learning anew the 
 meaning of visual impressions ; or, in other words, in acquiring 
 anew the stock of visual ideas that has been blotted out of the ani- 
 mal's mind by extirpating the cortical centre of sight. For if the 
 dog be carefully kept, for a long time, from any given kind of ex- 
 perience for example, from being struck with a whip or burned 
 with a coal it will give no sign of " psychical sight " in relation to 
 these particular objects. More remarkable still is the fact that, 
 according to Munk, 1 in certain cases, after the extirpation of A 1? a 
 single visual image or two for example, the motion of the hand 
 commanding the dog to hold out the foot may be retained. Ex- 
 tirpations of the cortical surface on the occipital lobe in the regions 
 marked A that is, before, beneath, or in front and above, the sight- 
 centre A, cause disturbances of sight in a less degree. Such phe- 
 nomena Munk considers explicable by the hypothesis that, while a 
 large part of the area of the occipital lobe is the seat of the percep- 
 tions (?) of sight, the visual images of memory are especially con- 
 nected with the so-called sight-centre A r When, then, all, or nearly 
 all, of the field of sight, in the widest sense, is extirpated from both 
 hemispheres, complete and permanent "soul-blindness" results. 
 The cortical projection-field corresponding to the entire retinas of 
 both eyes, its accumulations of old visual ideas, and capacity for 
 receiving new ones, has been wiped out. 
 
 Munk endeavors to establish a still more minute differentiation 
 of function in the cortical field of sight as corresponding to the ret- 
 inal field of sight. 3 Each retina, he holds, stands for the most part 
 in connection with the visual sphere of the cortex of the opposite side 
 of the brain ; only a small part namely, the extreme lateral por- 
 tion of the retina is in connection with the cortical sphere of the 
 same side. This lateral portion of the retina seems to be of differ- 
 ent dimensions in different races of dogs. Further, the retina is 
 
 1 Die Functioneu d. Grosshirnrinde, pp. 23, 34, 119 f. 
 
 2 See the " Fuiifte Mittheilung " of his Work, as cited before. 
 
288 CEREBRAL FIELDS OF SENSATION. 
 
 projected, as it were, on the cortical field of vision in and about A lf 
 in such manner that its lateral area corresponds to the lateral area 
 of the cortical sphere on the same side ; its inner area to the median 
 area of the cortical sphere on the opposite side ; its upper area to 
 the front area of the cortical sphere on the opposite side ; its lower 
 area to the hinder area of the cortical sphere on the opposite side. 
 
 In monkeys, as well as dogs, Munk finds that the sight-centre is 
 not, as Ferrier at first supposed, the gyrics angularis, but rather the 
 convex surface of the posterior lobes. Small circular extirpations, 
 of not more than two-fifths or three-fifths of an inch in diameter, 
 from this region are followed by disturbances of vision, and by these 
 alone. If the whole convex surface of one lobe is extirpated, the 
 animal has cortical blindness for those halves of both retinas that 
 are on the same side as the lesion. If the convex surfaces of both 
 posterior lobes are destroyed, the animal becomes entirely blind ; 
 no restoration of cerebral function subsequently takes place, unless 
 some considerable parts of the edges on the upper surface of at 
 least one lobe have escaped destruction. The cortical projection- 
 field for the visual impressions of the monkey differs from that of 
 the dog simply in having the lateral part of the retina, which cor- 
 responds to the cortical area of the same side, much more extended. 
 Accordingly, extirpation of the lateral half of the left cortical sight- 
 centre, and of the median half of the right cortical sight-centre, 
 produces in the monkey total cortical blindness of the left eye. 
 
 21. The searching examination which the views of Munk have 
 received has resulted in throwing doubt over some of his alleged 
 facts, and in discrediting several most important points in his hy- 
 pothesis. This is true especially of the work of Lob and Luciani, 
 both of whom have gone thoroughly over the ground covered by 
 Munk and come to conclusions dissenting from him. The former * 
 has minutely investigated the effects of destroying Munk's visual 
 centre A t , and even his entire visual sphere in the case of dogs. 
 He finds, contrary to Munk, that no blindness of the clear spot of 
 vision in the opposite eye is produced even by the most extensive 
 lesion of this area ; that losses of the cortical substance in the 
 area bordering on the lateral part of the visual sphere (i.e., in 
 Munk's auditory sphere) also produce disturbances of vision ; that 
 other disturbances of motion and intelligence also follow destruc- 
 tion of this area ; and that disturbances of sight may follow lesions 
 in other than the occipital lobes, especially in the frontal lobes. 
 This last conclusion agrees with the results obtained by other ob- 
 servers (Kriwotorow, Luciani and Tamburini, and especially Goltz), 
 1 See articles in Pfluger's Arcliiv, xxxiv. , pp. 67 ff. 
 
AND OTHERS. 289 
 
 and must be accepted as correct. The more permanent disturb- 
 ances which undoubtedly do follow injury of the occipital lobes 
 are thought by Lob to be clue to what is called a " homonymous 
 lateral hemiamblyopia " (or weakness of the corresponding lateral 
 half of the eye) on the opposite side. Munk's whole theory of 
 " psychical blindness " as due to the extirpation of visual percep- 
 tions and images, and of recovery from such blindness as due to 
 special education of the animal in forming new mental images, is 
 rejected by Lob. 
 
 The admirable observations of Luciani also tend to disprove 
 many of the particular conclusions of Munk, while at the same 
 time showing how relatively important are the occipital lobes in re- 
 spect to the cerebral and psychical elements of vision. These lobes, 
 together with the angular gyrus, are in a peculiar degree the re- 
 gion on which the animals are dependent for "psychical" vision 
 that is, for "discernment of things, and a right judgment con- 
 cerning their properties and their nature," by sight. 
 
 The foregoing general conclusions from experiment with the ani- 
 mals as to the especial importance of the occipital lobes for intelli- 
 gent (or " psychical ") vision are, on the whole, in accordance with 
 the indications from human pathology. Even Lob testifies that 
 after extirpating part of the occipital lobes he has never observed 
 a mere motor disturbance without one of vision also ; whereas after 
 extirpating part of the parietal lobes he has never observed a dis- 
 turbance of vision without a motor disturbance. 
 
 22. The answer of pathology to the question, whether the cere- 
 bral field especially connected with visual sensations and ideas is 
 the same in man as in the dog and the monkey, is not unambigu- 
 ous. The method of negative cases, according to Exner, 1 yields no 
 certain results ; no "absolute field" for vision can as yet be indi- 
 cated on the cerebral cortex. The methods of percentage and of 
 positive cases, however, point clearly to the occipital lobe as the 
 visual field, and to the upper end of the first gyrus occipitalis (Ol, in 
 Ecker's charts ; see p. 276 f.) as its most intensive portion. In six out 
 of seven cases of disturbances of vision due to cortical lesion the seat 
 of the lesion was here. The region of less intensity extends over 
 both' the first and second occipital convolutions, the cuneus, and 
 the adjacent part of the lobulus quadratics. Confirmatory evidence 
 may be found in the cases of several persons for a long time blind, 
 whose brains have been found on post-mortem to be atrophied 
 above the place where the parieto-occipital fissure emerges from 
 the median surface upon the convex surface of the occipital lobe. 
 
 1 Untersuchungen uber d. Localisation, etc., p. 60. 
 19 
 
290 CEREBRAL FIELDS OF SENSATION. 
 
 It should be said, on the other hand, that lesions of the occipital 
 lobes are very frequently latent, and that extensive injuries of this 
 cortical field in man are recorded which were followed by no 
 marked disturbance of sight. 
 
 23. Histology also has some evidence to contribute regarding 
 the nervous connections of the retinas of the eyes with the cerebral 
 cortex. The amount of crossing which the fibres of the optic nerve 
 undergo in the optic chiasm has been the subject of much debate. 
 It undoubtedly differs in different animals, and depends upon the 
 structures of both retina and brain, and upon the relations of the 
 two. The researches of von Gudden l and others have tended to 
 show that each optic nerve contains both a bundle of nerve-fibres 
 that is crossed and one that is uncrossed, in the optic chiasm or 
 beyond it, toward the cerebral connections of the nerve ; and that 
 the former bundle increases and the latter diminishes in size, on 
 the whole, in the higher orders of animals as compared with the 
 lower. Biesiadecki and others claim, on the contrary, that there 
 is total decussation of the optic nerves in the monkey and in man, 
 as well as the lower animals. Charcot 2 has propounded a yet more 
 elaborate scheme of decussation. In the case of man there is still 
 doubt, therefore, how far if at all the retina of each eye is repre- 
 sented on the cortical surface of both hemispheres of the brain. 
 That the cortical region especially concerned in the sensations, 
 perceptions, and images of sight is in the occipital lobe, and es- 
 pecially on its upper convex surface, is a highly probable conject- 
 ure. But for the settlement of further details we must await the 
 development of the evidence. In the work of this development, 
 experiment with animals can only suggest the question which a 
 more careful collation of a growing number of cases in human 
 pathology will perhaps finally answer ; meanwhile the evidence of 
 histology may be used to confirm or modify the conclusions estab- 
 lished, more or less conjecturally, on the basis of pathology. 3 
 
 g 24. The localization of other sensory functions in so-called 
 " fields" or " centres " on the hemispheres of man's brain of hear- 
 ing, taste, and smell is even more doubtful. Little confidence can 
 be placed in any conjectures thus far put forward. The tempta- 
 tion is naturally strong to suspect that those regions of the dortex 
 unoccupied by such motor and sensory functions as we are able to 
 
 ' Grafe's Archiv f. Ophthalmologie, 1874, Abth. ii.; 1875, Abth. iii.j 1879, 
 Abth. i. 
 
 '< Le Progres Medical, August, 1875. 
 
 9 For a further description of phenomena and cases, and for a defence of his 
 own views, see Ferrier, The Localization of Cerebral Disease, pp. 110 ff. 
 
CENTEES OF SMELL AND TASTE. 291 
 
 locate should have the other mental phenomena assigned to them. 
 In this way the entire brain appears to be made of some definite 
 value and use. Convolutions which are located where they are 
 unapproachable for purposes of experiment, and in which compar- 
 atively few cases of lesion occur, are peculiarly provocative of con- 
 jecture. In such fields of the cerebral cortex, theories of localiza- 
 tion may roam at will. The auditory centre is assigned by Terrier ' 
 to the superior temporo-sphenoidal convolution ; but the evidence 
 adduced in proof such as the pricking-up of the animal's ears, 
 etc. is highly unsatisfactory. The same centre is located by 
 Munk 2 at the region Bl, for its greatest intensity, and with less 
 intensity in the adjacent regions marked B ; but since the entire 
 region on both hemispheres must be extirpated (an almost certain- 
 ly deadly operation) in order that the animal may become wholly 
 " soul-deaf," and since we have no sure means for ascertaining 
 precisely to what deficiency we should ascribe the failure of the 
 animal to respond intelligently to sounds, Munk's experimental 
 proof is likewise unconvincing. Luciani, with much more proba- 
 bility, considers the "auditory sphere" to extend over the whole 
 cortical area of the temporo-sphenoidal lobe, and probably also the 
 cornu ammonis. 
 
 The centres of smell and taste are located by Ferrier close to- 
 gether in the subiculum and neighboring parts of the lower temporo- 
 sphenoidal convolutions ; the centre of touch in the gyrus hippo- 
 campi and hippocampus major. Munk, 3 however, regards these 
 centres of Ferrier as " phantasms." He is strongly inclined, on the 
 basis chiefly of one well-differentiated case, to localize smell in the 
 gyrus hippocampi. It is difficult to see how anything sufficiently 
 definite for scientific purposes can be known as to disturbances of 
 taste in a dog or a monkey. No adequate evidence is procurable as 
 yet for an induction from human pathological cases in regard to 
 the cortical fields of any of these so-called lower senses. 
 
 25. To the foregoing remark a possible exception must be 
 allowed for the sense of hearing. In this connection belongs the 
 noteworthy localization of the cerebral functions concerned in the 
 utterance and interpretation of articulate speech. The various de- 
 ficiencies in the power of producing and interpreting articulate 
 sounds, whether as spoken or written, which are due to lesions of 
 the cerebral cortex, may be grouped together under the general 
 
 ! The Functions of the Brain, p. 171 f. ; comp. The Localization of' Cerebral 
 Disease, p. 132 f. 
 
 2 Ueber d. Functionen d. Grosshirnrinde, p. 22 f . ; 40 f . 
 
 3 Ibid., p. 129. 
 
292 DISTURBANCES OF SPEECH. 
 
 term "aphasia." For about a decade 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 cere- 
 bral hemisphere were nearly all to which any advocate of the local- 
 ization of cerebral function could confidently appeal in behalf of 
 his theory. As long ago as 1825, Boillaud 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 gyms frontahs 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 " ' 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 to 
 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 
 phenomena of aphasia show some special connection of certain 
 cerebral centres with the complex process of apprehending and ex- 
 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. 
 
 26. 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. 2 A few of the more curious and instructive in- 
 
 1 Sur le si&ge de la facult6 du langage articule, etc., Bull, de la Soc. anat., 
 August, 1861 ; Du siege de la faculte du langage articule dans Phemisphere 
 gauche du cerveau, Bull, de la Soc. d'anthropol. , June, 1865. 
 
 8 For the whole subject, see the great monograph of Kussmaul, in Ziemssen's 
 Cyclopaedia, xiv., pp. 581-875. 
 
THE KINDS OF APHASIA. 293 
 
 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 serviceable 
 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 "Bon jour, monsieur," tolerably well, 
 could not pronounce the word "bonbon" at all. Another, whose 
 vocabulary was almost entirely limited to the meaningless syllables, 
 " 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, tou jours, and 
 Le Lo instead of Le Long." The first two and the last were used 
 with their appropriate meaning ; " tois " indicated all ideas of 
 number 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 sometimes 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. 
 "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 " 
 1 Ziemssen's Cyclopaedia, xiv. , p. 759. 
 
294 DISTURBANCES OF SPEECH. 
 
 and " word-blindness." Persons thus afflicted hear words as con- 
 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. 
 
 The agraphia, or inability to express thought in written language, 
 which not infrequently accompanies aphasia, may be incomplete, 
 or absolute and literal. Some patients, who have formerly been 
 highly cultivated, become unable to produce a single letter with 
 the pen. Others can write long rows of letters, but arrange them 
 for the most part in meaningless fashion, with an intelligible word 
 occurring here and there. In brief, the phenomena more or less 
 closely connected with the disturbance called aphasia are exceed- 
 ingly complex and various. 
 
 27. In the effort to classify so many complicated facts, and to 
 distinguish among them such as are of truly cerebral origin, Ex- 
 ner l makes the following distinctions : First, there are cases 
 where the understanding of the words is affected ; and such loss 
 may constitute the chief or the entire part of the aphasia. The 
 patient can then hear and articulate, but the "acoustic image " of 
 the word as the symbol of an idea has perished. In a second form 
 of aphasia, the inability concerns the clothing of the result of 
 thought (the idea) in words whether for purposes of spoken or of 
 written expression. In most such cases it is simply the appro- 
 priate word which is forgotten. In the third class of cases, the 
 aphasic person can form the idea and select the word appropriate 
 to express it, but cannot bring about those processes of central in- 
 nervation which are necessary to initiate the expression. All these 
 three forms may combine variously, and all may be connected with 
 disturbances of speech which are not to be localized in the cerebral 
 cortex, but which have their origin at some point in the extra-cor- 
 tical nerve-tracts concerned in speech. The very elaborate analysis 
 of Kussmaul leads him to make the following statement : "All 
 disturbances of speech can be brought under two great classes } 
 according as the connection between the conception and the word 
 is impeded in the direction from the former to the latter or, vice 
 versa, from the latter to the former. When the first happens, the 
 1 In Hermann's Handb. d. Physiol., II., ii., p. 343 f. 
 
CORTICAL AEEAS OF APHASIA. 295 
 
 expression suffers ; when the second, the understanding." By the 
 last word, however, we must mean the " understanding " as applied 
 especially to articulate speech. For aphasic persons are often very 
 intelligent in carrying on the trains of thought necessary to suc- 
 cess in games of skill, or in the expression of feeling in music ; and 
 if we accept, even with considerable allowances, the intelligent 
 testimony of Lordat concerning his own mental condition when 
 aphasic, they sometimes exercise the mind in abstract reasoning 
 of a high order, even when unable to recall a single word appro- 
 priate for the expression of their thoughts. 
 
 In all true aphasia, then, the connection between ideas and ar- 
 ticulate language is interrupted within the cerebral cortex. Is it 
 possible to indicate any region of this cortex, lesions in which are 
 regularly accompanied by aphasic symptoms ? or, in other words, 
 Can the function of articulate speech, so far as this consists in the 
 ability to apprehend and successfully to will its acoustic and visual 
 symbols, be localized in the cerebral hemispheres ? In answer to 
 this question it must be admitted that no absolute field for aphasia 
 can be pointed out ; that is, besides the region where lesions are 
 connected in by far the greater number of cases with aphasic dis- 
 turbances, other regions of the cerebral hemispheres only some- 
 times thus connected may be pointed out. 
 
 28. In a large percentage of cases of disturbance of speech due 
 to cerebral lesions, the posterior third of the third frontal convolu- 
 tion and the other regions bordering on the fissure of Sylvius 
 (island of Reil, and immediately adjacent parts of the parietal and 
 temporal lobes) are the seat of the lesions. Aphasia is far more 
 frequently due to changes in the left than in the right hemisphere 
 of the brain. Dr. Seguin, out of 260 cases, calculated the propor- 
 tion of aphasias due to lesions on the left side, as compared with 
 those due to lesions on the right, to be as 243 : 17 or 143 : 1. Such 
 disparity is far too great to be attributed to the comparative fre- 
 quency with which the left hemisphere in general is the seat of 
 lesions. In Exner's * collection of cases, out of 31 lesions resulting 
 in aphasia, all but one were on the left hemisphere (in three cases, 
 however, the right was also involved), and in that one the trouble 
 was only temporary. Such facts have led to the theory that, in all 
 but left-handed men, speech, like other motor functions, is chiefly 
 left-brained : remarkable cases of left-handed people who have be- 
 come aphasic through lesions on the right hemisphere are actually 
 recorded. 3 
 
 1 Functionen in d. Grosshirnrinde, p. 51 f . 
 
 2 See Kussmaul and his citations, p. 739 f. 
 
296 DISTURBANCES OF SPEECH. 
 
 Of the left hemisphere, the gyrus central-is anterior and the 
 adjacent convolutions of the frontal lobe, but especially the pos- 
 terior part of the third (lower) convolution, have much the highest 
 intensity as seats of aphasic lesions. In 53 carefully collected 
 cases by Lohmeyer, 1 50 were on the left hemisphere, 24 in the 
 lower frontal convolution, 34 in this convolution and neighboring 
 parts, 13 in the island and adjacent parts, C in the island alone. 
 Exner's collection, however, did not show that the "intensity" of 
 the lower is any greater than that of the middle frontal convolution, 
 or of the two upper temporal convolutions. This collection con- 
 tained, moreover, five cases in which lesions were seated in the 
 lower left frontal convolution without any resulting aphasia. Exner 
 therefore justly concludes that the " cortical field " of speech, like 
 the corresponding fields of all the motor functions, is really much 
 more extended than has generally been supposed. He is himself, 
 nevertheless, inclined to localize, yet more definitely, so-called 
 "ataxic aphasia" in the third frontal convolution, "word-deaf- 
 ness " in the middle gyrus temporally, and agraphia in the lower and 
 front part of the parietal lobe ; that is, in the neighborhood of 
 the motor region for the upper extremities. So specific localiza- 
 tion can hardly, however, be safely based on the restricted number 
 of cases which Exner considered. 
 
 Lohmeyer gives 2 cases of aphasia following lesions in the an- 
 terior portion of the frontal, 3 in the parietal, and 4 in the occipital 
 lobe. Exner gives 3 cases in which the central convolutions were 
 alone the seat of disease ; 2 in which the temporal and parietal 
 lobes were alone affected ; 1 in which the only lesion was in the oc- 
 cipital lobe. In the only sense in which the brain can be spoken 
 of as the " seat of the faculty of articulate language," we must ad- 
 mit that the evidence confirms the following assumption of Kuss- 
 maul : " It is, a priori, probable that an enormous association-tract 
 in the cortex has been assigned to speech, even though the key- 
 board of sound may be confined to the anterior cortical regions." 
 
 29. The more ardent and positive advocates of the theory 
 of locally specific cerebral functions find it exceedingly difficult to 
 refrain from seating general intelligence, or the powers of percep- 
 tion, memory, comparison, etc., as applied to all the objects of 
 cognition, in some particular so-called "field" or "area "of the 
 brain. At present the frontal lobes offer themselves as the most 
 convenient region for such pre-empting of the cerebral domain. 
 The general propriety of considering the connection which un- 
 doubtedly exists between the central nervous mechanism and men- 
 1 Archly f. Klin. Chirurgie, XIII., p. 309, as cited iu Kussmaul. 
 
IMPAIRMENT OF INTELLIGENCE. 297 
 
 tal phenomena, under any spatial terms whatever, will occupy our 
 attention later on. It is enough at present to say that the experi- 
 mental and pathological evidence do not warrant us in assigning 
 such pre-eminence to the frontal lobes. Extensive lesions may oc- 
 cur in these lobes with little or no diminution of so-called general 
 intelligence. On the other hand, small lesions in other regions of 
 the brain are not infrequently productive of comparatively prof ound 
 mental derangement or loss of function. Moreover, lesions localized 
 in those areas of the cerebral cortex which have thus far been con- 
 sidered namely, the parietal, occipital, and temporo-sphenoidal 
 lobes are, of necessity, connected with more or less impairment 
 of intelligence. 
 
 There can be no doubt that the mental processes which we describe 
 by the word "intelligence " are all closely related to the basic sensory 
 and motor activities that are chiefly localized elsewhere than in the 
 frontal lobes. An animal that is " soul-deaf " or " soul-blind " has, 
 so far forth, an impaired intelligence. The same thing is eminently 
 true of the man afflicted with aphasia in any of its severer forms. 
 The loss of intelligence is not necessarily (or even probably) due to 
 the partial destruction of that functioning of the hemispheres in 
 general which results in intelligence ; it is rather due to the fact 
 that the support which all ideation receives from the audible and 
 visible symbols of the idea whether chiefly as respects its forma- 
 tion or as respects its expression has become impossible. The 
 impairment of any considerable area of tactile sensations, especially 
 as localized in those parts of the body which are most used in per- 
 ception through such sensations (e.g., the hand), also occasions a 
 certain loss of intelligence. The restrictions which cerebral disease 
 introduces into the number and nicety of the sensory and motor 
 functions are, of course, much less important when they come upon 
 minds already furnished, as we say, " with a stock of ideas." Still, 
 even in such cases a basis of sensations and volitions constantly 
 underlies, as it were, all the higher and pre-eminently intellectual 
 mental processes. 
 
 30. In spite of the evidence adduced, a few experimenters still 
 either wholly reject the principle of the localization of cerebral 
 function, or else urge arguments against carrying it out even with 
 the limitations which the foregoing conclusions have observed. 
 Among such experimenters the most prominent is perhaps Goltz. ' 
 The method of extirpation practised by Goltz was that of wash- 
 ing away the substance of the cerebrum by streams of water sent 
 
 1 See especially his treatises as collected in the book, Ueber d. Verrichtungen 
 d. Grosshirns, Bonn, 1881 ; and Pfluger's Archiv for 1876, 1877, and 1879. 
 
298 SUMMARY OF EESULTS. 
 
 through orifices broken at selected places in the skulls of dogs. 
 This method has the advantage of saving bleeding ; it has the dis- 
 advantage of not definitely localizing the injury. Its author has 
 applied it with great care and skill to a large number of animals, 
 many of which he has succeeded in keeping alive for months, even 
 after the removal of considerable areas from both hemispheres 
 (in one instance the brain-substance, calculated to have weighed 
 originally 93 grammes, had been reduced to 13). The principal 
 conclusions drawn from his experiments by Goltz are adverse 
 to the theories of localization held by Terrier, Munk, Luciani, and 
 others. 
 
 Goltz's conclusions may be summarized as follows. No impair- 
 ment of intelligence follows the loss of a large amount of cortical 
 substance from one side of the brain ; but loss of any considerable 
 amount of substance from both sides whether in the frontal, 
 posterior, or temporal lobes produces a permanent impairment of 
 all the functions, which corresponds in a general way to the amount 
 of the loss. Every sense, and the intelligence of every sense, is 
 thus weakened ; for the cerebral elements of sense are impaired or 
 destroyed (Hirnsehschwdche, etc.). For example, a dog which has 
 been trained to give his paw on command loses the power to do so 
 in consequence of such loss of brain-substance, and never regains it. 
 It is not possible, by extirpating any amount of the substance of 
 the cortex on either side or on both sides, to produce a permanent 
 laming of any muscle of the body, or a total loss of sensibility in 
 any of its parts. It is, however, possible, according to Goltz, by 
 repeated removal of the cerebral substance on both sides, gradually 
 to reduce an animal to a condition of almost complete idiocy to 
 an elaborate eating, drinking, and walking "reflex-machine." The 
 removal of as much as 4 grammes from each hemisphere produces, 
 as he calculates, a considerable degree of idiocy. No part of the 
 cortex of the brain can, then, be called the exclusive organ or centre 
 of intelligence or feeling ; but the psychical functions of sensation, 
 volition, ideation, and thought are connected with all of its parts. 
 The quantity of the cerebral substance removed determines the 
 amount of the general impairment of mental powers, instead of the 
 locality from which the removal is made fixing the quality of mental 
 impairment. 
 
 It must be admitted that the facts discovered by Goltz, and the 
 
 conclusions which he reaches, seem at first strongly opposed to all 
 
 localization of cerebral function. But they are not really so ; nor 
 
 is it quite correct even to say, as Foster l does, that Goltz's results 
 
 1 A Text-book of Physiology, p. 649. New York, 1880. 
 
THE VIEWS OF GOLTZ. 299 
 
 are " absolutely opposed" to those of Munk. In fact, Goltz 1 him- 
 self asserts that destruction of the parietal lobes produces a greater 
 permanent disturbance of feeling, and destruction of the occipital 
 lobes a greater permanent disturbance of sight. In general, an 
 animal operated upon in the two hind-quarters of the cerebrum is 
 more stupid than one which has suffered loss of the fore-quarters ; 
 the former is duller of sight, hearing, smell, and taste ; the latter is 
 duller in respect to skin-sensations. The effect of injury to the 
 posterior parts of the brain is therefore much more marked in de- 
 pressing the intelligence of the animal (as shown in sense-percep- 
 tion). Moreover, Goltz 2 claims that he has never rejected the 
 possibility of a localization of the functions of the brain. He con- 
 firms 3 the conclusions of Fritsch and Hitzig, by saying that he has 
 often seen mechanical excitation of the parietal lobes produce mo- 
 tions in the limbs of the opposite side. His facts and arguments 
 are rather directed against that form of the hypothesis of localiza- 
 tion which seats the different functions in small circumscribed 
 areas * and then, when forced by facts, conceives of them as also ca- 
 pable of hopping about from one of these areas to another, like a 
 bird from twig to twig in the branches of a tree. Furthermore, a 
 detailed comparison of the experiments of Goltz with those even 
 of Munk shows that the results of the two are in the main recon- 
 cilable ; if only it be remembered that the former has not always 
 precisely denned the areas of brain-substance removed, nor suffi- 
 ciently taken into account the undoubted results obtained by others 
 from definitely circumscribed lesions ; and that the latter has, cer- 
 tainly, in many cases been more precise and confident than a fair 
 view of all the facts will warrant. 
 
 31. Three principles may be laid down as summing up the 
 results reached by inference upon the basis of experiment with 
 respect to the localization of function in the cerebral cortex. 5 The 
 first principle is to be accepted in the form of a general postulate 
 derived from a study of the other parts of the nervous system, and 
 confirmed on attempting to apply it to the cerebral hemispheres. 
 It may be stated as follows : the different elementary parts of the 
 nervous system are all capable of performing its different specific 
 functions when, and only when, they have been brought into the 
 proper connections and have been exercised in the performance of 
 
 1 Verrichtungen d. Grosshirns, pp. 114 f. and 150 f.; and art. in Pfliiger's 
 Archiv, xxxiv., pp. 450 ff. 
 
 2 Verriclitungen d. Grosshirns, p. 163. 3 Ibid., p. 165. 4 Ibid., p. 169. 
 
 5 Compare the five general laws of central functions given by Wundt, 
 Grundzlige d. physiolog. Psychologic, i., p. 224 f. 
 
300 SUMMARY OF RESULTS. 
 
 those functions. This principle includes two important laws which, 
 we know, hold good throughout the whole nervous mechanism, and 
 which lie at the physical basis of important psychical facts and laws ; 
 they are the law of Specific Energy and the law of Habit. Different 
 combinations of the elementary parts of the nervous system, form- 
 ing composite parts or organs, have different values and functions 
 in the general economy of the system. Every nerve-fibre, every ele- 
 ment of an end-organ, or of a central organ, may be said to have a 
 specific function, and to discharge that function in the exercise of 
 a specific energy. As to how far the capacity for this specific energy 
 is dependent upon the specific molecular structure of the element- 
 ary parts, we are only able to conjecture ; but about its depend- 
 ence upon the connections in which the elementary parts stand with 
 each other, there can be no doubt. 
 
 Moreover, the elementary parts of the nervous system, inasmuch 
 as they have the general adaptation necessary to the performance 
 of nervous functions, can " learn " (so to speak) to perform the 
 more specific of these functions but only in case they stand in 
 appropriate connections. The repeated action of the nervous ele- 
 ments in specific functions fits them the better to act in the same 
 functions. The effect of the exercise of any function in the past 
 may be " stored up " so as to increase the facility of the nervous 
 structure to exercise again every similar function. Thus, different 
 elementary parts of the nervous system, if at first forced by circum- 
 stances to become active in a given way, may by repeating the 
 activity gain a position of facility and value like that belonging to 
 other parts whose so-called normal action lies in this particular 
 way. This law of habit in the nervous system explains much of 
 the behavior of the nerve-muscle machine, or of the decapitated 
 frog, etc., under artificial stimuli ; it also underlies the theory of the 
 sensory-motor effects attributed to centres in the spinal cord and 
 basal ganglia ; it throws light upon the physical basis of our ex- 
 perience in learning to walk, talk, play upon musical instruments, 
 or handle tools, as well as upon the transmission from generation 
 to generation of minute bodily characteristics. Both the law of 
 specific energy and the law of habit undoubtedly apply to the func- 
 tions of the cerebral cortex. 
 
 The remaining two of the three principles alluded to above may 
 be said to follow from the first ; they are the principle of localized 
 function and the principle of substitution. The former asserts 
 that, in the normal condition of the nervous system, all parts have 
 not the same definite functions. Inasmuch as the functions of the 
 different elementary parts necessarily depend upon the manner 
 
NATURE OF THE CEREBRAL " FIELDS." 301 
 
 in which they are combined and connected, the composite parts 
 or organs thus formed must also have certain normal functions. 
 But such composite parts or organs have, of course, a definite local- 
 ity ; hence the functions of the nervous mechanism must be more 
 or less definitely localized. Nor can the principle be suspected 
 of a disposition to stop short off and abdicate its authority, when 
 we reach the region of the cerebral cortex. There is nothing in 
 the structure of the cortex to show why the general law of differen- 
 tiation of function should be inapplicable there. On the contrary, 
 everything in both its anatomy and physiology indicates that the 
 principle of localized function does apply, in some sort, to the cere- 
 bral hemispheres. 
 
 So-called "centres," or "areas," or "fields," of the cerebrum are 
 in no case, however, to be regarded as portions of its nervous sub- 
 stance that can be marked off by fixed lines for the confinement 
 of definite functions within rigid limits. These areas are some- 
 what different for different brains of the same species ; ihej widen 
 when a heightened energy is demanded of them; their centres 
 are neither mathematical points nor very minute collections of 
 cells. They are not composed of elements which have, each one, 
 a fixed and unchangeable value, and a definite function, as though 
 the number of mental operations assigned to a locality needed to 
 be precisely matched by the separate nerve-fibres and nerve-cells of 
 the locality. Nor are these areas perfectly isolated localities ; on 
 the contrary, they obviously overlap each other in certain cases. 
 According to the true statement of Luciani, " the single centres 
 in the sensory-motor zone are so completely bound up with, and, 
 so to speak, let into one another, that it is not possible to divide 
 them with a clear and definite line, such as is the case when the 
 cortex is incised and removed ; so that in destroying a centre one 
 necessarily eliminates a portion of the neighboring centres." Nev- 
 ertheless, there is no doubt that the cerebral functions connected 
 with the different sensations and motions of the peripheral parts of 
 the body are not all alike exercised by all parts of the cerebrum. 
 They are assigned specifically to those regions which alone have the 
 proper structure and stand in the proper relations. 
 
 Furthermore, the functions of the cerebrum are not absolutely 
 confined to those centres with which, under ordinary circumstances, 
 they are chiefly or wholly connected ; in which, that is to say, they 
 are localized. If such centres, for any reason, become incapacitated 
 or relatively unfitted to perform their normal functions, the same 
 functions may be performed by other areas of the cerebral cortex, 
 provided these areas also stand in the proper connections. This 
 
302 SUMMARY OF RESULTS. 
 
 is the principle of substitution. It is due to its working that 
 animals subjected to experiments in extirpation, as a rule, so 
 largely recover the powers of sensation or motion which they have 
 temporarily lost. It is on this class of phenomena that Goltz rests 
 much of his argument. In the cerebral hemispheres, however, the 
 principle of substitution does not overstep all limits, nor does it 
 operate arbitrarily. The portions of the same hemisphere that 
 are just adjacent to the so-called centres the larger areas sur- 
 rounding or contiguous to the smaller and, on account of its bi- 
 lateral structure, the corresponding portions of the other hemi- 
 sphere, are in general those best capable of exercising such substi- 
 tutive functions. It may be doubted whether these portions do 
 not, in all ordinary cases, cover the entire limits within which the 
 principle of substitution can act. Such substitutive functions im- 
 prove under the law of habit to which the organs of the cerebral 
 cortex are subjected. 
 
 The connections between the different cerebral areas and their 
 functions are so complex and subtile that physiological science will 
 need a long time to disentangle them ; it may be doubted whether 
 it will ever succeed in doing this completely. The connections 
 among the phenomena of conscious sensation, volition, ideation, 
 and thought are at least equally subtile and complex. Will psy- 
 chology ever disentangle these connections ? 
 
 The bearing of the subject on our conclusions concerning the 
 nature of the mind and its connection with the body will be con- 
 sidered elsewhere. 
 
CHAPTER III. 
 THE QUALITY OF SENSATIONS. 
 
 1. THE world of ordinary experience consists of a great number 
 of so-called '' things" that are known to us by their distinguishing 
 qualities. Although each one of these things is believed to be a 
 separate existence, they are all perceived as having certain common 
 characteristics, and as standing in certain relations to each other, 
 of space, time, and action. It is with the things, their common 
 qualities and mutual relations, that unreflecting practical life is 
 chiefly concerned. But even without special reflection, everyone 
 learns that his knowledge of such external objects depends upon 
 the kind and degree of the effect they exercise upon his conscious- 
 ness through the senses. Attention is thus turned from the things 
 themselves to the sensations produced in us by their action. The 
 variety of such sensations, at first bewilderingly great, is soon re- 
 duced to some order by a classification referring them to the dif- 
 ferent 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, hearing, 
 sight, and touch are the five classes of sensation, as the grouping 
 is made by the unprejudiced judgment of all. 
 
 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 x>f sensations arise through the skin is shown 
 
304 THE KINDS OF SENSATION. 
 
 by the popular use of the word to "feel." Things fed 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 estimated, however, is only ascribed in a very indefinite way to 
 the parts of the body that are chiefly concerned in passively sup- 
 porting, 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. 
 
 2. 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 in time, and as being connected 
 with the motion in space of the objects that occasion them. Of 
 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. 
 
 3. 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- 
 
THE ANALYSIS OF SENSATIONS. 305 
 
 spects conflicting. Yet it is precisely in these fields that modern 
 psychology has achieved its most brilliant successes. It has thrown 
 a flood of new light upon the essential nature and growth of hu- 
 man experience. It has profoundly influenced the current views 
 on metaphysics. It has contributed important factors toward the 
 solution of certain questions of interest to ethics and religion. It 
 has given us a new point of view for renewing the ancient debate 
 between Materialism and Spiritualism. 
 
 4. 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 two distinct kinds of sensation that have the skin for their 
 organ namely, temperature and pressure. 
 
 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 
 analysis. 
 
 5. 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 
 20 
 
306 THE KINDS OF SENSATION. 
 
 on the basis of several simultaneous affections of sense. As respects 
 developed experience, the simple sensation is a necessary fiction of 
 psycho-physical science. 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 w r hich, 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 
 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. 
 
 6. 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 
 
SPECIFIC ENERGY OF NERVES. 307 
 
 stimuli or immediate antecedents of the sensations. Moreover, our 
 knowledge of the intimate structure of the end-organs of sense, 
 and of the nature of the physical processes which excite them, is 
 still very incomplete. The detection of obscure but important dif- 
 ferences in the qualities of conscious 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 infre- 
 quently appear ; and language can only imperfectly 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, 
 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. 
 
 7. The authority of one great law is involved, as a silent 
 assumption, in all discussion of the quality of sensations. This law 
 is known as the law of the Specific Energy of the Nerves. It has 
 already been shown (Part I., chap. I., 35) that any such dis- 
 tinction of the kinds of nerve-fibres as denies their possession of 
 common functions cannot be maintained. But the phenomena 
 of sensation cannot be explained without a much more extended 
 application of this law than has thus far been found necessary. 
 Distinctions of quality in sensation depend upon the excitation of 
 specific corresponding elements of the nervous system. That only 
 the optic nerve is capable, when excited, of exercising the physio- 
 logical function upon which sensations of light and color depend, 
 does not admit of doubt ; the same specific quality cannot be 
 denied to the functional activity of the nerves of smell, taste, hear- 
 ing, and touch. Moreover, in the end-organs of each of these 
 senses, provision must be made for a further differentiation of 
 function. What is the nature of the evidence, and what conclu- 
 sions must be drawn from it, will be best appreciated at a later pe- 
 
308 SENSATIONS OF SMELL AND TASTE. 
 
 riod in the discussion. Meantime we find ourselves obliged to as- 
 sume the existence of some law of the specific energ} r of the nerves 
 of special sense. 
 
 8. Little of a scientific character is known concerning Sensa- 
 tions of Smell, considered as respects their quality. Anatomy, 
 chemistry, and physics fail to furnish definite information on this 
 point ; experimental physiology as applied to the lower animals is, 
 of course, unsatisfactory ; and the appeal to human consciousness 
 asks for an analysis of which it is incapable. It has already been 
 shown (Part L, Chap. V.) that the part of the mucous membrane 
 of the nasal passages known as regio olfactoria contains the end- 
 organs of smell ; the specific stimulus of the organs in this region 
 is applied as borne thither by the current of air, and almost, if not 
 quite, exclusively in the act of inspiration. In order that any sub- 
 stance may act through the end-organs on the nerve (olfactorius) 
 which is spread out in this region, it must either exist in gaseous 
 form or else be vaporizable under given conditions of temperature. 
 The degree of temperature at which different substances become 
 odorous therefore varies according to their physical characteristics. 
 For example, arsenic, which at ordinary temperatures is inodorous, 
 when raised to a dark-red heat is vaporized and the vapor excites 
 an intense sensation of smell. Fluid bodies which give off an 
 odorous reek, when brought in fluid form into contact with the 
 mucous membrane of the regio olfactoria have no smell ; if this 
 membrane is soaked in fluid of any kind whatever, it loses for a 
 time the capacity to be excited with olfactory impressions. E. H. 
 Weber ' discovered that if the head be placed with the nostrils 
 pointing upward, and the nasal passages be then filled with pure 
 water, sweetened water, or a mixture of water and eau de Cologne, 
 after these passages are emptied the sense of smell is, in all cases, 
 temporarily lost ; even when Cologne is used, with the exception 
 of the instant at which the fluid is poured in, no odor can be per- 
 ceived. Subsequent observers have confirmed the experiments of 
 Weber. One investigator 2 lost all sense of smell, even for acetic 
 acid and ammonia, for a period of half a minute ; another for five 
 minutes, and the sense in its full acuteness did not return for nearly 
 half an hour. Whether this effect of the fluid is due to impair- 
 ment of the end-apparatus of smell by soaking it (so Valentin), or 
 to the mechanical barrier which the layer of foreign substances 
 interposes between the odorous particles and this apparatus (so 
 Frohlich), we cannot say ; it may be due to both causes. Contrary 
 
 1 See Archiv f. Anat., Physiol., etc., 1847, p. 257. 
 
 Frohlich, in Sitzgsber. d. Wiener Acad., 1851, VI, p. 322. 
 
THE STIMULUS OF SMELL. 309 
 
 to the assertion of Wundt, 1 that probably no gases or vapors, except 
 atmospheric air and its constituents, are absolutely inodorous, so 
 far as we have present information a number of gaseous and va- 
 porizable substances are so ; and no reason is known for such 
 apparent exceptions to the rule. 
 
 9. The stimulus of smell is supposed to consist in certain ex- 
 ceedingly minute particles contained in the odorous gas or vapor 
 which is drawn in with the current of air over the mucous mem- 
 brane of the regio olfactoria. The question is as yet scarcely de- 
 cided, whether other forms of stimulus, besides these odorous 
 particles mechanical, electrical, thermic, or so-called subjective 
 can excite 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. Hitter (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. 2 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 
 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, show that compression of the olfactory nerve 
 
 1 Grundziige d. physiolog. Psychologie, i. , 384 ; comp. von Vintschgau, in 
 Hermann's Handb. d. Physiol., III., ii., p. 261 f. 
 
 2 See Rosenthal, in Arcliiv f. Anat, Physiol., etc., 1860, pp. 217 ff. 
 
310 SENSATIONS OF SMELL AND TASTE. 
 
 by tumors, etc., may produce sensations of smell. Disturbances of 
 the central organs, such as occur in certain cases of insanity, may 
 doubtless have the same result. The powerful effect which some 
 odors have upon the brains of certain persons, so that nausea, gid- 
 diness, and other disturbances of feeling result, scarcely needs 
 mention ; it cannot all be resolved into mental associations con- 
 nected with the sense-impressions. 
 
 10. No approach can be made toward a scientific classification 
 of the kinds of smells. 1 This specific sensation must, however, be 
 carefully distinguished from the other forms of feeling with which 
 it is most closely allied. Many supposed sensations of taste are 
 really sensations of smell. Substances like ammonia and acetic 
 acid powerfully excite the sensations of touch and common feeling 
 through their action on the trigeminus as well as the olfactory 
 nerve. Other sensations of touch and of the muscular sense are 
 reflexly occasioned in such cases, and blend with the specific sensa- 
 tions of smell in the total mental result. But of all the attempts to 
 classify the qualitatively pure sensations of this sense, none can be 
 said to have any scientific value. The division into pleasant and 
 unpleasant smells depends upon the idiosyncrasies of individuals ; 
 to some the smell of burning feathers, of assafoetida, of valerian, or 
 of rank cheese, is pleasant. Frohlich's 2 classification into those 
 which excite merely the olfactory nerve, and those which call out 
 other sensations reflexly through their action on the trigeminus, is 
 purely physiological and not psycho-physical ; moreover, it does 
 not apply to sensations of smell, as such. When we classify the sen- 
 sations according to the objects which produce them as practically 
 we are obliged to do we are not distinguishing the qualities of 
 our feeling ; the smell of a rose does not belong to a class of sen- 
 sations as does a sour taste or the color red. No known principle 
 will bring order out of the bewildering complexity of this sense. 
 
 Sensations of smell cannot, like those of pressure, hearing, and 
 sight, be schematized or represented as standing in any definite 
 local or mathematical relations to each other. Smells cannot be con- 
 ceived of as having a scale of pitch, or triangle of color-tones. As 
 Wundt 3 declares, the sensations of smell form " a discrete mani- 
 foldness which has an unknown arrangement." 
 
 11. The properties which any substance must possess in order 
 to be odorous, and the nature of the action of the odorous particles 
 
 'For the entire subject, see von Vintschgau, in Hermann's Handb. d. 
 Physiol., III., ii., p. 266 f. 
 
 2 Sitzgsber. d. Wiener Acad., 1851, VI., p. 322 f. 
 3 Physiolog. Psychologic, i., p. 386. 
 
NATURE OF ODOROUS SUBSTANCES. 311 
 
 upon the end-organ of smell, are wholly unknown as much so 
 now as when, more than a half-century since, Cloquet confessed the 
 complete ignorance of the scientific world on these matters. A 
 great variety of phenomena appear, but no known law has control 
 of them. Some plants are odorous by day alone, others by night 
 alone ; still others only in the morning. Some plants have a smell 
 when dried ; others give off only a weak odor when dry, but a 
 stronger one when moistened. Of course, the effect of any odorous 
 substance depends upon the ease with which it may be vaporized, 
 and the speed and extent of its diffusion through the atmosphere. 
 Camphor, musk, and other similar substances are distinguished for 
 their long-continued and far-reaching effects. 
 
 The discovery of Romieu, in 1756, that small bits of camphor on 
 the surface of water have a rotary motion, has called out various 
 investigations in the line suggested by this fact. Provost subse- 
 quently (1799) observed that other odorous bodies have a similar 
 motion on the surface of water, and that a very thin layer of water 
 on a perfectly clear plate or glass withdraws itself as soon as pul- 
 verized camphor is laid upon it. More recently still, Liegeois has 
 noticed the same phenomena, wholly or in part, exhibited by some 
 two hundred odorous substances of either vegetable or animal struct- 
 ure. Minerals, according to this observer, do not behave in the 
 same way. Some of these odorous substances seem to inhibit or 
 check the rotary motion in others. He concludes that we are jus- 
 tified in believing odorous substances to have the power, especially 
 when in contact with water, of setting up a motion of these outside 
 particles which distributes them through the atmosphere so that 
 they reach the mucous membrane of the nasal passages. Just how 
 they act upon the end-apparatus there it is impossible to say. The 
 researches of Tyndall l and others as to the influence which odorous 
 particles of different substances have upon the capacity of the air 
 to absorb heat may possibly be combined with the foregoing re- 
 searches in a way to suggest some tenable hypothesis touching the 
 nature and action of the stimuli of this sense ; but thus far, as has 
 been said, we cannot go beyond a confession of ignorance. 
 
 12. The condition of scientific attainment as to sensations of 
 taste and their stimuli is only little 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 
 1 Heat as a Mode of Motion, pp. 341 ff. New York, 1868. 
 
312 SENSATIONS OF SMELL AND TASTE. 
 
 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 have a taste. No known law regulates the relation between 
 the solubility of bodies and their power to excite sensations of this 
 class. It is disputed whether any of the gases are direct excitants 
 of the end-organs of taste. The monograph of A. Stick 1 maintains 
 the tastable character of certain gases, on the ground that a stream 
 of them, let fall upon the tongue when diy (so that they cannot well 
 be absorbed by the saliva), produces the peculiar sensations of 
 taste which these gases are known to possess. A stream of car- 
 bonic-acid gas, for example, when acting on the dry edge of the 
 tongue, has a taste which is described as sweetish sour. It is diffi- 
 cult, however, to secure such a degree of dryness of the tongue as 
 will not leave a moist capillary layer ; difficult, also, to exclude all 
 the connected sensations of smell and common feeling. 2 
 
 It is doubtful whether the sensation of taste can be excited by 
 mechanical means ; and there is no proof that heat can irritate the 
 gustatory nerves. Certain authorities of the first rank have indeed 
 described specific sensations of taste as mingled with the feelings 
 which follow rubbing, pricking, and pressing the tongue. Henle 
 observed a saltish taste to be excited by passing a current of air 
 over the tongue ; Wagner a bitter taste by pressing down the base 
 of the tongue with the dry finger ; Dr. Baly an acid or a saltish 
 taste by repeatedly and lightly tapping the end of the tongue. 
 
 The long-debated question as to the electrical stimulation of this 
 sense seems now to be decided affirmatively. 3 It was discovered in 
 1752 that the application of two different metals to the tongue is 
 followed by a peculiar sensation of taste. Volta recognized the 
 fact that the effect of the metals is due to the electrical current 
 called out between them. If the cathode is laid upon the upper 
 surface of the tip of the tongue, a sensation is produced by the cur- 
 rent passing out which is variously described as metallic, acid and 
 metallic, or bitter and metallic, etc. ; but if the anode is applied to 
 the same spot, the sensation produced by the entering current is 
 described as acid, or acid and metallic, or bitter and metallic. In 
 the former case, not infrequently, a strong current is needed to pro- 
 
 'Ueber d. Schmeckbarkeit d. Gase, Berlin, 1857; article in Annalen des 
 Charite-Krankenhauses. 
 
 2 See von Vintschgau, in Hermann's Handb. d. Physiol., III., ii., p. 196 f. 
 
 3 The whole question is discussed by von Vintschgau, ibid., p. 181 f.; and 
 Pfliiger's Archiv, xx., pp. 81 ff. 
 
ELECTRICAL STIMULUS OF TASTE. 313 
 
 duce any sensation at all. Since the discovery of electrolysis, it 
 has been objected that these effects are due to the decomposition 
 of the fluids of the mouth and the consequent accumulation of free 
 acid at the positive and free alkali at the negative pole ; they are 
 therefore not to be ascribed to the direct action of the electrical 
 current on the end-apparatus of sense. Experiments by du Bois- 
 Reymond, Rosen thai, 1 and others have been directed toward an- 
 swering this objection. The former showed that when a chain of 
 four persons is arranged in such manner as to send a current of 
 electricity through the tongue of one, the eyeball of another, and 
 the muscles of a frog-preparation held by two of the four, the same 
 current will cause simultaneously an acid taste, a flash of light, and 
 a movement of the animal's muscles. Rosenthal discovered that, 
 if two persons touch the tips of each other's tongues while one 
 holds in a moist hand the positive and the other the negative pole, 
 an electric current will cause the first person to have an alkaline and 
 the second an acid taste. Still other experiments confirm the 
 opinion that sensations of this sense may be directly clue to elec- 
 trical stimulation. Attempts have been made to prove the possi- 
 bility of exciting subjective sensations of taste by injecting tasta- 
 ble substances into the veins of animals ; but the psychology of 
 the subject has reaped no results from these attempts. Most of 
 the alleged cases of such subjective origin are probably due to 
 substances really brought to the tongue in the saliva. It is worth 
 remarking here that sensations of taste rarely or never mingle in 
 our dreams. 
 
 13. The question whether a tastable substance excites precisely 
 the same sensation when applied to all portions of the organs of taste 
 is a difficult one to answer satisfactorily (see Part I., Chap. V., 
 6). The tabulated results of different experimenters upon this 
 question disagree considerably. Such disagreement is suggestive 
 of idiosyncrasies of taste, and of doubt whether the different shades 
 of the same class of sensations are either nicely discriminated or 
 uniformly described by most persons. Descriptions which speak 
 of the sensations as prickly, piquant, cooling, etc., show, of course, a 
 combination of sensations of common feeling with those of special 
 sense. The minor varieties of taste may be occasioned in a manner 
 similar to that of the less important shades of color-sensations. It 
 seems tolerably well established that sweet and sour are tasted 
 chiefly with the tip of the tongue ; bitter and alkaline with its roots. 
 The experiments of two of the principal observers, Horn and Picht, 
 agree in the conclusion that nearly all substances (even sugar) call 
 1 Ueber d. elektrischen Geschmack ; Archivf. Anat., Physiol., 1860, p. 217 f. 
 
314 SENSATIONS OF SMELL AND TASTE. 
 
 out a bitterish taste when applied solely to the papillce circum- 
 vallatae. 
 
 14. Most of the different kinds of tastes admit of being con- 
 si'dered as compounds of a few simple sensations of this sense with 
 each other and with sensations of smell, touch, common feeling, and 
 muscular sense. Many so-called tastes are really chiefly smells. 
 Physiologists generally distinguish four principal classes of tastes 
 sweet, bitter, salt, and sour. Wundt ' adds to these four the alka- 
 line and the metallic. But possibly the alkaline may be considered as 
 a modification of the salt ; and the metallic is probably a compound 
 taste, although its analysis is by no means easy. The attempt has 
 been made by Valentin and others to reduce this number to two 
 the sweet and the bitter. The sour is thus considered as not a pure 
 sensation of taste, but as predominatingly a sensation of touch. 
 Acids in concentrated form certainly bring into action the nerves of 
 feeling.; but in very dilute form they seem to excite purely the sensa- 
 tion of taste. The same thing is true of saltish substances. The 
 bitter and the sweet are agreed by all to have the character of pure 
 sensations of this specific sense. Powerful reflex sensations of the 
 muscular sense are occasioned by strong stimulation of the nerves 
 of the tongue, and these sensations blend with the specific sensa- 
 tions of taste. There is no satisfactory reason to be given for 
 classing the sensation of nausea under the sense of taste. 
 
 The primary forms of taste are combined, in the greatest variety, 
 with an indefinite number of shades under each of them. The 
 hypothesis of four or more specifically different forms of the end- 
 apparatus corresponding to the primary forms of sensation for 
 example, " bitter- tasting " nerve-fibres, ' 'sweet- tasting " nerve- 
 fibres, etc. offers, under the law of the specific energy of the 
 nerves, an opportunity for explaining all the phenomena of this 
 sense somewhat similar to that embraced by the so-called Young- 
 Helmholtz theory of color-sensations. 
 
 15. Concerning that in tastable substances which fits them to 
 excite the end-apparatus of the gustatory nerves, or concerning the 
 molecular action of such substances, we have no information what- 
 ever. No scale of stimuli, considered as differing in the rapidity of 
 their vibration and corresponding to a scale of resulting sensations 
 differing in pitch or tone, can be made out for sensations of taste. 
 The great difficulties which accompany experiments upon this 
 sense, and the fact that the most fundamental questions concerning 
 its activities are still unanswered, place it in an unsatisfactory posi- 
 tion only less hopeless than that occupied by the kindred sense of 
 'Physiolog. Psychologic, i., p. 382. 
 
SUBJECTIVE CHARACTER OF SOUND. 315 
 
 smell. We have in the case of taste, however, the very great ad- 
 vantage of being able, at least loosely, to classify the sensations 
 whose quality we are considering. 
 
 16. 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, 1 who made the entire field peculi- 
 arly his own. Since the first appearance of this work, the subject 
 has also been greatly enriched by the original researches of Oetting- 
 en, 2 Mach, 3 Preyer, 4 Hensen, 6 Stumpf, 6 and others. In speaking 
 of the stimuli of these sensations, we are still compelled to refer 
 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 
 " sound " 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- 
 
 1 Die Lehre von d. Tonempfindungen als physiolog. Grundlage f. d. Theorie 
 d. Musik, Braunschweig, 1st edition, 1862; 3d edition, 1865; 3d edition, 
 1870 ; 4th edition, 1878. 
 
 2 Harmoniesystem in dualer Entwicklung, 1866. 
 
 3 Various contributions in the Archiv f . Ohrenheilkunde and elsewhere 
 (especially the Sitzgsber. d. Wiener Acad.). 
 
 4 Ueber d. Grenzen d. Tonwahrnehmung, 1876 ; Sitzgsber. d. Jen. Ge- 
 sellsch. f. Med., 1878 ; Akustische Untersuchungen, 1879. 
 
 5 In Hermann's Handb. d. Physiol., III., ii., pp. 3-142, and works by the 
 same author there referred to. 
 
 6 Tonpsychologie, Leipzig, 1883 (Vol I. only). 
 
316 SENSATIONS OF SOUND. 
 
 ure of the ear or of vibrating strings and particles of air, or of the 
 mathematics and physics of music. 
 
 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, objectively 
 considered, are wanting in the periodic regularity of stimulation 
 which characterizes all musical sounds, and, subjectively considered, 
 in the peculiar, pleasant modification of consciousness which the 
 latter produce. But noises accompany almost all tones ; and, con- 
 versely, 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, 1 
 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 a distinguishes 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. 
 
 17. 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 
 
 1 The Sensations of Tone, etc., p. 11 f. London, 1875. 
 
 2 Hermann's Handb. d. PLysiol., III., ii., p. 17. 
 
THE LIMITS OF PITCH. 317 
 
 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- 
 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 " color-tone ; " 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 color-tone is determined by the character of the 
 over-tones which are blended with the fundamental tone. 
 
 18. 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 ei- 
 ther 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. Indi- 
 vidual peculiarities are also very important in determining sensa- 
 tions of pitch. Some persons can hear tones below or above those 
 audible to most others. Helmholtz * thought that sensations of 
 1 The Sensations of Tone, p. 268. London, 1875. 
 
318 
 
 SENSATIONS OF SOUND. 
 
 tone begin to cease when the vibrations fall below 34 per second ; 
 some tuning-forks of great size, which vibrated only 28 times per 
 second, seemed to him, however, to have a trace of tone in the form 
 of a " weak drone." Preyer ' found that while 14 vibrations pro- 
 duced 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. Blake thinks that persons with defective ear-drums are 
 able to hear tones of higher pitch, reaching even 50,000 vibrations. 
 Vibrations slower than 28 to 30 per second produce in most ears 
 only a buzzing or groaning sound ; the more acute tones are unpleas- 
 ant, 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 rather more 
 than nine octaves, reaching from about A^ of the subcontra octave 
 (27 vibrations per second) to above c 7 of the seven-times-marked 
 octave (16,896 vibrations per second). 
 
 The following table a gives the pitch of all the musical tones audi- 
 ble 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 l of tfre contra, or from A^ of the 
 subcontra octave, to 6 4 namely, the seven or seven and a half 
 octaves of the modern piano. 
 
 
 C 
 
 D 
 
 E 
 
 F 
 
 Gr 
 
 A 
 
 B 
 
 
 Subcontra octave 
 
 16# 
 
 18 9 / 18 
 
 20% 
 
 22 
 
 24% 
 
 27X 
 
 30' 5 /i 
 
 C a , D 2 , etc. 
 
 Contra octave 
 
 33 
 
 trJJ 
 
 41 K 
 
 44 
 
 49 % 
 
 55 
 
 UK 
 
 C,, DU etc. 
 
 Great octave 
 
 6K 
 
 74^ 
 
 82V 
 
 88 
 
 99 
 
 110 
 
 123% 
 
 C. D, etc. 
 
 Small octave 
 
 132 
 
 148# 
 
 165 
 
 176 
 
 198 
 
 220 
 
 247^ 
 
 c, d, etc. 
 
 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 
 
 264 
 528 
 1,056 
 2,112 
 4,224 
 8.448 
 16,896 
 33 792 
 
 297 
 594 
 1,188 
 2,376 
 4,752 
 9,504 
 19,008 
 3b 016 
 
 330 
 660 
 1,320 
 2,640 
 5.280 
 10.560 
 21,1*) 
 42 240 
 
 352 
 704 
 1,408 
 2,816 
 5,632 
 11,264 
 22,528 
 
 396 
 792 
 1,584 
 3,168 
 6,336 
 12,672 
 25,344 
 
 440 
 880 
 1,760 
 3,520 
 7,040 
 14,080 
 28.1M 
 
 495 
 990 
 1,980 
 3,960 
 7,920 
 15,840 
 31,680 
 
 c. d>, etc. 
 c; d, etc. 
 C", d3, etc. 
 c, d, etc. 
 c*, d, etc. 
 c 8 , d, etc. 
 c 7 , d 7 , etc. 
 
 
 
 
 
 
 
 
 
 
 1 Grenzen d. Tonwahrnehmung, p. 23 f. 
 
 2 Taken from Stumpf, Tonpsychologie, I., p. xiv. , and giving the German 
 scale ; the French fixes a 1 at 435 vibrations ; the theoretical pitch in England 
 gives 512 for c 8 . 
 

 C4L 
 
 SENSITIVENESS TO PITCH. 319 
 
 19. The sensitiveness of the ear to differences of pitch varies 
 greatly with different individuals, and for the different octaves 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 
 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 1 
 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. 2 Persons insensitive to differences of a tone or 
 half-tone, who are sometimes said "not to know one note from 
 another," are by no means infrequently met with. Differences of 
 the two ears of the same person, in the fineness of this kind of per- 
 ception, are common enough ; in certain cases the difference may 
 amount to a half-tone or more. Sensitiveness to pitch is generally 
 capable of rapid cultivation, and may reach a high degree of per- 
 fection 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 differences in the sensations caused by 
 changing the number of vibrations not more than a third of a single 
 vibration per second, in the region of the scale between a 1 and c 2 . 
 In the octave from b l to b* more than 200 tones are distinguish- 
 able. But above and below this region the distinctions possible 
 are less fine ; above c 5 even well-trained ears commit errors in iden- 
 tifying two notes that differ by 100 or even by 1,000 vibrations. It 
 appears, then, that not only the musical quality of tones, but also 
 the power of distinguishing differences in them, diminishes rapidly 
 as we approach the upper and lower limits of the scale. 
 
 The fineness of the possible distinctions of purity of interval also 
 differs for different individuals and for different intervals. The 
 following table is compiled by Hensen 3 from data drawn from 
 Preyer's investigations. The bracketed numbers of the first column 
 indicate the proportion in which the vibrations of the different 
 intervals stand to those of the fundamental tone ; the quotient 
 n' : n = i, the variation from the pure interval which was found 
 detectable in each case ; F = the number of vibrations off from the 
 pure interval which is the least distinguishable ; and S is the de- 
 
 1 Reported by Grant Allen, in Mind, 1878, p. 157 f. 
 
 2 Comp. the lengthy and interesting discussion on ' ' Individualitat des Sin- 
 nes und Gedachtnisses fur Tonqualitaten," in Stumpf, Tonpsychologie I , pp. 
 262 ff. 
 
 3 See Hermann's Handb. d. Physiol., III., ii., p. 114. 
 
320 
 
 SENSATIONS OF SOUND. 
 
 nominator of the fraction which indicates the sensitiveness of the 
 ear to the purity of each interval 
 
 INTERVAL. 
 
 71. 
 
 ri. 
 
 i 
 
 V. 
 
 & 
 
 Fourth (1.333) 
 
 187.58 
 
 251.23 
 
 1.3396 
 
 1.02 
 
 211 
 
 Fifth (1.5) 
 
 167.68 
 
 251.23 
 
 1.4983 
 
 0.23 
 
 822 
 
 Minor Sixth (1.6) 
 
 143.66 
 
 231.41 
 
 1.6108 
 
 1.19 
 
 148 
 
 Major Third (1.25) j 
 
 139.60 
 
 163.68 
 
 1.2437 
 
 0.73 
 
 198 
 
 Minor Third (1.20) 
 
 139.62 
 207.54 
 
 175.53 
 251.23 
 
 1.2572 
 1.2102 
 
 0.89 
 1.90 
 
 193 
 117 
 
 Octave (20) 
 
 50040 
 
 1,001 
 
 2.0004 
 
 13 
 
 5 000 
 
 Whole Tone (1.125) 
 
 215.15 
 
 243 51 
 
 1.1291 
 
 085 
 
 274 
 
 
 
 
 
 
 
 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 
 exercised independently of everything except the two sensations 
 themselves. The latter judgment is the common power of mind 
 belonging to this sense ; the former is, as a rule, exercised only by 
 skilled persons, and by them only very imperfectly. Experiments 
 of Stumpf,' 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 1 to B^, to 15^-100$ of the trials ; in 
 the middle region (from a-y 1 , or from g-e*), to 0^-70$ ; in the 
 upper region (from g a -f*, or from/* 3 -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. 
 
 20. 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. 3 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- 
 
 'Tonpsjchologie, I., pp. 305 ff. 
 
 8 On this subject, comp. Lotze, Medicin. Psychologie, pp. 265 ff., 480 f. ; 
 Strieker, Studien uber d. Association d. Vorstellungen, 1883, p. 2 f.; G. E. 
 Miiller, Zur Grundlegung d. Psycho-physik, Berlin, 1878, pp. 276 ff. ; and 
 Stumpf, Tonpsychologie, I., pp. 134 ff. 
 
THE JUDGMENT OF TONES. 321 
 
 vate these organs (at least in an inchoate and partial way) that is, 
 \ve sound the note 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- 
 tect by ear a difference in quality between two tones of 400 and 
 4001 vibrations per second ; but the most skilful singer Jenny 
 Lind, for example scarcely succeeds in singing in quarter-tones. 
 Moreover, 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 color-tone, 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 sensations 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 j to e 3 (one 
 through ft 1 , c 2 , etc., and the other through g 1 , /', etc., around to 
 e 3 , c?, 3 and then c 8 ), as there are two ways of going from yellow to 
 blue (i.e., through green and blue-green, or through violet, red, 
 and orange). We speak, then, of the series of tones as a constant 
 and infinite series ; although, of course, no series of states of con- 
 sciousness is really infinite, and although the upper and lower 
 limits of the musical scale, as well as the limits of the least ob- 
 servable differences between two tones, are not constant but vari- 
 able for different 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 
 21 
 
322 SENSATIONS OF SOUND. 
 
 voice the organs are depressed ; in sounding the higher, they are 
 elevated. Low notes have a certain breadth and gravity which 
 corresponds 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. 
 21. We have seen that tones, like rays of light, come to us as 
 compounded into " clangs ; " these really composite tones being 
 esteemed as single notes in ordinary experience. The nature of 
 such composition determines the so-called "timbre," or "color- 
 tone," of the notes. Each sensation of a clang is a summing-up in 
 consciousness of several absolute qualities of musical sound ; the 
 stimulus which occasions this complex subjective state is a complex 
 sound-wave made up of the contrasts and coincidences of several 
 single waves that have the character of simple pendulum vibrations. 
 The quality of each clang depends upon the form of this complex 
 sound-wave. We need not consider in detail the physics and 
 mathematics of such complex waves. It is enough to observe that 
 those single tones whose vibrations stand in simple mathematical 
 relations to each other, when combined into a clang, cause a pe- 
 culiarly pleasant sensation ; those whose vibrations stand in com- 
 plex mathematical relations make, when combined, an unpleasant 
 sensation. In an octave of the musical scale the eight different 
 notes stand in the following ratios to each other. 1 
 
 C:D:E:F:G:A:B:C' 
 
 1 : f : f : f : f : f : -V- : 2 
 8 : 9 : 10 : lOf : 12 : 13J : 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. The acoustic waves which constitute the stimuli of each 
 complex sensation called a " clang," accordingly, also permit of 
 being regarded as the summing-up of the waves of a fundamental 
 tone and of certain partial tones belonging to the fundamental 
 tone. These partial tones, or " over-tones," are called "the har- 
 monics" of the " clang," or single compound tone. 
 
 22. When two or more "clangs "are sounded together, the re- 
 sult is what is called either a "chord " or a " discord." The former 
 
 1 FOT the mathematics and physics of tones, see Hensen, in Hermann's 
 Handb. d. Physiol., IIL, ii., pp. 4ff. 
 
CONSONANCE AND DISSONANCE. 323 
 
 is a pleasant, the latter an unpleasant, complex of sensations ; con- 
 sonance and dissonance are thus spoken of as qualities of sensations 
 of musical sound. Thus, if c and c l are struck together upon a 
 weil-tuned piano, the combination of clangs is a chord, or harmo- 
 nious musical sound ; but if c and d, or c and c sharp, or c and 
 its seventh, b above, are simultaneously sounded, then the com- 
 bination of tones is unpleasant. Cases of consonance and disso- 
 nance differ from those just considered under the term "clang" 
 only with respect to the relative strength of the partial tones as 
 compared with the fundamental tones : in the clang the over-tones 
 are weak as compared with the one fundamental tone ; but in the 
 chord or discord the fundamental tones of the other clangs are, of 
 course, strong, and stand in powerful relations of consonance or 
 dissonance both toward the fundamental tone of the lowest clang 
 and toward its partial tones. All the partial tones of the different 
 combined clangs enter into the formation of the total result pro- 
 duced. According to the table already given (p. 322), the Octave 
 is the most perfect possible consonance (1:2); then the Twelfth 
 (1 : 3), the Fifth (2 : 3), the Fourth (3 : 4), the Sixth (3 : 5), the ma- 
 jor Third (4 : 5), the minor Third (5 : 6). With the relation of the 
 Third we come upon the borders of dissonance ; indeed, the ancient 
 Greeks and Eomans considered the Third a dissonance, and avoided 
 it in singing, because, as Helmholtz supposes, their ears were more 
 sensitive to " beats " than ours. The consonance of the Sixth and that 
 of the Fourth have also been much disputed. The major Sixth 
 and major Third are called by Helmholtz "medial consonances ;" 
 the minor Third and minor Sixth, "imperfect consonances." 
 
 An analysis of the harmonics of these consonances yields the fol- 
 lowing results, 1 which show the amount of coincidence belonging 
 to the acoustic waves of the different tones when combined in a 
 chord with a fundamental tone. 
 
 Twelfth 
 Fifth 
 Fourth 
 Major Third 
 
 ( c 1 
 
 { 
 
 1 
 
 i; 
 i: 
 
 1 c 2 
 
 i i i 
 
 g* c 2 
 
 g 1 | g' 2 d 3 
 O i g i c s e 2 g 2 
 
 g 1 c 
 c' g' c* 
 
 I* g 1 
 e 2 g 2 
 
 f 1 c 2 
 c 1 g 1 c 2 
 
 fa 2 
 
 e 2 
 
 e 1 b 1 
 
 e 2 
 
 The major Sixth is similar in the form of its harmonics to the 
 major Third. 
 
 1 Comp. Helmholtz, The Sensations of Tone, p. 281 f. 
 
324 SENSATIONS OF SOUND. 
 
 Two psycho-physical causes for the characteristic feelings which 
 belong to sensations of consonance and dissonance, respectively, 
 may be assigned with more or less of probability. The first is that 
 proposed by Helmholtz. 1 The feeling of dissonance which is pro- 
 duced by sounding together two notes that differ only by a semi- 
 tone is found to be increased when the difference in the pitch of 
 the notes is still further diminished. Successive shocks called 
 " beats " occur, less frequently but more decidedly and unpleasantly, 
 as the pitch of the notes becomes more nearly the same. The feel- 
 ing of dissonance is found to reach its height when the number of 
 beats is about 30 per second. For example, if b l (495 vibrations) 
 and c 2 (528 vibrations) are struck together, the number of beats 
 is 33 (528 495=33), and the dissonance is very strongly marked. 
 In all marked dissonances such beats occur at the rate of from 20 
 to 40 in a second. The unpleasant effect in consciousness is an- 
 alogous to that produced by all sudden and rapid intermission of 
 stimulation ; as, for example, the flickering of light or the scraping 
 of uneven surfaces over the skin. The feeling of consonance is due 
 to the absence of beats. In addition to Helmholtz's negative reason, 
 Oettingen has proposed the positive one, that the pleasantness of 
 harmony is due to what he calls the " tonicity " and " phonicity " 
 of certain intervals and combined notes. " Tonicity " is the prop- 
 erty of being recognized as a constituent of a single fundamental 
 tone which is designated by the name "tonic." "Phonicity" is 
 that property of a chord or interval which consists in the possession 
 of certain partial tones that are common to all tones. The first of 
 these qualities of harmony seems to ally the pleasure it yields to 
 that which follows even the obscure and only half-conscious per- 
 ception, as it were, of all relations, as such, between our sensa- 
 tions. 
 
 23. In order that the physical apparatus of hearing may act 
 as the organ of those wonderfully fine discriminations which belong 
 to the most analytic of all the senses, it would seem that it must 
 possess an outfit of end-organs with structure sufficiently minute 
 to serve as a basis for a satisfactory development of "local signs." 
 The number of the cells of Corti, and of their separate terminal 
 auditory nerves, has been calculated by Hensen 2 at about 16,400 ; 
 by Waldeyer 3 at 20,000. It is doubtful, however, whether even 
 this large number will suffice to account for that niceness of audi- 
 tory discriminations which we have seen to be possible. 
 
 1 The Sensations of Tone, p. 255 f . 
 
 2 In Hermann's Handb. d. Physiol., III., ii., p. 115. 
 
 3 Strieker's Gewebelehre, II., p. 954. 
 
CHAPTEE IV. 
 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 as many 
 grades 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 quality of 
 such sensations. Thus defining the first problem 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 a 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 sen- 
 sations may be called (though somewhat ineptly) normal sensations 
 of color. When the foregoing question is answered 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 
 
326 SENSATIONS OF SIGHT. 
 
 rapid oscillations of luminiferous ether. Some forms of mechani- 
 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 bright- 
 er 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 the most gorgeous and varied coloring is often 
 seen 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 
 excitation 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 
 peculiar action of the ascending and descending electrical current 
 has been thought by some 1 to be due to its catelectrotonic or 
 anelectrotonic 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 
 
 1 See Fick, Physiolog. Optik, in Hermann's Handb. d. PhysioL, III., i., p. 
 
THE FINENESS OF VISION. 
 
 327 
 
 must be located at the back of the retina in the rods and cones 
 (see Part I., Chap. V., 18-22). The argument by which we have 
 connected 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. In order that two visual sensations 
 may be seen as separate, yet side by side, in an object, two neigh- 
 boring 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 histology and experimental physiology 
 agree fairly well. 
 
 The degree of accuracy which sight can attain is dependent 
 upon the size of the retinal elements directly affected by the 
 light. 1 Hooke observed that no one can distinguish two stars as 
 two, unless they are apart at least 30" ; few, indeed, can distin- 
 guish 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 ap- Flo> go._ A shows the appearance of Iine8 
 pear, not straight, but knotted and 
 nicked. This fact is due to the 
 action of the stimulus on the mo- 
 saic of rods and cones, as seen by the accompanying figure (No. 90). 
 The diminishing sharpness of vision as we move away on the sur- 
 face of the retina from its most central area corresponds to the 
 
 1 See Helmholtz, Haiidb. d. Physiolog. Optik, Leipzig, 1867, p. 215 ff.; 
 Fick, in Hermann's Haiidb. d. Physiol., III., i.,p. 152 f.; von Kries, Archiv 
 f. Anat. u. Physiol., Physiolog. Abth., 1882 (Appendix), p. 24 f. 
 
 drawn very closely together, which is sup- 
 posed to be due to their falling upon the 
 nervous elements of the retina in the man- 
 ner shown by B. 
 
328 SENSATIONS OF SIGHT. 
 
 comparative paucity of the nervous elements 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- 
 tensity of the light, and confining our attention to what has already 
 been defined as the normal action of the eye (comp. p. 325), 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 respect to the amounts of white (or colorless light) and 
 saturated light (or light of pure color-tone) which enter into them. 
 The foregoing distinctions in the quality of our color-sensations 
 may be confirmed by an appeal to experience. Ked 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 bright- 
 ness of a color is, scientifically speaking, dependent both upon 
 the degree of saturation which the color possesses and upon the 
 total intensity 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 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 following table ' 
 exhibits these facts on the scale of Fraunhofer's lines, which mark 
 those portions of the spectrum where its principal colors appear 
 most obvious to the normal eye. 
 
 'Taken from Pick, Physiolog. Optik, in Hermann's Handb. d. Physiolog., 
 III., i., p. 173. 
 
COLOR-TONES OF THE SPECTRUM. 
 
 329 
 
 Name of the line. 
 
 Number of vibra- 
 tions per second. 
 
 Wave-length in the air. 
 
 B 
 
 Billions. 
 450 
 
 Millimeters. 
 
 0.0006878 
 
 c . 
 
 472 
 
 0.0006564 
 
 D 
 
 526 
 
 0.0005888 
 
 E 
 
 589 
 
 0.0005260 
 
 F 
 
 640 
 
 0.0004843 
 
 G 
 
 722 
 
 0.0004291 
 
 H 
 
 790 
 
 0.0003928 
 
 
 
 
 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 
 increases beyond four hundred and seventy billions (C) 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 
 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 ultra-violet end of the 
 spectrum has been made visible for a certain extent by experiment ; * 
 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 
 ! See Helmholtz, Physiolog. Optik, p. 232 f . 
 
330 SENSATIONS OF SIGHT. 
 
 be much disturbed by the chromatic aberration of the refracting 
 apparatus of the eye. 1 
 
 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 
 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 quickly, then more slowly, and then more quickly 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, 
 
 G H 
 
 FIG. 91. (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 ordinates of 
 the dark curved line, the intensity of the rays as measured by calorific effect. 
 
 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. The sensitiveness of the 
 retina to slight variations in color-tone, as dependent upon differ- 
 ences 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- 
 ness with which the different colors of the spectrum appear to the 
 eye, as calculated by different methods and by two observers. It 
 will be seen that the results agree substantially, though by no 
 means perfectly. In the last two columns the letter sstand 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. 2 
 
 1 See Fick, Compendium d. Physiologie, 3d edition, p. 181 f . ; and Her- 
 mann's Handb. d. Physiol., III., i., p. 181 f. 
 
 2 See Helmholtz's Physiolog. Optik, p. 317 f.; von Kries, in Archiv f. Anat. 
 u. Physiol., Physiolog. Abth., 1882 (Appendix), pp. 56-76 ; Fick, in Hermann's 
 Handb. d. Physiol. , III. , i. , p. 174 f . ; Mandelstamm and Dobrowolsky, in Archiv 
 f. Ophthalmologie, XIII. , ii., p. 399, and XVIII. , i., p. 66. 
 
MIXED COLOR-IMPRESSIONS. 
 
 331 
 
 
 Praunhofer. 
 
 Vierordt. 
 
 Mandelstamm and Dobrowolsky. 
 
 Red, B 
 
 32 
 
 94 
 640 
 1,000 
 480 
 170 
 31 
 5.6 
 
 22 
 
 128 
 780 
 1,000 
 370 
 128 
 8 
 0.7 
 
 B 
 
 T*T 
 
 T*T 
 
 % 
 
 U4T) 
 if* 
 
 TiTT 
 72 
 
 T** 
 
 Orange, C 
 
 C 
 
 Reddish -yellow D 
 
 C-D 
 
 Yellow D-E 
 
 D 
 
 Green E 
 
 D-E 
 
 Blue-green, F . .... 
 
 E 
 
 Blue, G 
 
 E-F 
 
 Violet, H 
 
 F 
 
 
 G 
 
 H.... 
 
 
 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- 
 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 all possible distances along the spectrum, an indefinite 
 number of impressions of color may be obtained, which all differ 
 from those obtained by the homogeneous colors. These mixed 
 color-impressions, however, are not all different from each other ; 
 so that the number of the qualities of resulting sensations is far less 
 than that of the compound physical processes which stimulate the 
 retina. Their character depends both upon the place of the spec- 
 trum from which the simple color-tones are selected for mixture, 
 and also upon the relative intensity of the ones selected. For ex- 
 ample, 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- 
 
332 
 
 SENSATIONS OF SIGHT. 
 
 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 following 
 table ' is of interest in this connection. Where two colors are given 
 as resulting from the mixture, the variation is to be understood as 
 dependent upon the prevailing intensity of one of the two compo- 
 nents. 
 
 Components. 
 
 Tone of the color obtained by mixture. 
 
 Degree of 
 saturation. 
 
 Red and Yellow 
 
 Orange 
 
 Spectral 
 
 
 Yellow. 
 
 Spectral 
 
 Yellow and Green .... 
 
 Yellow-green . . . ... 
 
 Whitish. 
 
 Yellow-green and Blue-green.. .. 
 
 Green 
 
 Very whitish. 
 
 Green and Cyanic Blue 
 
 Blue-green ... 
 
 Whitish. 
 
 
 Cyanic Blue 
 
 Spectral 
 
 Cyanic Blue and Violet 
 
 Indigo 
 
 Spectral. 
 
 Red and Yellow-green . . 
 
 Orange or Yellow 
 
 Spectral. 
 
 Red and Green 
 Violet and Blue-green . . 
 
 Orange or Yellow or Yellow-green 
 Infligo or Cyanic Bine 
 
 Whitish. 
 Spectral. 
 
 Violet and Green 
 
 Indigo or Cyanic Blue or Blue-green 
 
 Whitish. 
 
 Violet and Orange 
 
 Red 
 
 Whitish 
 
 Red and Cyanic Blue 
 
 Indigo or Violet 
 
 Whitish 
 
 
 Violet 
 
 Slightly whitish 
 
 
 
 
 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 
 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 semitone, between blue and violet, 
 has perhaps no more real right to recognition than various other 
 intermediate color-tones. Donders 2 puts the number of color-tones 
 distinguishable in oil-colors at one hundred ; von Kries 3 the rec- 
 ognizable number of spectral tints at about two hundred and 
 thirty. But each of these yields different sensations of color ac- 
 cording to the degree of its saturation or purity, due to freedom 
 from admixture of white light. Another series of variations of sen- 
 sation must be allowed for, which are due to differences in " bright- 
 ness " 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 
 
 1 Made according to investigations by J. J. Miiller, and taken from Fick, in 
 Hermann's Handb. d. Physiol., III., i., p. 190. 
 8 Archiv f. Ophthalmologie, XXVII. 
 3 Archiv f. Anat. u. Physiol., Physiolog. Abth., 1882 (Appendix), p. 58 f. 
 
THE COMPLEMENTARY COLOKS. 
 
 333 
 
 about five hundred thousand to six hundred thousand. This num- 
 ber stands midway between the " many millions " of which Au- 
 bert speaks and the five thousand allowed by Donders. Herschel 
 thought that the 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 : * 
 
 Color. 
 
 Wave-length. 
 
 Complementary 
 color. 
 
 Wave-length. 
 
 Relation of 
 wave-lengths. 
 
 Red 
 
 2,425 
 
 Green-blue . . . 
 
 1,818 
 
 1,334 
 
 Oraneje 
 
 2 244 
 
 Blue 
 
 1 809 
 
 1,240 
 
 Gold-yellow 
 
 2 162 
 
 Blue . . 
 
 1,793 
 
 1,206 
 
 Gold-yellow 
 
 2,120 
 
 Blue 
 
 1,781 
 
 1,190 
 
 Yellow 
 
 2,095 
 
 Indigo-blue. . 
 
 1,716 
 
 1,221 
 
 Yellow 
 
 2,085 
 
 Indigo-blue. . . 
 
 1,706 
 
 1,222 
 
 Green-yellow 
 
 2 082 
 
 Violet 
 
 1 600 J ? nd 
 
 1,301 
 
 
 
 
 } less 
 
 
 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. 
 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 ei- 
 
 1 Taken from Helmholtz, Physiolog. Optik, p. 277. The numbers are given 
 in hundred-millionths of a Parisian inch, and may be reduced to millimetres 
 by multiplying by 27.07. 
 
334 SENSATIONS OF SIGHT. 
 
 ther a maximum or a minimum. At the maximum intensities of 
 the stimulus all sensations of color- tone cease, and even homoge- 
 neous rays appear white. Previous to reaching this maximum, 
 red and green pass over into yellow. 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. 
 The different colors appear and disappear, as such, at different 
 degrees of intensity of the stimulus green, among them all, re- 
 maining \isible in the weakest light. They all also change their 
 tone as the light which falls on them diminishes ; but it is scarcely 
 possible to describe the law of this change, on account of the great 
 difficulty of distinguishing color- tones in very weak light. 
 
 11. Changes of color also take place when the time of the 
 action 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. 1 
 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 2 has shown 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 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 dis- 
 tinguished. 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 3 
 has shown that the smaller the distance between the single perfo- 
 rations, the greater the distance at which the eye can recognize 
 colors through them. In general, then, two weak sensations, each 
 of which belongs to one eye, may fuse together into one strong 
 one. 
 
 1 According to Kunkel, in Pfliiger's Archiv, ix. , p. 207. 
 
 3 Pfltiger's Archiv, xvii., p. 152. 
 
 3 See Dobrowolsky, in Pfluger's Archiv, xxxv., p. 536 f. 
 
KINDS OF COLOR-BLINDNESS. 335 
 
 12. Very important changes in the visual sensations occur as 
 dependent on the place 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. 1 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. It follows, then, that whereas there is at the central zone 
 an indefinite number of color-tones possible, this number is re- 
 duced to comparatively few impressions at the middle zone ; while 
 all color-tones gradually become indistinguishable and are lost on 
 passing through the outer zone. These great changes in sensi- 
 tiveness to color are not accompanied by similar changes in sen- 
 sitiveness to colorless light ; it even appears that regions of the 
 retina distant about 30 from its centre are more sensitive to light 
 than is the polar zone. 
 
 A certain proportion of persons (perhaps one-twentieth or more) 
 appear to have a defective structure of the retina, which may be 
 described as corresponding in the polar zone to that of the normal 
 retina in the middle or even the outer zone. Such persons are said 
 to be " color-blind." The farther outward this imperfect condition 
 of the retina extends, the nearer does the defect approach to total 
 color-blindness. 2 In most cases of this defect there is a partial or 
 complete insensitiveness to the red rays ; these rays are especially 
 liable to be confused with the dark-green or the yellow. The spec- 
 trum is thus shortened at the red end. Cases of so-called violet- 
 blindness, as reported by Donders and Stilling, are much more rare 
 and doubtful. In total color-blindness only shades of gray from 
 
 1 See von Kries, Archiv f. Anat. u. Physiol., Physiolog. Abth., 1882 (Ap- 
 pendix), p. 90. 
 
 2 See Fick, Zur Theorie d. Farbenblindheit, p. 213 f. ; and in Hermann's 
 Handb. d. Physiol., III., i., p. 206 f. 
 
336 SENSATIONS OF SIGHT. 
 
 I 
 
 white to black are visible. In general, the attempts to make out a 
 spectrum for the color-blind are unsatisfactory, since we can only 
 be sure as to what color-tones appear like or unlike to them ; we 
 cannot, on the contrary, be sure that their abnormal sensations are 
 like any of our normal sensations in other words, that what they see 
 when red light falls on the retina corresponds to any of our color- 
 tones. The three or four cases reported where one eye of a person 
 has been normal and the other color-blind are, of course, especially 
 valuable ; since they offer an opportunity to compare immediately 
 the sensations of the normal with those of the pathological eye. 
 These cases, according to von Kries ' show that the two funda- 
 mental colors to which the color-blind are reduced may be con- 
 sidered as either red and blue-green or greenish-yellow and blue- 
 violet. 
 
 13. Important modifications of the normal action of the eye are 
 also caused by the previous condition of the retina, or by the contem- 
 poraneous condition of parts of it contiguous to those on which the 
 light falls. The former fact explains the phenomena of " inertia " 
 and " exhaustion ; " the latter, the phenomena of " contrast." The 
 reaction of the sense of sight is relatively very sluggish ; or in 
 other words the inertia of the eye is relatively great. This fact 
 is undoubtedly due to the chemical nature of the stimulus which 
 acts directly upon its end-organs. The light requires time in order 
 to effect those photo-chemical changes on whose action upon the 
 nervous elements of the retina our sensations of light and color 
 depend. On the other hand, if we close the eyes after looking 
 intently upon any bright object, the image of this object remains 
 for some time, and only slowly fades out of sight. Such an image 
 is called a "positive after-image," because its bright and dark lines 
 and surfaces correspond to those of the original object. The delay 
 which the sensations undergo, both in forming and in fading away, 
 is said to be due to the inertia of the retinal structure. It is, of 
 course, a law of all nervous excitation and action that it requires a 
 certain amount of time for beginning and for changing its char- 
 acter. 
 
 White positive after-images (as Fechner, Helmholtz, and oth- 
 ers have shown) pass quickly through greenish-blue to indigo- 
 blue and then to violet or rose-color. But " negative after-images " 
 are due to the exhaustion of the retina. If the eye be intently 
 fixed for some time on a small square of black lying upon a sheet 
 of white paper, and then suddenly turned upon the white surface, 
 a bright square appears, moves about with the eye, and slowly 
 1 Archiv f. Anat. u. Physiol., Physiolog. Abth., 1882 (Appendix), p. 152 f. 
 
PHENOMENA OF CONTRAST. 337 
 
 fades away. If we look for a long time at a green surface and then 
 direct the eye upon a white one, the latter appears for a moment to 
 be of a red color. In general, the color of the negative after-image 
 is such that, when combined with the color of the object, the two will 
 produce white. In other words, the color of such an image is u com- 
 plementary " of the color of the object. Such facts as the foregoing 
 must in some manner be brought under the law which applies to all 
 the elements of the nervous system, but especially to the end-organs 
 and the central organs ; these organs become wearied by continuous 
 use, and require time for recovery of their suspended or diminished 
 functions. Precisely how the application is to be made to the case 
 of the retina is, however, a matter of the general physiological the- 
 ory of vision which cannot as yet be stated with perfect certainty. 
 The phenomena of exhaustion are among the most important for 
 the formation of such a theory. Investigations in this direction 
 have led to the discovery that none even of the spectral colors are 
 perfectly saturated, since each of them can be made to appear more 
 so by looking at it with an eye wearied by the complementary 
 color. l Ked is most nearly saturated, blue and yellow next, and 
 green least of all. 
 
 14. 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 
 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 3 as deceptions of judgment, such as we are accustomed to in 
 our estimates of distances. To this explanation, however, Fick/ 
 Bering, 6 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. Such influence does not always 
 
 1 Comp. Helmholtz, Physiolog. Optik, p. 279 f. ; Exner, in Pfluger's Archiv, 
 i., p. 389; and see, especially, von Kries, Archiv f. Anat. u. Physiol., Phy- 
 siolog. Abth. , 1882 (Appendix), p. 115. 
 
 * Physiolog. Psychologic, i., p. 439. 
 
 3 Physiolog. Optik, pp. 388 ff. 
 
 4 In Hermann's Handb. d. Physiol., III., i., p. 231 f. 
 5 Sitzgsber. d. Wiener Acad., June, 1872, and December, 1873. 
 
 22 
 
338 SENSATIONS OF SIGHT. 
 
 take the form of depressing the excitability of the contiguous 
 parts ; on the contrary, stimulating certain elements for some time 
 may finally involve contiguous ones in a secondary way. This fact 
 they consider to be the true explanation of the spreading of a 
 bright object on a dark background, whose after-image becomes a 
 clear band of light around the dark image of the bright object. 
 When colored instead of white light is used in experimenting under 
 the law of contrast, phenomena similar to those of complementary 
 colors are obtained. 1 A small square of white on a surface 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. There 
 is the same dispute over these as over the other phenomena of 
 contrast. Shall they be considered as cases of deception of judg- 
 ment, or do they admit of a physiological explanation ? Mere 
 cases of deception they cannot well be. The theory which ascribes 
 to each part of the retina an influence upon other contiguous parts 
 is the most satisfactory form of a physiological explanation. But 
 guch physiological explanation seems to need supplementing by 
 reference to induced conditions of the central organs, concerning 
 the nature of which we are thus far almost entirely ignorant. 
 
 15. It will readily be seen that a theory which shall satisfac- 
 torily account for the complicated phenomena of visual sensations 
 is difficult to establish. Physiological optics will probably never 
 be able to explain in detail the individual sensations of light and 
 color. But each claimant to present such theory must, as Wundt 2 
 maintains, account for the following four main classes of facts : (1) 
 The subjective relations of the color-tones, and the fact that they 
 may all be graded downward, as it were, into colorless light ; (2) 
 the law of the mixing of all the colors from three (or more) funda- 
 mental color-tones ; (3) the phenomena of after-images ; and (4) 
 the phenomena of contrast. Among all the hypotheses hitherto 
 proposed to account for the quality of visual sensations, that brought 
 forward by Young, and elaborated and applied by Helmholtz, is by 
 far the most prominent. This hypothesis takes its point of start- 
 ing from the undoubted fact that, by admixture of a few so-called 
 fundamental color-tones, we can produce all the other colors, as 
 well as the sensation called "white." There are said to be three 
 such color-tones, because this is the smallest number which will 
 account for the facts. Of these three, green must be one, since, in 
 the spectrum of colors, this tone has no complementary color. Green 
 
 ' See Helmholtz, Physiolog. Optik, pp. 388 ff. 
 2 Physiolog. Psychologie, i., p. 450. 
 
THE YOUNG-HELMHOLTZ THEOEY. 
 
 339 
 
 being fixed, the other two color-tones must be chosen from near the 
 ends of the spectrum, and in such a way that, when combined with 
 spectral green, they will produce white. Red (carmine-red, ac- 
 cording to Fick) and either violet (so Young and Helmholtz) or blue 
 (indigo-blue, Fick) best fulfil the required conditions. It is, then, 
 assumed, by the Young-Helmholtz theory, that in every portion 
 of the retina which is susceptible to color there exist three kinds 
 of nervous elements, the excitation of which separately would pro- 
 duce three distinct kinds of sensations ; and that each kind of ele- 
 ment is capable of producing only that kind of sensation which is 
 peculiar to itself. It apparently follows that each of these three 
 kinds of nervous elements has its special form of end-apparatus, the 
 excitability of which differs from that of the others ; that is to say, 
 
 G H 
 
 PIG. 92. Diagram from Fick, illustrating the Young-Helmholtz Theory. (For explanation, see 
 
 the text.) 
 
 there are fibres of red color-sensation, whose end-apparatus responds 
 specifically to rays of small refrangibility ; fibres of green color- 
 sensation, whose end-apparatus responds to rays of medium re- 
 frangibility ; and fibres of violet or blue color-sensations, whose 
 end-apparatus responds to rays of great refrangibility. We must 
 suppose, however, since we cannot directly analyze into their com- 
 ponents the sensations which appear in consciousness, that no one 
 of the three kinds of elements is ordinarily excited alone. Every 
 actual sensation of color is therefore a complex affair, whose char- 
 acter is determined by the relations in which each one of the three 
 intensities of excitation stands to both the others. In explanation 
 of this assumption the following diagram is proposed. 1 (See Fig. 
 92.) The curved lines R, G, and B represent the three kinds of 
 
 1 Taken from Fick's Physiolog. Optik, in Hermann's Handb. d. Physiol., 
 III., i., p. 198; cornp. Helmholtz, Physiolog. Optik, p. 291. 
 
340 SENSATIONS OF SIGHT. 
 
 nerves sensitive to the three fundamental color-tones E to red, G to 
 green, B to blue (indigo). The curves described by them show the 
 strength of the excitation exercised by the stimulus, corresponding 
 to the colors of the spectrum, upon each kind of nerves. The per- 
 pendicular lines indicate the colors of the spectrum ; and the way 
 these lines cut the curves shows the relative strength of the excita- 
 tion of each kind of nerves which is combined to produce these 
 colors. 
 
 It should be gratefully acknowledged that the Young-Helmholtz 
 theory affords a brilliant explanation of a great many of the phe- 
 nomena of sensations of light and color. It is most successful with 
 those that relate to the mixing of colors and to complementary 
 color. The hypothesis cannot be said, however, to be wholly ade- 
 quate and satisfactory. One of its most intelligent advocates (Fick) 
 admits that it cannot explain the following cardinal fact : Every 
 ray of light which, so long as it is confined to a moderate extent of 
 the polar zone, makes the impression of a saturated color produces 
 a whitish impression, almost devoid of color-tone, as soon as it is 
 limited to an extremely minute portion of the retina. This is the 
 very opposite of what the hypothesis would lead us to expect ; for, 
 according to it, extremely minute impressions on the retina ought 
 to isolate the particular kind of fibres, and so yield the purest 
 possible color-tone. The facts of histology seem rather adverse 
 than favorable to the theory, although not much stress can be laid 
 upon them alone. Moreover, it does not satisfactorily explain the 
 facts of contrast of colors and of color-blindness. The most re- 
 cent investigations seem to indicate that cases of color-blindness 
 cannot be accounted for by dropping out one fundamental kind of 
 nerve-fibres, as the Young-Helmholtz theory supposes. 1 Various 
 other important objections are raised by its opponents (especially 
 by Bering, Wundt, and others). 
 
 16. In order to supply the alleged defects of the Young-Helm- 
 holtz theory of color- sensations, several other theories have been 
 devised notably those of Bering and of Wundt. The former 3 
 differs from most other investigators in his view of the nature of 
 the changes of sensation which take place as we, in experience, 
 run through all the different shades of gray from white to black. 
 All such changes Hering considers analogous to those alterations 
 in the quality of our sensations that would be produced by passing 
 
 1 See von Kries, Archiv f. Anat. u. Physiol., Physiolog. Abth., 1882 (Ap- 
 pendix), pp. 134-153. 
 
 5 E. Hering, Zur Lehre vom Lichtsinne, Sitzgsber. d. Wiener Acad., 6 papers, 
 1872-74. 
 
WUTSDT'S THEORY OF COLORS. 341 
 
 the eye over a surface on which the different color-tones almost 
 insensibly shaded into each other. Hering, therefore, proposes six 
 (or three pairs instead of three single ones) fundamental color- 
 tones namely, black and white, green and red, blue and yellow. 
 The changes which give rise to sensations of black, green, and blue 
 are ascribed to the process of " construction " of a so-called visual 
 substance ; those which give rise to white, red, and yellow are as- 
 scribed to the " destruction " of such visual substance. The three 
 pairs of color-tones are thus made antagonistic rather than com- 
 plementary. But the hypothesis of Hering appears to involve more 
 uncertain assumptions, and to explain fewer facts, than the one it 
 would displace. Moreover, the assumption that white, and its shades 
 down to black, may be considered as color-tones, instead of altera- 
 tions in the brightness of the true color-tones, is generally denied. 
 
 The theory of Wundt l emphasizes the difference in processes 
 rather than in the kinds of retinal elements. It involves the fol- 
 lowing principles : (1) In every excitation of the retina two dif- 
 ferent processes are set up, the variations of which follow differ- 
 ent laws ; one of these is a " chromatic " process (which gives us 
 color-tones), and is a function of the length of the waves of light ; 
 the other is " achromatic," and is also dependent upon the wave- 
 lengths, but varies only in intensity and remains in character the 
 same. (2) The achromatic excitation consists in a " uniform pho- 
 to-chemical process," which reaches its maximum at yellow and 
 falls off toward both ends of the spectrum. (3) The chromatic 
 excitation is a "polyform photo-chemical process," which changes 
 continuously with the wave-lengths of light. The extreme differ- 
 ences of this length are such as to produce effects that approximate 
 each other ; while the effects of certain different intervening wave- 
 lengths are related in such a way that opposed phases of one and 
 the same movement equalize each other perfectly. (4) Every pro- 
 cess of excitation of the retina outlasts .the stimulation for a certain 
 time, and exhausts the sensibility of the nerve-substance for that 
 particular form of stimulation. The positive after-images are to be 
 explained by the persistence of the retinal excitation, the negative 
 by exhaustion. (5) The difficult phenomena of contrast are to be 
 explained by the general principle that all impressions of light and 
 color are experienced in relation to each other. In other words, 
 they fall under the general law of relativity. 
 
 17. Von Kries 2 has subjected all the principal theories of color- 
 
 1 See Physiolog. Psychologic, i., pp, 450 ff. 
 
 2 See Archiv f. Anat. u. Physiol., Physiolog. Abtli., 1882, Appendix, pp. 
 1-178. 
 
342 
 
 SENSATIONS OF SIGHT. 
 
 sensations to a most searching criticism as considered in the light 
 of all the facts. He naturally finds serious defects in them all, but 
 arrives at the following highly important conclusions. The photo- 
 chemical facts concerned in vision compel us to adopt a theory of 
 component elements rather than one of changes qualitatively alike 
 and arranged in a continuous series. This would seem decisive 
 against the theory of Wundt. Only by the aid of assuming the 
 varied combination of such elements can we explain the phenomena 
 
 G 
 
 FIG. 93. Color-Triangle, from Fick. (For explanation see text.) 
 
 of exhaustion. Three series of components are apparently requisite : 
 one for the bright and dark, but colorless, sensations, and two 
 color-tone series a red-green series, and a yellow-blue series. 
 White is, nevertheless, not to be considered as belonging to the 
 three, since it corresponds to all the color-tones whenever they 
 reach a minimum of saturation. The processes corresponding to 
 these three series of components may be located at different places 
 in the nervous apparatus of vision either more centrally or more 
 
SYMBOLISM OF COLOR-TONES. 
 
 343 
 
 peripherally. The articulation and adjustment, as it were, of the 
 three processes von Kries would assign to the central organs. And 
 here we reach the extreme limits, not only of our assured knowl- 
 edge, but also of our power to frame a plausible theory ; for it ap- 
 pears that all theories must either leave certain important facts un- 
 explained, or else make further assumptions concerning nervous 
 processes especially in the central organs of vision of the exist- 
 ence and influence of which upon the sensations there can be no 
 doubt, but of the precise nature of which we are completely ig- 
 norant. 
 
 18. 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 differ- 
 ent 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 most successfully set forth by a triangle, which 
 may be constructed as in the foregoing figure ' (93). In this triangle 
 the different color-tones may be regarded as lying together along 
 the 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 the point W through the 
 points occupied on the curve by the 
 two color-tones. For example, the 
 difference between red and violet is 
 less than that between red and green, 
 as is indicated by the fact that the 
 angle R WHis smaller than the an- 
 gle R W G. 
 
 By Fig. 94 2 the relations of the 
 color-tones as contrasting with, and 
 complementary of, each other are rep- 
 resented. Of the two Concentric FIG. 94. Scheme for showing the Rela- 
 
 circles, each color in one corresponds tions of Color - tone ( see text )- 
 to the complementary color of the other. If the color inducing 
 the contrast is represented by a segment of the inner circle, the 
 coincident segments of the two circles represent the direction in 
 which the induced change is moving, as it were. For example, 
 
 1 Taken from Fick, in Hermann's Handb. d. Physiol., III., i., p. 184. 
 3 Taken from Wundt, Physiolog. Psychologic, i., p. 442. 
 
344 SENSATIONS OF THE SKIN. 
 
 since the segment green coincides with purple, and red coincides 
 with blue-green, green on a red ground is modified as it would be 
 if blue-green were mixed with it ; and red, as it would be if purple 
 were mixed with it. 
 
 19. At least two specifically different forms of sensation namely, 
 Pressure and Temperature have generally been admitted to have 
 their organ in the skin. ' The claims of various other kindred forms 
 of feeling to be considered as primitive factors of our sense-percep- 
 tions, arising from the activity of the skin as an end-organ of sense, 
 are more doubtful. Sensations of motion, of innervation and weari- 
 ness of the muscles, the so-called " common sensations " (or sensa- 
 tions of the sensus communis), the sensations of pain or pleasure, and 
 those delicate shadings of sensation, as it were, which constitute 
 the " local coloring" of all the feelings to which we assign a definite 
 place in the fields of sight and touch, are all closely allied to sensa- 
 tions of pressure and temperature. But some of these forms of 
 feeling as, for example, the so-called sensations of motion and of the 
 sensus communis are undoubtedly complex modifications of certain 
 simpler states of consciousness ; others of them, as the sensations. 
 of muscular weariness, of pain, of innervation, and " local coloring," 
 may possibly have, in part, a central origin. As a rule, they lack 
 the characteristic quality of being components of the " presenta- 
 tions of sense," as this quality belongs to all genuine sensations. 
 Sensations of " local coloring " have, indeed, a most important part 
 to take in the formation of the " presentations of sense ; " but they 
 are, in the realm of touch and of muscular feeling, as infinitely and 
 delicately varied (and even more difficult of description) as are the 
 finest shadings of musical tones or color-tones. 
 
 20. A sixth sense, however, and a sixth organ of sensations 
 must doubtless be recognized as constituted by the muscles and 
 the various kinds of feeling which their action occasions. These 
 muscular sensations, when combined with those of the skin, give 
 certain complex feelings of motion on which the adjustment of the 
 body to its environment is so dependent. The long-continued dis- 
 pute concerning the presence of sensory nerve-fibrils in the muscles 
 may be said to be settled affirmatively. 2 Certain subjective phe- 
 nomena cannot be accounted for by ascribing the so-called muscular 
 sensations to feelings of central innervation, or by identifying them 
 
 1 On the physiology of the skin, see Goldscheider, art. Neue Thatsachen 
 liber die Hautsinnesnerven, Archiv f. Anat. u. Physiol., Physiolog. Abth., 
 Supplement-Band, pp. 1-104. 
 
 * See, especially, Sachs, in Archiv f. Anat. u. Physiol., 1874, pp. 175 1, 491 
 f., and 645 f. 
 
THE FEELING OF PRESSURE. 345 
 
 with the sensations of pressure through the skin. 1 Bernhardt 2 found 
 that the degree of sensitiveness to different weights, when lifted by 
 the foot or the finger, was little or not at all diminished by exclud- 
 ing all central innervation of the muscles through an act of will. 
 The discrimination of differences of weight was riot greatly impaired 
 when the limb was bent by an induction-shock sent through the 
 muscles instead of by motor impulses arising in the brain. The 
 muscular sensations cannot, therefore, be due to such central activ- 
 ity. Investigation also shows that the muscular sensations sup- 
 plement those of pressure in the skin in all our estimates of the 
 position and motion of the limbs ; these two are, therefore, not 
 identical. Moreover, without assuming the existence and aid of 
 such sensations we cannot account for that nice control of the mus- 
 cles which, especially in the case of the eye, is so indispensable a 
 prerequisite, not only for adjusting their action to the ends desired, 
 but also for gaining an exact knowledge of the position and motion 
 of objects in the outside world. The precise manner, however, in 
 which the muscular sensations originate, through that stimulation 
 of the sensory nerves which the contraction of the muscular fibre 
 occasions, is as yet unknown. Nor can they easily be separated 
 and classified into kinds, apart from the sensations of pressure with 
 which they are in actual experience constantly allied. Their chief 
 interest to psychology centres in the help which they furnish to the 
 mind in forming its " presentations of sense." 
 
 21. Sensations of Pressure are dependent upon the excitation 
 of the sensory nerves of the skin through their appropriate end- 
 organs. The excitation of the trunk of any of these nerves at some 
 point along its course may produce the feeling of pain, but does not 
 produce those definite sensations of pressure which we are able to 
 localize so accurately and discriminate so nicely as to their degree. 
 Precisely which of these end-organs are specifically related to sen- 
 sations of pressure neither histology nor experimental physiology 
 has thus far been able to determine (see Part I., chap. V., 10). 
 The ordinary stimulus of the end-organs of the skin active in these 
 sensations consists in their compression or expansion by contact 
 with some external object which either rests upon them or upon 
 which they rest, or which is moved over or against them, or over or 
 against which they are moved. Such stimulus may, of course, vary 
 both in form and in degree. The quantity and succession of the 
 sensations of pressure, as well as the manner in which they com- 
 bine with one another and with sensations of the muscular sense, 
 
 1 Comp. Funke, in Hermann's Handb. d. Physiol., III., ii., p. 359 f. 
 
 2 Archiv f. Psychiatrie, III., p. 627. 
 
346 SENSATIONS OF THE SKIN. 
 
 have a marked effect in determining their characteristic "tone" 
 of feeling. In respect to quality pure and simple, sensations of 
 pressure scarcely admit of a scientific classification. We localize 
 them in the field of touch ; we make an important use of them in 
 connection with sensations of muscular origin, for constructing the 
 field of vision and for giving to different objects their respective 
 places in this field ; but in ordinary experience we do not directly 
 recognize kinds of the simple sensations of pressure as we do of 
 tastes, smells, tones, and colors. A distinction is sometimes made 
 between "light touch," or touch proper, and sensations of press- 
 ure or weight. But the distinction, so far as it leaves out of ac- 
 count the muscular sensations, has hitherto been one only of de- 
 gree and not of kind. 
 
 The more recent and thorough investigations of Goldscheider l 
 have led him to distinguish two specifically different sensations 
 which enter into what is ordinarily called the feeling of pressure. 
 This distinction is based upon facts experimentally ascertained. If 
 a very fine point of metal, wood, or cork, be touched lightly to the 
 skin, it will be found to awaken a definite sensation, such as is en- 
 titled to be called a "sensation of pressure," only at certain minute 
 spots in any given area of the skin. This sensation, when the 
 pressure is very light, is described as lively and delicate, often 
 accompanied by the feeling of being tickled. On increasing the 
 pressure upon these same spots the sensations change their char- 
 acter somewhat, and become as though some small, hard kernel 
 were pressed upon the skin (" Korniges Gefilhl "). Between these 
 distinctively "pressure-spots" it is not possible to excite by press- 
 ure the same characteristic sensation. Stimulation of the inter- 
 mediate spots, on the contrary, produces a dull, indefinable, "con- 
 tent-less " sensation ; and when the pressure is increased, a sense 
 of being pricked or stuck. Both of these kinds of sensation, when 
 the pressure is still further increased, pass over into painful feeling ; 
 but the character of the pain in the two is different. 
 
 The arrangement of the " pressure-spots " is analogous to that of 
 the temperature-spots (to be described subsequently). They occur 
 much more frequently in certain areas of the body than in others. 
 They are placed in chains, as it were, sometimes more and some- 
 times less thickly set. These chains ordinarily radiate from a kind 
 of central point, and run in such directions as to form either circu- 
 lar, longitudinal, or pyramidal figures. Their direction is seldom 
 identical with that of the temperature-spots. In the opinion of 
 
 1 Archiv f. Anat, u, Physiol., 1885, Physiolog. Abth., Supplement-Band, pp. 
 76 ff. 
 
ARRANGEMENT OF PRESSUKE-SPOTS. 
 
 347 
 
 Goldscheider the spots of both kinds correspond to the terminal 
 points of the nerve-fibres of two specifically different kinds of nerves 
 distributed over the skin. But whereas all the area of the skin is 
 well covered with such nerves as give us the general dull and in- 
 definite feeling of contact, the nerves of the sensation of pressure 
 are much more unevenly distributed. It need scarcely be said 
 that, other things being equal, they are most numerous in the areas 
 of the skin most sensitive to touch. The different pressure-spots 
 themselves differ in sensitiveness ; some are much more easily ex- 
 cited than others. The sensations themselves come under the 
 
 FIG. 95. Arrangement of Pressure- spots (Qoldscheider). 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. 
 
 general laws of exhaustion, practice, etc., as these laws apply to 
 the whole mechanism of sense. 
 
 The attempt has been made, on the other hand, to identify, in 
 kind, sensations of pressure (especially those of light touch) and 
 sensations of temperature. 1 E. H. Weber observed that cold bodies 
 resting on the skin appear heavier, and warm lighter, than they 
 really are. A single silver dollar of the temperature of 25-19.5 
 Fahr. appeared to be of the same weight as two dollars of the tem- 
 perature of 98.5-100.5 Fahr. Wunderli also argues the identity 
 of these two classes of sensations on the ground that, if certain parts 
 of the skin are lightly touched with cotton or slightly warmed by 
 approaching a heated surface to them, through a square opening in 
 
 1 For a discussion of this question, see Funke, in Hermann's Handb. d. 
 Physiol., HI., ii., p. 820 f. 
 
348 SENSATIONS OF THE SKIN. 
 
 a piece of paper laid upon the skin, the two sensations thus occa- 
 sioned are frequently mistaken for each other. But Szabadfoldi 
 has much weakened the force of Weber's experiment by showing 
 that small wooden disks when heated to 122 Fahr. often feel 
 heavier than those which are really larger but are not thus warmed. 
 And Wunderli's observation at best holds good only for compara- 
 tively obtuse parts of the skin, especially the back. Moreover, if 
 the same stimuli should serve to excite both the pressure-spots 
 and the temperature-spots, this would not prove the identity of the 
 two sensations. 
 
 Finally, the physiology of the sense of temperature re-enforces 
 the indubitable testimony of consciousness, and leads us to the con- 
 clusion that from beginning to end in the character of their stimuli, 
 of their nervous processes, and of the resulting modifications of 
 feeling the sensations of pressure and the sensations of tempera- 
 ture are qualitatively distinct. They have in common only the 
 organ in which their apparatus is located, and the fact that both 
 kinds of sensations are constantly associated most intimately in 
 time and space. 
 
 22. Sensations of Temperature, therefore, form a second dis- 
 tinct species which have their origin in the excitation of the nervous 
 end-apparatus of the skin. Whether their end-apparatus is locally 
 the same as that upon the excitation of which the sensations of 
 pressure are dependent, it has seemed until very lately impossible 
 to say. But recent investigations (especially of Blix, 1 Goldscheider, 2 
 and Donaldson 3 ) point unequivocally to the conclusion that certain 
 definite spots of the skin, and these only, are susceptible to irrita- 
 tions of a kind to result in sensations of temperature. Such spots 
 are insensible to pain (even the pain of temperature), and a needle 
 can be run into them without being felt ; they are probably also in- 
 sensible to pressure. What is more remarkable still, the existence 
 of " heat-spots " and " cold-spots " or minute localities of the skin 
 sensitive to heat but not to cold, and conversely seems demon- 
 strable. By using a machine which locates the stimulus micro- 
 metrically, the topography of the skin may be mapped out, and 
 extremely minute spots indicated which respond to irritation with 
 sensations of pain, of pressure, of cold, and of heat respectively. 
 These different kinds of sensation-spots appear never to be super- 
 
 1 Zeitschrift f. Biol., 1884, XX., pp. 141 ff. 
 
 2 Monatshefte f. prakt. Dermatol., 1884, III., Nos. 7, 9, 10 ; 1885, IV., No. 1 ; 
 and art. in Archiv f. Anat. u. Physiol., 1885, Physiolog. Abth., Supplement- 
 Band. 
 
 3 Research on the Temperature-sense, reprinted from Mind, No. XXXIX. 
 
ARRANGEMENT OF TEMPERATURE-SPOTS. 
 
 349 
 
 imposed. They are not located alike on the symmetrical parts of 
 the same individual, or on the corresponding parts of different in- 
 dividuals. An accurate mapping out of the different areas of the 
 skin, with respect to their temperature-spots, is difficult ; since ex- 
 periment soon blunts the sense, and even the approach of a heated 
 or cooled point raises or lowers the temperature over a considerable 
 area. But, in general, such spots occur in lines that radiate from 
 centres generally coincident with the roots of the hairs, in those 
 
 FIG. 96. Arrangement of Temperature-spots. A, heat-spots ; and B, cold- spots from the palm 
 of the left hand (Goldscheider). 
 
 regions of the skin where such appendages are found. These lines 
 often run so as to cross each other, forming figures of various shapes, 
 triangles with rounded corners, etc. Heat-spots are, on the 
 whole, less abundant than cold-spots ; but in parts of the body where 
 the skin is most sensitive to either heat or cold the corresponding 
 class of spots is relatively frequent. Temperature-spots may be 
 divided into first-class and second-class (so Goldscheider) according 
 to the strength with which they react on moderate stimulation. 
 Some spots are roused only by excessive temperatures. The same 
 object feels cool to one spot, ice-cold to another. 
 
 The electrical current when applied to these spots is thought to 
 call out the corresponding specific sensations. Goldscheider con- 
 siders that he has succeeded in exciting definite temperature-sensa- 
 tions by applying electricity to the trunks of the nerves distributed 
 to certain areas of the skin. This would appear to be almost a 
 demonstration that the nerves of this sense are specific, and of 
 two kinds nerves of heat-sensation and nerves of cold-sensation. 
 Puncturing a temperature-spot also gives rise to temperature-sen- 
 sations. The discriminative sensibility of the temperature-spots 
 is found to be much finer than that of the tactile sensations. 
 Everything which produces a change in the temperature of the 
 skin acts, of course, as a stimulus for the sensations of heat and 
 cold. 
 
 23. The above-mentioned discoveries as to the specific energy 
 of the nerves and end-apparatus of the skin, interesting as they are, 
 
350 SENSATIONS OF THE SKIN. 
 
 have not yet been completely brought into rational connection with 
 our experience of temperature-sensations and our knowledge of 
 the general laws of nervous action. It is obvious, however, that 
 the principles of contrast, of relativity, and of exhaustion, must 
 bear a large part in the explanation of all these sensations. Sen- 
 sations of temperature have apparently a certain dependence on 
 the temperature of the thermic apparatus itself. This law has been 
 elaborated and defended in detail by Hering, 1 in the following 
 form : " As often as the thermic apparatus at any spot in the 
 skin has a temperature which lies above its own zero-point we 
 have a sensation of heat ; in the contrary case, a sensation of cold. 
 Either sensation is so much the more marked, or stronger, the 
 more the temperature of the thermic apparatus at the time varies 
 from the temperature of its own zero-point." By the "zero-point'* 
 of any part of the skin is meant the exact objective temperature 
 which at that part will produce no sensation of either heat or cold. 
 Such zero-point is, of course, different for different parts of the 
 body, according as they are or are not exposed, and are or are not 
 well supplied with arterial blood, etc. It also changes in connection 
 with changes in the temperature of the surrounding air or of the 
 bodies with which the skin is in contact. By this principle a great 
 number of our ordinary sensations of temperature are explained 
 by Hering. The finger and the nose are colder than the inside 
 of the mouth, because they are exposed to radiation of their heat. 
 On passing from a room of a given temperature into one of either 
 higher or lower temperature we experience at first certain sensa- 
 tions of temperature while the zero-point of the thermic apparatus 
 is becoming adjusted to its new surroundings. After such adjust- 
 ment has taken place these sensations may cease to be renewed 
 in the reverse direction, however, on a return to the former sur- 
 roundings. This adjustment has its limits ; it is dependent chiefly 
 upon the evaporation of the skin and upon the circulation of the 
 blood. 
 
 If the surroundings are more than so hot or so cold, they may 
 excite constant sensations of temperature. Among the induce- 
 ments to sensations of heat at any locality of the skin, Hering men- 
 tions the following as prominent in our ordinary experience : All 
 checking of the radiation of heat, while the blood-supply remains 
 unaltered ; all contact with a medium or object of higher tem- 
 perature and this according to the ease with which such medium 
 or object parts with its heat ; and all increase of heat in the skin 
 
 1 In Hermann's Handb. d. Physiol., III., ii., p. 419 f. ; and Sitzgsber. d. Wie- 
 ner Acad., LXXV., Abth. 3, p. 101 f. 
 
DETERMINATION OF THE ZERO-POINT. 351 
 
 coming from the interior of the body, as in the sudden hypersemia 
 which takes place in blushing. Inducements to sensations of cold 
 are as follows : Increased convection of the heat of the skin by the 
 surrounding medium, while the blood-supply remains unchanged 
 (as when the wind blows over the hand or face, especially if the 
 skin be moist) ; contact with objects which have the same (or even 
 slightly higher) objective temperature as the surrounding air, but 
 convey the- heat from the skin more rapidly than it ; contact with 
 or proximity to objects colder than the skin ; lessening of the 
 interior warmth of the body for example, by contraction of the 
 blood-vessels which supply a given portion of the skin. Ordinary 
 experience makes us familiar with many of the phenomena which 
 come under all these cases. 
 
 The determination of the exact zero-point of different parts of the 
 body is a matter of great difficulty. The rise and fall of the tem- 
 perature of the thermic apparatus, in connection with that principle 
 of exhaustion which applies to all the nervous mechanism, and es- 
 pecially to certain of the end-organs of sense, doubtless account 
 (at least partially) in some way for the well-known phenomena of 
 contrast in temperature-sensations. Weber showed that if the 
 hand be held for a minute in water of the temperature 54.5 
 Fahr., and then in water of 64.4 Fahr., a sensation of heat will be 
 felt for a few seconds, although the latter would have felt cold to 
 the hand if placed in it at first. Moreover, if we hold one hand in 
 moderately cold water, and dip the other repeatedly in the same 
 water, the sensation of cold is stronger in the latter, although the 
 temperature of the hand held in the water is really lower. But, 
 according to an experiment of Goldscheider's, if one hand be left 
 for ten seconds in water of the temperature of 104 Fahr., and then 
 both hands immersed in cold water, the warmed hand will feel the 
 cold less distinctly than the other. This latter investigator, how- 
 ever, is inclined to dissent from Hering's theory, and return to the 
 theory of E. H. Weber. Weber held that the rising of the tem- 
 perature of the skin is felt as heat, and its sinking as cold. 
 
 After-images of temperature-sensations seem also to exist. But 
 when a surface of the skin has been warmed or cooled, and the 
 after-image has faded quite away, it is said that it can be called 
 back by light mechanical irritation ; this is especially true of sen- 
 sations of cold. The phenomena of exhaustion are noticed in sen- 
 sations of temperature. Our perception of the absolute degree of 
 temperature, and of minute variations in its degree, is most acute 
 for places in the scale lying close to the normal temperature of the 
 skin. It would seem, on the whole, as though the phenomena 
 
352 SENSATIONS OF THE SKIN. 
 
 of contrast of sensations of temperature, as well as of color, require 
 for their satisfactory explanation a knowledge (possibly of the 
 action of the central organs of the nervous system) which we do 
 not yet possess. 
 
 E. H. Weber also showed that the amount of the skin which is 
 stimulated has a marked influence on the quality of the resulting 
 sensation. The temperature of the same fluid does not feel pre- 
 cisely the same to a single finger and to the entire hand. This 
 experience is similar to that which has already been described in 
 the case of sensations of color. It appears explicable in the case 
 of the skin from what is now known about the existence of a cer- 
 tain variable number of heat-spots and cold-spots. In the same 
 way, in part, may we explain the fact that smooth objects, which 
 therefore come into contact with a larger portion of the skin like 
 leather, paper, wood, glass, and porcelain appear colder to the 
 whole hand even when they have the same objective temperature 
 with it. 
 
 24. Nothing whatever is known as to the exact manner in which 
 changes of temperature act upon the thermic apparatus to excite 
 it ; the recent discoveries appear to make such action all the more 
 difficult of conception and description. Since the terms "hot" 
 and " cold " are in physics only relative, it is hard to see why ab- 
 solutely different apparatus, with a distinct local position, should 
 be used (as Goldscheider's discoveries indicate) for the sensations 
 corresponding to each. Moreover, on Bering's hypothesis, how 
 are we to account for the fact that heat-spots and cold-spots are in 
 turn stimulated by the same objective temperature according to 
 the rise and fall of the zero-point of the entire region of the skin ? 
 Possibly it may be found that certain chemical or electrical changes, 
 dependent upon the increase or decrease of that mode of molecular 
 motion which physics calls " heat," are the proximate stimuli of 
 the two classes of end-organs of the temperature-sense. Gold- 
 scheider supposes that the difference in sensitiveness of different 
 areas of the skin to temperature must be ascribed to the anatomi- 
 cal distribution of the heat-sensitive and cold-sensitive fibres, re- 
 spectively. But he does not show us what kind of nervous con- 
 trivance would satisfy all the conditions which are imposed by the 
 complicated facts of experience. 
 
 A. Herzen ' considers himself to have demonstrated, by patho- 
 logical cases and experiment upon animals, that sensory impulses 
 of cold, like those of touch, pass along the posterior strands of the 
 spinal cord ; and that the same region of the brain (Gyrus sigmoi- 
 1 See Pfluger's Archiv, 1885, pp. 93 ff. 
 
SPECIFIC ENERGY OF THE NERVES. 353 
 
 deus) is the " centre " for both. Sensitiveness to heat can be re- 
 tained, it would seem, after sensitiveness to cold has been lost. 
 
 25. 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 to affirm as does 
 Fick, 1 for example that two sensations are distinguishable as re- 
 spects quality only in case they are occasioned by two individually 
 different elements of the nervous system. For we have seen that 
 the quality of sensations depends upon their quantity, upon their 
 relation to preceding and contemporaneous sensations, and upon 
 considerations other than merely the one of what particular nerve- 
 fibre or element of the end-apparatus was acted upon by the stim- 
 ulus. 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 func- 
 tion, and the assignment to specifically distinct apparatus of par- 
 ticular nervous impressions corresponding to particular mental 
 states, is carried to a great length in the special senses. In this 
 differentiation of function 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 corresponding regions of the central organs. In 
 accounting for those complex sensations which appear in ordinary 
 consciousness, the law of permutations and combinations has, of 
 course, to be considered.. A vast variety of such sensations maybe 
 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 sensations, which apparently must have their 
 physical basis in the excitation of specifically distinct elements of 
 the nervous mechanism. 
 
 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- 
 1 In Hermann's Handb d. Physiol., III., ii., p. 166. 
 
354 SENSATIONS OF THE SKIN. 
 
 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 ei- 
 ther direction. It may be objected to the law that it is absurd to 
 suppose a complexity of the end-organs of smell such as to corre- 
 spond to each specific kind of olfactory stimulus with a specific sen- 
 sation for example, the smell of musk, or of sulphuretted hydro- 
 gen. It may be replied to the objection that, in the case of the 
 ear, there are at least 16,000 or 20,000 distinct forms of auditory 
 end-apparatus corresponding to the different musical tones ; and 
 it is therefore by no means impossible that the entire regio ol- 
 factoria 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. It is thought 
 possible by most physiologists to reduce all the sensations of taste 
 to four, or at most six, different species. It is easy to suppose 
 as many specifically different forms of the nervous apparatus cor- 
 responding to the different classes of sensations sweet and sour, 
 salt and bitter, alkaline and metallic. In -spite of the fact that 
 such a classification appears satisfactory to most authorities, experi- 
 ence is reluctant to confirm it. Many of the complex tastes, even 
 when separated from their accompanying sensations of smell, are 
 scarcely resolvable into combinations of the above-mentioned 
 simple tastes. Into which of the six, for example, would experi- 
 ment resolve the gustatory sensations which come from chewing a 
 bit of chocolate, or of a nut from a black-walnut tree ? 
 
 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 ap- 
 parently dependent upon the excitation of a correspondingly large 
 number of distinct nervous elements. From sensations of sight, 
 although many points of the prevalent theory are still obscure and 
 unsatisfactory, a considerable force of evidence bearing in the same 
 direction may be obtained. It seems almost certain that the 
 numerous states of consciousness which result from stimulating 
 the different nervous elements of the retina are due to combina- 
 tions of a comparatively few kinds of such elements, each of which 
 responds in a specific way to a special order of stimulus. Yet this 
 is not precisely what the theory of specific energy seems to de- 
 mand. For the different color-sensations all appear as simple and 
 
SPECIFIC ENERGY OF THE NERVES. 355 
 
 unannlyzable states of consciousness. None of them are twofold, as 
 sensations. We are at a loss to say why, according to the theory 
 of specific energy, each sensation should not result from the ex- 
 citation of one, and only one, kind of nervous elements. 
 
 The recent discoveries as to the existence of pressure-spots, 
 heat-spots, and cold-spots in the skin add important evidence to 
 that already existing in favor of the law under discussion. It will 
 further appear, when we consider the process of localization in the 
 so-called " geometrical senses " of the eye and the skin, that the 
 very possibility of such a process demands a strict and far-reaching 
 application of the law of the specific energy of the nerves. Pre- 
 cisely how we are to state and limit this law, neither its opponents 
 nor its advocates have as yet been able satisfactorily 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 V. 
 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 obviously requires for its perform- 
 ance nothing beyond what is immediately given in consciousness. 
 All sensations appear there as differing among themselves, not only 
 with respect to the nature of the impression which serves to classify 
 them into groups (as sensations of sight, sound, etc.), but also with 
 respect 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 note 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 to, 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 de- 
 scribed 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 hot, our attention being 
 directed chiefly to the quantum of sensation which it calls forth. 
 In other words, it is generally the same kind of pressure and tem- 
 perature, with a varying degree of intensity, of which we are 
 conscious. 
 
QUANTITY AND QUALITY. 357 
 
 It is more difficult, however, even in the most indefinite way, to 
 separate the quantities of our sensations of smell and taste from 
 the changes in quality of the same sensations. A concentrated 
 sweet or acid so strongly excites a variety of forms of feeling which 
 mingle indistinguishably with the specific 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 sensa- 
 tion 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, atten- 
 tion is forcibly directed toward the kind of sensation which results 
 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 pos- 
 sess. 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, 
 however, with simply describing the varying degrees of intensity 
 possessed 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 grades of intensity. 
 
 2. That changes in the intensity of our sensations are not, in 
 fact, independent of changes in their specific nature has already 
 been proved (Chap. IV., 4). Only in the case of musical tones 
 are. we able at the same time to attend carefully to both the quan- 
 tity and quality of our sensations, and so discover with perfect 
 confidence that the former is changing while the latter remains un- 
 changed. 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 in- 
 crease in the brightness of a particular color invariably changes its 
 
358 MEASUREMENT OF SENSATIONS. 
 
 characteristic color-tone. A white of less intensity is not merely 
 less white, but becomes a gray ; and by constantly diminishing its 
 intensity white can be shaded through the different grays toward 
 black, which is certainly not a 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 would be 
 a mistake, 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. 1 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 senses for 
 forming an exact judgment as to its intensity. It would be difficult 
 under the most favorable circumstances to affirm that the sensa- 
 tion 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 esti- 
 mates of the quantity of sensation approach the point at which 
 they lose their meaning and tend to become absurd, when we com- 
 pare, for example, sensations of smell or taste with those of press- 
 ure, temperature, 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 somewhere in that scale of intensities through which 
 1 Comp. Stumpf, Tonpsycliologie, I., p. 347 f. 
 
PROBLEMS OF QUANTITY. 359 
 
 the different qualities may run ; each may, therefore, be compared 
 with every other, with respect to the general position which it oc- 
 cupies in its characteristic scale. 
 
 4 All things to which terms of quantity apply admit of some 
 kind of measurement and comparison with respect to their quantity. 
 Sensations, to be sure, are not "things," but rather modes of the ac- 
 tivity of mind, excited through the nervous mechanism of sense. 
 Nevertheless, since, like material things, they admit of some appli- 
 cation to themselves of the terms of quantity ; and since they vary 
 in their absolute and relative degrees of quantity, it is not strange 
 that experimental science has endeavored to measure sensations, 
 and to state laws for their comparison and mutual relations. The 
 general question of the quantity of sensation involves an answer to 
 two subordinate inquiries. Of these two the first concerns the 
 limits within which the different sensations may vary in quantity, 
 and yet remain sensations of the same sense ; the second concerns 
 the law of the relation which is maintained within the limits among 
 the various sensations compared. But neither of these questions 
 can be answered directly. Sensations cannot be kept constant in 
 quantity, and measured by the direct application of physical stand- 
 ards, whether with a view to fix their absolute or their relative 
 magnitude. They are all, however, under ordinary circumstances, 
 connected with the action of different forms of physical energy 
 upon the nervous system ; that is to say, they are caused by the 
 application of stimuli to the nerves, and the changes in the amount 
 of the sensations are dependent upon changes in the intensity of 
 the stimuli which occasion them. These stimuli admit of changes 
 in quantity, which, theoretically at least, are measurable objectively, 
 with more or less exactness. Resulting changes in consciousness 
 can only be measured by attentive judgment, which directly dis- 
 criminates the sensations as varying in intensity, and as being 
 greater or less, one than the other, in the scale of impressions 
 which experience has framed. 
 
 The problems of the measurement of sensation may then be 
 stated as follows : (1) To determine how little and how much of each 
 kind of stimulus will produce respectively the least and the 
 greatest quantity 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 consciousness, are dependent upon changes in the intensity of 
 the stimuli. 
 
 5. Unexpected and insuperable difficulties, however, stand in 
 
360 MEASUREMENT OF SENSATIONS. 
 
 the way of a direct solution of either of the two above-mentioned 
 problems, even in the modified form in which they were last stated. 
 For, in the first place, it is only with respect to sensations of press- 
 ure and of the muscular sense that we can measure objectively the 
 physical energies which act on the nervous end-organs, with much 
 approach to perfect exactness. 1 The amplitude of the acoustic waves 
 in the air which originate from a given source would indeed admit 
 of exact measurement ; but the modifications which these waves 
 undergo before they reach the nerve-cells and nerve-fibres of the 
 inner ear are so complicated as to make it impossible to calculate 
 accurately the amount of the physical stimulus which is directly 
 applied to the end-organs of hearing. The photo-chemical and 
 thermic effects of light may be measured objectively. But this 
 light is not the direct physical stimulus for the fibres of the optic 
 nerve, or even for the end-organs of the retina ; and we have no 
 sufficient means for estimating the amount of those chemical changes 
 in the visual -substances, or pigments of the eye, which are supposed 
 to be the immediate excitants of the terminal apparatus of vision. 
 The case is yet more hopeless with respect to the senses of taste 
 and smell ; inasmuch as we do not even know what properties smell- 
 able and tastable substances must possess in order to influence the 
 nerves of those senses. The objective measurement of the stimulus 
 for sensations of temperature also is made difficult by the fact that 
 its amount is dependent upon the zero-point of the skin itself, 
 since this point is different at different times and for different 
 areas of the entire surface, and is always difficult of precise deter- 
 mination. 
 
 Moreover, could we measure with perfect exactness the intensity 
 of the stimulus as it is applied directly to the appropriate end- 
 organs of sense, our knowledge of the intensity of the necessary 
 physical antecedents of the resulting sensations would be far 
 enough from complete. How do the end-organs modify the quan- 
 tities of the stimuli before they transmit their effect to the conduct- 
 ing nerve-fibres? Precisely how much further modification do 
 these quantities receive in transmission to the central organs, at 
 the hands of the conducting nerve-tracts ? What are the laws which 
 control the reception, diffusion, and modification of the different 
 intensities of the transmitted nerve-commotions, within those parts 
 of the nervous mechanism (the central organs), where they become 
 the immediate occasions of the rise and change of sensations in the 
 mind ? These are questions to which we are absolutely unable to 
 give any satisfactory answer. 
 
 1 Comp. Wundt, Philosophische Studien, 1883, II., hefti., pp. 10 ff. 
 
LEAST OBSERVABLE DIFFERENCE. 361 
 
 6. But if an exact objective measurement of the physical stim- 
 uli is intrinsically difficult, an exact subjective measurement of the 
 sensations themselves is inherently impossible. Such subjective 
 measurement can exist at all only in the form of a judgment which 
 compares two or more sensations with a view to pronounce whether 
 they are equal in intensity ; or, if unequal, which is the greater and 
 which the less of the two. But we have seen that the ordinary 
 estimate of the absolute strength of a sensation is able simply to 
 assign to it an indefinite position in the scale of its kind. With 
 certain exceptions, scientific analysis can do little to exclude the 
 uncertainties of the ordinary estimate. These exceptions are all of 
 the following kind : Where two sensations of the same quality are 
 produced, either simultaneously on different corresponding areas of 
 the same organ or successively (with the most favorable interval be- 
 tween) upon the same area, by amounts of stimulation that are very 
 nearly or precisely equal, the attentive mind can discriminate the 
 minute differences, or exact equality, of the intensities of these two 
 sensations, with a great degree of nicety. The problem of meas- 
 uring the quantity of sensations depends, therefore, upon obtaining 
 the least observable differences in intensity for each kind of sensa- 
 tions, and for every point along the scale of degrees of intensity. 
 
 But in this connection another occasion for doubt and debate 
 arises. Is "the least observable difference" of two sensations it- 
 self a constant quantity ? The affirmative answer to this question 
 is assumed by Fechner 1 and all strenuous advocates of the law 
 which he defends. It has even been argued that to hold another 
 than the affirmative view involves a contradiction in terms. 2 What 
 can be meant, it is asked, by a "least observable difference " in in- 
 tensity between two sensations, unless it be that this difference is 
 a constant unit for the measurement of those sensations of the 
 same kind which lie near the same point in the scale ? If the dif- 
 ference is more than just observable, then of course it is not the 
 least observable ; if it is less, then it is not observable at all that 
 is to say, there is no change in sensation. But to this argument 
 the following reply is pertinent: The "least observable difference" 
 is not itself a mental entity or a mental state, that can be measured 
 and used as a unit for measuring the quantity of other mental 
 states. For example, if the addition of n to the stimulus S is the 
 smallest amount that will produce such a change in the mental 
 state x as to cause it to pass over into x', which the mind recog- 
 
 1 Elemente d. Psychophysik (1860), i., p. 54 f. ; In Sachen d. Psychophysik 
 (1877), p. 45 f. ; Revision d. Hauptpunkte d. Psychophysik (1882), p. 18 f. 
 
 2 Comp. the first edition of Wundt's Physiolog. Psychologic, p. 294. 
 
362 MEASUEEMENT OF SENSATIONS. 
 
 nizes as having a greater quantity of sensation than x, such fact is 
 to be stated and accepted as a mere fact ; it does not follow, how- 
 ever, that we may conclude that x' x A, and that this A is en- 
 titled to a name (" least observable difference ") and a rank among 
 the mind's experiences by way of sensation. There are no sensa- 
 tions (whatever physical occasions of sensations may exist) except 
 those that appear in consciousness ; ex hypothesi, there appear in 
 consciousness only x and x', and no sensation whatever l}ing be- 
 tween the two in intensity. We judge, indeed, that the intensity 
 of x', now present in experience, is greater than was the intensity of 
 x, now remembered as an image of past experience ; but A (or x' x) 
 is a mere abstraction, a figment of the experimenter's brain, and not 
 a real experience of the person with whom he is experimenting. 
 
 Moreover, if A were capable in any case of being regarded as a 
 unit of subjective measurement, it would by no means follow that 
 its mental value is a constant. That n, or the amount of stimulus 
 which must be added to $ in order to produce an observable change 
 in the quantity of sensation, is not constant we know beyond doubt. 
 For the different senses, for different individuals, for different de- 
 grees of the absolute stimulus (i.e., value of S), for different con- 
 ditions of the organs of sense, this amount n is constantly varying. 
 The amount of A may also be held to vary, according to psycho- 
 logical changes in the means and power of mental discrimination, 
 such as we have no way of measuring objectively. For we must 
 again insist upon the fact that the real quantity of a sensation is 
 not the same thing as the estimated quantity of the same sensation. 
 The " least observable difference " would not, therefore, necessarily 
 be the same as the least real difference, between two sensations. 1 
 It is not the mind's custom to attend accurately to the changes in 
 quantity of its sensations as such. Properly speaking, many con- 
 siderable changes in our sensations, as we may judge by the guid- 
 ance they give to the bodily motions and the mental train, do not 
 appear in consciousness with a label of exact quantitative measure- 
 ment, as it were, attached to them. 
 
 It is therefore obvious, from the great difficulties which belong 
 inseparably both to the objective measurement of the stimuli of 
 sensation, and to the subjective measurement of the resulting sen- 
 sations, that any law of their relation can have only an indefinite 
 statement and a secondary value. 
 
 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 
 1 Comp. Stumpf, Tonpsychologie, I., p. 51 f. 
 
DETERMINING THE LIMIT. 363 
 
 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 
 begins 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 
 sensitiveness increases, of course, in inverse ratio to the amount of 
 stimulus necessary for producing any sensation at all, or for pro- 
 ducing 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 
 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. 1 
 
 The upper limit, or maximum amount of stimulus which the ner- 
 vous organism can receive, cannot be determined experimentally. 
 The use of excessive quantities of stimulus is not only too fatigu- 
 ing but also too dangerous to the structure of this organism (for 
 example, of blinding light upon the eye, stunning noise in the ear, 
 etc.) to admit of successful experiment in this direction. Moreover, 
 the application calls out so much of those varied forms of feeling 
 which are allied with all the specific sensations as to overwhelm 
 the latter with the former. Very concentrated, sour, or bitter solu- 
 tions, or very intense odors, are not simply tasted and smelled ; they 
 are also felt with all the adjoining parts of the body. Very strong 
 light and very loud noise do not simply heighten the specific sensa- 
 tions of sight and hearing, they rather destroy them in a flood of pain- 
 ful feeling. We may affirm in general, however, that the " capacity " 
 of each sense varies directly as the amount of stimulus which it can 
 receive. The " circuit " or range of the sensations of each sense 
 
 f 1 1 
 
 may then be said to be where _ stands for the measure of the 
 S S 
 
 sensitiveness, and C for the measure of the capacity, of each sense. 2 
 
 1 On the question whether absolute stillness is possible, and whether the ear 
 has any sensation comparable to the black of the eye, see Lotze, Medicinische 
 Psychologie, p. 218; Volkmann von Volkmar, Lehrbuch d. Psychologie, 
 1884, I., p. 273 ; and Stumpf, Tonpsychologie, I., p. 380 f. 
 
 2 See Wundt, Physiolog. Psychologie, i., p. 324. 
 
364 MEASUREMENT OF SENSATIONS. 
 
 8. There are three methods of determining experimentally the 
 least observable differences in sensations. These are called, (1) the 
 method of least observable difference ; (2) the method of average 
 errors ; (3) the method of correct and mistaken cases. Of the 
 three methods, the first bears the name which suggests the real 
 subject of investigation in them all. This method is divided by 
 Wundt ' and others into two namely, the method of mean grada- 
 tions of sensation, and the method of minimum changes of sensation. 
 But these are really only two modes of applying one method. In the 
 one case an attempt is made to form a scale of stimuli whose intervals 
 correspond to equally large intervals in our estimate of the resulting 
 sensations, by judging what amount of the stimulus produces a sensa- 
 tion (M) that lies exactly midway between two other sensations (A and 
 O) separated by a clearly perceptible interval (hence A : M : : M : O). 
 Between A and M another middle term, the sensation of magnitude K, 
 may then be sought and found ; and so on until the limit of observ- 
 able differences is reached. This mode is, however, less comprehen- 
 sive and fruitful than the second mode of applying the same principle. 
 The " method of minimum changes in sensation " seeks directly to 
 establish, all along the scale of intensities of the stimuli, that change 
 in their strength which is just enough (and no more than enough) 
 to produce a minimum change in sensation. Such minimum 
 change may be conceived of as standing just on the " threshold " 
 of our power to make distinctions in the degrees of strength with 
 which our sensations are apprehended in consciousness. 3 
 
 The *' method of average errors" (2) begins by fixing upon some 
 given sensation which is known to be caused by a given intensity 
 of stimulus ; the attempt is then made also to fix upon another 
 stimulus, by means of the sensation it produces, as being exactly 
 equal to the former. The trial results in a number of guesses that 
 are more or less out of the way. By averaging all the cases of trial, 
 the degree of sensitiveness to distinctions is discovered. In other 
 words, the method attempts to determine, at each point along the 
 scale and for each kind of stimulus, the differences in the strength 
 of stimuli that are just below the amount necessary to make an ob- 
 servable difference in the resulting sensations. 
 
 In the " method of correct and mistaken cases " (3) minute ad- 
 ditions or subtractions of the amount of stimulus are made, with 
 the intent of seeing how many cases of right and how many of 
 
 1 See Wundt, Physiolog. Psychologie, i., p. 325 f., and comp. his Philosoph- 
 ische Studien, 1881, p. 8 f. 
 
 2 Called "Unterschiedsschwelle" by Fechuer, Elemented. Psychophysik, 
 i., p. 242. 
 
CORRECT AND MISTAKEN GUESSES. 365 
 
 wrong guessing, respectively, will result for each of the different 
 positions in the scale of the stimuli, and for each kind of stimulus. 
 If, then, the proportion of the number of correct to mistaken guesses 
 is kept the same for all points of the scale, the amount of change 
 in the stimulus necessary for this may be held to measure the sen- 
 sitiveness to differences which belongs to each of these points. 
 Thus, let n = the whole number of guesses, and r = the number 
 
 T 
 
 of right guesses ; then = the sensitiveness to differences. But 
 
 the positive value of this quotient being kept unchanged, the 
 amount of stimulus added to or subtracted from the original 
 amount will measure the sensitiveness to differences for all points of 
 the scale. This method has been largely used and warmly defended 
 by Fechner l in experimenting with sensations of pressure. Much 
 doubt has, however, been thrown upon the use made of it by this 
 observer ; and especially upon the propriety of reckoning the doubt- 
 ful cases one-half to the right and one-half to the wrong guesses. 2 
 
 A comparison of the above-mentioned methods shows that they 
 are all simply different ways of measuring the sensitiveness of the 
 mind to minute differences in the quantity of its sensations as de- 
 pendent upon changes in the intensity of the stimuli. They should 
 never be employed, therefore, without taking into account the 
 fact that various other causes, besides such objective changes in 
 the stimuli, always co-operate to determine the degree of this 
 mental sensitiveness. To eliminate these other factors from the 
 calculation is by no means easy. 
 
 9. The one law which claims to be a scientific expression of the 
 relations between changes in the intensity of stimuli and changes 
 in the quantity of the resulting sensations is that known by the 
 name of E. H. Weber. This observer originally used the method 
 of least observable differences as applied to sensations of pressure 
 and to the measurement of lines by the eye. 3 " Weber's law" has 
 been elaborated, confirmed by a vast amount of experiment, and 
 defended as a psycho-physical principle of the widest application, 
 by Fechner (in the works referred to, note, p. 361). The significant 
 addition which Fechner has made to Weber's law consists in the 
 assumption that all just observable differences are equally great. 4 
 
 1 Elemente d. Psychophysik, i. , pp. 93-120. 
 
 8 On this point see, especially, G. E. Miiller, Grundlegung d. Psychophy- 
 sik, p. 36 f. ; and Wundt, Physiolog. Psychologie, i. , p. 330 f . 
 
 3 Especially in articles on the sense of touch, in R. Wagner's Handworterb. 
 d. Physiologie, III., ii. ; and Archiv. f. Anat, Physiol., etc., 1835, pp. 152 if. 
 
 4 On this point comp. Funke, in Hermann's Handb. d. Physiol. , III. , ii. , p. 
 349 f. ; and Wundt, Philosophische Studien, II., Heft 1, p. 6 f. 
 
366 THE LAW OF WEBER. 
 
 
 
 It is therefore also called " Fechner's law." As an empirical law 
 it attempts to put into scientific form, on the basis of experimen- 
 tal investigation, the truth of ordinary experience namely, our 
 estimate of the difference in amount between two sensations is 
 not directly proportioned to the real difference in their stimuli, 
 but the latter must increase faster than does the former. For ex- 
 ample, the difference in the intensity of the shadows cast by one 
 and by two wax tapers is very perceptible in a dimly lighted room, 
 but is altogether un observable in open sunlight ; or the strength 
 with which two clocks tick can be discriminated with much nicety, 
 but not the amount of noise made by two successive discharges of a 
 cannon. 
 
 In other words, if we assume that the least observable difference 
 in sensations may be regarded as a constant quantity, then, in 
 order to produce this increase or decrease in the amount of sen- 
 sation, the addition or subtraction of a much greater amount of 
 stimulus is needed for the higher than for the lower portions of the 
 scale. Weber's law undertakes to tell us how much greater such 
 required amount of stimulus must be. It admits of statement in 
 the several following ways : The difference between any two stimuli 
 is experienced as of equal magnitude in case the mathematical re- 
 lation of those stimuli remains unaltered ; or, If the intensity of 
 the sensations is to increase by equal absolute magnitudes, then 
 the relative increase of the stimulus must remain constant ; or, The 
 strength of the stimulus must ascend in a geometrical proportion 
 in case the strength of the sensation is to increase in an arith- 
 metical proportion. 1 
 
 1 See Wundt, Physiolog. Psychologie, i., p. 335. For the detailed mathe- 
 matical discussion and expression of Weber's law the reader is referred to 
 the technical works, especially of Fechner and G. E. Miiller. A simple state- 
 ment of Weber's principle may be given as follows : Let 11= the intensity of 
 the light of one-half of a white field ; -^j- = the smallest fraction of stimulus 
 added to H that will produce an observable increase in this intensity ; and 
 H + JL the intensity of the other half of the same field. Then let 8 = 
 the sensation produced by H; 8+ s = the sensation produced by H+ yf^; 
 and 8 will, of course, represent the so-called least observable difference at this 
 point in the scale. We have, then, H produces 8; J5T+ -^ t or |ft //, pro- 
 duces 8 + s ; |U H+ i-^l^ , or IgMSo & produces 8 + 8 + 8 ; and so on. 
 That is to say, if s is to be kept of the same magnitude, then ZTmust be mul- 
 tiplied by the same magnitude (ffto)- 
 
 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 2, and A/6' a just observable increase in this sen- 
 
QUANTITY OF TACTUAL SENSATIONS. 367 
 
 The empirical data upon which the advocates of Weber's law rely 
 are very numerous, but their value and trustworthiness are often 
 much diminished by the fact that most experimenters 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* 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 significance 
 and origin occur in all the senses. 
 
 10. In determining the least observable sensations of touch, 
 the result is largely dependent upon the presence of muscular sen- 
 sations also. It further depends upon the method in which the 
 comparison is made ; for, as Weber discovered, an actually present 
 sensation can be compared with the remembered image of one just 
 past better than two present sensations can be compared. The in- 
 terval of time and the locality of the organ have also a great influ- 
 ence/ Most persons observe a stronger sensation of pressure when 
 the weight is laid on the left than when it is laid on the symmetri- 
 cal place of the right side. The same amount of surface must be 
 covered, and the objects compared must have the same temperature, 
 in order to secure trustworthy results of experiment. Weber 
 found that, when the interval was fifteen to thirty seconds, under 
 the most favorable circumstances, 14J could be distinguished from 
 15 grammes, or 14 J from 15 ounces. That is, some persons can 
 distinguish weights which differ as 29 : 30, by the sensations of 
 pressure they occasion, when laid on the volar side of the last 
 phalanges. By raising the weights the nicety of discrimination can 
 be increased so as to be represented by the proportion 39 : 40. 
 
 sation which is caused by an increase of the stimulus = A2. Let C be a con- 
 stant dependent on the values chosen for 8 and 2. Then AS = =-. Let it 
 be further assumed that AS remains constant whatever values for S and A2 
 are assumed; then dS = Q d ~, and by integration 8G log. 2, which is 
 Fechner's "fundamental formula." But if the stimulus is just below the least 
 observable amount, and be = 2, then substituting in the above formula we 
 have C log. 2 ; from which Fechner derives formula No. 2 (the formula 
 
 of measurement), namely, 8= Clog. J, which means that the magnitude of 
 the sensation is " negative," in case the stimulus sinks below the least observ- 
 able = 2'. If two sensations (8 and 8) are observably different, then S S' 
 = C (log. 2 log. 2') ; 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. 
 
368 THE LAW OF WEBER. 
 
 By an extended series of experiments with weights ranging from 
 300 to 3,000 grammes Fechner ' employed the method of correct 
 and mistaken cases to confirm Weber's law as applied to combined 
 sensations of pressure and of the muscular sense. Some experi- 
 ments were made with both hands ; others with the right or left 
 separately. The weight used to add or subtract was either 0.04 or 
 0.08 of the absolute weight. The results showed that the law held 
 only approximately for all the series of experiments, and not abso- 
 lutely for any one series. As calculated by G. E. Mtiller a they 
 give, instead of a constant quotient to express the degree of sensi- 
 tiveness (as Weber's law requires), a quotient varying from T J ^ for 
 weights of 300 grammes to T 1 7 for weights of 3,000 grammes. Nor 
 can Fechner's effort to correct the variation, by introducing after- 
 ward a conjectural allowance for the weight of the arm itself, be 
 considered successful. 3 Biedermann and Lowit, by the method of 
 just observable differences, obtained results departing widely from 
 Weber's law. 4 By experimenting with weights varying from 10 to 
 500 grammes they found that the sensitiveness to pressure rose 
 with the increase of the weights from 10 to 400 grammes, and then 
 fell off rapidly, as the following table will show : 
 
 Absolute weight. Least observable difference. Quotient of sensitiveness. 
 
 Grammes. Grammes. 
 
 10 0.7 -fr 
 
 50 1.7 iV 
 
 100 2.4 -^ 
 
 200 3.6 -fa 
 
 300 4.6 T&- 
 
 400 5.2 Jr 
 
 450 6.5 ife 
 
 500 25.5 A 
 
 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 ^ for weights of 250 grammes, y^ for weights of 2,500 
 grammes, -^ for weights of 2,750 grammes. 
 
 That Weber's law does not hold good, near the lower limits, for 
 
 1 Elemente d. Psychophysik, i. , p. 183 f. 
 * Zur Grundlegung d. Psychophysik, p. 197. 
 
 3 In Sachen d. Psychophysik, p. 198. 
 
 4 See Bering, Sitzgsber. d. Wiener Acad., LXXII., Abth. iii., p. 342 f. 
 
QUANTITY OF TEMPERATURE-SENSATIONS. 369 
 
 sensations of pressure, and of muscular innervation and movement, 
 is admitted by all. The absolute sensitiveness of these sensations 
 differs greatly, as has been shown (p. 346 1), for different localities 
 on the surface of the body. Aubert and Kammler found the light- 
 est weight which produced a sensation of touch to be 0.002 gramme 
 on the forehead, temples, and dorsal side of the forearm and hands ; 
 0.003 gramme for the volar side of the forearm ; 0.005 gramme 
 for the nose, lips, chin, eyelids, and skin of abdomen ; 0.005-0.015 
 gramme for the volar side of the fingers ; and 1 gramme for the 
 fingernails and skin of the heel. This kind of sensitiveness has 
 been 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 
 localit} r . The foregoing and similar conclusions all need to be re- 
 vised in the light of Goldscheider's determinations of the pressure- 
 spots. 
 
 11. Extraordinary difficulties accompany the attempt to apply 
 Weber's law to sensations of temperature. As has already been seen 
 (p. 350 f.), 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 ' ^ sensitiveness of the hand to changes in 
 temperature when it ' e . cooling off; but he thinks the law 
 
 holds good approxi or degrees of warmth varying between 
 
 25 and 37.5 C. .5 Fahr.), if 18.71 C. (65.66 Fahr.) be 
 
 taken as the z * The assumption of this zero-point is, 
 
 however, arb : No general rule for the quantity of sensations 
 
 of tempera' . well be given except this : the skin is most 
 
 sensitive f ,s which lie near its own zero-point. In compar- 
 
 ing twr atures it is most favorable to nice discrimination 
 
 that f Ad lie slightly above, the other slightly below, this 
 
 poir degrees of the thermometer between which the maxi- 
 
 m ,ensitiveness is attainable are given differently by differ- 
 
 jrvers : By Nothnagel, 27-33 C. (80.6-91.4 Fahr.) ; by 
 
 .nann, 26-39 C.; by Alsberg, 35-39 C.; by Fechner, 12- 
 
 J. where it is so great as not to be easily measurable by a 
 
 od quicksilver thermometer (about | Fahr.). Cold and heat alike, 
 
 ,vhen 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 5 or f Fahr.; 
 
 1 So Heriug, see Hermann's Handb. d. Physiol., III., ii., p. 430. 
 24 
 
370 THE LAW OF WEBER. 
 
 by cold it can be rendered insensible to changes measuring from 
 2 to 5. 
 
 We have already seen (comp. p. 348 f.) 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 investiga- 
 tions have shown that the table of sensitiveness 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 heat-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 tem- 
 perature-sense had previously been investigated, Goldscheider has 
 given a lengthy statement 1 of the sensitiveness 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 intense, 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 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 dis- 
 tributory fibrils of the temperature-nerves), the thickness of the skin, 
 etc., are determining factors. 
 
 12. The possibility of executing or appreciating a musical 
 passage in which the intensity of the successive notes is brought 
 to a certain standard of memory, or in which these notes 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 
 
 1 See the Archiv f. Auat. u. Physiol., Physiolog. Abth.j 1885, Supplement- 
 Band, pp. 60 ff. 
 
MEASUREMENT OF SOUND. 371 
 
 notes previously executed, with about the same stress of tone. 1 
 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 
 general difficulty which belongs to investigating the intensity of 
 sensations, even under the most favorable circumstances, is here 
 enhanced by the facts, that the pitch and timbre of each clang have 
 much to do with our judgment of its strength ; that different ears 
 differ so widely in their organic susceptibility, while the mind is 
 peculiarly sensitive to changes of feeling and judgment connect- 
 ed with sensations of sound, and thus very weak sensations are 
 vacillating and unsteady in consciousness, and sounds appear and 
 disappear in the ear while the degree of stimulus and of attention 
 are unchanged ; that the reflection and interference of the acoustic 
 nerves, their distance and direction, and the absence or presence of 
 " entotic " sounds, are so influential ; and, finally, that it is impossi- 
 ble to discover a sounding apparatus of definitely ascertainable and 
 uniform intensity of action. 
 
 13. None of the means employed for determining the amount 
 of stimulus necessary to produce the weakest sensations of sound, 
 or the least observable differences in such sensations, are entirely 
 satisfactory. The method of listening to noises made by falling 
 weights is rendered uncertain by the fact that the character, both of 
 the weight and of the surface on which it strikes, has so much in- 
 fluence. Moreover, it is a matter of dispute whether the intensity 
 of the stimulus is to be measured by the product of the mass into 
 the height from which the body falls (ra x h) or into the square- 
 root of that height (in x Vh). It is possible that neither of these 
 measurements is exact. 2 Assuming the former to be correct (noise 
 = m x h\ by using a sound-pendulum A. W. Volkmann found that 
 diTerences in intensity are observable when they stand in the pro- 
 portion 3 : 4. Vierordt, on the other hand, concluded that the latter 
 measurement (noise = m y/h ) is more nearly correct; and by assum- 
 ing Vierordt's view, and using iron balls that fell vertically on a vi- 
 brating plate, Norr * attempted to fix a unit of measurement. This 
 unit he made = 1,500 milligramme-millimetres with the ear distant 
 50 ctm. from the source of the sound. Experimenting with sounds 
 
 1 Comp. Stumpf, Tonpsychologie, I., p. 345 f. 
 
 8 Comp. Wundt, Physiolog. Psychologie, i. , p. 341 and note ; and E. Tischer, 
 in Philosophische Studien, L, heft 4, p. 543 f. 
 Zeitschrift f. Biologie, 1879, XV., p. 297 f. 
 
372 THE LAW OF WEBER. 
 
 ranging in intensity from those a little above the least observable to 
 those of unpleasant strength (1.71-524167.8 times the unit), and 
 dividing the entire scale into 7 groups, within each of which about 
 1,000 experiments were conducted, he found that the proportion of 
 
 right guesses to the entire number made f ^remained approxi- 
 mately constant that is, that Weber's law holds for sounds of vary- 
 ing intensity. A large allowance, however, had to be made for 
 relations of time ; the percentage of correct guesses being about 8. 7 
 larger when the sound of greater intensity followed that of less 
 intensity. 
 
 More recently, E. Tischer 1 has apparently added some evidence 
 to the validity of Weber's law by experimenting with an improved 
 form of the method of Vierordt and Norr. Keeping one of the two 
 sounds to be compared at a constant intensity, he increased or 
 diminished the other until from 4 to 6 successive correct guesses 
 as to their relative value were obtained. But the fact that, when 
 the second stimulus was diminished until certainty of judgment 
 was obtained, very considerable unexplained variations from the 
 results expected by Weber's law occurred leaves much doubt still 
 hanging over the matter. In order to harmonize the conflicting 
 results, the proposal has been made to introduce another function 
 into the formula, noise = m h or ra\M. All the investigations, 
 therefore, still leave the question of the applicability of Weber's 
 law to sensations of sound in a rather uncertain condition. 
 
 Little or nothing has been accomplished by experiment to de- 
 termine whether the same law applies to the intensity of musical 
 tones. Among the various factors which enter into our judgment 
 of the intensity of tones, the " color-clang " is especially influential. 3 
 Absolute pitch and intervals of pitch are also very important. In 
 general, tones and noises of a higher pitch, with an equal objective 
 intensity of stimulus, are judged to be louder than those of a lower 
 pitch. Konig showed that a tuning-fork of the pitch c must hfve 
 about four times the amplitude of vibration required by one of the 
 pitch (7, in order to produce upon the ear the same effect from the 
 same distance. 
 
 14. The various attempts to determine the lower limit of sound 
 for the human ear have not resulted in any precise statement. 
 Schafhautl, after experiments in as near perfect stillness as possi- 
 ble, at midnight, fixed the limit at the noise made by a cork ball of 
 1 milligramme weight (about 0.0154 gr.) falling from a height of 1 
 
 1 Wundt's Philosophische Studien, 1883, I., heft 4, pp. 495 ff. 
 3 Comp. Stumpf, Tonpsychologie, I., p. 364 f. 
 
LOWER LIMIT OF SOUND. 373 
 
 millimetre (0.03937 inch). Boltzmann and Topler have reached 
 results which Hensen 1 considers to be as accurate as possible. By 
 measuring the compression of the air at the end of an organ-pipe of 
 181 vibrations per second, they calculated that, even under circum- 
 stances not as favorable as possible, the ear responds with sensation 
 to an amplitude in the vibration of the molecules of the air not more 
 than 0.00004 mm. at the ear, or about -f-$ the wave-length of green 
 light. The mechanical work done upon the ear-drum in a single 
 vibration of such small intensity is reckoned at not more than ^J-y 
 billionth kilogrammetre ; or only about ^ T of that done upon the 
 same surface of the pupils of the eye by a single candle at the 
 same distance. These calculations indicate that the motions in 
 the cochlea which excite the end-organs of sense are astonishingly 
 minute far too minute to be observed even by the microscope. 
 Yet the sharpness of hearing may be enormously increased by dis- 
 
 15. Judgments of the intensity of sounds are dependent also 
 upon practice, and upon other psycho-physical conditions such as 
 determine the nicety of all judgments of quality. Small impres- 
 sions of noise are apt to have their intensity underestimated ; the 
 inclination to do this has been attributed to the influence of our 
 custom of withdrawing attention from them altogether under ordi- 
 nary circumstances. 2 The fact that sounds to which we become 
 accustomed lose most of their intensity in consciousness must be 
 explained chiefly under the same law of mental habit ; it cannot, on 
 the other hand, be largely due to the physiological law of exhaus- 
 tion. 
 
 g 16. 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 preceding 
 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 -fa. 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 : : ^ : fa, 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 
 
 1 See Hermann's Handb. d. Physiol., III., ii , p. 117 f. 
 
 2 So Stumpf, Tonpsychologie, I., p. 388. 
 
374 THE LAW OF WEBER. 
 
 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. 
 
 17. It has already been shown (p. 326) 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 testing 
 Weber's law were first conducted by A. W. Volkmann and others, 
 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 
 T J 7 . 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 T j-j- to be observable. Later 
 experiments of the same observer yielded results less favorable to 
 Weber's law. 3 The quotient was found to vary from -^^ 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 yj ? ; 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 
 of T J 7 to T J-jj- 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 
 
 1 See Elemente d. Psychophysik, p. 148 f. 
 
 8 A. W. Volkmann, Physiolog. Untersuchungen, I., p. 56 f. 
 
 3 Physiologische Optik, p. 315 f. 
 
INTENSITY OF COLOR-TONES. 375 
 
 the limits showed much greater departures from Weber's law. Just 
 above the lower limit, an addition of even ^ to ^ 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. 
 
 A more accurate and carefully guarded series of experiments 
 than any of the foregoing is recently reported by Dr. Emil Krae- 
 pelin. 1 This experimenter used the method of just observable dif- 
 ferences as applied to Masson's disks when looked at through gray 
 glasses of varying intensity. The utmost care seems to have been 
 taken to exclude disturbances from changes in the adjustment of 
 the eye, retinal exhaustion, reflection of light from surrounding 
 objects, etc. Three groups of experiments were conducted one 
 by daylight, one by candlelight, one by lamplight. Both eyes 
 were experimented upon ; and both directions of alteration in the 
 intensity of the stimulus (stronger following weaker, and vice versa) 
 were employed. Kraepelin concludes that for the unexhausted eye, 
 with a good power of accommodation, the fraction which gives the 
 least observable difference remains constant, while the intensity of 
 the light varies between values of 1,000 and 9.61 of absolute inten- 
 sity as fixed for his experiments. That is, within these limits the 
 law of Weber holds good as expressing with closely approximating 
 accuracy the results of experiment. 
 
 The experiments of Dobrowolsky 2 and Lamansky 3 with light of 
 the different spectral color-tones shows that, with these sensations 
 also, Weber's law holds approximately good for moderate intensi- 
 ties, but is subject to considerable variations as we approach the 
 upper and the lower limits. The former used the method of com- 
 paring a white surface with one in which colored light had been 
 mixed with the white. On changing the absolute intensity of the 
 light between values of 1 and 0.0302, only a slight variation in the 
 quotient indicating the least observable difference of intensity ap- 
 peared for the color red. This quotient was found, by the same 
 observer, 4 however, to be very different for different color-tones : thus 
 for red, ^ ; yellow, V ; green, ^ ; blue, ^ ; violet, ^. Laman- 
 sky and others have made the sensitiveness to changes in the inten- 
 sity of color-tones greatest for green instead of violet ; and have ob- 
 tained other results different from those obtained by Dobrowolsky. 
 
 18. The minimum of the intensity of light appreciable by the 
 
 1 In Wundt's Philosophische Studien, 1884, II., heft 2, pp. 306 ff. 
 
 2 Pfluger's Archiv, xii., p. 441 f. 
 
 3 Archiv f. Ophthalmologie, XVII. , i., p. 123 f. 4 Ibid., XVIII., i., p. 74 f. 
 
376 THE LAW OF WEBER. 
 
 eye under the most favorable circumstances was fixed by Aubert at 
 3~^y- of that reflected from white paper in the light of the full-moon. 
 This result can only be considered as approximate. The individ- 
 ual factor in all such calculations must be held to be very large and 
 variable ; especially, perhaps, if we admit that there is a class of 
 so-called " sensitives " to whom the ends of an electro-magnet 
 when excited appear luminous, as Reichenbach's experiments seem 
 to show. Weber applied his own law to so-called extensive sensa- 
 tions of sight. 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 -jL- to T ^. Fechner ' defends 
 the validity of the law for lines of lengths varying between 10 and 
 240 mm. (f to 9 in.), with the eye removed from 1 ft. 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, 
 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. 
 
 19. 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 tastable 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 8 found by 
 
 1 Elemente d. Psychophysik, i , p 211 f. 
 
 2 See Zeitschr. f. "Biologic, 1870, VI., p. 440 f. 
 
MEASUREMENT OF TASTE. 377 
 
 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 papillae 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 
 bitter tastes. Keppler ! endeavored to test Weber's law by deter- 
 mining the sensitiveness to minute changes in the four principal 
 kinds of taste ; and arrived at a negative result. Fechner, how- 
 ever, considers that Keppler's experiments with common salt confirm 
 Weber'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. 
 
 20. The experiments of Valentin 3 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 gramme of a solution containing 1.2 per cent, of cane- 
 sugar excited the sensation of sweet ; a solution containing r |^ 
 part of common salt was scarcely detectable ; of sulphuric acid 
 T<roVo"o could be discerned, T o"o^o inr n t '> extract of aloes contain- 
 ing sWroTF cou ld l> e distinguished from distilled water; -^oTiT 
 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 nroizrrni f ^ia substance. In general, a smaller 
 absolute quantity of stimulus, when in a relatively concentrated 
 
 1 Pfluger's Archiv, 1869, ii., p. 449 f. 
 
 2 In his Lehrb. d. Physiol. d. Menschen, 2 ed., Abth. 2. 
 
378 THE LAW OF WEBER. 
 
 solution, will suffice to excite the end-organs of taste. 1 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. 
 
 21. 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 excited 
 by small quantities of stimulus, as distinguished from " fineness," 
 or power to distinguish minute variations in the sensations. It is 
 undoubtedly different in different species of animals, as dependent 
 upon unknown differences in their psycho-physical constitution ; 
 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 number 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 ^ oV^TT f va P or of bromine excited a strong un- 
 pleasant sensation. Atmosphere polluted with even -pnrfoiriT f 
 sulphuretted hydrogen could be detected. It was calculated by 
 this observer that ^--fro o"oT7) f a milligramme of alcoholic extract of 
 musk is about as little as can be perceived. The effect of constant 
 over-excitement of the organs of this sense, in deadening their sen- 
 sibility, is too well known to require illustration. No argument 
 for or against Weber's law can safely be drawn from sensations of 
 smell. 
 
 22. A review of the preceding facts confirms what was previ- 
 1 See Camerer's table in Pfluger's Archiv, ii., p. 322. 
 
INTERPRETATION OF THE LAW. 379 
 
 ously said as to the unsatisfactory nature of the evidence adduced 
 in proof of the principle which is thought to control the quantita- 
 tive relations of our sensations and their stimuli. At best, Weber's 
 law is only an approximately correct statement of what holds true 
 of the relative intensity of certain sensations of sight and hearing, 
 and, less exactly, of pressure and the muscular sense, when these 
 sensations are of moderate strength, and other causes for variations 
 in their intensity, besides objective changes in the amount of the 
 stimulus, are as far as possible excluded. In general, it is true that 
 the amount of matter pressing on the skin, or lifted by moving the 
 arm or leg, as well as the intensity of the waves of light and sound 
 acting on eye and ear, must increase much more rapidly than does 
 the intensity of the resulting sensations, as estimated by comparing 
 them with each other in consciousness. Within certain limits for 
 the above-mentioned four kinds of sensation, the latter scale of 
 quantities is ordinarily related to the former about as an arithmet- 
 ical to a geometrical series. But other conditions than mere increase 
 in the objective quantity of the stimulus largely determine its 
 effect upon the resulting amount of sensation. Stimuli and sensa- 
 tions are not connected quantitatively in such a simple manner that we 
 can measure one off in terms of the other ; so much feeling for so 
 much amplitude of wave-lengths, or work done on the end-organs 
 by mechanical pressure. Numerous factors, some of which are 
 individual and extremely obscure and variable, constantly mix 
 with the purely quantitative relations between sensations and their 
 stimuli. 
 
 23. The value of Weber's law ia 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 
 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 make any assumption, 
 it would be that the quantitative relation between the last ante- 
 cedent 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- 
 
380 THE LAW OF WEBER. 
 
 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 the explanation of so much as is true of Weber's law 
 lies largely in physiological causes ; but our knowledge of the struct- 
 ure and function of the end-organs of sense, and especially of their 
 cerebral representative elements, is so incomplete that no satisfac- 
 tory statements can be made on this point. In all of the senses, the 
 end-organs profoundly modify the intensity of the stimulus they re- 
 ceive. In the so-called chemical senses (smell, taste, sight) a pro- 
 found quantitative modification takes place, even before the stimu- 
 lus 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 modifications in the same direction by 
 reason of the interaction of different nerve-elements in the brain we 
 are yet more ignorant. And although we can have little confidence 
 in Wundt's 1 theory of an " apperception-centre " and its influence 
 in accounting for Weber's law, we cannot deny the general assump- 
 tion on which that theory is based. 
 
 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, 2 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 
 
 his Physiol. Psychologie, i., p. 351 f., and ii., p. 207 f . ; also 
 Philosophische Studien, 1883, II., heft 1, p. 31 f. 
 
 9 In proof, see his Revision d. Hauptpuncte d. Psychophysik, p. 13 f. 
 
THE LAW OF RELATIVITY. 381 
 
 states. It is not so much, then, a law of the absolute quantity of 
 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 the 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. Further 
 discussion of the significance and extent in application of the men- 
 tal law of relativity will appear in other connections. The subject 
 of the quantity of sensation as a matter of psycho-physical investi- 
 gation is fitly closed with the following quotation from Wundt : l 
 "In the imperfect condition of cerebral physiology, we are not 
 seldom in a position to recognize the psychological formulating of 
 certain laws, the physiological meaning of which still lies in ob- 
 scurity or belongs to the domain of hypothesis." 
 
 1 Physiol. Psychologie, i., p. 352. 
 
CHAPTEK VI. 
 THE PRESENTATIONS OF SENSE. 
 
 1. SENSATIONS are, primarily considered, modes of our being 
 affected ; but the objects of sense are known as real beings, which 
 are assumed to exist independent of the affections of our minds, 
 and to have their inherent qualities disclosed to us through the 
 operation of the senses. There is a wide interval, however, between 
 our consciousness of being ourselves affected and the perception 
 of " things " as having qualities resembling our mental states or re- 
 vealed by them. This interval is filled, in nature, by the develop- 
 ment of mind as conditioned upon its environment of sense-stimuli ; 
 it must be filled in psychological theory, by a description of the 
 process of development. Physiological Psychology constructs such 
 theory as much as possible on a basis of experiment to determine 
 how the various steps in the mental development are related to the 
 changes which the stimuli produce in the nervous system, espe- 
 cially, of course, in the organs of sense. Upon this work of con- 
 struction it has expended its choicest resources and utmost ingenu- 
 ity. Its efforts are yet far from being completely successful. Many 
 of the secondary principles, and even questions of fact, are still un- 
 settled ; no theory of perception that will account satisfactorily for 
 all the admitted truths has hitherto been discovered. Nor is this 
 lack of complete success surprising, when we consider how rapid 
 and complex are the processes which combine to form the world of 
 sensible objects ; as well as how entire is the loss suffered by memory 
 and consciousness of those details which served as a basis for the 
 earlier and most significant stages of the development. 
 
 Nor should we fail to take account of the fact that the mechan- 
 ism of both nervous system and mind operates as rendered native 
 to the individual by his inheriting the results of many ages of an- 
 cestral experience. The psychologist does not remember by what 
 stages he first learned to see or feel the extended and external ob- 
 jects of sense. The child cannot describe the process to the psy- 
 chologist ; the child is farther from his own infantile experience in 
 this regard than the philosopher is from that of the child. It is 
 
COMMON-SENSE VIEW OF PERCEPTION. 383 
 
 not even likely that, if the infant were endowed with the developed 
 power of searching his own consciousness, and of describing its 
 contents, he could discover and impart what is needed in order to 
 explain the process of his own mental development. 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. Scientific analysis of the process of perception corrects in 
 many particulars the so-called "common-sense" view. The convic- 
 tion which everyone has on opening the eyes upon a landscape, for 
 example, is undoubtedly that of being immediately impressed with 
 a faithful copy of extra-mental reality. Some of the objects are seen 
 as larger, others smaller, some in the foreground near by, and others 
 more remote ; but all have that solid, substantial character which 
 makes them things as distinguished from the images of revery or 
 dreaming. But it is precisely the acquired power thus to construct 
 the landscape which psychological science tries to explain. The ordi- 
 nary conviction accepts the apparent fact of an immediate and certain 
 knowledge of these things through the eyes, as though it were matter- 
 of-course and needed no explanation. We must begin by removing 
 certain assumptions obviously involved in the ordinary conviction. 
 
 The forms of being and happening in the world, outside of the 
 body, furnish in themselves no explanation whatever of the presen- 
 tations 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. What- 
 ever exists e^ra-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 within the mind, by 
 causing changes in our mental states, that any theory of knowledge 
 by the senses can make use of them. Centuries ago the popular 
 feeling was framed into a doctrine that semi-spiritualized copies 
 of the material realities enter the body through the senses and meet 
 the mind somewhere within ; or, that the mind itself, passing out 
 through the openings of sense in semi-materialized form, embraces 
 and so knows these realities. The time for all similar crude theo- 
 ries of knowledge by the senses ought, however, to have long gone 
 by. And yet fragments or suggestions of essentially the same 
 assumptions are still frequent enough. 
 
 What is true of all that exists and happens outside of the body 
 is just as true of all the bodily conditions and processes. Strictly 
 speaking, they can in themselves furnish no explanation for the rise 
 and development of the presentations of sense. Only mental factors 
 
 1 Comp. Volkmann von Volkmar, Lehrbucli d. Psychologie, Cothen, 1885, 
 II., p. 1 f . ; and Lotze, Medicinische Psychologie, Leipzig, 1852, p. 3251 
 
384 THE SYNTHESIS OF SENSATIONS. 
 
 can be built into mental products. The simple sensations are in 
 themselves always psychical phenomena, and are to be referred, as 
 modes of its being and action, to the subject called " mind." It is 
 only when considered in this way that they afford, by their charac- 
 teristic qualities and modes of combination, any explanation of the 
 resulting knowledge of things. The image on the retina, for exam- 
 ple, 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. Nothing in its construction, 
 in itself considered that is, as independent of the system of local- 
 ized sensations which result on the other or psychical side of the 
 transaction helps to explain the act of vision. What is true of the 
 peripheral is also true of the central organs of sense. There is no 
 image in the brain transmitted in exact copy from the retina by 
 the optic nerve to its central nerve-fibres and nerve-cells ; if there 
 were such a brain-image, we should need another eye connected 
 with a second brain and mind to read it. 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 arrange- 
 ment of the fibrils of sensory nerves within the skin or muscular 
 fibre. That such fibrils exist in the muscles has apparently been de- 
 monstrated by Sachs and others (comp. Chap. IV., 20) ; the nervous 
 impulses occasioned in them, when conveyed to the central organs, 
 are probably one main physical basis of those feelings of innerva- 
 tion, of being in the body, etc., which enter as essential factors into 
 the spatial perception -field of our own periphery. 1 But it is the 
 muscular sensations, as modes of the affection of mind, which per- 
 form this office. We have nothing approaching an immediate cog- 
 nition of the extended net-work 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 stretchings 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. 
 
 A further negative statement may be made with entire confidence. 
 The place at which each organ of sense is found in the periphery 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 and for distinguishing it from other places near or 
 1 See an article in Mind, by G. Stanley Hall, III. (1878), pp. 433 ff. 
 
PSYCHICAL NATURE OF SENSATIONS. 385 
 
 remote. To suppose this is virtually to return to an ancient, dis- 
 carded theory ; it is to regard the mind as well diffused through 
 the extended body, especially over its nervous periphery ; and as 
 thus constantly sensing the condition of this periphery with respect 
 to excitation, as well as the spatial relation of its various excited 
 and non-excited parts. 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. 
 
 3. In contrast to all theories like those just rejected, the fol- 
 lowing positive affirmations are to be held firmly. 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 is the 
 mind. The transference of these sensations from mere mental 
 states to physical processes located in the periphery of the body, or 
 to qualities of things projected in space external to the body, is a 
 mental act. It 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 development. The product of this act, the presentation 
 of sense (or, considered objectively, the "thing" as known by the 
 senses) has characteristics that do not belong to the simple sensa- 
 tions out of which scientific experiment and theory show it to be 
 composed. The presentation of sense has " space-form ; " it is ex- 
 tended, and consists of an indefinite number of visible or tangible 
 parts that are systematically arranged beside each other into a con- 
 tinuous whole ; it is related with respect to position, magnitude, 
 etc., to other similar objects of sense. Certain compound sensa- 
 tions as, of light and color, of smoothness and hardness are not 
 regarded merely as psychical acts whose cause lies in the extended 
 object ; they are regarded as qualities of its surface, and appear to 
 belong to it as forms of its objective being. Such a result, how- 
 ever, must be regarded as brought about by the action of mind, in 
 forms and according to laws of its own. 
 
 The one characteristic which the presentations of sense possess, 
 but which does not belong to the simple sensations that are their 
 factors, 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 psy- 
 chology has to solve in this direction may then be stated as follows : 
 On the basis of what combinations of physical processes of sense do the 
 different resulting sensations come to be combined into presentations of 
 sense under the new characteristic of space-form f 
 25 
 
386 THE SYNTHESIS OF SENSATIONS. 
 
 4. The most complete answer possible to the question just 
 raised is obliged to recognize the following particular truths : 
 
 (1) A combination (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. If our sense -perception were all by a 
 single organ, or by a single activity of one organ, the objects of 
 sense would largely, if not wholly, lose their present characteristics 
 of position and extension in space. 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, 
 temperature, 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 these characteristic differences 
 and this adaptability. We may then speak of peculiarly " spatial 
 series " of sensations, 1 and of other series of sensations as non- 
 spatial. By this term, however, it must not be understood that 
 any sensations, as such (or quoad sensations), are extended in space, 
 or can come by any process of theoretical manipulation to be en- 
 dowed with extension. 
 
 The sensations of smell are manifestly not fitted to form a so-called 
 spatial series ; indeed, they are incapable of being arranged in any 
 series at all. An experience consisting wholly of sensations of 
 smell would have no elements from which to construct objects of 
 sense. The same thing is true of pure sensations of taste and of 
 sound. On the contrary, the various series of complex sensations 
 that come through the eye and skin (including those of the mus- 
 cular sense) are qualitatively adapted to enter into such relations to 
 each other as shall give a ground in their combined existence for 
 a perception of things. Accordingly the eye and skin are the so- 
 called " geometrical senses." 
 
 (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 at these parts an extended object out- 
 side of the body is to be perceived (as in the case of both skin and 
 
 1 Comp. Volkmann von Volkmar, Lehrb. d. Psychologie, II., pp. 36 ff. ; 
 whose very interesting theory of the origin of space-intuitions can, how- 
 ever, no more be pronounced wholly satisfactory than the similar theory of 
 Bain. 
 
LOCALIZATION AND PROJECTION. 387 
 
 eye) must each have some mental representative in the sensations 
 which stimulation of each calls forth. As places in material sub- 
 stance, or as parts of the organism, in themselves, they have no im- 
 mediate significance for the mind. But a psychological equivalent 
 or representative they must have. To this principle is due the fact 
 that those senses are not " geometrical senses " in which we cannot 
 vary, largely at pleasure, the locality to which the stimulus is ap- 
 plied. It is therefore assumed that every sensation, besides its 
 general characteristic quality as sensation of this or that particular 
 sense, must have a peculiar "local stamp," or shade, or mixture of 
 quality, dependent upon the place of the organ at which the stimu- 
 lus is applied ; otherwise such sensation cannot serve as a factor 
 in the construction of an extended object of sense. Such peculiar 
 local stamp, or shade, or mixture of quality is a so-called " 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 presen- 
 tations of sense. The theory, as he truly says, is an indispensable 
 assumption of every satisfactory account of perception by the senses. ' 
 In what, precisely, these so-called local signs consist has been dis- 
 puted by those who agree in holding that the explanation of the 
 facts requires the assumption. 
 
 (4) Various stages in the process of elaborating the presenta- 
 tions of sense from the material of simple sensations must be recog- 
 nized. Thus the construction of a retinal field of vision is a less 
 elaborate work, both of the mechanism and of the mind, than the 
 construction of that more objective field in which real things are 
 seen with solid forms and set at varying distances. So, too, 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 discrimination 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 require a special 
 consideration. These are "localization" or the transference of the 
 composite sensations from mere states of the mind to processes 
 or conditions recognized as taking place at more or less definite- 
 ly fixed points or areas of the body ; and " eccentric projection " 
 (sometimes called "eccentric perception "), or the giving to these 
 sensations an objective existence (in the fullest sense of the word 
 " objective ") as qualities of objects situated within a field of space 
 and in contact with, or more or less remotely distant from, the 
 body. The law of eccentric projection is generally stated thus : 
 1 Medicin. Psychologic, p. 331. 
 
388 THE SYNTHESIS OF SENSATIONS. 
 
 Objects are perceived in space as situated in a right line off the 
 ends of the nerve-fibres which they irritate. 
 
 (5) The entire process of elaborating the presentations of sense 
 presupposes for its explanation a constant activity of the mind in 
 reacting, with sensations of different kinds, upon the stimuli which 
 produce various forms of molecular disturbance in the nervous 
 system ; and, furthermore, its activity in combining the sensations 
 into the more complex presentations of sense, according to modes 
 of behavior that belong to its own nature, as mind. This combin- 
 ing activity is best called "synthetic," l or constructive. It may, in- 
 deed, always have a physical basis in some central organic combi- 
 nation of the neural processes resulting from stimulating, simul- 
 taneously or in the right succession, the different end-organs and 
 areas of the end-organs of sense. About this, as a matter of scien- 
 tific knowledge, we are almost wholly in the dark. On general 
 grounds of our theory of the nervous mechanism we conjecture 
 that it is so. But if this organic combination takes place in each 
 instance as a physical basis for the psychical synthesis, the former 
 does not do away with the latter. Obscure as the latter is, and 
 doubtful as some of its elements and stages are, the former is more 
 doubtful and obscure. 2 
 
 Nor is the fact that this synthetic activity takes place to be con- 
 cealed by ascribing the product to the so-called " association " of 
 sense-impressions and of ideas. Ideas and sensations are not enti- 
 ties or real subjects of states ; they are only particular phenomena. 
 They cannot associate themselves ; nor are they things which may 
 be combined, after the analogy of material atoms, by the action 
 upon them of neural conditions. The term " association," as ap- 
 plied in all theories of sense-perception, is only an inadequate ex- 
 pression for this same synthetic activity of the mind. 
 
 5. It follows, then, that an analysis of the presentations of sense 
 leads us to find our explanation of certain primary facts and results 
 
 1 The word " synthesis " for this mental activity is employed and defended 
 by Wundt (Physiolog. Psychologie, ii., pp. 28 f., 164 1, 177), who justly ob- 
 jects 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." 
 
 2 Yet E. Montgomery, in Mind, 1885, pp. 227 ff. and 377 ff., speaks as 
 though the indubitable and clearly understood portion of the whole psycho- 
 physical process were the neural and organic part ; and as though it were 
 doubtful whether we have any right at all to refer the nature of the product 
 to the nature of the subject whose product it is namely, to mind. 
 
NATIVISM AND EMPIRICISM. 389 
 
 in the nature of the Mind itself. Physiological psychology can do 
 much toward giving a descriptive history of the process in which 
 these complex products have their rise. It can point out the ele- 
 ments which enter into the products both the more primary and 
 the derived and can state the laws which regulate the process. 
 But the forth-putting of these primary elements, and the modes of 
 the activity which are called the laws of the process, must all in the 
 last analysis be thought of as native to mind. It is in vain to object 
 that to do this leaves the subject, ultimately, still shrouded in mys- 
 tery. 1 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 most vigorously pushed. For physical science always has 
 to admit into its explanations the unexplained mystery of elements 
 of physical reality which behave in certain ways not simply be- 
 cause they are thus or thus circumstanced, but also because when 
 they are thus circumstanced it is their nature so to behave. 2 
 
 6. The foregoing remarks indicate what is the correct position 
 toward the two rival theories as to the nature and origin of presen- 
 tations of sense. These theories have been named the "nativistic" 
 (or intuitional) and the " empiristic " by Helmholtz, 3 the "nativis- 
 tic" and "genetic" by Wundt. 4 Properly speaking, they are not 
 two fundamentally different theories, but rather two tendencies 
 which appear in the attitude 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 ten- 
 dencies are largely due to differences of position on certain funda- 
 mental philosophical questions, especially the question as to the 
 reality and self-activity of Mind, which (however much the effort is 
 made to avoid them) inevitably have their bearing upon the re- 
 searches of physiological psychology. Thus influenced, the so- 
 called "Nativistic School" is inclined to depreciate the explanations 
 
 1 Sully thinks it an objection to Wundt's view that it " burdens us with the 
 mystery of what may be called a psychical form of spontaneous generation " 
 (see Mind, 1878, p. 192). But it is one chief merit of Wundt that he frankly 
 acknowledges the mystery and knows where to locate it. Doubtless he would 
 rejoice with us all, if Sully, or any other investigator, could push analysis 
 further. 
 
 In the article just referred to (Mind, 1878, p. 184) Sully speaks of Lotze 
 as "burdened with survivals of Herbart's metaphysics." We fail to see why 
 any recognition of the reality and self-activity of mind should be considered 
 as such a "burden" by certain English psychologists. 
 ' 3 Physiolog. Optik, p. 435. 4 Physiolog. Psychologie, ii., p. 23. I 
 
390 THE SYNTHESIS OF SENSATIONS. 
 
 offered by the other school as to how and why the presentations of 
 sense come to have the character they really bear ; the advocates of 
 this 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," and popularly look upon as immediately con- 
 scious of the reality of things by virtue of its native right and power, 
 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 ex- 
 plained as the result of a development. The other is inclined to look 
 upon space-form as wholly a form which " things " have come to ac- 
 quire, and which will be fully explained when science has described 
 the empirical process by which solely this acquisition is gained for 
 them. Inasmuch as the chief difficulties of a theory of perception 
 have hitherto been found in accounting for the construction of visual 
 space, the one party in the controversy has insisted, as a rule, on 
 the native power of the mind in and through the retinal image di- 
 rectly to intuit space of two, or even of three, dimensions ; the other 
 has denied this power, and has laid great stress upon the necessity 
 of motion, with its equipment of double images and of graded mus- 
 cular and tactual sensations. 
 
 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. 
 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 perfect 
 " immediateness," they are really extremely complex products, in- 
 volving 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 picture of a single object 
 when seen by instantaneous illumination, or of the unhesitating lo- 
 calization 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. 
 
GENESIS OF SPATIAL QUALITIES. 391 
 
 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 (the simple sensations) 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. The nature of the process in which what is sim- 
 ple and homogeneous becomes complex and heterogeneous, and ac- 
 quires the added characteristics of so-called space-form, can never 
 be regarded otherwise than as due to the constructive and synthetic 
 action of the mind. 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. 
 
 The only satisfactory course in considering this subject of per- 
 ception by the senses is, therefore, perfectly obvious. The differ- 
 ent simple sensations which enter as elements into the presentations 
 of sense, the method and laws of their combination, the correlations 
 of mind and nervous mechanism involved in the process, must all 
 be pointed out as fully as the present condition of psycho-physical 
 science will admit. But the existence of unexplained mysteries, 
 the fact that original and underived activities of the mind are 
 necessarily assumed, the truth that the entire complex process is 
 to be ascribed however occasioned or conditioned to the mind, 
 must also be admitted. Especially must we avoid all attempts, 
 whether avowed or concealed, to account for the spatial qualities of 
 the presentations of sense by merely describing the qualities of 
 the simple sensations and the modes of their combination. .It is 
 position and extension in space which constitutes the very pecu- 
 liarity of the objects as no longer mere sensations or affections of 
 the mind. As sensations, they are neither out of ourselves nor 
 possessed of the qualities indicated by the word " spread-out" As 
 objects of sense, they are both out and " spread-out." No manip- 
 ulation of their mental qualities and values can fully explain how it 
 is that when combined they acquire the new peculiarity of their 
 space-form. If it should be found that any system of simple sen- 
 sations is probably originally given as localized, it would be neces- 
 sary to acknowledge this, too, as an inexplicable matter of fact. 
 Science cannot explain, from previous experience, an event in ex- 
 
392 NATURE OF SPATIAL SERIES. 
 
 perieDce which is so fundamental as this. Such an event consti- 
 tutes an unsolvable datum of nature, so to speak. And if the fur- 
 ther question be asked, Of the nature of what ? the only possible 
 answer must be, Of the nature of the subject of the experience, of 
 the Mind. 
 
 8. Before proceeding to illustrate and confirm in detail the 
 five 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 sen- 
 sations belonging to some of the senses possess, and which adapts 
 them to combine into presentations of sense under space-form ? 
 In other words, what kinds of sensations are fitted to constitute a 
 so-called " spatial series f " Plainly, it is not necessary that those 
 elements of the complex 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. The mosaic of 
 nervous elements (rods and cones) set side by side in the ninth 
 layer of the retina is a physical pre-condition of the extended visual 
 object which the mind has when the retina is excited. According 
 as the irritation spreads over this mosaic the extent and shape of 
 the object are determined. But the extension of the object is not 
 a copy of the extension of the retinal nerve-expanse with its minute 
 parts set side by side. For example, the object under ordinary 
 circumstances has no gap corresponding to the blind-spot ; and 
 the nervous elements, on whose excitement the appearance of the 
 extended object depends, belong to the retinas of two eyes, and 
 are therefore not side by side at the periphery ; nor do histology 
 and physiology warrant us in assuming that the elements in the 
 brain corresponding to the retinal elements are set exactly side by 
 side there. Moreover, the different parts of the object appear side 
 by side as the result of motion of both eyes with its resulting mus- 
 cular and tactual sensations. The nervous elements whose irrita- 
 tion induces these latter sensations are not locally contiguous to 
 the elements of the retina whose excitation produces the sensations 
 of light and color ; yet the two kinds of sensations combine to 
 form one extended object. 
 
 In brief, we have no reason to assume that any two. kinds of sen- 
 sations require, in order to combine into one object, that the excited 
 nervous elements which form the physical bases of them both 
 should be set precisely side by side in the brain. If they were 
 we may ask what good would it do ? if they were not, what harm ? 
 Let the locally different sensation-elements of the light-and-color 
 series be represented by a, 6, c, d, etc. ; and those of the muscular 
 
REPETITION OF LIKE SENSATIONS. 393 
 
 and tactual series by a, j3, y, 8, etc. How shall we arrange the two 
 series so as to form within the brain a physical basis for one object 
 with its parts all set in visible spatial order ? Shall this be accom- 
 plished by interpolating one of the latter series between every ad- 
 joining pair of the former series thus a, a. ; b, /? ; c, y ; etc. ? 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 " spatial series." 1 
 
 9. 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 arrange- 
 ment. An order of sensations in time, however varied and fre- 
 quently repeated, can never of itself account for an order of parts 
 in an extended object set side by side in space. But the character 
 of the ordering possible for the simple sensations does determine 
 whether or not they can become elements of such an object. 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 
 position in consciousness. In adult experience the series is rapidly 
 concluded, and instantaneously interpreted as a whole. The indi- 
 vidual members of the series are then scarcely, or not at all, distin- 
 guished in consciousness ; that is, as sensations, many of them largely 
 or wholly drop out. But they may be reproduced in a measure by 
 slowly moving a limb in any direction, and endeavoring to pay strict 
 and exclusive attention to the succession of feelings which results. 
 Every motion of each limb, from about the same position a to about 
 the same position m, relative to the whole body, with similar energy, 
 speed, and other concomitant circumstances, yields a nearly iden- 
 tical series of sensations (a, /?, y, . . . /A). Other motions of dif- 
 ferent limbs, or differing otherwise (in energy, speed, point of start- 
 ing or of conclusion, etc.), yield series differing in the value and or- 
 dering of their individual members. What is true of the muscular 
 sensations that result from the movement of the limbs is also true 
 of the accompanying sensations of the skin, such as arise from 
 
 1 On this subject comp. the remarks of Volkmann von Volkmar, Lehrb. d. 
 Psychologic, II. , pp. 36 ff . 
 
394 NATURE OF SPATIAL SERIES. 
 
 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 mo- 
 tion of the eye 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 also of the 
 kind favorable to forming a spatial series. Slight changes of color- 
 tone and brightness, as the different parts of the surface of the 
 same object come successively in review, or more abrupt changes 
 on transition from one object to another, characterize the effect of 
 such movement. 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, /J, y, 8, . . . ^, 
 or in inverse order, as /*, X, *, . . . /?, a, but also in an endless 
 variety as an intersecting net-work of sensations. 
 
 10. It has been claimed that all the foregoing qualifications 
 belong to musical tones, and yet hearing is not a "geometrical 
 sense." In reply, it must be admitted that the foregoing are not 
 the only qualifications possessed by sensations of spatial series. 
 Yet in regard to these qualifications alone, the sensations of hear- 
 ing bear no comparison with those of the eye, skin, and muscles. 
 Few persons have more than a very imperfect and infrequent expe- 
 rience with series of musical tones. Such series, when arising in 
 consciousness, ordinarily have little variety, and are rarely or never 
 repeated in changed order of arrangement with a recognized quali- 
 tative value to the members of the different series. Few persons 
 have heard frequently more than half a score of tunes ; fewer still 
 have had any considerable number of the notes composing the 
 strains of these tunes repeated in recognizably inverted or other- 
 wise varied order. Most of our experience with sounds is that of 
 sudden shocks of noise which occur to interrupt the continuous 
 flow, as it were, of sensations of the eye, muscle, and skin. 
 
 In close connection with the foregoing stands also the fact that 
 the ear is not, like the eye and the hand, a movable organ ; nor is 
 
ASSOCIATION OF SENSATIONS. 395 
 
 the irritation which its nervous elements experience graded accord- 
 ing to the varying extent of the surface affected, except as the noise 
 becomes so loud or acute as to occasion decided forms of muscular 
 sensation and of common feeling. Compared with the almost un- 
 ceasing call made for attention to sensations of sight and touch, the 
 experience of men with sensations of musical sound (the only sen- 
 sations of hearing which easily admit of being arranged in any sort 
 of a graded series) seems meagre and trivial indeed. Music is, 
 therefore, of the nature of an indirect recreative and aesthetic ac- 
 quaintance with things, rather than of a necessary direct and practi- 
 cal acquaintance with them. 
 
 11. 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 associab/e 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. 1 Yet these two simultaneous series of qualitatively differ- 
 ent sensations do not combine into a succession of extended objects 
 of sense. For this fact there are several obvious reasons. Of the 
 three principal kinds of sensations involved namely, sensations of 
 musical sound, sensations localized in the organs with which the 
 sounds are executed, and sensations localized in or near the organs 
 with which the sounds are heard only the last two belong to spatial 
 series. The first of the three, however, is not a spatial series. It has 
 not been accustomed to give us direct knowledge of any part of our 
 own body ; nor does it combine with either of the other two so as 
 to form an object of sense. We know, indeed, 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 sensations 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 
 1 Comp. Lotze, Medicin. Psychologie, p. 382 f. 
 
396 NATURE OF SPATIAL SERIES. 
 
 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 pre-condition 
 of the latter. 
 
 12. The third characteristic of the spatial series of sensations 
 is the possession of a system of local signs. By a " local sign " we 
 understand that peculiar shading or mixture of quality which be- 
 longs to sensations, otherwise qualitatively similar, on account of 
 the locality of the organ at which the stimulus is applied. Further 
 as to their nature and origin it is not easy to give a satisfactory 
 account. While accepting the general theory of Lotze as to the 
 existence of such local signs, his more specific view as to what they 
 are must be held subject to doubts. The local signs of sight 
 Lotze conceived of in the following way. 1 In addition to the same 
 sensation (for example, red, r) which each color produces at all 
 places of the retina, it produces also an accessory impression a, /?, 
 y, etc., for each of its different places a, b, or c, etc. The existence 
 of such accessory data must be assumed, or the spatial differences 
 and relations among the retinal impressions could not be " compen- 
 sated for " by corresponding non-spatial and merely intensive rela- 
 tions among the impressions in the soul. 
 
 We can only conjecture, however, in what the accessory impres- 
 sions consist. Lotze's conjecture is that, since we involuntarily 
 and by reflex motion of the eye fixate its most sensitive spot 
 upon every especially luminous point, and in order to do this must 
 rotate the eye through an arc PE, or RE, etc., according to the 
 position from which it takes its start in the fixation of E, a great 
 number of series of changing "feelings of position " are developed 
 corresponding to each arc (thus, TTC, pe, etc.). When, then, both P 
 and R are simultaneously stimulated, although with equal intensity 
 and so that actual rotation of the eye cannot take place over both 
 of the opposed arcs PE and RE, the stimulation of P and R repro- 
 duces the series of feelings of position belonging to each (respec- 
 tively, Tre and pe). Thus there comes to be connected with every 
 excitation of the places P and R the mental presentation of the 
 magnitude and qualitative peculiarity of a series of changes which 
 consciousness " would have to experience in order that these exci- 
 
 1 The most mature expression of Lotze s theory is to be found in Outlines of 
 Psychology, Boston, 1886, p. 51 f.; and Metaphysik, Leipzig, 1884, in., chap. 
 4; the earlier, in Medicin. Psychologic, Book ii., chap. 4; and article, Seele 
 u. Seelenleben, Wagner's Handworterb. d. Physiol., 1850, III., 1. Abth. 
 
LOCAL SIGNS OF THE SKIN. 397 
 
 tations might fall upon the place of clearest vision." The physical 
 basis for this is to be found in the cerebral connection between the 
 sensory and the motor nerves ; it is the excitation of the latter at 
 their central endings which gives to every color-impression its local 
 character. 1 The local sign of sight is, then, a kind of feeling of ten- 
 dency to motion, mentally reproduced out of the associated series of 
 impressions that have previously accompanied the movement of the 
 eye in fixation from one position to another. The sign consists in 
 awakening definite "motor tendencies,"* or, rather, "associated 
 feelings of movement." 3 
 
 Various objections to Lotze's hypothesis may be brought forward. 
 To speak of "accessory" or "adjunct" impressions as attaching 
 themselves to the chief impressions of color-tone seems unfortu- 
 nate. Indeed, Lotze himself conceives of them rather as blending 
 in one mixture of feeling with the principal sensation. The color- 
 tone itself does indeed change with the position upon the retina 
 where the stimulus falls. 4 But the change is always a change into 
 another color-tone, which is, so far as it is color-tone at all, capable 
 of reproduction at any one of innumerable other points of the 
 retina. The question then recurs as to the origin and nature of 
 the individual members of the series of original so-called " feelings 
 of position " (TT to e, or p to e). Are they feelings which arise from 
 the changing condition of the muscles of the eye, or of the skin 
 surrounding the eyeball, when the eye is in motion ? Or are they 
 specific shades of feeling which naturally belong to each of the 
 nervous elements of the retina, and which are awakened by stimu- 
 lating these points without any dependence on the motion of the 
 eye? 
 
 13. Several views are possible as to the nature of the local signs 
 of the skin. It may be held that they are not qualitative differences 
 at all, but differences in the intensity and time -course of the tactual 
 sensations. Von Kries and Auerbach, 5 however, have shown that 
 locality on the skin is much more quickly discriminated than even 
 considerable differences in the quantity of sensations. We can tell 
 the point where we are touched easier than the point where the 
 amount of pressure is increased. Again, it may be held that the 
 local signs of touch are qualitative differences of sensation de- 
 
 1 Medicin. Psychologie, p. 360. 
 
 2 So in the earlier view, Medicin. Psychologie, p. 340. 
 
 3 Comp. Sully, in Mind, 1878, p. 181 f . 
 
 4 Bain makes the odd mistake of supposing that "both Lotze and Wundt re- 
 gard this series of changes in color-tone as constituting the local signs of vi- 
 sion. The Senses and the Intellect, p. 397 f . (note). New York, 1879. 
 
 6 Ajrchiv f. Anat. u. Physiol., Physiolog. Abth., 1877, p. 351 f. 
 
398 NATURE OF SPATIAL SERIES. 
 
 pendent upon the modifications which the stimulus undergoes on 
 account of the changing character of the skin with respect to ten- 
 sion, nature of the substance of muscle, tendon, and bone over 
 which it is stretched, etc. But experiment shows that stimulation 
 of the skin by electricity, and in such a way that these influences 
 could have no appreciable effect, is immediately localized. Finally, 
 it may also be held that the local signs of the skin are qualitative 
 differences of sensation peculiar to the different nervous elements 
 existing in different parts of this organ of 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 elements being 
 stimulated. It is to surrender explanation at this point, and accept 
 what takes place as mere datum of fact. 
 
 14. In view of all the evidence, it would seem that the general 
 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 
 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, ft, y, 
 S, etc., differs from that aroused by irritating the elements y, 8, , , 
 etc. The same thing holds true of locally related nervous elements 
 of the skin. How much of the local coloring is due, on the phys- 
 iological side, to differences in structure and how much to dif- 
 ferences in processes, how much to peripheral elements and how 
 much to central nervous connections, it is 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. ' The theory is thus opposed to that of Bain 2 and others of 
 
 1 Comp. Funke, in Hermann's Handb. d. Physiol., III., ii., p. 408 f. 
 8 The Senses and the Intellect, pp. 73 ff., 348 ff. New York, 1879. 
 
LOCAL MIXTURES OF FEELING. 399 
 
 the Association school, who are inclined wholly or largely to reduce 
 all the local signs to mere symbols of associated differences in the 
 muscular sense. They are thus made to become mere signs of signs 
 of "Things." 
 
 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 peculiar to 
 the series. Thus the spatial series of tactual impressions pro- 
 duced by moving an object from a to d on the hand differs from 
 that produced by moving it from a to n ; that belonging to move- 
 ment from a to d on the hand from that belonging to movement 
 from mtox on the back. The series of muscular sensations de- 
 veloped by raising one pound differs, with respect to the color-tone 
 of its members, from that developed by raising two pounds, with 
 the hand ; both differ from the series belonging to the raising of 
 the same weights with the foot. Every series of muscular and 
 tactual sensations produced by moving the eye also depends upon 
 the direction and amount of its motion ; the series of local signs 
 of the retina depends upon the direction and amount of the motion 
 of the object over the retinal field. 
 
 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 admixtures of feeling dependent upon the combined 
 specific energies of the nervous elements simultaneously excited, 
 with a given amount of energy and with given relations to preced- 
 ing conditions. Thus it comes about that the place where we locate 
 a visual object does not depend merely upon the place where its 
 image falls on the retina, but also on the feeling of the position of 
 the eye as indicated by its muscular and tactual local signs. In other 
 words, the real and complete " local sign " (that which signifies the 
 locality of the object to the mind, as it were) is the whole complex 
 of feeling which is combined from the local signs of the different 
 spatial series that are acting together. We define the local sign, 
 then, as that mixture of feeling which gives to the sensation its 
 peculiar coloring, and is dependent upon the combined result of 
 exciting the nerves of a given locality of the organ. 
 
 15. It may be objected that the foregoing theory of local signs 
 is too elaborate and artificial to be in fact true. But the nervous 
 elements themselves are indescribably numerous and varied ; and 
 so is, without doubt, the total complex of feelings which results 
 from their activity. Reflective consciousness finds itself baffled in 
 its effort to catch and fix, for purposes of analysis, all the various 
 
400 NATURE OF SPATIAL SERIES. 
 
 shadings of sensations which actually appear in adult life. Yet it 
 is characteristic of this adult life that its very development presup- 
 poses the loss from consciousness of many discriminations of shades 
 of feeling on which the development is itself based. The adult does 
 not, and cannot, recall the complex of sensations by which he 
 learned to talk ; nor can the skilful player of the violin reproduce 
 the " tact " with which he experimented among the innumerable 
 muscular and tactual sensations concerned in producing a required 
 musical tone. Let anyone, however, who imagines that the limit can 
 be easily fixed for the peculiar shadings which sensations ordinarily 
 classed together really possess, spend a half-hour, with one hand 
 lying motionless across the other, in the endeavor to pick out all 
 the different color-tones of tactual feeling which he can localize in 
 both hands. 
 
 The dependence of even developed experience upon these local 
 signs in consciousness, for its apparently instinctive localization 
 of sensation, may be faintly manifested by various experiments. 
 Select two portions of the body that are in structure and function 
 most nearly identical, and whose local signs will accordingly be 
 such mixtures of feeling as, when brought into consciousness, 
 will be most difficult to keep separate. The corresponding points 
 on the tips of the two middle fingers, perhaps, fulfil these con- 
 ditions best. With closed eyes rub these points gently together, 
 concentrating the attention, as much as possible, solely upon the 
 sensations thus produced. The tactual sensations may thus be made 
 to fluctuate in locality from one finger to the other ; at times they 
 appear almost to lose their objective character, and to resemble 
 musical tones heard without consciousness of the direction from 
 which they come, or of the extent and locality of the bodily affec- 
 tion through which they come. A similar nearly complete detach- 
 ment of sensations of light and color may be secured by closing the 
 eyes, letting all after-images die away, suppressing the tendencies 
 to motion of the organ, and directing attention, as much as pos- 
 sible, solely to the quality of our affection. Color seems then to be 
 felt (but nowhere in particular), rather than seen as localized in 
 space-form. 
 
 16. 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- 
 
TOUCHING AND BEING TOUCH 
 
 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 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 infini- 
 tesimal distance or to one remote. Localization and eccentric pro- 
 jection are rather two processes, largely unlike, which go on con- 
 temporaneously and are set up chiefly on the basis of different 
 classes of sensations. 
 
 Where two parts of the sensitive skin of our own bodies come 
 together the conditions for both of the above-mentioned processes 
 are fulfilled. Accordingly, one part has localized in it those com- 
 plex 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 considerations. Those mem- 
 bers of the body which are most used in active touch are generally 
 known as touching, and the less active parts as being touched. For 
 example, if with closed eyes the forehead be moved across the sta- 
 tionary tip of a finger, the latter will appear to be the active organ 
 of touch. Comparatively insensitive areas of the skin are less likely 
 to be presented to the mind as touching other more sensitive parts ; 
 callous spots, indurated surfaces, etc., seem, as a rule, to be touched. 
 Parts of the body which lose all sensitiveness come to be regarded 
 as external things. If the tip of a finger of normal sensitiveness be 
 brought into contact with the callous tip of the corresponding fin- 
 ger of the other hand, the former will be known as touching and 
 the latter as being touched. The direction of attention often deter- 
 mines the strife, as it were, between the motifs to localization and 
 those to eccentric projection. We ordinarily strive to gain knowl- 
 edge of the qualities of some outside object, rather than of the con- 
 dition of our own periphery with respect to the sensations localized 
 in it ; the attention is therefore directed to those series of sensations 
 which form the basis of eccentric projection, even when some part 
 of our own sensitive organism is the object known. But sensations 
 which are accompanied by obtrusive feeling of some kind furnish 
 superior grounds for localization. We locate pains, pricks, severe 
 pressure, sensations of creeping, and tickling, in the body. In gen* 
 26 
 
402 PERCEPTIONS OP THE LOWER SENSES. 
 
 eral, then, a strong tone of feeling with the sensation favors the 
 process of localizing ; tonelessness of sensation favors the process 
 of objectifying. ' 
 
 A system of localized sensations, gained chiefly by pressure of 
 the skin and muscles, and accompanied by a strong tone of feeling, 
 gives us the primary field of the body as known to touch. Certain 
 points of starting, as it were, must first be fixed in the process of 
 localizing ; this process then goes on by relating all other localized 
 sensations to these points of starting. But by eccentric projection, 
 the system of muscular sensations of movement and the system of 
 visual sensations are combined to develop our perceptions of ob- 
 jective space with its three dimensions. The sensations of touch 
 are subsequently projected into a space thus originally constituted 
 by combined muscular sensations and visual sensations. The eye 
 and hand in motion, therefore, project their extended objects into 
 a space which they develop themselves ; while the ear and the nose 
 project their perceptions into a space which they are compelled to 
 assume on the authority of the other senses. 
 
 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 sensa- 
 tions. 
 
 17. 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. They cannot even be said to be localized. 
 Fineness of smell, or power to make minute distinctions in quality, 
 and so infer the presence or direction of an object previously known 
 to excite such quality of sensations, differs greatly in different spe- 
 cies 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. 2 
 The direction and nature of the object which causes the sensations 
 are judged by variations of intensity on turning the head, or on 
 approaching or receding from the object. Sensations of smell are 
 known to come through the nose, by localizing there the accom- 
 
 1 Compare Volkmann von Volkmar, Lehrbuch der Psychologic, II. , p. 
 126 f. 
 
 2 See the articles of Donhoff, in Archiv f. Anat., Physiol., etc., Physiolog. 
 Abth , p. 7501 (1874) ; and Jager, in Zeitschr. f. wissensch. Zool., xxvii., p. 
 319 f. (1876). 
 
TASTE AND HEAEING. 403 
 
 panying 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 statement of Valentin, that the stronger 
 overwhelms the weaker. The power of discrimination may, of 
 course, be cultivated in this sense as in every other. 1 
 
 18. 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. Brticke 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. 
 
 19. Perceptions of Hearing next demand consideration. More 
 difficulty accompanies the effort to establish the proposition that 
 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. Such a proposition is, however, 
 undoubtedly true. The localizing of the area of the body which 
 serves as the organ of the sensations of sound, the knowledge that 
 we hear with the ear, is accomplished chiefly through those sen- 
 sations of shock to the muscles and skin of the region which come 
 from loud and massive or piercing sounds. Sensations of sound 
 originating through excitement within or very near to the ear itself 
 are called "entotic." A great part of such sounds, if not all of them, 
 are transmitted through the tympanum. Perceptions combined of 
 such sensations may be located either within the ear or at some dis- 
 
 1 On the whole subject see von Vintschgau's monograph in Hermann, Handb. 
 d. Physiol., III., ii., pp. 225 ff. 
 
 2 Lehrbuch der Physiol. d. Menschen, etc., Abth. ii., p. 308 (second edi- 
 tion). 
 
 3 Vorlesungen iiber Physiol. (ed. 1884), ii., p. 262. 
 
404 PERCEPTIONS OF THE LOWER SENSES. 
 
 tance from the body, according to previous associations and oppor- 
 tunities for judgment. The sound produced by the vibration of the 
 adjoining muscles, and heard as a low musical tone when the fin- 
 gers are pressed in the ears (especially if the teeth are tightly set 
 together), is located in the head by the help of its accompanying 
 sensations of other kinds. The same thing is true of the crackling 
 noise sometimes produced by yawning, or of the whirring occasioned 
 by the passing of the blood through the neighboring large blood- 
 vessels. In the same way we learn to hear the beating of our own 
 hearts, or the noise of air in our respiration. But the click of the 
 valves of the internal organ may, when experience gained through 
 tactual and muscular sensation fails us, be located in the watch 
 under our pillow; just as the singing or ringing "in the ears" 
 produced by quinine, or cerebral excitement otherwise occasioned, 
 may be located in a cricket supposed to be upon the sill of the open 
 window. In certain pathological cases the power to distinguish 
 between entotic sounds and those having an external origin is al- 
 most wholly lost. 
 
 20. We can orientate ourselves in space with reference to ex- 
 ternal sounds with great speed and considerable precision, but as 
 an acquired art differing in different individuals and dependent 
 upon attention and previous experience. E. H. Weber thought 
 that we tell the direction of sounds by the help of the feeling of 
 the swing of the ear-drum ; and instanced, in proof, that this 
 eccentric projection is hindered by filling the external passage 
 of the ear with water. When using both ears and moving the 
 head freely in space, we undoubtedly determine the direction of 
 sounds by differences in the intensity of the sensation dependent 
 upon changes in the relative position of both ears. Kayleigh l 
 found that, in a quiet place under favorable circumstances, the 
 direction of a word or letter uttered in a natural voice could be 
 given with considerable accuracy ; that of a musical tone much 
 less accurately. The direction of a sound from a tuning-fork 
 could not be given when it was held either behind or before, but 
 could be given if the fork was held to the right or to the left. It 
 is said that the conducting of an intermitting current from a tele- 
 phone through both ears causes a perception of tone localized in the 
 median plane of the head. 
 
 Such facts as the foregoing introduce us to the theory of "acous- 
 tic shadows," or of the amount of " covering " power which the 
 sound produced by the waves of a given intensity entering one 
 ear would have upon the sound produced by waves of a different 
 1 Nature, XIV., p. 32. 
 
THE EXPERIMENTS OF WEBEK. 405 
 
 intensity entering the other ear. It does not appear to Hensen, 1 
 however, that the prompt and accurate localization of direction 
 possible to some ears can be wholly accounted for by the theory of 
 acoustic shadows. Some other form of feeling, possibly connected 
 with the remarkable arrangement of the semicircular canals, may 
 blend with the estimate of differences in intensity to form a basis for 
 judgment. Von Kries and Auerbach found that the promptness 
 with which the direction of the noise from an electric spark can be 
 localized depends upon its relation to the circuit of the head. 2 
 
 Our perceptions of the absolute distance of sounding objects are 
 entirely dependent upon our knowledge of the quality and quantity 
 of the sounds ordinarily proceeding from them ; they are, that is 
 to say, not presentations of sense, but indirect estimates as to the 
 objective cause of -the sensations immediately experienced. It has 
 been claimed that a change in the relation of the partial tones to 
 the fundamental tone, dependent upon the remoteness of the place 
 of origin of the compound clang, aids our estimate of distance by 
 sound. 
 
 21. 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 established 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. s 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 local- 
 ized sensations is the measure of the sensitiveness to local distinction 
 of that region. The following table gives some of the results of 
 Weber's experiments; the figures indicate the number of milli- 
 meters * apart which the points of the dividers were when the given 
 area of the organ was just able to distinguish them : 
 
 1 In Hermann's Handb. d. Physiol., III., ii., p. 136. 
 
 2 Archiv f. Anat. u. Physiol., Physiolog. Abth., 1877, p. 331 f. 
 
 3 Annot. Anatom., vii., p. 4 f. ; Wagner's Handworterb. d. Physiol., III., 
 Abth. ii., p. 529 f. 
 
 4 The numbers were given by Weber in Parisian lines ; in the table they are 
 taken from Wundt, Physiolog. Psychologic, ii. , p. 7, who has reduced them to 
 even millimeters. 
 
406 THE FIELD OF TOUCH. 
 
 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 
 holds good when all the space between the impressions is filled up, 
 as it were, so as to make a continuum of localized sensations. Thus 
 Weber found that the circular form of a tube of only 1| Parisian 
 line in diameter could be recognized by pressure on the tip of the 
 tongue ; while on the skin of the abdomen the diameter of the tube 
 must reach 3f inch before its form was recognizable. Our estimate 
 of the length of lines of pressure marked out by laying rods upon 
 the skin follows the same principle. 
 
 22. Other important discoveries as to the skin's so-called 
 
NATUKE OF SENSATION-CIRCLES. 407 
 
 " sense of locality " have been made since those of Weber. Valen- 
 tin has called attention to the fact that enormous individual differ- 
 ences exist in the fineness of this kind of perception ; some per- 
 sons are not more than one-fourth as sensitive as are some others. 
 The relative degree of fineness belonging to different areas of the 
 skin remains, however, approximately the same in different persons. 
 A. W. Volkmann 1 showed the remarkable effect of exercise upon the 
 cultivation 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 
 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. 2 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 3 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. 
 
 23. The explanation of Weber's " sensation-circles " of the skin 
 has been the subject of much debate. It is natural at first to as- 
 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- 
 
 1 Berichte d. Sachsischen Gesellschaft d. Wissenschaften, 1858, p. 38 f. 
 
 2 Comp. Czermak, Sitzgsber. d. Wiener Acad., XVII., Abth. ii., p. 563 f. 
 a See Hermann's Handb. d. Physiol., III., ii., p. 382. 
 
408 THE FIELD OF TOUCH. 
 
 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 Gold sch eider'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 im- 
 pression of two localized 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 measure- 
 ment of distances on the skin. Other advocates of Weber's ex- 
 planation have spoken as though the brain could somehow become 
 conscious of the unexcited fibres lying between the two excited 
 ones, and so derive a support for its judgment from their num- 
 ber^ 2 Of course, all attempts to explanation which assume the 
 mind's knowledge of the condition of the minute subdivisions of 
 the nervous elements are wholly futile and illusory. Wundt 3 cor- 
 rectly calls attention to the fact that the differences in the so-called 
 sensation-circles of the skin are simply a special case under the 
 general psychological laws of the least observable differences in 
 sensations ; only in this case the differences are not pure differences 
 in intensity, but rather differences in the complex color-tone of the 
 quality of sensation. In other words, the sensation-circles represent 
 the local difference between the points at which stimulus must be 
 applied to the skin in order to produce enough of difference in the 
 color-tone of the resulting sensations to make them observable by 
 the mind. These local signs of the skin, as the organ of touch 
 proper, like all local signs, are complex mixtures of feeling belong- 
 ing to different localities ; as such they are dependent, not only 
 
 1 So Funke, in Hermann's Handb. d. Physiol., III., ii., p. 392 f. 
 
 2 Comp. Bernstein, The Five Senses of Man, p. 31 f. New York, 1876. 
 
 3 Physiolog. Psychologie, ii. , p. 10. 
 
THE THEORY OF VIERORDT. 409 
 
 upon original, anatomical, and physiological differences, but also 
 upon other peculiarities of the individual, upon habit, and upon 
 association with each other and with other spatial series of sensa- 
 tions of the skin. 
 
 24. Difficulty has been found 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, l this difference is chiefly due to the varying character of 
 the areas of the skin, with respect to richness in nerve-fibres, thick- 
 ness 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 
 determination of that mixture of feeling which characterizes the lo- 
 cal signs of the skin. The theory suggested by Vierordt, 2 on the 
 basis of experiments made by himself and his pupils, should also 
 be mentioned. This investigator concluded that the fineness of 
 the sense of locality belonging to any area of the skin increases 
 in direct proportion with the distance of that area from the axes 
 about which it 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 movableness of these different parts, it should be re- 
 membered that they not only all move in an enlarging circuit from 
 the shoulder- joint downward, but that each of them from the el- 
 bow-joint downward has its special increased circuit and more 
 numerous forms of motion. 
 
 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 3 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 
 exercise ; it therefore falls under the law of habit. Furthermore 
 as we have occasion to remark concerning many similar functions 
 
 1 See Medicin. Psychologic, p. 405 1 
 
 2 Pfluger's Archiv, 1869, ii., pp. 297 ff. ; and Zeitschr. f. Biologie, VI., VII., 
 IX.,X.,XI. 
 
 8 Hermann's Handb. d. Physiol. , III. , ii. , p. 384. 
 
410 THE FIELD OF TOUCH. 
 
 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 na- 
 tive to the individual. 
 
 25. 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 recent experi- 
 ments of Goldscheider * and others. We have already seen (p. 
 346 f.) that this experimenter distinguishes, more carefully than has 
 hitherto been done, the sensations of pressure from other closely 
 allied sensations coming through the same organ. 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 must be held, then, that 
 the sensations produced by laying a single blunted dividers' point 
 upon the skin, as in Weber's classical experiment, are really very 
 complex, and are composed of the sensations 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 depends, then, upon how all the 
 points irritated stand related to the specific pressure-spots. Gold- 
 scheider finds that only when two irritating points touch two press- 
 ure-spots are they felt as two. But 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 resting on the pressure-spot is 
 felt. 
 
 Moreover, the impression of being doubly touched may be ex- 
 cited by the points when lying much nearer together, in case they 
 rest upon pressure-spots that belong to two different chains of 
 such spots than when both spots belong to the same chain. This 
 is to say, pressure-spots thus located have a high degree of sensitive- 
 ness. Still further, the minimum distance required to produce a 
 sensation of being touched twice is surprisingly small, when one of 
 the touching points rests upon a pressure -spot from which the 
 chain radiates or at which it makes a sharp bend. 
 
 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 
 
 1 On this subject, see Goldscheider, Archiv f. Anat. u. Physiol., Physic-log. 
 Abth. , 1885, Supplement-Band, pp. 1-104 ; especially, p. 84 f . 
 
DIRECTION OF THE PKESSURE-SPOTS. 
 
 411 
 
 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 of l^nd 0.3-0.6 
 
 I. and II. phalanges (volar). . 0.2-0.4 
 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 
 
 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 0.51.0 
 
 From the foregoing data it would seem to follow that, as the con- 
 struction and relation of the chains of pressure-spots differ in the 
 different areas of the body, so will our sense of locality change. 
 The number, sensitiveness, and direction in the chains of these 
 spots determine the sensitiveness of a given area. Moreover, our 
 perception of the size and shape of objects in contact with the skin 
 depends upon the same conditions. This can be shown in an as- 
 tonishing way by comparing the apparent direction which the out- 
 lines of any small body moved across the skin seem to assume with 
 the way the pressure-spots are located in the different areas through 
 which it is moved. If the curve of the chain of pressure-spots, for 
 example, bends in the reverse direction from that of the outline of 
 the body moved, the effect may be to make this outline curve ir- 
 regularly or even straighten it out. 
 
 It need scarcely be said that Goldscheider regards the true ex- 
 planation of these phenomena to lie in the anatomical distribution of 
 the specific nerves of sense in the different areas of the skin. How- 
 ever this may be, it is certain that our sensations of pressure are 
 primarily "punctiform," and afterward massed into a tactual continu- 
 um ; and that what we primarily know is not the extended object as 
 such, but our sensations of pressure which are afterward objectified. 
 
 26. Closely connected with the foregoing is the difference in 
 power of different parts of the skin in giving to the mind 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 ' 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 sensi- 
 bility of the skin for motion is much greater than that for sepa- 
 rate touch, as determined by Weber's experiments. Thus, while at 
 
 1 Motor Sensations on the Skin, by Professor G. S. Hall and Dr. H. H. 
 Donaldson, in Mind, October, 1885, pp. 557 ff. 
 
412 THE FIELD OF TOUCH. 
 
 least a distance of 25 mm. between 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 aver the skin the liabil- 
 ity to error is always great ; but, as a rule, distances rapidly trav- 
 ersed 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, 1 we have a double liability to 
 error involved in regulating the muscular movement 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 millimeters, 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 centimeters. 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 the 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- 
 1 Mind, October, 1885, p. 564 f. 
 
FINENESS OF TEMPERATURE-SENSE. 
 
 413 
 
 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 touching 
 surface over the surface touched, we do not simply multiply, but 
 also diversify, our data for filling up the dermal blind-spots and 
 judging the nature of impressions. 
 
 27. 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. Eecent researches, already referred to (Chap. IV., 22), dem- 
 onstrate the fact that the relative number and arrangement of 
 heat-spots and cold-spots is different for different areas of the skin. 
 Goldscheider l has 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 sen- 
 sations 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 ap- 
 pears 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 distances are given in millimeters. 
 
 Part of the body. 
 
 Cold-spots. 
 
 Heat-spots. 
 
 Forehead, cheek, and chin .... 
 
 8 
 
 3-5 
 
 
 2 
 
 4-5 
 
 Abdomen . 
 
 1 2 
 
 4-6 
 
 Back . . . . .... 
 
 1520 
 
 4 6 
 
 Upper arm 
 
 1.5-2.0 
 
 2-3 
 
 
 2-3 
 
 2-3 
 
 Hollow of the hand 
 
 8 
 
 2.0 
 
 
 2-3 
 
 3-4 
 
 
 
 
 1 Archiv f. Anat. u. Physiol., Physiolog. Abth., 1885, Supplement-Band, pp. 
 70 ff. 
 
414 THE FIELD OF TOUCH. 
 
 Some basis seems to be laid in the foregoing facts for a system 
 of local signs of the skin, that consist in a mixture of color-tones of 
 temperature-sensations. Yet sensations of heat or cold, in them- 
 selves considered, differ chiefly, if not wholly, in intensity. In them- 
 selves, 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. 1 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 
 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 medium value of this distance is reached when these 
 points have a temperature of 20-40 C. (68-104 Fahr.) ; it dimin- 
 ishes on either raising or depressing their temperature greatly 
 above or below the zero-point of the skin. The fineness of our 
 sense of locality, as well as of our sensitiveness to motion (comp. 
 25), is increased by exciting sensations of temperature 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 ex- 
 ample, a cool hand and warm forehead, or a cool hand and a warm 
 one it is often difficult by strict attention to the sensations of tem- 
 perature 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 tem- 
 perature in the direction of the temperature of the other. It is 
 therefore induced to feel itself, as it were, as being of the tempera- 
 ture of that other. A confusion of the data for judgment, accord- 
 ingly, takes place. Any localization of the sensations which occurs 
 under such circumstances is largely dependent upon secondary con- 
 siderations, 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 sensations by changing the position of the body in space 
 as related to what we know to be hot and cold bodies or surround- 
 ing media. 
 
 1 See Czerraak, Sitzgsber. d. Wiener Acad., March, 1855, p. 500; confirmed 
 by Klug and others. 
 
JSTATUKE OF THE MUSCULAR SENSE. 415 
 
 28. 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 yei 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) So far as they are not tactual, they are to 
 be resolved into " central feelings 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 (so Wundt, and 
 others) ; (2) they are not specific sensations, but are due to interpre- 
 tations of those feelings in the skin which originate on account of 
 its changes of position, tension, etc., as the underlying muscles are 
 moved (so Schiff, and others) ; (3) they are specific sensations de- 
 pendent on a specific 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 manner dependent upon the kind, amount, and 
 direction of the muscular movement taking place (so Bell, Weber, 
 Funke, and others). 
 
 "We have already given certain reasons for rejecting the first 
 two and accepting the last of the foregoing views (see p. 344 f.) ; 
 other reasons will be mentioned subsequently in discussing the 
 so-called "feeling of innervation" or of "active energy." The 
 muscular sense, like all the other senses which contribute 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 combi- 
 nation of the muscles moved, and according to the extension 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. The series of sensations with 
 all the qualities of rapid and nice gradations which belong to 
 " spatial series " of sensations called out by moving one limb dif- 
 fers from that called out by moving another limb. At each step in 
 the flexing of the leg for example the color- tone of the muscular 
 sensations has a specific quality and value as a local sign, 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 sen- 
 sations are intimately, and even inextricably, combined with the 
 
416 THE FIELD OF TOUCH. 
 
 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 extent of the circuit of motion gone 
 through by any limb increases, or the intensity of the strain be- 
 comes greater, the quality of the mass of resulting muscular sensa- 
 tions is perpetually changing. These sensations are, accordingly, 
 localized over a broader area of the body and deeper in its sub- 
 stance, as it were. Everyone knows what new mixtures of sensa- 
 tion are produced in consciousness by calling into vigorous exer- 
 cise the unused more deeply lying muscles of the body. Bain 1 has 
 discussed these sensations at great length and with commenda- 
 ble acuteness. But the apparent assumption that these particu- 
 lar sensations can, by being associated, acquire of themselves the 
 quality of extension in space, and the accounting for all our other 
 perceptions of spatial qualities and relations as merely secondary 
 and symbolic of the associated muscular sensations, are in plain 
 contradiction of established psychological facts and principles. 
 
 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 a 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 
 utmost nicety of knowledge possible to " touch," in the widest 
 meaning of the word. 
 
 29. It is unnecessary to illustrate in further detail the process 
 by which the mind, with its native synthetic activity, and with 
 the help of qualitatively different sensations, constructs its field of 
 
 1 The Senses and the Intellect, especially pp. 57-100, 336-348, and 364- 
 398. 
 
 2 Mind, October, 1885, p. 567. 
 
RELATIONS OF EYE AND HAND. 417 
 
 touch. To multiply instances would neither explain the ultimate 
 mystery which enters into the processes of " localization " and " ec- 
 centric projection " by touch, nor add materially to our compre- 
 hension of known psycho-physical principles. The muscular sense 
 may probably be said to have the leading position in the develop- 
 ment of the perception of spatial objects and relations, so far as 
 attainable without the aid of sight. Perceptions of the magnitude, 
 distance, and primary spatial qualities such as the extension and 
 inertia of material things are largely dependent upon associated 
 sensations of the muscular sense, although these perceptions cannot 
 be said to be mere compounds of such sensations with secondary 
 and symbolic sensations of other kinds added to them. The activ- 
 ity of the hand, as it moves over various surfaces of the body, either 
 touching them itself or carrying with it something with which these 
 surfaces are touched, early combines with the different series of 
 muscular sensations other spatial series of tactual sensations. 
 
 The localization of certain points in the area of the body which 
 are of marked local characteristics, and frequently recurrent in ex- 
 perience, is the first achievement in constructing the field of touch. 
 To these landmarks, as it were, other points or areas, subsequently 
 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 mo- 
 tion fresh combinations of muscular and tactual sensations result. 
 
 30. But long before the entire field of touch has been con- 
 structed with any considerable approach to completeness, the eye 
 has already explored those parts of the body which are open to its 
 inspection. 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 
 27 
 
418 THE FIELD OF TOUCH. 
 
 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 
 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. 
 
 31. 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 w r ould 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 
 
FEELINGS OF DOUBLE CONTACT. 419 
 
 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 
 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 di- 
 rectly dependent upon the other end of the implement. Upon the 
 aesthetic and pleasurable uses of these feelings of double contact 
 Lotze ' 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. 
 
CHAPTEE VII. 
 THE PRESENTATIONS OF SENSE. [CONTINUED.] 
 
 1. THE application of the general principles which control the 
 development of our presentations of sense to the particular case 
 of the eye has many peculiar difficulties. The physiological psy- 
 chology 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. For here, as elsewhere in scientific research, one 
 chief result of extended examination is to raise unanswered in- 
 quiries. Peculiar difficulties, however, are intrinsic in the case of 
 the eye. These are due to the great complexity of its native 
 activities, and to 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 provided it with such constant stimulation as to cause 
 it to acquire an incomparable facility. But the character of its 
 structure, functions, and development is such as to make experi- 
 ment difficult in a way to disentangle the simple factors from those 
 complex forms into which the synthetic activity of the mind has 
 constructed them. 
 
 2. It is affirmed by one authority ' that no less than eight 
 different data, or motifs, are used 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 dependent on distance ; (3) the perspective elevation of 
 the bottom of distant objects above the horizon; (4) the covering 
 of known distant objects by those placed nearer ; (5) the alter- 
 ations 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 distance of the object ; (8) the muscular sen- 
 1 Volkmann von Volkmar, Lehrb. d. Psychologic, II. , p. 84. 
 
PROBLEM OF VISUAL PERCEPTION. 421 
 
 sations of the accommodation of the eye. To these eight data, two 
 others at least must be added for binocular vision ; namely (9), the 
 stereoscopic double_Jmages, and (10) the sensations arising from 
 convergence of the axes. These ten sets of variable experiences may 
 be cornbinecf7"6"Fcourse, 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 
 tactual, as well as muscular, sensations which accompany the move- 
 ment of the eyeballs in their sockets are not ineffective in giving 
 grounds for judgment in certain cases. The question must also 
 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. And 
 after all this cataloguing of data, the dispute as to the existence of 
 series of sensations of innervation that have a central origin and 
 differ only in intensity as directly dependent upon so-called acts of 
 will (so Wundt) remains unsettled. 
 
 Several of the data just enumerated, however, are plainly of only 
 secondary rank and value ; they do not necessarily enter into every 
 preception of a visual object as such. What does seem necessary 
 to the most elementary form of visual perception may be stated as 
 follows : Sensations of light and color, differing in intensity and qual- 
 ity, but simultaneously present in consciousness, must be systematically 
 arranged with reference to each other by being localized with the help 
 of retinal signs, and associated with other spatial series of muscular 
 sensations that arise from accommodation of the eye and from its 
 motion. The complexity of the combinations 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 sj'stems 
 of retinal signs, two images of each object, and two sets of motions. 
 But the two eyes are (as we shall see subsequently) 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. 
 
422 DATA OR MOTIFS OF VISION. 
 
 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. We should wish, before either eye had been 
 moved when open, to excite the nervous elements of a small area of 
 one stationary retina, and to ascertain how far the sensations of light 
 and color thus excited could be said to have any strictly "local" 
 arrangement with reference to each other. We should then wish 
 to try the effect of combining with these sensations other spatial 
 series, consisting of muscular sensations and arising from the ac- 
 commodation and motion in its orbit of the same eye. Finally, the 
 intricate process of putting the two eyes together both open and 
 both moving might be studied in detail. At present, however, 
 it is quite impossible to say what the experience of the subject of 
 such experiments would be. The testimony of the few blind per- 
 sons whose eyes have been couched is so meagre and unsatisfac- 
 tory, on account of its failure to comply with the conditions of 
 scientific investigation, that it can be used only to confirm con- 
 clusions arrived at on other grounds. Nothing remains, then, but 
 to employ the data which physiological optics has secured, in order 
 to make a theoretic reconstruction (confessedly imperfect and doubt- 
 ful) of the process that nature is all the while successfully com- 
 pleting. In this effort we naturally follow the order of nature, so 
 far as possible ; we begin with the simplest conceivable case, and 
 proceed from it to the explanation of the amazing complexity which 
 really belongs to our apparently simple daily experience of vision 
 with two trained eyes. This is substantially the course followed 
 by Wundt, 1 who finds 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 
 1 Physiolog. Psychologic, ii. , p. 62. 
 
NATURE OF THE RETINAL FIELD. 423 
 
 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 theo- 
 retically constructed in order to explain the process by which the 
 mind reaches the construction of the third and last. Indeed, the 
 question may be pressed, whether w r e 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 muscular sensations 
 of the eye, could furnish any presentations of sight. 
 
 4. The most nearly original experience of sensations of light 
 and color which can be easily produced for adult observation is 
 gained by closing and blindfolding both eyes, and then keeping 
 them as motionless as possible. Let time enough be allowed for 
 all the after-images, both positive and negative, to die wholly away. 
 Nothing is then seen but a small and undefined expanse or massive 
 aggregate of related color-sensations, which we will call the " ret- 
 inal field ; " it might almost be said that this is felt rather than 
 seen. Such " vision " (?) of a certain continuum of sensations can- 
 not be said to be either localized or projected in space, as a whole, 
 and by the eye alone. When we speak of it, for example, as " in 
 front of " the upper part of the body, we introduce terms that are 
 derived from experiences of touch. Now, without moving or un- 
 covering the eyes, let the head be turned to the right or to the 
 left, and the expanse of color-sensations will move in the same 
 direction ; or, if we turn the face upward, the retinal field seems 
 above us ; if downward, then it seems to sink toward our feet. But 
 each position of this field, as a whole, is entirely determined by the 
 fact that the customary muscular and tactual sensations assure us 
 of the posture of the head with reference to the rest of the body. 
 Such localization is accomplished chiefly by sensations in the neck. 
 So far as sight alone is concerned, the entire expanse of color-sen- 
 sations cannot be said to be perceived as anywhere in space. 
 
 The "retinal field" has no clearly defined limits, or boundary- 
 lines ; it may be described rather as having its expanse of sensa- 
 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- 
 
424: DATA OR MOTIFS OF VISION. 
 
 tions which are massed in the foregoing experience constitute a 
 true spatial expanse ; they are not simply recognized as differing 
 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 simultaneous!} 7 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, as a true perception of an extended object. The " ret- 
 inal field " may, then, be said to be extended in two dimensions ; 
 and the minima wsibilia 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 ^m^g^- i" 
 color-tone and brightness of the minute^jjpr tions of the field, 
 which has an effect somewhat like that we geton looking at a very 
 dens'eTjnist oFjjarticlea diSerentJy^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. 
 
 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 " (see p. 195 f.) be produced in either eye by pressing 
 upon its ball, the colored circle will be located in the correspond- 
 ing part of the field ; but the character of the entire field, as formed 
 by the activity of both retinas, will be changed. It is, of course, 
 impossible to suppress the action of one retina, and thus exam- 
 ine 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 motionless now be opened. Immediately a 
 
 1 Ueber d. physiolog. Ursprung d. Raumvorstellung. Leipzig, 1873. Stumpf 
 holds that "Space is just as originally and directly perceived as quality" (p. 
 115). 
 
 5 In Hermann's Handb. d. Physiol., IIL, i., p. 572 f. 
 
NATURE OF THE RETINAL FIELD. 425 
 
 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 determined according to the law r s of visual 
 perception. This monocular 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 " phosphenes " in the former. On 
 pressing the closed eye brightly colored circles are presented in 
 the corresponding part of the field ; and by using sufficient press- 
 ure 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 direc- 
 tion 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. The conclusion, then, 
 is, that certain muscular sensations constitute an indispensable 
 part of the data for localizing objects even in the retinal field at 
 least such as are only a slight distance from its centre. Moreover, 
 it will be found that the extent of this entire field and its pro- 
 longation, as it were, in any given direction are dependent upon 
 the accommodation and motion of the eyes. The explanation of 
 this fact can be found only in the same truth, namely : The per- 
 ception of localized areas in the two-dimensioned " retinal field " of the 
 closed eyes is dependent upon the revival of associated muscular sen- 
 sations. 
 
 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. They do not even prove the truth of Wundt's statement : ' 
 1 Physiolog. Psychologic, ii., p. 69. 
 
426 DATA OK MOTIFS OF VISION. 
 
 " Our sensations of light do not immediately possess spatial form." 
 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 pre- 
 vious 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 ret- 
 ina, 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. IV., 3) 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 the spa- 
 tial quality of our visual percepts depends upon its specific functions. 
 
NATTJEE OF THE RETINAL IMAGE. 427 
 
 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 motionless 
 eyes is only the residuum, as it were, of past sensations of a muscu- 
 lar kind. Such an appeal to consciousness could not be made, in- 
 deed, 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. 1 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. 2 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 
 
 1 See Le Conte, Sight, pp. 85 f. and 105. New York, 1881. Le Conte is 
 obliged to admit, however, that this " law of direction " is sometimes opposed 
 to the "law of corresponding points " (p. 258). 
 
 2 See Wundt, Physiolog. Psychologie, ii., p. 68. 
 
428 DATA OR MOTIFS OF VISION. 
 
 to irritations from light could not be said to have what we call 
 " vision." But, on the other hand, the spatial quality which be- 
 longs to the visual sensations of man cannot be all resolved into 
 muscular and tactual sensations of eye and hand ; these sensations, 
 quoad sensations of light and color, do have the quality which in- 
 sures 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 dif- 
 ferent 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 op- 
 tics cannot account for the phenomena of vision without assum- 
 ing 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 ca'se 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. 183), 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 
 accompanied by sensations of motion and strain ; they are accom- 
 plished by the six muscles of the eyeball. This wandering of 
 the point of regard over an object may be considered as accom- 
 plished by rotating the eye upon a pivotal point, or " centre of 
 rotation," by motions that have different axes of rotation. The 
 centre of rotation is, however, only theoretically a point, but is 
 really an interaxial space. It has been variously located for normal 
 eyes at about 13.45-13.73 mm. behind the cornea, and 1.24-1.77 
 
PRIMARY POSITION OF THE EYE. 
 
 429 
 
 mm. behind the middle of the optical axes. Of such axes of rota- 
 tion, 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 cen- 
 tre of rotation, there are, in vi- 
 sion with both eyes, two lines of 
 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 re- 
 gard of both eyes in this position 
 is the " primary plane of vision." 
 In this position for most eyes, 
 however, the line of vision is in- 
 clined somewhat below the hori- 
 zontal plane. 
 
 Starting from the primary po- 
 sition, one set of positions are suc- 
 cessively assumed by moving the eye upon its transverse and verti- 
 cal 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 " ascen- 
 sional 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 dis- 
 placement." In passing from the primary position to the foregoing 
 secondary position no rotation of the axis itself occurs. Another 
 order of positions is assumed by an apparent rotation on the antero- 
 posterior axis, combined with lateral or vertical displacements ; 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." 
 
 1 For the detailed theory of the movements of the eye, see Hering, in Her- 
 mann's Handb. d. Physiol., III., i., chaps. 9-11 ; Helmholtz, Physiolog. Op- 
 tik, 27-30 ; and Wundt, Physiolog. Psychologic, ii., p. 72 f. 
 
 FIG. 97. Diagram of the Attachments of the 
 Muscles of the Eye, and of their Axes of Ro- 
 tation 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. 
 
430 
 
 DATA OR MOTIFS OF VISION. 
 
 9. The various movements possible for the eye in all the direc- 
 tions just described are accomplished by the combined pull of the 
 muscles of the eye as summarized in the following table * (for the 
 muscles and their position, see Fig. 49 f., p. 174) : 
 
 Number of muscles active. 
 
 
 Direction of line of regard. 
 
 Muscles acting.. 
 
 On6 
 
 
 Inward 
 
 Internal rectus. 
 
 
 
 Outward 
 
 External rectus 
 
 
 
 Upward 
 
 j Superior rectus. 
 
 Two 
 
 
 
 / Inferior oblique. 
 
 
 
 Downward 
 
 j Inferior rectus. 
 
 Three 
 
 
 'Inward and upward, . . . 
 Inward and downward . . 
 
 (Superior oblique. 
 Internal rectus. 
 \ Superior rectus. 
 ( Inferior oblique. 
 ( Internal rectus. 
 j Inferior rectus. 
 ( Superior oblique. 
 
 
 
 Outward and upward . . 
 ^ Outward and downward. 
 
 ( External rectus. 
 s Superior rectus. 
 ( Inferior oblique. 
 ( External rectus. 
 A Inferior rectus. 
 ( Superior oblique. 
 
 10. The law which seems to govern all 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 a in the fol- 
 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 : 3 "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. Concerning 
 one important matter in the carrying out of Listing's law, there is 
 a direct conflict of view between authorities. According to Helin- 
 
 1 Given by Beaunis, and to be found in the Encyclopaedia Britannica, 
 ninth ed., VIII., p. 825. 
 
 2 Physiolog. Optik, p. 466. 3 Physiolog. Psychologic, ii., p. 79 f. 
 
THE EFFECT OF ROTATION. 431 
 
 holtz,' when the plane of vision is raised, lateral displacements to 
 the right produce rotation of the eye to the left, and lateral dis- 
 placements to the left produce rotation to the right ; when the 
 plane of vision is depressed, lateral displacements to the right pro- 
 duce rotation to the right, and vice versa. But according to Le 
 Conte, 2 in elevation of the visual plane the eyes both move and ro- 
 tate to the right or to the left ; in depression of this plane, motion 
 of the eyes to the right is accompanied with rotation to the left, 
 and motion to the left with rotation to the right. 
 
 More detailed statement of the laws of the eye's motion in vi- 
 sion is not necessary for the purposes of physiological psychology. 
 It need only 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 series of muscular 
 sensations that have spatial qualities and are fitted to combine with 
 the spatial series of light and color sensations. 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 pro- 
 gressive synthesis of the mind, stimulated and guided by means which 
 consist in varying states of consciousness, chiefly dependent upon 
 the local coloring of the two sets of sensations thus far described. 
 
 11. 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 
 retinal elements and nerve-fibres of the muscles of the eye. Both 
 the general form of the field of vision and the relative position of 
 the objects in it are determined by the movements of the eye. The 
 rule is, that 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 ; objects indirectly seen ap- 
 pear in the place which they would assume if their retinal images 
 were transposed to the point of regard and its immediately sur- 
 rounding points. 3 It follows, further, that all 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 
 
 1 Physiolog. Optik., p. 463. 
 
 8 Sight, pp. 173 ff. ; and American Journal of Science and Arts, xx. (1880), 
 pp. 83 ff. 3 Comp. Wuiidt, Physiolog. Psychologie, ii., p. 90 f. 
 
432 DATA OR MOTIFS OF VISION. 
 
 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 
 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-im- 
 
 e\ 
 
 FIG. 98 (From Herinpf. after Helmholtz). With the eye at the distance e-e, and fixated upon the 
 centre, the hyperbolic lines which limit the black and white surfaces show the eo-called ' ' right 
 lines " of the field of vision. 
 
 ages will themselves be found to suffer torsion. 1 Such images, re- 
 ceived upon the vertical meridian of the eye when it is in its pri- 
 mary position, lean to the right, thus / , when the visual plane is 
 elevated and the eye moved to the right ; but when, with this 
 plane elevated, the eye is moved to the left, the vertical image in- 
 clines to the left, thus V . With depression of the visual plane, 
 
 the inclination of the after-image is reversed. The image of a 
 
 perfect rectangular cross is distorted as follows by different torsions 
 
 1 See Le Conte, Sight, pp. 164 ff. 
 
SENSATIONS OF ACCOMMODATION. 433 
 
 of the eye : Upward and to the right, -^-". ; upward and to 
 
 i ' \ 
 
 the left, ^K ; downward and to the right, *^X^ ; down- 
 
 V. '/ \ 
 
 ward and to the left, -<jf%- The connected results of all the 
 
 possible torsions of the eyes in curving the lines of the field of vis- 
 ion is illustrated by the accompanying figure (98) ' ; the study of 
 this figure, as it appears at various distances, from arm's length to 
 contact with the nose and forehead, is an instructive exercise. The 
 dependence of the field of vision upon the positions and motions 
 of the eye is one principal source of the errors of this sense. 
 
 12. Besides the help from muscular sensations due to move- 
 ments of the eye in fixing its point of regard, account must be taken 
 of those which result from accommodation of the eye (for the 
 mechanism of accommodation, see p. 177 f.). As says Helmholtz : a 
 " There can be no doubt that anyone 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 this change of muscular feeling would not be 
 realized as indicating a third dimension of space, were it not com- 
 bined with sensations belonging to the use of both eyes in conjunc- 
 tion with the organs of touch. Even adult judgment of distance, 
 by accommodation alone, is extremely imperfect. Wundt 3 experi- 
 mented to determine the niceness 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 
 considerable accuracy by changes in accommodation ; and, as might 
 be expected, with more accuracy when the apparatus was called 
 into more active operation by approach of the object toward the 
 eye. Helmholtz 4 found that he required a stronger accommoda- 
 tion to see a red stripe clearly through a tube than was necessary 
 to see one of blue. 
 
 1 Taken from Hering (after Helmholtz), in Hermann's Handb. d. Physiol. , 
 III., i., p. 537. 
 
 2 Physiolog. Optik, p. 633. 
 
 3 Beitrage zur Theorie d. Shmeswahrnehmung, 1862, pp. 105-118. 
 
 4 Ibid., p. 634. 
 
 28 
 
434 
 
 BINOCULAR FIELD OF SIGHT. 
 
 13. 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 
 consideration of two sets of data in addition to those already enumer- 
 ated. These are the existence and relations of the two retinal images, 
 and the relations and laws of the binocular movements of the eyes. 
 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 direc- 
 tion intersect. J It may be assumed, 
 to begin with, that this point of in- 
 tersection is the same for accom- 
 modation to all distances of the 
 object. If the two retinas were 
 perfectly symmetrical all the ner- 
 vous elements which compose the 
 mosaic of each one might be re- 
 garded as situated at points identi- 
 cal with those occupied by the ner- 
 vous elements of the other. In 
 other words, the surfaces of the 
 two retinas might be regarded as capable of being perfectly super- 
 imposed. Upon such retinas, when the eyes were parallel, each sin- 
 gle point of an object would have its image formed upon two " iden- 
 
 1 See Bering, Physiolog. Optik, in Hermann's Handb. d. Physiol. , III., i., p. 
 349 f. 
 
 a 
 
 FIG. 99. Diagram to illustrate the theory 
 of corresponding retinal points. The im- 
 ages of objects at a" or b" or c" will fall 
 on corresponding points of the retina 
 a and a', 6 and &', c and c 7 and be seen 
 single. 
 
IDENTICAL AND CORRESPONDING POINTS. 435 
 
 tical " 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. If the eyes . be fixated upon any 
 very remote object without apparent magnitude for example, a 
 star the points of two retinas upon which its image falls when rfc 
 is seen as single are "corresponding." One image then exactly 
 covers the other. But 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 cor- 
 responding sometimes fail to cover each other. Hence, a distinc- 
 tion may be made between corresponding points and " covering 
 points ; " the latter term being used^ for those points whose im- 
 pressions, 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. 2 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. 
 
 14. 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. 3 If we hold 
 
 1 By Wundt, Physiolog. Psychologie, ii. , p. 122 f . 
 
 2 Here see Hering, in Hermann's Handb. d. Physiol., III., i., pp. 355 ff. ; 
 and Helmholtz, Physiolog. Optik, pp. 695 ff. 
 
 a See, especially, the ones described by Le Conte (Sight, pp. 92 ff.), from 
 which the immediately following are taken. 
 
436 BINOCULAR FIELD OF SIGHT. 
 
 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 trans- 
 parent images of the finger, rather than 
 one solid finger, will be seen. Many 
 persons may have difficulty in seeing 
 the 100 images, but none will fail to no- 
 tice their transparent character. Under 
 these circumstances the wall, sky, or 
 distant object, may readily be seen 
 through the finger. By experimental 
 methods the images of a single object 
 may be dissociated, and what is really 
 one be seen as two ; on the other hand, 
 images coming 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 press- 
 ure upon one eyeball to create for us 
 
 FIG. 100. Diagram to illustrate phe- . , , , , 
 
 nomena of double vision. If the the QOUDie Ol eacn One Ol a gl'OUp Ol 
 
 image of the point 6 fall in one eye .T -i > 11 j- n 
 
 on 6, and in the other on 7, the dis- friends, and to SC6 O116 body partially 
 
 *S through the transparent image of an- 
 
 other. 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- 
 gratingmay have all their images combined by slipping them, as 
 it were, simultaneously to one side. There is, then, a double-seeing 
 of what is really single and a single-seeing of what is really double ; 
 but the latter is much rarer than the former, and seldom occurs 
 except when brought about for purposes of experiment. 
 
 15. 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 sin- 
 gle. Points of the object lying near to the point of regard in any di- 
 rection, and thus having their images formed close to the centres of 
 
CALCULATION OF THE HOROPTER. 
 
 437 
 
 the two retinas, are also seen single. For the points of the retinas 
 on which the images are then formed, although not strictly identi- 
 cal, are corresponding ; that 
 is, they have habitually act- 
 ed together in seeing ob- 
 jects single by binocular 
 vision, and the slight incon- 
 gruousness of the two sets 
 of images is disregarded, as 
 it were, by the mind. But 
 all objects lying nearer or 
 more remote than the point 
 fixated by the eyes are liable 
 to be seen double ; for their 
 images do not fall on corre- 
 sponding points of the re- 
 tinas. 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 oc- 
 cupy 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. 
 
 There has been a great amount of calculation, experiment, and 
 discussion, to determine the exact nature of the horopter. It has 
 been held to be a surface (plane or curved), a circle, a line, a num- 
 ber of disconnected points. Its calculation as a matter of mathe- 
 matics is unsatisfactory, for the really corresponding points of any 
 two retinas are not to be determined by mathematics. Experiment 
 is made exceedingly difficult by the indistinctness with which we 
 see objects that do not lie near the point of regard. No conclusions 
 regarding the nature of the horopter are, perhaps, on the whole, 
 more trustworthy than Meissner's. 1 They are thus summarized by 
 Le Conte. 2 With the eyes in the primary position, the horopter is 
 
 1 Beitrage zur Physiologie d. Seuorgans, Leipzig, 1874 ; and Archives des 
 Sciences, iii. (1858),' p. 1601 
 
 2 Sight, p. 204. 
 
 FIG. 101 (From Hering). //, the sash of the window, 
 and p the black spot fixated. On the left line of 
 vinion / 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, therefore, on corresponding points of the two 
 retinas. 
 
438 BINOCULAR FIELD OF SIGHT. 
 
 a plane perpendicular to the median line of sight. For all nearer 
 points in the primary plane, it is a line which dips toward the ob- 
 server with an inclination to the visual plane, increasing with the 
 nearness of the point of regard. When the plane of vision is turned 
 upward, the inclination of the horopteric line increases ; when the 
 plane is turned downward, the inclination of the line decreases 
 until it becomes zero at 45, and the line expands into a plane. 
 The plane of the horopter, then, passes through the point of regard 
 perpendicular to the median visual line. With these conclusions 
 the careful experiments of Le Conte himself correspond in the 
 main ; but Le Conte considers that the inclination of the horopteric 
 line remains constant, and that its surface, when the horopter be- 
 comes a surface, is curved instead of a plane. 
 
 16. The existence and relation of the two images in binocular 
 vision is of the greatest importance for all perception of solid 
 objects set at varying distances from each other. It is largely by 
 their help that binocular perspective and stereoscopic vision are 
 explicable. But all such elaborate and complex presentations of 
 visual sense require for understanding them certain considerations 
 concerning binocular movements of the eyes. In binocular move- 
 ments the laws of parallel motion hold good only for the case when 
 the eyes, being in the primary position, are both turned equally in 
 the same direction. But in fixating the point of regard for the two 
 eyes for a near object the eyes move in opposite directions, so that 
 the lines of vision may converge upon the object. In convergence 
 the eyes rotate on the optic axis in opposite directions. 1 Since 
 divergence of the eyes in visual activity is, in all ordinary cases, 
 impossible, there are three customary indissoluble conjunctions of 
 motion which belong to the eyes as under control from the central 
 nervous organism ; these are, right and left together, up and down 
 together, or turning symmetrically inward. In lowering the plane 
 of vision, as well as in fixating the point of regard upon near 
 objects, convergence naturally takes place ; in elevating this plane 
 or in looking upon distant objects, the converging lines of regard 
 diverge toward a parallel position. Convergence may be " sym- 
 metrical " or " asymmetrical ; " in the former case the two lines of 
 regard are turned inward at equal angles and the point of regard 
 is kept in the median plane of vision ; in the latter case the point 
 of regard is outside of the median plane, and either the two eyes are 
 turned at unequal angles inward, or else one is turned inward, and 
 the other, at a smaller angle, outward. Both kinds of convergence 
 are possible at different angles of the elevation of the plane of vision. 
 1 See Le Conte, Sight, p. 178 f. 
 
THE INFLUENCE OF ATTENTION. 439 
 
 Listing's law does not hold for movements of the eyes in conver- 
 gence. 1 The principal points at which this law is abrogated for 
 converging motion of the eyes are stated thus by Le Conte : 2 
 When the right eye moves to the left in convergence, it rotates to 
 the right instead of to the left as in parallel motion ; so the left eye 
 rotates to the left when turning inward. Whereas in parallel mo- 
 tion the torsion of the eye increases with the angle of the depres- 
 sion of the plane of vision, in convergent motion it decreases to 
 zero at 45. These facts doubtless result in imparting variety of 
 local coloring to those sensations of strain, etc., which are produced 
 in the two kinds of motion of the eyes, and which serve the mind 
 as local signs in its synthesis of extended visual objects. 
 
 Changes of accommodation naturally accompany the changing 
 convergence of the eyes for near objects, and the resulting sensa- 
 tions enter into the spatial series out of which the presentations of 
 visual sense are constructed. In the alteration of the indices of re- 
 fraction, and in the contraction of the pupils, the eyes act together 
 under the influence of motor impulses from the central nervous 
 organs. 
 
 17. An effort to see, and a corresponding fixation of the atten- 
 tion upon the object lying at the point of regard, are implied in 
 the convergence of the eyes. The eyes of new-born children and 
 eyes that are recently couched after long-continued blindness move, 
 as a rule, in parallel lines. 3 Arrest of attention brings the two 
 eyes into use as one organ, and this necessitates the turning of the 
 lines of vision of both so that they shall meet at a common point 
 where lies their common object. It follows, also, that the sensa- 
 tions accompanying innervation of their muscles so as to produce 
 convergence are of capital importance in the construction of the 
 most elaborate and intelligent visual presentations. According to 
 Wundt, these " feelings of innervation " are the direct expression 
 in consciousness of the cerebral changes that accompany the initiat- 
 ing of motor impulses in the central organs. They differ only in 
 intensity or amount. It is by the " feeling " of this amount, as it 
 were, that our knowledge of the size and distance of the object 
 seen in convergence is obtained. Wundt's view 4 of the nature and 
 origin of the feelings of innervation, however, is unsatisfactory. 
 
 In the opinion of Hering, 6 the innervation of both eyes is equal, 
 
 1 Comp. Hering, in Hermann's Handb. d. Physiol., III., i., p. 497 f . ; and 
 Le Conte, Sight, pp. 177 ff. 2 Sight, p. 190. 
 
 3 Comp. Bonders in Pfliiger's Archiv, xiii., p. 383. 
 
 4 See Physiolog. Psychologie, ii., p. 118 f. 
 
 6 Physiolog. Optik, Hermann's Handb. d. Physiol., III., i., p. 519 f. 
 
440 BINOCULAR FIELD OF SIGHT. 
 
 however they are moved with relation to each other. Even when 
 the movements of the two are unequal, the law holds ; for each eye 
 is then under the influence of two innervations, one of which is 
 directed toward turning both eyes right or left, and the other 
 toward turning them inward or outward. As a result, in one eye 
 the two innervations would support, and in the other eye oppose, 
 each other thus bringing about a compensation. In this way the 
 will guides its pair of horses in either direction by a pull upon one 
 rein. The innervation for accommodation is also supposed to be in 
 like manner bilateral and uniform. Whatever view may be taken 
 of the foregoing theories as to the distribution of central innerva- 
 tion to the two eyes and as to the origin of so-called " feelings of 
 innervation," there can be no doubt that the mental representatives 
 of the different areas passed over and positions reached, in both 
 parallel and converging motions, are important factors in construct- 
 ing the presentations of sight. 
 
 18. By the various helps already described, stereoscopic vision 
 and the seeing of things in perspective are made possible. To one 
 eye acting alone and without previous experience, only one of the 
 spatial series possible could, in any event, serve as a suggestion of 
 depth ; this is the series of muscular sensations accompanying the 
 accommodation of the eye to near distances. How little such sen- 
 sations of themselves can accomplish, even at the end of years of 
 experience in binocular vision, the experiments of Wundt make 
 obvious (already alluded to, p. 433). Our localization of objects 
 by one eye, with respect to the third dimension of space, is con- 
 fessedly very imperfect even under the best of circumstances. It 
 is probable, then, that the field of monocular vision is directly 
 known only as a plane, and that all immediate perception of depth 
 depends upon the existence of double images and muscular sen- 
 sations derived from the movements, especially in convergence, of 
 the two eyes. 
 
 The stereoscopic and perspective vision which takes place, with 
 apparent immediateness, even when one eye is closed, is therefore 
 really mediate and indirect ; it is accomplished solely by second- 
 ary means of varying intensities of light and color, changes in 
 apparent magnitude, etc., on the basis of associations gained by 
 using both eyes and the hand. Accordingly, it is easy to reduce 
 all the objects seen in the field of monocular vision to one depth 
 to flatness outlined on the same plane by cutting off these 
 secondary helps and withdrawing attention as much as possible 
 from the influence of judgment based on experience. By nearly 
 closing one eye while the other is wholly shut, objects really 
 
THE PERCEPTION OF DISTANCE. 441 
 
 situated at different distances from the head may easily be made 
 to appear as patches of light and color blended, indistinguishably 
 to the visual perception, with other patches of the retina's own 
 light. That is to say, when the results of experience in interpret- 
 ing the secondary signs of the third dimension are withdrawn, the 
 field of monocular vision becomes as purely two-dimensioned as is 
 the "retinal field." 
 
 19. There is no doubt that the double images, and the muscu- 
 lar sensations resulting from binocular movement, furnish motifs 
 for the immediate perception of the distance and solidity of ob- 
 jects. In other words, these two spatial series are most important 
 data for constructing visual presentations of objects having the 
 third dimension. It is more doubtful just how this service is 
 rendered. Stereoscopy has made the fact familiar, that the two 
 images of each object are different as furnished by the two eyes. 1 
 The right eye sees the object farther around on its right side, the 
 left eye on its left. Every small portion of a solid object, as seen 
 in binocular vision, provided it lies a little way out of the point of 
 regard, instead of consisting of two exactly similar sets of lines 
 which might be superimposed, consists of two sets of minute 
 curves that are partial images of its lines and are different for each 
 eye. The constant and uniform objects of sense which appear 
 through the use of both eyes result, therefore, from uniting a great 
 number of varying partial images of these objects due to simul- 
 taneous excitation of both retinas. In some manner or other the 
 perception of solidity is substantially aided by the combination of 
 these partial images. 
 
 Furthermore, in ordinary binocular vision, our perception of the 
 solidity and distance of objects is accomplished largely by motion 
 of the eyes which successively unites and separates the double im- 
 ages of the objects seen. In viewing all objects of any size, whether 
 near or distant, we may readily become conscious of the fact that 
 we are engaged in sweeping over the field of vision with a moving 
 point of regard. Even when we suppose the eye to be looking at a 
 single point, with a perfectly fixed regard, it is actually making 
 short and rapid excursions in one direction and another around 
 this point. How difficult it is to keep the organ of vision perfectly 
 motionless, anyone knows who has tried to hold steady one of the 
 floating specks (muscce volitantes) situated in, and projected in the 
 
 1 For the study of the theory of stereoscopy as a matter of optics, the reader 
 is referred to treatises on this science ; a brief allusion to the fact is enough 
 for our purpose, which primarily is, of course, to illustrate the psychology of 
 visual perception. 
 
442 BINOCULAR FIELD OF SIGHT. 
 
 air before, this organ. Such facts strengthen the theory of Briicke 1 
 and others, that we gain our perception of depth by running the 
 point of regard back and forth with a varying degree of conver- 
 gence to the axes, and so combining successively the different parts 
 of the two pictures as seen by the two eyes. 
 
 But that motion is not necessary for stereoscopic vision with 
 adult eyes is proved by what is known as " Dove's experiment." 
 A field composed of different solid objects stationed at different 
 distances in space, or of two stereoscopic pictures, may be seen in 
 perspective when illuminated by the light of an electric spark. 
 Since the duration of this spark is perhaps not more than 2r!o"o 
 sec., it is plain that no change of convergence, or running back and 
 forth of the point of regard, has time to take place. It is asserted 
 by Le Conte 2 that the interpretation of the double images depends 
 upon the fact that such images of any object are different according 
 as the object lies nearer or more remote than the point of regard. 
 In the latter case, the double images are called " homonymous," 
 and are united by less convergence ; in the former case they are 
 called " heteronymous," and are united by greater convergence of 
 the optic axes. Now the observer knows, "instinctively and without 
 trial" whether greater or less optic convergence will be necessary 
 to unite the double images ; and accordingly refers the homony- 
 mous images to objects beyond, the heteronymous images to objects 
 this side of, the point of regard.- But the question arises, How does 
 this so-called " instinctive " knowledge come? It can scarcely be 
 by way of a native insight into the distinction between homony- 
 mous and heteronymous images, as such ; or through any seeing of 
 both retinal images by the mind's eye, 3 as it were. Since what is 
 needed to unite the images is motion of the eye, and since the 
 mind has always been accustomed to associate sensations of motion 
 with the double images of binocular vision, it is impossible to avoid 
 the conclusion that instantaneous binocular vision, like monocular 
 vision, of solidity and distance, is secondary and wholly dependent 
 upon previous experience acquired with both eyes in motion. 
 
 20. Localizing of the third dimension is, accordingly, much 
 more secure in binocular than in monocular vision ; and judgments 
 of distance are assisted greatly by movements of both eyes. If 
 
 1 Archives des Sciences, iii. (1858), p. 142. 
 
 2 Sight, p. 151 ; and Am. Journal of Science and Arts, ii., 1871, p. 425. 
 
 3 To say that "each eye, as it were, knows its own image, although such 
 knowledge does not emerge into distinct consciousness," is in plain contradic- 
 tion with all the fundamental laws which psychology has to propound con- 
 cerning the nature of visual perception. 
 
INTERPRETATION OF DOUBLE IMAGES. 443 
 
 no other motif for seeing depth of space is present, according to 
 Hering l the following law seems to hold : All the lines or points 
 whose images lie, with a given position of the point of regard, in the 
 vertical horopter, appear clearly defined on a surface which is either 
 plane or slightly cylindrical, and all the lines or points lying this 
 side of the surface of the vertical horopter and whose images have 
 a " crossed disparateness " (that is, the left one of the double im- 
 ages belongs to the right eye, and the right one to the left eye 
 making them " heteronyrnous "), appear in front of the surface ; 
 while those lying beyond the horopter and whose images have an 
 " uncrossed disparateness " (that is, the right image belongs to the 
 right eye, and the left image to the left eye making them " hom- 
 onymous "), appear behind the surface on which whatever lies in 
 the horopter is seen. But, as we have already learned, interpreta- 
 tion of the double images for the stationary eyes is an acquired art, 
 which is dependent upon previous association of the retinal signs 
 of both eyes with muscular sensations arising from the innervation 
 and movement of the eyes. It is also in perfection of practice de- 
 pendent, as all stereoscopic vision is, upon the so-called " second- 
 ary " means of such vision. 
 
 21. All stereoscopic vision, or vision of perspective for remote 
 objects, requires, in order to secure any considerable accuracy, the 
 larger use of " secondary helps." Five or more classes of such helps 
 may be mentioned. Vision, as accomplished by such means, is 
 often called judgment in distinction from immediate perception. 
 This should not be held to imply that activity of the mind in as- 
 sociation and discernment is not involved in all the presentations of 
 sense. The distinction lies between such a synthesis of the sensa- 
 tions into objects of sense as is inseparably connected with all nor- 
 mal binocular vision, and such other seeing (or judging) of the spa- 
 tial properties and relations of remote objects as depends for its 
 accuracy upon changing aspects of these objects. The increased 
 necessity for secondary helps when the objects of vision are remote 
 arises largely from the fact that the mind loses the data (or motifs) 
 that accompany strong convergence and accommodation of the nor- 
 mal eye for near objects. Changes in the tone and intensity of the 
 muscular sensations are comparatively slight on passing from vision 
 of objects 20-40 feet distant to vision at infinite distance. On the 
 contrary, such changes are relatively great on converging the eyes 
 to alter the point of regard from a distance of 20-40 feet to one 
 of 5-6 inches ; still greater on increasing the convergence for still 
 nearer vision. Hence the increased necessity, in vision of distant 
 1 Physiolog. Optik, Hermann's Handb. d. Physio!., III., i., 400 f. 
 
444 
 
 BINOCULAR FIELD OF SIGHT. 
 
 objects, for other secondary helps to take the place, as it were, of 
 the diminished value of the primary data or motifs of the eye. 
 
 22. The principal secondary helps of stereoscopic vision and 
 vision of perspective are the following : ' The course of the limiting 
 lines of the objects in the field of vision determines our perception 
 of their distance and form as lying in the third dimension of space. 
 In looking at a building, we connect together into vertical, horizon- 
 tal, or curved wholes, the successive fragments of the images of its 
 lines as the eyes are swept along in the requisite directions. If 
 these lines become confused in distinctness, or changed into direc- 
 tions that are contrary to our previous experience of how the 
 parts of a building appear to the eye, we are liable to errors in per- 
 ception. When the bottom lines of a distant object are covered, 
 its distance and shape in the third dimension become uncertain to 
 the eye. Mountains that tower behind each other seem to lie in 
 one surface, provided the presence of other secondary helps, such 
 as atmospheric perspective, etc., is excluded. The parts of un- 
 shaded geometrical figures drawn on a plane where the course of 
 their outlines does not define the matter to us as fixed in one way, 
 may often receive two or more interpretations ; they may thus 
 actually appear as subject to change from a nearer to a more re- 
 mote place in space. The same arrangement of lines may appear 
 
 either as a staircase or a 
 portion of an overhang- 
 ing wall (see Fig. 102). 
 The same angle of a poly- 
 gon may be made to seem 
 either the nearest or the 
 most remote. Indeed, 
 the whole stereoscopy of 
 certain figures may thus 
 easily be reversed. But 
 if the course of the limit- 
 ing lines of an object for- 
 bids more than one interpretation of the relations of its parts in 
 the third dimension of space, then the object must be seen as in- 
 terpreted in that one way. Objects of known size and shape are 
 seen as nearer or remote, according to the manner in which the 
 parts cover each other and are covered by each other. The con- 
 tour of an object, then, is one determining factor of its stereoscopic 
 appearance (see Fig. 103). 
 
 1 Comp. Wundt, Physiolog. Psychologie, ii., p. 145 f.; and Helmholtz, Phys- 
 iolog. Optik, pp. 622 ff. and 766 ff. 
 
 FIG. 102 (From Wundt). a can be made to appear either 
 nearer or farther off than b. 
 
INFLUENCE OF SECONDARY HELPS. 
 
 445 
 
 Mathematical perspective, or the size of the angle of vision which 
 is covered by near and far objects respectively, is one of the most 
 important secondary helps of stereoscopic vision and vision of per- 
 spective. In this way objects of known size are seen as placed at 
 a distance necessary to give them their apparent size. The street 
 appears narrower and more distant, the houses lower and more re- 
 mote, in the upper part of its visual picture. Parallel lines, like 
 the tracks of a railway, appear to converge from us more and more 
 toward a point ; the same thing is true of the sides of the table or 
 box at one end of which we are standing, or of the walls of the 
 
 Fia. 103 (From Wundt). The two rings A and B may be stereoscopically combined in either of 
 the following ways according as the vertical or horizontal contours prevail. 
 
 room. For although the perspective of visual experience is very 
 different from true " mathematical perspective," the latter affords 
 to the former one of the secondary helps. 
 
 More distant objects are also, on account of the amount of atmos- 
 phere through which the rays of light reflected from them have to 
 pass, more dim in outline and of changed shades of color. Such 
 alterations in the character of the image furnish another of the 
 secondary helps of our vision of perspective. Accordingly, things 
 are seen nearer in a clear atmosphere, more distant in one less 
 clear. This is sometimes called " aerial perspective." 
 
 The size and direction of the shadows also furnish data for the 
 
446 BINOCULAR FIELD OF SIGHT. 
 
 perception of the distance and shape in the third dimensions of vis- 
 ual objects. In the morning and evening light, when all shadows 
 are lengthened, the objects of the landscape appear more distant 
 from us and from each other. The direction of the shadows of 
 different objects with relation to each other and to the source from 
 which the light comes is also an aid to vision of perspective. The 
 arrangement of the lights and shadows is by far the most impor- 
 tant means for determining the relative position in space of differ- 
 ent parts of objects like intaglios or medallions. A change of the 
 arrangement of the lights and shadows of such an object, so as to 
 substitute the one for the other throughout, converts an intaglio 
 into a medallion or bas-relief, and vice versa. A medallion, placed 
 near a window, but shielded from its direct light, and lighted from 
 the other side by reflection from a mirror, has its relief reversed. 
 
 23. Other secondary helps to stereoscopic vision and vision of 
 perspective are derived from experience in a still more indirect 
 way. Within certain limits we see what we know to be in the 
 field of vision ; but, on the other hand, we are not infrequently 
 compelled to see what we know cannot be there. The account of 
 such phenomena depends upon laws of association and reproduc- 
 tion, the physical basis for which is exceedingly obscure. Since 
 the ultimate psycho-physical processes take place in the brain ; 
 and since the central processes come under the law of habit and 
 are in part determined by the tendencies embedded, as it were, in 
 the structure and customary functions of the central mechanism ; 
 the influence of changes in the peripheral organs of vision, the 
 shape and clearness of the retinal image, etc., cannot always deter- 
 mine just what the presentation of sight 
 will be. Many retinal images admit of 
 two or more interpretations which in- 
 terpretation will be chosen depends 
 upon a variety of circumstances that 
 perhaps cannot all be accurately denned. 
 The few lines drawn upon the black- 
 board, or employed by the skilful etch- 
 er, cause us to see what is not, but 
 rather ought to be, in the image formed 
 no. m-F^st one, then the other upon the retina. Anyone accustomed 
 
 to stud ying tne effect of tlie C( > lorecl 
 
 points and outlines which appear in the 
 image seen with closed eyes by the retina's own light, knows how 
 apparently lawless is the interpretation given to this image. This 
 is especially true when attention is somewhat relaxed as, for exam- 
 
INFLUENCE OF EXPERIENCE. 447 
 
 pie, on sinking into re very or sleep. Much of the "stuff" out of 
 which the usual phenomena of dreams are made, may be suggested 
 and controlled by the condition of the "retinal field." In all these 
 cases, only a sharper attention and more objective view of things is 
 needed to dispel the illusion and make us aware how scanty is the 
 schema, as it were, out of which, by association and reproduction, we 
 have constructed our presentations of sense. Similar experiences not 
 infrequently occur even with open eyes by day, in the dimly lighted 
 room, or in the obscurer nooks and recesses of vision on the street. 
 In this way numberless ghosts and apparitions have been most per- 
 spicuously seen. The face of a friend whom we know to be thou- 
 sands of miles distant may look at us from the window of a house ; 
 it is only after persistently trying to interpret the appearance in ac- 
 cordance with our knowledge that we finally succeed in resolving 
 the face into some chance combination of lights and shadows, of 
 window-sash, curtain, or other objects. 
 
 24. Phenomena like the foregoing recall once more the gen- 
 eral office of experience in determining the existence and character 
 of particular presentations of visual sense. We have seen that the 
 strife between the two rival theories of the origin and development 
 of sense-perception concerns the relative amount of what is to be 
 counted "native," on the one hand, or accredited to a process of 
 " learning " how to perceive on the other hand. 
 
 The analysis of the mind's data or motifs has made it apparent 
 that the influence of experience through the association and repro- 
 duction of its past forms is very great over the presentations of 
 sense. 1 The mind sees, not simply according to the objective 
 character of so-called " things," nor simply according to the retinal 
 images as connected with sensations of motions, but also accord- 
 ing to its custom in seeing. When, therefore, its habits are broken 
 up for the time, its interpretation of the sensations, as well as its 
 synthesis of them into recognized objects of sense, is liable to be 
 disturbed. Various experiments impress this truth in a vivid way. 
 For example, let one regard, with one eye, the reversed picture of 
 a landscape or the photograph of a friend turned upside down 
 after covering up nearly all of it except the face. The effect of the 
 pseudoscope, or optical instrument, which, by exchanging the two 
 stereoscopic pictures, changes convex into concave, and vice versa, 
 when applied to a complicated scene of landscape, streets, etc., is 
 very bewildering. The data with which the mind has been wont 
 
 1 Comp. the chapter,' Der Einfluss der Erfahrungsmotive auf die Localisi- 
 rung, in Heriug's Physiolog. Optik, Hermann's Handb. d. Physiol., IIL, i., pp. 
 564 ff. 
 
448 BINOCULAR FIELD OF SIGHT. 
 
 to deal may all be given, and the sensations localized according to 
 the laws of stereoscopic vision, but the relation of the parts is in- 
 explicable out of any previous experience. Similar effects are pro- 
 duced by the telestereoscope, 1 or optical instrument which enables 
 us to see a larger portion of a distant object than is possible with 
 two ordinary eyes, after the fashion of a pair of optical organs in 
 the sides of a gigantic head. Individual peculiarities of localizing, 
 such as are acquired by the practice of some trade or art, are also 
 accounted for under the principle of influence from experience over 
 those elements of reproduction that determine what object of sense 
 shall be constructed out of the various sensational data at command. 
 Indeed, all our estimates of visual size, shape, and distance, as well 
 as our " errors of sense," can be understood only in the light of 
 this same general principle. 
 
 25. Not only what we know, but what we choose, has an influ- 
 ence often a determining one upon what we see. This is true, 
 not simply because we can at will, within certain limits, decide the 
 area of the field of vision over which the point of regard shall move, 
 as well as the parts of this area upon which it shall be fixated, but 
 also because we can regulate the amount of attention which shall 
 be given to visual impressions and the manner of the distribution 
 of attention over the various parts of these impressions. Further- 
 more, it often lies with us to say how we will interpret the data, 
 and so see the complex product resulting from the act of mental 
 synthesis. This is especially true of geometrical figures in outline, 
 as in the cases already referred to under another head ( 22). 
 
 26. With the use of the foregoing data, and under the guidance 
 of past experience, we judge of the spatial extension and relations 
 of lines, angles, and solid bodies, of their shape, size, distance, and 
 relative situation. The position of lines and angles affects our es- 
 timate of their magnitude ; under this principle many errors of sense 
 originate. Distance and size are, of course, so related that they vary 
 inversely, and when one is known the other is immediately or readily 
 judged on the basis of such knowledge. But the size of the visual 
 object is measured by the magnitude of the visor angle covered by 
 its image, or the relative extent of the retinal surface simultaneously 
 excited by the rays of light reflected from the object. This is called 
 its " apparent magnitude." The real magnitude of any object is its 
 size as related to certain fixed standards of measurement formed on 
 the basis of generalizations from the use of both eye and hand. Dis- 
 tance, apparent magnitude, and real magnitude, are therefore con- 
 
 1 See Helmholtz, Physiolog. Optik, p. 646 f., for an account of these two in- 
 struments. 
 
INFLUENCE OF INTENSITY. 449 
 
 nected as three factors of one problem proposed by each presentation 
 of sight. Given the apparent magnitude and the real magnitude of 
 an object, we judge of the distance according to our experience of 
 how large an object of such size appears at an assumed distance. 
 The remote spot on which a human figure is standing seems nearer 
 or farther away according as we know the figure to be that of a 
 man or that of a boy. Distance and apparent magnitude being 
 given, the real magnitude of the object is judged as that which it 
 would need to have in order to appear so large at the given dis- 
 tance. When one of the two necessary data is lacking, no judg- 
 ment can be formed except upon the basis of other secondary helps, 
 such as aerial perspective, etc. Thus, no common standard for es- 
 timating the distance of the sun or moon being given, their size ap- 
 pears different according to the place where different observers are 
 inclined to locate them, or according to the standard of comparison 
 made necessary for the time by their position. These bodies ordi- 
 narily appear to some persons no larger than a saucer, to others 
 larger than a large cart-wheel. When the sun sets behind a tree, 
 the size of the spreading of whose branches is fairly well known, it 
 may be enormously magnified by being seen to fill its branches en- 
 tirely. 
 
 27. When the eye is in motion, as in all ordinary vision of ob- 
 jects not very minute and very near, the number, duration, and in- 
 tensity of the spatial series of sensations called forth by the motion 
 determine our estimate of the outline-form, magnitude, and dis- 
 tance of the objects. 1 Every spatial series of sensations contrib- 
 utes the larger magnitude to the object the greater the number of 
 members which enter into the series. For this reason the same 
 extension of a line or surface when broken up into parts by inter- 
 secting lines appears larger than when perceived as an uninter- 
 rupted whole. The repetition of similar figures in architecture, 
 upon walls, columns, etc., takes advantage of this effect. 
 
 The intensity of the sensations of a spatial series, and of the act 
 of attention necessary to comprehend them in one whole as a pres- 
 entation of sense, also has an influence on the size of the object. 
 When the movements of the eyes are made with lamed or tired 
 muscles, the size of the thing perceived by them is increased. 
 When the function of one of the muscles (for example, the rectus 
 externus) is impaired, so that the circuit of the eye in a given direc- 
 tion is shortened, objects lying at any position in the field of vi- 
 sion, as seen by the eye moving in the shortened circuit, are located 
 where they would have been if the same intensity of muscular sen- 
 
 'Comp. Volkmann von Volkmar, Lehrb. d. Psychologie, 1885, II, pp. 99 ff. 
 29 
 
450 BINOCULAR FIELD OP SIGHT. 
 
 sation had been necessary to bring them to this position with a 
 normal function of the muscles. A patient with paralysis which 
 prevents turning the eye more than 20 will locate an object 
 actually lying only 20 from the median plane much farther to 
 one side. Such a patient will reach beyond w r hen he tries to grasp 
 the visual object. The increased size which is given to objects 
 that are parti-colored or mottled, and so have an interrupted sur- 
 face and furnish greater difficulty to perception of them as wholes, 
 may be due to both the foregoing causes. Volkmann von Volk- 
 mar ' calls attention to the fact that both monotony and variety 
 may, under the working of these principles, be productive of the 
 same effect in magnifying the size of an object. For the size of 
 any visual surface is usually estimated by the application of some 
 standard of measurement selected from the field of vision. The 
 frequent repetition of this standard creates the impression of vast- 
 ness ; and the absence of any standard to apply, or a vague, unsuc- 
 cessful effort to find a standard, may produce the same impression. 
 Monotonous areas of unbroken snow, and stretches of streets 
 crowded with forms of men and animals, both seem of great ex- 
 tent. 
 
 The amount of time through which the spatial series of sensa- 
 tions endure has also an influence on the magnitude of the objects 
 perceived through those sensations. It is as enduring in time that 
 the changing qualities and quantities of sensation which belong- 
 to the perception of any complex object are expressed. The length 
 of the time-course, as well as the degree of the intensity of the spa- 
 tial series of sensations, may be interpreted as extensive magnitude 
 of the perceived object. 
 
 28. The laws which control our estimates of visual magnitudes 
 are psychological, and apply to all the action of the mind in con- 
 structing 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 objects are deliberately estimated and 
 expressed in terms of an accepted 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 
 spatial series of sensations which compose the visual objects. Of 
 such series, those most capable of exceedingly fine differentiation 
 are the local retinal signs and the muscular sensations accompan} 7 - 
 ing convergence of the eyes for near distances. It is difficult 
 1 Lehrb. d. Psychologie, II., p. 101 f. 
 
MEASURING POWER OF THE EYE. 451 
 
 to assign the exact proportion of help which these two series ren- 
 der in making the finest possible distinctions of visual magnitude. 
 Hering ' 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 as- 
 sistance of muscular sensations, 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 ob- 
 jects. Wundt 3 and others claim that it is by gradations in the so- 
 called " feelings of innervation " alone that we make the most accu- 
 rate of these estimates ; they deny that any " spatial sense " (in 
 Bering's meaning of the words) belongs to the retina. The evi- 
 dence seems to favor the view that both the muscular sensations and 
 the local retinal signs 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 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. V., 18). 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- ? i i i i i iiiiiiiii 
 able difference. [ ' 7 "~" 2ff 29-32 TT-TS Tf.TT TT-TJS ro-T* 43-30 
 
 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 
 lying at a vertical distance of 20 mm. are estimated as equally far 
 away with those lying at a horizontal distance of 25 mm. 4 Most 
 estimates of direction and distance are comparatively inaccurate 
 when only one eye is used. A vertical line drawn at right angles to 
 
 1 Hermann's Handb. d. Physiol., III., i., p. 533 f. 
 'Medicin. Psychologic, 384 f. 
 
 3 Physiolog. Psychologie, ii., pp. 85 flP. ; comp. i., pp. 375 ff. 
 4 So Wundt found, Physiolog. Psychologie, ii. , p. 96. 
 
452 BINOCULAR FIELD OF SIGHT. 
 
 a horizontal appears bent to monocular vision ; its apparent inclina- 
 tion is variable, and was found by Bonders l to vary between 1 and 
 3 of the angle within a short time. 
 
 Helmholtz 2 experimented to determine the accuracy of the bi- 
 nocular perception of depth by trying how small a deviation from a 
 perfectly straight line could be detected in a wire bent at one point 
 so that its two halves formed an extremely obtuse angle, when the 
 wire was looked at both in and out of the horopteric line ; he also 
 employed for the same purpose three nails, the heads of which could 
 be very slightly displaced from a straight line. Under the most 
 favorable circumstances, he found that a displacement of a nail by 
 a change of its distance corresponding to 60^ seconds of the angle 
 of vision, or 0.0044 mm. variation in the position of the retinal 
 image, could be detected. The latter distance corresponds so well 
 to the calculated size of the retinal elements (see p. 327) as to form 
 an argument in favor of the theory that estimates of size and dis- 
 tance are dependent upon the local signs attached to the excitation 
 of these elements. 
 
 But, on the other hand, it is claimed that Weber 3 showed the 
 muscular sense of the eye to be one of the finest of micrometric 
 apparatuses, since a distinct muscular sensation is attached to a 
 displacement of the most sensitive spot of the retina of not more 
 than -^y of a Parisian line. By experimenting with a black thread 
 stretched over against a white wall and moved in the median plane 
 toward and from both eyes, which looked at it through a horizon- 
 tal slit in an upright board, it was found possible to detect changes 
 in distance of 3.5 ctm. at an absolute distance of 180 ctm., and 
 changes of 1 ctm. at an absolute distance of 60 ctm. 4 
 
 29. 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 motion- 
 
 1 Archiv f. Ophthalmologie, XXI., iii., p. 100 f. 
 
 2 See Physiolog. Optik, p. 644 f. 
 
 3 In the Ber. d. sachs. Gesells., etc., for 1852, p. 130; cited by Volkmann 
 von Volkmar, Lehrb. d. Psychologic., II., p. 56. 
 
 4 See Wundt, Physiolog. Psychologic, ii., 94 f., and the reference there. 
 
THE PERCEPTION OF MOTION. 453 
 
 less and looking into the distance with the lines of vision parallel ; 
 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. 
 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 only primary data ; secondary 
 helps, and associated ideas of muscular sensations which have been 
 by experience found necessary to follow objects in motion, 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 
 position. When the eye and the object both move in such a way 
 that the point of regard remains fixed on the object, our percep- 
 tions of motion, and estimates of its direction and magnitude are 
 dependent 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 at about a given rate in 
 a given direction. If any of the links ordinarily belonging to this 
 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 
 
454 BINOCULAR FIELD OF SIGHT. 
 
 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 
 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 ; which 
 
 1 Physiolog. Optik, p. 486. 
 
 * In Hermann's Handb. d. Physiol., III., i., p. 495. 
 
 3 See Le Conte, Sight, p. 229. 
 
VISION AS INTERPRETATION. 455 
 
 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 direc- 
 tion from that in which we know the organ of vision to be turn- 
 ing. 
 
 30. 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. JE/rrors 
 of sense are only special instances where the mind makes its syn- 
 thesis unfortunately, as it were, out of incomplete data, instantane- 
 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 
 
456 ERRORS OF VISUAL PERCEPTION. 
 
 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. 
 
 31. 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. A chief condition of the single vision of solid objects 
 is that they shall be seen with two eyes. Whether anything what- 
 ever 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 
 what is two is seen as one, and what is one is seen as two all in 
 essentially the same way. 
 
 A still larger class of errors of the visual sense falls under the 
 laws which regulate the smallest observable differences in the 
 muscular sensations as related to the mathematical perspective of 
 
COMPAEISON OF MAGNITUDES. 
 
 457 
 
 lines, angles, and surfaces. 1 Reference has already been made to 
 
 the working of this principle in our ordinary perceptions of the 
 
 visual magnitude, contour, and distance of objects. 
 
 Vertical distances are regularly perceived as larger FlG 105 
 
 than equally large horizontal distances. On trying 
 
 to draw a cross with limbs of equal 
 
 length one is apt to get the vertical 
 
 dimension too small ; exact squares 
 are likely to appear higher than their 
 
 -H- 
 
 FIG. 106. 
 
 breadth. When comparing magnitudes in 
 the upper part of the field of vision with 
 those in its lower part, one is likely to over- 
 estimate the former. The upper and lower 
 half of a letter " S " or a figure " 8 " appear 
 of nearly the same size ; but when they are 
 inverted ("g " and " g ") the difference in the 
 size of the two halves becomes magnified. Under the same prin- 
 ciple in part at least may those errors be brought which are 
 
 FIG. 107. 
 
 FIG. 108. 
 
 determined by the way in which the field of vision is filled up. If 
 the horizontal distance between two points be exactly half filled 
 with a line, this line will ap- 
 pear longer than the remain- 
 ing empty space. A square 
 intersected by parallel hori- 
 zontal lines appears elongat- 
 ed upward, but one intersect- 
 ed by parallel vertical lines 
 appears elongated sideways. 
 If one of the two right angles 
 formed by drawing a vertical 
 perpendicular to a horizontal 
 line be filled with several lines diverging from the point of the an- 
 
 1 Comp. Wundt's discussion of such cases, Physiolog. Psychologie, II., pp. 
 92 ff. 
 
 FIG. 109. 
 
458 
 
 EERORS OF VISUAL PERCEPTION. 
 
 gle, the angle thus filled will appear the larger and the perpendic- 
 ular will seem bent. For essentially the same reason, when two 
 unequal angles together make 180, the obtuse angle appears rela- 
 
 Fio. 111. 
 
 Fio. 112. 
 
 tively too small, and the acute angle relatively too large. Many 
 surprising errors of sense result from the varied applications of this 
 principle. (See the Figs, on p. 457 f.) 
 
THE ILLUSIONS OF ART. 
 
 459 
 
 FIG. 113. 
 
 32. The influence of experience, which often corrects what 
 would otherwise be an instinctive interpretation of the data fur- 
 nished to the mind, is at 
 other times the cause of 
 errors. If the data will at 
 all permit it, we incline to 
 perceive any object as we 
 know that similar objects 
 are usually perceived. Such 
 errors of sense as result 
 from the vision of distant 
 objects through secondary 
 helps are too well known 
 and frequently remarked 
 upon to require extended 
 treatment. All the pleasant 
 illusions of art in archi- 
 tecture, drawing, and paint- 
 ing are obliged constantly 
 
 to take them into account. The very relations of light and shade, 
 the conjunction, separation, and covering of lines and surfaces, upon 
 which reliance is ordinarily placed for perception of fact, may be 
 employed by either nature or art to compel us to perceive what is 
 contrary to fact. Painting is successful according to the skill it 
 displays in furnishing to the eye its customary data so as to entice 
 it to regard things as other than they really are. That its success 
 is so good need not surprise us, when we remember that the mind 
 has never anything more than these same data out of which to 
 construct its objects of sense and to make its various judgments 
 concerning them. The many errors in our perceptions of motion 
 may, for the most part, be explained in the same way. It matters 
 not whether the data for such perceptions are furnished by actual 
 changes in the relative position of things in external space, or 
 whether the same sensations arise through changes confined to the 
 organs of sense. Past experience has great influence in all this 
 domain. We incline for this reason, when two objects are chang- 
 ing their relative position, to perceive the smaller of them as in 
 motion ; we also over-estimate the speed of small bodies in motion, 
 and under-estimate that of large bodies. 
 
 33. Some errors of visual perception differ from the foregoing 
 in that their explanation seems to be due to cerebral activity under 
 other laws as yet unknown to us. We have already seen (Chap. 
 IV., 14) that the phenomena of contrast of colors must be referred 
 
460 ERRORS OF VISUAL PERCEPTION. 
 
 to certain inexplicable activities of the central organs as related to 
 our sensations or states of consciousness. The same thing is ap- 
 parently true of 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 sur- 
 faces, 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 per- 
 ceived 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 bright- 
 ness 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 red- 
 der and sometimes bluer than that of the two side images, but in 
 no case exactly like either of them. By steady 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 bi- 
 nocular mixing of colors." If such a deception can be secured, it is 
 manifest that the mixing of colors on which it depends must take 
 place in the brain, and not 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 looking 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." 
 
 The peculiar perception of luminosity is due to a struggle be- 
 tween 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 produce 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 luminous. But when the surface of such bodies 
 as, for example, the surface of a sheet of water becomes ruffled 
 by ripples, it becomes luminous. The perception of luminosity 
 may be produced 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 combining to produce an equal tint of gray over the 
 
 1 So Hering asserts, Physiolog. Optik, in Hermann's Handb. d. Physiol., 
 III. , i. , p. 592. Binocular mixing of colors lias been denied by some authorities. 
 
LAWS OF CEKEBKAL ACTION. 461 
 
 whole surface, the images of the separate points on the two retinas 
 enter into a struggle with each other ; and the rapid alternation of 
 the prevalence, first of one and then of the other, gives rise to the 
 appearance of luminosity. 
 
 Such phenomena as the foregoing seem to require a reference to 
 certain unknown processes in the central organs as a physical basis 
 for the psychical experience. Some experimenters claim that in 
 these cases of contrast they are able to see either color at will by 
 giving attention, first to the image on one retina, and then to that 
 on the other. The words in which this claim is couched, however, 
 afford no explanation of the phenomena ; for, we repeat again, the 
 mind does not see the image on the retina, and cannot direct spe- 
 cial attention to it. It can only attend to this or that feature of 
 the "presentation of sense," which is in every case a subjective 
 affair. But the very question that we are unable to answer con- 
 cerns the reason why the presentations of sense are constructed as 
 they are in such cases ; the reply, so far as any reply can be given, 
 must be, that such data or motifs furnished by the spatial series of 
 sensations as we cannot connect with known laws of the peripheral 
 organs of sense must be referred to unknown laws of the central or- 
 gans of the same sense. Apparently this truth holds good of certain 
 optical illusions of motion. The fact that a steady succession of im- 
 ages (as in the case of watching a fall of water), passing over a partic- 
 ular region of the retina for a long time, sometimes ceases to be per- 
 ceived as a motion, and that the image of a stationary body on the 
 same retinal region may appear to be moving in the opposite direc- 
 tion, has been explained by "Thomson's law." This law refers the 
 phenomena to the principle of fatigue. Recent investigations, how- 
 ever, seem to show that the explanation is incorrect. They bring 
 out the 'remarkable 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 cere- 
 bral action would seem to afford the only possible explanation. 1 
 
 34. The fact that things are seen upright and in correct relations 
 horizontally, by means of data furnished through inverted retinal 
 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 f 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- 
 1 See Journal of Physiology, iii. , p. 299 f . 
 
462 DEVELOPMENT OF VISUAL PERCEPTION. 
 
 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 
 image for the data from which it is constructed ; the image is 
 dependent 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 primarily localized simply with reference to each other by 
 means of local retinal signs and of muscular sensations produced 
 by motion of the eyes. But as yet the field of vision has no locality 
 in objective 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 sur- 
 rounding 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 corresponding 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. 
 
 35. 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. 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 
 
LEAENING TO LOCALIZE. 463 
 
 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 
 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 spot 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 mind, afford the compara- 
 tively simple problems furnished by the primary position. In this 
 way a central point, 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. 
 
 The motifs or data which give to the mind its guidance in 
 achieving its more difficult tasks are the spatial series of muscular 
 
464 DEVELOPMENT OF PERCEPTION. 
 
 and tactual sensations which are caused by the motions of the eye 
 for parallel turning, for accommodation, and for convergence in 
 near vision. The general principle is, 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 the problem of depth, binocular vision with 
 moving eyes, and its resulting combination and separation of the 
 double images of objects, seems necessary. The existence and as- 
 sistance of those secondary helps, which are so important in per- 
 ceiving the solidity and distance of objects, imply a further devel- 
 opment of experience. In all these advances, however, 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. 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 availing themselves of 
 all possible secondary helps. It also depends upon what we have 
 learned to know of the nature and probable position and shape of 
 manifold objects of which the eye has already attained the mastery. 
 
 36. 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 
 
TRANSLATION OF PERCEPTIONS. 465 
 
 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. 
 
 37. 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 -f 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. 
 
 1 Physiolog. Optik, p. 650. 
 
 2 Archiv f. Ophthalmologie, XVII., ii., p. 55. 
 
466 DEVELOPMENT OF PERCEPTION. 
 
 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 recently been examined, experimentally, at 
 considerable length by J. Jastrow. 1 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 
 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," 2 the question arises 
 whether all the phenomena cannot be accounted for by a special 
 application of the law of habit in connection with the normal action 
 of the sensory apparatus. Each sense, when expressing the esti- 
 mate, tends tq 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 
 
 1 Art. on The Perception of Space by Disparate Senses, in Mind, October, 
 1886, pp. 539-554. 
 'Ibid., p. 549. 
 
MIND AS PSYCHICAL SUBJECT. 467 
 
 the spatial perceptions belonging to each one of the two senses 
 concerned when interpreting its own specific data in corresponding 
 terms, as it were. 
 
 38. 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 eatfra-mental 
 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 aftair, 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. 
 
CHAPTEK VIII. 
 TIME-RELATIONS OF MENTAL PHENOMENA. 
 
 1. " PRESENTATIONS of sense " appear in consciousness, not only 
 as having spatial qualities and relations, but also as occurring either 
 simultaneously or successively as respects Time-form. The clear- 
 est 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 notes of a melody, or to the rhythm of visual or mus- 
 cular impressions which accompanies a regularly recurrent motion 
 of some member of the body. What is true of the presentations 
 of sense is also true of all mental phenomena, of the reproduced 
 images of sense, of pure creations of fancy, and of thoughts. All 
 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 conscidus 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 Comp. Volkmann von Volkmar, Lehrb. d. Psychologic, II., p. 11 . 
 
METHODS OF MEASUREMENT. 469 
 
 over, they are nearly all merely restatements in more definite form 
 of 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 excit- 
 ing 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 are matters of 
 common observation. 
 
 2. It is not necessary to describe the construction of the 
 machines which have been used in experimenting upon the time- 
 relations of mental phenomena, or the methods of using them em- 
 ployed and commended by different observers. The general prob- 
 lem 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 
 pressing 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 
 complex process involved. Such experiments need to be repeated 
 many times upon the same individual, so as to eliminate the vari- 
 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 upon the so-called personal equation. 
 
470 THE FIXING OF KEACTION-TIME. 
 
 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. Keaction-time is " simple " when all the 
 elements which tend to complicate the processes involved in the re- 
 action, 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 quality, the instant of whose appearance is ex- 
 pected, by executing a single natural and easy motion, best fulfils 
 the conditions of simplicity. It is therefore requisite, for all ex- 
 periments 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 inter- 
 val. But even the simplest reaction-time is, of course, a very com- 
 plex 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 a 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 motion. 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 definiteness (see Part I., chap, iii., on the speed of nervous 
 processes). It is, then, theoretically possible to ascertain 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 3 assumes G2 meters 
 per second as the probable rate of conduction in both sensory 
 and motor nerves ; and in the spinal cord, 8 for the sensory and 
 
 1 Archiv f. Anat, Physiol., etc., 1868, p. 664. 
 
 2 See Hermann's Handb. d. Physiol., II., ii., p. 271. 
 ' 3 Ibid., p. 2721 
 
ELEMENTS OF PSYCHO-PHYSICAL TIME. 471 
 
 11-12 for the motor process. He thus calculates that about 0.0828 
 sec. is" the "reduced reaction-time," or interval occupied within 
 the cerebral centres in transforming the sensory into motor im- 
 pulsesin 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. " Psycho-physical time " (No. 4 of Exner's seven processes) 
 is analyzed by Wundt ' into three psycho-physical processes : (1) 
 Entrance into the visual field of consciousness, or simple perception ; 
 (2) entrance into the point of clear vision with attention, or apper- 
 ception (attentive and discerning perception) ; (3) the excitation of 
 the will, which sets free in the central organ the registrating mo- 
 tion. 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 indefi- 
 nitely 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 centrahs of 
 consciousness (Blickpunkt) where discerning attention is bestowed 
 upon them and they are apperceived. When they are apperceived, 
 further time is required to get up the corresponding molecular mo- 
 tion 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 physio- 
 logical 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. 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 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 intensity. The 
 1 Physiolog. Psychologie, ii.. pp. 220 ff. 
 
472 THE FIXING OF KEACTION-TIME. 
 
 position that the mental forms of perception, apperception, and 
 will, are exactly simultaneous with corresponding cerebral processes, 
 and that mental states are not to be regarded as forming them- 
 selves in a separate time, as it were, on top of these processes, may 
 also be admitted in a provisional way. It is probable theory, how- 
 ever, rather than demonstrated fact. Accordingly, the first problem 
 of psychometry is, to determine the simple reaction-time, and from 
 it to find the three factors of psycho-physical time namely, per- 
 ception-time, apperception-time (or discernment-time), and will- 
 time. 
 
 5. Any psycho-physical theory of the time-relations of mental 
 phenomena requires that account should be taken of the inertia 
 of the nervous system. As composed of moving molecules, it ne- 
 cessarily 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 
 excitement, 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 
 mechanism. It is difficult, however, to justify the assumption 
 experimentally, 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 T ^j- sec. for the muscle when electricity is used. 
 The case is different, however, with the end-organs of sense. They 
 do exhibit 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 
 widely. Preyer thought that 27.6-36.8 nervous shocks (per sec- 
 ond) of the skin fused into one continuous sensation ; but Valentin 
 put the limit at 480-640, and von Wittich ' succeeded in distinguish- 
 
 1 For his remarks onPreyer's experiments, see the article in Pfliiger's Archiv, 
 ii., pp. 329 ff. 
 
STATEMENT OF TALBOT'S PKINCIPLE. 473 
 
 ing about 1,000 separate excitations in this unit of time. Hearing 
 can receive 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 
 sensations of sound may then be placed at about 500 per second. 
 Mach, ' however, by using the click from a revolving toothed- wheel, 
 claims to have reduced the interval to 0.016 sec. The interval 
 is increased to about 0.064 sec. when the same auditory impres- 
 sions are heard by both ears. 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. Far fewer musical tones than noises can be heard in a 
 second ; and, indeed, a number of vibrations, occupying a consider- 
 able fraction of a second, must be secured before the sensation of 
 tone is established, as it were. 
 
 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 thefovea 
 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 (comp. p. 334) 
 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 b, and if the 
 time a + b is less than about 0.04 sec., then the sensation becomes 
 
 continuous, with a strength corresponding to the excitation , 
 
 1 Sitzgsber. d. Wiener Acad., LI., p. 142. 
 
 2 Comp. Exner, in Hermann's Handb. d. Physiol., II., ii., p. 256 f.; and 
 Sitzgsber. d. Wiener Acad., LXXIL, p. 156 f. 
 
 3 Archiv f. Anat., Physiol., 1863, p. 739 f.; and Hermann's Handb. d. Phy- 
 siol., III., i, p. 2121 
 
474 THE FIXING OF KEACTION-TIME. 
 
 If the inertia of the eye for the different color-sensations were 
 greatly different, we would see objects differently colored accord- 
 ing to the time that the rays from them were acting on the retina. 
 That the different parts of the spectrum do actually require slightly 
 different intervals of time to reach the maximum of their excitation 
 has been shown by Kunkel. l Equally bright light, as before stated 
 (p. 334), attains its maximum effect, for red rays in about 0.0573 
 sec., for green in 0.133 sec., for blue in 0.091G sec. With the same 
 color-tone, the greater the brightness the quicker the maximum 
 effect is reached. Thus for three degrees of brightness the time for 
 red rays is 0.0573-0.071 sec. ; for green, 0.0699-0.133 sec. ; for blue, 
 0.0916-0.102 sec. Accordingly, the spectrum may be reduced in 
 size and number of color-tones by diminishing the duration of the 
 action of the light which forms it. 
 
 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. 2 
 
 6. 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 3 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 (ztfovea centrcdis, by optical images). . 0.044 
 Between two sensations of light (at periphery of retina, by optical images) 0. 049 
 Between sensation of sight and sensation of touch (sight following). . . . 0.05 
 Between sensation of sight and sensation of hearing (sight following). . 0.06 
 
 Between two sensations of noises (each heard by one ear) 0.064 
 
 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 
 
 1 Pfluger's Archiv, ix. , p. 206 f. 
 
 2 Comp. von Vintschgau, in Hermann's Handb. d. Physiol., III., ii., p. 221. 
 
 3 By Exner, in Hermann's Haiidb. d. Physiol., II., ii., p. 262. 
 
REACTION FKOM HAND TO HAND. 475 
 
 7. The way that the intensity of sensations of light rises to a 
 maximum, continues there, and then falls off through exhaustion 
 of the retina, in time, has been 
 represented by Fick ' with the 
 use of the accompanying figure 
 (No. 114). 
 
 8. The point of starting for 
 determining experimentally all 
 the problems which concern the 
 durations and relations in time . 
 
 of mental phenomena is gained 
 
 * . FIG. 114. Curves showing the Rise and Fall of 
 
 by fixing' the " Simple reaction- the Intensity of Sensations of Light the ab- 
 it mi / -ii f cissas measured along a-d representing the 
 
 time. This is found to vary for time, 
 different persons, for the differ- 
 ent senses, and under different conditions of expectation, attention, 
 habit, etc. In its very simplest form, the question may 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 motion follows as the result of rec- 
 ognizing the fact, in consciousness, that such stimulation has taken 
 place ? In this form the three elements of psycho-physical time 
 (perception, discernment or apperception, and choice) are sup- 
 posed to be reducible to one namely, to simple perception. This 
 supposition is, of course, true only in case that, by practice in react- 
 ing upon an expected sensation in one definite way, the cerebral 
 sensory-motor processes have attained the highest possible rate of 
 speed, and the time ordinarily occupied in deciding what to do, and 
 in starting the voluntary motor mechanism, has been reduced al- 
 most, or quite, to zero. 2 The entire process then becomes reflex, 
 simply the sensory central part of it being represented by a con- 
 scious act of perception. To shorten the reaction-time as- much 
 as possible, the subject of the experiment must know what place of 
 the sensory organism is to be hit by the stimulus, and about when 
 to look out for 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. 
 
 The following table 3 gives the mean values of the reaction-time 
 "from hand to hand " (one hand being hit by the electrical current 
 
 1 Hermann's Handb. d. Physiol. III., i., p. 216. 
 
 2 Comp. Wundt, Physiol og. Psychologie, ii., p. 226 f. 
 
 3 Taken from Exner, in Hermann's Handb. d. Physiol., II., ii,, p. 263; the 
 two sets of numbers indicate values which were found in two series of exper- 
 iments. 
 
476 THE FIXING OF KEACTION-TIME. 
 
 and the other reacting, for example, to press a key), as determined 
 by various observers : 
 
 Hi^,, Koh^sc,, En.. 
 
 0.13776 sec. I 0.1733 sec. ) n 1RQ7 ) 0.153 sec. [ 0.1276 sec. ) 0. 1087 sec. I 0.117 sec. 
 0.12495 sec. j 0.1911 sec. J U ' 1D IC< f 0.166 sec. } 0.1283 sec. ( 0.1860 sec. J 0.146 sec. 
 
 The last two experimenters developed certain interesting results. 
 They found the reaction-time, when the stimulus was applied to 
 the middle finger, to be for Kries 0.117 sec. and for Auerbach 0.146 
 sec. ; but when applied to the back of the hand, to be for Kries 
 0.119 sec. and for Auerbach 0.147 sec. But the hand being about 
 16 ctm. nearer the brain than the middle finger, its reaction-time 
 should have been some 0.004 sec. shorter instead of longer, as a 
 matter of physiological time. Exner found that the reaction-time, 
 when the forehead is stimulated, is greater than when the stimulus 
 is applied to the hand. Bloch found the same thing true when the 
 nose is stimulated. The intercerebral relations, taken in connection 
 with the law of habit, probably account for the foregoing facts. 
 
 The value of the reaction-time also changes when the character 
 of the stimulus is changed on which the subject of experiment re- 
 acts. This fact is made apparent by the following table : l 
 
 Observer. Optical stimulus. Acoustic stimulus. Stimulus of touch. 
 
 Sec. Sec. Sec. 
 
 Hirsch ............... 0.200 0. 149 0.182 (hand). 
 
 Hankel ............... 0.225 0.151 0.155 
 
 Bonders ............... 0.188 0.180 0.154 (neck). 
 
 Von Wittich .......... 0.194 0.182 0.130 (forehead). 
 
 Wundt ................ 0.175 0.128 0.188 
 
 Exner ................ 0.1506 0.1360 0.1276 (hand). 
 
 Auerbach ............. 0.191 0.122 0.146 
 
 VonKries ............. 0.193 0.120 0.117 
 
 We conclude, then, that under the most favorable circumstances 
 the reaction-time can scarcely be reduced to T 1 of a second, while 
 it rarely rises much above -f$ of a second. 
 
 9. 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 veiy 
 good means of measuring stimuli of one sense in terms of another 
 sense. It has been proposed 2 to reduce them to a common standard 
 by referring the sensations to the point where they barely reach 
 the " threshold of excitation " (JReizschwelle) ; that is, where they 
 
 1 Taken from the article of von Kries and Auerbach, Archiv f. Anat. u. Phy- 
 siol., Physiolog. Abth., 1877, p. 359 f. 
 
 8 By Wundt, Physiolog. Psychologic, ii., p. 223 f. 
 
EFFECT OF INCKEASED INTENSITY. 
 
 477 
 
 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 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 pro- 
 cesses by which sensory impulses pass over into motor impulses, 1 
 is different. 
 
 10, The effect of increasing the intensity of the stimulus, in di- 
 minishing the reaction-time, has been studied by Wundt for sen- 
 sations of sound occasioned by the fall of a hammer or ball with 
 the following result : 
 
 Height of hammer. Eeaction-time. 
 
 Sec. 
 
 1 millimeter 0.217 
 
 4 millimeters 0.146 
 
 8 millimeters 0.133 
 
 16 millimeters . . 0.135 
 
 Height of ball. Eeaction-time. 
 
 Sec. 
 
 2 centimeters 0.161 
 
 5 centimeters 0.176 
 
 25 centimeters 0.159 
 
 55 centimeters . . . 0.094 
 
 This effect is obtained, of course, only within certain limits ; for 
 the sound must not be so loud as to startle and confuse. Thus, 
 also, when the length of the electric spark which stimulates the 
 retina is increased the reaction-time is diminished. Exner 2 found 
 that, while it was 0.1581-0.1502 sec. for sparks 0.5-1 mm. in length, 
 it was 0.1479-0.1384 sec. for those 2-5 mm. long, and diminished 
 to 0.1229 sec. for those of 7 mm. 
 
 The reaction-time is also diminished by indicating the instant 
 at about which the sensation may be expected, through some pre- 
 ceding signal. The interval for the sound caused by a ball falling 
 25 ctm., which without signal was 0.253 sec., was reduced by a 
 signal to 0.076 sec. ; and when the fall was 5 ctm. the interval was 
 reduced by the signal from 0.266-0.175 sec. 3 In order to secure 
 this effect, however, the interval between signal and impression 
 should be nearly constant, and not so long as to overstrain atten- 
 
 1 Comp. von Kries and Auerbach, Archiv f. Anat. u. Physiol., Physiolog. 
 Abth., 1877, p. 359 f. 
 
 8 Hermann's Handb. d. Physiol., II., ii., p. 2691 
 3 See Wundt, Physiolog. Physiol., ii., p. 238. 
 
478 
 
 THE FIXING OF REACTION-TIME. 
 
 tion or prevent the carrying of a definite mental image of this in- 
 terval. 
 
 The detailed investigations of Berger, 1 recently published, an- 
 nounce the following conclusions : (1) The reaction-time increases 
 in inverse ratio to the intensity of the stimulus, and so much the 
 faster the nearer we approach the " threshold " (or lower limit) of 
 the stimulus ; (2) discernment-time is related to alterations in the 
 intensity of the stimulus in the same way as simple reaction-time ; 
 and (3) will-time is independent of the intensity of the stimulus. 
 
 11. When the quality of the impression to be expected is 
 known, but its intensity is unknown, the reaction-time is increased. 
 The increase is greater if the alternation of intensities is very irreg- 
 ular. This fact is exhibited by the following table : 3 
 
 I. Uniform change of intensity. 
 
 Sec. 
 
 Loud sound 0.116 
 
 Feeble sound.. . 0.127 
 
 II. Irregular change of intensity. 
 
 Sec. 
 
 Loud sound 0.189 
 
 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. It is also 
 greatly lengthened when the impression is wholly unexpected by 
 the subject of the experiment being taken off guard, as it were ; in 
 such a case, also, it may reach 0.5 sec. As might be expected, it 
 takes longer to react in an unnatural and unaccustomed way. It 
 requires more time to react with the foot than with the hand ; a 
 mean reaction-time from eye to foot was found by Exner to be 
 0.1840 sec. 
 
 12. The reaction-time for the sense of taste varies in depend- 
 ence upon the part of the tongue to which the stimulus is applied, 
 and upon the character of the gustable substance. It also varies 
 greatly with different persons. Von Wittich fixed it at 0.167 sec. 
 from tongue to hand, by using the sour taste which the electrical 
 current excites. The reaction-time for sugar on the tip of the 
 tongue varied, for three different persons, from 0.1639 to 0.3502 
 sec. ; and for quinine, for two persons, from 0.2196 to 0.993 sec. 
 For the root of the tongue, it was found to be 0.552 sec. for sugar, 
 and 0.502 sec. for quinine. 3 Little has been done to determine the 
 reaction-time of smell. Some have maintained that it must be much 
 longer than the reaction-time of sight and hearing, and even reach 
 several seconds ; others have held that, although slower than these 
 
 1 Wundt's Philosoph. Studien, 1885, III., heft i., pp. 38 ff. 
 
 2 Wundt, Physiolog. Psychologie, ii., p. 241 f. 
 
 3 Von Vintschgau and Honigschmied, in Pfluger's Archiv, x., 1; xii., 2; 
 riv., 3. 
 
THE METHOD OF BONDERS. 479 
 
 senses, smell has probably a reaction-time of only a fraction of a 
 second. The latter view seems more recently to have been con- 
 firmed by the experiments of Moldenhauer, 1 who obtained the fol- 
 lowing among other figures : Oil of mentha, 0.203-0.362 sec. ; oil 
 of bergamot, 0.212-0.374 sec. ; camphor, 0.226-0.492 sec. ; musk, 
 0.319 sec. Taste and smell are much more subject to change in 
 the length of the reaction-time through individual peculiarities 
 of the exciting substances and of the subjects of experiment, than 
 are the senses of hearing, sight, and touch. 
 
 13. Having obtained the mean reaction-time for the different 
 senses under varying circumstances, the method of investigation 
 requires that it should be determined how much the reaction-time 
 is increased by increasing and complicating the psycho-physical 
 elements. One principal question to be answered by this method 
 is the following : How much time is required for " apperception" or 
 clear discernment of perceived objects in the central point of con- 
 sciousness under different conditions ? This question has been very 
 patiently and fully investigated, at first by Ponders, and since by 
 many observers, especially by von Kries and Auerbach. 3 
 
 Donders 9 and his pupils were the first to examine in detail the 
 speed of psychical processes, with a view to determine how long it 
 takes to recognize one of two or more different presentations of 
 sense ; and also how long to solve the dilemma of choosing one 
 of two means for making the reaction. For example, in one series 
 of experiments the eye was suddenly stimulated with either red or 
 white light (the subject of the experiment not knowing which to 
 expect), the signal for the former to be given with the right hand, 
 the signal for the latter with the left. In another series of experi- 
 ments, the quality of the light or sound was to be recognized before 
 reaction, but reaction was to take place only in case a particular 
 one of the two sensations was recognized ; in case the other sensa- 
 tion appeared in consciousness, no reaction was to take place. In 
 other words, discernment of the presentation of sense was to be 
 followed by the choice between reacting in a prescribed way and re- 
 fraining from reacting at all. In still other experiments the stimulus 
 consisted of a "vocal clang "called into a "phonautograph" by one 
 person, and the reaction consisted of the same clang repeated by 
 another person ; or, again, the recognition of one or more letters 
 seen was signalled by a movement of the hand. By such methods 
 
 1 Wundt's Philosoph. Studien, I, heft iv., p. 606 f. 
 
 2 See their article in Archiv 1 Anat. u. Physiol., Physiolog. Abth., 1877, pp. 
 297-378. 
 
 3 Archiv f. Anat, Physiol., etc., 1868, pp. 657-675. 
 
480 APPERCEPTION AND WILL TIME. 
 
 Donders made the mean reaction-time of five persons, for dis- 
 cernment between red and white light, with choice of the hand by 
 which to react, to be 0.154 sec.; the minimum, 0.122 sec.; the 
 maximum, 0.184. The mean reaction-time for two letters, with 
 discernment and signal by calling them out, was found to be 0.166 
 sec.; when the number of letters was increased to five, the mean 
 reaction-time rose to 0.170. It took 0.180 sec. to discern and 
 repeat a vocal clang when known, 0.268 sec. when unknown. With 
 the method of reacting only on one clang and keeping silent when 
 others were heard, the mean reaction-time varied from 0.201 to 
 0.284 sec. The investigations of Donders made obvious the fact 
 already stated (p. 475 f.), that the natural connection between the 
 sensation and the peculiar means chosen for reaction is of in- 
 fluence in determining the interval. Donders assigned 0.039 sec. 
 to the psychical process of the development of a presentation of 
 sound in his own case, and a little less to the formation of a 
 decision of will. 
 
 14. Another and ingenious method of determining the time 
 required for " apperception " was proposed by Baxt ;' it was based 
 upon the principle of the inertia of the senses, especially of sight. 
 Suppose the question raised, How long must an image act on the 
 optical apparatus in order to occasion a clear presentation of sense ? 
 It may be answered by discovering how quickly after a given im- 
 pression another stronger one must follow in order that the latter 
 may overtake the former, and quench it as it were before it 
 reaches the focus of apperception. Let then some image which 
 requires discernment to interpret it as the image of several letters, 
 or of a simple geometrical figure be thrown upon the retina, and 
 let this image be succeeded after a brief interval by the image of a 
 bright white disk ; then, if the interval be less than a certain time, 
 apperception (or clear vision, with discernment of the significance of 
 the image) will not take place at all. Baxt found that the time ne- 
 cessary under these circumstances for a presentation of visual sense 
 depends upon the intensity of the second excitation ; it increases 
 as this intensity increases. It depends also upon the complexity of 
 the apperception required ; to recognize three letters required only 
 about half the time necessary to recognize five or six. With an 
 interval of 0.0048 sec. between the two excitations, the first ap- 
 peared as scarcely a trace of a weak shimmer ; with an interval of 
 0.0096 sec., letters appeared in the shimmer one or two of which 
 could be partially recognized when the interval increased to 0.0144 
 sec. When the interval was made 0.0192 sec., the objects were a 
 1 See Pfluger's Archiv, iv., pp. 325 ff. 
 
CONCLUSIONS OF KEIES AND AUERBACH. 
 
 481 
 
 little more clearly discerned ; at 0.0336 sec. four letters could be 
 well recognized ; at 0.0432 sec., five letters ; and at 0.0528 sec. all 
 the letters could be read. 
 
 15. The method of Baxt was rejected by von Kries and Auer- 
 bach 1 as unsuitable to answer the question most interesting to 
 psycho-physical researches ; because it includes as an inextricable 
 factor the time used up in the peripheral nerves. Besides, we 
 have no means of estimating just to what stadium a psycho-physi- 
 cal process must have advanced when it becomes impossible for a 
 strong succeeding sensation to overwhelm it. These observers pre- 
 ferred, therefore, the method employed by Donders, and especially 
 in the form (called " Bonders' C-method ") in which the subject of ex- 
 periment reacts in one prescribed way or else refrains from reacting 
 at all. By this method they endeavored to answer the questions : 
 " How long time passes after the occurrence of a stimulus of sight 
 before I know what color it has ; and how long before I know at 
 what place in the field of sight I experience it," etc. ? Their results 
 may be summed up in the following table. a [The numbers give 
 the time assigned by them to the discernment involved in the va- 
 rious processes performed that is,, the psycho-physical time exclu- 
 sive of all solution of a dilemma by the will. This time is found 
 by subtracting the simple reaction-time, or time necessary for re- 
 acting when no discernment is required, from the whole time re- 
 quired for the process including such discernment.] 
 
 
 Auerbach. 
 
 Von Kries. 
 
 Discernment of the direction of light 
 
 Sec. 
 Oil 
 
 Sec. 
 
 017 
 
 Discernment between two colors ... 
 
 012 
 
 034 
 
 
 015 
 
 032 
 
 Discernment of tone when higher 
 
 019 
 
 049 
 
 
 0.021 
 
 036 
 
 Localization of distance by sight 
 
 022 
 
 030 
 
 Discernment between tone and noise . ... 
 
 022 
 
 046 
 
 Judgment of intensity of sensations of touch (strong). . . . 
 Discernment of tone when lower 
 
 0.023 
 034 
 
 0.061 
 054 
 
 Judgment of intensity of sensations of touch (weak) 
 Localization of sound (maximum) 
 
 0.053 
 0.062 
 
 0.105 
 0.077 
 
 
 
 
 16. 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 
 
 1 See Archiv f. Anat. u. Physiol., 1877, Physiolog. Abth., p. 298. 
 
 2 Ibid., p. 346 f. 
 
 31 
 
482 APPERCEPTION AND WILL TIME. 
 
 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 
 required of the two observers, the reaction-time of K. was relative- 
 ly 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 forK. 
 The character of our judgments of intensity is, perhaps, dependent 
 on the steepness, as it were, with which the curve rises in consci- 
 ousness ; 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 ex- 
 planation for these experiences requires reference again to the fact 
 that some 15-20 vibrations are necessary to start the organ so that 
 the sensation of musical tone can be received at all. ' 
 
 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 
 
THE EXPERIMENTS OF WUNDT. 483 
 
 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 35-2G. 
 
 Angle 26 -11. 
 
 A . . 
 
 Sec. 
 0.020 
 
 Sec. 
 
 0.033 
 
 Sec. 
 
 0.120 
 
 K 
 
 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. 
 
 17. Various strong objections have been raised to the validity 
 of the results reached by the observers last mentioned. For ex- 
 ample, Eichet 1 thinks that an interval so small as their " discern- 
 ment-time" (about 0.03 sec.) is below the limit of the experimental 
 error. But the constancy of the results obtained by such a large 
 number of experiments with reference to the mental peculiarities 
 of the two subjects of experiment, and to the different kinds of men- 
 tal processes involved (discernment of locality, quality, quantity, 
 etc.), is too great to allow of the results being summarily rejected. 
 The criticism of Wundt 2 as applied to the method employed (the 
 so-called "Bonders' C-method") is much more pertinent. This 
 veteran experimenter considers that the psycho-physical time re- 
 quired to discriminate between two or more possible presentations 
 of sense, and then react only in case one of them is apperceived, 
 cannot all be allotted to discernment-time. For an element of vo- 
 lition, or will-time, is as truly involved in the decision whether to react 
 or not to react as in the decision between two modes of reaction. 
 
 With a view, then, to analyze these elements more perfectly, and 
 so determine the exact duration of "apperception," in the sense 
 in which the word is used by Wundt, another method of experiment 
 has been employed by Friedrich. 3 In this method the subject of 
 experiment is warned when to expect one of two or more colors 
 
 1 See Revue philosophique, VI. , p. 395. 
 
 2 Physiolog. Psychologie, ii. , p. 251 f . 
 
 3 See Wundt's Philosoph. Studien, II., heft i., pp. 39 ff. 
 
484 
 
 APPERCEPTION AND WILL TIME. 
 
 to be discerned, but does not know which one to expect ; only one 
 way of reaction is employed for all cases ; and the judgment of the 
 subject is left to determine just when he clearly discerns, or " apper- 
 ceives," the object.. The time of apperception is then found by sub- 
 tracting the simple reaction-time (or time necessary to announce the 
 sensation without discernment) from the entire reaction-time thus 
 obtained. In this way the interval for apperception was fixed at 
 0.030 to 0.185 sec. for white, in the case of Wundt, and 0.044 to 
 0.146 for black ; in the case of Tischer, 0.046 to 0.112 sec. for white, 
 and 0.021 to 0.061 for black ; and in the case of Friedrich himself, 
 0.042 to 0.084 sec. for white, and 0.019 to 0.064 for black. The mean 
 duration of apperception, as derived from all the experiments with 
 two color-sensations, was 0.086 sec. for W., 0.047 for T., 0.050 for F. 
 18. The time required for discernment increases, of course, 
 when the other conditions are kept as nearly as possible the same, 
 but the number of objects is increased among which the discern- 
 ment is required. For example, Friedrich found that when four 
 (instead of two) colors black, white, green, and red were inter- 
 changed in an unknown order, both the reaction-time as a whole 
 and the duration of apperception were increased. The latter for 
 black as one of four colors was, when averaged for several series of 
 experiments, 0.081 to 0.141 in Wundt's case, 0.021 to 0.105 in Tisch- 
 er's, 0.076 to 0.197 in his own. With red as one of four colors the 
 duration of apperception was 0.049 to 0.247 for W., 0.024 to 0.124 for 
 T., and 0.066 to 0.234 for F. Experiments were also instituted with 
 a view to determine how long it takes to discern composite percep- 
 tions of sight. 1 For this purpose printed figures of 6 mm. high 
 and 3.8 mm. broad were employed, ranging from one to six places 
 the reaction being in the form of calling the number constituted by 
 the figures as arranged when displayed. The mean time required 
 for apperception by the three subjects of experiment is shown by 
 the following table : [The figures at the head of the columns indi- 
 cate the number of places which the numbers had in the different 
 series of experiments ; the letters, the subjects of experiment ; the 
 figures under the head, the seconds of apperception-time.] 
 
 
 l. 
 
 2. 
 
 8. 
 
 4. 
 
 5. 
 
 6. 
 
 W.. 
 
 Sec. 
 0.344 
 
 Sec. 
 
 0.361 
 
 Sec. 
 
 0.354 
 
 Sec. 
 0.459 
 
 Sec. 
 
 0.573 
 
 Sec. 
 
 0.817 
 
 T 
 
 0.290 
 
 0380 
 
 0493 
 
 709 
 
 0849 
 
 1 197 
 
 F 
 
 0320 
 
 0346 
 
 0344 
 
 0.481 
 
 0670 
 
 1.043 
 
 
 
 
 
 
 
 
 Wundt's Philosoph. Studien, I., heft i., p. 53 f. 
 
THE HEADING OF NUMBERS. 485 
 
 It will be noticed that the reaction- time for numbers of three 
 places is not much greater than that for numbers of one place ; but 
 when the complexity is increased to four places, the reaction-time 
 is suddenly and largely increased. This fact is probably due in 
 part to the habit of grasping numbers mentally in periods of three 
 places each. It is not certain, however, that the apperception-time 
 as calculated by this method is trustworthy ; for, although the sub- 
 ject of experiment reacts, and then notifies the number discerned, 
 the act of discernment may really be completed after the reaction 
 has taken place. Moreover, a certain time for accommodating the 
 eye must be allowed ; especially in the case of numbers of five or 
 six figures each. This time, in the opinion of one critic, may be 
 placed at 0.166 to 0.186 sec. A more recent investigator 1 concludes 
 that the true discernment-time in a composite perception is possibly 
 so short as to occupy only a few hundredths of a second. For 
 numbers of one to three places the interval between perception and 
 apperception is 0.015 to 0.035 sec. This conclusion accords with 
 and enforces that arrived at by the careful experiments of von Kries 
 and Auerbach. 
 
 Cattell 2 by assuming that perception-time and wdll-time are 
 about equal, and thus dividing into two parts the remainder ob- 
 tained by subtracting the simple reaction-time from the whole time 
 including both discernment and choice estimated the perception- 
 time for light, of B. at 0.030 sec., and of himself at 0.050. It took 
 longer, of course, to discern between two colors. Between red and 
 blue discernment required about 0.038 sec. for B., and 0.054 for C. 
 If one color among ten was to be discerned, the time rose to 0. 105 
 for B., and 0.117 for C. The time required to discern one letter 
 from all the others was found to vary somewhat for the different 
 letters ; but the legibility, or comparative accuracy of the quick 
 discernment, varied still more. In this sense the letter W is about 
 four times as " legible " as E. The discernment-time for a simple 
 picture of a familiar object was found to be about the same as that 
 for a color, and shorter than that for a letter or a word. 
 
 19. We have already seen (p. 480) that Donders assigned a 
 little less time (0.036 sec.) to the operation of will in setting free 
 the required impulse when a dilemma was presented to it than is 
 required for discernment between two objects of the visual sense. 
 The duration of " will-time" (the third element of psycho-physical 
 time) has since been investigated at greater length by Friedrich, 
 Buccola, Merkel, and others. The first of these experimenters 
 
 1 Tigerstedt, in Zeitschrift f. Biologie, 1883, XIX., p. 42 f. 
 3 See Mind, July, 1886. 
 
486 APPERCEPTION AND WILL TIME. 
 
 determined the time required for simple choice by finding how 
 much the reaction-time is increased when the subject of experiment 
 must decide whether to react or not. l For example, let the sensa- 
 tions of black and white follow each other in unknown order, it be- 
 ing understood that only white is to be followed by reaction. The 
 interval for simple choice was thus found by Friedrich to have a 
 mean value of 0.152 sec. for W., 0.184 for T., and 0.183 for him- 
 self ; it lay, therefore, between ^ and ^ sec. If choice was re- 
 quired, however, between the two hands reaction with one hand 
 being the sequence of the appearance of one color, and with the 
 other hand of the appearance of the other color the interval 
 assigned to this element of psycho-physical time increased to 0.188 
 sec. for W., 0.330 for T., and 0.287 for F. 2 
 
 Merkel 3 has perhaps investigated most thoroughly the time- 
 relations of activities of the will. The question he proposes for 
 answer is the following : How long does it take, under different 
 circumstances, to set free a voluntary impulse ? He assumes that, 
 in all cases where reaction follows discernment between two or 
 more impressions, some " will-time " is present, although the 
 amount of this time may become so minute as to escape detection. 
 The method of Merkel was suggested by Wundt. The simple 
 reaction-time (R), or time required when the nature of the stimulus 
 is known and the mode of reaction fixed the same for all cases, is 
 first found. The reaction-time required to discern clearly one of 
 two or more impressions, and announce the fact in some one way 
 previously determined upon (R d), is next found. Finally, the re- 
 action-time is found for cases where there is involved, in addition 
 to discernment, a choice between one or more ways of reacting, or 
 between reacting and not reacting (R d w). The difference RdRis 
 then held to give the "discernment-time;" the difference R dwR d 
 is held to give the " will-time." It is with the latter that we are 
 now concerned. For his experiments Merkel used figures of about 
 13 mm. altitude placed 250 mm. distant from the eye. To deter- 
 mine the reaction-time, including discernment (R d), but not includ- 
 ing choice, reaction is ordered invariably with the same finger. To 
 determine reaction-time, including choice (R d w), reaction is ordered 
 with some one finger of either hand previously assigned to each 
 number. Will- time is then found by subtracting the reaction - 
 time in the former case from that in the latter (R dwR d). The 
 
 1 Friedrich, in Wundt's Philosoph. Studien, I., heft i. , p. 57 f. 
 
 2 Only a few experiments of this kind were tried ; the numbers are those 
 given by Merkel in his article. 
 
 3 Article in Wundt's Philosoph. Studieu, II., heft i., pp. 73-127. 
 
DIFFERENCES OF INDIVIDUALS. 487 
 
 results showed that while the simple reaction-time for the different 
 fingers of both hands does not differ greatly, the difference in will- 
 time for the different fingers is much more marked. The latter 
 difference is greater among the fingers of the right than of the left 
 hand. For ten persons experimented upon, the mean interval 
 required for setting free a definite reaction, with a choice between 
 two possible courses, varied from 0.024 to 0.155 sec. This inter- 
 val increases for every additional course possible until, in case the 
 subject of the experiment is required to select one of his ten fingers 
 with which to react on receiving an impression corresponding to 
 that finger, the will-time becomes 0.298 to 0.448 sec. 
 
 Very interesting individual differences in the speed of decision 
 attained under different conditions of complexity are brought out 
 by Merkel's experiments. This fact may be shown by plotting the 
 curve of the will-time of each individual. In general, it was found 
 that the individual differences increased as the complexity of the 
 choice required was increased from one to five places, and that they 
 then fell off, being least at nine or ten places. That is to say, different 
 individuals differ much more markedly in the speed with which they 
 can choose one of two or five than one of nine or ten different pos- 
 sible ways of reaction. Merkel's value for will-time when the choice 
 lies between two courses (E w 0.024 to 0.155 sec.) may profitably be 
 compared with that given by Buccola for choice between motion and 
 rest with discernment of color-tone and locality (0.028 sec. and 0.066 
 sec. respectively), or with that given by Tischer 1 for choice between 
 motion and rest (for nine persons, 0.052 to 0.179 sec*.) or for choice 
 between two symmetrical motions (0.033 to 0.179 sec.). 
 
 20. A careful survey of the statistics and discussions furnished 
 by different experimenters shows that it is not as yet possible to 
 analyze with perfect confidence the different elements of psycho- 
 physical time. Wundt seems justified in holding that will-time is 
 necessarily involved in the choice between motion and rest. But, 
 on the other hand, von Kries and Auerbach appear to have reduced 
 this time almost or quite to zero, by practice and by arranging 
 their experiments under the most favorable conditions. By elimi- 
 nating will-time they have, perhaps, found about what is the least 
 possible interval required for simple acts of discernment. There is 
 other evidence (from the phenomena of rhythm, etc.), however, to 
 indicate that successive acts of discernment may attain a higher 
 rate of speed than is possible for successive acts of will. 
 
 21. Investigation has also been directed toward determining 
 how far series of events in consciousness correspond, as regards 
 1 See Wundt's Philosoph. Studien, I. , heft iv. , p. 533 f. 
 
488 APPERCEPTION AND WILL TIME. 
 
 time, to the series of excitations which occasion them. Something 
 may thus be accomplished toward fixing the time-rate of conscious- 
 ness, as well as the interval which it is possible for the mind to ap- 
 preciate with the nearest approach to perfect accuracy. In all the 
 preceding experiments the subject of them is uniformly aware that 
 a pause, as it were, takes place between the excitation and the re- 
 action ; this pause he is able to estimate with much accuracy, and 
 so to tell whether his effort to react promptly has been more or less 
 successful. Exner ' states that in 39 cases of reaction from eye to 
 foot, which had a mean of 0.184 sec., the reaction was always felt 
 (with a single exception) to be " too slow " when it reached 0.1994 
 sec., and pronounced " very good " when it fell below 0.1781 sec.; 
 its time was therefore estimated within about 0.01 sec. There is 
 abundant proof, however, that the speed and duration of our sensa- 
 tions, as estimated in consciousness, do not precisely correspond 
 with the series of stimulations of the organ of sense. Indeed, 
 under certain circumstances very remarkable errors may occur in 
 our estimate of both the rate and the interval of our mental acts. 
 
 To show the fact and amount of the error which takes place when 
 we are called to intercalate an excitation of any kind in a series of 
 impressions, Wundt 3 devised the following experiment : An indi- 
 cator is kept moving at a uniform rate over a graduated scale, and 
 so situated that the place of the needle can be clearly seen at each 
 instant of time. The action of the same clock which moves the 
 needle causes a sound at any moment, but in such a way that the 
 subject of experiment does not know when to expect it. With 
 what position of the needle, now, will the sensation of sound be 
 combined? Will the sound be heard exactly when it occurs as 
 indicated by the needle ; or later than its real time (" positive " 
 lengthening) ; or earlier than its real time (" negative " lengthen- 
 ing) ? The result shows that one rarely hears the sound without 
 either positive or negative displacement of it ; but most frequently 
 the lengthening is negative that is, one believes one hears the 
 sound before it really occurs as measured by the indicator. 
 
 22. Vierordt, 8 after experimenting upon our power to repro- 
 duce the interval as heard between two sensations of noise, con- 
 cluded that very small intervals are regularly overestimated and 
 greater ones underestimated. The minimum of error in estimating 
 intervals the duration that corresponds most perfectly in our con- 
 sciousness to the real duration as measured by objective methods 
 
 ! Hermann's Handb. d. Physiol., II., ii., p. 273 f. 
 2 Physiolog. Psychologic, ii., p. 264 f. 
 Der Zeitsinn. Tubingen, 1868. 
 
INTERVAL OF MINIMUM ERROR. 489 
 
 he placed at 1-1.5 sec. More recently (1881) Kollert ' published 
 the results of experiments instituted with a view to determine the 
 accuracy of our sense of time. Suppose that one metronome is 
 marking off time, by the sound it makes, with a normal interval 
 = t. Another metronome is at the same time at work with an in- 
 terval = 3, that may be made to vary with different experiments, 
 and that is set as either equal to or slightly greater or less than t. 
 Let T the time in our consciousness which is equivalent to t, and 
 A = a constant representing the mean error which is made in es- 
 timating the relations of the two intervals (t and <9) ; that is, A = 
 t $. Let 3 l the intervals of consciousness just observably 
 smaller, and $ 2 = those just observably greater, than the normal 
 time (t). Then the following table shows how A varies as t varies; 
 d l giving the experiments in which the interval of the variable 
 metronome was smaller, and d 2 those in which it was greater, than 
 t (^ - t = d l5 and S s - t = dj. 
 
 t= dj= d a = A= 
 
 0.4 -0.018 +0.090 +0.036 
 
 0.5 0.044 0.098 0.026 
 
 0.7 0.044 0.055 0.005 
 
 0.8 0.073 0.060 -0.006 
 
 1.0 0.107 0.063 0.022 
 
 1.2 0.206 0.074 0.066 
 
 It will be noticed that, according to these results, our sensitive- 
 ness to minute differences of time varies for different intervals so 
 that it is greatest at 0.7-0.8 sec. (A = o at about 0.755 sec.) ; while 
 it falls off quickly for intervals less than this, and more slowly for 
 intervals longer than this. Kollert confirms Vierordt's statement 
 that times above this most favorable time are estimated too small, 
 and those below too large. He also concludes that, as the normal 
 time increases, our sensitiveness to minute differences of diminution 
 is lessened, while our sensitiveness to minute differences of increase 
 grows greater. 
 
 The same method as that employed by Kollert has been applied 
 by another observer 2 to intervals longer than 0.4-1.5 sec. Using 
 intervals of 1.8-8.0 sec. for the normal time, the sensitiveness of 
 our estimate of minute differences was still found to diminish as 
 the normal time increases ; thus the mean error of all the individ- 
 uals engaged in the experiments (A m) increased from 0.0792- 
 0.0879 sec. for intervals of 1.8-2.0 sec. to 0.5988 sec. for intervals 
 
 1 Wundt's Philosoph. Studien, L, heft i., pp. 78 ff. 
 
 2 Estel, in Wundt's Philosoph. Studien, II. , heft i., pp. 37 ff. 
 
490 COMPLEX REACTION TIME. 
 
 of 8.0 sec. 1 Estel reaches the conclusion that "our ideas of time, 
 like our other sensations and ideas, are essentially conditioned upon 
 past impressions belonging to the same domain of sense ; and a 
 short time makes the one next succeeding appear longer, while a 
 long time shortens still more the succeeding shorter time." A 
 more recent investigator 2 places the interval which can be repro- 
 duced with greatest accuracy at 0.53-0.87 sec. ; but reaches the 
 conclusion that, with all other intervals, an error is made which is 
 plus for those above, and minus for those below, this so-called in- 
 difference-point. This conclusion would seem to need re-examina- 
 tion, since it is exactly the opposite of that of preceding investi- 
 gators. The difference in result may, however, be clue to difference 
 in the method of experiment, which in the one case consisted in 
 noticing the least observable difference in two series of intervals, 
 and in the other consisted in catching, mentally, a given interval, 
 retaining and reproducing it. 
 
 23. In this connection should be mentioned the results of in- 
 teresting " studies of rhythm " undertaken by G. Stanley Hall and 
 J. Jastrow. 3 These observers experimented to find the degree of 
 accuracy with which successive clicks having a constant interval 
 can be counted. It was found that persons most successful were 
 able to count 2-4 clicks with perfect accuracy, when the interval be- 
 tween them was 0.0895 sec. ; but if this interval was diminished to 
 0.0523 sec., they could not be sure of more than two clicks. When 
 the number of clicks was increased to 45, with the longer of these 
 intervals, the most successful estimates were 42 and 43 ; with the 
 shorter interval, for the same actual number, the best estimate was 
 32 and the worst was 17. The conclusion is thus reached that " count- 
 ing requires a series of innervations, if not of actual muscular 
 contractions," and that " attention discriminates sensation much 
 more rapidly than the will can generate impulses." If, then, the 
 interval between the acoustic sensations is less than the reduced 
 reaction-interval (or time necessary for starting successive impulses 
 of innervation) between ear and tongue, some of the sensations will 
 drop out of consciousness as a result of the blending, as it were, 
 of the later afferent with the earlier efferent stage of the complex 
 process. The time-sense for series of mental phenomena is then 
 different for different classes of these phenomena. The rate of 
 sensation may considerably surpass the rate of motor impulses ; 
 " we do not realize how far the fastest counting falls short of the 
 
 1 Estel, in Wundt's Philosoph. Studien, II., heft i., p. 43. 
 8 L. T. Stevens, in Mind, October, 1886, pp. 393-408. 
 8 Mind, January, 1886, pp. 55 ff. 
 
EEPEODUCTION OF COMPOSITE IMAGES. 491 
 
 fastest hearing." The same observers found that the most rapid 
 possible rate of pronouncing the names of letters was greater than 
 that of counting them ; the former being 0.248 sec. per letter for 
 50 letters, and the latter 0.283 sec. The reasons for this dif- 
 ference are apparently to be found in the fact that counting in- 
 volves a severer strain on the attention and more complex processes 
 of association and discernment in order to give to each number its 
 right name and place in a series of numbers. 
 
 24. We are thus led to consider certain researches for deter- 
 mining the reaction-time of yet more complex mental processes, 
 such as involve reproduction of composite images of memory and 
 the association of ideas. The mean duration of association that 
 is, " of the time which is required for the reproduction of an image 
 of memory by some apperceived presentation of sense " has been 
 investigated at great length by Trautscholdt ' under the direction 
 of Wundt. For purposes of experiment the following elaborate 
 classification of the possible kinds of association was adopted. All 
 associations are either (I) External or indirect, or else (II.) Inter- 
 nal or direct. The former are such as are induced by the habit 
 of perceiving objects together simultaneously or successively in 
 space or time, without any interior relation between them ; the 
 latter are such as imply kinship in common properties or other in- 
 herent relations. External associations may, then, be either simul- 
 taneous, as in the case of the parts of a single presentation of sense 
 or of the coexistence of independent presentations ; or they may be 
 successive, as in the case of repeated impressions of sound, of sight, 
 and of the other senses. Internal associations may either be such 
 as involve the relation of ranking one mental object below or above 
 another in terms of genus and species ; or they may follow relations 
 of co-ordination, as similar or contrasted ; or they may follow rela- 
 tions of dependence, as of cause and effect, means and end. To de- 
 termine the time required for a " word-reaction " ( Wr), the experi- 
 menter spoke aloud some word at the instant that he pressed down 
 his key ; and the person reacting indicated the instant at which he 
 apprehended the word. The time required for such reaction was 
 found to have the following mean value for the four persons ex- 
 perimented upon : For W., 0.303 sec.; for B., 0.285 ; for H., 0.280 ; 
 and for T., 0. 173. a The time required for the discernment of a single 
 word is obtained by subtracting from these numbers the simple 
 reaction-time (R) for each individual (Wr E). The result obtained 
 
 1 Experimented Untersuchungen iiber d. Association d. Vorstellungen, in 
 Wundt's Philosoph. Studien, I., heft ii., pp. 213-250. 
 1J Philosoph. Studien, I., heft ii., p. 236. 
 
492 
 
 COMPLEX KEACTION-TTME. 
 
 gave, for W., a "discernment-time "of 0.107sec.;forB., of 0.177; for 
 H., of 0.137 ; and for T., of 0.057. In experiments to determine the 
 duration of association, the person reacting does not press his key 
 until the instant when the reproduction of some idea called forth 
 by hearing the word spoken has fully taken place. For example, 
 on hearing the word " zero " the idea of " infinity " may arise in 
 the mind ; or "market-place" as associated with "market," "port- 
 folio" with "letter," etc. The mean reaction-time for such as- 
 sociation, both before and after subtracting the time for word- 
 reaction alone, was as follows : 
 
 
 W. 
 
 B. 
 
 H. 
 
 T. 
 
 Association-reaction (-47*) .... 
 
 Sec. 
 
 1.009 
 
 Sec. 
 
 1.037 
 
 Sec. 
 1 154 
 
 Sec. 
 
 0896 
 
 Association-time (Ar Wr). . . . 
 
 0.706 
 
 0.752 
 
 0.874 
 
 0.723 
 
 The mean duration of association for all the subjects of experi- 
 ment (excluding one for special reasons) is, therefore, placed at 
 0.727 sec. It takes, that is to say, about f of a second of psycho- 
 physical time to recall a familiar idea associated with a word we 
 hear. This association -time was most prolonged in certain cases 
 where the result may be looked upon as odd and unexpected, or 
 where a pause would seem to have taken place through hesitation 
 between several ideas simultaneously suggested. For example, the 
 association-time which elapsed between the word "pious" and the 
 idea " God-fearing " was 1.132 sec.; between " throne " and " king," 
 1.437 ; between the German word " Sieg " and " a person of this 
 name," 1.626 ; between "Karl" and "August," 1.662. The mini- 
 mum of association-time was reached with ordinary words where 
 the associated ideas were such as all individuals are likely to have in 
 common. Thus, from " gold " to " silver " required only 0.402 sec.; 
 from "storm" to "wind," 0.368 ; from "clear" to "dark," 0.507; 
 from "north" to "south, "0.502; from "duty" to "right," 0.415; etc. 
 
 25. Trautscholdt 1 also found as we should expect that 
 judgments involving subsumption, or the definition of the word 
 heard, required more time than mere association, when they were 
 at all complex. But when very simple, the minimum time for such 
 judgments was about that of association ; and the mean value of 
 " subsumption-time " (0.766) differed but very little from the mean 
 value of " association- time." Thus it required 1.403 sec. to judge 
 that a " ray " is a "'form of the motion of light ; " 2.023 to judge 
 1 Philosoph. Studien, I., heft ii., p. 245 f. 
 
MULTIPLICATION OF TWO NUMBERS. 493 
 
 that " fame " is a, " form of the ascription of praise ; " and 1.899 to 
 judge that " art " is an " aesthetic activity " of man ; but only 0.391 
 sec. to recognize a "mast "as a "part of a ship," and 0.469 to 
 identify "egg" and "cell." 
 
 The time necessary for multiplying, in the head, two numbers of 
 one figure each has also been investigated by von Vintschgau. 1 
 The psycho-physical processes involved in this achievement are 
 necessarily somewhat complex. The influence of association must 
 vary largely, according to the number of times we have previously 
 made the same or any very similar calculation. For example, the 
 whole process involved in answering the question, How much is 
 1x1? is different from that involved when the numbers are 8 x 12 ; 
 much more different when they are quite unfamiliar as, e.g., 
 76 x 89. The order of the numbers is not indifferent ; as a rule, 
 reaction is quicker and more correct when the smaller number pre- 
 cedes. The mean reaction-time for multiplying two numbers (1 to 
 10) was 0.211 sec. for L., 0.207 for P., and 0.259 for V., when repro- 
 duction was with the finger ; and 0.200 for L., 0.252 for P., 0.248 
 for V., when reproduction was with the lip. Of this, 0.049 for L. 
 reproducing with the finger, and 0.096 with the lip ; 0.051 for P. 
 with the finger, and 0.082 with the lip ; and 0.098 for V. with the 
 finger, and 0.087 with the lip are calculated to have been due to 
 the cerebral processes involved in the calculation. As might be 
 expected, any great increase in the speed was found to be accom- 
 panied by an increase in the number of mistakes. 
 
 Cattell, 2 after objecting on good grounds that the results of 
 the laboratory are always too artificial and often too incorrect to 
 " give the time it takes a man to perceive, to will, and to think," 
 attempted to do away with "involved methods and complicated 
 apparatus," and in simpler fashion determine the time we usually 
 require to see and name an object, such as a letter or a color. He 
 concludes that to see and name letters requires from J to ^ sec. 
 for each letter, and to see and name words that do not make sen- 
 tences requires J to J sec. When the words are connected into sen- 
 tences it requires only about one-half as much time to name them 
 the rate at which one can read varying from 0.138 to 0.484 sec. 
 for each word, according to one's degree of familiarity with the 
 language. Single letters can be named more rapidly when several 
 are in view at the same time ; nearly all persons are helped by hav- 
 ing as many as three, and most persons by having as many as four 
 
 1 See Pfliiger's Archiv, xxxvii., pp. 127 ff. 
 
 s See Mind, January, 1886, p. 63 f.; and Wundt's Philosoph. Studien, II.. 
 heft iv., pp. 635 ff. 
 
494 COMPLEX REACTION-TIME. 
 
 or five, letters in view at the same time. That is to say (using 
 Wundt's figure of speech), the time of perception, or field of con- 
 sciousness, covers the time of apperception, or clear spot of con- 
 sciousness ; while one presentation of sense is at the focus of 
 consciousness, several others may be coming toward the focus 
 from the background of consciousness. The second letter in view 
 shortens the time of apperception about ^ ; the third, -^ ; the fourth, 
 ^ ; the fifth, ^ sec. 
 
 26. Finally, the closing remark of the foregoing paragraph in- 
 troduces the inquiry into the "circuit of consciousness" that is, 
 the number of impressions which can exist within the field of con- 
 scious perception at any one time ? This question has been much 
 debated on abstract or metaphysical grounds touching the nature 
 of the soul. Since the soul is one and simple it has been claimed 
 there can be before it but one object at the same instant of time ; 
 but since all its knowledge is relative, the claim has also been 
 made that at least two impressions are always contemporaneous in 
 consciousness. Hamilton,' on the contrary, concluded, from ob- 
 serving his own mental activities, that the circuit of consciousness 
 could embrace as many as six or seven distinct simultaneous im- 
 pressions. It is evident that only an appeal to facts can decide 
 such a dispute ; it is also evident that the manner of appeal should 
 be more precise and scientific than the one proposed by Hamilton. 
 
 Experiments have therefore been instituted to determine the 
 " circuit of consciousness," by finding how many regularly recurrent 
 successive impressions of sound, for example, can be so far united 
 into one mental image as to have their likeness or unlikeness to 
 another similar series clearly discerned. The method of experi- 
 ment employed by Dietze 2 was the following : The stroke of a pen- 
 dulum, heard at regular intervals, was employed as the stimulus. 
 After a single stroke as a warning, a series of successive strokes 
 was given, which was begun and ended by the sounding of a clock- 
 bell simultaneously with a stroke of the pendulum. Another series 
 of strokes of the pendulum followed immediately upon the sound 
 which announced that the first series was closed. The second se- 
 ries was ended by simply stopping the pendulum. A fixed number 
 of strokes constituted the first series ; a variable number (either one 
 more or one less than the first series) constituted the second. The 
 question proposed for answer was : How many impressions of sound 
 can be received in the first series and the relation of the second 
 series to the first (as equal, greater, or less) be accurately discerned 
 
 1 Lectures on Metaphysics, p. 165. f., and elsewhere. Boston, 1860. 
 
 2 Article in Philosopli., Studien, IL, heft iii., pp. 362 ff. 
 
EFFECTS OF PRACTICE AND ATTENTION. 495 
 
 without counting? The effect upon the subject of such an experi- 
 ment may be described as that of sending through the focus of con- 
 sciousness a train of impressions, in regular succession, from the 
 obscure regions of perception at which they enter to the obscure 
 regions at which they depart from the field of consciousness. An 
 image of the whole field of consciousness, with the line of march of 
 the impressions, must be formed in order accurately to compare two 
 series like those described. The result of such experiments showed 
 that the number of successive impressions which can be comprised 
 within the circuit of consciousness depends upon the rate with 
 which they succeed each other. The most favorable interval was 
 found to be 0.2-0.3 sec. ; with this interval most of the subjects of 
 experiment attained a high degree of accuracy for even as many as 
 ten or twelve impressions. 1 Individual differences were marked, 
 however; thus, for one subject, the time of 0.21 sec. interval was 
 too great for the maximum of accuracy, and 2.0 sec. was so large 
 as to prevent his having any satisfactory apperception of even a 
 single impression. The manner of uniting the impressions was 
 also found to have a great effect upon the circuit of consciousness. 
 When the process of apprehension was allowed to have a rhythmic 
 form of grouping the impressions, the number possible in a single 
 field of consciousness was increased. Without grouping, 16 was 
 the maximum even number, 15 the maximum odd number, at- 
 tained. Rhythmic grouping raised this number to 40, or a little 
 more, as the maximum even number, and to 35, 37, and 39 as the 
 maximum odd numbers. It is possible, then, to apprehend a larger 
 even number than odd number of impressions in a single circuit 
 of consciousness. Certain numbers, perhaps those most familiarly 
 grouped in experience, seem also to have the preference over 
 others. 
 
 27. In all experiments to determine the time-relations of mental 
 phenomena, the effects of practice and attention in diminishing 
 psycho-physical time, and of fatigue to increase it, are made ap- 
 parent. Certain special experiments have also been instituted to 
 show how illness, old age, and drugs operate upon the speed of 
 psycho-physical time. Thus Merkel 9 found that the will-time ne- 
 cessary for choice between two motions was reduced by practice, for 
 three subjects of experiment, from 0.080 sec. to 0.050, from 0.097 
 to 0.0535, and from 0.098 to 0.062, respectively. For choice among 
 
 1 See Dietze's statement, Philosoph. Studien, III., heftiii., p. 386, and a 
 note, p. 384, correcting the erronous conclusion drawn by M. Ribot (German 
 Psychology of To-day, p. 278, New York, 1886). 
 
 * Wundt's Philosoph. Studien, II., heft L, p. 110 f. 
 
496 COMPLEX REACTION-TIME. 
 
 five and ten possible motions, the effect of practice was yet more 
 marked : thus, with five possible choices, the will-time of one per- 
 son was reduced by practice from 0.239 sec. to 0.083 ; and of an- 
 other, with ten possible choices, from 0.358 to 0.094 For each 
 single day's series of experiments, the time diminished faster at first 
 than subsequently ; but, in many cases, more distinctly on the second 
 than on the first day of experiment. Kries and Auerbach ' discov- 
 ered that, after a short time, further practice has no influence on sim- 
 ple reaction-time ; but some practice is necessary to give any reliable 
 value to such time. The effect of practice on discernment-time is 
 very different. In the experiments upon the localization of tactile sen- 
 sations, the discernment-time for A. was at first as great as 0.064 sec., 
 or even 0.117, but afterward fell to a mean of 0.021. Discernment- 
 time continues to decrease by practice, after all diminution of sim- 
 ple reaction-time has ceased ; it is also transferable to other regions 
 of the same sense. Trautscholdt 2 found that practice for fourteen 
 days reduced the "word-reaction-time" of one subject from 0.309 
 sec. to 0.149 ; association-time was found to be sensitive to practice 
 in a much smaller degree. The effect of attention must also al- 
 ways be taken into account. It was, apparently, by guarding this 
 carefully, and by practice, that Kries and Auerbach succeeded in 
 obtaining such small values for discernment-time. The effect of 
 distracting the attention was observed by Wundt, 3 who introduced 
 disturbing sensations of the same or of a different sense into series 
 of regularly recurring impressions. Thus, the mean reaction-time 
 for a weak impression of sound was lengthened from 0.189 to 0.313 
 sec., by a disturbing noise ; and for a strong impression of sound, 
 from 0.158 to 0.203 sec. The mean reaction-time for sight of an 
 electric spark was increased from 0.222 to 0.300 by a disturbing 
 noise occurring simultaneously. 
 
 All the experiments also make obvious the great influence of 
 individual peculiarities. But this influence may not be in the di- 
 rection in which it would most readily be supposed to lie. For 
 example, Exner 4 found that, of two young men, one of whom had a 
 very lively temperament and the other not, the former had much the 
 longer simple reaction-time (0.3311 sec. as compared with 0.1337). 
 The reaction-time of a man of seventy-seven, taken from the alms- 
 house, was at first 0.9952 sec. ; this was, however, reduced by practice 
 to 0.1866 sec. The enervation produced by a hot summer's day, or 
 
 1 Archiv f. Anat. u. Physiol., Physiolog. Abth., 1877, p 361 f. 
 8 Wundt's Philosoph. Studien, I., heft ii., p 237 f. 
 
 3 Physiolog. Psychologie, ii., p. 243. 
 
 4 Hermann's Handb. d. Physiol., II., ii., p. 268. 
 
THE RESULTS OF EXPERIMENT. 497 
 
 the exhaustion of a sleepless night, or bad news, etc., increases the 
 reaction-time. A small quantity of wine, slowly drunk, decreased 
 the reaction-time ; but a larger quantity increased it from 0. 1904 
 sec. to 0.2969, although the subject of the experiment considered 
 himself to be reacting more promptly than usual. Coffee begins to 
 decrease the reaction-time at 20-25 minutes after it is taken, and 
 continues this effect for about 2 hours. Subcutaneous injections 
 of morphine delay ' it ; but this effect does not last long unless the 
 injection is repeated. Obersteiner found that a subject whose re- 
 action-time in the first stage of paralysis was 0.166 sec., gradually 
 lost control of himself until, in the last stages when experiment was 
 possible, the interval was 0.281-0.753. Buccola, who has experi- 
 mented upon idiots, imbeciles, epileptics, etc., finds that the dura- 
 tion of perception is lengthened in all these cases, with the exception 
 of some forms of abnormal excitement. 
 
 28. On summing up the results of all the experiments hitherto 
 made in psychometry, we can only reiterate what we began by say- 
 ing : Experimental research does not explain the origin or nature 
 of our idea of time and its relations, nor has it succeeded in estab- 
 lishing many new principles of great moment for psychology. It 
 is, however, a vigorous and promising branch of psycho-physical 
 study. It has placed upon a scientific basis, and defined in accurate 
 mathematical terms, many of our ordinary impressions as to the 
 time-relations of mental phenomena. The attempt to analyze psy- 
 cho-physical time seems to show that its various elements of simple 
 perception, apperception, or clear discernment, and volition, occur in 
 the order named, and yet ordinarily overlap each other, as it were. 
 Practice and attention, under the most favorable circumstances, may 
 reduce either one nearly to zero. In this way, simple reaction-time 
 becomes most nearly equal to purely physiological or reflex time ; 
 reaction-time, with discernment, is almost reduced to simple per- 
 ception-time ; and the duration required by will-time for solving a 
 dilemma is wellnigh eliminated. 
 
 Conclusions as to the existence and intercerebral relations of 
 nervous centres of apperception and volition would be premature, 
 and probably misleading, in the present state of this science. Nor 
 have we as large hopes as to its ability "to solve many of the old 
 problems " in the future as have been expressed by some of its 
 enthusiastic students. 
 
 1 See von Vintschgau andDietl, in Pfliiger's Archiv, xvi., pp. 316 ff ; and 
 Exner, in Hermann's Handb. d. Physiol., II., ii., p. 270 f. 
 32 
 
CHAPTER IX. 
 FEELINGS AND MOTIONS. 
 
 1. FBOM this point onward the study of Physiological Psychol- 
 ogy is compelled to content itself with opinions much more in- 
 definite and uncertain even than those to which we have already 
 become accustomed. Theories of the localization of cerebral func- 
 tion, of the quantity and quality of sensations and their combina- 
 tion into presentations of sense, and of the time-relations of mental 
 phenomena, admit to a considerable extent of experimental tests. 
 But the feelings and their physical basis elude the efforts made to 
 subject them to the conditions of a strictly scientific investigation. 
 The same complaint may be justified concerning all opinions as 
 to the physical basis of the higher intellectual operations, and as 
 to the effect of age, temperament, sex, and race, upon the character 
 and development of the mind. No attempt whatever will be made 
 to conceal the meagre, obscure, and doubtful character of the evi- 
 dence upon which our conjectures must be based. Indeed, on 
 these matters nothing but the greatest caution is fitted to inspire 
 confidence ; the supreme wisdom is not infrequently a frank con- 
 fession of ignorance or uncertainty. 
 
 About our "Feelings" so-called their nature, origin, relation 
 to a physical basis and to sensations and ideas we know remark- 
 ably little. Nor has any classification of the feelings hitherto been 
 made which is entitled to command general assent. 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 sensations 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 unproductive 
 of assured results. The fact that their physiological conditions 
 are laid so largely in obscure, rapid, and infinitely varied changes 
 within the central organs, such as cannot be either directly ob- 
 served or indirectly subjected to experimentation, increases the 
 difficulties of the subject. What is the nature of feeling ? How do 
 
DIFFERENT THEORIES OF FEELING. 499 
 
 the different feelings differ, and what elements have they in com- 
 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 ? l These and 
 other similar questions may be asked of psycho-physical science 
 with little satisfactory result. 
 
 2. Many diverse views have been held as to the essential na- 
 ture of feeling. These views may, for the most part, however, be 
 classified under two heads : They are either such as emphasize the 
 dependence of the feelings on bodily conditions, and so resort to 
 physiological explanation of their origin and nature ; or else they 
 are such as emphasize their dependence upon relations that obtain 
 among the so-called " ideas," or purely mental states and products 
 of the mind. 1 The extreme form of one of these two theories holds 
 that feeling is always merely a consciousness of a certain condition 
 of the nervous elements. The extreme form of the other leads to 
 the position from which all feeling is regarded as a sort of sec- 
 ondary consciousness of the " furtherance " or " hindrance " of one 
 idea by another. The principal real ground for the former theory 
 lies in the fact that certain conditions of the nervous elements under 
 stimulation are, as a rule, followed by painful, and certain others by 
 pleasurable, feeling. The latter theory is based solely upon the truth 
 that certain mental states called " ideas " are, as a rule, accompanied 
 or followed by corresponding modes of being affected which have 
 the characteristic tone of all feeling that is, are either agreeable or 
 disagreeable, Neither of these views, however, serves to define the 
 essential nature of feeling, since to feel is as simple and funda- 
 mental an operation of mind as it is to know. Feeling can never 
 be stated in terms of knowledge. Inasmuch, then, as all definition 
 is only the expression of an elaborate and complex form of knowl- 
 edge> 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 physical sensation or to mental images, it is de- 
 prived of the very characteristic which makes it to be feeling rather 
 than sensation or idea. Both theories, however, have succeeded in 
 stating certain conditions or antecedents of the reaction of mind in 
 the way of feeling. 
 
 3. The foundations for a physiological theory of the feelings 
 
 1 Comp. Hortricz, Psychologische Analysen, i. , p. 21 f. 
 
500 NATUKE AND KINDS OF FEELING. 
 
 were laid by Lotze, ' with that blending of scientific caution and psy- 
 chological insight which characterize most of his work. He distin- 
 guished the feelings, as mental conditions of pain or pleasure, from 
 sensations as indifferent elements of our percepts of things. Yet, 
 iu fact, sensations are always, or usually, colored with feeling ; and 
 analysis is therefore obliged here to distinguish in theory what co- 
 exists in fact. Feelings are of two kinds : " sensuous," as corning 
 from bodily impressions ; and " intellectual," as flowing from the 
 relations of ideas. Pleasurable feelings always arise from the co- 
 incidence, and painful from the opposition, between the effects of 
 the stimulus and any one of those conditions to which the regular 
 expression of the bodily or spiritual life is attached. Yet even this 
 statement is not true without making further explanations and 
 limitations. For something bitter may be harmless, or even bene- 
 ficial ; and acetate of lead may be sweet and pleasant, but deadly. 
 More precisely, then, " feeling is, in general, only the measure of 
 the partial and momentary concord between the effect of the stimu- 
 lus and the conditions of vital activity." 2 Lotze was far too keen 
 a psychologist, however, to suppose that in laying down this law 
 he was explaining the nature of feeling as a secondary^ and derived 
 form of consciousness. He has himself vindicated its right to be 
 regarded as primitive, and not deducible from either sensations or 
 ideas. 3 But even in the way in which he understood his own 
 theory, much doubt may be thrown upon its truth. 
 
 It is admitted by all that certain intensities of nearly all forms of 
 stimuli are both productive of painful feeling and also antagonistic 
 tolhe vltaTconditions of the organism. Undoubtedly, also, suffering 
 'is both an indication and a cause of abnormal and injurious action of 
 the nervous mechanism. But that the feeling of pain measures the 
 degree of this antagonism, or that everything found at the time dis- 
 agreeable is in any degree demonsti'ably opposed to the vital wel- 
 fare of the organism, cannot be assumed ; and the alleged law even 
 seems incompatible with the individual peculiarities which charac- 
 terize what is agreeable or disagreeable to the senses of different 
 persons. The excessive stimulus of the surgeon's knife is not ren- 
 dered any more really in accord with the conditions of the vital 
 activity of the organ to which it is applied by the fact that anaes- 
 thetics prevent the pain which would otherwise result. An exces- 
 sive and immediately injurious stimulation of considerable portions 
 of the body may be accompanied by a large amount of positive 
 
 1 Medicin. Psych ologie, pp. 233 ff. 2 Ibid. 
 
 3 See Metaphysic, p. 474, Oxford, 1884; and Microcosmus, i. p. 1771, 
 Edinburgh, 1885. 
 
PHYSIOLOGICAL LAW OF FEELING. 501 
 
 pleasure or, at most, with very little pain, while a small and quite 
 harmless degree of another kind of stimulation may result in great 
 discomfort. Attention, association, and control of the will have 
 also much to do with determining the subjective state which is con- 
 nected with any given relation between the effect of the stimulus 
 and the conditions of vital activity. 
 
 4. More recent attempts to give a general physiological law for 
 the phenomena of pleasurable and painful feeling can scarcely be 
 said to be any more satisfactory. 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 pleasure are connected 
 with an increase, and states of pain with an abatement, 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 immediate result of pro- 
 ducing pleasure rather than pain. After objecting to Bain's state- 
 ment as being " too vague," etc., Grant Allen 2 declares the true 
 principle of connection to be the following : " Pleasure is the con- 
 comitant of the healthy action of any or all of the organs or mem- 
 bers supplied with afferent cerebro-spinal nerves, to an extent not 
 exceeding the ordinary powers of reparation possessed by the sys- 
 tem." ^Esthetic pleasure he provisionally defines as " the subjec- 
 tive 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, 
 pleasureable 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." Does the 
 man whose powers of nervous reparation are extraordinarily great 
 necessarily find his quinine any the pleasanter? The statement 
 that pain is a warning of danger from excessive or abnormal activ- 
 ity of the nervous system must, of course, be accepted as summing 
 up a large number of facts ; but there are other facts not easily 
 brought under such statement. Moreover, until we have some objec- 
 
 1 The Senses and the Intellect, p. 281 f. 
 
 2 Physiological ^Esthetics, London, 1877, p. 21. 
 
 3 Ibid., p. 34. 
 
502 NATURE AND KINDS OF FEELING. 
 
 tive means of determining what is the " normal amount" of function 
 in any tissue, the alleged law that pleasure is " the subjective con- 
 comitant " of such amount is of little or no value. ; The whole sub- 
 ject is left in that indefinite condition which invites us, on the one 
 hand, to consider pain as the proof that the function of the nervous 
 system which occasions it is destructive ; and then tells us, on the 
 other hand, that the essence of the pain is in its being the subjec- 
 tive concomitant of such function, j 
 
 5. But if purely physiological theories of feeling do not suc- 
 ceed in defining its nature, or in stating the relation between the 
 action of the nerves and the pleasureable or painful tone of the 
 feeling, the success of the second class of theories is no greater. 
 Of this class the views of Nahlowsky,' as set forth in his interest- 
 ing monograph, are perhaps the best example. This author begins 
 by drawing a sharp distinction between sensation and feeling. The 
 " tone " of sensations as pleasant or unpleasant he would not call 
 by the term "feeling ; " such tone is rather that which gives us 
 " the how " (" Wie ) of the sensation, and depends for its pleasant, 
 or unpleasant character upon whether the effect of the stimulus 
 furthers or inhibits the functions of the vegetative life. Even those 
 states of consciousness which are constituted from various ele- 
 ments due to stimulations of the nerves at various quarters not defi- 
 nitely localized, and ordinarily called " common feeling," Nahlow- 
 sky would define as " common sensations." Pain also is a sensation 
 and not a feeling. But feeling, according to this author, is neither 
 tone nor quality of sensation ; though it may be an elevation or de- 
 pression of mind produced indirectly by the sensations. Feelings> 
 properly characterized, comprise all the conditions resulting from 
 the simultaneous existence of ideas in the mind, which either sup- 
 port or interfere with each other. In the former case, they are 
 agreeable ; in the latter, disagreeable. They are, then, secondary 
 conditions of mind, dependent on ideas, recognized as not of bodily 
 origin, but as having a content of a mental rather than physical order. 
 Hunger, thirst, weariness, shivering, etc., are sensations ; sympathy 
 love, gratitude, reverence, admiration, etc., are feelings. According- 
 ly, feeling is defined as " the immediate consciousness of the momen- 
 tary rising or depression of one's own psychical activity" (that is, of 
 the movement of the ideas). Even to the affections this theory would 
 deny a place among the kinds of feeling for the former spring 
 frojn the latter under the mediating influence of organic effects. 
 
 *The foregoing view of the nature of feeling is adhered to sub- 
 
 1 Das Gefiihlsleben, in seiner wesentlichsten Ersclieinungen u. Beziigen, 
 2d ed. Leipzig, 1884. 
 
PSYCHOLOGICAL THEORY OF FEELING. 503 
 
 stantially by many others, especially by the followers of Herbart. 
 Thus Drbal ' holds that feelings are not primitive states of mind, 
 but result from the reciprocal action of ideas if this ideating ac- 
 tivity is one of reciprocal inhibition, the feeling which is the be- 
 coming conscious of the inhibition is unpleasant ; if the activity is 
 one of reciprocal combination, the conscious feeling of this fact is 
 pleasant. Feeling in general is therefore the immediate conscious- 
 ness of the rising or falling of one's power of ideating, as it were. ) 
 Beneke, 2 also, considers that two mental images excited belong to 
 every feeling ; of these, one is that which is felt, and the other is 
 one "against" which the first is felt. Feeling and no-feeling, or 
 this or that feeling, can therefore attach itself to one and the same 
 image. Volkmann von Volkmar, in his great work, 3 considers feeling 
 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. 
 
 6. The theory which makes feeling a derived consciousness de- 
 pendent upon the relations of the ideas as furthering or checking 
 each other is unsatisfactory. 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. 4 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 pleasur- 
 able or not, must predominate. Other forms of feeling of desire, 
 uneasiness, comfort, etc. are inseparably connected with its first 
 states of consciousness ; they belong to its inherited impulses and 
 
 1 Lehrbuch d. empirischen Psychologie, 2d ed., pp. 200 ff. Wien, 1875. 
 
 2 Lehrbuch d. Psychologie als Naturwissenschaft, pp. 170 ff. Berlin, 1877. 
 
 3 Lehrbuch d. Psychologie, II., pp. 298 ff. 
 
 4 This view of the feelings is maintained by Horwicz, and developed at length, 
 polemically, by Lotze (Horwicz, Psychologische Analysen, i., p. 168 f . ; Lotze, 
 Medicin. Psychologie, p. 235 f . ; and references already cited in note p. 500). 
 
504 NATURE AND KINDS OF FEELING. 
 
 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." 
 
 Moreover, although we are to distinguish sensation from feeling, 
 we must regard the feeling which inseparably accompanies sensa- 
 tion as feeling, strictly speaking, and not as tone of sensation ; or, 
 in other words, the tone of every sensation, as either pleasurable 
 or painful, is given to it by the feeling which accompanies and 
 blends with it. The sensation, as having a certain quality, quan- 
 tity, and locality, is capable of being built into a "Thing" which 
 the mind perceives as not-itself. But the feeling, the pleasurable 
 or painful tone of the sensation, is always recognized as purely and 
 simply a way in which the mind is affected. To refuse to speak of 
 sensations 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 matter the mistake which it is guilty of committing repeatedly ; 
 it regards the " ideas " as realities that have in some sort a sub- 
 stantial 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 is the manner of its ideating energy. Finally, 
 this theory wrecks itself upon the denial of all that which the phys- 
 iological theory maintains and establishes. The two theories, then, 
 supplement 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. 
 
 7. The truth appears to be as follows : Feeling is an original 
 and underlined form of consciousness, or mode of the operation of con- 
 scious mind. It can neither be defined by, nor deduced from, 
 sensation or ideation. To know what it is to feel, the highest in- 
 telligence of itself would be incapable. Such knowledge comes only 
 from having felt. Feeling accompanies all mental experience, both 
 that of sensation and of the higher- intellectual processes. It un- 
 doubtedly has a certain physical basis ; and certain laws may be 
 stated which discover some of the relations that hold good between 
 conditions of the nervous system and resulting conditions of feel- 
 ing. Certain other laws may be laid down which partially define 
 
CLASSIFICATION OF THE FEELINGS. 505 
 
 the relations existing between the purely intellectual and the feel- 
 ing activities, or reactions, of mind. These two sets of laws give 
 us the physical and the intellectual conditions of different tones 
 of feeling respectively. But no one law has yet been discovered 
 which covers all the facts of relation, either between feeling and 
 the bodily states or between feeling and the ideas. Nor is it likely 
 that any such law exists to be discovered. Manifold relations, as 
 determined by heredity, individual peculiarities, association, atten- 
 tion, etc., always exist, and contribute to the complex result. 
 
 8. The various attempts to establish fixed classes of the feelings 
 can scarcely be pronounced more satisfactory than the attempts 
 to define their nature. Very great difficulties stand in the way of 
 such a classification, of which the following are most important : 
 The phenomena are themselves very obscure, changeable, and multi- 
 form. ; they are also inextricably associated with the phenomena of 
 sensation and ideation. Moreover, the theory held by any inquirer 
 as to the origin and nature of the feelings is pretty sure to deter- 
 mine his classification of them. For example, two before-men- 
 tioned authorities (Nahlowsky ' and Drbal 2 ), as a result of the 
 " ideational " theory, divide the feelings into (1) such as are de- 
 pendent on the form of the course of ideas, and (2) such as are 
 conditioned by the content of the ideas. Besides these simple 
 classes of feelings, one of these writers speaks of certain " mixed 
 feelings," that are feelings of oscillation and change. Under the 
 " formal " feelings, or such as are dependent on the form of the 
 course of representation, the other writer finds four classes namely, 
 (a) feelings of expectation and impatience, (6) of hope, anxiety, sur- 
 prise, and doubt, (c) of tedium, and (d) refreshment and work all 
 according to the aspect of the ideas in time-form. Four kinds of 
 " qualitative " feelings are also distinguished these are the intel- 
 lectual, the sesthetic, the moral, and the religious according as 
 the ideas exciting the feelings refer to truth, beauty, morality, or 
 religion. But the foregoing attempts and all similar attempts at 
 classifying the feelings lay a false emphasis upon the dependence of 
 certain feelings on mental representation ; they thus overlook all 
 those considerations on which the physiological theories of feeling 
 rightfully insist. 
 
 But, on the other hand, the classifications made under the in- 
 fluence of the physiological theory are even more unsatisfactory. 
 In their desire to reduce all the phenomena of human feeling under 
 some one physical " law," so called, they bring the higher forms of 
 
 1 Das Gefiihlsleben, p. 44 f. 
 
 2 Lehrbucli d. empir. Psycliologie, p. 205 f. 
 
506 NATURE AND KINDS OF FEELING. 
 
 feeling into a much closer and more complete connection with the 
 feelings of sensation than the facts will warrant. Thus, with Grant 
 Allen, the aesthetic feelings are "the cumulative effect of many 
 infinitesimal physiological factors," 1 which differ from the pleasures 
 and pains of sensation chiefly in the fact that the activity of the 
 end-organs in them is " not directly connected with life-serving 
 function ; " 2 all the different tastes of different individuals, their 
 varying "perceptions of beauty and ugliness," are then boldly 
 stated to be " depending on the structural variations of the nervous 
 system." 
 
 Horwicz's more profound theory as to the nature of feeling leads 
 him to a more satisfactory classification of its forms. The variety 
 of feeling, he holds, is dependent on the natural organic variety in 
 the activities of the soul. 3 Thus we derive (1) the sensuous feelings, 
 or such as depend on the different qualities of the sensations of the 
 special senses and of common feeling ; (2) the aesthetic feelings, or 
 those agreeable or disagreeable forms of consciousness which cor- 
 respond to the mental images of perception and imagination ; (3) 
 the intellectual feelings, which correspond to the theoretic interests 
 called out by the higher forms of thinking ; (4) the moral feelings, 
 or those which correspond to the relations of desire and will. The 
 development of these feelings in varying relations to each other 
 gives rise to various mixed or complex forms. Certain moods and 
 characteristic affections result from the combined tone, color, and 
 rhythm, of the simple feelings, and the strength with which the 
 physical organism reacts. Higher feelings, or " feelings of feel- 
 ings," unfold themselves ; these are dependent upon the complex 
 relations of society as organized in its several existing forms. 
 . A recent writer in Mind,* after criticising all previous attempts 
 at classification of the feelings, proposes an exceedingly elaborate 
 substitute for them all ; but this substitute is so burdened with 
 uncouth terminology, is founded on so many false or doubtful psy- 
 chological assumptions, and involves so many artificial distinctions 
 and cross divisions, that it is little likely to meet with general ac- 
 ceptance. 
 
 9. Another and insuperable difficulty in the way of a strict 
 classification of the feelings is the fact that they are actually, and 
 as peculiar conditions of consciousness, unclassifiable. In other 
 words, no principle of classification can be suggested which will 
 undeniably apply to them all. For example, if we classify them 
 
 1 Physiological ^Esthetics, p. 42. a Ibid. , p. 34. 
 
 * Psychologische Analysen, ii., p. 82 f. 
 
 4 Mercier, July and October, 1884, and January, 1885. 
 
PRIMITIVE CHAEACTEK OF FEELING. 507 
 
 into pleasurable or painful, we indicate in this way only a quality 
 of tone which itself constantly varies in dependence on more per- 
 manent characteristics. Besides, it is not easy to demonstrate that 
 feelings are never indifferent (neither agreeable or disagreeable) in 
 tone. If we classify according to the anatomical part or phys- 
 iological function of the nervous system which chiefly gives con- 
 ditions to the feelings, we can carry our classification only a little 
 way without resorting to unwarrantable assumptions. Indeed, 
 there are grounds for supposing that the feelings are of central 
 origin that is, have their physiological basis in those regions of 
 the nervous system that have thus far almost wholly eluded sci- 
 entific research. If we classify according to the relation of each 
 feeling to other activities of the soul (as Horwicz does), we en- 
 counter the facts that sense, will, and intellect, doubtless all enter 
 into all the activities connected with our developed feeling ; but 
 that the measure of the degree in which they are influential upon 
 feeling is so uncertain and changeable as to render it unfit to serve 
 as a basis for classification. 
 
 No hard and fixed line can be drawn about the different so-called 
 classes of feelings. Feeling, with its color-tone of pain or pleasure, 
 enters into all conscious life. The aesthetic feelings cannot be 
 separated from the sensuous ; for example, the feeling which 
 accompanies the sensation of a musical chord, or of the color pur- 
 ple, 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 
 seldom or never so purely ideal as not plainly to involve in itself 
 feeling of sensuous and aesthetic sort ; hate not mixed with anger, 
 and so supported on some elements of that physical basis which 
 underlies the latter, is hard to discover in real life. All psycho- 
 logical analysis that would extend to the point of establishing fixed 
 classes of the psychical activities is difficult ; but in the special 
 case of the feelings, the character of experience is such as to make 
 strict classification impossible. 
 
 Accordingly, in treating of the feelings from the physiological 
 point of view we shall content ourselves with selecting certain ex- 
 
508 NATURE AND KINDS OF FEELING. 
 
 amples from the current classes which admit of being thus treated 
 most successfully. Such are, obviously, the so-called sensuous feel- 
 ings, the so-called common feeling, certain aesthetic feelings, certain 
 of the feelings known as emotions or affections, and certain feelings 
 connected with the functions of will especially the feeling of effort, 
 
 10. All feelings are characterized by tone, strength, rhythm, 
 and content. 1 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 simple, as is the case with the feeling 
 connected with the presentation of a colored surface (for example, 
 purple or green), or 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 especial^ 
 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- 
 pendence upon the rhythmic movement of the nervous processes 
 and of the train of ideas. No feeling is kept at a long continu- 
 ous 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 intensity 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 (comp. p. 108 f .) 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 
 1 Comp. Volkmann von Volkmar, Lehrb. d. Psychologie, II., p. 311 f. 
 
THE TONE OF ALL FEELING. 509 
 
 pain, we succeed only intermittently. But the course of the ideas 
 must also be taken into account as influencing the rhythm of feel- 
 ing. 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. 
 
 11. The tone of all feeling is either one of pleasure or of pain 
 (using these words in their widest possible meaning). 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 positive tone or, in other words, are not absolutely in- 
 different to us there can be no question. Is it agreeable or dis- 
 agreeable, at least in some slight degree and "in some more or less 
 indefinite manner ? is an inquiry which we can pretty readily an- 
 swer with respect to nearly all our sensations and ideas. The 
 question has been debated, however, whether this is necessarily true 
 of all our feelings. Is there any such thing as completely ''neutral" 
 feeling, or feeling that is in no respect or degree either agreeable 
 or disagreeable to us? Neutral or indifferent feelings were recog- 
 nized by Keid, but disputed by Hamilton. 1 Bain asserts it as un- 
 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." 2 Wundt 3 
 argues, on theoretical grounds, that pleasure and pain, as tones 
 of feeling belonging to sensation, 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 
 Xof 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 
 
 1 Hamilton's Works of Thomas Reid, p. 311. Edinburgh, 1854. 
 
 2 The Emotions and the Will, p. 13. 
 
 3 Physiolog. Psychologic, i. , p. 465 f. 
 
510 NATURE AND KINDS OF FEELING. 
 
 one of intense pain without any 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 intervening feel- 
 ing of indifference. We incline, then, to agree with Sully : in af- 
 firming that every feeling is either pleasurable or painful in some 
 degree. " We apply the name 'feelings,' " says Lotze, 2 " exclusively 
 to states of pleasure and pain, in contrast with sensations as indif- 
 ferent perceptions of a certain content." 
 
 12. Various questions may be raised as to the physical appar- 
 atus, the nervous elements and processes, for pleasurable or pain- 
 ful feeling, w ? hich cannot be answered satisfactorily. Are there 
 special nervous elements whether end-organs, or nerve-fibres, or 
 nerve-tracts and centres in the central organs which must be ex- 
 cited in order to give rise to the feeling of pain ? If the apparatus 
 for feeling is the same as that for the sensations to which the feel- 
 ing gives its color-tone, do the feeling and the sensations imply 
 different processes in these same elements as the physical basis on 
 which they rest ? Is not pleasure, rather, the result of a normal 
 and moderate amount of process in these elements ; and pain the 
 result of a process in the same elements whose amount has been 
 increased so as to be destructive or injurious to the tissue involved ? 
 Lotze 3 raised these questions, and answered them with the opinion, 
 somewhat doubtfully expressed, that sensation and feeling are due 
 to two forms of processes in the same nervous elements, and that 
 there is no need of assuming special organs of feeling, whether 
 peripheral or central. Probably the prevalent view hitherto has 
 been, that the same apparatus of end-organs, conducting nerve- 
 tracts, and central areas, which on moderate excitement produces 
 the simple sensations of pressure or of temperature, or the more 
 complex sensations of tickling, shuddering, etc., produces the feel- 
 ing 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 weariness 
 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 psychological 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 
 
 1 Outlines of Psychology, p. 449, New York, 1884 ; and comp. Volkmann 
 von Volkmar, Lehrb. d. Psychologie, II., p. 311 f. 
 * Outlines of Psychology, p. 73. Boston, 1886. 
 3 See Medicin. Psychologie, pp. 245 ff. 
 
PHYSICAL APPARATUS OF FEELING. 511 
 
 that comes from heavy pressure or burning, as it is that sensations 
 of light are unlike those of musical tone. 
 
 Besides the obvious difference which the results of exciting it 
 have in consciousness, there are other and physiological reasons 
 for doubting the complete identity of the nervous apparatus of 
 pleasurable and painful feeling with that of the sensations with 
 which the feeling is allied. The facts upon which Schiff and others 
 support the view that nervous impulses resulting in pain travel by 
 more or less distinct paths along the spinal cord have already been 
 stated (Part I., chap. HI, 32). The most recent experiments seem 
 to show that the end-organs of temperature, pressure, and pain are 
 locally separable in the different minute areas of the skin (Part II., 
 chap. IV., 21 f.). Pathological results indicating the same separa- 
 tion of the nervous elements of feeling also deserve a brief men- 
 tion. In certain cases the sensibility of the skin to pain is lost (a 
 condition called "analgesie" by Beau, and " analgie " by Lotze), 
 while its sensibility to touch is not weakened or is even increased. 
 The reverse condition also sometimes occurs. " Analgie," 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, lo- 
 cally, the central nervous processes which result in sensation with 
 those which result in feeling. In certain stages of narcosis, produced 
 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 dorsahs, a con- 
 stant difference seems to exist in the time at which the sensations 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 
 second slater. 1 The case of the eye, which responds with sensations 
 of light and color when the optic nerve is moderately excited, and 
 with the painful feeling of being blinded when the stimulus is in- 
 creased, is not perfectly clear. For cases of amaurosis are on 
 
 1 See Funke, in Hermann's Handb. d. Physiol., III., ii., p. 297 f. ; such 
 phenomena have been especially discussed by Osthoff, Die Verlangsamung d. 
 Schmerzempfindung bei Tabes dorsalis, Erlangen, 1874. 
 
512 NATURE AND KINDS OF FEELING. 
 
 record where the painful feeling persisted after the eye had lost all 
 power to distinguish light. It has been argued, therefore, that 
 while the specific sensations of light and color are due to the irri- 
 tation of the optic nerve, the excitement of feeling indicates a si- 
 multaneous 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 form the physical basis of painful and pleasurable feeling, 
 require further investigation. The tendency of the evidence, how- 
 ever, 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). 
 But how such a theory will reconcile itself with the other familiar 
 facts which appear, obviously, to make the tone of feeling depend 
 upon the degree of intensity which the nervous processes attain, it 
 is impossible to predict. 
 
 13. One kind of feeling, which has the tone of pleasure or pain 
 belonging to all feeling, is undoubtedly of central nervous origin ; 
 this is the so-called sensus communis, or "common feeling." 1 Such 
 feeling may have more or less of content of one kind or another, 
 according 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 
 
 1 Com p. Funke, Der Tastsinn u. d. Gemeiugefiihle, in Hermann's Haudb. d. 
 Physiol., III., ii., pp. 289 ff. 
 
COMPOSITION OF COMMON FEELING. 513 
 
 and mixed bodily feelings stand related to the mind's course of idea- 
 tion, to attention, association, etc. This relation often determines 
 whether such obscure impressions shall be definitely objectified or 
 not ; whether they shall not rather run together in the dark stream 
 of common feeling. Let anyone suspend for an instant a train of 
 interesting thought, which has up to the moment been interrupted 
 only by certain obscure feelings of uneasiness, and such one will be 
 able instantly to select and localize in the cramped chest, or op- 
 pressed limbs, or tired organs of special sense, most of the sensa- 
 tions whose painful tone has thus colored the stream of common 
 feeling. Separation from localized sensations is, then, the chief 
 negative characteristic of common feeling. 1 Under its different 
 principal forms we may distinguish different total results, according 
 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 coursing of the blood through every artery and 
 vein, as though mentally present in the extended tissues. 
 
 14. According to Strieker, 2 information derived from the pe- 
 ripheral nerves consists of either sensations or feelings ; the latter 
 implies self -reference, which may be of two kinds. If this reference 
 extends to the whole sensorium, and so to the whole organism, the 
 feelings are called " common " or ." collective ;" under certain cir- 
 cumstances they appear as fixing the mood of our consciousness. 
 Some of the organs, in their sound condition, have no organic feel- 
 ings ; others of them undoubtedly largely determine the character 
 of our common feeling by their condition, tension, action, etc. / If, 
 now, we extend the sensations of touch so as to include all the ob- 
 scurely localized organic and muscular sensations, we feel the neces- 
 sity of distinguishing such sensations from what we have called 
 the "common feeling." 
 
 One characteristic which the sensations have as sensations, strictly 
 speaking, concerns the method of their excitation. In general, the 
 stimulus must affect the nerves through the specific end-organs of 
 sense, in order to give rise to a proper sensation. E. H. Weber 
 sought to prove that, whenever nerves are irritated, not through the 
 
 1 Comp. Lotze, Medicin. Psychologie, p. 278 f. 
 
 8 Studien iiber d. Bewusstsein, p. 17 f. Wien, 1879. 
 
514 NATURE AND KINDS OF FEELING. 
 
 end-organs, but along the trunk, the irritation gives rise to feelings 
 of pain instead of sensations. One experiment for this purpose 
 consisted in dipping the point of the elbow into ice-cold water ; 
 when the sensation of cold is at once located in the skin, and the 
 feeling of pain arises as soon as the stimulus has penetrated to the 
 trunk of the nerve lying beneath. But other experiences seem to 
 show that tones of common feeling may be indirectly excited, which 
 are characterized by the massing of a great number of minute and 
 obscurely localized sensations of touch. For example, the prick of 
 a needle is felt at a given point as a circumscribed pain. The tick- 
 ling from a feather, confined to a small surface, may be regarded 
 as consisting of complex sensations of light pressure, with no fixed 
 locality for each one, but localized in general at about such a spot. 
 But the tickling may be continued until a general tone of painful 
 feeling is developed, which quite overwhelms all localized sensa- 
 tions. These phenomena may be considered as agreeing with the 
 other phenomena to show that common feeling is due to widely 
 extended and complex conditions of the central areas, in which 
 the results of a large number of separate peripheral stimulations 
 may unite so as to lose all their individual character, although each 
 one contributes something to the common result. 
 
 15. There are feelings so connected with the operation of the 
 organs of sense as to be called feelings of sensation. A certain tone 
 of feeling (a third element, as distinguished from its specific quality 
 and intensity) belongs to most sensations. We are scarcely war- 
 ranted, however, in asserting that every sensation, as such, possesses 
 some tone of feeling. 1 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 
 consciousness which they often carry, as inseparably connected with 
 them. The particular tone belonging to any sensation is, to a large 
 extent, dependent on its intensity. The laws of this dependence 
 have been ingeniously conjectured by Wundt. 2 Sensations of mod- 
 erate intensity 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 intensity of the stimulus increases. The curves which represent 
 the increase of feeling and the increase of sensation by no means 
 correspond. It is assumed by Wundt that the maximum point of 
 pleasure lies about the so-called "cardinal value " of the sensation, 
 
 1 As Wundt does, for example, Physiolog. Psychologie, i., p. 465. 
 
 2 Physiolog. Psychologie, i. , p. 469 ; comp. p. 360. 
 
TONE DEPENDENT ON INTENSITY. 515 
 
 or place where the sensation ceases to increase in simple proportion 
 to the strength of the stimulus. The amount of pleasurable feeling 
 is also dependent on the element of time. It is 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. I 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. Probably, also, the disagreeable tone of 
 feeling which almost universally attaches itself to certain qualities 
 of sensation, however moderate or unobtrusive their intensity, is 
 largely explicable 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 directly dependent on the quality of 
 sensation. This is a question upon which only consciousness can 
 pronounce. All degrees of some tastes and smells are disagreeable 
 to most persons. Bitter is a distinctive species of the quality 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 disagreeable, 
 become tolerable, or even pleasant, when the smell from them is 
 faint. But this faint smell is not the same, but a distinctly differ- 
 ent 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.). 
 
 16. 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 difference 
 in the way we are affected by major and minor chords, by succes- 
 sive tones having different musical intervals (for example, the di- 
 minished 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, 
 1 So Wundt, Physiolog. Psychologic, i., p. 470. 
 
516 HIGHER FORMS OF FEELING. 
 
 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 * called attention to the change in spiritual tone, as it were, 
 which harmonizes w r ith 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." 2 
 
 17. 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. Unhar- 
 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 widespread sense of organic discomfort. Pains 
 in the skin and interior organs, however, may take a character of 
 intense 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. 3 
 
 18. 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 
 secondary element of experience, which arises through certain ac- 
 quired 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 know nothing whatever ; and the subject 
 is not as yet one with which physiological psychology can success- 
 fully deal. 
 
 19. The consideration of the affections and the emotions, or 
 passions, involves at least three important particulars : (1) The 
 characteristic feeling which distinguishes each ; (2) its relations to 
 the train of ideas, and the changes induced by it in the ideas ; (3) 
 
 1 Farbenlehre, 763. 
 
 'Comp. Wundt, Physiolog. Psychologie, i., p. 477. 
 
 3 Comp. Lotze, Outlines of Psychology, p. 75 f . 
 
BODILY BASIS OF THE EMOTIONS. 517 
 
 the relations to the different bodily organs, and the reflex effect of 
 the changes in these organs upon both the feelings and the ideas. 
 
 Each of the various affections or passions is characterized by a 
 peculiar feeling, whose tone is either agreeable or painful, whose 
 intensity admits of various degrees, and whose content is determined 
 by the mental representations to which it has become attached. 
 Each may be considered as having its rise, psychologically, in spme_ 
 form of blind, instinctive impulse that needs to be connected with 
 a mental image of the object which experience has related to it as 
 corresponding to the impulse. The germ of the impulse is the 
 natural susceptibility of having desire awakened by an appropri- 
 ate stimulus, and the capacity of forming by experience the idea 
 which corresponds to, or gratifies, the impulse. 1 .Impulses may be 
 describexL 4is. of two kinds craving, or attraction, and repulsion. 
 When the feeling, whuj-iji.a rr|erf> impulse ia blind with respect to 
 tne object of gratiticaTionTbecomes connected in experience with 
 appropriate presentations of sense or mental pictures, the basis for 
 an affection or passion has been laid. Thus the germ of anger and 
 hate is found in that instinctive impulse of repulsion which is pro- 
 duced by all unpleasanj; resistance of effort, or painful excitement 
 of the nervous system.' Sudden and intense irritations as the 
 striking of one's hand against the table, the slamming of a door in 
 one's ear tend to arouse the feeling of resentment. The affection 
 of the child for the mother ultimately becomes far more than the 
 feeling of comfort it has in her arms or at her breast ; but the 
 former is cradled and nursed in the latter. By varied associations, 
 impulses of attraction or of repulsion become developed into a 
 great variety of affections, emotions, and passions, characteristic of 
 the different manifold relations in which the sentient soul finds it- 
 self standing toward things and persons. 
 
 20; 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." 2 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 
 
 1 Comp. Wnndt, Physiolog. Psychologie, ii., p. 33 f. 
 
 2 Comp. Nalilowsky, Das Gefiihlsleben, etc., Einleitung. 
 
518 HIGHER FORMS OF FEELING. 
 
 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. 
 
 21. But the wonderful, characteristic effect which these forms 
 of feeling produce upon certain of the vital organs is the most 
 noteworthy peculiarity of all affections, emotions, and passions. 
 Upon this point science has far less than we could wish of informa- 
 tion reaching beyond the observations of ordinary experience. Of 
 such information, perhaps the most important concerns the influ- 
 ence exerted through many groups of muscles, from the central 
 organs, upon the vaso-motor system. VThe effect of shame, fear, or 
 anger, for example, upon the circulation of the blood is matter of 
 common remark. \ But some grow pale and others red, when angry. 
 In 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 
 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 petard, 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 
 
 1 This subject has been investigated by Conty and Charpentier, by Cyon, 
 Heidenhain, and others; comp. Exner, in Hermann's Handb. d. PhysioL, II., 
 ii, p. 289 f. 
 
BODILY BASIS OF THE EMOTIONS. 519 
 
 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." l Un- 
 usual tension or relaxation of certain groups of muscles characterizes 
 all these forms of feeling. N 
 
 The marked effect which certain feelings have upon particular 
 organs of the body is complemented by the fact that such organic 
 effect has in turn a marked effect upon the feelings. The organic dis- 
 turbances 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 emo- 
 tion subsides. The organic changes are not merely an expression 
 of the mental ; they are its material cause and support. Professor 
 James has emphasized these facts with great skill and in an inter- 
 esting way. 2 The effect upon the emotions and passions of putting 
 the muscles or other organs of the body into certain conditions, 
 which is so remarkable in all cases of hypnotism, is also undoubted 
 in what we ordinarily consider normal states of body and mind. 
 "What kind of an emotion of fear would be left, if the feelings nei- 
 ther of quickened heart-beats nor of shallow breathing, neither of 
 trembling lips nor of weakened limbs, neither of goose-flesh nor 
 of visceral stirrings, were present, it is quite impossible to think. 
 Can one fancy the state of rage and picture no ebullition of it in 
 the chest, no flushing of the face, no dilatation of the nostrils, no 
 clinching of the teeth, no impulse to vigorous action, but in their 
 stead limp muscles, calm breathing, and a placid face ? " In view ' 1 
 of the foregoing facts, Professor James propounds the thesis, that / 
 the " bodily changes follow directly the perception of the exciting / 
 fact, and that our feeling of the same changes as they occur is the 
 emotion." s 
 
 To neglect, however, that element of feeling in eveiy emotion 
 which is immediately attached to certain perceptions and ideas 
 would be quite as faulty as to neglect the elements which are only 
 reflexly blended with the complex of feeling on account of the condi- 
 tion into which the bodily organs are thrown. The relation between 
 perception and feeling as a psychological fact is as certain and im- 
 mediate as any relation can be. What the physiological basis for 
 this connection is we do not know ; but there is every reason to 
 suppose that it is, at the same time, direct and of the nature of re- 
 ciprocal influence between the nervous elements and areas of the 
 
 1 Wundt, Physiolog. Psychologic, ii., p. 330. 
 
 2 Mind, 1884, IX., p. 1881 
 
 3 But since its author seems scarcely to have taken this thesis seriously, it 
 may be thought superfluous even to object in brief to it (see p. 205). 
 
520 HIGHER FORMS OF FEELING. 
 
 cerebral centres ; as well as indirect, through disturbances pro- 
 duced by perceptions and ideas within the remote bodily organs. 
 The influence of perception upon the feelings in the form of strong 
 emotion is partly, but not wholly, through the skin, muscles, blood- 
 vessels, organs of respiration, and viscera. At the same time the 
 characteristic tone which strong emotions have is largely colored by 
 the sensuous and common feelings occasioned by the disturbance 
 of the organs. When even the feelings called aesthetic, or intellect- 
 ual, or ethical and religious, are vehemently aroused, an ''emo- 
 tional " equality is imparted to them from the same source. 
 
 22. By mental " moods " is ordinarily understood those collec- 
 tive conditions of the mind which are characterized by some funda- 
 mental tone, but without any special feelings accompanied by clear 
 consciousness of their inducing causes. The principal elements that 
 enter into such moods consist of ill-localized sensations arising from 
 the internal organs especially due, perhaps, to disturbed or de- 
 pressed cerebral function ' and a throng of half -reproduced feel- 
 ings and ideas, or of vague single feelings, such as undefined fore- 
 boding, anxiety, fear, etc. Since these elements belong to the some- 
 what permanent equipment of the mind (at least until a marked 
 and lasting change in cerebral condition and the train of ideas is 
 brought about), their prevalent tone is characteristic of different 
 persons ; whereas the emotions and passions run their course 
 quickly, and give a color to the personality rather by the sudden- 
 ness and frequency with which this kind or that, respectively, is 
 present in consciousness. But mental moods also may be charac- 
 terized by emotions or affections of a low and lingering tone pale 
 and faded specimens of the type, as it were. 
 
 23. Of all the so-called " higher feelings " (aesthetic, intellectual, 
 ethical, religious), it is only certain elementary forms\of aesthetic 
 feeling concerning whose peculiar physical basis we have any as- 
 sured information. All these feelings, however, when they reach 
 a certain degree of intensity, tend to assume an emotional character. 
 They then come in part under the considerations which have already 
 been urged as applying to the emotions in general. A large portion 
 of the strong feelings of admiration for scientific objects, discoveries, 
 laws, and personalities, or of religious fervor, aspiration, and devotion, 
 or depression, is reflex ; it rests upon the physical basis of effects that 
 are produced in the muscles and vital organs especially the organs 
 of secretion, respiration, and circulation. But these facts do not 
 explain or annul the other class of facts, which leads the judicious 
 investigator also to emphasize the spiritual origin of such feelings 
 1 Comp. Strieker, Studien iiber d. Bewusstseiu, p. 63. 
 
FEELING OF HARMONY AND RHYTHM. 521 
 
 considered as complex reactions of the mind in view of the presence 
 in consciousness of certain classes of ideas. So far as such spiritual 
 reactions by way of feeling have any even conjectural physical basis 
 peculiar to them, this basis must be sought for in the central 
 organs of the nervous system. How far such a basis really exists, 
 and in what it consists, we have as yet scarcely a right to imagine 
 so complete is our ignorance. 
 
 24( The aesthetic 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 called laws 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 
 evolution are given in the work of Grant Allen on " Physiological 
 Esthetics." ' But even the most elementary aesthetic feelings can- 
 not be considered as on a par with the sensuous feelings, or as mere 
 aggregates of such feelings. 2 The tone of feeling which characterizes 
 the sensations furnishes a material, as it were, for genuinely aesthetic 
 feeling ; but the latter always implies also the working of certain in- 
 tellectual laws, and a union of the simple feelings of sensation under 
 time-form and space-form. ^Esthetic feelings, then, may be said to 
 spring from the manner of the combination of sensuous feelings ; 
 time and space furnish the framework in which they are arranged. 
 Hearing is 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 
 cesthetic feelings by other senses than the eye and ear is extreme- 
 ly limited. The agreeable and disagreeable feelings which come 
 through sensations 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 beween major 
 
 1 See pp. 30 ff. 
 
 2 On this point, see Wundt, Physiolog. Psychologic, ii., p. 179 f. 
 
522 HIGHER FORMS OF FEELING. 
 
 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 intense, 
 it may involve the pain of over-excitement ; the other, when inten- 
 sified, stirs a kind of agreeable pain of unrest. In musical time it is 
 the periodic nature of the excitation, with a change in the individ- 
 ual 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 ac- 
 centuated, 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 difference 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 
 loaves of different kinds of feeling are made by music to pass over 
 the soul. 1 
 
 The elementary aesthetic feelings which come through sight lead 
 to the consideration of the aesthetic effect of visual form. Such ef- 
 fect can be considered only very imperfectly from the physiological 
 point of view. In one important particular, however, pleasurable 
 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. Beautiful 
 form is determined by the course of the limiting lines ; and limit- 
 ing lines, in order to have the effect of arousing agreeable aesthetic 
 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 aesthetic effect. So also must the main lines of a 
 building lie in horizontal or vertical directions, preferably in the 
 
 1 For a treatment of aesthetic feeling in music, comp. Wundt, Physiolog. 
 Psychologic, ii., pp. 180 ff. 
 
THE GOLDEN DIAMETEK. 523 
 
 former direction. But long oblique lines for example, from a 
 lower right-band to an upper left-hand corner of a building are 
 scarcely tolerable. The ease with which the eye sweeps the lines, 
 in order to make that synthesis of successive 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 
 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. The rule, that the whole of a presentation 
 of sight shall be to the larger part as the larger part is to the small- 
 er part, has been called " the golden diameter " (x + 1 : x : : x : 1) ; 
 since the proportion thus determined has been supposed to be par- 
 ticularly favorable to pleasurable aesthetic feeling. Ease of the 
 mental apprehension with which the relations in proportion of the 
 different parts are presented is favorable to agreeable aesthetic feel- 
 ing. 
 
 25. But all the foregoing rules, and all others similar, are appli- 
 cable to the aesthetic feelings of form rather as coming under the 
 general class of intellectual feelings. That change in degree or kind 
 of activity, recognition of similarity or contrast, and mental appre- 
 hension of a law or principle as expressed in the presentations of 
 sense, determine the agreeable character of our intellectual feel- 
 ings, is recognized by psychologists ' generally ; but as to the 
 physical basis of mental facts of this order we are almost completely 
 ignorant. It is not unlikely, however, that the effects of monotony 
 and change upon the feelings of an intellectual order are connected 
 with the same law of the exhaustion of the nervous elements as ap- 
 plied to the cerebral areas which we know to hold good in other 
 parts of the nervous system. 
 
 26. The only other class of feelings which admit of considera- 
 tion from the physiological point of view is the so-called "feelings 
 of effort, or of innervation." These feelings are especially con- 
 nected with all the motions of the body considered as furnishing 
 
 1 See the judicious remarks of Sully, Outlines of Psychology, pp. 457 ff. 
 New York, 1884. 
 
524 HIGHER FORMS OF FEELING. 
 
 information concerning its position and the condition of tension or 
 strain to which its parts are subject ; as well as furnishing, through 
 revived mental images of such feelings, the means for reproducing 
 voluntarily the required definite modes of motion. Such feelings 
 also have a great psychological interest on account of their obvious 
 connection with the development and consciousness of acts of will. 
 The dispute as to whether they are of central or peripheral origin, 
 and as to that in which their precise nature consists, has already 
 been alluded to (pp. 344 and 415). 
 
 That we have a ' ' feeling of effort " is a fact, as says Professor 
 James, 1 "consecrated by the institution of the word effort, and its 
 synonyms exertion, striving, straining." The nervous process which 
 occasions this feeling the great physiologist Miiller ' 2 considered 
 to be purely central, and to consist in the discharge from a motor 
 centre into the motor nerves. This view has since been widely 
 adopted by physiologists ; it has also been used especially by Bain 
 and Wundt as an essential factor in a theory of sense-perception, 
 as of chief importance in accounting for our experience of solid re- 
 sisting objects of sense and of whatever belongs to the inertia of 
 matter in general. On the contrary, it has been maintained that 
 the feeling of effort, over and above what is purely " moral" (as in 
 the effort to remember to make a decision, etc.), is a complex of 
 afferent sensations " coming from the tense muscles, the strained 
 ligaments, squeezed joints, fixed chest, closed ' glottis, contracted 
 brow, clinched jaws, 3 etc." 
 
 Of the two views above mentioned, the latter has by far the most 
 in its favor. The argument from the consciousness of effort which 
 we may have when we intensely make believe use any limb, but 
 do not actually move it (as, for example, the pulling of a trigger 
 with the forefinger of an extended hand), has been answered by 
 Ferrier. 4 This observer calls attention to the fact that the feeling 
 of effort in such cases is due to keeping the glottis tightly closed, 
 and actively contracting the respiratory muscles. If we try, how- 
 ever, to make believe exert ourselves without actually contract- 
 ing the muscles of the limbs, and at the same time keep breathing 
 regularly, we shall not experience the slightest trace of the feeling 
 of effort, no matter how hard we fay. This feeling, then, when the 
 glottis is closed and the respiratory muscles are tense, is due to 
 centripetal impressions coming from the parts thus innervated. 
 
 The argument from the feelings of effort which determine our 
 
 1 The Feeling of Effort, Anniversary Memoirs of the Boston Soc. of Nat. 
 Hist., 1880, last monograph. 2 Physiologic d. Menschen, II., p. 500. 
 
 3 See James, Feeling of Effort, p. 4. * The Functions of the Brain, p. 222 f . 
 
THE FEELING OF EFFORT. 525 
 
 localization of objects has been presented in the most convincing 
 way possible by Helmholtz and Wundt, as applied to the case of 
 partial paralysis of the external rectus of one eye. Inasmuch as 
 the patient feels (so von Graefe ' showed) that he has moved his 
 lame eye much farther than he really has, the inference is drawn 
 that this exaggerated feeling of effort must originate in central 
 motor impulses which have followed upon the fiat of the will. This 
 argument, however, neglects to notice what goes on in the other 
 and sound eye. Since this eye, unlike the lame one, continues its 
 motion until the limit of motion and its corresponding condition 
 of peripheral strain is reached ; and since, as Hering 2 has shown 
 (comp. what has already been said, p. 439 f .), both eyes are innervated 
 by one common act, and their motor apparatus is to be regarded as 
 functioning as one organ the feeling of effort is probably due to 
 afferent sensory impulses occasioned by the condition of the sound 
 eye as well as of the other eye. 3 Moreover, the more critically we 
 examine those cases which occasionally occur, where, on account 
 of paralysis causing anaesthesia, the sense of position of the limbs 
 is impaired or lost, the more conclusive does the evidence appear 
 against the theory that the feeling of effort is of purely central ori- 
 gin. For, in general, it seems that, while the power of voluntary 
 motion remains unimpaired, if the sensations which have a pe- 
 ripheral origin are impaired or lost, the various feelings of effort 
 connected with the accomplishment of a given amount of motion, 
 or with the act of holding any member of the body against the pull 
 of gravity, are disturbed or disappear. 4 As far as the evidence re- 
 garding this obscure subject reaches at present, the feeling of effort 
 
 1 Handb. d. gesammten Augenheilkunde, VI., p. 18 f. 
 
 9 See Hermann's Handb. d. Physiol., III., i., pp. 512 f. and 520 f. 
 
 3 See James, Feeling of Effort, p. 10 f. 
 
 4 See a recent paper on " Le Sens musculaire et les Sensations musculaires,'' 
 by E. Gley, in the Revue Philosophique, 1885, pp. 601 ff. In this paper the 
 results of the investigations of M. Maguin, conducted upon paralytics in the 
 Hopital de la Pitie (Comptes rendus, Mars 1884, i.), are appealed to as con- 
 firmatory of the view that the stretching and rubbing of skin, ligaments, joints, 
 etc., enter into our so-called feelings of effort. M. Demeaux has reported the 
 case of a woman who could move her limbs, but could not tell whether they 
 were moved, or in what direction, or how far. No fewer than three similar 
 cases were reported by French physicians in the year 1885. The same view 
 of the complex peripheral origin of the feeling of effort seems also to be fa- 
 vored by the experiments of M. Bloch, who tried placing his hands symmet- 
 rically on a screen with two leaves covered with paper, divided into small 
 squares, under the guidance of the so-called muscular sense. The testimony 
 of persons who have lost their limbs, as to whether they can produce any feel- 
 ing of effort by the fiat of will to move the lost member, is conflicting. In all 
 
526 MATURE OF THE BODILY MOTIONS. 
 
 must be held to be complex, and so is akin to other forms of com- 
 mon feeling ; its constituent elements are the various obscurely lo- 
 calized sensations, with their characteristic tones of feeling, which 
 arise in the condition of skin, muscles, ligaments, joints, etc. 
 
 27. The feeling of effort is closely connected in experience 
 with the changing positions of the members of the body, and its 
 consideration therefore fitly introduces that of the bodily motions. 
 As concerns their relation to the phenomena of mind, these motions 
 may be divided into two great classes ' namely, such as are not 
 demonstrable connected with antecedent changes in the states of 
 consciousness, and such as, in addition to their physical condi- 
 tions and causes, require that their explanation should take account 
 also of preceding states of consciousness. The former are to be 
 regarded purely as activities of the physical mechanism, 2 and are 
 either automatic or reflex. Automatic motions are such as, with- 
 out any corresponding idea or fiat of will, originate from inner 
 excitations of the central nervous system ; the reflex are those in 
 which the central excitations resulting in motion are traceable to 
 the action of sensory n'erves which have been peripherally excited. 
 It is extremely difficult to distinguish between automatic and reflex 
 motions, and scarcely less so to distinguish between the automatic 
 and the impulsive. Indeed, while it is true, on the one hand, that 
 reflex and centrally co-ordinated movements form the basis upon 
 which all our developed life of voluntary motion takes place, it is 
 also true, on the other hand, that the more complex co-ordinated 
 movements are themselves originally voluntary motions which have, 
 as it were, become habitual and so dropped out of consciousness 
 into a statical and mechanical way of taking place. The sensations 
 and ideas of motions may then be said to tend constantly in two 
 directions either toward consciousness or out of it. It is by 
 means of these processes in two directions that all our learning of 
 complicated movements of the body, of feats of dexterity and skill 
 learning to handle tools, to play on musical instruments, etc. 
 takes place. The interest which psychology has in the automatic 
 
 such cases, however, it is probable that the alleged feeling of effort is to be 
 looked for in the actual condition of strain into which some existing part 
 of the body is thrown especially the apparatus of respiration (see Bastian, 
 British Med. Journal, 1869, p. 461). 
 
 1 Comp. Wundt, Physiolog. Psychologic, ii., p. 400 f . ; and Lotze, Medicin. 
 Psychologie, p. 286 f. 
 
 2 The mechanism of the bodily motions has already been treated at length : 
 for the nerve-muscle machine, see pp. 104 ff. ; for reflex motion, see pp. 132 ff.; 
 for automatic motion, see pp. 147 ff. ; for the sensory-motor areas of the cere- 
 brum, see p. 267 ff. 
 
BASIS OF VOLUNTARY MOVEMENT. 527 
 
 and reflex motions is chiefly on account of their relations to motions 
 which are actually preceded by conscious ideation and volition. 
 
 28. Such motions of the body as require us to take account of 
 antecedent or accompanying states of consciousness, in addition to 
 the connections of the physical mechanism, are either impulsive or 
 voluntary. But this distinction is one which admits of such a great 
 variety of degrees shading into each other, that, although it is 
 valid and necessary in principle, it cannot be carried through in 
 practice with any considerable precision. By an impulsive motion 
 we understand a motion which, without a conscious fiat of will, fol- 
 lows upon certain ideas and excited states of feeling. The motif 
 of the impulsive movement lies, then, in some form of feeling that 
 determines will one way without any proper choice. If we speak 
 of such motions as volitional or voluntary, it must be understood 
 that we are referring to activities of will of a lower order, psycho- 
 logically considered, than those which come into play in all cases of 
 conscious choice. Impulsive motions are, in general, more quickly 
 accomplished than are voluntary motions; because the reaction- 
 time is shortened through will-time proper having been dropped 
 out (comp. chap. VIII., 19 f.). 
 
 29. All voluntary movement has its basis laid, so to speak, in 
 impulsive movement, and in the reactionary effect which the latter 
 has upon the conditions of reflex and automatic bodily activities. 
 As laid in this basis, voluntary motions imply a development of in- 
 telligence and will. The infant finds itself equipped with a bod- 
 ily mechanism which, under the influence of external and internal 
 stimuli, is kept excited to unceasing activity of the peripheral mem- 
 bers. This activity results in certain sensations and feelings of 
 effort, in the manner previously described. The tone of these states 
 of consciousness is one of either pleasure or discomfort, under 
 those laws of relation between the nervous mechanism and conscious 
 feeling which can be only imperfectly stated ; and which, when 
 most perfectly stated, can only be accepted as ultimate matters of 
 fact. By nature the nervous mechanism is so arranged that certain 
 other bodily motions of peripheral origin are started on occasion of 
 the pleasant or painful feeling, and these motions are adapted to 
 enhance the feeling if pleasant and to relieve it if painful. The 
 feelings thus become further connected with the ideas of the mo- 
 tions that modify them ; yet the mechanism of the motions is not 
 to be regarded as originally dependent upon the ideas, but rather 
 as originated in connection with the feelings of pleasure or discom- 
 fort and naturally adapted to secure an increase of the one or a 
 diminution of the other. 
 
528 NATURE OF THE BODILY MOTIONS. 
 
 The voluntary movements of the body, accordingly, presuppose 
 the impulsive, and yet they reach far back into the obscurity of the 
 earlier development of consciousness. Strictly speaking, they imply 
 the presence in consciousness of two or more different or conflict- 
 ing ideas of motion, one of which rather than the others is realized 
 as a sequence of an act of conscious choice. They imply, then 
 as has already been said a considerable development of the men- 
 tal activities of ideation and volition. Moreover, those movements 
 which are ordinarily called voluntary are really so only with respect to 
 certain of their elements ; they all also contain elements which must 
 be classed as reflex, centrally co-ordinated, and impulsive. The term 
 " voluntary " fitly lays the emphasis upon the conscious act of choice ; 
 and this, in turn, implies ideas of various possible forms of bodily 
 motion gained by previous experience with the correlated states of 
 conscious feeling and conditions of the body as giving rise to or 
 modifying these states. 
 
 The voluntary motions, therefore, constitute the highest class of 
 motions, both because their conditions include all those which be- 
 long to the other classes, and other conditions besides, and also 
 because of their more direct connection with the development of 
 certain mental phenomena of supreme psychological interest and im- 
 portance. To move any part of the body voluntarily requires the fol- 
 lowing particulars : (1) The possession of an educated reflex-motor 
 mechanism, under the control of those higher cerebral centres which 
 are most immediately connected with the phenomena of conscious- 
 ness ; (2) certain motifs in the form of conscious feelings that have 
 a tone of pleasure or pain, and so impel the mind to secure such 
 bodily conditions as will continue or increase the one and discon- 
 tinue or diminish the other ; (3) ideas of motions and positions of 
 the bodily members, which previous experience has taught us an- 
 swer more or less perfectly to the motifs of conscious feeling ; (4) a 
 conscious fiat of will, settling the question, as it were, which of these 
 ideas shall be realized in the motions achieved and positions attained 
 by these members ; (5) a central nervous mechanism, which serves 
 as the organ of relation between this act of will and the discharge of 
 the requisite motor impulses along their nerve-tracts to the groups 
 of muscles peripherally situated. 
 
 As to the first and second of the foregoing particulars, nothing 
 further need be said ; and as to the definite nature of the physical 
 basis which underlies the connection of ideas of motion, fiat of will 
 adopting one idea, and the starting outward of the right motor im- 
 pulses, our ignorance is almost complete. It is more than probable 
 that we cannot will the movement of muscles, of the results of whose 
 
WILL AS A PSYCHICAL ACTIVITY. 529 
 
 actual movement in the induced motion of the limbs we have ac- 
 quired no idea from previous experience. The mental images of the 
 various feelings of motion and position which have been acquired 
 in the past are our guides in realizing again the same motions and 
 positions of the limbs. To say I will, refers to the future. But we 
 can never " will" motion in general motion, that is, of no partic- 
 ular members of the body, and without specific quality, direction, 
 and velocity of the motion. That certain nervous processes in the 
 central organs form a physical basis for the mental phenomena of 
 ideation and fiat of will there is sufficient ground for believing. 
 The phenomena of reaction-time show that interrelated cerebral 
 activities of more and more complicated sort are implied in the in- 
 creased time required for completing the mental actions of repre- 
 sentation and choice between two members of an alternative. It 
 would be a great mistake, however, to regard the mind as having 
 before it the cerebral machinery, all nicely laid out, together with 
 the acquired art of selecting and touching the right nervous ele- 
 ments in order to produce the desired motion, as a skilful player of 
 the piano handles his key-board. The mind has no native or ac- 
 quired knowledge of the different ideo-motor areas of the cerebrum. 
 Even less can we regard the mind, acting under the form of energy 
 of will, as bringing some stress to bear upon the right centres of 
 the brain, and thus setting them in motion by laying its own hand 
 to them, as it were. The activity of which we are directly con- 
 scious under the term " to will " is a purely psychical activity ; it 
 is marked by no transition of force from the spiritual realm to the 
 material molecules of the nervous structure. The feeling of effort, 
 which seems to us to accompany the active putting-forth of will, is 
 itself a resultant of mixed sensations that have a peripheral origin. 
 The whole description of such transactions of voluntary motion as 
 are constantly occurring for example, when we rise to close the win- 
 dow, take the pen in hand to write, etc. is as follows : We desire 
 to have something done ; mental images of the bodily motions and 
 positions involved in this doing arise in the mind ; the fiat of will 
 goes forth adopting one of them, and willing it, as we say ; an or- 
 der of nature which has correlated this fiat with certain cerebral 
 changes, but of which we know nothing whatever directly, and little 
 through the most searching investigations of science, runs its course, 
 and the transaction which we have ideated and willed takes place.' 
 The mind can represent the ideas in consciousness, and issue the fiat 
 of will ; it can do nothing more. Science can only conjecture at 
 
 1 This view of the subject has been repeatedly enforced by Lotze ; see, espe- 
 cially, the Microcosmus, i., pp. 283 ff. Edinburgh, 1885. 
 34 
 
530 NATURE OF BODILY MOTIONS. 
 
 present what then takes place. It is to its advancing theory of 
 nerve-physiology, 1 and of the localization of cerebral function, 3 that 
 we must look for more light on the question Wliat happens in the 
 brain when the fiat of will issues in consciousness f 
 
 30. Reflection on the foregoing principles makes it obvious 
 that the different concrete motions of ordinary experience cannot 
 be assigned with confidence to this or that class exclusively. In 
 the life of the infant we can trace a general progress from an 
 almost exclusive predominance of reflex and automatic motions, 
 through the impulsive, to more and more of the voluntary. But 
 even in the infant's case no hard and fixed lines can be drawn be- 
 tween the various classes of motions. It is impossible to say how 
 much of the constant movement of its legs and arms is reflex, how 
 much automatic. It is also doubtful how far and how long the 
 winking of the eyes, the grimaces of face accompanying the stimu- 
 lating of the tongue, the starting at sound, etc., are reflex rather 
 than impulsive. The same thing is true of its earlier cryings, mi- 
 metic and imitative movements, and various ways of thrusting out 
 and drawing in its limbs in a purposeful way. Nor can the earlier 
 voluntary motions be confidently distinguished from the impulsive. 
 
 This line of inquiry is especially interesting with respect to the 
 beginnings of articulate speech. A tolerably regular transition 
 from the sounds in which the earliest emotions express themselves 
 to the deliberate formation of words and sentences makes it impos- 
 sible to tell precisely when the child assumes control of its organs 
 of speech. But our difficulties with the unclassifiable phenomena 
 of infantile life do not seem so strange when we reflect upon the 
 fact that the complicated bodily motions of adult life partake at 
 one and the same time of all the four above-mentioned classes ; 
 and that precisely the same motions may pass rapidly out of one 
 class into another. The person, for example, who is balancing with 
 a pole on a tight-rope, or dancing to music, is involved at once in 
 motions which correspond to all four of these principal classes ; 
 and a quick change in circumstances may make any one of the 
 four more prominent than another. So perfectly may the nervous 
 mechanism be trained to its work that it may continue to play the 
 violin in an orchestra after the player has lost consciousness. Yet 
 the rise and fall of feeling usually serves as a guide to the artist, so 
 that impulsively his bowing draws nearer the bridge in the cres- 
 cendo, and nearer the key-board in the diminuendo, passages. If he 
 plays false or out of time, the sight of the leader's baton, or his own 
 
 1 Comp. Part I., chaps. III. and VII. 
 
 2 Comp. Part II., chaps. I. and II. 
 
MOTION AS EXPRESSIVE OF FEELING. 531 
 
 sensations, may decide him to the fiat of will which changes the 
 spacing with the left hand or the bowing with the right arm. 
 
 31. The origin and nature of those motions of the body that 
 are specifically expressive of certain ideas and feelings constitutes 
 one of the most interesting fields of inquiry. It is a field, however, 
 in which comparative psychology, by dealing with the facts of ani- 
 mal life under the theory of evolution, is particularly successful ; 
 whereas Physiological Psychology, strictly speaking, has little to 
 communicate. This little has been summarized by Wundt ' under 
 three general statements or principles namely, the principle of 
 the direct alteration of innervation, the principle of the association 
 of analogous sensations, and the principle of the relation of motion 
 to the presentations of sense. Under the principle of the direct 
 alteration of innervation are placed those facts which show that 
 strong emotions exercise an immediate reaction on the central parts 
 of motor innervation in such a way that many groups of muscles are 
 lamed at once, and others are excited to tense action followed by ex- 
 haustion. Hence the tremblings of limbs and organs of speech, the 
 changes in the blood-vessels and capillaries connected with secre- 
 tion, the paling of fear, the reddening of anger and shame, the 
 erect hairs under the influence of terror, etc. The principle of the 
 association of analogous sensations emphasizes such facts as imply 
 that sensations having a common tone of feeling are most easily 
 combined, and then operate mutually to strengthen each other. 
 Under this principle come the mimetic movements of mouth and 
 nose expressive of disgust or pleasant taste, the posturings of the 
 tongue in connection with ideas of sweet or bitter, the expressive 
 condition of the muscles due to certain sensations of the skin, etc. 
 The consideration of the third principle that of the relation of 
 motion to the presentations of sense brings before us the question 
 of the origin of all the gestures and pantomimic action not account- 
 ed for under the two foregoing principles. Gestures with eyes and 
 head and limbs, indicative of extension and relations in space ; the 
 arrangement of the muscles and skin of the countenance, and the 
 motions of the eyes under the influence of care, expectation, and 
 reflection ; the angles of the lines about the mouth and the open- 
 ings of mouth and nostrils when weeping or laughing, etc., all be- 
 long under this principle. But the physiology and psychology of 
 the comic, the science of physiognomy, and of articulation in expres- 
 sive speech, although properly coming in this connection, lead into 
 descriptive anatomy and the theory of aesthetics much beyond the 
 limits necessarily set to our investigation. 
 
 1 Physiolog. Psychologie, ii., chap. 20. 
 
CHAPTEE X. 
 PHYSICAL BASIS OF THE HIGHEK FACULTIES. 
 
 1. 'AN ardent advocate of "Psychology without a soul " affirms l 
 that "the study of abstract concepts (time, number, etc.) falls out- 
 side the province of physiological psychology, and has been made 
 incidentally only." To be sure, this author has previously 2 antici- 
 pated the time when the science of mind will succeed " in deter- 
 mining the (physical) conditions of all mental action, of whatever 
 sort, as well of pure thought as of perception and movement, "j will 
 in brief be " entirely physiological. " It is not necessary to in- 
 quire how these two sentences can be reconciled. But, undoubt- 
 edly, at present the statement of fact is far better founded than 
 the anticipation. It is not easy to predict how far psycho-physical 
 science will be able to push its discoveries in the future ; or just 
 where it will meet those insuperable barriers which surround all 
 fields of human inquiry. It is perfectly safe, however, to affirm 
 of all the phenomena of the so-called " higher faculties " of mind 
 what M. Kibot says of the study of abstract concepts they still 
 " fall outside the province of physiological psychology." Certain 
 difficulties are so obviously intrinsic and essential to the very nat- 
 ure of the facts with which this science attempts to deal when ap- 
 proaching these faculties that we cannot see how they will ever be 
 successfully met. 
 
 2. The foregoing conclusions apply most obviously to the for- 
 mation of abstract concepts, the conducting of trains of reason- 
 ing, the exercise of choice, and the activities of the creative imagi- 
 nation in artistic production, scientific discovery, or mechanical 
 invention. They apply only less obviously to the higher aesthetic, 
 ethical, and religious feelings ; although we have already pointed 
 out certain facts and laws which connect such feelings with a phys- 
 ical basis. We are also almost as much at a loss how to be " sci- 
 entific" (strictly speaking) in our treatment of the phenomena 
 which suggest some kind of physical basis for the action of will 
 
 See M. Ribot, German Psychology of To-day, p. 306. New York, 1886. 
 
 y^tfri/i p 15 
 
THE METHOD OF INQUIRY. 533 
 
 especially in the direction of attention for the apperception of 
 objects of sense, and for the control of the train of ideas or the 
 movement of the bodily organism. The same thing is true of the 
 phenomena of memory, whether considered as involving retention 
 merely or reproduction as well. All the attempts hitherto made to 
 explain or deduce consciousness, either in general or in the par- 
 ticular phase called self-consciousness, from cerebral functions and 
 activities, have been quibbling and wholly unsatisfactory. Yet 
 there are indubitable proofs of the dependence of consciousness for 
 its existence and modes upon the cerebral centres. 
 
 The inquiry after the physical basis of the mental phenomena 
 usually classed as " higher " is, therefore, although peculiarly in- 
 teresting, peculiarly unproductive of assured results. We may sus- 
 pect that there exist in the nervous elements of the gray matter 
 of the cerebral hemispheres inherited and acquired peculiarities of 
 molecular constitution and of dynamical combination, which, if we 
 could only get at them, would throw a flood of light upon such 
 mental phenomena. But after all, to speak soberly, we are obliged 
 to admit that the very existence of such peculiarities is still almost 
 wholly a matter of conjecture ; while the request for precise and 
 verifiable information as to their nature, and as to the laws which 
 connect them with undoubted facts of consciousness, can only be 
 met by evasion, confession of ignorance, or poetizing and declama- 
 tion under the garb of science. 1 Physiological Psychology has a 
 right to its own hypotheses ; it has, however, no right to introduce 
 myths about the genesis and marriage and "erethism" of nerve- 
 cells, and speculation as to nerve-fibres dynamically inclined, into 
 the domain of either physiological or psychological laws. 
 
 3. The only safe method of arriving at the few probable con- 
 clusions attainable concerning the subject of this chapter is, accord- 
 ingly, the following : The points of starting and the guides as to 
 the way must, in nearly every case, be taken from introspective psy- 
 chology. In studying the higher mental phenomena, physiological 
 psychology is obliged almost wholly to adopt, as the only direct 
 path open, the ?? cm-physiological method. Here, at any rate, we 
 start from that which appears to us as terra firma. We know what 
 it is to attend, to choose, to remember, and to reason in short, to 
 be conscious in some of the many modes or phases of conscious- 
 ness. Moreover, whatever may be said in disparagement of the 
 
 1 It is only by such terms as " poetizing" that we can truthfully characterize 
 the greater part of what is said, for example, by M. Luys, in his work on The 
 Brain and its Functions ; this, while admitting the skill and brilliancy with 
 which the author treats his own interesting conjectures. 
 
534 PHYSICAL BASIS OF VOLITION. 
 
 " old psychology," it cannot fitly be denied that it has most thor- 
 oughly and subtly analyzed the phenomena of judgment, memory, 
 and choice, as these phenomena appear connected with each other 
 in the flowing current of our conscious life. The result of such 
 analysis has been secured in the laws of logic, of the association of 
 ideas, etc., and in the various doctrines of the will and its relations 
 to motive and conduct. In fact, all study of these mental phenom- 
 ena from the physiological point of view is compelled to accept in 
 some form the conclusions of a study of the same phenomena from 
 the introspective point of view. For example, the reproduction of 
 ideas under the so-called laws of association is a general fact of 
 consciousness ; in the attempt to explain this fact according to 
 psycho-physical causes we are obliged to rely upon the results 
 reached by the introspective psychology. The application to men- 
 tal phenomena of uncouth terms derived from the physical sciences 
 such as " agglutination," " agglomeration," " cohesion," " organic 
 phosphorescence," " histological catalepsy," etc. has simply the 
 effect of repeating certain psychical facts and laws in a less appro- 
 priate way, without adding an item of information regarding the 
 *eal nature of their physiological basis. Ideas, or states and prod- 
 ucts of consciousness, cannot speaking literally cohere, or be- 
 come agglutinated or agglomerated ; and we need some better 
 proof than mere declamation to show that these states and prod- 
 ucts depend upon any physical processes resembling agglutina- 
 tion, phosphorescence, or catalepsy of the nerve-cell. Physiological 
 psychology is obliged, then, to accept certain conclusions of the 
 psychology of self-consciousness ; otherwise it has no motif or guide 
 in its investigation of the higher mental faculties. 
 
 But while our conscious psychical experience of the higher men- 
 tal activities is so far obvious as to make that side of the subject 
 capable of scientific statement, our knowledge of the physiological 
 processes connected with those activities is in precisely the oppo- 
 site condition. Over and over again the confession has been forced 
 from us that strictly speaking a scientific physiology of the 
 cerebral hemispheres does not yet exist. We can only dimly con- 
 jecture what takes place in the nerve-elements of the cortex of the 
 cerebrum as the physical basis of conscious sensation and percep- 
 tion. The molecular physics, or general nerve -physiology of the 
 nerve-muscle machine the simple peripheral nerve with muscle 
 attached is in a very unsatisfactory state. A science for the vast 
 complex of nerve-cells and nerve-fibres which exists in the gray 
 matter of the brain proper is at present scarcely a matter for even 
 hopeful anticipation. Faint and doubtful guesses, more or less 
 
AUTOMATISM OF CENTRAL ORGANS. 535 
 
 intimately connected with general principles of molecular physics 
 and physiology of the nervous system, are all that can appear in 
 the name of such a science. But the very business of physiological 
 psychology is to connect together under general laws the mental 
 phenomena, on the one side, and the ascertained facts of physiol- 
 ogy, on the other side. In this case, we are tolerably equipped 
 with information as to the former ; we have little but un verifiable 
 assumption to take the place of the latter. In attempting the in- 
 quiry into the physical basis of the higher faculties (the physio- 
 logical psychology of volition, memory, conception, etc.), no other 
 course is open but to accept the facts of consciousness, and then 
 speculate as to how they may, perhaps in part, be accounted for by 
 a conjectural extension of certain physical and physiological facts 
 to the cerebral hemispheres. This procedure certainly cannot be 
 called "science;" it is, however, the only one open instead of a 
 confession of complete ignorance. 
 
 4. The mental phenomena of the higher order, concerning whose 
 physical basis conjecture is most plausibly supported by a number 
 of related facts, may be divided into two great classes. One of 
 these covers the phenomena of Will, in the forms of attentive per- 
 ception and the effort determining the extent and character of the 
 field of consciousness ; the other covers the phenomena of Memory, 
 whether considered as the retention or the reproduction of ideas. 
 Certain conjectures as to the physical basis of both these kinds of 
 mental activities are in good degree warranted by the principles 
 discussed in the foregoing chapters. 
 
 The physiological basis (so far as such basis can be said to exist) 
 for those mental phenomena which appear in consciousness as "acts 
 of will " is laid, in general, in that power of automatism which is 
 concentrated, so to speak, in the nerve-cells of the central organs. 
 Automatism, or the power of originating motions which cannot be 
 explained as due to external stimuli, is indeed in some sort a 
 property of all living protoplasm ; but in that elaborate differentia- 
 tion of structure and function which the human body exhibits, the 
 nerve-cells of the central organs have absorbed this power and be- 
 come distinctively automatic. To them chiefly does it belong to 
 initiate within themselves the molecular changes which are neces- 
 sary to keep the body, both as a whole and in its several parts, 
 adjusted to the changes of its environment. It is sometimes said 
 that " an amoeba has a will of its own." Our only right to speak 
 in this manner is derived from the fact that many of its formal 
 changes seem to arise from within, and are quite inexplicable under 
 any known laws of merely reflex motion. If we raise the inquiry 
 
536 PHYSICAL BASIS OF VOLITION. 
 
 whether such automatic changes of its molecular structure are ac- 
 companied by anything which corresponds to what we call conscious 
 volition, it must be admitted that we are quite unable to answer 
 such an inquiry. We can easily imagine the amoeba, however, to 
 have a consciousness of an " act of will " as an accompaniment of 
 each automatic change in the arrangement of its molecules. A 
 large part of man's activity in the control of his bodily organism, 
 we know, is unaccompanied by any conscious volition. Such un- 
 conscious but purposeful activity belongs to the spinal cord and 
 to the lower cerebral centres, which act both reflexly and automat- 
 ically under the laws of acquired skill and of habit. In this way 
 many even of our so-called voluntary movements really take place. 
 But some sudden emergency as, for example, the sight of a 
 threatening object, a change in the character of the soil on which 
 the pedestrian treads, the parting of a rein in the rider's hand 
 may call for a succession of distinct and intense acts of will. And, 
 ordinarily, mild and rather obscure volitions connected with the 
 movement of the body intermingle with the succession of sensa- 
 tions and ideas which compose the principal material of con- 
 sciousness. 
 
 In all such cases as the foregoing we have reason to suppose 
 that, either through external or internal stimuli (either through 
 sensory impulses coming in along the centripetal nerve- tracts or as 
 started by changed conditions of blood-supply), the nerve-cells of 
 the cerebral hemispheres are called upon to exercise their peculiar 
 functions. Such functions we may well believe are always both 
 reflex and automatic ; that is to say, the nerve-commotions which 
 issue from the cells are dependent for their intensity and charac- 
 ter both upon the excitations coming to them from without and 
 also upon their own internal molecular structure and condition 
 especially as respects the blood-supply. Accordingly, it must be 
 held that volitions, or acts of will in consciousness, do not have 
 their physical basis in any special organ or area of the brain. 
 There is no special organ of ivill. All the central organs have pre- 
 eminently the property of automatism. But since, in the case of 
 man at least, it is only on occasion of a certain kind and degree of 
 activity of the cerebral hemispheres that what takes place in the 
 nervous system has any corresponding expression in conscious- 
 ness, the physical basis for acts of will in general is the automa- 
 tism of these hemispheres in general. 
 
 5. An act of will, however, is always an act of some special kind. 
 There can be no volition to motion in general, but only a volition 
 defined and limited to the movement of certain limbs, or of the 
 
SIGNIFICANCE OF SPECIAL AREAS. 537 
 
 trunk including the limbs, with a certain direction and degree of 
 motion. Thus also every act of will for the control of the mental 
 train, or for the apperception of an object of sense, through con- 
 centrated attention, is denned by some particular mental state or 
 modification upon which it is directed. We have seen good reason 
 to believe that certain areas of the cerebral cortex are especially 
 connected with certain corresponding sensory-motor activities 
 (comp. Chap. II. throughout). In the same areas, then, the physical 
 basis is laid for those acts of will that are concerned with the 
 corresponding activities. The acts of will which have to do with 
 the movement of the upper and lower limbs, for example, im- 
 ply the special activity of the cerebral areas on either side the 
 Fissure of Rolando ; those acts of will that have to do with the 
 movement of the organs used in articulate speech are especially re- 
 lated to the areas lying about the lower part of the Fissure of Syl- 
 vius the posterior third of the lower frontal convolution, etc. We 
 have no sufficient ground for locating in one circumscribed spot 
 the physical basis of such acts of voluntary attention as concern 
 the different presentations of sense and the images of memory de- 
 rived from them. The case is not as though the mind made a 
 transit, as it were, from some special seat of intelligence and will, 
 to contemplate with attention and pronounce upon the complicated 
 sensory impressions which have arrived and been elaborated in the 
 particular sensory areas ; or as though it travelled from adjacent 
 parts to lay its grip upon the right motor areas when sensation or 
 desire indicated to will that certain groups of muscles should be 
 innervated. Whenever an act of mill takes place, then at the cerebral 
 area which corresponds to the particular nature of the act (namely, 
 the will to attend to this object of sense, or to start in motion that 
 limb) the particular molecular changes arise in the nerve-cells which 
 are correlated with such mode of consciousness. 
 
 6. As to the relation in time which is maintained between the 
 conscious act of will and the particular form of automatic cerebral 
 excitation which we have called its physical basis, it is not possi- 
 ble to pronounce with confidence. But there is no good reason 
 to suppose that the conscious mental act is interpolated as an 
 independent element of time, so to speak, among the physiological 
 processes. The flow of consciousness from obscure sensation to 
 perception and clear attentive discernment, then to the act of de- 
 cision between two or more possible forms of appropriate movement, 
 and, finally, to the issue of the right fiat of will, all keep pace with 
 the corresponding physiological processes in the cerebral areas. 
 As to the exact nature of these processes, and as to how they fur- 
 
538 PHYSICAL BASIS OF VOLITION. 
 
 nish necessary conditions to the mental movement, there is no in- 
 formation to be imparted. 
 
 7. The problem is complicated when our consciousness becomes 
 one of deciding to which of several presentations of sense or im- 
 ages of memory we will to direct the attention. " Concerning the 
 physiological processes," saysExner, 1 " from which we abstract the 
 conception of attention we know absolutely nothing." This is true 
 even when attention seems determined or forced upon us by causes 
 over which the mind has no control ; it is, of course, more obviously 
 true when the mind is conscious of deliberation and choice. The 
 attention which directs to the single object and heightens the clear- 
 ness of our perception, converting it into an " apperception," may 
 properly be spoken of as an act of ivill ; but starting from the point 
 of view of consciousness, it must be admitted that, in the majority 
 of such activities of apperception, there is no consciousness of choice 
 the will is determined in one way. This is equally true of the 
 attachment of attention to certain particular images of the mental 
 train, as that train is conducted along under the laws of association. 
 Most things which we clearly perceive, or feel with any decided 
 pain or pleasure, or which are vividly brought before the mind as 
 images of memory and imagination, we cannot help attending to. 
 The sudden flashing of a light, the passing of a bright object across 
 the field of vision, the occurrence of a loud noise, or of a fainter 
 one with a character that interests us, the smells in the atmosphere 
 and the taste of our food, the sensations of the internal organs and 
 of the skin, when sufficiently intense all these compulsorily draw 
 after them the attention. They get themselves perceived by an im- 
 pulsive and involuntary act of will. So, too, do the revived images 
 of memory, in ordinary circumstances where the perception of ex- 
 ternal objects is relatively suppressed, appear to force themselves 
 upon the attention. 
 
 In view of the foregoing familiar facts of consciousness, we may 
 conjecture that when the cerebral centres are not preoccupied, as 
 it were, with contradictory forms and phases of nerve-commotion, 
 certain processes set up within them, whether due to external stim- 
 uli or to changes in the blood-supply, are necessarily followed by 
 the phenomena of conscious attention. Even when these centres 
 are largely thus preoccupied, similar changes may be rapidly forced 
 within them, by the action of some very strong excitation from the 
 end-organs of sense, or from some connected cerebral centre. Hence 
 the shock of surprise which sudden and vehement impressions cre- 
 ate. In all such cases of forced attention the resulting tone of feek 
 1 In Hermann's Handb. d. Physiol., II., ii., p. 283. 
 
THE DIRECTION OF ATTENTION. 539 
 
 ing in consciousness is different from that which prevails when the 
 choice to attend is being deliberately maintained or persistently 
 revived. The motifs of much of our activity of will in attention, 
 therefore, plainly lie in that state of the cerebral centres which is 
 compelled by the intensity of the stimulation they receive (either 
 from external or internal stimuli). 1 If there were no other phe- 
 nomena of will than those of forced attention, it would be necessary 
 to admit the probability that all the mental activities are purely 
 mechanical and absolutely dependent upon the action of the ner- 
 vous system under the exciting influence of stimuli. 
 
 8. Certain phenomena of will in the form of attention suggest 
 conclusions of a different order from the foregoing. Taking our point 
 of starting again from consciousness, we know by a manifold expe- 
 rience that the different degrees of clearness with which we perceive 
 objects or apprehend the images of memory implies a graded appli- 
 cation of attention. The grading of this application of attention is 
 by no means always determined solely by the intensity of the stim- 
 ulus, so far as we can measure such stimulus. It is a principle of 
 wide reach, that to quote the words of Wundt " the degree of 
 apperception is not to be measured according to the strength of the 
 external impression, but according to the subjective activity by 
 which the consciousness is applied to a definite sense-stimulus." 
 The subjective activity which applies the consciousness, as it were, 
 to this or that presentation of sense or image of memory, rather than 
 to some other, is an activity of will ; and the effect of the activity is 
 seen both in heightening the attention as directed to the object, and 
 also in adapting the attention to the particular object upon which it 
 is directed. Accompanying this twofold control of attention, and 
 indeed forming the very basis upon which it rests, when deliberately 
 exercised, is the consciousness of " choice " the activity of will in 
 deciding the direction and amount of attention bestowed upon one ob- 
 ject among several in the field of consciousness. Percepts and ideas 
 do not move from the various obscurer parts of the field of con- 
 sciousness into the focal point by virtue solely of a momentum be- 
 longing to them as such ; they are placed and kept there by an act 
 of will. This must be admitted as an indubitable fact of conscious- 
 ness, whether or not the physiological correlate or so-called explana- 
 tion of this fact can be discovered or even conjectured. 
 
 Many indisputably valid phenomena, both those accessible to 
 ordinary observation and those discovered by special experiment, 
 
 1 Comp. Wundt, Physiolog. Psychologie, ii., pp. 387 ff.; and Staude, Der Be- 
 griff d. Apperception in d. ueueren Psychologie, Philosoph. Studien, I., Heft 
 ii., p. 1941 
 
540 PHYSICAL BASIS OF VOLITION. 
 
 illustrate the foregoing principle. By an act of will attention may 
 be heightened and accommodated to the object, with a marked in- 
 fluence upon apperception and the association of ideas. Upon this 
 point we have to recall facts already mentioned. The effect of a 
 voluntary increase of attention upon the reaction-time is to diminish 
 it, of distracted attention to increase it or destroy its value alto- 
 gether. 1 If, simultaneously, the ear is stimulated by the periodic 
 strokes of a bell, and the retina by regularly recurring electrical 
 sparks, the attention will naturally be directed to the former ; the 
 image of the latter will then be located only very obscurely in the 
 flow of consciousness, and the time of its occurrence may scarcely 
 be noticed at all. We incline to attend to the stronger of two ex- 
 citations of sense ; to yield to the inclination depresses the weaker 
 still further perhaps below the plane of conscious perception. 
 But within certain limits we attend where we will. We also incline 
 to attend to objects lying in the point of regard of the field of 
 vision, but we can will to attend to objects lying in the outward 
 portions of this field. 2 The voluntary direction of attention in this 
 case determines the apperception of these objects to the neglect of 
 those lying in the more favorable parts of the field. We can at- 
 tend to the field of vision of one eye, neglecting the other, as skil- 
 ful microscopists do. We can see by voluntary attention the other- 
 wise invisible double images. It is claimed by some experimenters 
 with the "conflict of colors" in binocular vision, that, by the direc- 
 tion of attention, when a green image is formed on one eye and a 
 red upon the other, they can see either at will, or at will can com- 
 bine the two. 
 
 Experiments with instantaneous illumination by the electrical 
 spark also demonstrate in a marked way the effect of attention. 
 Objects which under ordinary circumstances are without great effort 
 seen, either as stereoscopic or as double images, can also be seen 
 in both ways by the electric spark, according to the direction of 
 attention. The first impression is ordinarily stereoscopic ; but if a 
 pause of 10 sec. be allowed for the after-images to die away, the 
 experimenter can at will see the double images, although the point 
 of fixation and the influence of the light remain absolutely the 
 same. 3 The effect of attention in analyzing composite musical 
 
 1 See the table of Obersteiner, Brain, I., p. 439, to show the fluctuations of 
 the reaction-time of a person reacting while an organ was playing in the same 
 room. The normal reaction-time of the person being 0.100 sec., it rose to 
 0.148, and even 0.215, while the instrument was heard, and fell to 0.095 and 
 0.087 during pauses in the playing. 
 
 - See Helmholtz, Physiolog. Optik, p. 740 f . 3 Ibid. , p. 741. 
 
EFFECT OF ATTENTION ON MEMORY. . 541 
 
 clangs into their elements is equally marked. 1 In addition to a 
 previous acquaintance with the character of such over-tones as are 
 to be expected and analyzed out of the clang, the analysis can take 
 place in no case without a " certain undisturbed concentration of 
 the attention." That changes in the clearness of perceptions take 
 place in dependence on the changes in the degree of attention is a 
 matter of the most ordinary experience. On waking gradually 
 from sleep our surroundings become less and less obscure to the 
 senses of eye, ear, and skin, as the grade of voluntary attention in 
 apperception progressively rises. On casting the glance casually 
 upon a landscape seen through a window, its objects are, at first, 
 scarcely perceived at all ; by gradual increase in the intensity of 
 attention (changing the casual glance into a steady look) these 
 objects become apperceived more and more clearly. The voluntary 
 concentration of attention (comp. p. 446 f.) often dissolves an error 
 of sense or changes the entire appearance of the visual object. On 
 the other hand, a great strain of attention may lead one to anticipate 
 an expected impression of sense, and perceive its occurrence before 
 it has actually taken place. It may also cause other illusions, as 
 when, on expecting eagerly the stroke of the clock, some weaker 
 sound may be mistaken for it. 2 
 
 9. Voluntary attention directed toward the images of memory 
 has also a marked effect upon their character and duration in con- 
 sciousness. In certain cases it may impart to them the vividness 
 of presentations of sense, although the power to bring this about 
 differs greatly for the different senses and in different individuals. 
 By an effort of will the player of a musical instrument can cause 
 himself to feel again the revived images of the muscular and tactual 
 sensations which accompanied a particular exercise of his skill. 
 The hearer of some impressive musical air may voluntarily set it 
 running, with its variation of tone and rhythm, through his mind's 
 ear, as it were. Not to speak of hallucinations and visions, most 
 men see sights in dreams, and even in reverie, that closely approach 
 the intensity of the presentations of sense in the waking state and 
 in broad daylight. Artists in forms of art involving a special sus- 
 ceptibility and activity of some one or more of the senses, are, of 
 course, gifted with a specialized creative energy of imagination. 
 Particular images of memory may be seized upon at will, as it were, 
 and the attention so concentrated upon them as to impart to them 
 much of the strength which their originals enjoyed. 
 
 Moreover, the effect of attention upon certain images of memory 
 
 1 Hehnholtz, Die Lehre von den Tonempfindungen, etc. , p. 84 f. 
 
 2 See Fechner, Elemente d. Psycho-physik, ii., p. 491 f. 
 
542 PHYSICAL BASIS OF VOLITION. 
 
 is such as apparently to localize them anew in the organs by which 
 these originals were formed. To try to revive a melody as distinctly 
 as possible produces a sense of strain (a feeling of being innervated) 
 in the region of the ear. We recall sounds, especially if we recall 
 them vividly, with the organs of hearing. The prolonged effort to 
 recall or image colors or visual forms tires the visual organs ; the 
 impression is as though the recollection or imagination were accom- 
 plished in and through these organs. The violin-player remembers 
 and goes over the solo he is to play, not only in his ear, but in his 
 arms and fingers. Upon such power of reproduction the power of 
 new production depends. Indeed, we may say that " the activity 
 of voluntary sensuous attention largely consists in a voluntary re- 
 production of earlier conditions of sensations." l It has even been 
 claimed that vivid representative images of color-sensations may be 
 followed by the corresponding negative after-images. 2 
 
 10. Concerning the physiological basis of the phenomena of 
 voluntary attention, little is known. Apparently, part of the effect 
 must be due to the changed condition which is brought about in 
 the end-organs of sense when especially innervated and so prepared 
 for receiving the stimulation appropriate to them. To this fact is 
 due the peculiar feeling of strain in the organ of attentive apper- 
 ception, or of vivid reproduction of the image of memory. But 
 the chief effect of attention is realized in the altered condition of 
 the cerebral centres. It is only obvious, according to Exner, 3 that 
 we have to do with changes in the central mechanism set up by act 
 of will, and that these changes vary quantitatively and concern the 
 circles of our psychical activity. Moreover, the point of attach- 
 ment, as it were, for the attention is found only after the impres- 
 sion of sense has been elaborated to a certain degree. We cannot \ 
 voluntarily attend without perceiving, at least obscurely, that to 
 which (presentation of sense or image of memory) attention is to 
 be directed. Still further, fluctuations of the cerebral activity 
 are constantly occurring ; for, as every one knows who has expe- 
 rimented with himself (for example in determining reaction-time), 
 it is impossible to keep attention on a perfectly steady stretch with 
 respect to its object. Waves .of consciousness in connection with 
 these fluctuations of attention rise and fall. 4 In the most suc- 
 cessful reactions the attention is- effectual in producing and inain- 
 
 1 Comp. G. E. Miiller, Zur Theorie d. sinnliclien Aufmerksamkeit, Leipzig, 
 p. 89. 
 
 2 Wundt, Vorlesungen uber Mensch u. Thier, I. , p. 387. 
 
 3 In Hermann's Handb. d. Physiol., II., iL, p. 283. 
 
 4 Comp. Feehner, Elemente d. Psvcho-physik, ii. , p. 452 . 
 
INFLUENCE OF THE FRONTAL REGIONS. 543 
 
 taining a state of strained expectation, in which the occurrence of 
 the expected stimulation sets the motor mechanism off without, and 
 even in spite of, a separate act of will. In such cases the cerebral 
 centres have apparently been thrown into an exalted and explosive 
 state of irritability. We all know that very little suffices to set the 
 muscles agoing when the mind is on the stretch. It is not unlikely 
 that the effect of attention is felt in depressing certain cerebral 
 areas not intimately connected with the production of the particular 
 image of memory or presentation of sense, as well as in heightening 
 the activity of others that are thus connected. 1 The phenomena, at 
 any rate, imply an increased difference of excitability and conductivity 
 for their specific forms of nerve-commotion in the different cerebral 
 areas. Under the influence of attention the cerebrum has become 
 more susceptible for certain impressions, less so for certain others. 2 
 Stored energy of the nerve-cells is being rapidly called forth. Con- 
 centrated voluntary attention implies a large amount of work being 
 done in the cerebral hemispheres. We recognize this fact in the ac- 
 companying feelings of strain and in the subsequent feelings of 
 brain -exhaustion. The subject of experiment to determine the 
 reaction-time under concentrated attention often, though sitting 
 quiet, sweats profusely. 
 
 11. What happens when two different excitations, arising either 
 from conflicting presentations of sense, or conflicting ideas, meet in 
 any single region of the brain ? We can only answer this question 
 with vague conjecture. The phenomena of the conflict of colors in 
 binocular vision seem to imply that either the more intense of the 
 two may prevail over the other, or the two may both persist and 
 interpenetrate as it were. In certain cases voluntary attention 
 may determine which event shall ensue. The sugar sweetens the 
 acid of the lemonade, not in the vessel which contains it or on the 
 tongue which tastes it, but in the brain. According to the con- 
 jecture of Wundt, 3 the frontal regions of the cerebrum are the 
 " bearers (Trdger) of the physiological processes which accompany 
 the apperception of the presentations of sense." In order, then, 
 that the process set up in any cerebral region by an excitation of 
 the organs of sense connected with it may result in clear discern- 
 ment of an object, certain physiological processes must be con- 
 ducted from the frontal regions to that region. Wundt's con- 
 jecture is plausible, at least it gives the frontal region something 
 to do, and answers in part the inquiry why so much of the cere- 
 
 1 See G. E. Mia Her, Zur Theorie d. smnlicheu Aufmerksamkeit, p. 52 f. 
 ' 2 See Exner, in Pfluger's Archiv, xi., p. 428. 
 3 Physiolog. Psycliologie, i. , p. 218. 
 
544 PHYSICAL BASIS OF VOLITION. 
 
 bral substance should seem merely negative as respects the phe- 
 nomena of sensation and motion. 
 
 12. Nothing thus far said, and nothing of scientific value which 
 physiological psychology has to offer, throws any clear light on the 
 problem of the "freedom of the will." When M. Luys, 1 for exam- 
 ple, maintains that to imagine " we think of an object by a spon- 
 taneous effort of mind is an illusion" and that, in fact, the object is 
 only forced on us by the cunning conjurer, the brain, " because the 
 cell-territory where that object resides has been previously set vi- 
 brating in the brain," he is controverting a plain and universal 
 dictum of consciousness by his private and unverifiable hypothesis 
 on a question of cerebral physiology where experts and novices are 
 alike ignorant. Physiology neither disproves nor verifies the post- 
 ulate of free will ; accordingly, this postulate must be raised and 
 discussed upon other grounds. Metaphysics and ethics cannot 
 properly dictate their facts and conclusions to the science of physi- 
 ological psychology ; but, in turn, this science cannot properly 
 dictate to metaphysics and ethics the conclusions which they shall 
 draw from facts of consciousness, by giving out its myths and fa- 
 bles in the garb of well-ascertained history of the cerebral pro- 
 cesses. 
 
 13. Consciousness, or the having any form of sentient life, in 
 distinction from being in a condition of dreamless sleep or swoon- 
 ing, and Self-consciousness, or the recognition of the states of con- 
 sciousness as states of the ego or subject of them all, are inti- 
 mately connected with the phenomena of will. By the amount 
 and speed of the energy expended in attention we measure in large 
 degree the extent and intensity of consciousness. The stir of feel- 
 ing through the presentation of some object of sense, or through 
 some idea, causes us, either voluntarily or involuntarily, to rouse 
 ourselves to what is then recognized as a wider and higher energy 
 of consciousness. But inasmuch as consciousness is the condition 
 of all internal experience whatsoever, we cannot deduce or explain 
 the essential nature of consciousness from other forms of such ex- 
 perience. 2 For the same reason we cannot define consciousness. 
 
 Concerning the physical basis of consciousness little can be added 
 to what has already been said concerning the physical basis of 
 the various forms of consciousness. Consciousness is never con- 
 sciousness in general neyer_an activity or state that may be sep- 
 arated from the individual states and processes of consciousness. 
 In the case of man, the cerebrum is apparently the sole, as it cer- 
 
 1 The Brain and its Functions, p. 254. 
 
 * Comp. Wundt, Physiolog. Psychologic, ii. , p. 195 f. 
 
CONDITIONS OF CONSCIOUSNESS. 545 
 
 tainly is the chief, organ of consciousness (comp. pp. 249 ff.). By 
 calling the cerebrum the " organ " of consciousness, however, little 
 more is meant than that the constitution and processes of the ner- 
 vous matter of t^ris organ are related in the most immediate and 
 special way to all mental phenomena, and that what takes place in 
 material elements outside of the cerebrum (including the elements 
 of the other portions of the nervous system) has an effect upon con- 
 sciousness only in case it gets itself represented, as it were, in the 
 corresponding cerebral processes. As to a special organ of con- 
 sciousness in the brain that is, a cerebral area where the mind 
 comes to consciousness it is not proper to speak. 
 
 Accordingly, the physical basis of the different forms of con- 
 sciousness is laid in those cerebral areas which have been found 
 to be or, though still undiscovered, actually are especially con- 
 nected with these forms. But if the question is further pressed as 
 to the physical basis for the activities of self-consciousness, no answer 
 can be given or even suggested. From its very nature that mar- 
 vellous verifying actus of mind in which it recognizes itself as the 
 subject of its own states, and also recognizes the states as its own, 
 can have no analogous or corresponding material substratum. It 
 is impossible to specify any physiological process representing this 
 unifying actus ; it is even impossible to imagine how the descrip- 
 tion of any such process (in case we knew what to attempt to de- 
 scribe) could be brought into intelligible relation with this unique 
 mental power. 
 
 In general, concerning the physical conditions of consciousness', it 
 is known that they are dependent upon the character and amount 
 of the blood-supply. To stop this supply is to put an end for the 
 time to consciousness ; to impede or corrupt it is to depress and to 
 disturb consciousness ; to alter its character is to affect, more or 
 less promptly and profoundly, the character of consciousness. The 
 character of the circulation in the cerebrum largely determines the 
 nature of the phenomena of consciousness. Quickened circulation 
 here accelerates or agitates the circuit and time-rate of conscious- 
 ness ; slower circulation diminishes and inhibits them. It has 
 been alleged by Mosso that certain changes in the relative circula- 
 tion of the human body occur when the attention is occupied in- 
 tensely either with external impressions or with psychical work. 
 Such observations are not as yet extended and accordant enough 
 to command unhesitating assent to the details of their results. 
 
 14. The other group of so-called higher mental phenomena 
 which admits of the most of probable conjecture regarding the nat- 
 ure of its physical basis comprises Memory, as retentive and re- 
 35 
 
546 PHYSICAL BASIS OF MEMORY. 
 
 productive, and the laws of the Association of Ideas. The experience 
 of consciousness is one of a constantly changing succession of states. 
 The rise and fall in voluntary or involuntary attention, and the 
 change of its direction, are accompanied by a continual alteration 
 of the phases and of the circuit of consciousness. Of these shifting 
 mental states certain ones bear the peculiar mark of a claim to rep- 
 resent previous states of consciousness, in some regard and to some 
 extent similar to themselves. The image of memory is itself a prod- 
 uct, a phase, of present consciousness ; it is not itself of the past, 
 and yet it claims, by virtue of its essential character, to stand for 
 the past. This claim can, of course, in no instance be verified by 
 carrying the consciousness back to that past ; we are never able by 
 attentive apperception to compare the image of memory with its 
 alleged original and thus make sure of the validity of the claim. 
 Experience also teaches us that the mental images do not come 
 and go wholly at random and irrespective of the characteristic con- 
 tent of such as are most closely connected in time. That these 
 images are associated in time is a part of the fundamental fact of 
 memory ; mental states are not states of memory without some 
 more or less definite localization of the ideas thus presented to the 
 mind with reference to its past. 
 
 Further examination of the particular character of the ideas 
 which most frequently occur simultaneously, or in closest succes- 
 sion, has given rise to the assumption that the images of memory 
 are associated in a regular way. Hence those general facts of psy- 
 chology called the "laws of the association of ideas." From the 
 phenomena of memory and reminiscence, as experienced in the 
 consciousness of the individual, arises the belief that the objects of 
 past experience are retained in the mind, and that they suggest each 
 other (at least ordinarily) in some orderly way. But properly 
 speaking, the " retention " of states of consciousness, whether of 
 ideas or of presentations of sense, is not a faculty or power of 
 mind. To ask, 'Where is the idea I once had, or the object I once 
 saw, between the time of the original experience and the time of 
 recall, is to ask a question that can have only one answer. Such 
 idea or presentation of sense is nowhere, for it does not exist in 
 any sense of the word whatever. Both presentation of sense and 
 image of memory are transitory phases of consciousness, each per- 
 ishes with that phase of consciousness in which, and as which, it 
 has its existence. It is the power of recall solely which induces us 
 to speak as though the mental object were retained or kept in the 
 Mind. It is only in the facts and laws of conscious reproduction that 
 any trace of the activity of mind, as memory, is to be found. Much 
 
PRIMARY 131 AGE OF MEMORY. 547 
 
 "cerebration" may be unconscious; there may be considerable 
 periods of complete unconsciousness in the daily life of every indi- 
 vidual, as there certainly are such periods occasionally in the lives 
 of some individuals. But of unconscious retention or reproduction 
 of ideas as an activity of mind, there is none. 
 
 15. Both ordinary observation and experiment in reaction-time 
 indicate that the speed with which the images of memory vanish 
 depends upon a variety of circumstances such as individual pe- 
 culiarities, intensity and frequency of the repetition of the original 
 impression, condition of the cerebral centres and of the conscious- 
 ness at the time of this impression, etc. The thousands of faint 
 impressions which enter into every-day life seem quickly to vanish, 
 without leaving a trace behind in either body or mind. But that 
 these impressions do linger for a time in memory, as we say, or 
 are reproducible in consciousness under the form of images of 
 memory, there can be no doubt. For example, if, while one per- 
 son is intently counting the lines of a page or adding a column of 
 figures, another near by makes some slight motion, the image of 
 the motion in the mind of the former may be recalled within a few 
 seconds of the occurrence ; a little later, however, such an image 
 may have so completely vanished that the observer will declare the 
 motion was not noticed at all. 1 The vividness of fresh images of 
 memory may be so great as even to make it difficult to distinguish 
 them from true presentations of sense. But even in the case of 
 impressions made clear and strong by the strenuous concentration 
 of attention, the vividness of the image of memory diminishes at 
 first very quickly. Even in such a case the so-called " primary " 
 image of memory may not last beyond a few minutes ; while in 
 cases where there is little attentive apperception this image van- 
 ishes in a few seconds. 2 Let a line of given length be regarded for 
 a brief time, then removed, and after a varying interval the effort 
 made to recall its image so as to compare it accurately with another 
 line of nearly the same length. It will be found that the clearness 
 of the image of memory, which quickly falls off at first, falls off 
 afterward more slowly, and finally approximates more nearly to a 
 stationary condition. Lotze 3 has insisted upon the caution that 
 we should not confuse the clearness of ideas with their intensity ; 
 the idea of the most intense brightness is not intensely bright. Yet 
 we cannot agree with Lotze in the opinion that the ideating activity, 
 when applied to the recall of sensations, does not differ in intensity 
 
 1 See Exner, in Hermann's Handb. d. Physiol., II., ii., p. 281 f. 
 
 2 Comp. Fechner, Elemente d. Psycho physik, ii., p. 491. 
 
 3 Outlines of Psychology, p. 28 f. 
 
548 PHYSICAL BASIS OF MEMORY. 
 
 as well as in clearness. Besides the difference in the clearness with 
 which two persons, for example, remember a mosaic of colors, as re- 
 spects all the details of what particular colors were arranged in what 
 particular order, there is also a difference in the strength which the 
 various revived images of the colors have in the two consciousnesses.. 
 
 16. That the mental phenomena which lead us to speak of the 
 retentive power of memory have a physical basis there can be no 
 doubt. This conclusion is warranted by the nature of the phenom- 
 ena themselves, by the impossibility (already alluded to) of con- 
 ceiving of a permanent modification of unconscious mind, and by 
 all that we know of the principles of biology in general and of hu- 
 man physiology in particular. Every sensory impulse, and every 
 combination of such impulses, must produce changes both in the 
 end-organs and in the central organs ; and although these changes 
 vanish, so far as their effect in the corresponding phenomena of 
 conscious mind is concerned, they nevertheless cannot fail to leave 
 the organs in different condition from that in which they found 
 them. As a matter of course, the effect of stimulus upon every 
 end-organ of sense consists in the production of molecular changes, 
 which, on account of the principle of inertia as applied to such or- 
 gan, continue for a time after the stimulus has been removed. Of 
 this fact the existence of the after-images on the retina is the most 
 notable example. The successive stages passed through by the af- 
 ter-images, both positive and subsequently negative, are themselves 
 indicative of a series of molecular changes set up by the action of 
 the stimulus. But the effects of stimulus must also be felt in the 
 production of molecular changes in the central organs, in the nerve- 
 elements (especially the nerve-cells) of the cerebral hemispheres, 
 if the sensory impulses are to result in conscious sensation and per- 
 ception. Experience would then lead us to infer, further, that each 
 combination of sensations produces changes in the cerebral hemi- 
 spheres which outlast the action of the stimulus upon the end-organ 
 of sense. A study of consciousness, simply as consisting of chang- 
 ing sensations and perceptions, might appear to indicate that the 
 after-forms of molecular changes themselves die out and leave the 
 end-organs and cerebral centres in precisely the same condition 
 as before. But the formation of habits of perception and motion, 
 the phenomena of conscious mental reproduction, and the general 
 principles of molecular science as applied to the nervous mechan- 
 ism, suggest and enforce another view. 
 
 Certain experiences in the use of the senses show that molecular 
 activities induced in the end-organs by stimulation may, under cer- 
 tain circumstances, persist much longer than we are at first inclined 
 
FORMING OF MOLECULAE TENDENCIES. 549 
 
 to suppose. For example, a study of the after-images left by 
 strong impressions on the retina shows that traces of them recur 
 again and again, even several minutes after the eyes have been 
 closed. Prolonged work with the microscope will cause the images 
 seen in its focus to " live in the fundus of the eye " so that, after 
 several hours, shutting the eyes will cause these images to reappear 
 with great distinctness. Of a similar kind is our experience with 
 sounds the rattle of the railroad-car after a long journey, the im- 
 pressive cries or words to which we have listened, the successive 
 notes and chords of the musical composition heard at a concert, 
 seem to be repeated in the ear for hours after the primary sensa- 
 tions have subsided. According to Dr. Moos, 1 after long musical 
 seances the sounds persisted for fifteen days in one patient ; and a 
 professor of music was accustomed to hear over again the notes 
 sounded, for several hours after each lesson. After startling and 
 impressive experiences with different kinds of sense-percepts 
 sounds, sights, etc. it is not an uncommon thing for the same 
 bodily affections to recur with such vividness as to make it almost 
 impossible at the instant to distinguish them from fresh experi- 
 ences of the same kind. 
 
 Moreover, all those inherited and acquired unconscious habits of 
 motion, with which the study of the nervous mechanism has already 
 made us familiar, imply that the effects of repeated stimulations 
 persist in the molecular constitution and tendencies to molecular 
 change of the nervous substance of the central organs. The puppy 
 which has inherited a brain and spinal cord embodying the habits 
 of his race, and the trained gymnast or skilled player on a musical 
 instrument, alike illustrate this principle of stored and organic ex- 
 perience as pertaining to the elements of the nervous system. The 
 general principle of molecular science, which finds numerous ex- 
 amples both in inorganic chemistry and in biology, compels a 
 similar conclusion as to the physical basis of memory. The estab- 
 lished practice of photography depends upon the fact that a plate 
 of dry collodion, after being briefly exposed to the sun's rays, re- 
 tains for weeks, in the darkness, the effects of the indescribably deli- 
 cate changes which have been brought about in it. The " latent 
 image " contained in it may be revived by proper treatment. The 
 phenomena of phosphorescence, also, show that the impressions of 
 the luminous undulations persist in certain bodies for a consider- 
 able time after these undulations themselves have ceased acting. 
 Niepce de Saint- Victor 2 has shown that such undulations may be 
 
 1 So Luys, The Brain and its Functions, p. 136. 
 
 - Comptes-rendus de I'Academie des Sciences, xlv. , p. 811 , and xlvi., p. 448. 
 
550 PHYSICAL BASIS OF MEMORY. 
 
 " to some extent garnered up in a sheet of paper," ready to be re- 
 vealed at the call of special reagents. Inasmuch as the nervous 
 system consists of an inconceivably complicated and delicate molec- 
 ular mechanism, every element of which may be regarded as a 
 highly complex molecular structure, it may well be expected to ac- 
 complish much more wonderful results than the plate of dry col- 
 lodion or sheet of paper, in the way of storing up for future de- 
 mand the results of the impressions made upon it. We may even 
 go so far as to say that the retentive power of this molecular mech- 
 anism is perf ect ; ' that it never loses entirely the effect of any im- 
 pression once made upon it. 
 
 17. In view of such considerations as the foregoing it has been 
 proposed by some writers to regard memory simply as one phase 
 of the general biological fact, as a particular form of synthesis " of 
 one of the primordial properties of the nervous elements." Con- 
 .scious memory is then considered as "a phosphorescence of the 
 nervous elements " plus consciousness ; and this power of these 
 elements itself is "to be called a power of "unconscious reminis- 
 cences " (M. Luys) or an " organic memory " (Hering and M. Ribot). 
 But, while admitting the general fact of molecular science, and the 
 application of it to the phenomena of habit in the nervous mechan- 
 ism as contributing something to the description of the physical 
 basis of conscious memory, it is wise to refuse to use such terms as 
 the foregoing. For these terms are not needed to state the facts ; 
 of themselves they lend nothing to the desired explanation, and 
 they are liable to lead to serious confusion. " Organic " memory, 
 or the habitual mode of the behavior of the nervous system, to- 
 gether with that tendency to reproduce the mode which belongs 
 to all habits, when minus consciousness, is not memory at all ; it 
 bears, indeed, no resemblance to memory. " Unconscious" reminis- 
 cence, regarded as a function of material elements, is not reminis- 
 cence at all. 
 
 Moreover, when we inquire as to precisely what constitutes this 
 wonderful power of conserving the results of molecular changes 
 induced by the action of stimuli, which the nervous system pos- 
 sesses, we find it impossible to give a wholly satisfactory answer. 
 The most plausible answer consists in inferences touching the 
 highly probable application of certain biological laws to the special 
 case of the nervous system, regarded as furnishing a physical basis 
 for the phenomena of memory. The general fact from which these 
 inferences take their point of starting is undoubted ; the entire 
 nervous mechanism must be regarded as a vast system of interrelated 
 elements (nerve-fibres and nerve-cells), each of which must also bo 
 
EFFECT OF ALTEREP NUTRITION. 551 
 
 regarded as a system of interrelated molecules. The excitation and 
 propagation of nerve-commotion consists in producing and continu- 
 ing changes in the atomic structure and mutual relations of these 
 molecules. In order to account for that " bent," direction, or ten- 
 dency to act in a certain way, which all habit, of the nervous system 
 presupposes, the internal molecular alteration of the nervous ele- 
 ments, especially of the individual nerve-cells, has to be assumed. 
 But the power of propagating their kind belongs to these individ- 
 ual nerve-cells ; and it is likely that all the essential principles of 
 heredity and evolution apply to the exercise of this power in the 
 case of these cells. 
 
 The biological laws which control the nutrition of living organ- 
 isms also have an application to the nervous elements. The exer- 
 cise of any of the nerve-cells or groups of nerve-cells of which the 
 end-organ or the central organ is composed tends to enlarge them 
 by appropriation of the nutriment brought to them in the blood- 
 supply. Such nutrition, however, will necessarily be dependent, 
 for the special type which must characterize its manner of building, 
 upon the acquired molecular character of the cells that build the 
 new material into themselves. And when the cells, thus enlarged 
 and molecularly altered according to the character and amount of 
 their exercise, multiply themselves, their offspring of new cells will 
 necessarily come under the general principle of heredity in its ap- 
 plication to all living cells. Accordingly, three things must be 
 taken into the account when considering what has been called 
 (ineptly, as we believe) " organic memory," namely : (1) The en- 
 largement of the single cells or fibres of which the organ is com- 
 posed ; (2) the multiplication of these elements so that new cells 
 and fibres originate under the laws of heredity ; (3) the internal 
 molecular alteration of the nerve-cells and nerve-fibres. 1 
 
 18. Furthermore, it is certain that the unity and continuity of 
 the nervous system is such, even with respect to its individual cells, 
 that alterations in one group of elements involve alterations in 
 other groups. Indeed, it is much more difficult to predict where 
 such sympathetic alteration will end than to affirm that it certainly 
 must begin and proceed to considerable lengths. The phenomena 
 of aphasia, for example (see chap. EL, 25 ff.), indicate how many 
 and intricate are the ways in which those elements of the central 
 organs must be internally connected and related that constitute the 
 physical basis of the memory as retentive and reproductive of the 
 ideas and symbols of articulate speech. The phenomena occurring 
 
 1 See the Vortrag of E. Hering, Ueber d. Gedaclitniss als eine allgemeine 
 Function d. organisirten Materie. Wien, 1876. 
 
552 PHYSICAL BASIS OF MEMORY. 
 
 in other diseases than aphasia, and in so-called " freaks " of mem- 
 ory, clearly indicate the same truth. But no attempt to bring 
 these phenomena under any strictly scientific formulas has hitherto 
 been rewarded with any considerable success. By continual cor- 
 related action on the part of groups and areas of nervous elements 
 more or less remotely situated, such elements become in some sort 
 associated together ; being thus associated together, they tend to 
 act together for mutual helpfulness and modification (that is, to 
 intensify, inhibit, or characteristically alter each other), whenever 
 either one is in any manner roused to yield up the energy it has in 
 store. 
 
 The physical basis of memory as retentive is therefore laid in 
 the habit, or acquired tendency, of the elements of the nervous sys- 
 tem both as respects the molecular constitution of the individual 
 elements, and also as respects the association of groups of these 
 elements more or less distant from each other. Each element of 
 this system, especially in the more significant of its central organs, 
 may be considered as a minute area intersected by an indefinite 
 number of curves of different directions and orders ; thus a molec- 
 ular commotion in any such area may, according to its character 
 and point of greatest intensity, run out into the system along any 
 one of these many curves. In every such small fragment " the 
 whole curve slumbers," although the microscope of the histologist 
 cannot detect the full significance of the fragment or distinguish 
 it from similar fragments of other curves intersecting each other in 
 the same area. 1 
 
 19. The nature of the physical basis of memory considered as 
 reproductive, under the so-called laws of association, is even more 
 purely conjectural than that of memory considered as retentive. 
 To speak of an excitation as imprinting itself upon the cerebral 
 cells, and " perpetuating itself in them in the form of persistent 
 vibrations," or to imply that mental reproduction is only the weaker 
 "echo" of these vibratory conditions, to persist in which is the 
 mysterious property of all the nervous elements as does M. Luys Q 
 is neither good physics nor good psychology. The nature of 
 nerve-commotion, so far as we know anything about it, is not such as 
 fitly to be described by the word " vibrations ;" and that the forms 
 of nerve-commotion, even if properly described by this word, do not 
 " persist " within the cells, there is every reason to believe. Much 
 more unobjectionable is the language used by M. Ribot 3 to describe 
 
 1 A figure of speech adapted from Hering, Ueber d. Gedachtniss, etc., p. 
 15 f. * See The Brain and its Functions, p. 147 f. 
 
 3 Diseases of Memory, p. 26 f. 
 
SO-CALLED DYNAMICAL ASSOCI 
 
 the conjectural physical basis of the same psychical phenomena. 
 According to the hypothesis of this authority, " determinate associa- 
 tions," or "dynamic affinities," are formed among the nervous 
 elements by their acting together; by repetition these affinities 
 may become as stable as are the primitive anatomical connections. 
 Such " dynamical associations have a much more important part to 
 play in conscious memory than in organic memory." A rich and 
 extensive memory is not a collection of impressions (for all such 
 terms as " impression," " imprint," " registration," etc., are inap- 
 plicable to the case), but an accumulation of those dynamical asso- 
 ciations that are " very stable and very responsive to proper stim- 
 uli." The recurrence of some ideas rather than others, as started 
 by this or that sensuous impression or other phase of conscious- 
 ness, would then depend upon the character, number, and strength, 
 respectively, of the different " dynamical associations." 
 
 20. According to the physiological theory of memory, forget- 
 fulness, or loss of memory, is to be accounted for as the result of 
 the process of dissolution. As says M. Eibot: * "To live is to ac- 
 quire and lose ; life consists of dissolution as well as assimilation. 
 Forgetfulness is dissolution." A large amount of such " forgetful- 
 ness " must then be considered as indispensable to the exercise of 
 memory ; for if all the alterations of the intermolecular constitution 
 of the nerve-cells were alike conserved and propagated, and if all 
 the dynamical associations among the different more or less remote 
 groups of these cells were equally stable, there could be no basis 
 laid for specific and characteristic reproduction of the images of 
 memory. The survival of any of these associations involves the 
 dissolution of many others. Under this general fact of the condi- 
 tions of forgetfulness the phenomena of those sudden losses and 
 disturbances which lesion of the cerebral substance often produces 
 must be brought. Temporary forgetfulness or disturbance of 
 memory may be assumed to be connected simply with such func- 
 tional derangement of the cerebral centres as interferes with the 
 working of the customary " dynamical associations " among the 
 nervous elements of which these centres are composed, or with 
 which they are regularly connected. 
 
 21. No good ground exists for speaking of any special organ or 
 seat of memory. Every organ indeed, every area and every ele- 
 ment of the nervous system has its own memory. This view be- 
 longs to the very essence of every theory which considers conscious 
 mental reproduction as only one form or phase of the biological 
 fact of " organic memory." We might properly speak, then, of the 
 1 Diseases of Memory, p. 61. 
 
554 PHYSICAL BASIS OF MEMORY. 
 
 memory of the end-organ of vision or of hearing, of the memory 
 of the spinal cord and of the different so-called " centres " of reflex 
 action belonging to the cord, of the memory of the medulla ob- 
 longata, the cerebellum, etc. But if only the cerebral hemispheres 
 are specially and directly related to the phenomena of conscious- 
 ness, then it is only the organic memory of these hemispheres 
 which can be spoken of as the physical basis of our memory. For 
 only the molecular constitution and dynamical associations of the 
 nervous elements of this organ can immediately determine the 
 character of conscious mental reproduction. 
 
 Much of the foregoing language arouses a protest against such a 
 misuse of psychological terms. The fact that repeated action un- 
 der stimulation of the nerve-cells of the cerebral cortex results in 
 a modification of their molecular constitution, and in the establish- 
 ment of certain tendencies to associated action among them, is 
 doubtless a biological fact. It is, perhaps, most important in lay- 
 ing the physical basis, in determining the physical antecedents and 
 concomitants of memory ; but it is not in any sense a fact of mem- 
 ory. It is no more fitly called " organic memory " than are the 
 molecular alterations produced by generations of use in the wood 
 of an old Cremona. The changes in nerve-cells are indeed far dif- 
 ferently related to memory from the changes that take place in the 
 molecules of the violin ; but it is only the addition of consciousness 
 to the whole transaction that gives us any right to characterize it by 
 the word " memory." 
 
 22. Just as there is no such experience as that of willing in 
 general, so there is no such experience as that of remembering in 
 general. The image of memory always possesses certain character- 
 istic features ; and if it be an image representing the percept of 
 some one of the special senses, its features are determined by the 
 nature of the percept which it represents. There is sound reason 
 for the customary form of speech which recognizes a good or bad 
 " memory of the ear," of the eye, etc. Such phrases might fitly be 
 extended to all the forms of sensation and perception ; and, indeed, 
 to all the mental experiences capable of being represented by the 
 images of memory. It would be equally fitting to speak of a good 
 memory of the fingers, of the tongue, of the larynx and other or- 
 gans of speech, etc. Inasmuch as the sensations which arise in, 
 and the movements which are imparted to, all these peripheral por- 
 tions of the body have their representatives in certain cerebral 
 areas, the physical conditions of the images of memory (the physical 
 basis of the different kinds of memory) are undoubtedly laid in these 
 same areas. There is no one place where memory, par excellence, 
 
PECULIARITIES OF MEMORY. 555 
 
 is at home in the brain ; or from which it rules the different organs 
 of expression by making involuntary or voluntary sallies forth, as it 
 were. Yet the memory of any one thing or event involves so many 
 complex and closely related activities of mind and doubtless also 
 of the brain that it is impossible to tell how far weakness or dis- 
 turbance set up at any single point may succeed in spreading itself. 
 
 As to the physical basis for characteristic weaknesses or excel- 
 lences of memory (such as the inability or marked ability to remem- 
 ber names, or dates, etc.), and for those apparent freaks of mem- 
 ory which some emotion or bodily disturbance may produce, little 
 can be affirmed with confidence. Where the mental peculiarity ex- 
 tends to rather a large range of subjects, such as come under one spe- 
 cial sense, or under one of the more general forms of the operation 
 of one sense, the natural constitution or acquired condition of the 
 particular organs involved may be assumed to be peculiar. The 
 phrases, " a good ear " for music, " a good eye " for form, color, 
 proportion, or whatever is visible, have doubtless both a psycho- 
 logical and a physiological significance. But what reason should 
 exist in the brain why some particular date or name should re- 
 peatedly slip away beyond the power of attention to recall it (Forbes 
 Winslow tells of a man who, after a fever, lost all knowledge of the 
 letter F), while other dates or names in which we have had little in- 
 terest cling so as to make it difficult to be rid of them, even conject- 
 ure fails to make evident. That there is such physical reason, how- 
 ever, the phenomena of aphasia, and of other diseases of memory, as 
 well as the results of experimentation upon animals for the locali- 
 zation of cerebral function, all seem strongly to indicate. The physi- 
 cal reason for those times of general depression or exaltation of con- 
 scious memory, with which almost all persons are familiar, is less 
 difficult to assign. Such reason is to be found chiefly in the 
 changes of character and quantity suffered by the blood-supply of 
 the cerebral areas especially in their effect upon the extremely 
 sensitive nerve-cells which abound there. 
 
 All calculations as to the possibility of representing all the in- 
 dividual ideas and images of memory by one or more nerve-cells 
 and nerve-fibres each, we regard as wholly useless whether the 
 number of nerve-cells in the cerebrum be, as Meynert calculates, 
 600,000,000, or even many more, as LionelBeale supposes. Every- 
 thing which psychology teaches as to the character of the mental 
 phenomena, and everything which physiology teaches as to the 
 nature of the cerebral functions, discourages the puerile attempt to 
 connect separate mental images or ideas with isolated nerve-cells 
 as their product. 
 
556 PHYSICAL BASIS OF MEMORY. 
 
 23. It will doubtless occur to thoughtful readers that nothing 
 which has thus far been said concerning the physical basis of will 
 and memory is, in any true sense of the word, an explanation of 
 these mental activities. In what sense physiological psychology 
 can be said to explain any mental phenomena we shall consider 
 elsewhere. But in the particular case of memory, for example, 
 none of the relations conjectured as probably existing between the 
 molecular constitution and dynamical associations of the cerebrum, 
 on the one hand, and the facts of conscious experience, on the 
 other hand, even on the supposition that these conjectured rela- 
 tions were all demonstrated facts of psycho-physical science, would 
 amount to anything approaching the character of an explanation. 
 For none of these physical conditions immediately concerns the 
 very mental activity which constitutes the essence of conscious mem- 
 ory. What is explained, if anything, is simply why I remember 
 one thing rather than another granted the mind's power to remember 
 at all. This power is a spiritual activity wholly sui generis, and in- 
 capable of being conceived of as flowing out of any physical condi- 
 tion or mode of energy whatever. 
 
 The truth of the position just taken may be enforced (among 
 other considerations) by certain conclusions which resulted from 
 our psycho-physical study of perception. In the study of percep- 
 tion psycho-physics can do much toward a scientific explanation. 
 It can tell what qualities of stimuli produce certain qualities of 
 sensations ; it can suggest a principle relating the quantity of the 
 stimuli to the intensity of the sensation ; it can investigate the laws 
 under which, by combined action of various excitations, the sensa- 
 tions are combined into presentations of sense ; it can show how 
 the time-relations of the sensations and percepts in consciousness 
 correspond to the objective relations in time of the stimulations. 
 But for that spiritual activity which actually puts together in con- 
 sciousness the sensations, it cannot even suggest the beginning of 
 a physical explanation. Moreover, no cerebral process can be con- 
 ceived of which in case it were known to exist could possibly 
 be regarded as a fitting physical basis for this unifying actus of 
 mind. Thus also, and even more emphatically, must we insist 
 upon the complete inability of physiology to suggest an explana- 
 tion for conscious memory, in so far as it is memory that is, in so 
 far as it most imperatively calls for explanation. 
 
 Any example of an act of memory will serve to illustrate the fore- 
 going truth. Let it be supposed that one has looked for a few 
 seconds steadily at certain pickets of a fence standing in the open 
 sunlight. On closing the eyes, the strong positive after-image of 
 
THE MYSTERY OF MEMORY. 557 
 
 the object remains for some seconds presented to the mind ; this 
 positive after-image is then succeeded by a succession of negative 
 after-images. When these latter have subsided, one can still recall 
 the image of the section of the fence seen some minutes since ; one 
 can recall the same image the next day, or the next week, or after an 
 indefinite length of time. But a very marked difference exists be- 
 tween that which is before consciousness in these cases of so-called 
 recollection and that which was before consciousness while the im- 
 pression of sense was going though its various phases of dying 
 away. Physiological explanations, having reference chiefly to the 
 action of the nervous elements in the retina, may be given as to 
 why the after-images are produced, and as to why they have the 
 order of succession which actually belongs to them. Other physio- 
 logical explanations, having to do chiefly with assumed activities of 
 the cerebral nervous elements, may attempt the problem, why the 
 image of memory is fainter than the original impression of sense, and 
 why this image rather than some other is represented at a particular 
 time. But all such psycho-physical explanation does not touch, 
 does not even approach, the real mystery of memory. The positive 
 after-image of the pickets of the fence is not the same as the per- 
 cept which preceded it ; nor is it the same as the negative after- 
 image, or the images of memory, which follow it. Yet all these 
 images are regarded by the mind as similar to the original object ; 
 indeed, as standing for it. How can they be regarded as similar 
 to one another, and to their common original, when no two of them 
 are at the same instant before consciousness in order to be com- 
 pared together there ? The very essence of the act of memory con- 
 sists in the ability to say : This after-image is the image of a per- 
 cept I had a moment since ; or this image of memory is the image 
 of the percept I had at a certain time I do not remember precisely 
 how long since. 
 
 It would, then, be quite contrary to the facts to hold that, when 
 the image of memory appears in consciousness, it is recognized as 
 belonging to a particular original percept on account of its per- 
 ceived resemblance to this percept. The original percept does not 
 exist, and will never be reproduced. Even more palpably false and 
 absurd would it be to hold that any similarity of the impressions 
 or processes in end-organs or central organs explains the act of 
 conscious memory. Consciousness, of course, knows nothing of 
 such similarity in impressions and processes ; knows nothing even 
 of the existence of nervous impressions and processes. Moreover, 
 we could never know two impressions or processes that are sepa- 
 rated in time to be similar, without implying this same inexplicable 
 
558 PHYSICAL BASIS OF JUDGMENT. 
 
 act of memory. It is a fact of consciousness, on which all possi- 
 bility of connected experience and of recorded and cumulative hu- 
 man knowledge is dependent, that certain phases or products of 
 consciousness appear with a claim to stand for (to represent) past 
 experiences to which they are regarded as in some respect similar. 
 It is this peculiar claim in consciousness which constitutes the es- 
 sence of an act of memory ; it is this which makes memory wholly 
 inexplicable as a mere persistence or recurrence of similar impres- 
 sions. It is this which makes conscious memory a spiritual phe- 
 nomenon, the explanation of which, as arising out of nervous pro- 
 cesses and conditions, is not simply undiscovered in fact, but 
 utterly incapable of approach by the imagination. When, then, we 
 speak of a physical basis of memory, recognition must be made of 
 the complete inability of science to suggest any physical process 
 which can be conceived of as correlated with that peculiar and 
 mysterious actus of the mind, connecting its present and its past, 
 which constitutes the essence of memory. 
 
 24. We decline to enter upon the discussion of a special physi- 
 cal basis for the mind's power to form generalized concepts, to 
 combine the elements of past experience into the creations of in- 
 vention and art, to discover laws, and to reason about a world of 
 reality assumed to exist e^ra-mentally, or about the nature and 
 order of the phenomena of its own consciousness. There is abso- 
 lutely no scientific ground on which to place such a discussion. 
 A physical basis of the logical faculty, so far as it is a subject of 
 either knowledge or conjecture, is laid in those general processes 
 of the nervous system that are correlated with the elementary forms 
 of mental activity upon which the higher forms are built, as it 
 were, or which they presuppose. Particularly important is the 
 function of articulate language in serving as a support for the logi- 
 cal processes. But that which is peculiar to all these forms of 
 psychical activities, and which causes them to be spoken of as 
 higher and more distinctively spiritual faculties, does not, as such, 
 admit of being made the subject of psycho-physical researches. 
 The attempt to deal with this subject psycho-physically only leads 
 to a tedious and meaningless repetition of the phrases and state- 
 ments which the psychology of consciousness correctly employs ; 
 but nothing is made the clearer by repeating words that are only 
 applicable to psychical phenomena in connection with conjectures 
 concerning related physical phenomena. " For all the higher 
 spiritual faculties," says Lotze, 1 " which consist in judgment of the 
 relations of given conceptions, we neither know how empirically to 
 1 Outlines of Psychology, p. 141 f. 
 
HIGHER SPIRITUAL FACULTIES. 559 
 
 demonstrate a definite bodily organ, nor should we know how to 
 conceive precisely what, that is of any use, such an organ could 
 contribute toward the solution of the most essential part of the 
 problem that is, the pronouncing of the judgment itself. It is 
 conceivable, on the other hand, that these higher activities might 
 presuppose the complete and clear representation of the content 
 about which the judgment is to be passed, and, consequently, also 
 the undisturbed function of those organs which contribute, first, to 
 perception by the senses ; then to its reproduction and combina- 
 tion with other perceptions ; and, finally, to the appropriate attach- 
 ment of feelings of value to each of them." 
 
CHAPTEE XL 
 
 CERTAIN STATICAL RELATIONS OF THE BODY AND 
 MENTAL PHENOMENA. 
 
 1. THE intimate relation between the constitution and func- 
 tional changes of the bodily structure, on the one hand, and the 
 character and course of the phenomena of consciousness, on the 
 other hand, is most easily made obvious by such alterations of ex- 
 perience as are connected with the use of the organs of sensation 
 and motion. These alterations are usually sudden. Where, on the 
 contrary, the relation of body and mind is stationary, or subject to 
 only very slow changes, it is far less obvious ; it may be, indeed, 
 completely hidden from our observation. The relation is not for 
 this reason, however, any the less certain and profoundly influential. 
 Indeed, it is just those physical conditions which are part of the 
 unchanging equipment of our lives that most surety, though most 
 stealthily, determine the development of conscious experience. 
 These are the influences of whose very existence we are for the 
 most part unaware, and over the effect of which we have compara- 
 tively little or no control. Within certain limits, one can deter- 
 mine the character and number of the excitations that fall upon 
 the end-organs of sense, and the resulting changes in the movable 
 parts of the body ; one can also regulate to some extent the suc- 
 cession of images of memory and fancy, and so the character and 
 intensity of the feelings and emotions that possess the field of con- 
 sciousness. But one cannot determine one's own age, or sex, or 
 race, including parentage and prenatal and infantile environment ; 
 nor can one choose one's temperament. Yet how pervasive, mighty, 
 and enduring are the unobserved influences that flow into the 
 conscious life of the individual from age, sex, race, and tempera- 
 ment! 
 
 The bearing of the foregoing remarks might be enforced by 
 many illustrations. No one is in need of technical information to 
 assure him that the character of his consciousness is every instant 
 dependent upon whether his eyes are closed or open. That we 
 hear with the ear, feel both the roughness and smoothness and 
 
NATURE OF POPULAR IMPRESSION. 561 
 
 also the heat and coolness of objects with the skin, taste with the 
 mouth, and smell with the nose are matters of experience belonging 
 to each moment of our work-a-day life. The pleasures and pains 
 of sense irresistibly demonstrate the dependence of our mental 
 states upon the condition of the body. Other common experiences, 
 although not arresting attention in the same obtrusive fashion, 
 nevertheless tend to confirm the same impression. The disturb- 
 ances of consciousness which follow the altered bodily condition of 
 sleep are too much an affair of daily experience wholly to escape 
 attention. The question, Why are dreams so queer ? taken in con- 
 nection with our observation of the abnormal state of the sleeper's 
 body, is necessarily answered in a way further to emphasize the 
 general relation between bodily and mental states. Almost every- 
 one has also particular times of experience when he is forced into 
 the admission that the physical system is to be blamed for the 
 altered conditions of his mental life. The loss of a night's rest 
 renders attention to work impossible on the following day ; a 
 slight fever sets the train of memory's images and fancy's creations 
 into accelerated and altered movement, or throws it into wild con- 
 fusion ; a settled melancholy comes as the obvious result of 
 chronic dyspepsia. In these ways the popular impression that the 
 body dominates the mind, and that bodily conditions determine 
 our feelings and thoughts, is strongly corroborated. 
 
 But other phenomena constantly tend to confirm the other popu- 
 lar impression, that the mind dominates the body and makes it the 
 servant of its feelings and thoughts. In the ordinary estimate, all 
 performance of physical work by the body, when accompanied by 
 the feeling of effort, is an indisputable proof of the immediate in- 
 fluence of the mind over the body. It is even more than this ; it 
 is a proof that the conscious self, the ego, is a source of physical 
 energy, which pours forth, as it were, into the limbs and braces 
 them to the appointed task. The thought of the aged but vig- 
 orous French philosopher, who insisted upon remarking how well 
 he carried his legs, rather than upon how well they carried him, 
 accords accurately with the popular impression. All the cus- 
 tomary language about looking, listening, recalling, etc., as dis- 
 tinguished from mere seeing, hearing, and happening to remem- 
 ber, also enforces the same impression. The average man or 
 woman, whose life is one of constant toil, is led to say with about 
 equal frequency" My limbs are tired, and / must stop trying," 
 or " 7 am tired, but my limbs must be made to go on with their 
 work." 
 
 Other phenomena of an abnormal kind tend to confirm still fur- 
 36 
 
562 GENERAL BODILY DIFFERENCES. 
 
 ther the above-mentioned vague popular impression. Among them 
 may be mentioned the wonderful cases of so-called diseases of mem- 
 ory or of will, and of double personality or other alienations from 
 normal self-consciousness. In such cases strange alterations of the 
 modes of the mind's behavior alterations which appear to involve 
 the suspension or reversal of some of those mental laws and activi- 
 ties which we are accustomed to consider among the most funda- 
 mental seem to be connected with certain alterations of the bodily 
 organs. The effect of certain drugs, through the body, upon the 
 feelings and mental train gives other occasions for insisting upon 
 the dependence of the phenomena of consciousness upon the state 
 of the body. On the other hand, hallucination, hypnotism, and the 
 yet more obscure phenomena of so-called " mind-reading," as well 
 as all the phenomena to which modern Spiritualism appeals, pre- 
 sent the question to us afresh from a somewhat different point of 
 view. 
 
 2. It requires more reflection upon wider experience to origi- 
 nate and confirm the impression that all the characteristic experi- 
 ences of the individual are built upon a solid and enduring basis of 
 common relations which universally maintain themselves between cer- 
 tain types of physical constitution and activity and certain correspond- 
 ing types of the character and action of the mind. The development 
 of the child is ordinarily regarded, whether from the physical or the 
 psychical point of view, in an isolated and disconnected wa} r . It is 
 perhaps noted that he has grown so much taller or heavier, or that 
 certain external features of the body are becoming relatively more 
 pronounced ; it is also noted that he is learning to walk, to talk, to 
 take an interest in certain things before unnoticed, and to remem- 
 ber what he has been taught. But the close relations between the 
 bodily changes of advancing years and the mental development of 
 the child are not (at least until the age of puberty is reached) apt 
 to be made the subject of careful observation. In somewhat the 
 same way does ordinary reflection deal with the question as to any 
 relation between the physical and the mental peculiarities of the 
 sexes. It is common enough to note that boys and girls do not, 
 even in selecting and conducting their plays, act precisely alike. 
 That the former are, as a rule, taller, heavier, coarser, than the lat- 
 ter is patent to all observers. But that the absolute and relative 
 development of all the organs of the male and female is different, 
 and that certain sexual peculiarities of perception, feeling, thought, 
 and action are constantly related to this difference, is something 
 which, few if they even suspect it take any pains accurately to 
 remark or describe. Moreover, while almost all agree that the psy- 
 
NATURE OF POPULAR IMPRESSION. 563 
 
 chical life of the adult male and female is distinguished by sexual 
 peculiarities, there is the widest diversity of opinion as to the pre- 
 cise nature and range of these peculiarities. 
 
 When the inquiry concerns characteristics of both mind and 
 body belonging to race and ancestry, the answers given by different 
 observers seem to lose all claim to strictly scientific quality. The 
 Frenchman does not describe himself as the Englishman describes 
 him ; and neither one of the two can be expected to agree with the 
 Russian as to what are the peculiarities that characterize this last 
 type of the human species. The history of the discussion regard- 
 ing the kinds and significance of so-called "temperaments," and 
 even regarding the very existence of temperament, shows clearly 
 how uncertain is this entire field of research. 
 
 Perhaps the most remarkable instance of firm conviction con- 
 cerning the general fact that intimate relations exist between mind 
 and- body, accompanied by the utmost vagueness concerning the 
 precise nature of the basis of such relations, may be derived from 
 the ordinary views as to heredity. It is constantly being remarked 
 of children that they resemble some one of their ancestors in one 
 or more physical characteristics. But this remark is scarcely more 
 frequent than the corresponding one with respect to mental con- 
 stitution or mental idiosyncrasies. Of course the obvious implication 
 is, that we are to look to the laws of heredity for an account of the 
 origin of both classes of qualities ; in other words, both physical and 
 mental qualities are regarded as inherited. Further than this ad- 
 mission ordinary reflection upon experience with facts of this order 
 does not lead most men. It is obvious, however, that even the 
 loose popular impression must be explained, if at all, by insisting 
 upon much more numerous and intimate relations between body 
 and mind than the impression would seem at first to imply. For 
 how can ancestral characteristics be transmitted, unless they are 
 potentially carried over in those living cells from the two parents 
 which actually fuse together in the production of the new life ; or 
 else are also due to the prenatal conditions that control the nutri- 
 tion of the infant's body before it separates from the body of its 
 maternal ancestor ? But to admit this is to insist upon the pro- 
 foundest connection between the molecular structure and dynam- 
 ical associations of the elements of the physical organism and the 
 development of conscious life. It is even to insist upon the 
 mysterious fact that the character of the conscious life is deter- 
 mined in no small degree by the statical peculiarities of the or- 
 ganism. 
 
 3. In general, it may be said, then, that while no doubt exists 
 
564 GENERAL BODILY DIFFERENCES. 
 
 in the popular impression as to the dependence of the mental life 
 upon the age, sexual differences, and inherited ancestral qualities of 
 the bodily organism, the greatest uncertainty and vagueness exist 
 as to the nature and extent of such dependence. When this kind 
 of inquiries is brought to the tests of science, it is found that all 
 the evidence confirms the positive part of the ordinary impression ; 
 but it cannot be said that any substitute for the uncertainty and 
 indefiniteness of the popular estimate has yet been found. The 
 reasons for this failure lie in the very nature of the subject. No 
 guidance by the immediate evidence of consciousness is possible 
 in determining the nature of this class of psychical phenomena. 
 What it is to will, to remember, or to reason, each one trusts him- 
 self to know as a matter of his own inner experience. But the in- 
 quiries, How are the mental peculiarities of the different ages, or 
 of the two sexes, to be distinguished from each other ? or, How 
 does a person of this or that race or temperament think, feel, and 
 act differently from a person of another race or temperament ? are 
 plainly not subjects for an appeal to consciousness. Few questions 
 can be raised, for example, about which a wider diversity of view is 
 likely to be evoked, than the question as to how man and woman 
 differ mentally. Yet this question must be answered, if we are to 
 have an answer to the further inquiry concerning the correlations 
 between sexual differences of organism and sexual mental differ- 
 ences. 
 
 The difficulty of simply getting at the anatomical and physio- 
 logical facts necessary for an induction is scarcely less unmanage- 
 able. A great amount of careful measurement and a vast array 
 of statistics are necessary even to tell how human beings differ 
 in the most external features, at different ages and as between 
 the two sexes. Certain data with respect to the height, weight, 
 relative size of the different external members of the body, and of 
 the brain, are obtainable ; but other data equally desirable are as 
 yet unattainable. Concerning the nature of the physical basis of 
 temperament and of personal idiosyncrasies we are wholly in the 
 dark. 
 
 There are therefore many gaps and deficiencies in both the 
 physical and the psychical series. But where the members of both 
 of any two series to be compared are in this condition, the laws of 
 their relation cannot be pointed out. Nothing remains, then, but 
 to guide ourselves as best we may by general observation of the 
 psychical facts, and by use of such few statistical results as are 
 available. On few points will precise conclusions be found attain- 
 able. But the one conclusion of greatest value concerns the main 
 
THE PHASES OF LIFE. 565 
 
 point called in question and this is the general fact of the corre- 
 lated action of the bodily organism and the mind as the subject of 
 the phenomena of consciousness. 
 
 4. Certain facts of general import, concerning the height, 
 weight, comparative growth of the members, size of the brain and 
 organs of sense, etc., which characterize the different Phases of 
 Life may be relied on with considerable confidence. The struct- 
 ural and physiological development of the prenatal human being 
 has been investigated with more or less success by embryology 
 (comp. Part I., chap. VI.) ; but scarcely any trustworthy data exist 
 for a comparative psychology of the fcetus. It cannot be held that 
 its sentient life keeps even pace with the formation and growth of 
 its bodily organs not even of those which, like the brain and the 
 end-organs of sense, are most intimately connected with the phe- 
 nomena of consciousness. 1 Large or elaborate structures, such as 
 the lungs, the eyes, the ears, etc., are formed under morphological 
 conditions and influences with which we are only very imperfectly 
 acquainted, without any corresponding psychical development. 
 The brain at birth is apparently little different from the same 
 organ a few weeks later ; but at this later period an important 
 psychical advance has been made through the activity of the end- 
 organs of sense. This psychical advance must be represented in 
 the cerebral areas by the formation of such molecular changes and 
 dynamical associations of the nervous elements as constitute the 
 physical basis of memory considered as a retentive and reproduc- 
 tive power. 
 
 It is a reasonable conjecture that the psychical life of the un- 
 born child consists wholly of sensations of pressure and temperature, 
 for the most part exceedingly transient and disconnected, occa- 
 sioned by the stimulus of its changing conditions and positions in 
 the womb of the mother. Such a low grade of mental experience 
 if, indeed, we are to speak of prenatal "consciousness" can as 
 little be accurately represented by any conscious state of the hu- 
 man adult as can the experience of the animals to which the struct-, 
 ure and functions of the body of the foetus, in succession, bear more 
 or less of resemblance. About such a matter it is safest to refuse to 
 speculate. About one principle, however, there can be little doubt 
 many of those structural and physiological factors which form the 
 most important and intimate foundation for the spiritual functions 
 are secured only indirectly in the central organs through the culti- 
 vation given to these organs by the use of the end-organs of sense. 
 
 1 Comp. the article of A. W. Volkmann, in Wagner's Handworterb. , L, p. 
 563 ; and the strictures of Lotze upon it, Medicin, Psychologie, p. 546 f. 
 
566 GENERAL BODILY DIFFERENCES. 
 
 According to Soltmann' and others, stimulation of the cerebral 
 areas, considered as " motor " by Hitzig, in new-born animals, 
 does not produce the usual localized movements (comp. chap. II., 
 5 f.). The use of his hand by the child, the use of his organs 
 of speech, etc., educates his brain. So that the dependence of 
 mind on brain is not whether with respect to the life before or 
 after birth merely direct and simple, but also indirect and com- 
 plex. 
 
 5. Chaussier considered that the growth of the foetus in length 
 for the six months preceding birth is regular, and that it averages 
 about 54 mm. a month. The mean height at birth of 100 infants 
 of both sexes, measured in Brussels, was found to be 0.501 m. for 
 the boys, 0.491 for the girls, 2 or about 19} and 19^- inches respec- 
 tively. The most rapid growth of the child takes place in the first 
 year after birth ; this amounts to an average of about 2 dcm. 
 (7.87 in.). The growth of the second year is about half of that of 
 the first year ; that of the third year about one-fourth. From the 
 fourth or fifth year until the age of puberty the annual increase 
 of height is nearly regular, and amounts to some 56 mm. During 
 or shortly before this period a sudden rise in the curve of growth 
 occurs ; but after this period the rate continues to diminish until 
 the age of about twenty-five, when the full height may be regarded 
 as attained. In most cases, however, a slight increase takes place 
 between this age and fifty, after which a decrease goes on espe- 
 cially in extreme old age. The average height attained by the 
 human being is an effect of race, of climate, of conditions of liv- 
 ing and work, etc. For nine hundred persons, measured in Brus- 
 sels, ranging from nineteen to thirty years of age, the mean was 
 1.6648-1.6841 m. 3 The average height of eighty students, at Cam- 
 bridge, was 1.768 m. To express the facts by 'the fraction of the 
 whole height previously attained which the growth of each year 
 amounts to for the first year it is about f ; for the second, \ ; 
 for the third, -f s ; for the fourth, -^ ; for the fifth, -fa ; for the 
 sixth, -jig- ; etc. 
 
 6. The weight of the newly born infant is said by Quetelet, 4 as 
 a rule, to remain about stationary, or even to diminish a little, for 
 some seven days after birth ; it then, like the height, grows with 
 its maximum rapidity during the first year of life. Like the height, 
 also, the weight at birth varies according to parentage, prenatal 
 
 1 Centrblt. Med. Wiss., 1875, p. 209. 
 
 * See Quetelet, Physique Sociale de 1'Homme, II., p. 13 f. Paris, 1869. 
 
 3 See Quetelet, Ibid., II., p. 19. 
 
 4 Ibid., II., p. 81 f. 
 
PROPORTIONATE SIZE OF ORGANS. 567 
 
 conditions as respects nutrition, etc. The average weight of 119 
 infants weighed at birth, in Brussels, was found to be 3.055 kilo., 
 or 6.735+ Ibs. avoirdupois. With this the number 3.059 kilo., 
 given in the " Dictionnaire des Sciences medicales," agrees very 
 closely. A year after birth infants of both sexes have, on the aver- 
 age, tripled their weight. Six years more are necessary to double 
 the weight attained at the end of the first year ; and thirteen more 
 to quadruple it. At about the age of nineteen the mean weight of 
 both sexes is nearly that of old age. The maximum weight of the 
 male is attained, as a rule, about forty ; that of the female, somewhat 
 later. At this time the weight is about twenty times that of the 
 infant at birth. At sixty the weight, like the height, begins to di- 
 minish. Quetelet ' has attempted to establish the empirical law 
 that, during the period of development, the square of the weight 
 at different ages is, on the average, as the fifth power of the height ; 
 while for fully developed individuals of both sexes the weight is as 
 the square of the height. 
 
 7. The proportions which exist among the different organs 
 and members of the human body are of interest in this connection. 
 These proportions vary greatly for the different ages of life, but 
 remain nearly the same for all individuals (not obviously deformed) 
 of the same age. The parts least subject to any departure from 
 the normal type are the most essential parts. The height of the 
 head at birth is about one-half that attained on complete devel- 
 opment or an average of about 111 mm. (4.37 in.). It attains 
 about 154 mm. by the end of the first year, and 173 by the end of 
 the second ; its growth of the first two years (62 mm.) is, there- 
 fore, more than all the subsequent growth up to complete develop- 
 ment (when it is, on the average, 228 mm.). The developed head 
 is about -J- to of the height of the entire body. The back of the in- 
 fant, however, has at birth only about J its subsequent length ; the 
 arm, J- ; the leg, up to the place of bifurcation, only about |. 2 The 
 foot of the infant (which will probably never afterward appear in 
 its natural form and proportions) is about -\ of the length of the 
 body. This member has naturally, for all ages and both sexes, 
 about the same length as the head. The hand is about -J- of the 
 length of the entire body. Unlike the head, the limbs grow rapidly 
 after the second year ; especially are they lengthened at the ex- 
 pense of their transverse dimensions at the age of puberty, when 
 the bony framework is outstripping the muscles, as it were. The 
 
 1 Physique Sociale, II., p. 92 f. 
 
 2 See Quetelet, Anthropometrie, ou Mesure des Differentes Facultes de 
 1'Homme, pp. 45 f., 1941 
 
568 
 
 GENERAL BODILY DIFFERENCES. 
 
 following table ' shows the relative weight of several internal or- 
 gans in the infant at birth and in the adult : 
 
 Organ. 
 
 Percentage of body-weight. 
 
 Eatio of the two, 
 the infant taken 
 asl. 
 
 Infant at birth. 
 
 Adult. 
 
 Skeleton 
 
 16.70 
 
 23.40 
 2.16 
 .89 
 11.30 
 .28 
 14.34 
 
 15.35 
 43.10 
 2.01 
 .52 
 6.30 
 .028 
 2.37 
 
 26 
 
 28 
 20 
 15 
 12 
 1.7 
 3.7 
 
 
 
 Heart 
 
 Skin 
 
 Eye 
 
 Brain 
 
 
 8. A survey of the physical changes which take place in de- 
 pendence upon the age of the human being shows that most of 
 them are only indirectly connected with the development of the 
 mind. The connection is, however, scarcely less strongly marked 
 and important on that account. The changing size and weight of 
 different members of the body, both absolutely and relatively, gives 
 conditions to the life of sensation and motion ; and it is by act- 
 ual use of these members in sensation and motion that the de- 
 velopment of the mental powers of discernment, memory, and will 
 takes place, and all the knowledge of the spatial qualities and re- 
 lations of things is acquired. The metabolic activities of the infant 
 are much more pronounced than are those of the adult ; and much 
 of this metabolism is directed toward the ends of construction. To 
 make the rapid growths of the first years, a great amount of food, 
 representing a great amount of potential energy, must be converted 
 into living tissue. More rapid metabolism is also demanded by the 
 necessity of keeping up the normal temperature of the infant's 
 body, which is slightly warmer (.3) than the body of the adult, 
 and which loses heat much faster on account of its extremely 
 vascular skin. The heart of the infant is, relatively to its body- 
 weight (see table above), considerably larger than that of the adult, 
 and the whole circuit of the circulatory system is traversed much 
 quicker (in about 12 seconds, instead of 22). Accordingly, the heart- 
 beat is more frequent namely, about 130-140 per minute, falling 
 off to about 110 in the second year and to about 90 in the tenth 
 year. The respiration is also more frequent it being about 35 per 
 minute at first, 28 in the second year, and 26 in the fifth. The 
 brain and organs of sense are relatively very much larger in the 
 6 See Vierordt, Grundriss d. Physiologie (5th ed.), p. 605. 
 
SENSES OF THE INFANT. 
 
 infant than in the adult, and accordingly grow less rapidly in early 
 life. 
 
 Everything in the infant indicates, therefore, a mobile, flexible, 
 changeable condition of the bodily organs, with a relatively large 
 development of the most important parts of the nervous mechanism. 
 Such a condition is significant of a paucity of bodily and mental 
 habits ; the lines of the habitual action of the mechanism, the 
 character and number of the dynamical associations among its 
 elements, have not yet been rigidly marked out and firmly fixed as 
 they subsequently are. But the advanced development of the brain 
 and end-organs of sense is significant of the potentialities, as it 
 were, rather than of the actual experience of the babe. There is 
 difficulty in tracing accurately the course of the earliest mental de- 
 velopment, if by " mental " we intend to designate the phenomena 
 of consciousness. The eyes of the child during the first days of its 
 life are seldom open for any length of time. Preyer 1 asserts that 
 some newly born children move the eyes with associated and co- 
 ordinated movements, others not ; but there is no fixation of eyes 
 such as indicates an act of will in attentive regard until much later, 
 and then in such way as to show a gradual unfolding of the power 
 of attention. 
 
 All newly born children are deaf ; the temporary deafness is 
 caused by lack of air in the tympanum previous to respiration. 
 Great individual differences exist as respects the age at which 
 children give unmistakable tokens of having sensations of sound. 
 It was not until the first half of the fourth day that one investigator 
 w r as satisfied his child could hear. From the conditions under which 
 the foetus grows we might suppose that the sense of touch, as re- 
 gards both pressure and temperature, would be well developed in 
 the infant. The reflex excitability of the different regions of its 
 skin is, however, inferior to that of the adult, and only gradually 
 approaches it under the influence of constant cultivation. Accord- 
 ing to Preyer, 9 it is highly probable that the sensations of sweet, 
 salt, sour, and bitter are distinguishable from birth. Taste may 
 then be said to be "instinctive" with it as with other young ani- 
 mals. There is more doubt about sensations of smell ; according 
 to some these are not experienced earlier than from four to eight 
 weeks, but according to others they belong to the first days of the 
 child's life. It is, of course, largely in connection with the unfolding 
 of the activity of these organs of sensation and motion that its en- 
 tire mental development takes place. 
 
 9. After full maturity has been attained, and the period of 
 
 1 Die Seele d. Kindes, p. 25 f. Leipzig, 1882. 2 Ibid., p. 76. 
 
570 GENERAL BODILY DIFFERENCES. 
 
 decline for the bodily powers has begun, the mental powers also 
 are, as a rule, less aggressive and acquisitive, or even begin to de- 
 cline. But the period of the more immediate dependence of the 
 latter upon the sensory and motor activities of the bodily organs 
 has passed ; the lines of spiritual as well as of corporal habit have 
 become firmly drawn, and both mechanism and mind may be said 
 to contain a great amount of stored experience ; judgment is 
 trained, and less liable to sudden action under the assaults of vari- 
 ous forms of impulse. If, then, no sudden accident or slow decay 
 impairs the cerebral centres, the fullest and most impressive ma- 
 turity of the mental powers may arrive and continue years after the 
 activities of sense and motion are past their prime. 
 
 10. That the two sexes differ in many ways, as respects both 
 physical and mental characteristics, is an almost universal im- 
 pression. As to what are the mental characteristics of either sex 
 a wide difference of opinion undoubtedly prevails. But the sta- 
 tistics of certain physical characteristics of the sexes are tolerably 
 complete. Besides the more obvious bodily differences of man and 
 woman, the two sexes differ from birth in average height, weight, 
 physical energy, proportion of parts, relative development of or- 
 gans, frequency of pulse, respiration, etc. They also differ in many 
 other subtler and less obvious characteristics. As we have already 
 seen (p. 566), the height of the male infant at birth slightly (about 
 0.01 m.) exceeds that of the female. The excess increases, but not 
 with perfect regularity, until full maturity is reached. At this time 
 the height of the man may be given as 1.467-1.890 m. (about 4 ft. 
 11 in. to 6 ft. 4 in.) ; that of the woman, 1.444-1.740 m. (about 4 ft. 
 10 in. to 5 ft. 10 in.). The curve of the growth of the two sexes from 
 birth onward runs somewhat differently ; although up to the age of 
 four or five the difference is scarcely perceptible. All the sexual 
 differences are, of course, least pronounced in the earliest years of 
 life. For these years the proportions of height remain about as 1 
 to 0.988 ; at complete development they are as 1 to 0.937, or about 
 as 16 to 15. But at sixteen or seventeen years of age the growth 
 of girls is relatively as far advanced as is that of boys at eighteen 
 or nineteen. Between five and fifteen years the former make an 
 annual growth of about 56 mm., the latter of about 52 mm. 1 
 
 11. The relative weight of the two sexes varies in somewhat the 
 same manner, but not precisely, as their height. Of 119 infants 
 weighed at birth, in Brussels (63 males and 56 females), the average 
 weight of the males was 3.20 kilo. (7.05 Ibs. avoirdupois) ; of the 
 latter, 2.91 kilo. (6.42 Ibs.). At the same age the male is, as a rule, 
 1 See Quetelet, Physique Sociale, II., p. 15 f. ; and Anthropomctrie, p. 176 f. 
 
GROWTH OF SEXUAL DIFFERENCES. 571 
 
 heavier than the female ; but although the boy is born heavier, 
 and in his earliest years makes a larger gain of weight, at about 
 twelve the two sexes have nearly the same average weight. The 
 limits of weight for persons normally formed are about 49.1- 
 98.5 kilo. (108-217 Ibs.) for man, 39.8-93.8 kilo. (98-207 Ibs.) for 
 woman. Woman attains her maximum weight several years later 
 than man. 
 
 The relative proportion of the bodily parts is different for the two 
 sexes. At about the age of four or five the sexual differences in 
 this regard become more observable. The bony framework of the 
 boy is relatively prominent, and the outlines of the limbs become 
 more clearly traced in a way to conform to agile and strong move- 
 ment. Soundness of limbs and amplitude of flesh concealing the 
 framework are more characteristic of the girl. At the age of puberty 
 these and other similar differences suddenly become more strongly 
 marked. Careful measurement of many individuals who have at- 
 tained the development of the adult shows certain noteworthy stati- 
 cal differences of sex. The head, which is contained about 7.4 
 times in the entire height of the man, is contained only 7. 2 times in 
 th^height of the woman ; it is, then, relatively a little longer in the 
 latp. The chest of the adult male is more developed. The length 
 of the arms stretched out is about 1.045 of his height ; of the female, 
 only 1.015. The relative length of the legs is greater in the man. 
 The circumferences of the different parts of the body are also rela- 
 tively different in the two sexes. The relative step is as 1,000 to 
 1,157; and the weight of the brain as about 1,272 to 1,424 (see 
 chap. L, 4). 
 
 There are also marked differences between the sexes in the forma- 
 tion of the pelvis, and in the part of the body on which the centre 
 of the line of length of the entire body falls. The costal mechan- 
 ism of respiration differs. The girl of five breathes with her ribs 
 as does the adult woman. The pulse of woman is quicker in about 
 the same proportion as her height is less. The physical energy of 
 which the male is capable, whether as measured by lifting weights, 
 by pressure with the hands, or other ways of producing a meas- 
 urable mechanical effect, is much greater than that of the female. 
 This follows, of course, from his larger brain and skeleton, and 
 from his superior equipment of muscles. Before puberty the dif- 
 ference has been estimated as expressed by the ratio 3:2; after 
 that age it is greater, and is measured by the figures 9 : 5, or is 
 perhaps double. The average boy of nine or ten can support his 
 own weight for some time with his hands ; the girl cannot. The 
 average man can, by using his disposable energy, lift some 154 
 
572 GENERAL BODILY DIFFERENCES. 
 
 kilo. ; the woman scarcely half as much. 1 The metabolism of the 
 female, whether measured by respiratory or other excreta, is not 
 only absolutely, but relatively less ; her blood is not only less in 
 quantity, but also of lighter specific gravity, and contains fewer red 
 corpuscles. 2 The woman is more inclined than the man to be "hy- 
 per aesthetic " (in the physiological meaning of the word) ; this in- 
 volves a tendency to many forms of cramping of the muscles, to sud- 
 den secretions, to the wide spreading of stimulation so as to involve 
 a considerable number of the bodily parts. 3 Many of the woman's 
 sensations are less sharply discerned as to their qualitative content, 
 but stir up accompanying forms of feeling with more energy. 
 
 12. In the description of those mental characteristics of sex 
 which undoubtedly exist, and which are dependent upon or con- 
 nected with the foregoing physical characteristics, a great diversity 
 of view prevails. We cannot enter into the details of the discus- 
 sion. It is plain, however, that the greater bulk of those nervous 
 and muscular masses which are involved in the conscious life of 
 sensation and motion both implies and necessitates great differ- 
 ences in the development of this life. But judgment and decision 
 are also involved in the conscious life of sensation and motion ; 
 they are dependent upon that life for the amount, direction, and 
 lower or higher order of their development. The superior strength 
 of the chest, shoulders, and hips of the male, in lifting and moving 
 heavy burdens, and the fitness of body and legs for walking firmly 
 and running swiftly, cannot fail to produce a marked consciousness 
 of ease, elasticity, and security, both of posture and of movement. 
 
 Other important sexual differences, consisting of variations in 
 the kind and amount of feeling sensuous, aesthetic, intellectual, 
 moral and especially of the so-called emotions, are undoubtedly 
 connected with the existence and development of those organs 
 specifically characteristic of sex. The differences in circulation, 
 respiration, metabolism, etc., are also the cause of characteristic 
 differences in sentiment and feeling. Especially important, and 
 even determinative, is the man's larger mass of nervous matter in 
 the cerebral centres. In active energy, whether as given out on 
 sudden call or in the form of sustained endurance of the strain of 
 labor, and in all pursuits and achievements requiring such energy, 
 the woman (however much she may seem to be superior in the 
 passive endurance of pain, etc.) can never compete successfully with 
 the man. 
 
 1 Quetelet, Anthropometrie, p. 359 f. 
 
 8 Foster, Text-book of Physiology, p. 713. 
 
 3 Lotze, Medicin. Psychologic, p. 559 f. 
 
SEXUAL MENTAL CHARACTERISTICS. 573 
 
 Other mental differences closely related to the more obvious 
 ones, and largely dependent upon them, are less obvious and easy 
 to demonstrate. Our purpose will be served sufficiently by citing, 
 concerning such differences, a few points from Lotze, 1 who has 
 treated the whole subject briefly, but with much insight and caution. 
 In Lotze's opinion, woman naturally adapts herself more easily to 
 new conditions of life ; while acquired habits have a stronger hold 
 on man. Her characteristics involve a mixture of the sanguine 
 temperament and the sentimental stage ; while varieties of educa- 
 tion conceal more of native qualities. This would seem to imply 
 a greater molecular immobility and stronger dynamical associations 
 among the elements of man's organism. The intellectual capacity 
 of the sexes, Lotze thinks, differs chiefly or solely in so far as 
 special emotional interests prescribe the course of the intellectual 
 life. It is characteristic of masculine philosophy to analyze strik- 
 ing phenomena ; it is characteristic of woman rather to hate analy- 
 sis. Masculine thought depends upon the conviction that whatever 
 is most great and beautiful in the world has its mechanical con- 
 ditions ; masculine effort upon a profound reverence for general 
 principles. The faith of woman is that no general principle or form 
 has an independent value, but that this value belongs to the living 
 reality founded upon such principle ; the sentiment of the feminine 
 mind is devout toward completeness. The notions of the two as to 
 spatial and mathematical relations, and their perceptions as to the 
 nature of the concrete realizations of the ideas of space and time, 
 are markedly different. In seeking for some physical basis for 
 these and similar differences in case their existence be once as- 
 sumed we are forced to admit that any such known basis can be 
 at best only indirectly related to the differences themselves. The 
 general truth holds, however, that certain intellectual differences 
 are intimately, and even necessarily, connected with certain emo- 
 tional differences ; and that the latter plainly have, in many cases, 
 their ground in the organic differences of the two sexes. 
 
 13. The different intellectual and emotional characteristics of 
 the different races, and the relations of such characteristics to defi- 
 nite variations of the bodily type of each race from that of our com- 
 mon humanity cannot be discussed without raising even more 
 obscure and doubtful inquiries. If external influences of soil, cli- 
 mate, food-supply, character of the prevalent civilization, etc., have 
 any observable influence upon the type of the bodily form and of 
 this there can scarcely be a doubt and if the laws of heredity are 
 to be allowed the scope and influence which belongs to them, the 
 1 Microcosmus, ii., p. 39 f. Edinburgh, 1885. 
 
574 DOCTRINE OF TEMPERAMENTS. 
 
 existence of both physical and mental characteristics of race must 
 be admitted. The popular impression confirms the assumption of 
 anthropological science. But there are few subjects concerning 
 which statistics and impressions are both more incomplete and 
 more unsatisfactory. 
 
 According to Quetelet's conclusions, the proportions of the aver- 
 age human body are such as to render it the type of manly beauty ; 
 and the limits of these proportions are the more fixed and un- 
 changeable the nearer we approach to perfection. In the special 
 features of height, weight, and relative form, rather than size of 
 the organs, certain differences appear which belong to different 
 peoples and races. Each people may be said to have its peculiar 
 type ; and among each people such type exists, not only in fact and 
 determinable by scientific means, but as vaguely established in the 
 general appreciation. According to Quetelet, 1 the principal pro- 
 portions of the human figure vary very little among different races 
 of men. " The real differences which the races present appertain 
 to characteristics which the eye seizes better than the compasses ; 
 in order to establish them firmly, an appreciation of minute differ- 
 ences is required, and a tact that presupposes a long experience in 
 such researches. One can see the difficulties with which phrenolo- 
 gists meet in making numerical estimates of the characteristics of 
 the skull ; nothing precise can be formulated in this regard " (comp. 
 chap. II., 4). This conclusion of Quetelet is formulated in view of 
 careful measurement, not only of many individuals from the modern 
 European peoples, and of certain selected cases among the North 
 American Indians, the Chinese, and the Kaffirs, but also of Egyp- 
 tian mummies, of Greek statues, and of other means for ascertain- 
 ing the proportions of. ancient man. 
 
 14. Few impressions are more firmly fixed than this, that dif- 
 ferent individuals (at least among all the more highly civilized 
 peoples) possess, each one, a characteristic "natural disposition." 
 Such disposition constitutes a predominating tendency to feel, 
 think, and act in certain forms rather than others among the many 
 that are conceivable. The conviction that the disposition of the 
 individual is innate and inherited, rather than the result of training 
 or environment, is doubtless due to the fact that it appears with 
 considerable strength in childhood, and generally maintains itself 
 under great alterations of circumstances, and against effort, to the 
 close of the individual's life. The so-called " disposition " can, in- 
 deed, be greatly modified, and even seem wholly changed ; but such 
 modification is invariably made at the expense of greater energy 
 1 Anthropometrie, p. 323. 
 
THE THEORY OF DR. GEORGE. 575 
 
 than is required to form and break those habits which are acquired 
 differently in different individuals after birth. Moreover, the 
 modification is often one of expression and power of Qontrol rather 
 than of disposition. 
 
 Patent facts like the foregoing have given rise to the theory of 
 Temperaments. Curiously enough, the number four has usually 
 been chosen as sufficient to designate the kinds or types of native 
 disposition, the varieties of temperament. The attempt has also 
 often been made to connect the different temperaments with a 
 bodily basis. As in several of the foregoing inquiries, so in this, 
 our reliance is mainly placed upon the correctness of certain wide- 
 spread but vague impressions. It is impossible to classify the tem- 
 peraments with the use of methods required by strict scientific in- 
 duction. The individual can judge of his own temperament only 
 by remembering his actions and the states of consciousness con- 
 nected with them. But upon such a point memory, and even the 
 immediate recognition of consciousness, are but little trustworthy. 
 Few things are more common than for the individual quite to mis- 
 conceive and misinterpret his own mental states and tendencies. 
 On the other hand, we have no means of judging the temperament 
 of others except by their action using the word action in its 
 broadest signification. A large part of such judgment is unavoid- 
 ably rnisjudgnient. But notwithstanding all the doubts and un- 
 certainties which attach themselves to the subject of temperaments, 
 those who are carefully observant of their fellows will continue to 
 believe that important and determining natural differences exist 
 among them. 
 
 15. Some of the older treatises on psychology contained elab- 
 orate discussions of the doctrine of temperament, in which many 
 well-observed facts and shrewd conjectures were united with no 
 little fanciful speculation. This is to some extent true of the treat- 
 ment (on the whole admirable) given to the subject, for example, 
 by Dr. Leopold George. ' According to Dr. George, the four tem- 
 peraments are defined by the nature of the interior relation which 
 exists between perception and the affections of the mind. Thus 
 the greater the mind's wakefulness to impressions, the greater is 
 also its susceptibility to the feelings of pleasure or pain which are 
 attached to the impressions. The " sanguine " temperament is 
 distinguished by strength in this interior relation. But the greater 
 the attention given to the objects before the mind, the greater are 
 the emotions of hope or fearful expectation which the objects excite ; 
 and from the emphasis being laid, as it were, on this relation the 
 1 Lehrbuch d. Psychologie, pp. 125-151. Berlin, 1854. 
 
576 DOCTRINE OF TEMPERAMENTS. 
 
 " melancholic " temperament results. A large degree of suscepti- 
 bility to sensation is naturally accompanied by feelings of attrac- 
 tion or dread toward the object of sensation. This fact forms a 
 basis for the " choleric " temperament. And, finally, the so-called 
 "phlegmatic" temperament depends upon the degree of mental 
 apprehension with which different objects are seized, and the con- 
 sequent emotions of satisfaction or disgust. The theory is then 
 developed that different races and peoples are distinguished by 
 some one of these four temperaments for example, the French 
 are sanguine, the English melancholic, the Spanish and Italians 
 choleric, the Germans phlegmatic. More generally still, the Cau- 
 casian race is sanguine, the Mongolian melancholic, the Negro 
 phlegmatic, the Malayan choleric. The four periods of life cor- 
 respond to the four temperaments, according to Dr. George ; and 
 this opinion has undoubtedly a certain basis in fact, as well as a 
 suggestion concerning the nature of the physical conditions which 
 may possibly underlie the existence of temperaments. Even dif- 
 ferent species of animals are, in the opinion of this writer, charac- 
 terized by predominance of one of these four great types. 
 
 16. Modern psychology, approaching the subject of temperament 
 from the physiological and biological points of view, is more likely 
 to be self-restrained and cautious in its conclusions. According to 
 Wundt, 1 the fourfold division of the temperaments is correct, be- 
 cause, in the case of every individual, there must be a certain com- 
 bination of the two factors of strength and speed in all change 
 which goes on in the mental movements. The various affections of 
 the mind are therefore classifiable as either strong and quick or 
 strong and slow, or else as weak and quick or weak and slow. 
 Choleric and melancholic persons are inclined to strong affections, 
 sanguine and phlegmatic to those that are weak. By crossing 
 these two principles of division the following scheme is derived : 
 
 Strong. Weak. 
 
 Quick Choleric Sanguine. 
 
 Slow Melancholic Phlegmatic. 
 
 The quick temperaments are directed rather toward the present, 
 the slow toward the future. The quick require additional strength, 
 and the weak additional time, in order to achieve the largest 
 amount of work possible for them. The choleric and phlegmatic 
 are temperaments of action rather ; while the sanguine and melan- 
 cholic are temperaments of feeling. 
 
 Wundt agrees with the observations of Dr. George respecting 
 1 Physiologische Psychologic, ii. , p. 345 f . 
 
THE THEOEY OF LOTZE. 577 
 
 the applicability of the conception of temperament to orders, fam- 
 ilies, and species of other animals as well as to man. He also 
 makes the penetrating observations that Pessimism generally rests 
 upon an individual peculiarity of temperament ; and that the true 
 art of life consists in not having one temperament, but in combining 
 them all. " One should be sanguine amid the petty sufferings 
 and joys of daily life, melancholy in the more serious hours of 
 life's more important events, choleric toward impressions- that fetter 
 one's profounder interests, phlegmatic in the execution of the re- 
 solves that have been reached." 
 
 Lotze's 1 treatment of the doctrine of temperaments is more ex- 
 tended than Wundt's, but no less cautious and suggestive. Va- 
 rieties of temperament, as of all other innate natural capacities, 
 appear to be most marked under the conditions of an advanced 
 civilization. By the term "temperaments," according to Lotze, we 
 understand : " (1) The differences, in kind and degree, of excitability 
 for external impressions ; (2) the greater or less extent to which 
 the ideas excited reproduce others ; (3) the rapidity with which the 
 ideas vary ; (4) the strength with which feelings of pleasure and 
 pain are associated with them ; (5) finally, the ease with which ex- 
 ternal actions associate with these inner states themselves." The 
 ancient fourfold division of temperaments is approved by Lotze 
 as, indeed, it must be by all who advocate intelligently any the- 
 ory upon the subject. The sanguine temperament is distinguished 
 by great rapidity of change and lively excitability. This indi- 
 cates a permanent excess of the general capacity for reciprocal ex- 
 citement among all the different psychical states, and an exces- 
 sive sensitiveness of the soul to all external stimuli. It is natural 
 in children and uncivilized tribes ; it is, on the whole, advantageous 
 to the beginnings of culture, and prevents the establishment of 
 narrow notions and attachment to ideas acquired accidentally. But 
 adults who are strongly marked by this temperament make the 
 impression of immaturity, of being " grown-up children." For the 
 temperament usually called "melancholic " Lotze prefers the term 
 sentimental This temperament is distinguished " by special recep- 
 tivity for the feeling of the value of all possible relations," but is 
 indifferent toward bare matter of fact. Here a lively appreciation 
 of the harmonies and discords of surrounding objects may be com- 
 bined with little inclination for hard work ; a great variety of 
 aesthetic feeling, of imaginative activity, may go with theoretical 
 vagueness and the disturbance of an established sense of duty by 
 
 1 Miorocosmus, ii. , pp. 24 ff. , Edinburgh, 1885 ; Medicin. Psychologic, 
 560 f. ; Outlines of Psychology, p. 137. 
 37 
 
578 DOCTRINE OF TEMPERAMENTS. 
 
 this aesthetic feeling. The sentimental temperament shows itself in 
 science among those who "spend their ingenuity in constantly de- 
 vising some new dress for the knowledge they have acquired ;" and 
 in art by dealing with " isolated lyric movements of emotion," with- 
 out being capable of grasping them and bringing them together 
 into a coherent whole. It is distinctive of youth and, in its most 
 pleasant form, of those who retain a youthful disposition on into 
 the later and the latest years of life. 
 
 The marks of the choleric temperament are " one-sided recep- 
 tivity and great energy in single directions." It is therefore dis- 
 tinguished by diminished susceptibility to excitement, but great 
 force and endurance in reaction when feeling has once been 
 aroused. Its fine effect is an apparent moral steadiness of char- 
 acter ; its uncomely effect is obstinate and narrow perseverance in 
 a path once entered upon, even when reasons exist for deviating 
 from or abandoning it. Its time of most natural development is in 
 adult manhood ; but its occurrence in a notable way even among 
 children shows it to be one of the native dispositions of the mind. 
 Finally, the phlegmatic temperament, which is the natural temper 
 of advanced age, is distinguished by slightly varied and slow, but 
 not necessarily weak, reactions. Sluggishness in youth and equa- 
 nimity in old age may both result from the action of this adjust- 
 ment of the feelings and impressions to external stimuli and the 
 train of ideas. 
 
 17. The permanent common features of the foregoing views as 
 to the nature of temperament illustrate sufficiently the real truth 
 of the case. The doctrine as a whole is one which in its main 
 principles is undoubtedly required by the most wide and varied 
 observation. On the other hand, the differences in the details with 
 which the different descriptions are filled out show the uncertainties 
 which belong to every attempt to elaborate it. Common impres- 
 sions, producing a common play of feeling and regulating the train 
 of associated ideas, belong to all individuals. But in each individual 
 there is something characteristic as to the mode, the intensity, the 
 speed with which these impressions arise when the stimulus acts, 
 then combine with one another, and so provoke feeling or regulate 
 the ideas. In a more or less definite way, all men generalize the 
 various individual examples and form them into classes which 
 have necessarily lost that variety and minuteness of peculiarities that 
 characterizes the individual and have been conformed to some idea 
 of a type. No real individual perfectly expresses such a typical 
 idea. But especially in those conditions of civilization where the 
 expression of individuality in a varied and impressive way is pos- 
 
PHYSICAL BASIS OF TEMPERAMENT. 579 
 
 sible most individuals are recognized as conforming more nearly 
 to some one rather than another of these types. 
 
 18. As to the exact nature of the physical basis of temperament 
 nothing is known. The influence of abnormal bodily conditions, 
 and of certain diseases, to produce or alter the disposition of the 
 mind in a manner resembling temperament would seem to indicate 
 that the original constitution of the brain is not the principal 
 determining factor. The nature of the excitation which external 
 stimuli produce upon the end-organs of sense, the strength of the 
 resulting reactions in the form of common feeling, the habitual con- 
 dition of the internal and visceral organs and the coloring they 
 impart to common feeling, seem to be of prime importance in de- 
 termining the temperament. Further than this it is difficult to be 
 more specific, even in conjecture. The fact that the different periods 
 of life are apt to be characterized by a predominance of one of the 
 four temperaments is not an argument against the physical nature 
 of the basis of temperament in general. Certain changes in the 
 nature, speed, and strength of the reactions derived from the end- 
 organs and the internal organs of the trunk necessarily accom- 
 pany the early development, riper maturing, and decay of the bod- 
 ily powers. These cannot fail to have a great, though indirect, 
 influence upon the activities of the cerebral centres. But where so 
 much already said is so uncertain, we refrain from adding further 
 conjectures. 
 
 19. What has hitherto been developed in detail respecting the 
 relations which maintain themselves between the structure and 
 activity of the nervous mechanism and the phenomena of conscious- 
 ness may now be summarized in somewhat the following way. We 
 seem warranted in insisting that the following five great groups of 
 correlations between body and mind are always maintained during 
 the mind's conscious existence : 
 
 1. The quality and intensity of the sense-element in our expe- 
 rience is correlated with the condition of the nervous system as 
 acted on by its appropriate stimuli. That the precise character 
 and amount of our sensations are dependent upon what and how 
 much of various forms of physical energy acts upon the organs of 
 sense there is scarcely need to say. But the phenomena which 
 demonstrate the effect of attention upon the sense-element itself 
 prevent us from regarding the relation as only one-sided and sim- 
 ple. The true state of the case is never represented by considering 
 the sensations as mere passive impressions depending solely upon 
 
580 CORRELATIONS OF MIND AND BODY. 
 
 the kind and degree of the action which the stimuli exert. These 
 sensations depend also on the condition of the mind at the time 
 the stimulating effect of the excited sensorium is realized in con- 
 sciousness, and in terms of consciousness. To represent the men- 
 tal condition of attention as itself simply and absolutely dependent 
 upon the condition of the centres of the brain is to cover up much 
 of our ignorance concerning the relation of body and mind with a 
 scanty stock of conjecture in cerebral physiology. There are many 
 facts to countenance the reverse statement the condition of the 
 centres of the brain depends upon the state of the mind with re- 
 spect to attention. For the present we content ourselves with this 
 expression : The sense-element in our experience is constantly 
 correlated with the condition of the nervous system as under excite- 
 ment from its appropriate stimuli. 
 
 2. The combination, whether simultaneous or successive, of our 
 conscious experiences is correlated with the combination of the im- 
 pressions made, from whatever source, upon the nervous organism- 
 That the number and form of the different sensations composing 
 any presentation of sense is dependent upon the number and qual- 
 ity of the different excitations of the nervous system which com- 
 bine in such presentation there can be no doubt. So, too, does 
 the order and time-rate of the phenomena of consciousness depend 
 upon the order and time-rate of the separate excitations of the 
 nervous system. But no object of sense can be considered merely 
 as a compound of the elements of sensation entering into it ; nor is 
 the nature of the mental product to be derived from the physical 
 laws according to which the different stimulations modify, support, 
 supplement, or inhibit each other. A mental synthesis, an activity 
 that combines under different laws from those which govern the 
 putting together of stimuli of various wave-forms and degrees of 
 intensity, must take place in order that one object of sense may be 
 constructed out of several sense-elements. Again, the order in 
 succession and time-rate of the conscious states is not a mere copy 
 of the order and time-rate of the impressions made upon the ner- 
 vous system. It is simple matter of fact that mental education in 
 the making of those syntheses which take place in all acts of per- 
 ception is necessary in order to see or touch extended " Things," 
 as distinguished from merely having visual and tactile sensations. 
 Whatever special form of activity in the cerebral centres is assumed 
 as the physical basis of this mental act of synthesis, it is by no 
 means certain that such cerebral activity does not as truly depend 
 upon the mental act as the mental act depends upon it. Here 
 again, at any rate, the word " correlation " seems best adapted to 
 
CORRELATIONS OF MIND AND BODY. 581 
 
 express the connection between the physical basis and the mental 
 phenomena. 
 
 3. Those phenomena of consciousness which we designate as 
 " memory " and " recollection" as well as the play of the repro- 
 duced images of representation in general, are correlated with the 
 molecular constitution and tendencies, and with the so-called " dy- 
 namical associations," of the elements of the nervous system. It is 
 not necessary to repeat in this connection what has already been 
 said in proof of the fact that these elements furnish, in part, the 
 necessary conditions of conscious acts of memory ; and, on the 
 other hand, that the enumeration of certain physical conditions 
 throws no light upon what is peculiarly mental in the phenomena. 
 To say that memory depends on the condition of the cerebral 
 centres emphasizes the relations involved in one class of facts ; to 
 say that memory depends upon the conscious act of attention, both 
 to the original object and to its reproduced image, presents the 
 relations involved in another class of facts. 
 
 4. The course of thought, and all the higher forms of self-con- 
 scious experience, are correlated ivith the condition of the cerebral 
 centres. The dependence of these conscious mental activities upon 
 the quantity and character of the blood-supply in the brain, and 
 upon the integrity and unimpeded activity of its tissues, cannot be 
 called in question. Of the exact nature of this dependence we can 
 form only a very inadequate picture ; and we have no means what- 
 ever of subjecting this dependence to a rational explanation. But, 
 on the other hand, many of our experiences would just as certainly 
 lead to the conclusion that the condition of the cerebral centres 
 depends upon the higher forms of self-conscious experience. This 
 is true of the results of all our voluntary acts accompanied by 
 conscious discernment and choice of one among several possible 
 courses of action. The facts of which we are sure in such cases 
 are these : An idea of something to be done, an idea of the means 
 (the parts of the body to be moved, and the sensations and feelings 
 of effort which are associated in experience with such movement), 
 a fiat of will, and a result in sensations and perceptions showing 
 that the movement has been accomplished. The actual movement 
 we trace back, under the guidance of physiological facts and laws, 
 to the starting of some form of nerve-commotion in the requisite 
 motor areas of the cerebrum. But unless we stoutly, and from 
 mere prejudice, refuse to acknowledge a possibility of .the mind 
 exercising any influence upon the body, we are warranted in 
 saying that this nerve-commotion in the cerebral motor areas de- 
 pends upon the preceding ideas ending in the fiat of will to exe- 
 
582 COKRELATIONS OF MIND AND BODY. 
 
 cute a certain form of external motion. At this stage of the discus- 
 sion, however, we prefer to use the vague term " correlation " to in- 
 dicate the mutual connections between phj'sical condition and this 
 class of so-called higher mental phenomena. It has already been 
 made clear that such connections are here much less patent, direct, 
 and susceptible of being stated in the form of general laws than 
 are those of the first two classes. 
 
 5. The statical condition of the body (by which we mean all 
 those inherited peculiarities of the organism, the sexual and tribal 
 bodily characteristics, the corporal constitution as dependent upon 
 age, which change only slowly and within narrow limits, or do not 
 change perceptibly at all) and the general tone or coloring of con- 
 scious experience are correlated. Upon this obscure subject we 
 may (at least at present) wisely decline to take either one of two 
 extreme and indefensible positions. It cannot be said to be re- 
 quired by the facts that all the phenomena of consciousness should 
 be regarded as strictly predetermined by the constitution, environ- 
 ment, and independent action of the corporal elements. The really 
 convincing argument for all such thorough-going organ icism is the 
 wish to have it so. At another extreme stands the fanciful philos- 
 ophy which considers the mind as the builder of the body as in 
 some way fashioning to its own inherent constitution and uses the 
 organs of the physical mechanism. This conclusion, also, we must 
 decline to accept without further testing. All the facts, however, 
 do obviously impress upon us the conclusion, how pervasive, inti- 
 mate, varied, and profound are the mutual relations the correla- 
 tions of the physical mechanism and the phenomena of con- 
 sciousness. Any further speculation as to the real nature of this 
 connection, and as to the nature of the subject of the mental phe- 
 nomena of the Mind must be reserved for the succeeding part of 
 our inquiry. 
 
PART THIRD. 
 
 THE NATURE OF THE MIND. 
 
CHAPTER I. 
 THE FACULTIES OF THE MIND, AND ITS UNITY. 
 
 1. UP to this point in our psycho-physical investigations we 
 have been content to speak of the mind simply as the " subject " of 
 the phenomena of consciousness. In other words, the phenomena 
 of consciousness may with equal propriety be spoken of as mental 
 phenomena ; for they are phenomena of, or appertaining to, what 
 all men indicate by the subject "I" (the ego) when describing their 
 different conscious experiences (comp. p. 31). This indefinite and 
 provisional recognition of the mind as an existence to which differ- 
 ent states, or conditions, or modes of activity, may belong with- 
 out destroying its unity, suffices for a simple description of the 
 constitution and activities of the nervous mechanism, and of the 
 relations between it and the phenomena of consciousness. But 
 psychology, even when pursued from the physiological point of 
 view, can scarcely be satisfied to push its inquiries no farther into 
 the nature of mind. Psychology, from whatever point of view it is 
 pursued, aims to perfect a science of mind. Like every other 
 science, it strives to discover the essential nature of that which it 
 investigates. 
 
 Pains must indeed be taken to avoid substituting words for 
 things, abstractions for realities. By the " essential nature " of 
 mind we mean just that nature which is duly inferred from the 
 phenomena as essential to their rational explanation. But there 
 are especial and unique reasons why psychology should not will- 
 ingly desist from renewed attempts at such rational explanation. 
 Is the subject of the phenomena of consciousness the so-called 
 Mind entitled to be considered as having unity and reality in any 
 defensible meaning of the words ? Is it not peculiarly entitled to 
 be considered as a real being, with a permanent and essential nature 
 of its own ? It is impossible for human reason not to attach the 
 greatest interest and importance to these ultimate psychological 
 inquiries. A sentient and rational life, without any self interest in 
 the examination of its own permanent characteristics, and of the 
 grounds upon which it rests, would be an absurdity. 
 
586 RELATIONS OF MIND AND BKAIN. 
 
 2. Various objections may be raised against allowing considera- 
 tions like the foregoing to apply to the researches of that branch of 
 psychology which is called " physiological." It may be claimed that 
 the rational explanation of the mental phenomena belongs to Meta- 
 physics rather than to Physiological Psychology. To a certain ex- 
 tent the force of such objections must be admitted. They are 
 not of a nature, however, to debar us from the inquiries that are to 
 be raised in the following chapters. On the contrary, the result of 
 the discoveries made by starting from the point of view held by 
 experimental science is such as irresistibly to urge upon us some 
 of these very inquiries. For example, it has been shown beyond 
 doubt that the construction of presentations of sense requires the 
 activity of both body and mind, considered as standing in peculiar 
 relations to each other with respect to the conditions which they 
 furnish for the spatial relations and spatial properties of these pres- 
 entations. But do body and mind themselves exist in spatial re- 
 lations ; and may the latter be spoken of as having spatial prop- 
 erties ? In other words : In what sense can we localize mind in 
 the bod}', or speak of the body as the seat or organ of mind? 
 Moreover, as we observe the two classes of phenomena (the organic 
 and the mental) the impression is inevitable that in some sort they 
 keep pace with each other in the order of development. This fact 
 unavoidably raises an inquiry as to the relation of the mind to the 
 body with respect to its origin and destiny. 
 
 As a matter of fact, moreover, it is found that those who are 
 most inclined to complain at the introduction of any " metaphysi- 
 cal " inquiries into the discussions of physiological psychology are 
 quite as apt as others to give grounds for the same complaint 
 against themselves. They themselves rarely escape the charge of 
 having a so-called metaphysical theory of the soul to maintain. 
 With such complainants, moreover, it is often from the very begin- 
 ning a foregone conclusion what the general nature of that theory 
 must be. They decry metaphysics and advocate a " psychology 
 without a soul." Yet they hold, as an unalterable but unverifiable 
 assumption, that psychological phenomena must not be so discerned 
 and interpreted as to seem to require for their explanation an ex- 
 istence called " a soul." 
 
 3. In order to arrive at any satisfactory conclusions regarding 
 the essential nature of the Mind, it is plainly necessary that we 
 should take our point of starting from a consideration of mental 
 phenomena. For these are the very phenomena for which an ac- 
 count is to be given ; and there is no safe way of concluding what 
 is the nature of any reality, or even of determining whether any 
 
YCH< 
 
 FAULTS OF THE OLD PSYCSlOLOGT. 587 
 
 assumed reality actually exists, except by considering the phenom- 
 ena which are attributed to it. The questions, how far mental 
 states and mental changes are explicable by referring them to ante- 
 cedent or concomitant states and changes of states in the nervous 
 system ; and how far such mental states and changes require us to 
 assume the existence of some other real being than the molecules 
 of the brain and spinal cord cannot even be properly approached 
 without a clear knowledge of what these states and changes in 
 themselves are. But the only way to know what mental phenom- 
 ena, as phenomena, in fact are, is through observation of such phe- 
 nomena by the method of introspection. We must then begin this 
 particular part of our general discussion by changing for the time 
 our point of view. 
 
 Much fault has been found of late with the failures of the so- 
 called "old " psychology. It has often been explained at length 
 that these failures were largely due to its wrong method ; and, as 
 is well known, its method was almost exclusively the method of in- 
 trospection or self-consciousness. The exclusive use of this method 
 resulted in confining the efforts of psychology very largely to the 
 rather barren task of classifying the different kinds of mental activ- 
 ities, and of discussing what so-called " faculties " must be assumed 
 to belong to the mind in order to account for so many kinds of 
 activities. Now, classification of phenomena is certainly one im- 
 portant part of the work of every science ; nor should it be forgot- 
 ten that much of the more recent progress in psychology is due to 
 previous painstaking observations of mental phenomena resulting 
 in their classification from the purely introspective point of view. 
 Classification, however, is not explanation ; and the " faculties " 
 into which the " old psychology " divided the mind were too often 
 mere names that repeated the bare fact of the observer having suc- 
 ceeded, to his own satisfaction, in classifying the phenomena. It 
 is demanded, however, in order to make real progress in psychology 
 as a science, that the correlations, under precise and definite laws, of 
 the mental phenomena with one another and with the events which 
 happen in external nature shall be ascertained. Nevertheless, even 
 after adopting this view of the problem we cannot dispense with 
 the method of introspection ; for we have no other way of ascer- 
 taining what are the phenomena that require explanation. If the 
 further question be raised, What is the real nature of that subject 
 of the mental phenomena popularly spoken of as the mind? we 
 surely cannot approach the answer to this question without calling 
 attention to the nature of the phenomena themselves. 
 
 So far as the necessities of the present discussion are concerned, 
 
588 RELATIONS OF MIND AND BRAIN. 
 
 it may be said that there are two rival and contrary ways of reply- 
 ing to the general inquiry into the nature of the Mind. One of 
 these denies that, in order to account for mental phenomena, we 
 need assume much less are able to prove the existence 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 phenomena 
 is possible without referring them to a non-material or spiritual 
 entity as the real subject or ground of them all. Both of these 
 ways of explanation admit of various modifications. The former, 
 as held by its different advocates, has used different terms to set 
 forth the relation in which it believes that the phenomena of con- 
 sciousness stand to the states and activities of the brain. The lat- 
 ter, also, has by no means always been self-consistent in its advo- 
 cacy of the unique and independent character of the subject of 
 mental phenomena. Even the power of immediately penetrating 
 in consciousness the secret of its own interior nature has sometimes 
 been claimed for the mind. The former of the foregoing views, in 
 whatever particular shape it may occur, has customarily been re- 
 garded as essentially the "materialistic," and the latter as the 
 " spiritualistic," theory of the human mind. 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 allusion. 
 
 4. In the remaining part of our discussion we shall be chief- 
 ly occupied with considering which one of the two theories just 
 stated 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 mechanism and the phenomena of con- 
 sciousness. The question before us may then be stated in the 
 following provisional form : Do the phenomena of consciousness 
 require for their explanation nothing more than a statement of 
 those changes in the material mechanism with which they are obvi- 
 ously correlated ; or do they also require the assumption of one 
 real and non-material being as the subject and ground of them all ? 
 To repeat a remark already made, the approach to this question 
 must be through the introspective study of mind ; for only such 
 study can tell us what the phenomena of consciousness actually are. 
 
 5. It is so obvious as scarcely to need or admit of debate, that 
 mental phenomena are not identical with the changing conditions 
 
CONSCIOUSNESS AND NERVE-COMMOTION. 589 
 
 and activities of the nervous system. However our states of con- 
 sciousness may be related to the states of the brain even if the 
 former are absolutely and without exception dependent upon the 
 latter the two are certainly not the same. What the exact states 
 of the brain are with which any of the mental states are correlated 
 we know only very imperfectly and by remote conjecture. But so far 
 as we do know anything about the particular molecular activities 
 of the central nervous system which are most directly connected with 
 the phenomena of consciousness, they do not differ essentially from 
 other molecular activities of this system not thus connected with 
 consciousness. The chemical constitution and structural form of 
 the nerve-fibres and nerve-cells of the brain do not differ from 
 those of the spinal cord in any such respect as, of itself, to account 
 for the difference in the relations in which the two stand to con- 
 scious mental states. They do not so differ even from the mole- 
 cules which enter into the living plant or animal of much lower 
 species, mentally, than man. It is a surprise, from which scientific 
 investigation can never recover, to find that the Connection between 
 our sensations, mental images, and volitions and the peculiar ma- 
 terial constitution and functions of the cerebral mass of nervous 
 matter should be so intimate as it undoubtedly is. The foregoing 
 fact shows that it is quite impossible to regard the two classes of 
 events the molecular changes of the central nervous mass and the 
 happenings of our conscious experience as one and the same. 
 
 All physical events are modes of motion alterations in the re- 
 lations of the material atoms or masses to each other in space. 
 This is as true of the human brain as it is of the clod of the valley. 
 Its atoms cannot be conceived of as doing anything, so long as they 
 remain material atoms, that does not essentially consist merely in 
 changing their relations in space to other material atoms. This 
 is the activity which chemistry supposes to be continually taking 
 place as the work of nutrition and depletion in the nervous cen- 
 tres accompanies the process of thought ; this is what, as general 
 "nerve-physiology" rightfully conjectures, occurs when any form 
 of stimulus acts upon the afferent nerve through the end-organs 
 of sense, and corresponding states of sensation arise in the mind. 
 But the conscious process of thinking ?s not the change in the 
 chemical constitution of the nervous mass ; the conscious sensa- 
 tions are not the wave-like movements of nerve-fibres and nerve- 
 cells. It is not simply true that to identify these two kinds of 
 phenomena phenomena of the motion of material atoms and 
 phenomena of change in mental states is difficult for the average 
 mind, but attainable by the scientific observer ; it is rather true 
 
590 RELATIONS OF MIND AND BRAIN. 
 
 that no mind can frame any intelligible idea of what could be 
 meant by identifying the two. 
 
 Moreover, the history of investigation shows that a man may be 
 highly trained, both in the. observation of the phenomena of the 
 animal body and of his own self-conscious mind (for example, 
 Aristotle), without even suspecting the important relation which 
 exists between the latter and the cerebral mass. Indeed, there is 
 no distinction which all men are compelled to make more clearly 
 than that between their own conscious states and the changing con- 
 ditions, by way of motion, of the masses and molecules of matter. 
 All theory which assumes the possibility of identifying molecular 
 motions of brain-atoms with the shifting forms of mental experi- 
 ence, or attempts to set forth the peculiar nature of the latter by 
 simply stating the conjectured laws which control the former, in- 
 creases the general confusion which tends to surround the whole 
 subject. 
 
 6. Furthermore, it is not easy to see what could possibly be 
 meant, that is worth serious consideration, by speaking of the phe- 
 nomena of consciousness as the product of the brain. By the word 
 "product "we ordinarily understand the new form into which a 
 material substance has been thrown by the action upon it of some 
 machine or mechanism. Thus we call certain secretions of the body 
 the "product" of the tissues where they are secreted in somewhat 
 the same way as that in which we speak of the products of the 
 field or of the loom. The function of the living molecular mech- 
 anism of which certain tissues consist is exercised in producing 
 from the pabulum brought to them by the blood the secretions of 
 gastric juice and bile. To speak of mental states and processes as 
 the " product " of the nervous mass of the brain, in any sense of 
 the words corresponding to that which we rightly apply to the 
 various secretions of the body, involves us at once in the grossest 
 absurdities. The secretory product of the brain is the fluid found 
 in certain of its cavities ; its nutritive product, so to speak, is the 
 new nervous tissue which is constantly being formed from the 
 blood by that activity of reproducing its own kind which this tis- 
 sue has in common with other living tissues. But this fluid and 
 these newly produced molecules or nerve-corpuscles in the brain 
 are in themselves no more like mental processes, and no more to be 
 identified with such processes, than are the tears that flow from the 
 tear-ducts or the pus that exudes from a wound. In so crass a 
 way of speaking it is difficult to distinguish what can properly be 
 meant by compaiing under the term " product " the relation of 
 conscious sensation and thought to the brain with the relation of 
 
CONSCIOUSNESS AND NEKVE-COMMOTION. 591 
 
 the gastric juice to the stomach or of the pancreatic juice to the 
 pancreas. 
 
 There is another and more plausible use of the word "product 5 ' 
 to describe the connection between the nervous matter of the brain 
 and the phenomena of consciousness. When a system of material 
 molecules is acting under relations to each other which are deter- 
 mined by their constitution, arrangement, and environment, we may 
 speak of the constantly new relations which they assume as the 
 product of their previous constitution and arrangement, and of 
 whatever influences act upon them from material molecules outside 
 of the system (the environment). Thus the functional activity of 
 the nervous centres, the complex and interacting nerve-commotions 
 of the brain, might be regarded as the product of the matter 
 constituting these centres. This manner of speaking has certain 
 marked advantages. It emphasizes the merely mechanical point of 
 view. It insists upon the valid assumption that the account of 
 every change that arises in the material particles out of which the 
 brain is composed must be sought for in the previous constitution 
 and arrangement of those same particles as acted upon by stimuli 
 either external or internal to the whole brain-mass. 
 
 Let us suppose, for the sake of illustration, an incredible increase 
 in our powers of observation. Let us suppose that it were possible 
 with the microscope to discern the exact chemical constitution of 
 every molecule of the nervous substance of the brain, to watch the 
 motion of all the atoms composing these molecules as chemical 
 changes take place or as waves of nerve-commotion in infinite va- 
 riety move hither and thither among its countless nerve-fibres and 
 nerve-cells. All this would be in itself absolutely nothing more than 
 an expansion under the eye of the observer of what he sometimes 
 sees in somewhat grosser form for example, in the amoeba and 
 of what he infers is constantly taking place in every kind of nervous 
 tissue. The status of the system of moving molecules at each in- 
 stant is to be explained so far as explanation is possible from its 
 status at the preceding instant, in connection with any influences 
 brought to bear upon it from the outside. Moreover, all such out- 
 side influences, so far as they are of a physical sort, are nothing but 
 modes of the molecular activity of other material particles. Looked 
 at in this way, the product of the brain is the molecular activity of 
 the brain. That is to say, the function of this unique system of 
 molecules is to be constantly in motion, in the form of activity 
 which we have already examined as "nerve-commotion." 
 
 But if the foregoing statement be admitted, how does it help us 
 in the least to understand the phenomena of consciousness regarded 
 
592 EELATIONS OF MIND AND BRAIN. 
 
 as the product of the brain ? In order to hold that mental phe- 
 nomena are related to the substance of the brain, in the same way 
 as that in which the nerve-commotions or molecular changes are 
 related to the same substance, we must identify mental phenom- 
 ena with molecular changes. But we have already seen that it is 
 impossible to identify the two classes of phenomena, as phenomena. 
 The phenomena of nerve-commotion may be regarded as the prod- 
 uct of the nervous mass in which they occur ; that is, they may 
 be attributed to the constitution and arrangement of the molecules 
 which compose this mass, as showing what the mass can do. But 
 the phenomena of consciousness cannot be regarded as the product 
 of the same nervous mass in any similar meaning of the word 
 "product." 
 
 7. Yet another unsatisfactory way of regarding the relation 
 between the brain and conscious mental phenomena requires a 
 more detailed and careful consideration. Everything in our pre- 
 vious examination has tended to show that the molecular changes 
 which go on in the brain, whether they are occasioned by afferent 
 impulses that originate in the application of stimuli to the end- 
 organs of sense, or by the modified amount and quality of the blood- 
 supply, are not only regular antecedents, but true causes of what 
 takes place in the mind. All the second part of this treatise was 
 occupied in pointing out the different classes of such relations 
 between physical antecedents and consequent mental phenomena. 
 [The objections which are ordinarily urged against speaking of 
 physical changes as the cause of the phenomena of consciousness 
 will be raised and answered later on.] 
 
 It is plain to every unprejudiced mind that, in some valid sense, 
 changes in the condition and activity of the substance of the brain 
 are specially related to certain of the shifting phenomena of con- 
 scious mental life. From this admission, which is enforced by the 
 entire study of Physiological Psychology, the temptation is strong 
 to proceed at once to the completion of an apparently simple and 
 comprehensive theory. This theory claims that all mental phenom- 
 ena, whatever their varied characteristic shading, have exact equiva- 
 lents, as it were, in specific forms of the nerve-commotion of the 
 living brain. Every such phenomenon, therefore, is only the man- 
 ifestation of what has previously taken place, or is simultaneously 
 taking place, in the physical molecular structure of the nervous 
 centres. To employ a figure of speech for every state and action 
 of the so-called mind in consciousness, that collection of nerve- 
 fibres and nerve-cells which we call the brain exacts a payment 
 in some special kind of coin. For example, with the molecular 
 
CEEEBEAL AND MENTAL COMBINATIONS. 593 
 
 changes in the substance of the brain which may be designated A, 
 B, C, D, etc., the mental states called a, ft y, 3, etc., are uniformly 
 and necessarily joined ; and with the combination of molecular 
 changes which may be described by A -h B -f C + D, etc., the 
 mental states a + /5 + y + S, etc., are as uniformly and necessarily 
 joined. When the molecular changes recur in a fainter or modi- 
 fied form, as A', B', C', D', then there must be a recurrence of the 
 corresponding mental states, only in fainter form, as a', ft, y', S'. 
 Finally, it is without exception true so this theory holds that 
 nothing happens in the mind by way of conscious sensation, pres- 
 entation of objects of sense, ideation, reproduction of mental im- 
 ages, and higher aesthetic feeling or intellectual processes or choice, 
 which does not find its only real explanation in the equivalent 
 changing states of the nervous system. 
 
 8. Our first impression on considering the foregoing way of ac- 
 counting for mental phenomena is that of a certain surprising audaci- 
 ty. The theory, standing on a slender basis of real fact, makes a 
 leap into the dark which carries it centuries in advance of where 
 the light of modern research is now clearly shining. Physiological 
 Psychology, as we have been compelled to regard it, has been seen 
 to be encompassed with difficulties at every step ; and some of 
 these difficulties appear absolutely insurmountable. It has achieved 
 its greatest triumphs in giving a physical and physiological expla- 
 nation of the variations in the quantity and quality of sensation, 
 and of the time-rate of the simpler mental processes. But even in 
 these domains of greatest achievement it is found that almost every- 
 thing needed for an exact science of the relations of the molecular 
 changes in the substance of the brain and the changes in states of 
 consciousness is lamentably deficient. In the first place, little prog- 
 ress has been made in framing a theory of the nature of the phys- 
 ical changes themselves. Physical science is not as yet able to deal 
 with the phenomena of nervous action, as shown even by a single 
 living nerve with a muscle attached when acted upon by any one 
 form of external stimuli ; how much less, then, with that vast com- 
 plex of nerve-fibres and nerve-cells which constitutes the human 
 brain ! As to purely physical explanations of the variations in the 
 quantity of sensations we are also in great doubt. No adequate 
 means exist for measuring accurately the changes in the amount of 
 all the stimuli which act on the end-organs of sense. We have less 
 knowledge of the laws which regulate the amount of excitation set 
 up in these organs by changes in the amount of stimuli; and 
 scarcely any real knowledge at all of what molecular changes take 
 place within the central organs when the afferent nerves have stirred 
 
594 MIND AS CONSCIOUS BEING. 
 
 them to their characteristic action. Hence Fechner's laws, con- 
 sidered as merely empirical statements of relations between the 
 amounts of certain external stimuli and our judgments under ex- 
 traordinary conditions as to how much we are affected thereby, are 
 readily disputed. But considered as setting fortli the essential re- 
 lations which exist between the physical changes in the brain and 
 the intensities of the resulting mental changes they are quite inde- 
 fensible. Moreover, the amount of our being affected, the quan- 
 tity of the sensation which results from the application of physical 
 stimulus, can be determined only by the judgment of the same 
 consciousness which is affected with the sensation. But judgment 
 itself is a form of mental phenomena for the essential part of 
 which no physical equivalent can be discovered or even conceived 
 of. 
 
 What has just been said concerning our inability to give a com- 
 plete physical explanation of variations in the quantity of sensa- 
 tions applies with equal force to their qualities and time-relations. 
 But even if the whole field of sensation, as respects the amounts, 
 kinds, and order in time of its phenomena, were already covered by 
 such purely physical explanation as refers them wholly to changes 
 in the molecular condition of the brain, the above-mentioned theory 
 would by no means be established. For in investigating the corre- 
 lations which undoubtedly exist between the nervous mechanism 
 and the phenomena of consciousness, it is found that some of these 
 phenomena imply activities of the mind which do not admit, in any 
 sense of the word, of being thus correlated. For an example upon 
 this point, we may refer to what was said (Part II, chapters VI. and 
 VIE.) as to a mental synthesis being implied in the formation of all 
 presentations of sense. " Things " are not mere loose aggregates 
 of sensations. They are the results of mental synthetic acts, the 
 laws of which cannot be attributed solely to the various ways in 
 which the physical molecules of the brain are made to move by the 
 action of stimuli reaching them. Let it be admitted that, with in- 
 creased information, we should find the scale of varieties in the 
 kinds of mental phenomena called sensations corresponding, point 
 for point, to the scale of varieties in the manner of the motion of the 
 waves of nervous excitation. Let it also be admitted that the scale 
 of the changes in the intensities of mental phenomena, that are of 
 the same kind, corresponds with equal exactness to the scale of the 
 changes in the vibratory swing of these waves. Let it be still 
 further admitted that, whenever any presentation of sense occurs, 
 there exists a kind and amount of various excitations simultane- 
 ously effected in the brain which corresponds exactly to this par- 
 
NATURE OF ALL "THINGS." 595 
 
 ticular presentation of sense. The mere congeries or common 
 occurrence of such sensations, as the necessary result of the exci- 
 tations of the brain, does not constitute a real " Thing." Each 
 "thing" implies, not simply a vast number of moving physical 
 molecules, on the kind and amount of whose motion the phenomena 
 of conscious sensation are dependent, but a uniting energy and a 
 unity in mind. Fifty million molecules, even when they are highly 
 complex and unstable phosporized compounds, gyrating in the most 
 wonderful fashion with inconceivable rapidity, certainly do not 
 constitute one thing. They do not, then, by molecular constitution 
 and activities, even constitute a physical basis which is conceivable 
 as a representative or correlate of one thing. Each molecule among 
 them all, even in order to be conceived of as being itself one such 
 thing among the other millions of more or less similar but not 
 identical molecules, is dependent upon this same synthetic activity 
 of the mind. 
 
 9. Still further, the study of metaphysics shows us that certain 
 assumptions, which are not of a sensuous character, or verifiable 
 at all by an appeal to the sensations, enter into every presentation 
 of sense. No such presentation of sense consists of a mere put- 
 ting together of individual sensations. Whatever account one may 
 choose to give of the nature and origin of this belief, there can be 
 no doubt that all men do believe that the "things" they perceive 
 are neither bare groupings of mental phenomena nor forms of the 
 molecular motion of a nervous mass ; men believe that things are 
 real existences set in space outside of their own bodies. Things 
 are known as real ; they are supposed to have attributes ; they act 
 on each other and on us who observe them ; they exercise force ; 
 they are extended and movable in space, and continue uninterrupt- 
 edly through more or less of time. To be all this is necessary in 
 order to be a "Thing." Now, the assumptions which enter into 
 the popular belief may be regarded as all true or all false, or as 
 partly false and partly true, in the form in which men ordinarily 
 hold them. But not one of them is capable of being justified, or 
 in any way accounted for, by an enumeration of the sensory states 
 into which consciousness is thrown by the action of the stimuli 
 on the nervous system. Much less is it accounted for by refer- 
 ence to certain hypothetical wave-like motions in the substance of 
 the brain. That such wave-like motions occur we have no doubt. 
 That the changes in the quantity and quality of the sensations are 
 related to, and dependent upon, the intensity and the kind of these 
 motions is a most reasonable conjecture. That the motions which 
 are correlated with the presentations of sense differ in kind or de- 
 
596 MIND AS CONSCIOUS BEING. 
 
 gree from those which are correlated with mere images of imagi- 
 nation may also be true. It is true, furthermore, that our percep- 
 tions and ideas of the extension and motion of things are dependent 
 for their characteristics in great measure upon the structure of 
 the physical organism. But it is impossible to conceive of any 
 form of molecular motion which could serve as the physical basis 
 or physical representative of any of those metaphysical assumptions 
 which enter into all knowledge of things. What kind of nervous 
 action can be the equivalent of an unchanging conviction or belief 
 in the reality and true causal energy of all things both visible and 
 invisible ? What splitting up of the chemical constitution of the 
 molecules of nervous substance, or difference in the character of 
 their agitations, can be conceived of as analogous to, or serving as 
 true cause of, the distinction which .is involved in our speaking of 
 each Thing as though it were a substance with attributes ? 
 
 Keference to what has already been said (Part II., chapter X.) 
 concerning the impossibility of assigning a physical basis to the 
 mental operations of voluntary recollection, with its recognition of 
 similarity, to attentive choice, and to all the discrimination which 
 underlies the work of the intellect proper, will furnish further oc- 
 casion for distrusting the above-mentioned theory. For example, 
 the new process of physical excitation, which serves as the " basis " 
 so to speak of any image of memory, may be similar to the 
 process which served as the basis of the original presentation of 
 sense. But the mental act of recognizing the similarity of the ob- 
 ject before the mind to one no longer before it, and yet of distin- 
 guishing the former from the latter as characterized by the time in 
 which it occurs, does not admit of being conceived of under any 
 analogy to such physical processes. The same thing may be said 
 of consciousness in itself considered, and so of every mental phe- 
 nomenon considered as being what it actually is a phase of con- 
 sciousness. But to make clear this aspect of the case requires that 
 we should resume the consideration of the so-called "material- 
 istic" theory of the relations between mental phenomena and ner- 
 vous substance from a slightly different point of view. 
 
 10. There can be no doubt that the popular and wellnigh uni- 
 versal belief regards the subject of the mental phenomena as a real, 
 non-material, and permanent being. This belief also undoubtedly 
 regards this subject as one indivisible being, a "unit-being." In 
 other words, the prevalent conception of the Mind is that of an 
 existence which is spiritual and is a unity in some unique sense. 
 The many objections which have been raised against the belief 
 may be divided into two classes one of which may be called 
 
INFLUENCE OF THE BLOOD-SUPPLY. 597 
 
 metaphysical, and the other physiological or physical. The meta- 
 physical objections arise, in part, from the difficulty which is felt 
 in defining what is meant by "reality," "spirituality" (or non- 
 materiality), and " unity " in that strict sense in which these terms 
 are thought to apply to the mind. The consideration, both of 
 these objections and of that way of considering the facts which es- 
 capes them as far as possible, will be for the present postponed. 
 But the other class of objections arises from the very facts with 
 which it is the special business of the science of Physiological 
 Psychology to deal. It may be summarily stated in the form of 
 questions like the following : What kind of permanent reality can 
 belong to a being whose essential characteristic of having various 
 states of consciousness can be temporarily laid aside when the brain 
 sleeps or is deprived of its blood-supply ; or can be wholly lost 
 when certain nervous centres are subjected to permanent pressure 
 or destroyed by disease or the surgeon's knife ? How can non- 
 materiality be affirmed of phenomena which so far as we can 
 trace them at all exist only in immediate dependence upon a cer- 
 tain chemical constitution, structural form and arrangement, and 
 functional activity of material atoms ? How can the claim of be- 
 ing the highest unity be made for that which exists at all only as it 
 is in a constant flux ; which, indeed, is possessed of its one charac- 
 teristic activity of being conscious, only on condition that it divides 
 itself into subject and object and experiences a constant change of 
 the forms in which it is conscious? 
 
 In spite of such objections from the physiological point of view 
 as are the foregoing, the popular assumption, when freed from its 
 crudities and interpreted intelligently, may be shown to be the only 
 one compatible with all the facts of observation. It may be shown 
 that it is demanded by these facts. On the contrary, the attrib- 
 uting of mental phenomena to the substance of the brain (with or 
 Without including the rest of the nervous substance) does not satisfy 
 the facts of observation. The relation between brain and mind is 
 not such that the former can be considered as a real being, of which 
 the phenomena of the latter may be regarded as activities. Another 
 real being must be assumed to exist as the subject of the mental 
 phenomena a being with a nature quite unlike that of material 
 molecules. 
 
 11. The phenomena whose relation to the molecules of the ner- 
 vous system is in dispute are phenomena of consciousness. Noth- 
 ing in regard to these phenomena is more impressive, upon first 
 subjecting them, as such, to introspective observation, than their 
 surprising complexity in unity. Or we may rather say, the way in 
 
598 MIND AS CONSCIOUS BEING. 
 
 which all states of consciousness, however different they may be 
 with respect to characteristic quality or origin, are attributed by 
 the conscious subject to one subject as his own states is the most 
 surprising of all facts. This fact underlies all the truths and laws 
 into which psychology inquires, whether starting from the physio- 
 logical or from some other point of view. 
 
 It has been customary for psychologists to classify all the mental 
 phenomena under a few so-called " faculties " of the mind. The 
 objection has often been made to such classification that it tends 
 to confuse or destroy a just appreciation of the unity of mind. A 
 more obvious objection to the ordinary psychological classification 
 is perhaps this, that it fails to take due account of the vast com- 
 plexity of mental phenomena. For it should never be forgotten 
 mental phenomena are always, primarily considered, nothing more 
 than events. This is true of sensations and perceptions, with all 
 their objective reference, as well as of acts of imagination and of 
 so-called pure thought. The yellow of the watch, the red of the 
 rose, which I see, are modes of the affection of my consciousness. 
 Excluding for the present all reference to any metaphysical assump- 
 tions, these colors are simply events in consciousness. The real 
 processes outside of consciousness the objective existences and 
 events to which the events in consciousness are referred by science 
 are in no sense similar to the events of consciousness themselves. 
 What the real process outside of consciousness is whether it 
 consist of rapidly vibrating waves of ether, or of photo-chemical 
 changes in the tissues of the retina, or of nerve-commotions prop- 
 agated along the optic nerve and in the upper occipital lobe of 
 the brain we know only by doubtful inference from certain con- 
 scious affections of our own to certain material existences assumed 
 to exist out of consciousness. Whether the right to make these 
 inferences be disputed or allowed, there can be no dispute over the 
 statement that the phenomena of conscious vision are not copies of 
 any of these external events. What is true of colors is also plainly 
 true of smells, and tastes, and sounds. These sensations are all 
 events in consciousness. Recent researches into the nature of the 
 sensations which come through the excitation of the skin show that 
 these also are, primarily considered, mental events of various kinds ; 
 such as having the feeling of cold, or of heat, or of pain, or of 
 pressure, or of motion, etc. 
 
 Moreover, the modern experimental view of the way in which the 
 sensations are localized and synthetically combined to form the 
 presentations of sense shows that the latter also must be consid- 
 ered as being, primarily, mental events. Of course there is a sense 
 
COMPLEXITY OF MENTAL PHENOMENA. 699 
 
 in which it is absurd to say, not simply, " Perceptions of things are 
 always merely mental events; "but also, "Things themselves are 
 merely mental events. " It has already been shown that certain, as- 
 sumptions enter into all our perceptions and conceptions of so- 
 called " things ; " but that even the attempt to account for these 
 assumptions, by assigning them to any conceivable form of a physi- 
 cal basis in the brain, leads to absurdity. Still there is also a sense 
 in which " things," that can certainly never be any different with 
 respect to their known qualities from the way in which they appear 
 to us when perceived, are mental events. Sensations are mental 
 events ; the discriminating, combining, and localizing of sensations 
 are mental events. Things to us are never more than discrimi- 
 nated, combined, and localized sensations, plus the metaphysical 
 assumptions to which reference has been made. 
 
 12. Reflection upon facts like those just stated leads us to won- 
 der at the enormous complexity of mental phenomena. For the 
 purposes of the practical life we are warranted in regarding the 
 surrounding world as composed of a limited number of material 
 existences that undergo little or no change from day to day. But 
 this way of regarding the objects of experience does not at all 
 satisfy the demands of psychological inquiry. Such inquiry con- 
 siders the knowledge of external nature, as well as of what is recog- 
 nized as pertaining most strictly to the world within, to involve an 
 unceasing change in the activities of consciousness. Indeed, with 
 the limitations already referred to, we may say that the entire 
 world, so far as it is our world, consists only of these changing 
 states. It follows, then, that the first truth of which we have to 
 take account is the following : Our entire conscious existence 
 whether regarded simply as being ourselves affected in a certain 
 way or as having a purely objective experience of the existence and 
 qualities of so-called Things is a continually shifting succession 
 of individual mental activities, no one of which is exactly like any 
 other or is to be considered as a mere repetition of any other. 
 
 It would be acknowledged by all that a mental state of pain on 
 account of toothache is different from a state of pleasure in smell- 
 ing a rose ; that the sensation of yellow is unlike the sensation of 
 red, and that these sensations, in common with all sensations of 
 color, are unlike those of touching a cold piece of iron or of hear- 
 ing a musical tone. All the practical activities of men are based 
 upon the conviction that the individual things with which they 
 deal differ from each other not simply in respect to the qualities 
 they have, and the degree in which they severally possess these 
 qualities, but also as separate real beings differ. But the truth 
 
600 MIND AS CONSCIOUS BEING. 
 
 now under consideration reaches further than this common ac- 
 knowledgment. Strictly speaking, from the point of view taken by 
 an analysis of consciousness, the same so-called " thing " is never 
 in experience twice the same. Every time that it exists before the 
 mind, as a presentation of sense, it is constituted anew by an ac- 
 tivity of the mind. Unless various localized sensations and re- 
 membered images of sensations are " synthesized " under the laws 
 which govern such kinds of mental phenomena, there can be no 
 presentation of sense ; and the only " thing " which has immedi- 
 ate existence for us is the presentation of sense. Accordingly, the 
 variety of the mental phenomena, when we begin the attempt to 
 classify them, appears as great as that of all the individual acts 
 and states of consciousness, whether those acts and states have 
 reference to comparatively unchanging beings outside of conscious- 
 ness or not. 
 
 Even a description of the different kinds of mental phenomena 
 which psychological science proposes to explain would be impos- 
 sible, if the foregoing truth were not merely one side of the whole 
 truth. For what is infinite in variety and always changing its kind 
 cannot be described. There could then be no science of the mental 
 phenomena, no classification of the states of consciousness, or ex- 
 planation of them by relating them to each other and to the physical 
 basis on which they are supposed to rest. 
 
 13. Observation of the phenomena of consciousness, however, 
 shows us that they are plainly classifiable, and that the authority on 
 which this classifying reposes is immediate and indisputable. For 
 the phenomena of consciousness are directly recognized in con- 
 sciousness as like or unlike with varying degrees of similarity and 
 dissimilarity. They are assigned by everyone possessed of a de- 
 veloped experience to this or that class, without any question that 
 the act of classification is legitimate and correct. The number of 
 distinguishable colors, or kinds of visual sensations, is indeed in- 
 definitely large ; and so is the number of sensations of musical 
 tone. The power to make all the necessary discriminations in these 
 sensations varies greatly with different persons. It is perfectly 
 proper to say that the number of kinds of the states of conscious- 
 ness arising through the senses of sight and sound varies greatly 
 in different individuals. There are many more colors and tones 
 for some men than for others. The qualities belonging to the sen- 
 sations which follow excitation of the skin are also diverse ; and, 
 in the case of sensations of smell, the means of classification for 
 each individual are limited chiefly by the number of the different 
 smellable things with which he has happened to come in contact. 
 
DIFFERENCES IN PHENOMENA. 601 
 
 It must also be confessed that the popular classifications of the 
 states of consciousness, as based on conscious experience itself, is 
 not infrequently erroneous. So far as the sensations are concerned, 
 this is partly due to the fact that they are ordinarily referred to some 
 thing or to some part of the body, which is only approximately cor- 
 rect, and yet correct enough for practical purposes. The taste of 
 the onion is no less clearly distinguished from other states of sensa- 
 tion arising through the mouth because the subject of the taste does 
 not know that his sensations are in fact largely sensations of smell. 
 Because one believes that one tastes the pepper instead of feels it 
 with the tongue, one has lost none of one's power to make clear dis- 
 criminations in the domain of consciousness. Nor is this power 
 taken from the color-blind, although the length of their color-spec- 
 trum is shortened so that they have a less extended domain of sen- 
 sations of this order within which to discriminate. 
 
 It appears, then, that likenesses and unlikenesses in the phe- 
 nomena of consciousness, and an immediate "awareness" so to 
 speak of these likenesses and unlikenesses, or direct discrimina- 
 tion by consciousness of the quality of the states of consciousness, as 
 such, are involved in that classification of the mental phenomena 
 which all men make. If the classification becomes more refined 
 and elaborate, it at no time ceases to repose on the same founda- 
 tion. All classifications of mental phenomena which have any other 
 foundation, or which introduce any other consideration than the 
 simple one of how I, the conscious subject, am affected, are not 
 really classifications of mental phenomena as such. Such alleged 
 classifications may indeed tell us much that scientific inquiry de- 
 sires to know about the origin or correlations of the mental phe- 
 nomena. They may assume to say what special part of the external 
 organ of sense, or what special area of the brain, is more direct- 
 ly concerned in producing the physical state which precedes or 
 causes the mental phenomena. They may demonstrate what kind 
 of physical stimuli result, respectively, in exciting such and such 
 states of consciousness. But they tell nothing whatever as to what 
 the phenomena really are, whether like or unlike, or as to how they 
 may be ranged and rated together. They are classifications of the 
 causes, or occasions, or physical excitements of the mental phe- 
 nomena ; they are not classifications of mental phenomena, as such. 
 
 14. Moreover, the principles which regulate the formation of 
 classes among the conscious states of the rnind are very different 
 from those which regulate the proposal to classify the elements of 
 the physical basis of these states, or the molecular activities of 
 these elements. To be sure, the Young-Helmholtz theory of vis- 
 
602 MIND AS CONSCIOUS BEING. 
 
 ual sensations proposes to distinguish reel, green, and yellow as 
 the three kinds of retinal elements ; and the opto-chemical theory 
 of vision speculates as to the classes of pigments connected with 
 the seeing of various colors. It has been held by some physiolo- 
 gists that the two main parts of the inner ear (vestibule and coch- 
 lea) correspond to the two principal classes of sensations of sound 
 (to noises and musical tones) ; or even that the vibration of an in- 
 dividual nervous element in the organ of Corti represents, in some 
 sort, each definite one among the many states of consciousness that 
 constitute our hearing of a melody. Sensations of feeling may be 
 classified as of peripheral or central origin, in reference to the 
 place from which the nervous impulses occasioning them are sup- 
 posed to take their rise. But one need not be deceived by all this. 
 The " red " retinal elements are not themselves red ; and if they 
 did appear of this color to external observation, it would not in the 
 least degree help us the better to understand what is the quality 
 of that state of consciousness which we call the sensation of red. 
 The different nervous elements of the organ of Corti may be capa- 
 ble of being arranged in a scale ; if so, they are classified as longer 
 or shorter, and as arranged in space one above the other. But this 
 classification is in no real respect like that which we make when we 
 arrange the states of consciousness called musical tones in a scale 
 of their pitch. 
 
 In general, the physical elements of the nervous system are di- 
 visible into classes according to their chemical constitution as, 
 for example, the phosphorized and the non-phosphorized proximate 
 principles of the brain ; or, according to their structural form, into 
 nerve-fibres and nerve-cells of a great variety of shapes, sizes, etc. ; 
 or, according to their physiological function, into afferent and effer- 
 ent, reflex or inhibitory or automatic. But no such principles of 
 division can be carried over to the mental phenomena. They have 
 absolutely no applicability to these phenomena, as such. We cannot 
 introduce distinctions of chemical constitution, structural form, and 
 physiological function into sensations and ideas, into feelings and 
 thoughts and acts of will. A phosphorized thought, or a stellate 
 feeling, or an afferent thought, are phrases that have no meaning. 
 
 Nor should we be any better able to apply the principles by which 
 we classify the different factors of the physical basis of conscious- 
 ness to the phenomena of consciousness itself, in case we had a 
 much more minute acquaintance with those factors. Let it be sup- 
 posed that psycho-physical science should so far advance as to be 
 able to tell precisely what kinds of molecular activities of what 
 nervous elements are correlated with all the various classes of 
 
PRINCIPLES OF CLASSIFICATION. 603 
 
 mental phenomena. The latter would still remain precisely such 
 as they are in kind ; they would still have to be classified, if at all, 
 on the ground of their likenesses and unlikenesses as directly 
 known in consciousness. If it should be discovered that the mo- 
 lecular agitation of a particular group of fibres and cells in the or- 
 gan of Corti is necessary in order to produce the sensation of the 
 musical note a, and that this agitation resembles that which some- 
 what similar fibres and cells in the retina undergo preceding the 
 sensation of a particular shade of green, the two sensations would 
 then be no easier to classify together than they are now. Both 
 stand near the middle of their respective scales. In that respect 
 it might be said that the sensation of the musical tone a' is like the 
 sensation of green. This point of likeness is really, however, one 
 that appertains solely to a physical quality or function in certain 
 bits of nervous matter more or less directly connected with the 
 physical antecedents of the sensations ; it cannot be conceived of 
 as appertaining to the sensations themselves. 
 
 15. Considerations derived from the theory of the quantity of 
 stimuli and their resulting intensities in the states of conscious- 
 ness might be urged in the same direction. The variations in the 
 amount of the molecular processes within the nervous system are 
 all measurable and classifiable, if at all, on principles with which 
 physics is familiar. The excitation in the nervous system always 
 consists simply of so many molecules, having such a chemical con- 
 stitution, moving so much in such a direction with relation to each 
 other. But the most nearly related mental phenomena are feelings, 
 and judgments comparing feelings, such as can be expressed only 
 somewhat as follows : " Now that shade of red seems to me a little 
 more intense ; " or, " now I think something has been added to the 
 sensation of pressure ; " or, " I should call this note the major 
 third of the other." In general, we may say that, quantitatively 
 considered, the changes in the nervous mechanism are all precisely 
 alike as to kind that is, are all that mode of motion called "nerve- 
 commotion " and admit of being measured off by a common phys- 
 ical standard or unit of measure. But the mental phenomena are 
 classifiable only as a succession of states, that are continually chang- 
 ing their quality and have no quantitative measure which we can 
 separate from these changes of quality. Nerve-commotions can be 
 conceived of as added and subtracted, switched to the right or to 
 the left, concentrated, distributed, dissipated. But, strictly speak- 
 ing, the resulting mental phenomena admit of no treatment of this 
 sort. Every sensation exists, if at all, as an indivisible qualitative 
 state of consciousness ; it cannot even, as such state, be retained to 
 
604 MIND AS CONSCIOUS BEING. . 
 
 be compared with some other succeeding state, in order to decide 
 whether it is more or is less in respect to quantity*. Without 
 memory and comparison, which are complex activities of mind dif- 
 ferent from the mere having of sensation, we could not affirm of 
 any mental state that it is more or less than another. 
 
 Still further, other forms of mental activity which have an un- 
 doubted reality do not admit of even the loose application to them 
 of terms of quantity which is proper when speaking of sensations 
 and feelings. A " weighty" judgment and a " high " ideal are not 
 to be measured by standards that admit of correlation with the in- 
 creasing or diminishing swing of nervous molecules. Yet if those 
 classes of the phenomena of consciousness which these words imply 
 have, as such, any physical basis whatever, this basis must consist 
 in some form of nerve-commotion as to the quantity of which an 
 exact measurement is conceivable. If, however, a wide swing of 
 the molecules were found to go with a good, sound, clear judg- 
 ment, and a contracted swing to be the physical cause of narrow- 
 ness of mental apprehension, such a fact would not help us in any 
 regard the better to revise the Aristotelian classification of the syl- 
 logism. Reasoning, as a mental activity, would be deductive or 
 inductive, analytic or synthetic, as before, after making the discov- 
 ery that the one process is connected with a continual diminution 
 of the cortical areas over which the nerve-commotions spread them- 
 selves, and the other with a noble diffusion of such commotions 
 over an ever-widening expanse of the brain. 
 
 16. It is an undoubted fact, however, that mental phenomena 
 admit of a fairly satisfactory arrangement into classes. The ar- 
 rangement can be made only upon the basis of their likenesses and 
 unlikenesses as known in consciousness and by consciousness. No 
 other arrangement of these phenomena, as such, is possible none 
 that is not founded upon the same ultimate facts thus distinguished. 
 All attempts at an arrangement of them by other principles, and 
 from other points of view, result in the classification of something 
 else (such as the physical antecedents, causes, or concomitants) 
 than the phenomena themselves. It is not strange, then, that the 
 " old psychology " won its principal triumphs, by the method of 
 introspection, within the field of classification. The crude begin- 
 nings of a physiological psychology in the phrenology of Gall and 
 Spurzheim were obliged at that time to accept from introspective 
 psychology its division of the mind's activities into so-called " fac- 
 ulties." In its more scientific and experimental form at the pres- 
 ent time physiological psychology is just as dependent as ever 
 upon introspective psychology for a classification of the mental 
 
KINDS OF MENTAL PHENOMENA. 605 
 
 phenomena. The introspective science of mind has already arrived 
 at very general agreement upon this point. 
 
 It is very generally agreed that all the mental phenomena are 
 classifiable under three great heads into phenomena of knowing, 
 phenomena of feeling, and phenomena called acts of will. The 
 distinction of the two latter classes of phenomena is, indeed, a mat- 
 ter established in comparatively recent times. A certain number 
 of investigators, who use the method of introspection, still venture 
 to affirm that acts of feeling in the form of desire are not to be dis- 
 tinguished as differing in kind from acts of will. In their view, 
 so-called acts of will are resolvable into phenomena of feeling. 
 But the opinion of the great majority of students of psychology 
 is decidedly in favor of adhering to the threefold division of the 
 mental faculties. There is also a large amount of agreement as to 
 the sub-classes that fall under these three at any rate, with the 
 exception of the varied efforts made to deal with the very complex 
 phenomena of feeling in its many forms. The more recent at- 
 tempts (made especially by certain English writers on psychology) 
 to depart from the accepted classifications of the " old psychology" 
 have attained little or no valuable result. These attempts have in- 
 troduced into psychology a great number of uncouth terms, derived 
 largely from false analogies of physical science, which tend to rep- 
 resent the case as though sensations could be weighed or measured 
 or compounded like the nervous shocks which cause them, and as 
 though ideas could become " agglutinated " or "agglomerated" 
 like globules of mercury or minute particles of water. Such at- 
 tempts, however, have thrown no light on the nature of the mental 
 phenomena, or on the question of their correct classification ; they 
 have not really succeeded in supplanting or discrediting the classi- 
 fications of the " old psychology." Nor is it at all likely that the 
 principal classes into which all mental phenomena are now thrown 
 will ever be changed. The " faculties" of the mind however the 
 term is to be understood will remain the same. At all events, if a 
 change should be made in these divisions, such change could only 
 be accomplished by the method of introspection. 
 
 17. The question may now be raised : How are we to account 
 for facts like the foregoing? In attempting an answer to this 
 question, the great significance of the facts themselves should be 
 recalled. It has been shown that mental phenomena cannot be 
 conceived of as identical with the molecular motions of the nervous 
 mass ; and that the fundamental relations between the two cannot 
 be expressed by the statement, the phenomena of consciousness are 
 the product of the brain in any meaning of the word " product " 
 
606 MIND AS CONSCIOUS BEING. 
 
 which can be clearly defined. Nor can the theory be accepted that 
 every mental state and process has its exact equivalent, with respect 
 to all its factors, in some antecedent or concomitant state or pro- 
 cess of cerebral nerve-fibres and nerve-cells, and that therefore all 
 mental phenomena are to be directly and exclusively referred to 
 these physical structures as their sole subject or ground. More- 
 over, it has been found necessary to admit that mental phenomena, 
 as such, can be classified only by introspection ; and that the prin- 
 ciples on which the classification must be based differ from those 
 applicable to the nervous substance, while the actual classes discov- 
 ered by the only available method do not correspond to any of 
 those with which physics and physiology make us familiar. And 
 yet there is no insuperable difficulty in classifying the phenomena 
 of mind. Introspective psychology furnishes us with a classifica- 
 tion, on the whole, tolerably satisfactory 
 
 What, then, is the fair inference from all these facts with respect 
 to a decision between the two theories of mind which were previ- 
 ously proposed ? (See 3. ) Plainly, such inference favors looking 
 toward some other subject or ground of the mental phenomena 
 than the nervous substance of the brain. Its result commends sub- 
 stantially the same view as that held by the great majority of man- 
 kind. We shall state and explain this view, however, in different 
 terms from those employed by this majority. 
 
 The phenomena of human consciousness must be regarded as activi- 
 ties of some other form of Eeal Being than the moving molecules of 
 the brain. They require a subject or ground which is in nature 
 unlike the phosphorized fats of the central masses, the aggregated 
 nerve-fibres and nerve-cells of the cerebral cortex. This real be- 
 ing, thus manifested immediately to itself in the phenomena of 
 consciousness, and indirectly to others through the bodily changes, 
 is the Mind. To it the mental phenomena are to be attributed as 
 showing what it is by what it does. The so-called mental "fac- 
 ulties " are only the modes of the behavior in consciousness of this 
 real being. We actually find, by the only method available, that 
 this real being called Mind behaves in certain perpetually recur- 
 ring modes ; therefore, we attribute to it certain faculties. The 
 mental faculties, then, are not entities that have an existence of 
 themselves ; nor are the individual behavings of the mind (the 
 so-called " ideas ") existences that can become " agglutinated " 
 or " associated " or " compounded " in any way. They are not 
 divisions of the mind ; nor are they pouters of the mind, if by 
 this word be meant some permanent recognizable reality, stored 
 up in a spiritual subject, or attached to it or inherent in it, after 
 
THE UNITY OF CONSCIOUSNESS. 607 
 
 the analogy of the relation of physical forces to their subjects, the 
 atoms. The faculties of the mind are the modes of the behavior, 
 in consciousness, of the mind. And the very nature of the classi- 
 fying acts which lead to their being distinguished is explicable 
 only upon the assumption that a real being called mind exists, and 
 is to be distinguished from the real beings known as the physical 
 molecules of the brain's nervous mass. 
 
 That the subject of the states of consciousness is a real being, 
 standing in certain relations to the material beings which com- 
 pose the substance of the brain, is a conclusion warranted by all 
 the facts. That the modes of its activity in consciousness are cor- 
 related under law with the activities of the brain-substance is a 
 statement which Physiological Psychology confirms ; one upon 
 which, indeed, it is largely based. It will be our task, in a subse- 
 quent chapter, to consider under what general terms such correla- 
 tion may best be expressed. All physical science, however, is based 
 upon the assumption that real beings may have an existence such 
 as is sometimes called " independent," and yet be constantly re- 
 lated to each other under known or discoverable laws. If this as- 
 sumption could not be made and verified, all the modern atomic 
 theory would stand for nothing but a vain show of abstractions. 
 Upon what grounds of reason or courtesy we may inquire at this 
 point does Materialism decline to admit the validity of similar as- 
 sumptions as demanded by mental phenomena ? 
 
 18. The foregoing view of the mind and its faculties is greatly 
 confirmed by another consideration. Consciousness has a certain 
 remarkable unity. If the complexity of mental phenomena is be- 
 wilderingly great, the unity of consciousness is striking and unique. 
 Many disputed questions may be raised touching the essential nat- 
 ure of this unity, the means we have for recognition of it, and the 
 inferences which may legitimately be drawn therefrom. Some of 
 these questions will be merely alluded to at this point, and their 
 further consideration postponed until later on. 
 
 All developed forms of consciousness involve an attribution of the 
 present -particular state of consciousness to a subject of the state. 
 It is for this reason, as has already been remarked, that we express 
 each state of our consciousness by saying : "/am in such or such 
 a condition ; " "/feel thus and so ;" "/ see, or hear, or smell, or 
 taste, or think, or plan," etc. Such language and all language de- 
 signed to describe our mental phenomena is such plainly shows 
 that some kind of distinction is made by everyone between the 
 state and the "ego" which is the subject of the state. It is no 
 adequate explanation of this fact to say that by the subject, in all 
 
608 MIND AS CONSCIOUS BEING. 
 
 these sentences (the "I" to which the states are attributed), we 
 mean to denote a mere concept of myself formed upon the basis 
 of past experience. Such a concept may indeed be formed. Its 
 completeness and correctness furnishes an excellent test of the 
 amount of development attained by each individual in self-knowl- 
 edge. Few individuals, however, would be found able to give a 
 statement, at all satisfactory to themselves or to others, of just what 
 characteristics are to be considered peculiar to their "self -hood." 
 We do not, then, merely designate the "self" when we thus con- 
 stantly refer to the " I " which is the subject of each state. Our 
 knowledge that the state is our state, or that we are in this definite 
 individual state, is perfectly clear and immediately conclusive with 
 reference to all the experience we have or can remember. 
 
 The clearness and imrnediateness of the reference which we are 
 continually making of our states to the subject of them all is in 
 marked contrast with the obscurity and indirect nature of the con- 
 cept we are able to form as to what manner of persons we are. 
 Moreover, all the ability we have to frame a concept of the " self " 
 is dependent upon this constant, primary, and inexplicable fact of 
 a possible reference of each state to the subject of the state (the 
 "I"). To explain this reference, we have to assume that it has 
 already been made ; we have to assume it in each attempt at ex- 
 planation. We may express the absurdity of the effort to think or 
 imagine ourselves out of the reach of this form of all consciousness 
 by asking ourselves such questions as follow : How can there be 
 a pain, a sensation, a thought, an act of will, that is not somebody's 
 pain, sensation, thought, or act of will ? What is a state of con- 
 sciousness considered as separable, or actually separated, from a 
 subject of such state ? That is to say, no state of consciousness 
 can even be conceived of that does not involve this same reference. 
 There may, indeed, be great doubt whether some of the lower 
 animals ever make any such reference. It may be that the amceba 
 or the oyster can have a sensation that is not, quoad sensation, the 
 conscious state of the amceba or the oyster. As to this we cannot 
 say. But we can say that if such a so-called sensation is possible 
 for any animal, it is impossible for us to imagine it. We cannot 
 imagine what we cannot bring under the unchanging forms of our 
 own consciousness. 
 
 The force of the foregoing remarks is not destroyed by calling 
 attention to the fact that the attribution of the state of conscious- 
 ness to the subject of the state, to the "I," is by no means con- 
 tinually being made. It is plain that some distinction must here 
 be drawn between being conscious and being se/f -conscious. The 
 
KEFERENCE TO THE EGO. 609 
 
 crowd intently watching a tragedy in a theatre, or a conflagration, 
 is certainly not unconscious, but is rather in an exalted state of 
 consciousness ; on the contrary, he who is intently watching the 
 spectacle is not at all, or is only in a slight degree, s^f-conscious. 
 There are considerable periods of every day when, so far as we can 
 remember, we have been "conscious" (not being in profound slum- 
 ber or having fainted away), but with little or no reference in con- 
 sciousness of its activities to the subject of them all. Nevertheless 
 there can be no doubt that we are capable of this reference ; that 
 it is found to be involved in every mental state just so soon as we 
 seek to determine the factors of such state ; and that to recognize 
 its being there is essential to any explanation of the nature of 
 mind. 
 
 19, All the different mental phenomena of an individual must 
 be regarded as states of one consciousness ; they are all said to occur 
 in the unity of consciousness. There can be no doubt that every 
 person (with the exception, at most, of certain rare cases of so- 
 called double consciousness) attributes all the forms of his con- 
 sciousness to one and the same subject. This is what is meant by 
 saying that he regards them all as his states. We cannot conceive 
 of ourselves as dubitating whether some particular pain or pleasure, 
 or act of memory or of imagination or of will, present in conscious- 
 ness, is to be ascribed to our ego or not. We cannot attribute any 
 such state to some other than our own ego. It is true that in cer- 
 tain cases of disease or lesion of the brain an abnormal condition 
 in this respect seems to occur. The one person sometimes seems 
 to pass back and forth between two mental lives, which are so 
 distinct from each other that they may well be said to belong to 
 two personalities. But reflection upon these abnormal cases only 
 makes the stronger and clearer our conviction as to the unity of 
 consciousness. Living two seemingly distinct mental lives is not 
 possible without its being assumed that each one of the two is lived 
 in the unity of its own consciousness. This would be as true of 
 twenty distinct lives, if they followed each other in the case of any 
 individual as the result of disease of the brain, as it is of two such 
 lives. 
 
 It is not at all surprising that the fact of the primary unity of 
 consciousness should be inexplicable ; for it is itself the fact im- 
 plied and assumed in all attempts at explanation of other mental 
 facts. Were this the line of thought necessary to introduce at this 
 point, it might be shown that all the unity possessed by " Things " 
 is dependent upon the unity of consciousness. Without memory 
 and judgment there can be no perception of Things. But the bind- 
 
610 MIND AS CONSCIOUS BEING. 
 
 ing force of memory is dependent upon this unity. We cannot 
 remember that which has not in some form or other been previously 
 present in consciousness, in our own consciousness, in one and the 
 same consciousness as that in which the remembered image is now 
 present. We cannot judge except by uniting two terms in one con- 
 sciousness. 
 
 Of course, all language as to the unity of consciousness, when 
 carefully examined, turns out to be figurative, and to have no mean- 
 ing except as interpreted over from entities and relations of a 
 material sort into terms of consciousness. By the " unity of con- 
 sciousness " it cannot be meant that consciousness is some kind of 
 an entity which remains one and unchangeable throughout, like 
 those atoms which physical science has 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 be- 
 haves itself in exactly the same way under precisely similar circum- 
 stances. But this mental picture would itself have any unity be- 
 longing to it only as it existed in the unity of consciousness. It is 
 this unity which makes each " Thing "to be one thing ; it is this 
 unity which imparts to all else that is one whatever unity it may 
 have. 
 
 W T hen, 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 understood to be states of one and the 
 same subject. This reference and this understanding enter into 
 ah 1 our experience ; they give conditions to experience and make it 
 possible. Whatever changes experience may be conceived of as 
 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 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 consciousness cannot be con- 
 
KNOWLEDGE OF SELF INDIRECT. 611 
 
 ceived of as occurring without being referable to some one subject 
 as its modes or states. 
 
 20. 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 immediate 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 look within myself, and by thus looking I can discern 
 what I really am. I immediately know (that is, know by the intro- 
 spective act of self-consciousness) that "7" am always, however my 
 states may change, one and the same real being. I am a real, self- 
 identical entity ; and if asked how I know that I am all this, my 
 appeal is to the indubitable evidence of the act of self-conscious- 
 ness. 
 
 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- 
 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 
 
612 MIND AS CONSCIOUS BEING. 
 
 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. 
 
 21. The grounds on which depends the assumption, that the 
 subject to which all the phenomena of consciousness are actually 
 referable is one real being, will be considered more in detail at 
 another point. For the present we merely adopt the assumption, 
 provisionally, as much more probable than any which accounts for 
 our conscious reference to such subject, by enumerating certain 
 possible relations into which the masses and molecules of the brain 
 may be thrown as conditions of the empirical unity of conscious- 
 ness. That there is such empirical unity of consciousness there 
 can be no dispute. Dispute itself would assume it. It scarcely 
 admits of more doubt that all physical theories to account for this 
 unity are wholly unsatisfactory. We know, indeed, certain of the 
 physical conditions and concomitants of consciousness. If ox} r gen- 
 ated blood is shut off from the cerebral substance, consciousness 
 disappears. If the blood has floating in it certain drugs, or prod- 
 ucts of the combustion of tissue, consciousness is disturbed. If 
 certain cerebral areas are injured or eradicated, the psycho-physi- 
 cal basis of certain forms of consciousness is altered. Still, all 
 this does not seem to bring us a step nearer a satisfactory physical 
 account of the unity of consciousness. The molecules of the brain 
 are infinitely numerous ; they are made up into structural forms of 
 indefinite number and variety ; the kinds of the relations they as- 
 sume toward each other are indescribably many. Consciousness, 
 so far as we know, has no special centre or seat within the cere- 
 brum ; and if it had, the constituents and activities of that centre 
 would have to be exceedingly manifold and complex. Now, all 
 this is precisely the opposite of what we should expect of a phys- 
 ical structure which should be called upon to exhibit the phenom- 
 ena of many conscious states, all referable to one subject. No help 
 toward solving the problem is derived from calling attention to 
 the fact that the different portions and elements of the brain are 
 all interconnected. The connecting structures only add still fur- 
 ther to its multiplicity and complexity of elements. It would be 
 easier to conceive of an atom as becoming conscious than the cer- 
 ebral cortex. 
 
 22. But surely the assumption that one real being is the sub- 
 ject of these states, which are certainly all referable in conscious- 
 
MIND A EEAL UNIT-BEING. 613 
 
 ness to one and the same subject, is not an impossible one. On 
 the contrary, it is the most natural assumption ; it is the assump- 
 tion instinctively made by men in general. Notwithstanding the 
 difficulties which encompass it as soon as we attempt to define it, 
 or to test the ground on which it rests, we shall find that it is de- 
 fensible and valid. 
 
 We conclude, then, from the previous considerations : The sub- 
 ject of all the states of consciousness is a real unit-being, called Mind ; 
 which is of non-material nature, and acts and develops according to 
 laws of its own, but is specially correlated with certain material mole- 
 cules and masses forming the substance of the Brain, 
 
CHAPTEE II. 
 THE DEVELOPMENT OF THE MIND. 
 
 1. 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. 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 value of this historical and 
 genetic way of studying everything. Any complex existence is cer- 
 tainly far better understood after it has been not simply analyzed 
 into its present component parts, but has also been traced back to 
 its most nearly primitive and undifferentiated stages. The history 
 of the egg explains the bird even more than the nature of the bird 
 explains the egg. 
 
 Both of the two subjects, with whose correlations Physiological 
 Psychology deals, require for their most satisfactory understanding 
 to be studied by this genetic method. The structure of the nervous 
 system appears in a new light when regarded as the result of a pro- 
 cess of evolution. Beginning with the unimpregnated 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 differentiated 
 from the other portions. By vital processes kept up through nutri- 
 tion 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 this modern 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 a 
 
MENTAL FACULTIES PROGRESSIVE. 615 
 
 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 exclude 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 in- 
 tervals in its entire history. Although we can never reproduce 
 in adult self-consciousness the forms of the earliest stages, we 
 can show that these forms differed greatly from those taken in 
 this adult self-consciousness. We can show that the earlier forms 
 must have been much the simpler. For example, an analysis of 
 the presentations of sense shows that the " things " of developed 
 experience are resolvable into certain elements of sensation which 
 the mind has learned to localize. In other words, perception 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. 
 
 2. So far as the connection of mental phenomena with the in- 
 creasing complexity of the nervous activities, and with the stored 
 energies and hardening habitus of the nervous elements, affords any 
 explanation of the development of the mind, we have already said 
 all that is necessary. The growth of the mind in the acquirement 
 and arrangement of sensations, in the recalling of ideas, in the 
 forming of judgments about objects of sense, etc., is plainly de- 
 pendent upon the evolution of the bodily members. But the nat- 
 ure of the relation which exists between the mental phenomena 
 and the nervous mechanism, so far as this can be learned by study- 
 ing 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. On the one hand, the 
 attempt is made to refer all the so-called development of the mind 
 to the evolution of the substance of the brain, under purely physi- 
 cal and mechanical causes. This attempt, then, denies that any 
 real unit-being called the Mind needs to be assumed as undergoing 
 a process of development according to laws of its own. It cannot 
 be disputed that many facts of experience tend to strengthen such 
 an attempt. 
 
 There is a general correspondence, with respect to the complex- 
 ity and quality of the work done, between the different stages in 
 
616 MIND AS A DEVELOPMENT. 
 
 the development of both body and mind. Nervous system and 
 mental condition are both immature in infancy ; both develop 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 keep pace in their decline. Moreover, cases of arrested 
 development of brain are cases of arrested development of mind. 
 Idiots are frequently microcephalic ; many of them have brains 
 weighing less than thirty ounces. Degeneracy of the tissues of the 
 cerebral hemispheres is commonly connected with increasing de- 
 generacy of the mind. As the tides of molecular nerve-commotion 
 rise and fall in the nervous mass, so rise and fall the tides of men- 
 tal vigor. A temporary increase of cerebral action, caused by a 
 glass or two of wine, is expressed in the form of mental phenomena 
 by a heightening of imagination, a quickened flow of ideas. What 
 need, then it is asked of assuming any permanent subject of 
 what we regard as mental development, other than the mechanism 
 of physical molecules with its evolution under the control of physi- 
 cal law ? 
 
 On the other hand, all attempts to account for the orderly in- 
 crease in complexity and comprehensiveness of the mental phe- 
 nomena by tracing the physical evolution of the brain are wholly 
 unsatisfactory to many minds. We have no hesitation in classing 
 ourselves among this number. Those facts of experience which 
 show a correspondence in the order of the development of the body 
 and the mind, and even a certain necessary dependence of the lat- 
 ter upon the former, are, of course, to be admitted ; but they are 
 equally compatible with another view of the mind's development. 
 This other view has the additional advantage that it makes room 
 for many other facts of experience which are very difficult of rec- 
 onciliation with any materialistic theory. On the whole, the his- 
 tory of each individual's experience is such as requires the assump- 
 tion that a real unit-being (a Mind) is undergoing a process of de- 
 velopment, in relation to the changing condition or evolution of 
 the brain, and yet in accordance with a nature and laws of its own. 
 
 3. That the development of a real non-material being is im- 
 plied in the history of the mental phenomena of each person mny 
 be argued on two principal grounds. In the first place, it may be 
 shown that the stages and laws of the development of mind do not 
 fully correspond to those which are observed on tracing the evolu- 
 tion of the nervous system. It may also be shown that certain ele- 
 ments necessarily enter into the development of mind, which have 
 nothing like them, or strictly correlated with them, in the evolu- 
 
I1EALITY OF MENTAL HISTORY. 617 
 
 tioii of the material mechanism. One real being may be dependent 
 on other beings for its starting, as it were, and for certain factors 
 that enter into its growth or furnish the indispensable conditions 
 of its growth. It may thus receive the form and direction of its 
 development, in large measure, from these other beings. And yet 
 this fact gives us no right whatever to refuse to such a being all 
 title to take rank among other real existences with a complex nat- 
 ure of its own. No existence loses or impairs its claim to reality 
 by being dependent for its development on other existences. The 
 mind, on the contrary, most indubitably establishes such a claim, 
 because the stages and laws of its unfolding, and some of the 
 factors which necessarily enter into this unfolding, are peculiar to 
 itself (swi generis). 
 
 4. That the words, "development of the Mind," stand for a 
 real process, there can be no reasonable doubt. The sum-total of 
 the conscious experience of each individual is something 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 much more which is companionable and 
 mutually intelligible between the adult man and his dog than be- 
 tween the adult man and his newly born child. The latter is, 
 however, raised at once above the most intelligent animal when 
 we consider the possibilities of its mental development. What the 
 human being is cannot be at all adequately described without con- 
 sidering the nature and limits of that process of becoming which 
 belongs to it. 
 
 There is no doubt, also, that the incomparable improvement of 
 the mental processes which distinguishes the adult from the infantile 
 human being is a true development. Each stage of this improve- 
 ment is dependent upon preceding stages. The changes are all in 
 some sort according to a plan. Thus the life of every individual's 
 mental experience is capable of being made into a history. A cer- 
 tain tolerably uniform order in the relative development of the dif- 
 ferent faculties is discernible. At first the senses are awakened to 
 a lively and varied activity ; then memory and imagination become 
 more prominent ; and, finally, judgment and the reasoning powers 
 assert their sway. Gradually, things become known and conduct 
 
618 MIND AS A DEVELOPMENT. 
 
 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 characteristics of unfolding 
 itself in a regular order, in which all that comes at all comes in 
 due sequence and acknowledged dependence upon what has pre- 
 ceded. This is the very essence of a true development. 
 
 5. Can the development of the mind be explained as merely 
 the resultant or expression of the physical evolution of the nervous 
 system this system being regarded as situated in the rest of the 
 bodily environment, and surrounded by the more extended envi- 
 ronment of the world of active physical energies outside ? Against 
 an affirmative answer to this inquiry stand many facts and laws of 
 all such mental development. In spite of what must be said con- 
 cerning the striking correspondence between the evolution of the 
 bodily organism and the development of the mental powers, it 
 must be held that there are marked divergences as well. At cer- 
 tain epochs of life the evolution of the brain seems to stand far in 
 advance of the mind ; at others, the mind appears to have over- 
 taken and passed by the stage reached by its physical substratum. 
 During a long period of life the growth of mental powers is con- 
 stant and solid, while the growth of the physical basis has nearly 
 ceased, and such changes as are taking place in it appear quite 
 inadequate to serve as correlates for the mental growth. More- 
 over, the most distinctly typical features in the development of the 
 mind remain the same when malformation or disease or accident 
 have largely changed the physical evolution of the brain. 
 
 6. 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 
 conscious states. But it is probable that its antenatal movements 
 are not all purely reflex, and neither accompanied nor directed by 
 conscious sensation, feeling, and volition. 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 compjex mental phenomena. 
 Taste, smell, hearing, and sight are, of course, not to be thought 
 of as entering into such a mental life. Touch, as we understand 
 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- 
 
THE AROUSING OF THE MIND. 619 
 
 ing equally obscure feelings of innervation, as its limbs are moved, 
 must constitute the great part of its experiences. As yet there is 
 no experience, properly so called ; no perception of things, no feel- 
 ing 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 evolution the nervous 
 system of the most intelligent adult animals. Previous 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 infancjr the same relation between the 
 relative development of the body and soul of the child is 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 occasionally 
 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 aroused 
 and controlled. But for some weeks there are only sensations and 
 impressions, without time perceptions ; there is as yet no knowl- 
 edge of any " Thing." This earliest relation of mind and brain, 
 with respect to the degree and rate of their development, is not 
 
620 MIND AS A DEVELOPMENT. 
 
 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 be furnished to the mind as the primary 
 factors in its development. 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 characteristics of spatial series (see Part II., chap. VI.), and so 
 are able to stimulate the mind, and afford it the requisite material 
 for the construction of true presentations of sense. 
 
 7. 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 little surrounding world of objects of sense. It has also 
 begun 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. The as- 
 sumptions 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. 
 
 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 
 development 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. 
 It consists rather in the forming of what has already been alluded 
 to (Part II., chap. X., 18 f.) as " dynamical associations " among 
 the existing elements. The statement that such is the nature of 
 the developing activities of the nervous mechanism, and the as- 
 sumption that such activities are an indispensable physical condi- 
 
FORMATION OF ASSOCIATIONS. 621 
 
 tion for the growth of the mind, may both be taken for granted. 
 But even then the argument is far from complete 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, is denied. The forma- 
 tion of the so-called "dynamical associations" among the mole- 
 cules of the nervous mass furnishes no adequate 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 association among the 
 molecules of nervous substance. It is rather a development which 
 for its very existence requires something different from such asso- 
 ciations. The child might go on forever merely associating together 
 affections of its own mind in correspondence to dynamical associa- 
 tions among the nervous molecules, and yet have no growth of ex- 
 perience 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 respecting 
 certain non-sensuous realities. To account for this boundless ex- 
 pansion 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 im- 
 potency. So far as we really know anything about the development 
 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. 
 
 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 
 imagination from the sudden unfolding of certain bodily organs 
 and powers. The indirect influence of these acts of feeling and 
 imagination 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- 
 
622 MIND AS A DEVELOPMENT. 
 
 cernible development. Nothing that microscope or electrometer 
 can detect distinguishes the brain of the man of twenty-five from 
 that of the man of fifty. A few grammes of weight have perhaps 
 been added to it during this long period of years. Anyone 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 curiosity, 
 attention, careful and comprehensive judgment, sound moral pur- 
 pose, etc., that most men fail to develop during adult life in their 
 mental powers. And these are mental activities for explaining 
 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, 
 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 speculations, 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. 
 
 8. 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- 
 
THE FINAL MENTAL STAGE. 623 
 
 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 mental 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 
 certain emotions and passions whose physical basis is of the most 
 obvious 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 
 decay 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. 
 
 9. 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 consideriDg 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 materialistic theory of mental de- 
 velopment (see Part II, chaps. I. and II). Extensive 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 effects may 
 in time wholly or partially disappear. When such disturbance is 
 permanent, it is not necessarily connected with loss in the power 
 of judgment, in the higher intellectual, aesthetic, and ethical activi- 
 ties of feeling, intellect, or will. Even where aphasia is so severe 
 
624 MIND AS A DEVELOPMENT. 
 
 as to include the loss of all power to utter or understand articulate 
 language, the patient may still show a good degree of mental acute- 
 ness by ability to make calculations or play games of skill. 
 
 On the other hand, the much more serious interruption or com- 
 plete loss of mental development may occur when no adequate ex- 
 planation can be detected in the disturbance or arrest of cerebral 
 development. It is, of course, natural to conjecture that, in all 
 this latter class of cases, more accurate information would show us 
 some diseased 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 detecting, 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. 
 
 10. All the foregoing considerations suggest the conclusion 
 that the mind is primarily and chiefly dependent in its develop- 
 ment upon the nervous mechanism for furnishing and directing 
 the combination and order in recurrence of those sensations which 
 enter into all presentations of sense. Let any person of normal 
 and sane brain and mind consider how intimate and extensive is 
 the connection between his sensations and his whole mental devel- 
 opment. Fickle and confused experience of sensation involves 
 fickleness and confusion of judgment on all matters of sense. Loss 
 of any class of sensations, as a whole or in part, involves a distinct 
 impairment of mental powers. Such loss necessarily changes to a 
 considerable extent the character of the subsequent mental life. 
 Such loss, however, is regularly compensated for, to some extent, 
 by an increase of mental activity along the lines which remain open 
 to the mind. The man blind from birth can never have the same 
 course of mental unfolding, with respect to his perceptions of 
 things, his idea of space, his feelings before the beautiful, etc., as 
 that open to his more fortunate fellow. But he is not necessarily 
 inferior in mental capacity and activity ; because the development 
 of his mind, as conditioned upon the other senses, proceeds with 
 the ordinary pace, although along a different path. 
 
 The mind is absolutely dependent upon the nervous organism for 
 its awakening and furnishing in the life of conscious sensation. 
 The case of Laura Bridgman, and others similar, show how large 
 mental development is possible with even a greatly diminished out- 
 fit of the senses. But, of course, if touch and muscular sensation, 
 as well as smell, taste, hearing, and sight, were lacking, no con- 
 
RELATION OF THE FACULTIES. 625 
 
 scious mental life would be possible in any form known to human 
 experience. The form in which the sensations shall combine, and 
 the time-order of their recurrence, are also dependent upon the 
 character, number, and succession of the cerebral excitations 
 caused by external or internal stimuli. But when once the mind 
 is started upon its career of unfolding its powers, it maintains a 
 relative independence of its physical basis. Not that sensations and 
 resulting presentations of sense, together with the reproduced im- 
 ages of such mental products, do not always continue to furnish 
 indispensable factors and conditions of all mental development. 
 In the most abstract thought, and in the highest nights of the im- 
 agination, the mind never wholly gets away from the world of sen- 
 sation and perception, with its immediate dependence upon the 
 activities of the physical and nervous basis. On the other hand, 
 the course and extent of its unfolding are such as to show that its 
 stages and laws do not all correspond to those which characterize 
 the evolution of this basis. Its general dependence upon such 
 basis, in all its development, is through the sensations and their 
 reproduced images. 
 
 11. Several references to the second argument (comp. 3) 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 mind which have nothing similar to 
 them, or strictly correlated with them, in the evolution of the ma- 
 terial mechanism. The mind can, indeed, undergo no development 
 except as conditioned upon these elements. But the elements 
 themselves cannot be regarded as the expression in consciousness 
 of merely physical causes, or as flowing necessarily from more 
 primitive activities of the mind which may possibly be regarded as 
 the expression of such causes. 
 
 All three of those fundamental forms of activity which are recog- 
 nized in the ordinary threefold division of the soul into faculties 
 namely, acts of feeling, acts of knowledge, and acts of will neces- 
 sarily enter into the development of the mind. Its development 
 consists in the unfolding of these three classes of acts, in their 
 mutual dependence and according to the laws which belong to each. 
 Among each of these three great classes of acts there are certain 
 subordinate kinds that defy all attempts whatever to correlate them 
 with the changes in the nervous mechanism, or to explain them as 
 necessarily or actually arising out of such physical changes. Such 
 are the feeling of moral obligation, the sentiment of justice, the 
 love of truth, and certain of the higher aesthetic feelings. Among 
 the acts of knowledge, such are the mind's relating activity, its use 
 40 
 
626 PSYCHICAL FACTORS SUPREME. 
 
 of the principle 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 manifestation of an invisible world of real beings, with per- 
 manent properties and forces, acting and reacting under law. 
 Such, also, are the acts of deliberate choice among courses of con- 
 duct, under the influence of moral considerations the 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 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 
 process which can account for the sui generis character of the re- 
 lating 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 
 indispensable elements of mental development and it is obvious 
 that they do this development cannot properly be accounted for 
 by 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 real being of another than the 
 physical order. This being must be thought of as stimulated by 
 the rise and recurrence of sensations and images of past sensations, 
 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. 
 
 12. The trial will doubtless be made to escape from the con- 
 clusions hitherto reached, by means of help derived from a certain 
 psychological theory of the development of mind. It may be ad- 
 mitted that the attempt to find, directly, an adequate physical basis 
 for all these so-called higher faculties, or modes of the behavior of 
 
MECHANICAL THEOEY OF MIND. 627 
 
 the mind in its development, must be abandoned. But the higher 
 faculties themselves it is said are to be regarded as develop- 
 ments of the simplest activities. These highest faculties of all may 
 then be directly connected with the evolution of the body, or of 
 the cranial mass, through the simplest mental activities. In this 
 way a kind of necessitated psychical mechanism is set up, which is 
 itself entirely explicable as a development from one kind of element 
 (the sensation) ; and then, by regarding this one kind of element 
 as connected with the motion of nervous molecules in a purely 
 mechanical way, the need is obviated of supposing any real being 
 called Mind as undergoing a process of mental development. 
 
 For example, it may be claimed that the one simple and undif- 
 ferentiated element of all psychical experience is the "nervous 
 shock." This nervous shock is merely the simplest expression or 
 result in consciousness of a nerve commotion set up by the action 
 on the nervous mechanism of external or internal stimulus. By 
 differentiation and combination of the nervous shocks, the so-called 
 simple sensations arise. By reproduction of similar combinations 
 of fainter shocks, the images of memory are produced. By " ag- 
 glutination " and " agglomeration " of the sensations and ideas, 
 judgments take place only, since some new kind of idea does cer- 
 tainly seem to be involved in the essence of judgment, it must be 
 held that a "feeling of relation" is somehow slipped in between the 
 agglutinated and agglomerated sensations and ideas. By still more 
 elaborate groupings of the simple ideas, systems of thought and so- 
 called ideas of the highest order of abstraction like the ideas of 
 space, time, etc. come into mental being. 
 
 In the foregoing way, all the so-called mental processes that con- 
 stitute the development of the mind are strictly correlated, under 
 laws analogous to those which control the relations of physical ele- 
 ments, with the processes that go on in the nervous system ; thus 
 so-called " psychology " results in bringing the mind of man into 
 the same strict subjection to the energy of outside nature, under 
 the law of the conservation and correlation of energy, that charac- 
 terizes all the phenomena with which modern physical science is 
 accustomed to deal. 
 
 The above-mentioned theory is doubtless admirably simple and 
 thorough-going. But its somewhat extreme simplicity and thor- 
 oughness constitute very important objections to it. In so crude a 
 form it scarcely deserves detailed consideration. It is enough in 
 this connection to call attention to the fact that the theory is built 
 throughout upon unverified assumptions ; and that, even granting 
 its assumptions, it affords no adequate description whatever of the 
 
628 PSYCHICAL FACTORS SUPREME. 
 
 real process of human mental development. No theory of nerve- 
 commotion has yet been devised to connect it with the external 
 stimuli under the law of the conservation of energy. If by " ner- 
 vous shock " be meant a psychical event, the break between such 
 shock and the nerve-commotion which is its antecedent is absolutely 
 impassable ; no physical energy, under the general law of its con- 
 servation and correlation, can pass this break. 
 
 Moreover, there is no actual or conceivable psychical event cor- 
 responding to the undifferentiated nervous shock. Sensations are 
 always, as such, and from their very nature, of this or that definite 
 quality. An undifferentiated psychical element is a pure abstrac- 
 tion. Nor do sensations and their remembered images constitute 
 such existences that they can be spoken of as " agglutinated " or 
 " agglomerated." The so-called " feelings of relation," slipped in 
 between the single ideas and sensations, if by this be meant any- 
 thing less than relating activities of the mind, are absurdities in no 
 way fitted to explain or represent the act of judgment. And, finally, 
 this entire account of the course of mental evolution is an utterly 
 inadequate description of what actually takes place in the history 
 of even the poorest and weakest human minds. 
 
 13. All theories of the mental development which account for 
 the different so-called faculties and stages of the growth of mind as 
 though they flowed necessarily from some one fundamental activity 
 are inadequate and misleading. The mind is indeed a unit-being, 
 but its unity is not of the kind alleged by these theories. Its dif- 
 ferent constitutional modes of behavior are not to be resolved into 
 each other, or into any one most primitive activity ; nor do they all 
 necessarily flow forth from such a primitive activity. They manifest 
 the rich variety of the mind's nature. They do, indeed, preserve a 
 certain order in time with regard to the relative amount of their 
 unfolding. The different periods of life are characterized by dif- 
 ferent stages of mental development ; these different stages of men- 
 tal development are characterized by a relative prominence of partic- 
 ular faculties, or modes of the behavior of the mind. But because 
 such a time-order is followed in the development of the mind, we 
 can by no means conclude that the faculties latest developed are 
 any the less native and essential to the character of the mind. Nor 
 is it true that these latest and highest faculties can be explained as 
 mere developments from, or modifications of, the earlier and sim- 
 pler. 
 
 Strictly speaking, none of the faculties, or constitutional modes 
 of the behavior of the mind, admit of being explained as mere de- 
 velopments of other faculties. This is true even of those minor 
 
NO DEDUCTION" OF FACULTIES. 629 
 
 forms of activity which it is customary to class under the same fac- 
 ulty. That I have the sensation " red " is no reason why I should 
 have the sensation " green ; " and that I have the sensations "red " 
 and " green " is no reason why I should also have the sensation 
 " blue." Neither does the existence of all these so-called funda- 
 mental color-tones, of itself, form any reason why the mind should 
 be affected with any of all the thousands of sensations supposed to 
 be compounded from them. None of these color-tones, psychologi- 
 cally considered, can be regarded as a development from the fun- 
 damental color-tones. That I am affected with a certain sensation 
 of color, lying at the bottom of the spectrum's scale, when several 
 billion vibrations of ether strike the retina, and with a qualitatively 
 different sensation when the number of vibrations is increased by 
 several billions more, cannot be explained as an evolution. The 
 same remark holds within the limits of each of the other senses. 
 Their scales of quality are not such that experiences at one place 
 of the scale can be evolved from those at other places of the scale. 
 Some of them, such as smell and taste, do not admit of being re- 
 ferred to any form of a scale or diagram representing relations of 
 quality. The feeling of heat is not another phase of the feeling of 
 cold ; neither of the feelings of temperature is to be explained as 
 arising out of feelings of pressure or motion. 
 
 When the sensations of the different senses are compared with 
 each other, the impossibility of considering any of the classes as 
 developing from any other becomes yet more apparent. That a 
 sentient being has an experience of hearing musical tones which rise 
 in pitch as the number of acoustic vibrations varies from thirty to 
 thirty thousand is no reason at all why the same being should have 
 an experience of seeing colors that change their " color-tone " as the 
 number of light-vibrations varies from about four hundred billions 
 to about seven hundred billions. In the development of the mind, 
 the senses may actually awaken in a certain order more or less defi- 
 nitely fixed. But this is very different from holding that they develop 
 out of each other, or that they are all developments of some undif- 
 ferentiated sense-element, the psychical correlate of the nervous 
 shock. Moreover, our percepts, or knowledges of " Things," cannot 
 be regarded as mere developments of sensations. That a sentient 
 being should have sensations of sound and smell and taste, and even 
 of light, color, temperature, and pressure, is not of itself a sufficient 
 reason for its having perceptions such as belong to human experi- 
 ence. The only way in which such perceptions can be regarded as 
 the necessary resultants of the sensations which enter into them as 
 component parts is by taking the nature of the mind into the ac- 
 
630 PSYCHICAL FACTORS SUPREME. 
 
 count. But this implies that perceptions are not developed forms 
 of sensations ; that they are rather advanced forms of the activity 
 of that real being which is developing under the experience of 
 sensation elaborate products of the synthetic activity of mind. 
 
 14. The knowledge of things by perception involves the activ- 
 ity of the mind as 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 "im- 
 age " of memory as though it were merely a faint or faded-out im- 
 pression of sense is quite unavailing ; it does not hit the real point 
 of inquiry, and consequently does nothing to explain the mystery 
 (comp. Part II., chap. X., 18 ff.). The vital element in memory, 
 that which makes it to be memory, is neither a sensation, nor a 
 modified form of sensation, nor a development of sensation. The 
 same statement is true of judgment. 
 
 The relating activity of mind, the power to bring two objects 
 together in the unity of consciousness, 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 start- 
 ling 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 " resultant " of two sensations or images of 
 sensations arising simultaneously in the mind. We cannot consider 
 judgment under the principle of the conservation and correlation of 
 energy. To treat it as such involves the grossest misapplication of 
 the laws which control the coincidence or conflict of physical forces. 
 Nor are the different forms of the relating activity of the mind 
 concept, judgment, deduction, induction to be legarded, strictly 
 speaking, as developments from each other or from any one mental 
 activity simpler than any of them. They may all, indeed, be consid- 
 ered as modes of the relating activity, because they involve discrim- 
 ination, the discernment of likenesses 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 of 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. 
 
 15. We may properly continue the foregoing line of remarks into 
 the consideration of the mind's most general activities. Modern 
 psychology, we have seen, is accustomed to distinguish faculties of 
 
UNITED ACTION OF FACULTIES. 631 
 
 knowing, feeling, and willing as belonging to the mind. But it is 
 emphatically true that no one of these three faculties can be 
 regarded as developed from any other one, or from any two com- 
 bined. 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. Con- 
 versely, a being is conceivable with the knowledge of an archangel, 
 but without experience of desire, emotion, or sentiment of attrac- 
 tion 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 con- 
 ceived of as taking place without this dependence. But acts of 
 will are not mere developments of those acts of knowledge and 
 feeling on which they undoubtedly depend. The act of choice in- 
 volves a new element, an element not to be necessarily evolved from 
 the other activities of mind. 
 
 16. We are so accustomed to the action in common, in the 
 unity of consciousness, of all the so-called faculties, that any 
 attempt to account for them as different modifications of one form 
 of energy meets with a favorable reception. Nothing thus far said 
 is, of course, to be construed to the prejudice of the unity of the 
 mind. But, on the other hand, the incomparable wealth in variety 
 of its natural achievements should not escape our notice. From 
 the beginning to the end of conscious life, the forth-puttings of the 
 mind continue. They are all actual concrete events, happenings in 
 consciousness which have no permanent existence and are never 
 twice precisely alike. That they are, however, alike in certain 
 particulars and unlike in others, we can observe in consciousness 
 itself. Indeed, it is upon this fact that the possibility of any 
 orderly progress, any true development of mind, depends. But 
 the different classes of mental activities are not to be regarded as 
 though they could themselves be explained each from the other, as 
 the different stages of the embryo of an animal or of the germinat- 
 ing and growing seed of a plant are successively evolved. 
 
 The development of mind, therefore, cannot be explained after 
 the analogy of the accretion of molecul.es within a germ, and the 
 resulting division, multiplication, and advancing arrangement of 
 the living cells into separate organs of the entire system. No real 
 
632 PSYCHICAL FACTORS SUPREME. 
 
 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 the same order or 
 the same laws as govern the material evolution ; and some of its 
 most important factors cannot be regarded as having any physical 
 correlate, or as evolved from other factors which have such a corre- 
 late. The development of Mind can only be regarded as the progres^ 
 sive manifestation in consciousness of the life of a real being which, 
 although taking its start and direction from the action of the physi- 
 cal elements of the body, proceeds to unfold powers that are sui generis, 
 according to laws of its own. 
 
CHAPTER III. 
 KEAL CONNECTION OF BEAIN AND MIND. 
 
 1. THAT certain uniform relations exist between the mental 
 phenomena and the action of stimuli upon the nervous system, is a 
 most general conclusion of Physiological Psychology. These rela- 
 tions are chiefly concerned with variations which take place in the 
 quality, intensity, combination, and time-order of the states of con- 
 sciousness, as dependent upon the varying amounts and order of 
 different modes of physical energy as applied to the end-organs of 
 sense. But evidence enough exists to show that the more ultimate 
 psycho -physical relations are those which exist between states of the 
 brain and states of the mind. The dependence of mental states on 
 physical events outside of the body, or at its periphery, is gained by 
 means of the central organs of the nervous system. In the case of 
 man, at least, what happens beyond the cerebral hemispheres is 
 significant for the states of consciousness only as the hemispheres 
 themselves are affected by it. What happens beyond the cerebral 
 hemispheres becomes the cause or antecedent of what happens in 
 consciousness, through this portion of the brain. If our informa- 
 tion were sufficient, then, the empirical science of the connection 
 of body and mind would comprise a statement of all the relations 
 which exist between the mental phenomena and the changes with 
 respect to chemical constitution, structural form, and physiological 
 function, which take place in the molecules of the cerebral areas. 
 
 But even if the conditions were already fulfilled for a complete 
 science of Physiological Psychology, we should scarcely find our 
 speculative inquiries satisfied by this science. The desire would 
 doubtless still be strong to discover some more general statement 
 for the real connection between physical and psychical phenomena. 
 The question would still be raised : What, then, is the one inclusive 
 proposition, or word, or term, which gives the essence of all. the re- 
 lations between the brain and the mind ? It would seem tedious and 
 disappointing to reply to this question by again enumerating all the 
 particulars which psycho-physical science has discovered. Let it be 
 taken for granted that, when lesions happen in certain areas of the 
 
634 MIND AND ITS OKGAN. 
 
 cerebral cortex, such or such disturbances of the phenomena of 
 consciousness take place ; that when so many molecular vibrations of 
 a given wave-form and intensity occur within the cerebral elements, 
 sensations of a certain fixed quality and quantity arise in the rnind ; 
 that when certain fainter vibrations of like wave-form return in the 
 same elements, reminiscences of the aforesaid sensations are ex- 
 perienced, etc., etc. We are still inclined to ask : What is the 
 meaning of all this ? or, How are brain and mind, actually and in 
 principle, related to each other ? 
 
 It is in deference to the raising of inquiries of the foregoing 
 rational sort inquiries which are perpetually repeated all the way 
 along the path of psychological research that, we speak further of 
 a real connection between brain and mind. Of course every such 
 form of speech involves the assumption that the mind is a real be- 
 ing which can stand in relation to other real beings, and not 
 merely the formal or grammatical subject of mental phenomena. 
 This assumption has already been made and partially verified. In 
 continuing to make it for the purposes of the present chapter, we 
 shall find it still further verified. 
 
 2. Various attempts have been made, from one or another 
 point of view, to sum up in some single word the relations that 
 maintain themselves between the body and the soul. Thus, the 
 body has frequently been spoken of as the " seat " or "organ " of 
 the soul. Looking at these relations from the more materialistic 
 point of view, we have already seen how mental phenomena may 
 be regarded as the " products " or "resultants " or "manifesta- 
 tions " of the functional activity of the brain. More highly figura- 
 tive terms even have often enough been employed. The body has 
 been called the " prison " or " tenement " or " tabernacle " of the 
 soul. Not seldom, also, has the mind been represented as master- 
 ing 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 impregnation of the ovum 
 in shaping its increasing molecules according to an unconscious or 
 dimly conscious plan. Much debate has also been held as to 
 whether the conception of " cause " is applicable to any of the rela- 
 tions in which body and soul stand to each other whether, indeed, 
 it must not rather be held that what happens in one is only the 
 " occasion " on which some underlying cause, common to both, 
 operates to produce a change in the other. 
 
 3. The inquiry in what sense, if at all, the brain can be said to 
 
BRAIN AS THE SEAT OF MIND. 635 
 
 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. Few 
 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 wander- 
 ing about among the nerve -molecules to find its temporary "seat " 
 where occasion seems to require its presence. And, although some 
 of the phenomena of mind and brain perhaps admit very well of 
 being brought under the conception of the 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 reality an atom. 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 the molecules of 
 the central nervous system. "We shall, then, briefly consider the 
 question in this form. 
 
 4. There can be no doubt that ordinary language justifies us 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 matter, assents to the neces- 
 sity of asserting a special relation in space between it and the body. 
 Hence some old-time philosophies represented the soul in percep- 
 tion as streaming out through the avenues of sense in order to get 
 the sensuous object into its embrace : or else pictured some ethe- 
 realized copy of this object as streaming into the soul by the same 
 avenues. 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 becomes known shows 
 that all our experience is connected with the establishing and jus- 
 tifying of this claim. There are no " things " known to experi- 
 ence except as our sensations, or modes of being affected, are both 
 localized and projected e^ra-mentally. Inducements and consid- 
 erations, such as have already been treated in great detail (Part II., 
 
636 MIND AND ITS ORGAN. 
 
 chaps. VI. and VII.) irresistibly urge the mind to arrange all its 
 phenomena into two great classes phenomena which are qualities 
 of outside things, and phenomena which are mere states of internal 
 experience. But the same inducements and considerations compel 
 us to look upon certain phenomena of the first class as related to 
 our mere states of consciousness in a peculiar way. The world of 
 things outside always (at least in ordinary experience) affects us 
 is perceived by us or modifies our consciousness through the body. 
 The mind is, therefore, said to be in the body. 
 
 The conclusion from the foregoing general experience is con- 
 firmed 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 the mind, are all connected with sensations 
 more or less definitely localized in the body. Hence men say, " My 
 nose is offended by this smell," "My tooth is aching," or "My 
 limb is suffering." So close is the connection between 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 localizing of sensa- 
 tions connected with feelings of strong " tone " is very indefinite, 
 as it is in cases where the feelings arise from the condition of large 
 areas of the internal organs, the soul seems to be suffering 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 parts of the body. The ancients located the soul in the 
 heart or lower viscera, because of certain marked connections be- 
 tween the states of the soul and the condition of these organs. 
 But to speak of the soul as seated in the heart or other viscera 
 plainly applies most pertinently only to the soul as an emotional 
 being ; the obvious connection of the head with most of the more 
 obtrusive sensations tends to confirm us in the belief that the 
 mind, as perceptive, has its " seat " in that region of the body. 
 For reasons already given in detail (see Book II., chaps I. and H 
 and elsewhere), modern scientific researches justify us in narrowing 
 more precisely the local domain within which we can affirm the 
 mind to have its seat. The mind is certainly in the nervous sys- 
 tem, 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 
 
NO MATERIAL SEAT OF MIND. 637 
 
 voluntary motor-impulses, here if anywhere must it be held 
 that the subject of the states of consciousness has its dwelling- 
 place and home. 
 
 5. At this point, however, the results of modern scientific in- 
 quiry become unfavorable to the effort yet more particularly to 
 designate a material " seat " for the mind. The eager imagination 
 having, as it were, hunted the soul down as it retreats inward and 
 upward to the higher regions of the supreme central organs, re- 
 quires some more precise information as to just where in these 
 regions its existence may be pointed out. Is there any one mathe- 
 matical 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 impor- 
 tant a place ? The answer of cerebral histology and physiology to 
 the foregoing questions is, on the whole, a decided negative (comp. 
 Book L, chap. II., and Book II., chaps. I. and II.). 
 
 Certain areas of the cerebral cortex do, indeed, appear to have a 
 particular connection with the execution of certain functions of the 
 mind ; the exact nature of this connection, however, cannot as yet 
 be clearly indicated. But the very phenomena on which reliance 
 is placed for establishing the foregoing 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. Considerable parts 
 of all the cerebral areas can Tpe destroyed without impairment of 
 any of the essential functions or faculties of mind. Moreover, both 
 gross and microscopic anatomy show us that the cortical part of 
 the brain, like all its other parts, is not constructed on the plan of 
 having its uses for the mind concentrated in any one minute cir- 
 cumscribed spot. In any sense in which the 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 com- 
 plexity of nerve-fibres and nerve-cells, be said to be the "seat " of 
 the mind. 
 
 6. 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? 
 No one is directly conscious of these organs. The subject of con- 
 
638 MIND AND ITS ORGAN. 
 
 sciousness is not a being which can be conceived of as " posturing " 
 within or amongst a certain larger or smaller group of material 
 molecules. And yet, plainly, in some sense the mind is to be 
 thought of as in the brain, as it is not in any object outside of the 
 body, or in any of the non-nervous organs of the body (bones, hair, 
 nails, fat, muscular tissue as such, etc.), or even in the remainder 
 of the nervous system. 
 
 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 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 
 project 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 contained in the cranial cavity. Our modes of being af- 
 fected are directly localized in space outside of the body, or in the 
 various peripheral parts of the body. The part of the body on 
 which the activity of having these percepts is immediately depend- 
 ent is localized by science in the brain. Other activities of mind 
 are probably also thus dependent on the brain. In no other sense 
 can the brain be said to be the seat of the mind. 
 
 As to the possibility of such a sui generis relation between 
 material elements which exist in space, and the localizing and other 
 activities 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 not to be disputed on any ao- 
 
MYSTEEY OF THE KELATION. 639 
 
 called a priori grounds whatever. Both the dicta which have some- 
 times been made to bear on the case are alike inapplicable. On 
 the one hand, it has sometimes been claimed that a being cannot 
 act where it is ; on the other hand, that a being cannot act where 
 it is not. Nothing, however, can be known as to how and where 
 beings can or cannot act, except through experience of how they 
 actually do act. Building our conceptions upon the basis of facts, 
 we should be inclined to say that beings act upon, and are acted 
 upon by each other, according to their differences in constitution 
 and relations in space. Gravitation keeps constantly before us the 
 example of all bodies acting unceasingly upon each other, in many 
 cases over distances that are immense. The amount of this action 
 depends, indeed, upon the distance over which it takes place ; but 
 the action at all is an instance of beings acting where they are not. 
 
 When material molecules are approached nearer to each other 
 than a given small distance we at once discover new modes of be- 
 havior set up, which depend upon what the molecules are, and 
 what their condition, etc. New laws, such as those of cohesion and 
 chemical affinity, have now to be taken into the account. But 
 gravitation, cohesion, and chemical affinity are all alike to be under- 
 stood as expressive simply of the regular modes of the behavior of 
 material elements, with reference to each other, under varying con- 
 ditions. All these modes of behavior modern physical science re- 
 duces to motions of various kinds, directions, durations, and veloci- 
 ties. What is true of all material elements is true of those of the 
 brain ; they can do nothing but move. 
 
 If, then, we are to speak of the mind as having its " seat " in the 
 brain, in a literal way, we must regard it as one among the many 
 molecules or atoms of which the brain is composed wandering 
 (that is, moving in a peculiar fashion) among the others, and so 
 variously acting on them, and being acted upon by them. But if 
 the mind were such a molecule or atom, the only affection it could 
 receive from the rest of the brain-molecules would be to change 
 the kind and direction of its own motion ; the only effect the 
 mind-atom could produce in the material atoms of the brain would 
 be to modify their motion with respect to kind and direction. But 
 it would still be just as difficult as before to understand how the 
 phenomena of consciousness should result from the movement of 
 one atom among other atoms no matter how peculiar in constitu- 
 tion each of these two kinds of atoms (the mind-atom and the 
 brain-atoms) might be. 
 
 It does not follow, however, that the relation of the mind to the 
 brain is any more ultimately mysterious than that of the molecules 
 
640 MIND AND ITS ORGAN. 
 
 of the brain to one another. Nor does it form an insuperable ob- 
 jection to the former relation that it is not, like the latter, a rela- 
 tion of changes of position in space. For who shall undertake to 
 affirm that beings which are not extended and movable in space, 
 because their very nature is of another order, cannot exist in rela- 
 tions of any kind to beings which are thus extended and movable ? 
 If the existence of the former kind of beings consists essentially in 
 states of consciousness, this fact does not prevent their dependence 
 upon the changing relations in space of extended and movable be- 
 ings. It is, in reality, in this way that the mind is related to the 
 brain. To speak of the mind as having its " seat " in the brain is 
 to reaffirm the reality of such relations. 
 
 7. 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 of 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 anal- 
 ogous 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 de- 
 sires 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 throw up the ground with spade 
 or shovel, cut down the tree with an axe, feed ourselves with knife 
 and fork, etc. We can, by means of much more 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 
 
THE USES OF THE BRAIN. 641 
 
 out of the composite clang. With the lenses of his spectacles the 
 man of defective vision sees what would otherwise be invisible ; 
 and with a prism the optician beholds the colors of the spectrum. 
 Remote objects are brought near with the use of the telescope, and 
 very minute objects near by are revealed by the microscope. 
 
 It is characteristic of all the most skilful use of tools and instru- 
 ments that they come to seem to the observer like a part of his 
 own bodily mechanism. By feelings of " double contact " (see 
 Part II, chap. VI, 31) the workman comes to know, with the 
 chisel, the wood or metal which he is carving 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. 
 
 8. The conception of an " organ " or instrument may with pro- 
 priety be extended so as to cover the relation which exists between 
 the nervous system and the muscular, and between the central and 
 the 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 ex- 
 presses, 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 the parts 
 that lie between the extremes may be looked upon as means or 
 media as instrumental to the completion of the process. For ex- 
 ample, in quick succession upon a certain idea of motion, and a vo- 
 lition to a definite motion, my arm is raised or my whole body 
 changes its position in space. How can this come about ? To the 
 inexperienced person the result seems to be an " immediate " ef- 
 fect of the will that is, no apparent media or instruments stand 
 between the volition and the subsequent changes in the relations of 
 the masses of the body to other objects. 
 
 The vulgar persuasion undoubtedly is, that a man immediately 
 41 
 
642 MIND AND ITS OKGAN. 
 
 knows himself to be the cause of the movement of his own arms or 
 legs ; that he knows that he can move them if he ivilL Little in- 
 vestigation, however, is needed to discover that such is in no respect 
 the state of the case. A thousand hidden links, any one of which 
 might drop out without our being immediately aware of it, inter- 
 vene between the volition and the actual motion. No one directly 
 knows that one can move as one will ; one knows that one can will, 
 and infers that, if one will, the movement will follow. In tracing 
 the line of physical sequences backward from the motion of the 
 limb toward the arising of the volition in consciousness, we bring 
 it to a termination in a hypothetical nerve-commotion in some (so- 
 called "motor") area of the brain. At this point the line of 
 sequences, considered as a succession of modes of motion, draw- 
 ing constantly nearer to the instant of the volition, comes to an 
 end. The connection beyond and into that state of consciousness 
 which is called a " volition " cannot be conceived of as the pro- 
 gressive propagation of a peculiar molecular motion called nerve- 
 commotion. 
 
 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 a 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. All 
 this system of instrumentalities or interdependent organs is- of one 
 nature. It consists of material particles having a definite chemical 
 constitution, structure, and arrangement in space ; its functions are 
 all modes of motion of such particles. One part serves as an instru- 
 ment or " organ " for another, because the changes in the former 
 effect changes elsewhere, not directly, but through contiguous and 
 connected parts. If the necessary contiguous parts are wanting 
 or their relations disarranged, if the connection is interrupted or 
 destroyed, then the work cannot be done; the "organ," "instru- 
 ment," or " means," is lacking. 
 
 - What is true of the relations described by the word " organ," 
 as these relations exist between any two parts of the nervous 
 mechanism, is only partially true of the relations which exist be- 
 tween any particular portion of this mechanism and the phenom- 
 
THE USES OF THE BRAIN. 643 
 
 ena of consciousness. In other words, only a part of the real rela-^ 
 tions existing between mind and brain can properly be described 
 under such terms as "organ," "instrument," etc. The brain, 
 with its appropriate functions, is an indispensable medium between 
 certain changes in the peripheral parts of the body and correspond- 
 ing changes in the states of consciousness. If ideas of motion and 
 volitions to move are to be followed by actual motions correspond- 
 ing to the ideas and volitions, then the brain must act with its ap- 
 propriate functions. The motions can be executed, if at all, only 
 through the brain. 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. If nothing 
 takes place here, nothing at the periphery of the body will come 
 from the volitions ; if anything wrong takes place here, all that 
 goes on at the periphery will be wrong, and the mind will not get 
 its volitions executed. In this sense, at least, the brain is the par- 
 ticular organ of the mind ; it is the most intimate and indispen- 
 sable 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. After experience in moving 
 a particular member of the body has once been gained, that mem- 
 ber may be lost ; and yet if the proper areas of the brain remain 
 unimpaired, the ideas, feelings, and volitions connected with the 
 movement of the lost member will still arise in the mind. The man 
 whose leg or arm has been amputated can still feel it, can form the 
 image of how it should be moved to be in this position or in that, 
 and even will to have it moved. The leg is not, then, the organ of 
 these ideas, feelings, and volitions, f 
 
 Experiments with animals, by extirpating the cortical areas, and 
 observation of human pathological cases especially, perhaps, in 
 certain forms of aphasia (comp. Part EL, chaps. I. and II.) seem 
 clearly to show that a much more intimate relation exists between 
 the brain and the mind. With the destruction or derangement of 
 certain of its areas, the power even to form such ideas and volitions, 
 or to have such feelings, seems to be impaired or lost. We cannot 
 
644 MIND AND ITS ORGAN. 
 
 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 
 to its power to frame a certain set of definite ideas and volitions as 
 respects the motion 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. Thus much, then, we 
 are also entitled to include in our declaration that the brain is the 
 " organ " of the mind. 
 
 There is another most important class of facts which may be par- 
 tially described under the same terms as the foregoing. The brain 
 is the indispensable means for furnishing the mind with its sensa- 
 tions, and so with its presentations of sense or perceptions of things. 
 This statement is not to be understood as though the brain 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. The brain is not the 
 " organ " of the mind in the sense in which a being that starts or 
 receives some mode of motion becomes instrumental for the pro- 
 duction of changes in another being. Nevertheless, no sensations 
 will arise in the mind unless the brain be affected in a certain way. 
 Looking at the chain of sequences as it runs from without 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. 
 
 What is properly meant by speaking of the brain as the mind's 
 organ of sensation is, strictly speaking, to be described as follows : 
 The brain is the last and most important physical antecedent to the 
 mind's being affected with the different sensations. The eye, ear, 
 nose, etc., are popularly called organs of sense. Nothing is more 
 obvious about the whole process of sensation than the part played 
 in it by the peripheral sensory organs. It is apparently through 
 these, by means of these, only on condition of the acting of these, 
 that sensations arise in the mind. Further examination shows, 
 
MATERIAL CONNECTION IMPOSSIBLE. 645 
 
 however, that the end-organs of sense are more remotely connected 
 with the origin of sensations than might at first be supposed. 
 Excitement of the afferent nerve-tracts between the end-organs 
 and the brain will also produce sensations. If these sensations are 
 not so refined and complex as those which are caused by stimu- 
 lating the end-organs themselves, the reason is to be found in the 
 fact that we cannot stimulate the afferent nerves in the way to pro- 
 duce such sensations except through the appropriate end-organs. 
 Still further examination shows that the value of the stimulus ap- 
 plied to the afferent nerves for the production of sensation is en- 
 tirely dependent upon what the conducting organs convey to the 
 brain, according to their specific nature and appropriate connections. 
 Sensations can be equally well produced by stimulating the cerebral 
 substance directly. When drugs, or gases, or diseases, or increased 
 action of the blood-vessels change the character of the blood -supply, 
 we see, and hear, and feel a world of things that has no existence 
 for the end-organs of sense. The brain is then the "organ" of 
 sensation for the mind, in the sense of being the indispensable and 
 most immediate means for the production of sensation. 
 
 Nothing that has just been said interferes in the least with the 
 valid claim for the mind, that it alone is the producer of every sen- 
 sation ; or, in other words, all sensations are modes of the behav- 
 ior of a being that is non-material and a unit-being, and is called 
 Mind. When the physical conditions are fulfilled in the brain, and 
 according to the way in which they are fulfilled, the mind itself 
 puts forth the phenomena of sensation. For the sensations are 
 not copies of outside material molecules, whether acting on the 
 end-organs of sense or acting as excited nervous substance in the 
 brain ; it can scarcely be repeated too often they are modes of 
 the conscious activity of mind. 
 
 10. 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. How are mind and brain connected? 
 What real tie binds them, so that they are obliged to have re- 
 gard to each other in the modes of their behavior ? 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, un- 
 less we are ready to concede that the material and the non-material 
 can stand connected without some special bond. In case this con- 
 
646 CONNECTION OF BRAIN AND MIND. 
 
 cession is once made, however, we cease to feel the need of a 
 special bond between the mind and the brain. 
 
 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. What bond connects 
 together the planets of the solar system so that each one moves in- 
 variably with reference to the position of all the others, and yet 
 in a path peculiarly its own ? We can only respond by talking of 
 the force and laws of gravitation. These " laws," however, are 
 simply a mathematical statement of the uniform modes of the be- 
 havior of certain physical beings ; this " force " is no entity or 
 bond connecting the individuals with each other, as the rods of 
 the orrery bind its parts to a common centre. Did such rods 
 exist to bind the planets to the sun, we should still have to inquire 
 for some bond between the particles of the rods ; and for another 
 bond to unite the atoms into these particles. Nor would it be an 
 answer to such inquiry to discourse of cohesion and of chemical 
 affinity, or of the laws which control the action of those forces. 
 For cohesion and chemical affinity are not special bonds ; they, 
 too, are but expressions for the facts that the elements of material 
 reality, under certain conditions and according to the kind to 
 which they belong, behave as though bound ; these elements be- 
 have, that is to say, according to what they are, and according to 
 the relations in which they stand to each other. 
 
 No more obscure and unsatisfactory is our knowledge concern- 
 ing the " bond " which unites body and soul, or, more especially, 
 the mind and the brain. The brain is a vast collection of material 
 molecules, connected together in a great variety of ways, which 
 always act, as it were, with their own chemical constitution, and 
 relations to other similarly constituted bodies, fully in mind. 
 Even the molecules are not bound to each other by any one dis- 
 coverable or conceivable bond. So far as we can speak of them as 
 " connected " at all, they are connected by a great variety of bonds. 
 Each of these bonds depends upon the nature of the molecules 
 which enter into it, and upon the manner in which each molecule 
 is related to other molecules. Essentially the same thing is true 
 and perhaps with no more of ultimate mystery in its truthful- 
 ness of the connection between mind and brain. The mind is a 
 conscious being, a being that perceives, feels, remembers, imagines, 
 thinks, and wills. In respect to certain classes of its activities, at 
 least, what it does depends upon what is done by the molecules of 
 the brain with which it is, as we say, specially connected. The con- 
 
BODY AS TENEMENT OF SOUL. 647 
 
 nection is not, however, such as can be explained by assuming 
 one special form of a " bond " between the two. An infinite variety 
 of relations, some of which are in a measure reducible under law, 
 and others of which elude all attempts thus to subject them, exists 
 between the unit-being called mind, and the composite structure 
 and varied functions of the brain. The connection is no less real, 
 however, because invisible ; no less valuable and certain, because 
 not one bond, but an infinite variety of relations. 
 
 11. 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 almost 
 purely figurative and poetic. The limited and defective nature of 
 our sense-perceptions, the misery of much of life, the unrealized 
 longings for knowledge and happiness, and the work of imagina- 
 tion in framing a picture 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 restraining 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 dissolve the bodily functions, together with 
 the persuasion that the thinking principle will have a continued 
 existence, suggest the reflection: the body is the "tenement "or 
 " tabernacle " of the soul. 
 
 However true and comforting the foregoing hopes and reflec- 
 tions may be, it cannot be claimed that they throw any clear light 
 on the subject of our investigation. Physiological Psychology 
 rests upon such facts as show a most intimate and unceasing cor- 
 relation between the body and the soul. It can never, therefore, 
 consider the ultimate connection of the two as though it were as 
 unimportant and superficial as that between the prisoner and the 
 prison which holds him, or between the tenant and the tenement or 
 tent which for the time being is his abode. We are not at present 
 engaged in considering the evidence that the mind is immortal, 
 and can exist apart from this body and in another body, if not 
 apart from all bodies ; nor even the proof that its nature is vastly 
 superior to that of all the material structures to which it might be- 
 come related. We are rather testing the assumption that the mind, 
 as connected with the brain, is a real being which, although depend- 
 ent upon what occurs in the brain for its character and the order 
 of its activities, has nevertheless that existence which belongs to 
 
648 CONNECTION OF BRAIN AND MIND. 
 
 all real beings a nature and a development of its own. This as- 
 sumption, indeed, is applied and confirmed in every attempt to 
 characterize the real connection which exists between the mind 
 and the brain whether the words "seat," " organ," " instrument/' 
 or other corresponding words be used. 
 
 12. Thus far little has been explicitly said as to the propriety 
 of applying the terms of " causation " (such as " energy," " action," 
 "force," "impulse," "effective agency," etc.) to the case of mind 
 and brain. Yet everything which has been said has implied that 
 these terms are really applicable. 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 instru- 
 ment 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. 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 cen- 
 tres and end-organs of sense. Now, unless we are ready to be 
 satisfied with simply stating the facts, without making the at- 
 tempt to find any rational account for them, we are obliged to con- 
 sider these correlated changes under the terms of cause and effect. 
 That is to say, we regard the mind as a real being with activities 
 called states of consciousness, and the brain as a collection of real 
 beings called moving molecules of nervous matter, and we as- 
 
INFLUENCE OF THE CEREBRAL CIRCULATION. 649 
 
 sume that the latter acts upon the former and is acted upon by it 
 in turn. In other words, brain and mind are conceived of as really 
 connected under the law of causation. 
 
 Were it not for the influence of prejudice derived from specula- 
 tion 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. The stoppage of the arteries leading 
 to the cerebrum, by outside pressure or by embolism, is speedily 
 and regularly followed by a disturbance or cessation of conscious- 
 ness. Who doubts that a man loses his senses as truly as he loses 
 a portion of his brain-mass, because he has been struck a blow upon 
 the head ? The falling of waves of light or sound upon the eye or 
 ear, the contact of the hand with the hard substance of the metal 
 or wood, the breathing of the air into the nostrils, are universally 
 regarded as the causes of the sensations and perceptions which 
 follow. The general impression undoubtedly is, that the act of 
 will is the cause of the motion of the different limbs or of the en- 
 tire body. In each of these cases more careful observation results 
 in supplying many links in the chain of causation which the popu- 
 lar account has overlooked. The result is a more minute and 
 careful picture of those molecular changes which take place in the 
 cerebral substance, as induced by the severe shock of the blow or 
 by the gentle stir of the stimulus acting through the end-organs of 
 sense. Science explains the way in which the visible changes in 
 the position of the ponderous masses of the body are due to ante- 
 cedent invisible changes in the molecules of the muscles, of the 
 efferent nerves, and of the lower and the supreme nervous centres. 
 But all this explanation implies the application of terms of causation 
 to the entire chain of physical events ; and if these events are to be 
 considered as in any measure explaining the psychical events with 
 which they are connected in time, the relation of the two classes of 
 events is also assumed to be one capable of statement in the same 
 terms. 
 
 How impossible 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. The character of the cerebral circulation 
 is said to have a great " influence " upon the condition of the mind. 
 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 condition of the thoughts, fan- 
 
650 CONNECTION OF BRAIN AND MIND. 
 
 cies, and sensations is a direct expression of the kind of work which 
 is going on, because of the accelerated heart-beat and the disor- 
 dered character of the blood, within the cerebral arteries. Schroe- 
 der 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 considerable extent, 
 by the position of the head and the way in which this position 
 affects the cranial circulation. Hallucinations not infrequently are 
 immediately made to cease, when the person having them assumes 
 the standing posture, or has leeches applied to the head. 
 
 13. Objections have arisen from various sources, and have been 
 urged with various degrees of skill and intensity, against applying 
 the conception of causation to the relations of mind and brain. So 
 far as these objections are more purely ethical or religious, it is not 
 consistent with the purpose of the present investigation to consider 
 them. But certain objections are more purely scientific, or per- 
 haps philosophical, upon a basis of observed physical and psycho- 
 logical facts. A brief examination of such objections is not only 
 consistent with the present investigation, but even required by it. 
 
 Among the followers of the Cartesian philosophy it was held 
 that body and soul cannot really act upon each other, because of 
 the obvious difference in the essential characteristics of the two. 
 The body is extended and material ; the soul, being non-material, 
 does not possess the characteristic most distinctive of all that 
 comes under the conception of matter. Matter and mind, as being 
 in their very essence opposed, are separated from each other by the 
 whole diameter of being. They cannot be regarded as united directly 
 through any real tie, but stand at the mutually exclusive poles of 
 being. That a certain marked correspondence exists between the 
 phenomena of the extended and material body and the phenomena 
 of the conscious non-material soul plainly cannot be denied ; and 
 some account for this correspondence must be given. No one can 
 doubt that his sensations, in their quality and order of succession, 
 are related to certain events in the physical organs of his own 
 body ; neither is it easy to persuade one's self that one's move- 
 ments are not, at least in some indirect way, " ordered " by one's 
 desires and volitions respecting them. 
 
 To account for the obvious regular relation between bodily 
 changes and mental phenomena, two or three somewhat different 
 
THEOKY OF OCCASIONALISM. 651 
 
 theories may be proposed. One of these is the so-called doctrine 
 of " Occasionalism." According to this doctrine body and mind 
 do not stand in the relation of cause and effect toward each other ; 
 neither one ever really acts upon the other. But on occasion of 
 some event of a definite kind happening in the bodily realm, a 
 corresponding event of its own definite kind happens in the domain 
 of consciousness ; and vice versa. To say this, however, is plainly 
 in itself nothing more than to repeat the facts of experience over 
 again, but without offering any explanation of them. Since some 
 real " ground " or reason that shall have causal efficiency seems 
 needed in order to explain why body and mind should take "occa- 
 sion " to act at all, in view of each other's action, theology readily 
 finds such ground in the Divine Being. God, it is said, on occasion 
 of an event occurring in either of the two diametrically opposed 
 spheres, causes the right corresponding event to occur in the other 
 sphere. Matter and mind are not causally connected immediately 
 with each other ; they are causally connected only through a com- 
 mon ground in God. Pure Occasionalism, however, seems to 
 make too large demands upon a pious credulity. To be always 
 observing mere "occasions," in order to cause body and mind to 
 keep the right pace with each other, may well be regarded as un- 
 worthy of Divine Being. To meet this difficulty the theory of 
 "Pre-established Harmony " is devised. According to this theory 
 God has eternally predestined the entire succession of events in 
 the world, down to every minutest detail. Body and mind, there- 
 fore, may be regarded as like two clocks which have been so con- 
 structed that, without either having any effect upon the other, they 
 go on exactly as though one were actually moved by the other. 
 
 It would scarcely be worth while to consider seriously these 
 older forms of the denial that any real causal relation exists be- 
 tween body and mind, were it not for the fact of their essential 
 agreement with more modern forms of the same denial. Two re- 
 marks upon the foregoing theories, in special, are 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 in the face of many of the most important psycho-physical 
 facts. We know nothing about what kind of beings can or can- 
 not act on each other, except through our experience of what 
 beings do actually act upon each other. The mystery involved in 
 any one being acting on any other is equally deep and unfathom- 
 able, in whatever direction we attempt to explore it. Before ex- 
 
652 CONNECTION OF BRAIN AND MIND. 
 
 perience 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, unbiased, 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 im- 
 mediately under law, that we are warranted in believing in the 
 action of each upon the other. 
 
 Moreover, the theory of Occasionalism, Pre-established Har- 
 mony, and all similar theories, do not in the least assist us to es- 
 cape the difficulties which attach themselves to every conception 
 of causation. We cannot regard the Divine Being as bringing 
 about a change in either mind or bod} r , on " occasion " of some 
 other change, without assuming that mind (the Infinite Mind) 
 stands in the causal relation to matter. Furthermore, we cannot 
 conceive of a " reason " why this Being should effect one change 
 rather than another, without regarding Him as subjecting himself 
 to the same relation. 
 
 14. It is interesting to notice certain relations, both of similar- 
 ity 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. Modern science 
 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 phe- 
 nomena, and the regular modes of their recurrence, if not the in- 
 dependent 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 ma- 
 terial reality (called " atoms ") are assumed to exist ; the univer- 
 sal form of their relation is held to be the law of the conservation 
 and correlation of energy. By " energy " we are to understand 
 that which moves or tends to move the elementary atoms, or their 
 
NO TRANSMISSION OF ENERGY. 653 
 
 aggregations, into molecules and masses. The energy which is re- 
 garded 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 en- 
 ergy 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, accord- 
 ingly, 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. 
 
 On attempting to account for the whole world of phenomena in 
 terms of motion, kinetic or potential, under the law of the conser- 
 vation and correlation of energy, we are met with insuperable diffi- 
 culty as soon as 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 correlation between physical and mental phenomena. 
 Just as the momentum 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 mathematical equivalent of the conception of home, of 
 the sense of obligation, or of the idea of God. In other words, it 
 seems impossible to regard any amount of physical energy as ab- 
 stracted 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 consciousness. 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, no pass- 
 ing back and forth of energy, can occur. 
 
 Representing the same truth in another way, we may declare : 
 The entire circuit of the transmission and distribution of energy is 
 complete within the brain itself. Not a single atom enters its sub- 
 stance that does not come forth unchanged, with all its forces in- 
 herent 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 
 
654 CONNECTION OF BRAIN AND MIND. 
 
 is not either used up there or returned to external nature in con- 
 nection with the constitution of the molecules separated from this 
 substance. The stricter we make our application of the law of the 
 correlation and conservation of energy within the physical realm, 
 the more impossible does it become to apply it at all to the rela- 
 tions of body and mind. 
 
 15. It is not surprising that, in the estimate of one who is un- 
 accustomed 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 
 extraordinary treatment. In any event, the facts of consciousness, 
 as facts, cannot be denied. Whether we can explain them or not, 
 with or without use of the law of the conservation and correlation 
 of energy, they are equally plain and persistent. Men perceive, 
 and imagine, and remember, and reason, and believe in the invisi- 
 ble, and choose, etc. All this they do, as possessed of a body 
 and, particularly, of a nervous 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 its host of inherent energies ; 
 and supreme is the law of the conservation and correlation of these 
 energies regarded as modifications of one fundamental form. 
 
 In view of so grave difficulties it has been of late customary to 
 escape from them in one of several different ways. The general 
 claim may be set up that all hopeful inquiry as to the nature of real 
 beings, which act upon and are acted upon by each other under the 
 Jaw of causation, must be abandoned. Knowledge, it is said, con- 
 sists simply in the relating of phenomena under certain constant 
 and regular forms of their recurrence, called "laws." This is sub- 
 stantially the position of Positivism. 
 
 It may also be held that all mental phenomena are to be re- 
 garded as merely transitory appearances shadows cast, as it were, 
 by the changing activities of the material molecules ; and that the 
 latter are the only realities. In this case the constitution and ac- 
 tivities of the molecules are all to be regarded as determined by 
 the interaction of the ultimate atoms which compose them, accord- 
 ing to their inherent and inseparable forces, under the law of the 
 conservation and correlation of physical energy. Whenever a cer- 
 tain constitution and consequent modes of activity are brought 
 about in the molecules, under this general law, then it is of their 
 own incomprehensible nature to exhibit, in addition to the various 
 forms of motion known as nerve-commotion, another peculiar class 
 of coexisting phenomena, called mental phenomena. The latter 
 
MATERIALISM AND MONISM. 655 
 
 phenomena do not require a new subject ; their appearance is the 
 necessary result simply of the special and unique constitution and 
 relations of the physical molecules of the brain. The mental phe- 
 nomena are one form of expressing the fact of the real existence 
 of these molecules, with such a constitution and in such relations. 
 And just as we do not require a new subject for the mysterious 
 and unique phenomena of magnetism, or of crystallization, but be- 
 lieve them to be only the expression of the new relations into which 
 the same subjects of all phenomena the imperishable atoms 
 have been brought under the one law of the conservation and cor- 
 relation of energy ; just so do we find no particular need of a 
 new kind of subject, other than the aggregated atoms, for the mys- 
 terious and unique phenomena of consciousness. This position is 
 Materialism. 
 
 Still further, the impossibility of binding together by a causal 
 tie, under the law of the conservation and correlation of energy, 
 phenomena so utterly incomparable as are those of mind and brain, 
 and the difficulty of assigning the mental phenomena to the same 
 subject as that which, otherwise, manifests itself only as modes of 
 motion rather than modes of thought, have led to more recondite 
 speculation. Hence a return to the "mediasval" view has been 
 made. The real connection of mind and brain has been found in 
 a third somewhat, which is neither mind nor brain, as we know 
 them, but is the ground of both. There is, then it is claimed 
 only one substance as the real subject for the two sets of proper- 
 ties. " The one substance, with two sets of properties, two sides, 
 the physical and the mental a double-faced unity would appear 
 to comply with all the exigencies of the case." This position may, 
 in general, be designated as that of Monism. 
 
 But immediately the inquiry arises and presses for an answer, 
 whether we may not know something as to the real nature of this 
 "double-faced unity," besides the mere fact that, phenomenally 
 considered, it has two faces, or sides the physical and the mental. 
 Why does it manifest itself both as physical motion and as mental 
 states one Being, in two utterly incomparable modes of manifes- 
 tation? Is it itself extended and movable, a material reality? or 
 is it unextended and conscious, a psychical reality ? To refuse to 
 attempt the answer to this question is to take refuge in Agnosti- 
 cismand that at a critical point, to which we have brought our- 
 selves unnecessarily through having been already overwise. For 
 no one who claims already to know enough about the nature of so- 
 called matter and of so-called mind to affirm confidently that they 
 cannot be two forms of real being, acting on each other and being 
 
656 CONNECTION OF BRAIN AND MIND. 
 
 acted upon by each other, is entitled, just beyond this advanced 
 line of knowledge, to make a run sideways into the refuge of con- 
 fessed ignorance. Furthermore, if the " double-faced unity" is held 
 to be, in reality, either matter or mind, we raise again all the diffi- 
 culties as to a real connection between two sets of phenomena so 
 incomparable. Both Materialistic Monism and Idealistic Monism 
 have, then, to undertake the task of showing how the one reality 
 can appear under these two phenomenal forms of being matter 
 and mind with its two sides causally connected. 
 
 16. So far as the theories, which are proposed in order to escape 
 the difficulty of admitting a direct causal connection between mind 
 and brain, involve the assumption that the phenomena of conscious- 
 ness can be regarded as modes of motion, or can be attributed to 
 the molecules of the brain as their sole subject, they have already 
 been refuted. So far as these theories resolve themselves into the 
 speculations proposed by different schools of philosophy concerning 
 the supreme philosophical inquiry, What is the nature of the Ulti- 
 mate Keality (the Absolute)? psycho-physical researches have no 
 direct answer to offer to them. But our present inquiry is a more 
 modest one, namely : What is the nature of the real connection 
 between human mind and human brain, so far as psycho-physical 
 science throws any light on such connection ? Our general reply 
 is : This connection is, in reality, such as we find between all so- 
 called real beings, to whichever of the tw r o supreme classes (material 
 or spiritual) such beings may belong. The molecules of the brain 
 (so far, at least, as psj'cho-physical science knows anything of them) 
 are composed of elements of material reality, called "atoms ; " these 
 atoms act by way of motion, according to their constitution and re- 
 lations to each other and to their environment. The mind, on the 
 other hand, is a real unit-being of another order than that of the 
 atoms. Its acts are the various modes or states of consciousness. 
 This being called mind is causally related to the beings called 
 atoms ; the relation is mutual. The mind behaves as it does be- 
 have because of the constitution and behavior of the molecules of 
 the brain. The molecules of the brain behave as they do behave 
 because of the nature and activities of the mind. Each acts in view 
 of the other. The action of each accounts for the action of the 
 other. But the action of neither is to be explained as solely due to 
 the action of the other; neither mind nor brain can be regarded as 
 the subject for the phenomena ordinarily ascribed to the other. 
 
 The position just taken is, of course, the most unmistakable 
 Dualism. It assumes two kinds of real beings for the two incom- 
 parable classes of phenomena. Whether this position is the ulti- 
 
THE VIEW OF DUALISM. 657 
 
 mate one attainable by human reason or not, the facts of Physiolog- 
 ical Psychology afford no basis for speculation. It is possible that 
 some higher point of view might enable us to resolve the Dualism, 
 and to discover a common ground for the body and soul of man, and 
 even for all physical and spiritual phenomena. But psycho-physical 
 science, simply observing the facts and building upon them, and 
 upon such assumptions as it, in common with all the sciences, is 
 compelled to make, establishes this Dualism of brain and mind, and 
 then hands the case over to philosophy for further consideration. 
 Moreover, there is nothing in any science, physical or psychological, 
 which offers a single valid reason why both mind and brain should 
 not be regarded as real beings, material and spiritual, mutually in- 
 teracting. This last statement we shall now justify by considering, 
 briefly, the objections to it, which have induced the resort to the 
 before-mentioned other theories. 
 
 17. The law of the conservation and correlation of energy as 
 far as it has been observed, or can reasonably be assumed to hold 
 good offers no valid objection to the existence of a real causal 
 connection between the mind and the brain. The present position 
 of this law is that of an empirical generalization, found to hold ap- 
 proximately true for a large number of classes of phenomena, and 
 presumably true for yet other classes. To exalt it to the place of a 
 universal and necessary relation among all phenomena of every class 
 mental as well as physical would be unwarrantably to extend 
 its application. Even in the sphere of physical events the law is as 
 yet demonstrably true only to a limited extent. The various forms 
 of physical energy in the inorganic world are as yet by no means 
 all reducible to the terms of this law. Gravitation, on the one 
 hand, and magnetism, or chemical affinity, or cohesion, or the forces 
 that act when every crystal is formed, on the other hand, cannot be 
 as yet related together so as to be expressed in these terms. No 
 mathematical formula, or picture framed by the imagination, has 
 thus far bridged over the gap between the molecular energy of 
 inorganic and that of organic structures. In discussing the phe- 
 nomena of general nerve-physiology, it was made obvious at every 
 turn that even the behavior of the vital nerve-muscle machine 
 under the influence of electrical or other excitation cannot be ac- 
 counted for by aoy conceivable application of the known laws of 
 those forces that move unorganized particles of matter. Nerve- 
 force what it is and what it will do ; what it is as judged by what 
 it will do cannot, at present, be correlated with any of the forms 
 of energy which act as nervous stimuli. Yet who would for a 
 moment hesitate to say that the action of the electrical current, or 
 42 
 
658 CONNECTION OF BRAIN AND MIND. 
 
 of the irritating acid, or of mechanical impulse when applied to 
 the nerve, is a "cause" of the contraction of the muscle? 
 
 The effort of certain scientific observers to bring all causal rela- 
 tion, all action of one being on another, under the law of the con- 
 servation and correlation of physical energy is mistaken, and must 
 prove unsuccessful. The discovery that all the action of physical 
 beings is to be understood only in terms of motion, and that all re- 
 lations of such beings are to be expressed as comparable quantities 
 of motion, either obvious or potential, has, of course, greatly stim- 
 ulated this effort. The effort is to a certain extent laudable. It 
 has unified the physical universe ; it has showed to us this universe 
 all alive, as it were, with unceasing, correlated, wondrous motions, 
 which it is indeed conceivable should be all commensurable one 
 with another. But it should never be forgotten that this picture 
 of an objective world composed of beings called atoms, eternally 
 moving with reference to each other and according to the law of 
 the conservation and correlation of energy, is itself a picture con- 
 structed by the imaging and reasoning mind. As such a mental 
 picture, it is, and must always remain, dependent on the imagina- 
 tion. Mind, as reasoning and imagining, follows the moving 
 beings into minutiae of forms and into places where observation 
 can never reach them. Hence the talk of atoms having "forces 
 inherent " in them, of energy " potential " as well as kinetic, of the 
 "influence" or "action" of molecule on molecule, and mass on 
 mass, under this one great law discovered by modern physics. 
 
 For the principle of causation is of far wider application, and of 
 far more secure foundation, than the law of the conservation and 
 correlation of energy. The one is a law which, in the form of the 
 principle of reason and consequent, is worked into the very struct- 
 ure of the rnind, and is of universal and necessary application to all 
 phenomena ; the other is an empirical generalization, of doubtful 
 import and uncertain extent of application. Indeed, we should not 
 accept the physical law at all, or seek to establish its further appli- 
 cation, were it not that the mental principle is already taken for 
 granted. It is in our search for causes, and as a result of our per- 
 suasion that real beings exist, which act on and are acted on by each 
 other, that we hit upon the hypothesis of the sum-total of their ener- 
 gies as shown by motion remaining unchanged, and of its different 
 kinds being all measurable one against the other. But no objection 
 exists, either in the nature of the mind or in the nature of things, so 
 far as we know, to the reverse of this being true. For example, a 
 world might be constructed in which a certain number of physical 
 beings, of a certain kind (molecules and masses), remained abso- 
 
NATURE OF THE CAUSAL NEXUS. 659 
 
 lutely motionless and unchanged, while all other beings were in 
 perpetual motion. Or a world might be constructed in which the 
 activities of different physical beings, as expressed by motion, should 
 be related in a totally different way from that formulated by the 
 present law of the conservation and correlation of energy. In this 
 imaginary world, some kinds of beings might put forth an amount 
 of energy which was proportional to that of all the energy of the 
 beings acting on them, as the square to the square-root, or as the 
 cube to the cube-root ; and other kinds of beings might act and be 
 acted upon under very different laws of relation with respect to the 
 quantity of energy. In fine, the fact that the law of the conserva- 
 tion and correlation of energy cannot hold true as to the connection 
 between physical and psychical phenomena furnishes no sort of 
 proof against the reality of the mind or of the causal connection 
 between it and the brain. 
 
 18. Nor is there anything in the nature of the so-called " cau- 
 sal nexus " itself which forms a reason why it should not be as- 
 sumed to exist between brain and mind. For what do we mean 
 when we speak of one thing or event as the cause of another? 
 What do we mean when we speak of "influence exerted," "force 
 transmitted" or "passing over" from one being to another, etc.? 
 Nothing that can be explained or illustrated after the analogy of 
 any series or collocation of phenomena, of any relation of one ob- 
 ject to another as discernible by sense or picturable by imagination. 
 Nothing passes from the match to the gunpowder which explains 
 why the latter explodes ; or from the bat to the ball which explains 
 why the latter, when struck by the former, changes the direction of 
 its motion. The proximity of the earth and the unsupported con- 
 dition of the apple, shaken by the wind from its stem, are indeed 
 spoken of as the cause* of the apple's fall ; but no invisible hands 
 are reached up from the ground to draw the apple down. Were 
 such feelers put out to clasp the smaller body and draw it to the 
 larger, the energy of the clasp and its effect would still as truly 
 need an explanation as does the action of the so-called force of 
 gravitation. None of the senses is capable of discovering or ap- 
 preciating the energy that is assumed to act ; the causes of an event 
 cannot be seen, handled, heard, smelled, or tasted. The world of 
 experience given to us by the activity of the senses is a world in 
 which a ceaseless change of objects takes place, but any evidence 
 of a tie connecting the physical phenomena with real beings as 
 their attributes, or connecting our minds with these physical real 
 beings, so that they may be said to affect us, is quite beyond the 
 range of the senses. 
 
660 CONNECTION OF BRAIN AND MIND. 
 
 In general, it may be said that the world of appearances is found 
 by an analysis of our adult experience to be assumed to rest, as it 
 were, upon an invisible world of reality. The popular and uncriti- 
 cal mode of the assumption is, that the world is made up of a great 
 number of real " Things ; " that these things exist e^ra-mentally, 
 just as they appear to us as objects of experience ; that our knowl- 
 edge of them is a more or less true copy, obtained through the 
 senses, of what they extra-mentally are ; and that these things are 
 constantly doing somewhat to each other acting on each other and 
 being acted on by each other. Scientific researches greatly modify 
 the character of the popular assumption. They show that it is de- 
 monstrably false in almost every particular ; and yet they re-estab- 
 lish it in other forms. Physics, by a series of careful observations 
 and subtle and remote inferences, constructs an extra-mental world 
 of moving atoms ; it shows us how these atoms always have regard 
 to each other when they move, and are ceaselessly moving with 
 reference to each other ; it strives to image the direction and veloc- 
 ity of the most infinitesimal of these motions, and to formulate 
 their laws or constant modes of relation. Psychology shows how 
 . the world of mental objects, the only world of immediate experi- 
 ences, is built up by the synthetic activity of mind ; it calls upon 
 the physicist to remember that he has no other way of reaching 
 these atoms, and of discovering the laws of their relations, except 
 by the path of mental activity ; and it reminds him that this activity 
 cannot escape the control of mental law. But both the popular 
 view and the scientific attainment are in substantial agreement as 
 to their fundamental view of the world. Both believe that our ex- 
 perience is explicable only on the general hypothesis of the exist- 
 ence of a vast number of real beings which perpetually act on each 
 other and are acted upon by each other. 
 
 19. The effort to restrict the working of the above-mentioned 
 assumption, in which common-sense and scientific analysis both 
 agree, just at the point where the relation of mind and brain is 
 subjected to scientific treatment, is as needless as it is unavailing. 
 Because it is bDth needless and unavailing it is often absurd. If it 
 be granted that the law of the conservation and correlation of phys- 
 ical energy cannot possibly be applicable to the connection of mind 
 and brain, and yet that all which we know of the nature and extent 
 of this law forms no valid objection to regarding both mind and 
 material atoms as real beings standing in certain relations to each 
 other What good reason can be urged for refusing to affirm a causal 
 connection between the two? The fear that either of the two parties 
 will suffer in dignity or integrity by such connection with the other 
 
CAUSAL INFLUENCE OF THE BRAIN. 661 
 
 may easily be laid to rest ; and if it could not be, it would have no 
 right to interfere with the only reasonable interpretation of psycho- 
 physical facts. Beings do not lose their reality, or characteristic 
 nature, or value in the universe of Being, because they are causally 
 connected with other beings. On the contrary, none but real be- 
 ings can be thus connected with each other ; none but real beings 
 can act and be acted upon. The so-called causal connection is no 
 bondage of such nature as to destroy the nature of the beings which 
 act under it. Only beings that have natures of their own can be 
 causally connected. In other words, all that appears to us as a 
 causal relation between the objects of our experience is, ultimately 
 considered, due to no material spur or whip which urges, or band 
 that represses, as though one kind of real being could thus domi- 
 nate and subdue another. No atom acts without being acted on ; 
 what it does depends both upon what it is and also upon how it 
 stands related to other atoms. 
 
 20. We affirm, then, that we are entitled to say : The changes 
 of the brain are a cause of the states of consciousness ; and the 
 mind behaves as it does behave, because of the behavior of the 
 molecules of the brain. Modify the constitution and functional 
 activity of the material atoms, and you make the activities of the 
 mind, its acts and states of consciousness, to be differently put 
 forth by the mind. The nature and extent of this " influence " of 
 the material basis upon the psychical subject can never be deter- 
 mined a priori, or brought under any general formula applicable 
 only to a restricted sphere of physical action, like the law of the 
 conservation and correlation of physical energy. The nature and 
 extent of such influence must be learned by investigation. It has 
 been the special task of this treatise on Physiological Psychology 
 to investigate and, as far as possible, to formulate the causal action 
 of brain on mind. Such action has been seen to consist chiefly 
 (if not wholly) in determining the intensity, quality, mode of com- 
 bination, and of recurrence in time, of the sensational elements of 
 the mind's activity, and of its other activities so far as dependent 
 upon the sensational elements. 
 
 The affirmation of a causal influence of the brain on the mind, 
 however, does not really work any prejudice to the claims of the 
 mind to be considered a real being, or to be spiritual and free. For 
 the sole account or cause of the mind's activities can, in no instance, 
 be found in the molecular condition and changes of the brain. 
 The simplest sensation must be referred also to the nature of the 
 mind as its cause. It must be considered, not simply as caused by 
 a certain form of nerve-commotion in the cerebral cortex, but also 
 
662 CONNECTION OP BRAIN AND MIND. 
 
 as a psychical activity put forth by the being called mind. There 
 is no incompatibility in these two ways of regarding each state of 
 sensation. Even in the case of some physical event, the nature of 
 each of the factors combining to form the event must be taken 
 into the account. For example, atoms of oxygen will, under cer- 
 tain circumstances, unite with atoms of hydrogen to form water ; 
 under other circumstances they will unite with atoms of iron to 
 form iron-rust ; they may also be mechanically mixed with nitro- 
 gen-atoms to form air, etc. In each case the cause of the result is 
 to be found in the presence with the oxygen, under certain definite 
 circumstances, of atoms of hydrogen, iron, nitrogen, etc. But in 
 each case the cause is also equally to be found in the nature of 
 the atom of oxygen. So every sensation, however closely it may 
 be correlated with the condition and functional activity of the 
 molecules of the brain, must be explained by referring it to the 
 nature of the mind which has the sensation. Nothing which Physi- 
 ological Psychology has ever discovered, or can hope to discover, 
 in the least mitigates the necessity of saying, when the question 
 is asked Why does the mind behave in this particular way under 
 such circumstances ? It is the nature of the mind so to behave 
 when its circumstances are such. In other words, our explanations 
 of the causes of mental phenomena, as lying in the physical basis 
 of such phenomena, does not at all satisfy the need of a real and 
 spiritual subject of the phenomena. 
 
 Moreover, we have seen that there are large and most important 
 classes of mental activities which can scarcely be conceived of as 
 standing in any direct relation to the nerve-commotions of the 
 cerebral cortex. These classes are indeed always allied with phe- 
 nomena of sensation and feeling for which we can trace a bodily 
 basis. But this fact only makes their connection with the brain 
 presumably more indirect. For the explanation of such classes of 
 mental phenomena we are driven much more imperatively and ex- 
 clusively to an appeal to the existence of a spiritual subject, with a 
 nature and laws of action very different from those ascribed to its 
 physical basis, the brain. 
 
 21. We affirm, also, that we are equally entitled to say : The 
 states of consciousness are a cause of the molecular condition and 
 changes of the nervous mass of the brain, and through it of the 
 other tissues and organs of the body. And just as no fear for the 
 reality, integrity, and dignity of the mind prevents us from accept- 
 ing its dependence for the mode of its activity upon the condition 
 of the brain, so no fear for the reality of physical substance, and 
 for the value and extension of physical law, prevents us from as- 
 
CAUSAL INFLUENCE OF MIND. 663 
 
 serting the dependence of the brain, for the mode of its activity, 
 upon the states of the mind. Of course it need scarcely be said 
 again no relation exists between these two kinds of beings which 
 can be represented as an interchange of physical energy, under 
 the law of the conservation and correlation of such energy. This 
 fact, however, affords no objection to our recognizing a true causal 
 connection between the two, unless we are ready to insist upon 
 the monstrous claim that modern physical science is entitled to 
 affirm the impossibility of any interaction (or conditional action) 
 taking place in the universe otherwise than between material atoms 
 under the aforesaid law. 
 
 The phenomena which indicate that mind operates as true cause 
 within the structure of the body are innumerable. They are quite 
 as numerous, though perhaps not so obvious and impressive, as 
 those which indicate the reverse relation. 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 measurement. External stimuli con- 
 stitute the causes of mental changes which we can most easily ob- 
 serve and estimate. Ideas, feeling, and acts of will arising in the 
 consciousness, and considered as causes of the resulting bodily 
 changes, cannot be treated by the same methods of experimental 
 science as apply to the physical stimuli. But that the mind acts 
 on the body is one of the most familiar of experiences. Such ac- 
 tion penetrates and modifies all the life of the body. Hence the 
 material mechanism of the animal structure can never be consid- 
 ered, 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 de- 
 pendent 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 dependent upon the states of the mind. 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 
 
664 CONNECTION OF BRAIN AND MIND. 
 
 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- 
 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 
 consciousness. 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. 
 
 22. No valid objection, therefore, can be urged against con- 
 ceiving of the connection between mind and brain in the following 
 way, at once most natural and most philosophical : The brain is a 
 vast collection of material molecules, whose constitution and ar- 
 rangement are such as to connect them, in a unique way, with cer- 
 
CAUSAL INFLUENCE OF MIND. 665 
 
 tain forms of physical energy outside of the body. Whenever these 
 appropriate forms of energy act upon the parts of the nervous sys- 
 tem lying below, and the impulses are transmitted to the brain, 
 or whenever the chemical character of its blood-supply is altered, 
 then the molecules of the brain are capable of undergoing very re- 
 markable and intricate changes of constitution and arrangement. 
 That is, the brain can be stimulated to certain of its peculiar com- 
 binations of nerve-commotion by external stimuli. Moreover, it is 
 constantly initiating other combinations of nerve-commotion that 
 are apparently not due to such stimuli. Some of its actions, that 
 is, are of a kind constantly arising within the system itself ; they are 
 called automatic. We have as yet no adequate means whatever 
 for making a quantitative statement of the relations which exist 
 between the energy of atoms thus constituted and arranged and 
 the energy of the masses or molecules that serve as stimuli of the 
 system composed of these atoms. Nervous energy is not an entity 
 to be dealt with by a sum in addition and subtraction of momenta. 
 For aught we know, it is of the nature of atoms, when they are 
 brought into relations so extraordinary as those which prevail in 
 the nervous system, to behave with reference to each other in a 
 way that is wholly irreducible to any simple formula like that of 
 the conservation and correlation of energy. If this should final- 
 ly appear to be indubitably true, the fact would not be specially 
 mysterious. All action and reaction of the atoms is mysterious ; 
 the methods of it are to be learned from experience as ultimate 
 and inexplicable facts. 
 
 Still further, the molecules of the brain are so constituted and 
 arranged as to be capable of standing in yet more surprising and 
 unique relations to a being of a different nature from their own 
 that is, to the mind. These relations involve a causal connection as 
 truly as any relations of real physical beings in which such beings, 
 as we are compelled to believe, act on each other and are acted on 
 by each other. That molecules thus constituted and arranged are 
 causally connected with the subject of consciousness is an ultimate 
 fact ; it involves the nature of both classes of beings thus connected 
 of the brain and of the mind ; it involves also the action of each 
 upon the other. In speaking, however, of mind and brain as act- 
 ing on each other, we accomplish nothing whatever for the comple- 
 tion of the picture by trying to introduce the conception of ener- 
 gy "transmitted" or "passed over" from one to the other. The 
 simple, ultimate fact remains, that how each behaves depends upon 
 the behavior of the other. It is the business of psycho-physical 
 science to discover, if possible, the general modes of this depend- 
 
666 CONNECTION OF BRAIN AND MIND. 
 
 ence that is, the laws of the relation between the mind and the 
 brain. 
 
 23. In more particular description of the connection between 
 the mind and the brain, it may be said that all intercourse between 
 material objects and the spiritual subject involves three processes 
 a physical, a physiological, and a psychical. In these processes 
 the perceived object and the perceiving subject mutually conation 
 each other. This fact, however, does not destroy the necessity, 
 under which all scientific investigation finds itself, of assuming that 
 both object and subject exist as real beings. The physical pro- 
 cess 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-com- 
 motion 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 mate- 
 rial 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. But that the law of the conservation and correlation of 
 energy, as formulated for much simpler cases of the relations of 
 forces between inorganic 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 confi- 
 dence. 
 
 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 pro- 
 cesses returning energy to the world outside of the body. 
 
MIND A REAL BEING. 667 
 
 When the mental process is a perception of some object, called 
 an " external " object, it is no less truly & 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. 
 
 24. 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 ex- 
 perience. There is nothing which we know about the nature of 
 material beings 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 forbids the aforesaid assumption. On the 
 contrary, everything which we actually know, as distinguished 
 from what we conjecture to be true, or would like to have true, for 
 the satisfaction of certain of our quasi-scientific or ethical impulses 
 favors this assumption. And no other assumption, substantially 
 different from this, is compatible with the facts of experience. 
 
CHAPTER IV. 
 THE MIND AS REAL BEING. 
 
 1. No attempt need be made to conceal the fact that the last 
 three chapters have given to the phenomena and laws of Physi- 
 ological Psychology a " metaphysical " treatment. In the intro- 
 duction (see 5) to the scientific discussion of the subject, the in- 
 tention finally to raise and answer certain metaphysical questions as 
 to the nature of Mind was frankly avowed. Indeed, since all dis- 
 cussion of those assumptions which underlie our experience of what 
 we call "reality" is metaphysical, it is not easy to see how the 
 science of mind from whatever point of view approached can be 
 thorough and conclusive without involving metaphysics. In this, 
 the concluding chapter, certain still more distinctively metaphysi- 
 cal inquiries must be briefly pursued. The mind has been spoken 
 of as "real," "spiritual" (or non-material), a "unit-being," etc. 
 These are terms which require further explication. What is meant 
 by speaking of the mind as a real being ? What is it to be, in 
 reality, spiritual rather than material? What is the real nature of 
 that unity which belongs to mind ; and on what grounds do we 
 affirm that the mind is a " unit-being f " 
 
 Thus far the effort has constantly been made to maintain a close 
 connection between the answer given to semi-metaphysical inquiries 
 and the facts of physiological psychology. In rendering such an- 
 swer the appeal has constantly been taken to the facts. Should 
 the facts, in any case of appeal, not bear out or, at least, should 
 they contradict the conclusions alleged to be based upon them, 
 then the conclusions must be modified, or change the basis on 
 which they assume to rest, or utterly fall. But in answering the 
 more distinctively metaphysical questions now raised as to the nat- 
 ure of mind, the psycho-physical facts are of little direct assist- 
 ance. Such questions are fitly raised at the conclusion of psycho- 
 physical researches only because these researches have led us to a 
 certain view as to the nature of the subject of the researches 
 namely, as to the nature of the mind. 
 
 2. The mind is a "real" being in the highest sense in which 
 
THE POPULAR IMPRESSION. 
 
 any finite being can be real. Indeed, its claim to be considered 
 real is more indisputable than the same claim as put forth for any 
 material thing ; it is unique. The reality of mind underlies and 
 makes possible all our knowledge of other real beings, and all our 
 assumptions as to the existence of such beings. It is only on con- 
 dition of granting its reality, in the highest sense of the word, that 
 we can affirm the reality of other beings. 
 
 There can be no doubt that the popular impression attributes a 
 reality to material things which it does not consider to be pos- 
 sessed by the mind. This impression makes the clearly visible 
 and hard, tangible substances, experience of which constitutes so 
 important a part of ordinary working-day life, the test and stand- 
 ard of the most indubitable reality. Substantial as a rock (that 
 is, a presentation of sense which consists of certain qualities made 
 known especially to the tactual and muscular senses) ; and unsub- 
 stantial as a day-dream (that is, a series of representative images 
 largely free from all admixture of presentations of sense) thus 
 does the popular estimate express itself with respect to the reality 
 of the phenomena referable to things as compared with the pure 
 states of mind. Materialistic objections to the reality of mind, 
 when made to rest upon scientific data, repeat and confirm the 
 popular impression. These objections ordinarily assume that no 
 doubt can be raised as to the reality of material " Things." Such 
 things as are real in the highest sense of the word, however, are 
 not now understood in the same way as the things indicated in the 
 popular impression. Only the atoms, or elemental and permanent 
 factors which enter into the composition of all the objects of sense, 
 are held from the scientific point of view to be real in the highest 
 sense of the word. 
 
 The things of experience by the senses are admitted to be con- 
 stantly changing, and at no time, extra-mentally, to resemble the 
 unchanging material realities with which science deals. Things, 
 as they appear to the eye and to touch, are spread-out, continuous, 
 without empty space between them, and for the most part motion- 
 less, except as they are moved in masses by application of external 
 energy. Things as they really are, however, are neither spread- 
 out, nor continuous, nor motionless. On the contrary, they consist 
 of a countless number of invisible and intangible real beings, called 
 atoms, that are ceaselessly moving, with incredible velocity and in- 
 tricateness of changing directions, in empty space, and according 
 to forces inherent in them. These atoms are real, and have always 
 been the sole element of all which appears as real ; so this form 
 of quasi-scientific metaphysics goes on to declare. 
 
670 THE REALITY OF MIND. 
 
 On the other hand, it is argued by certain advocates of the fore- 
 going view that the so-called mind is wanting in every characteristic 
 which could justly entitle it to be called a reality. Certainly it is 
 not adapted to win the popular respect as a hard and solid sub- 
 stance, which it is difficult to move, and impossible to remove from 
 the sphere of possible sensations. What is the mind, in reality f 
 It cannot be seen or touched, or apprehended by any of the senses. 
 It cannot be imaged as spread out in space, or as space-filling, by 
 virtue of some physical energy streaming uninterruptedly forth 
 from the mathematical point at which it is situated. It can do 
 nothing except through the body ; that is to say, all that is done, 
 which could possibly be referred to the mind, is really done by the 
 body. And the body is a material mechanism, which is nothing 
 except as it is constructed out of the same atoms, with their inherent 
 forces, which have composed the star, the crystal, the flower ; and 
 which can do nothing except as the ceaseless play of the energies 
 of great Nature (of which it is a point, a part) are kept playing 
 through it. Without the physical mechanism, as a real existence, 
 there is no manifestation of so-called mind, no manifestation actual, 
 possible, or conceivable. And when this mechanism is dissolved, 
 the mental phenomena, so far as appears, wholly cease. What, 
 then, is Mind ? What claim to reality can it possibly make valid ? 
 
 More particularly, certain puzzling questions regarding the nature 
 of the mind's behavior may be raised by the advocates of the same 
 foregoing view. Where is the real mind, it may be asked, when 
 consciousness is gone, as in swooning or deep sleep ? What becomes 
 of the mental Being when the mental faculties one by one drop 
 away, as in cases of general paralysis ? What worthy kind of real- 
 ity can belong to the subject of phenomena so evanescent and tem- 
 porary, so incapable of being measured, and weighed, and related 
 to the permanent forces and beings of the material world ? More- 
 over, if mind is a real being, what shall be said in answer to the 
 inquiry, why certain of the lower animals can apparently divide up 
 their souls by fission of their physical structure ? And cannot even 
 man's proud unity of real being become disturbed by the accident 
 or disease which results in a double consciousness, or in the loss 
 of all previously acquired knowledge of the mind as previously 
 existing and developed? 
 
 3. That many puzzling, and even unanswerable, questions can 
 be asked concerning the nature of the mind, we have no interest to 
 deny. Doubtless, if difficulties growing out of our inability wholly 
 to clear up our ideas of " real being," " self-identical and perma- 
 nent existence," etc., are objections to believing that any real beings 
 
THE POPULAR IMPRESSION. 671 
 
 exist, they are also objections to our believing in the reality of mind. 
 But certainly it is one thing to ask unanswerable questions regard- 
 ing the ultimate nature of any particular real beings, so called, and 
 another thing to prove that our belief in the existence of such real 
 beings is unfounded. Moreover, the fact that we cannot conceive 
 of or define the real being of the soul, in terms which apply to 
 material things, is cheerfully conceded. Indeed, it is this general 
 fact upon which chief reliance is placed to prove that the being of 
 the soul is unlike that of "Things" is non-material, or spiritual. 
 But how can this fact prejudice the claim of the soul to be real, un- 
 less it has previously been established that to be hard, and round, 
 etc., or to be a minute material bit (an atom) ceaselessly in move- 
 ment, is necessary in order really to be at all? The truth is, how- 
 ever, that both the popular impression and the more scientific 
 theory, just so far as they can cogently be urged against the reality 
 of spiritual being, themselves rest on the most unverifiable and ab- 
 surd assumptions. 
 
 The popular conviction of the indubitable, and, as it were, supe- 
 rior reality of certain classes of things is easily explained as the 
 necessary result of the development of experience. All things 
 which are pre-eminently real, in this meaning of the word, are cog- 
 nizable by means of tactual and muscular sensations marked by a 
 strong color-tone of feeling. Things merely smellable have no 
 "reality," in this sense of the word, because they are not apprecia- 
 ble by touch and offer no muscular resistance. We cannot put our 
 hands on the effluvia which excite the olfactory nerve ; the air when 
 laden with sweet and sickening odors is not tangible or impenetra- 
 ble. Real things that is, things which can be seen and handled 
 are, however, regarded as the sources of our sensations of smell. 
 What is true of sensations of smell is also true, in less degree, of 
 sensations of sound. But in the case of sound we are generally 
 able at once to refer the origin of the acoustic sensation to some 
 so-called real " Thing." Objects tasted are popularly regarded as 
 real, because they are objects which are handled before tasted, and 
 constantly touched as they are being tasted in the mouth. A "bad 
 taste in the mouth " is not regarded as giving evidence of the pres- 
 ence of any real thing ; it may simply be regarded as a sensation 
 located in that region. Accordingly, one does not consider one's 
 self to be tasting one's own mouth as a real thing, although one 
 may say that the mouth has a bad taste. 
 
 . Even when the presentation of sense is a clearly visible object, 
 it does not necessarily seem to have the characteristics of a real 
 thing. For the object of vision readily and quickly changes its 
 
672 THE REALITY OF MIND. 
 
 color, apparent magnitude, characteristics of superficies, and visible 
 outlines as a solid. Moreover, everybody knows that his eyes have 
 often deceived him ; even when they have been closed visual im- 
 ages have appeared before the mind, such as could not possibly 
 represent any so-called reality. But the nature of tactual and mus- 
 cular sensations is different from the visual in several important 
 particulars. Such sensations, on bestowing the requisite attention, 
 may ordinarily be brought strongly into consciousness. They do 
 not so readily change their quality regarded as coming to us from 
 an apprehension of the properties of things. The feeling of effort 
 colors them highly ; and the pain from being struck, pressed, 
 pinched, or impeded, is a frequent accompaniment. Therefore, 
 children are educated in their knowledge of, and belief in, a world 
 of reality by being constantly resisted by material things ; and adults 
 naturally suppose that when they can lay hands on an object they 
 know that it really is, and what it really is, with a certainty impos- 
 sible in any other way. 
 
 It wholly escapes the ordinary observation that the same assump- 
 tions whether they be deemed verifiable or unverifiable underlie 
 the conviction of the reality of things tangible which belong to 
 the operation of all the senses. It is true, as experience shows, 
 that tactual and muscular sensations are, from their very nature, 
 and from the manner and frequency of their recurrence, pecul- 
 iarly adapted to serve the mind well in those acts of synthesis 
 by which it constructs the real things of its experience. But this 
 fact does not in the least diminish the force of the other fact 
 namely, a certain assumption or postulate as to an extra-mental 
 reality (an X, which is not any one of the attributes a, b, c, etc. 
 of the " Thing," but which is the subject or ground of them all) 
 underlies and conditions all the apprehensions of sense. Without 
 granting and using this assumption we cannot affirm that even by 
 tactual and muscular sensations we know any reality whatever, 
 beyond the real fact that so our own minds stand affected with the 
 presentation of an object of sense. If the popular impression con- 
 cerning the reality of "things " does not extend beyond this simple 
 act of self-knowledge, as it were, it certainly forms no ground for 
 affirming the superior and undoubted nature of such reality. 
 
 4. The cause of the scientific objector to the reality of mind, as 
 standing on an equality, with respect to the cogency of its evidence, 
 with those material atoms about whose reality he tolerates no doubt, 
 is not a whit better off than is that of the popular impression. In- 
 deed, it is by no means so good. That ready-made " things " really 
 exist in independence of mind (meaning by such " things" the ob- 
 
THE SCIENTIFIC OBJECTION. 673 
 
 jects of everyone's immediate experience) is a proposition which it 
 involves fewer doubtful elements to maintain than the proposition 
 that so-called " atoms " have such existence. It is time to raise the 
 question : How can one know, so confidently, that those cerebral 
 molecules exist e.*tfnz-mentally, with all their incalculable and almost 
 inconceivable motions, on whose real being the phenomena of mind 
 are sometimes made to depend? The brain it is claimed by Ma- 
 terialism may be made responsible for mental phenomena, for the 
 latter are mere manifestations in consciousness of the changes which 
 are going on in the material constituents of which this organ is 
 really composed ; there is no need of a real non-material being as 
 the subject of the mental phenomena ; the physical phenomena, 
 however, must have some real being as their subjects ; such reality 
 is to be found in the molecules of the brain. But what are the 
 grounds and the nature of our knowledge of this wonderful con- 
 jurer styled the brain ? 
 
 The so-called scientific argument against the reality of mind, as 
 often applied, may be stated in terms somewhat like the foregoing. 
 Little examination is needed, however, to show that its conclusive- 
 ness involves certain assumptions which cannot themselves be vali- 
 dated without weakening or destroying the very ground on which 
 the argument is itself based. Let the case be tried by making a 
 beginning with that sort of testimony with which everyone is most 
 familiar. I know that I think, feel, will ; that is to say, phenomena 
 take place in consciousness which there is no conceivable way of 
 describing except by attributing them to the subject of all con- 
 sciousness to the self-conscious " me " called mind. But because I 
 cannot perceive this subject of all consciousness as an extended and 
 external somewhat 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. For if physical 
 science has established anything whatever with regard to a particu- 
 43 
 
674 THE EEALITY OF MIND. 
 
 lar organ or substratum of the mental phenomena, it is that such 
 organ or substratum is the brain. 
 
 But the inquiry must next be raised : How does one know that 
 one has a brain, which may serve as the real substratum of the 
 phenomena of one's consciousness ? It scarcely need be said that 
 no one has ever had 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. It may even be 
 that the 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. It is certain that no ego 
 has directly observed the molecular changes of any central nervous 
 mass, whether belonging to another or to itself, when such mass 
 was engaged in the activities whose resultant the phenomena of 
 consciousness are claimed to be. Since there is such scarcity of 
 direct ocular and tangible demonstration of a special relation be- 
 tween the brain and mental phenomena, it is plain that the testi- 
 mony of experts must be summoned. Resort must be had to the 
 great anatomists and experimental physiologists who have had most 
 experience as to the structure and functions of the brain-mass. 
 
 It must, of course, be confessed that no expert has any more 
 direct evidence than every self-conscious ego has of the existence 
 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 par- 
 ticular ego which each one calls " myself" is connected with a brain. 
 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 arn asked 
 to dispense with my confidence in the reality of the being which 
 
THE ATOMS AS AGENTgfc^C/} /^ 675 
 
 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. 
 
 5. 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. In all the first Part of our 
 investigation we saw how necessary physiology finds it to regard 
 the nervous centres as molecular mechanisms. Nothing that is in 
 itself of first importance appears to the eye of the observer who 
 looks upon the freshly extracted mass of the human brain. And 
 when this" mass has been skilfully prepared for investigation under 
 the microscope, the investigation itself does not reveal directly, to 
 the highest magnifying powers of the glass, the ultimate agents 
 in the wonderful drama it has been playing. These agents are the 
 atoms, to whose real being and so-called " inherent " forces all that 
 is done by the complicated mechanism must be referred. But the 
 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 stupendous 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- 
 ness. From these real beings and their relations there must be 
 derived, not only the activities which all ascribe to nervous matter, 
 but also those which some are 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 materialist 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 
 
676 THE REALITY OF MIND. 
 
 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. 
 
 The phenomena whose rational explanation seems most peremp- 
 torily to demand some hypothesis of atoms are the phenomena of 
 chemistry. When, however, the further inquiry is raised as to the 
 real nature of the atoms, it is found that modern physical science 
 is by no means satisfied with its own answer. Dynamical theories, 
 tending to resolve the atoms into mathematical points acting as 
 mere centres of force, contend with other more realistic theories 
 which regard the atoms as simply the smallest bits of matter into 
 which we can by any known means break up the larger collections. 
 What is meant by the forces being " inherent " in the atoms is a 
 still more difficult question to answer. Indeed, to this question no 
 answer can be given which gets much beyond the simple declara- 
 tion of the facts of experience ; that is to say, these hypothetical 
 and yet sole real material beings are always supposed to behave, 
 with respect to their motions, in the same way under the same re- 
 lations, and something can be done by science toward measuring 
 their various motions in terms one of the other. 
 
 Moreover, the best efforts of modern investigation to describe 
 the nature of the atoms 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 form 
 of being that binds them together, as it were, and makes them 
 work to a unity of plan. But here, again, we are reminded 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 phys- 
 ical 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 perceiving the things ; for the study of perception, from 
 the physiological point of view even, has enabled us to show that 
 
ATOMS AS DUE TO MIND. 677 
 
 no so-called "thing" is a ready-made material product, appre- 
 hended by mind in a form which is a copy of some extra-mental 
 being. In trying, therefore, to comprehend what is the nature of 
 those real beings (the atoms), on whose existence, activity, and rela- 
 tions all mental phenomena are assumed by Materialism to depend, 
 we find that the picture we frame of them is the work of the mind. 
 
 6. 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 
 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. 
 
 And now let the attempt of materialistic theory be made anew ; 
 let it be assumed that the phenomena of consciousness have no 
 real subject in the mind. Such phenomena must, accordingly, be 
 attributed to the peculiarly constituted and mutually interacting 
 molecules of the brain. But these supreme physical beings are 
 themselves, as far as they are the object of knowledge, pre-emi- 
 nently mental creations ; and the sole warrant for carrying them 
 over into the realm of extra-mental reality consists in certain irre- 
 sistible convictions or assumptions of mind. To make their real 
 being the account of the mental phenomena, and thus to deny the 
 real being of the subject of mental phenomena, is not onty to ex- 
 plain what is most direct and certain by what is most indirect and 
 uncertain ; it even involves the wonderful paradox, that the one be- 
 ing in whose active energizing all conceptions of all real being arise, 
 feels justified in denying its own reality in the supposed favor of cer- 
 tain of its most remote and doubtful conceptions. 
 
 7. 'What is meant by affirming the reality of mind may be 
 made obvious by pursuing the following train of reflections : In 
 the development of the mental life its phenomena come inevitably 
 
678 THE REALITY OF MIND. 
 
 to divide themselves into two great classes. As it appears to adulfc 
 experience, not only the unfolding, but even the very existence of 
 self-consciousness seems to involve the distinction between 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 unknown X which is not 
 I). It is only as involving all this mental process that any real 
 being is known or believed to exist ; but the mind in the develop- 
 ment of experience inevitably completes the process, which involves 
 the assumption that real beings do exist, and that all these real 
 beings are either " things," such as I know, or myself and other 
 conscious beings, such as I am. What any real being is can only 
 be told by an enumeration of its so-called attributes ; and this is as 
 true of myself as of the things which I know. It is also as true of 
 them as it is of myself. If the foregoing statements covered the 
 entire case, it would simply be true that I have no better reason 
 for attributing a real Being to any material thing than to the 
 subject of consciousness. But we have already seen that the pro- 
 cess by which we reach the real being of the molecules of the 
 brain is much more indirect and doubtful than that by which we 
 reach the affirmation of a real being for the things of daily experi- 
 ence and for the subject of all that experience. 
 
 8. Peculiar and cogent reasons may be given, however, which 
 further enforce and verify the assumption of a real existence for 
 the Mind. We have seen (comp. Part II, chap. X., and the pre- 
 ceding chaps, of Part III.) that there is a class of so-called mental 
 faculties, most important and distinctive, for the distinguishing 
 characteristic of which no physical analogies or correspondences 
 whatever can be discovered or imagined. This is true of memory 
 as active reminiscence, of the unity of consciousness, of voluntary 
 attention, and of the relating activity. The existence of these 
 modes of mental behavior requires the assumption of a charac- 
 teristic real being, other than the molecules of the brain, to which 
 they may be referred. Some of these modes of behavior are con- 
 spicuously 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 which does not 
 
TO BE AND BE CONSCIOUS. 679 
 
 involve its acceptance can possibly be devised. But the present 
 state of memory is a state of my consciousness, and the state which 
 it claims to represent was also a state of 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 
 represented 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 
 constitute the syllogism. 
 
 To "be really" and to be the one permanent subject of changing 
 states, are, in our conception, but different ways of expressing the 
 same truth. That really is which is such a subject of its own 
 states. It is for this reason that modern physical science regards 
 the atoms as having a permanent reality which does not belong 
 to the composite structures the things of our experience into 
 which the atoms enter. Every " Thing " may perish that is to 
 say, as such thing, it may cease to be the object of observation, 
 the subject of states. But the atoms are supposed to remain with 
 unchanged natures through all the changes of relation which they 
 may undergo toward other beings with somewhat similar natures. 
 Even if we were obliged to adopt the hypothesis of a constant 
 change of states in the interior of the atoms, since every atom 
 shows a variety of possible activities according to the relations in 
 which it stands for the time to other atoms, it is not considered 
 to have lost its real being or distinctive nature by changing its 
 states. For it the atom can be brought into the same relations 
 again, and then it will again display the same modes of behavior. 
 Its reality does not depend upon its interior rigidity, the unchang- 
 ing nature of its being ; it rather depends upon its capacity for 
 being the subject of so-called states, and for following a law or an 
 idea which recalls it, as it were, to the same states when the same 
 circumstances recur. 
 
 How can it be denied that all our conceptions of the atoms as 
 enduring subjects of various states are derived from our experience 
 with ourselves ? The " I " which is the subject of all consciousness 
 is accustomed to attribute to itself every state of that great variety 
 
680 THE REALITY OF MIND. 
 
 into which consciousness may be shaped. The states are changing ; 
 they have a transitory and phenomenal being. But they are all 
 states attributable to one subject. On what ground, then, shall one 
 undertake to deny the confidence which the soul comes to have in 
 itself as the real and permanent subject of its own states ? For we 
 can form no conception of real being at all which is not modelled 
 after this pattern. To have a variety of changing states attributed 
 to it as the subject of them all this is to demonstrate in conscious- 
 ness a claim to real Being. Unchanging rigidity, the permanence 
 of the mathematical point or of the material atom, on the supposi- 
 tion that the latter undergoes no interior changes whatever, if such 
 rigidity and permanence anywhere exist, constitutes no claim to the 
 title of real being. 
 
 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 belonging to itself. But its law is that of 
 development ; and, unlike all " things " which are subjects of va- 
 rious kinds of evolution, so called, the soul can recognize the law 
 of its own being. When, therefore, we are asked what the Mind 
 really is, we can respond by telling what it comes to be as the re- 
 sult of its unfolding under the fixed conditions of its native powers. 
 But these " powers " cannot be called native, as though they were 
 actual achievements of the mind's inborn faculties, or separate forms 
 of energy inherent in it, after the analogy of the forces said (some- 
 what unintelligibly, it must be admitted) to be " inherent " in the 
 atom. 
 
 But we do not define the nature of any real being simply by 
 stating how it appears and behaves in its most germinal and unde- 
 veloped form. The tree explains the seed ; the adult bird, the 
 egg ; the character of the highly differentiated product must be 
 studied in order to know the full description of the energies that 
 are potential in the simpler stages. It is an undoubted fact that 
 the mind has a history in each individual case ; and in each case 
 such history is a development. The great service which Physiolog- 
 ical Psychology has rendered to the general science of mind con- 
 sists in its desciiption of the nature and stages of this development, 
 so far as concerns the phenomena of sensation and perception by 
 the senses. This self-recognizing unity of development which be- 
 longs to the mind is a striking proof of the validity of its claim to 
 be considered a real being. As the being which acts and knows 
 itself as acting, which is acted upon and knows itself as affected, 
 which is the subject of states and itself attributes these states to 
 
NON-MATERIALITY OF MIND. 681 
 
 itself, which develops according to a plan and so remembers and 
 comprehends the significance of its past states that it can recognize 
 the fact of its own development as such a being the Mind is more 
 entitled to consider itself " real " than to consider real any of the 
 various objects that, immediately or indirectly, appear before it in 
 the course of its history. 
 
 9. The question whether the mind is to be spoken of as non- 
 material or " spiritual " scarcely merits the grave and lengthy dis- 
 cussion to which it has often been carried. Materiality, as predi- 
 cated of any real being, is only a complex term including a num- 
 ber 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 complex, general notion of "materi- 
 ality," no one would find it easy to think of giving this question an 
 affirmative answer. The mind attributes to "things" the qualities 
 of extension, impenetrability, and all the various subordinate mod- 
 ifications of these qualities. It perceives these things as colored, 
 cold, hot, rough, smooth, etc. But it does not attribute such quali- 
 ties to itself ; it can find nothing in the modes in which it mani- 
 fests 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 feeling. But when the suffer- 
 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-fero), 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. 
 
682 SPIRITUALITY OF MIND. 
 
 We have no way, however, of telling what is the nature of any 
 so-called real being except by enumerating its qualities, or those 
 modes of behavior which we attribute to it on account of its affect- 
 ing our consciousness in certain definite ways. To attempt to re- 
 gard the mind as material, when it manifests itself to 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 different 
 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 everyone who admits that mind is a real being with 
 any nature whatever. 
 
 10. 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 describes 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- 
 ual, in the only sense in which we'are entitled to affirm spirituality 
 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- 
 
MENTAL ACTIVITIES SPIRITUAL. 683 
 
 bodied mind, would perceive, and feel, and think, and will, is a ques- 
 tion toward the answer to which we can make no beginning. To 
 attempt its answer at all involves us in the vain effort to use the 
 very relations which are most inseparately connected with the con- 
 scious activity of the mind in such way as to escape from the con- 
 trol of the relations themselves. 
 
 It is true, nevertheless, that a marked difference exists in the 
 directness and intimacy which belong to different classes of mental 
 states, as regards their comparability to the classes of physical 
 stimuli which rouse the mind to its fundamental activities. Those 
 which appear to be most indirectly, and, as it were, loosely related 
 to these fundamental activities are fitly most relied upon to show 
 the spiritual nature of mind. To control the mental train as dis- 
 tinguished from being a passive member of a mental mechanism, 
 to reason so as to deduce conclusions and make inductions to gen- 
 eral laws, to recognize the call of duty, and to call up and classify 
 in the consciousness the lofty and complex ideas which answer to 
 words like " beauty," " truth," and " God " these and other simi- 
 lar operations of the mind pre-eminently emphasize its spirituality. 
 
 11. 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 act us 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. Wherever 
 in all space it might be moved, or whatever in all time might hap- 
 pen to it, it could ex hypothesi never be made two. If, now, such a 
 unit-atom were to be endowed with consciousness and spiritual be- 
 ing, how secure would its unity continue to be ! Unlike the mind 
 of man, it could not fear that some rude concurrence of other 
 atoms, not of the right affinities, or setting themselves in untoward 
 relations, would dissolve its complex material substratum and so 
 
684 THE UNITY OF MIND. 
 
 destroy its spiritual oneness. The molecules of the human brain 
 are in number beyond computation ; they are highly complex and 
 unstable compounds ; they are not so protected by their inclosure 
 in the cranial cavity as to make them invulnerable against all man- 
 ner of assaults. In how dangerous a position, then, is this so highly 
 valued unity of our present spiritual organization ! 
 
 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 hard- 
 ly 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 
 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. 
 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 variety and 
 nobler in quality the mental processes themselves become. More- 
 over, so far as we can ascertain, the highest unity of consciousness 
 belongs in connection with the greatest complexity of the material 
 
THE UNITY OF THE ATOM. 685 
 
 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. 
 
 12. No information derived from the study of Physiological 
 Psychology warrants us in affirming that a highly developed self- 
 conscious 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 
 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 representa- 
 tive ideas. What experience would be, if its basis were not laid 
 in such rise and combination and recurrence of sensations, we can- 
 not even conjecture. In the highest flights of imagination, in the 
 profoundest explorations of reflection, we never escape out of the 
 influences arising from this basis. The nature of this psychical 
 basis of sensation and perception depends upon the nature of the 
 physical basis of the living and acting brain. In other words, what 
 sensations and perceptions constitute, at least in part, the " stuff" 
 of all consciousness depends upon what the molecules of the cen- 
 tral 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. 
 
 13. Furthermore, the unity of a single indestructible and eter- 
 nally 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 consciousness. 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 
 
686 THE UNITY OF MIND. 
 
 psychological point of view we can conceive of the unit-atom as 
 having an experience resembling our own. We, in our conscious- 
 ness, 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 cus- 
 tomary mental behavior. But this is quite a different thing from ex- 
 plaining the consciousness 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 con- 
 sciousness is always some particular definite state ; and self-con- 
 sciousness is always the being aware of some particular definite 
 state. There is no consciousness in general ; there is no conscious- 
 ness which does not involve change of state. Indeed, change is a 
 reality in human consciousness, if nowhere else in the universe of 
 being. No particular state of consciousness, whether considered as 
 involving an attribution of that state to a subject or not, could be 
 explained by reference to the material nature or condition of such 
 a unit-atom. 
 
 14 The foregoing remarks have their value chiefly as a warn- 
 ing against supposing that the unity of the soul's real being suffers 
 any prejudice because it is not to be regarded or explained from a 
 point of view furnished by physical analogies. To be one, as a rigid 
 material atom may possibly be regarded as one, would be no ad- 
 vantage to the soul. Or if it be admitted that, in case it had such 
 unity, it could never lose its real being, it must also be admitted that 
 we are unable to see how it could ever gain any real being as a soul. 
 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 realty 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 
 
SELF-APPEAKANCE OF MIND. 687 
 
 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. 
 
 15. That the mind attributes its own conscious states to a sub- 
 ject of such states (the " I " in all sentences such as " I think," " I 
 feel,"^tc.), we have seen it necessary to admit from the very be- 
 ginning of our psycho-physical researches. As one result of the 
 study of perception by the senses, it was also found necessary to 
 recognize a certain unifying or synthetic activity in order to ac- 
 count for the way in which sensations combine to form the " pres- 
 entations of sense ; !' such unifying activity seems plainly to imply 
 the existence of a unit-being, the so-called Mind. Further argu- 
 ments in the same direction came to light as the phenomena of the 
 mind in memory, voluntary attention, and judgment were brought 
 under examination. More recently still, an examination of the 
 factors of self- consciousness, from the more purely introspective 
 point of view, has confirmed the same opinion. In this connection 
 we may add, finally, the argument for the existence of the mind as 
 a real unit-being, which has been so forcefully urged by one of the 
 greatest of modern psychologists (Lotze). The mind is a real unit- 
 being, not simply because it appears to itself to be such, but chiefly 
 because it appears to itself at all. Granted that all that which 
 only appears to another maybe mere seeming, it still remains indis- 
 putable that somewhat appears. The somewhat which merely ap- 
 pears may be really many when it appears as one ; this happens, 
 in some sort, in the case of all " Things " which appear many or 
 one according to the way we consider them. But how can that to 
 which all else appears, whether as one or as many, and that which 
 also appears to itself whether it appear to itself as one or as many 
 really be other than one, in the highest sense of the word unity ? 
 No twisting of imagination, or subtlety of argument, can show how 
 a mind not really one could appear to itself at all ; or break the 
 strength of the conviction inwrought into the very structure of 
 human self-consciousness, that the real and spiritual being, which 
 we call Mind, is not a fortunate confluence or phenomenal centre 
 of changing modes, but a unit-being, and a reason of all unity in 
 whatever becomes the object of its thought. 
 
 16. As to the first and last things of the Mind its origin and 
 destiny, its mortality or corruptibility Physiological Psychology 
 finds itself unable to pronounce. It cannot, indeed, explain the en- 
 tire being of the mind as arising out of the development of the 
 physical germ from which the bodily members unfold themselves. 
 It knows no decisive reason against the belief that such a non- 
 
688 THE UNITY OF MIND. 
 
 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 exist- 
 ence for the Mind. But, in general, if it remain faithful to its own 
 mission, within its own limits, it entrusts the full consideration of 
 these questions, after it has cleared the way from barriers of igno- 
 rance and prejudice, to Kational Psychology, to Ethics, to Meta- 
 physics, and to Theology. 
 
INDEX. 
 
 AGRAPHIA, nature of, 294 
 
 Allen, Grant, on nature of feeling, 501 f., 
 
 521 
 Aphasia, phenomena of, 292 f. ; kinds of, 
 
 294 f. 
 Aqueduct of Sylvius, 88 ; gray matter of, 
 
 91 
 
 Aqueous Humor, the, 173 
 Arachnoid, the, structure of, 63 f. 
 Attention, effect of, on reaction-time, 
 
 480 f., 495 f.; physical basis of, 538 f., 
 
 542 f.; effect of, on perception and 
 
 memory, 539 f . 
 
 Aubert, on measurement of light, 374 f. 
 Automatic Action, nature of, 49 f., 130 f.; 
 
 in spinal cord, 138 f . ; and brain, 144 f. ; 
 
 physical basis of volition, 535 f . 
 
 BAIN, on local signs, 397 (note) ; theory 
 of feeling, 501 ; feeling of effort, 524 
 
 Baxt, on reaction-time, 481 
 
 Bechterew, on the olivary bodies, 150, 
 161 f . ; and central gray matter, 161 f. 
 
 Bell, Sir Charles, discovery of, 123 f . 
 
 Ben eke, on nature of feeling, 503 
 
 Berger, on reaction-time, 478 
 
 Bernstein, on exhaustion of nerves, 109. 
 
 Betz, "giant-cells" of, 97, 283 
 
 Birge, E. A., on number of nervous ele- 
 ments, 46, 70 ; excitability of cord, 143 
 
 Blastoderm, the, 200 f.; layers of, 202 f.; 
 areas of, 202 f. 
 
 Blind-spot (papilla optica), 183 
 
 Body, general relations of, to mental 
 phenomena, 560 f.; early development 
 of, 562 f., 567; phases of, 565 f.; sexual 
 differences of, 570 ; relative proportions 
 in, 571 f . ; race-characteristics of, 573 f . 
 
 Brain, chemistry of, 25 f., 27 f.; mem- 
 branes of, 61 f.; structure of, 73 f., 85 
 44 
 
 f . ; ventricles of, 85, 149 f . ; ganglia of, 
 85 f.; hemispheres of, 91 f.; lobes of, 
 92 f.; cortex of, 95 f.; inhibitory in- 
 fluence of, 143 f.; as central organ, 
 143 f.; development of , 204 f.; general 
 functions of, 239 f.; temperature of, 
 242; comparative weight of, 243 f.; 
 weight of human, 244 f.; relation of, to 
 mind, 24J ,"606 f., 633 f., 640 f. 
 
 Broca, convolution of, 292 f . 
 
 Briicke, on neutralization of taste, 403 ; 
 perception of depth, 442 
 
 Byasson, on brain-waste, 242 
 
 CAMEREK, on measurement of taste, 
 376 f. 
 
 Capsule, the internal, 90, 91 
 
 Carville and Duret, on stimulation of 
 motor areas, 257 
 
 Cattell, on reaction-time, 485, 493 f . 
 
 Cells, the olfactory, 164f.; the gustatory, 
 167f.; the auditory, 192 
 
 Central Canal, 66 f . 
 
 Cerebellar tract, 71 f., 75, 77 
 
 Cerebellum, 74, structure of, 78 f.; pe- 
 duncles of, 79 ; arbor vitce of, 79 ; func- 
 tions of, 152 f.; lesions of, 153 f. 
 
 Cerebrin, 24, 25 
 
 Cerebro-spinal system, axis of, 62, 204 f . ; 
 development of, 204 f., 212 
 
 Cerebrum, 74. 82 ; shape of, 82 ; gyri of, 
 84, 91 f., 95 ; sulci of, 84, 91 f.; nervous 
 elements in, 91, 95 f . ; layers in its cortex, 
 95 f.; fibres of, 97 f.; nervous paths in, 
 127 f., 269 f.; functions of, 150 f., 156 
 f., 239 f.; development of. 204 f.; local- 
 ization in, 239 f., 250 f., 255 f., 269 f.; 
 significance of, 249 f . ; effects of injury 
 to, 258 f., 269 f. 
 
 Charcot, scheme of decussation, 290 
 
690 
 
 INDEX. 
 
 Charcot and Pitres, on localization of 
 cerebral function, 282 
 
 Chaussier, on growth of foetus, 566 
 
 Chemistry, of nervous system, 21 1, 217 
 1 ; of cells and fibres, 28 ; of physio- 
 logical function, 28 f., Ill f., 222; of 
 vision, 184 f. 
 
 Chodin, measuring power of the eye, 451 
 
 Cholesterin, 23 f. 
 
 Choroid, the, 171 f. 
 
 Clarke, columns of, 70 
 
 Cochlea, the, 191 
 
 Color, stimulus of, 328 f., 338 f.; saturated, 
 328 ; tones of, 329 ; brightness of, 330, 
 376 ; shades of, 331 f . ; complementary, 
 
 333 f., 343; dependence of, on time, 
 
 334 f.; and place of the retina, 335, 
 338; blindness to, 335 f.; contrast of, 
 337, 460 ; Young-Helmholtz theory of, 
 338 f.; symbolism of, 342 f.; sensitive- 
 ness to, 375 f. 
 
 Consciousness, the circuit of, 494 f.; 
 physical basis of, 544 f . ; possibility of 
 a prenatal, 565 1; psycho-physical ex- 
 planations of, 596 ; phenomena of, 597 
 f.; unity of, 607 f., 631 f. 
 
 Cornea, structure of, 171, 173, 175 f.; 
 index of refraction of, 176 ; function 
 of, 176 f. 
 
 Corona Radiata, 91 
 
 Corpus albicans, 83, 87 
 
 Corpus callosum, 82, 85; function of, 
 98 
 
 Corpus dentatum, of the medulla, 78 ; of 
 the cerebellum, 79 
 
 Corpus geniculatum, 87, 89 
 
 Corpus quadrigeminum, position of, 87- 
 structure of, 90 f. ; functions of, 156 1; 
 development of, 208 f. 
 
 Corpus striatum, 86 ; nuclei of, 86 f . ; 
 paths in, 129; functions of, 1581, 160 
 f.; development of, 208 f. 
 
 Corpus subthalamicon, 89 
 
 Cortex of Cerebrum, structure of, 95 f. 
 
 Crura Cerebri, 83, 87; crusta and teg- 
 mentum of, 87 f.; fibres in, 87 f.; func- 
 tions of, 156 
 
 Crusta, see Crura Cerebri 
 
 Crystalline Lens, the structure of, 173 
 
 DEITERS, processes of, 43, 70 ; conical 
 
 hair-cells of, 195, 197 
 Dietze, on the circuit of consciousness, 
 
 4941 
 
 Dobrowolsky, on measurement of color- 
 sensations, 375 f . 
 
 Bonders, on localization of depth, 465 ; 
 time of mental processes, 468 f., 479 f. 
 
 Dove, the experiment of, 442 
 
 Drbal, on nature of feeling, 503 ; kinds 
 of feeling, 505. 
 
 Du Bois-Reymond, discoveries of, 104, 
 112, 115, 117 ; theory of nervous ac- 
 tion, 227 f . 
 
 Dura Mater, structure of, 61 f . ; processes 
 of, 63. 
 
 EAR, 1851; the external, 185 ; the middle, 
 186 1 ; bones of, 187 f. ; tympanum of, 
 188 ; vibrations in, 188 ; the internal, 
 189 1; vestibule of, 189; canals of, 
 189 f . ; cochlea of, 191 ; nerve of, 191 1 ; 
 terminal apparatus of, 192 1, 324; 
 problem of, 195; development of , 211 1; 
 sensitiveness of, 317, 319 
 
 Ecker, view of cerebral cortex, 263 ; 
 charts of, 276 1 
 
 Eckhard, law of central mechanisms, 161 
 
 Electricity, " current of rest " in nerves, 
 104, 106 1, 117, 227 f. ; as stimulus of 
 nerves, 111, 112, 1141, 2281 ; "nega- 
 tive variation " in nerves, 118, 227 f. 
 
 Electrotonus, Pfl tiger's law of, 113, 115 1; 
 theory of, 222 f., 226 1 
 
 Embryo, knowledge of, 198 1, 212 f. ; of 
 the fowl, 199 1 ; development of, 200 1, 
 204 f., 212 1, 618 
 
 Encephalon, see Brain 
 
 End-organs of Motion, place in nervous 
 system, 60, 164 ; structure of, 197 
 
 End-organs of Sense, place in nervous 
 system, 60, 164 ; significance of, 1631; 
 end-organs of smell, 164 1 ; of taste, 
 106 1 ; of touch, 168 f. ; of sight, 
 171 1 ; of hearing, 185 1 
 
 Engelmann, on continuity of axis-cylin- 
 ders, 40 f . 
 
 Estel, on reaction-time, 490 
 
 Eustachian Tube, 186 1, 189 
 
 Exner, on speed of reflex action^ 135; 
 regio olfactoria, 165; nature of nerve- 
 commotion, 224 1 ; general function of 
 the brain, 240 ; cerebral physiology, 
 254, 267 ; views of, on localization, 267, 
 279 f., 284 1, 289 f. ; methods of, 276 1 ; 
 on aphasia, 294; reaction -time, 4701, 
 480, 496 f. ; attention, 538 
 
 Eye, structure of, 171 1; tunics of, 171 1 ; 
 
INDEX. 
 
 691 
 
 refracting media of, 173 f., 175 1 ; ap- 
 pendages of, 173 f., 177; muscles of, 
 174 f., 428 f. ; problem of, 174 f. ; ad- 
 justment of, 177 f., 433 ; pigments of, 
 184 ; development of, 210 f .; motion of, 
 428 f., 439 f. ; meridians of, 431 f. ; 
 torsions of, 432 f. ; innervation of, 
 439 f . ; stereoscopy of, 440 f. 
 
 FASCICULUS GRACILIS, 68, 72 
 Fechner, conception of psycho-physics, 
 12; on measurement of sensation, 361 f., 
 369 f. ; law of, 365 f., 374 f., 594 
 Feeling, mixture of, in local signs, 398 f. ; 
 of innervation or effort, 415 f., 523 1; 
 of " double contact," 417 f. ; nature of, 
 499 f ., 504 ; classes of, 505 f. ; intensity 
 of, 508 1 ; tone of, 509 f. ; physical ap- 
 paratus of, 510 f. ; common, 512; of 
 sensation, 514 f. ; the emotions, 5161, 
 519 f.; sthenic and asthenic, 518 f. ; the 
 higher aesthetic and intellectual, 520 f., 
 523 
 
 Ferrier, on corpora quadrigemina, 157 ; 
 and striate bodies, 159 ; experiments of, 
 254, 264 ; centres of, 268 f ., 285 f., 291 ; 
 on feeling of effort, 524 
 
 Fick, on muscle-contractions, 119 ; mi- 
 nute color-sensations, 334 f.; curve of 
 intensity, 475 
 
 Filum terminale, 64 
 
 Fissures, of Sylvius, 92, 94, 210, 267 f.; of 
 Rolando, 92, 94, 267 
 
 Flechsig, on tracts in spinal cord, 71 f. 
 
 Flourens, on respiratory centre, 147 f . ; op- 
 tic lobes, 156 ; localization of cerebral 
 functions, 253 
 
 Foramen magnum, 64 
 
 Formatio reticularis, in the medulla, 77 
 f.; in the tegmentum, 88 
 
 Foster, on the respiratory centre, 148 
 
 Fovea centralis, 183 
 
 Franck and Pitres, on stimulation of 
 brain, 257 
 
 Friedrich, on reaction-time, 483 f. 
 
 Fritsch, experiments of, 253 f., 264 
 
 Frohlich, classification of sensations oi 
 smell, 310 
 
 Funke, on Weber's "sensation-circles, 
 407 
 
 GALL, on cerebellum, 156; phrenological 
 theory of, 252 
 
 Gamgee, on chemistry of brain, 25 f. 
 janglia, the "basal," 88 f. 
 
 anglion-cell, see nerve-cells 
 George, theory of temperament, 575 f. 
 Gerlach. on intimate structure of the 
 
 cord, 70 ; and cerebral cortex, 96 
 urliky, on nerve-tracts, 261 
 Goldscheider, on "pressure spots," 346 
 
 f., 369, 410; temperature-spots, 348 f., 
 
 370, 413 
 Groll, column of (see fasciculus gracilis) 
 
 oltz, experiments of, on spinal cord, 140; 
 
 on optic lobes, 157 ; view of localiza- 
 tion, 264 f., 273 f.; experiments of, 
 
 297 f. 
 Grutzner, on nature of nerve-commotion, 
 
 225 f. 
 Gyri (or convolutions) of the cerebrum, 
 
 84, 92, 93 f.; development of, 210 
 
 HALL, G. STANLEY, on perception of mo- 
 tion, 411 f., 416 ; studies of rhythm, 
 490 
 
 Hamilton, on the circuit of consciousness, 
 494 ; tone of feeling, 509 
 
 Hearing, end-organ of, 185 f.; sensations 
 - of, 195, 315 f.; perceptions of, 403 f. 
 
 Helmholtz, on speed of nervous processes, 
 120 f.; index of refraction of cornea, 
 176; accommodation of eye, 177 f., 
 433; size of blind-spot, 183 f.; analysis 
 of sound, 196; nature of noises, 316; 
 consonances of tone, 323 f.; theory of 
 color-sensations, 338 f . ; and of percep- 
 tion, 389 f., 452 ; on Listing's law, 431 ; 
 movements of the head, 454 ; localiza- 
 tion of depth, 465 
 
 Hensen, on function of labyrinth, 194 f . , 
 196; nature of noises, 316 
 
 Bering, theory of color-sensations, 340 f . ; 
 of temperature-sensations, 350 f.; in- 
 nervation of the eye, 439 f., 451, 525; 
 movements of the head, 454 
 
 Hermann, on electrical phenomena, in 
 nerves, 118 f., 120; theory of nervous 
 action, 226 f. 
 
 Herscbel, on brightness of stars, 373 f. 
 
 Herzen, on sensations of temperature, 
 3521 
 
 Hill, A., view of "basal ganglia," 89 
 
 Hirsch, on reaction-time, 470 
 
 Hitzig, experiments of, 253 1, 264; on 
 localization of cerebral function, 253 1, 
 267 ; centres of, 267 f . 
 
692 
 
 INDEX. 
 
 Horopter, calculation of, 437 
 Horwicz, theory of feeling 
 
 INHIBITION, nature of, 51, 144; from 
 
 brain on cord, 143 f. 
 Iris, the, 172 
 Island of Reil, 92 ; layers in, 97 ; function 
 
 of, 295 f . 
 
 JAMES, PROFESSOR, theory of the emo- 
 tions, 519 f.; on the feeling of effort, 
 5241 
 
 Jastrow, on comparative judgments of eye 
 and hand, 466 ; studies of rhythm, 490 
 
 KANT, on sthenio and asthenic feeling, 
 518 f. 
 
 Keppler, on measurement of taste, 377 
 
 Klug, on localization by temperature, 414 
 
 Kolliker, on end-organs of touch, 170 
 
 Kollert, on reaction-time, 489 f . 
 
 Kraepelin, on measurement of visual sen- 
 sations, 375 
 
 Krause, end-bulbs of, 169 f.; on index of 
 refraction, 176 
 
 Kuhne, on chemistry of retina, 28, 184 f.; 
 function of nerve-fibres, 54; structure 
 of end-plates, 197 
 
 Kunkel, on inertia of the retina, 474 
 
 Kussmaul, on aphasia, 293 
 
 LAMANSKY, on measurement of color- 
 sensations, 375 
 
 Lecithin, 26 f. 
 
 Le Conte, on Listing's law, 431, 439; 
 torsions of the eye, 431 ; nature of the 
 horopter, 437 f . ; theory of double 
 images, 442 
 
 Listing, the law of, 430 f., 439 
 
 Lobes, of the cerebrum, 92 f . 
 
 Local Signs, theory of, 387 1, 396 , 398 
 f., 409 
 
 Lob, on visual areas, 288 f. 
 
 Lohmeyer, on cases of aphasia, 296 
 
 Lombard, on temperature of brain, 242 
 
 Longet, on columns of the cord, 125 f. ; 
 localization of cerebral function, 252 f . 
 
 Lotze, theory of local signs, 387, 396 f., 
 451 ; on distinctions by the skin, 409 ; 
 perception of magnitude by the eye, 
 451 ; errors of sense, 455 ; theory of 
 feeling, 499 f., 510; image of memory, 
 547 f.; differences of the sexes, 573 ; 
 kinds of temperament, 577 f. 
 
 Luchsinger, on reflexes of the cord, 138, 
 141 
 
 Luciani, on localization of cerebral func- 
 tion, 269 f., 285, 288, 301 
 
 Luys, on basal ganglia, 129 ; attention 
 and will, 544 ; memory, 552. 
 
 MACH, on fusion of nervous shocks, 473 
 
 Magendie, discovery of, 123 f. 
 
 Materialism, views of, 607 f. 
 
 Matteucci, on electrotonus, 228 
 
 Mechanism, nervous system as, 4 f., 19 f., 
 198, 214 f.; the nerve as, 104; develop- 
 ment of, 198 ; theory of the nervous, 
 2141, 2221, 2261 
 
 Medulla Oblongata, structure of, 74 1; 
 tracts of white matter in, 76 1; gray 
 matter in, 77 f.; nuclei of, 78; reflex- 
 motor functions of, 146 1; as auto- 
 matic, 147 1; centres of, 147 f., 150; 
 vaso-motor function of, 148 
 
 Meissner, calculation of the horopter, 
 4371 
 
 Membranes, of the brain, 63 f.; the bas- 
 ilar, of Reissner, 191 ; Kolliker, 194 
 
 Memory, reproduction of images of, 491 \ 
 1, 5461; physiological study of, 5351; / 
 physical basis of, 545 1, 550 1; as re- 
 tentive, 548 f.; the organic, 550 1; as 
 reproductive, 552 1 ; psychological nat- 
 ure of, 554 f . 
 
 Merkel, on reaction-time, 483 f., 486, 
 4951 
 
 Mesencephalon, development of, 207 1 
 
 Meynert, description of brain, 73, 98 f., 
 246; on layers of cerebral cortex, 95; 
 relation of brain to intelligence, 248 ; 
 nerve-tracts in cerebrum, 283 
 
 Mind, subject of phenomena, 3 1, 585 1, 
 596 1 ; relation to nervous mechanism, 
 235 f., 560 1, 579 f.; and to the brain, 
 247 1, 588 1, 592 1, 005 f., 633 1; syn- 
 thetic act of, in perception, 388 f., 416 
 1, 462 1, 467, 594 1; faculties of, 5S7 
 f., 600 1; physical explanations of, 593 
 1, 602 1, 625 1; as a unit-being, 5% 1, 
 668 1, 683 f.; phenomena of. 597 f.; as 
 a real being, 606 1, 611, 633 1, 656, 
 668 1; development of, 614 1, 62:5 1; 
 seat of, 634 1 ; physical organs of, 640 
 1; as a cause, 648 1; spirituality of, 
 681 f . 
 
 Moldenhauer, on reaction-time of taste, 
 479 
 
INDEX. 
 
 693 
 
 Moos, on duration of the image of mem- 
 ory, 549 
 
 Motions, the Bodily, classes of, 526 f.; the 
 impulsive, 537; the voluntary, 527 f., 
 530 ; the expressive, 531 
 
 Miiller, G. B., on measurement of sensa- 
 tions, 368 
 
 Miiller, J., on brain as measure of intelli- 
 gence, 248 
 
 Munk, experiments of, 270 f . ; on localiza- 
 tion of cerebral function, 272 ; motor 
 areas of, 272 f.; visual areas of, 286 f.; 
 auditory area of, 291 
 
 NAHLOWSKY, on nature of feeling, 502 ; 
 kinds of feeling, 505 f. 
 
 Nerve-cells, chemistry of, 28 ; elements of 
 nervous system, 30 f . ; kinds of, 31 f . ; 
 intimate structure of, 42 f. ; shapes of, 
 44 f . ; processes of, 44, 70 ; size of, 45 ; 
 as a typical element, 45 f . ; number of, 
 46; functions of, 49 f., 134, 2341; of 
 the embryo, 200 f., 212 
 
 Nerve-commotion, causes of, 48 ; condi- 
 tions of, 106 f., 122 ; phenomena of, 
 111 f., 230; nature of, 116 f., 122, 
 222; laws of, 118 f., 122, 230; speed of, 
 120 f., 122 f. ; paths of, 122 f., 1271, 
 261, 283 ; summation of, 223 f., 233 1 ; 
 facilitation of, 224 1 
 
 Nerve-fibres, chemistry of, 28; ele- 
 ments of nervous system, 30 f. ; kinds 
 of, 34 f. ; size of, 34, 41 1 ; structure of 
 the medullated, 35 1 ; fibrillated axis- 
 cylinder of, 38 f . ; origin of, 45 1 ; 
 number of, 46 ; in the cord, 681, 123 1, 
 134 ; of the embryo, 212 
 
 Nerve-muscle machine, 104 1 ; behavior 
 under electricity, 111 f . ; as a mechan- 
 ism, 215 1 
 
 Nerves, structure of, 33 f . ; general func- 
 tion of, 47 1, 54, 59 1, 106 f. ; excita- 
 bility of, 47, 106 1, 353 f.; conductivity 
 of, 47, 60, 102-122, 1181, 120; kinds 
 of, 51 1, 60, 353 f.; afferent, 52 1, 120 ; 
 efferent, 521, 120; the cranial, 1001; 
 the encephalic, 100 1 ; exhaustion of, 
 1081; mechanical properties of, 109; 
 thermic influences upon, 110 1; chemi- 
 cal influences on, 111 ; processes in, 
 1171; specific energy of, 300 1, 307 1, 
 3531 
 
 Nervous Matter, kinds of, 22 f . ; specific 
 gravity of, 22 f . 
 
 Nervous System, a mechanism, 4 1, 191, 
 1981, 214 f., 222 f., 226 1 ; general func- 
 tion of, 18 f., 57, 219 1; elements of, 21 
 f.,30 1, 216 1; chemistry of, 21 1,2171; 
 cells and fibres in, 30 f . ; structure of, 
 56-101 ; plan of, 57 1, 219 1 ; sets of 
 organs in, 59 1; the sympathetic, 60 f.; 
 the cerebro-spinal, 601; development 
 of the,- 198 f.; inertia of, 472 f. 
 
 Neuclein, 24 1 
 
 Neuroglia, nature of, 31 f. 
 
 Neurokeratin, 24 
 
 Nothnagel, on striate bodies, 159; fine- 
 ness of temperature-sense, 369 1 
 
 Nuclei of nerve-cells, 43 ; of the medulla, 
 78 ; of the corpus striatum, 86, 88, 90, 
 159 ; of the tegmentum (red nucleus), 
 88, 89 
 
 Nystagmus, 153 1 
 
 OLIVES, the, 75, 78 ; functions of, 150 
 
 Optic Thalami, position of, 86 1 ; struct- 
 ure of, 89 1; connections of, 90, 1271; 
 cells and fibres of, 90 ; paths in, 129 ; 
 functions of, 158 f . ; development of, 
 2081 
 
 Organ of Corti, 193 1 
 
 Organs, kinds in nervous system, 59 1 ; 
 the central, 60, 73 1 ; functions of, 
 130 f ., 224 f . 
 
 Ott, on centre of temperature, 161 
 
 PACINI, corpuscles of, 169 
 
 Paneth, on excitation of cerebral cortex, 
 2691 
 
 Papillae, circumvallatae, 166 1; fungi- 
 formes, 166 f. 
 
 Peduncles, of the cerebellum, 79 ; of the 
 cerebrum, 82, 871,971 
 
 Perception, nature of, 382 f., 462 1, 467, 
 538 1; nativistic and empiristic the- 
 ories of, 389 1; by smell, 402 1; taste, 
 403; hearing, 403 f.; touch, 405 1; of 
 motion, 411 1, 452 1; by temperature, 
 413 1; of sight, 421 1, 440 1, 448 1; 
 of depth, 441 1, 459, 464; of spatial 
 relations, 448 1, 464 1; development 
 of, 462 1; physical basis of, 538 1 
 
 Pfluger, table from, 113; law of, 113 1, 
 115 ; on reflexes of the cord, 137 f . 
 
 Physiological Psychology, definition of, 
 1 1, 4 1; combines two sciences, 6 f.; 
 divisions of, 8 1; method of, 9 f., 12, 
 532 ; claims of, 13 ; successes of, 304 f ., 
 
694 
 
 INDEX. 
 
 532 f., 593 f.; theory of perception of, 
 3821; limits of, 532 f. 
 
 Physiology, relation to psychology, 1 f. ; 
 of nerves in general, 103 f. 
 
 Pia Mater, structure of, 64 
 
 Pons Varolii, 74 ; structure of, 81 f. 
 
 Presentations of Sense, elements of, 304 
 f., 383 f., 468 f.; process of construc- 
 tion of, 382 f., 387 f., 416 f., 448 f.; 
 space-form belonging to, 385 f., 391 f., 
 448 f.; synthesis of, 386 f., 416 f., 467; 
 analysis of, 388 f., 595; nativistic the- 
 ory of, 389 1 ; empiristic theory of, 389 
 1; stages of, 400 f.; by smell, 402 f.; 
 taste, 403; hearing, 403 f.; by touch, 
 405 f.; by sight, 421 f., 433 f., 443 f.; 
 time-relations of, 468 f.; assumptions 
 entering into, 594 
 
 Pressure, sensations of, 345 f., 367 f.; 
 spots of, 346 
 
 Preyer, on sensitiveness to pitch, 319 f.; 
 fusion of nervous shocks, 472; sensa- 
 tions of new-born child, 569 
 
 Protagon, 25 f. 
 
 Psychology, conception of, 2 f.; method 
 of, 9 f., 587 f., 605 f.; classifications of, 
 5871, 6051 
 
 Psychometry, method of, 469 f.; elements 
 
 Retina, the, 172 f.; problem solved by, 
 174 1, 178 1, 183 1; layers of, 179 1, 
 183 f.; nervous elements of, 180 1, 326 
 1; rods and cones in, 181 f., 327 ; own 
 light of, 326 ; relation of, to sight, 335, 
 423 1; field of, 423 1; identical and 
 corresponding points of, 434 f . 
 Ribot, on physiological study of con- 
 cepts, 532 ; and of memory, 552 1 
 Ritter, on sensations of smell, 309 
 Rolando, f uniculus of, 77 ; tubercle of, 
 
 77 ; fissure of, 92, 267, 282 
 Romieu, on stimulus of smell, 311 
 Rosenthal, on speed of reflex action, 135 
 1; electrical taste, 313 
 
 SCHAFHAUTL, on limits of sound, 372 1 
 
 Schiff, on posterior columns of cord, 125 
 1; on excitability of cord, 141 1; on 
 cerebellum, 153, 155 f.; temperature of 
 brain, 242 ; localization of cerebral 
 function, 273 1, 283 1 
 
 Schultze, Hans, on structure of axis- 
 cylinder, 39 1 
 
 Schultze, Max, on varieties of nerve- 
 fibres, 34 1; and structure of nerve- 
 cell, 42 f.; on olfactory cells, 164 f.; 
 auditory cells, 192 
 
 Schwann, sheath of, 36 ; substance of, 36 
 
 of time in, 470 f . ; results of, 497 
 
 Psycho-physics, Fechner's conception of, j Sclerotic, the, 171 
 12, 380 1; method of, 359, 361 1, 365 I Seguin, on cases of aphasia, 295 
 
 1; the laws of, 359 1; 365 1; 379 1; 
 
 of sensations of touch, 367 1 ; of sound, 
 
 370 ; of light, 373 1; of smell and taste, 
 
 3761 
 
 Purkinje, cells of, 80 
 Pyramidal tract, 71 f., 77, 97 
 
 QOETELET, proportions of human body, 
 566 f., 574 
 
 RANVIER, nodes of, 36 1, 40 1; on struct- 
 ure of ganglion-cell, 43 
 
 Reaction -time, nature of, 475 f.; influ- 
 ences upon, 476 1, 495 1; methods of 
 determining, 479 f.; complex processes 
 of, 491 
 
 Reflex action, 50 f., 130 f.; kinds of, 131 ; 
 in spinal cord, 131 1, 136 f.; conditions 
 of, 134 f ., 136 f. ; speed of, 135 1 ; in the 
 brain, 143 f., 224 1 
 
 Regio olfactoria, 164 1, 308 f. 
 
 Reissner, membrane of, 191 
 
 Remak, fibres of, 34, 41 
 
 Semicircular canals, the, 189 1 
 
 Sensations, end-organs of , 1641; analysis 
 of auditory, 195, 324 ; quality of, 303 f., 
 325 1; simple, 305 1; conditions of, 
 307 f.; of smell, 3081, 376 1; of taste, 
 311 1, 376 1; of sound, 3151, 3701; of 
 sight, 3251, 3731; theory of the vis- 
 ual, 338 1 ; of temperature, 344, 309 1 ; 
 of pressure, 344, 345 f., 367 1; the mus- 
 cular, 3441; quantity of, 3561; meas- 
 urement of , 359 1, 364 1, 3691; least 
 observable difference in, 361 f., 364 1; 
 range of, 363 1; spatial series of, 386 f., 
 393 f.; localisation of, 387 1, 405 f. 
 
 Senses, organs of the, 164 f.; classifica- 
 tion of the, 303 f . ; the geometrical, 386 
 1 ; errors of the, 455 f . 
 
 Setschenow, on inhibitory centres, 144. 
 
 Sight, end-organs of, 171 1, 1741, 338 1; 
 photo-chemistry of, 178 1, 184 1, 326 ; 
 sensations of, 325 f.; stimulus of, 325 1; 
 after-images of, 336 f. ; elements in per- 
 ception of, 420 f., 447 1; motion of eye 
 
 
INDEX. 
 
 695 
 
 in, 428, 431 f . ; single and double images 
 in, 434 f . , 438 f. ; stereoscopic and per- 
 spective. 440 f . ; secondary helps of, 443 
 f., 4551 
 
 Smell, organs of, 164 f., 308 f.; nerve of, 
 165,310; stimulus of, 165 f., 308 f., 
 310; sensations of, 308 f.; kinds of, 
 310 ; measurement of, 378 f . ; percep- 
 tions of, 402 f . 
 
 Soul, see Mind 
 
 Sound, analysis of, 195 f., 324; sensa- 
 tions of, 315 f., 370 f.; kinds of, 316; 
 nature of the musical, 316 f. ; limits of, 
 317, 371 1; " entotic,"403 f.; direction 
 of, 404 
 
 Spinal cord, membranes of, 61 f . ; struct- 
 ure of, 64 f ., 143, 207 ; fissures of, 64 f.; 
 columns of, 66, 67, 125 f. ; commissures 
 of, 66 ; horns of, 67 ; white substance 
 of, 68; nerve-fibres in, 68 f., 123 1; 
 gray substance of, 69 f . ; nervous tracts 
 in, 71 f., 123 f.; as mechanism, 72, 122, 
 133 f., 144; nerves from, 100 f.; nervous 
 processes in, 122, 1341; roots of, 1231, 
 207; as a central organ, 1321, 138 1; 
 automatism, 138 f. ; "centres" of, 140 
 1; excitability as a whole, 141 1; " ses- 
 thesodic" and " kinesodic," 142; in- 
 fluence of brain on, 143 1; develop- 
 ment of, 207 
 
 Stimulus, kinds of, 48; heat as, 110 1; 
 electricity as, 111 1, 312 1; of smell, 
 165 1, 308 1, 376 1; of taste, 311 f., 
 3761; of hearing, 315, 370 f.; of sight, 
 3251, 3281, 373 1; measurement of, 
 359 1, 3671; limits of, 362 1, 367 1 
 
 Strabismus, 153 f. 
 
 Strieker, on common feeling, 513 
 
 Stumpf, on judgment of tone, 320 
 
 Substantia gelatinosa, 69 
 
 Substantia nigra, 87 
 
 Sulci, of the cerebrum, 84, 91, 92 1; de- 
 velopment of, 210 
 
 Sully, on tone of feeling, 511 
 
 Suspensory ligament, 173; function of, 
 1771 
 
 Sympathetic System, structure of, 60 1 
 
 TALBOT, the principle of, 473 
 
 Taste, end- organs of, 1661,313; nerve 
 of, 168, 314; sensations of, 311 f., 
 376 1 ; stimulus of, 3121 ; kinds of, 
 314; measurement of, 376 1 ; percep- 
 tions of, 403 1 
 
 Tegmentum, see Crura Cerebri 
 
 Temperament, theory of, 575 1, 579; 
 kinds of, 575 1 ; physical basis of, 
 579 
 
 Temperature, sensations of, 344, 3481, 
 369 1 ; after-images of, 351 1 ; measure- 
 ment of, 369 1 ; sense of locality by, 
 4131 
 
 Thalamen-cephalon, 208 
 
 Things, distinguished from sensations, 
 359 f . , 382, 594 ; results of mental syn- 
 thesis, 594 1, 609 ; unity of, 609 1 
 
 Thudichum, on chemistry of brain, 27 
 
 Tischer, on Weber's law, 372 
 
 Tones, the musical, 316; pitch of, 317 1; 
 table of, 318; sensitiveness to, 319, 
 3701; purity of, 319 1 ; judgments of, 
 320 ; relations of, 322 1 
 
 Touch, kinds of, 168, 345 1 ; end-organs 
 of, 168 f. ; sensations of, 345 1, 3671; 
 perceptions of, 405 1 ; the field of, 406 
 1,4161 
 
 Trautscholdt, reaction-time of complex 
 processes, 491 f.; on effect of practice, 
 496 
 
 Tiirck, method of, 71 ; on columns of 
 cord, 126 
 
 Tympanum, the, 186 1 ; membranes of, 
 186, 188 1 ; windows of, 186 ; muscles 
 of, 187 ; office of, 187 1 
 
 Tyndall, on stimulus of smell, 311 f. 
 
 VALENTIN, on nervous excitation, 223 1 ; 
 sensations of smell, 308 f. ; of taste, 
 314, 3771,403; on sense of locality, 
 406 1 ; fusion of nervous shocks, 472 
 
 Valli, principle of, 107 
 
 Vestibule, of the ear, 189 
 
 Vierordt, on measurement of sensation, 
 371 f . ; localization by touch, 409 1 ; 
 subjective estimate of time, 488 1 
 
 Vitreous Humor, the, 173 
 
 Volkmann, A. W., on measurement of 
 sound, 371 ; of light, 374 ; of length of 
 lines, 376 ; on sense of locality, 407 
 
 Volkmann von Volkmar, on motifs of 
 monocular vision, 430; nature of feel- 
 ing, 503 
 
 Volta, on electrical taste, 312 1 
 
 Von Gudden, on optic chiasm, 290 
 
 Von Kries, on the number of colors, 332 ; 
 on theories of color-sensations, 341 1; 
 sense of locality, 397 
 
 Von Kries and Auerbach, on sense of 
 
696 
 
 INDEX. 
 
 locality, 397 ; reaction-time, 476, 481 f., 
 487, 496 
 
 Von Vintschgau, on conduction in nerves, 
 121 ; reaction-time for multiplying, 493 
 
 Von Wittich, on fusion of nervous shocks, 
 473 ; reaction-time of taste, 478 
 
 Vulpian, on nervous function, 54 ; on ex- 
 citability of cord, 142 ; centres of the 
 medulla, 149 ; and cerebellum, 154 f. 
 
 WAGNER (H. and R.), corpuscles of, 170 ; 
 on measurement of brain-mass, 247 ; 
 effect of fear, 518 
 
 Waller, method of, 107 f . 
 
 Weber, E. H., on temperature-sensations, 
 110, 347, 351 ; smell, 308 ; law of, 365 
 f., 368 f., 374 f., 378 1; on direction of 
 sound. 404 ; perceptions of touch, 405 
 f.; "sensation-circles" of,' 406 f.; 
 measuring power of the eye, 452 
 
 Wertheim, on ductility of nerves, 109 
 Will, effect of, on bodily motions, 528 f.; 
 physiological study of, 535 f.; physical 
 basis of, 536 f.; in attention, 539 f. 
 Wundt, on columns of the cord, 126 ; on 
 cerebellum, 152 ; optic thalami, 158 f. ; 
 and striate bodies, 160; mechanical 
 theory of, 231 f.; kinds of taste, 314; 
 theory of color-sensations, 341 ; on com- 
 plementary colors, 343 ; theory of apper- 
 ception, 380, 539 f.; on Weber's law, 
 381 ; on theories of perception, 389 f.; 
 "sensation-circles," 408 ; visual percep- 
 tion, 422, 425 f., 451; judgment of 
 distance, 433; feelings of innervation, 
 439, 451, 524 ; psycho-physical time, 
 471, 477, 483 f., 488, 496 ; curve of feel- 
 ing, 514 ; theory of temperament, 576 f. 
 
 ZONULE of Zinn, 173 
 
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