^ 
 
9P 
 
AN AMERICAN TEXT-BOOK 
 
 OF 
 
 PHYSIOLOGY 
 
 HENRY P. BOWDITCH, M. D. 
 JOHN G. CURTIS, M.D. 
 HENRY H. DONALDSON, PH. D. 
 W. H. HOWELL, PH.D., M.D. 
 FREDERIC S, LEE, PH.D. 
 
 BY 
 
 WARREN P. LOMBARD, M.D. 
 GRAHAM LUSK, PH.D.,F.R.S. (EDIN. 
 W. T. PORTER, M.D. 
 EDWARD T. REICHERT, M.D. 
 HENRY SEWALL, PH.D.,M.D. 
 
 EDITED BY 
 
 WILLIAM H. HOWELL, PH.D., M.D. 
 
 Professor of Physiology in the Johns Hopkins University, Baltimore, Md. 
 
 SECOND EDITION, REVISED 
 
 VOL. II. 
 
 MUSCLE AND NERVE; CENTRAL NERVOUS SYSTEM; 
 
 THE SPECIAL SENSES; SPECIAL MUSCULAR 
 
 MECHANISMS; REPRODUCTION 
 
 PHILADELPHIA AND LONDON 
 W. B. SAUNDERS & COMPANY 
 
 J90J 
 
BIOLOGY 
 LIBRARY 
 
 GENERAL 
 
 COPYRIGHT, 1900, 
 BY W. B. SAUNDERS & COMPANY 
 
 PRESS OF 
 W. 8. SAUNDERS A COMPANY 
 
CONTRIBUTORS TO VOLUME IL 
 
 HENRY P. BOWDITCH, M.D., 
 
 Professor of Physiology in the Harvard Medical School. 
 
 HENRY H. DONALDSON, PH.D., 
 
 Professor of Neurology in the University of Chicago. 
 
 FREDERIC S. LEE, PH.D., 
 
 Adjunct Professor of Physiology in Columbia University (College of Physicians and 
 Surgeons). 
 
 WARREN P. LOMBARD, M.D., 
 
 Professor of Physiology in the University of Michigan. 
 
 HENRY SEWALL, PH.D., M.D., 
 
 Professor of Physiology in the Denver College of Medicine, Medical Department of the 
 University of Denver. 
 
 1 1 7283 
 
PREFACE TO THE SECOND EDITION. 
 
 ADVANTAGE has been taken of the necessity of issuing a second edition 
 of the American Text-Book of Physiology to alter somewhat its general 
 arrangement. The book has proved to be successful, and for the most part 
 has met only with kindly and encouraging criticisms from those who have 
 made use of it. Many teachers, however, have suggested that the size of 
 the book, when issued in a single volume, has constituted to some extent an 
 inconvenience when regarded from the standpoint of a student's text- book 
 that may be needecj daily for consultation in the lecture-room or the labora- 
 tory. It has been thought best, therefore, to issue the present edition in two 
 volumes, with the hope that the book may thereby be made more serviceable 
 to those for whose aid it was especially written. 
 
 This change in the appearance of the book has necessitated also some 
 alteration in the arrangement of the sections, the part upon the Physiology 
 of Nerve and Muscle being transferred to the second volume, so as to bring it 
 into its natural relations with the Physiology of the Central Nervous System. 
 
 The actual amount of material in the book remains substantially the same 
 as in the first edition, although, naturally, very many changes have been 
 made. Even in the short time that has elapsed since the appearance of the 
 first edition there has been much progress in physiology, as the result of the 
 constant activity of experimenters in this and the related sciences in all parts 
 of the world, and an effort has been made by the various contributors to keep 
 pace with this progress. Statements and theories that have been shown to 
 be wrong or improbable have been eliminated, and the new facts discovered 
 and the newer points of view have been incorporated so far as possible. Such 
 changes are found scattered throughout the book. 
 
 The only distinctly new matter that can be referred to specifically is found 
 in the section upon the Central Nervous System, and in a short section upon the 
 modern ideas and nomenclature of physical chemistry, with reference especially 
 to the processes of osmosis and diffusion. The section dealing with the Central 
 Nervous System has been recast in large part, with the intention of making 
 it more suitable to the actual needs of medical students ; while a brief presen- 
 tation of some of the elementary conceptions of physical chemistry seems to 
 be necessary at the present time, owing to the large part that these views are 
 taking in current discussions in physiological and medical literature. 
 
 The index has been revised thoroughly and considerably amplified, a table 
 of contents has been added to each volume, and numerous new figures have 
 been introduced. 
 
 AUGUST, 1900. 
 
PREFACE. 
 
 THE collaboration of several teachers in the preparation of an elementary 
 text-book of physiology is unusual, the almost invariable rule heretofore 
 having been for a single author to write the entire book. It does not seem 
 desirable to attempt a discussion of the relative merits and demerits of the two 
 plans, since the method of collaboration is untried in the' teaching of physi- 
 ology, and there is therefore no basis for a satisfactory comparison. It is a fact, 
 however, that many teachers of physiology in this country have not been 
 altogether satisfied with the text-books at their disposal. Some of the more 
 successful older books have not kept pace with the rapid changes in modern 
 physiology, while few, if any, of the newer books have been uniformly satis- 
 factory in their treatment of all parts of this many-sided science. Indeed, the 
 literature of experimental physiology is so great that it would seem to be 
 almost impossible for any one teacher to keep thoroughly informed on all 
 topics. This fact undoubtedly accounts for some of the defects of our present 
 text-books, and it is hoped that one of the advantages derived from the col- 
 laboration method is that, owing to the less voluminous literature to be 
 consulted, each author has been enabled to base his elementary account upon 
 a comprehensive knowledge of the part of the subject assigned to him. Those 
 who are acquainted with the difficulty of making a satisfactory elementary 
 presentation of the complex and oftentimes unsettled questions of physiology 
 must agree that authoritative statements, and generalizations, such as are fre- 
 quently necessary in text-books if they are to leave any impression at all upon 
 the student, are usually trustworthy in proportion to the fulness of informa- 
 tion possessed by the writer. 
 
 Perhaps the most important advantage which may be expected to follow 
 the use of the collaboration method is that the student gains thereby the point 
 of view of a number of teachers. In a measure he reaps the same benefit as 
 would be obtained by following courses of instruction under different teachers. 
 The different standpoints assumed, and the differences in emphasis laid upon 
 the various lines of procedure, chemical, physical, and anatomical, should 
 give the student a better insight into the methods of the science as it exists 
 
PREFACE. 
 
 to-day. A similar advantage may be expected to follow the inevitable over- 
 lapping of the topics assigned to the various contributors, since this has led 
 in many cases to a treatment of the same subject by several writers, who have 
 approached the matter under discussion from slightly varying standpoints, and 
 in a few instances have arrived at slightly different conclusions. In this 
 last respect the book reflects more faithfully perhaps than if written by a 
 single author the legitimate differences of opinion which are held by physi- 
 ologists at present with regard to certain questions, and in so far it fulfils 
 more perfectly its object of presenting in an unprejudiced way the existing 
 state of our knowledge. It is hoped, therefore, that the diversity in method 
 of treatment, which at first sight might seem to be disadvantageous, will prove 
 to be the most attractive feature of the book. 
 
 In the preparation of the book it has been assumed that the student has 
 previously obtained some knowledge of gross and microscopic anatomy, or is 
 taking courses in these subjects concurrently with his physiology. For this 
 reason no systematic attempt has. been made to present details of histology or 
 anatomy, but each author has been left free to avail himself of material of 
 this kind according as he felt the necessity for it in developing the physiolog- 
 ical side. 
 
 In response to a general desire on the part of the contributors, references 
 to literature have been given in the book. Some of the authors have used 
 these freely, even to the point of giving a fairly complete bibliography of the 
 subject, while others have preferred to employ them only occasionally, where 
 the facts cited are recent or are noteworthy because of their importance or 
 historical interest. References of this character are not usually found in ele- 
 mentary text-books, so that a brief word of explanation seems desirable. It 
 has not been supposed that the student will necessarily look up the references 
 or commit to memory the names of the authorities quoted, although it is pos- 
 sible, of course, that individual students may be led to refer occasionally to 
 original sources, and thereby acquire a truer knowledge of the subject. The 
 main result hoped for, however, is a healthful pedagogical influence. It is too 
 often the case that the student of medicine, or indeed the graduate in medicine, 
 regards his text-book as a final authority, losing sight of the fact that such 
 books are mainly compilations from the works of various investigators, and 
 that in all matters in dispute in physiology the final decision must be made, so 
 far as possible, upon the evidence furnished by experimental work. To enforce 
 this latter idea and to indicate the character and source of the great literature 
 from which the material of the text-book is obtained have been the main 
 reasons for the adoption of the reference system. It is hoped also that the 
 
PREFACE. 
 
 book will be found useful to many practitioners of medicine who may wish to 
 keep themselves in touch with the development of modern physiology. For this 
 class of readers references to literature are not only valuable, but frequently 
 essential, since the limits of a text-book forbid an exhaustive discussion of 
 many points of interest concerning which fuller information may be desired. 
 
 The numerous additions which are constantly being made to the literature 
 of physiology and the closely related sciences make it a matter of difficulty to 
 escape errors of statement in any elementary treatment of the subject. It can- 
 not be hoped that this book will be found entirely free from defects of this 
 character, but an earnest effort has been made to render it a reliable repository 
 of the important facts and principles of physiology, and, moreover, to embody 
 in it, so far as possible, the recent discoveries and tendencies which have so 
 characterized the history of this science within the last few years. 
 
CONTENTS OF VOLUME I. 
 
 INTRODUCTION (By W. H. HOWELL). 
 
 BLOOD (By W. H. HOWELL). 
 
 LYMPH (By W. H. HOWELL). 
 
 CIRCULATION (By JOHN G. CURTIS and W. T. PORTER). 
 
 SECRETION (By W. H. HOWELL). 
 
 CHEMISTRY OF DIGESTION AND NUTRITION (By W. H. HOWELL). 
 
 MOVEMENTS OF THE ALIMENTARY CANAL, BLADDER, AND 
 URETER (By W. H. HOWELL). 
 
 RESPIRATION (By EDWARD T. REICHERT). 
 
 ANIMAL HEAT (By EDWARD T. REICHERT). 
 
 THE CHEMISTRY OF THE ANIMAL BODY (By GRAHAM LUSK). 
 
CONTENTS OF VOLUME II. 
 
 PAGE 
 
 GENERAL PHYSIOLOGY OF MUSCLE AND NERVE (By WARREN 
 
 P. LOMBARD) 17 
 
 A. INTRODUCTION 17 
 
 The general property of contractility, 17 The movements of amosbee, leucocytes, 
 
 vorticella, etc., 19 The general property of irritability, 20 The general property of 
 conductivity, 20 The general distribution of the properties of conductivity and irri- 
 tability, 21. 
 
 B. IRRITABILITY OF MUSCLE AND NERVE 23 
 
 Definition of various irritants, 23 The persistence of irritability in excised organs, 
 24 Irritability of nerves, 24 Demonstration of irritability by various forms of 
 stimuli, 25 The independent irritability of muscle, 25 The curare experiment to 
 prove independent irritability, 26 Other proofs of direct irritability of muscle, 27 
 Conditions that determine the efficiency of irritants, 28 Irritating effect of the 
 electrical current, 28 Description of the apparatus used in electrical stimulation, 29 
 Effect of the rate of stimulation, 31 Du Bois-Reymond's law, 32 Irritating effect 
 of induced electric currents, 33 The myogram, 34 The make and break shocks of 
 the induction current, 35 kathodal and anodal contractions, 35 Description of the 
 commutator, 36 The closing contractions stronger than the opening contractions, 
 38 The effect of variations in the strength of stimuli, 39 The effect of density of 
 the electrical current, 41 The spread of the electric current in moist conductors, 
 41 The spread of electrostatic charges, 42 Means of preventing the spread of cur- 
 rent, 44 The unipolar method of excitation, 45 The effect of duration of current on 
 its stimulating action, 46 The effect of the angle at which the current enters, 48 
 The effect of the direction of the current (Pfliiger's law), 49 The effect of battery 
 currents on normal human nerves, 51 The conditions that determine the irritability 
 of nerves and muscles, 55 The effect of mechanical agencies, 55 The effect of tem- 
 perature, 56 The effect of chemicals and drugs, 58 The effect of the electrical cur- 
 rent on muscle, 61 The effect of the electrical current on nerve, 62 Electrotonus, 
 63 Effect of rapidity of stimulation, 65 The effect of varying the normal blood- 
 supply, 66 The effect of separation from the central nervous system, 69 The effect 
 of the fatigue of muscles, 70 The fatigue of nerves, 75 The effect of use and disuse, 
 76 The effect of enforced rest, 77. 
 
 C. CONDUCTIVITY 77 
 
 The necessity of protoplasmic continuity for conduction, 77 Isolated conduction 
 
 in nerve-trunks, 79 The distribution of the excitation by the branches of a nerve, 
 80 conduction in muscles, 80 The transmission of the excitation by means of end- 
 organs, 82 Conduction in both directions in muscles, 84 In nerves, 85 The rate 
 of conduction, 87 Transmission of the wave of contraction, 87 The length of the 
 contraction wave, 88 The rate of conduction in different kinds of muscles, 89 Eate 
 of conduction in motor nerves, 89 Rate of conduction in sensory nerves, 91 The 
 effect of death processes on the conduction, 91 The effect of mechanical conditions 
 on conduction, 92 The effect of temperature on conduction, 92 The effect of 
 chemicals and drugs on conduction, 93 The effect of a constant battery current 
 on conduction, 94 Practical application of the foregoing effect, 95 The relation 
 of conduction to the fatigue of nerves, 95 Nature of the conduction process (nerve- 
 impulse), 97. 
 
 D. CONTRACTILITY 99 
 
 Graphic records of simple muscle contractions, 99 The myograph, 100 The 
 
 chronograph, 100 The latent period, 102 Optical properties of muscle during rest 
 and contraction, 103 The elasticity of muscle, 105 The muscle contraction in dif- 
 ferent muscles, 108 The effect of tension on the curve of contraction, 109 The 
 effect of rate of excitation on the curve of contraction, 111 Introductory and stair- 
 case contractions, 112 The effect of fatigue from repeated stimulation, 113 The 
 effect cf repeated stimulations on the form of separate contractions, 115 The pro- 
 duction of tetanus by repeated stimulations, 117 The cause of summation in tetanic 
 contractions, 120 The effect of two excitations following rapidly, 121 The effect of 
 support on the height of contractions, 122 The effect of gradually increasing the 
 rate of excitation, 123 Summary of the factors producing tetanus, 124 The number 
 of stimulations necessary to produce tetanus, 125 The effect of very rapid rates of 
 excitation, 126 Relative intensity of tetanus and single contractions, 126 Con- 
 
 13 
 
12 CONTENTS. 
 
 PAGE 
 
 tractures and continuous contractions, 127 Contracture following frequent excita- 
 tions, 128 Contracture following single excitations, 129 The effect of fatigue on 
 contracture, 130 Effect of the constant current on the form of contraction, 131 
 The effect of death processes on the form of contraction, 132 Normal physiological 
 contractions, 132 Muscle sounds, tremors, etc., 132 Comparison of effects of normal 
 and artificial stimulation, 134 Fatigue of voluntary muscle contractions, 134 
 Effect of temperature on muscular contractions, 136 Effect of drugs and chemicals 
 upon muscular contractions, 137 Liberation of energy by the contracting muscle, 
 138 Conditions controlling the amount of work done by a muscle, 139 The thermal 
 energy given off by a contracting muscle, 141 Muscle-tonus and chemical tonus, 143. 
 
 E. ELECTRICAL PHENOMENA IN MUSCLE AND NERVE 144 
 
 Liberation of electrical energy during functional activity, 144 Description of the 
 
 galvanometer and capillary electrometer, 145 The current of rest, 147 Theories as 
 to the cause of the current of rest, 148 The current of rest in nerves, 149 Currents 
 of action in muscle, 150 Secondary tetanus, 150 The diphasic action currents, 152 
 Kelation of the action current to the muscle contraction, 153 Currents of action in 
 nerves, 153 Relation of the electrical phenomena of nerves to the physiological pro- 
 cesses, 157. 
 
 F. CHEMISTRY OF MUSCLE AND NERVE 159 
 
 The condition of rigor mortis, 159 Conditions influencing the development of 
 
 rigor mortis, 160 The cause and nature of the contraction of rigor mortis, 161 The 
 chemical changes occurring in rigor mortis, 162 Eigor caloris, 164 The constituents 
 of muscle serum, muscle proteids, 166 Nitrogenous extraction of muscle, 166 The 
 non-nitrogenous constituents of muscle, 167 The gases of muscle, 168 The chem- 
 istry of nerves, 169. 
 
 * 
 
 CENTRAL NERVOUS SYSTEM (By HENRY H. DONALDSON) 171 
 
 INTRODUCTION 171 
 
 The unity of the central nervous system, 171 Phenomena involving consciousness, 
 172 Growth and organization, 172 Plan of presentation of the subject, 172. 
 
 PART I. PHYSIOLOGY OF THE NERVE-CELL 173 
 
 A. ANATOMICAL CHARACTERISTICS OF THE NERVE-CELL 173 
 
 Form of nerve-cells, 173 Peculiarities in structure of nerve-cells, 174 The volume 
 
 relations of nerve-cells, 175 Size of nerve-cells in different animals, 175 Relation 
 between size and function, 175 The growth of nerve-cells, 176 Maturing of nerve- 
 cells, 177 Classification of cells by means of the form of the axone, 177 Growth of 
 the branches of the nerve-cell, 179 Internal structure of the neurones, 179 Medulla- 
 tion of nerve-fibres, 179 Growth of the medullary sheath in peripheral nerves, 180 
 Medullation in the central system, 181 Changes in the cytoplasm, 182 Old age of 
 the nerve-cells, 182. 
 
 B. THE NERVE IMPULSE WITHIN A SINGLE NEURONE 183 
 
 The nerve-impulse, 183 Direction of the nerve-impulse, 184 Double pathways for 
 
 the nerve-impulse, 185 Significance of cell branches, 186 The generation of nerve- 
 impulses, 187 The rate of discharge of nerve-impulses, 189 Points at which the 
 nerve-impulse can be aroused, 189 Irritability and conductivity, 189 Summation of 
 stimuli in nerve-cells, 190. 
 
 C. THE NUTRITION OF THE NERVE-CELL .......' 191 
 
 C.hemical changes in the nerve-cell, 191 Fatigue of the nerve-cell, 191 Atrophic 
 
 influences affecting the nerve-cell, 195 Effects of amputations in man on the nerve- 
 cells, 196 Degeneration of nerve-elements, 197 The nutritive control of the neurone, 
 198 Degeneration of the cell-body, 199 Regeneration of the axone, 199. 
 
 PART II. THE PHYSIOLOGY OF GROUPS OF NERVE-CELLS 202 
 
 A. ARCHITECTURE AND ORGANIZATION OF THE CENTRAL NERVOUS SYSTEM ... 202 
 General arrangement of the central nervous system, 202 Arrangement of the cells 
 
 forming the several groups, 205 Segmentation of the central nervous system, 205 
 ( Relative development of different parts, 206 The connections between cells, 206 
 Theories of the passage of the nerve-impulses, 207. 
 
 B. REFLEX ACTIONS 207 
 
 The conditions of stimulation controlling reflex actions, 207 The diffusion of cen- 
 tral impulses, 208 Simple reflex actions, 208 Influence of location of stimulus on 
 reflexes, 209 Segmental reflex actions, 210 The influence of the strength of stimulus 
 
 on reflex actions, 210 Continuance of the reflex response, 211 The latent period of 
 reflexes, 211 The summation of stimuli in reflexes, 211 Reflex reactions from frac- 
 tions of the cord, 212 Reflex reactions in other vertebrates, 212 Co-ordination of the 
 efferent impulses in reflex actions, 214 Purposeful character of reflex responses, 215 
 Reflexes in man, 216 Periodic reflexes, 216 Variations in diffusibility in reflexes, 
 217 Influence of strychnine on reflexes, 217 Peripheral diffusion of reflex impulses, 
 218 Reflexes in the sympathetic system, 218 Manner of diffusion of impulses in 
 
CO-V'/'AYV'/X 13 
 
 PAGE 
 
 the sympathetic system, -jilt -Evidence for continuous outgoing impulses from the 
 central nervous system. :_"_'<> -Kigor mortis as affected by t lie nervous system, 
 Modification of rellexes by simultaneous and successive afferent impulses, 221 Effects 
 of afferent impulses on reflexes, 223 Inhibit ion of reflexes, 223. 
 
 C. REACTIONS INVOLVING: TIIK KN KPHALON 226 
 
 The path of afferent impulses in the central nervous system, 226 Degenerations in 
 
 the spinal cord after hemisection, 228 Physiological observations on afferent pathways 
 in the central nervous system, 22!) The nerves of common sensation. -_':;o The nerves 
 of pain and their pathway in the cord, 231 The pathways of impulses in the cord, 
 233 The nuclei and courses of the cranial nerves, 236 The pathway of the fibres of 
 the optic nerve, 238 The pathway of the fibres of the olfactory nerve, 240. 
 
 D. LOCALIZATION OF CELL-GROUPS IN THE CEREBRAL CORTEX 241 
 
 The discovery of localization of function in the cortex, 241 Effects of stimulation 
 
 of the cortex, 241 Course of the descending impulses, 244 Mapping of the cortex, 
 247 The size of the cortical areas, 247 Subdivision of the cortical areas, 247 Sepa- 
 rateness of centres and areas, 248 Multiple control of muscles from the cortex, 250 
 Cortical control is crossed, 251 Course of impulses leaving the cortex, 251 Size of 
 the pyramidal tracts in different mammals, 252. 
 
 E. LOCALIZATION IN THE CEREBRAL CORTEX OF THE CELL-GROUPS RECEIVING THE 
 
 AFFERENT IMPULSES 252 
 
 Sensory regions of the cortex, 252 Delimitation of the sensory areas, 253 Hemi- 
 anopsia. 255 Association-fibres arid association-centres (Flechsig), 256 Aphasia, 257 
 Relative importance of the two hemispheres, 258 Composite character of incoming 
 impulses, 260 Variations in association, 260 Latent areas, 261. 
 
 F. COMPARATIVE PHYSIOLOGY OF THE DIVISIONS OF THE ENCEPHALON .^262 
 
 Methods of determining, 262 Removal of central hemispheres, 263 Functions of 
 
 the corpus callosum, 270 Functions of the corpora striata, 271 Functions of the 
 thalamus, 271 Functions of the cerebellum, 272. 
 
 PART III. PHYSIOLOGY OF THE NERVOUS SYSTEM TAKEN AS A WHOLE 274 
 
 A. WEIGHT OF THE BRAIN AND SPINAL CORD 274 
 
 Weight of the encephalon and spinal cord, 274 Weight of the encephalon, 275 In- 
 terpretations of weight, 277 Weights of different portions of the encephalon, 277 
 Effect of social environment, 277 Brain-weight of criminals, 277 Brain-weights of 
 different races, 278 Weight of the spinal cord, 278. 
 
 B. GROWTH CHANGES 278 
 
 Growth of the brain, 278 Relation between growth of body and that of encephalon, 
 
 280 Increase in the number of functional nerve-elements, 280 Increase in the fibres 
 of the cortex, 282 Significance of medullation, 283 Increase in the mass of the neu- 
 rones, 283 Number of cells, 283 Change in specific gravity with age, 284. 
 
 C. ORGANIZATION AND NUTRITION OF THE CENTRAL NERVOUS SYSTEM 285 
 
 Organization in the central system, 285 Defective development of the central system, 
 
 285 The central nervous system of laboratory animals, 286 The blood-supply of the 
 central system, 286 The influence of glands on the nervous system, 289 The influ- 
 ence of starvation on the nervous system, 289 Fatigue of the central nervous system, 
 289 Daily rhythms in the activity of the central nervous system, 289 The time 
 taken in central processes, 291. 
 
 D. SLEEP 291 
 
 Conditions favoring sleep, 291 Causes of sleep, 292 Condition of the central 
 
 nervous system in sleep, 293 Effect of loss of sleep, 295. 
 
 E. OLD AGE OF THE CENTRAL SYSTEM . . 295 
 
 Metabolism in the nerve-cells in old age, 295 Decrease in weight of the brain in old 
 
 age, 296 Changes in the encephalon in old age, 296 Changes in the cerebellum in 
 old age, 296. 
 
 THE SPECIAL SENSES 298 
 
 A. VISION (By HENRY P. BOWDITCH) 298 
 
 The general physiology of vision, 298 The mechanical movements of the eyeballs 
 around various axes, 298 The muscles of the eye, 299 The dioptric apparatus of the 
 eye, 300 The refracting media of the eye, 302 The optical constants^of the eye. 301 
 The mechanism of accommodation, 306 The range of accommodation. 312 Myopia 
 and hypermetropia, 313 Presbyopia, 314 Spherical aberration. 315 Chromatic 
 aberration, 316 Astigmatism, 317 Intraocular images. 320 Musc;e volitantes, .')-<> 
 The retinal vessels, 321 Circulation of blood in the retina, 322 The innervation 
 and movements of the iris, 322 The principle of the ophthalmoscope, 326 The 
 structure of the retina, 327 The blind-spot of the retina. 32S ( 'hantres produced in 
 the retina by light, 330 The production of the sensation of light. 331 The qualita- 
 tive modifications of light, 332 Color-sensations, 333 Means of producing color- 
 
14 CONTENTS. 
 
 mixtures, 333 Color-theories, 335 Color-blindness, 338 The intensity of light 
 sensations, 339 The luminosity of different colors, 340 The function of rods and 
 cones, 341 The saturation of color-sensations, 342 Retinal stimulation, 343 The 
 latent period of light-sensations, 343 The rise to maximum of light-sensations, 343 
 The fatigue of the retina, 344 The after-effect of stimulation, 345 After-images, 
 346 Color-contrasts, 346 The perception of space, 347 Irradiation, 349 The false 
 judgments of subdivided space, 350 The perception of distance, 354 Binocular 
 vision, 356 Pseudoscopic vision, 357 Binocular combination of colors, 358 Corre- 
 sponding points, 358 Visual illusions, 360. 
 
 B. THE EAR AND HEARING (By HENRY SEWALL) 362 
 
 The anatomy and histology of the ear, 362 The external ear, 362 The middle ear, 
 
 363 Movements of the ear-ossicles, 367 The Eustachian tube, 369 The muscles of 
 the middle ear, 369 The vibrations of the tympanic membrane, 370 The structure 
 of the internal ear, 371 The general anatomy of the cochlea, 374 The transmission 
 of vibrations through the labyrinth, 376 The membranous cochlea and the organ of 
 Corti, 376 The theory of auditory sensations, 380. 
 
 C. THE RELATIONS BETWEEN PHYSICAL AND PHYSIOLOGICAL SOUND 381 
 
 Production of sound-waves, 381 Loudness and musical pitch, 381 The tympanic 
 
 membrane as an organ of pressure-sense, 382 Overtones and quality of sound, 383 
 Analysis of composite tones by the ear, 384 Inharmonic overtones, 386 The produc- 
 tion of beats, 386 Harmony and discord, 387 Combinational tones, 387 Auditory 
 fatigue, 387 Imperfections of the ear, 388 Perceptions of time-intervals, 388 Musi- 
 cal tones and noises, 388 Functions of different parts of the ear, 388 The judgment 
 of direction and distance, 389. 
 
 D. CUTANEOUS AND MUSCULAR SENSATIONS 390 
 
 General importance of the cutaneous and muscular sensations, 390 Ending of 
 
 sensory nerve-fibres in the skin, 391 Relations of stimulus to the touch-sensations, 
 392 The localization of touch-sensations, 394 Pressure-points, 396 The importance 
 of the end-organ, 396 Touch-illusions, 396 The temperature-sense, 397 Cold and 
 warm points, 398 Common sensation and pain, 399 Transferred or sympathetic 
 pains ; allochiria, 400 Muscular sensation, 401 Hunger and thirst, 404. 
 
 E. THE EQUILIBRIUM OF THE BODY ; THE FUNCTION OF THE SEMICIRCULAR CANALS 404 
 The sense of equilibrium, 404 Disturbances of the sense of equilibrium, 405 
 
 Theory of the relation of the semicircular canals to equilibrium, 406 The relation of 
 the vestibular sacs to equilibrium, 407. 
 
 F. SMELL . 408 
 
 Structure of the olfactorv epithelium, 408 The production of olfactory sensations, 
 
 409. 
 
 G. TASTE 410 
 
 Structure of the taste-buds, 410 The production of taste-sensations, 411 Classifica- 
 tion of taste-sensations, 412 Specific energy of taste-nerves, 413. 
 
 PHYSIOLOGY OF SPECIAL MUSCULAR MECHANISMS 414 
 
 A. THE ACTION OF LOCOMOTOR MECHANISMS (By WARREN P. LOMBARD) 414 
 
 The articulations, 414 Sutures, 414 Symphyses, 414 Syndesmoses, 414 Diar- 
 
 throses, 415 The lever-action of muscles on bones, 417 The act of standing, 418 
 The act of locomotion, 420 Walking, 420 Running, 421. 
 
 B. VOICE AND SPEECH (By HENRY SEWALL) 421 
 
 Voice-production, 421 Functions of the epiglottis, 422 Ventricular bands and 
 
 ventricles of Morgagni, 422 The true vocal cords, 423 The cartilages of the larynx, 
 425 The muscles of the larynx, 425 Specific actions of the laryngeal muscles, 427 
 The nerve-supply of the larynx, 428 The laryngoscopic appearance of the larynx, 
 429 The production of voice, 430 Loudness and pitch of voice, 430 Quality of voice, 
 430 Arrangements for changing the pitch of the voice, 432 The vocal registers, 432 
 A whistling register, 433 Speech, 433 The classification of vowel sounds, 434 
 Whispering, 436 The production and classification of consonants, 436. 
 
 REPRODUCTION (By FREDERIC S. LEE) \ ... 439 
 
 A. REPRODUCTION IN GENERAL 439 
 
 Asexual reproduction, 439 Sexual reproduction, 440 Origin of sex and theory of 
 
 reproduction, 441 Primary and secondary sexual characters, 442 The sexual organs, 
 443. 
 
 B. THE MALE REPRODUCTIVE ORGANS . 443 
 
 Structure and properties of the spermatozoon, 443 Maturation of the spermatozoon, 
 
 445 The composition of semen, 445 The testis, 446 The urethra, 448 The prostate 
 gland, 448 Cowper's glands, 448 The penis, 448. 
 
CONTENTS. 15 
 
 PAGE 
 
 C. THE FEMALE REPRODUCTIVE ORGANS 449 
 
 The ovum, 449 Maturation of the ovum, 451 The ovary and ovulation, 454 The 
 
 Fallopian tuhes, 456 The uterus, 456 The act of menstruation. r>7 -Comparative 
 physiology of menstruation, 459 Theories of menstruation, 460 The vagina, 462 
 The vulva and its parts, 462 The mammary glands, 462 The internal secretion of 
 the ovaries, 1 )>_>. 
 
 D. THE REPRODUCTIVE PROCESS 463 
 
 The act of copulation, 463 Locomotion of the spermatozoa, 465 Fertilization of 
 
 the ovum, 466 Segmentation of the ovum, 467 Polysperiny, 471 The decidua 
 graviditatis, 471 The fetal membranes, 472 The placenta, 474 Nutrition of the 
 embryo, 475 Physiological effects of pregnancy on the mother, 477 The duration of 
 gestation, l?s Parturition, 479 First stage of labor, 479 Second stage of labor, 480 
 Third stage of labor, 480 Physiology of labor, 481 Multiple conceptions, 482 
 The determination of sex, 483. 
 
 E. EPOCHS IN THE PHYSIOLOGICAL LIFE OF THE INDIVIDUAL 486 
 
 Growth of the cells, the tissues, and the organs, 486 Growth of the body before 
 
 birth, 486 Growth of the body after birth, 487 The condition of puberty, 489 The 
 Climacteric, 490 Senescence, 490 Death, 491 Theory of death, 492. 
 
 F. HEREDITY 494 
 
 The facts of inheritance, 494 Latent characters, atavism or reversion, 495 Re- 
 generation, 496 The inheritance of acquired characters, 496 The inheritance of dis- 
 ease, 498 Theories of heredity, 498 Germ-plasm, 499 Variation, 500 Darwin's theory 
 
 of pangeuesis, 501 Weismann's theory of heredity, 502 Theory of epigenesis, 504. 
 
I. GENERAL PHYSIOLOGY OF MUSCLE AND 
 
 NERVE. 
 
 A. INTRODUCTION. 
 
 IT is seldom that the physical and chemical structure of a tissue, as revealed 
 by the microscope and the most careful analysis, gives even a suggestion as to 
 its function. No one would conclude from looking at a piece of beef, or even 
 microscopically examining a muscle, that it had once been capable of motion, 
 nor would the most exact statement of its chemical composition give indication 
 of such a form of activity. The most thorough histological and chemical 
 examination of the bundle of fibres which compose a nerve would fail to sug- 
 gest that a blow upon one end of it would cause to be transmitted to the other 
 end an invisible change capable of exciting to action the cell with which the 
 nerve communicated. To understand such a structure we must first learn the 
 forms of activity of which the tissue is capable, the influences which excite 
 it to action, and the conditions essential to its activity, and then seek an expla- 
 nation of these facts in its physical and chemical constitution. 
 
 Contractility. One of the most striking properties of living matter is 
 its power to move and to change its form. At times the movements occur 
 apparently spontaneously, the exciting cause seeming to originate within the 
 living substance, but more often the motions are developed in response to some 
 external influence. This power finds its best expression in muscle-substance. 
 In its resting form a muscle, such as the biceps, is elongated, and when it is 
 excited to action it assumes a more spherical shape, i. e. shortens and thickens, 
 whence it is said to have the property of contractility. It is the shortening, 
 the contraction, of the muscle which enables it to perform its function of 
 moving the parts to which it is attached, as the bones of the arm or leg, and 
 of altering the size of the structures of which it forms a part, as the walls 
 of the heart, intestine, or bladder. Ordinary muscle-substance is arranged 
 in fine threads, each one of which is enveloped in a delicate membrane, the 
 sarcolemma ; these muscle-fibres can be compared to long sausages of micro- 
 scopic proportions. A muscle is composed of a vast number of fibres 
 arranged side by side in bundles, the whole being firmly bound together by 
 connective tissue. Since isolated muscle-fibres have been seen under the 
 microscope to contract, each fibre can be looked upon as containing true muscle- 
 substance and being endowed with contractility. The movements of muscles 
 are the resultant of the combined activity of the many microscopic fibres of 
 which the muscles are composed. 
 
 The rate, extent, strength, and duration of muscular contractions are adapted 
 
 VOL. II. 2 17 
 
18 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 to the needs of the parts to be influenced, and it is found that the structure of 
 the muscles differs according to the work which they have to perform. Thus 
 we find two large classes_pf muscles : the one, like the muscles which move the 
 bones, remarkable for the rapidity with which they change their form, but 
 unsuited to long-continued action ; the other, occurring in the walls of the 
 intestine, blood-vessels, bladder, etc., sluggish of movement, but possessing great 
 endurance. The first of these, when examined with the microscope, is seen 
 to be composed of bundles of fibres, which are transversely marked by alter- 
 nating dark and light bands, and hence are called striated or striped muscles ; 
 the other, though composed of fibres, shows no such cross markings, and 
 therefore is known as smooth or non-striated muscle. " Striated muscles are 
 
 FIG. 1. Amoeba proteus, magnified 200 times : a, endosarc ; b, simple pseudopodium ; c, ectosarc ; d, 
 first stage in the growth of a pseudopodium ; e, pseudopodium a little older than d ; /, branched pseudo- 
 podium ; g, food-vacuole ; h, food-ball ; i, endoplast ; k, contractile vesicle (after Brooks : Handbook of 
 Invertebrate Zoology). 
 
 often called voluntary, because most of them can be excited to action by the 
 will, whereas non-striated muscles are termed involuntary, because in most 
 cases they cannot be so controlled. Within these two large classes of muscles 
 we find special forms presenting other, though lesser, differences in function 
 and structure. The muscle of the heart, though striated, differs so much from 
 other forms of striped muscle as almost to belong in a special class. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 19 
 
 Since contractility is possessed by all forms of muscle-tissue, it is evident that 
 it is independent of superficial structural differences. Nor is muscle the only 
 substance possessing this property. Even isolated microscopic particles of liv- 
 ing mutter are capable of making movements, both spontaneously and when 
 excited by external influences. As far back as 1755, Rosel von Rosenhof 
 described the apparently spontaneous changes in form of a living organism 
 composed of a single cell, a fresh-water amoeba. Moreover, he noted that, if 
 quiet, it could be excited to action by mechanical shocks. 
 
 The amoeba (Fig. 1) is a little animal, of microscopic size, which is found 
 in the ooze at the bottom of pools, or in the slime which clings to some of our 
 fresh-water plants. Under the microscope it is seen to be composed of jelly- 
 like, almost transparent matter, in which are a vast number of fine granules, a 
 delicate tracery of finest fibrils, a small round body, called the nucleus or 
 endoplast, a round hollow space termed the contractile vesicle, which is seen to 
 change in size, appearing or disappearing from time to time, and small parti- 
 cles, which are bits of food or foreign bodies. In the resting state the body 
 has a somewhat flattened, irregular form, which, if the slide on which it rests be 
 kept warm, is found to alter from minute to minute. Little tongue-like projec- 
 tions, pseudopods (false feet), are protruded from the surface like feelers, and 
 are then withdrawn, while others appear in new places. Evidently the little 
 creature, though composed of a single cell, is endowed with life and has the 
 power of making movements. Moreover, it may be seen to change its place, 
 the method of locomotion being a peculiar 
 one. One of the processes, or pseudopods, 
 may be extended a considerable distance, and 
 then, instead of being withdrawn, grow in size, 
 while the body of the animal becomes corre- 
 spondingly smaller ; thus a transfer of material 
 takes place, and this continues until the whole 
 of the material of the cell has flowed over to 
 the new place. This power of movement per- 
 mits the animal to eat. If when moving over 
 the slide it encounters suitable food material, a 
 diatom for instance, it flows round it, engulf- 
 ing it in its semifluid mass ; and in a similar 
 manner the animal gets rid of the useless sub- 
 stances which it may have surrounded, by flow- 
 ing away from them. These movements may 
 
 !^ p " , I'll j -! FIG. 2. Vorticella nebulifera, X 600: 
 
 result from changes which have occurred within a cilia of ciliated disk . 6> ciliated disk . 
 its own substance, and apparentlv independently c, peristome -, d, vestibule ; e, oesophagus ; 
 
 , . , f, contractile vesicle ; g, food-vacuoles ; 
 
 of any external influence. On the other hand, ht en dopiast; i, endosarc; k, ectosarc; i, 
 
 if its body be disturbed bv being touched, by cuticle; m, axis of stem (after Brooks: 
 * . , Handbook of Invertebrate Zoology). 
 
 an unusual temperature, by certain chemicals, 
 
 or by an electric shock, it replies by drawing in all of its pseudopods and 
 
 assuming a contracted, ball form. 
 
 The movements of the leucocytes of the blood resemble in many respects 
 
20 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 those of ^ the amoeba. 1 The property of contractility is possessed by a vast 
 variety of unicellular structures in lower forms of animal life. Another 
 example is the Vorticella (Fig. 2). 
 
 The vorticella, like the amoeba, is a little animal which, although consisting 
 of a single cell, possesses within its microscopic form all the physiological prop- 
 erties essential to life and the perpetuation of its species. It consists of a bell, 
 with ciliated margin, borne upon a contractile stalk. If touched with a 
 hair, or jarred, the cell rapidly contracts ; the edge of the bell is drawn in so 
 as to make the body nearly spherical, and the stalk is thrown into a spiral 
 and drags the body back toward the point of attachment. The contraction is 
 rapid; the relaxation, which comes when the irritation ceases, is gradual. An 
 interesting account of the movements of Vorticella gracilis is given by Hodge 
 and Aikins 2 under the title of " The Daily Life of a Protozoan." 
 
 Other examples of contractile power possessed by apparently simple organ- 
 isms are to be found in the tentacles of Actiniae, the surface sarcode of sponges, 
 the chromatoblasts of Pleuronectidse, 3 which are controlled by nerves and 
 under the influence of light and darkness change their size and so alter the 
 color of the skin, and the vast variety of ciliated- forms, including spermatozoa, 
 and some of the cells of mucous membranes. 4 
 
 Irritability. We have thus far referred to but one of the vital properties 
 of protoplasm, viz. contractility. Another property intimately associated with 
 it is irritability. Irritability is the property of living protoplasm which causes 
 it to undergo characteristic chemical and physical changes when subjected to 
 certain external influences called irritants. Muscle protoplasm is very irri- 
 table, and is easily excited to contraction by such irritants as electric shocks, 
 mechanical blows, etc. The muscles which move the bones rarely, if ever, in 
 a normal condition, exhibit spontaneous alterations in form, and cannot be said 
 to possess automatic power. By automatism is meant that property of cell- 
 protoplasm which enables it to become active as a result of changes which 
 originate within itself, and independently of any external irritant. Examples 
 of this power may perhaps be found in the movements of ciliated organisms 
 and the infusoria. Possibly the rhythmic movements of heart muscle are of 
 this nature. Still another property of protoplasm, closely allied to contractility 
 and irritability, and possessed by muscle-substance, is conductivity. 
 
 Conductivity is the property which enables a substance, when excited in 
 one part, to transmit the condition of activity throughout the irritable mate- 
 rial. For example, an external influence capable of exciting an irritable 
 muscle-fibre to contraction, although it may directly affect only a small part of 
 the fibre, may indirectly influence the whole, because the condition of activity 
 which it excites at the point of application is transmitted by the muscle-sub- 
 stance throughout the extent of the fibre. 
 
 1 An excellent description of these movements, accompanied by illustrations, is given in 
 Quain's Anatomy, vol. i., pt. 2, pp. 174-179. 
 
 2 Hodge and Aikins : American Journal of Psychology, 1895, vol. vi., No. 4, p. 524. 
 
 3 Krukenberg : Vergleichend-physiologische Vortrdge, 1886, Bd. i. S. 274. 
 
 4 A careful study of the different forms of movement exhibited by simple organisms has 
 been made by Engelmann: Hermann' Handbnch der Physiologic, 1879, Bd i., Th. 1, S, 344. 
 
L nrvsio: 
 
 Irritability and conductivity are not confined to contractile mechan- 
 isms. T 
 
 a^ tli not tlr n found to have -or of 
 
 rnov it \\ 'hieli with the growth of a cell, 
 
 near ! one or more branches. The bodies of the 
 
 nerv chiefly in the spinal cord ami brain, a smaller nu 
 
 being found in the spinal ganglia and in the ganglia of the so-called sympa- 
 thet ; . The branches of a neurone are of two kinds, an axis 
 
 T axone, which frequent! it its extremity a specially formed 
 
 liieli it is able to excite to action the cells with which it 
 comes in contact, and protoplasmic processes, or dendritos, which have no 
 such and are destined to receive excitation and trai; 
 
 it to the body of the nerve-cell. Outside the central nervous system, at 
 >ne and the dendrite acquire a delicate membranous sheath, 
 ;na, which invests it as the sarcolemma does the muscle-fibre. 
 The branches of nerve-cells together with their sheaths form the nerve- 
 - . There are two classes of nerve-fibres, medullated and non-medullated, 
 which are distinguished by the fact that the former has between the axis- 
 cylinder and the neurilemma another covering composed of fatty material, 
 called the medullary sheath, while in the latter this is absent. Just as 
 it is the special function of the muscle-fibre to change its form when it 
 is excited, so it is the special function of the nerve-fibre to transmit the 
 condition of activity excited at one end throughout its length, and to 
 \\aken to action the cell with which it communicates. Nerve-fibres are 
 the paths of communication between nerve-cells in the central nervous sys- 
 tem, between sense-organs at the surface of the body and the nerve-cells, 
 and between the nerve-cells and the muscle- and gland-cells. Nerve- 
 fibres are distinguished as afferent and efferent, or centripetal and centrifugal, 
 according as they carry impulses from the surface of the body inward or from 
 the central nervous system outward. Further, they receive names according 
 to the diameter of the activity which they excite: those which excite muscle- 
 to contract are called motor nerves; those distributed to the muscles 
 in the walls of blood-vessels, vaso-motor ; those which stimulate gland-cells to 
 action, secretory ; those which influence certain nerve-cells in the brain and so 
 cause sensation*, sensory. Still other names are given, as " trophic " to fibres 
 which are supposed to have a nutritive function, and " inhibitory " to those 
 which check the activities of various organs. The method of conduction is the 
 same in all tin dt depending wholly on the organ stimulated. 
 
 ve-fibres do not run for any distance separately, but always in company 
 with others. Thus large nerve-trunks may be formed, as in the case of the 
 s to the limbs, in which afferent and efferent fibres run side by side, the 
 whole being bound together into a compact bundle by connective tissue. The 
 separate fibres, though thus grouped together, are anatomically and physiologi- 
 cally as distinct as the wires of an ocean cable; that tit are 
 bound together is of anatomical interest, but has little physiological signifi< 
 The aci 'ance of the nerve-fibre does not show contractility, but this 
 
AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 does not prevent it from being classed with other irritable forms of. living cell- 
 substance as protoplasm. In spite of differences in structure and composition, 
 nerve protoplasm and muscle protoplasm are found to have many points of 
 resemblance. An explanation of the physiological resemblances may be found 
 in their common ancestry. All the cells of the many structures of the animal 
 body are descended from the two parent cells from which the animal is developed. 
 The fertilized ovurn divides, and two cells are formed, these new cells divide, 
 and so the process continues, the developing cells through unknown causes be- 
 coming arranged to form more or less definite layers and groups, which by means 
 of foldings and unequal growths develop into the .various structures and organs 
 of the fetus. At the same time that the division is going on, the total amount 
 of material is increasing. Each of the cells absorbs and assimilates dead food- 
 material, and this dead material is built into living substance. Daring this 
 process -of development and growth the cells of special tissues and organs 
 acquire special anatomical and chemical characters. This development of 
 specialized cells is termed cell-differentiation. Hand in hand with the ana- 
 tomical and chemical differentiation goes a physiological differentiation. The 
 protoplasm of each type of cell, while retaining the general characteristics of 
 protoplasm, has certain physiological properties developed to a marked degree 
 and other properties but little developed, or altogether lacking. The fertilized 
 ovum does not have all the anatomical and chemical characteristics of all the 
 cells which are descended from it, not at least in just the form in which they 
 are possessed by these cells, and it cannot be assumed that its living sub- 
 stance possesses all the physiological properties which are owned by its 
 descendants. Many of these properties it must have, for many of them are 
 essential to the continuance of life of all active cells, such as the power to take 
 in, alter, and utilize materials which are suitable for the building up and r< 
 of the cell-substance, the power of chemically changing materials possessing 
 potential energy so that the form of actual energy which is essential to the per- 
 formance of the work of the cell shall be liberated, and the power to give 
 off the waste materials which result from chemical changes. The protoplasm 
 of the ovum, to have these powers, has properties closely allied to absorption, 
 digestion, assimilation, respiration, excretion ; and, in consideration of the special 
 function of the ovum, we may add that it possesses the property of reproduc- 
 tion. The question of its possessing the characteristic properties of muscle and 
 nerve protoplasm cannot be answered off-hand. Careful study, however, has 
 shown the ovum of Hydra to possess irritability, conductivity, and contractility. 
 It undergoes amoeboid movements, as was first shown by Kleinenberg. 
 Balfour, 1 in writing of the development of the ova of Tubularidse, whi 
 of a type similar to Hydra, says: " The mode of nutrition of the ovum may 
 be very instructively studied in this type. The process is one of actual feed- 
 ing, much as an amoeba might feed on other organisms." Something similar 
 seems to be true of the ova of echinodermata. During impregnation various 
 movements are described implying the properties of irritability, conductivity, 
 and contractility. Thus in the case of Asterias glacialis, when the head of the 
 1 Comparative Embryology , pp. 17, 29. 
 
 
/'// } ' 
 
 murl!;i" 
 
 matozoon n uper- 
 
 i layer of pro until it coraes in contact with 
 
 u At ntnct between the sperma- 
 
 fi and the egg % , the outermost layer of protoplasm of the lair itself 
 
 up as a distinct membrane, which separates from the egg and prevents the 
 entrance of other spermatozoa." Some of the eggs of arthropods and other 
 forms have likewi observed to undergo amoeboid movements as a result 
 
 of the phy-- -timulus given by the Spermatozoon, 1 
 
 Although irritability and contractility of the ovum have thus far been made 
 out in but few forms, it is probable that they play an important part in all 
 during fertilization and division. It would seem, then, that the ovum has all 
 the principal properties which we ascribe to cell-protoplasm, and that 
 pro}). > inherited more or less completely developed by the many forms 
 
 of cells descended from it. The protoplasm of specialized cells, in spite of 
 their differences in structure, still retains its protoplasmic nature. Undoubtedly 
 structural peculiarities are intimately related to specialized functions, the 
 striped muscle, for example, is especially adapted for rapid movements, and 
 the nerve-fibre is remarkable for its power of conduction. 
 
 Physiological methods for the examination of individual cells are as yet in 
 their infancy, and we must still seek for exact knowledge of the functional 
 activity of cells by observing the combined action of many cells of the same 
 kind. 2 
 
 B. IRRITABILITY OF MUSCLE AND NERVE. 
 
 Irritability is the property of living protoplasm which causes it to undergo 
 characteristic physical and chemical changes when it is subjected to certain 
 influences, called irritants, or stimuli/ By an irritant is meant an external influ- 
 ence which, when applied to living protoplasm, as of a nerve or muscle, excites 
 it to action. Irritants may be roughly classed as mechanical, chemical, thermal, 
 and electrical. The normal physiological stimulus is. developed within some 
 of the nervous mechanisms of the body as the result of the activity of the 
 nerve-protoplasm, this having been excited as a rule by some form of irritant. 
 The degree of irritability of a given form of protoplasm is measured by the 
 amount of activity which it displays in response to a definite irritant, or by fhe 
 minimal amount of irritation required to excite it to action. If the irritant be 
 applied directly to a muscle, the height to which the muscle contracts and r 
 a given weight may be taken as an indication of its activity. A^ the nerve 
 gives no visible evidence of activity, the effect of the irritant upon it is usually 
 uted by the extent to which the organ stimulated by the nerve reacts : in 
 the case of motor nerves the strength of the contraction of the corresponding 
 le is taken as an imi 
 
 To determine the exact relation of an irritant to its irritating effect we should 
 
 1 Korschelt: Zoologiseht-. 1801, Anat, Urtheil., Bd. iv., -vig: 
 
 e Centralbl'i 
 
 OS of animal life, see <. 
 -lation by ! w York, ] - 
 
24 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 be able to accurately measure them. This' we cannot do. AW are unable to 
 state in irritation-units the relative value of different kinds of irritants, i 
 if we could accurately estimate the amount of energy which each form of irri- 
 tant can expend in iritation, we should have only one of the many factors 
 which determine its efficiency. It is equally difficult to compare the irritating 
 effect of irritants upon different forms of protoplasm ; e. g. we cannot state 
 what degree of activity of a nerve-fibre corresponds to a certain amount of 
 activity in a muscle-fibre. In spite of the lack of exact quantitative measure- 
 ments, we have gained a clear idea of the way different forms of irritants 
 act when applied to nerves and muscles in certain ways, and have learned to 
 control the methods of excitation sufficiently to permit the influences which 
 alter the irritability of nerves and muscles to show themselves. The effect of 
 irritants can best be studied upon the nerves and muscles of cold-blooded ani- 
 mals, because these retain their vitality and irritability for a considerable time 
 after they have been separated from the rest of the body. It is a common 
 observation of country folk that the body of a snake remains alive for a long 
 time after the head has been crushed, while the body of a chicken loses all signs 
 of life in a comparatively short time after it has been decapitated. More care- 
 ful examination would show that in neither case do all parts of the body die' J 
 simultaneously. Each of the myriad cells has a life of its own, which it 
 loses sooner or later according to its nature and to the alterations to which it 
 is subjected by the fatal change. The cells of cold-blooded animals, as the 
 snake and frog, are much more resistant than those of warm-blooded animals, 
 because the vital processes within the cells are less active, and the chemical 
 changes which precede and lead to the death of the part occur more slowly. 
 For instance, the nerves and muscles of a frog remain irritable for many hours, 
 or even days, after the animal has been killed and they have been removed 
 from the body. This fact is of the greatest use to the student. It enables him 
 to study the nerve or muscle by itself, and under such artificial conditions as 
 he cares to employ. Experience shows that the facts learned from the study of 
 the isolated nerve and muscle hold good, with but slight modification, for the 
 nerves and muscles when in the normal body. Moreover, it has been found 
 that the nerves and muscles of warm-blooded animals, and even man, resemble 
 physiologically as well as anatomically those of the frog. The correspondence 
 is by no means complete, but it is so great as to make the facts discovered by 
 a study of the nerves and muscles of the frog of the utmost importance to us. 
 We are driven to such sources of information because of the great difficulty of 
 keeping the muscles of warm-blooded animals alive and in a normal condition 
 after removal from the circulation. 
 
 Irritability of Nerves. The following preparation suffices to illustrate 
 the more striking effects of irritants upon a nerve. A frog is rapidly killed. 
 and then the sciatic nerve is cut high up in the thigh and dissected out from 
 its groove, the branches going to the thigh-muscles being divided. The leg is 
 then cut through just above the knee. This gives a preparation consisting of 
 the uninjured lower leg and foot, and the carefully prepared nerve supplying 
 the muscles of these parts. The leg may be placed foot upward, and fastened 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 
 
 in this position by a clamp which grasps the bones at the knee, tli. c 
 being supported by an upright (see Fig. 3). This preparation can then !*> 
 subjected to a variety of tests. 
 
 Mechanical Irritation. If the nerve be cut, pinched, suddenly stretched, or 
 subjected to a blow, the muscles of the leg will 
 contract and the foot will be quickly moved. 
 
 Chemical Irritation. If acid, alkalies, vari- 
 ous salts, glycerin, or some other chemical sub- 
 stances be placed upon the nerve, the muscles 
 of the leg begin to twitch irregularly, and as 
 the chemical enters more and more deeply into 
 the nerve the movements will become more 
 and more marked, until finally all the muscles 
 are actively contracted and the foot is held 
 straight up. 
 
 Thermal Irritation. If hot glass, or the * ^-Experiment for dete-- 
 
 the irritability of nerves. 
 
 name of a match, be applied to the nerve, a 
 
 condition of activity will be developed in the rapidly heated nerve-fibres, and 
 
 lr responded to by more or less vigorous muscular contractions. 
 
 Electrical Irritation. If the wires connected with the two poles .of a 
 galvanic cell, static machine, or induction apparatus be brought in contact 
 with the nerve, the muscles will twitch each time there is a sudden chan 
 potential. 
 
 Physiological Irritation. By all these methods the nerve was excited by 
 irritants applied to it from without, and the muscle was excited to action by 
 the physiological stimulus coming to it from the excited nerve. The irritant 
 produced no visible change in the nerve, but the movement of the mil 
 was an evidence that the nerve had undergone a change at the point of stim- 
 ulation, and thatthe active, state thus produced had been transmitted tin mgh the 
 length of the nerve, and had been sufficiently marked to stimulate the mnx-lo 
 to contraction. This condition of activity which was transmitted along the 
 nerve is called the nerve-impulse. The same condition is excited in the 
 nerve-fibre when the body of the cell becomes active. 
 
 Independent Irritability of Muscle. In the above instances the irritant 
 were applied to the nerve, and the muscle was indirectly stimulated. Miisd. 
 protoplasm, like nerve protoplasm, may be directly excited to action by variou 
 forms of irritants. A nerve after entering a muscle branches freely, and the 
 nerve-fibres are distributed quite generally through the muscle. An irritant, 
 if directly applied to muscle, would probably excite the nerve-fibres present a> 
 well as the muscle-fibres, and to obtain proof of independent irritability of 
 muscle-substance it would be necessary to prevent the nerves from stimulating 
 the muscle. This can be done by paralyzing the nerve-endings with cur. 
 
 Curare, the South^merican arrow-poison, \? used by the Indians in hunt- 
 ing. The bird shot by these poisoned arrows gradually becomes para' 
 and, losing power to move its muscles, is easily captured. The following 
 experiment reveals the method of the action of this drug, and at the - 
 
26 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 time shows, first, that the muscle protoplasm can be irritated directly, and 
 secondly, that the nerves do not communicate directly with the muscles, but 
 stimulate them through the agency of terminal end-organs, called motor end- 
 plates} 
 
 Curare Experiment. Rapidly destroy the brain of a frog with a slightly 
 curved, blunt needle, and, to prevent hemorrhage, plug the wound by thrust- 
 ing a pointed match through the foramen magnum into the brain-cavity. 
 Expose the sciatic nerve of the left thigh, carefully pass a ligature under it, and 
 tie the ligature tightly about all the tissues of the thigh excepting the nerve, 
 thus cutting off the circulation from all the leg below the ligature without in- 
 jury to the nerve. Inject into the dorsal lymph-sac or the abdominal cavity a 
 few drops of a 2 per cent, solution of curare. In from twenty to forty minutes 
 the drug will have reached the general circulation and produced its effect. 
 
 Although the brain has been destroyed and the frog is incapable of having 
 sensation, it will be found that muscular movements will be made if the skin 
 be pinched soon after the drug has been given. These are reflex movements, 
 and are due to excitation of the spinal cord by the nerves connected with the 
 skin. As the paralyzing action of the drug progresses, these reflex actions be- 
 come feebler and feebler until altogether lost in the parts exposed to the drug, 
 although they may still be shown by the parts from which the drug has been 
 excluded. The condition of the nerves and muscles can be examined as soon 
 as reflex movements of the poisoned parts cease. 
 
 To ascertain the action of the poison, expose the nerves of the two legs, 
 either high up in the thigh or inside the abdominal cavity, where they have 
 been subjected to the poison, and test their irritability by exciting them with 
 electric shocks. Stimulation of the motor nerve of the right leg (a, Fig. 4) 
 causes no contraction of the muscles of that leg, while stimulation of the motor 
 nerve of the left leg (6), results in active movements of the muscles of that 
 leg. The response of the left leg shows that nerve-trunks are not injured by 
 \ the poison, and that the paralysis of the right leg must find some other expla- 
 nation. - On testing the muscles it is found that they are irritable and contract 
 when directly stimulated. Since neither nerve-trunks nor muscles are poisoned, 
 t is necessary to assume that the cause of the paralysis is something which pre- 
 s the nerve-impulse from passing from the nerve to the muscle. Micro- 
 opic examination shows that the nerve-fibre does not communicate directly 
 ith the muscle-fibre, but ends inside the sarcolemma in an organ which is 
 ^P* called the motor end-plate (see Fig. 31). It appears that the nerve acts on the 
 muscle through this organ, and its failure to act on the side which was exposed 
 to the curare was because the end-plate had been paralyzed by the drug. By 
 the use of curare, therefore, we are enabled to prevent the nerve-impulse from 
 reaching the muscles, and, when we have done this, we find that the muscle 
 is still able to respond to direct excitation with all forms of irritants, viz., 
 
 1 Ch. Bernard : "Analyse physiolo iqne des Prnprietes des Systemes muscnlaires et nerveux 
 au moyen du Curare," Comptes-rendus, 1856, p. 825. Kolliker: " Physiologische Untersuch- 
 ungen iiber den Wirkungen einiger Gifte," Archiv fur pathologische Anatomie, 1856. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 27 
 
 electrical, mechanical, thermal, and ehemieal. Kvidently the muscle-proto- 
 plasm is irritable and is capable of developing a contraction independently of 
 the nerves. There are a number of natural plant bases that have a "curare- 
 like" action c. </., brucin, strychnin, leucin, nicotin, conin, etc. 1 I!' a 
 nerve-muscle preparation be dissected out and placed in a 0.7 per cent, solu- 
 tion of sodium chloride containing one of these drugs, sooner or later the nerve- 
 ends will be poisoned, and it will be found that excitation of the nerve has no 
 effect on the muscle, although the muscle responds well to direct excitation. 
 
 Other 7 Voo/x Ihnt tlie Muscle-protoplasm can be Directly Irritated. Mus- 
 cles with long parallel fibres, such as the sartorius of the frog, contain no 
 nerves at their extremities, the nerve-fibres joining the 
 muscle-fibres at some little distance from their ends. 
 The tip of such a muscle, where no nerve-fibres can 
 be discovered by the most careful microscopical exam- 
 ination, is found to be irritable. The fact that in some 
 of the lower animals there are simple forms of contrac- 
 tile tissue in which nerves cannot be discovered, and 
 which are irritable, is interesting as corroborative evi- 
 dence, although it is not a proof, of the independent 
 irritability of a highly differentiated tissue such as 
 striated muscle. Another similar piece of evidence is 
 to be found in the fact that the heart of the embryo 
 beats rhythmically before nerve appears to have been 
 developed. A proof can be found in the observation 
 that if a nerve be cut it begins to undergo degenera- FIG. 4 -curare experiment: 
 
 the shaded parts show the re- 
 
 tion and loses its irritability and conductivity in four gion of the body to which the 
 or five days, and the excitation of such a nerve has ^^SKSS^ 
 no effect upon the muscle although direct stimulation protected by the ligature 
 
 P ,1 i *L 11* .* 11 j i_ x- A from tne action of the drug. 
 
 Of the muscle itself. IS followed by contraction. As The unbroken lines represent 
 
 degeneration involves not onlv the whole course of the tlie sensory nerves which 
 
 , , . " , , . . carry sensory impulses from 
 
 nerve, but also the nerve end-plates, the contraction t he skin to the central nerv- 
 must be attributed to the irritability of the muscle- us system ; the broken lines 
 
 . . indicate the motor nerves, 
 
 substance. Another point of interest in this connection which carry motor impulses 
 
 is the behavior of a dying muscle. If it be struck, 
 
 instead of contracting as a whole it contracts at the Lauder Brunton: 
 
 place where it was irritated, the drawing together of SioT^ 
 
 the fibres at the part forming a local swelling, or welt. 
 
 If such a muscle be stroked, a wave of contraction spreads over it, following 
 
 the instrument, instead of extending, as under normal conditions, by means of 
 
 the excited nerve-fibres to other parts. Under these circumstances the circum- 
 
 scribed contraction would seem to show that the nerves had lost their irrita- 
 
 bility, or that the nerve-ends no longer transmitted the stimulus to the muscle, 
 
 and the response was due to the direct excitation of the dying muscle-fibres. 
 
 This phenomenon is known as an idiomuscular contraction. 
 
 1 Santesson : Archivfiir experimentdle Pathologic und Pharmakologie, l>95j Bd. 35, S. 23. 
 
28 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 CONDITIONS WHICH DETERMINE THE EFFECT OF EXCITATION. 
 
 The result of the irritation of nerve and muscle is dependent on two sets 
 of conditions namely, conditions which determine the irritability ; condi- 
 tions which determine the efficiency of the irritant. 
 
 It will be necessary for us to study the second set of conditions first for, 
 before we can judge of the irritability and the effect of various influences upon 
 it, we must consider how far the activity of the nerve and muscle is depend- 
 ent on the character, strength, and method of application of the irritant. 
 
 Conditions which Determine the Efficiency of Irritants. Some of 
 these conditions can be best studied on nerves, while others are more ap- 
 parent in their effects on muscles. The most useful irritant for purposes of 
 study is the electric current. Mechanical, thermal, and chemical irritants are 
 likely to injure the tissue, and are not manageable, whereas electricity, if not 
 too strong, can be applied again and again without producing any permanent 
 alteration, and can be accurately graded as to strength, place, time, duration 
 of application, etc. Of course, the results obtained by the use of a given 
 irritant cannot be accepted for others until verified. The conditions which 
 determine the effectiveness of the electric current as an irritant may be 
 classed as follows : (a) The rate at which the intensity changes, (b) The 
 strength of current, (c) The density of current, (d) The duration of 
 application, (e) The angle of application. (/) The direction of flow. 
 
 Irritating Effect of the Electric Current Luigi Galvani, Professor of 
 Physics at Bologna, 1791 (or, according to some, his wife Lucia), observed the 
 legs of frogs which had been prepared for the kitchen, and had been suspended 
 by brass hooks from an iron balcony, make convulsive movements every time 
 the wind blew them against the iron. He repeated the experiment in his 
 laboratory, and decided that the frogs had been excited to action by electric 
 currents developed within themselves ; he looked upon the metals which he 
 had used merely as conductors for this current. Yolta, Professor of Natural 
 Philosophy at Pavia, repeated Galvani' s experiment, and concluded that 
 there had been an electric current developed from the contact of the dissimilar 
 metals with the moist tissues of the frog. In accordance with this idea he con- 
 structed the voltaic pile, and this was the starting-point of the science of 
 electricity of to-day. 
 
 Although it is true that, under certain conditions, differences in electric 
 potential sufficient to excite muscles to contraction can be developed in the 
 animal body, the contractions of the frog's leg which Galvani observed were 
 due to the metals which he employed. The experiment can be easily per- 
 formed by connecting a bit of zinc to a piece of curved copper wire, and bring- 
 ing the two ends of the arc against the moist nerve and muscle of a frog. A 
 stronger and more efficient shock can be obtained from a Daniell or some other 
 voltaic cell. 
 
 A Daniell cell (Fig. 5) is composed of a zinc and copper plate, the former dipping 
 into dilute sulphuric acid, the latter into a strong copper-sulphate solution. Although 
 gravity will keep these liquids separated, if the cell is to be moved about it is better 
 
GENERAL PHYSIOLOGY OF 
 
 AND NERVI-:. 29 
 
 to enclose one of them in a porous cup. A common form of cell consists of a glass jar, 
 in the middle of which is a porous cup; outside the rup is the sulphuric arid and tlie 
 zinc plate, and inside the cup is the copper sulphate solution and the copper plan-. 
 The zinc plate is acted upon by the sulphuric acid, and, as a result of the chemical 
 change, a difference of electric potential is set up between the 
 metals, so that if the zinc and copper be connected by a piece of 
 metal, what we call an electric current flows from the zinc to the 
 copper inside the cell, and from the copper to the zinc outside the 
 cell. The zinc plate, being the seat of the chemical change, is 
 called the positive plate, and the copper the negative plate. 
 Several such cells may be connected together to form a battery, 
 each cell adding to the electro-motive force, and hence to the 
 strength of the current. As the current is alwaj's considered to 
 flow from + to , we call the end of the wire connected with the 
 copper (negative plate) the positive pole, or anode, and the end 
 of the wire connected with the zinc (positive plate) the negative 
 pole, or Ivctthode. If one of these wires be touched to a nerve, 
 under ordinary circumstances no effect is produced ; but when the 
 other wire is likewise brought in contact with the nerve, the 
 moist tissues of the nerve form a conductor, complete the cir- 
 cuit, and an electric current at once flows through the nerve from 
 
 the anode to the kathode. The effect of tha sudden flow of 
 , . . . FIG. 5. Darnell cell, 
 
 electricity into the nerve is to give it a shock as we say, it 
 
 irritates the nerve and the muscle which the nerve controls is seen to contract. 
 
 In the place of using ordinary wires for applying the electricity, we use electrodes. 
 These are practically the same thing, but have insulated handles, and have a form better 
 suited to stimulate nerves or other tissues. The two wires may be held in two different 
 
 FIG. 6. a, Ordinary electrode for exciting exposed nerves and muscles, consisting of two wires 
 enclosed, except at their extremities, in a handle of non-conducting material ; 6, c, non-polarizable elec- 
 trodes. When metals come in contact with moist tissues a galvanic action is likely to occur and polariz- 
 ing currents to be formed. These extra currents would complicate or interfere with the results of many 
 forms of experiment, and they are avoided by the use of non-polarizable electrodes. A simple form con- 
 sists of a short glass tube, at one end of which is a plug of china clay mixed with a 0.6 per cent, solution 
 of sodium chloride, and at the other end a cork through which an amalgamated zinc rod is thrust. The 
 zinc rod dips into a saturated solution of zinc sulphate, which is in contact with the clay. The clay plugs 
 touch the tissue to be excited, and the current passes from the zinc rods through the zinc-sulphate and 
 sodium-chloride solutions in the clay to the tissues ; d-f, electrodes for exciting human nerves and mus- 
 cles through the skin (after Erb) : these may be of various forms and sizes, and are arranged to screw 
 into handles (g), to which the wires are attached ; they are usually made of brass and covered with 
 sponge or other absorbent material wet with salt-solution. The smaller electrodes are used when a dense, 
 well-localized stream is required, and the larger electrodes when little action is wished and it is of 
 advantage to have the stream diffuse. 
 
 
 handles, in which case we speak of the positive and negative electrodes, or the anode 
 and the kathode, or they may be held in the same handle (Fig. 6). 
 
30 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 v Keys. It is not as convenient to stimulate a nerve by touching it with the electrodes as 
 it is to place it upon the electrodes and close the connection between the zinc and copper at 
 some other part of the circuit ; this may be done by what is called a key. Any mechanism 
 
 which can be used to complete the circuit could 
 receive this name, and there are a number 
 of convenient forms. The one most used by 
 physiologists is that devised by Du Bois-Rey- 
 mond, and which bears his name (see Fig. 7). 
 This has the advantage of being capable of 
 being used in two different ways one simply 
 as a means to close the circuit, and the other 
 to short-circuit the current. These two meth- 
 ods are shown in Figure 8. 
 
 By the former method the key supplies a 
 movable piece of metal by which contact be- 
 tween the two ends of the wires may be made 
 as in a (Fig. 8), or broken as in 6, and the 
 current be sent through the nerve, or prevented 
 from entering it. By the latter method the 
 battery is all the time connected with the 
 electrodes., and the key acts as a movable 
 bridge between the wires, and when closed gives a path of slight resistance by which 
 the current can return to the battery without passing through the nerve. The current 
 always takes the path of least resistance, and so, if the key be closed as in c, all the cur- 
 rent will pass through the key and none will go to the nerve, which has a high resistance, 
 whereas if the key be opened as in d, the bridge being removed, all the current will go 
 through the nerve. It is often better to let the cell or battery work a short time and to 
 get its full strength before letting the current enter the nerve, and the short-circuiting key 
 permits of this. Moreover, there are times when a nerve may be stimulated if connected 
 
 FIG. 7. Electric key. 
 
 FIG. 8. Electric circuiting. 
 
 with the source of electricity by only one wire ; when the nerve is so excited, it is called 
 unipolar stimulation ; this may be prevented by the short-circuiting key. 
 
 As has been said, a nerve is irritated if it be connected with a battery and 
 an electric current suddenly passes through it. Unless the current be very 
 strong the irritation is transient, however ; the muscle connected with the 
 
GENERAL rilYNTOLOOY OF MUSCLE AND NERVE. 31 
 
 uerve gives a single twitch at the moment that the current enters the 
 nerve, and then remains quiet; and thus we meet with the remarkable faet 
 that an electric current, though irritating a nerve at the moment that it 
 enters it, can flow through the nerve continuously without exciting it. Fur- 
 ther, although the current while flowing through the nerve does not excite 
 it, a sudden withdrawal of the current from the nerve irritates it, and causes 
 the muscle connected with it to contract. It is our custom to speak of 
 closing, or making, the circuit when we complete the circuit and let the 
 current flow through the nerve, and of opening, or breaking, the circuit 
 when we withdraw the current from the nerve. Since the closing of the 
 circuit acts as a sudden irritant to the nerve, we speak of this irritant as 
 a "making" or " closing " shock, and the corresponding contraction of the 
 muscles as a making or closing contraction ; similarly we speak of the effect 
 of opening the circuit as an " opening" or " breaking" shock, and the result- 
 ing contraction as an opening or breaking contraction. As we shall see later, 
 the making contraction excited by the direct battery current is stronger than the 
 breaking contraction : the explanation of this must be deferred (see page 38). 
 
 (a) Effect of the Rate at which an Irritant is Applied, Illustrated by the Elec- 
 tric Current. As has been said, an electric current of constant medium strength 
 
 FIG. 9. Rheonome. 
 
 does not irritate a nerve while flowing through it, but the nerve is irritated at 
 the instant that the current enters it, and at the instant that the current leaves 
 it. Is it the change of condition to which the nerve is subjected, or is it the 
 suddenness of the change, which produces the excitation ? Would it be possi- 
 ble to turn an electric current into a nerve and remove it from a nerve so 
 slowly that it would not act as an irritant ? 
 
 The experiment has been tried, and it has been found that if the nerve be 
 subjected to an electric current the strength of which is increased or decreased 
 very gradually, no change occurs in the nerve sufficient to cause a contraction 
 of the muscle. In this experiment, instead of using the ordinary key, we close 
 and open the circuit by means of a rheonome (see Fig. 9). 
 
 This instrument contains a fluid resistance, which can be altered at will, thereby per- 
 mitting a greater or less strength of current to pass from the battery into the circuit 
 
32 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 containing the nerve. The wires from the battery are connected with binding-posts, a, b 
 (Fig. 9), at opposite sides of a circular groove containing a saturated solution of zinc sul- 
 phate. Strips of amalgamated zinc connect the binding-posts with the fluid, and so com- 
 plete a circuit which offers much resistance to the passage of the current. From the centre 
 of the block containing the groove rises an upright bearing a movable horizontal bar, from 
 each extremity of which an amalgamated zinc rod, e and/, descends and dips into the zinc- 
 sulphate solution. The zinc rods are connected with binding-posts on the movable bar, and 
 from these wires pass to the electrodes on which the nerve rests. The bar revolves on a 
 pivot on the top of the upright, and thus the zinc rods can be readily approached to 
 or removed from the zinc strips, the poles of the battery. When the zinc rods hold a 
 position midway between these poles, the current all passes by the way of the fluid. As 
 the bar is turned, so as to bring the zinc rods nearer and nearer the two poles of the bat- 
 tery, the current divides, and more and more of it passes through the path of lessening 
 resistance of which the nerve is a part. When the zinc rods are brought directly opposite 
 the poles of the battery nearly all the current passes by the way of the nerve. If the bar be 
 turned more or less rapidly, the current is thrown into, or withdrawn from, the nerve more 
 or less quickly. 
 
 By this arrangement we can not only observe that the nerve fails to be irritated 
 when the current is made to enter or leave it gradually, and when it is flowing 
 continuously through it, but that sudden variations in the density of the cur- 
 rent flowing through the nerve, such as are caused by quick movements of the 
 bar, although they do not make or break the circuit, serve to excite. This 
 experiment shows that electricity, as such, does not irritate a nerve, but that a 
 sudden change in the density of the current, whether it be an increase or 
 decrease, produces an alteration in the nerve-protoplasm which excites it to 
 action and causes the development of what we call the nerve-impulse. 
 
 Du Bois-Reymond's Law. Du Bois-Reymond formulated the following 
 rule for the irritation of nerves by the electrical current : " It is not the abso- 
 lute value of the current at each instant to which the motor nerve replies by a 
 contraction of its muscle, but the alteration of this value from one moment to 
 another; and, indeed, the excitation to movement which results from this change 
 is greater the more rapidly it occurs by equal amounts, or the greater it is in 
 a given time." 
 
 We shall have occasion to see that this rule has exceptions, or rather that 
 there is an upper as well as lower limit to the rate of change of density of the 
 electric current which is favorable to irritation. 
 
 Similar observations may be made with other forms of irritants. Pres- 
 sure, if brought to bear on a nerve gradually enough, may be increased to the 
 point of crushing it without causing sufficient irritation to excite the attached 
 muscle to contract, although, as has been said, a very slight tap is capable of 
 stimulating a nerve. Temperature, and various chemicals, likewise, must be so 
 applied as to produce rapid alterations in the nerve-protoplasm in order to act 
 as irritants. The same rule would seem to hold good for the nerve-cells of the 
 central nervous system. It is a matter of daily experience that the nervous 
 mechanisms through which sensory impressions are perceived are vigorously 
 excited by sudden alterations in the intensity of stimuli reaching them, and but 
 little affected by their continuous application ; the withdrawal of light, a sudden 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 33 
 
 alteration of temperature, an unexpected noise, or the cessation of a monotonous 
 sound, as exemplified by the common experience that a sleeper is awakened 
 
 FIG. 10. Induction apparatus: a, primary coil; 6, secondary coil ; c, the automatic interrupter. 
 
 when reading aloud abruptly ceases, attract the attention, although a continu- 
 ous sensory irritation may be unnoticed. This physiological law of the nervous 
 system would seem to have a psychological bearing as well. 
 
 FIG. 11. Schema of induction apparatus. 
 
 Irritating Effect of Induced Electric Currents. Within certain limits, the 
 more rapid the change in intensity of an electric current the greater its power to 
 irritate. This probably accounts in part 
 for the fact that the induced current is a 
 more powerful irritant to nerves than the 
 direct galvanic current. Induced currents 
 are usually obtained by means of an induc- 
 tion apparatus (see Fig. 10). 
 
 The ordinary induction apparatus employed 
 in the laboratory (see Fig. 11) consists of a coil of 
 wire, p, which may be connected with the ter- 
 minals of a battery, 6, and a second coil, s, wholly 
 independent of the first, which is connected with 
 electrodes, e. At the instant that the key, &, in 
 the primary circuit is closed, and the battery cur- 
 rent enters the primary coil, an induced current 
 s developed in the secondary coil, and the nerve 
 resting on the electrodes is irritated. The in- 
 duced current is of exceedingly short duration, 
 suddenly rising to full intensity and falling to 
 zero. As long as the battery current continues to 
 flow constantly through the primary coil, there is 
 no change in the electrical condition of the sec- 
 ondary coil, but at the instant the primary current is broken another induced cuyent of short 
 duration is set up in the secondary coil, and again the nerve receives a shock. The rise and 
 VOL. II. 3 
 
 FIG. 12. Schema of the relative intensity 
 of induction currents (after Hermann, Hand- 
 buck der Physiologic, Bd. ii. S. 37) : P, abscissa 
 for the primary current; S, abscissa for the 
 secondary current; 1, curve of the rise of 
 intensity of the primary current when made ; 
 2, curve of the rise and fall of intensity of 
 the corresponding induced current; 3, curve 
 of fall of the intensity of the primary cur- 
 rent when it is broken; 4, curve of the rise 
 and fall of intensity of the corresponding in- 
 duced current. 
 
34 
 
 AN AMERICAN TEXT-BOOK OF PHYS1OLCGY. 
 
 fall of the density of the current in the secondary coil is very rapid, and this rapid double change 
 in density of the current causes the induction shock to be a very effective irritant. The break- 
 ing induction shock, as we call that which is produced by breaking the primary current, is 
 ^ found to act more vigorously than the making shock, which is the reverse of what is found 
 with direct battery currents. The cause of this lies iii the nature of 'he apparatus. At the 
 moment that the current begins to flow into the primary coil, it induces ftot only a current 
 in the secondary coil, but also currents in the coils of wire of the prmia'ry coil. These 
 extra induced currents in the primary coil have the opposite direction to the battery cur- 
 rent and tend to oppose its entrance, and thereby to prevent it from immediately gain- 
 ing its full intensity. This delay affects the development of the induced current in the 
 secondary coil, causing it to be weaker and to have a slower rise and fall of intensity than 
 would otherwise be the case. When the primary current is broken, on the other hand, 
 there is no opposition to its cessation, and the current induced in the secondary coil is 
 intense and has a rapid rise and fall. These differences are illustrated in Figure 12. 
 
 Myogram. To accurately test the effect of the making and breaking 
 induction shocks, it is necessary to record the reaction of the nerve ; this can 
 be done by recording the extent to which the corresponding muscle contracts 
 in response to the stimulus which it receives from the nerve. In such an 
 experiment it is customary to use what is known as a nerve-muscle prepara- 
 tion. The gastrocnemius muscle and sciatic nerve of a frog, for instance, are 
 carefully dissected out, the attachment of the muscle to the femur being pre- 
 served, and the bone being cut through at such a point that a sufficiently long 
 piece of it shall be left to fasten in a clamp, and so support the muscle (see 
 Fig. 13). 
 
 FIG. 13. Method of recording muscular contraction. 
 
 The simplest method of recording the extent of the muscular contraction 
 i*s to connect the muscle by means of a fine thread with a light lever, and let 
 the point of the lever rest against a smooth surface covered with soot, so that 
 when the muscle contracts it shall draw up the lever and trace a line of cor- 
 responding length upon the blackened surface. The combination of instru- 
 
GENERAL PHYSIOLOGY OF MUtiCLE AND NERVE. 35 
 
 raents employed to record the contraction of a muscle is called a myograph, and 
 the record of the contraction is termed a myogram. If, when the muscle of a 
 nerve-muscle preparation is thus arranged to write its 
 contractions, the nerve be irritated with alternating mak- 
 ing and breaking induction shocks of medium strength, 
 the muscle will make a series of movements, which, if 
 the surface be moved past the writing-point a short 
 distance after each contraction, will be pictured in the 
 record as a row of alternating long and short lines, the FIG 14 _ Effect of making 
 records of the breaking contractions being higher than and breaking induction 
 those of the making contractions (Fig. 14). Similar sl 
 results are obtained if, instead of irritating the nerve, we irritate the curarized 
 muscle directly. 
 
 Stimulating Effects of Making and Breaking the Direct Battery Current. 
 On account of the construction of the induction apparatus, breaking induction 
 shocks are more effective stimuli than making induction shocks. The reverse 
 is true of the stimulating effects which come from making and breaking the 
 direct battery current. The excitation which results from sending a galvanic 
 current into a nerve or muscle is stronger than that which is caused by the 
 withdrawal of the current. This difference is due to the physiological altera- 
 tions produced by the current as it flows through the irritable substance, and 
 is without doubt closely associated with changes in the irritability which occur 
 at the moment of the entrance and exit of the current. 
 
 The making contraction starts from the kathode, and the breaking contraction 
 from the anode. The irritation process which results from making the current 
 is developed at the kathode, and that which results from breaking the current 
 is developed at the anode. This was first demonstrated on normal muscles by 
 Yon Bezold, 1 and has since been substantiated for nerves as well as muscles 
 
 FIG. 15. Schema of Bering's double myograph : C, clamp holding middle of muscle ; P,P, pulleys to 
 the axes of which the recording levers are attached ; p,p, pulleys for the light weights which keep the 
 muscle under slight tension ; A, positive electrode ; K, negative electrode ; r, commutator for reversing 
 the current ; k, key ; 6, battery. 
 
 by the experiments of a great many observers. Perhaps the most striking 
 demonstration is to be obtained by Engelmann's method. The positive and 
 negative electrodes are applied to the two extremities of a long curarized sarto- 
 
 1 Untersuchungen uber die elektrische Emeguny von Muskdn und Nerven, 1861. 
 
36 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 rius muscle, which is clamped in the middle firmly enough to prevent the con- 
 tractions of one half from moving the other, but not enough to interfere with 
 the conduction-power of the tissue. The record of the contractions is best 
 obtained by the double myograph of Hering (Fig. 15), which permits the 
 recording levers attached to the two ends of the muscle to write directly under 
 each other, so that any difference in the beginning of the contraction of the 
 two halves of the muscle is immediately recognizable from the relative posi- 
 tions of the records of their contractions. * 
 
 The current is applied to the two extremities of the muscle by non-polarizable electrodes. 
 In all experiments with the direct battery current it is essential to employ non-polarizable 
 electrodes. The form devised by Hering is very useful where the current has to be applied 
 directly to the muscle, because the two electrodes are hung from pivots in such a way that 
 they move with the movements of the muscle, and hence do not shift their position when 
 the muscle contracts. Some kind of apparatus has to be employed for quickly reversing 
 the direction of the current. A convenient in- 
 strument for this purpose is Pohl's mercury com- 
 mutator (Fig. 16). This instrument consists of 
 a block of insulating material in which are six 
 little cups containing mercury, which is in con- 
 nection with binding-posts on the sides of the 
 block. Two of the mercury cups on the opposite 
 
 FIGS. 16, 17. Pohl's mercury commutator. 
 
 sides of the block a and b (Fig. 17, A), are connected by wires with the battery ; two others, 
 c and d, are connected with wires which pass to the electrodes ; the remaining two on the 
 opposite side of the block, e and /, are joined by movable good conducting wires with the 
 cups c and d in such a way that c connects with /, and d with e. Two anchor-like pieces of 
 metal are connected by an insulated handle, and are so placed that the stocks of the anchors 
 dip into the mercury cups a and b (Fig. 16). The anchors can be rocked to one side or the 
 other, so that the ends of the curved arms shall dip into the cups c and d (in which case 
 cup a will be connected with cup c, and cup b with cup d ), or so that the other ends of the 
 arms shall dip into cups e and / (in which case cup a will be connected with cup e, and by 
 means of the cross wire with cup d, and cup b will be connected with cup/, and by means 
 of the cross wire with cup c). By the arrangement shown in Fig. 17, A the current can 
 pass from the battery by way of a and c down the nerve, and by way of d and b back to 
 the battery ; or it can pass from the battery by way of a, e, d, and in the reverse direction, 
 up the nerve and back to the battery, by way of c, /, b. There are many other forms of 
 apparatus, generally known as pole-changers, which may be employed to reverse the 
 current. 
 
 The commutator can be used in another way (see Fig. 17, J5). If the battery be con- 
 nected with it as before, and the cross wires be removed, the current can be sent at will 
 into either one of two separate circuits. For instance, if the cups c, d be connected with 
 
GENERAL PHYSIOLOGY OF MUSCLE AMD NERVE. 37 
 
 the electrodes on one part of the nerve, and the cups e, f with the electrodes on another 
 part, the anchors have only to be rocked to one side or the other to complete the commu- 
 nication between the battery and one or the other of these pairs of electrodes. 
 
 In experiments with the double myograph, in which the making of the 
 current is used to irritate, records are obtained such as are shown in Figure 18. 
 
 FIG. 18. The making contraction starts at the kathode (after Biedermann). 
 
 In these records the beginning of the tuning-fork waves shows the moment 
 that the current was made and the irritation given. In the experiment from 
 which record a was taken the anode was at the knee-end of a curarized sartorius 
 muscle and the kathode at the pelvic end i. e. the current was ascending 
 through the muscle. The lower of the two curves was that got from the 
 kathode half, the arrangement being that shown in Figure 15, and the lower 
 curve began before that got from the anode half; i. e. the contraction originated 
 at the kathode and spread thence over the muscle. In b the current was 
 reversed, and the upper curve was obtained from the kathode half and the 
 lower from the anode half; in this also the kathode end contracted first. 
 In the above experiments the making of the current was used to irritate, 
 and the muscular contraction began at the kathode ; in experiments in which 
 the breaking of the current was employed the opposite was observed, the 
 anode end being seen to contract first, regardless of the direction of the cur- 
 rent. 
 
 If strong currents be used, the fleeting contractions which result from 
 opening and closing the current are followed by continued contractions, the 
 closing, Wundt's, and the opening, Hitter's tetanus, as they are called. These 
 continued contractions, which last for a considerable time, remain strictly 
 located at the region where they originate, and Engelmann proved by his 
 
38 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 experiments that the tetanus which results from closing a strong current 
 remains located at the kathode, and the tetanus following the opening of 
 the current remains located at the anode. 
 
 The same is true of the nerve as of the muscle; the irritating process which 
 is called out by the sudden entrance of a battery current into a nerve starts 
 from the negative pole, the kathode, and spreads thence throughout the nerve, 
 while the irritating process excited by the cessation of the flow of the current 
 starts from the region of the positive pole, the anode, and spreads from that 
 point throughout the nerve. A proof of this was obtained by Von Bezold, 
 who observed the difference in the time between the moment of excitation and 
 the beginning of the contraction of the muscle, when the nerve was excited 
 by opening and by closing the current, with the anode next to the muscle, 
 and with the kathode next to the muscle. He found the time to be longer 
 when the current was closed if the kathode was the more distant, and to be 
 longer when the current was opened if the anode was farther from the muscle. 
 Evidently in the case of the nerve as of the muscle, the irritable substance 
 subjected to the current is* not all affected alike. The current does not 
 set free the irritating process at every part of the nerve, but produces 
 peculiar and different effects at the two poles, the change which occurs at the 
 kathode when the current is closed being of a nature to cause the development 
 of the excitatory process which awakens the closing contraction, and the 
 change which occurs at the anode when the current is opened being such a? 
 to cause the development of the excitatory process which calls out the opening 
 contraction. 
 
 Closing contractions are stronger than opening contractions. The irritation 
 developed at the kathode is stronger than that developed at the anode. It is 
 true of both striated and unstriated muscles that an efficient irritation can be. 
 developed at the kathode with a weaker irritant than at the anode. Moreover, 
 a greater strength of current is required to produce opening than closing con- 
 tinued contractions. 
 
 The same may be said of nerves. If one applies a very weak battery cur- 
 rent to the nerve of a nerve-muscle preparation, he notices when he closes the 
 key a single slight contraction of the muscle, and when he opens the key, no 
 effect. If he then increases the strength of the current very gradually, and 
 tests the effects of the making and breaking of the current from time to time, 
 he observes that each time the strength of the current is increased the closing 
 contraction, which is due to irritation originating in the part of the nerve sub- 
 ject to the kathode, grows stronger, and finally contractions are also seen when 
 the circuit is broken, the irritation process developed at the anode having 
 become strong enough to excite the muscle. These opening contractions at 
 first are weak, but gradually increase in strength, until with a medium strength 
 of current vigorous contractions are seen to follow both opening and closing of 
 the current. If the strength of the current be still further increased, it is 
 found that either the closing or opening contraction begins to decrease in 
 size, and if a very strong current be employed, the closing or opening con- 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 39 
 
 traction will be absent. It has been ascertained that the direction in which 
 the current is flowing through the nerve determines which of these con- 
 tractions shall cease to appear. The cause of this will be explained a little 
 later. 
 
 ^ (6) Effect of Strength of Irritant. As a rule, the stronger an electric current 
 the greater its irritating effect. This can be readily tested upon a nerve with 
 the induction current, the strength of which can be varied at pleasure. The 
 strength of the induced current obtained from a given apparatus depends 
 upon the strength of the current in the primary coil, and on the distance of 
 the secondary from the primary coil. In ordinary induction machines (see Fig. 
 10, p. 33) the secondary coil is arranged to slide in a groove, and can be easily 
 approached to or removed from the primary coil, thus placing the coils of wire 
 of the secondary coil more or less under the influence of the magnetic field 
 about the primary coil. This permits the strength of the current to be graded 
 at will. The strength of the induced current does not increase, however, in 
 direct proportion to the nearness of the coils. As the secondary approaches 
 the primary coil, the induced current increases in strength at first very slowly, 
 and later more and more rapidly, reaching its greatest intensity when the 
 secondary coil has been pushed over the primary. 
 
 The relation of the strength of a current to the irritating effect upon a nerve 
 can be readily tested with such an induction apparatus. The secondary coils 
 can be connected with a pair of electrodes on which the nerve of a nerve- 
 muscle preparation rests (as in Fig. 11, page 33), and the muscle can be 
 arranged to record the height of its contractions (as in Fig. 13, p. 34). 
 The experiment can be begun by placing the secondary coil at such a dis- 
 tance from the primary that the making and breaking shocks are too feeble to 
 have any effect upon the nerve. Then the secondary coil can be gradually 
 approached to the primary, the primary current being made and broken at 
 regular intervals. At a certain point the breaking shock will excite a very 
 feeble contraction, the making shock producing no effect. If this contraction 
 is barely sufficient to be recognized, we call it the minimal breaking contraction 
 (see Fig. 19, a). In seeking the minimal contraction care must be taken not 
 to excite the preparation at too short intervals of time, for, as we shall see, 
 an irritation too slight to excite even a minimal contraction may, if repeated 
 at short intervals, increase the irritability of the preparation and so become 
 effective. By using a short-circuiting key in the secondary circuit we can 
 cut out the making, shocks, and test the effect of a further increase in the 
 strength of the current by the response of the muscle to the breaking shocks. 
 As the contractions become larger, care must be taken not to irritate the muscle 
 too frequently, lest it be fatigued and so fail to give the normal response. As 
 the current is strengthened the breaking contractions will become higher and 
 higher until a point is reached beyond which the strength of the current may 
 be increased to a considerable extent without any further heightening effect (Fig. 
 19, 6). If the current be still further increased, this first maximum is suc- 
 ceeded by a still further growth in the height of the contractions, until finally 
 
40 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 a second maximum (Fig. 19, d) is reached, beyond which no further increase 
 is to be obtained, however much the current may be strengthened. 1 
 
 FIG. 19. Effect of increase of strength of current on the efficiency of breaking induction shocks (after 
 Fick) : a, minimal contraction ; 6-c, first maximum ; d-e, second maximum. , 
 
 If both the making and breaking contractions be recorded, inasmuch as 
 the making shocks are weaker stimuli than the breaking (see p. 35), the mak- 
 ing contractions do not appear until after the breaking contractions have 
 acquired a considerable height. After the making minimal contraction has 
 been obtained, the making contractions rapidly gain in height as the current 
 is strengthened, and finally acquire the same height as the maximal breaking 
 shocks. 
 
 The relation of the strength of the electric current to its irritating power 
 can be demonstrated equally well by using the direct galvanic current. The 
 strength of the galvanic current depends upon the character and number of the 
 cells employed, and the total resistance in the circuit. The strength of the 
 current can be easily varied by altering the resistance, and there are a number 
 of forms of apparatus for this purpose. 
 
 FIG. 20. Rheostat. 
 
 A convenient instrument is the rheostat (Fig. 20). This is a box containing coils of 
 wire of known resistance. These coils are connected with a series of heavy brass blocks on top 
 of the box. The current enters the box by a binding-post attached to the first of the brass 
 blocks and passes thence from block to block, by going through the coils of wire connecting 
 them, until it reaches the binding-post at the other end of the series. The blocks can be 
 also connected by good conducting brass plugs, which can be pushed in between them, and 
 when this is done, as the current passes directly from block to block instead of going through 
 the resistance coils beneath, the resistance is reduced to a corresponding amount. 
 
 Another method of altering the strength of the current flowing through the 
 nerve is to employ some form of shunt to split the current so that only a part 
 of it shall pass by way of the nerve. A current takes the path of least resist- 
 
 1 Fick : Untersuchungen uber elektrische Nervenreizung, Braunschweig, 1864. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 41 
 
 ance, and if two paths are opened to it, more or less can be sent through one 
 of them by decreasing or increasing the resistance in the other. 
 
 A useful instrument for dividing the current is the rheocord. The schema given in 
 Figure 21 illustrates the way in 
 which it is used. The amount 
 
 of current passing to the nerve *>,. * " * 
 
 will vary with the relative re- (&) <^m e 3 
 
 sistance in , b, c, d, e, f, and ^ 
 
 in a, ft, <?, A, e, / The bridge 
 
 c, d can be slid along the fine 
 
 German -silver wires 6, i and 
 
 e, j, and thus the resistance 
 
 a, &, c, d, e. /, and the amount FIG. 21. Rheocord. 
 
 of current passing through the nerve, can be varied at pleasure. 
 
 With such an arrangement we should find that the irritating effect of the 
 current is largely dependent upon its strength. In the case of strong currents, 
 however, the results may be complicated by alterations in the irritability and 
 conductivity, which we will consider later. It is true also of other forms of 
 irritants, and of muscles as of nerves, that the effect of stimulation, up to a 
 certain limit, increases with the strength of the irritant. 
 
 (c) Effect of Density of the Current. Although the strength of the current 
 is an all-important factor in its excitatory action, the effectiveness of the cur- 
 rent as an irritant depends very largely on the density of the stream. When 
 the current enters into a conductor, it spreads widely through the conducting 
 substance, and though the larger part of it takes the path of least resistance, 
 which is usually the shortest path to the point of exit, many of the threads 
 of current make a comparatively wide circuit to reach the outlet. If the con- 
 ductor is equally good at all points, but is irregularly shaped, the density of 
 the stream will be greatest where the diameter of the conductor is least. Thus 
 it happens that if a current be made to flow from end to end of a muscle, like 
 the sartorius of the frog, which is smaller at the knee end than at the pelvic 
 end, the density of the current will be greater at the lower than at the upper 
 end, and the irritating power of the current will be greater at the lower end. 1 
 
 This question of the effect of the density of the current is important, as it 
 helps to explain the peculiar reactions to the electric stream obtained when a 
 current is applied under normal conditions through the skin to the human 
 nerve (see p. 51). 
 
 Spread of Electric Current. The tendency of electric currents to spread 
 widely through moist conductors is a common source of error in electrical 
 excitation, and should be always guarded against. For example, if it is 
 necessary to excite a nerve at the bottom of a deep wound, shielded elec- 
 trodes should be used i. e., electrodes in which the metal terminals are insu- 
 lated by vulcanite, except at the part which the nerve is to touch. More- 
 over, care should be taken that there is no fluid communication between the 
 electrodes and the surrounding tissues. If these precautions are not observed, 
 1 Biedermann : Elektrophysiologie, 1895, Bd. i. S. 185. 
 
42 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 the current may excite other parts than those which it is intended to excite 
 and false conclusions may be reached. 
 
 In case currents of high potential are employed, another source of error 
 may arise through electrostatic charging of distant parts. 
 
 Spread of Electrostatic Charges. If the primary coil of an induction 
 apparatus be connected with a battery by the closure of a key in the primary 
 circuit, the sudden flow of current through the coil is accompanied by a 
 transient change in the stress of the magnetic field about the coil. This change 
 in the magnetic field induces an alteration in the electrical condition of the 
 wire of the secondary coil of the apparatus, and the terminals of this coil 
 undergo a rapid change of electrical potential, the one becoming positive, the 
 other negative. If two electrodes be connected with the binding posts of the 
 secondary coil, they become the terminals of the coil and are given, one a 
 positive, the other a negative charge. The same thing happens when the key 
 in the primary circuit is opened. In both cases the change of potential is only 
 momentary in its duration. The effect of opening the primary circuit is con- 
 siderably stronger than that of closing the circuit, for reasons stated on page 33. 
 
 If the two electrodes are connected by a conducting material, an electric 
 current will flow from one to the other at the instant the change of potential 
 takes place. If the electrodes be connected by the nerve of a nerve-muscle 
 preparation, an electrical current will flow through the nerve ; the nerve will 
 be excited, a nerve-impulse will be developed and be transmitted along the 
 nerve to the muscle and cause it to contract. It not infrequently happens, 
 if the current entering the primary coil is strong and a large electromotive 
 force is developed in the secondary coil, that the exciting effect of the sudden 
 electrical change is not confined to the part of the nerve directly connecting 
 the electrodes, but spreads to distant parts of the nerve, and even to the 
 muscle. This is shown by the fact that the muscle will contract even after 
 a moist ligature, tied tightly about the nerve, has broken the continuity of 
 its protoplasm and so prevented the nerve impulse from reaching the muscle. 
 In such a case the contraction of the muscle is due to an irritation of the 
 nerve beyond the point to which the ligature was applied or to the direct 
 excitation of the muscle itself. 1 
 
 If it is found that the muscle will contract after the nerve has been 
 crushed by the ligature, it will also be found that it will contract in case one 
 electrode be removed from the nerve, so that it remains connected with only 
 one pole of the induction apparatus. To understand this, we must look upon 
 the muscle as the terminal of the pole of the secondary coil with which it is 
 in connection. When the potential of the poles of the secondary coils is 
 suddenly changed, the change of potential spreads through all conducting 
 bodies connected with these poles, and in the case in question it passes, by 
 way of the wire, electrode, and nerve, to the muscle. In short, the muscle, 
 like any conductor, is charged up, and in the process of charging there is a 
 flow of current which excites the nerve and muscle. 
 
 1 Du Bois-Keymond : Untersuchungen ilber thierische Electricitat, Bd. i. S. 423. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 43 
 
 A much stronger contraction is obtained if the muscle be connected with 
 a large conductor, such as the human body, a large surface of tin-foil, a con- 
 denser, or the earth, for in the process of charging and discharging these 
 bodies there is a large flow of current through the preparation. Further 
 uniting the free pole of the secondary coil with the earth, because increasing 
 the difference in the potential of the two poles, increases the effect. In case 
 the free pole of the secondary coil be united to a large insulated conductor, 
 and this be brought near the nerve-muscle preparation without touching it, 
 the amount of excitation will be increased through what is known as " influ- 
 ence" action. For example, if the observer touch the free pole of the 
 secondary coil with one hand and approaches the other to the nerve prepara- 
 tion a larger contraction will be seen when the primary current is made or 
 broken. The effect produced on the preparation by the presence of a con- 
 ductor, which is suddenly given an electrostatic charge of opposite sign, as in 
 the case just mentioned, cannot be discussed here ; suffice it to say, it is 
 analogous to the influence exerted by the primary coil of an induction appa- 
 ratus on the secondary coil at the time that the battery current is made and 
 broken. 1 
 
 In all cases which we have cited the excitation of the nerve and muscle 
 was caused not by the change of electric potential, but by the sudden flow of 
 current accompanying the change. The exciting effect of the current depends 
 not only on the quantity of current, but also on the density of the stream. In 
 the unipolar experiments thus far described the nerve-muscle was brought 
 into connection with the secondary coil only at the point where the nerve 
 touched the electrode ; the electrical charge had to pass the length of the 
 nerve to reach the muscle, and all the charging current had to flow in a 
 dense stream the length of the nerve. This can be obviated by greatly 
 enlarging the electrode and letting it come in contact with a large part of 
 the nerve and muscle e. g. y by using for an electrode a piece of thin tin-foil, 
 or better gold-foil, and applying this to a large part of the surface of the 
 nerve and muscle (see Fig. 22). By this arrangement the change in electric 
 potential will be transmitted practically instantaneously throughout the good 
 conducting foil, and the nerve and muscle will be charged from a vast 
 number of points of contact and will at no part be subjected to a large quan- 
 tity of electricity flowing in a dense stream. The whole of the nerve-muscle 
 preparation will be charged, as before, to the potential of the pole with which 
 it is connected, but it will not be stimulated. That the nerve-muscle receives 
 an electrostatic charge under the above conditions can be readily observed by 
 approaching the finger to the muscle at the time that the primary circuit is 
 closed or opened. If the body of the observer has a large capacity, a large 
 amount of current will flow through the nerve and muscle to the finger. This 
 current will pass in a dense stream from the muscle at the point of contact 
 with the finger, and the muscle-fibres at this part, because subjected suddenly 
 
 1 Hermann : Handbuch der Physiologic, Bd. ii. Thl. 1, S. 87 ; Biedermann : Electro-physiology 
 (translated by F. A. Welby), 1897, vol. ii. p. 219. 
 
44 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 to a dense flow of current, will be excited and the muscle will contract. 
 Unless the primary current is very strong the electromotive force developed in 
 the secondary coil on the closing of the primary circuit may be too weak to 
 cause contraction, and only the effect of opening the circuit may be observed ; 
 in any case, the effect of breaking the primary circuit will be the stronger. 
 More striking results will be obtained if the primary current be rapidly 
 made and broken by an automatic interrupter introduced into the primary 
 circuit; the muscle will then be excited by a series of rapidly following 
 shocks. 
 
 FIG. 22. Unipolar, localized excitation of nerve. By this arrangement a large part of the surface 
 of the nerve and muscle is brought into immediate connection with the secondary coil through the sheet 
 of gold-foil. The nerve is locally excited at the point that is touched by the needle, because the current 
 going to charge the tin-foil conductor passes out of the nerve at this point as a dense stream. The 
 muscle (a) is supported by an insulating clamp of lead-glass and vulcanite (6), and is connected to the 
 writing lever by a dry lead-glass hook (c) ; the nerve (d) lies on a sheet of gold-foil (), which is also 
 wrapped about the muscle, and which rests on a block of vulcanite (/) supported by a glass rod (g) ; the 
 gold-foil is in close contact with the binding-post (h), and this is connected with one terminal of the 
 secondary coil (i) of an induction apparatus, the other terminal being connected with a gas pipe (j), and 
 so with the earth ; in the primary induction circuit there are a battery (k) and a key (I) ; the needle (ra) is 
 connected with a large conductor (n), which is composed of a board covered with tin-foil, and is sus- 
 pended from glass hooks. 
 
 For the sake of simplicity we have thus far only spoken of the charging 
 of the preparation from the secondary coil. It must be borne in mind, how- 
 ever, that the change in the electrical condition of the secondary coil lasts 
 only an instant, and the terminals of the coil and the tissues connected with 
 them immediately return to their original potential, this change being accom- 
 panied by a backward surge of the electrical wave from the muscle through 
 the nerve, electrodes, and wire to the coil, and this reverse current acts like 
 the charging current to cause excitation. The charging and discharging 
 processes follow each other with such rapidity, however, that they act upon 
 the tissues as a single excitation. 
 
 To Prevent the Spread of Current. As we have seen when the nerve of a 
 nerve-muscle preparation is connected by two electrodes with the poles of 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 45 
 
 the secondary coil of an induction apparatus, if a large electromotive force is 
 developed in the secondary coil, a current not only passes through the part 
 of the nerve bridging the electrodes, but through the part of the nerve 
 between the electrodes and the muscle. This spread of current may be in 
 part prevented by connecting the electrode nearest to the muscle with a gas- 
 pipe and leading the charge through this to the earth (Hermann). Another 
 method which has been suggested is to connect the two sides of the nerve 
 beyond the electrodes by a loop of good conducting metal, so that the spread- 
 ing currents shall be short-circuited (Hering). 
 
 Application of the Unipolar-excitation Method to the Localization of Excita- 
 tion. The principle that a flow of current will excite at the point where the 
 current is dense can be employed to obtain definitely localized excitations by 
 the unipolar method of irritation. For example, Kiihne employs the follow- 
 ing arrangement to show isolated contraction of muscle-fibres by localized 
 excitation. A thin parallel-fibred sartorius muscle of a frog is curarized to 
 shut out the effect of excitation of the nerve so that only the muscle-fibres 
 which are directly excited will contract. The preparation is then placed 
 on an insulated copper plate, which is connected with one pole of the sec- 
 ondary coil of an induction apparatus,' the other pole being connected with 
 the earth (see Fig. 23). The muscle makes no contractions when the key in 
 
 FIG. 23 Unipolar localized excitation of the sartorius muscle. The muscle (a) rests on a sheet of 
 copper (6), which is on a plate (c), resting on a sheet of vulcanite (c/) ; the binding-post (e) on the copper 
 plate is connected with one terminal of the secondary coil of the apparatus (/), the other terminal being 
 connected with a gas pipe (g), and so with the earth ; the primary coil (h) is connected with a battery (i) 
 and a key (j); the muscle is locally excited by the current, which passes in a dense stream through it 
 to the needle (k) held in the hand. 
 
 the primary circuit is closed or opened. Its potential is undoubtedly changed, 
 but its capacity is small, and it is charged from many points ; the charging 
 current is at no place sufficient in quantity or density to excite. If now the 
 experimenter touch the top of the muscle near one side with the point of a 
 needle held in the hand, the muscle twitches on the side touched each time 
 the current is opened, and if the current is strong each time it is closed. 
 The contraction is limited to the fibres just below the needle, because this is 
 the point where the current charging the body of the observer passes through 
 the muscle in a dense stream. The effect is more striking if, instead of 
 using single shocks, the primary current be frequently interrupted. The 
 fibres on the side stimulated will then be continuously contracted and the 
 muscle will curl toward the stimulated side. 
 
46 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 In a like manner if a nerve-muscle preparation be isolated, as shown in 
 Fig. 22, and a needle, held in the hand or connected with a large metallic 
 conductor or a condenser, be brought in contact with some point of the nerve, 
 the excitation which occurs on the opening and, with a strong current, on the 
 closing of the primary circuit will be strictly limited to the part of the nerve 
 touched by the needle. This method can be used to advantage in studying 
 the rate of conduction in nerves or any problem which requires strict local- 
 ization of electric excitation. 
 
 (d) Effect of the Duration of the Electric Current on its Power to Irritate 
 Nerves and Muscles. As we have seen, a constant battery current, when flow- 
 ing uninterruptedly through a motor nerve, does not ordinarily excite it; very 
 slow variations in the strength of the current also fail to irritate ; but rapid 
 alterations in the strength, whether in the direction of increase or decrease, act 
 as vigorous stimuli. For example, medullated nerves are irritated more vigor- 
 ously by the rapid changes of intensity of induced currents than by the some- 
 what slower changes occurring at the make and break of battery currents. 
 Within certain limits, at least, the more rapidly the intensity of the current 
 changes, the greater the irritating effect upon nerves. That there is a limit 
 even for the rapidly reacting protoplasm of medullated nerves is shown by 
 the fact that by unipolar excitation the charging and discharging of the con- 
 densers through a nerve is the more effective the greater the capacity of the 
 condensers. The process is more prolonged if the condenser is large, and 
 the effect is greater. 1 Not all nerves are equally susceptible to rapid altera- 
 tions of the intensity of the current. Non-medullated nerves do not appear 
 to react as readily as medullated to electric currents of short duration. For 
 instance, the nerves of the claw muscles of the crab are not readily excited 
 by induced currents, and respond better to the more prolonged influence of 
 the closing and opening of battery currents. 2 
 
 The question now arises, Is the reaction of muscle to electric currents the 
 same as that of nerves ? Experiment shows that muscles which have been 
 removed from the action of nerves, by means of curare, differ from medullated 
 nerves in that they are excited more vigorously by the opening and closing of 
 battery currents ; less vigorously by making and breaking induction currents. 
 This latter fact is well seen in experiments in which two gastrocnemius 
 muscles from the same frog, one of which has been curarized and the other 
 not, are connected with an induction apparatus in series, so that the cur- 
 rent shall flow through them both in the same direction. If the primary 
 current be made and broken, the non-curarized muscle will respond to a 
 weaker induction shock than the curarized. By the curarized muscles the max- 
 imal contraction got on opening and closing a battery current is both higher 
 and more prolonged than that to be obtained with a single induction shock. 
 Unstriated muscles exhibit this difference to a still greater degree than 
 striated muscle ; they react well to the closing of battery currents of medium 
 
 1 Hermann : Handbuch der Physiologic, Bd. ii. Theil 1, S. 88. 
 
 2 Biedermann : Elektrophysiologie, 1895, Bd. ii. S. 546. 
 
GENERAL PMY8IOLOQY OF Ml'SClJ-: AND NERVE. 47 
 
 strength, provided these last some little time, but respond to induced currents 
 only when they are very strong. Thus the unstriated muscle which closes the 
 shell of some of the fresh-water mussels, as the Auodonta, gives larger and 
 larger contractions as the duration of the current is increased from one-quarter 
 of a second to three seconds. Much the same is true of the uustriated muscles 
 of the ureters; 1 the battery current must remain closed quite a while for the 
 closing contraction to be called out, the length of time depending upon the 
 strength of the current ; and induction shocks have little or no effect unless 
 very strong. Such a comparison makes it evident that the duration of the 
 current is an important element in the influence exerted by electric currents 
 on various forms of protoplasm. Unstriated muscles require that the current 
 shall last from one-quarter of a second to three seconds to produce maximum 
 contractions. Striated muscles require that a current shall last 0.001 second 
 (Fick), and even medullated nerves fail to react if the current lasts too short 
 a time. Various forms of irritable tissue can be arranged in series according 
 to their ability to respond to electric currents of short duration, viz. medul- 
 lated nerves, non-medullated nerves, striated muscles, non-striated muscles, 
 and the little-differentiated forms of protoplasm of many of the protozoa. 
 On the other hand these tissues are found to respond in the reverse order to 
 currents which are more prolonged and which change their intensity slowly. 
 It would seem as if the less perfectly differentiated the form of protoplasm, 
 the less its mobility and its susceptibity to passing influences. 
 
 The same form of tissue reacts differently in different animals. For instance, 
 the sluggish striated muscles of the turtle do not respond as well to induced 
 currents as the more rapid striated muscles of the frog. Further, the condition 
 of the tissue at the time is found to have an influence on its irritability and its 
 power to respond to stimuli of short duration. Von Kries reports that nerves, 
 if cooled, react better to slow variations in the intensity of the electric current, 
 and, if warmed, to rapid variations. Under pathological conditions the reac- 
 tion of nerve and muscle to electric currents may become blunted, and, as the 
 tissue degenerates, its power to respond to rapid changes of the electric current 
 is lessened. If a nerve be cut, the part which is separated from the influence 
 of the nerve-cells degenerates. The irritability at first increases and then 
 very rapidly decreases, in from three to four days being wholly lost. As the 
 nerve regenerates, the irritability is recovered very gradually, and the power 
 to respond to the relatively prolonged action of mechanical stimuli is regained 
 sooner than the ability to reply to changes as rapid as those of induced cur- 
 rents. Howell and Huber observed that regenerating nerve-fibres when they 
 have reached the stage resembling embryonic fibres, i. e. are strands of proto- 
 plasm without axis-cylinders, fail to respond to induction currents, though they 
 can be excited by mechanical stimuli. It was found that it is not until the 
 axis-cylinder has grown down into the regenerating fibres that the nerve is 
 capable of responding to induction shocks. 
 
 1 Engelmann : PflilgeSs Archiv, 1870, Bd. iii. S. 263. 
 
48 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 When human striated muscle undergoes degeneration as a result of an in- 
 jury to its nerve, the degenerating muscle comes to resemble normal unstriated 
 muscle in its reactions to electricity, responding feebly to induced currents, at 
 a time when irritability to mechanical stimuli and to direct battery currents 
 is even increased. This is used by clinicians as a means of diagnosis of the 
 condition of the nerve and muscle. 
 
 From what has been said it is evident that the rule laid down by Du Bois- 
 Reymond (see p. 32) must be modified in so far that there is for each tissue 
 a limit to the rate at which a change of intensity of the electric current acts 
 as an irritant. 
 
 (e) Effect of the Angle at which the Current Enters and Leaves the Muscle 
 and Nerve. The angle at which the current acts on the muscle-fibre has 
 been found to have a bearing upon its power to stimulate. Leicher * succeeded 
 in obtaining definite experimental evidence that when the current is so sent 
 through a muscle as to cross it at right angles to its fibres it has no irritating 
 effect, and that its power to stimulate increases as the angle at which the 
 threads of current strike the muscle-fibres decreases, being greatest when the 
 current passes longitudinally through the fibres. 
 
 Similarly, it was found by Albrecht and Meyer 2 that the irritating effect 
 of the electric current is most active when it flows longitudinally through the 
 nerve, and that it is altogether absent when it flows transversely through it. 
 This view is doubted by some observers, who would attribute the difference 
 observed to differences in the electrical resistance. It is true that the resist- 
 ance to cross transmission is greater than to longitudinal transmission, but it 
 is not likely that this difference suffices to explain the lack of response to cur- 
 rents applied at right angles to the nerve-axis. 
 
 Relative Efficacy of the above Conditions upon the Irritating Power of the 
 Electric Current. When a current is applied to an irritable part of a nerve 
 or muscle at an angle suitable to excitation, the stimulating effect of the current 
 depends upon the rate at which its intensity is changed, the strength and 
 density of the current, i. e. its intensity, and the duration of the current. 
 
 Fick 3 gives the following schema (Fig. 24) for the different ways in which 
 the intensity of the electric current may be varied, and compares the effects 
 of these different methods of application of the current. It must be re- 
 membered that a decrease of intensity acts no less than an increase to produce 
 excitation. In the above schema the abscissa represents the time, and the 
 ordinates the strength, of the current. Suppose the rise of intensity has a 
 form such as is represented in a, Figure 24 that is, that the strength of the 
 current increases to a considerable height, but very slowly. Such a rate of 
 change, even though the rise of intensity were continued until the strength of 
 current was very great, would have no exciting effect upon a nerve and might 
 
 1 Untersuchungen aus dem physiologischen Institut der Universitdt Halle, Heft i. S. 5. 
 
 2 Pfluger's Archiv, 1880, Bd. xxi. S. 462. 
 
 3 Beitrdge zur vergleichende Physiologic der irritablen Substanzen, Braunschweig, 1863. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 49 
 
 fail to irritate a striated or non-striated muscle. A more rapid rise, such as 
 is shown in 6, might irritate a non-striated muscle, but fail to irritate a nerve 
 or a striated muscle. With currents which rapidly gain their full intensity 
 and then return again to zero, the following cases would be possible: A 
 rapid rise and fall of intensity (see c), such as occurs by an induction shock 
 or by the momentary closure of a battery current, might suffice to excite 
 a nerve but not be an effective irritant to a striated, much less a non-striated 
 muscle, unless the short duration of the current were compensated for by 
 a considerable increase in the intensity (see d). On the other hand a^form 
 of variation such as is shown in e, where the rate of change is very rapid, 
 although the intensity is not great, might act. to irritate nerves, and, because 
 of the longer duration of the current, striated muscles, though having no effect 
 on non-striated muscles ; and the slower rate of change, and considerable dura- 
 
 10 20 SO 40 
 
 100 110 
 
 20 
 10, 
 
 o. 
 
 i 
 c 
 
 A 
 
 I 
 
 / 
 
 r\ 
 
 
 FIG. 24.. Schema of relation of the method of application of the electric current to the 
 
 irritating effect. 
 
 tion, illustrated by/, though not affecting nerves, might suffice for striated 
 muscles and be favorable to the excitation of non-striated muscles. 
 
 In the case of nerves, duration of current is less important than a rapid 
 change of intensity. In the case of striated muscles the advantage to be 
 gained by rapid variations can be easily overstepped, and the importance of 
 the duration of the current is greater ; while in the case of non-striated muscles 
 duration of current is of the first importance and rapid variation may fail to 
 excite. In the case of all tissues, strength and density of current, what we 
 may call intensity of current, is favorable to excitation. 
 
 (/) Effect of the Direction in which the Current flows along the Nerve. 
 The result of the irritating change produced in a nerve by a battery current 
 has been found to depend upon whether the current flows toward or away 
 from the organ stimulated by the nerve. This fact can be most readily ob- 
 served in the case of isolated motor nerves. In the case of these nerves, the 
 effects produced by opening and closing the current are different according as 
 the current is descending, i. e. flows through the nerve in the direction of the 
 muscle, or ascending, i. e. flows through the nerve in the opposite direction. 
 
 VOL. II. 4 
 
50 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 Moreover, by a given rate of change of intensity, the stimulating effect varies 
 with the strength of the current employed. Pfliiger in his celebrated mono- 
 graph, Untersuchungen uber die Physiologic des Elektrotonus, published in 
 Berlin, 1859, p. 454, formulated the following rule for the result of excitations 
 under varying conditions : 
 
 Pfliiger's Law of Contraction. 
 
 Ascending Current. 
 Closing. Opening. 
 
 Weak current Contr. Kest. 
 
 Medium " Contr. ' Contr. 
 
 Strong " Best. Contr. 
 
 Descending Current. 
 Closing. Opening. 
 
 Contr. Eest. 
 
 Contr. Contr. 
 
 Contr. Eest. 
 
 To understand this so-called " law of contraction " we must bear in mind 
 certain fundamental facts, namely : 
 
 a. When a nerve is subjected to a battery current, an excitatory process is 
 developed in the part of the nerve near the kathode when the current is 
 closed, and in the part of the nerve near the anode when the current is opened 
 (see p. 38). 
 
 6. The excitatory process developed at the kathode is stronger than that 
 developed at the anode (see p. 38). 
 
 c. A third fact which is of no less importance, and which will be considered 
 in detail when we study the effects of the constant current on the irritability 
 and conductivity of nerve and muscle (see p. 95), is the following : During 
 the time that a strong constant current is flowing through a nerve, the conduct- 
 ing power is somewhat lessened in the part to which the kathode is applied, and 
 is greatly decreased, or altogether lost, in the region of the anode ; moreover, 
 at the instant that the current is withdrawn from the nerve the conducting 
 power is suddenly restored in the region of the anode, and greatly lessened, or 
 lost, in the region of the kathode. 
 
 Ascending Current. 
 K A 
 
 Descending Current. 
 A K 
 
 Weak current. 
 
 Medium current. 
 
 Strong current. 
 
 FIG. 25. Diagram illustrating Pfluger's law. 
 
 The twelve cases included in the above table can be represented in the fol- 
 lowing diagram (Fig. 25), in which a cross is marked at the part of the nerve 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. :>1 
 
 from which the irritation that is effective in producing a contraction takes 
 its rise. 
 
 In the case of fresh motor nerves of the frog, when the current is weak, 
 only closing excitations, /. e., those originating at the kathode, are effective by 
 both directions of the current. As the strength of the current is increased, at 
 the same time that the closing kathodic contractions grow stronger, opening 
 anodic contractions begin to appear ; and with currents of medium strength 
 both closing and opening contractions are obtained with both directions of the 
 current* If the strength of the current be still further increased, a change is 
 observed; with a strong current, the closing of the ascending and the opening 
 of the descending current fails to excite a 
 muscular contraction. This fact is demon- 
 strated most clearly if we employ two 
 nerve-muscle preparations, and lay the nerves 
 in opposite directions across the non-polar- 
 izable electrodes, so that the current from 
 the battery shall flow through one of the 
 nerves in an ascending direction and through 
 the other in the descending direction (see 
 Fig. 26). If under these conditions a strong 
 battery current be employed, muscle a (through 
 the nerve of which the current is descending) 
 will contract only when the circuit is closed, 
 and muscle b (through the nerve of which the current is ascending) will con- 
 tract only when the circuit is opened. 
 
 Since in the case of currents of medium strength, both opening and clos- 
 ing the circuit, when the current is ascending and when it is descending, 
 develops a condition of excitation in the nerve sufficient to cause contractions, 
 the failure of the contraction by the closing of the strong ascending current, 
 and by the opening of the strong descending current, can scarcely be supposed 
 to be due to a failure of the exciting process to be developed in the nerve ; and 
 it would seem more likely that the nerve-impulse is for some reason prevented 
 from reaching the muscle which, as has been said, is the fact, the region of the 
 anode being incapable of conducting during the flow of a strong current, and 
 the region of the kathode losing its power to conduct at the instant such 
 a current is opened. 
 
 Effect- of Battery Currents upon Normal Human Nerves. In experi- 
 ments upon normal human nerves, the current cannot be applied directly to the 
 nerve, but has to be applied to the skin over the nerve. As it passes from the 
 anode, the positive electrode, through the skin, the threads of current spread 
 through the fluids and tissues beneath, somewhat as the bristles of a brush 
 spread out, and the current flows in a more or less diffuse stream toward the 
 point of exit, where the threads of current concentrate again to enter the 
 kathode, the negative electrode. This spread of the current is illustrated in 
 Figure 27. 
 
 V \) 
 
 A *> K 
 
 FIG. 26. Effect of direction of current 
 as shown by simultaneous excitation of 
 two nerve-muscle preparations. 
 
52 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 FIG. 27. Rough schema of active threads of current by 
 the ordinary application of electrodes to the skin over a 
 nerve (ulnar nerve in the upper arm). The inactive threads 
 are given in dotted lines (after Erb : Ziemssen's Pathologic und 
 Therapie, Bd. iii. S. 76). 
 
 The density of the current entering any structure beneath the skin will 
 depend in part upon the size of the electrode directly over it that is, the 
 
 amount to which the current is 
 concentrated at its point of en- 
 trance or exit in part on the 
 nearness of the structure to the 
 skin, and in part on the con- 
 ductivity of the tissues of the 
 organ in question as compared 
 with the tissues and fluids 
 about it. If the conditions be 
 such as are given in Figure 27, 
 the current will not, as in the 
 case of the isolated nerve, enter 
 the nerve at a given point, flow 
 longitudinally through it, and 
 then leave it at a given point ; 
 most of the threads of current 
 will pass at varying angles di- 
 agonally through the part of 
 the nerve beneath the positive 
 pole, then flow through the fluids and tissues about the nerve, until, at a point 
 beneath the negative pole, the concentrating threads of current again pass 
 through the nerve. A distinction is to be drawn between the physical and 
 physiological anode and kathode. The physical anode is the extremity of the 
 positive electrode, and the physical kathode is the extremity of the negative 
 electrode ; the physiological anode is the point at which the current enters the 
 tissue under consideration, and the physiological kathode is the point where it 
 leaves it. There is a physiological anode at every point where the current 
 enters the nerve, and a physiological kathode at every point where it leaves the 
 nerve; therefore there is a physiological anode and kathode, or groups of 
 anodes and kathodes, for the part of the nerve beneath the positive electrode, 
 and another physiological anode and kathode, or collection of anodes and 
 kathodes, for the part of the nerve beneath the negative electrode. 
 
 To understand the eifect upon the normal human nerve of opening and 
 closing the battery current, it is necessary to bear in mind three facts, viz.: 
 
 1. At the moment that a battery current is closed, an irritating process is 
 developed at the physiological kathode, and when it is opened, at the physio- 
 logical anode. 
 
 2. The irritating process developed at the kathode on the closing of the 
 current is stronger than that developed at the anode on the opening of the 
 current. 
 
 3. The effect of the current is greatest where its density is greatest. 
 
 The amount of the irritation process developed in a motor nerve is esti- 
 mated from the amount of the contraction of the muscle. The contraction 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 53 
 
 which results from closing the current, the closing contraction as it is called, 
 represents the irritating change which occurs at the physiological kathode, while 
 the contraction which results from opening the current, the opening contrac- 
 tion, represents the irritating change developed at the physiological anode. 
 Since there are physiological anodes and kathodes under each of the two elec- 
 trodes the physical anode and physical kathode (see Fig. 28) four possible 
 
 may arise, namely: 
 1. Anodic dosing contraction i. e. the effect of the change developed at 
 
 FIG. 28. Diagram showing physical and physiological anodes and kathodes : A, the physical anode, 
 or positive electrode ; K, the physical kathode, or negative electrode ; a, a, a, physiological anodes ; k, k, k, 
 physiological kathodes. 
 
 the physiological kathode, the place where the current leaves the nerve, 
 beneath the physical anode (the positive pole). 
 
 2. Anodic opening contraction i. e., the effect of the change developed at 
 the physiological anode, where the current enters the nerve, beneath the 
 physical anode (the positive pole). 
 
 3. Kathodic closing contraction. i. e. the effect of the change developed at 
 the physiological kathode, where the current leaves the nerve, beneath the 
 physical kathode (the negative pole). 
 
 4. Kathodic opening contraction i. e., the effect of the change developed at 
 the physiological anode, where the current enters the nerve, beneath the 
 physical kathode (the negative pole). 
 
 For convenience these four cases are represented by the abbreviations ACC, 
 AOC, KCC, and KOC. 
 
 Since the irritation process developed at a physiological kathode by 
 closing a current, is, other things being equal, stronger than that developed 
 at a physiological anode by opening the current, we should expect that the 
 two closing contractions, KCC and ACC, would be stronger than the two 
 opening contractions, KOC and AOC. This is the case, and as the current is 
 more dense in the region of the physiological kathode, beneath the physical 
 kathode, than at the physiological kathode, beneath the physical anode, KCC 
 is stronger than ACC. 
 
 Of the two opening contractions, AOC is stronger than KOC because 
 of the greater density of the current in the region of the physiological anode, 
 
54 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 beneath the physical anode, than in the region of the physiological anode, 
 beneath the physical kathode. 
 
 These differences in the strength of the irritation process developed in these 
 different regions is well shown by examining the reaction of nerves to cur- 
 rents of gradually increasing strength. The effect of the opening and closing 
 irritation is seen to be as follows : 
 
 Weak currents. Medium currents. Strong currents. 
 
 KCC KCC KCC 
 
 ACC AGO 
 
 AOC AOC 
 
 KOC 
 
 The natural order, therefore, would be KCC, ACC, AOC, KOC. Some- 
 times, however, AOC is stronger than ACC ; this happens when on account 
 of the relation of the surrounding tissues to the nerve the density of the cur- 
 rent at the physiological anode is great as compared with the density at the 
 physiological kathode. Borclier l tested the strength of battery current neces- 
 sary to awaken minimal sensations by unipolar excitations, and found the 
 effect to be greatest by KC, then AC, then AO ; and that it was least by KO 
 i. e.j sensory behave like motor nerves. 
 
 In testing the effect of the battery current on the nerves and muscles of 
 man, it is customary to use one small and one large electrode (Fig. 6, d, e,f). 
 The small electrode is placed over the part to be stimulated, while the large 
 electrode is put over some distant portion of the body. This arrangement 
 causes the current to be condensed, and hence efficient, when it enters or 
 leaves the small exciting electrode, and to be diffused, and hence ineffective, 
 at the large indifferent electrode. For example, the indifferent electrode 
 may be placed on the sternum or over the back of the neck, while the excit- 
 ing electrode may be put over the ulnar nerve at the elbow. The two poles 
 may be connected with the battery, a pole-changer, rheostat, milliamperemeter, 
 and exciting-key being introduced in the circuit. The pole-changer permits 
 the exciting pole to be made A or K at the wish of the operator, the rheostat 
 allows the strength of current to be raised gradually, and the milliampere- 
 meter shows the strength of the current employed. With this arrangement 
 the reaction of the nerve can be readily tested. 
 
 When the currents employed are strong, it occasionally happens in the 
 case of men that not only are the make and break followed by the usual rapid 
 contractions of short duration, but during the closure of the current there is 
 a continued contraction galvanotonous, as it is sometimes called. This is 
 especially seen under certain pathological conditions. 
 
 When the nerve or muscle is diseased we may have the above order 
 changed, and ACC obtained with weaker currents than KCC, and KOC than 
 AOC (Babinski) 2 . This is known as the reaction of degeneration. Under 
 
 1 Bordier : Archives de Physiologic normals et Pathologique, 1897, pp 543-553. 
 
 2 Babinski : Comptes rendus de la Sociele de Bioloyie, 1899, p. 343. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 55 
 
 such circumstances the nerve might respond well to the direct battery current 
 and yet fail to respond to the induced current. This would be still more 
 markedly the case with the muscle, which at the same time that it gave no 
 response to induction shocks would react better than normally to battery 
 currents. At such times galvanotonus is easily excited. Thus during 
 degeneration the irritability of the nerve and muscle approaches that of 
 slowly reacting forms of protoplasm (see p. 70). 
 
 Conditions which Determine the Irritability of Nerves and Muscles. 
 We have thus far considered the conditions which determine the efficiency 
 of such an irritant as the electric current. Other irritants are subject to like 
 conditions, their activity being controlled to a considerable extent by the sud- 
 denness, strength, density, duration, and, possibly, direction of application. It 
 is not necessary for us to consider how each special form of irritant is affected 
 by these conditions ; it will be more instructive for us to study how different 
 irritants alter the irritability of nerve and muscle, and the relation of irri- 
 tability to the state of excitation. 
 
 The power to irritate is intimately connected with the power to heighten 
 irritability for a condition of heightened irritability is difficult to distin- 
 guish from a state of excitation. The irritability of cell-protoplasm is very 
 dependent upon its physical and chemical constitution, and even slight altera- 
 tions of this constitution, such as may be induced by various irritants, 
 will modify the finely adjusted molecular structure upon which the normal 
 response to irritants depends. If this change be in the direction of increased 
 irritability, the result may be irritation. But we must defer the discussion of 
 the relation of irritability to irritation until we have considered the conditions 
 upon which the irritability of nerve and muscle depends. These conditions 
 can be best studied in connection with the influences which modify them 
 namely : 
 
 (a) Irritants. 
 
 (6) Influences which favor the maintenance of the normal physiological 
 condition. 
 
 (c) The effects of functional activity. 
 
 (a) The Influence of Irritants upon the Irritability of Nerve and Muscle. 
 Effect of Mechanical Agencies. A sudden blow, pinch, twitch, or cut excites 
 a nerve or muscle. All have experienced the effect of a mechanical stimulation 
 of a sensory nerve, through accidental blows on the ulnar nerve where it passes 
 over the elbow, " the crazy bone." The amount of mechanical energy required 
 to cause a maximal excitation of an exposed motor nerve of a frog is estimated 
 by Tigerstedt 1 to be 7000 to 8000 milligrammillimeters, which would corre- 
 spond roughly to a weight of 0.500 gram falling fifteen millimeters at least 
 a hundred times less energy than that given out by the muscles in response to 
 the nerve-impulse developed. Such stimuli can be repeated a great many 
 times, if not given at too short intervals, without interfering with the activity 
 
 1 Studien iiber mechanische Nervenreizung," Acta Societatis Scientiarum Fennictz, 1880, 
 Bd. xi. S. 32. 
 
56 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 of the nerve. A nerve can be irritated thirty to forty times, at intervals of 
 three to four minutes, by blows from a weight of 0.485 gram, falling 1 to 20 
 millimeters, the contractions of the muscle, weighted with 30 to 50 grams, 
 varying from minimal to from 3 to 4 millimeters in height. Rapidly following 
 light blows or twitches applied to a motor nerve, by the tetanomotor of Heiden- 
 hain or Tigerstedt, excite a series of contractions in the corresponding muscles 
 which fuse more or less into a form of continuous contraction, known as 
 tetanus. 
 
 Not only may a nerve be excited by bringing sudden pressure to bear on it, 
 but the sudden removal of weights or a sudden lessening of tension irritates. 1 
 Kiihne long ago called attention to the excitation of sensory fibres of the 
 ulnar nerve of man on the removal of pressure. The cause is probably the 
 irregular return of the semi-fluid parts of the nerve to their normal relations. 
 
 Mechanical applications to nerve and muscle first increase and later lessen 
 and destroy the irritability. Thus pressure gradually applied first increases 
 and later reduces the power to respond to irritants. Stretching a nerve acts in 
 a similar way, for this also is a form of pressure ; as Valentin said, the stretch- 
 ing causes the outer sheath of the nerve to compress the myelin, and this in 
 turn to compress the axis-cylinder. Tigerstedt states : 2 " From a tension of 
 up to 20 grams the irritability of the nerve is continually increased, but 
 it lessens as soon as the weight is further increased." 
 
 Surgically the stretching of nerves is sometimes employed to destroy their 
 excitability. Slight stretching heightens the excitability and even quite vigor- 
 ous stretching has only a temporary depressing effect unless it be carried to 
 the point of doing positive injury to the axis-cylinder, and of causing degen- 
 eration. As nerves have the power to regenerate, they may recover from even 
 such an injury. 
 
 The irritability of muscles is likewise increased by moderate stretching and 
 destroyed if it be excessive. Thus slight stretching produced by a weight 
 causes a muscle to respond more vigorously to irritants. Similarly tension of 
 the muscles of the leg, produced by slight over-flexion or extension, makes 
 them more irritable to reflex stimuli, as in the case of the knee-jerk and ankle- 
 clonus. Tension must be very marked to permanently alter the irritability of 
 the muscles. 
 
 Effect of Temperature. Changes in temperature, if sudden and extreme, 
 irritate nferves and muscles. If the nerve or muscle be quickly frozen or 
 plunged into a hot fluid it will be excited and the muscle be seen to contract 
 The cause of the irritation has been attributed to mechanical or chemical 
 alterations produced by the change of temperature. The ulnar nerve at the 
 elbow is excited if the part be dipped into ice-water and allowed to remain 
 there until the cold has had time to penetrate ; as is proved by the fact that in 
 addition to the sensations from the skin, pain is felt which is attributed by the 
 subject of the experiment to the region supplied by the nerve. As the effect 
 
 1 v. Uxhull: Zeitschrift fur Biologic, 1894, Bd. xxxi. S. 148 ; 1895, Bd. xxxii. S. 438. 
 
 2 Op. tit., S. 43. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 57 
 
 of the cold becomes greater the pain is replaced by numbness, both the irrita- 
 bility and power of conduction of the nerve being reduced. Gradual cooling 
 of motor nerves or muscles, and gradual heating, even to the point of death 
 of the tissue, fails to excite contractions. It is stated that if a frog whose 
 brain has been destroyed is placed in a bath the temperature of which is very 
 gradually increased, the heating may be carried so far as to boil the frog without 
 active movements having been called out. If a muscle be heated to 45 C. 
 for frogs and 50 C. for mammals, it undergoes a chemical change, which is 
 accompanied by a form of shortening different from the contraction induced by 
 irritants. This form of contraction, though extensive, is feeble and is asso- 
 ciated with a stiffening of the muscle, known as rigor caloris (see p. 164). 
 
 In general it may be said that raising the temperature above the usual tem- 
 perature of the animal increases, while cooling decreases, the irritability of the 
 nerves and muscles. This statement requires to be amplified, because the 
 character of the stimulus has a marked effect upon the result. Cooling 
 the nerve increases its irritability for mechanical and chemical stimuli, for 
 the constant current if it lasts at least 0.005 sec., for condenser discharges, 
 and for sine currents of at least 0.005-0.01 sec. duration : heating the nerve 
 increases its irritability for these forms of electrical excitation when of 
 shorter duration, and also for induced currents. 1 If a nerve be excited by 
 charging or discharging a condenser through it, the size of the condenser 
 plays an important part, because it determines the duration of the stimulus ; 
 for example a slow, prolonged rate of discharge may excite a nerve at 4 
 C. and fail to excite one at 30 C., while a rapid, brief fall of energy will 
 excite a nerve at 30 C. and fail to excite one at 4 C. 2 Not only does 
 temperature influence the ability of the nerve to take on the change which is 
 associated with the development of what we call the nerve impulse, but it 
 alters its power of recovery. This appears in experiments in which the 
 ability of the nerve to respond to two rapidly following stimuli is tested by 
 different temperatures. A nerve, like the heart-muscle, shows a "refrac- 
 tory period " for a short interval after excitation, and during this period it is 
 incapable of responding to stimulation. The length of the interval varies 
 with the temperature. If the two stimuli are separated by an interval of 
 0.001 sec., the second stimulus will be effective at 15 C., but it will fail at 
 3 C. ; at this temperature, even with an interval of 0.006 sec., the second 
 stimulus will be without effect, as much as 0.01-0.02 sec. being needed for the 
 recovery of nerve at this low temperature. 3 
 
 Cold, unless excessive and long continued, though it temporarily suspends, 
 does not destroy the irritability ; while heat, if at all great, so alters the 
 chemical constitution of the cell-protoplasm as to destroy its life. 
 
 The higher the temperature the more rapid the chemical changes of the 
 body and the less its power of resistance ; low temperature, on the other hand, 
 
 1 Gotch and Macdonald : Journal of Physiology, 1896, xx. p. 247. 
 
 a Waller: Ibid., 1899, xxiv. p. 1. 
 
 3 Gotch and Burch: Ibid., 1899, xxiii. p. 22 ; Boycott: Ibiff., 1899, xxiv. p. 144. 
 
58 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 slows chemical processes and increases the endurance. It is noticeable that 
 nerves and muscles remain irritable much longer than ordinarily in case the 
 body be cooled before their removal. In the case of a mammal, the irritability 
 may last from six to eight hours instead of two and a half, while in the case 
 of frogs it may be preserved at for ten days, although at summer heat it lasts 
 only twenty-four hours. In the case of frogs which have been kept at a low 
 temperature the irritability becomes abnormally high when they are warmed 
 to ordinary room-temperature. 
 
 Effect of Chemicals and Drugs. The irritability of nerve and muscle proto- 
 plasm is markedly influenced by even slight changes in its constitution. If 
 a nerve or muscle be allowed to lie in a liquid of a different composition from 
 its own fluid, and especially if such a liquid be injected into its blood-vessels, 
 an interchange of materials takes place which results in an alteration of the 
 constitution of the tissue and a change in its irritability. Indeed, the only 
 solutions which fail to alter the irritability are those which closely resemble 
 serum and lymph. Fluids having other than the normal percentage of salts 
 have a marked effect, while even the absence of proteids appears to have little 
 influence unless continued for a considerable time. 
 
 Pure water acts as a poison to protoplasm, soon destroying its life. 
 Through diffusion and osmosis it is imbibed into the cells at the same time 
 that the salts pass out, and the resulting change in the physical and chemical 
 condition of the tissue cause if rapid, first an increase, and in any case 
 later a decrease, and finally a total loss of irritability. Thus water injected 
 into the blood-vessels of muscles first excites contraction and later destroys 
 the irritability, and results in the condition known as water rigor. These 
 effects are prevented by the presence of small amounts of salt. A sodium 
 chloride solution, of a strength of 6 parts per 1000 of distilled water, has 
 been called the physiological solution, because it was supposed to have no 
 effect on the irritability of nerves and muscles of cold-blooded animals ; even 
 this solution, if long continued, gradually increases and later decreases the 
 irritability. A solution containing 7 parts of sodium chloride per 1000 is 
 more nearly isotonic to the fluids of cells of the frog, and one containing 9 
 parts per 1000 is approximately in osmotic equilibrium with the fluids of the 
 cells of the mammal. Such fluids cannot be properly regarded as physio- 
 logical solutions, however, for this would mean that they would cause no 
 change in constitution of the cells. They contain only one of the salts essen- 
 tial to the normal activity of the tissues, and the difference in the partial 
 pressure of the other salts of the muscle would cause the muscle cells to lose 
 some of each of these, and, as a result, to have their irritability altered. The 
 importance of the individual salts present in the fluids normally surrounding 
 the tissues, and the need that they should be present in definite proportions, 
 were most strikingly demonstrated by experiments, by Ringer and others, on 
 the nature of the fluid which is essential to the maintenance of the activity 
 of the isolated heart of the frog. These experiments have shown that not 
 only Na, but Ca and K are essential. The heart of the terrapin can be kept 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 59 
 
 beating for more than forty-eight hours in a solution containing NaCl, CaCl, 
 and KC1, even when there are no energy-giving substances present in the 
 fluid. Howell l says that NaCl is needed in the proportion in which it occurs 
 in the blood, to preserve the osmotic relations of the tissues, while Ca and K 
 are essential to the development of the rhythmic movements of the heart- 
 muscle. Loeb 2 made a careful study of the relation of salts of the blood to 
 the activity of striated and non-striated muscles. He found, as others had 
 done before, that a striated muscle if left in 0.7 per cent. NaCl solution in 
 time develops more or less rhythmic automatic contractions. He considers 
 that Na, Ca, and K are held in the muscle, not only as salts, but in combi- 
 nation with the proteids, and that all of these are necessary to the normal 
 functional activity of the protoplasm. A fluid to deserve the name of physio- 
 logical must contain all these ions. The muscle contracts rhythmically in a 
 solution of pure NaCl because the Na drives some of the Ca and K out of 
 their ion-proteid combinations. If Ca and K are present in the solution, 
 this cannot occur. He goes so far as to say that were it not for the Ca and 
 K in the blood, the human skeletal muscles would show rhythmic contraction. 
 
 A truly physiological solution would contain all the constituents of the 
 fluid of the blood, and a physiological salt solution would contain all the salts 
 of the blood, in the proportion in which they exist in the blood. The salts 
 would appear to have a twofold function : they would maintain the normal 
 imbibition relations of the cells, and they would supply the Na, Ca, and K 
 ions which are required for the ion-proteid compounds in the muscle. The 
 quantities of inorganic salts are different in different tissues of the same 
 animal, which shows that the presence of these inorganic substances is 
 dependent not merely on the amount presented to them by the fluids in 
 which they are bathed, but also on the chemical conditions within the cells, 
 each type of cell requiring a definite supply for its normal functional activity. 
 Howell reports a fact of interest in this connection : the muscle of the ven- 
 tricle of the heart of the terrapin does not make automatic rhythmic move- 
 ments in a Ringer's solution containing Na, Ca, and K in amounts equal to 
 those occurring in the blood, but the large veins at the base of the heart do 
 make such contractions and supply the excitation necessary to rhythmic con- 
 traction of the whole heart. 
 
 The presence of inorganic salts is essential to the normal functional 
 activity of nerves, as it is of muscles. If the nerve be subjected to distilled 
 water, it gradually loses its salts through osmosis, and imbibes water, and 
 the resulting chemical and physical change in its constitution is accompanied 
 by a loss of irritability. Likewise the withdrawal of water from a motor 
 nerve by drying, or by strong solutions of urea, glycerin, etc., causes a change 
 of irritability. The irritability is first increased, due to a concentration of 
 the salts within the nerve and to the mechanical excitation resulting from 
 the shrinkage of the tissue. If the change is a rapid one, it is frequently 
 accompanied by an active irritation, and the muscle connected with the nerve 
 
 1 American Journal of Physiology, 1898, ii. p. 47. z Ibid., 1900, iii. p. 383. 
 
60 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 shows irregular contractions, as the different fibres of the nerve are one after 
 the other affected. If the drying has not been continued too long, the normal 
 irritability may be restored by supplying water. Muscles behave like nerves 
 in these respects. 
 
 Most drugs and chemicals capable of altering the irritability of nerves and 
 muscles first increase and later destroy the irritability. If the change in the 
 chemical constitution of the nerve is sufficiently rapid, it may be accompanied 
 by the phenomena of excitation. For example, veratria, eserin, digitalis, 
 most mineral acids, and many organic acids, free alkalies, most salts of heavy 
 metals, destroy the irritability of nerves and muscles, as a rule after first pro- 
 ducing increased excitability. Potash salts, if concentrated, rapidly kill, but 
 excite less than soda compounds. Verworn says : Acids, alkalies, and salts 
 have a similar effect on the protoplasm of a thick pseudopod of one of the 
 rhizopods of the Red Sea ; they first excite and later paralyze, acting like 
 narcotics on the central nervous system. 
 
 Ammonia, carbon disulphide, and ethereal oils may destroy the irritability 
 of nerves without causing excitations, at least not in sufficient amount to 
 produce visible muscular contractions. If applied directly to the muscle, 
 however, these substances excite contractions. 
 
 The attempt to ascertain some exact relation between the molecular 
 weight of different salts and acids and their destructive power has encoun- 
 tered too many exceptions for the establishment of any definite rule ; in 
 general, however, the higher the molecular weight the stronger the effect on 
 the muscle. 1 Many gases and vapors have a marked effect on the irri- 
 tability and activity of protoplasm. 55 Carbonic-acid gas, tobacco-smoke, the 
 fumes of ether, alcohol, and chloroform, applied directly to exposed nerves, 
 first stimulate, later anaesthetize, and finally kill. CO 2 has a very powerful 
 effect, even a fiftieth of a milligram sufficing to influence profoundly the 
 activity of the protoplasm of the nerve, a fact of considerable importance if 
 we recall that this gas is produced by the normal oxidation of carbon within 
 the tissues of the body. Tobacco-smoke acts like CO 2 and probably because 
 of the CO 2 which it contains. Alcohol first excites and then paralyzes the 
 nerve. If the fumes of alcohol have not acted for too long a time, the para- 
 lyzed nerve may recover its function, and the same is true for ether and 
 chloroform. These vapors, if present in considerable quantities act rapidly 
 upon exposed nerves ; thus ether (diethyl oxide) will anaesthetize a nerve in 
 three minutes ; if the drug be then removed, the nerve can completely recover 
 in five minutes. Chloroform would appear to be a more dangerous anaes- 
 thetic than ether, as recovery of the nerve is less likely to occur in case the 
 anaesthetic action is somewhat prolonged. Many other gases and fumes 
 chemically irritate and kill nerve-muscle protoplasm. 
 
 From all these results it becomes evident that the normal irritability of 
 
 Bluraenthal : Pfluger's Archiv, 1896, Bd. 62, S. 513. 
 
 2 Waller: Lectures on Physiology, first series, " On Animal Electricity," London, 1897, pp. 
 42-46. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 61 
 
 nerves and muscles requires that a certain chemical constitution be maintained, 
 and that even slight variations from this suffice to alter, and, if continued, to 
 destroy, the irritability. Further, it is noticeable that in most cases the first 
 step toward deterioration is a rise of irritability, which, if sudden and marked, 
 is accompanied by a condition of irritation. If the cause of the increase in 
 irritability and excitation be continued, sooner or later exhaustion supervenes, 
 the irritability lessens, and finally is lost. 
 
 Effect of the Electric Current upon Muscles. If a constant-battery current 
 of medium strength be sent through a muscle for a short time, the muscle will 
 give a single short contraction at the moment that the current enters it, and 
 again when the current leaves it. If a strong current be used, the short 
 closing contraction may be followed by a prolonged contraction (Wundt's closing 
 tetanus), which, though gradually decreasing, may last as long as the current 
 is closed ; and when the current is broken, the usual opening contraction may 
 be likewise followed by a prolonged contraction (Hitter's opening tetanus), 
 which only gradually passes off. The closing contraction originates at, and 
 the closing continued contraction may be limited to, the region of the kathode; 
 and the opening contraction originates at, and the opening continued contrac- 
 tion may be limited to, the region of the anode. 
 
 In case a very weak current is used, no contraction will be observed ; 
 nevertheless, while the current is flowing through the muscle it modifies its 
 condition ; a state of latent excitation is produced at the kathode, which shows 
 itself in a considerable increase of irritability of that part of the muscle. On 
 the other hand, the irritability of the muscle at the kathode will be found to 
 be lessened after the withdrawal of the polarizing current, because the condi- 
 tion of excitation which it causes fatigues that part of the muscle. 
 
 The effects of the battery current at the region of the anode are just oppo- 
 site to those produced at the kathode. While the current is flowing, the irri- 
 tability at the anode is lessened, and when the polarizing current is removed, 
 irritability at the anode is found to be greater than it was before the battery 
 current was applied. 
 
 The lessened irritability which is produced at the anode during the flow 
 of the battery current may be shown by an inhibition of a condition of exci- 
 tation which may be present at the time that the current is applied to the 
 muscle. For example, in the case of unstriated muscles, not only does closing 
 the battery circuit never cause a contraction at the anode, but if the part of 
 the muscle exposed to the influence of the anode happens to be at the time in 
 a condition of tonic contraction, the entrance of the current causes that part 
 of the muscle to relax. The inhibitory influence exerted by the anode, as a 
 result of the lowering of the irritability, is seen to a remarkable degree in its 
 effect upon the heart. 1 If the anode rest on the ventricle of the frog's heart, 
 and the kathode at some indifferent point, relaxation is seen in the region of the 
 anode with each systole of the ventricle. Inasmuch as the rest of the ventricle 
 contracts, the pressure of the blood causes the wall of the ventricle to bulge 
 1 Biedermann: Elektrophyswlogie, 1895, S. 195. 
 
62 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 out, and make a little vesicle at the region of the anode. A similar inhibitory 
 influence may be observed upon an ordinary striated muscle at the point of 
 application of the anode, if it be in a condition of tonic contraction when the 
 battery current is sent into it. During the flow of the constant current through 
 a muscle, the irritability is increased in the region of the kathode and decreased 
 in the region of the anode. When the current is withdrawn from the muscle, 
 on the other hand, the irritability of the kathode is found to be decreased, and 
 at the anode to be increased. 
 
 Effect of the Electric Current upon Nerves. The polarizing effects of a con- 
 tinuous constant current are the same upon a nerve as upon a muscle, with the 
 exception that in the case of the nerve the condition of altered irritability is 
 not so strictly limited to the point of application of the anode and kathode, but 
 spreads thence throughout the part of the nerve between the two electrodes, the 
 intrapolar region, as it is called, and for a considerable distance into the parts 
 of the nerve through which the current does not flow, i. e. the extrapolar region. 
 The term electrotonus has been applied to the effects of battery currents on 
 nerves and muscles, and includes two sets of changes (1) manifested by the 
 alterations of irritability which we are considering ; (2) exhibited in changes 
 of the electrical condition of the tissue. 
 
 There can be little doubt that both of these sets of changes are the result 
 of electrolytic alterations of the nerve protoplasm, caused by the flow of the 
 polarizing current. We shall consider here only the former of these sets of 
 changes. The true nature of the electrotonic changes of the electrical condi- 
 tion of the nerve, and their relation to the nerve impulse, embrace a number 
 of difficult problems, which are still under discussion and cannot be profit- 
 ably considered here. 1 
 
 The most important work on the influence of the constant current on the 
 irritability of nerves was done by Pfliiger. 2 He ascertained the electrotonic 
 effects of the polarizing current to be most vigorous in the immediate vicinity 
 of the anode and kathode, and to spread thence in both directions along the 
 nerve. He called the change produced in the nerve in the region of the 
 anode " anelectrotonic," and the condition itself " anelectrotonus ;" while the 
 change at the kathode was termed " katelectrotouic," and the condition 
 " katelectrotonus." The same names are given to the effects of battery cur- 
 rents upon muscles. 
 
 To test the effect of a constant battery current upon the irritability of a 
 nerve, put the nerve of a nerve-muscle preparation upon two non-polarizable 
 electrodes (A, K, Fig. 29) which are placed at some little distance apart and 
 at a considerable distance from the muscle. Connect these electrodes with a 
 battery, introducing into the circuit a key (&), which permits the current to 
 be quickly thrown into or removed from the nerve, and a commutator ((7), 
 which allows the current to be reversed and to be sent through the nerve in 
 
 1 Waller: Lectures on Animal Electricity, London, 1897; Biedermann: Electrophysiology, 
 translated by F. A. Welby, 1898, vol. ii. 
 
 a Pfliiger : Untersuchungen iiber die Physiologie des Electrotonus, Berlin, 1859. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 63 
 
 either the ascending or descending direction. ( \nimvt the muscle with a myo- 
 graph lever, arranged so as to record the height of the muscle contractions. 
 Then apply to the nerve at some point between the polarizing electrodes and 
 the muscle a pair of electrodes (/) connected with the secondary coil of an 
 induction apparatus, which is placed near enough to the primary coil to cause 
 excitations of medium strength, and introduce into the secondary circuit a 
 short-circuiting key (S\ by which the closing shocks can be prevented from 
 reaching the nerve. 
 
 If, with this arrangement, a breaking induction shock of medium strength 
 be given, the nerve will be excited, and the height of the muscular contraction 
 which results may be taken as a test of the irritability of the nerve at /. 
 
 Pis. 29, Method of testing anelectrotonic and katelectrotonic alterations of irritability in nerves. 
 
 Now send the polarizing current through the nerve, in the ascending direction, 
 that is, with the anode nearer the muscle. At the moment the current is 
 closed, if it be of medium strength, a closing contraction will be observed ; 
 then comes a period during which the muscle is not contracting and the polar- 
 izing current is apparently producing no effect on the nerve ; if, however, after 
 the current has acted a short time, the irritability of the nerve at the point 
 / be again tested with a breaking induction shock, it will be found to be de- 
 creased, on account of the condition of anelectrotonus which has been induced. 
 If the key in the polarizing current be then opened, the usual opening con- 
 traction will be recorded. After the polarizing current has been removed, the 
 condition of the nerve at I can be again tested, and it will be seen that the 
 irritability has returned to the normal, or is even greater than it was at the 
 start. 
 
64 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 The effect of the kathode on the irritability may be tested in a similar way, 
 by reversing the polarizing current and again sending it into the nerve. This 
 time the current will be descending, i. e. the kathode nearest the muscle. As 
 before, a closing contraction will be seen when the circuit is made, but on test- 
 ing the irritability at /with an induction shock of the same strength as before, 
 it will be found to be increased, the shock causing a larger contraction. On 
 opening the polarizing current the usual opening contraction will be seen, and 
 if after the current has been removed the irritability be again tested, it will 
 be found to have returned to the normal, or to be decreased. The changes 
 in irritability described can be ascertained by using mechanical or chemical 
 stimuli as well as induction shocks. Alterations of the irritability induced by 
 anelectrotonic and katelectrotonic changes of the nerve-substance are to be 
 found not only in the part of the nerve between the point to which the polar- 
 izing current is applied and the muscle, but in the extrapolar region at the 
 central end of the nerve, and in the intrapolar region. The experimental 
 evidence of this is not so readily obtained, but there is no doubt of the fact. 
 
 The effect of the polarizing current is the greater, the better the condition 
 of the nerve ; moreover, the stronger the current employed, the more of the 
 nerve influenced by it. Of course, in the intrapolar region there is a point 
 where the effect of the anode to decrease the irritability comes into conflict with 
 the effect of the kathode to increase it, and where, in consequence, the irrita- 
 bility remains unchanged. This indifferent point may be observed to approach 
 the kathode as the strength of the current is increased. The following schema 
 is given by Pfluger to illustrate the way in which the irritability is changed in 
 the anelectrotonic and katelectrotonic regions as the strength of the current is 
 increased : 
 
 y 
 
 FIG. 30. Electrotonic alterations of irritability caused by weak, medium, and strong battery 
 currents : A and B indicate the points of application of the electrodes to the nerve, A being the anode, 
 E the kathode. The horizontal line represents the nerve at normal irritability ; the curved lines illus- 
 trate how the irritability is altered at different parts of the nerve with currents of different strengths. 
 Curve y l shows the effect of a weak current, the part below the line indicating decreased, and that above 
 the line increased irritability, at a: 1 the curve crosses the line, this being the indifferent point at which 
 the katelectrotonic effects are compensated for by anelectrotonic effects ; 7/ 2 gives the effect of a stronger 
 current, and y 3 , of a still stronger current. As the strength of the ciirrent is increased the effect becomes 
 greater and extends farther into the extrapolar regions. In the intrapolar region the indifferent point is 
 seen to advance with increasing strengths of current from the anode toward the kathode. 
 
 As in the case of the muscle, so of the nerve, the constant current leaves 
 behind it important after-effects. In general it may be stated that wherever 
 during the flow of the current the irritability is increased, there is a decrease 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 
 
 of irritability immediately after the removal of the current, and r'n-c 
 When the current is withdrawn from the nerve, the irritability in the reo-io;- 
 of the kathode is lowered, and in the region of the anode raised. It musi 
 added, however, that the decrease of irritability soon at the kathode gradual 
 passes over into a second increase of irritability, while the increase seen at the 
 anode upon the removal of the current continues a considerable time and is 
 not reconverted to a decrease ; therefore the total after-effect is an increase 
 of irritability. The effect of the battery current to alter the irritability of 
 the human nerve can be made out by the following experiment. 1 Test the 
 irritability of the nerve by giving it a series of light blows through the 
 medium of the electrode itself, which is made anodic or kathodic. The elec- 
 trode is pressed carefully upon the nerve, and is regularly tapped by a light 
 mallet just hard enough to give distinct twitches of the fingers; if while this 
 is going on the electrode is made kathodic, the twitches become stronger, 
 and if it be made anodic they are abolished. 
 
 The fact that when the current is closed the irritation starts from the kathode, 
 and when the current is opened from the anode, may well be associated with the 
 changes in irritability which take place at the kathode and anode upon the closing 
 and the opening of the current. The setting free of an irritation appears to be 
 associated only with an increase of irritability. When the current is closed the 
 establishment of the condition of katelectrotonus is accompanied by a rise of 
 irritability at the kathode, and when the current is opened the cessation of the 
 condition of anelectrotonus is likewise accompanied by a rise of irritability. In 
 the first case the irritability rises from the normal to something above the 
 normal, and in the second case thejirritability rises from the condition of 
 decreased irritability up to somethingpbove the normal irritability. The change 
 from the normal to the anelectrotouic condition of decreased irritability, or 
 from the katelectrotonic condition of increased irritability down to normal 
 irritability, does not irritate. As has often been said, it is hard to distinguish 
 between increase of irritability and irritation. 
 
 Effect of Frequency of Application of the Stimulus on Irritability. We have 
 seen that influences which act as irritants may also have an effect upon the irri- 
 tability of the nerve or muscle. In order to produce this change they must be 
 as a rule powerful, or act for a considerable time. Nevertheless, in the case 
 of muscles, at least, even a weak irritant of short duration, if repeated fre- 
 quently, tends to heighten irritability. For example, if a muscle be stimulated 
 by separate weak induction shocks at long intervals, the effect of each shock is 
 slight, and the change produced by it is compensated for by restorative pro- 
 cesses which occur within the living protoplasm during the following interval 
 of rest, and each of the succeeding irritations finds the mechanism in much the 
 same condition ; if, however, the shocks follow each other rapidly, each stimu- 
 lation leaves an after-effect which may have an influence upon the effectiveness 
 of the stimulus following it. As a result of this, induction shocks too feeble to 
 excite contractions may, if frequently repeated, after a "Tittle time cause a visible 
 
 1 Waller and de Watteville: Philosophical Transactions of the Royal Society, 1882. 
 VOL. II. 5 
 
66 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 movement, and shocks of medium strength, if given at short intervals, may 
 each cause a larger contraction than its predecessor, until a certain height of 
 contraction has been reached, beyond which there is no further increase pos- 
 sible. We shall consider these so-called " staircase contractions " more care- 
 fully later (see page 112). When irritations follow each other very rapidly 
 the whole character of the contraction is changed, and the muscle, instead of 
 making rapid single contractions, enters into the condition of apparently con- 
 tinuous contraction known as tetanus, during which it shortens considerably 
 more than it does when making single contractions. Increase in irritability 
 plays only a comparatively small part in the production of this remarkable 
 phenomenon, which we shall study more carefully when we come to the 
 mechanical problems involved in muscular contractions. 
 
 Rapidly repeated stimuli, though at first favorable to activity of a muscle, 
 soon exert an unfavorable influence by causing the lessened irritability which 
 is associated with fatigue. 
 
 When a nerve is excited there is a change in its electrical condition, and 
 the extent of the change is generally believed to be an indication of the 
 extent to which the protoplasm of the nerve has become active in response 
 to excitation. Waller, 1 taking the amount of change in the electrical 
 condition of the nerve as an evidence of the ability of the protoplasm to 
 react under varying conditions, found that repeated excitation increases the 
 activity of the nerve as it does of the muscle. Repeated excitation of a 
 nerve at suitable, regular intervals causes a staircase-like increase in the 
 strength of the electrical response, the record resembling that got by stair- 
 case contractions of muscles (see page 112). Moreover, if the electrical con- 
 dition of the nerve is tested by a series of excitations of equal strength 
 before and after it is subjected to a tetanizing current, the strength of the 
 variations is found to be increased. 
 
 If a second stimulus follows the first too soon, it may be wholly ineffec- 
 tive ; at least this has been found to be the case with certain forms of proto- 
 plasm. It has been shown that heart muscle has a " refractory period/ 7 as it 
 is called, responding very imperfectly to stimuli applied to it just before 
 and during its systole. 2 Apparently much the same is true of the nerve. 
 Boycott, 3 using contraction of muscle as a test, and Gotch and Burch, 4 using 
 the current of action as a test, have lately discovered that for a brief period 
 after the nerve has been stimulated it is incapable of responding to a second 
 stimulus. The length of the period of lessened excitability is greatly influ- 
 enced by temperature; at 4 C., with maximal stimuli, the "critical period " 
 may be 0.007-0.008 second ; at higher temperatures it is shorter. 
 
 (b) Influences which favor the maintenance of the Normal Physiological 
 Condition of Nerve and Muscle. Effect of Blood-supply on Nerve and Muscle. 
 The vascular system is a path of communication between the several organs 
 
 1 Waller : Lectures on Physiology, first series, 1897, p. 68. 
 
 2 Gushing : Journal of Physiology, 1897, vol. xxi. p. 214. 
 
 8 Boycott : Ibid., 1899, vol. xxiv. p. 144. * Gotch and Burch: Ibid., p. 410. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. (5, 
 
 and tissues, and the circulating blood is a medium of exchange. The blood 
 carries nutritive materials from the digestive organs and oxygen from the 
 lungs to all the tissues of the body, and it transports the waste materials which 
 the cells give off to the excretory organs. In addition to these functions it 
 has the power to neutralize the acids which are produced by the cells during 
 action, and so maintain the alkalinity essential to the life of the cell ; it sup- 
 plies all parts with moisture ; by virtue of the salts which it contains, it secures 
 the imbibition relations which are necessary to the preservation of the normal 
 chemical constitution of the cell-protoplasm ; it distributes the heat, and so 
 equalizes the temperature of the body ; finally, in addition to these and other 
 similar functions, it is itself the seat of important chemical changes, in which 
 the living cells which it contains play an active part. It is not strange that 
 such a fluid should exert a marked influence upon the irritability of the nerves 
 and muscles. Since the metabolism of muscles is best understood, we will 
 first consider the importance of the circulation to the muscle. Muscles, even 
 in the so-called state of rest, are the seat of chemical changes by which energy 
 is liberated, and when they are active these changes may be very extensive. 
 If the cell is to continue its work, it must be at all times in receipt of mate- 
 rials to replenish the continually lessening store of energy-holding compounds; 
 moreover, as the setting free of energy is largely a process of oxidation, a free 
 supply of oxygen is likewise indispensable to action. These oxidation pro- 
 cesses result in the formation of waste products such as carbon dioxide, water, 
 lactic acid and these are injurious to the muscle protoplasm, and if allowed 
 to accumulate would finally kill it. Of the services which the blood renders 
 to the muscle there are, therefore, two of paramount importance, viz. the 
 bringing of nutriment and oxygen and the removal of waste matter, and sur- 
 plus energy, as heat. 
 
 A classical experiment illustrating the effect of depriving tissues of blood 
 is that of Stenson, which consists in the closure of the abdominal aorta of a 
 warm-blooded animal by a ligature, or by compression. In the case of a 
 rabbit, for example, the blood is shut off, not only from the limbs but from the 
 lower part of the spinal cord. The effect is soon manifested in a complete 
 paralysis of the lower extremities, sensation as well as power of voluntary and 
 reflex movements being lost. The paralysis is due, in the first instance, to the 
 loss of function of the nerve-cells in the cord by which the muscles are nor- 
 mally excited to action. Later, however, the nerves and muscles of the limbs 
 lose their irritability. Of the peripheral mechanisms the motor nerve-ends 
 are found to succumb before the nerves and muscles. This is shown by the 
 fact that although the muscles are still capable of responding to direct irrita- 
 tion, they are not affected by stimuli applied to the nerve, although the nerve 
 at the time, to judge from electrical changes which occur when it is excited, 
 is still irritable. Since the nerve and muscle are irritable, the lack of response 
 must be attributed to the nerve-ends. The response to indirect stimulation 
 (i. e. excitation of a muscle by irritating its nerve) is lost in about twenty 
 minutes, while the irritability of the muscle, as tested by direct excitation, is 
 
6' 
 
 38 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 r 
 
 not lost for four or five hours. In this as in so many instances the loss of 
 irritability of the muscle is due primarily to the disturbance of the respira- 
 tion of the muscle. Of the substances supplied to the muscle by the blood, 
 oxygen is one the want of which is soonest felt. The muscle contains within 
 itself a certain store of oxygen, but one which is by no means equal to the 
 amount of oxidizable substances. Of this oxygen, that which is in the least 
 stable combinations, and which is available for immediate needs, is soon 
 exhausted. A continual supply of oxygen is required even for the chem- 
 ical changes which occur in the quiet muscle. Of the waste substances which 
 the blood removes from the cell, carbon dioxide is the one which accumu- 
 lates most rapidly and is the first to lessen the irritability. Lactic acid and 
 waste products from the breaking down of nitrogenous materials of the cell 
 are also injurious. 
 
 The dependence of nerve-fibres upon the blood-supply is by no means so 
 well understood. The nerve-fibre is a branch of a nerve-cell, and it seems as 
 if the nourishment of the fibre was largely dependent upon that of the cell- 
 body (see Fatigue of Nerve, pp. 75 and 95). Nevertheless, the nerve-fibre 
 requires a constant supply of blood for the maintenance of its irritability. 
 The irritability of the nerve cannot long continue without oxygen, and a 
 nerve which has been removed from the body is found to remain irritable 
 longer in oxygen than in air, and in air than in an atmosphere containing no 
 oxygen. Waste products liberated by active muscles have a deleterious 
 effect on nerves ; whether such substances are produced in the nerves them- 
 selves will be considered later. 
 
 The efficacy of the blood to preserve the irritability is to be seen in such 
 experiments as those of Ludwig and Schmidt j 1 they succeeded in maintaining 
 the artificial circulation of defibrinated, aerated blood through the muscles 
 of a dog, and kept them irritable for many hours after death of the animal. 
 If such an experiment is to be successful, the blood must be maintained at the 
 normal temperature, be plentifully supplied with oxygen, and be kept as free 
 from carbon dioxide as possible. Von Frey 2 made an elaborate experiment of 
 this nature. A dog was killed, the body was cut in halves, and the aorta and 
 inferior vena cava were quickly connected with an apparatus for pumping the 
 blood at a regular rate through the hind part of the body. Before the blood 
 entered the arteries it passed through coils in which it was warmed to the nor- 
 mal temperature, and an artificial lung, where it received a supply of oxygen 
 and was relieved of its carbon dioxide. Under these conditions the muscles 
 were kept alive for more than seven hours, and so far retained their normal 
 condition that throughout this period they were able to respond to stimuli 
 sent to them through their nerves and contract with sufficient vigor to raise a 
 considerable weight. H. N. Martin 3 made a similar experiment on the heart 
 
 1 Sitzungsberichte der math.-phys. Clause der k. sticks. Gesellsehoft der Wissenschaften , vol. xx., 1868. 
 
 2 Versuche iiber den Stoffwechsel des Muskels," Archiv fur Anatomic und Physiologic^ 
 1885; physiologische Abtheilung, S. 533. 
 
 3 Studies from the Biological Laboratory of Johns Hopkins University, 1882, vol. ii. p. 188. 
 
GENERAL PIIYS1* >/.m; \ r OF MUSCLE AND NE, 
 
 of a (loir- Th*' heart and lim^s were isolated from the rest of tl 
 heart was led with delihriuated blood from a Mariotte flask, and ; 
 were supplied with air by an artificial respiration apparatus. The hean, < 
 was kept moist and at the normal temperature, continued to beat for four hours 
 and more. Porter 1 has succeeded in keeping even small pieces of the 
 triele of the mammalian heart alive by maintaining a good circulation 
 well-oxygenated blood through its vessels (see Section on Nutrition of the 
 Heart). 
 
 Normally the blood-supply to the muscle is varied according to its needs. 
 When the muscle is stimulated to action its blood-vessels are at the same 
 time dilated, so that it receives a five supply of blood. 2 Moreover, if mus- 
 cular work is extensive, the heart beats faster and the respiratory movements 
 are quicker, so that a larger amount of oxygen is provided and the carbon 
 dioxide is removed more rapidly. The importance of the blood-supply to a 
 muscle can be best understood if we consider it in relation to the effects of 
 fatiguing work upon the muscles (see p. 74). The relation of special sub- 
 stances in the blood to the needs of the muscle can be best considered 
 together with the chemistry of the muscle (see p. 159). 
 
 Effect of Separation from the Central Nervous System. If a motor nerve 
 be cut, or if some part of it be so injured that the fibres lose their power of 
 conduction, the portion of the nerve thus separated from the central nervous 
 system sooner or later completely degenerates (see p. 77). Each of the 
 motor nerve-fibres is a branch of a motor cell in the anterior horns of the 
 spinal cord. These nerve-cells are supposed to govern the nutrition of their 
 processes, though how a microscopic cell can thus influence a nerve-fibre a 
 meter or so long is by no means clear. Soon after the nerve is separated 
 from its cell it exhibits a change of excitability. In general it responds 
 more readily to the kathode than the anode, which would imply an increased 
 irritability ; at the part near the cut, however, it responds best to the anode. 3 
 The increase is soon followed by a gradual decrease of irritability. In the case 
 of mammalian nerves loss of irritability may be complete at the end of three 
 or four days, but the nerves of cold-blooded animal may retain their irri- 
 tability for several weeks. The immediate cause of the loss of irritability is 
 the change in the chemical and physiological structure of the axis-cylinder. 
 The degenerative changes result finally in the complete destruction of the 
 nerve-fibres, and involve the motor end-organs as well, but do not imme- 
 diately invade the muscle, which may be considered a proof that nerve and 
 muscle protoplasm are not continuous. 
 
 Though no immediate change in the structure of the muscle is observable, 
 the irritability of the muscle soon begins to alter. At the end of a fortnight the 
 irritability of the muscle for all forms of stimuli is lessened. From this time 
 on, the irritability gradually undergoes a remarkable change, the excitability 
 
 1 Porter: American Journal of Physiology, 1899, vol. ii. p. 127. 
 
 2 Sczelkow: Sitzungsber, d. k. Akad. Wien, 1862, Bd. xlv. Abth. 1. 
 
 3 Blix: Skandinavisches Archiv fur Physiologic, 1889, Bd. i, S. 184. 
 
70 AN AMERICAN TEXT- BOOK OF PHYSIOLOGY. 
 
 for mechanical irritants and for direct battery currents (see p. 54) beginning to 
 increase, but the power to respond to electric currents of short duration, 
 as induction shocks, continuing to lessen; indeed, the reactions of the 
 muscle appear to take on more of the character of those of smooth muscle- 
 fibres. The condition of increasing irritability to direct battery currents and 
 mechanical irritants reaches its maximum by the end of the seventh week, 
 and from that time on the power to respond to all forms of stimuli lessens, 
 the excitability being wholly lost by the end of the seventh or eighth month. 
 During the stage of increased excitability fibrillary contractions are often 
 observed. 
 
 As in the case of a nerve, so of the muscle the loss of irritability is due to 
 degenerative changes which gradually lead to the destruction of the muscle 
 protoplasm. The cause of the change in the muscle is still a matter of doubt, 
 some regarding it as due to the absence of some nutritive, trophic influence 
 from the central nervous system, others consider it to be the result of cir- 
 culatory disturbances, consequent upon the lack of a proper regulation of the 
 blood-supply, due to the division of the vaso-motor nerves, and still others 
 attribute it to a lack of exercise, it being no longer stimulated to action. As 
 regards the second view, it may be said that muscles whose vaso-motor 
 nerves are intact, the vessels being innervated through other nerves than 
 those which supply the muscle-tissue proper, as is the case with some of the 
 facial muscles, undergo similar changes in irritability when their motor 
 nerves are cut. As regards the first and last views, it may be said that if 
 the muscles be artificially excited, as by electric stimuli, and thus are exer- 
 cised daily, the coming on of degeneration can be at least greatly delayed. 
 The question as to whether the anabolic processes within the muscle-cell 
 are dependent on the central nervous system, in the sense of their being 
 specific trophic influences sent from the nerve-cells to the muscles, is still 
 under discussion and need not be considered further in this place. Without 
 doubt the reflex tonus impulses which during waking hours are all the time 
 coming to the muscles are productive of katabolic changes and, indirectly at 
 least, favor anabolism. 
 
 (c) Effect of Influences which result from the Functional Activity of Nerves 
 and Muscles. Fatigue of Muscles. The condition of muscular fatigue is cha- 
 racterized by lessened irritability, decrease in the rate and vigor with which 
 the muscle contracts and liberates energy, and a still greater decrease in the 
 rate with which it relaxes and recovers its normal form. In a sense, whatever 
 induces such a state can be said to cause fatigue, but it is perhaps best to 
 restrict the term to the form of fatigue which is produced by excessive 
 functional activity. The cause of exhaustion which results from over- 
 work is in part the same as the cause of the loss of irritability and power 
 which follows the cutting off of the blood-supply. The working cell liberates 
 energy at the expense of its store of nutriment and oxygen, and through oxi- 
 dation processes forms waste products which are poisonous to its protoplasm. 
 The fatigue which results from functional activity has, therefore, a twofold 
 
GENERAL PHYSIOLOGY OF MUSCLE AND N^liVI-:. 71 
 
 cause, the decrease in energy-holding compounds available for work and the 
 accumulation of poisonous waste matters. 
 
 It is evident that the length of time that the cell can continue to work will 
 depend very much upon the rapidity with which the energy-holding explosive 
 compounds are formed by the cell-protoplasm and the waste products are 
 excreted. If a muscle is made to contract vigorously and continuously, as 
 when a heavy weight is held up, fatigue comes quickly ; on the other hand, a 
 muscle may be contracted a great many times if each contraction is of short 
 duration and considerable intervals of rest intervene between the succeeding 
 contractions. The best illustration of this is the heart, which, though making 
 contractions in the case of man at the rate of seventy or more times a minute, 
 is able to beat without fatigue throughout the life of the individual. Each 
 of the vigorous contractions, or systoles, is followed by an interval of rest, 
 diastole, during which the cells have time to recuperate. The same is true of 
 the skeletal muscles. It was found in an experiment that if a muscle of the 
 hand, the abductor inditis, were contracted at regular intervals, a weight being 
 so arranged that it was lifted by the finger each time the muscle shortened, a 
 light weight could be raised at the rate of once a second for two hours and a 
 half, i. e. more than 9000 times, without any evidence of fatigue. If, however, 
 the weight was increased, which required a greater output of energy, or if the 
 rate of contractions was increased, which shortened the time of repose, the mus- 
 cle fatigued rapidly. In general, the greater the weight which the muscle has 
 to lift, the shorter must be the periods of contraction in proportion to the inter- 
 val of rest if the muscle is to maintain its power to work. Maggiora, 1 in his 
 interesting experiments in Mosso's laboratory at Turin, made a very careful study 
 of this subject, and ascertained that for a special group of muscles there is for 
 each individual a definite weight and rate of contraction essential to the accom- 
 plishment of the greatest possible work in a given time. These experiments 
 were made on men, and the height of the succeeding contractions was re- 
 corded by an apparatus devised by Mosso, the ergograph, 2 which made it 
 possible to estimate the total amount of work done by the muscles studied. 
 Many forms of apparatus have since been devised to accomplish this. 
 Mosso's ergograph consisted of two parts, an arm rest equipped with suitable 
 clamps for fixing the arm and hand, and a writing mechanism arranged to 
 record the movements of the weight which was raised by the flexion of 
 the second finger. Either increasing the weight or the rate of contrac- 
 tion hastens the coming on of fatigue and so lessens the power and the 
 total amount of work. In such an exercise as walking the muscles are 
 not continually acting, but intervals of rest alternate with the periods 
 of work, and the time for recuperation is sufficiently long to permit the 
 protoplasm of the muscle-cells to prepare the chemical compounds from 
 w r hich the energy is liberated as fast as they are used, and get rid of the 
 
 1 Archiv fur Anatomic und Physiologic, 1890; physiologische Abtheilung, S. 191. 
 
 2 Mosso: Die Ermiidung, Leipzig, 1892, S. 90; Lombard: Journal of Physiology, 1892, vol. 
 xiii. Fig. 1, Plate 1. 
 
72 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 waste products of contraction, so that vigorous muscles can be employed 
 many hours before any marked fatigue is experienced. Sooner or later, 
 however, the vigor of the muscle begins to decrease. The reason for 
 this is not wholly clear. It is noticeable, however, that not only the 
 muscles employed in the work, but other muscles, such as those of the 
 arms for instance, even when purposely kept quiet, have their irritability 
 reduced. This would suggest that the fatigue which finally asserts itself is 
 due to some general rather than local influence. To understand this we must 
 recall the fact that all parts of the body are in communication by means of 
 the circulatory system. The ever-circulating blood as it is thrown out by the 
 heart is divided into minute streams, which, after passing through the many 
 organs of the body, unite again on their return to the heart. If materials be 
 taken from the blood by one part, they are lost to all the rest, and if materials 
 be added to the blood by any part, they are sooner or later carried to all the rest. 
 During the course of a long march, the muscles of the leg take up a great deal 
 of nutriment, and give off many waste products, and all the organs suffer in con- 
 sequence. Mosso, 1 in his experiments upon soldiers taking long forced marches, 
 found that lack of nutriment is not the only cause of the general fatigue 
 produced by long-continued muscular work. The soldiers, though somewhat 
 refreshed by the taking of food, did not recover completely until after a pro- 
 longed interval of rest. He attributed this to the fatigue-products which he 
 supposed the muscles to have given off, and concluded that they were only 
 gradually eliminated from the blood. To see if there were fatigue-products 
 in the blood of a tired animal capable of lessening the irritability of organs 
 other than those which had been working, he made the following experiment : 
 He drew a certain weight of blood from the veins of a dog, and then put back 
 into the animal an equal amount of blood from another completely rested dog. 
 The dog which was the subject of the experiment appeared to be all right after 
 the operation. On another day he repeated the experiment, but this time the 
 blood which was put back was taken from a dog that was completely tired out 
 by running. The effect of the blood from the fatigued animal was very 
 marked ; the dog receiving it showed all the signs of fatigue, and crept off into 
 a corner to sleep. Mosso concluded from this experiment, that during mus- 
 cular work fatigue-products are generated in the muscles, pass thence into the 
 blood, and are conveyed to other muscles, where they produce the lowered 
 irritability and loss of power characteristic of fatigue. Many years before, 
 Von Ranke extracted from the tired muscles of frogs substances which he 
 considered fatigue materials. Lee 2 would draw a sharp distinction between 
 fatigue and exhaustion. He considers the former to be a transient change 
 in the capacity for work induced by the presence of waste products, while 
 the latter is a far more serious condition and is due to a lack of nutritive 
 energy-giving substance. He considers that fatigue, by lessening the irri- 
 
 1 Archiv fur Anatomic und Physiologic, 1890; physiologische Abtheilnng. 
 
 2 Proceedings of the American Physiological Society, Dec., 1898, published in American 
 Journal of Physiology, 1899, vol. ii. p. 11. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NKHVi:. 73 
 
 lability, may exert a protective influence :m<l prevent the work from being 
 carried under ordinary conditions to the point of exhaustion. In favor of 
 this view he states that the muscles even of starving animals, although 
 incapable of long-continued work, do not make contractions of the type 
 characteristic of fatigued muscles (see p. 115); on the other hand, muscles 
 which have been subjected to lactic acid, one of the waste products resulting 
 from muscular work, whether it be free or combined, as it probably is in the 
 muscle, with potassium or sodium, do make contractions of the type shown 
 by fatigued muscles. Waller 1 has of late laid much stress upon the action 
 of CO 2 to stimulate protoplasm when present in small amounts and to anes- 
 thetize it when in larger quantities. CO 2 is also a waste product of muscle, 
 but it is doubtful whether the paralyzing effect of large amounts can be 
 regarded as a fatigue effect. 
 
 Maggiora, in his experiments upon the fatigue of special groups of mus- 
 cles, likewise found that the taking of food causes only a partial recovery of 
 the tired muscles, and that an interval of rest is essential to complete recovery. 
 In these experiments the irritability of the muscles was tested not only by 
 volitional impulses, but by the strength of the electric current required to 
 cause direct excitation. A curve of fatigue of human muscles by voluntary 
 contractions is shown in Fig. 59, and one resulting from electrical excitation 
 of the muscle in Fig. 58. In the case of vigorous men, one and a half hours 
 suffice to restore the muscles of the forearm which have been completely tired 
 out by raising a heavy weight many times. He also observed that the time 
 required for recovery can be greatly shortened if the circulation of the blood 
 and lymph in the muscles be increased by massage. This suggests that the 
 power of the cell to give off its waste products to the blood is sufficiently 
 rapid to keep pace with the ordinary production, but not with the more rapid 
 formation taking place during fatiguing work. This would seem to be the 
 case in spite of the fact that circulation of the blood and lymph in the mus- 
 cles is increased during action. This increase in the circulation through the 
 acting muscle is brought about in part by the fact that the muscle massages 
 itself by its own contractions. Jt is a pumping mechanism, which acts at the 
 time when the increased taking of oxygen and nutriment and giving off of 
 waste products make the rapid renewal of the restoring fluids imperative. 
 Every time the muscle contracts the swelling, tense fibres compress the lym- 
 phatics and blood-vessels between and about them, and when it relaxes the 
 valves in the lymph vessels and veins prevent the return of the fluid which 
 has been squeezed out. In addition to this, when muscles are stimulated to 
 action by impulses coming to them from the central nervous system, the mus- 
 cles in the walls of the blood-vessels of the muscle are acted upon by their 
 vaso-dilator nerves, and, relaxing, permit a greater flow of blood through the 
 muscle ; when the muscles cease to be excited the muscles in the vessel walls 
 gradually regain their tone, and the blood-supply to the muscle tissue is 
 correspondingly lessened. This arrangement would seem to suffice for the 
 1 Lectures on Physiology, first series, on Animal Electricity, London, 1897, p. 47. 
 
74 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 bringing of nutriment and oxygen and the removal of waste matters under 
 ordinary conditions. 
 
 Considerable difference of opinion exists as to which of three classes of 
 food-stuffs proteids, carbohydrates, and fats supply the energy used by the 
 muscle in ordinary and excessive work, and how these are employed by the 
 muscle. 
 
 The question has been studied by examining the character and quantity 
 of waste products liberated from the body during and after excessive mus- 
 cular work, as compared with those given off when the subject is at rest. 
 Another method has been to test the strength of the muscle in ergographic 
 experiments, and to find the effect of different kinds of food upon the time 
 required for its recovery. Experiments of Fick and Wislicenus, 1 Voit and 
 Pettenkofer, 2 Voit, 3 and others caused the view to become generally accepted 
 that the energy of the muscle by violent muscular work comes largely from 
 the non-proteid substances in the muscles. Later Pflliger and his pupils 
 have gone to the other extreme and conclude that proteid is the chief source 
 of energy. 4 
 
 Very many others have written on both sides of the subject and still a 
 final conclusion has not been reached. 5 
 
 Probably the sugars, and possibly after these the fats are employed by the 
 muscle as the most available form of energy, while the proteid forms a more 
 permanent part of the muscular machine, and is only made use of when the 
 work is exhaustive (see page 166). The taking of any one of these classes 
 of food hastens the recovery from fatigue, and the sooner the more readily it 
 is digested and assimilated (see Metabolism effect of muscular work). 
 
 Normally the muscles are never completely fatigued. It would seem 
 that as the muscles tire and their irritability is lessened, the central nerve- 
 cells which send the stimulating impulses to them have to work harder, 
 and that the nerve-cells give out sooner than the muscles. On the other 
 hand, certain experiments seem to show that the nerve-cells recover from 
 fatigue more rapidly than the muscles do, so that it is an advantage to 
 the organism that they should cease to excite the muscles before muscular 
 fatigue is complete. With the decreasing irritability of the muscle, a feeling 
 of discomfort in the muscle and an increasing sense of effort are experienced 
 by the individual, both of which tend to cause a cessation of contraction, and 
 prevent a harmful amount of work. That such an arrangement would be of 
 service was apparent in the experiments of Maggiora, in which he found that 
 if muscles are forced to work after fatigue has developed, the time of recovery 
 is prolonged out of all proportion to the extra work accomplished. 
 
 At the close of even exhaustive muscular work there is always a large 
 amount of energy-holding materials in the blood and tissues, and the rapid, 
 
 1 Viertetyahresschrift der naturforsche Gesellschaft in Zurich, 1865, Bd. x. S. 317. 
 
 2 Zeitschnft fur Biologic, 1866, Bd. ii. 3 Ibid., 1876, Bd. vi. S. 305. 
 
 4 Pfluger's Archiv, 1899, Bd. 77, S. 425. 
 
 5 Schafer's Text-book of Physiology, 1898, vol. i. p. 912. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 75 
 
 though partial, improvement in the condition on the taking of food is per- 
 haps best explained as the result of a stimulating effect on the central nerv- 
 ous system. This might be due to the change in the circulation which follow- 
 the taking of food, as well as the fact that a fresh supply of uncombined and 
 hence available energy-holding substances is being received. The effect of 
 the so-called stimulants, alcohol, tea, coffee, etc., to temporarily increase the 
 ability to do work, is probably chiefly through their action on the central 
 nervous system. Their influence is a temporary one, and only markedly 
 increases the amount of work when the body has a plentiful supply of nutri- 
 ment. 1 
 
 Fatigue of Nerves. Muscle-, gland- and nerve-cells in fact, almost every 
 form of protoplasm if excited to vigorous long-continued action, deteri- 
 orate and exhibit a decline of functional activity. As we have seen, in 
 the case of muscle there are a using up of available energy-holding com pounds 
 and a production of poisonous waste matters, and these two effects induce the 
 condition known as fatigue. A priori, we should expect similar changes to 
 occur in the active nerve-fibre ; almost all the experimental evidence is, how- 
 ever, opposed to this view. The form of activity which is most character- 
 istic of muscle is contraction ; that which is most characteristic of nerve is 
 conduction. In the case of the muscle it is exceedingly difficult to distin- 
 guish between the effects produced by the processes associated with the change 
 of form and those which result from the transmission of the excitatory change. 
 There is little doubt that fatigue is associated with the former ; whether 
 it is associated with the latter is not known. In the case of the nerve, where 
 the transmission process may be studied by itself, conduction does not seem 
 to fatigue (see p. 95). 
 
 Apparently the same may be said of the processes which result in the 
 development of what we call the nerve-impulse. We have already seen that 
 the nerve may undergo an alteration of irritability if subjected to artificial 
 irritants. Such a change at the point of application of the irritant is hardly 
 to be regarded as a fatigue effect, however, for in many cases, at least, it is 
 due to the direct effect of the irritant on the physical or chemical structure of 
 the nerve-protoplasm rather than to molecular changes which are peculiar to 
 the development of the nerve-impulse. Thus the change of irritability which 
 results from a series of light blows, such as may be given to a nerve by 
 Tigerstedt's tetanomotor, cannot properly be said to be the result of fatigue. It 
 has been found that a medullary nerve may be excited many times a second 
 for hours, by an induced current, and still be capable of developing at the 
 stimulated point what we call the nerve-impulse. The change which is de- 
 veloped at the point of excitation and which passes thence the length of the 
 nerve, would seem to be the expression of a form of energy liberated within 
 the nerve, and since the liberation of energy implies the breaking down of 
 chemical combinations, the apparent lack of fatigue of the nerve is incompre- 
 hensible. It is the more remarkable since the nerve-fibre is to be considered a 
 1 Schumburg: Archiv fur Analomie und Physiologic, 1899, supplement, S. 289. 
 
76 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 branch of a nerve-cell, and nerve-cells appear to fatigue if frequently excited 
 to vigorous action. Inasmuch as we have as yet no definite knowledge of the 
 nature of what we call the nerve-impulse, or of the character of the processes 
 by which it is transmitted along the nerve, we can afford to leave this question 
 open, and simply state that the evidence thus far obtained is opposed to the 
 view that nerve-fibres fatigue. 
 
 Effect of Use and Disuse. Different kinds of muscle- tissues possess very 
 different degrees of endurance. By endurance we mean the capacity to liber- 
 ate energy during long periods of time. This capacity is intimately associated 
 with irritability, for one of the first marks of failure of power is a decline of 
 irritability. In general, the more irritable a muscle the less its endurance, 
 because with an increase of irritability there is associated a more rapid and 
 extensive liberation of energy in response to irritants. For example, the rap- 
 idly responding and acting pale striated muscles of the rabbit have less resist- 
 ing power than the red striated muscles, while the sluggish unstriated muscle- 
 fibres can contract a long time without suffering from fatigue. 
 
 The endurance of muscles of even the same kind may differ very considera- 
 bly in the same individual, but the differences are more striking in the case of 
 different individuals. One of the causes of this is the extent to which the 
 muscles are employed. Use, exercise, is the most effective method of increasing 
 not only the strength, but the endurance of the muscle. Though this fact is 
 so well known as to scarcely need repeating, the explanation of it is by no 
 means so clear. Undoubtedly one of the causes is a more perfect circulation 
 in a muscle which is often used, but this is not all. It would seem as if the 
 protoplasm of the muscle-cell was educated, so to speak, to be more expert in 
 assimilating materials containing energy, in building up the explosive compounds 
 employed in its work, and in excreting deleterious waste matters. 
 
 The effect of exercise upon irritability has not been thoroughly worked out. 
 It would seem as if there were a normal degree of irritability for each special 
 form of muscle-tissue, and as if either an increase or decrease of the irritability 
 above or below this level was a sign of deterioration. Exercise, if not excess- 
 ive, is favorable to the maintenance of this normal physiological condition. 
 Without doubt many of the differences which we attribute to the muscles of 
 different men are really due to differences in the central nerve-cells, the action 
 of muscles, rightly interpreted, being rather an expression of central nervous 
 activity than the result of peculiarities of the muscles themselves. To vol- 
 untarily exercise the muscles is to exercise the nerve-cells, and the effects of 
 exercise upon these nervous mechanisms is of as much importance as the 
 effect upon the muscles. In admiring visible proportions we must always 
 bear in mind that though "beef" is of use to the athlete, the muscles are 
 merely the servants, and can accomplish nothing if the master is sick. The 
 nerve-cells always give out before the muscles, and the man preparing for a 
 contest should watch his nervous system more than his muscles. He who 
 forgets this can easily over-train, and do himself a permanent injury, besides 
 failing in the race. 
 
QEXEKAL PHYSIOLOGY OF MUSCLE AND NERVE. 77 
 
 Effect of Enforced Rest. Not only is the strength of the mu-cl<s 
 increased by exercise, but a lack of exercise soon results in a loss of 
 This is seen when an individual is confined to his bed for even a comparatively 
 short time, or when a limb is subjected to enforced rest by being placed in a 
 splint. The cause is to be sought in changes peculiar to the muscle proto- 
 plasm itself, although reduced circulation may also play a part. The effect of 
 prolonged rest on the irritability of muscles, is seen most markedly when they 
 are separated from the central nervous system by injuries of their nerves (see 
 p. 70). The lowered irritability which results from prolonged rest is not 
 peculiar to muscles, but is shared by all forms of protoplasm. 
 
 G. CONDUCTIVITY. 
 
 Conductivity is that property of protoplasm by virtue of which a condition 
 of activity aroused in one portion of the substance, by the action of a stimulus 
 of any kind, may be transmitted to any other portion. For example, if the 
 edge of the bell of a vorticella (see Fig. 2, p. 19) be irritated by a hair, not 
 only do the movements of the cilia cease, but the contractile substance of the 
 bell draws it into a more compact shape, and the fibrilla? of the stalk shorten 
 and pull the bell away from the offending irritant. In such a case an exciting 
 process must have been transmitted throughout the cell, and through several more 
 or less differentiated forms of protoplasm. This property of conductivity is not 
 known to be limited to any one peculiar structural arrangement of protoplasm 
 distinguishable with the microscope, but is exhibited by a vast variety of forms 
 of cell-protoplasm, and by plants as well as animals. The cytoplasm of cells, 
 the part of the protoplasm surrounding the nucleus, appears to be composed 
 of a semifluid granular material, traversed in all directions by finest fibrillse 
 which in some cases appear to form an irregular meshwork, the reticulum, and 
 in others to be arranged side by side as more or less complete fibrils. It is not 
 known whether the power of conduction is possessed by the whole of the pro- 
 toplasmic substance or is confined to the reticular substance, but there are cer- 
 tain reasons why the former view may be considered the more probable. The 
 rate and the strength of the conduction process varies greatly in different forms 
 of protoplasm, and there appear to be differences in the facility with which 
 the exciting process spreads through different parts of even the same cell. 1 Not 
 only are such differences to be noticed in many of the ciliated infusoria, but 
 even the substance of striated muscles seems to conduct in two different ways, 
 the sarcoplasm appearing to conduct slowly, and the more highly differentiated 
 fibrillary portion of the fibre rapidly. In general the process appears to be 
 more rapid and vigorous where a fibrillated structure is observable. Smooth 
 muscle-tissue, which has a somewhat simple structure, conducts comparatively 
 slowly; striated muscle, which is more highly differentiated, more rapidly, and 
 the fibrillated axis-cylinder of the nerve-fibre, most rapidly of all. 
 
 Protoplasmic Continuity is Essential to Conduction. Effect of a 
 Break in Profoj>/<txin<<' (.'out I nnily. A break of protoplasmic continuity in any 
 1 Biedermann : Elektrophysiologie, 1895, S. 137. 
 
78 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 part of a nerve- or muscle-fibre acts as a barrier to conduction. If a nerve be cut 
 through, the irritability and conductivity remain for a considerable time in the 
 severed extremities, but communication between them is lost, and remains absent 
 however well the cut extremities may be adjusted. The nerve-impulse is not 
 transmitted through the nerve-substance as electricity is transmitted along a 
 wire : join the cut ends of a wire, and the contact suffices for the passage of 
 the current ; join the cut ends of a nerve, and the nerve-impulse cannot pass. 
 Any severe injury to a nerve alters the protoplasmic structure and prevents 
 the chemical and physical processes through which conductivity is made 
 possible. It is probable that the same may be said of all forms of liv- 
 ing cells, and the absence of protoplasmic continuity would seem to be an 
 explanation of the fact that nerve- and muscle-fibres which lie close together 
 may physiologically act as separate mechanisms. 
 
 Even in the case of apparently homogeneous protoplasm there is probably 
 a definite structural relation of the finest particles, and upon this the physi- 
 ological properties of the substance depends. Slight physical and chemical 
 alterations suffice to change the rate and strength of the conduction process, 
 and the power to conduct is altogether lost if the protoplasm is so altered that 
 it dies. 
 
 The relation of conductivity to structure of cell-protoplasm is illustrated in 
 the effects of degeneration and regeneration upon the physiological properties of 
 the nerve-fibre (see p. 69). The life of the nerve-fibre is dependent on influ- 
 ences exerted upon it by the body of the cell of which it is a branch. When 
 any part of the fibre is injured it loses its power to conduct, and the portion 
 of the fibre separated by this block from the body of the cell soon dies. The 
 irritability and conductivity are wholly lost at the end of a period varying 
 from four days to several weeks, the time differing in different kinds of nerves, 
 and the fibre begins to undergo degeneration. The axis-cylinder and the 
 myelin are seen to break up and are then absorbed, apparently with the 
 assistance of the nuclei which normally lie just inside the neurilemma, 
 and which at this time proliferate greatly and come to occupy most of 
 the lumen of the tube. The process of absorption is nearly complete at 
 the end of a fortnight after the injuy. Under suitable conditions, however, 
 regeneration may occur, and as this takes place there is a recovery of physi- 
 ological activities. The order in which conductivity and irritability return is 
 instructive. Howell and Huber 1 made a careful study of this subject. They 
 found that the many nuclei which develop during degeneration are apparently 
 the source of new protoplasm, which is seen to accumulate in the old sheath until 
 a continuous band of protoplasm is formed. About this thread of protoplasm 
 a new membranous sheath develops, and thus is built up what closely resembles 
 an embryonic nerve-fibre. The embryonic fibre formed in the peripheral end 
 of the regenerating nerve joins that of the central end in. the cicatricial tissue 
 which has been deposited at the point of injury. Thus a temporary nerve- 
 fibre is formed and united to the undegenerated part of the old fibre, and this 
 1 Journal of Physiology, 1892, vol. xiii. p. 381. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 79 
 
 new structure, though possessing neither myelin nor axis-rylindrr, is found to 
 be capable of conduction and to have a low form of irritability, being ex- 
 citable to violent mechanical stimuli but not to induction currents. The pmvn- 
 of conduction appears to return before irritability, and may be observed first 
 at the end of the third week. Apparently sensation is recovered before the 
 power of making voluntary movements; this difference may well be due, not 
 to any essential difference between sensory and motor fibres, but to the fact 
 that extra time is required for the motor fibres to make connection with the 
 muscle. The embryonic fibre gradually gives place to the adult fibre, new 
 myelin being formed all along the fibre, and a new axis-cylinder growing down 
 from the old axis-cylinder. As the axis-cylinder grows down, the irritability 
 for induction shocks is recovered. Many months may be necessary for the 
 complete recovery of function. Langley l reports that medullated fibres of 
 the sympathetic, if cut, regenerate and recover the power to function before 
 they regain a medullary sheath. Such experiments show the axis-cylinder 
 to be the true conducting medium, and that the medullary sheath has a sub- 
 ordinate function. 
 
 The same is true of muscle as of nerve protoplasm, the power of con- 
 duction ceases with the life of the cell-substance ; thus, if the middle part of 
 a muscle-fibre be killed, by pressure, heat, or some chemical, the dead proto- 
 plasm acts as a block to prevent the state of activity which may be excited at 
 one end from being transmitted to the other, and the conduction power is only 
 recovered on the regeneration of the injured tissue. 
 
 Isolated Conduction is the Rule. (a) Conduction in Nerve-trunks. The 
 axis-cylinders of the many fibres which run side by side in a nerve-trunk are 
 separated from each other by the neurilemma, and in the case of the medullary 
 nerves by the myelin substance as well, so that there is not even contiguity, 
 much less continuity of nerve-substance. Thus the many fibres of a nerve- 
 trunk, some afferent and others efferent, though running side by side, conduct 
 independently of one another. For example, if the skin of the foot be pricked, 
 the excitation of its sense-organs is communicated to sensory nerve-fibres, and 
 is transmitted along them to the spinal cord, where the stimulus awakens cer- 
 tain groups of cells to activity ; these cells in turn, by means of their branches, 
 the motor nerve-fibres, transmit the condition of excitation down to the mus- 
 cle-fibres of the legs, which, when stimulated, contract and withdraw the foot 
 from the offending irritant. The sensory and motor nerves concerned in this 
 reflex act run for a considerable part of their course in the same nerve-trunk, 
 but the sensory impulses have no direct effect on the motor nerve-fibres, and 
 the roundabout course which has been described is the only way by which 
 they can influence them'. 
 
 Isolated conduction by separate fibres and their branches holds good within 
 the central nervous system, as elsewhere, otherwise we could scarcely explain 
 the localization of sensations, or co-ordinated movements. 
 
 The presence of a medullary sheath is not essential to isolated conduction, 
 1 Journal of Physiology, 1897, vol. xxii. p. 223. 
 
80 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 for it occurs in the absence of this sheath, both in the peripheral nerves and 
 in the central nervous system. The large class of non-raedullated nerves 
 have the power of isolated conduction, and Donaldson reports that new-born 
 rats can make co-ordinated movements, although the nerves of both the 
 peripheral and central nervous systems do not acquire a medullary sheath 
 until several days after birth. It is not likely that the neuroglia cells are 
 essential to isolated conduction within the central nervous system, for this 
 occurs in its absence in the peripheral nerves. Although the neurilemma, by 
 separating the axis-cylinders of adjacent fibres, may make the insulation more 
 complete, it is probably not the real cause of isolated conduction. A break 
 of the protoplasmic continuity of the nerve protoplasm stops conduction, and 
 conduction fails wherever protoplasmic continuity is lacking. 
 
 An apparent contradiction to the rule that absolute continuity of nervous 
 matter is essential to conduction by nerves, is to be found in the phenomenon 
 known as " Hering's Paradoxical Contraction." This will be explained 
 later (see p. 157, d). 
 
 (6) Distribution of Excitation by Branches of Nerves. Nerve-fibres some- 
 times branch in their passage along the peripheral nerves, but most of the 
 branches which are seen to be given off from the nerve-trunks are composed 
 of bundles of nerve-fibres which have merely separated off from the rest. 
 After the nerves have entered a peripheral organ, or the central nervous system, 
 the axis-cylinders may give off branches. Thus in muscles, and to a still greater 
 degree in the electric organs of certain fish, the nerve-fibre and its axis-cylinder 
 may divide again and again, or after entering the spinal cord the fibre may be 
 seen to give off a great many lateral branches collaterals, as they are called. 
 It is not known whether in such cases the fibrillae of the axis-cylinder give 
 off branches, or whether they simply separate, a part of them entering the 
 branch while the rest of them continue on in the main fibre. Though the 
 exciting process does not pass from fibre to fibre, it probably involves in a 
 greater or less degree all the elements of the same fibre, and passes into all its 
 branches. It is evident that where it is necessary for the irritation to be 
 localized, branching could not occur ; but where a more general distribution 
 is permissible, especially where several 4 parts of an organ ought to act at the 
 same instant, conduction through a single fibre which branches freely near its 
 termination would be useful. 
 
 (c) Conduction in Muscles. Each fibre of the muscles which move the 
 bones the skeletal muscles, as they are sometimes called is physiologically 
 independent of the rest. The sarcolemma prevents not only continuity, but 
 contiguity of the muscle-substance of the separate fibres, and there is no cross 
 conduction from fibre to fibre. That this is so is proved by an experiment, such 
 as was described on page 45, in which unipolar excitation of the part of the 
 fibres of a curarized sartorius muscle results in a contraction strictly con- 
 fined to the fibres which are subjected to the irritating current. Each of the 
 separate muscle-fibres is supplied by at least one nerve-fibre or a branch 
 of a fibre, and, under normal conditions, only acts when stimulated by 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 81 
 
 the nerve. Tn the case of plant-cells, and of certain forms of mus- 
 cle-cells, about which there is a more or less definite wall or sheath, there 
 arc little bridges of protoplasm binding the cells together. For example, 
 Engelmann describes the muscle of the intestines of the fly as composed of 
 striated cells, sheathed by sarcolemma, except where bound together by little 
 branches of sarcoplasma, which may act as conducting wires between the cells. 
 
 There are certain cells, however, which have been supposed to be exceptions 
 to the rule that protoplasmic continuity is essential to conduction. The stri- 
 ated muscle-fibres of the heart are quite different from those of ordinary 
 skeletal muscles, physiologically as well as anatomically. They are stumpy, 
 quadrangular cells, which are not known to have a sarcolemma, and which 
 are united not only by their broad ends, but by lateral branches. Engelmann 
 and lately Porter l and others have concluded that conduction takes place in 
 the heart from cell to cell, without the intervention of nerves, and may 
 occur in all directions. This question is considered at length in the section 
 on the conduction of excitation in the heart. 
 
 The cells of the contractile substance of some of the medusaB (as Aurelia), 
 have been supposed to communicate by contiguity rather than by continuity. 
 The same has been thought to be the case with many forms of unstriated 
 muscle-tissue ; 2 moreover, there are groups of ciliated cells, the members of 
 which act in unison although they have not been found to be connected either 
 directly or by nerves. These cells have apparently no membranous covering, 
 and though living as independent units, are so related that a condition of 
 activity excited in one seems to be transmitted to the rest by means of contact, 
 or through the mediation of cement-substance. 
 
 From what has been said it will be seen that protoplasmic continuity 
 ensures free communication between different cells ; that protoplasmic con- 
 tiguity, either directly or through the mediation of the cement-substance, may 
 possibly permit of conduction ; but that normally the intervention of a dif- 
 ferent tissue, even as delicate as the sarcolemma, suffices to cause complete 
 isolation of the cell from its neighbors. Under normal conditions there may 
 be a spread of excitation from muscle-fibre to muscle-fibre, even in the skel- 
 etal muscles. Kiihne's experiment with the sartorius muscle of the frog, 
 described on page 45, gives a good proof that the activity of a striated 
 muscle-fibre is not normally transmitted to its neighbors ; nevertheless, 
 Kuhne 3 has found that if the extremities of two sartorius muscles be 
 pressed firmly together by a suitable clamp, care being taken that the 
 pressure shall not be enough to destroy the physiological activity of the 
 protoplasm, excitation of one muscle may cause contraction of the other. 
 A satisfactory explanation is lacking. 
 
 Biedermann 4 reported that when a frog's muscle was partly dried a slight 
 
 1 Porter: Journal of Experimental Medicine, 1897, ii. p. 391 ; American Journal of Physi- 
 ology, 1899, ii. p. 127. Engelmann : Pfliiger's Archir, 1871, Bd. iv. 
 
 * Kiihne : Untersuchungen aus der physiologischen Institute in Heidelberg, 1880, Bd. 3, S. 1. 
 
 * Biedermann : Serichte der Wiener Akademie, 1888, Bd. 97, Abth. 3, S. 145. 
 
 VOL. II 6 
 
82 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 mechanical excitation of one part of it might lead to a contraction of the 
 whole. Similarly, a partly dried-up frog may be seen, if mechanically 
 excited, to make movements simulating life. The cause of these movements, 
 also, is not understood. Drying of the muscle in its early stages greatly 
 increases its irritability because of the concentration of the salts, but that 
 does not account for the loss of insulation. 
 
 Transmission of Excitation by Means of End-organs. In spite of the 
 rapid advances which have been made in the histology and physiology of the 
 nervous system during the past few years, we are still in doubt as to the exact 
 way that the axone, the exciting branch of the neurone, stimulates the cell 
 
 ~ to which it is distributed. In many 
 
 _ Vl____jL_^ -- ! cases ? at 'least, the axone terminates in an 
 
 ^yt/Vc end-organ which is physiologically dif- 
 
 r\~~* . /. ' ferent from the rest of the cell, and this 
 
 end-organ is the exciting agent. The 
 
 / ^ , d relation of the protoplasm of the end- 
 
 ^V^-rW^. /_ organ to the protoplasm of the cell which 
 
 v*u j 3 j it stimulates, whether one of continuity 
 
 ^^^ or contiguity, is not certain, but most 
 
 , histological and physiological observa- 
 
 FIG. si. Nerve-termination in voluntary tions are distinctly in favor of the latter 
 
 muscle of the rabbit, stained in methylen-blue 
 
 (intra vitam), fixed, sectioned, and counter- V16W. 
 
 stained in alum carmin. A, surface view; B, The physiology of the end-Organs of 
 
 longitudinal section through nerve-termina- ' . - 
 
 tion and muscle-fibre; C, cross-section : S, motor aXOUCS distributed to striated 
 
 sarcolemma; n. I., neurilemma. (From Text- musc l es j s best kllOWU. 
 
 book of Histology, Bohm and Davidoff, revised 
 
 by G. C. Huber. W. B. Sauiiders, Philadel- Fig. 31 shows a surface View and 
 
 phia, 1900). a l on gitudinal and cross-section of the 
 
 end-organ of an axone supplying a voluntary muscle of a rabbit. The axis- 
 cylinder loses its medullary sheath shortly before reaching the fibre, and the 
 neurilemma becomes continuous with the sarcolemma, so that the axis-cylin- 
 der on penetrating the sarcolemma comes into direct contact with the sarco- 
 plasma of the muscle. The sarcoplasma is heaped together at this place, 
 making a little mound, and the axis-cylinder, after dividing into a number 
 of fine terminal twigs, ends in the midst of this mass of sarcoplasma. Evi- 
 dently the nerve and muscle protoplasm come into very close relation. On 
 the other hand, nerve and muscle protoplasm retain each its peculiar reaction 
 to staining-fluids, and as far as these chemical reactions can show each main- 
 tains its peculiar chemical and histological structure. Moreover, the results 
 of physiological experimentation have shown that, although no definite histo- 
 logical boundary has been found between the axone and its terminal organ, 
 the exciting organ must be considered to be a specially differentiated struct- 
 ure, differing widely from the rest of the neurone. 
 
 The motor end-organ uses up more time in the excitation of the muscle 
 than would be required for transmission of the excitation through a like 
 amount of nerve- or muscle-substance. It is found by experiment that a 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 83 
 
 muscle does not contract so quickly if it be excited through its nerve as when 
 stimulated directly. Part of the lost time is spent in transmission of the 
 excitation through the nerve ; but after allowance has been made for this loss 
 there is a balance to be accounted for, and this is credited to the motor end- 
 organ. The time used by the motor end-plate is found to be 0.0032 second. 1 
 
 Motor end-organs are, as we have seen, poisoned by curara (see p. 26) and 
 a number of other drugs which have little influence on the rest of the axone 
 or on the muscle. 
 
 If a muscle is continuously excited for a considerable time by irritants 
 applied to its nerve, it will at last cease to contract. Direct excitation shows 
 that, though weakened, it is still capable of contraction, and we know that 
 the nerve-fibre does not fatigue. The cessation of contraction is due to 
 fatigue of the motor ends. 
 
 The motor end-organ is found to lose its vitality quicker than the muscle 
 or the nerve-fibre, if it be deprived of its normal blood-supply. 
 
 If a motor nerve be cut, the part of the axone separated from the body of 
 the nerve-cell and the terminal organ degenerates, but the degeneration proc- 
 ess stops at the muscle. 
 
 These facts show that the motor end-organ differs physiologically in many 
 respects from the rest of the axone and from the muscle. Moreover, they 
 favor the idea that excitation is not conducted directly from nerve to muscle 
 protoplasm. That this is the case is also made probable by the fact that though 
 a condition of excitation is transmitted in both directions through nerve and 
 muscle protoplasm as long as there is continuity, a condition of excitation in 
 muscle substance does not appear to be transmitted to the motor nerve. Ap- 
 parently the protoplasm of the end-organ and the muscle are in contact, but 
 are not physiologically continuous, and excitation of muscle protoplasm by the 
 end-organ occurs through some special process. Various views have been 
 advanced with reference to the probable nature of such a process, but as no 
 one of them has received general acceptance they need not be dwelt upon here. 
 One point more, of interest in this connection, is the fact that it is the sarco- 
 plasma rather than the fibrillary elements of the muscle that comes in contact 
 with the nerve end-organ, which would seem to show that this substance is 
 capable of being excited and conducting the excitation. If this be true of 
 muscle substance, it is likely that the semi-fluid part of the protoplasm of the 
 nerve, as well as perhaps the fibrillary part, may have the power of conduction. 
 
 As a result of a series of remarkable histological investigations on the 
 anatomy of the nervous system, the view has come to be generally accepted, 
 that the afferent nerve-fibres entering the spinal cord do not communicate 
 directly with the nerve-cells, but terminate in brush-like endings in close 
 contact with some part of the cells which they excite. A similar arrange- 
 ment has been found wherever nerve-cells are excited to action by nerve- 
 fibres. As in the case of the motor end-organ, it has remained a matter of 
 doubt whether the brush-like ends of the axones should be considered to be 
 1 Bernstein: Archir fur Anatomic und Physioloyie, 1882, S. 329. 
 
84 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 in contact with the bodies and dendrites of the cells to be excited, and whether 
 this relation would be sufficiently close for a transmission of excitation, or 
 whether they should be considered as specially differentiated exciting mechan- 
 isms, which do not simply transmit the condition of excitation by a proc- 
 ess of conduction, but which develop a special form of physiological stimulus, 
 and through this excite the second neurone to activity. 
 
 Of late, certain histologists claim to have traced the fibrilla} of the axone 
 of one neurone into the cell-body of another neurone, and have even suggested 
 that the nerve impulse from the first might be transmitted through the cell- 
 body of the second and into its branches without the intervention of the pro- 
 toplasm of the body of the cell. 
 
 It is possible that in some cases the axone of the exciting neurone may, 
 instead of ending close to the neurone to be excited, penetrate it and end in 
 its substance, just as the motor end-organ penetrates into the sarcoplasma 
 of the muscle-fibre. This could happen, and yet the protoplasms of the two 
 cells might preserve their individuality. 
 
 There are many facts which show that, physiologically at least, the two 
 neurones act as wholly independent mechanisms. These will be dealt with 
 more at length in the section devoted to the physiology of the central nervous 
 system. Suffice it to say, the end-brush at the extremity of the axone can 
 excite the cell body of another neurone, but cannot be excited by it. A 
 reflex act involving only two neurones requires more time than could be 
 used in simple conduction through the two cells. The character of the 
 impulse sent out of the spinal cord by the efferent cell may be very different 
 from that passing in along the afferent cell e. g., the efferent impulse may be 
 stronger than the afferent ; the strength of efferent discharge may vary greatly 
 within short intervals of time even when the strength of the afferent impulses 
 remains the same ; weak afferent impulses may, by summation, lead to a 
 strong efferent discharge, and continuous afferent stimulation may awaken 
 rhythmic efferent discharges. 
 
 In short, physiological facts are all opposed to the idea that there is con- 
 tinuity of protoplasm of different nerve-cells, and in favor of the view that the 
 end-brush, like the motor end-plate, acts as a specialized exciting mechanism. 
 
 Conduction in Both Directions. (a) In Muscle. Wherever proto- 
 plasmic continuity exists, conductivity would seem to be possible ; moreover, 
 the active change excited by an irritant would seem to be able to pass in all 
 directions, though whether with the same facility is not known. Where the 
 spread of the excitatory process is accompanied by a change in form, as is the 
 case in many of the lower organisms and in muscle-tissue, it is not difficult to 
 trace the process. The rate at which the excitation spreads through the irrita- 
 ble substance is very rapid, and special arrangements have to be employed to 
 follow it, but the change is not so swift that its course cannot be accurately 
 determined. It has been found that if a muscle-fibre be stimulated, as nor- 
 mally, by a nerve-fibre, the active condition produced at the point of stimula- 
 tion spreads along the muscle-fibre in both directions to its extremities ; if the 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 85 
 
 fibre be artificially irritated at either end, the exciting change runs the length 
 of the fibre, regardless of the direction, and stimulates every part of it to con- 
 traction. 
 
 (6) In Nerves. In the cases of nerves where excitation is accompanied by 
 no visible manifestation of activity, a definite answer to the question is not so 
 readily obtained. As long as a nerve is within the normal body, the activity 
 of the nerve-fibre can only be estimated from the response of the cell which the 
 nerve-fibre excites, and there is such an organ only at one extremity of the fibre. 
 
 Paul Bert made a well-known experiment, in which he tried to reverse a 
 sensory nerve in the living animal. He succeeded in bringing about union 
 of the end of the tail of a rat with the tissues of the back, and found, when 
 the union was complete, after the tail was cut off at its base, it was still capa- 
 ble of giving sensations of pain. The experiment failed to throw light on 
 the problem, however, for we now know that the peripheral part of the cut 
 nerve dies, and the conduction power manifested in this case was dependent 
 on new axis-cylinders which had grown down from the central nerve-stump 
 (see p. 79). 
 
 Efforts have been made to elucidate the problem by attempting to unite 
 the central part of a cut sensory nerve with the peripheral part of a divided 
 motor nerve, and observing, after the healing was complete, whether excita- 
 tion of the sensory nerve caused movements in the part supplied by the 
 motor nerve. Most of these experiments have given doubtful results, but 
 lately Budgett and Green l have succeeded where others have failed, and 
 have made cut sensory fibres grow down the degenerated trunk of a motor 
 nerve, and connect with muscle-fibres, so that the muscle contracted when 
 the peripheral end of the sensory fibres was stimulated. The impulse went 
 up the old sensory fibres, and then down the newly developed fibres in the 
 old motor trunk. Their method was to cut the left pneumogastric nerve 
 between the ganglion and the cranium, and to suture its peripheral cut end 
 to the peripheral cut end of the hypoglossal. All the fibres of the hypoglossal 
 and the efferent fibres of the pneumogastric must have degenerated, because 
 these fibres were separated from the bodies of the cells of which they were 
 branches. The sensory fibres of the pneumogastric, on the other hand, be- 
 cause still in connection with the nerve-cells of the ganglion, continued to 
 live, and the part connected with the peripheral stump of the cut hypoglossal 
 grew down this nerve and came into relation with the muscles of the tongue. 
 
 Two or three months after the operation the left pneumogastric was divided 
 just above the thorax, and the combined vago-hypoglossal nerve, together 
 with the tongue, was excised. When the peripheral end of the pneumogastric 
 was excited the muscles of the tongue were seen to contract. Mechanical as 
 well as electrical stimuli were effective, and there would seem to be no escape 
 from the conclusion that the sensory fibres of the pneumogastric had con- 
 ducted the impulse centripetally as far as the ganglion, and then centrifugally 
 down to the muscle of the tongue. 
 
 1 Budgett and Green: American Journal of Physiology, 1899, iii. p. 115. 
 
86 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 There is, however, an entirely different method of experimentation which 
 seems to prove that nerve-, like muscle-protoplasm, can conduct in both direc- 
 tions. This method is based on the fact that though nerve-fibres rarely branch 
 in the peripheral nerve-trunks on their way to an organ, they may divide very 
 freely after reaching it. Such branchings of fibres occur in muscle, and Kuehne 1 
 found that if one of these branches was stimulated, the irritation passed up 
 the branch to the nerve-fibre and then down the other branches to the muscle. 
 For example, he split the end of the sartorius muscle of a 
 frog by a longitudinal cut, and then found on exciting one 
 of the slips that the other contracted (see Fig. 32). Since 
 cross conduction between striated muscle-fibres does not 
 occur, no other explanation presents itself. Perhaps a still 
 more striking example is to be found in an experiment of 
 Babuchin 2 on the nerve of the electric organ of an electric 
 fish, the Malopterurus. The organ, consisting of many 
 thousand plates, is supplied by a single enormous nerve- 
 fibre which after entering the organ divides very freely so 
 
 FIG. 32. Kuehne's / / 
 
 preparation of sarto- as to supply every plate. In this case mechanical stimu- 
 rius to show double ] $ f th t T d f f th t branches of the 
 
 conduction in nerve. 
 
 nerve, sufficed to cause an electric discharge of the whole 
 organ. The irritation must have passed backward along the irritated branch 
 until the main trunk was reached and then in the usual direction down the 
 other branches to the electric plates. 
 
 Still another method is that which was employed by Du Bois-Reymond, 3 
 on the fibres of the spinal nerve-roots. When a nerve is excited to action it 
 undergoes a change in electrical condition, and this change progresses along 
 the fibre at the same rate and in same direction as the nerve-impulse. This 
 electrical change, though entirely different from the nerve-impulse itself, can 
 be taken as an indication of the direction of movement of the process of 
 conduction. Du Bois-Reymond found that if he stimulated the afferent fibres 
 of the posterior spinal nerve-roots of the sciatic nerve of the frog, a " nega- 
 tive variation current," as the current resulting from the change in the elec- 
 trical condition of the nerve is called, passed down the nerve in a direction 
 opposite to that which the normal impulse takes. Further, it was found 
 that if the sciatic nerve was excited, a negative variation current could be 
 detected in the anterior as well as the posterior roots. Normally the irritation 
 only passes up the posterior roots and down the anterior, for normally the 
 sensory fibres of the posterior roots are excited only at the peripheral end and 
 the motor fibres of the anterior roots only at the central end. The experiment 
 showed both sensory and motor fibres to be capable of conducting in both 
 directions. Normally a nerve is stimulated only at one end, and therefore 
 conducts in only one direction. 
 
 1 Archivfiir Anatomic und Physiologic, 1859, S. 595. 
 
 3 Ibid., 1877, S. 262. 
 
 3 Thierische Electricitdt, 1849, Bd. ii. S. 587. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 87 
 
 Rate of Conduction. The activity of the conduction process varies 
 greatly in different tissues. The nerves of warm-blooded animals conduct nioiv 
 rapidly than those of cold; in a given animal the nerve-fibres conduct more 
 rapidly than muscle-fibres ; striated muscle conducts more rapidly than smooth 
 muscle; and even within a single cell different portions may transmit the ex- 
 citing process at different rates ; thus the myoid substance of the contractile fibres 
 of one of the rhizopods conducts more rapidly than the less highly differen- 
 tiated protoplasm of the cell. In general, it may be said that, " the power to 
 conduct increases with increase of mobility and sensitiveness to external irri- 
 tants, a fact which reveals itself in the protozoa, by a comparison of the slowly 
 moving rhizopods with the lively flagellata and eiliata." 1 A study of different 
 classes of muscle-tissue supports this view. 
 
 (a) Rate of Conduction in Muscles. The conduction process is invisible, 
 hence we estimate its strength and rate by its effects. It is most readily fol- 
 lowed in such mechanisms as muscle, where the conducting medium itself 
 undergoes a change of form as the exciting influence passes along it. 
 
 Rate of Transmission of Wave of Contraction. If a muscle be excited to 
 action by an irritant applied to one end, it does not contract at once as a whole, 
 but the change of form starts at the point which is irritated and spreads thence 
 the length of the fibres. At the same time that the muscle shortens it 
 thickens, and under certain conditions the swelling of the muscle can be seen 
 to travel from the end which is excited to the further extremity. In the case 
 of normal, active, striated muscle, the rate at which the change of form spreads 
 over the muscle is far too rapid to be followed by the eye, and hence the 
 muscle appears to act as a whole. By suitable recording mechanisms, evidence 
 can be obtained of the rate at which the exciting influence and contraction pro- 
 cess pass along the fibre. Thus two levers can be so placed as to rest on the 
 two extremities of a muscle, at the same time that the free ends of the levers 
 touch a revolving cylinder, the surface of which is covered with paper black- 
 ened with lampblack. The writing-point of one lever must be directly under 
 the point of the other. If, when the cylinder is revolving, one end of the mus- 
 cle be stimulated, the record will 
 show that the lever resting on that 
 part is the first to move, making 
 it evident that that part of the mus- 
 cle begins to thicken first, and that 
 the contraction does not begin at 
 the further extremity of the mus- 
 cle until somewhat later. The re- 
 cord given in Figure 33 was ob- FIG. 33. Rate of conduction of the contraction pro- 
 
 tained in a similar experiment, but 
 
 One in which the Contraction of fork waves record T fo second (after Marey). 
 
 the muscle was registered by the 
 
 pince myographique and recording tambour of Marey (see Fig. 34). 
 1 Biedermann : Eleklrophysioloyie, 1895, Bd. i. S. 124. 
 
88 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 Bernstein l measured the rate at which the irritating process is transmitted 
 along the muscle by recording the latent period, the time elapsing between the 
 
 FIG. 84. Method of recording the rate of passage of the contraction- process along a muscle (after 
 Marey). The movements of the muscle are recorded by means of air-transmission. The pince myo- 
 graphique consists of two light bars, the upper of which acts as a lever, moving freely on an axis sup- 
 ported by the lower. When the free end of the upper bar is raised, the other end presses down on a 
 delicate rubber membrane which covers a little metal capsule,-which is carried on the corresponding 
 extremity of the lower bar. The capsule is in air-communication, by a stiff-walled rubber tube, with 
 another capsule which is similarly covered with rubber membrane. A light lever is connected with the 
 membrane of the second tambour, and records its movements on the surface of a revolving cylinder. 
 The muscle is placed between the free ends of the bars of the pince myographique, and, when the muscle 
 thickens in contraction, it raises one end of the lever, depresses the membrane at the other end, and 
 drives air into the recording tambour, and thus, by automatically raising the writing-point, records its 
 change in form on the cylinder. 
 
 moment of irritation and the beginning of the contraction (see p. 102). A 
 lever was so connected with one end of the muscle as to record the instant that 
 it began to thicken. The muscle was stimulated in one experiment at the end 
 from which the record of its contraction was taken, and in another immediately 
 following experiment it was stimulated near the other end. The distance 
 between the stimulated points being known, the rate of transmission was 
 reckoned from the difference in the latent periods. In his experiments he 
 found the rate of conduction in the semimernbranosus of the frog to be from 
 3.2 to 4.4 meters per second. Hermann found the rate to be 2.7 meters for 
 the curarized sartorius of the frog. The results obtained by Abey and some 
 others are a little lower, but probably 3 meters per second can be accepted as 
 the average normal rate for frog's muscle. 
 
 Length of Wave. By such experiments it becomes obvious that the con- 
 traction process passes over the muscle, in the form of a wave. In an experi- 
 ment, such as Bernstein's, in which the thickening of the muscle is recorded, 
 we can determine from the length of the curve written by the contracting 
 muscle how long the contraction remains at a given place. Knowing this, 
 and the rate at which the process passes along the fibre, we can estimate the 
 length of the contraction wave, just as we could reckon the length of a train 
 
 1 Untersuchungen ilber die elektrische Erregung von Muskeln und Nerven, 1871, S. 79. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 89 
 
 of cars if we knew how fast it was moving and how long it required to pass 
 a given station. Thus, if the contraction is found to last at a given point 
 on the muscle 0.1 second, and the rate at which the contraction process is 
 travelling is 3000 millimeters per second, the length of the wave is 300 milli- 
 meters. According to Bernstein's determinations, the length of the wave of 
 contraction in a frog's striated muscle is from 198-380 millimeters. The 
 length of a striated muscle-fibre is, at the most, scarcely more than 40 milli- 
 meters, and normally the muscle-fibre is stimulated, not as in the above ex- 
 periment at one end, but near its centre, at the point where the nerve joins 
 it ; the irritation process spreads along the fibre in both directions from this 
 point, and would pass over the distance 20 millimeters so quickly that practi- 
 cally the whole muscle-fibre would be in the same phase of contraction at the 
 same time. 
 
 Rate of Conduction in Different Kinds of Muscle. The rate of conduction 
 varies very considerably in the muscles of different animals, and in different 
 kinds of muscle in the same animal, just as the contraction process itself dif- 
 fers in its rate and strength. 
 
 Meters per second. 
 
 Smooth muscle-fibres of the ureters of the rabbit . . . 0.02-0.03 (Engelrhann). 
 
 Muscle of the heart- ventricle of the frog 0.1 (Waller). 
 
 Contractile substance of medusae 0.5 (Waller). 
 
 Neck-muscles of the turtle 0.1 -0.5 (Hermann and Abey). 
 
 Gracilis and semimembranosus of the frog ..... 3.2 -4.4 (Bernstein). 
 
 Cruralis (red muscle) of the rabbit 3.4 (Kollet). 
 
 Sterno-mastoid of the dog 3. -6 (Bernstein and Steiner). 
 
 Semimembranosus (white muscle) of the rabbit ... 5.4 -11.4 (Rollet). 
 
 Human muscle 10. -13 (Hermann). 
 
 (6) Rate of Conduction in Nerves. Conductivity is most highly developed 
 in the case of the nerve-fibre. The distances through which it acts and the 
 rapidity of the process excite our wonder. The process is accompanied by no 
 visible change in the nerve-fibre itself, and the strength and rate have to be 
 estimated by the effect produced on the organ which the nerve excites to action, 
 or by the change which takes place in the electrical condition of the nerve as 
 the wave of excitation sweeps over it. 
 
 Rate in Motor Nerves. Helmholtz was the first to measure the rate of con- 
 duction in nerves. 1 Originally he employed Pouillet's method for measuring 
 short intervals of time. The arrangement is illustrated in Figure 35. The 
 moment that a current was thrown into the coils of a galvanometer (see p. 
 145) the current in the primary coil of an induction apparatus was broken 
 and the nerve connected with the secondary coil received a shock. An instant 
 after, the contraction of the muscle which resulted from the stimulation of the 
 nerve broke the galvanometer circuit. The amount of deviation of the magnet 
 of the galvanometer varied with the time that the circuit remained closed, and 
 therefore could be taken as a measure of the interval elapsing between the 
 stimulation of the nerve and the contraction of the muscle. The nerve was 
 1 Helmholtz: Archivfiir Anatomic und Physiologic, 1850, S. 71-276; 1852, S. 199. 
 
90 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 excited in two succeeding experiments at two points, at a known distance apart, 
 and the difference in the time records obtained was the time required for the 
 transmission of the nerve-impulse through this distance. 
 
 V V\/\y^rv/\> 
 
 FIG. 35. Method of estimating rate of conduction in motor nerve of frog, as used ~by Helmholtz. The 
 horizontal bar a-b is supported on an axis in such a mariner that when the contact is made at a it is 
 broken at 6, therefore at the same instant a current is made in the galvanometer circuit g and opened in. 
 the primary circuit of the induction apparatus p. When the muscle contracts, the galvanometer circuit 
 is broken at c. The nerve was stimulated in two successive experiments at d and e. 
 
 Later, Helmholtz devised a method by which a muscle would record its 
 contractions on a rapidly moving surface, and employed this to measure the 
 rate of conduction in motor nerves. He stimulated the nerve as near as 
 possible to the muscle and let the contraction be recorded ; then he stimulated 
 the nerve as far as possible from the muscle, and again had the contraction 
 recorded. The difference in time between the moment of excitation and the 
 beginning of the contraction in the two experiments was due to the difference 
 in the distance that the nerve-impulse had to pass in the two cases, and, this 
 distance being known, the rate of conduction could be readily calculated. 
 By this means he found the rate of transmission in the motor nerves of the 
 frog to be 27 meters per second. In similar experiments upon men he 
 recorded the contractions of the muscles of the ball of the thumb, and noted 
 the difference in the time of the beginning of the contractions when the 
 median nerve was excited through the skin at two different places. He 
 found the average normal rate for man to be about 34 meters per second, a 
 rate which is considerably quicker than that of our fastest express trains, 
 but a million times less than the rate at which an electric current is trans- 
 mitted along a wire. These determinations are still accepted as approxi- 
 mately correct for human nerves, although they are found to vary very con- 
 siderably under different conditions, a high temperature and strong irritation 
 quickening the rate to 90 or more meters per second, while cooling may 
 gradually slow the rate and finally stop conduction. Moreover, considerable 
 differences exist in nerves controlling different functions, even in the same 
 animal. Thus Chauveau gives the rate for the fibres of the vagus nerve, 
 which supply the rapidly contracting striated muscles of the larynx, as 66.7 
 meters per second ; and the rate for vagus fibres, controlling the slower 
 smooth muscles of the oesophagus, as 8.2 meters per second. The rate of 
 transmission in the non-medullated nerves of invertebrates appears to be still 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 91 
 
 slower ; the nerve for the claw-muscles of the lobster conducts at a rate of 
 from 6 to 12 meters per second, according as the temperature is low or high 
 (Fredericq and Vandervelde). The rate in non-medullated nerves of the 
 Cephalopodia is 3.5-5.5 meters per second (Boruttau). 
 
 Contrary to the view frequently expressed (Pfliiger 1 and others), all parts 
 of the nerve have the same rate of conduction. 2 
 
 Rate in Sensory Nerves. We have no definite knowledge of the rate of 
 conduction in sensory nerves. The attempt has been made to measure it by 
 stimulating the sensory fibres of a nerve-trunk at two different points and 
 noting the difference in the time of the beginning of the resulting reflex acts ; 
 or, in experiments on men, the difference in the length of the reaction time 
 has been taken as an indication. By reaction time is meant the interval 
 which elapses between the moment that the irritant is applied and the signal 
 which is made by the subject as soon as he feels the sensation. Oehl found 
 the mean normal rate of conduction in the sensory nerves of men to be 36.6 
 meters per second. 3 Dolley and Cattell, 4 by employing the reaction-time 
 method, found the rate for the sensory fibres of the median nerve of one of 
 them to be 21.1 meters per second, and for the other 49.5 meters per second, 
 while the posterior tibial nerve gave rates, for one of them 31.2 meters, and 
 for the other 64.9 meters. They attribute these wide variations in part to 
 differences in the effectiveness of the irritant at different parts of the skin, 
 but chiefly to differences in the activity of the central nervous processes 
 involved in the act. 
 
 Schelske 5 observed similar differences in different men for one 25.3 
 meters, for another 32.6 meters, and for still another 31.05 meters per 
 second. 
 
 In spite of the great difficulty of getting definite measurements in experi- 
 ments on men, we may conclude from the work of these and other observers 
 that the rate of conduction in sensory fibres is about the same as in motor 
 fibres ; in the case of man about 35 meters per second. 
 
 Another method applicable to isolated nerves is based on the fact that the 
 passage of the nerve-impulse along a nerve is accompanied by a change in 
 its electrical condition. The rate of conduction can be ascertained by finding 
 the rate at which this electrical change is transmitted. 
 
 Influences which Alter the Rate and Strength of the Conduction-proc- 
 ess. (a) Effect of Death-processes. Normally, the rate of conduction in mus- 
 cle-fibres is so rapid that the whole muscle appears to contract at the same time ; 
 but there are certain conditions under which the transmission of the exciting 
 influence is very much slowed, or even altogether prevented, so that the stimu- 
 lation of a given part of the muscle results in a local swelling, or welt, limited 
 
 1 Pfliiger: Untersuchungen uber die Physiologic des Electrotonus, Berlin, 1859, S. 465. 
 
 2 R. du Bois-Reymond : CeniraJblatt fiir Physiologic, 1899, Bd. xiii. S. 513. 
 
 3 Oehl : Archives italiennes de Biologic, 1895, xxi. 3, p. 401. 
 
 4 Dolle3 r and Cattell : Psychological Review, New York and London, 1894, i. p. 159. 
 
 5 Schelske: Archiv fur Anatomic und Physiologic, 1864, S. 151. 
 
92 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 to the excited area. When a muscle is dying, the rate of conduction as well 
 as the rapidity of contraction is lessened. The muscles of warm-blooded ani- 
 mals exhibit more striking differences than those of cold-blooded, but both are 
 similarly affected. If a dying muscle be mechanically stimulated, as by a direct 
 blow, a localized swelling develops at the place ; and if the muscle be stroked 
 with a dull instrument, a wave of contraction may be seen to follow the instru- 
 ment, the contraction being quite strictly limited to the excited area, so that 
 one can write on the muscle. The strict localization of the contraction to the. 
 irritated parts makes it evident that the nerves take no part in it, hence Schiff 
 called it an idio-muscular contraction, in distinction from the normal neuro- 
 muscular contraction. In the dying nerve as in the dying muscle the rate of 
 transmission is found to be slowed. 
 
 (6) Effect of Mechanical Conditions. The effect of pressure to lessen the 
 conduction-power of nerves is one which everyone has had occasion to demon- 
 strate on himself. For example, if pressure be brought to bear on the ulnar 
 nerve where it crosses the elbow, the region supplied by the nerve becomes numb, 
 "goes to sleep," so to speak. It is noticeable that only a slightly greater 
 effort is required to move the muscles, at a time when no sensations are received 
 from the hand. For some unexplained reason the sensory nerve-fibres appear 
 to be less resistant than the motor. Gradually applied pressure may paralyze 
 the nerve without exciting it, but on the removal of the pressure the recovery 
 of function of the sensory fibres is accompanied by excitation processes, which 
 are felt as pricking sensations referred to the region supplied by the nerve. The 
 exact reason of the loss of functional power caused by pressure which is insuf- 
 ficient to produce permanent injury is not altogether clear. Stretching a nerve 
 may act to lessen, and if severe destroy, conductivity. It is in one sense another 
 way of applying pressure, since the calibre of the sheath is lessened and through 
 the fluids pressure is brought to bear on the axis-cylinder. Of course, if the 
 stretching were excessive, the nerve-fibres would be ruptured and degenerate. 
 
 Whether stretching can alter the rate of conduction in nerves is not known. 
 Apparently it does not do so in muscles, although because of the greater 
 length of the muscle it appears to do so. 1 
 
 (c) Effect of Temperature on Conduction. Helmholtz and Baxt found that 
 by cooling motor nerves they could lower the rate of conduction, and by heat- 
 ing them they could increase it even more markedly. By altering the tem- 
 perature of the motor nerves of man, they observed rates varying from 30 to 
 90 meters per second. The rate of the motor nerves of other animals is like- 
 wise greatly altered by heat and cold. This is true of the invertebrates as well 
 as the vertebrates ; the rate in the nerves of the claw-muscles of the lobster, 
 for example, changes from 6 to 12 meters per second as the temperature is 
 varied from 10 to 20 C. Sensory nerve-fibres are similarly influenced by 
 temperature. Oehl 2 found by cooling and heating the nerves of men, varia- 
 tions of from 30 to 25 meters per second on cooling, and from 30 to 50 meters 
 
 ^chenck : Pfliiger's Archiv, 1896, Bd. 64, S. 179. 
 
 3 Oehl: Archives italiennes de Biologie, 1895, xxiv. p. 231. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 93 
 
 a second on heating. Both the sympathetic and the vagus nerve-fibres in the 
 frog have their influence on the heart-beat decreased by cold and increased by 
 heat. 1 The favorable influence of heat on the conduction power seems common 
 to all nerves, but only within certain limits. The motor fibres of the sciatic of 
 the frog lose their power to conduct at 41 to 44 C., but may recover the power 
 if quickly cooled ; if the temperature has reached 50 C. conductivity is per- 
 manently lost. 
 
 Nerves of like function in different animals may lose the power of conduc- 
 tion at different temperatures. Thus the motor fibres of the sciatic nerve of 
 the dog cease to conduct at 6 C., those of the cat at 5 to 3 C., of the frog at 
 about C. The inhibitory fibres of the vagus nerve of the dog show dimin- 
 ished activity at 3 C., and become wholly inactive at C. ; the inhibitory 
 fibres of the vagus of the rabbit become inactive at 15 C. 
 
 Different kinds of fibres in the same nerve-trunk may be differently affected 
 by temperature, and this difference may be sufficiently marked in some cases to 
 be used as a means of distinguishing them. 2 For example, the temperature 
 limits at which the vaso-constrictor fibres of the sciatic of the cat can conduct 
 are 2-3 C. to 47 C., while the limits for the dilator fibres are both lower 
 , and higher than for the constrictors. If cold be applied to the sciatic nerve, 
 the fibres supplying the extensor muscles seem to fail before those which in- 
 nervate the flexors. 
 
 Further, it has been observed that if cold be applied locally to a nerve, the 
 part affected loses its power to conduct, and acts as a block to the passage of 
 the nerve-impulse generated in another part of the nerve. Application of 
 extreme cold to the ulnar nerve of man at the elbow results in a complete 
 loss of feeling in the parts which the nerve supplies. 3 On the other hand, 
 the strength of an impulse is increased by passage through a region which has 
 been warmed. These facts remind us of the effect of heat and cold on the 
 activity of other forms of protoplasm and would find a comparatively easy 
 explanation were we content to look upon conduction as the result of chemical 
 change in the axis-cylinder. The fact that conduction does not cause fatigue 
 is opposed to such an explanation, and so we take refuge in the idea that heat 
 is favorable and cold unfavorable to molecular activity in general. 
 
 (d) Effect of Chemicals and Drugs. The conductivity, like the irritability, 
 of nerve and muscle is greatly influenced by anything which alters the chemical 
 constitution of active substance. In general, influences which increase or 
 decrease the one have a similar effect upon the other. There are important 
 exceptions to the rule, however. The direct application of alcohol, ether, 
 etc., to the nerve may destroy the conductivity without greatly lessening the 
 irritability, while carbon dioxide 4 or cocain will destroy the irritability very 
 much sooner than the conductivity. Such observations suggest that con- 
 
 1 Stewart : Journal of Physiology, 1891, vol. xii. No. 3, p. 22. 
 
 'Howell, Budgett, and Leonard : Journal of Physiology, vol. xvi. Nos. 3 and 4, 1894. 
 3 Weir Mitchell: Injuries of Nerves and their Consequences, Philadelphia, 1872, p. 59. 
 *Griinhagen : Pjliiger's Archiv, 1872, vi. S. 180. 
 
94 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 ductivity is dependent on other properties of the nerve than irritability, and 
 there are some other facts pointing in the same direction ; for example, 
 regenerating nerves acquire the power to conduct before they recover their 
 irritability. The usual explanation of those who regard conduction as due 
 to the excitation of each succeeding part of the nerve by the one just pre- 
 ceding it is, that external excitation is a coarse affair as compared with the 
 normal internal excitation process, and the effect of the former may be lost 
 when the latter is still effective. 
 
 (e) Effect of a Constant Battery Current. A constant electric current, if 
 allowed to flow through a nerve or muscle, not only alters the irritability, but 
 also the conductivity. The change in the conductivity affects both the 
 strength and rate of the conduction process. Von Bezold 1 found that weak and 
 medium currents have little effect on the conductivity, but that strong currents 
 completely destroy the power of the nerve to transmit the nerve-impulse. As 
 the strength of the current is increased, the first effect is observed at the anode, 
 and shows itself in a slowing of the passage of the exciting impulse. This 
 action is the greater the more of the nerve exposed to the current, the stronger 
 the current, and the longer it is closed. The loss of conduction power is asso- 
 ciated with changes at the place where the current enters and where it leaves 
 the nerve rather than with alterations within the intrapolar region. Engelmann, 
 in his experiments on the smooth muscle-fibres of the ureter, saw a decline of 
 power of conduction at the anode by weak currents, which when the strength 
 of the current was increased appeared also at the kathode ; the conductivity 
 was wholly lost at both poles when the current was very strong. In the case 
 of a striated muscle, such as tne sartorius of the frog, the kathode has been 
 found to become impassable after strong currents have flowed through a muscle 
 for a considerable time. The same is true of nerves. 
 
 It is not surprising that a current which can greatly decrease the irritability 
 at the anode, and even inhibit a contraction which may be present when it is 
 applied, should be found to decrease the conductivity as well, but that the con- 
 ductivity should be decreased at the kathode, where the irritability is greatly 
 increased, was not to be expected. Rutherford 2 found that with weak currents 
 the rate of the conduction power at the kathode was increased rather than 
 diminished, and that it was only when strong currents acted a considerable 
 time that the conduction power lessened at the kathode. Biedermann explains 
 this on the ground that the increased excitability at the kathode leads in the 
 muscle to a condition of latent contraction and therefore to fatigue, and that 
 it is this which lessens the conductivity. The lessened power to conduct con- 
 tinues at the kathode after the removal of the current. There is little doubt 
 that fatigue interferes with the conduction power of muscle, but this explana- 
 tion would hardly apply to nerves which are not known to fatigue at the point 
 of stimulation, i. e. if we limit the term fatigue to changes resulting from 
 physiological activity. Undoubtedly chemical and physical alterations may 
 
 1 Untersuchungen iiber die ekktrische Erregung der Nerven und Muskeln, Leipzig, 1861. 
 
 2 Journal of Anatomy and Physiology, 1867, vol. 2, p. 87, 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 95 
 
 occur in nerves as a result of the passage of an electric current through them, 
 and it would seem as if the loss of conductivity which they show when sub- 
 jected to strong currents is to be accounted for by such electrolytic changes. 
 
 The changes produced in the conductivity of nerves by strong currents 
 explain the failure of the closing of the ascending current and opening of the 
 descending current to irritate the muscle (see Pfluger's law, p. 50). In the 
 former case the anode region of decreased conductivity intervenes between the 
 kathode, where the closing stimulus is developed, and the muscle. In the 
 latter case the irritation developed at the anode, on the opening of the current, 
 is unable to pass the region of decreased conductivity which is formed at the 
 kathode, and which persists after the current is opened. 
 
 Practical Application of Alterations produced by Battery Currents. The 
 alterations produced by strong battery currents in the irritability and conduc- 
 tivity of nerves and muscles may be made use of by the physician. If the 
 effect of only one pole is desired, it may be applied as a small electrode im- 
 mediately over the region to be influenced, while the other pole may be a large 
 electrode placed over some distant part of the body where there are no import- 
 ant organs. The size of the electrodes used determines the density of the 
 current leaving or entering the body and consequently the intensity of its 
 action. The application of the anode to a region of increased excitability, by 
 decreasing the irritability, may for the time lessen irritation; on the other 
 hand the kathode may heighten the irritability of a region of decreased 
 excitability. The sending of a strong polarizing current through a motor 
 nerve, by lessening the conductivity, may prevent abnormal motor impulses 
 from reaching muscles, and so stop harmful r " cramps ;" or the sending of 
 such a current through a sensory nerve may, during the flow of the cur- 
 rent, keep painful impulses from reaching the central nervous system. In 
 applying a strong battery current to lessen irritability or conductivity it 
 must be remembered that the after-effect of such a current is increased 
 irritability. 
 
 (/) Effect of Conduction and Fatigue of Nerves. Many experiments have 
 been made in the hope of detecting some form of chemical change as a result 
 of conduction. The nerve has been stimulated for many hours in succession 
 with an electric current, and then been examined with the utmost care to 
 find whether there had been an accumulation of some waste product, as 
 carbon dioxide, or some other acid body. The gray matter of the spinal 
 cord, which is largely composed of nerve-cells, is found to become acid as a 
 result of activity, 1 but this cannot be found to be the case with the white 
 matter of the cord, which is chiefly made up of nerve-fibres, nor has an acid 
 reaction been obtained with certainty in nerve-trunks. 2 
 
 1 Funke: Archivfur Anatomic und Physioloyie, 1859, S. 835. Ranker Centralblatt filr medicin- 
 ische Wissenschaft, 1868 and 1869. 
 
 2 Heidenhain : Studien aus dem physiologischen Inslitut zu Breslau, ix. S. 248 ; Cenlralblatt fur 
 Medicin, 1868, S. 833. Tigerstedt: " Studien iiber mechanische Nervenreizung," Acta Societatis 
 Scientiarum Fennicce, 1880, torn. xi. 
 
96 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 Not only has an attempt to discover this or other waste products which 
 might be supposed to result from chemical changes within the nerve-fibre 
 failed, but observers have been unable to obtain evidence of the liberation 
 of heat, which one would expect to find were the nerve-fibre the seat of chem- 
 ical changes during the process of conduction. 1 Stewart writes : " Speaking 
 quite roughly, I think we may say that in the nerves of rabbits and dogs there 
 is not even a rise of temperature of the general nerve-sheath of ^oW of a 
 degree during excitation. 7 ' 
 
 Many experiments have been made to ascertain whether a nerve would 
 fatigue if made to conduct for a long time. Most of these have been made 
 upon motor nerves, the amount of contraction of the muscle, in response to a 
 definite stimulus applied to the nerve, being taken as an index of the activity 
 of the nerve. Since the muscle would fatigue if stimulated continuously for 
 a long time, various means have been employed to block the nerve-impulse 
 and prevent it from reaching the muscle, except at the beginning and end of 
 the experiment. This block has been established by passing a continuous 
 current through the nerve near the muscle, thus inducing an electrotonic 
 change and non-conducting area ; 2 or the nerve-ends were poisoned with 
 curare (see p. 26), and the nerve excited until the effect of the drug wore off, 
 and the nerve-impulse was able to reach the muscle ; 3 or the part of the nerve 
 near the muscle was temporarily deprived of its conducting power by an 
 anesthetic, such as ether. Another method of experimentation consisted in 
 using the negative variation current of a nerve (see p. 150) as an indication 
 of its activity, the presence of the current being observed with the galvanom- 
 eter. 4 Other experimenters have examined the vagus nerve, to see if after 
 long-continued stimulation it was still capable of inhibiting the hearf, the 
 effect of the stimulation being prevented from acting on the heart muscle 
 during the experiment by atropin, 5 or by cold, applied locally to the nerve. 6 
 Still another method was to study the effect of long-continued stimulation on 
 the secretory fibres of the chorda tympani, the exciting impulse being kept 
 from the gland-cells by atropin. 7 Most of these experiments have yielded nega- 
 tive results, and it is doubtful whether nerves are fatigued by the process of 
 conduction. 
 
 These results, of course, do not show that the nerve-fibres can live and 
 function independently of chemical changes. As has been said, nerves lose their 
 irritability in time if deprived of the normal blood-supply, and undoubtedly 
 they are, like all protoplasmic structures, continually the seat of metabolic 
 
 1 Helmholtz : Archiv fur Anatomie und Physiologic, 1848, S. 158. Heidenhain : op. cit. 
 Rolleston: Journal of Physiology, 1890, vol. xi. p. 208. Stewart: ibid., 1891, vol. xii. p. 424. 
 
 * Bernstein : Pftilger's Archiv, 1877, xv. S. 289. Wedenski : Centralblatt fur die medicinischen 
 Wissenschaften, 1884. 
 
 * Bowditch : Journal of Physiology, 1885, vi. p. 133. 
 
 4 Wedenski : loc. cit. Maschek : Siteungsberichte der Wiener Academic, 1887, Bd. xcv. Abthl. 3. 
 
 5 Szana: Archiv fur Anatomie und Physiologic, 1891, S. 315. 
 
 6 Ho well, Budgett, and Leonard: Journal of Physiology, 1894, xvi. p. 312. 
 
 7 Lambert: Comptes-rendus de la Societe de Biologic, 1894, p. 511. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 97 
 
 processes. The normal function of the nerve, however, the conduction of the 
 nerve-impulse, seems to take place without any marked chemical change. 
 
 Nature of the Conduction Process. There have been a great many 
 views as to the nature of the conduction process, one after the other being 
 advanced and combated as physiological facts bearing on the question have 
 been accumulated. It has been suggested that the whole nerve moved like a 
 bell-rope ; that the nerve was a tube, and that a biting acid flowed along it ; 
 that the nerve contained an elastic fluid which was thrown into oscillations ; 
 that it conducted an electric current, like a wire ; that it was composed of 
 definitely arranged electro-motor molecules which exerted an electro-dynamic 
 effect on each other ; that it was made up of chemical particles, which like 
 the particles of powder in a fuse, underwent an explosive change, each in 
 turn exciting its neighbor ; that the irritant caused a chemical change, which 
 produced an alteration of the electrical condition of such a nature as to excite 
 neighboring parts to chemical change and thereby to electrical change, and 
 so alternating chemical and electrical changes progressed along the fibre in the 
 fo^m of a wave ; finally, that the molecules of the nerve-substance underwent 
 a form of physical vibration analogous to that assumed for light. 
 
 A discussion of these different theories, none of which can be regarded 
 as entirely satisfactory, cannot be entered upon here. 
 
 Although the exact nature of the conduction process is not determined, 
 there seems little doubt that it is of the same type in all forms of protoplasm. 
 In all cases it is a property of the living substance of the cell and is lost 
 when the cell dies : the state of activity spreads like a wave in all directions 
 through the living substance, and is markedly altered by physical and chem- 
 ical influences which change the irritability of the living substance, and in 
 much the same way as this is altered ; continuity of protoplasm is absolutely 
 essential to conduction, hence the spread of the excitation change is limited 
 to the one cell, unless the cell is connected by protoplasmic bridges with 
 other cells, or possesses a specially differentiated exciting end-organ. 
 
 In its details the conduction process exhibits many peculiarities in differ- 
 ent cells and even in the different parts of the same cell. The receiving 
 organs at the extremities of the dendrites of different classes of neurones 
 differ widely in respect to structure, and in their capacity to react to different 
 kinds of stimuli and to transmit the state of excitation to the dendrite. The 
 exciting organs at the extremities of the axones of different classes of neurones 
 are of different types, and behave differently, the discharge of the exciting 
 process upon a muscle, gland, or nerve-cell being adjusted to the capacity for 
 reaction possessed by the organ in question. In each neurone the strands of 
 protoplasm which connect these distant receiving and exciting mechanisms 
 with the cell body, and the body of the cell itself, work each according to its 
 own nature. For example, the time spent by the phase of activity in the 
 body of a ganglion-cell of the posterior spinal root-ganglion, is far longer 
 than that used in a corresponding length of protoplasm in the dendrite of the 
 cell. Although the conduction process differs in its details even in different 
 
 VOL. II. 7 
 
98 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 parts of the same neurone, the condition of activity which spreads through the 
 neurone and which we call the nerve impulse, has the same general character- 
 istics in all forms of nerves whether medullated or non-medullated, motor, 
 sensory, or secretory. The character of a movement or secretion depends on 
 the character of the organ excited, and not on the nature of the change trans- 
 mitted along the efferent nerve, and the specific character of a special sensa- 
 tion depends on the form of psychic activity developed in the central nervous 
 system, and not on the nature of the process of transmission in the afferent 
 neurone. This view that the nerve impulse is to be regarded merely as an 
 excitatory process, and that it has the same general characteristics in all 
 kinds of nerves, is strengthened by two sets of experiments which have been 
 reported lately. 
 
 One of these sets of experiments was reported by Langley. 1 He found 
 that preganglionic sympathetic fibres i. e. fibres between the ganglion and 
 the cord if cut centrally from the ganglion, after a time regenerate and make 
 new connections with the nerve-cells of the ganglion. In some cases they 
 unite with cells of their own class, and sometimes with other cells; for 
 example, pupillo-dilator fibres were found to have established connection 
 with pilo-motor neurones i. e. with ganglion-cells which send their axones to 
 the erector muscles of the hairs. Further, by section of post-ganglionic 
 fibres i. e. fibres between the ganglion and the periphery it was found, after 
 regeneration had occurred, that pilo-motor fibres can form nerve-endings in 
 the iris and become pupillo-dilator fibres. Evidently ganglion-cells and 
 muscle-fibres can be excited by nerve impulses developed in other nerves 
 than those normally connected with them. 
 
 A still more remarkable result was obtained by Budgett and Green. 
 A description of this experiment is given on page 85. They succeeded in 
 causing sensory fibres of the pneumogastric to grow down a degenerated 
 motor trunk, the hypoglossal, and connect with the muscles of the tongue. 
 In this case excitation of the peripheral part of the afferent nerve caused 
 muscular contractions. If we should think of the nerve which was excited, 
 we would be inclined to say that a sensory impulse was generated ; if we 
 should think of the effect on the muscle, we would call it a motor impulse, 
 and the latter would be the proper term. Evidently the condition of 
 activity which can be aroused in a sensory nerve is capable under suitable 
 conditions of exciting muscles, and sensory nerves cannot be considered to 
 be the seat of specific forms of energy different from those generated in 
 motor nerves. 
 
 D. CONTRACTILITY. 
 
 Contractility is the property of protoplasm by virtue of which the cell is 
 
 able to change its form when subjected to certain external influences called 
 
 irritants, or when excited by certain changes occurring within itself. The 
 
 change of form does not involve a change of size. The contraction is the 
 
 1 Langley : Journal of Physiology, 1897, xxii. p. 215. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 99 
 
 result of a change in the position of the more fluid parts of the cell-protoplasm, 
 and the effect is to cause the cell to approach a spherical shape. In the case of 
 an amoeba, for instance, excitation causes a drawing in of the pseudopods, and 
 as the material in them flows back into the cell the body of the cell expands 
 and acquires a globular form. In the simpler forms of contractile protoplasm 
 the movement does not appear to be limited to any special direction, but in the 
 case of the highly differentiated forms, such as muscle, both contraction and 
 relaxation occur on definite lines. 
 
 When a muscle is excited to action, energy is liberated through chemical 
 change of certain constituents of the muscle-substance, and this energy in some 
 unknown way causes a rearrangement of the finest particles of the muscle-sub- 
 stance, and the consequent change of form peculiar to the contracted state. 
 When the 'irritation ceases and relaxation takes place, there is a sudden return 
 of the muscle-substance to the position of rest, either because of elastic recoil or 
 of some other force at work within the muscle itself. That the recovery of 
 the elongated form peculiar to the resting muscle is not dependent on external 
 influences is evidenced by the fact that a muscle floating on mercury, and 
 subjected to no extending force, will on the cessation of irritation assume its 
 resting form. The relaxation no less than the contraction must be regarded 
 as an active process, but on account of their flexibility muscle-fibres are incap- 
 able of exerting an expansion force, therefore cannot by relaxing do external 
 work. 
 
 Both the histological structure and physiological action of the striated mus- 
 cles which move the bones show them to be the most highly differentiated, the 
 most perfect form of contractile tissue. It is by means of these structures that 
 the higher animals perform all those voluntary movements by which they change 
 their position with reference to external objects, acquire nourishment, protect 
 themselves, and influence their surroundings. An exact knowledge of the 
 method of action of these mechanisms and the influences which affect them is 
 therefore of the greatest importance to us. 
 
 1. Simple Muscle -Contractions Studied by the Graphic Method. 
 When a muscle makes a single contraction, in response to an electric shock or 
 other irritant, the change of form is too rapid to be followed by the eye. To 
 acquire an adequate idea of the character of the movement it is necessary that 
 we should obtain a continuous record of the alterations in shape which it un- 
 dergoes. This can be done by connecting the muscle with a mechanism which 
 enables it automatically to record its movements. 
 
 If one moves a pencil vertically up and^down on a piece of paper, a straight 
 line is written ; if while the vertical movements are continued the paper be 
 drawn along at a regular rate in a direction at right angles to the move- 
 ment of the pencil, a curve will be traced. If the paper be moved at a regular 
 rate, the shape of the curve will depend on the rate at which the pencil is 
 moved, and, if the speed of the paper be known, the rate of movement of the 
 pencil can be readily determined. This principle is employed in recording the 
 movements of muscles. The muscle is connected with a mechanism which 
 
100 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 rises and falls as the muscle contracts and relaxes, and records the movements 
 of the muscle on a surface which passes by the writing-point at a regular 
 speed (see Fig. 38) ; such a record is called a myogram. 
 
 The Myograph. The writing mechanism, together with the apparatus 
 which moves the surface on which the record of the movement of a contracting 
 muscle is taken is called a myograph. The writing mechanism has usually the 
 form of a light, stiff lever, which moves very easily on a delicate axis ; the 
 lever is so connected with the muscle as to magnify its movements. The point 
 of *he lever rests very lightly against a glass plate, or surface covered with 
 glazed paper, which is coated with a thin layer of soot. The point of the lever 
 scratches off the soot, and the movements are recorded as 
 a very fine white line. At the close of the experiment 
 the record is made permanent by passing it through a 
 thin alcoholic solution of shellac. The recording surface 
 in some cases is in the form of a plate, in others of a cyl- 
 inder, and is moved at a regular rate by a spring, pendu- 
 lum, falling weight, clockwork, electric or other motor. 1 
 The record which is traced with the myograph lever 
 by the muscle has the form of a curve. From the height 
 of the curve we can readily estimate the amount that 
 the muscle changes its length, but in order to accu- 
 rately determine the duration of the contraction process 
 and the time relations of different parts of the curve, 
 it is necessary to know the exact rate at which the 
 recording surface is moving. The shape of the curve 
 drawn by the muscle will depend very largely on the 
 rate of the movement of the surface on which the record 
 is taken. This is illustrated by the four records repro- 
 duced in Figure 36. These were all taken from the 
 same muscle within a few minutes of each other and 
 under exactly the same conditions, except that in the 
 successive experiments the speed of the drum on which 
 the record was traced was increased. 
 
 A glance at these records shows that a knowledge 
 of the rate of movement of the surface on which the record is taken is indis- 
 pensable to an understanding of the time relations of the different parts of the 
 curve written by the muscle. The rate of movement of the recording surface 
 can be registered by an instrument called a chronograph. 
 
 The chronograph (g, Fig. 37), consists of one or two coils of wire wound 
 round cores of soft iron, and a little lever bearing a strip of iron, which is 
 attracted to the soft-iron cores whenever they are magnetized by an elec- 
 tric current flowing through the coils of wire about them. When the current 
 ceases to flow and the iron ceases to be magnetized, a spring draws the lever 
 
 1 See O. Langendorff: Physiologische Graphik, Franz Deuticke, Leipzig, 1891 ; J. S. 
 Brodie : The Essentials of Experimental Physiology, London, 1898. 
 
 FIG. 36. Records of four 
 contractions of a gas- 
 trocnemius muscle of a 
 frog: a, recording sur- 
 face at rest; 6, surface 
 moving slowly ; c, sur- 
 face moving more rapidly ; 
 d, surface moving even 
 faster. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NP1RVE. 101 
 
 away from the iron. Many of the instruments employed for this purpose are 
 very delicate, and are capable of responding to very rapid interruptions of the 
 
 FIG. 37. Method of interrupting an electric circuit by a tuning-fork, and of recording the intci . up- 
 tions by means of an electro-magnet: a, battery; 6, tuning-fork, with platinum wire at the extremity 
 of one of its arms, which with each vibration of the fork makes and breaks contact with the mercury 
 in the cup below ; c, mercury cup ; e, electro-magnet which keeps the fork vibrating ; g, chronograph. 
 The current from the battery a, passes to the fork 6, then, by way of the platinum wire, to the mercury 
 in cup c, then to the binding-post d, where it divides, a part going through the coils of wire of the 
 chronograph g, and thence to the binding-post /, the rest through the coil of wire of electro-magnet 
 e, and then to the post/, from which the united threads of current flow back to the battery. The 
 electro-magnet e keeps the fork in vibration, because when the platinum wire enters the mercury 
 at c, the circuit is completed and the electro-magnet magnetizes its soft-iron core, which attracts the 
 arms of the fork, and thus draws the wire out of the mercury and so breaks the circuit. When the 
 current is broken the fork, being released, springs back, dips the wire into the mercury, and by 
 closing the circuit causes the process to be repeated. 
 
 current. The electric current is made and broken at regular intervals by a clock, 
 tuning-fork (6, Fig. 37), or other interrupting mechanism, and the lever of the 
 chronograph, which has a writing-point at its free end, moves correspondingly 
 
 FIG. 38. Myogram from gastrocnemius muscle of frog ; beneath, the time is recorded in 0.005 second : 
 a, moment of excitation ; b, beginning of contraction ; c, height of contraction ; d, end of contraction. 
 
 and traces an interrupted line on the recording surface of the myograph (see 
 Fig. 38). The space between the succeeding jogs marked by the chronograph 
 lever is a measure of the amount of the surface which passed the point of the 
 chronograph in one second, ^ second, or T ^g- second, as the case may be. 
 
 Myogram of Simple Muscle-contraction. The rate of the movement of the 
 muscle during every part of its contraction can be readily determined by com- 
 paring the record it has drawn with that of the chronograph. 
 
 Figure 38 is the reproduction of a single contraction of a gastrocnemius 
 muscle of a frog. The rise of the curve shows that the contraction began 
 comparatively slowly, soon became very rapid, but toward its close was again 
 gradual the relaxation began almost immediately, and took a similar course, 
 
102 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 though occupying a somewhat longer time. The electric current which 
 actuated the chronograph was made and broken by a tuning-fork which 
 made 200 complete vibrations per second, therefore the spaces between the 
 succeeding peaks of the chronograph curve each represents 0.005 second. A 
 comparison of the movements of the muscle with the tuning-fork curve 
 reveals that about yf^ second elapsed between the point 6, at which the muscle 
 curve began to rise, and c, the point at which the full height of the contraction 
 was reached, and that about yj^ second was occupied by the return of the 
 muscle curve from c to point d, at the level from which it started. The muscle 
 employed in this experiment was slightly fatigued, and the movements were 
 in consequence a little slower than normal. 
 
 Latent Period. The time that elapses between the moment that a stim- 
 ulus reaches a muscle and the instant the muscle begins to change its form is 
 called the latent period. In the experiment recorded in Fig. 38 the muscle 
 received the shock at the point a on the curve, but the lever did not begin to 
 rise until the point b was reached. The latent period as recorded in this ex- 
 periment was about 0.006 second. The latent period and the time relations of the 
 muscle-curve were first measured by Helmholtz, who introduced the use of the 
 myograph. 1 Helmholtz concluded from his experiments that the latent period 
 for a frog's "muscle is about yj-^ second, that the rise of the curve occupies 
 about yJ-Q-, and the fall about yjj-g- second, the total time occupying about -fa 
 second. These rates can be considered approximately correct, excepting for 
 the latent period, which has been found by more accurate methods to be con- 
 siderably shorter. Tigerstedt connected a curarized frog's muscle with a myo- 
 graph lever, which was so arranged as to break an electric contact at the 
 instant that the muscle made the slightest movement ; the break in the electric 
 circuit was recorded on a rapidly revolving drum, by an electro-magnet similar 
 to the chronograph. By this means he found the latent period of a frog's 
 muscle may be as short as 0.004 second. Tigerstedt 2 did not regard this as 
 the true latent period, however ; he expressed the belief that the muscle proto- 
 plasm must have begun to respond to the excitation much sooner than this. 
 The contraction of the whole muscle is the result of a shortening of each of the 
 myriad of light and dark disks of which each of the muscle-fibres is composed 
 (see Fig. 39). The distance to be traversed by the finest particles of muscle- 
 substance is microscopic, hence the rapidity of the change of form of the whole 
 muscle. Even such a change would require time, however, and it is probable 
 that the muscle protoplasm becomes active before any outward manifestation 
 occurs. That this view is correct has been proved by electrical observations. 
 
 When muscle protoplasm passes from a state of rest to one of action it 
 undergoes an alteration in electrical condition. This change can be detected by 
 the galvanometer (Fig. 62, p. 144) or by the capillary electrometer (Fig. 63, 
 p. 146). Burdon Sanderson 3 has found that by the aid of the latter instru- 
 
 1 Archivfiir Anatomie und Physiologic, 1850, S. 308. 
 
 2 Ibid., 1885, Suppl. Bd., S. 111. 
 
 3 Journal of Physiology, 1898, vol. xxiii. p. 350. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 103 
 
 ment an alteration of the electrical condition of the muscle of a frog can be 
 detected less than 0.001 second after the stimulus has been applied to it. 
 Since some slight interval of time must have been lost even by this delicate 
 method, it would seem that muscle protoplasm begins to be active at the 
 instant it is stimulated. 
 
 According to this view, muscle-substance has no latent period ; neverthe- 
 less we can still speak of the latent period of the muscle as a whole. It will 
 be necessary, however, to distinguish between the electrical latent period and 
 the mechanical latent period : by the former we mean the time which elapses 
 between the moment of excitation and the first evidence obtainable of a change 
 in the electrical condition of the muscle ; by the latter, the time between exci- 
 tation and the earliest evidence of movement which can be observed. In the 
 case of the striated muscles of a frog the electrical latent period is less 
 than 0.001 second, and the mechanical about 0.004 second. Mendelssohn l 
 estimated the mechanical latent period of the muscles of man to be about 
 0.008 second. There can be little doubt, however, that this figure is too 
 large. 
 
 Bernstein 2 found that if a normal frog's muscle be excited indirectly, 
 by the stimulation of its nerve, the mechanical latent period is somewhat 
 longer than when it is directly excited. Of course a certain length of time is 
 required to transmit the excitation through the length of nerve intervening 
 between the point stimulated and the muscle fibres. If this time be deducted, 
 there still remains a balance of about 0.003 second, which can only be ac- 
 counted for on the assumption that the motor nerve end-plates require time to 
 excite the muscle-fibres. The motor end-plates are therefore said to have a 
 latent period of 0.002-0.003 second. 
 
 The latent period, and the time required for the rise and fall of the myo- 
 graph curve, are found to be very different not only for the muscles of differ- 
 ent animals, but even for the different muscles of the same animal. Moreover, 
 the time relations of the contraction process in each muscle are altered by a 
 great variety of conditions. 
 
 Before considering the effect of various influences upon the character of the 
 muscle contraction, let us give a glance at the finer structure of the muscle, 
 and the change of form which the microscopic segments of the muscle-fibre 
 undergo during contraction. 
 
 2. Optical Properties of Striated Muscle during 1 Rest and Action. 
 An ordinary striated muscle is composed of a great number of very long 
 muscle-cells, fibres as they are called, arranged side by side in bundles, the 
 whole being bound together by a fine connective-tissue network. Each muscle- 
 fibre consists of a very delicate elastic sheath, the sarcolemma, which is com- 
 pletely filled with the muscle-substance. Under the microscope the fibres are 
 seen to be striped by alternating light and dark transverse bands, and on focus- 
 ing, the difference in texture which this suggests is found to extend through 
 
 1 Archives de Physiologic, 1880, 2d series, t. vii. p. 197. 
 
 2 Untemuchungen uber den Erregungsvorgang im Nerven und Muakelsystem, 1871. 
 
104 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 the fibres, i. e. the light and dark bands correspond to little disks of substances 
 of different degrees of translucency. More careful study with a high power, 
 
 shows under certain circumstances other 
 cross markings (see Fig. 39, A), the light 
 band is found to be divided in halves by 
 a fine dark line, Z, and parallel to it is 
 another faint dark line, n, while the dark 
 band, , is found to have a barely per- 
 ceptible light line in its centre. 
 
 The fine dark lines, Z, which run 
 through the middle of the light bands, 
 were for a time supposed to be caused by 
 delicate membranes (Krause's membrane), 
 which were thought to stretch through 
 the fibre and to divide it into a series of 
 little compartments, each of which had 
 exactly the same construction. Kuehne 
 FIG. 39.-schema of histoiogicai structure of cna nced to see a minute nematode worm 
 
 muscle-fibre : A, resting fibre as seen by ordinary 
 
 light ; B, resting fibre seen by polarized light ; C, moving along inside a muscle-fibre, and 
 
 '" ^served that it encountered no obstruc- 
 
 tion, such as a series of membranes, how- 
 
 ever delicate, would have caused. As it moved,, the particles of muscle-sub- 
 stance closed in behind it, the original structure being completely recovered. 
 This observation did away with the view that the fibre is divided into com- 
 partments, but the arrangement shown in Figure 39, A, repeats itself through- 
 out the length of the fibre and indicates that it is made up of a vast succession 
 of like parts. 
 
 Muscle-substance consists of two materials, which differ in their optical 
 peculiarities and their reaction to stains. If a muscle-fibre be examined by 
 polarized light, it is found that there is a substance in the dark bands which 
 refracts the light doubly, is anisotropic, while the bulkpf the substance in the 
 light bands is singly refractive, isotropic (, Fig. 39). The anisotropic sub- 
 stance is found to stain with ha3inatoxylin, while the isotropic is not thus 
 stained ; on the other hand, the isotropic substance is often colored by chloride 
 of gold, which is not the case with the anisotropic. By means of these reac- 
 tions it has been possible to ascertain something as to the arrangement of these 
 substances within the muscle-fibre, though the ultimate structure has not been 
 definitely decided. It appears that the isotropic material is the sarcoplasma, 
 which is distributed throughout the fibre and holds imbedded within it the 
 particles of the anisotropic substance, these particles having a definite arrange- 
 ment. Striated muscle-fibres present not only cross markings, but under 
 favorable conditions longitudinal striations, these being most evident in the 
 dark bands. These longitudinal striatious are looked upon with great interest 
 as indicating that the particles of anisotropic material are arranged in long 
 chains as incomplete fibrillaB. According to this view the muscle-fibre is com- 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 105 
 
 posed of semifluid isotropic substance, in which are the particles of anisotropic 
 material, arranged to form vast numbers of parallel fibrillse of like structure, 
 and so placed as to give the effect of transverse disks (Z, n, Q, Fig. 39). 
 
 When a striated muscle contracts, each of its fibres becomes shorter and 
 thicker, and the same is true of the dark and light disks of which the fibres 
 are composed. If we examine a muscle-fibre which has been fixed by ramie 
 acid at a time when part of it was contracting, we see that in the contracted 
 part the light and dark bands have both become shorter and wider, but that 
 the volume of the dark bands (, Fig. 39, C) has increased at the expense of 
 the light bands. 
 
 Further, the dark bands are seen to be lighter and the light bands darker 
 in the contracted part, while examination with polarized light shows that 
 though the anisotropic substance does not seem to have changed its position, 
 (Fig. 39, Z>), the original dark bands have less and the lighter bands greater 
 refractive power. These appearances would seem to be explained by Engel- 
 mann's view that contraction is the result of imbibition of the more fluid 
 part of the sarcoplasm by the anisotropic substance. He has advanced the 
 theory that the cause of the imbibition is the liberation of heat by chemical 
 changes which occur at the instant the muscle is excited. In support of 
 this theory Engelmaim 1 showed that dead substance containing anisotropic 
 material, such as a catgut string, can change its form, by imbibition of 
 fluid under the influence of heat, and give a contraction curve in many 
 respects similar to that to be obtained from muscle. This theory of 
 the method of action of the muscle-substance, though attractive, can be 
 accepted only as a working hypothesis, and is not to be regarded as proved. 
 Various other theories have been advanced to explain the connection between 
 the chemical changes which undoubtedly occur during contraction and the 
 alteration of form, but none have been generally accepted. Enough has been 
 said to show that the contraction of the muscle as a whole is the result of 
 a change in the minute elements of the fi brills, and that the various condi- 
 tions which influence the activity of the process of contraction must act chiefly 
 through alterations produced in these little mechanisms. 
 
 3. Elasticity of Muscle. The elasticity and extensibility of muscle are 
 of great importance, for by every form of muscular work the muscle is sub- 
 jected to a stretching force. Elasticity of muscle is the property by virtue of 
 which it tends to preserve its normal form, and to resist any external force 
 which would act to alter that form. The shape of muscles may be altered by 
 pressure, but the change is one of form and not of bulk ; since muscles are 
 largely made up of fluid, their compressibility is inconsiderable. The elasticity 
 of muscles is slight but quite perfect, by which is meant that a muscle yields 
 readily to a stretching force, but on the removal of the force quickly recovers 
 its normal form. Most of the experiments upon muscle elasticity have been 
 made after the muscle had been removed from the body, hence under abnormal 
 
 1 Ueber den Ursprung der Muskelkraft, Leipzig, 1893. 
 
106 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 FIG. 40. a, Curve of extensibility 
 and elasticity of a rubber band ; b, curve 
 of extensibility and elasticity of a sar- 
 torius muscle of a frog. The weights 
 employed were 10 grams each. The 
 same length of time was allowed to 
 pass between the adding and subtract- 
 ing of the weights. 
 
 conditions. Under these circumstances it is found that if a number of equal 
 weights be added to a suspended muscle, one after the other, the extension pro- 
 duced is not, like that of an inorganic body 
 such as steel spring, proportional to the weight, 
 but each weight stretches the muscle less than 
 the preceding. If the weights be removed 
 in succession, an elastic recovery is observed, 
 which, although considerable, is incomplete. 
 If the change in the length be recorded by 
 a lever attached to the muscle, the surface 
 being moved along just the same amount after 
 each weight is added or removed, a curve is 
 obtained such as is shown in Fig. 40, b. 
 Above this is a record taken in a similar way 
 from a piece of rubber (a). The rubber resem- 
 bles a steel spring in that equal weights stretch 
 it to like amounts, but the elastic recovery, 
 though more complete than that of the muscle, 
 is imperfect. 
 
 In such an experiment it is found that the 
 full effect of adding the weights, or removing 
 them from the muscle, does not occur immedi- 
 ately, but when a weight is added there is a 
 gradual yielding to the stretching force, and, on the removal of a weight, a 
 gradual recovery of form under the influence of the elasticity. This slow 
 after-action makes it difficult to say just what is to be considered the proper 
 curve of elasticity of muscle, especially as the physiological condition of the 
 muscle is always changing. The elasticity of muscles is dependent on normal 
 physiological conditions, and is altered by death, or by anything which causes 
 a change in the normal constitution of the muscles, as the cutting off of the 
 blood-supply. The dead muscle is less extensible and less elastic than the 
 normal living muscle. Heating, within limits, increases, and cooling decreases 
 the elasticity, possibly by altering the mobility of the semifluid materials of 
 the muscle, and hence changing the internal friction. 1 Contraction is accom- 
 panied by increased extensibility, i. e. lessened elasticity and the changes 
 caused by fatigue lessen the elasticity. It is interesting to note in this con- 
 nection that the elasticity is decreased by weak acid solutions and increased 
 by weak alkaline solutions (Brunton and Cash). 2 
 
 The elasticity of a muscle within the body is generally considered to be 
 more perfect than that of the isolated muscle, but even here one can observe 
 the after-stretching described by Weber and the contraction remainder 
 described by Hermann. Mosso 3 suggests the following experiment on man : 
 
 1 Blix : Skandinavisches Archivfur Physiologic, 1893, iv. S. 392. 
 
 2 Philosophical Transactions, 1884, p. 197. 
 
 3 Mosso : Archives italiennes de Biologic, 1895, xxv. p. 27. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 107 
 
 Place the subject in a sitting position, make the suspended leg immovable 
 by suitable clamps, strap a board to the bottom of the foot, and connect the 
 toe end of the board with a weight by means of a cord passing over a pulley. 
 As the weight is increased or decreased the foot is more or less flexed, and 
 the gastrocnemius muscle is stretched more or less. A pointer fastened to 
 the foot-board moves over a scale and indicates the amount the muscle 
 changes its length when subjected to various weights. Mosso reports that 
 though the curve of elasticity has about the same character as that of isolated 
 frog muscle, the curve of extensibility is different, each of the added weights 
 causing greater amount of stretching. This is probably due to the fact that 
 a muscle within the body is always being influenced by the central nervous 
 system. Its length at any given moment is due not only to its elasticity as 
 compared with that of its antagonist, but also to the strength of the nervous 
 impulses, reflex and voluntary (often unintentional), coming to it. The sub- 
 ject would have to be under an anaesthetic or in very deep sleep for such an 
 experiment to give a true picture of its elasticity. Mosso describes, in fact, 
 movements of the foot accompanying the respirations, due to variations in 
 the tonus impulses coming to the muscles in inspiration and expiration. In 
 spite of the innate difficulties of such an experiment, we can ascertain that 
 in general the conclusions arrived at by studying the isolated muscles of a 
 frog apply to the muscles when in the living body. 
 
 The elasticity of a muscle within the normal body suffices to preserve the ten- 
 sion of the muscle under all ordinary conditions. The muscles are attached to 
 the bones under elastic tension, as is shown by the separation of the ends in case 
 a muscle be cut. This elastic tension is very favorable to the action of the 
 muscle, as it takes up the slack and ensures that at the instant the muscle 
 begins to shorten the effect of the change shall be quickly imparted to the 
 bones which it is its function to move. The extensibility of the muscle is 
 a great protection, lessening the danger of rupture of the muscle-fibres and 
 ligaments, and the injury of joints when the muscles contract suddenly and 
 vigorously, or when they are subjected to sudden strains by external forces. 
 The importance of extensibility and elasticity to muscles which act as antag- 
 onists is evident. When a muscle suddenly contracts against a resisting force 
 such as the inertia of a heavy weight, the energy of contraction, which puts the 
 muscle on the stretch, is temporarily stored in it as elastic force, and as the 
 weight yields to the strain, is given out again ; thus the effect of the contrac- 
 tion force is tempered, the application of the suddenly developed energy being 
 prolonged and softened. Elasticity is very important to the function of the 
 non-striated muscles of the blood-vessels, bladder, intestine, etc. This is 
 especially true of the sphincter muscles, for it is an important factor in 
 securing the continued tension characteristic of their action. 
 
 4. Influences which Affect the Activity and Character of the Con- 
 traction. (a) The Character of the Muscle. Attention has been called to 
 the fact that irritability and conductivity may be different not only in different 
 kinds of muscle-tissue, and in muscles of different animals, but even in similar 
 
108 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 kinds of muscle-tissue in the different muscles of the same animal ; the same 
 may be said of contractility. Although irritability, conductivity, and contrac- 
 tility are to be regarded as different properties of muscle protoplasm, they are 
 usually found to be developed to a corresponding degree in each muscle. 
 Those forms of muscle which require for their excitation irritants of slow and 
 prolonged action, are found to conduct slowly and to make slow and long- 
 drawn-out contractions, and muscles which are excited by irritants acting 
 rapidly and briefly are noted for the quickness with which they contract 
 and relax. 
 
 Differences in the activity of the contraction process are made evident 
 by the duration of single contractions of different forms of muscle-tissue. 
 The duration of the contraction of the striated muscles of different animals 
 differs greatly, e. g. of the frog ^ second, of the turtle 1 second, of certain 
 insects only -^ second. Even muscles of apparently the same kind in the 
 
 Pectoralis major 
 Omohyoid. 
 
 FIG. 41. Records of maximal isotonic contractions of four different muscles from a turtle, each 
 weighted with 30 grams : Pectoralis major ; omohyoid ; gracilis ; palmaris. The dots mark i second, and 
 the longer marks seconds (after Cash). 2 J 
 
 same animal exhibit different degrees of activity. Cash l reports the following 
 differences in the duration of the contractions of different striated muscles of 
 a frog in fractions of a second: Hyoglossus, 0.205; rectus abdominis, 0.170; 
 gastrocnemius, 0.120; semimembranosus, 0.108 ; triceps femoris, 0.104. Sim- 
 ilar differences are found to exist between different muscles in other animals 
 in the turtle, for instance, as is shown by the myograms in Fig. 41. 
 
 It is interesting to connect the rate of the contraction process in different 
 muscles with their function. The omohyoid muscle of the turtle is capable of 
 comparatively rapid contractions, and the action of this muscle is to draw back 
 the head beneath the projecting shell ; the pectoralis, on the other hand, 
 although strong, contracts slowly; it is a muscle of locomotion and has to 
 move the heavy body of the animal. Unstriated muscles, which are remark- 
 able for the slowness and the duration of their contractions, are found chiefly 
 in the walls of the intestines, blood-vessels, etc., which require to remain in a 
 state of continued contraction for considerable periods and do not need to alter 
 rapidly. It is the business of the heart-muscle to drive fluids often against 
 considerable resistance, and a strong,' not too rapid, slightly prolonged contrac- 
 tion, such as is peculiar to it, would be best adapted to its function. The bulk 
 of the muscles of the bodies of warm-blooded animals are capable of rapid 
 contraction and relaxation, but the rate normal to the muscle is found to vary 
 with the form of work to be done. The muscles which control the vocal 
 organs, for instance, have a very rapid rate of relaxation as well as of con- 
 
 1 Archivfilr Anatomic und Physiologie, 1880, Suppl. Bd., S. 147. a Op. cit., S. 157. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 109 
 
 traction. The muscles which move the bones appear to have different rates 
 of contraction and relaxation according to the weight of the parts to be moved ; 
 those which control the lighter parts, as the hand, being capable of rapid con- 
 tractions, while those which have to overcome the inertia of heavier parts, to 
 which rapidity of action would be a positive disadvantage, react more slowly. 
 In general, where rapid, brief, and vigorous contractions are required, pale 
 striated muscles are found; where more prolonged contractions are needed, 
 red striated muscles occur. The accompanying myograms (Fig. 42) illustrate 
 
 100 
 
 50, 
 
 100 
 
 FIG. 42. A, maximal contractions of the gastrocnemius medialis of the rabbit (pale muscle), weighted 
 with 50, 100, 300, and 500 grams ; B, maximal contractions of the soleus of the rabbit (red muscle), weighted 
 with 50, 100, and 200 grams (after Cash). 
 
 the difference in the rate of contractions of pale and red striated muscles of 
 the rabbit. Ranvier says the latent period of red muscle of rabbit is four 
 times as long as that of the pale ; and Tigerstedt states the latent period of 
 red muscles of the frog to be 0.02 second and of the pale muscles 0.005 
 second. 
 
 Pale and red striated fibres are found united in the same muscle in certain 
 instances, and in these cases it is supposed that the former, which are capable 
 of very rapid and powerful but short-lived contractions, start the movement, 
 while the slower red muscles continue it. Bottazzi l would explain many of 
 the peculiarities of muscle contraction on the theory that both the isotropic 
 and anisotropic substances are contractile, and that they react differently 
 under varying conditions. The isotropic substance, the sarcoplasma, is 
 responsible for the slow, prolonged movements of the muscle and the aniso- 
 tropic substance for the rapid, brief movements. In ordinary contractions 
 they both act, though to different degrees. 
 
 (6) Effect of Tension Caused by Weights and Myograph-lever on the Extent 
 
 and Course of the Contraction. As we have seen, the rate of the contraction 
 
 of an ordinary striated muscle is much too rapid to be followed by the eye, 
 
 and to study the course of the change in form it is necessary to employ some 
 
 kind of recording mechanism. Every mechanical device for recording the 
 
 rnents of the muscle has inertia, and, if given motion, acquires moinen- 
 
 1 Bottazzi : Journal of Physiology, 1897, xxi. p. 1. 
 
110 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 turn. Both of these factors would tend to alter the shape of the record, and 
 the more the greater the weight of the recording apparatus. 
 
 A weight, or tension, can be applied to a muscle in various ways, and the 
 form of the contraction will be correspondingly changed. If a muscle is made 
 to work with a considerable weight hanging on it, we speak of it as loaded; 
 if the weight be connected with the muscle, but so supported that it does 
 not pull on it until the muscle begins to shorten, the muscle is said to be after- 
 loaded; if the weight is the same throughout the contraction, as when the 
 muscle has only to lift a light weight, applied close to the axis of the lever, the 
 contraction is said to be isotonic ; if on the other hand the contracting muscle 
 is made to work against a strong spring, so that it can shorten very little, i. e. 
 has almost the same length throughout the contraction, the contraction is said 
 
 to be isometric. 1 The shape of the 
 myogram recorded as a result of 
 the same stimulus would evidently 
 be very different in these four 
 cases. The effect of a weight to 
 alter the myogram is illustrated in 
 the record given in Figure 43. 
 Increasing the weight prolonged 
 the latent period, and lessened the 
 height and duration of the con- 
 tractions. 
 
 The alterations liable to occur 
 in the form of the myogram by 
 the isotonic method, as a result 
 of the mechanical conditions under 
 which the work is done, are 
 
 (1) Prolongation of the latent 
 period. There can be no move- 
 ment of the lever until the inertia 
 of the weight has been overcome, 
 and the first effect of the contrac- 
 tion is to stretch the muscle, a 
 part of the energy of contraction being changed to elastic force, which on the 
 recoil assists in raising the weight. Thus the myogram may fail to reveal 
 the instant that the contraction process starts. Indeed, inasmuch as tension 
 increases the activity of muscle protoplasm, it is probable that the presence 
 of the weight really hastens the liberation of energy at the same time that it 
 delays the recording of the contraction. 
 
 (2) Alteration in the shape of the ascending limb of the myograph curve. The 
 
 weight will either lessen the rate at which the curve rises and decrease the 
 
 height, or, if the weight be not great, it may acquire a velocity from the energy 
 
 suddenly imparted to it by the muscle, which will carry the record higher 
 
 1 Fick : Mechanische Arbeit und W armeentwickelung bei der Muskelthatigheit, Leipzig, 1882. 
 
 FIG. 43. Effect of the weight upon the form of the 
 myogram. The gastrocnemius muscle of a frog excited 
 by maximal breaking induction shocks five times, the 
 weight being increased after each contraction, and in the 
 intervals supported at the normal resting length of the 
 muscle ; i. e. the muscle was after-loaded : 1, muscle 
 weighted only with very light lever; 2, weight five 
 grams ; 3, ten grams ; 4, twenty -five grams ; 5, fifty grams. 
 The perpendicular line marks the moment of excitation. 
 The time is recorded at the bottom of the curve by a 
 chronograph, actuated by a tuning-fork vibrating 50 times 
 per second. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. Ill 
 
 than the absolute contraction of the muscle. The part of the myogram cor- 
 responding to the height of the contraction of the muscle can be distinguished 
 from that due to the throw of the lever by a method suggested by Kaiser. 1 
 If the rising lever strikes a check, it remains in contact with the check as 
 lonir as the muscle continues to contract, but falls immediately if not held 
 there by the contraction process. By varying the height of the check, the 
 point corresponding to the true contraction height can be ascertained. 
 
 (3) The fall of the curve may be altered. The weight, suddenly freed by the 
 rapidly relaxing muscle, may acquire a velocity in falling which will stretch 
 the muscle-tissue, carry the record lower than the actual relaxation of the 
 muscle would warrant, and lead to the development of artificial elastic after- 
 oscillations. It must not be supposed, however, that the relaxation of the 
 muscle is merely a passive aifair, and that it returns to its original shape 
 because, when it ceases to develop energy, it is stretched by the weight. The 
 relaxation, like the contraction process, is an active event, and it is antago- 
 nistic to the contraction process. 2 
 
 These sources of error can be in part overcome by the employment of an 
 exceedingly light, stiff writing-lever, and by bringing the necessary tension on 
 the muscle by placing the extending weight very near the axis of the lever, so 
 that it shall move but little and hence acquire little velocity. 
 
 (c) Effect of Rate of Excitation on Height and Form of Muscular Contrac- 
 tion. If a muscle be excited a number of times by exactly the same irritant 
 and under the same external conditions, the amount and course of each of 
 the contractions should be exactly the same, provided the condition of the 
 muscle itself remains the same. The condition of the muscle is, however, 
 altered every time it is excited to contraction, and each contraction leaves 
 behind it an after-effect. This altered condition is not permanent ; as we have 
 seen, increased katabolism is accompanied by increased anabolism, and, if the 
 excitations do not follow each other too rapidly, the katabolic changes occur- 
 ring in contraction are compensated for by anabolic changes during the suc- 
 ceeding interval of rest. Normally, a muscle, under the restorative influence 
 of the blood, rapidly recovers from the alterations produced by the contraction 
 process, and, therefore, if not excited too frequently, will give, other things 
 being equal, the same response each time it is called into action. The best 
 illustration of this is the heart, which continues to beat at a regular rate 
 throughout the life of the individual. Tiegel found that one of the skeletal 
 muscles of a frog, while in the normal body, can make more than a thousand 
 contractions in response to artificial stimuli without showing fatigue ; finally 
 the effect of the work shows itself in a lessening of the power to contract. 
 Every muscle contains a surplus of energy-holding compounds and also sub- 
 stances capable of neutralizing waste products, and even a muscle which has 
 been separated from the rest of the body retains for a considerable time the 
 abiln\ to recover from the effects of excitation. It is evident that when a 
 
 1 Kaiser: Zeitschrift fur Biologic, 1896, xxxiii. S. 157, 360. 
 'ick, v. Kries, and others. 
 
112 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 muscle is excited repeatedly, a certain interval of rest must be permitted 
 between the succeeding excitations if its normal condition is to be maintained, 
 and that the more extensive the chemical changes produced by the excita- 
 tions the longer must be the periods allowed for recovery. This being the 
 case, the rate of excitation and consequent length of the interval of rest will 
 have a great effect upon the condition of the muscle and its capacity for work. 
 
 (1) Effect of Frequent Excitations on the Height of Separate Muscular 
 Contractions. -Other things being equal, the height to which a muscle can con- 
 tract when excited by a given irritant can be taken as an index of its capacity 
 to do work, and if a muscle be excited many times in succession, the effect of 
 action upon the strength of the contraction process, the endurance, and the 
 coming on of fatigue can be estimated from the height of the succeeding con- 
 tractions. One might expect that every contraction would tend to fatigue and 
 to lessen the power of the muscle, but almost the first effect of action is to 
 increase the irritability and mobility of muscle protoplasm. 
 
 Introductory and Staircase Contractions. The peculiar effect of action to 
 increase muscular activity was first observed by Bowditeh, 1 when studying 
 the effect of excitations upon the heart. He found that repeated excitations 
 of equal strength applied to the ventricle of a frog's heart caused a series of 
 contractions each of which was greater than the preceding. If the contrac- 
 tions were recorded on a regularly moving surface, the summits of the succes- 
 sive contractions were seen to rise one above the other like a flight of steps. 
 This peculiar phenomenon received the name of the " staircase contractions " 
 (see Fig. 44). 
 
 FIG. 44. Staircase contractions of a frog's ventricle in response to a series of like stimuli, written on 
 a regularly revolving drum by the float of a water manometer connected with the chamber of the 
 ventricle (after Bowditeh). The record is to be read from right to left. 
 
 This effect of repeated excitations was later observed by Tiegel, 2 on the 
 skeletal muscles of frogs; by Eossbach, 3 on the muscles of warm-blooded 
 animals, and by Komanes 4 on the contractile tissues of Medusae. 
 
 The following series of contractions (Fig. 45), which closely resembles the 
 above, was obtained from the gastrocnemius muscle of a frog, excited at a 
 regular rate by a series of equal breaking induction shocks. 
 
 The contractions in Figure 45 did not begin to increase in height imme- 
 diately ; on the contrary, each of the first four contractions was slightly lower 
 than the one which preceded it. A decline in the height of the first three or 
 four contractions is the rule when a normal resting muscle is called into action 
 
 1 Berichte der koniglichen sdchsischen Gesellschaft der Wissenschaft, 1871. 
 
 3 Pftuger's Archiv, 1882, 1884, Bd. xiii., xv. 
 
 4 Romanes : Jelly-fish and Star-fish, International Science Series, p. 54. 
 
 Ibid., 1875. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 113 
 
 (see Figs. 46 and 49), and these contractions at the beginning of a series have 
 received the name of the " introductory contractions." The introductory con- 
 tractions appear to indicate that the first effect of action is to lessen irritability, 
 or that anabolic^ changes are too slow to compensate for katabolic changes, and 
 each of the first few contractions leaves behind it a fatigue effect. It is not 
 long, however, before the influence of activity to heighten anabolism and 
 increase irritability shows itself in the growth of the height of the succeeding 
 contractions, and the " staircase contractions" are observed. This growth of the 
 height of contractions must necessarily reach a limit, and the amount of 
 increase is found to gradually lessen until the succeeding contractions have the 
 same height. Sometimes the full height of the staircase is not reached before 
 more than a hundred contractions have been made. These maximal contractions 
 may be repeated many times ; sooner or later, however, an antagonistic effect of 
 the work manifests itself and the height of the contractions begins to lessen. 
 
 Effect of Fatigue. A. decline in the height of the contractions is an 
 evidence of fatigue, and indicates that anabolism is failing to keep pace with 
 
 FIG. 45. Staircase contractions of gastrocnemius muscle of a frog, excited once every two seconds by 
 strong breaking induction shocks. 
 
 katabolism, or that the waste products which result from the work are col- 
 lecting faster than they can be removed or neutralized and are exerting a 
 paralyzing influence on the muscle protoplasm (see p. 70). From this time 
 on, the height of the succeeding contractions continually lessens, and often 
 with great regularity, so that a line drawn so as to connect the summits 
 of the declining contractions, the "curve of fatigue," as it is called, may 
 be a straig t line. In the experiment, parts of the record of which are 
 .reproduced ir> Figure 46, an isolated gastrocnemius muscle of a frog was 
 excited with maximal breaking induction shocks at the rate of 25 times 
 a minute for about one and one-half hours ; the contractions were isotonic, and 
 the total weight of lever and load did not exceed 20 grams ; the records of 
 the succeeding contractions were recorded on a slowly moving cylinder. The 
 experiment counted of two parts in the first 66 contractions, in the second 
 over 1700 contractions were made; an interval of rest of five minutes was 
 permit t-d U-r t>n the two series. 
 
 In the first part cf the experiment there was a decline in the height of the 
 contractions for the first five contractions, the "introductory contractions," 
 then during the next sixty-one contractions a gradual rise in the height of the 
 
114 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 contractions, the " staircase contractions." These phenomena repeat themselves 
 in the second part of the experiment, that following the interval of rest. The 
 contractions at the beginning of the second series were not so high as those at 
 the end of the first series, though somewhat higher than those seen at the 
 beginning of the first series; the rest of five minutes was not sufficient to 
 entirely do away with the stimulating influence of the preceding work. The 
 contractions of the second series took the following course : The first four 
 introductory contractions gradually declined, then came the staircase contrac- 
 tions, which continued to rise until the 100th contraction, when a gradual 
 lessening of the height of the contractions began. This decline continued 
 
 600 
 
 700 
 
 800 
 
 900 
 
 1000 
 
 1100 1200 
 
 1300 
 
 1400 
 
 1500 
 
 1600 
 
 1700 
 
 FIG. 46. Effect of fatigue on the height of muscular contractions. The figure is a reproduction of 
 parts of a record of over 1700 contractions made by an isolated gastrocnemius muscle of a frog. The con- 
 tractions were isotonic, the weight being about 20 grams. The stimuli were maximal breaking induction 
 shocks, and were applied directly to the muscle, at the rate of 25 per minute. Between the first group of 
 66 contractions and the following groups a rest of five minutes was given ; after this r ,st the work was 
 continued without interruption for about one and a half hours. The second group of contractions, that 
 immediately following the period of rest, contains the first twenty contractions of tlte new series ; the 
 next group the 100th to the 110th ; the next the 200th to the 210th, and so on. 
 
 throughout the long series of more than 1 700 contractions g : ven in the record, 
 and, had the experiment been continued, would have undoubtedly gone on 
 until the power was completely lost. The curve of fatigue was not a straight 
 line, but fell somewhat more rapidly during the early part of the work than 
 toward the end. 
 
 That the peculiar changes in the height of the contractions which occur in 
 the early part of an experiment such as that which we have described are not 
 abnormal, and the result of the artificial conditions under which the work is 
 done, is shown not only by the fact that they are observed when a muscle 
 
GENERAL PHYSIOLOGY or J/r,SY7,/v AM) NEMVJE. 115 
 
 which has its normal blood-supply is rhythmically excited to a large number 
 of contractions, but by the personal experience of every one accustomed to 
 violent muscular exercise. Everyone is conscious that he cannot put out the 
 greatest muscular effort until he has "warmed up to the work." The- runner 
 precedes the race by a short run ; the oarsman takes a short pull before going 
 to the line ; in all the sports one sees the contestants making movements to 
 limber up" before they enter upon the work of the game. These prelim- 
 inary movements are performed not only to put the muscles in better condition 
 for action, but to ensure more accurate co-ordination that is to say, the facts 
 ascertained for the muscle can be carried over to the central nervous system. 
 The finely adjusted activities of the nerve-cells which control the muscles reach 
 their perfection only after repeated action. 
 
 In such experiments as that recorded in Figure 46 the record shows to 
 
 FIG. 47. Effect of excitation upon the form of separate contractions. In this experiment the records 
 of the muscular contractions were taken upon a rapidly revolving drum. The muscle was the gas- 
 trocnemius of the frog ; the contractions were isotonic ; the weight was very light, about 10 grams ; the 
 stimuli were maximal breaking induction shocks ; and the rate of stimulation was twenty-three per 
 minute. 1 marks the first contraction; 2, the 100th; 3, the 200th ; 4, the 300th. The muscle was excited 
 automatically by an arrangement carried by the drum, and the excitation was always given when a 
 definite part of the surface of the drum was opposite the point of the lever which recorded the con- 
 tractions. 
 
 a remarkable degree the fact that at any given time the muscle has a definite 
 capacity for work. A suitable explanation of this is lacking. The corre- 
 spondence in the height of the contractions of the same group, and the differ- 
 ence in the height of different groups of contractions, must be attributed to the 
 existence within the muscle-cell of some automatic mechanism which regulates 
 the liberation of energy and which has its activity greatly influenced by the 
 alterations which result from action. Whether this supposed automatic regu- 
 ^latory mechanism controls both the preparation of the final material from 
 which the energy displayed by the muscle is liberated, and the amount of the 
 explosive change which results from the application of the irritant, cannot be 
 definitely said. 
 
 (2) Effect of Frequent Excitations upon the Form of Separate Contractions. 
 -The effect of activity is not only observable in the change in the height 
 
116 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 of the muscular contractions, but in the length of the latent period, in the rate 
 at which the muscle shortens, and in the rate at which the muscle relaxes. 
 The effect of a large number of separate contractions, made in quick succes- 
 sion, upon the rate at which the muscle changes its form during contraction, 
 is illustrated in the myograms reproduced in Figure 47. 
 
 In Figure 47 only the 1st, 100th, 200th, and 300th contractions were re- 
 corded. The perpendicular line marks the point at which the stimulus was 
 given. In this experiment the latent period for each of the succeeding con- 
 tractions is seen to be longer ; the height is lessened ; the rise of the curve of 
 contraction is slowed and the curve of relaxation is even more prolonged. These 
 and certain other changes are to be observed in the records of Figure 48, which 
 were taken in an experiment made under the same conditions as the last, except 
 that the rate of excitation was 80 per minute, instead of 23, as in the preced- 
 ing experiment, and the record of every 50th contraction was recorded. 
 
 FIG. 48. Effect of frequent excitation on the form of separate contractions. The method employed 
 to obtain this record is the same as in the preceding experiment, except that the drum is revolving more 
 rapidly, and every 50th contraction is recorded: 1 marks the first contraction; 2, the 50th: 3, the 100th; 
 4, the 150th ; 5, the 200th ; 6, the 250th ; 7, the 300th. 
 
 A comparison of the first with the 50th contraction gives a number of 
 points of interest. The stimulating effect of action upon the contraction pro- 
 cess is shown by the fact that the latent period of the 50th (2 of Fig. 48) is 
 shorter than that of the first, the rise of the curve is somewhat steeper, and 
 the height is considerably greater. It is noticeable, however, that the crest 
 is prolonged, and consequently the total length of the contraction is increased. 
 Such a prolongation of the contraction is known as " Contracture." In con- 
 sidering the greater activity of the contraction process of this 50th contraction 
 as compared with the first, we must recall that it represents one of a series 
 of staircase contractions, such as we noticed in Figure 46. If we examine 
 the 1 00th contraction (3 of Fig. 48), we see the evidences of the beginning of 
 fatigue ; although the latent period is nearly as quick as in the first, the rise, 
 of the curve is less rapid, the height is less, and rate of relaxation is very 
 much slowed. These changes are to be seen in a more marked degree in the 
 150th contraction (4 of Fig. 48), and the prolongation of the crest of the 
 contraction and the decreased rate of relaxation are particularly noticeable. 
 The same sort of differences is to be observed in the later contractions. By 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 117 
 
 still more rapid rates of excitation these alterations in the contraction curve 
 are not only exaggerated, but develop more quickly, and play a very 
 important part in producing the peculiar form of continued contraction 
 known as tetanus. 
 
 Lee 1 states that the slowing of the contraction process, which is compara- 
 tively slight in the muscles of the frog, is very marked in the muscles of the 
 turtle, but practically absent from the white muscles of the cat. Moreover, 
 the prolongation of the relaxation which is very noticeable in the case of the 
 muscles of frogs and turtles, is very slight in those of the cat. Contracture 
 effects have, however, been seen on both the red and pale muscles of the 
 rabbit and on the muscles of man. Although the muscles of different animals 
 show certain peculiarities, the facts illustrated in the above experiments can 
 be considered as in general true of most striated muscles. 
 
 (3) Effect of Frequent Excitations to Produce Tetanus. As we have seen, the 
 normal muscle the first time that it is excited relaxes almost as quickly as it 
 contracts, but if it be excited rhythmically a number of times a minute, gradu- 
 ally loses its power of rapid relaxation. The tendency to remain contracted 
 begins to show itself in a prolongation of the crest of the contraction curve, 
 even before fatigue comes on, and increases for a considerable time in spite of 
 the effect of fatigue in lessening the height of the contractions. If a skeletal mus- 
 cle of a frog be excited many times, at a rate of about once every two seconds, 
 the gradual increase in the duration of the contractions will have the effect of 
 preventing the muscle from returning to its normal length in the intervals be- 
 tween the succeeding stimuli, for contraction will be excited before relaxation 
 is complete. As is shown in the record of the experiment reproduced in Figure 
 49, there will come a time in the work when the base-line connecting the lower 
 extremities of the succeeding myograms will be seen to rise in the form of a 
 curve, the change being at first gradual, then more and more rapid, and then 
 again gradual (see 6, Fig. 49). The effect of the change in the power to relax 
 is to make it appear as if the muscle were the seat of two contraction processes, 
 the one acting continuously, the other intermittently in response to the suc- 
 cessive excitations. Such a condition as that exhibited in section c, Figure 49, 
 is spoken of as an incomplete tetanus, complete tetanus being a condition of 
 continuous contraction caused by rhythmical excitations, in which none of the 
 separate contraction movements are visible. In complete tetanus the muscle 
 writes an unbroken curve. 
 
 The slowing of the relaxation of the muscle and consequent state of con- 
 tinued shortening which is to be seen in the latter part of the above experiment 
 is the result of the developing contracture. The amount of contracture 
 increases, within limits, with the increase in the strength and rate of exci- 
 tation. The intensity and rate of stimulation required for the production 
 of this condition depend very largely upon the character of the muscle and 
 its condition at the time. In the experiment recorded in Figure 50 the 
 development of the condition of contracture was more marked than in the 
 l Lee : American Journal of Physiology, 1899, ii. 3, p. 11. 
 
118 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 above experiment, and the resulting condition of continued contraction caused 
 first incomplete and finally complete tetanus. 
 
 Although frequent excitations appear to be essential to the development 
 of contracture, it is not to be considered a fatigue effect, since the contracted 
 state which it produces may be increasing at the time that fatigue is lessen- 
 ing the height of the ordinary contraction movements, and since the form of 
 contraction peculiar to contracture is itself seen to lessen as fatigue becomes 
 excessive. Both of these facts are illustrated in Figure 50, but are more 
 strikingly shown in Figure 51, in which a more rapid rate of excitation was 
 used. The effect of fatigue to prolong muscular contractions and the relation 
 of contracture to fatigue effects will be considered later (see p. 130). 
 
 The record in Figure 51 shows many points of interest : a to 6, a rapidly 
 
 PIG. 49. Effect of frequent stimuli to gradually produce incomplete tetanus. Series of isotonic con- 
 tractions of a gastrocnemius muscle of a frog, excited once every two seconds by strong breaking induc- 
 tion shocks. Only a part of the record is shown, 70 contractions have been omitted between the end of the 
 section marked a and the beginning of section b, and 200 contractions between the end of section band the 
 beginning of c. The increase in the extent of the relaxations seen at the close of the record was due 
 to the slowing of the rate of excitations at that time. 
 
 developing staircase, which is accompanied by a rising of the base line, which 
 indicates that contracture began to make itself felt from the moment the work 
 began ; 6 to c, a rapid and then a gradual fall in the height of contractions 
 due to fatigue effects ; c to d, a rise in the top of the curve in spite of the 
 lessening height of the contractions, due to the increasing contracture; d to e, 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NKIiVI-:. 11!> 
 
 a gradual fall of the curve of incomplete tetanus, due to the effect o 
 
 on the contracture ; e, complete tetanus, but continued gradual decline in the 
 
 height of the curve under the influence of fatigue. 
 
 FIG. 50. Effect of frequent excitations to gradually produce tetanus. Experiment on a gastrocnemius 
 muscle of a frog, similar to the last. The weight was only 10 grams. The rate of excitation was 100 per 
 minute. This muscle had been worked a short time before this series of contractions was taken, and, as 
 a result, the introductory and staircase contractions were absent and contracture began much sooner 
 than in the experiment recorded in Figure 48. The record in section 6 is a continuation of that in 
 section a. 
 
 The following experiment, Figure 52, differs from those which have preceded 
 it, in that the muscle, instead of being directly excited, was stimulated indirectly 
 by irritation of its nerve. Each shock applied to the nerve was represented 
 by a separate contraction process in the muscle. The experiment illustrates 
 well the combined effect of the staircase and the contracture to raise the height 
 
 FIG. 51. Development and fatigue of contracture. Experiment on a gastrocnemius muscle of a frog. 
 The weight was 10 grams. As in the preceding experiments strong maximal breaking induction shocks 
 were used to excite. The rate of excitation was 5 per second. The record appears as a silhouette for the 
 reason that the drum was moving very slowly. 
 
 of the contractions. On account of the more rapid rate of excitation, the 
 contracture came on more quickly than in the preceding experiments ; it did 
 
120 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 not become sufficient during the few seconds that this experiment lasted to 
 prevent the separate relaxations from being seen, and an incomplete tetanus 
 was the result. 
 
 In the experiment the record of which is given in Figure 53, the muscle was 
 directly stimulated, and the rate of excitation was rapid, 33 per second. Not 
 even this rate sufficed to cause complete tetanus, and the crest of the curve 
 
 FIG. 52. Development of incomplete tetanus and contracture, by indirect stimulation. A gas- 
 trocnemius muscle of a frog was indirectly stimulated by breaking induction shocks, of medium 
 strength, applied to the sciatic nerve. The rate was about 8 per second, as shown by comparison of the 
 seconds traced at the bottom of the figure with the oscillations caused by the separate contractions. The 
 weight was somewhat heavier than in the preceding experiment. The drum was revolving much faster 
 than in the other experiments, hence the difference in the apparent duration of the contractions. 
 
 shows fine waves, which represent the separate contractions the combined effect 
 of which resulted in the almost unbroken curve seen in the record. Had the 
 rate been a little more rapid, no waves could have been detected and the tetanus 
 would have been complete from the start. The effects of the staircase and con- 
 tracture are merged into one another, and a very rapid high rise of the curve 
 of contraction is the result. It is noticeable that the summit of the curve is 
 rising throughout the experiment, owing to the increasing contracture. 
 
 It is evident that the condition of contracture which is developed in a 
 rapidly stimulated muscle will tend to maintain a condition of continuous con- 
 
 FIG. 53. Effect of rapid excitations to produce tetanus. Experiment with a gastrocnemius muscle 
 of a frog, excited directly, with breaking induction shocks of medium strength, at the rate of 33 per 
 second. The weight was about 15 grams. The drum was moving much more slowly than in the pre- 
 ceding experiment. The time record gives fiftieths of a second. 
 
 traction, there being no opportunity for the muscle to relax in the intervals 
 between the succeeding excitations. 
 
 4. Explanation of the Great Height of Tetanic Contractions. We have 
 now to seek an explanation of the fact that a muscle when tetanized will con- 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. \'1\ 
 
 tract much higher than it will as a result of a single excitation. As we have 
 seen, repeated excitations cause, in the case of a fresh muscle, a gradual increase 
 in irritability and consequently a gradual rise in the height of the succeeding 
 contractions, but the staircase sooner or later reaches its upper limit, and will 
 not alone account for the great shortening which occurs in tetanus. 
 
 Effect of Two Rapidly Following Excitations. Helniholtz was the first to 
 investigate the effect of rate of excitation on the height of combined contrac- 
 tions. For the sake of simplicity, he excited a muscle with only two breaking 
 induction shocks, of the same strength, and observed the effect of varying the 
 interval between these two excitations. He concluded that if the second stim- 
 ulus is given during the latent period of the first contraction, the effect is the- 
 same as if the muscle has received but one shock ; if the second shock be applied 
 at some time during the contraction excited by the first, the second contraction 
 behaves as if the amount of contraction present when it begins were the resting 
 state of the muscle, i. e. the condition of activity caused by the first shock has 
 no influence on the amount of activity caused by the second, but the height 
 of the second contraction is simply added to the amount of the first contraction 
 present. Were this rule correct, as a result of this summation, if the second 
 contraction occurred when the first was at its height, the sum of the two con- 
 tractions would be double the height of either contraction taken by itself. 
 
 Helmholtz' conclusion, that the condition of activity awakened by the first 
 excitation has no effect upon that caused by the second excitation, has not been 
 substantiated by later observers. Von Kries * has found that the presence of* 
 the first contraction hastens the development of the contraction process result-' 
 ing from the second excitation ; and Von Frey 2 has ascertained that Helrn- 
 holtz's rule of summation applies only to weighted muscles. In the case of 
 unweighted muscles the summation effect is greatest when the second contrac- 
 tion starts during the period of developing energy caused by the first excita- 
 tion, i. e. during the rise of the first contraction. If the second contraction 
 
 FIG. 54. A schema of the effect of double excitations upon the gracilis muscle of a frog, by differ- 
 ent intervals of excitation. To obtain this figure, the results of different experiments were super- 
 imposed (after Von Frey). 
 
 starts during the period of relaxation of the first, the second may be not 
 even as high as when occurring alone (see Fig. 54). 
 
 1 Archiv fur Anatomie und Physiologic, 1888. 2 IKd., S. 213. 
 
122 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 The fact that the second contraction is higher if it starts during the ascent 
 of the first, may be explained as due to a summation of the condition of ex- 
 
 FIG. 55. Effect of support on height of contractions (after Von Frey) : a, gastrocnemius muscle of a 
 frog, separate contractions, tetanus, separate contractions, and group of supported contractions ; weight 
 10.5 grams ; b, the same, by weight of 0.5 grams. 
 
 citation awakened by the two irritants, and hence the liberation of a greater 
 amount of energy. Nevertheless, the increased irritability, indicated by stair- 
 case contractions, and the summation of excitation effects which occur by rapidly 
 repeated excitations, shown by the above experiment, do not suffice to wholly 
 explain the great shortening of the muscle seen in tetanus. Helmholtz' idea, 
 that there is a support afforded by the first contraction to the second, must 
 also play an important part, and we must turn to this for the completion of the 
 explanation of the great height acquired by the tetanus curve. 
 
 Effect of Support on the Height of Contractions. Von Kries l and Von 
 Frey 2 found that, in general, the shorter the distance the muscle has. to raise 
 a weight, the higher it can contract, and that if a muscle be excited at a regu- 
 lar rate, and the support for the weight be raised between each of the succeed- 
 ing contractions, at a certain height of the support the contractions may be 
 as high as during tetanus (see Fig. 55) This effect can be got with a fresh 
 muscle when the interval between the excitations is such that there can be no 
 summation in Helmholtz' sense. 
 
 The importance of this discovery to our understanding of tetanus is very 
 great, for it has been found that if an unsupported muscle be rapidly excited, 
 effects are observed which closely resemble those obtained by the aid of a sup- 
 port; this we have seen in the experiments recorded in Figures 50, 51, p. 119. 
 After a certain amount of excitation, a change occurs in the condition of a 
 muscle, owing to which it acts as if it had received an upward push, and as 
 if a new force had been developed within it, which aids the ordinary con- 
 traction process in raising the weight. The new aid to high contraction is 
 the support afforded by the developing condition of contracture. That con- 
 1 Archivf'dr Anatomie und Physiologic, 1886. 2 Ibid., 1887. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NKin'i:. \'2:\ 
 
 tracture offers an internal support to the muscle, and raises the total height 
 of the contraction curve just as von Frey found an external support to do, 
 can he seen in Figure 57. 
 
 5. Effect of Gradually Increasing the Rate of Excitation. One of the 
 most instructive methods of exciting tetanus is to send into the muscle a series 
 of breaking induction shocks of medium intensity, at a gradually increasing 
 rate. The record of such an experiment has been reproduced in Figure 5(\ 
 
 FIG. 56. Effect of a gradually increasing rate of excitation. Excitation of a gastrocnemius muscle 
 of a frog with breaking induction shocks of medium strength. The time was recorded directly, by n 
 tuning-fork making 100 vibrations per second. The rate of excitation was gradually increased, and 
 then gradually decreased. The ascending curve, a-b, shows the effect of increasing, and the descending 
 curve, c-d, of decreasing the rate of stimulation. Excitation was given by means of a special mechanism 
 for interrupting the primary circuit of an induction apparatus and at the same time short-circuiting the 
 making shocks. This interrupter was run by an electric motor which was allowed to speed up slowly, 
 and was slowed down gradually. 
 
 At the beginning of the experiment, a, one complete contraction with a 
 wave of elastic after- vibration was recorded ; this was followed by two con- 
 tractions of less height, " introductory contractions ;" then came three contrac- 
 tions each of which was higher than the preceding, "staircase contractions;" 
 these were followed by three contractions, which, in spite of the developing 
 contracture, were of less height, "fatigue effect." The rate of excitation at 
 this place was about 17 per second. From this point on, the developing con- 
 tracture supported the muscle more and more and the contraction waves became 
 less and less, until finally, when the rate had become 36 a second, the effect 
 of the separate stimuli could scarcely be detected, although the curve continued 
 to rise. This is as far as the record shows, but the rate was increased -till 
 further, and the contraction curve continued to rise, although less and less, 
 
124 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 until finally an almost straight, unbroken line was drawn. After a little time 
 this was seen to begin to fall, the contracture yielding to the effect of fatigue. 
 
 As the drum had nearly revolved to the place at which the experiment had 
 been begun, the rate of excitation was then slowly decreased. With the lessen- 
 ing rate, the curve fell more and more rapidly, and oscillations began to show 
 themselves. The character of the record during the rest of the experiment is 
 shown in the curve c-d, Figure 56. At c the rate was about 17, and at d it 
 was so slow that separate contractions were recorded, nevertheless the curve as 
 a whole kept up. Indeed, even after the excitation had altogether ceased, the 
 muscle maintained a partially contracted state for a considerable time, on 
 account of the contracture effect, which only gradually passed off. 
 
 6. Summary of the Effects of Rapid Excitation which produce Tetanus. 
 Muscle- tetanus is the result of the combined action of a great many different 
 factors, but the essential condition is that the muscle shall be excited at short 
 intervals, so that the effect of each excitation shall have an influence on the 
 one to follow it. This influence is exerted in several different ways : 1. In- 
 crease of irritability resulting from action, and leading to the production 
 of staircase contractions ; 2 1 _Summation of excitation effects, as when each 
 of the succeeding stimuli begins to act before the contraction process excited 
 by its predecessor has ceased ; 3. Support given by the contracting muscle to 
 itself, especially the support offered by contracture. 
 
 The experiment, the record of which is reproduced in Figure 57, was made 
 on the gastrocnemius muscle of a frog during the latter part of the winter, 
 and when the muscle had begun to show the effects of spring irritability. A 
 light weight was used. The muscle was first tested with four separate break- 
 ing induction shocks given at intervals of two seconds ; it was then subjected 
 for nine seconds to a tetanizing current; and in order that the condition of the 
 muscle during this period might be ascertained, the tetanizing current was 
 shut off from the muscle by a short-circuiting mechanism for a brief period 
 every two seconds. Finally, at the close of the tetanus, the condition of the 
 muscle was again tested by single-breaking shocks of the same intensity as 
 those used before the tetanus. The curve reveals many points of interest. 
 
 a. The first four single contractions show the " introductory " effect and 
 the beginning of a " staircase " effect such as is usually observed by serial 
 excitations. 
 
 b. Each of the short tetani starts with a sharp rise of the curve, making 
 what has been called the " introductory peak." These introductory peaks, 
 which are caused by the throw of the recording lever, give an evidence of the 
 intensity of the summation effects at these times. It is interesting to observe 
 that the first is high, the second low, and the third, fourth, and fifth show a 
 staircase-like growth, which is indication of the fact that excitation increases 
 the activity of the contraction processes. 
 
 c. The amount that the curve falls in the short interval separating the 
 succeeding periods of tetanus reveals the extent of the contracture present at 
 these times. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 125 
 
 d. The height of the base-line after the tetanus shows the persistence of the 
 contracture condition. 
 
 e. The height of the separate contractions following the period of tetanic 
 excitation was 22 mm., while the height of the first of the single contractions 
 preceding the tetani was 14 mm., which well illustrates how excitation may 
 increase irritability. 
 
 /. The total height to which the curve was carried by the separate shocks 
 after the period of tetanic excitation exhibits the effect of the support offered 
 
 FIG. 57. Effect of tetanizing excitations to increase the irritability of a muscle and at the same time 
 to produce a condition of contracture. The gastrocnemius muscle of a winter frog, connected with a 
 very light lever and a small weight, was arranged to write isotonic contractions on a slowly moving 
 kymograph drum. The time was recorded in seconds at the bottom of the record, and above this the 
 movement of the interrupter of the induction apparatus was written by an electric signal. The muscle 
 was excited four times by breaking induction shocks at intervals of two seconds ; then it was subjected 
 to a tetanizing current, this being short-circuited for brief periods at intervals of two seconds ; finally 
 it was again excited at two second intervals with breaking induction shocks of the same strength as 
 those used at the beginning of the experiment. 
 
 by the contracture to increase the total height of contraction, and corroborates 
 von Frey's statement that supported single contractions may carry the curve 
 as high as tetanus. 
 
 g. The rapid growth in the height of the crests of succeeding short tetani, 
 taken in connection with the lessening amount of relaxation during the inter- 
 val when the tetanizing current was shut off, and the curve of contraction 
 seen at the close of the tetani, all go to show how contracture may aid sum- 
 mation and staircase effects to give the great height to the tetanus curve. 
 Finally, it may be stated that the elasticity of the muscle gradually increases 
 as a result of tetanic excitations, and this may aid in the support of the weight 
 during long-continued tetanic contractions. 
 
 7. Number of Excitations required to Tetanize. The number of stimuli per 
 second required to tetanize a muscle depends largely on the nature of the 
 
126 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 muscle, for this decides the character of the separate contractions, and, through 
 them, the effect of their combined action. 
 
 The duration of the separate contractions, and the tendency of the muscle 
 to enter into contracture, are the predominant factors in determining the result. 
 Complete tetanus can only be obtained in the case of a fresh muscle, when the 
 interval between succeeding stimuli is shorter than is required for the muscle 
 to reach its maximal contraction by a single stimulus. Thus the prolonged 
 contractions of smooth muscles permit of the development of a form of tetanus 
 by successive closures of the galvanic current at intervals of several seconds. 
 The non-striated muscle of the bladder of the cat can be tetanized by induc- 
 tion shocks given at a rate of a little less than one in two seconds. 1 The 
 contraction of some of the muscles of the turtle may last nearly a second, and 
 two or three excitations a second suffice to tetanize. The muscles of mar- 
 mots during the winter sleep can be tetanized by 5 excitations per second 
 (Patrizi). Tetanus of the red (slowly contracting) striated muscles of the 
 rabbit can be obtained by 10 excitations per second, while 20-30 per second 
 are required to tetanize the pale (active) striated muscles (Kronecker and 
 Sterling) ; 100 stimuli per second are needed to tetanize the muscles of some 
 birds (Richet), and over 300 per second would be required to tetanize the 
 muscles of some insects (Marey). Any influence that will prolong the contrac- 
 tion process will lessen the rate of excitation required to tetanize. 
 
 8. Effect of Exceedingly Rapid Excitations. The question arises, Is there an 
 upper limit to the rate of excitation to which muscles will respond by tetanus? 
 There is no doubt that this is the case, but there is a difference of opinion as 
 to what the limit is, and how it shall be explained. 
 
 Striated muscles and nerves can be excited by rates at which our most deli- 
 cate chronographs fail to act. The muscle ceases to be tetanized by direct exci- 
 tation at a rate by which it can still be indirectly excited through its nerve. 
 The highest rate for the nerve has been placed at from 3000 to 22,000 by differ- 
 ent observers, 2 and this wide difference is probably attributable to the methods 
 of excitation employed. That such different results should have been reached 
 is not strange, if we recall the many conditions upon which the exciting power 
 of the irritant depends. That tetanus should be obtained by such high rates 
 does not show that the nerve responds to each of the separate shocks. As a 
 rule, when the rate of excitation is so high that tetanus fails a contraction is 
 observed when the current is thrown into the nerve, and often another when 
 it is withdrawn from the nerve that is, the muscle behaves as if it were sub- 
 jected to a continuous battery current. A satisfactory explanation for this, as 
 well as for the failure of the tetanus, is at present lacking. 
 
 9. Relative Intensity of Tetanus and Single Contractions. The amount that 
 a muscle is capacle of shortening, when tetanized by maximal excitations, and 
 
 1 C. C. Stewart: American Journal of Physiology, 1900, iii. p. 25. 
 
 2 Kronecker and Sterling: Archivfiir Anatomie nnd Physiologie, 1878, and Journal of Physi- 
 ology, 1880, vol. i. Von Frey und Wiedermann : Berichte der sachsischen Gesdlschaft der Wissen- 
 schaft, 1885. Both : Pfluga>8 Archiv, 1888. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 127 
 
 the strength of the tetanic contraction, depends very largely on the kind of 
 muscle. For example, pale striated muscles, although capable of higher and 
 more rapid single contractions than the red striated, do not show as great an 
 increase in the height and strength of contractions when tetanized as do thf 
 red; the latter, which are very rich in sarcoplasma, have likewise the givatoi 
 endurance. Gruetzner has called them " tetanus muscles," since they seem to 
 be particularly adapted to this form of contraction. Fick found that human 
 muscles when tetanized develop ten times the amount of tension, by isonirtric 
 contractions, that they give by single contractions ; and in this respect they 
 can be said to resemble red striated muscles. The following relations have 
 been found to exist between the strength of separate contractions and tetanus 
 in certain muscles : triceps and gastrocnemius of the frog, 1 : 2 or 3 ; the cor- 
 responding muscles of the turtle, 1:5; hyoglossus and rectus abdorainalis of 
 the frog, 1 : 8 or 9. 1 It is evident that no just estimate of the part played 
 by different groups of muscles in the movement of the body can be reached 
 without a careful analysis of the nature of the contractions peculiar to each 
 of the muscles participating in the movement. 
 
 Both the height and strength of the tetanus is controlled by the intensity 
 of the stimulus. A strong stimulus not only causes the separate contractions 
 of which the tetanus is composed to be higher, but is favorable to the develop- 
 ment of all the other factors which have been described as entering into the pro- 
 duction of tetanus. All normal physiological contractions are supposed to be 
 tetani, and everyone is conscious of the wonderful accuracy with which he can 
 grade the extent and strength of his voluntary movements. The remarkable 
 shading of the intensity of action observable in co-ordinated movements must 
 find its explanation in the adjustment of protoplasmic activity in the nerve- 
 cells of the central nervous system. 
 
 10. Continuous Contractions and Contractures. Under ordinary circum- 
 stances a striated muscle, % if excited by a single stimulus, gives a rapid con- 
 traction, followed almost immediately by a nearly equally rapid relaxation. 
 The duration and character of the period of relaxation are, however, subject to 
 great variation. In certain conditions the muscle may remain in a state of 
 continuous contraction for a considerable time, and then relax either slowly 
 or quite suddenly ; or it may begin to relax quickly and then suddenly stop^ 
 as if the relaxation process had received a sudden check ; or, after relaxing 
 quite rapidly for a short time, it may, without having received any visible 
 stimulus, contract again for a short distance and remain so contracted for a 
 considerable time. In any case when the relaxation period is unusually long, 
 the condition of prolonged contraction is termed " contracture." The form 
 of contracture which we are considering at present originates in the muscle 
 itself, and is to be sharply distinguished from a form of pathological contract- 
 ure, which originates in the central nervous system and in which the muscle 
 is kept continuously contracted by impulses coming from the spinal cord. 
 
 There are a great variety of conditions under which muscles respond to 
 
 1 Biedermann : Elektrophysiologie, S. 109. 
 
128 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 excitation by prolonged contractions. If a muscle be excited by frequent 
 induction shocks, even at a rate insufficient to produce tetanus, after a time 
 it will take on a condition of continuous contraction, which may be main- 
 tained for some time after the excitations have ceased (see Fig. 50). If the 
 muscle be very irritable, the contraction caused by a single irritation may be 
 long drawn out. A muscle poisoned by veratria and the same is true of 
 some other drugs (see p. 137) may show a remarkable degree of contracture 
 as a result of a single excitation. The contractions of fatigued muscles tend 
 to be greatly prolonged ; and this is very markedly the case with a dying 
 muscle, which gives well-defined, long-continued contractions, localized at 
 the point excited, called by Schiff the " idio-muscular contraction." The 
 contractions caused by the making and breaking of a strong battery current 
 applied to a muscle may likewise be followed by localized contractions which 
 last a considerable time. 
 
 In this connection one must bear in mind that the length of muscles 
 varies with their elasticity (see p. 105), and that this changes not a little 
 under varying conditions. Finally, it is necessary to recall that muscles when 
 entering into rigor mortis or rigor caloris take on a condition of contraction 
 which may last for days (see p. 159). 
 
 Contracture in Normal Muscles following Frequent Excitations. The con- 
 dition of prolonged after-contraction which results from frequent excitations 
 was first studied with care on the muscles of the frog, by Tiegel, 1 who gave 
 it the name of " contracture." 
 
 Richet found that the claw-muscles of the crab are particularly subject to 
 this form of contraction, Rossbach observed it in the muscles of the cat, and 
 Mosso 2 saw it in the muscles of man when vigorously excited either volun- 
 tarily or electrically. Mosso finds a teleological reason for its existence in 
 that it appears most marked under conditions when prolonged contractions 
 are desirable, and might offer a certain economy in^the innervation of muscle 
 by lessening the work of the nerve-cell. Richet 3 writes that normal con- 
 tracture is not to be confused with the prolonged relaxation of fatigued and 
 dying muscles, nor with the contraction of muscle substance in rigor mortis ; 
 it is best seen on muscles which are fresh and excitable. Although most 
 readily called out by strong direct electrical excitation of the muscle, it is 
 not due to the effect of the current as such, because it may be produced by 
 exciting the muscle indirectly through its nerve, and by voluntary muscular 
 contractions of man. On the other hand, the presence of the nerve is not 
 essential, for curarized muscles may exhibit contracture. 
 
 That a condition of increased excitability is favorable to the development 
 of contracture is made evident by the curve reproduced in Figure 57. In this 
 experiment the muscle was subjected to a tetanizing induction current for 
 nine seconds, the stimulation being interrupted for an instant every two 
 
 1 Tiegel : Pftiiger's Archiv, 1876, xiii. S. 71-84. 
 
 8 Mosso : Archives italiennes de Biologic, 1890, xiii. pp. 165-179. 
 
 8 Kichet: Dictionnaire de Physiologic, 1899, t. iv. pp. 391-393. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 129 
 
 seconds, to permit the contracture which was present at these times to show 
 itself. The effect was to increase the excitability of the muscle, as shown 
 by the increased height of the contractions recorded after the tetanus, and t<> 
 produce a marked contracture, as was shown by the fact that the muscle only 
 partially relaxed after the tetanizing current had ceased, and kept partially 
 contracted in the intervals between the succeeding separate contractions. The 
 fact that contracture can develop hand in hand with increasing excitability 
 shows that it may occur in the absence of fatigue. It is interesting to note 
 that the muscle made contraction and relaxation movements at the same 
 time that it remained continually, although incompletely, contracted ; and 
 finally, that the contracture offered a firm, elastic support to the separate 
 contraction movements, and that the relaxation movements following these 
 separate contractions were rapid, as is made evident by the character of the 
 elastic oscillations resulting from the rapid fall of the lever. 
 
 The fact that a muscle can remain continuously, though incompletely, con- 
 tracted, at the same time that it makes rapid contraction and relaxation move- 
 ments, suggests that it may at the same time be the seat of two independent 
 contraction processes. The observation recalls the action of the heart mus- 
 cle, for the ventricle maintains a condition of greater or less tonus, at the 
 same time that it makes separate beats ; it is therefore in harmony with a 
 well-known physiological process. 
 
 Contracture following Single Excitations. An examination of the contract- 
 ure effects sometimes seen to follow single excitations of irritable muscles 
 throws some light on the nature of the process. Richet observed on the 
 closing-muscle of the claw of the crab that a single excitation caused a rapid 
 contraction, which was followed by a rapid relaxation, and this in turn by a 
 second contraction movement which lasted a considerable time. 
 
 A similar curve may be obtained from the striated muscle of a frog incom- 
 pletely poisoned with veratria ; if a single shock be given, the curve rises 
 suddenly, and this quick rise is followed by an immediate fall, which is inter- 
 rupted by a second and slower rise, which is continued as a prolonged con- 
 traction. In both cases the curve suggests that the single excitation called 
 out two contraction movements, the first a rapid, short-lived contraction, the 
 second a slower, prolonged contraction. It has been suggested that the mus- 
 cle contains two kinds of muscle-fibres, which, like the pale (rapidly con- 
 tracting radialis externus) and red (slowly contracting radialis internus) 
 muscles of the rabbit, have two different rates of contraction. 1 This expla- 
 nation is not very satisfactory, because it has been found that both the pale 
 and the red muscles of the rabbit can give typical veratria contracture curves. 2 
 Moreover, both heart-muscle and non-striated muscles show independent tonus 
 and contraction movements though containing only one kind of muscle-fibre. 3 
 
 1 Griitzner: PJluger's Archiv, 1887, Bd. 41, 8. 256. 
 
 2 Carvallo and Weiss: Journal de Physiologic et Pathologic generale, 1899, t. i. p. 1. Bu- 
 cannan : Journal of Physiology, 1899, vol. xxv. p. 145. 
 
 3 Bottazzi : Journal of Physiology, 1897, vol. xxi. p. 1 
 
 VOL. II. 9 
 
130 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 It is hard to think* of one and the same kind of muscle substance con- 
 tracting and relaxing quickly at the same time that it is continuously con- 
 tracted, and the attempt has been made to explain the phenomenon on the 
 assumption that every muscle-fibre contains two kinds of contractile sub- 
 stance, and that the anisotropic fibrillary structures of the fibre are capable 
 of rapid contractions and the isotropic sarcoplasma of slow contractions. 
 According to this, the first quick rise of the contraction of the veratrinized 
 muscle, etc., is due to the anisotropic substance, and the prolonged after- 
 contraction to the sarcoplasma. This explanation offers much that is satis- 
 factory, but can scarcely be accepted until we are sure that anisotropic and 
 isotropic substances are capable of independent contractions. 
 
 The prolonged contraction of a muscle treated by veratria is an active 
 process, and not merely the result of a change in its physical condition, such 
 as an increase in elasticity. This is shown by the fact that during the stage 
 of contracture the muscle liberates more heat than when at rest. The heat 
 developed during a single veratria contraction may be as much as is given 
 off by a normal muscle excited to tetanus for two seconds. 1 The fact that 
 the prolonged contraction of the veratrinized muscle disappears on etheriza- 
 tion, and returns as the effect of the ether passes off, also favors the view 
 that it is dependent on physiological activity of the muscle protoplasm. 2 
 
 This conclusion is likewise indicated by the observation that the prolonga- 
 tion of the contraction is most marked at a moderate temperature, and fails 
 at very low or very high temperatures. 3 It has sometimes been thought that 
 it was an expression of fatigue, but this can hardly be the case, because it is 
 seen when the rate of the rise and the height of the curve of contraction are 
 normal, and it ceases in the case of the veratrinized muscle if the muscle is 
 worked for a time, and reappears when it has become rested. Moreover, 
 veratria in small doses strengthens the contractions of fatigued muscle and 
 increases its irritability, so that it responds to smaller stimuli by more work. 
 It would appear that we may conclude that the contracture of the veratrin- 
 ized muscle, like that of the normal muscle, is a true contraction process, but 
 that we must await further evidence before deciding as to the exact nature of 
 the contracture. 
 
 Effect of Fatigue. If a muscle be excited to contraction by frequent exci- 
 tations, its irritability for a time will be increased, the contractions will 
 become stronger, higher, and more prolonged. If a muscle be excited to 
 contraction at too slow a rate to cause an increase of irritability, it will 
 gradually fatigue, and, as it does so, its contractions will become weaker, 
 lower, and more prolonged. The prolongation of the contractions seen in 
 these two cases is probably due to quite different causes. 
 
 In the first experiment it was a true contraction process ; in the second it 
 
 1 Fick and Boehm : Verhand. der physikal-med. Gesellschaft in Wiirzburg, 1872, Bd. iii., N. 
 F., S. 198. 
 
 2 Locke : Journal of Experimental Medicine, 1896, vol. i. p. 630. 
 
 3 Brunton and Cash : Journal of Physiology, 1883, vol. iv. p. 237. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 131 
 
 was the result of inability to relax. Relaxation as well as contraction is to be 
 regarded as an active process, and in fatigue the power both to contract ami 
 relax is lessened. 
 
 The prolonged contraction of the fatigued muscle is chiefly caused by the 
 injurious effects of the waste products produced within it as a result of the 
 chemical changes accompanying its activity. One of these waste products, 
 sarcolactic acid, is known to have the effect to prolong muscular contractions, 1 
 and it is not unlikely that others may exert a similar influence. 
 
 In case the muscle be excited frequently and for a considerable time, 
 the contraction effect and the decreased power of relaxation due to fatigue, 
 toward the end of the experiment, may both be present at the same time, and 
 both act to prolong the curve of contraction. This was probably the case in 
 the experiments the records of which are given in Figures 47 and 48, and 
 many of the figures employed to illustrate the development of tetanus. 
 
 An example of this is to be seen in the effect of certain chemical sub- 
 stances on the muscle. For example, the withdrawal of water by drying, by 
 the application of glycerin, or by ^ strong solution of sodium chloride, may, 
 by rapidly altering the constitution of the protoplasm, cause an increase of 
 excitability which may pass over to a state of excitation, which will be man- 
 ifested by irregular but more or less continuous contractions. Such contrac- 
 tions are of the type of an incomplete tetanus. 
 
 Effect of Constant Battery Current. Attention has already been called to 
 the fact that under certain circumstances a form of continuous contraction 
 may be excited by a continuous constant electric current. If the current be 
 very strong, the short closing contraction may be followed by a more or less 
 continuous contraction the closing (or Wundt's) tetanus ; and the short open- 
 ing contraction may be followed by another continuous contraction, which 
 only gradually passes off the opening (or Hitter's) tetanus. This form of 
 contraction is quite readily excited in normal human muscles by both direct 
 and indirect excitation. The term " galvanotonus " is sometimes employed 
 for the continuous contraction of human muscles excited by the continuous 
 flow of a constant current. 
 
 Although a continuous contraction caused by the constant current is 
 spoken of as tetanus, it is a matter of doubt whether it is a true tetanic con- 
 dition, for the term tetanus is limited to a form of contraction which, though 
 apparently continuous, is really an interrupted process, and results from 
 many frequently repeated stimuli. Von Frey 2 expresses the view that the 
 continuous contraction which follows the closing of the continuous constant 
 current is a form of tetanus. It is certainly true that the closing tetanus 
 often shows irregular oscillations, suggestive of a more or less intermittent 
 excitation which might be explained on the supposition that the flow of the 
 current produces electrolytic decompositions within the tissue, and that the 
 liberated ions exciting the protoplasm of the different fibres irregularly lead 
 
 1 Lee: American Journal of Physiology, 1899, vol. ii. p. 11. 
 3 Archiv Jiir Anatomic und Physiologic, 1885, S. 55. 
 
132 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 to irregular contractions of the separate fibres, the combined action of which 
 
 produces more or less regular continued contraction. Another view would be 
 
 that contracture might be produced under the influence of the changes caused 
 
 - by the electric current, and a condition result similar to that which causes 
 
 . the prolonged contractions characteristic of poisoning with veratria, etc. 
 
 Effect of Death Processes. If a muscle be dying, it responds to excitations 
 by very slow, weak, and prolonged contractions, definitely localized at the 
 place excited. Such a form of contraction is often classed as contracture, in 
 spite of the fact that the irritability is greatly lessened. This form of con- 
 traction may be seen toward the end of prolonged wasting diseases in the case 
 of the muscles of men. They respond to mechanical stimulations by local- 
 ized, slowly developing contractions. 
 
 Pathological Contracture of Central Nervous Origin. In certain patholog- 
 ical conditions there may be contractures which do not depend upon the con- 
 dition of the muscle, but which originate in the central nervous system. In 
 these cases the muscles are in continuous receipt of nerve impulses from the 
 spinal cord cells, and are kept in continuous contraction, which varies in degree 
 from the amount observed during ordinary reflex muscle ton us to a state of 
 intense rigidity. The peculiarity of the condition is its endurance. The 
 muscle does not appear to fatigue ; moreover, it is said that it does not 
 develop the large amount of heat (Brissand et Regnard) which is always 
 formed as a result of the chemical changes which take place during the ordi- 
 nary contractions. 
 
 For these reasons, Richet * considers the shortening of the muscle to be 
 not a true contraction, but the result of an increase of elasticity. It is possi- 
 ble that some pathological contractions may be of different nature from those 
 which we have been considering, but they have not been studied sufficiently 
 to enable us to draw definite conclusions from them. 
 
 (d) Normal Physiological Contractions. All normal physiological con- 
 tractions of muscles are regarded as tetani. Even the shortest possible vol- 
 untary or reflex movements are considered to be too long to be single contrac- 
 tions. Inasmuch as we can artificially excite normal muscles to continuous 
 contraction only by means of a series of rapidly following stimuli, we find it 
 hard to explain continuous physiological contractions on any other basis, and 
 hence the view that the excitation sent by the nerve-cells to muscles has 
 always a rhythmic character, and that the normal motor-nerve impulse is a 
 discontinuous rather than continuous form of excitation. The view is prob- 
 ably correct, but cannot be considered as proved. The evidence in favor of 
 it is as follows : 
 
 Muscle-sounds, Tremors, etc. During voluntary muscular contractions the 
 muscle gives out a sound, which would imply that its finest particles are not 
 in a state of equilibrium, but vibrating. By delicate mechanisms it has been 
 possible to obtain records of voluntary and reflex contractions which showed 
 oscillations, although the contraction of the muscle appeared to the eye to be 
 
 1 Dictionnaire de Physiologic, 1899, iv. p. 393. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 133 
 
 continuous. If the surface of a muscle be exposed and be wet and glistening, 
 the light reflected from it during continued contractions is seen to flicker, as 
 if the surface were shaken by fine oscillations. In fatigue the muscle passes 
 from apparently continuous contraction to one exhibiting tremors, and mus- 
 cular tremors are observed under a variety of pathological conditions. 
 
 With these facts in mind, a number of observers have endeavored to dis- 
 cover the rate at which the muscle is normally stimulated. Experiments in 
 which muscles have been excited t6 incomplete tetanic contractions by induced 
 currents, interrupted at different rates, have shown that the muscle follows the 
 rate of excitation with a corresponding number of vibrations, and does not 
 show a rate of vibration peculiar to itself. Further, it has been ascertained 
 that the sound given out by a muscle excited to complete tetanus, I. e. an 
 apparently continuous contraction, corresponds to the rate at which it is ex- 
 cited. Apparently, any rate of oscillations detected in a muscle during normal 
 physiological excitation would be an indication of the rate of discharge of 
 impulses from the central nerve-cells. 
 
 Wollaston was the first to observe that a muscle gives a low dull sound 
 when it is voluntarily contracted, and that this sound corresponds to a rate of 
 vibration of 36 to 40 per second. It may be heard with a stethoscope placed 
 over the contracting biceps muscle, for instance, or if, when all is still and the 
 ears are stopped, one vigorously contracts his masseter muscles. Helmholtz 
 placed vibrating reeds consisting of little strips of paper, etc., on the muscle, 
 and found that only those which had a rate of vibration of 18 to 20 per 
 second were thrown into oscillation when the muscle was voluntarily contracted. 
 This observation indicated that the muscle had a rate of vibration of 18 to 20 
 per second, a rate too slow to be recognized as a tone. He concluded that the 
 tone heard from the voluntarily contracted muscle was the overtone, instead 
 of the true muscle-tone. The consideration that the resonance tone of the 
 ear itself corresponds to 36 to 40 vibrations per second, makes it question- 
 able whether the muscle-sound should be accepted as evidence of the rate of 
 normal physiological excitation ; nervetheless, the experiments with the 
 vibrating reeds remain to indicate 18 to 20 per second to be the normal 
 rate. 
 
 Within the last few years a number of researches bearing upon this question 
 have been published, and the results of these point to a still slower rate of vol- 
 untary excitation, varying from 8 to 12 per second according to the muscle on 
 which the experiment is made. Lov&i 1 discovered in the tetanus excited in 
 frogs poisoned with strychnia, and in voluntary contractions, both by mechani- 
 cal methods and by recording the electrical changes occurring during action 
 with the capillary electrometer, rates of 7 to 9 per second. Horsley and 
 Schafer 2 ex cited the brain cortex and motor tracts in the corona radiata and the 
 spinal cord of mammals by induction shocks, at widely differing rates, and 
 recorded the resulting muscular contractions by tambours placed over the 
 muscles. They observed oscillations in the myograms obtained which had a 
 
 1 Central blatt ftir die medicinischen Wissenscho/ten, 1881. 
 
 2 Journal of Physiology, 1886, vii. p. 96. 
 
134 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 rate of 8 to 12 per second, the average being 10. The rate of oscillations was 
 quite independent of the rate of excitation, and oscillations of the same rate 
 were seen by voluntary and by reflex contractions. Tunstall l found by the use 
 of tambours, in experiments on voluntary contractions of men, a rate of 8 to 13 
 per second, with an average of 10. Griffiths 2 likewise used the tambour 
 method, and studied the effect of tension on the rate of oscillations in voluntarily 
 contracted human muscles. He observed rates varying from 8 to 19, the rate 
 being increased with an increase of weight up to a certain point, and beyond this 
 decreased. The oscillations became more extensive as fatigue developed. Von 
 Kries by a similar method found rates varying with different muscles, but 
 averaging about 10. 
 
 It is not easy to harmonize the view that 8 to 13 excitations per second 
 can cause voluntary tetani, when it is possible for the expert pianist to make 
 as many as 10 or 11 separate movements of the finger in a second. It is, 
 indeed, a common observation that a muscle can be slightly and continuously 
 voluntarily contracted, and, at the same time, be capable of making additional 
 short rapid movements. Von Kries would explain this as due to a peculiar 
 method of innervation, while Biedermann favors Gruetzner's 8 view that the 
 muscle may contain two forms of muscle-substance, one of which is slow to 
 react, resembling red muscle-tissue, and maintains the continuous contraction, 
 the other, of more rapid action, being responsible for the quicker movements. 
 Although the evidence is, on the whole, in favor of the view that all normal 
 contractions of voluntary muscles are tetanic in character, there is a great deal 
 which remains to be explained. 
 
 Effect of Artificial compared with Normal Stimulation. Experiment shows 
 that, with the same strength of irritant, a muscle contracts more vigorously 
 when irritated indirectly, through its nerve, than when it is directly stimulated. 
 Rosenthal describes the following experiment : If the nerve of muscle A be 
 allowed to rest on a curarized muscle B, and an electric shock be applied in 
 such a way as to excite nerve A and muscle B to the same amount, muscle A 
 will be found to contract more than muscle B. 
 
 Further, it has been found that muscles respond more vigorously to volun- 
 tary excitations than to any artificial stimulus which can be applied to either 
 the nerve or muscle. This shows itself, not only in the fact that a muscle can 
 by voluntary stimulation lift much larger weights than by electrical excitation, 
 but that after a human muscle has been fatigued by electrical excitations it 
 can still respond vigorously to the will. An illustration of this is given in 
 Figure 58. 
 
 Fatigue of Voluntary Muscular Contractions. Mosso and his pupils have 
 done a large amount of work upon the fatigue of human muscles when excited 
 by voluntary and artificial stimuli under varying conditions (see p. 72). The 
 results at which they arrived all favor the view that human muscles differ 
 but little from those of warm-blooded animals, and that the facts which have 
 
 Journal of Physiology, 1886, vii. p. 114. 2 Journal of Physiology, 1888, ix. p. 39. 
 
 8 P/Higer** Archiv, 1887, Bd. 41, S. 277. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 1 :'.."> 
 
 been ascertained by experiments upon cold-blooded animals, such as the 
 frog, can be accepted with but slight modifications for the muscles <!' man. 
 In the experiment recorded in Figure 58 we see the effect of repeated tetanic 
 
 FIG. 58. Voluntary excitations are more effective than electrical. The flexor muscles of the second 
 finger of the left hand of a man were excited first voluntarily, a, then electrically, a-b, and then volun- 
 tarily, b. The electrical excitation consisted of series of induction shocks, which were applied once 
 every two seconds, during about half a second, the spring interrupter of the induction coil vibrating 
 23 times per second. Each time the muscle contracted it raised a weight of one kilogram. Each of the 
 contractions recorded, whether the result of electrical or voluntary excitation, was a short tetanus. 
 
 contractions, excited by electricity, to fatigue a human muscle. Normal 
 voluntary contractions, if frequently repeated, provided the muscle has to 
 raise a considerable weight, likewise cause fatigue. This was illustrated in 
 the experiment recorded in Figure 59. 
 
 FIG. 59. Effect of fatigue on voluntary muscular contractions. The flexor muscles of the second 
 finger of left hand were voluntarily contracted once every two seconds, and always with the utmost 
 force. The weight raised was four kilograms. 
 
 It is doubtful whether, in an experiment such as is shown in Figure 59, the 
 loss of the power to raise the weight is due to fatigue of the muscles. It is 
 more likely that the decline in power is due to fatigue of the central nerve- 
 
136 AN AMERICAN TEX7-BOOK OF PHYSIOLOGY. 
 
 cells by which the muscles are excited to action during the voluntary mus- 
 cular work. 1 This fact, that the nerve-cells give out before the muscles, ex- 
 plains the apparent contradiction, that a muscle fatigued by electric excitations 
 can be voluntarily contracted, and when the power to voluntarily contract the 
 muscles has been stopped by fatiguing voluntary work the muscles will respond 
 to electrical excitation. It is undoubtedly of advantage to the body that the 
 nerve-cells should fatigue before the muscles, for the muscles are thereby pro- 
 tected from the injurious effects of overwork, and are always ready to serve the 
 brain. 2 It may be added that nerve-cells not only fatigue more quickly, but 
 recover from fatigue more rapidly than the muscles. 
 
 (e) Effect of Temperature upon Muscular Contraction. Heat, within certain 
 limits, increases the irritability and conductivity of muscle-tissue, and at the 
 same time has a favoring influence upon those forms of chemical change which 
 liberate energy. The effect of a rise of temperature, as shown by the myo- 
 gram, is a shortening of the latent period, an increase in the height of contrac- 
 tion, and a quickening of the contraction and relaxation, the whole curve being 
 shortened. Of course there is an upper limit to this favoring action, since, at a 
 
 FIG. 60. Schema of effect of temperature on height and form of contraction curve : a, contraction at 
 19 C. ; b, c, d, e,f, contractions made at intervals, each one at a lower temperature; g, h, contractions 
 at higher temperatures than 19 C., h being made when the temperature was 30 C. ; i, k, I, show a different 
 series of contractions, made as the temperature was Increased from 30 C. toward the point at which the 
 muscle-substance coagulates (after Gad and Heymans). 
 
 certain temperature, about 45 C. for frog's muscle and about 50 C. for the 
 striated muscles of warm-blooded animals, 53-58 C. for the non-striated 
 muscles of the bladder of the cat, 3 heat-rigor begins, and this change is accom- 
 panied by a loss of all vital properties. Cold can be said, in general, to pro- 
 duce effects the opposite of those of heat; as the muscle is cooled, the latent 
 period, the contraction, and the relaxation are all prolonged. 
 
 Nevertheless, the effect of temperature is not a simple one (see Fig. 60). If 
 during the cooling process a striated muscle of a frog be irritated from time to 
 time with single induction shocks, the height of the contractions does not con- 
 tinually grow less as one would expect. 4 The maximal height is obtained at 
 30 C., the height above this point being somewhat less, the irritability les- 
 sening as the coagulation- point is approached; from 30 C. to 19 C. the 
 height continually decreases, but from 19 to C. the height increases, while 
 
 1 Lombard : Archives italiennes de Biologie, xiii. p. 1 ; of American Journal of Psychology, 
 1890, p. 1 ; Journal of Physiology, 1892, p. 1 ; 1893, p. 97. 
 'Waller: Brain, 1891, p. 179. 
 
 S C. C. Stewart: American Journal of Physiology, 1900, iii. p. 25. 
 4 Gad und Heymans : Archivfur Analomie und Physiologie, 1890, S. 73. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 137 
 
 below 0. it again becomes less, until at the freezing-point of muscle no con- 
 traction is obtained. The cause of these peculiar phenomena is not definitely 
 understood. 
 
 (/) Effect of Drugs and Chemicals upon Muscular Contraction. Certain drugs 
 and chemicals have a marked effect upon the irritability (see p. 58) and con- 
 ductivity (see p. 93) of muscles, and these effects must necessarily find expres- 
 sion in the amount of contraction which would be excited by a given irri- 
 tant. In addition to this, it is worthy of notice that the character of the con- 
 traction may be altered. 
 
 The drug which has the most striking effect upon the form of contraction is 
 veratria. A few drops of a 1 per cent, solution of the acetate of veratria, 
 injected into the dorsal lymph sac of a frog whose brain has first been 
 destroyed, in a few minutes alter completely the character of the reflex 
 movements : the muscles are still capable of rapidly contracting, but the con- 
 tractions are cramp-like, the power to relax being greatly lessened. The 
 poison acts upon the muscle-substance, and even a very small dose applied 
 
 FIG. 61. Myogram of muscle poisoned with veratria and that of a normal muscle : a, myogram from a 
 normal gastrocnemius muscle of a frog the waves at the close are due to the recoil of the recording lever ; 
 b, myogram from a gastrocnemius muscle poisoned with veratria, recorded at the same part of the drum. 
 
 directly to the muscle for a few hours e. g., a solution containing 1 part to 
 100,000 of 0.6 per cent, solution of sodium chloride suffices to greatly alter 
 the character of the contraction called out by various irritants. 1 If a muscle 
 poisoned with veratria be isolated and connected with a myograph, a contrac- 
 tion excited by a single induction shock will show a rise as rapid, as high, 
 and as strong as normal, but the fall of the curve will be greatly prolonged 
 (see Fig. 61). Often the crest of the curve will exhibit a notch, which shows 
 that relaxation may begin and be checked by a second contraction process 
 which carries the curve up again and holds it there for a considerable time. 
 In the above experiment the contracture effect followed the primary contrac- 
 tion immediately. The nature of the contracture of a muscle poisoned with 
 veratria has been considered (see p. 130). 
 
 There are a number of drugs which have an action on muscle-tissue simi- 
 lar to that of veratria e. g., cornutine 2 produces a similar effect on striated 
 muscles ; digitaline increases the tonus of heart muscle and of the smooth 
 
 ^ucannan: Journal of Physiology, 1899, xxv. p. 137. 
 * Cushny : Pharmacology and Therapeutics, 1899. 
 
138 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 muscle-tissue of the walls of the blood-vessels ; epinephrin, 1 the active prin- 
 ciple of the extracts obtained from the medullary part of the suprarenal 
 capsules, may be mentioned here, and is of especial interest because derived 
 from the animal body. If injected into the blood, it increases the strength 
 and prolongs the contraction of the muscles generally, and causes through its 
 effect on the muscle of the heart and the non-striated muscles of the blood- 
 vessels a marked rise of blood-pressure. 2 
 
 Barium salts, and to a less degree calcium and strontium, act similarly 
 to veratria to prolong the relaxation of the muscle, without lessening the 
 rapidity and height of contraction. 
 
 Potassium and ammonium salts and a large number of other chemical 
 substances and drugs act to kill the muscle, and as the death process develops 
 excitation produces prolonged localized contractions. This effect seems to be 
 quite different from that of veratria, for it is accompanied by a rapid lessen- 
 ing of the muscular power. 
 
 5. Liberation of Energy by the Contracting Muscle. The law of con- 
 servation of energy applies no less to the living body than to the inanimate 
 world in which it dwells. Every manifestation of life is the result of the 
 liberation of energy which was stored in the body in the form of chemical 
 compounds. When a muscle is excited to action it undergoes chemical 
 changes, which are accompanied by the conversion of potential into kinetic en- 
 ergy. This active energy leaves the muscle in part as thermal energy, in part 
 as mechanical energy, and, to a slight extent, under certain conditions, as^glec- 
 trical energy. In general, the sum of the liberated energy is given off as heat 
 or motion. The proportion in which these two forms of energy shall be pro- 
 duced by a muscle may vary within wide limits, according to the state of the 
 muscle and the conditions under which the work is done. Fick 3 states that 
 if the muscle works against a very heavy weight, possibly one-fourth of the 
 liberated energy may be obtained as mechanical work ; but if the weight be 
 light not more than one-twentieth of the chemical energy is given off in this 
 form, the muscle working no more economically than a steam engine. Zuntz 4 
 studied the work that the body as a whole could accomplish, and found that 
 somewhat more than one-third of the energy liberated can be obtained as 
 external mechanical work. The fact that always a part, and often the whole, 
 of the mechanical energy developed by the muscle is converted to thermal 
 energy within the muscle, and leaves it as heat, makes it the more difficult to 
 determine in what proportion these two forms of energy were originally pro- 
 duced. Moreover, if Engelmann's view be correct, that the change of form 
 exhibited by the muscle is the result of the imbibition of the fluid of the 
 isotropic substance by the anisotropic material, this change being brought 
 about by the heat which is liberated within the muscle, we must consider 
 potential energy to be set free first as heat, a part of which is afterward 
 
 Abel: Zeitschrift fur physiologische Chemie, 1899, Bd. xxviii. S. 354. 
 * Oliver and Schafer : Journal of Physiology, 1895, xviii. pp. 230-276. 
 3 Fick: Pfliiger's Archiv, 1878, xvi. S. 85. 
 */6rrf., 1897, Bd. Ixviii., S. 191. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 139 
 
 changed to mechanical energy, which in part, at least, is again changed to 
 heat. 
 
 Liberation of Mechanical Energy. The amount of work which a muscle 
 can do depends on the following conditions : 
 
 (a) The kind of muscle. The muscles of warm-blooded animals are stronger 
 than those of cold-blooded animals ; a human muscle can do twice the amount 
 of work of an equal amount of frog's muscle. The muscles of certain insects 
 have even greater strength. 1 Within the same animal there are great differ- 
 ences in the capacity of different forms of muscle tissue (see p. 107). Pale 
 striated muscle tissue, although more capable of rapid liberation of energy, 
 has not the endurance or the strength of the red striated muscle tissue ; and 
 different forms of non-striated muscle differ among themselves as well as 
 from striated in their capacity for work. 
 
 (6) The condition of the musck. Any of the influences which lessen the 
 irritability of the muscle lack of blood, fatigue, cold, etc. decreases the power 
 to liberate energy, and any influence which heightens the irritability is favora- 
 ble to the work. The effect of tension to heighten irritability has already been 
 referred to and is of especial interest in this connection, since the very re- 
 sistance of the weight is, within limits, a condition favorable to the liberation 
 of the energy required to overcome the resistance. This will be referred to 
 again. 
 
 (c) The strength and character of the stimulus. The liberation of energy is, 
 up to a certain point, the greater, the stronger the excitation. Furthermore, 
 rapidly repeated excitations are much more effective than single excitations, 
 because a series of rapidly following stimuli, both by altering the irritability and 
 by inducing the form of contraction known as tetanus, act to produce powerful 
 and high contractions. Bernstein states that the energy developed by the 
 muscle increases with the increase of the rate of excitation from 10 to 50 per 
 second, at which rate the contraction power may be double that called out by a 
 single excitation. 
 
 (d ) The method, of contraction and the mechanical conditions under which 
 the work is done. In estimating the amount of mechanical energy liberated 
 by a muscle, we observe the amount of external work which it accomplishes, 
 i. e. the amount of mechanical energy which it imparts to external objects. 
 If a muscle by contracting raises a weight, it gives energy to the weight, the 
 amount being exactly that which the weight in falling through the distance 
 which it was raised by the muscle can impart as motion, heat, etc., to the 
 objects with which it comes in contact. The measure of the mechanical 
 work done by the contracting muscle is the product of the weight into the 
 height to which it is lifted. For example, if a muscle raises a weight of 5 
 grams 10 millimeters, it does 50 grammillimeters of work. An unweighted 
 muscle in contracting does no external work ; a muscle, however vigorously 
 it may contract, if it be prevented from shortening, does no external work ; 
 finally, a muscle which raises a weight and then lowers it again when it 
 relaxes, does not alter its surroundings as the total result of its activity, and 
 1 Hermann : Handbuch der Physiologic, 1879, Bd. i. S. 64. 
 
140 ^4^ AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 hence does no external work. Although no external work is accomplished 
 under these circumstances, internal work is being done, as is evidenced by 
 the heat evolved by the muscle and the fatigue produced. Unquestionably 
 mechanical energy is developed within the muscle in all these cases, but it 
 is all converted to heat before it leaves the muscle. 
 
 The amount of weight is an important factor in determining the extent to 
 which a muscle will shorten when excited by a given stimulus, and, therefore, 
 the quantity of work which it will accomplish. If a muscle be after-loaded, 
 i. e. if the weight be supported at the normal resting length of the muscle, and 
 the muscle be excited to a series of maximal contractions, the weight being in- 
 creased to a like amount before each of the succeeding excitations, there is, in 
 general, a gradual lessening in the height of the contractions, but the de- 
 crease in height is not proportional to the increase of the weight. The 
 decrease in the height of contractions is, as a rule, more rapid at the beginning 
 of the series than later, though at times an opposite tendency may show itself 
 and the increasing weights temporarily increase the irritability and therefore 
 increase the amount of shortening. The effect of tension to increase the activ- 
 ity of the contraction process is seen if a muscle which is connected with a 
 strong spring or heavy weight be excited to isometric contractions and in 
 the midst of a contraction be suddenly released ; the muscle under such cir- 
 cumstances is found to contract higher than when excited by the same stimulus 
 without being subjected to tension. 1 The effect of tension on the activity of 
 muscular contractions is to be clearly seen in the case of the heart muscle. 
 A rise of pressure of the fluid within the isolated heart of a frog increases 
 the strength as well as the rate of the beat. 
 
 If the weight be gradually increased, although the height of the contrac- 
 tions is lessened, the work will for a time increase, and a curve of work (con- 
 structed by raising ordinates of a length corresponding to the work done, 
 from points on an abscissa at distances proportional to the weights em- 
 ployed), will be seen to rise. After the weight has been increased to a cer- 
 tain amount the decline in the height of contractions will be so great that the 
 product of the weight into the height will begin to decrease, and the curve of 
 work will fall, until finally a weight will be reached which the contrasting 
 muscle can just support at, but not rajse above, its normal resting length 
 This weight will be a measure of the absolute muscular force. 
 
 Load 
 (grams). 
 ... 
 
 Example. 
 
 Height of lift 
 (millimeters). 
 13 
 
 Work 
 (grammillimeters). 
 
 
 30 
 
 11 
 
 330 
 
 60 
 
 9 
 
 540 
 
 90 
 
 .7 
 
 630 
 
 120 
 
 .... 5 
 
 . . 600 
 
 150 
 
 3 
 
 450 
 
 180 
 
 2 
 
 360 
 
 210 . 
 
 . . 
 
 
 
 1 Ffck : Mechanische Arbeit, etc., S. 132. Santesson : Skandinavisches Archiv fur Physiologie, 
 1889, i. S. 56. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 141 
 
 In the above experiment 30 grams were added to the muscle after each 
 contraction ; as the weight was increased up to 90 grams the amount of 
 work was increased, with greater weights the amount of work was lessened. 
 
 It is evident that the absolute force of a muscle of a given type will depend 
 not only on the quantity, but also on the arrangement of the microscopic ele- 
 ments of which the muscle is composed. Each element of a fibre has to stand 
 the strain of the whole fibre ; so the force to be developed depends not on the 
 length of the fibres, but on the number of muscle elements which are arranged 
 side by side, i. e. the absolute force of a muscle will be proportionate to the 
 number of fibres. This can be stated for a muscle with parallel fibres in 
 terms of the cross section of the muscle. In the case of a muscle like the 
 gastrocnemius, where the fibres take an oblique course and are inserted into 
 a common tendon in the middle, the " physiological cross-section " has to be 
 estimated, i. e. the total section taken at right angles to the fibres. Such a 
 muscle is very strong in proportion to its thickness. Rosenthal estimated 
 the absolute force of striated muscles of the frog to be about 3 kilograms per 
 square centimeter, and Hermann l found the absolute force of striated muscle 
 of man to be 6.24 kilograms per square centimeter. 
 
 The physiological work of which a muscle is capable, on the other hand, 
 is dependent not only on the weight which it can lift, but also the height to 
 which the weight can be lifted. All the muscle elements, whether arranged 
 side by side or in chains, influence the result, and for purposes of comparison 
 one can state the capacity of the muscle for work in terms of the unit of 
 bulk, the cubic centimeter, or the unit of weight, the gram. Thus, Fick 
 states the maximal amount of external work of which frog's muscle is 
 capable, as one grammeter per gram of muscle substance. 
 
 From what has been said it is evident that the amount of muscle sub- 
 stance determines the amount of work of which the muscle is capable, while 
 the arrangement of the muscle substance decides the character of the work 
 which it is best fitted to perform. Muscles with long parallel fibres, even 
 though of small sectional area, such as the sartorius, are specially fitted to 
 produce extensive movements of the parts to which they are attached ; and 
 muscles which have a large number of fibres, even though these be short, as 
 in the case of the gastrocnemius, are adapted to move great weights. 
 
 Carvallo and Weiss 2 state that the gastrocnemius muscle of the frog 
 when at rest tears if subjected to a weight of 2 kilos. Its contraction power 
 is estimated to be half a kilo, and when it is contracting its resistance is cor- 
 respondingly increased, so that a weight of 2J kilos is required to rupture it. 
 The increased resistance can be best explained on the idea that, as Pfluger 
 thinks, a new chemical attraction force is developed in contraction. 
 
 Liberation of Thermal Energy. Energy leaves the body as mechanical 
 energy only when by its movements the body imparts energy to surrounding 
 objects. Most of the energy liberated within the body leaves it as heat ; 
 
 1 Fyiuger's Archiv, 1898, Bd. 73, S. 429. 
 
 1 Carvallo tvnd Weiss : Comptes rendus Societe de Biologie, 1899, p. 122. 
 
142 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 even during violent muscular exercise five times more energy may be ex- 
 pended as heat than as mechanical energy, and the disproportion may be 
 even greater than this. Rosenthal says that at the most not more than 30 
 per cent, of the energy developed in the muscle by oxidation and splitting 
 processes is to be got as available mechanical energy. So great is the pro- 
 duction of heat during exercise that, in spite of the great amount leaving the 
 body, the temperature of an oarsman has been found to be increased, during 
 a race of 2000 meters, from 37.5 C. to 39 or 40 C. 1 
 
 It is exceedingly difficult to ascertain with accuracy on the warm-blooded 
 animal the exact relation of heat-produccion to muscular contraction. The 
 best results have been obtained by experiments on isolated muscles of cold- 
 blooded animals. Helmholtz observed the temperature of a muscle of a 
 frog to be increased by tetanus lasting a couple of minutes 0.14 to 0.18 
 C. ; Heidenhain saw a change of 0.005 C. result from a single contraction ; 
 and Fick ascertained that a fresh, isolated muscle of a frog can by a single 
 contraction produce per gram of muscle-substance enough heat to raise 
 3 milligrams of water 1 C. 2 To obtain evidence of the slight changes 
 of temperature which occur in such small masses of muscle-tissue it is 
 necessary to employ a very delicate instrument, such as a thermopile or a 
 bolometer. 
 
 The thermopile consists of strips of two dissimilar metals, united at their extremities, 
 so as to form a series of thermo-electric junctions. If there be a difference of temperature 
 at two such junctions, a difference of electric potential is developed, -which causes the 
 flow of an electric current. If the current be passed through the coils of wire of a 
 galvanometer its amount can be measured, and the extent of the change in tempera- 
 ture at one of the junctions, the other remaining constant, can be estimated. In the 
 more sensitive instruments, several thermo-electric junctions are used. The amount of 
 current depends largely on the metals employed, antimony and bismuth being a very 
 sensitive combination. 
 
 The action of the bolometer is based on the fact that the resistance of a wire to the 
 passage of an electric current changes with its temperature. 
 
 The amount of heat developed within the muscle by direct conversion of 
 potential to thermal energy, and the amount formed indirectly, through con- 
 version of mechanical to thermal energy, has been made a subject of careful 
 study by Heidenhain, 3 Fick and his pupils, 4 and others, the experiments being 
 made chiefly with isolated muscles of frogs. 
 
 In general, the stronger the stimulus and the greater the irritability of the 
 muscle in other words, the more extensive the chemical changes excited in 
 the muscle the greater the amount, not only of mechanical, but of thermal 
 energy liberated. Increase of tension, which is very favorable to muscular 
 
 1 George Kolb : Physiolog i of Sport, translated from the German, second edition, London, 
 1892. 
 
 3 Fick : Pfliiger's Archiv, 1878, xvi. S. 80. 
 
 3 Mechanische Leistung, Warmeentwicklung and Stoffumsatz bei der Muskelthatigkeit, Leipzig, 
 1864. 
 
 * Myothermische Untersuchunyen aus den ph riologischen Laboratorium zv Zurich und Wurzburg, 
 Wiesbaden, 1889. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 143 
 
 activity, greatly increases the heat-production. For this reason isometric 
 contractions, that is, those in which the muscle works against a resistance 
 which is sufficient to prevent it from shortening, are accompanied by a 
 greater liberation of heat than isotonic contractions, in which the contracting 
 muscle raises a constant weight. As the weight is increased, both the 
 amount of heat developed and the work are increased, but the liberation of 
 heat reaches its maximum and begins to decline sooner than the amount of 
 work i. e., with large weights the muscle works more economically; simi- 
 larly, as the muscle is weakened by fatigue the heat-production lessens sooner 
 than the work. 
 
 Muscle-tonus and Chemical Tonus. During waking hours, the cells 
 of the central nervous system are continually under the influence of a shower 
 of weak nervous impulses, coming from the sensory organs all over the body ; l 
 moreover, activity of brain-cells, especially emotional forms of activity, leads 
 to an overflow of nervous impulses to the spinal cord and an increased irrita- 
 bility, or, if stronger, excitation of motor nerve-cells. If, when one is quietly 
 sitting and reading, he turns his attention to the sensory impressions which 
 are coming at every moment from all over the body to the brain, notes the 
 temperature of different parts of the skin, the pressure of the clothes, etc., 
 upon different parts, the light reflected from neighboring objects, and the slight 
 sounds about him, he will recognize that the central nervous system is all the 
 time subject to a vast number of excitations, which, because of their very 
 repetition, are ordinarily disregarded by the mind, but which are, nevertheless, 
 all the time influencing the nerve-cells. The effect of this multitude of affer- 
 ent stimuli, in spite of their feebleness, is to cause the motor cells of the cord 
 to continually send delicate motor stimuli to the muscles. These cause the 
 muscle to keep in the state of slight but continued contraction which gives the 
 tension peculiar to waking hours, and which is called muscle-tonus. That 
 such a tension exists is made evident by the change in attitude which occurs 
 when the relaxation accompanying sleep comes on. The effect of brain activ- 
 ity to cause muscular tension is, likewise, most easily recognized by observing 
 the relaxation of the muscles which occurs when mental excitement ceases. 
 
 Muscle-tonus, like every form of muscular contraction, is the result of chem- 
 ical change, and the liberation of energy. But little of this energy leaves the 
 body as mechanical energy, most of it being given off as heat. 
 
 This view is by no means universally accepted, and many physiologists 
 believe in a production of heat by the muscles, as a result of nervous influences, 
 independent of contraction. It is thought that a condition of slight but con- 
 tinuous chemical activity resulting in the production of heat maybe maintained 
 in the muscles by intermittent but frequent reflex excitations, a condition which 
 has been called chemical tonus. 2 That the chemical activity of muscles is kept 
 
 1 Brondgeest: Archiv fiir Anatomie und Physiologic, 1860, S. 703; Hermann, Ibid., 1861, 
 S. 350. 
 
 2 Koehrig und Zuntz : Pfliiger's Archiv, 1871, Bd. iv ; Pfliiger: Pfliiger's Archiv, 1878, xviii. 
 S. 247. 
 
144 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 up by small stimuli from the spinal cord is shown by the fact that if the nerves 
 be severed, or the nerve-ends be poisoned by curare, the muscle absorbs less 
 oxygen and gives off less carbon dioxide than when at rest under normal 
 conditions. 1 
 
 The theory of a reflex chemical tonus independent of contraction implies 
 the existence of special nervous mechanisms for the exciting of chemical 
 changes in the muscles which shall result in the liberation of energy as heat, 
 independent of the change of form of the muscle. The question of the exist- 
 ence of special nervous mechanisms controlling heat-production heat-centres, 
 as they are called will be considered in another part of this book. 
 
 E. ELECTRICAL PHENOMENA IN MUSCLE AND NERVE. 2 
 
 The active muscle liberates three forms of energy : mechanical work, heat, 
 and electricity. The active nerve makes no visible movements, gives off no 
 
 recognizable quantity of heat, but 
 exhibits changes in electrical condition 
 quite comparable to those observed in 
 
 ^ - ^^^^ the active muscle. The electrical 
 
 < \^' """-^ changes in nerves are the only evidence 
 
 of activity which we can observe, aside 
 from the effect of the nerve on the 
 organ which it excites ; they are there- 
 fore of great interest to us. 
 
 Electrical energy, like all forms of 
 active energy, is the result of a trans- 
 formation of potential or some form of 
 kinetic energy. In the case of the 
 muscle, as of an electric battery, we 
 find electricity to be associated with 
 chemical change, and believe it to be 
 liberated from stored potential energy. 
 In the case of nerves no chemical 
 change can be detected during action, 
 and hence we are at a loss to explain 
 the development of electricity. We 
 can only say that it is the result of 
 some chemical or physical process 
 which we have as yet failed to discover. 
 Although activity of nerve and 
 muscle is found to be associated with electrical change, we must not suppose 
 functional activity to be in any sense an electrical process. The movements 
 
 1 Zuntz : Pfliiger's Archiv, 1876, xii. 522 ; Colasanti, Ibid., 1878, xvi. S. 57. 
 2 Biederraann: Electrophysiologie, Jena, 1895, Bd. ii.; translation by F. A. Wei by, 1898; 
 Waller: Lectures on Animal Electricity, London, 1897. 
 
 FIG. 62. Schema of galvanometer : n, s, north 
 and south poles of astatic pair of magnets ; m, 
 compensating magnet, held by friction on the 
 staff, and capable of being approached to, or ro- 
 tated with reference to, the suspended magnet ; 
 X, mirror; /, fibre supporting the magnets; c, c, 
 c, c, coils of wire to carry the electric current 
 near to the magnets, the upper coils being wound 
 in the opposite direction to the lower ; e, e, non- 
 polarizable electrodes applied to the longitudinal 
 surface and cross section of a muscle. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 145 
 
 of a man may he interpreted from the movements of his shadow, but they 
 are very different phenomena; the activity of tin- nerve and muscle is indi- 
 cated by the electrical changes accompanying it, but they may be independent 
 processes. Certainly the irritating change which is transmitted along the 
 nerve and which excites the muscle to action, although accompanied by elec- 
 trical changes, is not itself an electric current. 
 
 Electrical energy is exhibited not only by active nerve and muscle, but 
 during the activity of a great variety of forms of living matter. It may be 
 detected in gland-cells, in the cells of many of the lower animal organisms, 
 and even in plant-cells. The amount of electrical energy developed in animal 
 tissues may be far from trivial. Although delicate instruments are necessary 
 to observe the electrical changes in nerve and muscle, as the great internal 
 resistance of the tissues causes the currents to be small, we find in certain 
 fish special electric organs, which appear to be modified muscle tissue, and 
 which are capable of discharging a great amount of electrical energy when 
 excited through their nerves. So intense is the action of this electrical 
 apparatus that it can be used as a weapon of defence and offence. Gotch 
 and Burch state that the electric organ of the malapterurus electricus can 
 give a shock having an electric potential of 200 volts. 1 
 
 1 . Methods of Ascertaining the Electrical Condition of a Muscle or a Nerve, 
 
 If the electric tension of any two parts of an object differs, the instant they are joined an 
 electric current will flow from the point where the tension is greater to that where it is less. 
 The presence, direction of flow, and strength of an electric current can be detected by an 
 instrument called a galvanometer. If any two parts of a muscle or nerve, as e, e, Figure 
 62, be connected by suitable conductors with the coils, c, c, of a galvanometer, and if there 
 be a difference in the electric potential of the two parts examined, an electric current will 
 be indicated by the instrument. In such tests all extra sources of electricity are to be 
 avoided, therefore the electrodes applied to the muscle must be non-polarizable. 
 
 The Galvanometer An ordinary form of galvanometer consists of a magnet suspended 
 by an exceedingly delicate fibre of silk, or quartz, and one or more coils, composed of many 
 windings of pure copper wire, placed vertically near the magnet and in the plane of the mag^ 
 netic meridian. If an electric current be allowed to flow through the wire, it influences the 
 magnetic field about it, and, if the coils be close to the suspended magnet, causes the 
 magnet to deviate from the plane of the magnetic meridian in one or the other direction, 
 according to the direction of the flow of the current. In the more delicate instruments the 
 influence of the earth's magnetism is lessened by the use of two magnets of as nearly as pos- 
 sible the same strength, placed so as to point in opposite directions, and fastened at the 
 extremities of a light rod. As each magnet tends to point toward the north, they mutually 
 oppose each other, and therefore the effect of the earth's magnetism is partly compensated. 
 Still another magnet may be brought near this "astatic" combination, and by opposing the 
 action of the earth's magnetism make the arrangement even more delicate. In the Thomp- 
 son galvanometer, the rod connecting the needles bears a slightly concave mirror, from 
 which a beam of light can be reflected on a scale. Or a scale may be placed so that its 
 image falls on the mirror, and the slightest movement of the magnet may be read in the 
 mirror by a telescope. 
 
 The ordinary galvanometer is influenced by changes in the magnetism of the earth r 
 and by earth currents which may be due to an escape of electricity from neighboring 
 street-car circuits, etc. These disturbances may interfere with the use of the instrument, 
 
 1 Proceedings of the Royal Society, 1900, vol. Ixv. p. 442. 
 VOL. II. 10 
 
146 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 because they may lead to uncontrollable movements of the magnet, and a consequent shift- 
 ing of the point. There are other forms of instruments, such as the Deprez-d' Arson val 
 mirror galvanometer, 1 which are not affected by such influences. 
 
 The galvanometer is very sensitive to the presence of electric currents. Another appa- 
 ratus which is even more responsive to changes in electric potential of short duration is 
 the capillary electrometer. 
 
 The capillary electrometer (Fig. 63) consists of a glass tube (a) drawn out to form 
 a very fine capillary, the end of which dips into a glass cup with parallel sides (/) contain- 
 ing a 10 per cent, solution of sulphuric acid. The 
 upper part of the tube is connected by a thick- 
 walled rubber tube with a pressure-bulb containing 
 mercury (c). As the pressure-bulb is raised, the 
 mercury is driven into the capillary, the flow being 
 opposed by the capillary resistance. By a suffi- 
 ciently great pressure, mercury may be driven to the 
 extremity of the capillary and all the air expelled. 
 When the pressure is relieved the mercury rises 
 again in the tube, drawing the sulphuric acid after 
 it. The column of mercury will corne to rest at a 
 point where the pressure and the capillary force just 
 balance. Seen through the microscope (e), the end 
 of the column of mercury, where it is in contact 
 with the sulphuric acid appears as a convex menis- 
 cus (d\ Any alteration of the surface tension of 
 the meniscus causes the mercury to move with 
 great rapidity in one direction or the other along 
 
 the tube ; and a very slight difference of electric 
 FIG. 63. Schema of capillary electrometer. , . . , , . n 
 
 potential suffices to cause a change in surface ten- 
 sion of the mercury-sulphuric acid meniscus. A platinum wire fused into the glass tube 
 (a), and another dipped into a little mercury at the bottom of the cup holding the acid, 
 permit the mercury in the capillary and the acid to be connected with the body the elec- 
 tric condition of which is to be examined. If the mercury and acid be connected with 
 two points of different electric potential, as g and h of muscle M, the mercury will instantly 
 move from the direction of greater to that of lesser tension, descending deeper into the 
 tube if the tension be raised on the mercury side, or lowered on the acid side, and vice 
 versa. As seen through the microscope the picture is reversed (tZ), and the movements 
 of the mercury appear to be in the opposite direction to that stated. The extent of the 
 movements of the mercury column can be estimated by a scale in the eyepiece. More- 
 over, the movement of the mercury can be recorded photographically, by placing a strong 
 light behind the column of mercury, and letting its shadow fall through a slit in the wall 
 of a dark chamber, upon a sheet of sensitized paper stretched over the surface of a revolv- 
 ing drum or a sensitized plate moved by clockwork or other suitable mechanism. This 
 instrument, of which there are a number of different forms besides that originally devised 
 by Lippmann, is very delicate, recording exceedingly slight differences in electrical poten- 
 tial. 
 
 The movements of a galvanometer may be recorded photographically by letting the 
 beam of light reflected from the mirror fall through a horizontal slit on a photographic 
 plate. If the plate be arranged to descend at a regular rate in a dark chamber behind the 
 screen holding the slit, the movements of the galvanometer magnet will be pictured as 
 black lines on a white ground. 
 
 The movements of the mercury column of a capillary electrometer may be recorded in 
 a similar manner, by placing the instrument in front of a vertical slit behind which a pho- 
 tographic plate or sheet of sensitized paper moves horizontally. If a strong light falls on 
 1 Bernstein : Pfliiger's Archiv, 1898, Bd. Ixxiii. S. 376. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 147 
 
 the slit, it will influence the sensitized surface except where the mercury column inter- 
 venes ; the movements of the mercury will be photographed as a silhouette. 
 
 2. Currents of Rest. A normal resting nerve or muscle presents no dif- 
 ferences in electric tension and gives no evidence of electric currents, wherefore 
 we say it is iso-electric. If any part of the structure be injured, its electrical 
 condition is forthwith changed, and if the injured portion and some normal 
 part be connected with a galvanometer, an electric current is 'observed to flow 
 from the normal region to the point of injury. These muscle-currents were 
 discovered at about the same time by Matteucci and Du Bois-Reymond, and 
 the latter wrote a now celebrated treatise upon the electrical phenomena to be 
 observed in the nerve and muscle under varying conditions. 1 
 
 Directions of Currents of Rest. If a striated muscle, with long parallel 
 fibres, such as the sartorius or the semimembranosus of a frog, be prepared 
 with care not to injure the surface, and then be given a cylindrical shape by 
 cutting off the two ends at right an- 
 gles to the long axis, the piece will 
 present two cross sections of injured 
 tissue and a normal longitudinal sur- 
 face (see Fig. 64). If non-polarizable 
 electrodes, connected with the coils of 
 wire of a galvanometer, be applied to 
 various parts of such a piece of mus- 
 cle, it will be found that all points on 
 the longitudinal surfaces are positive 
 in relation to all points on the cross 
 ^sections, but that the differences of 
 tension will differ according to the 
 points which are compared. Suppose 
 that the cylinder be divided into 
 equal halves by a plane parallel to the 
 cut ends. Points on the line bound- 
 ing this plane, the equator, show the 
 greatest positive tension, and the farther other points on the longitudinal sur- 
 face are from the equator the less their tension. Points on the cross section 
 show a negative tension, and this lessens from the centre to the periphery of 
 the cross section. Points on the cross section equidistant from the centre, or 
 on the longitudinal surface equidistant from the equator, have the same poten- 
 tial and give no current, while points placed unsym metrically give a current. 
 Splitting the cylinder by separation of the parallel fibres gives pieces of mus- 
 cle which show the same electrical peculiarities, and without doubt the same 
 would be true of separate muscle-fibres or pieces of fibres. 
 
 Samjloff 2 says that the electro-motive force of the currents ordinarily 
 
 1 Untersuchungen ilber thierische Elektricitat, Berlin, 1849. 
 
 2 Samjloff: Pftiiger's Archiv, 1899, Bd. Ixxviii. S. 1. 
 
 FIG. 64. Schema to show the direction of cur- 
 rents to be obtained from muscle. The schema 
 represents a cylindrical piece of muscle with nor- 
 mal longitudinal surface (a, c and 6, d), and two 
 artificial cross sections (a, b and c, d). The position 
 of the equator is shown by line e. The unbroken 
 lines connect points of different potential, and the 
 arrows show the direction which the currents 
 would take were these points connected with a 
 galvanometer. The broken lines connect points 
 of equal potential from which no current would be 
 obtained. 
 
148 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 obtainable from muscle, 0.060.08 volt, represents only about 80 per cent, of 
 the true electro-motive force of the muscle-currents, because only a part of 
 the current is led off to the galvanometer, the rest being short-circuited 
 through the fluids surrounding the muscle-fibres, and in the sheath of the 
 muscle. 
 
 The fact that there is a difference in electrical potential between the 
 normal longitudinal surface and the injured cross section of the muscle can 
 be ascertained by the use of the " physiological rheoscope," as the nerve- 
 muscle preparation is called. If the nerve of a fresh nerve-muscle prepara- 
 tion be allowed to fall so as to suddenly close a circuit between these two 
 parts of the muscle, an electric current will flow through it, it will be ex- 
 cited, and the muscle will contract. A muscle can even be made to stimu- 
 late itself by its demarcation current, if some point on the equator be sud- 
 denly connected with a fresh cross section by a good conductor. 
 
 Theories as to Cause of Currents of Rest. Du Bois-Reymond, impressed by 
 the facts which he had ascertained as to the direction of action of the electro- 
 motive forces exhibited by the muscle, tried to explain the difference in elec- 
 trical tension of the surface and cross section on the supposition that the 
 muscle was composed of electro-motive molecules which presented differences 
 in electric tension similar to those shown by the smallest particles of muscle 
 which it is possible to study experimentally. Further, he considered these dif- 
 ferences in tension, and the consequent electric currents, to exist within the 
 normal muscle the longitudinal surface and normal cross section, i. e. the 
 point where the muscle-fibre joins the tendon, having the same sort of differ- 
 ence in electric potential as the normal longitudinal surface and the artificial 
 cross section. When the muscle is injured the balance of the electro-motive 
 forces within is lost, and they are revealed. It is difficult to refute such a 
 theory by experiment, because our instruments only record differences in tension 
 at points on the surface of the muscle to which we can apply the electrodes. 
 We cannot say that there is an absence of electric tension or lack of electric 
 currents within the normal resting muscle ; we can only say that there is no 
 direct experimental evidence of the existence of such currents. 
 
 Another theory of the electrical phenomena observed in muscle, and one 
 which has found many adherents, was advanced by Hermann. 2 According to 
 Hermann's view there are no differences in electric potential and no electric 
 currents within the normal muscle ; the " current of rest " is a " current of 
 injury," a "demarcation current/' i. e. it is due to chemical changes occurring 
 in the dying muscle- tissue at the border line between the injured and living 
 muscle-tissue. 
 
 Although the greatest differences in potential are observed when many muscle- 
 fibres are injured, as when a cut is made completely through a muscle, injury 
 to any part causes that part to become negative as compared with the rest. 
 Even an injury to a tendon causes a difference in potential. It is exceedingly 
 
 1 Du Bois-Reyinond : Gesammelte Abhandlungen, 1877, Bd. ii. S. 319. 
 
 2 Hermann : Handbuch der Physiologic, 1879, Bd. i. S. 235. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 149 
 
 difficult, therefore, to expose a muscle without injuring it; but this can be done 
 in the case of the heart ventricle, and Engelmann showed that this gives no cur-' 
 rent when at iv>t, although a current is found as soon as any part is hurt, the 
 part becoming immediately negative in relation to other uninjured parts. In 
 experimental on isolated, long, parallel-fibred muscles, the current which is 
 caused by the injury of one extremity is found to fade away only very gradu- 
 ally (it may last forty-eight hours or more), and this current can be strength- 
 ened but little by new injuries. In the case of the heart-muscle the current 
 caused by cutting off a piece of the ventricle soon disappears, but another cur- 
 rent of equal strength is got if a new section be made by cutting off the tissue 
 injured by the first cut. In the case of the long-fibred muscles the death 
 process gradually progresses the length of the injured fibres, while in the case 
 of the heart-muscle, in which the cells are very short, the death processes are 
 limited to the injured cells, and on their death the current disappears; when a 
 new cut is made other cells are injured and again a strong current is obtained. 
 
 Dead tissue gives no current ; normal resting living tissue gives no current ; 
 dying tissue is electrically negative as compared with normal living tissue. 
 
 Hering 1 has carried Hermann's view that electrical change is the result of 
 chemical action still further. He considers that the condition of negativity is 
 an evidence of katabolic (breaking-down) chemical processes and that anabolic 
 (building-up) chemical processes are accompanied by a positive electrical change. 
 Like Du Bois-Reymond, he believes that the normal resting muscle may be 
 the seat of electro-motive forces which do not manifest themselves as long as 
 the different parts are in like condition. 
 
 Current of Rest of a Nerve. Nerves like muscles show no electric currents 
 if normal and resting, but give a demarcation current if injured, the dying por- 
 tion being negative to normal parts, and the direction of the currents is the 
 same as injured muscle. The current of injury of a nerve lasts only a short 
 time. The death process which is the immediate result of the injury pro- 
 ceeds along the nerve only a short distance, perhaps to the first node of 
 Ranvier, and when it has ceased to advance the current fails; a new injury 
 of the nerve causes another demarcation current as strong as the first. 
 Gotch and Horsley 2 ascertained the electro-motive force in the nerve of a 
 cat to be 0.01 of a Daniell cell and of an ape only 0.005, while in the spinal 
 nerve-roots of the cat it was 0.025, and in the tracts of the spinal cord of the 
 cat 0.046, and of the ape 0.029. Larger currents are obtained from non- 
 medullated nerves, probably because a non-medullated nerve contains a 
 larger number of axis-cylinders than a medullated nerve of the same size. 
 The olfactory nerve of the pike may give a current of 0.0215 to 0.0105 
 Daniell, while a piece of a frog's sciatic of equal diameter would give a cur- 
 rent of only 0.006 Daniell. 3 
 
 Hering found that an irritable nerve, like a muscle, could be excited by 
 
 1 Hering : Lotos, 1888, Bd. ix. ; translation in Brain, 1897. 
 
 2 Philosophical Transactions, 1891, B., vol. 182, pp. 267-526. 
 
 'Kuehne and Steiner: Heidelberger Untersuchungen, 1880, iii. S. 149-169. 
 
150 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 its own current, provided the longitudinal and fresh cross section were 
 united suddenly by a good conductor. 
 
 3. Currents of Action in Muscle. Just as the dying tissue of nerves 
 and muscles is electrically negative as compared with normal tissue, so active 
 nerve- and muscle-tissue is electrically negative as compared with resting 
 tissue. 
 
 Du Bois-Reymond discovered that if the normal longitudinal surface and 
 injured cut end of a muscle were connected with a galvanometer and the muscle 
 were tetanized, the magnet swung back in the opposite direction to the deflec- 
 tion which it had received from the current of rest. This backward swing of 
 the magnet was not due to a lessening of the current of rest, for if the effect 
 of the current of rest on the galvanometer were compensated for by a battery 
 current of equal strength and of opposite direction, so that the needle stood at 
 0, and the muscle were then tetanized, there was a deviation of the needle 
 in the opposite direction to that given it by the current of rest. Du Bois- 
 Reymond called this current of action the negative variation current. This 
 negative variation current was found to last as long as the muscle continued in 
 tetanus. On the cessation of the stimulus the current subsided more or less 
 rapidly and the needle returned more or less completely to the position given it \ 
 by the current of rest before the excitation. The return was rarely complete, ' 
 and by repeated excitations there was a gradual lessening of the current of 
 rest, the amount varying with the extent of the preceding irritation. The 
 strength of the current of action is influenced greatly by the condition of the 
 muscle, the temperature, etc. It increases with increasing strength of 
 excitation, in the same way as the strength of the contraction. 1 
 
 Secondary Tetanus. Matteucci and Du Bois-Reymond (1842) both dis- 
 covered the phenomenon which Du Bois-Reymond called secondary tetanus. 
 
 If two nerve-muscle preparations be 
 made, and the nerve of preparation B 
 be laid on the muscle of preparation 
 Ay when the nerve of A is stimulated, 
 not only the muscle of A but the 
 muscle of B will twitch (see Fig. 65). 
 If nerve A be excited by many 
 
 FIG. 65. -Secondary tetanus. 
 
 rapidly following induction shocks so 
 that muscle A enters into tetanus, 
 muscle B will also be tetanized. The phenomenon is not due to a spread of 
 the irritating electric current through nerve and muscle A to nerve J5, for the 
 tetanus of both muscles stops if nerve A be ligated ; moreover, a secondary 
 tetanus is obtained in case tetanus of muscle A is called out by mechanical 
 stimuli, such as a series of rapid light blows, applied to nerve A. 
 
 Du Bois-Reymond considered " secondary tetanus " a proof of the discon- 
 tinuity of the apparently continuous contraction of tetanus, for muscle F> could 
 only have been excited to tetanus by rhythmic excitations from A. Each of 
 
 1 Burdon-Sanderson : Journal of Physiology, 1898, xxiii. p. 323. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 151 
 
 the rapidly following excitations applied to A was the cause of a separate con- 
 traction process and a separate current of action in B ; the separate contractions 
 combined to produce the tetanus of B, but the separate currents of action did 
 not fuse, although they caused a continuous negative variation of the slowly 
 moving magnet of the galvanometer. 
 
 The correctness of this view has been shown by experiments with the capil- 
 lary electrometer, which approaches the " physiological rheoscope," as the 
 nerve-muscle preparation is called, in its sensitiveness to rapid changes in elec- 
 trical potential. 
 
 Burdon Sanderson 1 has obtained, by photographically recording the move- 
 ments of the column of mercury of the capillary electrometer (see Fig. 63, 
 p. 146), beautiful records of the changes of electric potential which occur when 
 an injured muscle is tetanized. 
 
 The record in Figure 66 shows, first, a series of negative changes resulting 
 from the separate stimuli. It is these which cause secondary tetanus and 
 which produced the negative variation current disclosed by the galvanometer 
 in the experiments of Dti Bois-Reymond. Second, there is a more permanent 
 negative change, likewise opposed to and lessening the effect of the negative 
 change at the part where the tissue is dying, and called by Sanderson " the 
 dhninutional effect." The continuous negative change is possibly attributable 
 to the presence of a continuous contraction process, perhaps the contracture 
 which we observed in studying the tetanus curve (see Fig. 52). This " diminu- 
 
 Fi<;. 66 Record of changes in electric potential in a tetanized injured muscle of a frog. The leading- 
 off non-polarizable electrodes connected with the capillary electrometer touched the normal longitud- 
 inal and injured cut surface of the muscle. The muscle was tetanized by an induction current applied 
 to its nerve, the rate of interruptions being 210 per second. A rise of the curve indicates an electrical 
 change of opposite direction to that caused by the injury. The diminution of the current of injury, 
 which was less than in some other experiments, was 0.008 volt. The time record at the bottom of the 
 curve was obtained from a tuning fork making 500 double vibrations per second (after Burdon San- 
 derson). 
 
 tional effect" is only to be observed upon an injured muscle, since it repre- 
 sents a difference in potential between the normally contracting and the injured, 
 imperfectly contracting muscle-substance. When all parts of the muscle are 
 normal and contracting to an equal amount, the electrical forces would be 
 everywhere of the same nature, balance one another, and give no external 
 evidence. Although the dimiuutional effect is only to be observed upon the 
 injured muscle, the temporary negative changes which follow each excitation 
 1 Journal of Physiology, 1895, vol. xviii. p. 717. 
 
152 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 are to be observed on the normal muscle. To understand this we must con- 
 sider the diphasic current of action. 
 
 Diphasic Current of Action. If a normal muscle be locally stimulated by 
 a single irritation, either directly or indirectly through its nerve, the part 
 excited will be the first to become active and electrically negative, and this 
 condition will be taken on later by other parts. Our methods only permit us 
 to observe the relative condition of the parts of the muscle to which the elec- 
 trodes are applied, the changes in the intermediate tissue failing to show them- 
 selves. If an electrode be applied near the place where the uninjured 
 muscle is stimulated, A, and another at some distant point, 13, and these 
 electrodes be connected with a capillary electrometer, a diphasic electrical 
 change will be observed to follow each stimulation. At the instant the irritant 
 is applied the muscle-substance at A will become suddenly negative with 
 respect to that at B ; when the spreading irritation wave has reached B, that 
 part too will tend to be negative, and an electrical equality will be temporarily 
 established ; finally, B continuing to be active after A has ceased to act, B 
 will be negative in respect to A. Since the wave of excitation spreads along 
 the fibres in both directions from the point irritated, each excitation will cause 
 diphasic electrical changes to either side of the place to which the irritant is 
 applied. 
 
 If the muscle has been injured at B, the dying fibres there will react but 
 poorly to the stimulus, and therefore the antagonistic influence of the negative 
 change at B will incompletely compensate for the negativity at A, and hence 
 only a single phase due to the condition of negativity at A will be seen. 
 
 The normally beating heart ventricle shows diphasic currents of action : 
 in the first phase the base, where the contraction process starts, is negative to 
 the apex, and in the second phase the apex is negative to the base. In case 
 the heart be injured, the negative change corresponding to action fails at the 
 injured part, and therefore a single and because not antagonized more pro- 
 longed negative change is observed. Under certain conditions a tri phasic 
 change is observed, which need not be discussed here. Waller l has succeeded 
 in recording the electrical changes which accompany the beat of the human 
 heart. 
 
 These diphasic changes of the electric condition are sufficiently strong and 
 rapid in the mammalian heart to excite the nerve of a nerve-muscle prepara- 
 tion, and the muscle will be seen to give one, or, if the heart is uninjured, 
 sometimes two, contractions every time the heart beats. 
 
 Bernstein 2 found the time between the two portions of diphasic change to 
 be proportional to the distance between the leading-off electrodes, and to cor- 
 respond to a rate of transmission the same as that of the wave of excitation, 
 as revealed by the spread of the contraction process (in the muscle of the frog 
 3 meters per second). Hermann, 3 by using cord electrodes on the human fore- 
 
 1 -Archiv fur Anatomie und Physiologic, 1890; physiol. Abtheil., S. 187. 
 
 2 Untersuchungen uber den Erregungsvorgang im Nerven- und Muskel-system, 1871. 
 
 3 Handbuch der Physiologic, 1879, i. 1, 8. 224. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVi:. ir,:>, 
 
 arm, found the rate of spread of the active process by the voluntary contraction 
 of human muscle to be from 10 to 13 meters per second. Du Bois-Reymond 
 dipped a finger of each hand into fluid contained in cups connected with a 
 galvanometer. If the muscles of one arm were vigorously contracted, a 
 deflection of the magnet was seen. This was probably due to electric currents 
 from the glands of the skin and not from the contracting muscles. 
 
 Relation of the Negative Variation Current to the Contraction. Bernstein 
 observed the negative variation of the demarcation current of the muscle 
 almost at the instant that the muscle was excited and before the contraction 
 began to be recorded i. e., during the mechanical latent period of the muscle 
 and concluded that it is associated with the excitation rather than the con- 
 traction process. This view is supported by others/ who find that the 
 highest point of the negative change is generally passed before the contrac- 
 tion shows itself. 
 
 On the other hand, it is reported that the negative state may continue 
 throughout the contraction. Sanderson and Page 2 saw the diphasic change 
 which accompanies the beat of the heart to last throughout the systole ; and 
 Lee 3 found the diphasic change which occurs when a skeletal muscle of 
 a frog is excited by a single stimulus, to continue as long as the muscle re- 
 mains active, including the period of relaxation ; in some cases it lasted from 
 0.05 to 0.06 second. Sanderson, as we have seen (see Fig. 66), tetanized 
 injured skeletal muscles of the frog, and found not only a series of negative 
 variations corresponding to the contraction processes which resulted from the 
 separate excitations, but also a continuous negative variation, the diminutional 
 effect, which developed comparatively slowly and lasted after the irritant 
 had ceased to act. 
 
 Still other observers, who claim that the electrical state of the muscle is 
 closely related to the contraction process, find sometimes a negative and 
 sometimes a positive change, according as the contractions are isometric or 
 iso tonic. 
 
 A further consideration of this subject would be out of place here. Suf- 
 fice it to say that there can be no doubt that the change which occurs in 
 muscle substance when it is excited is associated with a change in its elec- 
 trical condition ; whether the subsequent activity of the muscle protoplasm, 
 manifested in the change of form, has also an electrical change associated 
 with it, must be left an open question. 
 
 4. Currents of Action in Nerves. In general, the facts which have 
 been stated with regard to the current of action in muscles apply to nerves. 
 When a normal nerve is excited a negative change is forthwith developed at 
 the stimulated point and passes thence in both directions along the nerve at 
 the same rate as the nerve-impulse. This change is diphasic, first the part 
 excited and later distant parts showing the negative change. If the nerve 
 
 1 Jensen : PfliigeSs Archiv, 1899, Bd. Ixxvii. S. 107. 
 
 1 Journal of Physiology, 1879, vol. ii. p. 396. 
 
 8 Archiv fur Anatomic und Physiologic, 1887, S. 204. 
 
154 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 be injured, and the normal surface be compared with the dying or dead cross 
 section, the second phase is absent. If the nerve be frequently excited, each 
 excitation awakens a separate current of action. 
 
 Although nerves are excited most readily by electric currents, negative 
 variations of the demarcation current may be called out by various chemicals 
 e. g., salt or glycerin 1 and by mechanical excitation, such as a sharp cut 
 with the shears. 2 It is a physiological phenomenon, for a negative change 
 may be observed to accompany a nerve impulse which has been caused by 
 the spread of excitation from central neurones along their peripheral axones. 
 
 Du Bois-Reymond observed with the galvanometer a lessening (" negative 
 variation ") of the demarcation current (" current of rest ") when in strychnia- 
 poisoning the spinal motor neurones were sending vigorous impulses along 
 their axones and causing cramp-like tetanic muscular contractions. Gotch 
 and Horsley 3 applied electrodes connected with a capillary electrometer to 
 peripheral nerves, spinal nerve-roots, and tracts of motor fibres within the 
 spinal cord, and discovered that if the cortical brain-cells in the motor zones 
 were excited, the nerves showed currents of action corresponding in rate to 
 the discharge of motor impulses from these brain-cells e. g., if the epilepti- 
 form convulsions were occurring at the time, the capillary electrometer 
 revealed changes of potential of like rate in the nerves. 
 
 Macdonald and Reed 4 observed currents of action in the phrenic nerve 
 of mammals which corresponded in time with the respiratory movements. 
 These were due to the normal discharge of nerve impulses from the respira- 
 tory centers. When a condition of apnoea was established and the respiratory 
 movements ceased, the electrical change failed to appear ; when the respiratory 
 movements were quickened in dyspnoea, the rhythmic movements of the gal- 
 vanometer were quickened to correspond ; when during asphyxia the respi- 
 rations were of the Cheyne-Stokes type, the currents of action showed a like 
 rhythm. 
 
 Even physiological sensory nerve-impulses have been found to produce 
 negative variation currents. Steinach 5 observed currents of action to be 
 caused by mechanical pressure on the skin of the frog. If the pressure \vas 
 continued, the negative change gradually decreased, and a new negative 
 variation was seen if the pressure was suddenly removed. 
 
 Light falling on the retina of the eye of a frog has been seen to cause a 
 negative variation of the current of rest of the optic nerve. 
 
 The electrical change which we call the current of action can be thought 
 to sweep over the nerve as a wave, having in the medullated nerves of the 
 frog a length of 18 mm., and travelling at the rate of 28 meters per second. 
 The duration of the negative variation is different in different kinds of nerves 
 
 1 Kiihne and Steiner : Untersuchungen aus der physiologisehen Laboratoriitm in Heidelberg, 1880, 
 Bd. iii. S. 149. 
 
 2 Bering : Lotos, Neue Folge, 1888, Bd. 9, S. 35. 
 
 3 Physiological Transactions, 1891, vol. 182, pp. 267-526. 
 
 * Macdonald and Reed : Journal of Physiology, 1898, vol. xxiii. p. 100. 
 5 Steinach : Pfliiger's Archiv, 1896, Bd. Ixiii. S. 495. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 155 
 
 and in the same nerve under dilVnvnt conditions. In the medullated nerves 
 of the frog it lasts 0.008 to 0.024 second ; l in the non-medullated m-rvcs of 
 the pike the rise of negativity requires 0.08-0.029 second and the fall 0.40- 
 1.2 second. 2 The rate of conduction is slowed by cold, and this at the san it- 
 time lengthens the accompanying electrical change. This fact has been 
 made use of to ascertain that in the uninjured nerve, as in the muscle, there 
 is a diphasic current of action spreading in both directions from the point of 
 excitation. 
 
 The strength of the electrical change which takes place in a nerve when 
 it is excited is the best evidence which we have of the activity of the nerve. 
 "The physiological structures which the nerve normally excites obey laws 
 peculiar to themselves, and are liable to give results which are open to mis- 
 interpretation. For example, if a nerve be stimulated in the middle, the 
 condition of activity aroused spreads in both directions, and causes at the 
 one end a contraction of the muscle, and at the other a negative variation of 
 the current of rest, which may be observed with a galvanometer. If the 
 nerve be excited many times in succession, the height of the muscular con- 
 tractions is seen to decrease while the electrical changes show no sign of 
 fatigue. The decrease in the height of the contractions is really due to the 
 fatigue of the nerve ends and the muscle, and the constancy of the electrical 
 changes is the truer expression of the state of the nerve. 
 
 A difficulty presents itself here, however : the negative variation currents 
 observed in such an experiment may be so very regular as to suggest that 
 they are physical rather than physiological phenomena. That they are not 
 purely physical can be ascertained by subjecting the nerve to influences of a 
 type to alter the physiological activity of the protoplasm, without perma- 
 nently, or indeed markedly, altering its chemical and physical structure. A 
 study of the effect of anaesthetics on the nerve is especially instructive. In 
 general, anaesthetics are found iirst to heighten, later to lower, and finally to 
 destroy the irritability. 
 
 If the anaesthesia is not carried too far, the nerve may completely recover 
 its function on the removal of the drug. Waller 3 describes the following 
 experiment : A fresh nerve is placed on two pairs of non-polarizable electrodes 
 in a moist chamber. One pair is connected with the galvanometer which is 
 to record the current of action, the other pair is connected with an induction 
 apparatus, and brings the exciting current to the nerve. Induction shocks 
 of equal strength are sent into the nerve at regular intervals, and the extent 
 of the currents of action awakened is noted. After the electrical response 
 of the nerve has been tested, fumes of ether (diethyl oxide) are blown through 
 the chamber. At first the electrical response is found to be increased, later 
 it decreases, and at the end of three or four minutes it is altogether lost. If 
 air be now allowed to enter the chamber, the current of action reappears, 
 
 1 Biedermann : Electrophysiology, translation by F. A. Welby, 1898, vol. ii. p. 260. 
 
 2 Gartner: PflugeSs Archiv, 1899, Bd. Ixxvii. S. 498. 
 
 3 Waller: Lectures on Animal Electricity, 1897, Lecture II. 
 
156 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 and at the end of from five to ten minutes may be even stronger than it was 
 at the start. Chloroform, likewise, inhibits the activity of the nerve ; but it 
 is even more vigorous in its action and the nerve is less likely to recover. 
 The vapor of ethyl alcohol, after a preliminary exhilarating effect, paralyzes 
 the nerve ; under favorable conditions the nerve recovers in a few hours. 
 The action of CO 2 is particularly interesting, since this is one of the normal 
 waste products of the body. In small doses it is found to increase the 
 strength of the current of action, while in large doses it has an anaesthetic 
 effect. The nerve is so sensitive to CO 2 that even a fiftieth of a milligram 
 suffices to change its irritability : the amount that is contained in the expired 
 air, 4 per cent., suffices to do away with the current of action in three 
 minutes. 
 
 In general, whatever increases the irritability of the nerve increases the 
 negative variation currents which result from its excitation. For instance, 
 if a nerve be excited at regular short intervals with induction shocks of equal 
 strength, there is a staircase-like increase in the negative variation current ; 
 moreover, if it be subjected for a short period to tetanic excitation, the cur- 
 rent of action called out by a single shock is found to be increased. 1 Indeed 
 if one photographically records the movements of the galvanometer magnet 
 in such experiments, he obtains a curve closely resembling that obtained by 
 records of muscular contractions when the muscle is so excited (see Fig. 57). 
 Waller suggests that the effect of excitation to increase the irritability of the 
 nerve may be due to the production of CO 2 by the nerve, and a consequent 
 internal stimulation. The fact that the nerve does not fatigue he would 
 explain as the result of rapid restoration of the protoplasm and a rapid 
 neutralization of waste products. 
 
 Apparently the strength of the current of action can be taken as a fair 
 index of the activity of the nerve, and consequently of the strength of the 
 nerve impulse. This view corresponds with the fact that there is a close 
 relation between the strength of the current of action of the muscle and the 
 height of the contraction. Waller 2 and Green 3 found experimentally that a 
 current of action can be detected with difficulty with subminimal irritants ; 
 but as the strength of the current is raised above the threshold intensity the 
 strength of the electrical change increases proportionally to the strength of 
 the excitation, until a point is reached which is far beyond what is needed 
 to excite maximal muscular contractions. After this the increase is less and 
 less, until finally a maximal current of action is reached. This occurs with 
 a strength of excitation much stronger than is required to call out a maximal 
 muscular contraction, and probably beyond the limit of functional action. 
 It is doubtful whether the nerve-cell could excite such a condition in a 
 nerve. 
 
 On account of the great resistance of the nerve, and the short-circuiting 
 
 1 Waller : Lectures on Animal Electricity, 1897, p. 59. 
 
 8 Waller : Brain, xvii. p. 200. 
 
 3 Green : American Journal of Physiology, 1898, p. 104. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 157 
 
 of a part of the current by the lymph, etc., the currents which may be obtained 
 are usually small. Under especially favorable conditions the current may 
 be twice as great as the current of rest, and Gotch and Burch state that 
 they have found it in the sciatic nerve of the frog to have the excitatory effect 
 of 0.033 volt, 1 Hering 2 has shown that it may suffice to excite another nerve 
 in close contact with it, the experiment being of the same type as that de- 
 scribed as secondary tetanus, p. 150. 
 
 The fact that a negative variation of the demarcation current of a bundle 
 of nerve-fibres may excite other fibres lying beside them in the same nerve, 
 is the explanation of the so-called " paradoxical contraction." This may be 
 seen under the following conditions : Take a frog that has been kept in the 
 cold so that its nerves are very irritable ;' dissect out the branches of the 
 sciatic nerve at the knee, ligature, divide below the ligatures, and then dissect 
 up the nerve as far as the branches given off to the muscles of the thigh ; 
 expose the nervous plexus at the back of the abdominal cavity, and divide 
 close to the spinal cord ; remove all extra fluid, and avoid wetting the nerves 
 with salt solution. Kaise up the knee end of the nerve, and place on elec- 
 trodes connected -with the secondary coil of an induction apparatus : excite 
 with weak tetanizing current close to the end of the nerve, and see the thigh 
 muscles contract, As the fibres excited at the knee have no anatomical or 
 physiological connection with the fibres of the branches of the nerve dis- 
 tributed to the thigh muscles, the nerve impulse which passes up the nerve 
 could not reach these fibres, and the contraction could not occur were it not 
 for the spread of the -current of action under these abnormal conditions. To 
 make sure that the contraction is not due to a spread of the exciting current 
 or to unipolar stimulation effects, one can ligature the nerve above the excited 
 point. This would stop the progress of the nerve impulse and the accom- 
 panying current of action, and prevent the "paradoxical contraction." There 
 is still another possible source of error that has to be guarded against : the 
 exciting current, through electrolytic effects, may cause electrotonic currents 
 (see p. 62) to be developed in the nerve, and these may spread sufficiently 
 to excite branches of the thigh muscles. A ligature will stop the spread of 
 these currents, but their presence can be recognized from the fact that the 
 contractions will be the stronger the nearer the electrodes are brought to the 
 part of the nerve from which the branches to the thigh muscles are given 
 off. A contraction called out in this way is called the " electrotonic twitch." 
 A spread of the current of action to neighboring fibres never occurs when 
 the nerve is intact, the rule of isolated conduction by nerve-fibres holding 
 good. 
 
 Relation of the Electrical Phenomena of the Nerve to Physiological 
 Processes. Certain writers of the extreme mechanical school would explain 
 all the forms of functional activity of the nerve as purely physical processes, 
 which result from chemical change occurring within it. This point of view 
 
 1 Proceedings of the Royal Society, 1900, vol. Ixv. p. 441. 
 
 2 Hering: Sitzungsberichte der Wiener Akadamie, 1882, Ixxxv. 3, S. 237. 
 
158 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 is largely based on the remarkable results which have been obtained by the 
 study of artificial models of nerves, called core-conductors. These were first 
 carefully studied by Hermann, 1 and since then by Boruttau, 2 Waller, 3 and 
 others. Such a model can be made* by placing a platinum wire in a glass 
 tube, and surrounding it with a 0.6 per cent, salt solution. The wire represents 
 the axis-cylinder of the nerve, and the salt solution the medullary sheath. 
 In other models both core and sheath have been made of fluid electrolytes. 
 With such a core-conductor, one can observe electrical phenomena so closely 
 resembling those manifested by the normal nerve that the statement has 
 been made that all the electrical phenomena of the living nerve can be 
 explained if one will look upon it as a core-conductor. 4 According to this 
 idea, conduction in nerve depends* on the transmission of an electrical phase 
 caused by local differences in potential, resulting from chemical changes and 
 consequent polarization effects produced when the nerve is excited. 
 
 This extreme view is accepted by few physiologists. 5 The electrical effects 
 which follow excitation are exhibited not only by nerves, but also by a great 
 variety of protoplasmic structures ; they are stopped by anesthetics, which 
 do not alter the core-conductor-like structure of the nerve, and are greatly 
 modified by all influences which are capable of changing the irritability of 
 living protoplasm, even though they are too feeble to produce any recogniz- 
 able change in dead matter ; finally, they are called forth, not only by electric 
 currents, but by every form of stimulus capable of exciting living protoplasm 
 to action. 
 
 It is very easy to be led astray by the similarity of processes observable 
 on very different structures, and think to see the whole truth in what is only 
 a partial truth. As Engelmann has shown, the anisotropic substance in a 
 piece of catgut suspended in water will cause it to shorten and then lengthen 
 if quickly heated and then cooled, and if a lever be connected with it, to 
 write a curve strikingly like that of the contracting muscle. In muscle there 
 is anisotropic material surrounded by fluid, and heat is produced at the 
 instant of contraction ; it is doubtful, however, whether the physiological 
 contraction process is of the same type as that of the piece of catgut. Within 
 the body we have oxidation processes going on, and heat is liberated as it is 
 outside of the body in combustion, but the two sets of changes giving this 
 result are not identical. Similarly we may say that the heart is a pump, and 
 the eye a camera, but the behavior of these living organs is very different 
 from that of lifeless machines. 
 
 All physiological phenomena are to be regarded as of chemico-physical 
 nature, but many of them differ so widely from the chemical and physical 
 processes associated with dead matter that a sharp distinction should be 
 
 1 Pftuger* 8 Archiv, 1872, Bd. v., vi., vii.; also Handbuch der Physiologic, 1879, Bd. ii. S. 17. 
 
 2 Pfluger's Archiv, 1894-1897, Bd. Iviii., lix., Ixiii., Ixv., Ixvi., Ixix. 
 
 3 Lectures on Animal Electricity, London, 1897. 
 
 4 Boruttau : Pfliiger's Archiv, 1894, Bd. Iviii. S. 64. 
 
 5 Biedermann: Electrophysiology, translated by F. A. Welby, 1898, vol. ii. p. 303. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NEltVl-:. 159 
 
 made. Purely physiological phenomena are such as can he exhibited only 
 by a mechanism which has the chemical and physical structure of living 
 protoplasm, and such as cease with the life of the protoplasm. 
 
 The electrical phenomena of nerve are capable of being divided into two 
 classes, the one, purely chemico-physical, resulting from the core-conductor- 
 like structure, and the other, physiological, intimately dependent on the reac- 
 tions of the living protoplasm. The medullated nerve is not merely a core- 
 conductor. 
 
 It is too soon to try to separate these two classes of phenomena ; we must 
 wait not only for more work to be done on nerves, but on other irritable forms 
 of protoplasm, for many of these, although of entirely different structure from 
 the nerve, exhibit very similar electrical reactions. 
 
 F. CHEMISTRY OF MUSCLE AND NERVE. 
 I. CHEMISTRY OF MUSCLE. 
 
 Muscles consist of muscle-fibres bound together by connective tissue. 
 Between the fibres we find nerves, blood-vessels, and lymphatics. Fat-cells 
 containing considerable fat may also be found in the midst of the connective- 
 tissue network. Each fibre consists of a sheath, the sarcolemma, which 
 resembles elastin in its constitution, and within this the muscle-substance 
 proper, together with certain substances of nutritive value and waste prod- 
 ucts. 
 
 Muscle which has been freed as far as possible from blood, connective 
 tissue, and fat, has a mean specific gravity of 1.060; the extreme variations 
 found for the muscles of different animals being 1. 053-1. 074. 1 When it is 
 fresh the reaction is slightly alkaline. 
 
 It contains about 75 parts of water and 25 parts of solids ; nearly 20 
 parts of the solids are proteids, the remaining 5 parts consisting of fats, ex- 
 tractives, and salts. 
 
 Little is known concerning the chemistry of living muscle; the instability 
 of the complex molecules which makes possible the rapid development of energy 
 peculiar to muscles renders exact analysis impossible. The manipulations 
 essential to chemical analysis necessarily alter and kill the muscle protoplasm. 
 
 Death of the muscle is ordinarily associated with a peculiar chemical change 
 known as rigor mortis. To understand the chemical composition of muscle it 
 is necessary that we should consider the nature of this change. 
 
 1. Rigor Mortis. Rigor mortis, the rigidity of death, is the result of a 
 chemical change in the substance of a muscle by which it is permanently 
 altered, its irritability and other vital properties being irretrievably lost. The 
 change is manifested by a loss of translucency, the muscle becoming opaque, and 
 by a gradual contraction, accompanied by a development of heat and acidity, 
 and resulting in the muscle being stiff and firm to the touch, less elastic, and 
 less extensible. Whenever muscle dies it undergoes this change. 
 1 Carvallo and Weiss : Journal de Physiologic, 1899, i. p. 204. 
 
160 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 Conditions which Influence the Development of Rigor. Ordinarily on the 
 death of the body the muscle enters into rigor slowly the muscle-fibres are 
 involved one after the other, and through the gradual contraction and harden- 
 ing of the antagonistic muscles the joints become fixed and the body acquires 
 the rigidity which we associate with death. Rigor usually affects the different 
 parts of the body in a regular order, from above downward, the jaw, neck, 
 trunk, arms, and legs being influenced one after the other. The position taken 
 by the body is generally determined by the weight of the parts and the rela- 
 tive strength of the contractions of the muscles. 
 
 The time required for the appearance of rigor is very variable. It is deter- 
 mined in part by the nature of the muscle, its condition at the moment of 
 death, and the temperature to which it is subjected. The muscles of warm- 
 blooded animals enter into rigor more quickly than those of cold-blooded 
 animals ; of the warm-blooded animals, pale muscles more quickly than red, 
 and the flexors before the extensors ; of the cold-blooded animals, frog's muscles 
 more quickly than those of the turtle. In general, the more active the muscle 
 protoplasm, the more rapid are the chemical changes which it undergoes, and 
 amongst these the coagulation of rigor mortis. 
 
 The condition of the muscle plays a very important part in determining the 
 onset of rigor. If the muscles are strong and vigorous and death of the body 
 has come suddenly, rigor develops slowly ; if the muscles have been enfeebled 
 by disease or fatigued by great exertion shortly before death, it comes rapidly. 
 In the case of wasting diseases rigor comes quickly, is poorly developed, and 
 passes off quickly ; when the muscles are fatigued at the time of death, as in 
 the case of a hunted animal, it comes quickly. We hear of soldiers found dead 
 on the field of battle grasping the sword, as if the muscular contractions of life 
 had been continued by the contractions of death. In the case of certain dis- 
 eases of the spinal cord and brain, too, rigor may come so rapidly that the 
 limbs may maintain the position which they had at the time of death, " cata- 
 leptic rigor," as i't has been called. The coming on of rigor is particularly 
 striking in the case of diseases which, like cholera, are accompanied by violent 
 muscular cramps and lead to a rapid death. It is not uncommon, in such 
 cases, for the contractions of rigor to cause movements which may mislead a 
 watcher into supposing the dead man to be still alive. This idea is favored by 
 the fact that the body may remain warm, owing to the heat which is produced 
 in the muscles as a result of the chemical changes occurring during rigor. 
 The post-mortem muscular contractions and the rise of temperature observed 
 in such cases are only excessive manifestations of what always occurs on the 
 death of the muscle. The movements are probably due, in part, to the rapidity 
 with which the muscles contract in rigor, and in part to the fact that the 
 antagonistic muscles are not affected at the same time to the same degree. 
 Whether the contractions are partly excited by changes accompanying the 
 death of the motor nerve-cells in the central nervous system is uncertain, but 
 not impossible. Muscles are still able to respond by contractions to stimuli 
 coming to them through the nerve, even after rigor has become quite pro- 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 161 
 
 nounced, probably because the coagulation process attacks the different fibres 
 at different rates, and certain of the fibres are still alive and irritable after the 
 others are dead and coagulated. 
 
 Many observers favor the view that the central nervous system influences 
 muscles after the death of the body as a whole, and by weak stimuli resulting 
 from the changes in the nerve-cells excites chemical changes in the muscles 
 which favor the coming on of rigor. 1 In proof of this it is stated that cura- 
 rized muscles enter into rigor more slowly than non-curarized. Undoubtedly 
 stimulation of the nerve, or, indeed, anything which would excite a muscle to 
 action, tends to put it in a condition favorable to the coming on of rigor; 
 whether the influence exerted by the central nervous system is more than this 
 is very questionable. 
 
 Temperature has a marked influence on the development of rigor mortis. 
 Cold delays and warmth favors, 38 40 C. being most favorable. Since rigor 
 is the result of a chemical change, these effects of temperature are what one 
 would have expected. Other forms of chemical change which are attributable 
 to ferment action are found to be the most vigorous at a temperature of about 
 40 C. 
 
 In general, it may be said -that rigor in warm-blooded animals comes on 
 in from ten minutes to seven hours after death, although some state that it 
 may come as late as eighteen hours. It lasts anywhere from one to six days. 
 The sooner it comes on, the sooner it goes off. The stiffness can be broken up 
 artificially by forced movements of the parts, and when thus destroyed does 
 not return, provided the rigor was complete at the time. 
 
 The Cause and Nature of the Contraction of Rigor Mortis. The most likely 
 explanation of the contraction of the dying muscle is that it is the result of 
 the coagulation of a part of the semi-fluid muscle-substance within the sarco- 
 lemma. This was suggested by Bruecke, and Kuehne proved that such a 
 coagulation change takes place, by showing that the semi-fluid muscle-sub- 
 stance, " the muscle-plasma/ 7 if expressed from the frozen muscle, coagulates on 
 being warmed. The coagulation is the result of a chemical change, by which 
 two proteids of the plasma, paramyosinogen and myosinogen, are converted 
 into the coagulated proteid myosin, this change being produced by the action 
 of a ferment, the myosin ferment, which is thought to be formed at the death 
 of the muscle. 
 
 Another, though less generally accepted view, is that the contraction of the 
 muscle seen in rigor is of the same nature as ordinary muscular contractions. 2 
 Prolonged muscle contractions are observed under a great variety of condi- 
 tions (see p. 127), and there are many points of resemblance between the 
 contraction of normal and dying muscle viz., the change of form, the pro- 
 duction of heat, the formation of sarcolactic acid, the using up of oxygen and 
 the production of carbon dioxide, and the fact that the dying and presumably 
 coagulating muscle is, like normal contracting muscle, electrically negative 
 
 1 Brown-Se'quard : Archives de Physiologic, 1889, p. 675. 
 
 2 Hermann : Handbuch der Physiologic, 1879, Bd. i. S. 146. 
 VOL. II. 11 
 
162 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 as compared with normal resting muscle. To this may be added that, as has 
 been said, the muscle continues to be irritable even when rigor is quite 
 advanced, and that it enters into rigor more quickly if left in connection with 
 the central nervous system. 
 
 On the other hand, one cannot fail to be impressed with the differences 
 between the two forms of contraction. 
 
 Normal Contracting Muscle. 
 Contains uncoagulated proteids. 
 Is translucent. 
 Is soft and flexible. 
 Is no less elastic than in repose. 
 Is more extensible than in repose. 
 Contracts rapidly. 
 Fatigues rapidly and relaxes. 
 
 Muscle contracting by Rigor Mortis. 
 Contains coagulated myosin. 
 Is opaque. 
 Is firm and stiff. 
 Is less elastic than before. 
 Is less extensible than before. 
 Contracts very slowly, as a rule. 
 Remains contracted a long time. 
 
 Furthermore, it may be added that normal contractions only occur when 
 the irritable muscle is stimulated, while a muscle can enter into rigor when its 
 irritability has been taken away by subjecting it to oxalate solutions, 1 also, 
 when it has been curarized and so shut out from all nervous influences. 2 
 
 Rigor is not confined to the voluntary muscles, though it is less easily 
 observed in the case of most involuntary muscles. The heart enters rapidly 
 into rigor, with the formation of sarcolactic acid. The non-striated muscle 
 of the stomach and ureters, too, has been seen to undergo this change. 
 
 The passing off of rigor mortis is usually accompanied by beginning 
 decomposition, and, indeed, it has been thought that the decomposition is 
 the cause of softening of the muscle. This is denied by certain observers, 
 and it is stated that rigor may pass off when the presence of putrefactive 
 organisms is excluded by special aseptic precautions. Another explanation 
 is that a process of intramuscular digestion goes on. Pepsin, a proteolytic 
 ferment, has been found in small amounts in the muscle ; and acid, which is 
 necessary to the action of this digestive ferment, is formed in the muscle 
 when it coagulates. The presence of these substances would make the diges- 
 tion of proteid material possible, and the fact that proteoses and peptone, 
 products of such digestion, are to be found in the muscle after death, though 
 not present during life, favors the view. It cannot be considered, however, 
 to be definitely established. 
 
 The rigidity of muscles in rigor can be readily broken up by stretching 
 and massaging the muscles, and when this has been done it does not return. 
 
 The Chemical Changes which accompany the Development of Rigor. Rigor 
 mortis is characterized by the coagulation of a part of the muscle-substance ; 
 this can be prevented by a temperature a little below C. Cold, although 
 temporarily depriving the muscle of its irritability, does not, unless extreme and 
 long-continued, kill the muscle protoplasm. Frogs can be frozen stiff and 
 recover their activity when they thaw out. Indeed, this probably happens not 
 
 1 Howell : Journal of Physiology, 1893, vol. xiv. p. 476. 
 
 2 Nagel : Pftilger^s Archiv, Bd. Iviii. S. 279. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 163 
 
 infrequently to the frogs hibernating in holes in the banks of ponds. Since cold 
 prevents coagulation without destroying the life of the muscle protoplasm, we 
 can by its aid isolate the living muscle-substance from the nerves, blood- 
 vessels, connective tissue, and sarcolemna of the muscle, but as soon as we 
 begin to analyze it it loses its living structure. This method of obtaining 
 muscle-plasma was introduced by Kuehne 1 in the study of the muscles of frogs, 
 and was later employed with slight modifications by Halliburton 2 for the mus- 
 cles of warm-blooded animals. The blood was washed out of the vessels with 
 a stream of 0.6 per cent, sodium-chloride solution at 5 C. ; the irritable mus- 
 cles were then quickly cut out and frozen in a mixture of ice and salt. The 
 frozen muscle was then cut up fine in the cold, and a yellowish, some- 
 what viscid, and faintly alkaline muscle-plasma was squeezed out. This 
 fluid was found to coagulate in twenty to thirty minutes at a temperature of 
 40 C. ; if the temperature were lower the coagulation was slower. The clot, 
 which was jelly-like and translucent, contracted slowly and in a few hours 
 squeezed out a few drops of serum. The coagulated material formed in the 
 clot is called myosin. It dissolves readily in dilute neutral saline solutions, 
 as a 10 per cent, solution of sodium chloride or a 5 per cent, solution of mag- 
 nesium sulphate, and its saline solutions are precipitated in an excess of water 
 or by saturation with sodium chloride, magnesium sulphate, or ammonium 
 sulphate ; it has, in short, the characteristics of a globulin. Chittenden and 
 Cummins state that it has the following composition: C 52.82, H 7.11, N 
 16.17, S 1.27, O 22.03. 
 
 Halliburton, in studying the coagulation of muscle, followed for the sake of 
 comparison the methods which have been employed in the study of coagulation 
 of blood. He found that muscle-plasma, like blood-plasma, is prevented from 
 coagulating not only by cold, but by neutral salts, such as magnesium sulphate, 
 sodium chloride, and sodium sulphate; and further, that the salted plasma if 
 diluted coagulates. 
 
 The points of resemblance between the coagulation of myosin in the 
 muscle and fibrin in the blood suggest a similar cause, and Halliburton suc- 
 ceeded in obtaining from muscles coagulated by long standing in alcohol a 
 watery extract, which greatly hastened the coagulation of muscle-plasma. 
 He called the substance thus obtained myosin ferment. The extract obtained 
 contained an albumose which was either the ferment or held it in close com- 
 bination. The pure ferment has not been isolated. In the case of coagula- 
 tion of the blood, a proteid of the plasma, fibrinogen, is changed by coagula- 
 tion to fibrin, this change being brought about by the action of the fibrin 
 ferment, for the formation of which calcium is necessary. The calcium 
 does not enter into the chemical change independently, and it can go on in 
 the absence of calcium provided the ferment has been already formed. In 
 the case of coagulation of muscle, two proteids of the muscle plasma, 
 paramyosinogen and myosinogen, are changed by coagulation to myosin, or, 
 
 1 Untersuchungen ijber das Protoplaxma, Leipzig, 1864. 
 1 Journal of Physiology, 1887, vol. viii. pp. 133-202. 
 
164 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 as Fiirth says, to myogen-fibrin and myosin-fibrin l by the action of myosin 
 ferment. The change can go on in the absence of calcium, but whether this 
 is essential to the formation of the ferment is not yet known. The myosin 
 ferment is not the same as fibrin ferment, since neither can do the work of 
 the other. Moreover, fibrin ferment is destroyed at 75-80 C. and myosin 
 ferment is not destroyed till 100 C. 
 
 The chemical change which results in the formation of myosin is diiferent 
 from that which produces fibrin. The clotting of muscle-plasma and the 
 formation of myosins are accompanied or closely followed by the production 
 of an acid, while no such change occurs during the coagulation of blood-plasma. 
 In the earlier stages of clotting the acidity may be due in part to acid potas- 
 sium phosphate, but the final acidity is chiefly due to lactic acid. The source 
 of the lactic acid has not been definitely made out. The view that it comes 
 from glycogen is made questionable by Bohm's 2 observation that the amount 
 of glycogen is not lessened in rigor ; besides, the muscles of starving animals 
 become acid when entering into rigor, although, as Bernard found, they con- 
 tain no glycogen. Bohm concluded that the sarcolactic acid may be formed 
 from the proteids. Probably both glycogen and proteids can yield lactic 
 acid. 
 
 Some writers have thought that the coagulation of the muscle was due to 
 the formation of an acid by the dying muscle. This is unlikely, although the 
 presence of acid, like that of many other substances, quinine, caffein, digitalin, 
 veratrin, hydrocyanic acid, ether, chloroform, etc., which lead to altera- 
 tions in the conditions of the normal muscle-substance, may hasten the proc- 
 ess. Apparently, anything which causes a deterioration of the muscle-sub- 
 stance, chemical reagents, drugs, or the products of fatiguing work, hastens 
 the coming on of rigor. On the other hand, anything which helps maintain 
 the normal constitution of the muscle appears to postpone the change. Thus 
 Latimer 3 reports that the circulation of dextrose through fatigued muscle 
 largely does away with the effect of fatigue to hasten rigor mortis. Nor is 
 this because fatigue products are washed out of the muscle, for the circula- 
 tion of other fluids through the muscle, whether neutral, acid, or alkaline, 
 fails to have the effect. 
 
 Rigor Caloris. If a muscle be heated beyond its normal temperature, its 
 irritability is increased, and it undergoes rapid katabolic changes which lead 
 to its death. These changes, if sufficiently rapid, may bring about a con- 
 traction of the muscle, and this contraction, involving the diiferent fibres of 
 the muscle to different degrees, may be continued without break by the con- 
 traction that is peculiar to rigor mortis ; in addition to this, if the temperature 
 is raised sufficiently, there will be a heat precipitation of the various proteids 
 of the muscle, which will lead to a still further shortening, the contraction 
 
 1 Furth: Archivfur experimentelle Pathologic und Pharmakologie, 1895, xxvi. 231 ; and 1896, 
 xxxvii. 389. 
 
 2 Pfliiger's Archiv, 1880, Bd. xxiii. S. 44. 
 
 3 Latimer : American Journal of Physiology, 1899, ii. p. 29. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 165 
 
 of rigor caloris. The heated muscle may, therefore, be the seat of three 
 dill 'crent kinds of processes, each of which leads to a shortening. If frog's 
 muscle be gradually heated, it shows three separate contraction movements 
 at three separate temperatures, at about 34, 47, 58 C. The last two con- 
 tractions are due to heat coagulation of paramyosinogen (myosin of v. 
 Furth) at 47 C., and myosinogen (myogen of v. Furth) at 58 C. These 
 are undoubted effects of heat rigor. There is a difference of opinion as to the 
 nature of the first contraction. It has been generally attributed to the 
 coming on of rigor mortis i. e., to a post-mortem coagulation of para- 
 myosinogen and myosinogen. In case a muscle be rubbed between the 
 fingers, so that its anatomical condition is altered, although the chemical 
 structure remain the same, this form of shortening does not occur, and it is 
 not until the temperatures at which paramyosinogen and myosinogen are 
 coagulated by heat are reached that the muscle begins to shorten. 1 Probably 
 the rigor-mortis change occurs, but on account of the physical change in the 
 fibre it does not reveal itself. This suggests the well-known fact, that when 
 the rigidity of a muscle in rigor has been broken up by mechanical means it 
 does not return. The condition of the muscle has an important influence on 
 the temperature at which it will enter into rigor when heated. Latimer 2 
 reports that a fatigued muscle will go into rigor at a temperature 10 degrees 
 lower than a fresh muscle will. Probably both fatigue and high tempera- 
 ture are favorable to the formation of the myosin ferment, and heat hastens 
 the fermentation process, resulting in coagulation. Another view of the 
 nature of the first form of contraction has been advanced lately. According 
 to Brodie and Richardson, 3 this contraction in the case of frog's muscle may 
 be very considerable, and is due to heat coagulation of soluble myogen fibrin, 
 a form of proteid which v. Furth found to be formed from myosinogen (what 
 he termed myogen) at 30 C. Mammalian muscles do not show any marked 
 contraction at this temperature, and have not been found to contain myogen 
 fibrin. This form of shortening is seen only by light loads, for the coagula- 
 tion of the proteids of the muscle causes increased extensibility, in addition 
 to the tendency to contract. 4 The change of form in rigor caloris is more 
 in voluntary than in involuntary muscles, as much as 60 per cent, in the 
 former and 10 per cent, in the latter. The beginning of heat-rigor comes 
 at very different temperatures in the different muscles of different animals. 
 Mammalian muscle can stand several degrees higher than the muscles of 
 cold-blooded animals, heart-muscle can be heated two or three degrees higher 
 than the skeletal muscles, and skeletal muscles differ, e. g., the semimem- 
 branosus of the frog enters into rigor sooner than the gastrocnemius. 5 These 
 facts are brought out in experiments in which the temperature of the muscle 
 
 1 Stewart and Sollmann : 1899, xxiv. p. 428. 
 
 2 Latimer : American Journal of Physiology, 1899, ii. p. 29. 
 
 3 Philosophical Transactions of the Royal Society of London, 1899, Series B., vol. cxci. p. 353. 
 
 4 Gotschlich : Journal of Physiology, 1897, vol. xxi. p. 353. 
 
 5 Ward, H. C. : Unpublished experiments at the University of Michigan. 
 
166 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 is more or less rapidly raised, at the same time that the changes in the length 
 of the muscle are recorded on a slowly moving surface. Halliburton 1 gives 
 the following precipitation temperatures for muscle proteids. 
 
 Name. Temperature of coagulation. 
 
 Proteids obtained from I Paramyosinogen 47 C 
 
 the dissolved clot . . I Myosinogen 56 C. 
 
 Proteids obtained from f Myoglobulin 63 C. 
 
 muscle-serum ... 1 Myo-albumin 73 C. 
 
 v Myo-albumose (not coagulated by heat). 
 
 Constituents of Muscle-serum. Proteids. The fluid which can be 
 expressed from the coagulated fresh muscle is called muscle-plasma. This 
 undergoes a change in the process of coagulation, two of the globulins 
 present, the para -myosinogen and the myosinogen, being precipitated in the 
 form of myosin, which makes the substance of the clot. The fluid which 
 can be expressed from the clotted muscle, the muscle-serum, therefore lacks 
 at least two of the proteid constituents of the normal muscle. The proteids 
 of the muscle-serum are : myoglobulin, myo-albumin, and myo-albumose. 
 
 The myoglobulin resembles serum-globulin, although precipitated at 63 
 C. instead of 73 C. The myo-albumin is apparently identical with serum- 
 albumin. 
 
 To these proteids we must add the pigment haemoglobin. Another pig- 
 ment, myohsematin, is also found. It is not unlikely that these pigments have 
 here as elsewhere a respiratory function. 
 
 Although the proteids form the larger part of the solids of the muscle- 
 substance, but little is known as to the form in which they exist in the living 
 muscle or the part that they play in its activity. They seem to have a two- 
 fold function, they are at once the machine and the fuel. 2 Under normal 
 conditions they probably supply but a small part of the energy set free by 
 the muscle during ordinary work. In excessive muscular work they undergo 
 katabolic change, as is shown by the increased excretion of nitrogen and sul- 
 phur in the urine. In the case of an individual not in training it would 
 appear that during excessive muscular exercise, as in starvation, other parts 
 of the body may give up their proteids to the muscles, for under such cir- 
 cumstances uric acid and phosphorus-holding extractives, the waste products 
 of nuclein, appear in the urine, and the muscle contains but little nuclein. 3 
 This is much less the case if the individual is in training, from which it 
 would appear that through training muscles acquire the capacity of storing 
 more proteid or of utilizing their stock to better advantage. In any case if 
 a large amount of muscular work is to be done the amount of proteid in the 
 food should be increased. 
 
 Nitrogenous Extractives. The chief nitrogenous extractive is creatin ; in 
 
 1 Halliburton : Physiological Chemistry, p. 414. 
 
 2 Pfliiger: Pfliige/s Archiv, 1899, Bd. Ixxvii. S. 425. 
 
 3 Dunlop, Pat on, Stockman, Maccadam : Journal of Physiology, 1897, xxii. p. 67. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 167 
 
 addition to this we find small amounts of crcatinin and <>f various xanthin 
 bodies, as xanthin, hypoxanthin, carnin, carnic acid, and sometimes traces of 
 urea, uric acid, taurin, and glycocoll. The chemical nature of these bodies 
 need not be considered here. Physiologically they may be regarded as waste 
 products which result from the partial oxidation of the proteids of muscle 
 during the katabolic processes which are continually occurring even in the 
 resting muscle protoplasm. Monari has shown that the amount of creatin 
 and creatinin is increased by the wear and tear of muscular work, although 
 the proteids of the well-fed muscle probably supply but little of the energy 
 which is set free. 
 
 The non-nitrogenous constituents of muscle are fats, glycogen, inosit, dex- 
 trose, and lactic acid. 
 
 Fats are usually found in intermuscular connective tissue, and there is 
 some within the normal fibre. It is doubtful whether fat plays a direct part 
 in the ordinary metabolic processes involved in the action of muscles, although 
 it is probable that if more available sources of energy are lacking it may, like 
 the proteids, be altered and employed. Bogdanow l states that the fat which 
 is within the muscle-fibre is of different constitution from that between the 
 fibres ; the extracts contain more free fatty acid. He further found that the 
 fat within the fibre is used up during the work of the muscle, and is con- 
 tinually renewed from the blood. If a muscle of a frog be removed from 
 the circulation and tetanized, it stains much less with osmic acid than one 
 with its circulation unimpaired ; while a muscle which is curarized, and so 
 does no work, if it has a good blood-supply, stains much darker than ordinary 
 muscle. Under pathological conditions large amounts of fat may be found 
 inside the sarcolemma ; in phosphorus-poisoning the degenerated muscle-pro- 
 toplasm may be replaced by fat in the form of fine globules. 
 
 Glycogen is found jn very variable amounts in different muscles. The work 
 of many observers has shown that it is here, as in the liver, a store of carbo- 
 hydrate material, and is employed by the muscle, either directly or after con- 
 version into some other body, as a source of energy. The quantity, which is 
 rarely more than ^ per cent., lessens rapidly during starvation and muscle 
 work. 
 
 When it is required, it is changed to dextrose, and is finally oxidized to 
 OO 2 and H 2 O. If the action of the muscle is prevented by the cutting of 
 the nerve or of the tendon, the glycogen is found to accumulate. 
 
 Sugar is found in muscles in small quantities only ; nevertheless it probably 
 plays an important part, for Chauveau and Kaufmann, by studying the levator 
 labii superioris of the horse, found that the muscles take sugar from the blood, 
 and that they take more during action than rest. The sugar which the mus- 
 cle takes during rest is for the most part stored as glycogen. 2 Although sugar 
 is considered a source of muscle-energy, the exact way in which it is employed 
 is doubtful. 
 
 1 Archiv fur Anatomic und Physiologic, 1897, S. 149. 
 
 2 Comptes rendus de la Societe de Biologic, 1886, civ. 
 
AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 Ergographic experiments on the human subject have proved that muscles 
 which have been fatigued by long-continued voluntary work recover much 
 more rapidly if sugar be eaten. Curiously enough, Waller and Miss Sowton 
 observed that the endurance of an isolated frog's nerve was increased, or at 
 least its capacity to develop strong currents of action was enhanced, if it was 
 put for a time in a 0.6 per cent, solution of sodium chloride containing dex- 
 trose. 
 
 Lactic Acid. This is formed in the muscle during work and during 
 coagulation. It has the form of para-lactic acid or sarco-lactic acid, though 
 it is doubtful whether it exists in a free state. It is a decomposition product 
 of the carbohydrates and perhaps of proteid or some complex muscle-sub- 
 stance of which proteid forms a part. It is only partly responsible for 
 acidity of the muscle which has been worked. The acidity may well be in 
 part caused by acid potassium phosphate produced from alkaline phosphates 
 as a result of the formation of phosphoric acid from lecithin, nuclein, etc. 1 
 Rohmann 2 attributes the acidity of worked muscles to monopotassium phos- 
 phate, and the alkaline reaction of the resting muscle to dipotassium phos- 
 phate and sodium bicarbonate. 
 
 Inorganic Constituents of Muscle. Among the bases, potassium has the 
 greatest prominence, and sodium next; magnesium, calcium, and small amounts 
 of iron are also found. Of the acids, phosphoric is present in the largest quan- 
 tities. 
 
 The quantity of a given substance present in a tissue is not an evidence 
 of its value, and the salts in the muscles, although present in comparatively 
 small quantities, are absolutely essential, not only to their functional activity, 
 but to their life. According to Loeb, 3 salts are not only present, as such, in 
 the muscle, but the ions Na, Ca, K, and Mg, are in combination with the 
 proteids, and these ion-proteid compounds are essential to its irritability (see 
 p. 58). 
 
 Gases of Muscle. No free oxygen can be extracted, but carbon dioxide 
 may be obtained, in part free and in part in combination. A little nitrogen 
 can also be extracted, but apparently it has no physiological significance. 
 The amount of carbonic acid developed varies greatly with the condition of 
 the muscle ; for instance, it is much increased by muscle work. Muscles 
 take up oxygen from the blood freely, especially when active, and when 
 removed from the body may absorb small amounts from the air. Moreover, 
 a certain amount of oxygen comes to the muscle from the food. More oxy- 
 gen is taken up by the muscle during rest than is liberated as carbon dioxide, 
 but during action the reverse is the case. 4 Oxygen is not retained as free 
 
 1 Weyl und Seither: Zeitschrift fur physiologische Chemie, vi. S. 557. 
 2 Rohmann: Pflugei j s Archiv, 1892, 1. S. 84, and 1893, Iv. 589. 
 
 3 American Journal of Physiology, 1900, vol. iii. p. 327. 
 
 4 Ludwig und Sczelkow : Sitzungsberichte der 'k. Akad. Wien, 1862, Bd. xlv. Abthl. 1 ; and 
 Ludwig und Schmidt : Sitzungsberichte der math.-phys. Classe d. k, Sachs. Gesellschaft der Wissen- 
 schaft, 1868, Bd. xx. ; Eegnault and Reiset : Annales de Chimie et de Physique, 1849, 3me s6r., 
 xxvi. ; Pfliiger : Pfliiger's Archiv, 1872, vi. ; and others. 
 
GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 169 
 
 oxygen, but is stored in some combination more stable than oxyhsemoglnbin. 
 It is by virtue of the combined oxygen that the muscle is enabled to do its 
 work, but the process is not one of simple oxidation. That muscles hold oxygen 
 in available combinations was shown by Hermann, who ascertained that a 
 muscle can contract hundreds of times in an atmosphere free from oxygen, 
 and produce water and carbon dioxide. If a muscle be thus fatigued, it will 
 recover somewhat in case it be supplied with oxygen, but not otherwise 
 (Joteyko et Richet). 
 
 Zuntz l found that the amount of oxygen absorbed by the body during 
 muscular work gives a proportional measure of the energy expended. He 
 gives the following figures for bicycle-riding : 
 
 Rate per hour. Oxygen absorption per meter. 
 9 kilometers 4.5 cu. cm. 
 
 15 " 4.8 
 
 21.5 " 5.76 " 
 
 A comparison of bicycling and walking showed that by moderate speeds 
 (riding 15 kilometers and walking 6 kilometers per hour) about double the 
 amount of oxygen was used for like distances in walking, but about 22 per 
 cent, more was required during like periods of time in riding. 
 
 II. CHEMISTRY OF NERVES. 
 
 Most of our ideas concerning the chemistry of nerves are based on analysis 
 of the white and gray matter of the central nervous system. The white matter 
 is largely made up of nerve-fibres and supporting tissue, and the gray matter 
 of the bodies of nerve-cells. The peripheral nerve-fibres are a continuation 
 of the structures in the central nervous system, or are composed of similar 
 elements ; the active part of the fibre, the axis-cylinder, is an outgrowth of 
 the cytoplasm of the body of a nerve-cell, and the surrounding medullary 
 sheath is a continuation of the material which sheathes the axis-cylinder while 
 in the brain and cord. It is probable, therefore, that the chemistry of the 
 axis-cylinder approaches to that of the cytoplasm of the body of the nerve- 
 cell of which it is a branch, and that the chemistry of the medullary substance 
 is the same outside as inside the central nervous system. 
 
 The white matter of the brain of the ox, which is largely made up of nerye- 
 fibres, is composed of about 70 parts water and 30 parts solids, about one-half 
 the latter being cholesterin, about a quarter proteids and connective-tissue sub- 
 stance, and about a quarter complex fatty bodies, neuro-keratin, salts, chiefly 
 potassium salts and phosphates, and traces of xanthin, hypoxanthin, etc. 
 
 Analysis of human sciatic nerve gives the following percentage for the 
 principal organic constituents : Proteids, 36.8 ; lecithin, 32.57 ; cholesterin 
 and fat, 12.22; cerebrins, 11.30; neurokeratin, 3.07; other substances, 4.O. 2 
 The nerve-fibre has a delicate sheath, the neurilemma, the exact constitution 
 
 1 Zuntz : Pfliiyer's Archiv, 1897, Bd. Ixx. S. 346. 
 
 2 J. Chevalier : Zeitschrift fur physiologische Chemie, x. 8. 97. 
 
170 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 of which is unknown, but which is supposed to resemble the sarcolemma and 
 to be composed of a substance similar to elastin. The fibres are bound together 
 by connective tissue which on boiling gives gelatin. Within the neurilemma is 
 the medullary sheath, which is composed of two elements viz. (1) neuro-kera- 
 tin, a material similar to the horny substance of epithelial structures, which 
 forms a sort of loose trellis, or network, and probably acts as a supporting 
 framework to the fibre ; (2) a white, highly refracting, semi-fluid material, 
 which fills the meshes of the neuro-keratin network, and which is composed 
 largely of protagon and cholesterin combined with fatty bodies. Protagon is 
 a complex phosphorized nitrogenous compound, which many observers believe 
 to contain lecithin and cerebrin. According to Noll, it makes up 7.47 per 
 cent, of the dried nerve. Both lecithin and cerebrin are fatty bodies possess- 
 ing nitrogen, and the former phosphorus. These -and some other complex 
 fatty bodies probably exist in addition to protagon in the medullary sub- 
 tance. The formation of the "myelin forms 77 seen in the medulla of dead 
 nerves is attributed to lecithin. The axis-cylinder probably contains most of 
 the proteids of the fibre, chiefly globulins, mixed with complex fatty bodies. 
 
 The reaction of the normal living fibre is neutral or slightly alkaline. It 
 is said to become acid after death, but this change is not known to accompany 
 functional activity. Indeed, nothing is known of the physiological import of 
 the chemical constituents of the nerve-fibre or of the chemical changes which 
 occur in the axis-cylinder when it develops or transmits the nerve impulse. 
 The peculiar chemical composition of the medullary substance would suggest 
 that it has a more important function than simply to protect the axis-cylinder. 
 Some have attributed to it nutritive powers, and others have supposed it helped 
 to insulate : it is certain that the axis-cylinder can develop and transmit the 
 nerve impulse without the aid of the medullary sheath, for there is a large 
 class of important nerves the non-medullated nerves in which it is lacking. 
 
II. CENTRAL NERVOUS SYSTEM. 
 
 INTRODUCTION. 
 
 The Unity of the Central Nervous System. The human nervous 
 system is formed by a mass of separate but contiguous nerve-cells. Indeed, 
 a group of nerve-cells disconnected from the other nerve-tissues of the body, 
 as the muscles or glands are disconnected from each other, would be without 
 physiological significance. To understand, therefore, the physiology of the 
 nervous system, it is important to keep in mind the fact that by dissection it 
 is found to be continuous throughout its entire extent. 
 
 When the nervous system is described as formed of a central and a 
 peripheral portion, and the peripheral portion is further divided into its 
 spinal and sympathetic components, the parts distinguished are found to 
 have no sharply marked boundaries separating them, but really to merge one 
 into the other. 
 
 For topographical descriptions the convenience of such subdivisions is 
 undoubted ; but the physiological processes which it is our purpose to study 
 overstep in so large a measure these limits that the picture of events in the 
 central nervous system would be very incomplete, should they be separately 
 traced only within such prescribed anatomical boundaries. 
 
 By virtue of its continuity the nervous system puts into connection all the 
 other systems of the body. Conforming as it does in shape to the frame- 
 work of the body, its branches extend to all parts. These branches form 
 pathways over which nerve-impulses travel toward the central system the 
 brain and spinal cord and, in consequence of the impulses that come in, 
 there pass out from the central system other impulses to the muscles and 
 glands. * 
 
 All incoming impulses must reach the central system. It is a fact of the 
 greatest significance that until they have entered the central system the 
 incoming impulses do not give rise to those outgoing, for thus all incoming 
 impulses are first brought to the spinal cord and brain, where the outgoing 
 impulses are co-ordinated. 
 
 By means of the central system reactions are established in parts of the 
 body not directly affected by the variation of the external conditions. Owing 
 also to the wide connections of the nervous system and the conduction of all 
 incoming impulses to its central parts, a measure of harmony is maintained 
 
 171 
 
172 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 between the activities of the several systems composing the body. Thus, not 
 only the systems forming the body are in this manner controlled, but the 
 body as a whole, in relation to all things outside of it and forming its 
 environment, is even more plainly under the guidance of these administra- 
 tive cells. 
 
 Phenomena Involving- Consciousness. It is with the brain and espe- 
 cially the cerebral hemispheres that the phenomena of consciousness are most 
 closely linked. Strictly, physiology concerns itself, at present, with the reac- 
 tions of the nervous system which can be studied without an appeal to con- 
 sciousness. A moment's consideration shows, however, that in the physiology 
 of the brafa the assistance to be obtained by passing beyond the limit thus 
 laid down is of more value than any boundary, and hence, although the field 
 of consciousness is sacred to psychology, physiology should not be deprived 
 of any of the advantages which come from the privilege of occasional 
 trespass. 
 
 Growth and Organization. The physiological connections existing be- 
 tween the nerve-elements in infancy are very incomplete and poorly estab- 
 lished, more so than in any other system of the body ; in the history of 'the 
 growth of the nervous system, the increase in weight and change in shape 
 run parallel with an increase in its complexity. This increase in complexity 
 is accompanied by more perfect organization, which results in better and 
 more numerous physiological pathways, thus permitting the system, as a 
 whole, not only to do more perfectly at maturity those things which it could 
 do in some degree at an earlier age, but also, by virtue of its increased com- 
 plexity, to do at maturity those things which previously it could not do at all. 
 
 Growth /in the case of this system implies, therefore, an increase in com- 
 plexity such as nowhere else occurs, and, since this growth can be modified 
 by the experience of the individual during the growing period, the impor- 
 tance of jinderstanding it and its relation to organization is evident. 
 
 Plan of Presentation. Our subject may be discussed under three heads 
 dealing respectively with the physiology of the (1) single cells, (2) groups of 
 cells, and (3) the entire central system. 
 
 Part I. The physiology of the nerve-cells, considered as a peculiar kind 
 of tissue-element, endowed with special physiological characters. 
 
 Part II. The activities of groups of these elements. The physiological 
 grouping is, of course, mainly dependent on the anatomical arrangement. At 
 the same time, the activities of one group modify those of others. Stated in 
 general terms, the problem in this part is that of the pathway of any impulse 
 through the central system. 
 
 Part III. The reactions of the system taken as a whole. Here its capa- 
 bilities as a unit are contrasted with those of the other systems, and its 
 growth, organization, and rhythms of rest and activity are most readily de- 
 scribed. 
 
CENTRAL NERVOUS SYSTEM. 
 
 173 
 
 PART I. PHYSIOLOGY OF THE NERVE ; CELL. 
 
 A. ANATOMICAL CHARACTERISTICS OP THE NERVE-CELL. 
 
 Form of Nerve-cells. Morphologically the mature nerve-cell is regarded 
 as composed of a cell-body, containing a nucleus, together with other modified 
 inclusions, and possessed of one or more outgrowths or branches. Some of 
 these branches may be very long, such, for instance, as those which form 
 nerve-fibres ; other branches are sjiort and differ from the nerve-fibres in 
 their structure. 
 
 The terms employed in describing the nerve-elements are as follows : To 
 the entire mass under the control of a given nucleus and forming both cell- 
 body and branches, the term neurone is applied. The inclusions within the 
 
 FIG. 67. A group of human nerve-cells, all drawn to the same scale, from preparations according to 
 Nissl's method, made by Dr. Adolph Meyer, and kindly furnished for this purpose ; X 300: a, small motor 
 cell from ventral horn of cervical spinal cord ; b, cell from " Clarke's column," thoracic cord ; r, small 
 nerve-cell from tip of dorsal horn, thoracic cord ; d, sr?l nal ganglion-cell . cervical root; e, three granules 
 from the granular layer of the cerebellum; /, Purkinje's cell from the same preparation as e; g, small 
 pyramidal cell from the second layer of the cerebral cortex of the central gyri ; h, giant pyramidal cell 
 from the same region. * 
 
 % 
 
 cell-body have the usual designations. Nerve-cells differ greatly in the 
 number of the branches arising from them. In some cells there appear to be 
 two nerve-fibres arising from the cell-body, in others, only one. For conve- 
 nience the descriptions about to be given will' apply to the latter group only. 
 From most' cells there arises one principal branch, which when considered 
 alone is described as a nerve-fibre, but when considered as the outgrowth of 
 the cell-body from which it originates is called the axone. The axotie, in 
 many cases, has branches, both near its origin from the cell-body and also 
 along its course. These branches are designated as collaterals. - At their 
 distal ends the main stem of the axone and also the collaterals subdivide 
 
174 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 into finer twigs, forming the terminal arborizations. Contrasted with this 
 principal outgrowth are the other branches of the cell, which are individu- 
 ally much less extensive and which divide dichotomously at frequent in- 
 tervals. From the tree-like form which they thus acquire they have been 
 designated dendrites. 
 
 The accompanying illustration (Fig. 67) shows the cell-bodies of several 
 neurones, together with the beginning branches, and also gives some idea of 
 the variations in the size of the cell-bodies as found in man. The nerve-cell 
 body is at first ovoid in shape, although this type is much modified in many 
 cases. As will be seen from Fig. 67, the diameters of nerve-cells range 
 from 4 fj. to 65 n* though in some instances in the spinal cord cell-bodies of 
 even larger diameter are found. 
 
 Peculiarities of Nerve-cells. As compared with the other cells of the 
 body, the best developed nerve-cells are of large size, but the nucleus, pro- 
 portionately to the remainder of the 
 neurone, is not large. Moreover, its pro- 
 portional value decreases with the increase 
 in the size of the entire cell. The most 
 striking feature of the nerve-cell, how- 
 ever, is the great length to which its chief 
 branch, the axone, may attain, for in no 
 other tissue does anything like so great a 
 proportion of the cell-substance occur as 
 a branch. Since the axone is the direct 
 outgrowth of the cell-body and can have 
 attained its length only gradually, it is 
 not surprising to find that all varieties of 
 length occur. 
 
 The form of cell represented in Fig. 
 68 is one in which the axone shows a 
 very short stem between the cell-body and 
 its terminal twigs. In such an instance 
 the entire extension of the axone may be 
 less than a millimeter. With this are to 
 be contrasted those forms in which the 
 axone is very long and its mass great. 
 What its greatest length may be is easily determined. Within the central 
 system there are cells whose axones extend from the cerebral cortex to the 
 lumbar enlargement (60 centimeters), and again in the peripheral system 
 there are cell-bodies in the lumbar enlargement of the spinal cord, the axones 
 of which extend to the skin and muscles of the foot, a distance of 100 centi- 
 meters or more. Among the neurones found in the spinal ganglia of the 
 lumbar region, some cells send one axonic branch to the level of the nuclei 
 of the dorsal funiculi in the bulb and the other branch to the skin of the 
 
 1 i = 0.001 of a millimeter. 
 
 Fro. 68. A cell, with a short axone, 
 giving off many branches. In such a cell 
 the axone is less in volume than the cell- 
 body. This is the extreme form of the 
 "central cell" of Golgi (Ram6n y Cajal). 
 D., dendrites ; N., axone. 
 
CENTRAL NERVOUS SYSTEM. 175 
 
 toes, thus spanning nearly the entire length of the human body. These are 
 the extreme cases, but as the axoues are distributed to all intermediate points 
 both in the eentral and peripheral system every intermediate length is to be 
 found. 
 
 Volume Relations. Calculation shows that the volume of the axone of 
 a large motor cell of the spinal cord when it extends from the lumbar 
 enlargement to the foot, may be 1500 times that of the cell-body. This 
 would include in the term axone both the axis-cylinder and the medullary 
 sheath. If either of these is taken alone, the volume is reduced by one-half. 
 
 It is extremely difficult to estimate the volume of the dendrites. In 
 some instances, as in the cells of the spinal ganglia (Fig. 71), they are 
 rarely present, while in the large cells of the cerebellum (Purkinje's cells) 
 they form a mass which must be several times greater than that of the cell- 
 body proper. In most neurones, however, the dendrites have, at best, a 
 volume as great as that of the cell-body. 
 
 Size of Nerve-cells in Different Animals. In considering the size and 
 form of cells in man it becomes of interest to determine how far the obser- 
 vations apply to the lower mammals. The facts are briefly these : It can 
 be said that the smaller mammals usually have the smaller nerve-cells, but 
 the decrease in the volume of the nerve-cells is not proportional to the 
 decrease in the volume of the entire body. For example, Kaiser l has shown 
 that the cell-bodies occupying the ventral horn in the cervical enlargement 
 of the spinal cord of the bat, the rabbit, and the monkey are in many cases 
 as large or larger than those found in man. 
 
 Though the volume of the cell-body and the diameter of the associated 
 axone are approximately similar in any two animals of different size, as, for 
 instance, in a bat and in man, it is evident that the axones could not have 
 the length in the bat that they do in man, and that in this last dimension at 
 least there is a diminution corresponding to the size of the animal. 
 
 The bearing of this fact on the comparative physiology- of the nervous 
 system is evident, for, under these conditions, as the volume of the entire 
 nervous system is diminished, the number of cell-elements constituting it 
 must also be diminished, and thus the structure of this system in the smaller 
 mammals becomes numerically simplified. 
 
 Size ancUFunction. A nerve-cell contains a mass of living substances 
 capable of being broken down and built up chemically, and there is nothing 
 against the inference that the larger the cell the greater is the quantity of 
 these living substances, and hence the larger the amount of stored energy 
 represented by it. The larger cells are therefore those capable rfH^^ing 
 free the greater amount of energy. The energy-producing change!^ | be 
 associated with the cell-body rather than with any of the branches^^Kore- 
 over, the nerve-cells with large cell-bodies, sending out, as they do, branches 
 which are more voluminous than those the cell-bodies of which are small, 
 furnish a greater amount of material to form the terminal twigs into which 
 
 1 Die Funktionen der Ganglienzellen des Halsmarkes, Haag, 1891. 
 
176 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 these branches finally split. From this it follows that, in general, the large 
 nerve-cells have more points of connection with the structures about them, as 
 well as the capacity for the liberation of a greater amount of energy. 
 
 Growth of Nerve-cells. The nerve-elements are derived from germinal 
 cells found in the epiblast of the embryo (Fig. 69). 
 
 These divide rapidly and in such a way that one daughter cell continues 
 as the germinal cell, while the other moves away from the primitive surface 
 of the body and becomes, without further division, a young neurone, or 
 neuroblast. The formation of neuroblasts in man ceases, or becomes very 
 slow and unimportant, by the end of the third month of fetal life. 
 
 Two characters of the neuroblast are worthy 
 of careful consideration. First, there is good 
 indirect evidence that, in early life at least, 
 and before their branches have been formed, 
 they are migratory, moving in an amoeboid 
 manner. This being so, the perfection with 
 which they arrange themselves in the adult 
 system depends on the accuracy with which 
 they respond to those conditions that deter- 
 mine their migration as well as upon the 
 normal character of these directing influ- 
 ences (mechanical strain ; * chemotaxis ; nu- 
 tritive attraction or electrical influences). 2 
 But with so much liberty of movement and 
 with directing influences that are so compli- 
 cated, the chances for deviation from a fixed 
 arrangement are much enhanced. 
 
 Second, very early in the history of the 
 neuroblast the point on the cell-body from 
 which the axone will grow appears in many 
 cases to be determined, and the cell is thus 
 physiologically polarized. 3 This polarity being 
 established, the direction in which the axone first grows is determined, and 
 where the cells are misplaced this polarization can lead to a confusion of 
 arrangement. 
 
 The volume of either the germinal cell or of the first form of the neuro- 
 blast was found by His 4 to be 697 cubic // in a human fetus (embryo R- 
 length 5.5 millimeters), aged 3-3.5 weeks ; and it can be shown that the mature 
 neurone must often attain a volume more than 50,000 times that of the orig- 
 inal neuroblast. 
 
 1 His : Unsere Korperform, 1874. 
 
 2 Davenport: Bulletin of the Museum of Comparative Zoology, Harvard College, Nov., 1895 ; 
 Herbst: Biologisches Centralblatt, 1 894, Bd., xiv.; H. Strasser: Ergebnisse der Anatomic u. Ent- 
 wickelungsgeschichte, Merkel and Bonnet, 1891, Bd. i. S. 731. 
 
 3 Mall : Journal of Morphology, 1893, vol. viii. 
 "* Archiv fiir Anatomic und Physiologic, 1889. 
 
 FIG. 69. Portion of developing medul- 
 lary tube (human) seen in frontal section. 
 X 1100 diameters (His) : G, germinal cell ; 
 N, neuroblasts. 
 
CENTRAL NERVOUS SYSTEM. 
 
 177 
 
 Maturing of the Nerve-cell. The maturing of the nerve-cell involves 
 several changes. First, the outgrowth of the axone or axones ; next, the 
 formation of the dendrites ; and, finally, in some cases, the medullation of 
 the axone, while simultaneously and with greater or less rapidity the absolute 
 amount of substance in both cell-body and axone is being increased, together 
 with a chemical differentiation of the cytoplasm and the nucleus. The time 
 in the life-history of the individual at which these several events occur is 
 variable, and may be delayed beyond puberty at least, while the rate at which 
 they occur is different in different cases. Furthermore, many nerve-cells 
 never develop beyond the first stage of immaturity (Fig. 70). 
 
 showing the phylogenetic development of mature nerve-cells in a series of vertebrates ; 
 a-e, the ontogenetic development of growing cells in a typical mammal ; in both cases only pyramidal 
 cells from the cerebrum are shown ; A, frog ; B, lizard ; C, rat ; D, man ; a, neuroblast without dendrites ; 
 6, commencing dendrites ; c, dendrites further developed ; d, first appearance of collateral branches ; c, 
 further development of collaterals and dendrites (from S. Ram6n y Cajal). 
 
 Form of the Axone as a Means of Classification. Of the various de- 
 vices used to t classify nerve-cells, the form of the axone is the most useful. 
 
 Physiologically, the nerve-cell is significant as the source and pathway for 
 the nerve-impulses. The current conception of the change called the nerve- 
 impulse is that it begins at one point of the cell and travels from there to the 
 other parts ; one of the other parts is the axone, and along this the impulse 
 can be shown to pass. Indeed, the nerve-cell body stimulated at any point 
 may be responsive just an an amoeba is responsive at any portion of its sur- 
 face. When, however, the branches are formed they become the channels 
 through which the impulses pass, and hence assume a special significance 
 
 VOL. II. 12 
 
178 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 without indicating any fundamental change in the structure of the cell. 
 Where the cell has well-developed branches we explain the arrangement by 
 assuming that the impulse enters the cell-body by one branch and leaves it 
 by another. 
 
 On examining the mature nerve-cells of man with this idea in mind, two 
 types are found. The first type may be exemplified by the pyramidal corti- 
 cal cells shown in Fig. 70. Here, from a pyramidal body (D) there arise 
 a number of dendrites, while from the lower portion of the cell the axone 
 
 FIG. 71. Spinal ganglion of an embryo duck; composed of diaxonic nerve-cells (van Gehuchten). 
 
 gro vvs out and branches. In the other type the axone alone grows out. Its 
 branches are but two in number and both are medullated. They pass in oppo- 
 site directions, and in this type there are, as a rule, no dendrites. Such 
 are the typical spinal ganglion-cells of the mammal. To understand the 
 arrangement in these cases, recourse must be had to the facts of develop- 
 ment. The second type begins its development as a diaxonic cell, an axone 
 growing from each pole (Fig. 71). In the adult spinal ganglion of the 
 higher mammals, however, such diaxonic cells are rarely found, the great 
 majority having a single axone which soon divides into two branches. 1 
 
 FIG. 72. Diaxonic changing into monaxonic cells : from the Gasserian ganglion of a developing 
 
 guinea-pig (van Gehuchten). 
 
 Fig. 72 beautifully illustrates the phases of this change as seen in a sin- 
 gle section. At first one axone arises from each pole of the ovoid cell-body. 
 Later the cell-body occupies a position at the side of the two axones, which 
 appear to run into one other. Finally the cell-body is separated from the 
 two axones by an intervening stem. The stem has the characters of a medul- 
 lated nerve-fibre, and from the end of it the two original axones pass off as 
 branches. 
 
 1 Dogiel: Anat. Anz. Jena, 1896, Bd. xii. S. 140-152, describes the several kinds of neu- 
 rones which take part in the formation of the spinal ganglion. 
 
CENTRAL NERVOUS SYSTEM. 179 
 
 From this mode of development it is plain that the single stem must be 
 looked upon as containing a double pathway, although it appears to be in all 
 ways a single fibre, for on the one hand it contains the path for the incoming 
 and on the other for the outgoing impulses. Recent investigations have shown 
 in a striking way that cells modified in this manner are by no means limited 
 to the spinal ganglia, but occur in the cortex of the cerebellum and elsewhere. 
 Classifying the nerve-cells, therefore, in the light of these facts, we find : 
 (1) The pyramidal type, in which the dendrites and axone are both well 
 developed, and in which the greater number of the impulses most probably 
 enter the cell by way of the dendrites and leave by way of the axone ; (2) The 
 spinal ganglion type, in which originally the impulse passes in at one pole of 
 the cell and out at the other, but in the course of development the two axones 
 become attached to the cell-body by a single stem, and by inference there 
 must be in this stem a double pathway. In this latter case there are usually 
 no dendrites. 
 
 Growth of Branches. After the cells have taken on their type-form the 
 branches still continue to grow, not only in length, but also in diameter. In 
 man, for example, the diameter of the nerve-fibres (axones) taken from the 
 peripheral nerves at birth is 1.2-2 p. for the smallest, up to 7-8 p. for the 
 largest, with an average of 3-4 //, while at maturity it is 10-15 // for the 
 larger fibres. 1 
 
 Internal Structure of the Neurones. The status of this problem has 
 been admirably summarized by Barker, 2 to whose book the reader is referred. 
 For our purpose it is sufficient to state that the cytoplasm of nerve-cells is 
 composed of fibrils (the character of which is much discussed), and an inter- 
 mediate, non-fibrillar material. These constituents are distributed in different 
 proportions in the several parts of the neurone. The axone contains the 
 fibrils most closely packed. The intermediate substance is most evident in 
 the body of the cell, and in general the dendrites more closely resemble in 
 their structure the cell body. Part, at least, of the intermediate material 
 forms the "stainable substance" of Nissl, also called "tigroid," which, in its 
 susceptibility to change under disturbed nutritive conditions, acts like a stored 
 food material. But which portion of the cell acts to conduct the nerve im- 
 pulse is not known, and the contention that one or the other of the compo- 
 nent structures^ is the conductor of the nerve impulses rests on histological 
 evidence alone. For the present it is sufficient to know that the neurone 
 appears to be conductive in all its gross parts. 
 
 While the axone is growing as a naked axis-cylinder, it is usually slightly 
 enlarged at the tip (Cajal), suggesting that it is specially modified at that 
 point. The nutritive exchange on which the increase of the entire axone 
 depends appears to take place along its whole extent, and not to be entirely 
 dependent on material passed from the cell-body into the axone. 
 
 Medullation. After the production of its several branches, the next step 
 
 1 Westphal: Neurologisckes Centralblatt, 1894, No. 2. 
 J Barker: The Nenous System, 1899, pp. 101-114. 
 
180 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 in the growth of the cell is the formation of the medullary sheath about the 
 axone. Not all axones have a medullary sheath, nor is any axone com- 
 pletely medullated. In the sympathetic system there is a very large pro- 
 portion of unmedullated axones. In the central system the number is 
 also very large, although their mass is small. Of the significance of the 
 medullary sheath we know nothing. The suggestion that it acts to insulate 
 the nerve impulse within a given axis-cylinder has little or no evidence 
 in its favor. The suggestion that it is nutritive is plausible, but important 
 differences in the physiological reactions of the two classes of nerve-fibres 
 have not yet been found, if we except the observation that the non -medullated 
 nerves rapidly lose their irritability at the point of stimulation with the 
 faradic current, thus exhibiting a "stimulation fatigue" not found in nerves 
 unquestionably medullated. 
 
 Growth of Medullary Sheath in Peripheral Nerves. Whatever may 
 be the significance of the medullary sheath, it is usually formed before the 
 neurone has attained its full size. In the peripheral system it depends on 
 the presence of mesodermal cells which envelop the axis-cylinder, forming a 
 tube about it. Each ensheathing cell is physiologically controlled by a nu- 
 
 FIG. 73. Longitudinal (I?) and transverse (A) sections of nerve-fibres. The heavy border represents 
 the medullary sheath, which is thicker in the larger fibres. Human sciatic nerve. X200 diameters 
 (modified from van Gehuchten). 
 
 cleus which becomes situated about midway between its extremities. Accord- 
 ing to Ranvier and his school, the cell-substance is largely transformed into 
 myelin, and the line of junction between two of these sheathing cells forms a 
 node of the nerve-fibre. In the sheath of a growing axone at least two changes 
 can be readily followed : As the axis-cylinder increases in diameter the medul- 
 lary sheath becomes thicker. The change is such that in the mammalian 
 peripheral system the areas of the axis-cylinder and of the medullary sheath, 
 as shown in cross-sections of osmic acid preparations, remain nearly equal 
 (Fig. 73). On the other hand, the length of the internodal segments tends to 
 increase with an increase in the diameter of the nerve-fibre, and for nerves of 
 the same diameter it is less in man than in the lower animals. In a given 
 fibre the segments are shorter at the extreme peripheral end (Key and Retz- 
 ius). In the young fibres, also, they are shorter and increase in length 
 with age. 
 
 A physiological significance attaches to these segments, because, as Ran- 
 vier long since pointed out, it is at the nodes that various staining reagents 
 easily reach the axis-cylinder. This suggests that normal nutritive exchanges 
 may follow the same path, and thus short internodal segments giving rise to 
 
CENTRAL NERVOUS SYSTEM. 181 
 
 many nodes would represent the condition most favorable to exchange be- 
 tween the axis-cylinder and the surrounding plasma. Thus far, histological 
 observation shows the more numerous nodes where the physiological processes 
 are presumptively most active, and hence supports the hypothesis suggested. 
 
 In the peripheral nervous system the nerve-fibres conduct impulses before 
 they acquire their medullary sheaths : witness the activities of new-born 
 rats, in which the whole nervous system is entirely unmedullated. Moreover, 
 Langley l has reported, in the regenerating cervical sympathetic nerve, a 
 return of function, while the majority of the fibres are still without their 
 medullary sheaths. 
 
 Medullation in Central System. Concerning the relation of the medul- 
 lary sheath to the axis-cylinder in the central system, our information is less 
 complete. The elements which give rise to the medullary substance are not 
 known and the myelin is not enclosed in a primitive sheath. There are no 
 internodal nuclei regularly placed, yet Porter 2 has demonstrated in both the 
 frog and the rabbit the existence of nodes in some fibres taken from the spinal 
 cord. The conditions which there exist must be further studied before any 
 general statements concerning the development of the medullary substance in 
 the nerve-centres can be ventured. Yet, it is an important observation that 
 whereas medullation in the peripheral system is mainly completed during the 
 first five years of life, the process continues in the central system, and espe- 
 cially in the cerebral cortex, to beyond the fiftieth year. 3 
 
 Whatever views may be held concerning the capacities of a medullated 
 fibre, it is to be remembered that the medullary sheath does not cover the 
 first part of the axone on its emergence from the cell-body, nor are ultimate 
 branches of the axone medullated in the region of their final distribution. 
 
 What has just been said applies to the main stem of the axone. As shown 
 in Fig. 70, the axone often has branches near its origin, the collaterals, and 
 according to the observations of Flechsig 4 these also become medullated. 
 Concerning the time of the medullation of these branches there are no direct 
 observations ; but if it is controlled by the same conditions which appear to 
 control the process in the main stem, then, as the branches form their physio- 
 logical connections later than the main stem, it would follow that their 
 medullation should also occur later, and the studies on the progressive medul- 
 lation of the cerebral cortex favor such a view. 
 
 The acquisition of this sheath occurs in response to a physiological change 
 that appears at the same time along the entire length of the fibre. The proc- 
 ess, therefore, is not a progressive one, but is practically simultaneous. 
 
 From the observations of Ambronn and Held 5 on rabbits a day or two old, 
 it appears that the efferent (motor) spinal and cranial nerves acquire their 
 
 1 Langley : Journal of Physiology, 1897, xxii. p. 223. 
 
 2 Quarterly Journ.al of Microscopical Science, 1 890. 
 
 3 Vulpius : Archivfiir Psychiatric und Nervcnkrank., 1892, Bd. xxiii. 
 * Archiv fur Anatomic und Physiologic, 1889. 
 
 5 Ambronn and Held : Archivfiir Anatomic und Physiologic, Anatom. Abthl., 1896, S. 208. 
 
182 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 sheaths before the corresponding afferent (sensory) nerves are medullated 
 (except in the case of the vestibular branch of the auditory nerve, which is 
 medullated at the same time as the motor nerves). In the central system 
 the continuations of the afferent pathways become medullated before the 
 pyramidal tracts, while in the cerebral hemispheres medullation of the com- 
 missural and association fibres follows immediately that of the afferent tracts. 
 
 Ambronn and Held l have also shown that when the eyelids are prema- 
 turely opened in animals born blind, such as the rabbit, dog or cat, and the 
 animal is then exposed to the light, the medullary sheaths are more rapidly 
 formed in the optic nerves exposed to stimulation than in those developing 
 normally. 
 
 Changes in the Cytoplasm. While the nerve-cell is passing from the 
 immature to the mature form, increasing in mass and in the number of its 
 branches, as well as acquiring its medullary sheath, it also undergoes various 
 chemical changes. The stainable substance in the cytoplasm becomes more 
 abundant at maturity and the pigment-granules increase in quantity. 2 
 
 FIG. 74. To show the changes in nerve-cells due to age: A, spinal ganglion-cells of a still-born male 
 child ; B, spinal ganglion-cells of a man dying at ninety-two years ; n, nuclei. In the old man the cells 
 are not large, the cytoplasm is pigmented, the nucleus is small, and the nucleolus much shrunken or 
 absent. Both sections taken from the first cervical ganglion, X 250 diameters (Hodge). 
 
 Old Age of the Nerve-cells. But the nerve-cell, though possessing in 
 most cases a life-history coextensive with that of the entire body, eventually 
 exhibits retrogressive changes. These changes of old age consist, in some 
 measure, in a reversal of those processes most evident during active growth. 
 The cell-body, together with the nucleus and its subdivisions, becomes smaller, 
 the stainable substance diminishes and becomes diffused instead of appearing 
 in compact masses, 3 the pigment increases, the cytoplasm exhibits vacuoles, 
 the dendrites atrophy, and the axones also probably diminish in mass. In 
 some instances the entire cell is absorbed. Some of these facts are illustrated 
 by the observations of Hodge 4 on the spinal ganglion-cells of an old man of 
 ninety-two years as compared with those of a new-born child (see Fig. 74). 
 
 Since the chemical and morphological variations which occur during the 
 entire growth-cycle are accompanied by variations in the physiological powers, 
 1 LOG. cit, S. 222. J Vas : Archiv fur mikroskopische Anatomic, 1892. 
 
 3 Marinesco : Revue neurologique, October, 1899, No. 20. 
 
 4 Journal of Physiology, 1894, vol. xvii. 
 
CENTRAL NERVOUS SYSTEM. 183 
 
 we are led to anticipate in old age a correlation, on the one hand, between 
 the decrease in the quantity of functional substance in the cytoplasm, and a 
 decrease in the energy-producing power of the cells, and, on the other, 
 between the absorption of the cell-branches and a limitation in the extent t<> 
 which the neurones may influence one another. Both of these condition.- are 
 characteristic of the nervous system during old age. 
 
 B. THE NERVE-IMPULSE WITHIN A SINGLE NEURONE. 
 
 The Nerve-impulse. Neurones form the pathways along which nerve- 
 impulses travel. As introductory, therefore, to the study of the composite 
 pathways in the central system, comprising, as they do, several elements ar- 
 ranged in series, it becomes important to study the behavior of the nerve- 
 impulse within the limits of a single cell-element. 
 
 Experimentally, the passage of the nerve-impulse is revealed by a wave of 
 change in the form of an electrical variation which passes along the nerve- 
 fibre in both directions from the point of stimulation. Under normal condi- 
 tions, the intensity of the electrical change does not vary in transit, though 
 for moderate electrical stimuli the strength of the electrical change ("action 
 current ") is proportional to the strength of the stimulus. 1 It moves in the 
 peripheral nerves of the frog in the form of a wave some 1 8 millimeters in 
 length, at the mean rate of 30 meters per second, and this rate can be some- 
 what retarded by cooling the nerves and accelerated by warming them. In 
 mammals the rate in the peripheral nerves has been found by Helmholtz and 
 Baxt to be 34 meters per second, The nerve-impulse can be aroused at any 
 point on a nerve-fibre provided a sufficient length of fibre be subjected to 
 stimulation. Mechanical, thermal, chemical, and electrical stimuli may be 
 used to arouse it, but just how the impulse thus started differs from that 
 normally passing along the fibres, as a consequence of changes in the cell- 
 bodies of which these fibres are outgrowths, is not known. It appears, how- 
 ever, that the impulses aroused by artificial stimuli are usually accompanied 
 by a much stronger electrical variation than accompanies the normal impulses. 
 
 In the peripheral system the nerve-impulse, when once started within a 
 fibre or axone, is confined to that track and does not diffuse to other fibres 
 running parallel with it, although it does, of course, extend to all the branches 
 of the axone, whatever their distribution. 
 
 The above-mentioned relations have been deduced from the study of the 
 peripheral nerves, and these morphologically are but parts of the axones, the 
 cell-bodies of which are located either in the central system proper or in the 
 spinal or sympathetic ganglia. 
 
 The observations apply therefore to but one portion of the nerve-cell, and 
 our present purpose is to determine how far it is possible to apply them to 
 the entire nerve-cell, noting at the same time the modifications thus intro- 
 duced. 
 
 Owing to the small size of nerve-cell bodies there are, of course, very few 
 
 1 Greene: American Journal of Physiology, 1898, vol. i. p. 115. 
 
184 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 instances in which a single nerve-cell, or part of such a cell, has been the 
 object of direct physiological experiment. We shall therefore approach the 
 question indirectly by showing what the histological relations have to suggest. 
 
 Direction of the Nerve-impulse. In the case of a given nerve-cell the 
 impulses which we usually consider, pass in one direction only. For instance, 
 along the ventral nerve-roots of the spinal cord, the impulses pass from the 
 cord to the periphery, while in the dorsal roots, so far as the fibres take 
 origin from the cells of the spinal ganglia, these impulses travel in the oppo- 
 site direction. At the same time experiment has shown that if a nerve- 
 trunk be stimulated at a given point, then the nerve-impulse can be demon- 
 strated as passing away from the point of stimulation in both directions. 
 
 We are therefore led to inquire what limits are set to the passage of 
 impulses in a direction opposite to the usual one. The narrowest limits, it 
 appears, are those of the single cell in which the impulse has originated. 
 The* experimental observations are as follows : When the fibres forming the 
 ventral root of a spinal nerve are stimulated electrically, and the cross- 
 section of the spinal cord, somewhat cephalad to the level at which the root 
 joins it, is explored with an electrometer, there is not found any evidence of 
 nerve-impulses passing cephalad in the substance of the cord. The arrange- 
 ment of the cells in the cord is such, however, that the cell-bodies which give 
 origin to the fibres forming the ventral root are physiologically controlled by 
 fibres running toward them from every portion of the cord, and under normal 
 conditions these fibres convey impulses to the cell-bodies in question. The 
 experiment shows that when an impulse enters the cell-body by way of the 
 ventral root-fibre, to which it gives origin, the impulse does not stimulate the 
 other elements of the cord. 1 
 
 With the elements forming the dorsal spinal root the case is at first 
 glance apparently diiferent, though in reality it is the same. These elements 
 have the cell-body located in the spinal ganglion. The cells are essentially 
 diaxonic (Fig. 72); one axone extends from the point of division toward 
 the periphery and the other enters the spinal cord, where it forms two 
 branches, both of which course longitudinally for some distance within it 
 (see Fig. 75). In this case, therefore, the normal direction of the effective 
 impulses is from the periphery toward the cord, and within the cord they are 
 delivered to other elements, which carry them in all directions. It is there- 
 fore to be expected that the stimulation of the dorsal root-fibres would give 
 rise to impulses passing in both directions in the dorsal columns of the cord. 
 When, however, the dorsal columns of the cord are electrically stimulated 
 in a cross-section made just above the level of the entrance of a dorsal root, 
 then it is 'found that the electrical variation is to be detected in the nerve- 
 fibers on the distal side of the spinal ganglion. These impulses have there- 
 fore passed in a direction the reverse of that usually taken. The fibres 
 which in this instance are stimulated in the cross-section of the cord are, 
 however, outgrowths of the spinal ganglion-cells, and thus, although the 
 1 Gotch and Horsley : Proceedings of the Royal Society, 1888. 
 
CENTRAL NERVOUS SYSTEM. 
 
 185 
 
 stimulation of the cord does give rise to an impulse in the afferent spinal 
 nerve, nevertheless the impulse is continually within the limits of one cell- 
 element. This shows that the reversed impulse can pass the spinal ganglion, 
 and in doing this it probably traverses the cell-bodies there located. There 
 is, however, no evidence that the stimulation of 
 the dorsal columns of the cord produces out- 
 going impulses in the dorsal nerve-roots except 
 when the stimulus is applied to the axones, 
 which are outgrowths of the cells of the spinal 
 ganglia. 
 
 In the case of the interpolation of the cell- 
 body in the course of the axones there is every 
 reason to think that the nerve-impulse traverses 
 the body of the cell itself. This is suggested 
 by the changes caused in the cell-body of the 
 spinal ganglion-cells as the result of stimulating 
 the peripheral axone. Moreover, some observers 
 report an appreciable delay (0.036 second) in 
 the passage of the nerve-impulse through the 
 cell-body in the case of those cells which form 
 the spinal ganglion. 1 This delay has recently 
 been denied. 2 
 
 The observations of Steinach, 3 on the capacity 
 of the afferent nerves of the frog to conduct the 
 centripetal impulses through the region of the 
 spinal ganglion, indicate that impulses may pass 
 this region when the cell-bodies are very prob- 
 ably excluded from forming a part of the possi- 
 ble pathways, thus showing that the two branches 
 of the T-process are physiologically continuous. 
 These results do not show, however, that the 
 centripetal impulses fail, under normal condi- 
 tions, to pass to the cell-bodies also. It may be 
 pointed out that this is another piece of evi- 
 dence in favor of the view that within the 
 limits of a single neurone or fraction of a neurone there is no limitation to the 
 passage of a nerve-impulse in all directions, wherever it is started. 
 
 Double Pathways. If the view is correct, that in passing through the 
 spinal ganglion the normal impulse traverses the cell-body, then the nerve- 
 
 1 Gad and Joseph : Archivf. Andtomie u. Physiologic, 1889. 
 
 2 Moore and Reynolds : Proceedings of the Fourth International Physiological Congress, 
 held at Cambridge, 1898. Supplement, vol. xxiii, Journal of Physiology. These authors deny 
 the delay. 
 
 3 Steinach: " Ueber die centripetale Erregungsleitung im Bereiche des Spinalganglions," 
 PflugeSs Archiv, 1899, Bd. 78. 
 
 Fio. 75. A longitudinal section 
 of the cord to show the branching 
 of incoming root-fibres in dorsal 
 columns. At the left are three (D R) 
 root-fibres, each of which forms two 
 principal branches. These give off 
 at right angles other branches, col- 
 laterals, Col, which terminate in 
 brushes. C C, central cells, whose 
 axones give off similar collaterals 
 (Ramon y Cajal). 
 
186 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 impulse passes to and fro along the common stem which joins the cell-body 
 with the two branches (vide Fig. 72), the stem itself having all the characters 
 of a medullated fibre. 
 
 The study of this modification brings with it the following suggestion : 
 If the single stem in the modified spinal ganglion-cells must by virtue of its 
 development contain a double pathway, it is fair to inquire whether the same 
 may not be true of the other forms of the nerve-cell in which the axone also 
 appears to be single. Among the cortical cells the arrangement of the 
 branches is such that, for aught that is known, the stem of the axone may 
 functionate in the manner suggested, and contain more than one pathway. 
 
 The same arrangement must exist in the case of cells like those repre- 
 sented in Fig. 76, in which the axone arises from the base of a dendrite 
 
 FIG. 76. Showing the relations between the terminal branches of the dendrites (D) and of the- 
 axones (N f ) of the optic fibres where they come together in the superficial layer of the optic lobe of the 
 chick ; also showing the origin of the axone (N) from a dendrite (van Gehuchten). 
 
 at some distance from the cell-body, and in which nerve-impulses arriving 
 over the dendrites and leaving by the axone must normally follow the por- 
 tion of the cell-branch which is common to both, passing along it first in one 
 direction and then in the other. This last result has been extended by 
 Sherrington, 1 who found that he could produce movements of the hind limb- 
 in both monkeys and cats when the cord had been sectioned just below the 
 bulb, and the stimulus was applied to the fibres in the fasciculi graciles at 
 that level. The reaction is explained by the passage of impulses down the 
 dorsal columns (in a direction reverse to the normal), and their distribution 
 by way of the collaterals to the efferent elements located in the ventral horns. 
 Significance of Cell-Branches. Since the outgoing nerve-impulses are 
 isolated in the axone until they reach the terminal twigs, it follows that the 
 impulses destined to produce an effect beyond the cell limits will do so at the 
 extremities of the branches. This leads to the question how far the posses- 
 sion of branches is necessary to the functional activity of a nerve-cell either 
 fur the reception or transmission of an impulse. Since it has been pointed 
 out that the spinal cord of the newt and fish is capable of conducting impulses 
 even before the dendrites are developed, it follows that the transmission of 
 impulses is in some way dependent on the condition of the cell-wall, inde- 
 pendent of cell-branches. This modification may exist at points where there 
 are no branches, or during this early period be a general property of the 
 1 Proceedings of the Royal Society, 1897, Ixi. 243-246. 
 
CENTRAL NERVOUS SYSTEM. 
 
 187 
 
 wall, and only later become the peculiar property of those portions which are 
 drawn out to form the tips of the branches. But not only the capacity to 
 receive, but also the capacity to deliver impulses is a function of the ends 
 of the branches, and the cell-wall at these points must therefore be peculiarly 
 modified with a still further differentiation, determining the direction in which 
 the impulses may pass. Each dendrite may represent at least one pathway 
 by which impulses reach the cell-body. If, then, there are many dendrites, 
 the cell-body is subject to a more complicated series of stimuli than if the 
 branches are few. It will be remembered that the young nerve-cell has no 
 dendrites, that the first branch to be formed is the axone, and that the com- 
 pletion of the full number of dendrites is a slow process. The pathways 
 formed by the dendrites are therefore continually increasing up to maturity 
 
 FIG. 77. Climbing fibre from human brain : a, nerve-fibre ; 6, Purkinje's cell (Cajal). 
 
 (Fig. 77). The relation between the " climbing fibre " and the dendrites of 
 the Purkinje cell illustrates this arrangement. 
 
 Generation of Nerve-impulses. The impulses which arrive at the cell- 
 body produce there chemical changes. These changes, when they reach a 
 given volume, cause a nerve-impulse which leaves the cell-body by way of 
 the axone. If the nerve-impulse is, as we assume, dependent on the chem- 
 ical changes occurring in the cytoplasm, then it must vary according to these 
 changes, which in turn can hardly be similar when the incoming impulses 
 that arouse them arrive along different dendrites. We know that a stimulus 
 applied directly to the axone will give rise to a nerve-impulse ; but, as we 
 shall see later, the chemical changes accompanying the passage of this 
 impulse are too slight to be detected. Whether in the cell-body equally 
 slight changes would give rise to an impulse cannot be determined. 
 
 Birge l found upon stabbing the spinal cord of a frog with a needle in 
 1 Birge: Arch.f. Anat. u. PhysioL, Physiol. Abthl., 1882, S. 471. 
 
188 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 the region where t*he efferent cell-bodies are 
 clustered together, that not only were impulses 
 sent out of the cord, causing the muscles to 
 contract, but they continued to be sent out for 
 some seconds after the injury. 
 
 When an electrical stimulus is applied to the 
 cerebral cortex so as to stimulate the cells there 
 present, the discharging cells may also continue 
 to send out impulses for some time after the 
 cessation of the stimulus. 
 
 Experiments showing the multiple character 
 of the impulses aroused within the central 
 system have been made by Gotch and Horsley. 1 
 When the motor cortex of a monkey was 
 stimulated (Fig. 78) by means of the faradic 
 current, the muscles which by this means were 
 made to respond showed a long tonic contraction 
 followed by a series of shorter clonic ones (Fig. 
 79, D). When the spinal cord had been cut 
 across, the cortex was again stimulated, and the 
 electrical changes in the fibres of the cord which 
 convey the impulses from the cortex to the 
 spinal centres were investigated by means of the 
 capillary electrometer. By this means a curve 
 (Fig. 79, D) was obtained as a record of the 
 negative variations passing along these fibres. 
 This latter curve corresponds with the record 
 for the muscular contraction, and hence the 
 relation between the two series of events is 
 evident. It appears, therefore, that the cortical 
 
 cells after the cessa- 
 tion of the stimulus 
 still continue to dis- 
 charge in a rhyth- 
 mical manner. When 
 the cortex had been 
 removed, and the 
 electrodes were ap- 
 
 Mercury. 
 
 ,- Sulphuric acid 10%. 
 
 Microscope. 
 
 --Mercury. 
 
 FIG. 78. Schema illustrating the experiment for determining the num- 
 ber of separate nerve-impulses passing down the spinal cord upon stimula- 
 tion of the cortex (from experiments on the monkey ; Horsley) : E, E, elec- 
 trodes, intended to be on the " leg area." Where the cord is interrupted, plied directly to the 
 one non-polarizable electrode is placed over the cut end of the pyramidal i , . fl , ,, 
 
 fibres going to the lumbar enlargement ; the other, on the side of the cord. ' r V 1D 5 n eb ? tr 
 
 These lead to the capillary electrometer, in which the column of mercury discharge of the im- 
 moves each time an impulse passes. ^ -, 
 
 pulses was tound to 
 
 cease with the stoppage of the stimulus. The presence of the cortex was 
 therefore necessary for the continued discharge (Fig. 79, C). The attempt 
 
 1 Proceedings of the Royal Society, London, 1888. 
 
CENTRAL NERVOUS SYSTEM. 189 
 
 was also made to determine the rhythmic character of the negative varia- 
 tions in the motor nerve-trunk between the cord and the contracting muscle, 
 but the changes there present, though sufficient to cause contractions of the 
 muscle, were not strong enough to be recorded by a delicate capillary electrom- 
 eter. This result suggests that the impulses sent out from the spinal cord 
 by the normal discharge of the motor nerve-cells may differ from the 
 impulses artificially aroused in the lesser intensity of the electrical changes 
 that accompany them. 
 
 Rate of Discharge. The rate at which the nerve-cells discharge, as 
 shown by the number of impulses which produce tetanus of a muscle indi- 
 rectly excited, either by artificial stimulation of the nerve-elements in animals 
 or by voluntary impulses in man, is about ten impulses per second. It 
 appears that at least the cortical cells and those of the spinal cord have the 
 same rate of discharge, and that this rate is the same in some mammals 
 (dogs, cats, rabbits, and monkeys) as in man. Hence a tendency to discharge 
 about ten times a second may be assumed as characteristic of the mammalian 
 nerve-cell. 1 
 
 Points at which the Nerve-impulse can be Aroused. It is probable 
 that the excitation of any part of a nerve-cell is capable of producing a nerve- 
 
 UXCll 
 
 ation. | 
 
 < 1 1 1 1 1 1 1 
 
 1 1 Sec. | 
 
 fl ^ \ 
 
 1 
 
 
 I I I I I I I I \ ISec. \ 
 
 FIG. 79. From a photographic record of the movements of the column of mercury in a capillary 
 electrometer (Gotch and Horsley). The arrow shows the direction in which the record is to be read. 
 The upper curve (D) shows the period of excitation by the interrupted current ; this is followed by a 
 series of waves in the record showing a number of separate impulses sent down from the cortex after 
 electrical stimulation has ceased. In the lower curve (C'),the exciting electrodes were applied to the 
 white matter directly, the cortex having been removed. The record shows that in this case no impulses 
 pass after the stimulation has ceased. 
 
 impulse, whether the stimulus be applied at the tips of the dendrites or to 
 the axone in its course. 
 
 Irritability and Conductivity. In general, parts of the system which 
 are irritable are also conductive, but there are special cases in which the 
 irritability of the nerve-ftbre can be distinctly separated from its conductivity, 
 the latter being present while the former is absent. 
 
 It is an old observation that on stripping down the phrenic nerve by 
 compressing it between the thumb and forefinger and sliding these along the 
 nerve, a contraction of the diaphragm is caused. The part of the nerve thus 
 stimulated is soon exhausted. If, now, the same operation is repeated on a 
 portion of the nerve lying nearer the spinal cord, contraction of the diaphragm 
 again follows. This result was originally used to support the theory of a 
 ^chafer and Horsley: Journal of Physiology, 1885, vol. vii.; Schiifer, Ibid. 
 
190 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 nerve-fluid, and was held to demonstrate that after the nerve-tubes in the 
 portion of the trunk compressed had been emptied so that no reaction 
 followed further pressure, then if the pressure were applied still nearer the 
 cord, the fluid from that part of the nerve could be driven forward and a 
 contraction of the diaphragm would result. The notion of a nerve-fluid in 
 the sense in which that term was used by the earlier physiologists has long 
 since been abandoned ; but for our purpose, the experiment is important as 
 showing that under such treatment irritability and conductivity do not dis- 
 appear at the same time, but that the fibres remain conductive after they 
 cease to be irritable. 
 
 It has been shown also that 1 in young regenerating motor-fibres it often 
 happens that while no response is to be obtained by the direct stimulation 
 of the regenerated peripheral portion, yet the stimulation of the central and 
 fully grown portion does cause a contraction of the muscles controlled by 
 these fibres. In this case the newly formed fibres can conduct an impulse 
 which gives rise to a contraction, although such an impulse cannot be aroused 
 by directly stimulating them. 
 
 Summation of Stimuli in Nerve-cells. In an axone a single stimulus 
 if followed by a single nerve-impulse ; on the other hand, the studies which 
 have been made to determine the number of weak stimuli necessary to dis- 
 charge afferent cell-elements, when stimulated by way of the afferent nerves, 
 indicate that there may be a summation of stimuli i. e., the discharge does 
 not follow until a series of stimuli has been given. 2 
 
 Whether, however, the delay in the response is due to the failure of the 
 cytoplasm of the receiving cell to discharge until repeated impulses have 
 reached it, or whether the modification of the cell which causes the delay is 
 a process taking place at the point where the impulse passes over from the 
 branches of one cell to those of another, is not directly determined by the 
 experiments. The indirect .evidence is, however, entirely in favor of the 
 view that the delay which is notable in the arousal of a reflex response 
 occurs at the point where the impulse passes from one cell to another. 
 
 0. THE NUTRITION OF THE NERVE-CELL. 
 
 The metabolic processes within the nerve-cell are continuous, and the 
 chemical changes there taking place involve not only those prerequisite to 
 the enlargement of the cell during growth, but also those leading to the 
 formation of such substances as by their katabolism cause the nerve-impulse. 
 The passage of the nerve-impulses probably alters the osmotic powers of the 
 cell-wall toward the surrounding plasma, and this of course is fundamental 
 to the nutritive exchange. It follows, therefore, that the passage of nerve- 
 impulses is one factor determining the nutrition of these cells. 
 
 Histologically we look upon the cell-bodies as the part in which the most 
 
 1 Howell and Huber : Journal of Physiology, 1892, vol. xiii. 
 
 1 Ward : Archiv f. Anatomic u. Physiologic, 1880 ; Stirling : Arbeiten aus den physiologischen 
 Anstalt in Leipzig, 1874. 
 
CENTRAL NERVOUS SYSTEM. 191 
 
 active changes occur, since the network of blood-vessels is most dense about 
 them, indicating that the metabolic processes are here most active 1 (Fig. 80). 
 
 Chemical Changes. For the direct microchemical determination of 
 special substances within the nerve-cells there are but few methods, though 
 some phosphorus-bearing substances (nucleins) can be demonstrated, 2 and the 
 occurrence of chemical changes due to activity and to age are very evident. 
 Macallum 3 has demonstrated the presence of iron in the stainable substance 
 of Nissl. There is general consensus that the alkalinity of the nerve-tissues 
 is decreased during activity, and this decrease in alkalinity may amount at 
 times to a positively acid reaction. This change, too, is better supported by 
 the observations made where the cell-bodies are numerous than by those 
 made where the fibres are alone present. 
 
 Fatigue. Not only is the food-supply to the nerve-cells, as represented 
 by the quality and quantity of the plasma, variable, but the cells themselves 
 
 FIG. 80. Frontal section through the human mid-brain at the level of the anterior quadrigeminum 
 (Shimamura). On the left side the blood-vessels have been injected; on the right the gray matter is 
 indicated by the heavy lines. It appears by this that the blood-vessels are most abundant in the gray 
 matter. 
 
 are subject to wide variations in their power to utilize these food materials, 
 and deviations from the normal in either of these respects means a diminu- 
 tion in the physiological powers of the cell, which we may call fatigue. In 
 the nervous system the signs of fatigue are both physiological and histological, 
 but it is to the latter changes only that attention will be here directed. 
 
 If a faradic current is applied intermittently to the mixed nerve-trunk 
 going to a limb, changes are to be observed in the cell-bodies belonging to 
 the spinal ganglia of the several roots forming the nerve (Hodge). 
 
 When this experiment is made on a cat, and, after death, the sections 
 from the stimulated are compared with those of the corresponding, but un- 
 stimulated, spinal ganglion, a picture like that represented by Fig. 81, is 
 obtained. 4 
 
 1 Shimamura : Neurologisches Centralblatt, 1894, Bd. xiii. 
 
 2 Lilienfeld and Monti : Zeitschrift fur physiologische Chemie, 1892, Bd. xvii. 
 
 3 Macallum : British Medical Journal, London, 1898, vol. ii. p. 778. 
 
 4 Hodge : Journal of Morphology, 1892. 
 
192 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 The sections indicate that the cytoplasm, together with the enclosed nucleus 
 and nucleolus, as well as the nuclei of the enclosing capsule of the cell, have 
 all suffered change by this treatment. The stimulus was applied for only 
 fifteen seconds of each minute, the remaining forty-five seconds being given 
 to rest. In this way the cells here figured had been stimulated over a period 
 of five hours. The nuclei of the sheath are flattened, the cytoplasm of the 
 
 FIG. 81. Two sections, A and B, from the first thoracic spinal ganglion of a cat. B is from the gan- 
 glion which had been electrically stimulated through its nerve for five hours. A, from the correspond- 
 ing resting ganglion, The shrinkage of the structures connected with the stimulated cells is the most 
 marked general change. , nucleus ; n, s, nuclei of the capsule ; v, vacuole ; X 500 diameters (Hodge). 
 
 nerve-cells somewhat shrunken and vacuolated. With osmic acid the nuclei 
 of the stimulated cells stain more darkly and the cytoplasm less darkly than 
 in a resting cell. In the nerve-cells the nucleus is shrunken and crenated, 
 and the nucleolus is also diminished in size. 
 
 In the first experiments the attempt was made to demonstrate a measur- 
 able change within the nerve cell-bodies as the result of stimulation. Assum- 
 ing the nuclei of these cells to be approximately spherical, and calculating 
 their volume as spheres, the shrinkage amounted to that .shown in the follow- 
 ing table : 
 
CENTRAL NERVOUS SYSTEM. 
 
 193 
 
 Table showing the Decrease in the Volume of the Nucleus / Stimulated Spinal 
 Ganglion-cells of Cats. Stimulation for fifteen seconds alternating with 
 rest /or forty-jive seconds (Hodge). 
 
 Stimulation continued for 
 1 hour. 
 2.5 hours. 
 5 
 10 
 
 Shrinkage in the volume of the 
 nuclei of the stimulated cells. 
 
 22 per cent. 
 
 21 " 
 
 24 " 
 
 44 " 
 
 This table further shows that the shrinkage is greater, the greater the 
 time during which the stimulus was applied. There is thus established not 
 only the fact of a change in the cell, but also a relation between the amount 
 of this change and the length of time during which the stimulus was allowed 
 to act. The results when expressed by a curve yield the following : 
 
 Per 
 cent. 
 100 
 
 90 
 
 80 
 
 70 
 
 50 
 
 17 
 
 23 
 
 29 
 
 Hours 1 2i 5 10 Hi 
 
 FIG. 82. The broken line indicates the volume of the nuclei of the spinal ganglion-cells of a cat 
 after stimulation for the times indicated. The solid line indicates the volume of the nuclei, first after 
 severe stimulation for five hours, and then in other cats, also stimulated for five hours, but subsequently 
 allowed to rest for different periods of time. The period of rest is found by subtracting five hours from 
 the time at which the record is made. After twenty-four hours of rest the nucleus is seen to have 
 regained its normal volume (Hodge). 
 
 Whether these changes could be considered similar to the normal physio- 
 logical variations depended on whether it was possible to demonstrate recovery 
 from them. This w r as accomplished in the following manner : 
 
 Under fixed conditions a cat was stimulated in the usual way and the 
 amount of shrinkage in the nuclei of the spinal ganglion-cells was determined. 
 This was found to be almost 50 per cent. Four other cats were similarly 
 treated and then allowed various periods (six and a half, twelve, seventeen, 
 and twenty-four hours) in which to recover. The results appear in Fig. 82. 
 
 Having thus shown that the change was physiological in the sense that it 
 was one from which the cells could recover, it remained to be shown that the 
 features of the change were discernible in the living cell, and were not caused 
 secondarily by the actions of the reagents employed in preparing the sections. 
 
 For the study of the living cell, frogs were chosen, and the cells of the 
 sympathetic ganglia examined. In these experiments, cells from different 
 frogs were prepared under two different microscopes and kept alive in the 
 
 VOL. II. 13 
 
194 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 rs.O-OO 
 
 same way by irrigation with a nutrient fluid. In one case, however, the cell 
 
 was stimulated by electricity, while in the other no 
 stimulation was applied. During the time of the 
 experiment the cell which was not stimulated re- 
 mained unchanged, while the stimulated cell went 
 through the series of changes exhibited in Fig. 83. 1 
 
 It followed that if these changes were really 
 significant of normal processes they should be found 
 in the nerve- cells of those animals which show 
 well-marked periods of activity, alternating with 
 periods of rest. To determine this, birds and bees 
 were examined, one set of preparations being made 
 from animals which were killed at the beginning of 
 the day, after a night of rest, and the other from 
 those killed at the end of the day, after a period of 
 activity. The cells from the latter animals were 
 found altered in a way similar to that following 
 direct stimulation of the axone. The changes were 
 demonstrated in the cells of the spinal ganglia of 
 English sparrows, of the cerebrum of pigeons and 
 cerebellum of swallows, and of the an ten nary lobes 
 of bees. These observations therefore support the 
 conclusions drawn from the appearances following 
 direct stimulation. 
 
 Other observers 2 have obtained similar results. 
 
 The motor cells of the spinal cord and cells of 
 the retina (dogs, Mann) have been added to the list 
 of those showing fatigue changes. In the sympa- 
 thetic cells of the rabbit, both Yas and Mann 
 found, after a short period of stimulation, a pre- 
 liminary swelling of the cell-body, and the same 
 has been noted by Mann in the case of retinal cells 
 in the dog. 
 
 The application of these observations to changes 
 in the human nervous system has thus far been 
 made only in a casual way, but enough has been 
 already observed to make certain that the results 
 are applicable. 
 
 It will be noted that the changes above described 
 follow variations in the amount of stimulation, the 
 nutrient conditions represented by the surrounding 
 plasma remaining nearly constant. This latter, however, may undergo 
 
 FIG. 83. Showing the changes 
 in the form of the nucleus re- 
 sulting from the direct electrical 
 Stimulation of the living sym- 
 pathetic nerve-cell of a frog. 
 The hour of observation is given 
 within each outline. The ex- 
 periment lasted six hours and 
 forty-nine minutes. A control 
 cell treated during this time in 
 the same manner, except that 
 it was not stimulated, showed 
 no changes (Hodge). 
 
 1 Hodge: Journal of Morphology, 1892, vol. vii. 
 
 2 Vas: Archiv fur mikroskopische Anatomic, 1892 ; Mann : Journal of Anatomy and Physiology, 
 1894. 
 
CENTRAL NERVOUS SYSTEM. 195 
 
 alteration, and recent observations show that in various forms of poisoning 
 by inorganic substances or in zymotic diseases the nervous system and espe- 
 cially the cell-bodies are affected early and in a profound manner. 1 
 
 Fatigue in Nerve-fibres. There is no evidence for fatigue changes in 
 nerve-fibres. For the full discussion of this .question the reader is referred 
 to page 96. 
 
 Atrophic Influences. When a nerve-cell is not kept active by the 
 impulses passing within it, it usually atrophies and may degenerate. The 
 reason for this appears to be that the loss of those changes which accompany 
 the nerve-impulses decrease the vigor of the nutritive processes. 
 
 For the detailed study of metabolic changes within the cell-body the 
 method of Nissl 2 has been of prime importance. This method consists in 
 fixing and hardening the nerve-tissue in 96 per cent, alcohol and staining 
 with hot methylene blue. As a result, the cell-bodies especially, retain the 
 stain, and in the cells there is a "stainable substance" characteristically 
 arranged in small masses. 
 
 For a given animal the arrangement of the " stainable substance " is char- 
 acteristic of the cells from different divisions of the nervous system. In 
 a general way, too, cells occupying homologous positions in the central 
 system of mammals tend to have the substance arranged in a similar manner. 
 But the characteristic picture is modified in any given case by the age of the 
 animal and by the pathological conditions which may have surrounded the 
 cell chosen for study. The changes in the picture may be described as 
 variations in (1) the stainable substance; (2) in the non-stainable fibrillar 
 framework which appears to enclose the former. 
 
 In both of these, variations may be accompanied by gross physical 
 changes, i. e., alterations in the size of the cell-body, the nucleus and its parts, 
 and alterations in the position of the nucleus, which may appear pushed to 
 the periphery of a swollen cell, or even extruded from it. These physical 
 changes are, of course, the effects of the action of the alcohol and other 
 reagents employed on the cells altered from the normal, and while these 
 physical changes serve most admirably to distinguish the normal from the 
 abnormal cells, they do not necessarily represent the condition of the abnormal 
 cells during life, a cell with an extruded nucleus, for example, being a case 
 in point. These changes may ultimately cause the death of the element. 
 
 The stainable substance is found to be extremely sensitive to variations 
 in the physiological conditions surrounding the cell, and therefore to be most 
 important for the revealing of the effect of all sorts of changed conditions, 
 such as starvation, activity, fatigue, injury to the axone, or injury to the 
 afferent neurones bringing impulses to this particular cell, and, finally, the 
 effects of toxins circulating in the blood. 
 
 ^chaffer: Ungarisches Archivfiir Medicin, 1893; Pandi : Ibid., 1894; Popoff: Virchow's 
 Archiv, 1894; Tschistowitsch : Petersburger medicinische Wochenschrift, 1895. 
 
 2 The publications of Nissl have not yet been printed in a compact form. The voluminous 
 bibliography of the author is given by Barker : The Nervous System, 1899, pp. 105, 106. 
 
196 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 Amputation in Man. When the nerves to a limb have been severed, 
 the consequent changes in the spinal cord depend on the age of the patient at 
 the date of operation, the length of time elapsing between the operation and 
 death, and the level on the limb at which the amputation was made. When 
 the amputation occurs early in life, and the time before death is long, and 
 the level of the amputation high, the alterations are maximum, and consist 
 in an atrophy in the peripheral efferent nerve-fibres, slight atrophy (or some- 
 times complete disappearance) of the spinal ganglion-cell bodies, atrophy of 
 dorsal root-fibres and their continuations within the cord, and, on the ventral 
 side, disappearance or atrophy of the motor (efferent) cell-bodies in the 
 ventral horn of the cord, together with their axonic outgrowths, the ventral 
 root-fibres, the effect extending outward through the peripheral nerve to the 
 point of section (see Fig. 84). The final appearances are brought about by 
 slow changes, often requiring years for their completion, and hence most of 
 the cases examined tend to show less change than is here described. 1 
 
 FIG. 84. Cross-section of the spinal cord of the chick, X 100 diameters (van Gehuchten) ; D, dorsal sur- 
 face ; V, ventral surface ; d. r, dorsal root ; v. r, ventral root ; g, spinal ganglion. On the left the arrows 
 indicate the direction of the larger number of impulses in the dorsal and ventral roots respectively. 
 The small arrow on the right dorsal root calls attention to the fact that so^me axones arising in the ven- 
 tral lamina emerge through the dorsal root and convey impulses in the direction indicated. 
 
 The disturbance caused in the two sets of cells is, however, not the same. 
 In the case of the cells of the spinal ganglion, the chief pathway by which 
 they are stimulated under normal conditions is so far mutilated that probably 
 only a small number of impulses passes over them. That some do pass is 
 indicated by the sensations apparently coming from the lost limbs sensations 
 which are often very vivid and minutely localized. 2 
 
 Thus the cell-bodies located in the spinal cord are to a great degree 
 deprived by such an operation of one principal group of incoming impulses, 
 namely those which arrive through the dorsal root-fibres that are most 
 closely associated with them ; but at the same time there remain many other 
 ways in which these same cells are normally stimulated. The efferent path- 
 
 1 Marinesco: Neurol. Ceniralbl., 1892 (reviews the literature); Gregoriew : Zeilschrift /. 
 Heilkunde, 1894, Bd. xv. 
 
 2 Weir Mitchell : Injuries of Nerve*, Philadelphia, 1872. 
 
CENTRAL NERVOUS SYSTEM. 197 
 
 way from these cells is incomplete, and the impulses which must pass along 
 the stumps are inefficient. That impulses do pass alon-r the stumps of the 
 efferent roots is beyond question, since, when the distal portion of an effer- 
 ent nerve is cut off, the cell can be shown to still discharge through tin- por- 
 tion of the fibres connected with the cell-bodies, and, finally, there is always 
 a tendency for the cut fibre to regenerate, which indicates activity tli rough 
 its entire length. 
 
 Wherever in the central system a group of fibres forms the chief pathway 
 for the impulses arriving at a given group of cells then the destruction of 
 these afferent fibres brings about the more or less complete atrophy of the 
 cells about which they terminate, and this effect is the more marked the 
 younger the animal at the -time of injury. Examples of this relation are 
 found in the behavior of the nuclei of the sensory cranial nerves. 
 
 Thus the activity of a given cell contributes to the strength of its own 
 nutritive processes, and different cell-elements, so far as they are physiologi- 
 cally associated, stand in a nutritive or trophic relation to one another such 
 that the receiving cell is in some measure dependent for its nutrition on the 
 cell which stimulates it. 
 
 Degeneration of Nerve-elements. All parts of a nerve-cell are under 
 the control of that portion of the cell-body which contains the nucleus ; in 
 this respect the nerve-elements are similar to other cells which have been 
 studied, and in which the nucleated portion of the cell is found to be alone 
 capable of further growth. It was shown by Waller 1 that when sepa- 
 rated from the cell-body of which it was an outgrowth, a nerve-fibre belong- 
 ing to the peripheral nerve soon degenerate from the point of section to its 
 final distribution. The process is designated as secondary or " Wallerian 
 degeneration." According to recent studies on this subject, 2 this degener- 
 ative change occurs practically simultaneously along the entire length of the 
 portion cut off. The changes following the section of medullated nerve- 
 fibres consist in a fragmentation of the axis-cylinder followed by its dis- 
 appearance ; enlargement and multiplication of the nuclei of the medullary 
 sheath, and absorption of the medullary substance, so that in the course of 
 the fibres there remains at the completion of the process the primitive sheaths 
 together with the sheath-nuclei. In the early stages of this process the 
 medullary sheath, moreover, undergoes some changes, the result of which is 
 that it stains more deeply with osmic acid, and hence appears very black in 
 comparison with the normal fibres about it (Marchi). These changes, as 
 shown by the method of Marchi, may follow even slight injuries to the nerve- 
 fibres such as compression for a shor,t time. 
 
 Concerning the progress of degenerative changes in the non-medullated 
 fibres information is scanty. Bo wd itch and Warren 3 observed that when 
 the sciatic nerve of the cat was sectioned, degeneration of the motor and 
 
 1 Nouvelle methode anatomigue pour I' investigation du Systime nerveux, Bonn, 1851. 
 1 Howell and Huber : Journal of Physiology, 1892, vol. xii. 
 3 Journal of Physiology, 1885, vol. vii. 
 
198 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 vase-constrictor fibres in the peripheral portion went on at about the same 
 rate. Stimulation of the peripheral part of the nerve gave a vaso-dilator 
 reaction after the vaso-constrictor reaction had entirely disappeared, suggest- 
 ing that the constrictor fibres degenerate more rapidly than do the dilators, 
 although it is not improbable that the dilator fibres in this location really 
 belong to the medullated class (Howell). After five days no vaso-motor 
 reaction at all could be obtained. In a recent study by Tuckett 1 of the 
 degeneration of the non-medullated fibres contained in the branches springing 
 from the superior cervical ganglion, it is stated that the degeneration, as 
 traced by histological and physiological methods is complete within thirty 
 to forty hours after section of the fibres, and that the degenerative changes 
 involve only the core of the fibres, the outside sheath and nuclei being un- 
 affected. 
 
 In the central system, the distal portion of the. fibres separated from the 
 cell-body degenerate, as at the periphery, and this reaction has therefore 
 formed a means by which to study the architecture of the central system. 
 The details of the process are, however, not clear. 
 
 Nutritive Control. So far, then, as the principal outgrowth of the nerve- 
 cell is concerned, it is found to be always under the nutritive control of the 
 cell-body from which it springs. The changes which take place when the 
 spinal roots are cut will serve to illustrate this control (see Fig. 85). Sec- 
 
 Mr. 
 
 FIG. 85. Schema of a cross-section of the spinal cord, showing the dorsal and ventral roots and the 
 points at which they may be interrupted : D. r., dorsal root ; V. r., ventral root; G, ganglion ; M, muscle ; 
 S, skin ; 1, lesion between ganglion and cord ; 2, lesion between muscles and cord ; 3, lesion between skin 
 and ganglion ; 4, combination of 2 and 3. 
 
 tion of the dorsal root at the distal side of the spinal ganglion at 3, causes a 
 degeneration of all the fibres which form the dorsal nerve-root distal to the 
 ganglion. Section of the dorsal root at 1, causes degeneration, central to the 
 section, of those nerves which are outgrowths from the cell-bodies of the 
 spinal ganglion. Section of the ventral root at 2, causes a degeneration distal 
 to the point of section in those fibres which form the ventral root and which 
 arise from the cells within the spina-1 cord. In each case, therefore, the 
 necessary degeneration occurs on the side of the section away from the cell- 
 body. The fraction of the neurone on the other side of the section may also 
 degenerate under certain conditions, but the degeneration is not inevitable. 2 
 
 1 Tuckett: Journal of Physiology, 1896, vol. xix. 
 
 2 Bregmann : Arbeiten am dem Insiitutfur Anatomie und Physiologie des Centralnervensystems 
 an der Wiener Unwersildt, 1892-93. 
 
CENTRAL NERVOUS SYSTEM. 199 
 
 It is sometimes stated that degeneration takes place in the direction of 
 the nerve-impulse. In a general way this is true, since the impulses usually 
 travel from the cell-body along the axone. In the case of the fibres arising 
 from the cells of the spinal ganglion it is not true, since the section at the 
 distal side of the ganglion causes degeneration away from the spinal cord, 
 while that on the proximal side of the ganglion causes degeneration toward 
 the spinal cord; yet in both axones the impulse is in the same direction 
 namely, toward the cord (see Fig. 85). 
 
 Degeneration of the Cell-body. It was discovered by .von Gudden 1 
 that when the nerves of young animals are pulled away from their attach- 
 ment with the central system, they most frequently break just at the point 
 where they emerge from the cord or brain axis c When an efferent nerve is 
 thus broken, in animals just born or very young, the remaining portion i. e., 
 the cell-bodies with so much of their axones as lie within the central system 
 may atrophy to complete disappearance. 
 
 The bearing of such a fact is very direct. If in man there is reason to 
 think that an injury was suffered during fetal life, there is a possibility that 
 the injury may not only have prevented the further development of the cells 
 involved, but may also have caused the complete destruction of some of them, 
 in which case, of course, the architecture of the damaged region is necessarily 
 abnormal. 
 
 Such complete disappearance as the result of early injury has not been 
 shown for cells which lie entirely within the central system. Those forming 
 the spinal ganglia may die, however, after interruption of the- axones, even 
 when the animal is mature (van Gehuchten). In the case of those central 
 cells which form the sensory nuclei, like the sensory nucleus of the fifth 
 nerve, or of the vagus, pulling out the nerve-trunk formed by the axones of 
 the afferent ganglion cells, causes only an atrophy of the central cells, and 
 not their complete disappearance. 2 
 
 Regeneration. When the two ends of the sectioned nerve are brought 
 together under favorable conditions, the peripheral portion may be regener- 
 ated. This regeneration occurs only in axones possessing a nucleated (medul- 
 lated or unmedullated) sheath, or in the anatomical prolongations of these, 
 such as the dorsal root-fibres which penetrate the spinal cord. 3 In the 
 typical medullated peripheral nerve this process occurs in the following 
 steps as described by Howell and Huber : 4 
 
 "While the fragmentation and absorption of the myelin in the distal portion 
 of the cut nerves is going on, the protoplasm in the neighborhood of the 
 sheath-nuclei tends to increase. These enlarged masses of protoplasm then 
 appear as a thread of substance within the old nerve-sheath. A new sheath 
 is, however, soon formed on the protoplasmic thread, and the whole consti- 
 
 1 Archivfiir Psychiatrie, 1870, Bd. ii. 
 
 2 Fore 1 : Festschrift zur von Nagdi und von Kolliker, Zurich, 1891. 
 
 3 Baer, Dawson, and Marshall : Journal of Experimental Medicine, 1899, vol. iv. No. 1. 
 
 4 Journal of Physiology, 1892, vol. xiii. 
 
200 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 tutes an " embryonic fibre." The embryonic fibres lying on one side of the 
 cut, unite with those on the other, union taking place in the intervening 
 cicatricial tissue. Next the myelin appears in isolated drops, usually near 
 the nuclei, and these subsequently unite to form a continuous tube, the for- 
 mation of the myelin proceeding centrifugally from the wound. Then follows 
 the outgrowth of the new axis-cylinder slightly behind the organization of 
 the myelin into the tubular form. 
 
 It must not be forgotten that the last act, the formation of the axis- 
 cylinder, is the important event ; and while the whole process of repair may 
 require many months, the rate at which the axis-cylinder, when started, 
 grows out from the central end may be comparatively rapid. As a rule, 
 regeneration does not occur in the central system, 1 and thus the method of 
 experimentally causing degeneration has been one used for the study of the 
 architecture of both the brain and cord. 
 
 That the regeneration is due to an outgrowth from the central stump has 
 been clearly shown by Huber, 2 who inserted a bone tube between the cut 
 ends of the sciatic nerve of the dog, and obtained regeneration of the nerve 
 with a return of function, although the initial interval bet ween the two parts 
 of the nerve was more than three centimeters. The rate of growth from the 
 central end has been specially studied by Yanlair. 3 In the facial nerve of 
 the rabbit, function was restored in eight months after section, and in the 
 pneumogastric and ischiadic nerves of the dog in about eleven months. In 
 the latter case, this gives an average rate of growth of about 1 millimeter a 
 day. In the scar-tissue between the two parts of the nerve the rate is not 
 more than 0.25 millimeter a day, and hence the return of function tends to 
 be delayed by any increase in the distance between the cut ends of the nerve. It 
 appears also that the return of the cutaneous sensibility is more rapid than 
 the return of motion (Howell and Huber), from which we infer that the 
 aiferent fibres (from the skin) regenerate more rapidly than the efferent fibres 
 to the muscles. 
 
 Vanlair found that when the regenerated sciatic nerve of a dog was cut 
 a second time, it not only again regenerated, but did so more rapidly than in 
 the first case. 
 
 Much interest has always attached to the exact course taken by the re- 
 generating fibres. They appear in a general way to be guided by the old 
 sheaths of the peripheral portion. But the peripheral nerves contain both 
 afferent and efferent fibres, and it would appear most probable that in the 
 process of reformation the new fibres should undergo much rearrangement. 
 Since the peripheral portion of the nerve acts as a guide to the growing fibres, 
 the experiment has been tried of cross-suturing two mixed nerves. This has 
 been done with the median and ulnar nerves in dogs. Reunion of the crossed 
 
 1 Worcester : Journal of Experimental Medicine, 1898, vol. iii. p. 597, describes a case of 
 apparent regeneration of a fibre-bundle in the mid-brain, and cites the literature. 
 J Journal of Morphology, 1895, vol. xi. 
 3 Archives de Physiologic normale et pathologique, 1894. 
 
CEXTH. 1 L M-:ii \ '() I 'S HYSTl-IM. 201 
 
 nerves occurred and sensation and motion returned to the affected parts of 
 the limbs. 1 It is plain that by this arrangement the skin and muscles at the 
 periphery must have acquired central connections with the spinal cord very 
 different from those normal to them. 
 
 From the experiments of Cunningham, 2 it appears that the results of the 
 cross-suturing of nerve-trunks are about what other facts would lead us to 
 expect. If the trunks concerned control muscles acting in a similar manner, 
 then cross-suturing produces but slight incoordination as a result; where, 
 however, the central trunks normally innervate antagonistic muscles, then 
 incoordination follows and persists. The stimulation of the cerebral cortex at 
 the centre for a given muscle group always causes impulses to pass along the 
 efferent fibres which normally innervate that group, no matter to what muscles 
 these fibres may have been secondarily attached by cross-suturing. More- 
 over, striped muscles which normally exhibit rhythmic contractions lose this 
 function when their innervation is changed by cross-suturing to a nerve-trunk 
 which normally innervates an arhythmic muscle. Thus the central nervous 
 system, in dogs, at least, does not adapt itself to the changed conditions 
 introduced by cross-suturing. 
 
 In a series of investigations, Langley 3 has been able to show that when 
 the preganglionic fibres of the thoracic nerves, which send branches to differ- 
 ent groups of cells in the superior cervical ganglion, are allowed to regenerate 
 after section, the several bundles of fibres appear to find and become attached 
 to the cell-group which they normally controlled, since stimulation of the 
 several roots after regeneration gave the reactions which were characteristic 
 for them. However, there is reason to think that the arrangement after 
 regeneration is not exactly the same as that before, and that some fibres have 
 strayed from their original connections. Further, Langley 4 has been able by 
 cross-suturing to establish a connection of the lingual and the vagus nerves 
 respectively with the cervical sympathetic nerve, and so with the superior 
 cervical sympathetic ganglion. Thus we have evidence that fibres other than 
 those normally associated with the ganglion cells can at times form functional 
 connections with them and carry impulses which excite them to their normal 
 functions. This result has an important bearing on the theory of the stimu- 
 lation of one element by another. The reaction following the indirect excita- 
 tion of these cells depends, therefore, on the connections made by their axones, 
 and not on the source of the fibres which excite them. The regeneration 
 thus far described has been that of the axone by the cell-body or perikaryon. 
 Concerning the regeneration of the dendrites, we have no information. 
 
 The possibility of the formation in mammals of new nerve-cells by the 
 division of nerve-elements which are already mature and have been func- 
 tional, has been claimed. 
 
 1 Journal of Physiology, 1895, vol. xviii. 
 
 2 Cunningham : American Journal of Physiology, 1898, vol. i. 
 
 3 Langley : Journal of Physiology, 1897, vol. xxii. p. 215. 
 
 4 Langley : Ibid., 1898-9, vol. xxiii. p. 240. 
 
202 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 Karyokinetic figures in mature nerve-cells after injury have been demon- 
 strated, but we have yet to learn exactly what cells can exhibit this reaction, 
 and what becomes of them at the end of the nuclear changes. As there is no 
 reason to think that in mammals such a neoformation of neurones in the 
 nervous system has any significance for the general physiology of the animal,, 
 we shall pass the point with a mere reference to the literature. 1 
 
 PART II. THE PHYSIOLOGY OF GROUPS OF NERVE-CELLS. 
 
 A. ARCHITECTURE AND ORGANIZATION OF THE CENTRAL NERVOUS 
 
 SYSTEM. 
 
 Since the nerves form the pathways by which the sensory surfaces of the 
 body are put into connection with the central system, and also the pathways 
 by which this system in turn is rendered capable of controlling the tissues of 
 
 D.p 
 
 FIG. 86. Schema of the arrangement of the human spinal cord as seen in cross-section ; for clearness 
 the afferent fibres are shown on the left side only, efferent and central cells on the right side only (von 
 Lenhossek) ; D. R., dorsal root ; V. R., ventral root ; D. P., direct pyramidal fibres ; C. P., crossed pyramidal 
 fibres ; C., direct cerebellar tract ; A. L., antero-lateral tract ; D. C., dorsal columns. The various classes 
 of cell-bodies are indicated by the manner of drawing. 
 
 expression, it becomes at once important to determine over what nerves the 
 impulses arrive, how they travel through that system, and by what other 
 nerves they are again delivered at the periphery. The arrangement of these 
 paths as found in the adult human nervous system is our principal object; at 
 the same time it should not be forgotten that the reactions of simpler mam- 
 malian systems have furnished the greater number of facts, and to them we 
 must constantly refer. 
 
 General Arrangement of the Central Nervous System. As the 
 typical arrangement of the neurones is found in the spinal cord, the schematic 
 representation (Figs. 86, 87) of a cross-section through this part will most 
 readily illustrate it. 
 
 In accordance with this arrangement of the nervous system, as shown in 
 
 1 Tedeschi, A. : Anatomisch-experimentellen Beitrag zum Studien der Regeneration des Gewebe des 
 Centralnervensystems. Beitrdge zur pathologischen Anatomie und zur allegemeinen Pathologic, Jena, 
 1897, xxi. 43-72, 3 pi. * 
 
CENTRAL NERVOUS SYSTEM. 
 
 203 
 
 Figs. 86, 87, the elements which compose it fall into three groups: (1) 
 Ttie afferent neurones; those whose function it is to convey impulses due to 
 external stimuli from the periphery, including the muscles and joints, to the 
 central system. The expression 
 " external stimuli " is in this case 
 intended to include beside those 
 outside of the body, also such 
 stimuli as act within the tissues 
 of the body but outside of the 
 central nervous system ; for ex- 
 ample, those acting on tendons 
 and muscles, and affecting the 
 afferent nerves which terminate 
 in them. The dorsal roots of 
 the spinal cord arise from the 
 cell bodies in the spinal ganglion. 
 Sir Charles Bell (1811) showed 
 that these roots are sensory, since 
 in animals stimulation of the 
 central end of the severed root 
 causes reflex movements and ex- 
 pressions of pain, while in man 
 stimulation of these fibres in the 
 stump of an amputated limb 
 may give rise to all the sensa- 
 tions which would be derived 
 from their stimulation in the 
 normal limb. 
 
 In some vertebrates a few 
 efferent axones leave the spinal 
 cord by the dorsal roots. These 
 fibres can be seen in the chick 
 (Fig. 87). In the frog stimula- 
 tion of the peripheral end of the 
 
 i i i FIG. 87. Schema of the distribution of the efferent 
 
 Severed dorsal root may cause fibres of the spinal roots : A afferent fibres in the dorsal 
 
 Contraction of the skeletal mils- root onlv E > E > efferent fibres in both dorsal and ventral 
 
 , j . . , . roots. In the ventral root one group of efferent fibres 
 
 Cles. lliere IS no good evi- g0 es to M, the striped muscles ; another group to ganglion- 
 
 dence, however, that these fibres cells - s - formin * a sinj?le sy m P athetic ganglion, or to s< 
 
 cells located in more than one sympathetic ganglion, but 
 
 are present in mammals. 
 
 (2) The central neurones; 
 those the axones of which never 
 
 leave the central system, and the function of which is to distribute within 
 this system the impulses which have there been received. 
 
 (3) The efferent neurones; or those the axones of which pass outside of 
 
 1 R. J. Horton-Smith : Journ. Physiol., vol. xxi. p. 101. 
 
 all connected with one efferent fibre by means of its col- 
 laterals; P, peripheral plexuses into which the axones 
 of some sympathetic cells run. 
 
204 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 the central system, and which carry impulses to the periphery. In this last 
 group, again, two minor divisions may be made, namely : (a) the efferent ele- 
 ments, the cell-bodies of which lie within the central system, as is the case with 
 those giving rise to the ventral roots ; (b) those forming the peripheral ganglia 
 entirely outside of the central system the sympathetic ganglia and the more 
 or less solitary cells which take part in the formation of the peripheral plexuses. 
 It was Sir Charles Bell who also showed the motor character of the 
 ventral roots. Nevertheless the observation was soon made, that while 
 stimulation of the central end of the severed ventral root was always 
 without apparent effect, the stimulation of the peripheral end in addition 
 to the typical motor responses might sometimes cause expressions of pain. 
 This latter result was obtained even when the mixed nerve trunk, beyond 
 
 the union of the two roots, had been severed, so 
 that the only possible pathway for the impulses 
 was through the junction of the two roots to 
 the spinal ganglion, and so by the dorsal root 
 to the cord. It appears probable from studies 
 on the degeneration of the root fibres that the 
 peripheral axones of some afferent neurones on 
 their way to the meninges do run back toward 
 the cord within the ventral root, and that it is 
 the stimulation of these fibres which gives rise 
 to the phenomenon of " recurrent sensibility " 
 as it is called. 
 
 The "afferent neurones" (1) have their 
 cell-bodies collected to form the spinal gan- 
 glia. 1 The distal branches of these cells form 
 the peripheral sensory nerves, and the prox- 
 imal branches combine to form the dorsal 
 nerve roots. On entering the walls of the 
 
 spinal cord these latter fibres divide into two principal longitudinal branches 
 which lie about the dorsal horns and form the major part of the dorsal 
 columns. From time to time the ends of these branches, or their collaterals, 
 enter the gray matter of the cord. 
 
 Thus, in a cross-section of the cord, the dorsal column and the gray matter 
 represent the localities where the axones of the afferent elements are found. 
 The afferent cranial nerves which can be homologized with the afferent 
 spinal nerves have a corresponding distribution in the bulb. 
 
 The "efferent neurones" (3) have their cell-bodies only in the ventral 
 horns of the spinal cord, or the homologous localities in the bulb and brain 
 stem. See Figs. 88, 89, which show the part of the medullary tube divided 
 
 1 Recent work on the spinal ganglia shows that in addition to the elements usually described, 
 they probably contain cells, the axones of which are distributed entirely within the spinal gan- 
 glion, and also cells which send their branches to the distal side only of the ganglion. See 
 Dogiel: Anat. Am. Jena, 1896, Bd. xii. 
 
 FIG. 88. Cross-section in the cer- 
 vical region of a fetal human spinal 
 cord at the sixth week ; X 50 diameters 
 (v. Kolliker) : c. central canal ; a. a., 
 groove separating the two laminae : d.p., 
 dorsal lamina ; v, p., ventral lamina, in 
 which alone are located nerve-cells 
 the axones of which leave the central 
 system; d. r., dorsal root; v. r., ventral 
 root. 
 
CENTRAL NEin'Ol'S HYXTEM. 205 
 
 by His into the ventral and dorsal lamina* during development. The ventral 
 horns of the gray substance form part of the ventral laminae. 
 
 The. cells of the sympathetic system \\hieh are interpolated in one portion 
 of the pathway formed by the efferent elements lie, of course, entirely out- 
 side of the central system. (See Fig. 87.) The central neurones (2) occupy 
 all parts of the central system, and hence where the bodies or branches of the 
 first two groups are absent, the system is composed of central neurones only. 
 
 Arrangement of the Cells Forming- the Several Groups. All three 
 groups of elements are grossly arranged so as to be bilaterally symmetrical 
 with reference to the dorso-ventral median plane of the body. There are 
 some minor exceptions to this general statement, but these are not known to 
 have any physiological significance. 
 
 FIG. 89. Schema showing the encephalon and cord ; the unshaded portion is that derived from the 
 dorsal lamina ; the shaded that from the ventral (from Minot) : C, cerebrum ; Cb, cerebellum ; F, foramen 
 of Monro; /, infundibulum ; M, bulb; 0, olfactory lobe; P, pons; Q, quadrigemina ; Sp.c, spinal cord; 
 III, third ventricle ; IV, fourth ventricle. 
 
 The main axones of the afferent elements are distributed almost entirely 
 to the dorsal columns, and to the gray matter of that side of the cord on 
 which they enter, though some crossing occurs in the dorsal commissure. In 
 the cord, the efferent elements have their cell-bodies mainly on the side of 
 the cord from which the efferent fibres emerge. In the case of some cranial 
 efferent nerves the arrangement is different. There is found, for instance, a 
 partial decussation of the fibres of the oculo-motorius ; complete decussation 
 in the case of the patheticus, and no decussation in the case of the abducens. 
 It is the central cells which furnish almost all the axones forming the com- 
 missures, the decussating bundles and the projection systems, while the 
 association tracts arise entirely from them. 
 
 Segmentation. The grouping of the cell-bodies of the afferent fibres is 
 originally segmental, one spinal ganglion corresponding to each segment of 
 the trunk. In the brain, the original segmental arrangement has been 
 greatly modified. In the trunk, too, the distribution of the distal portion of 
 
206 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 the afferent nerves is segmental, the area of skin involved forming a band 
 about the body. On the other hand, the distribution of the proximal branches 
 forming the dorsal roots is such that while part of the axones and their 
 collaterals establish connection with the cord and bulb near the level at which 
 the axone joins it, the principal divisions of the axone often pass along the 
 cord a greater or less distance in both directions, and thus a long stretch of 
 the cord may receive impulses by way of a single afferent element. 
 
 In some of the lower vertebrates the arrangement of the " efferent " cells 
 is plainly segmental, but in man and the higher mammals this is hardly to be 
 demonstrated. In the least modified parts of the cord, the efferent fibres do 
 arise from cell-bodies mainly within the segment from which the ventral root 
 emerges. But this massing of the efferent cell-bodies is largely obscured by 
 the presence of central cells through the entire length of the ventral horns, 
 while in the portions of the cord controlling the limbs the columns of cells 
 furnishing fibres to a given ventral root may extend through as many as three 
 segments of the cord. The distribution of the efferent fibres is evidently 
 segmental in plan, though highly modified everywhere except in the thoracic 
 cord supplying the portion of the trunk between the limbs. The principal 
 peculiarity in the group of central cells is the great increase in the mass of 
 them as we pass from the cord cephalad, the cerebrum, for example, being 
 composed entirely of central cells. 
 
 Relative Development of Different Parts. The bulk of the three 
 subdivisions which have been named is very unequal. The central system 
 is far more massive than the afferent and efferent taken together, but the 
 relation cannot be stated with any exactness, since the mass of the peripheral 
 system is not definitely known. 
 
 Connections between Cells. In determining the connection between 
 cells which permits a nerve impulse in one cell to stimulate another, the fact 
 that the axone is the outgrowth of a cell-body, and that each cell is an inde- 
 pendent morphological unit, forms the point of departure. Under these 
 circumstances the question of the connection between cells takes the more 
 explicit form of the question whether cell-branches may become continuous 
 by secondary union. In several vertebrates there is good histological evidence 
 that such secondary union occurs in a few cases in the central system. 
 
 In one type the axone of one element spreads out and encloses the cell- 
 body of a second after the manner of a cup holding a ball. In other cases 
 it appears that the terminals of a given axone may even penetrate the cell 
 substance of the receiving neurone. 
 
 These are examples of concrescence. In the majority of cases, however, 
 a close approximation of the parts of two nerve-cells is alone to be seen 
 (Fig. 90). The termination of the discharging axone may be by fibrils or 
 expanded disks, and occur either close to or upon the body, dendrites, or 
 even collaterals l of the receiving neurone. If, as seems probable, the 
 dendrites form an important pathway by which the receiving neurone is 
 1 Held: Archivf. Anal, u. PhysioL, Anat. Abthl., Leipzig, 1897. 
 
(i:\TRAL NERVOUS SYSTEM. 
 
 207 
 
 excited, then a cell with many dendrites should oiler more receiving points 
 than one with few. It is perfectly evident, however, that in inanv cases the 
 dendrites are not the only pathway hy which impulses may travel toward 
 the cell -body. 
 
 Theories of the Passage of the Nerve-Impulses. Accepting the view 
 that, with the exceptions just noted, the nervous system is composed of dis- 
 continuous but closely approximated 
 cell-elements, it remains to explain 
 how impulses arising within the limits 
 of one element are able to influence 
 others. The relation between two 
 neurones is quite comparable to that 
 between a muscle and the nerve-fibres 
 controlling it, but the recognition of 
 that .fact does not afford us much 
 assistance. 
 
 As an hypothesis the passage of 
 the stimulus may be assumed to 
 depend on chemical changes set up 
 at the tips of the terminals and affect- 
 ing the surrounding substance, which, 
 thus affected, acts to stimulate some 
 point on the wall of the neighboring 
 cell, either along a dendrite or on the 
 cell-body itself. 
 
 The suggestion has been made that 
 in some cases the space between two neurones may be varied by amoeboid 
 changes in the dendrites and terminals of the elements concerned. Although 
 much may be said a priori in favor of this hypothesis, good histological evi- 
 dence is still wanting. 
 
 The structural changes which permit the stimulation of one element to 
 affect another are completed slowly, and, as we shall later see, these changes 
 continue in some parts of the human nervous system up to middle life. 
 
 From what has just been stated, it follows that the nervous system of the 
 immature person is quite a different thing from that of one mature, since in 
 the former it is more schematic, more simple, the details of the pathways not 
 having been as yet filled out. Moreover, considering the slow and minute 
 manner in which the central system is organized by the growth of the cell- 
 branches, it is the list place where there should be expected structural uni- 
 formity in the details of arrangement. 
 
 B. REFLEX ACTION. 
 
 Conditions of Stimulation. The conditions necessary for the generation 
 of a nerve impulse are an external stimulus acting on an irritable neurone. 
 While life exists, stimulation of varying intensity is always going on, and 
 
 FIG. 90. Showing at the lower edge of the 
 figure a series of basket-like terminations of axones 
 which surround the bodies of the great cells of 
 Purkinje in the cortex of the cerebellum (Ramon 
 y Cajal) : C, cell-body ; N, axones ; B, basket-like 
 terminations arising from cell C, and enclosing the 
 cells of Purkinje. 
 
208 AN AMERICAN TEXT-BOOK ^>F PHYSIOLOGY. 
 
 hence there is no moment at which the nerve is system is not stimulated, and 
 no moment at which the effectiveness of this stimulus is not varied. The 
 response to this continuous and ever-varying stimulation is not necessarily 
 always evident, but occasionally intensification of the stimuli renders them so- 
 strong that an evident reaction follows. 
 
 Though the foregoing statements suggest that the chief variable is 
 that represented by the stimulus, the strength of which changes, yet as 
 a matter of fact the variations in the pi ysiological (chemical) condition 
 of the nerve-cells are equally important ; but neither factor can be studied 
 independently. 
 
 The term "central stimulation " has been sometimes employed. For ex- 
 ample, the spasmodic movements of the young child, when there is no change 
 noticeable in the external stimuli acting upon it, are sometimes attributed ta 
 this cause ; but these, although doubtless due to central changes, altering the 
 irritability of the cells, are most properly classed with the reactions which 
 follow the external stimulus. The misconceptions here to be avoided are 
 those of supposing that the nervous system is at any time unstimulated, and 
 that the evident responses follow a change of the external stimulus only. 
 
 When the impulse in one cell-element is used to arouse an impulse in 
 another, as in all experiments where the nerve-cells are examined in a physi- 
 ological series, the strength of the impulse from the second, is not easily pre- 
 dicted. This is explained as due to variations in the ease with which the 
 impulse in one element stimulates the next, and also to the variations in the 
 second cell of those conditions which determine the intensity with which it 
 shall discharge. 
 
 When an impulse has once entered the central system by way of a dorsal 
 nerve root, it is found to follow the course of the afferent axones within the 
 central system, and thus must be distributed almost simultaneously to a 
 length of cord coextensive with that of the branches of the afferent axones. 
 
 The arrangement makes possible the stimulation of a large number of 
 central cells, and thus greatly increases the distribution of the initial disturb- 
 ance. In the case of some of the cells about which the branches of the axone 
 end, the impulse will not be adequate to cause in them a discharge, although 
 it may still produce a certain amount of chemical change in them. The im- 
 pulse thus tends to disappear within the system by producing, in part, chemi- 
 cal changes strong enough to cause a discharge of the next element in the 
 series and, in an increasing number, similar changes of a less intensity. 
 
 Diffusion of Central Impulses. If the previous description has been 
 correct, two very important events occur : in the first place, the impulse 
 reaches a far greater number of cells than evidently discharge, and in the 
 second, the pathway followed by the impulses which do produce the discharge 
 is by no means the only pathway over which the impulses can or do travel. 
 
 Simple Reflex Actions. We turn next to an examination of these groups 
 of neurones in action. 
 
 The simplest and most constant of the co-ordinated reactions of the nerv- 
 
CEX'IT .: NERVOUS 8Y8TK.M. 209 
 
 ous system are termed reflex.'' The term involves tin- idea that the response 
 is not accompanied by consciousness, and is dependent on anatomical Condi- 
 tions in the central system which are only in a slight decree subject to phvsi- 
 ological modifications. This view of reflex activities is in a large mea-un 
 justified by the facts, but at the same time it must be held subject to many 
 modifications, and it is not possible to make a hard and fast line between 
 reflex and voluntary reactions. 
 
 The principal features of a ivflex act may be illustrated by following a 
 typical experiment : 
 
 If the central nervous system of a frog be severed at the bulb, so as to 
 separate from the spinal cord all the portions of the central system above it, 
 and the brain be destroyed, the animal is fora time in a condition of collapse. 
 If. after recovering from the immediate shock, such a frog be suspended by 
 the lip, it will remain motionless, the fore legs extended and the hind ICL-S 
 pendent, though very slightly flexed. If such a frog were dissected down to 
 the nervous system, there would be found the following arrangement : 
 Afferent fibres running from the skin, muscles, and tendons, and entering the 
 cord by way of the dorsal nerve-roots. The central mass of the spinal cord 
 itself in which these roots end, each root marking the middle of a segment. 
 Within the cord and stretching its entire length are to be found the central 
 <r//x, interpolated more or less numerously between the terminals of the 
 afferent neurones and the cell-bodies of the efferent neurones. From each 
 segment of the cord go the ventral root-fibres passing in part to the muscles 
 and in part to the ganglia of the sympathetic system. The mechanism 
 demanded for a reflex response is an afferent path leading to the cord ; cells 
 in the cord by which the incoming impulses shall be there distributed ; and 
 a third set of efferent elements to carry the outgoing impulses to the terminal 
 organ which gives the response. It is important to consider in detail what 
 occurs in each portion of this reflex arc. 
 
 In a frog thus prepared, stimulation of the skin in any part supplied by 
 the sensory nerves originating from the spinal cord causes a contraction of 
 some muscles. 
 
 Influence of Location of Stimulus. The muscles which thus contract 
 tend to be those innervated from the same segments of the cord which receive 
 the sensory nerves that have been stimulated. Thus stimulation of the skin 
 of the breast causes movements of the fore limbs, and stimulation of the rump 
 or legs corresponding movements of the hind limbs. It is noticeable, how- 
 ever, that wherever the stimulus is applied the hind limbs have a tendency 
 to move at the same time that the muscles most directly concerned contract. 
 
 If the attempt is made to correlate these variations in reaction with varia- 
 tions in the structure of the cord, we have to picture the simplest reactions 
 (from the same level) as dependent on the formation of terminals on the 
 afferent fibre just after its entrance into the cord and in the immediate neigh- 
 borhood of an efferent neurone. In the second case either the afferent axone 
 is extended some distance through the cord forming several terminations by 
 VOL. II. H 
 
210 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 its collaterals, or a central cell is excited and serves to carry the impulse to a 
 distance. 
 
 Segmental Reactions. In attempting to explain this associated con- 
 traction of the leg muscles, it must be remembered that the hind legs are, 
 par excellence, the motile extremities of the frog, and therefore all general 
 movements involve their use. We infer from this, moreover, that the arrange- 
 ment in the spinal cord of the frog is not such that the sensory impulse- 
 ing into any segment tend to rouse exclusively the muscles innervated by 
 that segment, but that these incoming impulses are diffused in the cord 
 unevenly and in such a way as to easily involve the segments controlling the 
 legs. As reflex co-ordinating centres, therefore, the several segments of the 
 ^cord have not an equal value. 
 
 When the stimulus is applied on one side of the median plane, the responses 
 first appear in the muscles of the same side; and if the stimulus is slight they 
 may appear on that side only. The incoming impulses are therefore first and 
 most effectively distributed to the efferent cells located on the same side of 
 the cord as that on which these impulses enter. Such a statement is most 
 true, however, when the stimulus enters the cord at the level where the nerves 
 to the limbs are given off. At other levels the diffusion to the limb centres 
 may take place more, readily than to the cells in the opposite half of the same 
 segment. When the muscles on the side opposite to the point of stimulation 
 contract it is found that they correspond to the group of muscles giving the 
 initial response on the side of the stimulus. The diffusion then tends to cross 
 the cord and to involve the cells located at the same level as that at vvhiVh 
 the incoming impulses enter it. 
 
 There is some reason to think that when the impulses enter the cord 
 toward the lumbar end the path by which the diffusion takes place with 
 least resistance is not the shortest one between the two groups of cells, but 
 a path toward the cephalic end of the cord, so that the impulses tend to pass 
 up the cord on one side and down on the other. 1 
 
 Strength of Stimulus. In a reflex response the strength of the stimulus 
 influences the extent to which the muscles are contracted, the number of 
 muscles taking part in the contraction, and the length of time during which 
 the contraction continues. That the strength of the stimulus influences the 
 extent to which the contraction of a given group of muscles takes place is 
 easily shown when, for example, the toe of a reflex frog which has been sus- 
 pended is stimulated by pinching it or dipping it in dilute acid. In this ease, 
 if the stimulus be slight, the leg is but slightly raised, whereas, if the stimu- 
 lus be strong, it is drawn up high. In the same way by altering the stimulus 
 the muscles which enter into the contraction may be only those controlling 
 the joints of the foot, whereas, with stronger stimuli, those for the knee and 
 hip are successively affected, thereby involving a much larger number of 
 muscles. Here, too, we infer a spread of the incoming impulses which is 
 orderly, since the several joints of the limb are moved in regular sequence. 
 1 Rosenthal und Mendelssohn : Neurologisches Ccntralblatt, 1897, Bd. xvi. S. 978. 
 
 
CENTRAL NERVOUS SYSTEM. 1>11 
 
 The responses which are thus obtained are not spasmodic, but are con- 
 tractions of muscles in regular series, giving the appearance <>!' a carefully co- 
 ordinated movement a movement that is modified in accordance both with the 
 strength of the stimulus and its point of application. Moreover, such a move- 
 ment may occur not only once, but a number of times, the leg being alter- 
 nately flexed and extended during an interval of several seconds, although 
 the stimulus is simple and of much shorter duration. 
 
 Continuance of Response. The continuance of the response after the 
 stimulus has been withdrawn must be, of course, the result of a long-continued 
 chemical change at some point in the pathway of the impulse, and it appears 
 probable by analogy with the results obtained from the direct stimulation of 
 the central cortex, or the spinal cord, that in these cases the stimulating 
 changes are taking place (p. 188) in the central cells or efferent cells 1 as well 
 as in the skin supplied by the afferent nerves. 
 
 Latent Period. It has been observed that in the case of a reflex frog 
 that is, a frog prepared as described above, with the spinal cord separated 
 from the brain an interval of varying length elapses between the application 
 of a stimulus and the appearance of a reaction. The modifications of the 
 interval according to variations in the stimulus have been carefully studied. 
 When dilute acid applied to the skin is used as a stimulus, this latent interval 
 decreases as the strength of the acid is increased. When separate electrical 
 or mechanical stimuli are employed, the reaction tends to occur after a given 
 number of stimuli have been applied, although the time intervals between the 
 individual stimuli may be varied within wide limits. The experimental 
 evidence for electrical stimuli shows that the time intervals may range 
 between 0.05 second and 0.4 second, 2 while the number of stimuli required 
 to produce a response remains practically constant. 
 
 Summation of Stimuli. A single stimulus very rarely if ever calls forth 
 a reaction if the time during which it acts is very short, and hence there has 
 developed the idea of the summation of stimuli, implying at some part of the 
 pathway a piling up of the effects of the separately inefficient stimuli to a 
 point at which they ultimately become effective. 3 
 
 The details of the changes involved in this summation and the place at 
 which the changes occur, are both obscure, but it w /uld seem most probable 
 that summation is an expression of changes in the relations between the final 
 twigs of the afferent elements and the cell-bodies of the central or efferent 
 elements, which permit the better passage of the impulse from one element to 
 the other, for the evidence strongly indicates that the course of the impulse 
 can be interrupted at these junctions. 
 
 The foregoing paragraphs have been concerned mainly with changes 
 occurring in the afferent portions of the pathway. Next to be considered is 
 
 1 Birge: Archivfilr Anatomieund Physiologic (Physiol. Abthl.), 1882, S. 484. 
 
 2 Ward : Ibid., 1880. 
 
 3 Gad uud Goldscheider: " Ueber die Summation vou llautreizen," Zeitschrift fur klinische 
 Medicin, 1893, Bd. xx. Hefte 4-6. 
 
212 AN- AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 the amount of central nervous matter which must be present in the frog's 
 spinal cord in order that the reactions can take place. 
 
 Reactions from Fractions of the Cord. If the construction of the cord 
 was strictly segmental (a condition nearly approached in some worms and 
 arthropods), in the sense that each segment contained the associated nerves 
 for a given band of skin and muscle, there should be no disturbance on 
 dividing the cord into its anatomical segments ; and practically among the 
 invertebrates, where the ganglionic chain is thus arranged, the single segments 
 can perform alone all the reactions of which they are capable under normal 
 conditions. 1 In such invertebrates the only change effected by the combina- 
 tion of the segments is that of co-ordinating in time and in intensity the 
 reactions of the series. If, on the other hand, the segments of the cord were 
 more or less dependent upon one another, and not physiologically equivalent, 
 modifications of various degrees would arise according to the segments isolated. 
 It has been found that the spinal cord of the frog may, under special condi- 
 tions, be reduced to three segments and reactions still be obtained. 
 
 During the breeding season the male frog, by means of his fore legs, clasps 
 the female vigorously and often for days. If, at this season, there is cut out 
 from the male the region of the shoulder girdle bearing the fore limbs together 
 with the connected skin and muscles, and the three upper segments of the 
 spinal cord, then an irritation of the skin will cause' a reflex clasping move- 
 ment similar to that characteristic for the normal male at this season. 
 
 Reactions in Other Vertebrates. It must not be thought, however, 
 because it is the custom to emphasize the reflex activities of the lower verte- 
 brates, and to show that these reflexes can be carried out even by fractions 
 of the spinal cord alone, that, therefore, the spinal cord is particularly well 
 developed in them. Comparative anatomy shows in the lower vertebrates a 
 simplicity in the structure of the cord quite comparable with that found in 
 the brain, and, as we ascend the vertebrate series, both parts of the central 
 system /increase in complexity. In this increase, however, the cephalic divi- 
 sion takes the lead ; and further, by means of the fibre-tracts from it to the 
 cord, the cell-groups, in the cord are more and more brought under the influence 
 of the special sense-organs which connect with the encephalon: The physio- 
 logical reactions of the higher vertebrates are especially modified by this 
 latter arrangement. It is, therefore, true that the cord, as well as the brain, 
 is in man more complicated anatomically than in any of the lower forms, and 
 this is true in spite of the fact that the independent reactions of the human 
 cord are so imperfect. 
 
 When an amphioxus is cut into two pieces and then put back in the water, 
 a slight dermal stimulus causes in both of them locomotor .movements, such 
 as are made by the entire animal. 
 
 When a shark (Scyllium eaniculai) is beheaded, the torso swims in a co-or- 
 dinated manner when returned to the water. Separation of the cord from the 
 
 1 Loeb: Einleitung in die vergkichende Gehimphysiologie vr < nde Psychologie, Leip- 
 
 zig, 3899. 
 
Acuslicus 
 Vestibularis 
 
 Gustus 
 (cum Trigemino] 
 
 Pharynx 
 
 Oesophagus 
 Larynx, Trachea 
 
 Intestina 
 
 (Thorax, Abdomen) 
 
 Regio occipitalis 
 
 Regio colli 
 
 Regio nuchae 
 
 Regio Nervi radialis 
 Regio N. mediani 
 
 Thorax 
 
 Abdomen 
 
 anterior 
 mediana 
 j laleralis 
 ( posterior 
 
 Ill 
 
 Musciili laciei 
 Mm. phafyngis. palati 
 Mm. laryngis 
 Mm. linguae 
 Oesophagus 
 
 Sternocleidomastoideus 
 
 Musculi colli et nuchae 
 Scaleni, Splenius 
 
 Cucullaris 
 
 Rhomboidei, Levator anguli scapulae 
 
 i\ 
 
 v 
 
 ex.scapular.\ 
 
 Supra-, infraspinatus 
 
 Supinalor longus 
 
 vi 
 
 Ml 
 
 Mil 
 
 vn 
 
 {MOW MM 
 
 cruris 
 
 ' lateralis 
 Scrotum, penis 
 Perineum 
 
 Serratus anticus, Supinator brevis 
 
 Pecloralis major (portio ciavicuLj 
 
 Teres minor 
 
 Pronatores, Brachialis internus 
 
 Triceps 
 
 Extenspres carpi el digilor. longi 
 Pectoralis major jportio costalisj 
 Latissimus dor si. teres major 
 
 Tlexores carpi et digilor. longi) 
 Eitensores pollicis 
 
 Inlerossei lumbricales / 
 
 Thenar Hypothenar ) 
 
 II? 
 
 Glutaei, tensor fasc.lat. 
 Adductores femoris 
 Abductores femoris 
 Tibialis anticus 
 Gastrocnemius, Soleus 
 Biceps Semitendi Semimembranosus 
 Obturator, pyriformis, quadratus femor. 
 Flexores pedis 
 
 Peronei 
 
 Rectum:::: 
 
 Anus 
 
 Interossei 
 Mm. perineales 
 Mm. vesicales 
 Mm. rectales 
 
CENTRAL NERVOUS SYSTEM. 213 
 
 brain does not deprive a ray (Torpedo oculata) of the power of perfect loco- 
 motion. The same is true of the ganoid fish. In the case of the cyclostome 
 fish (Petromyzon) the beheaded trunk is, in the water, inactive, yet, on gentle 
 mechanical stimulation, it makes inco-ordinated responses; but, put in a bath 
 formed by a 3 percent, solution of picro-sulphuric acid, locomotion under 
 the influence of this strong and extensive dermal stimulus is completely per- 
 formed. In the case of the eel the responsiveness even to the picro-sulphorio 
 acid bath is evident in the caudal part of the body alone. In the bony fish this 
 capability of the spinal cord to control locomotion has not been observed. 1 
 
 In these experiments the central system is represented by the entire spinal 
 cord with the associated nerves, or by some fraction of it ; but so simple, 
 constant, and independent are the reactions of the cord under normal condi- 
 tions that a strong stimulus is able to elicit the characteristic responses from 
 even a fragment of the system. The higher we ascend in the vertebrate 
 series the less evident do the independent powers of the cord become. 
 
 Tarchanow 2 has shown that beheaded ducks can still swim and fly in a 
 co-ordinated manner, and among mammals (dog and rabbit) Goltz and Ewald 3 
 and others have demonstrated that if the lumbar region be separated from the 
 rest of the cord by a cut and the animal allowed to recover from the opera- 
 tion it will with proper care live for many months, and not only are the legs 
 responsive to stimulation of the skin, but the reflexes of defecation and urina- 
 tion are easily induced by slight extra stimulation. An instructive reaction 
 occurs when such an animal is held up so that the hind legs hang free. 
 When thus held, the legs slowly extend by their own weight and then are 
 flexed together. The reaction becomes rhythmic and may continue for a long 
 time. It is assumed in this case that the stretching of the skin and tendons 
 due to the weight of the pendent legs acts as a stimulus, and in consequence 
 the legs are flexed. This act in turn removes the stimulus, and as a result 
 they extend again, to be once more stimulated and drawn up. 
 
 In man, as a rule, death rapidly follows the complete separation of any 
 considerable portion of the cord from the rest of the central system, especially 
 if the separation be sudden, as in the case of a wound. But Gerhardt 4 has 
 recorded the retention of the reflexes in a case of compression of the cord by 
 a tumor, the case having been under observation for four and a half years ; 
 and Hitzig, 5 a case in which a total separation between the last cervical and 
 first thoracic segments had been survived for as long as seven years. The 
 principal reaction to be observed in such cases is a contraction of the limb 
 muscles in response to stimulation of the skin, such as a drawing up of the 
 legs when the soles of the feet are tickled. No elaborate reflexes are, how- 
 ever, retained such as would be necessary in acts of locomotion. 
 
 1 Steiner : Die Fanctionen des Centralnervensystems und ihre Phylogenese, 2te Abth., "Die 
 Fische," 1888. 
 
 2 Tarchanow: Pfliiger's Archiv, 1885, Bd. xxxiii. 
 8 Coltz und Ewald: Ibid,, 1896, Bd. Ixiii. 
 
 4 Neurologisches Centralblatt, 1894, S. 502. 5 Loc. cit. 
 
214 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 It thus appears that the reflex responses, namely, simple reactions unac- 
 companied by consciousness, are in man mainly given by the unstriped 
 muscle-tissue and by glands, and only in a minor degree by the striped 
 muscles. Moreover, while the typical reflex is a reaction over which we 
 cannot exercise direct control, the normal individual has some power over 
 many of these reactions ; for example, the impulse to micturition or defeca- 
 tion can be thus delayed, respiration arrested, and, in some instances, so 
 remote a reaction as the beat of the heart either accelerated or slowed at will. 
 
 It is of interest to note that many reflexes which in the young are not 
 controlled, as micturition, for instance, become so gradually, a change most 
 probably dependent on the growth of axones from the cephalic centres into 
 the cord, thus subjecting the cord-cells to a new set of impulses which modify 
 their reactions. That such is the case is indicated by the fact that extreme 
 fright or anaesthetics, which diminish the activities of the higher centres, often 
 cause these reactions to take place involuntarily. Other reflexes are present 
 in early life, but disappear later ; such are the sucking reflex of the infant, 
 and the remarkable clinging power of the hands, by which a young child is 
 enabled to hang from a bar, thus supporting the weight of its entire body, 
 often for several minutes. This last capacity soon begins to wane, and usually 
 disappears by the second month of life. 1 
 
 Co-ordination of the Efferent Impulses. Incessantly the efferent im- 
 pulses pass out from the cord to the muscles and glands. With each fresh 
 afferent impulse those which go out are modified in strength and in their 
 order, but just how they shall be co-ordinated is dependent on so many and 
 such delicate conditions that even in the simplest case the results are to be 
 predicted only in a general way. 
 
 The attempt to determine the spread of the impulse in the cord by 
 observing the order in which the various muscles of the thigh and leg con- 
 tract in response to thermal stimuli was made by Lombard. 2 In a reflex 
 frog the tendons of the leg and thigh muscles were exposed at the knee, and 
 each attached to a writing-rod in so ingenious a manner that simultaneous 
 records of fifteen muscles could sometimes be obtained. The stimulus was a 
 metal tube, filled with water at 47-61 C., which was applied to the skin. 
 Under these conditions, it was remarkable that a continuous stimulus was 
 often followed, not by a single contraction of the muscles, but by a series of 
 contractions, suggesting that in the central system the-eelU-were roused to a 
 discharge and then for a time concerned with the preparation for sending out 
 new impulses, and that during this latter period the muscles were relaxed. 
 
 Apparently a high degree of uniformity in the conditions was obtained in 
 these experiments, but at the same time the reactions were far from uniform, 
 in either the latent time of contraction or the order in which the contraction 
 of the several muscles followed, although certain muscles tended to contract 
 first, and certain series of contractions to reappear. The co-ordination of the 
 
 Robinson: Nineteenth Century, 1891. 
 
 2 Archivfiir Anatomie und Physiologic, 1885. 
 
CENTRAL NERVOUS SYSTEM. 215 
 
 contractions is therefore variable in time, even under these conditions. These 
 variations are probably due either to the fact that the impulses are not dis- 
 tributed in the centre in the same manner on cadi occasion ; or if they are 
 thus distributed, the central and efferent cells vary from moment to moment 
 in their responsiveness. That these cells should so vary is easy to compre- 
 hend, for all the cell-elements in such a reflex frog are slowly dying. In this 
 process they are undergoing a destructive chemical change, and with these 
 destructive changes are generated weak impulses sufficient to cause their 
 physiological status continually to vary, thus modifying the effects of any 
 special set of incoming impulses acting upon them. 
 
 It is not to be overlooked also that the dissection of the muscles tested, 
 and the removal of the skin about them, deprived the spinal cord of the 
 incoming impulses due to the stretching of the skin by the swelling of the 
 contracting muscles and disturbed the order and intensity of such sensory 
 impulses as come in from the tendons and the muscles themselves. The 
 observations of both Bickel l and Hering 2 show that these impulses are not 
 necessary for accurate reflex movements of the frog's leg, and thus weaken 
 the force of the suggestion just made. However much these impulses may 
 add to the regularity of the muscular responses, Lombard concludes that 
 the discharge of one efferent cell is not necessary in order that another 
 efferent cell may discharge, but that each discharging cell stands at the end 
 of a physiological pathway and may react independently. 
 
 Purposeful Character of Responses. When the muscular responses of 
 a reflex frog to a dermal stimulus are studied, they are seen to have a pur- 
 poseful character, in that they are often directed to the removal of irritation. 
 This is demonstrated by placing upon the skin on one side of the rump a 
 small square of paper moistened with dilute acid. As a result, the foot of 
 the same side is raised and the attempt made to brush the paper away ; if the 
 first attempt fails, it may be several times repeated. When the irritation has 
 been removed the frog usually becomes quiet. If the leg of the same side 
 be held fast after the application of the stimulus, or if the first movements 
 fail to brush away the acid paper, then the leg of the opposite side may be 
 contracted and appropriate movements be made by it. Emphasis has been 
 laid by various physiologists upon reactions of this sort as showing a capa- 
 bility of choice on the part of the spinal cord, thus granting to the cord 
 psychical powers. Against such a view it must be urged that the movements 
 of the leg on the side opposite to the stimulus do not occur until after the 
 muscles of the leg on the same side have responded. When these responses 
 are inefficient because the leg is prevented from moving or because they fail 
 to remove the stimulus, the prime fact remains that the stimulus continues to 
 act and the diffusion of the impulses in the cord goes on, involving in either 
 case the nerve-cells controlling the muscles of the opposite leg. The adjust- 
 ment of the reaction of the leg, on whichever side it occurs, is, however, far 
 
 1 Bickel : Pjluger's Archiv, Bd. Ixvii. 
 
 * Hering: Archiv fur experimentelle Pathologic und Pharmakologie, Bd. xxxviiL 
 
216 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 from precise ; and although the movements of the leg, when the stimulus is 
 applied far up on the rump, differ from those which follow the application 
 of the stimulus to the lower part of the thigh, yet in either case they are very 
 wide, and in both cases the foot is brushed across a large part of both the 
 rump and leg. Considering, therefore, the rather general character of these 
 movements, and the fact that the movements of the opposite leg only follow 
 after a continued stimulus to the leg of the same side has produced an inef- 
 fective response, it is best to explain the result by the diffusion of the impulses 
 within the cord, leaving quite to one side the psychical element. Such reflex 
 actions are in a high degree predictable, but in reality this has li+tle signifi- 
 cance, since there is but one general movement that a frog in such a condition 
 can make, whether the stimulus be applied to the toes or the rump namely, 
 the flexion of the leg so that under these circumstances the prediction of the 
 kind of movement is a simple matter. The extent of the contraction is related 
 to the intensity of the. stimulus, and is in turn dependent on the excitability 
 of the central system, which can be increased or diminished in various ways. 
 The modification of the reaction as dependent on the location of the stimulus 
 can be in a measure predicted, but the modification is wanting in precision 
 just in so far as the movements themselves are wanting in this quality. 
 
 Reflexes in Man. In the normal individual reflexes involving striped 
 muscles are found in the tendon reflexes, of which the knee-kick is an exam- 
 ple, in winking, and the whole series of reflex modifications of respiration, 
 such as coughing, sneezing, and the like. 
 
 The activities of the alimentary tract are examples of reflex actions involv- 
 ing the contraction of muscles in deglutition, defecation, and similar peristaltic 
 movements in other hollow viscera. These muscle-fibres are for the most part 
 unstriped. So, too, micturition, the cremaster reflex, emission and vaginal 
 peristalsis, and the reactions of parturition are to be classed here. Moreover, 
 the entire vascular system is controlled in this manner, the contraction and 
 distention of the small arteries being in a large measure in response to stimuli 
 originating at a distance ; while as a third group, we have the glands, the 
 activity of which is almost entirely reflex. For the discussion of the various 
 reflexes mediated by the cranial nerves, the reader is referred to the special 
 sections dealing with the cardiac, vasomotor, and respiratory centres in tho 
 bulb and the pupillary centres in the mid-brain. 
 
 Periodic Reflexes. Not all reflexes are to be obtained from the same 
 animal with equal intensity at all times. In general, frogs in the spring-time 
 and in early summer, after reviving from their winter sleep, are highly 
 irregular in their reflex responses so irregular that students are advised 
 not to attempt the study of these reactions at this season. On the other 
 hand, it is during the spring that mating occurs, and during this period the 
 male clasps the female and exhibits the peculiar reflex which has already 
 been described. Comparable with this variation in the frog must be the 
 changes which occur in the spinal cords of migratory birds, which, both in 
 the spring and in the fall, are capable of such extended flights, or in the 
 
hibernating 
 vari; tlu-ir h:: 
 
 Variations in Diffusibility. The 
 impul- s is d 
 
 :u the r on the phy 
 
 in which 
 Sis happ i-l. It d that by m 
 
 - 
 .linin aiul drug- with a similar pi ,1 have i, 
 
 Influence of Strychnin. The exp- study, uing 
 
 shows the followi V trog poisoned 1>\ the inj. 
 
 Imnvn ; -rain is intaet or lias 
 
 the injection. T -mid to have accumulated in the 
 
 tin- <pinal cord. 1 The p lit in th 
 
 timnlus will cause an extended and prolonged 
 n ot' t! the diffusion ot' imp 
 
 '.ent to an nnnsual degree. '1 
 
 appl uin to the spinal cord has been carefully studied by 
 
 i and Mnirhead.- When the Mrvehnin si>lution was applied locally 
 nt of the spinal cord of a brain i 
 
 : of the arms produced tetan \s nf 
 
 i he poison luui acted tor a time, of th runk 
 
 and i ier hand, stimulation of the lc_ 
 
 ill. Siiu-e. in order to can- ontraetion of th- 
 n trolling the mi -rust bed 
 
 when the stimulus was applied to the arm region tl 
 
 cells .iiise a tetanic spasm, while in tli :i the 
 
 stim ipplied t> the ICL d onlv slightly the alt< ' 
 
 in the cord prodm-ed by the drug must atleet some other orronp than these 
 
 nns of the leirs ->tdi! --d l>y the 
 
 - :n of the arm, the application ug to the 
 
 iial enlargement only, it iiat the ct-nt 
 
 by the dorsal root-fibres in the braehial region to 
 the lumbar enlargement, art- pmbablv atlVct^d : and, further, 
 
 cells that the drng mi, 
 
 aloiu vhieh the di -plication 
 
 of tl between the 
 
 
 
 = 
 
 within tl: with 
 
 which tli- 
 
 >r only in on now be determined. 
 
 i. ix. 
 5 .!/ 
 
218 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 The diffusion of impulses in the central system depends anatomically not 
 only on the amount of branching among the axones of the individual central 
 cells, but also on the association of many cells together, so as to accomplish 
 a wide distribution of the impulses. In the case of the afferent elements, 
 as we have seen, the diffusion depends on the branching of the axones alone. 
 
 Peripheral Diffusion. Turning next to the efferent system, we find the 
 conditions for diffusion dependent on the arrangement of several cells in 
 series. When a group of efferent cells discharges, we know from the 
 arrangement of the ventral roots that the impulses leave the cord mainly 
 along the fibres which comprise these roots ; but where the lateral root i& 
 present they may also pass out over it, as well as over the few efferent fibres 
 found in the dorsal roots. These axones carrying the outgoing impulses have 
 two destinations : (1) The voluntary or striped muscle-fibres ; (2) the 
 sympathetic nerve-cells, grouped in masses to form the vagrant ganglia (see 
 Fig. 93). 
 
 When the impulses are thus sent out there is in the case of motor nerves 
 no diffusion, the effect being limited to the peripheral distribution of the 
 efferent axones, by way of which the impulses leave the central system. 
 The fibres going to the voluntary muscles form, however, but one portion, 
 which has just been indicated as group (1). The connections of the remain- 
 ing group (2), passing to the sympathetic ganglia, are still to be examined. 
 
 Sympathetic System. Associated with the cerebro-spinal system by the 
 efferent axones, and by these alone, is the series of vagrant ganglia and also- 
 of peripheral plexuses containing ganglion-cells, which taken together form 
 the sympathetic system. 1 This system is composed of neurones always 
 monaxonic, but sometimes with, and sometimes without well-marked den- 
 drites. The cells are more or less grouped in ganglia, and these ganglia 
 interpolated between the efferent axones of the spinal nerve-roots on the 
 one hand and the peripheral plexuses or terminal tissues on the other. The 
 number of cells in the ganglia is greater than the number of spinal root axones 
 going to them, and hence their interpolation in the course of the ventral root- 
 fibres increases the number of pathways toward the periphery, as is shown in 
 Fig. 93. In speaking of the fibres concerned, it is desirable to distinguish 
 between the pre-ganglionic, or those originating in the medullary centres and 
 passing to the ganglia ; and the post-ganglionic fibres, or those originating in 
 the cells of the ganglia and passing to the periphery. 
 
 Following the histological observations of Gaskell, 2 previously quoted, and 
 the physiological studies of Langley, 3 an outline of the relations of the sym- 
 pathetic cells, based on the arrangement found in the cat, is briefly as follows : 
 
 Pre-ganglionic fibres i. e., those growing out of cell-bodies located in the 
 
 1 Gaskell : Journal of Physiology, 1885, vol. vii. ; von Kolliker, " Ueber die feinere Anatomie 
 und die physiologische Bedeutnng des sympathischen Nervensystems," Verhandlungen Gesell- 
 schaft deutscher Naturforscher und Aertze, 194, Allgemeiner Theil, 1894. 2 Loc. cit. 
 
 3 Langley : "A Short Account of the Sympathetic System," Physiological Congress, Berne,. 
 1895. 
 
CENTRAL NERVOUS SYSTEM. 219 
 
 cord arise from the first thoracic to the fourth or fifth lumbar, and from these 
 segments only (Gaskcll). The fibres are mcdullatcd. Lan^ley's experiments 
 indicate that no sympathetic cell sends a branch to any other sympathetic 
 cell, but other observers do not admit his results as conclusive. It has been 
 shown that the pre-ganglionic fibres are interrupted in the ganglia. The 
 post-ganglionic fibres are in part medullated, though sometimes medullation 
 occurs only at intervals, but in the main they are gray or unmedullated. 
 
 The cerebro-spinal axones end in the ganglia in such a manner that the 
 branches of the pre-ganglionic axone are distributed to a number of the 
 ganglion cell-bodies, and these cells in turn send their axones either directly 
 to the peripheral structures controlled by the sympathetic elements or to the 
 plexuses such as are to be found in the intestine and about the blood-vessels. 
 
 The same pre-ganglionic fibre may have connections with several cells in 
 one ganglion, or, by means of collaterals, connect with one or more cells in a 
 series of ganglia (Langley). 
 
 Manner of Diffusion. It has been found that while the cells in a sympa- 
 thetic ganglion are so arranged that one pre-ganglionic fibre may be in 
 connection with a group of cells, and thus the impulses which pass out of the 
 ganglion be more numerous than those which entered it, yet the several 
 groups of cells within the ganglion are not connected. In the peripheral 
 plexuses there appears to be a different arrangement. 1 
 
 It has been observed upon stimulation of the branches of the coeliac plexus 
 in the dog that the several branches, though unlike in size, bring about nearly 
 the same quantitative reaction in the constriction of the veins, from which we 
 infer that though entering the peripheral plexus by different channels, the 
 impulses find their way to the same elements at the end, owing to a multi- 
 plicity of pathways within the plexus. 2 
 
 Experiments with strychnin on the more proximal sympathetic ganglia do 
 not show any increased diffusibility following the application of the drug ; but, 
 on the other hand, Langley and Dickinson 3 have shown that nicotin applied 
 to the superior cervical sympathetic ganglion of the cat produces a condition 
 whereby electrical stimulation below the ganglion, which in the normal animal 
 is followed by dilatation of the pupil, is without effect. Since the application 
 of the drug to the nerve-fibres on either side of the ganglion is ineffective, 
 when, at the same time, the application to the ganglion itself is effective, it i& 
 inferred that the drug acts by altering some peculiar relation existing within 
 the ganglion, and the relation which is assumed to be thus modified is that 
 between the fibres terminating in the ganglion and the cells which they there 
 control. The passage of the efferent impulses through other sympathetic 
 ganglia is likewise blocked by nicotin. 
 
 Evidence for Continuous Outgoing- Impulses. Under normal condi- 
 tions striped and unstriped muscular tissues are always in a state of slight 
 
 1 Berkeley: Anatomischer Anzeiger, 1892. 
 
 2 Mall: Archivf. Anatomie u. Physiologic, 1892. 
 
 3 Proceedings of the Royal Society, 1889, vol. xlvi. 
 
220 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 contraction or tonus. When the nerves controlling any such set of muscles 
 are cut, or their central connections injured, the muscles at first relax. 
 
 If a frog be hung up vertically after removing the brain, the cord remain- 
 ing intact, it is found that the legs are slightly flexed at the hip and knee. 
 If now the sciatic nerve be cut upon one side, the leg on the side of the 
 section hangs straighter, indicating that the muscles have relaxed a little as 
 the result of the section of the nerve ; if, in the same animal, the smaller 
 arteries in the web of the foot be examined both before and after the section, 
 it is found that after the section they have increased in diameter. Con- 
 versely, artificial stimulation of the peripheral stump causes a contraction 
 of the vessels, but it is not possible in so rough a way to imitate the tonic 
 contraction of the skeletal muscles. 
 
 It is inferred from these experiments that normally there pass from the 
 central system along some of the nerve-fibres impulses which tend to keep 
 the muscles in a state of slight contraction. Destruction of the entire cord 
 abolishes all outgoing impulses, ancl produces a complete relaxation of these 
 muscles. 
 
 Though the intensity of these outgoing impulses is normally always small, 
 yet it is subject to significant variations. The difference between the tone 
 of the muscles of an athlete in prime condition and those of a patient recover- 
 ing from a prolonged and exhausting illness is easily recognized, and this 
 difference is in a large measure due to the difference in the intensity of the 
 impulses passing out of the cord. Among the insane, too, the variations in 
 this tonic condition follow in a marked way the nutritive changes in the 
 central system, and both facial and bodily expression have a value as an 
 index of the strength and variability of those impulses on which the tone of 
 the skeletal muscles depends. Indeed, so wide in the insane is the variation 
 thus brought about that when the expressions of an individual at one time in 
 a phase of mental exaltation, and at another in that of mental depression, are 
 compared, it appears hardly possible that they can be those of the same person. 
 
 This continuous outflow of impulses from the central system is indicated 
 also by the continuous changes within the glands, and the variations in these 
 metabolic processes according to the activities of the central system. 
 
 Rigor Mortis. Even in the very act of dying the influence of these 
 impulses can be again traced. The death of the central nerve-tissues being 
 expressed as a chemical change, causes impulses to pass down the efferent 
 nerves, and these impulses modify those chemical changes which, in the 
 muscles of a frog's leg, for example, lead to rigor mortis. It thus happens 
 that a frog suddenly killed and then left until the onset of rigor, will under 
 ordinary circumstances show rigor at about the same time in both legs. If, 
 however, the sciatic nerve on one side be cut immediately after the death of 
 the animal, the beginning of rigor in that leg is much delayed, thus showing 
 that the nervous connection is an important factor in modifying the time of 
 this occurrence (Hermann). 
 
 The Nervous Background. We return now to the conditions which 
 
CENTRAL NERVOUS SYSTEM. 221 
 
 modify the spread of the impulses within the central system, when this sys- 
 tem is represented by the spinal cord of a reflex frog. Admitted I v, then- is 
 in the rase chosen but a fraction of the central system. Jt has been shown that 
 all incoming impulses tend to spread over a large part of the central system. 
 In a reflex frog, therefore, the cord is cut off from the remote effect of im- 
 pulses which normally enter the system by way of cells located in the portion 
 removed. Moreover, in the complete nervous system the incoming impulses 
 tend to be transmitted to the cephalic end, and in some measure give rise to 
 impulses returning within the central system and affecting the efferent cells. 
 In a fragment of the central system like the cord such impulses taken up by 
 the central cells must pass so far as the axones are intact; but as these for the 
 most part end at the level of the section, such impulses are lost, in the physi- 
 ological sense, at that point. 
 
 The fact, therefore, that the experiments with spinal reflexes are con- 
 ducted on a portion of the central system has two important physiological 
 consequences. In the first place, there are wanting incoming impulses, direct 
 or indirect, from the portion removed ; on the other hand, through the sec- 
 tion of the afferent axones, in their course within the central system, there is 
 a direct diminution in the number of the pathways by which the impulses 
 arriving at the cord may be there distributed. It is most probable that in 
 the frog, at least, the reduction of the central mass does not so much dimin- 
 ish the number of pathways by which the impulses may be immediately 
 distributed by way of the afferent and central elements, as it diminishes the 
 number of impulses which by way of the portion removed arrive at the 
 efferent cells and modify their responsiveness. 
 
 The modification of the responsive cells tinder more than one impulse is 
 well illustrated by an experiment of Exner : l A rabbit was so prepared that 
 an electric stimulus could be applied to the cerebral cortex at a point the 
 excitation of which caused contraction of certain muscles of the foot. One 
 of these muscles was attached to a lever so that its contraction could be 
 recorded, and a second electrode applied to the skin of the foot oyerlying the 
 muscle. The discharging efferent cells in the cord were in this case subject 
 to impulses from two directions, one from the cortex and one from the skin 
 of the foot. With a current of given strength stimulation of the cortex alone 
 caused a contraction of the muscle, and stimulation of the skin of ilie foot 
 alone, a similar contraction. When both were stimulated simultaneously the 
 extent of the contraction was greater than when either was stimulated alone. 
 If now the strength of the stimulus applied to the skin of the paw was so 
 reduced that alone it was inefficient, then a stimulus from the cortex which 
 produced a reaction as indicated by the first cortical stimulus in Fig. 94 
 (-4, a), put the efferent cells in such a condition that the stimulus from the 
 skin (A, 6, Fig. 94), applied within 0.6 of a second, produced a second con- 
 traction of the muscle, although alone the stimulus from the skin had proved 
 inefficient. Here the first efficient stimulus from the cortex had rendered 
 1 Exner: Archivfilr die gesammte Physiologic, Bd. xxvii. 
 
222 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 the discharging cell for a short period of time more excitable. In the same 
 figure the record shows that if a longer interval here more than three seconds 
 be allowed to elapse, then the second stimulus from the skin remains im'tti- 
 cient. A similar relation between the two incoming impulses is also found 
 to hold when the stimulus from the skin is made to precede. The curv< />, 
 Fig. 94, shows the results when both stimuli are inefficient. In this the 
 stimuli (I) and a) produce no effect when given several seconds apart, but when 
 they occur within a short interval (l* f and a') in this case 0.13 of a second 
 a contraction of the muscle follows. These various experiments, taken 
 together, show in a beautiful way that in the cases chosen the two sets of 
 impulses tend to reinforce each other, whether they are efficient or inefficient, 
 and without regard to the order in which they come. 
 
 This relation between the discharging cell and those by which it is 
 stimulated can be illustrated in still another way. It was observed by 
 Jendrassik l that when a patient was being tested for the height of his kmv- 
 kick, a voluntary muscular contraction, or an extra sensory stimulus, occur- 
 
 Movement of paw. 
 
 Wiin illation of cortex. A a' 
 
 b' " paw. 
 
 Time in seconds. 
 
 * 
 
 
 
 r\rVHrHr\r\r\r-\j 
 
 FIG. 94. To show the reinforcing influence of stimuli applied to the cerebral cortex and to the skin 
 of the paw, on the movements of the paw of a rabbit (Exner). The arrows indicate the direction in 
 which the curves are to be read. In curve A the cortical stimulus at o causes a movement of the paw. 
 Dermal stimulus, within a second, at b causes a movement of the paw. Cortical stimulus at o' causes a 
 movement of the paw. Dermal stimulus several seconds later at V is ineffective. In curve If dermal 
 stimulus at b is ineffective. The cortical stimulus at a several seconds later is also ineffective. The 
 dermal stimulus at V is ineffective, but if followed within 0.18 second by a cortical stimulus at a' a move- 
 ment of the paw occurs. 
 
 ring about the same time that the tendon was struck, had the effect of 
 increasing the height of the kick. This relation was studied in detail by 
 Bowditch and Warren, 2 and they were able with great exactness to measure 
 the interval between the contraction of the muscle used for reinforcement 
 and the time at which the tendon was struck. The curve shown in Fig. 95 
 represents the results of these experiments. It indicates that, up to 0.4 of a 
 second, the closer together these two stimuli occur the greater the reinforce- 
 ment. At an interval of 0.4 of a second no effect is produced by the muscu- 
 lar contraction. Increasing the interval only very slightly has, however, the 
 effect of greatly diminishing the height of the knee-kick i. e., decreasing 
 the strength of the discharge of the efferent cells and this effect is not lost 
 until the interval is increased to 1.7 seconds, when the voluntary muscular 
 contraction ceases to modify the response. A given efferent cell is thus 
 modified in its discharge according to the several stimuli that act upon it. 
 
 1 Deutsches Archiv fur klinisehe Medizin, Bd. xxxiii. 
 Journal of Physiology, 1890, vol. xi. 
 

 
 Effects of Afferent Impulses.- 
 
 . 
 
 !!<<!. 
 
 .ito a sol; 
 sponsive; 
 
 En this cai on of tl of 
 
 -pcciaj fi 
 
 lie .-kin, and th .11 of those inipi ~es a 
 
 iiition in central P-JV.I,- which can thus be a<- 
 
 by cutting oil' the afferent im: the skin 
 
 \v diminution in the<c inmnl-cs althoiiLfh all 
 
 40- 
 
 30- 
 
 20- 
 
 K) 
 
 
 
 10-1 
 
 EO- 
 
 30- 
 
 V M 
 
 ft , - 
 
 
 or 0.2- o>r 
 
 0:7- 
 
 ur 
 
 millimeters, the amount by which the " reinforced " knee-kick vark<l from the 
 normal, the ; nich is represented by the horizontal line at 0, "normal." '\ tervals 
 
 . between the clinching of the hand (which constituted the reinforcement) and the tap 
 tendon are marked below. The reinforcement is greatest when the two events are nearly simulta 
 
 nterval of 0.4" it amounts to nothing; during the next 0.6" the height of the kick is actually 
 diminished the longer the interval, after which the negative reinforcement tends to disappear; and 
 allowed to elapse the height of the kick ceases to be affected by the clinching of the hand 
 rren). 
 
 slow changes are much more likely to be accompa some sort of 
 
 reby other afferent impulses in a measure take the place of 
 
 been suppressed. The loss of these impi ic-h rouse 
 
 'isuallx nclition than i iiitri- 
 
 * kept in view. 
 
 Inhibition. Evidence if Dilating to show that all the a -lies 
 
 of t! -heir nerves in two oppo- 
 
 esses in the gland.- res, or muscles <: 
 
 -es, nerves which cause inhibition ;n . 
 
 ' 
 
 : A nmltBe Patkologit trmakologif,, l& 
 
 h ! ..y, 1899. 
 
224 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 chemical changes in the inhibited muscle of the heart are peculiar and 
 different from those occurring during excitation. For instance, Gaskell* 
 found that a positive variation of the muscle current occurred in the hearts 
 of both the tortoise and crocodile upon stimulating the peripheral end of the 
 vagus ; while stimulation of the accelerator nerves caused the usual negative 
 variation. Further, Gaskell pointed out that during the inhibition of the 
 heart-muscle the anabolic processes were in excess, so that the cessation of 
 inhibition was followed by an increase in the strength of the heart-beats. 
 
 If we now turn to the observations bearing on inhibition in the central 
 nervous system, there are to be found numerous experiments, of which the 
 following is a type : Let one leg of a reflex frog be stimulated by pinching 
 it, or by dipping the toe in weak acid : a withdrawal of the stimulated leg 
 will follow. Now repeat the experiment, at the same time pinching or other- 
 wise stimulating the skin on the opposite leg. It will then be found that 
 either the latent period of the reacting leg is much prolonged or that the 
 reaction fails completely. This is a very simple example of a type of 
 inhibition which is continually occurring. 
 
 The inhibitory effects are, however, not limited to the motor responses of 
 the central system. It is an observation of the ancients that the greater 
 obscures the lesser pain, and, in a general way, all strong sensations prevent 
 the appreciation of the weaker ones, whether they be in the terms of the 
 same or of a different sense. 
 
 Within the central nervous system very remarkable examples of inhibitory 
 of lenomena have been investigated, chiefly by Sherrington. 2 Boubnoff and 
 w leidenhain 3 were the first to record the observation that under certain 
 Conditions stimulation of the cerebral cortex might cause a relaxation of 
 F some extensor muscles of the limbs when these were in a state of tonic 
 contraction. 
 
 Sherrington was able to show that the stimulation of the cortical area for 
 the flexors of the arms also gave rise to impulses leaving the cortex and 
 causing a (inhibition) relaxation of the antagonistic extensors. 
 
 On stimulating the cortical area for the extensor muscles a corresponding 
 relaxation of the flexors could be observed: Thus the cortical area for the 
 contraction of a given group of muscles coincides with the area for the inhi- 
 bition of the group antagonistic to it. Sherrington has also demonstrated the 
 important role of this inhibitory process in mediating muscular co-ordination 
 shown in movements of the eye. When all the muscles of the eye are para- 
 lyzed, the eyeball held by the connective tissues about it looks straight 
 ahead. Sherrington cut the nerves to all the muscles of the left eyeball 
 (monkey) except the external rectus. Under these conditions the eye, when 
 at rest, looked toward the left. Stimulation of the cortical centers, which 
 cause a conjugate deviation of both eyes to the right, was followed by a 
 
 1 Gaskell : Journal of Physiology, vol. vii. 
 
 2 Sherrington : 76id., vol. xvii. 
 
 3 Boubnoff und Heidenhain: Pftiiger's Archiv, xxvi. 
 
CENTRAL NERVOUS S VST KM. 225 
 
 movement of the operated eye toward the median plane (ilic ri^lit i, and to 
 (he position in which it would he held by the elastic tissues alon<-. This 
 conld he explained only through a relaxation or inhibition of the external 
 rcctns muscle, as a consequence of the cortical stimulation. Further experi- 
 ments support the explanation, and also show that the cells, the activity of 
 which is thus inhibited, must lie below the cerebral cortex, lor the inhibition 
 follows when the fibre-bundles below the cortex are directly stimulated, the 
 cortex having been first removed. 
 
 The general bearing of these results is of the greatest importance. As 
 has been pointed out by Hughlings-Jackson, 1 damage of any sort to a portion 
 of the nervous system may, in the simplest case, decrease the activity of the 
 group of neurones controlled by the damaged part by cutting off the stimulat- 
 ing impulses from them ; or, on the other hand and this is often overlooked 
 it may permit them to become abnormally active by the stoppage of some 
 impulses exerting an inhibitory control. Further, whether impulses from a 
 given set of cells shall prove stimulating or inhibitory depends on the o///o- 
 impulses affecting the receiving cell group, and on the time relations between 
 these several sets. This consideration serves to indicate the complex rela- 
 tions which may underlie the manifestations of disease in the central nervous 
 system. 
 
 As to the mechanism for these inhibitory reactions, it can be safely said 
 that for the most part the effects are not dependent on the existence of a 
 special class of inhibitory nerves, and the most we can think of structurally 
 is a different but not necessarily constant dendritic pathway for the cellu- 
 lipetal impulses causing inhibition. 
 
 0. REACTIONS INVOLVING THE ENCEPHALON. 
 
 On attempting to distinguish between a voluntary and a reflex act from the 
 physiological standpoint we find the chief difference to be that the voluntary 
 act is not predictable, because, according to the capabilities of the animal, it 
 may be more variable in form than is the reflex response, and also because, 
 instead of necessarily occurring within a short interval after the stimulus, as 
 does the reflex, the voluntary response may be delayed even for years. 
 
 Reflexes have been illustrated by the reactions from a portion of the spinal 
 cord. It is to be remembered, however, that any of the sensory cranial or 
 spinal nerves can serve as a pathway for the afferent impulses, and any of the 
 groups of efferent cells situated in the ventral horns or their homologues in 
 the brain stem, can carry the efferent impulses needed. Further, it must be 
 remembered that it is these same afferent cells which always furnish the first 
 set of impulses, and the efferent cells controlling the muscles and glands 
 which furnish the last set of impulses in both reflex and voluntary reactions. 
 The processes then which distinguish the two forms of reactions must take 
 place in the central cells. We turn, therefore, to the nervous connections of 
 the encephalon with the cord, since it is by means of these connections that 
 
 1 Hugh lings- Jackson : Lancet, 1898, vol. i. 
 VOL. II. 15 
 
226 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 the impulses travel to the many series of central cells which are concerned in 
 the simplest voluntary responses. 
 
 For the most complex voluntary reactions the entire central system is 
 necessary, and especially the cortex of the cerebral hemispheres, while it has 
 
 already been shown that the impulses 
 which cause reflex actions can make 
 their circuit in a very limited portion 
 of the spinal cord. In the case of 
 voluntary reactions the impulses take 
 a longer pathway and involve a larger 
 series of central nerve-elements, since 
 from the point at which they enter the 
 system they must pass to the cephalic 
 end and back again to the efferent 
 elements. At the same time, in a 
 voluntary reaction, a greater number 
 of impulses combine to modify the 
 discharge from the efferent cells. 
 
 In order that the encephalon may 
 be included in the pathway of the im- 
 pulses entering the cord, it is necessary 
 that pathways formed by axones 
 should, on the one hand, extend up 
 to the encephalon, and, on the other, 
 back from it to the cord. 
 
 Fig. 96 indicates how the first 
 part of this path is composed of the 
 afferent elements of the dorsal spinal 
 nerve-roots. The long paths in the 
 dorsal funiculi of the cord are formed 
 by the ascending branches of the af- 
 ferent axones, and these terminate, for 
 the most part, about the cell-bodies 
 which form the nuclei of the dorsal 
 funiculi at the junction of the cord 
 and bulb. From these nuclei a second series of axones passes out, decussates 
 at once, and then the axones pass forward in the medial lemniscus to find 
 a second ending in the ventral cell masses of the thalamus, or possibly to 
 continue up to the cortex. From this point a third group of neurones, with 
 their cell-bodies in the thalamus, send out their axones to the cerebral cortex. 
 The cranial afferent nerves, which are not nerves of specific sensations 
 (i. e., the fifth, the vestibular portion of the eighth, the ninth, and tenth), 
 probably have corresponding connections in the central system. 
 
 The impulses which are brought in by the afferent fibres also pass, in a 
 large measure, to cells in the dorsal horn of the spinal cord by way of the 
 
 FIG. 96. Schema showing the smaller pathway 
 of the sensory impulses. On the left side, S, S', 
 represent afferent spinal nerve-fibres ; C, an afferent 
 cranial nerve-fibre. This fibre in each case termi- 
 nates near a central cell, the axone of which 
 crosses the middle line and ends in the oppo- 
 site hemisphere. The interruption of the larger 
 pathway in the thalamus is not indicated (van 
 Gehuchten). 
 
CENTRAL NERVOUS SYSTEM. 227 
 
 collaterals and the ascending branches of the allen-nt a \ones, \\hi.-h end 
 before they reach the nuclei of the dorsal fimieuli. The cells in the dorsal 
 horns send their axones in large numbers across the cord to the lateral 
 columns of the opposite side, to reach the thalamus through the medial 
 lemniscus, and thence to the cortex. 
 
 Of the many disputed points in this pathway, the most important relates 
 to the interruption of the axones of the lemniscus in the ventral portion of the 
 thalamus. The recent researches of Tschermak 1 indicate that probably there 
 are two groups of neurones concerned, one of which sends its axones without 
 interruption to terminate in the cortex, while the axones from the other are 
 interrupted at the level of the thalamus. The latter group is the larger and 
 probably the more important for the general reactions of the central system. 
 
 The pathways which are here sketched have been worked out mainly by 
 the study of degenerations, in large part resulting from experimental lesions. 
 
 When the dorsal roots are crushed or sectioned between the spinal gan- 
 glion and the cord, the prolongation of the afferent fibre within the cord 
 degenerates throughout its entire extent. The degeneration extends in the 
 dorsal columns down the cord two or three centimeters from the level of the 
 section, and also up the cord as far as the nuclei of the dorsal columns located 
 at the commencement of the bulb. If the section is made near the caudal 
 end, the degeneration may in consequence run through the entire length of 
 the cord. Moreover, it occurs mainly on the side of the cord to which the 
 sectioned nerves belong. Take, for example, the area of degeneration caused 
 by the section in a dog of the dorsal roots on the left side between the sixth 
 lumbar and second sacral nerves. The degeneration in the lower lumbar 
 region is represented in Fig. 97, a, in the upper lumbar region in 6, and in 
 the thoracic region in c and d. The section e passes through the cervical 
 enlargement. On passing cephalad the area of degeneration becomes smaller. 
 This is interpreted to mean that all along, between the caudal and cephalic 
 limits, fibres are given off from the main bundle to the intermediate levels 
 of the cord. Here is evidence of an arrangement that is always to be kept 
 in view. Though a number of fibres among those degenerating after section 
 of the dorsal roots may run the longer course to the bulb, the larger portion 
 run a short or an intermediate course, and are, therefore, distributed at dif- 
 ferent points between the termini. Injury to the dorsal roots at di Hi rent- 
 levels shows, moreover, that the fibres from a given level which run the 
 length of the dorsal columns do not mingle indiscriminately with those from 
 other levels, but form a bundle ; and as that bundle passes cephalad in the 
 cord, it tends to lie nearer the middle line. Hence in the upper cervical 
 cord, where the bundles from all levels are present, a cross-section >hmvs the 
 bundles which entered lowest down to be located nearest the dorsal surface 
 and the median septum. 
 
 From these relations it is evident that comparatively few of the dorsal 
 root-fibres run the entire length of the dorsal fimiculi, since the majority 
 1 Tschermak: Archivfur Anatomieund Physiologic, Anat. Abthl., 1898, S. 291-400. 
 
228 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 terminate somewhere between this extreme limit and their point of en- 
 trance. 
 
 Since the fibres l in the dorsal funiculi of the cord degenerate on destruc- 
 tion of the dorsal roots, it is inferred that they must be morphologically con- 
 tinuous with certain fibres in the roots, and, since the dorsal roots are afferent 
 pathways, they too must form part of the afferent pathway in the cord. 
 
 The continuation of the other paths for the afferent impulses must, how- 
 ever, be formed by the axones of the central cells with which the dorsal 
 root-fibres connect as they terminate at the several levels of the cord. 
 
 FIG. 97. The sections are from five levels of the spinal cord of a dog. The dorsal roots on one side 
 had been sectioned in two groups : first, the twenty-eighth, twenty-seventh, and twenty-sixth spinal 
 nerves ; and second, the twenty-second and twentieth : a, shows a schematic picture, representing a 
 cross-section of the spinal cord taken just below the level of the twenty-second spinal root. The black 
 spot represents the principal bundle of degenerated fibres as it appears in the dorsal column. At this 
 level the bundle is rather near the median septum, but if sections further caudad were examined in series 
 it would be found that the bundle constantly approached the dorsal horn, and finally fused with it at the 
 level where the injured nerves joined the cord. If, on the other hand, a section be taken from the level 
 between the twenty-second and the twentieth nerves that is, after passing the level at which the second 
 group of sectioned nerves joins the cord there are to be seen two bundles of degenerated fibres marked 
 by black spots in the sections (b, c, d). The last bundle to enter the cord, and the one lying nearer the 
 dorsal horn, is, of course, formed by the degenerating fibres from the second group of roots. In the sec- 
 tions c, d, e, taken respectively at the level of the eighteenth nerve, the middle of the thoracic cord and 
 the cervical enlargement, it is seen that both degenerated bundles grow smaller ; that they shift toward 
 the median septum and approach one another ; and, finally, that they completely fuse in the cervical 
 region (e). 
 
 Degeneration after Hemisection of Cord. Upon hemisection of the 
 cord involving one lateral half, the ascending fibres which degenerate appear 
 in the dorsal columns, in the dorso-lateral ascending tract, and in the ventro- 
 lateral ascending tract. The number of degenerated fibres is large on the 
 side of the lesion, but on the opposite side there are also degenerated fibres in 
 all these localities, although they are by no means so numerous. It is inferred 
 that all the fibres which thus degenerate form paths for the afferent impulses. 
 
 The impulses which come in over a dorsal root on one side can, therefore, 
 find their way cephalad either by the direct continuations of the dorsal root- 
 fibres running in the dorsal column mainly on the side of the lesion or 
 through the interpolation of central cells, the axones of which appear degen- 
 
 1 The bundles of "endogenous fibres" not arising from spinal ganglion-cells are neglected 
 here. 
 
CENTRAL NERVOUS SYSTEM. 229 
 
 erated in both lateral columns, but more numerously on the side of the 
 lesion. 1 
 
 The tracts which undergo secondary degeneration after this treatment 
 include, therefore, those formed by the axones arising from central cells. 
 These neurones have their cell-bodies arranged in columns running the length 
 of the cord. In the neighborhood of these columns some of the dorsal root- 
 fibres terminate. In the bulb we are familiar with such groups of cells, well 
 marked as the "nuclei of the dorsal funiculi or columns," and the corresponding 
 cells in the cord, though far less clearly segregated, are the homologues of 
 those in the bulb. If this is granted, then the fibres which are outgrowths 
 from these central cell-groups, whether in the cord or bulb, are also homologous. 
 
 Corroborative of what has been said on the subject of afferent pathways 
 in the cord are the results of Pelizzi. 2 He studied dogs, making use of the 
 method of Marchi, whereby the nerve-sheaths of fibres beginning to degene- 
 rate or the nutrition of which is disturbed give a characteristic reaction. He 
 found, after hemisection of the cord, the same lesions that have been described 
 above, with the addition that the changes could also be followed in some of 
 the fibres of the ventral roots. More significant, however, is the fact that 
 section of the lumbar and sacral dorsal roots, without direct injury to the 
 cord, gave rise to modifications of the medullary sheaths, detectable by the 
 method of Marchi, in all the localities just named. 
 
 A distinction must be made at this point. Secondary degeneration in the 
 central system means eventual destruction of the severed fibre. The method 
 of Marchi shows a characteristic change in fibres entering upon this degenera- 
 tion, but this method also shows changes in the sheaths of elements which 
 are only physiologically connected with those about to undergo secondary 
 degeneration, but which themselves are, as a rule, not ultimately destroyed. 
 Under the usual conditions of experiment, complete degeneration is confined 
 within the morphological limits of a single cell-element, but the physiological 
 changes in the cells overstep this limit, as shown by Marches reaction. 
 
 Physiological Observations on Afferent Pathways. Making use of 
 the fact that strong stimulation of the sensory fibres, such as those in the 
 sciatic nerve, causes a rise in blood-pressure, Woroschiloff 3 sought to block 
 the passage of the impulses causing this reaction by section of the cord in 
 different ways in the upper lumbar region of the rabbit. It appears that in 
 this animal the reaction was most diminished that is, stimulation of the 
 sciatic produced least rise in the blood-pressure when the lateral columns 
 of the cord had been cut through ; and that the effect of section of the lateral 
 column on the side opposite to that on which the stimulus was applied was 
 greater than the following section of the column on the same side. These 
 experiments form a very definite part of the evidence which directs our 
 attention to the lateral columns of the cord as a principal afferent pathway. 
 
 1 Kohnstaram: Neurologisches CentralblaU, 1900, S. 242. 
 
 2 Archives ilaliennes de Biologie, 1895, t. xxiv. 
 
 3 Berichte der math.-phys. Classe d. k. Gesellwh. d. Wissen. zu Leipzig, 1874. 
 
230 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 The physiological observations of Gotch and Horsley 1 indicate that when 
 in a monkey a dorsal root is stimulated electrically 80 per cent, of the 
 impulses pass cephalad on the same side of the cord, while the remainder 
 cross. Of the 20 per cent, that cross, some 15 per cent, pass up in the 
 dorsal columns. This leaves only 5 per cent, of the impulses to pass up by 
 the contra-lateral columns. These experiments, therefore, give less impor- 
 tance to the lateral columns than was to be expected from the observations 
 of Woroschiloff. The dorso-ventral median longitudinal section of the cord 
 in the monkey (sixth lumbar segment) 2 shows an ascending degeneration in a 
 small part of the dorsal area of the direct cerebellar tracts and of the ventro- 
 lateral tracts, as well as in the columns of Goll. This would indicate that 
 the section had cut fibres which crossed the middle line and ran cephalad in 
 these localities. 
 
 Osawa 3 found that when the cord in a dog was hemisected (in the upper 
 lumbar or lower thoracic region) the animal showed for the most part no 
 permanent disturbance of sensation or motion. 
 
 If the cord was first hemisected on one side, and later on the other side, the 
 second hemisection being made a short distance above or below the first, sen- 
 sation and motion persisted behind the section, although they were somewhat 
 damaged. After three hemisections, alternating at different levels, there still 
 remained a trace of co-ordinated movement possible to the hind legs, although 
 the sensibility of the parts could not be clearly demonstrated. The path 
 thus marked out for some afferent impulses is certainly a tortuous one, and 
 at present not readily to be explained. It must be remembered, however, 
 that our information concerning the short pathways in the cord is very slight. 
 
 Nerves of Common Sensation. In order to analyze the afferent path- 
 ways still further, we next inquire whether among the dorsal nerve-roots 
 which pass between the cord and periphery there are separate nerve-fibres 
 for each of the modes of sensation represented by pressure, heat, cold, pain, 
 and the muscle-sensations. The data available for determination of this 
 question are not of the best, but are still of some value. 
 
 The number of dorsal root nerve-fibres on both sides was estimated (in a 
 woman twenty-six years of age) by Stilling to be approximately 500,000. 4 
 Stilling's estimate for the ventral root fibres in the same individual was 
 300,000. 
 
 The area of the skin in a man of 62 kilograms (136 pounds), and twenty- 
 six years of age, was found by Meeh to be 1,900,000 square millimeters. 5 
 
 From the study of the nerves going to the muscles of the dog, Sherring- 
 
 Croonian Lectures : Philosophical Transactions of the Royal Society, 1891. 
 
 Griinbaura : Journal of Physiology, 1894, vol. xvi. 
 
 Untersuchungen uber die Leitungsbahnen im Riickenmark des Hundes, Strassburg, 1882. 
 
 It seems probable that both these estimates were too low. 
 
 A slight correction is called for here, owing to the fact that the area of skin includes that 
 for the head, while the sensory nerves enumerated do not include the fibres going to the head. 
 The general relations given below would not, however, be significantly modified by the altera- 
 tion of the data. 
 
CENTRAL NERVOUS SY8TXM. 231 
 
 ton 1 reports that from one-third to one-half the number of tin -< muscular 
 fibres arise from the dorsal root iran^lion, and are the re- fore afferent in function. 
 
 If we assume that two-fifths of the number of sensory fibres, or 200,000, 
 go to the muscles and joints, this would leave but .'*()(.),( KM) sensory iibn-s 
 remaining, or one nerve-fibre to innervate, on the average, about six square' 
 millimeters of skin. 
 
 The experiments on tactile and temperature discrimination all indicate 
 that the innervation of the skin is very unequal. The average distribution 
 which has just been suggested must therefore be subject to local modifications 
 that are very wide. Woischwillo 2 has determined that in man the skin of 
 the arm is three times better supplied with sensory nerves than that of the 
 leg. In both arm and leg the relative abundance of the sensory nerves 
 increases toward the extremity of the limb. This increase is special lv 
 marked in the leg. Assuming, however, one nerve-fibre to six squa*re milli- 
 meters of the skin to be the average relation, it becomes a serious matter to 
 postulate separate groups of fibres for each mode of dermal sensation, since 
 each time a new set of fibres is admitted the area of the skin innervated by 
 any other set with a given function is thereby increased. 
 
 This being the case, there are good anatomical reasons for limiting the 
 number of categories of nerve-fibres. 
 
 In every case the fibres carrying the impulses which come from the skin 
 arise as outgrowths of the spinal ganglion-cells. Trophic nerves as a special 
 category are not recognized, nor reflex nerves, the functions attributed to the 
 latter being now explained by the collaterals of the afferent fibres. At 
 present it is sometimes maintained that there must be special nerves for pain, 
 pressure, heat, and cold. The evidence for those of pressure and heat and 
 cold is the most satisfactory. 
 
 Pain. Upon severe stimulation of the skin or muscles the normal person 
 experiences a distinct sensation of pain. There is, however, great variation 
 in the intensity of this sensation when the same stimulus is applied to differ- 
 ent persons. 
 
 If we include abnormal persons, it is found that while in a few cases com- 
 plete absence of painful sensations has been noted the other sensations 
 remaining normal there are at the other end of the scale those cases in 
 which pain is produced by many stimuli which would not have this effect on 
 persons in ordinary health. The capability of a given stimulus to produce 
 pain is therefore subject to wide variations acpording to the general condition 
 of the subject. 3 The same stimulus has different effects in a given individual 
 according to several circumstances. Peripheral irritation, such as an inflam- 
 matory process in the skin, greatly increases the intensity of the pain caused 
 by the stimulation of the nerves supplying the locality. Continued stimula- 
 
 1 Sherrington : Journal of Physiology, 1894-5, vol. xvii. 
 
 2 " Ueber das Verhaltniss des Kalibers der Nerven zur Haul und den Muskeln des Men- 
 schen," Inang. Diss. (Russian), 1883; vide Centrnlblatt fur Nervenheilkunde, 1883, Bd. vi. 
 s Strong : Psychological Review, 1895, vol. ii. No. 4. 
 
232 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 tion of the sensory nerves of the muscles and viscera has the same effect. 1 
 Local anesthetics, such as cocaine, may reduce the sensibility to zero, and 
 the same follows the general anesthesia produced by chloroform, ether, nitrous 
 oxide, morphine, and similar drugs. Painful sensations are distinct and 
 powerful only when the stimulus is applied to general sensory nerve-trunks 
 i. e. } those mediating cutaneous, muscular, and visceral sensibility while the 
 nerves which mediate the special sensations of light, sound, taste, and smell 
 do not give pain even on excessive stimulation. 
 
 Limiting our observation, therefore, to the nerves of cutaneous sensibility, 
 it is found in exceptional cases that the sensations of pressure, heat, and cold 
 may all be present to a normal degree, and yet increasing the stimulus be 
 without effect in causing any painful sensations whatever. This would 
 represent a condition of complete analgesia. Moreover, the capacity of the 
 skin to cause abnormal painful sensations upon the adequate stimulation of 
 each of these groups of nerves may be associated (in lesions of the central 
 system) with any one group alone, the abnormal pain-sensations thus pro- 
 duced being either excessive or deficient. 
 
 We advance the hypothesis, therefore, that each of these three sensations, 
 if pushed to excess, is usually accompanied by pain of gradually increasing 
 intensity. Therefore it is most probable that these nerves when slightly 
 stimulated mediate their proper sensations, but when this stimulus is pushed 
 to excess they can give rise to pain also, and that in the last instance this 
 sensation of pain may prove exclusive of any other. If this view is correct, 
 it appears improbable that special pain-nerves exist. 
 
 As various experiments show, increasing either the strength of the periph- 
 eral stimulus, the number of fibres to which it is applied, or the irritability 
 of the terminals of the fibres, will assist in arousing painful sensations. In 
 the last analysis the physiological condition for pain is excessive stimulation, 
 which by all analogy must mean excessive discharge within the central 
 system. The changes following this discharge into the central system are 
 not such as lead to co-ordinated muscular responses, but to convulsive reac- 
 tions of a very irregular character. Where this process takes place in the 
 central system we do not know. As to normal analgesia, it must be looked 
 upon as dependent on a condition in which excessive stimulation cannot be 
 produced ; and we find this condition normally present only in the case of 
 the nerves of special sense. 
 
 Since in the pathological conditions one sort of sensibility may be lost 
 while the others remain, it has been inferred that there are separate fibres 
 for the conveyance of each sort of sensation. This idea was expressed in the 
 law of the specific energies of nerves as formulated by Joannes Miiller, who 
 pointed out that in many cases the same nerve might be stimulated in any 
 way mechanically, electrically, or chemically, as well as in the normal physi- 
 ological manner; and that in all cases the mode of the response was the same 
 a sensation of light or taste or contact, as the case might be. Hence it was 
 
 1 Gad und Goldscheider : Zeitschrift fur klinische Medicin, Bd. xx. 
 
CENTRAL NERVOUS SYSTEM. 233 
 
 argued that the mode of the sensation was independent of the kind of stimu- 
 lus, but dependent on the nature of the central cells among which the idle rent 
 fibres terminated. It will be seen, however, that this argument does not 
 touch the character of the nerve-impulses in any two sets of nerves, and we 
 have no observations by which to decide whether the nerve-impulses pa: in- 
 along the optic nerve-fibres are, for example, similar or dissimilar to th<>>r 
 which pass along the auditory fibres. 
 
 If the nerve-impulses are always all alike, there seems no escape from the 
 inference that separate nerve-fibres convey the impulses destined to give rise 
 to different sensations. At the same time, it is just possible that the nature 
 of the impulses and of the resultant sensation is, in the nerves of cutaneous 
 sensibility, determined by the form of the peripheral stimulus, and that, as a 
 consequence, different branches of the same nerve-fibres may be conceived of 
 as susceptible to different forms of stimulation, and thus the two different 
 sensations follow from the partial stimulation of the same nerve-fibres. 
 
 The second possibility, that the nerve impulse has different characters in 
 different afferent nerves, and further may be modified by the nature of the 
 normal stimulus (pressure or temperature), is not to be too readily rejected, as 
 Hering at least argues in favor of such a view. 1 
 
 Pathway of Impulses in the Spinal Cord. The question arises how 
 these impulses are distributed among the afferent tracts which are recognized 
 in the cord, and whether these tracts form special paths for the impulses that 
 rouse the several sensations of pressure, temperature (heat and cold), and 
 pain. Since it is necessary to know the sensations of the subject, this prob- 
 lem can be, in some ways, best studied in man. Here, owing to wounds or 
 disease, it may so happen that some of these sensations are lost or greatly 
 diminished, and it is to be determined whether this loss is constantly associ- 
 ated with the interruption of definite tracts. Unfortunately, however, the 
 material for such a study is very meagre. 
 
 In man the typical group of symptoms following hemisection of the spinal 
 cord above the lumbar region has long been known as Brown-Sequard's 
 paralysis. The clinical observations on cases suffering from such a lesion 
 have been recently summarized by Oppenheim 2 as follows : 
 
 1. A paralysis of the homo-lateral muscles. In the case of the leg, the 
 effects are most intense and persistent in the flexors of the thigh and shank, 
 and the extensors of the foot. 
 
 2. When the two sides of the body are contrasted, there appears to be a 
 homo-lateral hyperaBsthesia, accompanied by contra-lateral anesthesia. 
 
 3. As to the several forms of sensation, the following may be stated : 
 
 (a) The muscle sensations (Bathyasthesia, Oppenheim) : the defect is 
 never contra-lateral ; sometimes, however, it is bilateral, but in most cases is 
 homo-lateral. 
 
 (b) The contact sensations are very often not affected at all sometimes 
 
 1 Hering: "Zur Theorie der Nerventhatigkeit," Akademischer Vortrag, Leipzig, 1899. 
 'Oppenheim : Archivfur Physiologic, Physiol. Abthl., Suppl. Bd., 1 Heft, July, 1899. 
 
234 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 slightly, and in the latter case the hypersesthesia may appear on either or 
 both sides. 
 
 (c) The prominent and almost constant sensory symptom is the contra- 
 lateral loss of the sensation for pain and temperature. 
 
 On the basis of a case l in which the lateral columns of the cord and the 
 gray matter of both horns on the same side were the seat of damage, and in 
 which there was a total loss of pain on the opposite side of the body without 
 impairment of tactile sensibility, it may be inferred that the pain-impulses 
 cross soon after entering the cord, and pass cephalad by some path lying 
 within the damaged area. 
 
 A second case 2 is recorded in which a stab-wound divided all of one-half 
 of the cord plus the dorsal column of the other half. There was here a loss 
 of sensibility to pain on the side opposite the lesion, together with the loss of 
 tactile sensibility on both sides, pointing, therefore, to the dorsal columns as 
 the paths for the tactile impulses. The experiments on the lower animals 
 contradict this conclusion. 
 
 The observations of Turner 3 on monkeys, in which hemisection of the 
 cord had been made in the lumbar and thoracic regions indicate that all sen- 
 sory impulses cross immediately after entering the cord, yet section in the 
 cervical region showed that the impulses roused by touching the skin pass in 
 part on the same side of the cord as the section, the other sensory impulses 
 being, however, completely crossed. 
 
 On the other hand, from his work on hemisection of the thoracic cord of 
 the monkey at different levels, 4 Mott found the disturbance of sensibility of 
 all forms mainly on the side of the section. 
 
 Hemisection of the Cord. From experiments on monkeys and a few 
 cats Schafer reports the following physiological changes after hemisection of 
 the spinal cord in animals : " In the first few days complete motor paralysis 
 of all parts supplied with nerves below the section. The limb or limbs on 
 the paralyzed side swollen and warm (vasomotor paralysis) and lessened out- 
 flow of lymph and the skin dry (diminution of sweat). Knee-jerk exag- 
 gerated. Sensation not lost on the same side as the lesion, but at first appears 
 dulled. (There is a difficulty in arriving at a clear decision on account of 
 the motor paralysis rendering the animal unable to move the limb.) After 
 a few days, unmistakable signs of feeling and localizing even a slight touch, 
 and this long before the motor paralysis has passed off. The animals gener- 
 ally disregard a clamp-clip on the skin of the paralyzed limb, but not always ; 
 this phenomenon usually lasts until the return of movement in the muscles 
 of the limb. 5 I have seen no signs of paralysis either motor or sensory on 
 
 1 Gowers : Clinical Society's Transactions, 1878, vol. xi. 
 
 2 Miiller : Beitrdge zur pathologische Anatomic und Physiologic des Ruckenmarkes, Leipzig, 1871. 
 
 3 Brain, 1891. 4 Mott : Journal of Physiology, 1891, vol. xvii. 
 
 5 It will be seen that my observations on this point agree generally with those of Mott 
 (Phil. Trans. B. 1892), although ray conclusions are somewhat different. Mott, in my opinion, 
 lays too much stress on the results of the clip test (Schafer). 
 
CENTRAL NERVOUS SYSTEM. 235 
 
 the side opposite to the hemisection in any case in which this has been strictly 
 confined to the one half of the cord. 1 Sometimes the adjacent posterior 
 column of the other half is injured, and in that event there is impairment of 
 sensation for a time on both sides below the lesion. The motor paralysis, at 
 first complete, becomes gradually incomplete, and finally is difficult or impos- 
 sible to determine. But purely voluntary movements are not recovered <>r 
 but very imperfectly, although all the ordinary associated movements of the 
 limb are recovered. After about three or four weeks it is difficult to detect 
 any sort of paralysis, but the limb which has been paralyzed is thinner than 
 the other." 
 
 If the hemisection is made above the level of the eighth cervical nerve, 
 the pupil on the same side is relatively contracted and remains so. The 
 dilator fibres and the pilomotor fibres in the cervical sympathetic do not 
 degenerate, but remain excitable. The pupil reacts to light and shade in 
 spite of its being persistently smaller than the other. Excitation of the 
 motor cortex of the opposite cerebral hemisphere produces, as a rule, no move- 
 ments in the limbs which have been paralyzed, even if the associated move- 
 ments have long returned. 
 
 As will be seen from the foregoing paragraphs bearing on the aiferent 
 pathways found in the spinal cord of man and the higher mammals, the evi- 
 dence for the path of the cutaneous impulses is decidedly contradictory. 
 
 In addition to the cutaneous impulses there are the sensory impulses from 
 the viscera, muscles, and tendons, which find their path cephalad probably 
 along the direct cerebellar tract as well as by the long pathways in the dorsal 
 columns. After hemisection of the cord the "muscular" sensations are 
 usually lost on the side of the section, and the observations of Tschermak, 
 already mentioned, point to the long fibres in the dorsal funiculi as the path- 
 way for the impulses from the muscles and joints. 
 
 Indeed, the lack of good evidence for the conduction of any impulses 
 save those from the muscles and joints by long tracts in the cord, has led 
 Starr 2 to suggest that the dermal impulses are transmitted by short path- 
 ways through the cord. 
 
 Since, then, the dorsal and lateral columns of the cord appear to contain 
 the chief afferent paths for the sensory impulses, the next step in following 
 the pathway is to find the terminations of these tracts, whether long or short. 
 Of the latter nothing can be said. The long tracts in the dorsal columns are 
 connected with the nuclei of those columns (nuclei of Goll.and of Burdach) 
 on the same side. The cells of these nuclei send their axones cephalad ; in 
 part they decussate in the sensory crossing and contribute to the formation 
 of the lemniscus, by way of which they pass either directly to the cerebral 
 cortex about the central gyri, or reach this only after interruption in the 
 
 1 Ferrier and Turner (Brain, 1891) describe loss of sensibility in the opposite hind limb in 
 the monkey. Brown-Sequard, as is well known, obtained this result in the rabbit. (See also 
 Ferrier, Functions of the Brain, and Croonian Lectures, 1890.) 
 
 2 Starr : Transactions of the American Neurological A ssociation, Twenty-third Meeting, 1897, p. 7. 
 
236 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 thalamus. It will be remembered that these fibres of the dorsal columns 
 are physiologically joined with the contra-lateral thalamus and hemisphere. 
 In part, however, the axones from the dorsal nuclei enter the cerebellum by 
 the inferior peduncle of the same side, and we shall refer to this when con- 
 sidering the cerebellum. 
 
 Cranial Nerves. We shall next consider y briefly the relations of the sev- 
 eral afferent cranial nerves, beginning with the vagus and working cephalad. 
 
 Nervus Vagus (Tenth Nerve). The nucleus of termination for the afferent 
 fibres of the tenth nerve (vagus) are shown in Fig. 91. The afferent fibres 
 of this nerve are found to convey impulses which arise in the pharynx, oesoph- 
 agus, stomach, liver, pancreas, spleen, larynx, bronchi, and lungs. Further, 
 this nerve contributes afferent fibres to the nervus laryngeus superior. The 
 location of the nucleus of termination (N. alee cinerse) falls within the area of 
 the noeud vital. Concerning the axones of the neurones forming the nucleus 
 of termination, it can only be said that they are continued cephalad in the 
 medial lemniscus and the fasciculus longitudinalis medial is. 
 
 Nervus Glossopharyngeus (Ninth Nerve). The ninth nerve (glossopharyn- 
 geus, Fig. 91) is represented in the bulb by the tractus solitarius, the fibres 
 of which find their principal nucleus of termination in the cell-group lying 
 just to the medial side of the tract. The neurones of this nucleus send their 
 axones cephalad by way of the medial lemniscus. The afferent fibres of N. 
 glossopharyngeus mediate general sensations for the tonsils and pharynx, the 
 tympanic cavity and Eustachian tube, while by way of the ramus lingualis 
 it innervates the taste-organs of the posterior part of the tongue and those in 
 the pharynx. In addition to these fibres mediating the sense of taste, patho- 
 logical evidence points to some additional fibres with the same function (not 
 belonging to the ninth nerve) which reach the bulb by way of the fifth nerve 
 and the nervus intermedius. The nuclei of termination for these three nerves 
 are very close to one another in the bulb, and hence the innervation of a 
 special sense-organ from three cranial nerves, which, in the first instance, 
 seems anomalous, becomes more intelligible when it is recognized that the 
 nuclei concerned are practically continuous. 
 
 Nervus Intermedius. In this connection the afferent fibres in the nervus 
 intermedius (of Wrisberg) should be mentioned. These fibres arise from the 
 cell-bodies of the ganglion geniculatum, enter the bulb between the super- 
 ficial origin of the seventh and the vestibular root of the eighth nerve, and, 
 running caudad along the dorso-medial tip of the ascending root of the fifth, 
 finally terminate with the fibres of the glossopharyngeus in the cells of ter- 
 mination found along the tractus solitarius. The longitudinal extension of 
 these fibres of the nervus intermedius in the bulb closely matches that of the 
 nucleus of the eighth nerve, and at the periphery the fibres from the genicu- 
 late ganglion are distributed, in part at least, with those of the seventh nerve. 1 
 
 Nervus Auditorius (Eighth Nerve). A. Cochlear Root. The eighth nerve 
 
 1 Van Gehuchten : " Eecherches sur la Terminaison centrale des Nerfs sensible pe'riphe'ri- 
 ques le Nerf interme'diare de Wrisberg." Le Niuraxe, Mars, 1900, t. i. fasc. L 
 
CENTRAL NERVOVS SYSTEM. 237 
 
 goes to the inner ear. 'Flu 1 oochlear portion of the inner ear mediates sensa- 
 tions of sound and is connected with the bulb by means of the nervus coch- 
 leae; the cochlear branch of the eighth nerve. The cell-bodies of the nervus 
 cochleae are located in the spiral ganglion of the cochleae, which is homologous 
 with the dorsal root ganglion of n spinal nerve. The ganglion cells are 
 bipolar or diaxonic, one axone passing toward the organ of Corti in the 
 cochlea, and the other toward the bulb. 
 
 On reaching the bulb, the nerve formed by the latter ax ones enters in a 
 large measure the nucleus nervi cochleae ventralis, 1 and to a less extent the 
 nucleus nervi cochleae dorsalis. According to Held, 2 some of the root-fibres 
 entering the ventral nucleus may be continuous as far as the superior quad- 
 rigemina, reaching that level by way of the trapezoideum, the superior olive, 
 the lateral lemniscus, and the colliculus inferior ; to all of which gray masses, 
 including the nucleus nervi cochleae dorsalis, these axones may give collat- 
 erals. Further, some fibres may terminate in any of the localities reached 
 by the collaterals. Besides the direct continuations of the afferent axones by 
 way of the ventral nucleus, each one of the localities mentioned above, includ- 
 ing both the dorsal and ventral cochlear nuclei, contains cell-bodies forming, 
 on the one hand, nuclei of termination, and on the other by their axones con- 
 tinuing the auditory pathway even to the cerebral cortex (Held). A group 
 of central cells with their bodies in the nucleus nervi cochleae dorsalis send 
 their axones across the floor of the fourth ventricle, forming the striae acusticae. 
 These axones in part decussate with the corresponding fibres the crossing 
 occurring in the raphe and then either as direct or crossed fibres find their 
 way cephalad by the same path (with some additions) as that described in 
 connection with the ventral nucleus. 
 
 B. Vestibular Root. Quite separate from the cochlear is the vestibular 
 division of the eighth nerve, and this separateness is a strong argument against 
 the suggestion sometimes made that the portions of the labyrinth innervated 
 by the vestibular nerve, may also mediate sensations of sound. The best 
 evidence shows the nerve to convey those impulses from the macula acustica 
 utriculi and the cristae ampullares, which are largely utilized in the mainten- 
 ance of the equilibrium and in arousing the sensations of the movement of 
 the body as a whole. The peripheral neurones which give rise to the vestib- 
 ular fibres have their cell-bodies collected in the vestibular ganglion. The 
 peripheral axones of the ganglion-cells end among sensory epithelium of the 
 parts just named, while the central axones, forming a larger root than that 
 associated with the cochlea, join the bulb at the caudal edge of the pons, the 
 vestibular root lying to the cephalic side of the cochlear root. Having 
 entered the bulb, the axones divide, after the manner of dorsal root fibres, 
 into an ascending and a descending branch, which find their nuclei of termina- 
 tion in 3 (1) the nucleus nervi vestibuli spinalis (the radix descendens); (2) in 
 
 1 Barker : The Nervous System and its Constituent Neurones, 1899, pp. 544-555. 
 
 2 Held: Archivfiir Physiologie, Anat. Abth., Leipzig, 1893. 
 
 3 Barker: The Nervous System and its Constituent Neurones, 1899, p. 627, et seq. 
 
238 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 the nucleus nervi vestibuli medialis ; (3) the nucleus nervi vestibuli lateralis 
 (nucleus of Deiters) ; and (4) the nucleus nervi vestibuli superior. Finally, 
 among other connections are to be specially mentioned those with the cere- 
 bellum (nuclei fastigii, nucleus dentatus, and the cerebellar cortex). Nothing 
 definite is known concerning the pathways by which the impulses entering 
 over the radix vestibularis reach the cortex. 
 
 Nervus Trigeminus (Fifth Nerve). The neurones of this nerve have their 
 cell-bodies located in the ganglion semilunari (Gasseri), and the peripheral 
 axones act as the nerves of common sensation for the skin of the face and 
 tongue and the mucous membranes of the mouth. The central axones which 
 branch on reaching the bulb send their shorter divisions cephalad for a little 
 distance, and the longer caudad, in both cases finding cells of reception in 
 the region of the substantia gelatinosa, and in the latter instance extending 
 caudad at least as far as the first segment of the spinal cord. Possibly, one 
 set of neurones of the fifth passes directly into the cerebellum. The path- 
 way from the nucleus of termination to the cortex has not been determined. 
 
 Second Nerve, Optic. As has long been recognized, the optic nerve, so 
 called, is a cerebral tract morphologically equivalent to such tracts as con- 
 nect any portion of the cerebral cortex with a primary centre, the retina 
 being in part the representative of the cerebrum ; and the pulvinares, the 
 quadrigernina, and geniculata externa being the primary centres. 
 
 At the chiasma where the two optic nerves come together their fibres 
 intermingle, and then emerge as the optic tracts, which contain not only the 
 fibres connected with the retina, but others added from the superposed parts 
 of the brain, and forming the commissures of Meynert and von Gudden. 
 
 In the rabbit it was shown by von Gudden 1 that in the chiasma the 
 majority of the fibres forming one optic nerve pass to the tract of the 
 opposite side, but that a portion of the fibres remains in the tract of the 
 same side. 
 
 This was inferred because removal of one eyeball caused in young rab- 
 bits a degeneration in the associated optic nerve and also in both optic tracts 
 most marked, however, in the tract of the side opposite to the lesion. Con- 
 versely, the section of one optic tract causes a degeneration in both optic 
 nerves, the nerve of the side opposite to the lesion being most affected, and a 
 smaller degeneration appearing in the nerve of the same side (see Fig. 98). 
 
 In the fish, amphibia, reptiles, and birds except the owls 2 the decussa- 
 tion appears to be complete. 3 For the partial decussation in the owls the 
 evidence is physiological. This distribution of the optic fibres was associated 
 by von Gudden with the position of the eyes in the head. The extreme 
 lateral position of the eyes as it occurs in the lower mammals permits of but 
 little combination of the two visual fields ; whereas the position in man, in a 
 
 1 von Gudden : Gesammelte und hinterlassene Abhandlunyen, Wiesbaden, 1889. 
 ' 2 Fcrrier : The Croonian Lectures on Cerebral Localization, London, 1890, p. 70. 
 3 Singer and Munzer : Denkschriften der math.-naturwiss. Olasse der kais. Akademie der Wissen- 
 schaften, 1888, Bd. iv. 
 
CENTRAL NERVOUS SYSTEM. 
 
 239 
 
 frontal plane, permits a combination of the fields to a much greater degree. 
 It was in accordance with this principle that partial dccussntioii of these 
 nerves was anticipated by von Gudden in the owl, although the histological 
 evidence for it was not obtained by him. 
 
 The most recent researches on mammals have so increased the number in 
 which a partial decussation occurs that we are justified in regarding this 
 
 / 
 
 
 
 f// 
 
 Uncrossed. /' / 
 
 [Crossed. 
 
 \ 
 
 C.Q 
 
 / 
 
 FIG. 98. Illustrating the relations 01 the afferent fibres in the optic nerve. The crossed fibres are 
 indicated by solid lines, the uncrossed fibres by broken lines : N, nasal side of the right eye ; T, temporal 
 side of the same ; G. E, geniculatum externum ; P. pulvinar ; C. Q. quadrigeminum anterius. 
 
 arrangement as the rule, although the proportion of the uncrossed fibres is 
 small in those mammals in which the eyes are placed laterally. 1 
 
 In man the evidence from degeneration in the optic nerve points to the 
 presence of a crossed and an uncrossed bundle of fibres in each optic nerve, 
 the uncrossed being much the smaller of the two bundles. The contrary 
 
 1 Cajal : Die Structur des Chiasma opticum nebst einer Allgemeinen Theorie der Kreuzung der 
 Nervenbdhnen, Leipzig, 1899. 
 
240 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 view of complete decussation has been maintained by Michel. 1 The central 
 ends of the afferent optic fibres forming an optic tract are for the most part 
 distributed between the anterior quadrigeminum, the geniculatum extern urn, 
 and the pulvinar of the same side. By central cells located in these latter 
 structures the pathway is continued to the occipital cortex of the hemisphere 
 of the same side, their axones passing in the occipital end of the internal 
 capsule and forming the optic radiation. It must be remembered, however, 
 that between the cortex and the primary centres, and again between these 
 centres and the retina, there are pathways conducting from the cortex to the 
 primary centres, and also/rom the primary centres to the retina. 2 
 
 As the result of partial decussation it will be seen that the relations of the 
 two retinae to the cortex is this : The nasal or crossed bundle of the contra- 
 lateral retina and the temporal or uncrossed bundle of the retina of the same 
 side come together in the optic tract of one side and are associated with the 
 occipital lobe of that side. Hence it would appear that hemianopsia or 
 blindness in the corresponding halves of the two eyes following a lesion of 
 the optic pathway anywhere behind the chiasm would be, in some measure, 
 explained by this anatomical arrangement. If strictly interpreted, an ap- 
 proximately equal number of fibres would be expected for each half of the 
 retina. Such, however, has not been established as the relation between the 
 areas of the bundles. It is to be added, nevertheless, that anatomical arrange- 
 ments such as decussations are probably open to wide individual variations, 
 and hence that many more observations are required before we can say what 
 is the usual relation between these two bundles. 
 
 With a view to determining the exact location of the cortical centres in 
 man, many observations have been made. The cuneus and immediately sur- 
 rounding parts of the cortex are those most concerned. Henschen 3 indicates 
 the calcarine fissure and its immediate neighborhood as the most important 
 locality. Observations on the arrest in the development of the cortex due to 
 early blindness following destruction of the retina in the case of the blind 
 deaf-mute Laura Bridgman, show the entire cuneus to be the central and 
 fundamental portion, while the associated portions extend some distance on- 
 to the convex surface of the hemisphere. 4 
 
 First Nerve. Comparative anatomy indicates that the parts of the enceph- 
 alon mediating the sense of smell are most closely connected with the cerebral 
 hemispheres, in the sense that phylogenetically the first development of the 
 cortex was in connection with the central terminations of the olfactory tracts. 5 
 It happens in man, however, that although the cerebral hemispheres are pro- 
 portionately much more massive than in the lower mammals, yet the olfactory 
 bulbs and tracts are at the same time but poorly developed. The pathway 
 
 1 K olliker's Festschrift, Wiirtsburg, 1887. 
 
 2 von Monakow: Archivfur Psychiatrie, 1890, Bd. xx. H. 3. 
 
 3 Henschen : Klinische und anatomische Beitrdqe zur Pathologic des Gehirns, Upsala, 1890-92.. 
 * Donaldson : American Journal of Psychology, 1892, vol. iv. No. 4. 
 
 5 Sir William Turner : Journal of Anatomy, 1890; Edinger: Anatomischer Anzeiger, 1893. 
 
CENTRAL NERVOUS SYSTEM. 241 
 
 of the olfactory impulses is from the olfactory area in the nose to the olfactory 
 bulb of the same side, thence via the olfactory tract to its termination in front 
 of the anterior perforated space, one branch of the tract passing directly into 
 the substance of the gyrus fornicatus at this point, and the other going into 
 the more lateral portion represented in man by the temporal end of the gyrus 
 hippocampi. The cortical areas, together with the olfactory lobe and tract, 
 form the rhineucephalon of the comparative anatomists. It has been shown, 
 nevertheless, by Hill l that in anosmic mammals the fascia dentata alone 
 varies with the development of the olfactory apparatus. The experimental 
 pathological evidence is very meagre in relation to these nerves, but, on the 
 other hand, the anatomical evidence is of the best. 2 
 
 D. LOCALIZATION OF CELL-GROUPS IN THE CEREBRAL CORTEX. 
 
 The foregoing section has brought to light the fact that groups of incom- 
 ing impulses find their way to the cerebral cortex. The significance of this 
 is evident only when in response to those impulses arriving at the cortex 
 others leave it, and finally affect some expressive tissue or instrument by the 
 aid of which we can interpret them. Since the cerebral hemispheres with 
 their cortex become increasingly developed as we pass up the mammalian 
 series, it naturally follows that the pathways connecting the cortex with the 
 lower parts of the system are correspondingly increased. Using the reactions 
 of the expressive tissues as a guide, it is our present purpose to trace the 
 impulses in those cases in which the cortex forms part of the path. We turn, 
 therefore, to the study of those parts of the cerebral cortex the direct stimu- 
 lation of which produces impulses that pass to cell-groups lying more or less 
 caudad in the central system. 
 
 Earlier Observations. It was demonstrated by Fritsch and Hitzig in 
 1870 3 that if a constant current is applied to the surface of the dog's cere- 
 brum, it is possible, by interrupting it, to obtain movements of the limbs 
 and face when the electrodes are placed on the parts of the cerebral cortex 
 about the sulcus cruciatus. The reaction varies according to the place of 
 stimulation, a constant relation subsisting between the two. From this time 
 on, active investigations of the relations thus suggested have been pursued, 
 both by stimulating small areas in the cortex of various animals, including 
 the monkey and man, and by the removal of various parts of the cerebral hemi- 
 spheres and cortex, together with the study of the effects of pathological lesions 
 in man. The results following removal of the parts are complicated by the 
 transitory effects of the lesion, and can best be treated by themselves later on. 
 The results following the stimulation of the cortex are the simplest, and will 
 next be described. 
 
 Stimulation of the Cortex. The common method of experiment is to 
 
 1 Philosophical Transactions of the Royal Society, 1893, vol. clxxxiv. 
 
 2 For a description of the very complicated pathways associating the olfactory bulb with 
 the other portions of the cerebrum, the reader is referred to Barker's The Nenous System, 1899. 
 
 3 Archivfur Anatomie und Physioloyie, 1870. 
 
 VOL. II. 16 
 
242 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 apply the faradic current by means of fine but blunt electrodes, the ends of 
 which are but two or three millimeters apart, to the exposed surface of the 
 cerebral hemispheres, the pia being undisturbed. Rabbits, dogs, and monkeys 
 have been the animals most commonly studied. 
 
 If the current be slight, its application for one or more seconds causes a 
 response in the shape of movements of muscles, which are thrown into co-ordi- 
 nated contraction. The contraction continues for some time after the stim- 
 ulus has been removed. When the stimulus is very strong, instead of a lim- 
 ited and co-ordinated response, there may be a widespread contraction of 
 
 FIG. 99. Brain of the macaque monkey, showing the sensory and motor areas. In the sensory region 
 the name of the sensation is over the locality most closely associated with the corresponding sense-organ ; 
 in the motor region the name of the part is written over the portion of the cortex which controls it. The 
 upper figure gives a lateral view of the hemisphere, and the lower a dorsal view (Beevor and Horsley). 
 
 many muscles, resembling an epileptic convulsion. This, however, occurs 
 more commonly in the lower than in the higher mammals. On the other 
 hand, the irritability of the cortex is easily reduced, so that it becomes irre- 
 sponsive, and often immediately after the first exposure of the brain there is 
 a time during which no response can be obtained. 
 
 Deferring for a moment the evidence by which the sensory characters of 
 the several areas have been established, and also the arrangements within the 
 cortex by which any group of muscles can be made to respond to stimuli 
 arriving at any sensory area, we shall follow out the distribution of those 
 
CENTRAL NERVOUS SYSTEM. 
 
 243 
 
 cortical cells which, on direct stimulation, cause contractions of the skeletal 
 muscles. 
 
 The results here presented were obtained from the electrical stimulation 
 of the monkey's brain by Beevor and Horsley l (see Figs. 99, 100). These 
 experimenters explored the exposed surface of the hemisphere with the elec- 
 trodes, moving them two millimeters at a time, and at each point noting the 
 muscle-group first thrown into contraction. 
 
 As the result of many observations on the monkey, it is possible to map 
 out the cerebral cortex in the following way : The surface of the hemispheres 
 is divided into regions (motor and sensory regions), which are the largest 
 subdivisions. These are subdivided into areas for the muscle-groups belong- 
 ing to different members of the body arms, head, trunk, etc. as well as 
 those areas within which all the impulses from a given sense-organ reach the 
 
 PoF. 
 
 FIG. 100. Mesial surface of the brain (monkey). The localization of motor functions is indicated 
 along the shaded portion of the marginal gyrus. The location of the visual area is indicated at the tip of 
 the occipital lobe, and the location of the olfactory area at the tip of the temporal (Horsley). 
 
 cortex. The areas in turn are subdivided into centres, comprising the groups 
 of cells, which, for example, control the smaller masses of muscle belonging 
 to a given segment of a limb, or in the visual area constitute those cells espe- 
 cially connected with one part of the retina. There is thus a motor region, 
 the stimulation of which gives rise to the more evident bodily movements. 
 Within this are several subdivisions, the stimulation of one of which is fol- 
 lowed by movements of groups of muscles for instance, those controlling the 
 arm and within such an area in turn come the smaller centres, or those the 
 stimulation of which is first followed by movements at one joint only. 
 
 The physiological characters of these cortical motor centres have been 
 determined by the following observations : 
 
 If a vertical incision be carried around such a centre so as to isolate it 
 from the other parts of the cortex, the characteristic reactions still follow the 
 stimulation of it, indicating that the special effect can be produced by the 
 1 Beevor and Horsley : Philosophical Transactions of the Royal Society, 1888-90. 
 
244 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 passage of impulses from the point of stimulation toward the infracortical 
 structures. If, in addition, a cut be made below the cortex and parallel with 
 its surface, then stimulation of the cortex above this section is ineffective, thus 
 indicating that the impulses pass from the cortex directly into the substance 
 of the hemisphere along certain nerve-tracts, which by this operation were 
 sectioned. Further, if the bit of cortex thus separated from the underlying 
 white substance be removed and the faradic current be applied to the white 
 substance beneath, a reaction of the same type and involving the same mus- 
 cles can be obtained, although it differs from that to be gotten from the cor- 
 tex itself, in the first place by being less co-ordinated, in the second by con- 
 tinuing only so long as the stimulus lasts, and in the third place by giving 
 rise to less intense electrical changes connected with the passing impulse. 
 By careful exploration the bundle of fibres which is thus picked out can be 
 followed, as the brain substance is cut away, through the internal capsule and 
 the cerebral peduncles. 
 
 These facts taken together lead to the conclusion that when the cortex is 
 stimulated the impulses concerned in producing the muscular contractions 
 traverse cell-bodies at the point of stimulation, and are transmitted thence 
 through the underlying fibres. We shall see later that this direct course 
 probably does not represent the sole pathway for these impulses. 
 
 Course of the Descending 1 Impulses. The course of the impulses is 
 next inferred from the relation between the removal of different parts of the 
 cortex and the consequent secondary degenerations throughout the length of 
 the central nervous system. When the part of the cortex removed is taken 
 from the motor area then the degeneration occurs in the internal capsule and 
 in the callosum. The path of the fibres forming outgrowths of the cortical 
 cells can be followed thence through the crusta and pyramids to the spinal 
 cord. 
 
 After removal of the motor region of one cerebral hemisphere the degen- 
 eration is mainly in the internal capsule and crusta of the same side, though 
 by way of fibres crossing in the callosum it may be traced to the other side 
 also. At the decussation of the pyramids the fibres occupying the internal 
 capsule of the same side as the lesion for the most part cross the middle line. 
 The portion which remains uncrossed passes as the direct pyramidal tract of 
 the ventral columns in man, while the crossed bundle, which is much the 
 larger, lies in the dorsolateral field of the lateral column, forming the crossed 
 pyramidal tract. Since the observations of Pitres 1 in 1881-82 evidence has 
 been accumulating to show that in man a lesion of the motor cortex of one 
 cerebral hemisphere is followed by a degeneration of the crossed pyramidal 
 tract on both sides of the cord. Of course, the degeneration in the hetero- 
 lateral tract is much the larger of the two. That the fibres degenerating in 
 the homolateral tract remain on the same side throughout their entire course 
 is shown by the physiological experiments of Wertheimer and Lapage 2 on 
 
 1 ProgrZs medicale, Paris, 1882, x. 528. 
 
 2 Archives de Physiologic, 1897, No. 1, p. 168. 
 
CENTRAL NERVOUS SYSTEM. 
 
 245 
 
 dogs, and by the studies of Mellus l on secondary degenerations occurring in 
 the cord after very limited lesions of the motor cortex of monkeys. 
 
 The direct pyramidal tracts are well marked only in man. They usually 
 disappear in the mid-thoracic region, having entered the gray substance by 
 way of the ventral commissure, in which they undergo decussation. The 
 crossed pyramidal tract shows the greatest diminution in area after passing 
 caudad of the cervical and lumbar enlargements respectively, and hence it is 
 inferred that the pyramidal Hbres largely terminate at these levels of the 
 cord. 
 
 FIG. 101. Schema of the projection-fibres within the brain (Starr) ; lateral view of the internal cap- 
 sule : A, tract from the frontal gyri to the pons nuclei, and so to the cerebellum ; B, motor tract ; C, sen- 
 sory tract for touch (separated from B for the sake of clearness in the schema) ; D, visual tract ; E, audi- 
 tory tract ; F, G, H, superior, middle, and inferior cerebellar peduncles ; J, fibres between the auditory 
 nucleus and the inferior quadrigeminal body ; K, motor decussation in the bulb; Vt, fourth ventricle. 
 The numerals refer to the cranial nerves. The sensory radiations v are seen to be massed toward the 
 occipital end of the hemisphere. 
 
 Sherrington has put forward the view that the pyramidal fibres recross in 
 the cord, these recrossing fibres being derived in large part from a division 
 of the pyramidal fibres into two branches, one of which may cross to the 
 opposite side of the cord, while the other continues its first course. Such 
 dividing fibres he designates as " geminal fibres ;" and the number of them 
 is by no means small. 
 
 The observations of Sherrington were made on monkeys (Macacus) and 
 
 1 Proceedings of the Royal Society, London, 1894 and 1895. 
 
246 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 dogs, and probably the arrangement of these fibres in man is similar. The 
 observations are particularly significant as gi\ r ing another anatomical basis 
 for the control of the movements in both halves of the body from each 
 cerebral hemisphere. 
 
 The continuous degeneration, coupled with the histological evidence for 
 the absence of intervening nerve-cells, indicates that the cell-bodies in the 
 cortex have axones that extend all the way to the cell-groups of the spinal 
 cord, even as far as the sacral region. The usual picture of the final connec- 
 tions of the pyramidal fibres shows the collaterals as coming into contact 
 with the large cells in the ventral horns of the cord. On the ground of 
 recent experiments on monkeys and cats, Schafer 1 denies the existence of 
 such a direct association of the two sets of elements. He finds that a large 
 mass of collaterals which degenerate when the pyramidal tract is interrupted 
 end about the large cells in the column of Clarke. 
 
 Returning to consider the arrangement of these cells in the cortex, we find 
 that the axones of one group of these cortical cells pass to the cell-groups in 
 the cervical enlargement, while those from others pass to the groups in the 
 lumbar enlargement. It thus happens that if the spinal cord be cut across 
 in the middle of the thoracic region, and then the leg area (see Fig. 78) be 
 stimulated, an electrometer applied to the cut end of the cord will show the 
 passage of nerve-impulses, because the electrometer is applied to a tract of 
 fibres on their way to the lumbar enlargement, and the fibres originate from 
 cortical cells within the region stimulated. 
 
 When, however, the cortical stimulus is made in the arm-area, the electrom- 
 eter being applied as before, no electric change occurs, for the axones of 
 the cells in the arm terminate in the part of the cord containing the cell- 
 groups which control the muscles of the arm, and these groups all lie cephalad 
 to the point of section of the cord. It is evident, therefore, that the arrange- 
 ment is a comparatively simple one namely, an extension of the axones of 
 the several groups of cortical cells from the different areas for the leg, arm, 
 face, etc., to the axial cell-groups which control the muscles of these parts, 
 and which are situated in the cord. 
 
 The cortical cells in the motor region belong to the group of central cells 
 i. e., their axones never leave the central system and hence they are en- 
 gaged in distributing impulses within it. To the axial cell-groups in the cord 
 they bring impulses, and therefore, from the standpoint of these latter, may 
 be considered as afferent, Avhereas, owing to the fact that they carry impulses 
 away from the cortex, they are sometimes called efferent. Just how these 
 two sets, the cortical and the cord elements, are numerically related still 
 requires to be worked out. According to one estimate, there are for the arm, 
 trunk, and leg, in man, 79,111 pyramidal fibres in each half of the cord, or 
 158,222 in the entire cord. 2 The number of fibres in the pyramidal tracts 
 indicates* that there certainly is not one fibre for each cell in the axial cell- 
 
 1 Journal, of Physiology, vols. xxiii. and xxiv.; Proceedings of the Physiological Society. 
 3 Blocq et Ozanoff: Gaz. des HopiL, 1892. 
 
CENTRAL NERVOUS SYSTEM. 247 
 
 groups, because the number of pyramidal fibres is very much less than is the 
 number of cells which they control. This discrepancy is in some measure 
 relieved by the formation of "geminal" fibres already described. Moreover, 
 the branching of the pyramidal fibres near their termination is very probable, 
 and the most plausible view at present is that each pyramidal fibre by means 
 of its collaterals controls, perhaps indirectly, a considerable number of cord 
 cells, and probably the cells controlled by any one fibre form a more or less 
 compact group. 
 
 Mapping- of the Cortex. Having sketched the relations of the pyramidal 
 cells forming the motor region of the cerebral cortex to the parts lying below, 
 we turn to study the arrangement, size, subdivisions, and comparative anatomy 
 of this region, and then to examine the relation of it to the other parts of the 
 cortex. The observations here quoted are those on the monkey only. 
 
 On glancing at Fig. 99 it is evident, first, that the areas for the face and 
 leg are widely separated from each other, that the arm-area lies between 
 them, and that the area for the trunk, though less schematically placed, is 
 located between that for the arm and leg. This arrangement is more typi- 
 cally represented on the mesial (Fig. 100) than on the convex surface of the 
 hemisphere, and in the former locality the serial order of the cortical areas 
 corresponds with the order of the muscle-groups which they control. 
 
 The Size of the Cortical Areas. Evidently there is no direct relation 
 between the extent of a cortical area and the mass of muscles which it con- 
 trols. Certainly in man the mass of muscles in the leg is three times greater 
 than that in the arm, and this latter many times greater than that of the face 
 and head ; yet it is for the last area that the greatest cortical extent is found. 
 Mass of muscle and extent of cortical area do not therefore go together. 
 
 When the movements effected by the muscles represented in these several 
 areas are considered, we find that such movements become more complex and 
 more accurate as we approach the head, and it therefore accords with the facts 
 to consider the extent of the motor areas as correlated with the refinement 
 of the movements which they control a relation which may depend even 
 more on the multiplication of the pathways bringing in impulses than on 
 those which send them out. 
 
 Subdivision of Areas. The areas which have been described are further 
 subdivided, the subdivisions in the arm-area being the clearest. Here it is 
 found that the stimulation of the upper part of the arm -area gives rise to 
 movements which start at the shoulder, while stimulation at the lower part 
 of this area gives rise to movements first involving the fingers, and especially 
 the thumb. The centres from which these several reactions may be obtained 
 occupy, as Fig. 99 shows, narrow fields across the cortex in a fronto-occipital 
 direction. Moreover, the centre for the most proximal joint of the arm is 
 farthest removed from that for the most distal, while the intermediate joints 
 are represented by their several centres lying in regular order between these 
 two. A similar arrangement appears in the subdivisions of the cortex con- 
 trolling the leg, and in the face-area as well. 
 
248 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 FIG. 102. Horizontal section of the human cere- 
 brum, showing the internal capsule on the left 
 side : F, frontal region ; 6, knee of the capsule ; 
 NC, NC, caudate nucleus ; NL, lenticular nucleus ; 
 O, occipital lobe ; TO, thalamus ; X, X, lateral ven- 
 tricle. In the internal capsule the letters indicate 
 the probable position of the bundles of fibres which 
 upon stimulation give rise to movements of the 
 parts named or which convey special sets of in- 
 coming impulses : E, eyes ; H, head ; T, tongue ; M, 
 mouth; L, shoulder; B, elbow; D, digits ; A, abdo- 
 men ; P, hip ; K, knee ; U, toes ; S, temporo-occip- 
 ital tract ; OC, fibres to the occipital lobe ; OP, optic 
 radiation (based on Horsley). 
 
 leg-area. In the orang-utang, 2 and to 
 
 1 Mann : Journal of Anatomy and Physiolo( 
 
 2 Beevor and Horsley : Proceedings of the 
 
 Interpreting these facts in the terms 
 of nerve-cells and their arrangement, 
 it appears that in the shoulder-centre 
 the axones of the cortical cells that 
 discharge downward affect predomi- 
 nantly the efferent cell-groups which 
 in the spinal cord directly control the 
 muscles of the shoulder, and that a 
 similar arrangement obtains for the 
 other centres in this region with the 
 corresponding cell-groups in the cord. 
 The stimulation of the different por- 
 tions of the internal capsule where it 
 is composed of bundles of fibres com- 
 ing from the motor region shows (ob- 
 servations on orang-utang) that the 
 fibres running to the several lower 
 centres are there aggregated and 
 arranged in the same order as the 
 cortical centres from which they arise 
 (see Fig. 102). 
 
 Separateness of Areas and Cen- 
 tres. As we ascend in the mammalian 
 series there is an increase in the per- 
 fection with which cells forming the 
 several centres are segregated, though 
 the areas in the different orders tend 
 to hold the same relative positions. 1 
 
 Figs. 103, 104 give the localizations 
 obtained in the rabbit's brain by stim- 
 ulation (Mann). The various areas 
 occupy a large proportion of the cortex, 
 and in some cases come very close to- 
 gether, so that they are not easily sepa- 
 rated by experiment. 
 
 In the lower monkeys (Macacus 
 sinicus) these cell-groups are segre- 
 gated, so that those associated with the 
 cervical portion of the cord and form- 
 ing the arm-area are much more to- 
 gether and quite separate from those 
 associated with the lumbar region, the 
 a greater extent in man, a further sepa- 
 
 gy, 1895, vol. xxx. 
 
 Royal Society, London, 1890-91, vol. xlviii. 
 
CENTRAL NERVOUS SYSTEM. 
 
 249 
 
 FIG. 103. Rabbit's brain, dorsal view. The 
 areas indicated are those the stimulation of which 
 causes a movement of the parts named (Mann). 
 
 FIG. 104. Rabbit's brain, lateral view. The 
 areas indicated are those the stimulation of which 
 causes a movement of the parts named (Mann). 
 
 FIG. 105. Lateral view of the left hemisphere of an orang-utang, showing the motor area about the cen- 
 tral fissure (Beevor and Horsley). 
 
 FIG. 106. Lateral view of a left human hemisphere, showing the motor areas in man. The schema 
 is based on the observations on the monkey, on pathological records (human), and on direct experiments 
 on man. It is to be remembered that in the human brain the excitable localities are surrounded by 
 rather extensive areas not directly excitable (Dana). 
 
 ration occurs, so that these centres come to be surrounded by parts of the cortex 
 from which no response can be obtained upon direct stimulation (see Fig. 105). 
 
250 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 By a few direct experiments and by many pathological observations some- 
 thing is known of the motor centres in the human cerebral cortex. When 
 
 FIG. 107. Mesial view of a human hemisphere, showing motor areas. 
 
 Fig. 106. 
 
 Formed in the same way as 
 
 the results are plotted they give a distribution such as is shown in Fig. 106. 
 
 At the same time all such figures are largely based on results obtained from 
 
 the monkey. It is here seen that 
 the two central gyri are the princi- 
 pal seat of these areas, and that it 
 is only along the great longitudinal 
 fissure separating the hemispheres 
 that the motor areas extend beyond 
 this limit in a cephalo-caudad direc- 
 tion. Perhaps the relation most 
 worthy of remark is the compara- 
 tively small fraction of the cortex 
 concerned with the direct control 
 of the spinal cord cells. The motor 
 areas in man are elaborated not so 
 much by the increase in the number 
 of the cells controlling the lower 
 centres, as by an increase in the 
 number of those cells under the in- 
 fluence of which these areas react. 
 According to the estimates of Miss 
 Thompson, 1 there are in man 9200 
 millions of cells in the entire cere- 
 bral cortex of both hemispheres. 
 In the motor region about the cen- 
 tral fissures there appear to be only 
 159,600 cells concerned in the pro- 
 duction of pyramidal fibres going 
 
 to the cord. The relation of the areas in a frontal section is shown in Fig 108. 
 Multiple Control from the Cortex. It has been found that stimulation 
 1 Helen B. Thompson : Journal of Comparative Neurology, 1899, vol. ix. No. 2. 
 
 FIG. 108. Frontal section of human cerebrum on 
 the left side. The fibres forming the internal capsule 
 
 ( ), the callosum ( ), and the anterior 
 
 commissure (. . . ) have been indicated : T, cor- 
 tical area for the trunk ; L, cortical area for the leg; A, 
 cortical area for the arm ; F, cortical area for the face ; 
 A, anterior commissure ; C, callosum ; CO, optic chias- 
 ma ; NC, caudate nucleus ; NL, lenticular nucleus ; R, 
 fornix ; TO, thalamus ; X, lateral ventricle. 
 
CENTRAL NERVOUS SYSTEM. 251 
 
 of the cortex in the region of the frontal lobes marked " eye " (Fig. 99) was 
 followed by movements of the eye. Schafer l has shown that very precise 
 movements of the eye also follow the stimulation of the temporal and 
 various parts of the occipital cortex. Since the efferent fibres which control 
 the muscles concerned start from the cell-groups forming the nuclei of the 
 third, fourth, and sixth cranial nerves, it would appear most probable that in 
 both parts of the cortex there are located cells the axones of which pass to 
 those groups and are capable of exciting them. An alternative hypothesis 
 namely, that the cortical impulse always travels first to the cortical cells in 
 the frontal lobe and thence, by way of them, to the efferent cell-groups was 
 at one time considered, for the latent period of contraction of the eye muscles 
 was less by several hundredths of a second when the stimulus was applied in 
 the frontal region than when applied elsewhere. The experiments of Schafer 
 show, however, that when the occipital and frontal lobes are separated from 
 one another by a section severing all the association-fibres the reactions can 
 still be obtained by stimulation in the former locality showing that the con- 
 nections of the two cortical areas with the cell-groups controlling the muscles 
 of the eye are independent of each other. 
 
 This instance of the direct control of the same efferent cell-groups from dif- 
 ferent areas of the cortex is analogous to the control of efferent cell-groups in 
 the spinal cord, either by impulses coming down from the cerebrum or by 
 those entering the cord through the dorsal roots, and the instance here cited 
 is typical of a general arrangement. 
 
 Cortical Control Crossed. Where the stimulation of the cerebral cortex 
 causes a response on one side only, that response is on the side opposite to 
 the stimulated hemisphere. It sometimes happens, however, that two groups 
 of symmetrically placed muscles both respond to the stimulus applied to one 
 hemisphere only ; but these cases the conjugate movements of the eyes, 
 movements of the jaw muscles or those of the larynx usually depend on the 
 response of muscles which are naturally contracted together. 
 
 This last reaction must be determined by the arrangement of the fibres in 
 the cord, since in lower mammals (dog and rabbit, for example) it is not 
 seriously disturbed by the removal of one hemisphere. 
 
 Here should be added the very important observations of Sherrington 2 
 already mentioned on p. 224, which show that a stimulus which applied to 
 the cortex will cause one set of muscles, the flexors of the arm, for example, 
 to contract, causes at the same time a relaxation of the antagonistic muscles, 
 thus rendering co-ordination possible in the movements of the limb. 
 
 Course of Impulses Leaving the Cortex. In the higher mammals, as 
 well as in man, it is by way of the pyramidal fibres that impulses travel from 
 the cortex to the efferent cell-groups of the cord. The pyramidal tracts by 
 definition form, in part of their course, the bundles of fibres lying on the ven- 
 tral aspect of the bulb, caudad to the pons, ventrad to the trapezium, and 
 
 1 Proceedings of the Royal Society, 1888, vol. xliii. 
 
 2 Sherrington : Journal of Physiology, 1897-1898, vol. xxii. 
 
252 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 between the olivary bodies. According to Spitzka, 1 these bundles are absent 
 in the case of the elephant and porpoise, a condition which is correlated with 
 the slight differentiation of the limbs, which are not modified either for fine 
 movements or for tactile purposes. It has been pointed out, too, that removal 
 of a hemisphere causes in the dog and most rodents a degeneration of other 
 parts of the cord (dorsal columns) than those occupied by the pyramidal tracts 
 in man. 2 The fibres passing from the cortex to the efferent cell-groups in the 
 cord do not, therefore, hold exactly the same position in various mammals. 
 
 Size of Pyramidal Tracts. It has been clearly shown that if the cross- 
 sections of the cords of the dog, monkey, and man be drawn of the same size, 
 the pyramidal fibres being indicated, then the area of this bundle is propor- 
 tionately greatest in man and least in the dog, the monkey being inter- 
 mediate in this respect. The relations thus indicated are evident namely, 
 that the number of fibres controlling the cell-groups in man is the largest, 
 and also is much larger than that in the lower animals. 
 
 The relative areas of the pyramidal tract at corresponding levels, the 
 area of the entire cord being taken as 100 per cent., are given by v. Len- 
 hossek 3 for the following animals : 
 
 Mouse 1.14 per cent. 
 
 Guinea-pig 3.0 " 
 
 Kabbit 5.3 " 
 
 Cat 7.76 " 
 
 Man 11.87 " 
 
 This relation is to be carefully noted, for with it is correlated the degree 
 of the disturbances in the reactions of the entire nervous system following 
 removal of parts of the cerebrum, the effect being slight when the cerebrum 
 is connected with the cord by a small number of fibres, and serious when the 
 connection is by many fibres, as in the case of man and the highest mammals. 
 
 E. LOCALIZATION IN THE CEREBRAL CORTEX OP THE CELL-GROUPS 
 
 RECEIVING THE AFFERENT IMPULSES. 
 
 Sensory Regions. If an attempt is made to unify the construction of 
 the entire cortex by bringing the motor and sensory areas under a common law, 
 it must be based on the fact that the system of axones bringing impulses to 
 the motor region forms part of the pathway for conducting the afferent im- 
 pulses from the skin and muscles back to some organ controlled by the 
 efferent nerves. To Munk 4 is due the credit of having from the first looked 
 upon the responsive cortex as marked off into areas within which certain 
 groups of these fibres terminate, so that apart from the sensory areas named 
 from the special senses, he calls the area which controls the skeletal muscles 
 the " Fuhlsphare " or body-sense area, on the assumption that in it end the 
 
 1 Journal of Comparative Medicine and Surgery, 1886, vol. vii. 
 
 a von Lenhossek : Anatomischer Anzeiger, 1889. 
 
 3 Diefeiner Ban des Nervensystems im Lichte neuester Forschungen, Basel, 1893. 
 
 * Ueber die Functionen der Grosshirnrinde, 1881. 
 
CENTRAL NERVOUS SYSTEM. 253 
 
 fibres bringing in impulses which arise through the stimulation of the skin and 
 muscles. It has been suggested, to be sure, that separate localities form the 
 seat for the dermal and muscular sensations. Ferrier indicated the limbic 
 lobe, especially the hippocampal gyrus, while Horsley and Schiifer argued 
 for the gyrus fornicatus. At present, the weight of evidence is in favor of 
 the location of the centres for dermal and muscular sensations in the central 
 gyri, a part just caudad to and a part overlapping the area stimulation of 
 which causes the muscles of the trunk and limbs to contract. Both in 
 monkeys and in man defects in sensation are not permanent after limited 
 lesions of the cortex, but, as suggested by Mott, the wide distribution of the 
 incoming impulses would explain this result. 
 
 Thus the entire portion of the cortex to which a definite function can be 
 assigned must be looked upon as containing fibres which bring impulses into 
 it, and cell-bodies which by their discharge send impulses to other divisions 
 of the central system as well as to other parts of the cortex itself. All parts 
 of the cortex having assigned functions give rise on stimulation to move- 
 ments ; but in the case of the sensory areas, so called, they involve the con- 
 tractions of only those muscles controlling the external sense organ, as the 
 eyeball, external ear, tongue, and nostrils. 1 Though physiologically impor- 
 tant, and in the case of the eye reaching a high degree of refinement, they 
 are quantitatively very insignificant when compared with the responses to be 
 obtained from stimulating the '' motor region/' from which contractions of 
 the larger skeletal muscles are obtained. Hence the usual terms " sensory " 
 and " motor " do not completely characterize the corresponding regions, 
 though they emphasize their most striking features. 
 
 Determination of the Sensory Areas. Using as a guide the appearance 
 of the medullary sheaths upon the projection-fibres of the cerebral cortex of 
 man during the last months of foetal life and shortly after birth, Flechsig 2 
 has been able to outline the sensory areas in the cortex with great clearness. 
 
 The illustrations from Flechsig (Figs. 109, 110) show the parts of the 
 brain where the projection-fibres can be determined at a time when these 
 fibres constitute all or almost all the medullated fibres connecting the cortex 
 with the stem and basal ganglia. By thus marking out in color on the 
 developing cortex the portions concerned, there are seen to be four main 
 areas : First, the area connected with the olfactory tract (olfactory area), 
 involving the uncinate gyrus, the gyrus hippocampi, and the part of the gyrus 
 fornicatus nearest the callosum. Second, the area connected with the optic 
 radiation (visual area), where the fibres in question are most abundant about 
 the calcarine fissure. They appear, however, all through the cuneus and 
 extend to the cortex which surrounds it on the ventral and lateral aspects of 
 the occipital lobe. Third, we have (auditory area) the portion of the cortex 
 which covers the transverse gyri in the Sylvian fissure and the first temporal 
 gyrus where the former join it. This area is occupied by the projection- 
 
 1 Ferrier : Functions of the Brain, 1876. 
 "Flechsig: Gehirn und Seele, Leipzig, 1896. 
 
254 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 fibres which convey the impulses arriving over the auditory nerve (the coch- 
 lear branch of the eighth). Finally the fourth area is seen (area for " body- 
 
 FIG. 109. The colored portion about the cuneus, especially that more deeply colored about the cal- 
 carine fissure, shows the msual area as seen from the mesial surface. The portion comprising the deeply 
 colored tip of the hippocampal gyrus, the dorsal portion of the hippocampal gyrus, and the edge of the 
 gyrus fornicatus through its entire extent, marks the olfactory area. The remaining portion, occupying 
 the paracentral gyrus and the mesial aspect of the first frontal gyrus, marks the mesial extension of 
 the body-sense area. The uncolored portions of the cortex form the association centres of Flechsig. F, pes ; 
 HS, crura ; Z, pineal body ; 1, corpus albicans ; 2, chiasma ; 3, anterior commissure ; 4, quadrigemina ; 
 5, callosum ; 6, fornix ; 7, septum lucidum (from Flechsig). 
 
 FIG. 110. The colored portion at the tip of the occipital lobe represents the postero-lateral extension 
 of the visual area. The colored portion about the central fissure and the neighboring parts of the frontal 
 lobe represents the lateral extension of the body-sense area. The colored portion about the caudal end of 
 the first temporal gyrus, and extending over the transverse gyri within the Sylvian fissure, represents the 
 auditory area. In all three areas the portions most deeply colored represent the areas where the projec- 
 tion-fibres are most abundant. The uncolored portions of the cortex form the association centres of Flechsig 
 (from Flechsig). 
 
 sense "), which is most richly supplied with projection-fibres about the central 
 fissure, in the two central gyri but also extends forward on the lateral sur- 
 
CENTRAL NERVOUS SYSTEM. 255 
 
 face to include part of all the frontal gyri, while on the mesial surface the 
 cortex concerned extends forward from the precuneus over more than half 
 of the mesial surface. In this last area are delivered the afferent impulses 
 from the skin, muscles, joints, viscera, and the lining walls of the alimentary 
 tract. Flechsig points out that the projection-fibres, according to which these 
 areas have been defined, are composed of axones bringing impulses to the 
 cortex, and hence are sensory, in the usual terminology. The areas thus 
 bounded are found to coincide with the areas which (in animals) respond to 
 direct stimulation by the contraction of definite groups of muscles. 
 
 The earlier determinations of the sensory areas in man were made from 
 the study of brains modified by destructive lesions or congenital defects. 
 
 The cortical centre for smell, inferred from comparative anatomy and 
 physiology to be closely connected with the hippocampal and fornicate gyri 
 and the uncus, has been similarly located in man on the basis of pathological 
 observations ; but the evidence lacks precision. Concerning the location of 
 taste sensations, very little is known. Both of these senses, it must be 
 remembered, are insignificant in man, and hence their central connections are 
 not easily studied. 
 
 On the other hand, the cortical areas for hearing and sight have been 
 located with much more precision and certainty. 
 
 Damage to the posterior third of the first temporal gyrus and the asso- 
 ciated gyri transversi causes in man deafness in the opposite ear, and con- 
 cordantly conditions of the ear which early in life lead to deafness and deaf- 
 mutism are accompanied by a lack of development in these gyri. 1 Destruc- 
 tion of this area on one side causes slight deafness mainly in the opposite 
 ear, while complete deafness follows a cortical lesion only when it is double. 
 
 In the case of the visual areas in man there is the same sort of evidence, 
 but somewhat more exact. The destruction of the area represented by the 
 cuneus and the surrounding cortex (Figs. 109 and 110) always injures vision, 
 the maximum disturbance following injury to the cortex of the calcarine 
 fissure. Conversely, the failure of the eyes to grow, arrests the development 
 of this portion of the hemisphere. 
 
 Hemianopsia. It is found, moreover, that injury to the visual area in 
 one hemisphere usually produces a hemianopsia or partial defect of vision in 
 both retinae. The homonymous halves are affected on the same side as the 
 lesion and the dividing line is usually vertical. The clinical picture corre- 
 sponds to a semi-decussation of the optic tract and the representation of the 
 homonymous halves of each retina in both hemispheres. At the same time 
 the relation is much more complicated than at first sight appears, for the 
 point of most acute vision is often unaffected in such casesl This peculiarity 
 depends apparently on the fact that there is a binocular centre for macular 
 vision in the cortex lining the sides and bottom of the posterior portion of 
 the calcarine fissure. 2 
 
 1 Donaldson : American Journal of Psychology, 1891-2, vol. iv. 
 
 2 Laqueur and Schmidt : Virc.how's Archiv, 1899, Bd. 158, Heft 3, S. 467. 
 
256 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 In the case of neither vision nor hearing do we find in man any subcortical 
 cell-groups capable of acting as centres ; that is, after the destruction of the 
 appropriate cortical region the corresponding sensations and reactions to the 
 stimuli which arouse these sensations are completely and permanently lost. 
 
 From these facts, therefore, it appears that the impulses which give 
 rise to visual and auditory sensations are delivered in certain parts of the 
 cerebral cortex, and unless they arrive there the appropriate sensations are 
 wanting. 
 
 Association Fibres and Association Centres. Common experience 
 shows us that we can voluntarily contract any group of muscles in response 
 to any form of 'stimulus dermal, gustatory, olfactory, auditory, or visual. 
 When, therefore, the hand is extended in response to a visual stimulus, the 
 
 FIG. 111. Lateral view of a human hemisphere, showing the bundles of association-fibres (Starr) : 
 A, A, between adjacent gyri; B, between frontal and occipital areas; C, between frontal and temporal 
 areas, cingulum; D, between frontal and temporal areas, fasciculus uncinatus; E, between occipital and 
 temporal areas, fasciculus longitudinalis inferior ; C, N, caudate nucleus; 0, T, optic thalamus. 
 
 nerve-impulses pass first to the visual area, and then in an indirect manner 
 arouse the cortical cells controlling the muscles of the hand. This connec- 
 tion of the two areas is accomplished through the so-called association-fibres 
 of the cortex. These fibres are formally defined as those which put into 
 connection different parts of one lateral half of any subdivision of the central 
 system (see Fig. 111). 
 
 The bundles which are thus shown in the cerebral hemisphere must be 
 looked upon as typical of the arrangement throughout the entire cortex, and, 
 further, the arrangement in the cortex is typical of that in other parts of the 
 central system. Anatomy would suggest, and pathology would bear out the 
 suggestion, that it is by these tracts that the impulses travel from one area to 
 another. 
 
CENTRAL NERVOUS SYSTEM. 257 
 
 The term " association centres " is applied by Flechsig ' to those portions 
 of the cerebral cortex that lie between the sensory centres which he has been 
 able to demonstrate. The functions of the association centres are first to 
 furnish pathways, more or less intricate, between the several centres, and 
 second, to retain as memories previous sense impressions, so that in acting 
 they also modify the impulses sent into them, and by these modifications 
 shade and adjust to an almost infinite degree the form of the final response. 
 
 On looking at Figs. 109, 110, we note two well-defined areas : (1) that 
 occupying the frontal lobe and forming the great anterior association centre, 
 and (2) the area in the parieto-temporal region which forms a second, the 
 posterior association centre. The third, the middle association centre coin- 
 cides with the Island of Reil, and is much less in evidence. On comparison 
 it will be seen that these regions correspond to what have been called the 
 "latent areas" of the cortex, because no evident response follows the direct 
 stimulation of them. When we compare the extent of these association 
 centres in man with that in other mammals, even the apes, we find the 
 human brain characterized by the high development of these portions. 
 Thus Flechsig feels justified in speaking of these association centres as the 
 " organs of thought," and in pointing out how by means of them the incom- 
 ing sense impressions are made to interact on one another, and in combination 
 with the memory images which are thus aroused give rise to new ideas. 
 
 The association processes carried on by these several centres, are modified 
 by their location, so that the several centres have different and distinct values. 
 With the disturbance of these association centres are correlated the several 
 sorts of mental defects which have been gathered under the term aphasia. 
 
 Aphasia. The development of the ideas bearing on this subject has been 
 slow. After the publication of the great work of Gall and Spurzheim (1810- 
 19) on the brain, some pathologists (Bouillaud, 1825; Dax, 1836), especially 
 in France, were in seach of evidence touching the doctrine of the localization 
 of function. At the same time the subject of phrenology, as put forward by 
 Gall and Spurzheim, was not in good repute, and anything which looked 
 that way, even in a slight degree, was generally scouted. Broca, however, 
 published (1861) the important observation that when the most ventral or the 
 third frontal convolution in the left hemisphere (often designated Broca's con- 
 volution) was thrown out of function, the power of expression by spoken 
 words was lost. For this reason, the name of " speech-centre " has been 
 applied to this convolution. 
 
 Since this discovery which links the neurology of the first part of the cen- 
 tury with that of to-day, and also forms a fundamental observation in the 
 modern doctrine of cerebral physiology, many steps have been taken. 
 
 It was early observed that although in such cases the capacity for spoken 
 language was lost, nevertheless the muscles which were used in the act of 
 phonation were by no means paralyzed. This relation is due probably to the 
 fact that the muscles are innervated from both hemispheres and possibly also- 
 
 1 Flechsig : Loc. cit. 
 VOL. II. 17 
 
258 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 from localities outside the third frontal gyms. Experiments show that in 
 animals stimulation of the cortex in the region corresponding to the third 
 frontal gyrus causes contractions of many of the muscles employed in 
 speech. 1 
 
 The interesting observation was also made that in the normal right-handed 
 person the muscles of phonation could not be co-ordinated for speech from 
 the right hemisphere alone. Thus the symmetrical portion of the right 
 hemisphere has not the same physiological value. 
 
 Besides this lesiop, which involves the cortex in front of the motor region 
 proper, numerous other lesions namely, those which involve the tracts run- 
 ning between the areas of special sensation (vision and hearing, for example), 
 and the motor or expressive region produce corresponding disturbances (see 
 Fig. 112). 
 
 An individual in whom the association tracts between the visual and motor 
 areas have been interrupted can, for instance, see an object presented to him 
 in the sense that he gets a visual impression ; but because of the interruption 
 of the association fibres the object is not recognized, and the impulses reach- 
 ing this sensory area elicit no re- 
 sponse from those muscles the 
 motor centres for which are 
 located outside of the receiving 
 cortex. 
 
 Upon attempting to picture 
 the anatomical arrangement in 
 anything like the completeness 
 demanded by the physiological 
 reactions, it is necessary to 
 postulate the existence of asso- 
 ciation pathways between each 
 area, whether sensory or motor, 
 and all the others. This arrange- 
 ment is to be regarded as modi- 
 fied in several ways. 
 
 In the first place, the connec- 
 tion between a given sensory and a given motor area differs widely accord- 
 ing to the areas concerned. The connection, for example, between the visual 
 area and the motor area for the arm is probably represented by more nerve- 
 elements, and these better organized, than the connection between the gusta- 
 tory area and that for the movements of the leg. 
 
 When, therefore, it is said that such connections exist, it must be added 
 always that the nexus is different for the several regions concerned, and what 
 is more, that in man, at least, it is different for the two hemispheres. 
 
 Relative Importance of the Two Hemispheres. The cerebral cortex 
 
 ii 
 
 FIG. 112. Lateral view of a human hemisphere ; cor- 
 tical area V, damage to which produces " mind-blind- 
 ness"; cortical area H, damage to which produces 
 "mind-deafness"; cortical area S, damage to which 
 causes the loss of audible speech ; cortical area W, dam- 
 age to which abolishes the power of writing. 
 
 1 Semon and Horsley : Philosophical Transactions of the Royal Society, 1890, vol. 181. 
 Deutsche medicinische Wochenschrift, No. 31, 1890. 
 
 Also 
 
CENTRAL NERVOUS SYSTEM. 259 
 
 is always active during our periods of consciousness, and it is to be thought 
 of as a region over which the focal point of intensest activity is continually 
 shifting this focal point, wherever it may be, having about it a halo of less 
 active cells as extensive as the cortex itself. 
 
 When the subject is right-handed, it appears that injury to the left cere- 
 bral hemisphere is productive of more disturbance than injury to the right 
 hemisphere. At the same time, lesion of the left hemisphere is far more fre- 
 quent than that of the right. So far as can be judged from experiments on 
 man, the higher sense-organs, the eye and the ear, are more perfect, physio- 
 logically, on the right side. Since the connection of the sense-organs is largely 
 with the cortex of the contra-lateral hemisphere, this means that the impulses 
 going mainly to the left hemisphere are better differentiated than those going 
 to the right, and it would appear to be easier for these impulses to reach a 
 motor area in the same hemisphere than to reach the corresponding area on 
 the opposite side. It is further true that in right-handed persons the cortical 
 activities of the left hemisphere in the region of the body sense-area, must 
 always be greater than those of the opposite hemisphere, and these two 
 circumstances cannot fail to have a profound influence. The observations of 
 Flechsig 1 on the pyramidal tracts also show that these tracts, before medulla- 
 tion at least, may be unevenly developed on the two sides of the cord, and the 
 ease of control may thus be rendered unequal a condition which must be 
 dominant in the determination of the side of the body which shall be most 
 exercised. Be this as it may, the lesions which cause aphasia or apraxia 
 (inability to determine the meaning and use of objects), are predominantly 
 in the left hemisphere in persons who are right-handed, while there is 
 some evidence that the right hemisphere is more important in left-handed 
 persons. 
 
 In the adult, damage to one hemisphere is usually followed by a perma- 
 nent loss of function, to a greater or less degree, but this loss may be more 
 transient and less serious when the lesion occurs in the very young subject, 
 so that during the growing period the sound hemisphere can in a measure 
 replace the one that has been injured. 
 
 Assuming this general plan for the arrangement of the cortex to be cor- 
 rect, it is evident that a given cell, the axone of which forms part of the 
 pyramidal tract, must in the human cortex be subject to a large series of 
 impulses coming to it over as many paths. Schematically, it would be as 
 represented in Fig. 113. 
 
 The discharging cell may be destroyed; then, of course, the muscles con- 
 trolled by it become paralyzed for voluntary movements. The discharging 
 cell may, however, remain intact, but the pathways by which impulses arrive 
 at it be damaged. This is the type of lesion which produces symptoms of 
 aphasia. When an interruption of associative pathways occurs some one or 
 more of these tracts is broken, and hence the discharging cell does not receive 
 a stimulus adequate to cause a response. 
 
 1 Leitunysbahnen im Gehrin und Ruckenmark, 1876. 
 
260 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 The physiological complexity of the elements in any part of the central 
 system, either when different portions of the system from the same animal or 
 when the corresponding portions of different animals are compared, depends 
 on the number of paths by which the impulses are brought to the discharging 
 cells. 
 
 Composite Character of Incoming- Impulses. To these conclusions 
 based on the anatomy are to be added others suggested by clinical observa- 
 tions. In order that a patient suffering from a lesion between the visual and 
 motor areas may be able to recognize an object and to indicate its use, it is 
 sometimes necessary that the object shall appeal to several senses. For 
 example, the name and use of a knife, when seen alone, may not be recalled, 
 
 FIG. 113. Schema showing in a purely formal manner the different sort of afferent impulses which may 
 influence the discharge of a cortical cell. 
 
 but when it is taken into the hand that is, when the dermal and muscular 
 sensations are added to the visual one the response is made, though, acting 
 alone, any one set of sensations is inadequate to produce this result. 
 
 Just where the block occurs in such a case it is not possible to say with 
 exactness, but the lesion lies, as a rule, between the sensory and motor areas 
 concerned, and by the damage to the pathway, it is assumed that one or more 
 groups of impulses are so reduced in intensity that they are alone insufficient 
 to produce a reaction ; and therefore it is only when the impulses from several 
 sources are combined that a response can be obtained. 
 
 Variations in Association. It is a familiar fact that individuals differ 
 
CENTRAL NERVOUS SYSTEM. 261 
 
 in no small degree in the acuteness of their senses i. e., in the power to dis- 
 criminate small differences, and this, too, when the sense-organs are normal. 
 Further, the powers of those best endowed are by no means to be attained by 
 others, however conscientious their training. Moreover, the sensory path- 
 ways differ widely. The inference is fair, therefore, that those who think in 
 terms of visual images, as compared to those who think in auditory images, do 
 so by virtue of the fact that in the former case the central cells concerned in 
 vision are distinctly the better organized, while in the latter case it is those 
 concerned in hearing. 
 
 In the same way, the power of expression varies in an equally marked 
 degree, and the capacity for the expression of ideas by means of the hand, in 
 writing, is by no means necessarily equal to the power of expression by means 
 of spoken words. In the former case we have the results of the play of 
 impulses from the several sensory centres on the motor area for the hand, and 
 this is reinforced by the sight of that which has been written, whereas in the 
 latter case impulses from these same sensory centres play upon the area which 
 controls the muscles of phonation, and this reaction is reinforced by the sound 
 of the words uttered. Of course, in the case of a defective, like a blind-deaf- 
 mute, the expression of thought is by movements of the fingers, and this is 
 reinforced by the tactile and muscular sensations which follow these move- 
 ments. 
 
 It is not by any means to be expected that the anatomical connections 
 which render such reactions possible will be equally perfect for the different 
 sensori-motor combinations, or the same combinations in different persons, 
 and hence the powers of the individual will be modified by the varying per- 
 fection of these paths. From this it also follows that the same lesion as 
 grossly determined, will not produce identical results in the two persons, for 
 it will not effect the damage of structural elements which are strictly com- 
 parable. 
 
 Latent Areas. It has been plain from an examination of the foregoing 
 figures, as well as from the descriptions, that there must be a large portion of 
 the cortex which, so far as has been observed, may be called latent. These 
 areas, which include nearly the entire ventral surface of the hemispheres, a 
 large part of the mesial surface, and on the dorsal and lateral aspects a large 
 portion of the frontal, parietal, and temporal lobes together with the island, 
 certainly require a word. 
 
 These last correspond with the " association centres" as described by 
 Flechsig. To direct stimulation they give no response. From any one 
 portion of the latent area the connections are not massive enough to 
 permit of impulses which will cause a muscular contraction, and hence 
 these impulses coming from one locality to a discharging cell form only a 
 fraction of the impulses which control it. For this reason the significance 
 of these parts fails to be clearly evident upon direct experiment. 
 
 The cortex of the frontal lobes has some connections with the nuclei of 
 the pons, and so with the cerebellum. The more recent experiments on th 
 
262 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 functions of this region are by Bianchi ] and Grosglik, 2 the former on monkeys 
 and dogs and the latter on dogs alone. 
 
 These experimenters found that the removal of one frontal lobe is com- 
 paratively insignificant in its effects, while when both are removed the change 
 is profound. On removing the frontal lobe on one side only there is no dis- 
 turbance of vision, hearing, intelligence, or character. There do occur both 
 sensory and motor disturbances, but these are for the most part transient. 
 On the side opposite to the lesion there is in the limbs a blunting of all sen- 
 sations and some paresis. Moreover, there is a hyperaesthesia combined with 
 a paresis of the muscles of the neck and trunk which move these parts 
 away from the side of the lesion. 
 
 These several effects of the operation tend to pass off, and if then the 
 remaining frontal lobe be removed from a dog or monkey, not only do the 
 symptoms just described appear on the other side of the body, but still more 
 fundamental changes occur. A ceaseless wandering to and fro, such as 
 Goltz 3 observed in those dogs in which the anterior half of the brain had 
 been removed, characterizes the animals ; curiosity, affection, sexual feeling, 
 pleasure, memory, and the capacity to learn are at the same time abolished, 
 and the expressions of the animal are those of fear and excessive irritability. 
 That, therefore, the frontal lobes play an important role in the total reactions 
 of the central system is amply evident, but this by no means justifies the 
 conclusion that they are the seat of the intelligence. 
 
 F. COMPARATIVE PHYSIOLOGY OF THE DIVISIONS OF THE ENCEPHALON. 
 
 For the better comprehension of the conditions found in man and the 
 monkeys, it will be of importance to briefly review the comparative physi- 
 ology of the parts of the encephalon in vertebrates below the monkeys. The 
 encephalon in the lower vertebrates is usually composed of a very much 
 smaller number of cells than is found in that of man, and also the massing 
 of the elements toward the cerebral cortex and in connection with the princi- 
 pal sense-organs has gone on to a far less extent. 
 
 For the determination of the functions of the several parts of the enceph- 
 alon it is possible to employ in animals the method of removal as well as the 
 method of stimulation. The doctrine of cerebral localization was at one time 
 crudely expressed by the statement that a cortical centre was one the stimu- 
 lation of which produced a given reaction, and the removal of which abolished 
 this same reaction. Goltz soon showed that in the dog the removal of even 
 an entire hemisphere did not cause a paralysis of the muscles on the opposite 
 side of the body, although others had shown that a stimulation of certain 
 portions of the cortex of the hemisphere would cause the muscles to contract. 
 It was argued, therefore and quite rightly that the cortical centres of the 
 dog did not completely answer to the definition. 
 
 1 Archives italiennes de Biolagie, 1895, t. xii. 
 
 2 Archivfilr Anatomic und Physiologic, 1895. 
 8 Ueber die Verichtungen des Grosshirns, 1881. 
 
CENTRAL NERVOUS SYSTEM. 263 
 
 From the experimental work of the strict localizationists like Hitzig, 1 
 Munk, 2 and Ferrier, 3 and from the work of those who, like Goltz 4 and Loeb, 5 
 denied a strict localization in the cerebral cortex, several important points 
 of view have been developed. 
 
 In the first instance, anatomy indicates that in the central system there 
 are but few localities which consist only of one set of cell-bodies, together 
 with the fibres coming to these bodies and going from them. Almost every 
 part has both more than one set of connections with other parts and also 
 fibres passing through it, or by way of it, to other localities. Hence in re- 
 moving any part of the hemispheres, for instance, not only are groups of cell- 
 bodies taken away, but a number of other pathways are interrupted at the 
 same time, and thus the damage extends beyond the limits of the part re- 
 moved. Moreover, when any portion of the central system has been removed 
 there is a greater or less amount of disturbance of function following imme- 
 diately after the operation ; but this disturbance partially passes away. 
 There are thus "temporary " as contrasted with " permanent " effects of the 
 lesion, and these require to be sharply distinguished, because it is a per- 
 manent loss which is alone significant in these experiments. Finally, it has 
 been made clear that neither the relative nor the absolute value of any 
 division of the central system is fixed, but depends on the degree to which 
 centralization has progressed, or, to use the more common measure, the grade 
 of the animal in the zoological series, both expressions implying an increase 
 in the connections between the cerebrum and the lower centres. The age of 
 the animal on which the operation has been made is also of no small impor- 
 tance in this respect. These relations can be illustrated by reference to several 
 experiments. 
 
 Removal of Cerebral Hemispheres. If from a bony fish the cerebral 
 hemispheres (including the corpora striata as well as the mantle) be removed, 
 the animal apparently suffers little inconvenience. The movements are un- 
 disturbed ; such fish play together in the usual manner, discriminate between 
 a worm and a bit of string, and among a series of colored wafers to which 
 they rise always select the red ones first. 6 In these fish the eye is the con- 
 trolling sense-organ, and, as will be recognized (see Fig. 114), the operation 
 has by no means damaged the primary centres of vision. 
 
 Quite different is the result when the cerebrum is removed from a shark. 7 
 In this case, although the eyes are intact, the animal is reduced to complete 
 quiescence ; yet, on the whole, the nervous system of the shark is rather less 
 well organized and more simple than that of the bony fish. 
 
 The astonishing effect produced is explained by a second experiment (see 
 Fig. 115). 
 
 1 Untersuchungen ueber das Gehirn, Berlin, 1874. 
 
 * Ueber die Functionen der Grosshirnrinde, Berlin, 1881. 
 3 The Functions of the Brain, London, 1876. 
 
 * Ueber die Verichtungen des Grosshirns, Bonn, 1881. 
 
 5 Archivfur die gesammte Pkysiologie, 1884, Bde. 33 n. 34. 
 
 6 Steiner : Die Functionen der Centralnervensystems, 1888. 7 Steiner : Loc. cit. 
 
264 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 If the olfactory tract be severed on one side, no marked disturbance in 
 the reactions of the shark is to be noticed ; when, however, both tracts are 
 severed, the shark acts as though deprived of its cerebrum. From this it 
 appears that the removal of the principal sense-organ, that of smell, is the 
 
 FIG. 114. Schema of the encephalon of a bony fish embryonic (Edinger). The vertical black line marks 
 off the structures in front of the thalamus. 
 
 real key to the reactions, and that the responsiveness of the fish is reduced in 
 the first instance, because in this case it has been deprived of the impulses 
 coming through the principal organs of sense, and in the second, the removal 
 of the cerebrum contains the pathway for the impulses from the olfactory 
 bulbs to the cell-groups which control the cord. 
 
 FIG. 115. Schema of the encephalon of a cartilaginous fish (Edinger). The vertical black line marks off 
 the striatum and pars olfactoria which lie in front of the thalamus. 
 
 I Passing next to the amphibia as represented by the frog, there are several 
 series of observations on the physiological value of the divisions of the cen- 
 tral system. Schrader 1 finds the following : Removal of the cerebral hemi- 
 spheres only, the optic thalami being uninjured, does not abolish the sponta- 
 neous activity of the frog. It jumps on the land or swims in the water, and 
 1 Archivfur die gesammte Physiologic, 1887, Bd. xli. 
 
CENTRAL NERVOUS SYSTEM. 
 
 265 
 
 changes from one to the other without special stimulation. It hibernates like 
 a normal frog, retains its sexual instincts, and can feed by catching passing 
 insects, such as flies (see Fig. 116). A frog without its hemispheres is tin -re- 
 fore capable of doing several things apparently in a spontaneous way. Such 
 frogs balance themselves when the support on which they rest is slowly turned, 
 moving forward or backward as the case demands, in order to maintain their 
 equilibrium. In doing this the frog tends first to move the head in the direc- 
 
 D 
 
 E 
 
 FIG. 116. Frog's brain; the parts in dotted outline have been removed : A, brain intact ; B, cerebral 
 hemispheres removed; C, cerebral hemispheres and thalami removed; D, cerebellum removed; E, two 
 sections through the optic lobes ; F, two sections through the right half of the bulb (Steiner). 
 
 tion opposite to the motion of the support, and then to follow with move- 
 ments of the body. If the optic thalami are removed (Fig. 116, C), the power 
 of balancing is lost, because, although movements of the head still occur, those 
 of the body are abolished. A frog thus operated on and deprived of the hem- 
 ispheres and thalami exhibits the lack of spontaneity which is usually described 
 as following the loss of the hemispheres alone, but which is not a necessary 
 consequence of this operation, as the preceding experiments show. 
 
 A frog possessed of the mid-brain and the parts behind it (Fig. 116, C) 
 
266 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 will croak when stroked on the back. When the optic lobes have been 
 removed this reaction becomes more difficult to obtain, but it is not necessa- 
 rily abolished, neither is the characteristic fling of the legs in swimming. At 
 the same time, a frog with its optic lobes can direct both its jumping and 
 swimming movements according to light stimuli acting through the eye, jump- 
 ing around and over obstacles which form a shadow in its path, and climbing 
 out of the swimming tank on the lighter side. This power is lost when the 
 optic lobes have been removed. 
 
 When the anterior end of the bulb (pars commissuralis Stieda) has been 
 also removed then the frog becomes incessantly active, creeping about and 
 not coming to rest until he has run himself into some corner. Schrader found 
 such frogs capable of clambering over the edge of a box eighteen centimeters 
 high. They are at a loss when the edge of the box has been finally attained, 
 and vainly reach into space from this position. In the water they swim " dog- 
 fashion," but only upon special stimulation do they make a spring. 
 
 If more of the bulb is removed, the bearing of the frog departs more and 
 more from the normal, and is only temporarily regained in response to strong 
 stimulation ; nevertheless, co-ordinated movements can be obtained when the 
 bulb down to the calamus scriptorius has been removed, and only when the 
 movements of the arms are directly affected by the damage of the upper end 
 of the cord does the inco-ordination become constant. 
 
 A section through the optic lobes at a (Fig. 116, E) puts the frog in a con- 
 dition similar to that following the isolated removal of the lobes, while a sec- 
 tion at 6 has the curious effect of causing the animal to move backward upon 
 stimulation of the toes. 
 
 When the small ridge which forms the cerebellum in the frog has been 
 removed a slight tremor of the leg-muscles and a loss of precision in jumping 
 are the only defects noted (Fig. 116, D). These results hold for symmetrical 
 removal of the divisions of the encephalon. When the removal is unsym- 
 metrical in the inter-brain, mid-brain, or bulb (Fig. 116 F, a and 6), there is 
 more or less tendency to forced positions or forced movements. 
 
 As a rule, action is most vigorous on the side of the body associated \vith 
 the greater quantity of nerve-tissue. This relation appears as a natural result 
 of the greater effectiveness of the incoming impulses when entering a larger 
 group of central cells. Indeed, the removal of the different portions of the 
 central system in the frog is accompanied by a progressive loss in responsive- 
 ness, stronger and stronger stimuli being required to induce a reaction. This 
 holds true down to the anterior end of the bulb, the removal of which, on the 
 contrary, sets free the lower centres, so that the frog becomes incessantly 
 active. Just how this release is effected is not easy to explain, but further 
 removal is again followed by the loss of responsiveness. 
 
 Passing next to the bird, as represented by the pigeon, the observations of 
 Schrader are the most instructive. 1 The removal of the hemispheres from 
 the bird involves taking away the mantle and the basal ganglia, the chiasma 
 1 Archiv fur die gesammte Physiologie, 1888, Bd. xliv. 
 
CENTRAL NERVOUS SYSTEM. 267 
 
 and the optic nerves being left intact. For the first few days after the 
 operation the bird is in a sleep-like condition. Next the sleep becomes 
 broken into shorter and shorter periods, and then the bird begins walking 
 about the room. From the beginning its movements are directed by vision ; 
 slight obstacles it surmounts by flying up to them, larger ones it goes around. 
 In climbing, its movements are co-ordinated by the sense of touch, and the 
 normal position of the body is maintained with vigor. The birds which walk 
 about by day remain quiet and asleep during the night. In flying from a 
 high place the operated pigeon selects the point where it will alight, and pre- 
 fers a perch or similar object to the floor. 
 
 A reaction to sound is expressed by a start at a sudden noise, like the 
 explosion of a percussion-cap. 
 
 Pigeons without the cerebrum do not eat voluntarily, though the presence 
 of the frontal portions of the hemispheres is sufficient to preserve the reac- 
 tion. 
 
 In a young hawk slight damage to the frontal lobes abolished for the time 
 the use of the feet in the handling of food, and thus abolished in this way the 
 power of feeding as well as that of standing. 
 
 With the loss of the cerebrum the pigeon does not lose responsiveness to 
 the objects of the outer world, but they all have an equal value. The bird is 
 neither attracted nor repelled, save in so far as the selection of the points 
 toward which it will fly is an example of attraction. Sexual and maternal 
 reactions both disappear, and neither fear nor desire is evident. 
 
 In ascending the mammalian series, the removal of the cerebrum becomes 
 a matter of increasing difficulty. The reasons for this are several, and reside 
 in the increasing size of the blood-vessels and the nutritive complications 
 dependent on the increase in the mass of the cerebrum, as well as in the 
 greater physiological importance of this division. Goltz l has been able by 
 repeated operations to remove the entire cerebrum of a dog, and still to keep 
 the animal alive and under observation for eighteen months, at the end of 
 which time the animal, though in good health, was killed for further exami- 
 nation. This dog was blind, though he blinked when a very bright light 
 was suddenly flashed in his face. He could be awakened by a loud sound, 
 and when awake responded to such sounds, when intense, by shaking the head 
 or ears. This would not, however, be complete proof that he could hear. 
 The sense of taste was so far present that meat soaked in quinine was rejected 
 after tasting. Tactile stimuli and those involving the muscle sense, as in the 
 case where the animal was lifted, caused him to struggle and to bite in the 
 direction of the irritation. These reactions were modified according to the 
 locality of the stimulus. The power to make movements expressive of pain 
 was still present. 
 
 On the motor side the dog was capable of such highly complicated acts 
 as walking, standing, and eating, and in these operations was guided by the 
 muscle-sense and that of contact. The sexual instincts were lost, but the 
 
 1 Archivfiir die gesammte Physiologic, Bd. xli. 
 
268 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 animal was excessively active, and became more and more excited when 
 ready to defecate or when hungry. 
 
 The examination of the brain showed that in front of the mid-brain the 
 important structures had been removed or were degenerated, only small por- 
 tions of the corpora striata remained, mainly parts of the caudal portions 
 of the nucleus caudatus. The frontal portion of the thalamus had been re- 
 moved and the nuclei in the remainder were highly atrophic, so that the 
 defects were due to a removal of rather more than the cerebrum proper. 
 
 Emotions, feelings, conscious sensations, or the capacity to learn were 
 entirely wanting in this dog, and its reactions were those of a very elaborate 
 machine. 
 
 If we compare, now, the effects of the removal of the cerebral hemispheres 
 in the bony fish, the pigeon, and the dog, we see that the results of the 
 operation are progressively more disturbing as we pass up the series. In 
 the higher animals the effects are more often fatal, the disturbance imme- 
 diately following is much more severe, the return of function slower, and the 
 permanent loss greater. As a partial exception to the above statements is 
 the observation that after operation the general health of pigeons always 
 declines, and it is not possible to keep them alive more than about six weeks. 
 On the contrary, a dog could be kept in good health for some eighteen 
 months ; but there is this difference between the experiments, that the 
 removal in the case of the dog was made by several successive operations. 
 
 By removal of the cerebrum the higher animal tends to lose just those 
 capacities which best serve to distinguish it from the lower forms. When, 
 therefore, the inquiry is made why the results obtained in the dog are not 
 obtainable in the monkey or in man, there are several replies. In the first 
 place, no such extensive experiments have been made on monkeys of the 
 right age and under equally favorable conditions. If the mature animal is 
 taken, the secondary degenerations are so massive that they certainly cause 
 great disturbance in the remaining part of the system. This is not equivalent 
 to an assertion that the same results could be obtained in the monkey by more 
 extensive experiments, but a suggestion of one difference behind the results 
 thus far reported. There is no reason for assuming any deep-seated differ- 
 ence in the arrangement of the central system of the highest mammals as 
 compared with that in the lower. Indeed, in some human microcephalic 
 idiots the proportion of sound and functional tissue in the encephalon is less 
 than one-fourth that found in a normal person ; yet, on the other hand, no 
 normal adult could lose anything like the amount of tissue which is out of 
 function in these microcephalic brains and at the same time live. 
 
 The central system, therefore, even in man, is to be looked upon as pos- 
 sessed of some power to adapt itself when portions have been lost, but this 
 is most evident when the defect begins early and develops slowly. 
 
 Keeping the cerebrum still in view, it is possible to go into further 
 details. In forms below the monkey the loss of portions of the cerebral 
 cortex from the motor area is accompanied by a greater or less paralysis of 
 
CENTRAL NERVOUS SYSTEM. 269 
 
 tin' muscles represented. This, however, is an initial symptom only, and 
 gradually disappears, though not always with the same completeness. In 
 man, of course, the tendency to recover is least. 
 
 The anatomical relations behind this ditl'ereiice are the following: The 
 efferent cells in the ventral horns are dominated principally by two sets of 
 impulses, those arriving directly over the dorsal roots of that segment in 
 which they are located, and those coming over the long paths by way of the 
 cerebral cortex and pyramidal tracts. In the lower mammals this second 
 pathway is insignificant, and when interrupted, therefore, the disturbance in 
 the control of the ventral-horn cells is but slight. Passing up the scries, 
 however, this pathway tends to become more and more massive and important, 
 as the figures previously given show (see p. 252), until in man and the 
 monkey a damage of it such as is effected by injury to the cortex causes a 
 high degree of paresis if not permanent paralysis, because by this injury a 
 large proportion of the impulses is thus cut off from the efferent cells. 
 
 It has been previously shown that the cortical areas do not vary according 
 to the mass of the muscles which they control. Experiments also show that 
 it is the fore-limbs which are most disturbed in their reactions when the 
 lesion involves the cortical centres for both fore- and hind-limbs, and this falls 
 under the law that the more highly adaptable movements (/. e., those of the 
 fore-limb as contrasted with those of the hind-limbs) are most under the con- 
 trol of the cortex. If the examination be restricted to the fore-limb alone, it 
 is found that the finger and hand movements or those of the more distal seg- 
 ments are in turn the ones most disturbed. Thus, in the limbs the more 
 distal groups of muscles are those best controlled from the cortex. It fol- 
 lows, then, that for the arm, paralysis of shoulder movements as the result 
 of cortical lesion is least complete, while as we travel toward the extremity 
 of the arm the liability to disturbance of its function as the result of cortical 
 injury increases steadily. 
 
 Turning now to the " sensory " areas of the cortex, the principles under- 
 lying their physiological significance and connections appear to be similar. 
 The lower the animal in the vertebrate series the more probable that its reac- 
 tions can be controlled by the afferent impulses which have not passed through 
 the cerebral cortex. 
 
 None of the senses except vision can be analyzed sufficiently to bring out 
 the significance of the subdivisions of the cortical area ; hence the illustra- 
 tions are taken from that sense alone. 
 
 It has already been shown that without cerebral hemispheres a bony 
 fish can distinguish the colors of wafers thrown on water and discriminate 
 between a bit of string and a worm. In the same case, a frog is able to direct 
 its movements and to catch flies i. e.j to detect objects in motion and react 
 to them normally. A pigeon can direct its movements in some measure, and 
 even select a special object as a perch ; but it is not able to respond to the sight 
 of food or its fellows, or those objects which might be supposed to excite the 
 bird to flight. In the dog the vision which remains permits only the response 
 
270 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 of blinking when the eye is stimulated by the flash of a strong light. The 
 progressive diminution in the response which follows visual stimuli in these 
 animals is open to the interpretation that the path by which the impulses 
 may pass over to the cells forming the primary centres intermediate between 
 the sense-organ and the cortex is progressively diminished. Thus, as the 
 pathway to the cortex becomes more permeable, the impulses arriving at the 
 primary optic centres are in a less and less degree reflected toward the cord. 
 When, therefore, the cortex has been removed, the reactions taking place by 
 way of it are disturbed in proportion to their normal importance. 
 
 In the first instance, when the reflection occurs in the primary centres the 
 incoming impulses are distributed toward the cord by paths not known, while 
 in the second they pass from the cortex along the pyramidal tracts. 
 
 In the cortex of the dog subdivisions of the visual area have been made 
 by Munk. 1 He found that the more anterior portions of the visual area were 
 associated with the superior parts of the retina, and the more posterior por- 
 tions with the inferior, while the area in one hemisphere corresponded with 
 the nasal portion of the retina of the opposite eye, and to a less degree with the 
 temporal portion of the retina of the same side. The determination of these 
 relations was made by the removal of parts of the visual area (dogs) and the 
 subsequent examination of the field of vision. It appears, therefore, that the 
 incoming impulses from certain portions of the retina are delivered at definite 
 parts of the cortex, and that when the parts are injured in the dog or higher 
 mammals these impulses are blocked. By stimulation, it will be remem- 
 bered, Schafer determined similar relations in the monkey. 
 
 Before leaving the cerebral hemispheres, mention of the fact should be 
 made that still other functions, control of the sphincter ani (Fig. 103), secre- 
 tion of saliva, and micturition, can be roused by the stimulation of the cortex 
 in the appropriate region namely, in the region where the muscles and glands 
 concerned might be expected to have representation if they followed the gen- 
 eral law of arrangement. Changes in the production a:id elimination of heat 
 from the body follow interference with the motor region of the cerebrum, and 
 the removal of portions of the cortex in this region is followed by a rise in 
 the temperature of the muscles affected and an increased blood-supply to them. 
 
 In the encephalon, the cerebrum, and especially its outer surface, is the 
 portion the functions of which have been studied. The significance of the 
 other portions of the encephalon can be far less well determined. The dis- 
 turbances caused by the section and stimulation of the callosum have been 
 studied by Koranyi 2 and by Schafer. 3 It was found that complete section of 
 the corpus callosum was not followed by any perceptible loss of function. 
 On the other hand, stimulation of the uninjured callosum from above gave 
 symmetrical bilateral movements, while if the cortex on one side was removed 
 stimulation of the callosum gave unilateral movements on the side controlled 
 
 1 Ueber die Functionen der Grosshirnrinde, Berlin, 1881. 
 
 2 Archiv fur Anatomie und Physiologie, Bd. xlvii. 
 
 3 Brain, 1890. ' 
 
CENTRAL NERVOUS SYSTEM. 271 
 
 by the uninjured hemisphere. These results seem to corroborate the conclu- 
 sion derived from liistological work to the effect that the system of the callo- 
 stim is composed only of commissural fil> res, and that it sends no fibres directly 
 into the internal capsule of either side. Concerning the corpora striata and 
 the optic thalami, very little is known. In the case of the corpora striata 
 injury causes in man no permanent defect of sensation or motion, although 
 both forms of disturbance may at the outset be present in the case of acute 
 lesions. Lesions of the corpora striata cause a rise in body-temperature. 1 
 Following a puncture of one corpus striatum there occurs in rabbits a rise 
 amounting to some 3 C.; it begins a few minutes after the operation and 
 may last a week, but the temperature tends to return to the normal. The 
 most striking feature in these experiments is the very wide effects produced 
 by an extremely small wound, like the puncture of a probe. 
 
 In the cases where lesion of the striatum on one side causes in man a rise 
 of temperature, it appears mainly on the side of the body opposite the lesion. 2 
 A vaso-motor dilatation occurs over the parts of the body where the temper- 
 ature is high. 
 
 In less degree a rise of temperature follows injury of the optic thalamus 
 at least such is the result of experiments on rabbits ; but the effect of the 
 lesion is never so marked as in the case of the striatum. Owing to the dis- 
 proportion between the area of the lesion and the extent of the effects, it is 
 difficult to conceive of the anatomical relations which permit the reaction. It 
 is of interest to note, however, that similar relations hold for the vaso-motor 
 centre in the bulb, in which case the vessels supplying a great area are con- 
 trolled by a small group of cells. 
 
 The difficulty of an anatomical explanation is increased by the fact that Ott 3 
 enumerates in animals six heat-centres: 1. The cruciate, about the Rolandic 
 fissure ; 2. The Sylvian, at the junction of the supra- and post-Syl vian fissures ; 
 3. The caudate nucleus ; 4. The tissues about the striatum ; 5. A point be- 
 tween the striatum and the thalamus, near the median line ; 6. The anterior 
 mesial end of the thalamus. 
 
 Thalamus. In considering the thalamus, we find that the various cell- 
 groups forming it are connected with distinct portions of the cerebral cortex 
 by double pathways one set of axones having their origin in cell-bodies 
 located in the cortex, and the other in cell-bodies in the subdivisions of the 
 thalamus. The relations between these two divisions have been specially 
 studied by v. Monakow, 4 who finds by experiment that lesion of one part, 
 either cortex or the thalamic nuclei, is followed by degeneration in the other 
 part, and that the location of the degeneration depends on that of the lesion. 
 
 Further, it has been observed by Mellus 5 that the axones passing from the 
 
 1 Aronsohn und Snchs: Archiv fiir die gesammte Physioloyie, 1885, Bd. xxxvii.; Richet: 
 Comptes rendus de V Acad. des Sciences, 1884; Ott: Brain, 1889, vol. xi. 
 
 2 Kaiser: Neurologisches Centralblatt, 1895, No. 10. 3 Ott: Loc. tit. 
 * Archiv fiir Psychiatric und Nervenkrankheiten, 1893, Bd. xxvii. 
 
 5 Proceedings of the Royal Society, London, 189-4 and 1895. 
 
272 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 motor cortex of the monkey toward the thalamus are fibres of smaller calibre 
 than those destined for the pyramidal tracts. 
 
 Moreover, the studies of Tschermak 1 on the termination of the tracts 
 which continue the dorsal columns of the spinal cord in the interbrain, show 
 an abundant connection of the fibres, especially with the ventral cell-groups 
 of the thalamus. The connection may be either an actual ending of the fibre 
 or a termination by means of collaterals. 
 
 When these anatomical observations are considered in connection with the 
 differences in the reactions of the frog with and without its thalami, it appears 
 that cell-groups which increase the responsiveness of the central system 
 must be located here. On the other hand, in the case of Goltz's dog without 
 its fore-brain, the thalami (interbrain) were so largely damaged that it hardly 
 seems possible that they could have been much utilized in the reactions which 
 were made by that animal. 
 
 Human pathology throws little light on the functions of the thalami 
 though lesion of it is often accompanied by loss of power to express the 
 emotions through the muscles of the face a symptom to which attention has 
 been repeatedly drawn. 
 
 The Cerebellum. The only other division of the encephalon, the func- 
 tions of which can properly be described apart, is the cerebellum. This 
 portion is among vertebrates almost as variable in its development as the 
 mantle of the cerebral hemispheres, and in many fish and mammals is asym- 
 metrical in its gross structure. 
 
 Observation on this subdivision has been carried out in the first instance 
 by Luciani, 2 and later by Russell 3 and by Ferrier. 4 
 
 The cerebellum is not concerned with psychical functions. The removal 
 of it does not cause permanently either paralysis or anaesthesia, but the im- 
 mediate effects of an extensive injury are (in dogs and monkeys) a paresis 
 and analgesia as well as anesthesia mainly in the hind-legs, and in conse- 
 quence a high degree of inco-ordination in locomotion. A distinct series of 
 symptoms, however, follows injury to this organ, and these are modified 
 according to the locality and nature of the lesion. Removal of one-half 
 (cerebellar hemisphere plus half the vermis) of the cerebellum in the dog 
 causes a deviation outward and downward of the optic bulb on the opposite 
 side, a proptosis of the bulbs on both sides, nystagmus and contracture of 
 the muscles of the neck on the side of the lesion, and an increase of the 
 tendon-reflexes in the limbs. In walking the dog wheels toward the side 
 opposite to the lesion, and tends to fall toward the side of the lesion. 
 
 The symptoms are chiefly unilateral, and, caudad from the cerebellum^ 
 are on the side of the lesion. The symptoms are less severe when only one 
 hemisphere, instead of an entire half of the cerebellum, has been removed. 
 
 1 "Notiz betreffs des Kindenfeldes der Hinterstrangsbahnen," Neurologisches Centralblatt, 1898, 
 No. 4. 
 
 8 Archives italiennes de Biologie, 1891-92, xvi. 
 
 3 Philosophical Transactions of the Royal Society, 1894. * Brain, 1893, vol. xvi. 
 
CENTRAL NERVOUS SYSTEM. 273 
 
 The existing symptoms are not intensified by the removal of the remaining 
 half. The permanent condition of the muscles after operation is expressed 
 l>v an atonia, or lack of tonus, in the resting muscle- ; an asthenia, or loss of 
 Strength, which was measured by Lueiaui,aud was most marked in the hind- 
 leg; an astasia, or a lack of steadiness in the muscles during action; and 
 finally an ataxia, or a want of orderly sequence, in the contractions of the 
 muscle-groups. The general expression of these symptom is a twist of the 
 trunk, the concavity being toward the operated side, combined with a dis- 
 orderly gait. At the same time there is no demonstrable permanent dis- 
 turbance of tactile or muscular sensibility. 
 
 Though the two halves of the cerebellum are united by strong commis- 
 snral fibres, the complete division of the organ in the middle line is followed 
 by a disturbance of the gait which is only transitory. Hence it is inferred 
 that the connections of the cerebellum are mainly with the same side of the 
 bulb and spinal cord. Cephalad of the cerebellum the connection, however, 
 is a crossed one, each cerebellar hemisphere being associated with the contra- 
 lateral cerebral hemisphere. Throughout these connections, both cephalad 
 and candad to the cerebellum itself, it appears that there is always a double 
 pathway, and the cerebellum not only sends impulses to, but receives them 
 from, the regions with which it is associated. 
 
 One effect of removal of one-half of the cerebellum is to increase the re- 
 sponsiveness of the cortex of the contra-lateral cerebral hemisphere to electri- 
 cal stimulation, thereby making it possible with a weaker stimulus to obtain 
 a reaction which could be obtained from the other hemisphere only with a 
 stronger one. When an irritative lesion is made, instead of a merely de- 
 structive one, the rotation and falling are away from the side of the lesion, 
 instead of toward it. 
 
 The experiments altogether show the cerebellum to be closely associated 
 with the proper contraction of the muscles, and this is so directly connected 
 with the maintenance of equilibrium 1 that it is not surprising to find that 
 stimulation or removal of the cerebellar cortex, besides producing nystagmus, 
 may give rise to deviations of the eyes similar to those found on injury of the 
 semicircular canals or stimulation of their nerves in fishes. 2 
 
 PART III. PHYSIOLOGY OF THE NERVOUS SYSTEM TAKEN 
 
 AS A WHOLE. 
 
 A. WEIGHT OF THE BRAIN AND SPINAL CORD. 
 
 In attributing a value to the mass of the nervous system we assume that 
 the elements which compose it possess potential energy. This energy varies 
 for any given element in accordance with a number of conditions, but for the 
 moment it will be sufficient to point out that if the mass of the entire system 
 is significant the masses of its respective subdivisions are also significant, as 
 
 1 A. Thomas: " Le Cervelet," Etn<]<> mminmique, dinique et physiologique, Paris, 1897. 
 
 2 Lee : Journal of Physiology, 1893, vol. xv. ; 1894, vol. xvii. 
 
 VOL. II. 18 
 
274 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 showing in some measure the relative physiological importance of the several 
 parts. 
 
 Weight of the Encephalon and Spinal Cord. When the weight of any 
 portion of the nervous system is taken, the final record represents, in addi- 
 tion to the weight of the nerve-tissues proper, that of the supporting and 
 nutritive tissues normally associated with them, together with the enclosed 
 blood and lymph. It is, however, assumed that under normal conditions the 
 relation between the nervous and non-nervous tissues is nearly a constant one, 
 and that the results of different weighings are therefore comparable among 
 themselves. 
 
 Outside of the nervous tissue proper are the pia and the fluid contained in 
 the vessels and ventricular cavities. Sometimes the encephalon is freed from 
 
 FIG. 117. Showing the principal divisions of the encephalon made for the study of its weight: 1, 
 hemisphere seen from the side, fissuration according to Eberstaller ; 2, mid-brain, region of the quad- 
 rigemina : 3, pons ; 4, cerebellum, or hind-brain ; 5, bulb, or after-brain. Divisions 2, 3, and 5, taken 
 together, form what is designated the " stem " in the tables of Boyd (modified from Quain's Anatomy). 
 
 the pia and fluid, and at others they are weighed all together. According to 
 Broca, 1 the weight of the pia covering the encephalon is, in normal males, as 
 follows : 
 
 20 to 30 years 45 gms. 
 
 31 to 40 " " 50 " 
 
 60 " 60 " 
 
 The cast of the ventricles as made by Welcker displaces 26 c.c. of water, 
 which gives an idea of the normal capacity of these cavities. In man, the 
 gray matter of the cerebrum has, on the average, 81 per cent, of water ; while 
 the white matter from various parts of the central system has 70 per cent. 2 
 
 1 Broca, quoted by Topinard : Elements tf Anthropologie generate, 1885. 
 
 2 Halliburton : Journal of Physiology, 1894. 
 
CENTRAL NERVOUS SYSTEM. 
 
 275 
 
 The specific gravity of the entire encephalon is for the male, 1036.3, and 
 for the female, 1036. 
 
 Weight of the Encephalon. The encephalon is that portion of the 
 central nervous system contained within the skull. The accompanying dia- 
 gram (Fig. 117) shows the encephalon, together with one manner of sub- 
 dividing it. Its weight has usually been taken while it was still covered by 
 the pia, but after allowing the fluids to drain away for five minutes or more. 
 Sometimes drainage has been facilitated by cutting into the brain ; hence, 
 when the brain-weight records by any observer are to be discussed, the first 
 question concerns the method according to which the brains were examined, 
 for the weights may be either with or without the pia and with or without 
 drainage. 
 
 The anthropologists classify encephala according to weight in the follow- 
 ing manner : 
 
 TJie Nomenclature of the Encephalon according to Weight. Weight in Grams. 
 
 ( Topinard.) 
 
 Classes. Males. Females. 
 
 Macrocephalic From 1925 to 1701 From 1743 to 1501 
 
 Large " 1700 to 1451 " 1500 to 1351 
 
 Medium " 1450 to 1251 " 1350 to 1151 
 
 Small " 1250 to 1001 " 1150 to 901 
 
 Microcephalic " 1000 to 300 " 900 to 283 
 
 From the observations of Dr. Boyd, in England, on the weight of the brain 
 the following table has been compiled : 
 
 Table showing the Weight of the Encephalon and its Subdivisions in Sane 
 Persons, the Records being arranged according to Sex, Age, and Stature 
 (from Marshall's tables based on Boyd's records). 1 
 
 MALES. 
 
 FEMALES. 
 
 
 S3 
 
 o 
 
 
 a 
 
 
 
 | 
 
 
 1 
 
 
 
 
 a' 
 
 3 
 
 
 
 s 
 
 H 
 
 
 
 S 
 
 1 
 
 1 
 
 1 
 
 a 
 
 d 
 
 I 
 
 JQ 
 
 2 
 
 1 
 
 1 
 
 1 
 
 S 
 
 5 
 
 a 
 
 1 
 
 i 
 
 8 
 
 8 
 
 i 
 
 H 
 < 
 
 Stature 175 cm. and upward. 
 
 Stature 163 cm. and upward. 
 
 20-40 
 
 1409 
 
 1232 
 
 149 
 
 28 
 
 23 
 
 134 
 
 1108 
 
 1265 
 
 20-40 
 
 41-70 
 
 1363 
 
 1192 
 
 144 
 
 27 
 
 23 
 
 131 
 
 1055 
 
 1209 
 
 41-70 
 
 71-90 
 
 1330 
 
 1167 
 
 137 
 
 26 
 
 24 a 
 
 130 
 
 1012 
 
 1166 
 
 71-90 
 
 Stature 172-167 cm. 
 
 Stature 160-155 cm. 
 
 20-40 
 
 1360 
 
 1188 
 
 144 
 
 28 
 
 26s 
 
 137s 
 
 1055 
 
 1218 
 
 20-40 
 
 41-70 
 
 1335 
 
 1164 
 
 144 
 
 27 
 
 26s 
 
 131 
 
 1055 
 
 1212s 
 
 41-70 
 
 71-90 
 
 1305 
 
 1135 
 
 142s 
 
 28as 
 
 24 
 
 128 
 
 969s 
 
 1121 
 
 71-90 
 
 Stature 164 cm. and under. 
 
 Stature 152 cm. and under. 
 
 20-40 
 
 1331 
 
 1168 
 
 138 
 
 25 
 
 24s 
 
 130 
 
 1045 
 
 1199 
 
 20-40 
 
 41-70 
 
 1297 
 
 1123 
 
 139a 
 
 25 
 
 25as 
 
 129 
 
 1051 a 
 
 1205 a 
 
 41-70 
 
 71-90 
 
 1251 
 
 1095 
 
 131 
 
 25 
 
 25 as 
 
 123 
 
 974 
 
 1122 
 
 71-90 
 
 1 a indicates that a record considered according to age is too large ; s indicates that a record 
 considered according to stature is too large. 
 
276 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 The method of weighing the brain used by Dr. Boyd * was as follows : 
 The skull-cap being removed and the pia intact, the hemispheres were sliced 
 away by horizontal sections as far down as the tentorium. The parts of the 
 hemispheres still remaining were then removed by a section passing in front 
 of the quadrigemina. The cerebellum was next separated from the stem, this 
 latter being represented by the quadrigemina, the pons, and the bulb. Each 
 hemisphere, the cerebellum, and the stem was then weighed separately. 
 
 If groups of similar ages and corresponding statures, as entered in the 
 Table, are compared according to sex, it is at once seen that the male pos- 
 sesses the heavier encephalon, and that all the subdivisions of it are likewise 
 heavier. 
 
 When individuals of the same sex and falling within the same age-limits 
 are compared according to stature, those having the greater stature are found 
 to have the greater brain- weight, though in the case of the subdivisions of the 
 encephalon, and especially among the females, there are some irregularities, 
 but these would probably disappear could the number of observations be 
 increased. Finally, within the groups of those having the same stature, but 
 different ages, the weight decreases with advancing age. The middle group, 
 forty-one to seventy years of age, is in one way unfortunate, because, while 
 the brain is probably still growing (see curve of growth, Fig. 118) during the 
 first third of that period, and is nearly stationary (males especially) during 
 the second, it begins to diminish so rapidly during the last third that the 
 average weight is lower for the cases between sixty-one and seventy years 
 than for the twenty years between forty-one and sixty years. Between 
 seventy -one and ninety years the involutionary changes in the central system 
 are most marked, and the decrease in weight during this period is clearly 
 indicated. 
 
 Before suggesting an explanation of these variations according to age, sex, 
 and stature, it is to be noted that they occur in other mammals as well as in 
 man. As regards the difference in the weight of the encephalon due to sex, 
 it has been shown to obtain among the apes, 2 the male having the heavier 
 brain ; and from the general relation of size according to sex among the mam- 
 malia, where the male as a rule has the greater body- weight and larger skull, 
 it is to be anticipated that a similar difference in the weight of the brain will 
 be shown in other genera. 
 
 Among individuals of the same species, but of different races or of different 
 statures and weights, the law holds good that the larger races have the heavier 
 brains, as do the larger and heavier individuals. 3 Here, as in the case of 
 man, it is always assumed that the differences in body-weights are mainly 
 correlated with the active tissues like muscle, and not with fat. As to the 
 
 1 Philosophical Transactions of the Royal Society, London, 1860 ; see also Marshall: Journal 
 of Anatomy and Physiology, 1892. 
 
 2 Keith : Journal of Anatomy and Physiology, 1895. 
 
 3 Du Bois, in the Arch, fur Anthropol., Bd. xxv., maintains that among forms which may be 
 fairly compared, the formula E S' 56 , will give the weight of the encephalon, E being the 
 encephalic weight and S the body-weight. 
 
CENTRAL NERVOUS SYSTEM. 277 
 
 loss of the brain in weight after maturity, observations on animals are scanty, 
 but point to decrease in weight toward the natural close of life. 
 
 Interpretations of Weight. Assuming as the simplest case that the 
 number of the nerve-elements composing a given portion of the central system 
 is constant within the limits of the same species, then differences in the weight 
 of these portions in different individuals imply variations in the size of the 
 component cells. The significance of variations in the size of the nerve-ele- 
 ments must be, primarily, that the larger the cells, and especially the larger 
 the cell-bodies, the greater the mass of cell-substance ready at any moment to 
 undergo chemical change leading to the release of energy, and the more nu- 
 merous the probable connections. On the other hand, if the number of ele- 
 ments is variable, an increase in the number must, in view of the law of 
 isolated conduction, also provide a larger number of conducting pathways. 
 Whether this increase in the number of pathways shall further add to the 
 complication of the system depends on the localities at which it occurs. 
 
 In the absence of fuller data, the explanation of the series of differences 
 shown in Boyd's table is in a very high degree tentative. The loss of weight 
 in advanced years appears to be due to a general atrophy of the nerve-ele- 
 ments. The greater brain-weight associated with greater stature appears to 
 depend on the variations in the size of the elements rather than in their num- 
 ber, and, so far as can be seen, the distinction according to sex is also sus- 
 ceptible of the latter explanation. 
 
 Weights of Different Portions. A study of the proportional weights of 
 the several subdivisions of the encephalon according to the sex, stature, and 
 age, shows that there is very little difference caused by variations in these 
 conditions. This, too, so far as it goes, suggests that the absolute weight is 
 dependent rather on variations in the size than in the number of the elements, 
 since an harmonious variation in number would be less probable than an har- 
 monious variation in size. 
 
 Social Environment. It is not to be expected that the weight of the 
 brain among the least-favored classes in any community will be the same as 
 that of those who, during the years of growth, are under favorable conditions. 
 All extensive series of observations which we possess relate to the least- 
 favored social classes, and hence it is not improbable that the figures in the 
 foregoing tables, which are based on data obtained mainly at the Marylebone 
 Workhouse in London, are decidedly below those which would be obtained 
 from the more fortunate classes in the same community. We have a list of 
 brain-weights which contains the records for a number of men of acknowl- 
 edged eminence, and also for others who attained recognition as able persons 
 without being exceptionable remarkable. This list shows the persons thus 
 selected to have brains on the average heavier than the usual hospital sub- 
 ject. 1 
 
 Brain-weight of Criminals. The observations of Manouvrier have shown 
 that among French murderers the brain-weight is similar to that of the incli- 
 1 Donaldson : The Growth of the Brain, 1895. 
 
278 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 viduals usually examined in the Parisian hospitals. In the same manner, the 
 observations on the brain-weight among the insane indicate, according to the 
 records of Boyd and others, that the insane as a class (the microcephalies 
 being excluded) are not characterized by a special brain-weight. When, 
 however, the insane are grouped according to the form of disease from which 
 they have suffered, it is evident that those in which the brain was congested 
 at death exhibit the higher weight, while those in which the pathological 
 processes caused destructive changes, exhibit a low weight. The differences 
 in these cases are rather the results of disease than the cause of it. 
 
 Brain-weights of Different Races. Concerning the weights of the brain 
 in different races there are no extensive observations which have been made 
 directly on the brain itself. Davis, 1 however, has determined the cranial 
 capacities of a series of skulls belonging to different races, and the brain- 
 weights have been calculated from these. 2 This calculation gives the largest 
 brain-weights to the western Europeans, but for a proper interpretation of 
 the results there are needed at least the data concerning stature and age of 
 the cases studied, both of which are here lacking. 
 
 "Weight of the Spinal Cord. Comparatively few observations are avail- 
 able for the spinal cord : Mies 3 found that in adults it weighed 24 to 33.3 
 grams, with an average weight of 26.27 grams ; this for the cord deprived of 
 the nerve-roots but covered by the pia. The variations due to sex and stat- 
 ure have not been determined. It seems probable, however, that the cord, 
 like the brain, will be found lighter in females and in short persons : Mies 
 states that its decrease in old age is proportionately less than that of the brain. 
 
 B. GROWTH-CHANGES. 
 
 The characters of the brain and cord thus far described have been those 
 found in the adult. Between birth and the natural end of life, however, 
 great changes take place, and, as it is necessary to consider the functions of 
 the central system at all times in its history, the importance of knowing the 
 direction in which the growth-changes are probably occurring is obvious. 
 
 Growth of the Brain. The weight of the brain from birth to the 
 twenty-fifth year is shown in Fig. 118. The curve is based on the 
 table of Vierordt. 4 
 
 The curve beyond the twenty-fifth year is continued on the basis of the 
 observations by Bischoff, 5 and for comparison the curve representing the en- 
 cephalic weights of a series of eminent men, forty-five in number, is drawn 
 in a dotted line, the averages for decennial periods being alone dotted. 
 
 These records exhibit the fact that at birth the weight of the brain is about 
 one-third of that which it will attain at maturity. The increase is very rapid 
 
 1 Journal of the Academy of Natural Sciences, Philadelphia, 1869. 
 
 3 Donaldson : Growth of the Brain, 1895, p. 115. 
 
 3 Neurologisches Centralblatt, 1893. 
 
 * Archiv fur Anatomic und Physiologic, 1890. 
 
 5 Hirngenncht des Menschen, Bonn, 1880. 
 
CENTRAL NERVOUS SYSTEM. 
 
 279 
 
 during the first year, and vigorous for the first seven or eight years, after 
 which it becomes comparatively slow. The maximum weight is indicated 
 
 !fo 
 
 re 3 
 
 ~ 2 o 
 
 cr 
 5,3=0 
 
 lil 
 
 CD 3 p 
 O O g 
 
 
 o 
 
 ?s 
 
 A 
 
 O. a> 
 
 2 B 
 
 111 
 
 ill 
 
 3 3 
 
 S3 
 
 3 " 
 
 ill 
 
 hi 
 
 I I B 
 
 CT5 A C 
 
 f^ O p 
 
 / 
 
 X 
 
 55 
 
 8 
 
 
 
 in the fifth decade (males), fourth (females), although there is a " premaxi- 
 mum " in the middle of the second decade (at thirteen and fifteen years for 
 males and fourteen years for females), in which the too early and too vigorous 
 
280 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 growth of the encephalon appears to be an important factor in the cause of 
 death ; hence the larger brain-weight found at autopsies during these years. 
 While, in general, the individual may be supposed to follow, in the develop- 
 ment of his encephalon, the course here indicated by the curve, this premax- 
 imal increase must be excepted for the reasons given. 
 
 Relation between Growth of Body and that of Encephalon. On com- 
 paring the growth of the entire body with that of the encephalon, it is evi- 
 dent that the growth is more rapid in the central nervous system than in the 
 body at large, and that it is almost completed in the former at the end of the 
 eighth year, whereas the body has at that time reached but one-third of the 
 weight which it will attain at maturity. 
 
 A causal relation between a well-developed central system and the subse- 
 quent growth of the entire body is thus suggested, and also it is evident that 
 conditions which influence growth will at any time find the body on the one 
 hand, and the central system on the other, at quite different phases in their 
 development. 
 
 The long-continued growth of the body brings it about that the central 
 system, which at birth may form 12 per cent, of the total weight of the indi- 
 vidual, is at maturity about 2 per cent, or less. For this change in propor- 
 tion the increase of the muscular system is mainly responsible. 
 
 Further, the much smaller mass of the muscular system in the female is the 
 chief cause of the higher percentage value of the central system in the female 
 a relation which has been much emphasized, but which is really not signifi- 
 cant, since in both sexes this high percentage value of the central system is 
 most developed at birth, and becomes steadily less marked as maturity is 
 approached. 
 
 Increase in the Number of Functional Nerve-elements. Having thus 
 briefly indicated the facts of growth so far as they can be detected by the 
 balances, it still remains to mention the series of changes which may be 
 studied by other means, such as micrometric measurements or enumeration. 
 The results obtained by these methods are somewhat complex, and must be 
 treated with great care. Human embryology indicates that after the third 
 month of foetal life the number of cells in the central system is not increased. 
 With the cessation in the production of new cells the only remaining means 
 of increase in size is by enlargement of those cells already present. 
 
 How this occurs is well indicated by the accompanying table (p. 281), 
 which shows the change in the size of cell -bodies in a given locality in 
 man. 
 
 All vertebrates are not similar in respect to the manner of this change. 
 Birge l has shown that in frogs there is a gradual increase in the number of the 
 fibres forming the ventral and dorsal spinal roots, and that this goes on at the 
 rate of about fifty additional fibres in the ventral roots and seventy in the 
 dorsal, for each gram added to the total weight of the frog. The increase 
 was still apparent in a frog weighing 112 grams. In the case of the ventral 
 
 1 Birge : Archivfur Anatomie und Physiologic, suppl., 1882. 
 
CENTRAL NERVOUS SYSTEM. 
 
 281 
 
 root-fibres it was also determined that the cells of origin in the ventral horns 
 of the spinal cord increased at a corresponding rate. Here is exemplified an 
 instance of long-continued enlargement of the nervous system by the regular 
 development of immature cells, a method of growth most marked probably 
 in those animals which increase in size so long as they live. 
 
 Volumes of the Largest Cell-bodies in the Ventral Horn of the Cervical Cord of 
 Han (based on Kaiser's records of the mean diameters). 
 
 The volume 700// 3 , in the fetus of four weeks, is taken from His, and the figures represent 
 multiples of that volume. 
 
 Subject. 
 
 Age. 
 
 Proportional 
 volume of 
 the cell-bodies 
 l=7(XVs. 
 
 Time interval. 
 
 Fetus 
 
 4 weeks 
 
 11 
 
 
 ii 
 
 20 " 
 
 17 
 
 
 u 
 
 24 " 
 
 31 ! 
 
 36 weeks 
 
 (i 
 
 28 " 
 
 67 1 
 
 
 < 
 
 36 " 
 
 81 
 
 
 Child at birth . .... 
 
 
 124 ) 
 
 
 Boy at fifteen years . 
 
 
 124 ) 
 
 15 years. 
 
 ]VIiin adult . 
 
 
 160 
 
 15 " 
 
 
 
 
 
 It is believed that in this case the new cells and new fibres are not, 
 strictly speaking, new morphological elements, but are the result of devel- 
 opmental changes taking place in the cells present in the system from an 
 early period. 
 
 A distinction is thus to be made between cell-elements which, because 
 they are not developed, are therefore not a part of the system already physio- 
 logically active, and those cells already organized together and which are 
 fully functional. When, therefore, it is said that the cells of origin for the 
 ventral root-fibres increase in number, the increase refers to the latter group, 
 and not to the total number of elements of both kinds present in the cord. 
 In other words, the number of cells appears to increase because the number 
 of developed cells becomes greater. 
 
 In support of this suggestion the observations on the growth of the fibres 
 in the roots of the frog's spinal nerves may be cited. 1 Hardesty found the 
 greatest number of medullated fibres in the ventral roots, nearest the cord, 
 and in the dorsal roots, nearest the spinal ganglion. Thus in each the 
 greatest number was nearest the cells of origin, an arrangement which is 
 most readily explained by assuming that some of the fibres had grown but a 
 short distance from their cells of origin at the time the frog was killed. 
 
 On the other hand, Schiller 2 counted the number of nerve-fibres in the 
 oculo-motor nerves of cats, and found but a very slight difference in this 
 number between birth and maturity. So far, then, as this nerve is concerned, 
 it is found in the cat to be nearly complete at the time of birth. 
 
 In man there are very few observations on the increase in the number of 
 
 1 Hardesty : Journal of Comparative Neurology, 1899, vol. ix. 
 
 2 Schiller: Comptes rendus de I' Academic des Sciences, Paris, 1889. 
 
282 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 functional nerve-cells with age. Kaiser, 1 as is shown in the accompanying 
 table, found in man increasing numbers of large nerve-cells in the ventral 
 horns of the spinal cord at the ages named : 
 
 Number of Developed Cells in the Cervical Enlargement of Man at Different 
 
 Ages (Kaiser): 
 
 Age. Number of Nerve-cells. 
 
 Fetus, 16 weeks 50,500 
 
 " 32 " 118,330 
 
 New-born child 104,270 
 
 Boy, 15 years 211,800 
 
 Male, adult 221,200 
 
 Here, as in the frog, the apparent increase must be looked upon as due to 
 the gradual development of elements present from an early date. And it 
 must be further remembered that in this case the cells thus maturing after 
 birth probably belong in a large measure to the group of " central cells," 
 the function of which is to increase the complexity of the pathways within 
 the cord. 
 
 FIG. 119. Diagram illustrating the extent of the cerebral cortex. The outer square v - > shows a sur- 
 face approximately one-fiftieth of 2352 sq. cm. in extent ; the inner square (A) has two-thirds of this area, 
 and is the proportion of the cortex sunken in the fissures. 2352 sq. cm. are approximately the area of the 
 entire cortex in a male brain weighing 1360 grams. 
 
 Increase in the Fibres of the Cortex. The area of the cerebral cortex 
 (see Fig. 119) varies according to several conditions, but in general the more 
 voluminous the cerebral hemispheres the greater its extent. That which 
 covers the walls of the sulci, the sunken cortex has in man about twice 
 the extent of that directly exposed on the surface of the hemispheres. 
 
 In the cortex of the human cerebral hemispheres it has been shown by 
 Vulpius 2 that the number of fibres in the different layers is greater in the 
 
 1 Die Fanctionen der Ganglienzelien des Halsmarkes, Haag, 1891. 
 3 Vulpius: Archivfiir Psychiatric und Nervenlcrankheiten, 1892. 
 
CENTRAL NERVOUS SYSTEM. 283 
 
 thirty-third year than ai earlier periods, hut in old age this number is 
 decreased. At exactly what age decrease sets in, is not to be determined 
 from these observations. They show, simply, that in general the number of 
 fibres was less at seventy-nine years than at thirty-three year-. 
 
 In a similar way Kaes has shown l that the association fibres of the human 
 cerebral cortex form three parallel systems. In general it is the deepest 
 layer i. e., that farthest from the surface of each system which first becomes 
 medullated. The first fibres appear at about the fourth month of life in the 
 deepest portion of the deepest layer. The middle system is the last to be 
 completely medullated, this process continuing in it up to the forty-fifth 
 year of life. 
 
 Passow 2 has shown that at maturity the cortex of the central gyri exhibits 
 association fibres which increase in abundance as we pass from the great 
 longitudinal fissure (leg area) toward the Sylvian fissure, these fibres being 
 most abundant in the areas for the hand and fingers. On the other hand, in 
 the central gyri of a child fifteen months of age, these fibres are equally 
 abundant in these two localities. From this it appears that the differentia- 
 tion takes place after the first year of life. 
 
 Significance of Medullation. Two sorts of nerve-fibres are described 
 those with and those without a medullary sheath. Both have the power of 
 isolated conduction, but in the peripheral system the non-medullated fibres 
 are found in connection with the sympathetic system, where less specialized 
 functions are carried on, and also in a large but varying degree in the central 
 system. The wider significance of this difference in medullation is at the 
 moment quite obscure. 
 
 The first suggestion, that absence of the medullary sheath is an immature con- 
 dition which persists in various parts of the nervous system, brings us at once 
 to the question of the physiological difference thus implied, but not explained. 
 
 It is known that the central system is at birth very imperfectly medul- 
 lated, and the growth of these medullary sheaths must form a large part of 
 the total increase in its bulk. In the mature nerve-fibre the axis-cylinder 
 and the medullary sheath have nearly equal volumes, and therefore approxi- 
 mately equal weights. The medullated fibres form probably not less than 
 97 per cent. 3 of the total weight of the nerve-tissues composing the encepha- 
 lon, and of this nearly one-half would be medullary substance. 
 
 Increase in the Mass of the Neurones. The amount of this increase 
 under various conditions has already been discussed, and been found to range 
 between zero. and fifty-thousand-fold (p. 176). 
 
 Number of Cells. Any attempt to determine the entire number of 
 nerve-cells, the bodies of which lie within the walls of the neural tube, must 
 be open to many sources of error. 
 
 A Kaes: Wiener med. Wochenschrift, 1895, Nos. 41, 42; Kaes: Jahrbuchern der Hamburg, 
 Stoats Krankenanstalten, Jahrgang, 1893-94, Bd. iv. 
 
 2 Passow : Archivfiir Psychiatric, Bd. 31, S. 859, 860. 
 
 9 Thompson : Journal of Comparative Anatomy, 1899, vol. ix. ; Donaldson: Ibid., No. 2. 
 
284 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 A careful study of the cortex l based on Hammarberg's records, gives 
 9200 million cell-bodies in this region alone. Considering the amount of 
 gray matter present in the rest of the central system, an estimate of 13,000 
 million for the total number in the entire central nervous system is probably 
 a conservative calculation. 
 
 From the foregoing facts, together with those bearing on the cell-elements, 
 it is possible to get some conception of the growth-processes in the central 
 system, and to see how they are due to an enlargement of the nerve-elements 
 which have been formed at a very early stage in the life-history of the indi- 
 vidual. In such enlargements the chief increase is due to the formation of 
 the axones, and in them, in turn, about half the substance is represented by 
 the medullary sheaths. 
 
 In all probability these sheaths are no exception to the rule according to 
 which all parts of the body are variable, not only in their absolute, but also in 
 their relative size, and therefore it is possible that the quantitative variation 
 in this constituent is a very important factor in modifying the weight of the 
 central system, perhaps accounting in some cases for the very heavy brains 
 occasionally reported. 
 
 Change in Specific Gravity with Age. During fetal life and at birth 
 the percentage of water in the nerve-tissues is high, but becomes less at 
 maturity. In white rats at birth the percentage of water for the encephalon 
 is 89 per cent, and for the spinal cord 85.3 per cent. At maturity it is about 
 78 per cent, for the encephalon and 70.1 per cent, for the cord. This change 
 is correlated in some measure with the development of the medullary sub- 
 stance. 
 
 For the gross physical changes which have thus been indicated as occur- 
 ring during growth, an explanation is to be found in the changes affecting the 
 constituent elements, and these have been set forth when describing the 
 growth of the individual cells. 
 
 O. ORGANIZATION AND NUTRITION OF THE CENTRAL NERVOUS SYSTEM. 
 
 What is here meant by organization may be easily illustrated. When, for 
 example, by later growth new tissue is added to the liver, or the skin is in- 
 creased in area or a muscle enlarged, there is caused by the addition of new 
 substance a change in the pow r ers of these tissues, which is mainly quantita- 
 tive. The larger organ exhibits the same capabilities that the smaller organ 
 exhibited, but does so in a greater degree. 
 
 In the central nervous system, on the other hand, it appears that with 
 growth the system becomes capable of new reactions in the sense that its 
 various responses are controlled and directed by a larger number of incoming 
 impulses, and thus the number, complexity, and refinement of the reactions 
 are increased, and in this sense it really attains new powers. 
 
 With the change in the age of the central system there occurs from birth 
 up to the prime of life, if we may judge from general reactions, an increase 
 1 Thompson : Journal of Comparative Neurology, 1899,, vol. ix. 
 
CENTRAL NERVOUS SYSTEM. 285 
 
 in this organization. This is maintained for a time, and then in old age it 
 breaks down, at first gradually, and later rapidly. It becomes important, 
 therefore, to examine the manner in which this organ i/at ion is accomplished. 
 
 Organization in the Central System. When first formed the cells com- 
 posing the central system are completely separated from one another. In the 
 mature nervous system the impulses, as has been pointed out, probably travel 
 for the most part from the axones of one unit to the dendrites of another. 
 
 For organization the most important changes, however, are those affecting 
 the cell outgrowths, both dendrites and axones. During growth both of these 
 increase in the length of their main stems and of their respective branches. 
 In picturing the approach of two elements within the central system the pro- 
 cess is usually described as that of the outgrowth of the axone toward the 
 dendrites or bodies of those cells which are destined to receive the impulse, 
 but it must not be forgotten that the dendrites are also growing, and the 
 question of the approximation of the branches of these latter to those of the 
 axone depends in part on their own activities. 
 
 The conditions modifying this process are, however, obscure. It is evi- 
 dent that medullation outside of the central system is not necessary to the 
 functional activity of a fibre, and therefore probably in the central system 
 unmedullated fibres are also in many cases functional. Whatever may be 
 the relation of the establishment of new pathways to the acquisition of medul- 
 larv sheaths by the axone and its branches, it is also found that all fibres 
 which when mature are medullated begin as unmedullated fibres, and that 
 the increase in medullation throughout the central system is an index of the 
 increase in organization. A consideration of the facts of growth in the layers 
 of the cortex, for instance, will show them to be open to this interpretation. 
 
 Applying these ideas concerning organization to the three classes of cells, 
 afferent, central, and efferent, which compose the nervous system, we find the 
 following : In the central system the afferent cells contribute to organization 
 by the multiplication of the collaterals. At the periphery the division of the 
 branches of the axone increases the number of opportunities for excitation 
 which such an element offers. These cells are, for the most part, without 
 dendrites. Among the central cells all possible modes of growth are con- 
 tributory ; that is, the branches of both kinds add directly to the complexity 
 of the central pathways. On the other hand, the efferent group contributes 
 to this complexity almost solely by the formation of dendrites, the collaterals 
 which come from the axones of these cells forming but an insignificant con- 
 tribution. Not only, therefore, is organization in large part dependent on 
 changes in the central cells by reason of their numerical preponderance, but 
 also by reason of the fact that to them a multiplication of pathways both by 
 elaboration of the axones and the dendrites is alone possible. 
 
 Defective Development. In view of these facts, defective development 
 in the nervous system may depend on failure in one or more of these several 
 processes by which the system is organized, and it should be possible to corre- 
 late defective development involving mainly one set of elements with a dis- 
 
286 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 tinct clinical picture. The results of defective development are not merely 
 an absence of certain powers, but in some measure a diminution in the strength 
 and range of those that remain (Hammarberg T ). 
 
 Laboratory Animals. The bearing of these facts on the conception 
 which we form of the nervous systems of those animals commonly employed 
 for laboratory .experiments may be here mentioned. The frog, pigeon, rabbit, 
 cat, and dog form a series in which the total mass of the central system in- 
 creases from the beginning to the end of the series. 
 
 The number of cells in the largest system, that of the dog, is many times 
 greater than that in the smallest, the frog ; and it is probable that the others 
 are in this respect intermediate. Organization is apparently more rapidly 
 completed and more nearly simultaneous throughout the entire system in 
 forms like the frog and pigeon, and also in these latter the organization is 
 least elaborate. While the educability of the nervous system of the dog may 
 depend on several conditions, the comparative slowness of organization is 
 undoubtedly one of them, and a very important one. Where the organiza- 
 tion is early established it is also simple, and thus portions of the system 
 retain through life a greater capacity for acting alone. In selecting an ani- 
 mal, therefore, on which to make a series of experiments, these several facts 
 must be kept in view, for the choice is by no means a matter of indifference. 
 
 Blood-supply.- For the general distribution of the blood-vessels in rela- 
 tion to the gross subdivision of the brain the student is referred to the works 
 on anatomy. The finest network of vessels is, however, to be found where 
 the cell-bodies are most densely congregated, and indeed the distinction 
 between the masses of gray and white matter in the central system is as 
 clearly marked by the relative closeness of the capillary network as in any 
 other way (see p. 191). One result of this relation between the blood-sup- 
 ply and the cell-bodies which form the gray matter is a general arrangement 
 of the vessels along the radii of the larger subdivisions of the brain, as the 
 cerebral hemispheres and the cerebellum. 
 
 The conditions which control the circulation within the cranium and 
 spinal canal are not exactly the same at all periods of life, but the variations 
 occur in minor points only. 
 
 The studies of Huber 2 show that in the cat, dog, and rabbit at least, the 
 vessels in the pia of the cerebral hemispheres are supplied with both medul- 
 lated and unmedullated nerves. The former are probably sensory in func- 
 tion ; the latter, possibly, vaso-motor. These latter nerves have been fol- 
 lowed to arteries so small as to possess but two layers of muscle-cells, but 
 were not traced by Huber to vessels actually penetrating the nervous sub- 
 stance of the hemispheres, von Kolliker, however, claims to have followed 
 them even there. 
 
 These observations make the existence of a corresponding vaso-rnotor 
 
 1 Hammarberg : Studien ueber Klinik und Pathologic der Idiotic nebst Untersuchungen ueber 
 die normale Anatomic der Hirnrinde, Upsala, 1895. 
 
 2 Journal of Comparative Neurology, 1899, vol. ix. No. 1. 
 
CENTRAL NERVOUS SYSTEM. 287 
 
 .supply to the pial vessels in man probable. Nevertheless the efficiency of 
 this vaso-motor mechanism does not appear to be great, since various authors 
 fail to find physiological evidence for a local control of the arterioles. 
 
 The reactions of the central vessels are broadly those of a system of elastic 
 tubes in a closed cavity. As a result, it is found that the quantity of blood 
 in the central system is subject to very slight variations only. A rise in the 
 arterial pressure causes a more rapid flow of the blood through the encepha- 
 lon. It also causes a rise in the venous pressure, and with this a correspond- 
 ing rise in the intracranial pressure, the last two varying in the same sense 
 and to the same extent. 1 
 
 The flow through the central system is subject to the influence of gravity, 
 and takes place the more readily the more the resistance is diminished. 2 
 The principal controlling mechanism is in the splanchnic area. According 
 to the condition of the vessels in this area, the intracranial blood-pressure 
 varies. 
 
 It is to be noted in passing that when a person lying on a table is bal- 
 anced on a transverse axis, this axis is about 8.77 cm. to the cephalic side of 
 the line which joins the heads of the femurs. 3 This leaves, of course, the 
 splanchnic area mainly on the cephalic side of this axis, and hence any inflow 
 of blood from the extremities would tend to make the head end of the person 
 thus balanced dip down. This dip will occur even when the splanchnic area 
 alone is filled, and hence the dipping as such would not necessarily indicate 
 an increase in the quantity of blood in the encephalon. 
 
 In the adult the cranial cavity is almost rigidly closed. There is an 
 opportunity for the escape of a small quantity of cerebro-spinal fluid through 
 the foramen magnum into the vertebral canal. When, as the result of in- 
 creased arterial pressure, the brain has distended so as to drive out the sub- 
 dural fluid, the brain is forced against the walls of the cranium and blocks 
 the outflow into the spinal canal. On the other hand, it has been found 
 that if a mass displacing from 2 to 3 c.c. be introduced into the subdural 
 space of a dog, the brain will adjust itself without rise of intracranial pres- 
 sure. If in this case the volume of the mass introduced is increased, there 
 follows a rise of intracranial pressure, and this rise in every instance tends to 
 impede the circulation through the brain. While the fontanelles are open 
 the brain normally pulsates, and we recognize in its variations in volume all 
 the different variations in blood -pressure with which we are familiar. The 
 pulsation of the brain is doubtless an important aid to the movements of the 
 fluids within, and hence tends to facilitate nutrition during the earlier stages 
 of growth. 
 
 In pathological cases where the cranial wall has been destroyed there is 
 a similiar variation in volume to be observed in the adult, and it is possible 
 that the beneficial effects which in so many instances follow trephining of 
 
 1 Ho well : American Journal of Physiology, 1898, i. No. 1. 
 
 2 Hill : Journal of Physiology, 189o, vol. xviii. 
 
 3 W. und Ed. Weber: Mechanik der menschlichen Gehwerkzeuye, 1836. 
 
288 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 the skull may depend upon this mechanical release. Of course, in cases with 
 a defective skull-wall an increase in arterial pressure causes a more decided 
 increase in the volume of blood in the brain ; this, however, is much more 
 marked than it would be under ordinary conditions, and is not to be regarded 
 as the main eifect, which is an increase in the quantity of the blood passed 
 through the central system in a unit of time. Mosso * has found the tempera- 
 ture of the blood coming from the brain (dog's) slightly higher than that of 
 the rectum and of the arterial blood. The differences are very small, but he 
 draws the conclusion that the metabolic processes in the brain are sufficiently 
 intense to raise the temperature of the blood passing through it. 
 
 As against the intensity of the metabolism in the'central system, it has 
 been observed that blood taken from the torcular Herophili of the dog was 
 intermediate in gaseous content between arterial blood and that taken from 
 the femoral vein, thus indicating that the arterial exchange was less intense 
 in the brain than in the muscles of the leg. The following is a condensed 
 statement of the figures : 
 
 Percentages of Oxygen and Carbonic Acid in various Samples of Dogs' Blood 
 
 (Hill): 1 
 
 f CO, . . 37.64 per cent. 
 Average of 52 arterial samples 1 O 18 25 ' 
 
 f CO 2 . . 41.65 
 Average of 42 torcular samples 1 O 13 49 
 
 Average of 28 femoral vein samples { O 2 6 34 
 
 The absolute quantity of the blood in the brain and cord is certainly 
 small ; if we may judge from the observations on animals, it is not more 
 than 1 per cent, of the entire blood in the body. It is to remembered, how- 
 ever, that the cell-bodies, which alone are well supplied with blood, probably 
 represent less than 2 per cent, of the entire encephalic mass. 
 
 With general rise and fall of pressure elsewhere, there is a rise and a fall 
 of pressure within the central system. During the first phases of mental 
 activity blood is withdrawn from the limbs ; the blood thus withdrawn can 
 be shown to pass toward the trunk and head, for when a person lying on a 
 horizontal table supported at the centre on a transverse knife-edge is just 
 balanced, then increased activity of the cerebral centres causes the head to 
 dip down (Mosso), and if the skull-wall is defective the brain is seen to swell. 
 
 In the later stages of fatigue the blood-supply to the nerve-centres dimin- 
 ishes owing to a decrease in force of the heart-beat and the tonicity of the 
 splanchnic vessels, so that the brain in birds exhausted by a long flight 
 has been found by Mosso to be in a high degree anaemic. There is much 
 reason to think that in man a similar reaction occurs. 
 
 The study of the cerebral circulation in the case of those in whom the 
 skull-wall is at some point deficient shows a bulging of the skin over the 
 
 1 Die Temperatur des Gehirns, Leipzig, 1894. 
 
 2 Journal of Physiology, 1895, vol. xviii. 
 
CENTRAL NERVOUS SYSTEM. 289 
 
 opening into the cranial cavity as a result of mental effort or emotion. In 
 the normal adult this bulging cannot, of course, occur to anything like such an 
 extent, and the space for the arterial blood must be gained both by driving 
 out the blood from the cerebral veins within the cranium and through the 
 expulsion of the subdural fluid. 
 
 Influence of Glands. In the growth of the nervous system it is not only 
 the quantity, but the peculiar qualities of the blood that are important, and 
 among the various glands the activity of which is so necessary for the growth 
 of the nervous, as well as the other systems, and is also needed for its full 
 maintenance, the thyroid appears as very important. In sporadic cretinism, 
 associated as it is with atrophy of the thyroid, the feeding of sheep's thyroids 
 has produced remarkable growth-changes in all parts of the body the nerv- 
 ous system included. 
 
 At the same time, experimental extirpation of the thyroid is followed by 
 destructive changes in the central system, caused by disturbances in its nutri- 
 tion. The future will doubtless reveal other forms of internal secretion which 
 also have a significance for the activity of the central system. 
 
 Starvation. In starving animals the nervous system loses but very little 
 in weight. 1 This small loss is most striking in view of the fact that exten- 
 sive histological changes occur in the cell-bodies. However, if we consider 
 the cell-bodies as the part mainly affected during starvation, then the small 
 mass of the cell-bodies would go far toward explaining the result, but it does 
 not explain why the myeline is so resistant. 
 
 Fatigue. The histological basis of fatigue as expressed by the changes in 
 the individual cells, has already been discussed. The fatigue of the system as 
 a whole is but the expression of fatigue in large numbers of its elements, but 
 the manner in which the changes show themselves is somewhat complicated. 
 
 When the attempt is made to raise a weight by the voluntary contractions 
 of the muscles of the index finger at regular intervals, say once a second, it 
 is found that if the weight be heavy the power of the finger decreases, and 
 the weight soon ceases to be lifted as high as at first. Finally a point is 
 reached when the voluntary effort produces little or no elevation of the weight. 
 If, however, despite this failure, the effort is still made at regular intervals, 
 it happens, in some persons, that this power returns gradually, and a few sec- 
 onds later the contractions are very nearly as high as at the beginning of the 
 experiment (Mosso). This phenomenon may repeat itself many times, giving 
 a record formed by groups of contractions most extensive near the centre of 
 each group, these latter being separated by portions of the curve in which 
 the contractions are very small or wanting (see Fig. 120). (See General 
 Physiology of Nerve and Muscle, p. 135.) 
 
 Daily Rhythms. Within the cycle of the astronomical day the progress 
 of events leading to fatigue is not a steady one. Lombard 2 found that if the 
 capacity for voluntary effort was measured by the amount of work which 
 
 1 Voit: Zeitsclmft far Biologic, 1894, P.d. xxx. 
 
 2 Journal of Physiology, 1892, vol. xiii. 
 VOL. IT. 19 
 
290 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 could be done by voluntarily contracting the flexor muscles of the index 
 finger before the first failure to respond to a voluntary stimulus appeared, then 
 
 FIG. 120. A record of the extent of the flexions of the forefinger lifting a weight at regular intervals. 
 The light lines are those for the voluntary contraction ; the heavy lines, those for contractions following 
 the direct stimulation of the flexor muscles by electricity. In the former there are periods, in the latter 
 none. The arrow shows the direction in which the record is to be read (Lombard). 
 
 the curve expressing this capacity for voluntary work throughout the day 
 was represented as in Fig. 121. Briefly, the curve shows two maxima, at 10 
 P. M., and 10 A. M., with two minima midway between them. In general 
 
 80 
 
 40 
 
 FIG. 121. Showing at each hour of the day and night how many centimeters a weight of 3000 grams 
 could be raised by repeated voluntary contractions of the forefinger before fatigue sets in. The curve is 
 highest at 10 to 11 A. M , and 10 to 11 p. M.; lowest, 3 to 4 p. M., and 3 to 4 A. M. Circle with dots, observa- 
 tion made just after taking food ; square with dot, smoking ; *, work done 8 minutes after drinking 15 
 c.c. of whisky (Lombard). 
 
 the immediate effect of taking food is to increase the work done by the sub- 
 ject. Alcohol has the same effect, while smoking produces a decrease. 
 
 Further, from day to day this capacity for work is influenced by a num- 
 ber of external conditions temperature, barometric pressure, etc. 
 
CENTRAL NERVOUS SYSTEM. 291 
 
 Time Taken in Central Processes. All processes in the nervous system 
 take time, and are for the most part easy to measure. The rate of the nerve- 
 impulse has already been given. When, however, it passes from one element 
 to another, the delay is even more marked, and it is plausible to assume that 
 this detention occurs at the juncture of the elements. Thus in those parts of 
 the central system where the cell-elements and also the cell-junctions are most 
 numerous, the time taken is longest. 
 
 Fig. 122 shows this very well. Between the middle of the cerebral hem- 
 isphere and the optic lobes, although the distance is short, the impulse takes 
 twice as long to travel as between the bulb and the lumbar enlargement. 
 When this time is measured in the conscious individual it is, of course, open 
 to a long series of modifying conditions, and these appear to be in part the 
 same conditions which modify the muscular endurance of the individual at 
 different portions of the day. Thus it has been determined that the speed 
 with which reactions can be made as indicated by the reaction time, is subject 
 to variations, and does not steadily decrease from the morning to the evening. 
 
 0.5 Sec. 
 
 FIG. 122. To show the rate at which impulses pass through the nervous system of a frog. At the 
 extreme left the vertical has the value of 0.5 second and the other verticals are compared with it; thus 
 between the cerebrum and the optic lobe requires about 0.25 second ; between the bulb and the lumbar 
 enlargement a greater distance only about half the time ; and for the still greater distance represented 
 by the length of the sciatic nerve even less time is needed (Exner). 
 
 It has been the purpose of the paragraphs just preceding to indicate that 
 through the day it is not possible to demonstrate a steady decline of power in 
 the nervous system. We begin the morning, to be sure, feeling fresh, and 
 are fagged in the evening, but the course by which this condition has been 
 attained is not a simple or direct one. 
 
 D. SLEEP. 
 
 Conditions Favoring Sleep. To recover from fatigue sleep is required. 
 The prime condition favoring sleep is the diminution of nerve-impulses pass- 
 ing through the central system. This is accomplished in two ways. In the 
 first instance it is usual to reduce all incoming stimuli to a minimum. This 
 is most directly under our own control. On the other hand, the permeability of 
 the nervous system and the intensity with which it responds are decreased as 
 the result of the beginning fatigue. How these conditions are brought about 
 has been a matter of much speculation and some experiment. 
 
 The parts played by the sensory and that by the central cells vary some- 
 what at different times of life, for impulses are much less widely diffused in 
 early years than at maturity. Moreover, in childhood the amount of stored 
 material is small, large at maturity, and small again in old age, and this holds 
 
292 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 true for all the groups of cells. Hence the cells would, by reason of this fact, 
 have the greatest capability for work in the middle period. Between child- 
 hood and old age there is, however, this difference that while in the former 
 the non-available substances in the cell are developing, not yet having ma- 
 tured, those in the latter have in some way become permanently useless. The 
 degree to which the blood-supply can be controlled varies with age, and the 
 amounts of substance capable of yielding energy at various periods of life are 
 different ; so that, considering these factors alone, though there are probably 
 others, it may be easily appreciated that the sleep of childhood, maturity, and 
 old age should be quite distinguishable. 
 
 Cause of Sleep. -It is recognized that local exercise is capable of pro- 
 ducing general fatigue, and the fatigued portions give rise to afferent impulses 
 which, reaching the central system, cause some of the sensations of fatigue; 
 moreover, the active tissues (nerve-cells and muscles) yield as the result of 
 their activity some by-product which is carried by the blood through the cen- 
 tral system and becomes the chief cause of sleep. It has been shown by 
 Mosso that if a dog be thoroughly fatigued, giving all the signs of exhaustion, 
 and the blood from this dog be transfused to one that has been at rest, then 
 after the transfusion, the dog which has received the blood from the exhausted 
 animal will exhibit the symptoms of fatigue in full force. The inference is 
 that from the tired animal certain by-products have thus been transferred, 
 and that these are responsible for the reactions. We know, further, that we 
 can distinguish in ourselves different forms of the feeling of fatigue, and that 
 the sensations which follow the prolonged exercise of the muscular system 
 differ from those following the exercise of the higher nerve-centres. 
 
 Two things appear as highly probable : First, that there is a wide individual 
 variation in the condition designated as normal sleep. Second, that normal 
 sleep is the result of several sets of influences which need not necessarily be 
 active to the same degree during each period of sleep. Excluding the factor 
 represented by diminution of the external stimuli, sleep has been attributed 
 more or less exclusively to one of the three following influences : 
 
 1. Chemical Influences. The theories emphasizing the chemical factor 
 point out that during the normal activity of the body there are formed and 
 taken up by the blood substances which may directly diminish the activity 
 of the nerve-cells and directly or reflexly affect the circulation so as to 
 diminish the supply of blood to the brain, and especially to the cerebral 
 cortex. 
 
 2. Circulatory Influences. The vaso-motor theories look upon the changes 
 in the blood-supply as a prime cause of sleep ; these changes to be referred 
 in the last instance to the fatigue of the vaso-motor centre in the bulb. 
 
 3. Histological Influences. These are made dependent on the shrinkage 
 of nerve-cells during fatigue, the retraction of the dendrites of the cortical 
 cells interrupting the nerve-pathways, or the mechanical separation of the 
 nerve-elements through the intrusion of the neuroglia-cells between them 
 (Cajal). The vaso-motor and chemical theories combined are at present most 
 
CENTRAL NERVOUS SYSTEM. 293 
 
 worthy of attention, and Howell, 1 after carefully reviewing the several 
 theories of sleep, emphasizes the fatigue of the vaso-motor centre in the 
 bulb as the important cause of the diminished blood-supply to the brain, 
 this fatigue in turn being caused by the continuous activity of this centre 
 during the waking hours. - 
 
 Cessation of stimuli, decreased responsiveness of the active tissues, a 
 change in the composition of the blood, and a diminution of the blood-supply 
 to the brain are the preliminaries to sleep. 
 
 A condition superficially resembling sleep can be induced in various ways. 
 Removal of all external stimuli, extreme cold, anesthetics, hypnotic sugges- 
 tion, compression of the carotids, a blow on the head, loss of blood, all pro- 
 duce a state of unconsciousness which, in so far, has the similitude of sleep. 
 These conditions produce this state, however, by mechanically decreasing the 
 blood-supply or cutting off the peripheral stimuli. 
 
 3 
 
 '"'\ """"""'"\i 
 
 / 
 
 M'"' 
 
 I'l'U. .il.ll" 1 ' 
 
 2 
 
 'i ill,. 
 
 'i, .,/ 
 
 I ~ 
 
 I, , il 1 m\\\\\\ 
 
 'In,,. , .Hi' 1 ' 1 mm 1 
 
 FIG. 123. Plethysmographic record taken from the arm of a person sleeping in the laboratory. A fail 
 in the curve indicates a decrease in the volume of the arm. The curve is to be read in the direction of 
 the arrow. 1. The night watchman entering the laboratory, waking the subject, who shortly fell asleep 
 again; 2, the watchman spoke ; 3, watchman went out; these changes (2 and 3) occurred without awak- 
 ening the subject (from experiments made by Messrs. Bardeen and Nichols, Johns Hopkins Medical 
 School). 
 
 Normal sleep is tested by the fact that during its progress the changes 
 that occur in the central system are recuperative, whereas this feature may 
 be almost absent in the states which nearly resemble it. 
 
 Condition of the System During- Sleep. It appears that during sleep 
 the capacity of the central system to react is never lost. Were such the case 
 it would not be possible to awaken the sleeper. The reactions most depressed 
 during sleep are those which require the full activity of the cerebral cortex 
 for their occurrence. Conversely, it is the spinal cord which is least affected. 
 Moreover, the sleeping person is far more responsive to stimuli from without 
 than at first might be thought. The close relations between dreams and ex- 
 ternal stimuli have been recognized, and plethysmographic studies show still 
 more clearly how the matter stands. 
 
 It was found that when a subject fell asleep with the arm in a plethys- 
 mograph, various stimuli which did not waken the sleeper still served to 
 cause a diminution in the volume of the arm which was certainly due to the 
 
 1 Howell : Journal of Erperimental Medicine, 1897. 
 
 2 De Manaceine : " Sleep : Its Physiology, Pathology, Hygiene, and Psychology," Contem- 
 porary Science Series, London, 1897. 
 
294 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 withdrawal of blood from it, the blood supplied to the brain being probably 
 at the same time increased (see Fig. 123). 
 
 This experiment shows that during sleep the nervous system is capable 
 of reactions which are not remembered in any way, but which naturally 
 form a feature of the condition intermediate between full consciousness and 
 deep slumber. 
 
 The depth of sleep as determined by the strength of the stimulus necessary 
 to elicit an efficient response has been measured. The stimulus in these ex- 
 periments was the sound caused by the fall of a ball upon a plate, and the 
 measure was the height from which the ball must fall in order to produce a 
 sound loud enough to awaken a sleeping person. The results of the observa- 
 tions are shown in Fig. 124. 
 
 Strength of Stimulus. 
 800 
 
 700 
 
 Hours QJ5 1.0 1.5 ZQ 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6-5 7.0 7.5 7.8 
 FIG. 124. Curve illustrating the strength of an auditory stimulus (a ball falling from a height) neces- 
 sary to waken a sleeping person. The hours marked below. The tests were made at half-hour intervals. 
 The curve indicates that the distance through which the ball required to be dropped increased during 
 the first hour, and then diminished, at first very rapidly, then slowly (Kolschutter). 
 
 It is seen from this that the period of deep slumber is short, less than two 
 hours ; and is followed by a long period, that of an average night's rest, dur- 
 ing which a comparatively slight stimulus is sufficient to awaken. A some- 
 what different curve has been more recently obtained by Monninghoff and 
 Piesbergen. 1 
 
 It is evident that the effectiveness of such a stimulus is, however, no 
 measure of the recuperative processes in the central system. Repair is by no 
 means accomplished during the interval of deep sleep, and experience has 
 shown, as in the case of persons undertaking to walk a thousand miles in one 
 thousand hours that although such an arrangement left the subject with two- 
 
 1 Zeitschrift fur Biologie, 1893, Bd. xix. 
 
CENTRAL NERVOUS SYSTEM. 295 
 
 thirds of the total time for rest and refreshment, yet the feat was most 
 difficult to accomplish by reason of the discontinuity in the sleep. 
 
 The changes leading to recuperation needed longer periods than those 
 permitted by the conditions of the experiment. 
 
 Loss of Sleep. Loss of sleep is more damaging to the organism as a 
 whole than is starvation. It has been found (Manaceine) that in young dogs 
 which can recover from starvation extending over twenty days, loss of sleep 
 for five days or more was fatal. Toward the end of such a period the body- 
 temperature may fall as much as 8 C. below the normal and the reflexes 
 disappear. The red blood-corpuscles are first diminished in number ; to be 
 finally increased during the last two days, when the animal refuses food. 
 The most widespread change in the tissues is a fatty degeneration, and in the 
 nervous system there were found capillary hemorrhages in the cerebral hemi- 
 spheres, the spinal cord appearing abnormally dry and anaemic. 
 
 Patrick and Gilbert 1 have studied the effects of loss of sleep in man 
 (three subjects, young men, observed during ninety hours without sleep). 
 All the subjects gained slightly in weight during the period, but lost this 
 excess in the course of the first sleep following the experiment. The excre- 
 tion of nitrogen and phosphoric acid was increased during the period, the 
 increase being relatively greater in the case of the phosphoric acid. There 
 was a marked tendency to a decrease in the pulse-rate, and some tendency 
 for the body-temperature to fall. During these ninety hours the subjects were 
 tested at intervals of six hours (the tests required some two hours on each 
 occasion), to determine variations in the muscular and mental powers. 
 
 In brief, it may be said that most tests revealed a loss, which early 
 appeared in the reactions of the muscular system, and later in those of the 
 nervous system. In the test for the acuteness of vision (measured by the 
 distance at which the subject could read a printed page illuminated by the 
 light of one standard candle at a distance of 25 cm.) there was, however, 
 an increase in capability in all the subjects. At the end of the experiment a 
 small number of hours of sleep in excess of that customarily taken appeared 
 to bring about a complete restoration of the subject. The disproportion 
 between this amount of extra sleep and the amount lost during the period 
 of experiment is noted by the authors, though it still lacks satisfactory 
 
 explanation. 
 
 E. OLD AGE OF THE CENTRAL SYSTEM. 
 
 Metabolism in the Nerve-cells. Connected closely with fatigue are 
 those alterations both of the constituent nerve-cells and of the entire system 
 found in old age. The picture of the changes in the living cells is that of 
 anabolic and catabolic processes always going on, but varying in their absolute 
 and relative intensity according to several conditions. Of these conditions 
 one of the most important is the age of the individual. In youth and during 
 the growing period of life the anabolic changes appear within the daily cycle 
 of activity and repose to overbalance the katabolic, the total expenditure of 
 1 Patrick and Gilbert : Psychological Review, 1896, vol. iii. No. 5. 
 
296 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 energy increasing toward maturity. During middle life the two processes 
 are more nearly in equilibrium, though the total expenditure of energy is 
 probably greatest then ; and finally in old age the total expenditure of energy 
 diminishes, while at the same time the anabolic processes become less and 
 less competent to repair the waste. The question why in the nervous system 
 the energies wane with advanced age is but the obverse of the question why 
 they wax during the growing period. The essential nature of these changes 
 is in both instances equally obscure. 
 
 Decrease in Weight of the Brain. Between the fiftieth and sixtieth years 
 of life there is a decrease in the bulk of the encephalon in those persons 
 belonging to the classes from which the greater number of the records have 
 been obtained. So far as can be seen from the present records, there is no 
 marked change in the proportional development of the encephalon in old 
 age, though the loss appears to be slightly greater in the cerebral hemi- 
 spheres than in the other portions. 
 
 Changes in the Encephalon. The thickness of the cerebral cortex 
 diminishes in harmony with the shrinkage of the entire system. In large 
 measure this must depend on the loss of volume in the various fibre-systems, 
 which, according to the observations of Vulpius, show a senile decrease in 
 the number of fibres composing them. This decrease is more marked in the 
 motor than in the sensory areas. The time at which it commences cannot, 
 however, be accurately stated, owing to the small number of records after the 
 thirty-third year. Where records have been made between this and the 
 seventy-ninth year it appears that there is no decided diminution until 
 after the fiftieth year, though at the seventy-ninth year the decrease is 
 clearly shown. Engel has shown that the branches of the arbor vitse of 
 the human cerebellum decrease in size and number in old age. 1 
 
 Changes in the Cerebellum. In the case of a man dying of old age 
 (Hodge) some cells in the cerebellum were found shrunken and others (cells 
 of Purkinje) had completely disappeared. In the antennary ganglion of 
 bees a very striking difference appears between those dying of old age and 
 the adult just emerged from its larval skin. These changes are comparable 
 with those described in mammals, and it further appears that in passing from 
 the youngest to the oldest forms cells have disappeared from the ganglia^ 
 and that in the young form of the bee there are some twenty-nine cells 
 present for each one found at a later period. 
 
 To the anatomy of the human nervous system in old age contributions 
 have been made by studies on the pathological anatomy of paralysis agitans. 2 
 
 In subjects suffering from this affection the bodies of the nerve-cells are 
 shrunken, pigmented, and show r in some cases a granular degeneration ; the 
 fibres in part are atrophied and degenerated ; the supporting tissues increase, 
 and the walls of the small blood-vessels are thickened. These changes have 
 been found principally in the spinal cord, being most marked in the lumbar 
 
 1 Engel: Wiener medicinische Wochenschrift, 1863. 
 
 2 Ketcher : Zeitschrift fiir Heilkunde, 1892 ; Redlich : Jahrbuchfur Psychiatrie, 1893. 
 
CENTRAL NERVOUS SYSTEM. 297 
 
 region. But the cords of aged persons who do not exhibit the symptoms 
 of paralysis agitans show similar changes, though usually they are not so 
 evident, and hence the pathological anatomy of this disease resolves itself into 
 a somewhat premature and excessive senility of the central system. 
 
 Shrinkage, decay, and destruction mark the progress of senescence, and 
 the nervous system as a whole becomes less vigorous in its responses, less 
 capable of repair or extra strain, and less permeable to the nervous impulses 
 that fall upon it ; and it thus breaks down, not into the disconnected elements 
 of the fetus, but into groups of elements, so that its capacities are lost in a 
 fragmentary and uneven way. 
 
III. THE SPECIAL SENSES. 
 
 A. VISION. 
 
 The Physiology of Vision. The eye is the organ by means of which 
 certain vibrations of the luminiferous ether are enabled to affect our conscious- 
 ness, producing the sensation which we call " light." Hence the essential part 
 of an organ of vision is a substance or an apparatus which, on the one hand, 
 is of a nature to be stimulated by waves of light, and, on the other, is so con- 
 nected with a nerve that its activity causes 'nerve-impulses to be transmitted to 
 the nerve-centres. Any animal in which a portion of the ectoderm is thus 
 differentiated and connected may be said to possess an eye i. e. an organ 
 through which the animal may consciously or unconsciously react to the exist- 
 ence of light around it. 1 But the human eye, as well as that of all the higher 
 animals, not only informs us of the existence of light, but enables us to form 
 correct ideas of the direction from which the light comes and of the form, color, 
 and distance of the luminous body. To accomplish this result the substance 
 sensitive to, light must form a part of a complicated piece of apparatus capable 
 of very varied adjustments. The eye is, in other words, an optical instrument, 
 and its description, like that of all optical instruments, includes a consideration 
 of its mechanical adjustments and of its refracting media. 
 
 Mechanical Movements. The first point to be observed in studying the 
 movements of the eye is that they are essentially those of a ball-and-socket 
 joint, the globe of the eye revolving freely in the socket formed by the capsule 
 of Tenon through a horizontal angle of almost 88 and a vertical angle of about 
 80. The centre of rotation of the eye (which is not, however, an absolutely 
 fixed point) does not coincide with the centre of the eyeball, but lies a little 
 behind it. It is rather farther forward in hypermetropic than in myopic eyes. 
 The movements of the eye, especially those in a horizontal direction, are sup- 
 plemented by the movements of the head upon the shoulders. The combined 
 eye and head movements are in most persons sufficiently extensive to enable 
 the individual, without any movement of the body, to receive upon the lateral 
 portion of the retina the image of an object directly behind his back. The 
 rotation of the eye in the socket is of course easiest and most extensive when 
 the eyeball has an approximately spherical shape, as in the normal or emme- 
 tropic eye. When the antero-posterior diameter is very much longer than those 
 
 1 In certain of the lower orders of animals no local differentiations seem to have occurred, 
 and the whole surface of the body appears to be obscurely sensitive to light. See Nagel : Der 
 Lichtisinn augenloser Thiere, Jena, 1896. 
 298 
 
THE SENSE OF VISION. 299 
 
 at right angles to it, as in extremely myopic or short-sighted eyes, the rotation 
 of the eyeball may be considerably limited in its extent. In addition to the 
 movements of rotation round a centre situated in the axis of vision, the eye- 
 ball may be moved forward and backward in the socket to the extent of about 
 one millimeter. This movement may be observed whenever the eyelids are 
 widely opened, and is supposed to be effected by the simultaneous contraction of 
 both the oblique muscles. A slight lateral movement has also been described. 
 
 The movements of the eye will be best understood when considered as 
 referred to three axes at right angles to each other and passing through the 
 centre of rotation of the eye. The first of these axes, which may be called 
 the longitudinal axis, is best described as coinciding with the axis of vision 
 when, with head erect, we look straight forward to the distant horizon ; the 
 second, or transverse, axis is defined as a line passing through the centres of 
 rotation of the two eyes ; and the third, or vertical, axis is a vertical line nec- 
 essarily perpendicular to the other two and also passing through the centre of 
 rotation. When the axis of vision coincides with the longitudinal axis, the eye 
 is said to be in the primary position. When it moves (from the primary posi- 
 tion by revolving around either the transverse or the vertical axis, it is said to 
 assume secondary positions. All other positions are called tertiary positions, 
 and are reached from the primary position by rotation round an axis which 
 lies in the same plane as the vertical and horizontal axis i. e. in the " equato- 
 rial plane " of the eye. When the eye passes from a secondary to a tertiary 
 position, or from one tertiary position to another, the position assumed by the 
 eye is identical with that which it would have had if it had reached it from 
 the primary position by rotation round an axis in the equatorial plane. In 
 other words, every direction of the axis of vision is associated with a fixed 
 position of the whole eye a condition of the greatest importance for the easy 
 and correct use of the eyes. A rotation of the eye round its antero-posterior 
 axis takes place in connection with certain movements, but authorities differ 
 with regard to the direction and amount of this rotation. 
 
 Muscles of the Eye. The muscles of the eye are six in number viz : 
 the superior, inferior, internal and external recti, and the superior and inferior 
 oblique. This apparent superfluity of muscles (for four muscles would suffice 
 to turn the eye in any desired direction) is probably of advant^v in reducing 
 the amount of muscular exertion required to put the eye ; ^to any given posi- 
 tion, and thus facilitating the recognition of slight differences of direction, for, 
 according to Fechner's psycho-physic law the smaller perceptible difference in 
 a sensation is proportionate to the total amount of tbe sensation. Hence if the 
 eye can be brought into a given position by a sliglt muscular effort, a change 
 from that position will be more easily perceived thui if a powerful effort were 
 necessary. 
 
 Each of the eye-muscles, acting singly, tends t> rotate the eye round an axis 
 which may be called the axis of rotation of th:-t muscle. Now, none of the 
 muscles have axes of rotation lying exactly ii the equator of the eye i. e. 
 in a plane passing through the centre of rota ion perpendicular to the axis 
 
^00 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 ofVision. 1 But all movements of the eye from the primary position take place, 
 as \ ;e have seen, round an axis lying in this plane. Hence all such movements 
 mu&v\be produced by more than one muscle, and this circumstance also is prob- 
 ably o advantage in estimating the extent and direction of the movement. In 
 this connection it is interesting to note that the eye-muscles have an exception- 
 ally abundant nerve-supply a fact which it is natural to associate with their 
 power of extremely delicate adjustment. It has been found by actual count 
 that in the muscles of the human eye ea^* nerve-fibre supplies only two or three 
 muscle-fibres, while in the muscles of the Climbs the ratio is as high as 1 to 
 40-1 25. 2 
 
 Although each eye has its own supply of muscles and nerves, yet the two 
 eyes are not independent of each other in their movements. The nature of 
 their connections with the nerve-centres is such that only those movements are, 
 as a rule, possible in which both axes of vision remain in the same plane. This 
 condition being fulfilled, the eyes may be together directed to any desired point 
 above, below, or at either side of the observer. The axes may also be con- 
 verged, as is indeed necessary in looking at near objects, and to facilitate this 
 convergence the internal recti muscles are inserted nearer to the cornea than the 
 other muscles of the eye. Though in the ordinary use of the eyes there is never 
 any occasion to diverge the axes of vision, yet most persons are able to effect a 
 divergence of about four degrees, as shown by their power to overcome the ten- 
 dency to double vision produced by holding a prism in front of one of the eyes. 
 The nervous mechanism through which this remarkable co-ordination of the 
 muscles of the two eyes is effected, and their motions limited to those which 
 are useful in binocular vision, is not completely understood, but it is supposed 
 to have its seat in part in the tubercula quadrigemina, in connection with the 
 nuclei of origin of the third, fourth, and sixth cranial nerves. Its disturbance 
 by disease, alcoholic intoxication, etc. causes strabismus, confusion, dizziness, 
 and double vision. 
 
 A nerve termination sensitive to light, and so arranged that it can be turned ^ 
 in different directions, is sufficient to give information of the direction from 
 which the light comes, for the contraction of the various eye-muscles indicates, 
 throngl] the nerves of muscular sense, the position into which the eye is nor- 
 mally brougiitj unorder to h@st_receive_Lbe luminous rays, or, in other words, 
 the direction of the luminous body. The eye, however, informs us not only of 
 the direction, but of the form of the object from which the light proceeds ; and 
 to understand how this is Affected it will be necessary to consider the refracting 
 media of the eye by meant of which an optical image of the luminous object 
 is thrown upon the expanded termination of the optic nerve viz. the retina. 
 
 Dioptric Apparatus of the Eye. For the better comprehension of this 
 portion of the subject a few Definitions in elementary optics may be given. A 
 
 1 The axes of rotation of the internal and external recti, however, deviate but slightly from 
 the equatorial plane. 
 
 ? P. Tergast : " Ueber das Verhainiss voa Nerven and Muskeln," Archiv fur mikr. Anat.. 
 ix. 36-46. 
 
THE SENSE OF VISION. 
 
 301 
 
 dioptric system in its simplest form consists of two adjacent media which have 
 different indices of refraction and whose surface of separation is the segment 
 of a sphere. A line joining the middle of the segment with the centre of the 
 sphere and prolonged in either direction is called the axis of the system. Let 
 the line APE in Figure 125 represent in section such a spherical surface the 
 
 FIG. 125. Diagram of simple optical system (after Foster). 
 
 centre of which is at N 9 the rarer medium being to the left and the denser me- 
 dium to the right of the line. Any ray of light which, in passing from the rarer t- 
 to the denser medium, is perpendicular to the spherical surface will be unchanged 
 iii its direction i. e. will undergo no refraction. Such rays are represented by 
 the lines P, M D, and M f E. If a pencil of rays having its ^origin in the rarer 
 medium at any point in the axis falls upon^the spherical surface, there will be 
 one ray viz. the one which coincides with the axis^of the system, which will 
 pass into the second medium unchanged in its direction. This ray is called 
 the principal ray (OP), and its point of intersection (P) with the spherical 
 surface is called the principal point. The centre of the sphere (N) through 
 which the principal ray necessarily. passes is called the nodal point. All the 
 other rays in the pencil are refracted toward the principal ray by an amount 
 
 FIG. 126. Diagram to show method of finding principal foci (Neumann). 
 
 which depends, for a given radius of curvature, upon the difference in the 
 refractive power of the media, or, in other words, upon the retardation of light 
 in passing from one medium to the other. If the incident rays have their 
 origin at a point infinitely distant on the axis i. e. if they are parallel to each 
 ot^cr--they will all be refracted to a point behind the spherical surface known 
 
302 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 as the principal focus, F. There is another principal focus (F') in front of the 
 spherical surface viz. the point from which diverging incident rays will be 
 refracted into parallelism on passing the spherical surface, or, in other words, 
 the point at which parallel rays coming from the opposite direction will be 
 brought to a focus. The position of these two principal foci may be deter- 
 mined by the construction shown in Figure 126. Let CA C f represent a sec- 
 tion of a spherical refracting surface with the axis A N, the nodal point N, and 
 the principal point A. The problem is to find the foci of rays parallel to the 
 axis. Erect perpendiculars at A and N. Set off on each perpendicular dis- 
 tances No, Np, Ao f , Ap f proportionate to the rapidity of light in the two media 
 (e. g. 2 : 3). The points where the lines p f o and p o' prolonged will cut the 
 axis are the two principal foci F and F' i. e. the points at which parallel rays 
 coming from either direction are brought to a focus after passing the spherical 
 refracting surface. If the rays are not parallel, but diverging i. e. coming 
 from an object at a finite distance the point where the rays will be brought to 
 a focus, or, in other words, the point where the optical image of the luminous 
 object will be formed, may be determined by a construction which combines 
 any two of the three rays whose course is given in the manner above described. 
 Thus in Figure 127 let AN be the axis, and F and F' the principal foci of 
 
 \ 
 
 FIG. 127. Diagram to show method of finding conjugate foci. 
 
 the spherical refracting surface CA C', with a nodal point at N. Let B be 
 the origin of a pencil of rays the focus of which is to be determined. Draw 
 the line B C representing the course of an incident ray parallel to the axis. 
 This ray will necessarily be refracted through the focus F, its course being 
 represented by the line CF and its prolongation. Similarly, the incident ray 
 passing through the focus F f and striking the spherical surface at C' will, after 
 refraction, be parallel to the axis i. e. it will have the Direction C r b. The 
 principal ray of the pencil will of course pass through the spherical surface and 
 the nodal point N without change of direction. These three rays will come 
 together at the same point 6, the position of which may be determined by con- 
 structing the course of any two of the three. The points B and b are called 
 conjugate foci, and are related to each other in such a way that an optical image 
 is formed at one point of a luminous object situated at the other. When the 
 rays of light pass through several refracting surfaces in succession their course 
 may be determined by separate calculations for each surface, a process which 
 is much simplified when the surfaces are " centred " i. e. have their centres 
 of curvature lying in the same axis, as is approximately the case in the eye. 
 
 Refracting- Media of the Eye. Rays of light in passing through the eye 
 penetrate seven different media and are refracted at seven surfaces. The media 
 
THE SENSE OF VISION. 303 
 
 are as follows : layer of tears, cornea, aqueous humor, anterior capsule of lens, 
 lens, posterior capsule of lens, vitreous humor. The surfaces are those which 
 separate the successive media from each other and that which separates the tear 
 layer from the air. For purposes of practical calculation the number of sur- 
 faces and media may be reduced to three. In the first place, the layer of tears 
 which moistens the surface of the cornea has the same index of refraction as 
 the aqueous humor. Hence the index of refraction of the cornea may be left 
 out of account, since, having practically parallel surfaces and being bounded 
 on both sides by substances having the same index of refraction, it does not 
 influence the direction of rays of light passing through it. For this same 
 reason objects seen obliquely through a window appear in their true direction, 
 the refraction of the rays of light on entering the glass being equal in amount 
 and opposite in direction to that which occurs in leaving it. For purposes of 
 optical calculation we may, therefore, disregard the refraction of the cornea 
 (which, moreover, does not differ materially from that of the aqueous humor), 
 and imagine the aqueous humor extending forward to the anterior surface of 
 the layer of tears which bathes the corneal epithelium. Furthermore, the cap- 
 sule of the lens has the same index of refraction as the outer layer of the lens 
 itself, and for optical purposes may be regarded as replaced by it. Hence 
 the optical apparatus of the eye may be regarded as consisting of the fol- 
 lowing three refracting media: Aqueous humor, index of refraction 1.33; 
 HppSj average index of refraction 1.45 ; vitreous humor, index of refraction 
 1.33. The surfaces at which refraction occurs are also three in number : An- 
 terior surface of cornea ? radius of curvature 8 millimeters; anterior surface 
 .ojfjens. radius of curvature 10 millimeters ; posterior surface of lens, radius of 
 curvature 6 millimeters. It will thus be seen that the anterior surface of the 
 lens is less and the posterior surface more convex than the cornea. 
 
 To the values of the optical constants of the eye as above given may be 
 added the following : Distance from the anterior surface of the cornea to the 
 anterior surface of the lens, 3.6 millimeters ; distance from the posterior sur- 
 face of the lens to the retina, 15. millimeters ; thickness of lens, 3.6 millimeters. 
 
 The methods usually employed for determining these constants are the fol- 
 lowing : The indices of refraction of the aqueous and vitreous humor are 
 determined by filling the space between a glass lens and a glass plate with the 
 fresh humor. The aqueous or vitreous humor thus forms a convex or concave 
 lens, from the form and focal distance of which the index can be calculated. 
 Another method consists in placing a thin layer of the medium between the 
 hypothenuse surfaces of two right-angled prisms and determining the angle at 
 which total internal reflection takes place. In the case of the crystalline lens 
 the index is foi.nd by determining its focal distance as for an ordinary lens, 
 and solving the equation which expresses the value of the index in terms of 
 radius of curvature and focal distance, thickness, and focal length. The 
 refractive irdex of the lens increases from the surface toward the centre, a 
 peculiarity which tends to correct the disturbances due to spherical aberration, 
 as well as to increase the refractive power of the lens as a whole. 
 
304 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 The curvature of the refracting surfaces of the eye is determined by an 
 instrument known as an ophthalmometer, which measures the size of the 
 reflected image of a known object in the various curved surfaces. The 
 radius of curvature of the surface is determined by the following formula : 
 
 T 
 
 B :b = A :- or r = 75-, in which B = the size of the object, b = the size of 
 2 B 
 
 the image, A = distance between the object and the reflecting surface, and 
 r = the radius of the reflecting surface. The distances between the various 
 surfaces of the eye are measured on frozen sections of the organ, or can be 
 determined upon the living eye by optical methods too complicated to be here 
 described. It should be borne in mind that the above values of the so-called 
 "optical constants" of the eye are subject to considerable individual variation, 
 and that the statements of authors concerning them are not always consistent. 
 The refracting surfaces of the eye may be regarded as still further sim- 
 plified, and a so-called " reduced eye " constructed which is very useful for 
 purposes of optical calculation. This reduced eye, which for optical purposes 
 is the equivalent of the actual eye, is regarded as consisting of a single refract- 
 ing medium having an index of 1.33, a radius of curvature of 5.017 milli- 
 meters, its principal point 2.148 millimeters behind the anterior surface of the 
 cornea, and its nodal point 0.04 millimeter in front of the posterior surface 
 of the lens. 1 The principal foci of the reduced eye are respectively 12.918 
 millimeters in front of and 22.231 millimeters behind the anterior surface of 
 the cornea. Its optical power is equal to 50.8 dioptrics. 2 The position of thi& 
 imaginary refracting surface is indicated by the dotted line p in Figure 1 28. The 
 
 FIG. 128. Diagram of th* 1 formation of a retinal image (after Foster). 
 
 nodal point, n, in this construction may be regarded as the crossing-point of all 
 the principal rays which enter the eye, and, as these rays are unchanged in their 
 direction by refraction, it is evident that the image of the point whence they 
 proceed will be formed at the point where they strike the retina. Hence to 
 determine the size and position of the retinal image of any external object 
 e. g. the arrow in Figure 128 it is only necessary to draw Iin3s from various 
 
 1 Strictly speaking, there are. in this imaginary refracting apparatus which is regarded as 
 equivalent to the actual eye two principal and two nodal points, each pair about 0.4 millimeter 
 apart. The distance is so small that the two points may, for all ordinary constructions, be 
 regarded as coincident. 
 
 2 The optical power of a lens is the reciprocal of its focal length. The dioptry or unit of 
 optical power is the power of a lans with a focal length of 1 meter. 
 
THE SENSE OF VISION. 
 
 305 
 
 points of the object through the above-mentioned nodal point and to prolong 
 them till they strike the retina. It is evident that the size of the retinal image 
 \vill be as much smaller than that of the object as the distance of the nodal 
 point from the retina is smaller than its distance from the object. 
 
 According to the figures above given, the nodal point is about 7.2 milli- 
 meters behind the anterior surface of the cornea and about 15.0 millimeters in 
 front of the retina. Hence the size of the retinal image of an object of known 
 size and distance can be readily calculated a problem which has frequently to be 
 solved in the study of physiological optics. The construction given in Figure 
 128 shows that from all external objects inverted images are projected upon the 
 retina, and such inverted images can actually be seen under favorable condi- 
 tions. If, for instance, the eye of a white rabbit, which contains no choroidal 
 pigment, be excised and held with the cornea directed toward a window or 
 other source of light, an inverted image of the luminous object will be seen 
 through the transparent sclerotic in the same way that one sees an inverted 
 image of a landscape on the ground-glass plate of a photographic camera. 
 The question is often asked, " Why, if the images are inverted in the retina, 
 do we not see objects upside down ?" The only answer to such a question is 
 that it is precisely because images are inverted on the retina that we do not see 
 objects upside down, for we have learned through lifelong practice to asso- 
 ciate an impression made upon any portion of the retina with light coming 
 from the opposite portion of the field of vision. Thus if an image falls upon 
 the lower portion of the retina, our experience, gained chiefly through mus- 
 cular movements and tactile sensations, has taught us that this image must cor- 
 respond to an object in the upper portion of our field of vision. In whatever 
 way the retina is stimulated the same effect is produced. If, for instance,^ 
 gentle pressure is made with the finger on the lateral portion of the eyeball 
 through the closed lids a circle of light known as a phosphene immediately 
 appears on the opposite side of the eye. Another good illustration of the 
 same general rule is found in the effect of throwing a shadow upon the retina 
 from an object as close as possible to the eye. For this purpose place a card 
 
 B p 
 
 FIG. 129. Diagram illustrating the projection of a shadow on the retina. 
 
 with a small pin-hole in it in front of a source of light, and three or four 
 centimeters distant from the eye i. e. within the near point of distinct 
 vision. Then hold some object smaller than the pupil e.g. the head 
 of a pin as close as possible to the cornea. Under these conditions 
 neither the pin-hole nor the pin-head can be really seen i. e. they are 
 
 VOL. II. 20 
 
306 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 both too near to have their images focussed upon the retina. The pin-hole 
 becomes itself a source of light, and appears as a luminous circle bounded by 
 the shadow thrown by the edge of the iris. Within this circle of light is seen 
 the shadow of the pin-head, but the pin-head appears inverted, for the obvious 
 reason that the eye, being accustomed to interpret all retinal impressions as 
 corresponding to objects in the opposite portion of the field of vision, regards 
 the upright shadow of the pin-head as the representation of an inverted object. 
 The course of the rays in this experiment is shown in Figure 129, in which 
 A B represents the card with a pin-hole in it, P the pin, and P' its upright 
 shadow thrown ,pn the retina. 
 
 Accommodation. From what has been said of conjugate foci and their 
 relation to each other it is evident that any change in the distance of the object 
 from the refracting media will involve a corresponding change in the position 
 of the image, or, in other words, only objects at a given distance can be 
 focussed upon a plane which has a fixed position with regard to the refracting 
 surface or surfaces. Hence all optical instruments in which the principle of 
 conjugate foci finds its application have adjustments for distance. In the 
 telescope and opera-glass the adjustment is eifected by changes in the distance 
 between the lenses, and in the photographic camera by a change in the posi- 
 tion of the ground-glass plate representing the focal plane. In the microscope 
 the adjustment is effected by changing the distance of the object to suit the 
 lenses, the higher powers having a shorter " working distance." 
 
 We must now consider in what way the eye adapts itself to see objects dis- 
 tinctly at different distances. That this power of adaptation, or " accommo- 
 dation," really exists we can easily convince ourselves by looking at different 
 objects through a network of fine wire held near the eyes. When with normal 
 vision the eyes are directed to the distant objects the network nearly disappears, 
 and if we attempt to see the network distinctly the outlines of the distant 
 objects become obscure. In other words, it is impossible to see both the 
 network and the distant objects distinctly at the same time. It is also evident 
 that in accommodation for distant objects the eyes are at rest, for when they 
 are suddenly opened after having been closed for a short time they are found 
 to be accommodated for distant objects, and we are conscious of a distinct 
 effort in directing them- to any near object. 1 
 
 From the optical principles above described it is clear that the accommo- 
 dation of the eye for near objects may be conceived of as taking place in three 
 different ways : 1st, By an increase of the distance between the refracting sur- 
 faces of the eye and the retina ; 2d, By an increase of the index of refraction 
 of one or more of the media ; 3d, By a diminution of the radius of curvature 
 of one or more of the surfaces/ Thejirst of these methods was formerly sup- 
 posed to be the one actually in use, a lengthening of the eyeball under a pres- 
 
 1 It has been shown by Beer (Archivfilr die gesammte Physiolngie, Iviii. 523) that in fishes 
 the eyes when at rest are accommodated for near objects, and that accommodation for distant 
 objects is effected by the contraction of a muscle for which the name "retractor lentis" is pro- 
 posed. 
 
THE SENSE OF VISION. 307 
 
 sure produced by the eye-muscles being assumed to occur. This lengthening 
 would, in the case of a normal eye accommodating itself for an object at a 
 distance of 15 centimeters, amount to not less than 2 millimeters a change 
 which could hardly be brought about by the action of any muscles connected 
 with the eye. Moreover, accommodation changes can be observed upon elec- 
 trical stimulation of the excised eye. Its mechanism must, therefore, lie within 
 the eye itself. As for the second of these methods, there is no conceivable way 
 by which a change in the index of refraction of the media can be effected , and 
 we are thus forced to the conclusion that accommodation is brought about by 
 a change in the curvature of the refracting surfaces i. e. by a method quite 
 different from any which is employed in optical instruments of human con- 
 struction. Now, by measuring the curvature of the cornea of a person who*. 
 looks alternately at near and distant objects it has been shown that the cornea 
 undergoes no change of form in the act of accommodation. By a process of 
 exclusion, therefore, the lens is indicated as the essential organ in this function 
 of the eye, and, in fact, the complicated structure and connections of the lens 
 at once suggest the thought that it is in the surfaces of this portion of the eye 
 that the necessary changes take place. Indeed, from a teleological point of 
 view the lens would seem somewhat superfluous if it were not important to ^ 
 have a transparent refracting body of variable form in the eye, for the amount 
 of refraction which takes place in the lens could be produced by a slightly 
 increased curvature of the cornea. Now, the changes of curvature which occur 
 in the surfaces of the lens when the eye is directed to distant and near objects 
 alternately can be actually observed and measured with considerable accuracy. 
 For this purpose the changes in the form, size, and position of the images of 
 brilliant objects reflected in these two surfaces are studied. If a candle is held 
 in a dark room on a level with and about 50 centimeters away from the eye in 
 which the accommodation is to be studied, an observer, so placed that his own 
 axis of vision makes about the same angle (15-20) with that of the ob- 
 served eye that is made by a line joining the observed eye and the candle, will 
 readily see a small upright image of the candle reflected in the cornea of the 
 observed eye. Near this and within the outline of the pupil are two other 
 images of the candle, which, though much less easily seen than the corneal 
 image, can usually be made out by a proper adjustment of the light. The 
 first of these is a large faint upright image reflected from the anterior surface 
 of the lens, and the second is a small inverted image reflected from the pos- 
 terior surface of the lens. It will be observed that the size of these images 
 varies with the radius of curvature of the three reflecting surfaces as given on 
 p. 303. The relative size and position of these images having been recog- 
 nized while the eye is at rest i. e. is accommodated for distance let the 
 person who is under observation be now requested to direct his eye to a near 
 object lying in the same direction. When this is done the corneal image and 
 that reflected from the posterior surface of the lens will remain unchanged, 1 
 
 1 A very slight diminution in size may sometimes be observed in the image reflected from 
 the posterior surface of the lens. 
 
308 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 while that reflected from the anterior surface of the lens will become smaller 
 and move toward the corneal image. This change in the size and position of 
 the reflected image can only mean that the surface from which the reflection 
 takes place has become more convex and has moved forward. Coincident 
 with this change a contraction of the pupil will be observed. 
 
 An apparatus for making observations of this sort is known as the phako- 
 scope of Helmholtz (Fig. 130). The eye in which the changes due to accom- 
 modation are to be observed is placed at an opening 
 in the back of the instrument at C, and directed al- 
 ternately to a needle placed in the opening D and 
 to a distant object lying in the same direction. Two 
 prisms at B and B r serve to throw the light of a 
 candle on to the observed eye, and the eye of an 
 observer at A sees the three reflected images, each 
 as two small square spots of light. The movement 
 and the change of size of the image reflected from 
 the anterior surface of the lens can be thus much 
 better observed than when a candle-flame is used. 
 The* course of the rays of light in this experi- 
 ment is shown diagrammatically in Figure 131. 
 The observed eye is directed to the point A, while 
 the candle and the eye of the observer are placed 
 symmetrically on either side. The images of the candle reflected from the various 
 surfaces of the eye will be seen projected on the dark background of the pupil 
 
 FIG. 130. Phakoscope of 
 Helmholtz. 
 
 FIG. 131. Diagram explaining the change in the position of the image reflected from the anterior surface 
 of the crystalline lens (Williams, after Bonders). 
 
 in the directions indicated by the dotted lines ending at a, 6, and c. When the 
 eye is accommodated for a near object the middle one of the three images moves 
 nearer the corneal image i. e. it changes in its direction from b to 6', showing 
 that the anterior surface of the lens has bulged forward into the position indi- 
 
THE SENSE OF VISION. 
 
 309 
 
 cated by the dotted line. The change in the appearance of the images is 
 represented diagrammatically in Figure 132. On the left is shown the appear- 
 ance of the images as seen when the eye is at rest, a representing the corneal 
 image, b that reflected from the anterior, and c that from the posterior surface 
 of the lens when the observing eye and the candle are in the position repre- 
 
 FIG. 132. Reflected images of a candle-flame as seen in the pupil of an eye at rest and accommodated 
 
 for near objects (Williams). 
 
 sented in Figure 131. The images are represented as they appear in the dark 
 background of the pupil, though of course the corneal image may, in certain 
 positions of the light, appear outside of the pupillary region. When the eye 
 is accommodated for near objects the images appear as shown in the circle on 
 the right, the image 6 becoming smaller and brighter and moving toward the 
 corneal image, while the pupil contracts as indicated by the circle drawn round 
 the images. 
 
 The changes produced in the eye by an effort of accommodation are indi- 
 cated in Figure 133, the left-hand side of the diagram showing the condition 
 
 FIG. 133. Showing changes in the eye produced by the act of accommodation (Helmholtz). 
 
 of the eye at rest, and the right-hand side that in extreme accommodation for 
 near objects. 
 
 It will be observed that the iris is pushed forward by the bulging lens and 
 that its free border approaches the median line. In other words, the pupil is 
 contracted in accommodation for near objects. The following explanation of 
 the mechanism by which this change in the shape of the lens is effected has 
 been proposed by Helmholtz, and is still generally accepted. The structure 
 of the lens is such that by its own elasticity it tends constantly to assume a 
 more convex form than the pressure of the capsule and the tension of the sus- 
 pensory ligaments (s, , Fig. 133) allow. This pressure and tension are dimin- 
 ished when the eye is accommodated for near vision by the contraction of the 
 ciliary muscles (c, c, Fig. 133), most of whose fibres, having their origin at the 
 
310 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 point of union of the cornea and sclerotic, extend radially outward in every 
 direction and are attached to the front part of the choroid. The contrac- 
 tion of the ciliary muscle, drawing forward the membranes of the eye, will 
 relax the tension of the suspensory ligament and allow the lens to take 
 the form determined by its own elastic structure. According to another 
 theory of accommodation proposed by Tscherning, 1 the suspensory liga- 
 ment is stretched and not relaxed by the contraction of the ciliary muscle. 
 In consequence of the pressure thus produced upon the 
 lens, the soft external portions are moulded upon the 
 harder nuclear portion in such a way as to give to the 
 anterior (and to some extent to the posterior) surface a 
 hyperboloid instead of a spherical form. A similar theory 
 has been recently brought forward by Schoen,* who com- 
 pares the action of the ciliary muscle upon the lens to that 
 of the fingers compressing a rubber ball, as shown in Fig- 
 ure 134. These theories have an advantage over that 
 oifered by Helmholtz, inasmuch as they afford a better 
 explanation of the presence of circular fibres in the ciliary 
 muscle. They also make the fact of so-called "astig- 
 matic accommodation" comprehensible. This term is 
 FIG. 134 TO illustrate applied to the power said to be sometimes gradually 
 
 Schoen's theory of ac- -, -, .,, ,. ,. > c 
 
 commodation. acquired by persons with astigmatic 3 eyes of correcting 
 
 this defect of vision by accommodating the eye more 
 strongly in one meridian than another. The theory of Tscherning is sup- 
 ported by Crzellitzer 4 as the result of investigations into the hyperboloid 
 form of the lens in accommodation. On the other hand, it is maintained by 
 Priestley Smith 5 that this form of the lens is not inconsistent with the Helm- 
 holtz theory. Moreover, it has been shown by Hess 6 and Heine 7 that in 
 extreme accommodation the lens drops slightly toward the lower part of the 
 eye, a movement which seems to indicate a relaxation of the suspensory liga- 
 ment. The weight of evidence seems, therefore, on the whole, to be on the 
 side of the theory of Helmholtz. 
 
 Whatever views may be entertained as to the exact mechanism by which its 
 change of shape is brought about, there can be no doubt that the lens is the 
 portion of the eye chiefly or wholly concerned in accommodation, and it is 
 accordingly found that the removal of the lens in the operation for cataract 
 destroys the power of accommodation, and the patient is compelled to use 
 convex lenses for distant and still stronger ones for near objects. 
 
 It is interesting to notice that the act of accommodation, though distinctly 
 voluntary, is performed by the agency of the unstriped fibres of the ciliary 
 muscles. It is evident, therefore, that the term " involuntary " sometimes 
 
 1 Archives de Physiologic, 1894, p. 40. 2 Archivfilr die gesammte Physiologic, lix. 427. 
 
 3 See p. 317. * Archivfilr Ophthcdmologie, xlii. (4) S. 36. 
 
 5 Ophthalmic Review, xvii. p. 341. 
 
 6 Archivfilr Ophthalmologie, xlii. S. 288, and xliii. S. 477. 
 
 7 Ibid., xliv. (2) S. 299, and xlvii. (2) S. 662. 
 
THE SENSE OF VISION. 311 
 
 applied to muscular fibres of this sort may be misleading. The voluntary 
 character of the act of accommodation is not affected by the circumstance that 
 the will needs, as a rule, to be assisted by visual sensations. The fact that 
 most persons cannot affect the necessary change in the eye unless they direct 
 their attention to some near or far object is only an instance of the close rela- 
 tion between sensory impressions and motor impulses, which is further exem- 
 plified by such phenomena as the paralysis of the lip of a horse caused by 
 division of the fifth nerve. It is found, moreover, that by practice the 
 power of accommodating the eye without directing it to near and distant 
 objects can be acquired. The nerve-channels through which accommodation 
 is affected are the anterior part of the nucleus of the third pair of nerves 
 lying in the extreme hind part of the floor of the third ventricle, the most 
 anterior bundle of the nerve-root, the third nerve itself, the lenticular ganglion, 
 and the short ciliary nerves (see diagram p. 323). 
 
 The mechanism of accommodation is affected in a remarkable way by drugs, 
 the most important of which are atropia and physostigmin, the former para- 
 lyzing and the latter stimulating the ciliary muscle. As these drugs exert a 
 corresponding effect upon the iris, it will be convenient to discuss their action 
 in connection with the physiology of that organ. 
 
 The changes occurring in the eye during the act of accommodation are 
 indicated in the following table, which shows, both for the actual and the 
 reduced eye, the extent to which the refracting media change their form and 
 position, and the consequent changes in the position of the foci : 
 
 Accommodation for 
 Actual Eye. distant objects. near objects. 
 
 Kadius of cornea 8 mm. 8 mm. 
 
 Radius of anterior surface of lens 10 " 6 
 
 Radius of posterior surface of lens 6 5.5 " 
 
 Distance from cornea to anterior surface of lens . . 3.6 " 3.2 " 
 
 Distance from cornea to posterior surface of lens . 7.2 " 7.2 " 
 
 Reduced Eye. 
 
 Radius of curvature 5.02 
 
 Distance from cornea to principal point 2.15 
 
 Distance from cornea to nodal point 7.16 
 
 Distance from cornea to anterior focus 12.918 
 
 4.48 
 
 2.26 
 
 6.74 
 
 11.241 
 
 20.248 
 
 Distance from cornea to posterior focus 22.231 
 
 It will be noticed that no change occurs in the curvature of the cornea, and 
 next to none in the posterior surface of the lens, while the anterior surface of 
 the lens undergoes material alterations both in its shape and position. 
 
 Associated with the accommodative movements above described, two other 
 changes take place in the eyes to adapt them for near vision. In the first 
 place, the axes of the eyes are converged upon the near object, so that the 
 images formed in the two eyes shall fall upon corresponding points of the 
 retinas, as will be more fully explained in connection with the subject of 
 binocular vision. In the second place, the pupil becomes contracted, thus 
 reducing the size of the pencil of rays that enters the eye. The importance 
 of this movement of the pupil will be better understood after the subject of 
 
312 ^1^ AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 spherical aberration of light has been explained. These three adjustments, 
 focal, axial, and pupillary, are so habitually associated in looking at near objects 
 that the axial can only by an effort be dissociated from the other two, while 
 these two are quite inseparable from one another. This may be illustrated 
 by a simple experiment. On a sheet of paper about 40 centimeters distant 
 from the eyes draw two letters or figures precisely alike and about 3 centimeters 
 apart. (Two letters cut from a newspaper and fastened to the sheet will answer 
 the same purpose.) Hold a small object like the head of a pin between the 
 eyes and the paper at the point of intersection of a line joining the right eye 
 and the left letter with a line joining the left eye and the right letter. If the 
 axes of vision are converged upon the pin-head, that object will be seen dis- 
 tinctly, and beyond it will be seen indistinctly three images of the letter, the 
 central one being formed by the blending of the inner one of each pair of 
 images formed on the two retinas. If now the attention be directed to the 
 middle image, it will gradually become perfectly distinct as the eye accommo- 
 dates itself for that distance. We have thus an axial adjustment for a very 
 near object and a focal adjustment for a more distant one. If the pupil of the 
 individual making this observation be watched by another person, it will b 
 found that at the moment when the middle image of the letter becomes distin^r 
 the pupil, which had been contracted in viewing the pin-head, suddenly dilates. 
 It is thus seen that when the axial and focal adjustments are dissociated from 
 each other the pupillary adjustment allies itself with the latter. 
 
 The opposite form of dissociation viz. the axial adjustment for distance 
 and the focal adjustment for near vision is less easy to bring about. It may 
 perhaps be best accomplished by holding a pair of stereoscopic pictures before 
 the eyes and endeavoring to direct the right eye to the right and the left eye to 
 the left picture i. e. to keep the axes of vision parallel while the eyes are 
 accommodated for near objects. One who is successful in this species of ocular 
 gymnastics sees the two pictures blend into one having all the appearance of 
 a solid object. The power of thus studying stereoscopic pictures without a 
 stereoscope is often a great convenience to the possessor, but individuals differ 
 very much in their ability to acquire it. 
 
 Range of Accommodation. By means of the mechanism above described 
 it is possible for the eye to produce a distinct image upon the retina of objects 
 lying at various distances from the cornea. The point farthest from the eye 
 at which an object can be distinctly seen is called the far-point, and the nearest 
 point of distinct vision is called the near-point of the eye, and the distance 
 between the near-point and the far-point is called the range of distinct vision 
 or the range of accommodation. As the normal emmetropic eye is adapted, 
 when at rest, to bring parallel rays of light to a focus upon the retina, its far- 
 point may be regarded as at an infinite distance. Its near-point varies with age, 
 as will be described under Presbyopia. In early adult life it is from 10 to 
 13 centimeters from the eye. For every point within this range there will be 
 theoretically a corresponding condition of the lens adapted to bring rays pro- 
 ceeding from that point to a focus on the retina, but as rays reaching the eye 
 from a point 175 to 200 centimeters distant do not, owing to the small size of 
 
THE SENSE OF VISION. 
 
 313 
 
 the pupil, differ sensibly from parallel rays, there is no appreciable change in 
 the lens unless the object looked at lies within that distance. It is also evi- 
 dent that as an object approaches the eye a given change of distance will 
 cause a constantly increasing amount of divergence of the rays proceeding from 
 it, and will therefore necessitate a constantly increasing amount of change in 
 the lens to enable it to focus the rays on the retina. We find, accordingly, that 
 all objects more than two meters distant from the eye can be seen distinctly at 
 the same time i. e. without any change in the accommodative mechanism 
 but for objects within that distance we are conscious of a special eifort of 
 accommodation which becomes more and more distinct the shorter the distance 
 between the eye and the object. 
 
 Myopia and Hypermetropia. There are two conditions of the eye in 
 which the range of accommodation may differ from that which has just been 
 described as normal. These conditions, which are too frequent to be regarded 
 (except in extreme cases) as pathological, are generally dependent upon the 
 eyeball being unduly lengthened or 
 shortened. In Fig. 135 are shown 
 diagram matically the three conditions 
 known as emmetropia, myopia, and 
 hypermetropia. In the normal or 
 emmetropic eye, A, parallel rays are 
 represented as brought to a focus on 
 the retina; in the short-sighted, or 
 myopic, eye, B, similar rays are 
 focussed in front of the retina, since 
 the latter is abnormally distant; while 
 in the over-sighted, or hypermetropic, 
 eye, C, they are focussed behind the 
 retina, since it is abnormally near. 
 
 It is evident that when the eye is 
 at rest both the myopic and the hy- 
 permetropic eye will see distant ob- 
 jects indistinctly, but there is this 
 important difference : that in hyper- 
 metropia the difficulty can be cor- 
 rected by an effort of accommodation, 
 while in myopia this is impossible, 
 since there is no mechanism by which 
 the radius of the lenticular surfaces can be increased. Hence an individual 
 affected with myopia is always aware of the infirmity, while a person with 
 hypermetropic eyes often goes through life unconscious of the defect. In this 
 case the accomodation is constantly called into play even for distant objects, and 
 if the hypermetropia is excessive, any prolonged use of the eyes is apt to be 
 attended by a feeling of fatigue, headache, and a train of nervous symptoms 
 familiar to the ophthalmic surgeon. Hence it is important to discover this defect 
 where it exists and to apply the appropriate remedy viz. convex lenses placed 
 
 FIG. 135. Diagram showing the difference betweer 
 normal, myopic, and hypermetropic eyes. 
 
314 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 in front of the eyes in order to make the rays slightly convergent when they 
 enter the eye. Thus aided, the refractive power of the eye at rest is sufficient 
 to bring the rays to a focus upon the retina and thus relieve the accommoda- 
 tion. This action of a convex lens in hypermetropia is indicated by the dotted 
 lines in Fig. 135, (?, and the corresponding use of a concave lens in myopia is 
 shown in Fig. 135, B. 
 
 The detection and quantitative determination of hypermetropia are best 
 made after the accommodation has been paralyzed by the use of atropia, by 
 ascertaining how strong a convex lens must be placed before the eye to pro- 
 duce distinct vision of distant objects. 
 
 The range of accommodation varies very much from the normal in myopic 
 and hypermetropic eyes. In myopia the near-point is often 5 or 6 centimeters 
 from the cornea, while the far-point, instead of being infinitely far off, is at a 
 variable but no very great distance from the eye. The range of accommoda- 
 tion is therefore very limited. In hypermetropia the near-point is slightly 
 farther than normal from the eye, and the far-point cannot be said to exist, 
 for the eye at rest is adapted to bring converging rays to a focus on the retina, 
 and such pencils of rays do not exist in nature. Mathematically, the far-point 
 may be said to be at more than an infinite distance from the eye. The range 
 of effective accommodation is therefore reduced, for a portion of the accommo- 
 dative power is used up in adapting the eye to receive parallel rays. 
 
 Presbyopia. The power of accommodation diminishes with age, owing 
 apparently to a loss of elasticity of the lens. The change is regularly pro- 
 gressive, and can be detected as early as the fifteenth year, though in normal 
 eyes it does not usually attract attention until the individual is between forty 
 and forty-five years of age. At this period of life a difficulty is commonly 
 experienced in reading ordinary type held at a convenient distance from the 
 eye, and the individual becomes old-sighted or presbyopic a condition which 
 can, of course, be remedied by the use of convex glasses. According to 
 Helmholtz, the far-point also recedes somewhat after fifty years of age. 
 Hence emmetropic eyes may become hypermetropic and slightly myopic 
 eyes emmetropic. Cases are occasionally reported of persons recovering 
 their power of near vision in extreme old age and discontinuing the use of 
 the glasses previously employed for reading. In these cases there is apparently 
 not a restoration of the power of accommodation, but an increase in the refrac- 
 tive power of the lens through local changes in its tissue. A diminution in 
 the size of the pupil, sometimes noticed in old age, 1 may also contribute to 
 the distinctness of the retinal image, as will be described in connection with 
 spherical aberration. 
 
 Defects of the Dioptric Apparatus. The above-described imperfections 
 of the eye viz. myopia and hypermetropia being generally (though not 
 invariably) due to an abnormal length of the longitudinal axis, are to be 
 regarded as defects of construction affecting only a comparatively small 
 
 1 The average diameter of the pupil is said to be in youth 4.1 mm. and in old age 3 mm. 
 Silberkuhl: Archivfiir Ophthalmologie, xlii. (3) S. 179.' 
 
THE SENSE OF VISION. 
 
 315 
 
 number of eyes. There are, however, a number of imperfections of the diop- 
 tric apparatus, many of which affect all eyes alike. Of these imperfections 
 some affect the eye in common with all optical instruments, while others are 
 peculiar to the eye and are not found in instruments of human construction. 
 The former class will be first considered. 
 
 Spherical Aberration. It has been stated that a pencil of rays falling 
 upon a spherical refracting surface will be refracted to a common focus. 
 Strictly speaking, however, the outer rays of the pencil i. e. those which fall 
 near the periphery of the refracting surface will be refracted more than those 
 which lie near the axis and will come to a focus sooner. This phenomenon, 
 which is called spherical aberration, is more marked with diverging than with 
 parallel rays, and tends, of course, to produce an indistinctness of the image 
 which will increase with the extent of the surface through which the rays 
 pass. % The effect of a diaphragm used in many optical instruments to reduce 
 the amount of spherical aberration by cutting off the side rays is shown dia- 
 grammatically in Fig. 136. 
 
 FIG. 136. Diagram showing the effect of a diaphragm in reducing the amount of spherical 
 
 aberration. 
 
 The r6le of the iris in the vision of near objects is now evident, for when 
 the eye is directed to a near object the spherical aberration is increased in con- 
 sequence of the rays becoming more divergent, but the contraction of the 
 pupil which accompanies accommodation tends, by cutting off the side rays, to 
 prevent a blurring of the image which otherwise would be produced. It must, 
 however, be remembered that the crystalline lens, unlike any lens of human 
 construction, has a greater index of refraction at the centre than at the periph- 
 ery. This, of course, tends to correct spherical aberration, and, in so far as it 
 does so, to render the cutting off of the side rays unnecessary. Indeed, the 
 total amount of possible spherical aberration in the eye is so small that its 
 effect on vision may be regarded as insignificant in comparison with that caused 
 by the other optical imperfections of the eye. 
 
316 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 Chromatic Aberration. In the above account of the dioptric apparatus 
 of the eye the phenomena have been described as they would occur with mono- 
 chromatic light i. e. with light having but one degree of refrangibility. But 
 the light of the sun is composed of an infinite number of rays of different 
 degrees of refrangibility. Hence when an image is formed by a simple lens 
 the more refrangible rays i. e. the violet rays of the spectrum are brought 
 to a focus sooner than the less refrangible red rays. The image therefore 
 appears bordered by fringes of colored light. This phenomenon of chromatic 
 aberration can be well observed by looking at objects through the lateral por- 
 tion of a simple lens, or, still better, by observing them through two simple 
 lenses held at a distance apart equal to the Sum of their focal distances. The 
 objects will appear inverted (as through afn astronomical telescope) and sur- 
 rounded with borders of colored light. N/ow, the chromatic aberration of the 
 eye is so slight that it is not easily detectecj, and the physicists of the eighteenth 
 century, in their efforts to produce an Achromatic lens, seem to have been 
 impressed by the fact that in the eye a ^combination of media of different 
 refractive powers is employed, and to havk sought in this circumstance an 
 explanation of the supposed achromatism of the eye. Work directed on this 
 line was crowned with brilliant success, for by combining two sorts of glass of 
 different refractive and dispersive powers it was found possible to refract a ray 
 of light without dispersing it into its different coloredrays, and the achromatic 
 lens, thus constructed, became at once an essential part Nof every first-class opti- 
 cal instrument. Now, as there is not only no evidenceHhat the principle of 
 the achromatic lens is employed in the eye, but distinct evidence that the eye 
 is uncorrected for chromatic aberration, we have here a remarkable instance of 
 a misconception of a physical fact leading to an important discovery in physics. 
 The chromatic aberration of the eye, though so slight as not to interfere at all 
 with ordinary vision, can be readily shown to exist by the simple experiment 
 of covering up one half of the pupil and looking at a bright source of light 
 e. g. a window. If the lower half of the pupil be covered, the cross-bars of 
 
 FIG. 137. Diagram to illustrate chromatic aberration. 
 
 the window will appear bordered with a fringe of blue light on the lower and 
 reddish light on the upper side. The explanation usually given of the way in 
 which this result is produced is'illustrated in Fig. 137. Owing to the chromatic 
 aberration of the eye all the rays emanating from an object at A are not 
 focussed accurately on the retina, but if the eye is accommodated for a ray of 
 medium refrangibility, the violet rays will be brought to a focus in front of 
 the retina at I 7 , while the red rays will be focnssed behind the retina at R. 
 
THE SENSE OF VISION. 
 
 317 
 
 On the retiiiii itself will be formed not an accurate optical image of the point 
 A, but a smull circle of dispersion in which the various colored rays are mixed 
 together, the violet rays after crossing falling upon the same part of the retina 
 as the red rays before crossing. Thus by a sort of compensation, which, how- 
 ever, cannot be equivalent to the synthetic reproduction of white light by the 
 union of the spectral colors, the disturbing effect of chromatic aberration is 
 diminished. When the lower half of the pupil is covered by the edge of a 
 card held in front of the cornea at D, the aberration produced in the upper 
 half of the eye is not compensated by that of the lower half. Hence the 
 image of a point of white light at A will appear as a row of spectral colors 
 on the retina, and all objects will appear bordered by colored fringes. Another 
 good illustration of the chromatic aberration of the eye is obtained by cutting 
 two holes of any convenient shape in a piece of black cardboard and placing 
 behind one of them a piece of blue and behind the other a piece of red glass. 
 If the card is placed in a window some distance (10 meters) from the observer, 
 in such a position that the white light of the sky may be seen through the col- 
 ored glasses, it will be found that the outlines of the two holes will generally 
 be seen with unequal distinctness. To most eyes the red outline will appear 
 quite distinct, while the blue figure will seem much blurred. To a few indi- 
 viduals the blue figure appears the more distinct, and these will generally be 
 found to be hypermetropic. 
 
 FIG. 138. Model to illustrate astigmatism. 
 
 Astigmatism. The defect known as astigmatism is due to irregularities 
 of curvature of the refracting surfaces, in consequence of which all the rays 
 proceeding from a single point cannot be brought to a single focus on the 
 retina. 
 
 Astigmatism is said to be regular when one of the surfaces, generally the 
 
318 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 cornea, is not spherical, but ellipsoidal i. e. having meridians of maximum 
 and minimum curvature at right angles to each other, though in each meridian 
 the curvature is regular When this is the case the rays proceeding from a 
 single luminous point are brought to a focus earliest when they lie in the 
 meridian in which the surface is most convex. Hence the pencil of rays will 
 have two linear foci, at right angles to the meridians of greatest and least 
 curvature separated by a space in which a section of the cone of rays will be 
 first elliptical, then circular, and then again elliptical. This defect exists to a 
 certain extent in nearly all eyes, and is, in some cases, a serious obstacle to dis- 
 tinct vision. The course of the rays when thus refracted is illustrated in Fig. 138, 
 which represents the interior of a box through which black threads are drawn 
 to indicate the course of the rays of light. The threads start at one end of the 
 box from a circle representing the cornea, and converge with different degrees 
 of rapidity in different meridians, so that a section of the cone of rays will be 
 successively an ellipse, a straight line, an ellipse, a circle, etc., as shown by the 
 model represented in Fig. 139. It will be noticed that this and the preced- 
 
 FIG. 139. Model to illustrate astigmatism. 
 
 ing figure are drawn in duplicate, but that the lines are not precisely alike on 
 the two sides. In fact, the lines on the left represent the model as it would 
 be seen with the right eye, and those on the right as it would appear to 
 the left eye, which is just the opposite from an ordinary stereoscopic slide. 
 The figures are drawn in this way because they are intended to produce a 
 " pseudoscopic " effect in a way which will be explained in connection with 
 the subject of binocular vision. For this purpose it is only necessary to cross 
 the axes of vision in front of the page, as in the experiment described on page 
 312, for studying the relation between the focal, axial, and pupillary adjust- 
 ments of the eye. As soon as the middle image becomes distinct it assumes a 
 
THE SENSE OF VISION. 319 
 
 stereoscopic appearance, and the correct relations between the different parts of 
 the model are at once obvious. 
 
 This imperfection of the eye may be detected by looking at lines such as are 
 shown in Figure 140, and testing each eye separately. If the straight lines 
 drawn in various directions through a common point cannot be seen with equal 
 distinctness at the same time, it is evident that the eye is better adapted to focus 
 rays in one meridian than in another i. e. it is astigmatic. The concentric 
 
 FIG. 140. Lines for the detection of astigmatism. 
 
 circles are a still more delicate test. Few persons can look at this figure attentively 
 without noticing that the lines are not everywhere equally distinct, but that in 
 certain sectors the circles present a blurred appearance. Not infrequently it 
 will be found that the blurred sectors do not occupy a constant position, but 
 oscillate rapidly from one part of the series of circles to another. This phe- 
 nomenon seems to be due to slight involuntary contractions of the ciliary 
 muscle causing changes in accommodation. 
 
 The direction of the meridians of greatest and least curvature of the cornea 
 of a regularly astigmatic eye, and the difference in the amount of this curvature, 
 can be very accurately measured by means of the ophthalmometer (see p. 304). 
 These points being determined, the defect of the eye can be perfectly corrected 
 by cylindrical glasses adapted to compensate for the excessive or deficient 
 refraction of the eye in certain meridians. 
 
 By another method known as " skiascopy," which consists in studying the 
 light reflected from the fundus of the eye when the ophthalmoscopic mirror is 
 moved in various directions, the amount and direction of the astigmatism of 
 the eye as a whole (and not that of the cornea alone) may be ascertained. 
 
 Astigmatism is said to be irregular when in certain meridians the curvatures 
 of the refracting surfaces are not arcs of circles or ellipses, or when there is a 
 lack of homogeneousness in the refracting media. This imperfection exists to 
 a greater or less extent in all eyes, and, unlike regular astigmatism, is incapable 
 of correction. It manifests itself by causing the outlines of all brilliant objects 
 to appear irregular. It is on this account that the fixed stars do not appear to 
 us like points of light, but as luminous bodies with irregular " star "-shaped 
 outlines. The phenomenon can be conveniently studied by looking at a pin- 
 hole in a large black card held at a convenient distance between the eye and a 
 strong light. The hole will appear to have an irregular outline, and to some 
 eyes will appear double or treble. 
 
320 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 Intraocular Images. Light entering the eye makes visible, under certain 
 circumstances, a number of objects which lie within the eye itself. These 
 objects are usually opacities in the media of the eye which are ordinarily invisi- 
 ble, because the retina is illuminated by light coming from all parts of the 
 pupil, and with such a broad source of light no object, unless it is a very large 
 one or one lying very near the back of the eye, can cast a shadow on the retina. 
 Such shadows can, however, be made apparent by allowing the media of the 
 eye to be traversed by parallel rays of light. This can be accomplished by 
 holding a small polished sphere e. g. the steel head of a shawl-pin illuminated 
 by sunlight or strong artificial light in the anterior focus of the eye i. e. 
 about 22 millimeters in front of the cornea, or by placing a dark screen with a 
 pin-hole in it in the same position between the eye and a source of uniform 
 diffused light, such as the sky or the porcelain shade of a student lamp. In 
 either case the rays of light diverging from the minute source will be refracted 
 into parallelism by the media of the eye, and will produce the sensation of a 
 circle of diffused light, the size of which will depend upon the amount of dila- 
 tation of the pupil. Within this circle of light will be seen the shadows of any 
 opaque substances that may be present in the media of the eye. These shadows, 
 being cast by parallel rays, will be of the same size as the objects themselves, 
 as is shown diagrammatical ly in Figure 141, in which A represents a source 
 
 FIG. 141. Showing the method of studying intraocular images (Helmholtz). 
 
 of light at the anterior focus of the eye, and b an opacity in the vitreous humor 
 casting a shadow B of the same size as itself upon the retina. It is evident that 
 if the source of light A is moved from side to side the various opacities will be 
 displaced relatively to the circle of light surrounding them by an amount de- 
 pending upon the distance of the opacities from the retina. A study of these 
 displacements will therefore afford a means of determining the position of the 
 opacities within the media of the eye. 
 
 Muscse Volitantes. Among the objects to be seen in thus examining the 
 eye the most conspicuous are those known as the muscce volitantes. These pre- 
 sent 'themselves in the form of beads, either singly or in groups, or of streaks, 
 patches, and granules. They have an almost constant floating motion, which 
 is increased by the movements of the eye and head. They usually avoid the 
 line of vision, floating away when an attempt is made to fix the sight upon 
 them. When the eye is directed vertically, however, they sometimes place 
 themselves directly in line with the object looked at. If the intraocular object 
 is at the same time sufficiently near the back of the eye to cast a shadow which 
 
THE SENSE OF VISION. 321 
 
 is visible without the use of the focal illumination, some inconvenience may 
 thus be caused in using a vertical microscope. 
 
 A study of the motions of the musccc volitantes makes it evident that the 
 phenomenon is due to small bodies floating in a liquid medium of a little 
 greater specific gravity than themselves. Their movements are chiefly in 
 planes perpendicular to the axis of vision, for when the eye is directed verti- 
 cally upward they move as usual through the field of vision without increasing 
 the distance from the retina. They are generally supposed to be the remains 
 of the embyronic structure of the vitreous body i. e. portions of the cells and 
 fibres which have not undergone complete mucous transformation. 
 
 In addition to these floating opacities in the vitreous body various other 
 defects in the transparent media of the eye may be revealed by the method of 
 focal illumination. Among these may be mentioned spots and stripes due to 
 irregularities in the lens or its capsule, and radiating lines indicating the stel- 
 late structure of the lens. 
 
 Retinal Vessels. Owing to the fact that the blood-vessels ramify near the 
 anterior surface of the retina, while those structures which are sensitive to light 
 constitute the posterior layer of that organ, it is evident that light entering the 
 eye will cast a shadow of the vessels on the light-perceiving elements of the 
 retina. Since, however, the diameter of the largest blood-vessels is not more 
 than one-sixth of the thickness of the retina, and the diameter of the pupil is 
 one-fourth or one-fifth of the distance from the iris to the retina, it is evident 
 that when the eye is directed to the sky or other broad illuminated surfaces it 
 is only the penumbra of the vessels that will reach the rods and cones, the umbra 
 terminating conically somewhere in the thickness of the retina. But if light 
 is allowed to enter the eye through a pin-hole in a card held a short distance 
 from the cornea, as in the above-described method of focal illumination, a 
 sharply defined shadow of the vessels will be thrown on the rods and cones. 
 Yet under these conditions the retinal vessels are not rendered visible unless 
 the perforated card is moved rapidly to and fro, so as to throw the shadow 
 continually on to fresh portions of the retinal surface. When this is done the 
 vessels appear, ramifying usually as dark lines on a lighter background, but 
 the dark lines are sometimes bordered by bright edges. It will be observed 
 that those vessels appear most distinctly the course of which is at right angles 
 to the direction in which the card is moved. Hence in order to see all the 
 vessels with equal distinctness it is best to move the card rapidly in a circle 
 the diameter of which should not exceed that of the pupil. In this manner 
 the distribution of the vessels in one's own retina may be accurately observed, 
 and in many cases the position of the fovea centralis may be determined by the 
 absence of vessels from that portion of the macula lutea. 
 
 The retinal vessels may also be made visible in several other ways e. g. y 
 1. By directing the eye toward a dark background and moving a candle to and 
 fro in front of the eye, but below or to one side of the line of vision. 2. By 
 concentrating a strong light by means of a lens of short focus upon a point 
 of the sclerotic as distant as possible from the cornea. By either of these 
 methods a small image of the external source of light is formed upon the 
 
 TT. 9.1 
 
322 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 lateral portion of the eye, and this image is the source of light which throws 
 shadows of the retinal vessels on to the rods and cones. 
 
 Circulation of Blood in the Retina. When the eye is directed toward a 
 surface which is uniformly and brightly illuminated e. g. the sky or a sheet 
 of white paper on which the sun is shining the field of vision is soon seen to 
 be filled with small bright bodies moving with considerable rapidity in irregu- 
 lar curved lines, but with a certain uniformity which suggests that their 
 movements are confined to definite channels. They are usually better seen 
 when one or more sheets of cobalt glass are held before the face, so that the 
 eyes are bathed in blue light. That the phenomenon depends upon the circu- 
 lation of the blood globules in the retina is evident from the fact that the 
 moving bodies follow paths which correspond with the form of the retinal 
 capillaries as seen by the methods above described, and also from the corre- 
 spondence between the rate of movement of the intraocular image and the 
 rapidity of the capillary circulation in those organs in which it can be di- 
 rectly measured under the microscope. The exact way in which the moving 
 globules stimulate the retina so as to produce the observed phenomenon must 
 be regarded as an unsettled question. 
 
 We have thus seen that the eye, regarded from the optician's point of view, 
 has not only all the faults inherent in optical instruments generally, but many 
 others which would not be tolerated in an instrument of human construction. 
 Yet with all its imperfections the eye is perhaps the most wonderful instance 
 in nature of the development of a highly specialized organ to fulfil a definite 
 purpose. In the accomplishment of this object the various parts of the eye 
 have been perfected to a degree sufficient to enable it to meet the requirements 
 of the nervous system with which it is connected, and no farther. In the 
 ordinary use of the eye we are unconscious of its various irregularities, shadows, 
 opacities, etc., for these imperfections are all so slight that the resulting inac- 
 curacy of the image does not much exceed the limit which the size of the 
 light-perceiving elements of the retina imposes upon the delicacy of our visual 
 perceptions, and it is only by illuminating the eye in some unusual way that 
 the existence of these imperfections can be detected. In other words, the eye 
 is as good an optical instrument as the nervous system can appreciate and 
 make use of. Moreover, when we reflect upon the difficulty of the problem 
 which nature has solved, of constructing an optical instrument out of living 
 and growing animal tissue, we cannot fail to be struck by the perfection of the 
 dioptric apparatus of the eye as well as by its adaptation to the needs of the 
 organism of which it forms a part. 
 
 Iris. The importance of the iris as an adjustable diaphragm for cutting 
 off side rays and thus securing good definition in near vision has been described 
 in connection with the act of accommodation. Its other function of protecting 
 the retina from an excess of light is no less important, and we must now con- 
 sider how this pupillary adjustment may be studied and by what mechanism 
 it is effected. The changes in the size of the pupil may be conveniently ob- 
 served in man and animals by holding a millimeter scale in front of the eye 
 and noticing the variations in the diameter of the pupil. It should be borne 
 
THE SENSE OF VISION. 
 
 323 
 
 Lc 
 
 in mind that the iris, seen in this way, does not appear in its natural size and 
 position, but somewhat enlarged and bulged forward by the magnifying effect 
 of the cornea and the aqueous humor. The changes in one's own pupil may 
 be readily observed by noticing the varying size of the circle of light thrown 
 upon the retina when the eye is illuminated by a point of light held at the 
 anterior focus, as in the method above described for the study of intraocular 
 images. 
 
 The muscles of the iris are, except in birds, of the unstriped variety, and 
 are arranged concentrically around the pupil. Radiating fibres are also recog- 
 nized by many observers, though their existence has been called in question 
 by others. The circular or constricting muscles of the iris are under the con- 
 trol of the third pair of cranial nerves, 
 and are normally brought into activity 
 in consequence of light falling upon 
 the retina. This is a reflex phenom- 
 enon, the optic nerve being the affer- 
 ent, and the third pair, the ciliary 
 ganglion, and the short ciliary nerves 
 the efferent, channel, as indicated in 
 Figure 142. This reflex is in man 
 and many of the higher animals bi- 
 lateral i. e. light falling upon one 
 retina will cause a contraction of both 
 pupils. This may readily be observed 
 in one's own eye when focally illumi- 
 nated in the manner above described. 
 Opening the other eye will, under 
 these conditions, cause a diminution, 
 and closing it an increase, in the size 
 of the circle of light. This bilateral 
 character is found to be dependent 
 upon the nature of the decussation of 
 the optic nerves, for in animals in 
 which the crossing is complete the 
 reflex is confined to the illuminated 
 
 I-M t i i \ 1HJIVC, c. y, viiia-i j ga-ngnwu, f i . u t ito OJ-iiri b i. WK J.&VTAU 
 
 eye. Ine arrangement ot the nbres j// )mo tor-ocuii nerve -.gym, its sympathetic root -,r.i, 
 
 in the Optic Commissure is in general its long root from V.ophthalmo-nasal branch of oph- 
 \ . . . . . . thalmic division of fifth nerve ; . c. short ciliary 
 
 associated With the position of the nerv es ; I. c, long ciliary nerves. 
 
 eyes in the head. When the eyes 
 
 are so placed that they can both be directed to the same object, as in man 
 and many of the higher animals, the fibres of each optic nerve are usually 
 found to be distributed to both optic tracts, while in animals whose eyes 
 are in opposite sides of the head there is complete crossing of the optic nerves. 
 Hence it may be said that animals having binocular vision have in general 
 a bilateral pupillary reflex. The rule is, however, not without exceptions, 
 for owls, though their visual axes are parallel, have, like other birds, a corn- 
 
 Course of constrictor nerve-fibres 
 
 " dilator " 
 
 FIG. 142. Diagrammatic representation of the 
 nerves governing the pupil (after Foster) : II, optic 
 nerve; l.g, ciliary -ganglion ; r. b, its short root from 
 
324 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 plete crossing of the optic nerves, and consequently a unilateral pupillary 
 reflex. 1 
 
 A direct as well as a reflex constriction of the pupil under the influence of 
 light has been observed in the excised eyes of eels, frogs, and some other ani- 
 mals. As the phenomenon can be seen in preparations consisting of the iris 
 alone or of the iris and cornea together, it is evident that the light exerts its 
 influence directly upon the tissues of the iris and not through an intraocular 
 connection with the retina. The maximum effect is produced by the yellowish- 
 green portion of the spectrum. 
 
 Antagonizing the motor oculi nerve in its constricting influence on the 
 pupil is a set of nerve-fibres the function of which is to increase the size of 
 the pupil. Most of these fibres seem to run their course from a centre which 
 lies in the floor of the third ventricle not far from the origin of the third pair, 
 through the bulb, the cervical cord, the anterior roots of the upper dorsal 
 nerves, the upper thoracic ganglion, the cervical sympathetic nerve as far as 
 the upper cervical ganglion ; then through a branch which accompanies the 
 internal carotid artery, passes over the Gasserian ganglion and joins the oph- 
 thalmic branch of the fifth pair ; then through the nasal branch of the latter 
 nerve and the long ciliary nerves to the eye 2 (see diagram, p. 323). These 
 fibres appear to be in a state of tonic activity, for section of them in any part 
 of their course (most conveniently in the cervical sympathetic) causes a con- 
 traction of the pupil which, on stimulation of the peripheral end of the divided 
 nerve, gives place to a marked dilatation. Their activity can be increased in 
 various ways. Thus dilatation of the pupil may be caused by dyspnea, vio- 
 lent muscular efforts, etc. Stimulation of various sensory nerves may also 
 cause reflex dilatation of the pupil, and since this phenomenon may be observed, 
 though greatly diminished in intensity, after extirpation of the superior cervi- 
 cal sympathetic ganglion, it is probable that the dilatation depends in part 
 upon a reflex inhibition of the constrictor nerves. 
 
 Since the cervical sympathetic nerve contains vaso-constrictor fibres for the 
 head and neck, it has been thought that its dilating effect upon the pupil might 
 be explained by its power of causing changes in the amount of blood in the 
 vessels of the iris. There is no doubt that a condition of vascular turgescence 
 or depletion will tend to produce contraction or dilatation of the pupil, but it is 
 impossible to explain the observed phenomena in this way, since the pupillary 
 are more prompt than the vascular changes, and may be observed on a bloodless 
 eye. Moreover, the nerve-fibres producing them are said to have a somewhat 
 different course. Another explanation of the influence of the sympathetic on 
 the pupil is that it acts by inhibiting the contraction of the sphincter muscles, 
 and that the dilatation is simply an elastic reaction. But since it is posssible to 
 produce local dilatation of the pu<pil by circumscribed stimulation at or near 
 
 1 Steinach : Archiv fur die gesammte Physiologic, xlvii. 313. 
 
 2 Langley : Journal of Physiology, xiii. p. 575. For the evidence of the existence of a 
 "cilio-spinal" centre in the cord, see Steil and Langendorff: Archiv fur die gesammte Phys- 
 iologic, Iviii. S. 155 ; also Schenck : Ibid., Ixii. S. 494. 
 
THE SENSE OF VISION. 325 
 
 the outer border of the iris, it seems more reasonable to conclude that the 
 dilator nerves of the pupil act upon radial muscular fibres in the substance of 
 the iris, in spite of the fact that the existence of such fibres has not been uni- 
 versally admitted. 
 
 Whatever view may be taken of the mechanism by which the sympathetic 
 nerves influence the pupil, there is no doubt that the iris is under the control 
 of two antagonistic sets of nerve-fibres, both of which are, under normal cir- 
 cumstances, in a state of tonic activity. Therefore, when the sympathetic 
 nerve is divided the pupil contracts under the influence of the motor oculi, and 
 section of the motor oculi causes dilatation through the unopposed influence of 
 the sympathetic. 
 
 The movements of the iris, though performed by smooth muscles, are more 
 rapid than those of smooth muscles found elsewhere e. g. in the intestines 
 and the arteries. The contraction of the pupil when the retina of the oppo- 
 site eye is illuminated occupies about 0.3" ; the dilatation when the light is cut 
 off from the eye, about 3" or 4". The latter determination is, however, diffi- 
 cult to make with precision, since dilatation of the pupil takes place at first 
 rapidly and then more slowly, so that the moment when the process is at an 
 end is not easily determined. After remaining a considerable time in absolute 
 darkness the pupils become enormously dilated, as has been shown by flash- 
 light photographs taken under these conditions. In sleep, though the eyes are 
 protected from the light, the pupils are strongly contracted, but dilate on 
 stimulation of sensory nerves, even though the stimulation may be insufficient 
 to rouse the sleeper. 
 
 Many drugs when introduced into the system or applied locally to the con- 
 junctiva produce effects upon the pupil. Those which dilate it are known as 
 mydriatica, those which contract it as myotics. Of the former class the most 
 important is atropin, the alkaloid of the Atropa belladonna, and of the latter 
 physostigmin, the alkaloid of the Calabar bean. In addition to their action 
 upon the pupil, mydriatics paralyze the accommodation, thus focussing the eye 
 for distant objects, while myotics, by producing a cramp of the ciliary muscle, 
 adjust the eye for near vision. The effect on the accommodation usually 
 begins later and passes off sooner than the affection of the pupil. Atropin 
 seems to act by producing local paralysis of the terminations of the third pair 
 of cranial nerves in the sphincter iridis and the ciliary muscle. In large 
 doses it may also paralyze the muscle-fibres of the sphincter. With this para- 
 lyzing action there appears to be combined a stimulating effect upon the dilator 
 muscles of the iris. The myotic action of physostigmin seems to be due to a 
 local stimulation of the fibres of the sphincter of the iris. 1 
 
 Although in going from a dark room to a lighter one the pupil at first con- 
 tracts, this contraction soon gives place to a dilatation, and in about three or 
 four minutes the pupil usually regains its former size. In a similar manner 
 the primary dilatation of the pupil caused by entering a dark room from a 
 lighter one is followed by a contraction which usually restores the pupil to its 
 original size within fifteen or twenty minutes. It is thus evident that the 
 1 See Paul Schultz : Archiv fur Physiologic, 1898, S. 47. 
 
326 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 amount of light falling upon the retina is not the only factor in determining 
 the size of the pupil. In fact, if the light acts for a sufficient length of time 
 the pupil may have the same size under the influence of widely different 
 degrees of illumination. 1 
 
 This so-called " adaptation " of the eye to various amounts of light seems 
 to be connected with the movements of the retinal pigment-granules and with 
 the chemical changes of the visual purple, to be more fully described in con- 
 nection with the physiology of the retina. 
 
 The Ophthalmoscope. Under normal conditions the pupil of the eye 
 appears as a black spot in the middle of the colored iris. The cause of this 
 dark appearance of the pupil is to be found in the fact that a source of light 
 and the retina lie in the conjugate foci of the dioptric apparatus of the eye. 
 Hence any light entering the eye that escapes absorption by the retinal pig- 
 ment and is reflected from the fundus must be refracted back to the source 
 from which it came. The eye of an observer who looks at the pupil from 
 another direction will see no light coming from it, and it will therefore appear 
 to him black. It is therefore evident that the essential condition for perceiving 
 light coming from the fundus of the eye is that the line of vision of the 
 observing eye shall be in the line of illumination. This condition is fulfilled 
 by means of instruments known as ophthalmoscopes. The principles involved 
 in the construction of the most common form of ophthalmoscope are illustrated 
 diagrammatically in Figure 143. 
 
 M 
 
 FIG. 143. Diagram to illustrate the principles of a simple ophthalmoscope (after Foster). 
 
 The rays from a source of light L, after being brought to a focus at a by 
 the concave perforated mirror M M, pass on and are rendered parallel by the 
 lens I. Then, entering the observed eye B, they are brought to a focus on the 
 retina at a f . Any rays which are reflected back from the part of the retina 
 thus illuminated will follow the course of the entering rays and be brought to 
 a focus at a. The eye of an observer at Aj looking through the hole in the 
 mirror, will therefore see at a an inverted image of the retina, the observation 
 of which may be facilitated by a convex lens placed immediately in front of 
 the observer's eye. 
 
 1 Schirmer : Archivfiir Ophthalmologie, xi. 5. 
 
THE SENSE OF VISION. 327 
 
 The fundus of the eye thus observed presents a reddish background on 
 which the retinal vessels are distinctly visible. 
 
 Retina. Having considered the mechanism by which optical images of 
 objects at various distances from the eye are formed upon the retina, we must 
 next inquire what part of the retina is affected by the rays of light, and in 
 what this affection consists. To the former of these questions it will be found 
 possible to give a fairly satisfactory answer. With regard to the latter nothing 
 positive is known. 
 
 The structure of the retina is exceedingly complicated, but, as very little 
 is known of the functions of the ganglion cells and of the molecular and 
 nuclear layers, it will suffice for the present purpose of physiological descrip- 
 tion to regard the retina as consisting of fibres of the optic nerve which are 
 connected through various intermediate structures with the layer of rods and 
 
 FIG. 144. Diagrammatic representation of the retina. 
 
 Figure 144 is intended to show, diagram matically, the mutual relation of 
 these various portions of the retina in different parts of the eye, and is not 
 drawn to scale. It will be observed that the optic nerve 0, where it enters the 
 eye, interrupts the continuity of the layer of rods and cones R and of the 
 intermediate structures /. Its fibres spread themselves out in all directions, 
 forming the internal layer of the retina N. The central artery of the retina 
 A accompanying the optic nerve ramifies in the layer of nerve-fibres and in 
 the immediately adjacent layers of the retina, forming a vascular layer V. In 
 the fovea centralis F of the macula lutea (the centre of distinct vision) the 
 layer of rods and cones becomes more highly developed, while the other layers 
 of the retina are much reduced in thickness and the blood-vessels entirely dis- 
 appear. This histological observation points strongly to the conclusion that 
 the rods and cones are the structures which are essential to vision, and that in 
 them are found the conditions for the conversion of the vibrations of the 
 luminiferous ether into a stimulus for a nerve-fibre. This view derives con- 
 firmation from the observations on the retinal blood-vessels, for it is found 
 that the distance between the vascular layer of the retina and the layer 
 of rods and cones determined by histological methods corresponds with that 
 which must exist between the vessels and the light-perceiving elements of the 
 retina, as calculated from the apparent displacement of the shadow caused by 
 given movements of the source of light used in studying intraocular images l as 
 1 " Dimmer: Verb. d. phys. Clubs zu Wien, 24 April, 1894," CenlralbL fur Physiolofa; 1894, 159. 
 
328 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 described on p. 321. Another argument in favor of this view is found in the 
 correspondence between the size of the smallest visible images on the retina and 
 the diameter of the rods and cones. ' A double star can be recognized as double 
 by the normal eye when the distance between the components corresponds to 
 a visual angle of 60". Two white lines on a black ground are seen to be dis- 
 tinct when the distance between them subtends a visual angle of 64"-73". 
 These angles correspond to a retinal image of 0.0044, 0.0046, and 0.0053 mil- 
 limeter. Now, the diameter of the cones in the macula lutea, as determined 
 by Kolliker, is 0.0045-0.0055 millimeter, a size which agrees well with the 
 hypothesis that each cone when stimulated can produce a special sensation of 
 light distinguishable from those caused by the stimulation of the neighboring 
 cones. The existence of the so-called blind spot in the retina at the point of 
 entrance of the optic nerve is sometimes regarded as evidence of the light- 
 perceiving function of the rods and cones, but as the other layers of the retina, 
 as well as the rods and cones, are absent at this point, and the retina here 
 consists solely of nerve-fibres, it is evident that the presence of the blind spot 
 
 FIG. 145. To demonstrate the blind spot. 
 
 only proves that the optic nerve-fibres are insensible to light. Figure 145 is 
 intended to demonstrate this insensibility. For this purpose it should be held 
 at a distance of about 23 centimeters from the eyes (i. e. about 3.5 times the dis- 
 tance between the cross and the round spot). If the left eye be closed and the 
 right eye fixed upon the cross, the round spot will disappear from view, though 
 it will become visible if the eye be directed either to the right or to the left of 
 the cross, or if the figure be held either a greater or a less distance from the 
 eye. The size and shape of the blind spot may readily be determined as 
 follows : Fix the eye upon a definite point marked upon a sheet of white 
 paper. Bring the black point of a lead pencil (which, except the point, has 
 been painted white or covered with white paper) into the invisible portion of 
 the field of vision and carry it outward in any direction until it becomes vis- 
 ible. Mark upon the paper the point 
 at which it just begins to be seen, and 
 by repeating the process in as many 
 different directions as possible the out- 
 line of the blind spot may be marked 
 out. Figure 146 shows the shape of 
 the blind spot determined by Helm- 
 holtz in his own right eye, a being 
 
 FIG. 146,-Porm of the blind spot (Helmholts). the P int f fixati n f th f *?*> and 
 
 the line A B being one-third of the 
 
 distance between the eye and the paper. The irregularities of outline, as at 
 
THE SENSE OF VISION. 
 
 329 
 
 Rods. 
 
 Cones. 
 
 d, are due to shadows of the large retinal vessels. During this determination 
 it is of course necessary that the head should occupy a fixed position with 
 regard to the paper. This condition can be secured by holding firmly between 
 the teeth a piece of wood that is clamped in a suitable position to the edge of 
 the table. The diameter of the blind spot, as thus determined, has been found 
 to correspond to a visual angle varying from 3 39' to 9 47', the average 
 measurement being 6 10'. This is about the angle that is subtended by the 
 human face seen at a distance of two meters. Although a considerable por- 
 tion of the retina is thus insensible to light, we are, in the ordinary use of the 
 eyes, conscious of no corresponding blank in the field of vision. By what 
 psychical operation we " fill up " the gap in our subjective field of vision 
 caused by the blind spot of the retina is a question that has been much dis- 
 cussed without being definitely settled. 
 
 The above-mentioned reasons for regarding the rods and cones as the light- 
 perceiving elements of the retina seem sufficiently conclusive. Whether there 
 is any difference between the rods and the cones with regard to their light- 
 perceiving function is a question which may be best considered in connection 
 with a description of the qualitative modifications of light. 
 
 The histological relation between the various layers of the retina is still 
 under discussion. According to recent observations of Cajal, 1 the connection 
 between the rods and cones on the one 
 side and the fibres of the optic nerve 
 on the other is established in a man- 
 ner which is represented diagram- 
 matically in Figure 147. The pro- 
 longations of the bipolar cells of the 
 internal nuclear layer E break up into 
 fine fibres in the external molecular 
 (or plexiform) layer (7. Here they are 
 brought into contact, though not into 
 anatomical continuity, with the termi- 
 nal fibres of the rods and cones. The 
 inner prolongations of the same bipolar 
 cells penetrate into the internal molec- 
 ular (or plexiform) layer F, and there 
 come into contact with the dendrites 
 coming from the layer of ganglion-cells 
 
 G. These cells are, in their turn, con- 
 
 * ' 
 
 nected by their axis-cylinder processes 
 with the fibres of the optic nerve. The 
 bipolar cells which serve as connective 
 links between the rods and the optic 
 nerve-fibres are anatomically distin- 
 guishable (as indicated in the diagram) 
 
 FIG. 147. Diagrammatic representation of the 
 structure of the retina (Cajal) : A, layer of rods 
 and cones ; B, external nuclear layer ; C, external 
 molecular (or plexiform) layer; E, internal nu- 
 clear layer ; F, internal molecular (or plexiform) 
 layer; 6, layer of ganglion-cells; H, layer of 
 nerve-fibres. 
 
 Die Retina der Wirbeltkiere, Wiesbaden, 1894. 
 
330 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 from those which perform the same function for the cones. Whatever be the 
 precise mode of connection between the rods and cones and the fibres of the 
 optic nerve, it is evident that each retinal element cannot be connected with 
 the nerve-centres by a separate independent nerve-channel, since the retina 
 contains many millions of rods and cones, while the optic nerve has only 
 about 438,000 nerve-fibres, 1 though of course such a connection may exist in 
 the fovea centralis, as Cajal has shown is probably the case in reptiles and birds. 
 Changes Produced in the Retina by Light. We must now inquire 
 what changes can be supposed to occur in the rods and cones under the influ- 
 ence of light by means of which they are able to transform the energy of the 
 ether vibrations into a stimulus for the fibres of the optic nerve. Though in 
 the present state of our knowledge no satisfactory answer can be given to this 
 question, yet certain direct effects of light upon the retina have been observed 
 which are doubtless associated in some way with the transformation in 
 question. 
 
 The retina of an eye which has been protected from light for a considerable 
 length of time has a purplish-red color, which upon exposure to light changes 
 to yellow and then fades away. This bleaching occurs also in monochromatic 
 light, the most powerful rays being those of the greenish-yellow portion of 
 the spectrum i. e. those rays which are most completely absorbed by the pur- 
 plish-red coloring matter. A microscopic examination of the retina shows 
 that this coloring matter, which has been termed visual purple, is entirely con- 
 fined to the outer portion of the retinal rods and does not occur at all in the 
 cones. After being bleached by light it is, during life, restored through the 
 agency of the pigment epithelium, the cells of which, under the influence of 
 light, send their prolongations inward to envelop the outer limbs of the rods 
 and cones with pigment. If an eye, either excised or in its natural position, 
 is protected from light for a time, and then placed in such a position that the 
 image of a lamp or a window is thrown upon the retina for a time which may 
 vary with the amount of light from seven seconds to ten minutes, it will be 
 found that the retina, if removed and examined under red light, will show the 
 image of the luminous object impressed upon it by the 
 bleaching of the visual purple. 
 
 If the retina be treated with a 4 per cent, solution of 
 alum, the restoration of the visual purple will be pre- 
 vented, and the so-called " optogram " will be, as pho- 
 tographers say, " fixed." 2 
 FIG. i48.-optogram in eye Figure 148 shows the appearance of a rabbit's retina 
 
 of rabbit (Kuhne). * j 11 
 
 on which the optogram of a window has been impressed. 
 
 Although the chemical changes in the visual purple under the influence of 
 
 light seem, at first sight, to aiford an explanation of the transformation of the 
 
 vibrations of the luminiferous ether into a stimulation for the optic nerve, yet 
 
 the fact that vision is most distinct in the fovea centralis of the retina, which, 
 
 1 Salzer : Wiener Sitzungsberichte, 1880, Bd. Ixxxi. S. 3. 
 
 2 Kiihne : Untersuchungen a. d. phys. Inst. d. Universitdt Heidelberg, i. 1. 
 
THE SENSE OF VISION. 331 
 
 as it contains no rods, is destitute of visual purple, makes it impossible to 
 regard this coloring matter as essential to vision. The most probable theory 
 of its function is perhaps that which connects it with the adaptation of the 
 eye to varying amounts of light, as described on p. 326. 
 
 In addition to the above-mentioned movements of the pigment epithelium 
 cells under the influence of light, certain changes in the retinal cones of frogs 
 and fishes have been observed. 1 The change consists in a shortening and thick- 
 ening of the inner portion of the cones when illuminated, but the relation of 
 the phenomenon to vision has not been explained. 
 
 Like most of the living tissues of the body, the retina is the seat of electri- 
 cal currents. In repose the fibres of the optic nerve are said to be positive in 
 relation to the layer of rods and cones. When light falls upon the retina this 
 current is at first increased and then diminished in intensity. 
 
 Sensation of Light. Whatever view may be adopted with regard to the 
 mechanism by which light is enabled to become a stimulus for the optic nerve, 
 the fundamental fact remains that the retina (and in all probability the layer 
 of rods and cones in the retina) alone supplies the conditions under which this 
 transformation of energy is possible. But in accordance with the " law of 
 specific energy " a sensation of light may be produced in whatever way the 
 optic nerve be stimulated, for a stimulus reaching the visual centres through 
 the optic nerve is interpreted as a visual sensation, in the same way that 
 pressure on a nerve caused by the contracting cicatrix of an amputated leg 
 often causes a painful sensation which is referred to the lost toes to which the 
 nerve was formerly distributed. Thus local pressure on the eyeball by stimu- 
 lating the underlying retina causes luminous sensations, already described as 
 " phosphenes," and electrical stimulation of the eye as a whole or of the stump 
 of the optic nerve after the removal of the eye is found to give rise to sensa- 
 tions of light. 
 
 Vibrations of the luminiferous ether constitute, however, the normal stirn- 
 ulusof the retina, and we must now endeavor to analyze the sensation thus 
 produced. In the first place, it must be borne in mind that the so-called ether 
 waves differ among themselves very widely in regard to their rate of oscilla- 
 tion. The slowest known vibrations of the ether molecules have a frequency 
 of about 107,000,000,000,000 in a second, and the fastest a rate of about 
 40,000,000,000,000,000 in a second a range, expressed in musical terms, of 
 about eight and one-half octaves. All these ether waves are capable of warm- 
 ing bodies upon which they strike and of breaking up certain chemical com- 
 binations, the slowly vibrating waves being especially adapted to produce the 
 former and the rapidly vibrating ones the latter effect. Certain waves of 
 intermediate rates of oscillation viz. those ranging between 392,000,000,- 
 000,000 and 757,000,000,000,000 in a second not only produce thermic and 
 chemical effects, but have the power, when they strike the retina, of causing 
 changes in the layer of rods and cones, which, in their turn, act as a stimulus 
 to the optic nerve. The ether waves which produce these various phenomena 
 are often spoken of as heat rays, light rays, and actinic or chemical rays, but 
 1 Engelmann : Archiv fur die yemmmte Physiologic, xxxv. 498. 
 
332 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 it must be remembered that the same wave may produce all three classes of 
 phenomena, the effect depending upon the nature of the substance upon which 
 it strikes. It will be observed that the range of vibrations capable of affecting 
 the retina is rather less than one octave, a limitation which obviously tends to 
 reduce the amount of chromatic aberration. 
 
 In this connection it is interesting to notice that the highest audible note is 
 produced by about 40,000 sonorous impulses in a second. Between the high- 
 est audible note and the lowest visible color there is a gap of nearly thirty-four 
 octaves in which neither the vibrations of the air nor those of the luminifer- 
 ous ether affect our senses. Even if the slowly vibrating heat-rays which 
 affect our cutaneous nerves are taken into account, there still remain over 
 thirty-one octaves of vibrations, either of the air or of the luminiferous ether, 
 which may be, and very likely are, filling the universe around us without in 
 any way impressing themselves upon our consciousness. 1 
 
 Qualitative Modifications of Light. All the ethereal vibrations which 
 are capable of affecting the retina are transmitted with very nearly the same 
 rapidity through air, but when they enter a denser medium the waves having 
 a rapid vibration are retarded more than those vibrating more slowly. Hence 
 when a ray of sunlight composed of all the visible ether waves strikes upon a 
 
 plane surface of glass, the greater 
 retardation of the waves of rapid 
 vibration causes them to be more 
 refracted than those of slower vibra- 
 tion, and if the glass has the form 
 of a prism, as shown in Figure 149, 
 this so-called "dispersion" of the 
 rays is still further increased when 
 the rays leave the glass, so that the 
 emerging beam, if received upon a 
 
 FIG. 149.-Diagram "^straUngthe dispersion of light whUe ^^ ^^ of forming a 
 
 spot of white light, produces a band 
 
 of color known as the solar spectrum. The colors of the spectrum, though 
 commonly spoken of as seven in number, really form a continuous series from 
 the extreme red to the extreme violet, these colors corresponding to ether vibra- 
 tions with rates of 392,000,000,000,000 and 757,000,000,000,000 in 1 second, 
 and wave lengths of 0.7667 and 0.3970 micromilli meters 2 respectively. 
 
 Colors, therefore, are sensations caused by the impact upon the retina of 
 certain ether waves having definite frequencies and wave-lengths, but these 
 are not the only peculiarities of the ether vibration which influence the retinal 
 sensation. The energy of the vibration, or the vis viva of the vibrating mole- 
 cule, determines the " intensity " of the sensation or the brilliancy of the light. 3 
 
 1 The vibrations of electrical energy utilized in wireless telegraphy are probably inter- 
 mediate in their rate between those of sound and light. 
 
 2 One micromillimeter = 0.001 millimeter one /". 
 
 8 The energy of vibration capable of producing a given subjective sensation of intensity 
 varies with the color of the light, as will be later explained (see p. 340). 
 
THE SENSE OF VISION. 333 
 
 Furthermore, the sensation produced by the impact of ether waves of a definite 
 length will vary according as the eye is simultaneously affected by a greater or 
 less amount of white light. This modification of the sensation is termed its 
 degree of " saturation/ 7 light being said to be completely saturated when it is 
 " monochromatic " or produced by ether vibrations of a single wave-length. 
 
 The modifications of light which taken together determine completely the 
 character of the sensation are, then, three in number viz. : 1. Color, depend- 
 ent upon rate of vibration or length of the ether wave ; 2. Intensity, dependent 
 upon the energy of the vibration ; 3. Saturation, dependent upon the amount 
 of white light mingled with the monochromatic light. These three qualitative 
 modifications of light must now be considered in detail. 
 
 Color. In our profound ignorance of the nature of the process by which, 
 in the rods and cones, the movements of the ether waves are converted into a 
 stimulus for the optic nerve-fibres, all that can be reasonably demanded of a 
 color theory is that it shall present a logically consistent hypothesis to account 
 for the sensations actually produced by the impact of ether waves of varying 
 rates, either singly or combined, upon different parts of the retina. Some of 
 the important phenomena of color sensation of which every color theory must 
 take account may be enumerated as follows : 
 
 1. Luminosity is more readily recognized than color. This is shown by 
 the fact that a colored object appears colorless when it is too feebly illuminated, 
 and that a spectrum produced by a very feeble light shows variations of inten- 
 sity with a maximum nearer than normal to the blue end, but no gradations 
 of color. A similar lack of color is noticed when a colored object is observed 
 for too short a time or when it is of insufficient size. In all these respects 
 the various colors present important individual differences which will be con- 
 sidered later. 
 
 2. Colored objects seen with increasing intensity of illumination appear 
 more and more colorless, and finally present the appearance of pure white. 
 Yellow passes into white more readily than the other colors. 
 
 3. The power of the retina to distinguish colors diminishes from the centre 
 toward the periphery, the various colors, in this respect also, differing mate- 
 rially from each other. Sensibility to red is lost at a short distance from the 
 macula lutea, while the sensation of blue is lost only on the extreme lateral 
 portions of the retina. The relation of this phenomenon to the distribution 
 of the rods and cones in the retina will be considered in connection with the 
 perception of the intensity of light. 
 
 Color-mixture. Since the various spectral colors are produced by the dis- 
 persion of the white light of the sun, it is evident that white light may be 
 reproduced by the reunion of the rays corresponding to the different colors, and 
 it is accordingly found that if the colored rays emerging from a prism, as in 
 Fig. 149, are reunited by suitable refracting surfaces, a spot of white light will be 
 produced similar to that which would have been caused by the original beam 
 of sunlight. But white light may be produced not only by the union of all 
 the spectral colors, but by the union of certain selected colors in twos, threes, 
 
334 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 fours, etc. Any two spectral colors which by their union produce white are 
 said to be " complementary " colors. The relation of these pairs of comple- 
 mentary colors to each other may be best understood by reference to Figure 150. 
 
 p 
 
 FIG. 150. Color diagram. 
 
 Here the spectral colors are supposed to be disposed around a curved line, 
 as indicated by their initial letters, and the two ends of the curve are united 
 by a straight line, thus enclosing a surface having somewhat the form of a tri- 
 angle with a rounded apex. If the curved edge of this surface be supposed to 
 be loaded with weights proportionate to the luminosity of the different colors, 
 the centre of gravity of the surface will be near the point W. Now, if a 
 straight line be drawn from any point on the curved line through the point 
 W and prolonged till it cuts the curve again, the colors corresponding to the 
 two ends of this straight line will be complementary colors. Thus in Figure 
 150 it will be seen that the complementary color of red is bluish-green, and 
 that of yellow lies near the indigo. It is also evident that the complementary 
 color of green is purple, which is not a spectral color at all, but a color 
 obtained by the union of violet and red. The union of a pair of colors 
 lying nearer together than complementary colors produces an intermediate color 
 mixed with an amount of white which is proportionate to the nearness of the 
 colors to the complementary. Thus the union of red and yellow produces 
 orange, but a less saturated orange than the spectral color. The union of two 
 colors lying farther apart than complementary colors produces a color which 
 borders more or less upon purple. 
 
 The mixing of colors to demonstrate the above-mentioned effects may be 
 accomplished in three different ways : 
 
 1. By employing two prisms to produce two independent spectra, and then 
 directing the colored rays which are to be united so that they will illuminate 
 the same white surface. 
 
 2. By looking obliquely through a glass plate at a colored object placed 
 behind it, while at the same time light from another colored object, placed in 
 front of the glass, is reflected into the eye of the observer, as shown in Figure 
 151. Here the transmitted light from the colored object A and the reflected 
 light from the colored object B enter the eye at C from the same direction, 
 and are therefore united upon the retina. 
 
 3. By rotating before the eye a disk on which the colors to be united are 
 painted upon different sectors. This is most readily accomplished by using 
 
THE SENSE OF VISION. 335 
 
 a number of disks, each painted with one of the colors to be experimented 
 with, and each divided radially by a cut running from the centre to the circum- 
 ference. The disks can then be lapped over each other and rotated together, and 
 in this way two or more colors can be mixed in any desired proportions. This 
 method of mixing colors depends upon 
 the property of the retina to retain an 
 impression after the stimulus causing JV 
 
 it has ceased to act a phenomenon of / 
 
 great importance in physiological optics, / 
 
 and one which will be further discussed / 
 
 in connection with the subject of " after- / \ 
 images." ^/ \fl 
 
 The physiological mixing of Colors FIG. 151. Diagram to illustrate color mixture by 
 
 cannot be accomplished by the mixture 
 
 of pigments or by allowing sunlight to pass successively through glasses of 
 different colors, for in these cases rays corresponding to certain colors are 
 absorbed by the medium through which the white light passes, and the phe- 
 nomenon is the result of a process of subtraction and not addition. Light 
 reaching the eye through red glass, for instance, looks red because all the rays 
 except the red rays are absorbed, and light coming through green glass appears 
 green for a similar reason. Now, when light is allowed to pass successively 
 through red and green glass the only rays which pass through the red glass 
 will be absorbed by the green. Hence no light will pass through the combi- 
 nation of red and green glass, and darkness results. But when red and green 
 rays are mixed by any of the three methods above described the result of this 
 process of addition is not darkness, but a yellow color, as will be understood 
 by reference to the color diagram on p. 334. In the case of colored pigments 
 similar phenomena occur, for here too light reaches the eye after rays of cer- 
 tain wave-lengths have been absorbed by the medium. This subject will be 
 further considered in connection with color-theories. 1 
 
 Color-theories. From what has been said of color-mixtures it is evident 
 that every color sensation may be produced by the mixture of a number of 
 other color sensations, and that certain color sensations viz. the purples can 
 be produced only by the mixture of other sensations,- since there is no single 
 wave-length corresponding to them. Hence the hypothesis is a natural one 
 that all colors are produced by the mixture in varying proportions of a certain 
 number of fundamental colors, each of which depends for its production upon 
 the presence in the retina of a certain substance capable of being affected 
 (probably through some sort of a photo-chemical process) by light of a certain 
 definite wave-length. A hypothesis of this sort lies at the basis of both the 
 Young-Helmholtz and the Hering theories of color sensation. 
 
 The former theory postulates the existence in the retina of three substances 
 capable of being affected by red, green, and violet rays, respectively i. e. by 
 the three colors lying at the three angles of the color diagram given on p. 334 
 
 1 For an interesting discussion of modern theories of color-vision, see the address of Professor 
 Frank P. Whitman on " Color-vision," Science, Sept, 9, 1898. 
 
336 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 s . 
 
 and regards all other color sensations as produced by the simultaneous affec- 
 tion of two of these substances in varying proportions. Thus when a ray of 
 blue light falls on the retina it stimulates the violet- and green-perceiving sub- 
 stances, and produces a sensation intermediate between the two, while simul- 
 taneous stimulation of the red- and green-perceiving substances produces the 
 sensations corresponding to yellow and orange ; and when the violet- and red- 
 perceiving substances are affected at the same time, the various shades of 
 purple are produced. Each of these three substances is, however, supposed to 
 be affected to a slight extent by all the rays of the visible spectrum, a suppo- 
 sition which is rendered necessary by the fact that even the pure spectral 
 colors do not appear to be perfectly saturated, as will be explained in connec- 
 tion with the subject of saturation. Furthermore, the disappearance of color 
 when objects are very feebly or very brightly illuminated or when they are 
 seen with the lateral portions of the retina (as described on p. 333) necessitates 
 the additional hypotheses that these three substances are all equally affected by 
 all kinds of rays when the light is of either very small or very great intensity 
 or when it falls on the extreme lateral portions of the retina, and that they 
 manifest their specific irritability for red, green, and violet rays respectively 
 only in light of moderate intensity falling not too far from the fovea centralis 
 of the retina. 
 
 The modifications of the Young- Hem holtz theory introduced by these sub- 
 idiary hypotheses greatly diminish the simplicity which was its chief claim to 
 acceptance when originally proposed. Moreover, there will always remain a 
 psychological difficulty in supposing that three sensations so different from each 
 other as those of red, green, and violet can by their union produce a fourth 
 sensation absolutely distinct from any of them viz. white. 
 
 The fact that in the Hering theory this difficulty is obviated has contributed 
 greatly to its acceptance by physiologists. In this theory the retina is supposed 
 to contain three substances in which chemical changes may be produced by etherj 
 vibrations, but each of these substances is supposed to be affected in two op] 
 site ways by rays of light which correspond to complementary color sensa-' 
 tions. Thus in one substance viz. the white-black visual substance kata- 
 bolic or destructive changes are supposed to be produced by all the rays of the 
 visible spectrum, the maximum effect being caused by the yellow rays, while 
 anabolic or constructive changes occur when no light at all falls upon the 
 retina. The chemical changes of this substance correspond, therefore, to the 
 sensation of luminosity as distinguished from color. In a second substance red 
 rays are supposed to produce katabolic, and green rays anabolic changes, while 
 a third substance is similarly affected by yellow and blue rays. These two 
 substances are therefore spoken of as red-green and yellow-blue visual sub- 
 stances respectively. 
 
 It has been sometimes urged as an objection to this theory that the effect of 
 a stimulus is usually katabolic and not anabolic. This is true with regard to 
 muscular contraction, from the study of which phenomenon most of our know- 
 ledge of the effect of stimulation has been obtained, but it should be remem- 
 
THE SENSE OF VISION. 337 
 
 bered that observations on the augmentor and inhibitory cardiac nerves have 
 shown us that nerve-stimulation may produce very contrary effects. There 
 seems to be, therefore, no serious theoretical difficulty in supposing that light 
 rays of different wave-lengths may produce opposite metabolic effects upon the 
 substances in which changes are associated with visual sensations. 
 
 A more serious objection lies in the difficulty of distinguishing between the / j 
 sensation of blackness, which, on Hering's hypothesis, must correspond to active I ^ 
 anabolism of the white-black substance, and the sensation of darkness (such as ) \ 
 we experience when the eyes have been withdrawn for some time from the / ^ 
 influence of light), which must correspond to a condition of equilibrium of ) 
 the white-black substance in which neither anabolism nor katabolism is / 
 occurring. 
 
 Another objection to the Hering theory is to be found in the results of 
 experiments in comparing grays or whites produced by mixing different colored 
 rays under varying intensities of light. The explanation given by Hering of 
 the production of white through the mixture of blue and yellow or of red and 
 green is that when either of these pairs of complementary colors is mixed 
 the anabolic and the katabolic processes balance each other, leaving the corre- 
 sponding visual substance in a condition of equilibrium. Hence, the white- 
 black substance being alone stimulated, the result will be a sensation of white 
 corresponding to the intensity of the katabolic process caused by the mixed 
 rays. Now, it is found that when blue and yellow are mixed in certain pro- 
 portions on a revolving disk a white can be produced which will, with a certain 
 intensity of illumination, be undistinguishable from a white produced by mix- 
 ing red and green. If, however, the intensity of the illumination is changed, 
 it will be found necessary to add a certain amount of white to one of the mix- 
 tures in order to bring them to equality. On the theory that complementary 
 colors produce antagonistic processes in the retina it is difficult to understand 
 why this should be the case. 1 
 
 A color theory which is in some respects more in harmony with recent/ 
 observations in the physiology of vision has been proposed by Mrs. C. LJ jh / 
 Franklin. In this theory it is supposed that, in its earlier periods of deA 
 velopment, the eye is sensitive only to luminosity and not to color i. e. it 
 possesses only a white-black or (to use a single word) a gray-perceiving sub- 
 stance which is affected by all visible light rays, but most powerfully by those 
 lying near the middle of the spectrum. The sensation of gray is supposed to 
 be dependent upon the chemical stimulation of the optic nerve-terminations by 
 some product of decomposition of this substance. 
 
 In the course of development a portion of this gray visual substance becomes 
 differentiated into three different substances, each of which is affected by rays 
 of light corresponding to one of the three fundamental colors of the spectrum 
 viz. red, green, and blue. This differentiation may be supposed to occur in \ 
 the cones rather than in the rods, which thus become organs specially adapted j 
 
 1 The renewal of the rod pigment in a dim light may afford an explanation of this phenom- 
 enon (see C. Ladd Franklin : Psychological Review, v. 311). 
 VOL. II. 22 
 
338 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 for the perception of color (see p. 342). When a ray of light intermediate 
 between two of the fundamental colors falls upon the retina, the visual sub- 
 stances corresponding to these two colors will be affected to a degree pro- 
 portionate to the proximity of these two colors to that of the incident ray. 
 Since this effect is exactly the same as that which is produced when the retina 
 is acted upon simultaneously by light of two fundamental colors, we are incap- 
 able of distinguishing in sensation between an intermediate wave-length and 
 a mixture in proper amounts of two fundamental wave-lengths. 
 
 When the retina is affected by two or more rays of such wave-lengths that 
 all three of the color visual substances are equally affected, the resulting decom- 
 position will be the same as that produced by the stimulation of the gray visual 
 substance out of which the color visual substances were differentiated, and the 
 corresponding sensation will therefore be that of gray or white. 
 
 It will be noticed that the important feature of this theory is that it pro- 
 vides for the independent existence of the gray visual substance, while pt the 
 same time the stimulation of this substance is made a necessary result of the 
 mixture of certain color sensations. 
 
 Another color theory has recently been brought forward by Prof. G. E. 
 Miiller, 1 who substitutes for Hering's antagonistic processes of assimilation and 
 dissimilation the conception of " reversible chemical actions " i. e. actions 
 in which the products of a chemical change can be used for the reconstruc- 
 tion of the original substance. 
 
 Color-blindness. The fact that many individuals are incapable of distin- 
 guishing between certain colors i. e. are more or less " color-blind " is one 
 of fundamental importance in the discussion of theories of color vision. By 
 far the most common kind of color-blindness is that in which certain shades 
 of red and green are not recognized as different colors. The advocates of the 
 Young-Helmholtz theory explain such cases by supposing that either the red 
 or the green perceiving elements of the retina are deficient, or, if present, are 
 irritable, not by rays of a particular wave-length, but by all the rays of the 
 visible spectrum. In accordance with this view these cases of color-blindness 
 are divided into two classes viz. the red-blind and the green-blind the basis 
 for the classification being furnished by more or less characteristic curves repre- 
 senting the variations in the luminosity of the visible spectrum as it appears 
 to the different eyes. There are, however, cases which cannot easily be brought 
 under either of these two classes. Moreover, it has been proved in cases of 
 monocular color-blindness, and is admitted even by the defenders of the Helm- 
 holtz theory, that such persons see really only two colors viz. blue and yellow. 
 To such persons the red end of the spectrum appears a dark yellow, and the 
 green portion of the spectrum has luminosity without color. 
 y A better explanation of this sort of color-blindness is given in the Hering 
 theory by simply supposing that in such eyes the red-green visual substance is 
 deficient or wholly wanting, but the theory of Mrs. Franklin accounts for the 
 phenomena in a still more satisfactory way ; for, by supposing that the differ- 
 \f l Zeitschrift fur Psychohgie und Physiologic der Sinnesorgane, 1875 and 1897. 
 
THE SENSE OF VISION. 339 
 
 entiation of the primary gray visual substance has first led to the formation 
 of a blue and a yellow visual substance, and that the latter has subsequently 
 been differentiated into a red and a green visual substance, color-blindness is 
 readily explained by supposing that this second differentiation has either not 
 occurred at all or has taken place in an imperfect manner. It is, in other 
 words, an arrest of development. 
 
 Cases of absolute color-blindness occasionally occur. To such persons 
 nature appears colorless, all objects presenting simply differences of light and 
 shade. 
 
 In whatever Avay color-blindness is to be explained, the defect is one of 
 considerable practical importance, since it renders those affected by it incapable 
 of distinguishing the red and green lights ordinarily used for signals. Such 
 persons are, therefore, unsuitable for employment as pilots, railway engineers, 
 etc., and it is now customary to test the vision of all candidates for employment 
 in such situations. It has been found that no satisfactory results can be 
 reached by requiring persons to name colors which are shown them, and the 
 chromatic sense is now commonly tested by what is known as the " Holmgren 
 method," which consists in requiring the individual examined to select from a 
 pile of worsteds of various colors those shades which seem to him to resemble 
 standard skeins of green and pink. When examined in this way about 4 per 
 cent, of the male and one-quarter of 1 per cent, of the female sex are found to 
 be more or less color-blind. The defect may be inherited, and the relatives 
 of a color-blind person are therefore to be tested with special care. Since 
 females are less liable to be affected than males, it often happens that the 
 daughters of a color-blind person, themselves with normal vision, have sons 
 who inherit their grandfather's infirmity. 
 
 Although in all theories of color vision the different sensations are supposed 
 to depend upon changes produced by the ether vibrations of varying rates 
 acting upon different substances in the retina, yet it should be borne in mind 
 that we have at present no proof of the existence of any such substances. The 
 visual purple or, to adopt Mrs. Franklin's more appropriate term, " the rod 
 pigment" was at one time thought to be such a substance, but for the reasons 
 above given cannot be regarded as essential to vision. 1 
 
 That a centre for color vision, distinct from the visual centre, exists in the 
 cerebral cortex is rendered probable by the occurrence of cases of hemianopsia 
 for colors, and also by the experiments of Heidenhain and Cohn on the influ- 
 ence of the hypnotic trance upon color-blindness. 
 
 Intensity. The second of the above-mentioned qualitative modifications of 
 light is its intensity, which is dependent upon the energy of vibrations of the 
 molecules of the luminiferous ether. The sensation of luminosity is not, how- 
 ever, proportionate to the intensity of the stimulus, but varies in such a way 
 that a given increment of intensity causes a greater difference in sensation with 
 
 1 In a recently developed theory by Ebbinghaus (Zeitschrift fur Psychologic und Physiologic 
 der Sinnesorgane, v. 145) a physiological importance in relation to vision is attached to this 
 substance in connection with other substances of a hypothetical character. 
 
340 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 feeble than with strong illuminations. This phenomenon is illustrated by the 
 disappearance of a shadow thrown by a candle in a darkened room on a sheet 
 of white paper when sunlight is allowed to fall on the paper from the opposite 
 direction. In this case the absolute difference in luminosity between the 
 shadowed and unshadowed portions of the paper remains the same, but it 
 becomes imperceptible in consequence of the increased total illumination. 
 
 Although our power of distinguishing absolute differences in luminosity 
 diminishes as the intensity of the illumination increases, yet with regard to 
 relative differences no such dependence exists. On the contrary, it is found 
 within pretty wide limits that, whatever be the intensity of the illumination, 
 it must be increased by a certain constant fraction of its total amount in order 
 to produce a perceptible difference in sensation. This is only a special case of 
 a general law of sensation known as Weber's law, which has been formulated 
 by Foster as follows : " The smallest change in the magnitude of a stimulus 
 which we can appreciate through a change in our sensation always bears the 
 same proportion to the whole magnitude of the stimulus." 
 
 Luminosity of Different Colors. When two sources of light having the 
 same color are compared, it is possible to estimate their relative luminosity 
 with considerable accuracy, a difference of about 1 per cent, of the total 
 luminosity being appreciated by the eye. When the sources of light have 
 different colors, much less accuracy is attainable, but there is still a great differ- 
 ence in the intensity with which rays of light of different wave-lengths affect 
 the retina. We do not hesitate to say, for instance, that the maximum 
 intensity of the solar spectrum is found in the yellow portion, but it is import- 
 ant to observe that the position of this maximum varies with the illumina- 
 tion. In a very brilliant spectrum the maximum shifts toward the orange, 
 and in a feeble spectrum (such as may be obtained by narrowing the slit of 
 the spectroscope) it moves toward the green. Hence changes of intensity are 
 associated with changes of color, and, as Haycraft l has observed, " we cannot 
 abstract ' brightness ' from our sensations of light as we can abstract ' loud- 
 ness' from our sensations of sound." The curves in Figure 152 illus- 
 trate this shifting of the maximum of luminosity of the spectrum with vary- 
 ing intensities of illumination. The abscissas represent wave-lengths in 
 millionths of a millimeter, and the ordinates the luminosity of the different 
 colors as expressed by the reciprocal values of the width of the slit necessary 
 to give to the color under observation a luminosity equal to that of an arbi- 
 trarily chosen standard. The curves from A to H represent the distribution 
 of the intensity of light in the spectrum with eight different grades of illumi- 
 nation. This shifting of the maximum of luminosity in the spectrum 
 explains the so-called " Purkinje's phenomenon " viz. the changing rela- 
 tive values of colors in varying illumination. This can be best observed 
 at nightfall, the attention being directed to a carpet or a wall-paper 
 the pattern of which is made up of a number of different colors. As 
 the daylight fades away the red colors, which in full illumination are 
 
 1 "Luminosity and Photometry," by John Berry Haycraft: Journal of Physiology, xxi. 126. 
 
THE HENSE OF VISION. 
 
 341 
 
 the most intense, becomes gradually darker, and are scarcely to be distin- 
 guished from black at a time when the blue colors are still very readily 
 distinguished. 
 
 Function of Rods and Cones. There is, as mentioned on p. 337, 
 some reason to suppose that the rods and cones have different functions. 
 That color sensation and accuracy of definition are most perfect in the 
 central portion of the retina is shown by the fact that when we desire to 
 obtain the best possible idea of the form and color of an object we direct 
 
 3.8- 
 
 3.4- 
 3.2 
 3. 
 
 2.8- 
 
 2.6 
 
 12.4 
 
 2.2 
 
 2. 
 
 1.8 
 
 UK 
 
 1.4 
 
 1.2 
 
 1. 
 
 0.8 
 
 0.6 
 
 0.4 
 
 0.2 
 
 Intensity H 
 O 
 F 
 E 
 D 
 
 B 
 
 A 
 
 670 650 625 605 590 575 555 535 520 505 
 BCD E 
 
 490 470 450 430 
 
 F O 
 
 FIG. 152. Diagram showing the distribution of the intensity of the spectrum as dependent upon the 
 
 degree of illumination (Konig). 
 
 our eyes in such a way that the image falls upon the foxeajcentralis of the 
 retina. The 1nmi'r)nai'fry of a faint object, however, seems greatest when we 
 look not directly at it, but a little tn ^n^ sH Q of it. This can be readily 
 observed when we look at a group of stars, as, for example, the Pleiades. 
 When the eyes are accurately directed to the stars so as to enable us to count 
 them, the total luminosity of the constellation appears much less than when 
 the eyes are directed to a point a few degrees to one side of the object. Now, 
 an examination of the retina shows only cones in the fovea centralis. In the 
 immediately adjacent parts a small number of rods are found mingled with 
 the cones. In the lateral portions of the retina the rods are relatively more 
 numerous than the cones, and in the extreme peripheral portions the rods alone 
 exist. Hence this phenomenon is readily explained on the supposition, | 
 which is supported by Ramon y CajaPs l recent observations, that the rods! 
 are a comparatively rudimentary form of visual apparatus, taking cognizance II 
 1 Zeitschrift fur Psychologic und Physiologic der Sinnesorgane, xvi. S. 161. 
 
 f 
 
342 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 of the existence ^ light Avith spfHnl reference to its varying intensity. 
 and that the cones are organs specially modified for the localization of 
 stimuli and for the perception of differences of wave-lengths. The view 
 that the rods are specially adapted for the perception of luminosity and the 
 cones for that of color derives support from the fact that in the retina of cer- 
 tain nocturnal animals e. g. bats and owls rods alone are present. This 
 theory has been further developed by Von Kries, 1 who in a recent article 
 describes the rods as differing from the cones in the following respects : (1) 
 They are color-blind i. e. they produce a sensation of simple luminosity 
 whatever be the wave-length of the light-ray falling on them ; (2) they are 
 more easily stimulated than the cones, and are particularly responsive to light- 
 waves of short wave-lengths ; (3) they have the power of adapting themselves 
 to light of varying intensity. 
 
 On this theory it is evident that we must get the sensation of white or 
 colorless light in two different ways : (1) In consequence of the stimulation 
 of the rods by any sort of light-rays, and (2) in consequence of the stimula- 
 tion of the cones by certain combinations of light-rays i. e. complementary 
 colors. In this double mode of white perception lies perhaps the explanation 
 of the effect of varying intensity of illumination upon the results of color- 
 mixtures which has been above alluded to (see p. 337) as an objection to the 
 Hering theory. The so-called " Purkinje's phenomenon," described on p. 340, 
 is readily explained in accordance with this theory, for, owing to the greater 
 irritability of the rods, the importance of these organs, as compared with the 
 cones, in the production of the total visual sensation is greater with feeble 
 than with strong illumination of the field of vision. At the same time, the 
 power of the rods to respond particularly to light-rays of short wave-length 
 will cause a greater apparent intensity of the colors at the blue than at the red 
 end of the spectrum. In this connection it is interesting to note that the phe- 
 nomenon is said not to occur when the observation is limited to the fovea 
 centralis, where cones alone are found. 2 
 
 (Saturation. The degree of saturation of light of a given color depends, as 
 above stated, upon the amount of white light mixed with it. The quality of 
 light thus designated is best studied and appreciated by means of experiments 
 with rotating disks. If, for instance, a disk consisting of a large white and a 
 small red sector be rapidly rotated, the effect produced is that of a pale pink 
 color. By gradually increasing the relative size of the red sector the pink 
 color becomes more and more saturated, and finally when the white sector is 
 reduced to zero the maximum of saturation is produced. It must be borne 
 in mind, however, that no pigments represent completely saturated colors. 
 Even the colors of the spectrum do not produce a sensation of absolute 
 saturation, for, whatever theory of color vision be adopted, it is evident that 
 all the color-perceiving elements of the retina are affected more or less by all 
 the rays of light. Thus when rays of red light fall upon the retina they will 
 
 1 Zeitschrift fur Psychologic und Physiologic der Sinnesorgane, ix. 81. 
 
 2 von Kries: Centralblatt fur Physiologic, 1896, i. 
 
THE SENSE OF VISION. 343 
 
 I stimulate not only the red-perceiving elements, but to a slight extent also (to 
 use the language of the Helmholtz theory) the green- and violet-perceiving 
 elements of the retina. The effect of this will be that of mixing a small 
 amount of white with a large amount of red light i. e. it will produce the 
 sensation of incompletely saturated red light. This dilution of the sensation 
 
 (can be avoided only by previously exhausting the blue- and green-perceiving 
 elements of the retina in a manner which will be explained in connection with 
 the phenomena of after-images. 
 
 Retinal Stimulation. Whenever by a stimulus applied to an irritable 
 substance the potential energy there stored up is liberated the following phe- 
 nomena may be observed : 1. A so-called latent period of variable duration 
 during which no effects of stimulation are manifest ; 2. A very brief period 
 during which the effect of the stimulation reaches a maximum ; 3. A period 
 of continued stimulation during which the effect diminishes in consequence of 
 the using up of the substance containing the potential energy i. e. a period 
 of fatigue ; 4. A period after the stimulation has ceased in which the effect 
 slowly passes away. 
 
 FIG. 153. Diagram showing the effect of stimulation of an irritable substance. 
 
 The curve drawn by a muscle in tetanic contraction, as shown in Figure 
 
 153, illustrates this phenomenon. Thus, if A D represents the duration of the 
 stimulation, A B indicates the latent period, B C the period of contraction, 
 C D the period of fatigue under stimulation, and D E the after-effect of 
 stimulation showing itself as a slow relaxation. When light falls upon the 
 retina corresponding phenomena are to be observed. 
 
 Latent Period. That there is a period of latent sensation in the retina 
 (i. e. an interval between the falling of light on the retina and the beginning 
 of the sensation) is, judging from the analogy of other parts of the nervous 
 system, quite probable, though its existence has not been demonstrated. 
 
 Rise to Maximum of Sensation. The rapidity with which the sensation of 
 light reaches its maximum increases with the intensity of the light and varies 
 with its color, red light producing its maximum sensation sooner than green 
 and blue. Consequently, when the image of a white object is moved across 
 the retina it will appear bordered by colored fringes, since the various con- 
 stituents of white light do not produce their maximum effects at the same 
 time. This phenomena can be readily observed when a disk on which a 
 black and a white spiral band alternate with each other (as shown in Figure 
 
 1 54, A) is rotated before the eyes. The white band as its image moves out- 
 ward or inward over the retinal surface appears bordered with colors which 
 
344 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 vary with the rate of rotation of the disk and with the amount of exhaustion 
 of the retina. Chromatic effects due to a similar cause are also to be seen 
 when a disk, such as is shown in Figure 154, B (known as Benham's spectrum 
 
 A B 
 
 FIG. 154. Disks to illustrate the varying rate at which colors rise to their maximum of sensation. 
 
 top), is rotated with moderate rapidity. The concentric bands of color appear 
 in reverse order when the direction of rotation is reversed. The apparent 
 movement of colored figures on a background of a different color when the 
 eye moves rapidly over the object or the object is moved rapidly before the 
 eye seems to depend upon this same retinal peculiarity. The phenomenon 
 may be best observed when small pieces of bright-red paper are fastened upon 
 a bright-blue sheet and the sheet gently shaken before the eyes. The red 
 figures will appear to move upon the blue background. The effect may be 
 best observed in a dimly-lighted room. 
 
 In this connection should be mentioned the phenomenon of " recurrent 
 images " or " oscillatory activity of the retina." l This may be best observed 
 when a black disk containing a white sector is rotated at a rate of about one 
 revolution in two seconds. If the disk is brightly illuminated, as by sunlight, 
 
 and the eye fixed steadily upon the axis of rota- 
 tion, the moving white sector seems to have a 
 shadow upon it a short distance behind its ad- 
 vancing border, and this shadow may be followed 
 by a second fainter, and even by a third still 
 fainter shadow, as shown in Figure 155. The 
 distance of the shadows from each other and 
 from the edge of the sector increases with the rate 
 of rotation of the disk and corresponds to a time 
 FIG. i55.-To illustrate the oscillatory interval of about 0.015". It thus appears that 
 
 activity of the retina (Charpentier). 
 
 when light is suddenly thrown upon the retina 
 
 the sensation does not at once rise to its maximum, but reaches this point by 
 a sort of vibratory movement. The apparent duplication of a single very 
 brief retinal stimulation, as that caused by a flash of lightning, may perhaps 
 be a phenomenon of the same sort. 
 
 Fatigue of Retina. When the eye rests steadily upon a uniformly illu- 
 1 Charpentier: Archives de Physiologic, 1892, pp. 541, 629 ; and 1886, p. 677. 
 
THE SENSE OF VISION. 345 
 
 rninated white surface (e. g. a sheet of white paper), we are usually unconscious 
 of any diminution in the intensity of the sensation, but it can be shown that 
 the longer we look at the paper the less brilliant it appears, or, in other words, 
 that the retina really becomes fatigued. To do this it is only necessary to place 
 a disk of black paper on the white surface and to keep the eyes steadily fixed 
 for about half a minute upon the centre of the disk. Upon removing the disk 
 without changing the direction of the eyes a round spot will be seen on the 
 white paper in the place previously occupied by the disk. On this spot the 
 whiteness of the paper will appear much more intense than on the neighboring 
 portion of the sheet, because we are able in this experiment to bring into direct 
 contrast the sensations produced by a given amount of light upon a fresh and 
 a fatigued portion of the retina. 1 
 
 The rapidity with which the retina becomes fatigued varies with the color 
 of the light. Hence when intense white light falls upon the retina, as when 
 we look at the setting sun, its disk seems to undergo changes of color as one 
 or another of the constituents of its light becomes, through fatigue, less and 
 less conspicuous in the combination of rays which produces the sensation of 
 white. 
 
 The After-effect of Stimulation. The persistence of the sensation after the 
 stimulus has ceased causes very brief illuminations (e. g. by an electric spark) to 
 produce distinct effects. On this phenomenon depends also the above-described 
 method of mixing colors on a revolving disk, since a second color is thrown 
 upon the retina before the impression produced by the first color has had time 
 enough to become sensibly diminished. The interval at which successive stim- 
 ulations must follow each other in order to pro- 
 duce a uniform sensation (a process analogous 
 to the tetanic stimulation of a muscle) may be 
 determined by rotating a disk, such as repre- 
 sented in Figure 156, and ascertaining at what 
 speed the various rings produce a uniform sen- 
 sation of gray. The interval varies with the 
 intensity of the illumination from 0.1" to 
 0.033", and may, therefore, be used as a 
 measure of the intensity, as in the method of 
 u flicker photometry." 2 The special advan- 
 tage of this method is that it affords a means FIG. 156.-Disk to illustrate the persistence 
 ... . . . of retinal sensation (Helmholtz). 
 
 of determining the relative intensity of lights 
 
 of different colors. The duration of the after-effect depends also upon the 
 length of "the stimulation and upon the color of the light producing it, the 
 most persistent effect being produced by the red rays. In this connection it 
 is interesting to note that while with the rapidly vibrating blue rays a less 
 
 1 Although the retina is here spoken of as the portion of the visual apparatus subject to 
 fatigue, it should be borne in mind that we cannot, in the present state of our knowledge, dis- 
 criminate between retinal fatigue and exhaustion of the visual nerve-centres. 
 
 2 Rood : American Journal of Science, Sept., 1893. 
 
346 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 intense illumination suffices to stimulate the eye, the slowly vibrating red 
 rays produce the more permanent impression. 
 
 After-images. When the object looked at is very brightly illuminated the 
 impression upon the retina may be so persistent that the form and color of the 
 object are distinctly visible for a considerable time after the stimulus has ceased 
 to act. This appearance is known as a " positive after-image/' and can be best 
 observed when we close the eyes after looking at the sun or other bright source 
 of light. Under these circumstances we perceive a brilliant spot of light which, 
 owing to the above-mentioned difference in the persistence of the impressions 
 produced by the various colored rays, rapidly changes its color, passing gen- 
 erally through bluish green, blue, violet, purple, and red, and then disappear- 
 ing. This phenomenon is apt to be associated with or followed by another 
 effect known as a " negative after-image." This form of after-image is much 
 more readily observed than the positive variety, and seems to depend upon the 
 fatigue of the retina. It is distinguished from the positive after-image by the 
 fact that its color is always complementary to that of the object causing it. In 
 the experiment to demonstrate the fatigue of the retina, described on p. 345, 
 the white spot which appears after the black disk is withdrawn is the " nega- 
 tive after-image" of the disk, white being complementary to black. If a 
 colored disk be placed upon a sheet of white paper, looked at attentively for a 
 few seconds, and then withdrawn, the eye will perceive in its place a spot of 
 light of a color complementary to that of the disk. If, for example, the disk 
 be yellow, the yellow-perceiving elements of the retina become fatigued in 
 looking at it. Therefore when the mixed rays constituting white light are 
 thrown upon the portion of the retina which is thus fatigued, those rays which 
 produce the sensation of yellow will produce less effect than the other rays for 
 which the eye has not been fatigued. Hence white light to an eye fatigued for 
 yellow will appear blue. 
 
 If the experiment be made with a yellow disk resting on a sheet of blue 
 paper, the negative after-image will be a spot on which the blue color will 
 appear (1) more intense than on the neighboring portions of the sheet, owing 
 to the blue-perceiving elements of that portion of the retina not being fatigued ; 
 (2) more saturated, owing to the yellow-perceiving elements being so far 
 exhausted that they no longer respond to the slight stimulation which is pro- 
 duced when light of a complementary color is thrown upon them, as has been 
 explained in connection with the subject of saturation. 
 
 Contrast. As the eye wanders from one part of the field of vision to 
 another it is evident that the sensation produced by a given portion of the 
 field will be modified by the amount of fatigue produced by that portion on 
 which the eye has last rested, or, in other words, the sensation will be the result 
 of the stimulation by the object looked at combined with the negative after- 
 image of the object previously observed. The effect of this combination is to 
 produce the phenomenon of successive contrast, the principle of which may be 
 thus stated : Every part of the field of vision appears lighter near a darker 
 part and darker near a lighter part, and its color seen near another color 
 approaches the complementary color of the latter. A contrast phenomenon 
 
THE SENSE OF VISION. 
 
 347 
 
 similar in its effects to that above described may be produced under conditions 
 in which negative after-images can play no part. This kind of contrast is 
 known as simultaneous contrast, and may perhaps be explained on the theory 
 that a stimulation of a given portion of the retina produces in the neighboring 
 portions an effect to some extent antagonistic to that caused by direct stimulation. 
 
 A good illustration of the phenomenon of contrast is given in Figure 157, 
 in which black squares are separated by white bands which at their points of 
 intersection appear darker than where they are bordered on either side by the 
 black squares. 
 
 A black disk on a yellow background seen through white tissue-paper 
 appears blue, since the white paper makes the black disk look gray and the 
 yellow background pale yellow. The gray disk in contrast to the pale yellow 
 around it appears blue. 
 
 FIG. 157. To illustrate the phenomenon of contrast. 
 
 The phenomenon of colored shadovjs also illustrates the principle of con- 
 trast. These may be observed whenever an object of suitable size and shape 
 is placed upon a sheet of white paper and illuminated from one direction by 
 daylight and from another by gaslight. Two shadows will be produced, one 
 of which will appear yellow, since it is illuminated only by the yellowish gas- 
 light, while the other, though illuminated by the white light of day, will 
 appear blue in contrast to the yellowish light around it. 
 
 Space-perception. Rays of light proceeding from every point in the 
 field of vision are refracted to and stimulate a definite point on the sur- 
 face of the retina, thus furnishing us with a local sign by which we can 
 recognize the position of the point from which the light proceeds. 
 Hence the size and shape of an optical image upon the retina enable us to 
 judge of the size of the corresponding object in the same way that the cutane- 
 
348 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 cms terminations of the nerves of touch enable us to judge of the size and 
 shape of an object brought in contact with the skin. This spatial perception 
 is materially aided by the muscular sense of the muscles moving the eyeball, 
 for we can obtain a much more accurate idea of the size of an object if 
 we let the eye rest in succession upon its different parts than if we gaze fixedly 
 at a given point upon its surface. The conscious effort associated with a given 
 amount of muscular motion gives, in the case of the eye, a measure of distance 
 similar to that secured by the hand when we move the fingers over the surface 
 of an object to obtain an idea of its size and shape. 
 
 The perception of space by the retina is limited to space in two dimensions 
 i. e. in a plane perpendicular to the axis of vision. Of the third dimension 
 in space i. e. of distance from the eye the retinal image gives us no know- 
 ledge, as may be proved by the study of after-images. If an after-image of 
 any bright object e. g. a window be produced upon the retina in the man- 
 ner above described and the eye be then directed to a sheet of paper held in 
 the hand, the object will appear outlined in miniature upon the surface of the 
 paper. If, however, the eye be directed to the ceiling of the room, the object 
 will appear enlarged and at a distance corresponding to that of the surface 
 looked at. 1 Hence one and the same retinal image may, under different cir- 
 cumstances, give rise to the impression of objects at different distances. We 
 must therefore regard the perception of distance not as a direct datum of vision, 
 but, as will be later explained, a matter of visual judgment. 
 
 When objects are of such a shape that their images may be thrown suc- 
 cessively upon the same part of the retina, it is possible to judge of their rela- 
 tive size with considerable accuracy, the retinal surface serving as a scale to 
 which the images are successively applied. When this is not the case, the 
 error of judgment is much greater. We can compare, for instance, the relative 
 length of two vertical or of two horizontal lines with a good deal of precision, 
 but in comparing a vertical with a horizontal line we are liable to make a con- 
 siderable error. Thus it is difficult to realize that the vertical and the hori- 
 zontal lines in Figure 158 are of the same length. The error consists in an 
 
 over-estimation of the length of the vertical 
 lines relatively to horizontal ones, and appears to 
 depend, in part at any rate, upon the small size 
 of the superior rectus muscle relatively to the 
 other muscles of the eye. The difference amounts 
 to 30-45 per cent, in weight and 40-53 per cent, 
 in area of cross section. It is evident, therefore, 
 that a given motion of the eye in the upward 
 direction will require a more powerful contraction 
 . of the weaker muscle concerned in the movement 
 
 FIG. 158,-To illustrate the over-esti- than will be demanded of the stronger muscles 
 
 mation of vertical lines. 
 
 moving the eye laterally to an equal amount. 
 
 1 This power of the surface of projection to determine the apparent size and distance of the 
 after-image may be to some extent influenced by the will. Jeffries : Journal of Boston Society 
 of Medical Sciences, vol. i. No. 9. 
 
THE SENSE OF VISION. 349 
 
 Hence we judge the upward motion of the eye to be greater because to accom- 
 plish it we make a greater effort than is required 
 for a horizontal movement of equal extent. 
 
 The position of the vertical line bisecting the 
 horizontal one (in Fig. 158) aids the illusion, as 
 may be seen by turning the page through 90, so 
 as to bring the bisected line into a vertical posi- 
 tion, or by looking at the lines in Figure 159, in 
 which the illusion is much less marked than in 
 Figure 158. 
 
 The tendency to over-estimate the length of 
 vertical lines is also illustrated by the error 
 commonly made in supposing the height of the 
 crown of an ordinarv silk hat to be greater 
 
 , . , , , FIG. 159. To illustrate the over-estima- 
 
 tnan its breadth. tion of ver ticai lines. 
 
 Irradiation. Many other circumstances 
 
 affect the accuracy of the spatial perception of the retina. One of the most 
 important of these is the intensity of the illumination. All brilliantly illumi- 
 nated objects appear larger than feebly illuminated ones of the same size, as is 
 well shown by the ordinary incandescent electric lamp, the delicate filament of 
 which is scarcely visible when cold, but when intensely heated by the electric 
 current glows as a broad band of light. The phenomenon is known as " irra- 
 diation," and seems to depend chiefly upon the above-described imperfections 
 in the dioptric apparatus of the eye, in consequence of which points of light 
 produce small circles of dispersion on the retina and bright objects produce 
 
 FIG. 160. To illustrate the phenomenon of irradiation. 
 
 images with imperfectly defined outlines. The white square surrounded by 
 black and the black square surrounded by white (Figure 160), being of the 
 same size, would in an ideally perfect eye produce images of the same size on 
 the retina, but owing to the imperfections of the eye the images are not sharply 
 defined, and the white surfaces consequently appear to encroach upon the darker 
 portions of the field of vision. Hence the white square looks larger than the 
 black one, the difference in the apparent size depending upon the intensity of 
 the illumination and upon the accuracy with which the eye can be accommo- 
 dated for the distance at which the objects are viewed. The effect of irradi- 
 ation is most manifest when the dark portion of the field of vision over which 
 the irradiation takes place has a considerable breadth. Thus the circular white 
 
350 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 spots in Figure 161, when viewed from a distance of three or four meters, 
 appear hexagonal, since the irradiation is most marked in the triangular dark 
 space between three adjacent circles. A familiar example of the effect of irra- 
 
 FIG. 161. To illustrate the phenomenon of irradiation. 
 
 diation is afforded by the appearance of the new moon, whose sun-illuminated 
 crescent seems to be part of a much larger circle than the remainder of the 
 disk, which shines only by the light reflected upon it from the surface of the 
 earth. 
 
 D E 
 
 FIG. 162. To illustrate the illusion of subdivided space. 
 
 Subdivided Space. A space subdivided into smaller portions by inter- 
 mediate objects seems more extensive than a space of the same size not so sub- 
 divided. Thus the distance from A to B (Fig. 162) seems longer than that from 
 B to (7, though both are of the same length, and for the same reason the square 
 
THE SENSE OF VISION. 351 
 
 D seems higher than it is broad, and the square E broader than it is high, the 
 illusion being more marked in the case of D than in the case of E, because, as 
 above explained, vertical distances are, as a rule, over-estimated. 
 
 The explanation of this illusion seems to be that the eye in passing over a 
 subdivided line or area recognizes the number and size of the subdivisions, 
 
 \ \ \ \ \ \ \ 
 
 \\N\\\\\\\\ 
 
 FIG. 163. Zollner's lines. 
 
 and thus gets an impression of greater total size than when no subdivisions 
 are present. 
 
 A good example of this phenomenon is afforded by the apparently increased 
 extent of a meadow when the grass growing on it is cut and arranged in hay- 
 cocks. 1 
 
 The relations of lines to each other gives rise to numerous illusions of 
 spatial perception, among the most striking of which are those afforded by the 
 so-called " Zollner's lines/' an example of which is given in Figure 163. Here 
 the horizontal lines, though strictly parallel to each A 
 
 other, seem to diverge and converge alternately, their 
 apparent direction being changed toward greater per- 
 pendicularity to the short oblique lines crossing them. 
 This illusion is to be explained in part by the tendency 
 of the eye to over-estimate the size of acute and to 
 under-estimate that of obtuse angles a tendency which, 
 according to Filehne, 2 is due to the fact that we are 
 constantly surrounded by square-cornered objects 
 (houses, furniture, etc.), the right angles of which, 
 being seen obliquely, are projected onto our retinas 
 as acute or obtuse angles. Knowing these angles 
 
 . . , Fio. 164. To illustrate illusion 
 
 be right angles, we are constantly applying mental of space-perception. 
 corrections to our visual data, and the habit thus 
 
 acquired forces us to regard all acute and obtuse angles as nearer to right 
 angles than they really are. The illusion in Zollner's lines is more marked 
 when the 'figure is so held that the long parallel lines make an angle of about 
 
 1 It is interesting to note that a similar illusion has been observed when an interval of time 
 subdivided by audible signals is compared with an equal interval not so subdivided (Hall and 
 Jastrow : Mind, xi. 62). 
 
 2 Zeitschrift far Psychologic und Physiologic der Sinnesorgane, xvii. S. 16. 
 
352 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 45 with the horizon, since in this position the eye appreciates their real 
 position less accurately than when they are vertical or horizontal. It is 
 diminished, but does not disappear, when the eye, instead of being allowed 
 
 FIG. 165. To illustrate contrast in space-perception (Muller-Lyer). 
 
 to wander over the figure, is fixed upon any one point of the field of vision. 
 Hence the motions of the eye must be regarded as a factor in, but not the 
 sole cause of, the illusion. 
 
 PIG. 166. To illustrate contrast in space-perception (Muller-Lyer). 
 
 The illusion in Fig. 164, where the line d is the real and the line / the 
 apparent continuation of the line a, is to be explained partly by the over- 
 estimation of acute angles and partly, according to Helmholtz, by irradiation. 
 
 FIG. 167. To illustrate contrast in space-perception (Muller-Lyer). 
 
 The fact that the illusion is greatly diminished by turning the figure on its 
 side seems to show that the tendency to over-estimate vertical dimensions 
 also plays a part in its production. 
 
THE SENSE OF VISION. 353 
 
 Our estimate of the size of given lines, angles, and areas is influenced by 
 neighboring lines, angles, and areas with which they arc compared. This 
 influence is sometimes exerted in accordance with the principle of contrast, 
 and tends to make a given extension appear larger in presence of a smaller. 
 and smaller in presence of a larger extension. This effect is illustrated in 
 
 FIG. 168. To illustrate so-called " confluxion " in space-perception (Miiller-Lyer). 
 
 Figure 165, in which the middle portion of the shorter line appears larger 
 than the corresponding portion of the longer line, in Figure 166, in which a 
 similar effect is observed in the case of angles, and in Figure 167, in which 
 the space between the two squares seems smaller than that between the two 
 oblong figures. 
 
 FIG. 169. To illustrate so-called "confluxion" in space-perception (Miiller-Lyer). 
 
 In some cases, however, an influence of the opposite sort 1 seems to be 
 exerted, as is shown in Figure 168, in which the middle one of three parallel 
 lines seems longer when the outside lines are longer, and shorter when they 
 are shorter than it is itself, and in Figure 1 69, in which a circle appears larger 
 if surrounded by a circle larger than itself, and smaller if a smaller circle is 
 shown concentrically within it. 
 
 FIG. 170. To illustrate the influence of angles upon the apparent length of lines (Mtiller-Lyer). 
 
 Lines meeting at an angle appear longer when the included angle is large 
 than when it is small, as is shown in Figure 170. This influence of the 
 included angle affords a partial explanation of the illusion shown in Figure 
 171, in which the horizontal line at B seems longer than at A ; but the distance 
 
 1 For this influence the name "confluxion " has been proposed by Miiller-Lyer, from whose 
 article in the Archivfur Physiologic, 1889, Sup. Bd., the above examples are taken. 
 VOL. II. 23 
 
354 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 between the extremities of the oblique lines seems also to affect our estimate 
 of the horizontal line in the same way as the outside lines in Figure 168 
 influence our judgment of the length of the line between them. 
 
 B 
 
 FIG. 171 Illusion of space-perception. 
 
 Einthoven 1 has recently explained this phenomenon as dependent upon 
 indistinct vision in the lateral portions of the retina which causes the blurred 
 images of the ends of the line a to appear nearer together than those of the 
 
 line b. This effect of indistinctness 
 of outline can be illustrated by photo- 
 graphing the lines more or less out 
 of focus as shown in Figure 172 a. A 
 similar explanation is given by Ein- 
 thoven for the illusion of subdivided 
 space described on p. 351. 
 
 Perception of Distance. The 
 retinal image gives us, as we have 
 seen, no direct information as to the 
 distance of the object from the eye. 
 This knowledge is, however, quite as 
 important as that of position in a plane 
 perpendicular to the line of vision, and 
 we must now consider in what way it 
 is obtained. The first fact to be noticed 
 is that there is a close connection be- 
 tween the judgments of distance and 
 of actual size. A retinal image of 
 a given size may be produced by a 
 small object near the eye or by a 
 large one at a distance from it. 
 Hence when we know the actual size 
 of any object (as, for example, a 
 human figure) we judge of its distance by the size of its image on the retina. 
 Conversely, our estimate of the actual size of an object will depend upon 
 our judgment of its distance. The fact that children constantly misjudge 
 both the size and distance of objects shows that the knowledge of this rela- 
 tion is acquired only by experience. If circumstances mislead us with regard 
 to the distance of an object, we necessarily make a corresponding error with 
 regard to its size. Thus, objects seen indistinctly, as through a fog, are 
 judged to be larger, because we suppose them to be farther off than they 
 really are. The familiar fact that the moon seems to be larger when near the 
 horizon than when near the zenith is also an illustration of this form of illu- 
 
 1 Pftuyer's Archiv, Ixxi. S. 1. 
 
 
 FIG. 172. Illustrating Einthoven's explanation of 
 space illusions through indistinct vision. 
 
THE SENSE OF VISION. 355 
 
 sion. When the moon is high above our heads we have no moans of esti- 
 mating its distance from us, since there are no intervening objects with which 
 we can compare it. Hence we judge it to be nearer than when, seen on the 
 horizon, it is obviously farther off than all terrestrial objects. Since the size 
 of the retinal image of the moon is the same in the two cases, we reconcile 
 the sensation with its apparent greater distance when seen on the horizon by 
 attributing to the moon in this position a greater actual size. 
 
 If the retinal image have the form of a familiar object of regular shape 
 c. (j. a house or a table we interpret its outlines in the light of experience 
 and distinguish without difficulty between the nearer and more remote parts of 
 the object. Even the projection of the outlines of such an object on to a plane 
 surface (?'. e. a perspective drawing) suggests the real relations of the different 
 parts of the picture so strongly that we recognize at once the relative distances 
 of the various portions of the object represented. How powerfully a familiar 
 outline can suggest the form and relief usually associated with it is well illus- 
 trated by the experiment of looking into a mask painted on its interior to 
 resemble a human face. In this case the familiar outlines of a human face 
 are brought into unfamiliar association with a receding instead of a projecting 
 form, but the ordinary association of these outlines is strong enough to force 
 the eye to see the hollow mask as a projecting face. 1 The fact that the pro- 
 jecting portions of an object are usually more brightly illuminated than the 
 receding or depressed portions is of great assistance in determining their rela- 
 tive distance. This use of shadows as an aid to the perception of relief pre- 
 supposes a knowledge of the direction from which the light falls on an object, 
 and if we are deceived on the latter we draw erroneous conclusions with 
 regard to the former point. Thus, if we look at an embossed letter or figure 
 through a lens which makes it appear inverted the accompanying reversal of 
 the shadows will cause the letter to appear depressed. The influence of 
 shadows on our judgment of relief is, however, not so strong as that of the 
 outline of a familiar object. In a case of conflicting testimony the latter 
 usually prevails, as, for example, in the above-mentioned experiment with the 
 mask. 
 
 Aided by these peculiarities of the retinal picture, the mind interprets it as 
 corresponding in its different parts to points at different distances from the eye, 
 and it is interesting to notice that painters, whose work, being on a plane sur- 
 face, is necessarily in all its parts at the same distance from the eye, use similar 
 devices in order to give depth to their pictures. Distant hills are painted with 
 indistinct outlines to secure what is called " aerial perspective." Figures of 
 men and animals are introduced in appropriate dimensions to suggest the dis- 
 tance between the foreground and the background of the picture. Landscapes 
 are painted preferably by morning and evening light, since at these hours the 
 marked shadows aid materially in the suggestion of distance. 
 
 1 In the experiment the mask should be placed at a distance of about two meters and one 
 eye closed. Even with both eyes open the illusion often persists if the distance is increased to 
 five or six meters. 
 
356 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 The eye, however, can aid itself in the perception of depth in ways which 
 the painter has not at his disposaL By the sense of effort associated with the 
 act of accommodation we are able to estimate roughly the relative distance of 
 objects before us. This aid to our judgment can, of course, be employed only 
 in the case of objects comparatively near the eye. Its effectiveness is greater 
 for objects not far from the near-point of vision, and diminishes rapidly as the 
 distance is increased, and disappears for distances more than two or three meters 
 from the eye. 
 
 When the head is moved from side to side an apparent change in the rela- 
 tive position of objects at different distances is produced, and, as the extent of 
 this change is inversely proportional to the distance of the objects, it serves as 
 a measure of distance. This method of obtaining the " parallax " of objects 
 by a motion of the, head is often noticeable in persons whose vision in one 
 eye is absent or defective. 
 
 Binocular Vision. The same result which is secured by the comparison 
 of retinal images seen successively from slightly different points of view is 
 obtained by the comparison of the images formed simultaneously by any object 
 in the two eyes. In binocular vision we obtain a much more accurate idea of 
 the shape and distance of objects around us than is possible with monocular 
 vision, as may be proved by trying to touch objects in our neighborhood with 
 a crooked stick, first with both eyes open and then with one eye shut. When- 
 ever we look at a near solid object with two eyes, the right eye sees farther 
 round the object on the right side and the left eye farther round on the left. 
 The mental comparison of these two slightly different images produces the 
 perception of solidity or depth, since experience has taught us that those objects 
 only which have depth or solidity can affect the eyes in this way. Conversely, 
 if two drawings or photographs differing from each other in the same way that 
 the two retinal images of a solid object differ from each other are presented, 
 one to the right and -the other to the left eye, the two images will become 
 blended in the mind and the perception of solidity will result. Upon this fact 
 depends the effect of the instrument known as the stereoscope, the slides of 
 which are generally pairs of photographs of natural objects taken simultaneous- 
 
 FIG. 173. To illustrate stereoscopic vision. 
 
 ly with a double camera, of which the lenses are at a distance from each other 
 equal to or slightly exceeding that between the two axes of vision. The prin- 
 ciple of the stereoscope can be illustrated in a very simple manner by drawing 
 circles such as are represented in Figure 173 on thin paper, and 'fastening each 
 
THE SENSE OF VISION. 
 
 357 
 
 pair across the end of a piece of brass tube about one inch or more in diameter 
 and ten inches long. Let the tubes be held one in front of each eye with the 
 distant ends nearly in contact with each other, as shown in Figure 174. If 
 the tubes are in such a position that the small circles are brought as near to 
 each other as possible, as shown in Figure 173, the retinal images will blend, 
 
 the smaller circle will seem to be much 
 nearer than the larger one, and the eyes will 
 appear to be looking down upon a truncated 
 cone, such as is shown in Figure 175, since 
 a solid body of this form is the only one 
 
 FIG. 174. To illustrate stereoscopic vision. 
 
 FIG. 175. To illustrate stereoscopic vision. 
 
 , bounded by circles relied to each other as those shown in this experiment. 
 
 Stereoscopic slides often serve well to illustrate the superiority of binocular 
 *>ver monocular vision*. If the slide represents an irregular mass of rocks or 
 ice, it is often-very Difficult by looking at either of the pictures by itself to 
 determine the relative distance of the various objects represented, but if the 
 slide is placed in the stereoscope the true relation of the different parts of the 
 picture becomes at once apparent. 
 
 Since the comparison of two slightly dissimilar images received on the two 
 retinas is the essential condition of stereoscopic vision, it is evident that if the 
 two pictures are identical no sensation of relief can be produced. Thus, when 
 two pages printed from the same type or two engravings printed from the same 
 plate are united in a stereoscope, the combined picture appears as flat as either 
 of its components. If, however, one of the pictures is copied from the other, 
 even if the copy be carefully executed, there will be slight differences in the 
 distances between the lines or in the spacing of the letters which will cause 
 apparent irregularities of level in the different portions of the combined pic- 
 ture. Thus, a suspected banknote may be proved to be a counterfeit if, when 
 placed in a stereoscope by the side of a genuine note, the resulting combined 
 picture shows certain letters lying apparently on different planes from the rest. 
 
 Pseudoscopic Vision. If the pictures of an ordinary stereoscopic slide be 
 reversed, so that the picture belonging in front of the right eye is presented to 
 the left eye, and vice versd, the stereoscopic gives place to what is called a pseudo- 
 scopic effect /. e. we perceive not a solid but a hollow body. The effect is best 
 
358 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 obtained with the outlines of geometrical solids, photographs of coins or medals 
 or of objects which may readily exist in an inverted form. Where the photo- 
 graphs represent objects which cannot be thus inverted, such as buildings and 
 landscapes, the pseudoscopic effect is not readily produced another example 
 of the power (see p. 355) of the outline of a familiar object to outweigh other 
 sorts of testimony. 
 
 A pseudoscopic effect may be readily obtained without the use of a stereo- 
 scope by simply converging the visual axes so that the right eye looks at the 
 left and the left eye at the right picture of a stereoscopic slide. The eyes may 
 be aided in assuming the right degree of convergence by looking at a small 
 object like the head of a pin held between the eyes and the slide in the manner 
 described on p. 312. Figure 173, viewed in this way, will present the appear- 
 ance of a hollow truncated cone with the base turned toward the observer. A 
 stereoscopic slide with its pictures reversed will, of course, when viewed in this 
 way, present not a pseudoscopic, but a true stereoscopic, appearance, as shown 
 by Figures 138 and 139. 
 
 Binocular Combination of Colors. The effect of binocularly combin- 
 ing two different colors varies with the difference in wave-length of the colors. 
 Colors lying near each other in the spectrum will generally blend together 
 and produce the sensation of a mixed color, such as would result from the 
 union of colors by means of the revolving disk or by the method of reflected 
 and transmitted light, as above described. Thus a red and a yellow disk 
 placed in a stereoscope may be generally combined to produce the sensation 
 of orange. If, however, the colors are complementary to each other, as blue 
 and yellow, no such mixing occurs, but the field of vision seems to be occupied 
 alternately by a blue and by a yellow color. This so-called " rivalry of the 
 fields of vision " seems to depend, to a certain extent, upon the fact that in 
 order to see the different colors with equal distinctness the eyes must be dif- 
 ferently accommodated, for it is found that if the colors are placed at different 
 distances from the eyes (the colors with the less refrangible rays being at the 
 greater distance), the rivalry tends to disappear and the mixed color is more 
 easily produced. 
 
 An interesting effect of the stereoscopic combination of a black and a 
 white object is the production of the appearance of a metallic lustre or polish. 
 If, for instance, the two pictures of a stereoscopic slide represent the slightly 
 different outlines of a geometrical solid, one in black upon white ground and 
 the other in white upon black ground, their combination in the stereoscope 
 will produce the effect of a solid body having a smooth lustrous surface. 
 The explanation of this effect is to be found in the fact that a polished surface 
 reflects the light differently to the two eyes, a given point appearing bril- 
 liantly illuminated to one eye and dark to the other. Hence the stereoscopic 
 combination of black and white is interpreted as indicating a polished surface, 
 since it is by means of a polished surface that this effect is usually produced. 
 
 Corresponding 1 Points. When the visual axes of both eyes are directed 
 to the same object two distinct images of that object are formed upon widely 
 
THE SENSE OF VISION. 359 
 
 separated parts of the nervous system. Yet but a single object is perceived. 
 The phenomenon is the same as that which occurs when a grain of sand is 
 held between the thumb and finger. In both cases we have learned (chiefly 
 through the agency of muscular movements and the nerves of muscular sense) 
 to interpret the double sensation as produced by a single object. 
 
 Any two points, lying one in each retina, the stimulation of which by rays 
 of light gives rise to the sensation of light proceeding from a single object are 
 said to be " corresponding points." Now, it is evident that the /owes centrcdes 
 of the two eyes must be corresponding points, for an object always appears 
 single when both eyes are fixed upon it. That double vision results when the 
 images are formed on points which are not corresponding may be best illus- 
 trated by looking at three pins stuck in a straight rod at distances of 35, 45, 
 and 55 centimeters from the end. If the end of the rod is held against the 
 nose and the eyes directed to each of the three pins in succession, it will be 
 found that, while the pin looked at appears single, each of the others appears 
 double, and that the three pins therefore look like five. 
 
 The two fovece centrales are not, of course, the only corresponding points. 
 In fact, it may be said that the two retinas correspond to each other, point for 
 point, almost as if they were superposed one upon the other with the fovea? 
 together. The exact position of the points in space which are projected on to 
 corresponding points of the two retinas varies with the position of the eyes. 
 The line or surface in which such points lie is known as the " horopter." A 
 full discussion of the horopter would be out of place in this connection, but 
 one interesting result of its study may be pointed out viz. the demonstration 
 that when, standing upright, we direct our eyes to the horizon the horopter is 
 approximately a plane coinciding with the ground on which we stand. It is 
 of course important for security in walking that all objects on the ground 
 should appear single, and, as they are known by experience to be single, the 
 eye has apparently learned to see them so. 
 
 Since the vertical meridians of the two eyes represent approximately rows 
 of corresponding points, it is evident that when two lines are so situated that 
 their images are formed each upon a vertical meridian of one of the eyes, the 
 impression of a single vertical line will be produced, for such a line seen bin- 
 ocularly is the most frequent cause of this sort of retinal stimulation. This 
 is the explanation commonly given of the singular optical illusion which is 
 produced when lines drawn as in Figure 176 are looked at with both eyes fixed 
 upon the point of intersection of the lines and with the plane in which the 
 visual axes lie forming an angle of about 20 with that of the paper, the dis- 
 tance of the lines from the eyes being such that each line will lie approximately 
 in the same vertical plane with one of the visual axes. Under these circum- 
 stances each line will form its image on a vertical meridian of one of the eyes, 
 and the combination of these images results in the perception of a third line, 
 not lying in the plane of the paper, but apparently passing through it more or 
 less vertically, and swinging round its middle point with every movement of 
 the head or the paper. In this experiment it will be found that the illusion 
 
360 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 of a Hue placed vertically to the plane of the paper does not entirely dis- 
 appear when one eye is closed. Hence it is evident that there is, as Mrs. 
 
 FIG. 177. Monocular illusion of vertical lines. 
 
 C. L. Franklin has pointed out, 1 a strong tendency to regard 
 lines which form their images approximately on the vertical 
 meridian of the eye as themselves vertical. This tendency 
 is well shown when a number of short lines converging 
 toward a point outside of the paper on which they are 
 drawn, as in Figure 177, are looked at with one eye held 
 a short distance above the point of convergence. Even 
 when the lines are not convergent, but parallel, so that their 
 images cannot fall upon the vertical meridian of the eye, the 
 illusion is not entirely lost. It will be found, for instance, 
 that when the Zollner lines, as given in Figure 163, are 
 looked at obliquely with one eye from one corner of the 
 FIG. 176. Binocu- figure, the short lines which lie nearly in a plane with the 
 ttcaUine 011 OJ & Ver vlsus ^ ax is appear to stand vertically to the plane of the 
 
 paper. 
 
 In this connection it may be well to allude to the optical illusion in conse- 
 quence of which certain portraits seem to follow the beholder with the eyes. 
 This depends upon the fact that the face is painted looking straight out from 
 the canvas i. e. with the pupil in the middle of the eye. The painting being 
 upon a flat surface, it is evident that, from whatever direction the picture is 
 viewed, the pupil will always seem to be in the middle of the eye, and the 
 eye will consequently appear to be directed upon the observer. The phenom- 
 enon is still more striking in the case of pictures of which the one repre- 
 sented in Figure 178 may be taken as an example. Here the soldier's rifle 
 
 1 Am. Journal of Psychology, vol. i. p. 99. 
 
THE SENSE OF VISION. 361 
 
 is drawn as it appears to an eye looking straight down the barrel, and, as this 
 
 foreshortening is the same in all positions of the observer, it is evident that 
 
 when such a picture is hung upon the wall 
 
 of a room the soldier will appear to be 
 
 aiming directly at the head of every person 
 
 present. 
 
 In concluding this brief survey of some 
 of the most important subjects connected 
 with the physiology of vision it is well to 
 utter a word of caution with regard to a 
 danger connected with the study of the sub- 
 ject. This danger arises in part from the 
 fact that in the scientific study of vision it 
 is often necessary to use the eyes in a way 
 quite different from that in which they are 
 habitually employed, and more likely, there- 
 fore, to cause nervous and muscular fatigue. 
 We have seen that in any given position of 
 the eye distinct definition is limited to an FIG. iTs.-niusion of lines always pointing 
 
 * toward observer. 
 
 area which bears a very small proportion to 
 
 the whole field of vision. Hence in order to obtain an accurate idea of the 
 appearance of any large object our eyes must wander rapidly over its whole 
 surface, and we use our eyes so instinctively and unconsciously in this way 
 that, unless our attention is specially directed to the subject, we find it diffi- 
 cult to believe that the power of distinct vision is limited to such a small 
 portion of the retina. In most of the experiments in physiological optics, 
 however, this rapid change of direction of the axis of vision must be carefully 
 avoided, and the eye-muscles held immovable in tonic contraction. 
 
 Our eyes, moreover, like most of our organs, serve us best when we do not 
 pay too much attention to the mechanism by which their results are brought 
 about. In the ordinary use of the eyes we are accustomed to neglect after- 
 images, intraocular images, and all the other imperfections of our visual appa- 
 ratus, and the usefulness of our eyes depends very much upon our ability thus 
 to neglect their defects. Now, the habit of observing and examining these 
 defects that is involved in the scientific study of the eye is found to interfere 
 with our ability to disregard them. A student of the physiology of vision 
 who devotes too much attention to the study of after-images, for instance, may 
 render his eyes so sensitive to these phenomena that they become a decided 
 obstacle to ordinary vision. 
 
362 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 B. THE EAR AND HEARING. 
 
 Anatomy and Histology of the Ear. The organ of hearing may con- 
 veniently be divided into three parts : (1) The external ear, including the 
 pinna or auricle and the external auditory meatus ; (2) the middle ear, called 
 the " tympanic cavity " or tympanum ; and (3) the internal ear, or labyrinth. 
 The labyrinth is situated in the dense petrous bone, and it contains a mem- 
 branous sac of complex form which receives the peripheral terminations of the 
 auditory nerve. This sac, therefore, is to the ear what the retina is to the eye 
 as the lens, cornea, etc. of the eye are simply physical media for the production 
 of sharp images on the retina, so all parts of the organ of hearing are devoted 
 solely to the accurate transmission of the energy of air-waves to the internal 
 ear. 
 
 The External Ear. The pinna or auricle, commonly known simply as 
 the " ear" (Fig. 179), is a peculiarly wrinkled sheet of tissue, consisting essen- 
 
 FIG. 179.-Diagram of organ of hearing of left side (Quain, after Arnold) : 1, the pinna; 2, bottom of 
 concha ; 2-2', meatus externus ; 3, tympanum ; above 3, the chain of ossicles ; 3', opening into the mastoid 
 cells ; 4, Eustachian tube ; 5, meatus internus, containing the facial (uppermost) and auditory nerves ; 
 6, placed on the vestibule of the labyrinth above the fenestra ovalis ; a, apex of the petrous bone ; b, 
 internal carotid artery ; c, styloid process ; d, facial nerve, issuing from the stylo-mastoid foramen ; e, 
 mastoid process ; /, squamous part of the bone. 
 
 tially of yellow elastic cartilage covered with skin, and forming at the entrance 
 of the auditory meatus a cup-shaped depression called the " concha." 
 
 The concha, and to some extent the whole auricle, serves a useful purpose 
 in collecting, like the mouth of a speaking-trumpet, the waves of sound falling 
 upon it ; but in many of the lower animals the concha is relatively larger than 
 in man, and, their ears being freely movable, the auricle becomes of greater 
 physiological importance. 
 
 External Auditory Meatus. In man the external auditory meatus or audi- 
 tory canal is about one and a quarter inches in length, and it extends from 
 
THE SENSE OF HKMtlNG. 
 
 363 
 
 the bottom and anterior edge of the concha to the membrana tympani, or 
 
 tympanic membrane. Starting 
 from the bottom of the concha, 
 the general direction of the audi- 
 tory canal is first obliquely up- 
 ward and backward for about 
 half an inch, and then inward 
 and forward. Therefore, to look 
 into the ear or to introduce the 
 aural speculum the canal must be 
 straightened by pulling the pinna 
 upward and backward. The 
 
 14 
 
 FIG. 180. Tympanum of left ear, with ossicles in situ 
 (after Morris) : 1, suspensory ligament of malleus ; 2, head 
 of malleus ; 3, epitympanic region ; 4, external ligament 
 of malleus ; 5, processus longus of incus ; 6, base of stapes ; 
 7, processus brevis of malleus; 8, head of stapes; 9, os 
 orbiculare; 10, manubrium ; 11, Eustachian tube ; 12, exter- 
 nal auditory meatus: 13, membrana tympani; 14, lower 
 part of tympanum. 
 
 canal-wall is cartilaginous 
 movable for about half an inch 
 from the exterior, but is osseous 
 for the rest of its extent; it is 
 lined by a reflexion of thin skin, 
 on whose surface, in the cartilag- 
 inous part of the canal, open the 
 ducts of numerous sebaceous and 
 ceruminous glands. 
 
 Tympanum. The middle ear, 
 or tympanum (Figs. 179, 180), is 
 shut off from the auditory canal 
 by the tympanic membrane. It 
 
 is an air-holding cavity of irregular shape in the petrous bone, and it is broader 
 
 behind and above than it is below and 
 
 in front. Posteriorly it is in open com- 
 munication with the complex system of 
 
 air-cavities in the mastoid bone known 
 
 as the mastoid antrum and the mastoid 
 
 ectk. Anteriorly it is continuous with the 
 
 pharynx through the Eustachian tube. 
 
 The inner wall slants somewhat outward 
 
 from top to bottom, and it is formed 
 
 chiefly by part of the bony envelope of 
 
 the internal ear. The surface of this wall 
 
 is pierced by two apertures, the fenestra 
 
 GW//X, or oval window, and the fenestra 
 
 rotunda, or round window, leading into 
 
 the cavity of the bony labyrinth ; in life 
 
 each fenestra is covered by a thin sheet 
 
 of membrane, and the foot of the stapes 
 
 is fastened by a ligamentous fringe in the 
 
 oval window. The outer wall of the middle ear is made up of the tympanic 
 
 FIG. 181. Otoscopic view of left membrana 
 tympani (Morris): I, membrana flaccid a ; 2,2', 
 
 foldg bounding the former ; 3, reflection from 
 processus brevis of malleus ; 4, processus lon- 
 
 gus of incus (occasionally seen): ft, mem- 
 
 brana tympani . 6f umbo an d end of manu- 
 t>rium; 7, pyramid of light. 
 
364 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 membrane and the ring of bone into which this membrane is inserted. 
 The roof is formed by a thin plate of bone, the tegmen, which separates it 
 from the cranial cavity, and the narrow floor, concave upward, is just above 
 the jugular fossa. The cavity is lined by mucous membrane continuous with 
 that of the Eustachian tube and the pharynx, and the membrane, like that 
 of the Eustachian tube, is ciliated except over the surfaces of the ossicles and 
 the tympanic membrane. Suppurative inflammation of the middle ear may 
 not only involve the mastoid cells, but may also cause absorption of the thin 
 plate of bone forming the roof of the tympanic cavity and the mastoid 
 antrum. In this and in other ways inflammation may extend from the tym- 
 panic to the cranial cavity, making otitis media, or inflammation of the middle 
 ear, the commonest source of pyogenic affections of the brain. 1 
 
 Tympanic Membrane, or Drum-skin. The membrana tympani (Figs. 181, 
 182) is a somewhat oval disk whose longer axis is directed from behind and above 
 
 downward and forward, and 
 whose length is about nine 
 millimeters. The membrane 
 is inserted obliquely to the 
 axis of the auditory canal, 
 so that the floor of the canal 
 is longer than its roof. The 
 membrana tympani, though 
 so thin as to be semi-trans- 
 parent, is composed of three 
 layers of tissue. Externally 
 it is covered by a thin plate 
 of skin ; internally, by mu- 
 cous membrane ; and between 
 these lies the proper sub- 
 
 FIG. 182.2 Tympanum of right side with ossicles in place, viewed 
 from within (after Morris) : 1, body of incus ; 2, suspensory ligament 
 of malleus ; 3, ligament of incus ; 4, head of malleus ; 5, epitym- 
 
 panic cavity ; 6, chorda tympani nerve ; 7, tendon of tensor tympani FIG. 183. The chain of auditory 
 
 muscle; 8, foot-piece of stirrup; 9, os orbiculare; 10, manubrium ; ossicles, anterior view (after Tes- 
 
 11, tensor tympani muscle; 12, membrana tympani; 13, Eustachian tut) : 1, head of malleus; 2, long 
 
 tube - process of incus ; 3, stapes. 
 
 stance (membrana propria) of the membrane, made up chiefly of fibrous tissue. 
 The greater number of the fibres of the membrana propria radiate from near 
 the centre to the periphery of the membrane ; but there are also circular fibres 
 of elastic tissue which are most numerous in a ring near the attached margin 
 of the membrane. The surface of the tympanic membrane is not flat, but is 
 funnel-shaped, with the apex of the funnel pointing inward. Moreover, lines 
 1 Macewen : Pyogenic Diseases of the Brain and Spinal Cord, 1893. 
 2 Figs. 180, 181, and 182 are taken by permission from Morris's Text-Book of Anatomy, Phila., 1893. 
 
TKE SENSE OF HEARING. 
 
 365 
 
 drawn from the centre to the margin of the membrane would not be straight, 
 but would be curved slightly, with the convexity outward, this shape being 
 due to the tension of the elastic circular fibres of the membrane. The mem- 
 brane, throughout the greater part of its circumference, is inserted in a groove 
 in a bony ring set in the wall of the auditory canal, but a small arc at its 
 superior portion is attached directly to the wall of the canal. The segment of 
 membrane corresponding to this arc, known as the membrana flacrida, lacks 
 the tenseness of the rest of the drum-skin. 
 
 Viewed through the aural speculum, the normal tympanic membrane has 
 a pearly lustre (Fig. 181). The handle of the malleus, or manubrium, inserted 
 within its fibrous layer, can be seen as an opaque ridge running from near the 
 upper anterior margin downward and backward and ending in the umbo, or 
 central depression, where the membrane is drawn considerably inward by the 
 tip of the manubrium. It is from this point that the radial fibres of the mem- 
 brana propria diverge. 
 
 At the top of the manubrium is a shining spot which is the reflection 
 from the short process of the malleus where it presses against the membrane. 
 From this point two delicate folds of the membrane run to the periphery 
 one forward and the other backward. They form the lower border of the 
 membrana flaccida, or ShrapneWs membrane, in which there is less fibrous tissue 
 than in the remaining part of the membrane, and the cutaneous and mucous 
 layers are also less tense than elsewhere. A bright reflection of triangular 
 shape, known as the " pyramid of light," is seen in the lower quadrant of the 
 tympanic membrane. The apex of this 
 bright triangle is at the tip of the manu- 
 brium, and its base is on or near the 
 periphery of the membrane. 
 
 Auditory Ossicles. The tympanic 
 membrane is put into relation with the 
 internal ear by a chain of bone, the 
 auditory ossicles, known as the malleus, 
 the incus, and the stapes, so called from 
 their fancied resemblance to a hammer, an 
 anvil, and a .stirrup (Figs. 180, 182, 183). 
 The malleus (Fig. 184) is 18 to 19 milli- 
 meters long; it presents a rounded head, FlG . 184 ._ Malleu80ftherightside:A>anterior 
 
 grooved O11 one side for artfculation with face; B, internal face (after Testut): 1, capitu- 
 . i , -i -11 iji lum or head of malleus ; 2, cervix or neck ; 3, 
 
 the incus, a short neck, and a long handle procewmg brevis . 4) pr0 ce S 8us graciiis ; 5, manu- 
 
 Or manubrium, which is inserted in the brium; 6, grooved articular surface for incus; 
 
 . ,> 7, tendon of m. tensor tympani. 
 
 tissue oi the tympanic membrane irom 
 
 a point on its upper periphery to a little below its centre. The processus 
 brevis of the malleus is a low conical projection which springs from the top 
 of the manubrium and presses directly against that segment of the tympanic 
 membrane known as the membrana flaccida, through which it can be seen 
 shining on inspection with the ear-speculum. The processus graciiis, or pro- 
 
366 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 cessus Folianus, long and slender, arises from an eminence just below the 
 neck of the malleus, and, passing forward and outward, is inserted in the 
 Glaserian fissure in the wall of the tympanum. The malleus is held in posi- 
 tion partly by ligaments; the suspensory or superior ligament passes downward 
 and outward from the roof of the tympanum to be inserted into the head of 
 the malleus. The main portion of the anterior ligament is attached to the 
 neck of the malleus just above the processus gradlis ; it embraces the latter, 
 
 and, passing forward, finds its origin 
 in the anterior wall of the tympanum 
 and in the Glaserian fissure. Another 
 division of this ligament, the external 
 ligament, arises and is attached more 
 externally than that just described. 
 
 Ig.inc 
 
 represents a nearly horizontal section of the tym- 
 panum, carried through the heads of the malleus 
 and incus : M, malleuf ; 1, incus ; t, articular tooth 
 
 . of the malleus serve to 
 
 keep its head in position. The exter- 
 
 i i .. i uiu j.i 
 
 na } lament, being attached above the 
 
 of incus ; Ig.a and Ig.e, external ligament of mal- axis of rotation of the hammer, pre- 
 leus; Ig.inc, ligament of the incus ; the line a-x rep- ,i u j / j.v i p 
 
 resents the axis of rotation of the two ossicles. vents tne head - oi this bcme * rom 
 
 moving too far inward, and the manu- 
 
 brium from being pushed too far outward. The superior ligament, owing to 
 its oblique course, restrains the head of the hammer from moving too far 
 outward. 
 
 The incus, ambos, or anvil-bone (Fig. 186) is shaped somewhat like a bicus- 
 pid tooth. Its thicker portion is hollowed on the surface and is covered with 
 
 cartilage for articulation with the 
 head of the malleus. It has two 
 processes, a long and a short, 
 which project at right angles to 
 
 FIG. 186. The incus of the right side : A, anterior face ; B, 
 internal face (after Testut) : 1, body of incus ; 2, processus 
 brevis ; 3, processus longus ; 4, articular curface for the mal- 
 leus ; 5, a convex tubercle, processus lenticularis, for articu- 
 lation with stapes ; 6, rough surface for attachment of the 
 ligament of the incus. 
 
 FIG. 187. The stapes (after Testut) : 1, 
 base 1 ; 2, anterior cms ; 3, posterior crus ; 
 4, articulating surface of head of the 
 bone ; 5, cervix or neck. 
 
 each other; the former has a length of 4J millimeters, and the latter a length 
 of 3 to 3J millimeters. When in position the long process descends nearly 
 parallel with the manubritim, but it has less than three-fourths the length of 
 the latter. The free end of the long process is turned sharply inward at right 
 angles, and terminates in a round projection, the os orbiculare, which is provided 
 with cartilage for articulation with the head of the stapes. The short process is 
 
THE SENSE OF HEAI!IX<;. 367 
 
 conical in shape and is thicker than the long process. It has a horizontal posi- 
 tion, and is attached by a thick ligament to the posterior wall of the tympanum. 
 
 The stapes (Fig. 187) articulates with the end of the long process of the 
 incus ; its plane is horizontal and about at right angles to that process. It 
 measures 3 to 4 millimeters in length and about 2J millimeters in breadth. 
 The base of the stapes is somewhat oval in shape, the superior margin being 
 convex and the inferior being slightly concave. It is set in the fenestra ovalis, 
 an aperture measuring about 3 millimeters by 1J millimeters, and is held in 
 place by a narrow membrane made up of radial fibres of connective tissue. 
 When in position, the inner face of the base of the stirrup is covered with 
 lymphatic endothelium and is washed by the perilymph of the internal ear; 
 the outer face, like the other tympanic bones and the wall of the cavity, is 
 covered by thin mucous membrane. 
 
 Movement of the Ossicles. The malleus-incus articulation is so arranged 
 that with outward movements of the manubrium the head of the malleus 
 glides freely in the joint ; but the lower margins of the articulating surfaces 
 project in such a way that the prominences lock together when the manubrium 
 moves inward. Thus, in inward movements of the tympanic membrane and 
 its attached manubrium, the malleus and the incus move together like one 
 rigid piece of bone, the motions of the manubrium and the long process of the 
 incus being parallel. Of the malleus-incus articulation Helmholtz 1 says: 
 " In its action it may be compared with the joints of the well-known Breguet 
 watch-keys, which have rows of interlocking teeth, offering scarcely any resist- 
 ance to revolution in one direction, but allowing no revolution whatever in the 
 other." In the outward movements the locking teeth or projections are prob- 
 ably still kept in apposition, under ordinary circumstances, through the elastic 
 reaction of the ligament and the stapedial attachment of the incus. Should, 
 however, the tympanic membrane be forced unduly outward, as by increase of 
 pressure within the tympanum or by rarefaction of air in the auditory meatus, 
 the incus only follows the malleus for a certain distance, the latter completing 
 its motion by gliding in the joint. There is thus no danger of the stapes being 
 torn out of the oval window. The hammer and the anvil, suspended by their 
 ligaments, move freely about an axis one end of which is found at the origin of 
 the anterior part of the anterior ligament of the malleus, and the other end in 
 the origin of the ligament which is continuous with the short process of the incus 
 (Fig. 185). In inward motions of the tympanic membrane the ossicles move like 
 a single bone around the axis of suspension ; and as the distance measured from 
 the axis of rotation to the tip of the manubrium, where the power is applied, is 
 about one and one-half times the distance to the end of the long process of the 
 incus, where the effect is produced, the motions transmitted to the stapes can have 
 but two-thirds the amplitude of the movements of the tip of the manubrium, but 
 have one and one-half times their force. It will be noticed that a large pro- 
 portion of the mass of both anvil and hammer is found above their axis of rota- 
 tion ; this upper portion acts as a counterpoise to the parts below which are directly 
 1 Sensations of Tone, trans, by Ellis, 1885, p. 133. 
 
368 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 concerned in the lever action. The bony lever being thus balanced, it is less 
 difficult to understand its known sensitiveness to impulses that are inconceivably 
 weak. The tense tympanic membrane, by reason of its funnel shape, resists 
 strong inward compression ; hence the stapes is prevented from being pressed 
 too far inward. The maximum amplitude of motion of the stapes in the 
 fenestra is very small, being only about -fa millimeter to -fa millimeter, while 
 that of the centre of the tympanic membrane is about fa millimeter to ^ 
 millimeter. 
 
 The functional movements of the auditory ossicles are not molecular but 
 are molar vibrations, the chain of bones moving in a body. The sole purpose 
 of this apparatus of the middle ear is to transmit exactly the variations of 
 pressure in the air of the external auditory meatus to the peri lymph which 
 bathes the foot of the stapes in other words, to convert air-waves into a 
 similar series of water-waves. In the words of Helmholtz, 1 " The mechanical 
 problem which the apparatus within the drum of the ear had to solve was to 
 transform a motion of great amplitude and little force, such as impinges on 
 the drum-skin, into a motion of small amplitude and great force, such as had 
 to be communicated to the fluid in the labyrinth. " 
 
 The adaptation of the apparatus of the middle ear to this end is worthy 
 of careful consideration. In the first place, it will be noticed that the area 
 of the fenestra ovalis which receives the impulses of the stapes is but a small 
 fraction of the surface of the tympanic membrane on which the air-waves 
 impinge, the latter area being some fifteen to twenty times greater than the 
 former, so that the energy of air-motion is, in a fashion, concentrated. In the 
 second place, as previously observed, the lever mechanism of the auditory 
 ossicles is such that the movements of the end of the long process of the incus 
 have two-thirds the amplitude of those of the tip of the manubrium, but 
 about one and one-half times their force. It should also be noticed that the 
 membrane fastening the foot of the stapes in the fenestra is somewhat less 
 tense on the upper side, so that the top of the oval foot-piece has a freer 
 motion than the bottom, and the head of the stirrup rises slightly with inward 
 motions. In the third place, it has been demonstrated by Helmholtz 2 that the 
 shape of the tympanic membrane peculiarly adapts it for transforming weak 
 movements of wide amplitude into strong ones of small compass. For this 
 membrane is not a simple funnel depressed inwardly, but the radii are slightly 
 curved with the convexity outward, a shape chiefly due to the tension of the 
 elastic circular fibres of the membrane on its inner face, these being most 
 numerous toward the circumference. Air-waves beating upon this convexity 
 flatten the curve somewhat, and their whole energy must be concentrated, with 
 increased intensity but loss of motion, at the central point of the membrane. 
 This effect may be illustrated by holding a slightly-curved brass wire, several 
 inches in length, with its plane perpendicular to the surface of a table and 
 supported on its ends. When one end of the wire is held immovable, up-and- 
 down motions of the arch are transferred to the free end with diminished 
 1 Op. cit., p. 134. 2 Op. cit. 
 
TEE SENSE OF HEARING. 369 
 
 amplitude. The wire represents a single radial fibre of the tympanic mem- 
 brane, and the funnel shape of this membrane is adapted to concentrating this 
 motion of the radial fibres upon the manubrium. The same effect is illus- 
 trated by the fact that when a string or a rope is stretched between two points, 
 no matter how tightly, it always sags at its middle; the weight of the cord, 
 however slight, is sufficient to give it a curved course, and produces a corre- 
 sponding traction on the points of support. 
 
 Eustachian Tube. That the tympanic membrane may maintain its 
 freedom of motion, it is obviously necessary that the average atmospheric 
 pressure on each side of it should remain the same. This equality of pressure 
 is maintained through the medium of the Eustachian tube, a somewhat trumpet- 
 shaped canal which, beginning in the lower anterior walls of the tympanum, 
 runs downward, forward, and inward, and terminates in a slit in the side of 
 the upper part of the pharynx. The Eustachian tube is lined, like the walls 
 of the tympanum, with ciliated epithelium, the cilia working in such a way 
 as to carry into the pharynx such secretions as may arise from the mucous 
 membrane of the middle ear. The pharyngeal opening of the Eustachian tube 
 is probably normally closed, but it may easily be made to open by increase or 
 decrease of air-pressure within the pharynx, as may be produced by closing 
 the nose and mouth and either forcing air into the pharynx by strong expiration 
 or rarefying it by suction. In the former case the air-pressure within the 
 tympanum is increased, and in the latter it is diminished. When air is thus 
 made to enter or to leave the tympanum, a sensation of a sudden snap and 
 a dull crackling noise in the ear is experienced. The lower end of the tube 
 is normally opened during the act of swallowing, and it is at this moment that 
 the intra- and extra-tympanic air-pressures are equalized. 
 
 Muscles of the Middle Ear. Two muscles are devoted to adjusting the 
 tension of the auditory mechanism of the middle ear. The tensor tympani is 
 lodged within a groove which is just above and about parallel with the Eusta- 
 chian tube. It terminates externally in a long tendon which bends nearly at 
 right angles round the outer edge of the groove and is inserted into the 
 handle of the malleus near the neck. Contraction of the tensor tympani 
 thus pulls inward the tympanic membrane, increases its tension, and some- 
 what dampens its vibrations. At the same time a strain is put upon the 
 chain of ossicles, the toothed processes of the malleus and incus are brought 
 more closely together, and the foot of the stapes is pressed into the oval win- 
 dow, increasing the pressure upon the fluids of the internal ear. It is si id 
 that the relaxed tympanic membrane, particularly after section of the tensor 
 tympani muscle, is thrown into sympathetic vibration with comparative ease, 
 and is in this condition best adapted to respond to weak ai'-rial impulses and 
 to the periodic waves of musical notes. When the membrane is tense its 
 vibrations are damped, and it is particularly fitted to transmit noises and con- 
 sonantal sounds, and thus the muscle involved would seem important to the 
 clear transmission of ordinary speech, though its effect would be to decn u-<- 
 the acuteness of hearing. According to Hensen, 1 the tensor tympani muscle 
 
 1 Hermann's Handbuch der Physioloyie, 1880. 
 VOT,. TT. 24 
 
370 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 is excited to reflex contraction by the initial waves of a sound, resulting in a 
 closer union of the toothed processes of the malleus and incus, so that there is 
 less loss of motion in the subsequent vibrations. But Ostermann l believes 
 the muscle to be chiefly a protective mechanism which by its contraction 
 prevents oscillation of so wide an amplitude as to be hurtful, and that its 
 reflex action is called forth chiefly by very loud noises (PL 1, Fig. 1). The 
 stapedius is a small muscle imbedded in the inner wall of the tympanum, 
 near the fenestra ovalis. Its tendon, passing forward, is inserted into the 
 neck of the stapes. Contraction of the muscle would cause a slight rotation 
 of the stapes round a vertical axis, so that the hinder part of the foot of the 
 ossicle would be pressed more deeply into the fenestra, while the remaining 
 portion would be drawn out of it. Its action probably reduces the pressure 
 in the cavity of the perilymph, and thus is antagonistic to that of the tensor 
 tympani (PL 1, Fig. 2, A, B). 
 
 Vibrations of the Tympanic Membrane. It is a general physical law 
 that every elastic body can be made to vibrate more easily at one definite rate 
 than at any other. The musical tone represented by this rate of vibration is 
 known as the prime or fundamental tone of the body. Membranes have funda- 
 mental tones (see p. 383), whose pitch is determined by their area, thickness, 
 and tension, but they differ from rods and strings in being less strictly confined 
 to a single fundamental tone in their vibration. The tympanic membrane is 
 quite peculiar in that it can hardly be said to have a definite fundamental tone. 
 It would obviously be a great imperfection in an organ of hearing were cer- 
 tain sounds intensified by it out of proportion to others, as would be the case 
 if the tympanic membrane had a marked fundamental tone of its own. This 
 is prevented in the case of the membrana tympani probably both by reason of 
 the peculiar form of its surface and its structure, and also because its oscilla- 
 tions are damped by the pressure of the malleus held in position by the other 
 mechanisms of the tympanum. When the tympanic membrane is perforated 
 or is wholly removed, without destructive inflammatory changes in the middle 
 ear, sounds are still heard, though usually with diminished loudness. A 
 musician who had suffered this accident was no longer able to play his violin, 
 probably because sounds of different pitch ceased to be perceived in their true 
 relations of loudness. We may thus conclude that the function of the tym- 
 panic membrane is not only to guard against injury to the delicate mem- 
 branes of the fenestra} and the internal ear, but also to transmit to the ossicles 
 sonorous vibrations with their true proportion of intensity. The membranes 
 covering the round and oval windows of the internal ear have no means of 
 damping sympathetic vibrations (see p. 385), and, should complex air-waves 
 strike directly upon them, they would, probably, by sympathetic resonance, 
 respond more powerfully to tones of certain pitch than to any others. 
 
 The sensation of sound may be excited by conduction through the bones 
 of the skull as well as in the ordinary way. Thus, a tuning-fork set vibrating 
 and held between the teeth or on the forehead is heard perfectly, and more 
 
 1 Archivfur Anatomic und Physiologic, 1898, S. 75. 
 
THE SENSE OF HEARING. 
 
 371 
 
 loudly when the ears are closed than when open. The vibrations thus con- 
 ducted probably partly affect the internal ear directly, and partly indirectly by 
 setting in oscillation the tympanic membrane. It is said that when the sound 
 of a tuning-fork held close to the ear dies away, it may again be heard if the 
 handle of the fork be prosed against the teeth. When the tone now fails, 
 it once more becomes audible if one of the ear-passages is lightly closed, and 
 the sound seems to be on the side which is closed. The sensation failing, it 
 may again be aroused if the appropriately formed handle of the fork be 
 inserted in the auditory meatus. 1 
 
 Normal individuals differ greatly in their keenness of hearing, and tests 
 show frequently disparity in the sensibility of the two ears. The hearing 
 ability of children is said to improve up to the age of twelve years. There 
 is no functional relation between keen hearing and sensibility to pitch. 2 
 
 The Internal Ear, or Labyrinth. The internal ear is the site of the true 
 organ of hearing. The membranous labyrinth (PL 1, Fig. 4 ; Fig. 191) is a com- 
 plicated system of membranous tubes and sacs, in which terminate at particular 
 points the filaments of the auditory nerve ; it is contained within a chamber, 
 the bony labyrinth, hollowed out in the petrous bone. The cavity of the bony 
 labyrinth (Figs. 188, 189) consists of a median part, the vestibule, which is pro- 
 longed posteriorly in the system of semicircular canals and anteriorly in the 
 cochlea. The vestibule is a space which measures about one-fifth of an inch 
 in diameter, and it is perforated in its outer wall by an oval opening known 
 as the fenestra ovalis. The semicircular canals are three tubes of circular 
 
 5 9 10 
 
 FIG. 188. Right bony labyrinth, viewed from 
 outer side : the figure represents the appearance 
 produced by removing the petrous bone down to 
 the denser layer immediately surrounding the 
 labyrinth (from Quain, after Sommering) : 1, 2, 3, 
 the superior, posterior, and horizontal semicir- 
 cular canals ; 4, 5, 6, the ampullae of the same ; 
 7, the vestibule ; 8, the fenestra ovalis ; 9, fenestra 
 rotunda; 10, first turn of the cochlea; 11, second 
 turn ; 12, apex. 
 
 FIG. 189. Interior view of left bony labyrinth after 
 removal of the superior and external walls (from 
 Quain, after Sommering) : 1, 2, 3, the superior, pos- 
 terior, and horizontal semicircular canals ; 4, fovea 
 hemi-elliptica ; 5, fovea hemispherica ; 6, common 
 opening of the superior and posterior semicircular 
 canals ; 7, opening of the aqueduct of the vestibule ; 
 8, opening of the aqueduct of the cochlea; 9, the 
 scala vestibuli ; 10, scala tympani ; the lamina spiralis 
 separating 9 and 10. 
 
 section, known respectively as the anterior or superior, the posterior, and the 
 
 1 Rinne, quoted by Ilensen : Hermann 's Handbuzh der Physiologic, 3880, Bd. iii. Th. 2, S. 26. 
 
 2 Seashore : "Studies in Psychology," Bulletin University of Iowa, 1899. 
 
372 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 external or horizontal semicircular canal. Their planes are at right angles to 
 one another, so that they occupy the three possible dimensions of space. The 
 external canal lies in a nearly horizontal plane, while the other two approach 
 the vertical. Each canal is dilated at one extremity into a globular cavity 
 
 which is more than twice the diameter of the 
 canal itself, and which is known as the am- 
 pulla. The anterior and posterior canals 
 unite near the ends not provided with am- 
 pullae, and they enter the vestibule as a com- 
 mon tube. Anteriorly the cavity of the 
 4 vestibule is continued as a tube of complex 
 internal structure which is coiled upon itself 
 two and one-half times, and which, from its 
 resemblance to the shell of a snail, is known 
 as the cochlea (PL 1, Fig. 3). The osseous 
 cochlea may be conceived as formed by a bony tube turned about a bony central 
 pillar, the modiolm, which diminishes in diameter from the base to the apex 
 of the cochlea. From the modiolus a bony shelf stretches into the cavity of 
 the tube, incompletely dividing it into two tubular chambers, winding round 
 the modiolus like a circular staircase, the upper of which chambers we shall 
 
 FIG. 190. Diagram of the osseous cochlea 
 laid open (after Quain) : 1, scala vestibuli ; 
 2, lamina spiralis ; 3, scala tympani ; 4, cen- 
 tral pillar or modiolus. 
 
 9 
 
 FIG. 191. Diagram of right membranous labyrinth seen from the external side (after Testut) : 1, utri- 
 cle ; 2, 3, 4, superior, posterior, and horizontal semicircular canals ; 5, saccule ; 6, ductus endolymphat- 
 icus, with 7, 7', its twigs of origin ; 8, saccus endolymphaticus ; 9, canalis cochlearis, with 9', its vestibular 
 cul-de-sac, and 9", its blind extremity ; 10, canalis reuniens. 
 
 soon learn to know as the scala vestibuli, and the lower chamber as the scala 
 tympani (Fig. 190 ; PI. 1, Fig. 3). The bony shelf mentioned above as partly 
 bisecting the cochlear tube has, of course, like the latter, a spiral course, and is 
 known as the lamina spiralis ; its importance as a supporter of the auditory- 
 nerve filaments will soon be seen. 
 
 Contained within the cavity of the bony labyrinth, and parallel with its walls, 
 is the membranous labyrinth, in which are found the essential structures of the 
 organ of hearing (PL lj Fig. 4 ; Fig. 191). The membranous labyrinth is filled 
 with a somewhat watery, mucin-holding fluid, the endolymph, while a similar 
 fluid, the perilymph, is found outside it and within the osseous labyrinth. The 
 
y///: ,s7-;.v.sv-; or 
 
 I'l-ATK 1, 
 
THK XEXHi: o/' y//-;.i/,'/.v<v. ;;:;; 
 
 perilymph space, which is lined by lymphatic epithelium, is in communication, 
 along the sheath of the auditory nerve, with the subdiiral and subarachnoid 
 lymph-areas of the brain. Numerous sheets and bars of connective tissue cross 
 from the wall of the bony to that of the membranous labyrinth and help support 
 the latter. That part of the membranous labyrinth lying within the vestibule 
 is composed of two separate sacs a larger posterior, known as the utricle or 
 utriculus, and a smaller, more anterior, known as the saccule or sacculus. The 
 plane of division between the two sacs ends opposite the fenestra ovalis (PI. 1, 
 Fig. 4). Though the sacs are quite separate, their cavities are indirectly continu- 
 ous, through the union of two small tubes arising from either sac, which tubes 
 unite to form the ductus endolymphnticits, a tube running inward through a 
 canal in the petrosal bone and ending blindly in a dilated flattened extremity, 
 the saccus endofymphaticus, this being supported between the layers of the 
 dura mater within the cavity of the skull (PL 1, Fig. 4). Bundles of audi- 
 tory-nerve fibres penetrate the wall of each sac. The utricle gives rise to the 
 membranous semicircular canals, which communicate with it at five points, 
 it being remembered that the anterior and posterior canals fuse into a single 
 tube at the ends not provided with ampullae, and that they have a common 
 entrance into the utricle. The saccule is continuous by a narrow tube, the 
 canalis reuniens, with that division of the membranous labyrinth contained 
 within the cochlea and known as the canalis cochlearis. The auditory nerve 
 really consists of two distinct divisions having separate origins and different 
 distributions. One of these branches passes finally to the cochlea, and the 
 other to the vestibule and the semicircular canals. The nerve approaches the 
 labyrinth by way of a canal known as the meatus audilorius internus y and 
 on reaching the angle between the vestibule and the base of the cochlea the 
 cochlear division passes to the cochlea. The remainder of the nerve consists 
 of two divisions, the superior of which is distributed to the utricle and to the 
 ampullae of the anterior and horizontal semicircular canals ; the inferior branch 
 supplies the saccule and the posterior semicircular canal. The inner wall of 
 both utricle and saccule is developed at a particular spot into a low mound, 
 the macula acustica, made up of an accumulation of the connective-tissue ele- 
 ments of the membranous wall and covered by a peculiarly modified epithe- 
 lium, the auditory epithelium (Fig. 192). All the auditory-nerve filaments that 
 enter the saccule and utricle respectively pass to these mounds and there enter 
 into relation with the auditory epithelium. 
 
 As the auditory-nerve endings are confined to a particular area in the 
 utricle and the saccule, so the nerve-fibres supplying the semicircular canals 
 arc limited to a certain part of the ampulla of each canal. The tissue of the 
 wall of the ampulla is developed into a ridge projecting into the cavity in a 
 direction across its long axis. This ridge, present in each ampulla, is called 
 the crista acustica; it is capped by a thick layer of columnar epithelial cells, 
 the auditory epithelium, which thins away at the border of the crista into 
 the sheet of flattened cells by which the rest of the ampulla is lined. The 
 auditory cells (Fig. 192) are said to be of two kinds one, cylindrical in 
 
374 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 shape and reaching only part way to the basement membrane, the hair-cells ; 
 the other, narrow and elongated, the supporting or sustentacular cells. The 
 former are peculiar in the fact that from their free ends there project long, 
 stiff, hair-like processes. The filaments of the ampullary-nerve branches 
 pass through the cristae and encircle the bodies of the hair-cells. The cells 
 covering the maculae acusticce have 
 
 essentially the same structure as those 
 just described, though in the maculae 
 the auditory hairs are shorter than in 
 the cristae. Seated on the free surface 
 of the macular epithelium is a fibrous 
 mass which is said to be a normal 
 structure, and not, like a somewhat 
 similar mass found covering the eristse 
 in post-mortem section, a coagulum 
 due to the method of preparation. 
 Imbedded in the membrane over the 
 maculae of both saccule and utricle 
 small crystals, otoliths or oto- 
 
 are 
 
 FIG. 192. Diagram showing the epithelial cells of 
 a macula or a crista (after Foster) : 1, cylinder or 
 hair-cell ; 2, the same, enveloped in a nest of nerve- 
 fibrils ; 3, 4, 5, forms of rod- or spindle-cells. 
 
 coma, composed chiefly of carbonate 
 of lime. Otoconia are also found 
 less constantly in the ampullae and 
 even in the perilymph space of the cochlea. In fishes there are large masses 
 of calcareous matter, otoliths, attached to the wall of the auditory sac. 
 
 General Anatomy of the Cochlea. By far the most complex structure of 
 the ear is found in the cochlea (PI. 1 . Figs. 1, 3, 4 ; Figs. 1 88-191). The bony 
 cochlea continues from the anterior wall of the vestibule, and in the upright posi- 
 tion of the head the axis of the modiolus is nearly horizontal, pointing, from base 
 to apex, outward and slightly down and forward, the base of the cochlea being 
 formed by the inner surface of the petrous bone. The membranous cochlea, 
 canalis or ductus cochlearis, is a tube of nearly triangular cross-section which 
 winds round the modiolus from base to apex (Fig. 193). The base or outer side 
 of this triangle is attached closely to the bony wall of the cochlea ; the upper 
 side, supposing the modiolus to be vertical with its apex above, is made of a thin 
 sheet of cells known as the membrane of Reissner ; the lower side is made up 
 partly of the bony margin of the lamina spiralis and partly of a membrane, 
 radially striated, stretched across from the edge of the spiral lamina to the side 
 wall of the cochlea ; this is called the basilar membrane, membrana basilaris. 
 The coiled tube forming the bony cochlea is thus divided by the lamina spiralis 
 and the canalis cochlearis into three tubes which wind spirally and parallel 
 round the modiolus. The canalis cochlearis contains endolymph, and its cav- 
 ity ends blindly above and below, but is continuous by way of the narrow 
 canalis reuniens with that of the saccule. The tubes above and below the 
 canalis cochlearis are perilymph-spaces ; it will be noticed that there is no 
 such space on the outer side of the membranous cochlea. 
 
THE SENSE OF HEARING. 
 
 375 
 
 The upper tube, when followed down to the base of the cochlea, is found 
 to open freely into the vestibule of the labyrinth ; it is therefore known 
 as the scala vestibuli. The lower tube ends blindly at the base of the 
 oK-hlea, but, where this part bulges into the tympanum as the "promontory" 
 of its inner wall, it is perforated by the aperture known as the feneatra 
 rotunda, whose proper membrane alone prevents the perilymph from escaping 
 
 
 FIG. 193. Diagram of a transverse section of a whorl of the cochlea (after Foster) : Sc. V, scala vestib- 
 uli; Sc. T, scala tympani; C.Chi, canalis cochlearis; Lam.sp, lamina spiralis; Gg.sp, ganglion spirale; 
 n.aud, auditory nerve ; m. R, membrane of Reissner ; Str.v, stria vascularis ; Lg.sp, ligamentum spirale ; 
 U, lymphatic epithelioid lining of basilar membrane on the tympanic side ; m.b, basilar membrane ; 
 Org.C, organ of Corti; L.t, labium tympanicum; Ib, limbus ; L.v, labium vestibulare; m.t, tectorial 
 membrane. 
 
 into the middle ear. This tube is therefore known as the scala tympani. 
 From its central position the membranous cochlear canal is frequently known 
 as the scala media. The scala vestibuli and the scala tympani both decrease in 
 size as they wind from the base to the apex or cupola of the cochlea ; the 
 membranous cochlear canal, on the contrary, increases in section from base to 
 apex until near the top ; hence the width of the basilar membrane and the 
 
376 AN AMERICAN TEXT-BOOK .OF PHYSIOLOGY. 
 
 length of its radial fibres increase from below upward. The scala vestibuli 
 and the scala tympani have no communication except through a small aperture 
 under the cupola of the cochlea, known as the helicotrema ; this is bounded 
 by the hook-like termination, the hamulm, of the bony lamina spiral is, which 
 forms the greater part of a ring completed by the pointed blind extremity of 
 the canalis cochlearis fastened above it to the cupola. 
 
 The Transmission of Vibrations through the Labyrinth. Vibrations 
 of the tympanic membrane are transmitted as pulses of very small amplitude to 
 the membrane covering the fenestra ovalis. The relatively considerable body of 
 perilymph bathing the inner face of this membrane must be thus set in motion, 
 and there starts a fluid-wave which is free to make its way throughout the 
 perilymph-spaces of the vestibule and the semicircular canals. It may pass 
 from the vestibule along the scala vestibuli to its top, through the helicotrema, 
 and back by way of the scala tympani, at whose bottom it finally surges 
 against the membrane covering the fenestra rotunda; or the wave may be 
 transmitted directly across the membranous cochlea. The fluids of the laby- 
 rinth being physically incompressible, the function of the fenestra rotunda as 
 a sort of safety-valve seems evident. Politzer inserted a glass tube in the 
 round window, and found that fluid in the tube rose when strong air-pressure 
 was brought to bear on the outer side of the tympanic membrane. The cavity 
 of the membranous labyrinth (PI. 1, Fig. 4) is nowhere in communication with 
 the perilymph-space about it, and we must therefore assume that the irritation 
 of the auditory cells seated in its wall must depend on vibrations transmitted 
 from the perilymph directly through the membranous sacs and tubes. 
 
 Like the perilymph-space, the cavity of the membranous labyrinth is in 
 communication throughout, though in certain situations the connection of 
 adjacent parts is very indirect. Thus, though the semicircular canals open 
 freely at both ends into the utricle, the utricle and saccule are only brought 
 into union by the two narrow tubes that unite to form the ductus endolym- 
 phaticus. It will be noted that by means of this duct the membranous laby- 
 rinth is really continued into the cranial cavity. The saccule in turn is 
 continuous with the scala media of the cochlea by way of the canalis reuniens. 
 
 The Membranous Cochlea and the Organ of Corti (Figs. 193-195). 
 The cochlear division of the auditory nerve, together with the nutrient blood- 
 vessels, penetrates the modiolus at its base and runs up through the spongy 
 interior of the bony pillar. As the nerve ascends through the modiolus its 
 fibres are gradually all diverted to run in a radial direction between the bony 
 plates of the lamina spiralis, to terminate in the organ of Corti of the canalis 
 cochlearis. A collection of nerve-cells is interposed in the course of the audi- 
 tory fibres at the base of the lamina spiralis. 
 
 A complete view of the nerves of the cochlea would show a central pillar 
 of nerve-fibres diminishing in thickness from below upward, and winding 
 round this pillar a spiral sheet of radially-disposed nerve-fibres containing, 
 near their point of departure from the central pillar, a spiral line of ganglion- 
 cells ; this collection of cells is therefore known as the ganglion spirale. The 
 
THE SENSE OF HEARING. -,77 
 
 thin, free edge of the bony hnnimr x/;/m//.s- is, in the recent state, thickened by 
 a development of connective tissue forming a promontory known as (he limbus. 
 The free edge of the limbus is in turn shaped in such a way as to make a short, 
 sharp projection in the plane of the upper surface of the lamina and a longer 
 projection in the plane of its lower surface, leaving the free margin between 
 them hollowed out. The upper projection, which is known as the vestibular 
 lip, labium vcdibn/are, serves for the attachment of the tectorial membrane, 
 membrana tectoria, presently to be described. The lower projection is called 
 the tympanic lip (labium tympanicum) ; to it is attached the inner margin of 
 the basilar membrane, on whose inner half is seated the very complex struct- 
 ure known as the organ of Corti. 
 
 The basilar membrane is a thin sheet of fibrillated connective tissue stretched 
 tightly between the tympanic lip of the limbus on the inside and the spiral 
 ligament (see p. 379) on the outside. The more median part of the membrane, 
 which supports the organ of Corti, is thin and rigid and is fibrillated in a 
 radial direction. The outer part, which is first thicker and then thinner again 
 near its point of attachment, is distinctly composed of radial fibres cemented 
 together ; the isolated fibres are characterized by being stiff and brittle. 
 
 The organ of Corti (Figs. 193, 194) has as its supporting basis a series of 
 peculiarly modified epithelial cells, known as the rods of Corti (Fig. 195, B, B r ), 
 which are disposed along the edge of the spiral lamina in two rows, an inner 
 and an outer. The inner rods have their feet on the basilar membrane near its 
 median attachment ; they lean outward and upward, and at their upper extrem- 
 ity join or articulate with the heads of the outer rods, whose feet are fastened to 
 the basilar membrane more externally. The two rows of rods are thus joined 
 together like the rafters of a house, and enclose beneath them a canal known 
 as the tunnel of the organ of Corti. The inner rods are more numerous than 
 the outer, so that the latter are fastened rather between than to the ends of the 
 former. Leaning against the inner or median side of the inner row of rods 
 is a single row of hair-cells (Fig. 194), much like those described as seated on 
 the maculae and cristse of the labyrinth, to which hair-cells filaments of the 
 auditory nerve are distributed. Closely applied to the single row of hair- 
 cells, on the inner side, are several rows of columnar cells gradually decreas- 
 ing in size toward the median line, and beneath the whole is a group of nuclei. 
 External to the outer row of rods, and separated from it by a space, are four 
 parallel rows of hair-cells known as the cells of Corti ; their bodies do not 
 reach downward as far as the basilar membrane, and just below each row is a 
 bundle of nerve-fibres which have traversed the tunnel of Corti and then have 
 changed their direction from a radial to a longitudinal or spiral one. These 
 fibres, and others having a more direct course, one by one end in clusters 
 encircling the individual hair-cells. 
 
 Four rows of peculiarly-modified columnar cells, the cells of Deiters, are 
 inserted closely between the cells of Corti, the outermost row being external 
 to the fourth row of Corti. These cells rest below on the basilar membrane. 
 Still external to these groups of cells is a series of rows of tall columnar cells 
 
378 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 of simple character supported upon the basilar membrane, and rapidly decreas- 
 ing in height externally into a layer of cuboidal epithelium covering the outer 
 part of the basilar membrane. The rods of Corti are peculiarly shaped at the 
 top, the upper extremity of each being bent at an angle so as to project exter- 
 nally and parallel with the basilar membrane ; these projections are the pha- 
 langar processes of the rods, the phalanges of the inner row overlapping those 
 of the outer row. These phalangar processes of the rods form the points of 
 attachment in fact, the beginning of the reticulate membrane (membrana 
 reticulata), a peculiar cuticular, network-like structure formed of rings and 
 cross-bars, having the appearance of certain vegetable tissues seen under the 
 microscope. The reticulate membrane stretches across the outer rows of hair- 
 
 m.t 
 
 iig 
 
 n.aud l.t i.spn t.spn o.spn ^ __ _ . 
 
 Ig.sp 
 
 FIG. 194. Diagram of the organ of Corti (from Foster, after Retzius) : i.r, inner rod of Corti ; o.r, outer 
 rod of Corti; i.hc, inner hair-cell; n.c, the group of nuclei beneath it; o.hc, outer hair-cells, or cells of 
 Corti ; C.D, the twin cells of Deiters (four rows) ; n.aud, the auditory nerve perforating the tympanic lip, 
 l.t, and lost to view among the nuclei beneath the inner hair-cells ; i.spn, the inner spiral strand of nerve- 
 fibrils ; t.spn, the spiral strand of the tunnel ; o.spn, the outer spiral strand belonging to the first row of 
 outer hair-cells ; the three succeeding spiral strands belonging to the three other rows are also shown ; 
 nerve-fibrils are shown stretching radially across the tunnel ; H.c, Hensen's cells ; Cl.c, Claudius' cells ; 
 U, lymphatic epithelioid lining on the side toward the scala tympani ; Icj.sp, ligamentum spirale ; c, cells 
 lining the spiral groove, overhung by the vestibular lip, l.v ; m.t, tectorial membrane ; a fragment, torn 
 from it, remains attached to the organ of Corti just outside the outermost row of hair-cells. 
 
 cells, the body of each of which is enclosed and is held at its top within a ring 
 of the network (Fig. 195, D). 
 
 Each of the cells of Deiters, described above, is continued upward in a 
 process which is attached to a cross-bar or a ring of the reticulate membrane 
 next outside its companion-cell of Corti. The inner or median line of the 
 Deiters cell is also modified into a cuticular thread fused below to the basilar 
 membrane and above to a ring of the reticulate membrane. Thus the audi- 
 tory hair-cells of Corti may be regarded as suspended from the reticulate mem- 
 brane, which in turn is supported by the cuticular processes of the cells of 
 Deiters. which rest upon the basilar membrane, and by the phalangar pro- 
 cesses of the rods of Corti. The physical contact of the cells of Corti with 
 those of Deiters is so intimate if, indeed, their substance is not continuous 
 that impulses generated in the one can probably easily be communicated to 
 the other. 
 
 The upper wall of the canalis cochlearis is made of a sheet of homogenous, 
 fibrillated connective tissue covered with flat cells, and stretches from the 
 limbus of the spiral lamina outward and upward to the side wall of the 
 
THE SENSE OF HEARING. 
 
 379 
 
 cochlea. It is known as the membrane of Reismer. The periosteal con- 
 nective tissue of the bony wall of the cochlea is generally well developed 
 within the area enclosed between the membrane of Reissner and the membrana 
 basilaris; it is particularly thick at the line of division between the scala media 
 and the scala tympani, where it forms a projecting ridge at the outer attach- 
 ment of the basilar membrane. This ridge is the spiral ligament ; an exten- 
 
 m.b 
 
 FIG. 195. Diagram of the constituents of the organ of Corti (from Foster, after Retzius) : A, inner hair 
 cell; A', the head, seen from above; B, inner, B', outer, rod of Corti; ph, in each, is the phalangar pro- 
 cess ; c, the twin outer hair-cell ; C.c, the cell of Corti ; h; its auditory hairs ; n, its nucleus ; x, Hensen's 
 body ; D.c, cell of Deiters ; n', its nucleus ; ph.p, its phalangar process ; fil, the cuticular filament ; m.b, 
 basilar membrane ; m.r, reticulate membrane ; c', the head of a cell of Corti, seen from above ; D, the 
 organ of Corti, seen from above ; i.hc, the heads of the inner hair-cells ; i.r.h, the head and phalangar pro 
 cess of the inner rod ; o.r.h, the head of the outer rod, with ph.p, its phalangar process, covered to the left 
 hand by the inner rods, but uncovered to the right ; o.h.c, the heads of the cells of Corti, supported by 
 the rings of the reticulate membrane ; ph, one of the phalangse of the reticulate membrane. 
 
 sion from it, gradually decreasing in thickness, reaches into both the vestibular 
 and the tympanic scala. 
 
 A thick layer of both columnar and cuboidal epithelium lines the con- 
 nective tissue forming the outer wall of the canalis cochlearis. This epithe- 
 lium is peculiar in that the blood-vessels of the underlying connective tissue 
 penetrate between the epithelial cells themselves. The tectvrial membrane 
 (membrana tectoria) is a sheet of radial ly-fibri Hated tissue, thin at its point of 
 attachment to the vestibular lip of the limbus, and becoming thicker and then 
 thinner again as it stretches out over the organ of Corti, reaching as far as the 
 most external row of hair-cells. It is said to lie in actual contact with the 
 rods of Corti and the free ends of the hair-cells, and it has been presumed to 
 serve as a damper for the vibrations imparted to the organ of Corti. 
 
380 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 The researches of Howard Ayers l have led him to conclusions concerning 
 the minute anatomy of the ear materially different from those just presented. 
 Thus, Ayers asserts that the so-called membrana tectoria is nothing more 
 than the matted mass of hairs " which spring from the tops of the hair-cells 
 and form a waving plume on the crest of the ridge of the organ of Corti." 
 He also holds the membrana reticulata and several other structures 
 described by different authors to be nothing more than artefacts produced by 
 the methods of preserving and manipulating the specimens. According to 
 Ayers, the cochlear nerves end in the hair-cells and not freely between them, 
 and they are probably continuous with the auditory hairs. 
 
 Theory of Auditory Sensation. It can hardly be doubted that the 
 nervous structures of the cochlea form an organ of special sense for the per- 
 ception of musical tones and probably of noises as well. But no trustworthy 
 conclusion can be maintained as to the precise mode of action of the auditory 
 apparatus. The fact that the rods of Corti are absent from the cochleae of 
 birds, which evidently are capable of appreciating musical tones, shows that 
 these structures may be accessory, but are not essential parts of the sensory 
 apparatus. Starting from the fact that the basilar membrane splits readily 
 in a radial direction, in which, moreover, it is tightly stretched between its 
 attachments, Helmholtz 2 long ago proposed the theory that the basilar 
 membrane behaves toward vibrations reaching it like a series of stretched 
 strings. As the wires of a piano have different rates of vibration according 
 to their length, and respond sympathetically to correspondingly different 
 notes sounded in their neighborhood, so it has been supposed that different 
 radial fibres of the basilar membrane are set into sympathetic vibration by 
 different rates of vibration in the fluids bathing them. These vibrations 
 must be imparted to the structures in the organ of Corti, and the irritation 
 of the nerves connected with the cells of Corti is a natural sequel. It may 
 be repeated that, though the canal of the bony cochlea as a whole diminishes 
 in diameter from base to cupola, the canal of the membranous cochlea, the 
 scala media with its lower wall or basilar membrane, increases in diameter. 
 Thus the radial fibres of the basilar membrane are longest near the apex of 
 the cochela. The radial width of the basilar membrane, measured near the 
 bottom, middle, and top, respectively, is given as 0.21 millimeter, 0.34 milli- 
 meter, and 0.36 millimeter. The waves of physical sound are thus supposed 
 to be analyzed in the peripheral sense-organ, each auditory nerve-fibre excit- 
 ing in consciousness a tone of a particular pitch, and the mind perceiving 
 the simultaneous effects of different pendular vibrations as notes of different 
 quality. 
 
 1 Avers : Journal of Morphology, May, 1892. 
 
 2 Helmholtz : Tonempfindungen, 1877, S. 240. 
 
THE SENSE OF HEARING. 381 
 
 0. THE RELATION BETWEEN PHYSICAL AND PHYSIOLOGICAL SOUND. 
 
 Production of Sound-waves. Sound, in its physiological meaning, is a 
 sensation which is the conscious appreciation of internal changes occurring in 
 certain cells of the cerebral cortex. Fibres of the auditory nerve come into 
 close relation with these cells, and in whatever way those fibres are excited 
 the result is one and the same, a sensation of sound. 
 
 The elaborate apparatus of the middle and internal ear is so constructed 
 that the energy of mechanical oscillations in the external air is transmitted to 
 the terminations of the auditory nerves in a manner to excite them. 
 
 Sound, in a physical sense, consists in waves of alternate condensation and 
 rarefaction travelling in the air from the point of origin of the sound, much as 
 waves radiate over the surface of water from the point where a stone is dropped. 
 Any sudden impulse, such as a puff of air, or the vibration of a solid body, 
 as a stretched string or a tuning-fork, pushes the adjacent molecules of air 
 against those further removed, and this impulse produces an area, or aerial 
 shell, of increased density or condensation. The air being perfectly elastic, 
 the molecules, relieved from pressure, spring back even beyond the position 
 of equilibrium, and leave an area of decreased density or rarefaction. Thus 
 a wave, consisting of a shell of condensation succeeded by a shell of corre- 
 sponding rarefaction, moves through the air. This single air-wave is the 
 simplest element of physical sound. When a number, no matter how great, 
 of sound-waves simultaneously excite the same particle of air, the resultant 
 motion of that particle is the algebraic sum of all the motions imparted to it 
 by the single sound-waves considered separately. As any elastic body, when 
 set vibrating, continues its oscillations for a time, so is it probable that strictly 
 isolated air-waves do not occur. Any elastic body, such as a stretched string, 
 or a tuning-fork, when set in vibration, sends out from itself a series of air- 
 waves which succeed one another at a rate identical with the rate of vibration 
 of the elastic body. Such a regular succession of air-waves striking upon the 
 tympanic membrane sets the latter into correspondingly regular oscillations 
 and produces in the auditory apparatus the sensation of musical tone. 
 
 Loudness and Musical Pitch. The more vigorous the vibrations of the 
 oscillating body, the more forcibly are the air-molecules which are struck by it 
 driven forward ; and the greater their excursion or amplitude of movement, 
 the greater is the force with which the tympanic membrane is driven inward 
 when the moving air-wave strikes it. The loudness of the tone manifestly 
 depends upon the extent of motion of the tympanic membrane, as does this on 
 the amplitude of air-motion. Different elastic bodies have different natural 
 rates of oscillation. The more rapid the rate, the more frequent is the succes- 
 sion of air-waves that strike upon the ear. It is said that the apparent pitch 
 of a tone is raised when its intensity is lowered, and that such an elevation 
 of pitch may equal one-fifth of a tone. 1 Musical pitch is determined by the 
 number of air-waves which pass a given point in a unit of time, or, in 
 other words, by the rate of vibration of the sound-producing body. When 
 
 1 Broca: Jahresbericht der Physioloyie, 1897, S. 111. 
 
382 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 the vibration-rate increases the pitch is elevated, and vice versd. If some body 
 capable of producing sound should have its rate of vibration changed grad- 
 ually from 5 or 10 vibrations per second to 50,000 per second, no sensation 
 of sound would be aroused until the vibrations reached the rate of about from 
 16 to 24 per second. The droning note of the 16-foot organ-pipe and the 
 lowest bass of the piano represent a vibration-rate of 33 per second. In 
 most persons sounds cease to be audible when the air-waves have a fre- 
 quency of 16,000 per second, though to some the note produced by 40,000 
 vibrations is perceptible. It seems clear that some animals hear tones whose 
 pitch is so elevated as to make them inaudible to human ears. When a mov- 
 ing bell or whistle, as of a locomotive, rapidly approaches, its pitch seems to 
 rise, and then to fall as it recedes. The reason for this variation is that the 
 motion of the locomotive adds to or subtracts from the number of sound- 
 waves reaching the ear in a given time. In musical execution and in the 
 ordinary uses of life the limits in the pitch of sounds are much narrower. 
 Thus, as just stated, the lowest bass of the piano (C^ represents a vibration- 
 rate of 33 in a second, while the highest treble (c r//// ) has that of 4224. As 
 to the absolute number of vibrations necessary to produce the sensation of 
 sound, it has been found that 2 or 3 vibrations excite the sensation of a mere 
 stroke ; 4 or 5 vibrations are necessary to give a tone ; and some 20 or 40 are 
 required to develop the full musical qualities of a tone. 1 That is to say, when 
 a musical tone falls upon the ear its characteristics cannot be appreciated until 
 20 to 40 vibrations have been completed. 
 
 Thus, from a physical scale representing aerial vibrations of indefinitely 
 various rapidity the mind selects and appreciates as sound a very small 
 fraction. 
 
 Tympanic Membrane as an Organ of Pressure-sense. There is good 
 reason to suppose that variations in air-pressure succeeding one another too 
 slowly or too irregularly to produce sound-sensation are still of great import- 
 ance in the extensive realm of sensations which but obscurely excite our con- 
 sciousness. Slow inward movements of the tympanic membrane may still 
 give rise to a perception of external changes. Thus, a blind man has been 
 able to say correctly that he has passed by a fence, and whether it be of solid 
 board or of open picket. If any one with closed eyes holds a book at half-arm's 
 length in front of the ear, a different sensation will be experienced according 
 as the book is turned flat or edgewise to the face ; the feeling is one of " shut- 
 in-ness " or " open-ness/ 7 respectively. The air is in ceaseless agitation, and 
 its waves, striking against various objects, must be reflected to the ear with an 
 intensity dependent on the position and the physical character of the reflecting 
 media. We may assert that the tympanic membrane is the peripheral organ 
 of a pressure-sense by which we become more or less accurately aware of the 
 nature and position of surrounding objects, irrespective of the sensations of 
 sight and hearing. Whether that group of sensations depends on the excite- 
 
 1 Mach: Physikalischen Notizen Lotos, Aug., 1873; V. Kries und Auerbach : Du Bois-Rey- 
 montfs Archiv fur Physiologic, 1877, S. 297 ; Helmholtz : Sensations of Tone, translated by Ellis. 
 
THE SENSE OF HEARING. 383 
 
 ment of tactile nerves in the tympanic membrane or of the auditory filaments 
 in the internal ear is yet uncertain. 1 Such sensations probably form an import- 
 ant quota of that complex system of sensations which do not obtrude themselves 
 on consciousness, but which, nevertheless, bring information from the outer 
 world, and have an intimate association with the more or lass reflex move- 
 ments that preserve the equilibrium of the body. 
 
 Overtones and Quality of Sound. We have thus far considered only 
 simple tones produced by simple vibrations of elastic bodies. Thus, a stretched 
 string plucked at its middle vibrates throughout its whole length, the greatest 
 amplitude of movement being at the middle point, which moves to and fro 
 like a pendulum. It is very rare that a body set vibrating confines itself to 
 a single pendular movement. Thus, a stretched string when struck not only 
 moves as a single cord, but the string may break up, as it were, into two halves, 
 each vibrating independently, but with twice the rate of movement of the 
 whole length of string. Not only is this the case, but the string in its vibra- 
 tion also breaks up into chords of one-third, one-fourth, one-fifth, etc. of its 
 original length, giving rise to vibrations three, four, and five times as rapid as 
 those produced by the whole string. In musical phrase, the middle c of the 
 piano, when this key is struck, gives not only a note c representing 132 vibra- 
 tions, but also its octave c f of 264 vibrations, the fifth above this of 396 
 vibrations, the second octave, 528, the third above this, 660, and so on. The 
 vibration of a string, then, sends to the ear a complex series of tones each of 
 which represents a simple pendular motion of the air. The lowest tone, that 
 produced by the slowest rate of vibration of the string as a whole, is known 
 as the fundamental tone. 
 
 The pitch of the fundamental tone determines our estimate of the pitch 
 of the whole complex note. The other tones produced by segmental vibration 
 of the string are known as partial tones, upper partials, or^-overtones The 
 fundamental tone is usually stronger than its accompanying Wertoties, the 
 successively higher upper partials diminishing rapidly in intensity. Some 
 musical instruments produce notes with a longer series of overtones than do 
 others ; the human voice is particularly rich in overtones. Instruments differ 
 also in the greater or lesser strength and in the relative prominence of the 
 individual overtones accompanying the fundamental. It is the number and the 
 relative prominence of the overtones in a musical note that determine its quality. 
 Thus, a violin, a cornet, and a piano, though sounding a note of the same 
 pitch, would never be mistaken the one for the other ; our discrimination of 
 their notes depends simply upon the difference in the relative strength and the 
 number of their overtones, the fundamental tone being the same throughout. 
 The brilliancy and richness of musical notes is dependent on their wealth of 
 upper partials. It is believed that a sound-producing body, like a stretched 
 string, does not send to the ear a separate set of waves representing each of its 
 segmental vibrations, but that all the waves aroused by it fuse together into 
 a single series of waves of peculiar form. Such a composite wave may be 
 1 W. James : Psychology, 1890, vol ii. p. 140. 
 
384 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 represented graphically by depicting under one another a series of waves having 
 two, three, four, etc. times the rate of succession of the curve indicating the 
 fundamental tone. If a vertical line be drawn across the series representing 
 the vibration-rates of the various tones, and an algebraic addition be made of 
 the distance of each point of intersection above or below the line of rest, the 
 result will determine the position of the composite curve on the same vertical 
 (Fig. 196). It is evident that the form of the composite wave must change 
 with every change in the number and relative prominence of musical overtones, 
 and the movement imparted by it to the tympanic membrane and the wave 
 
 FIG. 196. The curve B represents twice the vibration-rate of A. When the two curves are combined 
 by the algebraic addition of their ordinates, the result is the periodic curve c (solid line), having a dif- 
 ferent form ; the dotted line of c is a reproduction of A. If B is displaced to the right until e falls under 
 d in A (change of phase), the combination of A and B will give the curve D, the dotted line in D repre- 
 senting A as before. (After Helmholtz.) 
 
 generated in the perilymph must have corresponding differences. Notes of 
 different quality are produced by composite air-waves of different forms. But 
 waves differing in form may still produce notes of the same quality ; for if, 
 in the graphical figure, one or more of the curves representing simple tones 
 be slid to the right or the left, the form of the composite wave will thereby be 
 changed, but not the quality of the sound produced by it. In other words, 
 change of phase of the partial tones does not alter the quality of the note. 1 
 The quality of any complex note may be reproduced by sounding together 
 a series of tuning-forks which have, respectively, the vibration-rate of the 
 fundamental tone and that of one of the overtones of the complex note. 
 
 Analysis of Composite Tones by the Bar. According to the theory 
 outlined on page 380, the composite wave, beating against the sensitive organ 
 of the cochlea, is again analyzed into the elements composing it, one part of 
 the basilar membrane vibrating sympathetically with one partial tone, another 
 with another. The isolated irritation of each nerve-element arouses in the 
 mind the idea of a tone of a certain pitch and loudness ; but when a number 
 
 1 Helmholtz, op. cit., pp. 30-34. 
 
THE SENSE OF HEARING. :isr, 
 
 of such elements are simultaneously stimulated, the mind takes note, not of the 
 individual sensations thereby aroused, but of a resultant sensation formed by 
 the fusion of these. 
 
 That apparently simple tones are actually made up of a number of partials, 
 having rates of vibration which form simple multiples of the fundamental 
 tone, may easily be demonstrated at the open piano. If any note, as c in the 
 bass clef, be struck while the key of its octave c is depressed, and then the 
 struck string be damped, it will be found that the octave c rings out with its 
 proper note. So in turn the g above that, the second octave and the e above 
 that, may be made to sound when the lower c is struck, because each of these 
 strings is so tuned that its fundamental note has the same vibration-rate as 
 one of the overtones of the lower c. A note sung near the piano may in 
 the same way be analyzed more or less completely into its component tones. 
 The organ of hearing certainly has some such power of musical analysis, for 
 some cultivated ears can not only follow any special instrument in a play- 
 ing orchestra, but can even distinguish the overtones in a single musical 
 note. 
 
 The ear has little or no power of distinguishing difference of pitch in tones 
 of less than 40 or more than 4000 vibrations per second ; but in the upper 
 median parts of the musical scale the sensitiveness to change of pitch is very 
 acute. Thus, according to Preyer, 1 in the double-accented octave a difference 
 of pitch of one-half vibration in a second can be detected ; that is, in the 
 octave included between 500 and 1000 vibrations per second, 1000 degrees of 
 pitch can be perceived. 
 
 Every elastic body is capable of sympathetic vibration ; that is, air- waves 
 beating upon it at its own natural rate of vibration set it into corresponding 
 motion. In the same manner a heavy pendulum may be forced into violent 
 movement by exceedingly light taps with the finger, the only necessary condi- 
 tion being that the impulses imparted by the finger be exactly timed to the 
 periodic motion of the pendulum or to some multiple of it. A body capable 
 of sympathetic vibration with some particular tone is set into vibration by that 
 tone, and reinforces or magnifies it, whether the tone exists alone or as the 
 fundamental of a complex note, or is contained in the latter simply as an 
 upper partial. 
 
 The analysis of musical sounds is usually carried out by the use of resona- 
 tors, which are hollow cylinders or spheres of glass or of metal, rather widely 
 open at one pole, and narrow-pointed at the opposite end for insertion into 
 the ear. The mass of enclosed air vibrates, according to its size and shape, 
 at some particular rate, and it is very readily set into sympathetic vibration 
 whenever its fundamental tone is contained in any sound reaching it. By this 
 means it is possible strongly to magnify, and thus select, the individual over- 
 tones contained in a note. The vowel sounds of human speech owe their 
 difference of quality to the adjustment in size and shape of the resonant air- 
 chambers above the vocal cords. 
 
 1 Ueber die Grenzen der Tonwahrnehmung, June, 1876. 
 VOL. II. 25 
 
386 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 Inharmonic Overtones. It will be remembered that all the overtones con- 
 tained in a musical note are produced by vibrations which are simple multiples 
 of the rate of the fundamental tone. These overtones are properly called 
 harmonic upper partials ; they are, according to Helmholtz, particularly charac- 
 teristic of stretched strings and narrow organ-pipes. But most elastic bodies 
 have proper tones which are not exact multiples of the fundamental, and 
 which may be termed inharmonic upper partials. The high-pitched jingle 
 heard when a tuning-fork is first struck represents the inharmonic upper par- 
 tials of the fork. Stretched membranes have a great number of such inhar- 
 monic overtones. Inharmonic upper partials, as might be expected, rapidly 
 die out in a note of which they form a part. It is evident that inharmonic 
 proper tones, when nearly of the same pitch, must interfere with one another 
 and repress the development of a well-marked fundamental tone. 
 
 Production of Beats. When two tones of slightly different pitch are 
 sounded together, the more rapid vibrations overtake the slower, so that at 
 certain periods the crests, or phases of condensation, of two waves fall together, 
 and the result is a phase of increased condensation and louder sound. The 
 waves immediately cease to correspond, and diverge more and more until the 
 crest of one falls upon the trough of another, the result being silence, or at 
 least great diminution in the intensity of the sound. Such alternate augmenta- 
 tion and diminution of the waves give rise to pulses in the sound, known 
 technically as beats. This is one of the most familiar and important phenom- 
 ena of musical art. If two tuning-forks on resonance-boxes vibrate in unison, 
 a piece of wax stuck to the prong of one fork will lower its tone and give rise 
 to beats. The undulating sound caused by striking a bell or the rim of a thin 
 glass tumbler is due to beats. When two notes not included in a perfect chord 
 are sounded on the piano, beats are heard not only from the interference of the 
 fundamental tones, but of the upper partials as well. It is the absence of beats 
 in notes which should be in harmony, as those of the major chord, that deter- 
 mines the instrument to be in tune. When two tones produce beats, the 
 number of beats in a given time is equal to the difference between the number 
 of vibrations involved in the two tones in the same time. For example, a tone 
 produced by 256 vibrations in a second sounded with one of 228 vibrations 
 would give 28 beats in a second. It is evident that the frequency of beats 
 may be increased either by increasing the interval between the tones or by 
 striking tones of the same interval in a higher part of the scale. Beats which are 
 not too frequent from four to six in a second have important musical value, 
 but when they number thirty or forty in a second they become exceedingly dis- 
 agreeable, irritating the ear in a manner analogous to the effect of a flickering 
 light on the eye. When sufficiently near together the beats no longer produce 
 an intermittent sensation. The number of beats in a second required to result 
 in this fusion increases as we ascend the musical scale, varying from 16 beats 
 at c of 64 vibrations per second to 136 beats at c'" of 1024 vibrations. 1 The 
 reason for this variation lies in the progressive shortening of the waves as the 
 
 1 Mayer: Sound, 1891. 
 
THE SENSE OF HEARING. 387 
 
 sound becomes higher in pitch; for it is obvious that as we ascend the scale, 
 and the waves of sound become progressively shorter, spaces would be left 
 between the individual waves unless their number were proportionately- 
 increased. 
 
 Harmony and Discord. Tones are concordant, or harmonize, when they 
 produce no beats on being sounded together ; they are discordant when beats 
 are produced, and the painful sense of dissonance increases in intensity up to 
 about 33 beats per second. Perfect concord is obtained by blending notes 
 whose vibrations are to each other as small whole numbers. 
 
 Thus, in the major cord c E G c 
 
 the vibration-numbers are 132 165 198 264 
 
 their ratios are 4568 
 
 If notes the ratios of whose vibration-rates can be represented only by large 
 whole numbers are combined, a discord is formed, for the reason that their 
 upper partials interfere with one another and cause beats ; there is no especial 
 virtue in the small integer. 1 
 
 Thus, in the discord C D E 
 
 the vibration-numbers are 132 148.5 165 
 
 which are not reducible to small whole numbers. 2 
 
 Combinational Tones. When two tones are sounded together, there is 
 produced a new, usually weaker, tone, whose vibration-number is the numerical 
 difference between the vibration-rates of the original tones. It is therefore 
 known as a differential tone. Such tones may arise from upper partials as well 
 as from the fundamentals ; they do not appear to be formed, as might be sup- 
 posed, by the fusion of beats. Other " combinational " tones of more intricate 
 relations, as well as beats, arise from the interaction of vibrations when many 
 different notes, as those of an orchestra, are sounded together. To calculate 
 the physical result of the combination of these impulses, which it is the duty 
 of the tympanic membrane to transmit, is a problem of exceeding complexity. 
 
 Resume. To sum up the subject, musical sounds are distinguished in sen- 
 sation by the three factors, loudness, pitch, and quality, sometimes called color 
 or timbre. These sensations depend in turn on definite physical characters of 
 air-waves : their amplitude, or the extent of motion of the air-molecules ; their 
 frequency, or rate of succession of the waves; their form, which is deter- 
 mined by the pitch and relative predominance of the upper partials combined 
 with the fundamental tone. 
 
 Fatigue. That the ear is subject to fatigue toward a note that has been 
 sounded is easily demonstrated in the following way : Strike a single note of, 
 say, a major chord on the piano, and immediately afterward sound the full 
 chord; the quality of the latter will be altered from its normal character, 
 owing to the lessened prominence of the note which had been struck. 3 We 
 may therefore not improperly speak of a successive contrast in auditory sensa- 
 
 1 Tyndall : Sound. 2 Waller : Human Physiology, 1891. 
 
 3 Foster : Text-book of Physiology, 5th ed., 1891. 
 
388 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 tions, analogous to visual successive contrast, by which our perception of every 
 sound is colored by the sounds which have preceded it. 
 
 Imperfections of the Ear. Notwithstanding the mechanical provisions 
 for making the external and middle ear a perfect transmitting apparatus, 
 sound-perception is more or less modified by the action of the mechanism 
 under certain conditions. Thus, Helmholtz believed that various combina- 
 tional tones owe their origin chiefly to a periodic clicking in the joint between 
 the malleus and incus bones. The resonance of the ear is a familiar fact, 
 and through it high-pitched tones between e"" and g"" are reinforced and 
 heard with undue loudness. Certain hissing sounds, the chirp of a cricket or 
 the note of a locust, thus gain their intensity. This resonance probably is a 
 feature of the external auditory meatus, since it is at once destroyed by apply- 
 ing a small resonator to the ear (Helmholtz). 
 
 Perception of Time Intervals. The ear is eminently the sense apparatus 
 for determining small intervals of time. Flashes of light succeeding each 
 other at the rate of twenty-four in a second are fused in a continuous luminous 
 impression by the eye, but by the ear at least one hundred and thirty-two audi- 
 tory impulses as beats may be heard separately in a second. The power which 
 the ear possesses of resolving complex air-waves into the host of pendular 
 vibrations which may enter into their formation finds no analogy in the eye 
 (Helmholtz). 
 
 Musical Tones and Noises. The important feature of the physical 
 processes which give rise to musical tones is their periodicity. Every musical 
 tone is produced by a regular succession of alternate rarefactions and condensa- 
 tions in the air. The remaining class of sounds, known as noises, differs from 
 musical sounds in the respect that such sounds are produced by an irregular 
 succession of air- waves one in which the interval between phases of conden- 
 sation and rarefaction does not remain constant as in a musical note. Noises 
 are for the most part made up of short musical notes so associated as not to 
 " harmonize " with one another. As expressed by Helmholtz, the sensation 
 of a musical tone is due to a rapid periodic motion of a sonorous body ; the 
 sensation of a noise, to non-periodic motions. 
 
 Functions of Different Parts of the Ear. Concerning the functions of 
 the different parts of the internal ear in their relation to sound-perception, it 
 is generally believed, as previously stated, that the basilar membrane of the 
 cochlea, with the nervous elements seated on it, is the organ concerned in the 
 reception and transmission of musical sounds. There are a sufficient number 
 of fibres in the basilar membrane to allow several to vibrate with every 
 audible tone. 
 
 It cannot, however, too strongly be impressed that no theory of physiolog- 
 ical action should be accepted definitively without rigid experimental proof, and 
 such evidence concerning the definite functions of the cochlea is almost wholly 
 wanting. The sensory hair-cells on the macula of the saccule and the utricle 
 have been thought to have the duty of vibrating in response to any agitation 
 imparted to the perilymph, without regard to its periodic character; they 
 
THE SENSE OF HEARING. 389 
 
 might thus be termed sense organs for the perception of noises. Evidence 
 will be adduced later (p. 407) for the belief that they are peripheral organs 
 for the preservation of static equilibrium. 
 
 The hair-cells on the cristae of the ampullae of the semicircular canals seem 
 to have a special function in giving rise to sensations caused by changing the 
 position of the head ; they thus are organs concerned with the preservation of 
 the equilibrium of the body. 
 
 Judgment of Direction and Distance. The distance and direction from 
 which sounds come to the ear are not perceived directly, but our estimate of 
 them is a judgment based on the loudness and quality of the sound sensation, 
 combined with a power of reasoning from past experience. Thus, in seeking to 
 discover the direction whence a sound comes, it is usual for an observer to turn 
 
 FIG. 197,-End-bulbs from human conjunctiva (from Quain, after Longworth) : A, ramification of nerve- 
 fibres in the mucous membrane, and their termination in end-bulbs, as seen with a lens ; B, end-bulb, 
 highly magnified; a, nucleated capsule ; b, core, the outlines of its component cells not seen; c, entering 
 nerve-fibre branching, its two divisions to end in the bulb at d. 
 
 the head to the position in which the sound is heard loudest, and thus to form 
 an opinion as to the direction whence it comes. Errors of judgment as to the 
 direction are frequent, owing to the sound reflected from some object appearing 
 louder than that coming in a direct line from its source. It is said that when 
 there is total deafness in one ear every sound seems to have its origin on the 
 side of the healthy ear. When the eyes are closed, sounds originating in 
 the median plane of the head are very imperfectly localized, but tend to be 
 projected upward, and somewhat in front, since this is the space from which 
 most sounds come to us. 1 The quality as well as the loudness of a sound varies 
 according to the distance of its source. Thus the lower tones die away earliest 
 as a sound recedes, bringing the overtones into undue prominence. The art of 
 
 1 Seashore: Loc. tit. 
 
390 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 the ventriloquist consists largely in altering the quality of the sounds he pro- 
 duces to imitate the quality they would naturally have if arising under the 
 conditions which he would lead his hearers to believe to be their origin. A 
 comparatively feeble sound near at hand may have the same quality as a loud 
 one heard at a distance ; thus, a frog croaking in an adjoining room was once 
 mistaken by the writer for a large dog barking outside the building. 
 
 D. CUTANEOUS AND MUSCULAR SENSATIONS. 
 
 General Importance of the Cutaneous and Muscular Sensations. 
 Cutaneous sensations are aroused by the operation of some form of energy on 
 the skin, and they include the sensations of touchy of temperature, and of 
 
 pain. By muscular sensation is meant the ap- 
 preciation which we have of the intensity and 
 direction of muscular effort. Closely allied to 
 this sensation is a general sensibility through which 
 we gain a knowledge of the relative position of 
 the parts of our bodies, irrespective of movements. 
 The direction, size, distance, and surface features 
 of external objects are usually made known to us 
 through the sense of sight or of hearing. Yet these 
 fundamental facts regarding the things about us 
 do not become a part of knowledge through direct 
 visual and auditory perception. Such knowledge 
 is based on complex judgments concerning the 
 meaning of auditory and visual phenomena ac- 
 cording as they have, in past experience, been 
 interpreted by tactile and muscular perceptions. 
 That is, when reduced to its simplest terms, out- 
 most practical and important knowledge of the 
 world is the outgrowth of tactile and muscular 
 perceptions ; by and with them all other sense- 
 perceptions of objects have been corrected and compared. Thus, so simple 
 a feat as the estimate of the size of a distant object is the result of 
 
 FIG. 198. Tactile corpuscle 
 within a papilla of the skin of 
 the hand (from Quain, after Ran- 
 vier) : n, n, two nerve-fibres pass- 
 ing to the corpuscle ; o, a, ter- 
 minal varicose ramifications of 
 the axis-cylinder within the cor- 
 puscle. 
 
 FIG. 199. Semi-schematic figure of a neuromuscular spindle of the first type, namely, with complex 
 nerve-ending ; adult cat : c., capsule ; m. n. &., motor nerve-bundle ; pi. e., plate-ending; n. tr., nerve-trunk ; 
 pr. e., primary ending ; s.e., secondary ending ; 6. w., axial muscle-fibres. (From Ruffini. Journal of Physi- 
 ology, vol. xxiii.) 
 
 a complex judgment based on tactile and muscular experience. Through 
 the sense of sight we perceive the ratio of the visual angle subtended by 
 
CUTANEOUS AND MUSCULAR SENSATIONS. 
 
 391 
 
 the object to that of the whole Held of vision ; but as objects of different 
 size may fill the same visual angle when at different distances from the eye, 
 our estimate of their size depends upon the distance at which we suppose 
 them to be situated. The distinctness of the surface features of the body 
 afford the mind an important clue, since experience shows that details of 
 sin-face in a body become more obscure as we recede from that body. But 
 more important data concerning distance come from the sense of muscular 
 innervatiou, or feeling of the intensity of muscular contraction, by which we 
 estimate the degree of convergence of the optic 
 axes when the object is focussed, and still more by 
 the perception of the amount of muscular effort 
 necessary to sweep the optic axes over the ground 
 surface intervening between the observer and the 
 object. When objects approach the near-point of 
 vision the sense of innervation of the pupillary 
 muscles affords important evidence of their distance. 
 
 That fundamental education concerning the outer 
 world which engages the earliest years of every child 
 consists in accumulating and systematizing with 
 other sense-perceptions tactile and muscular im- 
 pressions of objects. A sensation is no sooner felt 
 than some muscular movement involving a definite 
 muscular feeling is made by which the character of 
 the sensation is changed and experimentally tested 
 under different conditions. The physiological pro- 
 cess involved in building up sense-knowledge, there- 
 fore, embraces in alternation sensation excited by 
 external objects, motion accompanied by muscular 
 sensation, and change in the original sensation. In 
 other words, the motor and sensory impulses form a 
 sort of balance, and both are necessary. 
 
 Ending- of Sensory Nerve-fibres in the Skin. 
 The afferent nerves supplied to the skin have several 
 modes of termination. In the commonest form the 
 plexus of medullated nerve-fibres found in the dermis 
 close under the epidermis gives off twigs which, losing the medullary sheath, 
 pierce the epidermis and here form a network among the cells of the Mal- 
 pighian layer, the single fibres ending freely in this position (Fig. 207). So 
 numerous are they that it would appear that every epithelial cell (of mucous 
 membrane as well as skin) is in contact with one or more nerve-fibrils. These 
 axis-cylinder threads are often varicose and usually end freely among the 
 epithelial cells, but in some cases they are expanded at their terminations 
 into well-defined sensory end-plates. In the corium and subcutaneous con- 
 nective tissue (mesoblastic) sensory nerves may terminate in the manner just 
 described. But they are frequently modified into or form a part of definite 
 
 FIG. 200. -Magnified view of a 
 Pacinian body from the cat's 
 mesentery (from Quain, after 
 Ranvier): n, stalk with nerve- 
 fibre enclosed in sheath of Henle, 
 passing to the corpuscle ; n', its 
 continuation through the coil, ra, 
 as a pale fibre ; a, termination of 
 the nerve in the distal end of the 
 core (the terminations are not 
 always arborescent); rf, lines 
 separating the tunics of the cor- 
 puscles; /, channel through the 
 tunics, traversed by the nerve- 
 fibre ; c, external tunics of the 
 corpuscle. 
 
392 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 structures of various forms, which may be regarded as peripheral sense-organs 
 of the skin. 1 Some of these terminal organs are known respectively as end- 
 bulbs, touch-corpuscles, and Pacinian bodies (Figs. 197-200). Each organ 
 consists of a more or less conical body in which a nerve-fibre terminates. 
 The end-bulbs are found only on the dermis of the conjunctiva and the lips, 
 and in modified form on the sensitive surfaces of the genital organs (Fig. 
 197). The touch-corpuscles, though apparently absent from the greater part 
 of the body, occur in great numbers in the skin of the palmar surface of the 
 hand and that of the fingers, especially at their tips ; at the edge of the eye- 
 lids and the lips ; on the soles of the feet and the toes ; and on the surface 
 of the genital organs. The touch- corpuscle often occupies a papilla of the 
 dermis directly under the epidermis (Fig. 198).. The Pacinian bodies, which 
 are oval corpuscles, larger than the foregoing, and easily visible to the 
 unaided eye, are found not in the skin proper, but in the subcutaneous con- 
 nective tissue beneath it. They are found in abundance beneath the skin of 
 the alm of the hand and the sole of the foot ; they are also numerous along 
 th/nerves of the joints, and even among the sympathetic nerves supplying 
 th* abdominal organs (Fig. 200). " Ruffini's endings, found in the subcu- 
 taneous tissue of the finger, are formed by the branching and anastomosis of 
 terminal axis-cylinders inclosed within a special connective-tissue envelope. 
 Various other modifications of sensory nerve termination have been described. 
 
 1. Sense of Touch. The Relations between Sensation and Stimulus. 
 Many so-called " tactile sensations," such as wetness, hardness, roughness, etc., 
 are not simple sensations at all, but are complex judgments built up out of the 
 association of certain tactile, temperature, and muscular sensations, and con- 
 veying to us a knowledge of the surface, substance, and form of bodies. 
 
 When analyzed, the sense of touch is nothing more than a sense of pressure 
 applied to the skin. To test the pressure sensibility of the skin the object 
 whose weight is to be estimated must not be lifted in the ordinary way, for 
 that would bring into play the muscular sensations. If the skin of the hand 
 is to be tested, the hand must be placed upon some firm support, such as a 
 table, and the weights be laid upon the skin. The smallest perceptible weight 
 that can thus be felt varies with the situation to which it is applied. Thus, 
 the greatest sensitiveness to pressure is found on the forehead, the temples, 
 the back of the hand, and the forearm, where a weight of .002 gram (-^ 
 grain) can be perceived. The weight must be increased to .005 to .015 gram 
 to be felt by the fingers, and to 1.0 gram when laid on the finger-nail. 2 
 
 The power of discriminating differences of pressure applied to the skin is 
 tested by finding the smallest increase that must be added to a weight in order 
 that it may be perceived as being heavier. This increment is not, as might 
 be supposed, the same for weights of different value, but it bears a distinct 
 proportion to them. Thus, a weight of 11 grains may just be perceptibly 
 heavier than one of 10 grains; but if we start with a weight of 100 grains, 
 
 1 Of. Barker : The Nervous System, 1899, pp. 361-421. 
 
 2 Aubert und Kammler : Moleschott's Untersuchungen, 1859, Bd. v. S. 145. 
 
THE SENSE OF PRESSURE. 393 
 
 a single grain added to it will arouse no difference of sensation, an increment 
 of 10 grains being necessary in order that one weight may appear heavier 
 than the other. This fact is the basis for Weber's law of the relation between 
 stimulus and sensation; this law may be formulated as follows: T7ie amount 
 of stimulus necessary to provoke a perceptible increase of sensation always bears 
 the same ratio to the amount of stimulus already applied. This law is found 
 to be only approximately correct, especially when very small and very large 
 weights are compared. Fechner attempted to express more exactly the relation 
 between the intensity of stimulus and sensation in his "psycho-physical law," 
 thus : T/ie intensity of sensation varies with the logarithm of the stimulus. In 
 other words, the sensation increases in arithmetical progression, while the 
 stimulus increases in geometrical progression. With moderate weights a 
 difference of pressure is perceptible when the ratio of increase is smaller than 
 when either very small or very large weights are used ; that is, sensitiveness 
 to pressure-change is keenest under moderate stimulation. 
 
 It is said that the forehead, the lips, and the temples appreciate an increase 
 of ^5- to ^5- of the weight estimated, while the skin of the head, the fingers, 
 and the forearm requires an increase of fo to -fa for its perception. In this 
 as in other kinds of sensation it is the difference, or variation of intensity, of 
 the sensation of which the mind takes particular cognizance. One touch- 
 sensation is more acutely perceived when contrasted with another than when 
 felt alone. Weber 1 found the discrimination of pressure-differences to be 
 finer when two weights were laid in rapid succession on the same skin-area 
 than when the weights were applied either simultaneously or successively to 
 different parts. If a finger be dipped in a cup of mercury or of water having 
 the same temperature as the skin, the pressure will be marked only at the 
 margin between the air and the fluid, and if the finger be moved up and 
 down it will seem as if a ring were being slid back and forth upon it. The 
 constant pressure of the mercury upon the submerged finger is not felt. The 
 fingers are particularly sensitive to intermittent variations of pressure a 
 facility the use of which is manifest when the function of these parts is 
 considered. 
 
 Two weights, in being tested, should press upon equal areas of skin ; 
 according to Weber, 2 if two equal weights have different superficial expanse, 
 that which touches the larger skin-surface, and thereby excites the greater 
 number of touch-nerves, will appear to be the heavier. The important part 
 played by judgment and mental inference in such experiments is shown by 
 the facts that when it is sought to compare weights by lifting them and with 
 the aid of sight, the smaller of two equal weights seems to be the heavier ; 
 and of two objects having the same size and weight, that which appears to 
 be the smaller seems heavier. 3 The simultaneous excitement of other sen- 
 sations may modify that of pressure ; thus, when two coins of equal weight, 
 
 1 '' Tastsinn und Gemeingefuhl," Wagner's Handworterbuch der Physiologic, 1846. 
 
 2 Quoted in Hermann's Handbuch der Physiologic, Bd. iii. 2, S. 336. 
 
 3 Dressier : American Journal of Psychology, 1894, vol. vi. No. 3. 
 
394 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 but one warm and the other cold, are laid upon the hand or the forehead, 
 the cold one appears to be much the heavier. 
 
 There is a sensation of after-pressure depending for its strength on the 
 amount of the weight and the length of time this weight has been applied. 
 In fact, this after-sensation may produce a striking eifect on consciousness, 
 a familiar example of which is the persistence of the sense of pressure of 
 the hat-band after the head-covering is removed. Even light weights leave 
 an after-sensation, and, in order to be perceived as separate, must be applied at 
 intervals of not less than -^$ to g-j-^- of a second. It is said that when the 
 finger is applied to the rim of a rotating wheel provided with blunt teeth, the 
 separate teeth are no longer felt, and the margin seems smooth, when the con- 
 tacts succeed each other at the rate of 500 to 600 in a second. 1 Vibrations of 
 a string cease to be appreciated by the finger when they have a rate of between 
 1500 and 1600 per second. 
 
 The Localization of Touch-sensation. When a touch-sensation is felt, the 
 mind inevitably refers the irritation to some particular part of the surface 
 of the body, and the sensation seems to be localized in this area. On the 
 accurate localization of tactile sensations depends not only the safety of the 
 individual, but also the performance of the ordinary acts of life. 
 
 We may suppose that to each area of peripheral distribution of tactile 
 nerve-fibres in the skin there corresponds an area of tactile nerve-cells in the 
 brain. It can hardly be doubted that the nerve-cells are divided into physio- 
 logical groups characterized by inherent and inborn quality-differences in the 
 sensations aroused by their respective excitements. The reference of the sen- 
 sations aroused by the excitement of definite nerve-cells to definite parts of the 
 periphery is a power acquired through the physiological experiences of the 
 earliest months of life. Through the sense of sight the seat of irritation is 
 recognized, and through muscular sensation its relation to surrounding parts 
 is experimentally explored, so that cumulative harmonious experiences of tactile, 
 visual, and muscular sensations finally bring into correspondence the various 
 areas with definite varieties of touch-sensation, or, to use an expression of 
 Lotze's, 2 every area of the skin acquires a " local sign " by which it 'is dis- 
 tinguished in consciousness. 
 
 This power of localization differs widely for different parts of the skin. 
 The fineness of the localizing sense for any skin-area is easily estimated by 
 determining how far apart the tips of a pair of compasses, applied to the skin, 
 must be separated in order to be felt as two. For this experiment the compass- 
 points must be smooth, and they should not be applied heavily. The general 
 result of such an inquiry is that the compass-points may be nearer together, 
 and still be distinguished as two, in proportion as the surfaces to which they 
 are applied have greater mobility. Since it is just such parts of the body as 
 the tips of the tongue and the fingers that are chiefly used in determining the 
 position of objects, the advantage of such an arrangement is obvious. The 
 
 1 Landois and Stirling : Human Physiology, 1886. 
 
 2 Funke, in Hermann's Handbuch der Physiologic, Bd. iii. 2, S. 404. 
 
THE SENSE OF PRESSURE. 395 
 
 skin can thus be marked out in areas (tactile area,?), within each of which the 
 compass-points are felt as a single object, but if they are separated so as to fall 
 beyond the borders of these areas, they are at once perceived to be two. 
 
 The following figures 1 represent the distances at which the compass-points 
 can just be distinguished as double when applied to various parts of the body : 
 
 Tip of tongue 1.1 mm. 
 
 Palm of last phalanx of finger 2.2 " 
 
 Palm of second phalanx of finger 4.4 " 
 
 Tip of nose 6.6 " 
 
 Back of second phalanx of finger 11.1 " 
 
 Back of hand 29.8 
 
 Forearm 39.6 . " 
 
 Sternum 44 " 
 
 Back 66 " 
 
 It will be observed that accuracy of localization and sensitiveness to pressure 
 find their most perfect manifestations in widely separate regions of the skin.. 
 
 Tactile areas are found to have a general oval form with the long axis 
 parallel with the long axis of the member investigated. If the compass-points, 
 separated, say, half an inch apart, be passed over the skin of the palm from 
 the middle of the hand to the finger-tips, the sensation will be that of a single 
 line gradually separating into two diverging lines. The result, of course, 
 depends on the compass-points passing successively through areas of finer 
 localization. If an area be marked out on a part of the skin where localiza- 
 tion is poor, within which area two points simultaneously applied appear to be 
 one, a single point moved within it is still perceived to change its place, and 
 two points successively applied may be perceived to occupy different positions. 
 The mental fusion or separation of the two compass-points cannot depend 
 altogether on their being placed over the terminal twigs of the same or of two 
 adjoining nerve-fibres, for, were this the case, the points could be discriminated 
 when separated by a very small distance across the line drawn between the 
 endings of adjoining nerve-fibres, while on either side the points would have 
 to be much more widely separated in the area of distribution of a single fibre. 
 The important factor in the mental separation of two stimulated points is, that 
 between such points there shall be found a certain number of sensory elements 
 which are unstimulated. 2 Practice in such experiments greatly increases the 
 power to localize impressions. This improvement is evidently due not to 
 the establishment of new nerves, but to a more perfect discrimination of sen- 
 sations in the nerve-centres. Dressier 3 found that after practice for four 
 weeks, the compass-points which at the beginning had to be separated 18 
 millimeters on the skin of the forearm to be distinguished, could, at the end of 
 the period, be recognized as two when only about 4 millimeters apart. Almost 
 as great an improvement of localizing power was gained by the unexercised 
 
 1 Foster's Physiology, 5th ed., 1891. 
 
 2 Weber: " Tastsinn und Gemeingefiihl," Wagner's Handworterbuch der Physiologic, 1846. 
 
 3 Dressier : Loc. cit. 
 
396 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 corresponding area of the skin of the opposite arm, but not by adjacent areas ; 
 in other words, the localizing power is central, not peripheral. Practice 
 aroused in both tactile areas a peculiar quality of sensation by which the 
 area was recognized. The improvement in localizing power is gradually lost 
 if unexercised. 
 
 Pressure-points. It has been found that if a light object, such as a lead- 
 pencil, be allowed to rest by a narrow extremity successively on different parts 
 of the skin, its weight will appear very different according to the part which 
 is touched. If the spots on which the weight appears greatest be marked with 
 ink, they will be found to have a constant position, and the skin may therefore 
 be mapped out in areas of pressure-points, which are believed to indicate the 
 place of ending of pressure-nerve filaments. The pressure-points are rela- 
 tively few in number and are principally collected about the hair-follicles. 
 
 The Importance of the End-organ. The sense of touch or pressure is a 
 special sense ; that is, any irritation conveyed to the nerve-centres in which 
 the nerves of pressure terminate gives rise to a feeling of touch, just as dis- 
 turbance in the visual or the auditory centre is recognized in consciousness as 
 a sensation of sight or of sound. The complex anatomical structures known as 
 sense-organs may be considered as instruments each of which is differentiated 
 in a manner to make it particularly irritable toward some special form of 
 energy. Thus, the retina is most sensitive to the luminiferous ether ; the organ 
 of Corti, to waves of endolymph, etc. To this differentiation of structure the 
 sensitiveness of the body to the forces of nature is chiefly due. The peripheral 
 ending of the pressure nerve, whether a naked axis-cylinder or a touch-corpus- 
 cle, is no doubt modified to be particularly irritable toward that form of energy 
 manifested in the molecular, vibration of the tissue solids, brought about by 
 contact with foreign objects. Hairs, particularly those in certain localities of 
 some animals, as the whiskers of the cat, appear to have the function of trans- 
 mitting mechanical vibrations to the nerve-endings in greater intensity than 
 could be accomplished through the skin alone. 
 
 No true sense of touch is aroused by direct irritation of a nerve-trunk or 
 exposed tissue, and touch-sensations do not arise from irritation of the internal 
 surfaces of the body. A fluid of the temperature of the body gives, when 
 swallowed, no sensation in the stomach ; when cooler or warmer than the 
 body, there is a sensation due, probably, to a transmission of temperature 
 change to the skin of the abdomen. 
 
 Touch, Illusions. Certain peculiar errors in judgment may arise when 
 tactile sensations are associated in a manner unusual in experience. Thus, in 
 an experiment said to have been devised by Aristotle, if the forefinger and 
 the middle finger be crossed, a marble rolled between their tips will appear to 
 be two marbles ; if the crossed finger-ends be applied to the tip of the nose, 
 there seems to be two noses. The illusion is due to the fact that under 
 ordinary circumstances simultaneous tactile sensations from the radial side of 
 the forefinger and the ulnar side of the middle finger are always caused by 
 
THE KKxxi: o/-- TI-:MI>I-:I;.[TURE. 397 
 
 two different objects. It is a not uncommon surgical operation to replace a 
 loss of skin on the nose by cutting a flap in the skin of the forehead, without 
 injury to the nerves, and sliding the flap round upon the nose. Touching 
 the piece of transplanted skin gives the patient the sensation of being touched, 
 not upon the nose, but upon the forehead ; after a time, however, a new fund 
 of experience is accumulated, and the sensation of contact with the transplanted 
 flap is rightly referred to the nose. Persons who have suffered amputation of 
 a lower limb often complain of cramps and other sensations in the lost toes. 
 The illusion no doubt comes from irritation, in the nerve-stump, of fibres 
 which previously bore irritations from the toes. 
 
 2. Temperature Sense. The skin is also an organ for the detection of 
 changes of temperature in the outep world. Such temperature differences prob- 
 ably make themselves manifest by raising or lowering the temperature of the 
 skin itself, and thus in some way irritating the terminal parts of certain sensory 
 nerves, the temperature nerves. The sensitiveness of the skin to temperature 
 variations is not the same in all parts; thus, it is more acute in the skin of the 
 face than in that of the hand ; in the legs and the trunk the sensibility is least. 
 We refer temperature sensations, somewhat like those of touch, to the periphery 
 of the body, and localize them on the surface. The skin over various parts 
 of the body may have different temperatures without exciting corresponding 
 local differences of sensation. Thus, the forehead and the hand usually seem 
 to be of the same temperature, but if the palm be laid upon the temples, 
 there is commonly felt a decided sensation of temperature change in one or 
 both surfaces. As in other sensations, fatigue and contrast play an important 
 part in the sense perceptions of temperature, and stimuli of rapidly-changing 
 intensity provoke the strongest sensations; thus, when two fingers are both 
 dipped into hot or cold water, the fluid seems hotter or colder to that finger 
 which is alternately raised and lowered. 
 
 In changing to a place of different temperature the skin for a time seems 
 warmer or cooler, but soon the temperature sensation declines, and on return- 
 ing to the original temperature the reverse feeling of cold or of warmth is 
 experienced. For every part of the skin, then, there is a degree of tempera- 
 ture, elevation above or depression below which arouses respectively the 
 feeling of warmth or of cold, and the temperature of the skin determining 
 the physiological null-point may vary within wide limits. 
 
 The smallest differences of temperature that can be perceived fall, for most 
 parts of the skin, within 1 C. The skin of the temples gives perception of 
 differences of 0.4-0.3 C. The surface of the arm discriminates 0.2 ; the 
 hollow of the hand, 0.5-0.4 ; the middle of the back, 1.2 . 1 
 
 The size of the sensory surface affected modifies the intensity of temperature 
 sensation : if the whole of one hand and a single finger of the other hand be 
 dipped into warm or cold water, the temperature will seem higher or lower to 
 the member having the greatest surface immersed. 
 
 1 Nothnagel : Deutschcs Arch ///,- ///// W-/K M^lid,,, 1866, ii. S. 284. 
 
398 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 FIG. 201. Cutaneous " cold " spots 
 (vertical shading) and " hot " spots 
 (horizontal shading), anterior sur- 
 face of the thigh (from Waller, after 
 Goldscheider). 
 
 Cold and Warm Points. The skin is not uniformly sensitive to tem- 
 perature changes, but its appreciation of them seems to be limited to certain 
 
 points distributed more or less thickly over the 
 surface. These spots appear to be the places of 
 termination of the temperature nerves in the epi- 
 dermis (Fig. 201). There is little doubt that there 
 are two distinct varieties of temperature nerves, 
 one of which appreciates elevation of temperature, 
 or heat, and the other diminution of temperature, 
 or cold. Thus, if a blunt-pointed metal rod be 
 warmed and be touched in succession to various 
 parts of the skin, at certain spots it will be felt as 
 very warm, while at others it will not seem warm 
 at all. If, on the contrary, the rod be cooled, a 
 series of cold points may in the same way be made 
 out. The point of an ordinary lead-pencil may 
 be used with some success to pick out the cold 
 spots. The "cold points" are more numerous 
 than the "hot," and those of each variety are 
 more or less distinctly grouped round centres, as 
 would be expected from the manner of nerve-distribution, though the groups 
 overlap to some extent (Fig. 201). Certain substances appear to act, prob- 
 ably by chemical means, as specific excitants of the two sets of nerves. 
 Thus, menthol applied to the skin gives a sensation of cold, while an atmo- 
 sphere of carbon dioxide surrounding an area of skin gives a sensation of 
 warmth. 1 
 
 The specific difference of the two sets of temperature nerves is indicated by 
 the fact that when a warm and a cold body held close together are simulta- 
 neously brought near the skin, the sensation is either one of both warmth and 
 cold, or now one and now the other sensation predominates. 2 Any stimulation, 
 whether mechanical or electrical, applied to the sensitive points thus far de- 
 scribed in the skin, for the appreciation of either pressure, heat, or coldj pro- 
 vokes, when effective, only the proper sensation of that point : any irritation 
 of a cold, hot, or pressure point gives rise, respectively, to the sensation of 
 cold, heat, or pressure alone. 
 
 As in other organs of special sense, the peripheral terminations of the 
 temperature nerves seem modified to be especially irritable toward their appro- 
 priate form of physical stimulus. Cold or heat directly applied to the nerve- 
 trunk excites no temperature sensation. Thus, if the elbow be dipped into a 
 freezing mixture, as the lowered temperature penetrates to the ulnar nerve the 
 sensation will be one, not of cold, but of dull pain, and it will be referred to 
 
 1 Goldscheider: Du Bois-Reymond's Archiv fur Physiologie, 1886, 1887 ; Blix: Zeifschrift filr 
 Biologic, 1884 ; Donaldson : Mind, 1885, vol. xxxix. 
 
 2 Czermak : Sitzungsberichte d. Wiener Akad., 1855, S. 500 ; Klug : Arb. d. physioL Anstalt zu 
 Leipzig, 1876, S. 168. 
 
Tin: ,x7-:.v,s7-; OF PAIN. 399 
 
 the hand and the fingers. The internal mucous surfaces of the body, from 
 the oesophagus to the rectum, inclusive, have no power of discriminating 
 temperature sensations ; a clyster of water cooled to from 7 to 16 C., if not 
 held too long, is only perceived as cold when the water escapes through the 
 skin of the anus. 
 
 The doctrine of specific nerve energy, enunciated by E. H. Weber, was 
 intended to convey the idea elaborated above, that each nerve of special sense, 
 however irritated, gives rise to its own peculiar quality of sensation. But it 
 seems clear that the existence and quality of the sensation are, respectively, 
 properties of the activity, not of the nerve-fibre, but of the peripheral end- 
 organ and the nerve-centres. 
 
 3. Common Sensation and Pain. The sensations thus far considered 
 have been called special sensations, because each affects the consciousness in 
 quite a different way, and any irritation which excites the sense apparatus 
 provokes a sensation of definite quality and measurable intensity. 
 
 Pain is a sensation which, according to a common but unproved belief, is 
 the result of sufficiently intensifying any of the simple sensations. 
 
 Pains have received various names to distinguish their quality, according to 
 the mode in which experience shows they may have been produced, as cutting, 
 tearing, burning, grinding, etc. One peculiar mark that distinguishes painful 
 sensations is the lack of complete localization. While lesser pains are referred 
 with fair exactness to different parts of the body, and even to those internal 
 parts devoid of tactile sensibility, greater pains radiate and seem diffused over 
 neighboring parts. Pain also differs from special sensation in the long latent 
 period preceding its development. The evidence of physiological experiment 
 is against the belief that any irritation of the nerves of so-called "special 
 senses " can produce pains, but it teaches that this sensation is the result of the 
 excessive or unnatural stimulation of a group of nerves whose function is to 
 give rise to what is indefinitely called " common sensation." By this term is 
 designated that consciousness which we more or less definitely have, at any 
 moment, of the condition and position of the various parts of our bodies. 
 When tactile, temperature, and visual sensations are eliminated, we are still 
 able to designate with considerable accuracy the position of our limbs, and we 
 become aware with extraordinary exactness of any change in that position, 
 indicating the possession of a posture sense. The nerves of common sensation 
 must, then, be continuously active in carrying to the sensorium impulses 
 which, though they do not excite distinct consciousness, probably are of the 
 utmost importance in keeping the nerve-centres informed of the relative posi- 
 tions and physiological condition of the various parts of the organism, and it 
 is not improbable that they are the afferent channels for many reflex acts 
 which tend to preserve the equilibrium of the body. The sudden failure of 
 these sensations in a part of the body would probably be felt as acutely as the 
 silence which succeeds a loud noise to which the ear has become accustomed. 
 Pain is thought to be the result of excessive stimulation of the nerves of com- 
 mon sensation, though it must be admitted that we know next to nothing 
 
400 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 of the anatomical and physiological conditions on which this sensation is 
 dependent. It is said not only that most internal organs possess no def- 
 inite tactile or thermal sensibility, but that, when normal, such irritation as 
 is caused by cutting, burning, and pinching seems to cause no pain; 1 
 let them, however, become inflamed, and their sensitiveness to pain is suf- 
 ficiently acute. The facts of labor-pains, of colic, and other visceral dis- 
 turbances which are attended by no inflammatory condition show, however, 
 that the factors on which the existence of pain depends are not as yet fully 
 understood. 
 
 The physiological facts on which is based the belief in " common sensa- 
 tion " are indisputable, but the evidence for a special nervous apparatus for 
 such sensibility is based rather on exclusion of known nerve-organs than on 
 positive demonstration. In the category of common sensations have been 
 included also such feelings as " tickling," shivering, hunger, thirst, and sexual 
 sensations. The feeling of fatigue which follows either muscular or mental 
 exertion may be placed in the same group. 
 
 A general feature of common sensations is their subjective character ; they 
 are not definitely localized within the body, nor are they projected external to 
 it, as in the case of the " special senses." 
 
 Between the common sensation and its existing cause there is no measurable 
 proportion, as is found, for instance, in the study of the pressure sense. It 
 may be stated that pressure and temperature sensations were within a recent 
 period grouped among common sensations, and future investigations may pos- 
 sibly limit each of the feelings now classed together as " common sensations " 
 to definite anatomical structures. 
 
 When the punctiform distribution of various sensations in the skin is inves- 
 tigated, some points are found in which no other sensation than that of pain 
 can be excited, and it has been thought that such spots mark the place of 
 ending of nerves of common sensibility. 
 
 According to v. Frey, the pain-points are much more numerous than the 
 pressure points, more than 100 falling within a square centimeter of skin, and 
 their nerves are probably more superficial. They require about 1000 times 
 as great an intensity of stimulus for their excitement as do the pressure- 
 nerves ; they have a long, latent period of stimulation and are inert to rapid 
 changes in the stimulus. This author believes that the free nerve-endings 
 are sense-organs for pain, the end-bulbs for cold, the terminal coils, or net- 
 works, for heat, and the tactile corpuscles for pressure-sensations. 2 
 
 Transferred or " Sympathetic " Pains ; Allochiria. It has long been a 
 matter of clinical observation that disease seated in certain internal organs is 
 often accompanied by superficial pain and tenderness in widely removed parts 
 of the body ; for example, a decayed tooth frequently causes intense pain in 
 the ear ; disease of heart or of aorta may cause pain between the shoulders, 
 
 1 Foster's Physiology, 1891, p. 1420. 
 
 2 Hermann's Jahresbericht ii. Physiologic, 1897, Bd. v. S. 115; 1896, Bd. iv. S. 113 ; 1895, 
 Bd. iii. S. 111. 
 
MUSCULAR SENSATION. 401 
 
 etc. The subject has received most accurate investigation from Head, 1 who 
 has shown that there is an intimate nervous connection between the internal 
 organs and definite areas of the skin, manifested by pain and tenderness 
 appearing in sharply-localized regions on the surface when definite organs 
 become disordered. He has also demonstrated that disorders of the thoracic 
 and abdominal viscera not only produce pain and tenderness on the surface of 
 the body, but also cause pain and tenderness over certain areas of the scalp. 
 Head is inclined to explain the topographical association of skin-tenderness 
 with visceral disorders by the assumption that the nerve-supplies of the parts 
 so related find their origin within the same segment of the spinal cord. The 
 sensory result of visceral irritation may be summarized in the following way : 
 " When a painful stimulus is applied to a part of low sensibility in dose cen- 
 tral connection with a part of much greater sensibility, the pain produced is 
 felt in the part of higher sensibility rather than in the part of lower sensibility 
 to which the stimulus was actually applied." 
 
 Certain transferred pains are explained by Meltzer 2 in the following man- 
 ner : An inflamed or irritated organ originates a succession of sensory 
 stimuli, which do not awake consciousness because they are continuous. 
 There is, nevertheless, a summation of such irritations within the central 
 organ which elevates its plane of irritability to such an extent that sensory 
 impulses reaching the implicated nerve-centre from any part of the body 
 arouse it above the threshold of consciousness and give rise to sensations 
 which are referred to the seat of peripheral inflammation or constant irrita- 
 tion. For example, the subject of a mild alveolitis may feel in the teeth a 
 stronger pain than is felt in the nose when a concentrated solution is thrown 
 into the latter organ. 
 
 That this transferred localization may characterize other sensations than 
 those of pain has been definitely observed by Obersteiner, 3 who found that in 
 patients suffering from certain central nervous lesions tactile irritation of a cer- 
 tain point on the skin was referred by them to some other part of the body, 
 usually the corresponding point on the other side. He designated this trans- 
 ference of sensation by the term allochiria, meaning a confusion of sides. 
 
 4. Muscular Sensation. Closely allied to common sensation, if not a 
 part of it, is muscular sensation. If two weights are to be compared, we 
 naturally do not lay them on the skin to determine their pressure-difference, 
 but we lift and weigh them in the hands, and experience shows that a much 
 more accurate estimate may thus be made. 
 
 We undoubtedly have a keen perception of the tension of a muscle, and 
 therefore of the amount of resistance against which it is contracting. This per- 
 ception may be the outcome of a direct consciousness of the amount of motor 
 energy sent out from the motor cells, or it may be due to the inflow of sensory 
 impulses which show the tension to which the muscles have been subjected. 
 The latter view has more to be said in its favor. 
 
 1 Brain, 1893-4. 
 
 J S. J. Meltzer : Philadelphia. Medical Journal, August 5, 1899, p. 12. 9 Brain, 1881. 
 VOL. II. 26 
 
402 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 Recent researches have demonstrated the existence of an abundant supply 
 of sensory nerves, whose excitement must depend upon the exercise of 
 jkej^iaLimiacle^. Ciaccio l has described the termination of sensory nerves 
 in tendons as a splitting up of the nerve-fibres whose axis-cylinders, in the 
 form of varicose threads, end freely as spirals or rings around the tendon- 
 bundles. The joints seem to be particularly rich in sensory nerve-supply. 
 Golgi 2 first described certain special modes of ending of sensory nerves just 
 at the junction of the voluntary muscle with its tendon. This terminal organ 
 is a fusiform corpuscle consisting of several delicate connective-tissue 
 envelopes with nuclei, and is situated on the surface of the tendon. One to 
 several nerve-fibres enter each corpuscle, and, dividing and losing their 
 medullary sheaths, break up into an arborization of naked axis-cylinders. 
 The skeletal muscles themselves are extraordinarily rich in sensory nerve- 
 supply. According to Sherrington, 3 " the proportion of afferent-fibres to total 
 myelinate fibres ranges from a little more than one-third in some muscular 
 nerves to a full half in others." These sensory fibres end, for the most part, 
 in the so-called " muscle-spindles," which are fusiform bodies, usually just 
 visible to the unaided eye (Fig. 199, p. 390). The spindles are for the most 
 part scattered between the ordinary muscle-fibres, though many abut upon 
 intramuscular septa or are in the immediate vicinity of aponeuroses. As 
 many as thirteen spindles have been counted in one cross-section of the genio- 
 glossus muscle. Sherrington 4 calculates that the number of spindle-organs 
 is sufficient to account for nearly or quite two-thirds of all the afferent fibres 
 demonstrated to exist in the nerve-trunks of the limb muscles. It is worth 
 observing that the spindle-organs have not been demonstrated in the eye 
 muscles nor in the intrinsic muscles of the tongue. The muscle-spindle con- 
 sists of a central core of modified muscle-fibres inclosed in an outer capsule 
 formed of several layers of concentrically disposed membranous lamellae 
 composed of connective tissue. Between the capsule and the central muscle- 
 bundle is a wide lymph-space traversed by a network of delicate filaments. 
 In forming the spindle two or three ordinary muscle-fibres of the red variety 
 become invested at the proximal end of the organ by a definite sheath of 
 connective tissue. As they penetrate further into this envelope the muscle- 
 fibres tend to split lengthways, each fibre giving rise to perhaps three 
 " daughter "-fibres, which are proportionally of less diameter. The striation 
 and fibrillation are frequently confined to the outer portion of these daughter- 
 fibres, some of which are devoid of sarcolemma. For the middle third of* 
 its course in the muscle-spindle each daughter-fibre becomes thickly crusted 
 with a sheet of nuclei. Toward the distal end of the spindle the muscle- 
 fibres often merge in tendon-bundles, which finally combine with the fibrous 
 tissue forming the capsule of the spindle ; so that of the two ends of the axial 
 bundle within the spindle, one is muscular and the other is tendinous. 
 
 According to Ruffini, 5 sensory nerves may end upon the axial muscle- 
 
 1 Barker : The Nervous System, 1899, p. 405. 2 Ibid. 
 
 3 Sherrington : Journal of Physiology, 1895, vol. xvii. p. 229. 4 Ibid. 
 
 6 Ruffini : Ibid., 1898, xxiii. 190. 
 
MUSCULAR SENSATION. 403 
 
 fibres of the spindle in either or all of three different modes (Fig. 199): 1. 
 The axis-cylinder may flatten out and twine in rings and spirals about the 
 muscle-fibre. 2. The axis-cylinder may break up into a number of leaflets 
 applied to the muscle-fibre (secondary mode). 3. The axis-cylinder may 
 end in a plate of varicose fibrils resembling the motor end-plate. 
 
 When we consider that it is through muscular sensation that we derive our 
 most accurate conceptions of the form, weight, and position of objects, and through 
 which we explore our own body-surface and distinguish its areas of localization ; 
 that this is the fundamental sense by which the sensations arising in most 
 other organs are tested and verified ; and that it is from the sense of muscular 
 movement that we can form ideas of time and space, it may well be regarded 
 as the mother of all sense-perceptions. Normal muscles, even when function- 
 ally inactive, are still in a state of tonic contraction ; it is not improbable that 
 this tone is a reflex action whose sensory element is formed by the impulses 
 travelling along nerves of muscular sensation. Such impulses are probably 
 indispensable to the preservation of the equilibrium of the body. 
 
 Sherrington found that if he separated the aponeurosis belonging to the 
 distal portion of the vastus medialis muscle, under which the muscle-spindles 
 are numerous, the knee-jerk could no longer be excited through the muscle. 
 
 Our appreciation of the weight of bodies is determined by lifting them. 
 But even in so simple an exercise of the muscular sense as this the judgment 
 is subject to extraordinary illusions depending on the preconception of the 
 weight of a body, and consequent muscular effort put forth in lifting it. 
 When bodies having the same weight and size, such as appropriately loaded 
 pieces of iron, cork, and wood, are compared, the specifically lighter body will 
 seem to be heavier. "Before lifting an object we normally estimate the 
 approximate weight by sight, and the effort to be exerted in lifting is adjusted 
 semi-automatically upon the basis of this preliminary estimate. If insufficient 
 effort is put forth at the beginning of the lifting, the weight of the object will 
 be overestimated. If too great effort is put forth, the weight of the object 
 will be underestimated." 1 In comparing the weight of objects having 
 different sizes the illusion takes another direction. Thus an inflated paper 
 bag may be estimated to have the same weight as a piece of lead weighing 
 sixty times as much. 2 
 
 The clinical study of disease in the central nervous system affords strong 
 evidence of the functional independence of the sense organs involved in the 
 appreciation of touch, heat, cold, and pain. In certain diseases of the spinal 
 cord, areas of skin may be mapped out in which sensations of pressure are 
 lost, but those of temperature remain, and vice versd. In other diseases the 
 patient can appreciate warmth applied to the skin, but not cold. 
 
 The sensations of cold and pressure seem to be usually lost or retained 
 together, while those of warmth and pain have a similar connection. It is a 
 peculiar fact that sometimes in the early stages of ether and chloroform narco- 
 sis the sense of touch remains while that of pain is abolished. Funke 3 refers 
 
 1 Seashore : Op. cit. 2 Wolfe : Psychological Review, 1898, p. 25. 
 
 3 " Der Tastsinn," Hennann's Handbuch der Physiologic, Bd. iii. S. 2. 
 
404 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 to two cases in which, while the tactile sense was preserved, muscular sensation 
 was lost, and an object could be held in the grasp only while the eyes were 
 turned upon it. 
 
 Hunger and T/iirst. Hunger and thirst are peculiar sensations which 
 depend partly on local and partly on general causes. Diminution in the bulk 
 of water and of circulating aliment in the body no doubt causes excitement 
 of sensory nerves on which depend the feelings of thirst and hunger, but in 
 ordinary life these feelings are dependent on the physical condition of certain 
 mucous surfaces. Any circumstance which causes drying of the lining mem- 
 brane of the mouth provokes thirst, and some condition of the empty stomach 
 arouses hunger. Thirst may be assuaged by introducing water directly into 
 the stomach through a gastric fistula, though to effect the purpose a larger 
 quantity must be employed in this way than by the mouth. Hunger in a 
 somewhat similar manner may be appeased by rectal alimentation. It seems 
 probable, however, that these sensations as usually felt are the result of a 
 sort of habit, depending on the physiological condition of the secreting and 
 absorbing mechanisms of the alimentary canal. 
 
 Clinical observation has shown that " bulimia," or voracious appetite, is 
 frequently a result of disease in certain parts of the central nervous system. 
 We are therefore justified in speaking of a "hunger-centre." 1 
 
 E. THE EQUILIBRIUM OF THE BODY; THE FUNCTION OF THE 
 SEMICIRCULAR OANALS. 
 
 The term equilibrium, as applied to the condition of the body, whether at 
 rest or in motion, indicates a state in which all the skeletal muscles are under 
 control of nerve-centres, so that they^ combine, when required, to resist the 
 effect of gravity or to execute some co-ordinated motion. The preservation 
 of equilibrium is manifestly of fundamental importance in animal life, and we 
 find, accordingly, several mechanisms sharing in this function. That the motor 
 co-ordinating centres may act properly, they must receive sensory impres- 
 sions conveying information of the relative position of the body at any given 
 moment. The sum-total of these sensations may be characterized as the sense 
 of equilibrium, and it is probably not going too far to assume that every known 
 sensation contributes to this fund of information. Thus, in ordinary life the 
 position of objects is commonly determined by the sense of sight : when one 
 tries to walk while looking through a prism, objects are not properly localized 
 by vision, and improper co-ordination results. The contact of the soles of the 
 feet with the ground, and that of the surface of the body with various objects, 
 are common sources of information as to our relation with the environment. 
 Standing upright, and still more when in motion, the muscular sense is active 
 in appreciating the tension, active or passive, of the muscles. In the erect 
 position, with eyes closed, a writing point attached to the head will show that 
 the body sways in a peculiar manner indicating successive contraction of differ- 
 ent groups of muscles ; and a person with failure of muscular and tactile sen- 
 
 \ Ewald : Diseases of the Stomach, p. 397. 
 
THE SENSE OF EQUILIBRIUM. 405 
 
 sibility, as in locomotor ataxy, cannot stand with eyes closed, and his move- 
 ments, even when sight is employed, are exaggerated and unnatural. Attention 
 has previously been called to the fact that air-waves, irrespective of those 
 producing sound-sensations, exert an influence upon the tympanic membrane 
 by which we are capable of appreciating the presence and, to some extent, the 
 physical character of objects. Whether this sensation involves the nerves of 
 touch, those of common sensibility, or those distributed to the internal ear, is 
 uncertain. 
 
 In the absence of any of these sensations the loss may be made up by more 
 perfect development of others. Ordinarily, the sensory information from all 
 these sources, when compared in consciousness, harmonizes and gives rise to 
 a concrete idea of position. Frequently, however, one of the sources of sense- 
 impression suddenly fails us or its testimony conflicts with that of other sense 
 organs ; the result is disturbance of equilibrium. A very common outcome 
 of this conflict of sensations is dizziness or nausea. The distress arising from 
 wearing ill-fitting glasses and the sensations experienced when one looks down 
 from a high eminence are examples in point. Internal disorders exciting nerves 
 of common sensation have the same effect, though the relation borne by visceral 
 sensations to equilibrium is very ill known. A false idea of position of the 
 body, a sense of falling in one direction or another, may lead to sudden effort 
 of recovery by which the person is precipitated to the opposite side. Thus, 
 when looking at rapidly-moving water erroneous ideas of equilibrium are 
 gained through the visual sense, and there is a strong tendency for the body 
 to precipitate itself in one direction or another. When, in going up a stair- 
 case, one miscalculates the number of steps, a peculiar sensation of want of 
 equilibrium is aroused through the muscular sense. It is clear, then, that- 
 the sense of equilibrium is served by various sense organs, and a complete 
 discussion of this function would entail a consideration of the whole field of 
 nerve-muscle physiology. There is, however, good reason for believing that 
 there is a special sense organ for determining the position and direction of 
 movement of the head and, by inference, of the whole body. The terminal 
 organ of this sense apparatus of equilibrium is found in the system of semi- 
 circular canals of the internal ear. 
 
 Experiments on the lower animals, chiefly performed on birds, show a con- 
 stant motor disturbance to follow division of any or all of the semicircular 
 canals. These disturbances are of two 'kinds. When the animal is at rest it 
 does not stand in a natural fashion, but sprawls in a more or less exaggerated 
 degree. It holds its head in an unnatural position, as with the vertex touch- 
 ing the back, or with the beak turned down toward the legs or bent over to 
 one side. Immediately after the operation, and whenever it is disturbed, the 
 animal goes through peculiar forced movements, together with rolling or 
 twitching of the eyes, of various kinds and degrees of violence, depending on 
 the position and number of canals severed. The disturbance varies from 
 simple unsteadiness in gait, with swaying motions of the head, to complete 
 lack of co-ordination and a violence of movement almost comparable to that 
 
406 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 of a chicken whose head has been cut off. Essentially the same results have 
 been determined to follow injury of the semicircular canals of widely different 
 groups of animals. 
 
 These results have been explained by the assumption that the hair-cells on 
 the cristce acusticce of the ampullae of the semicircular canals are irritated by 
 increase or decrease of pressure of the endolymph upon them, and thus give 
 rise to sensory impressions from which ideas of change of position are derived. 
 Section of the canal, by draining off the endolymph, would cause abnormal 
 pressure-irritation. The anatomical relations of the semicircular canals afford 
 an obvious basis for this view, for the canals of each ear are almost exactly at 
 right angles to one another, occupying the three planes of space ; considering 
 the two ears, the horizontal canals are nearly in the same plane, and the ante- 
 rior vertical canal of one side is nearly parallel with the posterior vertical canal 
 of the other side. Any possible movement of the head would thus produce 
 an increase of endolymph-pressure upon the hair-cells in one ampulla and a 
 decrease of pressure in the ampulla of the parallel canal, and every change of 
 position would be accompanied by the irritation of definite ampulla with defi- 
 nite degrees of excitement (Fig. 202). Experiments on man afford considerable 
 
 support to this theory of the function of the 
 semicircular canals. A person with eyes closed 
 and with muscular and tactile sensations elimi- 
 nated, supported on a table which can be rotated 
 in all directions, can determine with consider- 
 able accuracy not only that he is moved, but 
 in what direction and, to some extent, through 
 how great an angle. Further, when brought 
 
 FIG. 202. Diagrammatic horizontal & " 
 
 section through the head to illustrate to rest after a series of rotations the person 
 the planes occupied by the semicircu- d observation feels a sensation of motion 
 
 lar canals (after Waller) : s, superior 
 
 canal ;p, posterior canal ;H, horizontal in the opposite direction. Each of these re- 
 sults should be expected to follow were the 
 
 theory in question correct. The observations of James have shown that 
 with deaf mutes in whom the internal ear was at fault rapid rotation in 
 an ordinary " swing " failed to produce the dizziness which is the common 
 effect in ordinary individuals. On the other hand, diseases which may be sup- 
 posed to alter the intra-labyrinthine pressure are characterized by the symp- 
 toms of vertigo and inco-ordination of movement. The presumable effect of 
 cutting the semicircular canals is that the escape of endolymph changes the 
 pressure upon the sensory hair-cells and gives the animal the sensation of 
 falling in one direction or another, so that he is impelled to make compensa- 
 tory or j 'weed movements to counteract this imaginary change of position. In 
 birds and in fishes, whose life is passed more or less exclusively in a medium 
 in which tactile and muscular sensation can contribute little to the sense of 
 equilibrium, the semicircular canals are especially well developed. 1 In fishes, 
 though section of the canals themselves produces no disturbance, division of 
 
 1 Sewall : Journal of Physiology, 1884, iv. p. 339. 
 
THE SENSE OF EQUILIBRIUM. 407 
 
 the nerves supplying the ampullae usually gives rise to marked forced move- 
 ments, as shown in somersaults, spiral swimming, etc., when set free in the 
 water. When, however, the nerves are cut with great care, with sharp scis- 
 sors, so as to avoid traction on or crushing of the nerves, such forced move- 
 ments do not follow. 
 
 Lee l found that when a fish is turned in different positions there is a 
 compensatory change in the direction of the fins and the optic axes determined 
 by the semicircular canal in whose plane the movement is made. He con- 
 cludes that " Each canal has a principal and a subordinate function. The 
 former is the appreciation of rotational body movements in its own plane and 
 toward its side of the body ; the latter is the appreciation of similar move- 
 ments, but in the opposite direction." Electric stimulation of the ampullary 
 nerves or mechanical pressure upon the ampullae excites equally definite 
 movements of eyes and fins, and the ocular result of nerve-irritation is the 
 exact opposite of that of nerve-section. 
 
 The difference in function between the divisions of the internal ear is 
 indicated by investigations on albinos. White animals with blue eyes are 
 deaf, but possess the normal power of equilibration. Rawitz 2 found the 
 cochlea in such creatures to be much reduced and the organ of Corti atro- 
 phied, while the semicircular canals were normal. 
 
 According to Lee 3 and others, the equilibrium of rest and motion, or static 
 and dynamic equilibrium, depends upon the irritation of different nerve-ter- 
 minals. The manner of action of the latter has been considered. As to the 
 nervous mechanism on which static equilibrium depends, Lee is of the opinion 
 that the knowledge of the position of the head while at rest comes from the rela- 
 tion of the otoliths in the vestibular sacs to the nerve-endings on the maculoe 
 acusticce. These otoliths form considerable masses in the ears of fishes, and the 
 intensity and direction of their pressure upon hair-cells must vary with the 
 spatial relations of the head, and thus be comparable, in the sense of posi- 
 tion which they arouse, to the tactile sensations derived from the soles of 
 the feet in man. 
 
 The opinion may be ventured that in the semicircular canals we have a 
 sense-organ of a peculiar kind. The evidence is satisfactory that impulses 
 generated in the nerves of the ampulla, and probably of the vestibular sacs 
 also, give rise to sensations of position both dynamic and static. And it is 
 highly probable that such sensations form a constant basis for our notion of 
 the spatial relations of the head. But the preservation of equilibrium does 
 not depend wholly upon the special sense-organ, as does sight upon the eye. 
 For the muscular and tactile, not to speak of the visual and other senses, 
 supply information in the same direction, and, no doubt, these may to a cer- 
 tain extent vicariously fill the function of the semicircular apparatus when 
 this is abolished. 
 
 1 Lee: Journal of Physiology, xv. p. 311 ; xvi. p. 192. 
 
 2 Rawitz : Zoologucher Jahresbericht, 1896. 
 
 3 Journal of Physiology, xv. p. 311, xvi. p. 192. 
 
408 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 P. SMELL. 
 
 The complex paired cavity of the nose is divisible into a lower respiratory 
 and an upper olfactory tract, the mucous membrane over each of which is 
 distinctive. The covering of the respiratory tract is known as the Schneider- 
 ian or pituitary membrane; its surface is overlaid with cylindrical ciliated 
 epithelium, the ciliary current of which is directed posteriorly toward the 
 pharynx. 
 
 The Schneiderian membrane lines the lower two-thirds of the septum, the 
 middle and inferior turbinated bodies, and the bony sinuses which communi- 
 cate with the nasal chamber. The mem- 
 brane upon the turbinated bodies and 
 the lower part of the septum is composed 
 largely of erectile tissue. 
 
 The function of the respiratory tract 
 is threefold : it restrains the passage 
 of solid particles into the lungs; it 
 warms the air inspired to approximately 
 
 FIG. 203. Section of olfactory mucous mem- 
 brane (after V. Brunn) : the olfactory cells are in 
 black. 
 
 FIG. 204. Cells of the olfactory region (after V. 
 Brunn) : a, olfactory cells ; 6, epithelial cells ; n, 
 central process prolonged as an olfactory nerve- 
 fibril ; I, nucleus ; c, knob-like clear termination 
 of peripheral process ; h, bunch of olfactory hairs. 
 
 the temperature of the body ; and it gives up moisture sufficient nearly to 
 saturate the air. 
 
 The olfactory mucous membrane, which alone is the peripheral organ for 
 smell, is seated in the upper part of the nasal chamber, away from the line 
 of the direct current of inspired air. The membrane is thick and is covered 
 by an epithelium composed of two kinds of cells, columnar and rod cells. 
 The latter are the true olfactory cells (Figs. 203, 204), with which the fibres 
 of the olfactory nerve are known to be connected. These olfactory cells, in 
 fact, are comparable to nerve-cells in that the fibres connected with them, the 
 fibres composing the olfactory nerve, are direct outgrowths from the cells 
 (Fig. 205), essentially similar in every way to the nerve-fibre processes springing 
 from nerve-cells in the nerve-centres. In this respect the olfactory cells differ 
 from the sensory cells in other organs of special sense. The membrane 
 
THE SENSE OF SMELL. 
 
 409 
 
 appears to be not ciliated except near its juncture with the Schneiderian 
 membrane, where the columnar cells acquire cilia and gradually pass over 
 into the cells covering the respiratory tract. Substances exciting the sense 
 of smell exist as gases or in a fine state of division in the air inspired. 
 They reach the olfactory mucous membrane by diffusion, assisted by the 
 modified inspiratory movements of "sniffing" and "smelling," and are 
 most acutely perceived when the air containing them is warmed to the 
 body-temperature. The amount of odoriferous matter that may thus be 
 recognized is extraordinarily small ; thus, it is said that in one liter of air 
 the odor of 0.000,005 gram of musk and of 0.000,000,005 gram of oil of 
 peppermint can be perceived. 1 The odoriferous particles probably excite the 
 
 olf.c 
 
 FIG. 205. Diagram of the connections of cells and fibres in the olfactory bulb (Schafer, in Quain's Anat- 
 omy) : otf.c, cells of the olfactory mucous membrane ; olf.n, deepest layer of the bulb, composed of the 
 olfactory nerve-fibres which are prolonged from the olfactory cells ; gl, olfactory gloineruli, containing 
 arborization of the olfactory nerve-fibres and of the dendrons of the mitral cells ; m.c, mitral cells ; 
 a, thin axis-cylinder process passing toward the nerve-fibre layer, n.tr, of the bulb to become continuous 
 with fibres of the olfactory tract ; these axis-cylinder processes are seen to give off collaterals, some of 
 which pass again into the deeper layers of the bulb ; n', a nerve-fibre from the olfactory tract ramifying 
 in the gray matter of the bulb. 
 
 sense of smell by coming into contact with the olfactory epithelium after solu- 
 tion in the layer of moisture covering it. This epithelium is easily thrown 
 out of function, as the com-mon loss of smell when there is a " cold in the 
 head " testifies. When the nostril is filled with water in which an odorous 
 substance is dissolved, no sensation of smell is excited, but it is said that if 
 normal salt-solution, which injures the living tissues less than water, be used 
 as the solvent, the odor can still be perceived. In many lower animals the 
 sense of smell has an acuteness and an importance in their economy unknown 
 in the human race. It is probable that not only do different races have their 
 distinctive odors, but that each individual exhales an odor peculiar to himself, 
 distinguishable by the olfactory organs of certain animals. The classification 
 1 Passy : Comptes-rendus de la Societe de Biologie, 1892, p. 84. 
 
410 AN AMERICAN TEXTBOOK OF PHYSIOLOGY. 
 
 of odors is not very definite, and the relation of odors to one another in the 
 way of contrast and harmony is ill understood. No limited number of pri- 
 mary sensations, as in vision, have been discovered out of which other sen- 
 sations can be composed. Certain sensations, as those due to the inhalation 
 of ammonia and other irritant gases, are thought to be due to excitement of 
 the nasal filaments of the fifth nerve, and not of the olfactory. 
 
 Subjective sensations of smell are sometimes experienced, the result of some 
 irritation arising in the olfactory apparatus itself. 
 
 Finally, in man sensations of smell have their most important uses in con- 
 nection with taste ; many so-called " tastes " owe their character wholly or 
 partly to the unconscious excitement of the sense of smell. 
 
 G. TASTE. 
 
 The peripheral surfaces concerned in taste include, in variable degree, the 
 upper surface and sides of the tongue and the anterior surfaces of the soft 
 palate and of the anterior pillars of the fauces. Other parts of the buccal 
 and pharyngeal cavities are, in most persons, devoid of taste. 1 
 
 The chief peripheral sensory organs of taste are groups of modified epi- 
 thelial cells, known as t(jiste-buds (Fig. 206), seated in certain papilla? of the 
 tasting surfaces. According to some authors, only parts provided with taste- 
 buds can give taste-sensations. 2 
 
 The structure of taste-buds is most easily studied in the papilla foliata of 
 the rabbit, a patch of fine, parallel wrinkles found on each side of the 
 back part of the tongue of the animal. The taste-bud is a somewhat globular 
 body seated in the folds of mucous membrane between the furrows of the 
 papilla. It is made up of a sheath of flattened, fusiform cells enclosing a 
 number of rod-like cells each of which terminates in a hair-like process. These 
 cells surround a central pore which opens into a furrow of the papilla. 
 The hair-bearing cells recall the appearance of the olfactory rod-cells, and 
 are probably the true sensory cells of taste, since between them terminate the 
 filaments of the gustatory nerve. In the human tongue taste-buds are con- 
 fined to the fungiform papillae, seen often as red dots scattered over the upper 
 surface ; to the circumvallate papillas, the pores of the buds opening into the 
 groove around the papilla ; and to an area just in front of the anterior pillar 
 of the fauces, which somewhat resembles the papilla foliata of the rabbit. 
 
 The sensory nerves distributed to the tongue include filaments from the 
 glosso-pharyngeal, the lingual branch of the fifth, and the chorda tympani. 
 The relation of these nerves to the sense of taste has been the occasion of 
 much dispute. The weight of evidence probably favors the belief that the 
 glosso-pharyngeal is the nerve of taste for the posterior third of the tongue, 
 while the lingual and, to some extent, the chorda carry taste-impressions from 
 the anterior two-thirds. Clinical cases have been cited to show that all the 
 
 1 V. Vintschgau : " Geruchsinn," Hermann's Handbuch der Physiologie, iii. 2, 1880. 
 2 Camerer: Zeitschri/t fiir Biologic, 1870, vi. S. 440; Wilczynsky : Hofmann und Schwalbe's 
 Jahresbericht der PhysioL, 1875. 
 
THE HKNXE OF TAXTK. 
 
 411 
 
 FIG. 206. Section through one of the taste-buds 
 of the papilla foliata of the rabbit (from Quain, 
 after Ranvier), highly magnified: p, gustatory 
 pore ; , gustatory cell ; r, sustentacular cell ; m, 
 leucocyte containing granules ; e, superficial epi- 
 thelial cells ; n, nerve-fibres. 
 
 gustatory fibres arise from the brain as part of the glosso-pharyngeal nerve, 
 whatever may be their subsequent course to the tongue. On th<> mntrary, 
 other cases have shown a marked loss 
 of taste-sensation following upon lesions 
 of the fifth nerve at or near its origin 
 from the brain, while still others indi- 
 cate that some of the taste-fibres may 
 arise in the seventh nerve. The point 
 is of practical importance in diagnosis, 
 in the interpretation of loss of taste 
 over any given part of the tongue, but 
 the contradiction in the clinical cases 
 reported has led to the general belief 
 that the origin and course of the gusta- 
 tory fibres are subject to considerable 
 individual variations. 
 
 Our taste-perceptions are ordinarily 
 much modified by simultaneous olfac- 
 tory sensations, as may easily be dem- 
 onstrated by the difficulty experienced 
 in distinguishing by taste an apple, an 
 onion, and a potato, when the nostrils are closed. In the condition of anosmia 
 the ability to discriminate between tastes is much below par. Sight has also 
 an important influence, at least in quickening the expectancy for individual 
 flavors. Every smoker knows the blunting of his perception for burning 
 tobacco while in the dark ; various dishes having distinctive flavors are said 
 to lose much of their gustatory characteristics when the eyes are bandaged. 1 
 
 The intensity of gustatory sensation increases with the area to which the 
 tasted substance is applied. The movements of mastication are peculiarly 
 adapted to bring out the full taste-value of substances taken into the mouth, 
 and the act of swallowing, by which the morsel is rubbed between the tongue 
 and the palate, has been proved to develop tastes not appreciable by simple 
 contact with the sensory surface. A considerable area in the mid-dorsum of 
 the tongue is said to be devoid of all taste-sensibility. 2 
 
 The sensitiveness of taste-sensation is greatest when the exciting substance 
 is at the temperature of the body. Weber 3 found that when the tongue was 
 dipped during one-half to one minute in water either at the freezing tempera- 
 ture or warmed to 50 C., the sweet taste of sugar could no longer be appre- 
 ciated by it. It is probable that sapid substances reach the sensory endings 
 of the nerves of taste only after being dissolved in the natural fluids of the 
 mouth, and any artificial drying of the buccal surfaces or alteration of their 
 secretion must affect taste-perceptions. 
 
 1 Cf. Patrick : " Studies in Psychology," Univ. Iowa, 1899, vol. ii. 
 
 2 Shore: Journal of Physiology, 1892, vol. xiii. p. 191. 
 
 3 Archivfiir Anatomic und Physiologic, 1847, S. 342. 
 
412 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 The excitement of the taste-nerves appears to depend not so much on the 
 absolute amount of the substance to be detected as on the concentration of the 
 solution containing it. Thus, when, 1 part of common salt to 213 of water 
 was tasted by Valentin/ 1^- cubic centimeters of the fluid was sufficient to give 
 a saltish taste ; when diluted so that the ratio of salt to water was 1 to 426, 
 12 cubic centimeters taken in the mouth scarcely gave the salt taste. Sulphate 
 of quinine dissolved in the proportion 1 to 33,000 gave a decided bitter taste, 
 but a solution 1 to 1,000,000 was with difficulty perceived as bitter. 
 
 It has generally been conceded that all gustatory sensations may be built 
 up out of four primary taste-sensations namely, bitter, sweet, sour, and salt. 
 Some authors even limit the list to tastes of bitter and sweet (V. Vintschgau). 
 
 Auditory. 
 
 Gustatory, 
 
 Tactile. 
 
 FIG. 207. Diagram showing the mode of termination of sensory nerve-fibres in the auditory, gustatory, 
 and tactile structures of vertebrata (from Quain, after Retzius). Each sense organ may be considered as 
 essentially constructed of a nerve-cell with two processes, one finding its way centrally to cluster round 
 other nerve-cells or their processes, and the other to terminate in the periphery. In the organ of smell 
 the peripheral process is very short and is directly irritated by foreign particles, the original nerve-cell 
 being represented by the olfactory cell (Fig. 291). In the organs of touch the nerve-cell is found in the 
 ganglion of the posterior spinal nerve-root ; the peripheral process is very long and is acted on indirectly 
 through the modified epithelium round which it clusters. The same may be said of the other sense 
 organs. See Quain's Anatomy, 10th ed., vol. iii. pt. 3, p. 152. 
 
 There is strong reason to believe that corresponding to the four primary taste- 
 sensations there are separate centres and nerve-fibres, each of which, when 
 excited, gives rise only to its appropriate taste-sensation. Substances which 
 arouse the sense of taste are not appreciated in uniform degree over the surface 
 of the tongue. Thus, to V. Yintschgau, at the tip of the tongue acids were 
 perceived acutely, sweets somewhat less plainly, and bitter substances hardly 
 at all. It is generally admitted that sweet and sour tastes are recognized 
 chiefly at the front, and bitter, together with alkaline tastes, by the posterior 
 
 1 Lehrbuch qjer Physiologic, 1848. 
 
THE SENSE OF TASTE. 413 
 
 part of the tongue. Strong evidence in favor of the specific difference between 
 various taste-nerves is found in the fact that the same substance may excite a 
 different gustatory sensation according as it is applied to the front or the back 
 of the tongue. Thus, it has been demonstrated that a certain compound of 
 saccharin (para-brom -ben zoic sulphimide) appears to most persons to be sweet 
 when applied to the tip of the tongue, but bitter in the region of the circum- 
 vallate papilla. 1 
 
 Oehrwall 2 has examined the different fungiform papillae scattered over the 
 tongue with reference to their sensitiveness to taste-stimuli. One hundred and 
 twenty-five separate papillae were tested with succinic acid, quinine, and sugar. 
 Twenty-seven of the papilla? gave no response at all, indicating that they were 
 devoid of taste-fibres. Of the remaining ninety-eight, twelve reacted to suc- 
 cinic acid alone, three to sugar alone, while none were found which were acted 
 upon by quinine alone. The fact that some papillae responded with only one 
 form of taste-sensation is again evidence in favor of the view that there are 
 separate nerve-fibres and endings for each fundamental sensation; but the 
 figures given in the experiments show that the majority of the papilla? are 
 provided with more than one variety of taste-fibre. 
 
 An extract of the leaves of a tropical plant, Gymnema silvestre, when 
 applied to the tongue, renders it incapable of distinguishing the taste of sweet 
 and bitter substances; it probably paralyzes the nerves of sweet and bitter 
 sensations. When a solution of cocaine in sufficient strength is painted on 
 the tongue, the various sensations from this member are said to be abolished 
 in the following order: (1) General feeling and pain; (2) bitter taste; (3) 
 sweet taste ; (4) salt taste ; (5) acid taste ; (6) tactile perception (Shore). 
 
 That there are laws of contrast in taste-sensation has long been empirically 
 known. Thus, the taste of cheese enhances the flavor of wine, but sweets 
 impair it (Joh. Miiller). It is unfortunate, from a hygienic standpoint at 
 least, that in this most important department of the physiology of sensation 
 investigations are almost wholly wanting. 
 
 Certain tastes may disguise others without physically neutralizing them; 
 when, for example, sugar is mixed with vinegar, the overcoming of the acid 
 taste is probably effected in the central nerve-organ. 3 
 
 1 Howell and Kastle : Studies from the Biological Laboratory of Johns Hopkins University. 
 1887, iv. 13. 
 
 8 Sfcandinavisches Archivfilr Physwlogie, 1890, vol. ii. S. 1. 
 3 Briicke : Vorlesungen uber Physiologic, 1876. 
 
IV. PHYSIOLOGY OF SPECIAL MUSCULAR 
 MECHANISMS. 
 
 A. THE ACTION OP LOCOMOTOB MECHANISMS. 
 
 The Articulations. The form, posture, and movements of vertebrates 
 are largely determined by the structure of the skeleton and the method of 
 union of the bones of which it is composed. There are two hundred bones in 
 the human skeleton, and they are so connected together as to be immovable, 
 or to allow of many varieties and degrees of motion. There are four prin- 
 cipal methods of articulation : 
 
 1. Union by Bony Substance (Suture). This form of union occurs 
 between the bones of the skull. These bones, which at birth are independent 
 structures connected by fibrous tissue, gradually grow together and make 
 a continuous whole, only a more or less distinct seam remaining as witness 
 of the original condition. 
 
 2. Union by Fibro- Cartilages (Symphysis). The bodies of the verte- 
 brae and the sacro-iliac and pubic bones are closely bound together by disks 
 of fibre-cartilage. This material, which is very strong, but yielding and 
 elastic, acts as a buffer to deaden the effect of jars, permits of a slight 
 amount of movement when the force applied is considerable, and restores 
 the bones to their original position on the removal of the force. The spinal 
 column can be thought of as an elastic staff; the capacity for movement 
 differs greatly in different regions, however, partly on account of differences 
 in the thickness of the intervertebral disks as compared with the antero- 
 posterior and lateral diameters of the bodies of the vertebrae, and more espe- 
 cially on account of the method of contact of the superior and inferior verte- 
 bral processes. In the cervical region the disks are thick and the diameter 
 of the vertebrae is small, and this permits of considerable bending in all 
 directions and a certain amount of rotation. In the dorsal region a slight 
 amount of bending from side to side and a slight amount of rotation are pos- 
 sible ; but backward bending is inhibited by contact of the articular processes, 
 and forward bending is prevented by the strong articular ligaments. In the 
 lumbar region bending in all directions is more free, but rotation is made 
 impossible by the interlocking of the articular processes. 1 
 
 3. Union of Fibrous Bands (Syndesmosis). Some of the bones, as those 
 of the carpus and tarsus, are connected by interosseous ligaments which, at the 
 same time that they bind the bones together, admit of a certain amount of 
 
 1 Fick : Compendium der Physiologic des Menschen, Wien, 1891. 
 414 
 
THE ACTION OF LOCOMOTOR MECHANISMS. 415 
 
 play, the extent of the movement varying with the character of the surfaces 
 and the length of the ligaments, 
 
 4. Union by Joints (Diarthrosis). The adjacent surfaces of most of the 
 bones are so formed as to permit of close contact and freedom of movement in 
 special directions. The parts of the bones entering into the joint are clothed with 
 very smooth cartilage, and the joint-surfaces are lubricated by synovial fluid, 
 a viscid liquid secreted by a delicate membrane which lines the fibrous capsule 
 by which the joint is surrounded. The joint-capsule is firmly attached to the 
 bones at the margin of the articular cartilages, and, at the same time that it 
 completely surrounds and isolates the joint-cavity, it helps to bind the bones 
 together. The bones are further united by strong ligaments, in some cases 
 within and in other cases without the capsule. These ligaments are so placed 
 that they are relaxed in certain positions of the joints and tightened in others ; 
 they guide and limit the movements of the joints. The joint-surfaces always 
 touch, although usually the parts in contact change with the position of 
 the joint. If continuous contact of the joint-surfaces is to be maintained 
 and free movement is to take place in special directions, it is evident that the 
 opposing surfaces must not only be so constructed that they shall fit each 
 other with great accuracy, but also have forms especially adapted to the move- 
 ments peculiar to each of the joints. 
 
 The different joints exhibit a great variety of movements and may be clas- 
 sified as follows : gliding joints, hinge joints, condyloid joints, saddle joints, 
 ball-and-socket joints, pivot joints. For a description of the structure and 
 the peculiarities of these joints the student is referred to works on anatomy. 1 
 The contact of the surfaces of the joint is secured in part by the fibrous capsule, 
 in part by the joint ligaments, and in part by the tension of the muscles. The 
 elastic muscles are attached under slight tension, and, moreover, during wak- 
 ing hours are kept slightly contracted by touus impulses of reflex origin. 
 Another less evident but no less important condition is the atmospheric pres- 
 sure. The capsule fits the joint closely and all the space within not occupied 
 by the bones is filled by cartilages, fibrous bands, fatty tissues and synovial 
 fluid. The joint is air-tight, and, as was first demonstrated by the Weber 
 brothers, the atmospheric pressure keeps all parts in close apposition. This 
 force is sufficiently great in the case of the hip-joint to support the whole 
 weight of the leg even after all the surrounding soft parts have been cut 
 through. The proof that the air-pressure gives this support is found in the 
 fact that the head of the femur maintains its place in the acetabulum after 
 all the soft parts which surround the joint have been divided, but falls out 
 of its socket if a hole be bored in the acetabulum and air be permitted to 
 enter the cavity of the joint. Though the air-pressure keeps the bones in 
 constant contact it offers no resistance to the movements peculiar to the joints. 
 
 The movements of the bones are effected chiefly by muscular contractions, 
 but the direction and extent of the movements are for the most part deter- 
 mined by the form of the joint-surfaces and the limitations to movement 
 
 1 Quain's Anatomy, vol. ii. pt. 1 ; Gray's Anatomy ; Morris's Anatomy. 
 
416 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 which result from the method of attachment of the ligaments. The follow- 
 ing kinds of movement are possible : (a) angular, in which the angle formed 
 by the longitudinal axis of two bones changes, as in flexion and extension or 
 abduction and adduction ; (b) circumduction, in which the longitudinal axis 
 of a bone describes the sides of a cone, the apex of which is in the joint ; 
 (c) rotation, in which a bone moves about its longitudinal axis ; (d) gliding, 
 in which a bone so moves as to change its position with reference to its 
 neighbor, without rotation or change of angle. As a matter of fact, most of 
 the movements that are made are the resultant of two or of all of these simple 
 motions. In the gliding joints, in which the articular surfaces are nearly 
 flat (as in the case of the joints between the articular processes of the verte- 
 brae, and the carpal and tarsal joints), a sliding movement may occur in various 
 directions, and a rotation movement is possible ; but the extent of these 
 movements is very slight, being limited by the strong capsule and ligaments. 
 Hinge joints have but a single axis of motion, because the convex and some- 
 what cylindrical surface of one bone fits quite closely the concave surface of 
 the other, and because of tense lateral ligaments which permit of movements 
 in only a single plane. The joint between the humerus and the ulna at the 
 elbow is an example. In this case only flexion and extension are possible, 
 although a slight obliquity of the surfaces causes the head to move in flexion 
 toward the middle line of the body, which is interpreted by some as a screw 
 movement. In this joint the limits of motion are determined by the contact 
 of the coronoid and olecranon processes of the ulna with the bone in the cor- 
 responding fossae of the humerus, as well as by the resistance of capsule and 
 ligaments. The knee-joint l is a less simple form of hinge joint. The pres- 
 ence of the semilunar cartilages and the shape of the joint-surfaces cause 
 flexion to be produced by the combined action of sliding, rolling, and rotation 
 movements. In complete extension the lateral ligaments and the posterior 
 and anterior crucial ligaments are put on the stretch, and there is a locking 
 of the joint, no rotation being possible ; in complete flexion, on the other 
 hand, the posterior crucial ligament is tight, but the others are sufficiently 
 loose to allow of a considerable amount of pronation and supination. In the 
 condyloid joint the articulating surfaces are spheroidal, as in the case of the 
 metacarpo- and metatarso-phalangeal joints. These exhibit all forms of 
 angular movement and circumduction. In the saddle-joint there is a double 
 axis of motion e. g., the articulation of the trapezium with the first meta- 
 carpal bone of the thumb permits of movement about an axis extending from 
 before backward, and another, at nearly right angles to this, extending from 
 side to side. All modes of angular movement are possible with such a joint. 
 The ball-and-socket joint, of which the shoulder- and hip-joints are exam- 
 ples, permits of the greatest variety of movements, any diameter of the head 
 
 1 W. Braunne and Fischer have studied with mathematical accuracy the construction and 
 movements of many of the joints of the human body. Their articles are published in the 
 Abhandlungen der math.-phys. Classe der konigl. Sdchsischer Gesellschaft der Wissenschaften, Bd. 
 xvii., and others. 
 
THE ACTION OF LOCOMOTOR MECHANISMS. 417 
 
 of the bone serving as an axis of rotation. The /th-of-jo/nf allows of rotation 
 only; the atlanto-uxial and nulio-ulnar joints inav he placed in this class. 
 
 Method of Action of Muscles upon the Bones. The bones can be 
 looked upon as levers actuated by the forces which arc applied at the points 
 of attachment of the muscles. All three forms of levers are represented in 
 the body ; indeed, they may be illustrated in the same joint, as the elbow. 
 
 An example of a lever of the first class, in which the fulcrum is between 
 the power and the resistance, is to be found in the extension of the forearm in 
 such an act as driving a nail : the inertia of the hammer, hand, and forearm 
 offers the resistance, the triceps muscle acting upon the olecranon gives the 
 power, and the trochlea, upon which the rotation occurs, is the fulcrum. The 
 balancing of the head upon the atlas is another example : the front part of the 
 head and face is the resistance, the occipito-atlantoid joint the fulcrum, and 
 the muscles of the neck the power. 
 
 In the case of a lever of the second order, the resistance is between the ful- 
 crum and the power; for example, when the weight of the body is being 
 raised from the floor by the hands : the fulcrum is where the hand rests on the 
 floor, the weight is applied at the elbow-joint, and the power is the pull of the 
 triceps on the olecrauon. The raising of the body on the toes is another ex- 
 ample : the fulcrum is at the place where the toes are in contact with the 
 floor, the resistance is the weight of the body transmitted through the tibia to 
 the astragalus, and the power is applied at the point of attachment of the 
 tendo Achillis to the os calcis. 1 
 
 The raising of a weight in the hand by flexion of the forearm through 
 contraction of the biceps gives an example of a lever of the third order, in 
 which the power is applied between the fulcrum and the weight. This form 
 of lever, because of the great length of the resistance arm, as compared with 
 the power arm, is favorable to extensive and rapid movements, and is the 
 most usual form of lever in the body. 
 
 The power is applied to best advantage when it is exerted at right angles 
 to the direction of a lever, as in the case of the muscles of mastication and of 
 the calf of the leg. If the traction be exerted obliquely, the effect is the less 
 the more acute the angle between the tendon of the muscle and the bone ; for 
 example, when the arm is extended the flexor muscles work to great disad- 
 vantage, for a large part of the force is expended in pulling the ulnar and 
 radius against the humerus, and is lost for movement, but as the elbow is 
 flexed the force is directed more and more nearly at right angles to the bones 
 of the forearm, and there is a gain in leverage, which is of course again 
 decreased as flexion is completed. This gain in leverage which accompanies 
 the shortening of the muscles is the more important, since the power of the 
 muscle is greatest when the muscle has its normal length, and continually 
 lessens as the muscle shortens in contraction. There are a number of special 
 arrangements which help to increase the leverage of the muscles by lessening 
 the obliquity of attachment viz. the enlarged heads of the bones, and in some 
 
 1 Certain observers would class this movement as an example of a lever of the first class. 
 (Ewald : Pfliiger's Archiv, 1896, Bd. Ixiv. S. 53). 
 VOL. II. 27 
 
418 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 special processes projecting from the bones, the introduction of sesaraoid 
 bones into the tendons, and the presence of pulley -like mechanisms. 
 
 The contraction of a muscle causes the points to which it is attached to 
 approach one another, and the direction of the movement is often determined 
 by the direction in which the force of the contracting muscle is applied to the 
 bones. In the case of certain joints, however, the form of the joint-surfaces 
 and the method of attachment of the ligaments limits the direction of move- 
 ment to special lines; and when this is not the case the movement is usually 
 the resultant of the action of many muscles rather than the effect of the con- 
 traction of any one muscle. This question has been made the subject of 
 careful study by Fick. 1 
 
 In the case of many muscles, both of the bones to which they are attached 
 are movable, and the result of contraction depends largely on which of the 
 extremities of the muscles becomes fixed by the contraction of other muscles. 
 Though most muscles have direct influence over only one joint, there are certain 
 muscles which include two joints between their points of attachment, and pro- 
 duce correspondingly complex effects. The accurate adjustment and smooth 
 graduation of most co-ordinated muscular movements is due to the fact that not 
 only the muscles directly engaged in the act, but the antagonists of these mus- 
 cles take part in the movement. It would appear from the observations of 
 certain writers 2 that antagonistic muscles may be not only excited to contrac- 
 tion, but inhibited to relaxation, and that the tension of the muscles is thereby 
 accurately adjusted to the requirements of the movement to be performed. 
 The importance of the elastic tension and reflex tonic contractions of muscles 
 to ensure quick action, to protect from sudden strains, and to restore the parts 
 to the normal position of rest has been referred to elsewhere. 
 
 The shape of the muscle has an important relation to the work which it 
 has to perform. A muscle consists of a vast number of fibres, each of which 
 can be regarded as a chain of contractile mechanisms. The longer the fibre, 
 the greater the number of these mechanisms in series and the greater the total 
 shortening effected by their combined action ; consequently, a muscle with 
 long fibres, such as the sartorius, is adapted to the production of extensive 
 movements. In order that a muscle shall be capable of making powerful 
 movements it is necessary that many fibres shall be placed side by side, as in 
 the case of the gluteus : " Many hands, light work." 
 
 Standing-. In spite of the ease with which the many joints of the body 
 move, the erect position is maintained with comparatively little muscular 
 exertion. It is an act of balancing in which the centre of gravity of the 
 body is kept directly over the base of support. In the natural erect position 
 of the body the centre of gravity of the head is slightly in front of the oc- 
 cipito-atlantoid articulation, so that there is a tendency for the head to rock 
 forward, as is seen from the nodding of the head of one falling asleep. The 
 centre of gravity of the head and trunk together is such that the line of 
 gravity falls slightly behind a line drawn between the centres of the hip- 
 
 1 Hermann's Handbuch der Physiologic, 1871, Bd. i. pt. 2. S. 241. 
 
 2 Sherrington : Proceedings of the Royal Society, Feb., 1893, vol. liii. 
 
THE ACTION OF LOCOMOTOIl MECHANISMS. 419 
 
 joints, which would incline the body to fall backward. The line of gravity 
 of the head, trunk, and thighs falls slightly behind the axis of the knee- 
 joints, and the line of gravity of the whole body slightly in front of a line 
 connecting the two ankle-joints, so that the weight of the body would tend to 
 flex the knee- and ankle-joints. 
 
 We cannot here consider in detail the mechanical conditions which limit 
 the movements possible to the different joints in the erect position of the body. 
 Although these conditions help to support the body in the upright position, 
 they are not alone sufficient to the maintenance of this posture, as is shown by 
 the fact that the cadaver cannot be balanced upon its feet. That standing 
 requires the action of the muscles is further proved by the fatigue which is 
 experienced when one is forced to stand for a considerable time. The body 
 may be supported in the standing position in various attitudes. Thus, the 
 soldier standing at " attention " places the heels together, turns the toes out, 
 makes the legs straight and parallel, so as to extend the knees to their utmost, 
 tilts back the pelvis, straightens the spine, and looks directly forward. In 
 this position many of the muscles are relieved from action by the locking 
 of the hip- and knee-joints. The tilting backward of the pelvis causes the 
 line of gravity to fall slightly behind the axis of rotation of the hip-joint 
 and puts the strong ilio-femoral ligament on the stretch, which balances 
 the tendency of the weight of the body to extend the hip. The line of 
 gravity would fall slightly behind the axis of rotation of the knee, and tend 
 to cause flexion ; but when the joint is extended, the thigh, because of the 
 horizontal curvature of the internal condyle, receives a slight inward rota- 
 tion, and the knee cannot be flexed without a corresponding outward rota- 
 tion. In standing with the feet turned out, this rotation movement is 
 prevented by the same ilio-femoral ligament that locks the hip-joint. The 
 ankle-joint cannot be locked, and the tendency of the body to fall forward is 
 resisted by the strong muscles of the calf of the leg. The erect position of 
 the spine and the balancing of the head have likewise to be maintained by 
 the action of muscles. Although this position gives great stability, it cannot 
 be long maintained with comfort. It is less fatiguing to allow the joints to 
 be a little more flexed, and to keep the balance by the action of the muscles, 
 the position being frequently changed so as to bring fresh muscles into action. 
 .Perhaps the most restful standing position is found in letting the weight of 
 the body be supported on one leg, the pelvis being tilted so as to bring the 
 weight of the body over the femur, and the other being used as a prop to pre- 
 serve the balance. Absolute stability in standing is impossible for any length 
 of time ; the body is continually swaying, and a pencil resting on a writing 
 surface placed upon the head is found to write a very complicated curve. 
 There is a normal sway for every individual, and this may become markedly 
 exaggerated under pathological conditions. The maintenance of equilibrium 
 requires that afferent impulses shall continually pass to the co-ordinating cen- 
 tres which control the muscles involved in this act, and if any of these normal 
 impulses fail the sway of the body is increased ; for example, it is more diffi- 
 cult to stand steadily when the eyes are closed than when they are open ; the 
 
420 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 absence of the normal sensory impulses from the skin of the feet, the muscles, 
 joints, etc., also makes standing more difficult and tends to increase the sway. 
 The effect of the normal sway of the body is to shift the pressure and strain 
 from point to point and to relieve the different muscles from continuous action. 
 
 Locomotion. 1 The movements of animals were first studied by careful 
 observation, accompanied by more or less accurate direct measurements, and 
 by these simple methods the Weber brothers 2 arrived at quite accurate con- 
 clusions as to the nature of the processes, walking, running, jumping, etc. 
 These results were greatly extended by Marey, 3 who employed elaborate 
 recording methods, and exact pictures of all stages of these processes were 
 later obtained through the remarkable revelations of instantaneous photog- 
 raphy. 4 
 
 Walking. During the act of walking, at the same time that the body is 
 propelled forward it is continually supported by the feet, one or the other of 
 which is always touching the ground. Preparatory to beginning the move- 
 ment the weight of the body is thrown upon one leg, while the other leg is 
 placed somewhat behind it, the knee and ankle being slightly flexed. At the 
 start the body is given a slight forward inclination, then the back leg is ex- 
 tended and impels the body forward. As the centre of gravity progresses so 
 as to be no longer over the supporting leg, it would fall were it not that the 
 back leg is at the same instant swung forward to sustain it. As the body 
 moves forward and its weight is received by the leg which has just been 
 advanced, the leg which has been its support is freed from the weight 
 and becomes inclined behind it. This leg and foot are next extended, the 
 body thereby receiving another forward impulse, and then the hip-, knee-, 
 and ankle-joints flexing slightly, the leg swings forward past the supporting 
 leg and again becomes the support of the body. The forward movement of 
 the body is due in part to a slight inclination which tends to cause it to fall 
 forward, and in part to a push given it by each leg in turn as it leaves the 
 ground. 
 
 The amount of work performed by the legs in ordinary walking is com- 
 paratively slight, since the swing of the leg is, like that of a pendulum, largely 
 a passive act. Speed in walking is attained by inclining the body somewhat 
 more, by which it is better able to oppose the resistance offered by the air, and 
 by flexing the legs somewhat more, which, by lessening the distance between 
 the hip-joints and the ground, lengthens the step at the same time that it per- 
 mits the propelling limb in extending to push the body forward with greater 
 force. The more rapid movement of the body is also accompanied by a 
 more rapid forward swing of the leg, the muscles aiding the force of gravity. 
 
 The transfer of the weight of the body from one leg to the other in walk- 
 
 1 Beannis : Physiologic humaine, 1888, vol. ii. p. 269, gives many references to the litera- 
 ture of this subject. 
 
 2 W. and E. Weber : Mechanik der menschlichen Gehewerkzeuge, 1836. 
 
 3 La Methode graphique, 1885. 
 
 4 Marey : Methode graphique (supplement), 1885 ; Muybridge : The Horse in Motion, as Shown 
 by Instantaneous Photography, 1882. 
 
VOICE AND SPEECH. 421 
 
 ing causes an up-and-down and a lateral sway with cadi step. Were the legs 
 without joints, like stilts, these oscillations would be very great, especially 
 when the step was long; as a matter of fact, they arc slight. The tendency 
 for the centre of gravity to move from side to side as the le-s alternately 
 push the body forward is partly balanced by the swing of the opposite arm ; 
 and the vertical oscillations are minimized by the fact that the leg which is 
 about to receive the weight flexes as the centre of gravity moves forward and 
 comes over it, and extends as it passes on to be received by the other leg. 
 The path taken by the centre of gravity during walking is a complicated one. 
 If referred to the plane in which the body is moving, it describes for one 
 double step an oval ; projected on the horizontal and frontal planes, its path 
 has the form of the sign of infinity, oc . The rate of movement influences its 
 position in special parts of the curve. 1 
 
 In running, the body is inclined more than in walking, and the legs are 
 more flexed in order that the extension movement of the back leg, which 
 drives the body forward, may be more effective. In running, the body is pro- 
 pelled by a series of spring-like movements and there are times when both 
 feet are off the ground, the back leg leaving the ground before the other 
 touches it. 
 
 B. VOICE AND SPEECH. 
 1. STRUCTURE OF THE LARYNX. 
 
 Voice-production. The human vojee is produced by vibration of the 
 true vocal cords*, normally brought about by an expiratory blast of air passing 
 between them while they are approximated and held in a state of tension by 
 muscular action. Mere vibration of the cords could produce but a feeble 
 sound ; the voice owes its intensity both to the energy of the expiratory blast 
 (Helmholtz) 2 and to the reinforcement of the vibrations by the resonating 
 cavities above and below the cords. 
 
 A true conception of the action of the larynx can only be gained by a pre- 
 liminary study of the organ in situ, in its relations with the trachea, pharynx, 
 tongue, extrinsic muscles, and hyoidean apparatus. Removed from its con- 
 nections, the larynx, in vertical transverse section, is seen to be shaped some- 
 what like an hour-glass, the true vocal cords forming the line of constriction 
 half way between the top of the epiglottis and the lower border of the cri- 
 coid cartilage (Fig. 208). In median vertical section the axis of the larynx 
 above the vocal cords extends decidedly backward, and below the cords the 
 axis is nearly perpendicular to the plane in which they lie. The epiglottis is an 
 ovoid lamella of elastic cartilage, shaped like a shoe-horn, that leans backward 
 over the laryngeal orifice so that the observer must look down obliquely in 
 order to inspect the cavity of the larynx (Fig. 212.) The mucous membrane 
 is thickened into a slight prominence, known as the " cushion," at the base of 
 
 1 Fischer: Abhandl. d. math.-physik. CL d. Sachs. Gesellsch. d. Wissensch., xxv. Nr. i. 
 
 2 Quoted by Grutzner: Hermann's Handb. der Physiologic, 1879, J3d. 11, Th. 2, S. 14. 
 
422 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 the epiglottis. The epiglottis, which is extremely movable in a median plane, 
 may be tilted backward so as to close completely the entrance into the larynx. 
 
 Functions of the Epiglottis. One 
 function of the epiglottis seems obviously 
 to serve as a cover for the superior entrance 
 of the larynx, over which it is said to shut 
 in the act of swallowing. But it is found 
 that deglutition occurs in a normal manner 
 when the epiglottis is wanting or is too small 
 to cover the aperture, the sphincter muscles 
 surrounding the latter being capable of pro- 
 tecting the larynx against the entrance of 
 foreign substances. It is held by some 
 that the epiglottis has an important influ- 
 ence in modifying the voice according as it 
 more or less completely covers the exit to 
 the column of vibrating air. It is also held 
 that the epiglottis acts as a sort of sounding- 
 board, taking up and reinforcing the vibra- 
 tions of the air-column impinging against it. 1 
 Sweeping downward and backward from 
 FIG. 208. -vertical transverse section of each edge of the epiglottis is a sheet of 
 
 the larvnx (after Testut) : 1, posterior face of -, . , . 7 , . . /. 7 7 
 
 epiglottis, with i', its cushion; 2, aryteno- mucous membrane, the ary-epiglottic fold, 
 epigiottic fold; 3, ventricular band, or false w hich forms the lateral rim of the superior 
 
 vocal cord; 4, true vocal cord; 5, central . . 
 
 fossa of Merkei ; 6, ventricle of larynx, with aperture oi me larynx and which ends in, 
 6', its ascending pouch; 7, anterior portion and covers posteriorly, the arytenoid carti- 
 
 of cricoid ; 8, section of cricoid ; 9, thyroid, J J . J 
 
 cut surface; 10, thyro-hyoid membrane; ii, lages. The rounded prominence on the pos- 
 Sefls, SSS^raFES ** corner of this fold is made by the car- 
 13', its inner division, contained in the vocal tilage of Santorini, and a second, less marked, 
 
 cord ; 14, crico-thyroid muscle ; 15, subglottic n , i j. *j_ r ji >i r 
 
 portion of larynx; 1$, cavity of the trachea, swelling external to it, by the cartilage of 
 
 Wrisberg (Fig. 215). Looking down into 
 
 the larynx, it is seen that its lateral walls approach each other by the develop- 
 ment on each side of a permanent ridge of mucous membrane, known as the 
 ventricular band or false vocal cord (Fig. 208). 
 
 Ventricular Bands and Ventricles of Morgagni. The ventricular bands 
 or false vocal cords arise from the thyroid cartilage near the median line, a 
 short distance above the origin of the true cords. They are inserted into the 
 arytenoid cartilages somewhat below the apices of the latter. Their free bor- 
 der is more or less ligamentous in structure. They are brought into contact 
 by the sphincter muscles of the larynx, and thus protect the glottis. It has 
 even been stated that, in paralysis of the true cords, they may be set in vibra- 
 tion and be the seat of voice-formation. So-called " oedema of the glottis " is 
 chiefly due to accumulation of fluid in the wide lymph-spaces found in the 
 false cords. 
 
 1 Mills: Journ. of Physiology, 1883, vol. iv. p. 135. 
 
VOICE AND SPEECH. 
 
 The ventricular bands are parallel with and just above the true vocal cords, 
 from which they are separated by a narrow slit. They do not, however, reach 
 so near the middle line as the true cords, which can be seen between and below 
 the bands. The ventricular bands project more or less into the cavity of the 
 larynx like overhanging lips, so that each band forms the inner wall of a 
 space closed by the true vocal cords below, and communicating with the cavity 
 of the larynx through the narrow slit above mentioned. The spaces thus 
 bounded internally by the false cords are known as 
 
 The Ventricles of Morgagni (Fig. 208). No complete explanation has been 
 offered as to the purposes served by the ventricles of Morgagni and the false 
 vocal cords. Numerous mucous and serous glands seated in the ventricular 
 bands pour their secretions into the ventricles, whence the fluid may be trans- 
 mitted by the overhanging lips of the ventricular bands to the true vocal 
 cords ; hence, an important function of the former structure, probably, is to 
 supply to the vocal cords the moisture necessary to their normal action. The 
 secretion contained within the ventricle is protected by the ventricular band 
 from the desiccating influence of the passing air-currents. The existence of 
 the ventricular spaces also permits free upward vibration of the true cords. 
 The ventricles of Morgagni in some of the lower animals, as the higher apes, 
 communicate with extensiye cavities which serve an obvious purpose as reso- 
 nating chambers for the voice, and perhaps the preservation of this function in 
 the ventricles themselves is still of importance in the human being. It is not 
 improbable that the ventricular bands find their most important function as 
 sphincters of the larynx, the superior opening of which may be firmly occluded 
 by their approximation. The well-known fact that during strong muscular 
 effort the breath is held from escaping is, according to Brunton and Cash,' 
 due to the meeting of the false cords in the middle line. The overhanging 
 shape of the cords allows them to be readily separated by an inspiratory blast, 
 but causes them to be more firmly approximated by an expiratory effort. This 
 mechanism recalls the mode of action of the semilunar valves of the heart. 
 
 The true vocal cords arise from the angle formed by the side;/ of the thyroid 
 cartilage where they meet in front, a little below its middle point, and, passing 
 backward, are inserted into the vocal processes of the arytenoid cartilages. 
 The aperture between the vocal cords and between the vocal processes of the 
 arytenoids is known as the glottis or rima glottidis (Figs. 214, 215). Since, as 
 will be seen later, the vocal cords may be brought together while the vocal pro- 
 cesses of the arytenoids are widely separated at their bases, the space between 
 the cords themselves is sometimes called the rima vocalis and that between the 
 vocal processes the rima respiratoria. 
 
 In the adult male the vocal cords measure about 15 millimeters in length 
 and the vocal processes measure 8 millimeters in addition. In the female the 
 cords are from 10 to 11 millimeters in length. The free edges of the cord are 
 thin and straight and are directed upward ; their median surfaces are flattened. 
 Each cord is composed of a dense bundle of fibres of yellow elastic tissue, 
 1 Brunton and Cash : Journ. Anat. and Phys., 1883, vol. xvii. 
 
424 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 which fibres, though having a general longitudinal course, are interwoven, and 
 send off shoots laterally into the subjacent tissue. The compact ligament, 
 known commonly as the " vocal cord," forms only the free edge of a reflexion 
 from the side wall of the larynx. This reflexion is wedge-shaped in a vertical, 
 transverse section and contains much elastic tissue and the internal and part 
 of the external thyro-arytenoid muscle (Fig. 208). This whole structure 
 
 properly forms the vocal cord, and by 
 contraction of its contained muscle its 
 thickness and vibrating qualities may 
 be greatly modified. 
 
 Like the trachea, the larynx, with the 
 exception of the vocal cords, is lined 
 
 FIG. 209. Cartilages of the larynx, separated 
 (Stoerk) : 1, epiglottis ; 2, petiolus ; 3, median 
 notch of thyroid ; 4, superior cornu of thyroid ; 
 5, attachment of stylo-pharyngeus muscle; 6, 
 origin of thyro-epiglottic ligament ; 7, origin of 
 the thyro-arytenoid muscle ; 8, origin of true 
 vocal cord ; 9, inferior cornu of thyroid ; 10, car- 
 tilage of Wrisberg ; 11, cartilage of Santorini ; 12, 
 12', arytenoid cartilages, showing attachments of 
 the transverse arytenoid muscle; 13, 13', pro- 
 cessus muscularis, showing attachments of the 
 posterior and lateral crico-arytenoid muscles ; 
 14, base of the arytenoid cartilage ; 15, vocal pro- 
 cesses of the arytenoids ; 16, articular surface for 
 the base of the arytenoid cartilage ; 17, posterior 
 view of cricoid cartilage, with outline of attach- 
 ment of the posterior crico-arytenoid muscle ; 
 18, articular surface for inferior cornu of thyroid 
 cartilage. 
 
 FIG. 210. Cartilages and ligaments of the 
 larynx, posterior view (after Stoerk) : 1, epiglot- 
 tis; 2, cushion of the epiglottis; 3, cartilage of 
 Wrisberg ; 4, ary-epiglottic ligament ; 5, 8, mucous 
 membrane ; 6, cartilage of Santorini ; 7, arytenoid 
 cartilage ; 9, its processus muscularis ; 10, crico- 
 arytenoid ligament; 11, cricoid cartilage; 12, in- 
 ferior cornu of thyroid cartilage ; 13, posterior 
 superior cerato-cricoid ligament; 13', posterior 
 inferior cerato-cricoid ligament; 14, cartilages 
 of the trachea; 15, membranous portion of 
 trachea. 
 
 by columnar, ciliated epithelium, the direction of whose movement is upward 
 toward the pharynx. The vocal cords are covered by thin, flat, stratified epi- 
 thelium. The inner surface of the epiglottis, the walls of the ventricles, and 
 the ventricular bands contain much adenoid tissue, the spaces of which are apt 
 to become distended with fluid, giving rise to oedema of those parts. The 
 whole mucous membrane of the larynx, except over the vocal cords, is richly 
 supplied with glands both mucous and serous in character. 
 
VOICE AND SPEECH. .425 
 
 Cartilages of the Larynx. The mechanism of the larynx is supported 
 by a skeleton composed of several pieces of cartilage. The lowermost of these 
 cartilages is the cricoid cartilage, so called from its resemblance to a signet ring 
 (Fig. 209). The cricoid cartilage is situated above the topmost ring of the 
 trachea to which it is attached by a membrane. The vertical measurement of 
 the cricoid cartilage is about one inch on its posterior, and one-quarter inch on 
 its anterior surface. Superior to, and partly overlapping the cricoid, is the 
 thyroid cartilage, which forms an incomplete ring, being deficient posteriorly 
 (Fig. 209). The free corners of the thyroid behind are prolonged upward or 
 downward into projections known as the cornua. The upper pair are attached 
 to the extremities of the greater cornua of the hyoid bone, while by the inner 
 surface of the ends of the lower cornua the thyroid is articulated with the 
 cricoid cartilage and rotates upon it around an axis drawn through the points 
 of articulation. The lower anterior border of the thyroid cartilage is evenly 
 concave, but its upper border has a deep narrow notch in the middle line. 
 The upper half of the thyroid in front projects sharply forward in an elevation 
 known as Adam's apple (pomum Adami\ which is much more marked in adult 
 males than in females. The elliptical space between the cricoid and thyroid 
 cartilages in front is covered by a membrane. Adam's apple, the anterior part 
 of the cricoid ring, and the space between the two, can easily be felt in the liv- 
 ing subject ; they rise perceptibly toward the head with each swallowing movement. 
 
 The arytenoid cartilages are two in number and are similar in shape (Figs. 
 209, 210). Each cartilage, which has somewhat the form of a triangular 
 pyramid, is seated on, and articulates with, the highest point on the posterior 
 part of the cricoid cartilage some distance from the middle line. Of the free 
 faces of the pyramid, one looks backward, one toward the middle line, and the 
 third outward and forward. Each face is more or less concave. The apex of 
 each arytenoid cartilage is capped by a small body called the cartilage of San- 
 torini or, from its bent shape, corniculum laryngis (Figs. 209, 210). Outside 
 and in front of the latter is the minute cuneiform cartilage or cartilage of 
 Wtisberg, enclosed in the ary-epiglottic fold. The lateral posterior corner of 
 the arytenoid cartilage forms a blunt projection which serves for the attach- 
 ment of muscles, the processus muscularis. The anterior, lower, and median 
 part of each cartilage is of especial interest, since it serves for the posterior 
 attachment of the vocal cord ; it is known as the processus vocalis. 
 
 The thyroid and cricoid cartilages and the body of the aiytenoids are of 
 hyaline cartilage, and tend to become ossified in middle life. The other carti- 
 lages and the vocal processes of the arytenoids are composed of the elastic 
 variety. 
 
 The Muscles of the Larynx may be divided into two classes the extrinsic 
 and the intrinsic ; the former find their origin outside the larynx, and the latter 
 both arise and are inserted within it. 
 
 Extrinsic Muscles. To this group belong the sterno-hyoid, the stemo-thy- 
 roid, and the omo-hyoid muscles, which depress the larynx or hyoid bone; the 
 thyro-hyoid muscle, which depresses the hyoid bone or elevates the thyroid 
 
426 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 cartilage. To the elevators of the larynx belong the genio-hyoid, the mylo- 
 hyoid, the digastric, the stylo-hyoid, and the hyo-glossus. The muscles of the 
 palate and the constrictors of the pharynx enter into coordinated action with the 
 above. When food is passing through the pharyx in the act of swallowing, 
 the hyoid bone is drawn upward and forward, raising the larynx with it ; the 
 tongue is thrown backward so that the epiglottis covers the entrance into the 
 lairynx, and the constrictors of the larynx contract, completely closing the 
 entrance into that organ. 
 
 The intrinsic muscles of the larynx are the crico-thyroids, the lateral crico- 
 arytenoids, the posterior crico-arytenoids, the arytenoid, the aryteno-epiglot- 
 tideans, and the thyro-arytenoids ; all being in pairs except the arytenoid, 
 which crosses the middle line. The crico-thyroid muscle arises from the front 
 and side of the cricoid cartilage and, passing upward and backward, is inserted 
 into the lower edge of the thyroid cartilage (Fig. 211). The action of the crico- 
 thyroid muscle is to diminish the distance between the thyroid and cricoid car- 
 tilages in front, either by depressing the front of the thyroid or by elevating 
 that of the cricoid cartilage, or both. In the first case the distance between 
 the anterior attachment of the vocal cords and the vocal processes of the 
 
 arytenoid cartilages is increased by movement of 
 the thyroid, and in the second case the same effect 
 is produced by backward rotation of the edge of 
 the cricoid upon which the arytenoid cartilages are 
 seated (Fig. 210). The muscle, therefore, is a 
 tensor of the vocal cords. It is, probably, the 
 mechanism we ordinarily use in raising the pitch 
 of the voice when the vocal machinery has been 
 " set " by the other muscles (see below). If the 
 fingers be placed on the cricoid ring and on the 
 pomum Adami while the ascending scale is sung 
 in the middle chest register, both descent of the 
 front of the thyroid and ascent of the cricoid can 
 be made out. The lateral crico-arytenoid muscle 
 arises from the upper, lateral border of the cricoid 
 
 FIG. 211. Lateral view of the , i i i i 1^1 
 
 cartilages of larynx with the crico- cartilage, and passes upward and backward to be 
 thyroid muscle (Quain's Anatomy, i nser t e d into the outer edge of the arytenoid car- 
 after Willis) : 1, crico-thyroid mus- . /T i i i 
 
 cie; 2, crico-thyroid membrane; 3, tilage, on and in front oi the lateral prominence 
 
 (Fig. 212). Its main action is to wheel the 
 vocal process of the arytenoid toward the middle 
 line and thus approximate the vocal cords. The posterior crico-arytenoid is a 
 large muscle, which rises from the median posterior surface of the cricoid car- 
 tilage and passes upward and outward to be inserted into the outer surface of the 
 arytenoid cartilage, behind and above the insertion of the lateral crico-arytenoid 
 (Fig. 213). Its action is to turn the vocal processes outward and thus abduct the 
 vocal cords. The posterior crico-arytenoid occupies an important position in the 
 group of respiratory muscles; during vigorous inspiration it is brought into action 
 
 cricoid cartilage ; 4, thyroid carti- 
 lage ; 5, upper rings of the trachea. 
 
VOICE AND SPEECH. 
 
 and widens the glottis. Paralysis of this muscle is a most serious condition, since 
 it is followed by approximation of, and inability to separate, the vocal cords. 
 The arytenoid, or transverse or posterior aryteno'xl muscle, the single unpaired 
 
 -11 
 
 17 - 
 
 -13 
 
 FIG. 213. Larynx with its muscles, posterior 
 view (Stoerk) : 1, epiglottis ; 2, cushion ; 3, ary- 
 epiglottic ligament; 4, cartilage of Wrisberg; 
 5, cartilage of Santorini ; G, oblique arytenoid 
 muscles; 7, transverse arytenoid muscle; 8, 
 posterior crico-arytenoid muscle ; 9, inferior 
 cornu of thyroid cartilage; 10, cricoid car- 
 tilage ; 11, posterior inferior cerato-cricoid lig- 
 ament; 12, cartilaginous portion; 13, mem- 
 branous portion of trachea. 
 
 FIG. 212. Larynx and its lateral muscles after 
 removal of the left plate of the thyroid cartilage 
 (Stoerk) : 1, thyroid cartilage ; 2, thyro-epiglottic mus- 
 cle ; 3, cartilage of Wrisberg ; 4, ary-epiglottic mus- 
 cle ; 5, cartilage of Santorini ; 6, oblique arytenoid 
 muscles; 7, thyro-arytenoid muscle; 8, transverse 
 arytenoid muscle ; 9, processus muscularis of aryte- 
 noid cartilage ; 10, lateral crico-arytenoid muscle ; 11, 
 posterior crico-arytenoid muscle ; 12, crico-thyroid 
 membrane ; 13, cricoid cartilage ; 14, attachment of 
 crico-thyroid muscle ; 15, articular surface for the 
 inferior cornu of the thyroid cartilage; 16, crico- 
 tracheal ligament; 17, cartilages of trachea; 18, 
 membranous part of trachea. 
 
 muscle of the larynx, is a considerable band passing across the middle line from 
 the posterior surface of one arytenoid cartilage to that of the other (Fig. 213). 
 Its action is to draw the arytenoid cartilages together in the middle line and 
 approximate the vocal processes ; its action is essential in closing the glottis. In 
 the resting larynx the arytenoid cartilages are kept apart by the elastic tension 
 of the parts. The aryfeno-epiglottidean, sometimes called the oblique arytenoid, 
 muscles consist of two bundles of fibres seated upon the surface of the arytenoid 
 muscle (Fig. 213). Each muscle arises from the outer posterior angle of the 
 arytenoid cartilage, and, passing upward and inward, crosses in the rnicMle line 
 partly to be inserted into the outer and upper part of the opposite cartilage, 
 partly to penetrate the ary-epiglottic fold as far as the epiglottis, and the 
 remainder to join some fibres of the thyro-arytenoid muscle. The action of 
 the aryteno-epiglottidean muscles is to close the glottis. The thyro-arytenoid 
 is a muscle of complex mechanism, usually described as formed of two parts, 
 an external and an internal. The external thyro-arytenoid arises from the lower 
 
428 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 part of the angle of the thyroid cartilage ; its fibres pass, for the most part, 
 backward and somewhat upward and outward to be inserted into the outer 
 edge of the arytenoid cartilage and its lateral processus muscularis (Figs. 208, 
 214). Some of its bundles of fibres, however, have different directions, and 
 a portion of them pass upward into the ventricular bands. The internal thyro- 
 arytenoid, wedge-shaped in transverse section, lies between the muscular divis- 
 ion just described and the vocal ligament, by which its thin median edge is 
 covered. The internal thyro-arytenoid arises from the anterior angle of the 
 thyroid cartilage and is inserted into the processus vocalis and the outer face of 
 the arytenoid cartilage. Certain fibre-bundles of this, as of the external 
 division of the muscle, pass in various directions, some of them being inserted 
 into the free border of the vocal cord. The action of the muscle is, on the 
 whole, to draw the arytenoids forward and thus relax the vocal cords ; but, by 
 its contraction, the cords may also be approximated and their thickness, and 
 probably their elasticity, extensively modified. 
 
 Specific Actions of the Laryng-eal Muscles. To sum up the various 
 effects of the muscular action on the larynx : A sphincter action of the larynx 
 is brought about by the combined contraction of all the muscles with the 
 exception of the crico-thyroids and the posterior crico-arytenoids ; the vocal 
 
 cords are adducted and the glottis nar- 
 rowed by the transverse and oblique ary- 
 tenoids, the external thyro-arytenoids, 
 and the lateral crico-arytenoids; the 
 i.thy.ar.e. vocal cords are abducted and the glottis 
 -\m.thy.ar. widened chiefly or wholly by the poste- 
 rior crico-arytenoids ; the vocal cords 
 are made tense by contraction of the 
 crico-thyroids ; the vocal cords are slack- 
 ened by the combined action of the 
 FIG. 214. Diagram to illustrate the thyro-aryte- sphincter group and especially by the 
 
 nold muscles; the figure represents a transverse -. , . i 
 
 section of the larynx through the bases of the external thyro-arytenoids. 
 
 arytenoid cartilages (redrawn from Foster) : Ary, J^ w iH easily be Seen that in the 
 
 arytenoid cartilage; p.m, processus muscularis; i i i i u 
 
 P.V, processus vocalis: Th, thyroid cartilage; c.v, larynx, as in the skeleton at large, the 
 
 vocal cords; <E is placed in the oesophagus; efficiency of any single muscle involves 
 
 m.thy.ar.i, internal thyro- arytenoid muscle; ' 
 
 m.thy.ar.e, external thyro-arytenoid muscle; the action of accessory muscles ; thus, 
 
 of the crico-thyroid could 
 
 verse arytenoid muscle. have little effect in tightening the vocal 
 
 cords were not the arytenoid cartilages 
 
 fixed by contraction of the posterior crico-arytenoid and arytenoid muscles. 
 Nerve-supply of the Larynx. The larynx receives its nerve-supply from 
 the superior and the inferior or recurrent laryngeal nerves. The extremely 
 sensitive surface of the mucous membrane of the organ above the vocal cords 
 is supplied by sensory filaments of the superior laryngeal nerve. The superior 
 laryngeal also supplies motor fibres to the crico-thyroid muscle, whose action 
 as a tightener of the vocal cords is peculiar. All the other muscles of the 
 
VOICE AND SPEECH. 
 
 larynx receive their motor impulses from the inferior larynirral nerve. Much 
 of the nervous mechanism of the larynx is still in dispute. 
 
 Laryngoscopic Appearance of the Larynx. Much may be learned by 
 inspection of the larynx during life by means of the laryngoscopic mirror. It 
 is not difficult for an observer to examine his own larynx by placing himself 
 before a second mirror in which may be seen the image reflected from the 
 laryngoscope. To inspect the larynx the tongue must be held well out so 
 as to pull forward the epiglottis, then the structures below appear in the 
 laryngoscopic mirror in reversed position. Beneath the middle of the epiglottis 
 the cushion may be seen as a slight swelling, and continuing downward and 
 backward from the edges of the cartilage, may be seen the ary-epiglottic folds, 
 each marked at its extremity by tw r o rounded nodules, the cartilages of Wris- 
 bergand Santorini (Fig. 215). In quiet breathing the glottis is nearly stationary 
 and opened to the extent of from 3 to 5 millimeters. The vocal cords bounding 
 it look white and glistening in contrast with the red color of the general mucous 
 membrane. The cartilages of Santorini are several millimeters apart, and a 
 sheet of mucous membrane reaches from one to the other. The ventricular 
 
 15 
 
 FIG. 215. The laryngoscopic image in easy breathing (Stoerk) : 1, base of the tongue ; 2, median 
 glosso-epiglottic ligament ; 3, vallecula ; 4, lateral glosso-epiglottic ligament ; 5, epiglottis ; 6, cushion of 
 epiglottis; 7, cornu major of hyoid bone; 8, ventricular band, or false vocal cord; 9, true vocal cord; 
 opening of the ventricle of Morgagni seen between 8 and 9 ; 10, folds of mucous membrane ; 11, sinus 
 pyriformis ; 12, cartilage of Wrisberg ; 13, aryteno-epiglottic fold ; 14, rima glottidis ; 15, arytenoid carti- 
 lage ; 16, cartilage of Santorini ; 17, posterior wall of pharynx. 
 
 bands are seen as red shelves reaching to the outer margin of the shining 
 cords and separated from the latter by a dark line which is the entrance into 
 the ventricles of Morgagni. 
 
 When a deep inspiration is taken the glottis is widely opened, even to the 
 extent of half an inch ; an angle is formed between the vocal process of the 
 arytenoid and the vocal cord, the space between the cartilages of Santorini is 
 widened, and the rings of the trachea, and even its bifurcation may be seen 
 below. With the succeeding expiration the glottis again becomes narrow. 
 When the voice is sounded the picture at once changes. The space between 
 the cartilages of Santorini is obliterated, the vocal processes and cords are 
 
430 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 brought together, and the whole rim of the glottis or the vocal cords alone, 
 according to the pitch of the note, may be seen to vibrate. 
 
 2. THE VOICE. 
 
 The vocal machinery consists of (1) the motive power or breath ; (2) 
 the larynx, which forms the tone ; (3) the chest, the pharynx, the mouth, and 
 the nose, which color the tone ; and (4) the organs of articulation. 1 
 
 The production of voice is undoubtedly accomplished by the vibration of 
 the vocal cords which have previously been approximated in the middle line 
 and made tense through action of the nerve-muscular apparatus already de- 
 scribed. A blast of air from below pressing against the cords so adjusted, 
 causes them to separate and fall into vibration. We have to distinguish in 
 voice the three features of loudness, pitch, and quality. 
 
 The loudness of the tone depends on two factors: (1) the strength of the 
 tone-producing blast as determining not only the amplitude of vibration of 
 the vocal cords, but also the energy with which the air is expelled; (2) 
 the resonance of the two chambers between which the vocal cords are sus- 
 pended, the chest below and the cavities of the head above, whose walls and 
 contained air, by their sympathetic vibration, powerfully reinforce the oscilla- 
 tions imparted to them. 
 
 The pitch of the voice is determined by the thickness, tension, and length 
 of the vocal cords, conditions which regulate the pitch of the note obtained 
 from any vibrating string. The thickness and the elastic quality of the cords 
 are probably largely under the control of the thyro-arytenoid muscle. The 
 principal tensor of the cords is the crico-thyroid muscle. Other muscles, as 
 described above, may so fix the arytenoid cartilages that their vocal processes 
 may be prevented from taking part in the vibration of the cords throughout 
 the whole and also, possibly, throughout part only of their length. This 
 dampening of the vocal processes of the arytenoids may be accomplished either 
 by pressure applied to them throughout their whole length, in which case the 
 posterior part of the glottis is closed, or they may be pressed together at the 
 tips alone, leaving the respiratory glottis open as a triangular aperture. 
 
 Quality. Variation in the quality of the voice depends on the fact that 
 vibrations of the vocal cords are composite in character, giving rise to notes 
 made up of a fundamental tone combined with upper partial tones (see p. 383). 
 By reason of the varied adjustments that may be imparted to it, the larynx is 
 capable of producing many more qualities of tone than is any artificial instru- 
 ment. 2 Change in the size and shape of the resonance-chamber above and 
 below the vocal cords produces a corresponding change in their fundamental 
 notes and, therefore, in the partial tones of the voice which they reinforce by 
 sympathetic vibration (see p. 385). According to Helmholtz, 3 the difference 
 in quality between the various vowel sounds of the human voice depends on 
 
 1 C. H. Davis: The Voice, 1879. 
 
 2 Helmholtz: Sensations of Tone, trans, by Ellis, 1885, p. 98. 
 
 3 Op. tit., p. 104. 
 
VOICE AND SPEECH. 431 
 
 the number and relative prominence of the various overtones determined by 
 altering the shape and size of the nasal and buccal resonance-chambers. 
 
 By a simple experiment the production of voice by the vocal cords can 
 easily be illustrated. Take a glass tube, about J inch in diameter and of con- 
 venient length, and press one end firmly against the palmar surfaces of the 
 proximal phalanges of two fingers at their line of division when thiy are 
 brought together. By blowing smartly into the other end of the tube, a 
 musical note will be produced by the vibration of the folds of the skin be- 
 tween which the air is forced. By relaxing the pressure with which the 
 fingers are held together, the length of the vibrating segment of skin is in- 
 creased and its tension diminished ; its note is accordingly lowered. The 
 reverse conditions are produced when the fingers are held together tightly and 
 the tube applied firmly ; the pitch of the note is then raised. In these ways 
 the pitch of the note may be varied through two octaves, which is the range 
 of a good singing voice. Various upper partials of the note so produced may 
 be made prominent by sympathetic resonance, if the vibrating air-stream is 
 sent across the opening of a wide-mouthed bottle, of about a pint capacity. 
 The air within the bottle is thrown into sympathetic vibration when its funda- 
 mental tone is contained in the note emitted through the fingers ; when the 
 volume of the air is diminished by slowly pouring water into the bottle, the 
 fundamental tone of the resonator is changed, and it responds to one after 
 another of the partials contained in the musical note. 
 
 The marvellous adjustment of muscular action by which, at will, notes 
 may be struck of definite pitch and quality, is evidence of an elaborate 
 nervous machinery for the larynx, not only on the efferent side but, possibly 
 through a muscular sense, on the afferent side as well. The various phe- 
 nomena of aphasia, and the anatomical importance of the cerebral areas 
 devoted to the elaboration of speech, point in the same direction. The 
 relations between the centres for speech and hearing are most intimate. The 
 ear plays a constant part, as a critical medium, in the tuition of the vocal 
 organs in either speech or song. So-called " dumbness " is the result, usually, 
 not of defects in the vocal organs, but of lack of hearing and, hence, of 
 inability to control by the ear the pitch or quality of the vocal notes. 
 
 The voice and the larynx of the child fall naturally in a group with those 
 of the female as contrasted with the adult male. At the age of puberty a 
 boy's larynx becomes congested and undergoes rapid development. The voice 
 changes rapidly from the juvenile to the adult quality. During this change, 
 the voice frequently "breaks" or rapidly returns from the newly-acquired 
 chest register to the head or falsetto notes of childhood (see p. 433). In boy!T\ 
 who are castrated a good while before the age of puberty is reached, the larynx ( 
 does not undergo its characteristic development, and the voice -remains of a 
 peculiar quality, much valued in some countries in the rendition of vocal 
 music. The practice of castration for aesthetic purposes has, accordingly, in 
 certain districts, long been in vogue. In the female the changes in the larynx 
 and in the voice at puberty are much less marked than in the male. 
 
432 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 Arrangements for Chang-ing- the Pitch of the Voice. As has frequently 
 been mentioned, the vocal cords are stretched, and the pitch of their note is 
 elevated, by contraction of the crico-thyroid muscle. But the change that is 
 thus produced in the tension of the vocal cords is by no means capable of 
 accounting for the full range of pitch which falls within the compass of the 
 voice. When the arytenoid and the crico-arytenoid muscles sufficiently con- 
 tract, the vocal processes are brought tightly together and their vibration is 
 prevented. Voice-production must then be limited to the vocal cords them- 
 selves, and the stretching action of the crico-thyroids may begin anew and 
 reach its maximum with the glottis so set that only its ligamentous borders can 
 vibrate. It can also be seen that the vocal cords themselves may be shortened 
 functionally, or even be broken up into segments, or the main body of the cord 
 be changed in thickness, by contraction of the complex thyro-arytenoid muscles; 
 each such condition would be accompanied by a change in the rate of vibra- 
 tion. We are probably justified in assuming that, when the musical scale is 
 sung, the lowest notes are produced by vibration of the glottic borders through- 
 out their full length, and the elevation of pitch is affected by the gradually- 
 increased tension of the vocal ligaments through the action of the crico-thyroid 
 muscle. This contraction having reached its maximum, the muscle probably 
 relaxes, only to contract again after the vibrating segments of the glottis are 
 shortened by a partial or complete clamping together of the vocal processes 
 in the manner described above. There are thus two or three, or more, 
 adjustments which may be imparted to the vibrating mechanism of the lar- 
 ynx, each of which is distinguished by giving rise to a note of different 
 pitch that may further be altered by action of the crico-thyroid muscle. 
 It might be anticipated that the voice whose pitch was gradually ele- 
 vated in the manner described would suffer some alteration in quality 
 at those points in the scale where there is a change in the set of the lar- 
 ynx producing a shortening of the, vibrating segment. Such, indeed, is 
 the fact. 
 
 Registers. Long before the invention of the laryngoscope, and before any- 
 thing definite was known of the method of voice-production, it was recognized 
 that in ascending the musical scale there occur certain breaks, as it were, where 
 the voice changes in quality as well as in pitch. It is an object in musical 
 education to render these breaks as little prominent as possible. The kinds of 
 voice included between these breaks were distinguished as the vocal " registers." 
 There is no general agreement among musicians as to how many registers are 
 compassed by the voice, and the nomenclatures used to distinguish them differ 
 in the most confusing fashion. According to some authors, the range of the 
 voice is included within two registers only; more commonly three distinct 
 registers are described, to which, in certain cases, a fourth is said to be prob- 
 ably added. The most common designation of the lowest register is the " chest 
 voice/' though it has also been called " thick" 1 as distinguished from the 
 " thin " register ; another term applied to it is the " long-reed " register as con- 
 1 Browne and Behnke : Voice, Song, and Speech, 1890, p. 135. 
 
VOICE AND SPEECH. 433 
 
 trasted with the " short-reed " register. 1 The middle register of all voices is 
 by some authors (Garcia, 2 Mme. Seiler 3 ) denominated the "falsetto," while 
 other writers use this term to distinguish certain higher notes of the male 
 voice of a peculiar quality not in ordinary use. The third and highest series 
 of vocal sounds is usually known as the " head " register. 
 
 The lowest or chest register is that used in ordinary life. It is so called 
 from the strong vibrations of the chest-wall which may be felt while the voice 
 is sounded. In passing to the higher register the chest vibration is found to 
 diminish and that of the head bones to increase; in the one case the cavity of 
 the head acts strongly as a resonance chamber, and in the other that *of the 
 thorax. According to Madame Seiler, in the lowest register both the vocal 
 ligaments and the vocal processes of the arytenoids vibrate. In the middle 
 register the vocal processes are clamped together and the vibration of the liga- 
 ments seems confined chiefly to their sharp edges ; while in the highest register 
 the ligaments themselves appear to be damped throughout the greater part of 
 their length, the vibrations being confined to the edges of an oval slit at their 
 
 FIG. 216. The voicing (female) larynx (after Browne and Behnke). A, Small or highest register. B, 
 Upper thin or middle register. C, Lower thin or middle register: T,T, tongue; F,F, false vocal cords; 
 S,S, cartilages of Santorini ; W, W, cartilages of Wrisberg; V, V, vocal cords. 
 
 anterior ends (Fig. 216). Within any definite register the quality of individual 
 voices is determined by the size and elasticity of the parts of the larynx, and 
 probably also by peculiarities of the resonating chambers ; voices are accord- 
 ingly classified as base, tenor, alto, and soprano. 
 
 A Whistling Register. A friend and former pupil of the author's has the remark- 
 able power of emitting from the larynx notes which are indistinguishable in quality from 
 an ordinary whistle. He writes, " The whistle cannot be made to ' slide ' into vocal tones 
 of any sort, nor can any other tones be produced simultaneously with it. Its range is 
 about one and a half octaves, or half an octave less than my singing voice. 
 
 "The lips have nothing to do with the sound except as their position changes the reso- 
 nance-quality of the tone by ' reinforcement ' or otherwise, for I can whistle almost as read- 
 ily with the teeth closed and the lips wide parted as with the jaws and lips firmly closed as 
 in the ordinary position. Any other movement of the air-column destroys the sound at 
 once." Some years ago the author made a laryngoscopic examination of this larynx while 
 it was in the act of whistling. No notes were written at the time, but the picture remem- 
 bered is that of vocal cords closely approximated, except for an oval slit between their 
 anterior and middle portions, as in singing head tones, the cords vibrating chiefly along 
 their free edges. 
 
 Speech. Language consists, in general, of a combination of short musical 
 sounds, voweh or sonants, which are produced purely by vibration of the vocal 
 
 1 Mackenzie : Hygiene of the Vocal Organs, 1891, p. 55. 
 
 2 Garcia: Lond., Edin., and Dub. Mag., vol. x. 1855, p. 218. (Quoted by Seiler.) 
 
 3 Seiler : op. cit. 
 VOL. II. 28 
 
434 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 cords, together with superadded noises or modes of obstruction, con-sonants, 
 produced by action of the mouth-parts. The vowel sounds usually carry the 
 accent of syllables, and the consonants, for the most part, are sounded only 
 with, or represent peculiar modes of obstructing the former. No classification 
 of vocal signs can be made in which exceptions do not form important addenda 
 to general rules. 
 
 Articulation is the modification of sound in speech, usually effected by action 
 of the lips, the tongue, the palate, or the jaws, and the place of articulation 
 depends, in any definite case, on the mode in which a sound is formed. Its use 
 as an expression of thought is the chief physiological distinction between man 
 and the lower animals. Distinctness of articulation, so essential to clearness 
 of language, not to mention its aesthetic value, depends on the accuracy of the 
 muscular adjustments used in forming sounds, especially consonantal sounds. 
 
 The speaking is distinguished from the singing voice partly by the fact that 
 most sounds in the first case are articulate or formed in the mouth, while in 
 the latter their quality is only there modified. In singing the tone is sustained 
 at the same pitch for a considerable interval, while in speaking the voice is con- 
 tinually sliding up and down on the vowel sounds. In speaking the conso- 
 nantal noises and obstructions are more prominent because of their more abrupt 
 formation. 1 - 2 
 
 Vowel sounds owe their origin to vibration of the vocal cords, and their 
 quality to the selective resonance of the cavities above the cords. In sounding 
 the series of vowels, a, e, i, o, u (pronounced ah, a, e, o, oo), it is found that the 
 
 U 
 
 FIG. 217. Section of the parts concerned in phonation, and the changes in their relations in sound- 
 ing the vowels AC*), I (), V () (after Landois and Stirling) : T, tongue ; p, soft palate ; e, epiglottis ; g, glot- 
 tis ; h, hyoid bone ; 1, thyroid ; 2, 3, cricoid ; 4, arytenoid cartilage. 
 
 form and size of the mouth-cavity, the position of the tongue, the position of 
 the soft palate separating or allowing communication between the nasal and 
 pharyngeal cavities, undergo a progressive change (Fig. 217). Helmholtz has 
 shown that the vowel sounds owe their differences of quality to the varied 
 resonance of the mouth-cavity, dependent on its shape, through which now one, 
 now another, of the overtones in the note produced by vibration of the'Yocal 
 cords is reinforced. 3 This result is dependent on the fact that when the mouth 
 is set in position for the formation of the various vowel sounds the pitch of its 
 
 1 Browne and Behnke : op. cit., p. 28. 
 
 2 Monroe: Manual of Physical and Vocal Training, 1869, p. 51. 
 
 3 Helmholtz : loc. cit. 
 
VOICE AND SPEECH. 435 
 
 fundamental note, or the rate of vibration to which it sympathetically responds, 
 varies accordingly. 1 That the resonance of the mouth cavity changes with 
 its shape is illustrated in the various pitch of the notes produced by flipping 
 the edge of an incisor tooth, the cheek, or Adam's apple with the finger-nail, 
 while the mouth assumes the positions for production of the different vowels. 
 
 Vowels whose normal pitch is low, as o, u, cannot be sounded easily in the 
 higher part of the musical scale ; conversely, high-pitched vowels, as e in feet, 
 lose their character in the lower part of the scale. Language is, therefore, 
 much less distinct in song than in speech. 2 
 
 It has already been stated that the difference in quality of musical notes 
 depends upon the number and relative intensity of their partial tones, each 
 of which is separated from the fundamental tone by a fixed interval. Since 
 the mouth parts have a fairly fixed position for each vowel sound, the buccal 
 cavity reinforces by sympathetic resonance tones of definite vibration rates. 
 When a given vowel is sounded in different parts of the musical scale, now 
 one, now another partial tone is reinforced, according as its pitch harmonizes 
 with the prime tone of the mouth cavity, so that the interval between the 
 resonated partials and their fundamental tone may change, with corre- 
 sponding change in the quality of the vowel sound. That is, the resonated 
 partial depends not only on its relation to the fundamental, but also on its 
 vibration rate. 3 This feature of vocal resonance distinguishes the human 
 larynx from most musical instruments. That the ground is not covered by 
 these facts was shown by Auerbach, 4 who demonstrated that the strength of 
 upper partials in vowel sounds depends also on the strength of their production 
 by the vocal cords and, therefore, upon their relation to the fundamental tone. 
 That is to say, the quality of a vowel is dependent not only on the absolute 
 vibration numbers of its upper partials, according to which they are or are not 
 reinforced by the position of the mouth, but also on the relative position of these 
 upper partials as compared witli the fundamental tone. 
 
 The peculiar aesthetic value of the human voice is dependent on the fact 
 that, on account of its varied powers of adjustment, the larynx is capable of pro- 
 ducing many more kinds of tone-quality than any artificial instrument. Helm- 
 holtz 5 found no less than sixteen overtones to accompany the fundamental. 
 
 The posture of the mouth-parts differs markedly when set for the various 
 principal vowel sounds ; but as we know that each vowel sound has several 
 modifications or gradations so that a tone may pass by an easy glide from one 
 to another, so the form of the mouth passes by insensible steps from one vowel 
 position to another. It will be seen later that several articulate sounds play 
 the part now of vowels, now of consonants, according to their position in the 
 syllable or mode of formation. There has also been shown reason for believ- 
 ing that the form of the chest cavity and the tension of its walls are factors in 
 determining the pitch of its fundamental tone; so that through the varied 
 
 1 Helmholtz : op. tit., p. 108. 2 Op. tit., p. 114. 3 Op. cit., p. 118. 
 
 4 Quoted by Grutzner: op. cit., p. 179. 6 Op. cit., p. 103. 
 
436 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 sympathetic resonance of the thorax the reinforcement of laryngeal tones may 
 here be altered somewhat, as in the mouth itself. 1 ' 2 
 
 Whispering- is a mode of speech in which noise largely replaces pendular 
 musical vibrations. The glottis remains more or less widely open and the 
 vocal cords are not tense ; the vibrations are produced both in the larynx and 
 in the buccal-pharyngeal chambers. Vowel sounds may be produced in whis- 
 pering as well as in true voice because, from the multitude of irregular vibra- 
 tions, those waves are reinforced which make up the vowel sounds determined 
 by the set of the mouth. Gentle whispering requires much less effort than does 
 speaking, and inspiratory whispering is less easily distinguished from expiratory 
 than is the strained voice of inspiration from the natural sound of expiration. 
 Consonants, as already indicated, may sometimes play the part of vowels, but 
 pure consonants do not appear in syllables except in combinations with vowels, 
 which combinations always carry the syllable accent. 
 
 Consonants. The distinction between consonants and vowels lies in the 
 fact that the tones of the latter are produced by vibration of the vocal cords, 
 the parts above which act only as resonance-boxes and modify the sound, and 
 never offer marked obstruction to the exit of air ; whereas in the formation of 
 consonants there is some adjustment in the mouth-passage either in the nature 
 of a local narrowing, by which a peculiar noise is added to the vocal sound, or 
 in the nature of a sudden closing or opening of the air-channel by which a 
 characteristic noise is likewise added to the vocal sound. In other words, the 
 parts above the larynx make the sounds of consonants but only modify those 
 of vowels. 3 No sharp line of separation can be drawn between vowels and 
 consonants, since certain characters, according to their associations, now fall 
 into one, now into another class. In the classification of consonantal sounds 
 much confusion exists, dependent chiefly on the fact that several letter charac- 
 ters change their modes of formation and expression with their place in the 
 syllable. The same facts, also, are expressed by different authors by different 
 nomenclatures, and sounds occur in one language that are not found in another. 
 Adopting the general classification of Grutzner, 4 we may divide consonants 
 into the following three groups: 
 
 1. Semi-vowels or liquids, which can be used either as. vowels or consonants; 
 this group includes the sounds m, n, ng, I, and r. In expressing the function 
 of a consonant, the letter is not to be sounded as if it stood alone, but its cha- 
 racter given as actually expressed in a syllable ; thus the sound of p is not pee, 
 but is the abbreviated labial expression, as [unpack or piece when all the letters 
 are eliminated after the first. Of the liquids the n, m, and ng (sometimes 
 called " resonants ") have the nature of vowels when final (as in him, hen, 
 being}, and are then produced by vibration of the vocal cords, the lips having 
 previously been closed for the m, and the tongue applied to the roof of the 
 mouth to cut off the exit of air for n and ng ; the expelled air escapes alto- 
 gether through the nose, which acts as a resonance-chamber. Used as conso- 
 
 1 Op. tit., p. 93. 2 Sewall and Pollard : Journal of Physiology, 1890, vol. xi., p. 159. 
 3 Grutzner: op. tit., p. 196. * Op. tit., p. 197. 
 
VOICE AND SPEECH. 
 
 437 
 
 nants, as in make and no, m and n are seen to have the characters of the second 
 group, Explosives. L is pronounced somewhat like n, but air is allowed t<> 
 escape through the mouth on each side of the tongue ; it may be produced 
 either with voice or without voice (in whispers). It may have vowel charac- 
 ters as in play. R is characterized as a vibrative and may have several seats 
 of articulation, as by the thrill of the tip of the tongue against the hard 
 palate, or that of the hind part of the tongue against the soft palate, or even 
 by the coarse vibration of the vocal cords themselves. In the first two cases 
 it may be sounded either with or without voice. Its vowel nature is shown in 
 such words as pray. 
 
 2. Explosives, which are produced either when an obstruction is suddenly 
 offered to or removed from the exit of air from the mouth ; at the same time 
 a characteristic noise is produced. They may be subdivided according to the 
 place of articulation into labials (p, v) ; linguo-palatals (t, d) ; gutturals (k, g). 
 The similarity in the method of formation of p and b, t and d, k and g, is 
 striking. They are frequently characterized as being formed with or without 
 voice ; that is, b, d, and g require voice for their distinct recognition, and when 
 whispered they are easily mistaken for p, t, k, which latter do not require voice 
 (vibration of the vocal cords) for their recognition. A consonant, then, is said 
 to be formed with voice when it can be rendered distinctly only by an accom- 
 panying vibration of the vocal cords, without voiee when articulated clearly 
 without laryngeal aid. The former are sometimes called sonants, the latter 
 surds. This classification only approximates the truth, for the suddenness and 
 energy with which the obstruction to the breath is removed determines our 
 recognition of the consonant irrespective of voice. 1 
 
 Table of Consonantal Elements? 
 
 PLACE OF ARTICULATION. 
 
 ORAL. 
 
 NASAL. 
 
 Momentary. Continuous. 
 
 Continuous. 
 
 Surds 
 (without voice). 
 
 Sonants 
 (with voice). 
 
 Surds 
 (without voice). 
 
 Sonants 
 (with voice). 
 
 Sonants 
 (with voice). 
 
 Lips 
 
 P 
 
 b 
 
 
 w 
 th(y) 
 z, r 
 zh, r 
 7,1 
 
 ra 
 n 
 
 ng 
 
 Lips and teeth 
 
 f 
 th(in) 
 
 s 
 sh 
 
 Tongue and teeth . . . 
 
 
 
 Tongue and hard palate 
 (forward) 
 Tongue and hard palate 
 (back) . . . 
 
 t 
 ch 
 
 d 
 
 j 
 
 Tongue, hard palate, and 
 soft palate . 
 
 Tongue and soft palate . 
 Various places 
 
 k 
 h 
 
 g 
 
 
 
 
 3. Friction sounds or frictionals, often called aspirates, are all noises pro- 
 duced by the expired blast passing through a constriction in its passage, at 
 which point a vibration is set up. No obstruction being offered to the sound, 
 they are known as continuous as distinguished from the momentary sounds of 
 
 1 Griitzner, op. cit, pp. 211, 213. 
 
 2 Webster s International Dictionary, 1891, p. Ixvi. 
 
438 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 group 2. They may be divided into labio-dental frictionals, f (without voice) ; 
 v, w (with voice) ; the lingual frictionals s, th (as in them) ; sh, ch soft (with- 
 out voice) ; 2, j (with voice). The sound of h may be regarded as due to the 
 vibration of the separated vocal cords. It is peculiar, however, in appearing 
 to be formed in any part of the vocal chamber ; when it is formed the mouth 
 parts take on no peculiar position, but assume that of the vowel following the 
 h t as hark 9 hear, etc. 
 
V. REPRODUCTION. 
 
 THE principles and problems of Physiology that have been already pre- 
 sented in this work, comprising nutrition and the functions of the muscular 
 and the nervous systems, have reference to the individual man or woman. 
 Through the normal activity of those functions and their appropriate co- 
 ordination the individual lives his daily life and performs his daily tasks as 
 an independent organism. But man is something more than an independent 
 organism ; he is an integral part of a race, and as such he has the instincts of 
 racial continuance. The continuance of the race is assured only by the pro- 
 duction of new individuals, and the strength of the human reproductive 
 instinct is indicated in some measure by the large proportion of energy that is 
 expended by woman in the bearing of children and by both sexes in the nur- 
 ture and education of the young. The function of reproduction is not limited 
 to the daily life and well-being of independent organisms. It has a deeper 
 significance than that. Its essence lies in the fact that it has reference to the 
 species or race. Many of its problems are, therefore, broad ones ; they in- 
 clude not only the immediate details of individual reproduction, but larger 
 ones relative to the nature and significance of reproduction and of sex, and to 
 heredity. In the following discussion, while attention will be given chiefly 
 to the facts of individual reproduction, some of the broader applications of 
 the facts will be indicated. 
 
 A. REPRODUCTION IN GENERAL. 
 
 In all forms of organic reproduction the essential act is the separation from 
 the body of an individual, called the parent, of a portion of his own material 
 living substance, which under suitable conditions is able to grow into an inde- 
 pendent adult organism. 
 
 Among living beings two methods of reproduction are recognized, the 
 asexual and the sexual methods. Both are widespread among animals and 
 plants, but the asexual method is the more primitive of the two and is rela- 
 tively more frequent in low organisms. The sexual method, the only one 
 present in the production of new individuals among the higher animals, has 
 evidently been acquired gradually, and has probably been developed from the 
 asexual method. 
 
 Asexual Reproduction. Asexual reproduction, or agamogenesis, is the 
 chief method of reproduction among unicellular plants and animals, and 
 throughout the plants und in the lower nr 7T ;^ar animals it is important. 
 Among various species it takes various foi < n a-> fi ion or division, 
 
 gemmation or budding, exogenous cell-fun, -tion or spore-formation or multi- 
 
 439 
 
440 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 pie fission ; but all the varieties are modifications of the simplest form, fission 
 or division. In fission, found only in unicellular organisms and typified in 
 Amoeba, the protoplasm of the single cell, together with the nucleus, becomes 
 divided into two approximately equal portions which separate from one 
 another. In the process no material is lost, and two independent nucleated 
 organisms result, each approximately half the size of the original. The 
 parent has become bodily transformed into the two offspring, which have only 
 to increase in size by the usual processes of assimilation in order themselves 
 to become parents. In higher organisms, even where sexual processes alone 
 prevail in the production of new individuals, the asexual method has per- 
 sisted in the multiplication of the individual cells that constitute the body ; 
 embryonic growth is an asexual reproductive process, a continued fission, dif- 
 fering from the amoeboid type in the facts that the resulting cells do not sepa- 
 rate from one another to form independent organisms, but remain closely 
 associated, undergo morphological differentiation and physiological specializa- 
 tion, and together constitute the individual. Likewise in the adult the pro- 
 duction of blood-corpuscles and of epidermis, the regrowth of lost tissues, and 
 the healing of wounds are examples of asexual cell-reproduction. From the 
 standpoint of multicellular growth Spencer and jHaeckel have happily termed 
 the process of asexual reproduction in unicellular organisms " discontinuous 
 growth." 
 
 Sexual Reproduction. Sexual reproduction, or gamogenesis, -occurs in 
 unicellular organisms, where it is known as conjugation, and it is the prevail- 
 ing form of reproduction in most of the multicellular forms. In most of the 
 invertebrate and vertebrate animals it is the sole form of reproduction of 
 individuals. In its simple form of conjugation, typified in the minute monad, 
 Hderomita, it consists of a complete fusion of the bodies of two similar indi- 
 viduals, protoplasm and nuclei, followed by a division of the mass into 
 numerous spore-like particles, each of which grows into an adult Heteromita. 
 In the higher infusorian, Paramceeium, the fusion of the two similar individ- 
 uals is a partial and temporary one, during which a partial exchange ->f 
 nuclear material takes place; this is followed by separation, after which eai h 
 individual proceeds to live its Ordinary life and occasionally to multiply by 
 simple fission. 
 
 In the highly specialized sexual reproduction of higher animals, including 
 man, the individuals of the species are of two kinds or sexes, the male m 1 
 the female, with profound morphological and physiological differences between 
 them ; in each the protoplasm of the body consists of two kinds of cells, somatic 
 cells and germ-cells, the former subserving the nutritive, Min-riil i vous 
 
 functions of daily life, the latter subserving reproduction. The germ-cell.^ of 
 the male, called spermatozoa, are relatively small and active, thos of the 
 female, called ova, are relatively large and passive; the i**producti\ 
 consists of a fusion of a male and a female germ-eel ! -ontiaJ part being 
 
 a fusion of their nuclei ; and this ik followed by continued asexual c<:ll-r!ivision 
 and growth into a new individual. Among both p;auts aim! an- niuls it is not 
 
REPRODUCTION. 441 
 
 difficult to find a series of forms showing progressively greater and greater 
 deviations from the typical asexual toward the typical sexual method of 
 reproduction, and the existence of such a series is indicative of the derivation 
 of the latter from the former type. 
 
 Origin of Sex, and Theory of Reproduction. It is obvious that the 
 production of new individuals is necessary to the continued existence of any 
 species. It would be interesting to know the origin and significance of the two 
 existing methods of reproduction. Apropos of the asexual process, Leuckart, 
 and especially Herbert Spencer, have pointed out that during the growth of 
 a cell the mass increases as the cube, but the surface only as the square, of 
 the diameter i. e. the quantity of protoplasm increases much more rapidly 
 than the absorptive surface. It follows from this that during the growth of a 
 unicellular organism a size will ultimately be reached beyond which the cell 
 will not be able to absorb sufficient food for the maintenance of the proto- 
 plasm. In order that growth may continue beyond this point, a division of 
 the cell, which ensures a relative increase of surface over mass, is absolutely 
 necessary. Fission is, therefore, a necessary corollary of growth, and, although 
 we are ignorant of the details of its mechanism, it is conceivable that the method 
 of asexual reproduction arose through causes connected with growth. 
 
 The explanation of sexual reproduction is much more difficult, for here, in 
 addition to the budding off of the germ-cells from the parental bodies, which 
 has probably the same fundamental cause as fission in unicellular forms, we 
 must account for the differentiation into sexes, the existence of special sexual 
 cells, and the fusion of the male and the female germinal substance ; in short, 
 we must account for the conception of sexuality itself and all that it implies. 
 
 Regarding the origin of sexuality itself, as to the question whether sexuality 
 is an original and fundamental attribute of protoplasm or has been acquired, 
 we may say at once that at present we know really nothing. Yet, whatever 
 view is held as to the origin of sexuality, it seems entirely probable that the 
 method of reproduction known as sexual is a derivative of the method known 
 as asexual the latter is primitive, the former has arisen from it. From the 
 wide distribution and prominence of the former among vital phenomena we 
 must believe, with biologists generally, that sexual differentiation and sexual 
 processes have arisen from natural causes, and for the reason that sexual repro- 
 duction is of advantage to living beings and to species. In what way it is of 
 advantage, however, is disputed. Three views, all of which have evidence in 
 their favor and which are not mutually exclusive, are at present engaging the 
 attention of scientific men. The first to be mentioned is the theory advocated 
 by Hensen, Edouard van Beneden, and Butschli, according to whom the fusion 
 of the cells in sexual reproduction exists for the purpose of rejuvenating the 
 living substance. The power possessed by cells of dividing asexually is 
 limited; in time the protoplasm grows old and degenerates; its vital powers are 
 weakened, and without help the extinction of the race must follow. But the 
 mingling of another strain with such senescent protoplasm gives it renewed 
 youth and vig^r, restores the power of fission, and grants a new lease of life to 
 
442 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 the species. From his observations upon the Infusoria, Maupas 1 has brought 
 forward valuable evidence which has been quoted in favor of this view. Sty- 
 lonychia normally produces by fission 130 to 180 generations or individuals, 
 Onychodromus 140 to 230, and Leucophrys patula 300 to 450, after which con- 
 jugation is necessary to continued division. If conjugation be prevented, the 
 individuals become small, their physiological powers become weakened, their 
 nuclei atrophy, and the chromatin disappears; all of which changes are evidence 
 of the oncoming of senile degeneration, and this ultimately results in death. 
 Analogous to this is doubtless the fact, pointed out by Hertwig, 2 that in sexual 
 animals an unfertilized ovum within the oviduct soon becomes over-mature 
 and enfeebled, and subsequent fertilization, even though possible, is abnormal. 
 Even if the idea of " rejuvenescence" be regarded as fanciful and as a com- 
 parison rather than an explanation, it seems to be a principle of nature that 
 occasional fusion of one line of descent with another is necessary to continued 
 reproduction and continued life. 
 
 A second theory, defended by Hatschek and Hertwig, argues that sexual 
 reproduction prevents variation, and thus preserves the uniformity of the race. 
 The mingling of two different individuals possessing different qualities must 
 give rise to an individual intermediate between the parents, but differing from 
 them. Such differences between parents and offspring are numerous, but in a 
 single generation are minute, and they are easily obliterated by a subsequent 
 union, which latter in turn gives rise to other minute differences. Hence sexual 
 reproduction, although constantly producing variations, as constantly eradicates 
 them, and, by striving always toward the mean between two extremes, tends 
 toward homogeneity of the species. The essential truth of such a view seems 
 obvious. 
 
 A third theory, advocated by Weismann and Brooks, is quite the opposite 
 of the last, and maintains that the meaning of sexual reproduction lies in the 
 production of variations. " The process furnishes an inexhaustible supply of 
 fresh combinations of individual variations." These minute variations, seized 
 upon by natural selection, are augmented and made serviceable, and a variety, 
 better able to cope with the conditions of existence, results. The transformation, 
 not the homogeneity, of the species is thereby assured. The two latter views are 
 not necessarily mutually exclusive. Both claim that fertilization brings into 
 evidence variations. It is quite conceivable that subsequent fertilizations may 
 obliterate some and augment others, the result of union being the algebraic sum 
 of the characteristics contributed by the two sexes. 
 
 Primary and Secondary Characters. In the human species, as in all 
 the higher sexual animals, the characters of sex, anatomical, physiological, and 
 psychological, are divisible into two classes, called primary and secondary. 
 Primary sexual characters are those that pertain to the sexual organs them- 
 selves and to their functions. They are naturally the most pronounced of all 
 
 1 E. Maupas : Archives de Zoologie experimentale et generate, 2e srie, vii., 1889. 
 
 2 O. und K. Hertwig : Experimentelle Studien am thierischen Ei vor, wdhrend und nach der 
 Befruchtung, L, 1890. 
 
REPRODUCTION. 443 
 
 sexual attributes. Secondary sexual characters comprise those attributes that 
 are not directly connected with the sexual organs, but that, nevertheless, con- 
 stitute marked differences between the sexes; such are the greater size and 
 strength of man's body as compared with woman's, the superior grace and 
 delicacy of woman's movements, the deeper, rougher voice of man, and the 
 higher, softer voice of woman. In reality, all secondary sexual characters are 
 accessory to the primary ones, and the greater portion of the present article 
 will be devoted to a discussion of the latter. The primary sexual characters 
 of the male centre in the production of spermatozoa and the process of impreg- 
 nation, those of the female in the production of ova and the care of the devel- 
 oping embryo. 
 
 Sexual Organs. Sexual organs are classified into essential and accessory 
 organs. The essential organs are the two testes of the male and the two 
 ovaries of the female. The accessory organs of the male comprise the vasa 
 deferentia, the seminal vesicles, the urethra, the penis, the prostate gland, Cow- 
 per's glands, and the scrotum and its attached parts. The accessory organs of 
 the female comprise the oviducts or Fallopian tubes, the uterus, the vagina, the 
 various external parts included in the vulva, and the mammary glands. During 
 the greater part of life the sexual organs perform but a portion of their duties ; 
 only at intervals, and in some individuals never, do they complete the cycle 
 of their functions by engaging in the reproductive process itself. In the fol- 
 lowing account we shall discuss first the habitual physiology of the organs of 
 the male and of the female, and later their special activities in the repro- 
 ductive process. 
 
 B. THE MALE REPRODUCTIVE ORGANS. 
 
 The male reproductive organs, already mentioned, have as their specific 
 functions the production of the essential male germ-cells, the spermatozoa, the 
 production of a fluid medium in which the spermatozoa can live and undergo 
 transportation, the temporary storing of this seminal fluid, and its ultimate 
 transference to the outside world or to the reproductive passages of the female. 
 
 The Spermatozoon. Spermatozoa were first discovered by Hamm, a 
 student at Leyden, in 1677. Hamrn's teacher, Leeuwenhoek, first studied 
 them carefully. They were long believed to be parasites, even until near the 
 middle of the present century, when their origin and fertilizing function were 
 established. Spermatozoa are cells modified for locomotion and entrance into 
 the ovum. Human spermatozoa are slender, delicate cells, averaging 0.055 
 millimeter (-^^ of an inch) in thickness, and consisting of a head, a middle- 
 piece, and a tail (Fig. 218). The head (h) is flattened, egg-shaped, with a thin 
 anterior edge and often slightly depressed sides. It terminates anteriorly in a 
 slender, projecting, and sharply pointed thread or spear. It consists of a 
 nucleus composed of a dense mass of chromatin and covered by an excessively 
 thin layer of cytoplasm, von Bardelebon ! claims the number of chromo- 
 somes in the chromatin after maturation to be eight. 
 
 1 K. v. Bardeleben : Verhandlungen der anatomischen Gesdlschaft ; Anatomischer Anzeiger ; 
 1892, vii. 
 
444 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 The middle-piece (m) is a short, cytoplasmic rod, probably containing a cen- 
 trosome. The tail (t) is a delicate filiform, apparently cytoplasmic structure, 
 and analogous to a single cilium of a ciliated cell. The tail is tipped by an 
 excessively fine, short filament, the end-piece (e). The most 
 abundant of the solid chemical constituents of the spermato- 
 zoon is nuclein, probably in the form of nucleic acid, which 
 is found in the head. Other constituents are proteids, prota- 
 mine, lecithin, cholesterin, and fat. 
 
 The structure and power of movement of the spermatozoon 
 plainly show it to be adapted to activity. It is not burdened 
 by the presence of food-substance within its protoplasm. It 
 is the active element in fertilization ; it seeks the ovum, and 
 it is modified from the form of the typical cell for the special 
 purpose of fertilization. The nucleus is the fertilizing agent. 
 The head is plainly fitted for facilitating entrance into the 
 ovum. The tail is a locomotor organ capable of spontaneous 
 movements, and, after expulsion of the semen, it propels the 
 cell, head forward, through the liquid in which it lies. The 
 J movement is a complex one, and is effected by the lashing 
 
 r I of the tail from side to side, accompanied by a rotary move- 
 ment about the longitudinal axis. The rate of movement has 
 FIG. 2i8.-Human been variously estimated at from 1.2 to 3.6 millimeters in the 
 spermatozoa (after m i nu t e . Spermatozoa taken directly from the testis are 
 
 Retzius) : A, seen en 
 
 face; h, head|; m, quiescent ; normally they begin to move when mixed with 
 
 ^C *^ e secretions of the accessory sexual organs. 1 Toward heat", 
 seen from the side, cold, and chemical agents spermatozoa behave like ciliated 
 cells. 
 
 Ripe spermatozoa appear to be capable of living for months within the male 
 genital passages, where they are probably quiescent. Outside of the body 
 they have been kept alive and in motion for forty-eight hours. It is not 
 certain how long they may remain alive within the genital passages of the 
 human female. Diihrssen 2 claims to have found motile spermatozoa in the 
 oviduct at least three and one half weeks after coition. It seems not improb- 
 able that within the female organs their environment is favorable to a some- 
 what prolonged existence. In this connection it is of interest to know that 
 spermatozoa capable of fertilizing have been known to live within the recep- 
 taculum seminis of a queen bee for three years. 
 
 Spermatozoa are produced in large numbers. Upon the basis of observa- 
 tions in several individuals, Lode 3 computes the average production per week 
 as 226,257,000, and In the period of thirty years from twenty-five to fifty- 
 five years of age the total production as 339,385,500,000. This excessive 
 production is an adaptation by nature that serves as a compensation for the 
 
 1 Cf. Walker: Archivfiir Anatomie und Physiologie, Anatomischer Abtheilung, 1899, S. 313. 
 2 Diihrssen: Centralblatt fur Gynakologie, 1893, xvii. S. 592. 
 8 A. Lode: Pftiige^s Archivfiir die gesammte Physiologic, 1891, 1. 
 
REPR OD UCTION. 445 
 
 small size of the cells and the small chance of every cell finding an ovum. 
 Without large numbers fertilization would not be ensured and the continu- 
 ance of the species would be endangered. 
 
 Maturation of the Spermatozoon. Considerable theoretical inter, st 
 attaches to the question of the real morphological value of the spermatozoon. 
 It is undoubtedly a cell, and has arisen by division from one of the testicular 
 cells, called the spermatocyte or sometimes the mother-cell of the spermato- 
 zoon. l>ut is it the morphological equivalent of one of the mother-cells? 
 In most animals, and probably also in man, each spermatocyte gives rise to 
 four spcrmatids, which grow directly into four spermatozoa. The process of 
 derivation of the spermatozoa may be called, by analogy with the process in 
 the ripening of the ovum, maturation. The details and essence of the process 
 have been much discussed. Van Beneden found in an interesting worm, 
 Ascaris, that the number of chromosomes in the nucleus of a single sperma- 
 tozoon is only half that in the original testicular cell ; that is, the process of 
 maturation of the spermatozoon consists in a reduction of the chromosomes 
 by one-half. This discovery has since been extended to many other forms, 
 including mammals and man, 1 and it has been shown further that the mature 
 spermatozoon contains only one-half of the number of chromosomes charac- 
 teristic of the tissue-cells of the species in question. In the light of the sub- 
 sequent process of fertilization these facts are interesting. Following Hert- 
 wig and Strasburger, who regard the chromatic substance of the nucleus as 
 the bearer of the hereditary qualities, many biologists now interpret this 
 halving of the chromatin as a provision for the reduction of the hered- 
 itary mass, which later will he restored to its full amount by union with 
 the egg. As we shall see, the maturation of the ovum follows a some- 
 what similar course, and, since the process has been more fully studied 
 there, we shall reserve further discussion until that subject is reached 
 (p. 451). 
 
 Senien. Semen consists of spermatozoa, together with liquid and dissolved 
 solids, coming partly from the testes themselves, but secreted chiefly by the 
 accessory sexual glands namely, the glands within the vam deferentia, the 
 seminal vesicles, the prostate gland, and Cowper's glands. It is a whitish, 
 viscid, alkaline fluid, with a slight characteristic odor. The amount passed out 
 at any one time has been estimated at between 0.5 and 6 cubic centimeters. Its 
 chemical composition has not been examined exhaustively. Besides water, it 
 contains approximately 18 per cent, of solid substances, which comprise nuclein, 
 protamine, proteids, xanthin, lecithin, cholesterin, and other extractives, fat, and 
 sodium and potassium chlorides, sulphates, and phosphates. Under proper treat- 
 ment colorless crystals, called Bottcher's crystals, may be obtained from semen. 
 They appear to be a phosphate of a nitrogenous base, which has been called sperm- 
 
 1 : - ' - in it histolo^K" 1 ! i-Mthcr tli.-m its chemical 
 
 features. The fluid ] .;>, .i-n s< : i the trar m of an<- 
 
 sibly also for the nutrition of the ripe spermatozoa. C-- 
 
 1 v. Burdeleben : U-<- 
 
446 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 seminal granules, exist in semen. They are possibly parts of nuclei of disin- 
 tegrated cells. Comparatively little is known of the composition or the specific 
 function of the individual secretions contributed by the various organs. The 
 disintegration of the nutritive cells of the testis probably furnishes some of the 
 nutritive substance of the liquid. Prostatic secretion is viscid, opalescent, and 
 usually alkaline, and contains 1.5 per cent, of solids, comprising mainly pro- 
 teids and salts. It contributes at least a portion of the substance of Bottcher's 
 crystals to the semen, and their partial decomposition is said to be responsible 
 for the characteristic odor of the seminal fluid. The secretion from the 
 seminal vesicles is fairly abundant, is albuminous, .and in some animals at 
 least, such as the rodents, seems to contain fibrinogen. This enables the fluid 
 to clot after its reception in the female passages, and thus to prevent loss of 
 spermatozoa. Camus and Gley l find that this coagulation is caused by a 
 specific ferment present in the prostatic fluid. Cowper's glands secrete a 
 mucous fluid. By careful experiments upon white rats Steinach 2 has shown 
 that removal of the seminal vesicles and the prostate gland, while not dimin- 
 ishing sexual passion and the ability to perform the sexual act, including 
 the actual discharge of spermatozoa, prevents entirely the fertilization of the 
 ova ; removal of the seminal vesicles alone markedly weakens the fertilizing 
 power of the semen. Under normal circumstances the secretions of these 
 accessory glands are essential to the motility of the spermatozoa, 3 and they 
 may have other important functions. Ivanoff, 4 however, has been able to 
 impregnate dogs, rabbits, and guinea-pigs artificially by injecting into the 
 vagina spermatozoa taken directly from the epi<l lymis and mixed with a 0.5 
 per cent, solution of sodium carbonate. 
 
 The Testis. The testes (Fig. 219, t) are com pound tubular glands with 
 a unique structure. Formed early in embryonic life as solid structures, with 
 th "5 seminiferous tubules (ts) represented by solid co'rdjS of cells, they remain 
 in the embryonic condition until the time of puberty. Some of the cells, 
 the mother-cells of the spermatozoa, then begin actively to divide, and the 
 result of division with differentiation is the mature spermatozoa. These 
 latter accumulate at the centre of the tubules, the walls being formed largely 
 of the dividing cells or immature spermatozoa. Other cells do not produce 
 spermatozoa., but seem to disintegrate and give rise to the nutritive fluid .and 
 nuclear particles that are found mixed with the sperm-cells. From the time 
 of puberty on, usually throughout life, this cellular activity proceeds, the 
 rate and regularity probably varying greatly with individuals m and Depend- 
 ing largely on the frequency of discharge of the semen. Spermatozoa may 
 be wanting in old men, but they have been found in individuals at eighty 
 or ninety years of age. The spermatozoa accumulate within the seminal 
 
 1 damns and Gley : Comptes rendus de la Societe de -Biologic, 1896, p. 787, and 1897, p. 787. 
 
 2 E. Steinach: Pfluget's Archiv fur die gesammle Physiologit, 1894, Ivi. Cf. also Kehfisch : 
 Deutsche medicinische Wochenschri/t, 1896,'xxii. S. 24? ; and Lode: Siteu-ngsber. d. Kais. . I 
 
 Wiss. Wien. Math, naturw. CL, 1895, civ., Abth. iii. 
 
 *df. Walker: Archiv fur Anatomie und Physiologic, Anatomisclier Abtheilung, 1899, S. 313. 
 
 * Ivanoff: Journal de Physiologie et de Pathologic generate, 1900, ii. p. 95. 
 
REPRODUCTION. 449 
 
 and is covered by a layer of muscular fibres constituting a distinct muscle the 
 bulbs of the corpora cavernosa by the ischio-cavemosi (erectores penis), that <>f 
 the corpus spongiosum (called bulbus urethras) by the bulbo-cavemosus (accel- 
 erator urince). At its distal end each corpus cavernosum terminates bluntly, 
 while the corpus spongiosum projects farther and enlarges to form the extrem- 
 ity of the organ, the glans penis. Each corpus is spongy in consistence, being 
 formed of a trabecular framework of white and elastic connective tissue and 
 plain muscular fibres, with cavernous venous spaces, and it is covered by a 
 tough fibrous tunic. When the spaces are distended with blood the whole 
 organ becomes hard, rigid, and erect in position. The mechanism of erection 
 will be studied more in detail later (p. 463). The penis, especially toward 
 its termination, is beset with end-bulbs, Pacinian bodies, and other nerve-ter- 
 minations, which make it particularly sensitive to external stimulation. 
 
 0. THE FEMALE REPRODUCTIVE ORGANS. 
 
 The female reproductive organs, already mentioned, have as their specific 
 functions the production of the essential female germ-cells, the ova, and their 
 transference to the uterus, and, if unfertilized, to the outside world ; if the 
 ova are fertilized, other specific functions are the protection and nutrition of 
 the developing embryo, its ultimate transference to the outside world, and 
 the nutrition of the child during early infancy. 
 
 The Ovum. The human ovum was discovered in 1827 by von Baer, and 
 it was he who first completely traced the connection between ova in the gene- 
 rative passages and ova in the Graafian 
 follicles of the ovary. The conception 
 of ova as the essential female element 
 
 had, however, long been held, and Har- _ 
 
 vey's dictum of the seventeenth century, f-- \ 
 
 that everything living is derived from / -< l 
 
 an egg (omne vivum ex ovo), is well 
 known. The human ovum, as it comes 
 from the ovary, is a spherical, proto- 
 plasmic cell (Fig. 220), averaging with 
 the zona radiata, approximately 0.2 milli- 
 meter (y^-y inch) in diameter. As in 
 other cells, the cell-body may be distin- 
 gnished from the nucleus, the proto- 
 
 plasm of the former being Called Cyto- the amoeboid nucleolus (germinal spot) ; d, deu- 
 7 -r ., ft ,i toplasmic zone; p, protoplasmic zone; z, zona 
 
 plasm. In its finer structure the cyto- ra diata ; , peri^iteiiL s^ce. 
 plasm consists of an excessively delicate 
 
 network of protoplasmic substance. As in other mammalian eggs, it proba- 
 bly contains, adjoining the nucleus, a minute, specially differentiated portion, 
 consisting of a single or double centrosome surrounded by an attraction sphere 
 (Fig. 221, A). For some distance inward from the border the cytoplasm is 
 pure and transparent, and this portion is often called the protoplasmic zone 
 
 Vol.. II. 29 
 
450 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 (Fig. 220, p). Throughout the centre of the cell, however, it is obscured by 
 the presence of an abundance of yolk-substance, or deutoplasm, from which 
 the corresponding part of the ovum is sometimes called the deutoplasmic 
 zone (d). Deutoplasm is non-living substance; it consists of granules of 
 yolk imbedded in the meshes of the cytoplasmic network, and, like its ana- 
 logue, the yolk of the hen's egg, it serves as food for the future cells of the 
 embryo. 
 
 A comparison of the respective amounts of food in the human and the 
 fowl's egg, with the manner of embryonic development, is suggestive. The 
 chick develops outside the body of the hen, and, therefore, requires a large 
 supply of nutriment, which it finds in the yolk and the white of the egg. The 
 child develops within the mother's body and receives its nourishment from the 
 maternal blood ; hence the supply of food within the egg is only enough to 
 ensure the beginning of growth, special blood-vessels being formed to facilitate 
 its continuance. 
 
 The nucleus (n), frequently called by its early name, the germinal vesicle, is 
 spherical, and usually occupies a slightly eccentric position. Its protoplasm 
 consists of a network composed of two kinds of material : the more delicate, 
 slightly staining threads are the achromatic substance, the coarser, deeply 
 staining portion, the chromatic substance or chromatin. The former is con- 
 tinuous with, and probably of exactly the same nature as, the cytoplasm. 
 The chromatin is peculiar to the nucleus, and at certain stages in the nuclear 
 history is resolved into distinct granules or filaments, the chromosomes (Fig. 
 221, A), the number of which in the human ovum before maturation is 
 thought to be sixteen. There is every reason for believing that the chromatin 
 is the bearer of whatever is inherited from the mother. The nucleus is 
 limited by a nuclear membrane, and contains a strongly marked nucleolus, 
 which has likeAvise retained its original name of germinal spot. There is 
 probably no proper cell-wall, or vitelline membrane, such as is said to exist in 
 many mammalian and other eggs. The ovum is, however, surrounded by a 
 thick, tough, transparent membrane of ovarian origin, about 0.02 millimeter 
 (TTTO i ncn ) i n thickness, and called the zona radiata or zona pellucida (Fig. 
 220, z). It is pierced by a multitude of fine lines radiating from the surface 
 of the zona to the ovum ; these are thought to represent pores, to contain fine 
 protoplasmic processes of the surrounding ovarian cells, and thus to serve as 
 channels for the passage of nutriment to the egg. Between the zona radiata 
 and the ovum a narrow space, the perivitelline space (s), exists. Attached to 
 the outside of the zona radiata are usually patches of cells derived from the 
 discus proligerus of the Graafian follicle of the ovary, which may form a com- 
 plete covering and constitute the corona radiata. They disappear soon after 
 the egg is discharged from the ovary. 
 
 Regarding the chemistry of the mammalian ovum little is known definitely, 
 and of the human ovum nothing whatever except by inference from the eggs 
 of lower animals. The protoplasmic basis undoubtedly resembles other undif- 
 ferentiated protoplasm in its general composition, with an abundance of proteid 
 
REPR OD UCTION. 45 1 
 
 among its solid constituents. Dentoplasm is a rich mixture of food-substance 
 in concentrated form, and contains among its solids probably vitellin, nuclein, 
 albumin, lecithin, fats, carbohydrates, and inorganic salts. 
 
 The form and the structure of the egg suggest the part that it plays in 
 reproduction. It is not locomotor ; in fertilization it is the passive element ; 
 it remains in its place and is sought by the spermatozoon. Its nucleus is the 
 equivalent of that of the spermatozoon. Its form renders easy the entrance of 
 the male element. Its bulk consists largely of food in a very concentrated 
 form, and, as development proceeds, it supplies this food to the growing cells. 
 
 In lower forms of animal life, where eggs are fertilized outside the body 
 of the parent in the water into which they are set free, they are usually pro- 
 duced in enormous numbers. Some fail of fertilization, while others are 
 destroyed by enemies, and the large number is a compensatory adaptation by 
 nature for their poor chance of survival. In mammals and man, however, 
 ova have a much better opportunity of being fertilized and of developing into 
 adults, and their number is correspondingly reduced. Their relative fewness, 
 as compared with the spermatozoa, is in harmony with their larger size and 
 the fact that, while awaiting fertilization, they are carefully protected within 
 the body of the mother. 
 
 Maturation of the Ovum. Attention has been called to the maturation 
 of the spermatozoon. The ovum undergoes an analogous process of ripening, 
 which has been studied very carefully, and from its theoretical interest has 
 given rise to a large amount of discussion. Maturation begins approximately 
 as the ovum is leaving the ovary, and is not completed until after the ovum 
 has received the spermatozoon, although the exact time- relations in the 
 human species are not yet determined. It consists of a mitotic division of 
 the nucleus, essentially like mitosis (karyokinesis) in ordinary cell-division, and 
 an expulsion of one portion from the cell. This occurs twice in succession. 
 The cast-off bits of protoplasm are known as polar bodies. The details of the 
 process of maturation are as follows (Fig. 221) : In all animals the nucleus 
 of the ovarian ovum, or oocyte, at the time of its formation receives from its 
 mother-cell the same number of chromosomes as the ordinary tissue-cells con- 
 tain. These constitute its chromatic reticulum. As the oocyte prepares for 
 maturation the chromatin is resolved into masses, the number of which is one- 
 half that of the somatic chromosomes. In a large number of species, such as 
 many of the worms, the insects, and the crustaceans, each chromatic mass 
 constitutes a group of chromosomes, usually four in each group, which is 
 called a "quadruple-group" or "tetrad" (E). The number of tetrads is 
 hence one-half the number of original chromosomes, while the total number 
 of chromosomes in the nucleus at this stage is double the original number. 
 The nucleus moves from its position in the interior of the egg toward the sur- 
 face, and the nuclear membrane begins to disappear. At the same time the 
 two minute cytoplasmic structures, the centrosomes, which lie close beside 
 the nucleus, separate and take up positions at a considerable distance apart 
 from each other, in some cases even upon opposite sides of the nucleus. The 
 
452 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 FIG. 221. Stages in the maturation of the ovum ; diagrammatic (mainly from Wilson) : A, the orig- 
 inal ovarian ovum ; n, its nucleus, containing four chromosomes ; c, its double centrosome, surrounded 
 by the attraction sphere ; in if much of the chromatin has begun to degenerate ; the rest has become 
 arranged into two quadruple groups of chromosomes, or tetrads ; the formation of the spindle and the 
 asters has begun; in Cthe first polar amphiaster, bearing the chromosomes, is completed ; in D the am- 
 phiaster has become rotated and has travelled toward the surface of the ovum ; g. v, the degenerated 
 remains of the nucleus ; in E the division of the tetrads into double groups of chromosomes, or dyads, 
 has begun, and the first polar body, p. 6, is indicated ; in JPthe first polar body, containing two dyads, has 
 been extruded ; the formation of the second polar amphiaster has begun ; in G the first polar body is pre- 
 paring to divide ; the second poter amphiaster is fully formed ; in Hthe division of the dyads into single 
 chromosomes in both the first polar body and the egg has begun, and the second polar body, p. 6 3 , is in- 
 dicated; in I the formation of the polar bodies is completed ; 9, the egg-nucleus, containing two small 
 chromosomes, one-half the original number. In fertilization the spermatozoon will bring in two addi- 
 tional chromosomes, thus restoring the total number of four. 
 
REPROD UCTION. \ 53 
 
 substance lying between them either the cytoplasm!*- network or the achro- 
 matic substance of the nucleus loses its reticular appearance, becomes fila- 
 mentous, and arranges itself in the form of a spindle with the threads extend- 
 ing from pole to pole (C, I>). The groups of chromosomes become attached to 
 the spindle threads midway between the poles. At each pole there mav lie a 
 centrosome, and about it the cytoplasm may become arranged in the form of 
 a star, the ,s/<r, though these structures are not universal among species. The 
 spindle with the two asters is known as the polar ampkiaxta-, and the com- 
 plicated structure seems to be formed, as in ordinary cell-division, for the 
 sole purpose of dividing the nucleus into two portions. This is now per- 
 formed (E) : each quadruple-group of chromosomes splits into two, and these, 
 known as "double-groups," or "dyads," separate from each other and pass 
 toward the poles of the spindle. The nucleus is thus divided into halves. 
 While the division has been proceeding, the spindle has wandered halfway 
 outside the egg, and, when it is completed, one of the resulting nuclear halves, 
 comprising one-half of the full number of dyads, together with the centro- 
 some and the aster, finds itself entirely extruded from the egg and lying 
 within the perivitelline space. It is known as the first polar body (F, p. 6 1 ). 
 The diminished nucleus within the ovum proceeds at once to undergo a second 
 rnitotic division (G, H, /) ; each of the remaining dyads divides into two 
 single chromosomes, which are separated from each other ; and a second polar 
 body (p. 6 2 ), containing one-half the number of single chromosomes charac- 
 teristic of the tissue-cells, is extruded. Apparently the two 'polar bodies are 
 of no further use. In many animals the first divides into two, but sooner or 
 later both degenerate and disappear. The remnant of the nucleus left within 
 the egg, much reduced in size, wanders back to the interior. In the mam- 
 mals no true tetrads are formed, and a considerable interval of time elapses 
 between the formation of the two polar bodies, during which the sperma- 
 tozoon enters the egg. But in them the process of maturation is the same in 
 essence as in the lower animals. In all species the chromosomes are reduced 
 to one-half the number belonging to the ovarian ovum ; in many species they 
 are then resolved again into scattered chromatic substance. The nucleus 
 develops a membrane and again enters the resting stage. It is known hence- 
 forth as the egg-nucleus, or female pronucleus, and it awaits the coming of the 
 spermatozoon. According to most observers, its centrosome gradually degen- 
 erates and disappears. 
 
 Thus the curious process of maturation of the ovum is different in detail 
 from that of maturation of the spermatozoon. In the latter the spermatoey t- 
 divides into four functional spermatozoa; in the former the oocyte divides 
 into two functionless polar bodies (or, by subdivision of the first, three, which 
 have been called abortive eggs) and one functional ovum. It is entirely 
 probable, however, that the essence of the process is exactly the same in the 
 two eases, and lies in the reduction of the number of the chromosomes. The 
 three import. LOW been demonstrated in a large number of 
 
 species, viz. --that in tin' maturation of both the ovum and the spermatozoon 
 
454 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 the number of chromosomes is halved, that the number in the two mature 
 germ-cells is the same, and that this number is one-half that of the chromo- 
 somes of the somatic cells. It is wholly probable that these facts are uni- 
 versal in sexual reproduction. Each mature germ-cell, therefore, while in 
 reality a cell, is, when compared with the somatic cells, incomplete. The 
 subsequent union of the two in fertilization restores the chromosomes to their 
 normal number. Inasmuch as the chromatin is probably the all-important 
 constituent of the germ-cells, the bearer of the paternal and the maternal 
 inherited characteristics, the phenomena of maturation are of great interest. 
 Many biologists follow Hertwig and Strasburger in regarding maturation as 
 an adaptation for the prevention of the constant increase in quantity of the 
 hereditary substance that would otherwise take place with ev^ry union of 
 ovum and spermatozoon. Without a reducing process the quantity of chro- 
 matin in cells would become in a very few generations inconveniently great. 
 The most striking feature of maturation, however, is the halving of the num- 
 ber of chromosomes. The significance of this is not clear. Nevertheless it 
 is evident that maturation is a preparation of each germ-cell for union with 
 its mate. 1 
 
 The Ovary ; Ovulation. The ovaries (Fig. 222, o) are often spoken of 
 as glands, but they are not glands according to the ordinary histological and 
 physiological use of the term. They are solid organs with a structure 
 peculiar to themselves, and their function is the production of ova. Their 
 stroma consists of fine connective tissue with numerous connective-tissue 
 
 ^x cells. The ova are developed in the interior within cavities called, from 
 their discoverer, Graafian follicles (Gf\ from primitive ova that are modified 
 cells of the germinal epithelium of the embryo. It has been calculated 
 that a single human ovary at the age of seventeen years contains 17,600 
 primitive ova, 2 but that not more than 400 of these arrive at maturity. 3 
 Each Graafian follicle is lined by an epithelial layer several cells thick, 
 the membrana granulosa, and is filled with a clear, serous, viscid liquid, the 
 liquor folliculi. Imbedded in the epithelium upon one side is usually a 
 single ovum, completely surrounded by the cells and forming a prominent 
 hillock which projects well into the cavity of the follicle. The epithelium 
 immediately surrounding the ovum is the discus proligerus. Within the 
 discus the ovum grows and becomes surrounded by the zona pellucida. In 
 the process of growth the Graafian follicle approaches the surface of the ovary, 
 and finally comes to form a minute rounded vesicular projection covered only 
 
 - -Jjy the ovarian epithelium. When fully ready for discharge, the wall of the 
 follicle becomes ruptured, probably by the increasing pressure of the contained 
 liquid, and the ovum with its zona pellucida and a portion or all of the discus 
 proligerus, now called the corona radiata, is cast out upon the surface of the 
 
 1 For a critical discussion of maturation, see Wilson : The Cell in Development and Inheiii- 
 ance, 1900, 2d ed., New York. 
 
 2 Heyse : Archiv filr Gynakologie, 1897, liii. S. 321. 
 
 3 Henle : Handbuch der Anatomie, 1873. 
 
REPRODUCTION. 
 
 455 
 
 ovary to be taken up by the Fallopian tube. The empty follicle undergoes 
 changes and becomes the corpus luteum (c.l). Usually the corpus luteum de- 
 generates within a few days and ultimately disappears. If, however, pregnancy 
 follows ovulation, it grows very large, perhaps because of the congested state 
 of the reproductive organs, and remains for months before the retrograde 
 metamorphosis sets in. Not all Graafian follicles reach maturity and burst, 
 for many, after developing to a considerable size, undergo degenerative 
 changes, characterized by liquefaction and disappearance of their contents. 
 
 The discharge of the ovum is known technically as ovulation. In most 
 animals ovulatiou is a periodic phenomenon accompanying certain seasons, and 
 is marked by general sexual activity. In woman and many domesticated ani- 
 mals the relation to the seasons no longer exists, but too little is known of the 
 causes and time-relations of the phenomenon and its general bearings upon 
 other physiological processes, notably upon menstruation in woman. A large 
 
 FIG. 222. Diagram of the female reproductive organs (modified from Henle and Symington) : o, ovary; 
 G.f, Graafian follicle containing an ovum; c.l, corpus luteum; p, parovarium ; /, fimbriated end of F.t* 
 Fallopian tube ; u, body, and c, cervix of uterus ; o.e, os uteri externum ; vg, vagina; h, hymen ; u, open- 
 ing of urethra ; v, vulval cleft ; n, labia minora, or nymphse ; l.m, labia majora. 
 
 but not wholly decisive literature upon the subject in the human being has 
 been written. It is a common belief, originating in the seventeenth century, 
 that ovulation in woman is a periodic phenomenon occurring regularly every 
 month and contemporaneous with the occurrence of the menstrual flow, and 
 numerous post-mortem observations of the preseiv 
 discharged Graafian follicles at the menstrual ]>< 
 frequent coincidence of the two phenomena. Bu 
 menstruation, though probably usual, i 
 
456 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 times, for inter menstrual observations of fresh ovarian scars are not rare, and 
 prove without doubt that discharge of an ovum may occur at any time between 
 two successive periods (see under Menstruation, p. 457). Graafian follicles 
 develop even during infancy ; most of them, and perhaps all, retrograde with- 
 out discharging their ova, but the occasional instances of pregnancy at the ages 
 of seven, eight, or nine, prove that ovulation may occur during childhood. 
 Ovulation usually begins at puberty, its commencement thus coinciding with that 
 of menstruation, and continues until the climacteric. After -the climacteric it 
 may occur in exceptional cases, although here, as before puberty, retrogressive 
 degeneration of the Graafian follicles is the rule. It is commonly believed that 
 ovulation is at a standstill during both pregnancy and lactation. The un- 
 doubted possibility of a pregnancy originating during lactation would, how- 
 ever, seem to prove the possibility of ovulation during the latter period. It is 
 not decided whether removal of the uterus does away wholly with ovulation. 
 
 The Fallopian Tube. Each of the Fallopian tubes (Fig. 222, F. f), or 
 oviducts, opens into the peritoneal cavity about one inch from the correspond- 
 ing ovary. Around the opening is an expanded fringe of irregular processes, 
 thefimbrice (/), one of which is attached to the ovary. The length of the tube is 
 between three and four inches, and the opening into the uterus is extremely 
 small. The chief structures in the walls of the oviducts that are of physio- 
 logical interest are the double layer of plain muscle, an outer longitudinal 
 and an inner circular coat, longitudinal fibres from which pass also into the 
 fimbrise ; and the cilia with which the tube is lined throughout, and which are 
 present also upon the inner side of the fimbriaB. The direction of the ciliary 
 movement is from the ovary toward the uterus. The primary function of the 
 Fallopian tubes is to convey ova from the ovary to the uterus ; they also con- 
 vey spermatozoa in the reverse direction ; and within them the union of ovum 
 and spermatozoon usually takes place. 
 
 The mechanism of the receipt of the ovum by the tube is not fully under- 
 stood. After ovulation the ovum is slightly adherent to the surface of the 
 ovary by the agency of the viscid liquor folliculi. It is possible, but it has 
 not been proved, that in the human being, as has been seen in some animals, 
 the expanded, fimbriated end of the Fallopian tube clasps the ovary when 
 the egg is discharged. The passage of the ovum into the tube is probably 
 brought about by the cilia lining the fimbriae. Once within the tube, the 
 ciliary action, assisted perhaps by contraction of the muscular fibres in the 
 walls, carrias the ovum slowly along toward and finally into the uterus. In 
 some mammals the passage occupies three to five days ; the time in woman is 
 not definitely known, but is thought to be from four to eight days. 
 
 The Uterus. The uterus (Fig. 222, u), or womb, receives the ovum from 
 the Fallopian tube and passes it on, if unimpregnated, to the vagina; on the 
 r hand, it receives from the vagina spermatozoa and transmits them to the 
 Ira tubes; it is the seat of the function of menstruation ; when impreg- 
 ken place, it retains and nourishes the growing embryo, and ulti- 
 he child from tJik bocryy Its structure accords with these func- 
 
REPRODUCTION. i:,7 
 
 tions. Its thick walls consist largely of plain muscular tissue arranged 
 roughly in the form of three indistinctly marked layers. Of these, the exter- 
 nal and the middle coats are thin ; the fibres of the former are arranged in 
 general longitudinally, those of the latter more circularly and obliquely. 
 The third, most internal layer, which is regarded by some as a greatly hyper- 
 trophied muvcularis mucosce, forms the greater part of the uterine wall. Its 
 fibres are arranged chiefly circularly ; toward the upper part they become trans- 
 verse to the Fallopian tubes, and at the cervix longitudinal fibres lie within 
 the circular ones. The individuality of the muscular layers and uniformity 
 in the course of the fibres is largely interfered with by the numerous blood- 
 vessels of the uterine walls. The uterus is lined by an epithelium composed 
 of columnar ciliated cells, except in the lower half of the cervix, where a stratified 
 non-ciliated epithelium exists. The direction of the ciliary movement in woman, 
 as in other mammals, is toward the os uteri. 1 The mucous membrane is thick, 
 and contains very numerous, branching, tubular glands, which are lined by 
 ciliated epithelium and have a tortuous course, terminating in the edge of the 
 muscular layer. They secrete a viscid, mucous liquid. Between the glands 
 are branched connective-tissue cells, which are not unlike the connective-tissue 
 cells of embryonic structures, and wandering cells. Lymph-spaces and blood- 
 capillaries exist. The development of the tissue goes on slowly up to the 
 time of puberty, and, as we shall see, after puberty the mucous membrane is 
 subject to constant change. 
 
 Menstruation. Except during pregnancy the most striking activities of 
 the uterus are associated with that peculiar female function which, from its 
 monthly periodicity, is called menstruation. The most obvious external fact 
 of this phenomenon is the discharge every month of a bloody, mucous liquid 
 through the vagina ; the most obvious internal facts are the bleeding and the 
 degeneration and disappearance of a portion of the mucous membrane of the 
 body of the uterus. This curious process, though having analogies in lower 
 animals, occurs most markedly in the human female, and from before the time 
 of Aristotle to the present, among both primitive and civilized races, its signifi- 
 cance has been the cause of much speculation. The detailed phenomena of 
 menstruation are not as well known as they should be. Experimentation is 
 practically out of the question, and the opportunities of careful post-mortem 
 study of normal healthy uteri at different stages are rare. The main facts are 
 as follows : 
 
 Some days before the flow occurs the mucous membrane of the body of the 
 uterus begins to thicken, partly by an active growth of its connective tissue 
 elements and partly by an excessive filling of its capillaries and veins with 
 blood. The cause of this swelling is not known. It continues until the 
 membrane has doubled or trebled in thickness, and, according to some authori- 
 ties, the uterine cavity becomes a mere slit between the walls. Then occurs an 
 infiltration of blood-corpuscles and plasma, probably largely by diapedesis, 
 although possibly assisted by rupture, through the walls of the swollen capil- 
 1 Hofmeier: CenlrcUblatt fiir Gyndkolugie, 1893, xvii. S. 764. 
 
458 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 laries into the connective-tissue spaces beneath the epithelial lining of the 
 uterine wall. The epithelium is thus pressed up from beneath, and begins 
 rapidly to undergo fatty degeneration in places, and to disappear. The 
 immediate cause of the degeneration is not definitely known. The connective- 
 tissue elements and the upper portion of the glands are involved to some extent 
 in the degenerative change. The capillaries, thus laid bare, burst, and the dark 
 blood oozes forth and, mixed with disintegrated remains of the uterine tissues, 
 with the mucous secretion of the uterus and the vagina, and with the escaped 
 lymph, passes away, drop by drop, from the body. There is great difference of 
 opinion as to the extent of the destruction of uterine tissue. On the one extreme 
 side are those who claim that the loss of tissue is normally wholly trivial and 
 secondary, the hypersemia and the bloody glandular discharge being the 
 important events. Other authorities, equally extreme, have observed a disap- 
 pearance of the whole mucous membrane except the deepest layers containing 
 the bases of the glands ; this is probably pathological. From all the evidence an 
 opinion inclining toward the former view seems most reasonable namely, 
 that usually and physiologically only the superficial portion of the mucous 
 membrane disintegrates, and this only in spots. 1 Differences in the amount 
 undoubtedly occur. Occasionally it happens that the membrane, instead of 
 disintegrating, comes away in pieces of considerable size. The term decidua 
 menstrualis is applied to the lost coat. The flow continues upon an average 
 four days or more. From observations upon 2080 American women Emmet 2 
 finds the average duration of the flow at puberty to be 4.82 days, the average 
 in later life 4.66 days. The amount of blood discharged can be determined 
 only with great difficulty. It probably varies greatly, but is commonly 
 estimated at from 1 00 to 200 cubic centimeters (4 to 5 ounces). The blood is 
 slimy, with abundant mucus ; usually it does not coagulate. Epithelium 
 cells, red corpuscles, leucocytes, and detritus from the disintegrated tissues, 
 occur in it, and it has a characteristic odor. As the flow ceases a new growth 
 of connective-tissue cells, capillaries, glands, and from the glands superficial 
 epithelium, begins, and the mucous membrane is restored to its original 
 amount. Whether a resting period follows before the succeeding tumefaction 
 occurs is not definitely known, but it seems probable. The durations of the 
 various steps in the uterine changes are not well known, and probably vary 
 in individual cases. Minot 3 suggests the following approximate times : 
 
 Tumefaction of the mucosa, with accompanying structural changes 5 days. 
 
 Menstruation proper 4 
 
 Eestoration of the resting mucosa 7 
 
 Besting period 12 
 
 Total ../..:. 28 days. 
 
 The menstrual changes in the uterus are accompanied by characteristic 
 phenomena in other parts of the body. The Fallopian tubes are congested, 
 ^ee Westphalen : Archivfilr Gynakologie, 1896, lii. S. 35 ; and Mandl: Ibid.. S. 557. 
 *T. A. Emmet The Principles and Practice of Gynaecology; 1880, 2d ed. 
 *C. S. Minot: Human Embryology, 1892. 
 
REPRODUCTION. 459 
 
 and, according to some authorities, their mucous membrane degenerates and 
 bleeds like that of the uterus. The ovaries are likewise congested. As has 
 been stated, it is commonly believed, but not definitely proved, that ovulation 
 accompanies each period. Frequent accompaniments are turgescence of the 
 breasts, swelling of the thyroid and the parotid glands and the tonsils, cortges- 
 tion of the skin, dull complexion, tendency toward the development of pigment, 
 and dark rings about the eyes. The skin and the breath may have a character- 
 istic odor. In singers the voice is often impaired, which is one instance of 
 a general nervous and muscular enervation. Mental depression often exists. 
 Pain is a frequent accompaniment, and nervous and congestive pathological 
 phenomena may, at times, become very pronounced. Recent work has shown 
 that the various phenomena accompanying menstruation are evidences of a 
 profound physiological change, with a monthly periodicity, that the female 
 human organism undergoes, and of which the uterine changes are only a part. 
 Thus, during the intermenstrual period there is a gradual increase of nervous 
 tension and general mobility, of vascular tension manifested by turgescence of 
 the blood-vessels, a gradual increase of nutritive activity manifested by 
 increased production and excretion of urea and increased temperature, and 
 a gradual increase of the heart's action in strength and rate. 1 These various 
 activities of the organism usually attain a maximum a few days before the 
 menstrual flow begins and then undergo a rapid fall, which reaches a minimum 
 toward the close of the flow ; a second lesser maximum may occur a few days 
 after the flow ceases. All organic activities that have been carefully investi- 
 gated show evidences of such a monthly rhythm. It is not known that the 
 male possesses such a period. 
 
 The first menstruation is usually regarded as the index of the oncoming of 
 puberty or sexual maturity, and in temperate climates occurs usually at the age 
 of fourteen to seventeen. Its onset is earlier in warm than in cold climates, in 
 city than in country girls, and varies in time with food, growth, and environ- 
 ment. Exceptionally menstruation may begin in infancy or later than puberty, 
 and it has even been known to be wholly wanting in otherwise normal women. 
 Normally, it ceases during pregnancy, and probably usually during lactation, 
 although there are frequent exceptions to the latter rule. In nearly all cases 
 complete removal of the ovaries puts an end to menstruation. Removal of the 
 ovaries and Fallopian tubes diminishes the number of exceptional cases. The 
 final cessation of menstruation, which is a gradual process extending over 
 several months, usually marks the climacteric (menopause), or end of the 
 sexual life, and occurs usually at the age of forty-five to forty-eight. Excep- 
 tionally the flow may cease early in life or extend to extreme old age. 
 
 :n.jKirative Physiology of Menstruation. The comparative physiology of 
 menstruation, although it has been studied only incompletely in a few domesti- 
 
 1 Cf. Mary Putnam Jacob! : "The Question of Rest for Women during Menstruation," 
 Boylston Prize Essay, 1876 ; C. Keinl : Sammlung klinische Vortrage, 1884, No. 243 ; O. Ott : 
 JVbttrv. ea <f obstetrique et de gynecologic, 1890, v. ; and A. E. Giles: Transactions of the 
 
 Obsh* '(y of London, 1897, xxxix. p. 115. 
 
460 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 cated animals and some monkeys, 1 sheds some valuable light upon the phe- 
 nomenon in woman. In animals lower than man, in a wild state, the desire 
 and power of reproduction are usually limited to seasonal periods. At such 
 times conception is possible, and probably usually takes place. Such periods 
 are known as "rut," "heat," and "oestrus." During the rest of the year 
 sexual activities are in abeyance. Domestication, with its artificial condi- 
 tions of regular food-supply, warmth, and care, has increased productiveness 
 (Darwin) and rendered the reproductive periods more frequent. If impregna- 
 tion be prevented, as is often the case in domesticated animals, the periods of 
 " heat " appear for a while with great frequency and regularity (monkey, 
 mare, buffalo, zebra, hippopotamus, at intervals of four weeks ; cow, three 
 weeks ; sow, fifteen to eighteen days ; sheep, two weeks ; bitch, twelve to 
 sixteen weeks). They are characterized by general nervous excitement, desire 
 and power of conception, congestion and swelling of the external genital 
 organs, and a uterine discharge. The latter is scanty, mucous, and bloody, 
 the amount of blood increasing in ascending the evolutionary scale. The 
 histological processes occurring in the uterus have been studied carefully by 
 Retterer in the dog and by Heape in the monkey. In the latter the proc- 
 esses seem to be nearly identical with those of man. In the dog, growth 
 and congestion of the mucosa occur, and are followed by rupture of the capil- 
 laries, extravasation of blood, and degeneration of the tissues ; but it is doubt- 
 ful whether the epithelium is actually shed. It is generally believed that 
 " heat " in the lower mammals is accompanied by ovulation. It is not neces- 
 sarily so in monkeys. The phenomena of " heat " are thus closely similar to 
 those of human menstruation, the similarity being most marked in the 
 monkeys. In addition to these more hidden phenomena there is present 
 sexual desire, which in the human female is largely absent at such periods, 
 although it may be pronounced just before and just after the actual flow. 
 
 Theory of Menstruation. The significance of menstruation is in great dis- 
 pute. All modern theories agree in regarding it as associated in some way 
 with the function of childbearing. The flow was early believed to be a means 
 employed by the body to get rid of a plethora of nutriment. This was 
 followed by the well-known hypothesis, put forward especially by Pfliiger 
 (1865). According to this hypothesis, 2 the menstrual bleeding and the 
 uterine denudation occur for the purpose of providing a fresh uterine surface 
 to which the egg, if impregnated, can readily attach itself, just as, in graft- . 
 ing, the gardener provides a wounded surface upon which the young scion is 
 set, or, in uniting two membrane-covered tissues, the surgeon first wounds or 
 freshens their surfaces. This conception of menstruation is not now commonly 
 accepted. Pfliiger regards the mechanism of the uterine process to be as fol- 
 lows : The constant growth of the ovarian cells and the consequent swelling of 
 
 1 Cf. A. Wiltshire: British Medical Journal, March, 1883; E. Eetterer: Comptes rendus des 
 stances et memoires de la societe de biologie, 1892 ; W. Heape : Philosophical Transactions of the 
 Royal Society, 1894. (B), vol. 385, pt. i. ; and Proceedings of the Royal Society, 1897, Ix p. 202. 
 
 2 E. F. W. Pfliiger : Untersuchungen cms dem physiologischcn Laboratorium zu Bonn, 1865, 
 
REPRODUCTION. I'll 
 
 the ovary subject the ovarian nerve-fibres, and through them the spinal cord, 
 to a constant slight stimulation. Through the summation of the stimuli within 
 the cord a reflex dilatation of the vessels in the genital organs is produced. 
 The excessive blood-supply leads in turn to the tumefaction of the uterus, and 
 frequently to the ripening of a Graafian follicle. The bleeding follows, and 
 at the same time or slightly later the rupture of the follicle occurs, provided 
 the latter be sufficiently advanced in growth. The menstrual flow and ovulation 
 are, therefore, two phenomena conditioned usually by the same cause, namely, 
 the menstrual congestion, yet either may occur without the other. Pfliiger's 
 hypothesis accounts clearly for the absence of menstruation after removal of 
 the ovaries. Numerous other theories have been proposed, no one of which 
 can be said to be widely and generally accepted. The present tendency in 
 belief is as follows : Ovulation and menstruation are in great part independent 
 phenomena ; they may or they may not coexist ; the uterine growth is a prep- 
 aration for the future embryo ; the tissue of the deddua menstrualis is the fore- 
 runner of the deddua graviditatis (p. 471) ; if an ovum, whenever it is discharged, 
 be fertilized, it attaches itself to the thickened uterine wall, the tissues become 
 the deddua graviditatis, pregnancy follows, and the deddua is not discharged 
 until the time of parturition ; if, however, fertilization does not take place, 
 there is no attachment, the tissues degenerate and become the deddua men- 
 strualis, and the flow occurs. The suggestion of Jacobi l is not an extreme 
 one : " The menstrual crisis is the physiological homologue of parturition." 
 Its periodicity, which is approximately that of a tropical month (27.32 days), 
 has been the subject of much hypothesis. In a suggestive paper based upon 
 much careful statistical study Arrhenius 2 ascribes it to the influence of 
 atmospheric electricity, which he finds to undergo a periodic variation of 
 similar length. Regarding the mechanism of menstruation the above hy- 
 pothesis of Pfluger seems not unreasonable, and, moreover, seems to be sup- 
 ported by the experiments of Strassmann, 3 who by pressure artificially pro- 
 duced in the ovary by means of injections into it of salt solution, produced 
 hypersemia and swelling of the uterine mucous membrane, congestion of the 
 external genitals, and mucous and bloody discharges. 
 
 The mystery of menstruation largely ceases when we recognize what is un- 
 doubtedly a fact, that the phenomenon is a highly developed inheritance from 
 our mammalian ancestors, and that, although in the human race under the 
 influence of civilization and social life it has largely lost its technical sexual 
 significance, it is, nevertheless, primarily a reproductive phenomenon derived 
 directly from the lower females. Nature has endowed the latter, in a manner 
 yet unknown, with reproductive periods that are pronounced in the wild state 
 and are coincident with certain of the seasons. A primitive seasonal period 
 may perhaps still be shown in woman by the greater proportion of births that 
 take place during the winter months than at other times of the year : this sig- 
 
 1 Mary Putnam Jacobi: American Journal of Obstetrics, 1885, xviii. 
 a Ar^ienius : Skandinavisches Archivfur Physiologic, 1898, viii. S. 367. 
 3 dtrassmann : Archivfur Gynakologie, 1896, lii. S. 134. 
 
462 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 nifies greater sexual activity during the months of spring, as is the case in 
 most animals. 1 
 
 Domestication has, however, interfered with the original plan of nature. 
 It has rendered the lower forms more prolific and has made more frequent 
 their reproductive periods. Civilization has done exactly the same for woman. 
 It has rendered her more prolific and has made more frequent her reproduc- 
 tive periods. It is wholly probable that the menstrual periods of woman are 
 the homologues of the frequent reproductive periods of the lower forms. It 
 has been seen that the latter are characterized by the same kind of phenomena 
 that exist in the former ; the characteristic human menstrual phenomena are 
 least developed in the lower mammals, much more so in the monkey, and are 
 most pronounced in the human female. For what purpose this evolution of 
 function has taken place we do not know. Below the human species concep- 
 tion is confined to these times of " heat ; " in woman it is possible at other 
 than her menstrual periods. In this respect woman is more highly endowed 
 than her mammalian ancestors. 
 
 The Vagina. The vagina (Fig. 222, vg) is the broad passage from the 
 uterus to the external organs. Its walls consist of smooth muscle fibres, 
 arranged both circularly and longitudinally. It is lined by stratified scaly 
 epithelium and is surrounded by erectile tissue. Its walls contain few glands. 
 Its specific functions are connected solely with the reproductive process ; in 
 copulation it receives the penis and the semen. Its cavity is the pathway out- 
 ward for the products of menstruation and, in parturition, for the child. 
 
 The Vulva and its Parts. The vulva (Fig. 222) comprises the genital 
 organs that are visible externally viz. the mons Veneris, the labia majora (l.m), 
 the labia minora or nymphce (n), the clitoris, which is the diminutive homologue 
 of the penis of the male, and the hymen (h), or perforated curtain that guards the 
 entrance to the vagina and is usually ruptured at the time of the first coition. 
 The vulva receives the openings of the vagina, the urethra (u), and the ducts of 
 Bartholini's glands. Its parts are capable of turgidity through its rich vas- 
 cular supply, and perform minor ill-defined, adaptive, and stimulating func- 
 tions in copulation. Their surface is covered by mucous membrane which is 
 moistened and lubricated by a secretion from numerous mucous follicles, seba- 
 ceous glands, and the glands of Bartholini. The latter are comparable to 
 Cowper's glands of the male and secrete a viscid liquid. 
 
 The Mammary Glands. The mammary glands, being active only during 
 the period of lactation, may best be studied in connection with that function 
 (see Vol. I., p. 261). 
 
 Internal Secretion. A priori, the reproductive organs can scarcely be 
 regarded as organs that are quiescent during the greater part of life and pas- 
 
 1 " The largest number [of human births] almost always falls in the month of February, 
 
 .... corresponding to conceptions in May and June Observations tend to show the largest 
 
 number of conceptions in Sweden falling in June ; in Holland and France, in May-June ; in 
 Spain, Austria, and Italy, in May; in Greece, in April. That is, the farther south the earlier 
 the spring and the earlier the conceptions." Mayo- Smith : Statistics and Sociology, 1895. 
 
REPRODUCTION. 463 
 
 sively await the reproductive act. The view that they are more than this is 
 supported by some, although slight, experimental evidence. Notwithstanding 
 the fact that removal of the testis or the ovary in adult life is often unaccom- 
 panied by great somatic changes, the profound effects of early castration upon 
 development, in both the male and female, show that upon the presence of the 
 sexual organs depends the appearance of many of the secondary sexual cha- 
 racters characters which apparently are independent of those organs, and yet 
 of themselves distinguish the individual as specifically masculine or feminine. 
 The mode of dynamic reaction of the sexual organs upon the other organs can 
 at present be little more than hinted at. It is entirely probable that such 
 reaction is either nervous or chemical, or perhaps it is both combined. 
 Regarding the former little is known. Regarding the latter certain facts 
 point to a possible normal and constant contribution of specific material by 
 the reproductive glands to the blood or lymph, and thus to the whole body. 
 Such a process is spoken of as internal secretion. This subject is discussed 
 more fully in Vol. I. p. 273. 
 
 D. THE REPRODUCTIVE PROCESS. 
 
 Thus far attention has been given to the general functions of the' repro- 
 ductive organs. We come now to the special phenomena connected with the 
 reproductive process itself, and have to trace the history of the spermatozoon, 
 the ovum, and the embryo. It should be borne clearly in mind that the 
 essential part of the reproductive process is the fusion of the nuclei cf the two 
 germ-cells. Investigation is making it more and more probable that the 
 spermatozoon and the ovum, although so different in appearance a id general 
 behavior, are fundamentally and in origin both morphologically aid physi- 
 ologically equivalent cells. In the processes of their growth and maturation 
 they are secondarily modified, the one into an active locomotive body, the other 
 into a passive nutritive body. The modifications in both are confined, how- 
 ever, to the cell-protoplasm (cytoplasm and centrosome) ; the essential parts, 
 the nuclei, remain unmodified and both morphologically and physiologically 
 equivalent down to the time of their fusion in the process of fertilization. 
 The many and complex details of the reproductive process exist for the sole 
 purpose of bringing together these two minute masses of chromatin. 1 
 
 Copulation. Copulation is the act of sexual union, and has for its object 
 the transference of the semen from the genital passages of the male to those of 
 the female. It is preceded by erection of the penis and turgidity of the organs 
 of the vulva. These latter occurrences are in the main vascular phenomena, 
 and are brought aboui by a distention of the cavernous spaces of the erectile 
 tissues with blood. The vascular phenomena are, however, accompanied by 
 complex nervous and muscular activities. As regards the penis, the arteries 
 supplying the organ relax and allow blood to flow in quantity to the corpora 
 cavemosa and the cjrpus spongiosum. Simultaneous relaxation of the smopth 
 
 1 Compare Th. B_>veri : " Befruchtung," Merkel und Bonnet's Ergebnisse der Anatomic und 
 Entwickelungsgeschifhte, 1892, i. 
 
464 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 muscle fibres scattered throughout the trabecular framework of the corpora 
 increases the capacity of the blood-spaces. Furthermore, the ischio-cavernosus 
 (erector penis) and bulbo-cavernosus muscles contract and compress the 
 proximal or bulbous ends of the corpora and the outgoing veins. The result 
 of this combined muscular relaxation and contraction is a free entrance of 
 blood into and a difficult exit from the vascular spaces ; this leads to a swelling 
 and distention which aid further in compressing the venous outlets and, being 
 limited by the tough, fibrous tunics of the corpora, result in making the organ 
 stiff", hard, erect in position, and well adapted to its specific function. During 
 the process of erection the cresta of the urethra or caput gallinaginis, which is 
 an elevation extending from the cavity of the bladder into the prostatic por- 
 tion of the urethra and containing erectile tissue, becomes turgid and, by the 
 aid of the contraction of the sphincter urethra;, effectually closes the passage 
 into the bladder. Erection is a complex reflex act, the centre of which lies 
 in the lumbar spinal cord and may be aroused to activity by nervous impulses 
 
 om different directions. Impulses may originate in the walls of the 
 
 ducts of the testis from the pressure of the contained semen or in the penis 
 
 from external stimulation of the nerve-endings in the skin, in both cases 
 
 - the sensory nerves of the organs to the spinal centre ; or they 
 
 mite in the brain and pass downward through the cord, the impulses 
 
 orrespoiiding to sexual emotions. The centrifugal paths for the 
 
 plHp V-i along th: nervi erigente ,< ?, which are true vaso-dilator nerves, and 
 
 in tin where experiment has proven! their existence, pass from the 
 
 spinal < a along the posterior lumbar (monkey) or anterior sacral (monkey, 
 
 . >erves to their arterial distribution. The ischio- and bulbo-cavwno- 
 su$ muscles are under the control of their motor nerve supply, consisting of 
 branches of the perineal nerve. 
 
 In the female, anatomists recognize the hornologues of the male erectile 
 
 as follows : the clitoris with its corpora cavemosa and glans as the homo- 
 
 ^^B of the penis, the two bulbi vestibuli as that of the bulb of the corpus 
 
 iKngiosum, the pars intermedia perhaps as that of the corpus spongiosum 
 
 \ the erector clitoridis muscle as the homologue of the erector penis 
 
 rcrnosus). The mechanism of erection is similar to that in the male, 
 and the result is a considerable degree of firmness in the external genital 
 organs. 
 
 The sexual excitement attendant upon copulation is usually much greater 
 in man than in woman, and culminates in the sexua> orgasm, when the emis- 
 sion of semen from the penis into the vagina c 't will be remembered 
 that the prepared semen is stored in the ducts of the testes. The discharge 
 of the fluid is a muscular act which begins probably in the vasa efferentia 
 he canal of the epididymis, and sweeps along the powerful muscular 
 walls of the vasa defer entia in the form of a series of peristaltic waves. The 
 '^> contract, and the mixed liquid and spermatozoa are poured 
 through the ejaculatory ducts into the prostatic portion of the urethra. The 
 he prostate expel the prostatic iiuid and help to *ass the semen 
 
RE PR OD UCTION. 1 1 J 5 
 
 onward. The glands of Cowper possibly add their contribution. But the 
 final urothral discharge is effected especially by powerful rhythmic contractions 
 of the already partially contracted striped muscles, viz. the ischio- and hulho- 
 cavernosi, the constrictor urethras, and probably the anal muscles, the result of 
 the complex series of actions being to expel the semen with some force into 
 the upper part of the vagina close to the os utei*i. Ejaculation is a reflex net. 
 The centre lies in the lumbar spinal cord ; the centripetal nerves are the sen- 
 sory nerves of the penis, stimulation of the glans being especially effective ; 
 the centrifugal nerves are the nerves to the various muscles. In the female 
 during ejaculation the glands of Bartholini pour out a mucous liquid upon the 
 vulva. There is possibly a downward movement of the uterus, brought about 
 by contraction of its round ligaments and accompanied perhaps by a contrac- 
 tion of the uterine walls themselves. But all muscular and erectile activity, 
 as well as sexual passion, is usually less pronounced in woman than in man. 
 Locomotion of the Spermatozoa. The union of the spermatozoon and 
 the ovum probably takes place usually in the Fallopian tube not far from its 
 ovarian end, and to this place the spermatozoa at once proceed. Their mode 
 of entrance into the uterus is not wholly clear ; it is quite generally believed, 
 but without conclusive experimental proof, that relaxation of the uterus im- 
 mediately after copulation exerts a suction upon the liquid which aids in its 
 passage through the os and the cervix. It is possible that active contraction 
 of the vaginal walls assists. Spermatozoa have been found in the uterus a 
 half hour after coition. 1 The main agency in the locomotion of the sper- 
 matozoa through the body of the uterus and the Fallopian tubes, and prob- 
 ably also from the vagina into the uterus, is the spontaneous movement of 
 the spermatozoa themselves. By the lashing of their tails they wriggle their 
 way over the moist surface, being stimulated to lively activity probably by the 
 opposing ciliary movements in the epithelium lining the passages. Kraft 2 has 
 shown in the rabbit that, when spermatozoa in feeble motion are placed upon 
 the inner surface of the oviduct, not only are they thrown into active contrac- 
 tions, but they move against the ciliary movement, *. e. up the oviduct. The 
 capacity of the male cells thus to respond by locomotion in the opposite direc- 
 tion to the stimulating influence of the ciliary cells over which they have to 
 pass, is an interesting adaptation. Probably this is the directive agency that 
 enables the spermatozoa to follow the right path to the ovum, while the ovum, 
 being in itself passive, is by the same ciliary movement brought toward the 
 active male cell. The time occupied in the passage of the spermatozoa is un- 
 known in the human female, but is probably short ; in the rabbit spermatozoa 
 have been known to reach the ovary within two and three-quarter hours after 
 copulation. As has been seen, spermatozoa are probably capable of living 
 within the genital passages for several days, when, if ovulation has not taken 
 place, they perish. If, however, an ovum appears, they at once approach and 
 surround it in great numbers, being apparently attracted to it in some myste- 
 
 ^chuworski: Abstract in Mnnntnnrh rift fitr flrtmrfdiiilf*- u,,<l (lyniihnlnijie, 1896, iv. S. 275. 
 2 H. Kraft : Pfliiger's Archivfiir die gesammte Physiologic, 1890, xlvii. 
 VOL. II. 30 
 
466 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 rious manner. The work of Pfeffer, 1 who found that in the fertilization of 
 ferns malic acid within the female organs attracts the spermatozoids to their 
 vicinity, suggests strongly that also among animals the attraction may be a 
 chemical one, the ovum containing or producing something for which the sper- 
 matozoon has an affinity. If so, the meeting of the two germ-cells is an illus- 
 tration of a widespread principle of nature known as chemotaxts, or chemo- 
 tropism. Experimental evidence upon the subject in animals is wanting. 
 
 Fertilization. It will be remembered that the ovum and the spermatozoon 
 undergo in their growth the process of maturation, and that this process con- 
 sists essentially of a loss of one-half of the chromosomes of their nuclei. The 
 germ-cells thus matured meet, as we have seen, in the distal half of the Fal- 
 lopian tube and fuse into one cell, the process of fusion being called fertilization 
 or impregnation. The details of fertilization have not been observed in the 
 case of the human being, and the following account is generalized from our 
 knowledge of the process in other mammals and lower animals. In its broad 
 outlines fertilization is probably the same in all animals, the differences being 
 confined to details. 
 
 The ovum at the time of fertilization is surrounded by the zona radi- 
 ata alone, the corona radiata having been lost. The spermatozoa swarm 
 about the zona, lashing their tails and attempting to worm their way through 
 it. Several may succeed in reaching the perivitelline space, but for some 
 unknown reason in most cases one only penetrates the substance of the ovum ; 
 the others ultimately perish. In mammalian ova there is no micropyle, and 
 apparently the successful spermatozoon may enter at any point, the protoplasm 
 of the egg rising up as a slight protuberance to meet it (Fig. 223, A). In some 
 animals the tail is left outside to perish ; in others it enters, but then disap- 
 pears ; in no case does it appear to be of further use. The head and probably 
 the middle-piece are of vital importance. The head, now known as the sperm- 
 nucleus or male pronucleus, proceeds by an unknown method of locomotion 
 toward the centre of the egg, and becomes enlarged by the imbibition of liquid 
 (Fig. 223, B). The matured nucleus of the ovum, or egg-nucleus, also moves 
 slowly toward the future meeting-place of the two nuclei, which is near the 
 centre of the egg. The two finally meet (Fig. 224, c) and together form a 
 new and complete nucleus, called the first segmentation-nucleus (Fig. 224, D). 
 This body has the conventional nuclear structure namely, an achromatic 
 network with the chromatic reticulum mingled with it and the whole is 
 covered by a nuclear membrane. From the observations of Van Beneden, 
 Riickert, 2 Zoja, 3 and others, it seems to be a fact that the male and the female 
 chromosomes do not fuse together, but remain distinct from each other, per- 
 haps throughout all the tissue-cells. The chromosomes, it will be perceived, 
 are now restored to the original number present in either germ-cell before its 
 maturation, hence in the human being perhaps sixteen, one-half of them 
 
 1 W. Pfeffer : Untersuchungen aus dem Botanischen Institut zu Tubingen, 1884, i. 
 2 J. Riickert: Archiv fur mikroskopische Anatomie, 1895, xlv. 
 3 R. Zoja: Anatomischer Anzeiger, 1896, xi. 
 
REPR OD UCTION. 467 
 
 having come, however, from the male cell and one-half from the female cell. 
 On the commonly accepted theory that they constitute the hereditary sub- 
 stance, the first segmentation-nucleus contains within itself potentially all the 
 inherited qualities of the future individual. 
 
 While the head of the spermatozoon is making its way through the sub- 
 stance of the egg there appears beside it a minute cytoplasmic body, the 
 centrosome, and around the latter cytoplasmic filaments arrange themselves 
 in the form of a star, the whole body being known as the sperm-aster (Fig. 
 223, B). We have previously recognized such a structure in the ovum at the 
 time of maturation, and have found it functional in the formation of the 
 polar bodies ; after maturation it disappears. The sperm-aster accompanies 
 the sperm-nucleus, becomes gradually enlarged, and finally comes to lie, a 
 large and prominent body, beside the segmentation-nucleus. The origin of 
 its centrosome has been greatly disputed. Some investigators maintain that 
 
 
 
 <--,. 
 
 *. 
 
 FIG. 223. Stages in the fertilization of the egg (after Wilson). The drawings were made from sections 
 of the eggs of the sea-urchin, Toxopneustes variegatus, Ag. 
 
 A. The surface of the egg has become elevated to form c, the entrance-cone for the spermatozoon ; the 
 head (/i> and the middle-piece (m) of the latter have entered the egg. 
 
 B. Five minutes after entrance of the spermatozoon. The head, now the male pronucleus, has rotated 
 180 degrees, and has travelled deeper into the ovum. The cytoplasm of the latter has become arranged 
 in a radiate manner about the middle-piece of the spermatozoon, now the centrosome, to form the sperm- 
 aster ; the egg-nucleus, now the female pronucleus, is approaching the sperm-nucleus ; its chromatin 
 forms an irregular reticulum. 
 
 it is formed anew in the egg ; but the prevalent opinion at present seems to 
 be that it comes from the spermatozoon in immediate relation to the middle- 
 piece, and hence is exclusively of male origin. 
 
 There results from fertilization, it is perceived, a single cell complete in 
 all its essential parts. This is the starting-point of the new individual. A 
 pause or resting period usually follows fertilization, and then growth begins. 
 
 Segmentation. The process of growth is a complex process of repeated 
 cell-division, increase in bulk, morphological differentiation, and physiological 
 division of labor. 
 
 Cell-division is largely, if not wholly, indirect or mitotic. The term seg- 
 mentation, or cleavage, of the ovum is conveniently applied to the first few 
 divisions, although the details of segmentation are not different fundamen- 
 tally from those manifested later in the division of more specialized cells. 
 Each division may be resolved into three definite acts, which, however, 
 overlap each other in time. The first act is characterized by the appearance 
 of two centrosomes, each with its astral rays, in place of the one already 
 
468 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 existing (Fig. 224, c). The two take positions beside the nucleus (Fig. 224, 
 D) and await the time when they can exert their specific function. We have 
 spoken of the difference of opinion regarding the origin of the original cen- 
 trosome of fertilization. The origin of the two centrosomes present in seg- 
 mentation has likewise been disputed. The question is of considerable the- 
 oretical interest in connection with the problem of the physical basis of 
 
 FIG. 224. Stages in the fertilization of the egg (continued from Fig. 223). 
 
 c. Ten minutes after entrance of the spermatozoon. The male and the female pronuclei have met 
 near the centre of the egg and the fusion has begun ; the former has become enlarged and its chromatin 
 has become loosely reticulated. The sperm-aster has become enormously enlarged. The single centro- 
 some has been divided into two, which lie upon either side of the sperm-nucleus. 
 
 D. Still later after entrance of the spermatozoon. The two pronuclei have united to form the first 
 segmentation-nucleus. The sperm-aster has become divided into two asters, which have moved to 
 opposite poles of the nucleus. The egg is now ready to undergo segmentation. 
 
 inheritance. Certain observers have claimed that the centrosomes have a 
 double origin, one being derived from the male and one from the female 
 germ-cell. Upon this theory sexuality is shown by the cytoplasmic centro- 
 somes as well as by the nuclear chromosomes, and the inference is possible that 
 cytoplasm, as well as nucleus, transmits hereditary qualities. This double 
 origin of the centrosomes is not supported by trustworthy evidence. Other 
 observers, following Boveri, maintain that the centrosome of the sperma- 
 
REPRODUCTION. 
 
 469 
 
 \ i 
 
 tozoon divides into the two segmentation-centrosomes, the latter hence 
 exclusively of male origin. Still 
 others believe that the sperm- 
 centrosome disappears, its place 
 being taken by two new centro- 
 somes derived from the cyto- 
 plasm of the egg. The evi- 
 dence available at present does 
 not allow a decision to be made 
 between these two latter views. 1 
 According to both of them, 
 however, the cleavage-centro- 
 somes are not male and female, 
 and cannot be regarded as 
 bearers of inherited character- 
 istics. These observations not 
 only allow, but tend to 
 strengthen, the prevailing view 
 of the exclusive hereditary role 
 of the nucleus. (See below 
 under Heredity, p. 493.) 
 
 The second act of segmenta- 
 tion is more complicated than 
 the first, and consists of a halv- 
 ing of the nucleus. The nuclear 
 membrane gradually disappears. 
 The achromatic network resolves 
 itself into long cytoplasmic fila- 
 ments arranged in the form of a 
 spindle, and meeting at the two 
 centrosomes (Fig. 225, A). The 
 spindle, centrosomes, and asters 
 form the body known as the 
 amphiaster. The chromatic sub- 
 stance becomes changed into the 
 definite rod-like chromosomes, 
 
 FIG 225. Stages in the segmentation of the egg (after 
 Wilson). The drawings were made from sections of eggs 
 of the sea-urchin, Toxopneuntes variegatus, Ag. 
 
 A. The nuclear membrane has disappeared. Within the 
 nucleus a distinction between the chromatic and the achro- 
 matic substance has been made, the former existing as 
 clearly denned chromosomes aggregated in the centre to 
 form the equatorial plate, the achromatic substance exist- 
 wllich are collected in the eqiia- ing as delicate filaments extending in the form of a spindle 
 
 from pole to pole. 
 
 tonal zone OI the spindle, and B. Each chromosome has become split into two. and the 
 
 latter are being pulled toward the poles. 
 
 c. The divergence of the chromosomes has ceased and 
 the latter are becoming converted into vesicular masses 
 beside the centrosomes. The spindle is becoming resolved 
 into ordinary cytoplasm. 
 
 constitute the equatorial plate. 
 Each chromosome proceeds to 
 split lengthwise, and the two 
 halves move toward the two 
 centrosomes (Fig. 225, B). The cause of this movement is not known. 
 
 The 
 
 1 For a critical review of this and other problems in fertilization and segmentation see E. 
 B. Wilson: The Cell in Development and Inheritance, 1900, 2d ed., New York. 
 
470 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 original idea of Van Beneden, 1 that the astral rays are contractile and 
 mechanically pull apart the half-chromosomes, is supported by considerable 
 but unconvincing evidence. The idea appears to be growing that by reason 
 of chemical changes taking place in the centrosomes the half-chromosomes 
 are attracted to the two poles of the spindle. 2 Strasburger 3 suggests that 
 this attractive influence is chemotaxis. In the process of division each 
 nuclear half obtains half of the original male and half of the original 
 female chromatin, and hence contains inherited potentialities of both parents. 
 After division each half gradually assumes the structure of a typical resting 
 nucleus with its accompanying aster (Fig. 226). 
 
 The third act of segmentation consists of a simple division of the cytoplasm 
 into two equal parts, the separation taking place along the plane of nuclear 
 
 FIG. 226. Stages in the segmentation of the egg (continued from Fig. 225). 
 
 D. The vesicular chromatic masses have become converted into two typical resting nuclei, each with 
 a chromatic network. The single aster, formerly connected with each nuclear mass, has become divided 
 into two, which have taken positions at opposite poles of the nuclei. The division of the cytoplasm is 
 complete, and the two resulting cells, or blastomeres, are resting, preparatory to a second division in a 
 plane at right angles to that of the first. 
 
 division (Fig. 226, D). Each part contains one of the new nuclei, and the 
 result of the first division is the existence of two cells, two blastomeres, in 
 place of the one fertilized ovum. The beginning of differentiation is often 
 shown even as early as this, for one blastomere is often somewhat larger and 
 less granular than the other. 
 
 Each blastomere proceeds now to divide by a similar mitotic process into 
 two, the result being four in all, and by subsequent divisions, eight, sixteen, 
 and more, the divisions not proceeding, however, with mathematical regu- 
 larity. By such repeated mitotic processes the original fertilized ovum 
 becomes a mass of small and approximately similar cells, the morula, from 
 which by continued increase in the number of the cells, morphological differ- 
 entiation, and physiological division of labor, the embryo with all its functions 
 is destined to be built up. 
 
 1 Van Beneden : Archives de Biologie^ 1883, iv. 
 
 *Cf. Biitschli: Verh. Naturhist. med. Ver. Heidelberg, 1891 ; and'E. B. Wilson, op. cit. 
 
 3 Strasburger : Analomischer Anzeiger, 1893, viii. 
 
REPROD UCT1<>\. \ 7 \ 
 
 Polyspermy. It happens occasionally that two or more spermato/na enter 
 the ovum ; such a phenomenon is known as tli^tn-mi/ or /W//x/>o-///v, according 
 to the number of entering sperms. Each sperm with its nucleus and centro- 
 some becomes a male pronucleus and proceeds to conjugate with the female 
 pnmncleiis. In the case of dispermy the one female and the two male pro- 
 nuclei fuse together; each centrosome gives place as usual to t\\<>. making 
 four iu all, which take up a quadrilateral position about the first segmenta- 
 tion-nucleus ; the chromatic figure consists of two crossed spindles ; and tin- 
 egg segments at once into four instead of two blastomeres. Analogous phe- 
 nomena result from more complex cases of polyspermy. In such double- or 
 multi-fertilized eggs development may proceed to some distance, but typical 
 larval forms are not produced, and death occurs early. 
 
 During cleavage the ovum proceeds, after the manner of the non-fertilized 
 ovum, slowly along the Fallopian tube and enters the uterus. Unlike the non- 
 fertilized ovum, however, the morula is not cast out of the body, but remains 
 and undergoes further development. The morphological development of the 
 embryo in utero does not fall within the scope of the present article. Some 
 attention may, however, be given to the immediate environment of the develop- 
 ing child and its relations to the maternal organism. 
 
 Decidua Graviditatis. While the segmentation of the ovum is proceed- 
 ing within the Fallopian tube, the uterus prepares for the future guest by begin- 
 ning to undergo a profound change, probably being stimulated to activity re- 
 flexly by centripetal impulses originating in the walls of the tube through con-, 
 tact with the ovum. This change comprises an enlargement of the whole uterus 
 and a great and rapid growth in thickness of its mucosa and its muscular 
 coat. At first the alterations are not unlike the phenomena of growth pre- 
 ceding the menstrual flow, but, as they proceed, they become much more pro- 
 found than those. The supply of blood to the walls is greatly increased, the 
 vessels forming large irregular sinuses within the mucosa. The supply of lymph 
 is increased. The glands become tortuous and dilated into flattened cavernous 
 spaces, and their walls atrophy, the epithelium breaking down except in their 
 deepest parts. The mucosa is thus converted into a spongy tissue, the frame- 
 work of which contains numerous large irregular cells, derived probably from 
 the original connective tissue and called decidual ceils. The musculature is 
 greatly thickened by an increase, partly in number and partly in size, of its 
 constituent fibres, and the nerve-supply is increased. These general structural 
 changes proceed through the early part of gestation and are accompanied by 
 special changes to be discussed later. It is not definitely known how far the 
 alterations have gone before the advent of the. segmented ovum into the uterus. 
 With the latter instead of the unimpregnated ovum present in the Fallopian 
 tube, the hypertrophied uterine mucosa does not break awav a.s in menstrua- 
 tion, but remains, and henceforth is called the decidua graviditatis, special 
 names being given to special parts. Entering the uterus, the ovum attaches 
 itself in an unknown manner to the wall of the womb. The part of the mucous 
 
472 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 membrane that forms its bed is henceforth known as the decidua serotina ; as 
 the seat of the future placenta, it is physiologically the most interesting and 
 important portion of the uterine inucosa. The surrounding cells and tissues 
 are stimulated to active proliferation and grow around and over the ovum, 
 completely covering it with a layer, the decidua reflexa. The remainder of 
 the uterine lining membrane constitutes the decidua vera. Between the reflexa 
 and the vera is the uterine cavity. At first thickened, the reflexa later thins 
 away as the embryo grows, and approaches close to the vera ; finally it touches 
 the latter, and the original cavity of the body of the uterus becomes oblit- 
 erated. By the sixth month the reflexa disappears, either coalescing with the 
 vera or undergoing total degeneration (Minot). During the latter half of 
 gestation 'the vera itself thins markedly. This atrophy of the comparatively 
 unimportant reflexa and vera, in contrast to the placental hypertrophy of the 
 serotina, is interesting. The arrangement of the parts is well shown in the 
 accompanying illustration (Fig. 227). 
 
 The Petal Membranes. The segmented ovum absorbs nutriment at first 
 directly from its surrounding maternal tissues, and later through the mediation 
 of the placenta. Its growth and cell-division are active, and it increases in 
 size and complexity. It early takes the form of a generalized vertebrate em- 
 bryo, and by the fortieth day begins to assume distinctly human characteristics. 
 It becomes surrounded early by the fetal membranes, which are two in num- 
 ber, the omnion and the chorion or, as it is usually called in other vertebrates, 
 false amnion. The amnion is a thin, transparent, non-vascular membrane imme- 
 diately surrounding the embryo (Fig. 227). In origin a derivative of the embry- 
 onic somatopleure, later it becomes completely separated from the body of the 
 embryo. The space enclosed by the amnion, the amniotic cavity, within which 
 the embryo lies, is traversed by the umbilical cord and contains a serous liquid, 
 the liquor amnii. This liquid, highly variable in quantity, averages at full 
 term nearly a liter (1J pints). It has in general the composition of a serous 
 liquid. It contains between 1 and 2 per cent, of solids, consisting of proteids 
 (0.06-0.7 per cent.), mucin, a minute and variable quantity of urea, and inor- 
 ganic salts. Its origin, whether from the fetus, especially from the fetal 
 kidneys, or from the mother, has been much discussed. It may possibly 
 come in small part from the former, but its chief origin is doubtless by trans- 
 udation from the maternal blood, as is indicated by the ready appearance 
 within the amniotic cavity of solutions injected into the maternal veins, and 
 the fact that the amniotic liquid of diabetic women contains sugar. It bathes 
 the entire surface of the embryonic body, and is, moreover, apparently swal- 
 lowed into the stomach, as the presence of fetal hairs and epidermal scales 
 within the alimentary canal attests. Its chief functions appear to be those 
 of protecting the fetus from sudden shocks and from pressure, maintaining a 
 constant temperature, and supplying the fetal body with water. The pro- 
 teid possibly confers upon it a very slight nutritive value, and the minute 
 quantity of urea is perhaps indicative of an unimportant excretory function 
 of the fetal kidneys. As growth proceeds, the amnion expands and becomes 
 loosely attached to the outer fetal membrane, the chorion. 
 
REP ROD UCTION. 
 
 473 
 
 The chorion (Fig. 227), or false amnion, is formed simultaneously with the 
 true amnion, and like it from somatopleure. It is a thickened vascular mem- 
 brane, completely surrounding the amnion with the contained embryo. Be- 
 tween it and the amnion there is at first a considerable space, traversed by the 
 umbilical cord and filled with the chorionic fluid (which is probably of the 
 same general nature as the amniotic fluid). But later this space is obliterated 
 
 Decidua terotina. 
 Chorion Jrondosum. 
 
 Muscle. 
 
 Uterine glands. 
 Chorion Iseve. 
 
 Mucous plug within cervical canal. 
 
 FIG. 227. Diagram of the human uterus at the seventh or eighth week of pregnancy (modified from 
 Allen Thompson). The fetal villi are shown growing into the sinuses of the decidua serotina and the 
 decidua reflexa ; in the latter they are becoming atrophied. They are marked by the black fetal vessels, 
 which can be traced backward along the umbilical cord to the embryo. The placenta comprises the 
 decidua serotina and the chorion frondosum. 
 
 by the enlargement of the amnion. Externally the chorion presents, at first, 
 a shaggy appearance due to the existence of very numerous columnar pro- 
 cesses, called vitti, extending outward in all directions and joining by their 
 tips the decidua serotina and the decidua reflexa. Later the villi are aborted 
 except in the region of the serotina, where they become more prominent and 
 constitute an important part of the placenta. The blood-vessels of the chorion 
 
474 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 Amnion. 
 
 are fetal vessels coming from the embryonic structure, the allantois. They 
 comprise the branches and uniting capillaries of the two allantoic or umbilical 
 arteries, and the one (at first two) allantoic or umbilical vein. They are 
 especially well developed within the villi. As growth proceeds, the chorion 
 comes into close contact with the deddua reflexa, and, as the latter disappears, 
 with the deddua vera; this portion of it is called chorion Iceve. In the region 
 of the deddua serotina it enters into the formation of the placenta, and is 
 here called chorion frondosum. 
 
 The Placenta. The placenta (Fig. 227), or organ of attachment of mother 
 and fetus, is a disk-shaped body, approximately 20 centimeters (78 inches) in 
 diameter, attached to the inner surface of the uterine wall, usually either upon 
 the dorsal or the ventral side, and connected by the umbilical cord with the 
 
 navel of the fetus. It consists of 
 a maternal part, the modified 
 deddua serotina, and a fetal part, 
 the modified chorion, intimately 
 united together. The modifica- 
 tions of the serotina consist of a 
 degeneration of the superficial 
 layers of the mucosa, especially of 
 the epithelium and the glands, and 
 the development of very large 
 irregular sinuses at the surface, 
 into which the uterine arteries and 
 veins freely open. It is a disputed 
 question among histologists whether 
 the sinuses are maternal or fetal in 
 origin, or really spaces between 
 maternal and fetal tissues. The 
 modifications of the chorion con- 
 sist of a great increase in length 
 and complexity of branching of 
 the villi, a great development of 
 their contained blood-vessels, and 
 a firm attachment of their tips to 
 the uneven surface of the serotina, 
 
 S ** *^ <=Ome to float 
 
 F,G. 228,-Diagran, of the placenta (Schafer) : 8 , pla- 
 
 cental sinuses, into which project the fetal villi, con- freely Within the uterine sinuses 
 
 taining the red fetal vessels; ds, decidua serotina; sp. i *, i i ,1 i i i J 
 
 spongy layer, and m, muscular layer, of the uterus; a, aild to be bathed m terine blood 
 
 uterine artery, and v, uterine vein, opening into the (Fig. 228). The analogy between 
 
 placental sinuses. ,. , , , .. 
 
 the mammalian placental villi and 
 
 the gills of a fish, also highly vascular and floating in liquid, is striking. 
 We shall see later that the analogy is not only morphological but also 
 physiological, inasmuch as the villi have important respiratory functions. 
 The bulk of the placenta is this intravillous portion, of spongy consistence, 
 
REPRODUCTION. 475 
 
 comprising the maternal sinuses permeated hy the fetal villi ; this is in con- 
 tact upon the fetal side with the thin unmodified ehorion covered within l>v 
 the amnion, and upon the maternal side with the thin relatively unmodified 
 serotina covered without l>y the uterine muscle. The pure maternal blood 
 brought by the uterine arteries moves slowly through the sinuses and retires 
 by the uterine veins; the fetal blood is propelled by the fetal heart alono- the 
 umbilical cord within the allantoie arteries and through the villous capillaries, 
 and returns by the allantoie vein. The two kinds of blood never mix, but 
 are always separated by the thin capillary Avails and their thin villous invest- 
 ment of connective tissue and epithelium. Thus the anatomical conditions 
 for ready diffusion are present, and this is the chief means of transfer 
 of nutriment and oxygen from mother to child, and of wastes from child to 
 mother. The physiological role of the placenta is, therefore, an all-important 
 and complicated one. The placenta is, technically, the nutritive organ of the 
 embryo. 
 
 Nutrition of the Embryo. We have seen that a fundamental and most 
 striking difference between the minute human ovum and the large egg of the 
 fowl lies in the relative quantity of food contained in the two. The fowl has 
 retained the primitive habit of discharging the ovum from the maternal body, 
 and discharges within its shell at the same time sufficient food for the needs of 
 the developing chick. Evolution has endowed the human mother, in common 
 with other mammals, with the peculiar custom of retaining the offspring within 
 her body until its embryonic life is completed, and of doling out its nutriment 
 molecularly throughout the period of gestation. The store of nutritive deuto- 
 plasm with which the egg leaves the ovary is, therefore, only sufficient for the 
 early segmentative activities. Within the Fallopian tube absorption from the 
 surrounding walls doubtless goes on. Arrived in the uterus and imbedded in 
 its decidual wall, the segmented ovum continues to take nutriment from its 
 immediate environing cells. It has been suggested, but without much basis 
 of fact, that the uterine glands, which at this time are greatly dilated, may 
 furnish a nutritive secretion for the use of the embryo; but, a priori, it would 
 seem more reasonable that, just as the ovum within the Graatian follicle 
 obtains its food from its surrounding stroma, so within the highly vascular 
 decidua it absorbs directly from the decidual tissue. But that this source 
 soon proves insufficient for the rapid growth is indicated by the early develop- 
 ment of the ehorion with its villi and the embryonic vascular system. In 
 the youngest known human embryo, 1 believed to be scarcely seven days old, 
 the villi are alreadv well marked. From this time onward throughout ges- 
 tation the ehorion takes an important part in the embryonic nutrition, becom- 
 ing, as we have seen, an integral part of the placenta. The placenta is jm/- 
 <'.,<< 11,'ncc the medium of nutritive communication between mother and child. 
 
 Let us consider briefly the needs of the embryo. The fetal energies must 
 be directed almost wholly to the all-important functions of growth and prepa- 
 ration for the future independent existence. The organism requires, therefore, 
 deters: Verhnndltingen der deutschen GewtMi'ift fiir Gynakohgie, 1897, vii. S. 264. 
 
476 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 an abundance of food containing all the chief kinds of food-stuffs. With the 
 alimentary canal in its embryonic and functionless state, this food, when it 
 reaches the embryo, must necessarily be already digested and ready for absorp- 
 tion by the cells. A supply of oxygen, not necessarily great in quantity, is 
 also needed. The fetal lungs are not ready for respiration, and the oxygen 
 must come to the blood by another channel than them. Carbonic acid must 
 be got rid of, and through other than pulmonary paths. Urea and its fore- 
 runners and other wastes, probably not in great quantity, must be excreted. 
 The fetal kidneys and the skin are probably never very active, as is made rea- 
 sonably certain by the late external opening of the male urethra, the late 
 development of the cutaneous glands, and the composition of the amniotic 
 liquid, into which they would naturally pour their secretions. Thus the paths 
 of income and outgo that are normal to the individual after birth are only 
 partially open during fetal life ; nevertheless, the processes of income and 
 outgo must be performed. The placenta, with* its close relationship but non- 
 communication of maternal and fetal blood-vessels, has, therefore, been evolved 
 phylogenetically, and appears early in the course of ontogeny. There is 
 brought to it on the part of the embryo and discharged into the villous 
 capillaries a mixed blood, comprising venous blood from the various capil- 
 lary systems of the body, and containing, therefore, the carbonic acid and 
 other wastes of venous blood, and a certain proportion of purified blood 
 which has passed directly by way of the ductus venosus, the inferior vena 
 cava, the right auricle, the foramen ovale, and the left side of the heart to the 
 aorta and the umbilical arteries. There is brought to the placenta on the 
 part of the mother and discharged into the sinuses pure arterial blood, 
 laden with food and oxygen. Through the membrane intervening between 
 maternal and fetal vessels there pass from the fetus carbonic acid and other 
 wastes, and from the mother food (sugar, fats, proteids, etc.) and oxy- 
 gen. Back to the fetal liver and heart goes the nutritive and arterialized 
 blood, and back to the maternal excretory organs the vessels convey the fetal 
 wastes. The placenta is thus a peculiar organ intermediate between the living 
 cells of the embryo on the one hand and the digestive organs, lungs, kidneys, 
 and skin, of the mother on the other. Little is known of the actual details 
 of the placental process. The structure of the intervening cells indicates that 
 the interchange may be after a manner analogous to that taking place in the 
 lungs, rather than to that of a typical secreting gland i. e. that known physi- 
 cal processes, such as diffusion and filtration, play a prominent role. It has 
 been shown by several investigators that the fetus may be poisoned by car- 
 bonic oxide and strychnine, and may receive other harmless diffusible sub- 
 stances that are introduced in solution into the maternal circulation. The 
 mother may be affected similarly from the fetal circulation. But, as in the 
 case of the lungs, so the placental membrane can scarcely be regarded as 
 acting in the same passive way as a lifeless membrane would act (compare 
 Respiration). As accessory to the main nutritive source it has been sug- 
 gested that a diapedesis of maternal leucocytes into the fetus may take place. 
 
RE PR OD UCTION. 477 
 
 The uterine "-lands arc thought by some to afford a nutritive secretion to the 
 sinuses, and to the amniotic liquid lias been asrribed a nutritive function. 
 Theoretically, these various means are not impossible, but true plaeental dillu- 
 sibn must be regarded as the ehief principle at work. The result is that the 
 mother relieves the child of all the labor of nutrition except that connected 
 directly with the latter's own cellular and protoplasmic metabolism. The 
 fetal energies are, therefore, free to be expended in the process of growl h, 
 while gestation profoundly affects the maternal organism. 
 
 Physiological Effects of Pregnancy upon the Mother. As might have 
 been expected, there is probably not one organic system within the mother's 
 body that is not more or less altered by pregnancy, often morphologically, but 
 especially in regard to function. And such normal alterations pass so gradu- 
 ally and so frequently into genuine pathological conditions that it is sometimes 
 difficult to draw the line between the two. The most marked changes are 
 connected with the body of the uterus, and have already been described. The 
 walls of the cervix uteri become hypertrophied, though to a less degree than 
 the body, and their glands secrete a quantity of mucus that forms a plug com- 
 pletely closing the passage-way of the cervix (Fig. 227). The rest of the 
 reproductive organs from the uterus outward become involved in the increased 
 venous hypera3mia. The walls of the vagina become infiltrated with serous 
 liquid. The parts of the vulva partake in the general tumefaction. From the 
 second month of gestation onward the mammary glands undergo gradual devel- 
 opment as a preparation for the post-partum lactation. The increase in size of 
 the laden uterus brings gradually increasing pressure to bear upon the abdom- 
 inal viscera, and thus mechanically causes functional derangements of the 
 digestive and the urinary organs. The stretching of the abdominal skin 
 results in localized ruptures of the connective tissue of the cutis, the charac- 
 teristic scars forming the strice gravidarum, which persist after pregnancy. 
 Other organic changes are, however, more profound than these mechanical ones. 
 In accordance with the increased nutritive labor thrown upon the mother, the 
 total quantity of blood in her body is increased, if we can reason from deter 
 minations made upon the lower animals. 1 The condition of the blood has 
 been disputed. The old belief was that the blood of pregnancy is more waterv 
 and contains less hemoglobin than at other times. This is perhaps true for 
 the earlier months, but Schroeder 2 and others have shown that the proportion 
 of haemoglobin and the number of red corpuscles rise above the normal 
 during the later stages. The work of the maternal heart is increased during 
 gestation. It is maintained by some that the heart beats more rapidly- 
 according to Kehrer, 3 over eighty times in the minute. It has also been 
 thought, mainly from the results of percussion and from sphvomographic 
 tracings, that the left ventricle is hypertrophied during pregnancy. IVt- 
 mortem examination confirms this inference. Pregnancy necessarily throus 
 
 1 O. Spiegelberg und R. Gscheidelen : Archivfiir Gynakologie, 1872, iv. 
 
 2 R. Schroeder: Ibid., 1890-91, xxxix. ; Wild: Ibid., 1897, liii. S. 363. 
 
 8 F. A. Kehrer : Ueber die Verdnderungen der Pulseurve im Puerperium, 1886. 
 
478 AN' AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 increased labor upon both the liver and the kidneys, and these organs are prone 
 to functional disorders. Gastric disturbances are marked by frequent vomit- 
 ing. A tendency to increased pigmentation in the skin is present. The ner- 
 vous system is affected, manifesting its alterations both by nutritional disturb- 
 ances and by mental irritability, depression of spirits, disordered senses, easily 
 passing into temporary pathological states, and occasionally by feelings of 
 heightened well-being. The body-weight usually increases independently of 
 the added weight of the embryo. 
 
 Duration of Gestation. For centuries the duration of gestation in 
 woman has been commonly regarded as 280 days. The beginning of preg- 
 nancy, the union of the ovum and the spermatozoon, however, presents no 
 obvious signs by which it may be recognized, and hence the actual length of 
 pregnancy in the human female is no more known than in other mammals. 
 The obstetrician is obliged, therefore, to use artificial schemes in computing its 
 probable length. Several tables have been published of the time elapsing 
 between a single coition resulting in pregnancy and the terminal parturition. 
 "Veit, 1 in collecting 503 such cases reported by several obstetricians, finds the 
 duration to be from 265 to 280 days in 396 cases, and longer in the remaining 
 107 cases, the variation thus being marked. It is obvious that the date of the 
 effective coition can rarely be known. One of the first and most evident signs 
 of pregnancy is the non-appearance of the menses, and, probably largely from 
 the long-prevailing idea of the close relation existing between ovulation and 
 menstruation, it has been customary to regard gestation as dating from the last 
 menstruation. Following Naegele, obstetricians estimate the date of parturi- 
 tion as 280 days from the first day of the last menstruation; and this simple 
 but artificial rule is doubtless approximately correct. 
 
 In accordance with modern biological theories, it must be supposed that for 
 each species there has been developed a gestative period of a length most 
 favorable to the continuance of the species ; this has been a matter of natural 
 selection. But this principle does not account for the termination of the period 
 in any individual case. The proximate cause of the oncoming of birth must 
 be sought in more specific anatomical or physiological phenomena. This cause 
 has been sought long, and not wholly successfully. Among the agents sug- 
 gested may be mentioned the pressure which the uterine tissues, the ganglia 
 of the cervix, and the adjacent nerves, receive between the fetal head and the 
 pelvic wall, the stretching of the uterine wall, the fatty degeneration of the 
 deciduse, the thrombosis of the placental vessels, the venosity of the fetal 
 blood due to the growing functional importance of the fetal right ventricle 
 acting as a stimulus to the placental area, and a gradual increase in irritability 
 of the uterus as the nerve-supply of the organ increases. Some of these, such 
 as the fatty degeneration of the deciduse and the placental thrombosis, are not 
 constant phenomena, and the others are not definitely proved to be efficient 
 causes. It is probable that, with the uterus undoubtedly irritable, in different 
 1 J. Veit : Mutter's Handbuch der Geburtshiilfe, 1888, 1. 
 
REPRODUCTION. 479 
 
 cases different stimuli act to inaugurate the proocss of birth, and a priori 
 several of the above causes scciu not improbable ones. 
 
 Parturition in General. Parturition, birth, or labor, is the process of 
 expulsion of the developed embryo, the membranes, and the placenta from the 
 body of the mother. It is executed by contraction of the muscles of the so- 
 called upper segment of the uterus and those of the abdominal walls. The 
 lower segment of the uterus, comprising approximately that portion of the 
 body lying below the attachment of the peritoneum, the cervix, the vagina, 
 and the vulva, are largely, if not wholly, passive in parturition. The obstet- 
 ricians have found it convenient to divide labor into three stages, although 
 physiologically these are not sharply differentiated from each other. The first 
 stage is characterized by the dilatation of the os uteri, the second by the expul- 
 sion of the fetus, the third by the expulsion of the after-birth. The customary 
 position of the fetus within the uterus at the end of pregnancy is that in which 
 the head is downward or nearest the os, the back toward the ventral and left 
 side of the mother, and the arms and legs folded upon the trunk. 
 
 First Stage of Labor. For several weeks toward the close of pregnancy 
 there are occasional periods when rhythmic muscular contractions pass over the 
 uterine walls. These are mostly painless, and apparently are not in themselves 
 of special functional importance. The first stage of labor is ushered in by 
 various phenomena, prominent among which are an increase in the intensity 
 of the contractions, their painfulness, and their frequency and continuance. 
 In women they are confined practically to the upper segment of the uterus and 
 its attached ligaments, ceasing at a circular ridge that projects inward and is 
 called the "contraction ring." For some reason, at present disputed, the 
 lower segment of the uterus, and the cervix, are passive. The contractions 
 are probably peristaltic in character, as in lower animals. Schatz 1 has graphi- 
 cally recorded the uterine movements by means of a bladder filled with water 
 and introduced into the uterus. During the earlier part of parturition the 
 contractions gradually increase in intensity up to a maximum which they 
 then maintain. Their rhythm is somewhat irregular ; the duration of each 
 contraction averages about one minute, and a pause, which ensues between suc- 
 cessive contractions, extends from one and one-half to several minutes. The 
 relaxation of the muscle-fibres during the period of rest is incomplete, the 
 result being that the fibres enter gradually into a tonically contracted state. 
 Each contraction is accompanied by a pain, localized in the early part of labor 
 in the uterus alone, but later extending outward, upward into the abdomen, 
 and downward into the thighs. The pains of labor vary greatly in intensity 
 in individuals, but are usually more intense during the first gestation than 
 during later ones. They are due chiefly to direct mechanical stimulation of the 
 sensory uterine and other nerves by compression, tension, and even laceration. 
 
 ^.Schatz: Archiv fiir Gynakologie, 1885-86, xxvii. Compare O. Schaeffer : Experimentetle 
 Untersuchungen iiber die Wehenthatigkeit des menschlichen Utems, ausgefiihrt mitteht einer neuen 
 Pelotte und eines neuen Kymoyraphion, Berlin, 1896 ; abstract in Centralblatt fiir Gynakoloffif, 
 1896, xx. S. 85. 
 
480 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 As a result of the tonic contraction of the uterine walls, gradually increas- 
 ing with each new peristaltic wave, the uterus becomes gradually narrower in 
 diameter and longer, and the walls press more and more firmly upon the bag 
 of amniotic liquid containing the embryo. Schatz finds that the uterine pres- 
 sure under the uterine contractions rarely reaches and never exceeds 100 milli- 
 meters of mercury. The direction of least resistance to this pressure lies 
 along the cervical canal, the walls of which do not take part in the uterine labor. 
 With each succeeding contraction this canal is forced wider open and the uterine 
 contents are pressed tightly downward and into the cervix. The head of the 
 embryo is preceded by a bulging portion of the membrane, filled with liquid 
 and forming a distinct bladder-like advance-guard. This bag appears at the 
 os uteri, its contents increase under the increasing pressure, and in the majority 
 of cases, when the os is fully expanded, it bursts and allows the amniotic liquid 
 to escape to the exterior. In some cases the rupture is delayed until the sec- 
 ond stage of labor, and rarely the child is born with the membranes intact. 
 
 Second Stage of Labor. The uterine contractions frequently cease for a 
 period following the rupture of the membrane. They then begin anew with 
 increased force, and are accompanied by a new feature, namely, analogous 
 vigorous rhythmic contractions of the muscles of the abdominal walls. These, 
 following deep inspiration and accompanied by forced attempts at expiration 
 with a closed glottis, diminish the longitudinal and the lateral diameters of the 
 abdominal cavity, compress the abdominal organs, and help to augment greatly 
 the uterine pressure. At the beginning of the second stage the force of the 
 contractions is expended mainly upon the head of the embryo, which lies like 
 a plug in the cervical canal. This is squeezed gradually through the os into 
 the vagina, followed by the more easily passing trunk and limbs. The con- 
 tractions are frequent, vigorous, and painful, the pains reaching a maximum 
 as the sensitive vulva is put upon the stretch and traversed. The vertex is 
 usually presented first to the exterior, the head and body following as the suc- 
 cessive contractions of the maternal muscles develop sufficient power to over- 
 come the resistance offered to their passage by the surrounding walls. In 
 the human female the vaginal muscles do not appear to engage in the expel- 
 ling act, the uterine and the abdominal muscles alone sufficing and finally 
 forcing the child wholly outside the mother's body. In this gradual manner, 
 painful and dangerous alike to mother and child, the maternal organism forces 
 the offspring to forsake its sheltering and nutritive walls and begin its inde- 
 pendent existence. 
 
 Third Stage of Labor. During the later expulsive contractions of the 
 second stage the placenta, being greatly folded by the diminution in the uterine 
 surface of attachment, is loosened from the uterine wall by a rupture taking 
 place through the loose tissue in the region of the blood-sinuses. The child, 
 when born, is joined to the loosened placenta by the umbilical cord, until the 
 latter is tied and cut by the obstetrician. The muscular contractions, now 
 almost painless, continue through the third stage, and the placenta is torn 
 from its attachment, everted, and carried gradually outward. The lining 
 
4X1 
 
 membrane of the uterus from the placenta outward and for a consi<l< -ruble 
 depth is gradually torn free from the deeper parts through the spongy lavcr, 
 and with the attached chorion and ainnion follows the placenta. As a rule, 
 this after-birth appears at the vulva within fifteen minutes alter the cxpuUinn 
 of the child ; it consists of the placenta, the am n ion, the chorion, the deeidua 
 /r/A'.m, and a considerable portion of the deddua vera. 
 
 Previous to the third stage slight bleeding from laceration of the passages 
 occurs. But with the loosening of the placenta and the accompanying rupture 
 of the placenta! vessels the maternal blood flows freely and continues to flow 
 from the uterine wall, chiefly from the placental area, until the after-birth is 
 discharged. The average loss of blood amounts to about 400 grams. At the 
 close of the third stage of labor the uterine contractions have so far proceeded 
 that the organ is compressed into a hard compact mass, the ruptured vessels 
 are contorted and compressed, and the bleeding is thereby largely stopped. 
 For several hours, however, slight hemorrhage continues as an accompaniment 
 of the post-partum contractions, but finally this ceases with the formation of a 
 blood-clot over the wounded surface. 
 
 The third stage of labor may continue through one or two hours. It is 
 customary, however, for the obstetrician speedily to put an end to it by assist- 
 ing the removal of the after-birth. 
 
 Nature of Labor. Our knowledge of the nature of the muscular phe- 
 nomena of labor is incomplete. The uterine contractions are in part automatic 
 and in part reflex, but to what extent the former, and to what the latter, is not 
 known. Rein 1 found that in the rabbit after section of all uterine nerves 
 normal conception, pregnancy, and birth may occur. In some animals uterine 
 movements may continue after removal of the organ from the body. Such 
 and other observations indicate the existence of an automatic contractile power 
 resident in the organ itself. Since nerve-cells are not found in its walls, it 
 seems probable that the automatism resides in the muscle tissue. The uterus 
 is, moreover, very sensitive to direct stimulation, even after excision. In ani- 
 mals higher than rabbits a connection with the lumbar spinal cord seems 
 essential to normal labor. Goltz 2 obtained in dogs conception, pregnancy, and 
 delivery after section of the spinal cord at the height of the first lumbar 
 vertebra. In paraplegic women, with conduction in the cord broken in the 
 dorsal region, delivery is possible. A centre for uterine contraction must 
 hence be supposed to exist in the lumbar cord. Centripetal and centrifugal 
 fibres exist in both sympathetic and spinal nerves, and reflex uterine contrac- 
 tions are readily obtained by stimulation of the central ends of the divided 
 nerve-trunks. According to Langley and Anderson, 3 in the cat and the rab- 
 bit both the longitudinal and the circular muscular a>:\{~ and the arterie- <>!' 
 the uterus are supplied with motor nerve-fibres mainly by the third, fourth, 
 and fifth lumbar nerves ; the fibres pass to the sympathetic system, and 
 
 1 G. Rein : Pftiiger's Archivfiir die gesammte Physiologie, 1880, xxiii. 
 2 Fr. Goltx. : l\n<\.. 1>74. ix. 
 
 3 Langley and Anderson: Jo nnml of Physiology, 1895-96, xix. p. lL"J. 
 VOL. II. 31 
 
482 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 nearly all of them go to the inferior mesenteric ganglia and thence by the 
 hypogastric nerves to the uterus. Stimulation of the uterus itself, the vagina, 
 the vulva, the sciatic and the crural nerves, and various sensory regions, 
 notably the nipples, causes reflex contractions of the uterus. The same result 
 occurs upon stimulation of various portions of the brain, such as the medulla 
 oblongata, the cerebellum, the pons, the corpora qtiadrigemina, the optic thala- 
 mus, the corpus striatum, and even the corpus callosum. In woman psychic in- 
 fluences may call forth or inhibit uterine contractions. How largely the well- 
 known stimulating effects of the blood in asphyxia and of drugs, like ergot, 
 are due to central, and how largely to direct uterine influence is undecided. 
 The regular co-ordinated course of labor and many experimental facts make 
 it probable that, normally, reflex influences constitute a large part of the proc- 
 ess, the centripetal impulses arising within the uterus itself, probably largely 
 from the pressure upon the walls of the lower segment and the cervix. 
 In fact, it is customary to speak of labor as a complex reflex action. The 
 undoubted automatism of the uterine muscle-fibres must, however, be taken 
 into account, and the act should be regarded as composed of both automatic 
 and reflex elements. We have here to deal with that variety of contractility 
 peculiar to smooth muscle, in which central and peripheral influences work 
 together to bring about the result. It is perhaps not going too far to regard all 
 such actions, like that of the heart, as primarily automatic and called out by 
 direct stimulation, but as modified and controlled by reflex influences. The 
 parturitive contractions of the striated muscles of the abdominal walls are 
 probably more generally reflex in nature, modified, however, by voluntary 
 efforts. 
 
 Multiple Conceptions. According to the records given by different stat- 
 isticians, the frequency of twin births varies considerably in different coun- 
 tries. In 13,000,000 births in Prussia, G. Veit 1 found the number of twins 
 to be 1.12 per cent., or 1 in 89 births. In the cities of New York and 
 Philadelphia recent reports give the ratio of twins to single births as 1 : 120, 
 or 0.83 per cent- 
 Observations of discharged Graafian follicles in cases of multiple concep- 
 tions show that twins may arise either from separate eggs or from a single egg. 
 The presence at birth of a double chorion is commonly regarded as diagnostic 
 of the former origin, that of a single chorion of the latter. In the former 
 case the two ova may come from a single Graafian follicle, or from two folli- 
 cles situated within one ovary, or from both ovaries, direct observation of the 
 ovaries themselves being required to determine the origin in any particular 
 case. The two ova are discharged and fertilized probably at approximately 
 the same time. There are two distinct amnions. The two placentas may be 
 either fused into one or wholly separated from each other, and accordingly the 
 decidua reflexa may be single or double. The two offspring may be of sep- 
 arate sexes, and do not necessarily closely resemble each other. In cases 
 where the two embryos come from a single ovum their origin is little under- 
 
 1 G. Veit : Monatsschrift fur Geburtskunde und Frauenkrankheiten, 1855, vi. 
 
REPRODUCTION. 483 
 
 stood. It is conceivable that it may arise from the presence of two nuclei 
 within the one ovum. It is more probable, however, that it is due to a 
 mechanical separation of the blastomeres after the first cleavage or later in 
 segmentation. 1 Driesch, 2 Wilson, 3 Zoja, 4 and others have shown that in various 
 invertebrates and the low vertebrate Amphioxus, single blastomeres, isolated 
 from the rest by shaking or other unusual treatment, are capable of develop- 
 ing into small but otherwise normal and complete embryos. No reason is 
 obvious why such an occurrence cannot take place in human development, if 
 in any accidental manner within the Fallopian tube the blastomeres become 
 separated. Driesch observed in the sea-urchins and Wilson in Amphioxus 
 that incomplete separation of blastomeres produced two incomplete organisms 
 more or less united together. It is not improbable that even in man cases 
 like the Siamese Twins, and greater monstrosities, may be similarly accounted 
 for. In cases of double pregnancy from a single ovum the two amnions are 
 usually separate, in rare cases a breaking away of their partition wall throwing 
 them into one ; the two placentas usually fuse more or less into one, the blood- 
 vessels of the two halves always anastomosing ; and a single deddua reflexa 
 covers both. The two offspring are uniformly of the same sex and their per- 
 sonal resemblance is always close. 
 
 In Veit's statistics of 13,000,000 births in Prussia, triplets occur with a 
 frequency of 0.012 per cent, or 1 in 7910, and quadruplets 1 in 371,126 births. 
 There are well-authenticated cases of quintuplets. In all of these cases a 
 single ovum rarely, if ever, contributes more than two embryos, and these 
 are characterized, as in the case of twins, by being of similar sex, by pos- 
 sessing a single chorion, and by close personal resemblance. 
 
 The Determination of Sex. In most, if not all, civilized races more boys 
 are born than girls. This is shown in the following table : 6 
 
 Boys bom to 1000 Girls born (1887-91). 
 Italy 1058 England 1036 
 
 Ireland 1055 
 
 German Empire 1052 
 
 France . . 1046 
 
 Connecticut 1072 
 
 Rhode Island . . 1049 
 
 Massachusetts 1046 
 
 The proportional birth-rate of the two sexes is usually fairly constant from 
 year to year. This means that constant regulating factors are at work. 
 What determines sex in any one individual is ill understood. The sexual 
 organs in the human embryo are well differentiated at the eighth week of 
 intra-uterine life, hence the sex of the child must be settled previously to this 
 time. It is at present quite impossible to say whether it is settled in the 
 germ-cells previous to their union, in the act of fertilization, or during the 
 early uterine life. Many facts, both observational and experimental, and 
 
 1 Of. Fr. Ahlfeld : Archiv fur Gynakologie, ix., 1876. 
 
 2 H. Driesch: Zeiischrift fur wissenschaftliche Zoologie, liii., 1892; lv., 1893; Mittheilungen 
 mis der Zooloyischen Station zu Neapd, xi., 1893. 
 
 3 E. B. Wilson : Journal, of Morphology, viii., 1893. 
 
 4 R. Zoja : Archiv fiir k'nfi'-irkelungsmechanik der Organismen, ii., 1895. 
 
 5 Bulletin de Vinstitut international de statistique, vii. 
 
484 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 more hypotheses, bearing upon the determination of sex, have been brought 
 forward. The Hofacker-Sadler law (Hofacker, 1828; Sadler, 1830) is well 
 known, as follows : If the father be older than the mother, more boys than 
 girls will be born ; if the parents be of equal age, slightly more girls than 
 boys ; if the mother be older than the father, the probability of girls is still 
 greater. Since the promulgation of this so-called law facts for and against it 
 have been brought forward, but the balance of evidence seems to be in favor 
 of its truth. Thury in 1863 claimed that the degree of "ripeness" of the 
 ovum is the determining factor the female resulting from the less ripe ovum, 
 hence the earlier after its liberation the egg is fertilized, the greater is the 
 tendency to the production of a female ; the later the fertilization, the greater 
 the probability of a male. While it is not at all clear in what the " ripeness " 
 or " unripeness " of an ovum consists, breeders have made use of this prin- 
 ciple apparently with success offspring conceived at the beginning of " heat " 
 seem to be more usually females. Likewise, it is frequently believed that in 
 human beings conceptions immediately after menstruation produce a larger 
 proportion of females than later conceptions. Schenk 1 also bases his view 
 on the condition of ripeness of the ovum. He regards the presence of sugar 
 in the urine of the pregnant woman as evidence of incomplete metabolism 
 in the body, thus of incomplete nutrition or unripeness of the ovum, and 
 hence of tendency toward femaleness in the offspring. By means of a highly 
 nitrogenous diet, which eliminates the sugar from the urine and increases the 
 proportion of reducing substances, he claims to make the metabolism more 
 complete, to insure a riper ovum, and hence to make it probable that the 
 offspring will be a male. Schenk's reasoning is excessively hypothetical, and 
 his present facts are too few to substantiate his claims. Diising 2 accepts 
 Thury's view and extends it to the male element the younger the spermato- 
 zoon the greater the tendency toward the production of males. Hence among 
 animals the scarcity of one sex leads to the more frequent exercise of its 
 reproductive function, the employment of younger germ-cells, and therefore 
 the relative increase of that sex. Further, the nearer a parent is to the 
 height of his reproductive capacity the less will be the probability of trans- 
 mitting his own sex to the offspring. Nutrition seems to have some obscure 
 relation to the question of sex. Thus, by feeding tadpoles with highly 
 nutritious flesh Yung a increased the percentage of females from 56 to 92. 
 Mrs. Treat 4 showed that the butterflies of well-fed caterpillars became 
 females, those of starved caterpillars males. Statistics among mammals and 
 human beings indicate that the proportion of male to female offspring varies 
 inversely with the nutrition of the parents, especially of the mother. Thus, 
 
 1 L. Schenk : Einfluss auf das Geschlechtverhdltniss, Magdeburg, 1898. Authorized transla- 
 tion : The Determination of Sex, London, 1898. 
 
 2 K. Diising: Jenaische Zeitschrift fur Naturwissenschaft, 1883, xvi., and 1884, xvii.; also 
 published separately, Die Regulierung des Geschlectverhdltnisses bei der Vermehrung der Menschen, 
 Tiere und Pflanzen, Jena, 1884. 
 
 3 E. Yung: Oomptes rendus de I' academic des sciences, Paris, 1881, xcii. 
 4 Mrs. Mary Treat: The American Naturalist, 1873, vii. 
 
HE PR OD UCTION. 485 
 
 more boys are born in the country than in the city, and in poor than in pros- 
 perous families; the relative number of boys is said to vary even with the 
 price of food. It is contended, moreover, and with some statistical support, 
 that in the human race an epidemic or a war, either of which affects adversely 
 the well-being of the people, is followed by a relative increase of male births. 
 Statistics indicate also that the proportion of females is high in warm climates, 
 that of males high in cold climates. Maupas l found that sex in the rotifer, 
 lli/i/dtiiut wntd, could be controlled by altering the temperature of the medium 
 surrounding the egg-laying females. In various experiments at a tempera- 
 ture of 26-28 C. 81-100 per cent, of the eggs gave rise to males, the rest 
 to females; at 14-15 C. only 5-24 per cent, were males, the much lai-^-r 
 majority females. Nussbaum 2 has brought Maupas's facts into harmony 
 with the facts regarding nutrition by showing that the higher temperature 
 carries with it a higher birth-rate and more rapid development, hence a 
 greater need of food and relative lack of it for the individual ; the result is 
 poor nutrition and the production of an excess of males over females. It is 
 claimed, further, that ethnic intermixture causes a decrease in the relative 
 number of males born. This is strongly supported by a statistical study by 
 Ripley 3 of the two races inhabiting Belgium, the Walloons, of the same 
 origin as the Kelts in France, and the Flemish of German stock. AY here 
 these races are purest, the number of boys born to 1000 girls is 1064 ; along 
 the region where the two races come into contact, however, the number may 
 fall as low as 1043. 
 
 The above considerations are highly interesting and suggestive, but they 
 have not yet been brought under general laws sufficiently to make their bear- 
 ing upon the main problem wholly clear. It is probable that numerous 
 factors are of influence in the determination of sex. 4 The general deduction 
 from all the facts seems justified that unfavorable nutritive conditions sur- 
 rounding the parents tend to the production of males, favorable conditions 
 to the production of females. The experimental results indicate, moreover, 
 that the conditions surrounding the parents or the developing embryo are 
 largely responsible for the resulting sex. Watase 5 regards the embryo as 
 neutral as regards sex from the time of fertilization up to a certain stage in 
 its development ; external conditions act as a stimulus to the sexless proto- 
 plasm, and the resulting response is a development in the direction of either 
 maleness or femaleness according to the nature of the stimulus. How largely 
 and in what manner this may be true of the human species is wholly unknown. 
 Diising 6 urges that the various factors determining sex have arisen through 
 natural selection ; they are conducive to the continuance of the species, and 
 
 *E. Maupas: Comptes rendus de I'academie des sciences, Paris, 1891, cxiii. 
 
 2 Xussbaum : Archiv fur mikroskopische Anatomic, 1897, xlix. S. 227. 
 
 3 W. Z. Ripley : Quarterly Publications of the American Statistical Association, March, 1896, v. 
 
 4 For a critical review of the various theories see L. Cohn : Die willkurliche Bestimmung des 
 Geschiecht*. 2d ed., Wurzburg, 1898. 
 
 5 S. Watase: Journal of Morphology, 1892, vi. 
 6 Diising : Lc. fit. 
 
486 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 they act in such a way that sex is in a certain sense self- regulating the 
 scarcity of one sex tends to the greater production of individuals of that sex ; 
 this is instanced by the fact mentioned above that after the destruction of 
 males by war relatively more males are born than previously. 
 
 E. EPOCHS IN THE PHYSIOLOGICAL LIFE OF THE INDIVIDUAL. 
 
 Fertilization begins, somatic death ends, the physiological life of the indi- 
 vidual. Between these two events the life-processes go on gradually, and, 
 with the exception of birth, are marked by few abrupt changes. It is some- 
 times convenient to divide the individual life into a number of successive 
 stages, as follows : the embryonic period, the fetal period, infancy, childhood, 
 youth or adolescence, maturity, and old age or senescence. Such a division, 
 however, is not physiologically exact, the stages are not sharply limited, and 
 the terms are employed in very different senses by different writers. Between 
 fertilization and birth the functions originate and are developed gradually. 
 At birth the environment of the individual is abruptly changed, organic 
 connection with the mother suddenly ceases, and profound physiological 
 changes occur. At this time, or shortly after it, the individual is capable 
 of performing all the functions of adult life with the exception of reproduc- 
 tion, the functions needing, however, to be exercised and improved before 
 they are at their best. From birth to maturity, therefore, the physiological 
 history is mainly a history of progressive modifications of function modi- 
 fications, indeed, of great importance, but secondary to the primary fact of 
 function itself. The same may be said of the period of old age, with the dif- 
 ference that here the modifications of function are retrogressive. In the present 
 book, devoted mainly to the physiology of the adult at the time of maturity, 
 little can be said of the origin and development of function in the embryo ; 
 the modifications of function at different periods of life have been discussed in 
 connection with the various functions themselves ; certain topics of special physio- 
 logical significance have, however, been left for brief treatment in this chapter. 
 
 Growth of the Cells, the Tissues, and the Organs. All growth, 
 whether of the cells, the tissues, or the organs, is the result of no more than 
 three processes, viz. multiplication of cells, enlargement of cells, and deposition 
 of intercellular substance, the first two processes being the most potent of all. 
 Increase in the number of cells is largely, although not wholly, an embryonic 
 phenomenon ; increase in the size of cells and deposition of intercellular sub- 
 stance are especially important from the later embryonic period through the 
 time of birth and up to the cessation of the body-growth. The periods of 
 growth of the several tissues differ ; in view of this it is quite impossible to 
 designate any period except that of death at which the growth of the tissues 
 wholly terminates. Detailed statistics of the growth of organs are wanting. 
 
 Growth of the Body before Birth. The most obvious result of growth 
 of the cells, the tissues, and the organs, is growth or increase in size of the 
 body. Growth of the body continues actively from the beginning of the seg- 
 mentation of the ovum up to about the age of twenty-five years, and results in 
 
REPRODUCTION. 487 
 
 an increase in all dimensions and in weight. In determining the extent of 
 growth, the two most convenient and most commonly used measurement- mv 
 those of length, or height, and weight. For the embryo the following 
 has been compiled by Hecker: 1 
 
 Table showing the Average Length and Weight of the Human Embryo at 
 
 Different Ages. 
 
 Month. 
 Third 
 
 Length of embryo in centimeters. 
 4 to 9 
 
 Weight of embryo in grams. 
 
 Fourth 
 
 10 to 17 
 
 57 
 
 Fifth 
 
 18 to 27 
 
 984 
 
 Sixth 
 Seventh . 
 
 28 to 34 
 35 to 38 
 
 634 
 
 1218 
 
 Eighth . 
 
 39 to 41 
 
 1569 
 
 Ninth 
 
 42 to 44 
 
 1071 
 
 Tenth 
 
 45 to 47 
 
 9334 
 
 The length and the weight at birth vary very greatly. The average measure- 
 ments, as given for over 450 infants in Great Britain, are, for height, males 
 19.5 inches, females 19.3 inches; for weight, males 7.1 pounds, females, 6.9 
 pounds. The weight at birth is said to be greater the nearer the mother's 
 age is to thirty-five years, the greater the weight of the mother, the greater 
 the number of previous pregnancies, and the earlier the appearance of the first 
 menstruation. Race and climate are also of influence. Minot 2 believes that all 
 of these influences work principally through prolonging or abbreviating the 
 period of gestation, and that the variations at birth depend partly upon the 
 duration of gestation and partly upon individual differences of the rate of 
 growth in the uterus. 
 
 Growth of the Body after Birth. In studying the growth of the body 
 after birth two methods have been employed, named the " generalizing " and 
 the "individualizing" methods. The former consists in deducing the course 
 of growth by averages or other central values from statistics taken from a 
 large number of individuals at different ages. It is the method more com- 
 monly employed ; it shows the course of growth of the typical child, but is 
 inexact in enabling future growth to be predicted in individual cases. The 
 individualizing method consists in measuring the actual growth of the same 
 individual through successive years ; it shows well the relation of the indi- 
 vidual to the type throughout the period of growth. The course of growth 
 of British boys and girls from birth up to the age of twenty-four is graphically 
 shown in the accompanying diagram (Fig. 229). Growth is here seen to be 
 rapid during the first five years of life, then slower up to the tenth or 
 the twelfth year. From thence up to the fifteenth or the seventeenth year 
 that is, preceding and including puberty marked acceleration occurs, 
 which in turn is followed by slow increase up to the twentieth or the 
 twenty-fifth year. For from five to ten years thereafter slight increase in 
 
 1 C. Hecker: Monatsschrift fur Gebnrtxknnde und Franenkrankln /'///. 
 
 2 C. S. Minot : Human Embryology, 1892. 
 
488 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 height occurs, while from the accumulation of fat the weight usually rises 
 markedly up to the fiftieth or the sixtieth year. One of the most interesting 
 results revealed by statistics is the relative growth of the two sexes. From 
 birth up to about the age of ten or twelve, boys show a slight and increasing 
 preponderance over girls, but the two curves are nearly parallel. The prepu- 
 bertal acceleration of growth in girls, however, precedes that of boys, and is 
 even accompanied by some check in the male growth, with the result that 
 between the ages of twelve and fifteen girls are actually heavier and taller 
 than boys. This fact, first pointed out in 1872 by Bowditch l from observa- 
 
 Age. 
 
 10 
 
 15 
 
 20 
 
 Years. 
 
 25 
 
 10 
 
 1 
 
 I 
 
 140 
 
 120 
 
 100 
 
 80 
 
 60 
 
 40 
 
 20 
 
 Males. 
 
 Females. 
 
 FIG. 229. Diagram showing increase of stature and weight of both sexes, as determined by the Anthropo- 
 metric Committee of the British Association. 2 
 
 tions on several thousand Boston school children, has been abundantly con- 
 firmed by Pagliani in Italy, Key in Sweden, Schmidt in Germany, Porter in 
 St. Louis, and others. At about fifteen years boys again take the lead and 
 maintain it throughout life. Boys grow most rapidly at sixteen, girls at thir- 
 teen or fourteen, years of age ; the former attain their adult stature approxi- 
 mately at twenty-three to twenty-five, the latter at twenty to twenty-one years. 
 The details of growth and the actual measurements vary considerably with 
 race ; thus the supremacy of the American girl over her brother appears to be 
 less marked and to cover a shorter period than that of the English, German, 
 Swedish, or Italian girl. Children of well-to-do families are superior to 
 
 1 H. P. Bowditch : Eighth Annual Report of the State Board of Health of Massachusetts, 1877. 
 
 2 Roberts : Manual of Anthropometry, 1878. 
 
REPRODUCTION. 1S<) 
 
 others in both weight and stature. Beyer 1 has shown that systematic exercise 
 may markedly increase both height and weight. Disease may alter the form 
 of the curve of growth. But the final result seems to depend le>s upon 
 external conditions than upon race and sex. As an interesting accessory 
 fact it was found by IWter- that well-developed children take a higher r.-mk 
 in school than less-developed children of the same age. If the percentage 
 annual increase of the total weight be computed, it is found to diminish 
 throughout life, very rapidly during the first two or three years, later more 
 slowly and with minor variations of increase and decrease; that is, as growth 
 proceeds and the powers of the individual mature, the power to grow becomes 
 rapidly less. This is a peculiar and most interesting fact, and has not been 
 explained. It would seem to signify that the sum of the vital powers 
 declines from birth onward. Many facts indicate that the common concep- 
 tion, dating from the time of Aristotle, of human life as consisting of the 
 three periods of rise, maturity, and decline, must give way to a more rational 
 idea of a steady decline from birth. 
 
 Puberty. By puberty is meant the period of sexual maturity, at which 
 the individual becomes able to reproduce. In the male the exact time of its 
 onset, characterized primarily by the appearance of fully ripe spermatozoa, is 
 not well known, but is believed to be about one year later than in the female. 
 In temperate climates, therefore, it usually appears in boys not before the age 
 of fifteen ; it is earlier in warmer regions. It is preceded and accompanied by 
 acceleration in bodily growth, already spoken of. Other bodily changes, such 
 as general maturation of the functions of the reproductive organs, alterations in 
 the bodily proportions, increase of strength, and growth of the beard, all of which 
 are elements of the transformation from boyhood to manhood, either occur at 
 that time or follow soon after. One of the most obvious external changes is 
 that of the voice. Its tone may fall permanently an octave, and for the time being 
 become rough, broken, and uncontrollable. This is due to a rapid general 
 enlargement of the laryngeal cartilages and a lengthening of the vocal cords. 
 
 In the girl the oncoming of puberty is marked more exactly than in the 
 boy by the appearance of menstruation, in the majority of girls in temperate 
 climates at the age of fourteen to seventeen. But other characteristic anatom- 
 ical and physiological changes in the body occur. The uterus, the external 
 reproductive organs, and the breasts become larger, while the pelvis widens. 
 The prepubertal acceleration of growth has been mentioned. Nervous disor- 
 ders are especially prone to make their appearance at this time. The subcuta- 
 neous layer of adipose tissue develops and confers upon the outlines the grace- 
 ful curves characteristic of the woman's body. The mental faculties mature, 
 and the girl becomes a woman earlier and more rapidly than the boy a man. 
 
 1 H. G. Beyer : "The Influence of Exercise on Growth," Journal of Experimental Medicine, 
 1896, i. p. 546. See also " The Growth of U. S. Naval Cadets," Proceedings of the United States 
 Naval Institute, 1895, xxi. p. 297. 
 
 2 W. T. Porter: "The Physical Basis of Precocity and Dulness," Transactions of the Acad- 
 emy of Science of St. Louis, 1893, vi., No. 7. See also "The Growth of St. Louis Children," 
 Ibid., 1894, vi. No. 12. 
 
490 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 Climacteric. At the sixtieth year the power of producing spermato- 
 zoa, and, therefore, the reproductive power of man, begins to wane. It con- 
 tinues, however, in a diminishing degree, even to extreme old age, and there 
 is no recognized period of ending of the male sexual life. 
 
 In woman, on the other hand, the sexual period continues for only thirty 
 to thirty-five years, and the climacteric, menopause, or change of life, marks a 
 definite ending of the power of reproduction. In temperate climates it occurs 
 usually between the ages of forty-four and forty-seven ; in warmer regions it 
 comes early, in colder late. It is earlier in the laboring classes, and later 
 where menstruation has first appeared early. Its most characteristic feature is 
 the cessation of menstruation, which is a gradual process extending over a 
 period of two or three years and characterized by irregularity in the oncoming 
 and the quantity of the flow, and by gradual diminution. But the cessation 
 of the menses is but one phenomenon in a long series of changes that pro- 
 foundly affect the whole organism and endanger life. The reproductive organs 
 and the breasts diminish in size, and ovulation ceases. The changes in the 
 pelvic organs are in general the reverse of those occurring at puberty. The 
 organic functions generally are rendered irregular ; dyspepsia, cardiac palpi- 
 tation, sweating, and vasomotor changes are frequent ; vertigo, neuralgia, 
 rheumatism, and gout are not rare ; a tendency to obesity occurs, though 
 sometimes the reverse ; irritability, fear, hysteria, and melancholia may be 
 present ; the disposition may be temporarily altered ; all of which changes 
 indicate that the female organism at this time suffers a profound nervous 
 shock. The loss of the weighty function of reproduction and the adaptation 
 to the new order of events are not accomplished quietly. 
 
 Senescence. The progressive diminution in the power of growth from 
 birth onward throughout life has been mentioned, and may be interpreted as 
 indicating that the process of senescence begins with the beginning of life. 1 
 In the broadest sense this is true, and is confirmed by a study of various 
 organic functions. In the more restricted sense senescence or old age com- 
 prises the period from about fifty years (in woman from the climacteric) 
 onward, during which there is a noticeable progressive waning of the vital 
 powers. The leading somatic changes accompanying old age are atrophic and 
 degenerative, but detailed statistics of this period are almost wholly wanting. 
 A marked cellular difference between the young and the old, which is shown 
 by nearly if not quite all tissues, is the relatively large nucleus and small 
 quantity of cytoplasm in the young, the proportions being reversed in the old. 
 This has been pointed out as follows by Hodge 2 in the nerve-cells of the first 
 cervical spinal ganglion : 
 
 Volume of Nucleoli observ- Pigment Pigment 
 
 nucleus. able in nuclei. much. little. 
 
 Fetus (at birth) 100 per cent. in 53 per cent. 
 
 Old man (at ninety -two years) 64.2 " in 5 " 67 per cent. 33 per cent. 
 
 1 Of. C. S. Minot : Journal of Physiology, 1891, xii. 
 
 2 C. F. Hodge : Anatomischer Anzeiger, 1894, ix. : Journal of Physiology, 1894, xvii. 
 
REPR OD UCTION. 491 
 
 Thus with the progress of age the nuclei become small and irregular in out- 
 line, and the cytoplasm pigmented, while the nucleoli arc often waiitiu^. The 
 nuclear differences are even more marked in the cerebral ganglia of bees, where 
 moreover, aired individuals possess a smaller number of nerve-cells than the 
 young. The nuclear differences accord with the common belief that the nucleus 
 is the formative centre of the cell. It has been shown that a decrease 
 in the weight of the whole brain occurs in both men and women, beginning in 
 the former at about fifty-five years, in the latter at about forty-five years. In 
 eminent men the decrease begins later. The thickness of the cortex and the 
 number of tangential fibres in it diminish especially after fifty years, and this 
 probably signifies a loss of cells. There is a decrease in general brain-power, 
 in power of origination, in the power to map out new paths of conduction and 
 association in the central nervous system and thus to form habits. Reaction- 
 time is lengthened. The delicacy of the sense-organs is noticeably less, and in 
 the eye the hardening of the crystalline lens and the weakening of the ciliary 
 muscle diminish the power of accommodation. The muscles atrophy and mus- 
 cular strength is reduced. The pineal gland, ligaments, tendons, cartilage, and 
 the walls of the arteries, show a tendency toward calcification, and the bones 
 become more brittle. Subcutaneous adipose tissue disappears, but a fatty de- 
 generation of cells is not uncommon, notably in all varieties of muscle-cells, 
 in nerve-cells, and probably in gland-cells. The pigment of the hairs disap- 
 pears. The size of the muscles, the liver, the spleen, the lymphatic and prob- 
 ably the digestive glands, decreases. The heart and the kidneys seem to retain 
 their adult size. The vital capacity of the lungs, the amounts of carbonic acid 
 and of urine excreted, diminish. The rate of respiration and of the heart-beat 
 rises slightly. Ovulation is wanting, and the power of producing spermatozoa 
 is lessened. The stature undergoes a slight and steady decrease. Boas l has 
 shown that in the North American Indian this continues from about thirty 
 years of age onward. All of these changes, the details of which should be care- 
 fully studied and reduced to anatomical and physiological exactness, demonstrate 
 that senescence is characterized by a steady diminution of vitality. 
 
 Death. Sooner or later vitality must cease and the change that is called 
 death must come. The term " death " is used in two senses, according as it is 
 applied to the whole organism or to the individual tissues of which the organ- 
 ism is composed. The former is distinguished as somatic death, or death 
 simply, the latter as the death of the tissues. 
 
 Somatic death occurs when one or more of the organic functions is so dis- 
 turbed that the harmonious exercise of all the functions becomes impossible. 
 Thus, if the brain receives a severe concussion, the co-ordination of the organs 
 may be interrupted ; if the respiration ceases, the necessary oxygen is withheld ; 
 if the heart fails, the distribution of oxygen and food and the collection of 
 wastes come to an end ; if the kidneys are diseased, the poisonous urea is 
 retained within the tissues. A continuation of any one of these profound 
 abnormal conditions, which may be brought about by accident or disease, or a 
 
 1 F. Boas : Verhandlungen der Berliner Anthropologischen Gesellschaft, 1895. 
 
492 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 simultaneous occurrence of several slight disturbances of function, such as is 
 not infrequent in aged persons, may prevent the restoration of that concordance 
 among the organs without which the individual cannot live. The most con- 
 venient and most certain sign by which somatic death may be recognized is the 
 absence of the beat of the heart, and in nearly all cases this is the criterion 
 employed. But it should be borne in mind that the failure of the heart to 
 beat is but one of the causes, and frequently a very secondary one, the primary 
 cause being then associated with other functions. It is at present in most cases 
 quite impossible to trace the course of events by which the derangement of one 
 function leads to the ultimate cessation of individual life. 
 
 Death of the tissues or of the living substance is neither necessarily nor 
 usually simultaneous with somatic death. Constantly throughout life the mole- 
 cules of living matter are being disintegrated, and whole cells die and are cast 
 away ; life and death are concomitants. With the cessation of the individual 
 life the nervous system dies almost immediately. "With the muscular tissue it 
 is very different. The stopping of the beat of the heart is a gradual process, 
 and, as Harvey long ago pointed out, the last portion to beat, the ultimum 
 moriens, is the right auricle. For many minutes after death the heart, if 
 exposed, will be found to be excitable and to respond by single contractions to 
 single stimuli. Irritability is said to continue in the smooth muscle of the 
 stomach and the intestines for forty-five minutes, and considerably later than 
 this the striated muscles of the limbs can still be made to twitch by proper 
 stimuli, in the cat and rabbit after twelve or fourteen hours. 1 Gland-cells 
 die probably within a few minutes. As to the chemical changes undergone 
 by the protoplasm in the process of dying, little can be said. The composi- 
 tion of dead protoplasm is comparatively well known, that of living proto- 
 plasm is at present largely a blank ; and, although investigation has gone suf- 
 ficiently far to offer a basis for several suggestive hypotheses, the latter are too 
 abstruse for lucid discussion in the present space. Neither in somatic death 
 nor in the death of the tissues does the body lose weight. Within fifteen or 
 twenty hours it cools to the temperature of the surrounding medium. Rigor 
 mortis, due to the coagulation of the muscle-plasma within the muscle-cells, 
 begins within a time varying with the cause of death from a half hour to 
 twenty or thirty hours, and continues upon an average twenty-four to thirty- 
 six hours. Then the tissues soften, and soon putrefactive changes begin. 
 
 Theory of Death. It has been intimated that all the tissues are destined 
 to die. An exception must be made in the case of those germ-cells, both male 
 and female, that are employed in the production of new individuals. They 
 pass from one individual, the parent, to another, the offspring, and thus cannot 
 be said to undergo death. This is the basis of Weismann's theory of the 
 origin and significance of death in the organic world. 2 According to Weis- 
 mann, primitive protoplasm was not endowed with the property of death. 
 As found in the simplest individuals, like the Amoeba, even at the present 
 
 1 Lee, Adler, and Bulkley : American Journal of Physiology, 1900, iii. p. xxix. 
 
 2 A. Weismann : Essays upon Heredity, 1889, i. 
 
REPRODUCTION. \\\:\ 
 
 day, with a continuance of the proper nutritive condition protoplasm does not 
 grow old and die; the single individual divides into two and life continues 
 unceasing, unless accident or other untoward event interferes. With the 
 progress of evolution, however, the cells of the individual body have become 
 differentiated into germ-cells and somatic cells, the former subserving the 
 reproduction of the species, the latter all the other bodily functions. Germ- 
 cells are passed on from parent to offspring; they never die, they are immor- 
 tal. Somatic cells, on the other hand, grow old, and at last perish. Death 
 was, therefore, in the beginning, not a necessary adjunct to life ; it is not inhe- 
 rent in primitive protoplasm, but has been acquired along with the differen- 
 tiation of protoplasm into germ-plasm and somatoplasm, and the introduction 
 of a sexual method of reproduction. It has been acquired because it is to the 
 advantage of the species to possess it ; in the simplest cases it should occur at 
 the close of the reproductive period, and in fact it frequently does occur then. 
 A superabundance of aged individuals, after they have ceased to be reproduc- 
 tive, would be detrimental to the race ; it is to the advantage of the species that 
 they be put out of the way. Death of the individual in order that the species 
 may survive has, therefore, become an established principle of nature. But 
 the higher animals are better able to protect themselves from destruction than 
 the lower, and, moreover, they are needed to rear the young ; hence in them 
 the duration of life is frequently prolonged beyond the reproductive period. 
 
 Weismann's theory has been the cause of much discussion, and the pros 
 and cons have been set forth by eminent biological authorities. In its appli- 
 cation to the human race it would seem that the factors of social evolution 
 have brought it about that the aged are protected in the struggle for existence 
 for long after their reproductive usefulness has ceased, and thus the working 
 of a pitiless biological law has become modified. 
 
 F. HEREDITY. 
 
 Biologists are accustomed to recognize two factors as responsible for the 
 character and actions of the living organism. These are heredity and the 
 environment. Heredity includes whatever is transmitted, either as actual or 
 as potential characteristics, by parents to offspring. The environment com- 
 prises both material and immaterial components, such as food, water, air, or 
 other substances that surround the organism, and the forces of nature, such as 
 light, heat, electricity, and gravity, that act as conditions of existence or as 
 stimuli to action. The same principles apply to the character and actions of 
 everv cell of a many-celled organism, but here we must include in the envi- 
 ronmental factor the mysterious influences that are exerted upon the cell by 
 the other cells of the body. Of these two factors heredity acts from within, 
 the environment from without the living substance. Among unicellular or- 
 ganisms the individual begins his career when the bit of protoplasm that con- 
 stitutes his body is separated from the parent bit of protoplasm. Among 
 higher forms, including man, the term individual may be applied to the fer- 
 tilized ovum ; the union of the ovum and the spermatozoon inaugurates the 
 
494 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY, 
 
 new being. From the inception to the death of the individual, life consists 
 partly of manifestations of the powers conferred by the germ-cells and partly 
 of reactions to environmental influences. In considering the details of vital 
 action we are apt to overlook these fundamental facts and to evolve narrow 
 and erroneous views as to the causes of vital phenomena. Biologists are 
 seeking with increasing vigor to determine the relative importance of the parts 
 played by these two principles in development and in daily life. It is need- 
 less to say that the problem is a difficult one and is still far from solution. 
 In previous chapters of this book attention has been directed more especially 
 to the external than to the hereditary factor. A work upon physiology would 
 be incomplete, however, if it did not include an examination of the latter, 
 especially since at the present time heredity is one of the leading subjects of 
 biological research and discussion. It is proposed, therefore, in this section 
 to present a brief outline of the facts, the principles, and the attempted ex- 
 planations of the modes of working of heredity. It should be premised that, 
 because of the present incomplete state of our knowledge of the facts, the 
 highly speculative and involved character of most of the theories, and the con- 
 stant, active shifting of ideas and points of view, such an outline must neces- 
 sarily be incomplete and in many respects unsatisfactory. 
 
 Facts of Inheritance. It is not proposed in this paragraph to enter into 
 a discussion of the question as to whether a particular vital phenomenon is a 
 fact of inheritance or a reaction to external influences. For our present pur- 
 poses it is sufficient to record the common facts of resemblance to ancestors, 
 and to assume that such resemblance, when present, has been inherited. 
 Resemblances are strongest between child and parents, and appear in a dimin- 
 ishing ratio backward along the ancestral line. Galton l has computed that, 
 of the total heritage of the child, each of the two parents contributes one- 
 fourth, each of the four grandparents one-sixteenth, and the remaining one- 
 fourth is handed down by more remote ancestors. The correctness of this 
 estimate has been disputed by Weismanu. The fact must not be overlooked 
 that, in addition to and back of all the particular individual features that are 
 inherited, a host of racial characteristics are transmitted the progeny of a 
 given species belongs to that species ; the human being is the father of the 
 human child, the child of Caucasian parents is a Caucasian, of negro parents 
 a negro. 
 
 Congenital resemblances may be anatomical, physiological, or psychological, 
 and in each of these classes they may be normal or pathological. Anatomical 
 resemblances are the most commonly recognized of all : facial features, stature, 
 color of eyes and of hair, supernumerary digits, excessive hairiness of body, 
 cleft palate, monstrosities, and* various defects of the eye, such as those that 
 give rise to hypermetropia, myopia, cataract, color-blindness, and strabismus, 
 are all known examples. Physiological peculiarities that may be transmitted 
 include the tendency to characteristic gestures, locomotion and other muscular 
 movements, longevity or short life, tendency to thinness or obesity, handwriting, 
 1 Francis Galton : Natural Inheritance, 1889, p. 134. 
 
REPRODUCTION. 
 
 voice, haeraatophilia or tendency to profuse hemorrhage from slight wounds, 
 
 gout, epilepsy, and asthma. Psychological inheritance- ( iprise habits of 
 
 mind, talent, artistic and moral qualities, tastes, traits of character, tempera- 
 ment, ambition, insanity and other mental diseases, and tendencies to crime 
 and to suicide. 
 
 Latent Characters ; Revision. Characters that never appear in the parent 
 may yet he transmitted through him from grandparent to child ; such charac- 
 ters are called latent. Among the most striking latent characters are those con- 
 nected with sex. Darwin l says : " In every female all the secondary male 
 characters, and in every male all the secondary female characters, apparently 
 exist in a latent state, ready to be evolved under certain conditions." Thus, a 
 girl may inherit female secondary sexual peculiarities of her paternal grand- 
 mother that are latent in her father, or a boy may inherit from his maternal 
 grandfather characteristics that never show in his mother. An excellent 
 example of such transmission, taken from the herbivora, is the common one 
 of a bull conveying to his female descendants the good milking qualities of 
 his female ancestors. In the human species hydrocele, necessarily a disease of 
 the male, has been known to be inherited from the maternal grandfather, and 
 hence must have been latent in the mother's organism. That in such cases the 
 character is really potential, though latent in the intermediate ancestor, is 
 rendered probable by such well-known facts as the appearance of female cha- 
 racteristics in castrated males, and of male characteristics in females with dis- 
 eased ovaries or after the end of the normal sexual life. 
 
 Latency may be offered as the explanation of the numerous cases of 
 atavism, or reversion, by which is meant the appearance in an individual of 
 peculiarities that were formerly known only in the grandparents or more 
 remote ancestors, but not in the parents of the individual. This subject is one 
 of the most important in the whole field of heredity. Almost any character 
 may reappear even after many generations. In the human species stronger 
 likeness to grandparents than to parents is a frequent occurrence. The 
 majority of the frequent anomalies of the dissecting-room are regarded as 
 reversions toward the simian ancestors of the human race. The crossing of 
 ,two strains develops a strong tendency to reversion, and because of this the prin- 
 ciple of atavism must constantly be taken into account by breeders of animals 
 and growers of plants. As an example of reversion after crossing may be 
 mentioned the well-known one, studied by Darwin, of the frequent appear- 
 .ance of marked stripes upon the legs of the mule, the mule being a hybrid 
 from the horse and the ass, both of which are comparatively unstriped but 
 are undoubtedly descended from a striped zebra-like ancestor. Here the 
 capacity of developing stripes is regarded as latent in both the horse and the 
 ass, but as made evident in the mule by the mysterious influence of crossing. 
 Darwin thinks likewise that the customary degraded state of half-castes may 
 be due to reversion to a primitive savage condition which, usually latent in 
 
 1 Charles Darwin: The Variation of Anhnnl* mid Plants under Domestication, 1892, vol. ii., 
 :2d ed. 
 
496 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 both civilized and savage races, is rendered manifest in the offspring that 
 results from the union of the two. Reversionary characters are often more 
 prominent during youth than during later life a fact that has been quoted in 
 favor of their explanation on the theory of latency. 
 
 Regeneration. The facts of regeneration of lost parts must also be taken 
 into account in a theory of heredity. Such regeneration may be either physi- 
 ological or pathological. Physiological or normal regeneration has reference 
 to the reproduction of parts that takes place during the normal life of the 
 individual, such as the constant growth of the deeper layers of the epidermis 
 to replace the outer layers that are as constantly being shed. Pathological 
 regeneration refers to the replacement of parts lost by accident, and presents 
 the more interesting and striking examples. The power of pathological 
 regeneration in man and the higher mammals is limited. A denuded surface 
 may be re-covered with epithelium ; the central end of a cut nerve may grow 
 anew to its termination ; the parts of a broken bone may reunite ; muscle may 
 reappear ; connective-tissue, blood-corpuscles, and blood-vessels may develop 
 readily; and in the healing of every wound a regeneration of parts takes 
 place. But in descending the scale of animal life the regenerative power 
 becomes progressively stronger, and in many plants and low animals it is 
 marvellous. Thus, the newt may replace a lost leg, the crab a lost claw, the 
 snail an eyestalk and eye. If an earth-worm be cut in two, one half may 
 regenerate a new half, complete in all respects. A hydra may be chopped 
 into fragments and each fragment may re-grow into a complete hydra. From 
 a small piece of the leaf of a begonia, planted in moist earth, a new plant 
 with all its parts may arise. It is evident that the existing parts of an organ- 
 ism, if not too specialized, possess the power of restoring parts that are lost ; 
 under ordinary circumstances this power is latent. The growth of tumors is 
 perhaps allied in nature to regeneration. A study of regeneration shows that 
 in many cases the process of building anew follows the same course as the 
 original embryonic growth. It is properly a phenomenon of heredity. 
 
 The Inheritance of Acquired Characters. No topic in heredity has been 
 more debated during the past twenty years than that of the possibility of the 
 transmission to the offspring of characteristics that are acquired by the parents 
 previous to the discharge of the germ-cells, or, in the case of the mammalian 
 female, previous to parturition. Obviously, no one denies this possibility in 
 the unicellular organisms, where reproduction by fission prevails, for there the 
 protoplasm of the body of one parent becomes the substance of two offspring ; 
 in the transformation nothing is lost, and hence whatever peculiarities the ances- 
 tral protoplasm has acquired are transferred bodily to the descendants. But 
 in multicellular forms, where sexual reproduction exists, the case is very dif- 
 ferent, for here whatever is transmitted is transmitted through germinal cells, 
 or germ-plasm, as the hereditary substance contained in the germ-cells is now 
 commonly called. The problem then resolves itself into that of the relation 
 of the germ-plasm to the protoplasm of the rest of the body, the so-called 
 somatoplasm ; and the question to be answered is this : Are variations in the 
 
REPRODUCTION. 497 
 
 
 
 parental somatoplasni capable of inducing such changes in the gerra-plasm that 
 somatic peculiarities appear in the offspring similar to those possessed by the 
 parent? Weismann classifies all somatic variations according to their origin 
 into three groups viz. injuries, functional variations, and variations, mainly 
 climatic, that depend upon the environment. The problem of their inherit- 
 ance is a far-reaching one, and upon its correct solution depend principles that 
 are of much wider application than simply to matters of heredity; for if 
 acquired characters can be inherited, there is revealed to us a most potent fac- 
 tor in the transformation of species, and the whole question of the possibility 
 of use and disuse as factors of evolution is presented. The larger evolutionary 
 problem need not here be considered. 
 
 Regarding the problem of the inheritance of acquired characteristics we may 
 say at once that it is not yet solved. To the lay mind this may seem strange, 
 for at first thought it appears self-evident that parents may transmit to their 
 children peculiarities that they themselves have acquired. Affirmative evidence 
 seems all about us, as witness the undoubted cases of inheritance of artistic 
 tastes, of talent, of traits valuable in professional life, which seem to originate 
 in the industry of the parent. But scientific analysis by Weismann and others 
 of popular impressions, popular anecdotes and hearsay evidence, and accurate 
 original observation, have revealed little that cannot as well be explained on 
 other hypotheses. Anatomical and functional peculiarities of the body that are 
 apparently new often reappear in successive generations, but to assume that 
 they are acquired by the somatoplasm and have become congenital, rather than 
 that they are germinal from the first, is unwarranted. Direct experiments by 
 various investigators are almost as inconclusive. Weismann l has removed the 
 tails of white mice for five successive generations, and yet of 901 young every 
 individual was born with a tail normal in length and in other respects. Bos 2 
 has experimented similarly upon rats for ten generations without observing any 
 diminution of the tails. The practice of circumcision for centuries has resulted 
 in no reduction of the prepuce. The binding of the feet of Chinese girls has 
 not resulted in any congenital malformation of the Chinese foot. Brown- 
 S6quard, 3 and later Obersteiner, 4 have artificially produced epilepsy in guinea- 
 pigs by various operations upon the central nervous system and the peripheral 
 nerves, and the offspring of such parents have been epileptic. At first this 
 would seem to amount to proof of the actual hereditary transmission of mutila- 
 tions, yet in these cases the mutilation itself was not transmitted ; the offspring 
 were weak and sickly and exhibited a variety of abnormal nervous and nutri- 
 tional symptoms, among which was a tendency toward epileptiform convulsions, 
 the cause of which is still to be explained. Evidence from palaeontology 
 regarding the apparent gradual accumulation of the effects of use and disuse 
 throughout a long-continued animal series seems to require the assumption of 
 
 1 A. Weismann : Essays upon Heredity, vol. i., 1889, p. 432. 
 
 2 J. B. Bos: Biologisches Centralblatt, xi., 1891, S. 734. 
 
 3 E. Brown-Sequard : Researches on Epilepsy, etc., Boston, 1857; also various later papers. 
 
 4 H. Obersteiner : Medizinische Jahrbikher, Wien 1875,8.179. 
 
 Vor,. IT. 32 
 
498 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 such a principle as the inheritance of acquired characters, but even here the 
 principle of natural selection may perhaps be equally explanatory. 
 
 The Inheritance of Diseases. The question of the inheritance of diseases 
 has also been much discussed. The same general principles apply here as in 
 the inheritance of normal characteristics. The fact has been mentioned above 
 that pathological characters, whether anatomical, physiological, or psycholog- 
 ical, are capable of transmission. If, however, a pathological character has 
 been acquired by the parent and is not inherent in his own germ-cells, it 
 is extremely doubtful whether it can be passed on to the child. A diseased 
 parent, on the other hand, may produce offspring that are constitutionally 
 weak or that are even predisposed toward the parental disease, and such off- 
 spring may develop the parent's ailment. In such cases constitutional weakness 
 or predisposition, and not actual disease, is inherited ; the disease itself later 
 attacks the weak or predisposed body. Proneness to mildness or severity of, 
 and immunity toward, certain diseases seem to be transmissible. These sub- 
 jects, however, are so little understood, and the real meaning of such terms as 
 predisposition, inherited constitutional weakness, and inherited immunity, is so 
 little known, that it is idle to discuss them here. 
 
 Considerable experimental work has been performed recently upon the 
 transmissibility of infectious diseases. Undoubtedly infectious diseases cling 
 to a particular family for generations. The transmitted factor is probably fre- 
 quently, if not usually, simple predisposition. But in an increasing number 
 of cases there appears to be transmission of a specific micro-organism. Such 
 transmission is called germinal when the micro-organism is conveyed in the 
 ovum or the semen, and placental or intra-uterine when the micro-organism 
 reaches the fetus after uterine development has begun, and chiefly through the 
 circulation. Of germinal infections syphilis seems undoubtedly capable of 
 transmission within either the ovum or the semen. The possibility of germinal 
 transmission of tuberculosis has been maintained, but is not fully proven. Of 
 intra-uterine infections there have been observed in human beings apparently 
 undoubted cases of typhoid fever, relapsing fever, scarlatina, endocarditis, 
 small-pox, measles, croupous pneumonia, anthrax, syphilis, and possibly 
 tuberculosis and Asiatic cholera. It is obvious that neither germinal nor 
 placental inheritance, both taking place through the medium of a specific micro- 
 organism, and not through the modification of germ-plasm, is comparable to 
 inheritance in the customary sense. 
 
 Theories of Inheritance. From early historical times theories of inher- 
 itance have not been wanting. Physical and metaphysical, materialistic and 
 spiritualistic theories have had their day. Previous to the discovery of the 
 spermatozoon (Hamm, Leeuwenhoek, 1677) all theories were necessarily 
 fantastic, and for nearly two hundred years later they were crude. The 
 theories that are now rife may be said to date from 1864, when Herbert 
 Spencer published his Principles of Biology. Since that date they have 
 become numerous. Even the modern theories are highly speculative ; none 
 can be regarded as being accepted to the exclusion of all others by a large 
 
REPRODUCTION. 499 
 
 majority of scientific workers, and the excuse for introducing them into a 
 text-book of physiology is the hope that a brief discu.-sion of them mav 
 prove suggestive, stimulating, and productive of investigation. 
 
 Germ-plasm. Germinal substance, germ-plasm (Weismann), or, as it is some- 
 times called, idioplasm (Nageli), must lie at the basis of all scientific theories 
 of heredity. The father and the mother contribute to the child the >]>< rma- 
 tozoon and the ovum respectively, and within these two bits of protoplasm 
 there must be contained potentially the qualities of the two parents. Tlinv 
 is the strongest evidence in favor of the prevailing view that the nucleus alone 
 of each germ-cell is essentially hereditary, or, more exactly, that the chromatic 
 substance of the nucleus is the sole actual germinal substance. We have seen 
 that the tail of the spermatozoon is a locomotive organ, and that the body of 
 the ovum is nutritive matter. We have seen also that the essence of the 
 whole process of fertilization is a fusion of the male and the female nuclei, or, 
 more exactly, a mingling of male and female chromosomes. Hence most 
 physiologists agree with Strasburger and Hertwig that the chromatic substance 
 of the nuclei of the germ-cells transmits the hereditary qualities. 
 
 As to the origin of the germ-plasm, two hypotheses have been suggested. 
 Spencer, Darwin, Galton, and Brooks have argued in favor of a production 
 of germ-plasm within each individual by a collocation within the reproductive 
 organs of minute elementary vital particles " physiological units " (Spencer), 
 "gemmules" (Darwin) which come from all parts of the body; hence each 
 part of the body has its representative within every germ-cell. This hypothesis 
 affords a ready explanation of numerous facts, but its highly speculative cha- 
 racter, the entire absence of direct observational or experimental proof of its 
 truth, and the demand that its conception makes upon human credulity, mili- 
 tate against its general acceptance. Weismann, the promulgator of the second 
 hypothesis, denies altogether the formation of the germ-plasm from the body- 
 tissues of the individual, and maintains its sole origin from the germ-plasm of 
 the parent of the individual. Through the parent it comes from the grand- 
 parent, thence from the great-grandparent, and so may be traced backward 
 through families and tribes and races to its origin in simple unicellular 
 organisms. According to Weismann, therefore, germ-plasm is very ancient 
 and is directly continuous from one individual to another ; the parts of an 
 individual body are derivatives of it, but they do not return to it their repre- 
 sentatives in the form of minute particles. The general truth of Weismann's 
 conception can hardly be denied. 
 
 As to the morphological nature of germ-plasm, two views likewise are held. 
 One school, led by His and Weismann, holds that germ-plasm possesses a 
 complicated architecture ; that the fertilized ovum contains within its structure 
 the rudiments or primary constituents of the various cells, tissues, and organs 
 of which the body is destined to be composed ; and that growth is a develop- 
 ment of these already existing germs and largely independent of surrounding 
 influences. In accordance with this idea, segmentation of the ovum is specifi- 
 cally a qualitative process, one blastomere representing one portion of the 
 
500 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 future adult, another blastomere another portion, and so on. This theory 
 recalls in a refined form the crude theory of Preformation that was advocated 
 during the seventeenth and eighteenth centuries by Haller, Bonnet, and many 
 others, according to which the germ-cell was believed to contain a minute but 
 perfectly formed model of the adult, which needed only to be enlarged and 
 unfolded in growth. The other modern school, in which Oscar Hertwig is 
 prominent, maintains that the fertilized egg is isotropous that is, that one 
 part is essentially like another part that the architecture of the egg is rela- 
 tively simple, and that growth is largely a reaction of the living substance to 
 external influences. The idea of isotropy is based largely upon the experi- 
 mental results of Pfluger, Chabry, Driesch, Wilson, Boveri, and the brothers 
 Hertwig, who by various methods and in various animals have found that 
 single blastomeres of a segmenting ovum, when separated from the others, will 
 develop into normal but dwarfed larvae ; that is, a portion of the original germ- 
 plasm is capable of giving rise to all parts of the animal. These results are 
 interpreted to signify that segmentation, instead of being qualitative, is quanti- 
 tative, each blastomere being like all the others. The second theory, like the 
 first, resembles in some degree a theory of the past two centuries, advocated 
 by Wolff and Harvey, and known as the theory of Epigenesis. According to 
 this there was no preformation in the germ-cells, but rather a lack of organi- 
 zation which during growth, under guidance of a mysterious power supposed 
 to be resident in the living substance, gave place to differentiation and the 
 appearance of definite parts. 
 
 Modern microscopes have revealed no miniature of the adult in the egg, 
 nor has modern physiology found necessary an assumption of extra-physical 
 forces within living matter. With the increase of knowledge the old and 
 crude preformation of Haller and Bonnet and the speculative epigenesis of 
 Wolff and Harvey have given place to the new preformation and epigenesis 
 of the present time, and all modern theories of heredity may be classed in 
 the one or the other category or as intermediate between them. The mod- 
 ern advocates of preformation explain hereditary resemblance by the supposed 
 similarity of all germ-plasm in any one line of descent. The modern advocates 
 of epigenesis, while allowing the necessity of a material basis of germ-plasm, 
 ascribe hereditary resemblance to similarity of environment during develop- 
 ment. 
 
 Variation. It is a commonplace in observation that, however close hereditary 
 resemblance may be, it is never absolute ; the child is never the exact image 
 of the parent either physically or mentally. Variations from the parental type 
 may be either acquired by the offspring subsequent to fertilization or to birth, 
 and hence are to be traced to the action of the environment ; or they may be 
 congenital, that is, inherent in the germ-plasm. Although it is not always 
 easy in the case of any one variation to determine to which class it belongs, 
 yet the fact remains that the two classes exist; and a complete theory of 
 heredity must recognize and explain congenital variation as fully as congenital 
 resemblance. It is unnecessary to say that the origin of congenital variation 
 
RE PR OD UCTION. 501 
 
 is one of the much discussed and still unsettled questions. At least two causes 
 of congenital variations are commonly recognized, although opinions differ as 
 to the relative importance of the r6le played by each. These causes are differ- 
 ences in the nutrition of the germ-plasm, and sexual reproduction. As to the 
 former, it is evident that the germ-plasm in no two individuals, even father 
 and son, has exactly identical nutritional opportunities. Since the life of one 
 individual is not the exact counterpart of the life of another, the germ-plasm 
 of one individual has a different nutrition from that of another. It would 
 hence be strange, even although we regard the germ-plasm as relatively stable, 
 if with succeeding generations there did not appear variations that are sufficient 
 to give rise to unlikeness in relatives. Differences in the nutrition of the germ- 
 plasm in different individuals are, therefore, a true cause of variations. As 
 regards sexual reproduction, it must be remembered that a new individual is 
 the product of two individuals, that the two individuals have descended along 
 different genealogical lines, and hence that the two conjugating masses of germ- 
 plasm are different in nature. It is only to be expected, therefore, that the 
 resulting individual shall be different from the two contributing parents. Thus 
 sexual reproduction is a true cause of variations. 
 
 Having outlined the main facts and principles of heredity, let us now review 
 a few of the specific theories that have been of value in clearing the clouded 
 atmosphere. 
 
 Darwin's Theory of Pangenesis. Darwin's " Provisional Hypothesis of 
 Pangenesis" was published in 1868 as chapter xxvii. of his work on The Vari- 
 ations of Animals and Plants under Domestication. It was the first of the 
 modern theories to attempt to cover the whole ground of heredity ; it was 
 accompanied by a most exhaustive presentation and analysis of facts, and it 
 stimulated abundant discussion and investigation. In Darwin's own words 
 the hypothesis was formulated as follows : " It is universally admitted that the 
 cells or units of the body increase by cell-division or proliferation, retaining 
 the same nature, and that they ultimately become converted into the various 
 tissues and substances of the body. But besides this means of increase I assume 
 that the units [cells] throw off minute granules which are dispersed throughout 
 the whole system ; that these, when supplied with proper nutriment, multiply 
 by self-division, and are ultimately developed into units like those from which 
 they were originally derived. These granules maybe called gemmules. They 
 are collected from all parts of the system to constitute the sexual elements, and 
 their development in the next generation forms a new being; but they are 
 likewise capable of transmission in a dormant state to future generations, and 
 may then be developed. Their development depends on their union with other 
 partially developed or nascent cells which precede them in the regular course 
 
 of growth Gemmules are supposed to be thrown off by every unit, 
 
 not only during the adult state, but during each stage of development of 
 every organism ; but not necessarily during the continued existence of the 
 same unit. Lastly, I assume that the gemmules in their dormant state have a 
 mutual affinity for each other, leading to their aggregation into buds or into 
 
502 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 the sexual elements. Hence, it is not the reproductive organs or buds which 
 generate new organisms, but the units of which each individual is composed. 
 These assumptions constitute the provisional hypothesis which I have called 
 Pangenesis." 
 
 Since the cells of the body are represented by gem mules within the germ- 
 cells, Darwin's theory is a theory of Preformation. It explains the facts of 
 the regeneration of lost parts by the assumption that the gem mules of the part 
 in question are disseminated throughout the body and have only to unite with 
 the nascent cells at the point of new growth. Pangenesis explains reversion, 
 since gemmules may lie dormant in one generation and develop in the next. 
 It explains congenital variation, since the mixture of maternal and paternal 
 gemmules is plainly different from the two kinds taken separately. It explains 
 how acquired variations may become congenital, since an altered part throws 
 off altered gemmules, and by the collocation of these in the germ-cells the 
 alteration may be transmitted. It thus allows the transmission of acquired 
 characters. 
 
 Darwin's assumptions of gemmules and their behavior are pure assump- 
 tions, for which subsequent investigation has not provided a basis of facts. 
 As we have seen, also, the inheritance of acquired characters is greatly in 
 doubt, and, if they are heritable at all, they can be so only comparatively 
 feebly. Besides these objections it was early found that, with the increase 
 of knowledge of the facts of heredity, it was necessary to modify very mate- 
 rially the theory of Pangenesis. This has been ably done successively by 
 Galton, 1 Brooks, 2 and de Vries. 3 But neither the original theory nor its 
 modifications have been generally accepted. 
 
 Weismann' s Theory. Since 1880, Professor Weismann 4 of Freiburg has 
 published numerous essays upon heredity and allied subjects, in which, besides 
 reviewing the views of others, he has developed in detail a new and elaborate 
 theory of his own, that is the most ambitious attempt yet made to solve the 
 problem of inheritance. In the course of their development Weismann's 
 ideas have undergone some modification. Their leading features are as 
 follows : 
 
 The essential hereditary substance, or germ-plasm, is the chromatin of the 
 nucleus of the germ-cells. One of the fundamental tenets of Weismann's 
 system is expressed by his own phrase, " the continuity of germ-plasm." By 
 this is meant that the germ-plasm of one individual, instead of arising de novo 
 in the individual by the collocation of multitudinous " gemmules " derived 
 from the body-cells, originates directly from the germ-plasm of the parent, 
 thence from that of the grandparent, and so on backward through all genera- 
 tions to the origin of all germ-plasms that took place simultaneously with the 
 
 1 Francis Galton : "A Theory of Heredity," Journal of the Anthropological Institute, 1875. 
 
 2 W. K. Brooks: The Laws of 'Heredity, 1883. 
 
 3 H. de Vries : Die Intracelluldre Pangenesis, 1889. 
 
 4 August Weismann: Essays upon Heredity and Kindred Biological Problems, authorized 
 translation, vol. i., 1889; vol. ii., 1892; The Germ-plasm, authorized translation, 1893; The 
 Effect of External Influences upon Development, the Romanes Lecture, 1894. 
 
REPR OD UCTION. r,( ); j 
 
 origin of sex germ-plasm is continuous from individual to individual along 
 any one line of descent. Wcismaim draws a sharp line between </rnn.-j>la*m 
 and somatoplasm, or body-plasm, which latter comprises all protoplasm that 
 the body contains except the germ-plasm. Germ-plasm once originated con- 
 tinues from generation to generation ; somatoplasm develops anew in each gen- 
 eration from germ-plasm by growth and differentiation, resulting in a loss of its 
 specific germinal character. Germ-plasm is stable in composition ; somatoplasm 
 is variable. Germ-plasm, being passed on from parent to offspring, is immortal ; 
 somatoplasm dies when the individual dies. Weismann believes that "the 
 germ-plasm possesses a fixed architecture, which has been transmitted histori- 
 cally " and which represents the parts of the future organism. It consists of 
 material particles or hereditary units called determinants, each of which has a 
 definite localized position within the germ-plasm. The determinants are sug- 
 gestive of Darwin's gemmules, yet they are not the same, for, while gem mules 
 were supposed to represent individual cells, determinants are representatives 
 of cells or groups of cells that are variable from the germ onward. Deter- 
 minants consist of definite combinations of simpler units, or biophors, which 
 are the smallest particles that can exhibit vital phenomena. Below biophors 
 there come in order of simplicity of material structure the molecules and 
 the atoms of the physicist. Above biophors and determinants Weismann 
 finds it necessary to assume the existence of higher units, named in order ids 
 and idants, the former being groups of determinants, and actually visible as 
 granules of chromatin, the latter being the chromosomes of the nucleus. Each 
 one of these various units is possessed of the fundamental vital properties of 
 growth and multiplication by division. Such a complex system is Preforma- 
 tion in an extreme form. In fertilization idants of the sperm join with idants 
 of the ovum, and the resulting segmentation nucleus consists of a mixture of 
 paternal and maternal determinants. Within this mixture there exist in a 
 potential state the primary constituents of a considerable number of forms 
 which the future individual may assume. In ontogeny, or development of 
 the individual, these primary constituents take two paths : some of the ids 
 remain inactive and enter the germ-cells of the embryo for the production of 
 future generations ; other ids disintegrate into determinants, the determinants 
 enter the embryonic cells that result from segmentation, and there themselves 
 disintegrate and set free into the cytoplasm their constituent biophors ; thus 
 they determine the future character of the cells of the organism. The division 
 of primary constituents into those that shall remain latent and those that shall 
 become active is effected largely by the stimulation of external influences; 
 hence, given several potential formations in the germ, external influences 
 decide which one shall become the actual structure in the adult or^ani-m. 
 Once set free and having become somatoplasm, neither the biophors nor the 
 determinants are able to return to the germ-cells. In the adult, germ-plasm is 
 never capable of reflecting in any way the characteristics of the BOOMtOplasm 
 which surrounds it on all sides. With its ancient ancestry it leads a charmed 
 existence, largely independent of environmental changes. It follows that 
 
504 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 characters acquired by the adult are incapable of acquisition by the germ- 
 plasm, and hence may not be transmitted. The non-inheritance of acquired 
 characters is thus another of the fundamental tenets of Weisrnanu's theory, 
 and one about which he is most positive. 
 
 If these two principles of continuity of stable germ-plasm and non-inheri- 
 tance of acquired characters be true, why are not all individuals in any one 
 line of descent exactly like one another? How is congenital variation possible? 
 In the first place, Weismann allows that germ-plasm, while eminently stable, 
 is not absolutely so ; it is subject to slight continual changes of composition 
 resulting from inequalities in nutrition ; and " these very minute fluctuations, 
 which are imperceptible to us, are the primary cause of the greater deviations 
 in the determinants which we finally observe in the form of individual varia- 
 tions.'' The accumulation of minute deviations may be aided greatly by sex- 
 ual reproduction, or, to use Weismann's more exact term, which is equally 
 applicable to the combination of sexual elements in sexual organisms and to 
 the process of conjugation in the asexual forms, amphimixis. Given the in- 
 finitesimal beginning of a variation, the mingling of two lines of descent, with 
 different past surroundings, may be a most powerful factor in strengthening 
 the deviation and bringing it into recognition as a new character. Moreover, 
 natural selection becomes here also potent as soon as the variation has assumed 
 sufficient proportions to be seized upon by this important factor of evolution. 
 In cases of reversion Weismann supposes the determinants to remain inactive 
 in the germ-plasm for one or more generations and later to develop. The 
 theory accounts for the regeneration of lost parts by the assumption that the 
 cells in the vicinity of the wound, by the proliferation of which the new part 
 grows, contain, besides the active determinants that have given them their 
 specific character, other determinants that are latent until the opportunity for 
 regeneration arrives. Some cells do not possess such latent determinants, and 
 hence some parts of a body are incapable of reproducing lost parts. 
 
 Such are the main features of Weismann's theory a germ-plasm of highly 
 complex architecture and independent of somatoplasm ; continuity of germ- 
 plasm and non-inheritance of acquired somatic characters tending to preserve 
 the uniformity of the species ; slight nutritional variation of germ-plasm and 
 sexual reproduction tending to destroy that uniformity ; the result is inherited 
 resemblance and congenital variation. The theory is now being most actively 
 discussed. 
 
 Theory of Epigenesis. Among epigenesists no one theory may be said to 
 be pre-eminent. The main features of the epigenetic conception, already 
 referred to, may be summarized as follows : The fertilized ovum is isotropous, 
 i. e. all parts are essentially alike; germ-plasm probably consists of minute 
 particles, but these particles do not represent definite cells or groups of cells 
 of the adult; segmentation is a quantitative process; the early blastomeres 
 are essentially alike, and any one of them, if isolated from the rest, may 
 give rise to a whole organism, although under ordinary circumstances they 
 react upon each other in bringing about the resultant individual ; there is 
 
REPRODUCTION. 505 
 
 no predetermination, either in the germ-cells or in the segmenting ovum, 
 of the ultimate form or function of the various constituent parts ; morpho- 
 logical differentiation and physiological specialization are phenomena of 
 comparatively late embryonic life, and the prospective character of any one 
 cell, whether it is to be a muscle-cell, gland-cell, nerve-cell, or germ-cell, is 
 determined by the influence of the surrounding cells and the surrounding 
 physical and chemical conditions " the prospective character of each cell is a 
 function of its location." Extreme epigenetic views are not so numerous as 
 those of preformation. 1 
 
 The more moderate thinkers of the present time recognize truth in both 
 preformation and epigenesis, and are endeavoring by experimental methods to 
 determine how much share in the production of the characteristics of the off- 
 spring is to be ascribed to the original qualities of the germ-plasm and how 
 much to the physical, chemical, and physiological phenomena of the immediate 
 environment of the developing embryo. Such experimental work is per- 
 formed at present upon the simpler and lower animals, mostly marine inverte- 
 brates, and has reference to the effect of changes in the composition of the water 
 surrounding the embryo, the effects of various salts, of changes in temperature, 
 of pressure, of electricity, etc., etc. Such work is now in its infancy, but it is 
 doubtless destined to yield results of the highest value in an understanding of 
 the true nature of heredity. 
 
 1 The best statement of a moderate epigenetic theory is to be found in O. Hertwig : The 
 Biological Problem of To-day ; Preformation or Epigenesis f Authorized translation. 
 
INDEX TO VOLUME II. 
 
 ABSOLUTE muscular force, 141 
 Accelerator u rinse muscle, 449 
 Accommodation, 306 
 
 associated movements of, 311 
 
 astigmatic, 310 
 
 dissociation of, from convergence, 312 
 
 influence of drugs on, 311 
 
 in old age, 314 
 
 mechanism of, 309 
 
 nervous mechanism of, 311 
 
 normal range of, 312 
 
 range of, in hypermetropia, 314 
 in myopia, 314 
 
 relation of, to perception of distance, 356 
 
 voluntary character of, 311 
 Achromatic lenses, 316 
 Achromatism of the eye, 316 
 Acid salts of muscle, 168 
 Acidity of worked muscles, 168 
 Acids, action of, on nerves and muscles, 60 
 Acquired characters, inheritance of, 496 
 
 variations, 500 
 
 Actinic rays of the luminiferous ether, 331 
 Action current, diphasic, 152 
 in the heart, 152 
 in the muscles, 150 
 in the nerves, 153, 183 
 Adam's apple, 425 
 
 Adrenal extract, action of, on muscles, 138 
 Aerial perspective, 355 
 
 Afferent impulses, effect of, on irritability of the 
 central nervous system, 223 
 
 neurones of t he spinal cord, 203 
 
 paths in the cord traced electrically, 230 
 traced histologically, 229 
 traced physiologically, 229 
 After-birth, 481 
 
 After-effect of retinal stimulation, 345 
 After-images, 346 
 After-loading of muscles, 110 
 After-pressure, sensation of, 394 
 Agamogenesis, 439 
 
 Age, changes in organization of the central 
 nervous system with, 284 
 
 influence of, on nerve-cells, 490 
 on visual accommodation, 314 
 
 relation of brain -weights to, 276 
 of menstruation to, 459 
 
 specific gravity of the nervous system with 
 
 changes in, 284 
 
 Albinos, condition of the internal ear in, 407 
 Alcohol, action of, on conductivity of nerves, 
 93 
 
 effect of, on nerve-currents, 156 
 
 fumes, action of, on nerves, 60 
 
 stimulating action of. ?r> 
 
 Alkalies, action of, on nerves and muscles, 60 
 Allan toic arteries, 474 
 
 vein, 474 
 Allantois, 474 
 Allochiria, 400 
 
 Ammonia, action of, on nerves, 60 
 Ammonium salts, action of, on muscles, 138 
 Amnion, 47:2 
 
 Amniotic cavity, 472 
 Amoeba, contractility of, 19 
 Amoeboid movement, 19 
 in neuroblasts, 176 
 in ova, 82 
 Am phi aster, 469 
 Amphimixis as cause of congenital variation, 
 
 504 
 
 Amphioxus, reflexes in, 212 
 Amplitude of sound-waves, 381 
 Ampulla of Heule, 447 
 Ampullae of the semicircular canals, 372 
 Ampullary nerves, stimulation of, 407 
 Amputation in man, effects of, on neurones, 196 
 Anaemia of the brain during fatigue, 288 
 Anaesthesia, contralateral, after hemisectiou of 
 
 the cord, 233 
 
 Anaesthetics, action of, on nerve-currents, 155 
 Analgesia, 232 
 
 following removal of the cerebellum, 272 
 Analysis of composite tones, 384 
 Anatomy of the ear, 362 
 Anelectrotonus, 62 
 Angular movements of joints, 416 
 Anisotropic substance of muscle-fibres, 104 
 A nodal contraction, 36 
 Anode, physical, definition of, 52 
 
 physiological, definition of, 52 
 Anosmia, 411 
 Anterior association centre, 257 
 
 roots, recurrent sensibility of, 204 
 Aphasia, 257 
 Apraxia, 259 
 
 Aqueous humor, index of refraction of, 303 
 Arteries, calcification of, in old age, 491 
 Arthropods, segmental nervous system of, 212 
 Articular cartilages, 415 
 Articulation, 434 
 Articulations, varieties of, 414 
 Artificial circulation through the heart, 69 
 through the muscles, 68 
 
 stimulation of muscle compared with normal, 
 
 134 
 
 Aryepiglottic fold, 422 
 Aryteno-epiglottidean muscle, 426 
 Arytenoid cartilages, 425 
 
 muscle, 426 
 Asexual reproduction, 439 
 
 theory of the origin of, 441 
 Aspirates, 437 
 
 Association centre, anterior, 257 
 middle, 257 
 posterior, _'.">? 
 
 cerebral, variations in. -.'(in 
 
 fibres and centres of the cortex, rjr.i; 
 
 tracts, origin of. from central cells. _'<>."> 
 Astasia after removal of the cerebellum. -.'?:', 
 Asthenia from removal of the cerebellum. :*!'.'> 
 Astigmatic accommodation, 310 
 Astigmatism, 317 
 
 detection of, 319 
 
 irregular, 319 
 
 Astral rays, contractility of, 470 
 Atavism. !!.") 
 
 507 
 
508 
 
 INDEX TO VOLUME II. 
 
 Ataxia after removal of the cerebellum, 273 
 Atonia after removal of the cerebellum, 273 
 Atrophy of the nerve-cells from disuse, 195 
 Atropia, action of, on accommodation, 311 
 Atropin, action of, on the eye, 325 
 Attraction sphere of the ovum, 449 
 Auditory area of the cortex, 253 
 canal, 363 
 
 epithelium of the utricle and saccule, 373 
 judgments, 389 
 meatus, external, 362 
 nerves, central paths of, 237 
 cochlear division of, 376 
 subdivisions of, 373 
 ossicles, 365 
 
 movements of, 367 
 sensations, limits of, 382 
 successive contrast in, 388 
 theory of, 380 
 
 Auricle of the external ear, 362 
 Automatism, definition of, 20 
 Axoues, definition of, 21, 173 
 growth in diameter of, 179 
 length of, 174 
 
 BALL-AND-SOCKET joint, 416 
 
 Barium salts, action of, on muscles, 138 
 
 Bartholini, glands of, 462 
 
 Basilar membrane, structure of, 377 
 
 width of, 380 
 Bathyasthesia, 233 
 
 Beats in musical tones, production of, 386 
 Benham's spectrum top, 344 
 Binocular combination of colors, 358 
 vision, 356 
 
 illusions in, 359 
 
 rivalry of the fields of vision in, 358 
 Biophors of the germ-plasm, 503 
 Birds, removal of cerebral hemispheres in, 267 
 Birth, size of the child at, 487 
 Birth-rate of the two sexes, 483 
 Births, multiple, 482 
 
 ratio of male to female, 483 
 Blastomeres, 470 
 Blind spot, 328 
 Blood, amount of, in the central nervous system, 
 
 288 
 
 changes in, during pregnancy, 477 
 Blood-supply of the central nervous system, 286 
 
 relation of, to irritability, 67 
 Body-sense area of the cortex, 252, 254 
 Body-temperature, rise of, from injury to the 
 
 optic thalami, 271 
 
 from lesions in the corpora striata, 271 
 Bolometer, 142 
 
 Bones, action of muscles on, 417 
 Bottcher's crystals, 445 
 Brain, curve of growth of, 279 
 growth of, 278 
 
 number of nerve-elements during, 280 
 relation of, to growth of the body, 280 
 size of neurones during. 281 
 metabolic activity in, 288 
 regulation of circulation in, 287 
 weight of, 273 
 Brain -stem, 274 
 
 Brain- ventricles, capacity of, 274 
 Brain- weight, decrease of, in old age, 296 
 relation of, to insanity, 278 
 to sex, 276 
 
 to social environment, 277 
 Brain-weights of diiferent races, 278 
 Breaking contraction, point of origin of, 35 
 " Breaking " shock, 31 
 Broca's convolution, 257 
 Brown-Sequard's paralysis, 233 
 Brucin, action of, on end-plates, 27 
 
 Bulbo-cavernosus muscle, 449 
 action of, in erection, 464 
 Bulimia, 404 
 
 CAFFEIN, action of, on coagulation of muscle- 
 plasma, 164 
 Calcium salts, action of, on the muscles, 138 
 
 relation of, to irritability, 59 
 Canalis cochlearis, 373 
 
 reunions, 373, 374 
 Capillary electrometer, 146 
 Caput gallinaginis, 464 
 Carbon dioxide, action of, on conductivity in 
 
 nerves, 93 
 011 the nerves, 60 
 on warm spots, 398 
 effect of, on nerve-currents, 156 
 of the muscle, 168 
 production of, in nerves, 95 
 disulphide, action of, on nerves, 60 
 Cardiac palpitation at the climacteric, 490 
 Carnic acid of muscles, 167 
 Carnin of muscles 167 
 Castration, effects of, 463 
 
 on the voice, 431 
 Cataleptic rigor, 160 
 Caudate nucleus, heat centre of, 271 
 Cell, galvanic, 29 
 Cell-differentiation, 22 
 Cells, growth of, 486 
 
 Central cells, importance of, in relation to in- 
 crease of organization, 285 
 nervous system, amount of blood in, 288 
 arrangement of cell groups in, 205 
 blood-supply of, 286 
 change in specific gravity of, with age, 
 
 284 
 
 condition of, in sleep, 293 
 conscious phenomena of, 172 
 daily rhythms of, 289 
 development of, 172 
 fatigue of, 289 
 general arrangement of, 202 
 
 functions of, 171 
 
 influence of the thyroid on growth of, 289 
 in old age, 295 
 
 intensity of metabolism in, 288 
 medullation of nerves in, 181 
 operations on, in frogs, 265 
 organization of. at different ages, 284 
 neurones of the spinal cord, 203 
 stimulation of the nervous system, 208 
 Centre of hearing, cortical, 253 
 of rotation of the eye, 298 
 of smell, cortical, 253 
 of vision, cortical, 253 
 spinal, of ejaculation, 465 
 of erection, 464 
 of parturition, 481 
 Centres, association, 256 
 Centrosome of human spermatozoa, 444 
 of the fertilized egg, 468 
 of the ovum, 449 
 
 Cerebellum, anatomical connections of. 273 
 effects of injury to, 272 
 functions of, 272 
 senile changes in, 296 
 
 Cerebral circulation, conditions affecting, 288 
 hemispheres, effect of removal of, 263 
 relative physiological values of, 259 
 removal of, in birds, 266 
 
 in dogs, 262, 267 
 Cerebrin of nerves, 170 
 Cerebrum, heat-regulating functions of, 270 
 
 removal of, in dogs, 267 
 Characters, acquired, inheritance of, 496 
 Chemical reagents, action of, on irritability, 58 
 
INDEX TO VOLUME II. 
 
 509 
 
 Chemical stimulation of nerve, 25 
 
 ton us, 143 
 
 < 'liniiistry of nerves, !<>} 
 Chemotaxis, 466 
 
 influence of, on neuroblasts, 176 
 Cherao-tropism, -Hit 5 
 Chest-voice, 432 
 
 Chloroform, action of, on coagulation of muscle- 
 plasma, 164 
 
 effect of, on nerve-currents, 156- 
 
 vapor. action of, on nerves, 60 
 Cholesterin in nerve, 169 
 Chorda tympani nerve, gustatory function of, 
 
 410 
 Chorion, \1'.\ 
 
 frondostun. 171 
 
 laeve, 474 
 Cluirionic fluid, 473 
 
 villi, 473 
 
 Chromatic aberration, 316 
 Chromatoblasts of pleuronectidse, 20 
 Chromosomes, number of, in the segmentation 
 nucleus, 466 
 
 of germ-cells, hereditary function of, 499 
 
 of human spermatozoa. 443 
 
 of the sexual element, reduced number of, 
 454 
 
 ovarian, changes in, during maturation, 451 
 number of, 450 
 
 reduction of, in maturation of spermatozoa, 
 
 445 
 
 Chronograph, description of, 100 
 Ciliary ganglion, 323 
 
 muscles, action of, in accommodation, 309 
 
 nerves, long, 324 
 short, 311, 323 
 
 Circulation, artificial, through isolated organs, 
 68 
 
 of blood in the retina, 322 
 
 of the brain and cord, 286 
 Circumduction, movement of, 416 
 Climacteric, 459, 490 
 
 ovulation after, 456 
 Climate, influence of, on age of puberty, 489 
 
 on time of climacteric, 490 
 Clitoris, 462 
 
 homology of, 464 
 
 Coagulation of muscle-plasma, action of drugs 
 on, 164 
 
 of myosin, 163 
 Cocaine, action of, on conductivity in nerves, 93 
 
 on the tongue, 413 
 Cochlea, anatomy of, 374 
 
 bony, 372 
 
 membranous, structure of, 376 
 Cochlear root of the auditory nerve, central 
 
 paths of, 237 
 
 Coffee, stimulating action of, 75 
 Cold and warm points of the skin, 398 
 Collaterals of axones, 173 
 
 Color of objects, relation of, to intensity of illu- 
 mination, 333 
 
 sensations in indirect vision, 333 
 phenomena of, 333 
 
 theories, 335 
 
 triangle, 334 
 
 vision, theories of, 335 
 Color-blindness, 338 
 
 hereditary transmission of, 494 
 
 of the rods, 342 
 
 Colored shadows from simultaneous contrast, 347 
 Color-mixture, 333 
 Colors, binocular combination of, 358 
 
 complementary, 334 
 
 physical basis of, 332 
 
 relative luminosity of, 340 
 
 saturation of, 342 
 
 Combinational tones, 387 
 ('oinniH>iiiv. Mcyiu-rt's, 238 
 
 von ( Jiddrn's, 238 
 
 Commissures, origin of, from central cells, 205 
 Common sensation, definition of, 399 
 
 sensibility, afferent paths of tin- nerves of, 230 
 Commutators, method of using. :;; 
 Complementary colors, 334 
 
 Composite tones, analysis of, 384 
 Conceptions, multiple, 4H2 
 Concha of the external ear, 362 
 Conduction by contiguity, 81 
 
 directions of, 84 
 
 from neurone to neurone, 84 
 
 in branching nerves, 80 
 
 in ganglion-cells, 97 
 
 in muscles, 80 
 
 in nerves, effects of, 95 
 
 in nerve-trunks, 79 
 
 of nerve-impulses, direction of, 184 
 from neurone to neurone, 207 
 
 process, nature of, 97 
 
 rate of, 87 
 Conductivity, action of drugs on, 93 
 
 definition of, 20, 77 
 
 dependence of, on protoplasmic continuity, 77 
 
 effect of constant current on, 94 
 
 influences affecting, 91 
 
 of muscle, 20 
 
 of nerves, 21 
 
 of the neurone, 189 
 
 of ova, 22 
 
 Condyloid joints, 416 
 Cones, retinal, function of, 341 
 
 movements of, 331 
 Confluxion in space-perception, 353 
 Congenital resemblances, 494 
 
 variations, 500 
 
 Conjugate foci in a dioptric system, 302 
 Conjugation, 440 
 
 Conin, action of, on end-plates, 27 
 Consciousness, cerebral origin of, 172 
 Consonants, 436 
 
 Constant current, contracture effect of, in mus- 
 cles, 131 
 
 effect of, on conductivity, 94 
 on muscles, 61 
 on nerves, 62 
 
 Constrictor nerves of the iris, 323 
 Continuous contractions, 127 
 Contractility, definition of, 17, 98 
 
 in vorticella, 20 
 
 occurrence of, 20 
 
 of amoebae, 19 
 
 of muscle, 17 
 
 of muscles, adaptation of, to their normal 
 functions, 108 
 
 of ova, 22 
 
 of the astral rays, 470 
 
 Contraction curve of muscle, effect of frequent 
 excitation on, 115 
 
 idio-muscular, 92 
 
 of muscles, post-mortem, 160 
 relation of, to structure, 107 
 
 remainder, 106 
 
 wave in muscle, rate of transmission, 87 
 
 length of, 88 
 Contractions from repeated single stimuli, 112 
 
 introductory, 113 
 
 isometric, 110 
 
 isotonic, 110 
 
 normal, tetanic nature of, 132 
 
 of rigor caloris, 165 
 Contracture after frequent excitation, 128 
 
 after single excitation, 129 
 
 definition of, 116 
 
 from fatigue, 130 
 
510 
 
 INDEX TO VOLUME II. 
 
 Contracture in dying muscles, 132 
 in rigor mortis, 128 
 in veratria poisoning, 128 
 normal, 129 
 of the neck muscles after cerebellar injury, 
 
 272 
 
 pathological, 127, 132 
 relation of, to tetanus, 117, 122, 124 
 Contractures, 127 
 Contrast, visual, 346 
 
 in space perception, 352 
 Convergence of the eyes in accommodation, 311 
 
 muscular mechanism of, 300 
 Co-ordination of the efferent impulses in re- 
 flexes, 214 
 Copulation, 463 
 Core-conductors, 158 
 Cornea, curvature of, 303 
 Corniculum laryngis, 425 
 Corn u tine, action of, on muscles, 137 
 Corona radiata, 454 
 
 of the ovum, 450 
 Corpora cavernosa of the penis, 448 
 
 striata, functions of, 271 
 Corpus callosum, functions of, 270 
 
 luteum, 455 
 
 * spougiosum of the penis, 448 
 Corresponding points of the retinas, 359 
 Cortex cerebri, effects of localized electrical 
 
 stimulation of, 241 
 electrical stimulation of, 242 
 number of nerve-cells in, 284 
 course of efferent impulses from, 251 
 latent areas of, 261 
 Corti, cells of, 377 
 
 organ of, structure of, 377 
 rods of, 377 
 Cortical areas, 243 
 
 motor, in man, 250 
 size of, 247 
 centres, 243 
 motor control, crossed, 251 
 
 multiple character of, 250 
 regions, 243 
 
 stimulation, inhibitory effects of, 224 
 Cowper's gland, 443 
 histology of, 448 
 secretion of, 446 
 
 Crabs, regeneration of lost parts in, 496 
 Cranial nerves, afferent, 236 
 Creatin in muscle, 166 
 Creatinin of muscle, 167 
 Cretinism, sporadic, 289 
 Crico-arytenoid muscle, lateral, 426 
 
 posterior, 426 
 Cricoid cartilage, 425 
 Crico-thyroid muscles, 426 
 Criminals, weight of the brain in, 277 
 Crista acustica of the semicircular canals, 373 
 Critical period of nerves, 66 
 Cross-suturing of nerve-trunks, 201 
 Cruciate heat centre, 271 
 Cupola of the cochlea, 375 
 Curare, action of, 26 
 Currents of action in muscle, 150 
 
 in nerves, 153 
 of rest, 147 
 
 theories as to their cause, 148 
 Curve of fatigue with repeated single contrac- 
 tions, 113 
 
 of intensity of sleep, 294 
 of muscle contraction, effect of frequent exci- 
 tation on, 115 
 
 of muscular contractions, 100 
 of work for muscles, 140 
 Cutaneous sensations, cortical area for, 253 
 disturbance of, in disease, 403 
 
 Cutaneous sensations, varieties of, 390 
 
 temperature points, 398 
 Cytoplasmic changes in nerve-cells, 182 
 
 DANIELL cell, 28 
 
 Darwin's theory of heredity, 501 
 
 Death, definition of, 491 
 
 of the tissues, 492 
 
 somatic, 491 
 
 theory of, 492 
 Decidua graviditatis, 461, 471 
 
 menstrualis, 458, 461 
 
 reflexa, 472 
 
 serotina, 472 
 
 vera, 472 
 
 Decidual cells, 471 
 Defecation, cerebral control of, 270 
 
 reflex character of, 213 
 
 Degeneration after hemisection of the spinal 
 cord, 228 
 
 following removal of motor cortical areas, 244 
 
 of cut nerves, 78 
 
 of muscle after section of its nerve, 70 
 
 of nerve-cells, 199 
 
 of nerve-elements, 197 
 
 of nerves after section, 69 
 
 reaction of, 47, 54, 70 
 
 Deglutition, action of the epiglottis in, 422 
 Deiters' cells of the organ of Corti, 377 
 
 nucleus, 238 
 
 Demarcation currents of injured muscle, 148 
 Dendrites, definition of, 174 
 Dermal sensations, cortical area for, 253 
 path of conduction for, in the cord, 235 
 
 sensibility, area of distribution of the nerves 
 
 of, 231 
 
 Descending impulses, course of, 244 
 Desiccation of nerve, 59 
 Determinants of the germ-plasm, 503 
 Deutoplasm of the ovary, 450 
 
 ovarian, composition of, 451 
 Development of nerve-cells, 176 
 Dextrose, action of, in delaying rigor mortis, 
 
 164 
 Diapedesis of maternal leucocytes into the 
 
 fetus, 476 
 Diarthrosis, 415 
 Diaxonic nerve-cells, 178 
 Differential tones, 387 
 Diffusion of central nerve-impulses, 208 
 
 of impulses in the cord, influences affecting, 
 217 
 
 of nerve-impulses, peripheral, 218 
 Digastric muscle, 426 
 
 Digitalin, action of, on coagulation of muscle- 
 plasma, 164 
 on muscles, 137 
 
 Digitalis, action of, on nerves and muscles, 60 
 Digits, supernumerary, 494 
 Dioptric apparatus of the eye, defects of, 314 
 Dioptrics of the eye, 300 
 Dioptry, definition of, 304 
 Diphasic current of action, 152 
 Direction, judgments of, by means of auditory 
 sensations. 389 
 
 of the nerve-impulse, 184 
 Discord, 387 
 
 Discriminating sensibility of the skin for pres- 
 sure, 392 
 
 Discriminative sensibility for differences of tem- 
 perature, 397 
 
 Discus proligerus, 450, 454 
 Diseases, inheritance of, 498 
 Dispermy, 471 
 Dispersion of light, 316 
 
 Dissociation of the axial and focal adjustments 
 of the eye, 312 
 
INDEX TO VOLUME II. 
 
 511 
 
 Distance, judgments of, by means of auditory 
 sensations, 389 
 
 pen-option of, 354 
 
 visual perception of, 348 
 Disuse, effect of, on muscles, 77 
 Dizziness, 405 
 
 Dogs, removal of cerebrum in, 2<>7 
 Domestication, effect of, on menstruation in 
 
 animals, 460, !;.' 
 Dorsal nerve-roots, efferent fibres in, 203 
 
 roots, degeneration resulting from injury to, 
 287 
 
 spinal nerve-roots, number of fibres of, 230 
 Dreams, 293 
 I)u Bote-Reymond'i key, 30 
 
 law of stimulation, 32 
 
 theory of currents of rest, 148 
 Ductus cochlearis, structure of, 374 
 
 endolymphaticus, 373 
 
 venosus of the embryo, 476 
 Duration of electric currents, effect of, on their 
 
 irritating power, 46 
 Dynamic equilibrium, organs of, 407 
 Dyspepsia accompanying the climacteric, 490 
 Dyspnoea, effect of, on the iris, 324 
 
 EAR, analysis of composite tones by, 384 
 anatomy of, 362 
 
 discriminative sensibility of, for pitch, 385 
 fatigue of, 387 
 imperfections of, 388 
 membranous labyrinth of, 372 
 ossicles of, 365 
 sensibility of, in perception of time intervals, 
 
 388 
 
 Earth-worms, regeneration of lost parts in, 496 
 Efferent fibres of the optic nerves, 240 
 impulses in reflexes, co-ordination of, 214 
 neurones of the dorsal spinal nerve-roots, 203 
 
 of the spinal cord, 203 
 paths from the cortex, course of, 244 
 Ejaculation, 465 
 Ejaculatory duct, 447 
 Elasticity of muscle, 105 
 Electric currents, correlation of their duration 
 
 with histological structures, 47 
 effect of, on muscles, 61 
 on nerves, 62 
 the duration of, 46 
 their density, 41 
 galvanic, effect of, on normal human nerves, 
 
 51 
 influence of their direction in nerves, 48 
 
 of varying duration of, 47 
 methods of detecting, 145 
 spread of, in moist conductors, 41 
 stimulating effect of, 28 
 organ, 145 
 
 Electrical changes in the retina, 331 
 phenomena of muscle and nerve, 144 
 
 of nerves, interpretation of, 158 
 stimulation of nerve, 25 
 
 of nerves, law of, 32 
 Electrodes, shielded, 41 
 
 varieties of, 29 
 
 Electrostatic changes, stimulating action of, 42 
 Electrotonic changes of conductivity, 50 
 of irritability, 64 
 
 in human nerves, 65 
 twitch, 157 
 Electrotonus, 62 
 Embryo, nutrition of, 475 
 
 rate of growth of, 487 
 Emmetropia. 313 
 Emruetropic eye, 312 
 
 Encephala, classification of, according to 
 weight, 275 
 
 Encephalon, specific gravity of, 275 
 
 weight of, 274, 27.', 
 Kinl-hulhs. M-IISOIA . 388 
 Endolymph, 372 
 End-organs, importance of, in touch sensations, 
 
 386 
 
 transmission of excitation by means of, 82 
 Energy liberated in contracting museles, 138 
 Engelmann's theory of the nature of muscular 
 
 contraction, 105 
 
 Environment, influence of, on organisms, 493 
 Epididymis, 447 
 Epigenesis, theory of, 500, 504 
 Epiglottis, 421 
 
 Epinephrin, action of, on muscles, 138 
 Equilibrium of the body, definition of, 404 
 relation of the cerebellum to, 273 
 sense of, 404 
 Erection, 464 
 
 spinal centre for, 464 
 Erector clitoridis muscle, 464 
 penis, action of, in erection, 464 
 
 muscle, 449 
 
 Eserin, action of, on nerve and muscle, 60 
 Ether, action of, on coagulation of muscle- 
 plasma, 164 
 
 on conductivity of nerves, 93 
 effect of, on nerve-currents, 155 
 vapor, action of, on nerves, 60 
 Eustachian tube, 363 
 function of, 369 
 
 Excitability, changes in, during Wallerian de- 
 generation, 69 
 Exercise, effect of, on growth, 489 
 
 on muscular endurance, 76 
 Exhaustion of muscles, 72 
 Explosive consonants, 437 
 Extensibility of muscle, 105 
 External auditory meatus, 362 
 ear, anatomy of, 362 
 rectus muscle, 299 
 
 Extractives, nitrogenous, of muscle, 166 
 Extrapolar region, 62 
 Eye, abnormal positions of, after cerebellar 
 
 injury, 272 
 
 adaptation of, to light, 326 
 axes of rotation of, 299 
 chromatic aberration of, 316 
 constants, changes in, during accommodation, 
 
 311 
 
 defects in the dioptric apparatus of, 314 
 dioptric apparatus of, 300 
 mechanical movements of, 298 
 movements, binocular co-ordination in, 300 
 
 extent of, 298 
 muscles of, 299 
 
 in nervation of, 300 
 optical constants of, 303 
 
 power of, 304 
 positions of, 299 
 refractive media of, 302 
 
 surfaces of, 303 
 spherical aberration of, 315 
 
 FALLOPIAN tubes, 443, 456 
 False amnion, 473 
 Falsetto register of the voice, 433 
 Far-point.of vision, 312 
 Fatigue, cerebral anaemia from, 288 
 curve with repeated single contractions, 113 
 effect of, on height of contraction, 113 
 on muscular contraction, 130 
 on rigor caloris, 165 
 
 mortis, 160 
 
 from voluntary muscular contraction, 134 
 in nerve-fibres, lit") 
 of central nervous system, 289 
 
512 
 
 INDEX TO VOLUME II. 
 
 Fatigue of motor end-organs, 83 
 
 of muscle, 66, 70 
 recovery from, 73 
 
 of nerve-cells, 136, 191 
 
 of nerves, 75, 96 
 
 of retina, 344 
 
 relation of, to sleep, 291 
 
 theories of, 72 
 
 to auditory sensations, 387 
 Fats of muscle, 167 
 
 relation of, to muscular work, 74 
 Female births, relative number of, 483 
 
 pronucleus, 453 
 
 Females, rate of growth in, 488 
 Fenestra ovalis, 363 
 
 rotunda, 363, 375 
 
 use of, 376 
 
 Ferment, myosinogen-coagulating, 161 
 Fertilization, 466 
 Fetal membranes, 472 
 Field of vision, binocular rivalry of, 358 
 Fimbriae of the Fallopian tube, 456 
 Fish, bony, removal of cerebral hemisphere in, 
 
 263 
 Fishes, semicircular canals in, 407 
 
 visual accommodation in, 306 
 Flicker photometry, 345 
 Focal illumination of the eye, 320 
 Foci, conjugate, 302 
 
 principal, 302 
 
 Foramen ovale of the foetal heart, 476 
 Forced movements after section of the semi- 
 circular canals, 405 
 in frogs, 266 
 Fovea centralis, 327 
 Franklin's theory of color vision, 337 
 Frictionals, 437 
 Frogs, removal of cerebral hemisphere in, 264 
 
 striped muscle, time of single contraction in, 
 
 108 
 
 Frontal lobes of the hemispheres, effect of re- 
 removal of, 262 
 Fuhlsphare, cortical, 252 
 Fundamental tone, definition of, 383 
 
 GALVANI, LUIGI, 28 
 
 Galvanic current, action of, on conductivity, 94 
 contracture effect of, on muscles, 131 
 effect of, on muscles, 61 
 on nerves, 62 
 
 of making and breaking, 31 
 on normal human nerves, 51 
 opening and closing contractions with, 38 
 Galvanometers, 145 
 Galvanotonus, 54, 131 
 Gamogenesis, 440 
 Ganglion spirale of the ear, 376 
 Ganglion-cells, conduction in, 97 
 Gaseous exchanges in the brain, 288 
 Gases of muscle, 168 
 
 Geminal fibres of the pyramidal tracts, 245 
 Gemmules of the germ-plasma, 499 
 Genio-hyoid muscle, 426 
 Germinal spot of the ovary, 450 
 
 transmission of infectious diseases, 498 
 
 vesicle, structure of, 450 
 Germ -plasm as a basis of heredity, 499 
 
 continuity of, 502 
 
 definition of, 496 
 
 morphological nature of, 499 
 
 origin of, 499 
 
 Gestation, duration of, 478 
 Gland-cells, electric currents in, 145 
 Glands of Bartholini, 462 
 
 of Littre, 448 
 Glans penis, 449 
 Gliding movements in joints, 416 
 
 Glossopharyngeal nerve, gustatory function of, 
 
 410 
 Glossopharyngeus, central conducting paths of, 
 
 236 
 
 Glottis, 423 
 oedema of, 422 
 
 respiratory movements of,A29 
 Glycocoll in muscles, 167 ( 
 Glycogen of muscles, 167 x^^ 
 Golgi, organ of, in tendons, 402 
 Graafian follicle, 454 
 
 Graphic method of studying muscular contrac- 
 tions, 99 
 
 Gravity, influence of, on cerebral circulation, 287 
 Gray matter of the cerebrum, water contents of, 
 
 274 
 
 Growth after birth, 487 
 before birth, 486 
 increase of fibres of the cortex during, 282 
 
 of functional neurones during, 282 
 influence of sex on the rate of, 488 
 influences which modify, 489 
 of nerve-cells, 176 
 Gustatory nerves, 410 
 
 sensations, 411 
 Guttural consonants, 437 
 
 Gymnema silvestre, action of, on taste-nerves. 
 413 
 
 HEMOGLOBIN of muscle-serum, 166 
 Hair-cells of the crista acustica, 374 
 
 of the organ of Corti, 377 
 Hamulus, 376 
 Harmonic overtones, 386 
 Harmony, 387 
 
 Head register of the voice, 433 
 Hearing, 362 
 keenness of, 371 
 relation of, to speech, 431 
 Heart, changes in, due to pregnancy, 477 
 diphasic action currents in, 152 
 isolation of, 69 
 muscle, rate of conduction in, 89 
 
 rigor mortis of. 162 
 Heart-beat, voluntary control of, 214 
 Heat-centres, 271 
 
 Heat-production in contracting muscles, 138 
 in muscles, 142 
 in nerves, 96 
 in rigor mortis, 160 
 Heat-rays of ether, 331 
 Height of contraction, dependence of, on the 
 
 load, 111 
 
 effect of temperature on, 136 
 Helicotrema, 376 
 Hemianopsia, anatomical basis for, 240 
 
 from cortical lesions, 255 
 
 Hemisections of the cord alternating at differ- 
 ent levels, 230 
 
 Brown-Sequard's paralysis from, 233 
 degeneration resulting from, 228 
 effect of, on man, 233 
 
 on sensation and motion, 230 
 in animals, 234 
 physiological effects of, 234 
 Heredity, definition of, 493 
 
 theories of, 498 
 
 Hering theory of color vision, 336 
 Hermann's theory of currents of rest, 148 
 Heteromita, reproduction in, 440 
 Hinge-joints, 416 
 Histology of striped muscle, 104 
 Hofacker-Sadler law, 484 
 
 Holmgren method for testing color vision, 339 
 Horopter, 359 
 
 Human muscles, fatigue of, with artificial stim- 
 ulation, 134 
 
INDEX TO VOLUME II. 
 
 513 
 
 Hunger, 404 
 
 Hunger-centre, clinical evidence for, 404 
 Hydra, regeneration (if lost parts in, !!; 
 Hydrocyanic acid, action of, on coagulation of 
 
 muscle-plasma, 164 
 Hymen. lli'J 
 
 Hyo-glossus muscle, -I'.'ti 
 Hypenesthesia, homolateral, after hemisection 
 
 of the cord, 233 
 Hypermetropia. :u:{ 
 
 range of accommodation in, 314 
 Hypoxanthin of muscles, 167 
 
 IDANTS of the germ-plasm, 503 
 Idiomuscnlar contraction, 27, 92, 128 
 Idioplasm as a basis of heredity, 499 
 Ids of the germ -pi asm. ."><):; 
 Illusions, visual, in sizes of objects, 354 
 
 of space perception, 351 
 Immunity, inherited, 498 
 Impregnation, 466 
 Incus, 366 
 
 Independent irritability of muscle, 25 
 Index of refraction of the aqueous humor, 303 
 of the lens, 303 
 of the vitreous humor, 303 
 Indifferent point of polarized nerves, 64 
 Indirect vision, color sensations in, 333 
 Induced currents, making and breaking shocks 
 
 with, 40 
 prevention of spread of, 44 
 
 electric currents, stimulating effect of, 33 
 Induction apparatus, schema of, 33 
 Infections, intra-uterine, 498 
 Infectious diseases, germinal transmission of, 
 
 498 
 Inferior oblique muscle, 299 
 
 rectus muscle, 299 
 Inharmonic overtones, 386 
 Inheritance, facts of, 494 
 
 of acquired characters, 496 
 
 of diseases, 498 
 
 theories of, 498 
 Inhibition from cortical stimulation, 224 
 
 in the central nervous system, 224 
 Inorganic salts, relation of, to irritability, 59 
 Insanity, relation of brain-weight to, 278 
 
 variations of muscular tonus in, 220 
 Insect muscle, time of contraction in, 108 
 Intensity of visual sensations, 339 
 Intermedius nerve of Wrisberg, central path of, 
 
 236 
 Internal capsule, grouping of fibres in, 248 
 
 ear, anatomy of, 371 
 
 rectus muscle, 299 
 
 Intracranial pressure, relation of, to blood- 
 pressure, 287 
 Intraocular images, 320 
 Intrapolar region, 62 
 
 'Introductory contractions of a contraction 
 scries, 113 
 
 peak of tetanus curves, 124 
 Inversion of retinal images, 305 
 Invertebrates, conduction in the nerves of, 91 
 Involuntary muscles, rigor mortis of, 162 
 lon-proteid compounds of muscle, 168 
 Iris, dilator nerves of, 324 
 
 direct response to light by. :;_'! 
 
 innervation of. :;_':; 
 
 movements of. in accommodation, 309 
 rate of. :5-.Ti 
 
 muscles of. :',-}'.} 
 
 relation of. to spherical aberration, 315 
 Irradiation in the retina, .'>!!' 
 
 of nerve-impulses in the central nervous sys- 
 tem, 208 
 Irritability, definition of, 20, 23 
 
 33 
 
 Irritability, effect of Mood-supply on,r,t; 
 of constant currents on. ti-j 
 of repeated stimuli on, jr> 
 of muscle. _'."> 
 of nerve-fibres, 21 
 of nerves, 24 
 
 and muscles, conditions affecting, 55 
 effect of section on, <i! 
 of ova, 22 
 
 Irritants, classification of, .'.; 
 conditions determining their efficiency, 28 
 effect of, on irritability. v> 
 
 of variations in strength of, 39 
 relation of, to the response, 24 
 Ischio-cavernosi muscles, 449 
 Ischio-cavernosus, action of, in erection, 464 
 Isolated conduction in nerve-trunks, 79 
 Isometric contractions, definition of, 110 
 Isotonic contractions, definition of, 110 
 Isotropic substance of muscle-fibres, 104 
 
 JOINTS, classification of, 415 
 Jumping, 420 
 
 KARYOKINETIC figures in mature nerve-cells, 
 
 202 
 
 Katelectrotonus, 62 
 Kathodal contraction, 35 
 Kathode, physical, definition of, 52 
 
 physiological, definition of, 52 
 Keys, electric, 30 
 Knee-kick, reinforcement of, 222 
 Krause's membrane, 104 
 
 LABIA majora, 462 
 
 minora, 462 
 Labial consonants, 437 
 Labio-dental frictionals, 438 
 Labium tympanicum of the internal ear, 377 
 
 vestibulare of the limbus, 377 
 Labor, nature of, 481 
 
 stages of, 479 
 Labor-pains, 479 
 
 Labyrinth of the ear, anatomy of, 371 
 Lactation, ovulation during, 456 
 Lactic acid of muscles, 168 
 Lamina spiralis, 372 
 
 Laminae of medullary tube in the foetus, 20S 
 Laryngeal muscles, specific actions of, 428 
 
 nerve, recurrent, 428 
 
 superior, 428 
 Laryngoscope, 429 
 Larynx, cartilages of, 425 
 
 closure of, during muscular effort, 423 
 
 muscles of, 425 
 
 nerves of, 428 
 
 structure of, 421 
 Latent areas of the cortex, 261 
 
 characters, hereditary transmission of, 495 
 
 period, effect of temperature on, 136 
 
 of tension on, 110 
 of motor end-plates, 103 
 of muscle, in:; 
 of red muscle. 109 
 of retinal stimulation, 343 
 of simple muscular contraction, 102 
 "Law of .contraction," Pfluger's, 50 
 Law of stimulation of human nerves by battery- 
 currents, 54 
 Lecithin of nerves, 169 
 Lemniscus, medial, 226 
 
 sensory paths entering. -'':"> 
 Lens, changes in. during accommodation, 307 
 
 crystalline, changes in, with old age, 314 
 
 curvature^ ,,f. :\i\:\ 
 
 opacities in. :;_'! 
 
 refractive index of, M".': 
 
514 
 
 INDEX TO VOLUME II. 
 
 Lens, thickness of, 303 
 
 Lenticular ganglion, 311 
 
 Leucin, action of, on end-plates, 27 
 
 Leucocytes, movements of, 19 
 
 Leucophrys patula, reproduction of, 442 
 
 Life of the individual, stages of, 486 
 
 Ligaments of the malleus, 366 
 
 of the incus, 367 
 Light, changes in the retina produced by, 330 
 
 definition of, 298 
 
 dispersion of, 316, 332 
 
 monochromatic, 316 
 
 physical theory of, 331 
 
 rays of the luminiferous ether, 331 
 
 sensations, intensity of, 332, 339, 
 
 mechanism for the production of, 331 
 quality of, 332 
 Light-waves, lengths of, 332 
 Limbus of the spiral lamina, 377 
 Lingual frictiouals, 438 
 
 nerve, gustatory function of, 410 
 Linguo-palatal consonants, 437 
 Liquids, 436 
 Liquor amnii, 472 
 
 folliculi, 454 
 Listing's law, 299 
 Littre, glands of, 448 
 Load, effect of, on the contraction curve, 111 
 
 of muscle, effect of, on latent period, 110 
 Local signs of sensations, 394 
 Localization, cutaneous, variations of, 395 
 
 in the skin, theory of, 395 
 
 of cell-groups in the cerebral cortex, 241 
 
 of cortical cell-groups for different impulses, 
 252 
 
 of pain sensations, 399 
 
 of touch sensations, 394 
 
 power, relation of, to mobility, 394 
 Locomotion, 420 
 
 Locomotor ataxy, disturbance of equilibrium 
 in, 405 
 
 mechanisms, action of. 414 
 Long tracts of the cord, terminations of, 235 
 Long-reed register of the voice, 432 
 Loudness of the voice, factors determining, 430 
 
 physical cause of, 381 
 
 Luminiferous ether, rates of vibration of, 331 
 Luminosity, relative, of spectral colors, 340 
 Luminous sensations, intensity of, 339 
 Lustre in visual sensations, explanation of, 358 
 
 MACROCEPHALIC brains, weight of, 275 
 Macula acustica, 373 
 
 lutea, 327 
 
 Maculae acusticae, relation of, to static equilib- 
 rium, 407 
 
 Making contraction, point of origin, 35 
 " Making " shock, 31 
 Male births, relative number of, 483 
 
 pronucleus, 466 
 Males, rate of growth in, 488 
 Malleus, 365 
 
 ligaments of, 366 
 Mammary glands, 443, 462 
 
 in pregnancy, 477 
 Manubrium of the malleus, 365 
 Masticatory movements, effect of, on taste sen- 
 sations, 411 
 Mastoid antrum, 363 
 Maturation of germ-cells, significance of, 454 
 
 of nerve-cells, 177 
 
 of spermatozoa, 445 
 
 of the ovum, 451 
 Meatus auditorius internus, 373 
 Mechanical stimulation of nerve, 25, 56 
 
 strain, influence of, on neuroblasts, 176 
 
 work of muscular contraction, 138 
 
 Medial lemniscus, 226 
 
 Medullary sheath, development of, in the cen- 
 tral nervous system, 181 
 in the peripheral nerves, 180 
 significance of, 180 
 tube, fetal, 204 
 
 laminae of, in the foetus, 205 
 Medullation, central, progressive character of, 
 
 181 
 
 of nerve-fibres, significance of, 283 
 peripheral, 180 
 Medusae, rate of conduction in, 89 
 
 staircase contractions in, 112 
 Membrana basilaris, 374 
 flaccid a, 365 
 
 granulosa of the Graafian follicle, 454 
 reticulata, 378 
 tectoria, 377, 379 
 tympani, 364 
 
 Membrane of Reissner, 374, 379 
 Membranous labyrinth of the ear, 372 
 Menopause, 459, 490 
 Menstruation, 457 
 age of onset of, 459 
 cessation of, at the climacteric, 490 
 general disturbances accompanying, 459 
 in animals, 460 
 relation of ovulation to, 456 
 theory of, 460 
 Mental activity, relation of cerebral circulation 
 
 to, 288 
 
 Menthol, action of, on cold nerves, 398 
 Metabolism, intensity of,.in the brain, 288 
 Meynert's commissure, 238 " 
 Microcephalic brains, weight of, 275 
 Microcephalies, 268 
 Micturition, cerebral control of, 270 
 
 reflex character of, 213 
 Middle ear, 362 
 
 inflammatory disease of, 364 
 mechanism of, 368 
 muscles of, 369 
 Migration of neuroblasts, 176 
 Modiolus, 372 
 Molecular weight, relation of, to physiological 
 
 action, 60 
 
 Monochromatic light, 316 
 Mons Veneris, 462 
 Monstrosities, congenital, 494 
 
 origin of, 483 
 
 Morgagni, ventricles of, 422 
 Morula, 470 
 
 Motor areas, cortical serial arrangement of, 247 
 degeneration after removal of, 244 
 paralysis following removal of, 269 
 physiological characters of, 243 
 subdivision of, into centres, 247 
 cen-tres, degree of separateness of, 248 
 
 of the human cortex, 250 
 disturbance from hemisection of the cord, 230 
 end-plates, latent period of, 103 
 
 transmission of excitation by means of, 82 
 nerves, fatigue of, 96 
 
 rate of conduction in, 89 
 Movements of joints, varieties of, 416 
 of spermatozoa, 444 
 of the eyeball, 298 
 Multiple conceptions, 482 
 
 control from the cortex, 250 
 Muscae volitantes, 320 
 Muscle, accelerator urinae, 449 
 aryteno-epiglottidean, 426 
 arytenoid, 426 
 bulbo-cavernosus, 449 
 chemistry of, 159 
 ciliary, in accommodation, 309 
 crico-arytenoid, lateral, 426 
 
INDEX TO VOLUME II. 
 
 515 
 
 Muscle, crico-thyroid, 426 
 digastric, 426 
 elasticity of, 105 
 erector clitoridis, 464 
 external rectus, 299 
 fatigue of, 70 
 
 frog's, rate of conduction in, 89 
 gases of, 168 
 
 general physiology of, 17 
 geuio-hyoid, 4'_'<i 
 
 independent irritability of, 25 
 inferior oblique, 299 
 
 rectus, _'!! i 
 
 inorganic constituents of, 168 
 internal rectus, 299 
 
 limitation of the rate of stimulation in, 126 
 mylo-hyoid, 426 
 nitrogenous extractives of, 166 
 non-nitrogenous constituents of, 168 
 omo-hyoid, 425 
 posterior crico-arytenoid, 426 
 reaction of, 159 
 
 red, capacity for tetanic contraction, 127 
 retractor lentis, of fishes, 306 
 skeletal, sensory nerve-endings in, 402 
 specific gravity of, 159 
 sterno-hyoid, 425 
 sterno-thyroid, 425 
 striated, histology of, 104 
 
 optical properties of, 103 
 stylo-hyoid, 426 
 superior oblique, 299 
 
 rectus, 299 
 thyro-arytenoid, 426 
 external, 424 
 internal, 424 
 thyro-hyoid, 425 
 
 Muscle-contraction, Engelmann's theory of, 105 
 Muscle-plasma, 161, 163 
 
 Muscle-proteids, precipitation temperature of, 166 
 Muscles, absolute force of, 141 
 action of, upon the bones, 417 
 classification of, 18 
 currents of action in, 150 
 
 of rest in, 147 
 degeneration of, after section of motor nerves, 
 
 48, 54, 70 
 endurance of, 76 
 erectores penis, 449 
 human, fatigue of, with artificial stimulation, 
 
 134 
 
 rate of conduction in, 89 
 ischio-cavernosi, 449 
 of the eye, 299 
 of the iris, 323 
 of the middle ear, 369 
 rate of conduction in, 89 
 stapedius, 370 
 tensor tympani, 369 
 Muscle-serum, 166 
 Muscle-sounds, 132 
 Muscle-spindles, 402 
 Muscle-structure, relation of, to its contractility, 
 
 107 
 
 Muscle-tonus, 143 
 
 Muscular contractions, effect of drugs on, 137 
 of support on the height of, 122 
 of temperature on, 136 
 graphic record of, 99 
 influences affecting, 107 
 post-moi". 'in. 160 
 source of energy in, 74 
 effor: of larynx in, 423 
 
 inhi i cortical stimulation, 224 
 
 itions of antagonistic muscles 
 in, 418 
 
 Muscular sensations, cortical area for, 253 
 definition of, :;!o 
 
 effect of lieniisect ion of the cord on, '.'.;.") 
 in estimation of weights, -In:; 
 nature of, 401 
 
 path of conduction for, in the cord, 235 
 psychological value of, 391 
 work, effect of stimulants on. 7.~> 
 Musical sounds, characteristics of, 387 
 tones, beats of, 386 
 limits in the pitch of, 382 
 production of, 381 
 Mydriatics, 325 
 Mylo-hyoid muscle, 426 
 Myo-albumin, 166 
 Myo-albumose, 166 
 Myogen-fibrin, 164 
 Myoglobulin, 166 
 Myogram, 34 
 definition of, 100 
 
 effect of temperature on the form of, 136 
 Myograph, 35 
 description of, 100 
 double, of Heriug, 36 
 Myohffiinatiu, 166 
 Myopia, 313 
 
 range of accommodation in, 314 
 Myosin, 163 
 
 ferment, 161, 163 
 Myosin-fibrin, 164 
 Myosinogen, 163 
 
 temperature of heat coagulation of, 165 
 Myotics, 325 
 
 NAUSEA from disturbance of equilibrium, 405 
 Near-point of vision, 312 
 Negative after-images, 346 
 
 variation in muscles, rate of propagation, 152 
 
 relation of, to the contraction, 153 
 of muscle-currents, 150 
 of nerve-currents, 154 
 
 Nerve, chorda tympani, gustatory function of, 
 410 
 
 general physiology of, 17 
 
 glossopharyngeal, gustatory function of, 410 
 
 oculomotor, 323 
 
 recurrent laryngeal, 428 
 
 superior laryngeal, 428 
 Nerve-cells, atrophy of, from disuse, 195 
 
 changes with age in, 490 
 
 chemical changes in, 191 
 
 classification of, 177 
 
 connections between, 206 
 
 degeneration of the cell-bodies of, 199 
 
 diaxouic, 178 
 
 effect of exercise on, 76 
 
 fatigue of, 191 
 
 generation of impulses in, 188 
 
 growth of, 17o' 
 
 histological changes due to functional activity 
 in, 192 
 
 human, size of, 174 
 
 internal structure of, 179 
 
 maturation of. 177 
 
 morphology of, 17-'! 
 
 number of, in the central nervous system, 283 
 
 nutrition of, 190 
 
 nutritive control of, over nerve-fibres, 198 
 
 of animals, size of, 175 
 
 of spinal ganglia, development of, 178 
 
 peculiar!! ies of. 174 
 
 pyramidal. 17- 
 
 rate of discharge from, 189 
 
 regeneration of, -J"! 
 
 relation of size and function in, 17.~> 
 
 senescence of. i^-_>. nm 
 
 significance of the branches of, 186 
 
516 
 
 INDEX TO VOLUME II. 
 
 Nerve-cells, summation of stimuli in, 190 
 
 volume relations of, 175 
 
 Nerve-elements, primitive segmental arrange- 
 ment of, 205 
 
 Nerve-endings in the skin, 392 
 Nerve-fibres, classification of, 21 
 
 cortical, increase in the number of, during 
 growth, 282 
 
 fatigue in, 195 
 
 functions of, 21 
 
 reaction of, 170 
 Nerve-impulse, definition of, 25 
 
 direction of the passage of, 184 
 
 electrical variation accompanying, 183 
 
 generation of, 187 
 
 in peripheral nerves, 183 
 
 peripheral diffusion of, 218 
 
 reversed, in spinal ganglion-cells, 185 
 
 theories of, 97 
 
 transmission of, from neurone to neurone, 207 
 Nerve-muscle preparation, 34 
 Nerve-trunks, isolated conduction in, 79 
 Nerves, action currents in, 153 
 
 auditory, central path of, 237 
 
 chemistry of, 169 
 
 cross-suturing of, 200 
 
 current of rest in, 149 
 
 degeneration of, after section, 69, 78 
 
 fatigue of, 75 
 
 glossopharyngeal, central conduction paths 
 for, 236 
 
 in man, stimulation of, 51 
 
 law of stimulation of, with galvanic current, 50 
 
 limitation of the rate of stimulation in, 126 
 
 lingual, gustatory function of, 410 
 
 medullation of, 180 
 
 non-medullated, rate of conduction in, 90 
 
 of common sensation, central conduction paths 
 of, 230 
 
 of cutaneous sensation, central conduction 
 paths of, 233 
 
 of dermal sensation, area of distribution of, 
 231 
 
 of invertebrates, rate of conduction in, 91 
 
 of taste, nuclei of origin of, 236 
 
 of temperature, 397 
 
 of Wrisberg (intermedius), central path of, 
 236 
 
 olfactory, central paths of, 241 
 
 optic, central paths of, 238 
 
 rate of conduction in, 89 
 
 secondary degeneration of, 197 
 
 specific energy of, 232 
 
 trigeminal, central paths of, 238 
 
 vagus, course of the afferent fibres in, 236 
 Nervi erigentes, 464 
 Neuroblast, development of, 176 
 Neuro-keratin of nerves, 169 
 Neuromuscular spindle, 390 
 Neurone, definition of, 173 
 Neurones, 21 
 
 afferent, to the spinal cord, 203 
 
 changes in number and size of, 280 
 
 conduction in, 97 
 
 connection between, 206 
 
 double conduction in, 185 
 
 increase in number of, during growth, 282 
 
 internal structure of, 179 
 
 polarity of, 184 
 
 total number of, 283 
 Nicotin, action of, on end -plates, 27 
 
 on sympathetic ganglia, 219 
 Nissl method for study of nerve-cells, 195 
 
 substance, iron in, 191 
 
 of nerve-cells, 179 
 Nitrogen, free, of muscles, 168 
 Nitrogenous extractives of muscle, 166 
 
 Nodal point in the simplest dioptric system, 301 
 
 Noaud vital, 236 
 
 Noises, definition of, 388 
 
 Non-medullated nerves, rate of conduction in. 90 
 
 stimulation fatigue of, 180 
 Non-polarizable electrodes, 29 
 Nose, anatomy of, 408 
 
 respiratory tract of, 408 
 Nutrition of nerve-cells, 190 
 
 of the embryo, 475 
 Nutritive control of nerve-cell bodies over 
 
 nerve-fibres, 198 
 Nymphse, 462 
 Nystagmus after cerebellar injury, 272 
 
 OCULOMOTOR nerve, ciliary fibres of, 311 
 
 relation of, to the iris, 323 
 Odors, 410 
 
 (Edema of the glottis, 422 
 Old age of the central nervous system, 295 
 Olfactory area of the cortex, 253 
 
 cells, 408 
 
 epithelium, 408 
 
 nerves, central paths of, 241 
 
 paths to the brain, 409 
 
 stimuli, conditions affecting, 409 
 
 tracts, section of, in sharks, 264 
 Omo-hyoid muscle, 425 
 
 Ontogenetic development of nerve-cells, 177 
 Onychodromus, reproduction of, 442 
 Oocyte, 451 
 Ophthalmometer, 304 
 Ophthalmoscope, 326 
 
 Optic commissure, decussatiou of optic fibres in, 
 238 
 
 nerve, currents of action in, 154 
 
 nerve-fibres, number of, 330 
 
 nerves, central paths of, 238 
 cortical centres of, 240 
 efferent fibres of, 240 
 
 thalami, functions of, 271 
 Optical constants of the eye, 303 
 
 illusions in binocular vision, 359 
 of space perceptions, 351 
 
 properties of striated muscle, 103 
 Optograms, 330 
 Organ of Corti, structure of, 377 
 
 of Golgi in tendons, 402 
 Organization in the central nervous system, 285 
 
 relation of educability to the establishment 
 
 of, 286 
 
 Organs, growth of, 486 
 Oscillatory activity of the retina, 344 
 Os orbiculare of the incus, 366 
 Ossicles, auditory, 365 
 
 of the ear, movements of, 367 
 Otitis media, 364 
 Otoconia, 374 
 Otoliths, 374 
 Ova, 440 
 
 number of, in human ovary, 451 
 Ovaries, 443 
 
 effect of removal of, on menstruation, 459 
 
 structure of, 454 
 Overtones, definition of, 383 
 
 inharmonic, 386 
 Oviducts, 443, 456 
 Ovulation, 455 
 Ovum, chemistry of, 450 
 
 fertilization of, 466 
 
 human, structure of, 449 
 
 maturation of, 451 
 
 physiological properties of, 22 
 
 segmentation of, 467 
 
 stages in the maturation of, 452 
 Oxygen, storage of, in muscle, 169 
 
 supply, relation of, to irritability, 68 
 
INDEX TO VOLUME //. 
 
 517 
 
 PACINIAN body, 391 
 of the penis, 449 
 Pain nerves, evidence for the existence of, 232 
 
 points of the skin, 400 
 
 sensations of, 399 
 
 transferred or sympathetic, 400 
 Pale striped musele, physiological peculiarities 
 
 of, 109 
 
 Pangenesis, Darwin's theory of, 501 
 Papilla fpliata of rabbits, 410 
 Paradoxical contraction, 157 
 Parallax, use, of, in estimation of distance, 356 
 Paralysis after removal of motor areas, 269 
 
 agitans, 296 
 
 Hroxvu-Sequard's, 233 
 
 homolateral, after hemisection of the cord, 
 
 233 
 
 Paramo3cium, reproduction in, 440 
 Paramyosinogen, 163 
 
 temperature of heat coagulation of, 165 
 Paresis following removal of the cerebellum, 272 
 
 from inj ury to motor areas, 269 
 Partial tones, definition of, 383 
 Parturition, 479 
 
 spinal centre of, 481 
 
 Paths of conduction in the cord, clinical evi- 
 dence on, 234 
 Pendular vibrations, 381 
 Penis, 443 
 
 structure of, 448 
 Perilymph, 372 
 Periodic reflexes, 216 
 Peripheral nerves, medullation of, 180 
 
 reference of special sensations, 400 
 Pfliiger's law of contraction, 50 
 Phakoscope, 308 
 
 Phalangar process of the rods of Corti, 378 
 Phosphenes, pressure, 305, 331 
 Photometry, 345 
 
 Phrenic nerve, currents of action in, 154 
 Phylogenetic development of nerve-cells, 177 
 Physiological anode, definition of, 52 
 
 kathode, definition of, 52 
 
 observations on afferent paths in the cord, 229 
 
 rheoscope, 148, 151 
 
 salt solution, 59 
 Physostigmin, action of, on accommodation, 
 
 311 
 
 on the eye, 325 
 Pia mater, weight of, 274 
 Pigment epithelium, retinal, movements of, 330 
 
 retinal, relation of, to adaptation of the eye, 
 
 326 
 
 Pince myographique, 87 
 Pineal gland, calcification of, in old age, 491 
 Pinna of the ear, 362 
 Pitch, limits of perception of, 382 
 
 of musical tones, 381 
 
 of the voice, 430, 432 
 Pituitary membrane, 408 
 Pivot-joint, 417 
 Placenta, 474 
 
 Placenta! transmission of infectious diseases, 
 498 
 
 villi, 474 
 
 Plants, regeneration of lost parts in, 496 
 Pohl's mercury commutator, 36 
 Polar amphiaster of the ripening egg, 453 
 
 bodies, 451 
 
 of the ovum, 453 
 Polarity of neurones, 184 
 Polarization, after-effects of, 65 
 
 Shysiological, of neuroblasts. 176 
 arizing current, effect of, on conductivity, 50 
 on muscles. *!! 
 on nerves, ir.' 
 Pole-changers, 36 
 
 Polyspermy, 471 
 Pomum A 'lam i. l _' ". 
 Positive after-images, :;n; 
 Posterior association-centre, L'.")? 
 Post-gangliouic fibres of the sympathetic sys- 
 tem, 219 
 
 Posture sense, 399 
 Potassium salts, action of, on muscles, 138 
 
 relation of, to irritability, 59 
 Preformation theory of heredity, 500 
 Pre-ganglionic fibres of the sympathetic system, 
 
 Pregnancy, effects of, on the mother, 477 
 
 Presbyopia, 314 
 
 Pressure, effect of, on irritability of nerves, 56 
 
 influence of, on conductivity, 92 
 Pressure-points of the skin, 396 
 Pressure-sensations, fusion of, 394 
 Pressure-sense, delicacy of, 392 
 
 of the tympanic membrane, 382 
 Primary position of the eye, 299 
 
 taste-sensations, 412 
 Principal foci in a dioptric system, 302 
 
 point of the simplest dioptric system, 301 
 
 ray in the simplest dioptric system, 301 
 Processus brevis of the malleus, 365 
 
 gracilis sive folianus of the malleus, 365 
 Projection system of fibres, origin of, from cen- 
 tral cells, 205 
 Pronucleus, female, 453 
 
 male, 466 
 
 Proptosis after cerebellar injury, 272 
 Prostate glands, 443 
 histology of, 448 
 secretion of, 446 
 Prostatic fluid. 446, 448 
 Protagon of medullary substance, 170 
 Proteids of muscle, precipitation temperature 
 of, 166 
 
 of muscle-serum, 166 
 
 relation of, to muscular work, 74 
 Pseudoscopic vision, 318, 357 
 Psychical powers of the spinal cord, 215 
 Psycho-physic law, 340 
 
 of Fechner, 393 
 Puberty, 489 
 
 Pupil, changes during accommodation in, 311 
 in size of, 323 
 
 condition of, in sleep, 325 
 
 dilator nerves, 324 
 
 size of, in old age, 314 
 Pupillary reflex to light, 323 
 Purkinje-Sanson's images, 307 
 Purkinje's figure, 321 
 
 phenomenon, 340 
 
 explanation of, 342 
 Purposeful reflexes, 215 
 Pyramidal fibres, number of. 246 
 
 tracts in the cord, 245 
 geminal fibres of, 245 
 size of, 252 
 
 nerve-cells, development of, 178 
 
 QUADKUPLETS, 483 
 
 Quality of musical tones, 383 
 
 of the voice, 430 
 
 Quinine, action of, on coagulation of muscle- 
 plasma, 164 
 Quintuplets, 483 
 
 RACE, relation of brain-weight to, 278 
 Range of accommodation, normal, 312 
 Rate of conduction in muscles, 87 
 
 in nerves, 89 
 of excitation, effect of, on the contraction 
 
 curve. 111 
 of stimulation in voluntary contractions, 133 
 
518 
 
 INDEX TO VOLUME II. 
 
 Rate of stimulation, limitations of, for muscle, 
 
 126 
 
 required to tetanize, 125 
 Eeaction of degeneration, 48, 54, 70 
 
 of muscles, 159 
 
 of nerve-cells, 191 
 
 of nerve-fibres, 170 
 
 time, 291 
 
 in old age, 491 
 Eecurrent laryngeal nerves, 428 
 
 sensibility of the anterior roots, 204 
 Eed-striped muscles, physiological properties of, 
 
 109 
 
 Eeduced eye, 304 
 Eeflex actions, simple, 208 
 
 arc, 209 
 
 frog, 209 
 
 segmental reaction, 210 
 
 stinmlation of the nervous system, 208 
 
 tonus of muscular tissues, 220 
 Eeflexes, co-ordinated, 211 
 
 co-ordination of the efferent impulses in, 214 
 
 effect of location of stimulus on, 209 
 of strength of stimulus on, 210 
 
 from the isolated cord in man, 213 
 lumbar cord, 213 
 
 in different vertebrates, 212 
 
 in man, 216 
 
 latent period of, 211 
 
 of a purposeful character, 215 
 
 periodic, 216 
 
 simple, 208 
 
 spinal, 212 
 
 reinforcement of, 222 
 
 summation of stimuli in, 211 
 
 voluntary control of, 214 
 Eefractive index of the aqueous humor, 303 
 of the lens, 303 
 of the vitreous humor, 303 
 
 media of the eye, 302 
 
 surfaces of the eye, 303 
 "Eefractory period" of nerves, 57, 66 
 Eegeneration of lost parts, 496 
 
 of nerves, 78, 199 
 Eegisters of the voice, 432 
 Eegular astigmatism, 317 
 Eeinforcement of reflexes, 222 
 
 of the knee-kick, 222 
 Eeissner, membrane of, 374 
 Eejuvenescence by sexual reproduction, 442 
 Eelaxation of muscle, nature of, 99 
 Eeproduction, asexual, 439 
 
 of leucophrys patula, 442 
 
 of onychodromus, 442 
 
 of stylonychia, 442 
 
 sexual, 440 
 
 elements of, 440 
 
 theory of, 441 
 
 Eeproductive organs, female, 449 
 internal secretions of, 462 
 male, 443 
 
 process, 463 
 
 Eesonance of the ear, 388 
 Eesonants, 436 
 
 Eesonators, analysis of sounds by, 385 
 Eete vasculosum of the testis, 447 
 Eetina, changes produced by light in, 330 
 
 circulation in, 322 
 
 histology of, 329 
 
 oscillatory activity of, 344 
 
 space perceptions by, 348 
 
 structure of, 327 
 Eetinal currents, 331 
 
 images, inversion of, 305 
 size of, 305 
 
 stimulation, after-effect of, 345 
 fatigue in, 344 
 
 Eetinal stimulation, latent period of, 343 
 laws of, 343 
 
 rise to maximum for different colors, 343 
 vessels, demonstration of, 321 
 Eeversion to ancestral characters, 495 
 Eheocord, 41 
 Eheonome, 31 
 
 Eheoscope, physiological, 148 
 Eheoscopic frog, 148 
 Eheostat, 40 
 Ehinencephalon, 241 
 Eigor caloris, 57, 164 
 contracture in, 128 
 effect of fatigue on, 165 
 mortis, 159 
 
 chemical changes accompanying, 162 
 contracture of, 128 
 disappearance of, 162 
 influence of the nervous system on, 220 
 nature of changes in, 161 
 Eima glottidis, 423 
 respiratoria, 423 
 vocalis, 423 
 Eitter's opening tetanus, 37, 61 
 
 tetanus, 131 
 
 Eod-and-cone layer, function of, 327 
 Eod-pigment. 339. See also Visual purple. 
 Eods and cones, function of, 341 
 
 number of, 330 
 of Corti, 377 
 retinal, function of, 341 
 Eotation, movements of, 416 
 Eunning, 421 
 Eut of animals, 460 
 
 SACCULUS of the internal ear, 373 
 
 Saccus endolymphaticus, 373 
 
 Saddle-joint, 416 
 
 Salivary secretion, cerebral control of. 270 
 
 Salts, inorganic, relation of, to irritability, 58 
 
 of muscle, 168 
 of heavy metals, action of, on nerve and 
 
 muscle, 60 
 
 Santorini, cartilage of, 422, 425 
 Sarcode of sponges, contractility of, 20 
 Sarcolactic acid, formation of, in rigor mortis, 
 
 161, 162 
 
 in clotting of muscle-plasma, 164 
 relation of, to fatigue contracture, 131 
 Sarcoplasm, 104 
 Saturation of colors, 342 
 Scala media, 375 
 tympani, 372, 375 
 vestibuli, 372, 375 
 Schneiderian membrane, 408 
 Scrotum, 443 
 
 Secondary degeneration of nerves, 197 
 position of the eye, 299 
 tetanus, 150 
 
 Secretion, salivary, cerebral control of, 270 
 Secretory nerves, fatigue of, 96 
 Segmental arrangement of nerve-elements, 206 
 
 reactions, reflex, 210 
 Segmentation, 467 
 Segmentation -centrosomes, 469 
 Segmentation-nucleus, 466 
 Semen, composition of, 445 
 Semicircular canals, membranous, 373 
 of the bony labyrinth, 371 
 relation of, to equilibrium, 405 
 section of, 405 
 Seminal vesicles, 443 
 function of, 448 
 secretion of, 446 
 Semi-vowels, 436 
 Senescence of nerve-cells, 182 
 
 of the central nervous system, 295 
 
INDEX TO VOLUME II. 
 
 519 
 
 Senescenee, phenomena of, 486 
 Sensation, cutaneous, definition of, 390 
 imiMMilar, definition of, 390 
 of after- pressure, :'>!'! 
 of light, mechanism for the production of, 
 
 331 
 
 of temperature. .">!'? 
 Sense of equilibrium, 404 
 
 of touch, 392 
 Sensory areas of the cortex, determination of, 
 
 253 
 conducting paths in the spinal cord, 234 
 
 continuation of, in the brain, 235 
 cortical areas in man, 255 
 
 motor responses from. _'.".'! 
 relative functional importance of, 270 
 disturbance from hemisection of the cord, 230 
 impulses, path of, in the central nervous sys- 
 tem, 2-,'U 
 relation of, to the maintenance of the erect 
 
 posture, 419 
 
 nerve-endings in skeletal muscle, 402 
 in tendon, 402 
 in the skin, 391 
 
 nerves, rate of conduction in, 91 
 paths, degeneration of, after section of the 
 
 dorsal roots, 227 
 
 in the central nervous system, 226 
 regions of the cortex cerebri, 252 
 stimulation, relation of, to sleep, 291 
 Sex, characters of, 442 
 
 of offspring, determination of, 483 
 origin of, 441 
 
 relation of brain-weight to, 276 
 Sexual characters, 442 
 glands, accessory, 445 
 organs, 443 
 reproduction, 440 
 
 congenital variations resulting from, 501 
 theory of origin of, 441 
 Shark, reflexes in, 212 
 
 removal of cerebral hemispheres in, 263 
 Shrapnell's membrane, 365 
 Siamese twins, 483 
 Simple muscular contraction, duration of, 102, 
 
 108 
 
 explanation of, 101 
 Simultaneous contrast, 347 
 Singing voice, 434 
 Size, increase of the embryo in, 487 
 
 of nerve-cells, 175 
 Skiascopy, detection of astigmatism by means 
 
 of, 319 
 
 Skin, tactile areas of, 395 
 Sleep, 291 
 cause of, 292 
 
 condition of the pupils in, 325 
 curve of intensity of, 294 
 effects of loss of, 295 
 responsiveness to stimuli in, 293 
 Smell, 408 
 
 comparative physiology of, 409 
 subjective sensations of, 410 
 Smooth muscle, rate of conduction in, 89 
 Snails, regeneration of lost parts in, 496 
 Somatic death, 491 
 Somatoplasm, definition of, 496 
 Somatopleure, 472 
 Sound, physical, 381 
 Sounds, quality of. 383 
 Sound-waves, amplitude of, 381 
 composite, 384 
 limits of perception of, 382 
 production of, 381 
 Space illusions, 354 
 
 Space-perception from visual sensations, 347 
 Specific energies of nerves, doctrine of, 232, 399 
 
 Specific energy of the optic nerve. :;:;i 
 gravity of muscle, !.">!) 
 of the eneephalon, -JT.^ 
 
 of the nervous system at different ages, 284 
 nerve-energy, doctrine of. _>:;_', :;i! 
 Spectral colors, incomplete saturation of, 848 
 Spectrum, 3:;-j 
 
 luminous intensity of the colors of, 340 
 top, Benham's, 344 
 Speech, dependence of, on hearing, 431 
 
 elements of, 433 
 Speech-centre, 257 
 Sperm atids, 445 
 Spermatocytes, 445 
 Spermatozoa, 440 
 contractility of, 20 
 discovery of, 443, 498 
 entrance of, into the uterus, 465 
 locomotion of, 465 
 maturation of, 445 
 movements of, 444 
 structure of, 443 
 Sperm-aster, 467 
 Sperm-nucleus, 466 
 Spermiue, 445 
 Spherical aberration, 315 
 Sphincter aui, cerebral control of, 270 
 iridis, 325 
 
 urethrtE, contraction of, in erection, 464 
 Spinal cord, afferent paths of, 229 
 central neurones of, 203 
 degeneration of, from hemisection, 228 
 efferent neurones of, 203 
 motor tracts of, 245 
 reflexes in man, after section of, 213 
 schematic cross-section of, 202 
 weight of, 274 
 
 ganglion-cells, development of, 178 
 nerve-roots, section of, 198 
 Spiral ganglion of the ear, 376 
 ligament of the cochlea, 379 
 Staircase contractions, 66, 112 
 relation of, to tetanus, 124 
 Standing, 418 
 Stapedius muscle, 370 
 Stapes, 367 
 
 Starvation, effect of, on the nervous system, 289 
 Static equilibrium, organs of, 407 
 Stature, relation of brain -weight to, 276 
 Stenson's experiment, 67 
 Stereoscope, 356 
 Sterno-hyoid muscle, 425 
 Sterno-thyroid muscle, 425 
 Stimulants, effect of, on muscular work, 75 
 Stimulation fatigue of non-medullated nerves, 
 
 180 
 
 of the cortex, 241 
 Stimuli, chemical, of muscle, 131 
 classification of, 23 
 
 conditions determining efficiency of, 28 
 effect of changing intensity of, 32 
 of their repetition on irritability, 65 
 of varying strength of, 39 
 galvanic, con tractu re effect of, in muscles, 131 
 variations in intensity of, 31 
 Strabismus, 300 
 Striae acusticse, 237 
 gravidarum, 477 
 Strychnin, action of, on diffusion of impulses in 
 
 the cord, 217 
 on end-plates. 27 
 on sympathetic ganglia, 219 
 tetanus, 217 
 Stylo-hyoid muscle, 426 
 Stylonychia, reproduction of, 442 
 Successive contrast, 346 
 Sugar of muscles, 167 
 
520 
 
 INDEX TO VOLUME II. 
 
 Sugar, use of, in muscular work, 74 
 Summation of contractions in muscle, 121 
 
 of stimuli in nerve-cells, 190 
 
 in reflex action, 211 
 Superior laryugeal nerve, 428 
 
 oblique muscle, 299 
 
 rectus muscle, 299 
 Supernumerary digits, 494 
 Sustentacular cells of the crista acustica, 374 
 Suture of skull-bones, 414 
 Swallowing, action of the epiglottis in, 422 
 Sylvian heat-centre, 271 
 
 Sympathetic ganglia, action of nicotin on, 219 
 of strychnin on, 219 
 
 nerves to the iris, 324 
 
 pains, 400 
 
 system, connection of, with the cerebro-spinal, 
 
 218 
 
 post-ganglionic fibres of, 219 
 pre-ganglionic fibres of, 218 
 
 vibration, 385 
 Symphysis, 414 
 Syndesmosis, 414 
 Synoyial fluid, 415 
 Syphilis, hereditary transmission of, 498 
 
 TACTILE areas of the skin, 395 
 
 corpuscle, 390, 392 
 Taste, nerves of, 410 
 
 organs of, 410 
 Taste-buds, 410 
 
 Taste-nerves, nuclei of origin of, 236 
 Taste-perceptions, conditions affecting, 411 
 Taste-sensations, conditions which influence, 411 
 
 primary, 412 
 
 distribution of, 413 
 Taurin in muscles, 167 
 Tea, stimulating action of, 75 
 Tectorial membrane, 379 
 Tegmen of the tympanum, 364 
 Temperature, effect of, on muscular contraction, 
 136 
 
 influence of, on conductivity, 92 
 on irritability, 56 
 on rigor mortis, 161 
 
 limits of muscular contraction, 136 
 
 nerves, 397 
 
 of the blood from the brain, 288 
 
 rise of, from lesions of corpora striata, 271 
 
 sense, 397 
 
 spots of the skin, 398 
 Tendon reflexes after cerebellar injury, 272 
 
 sensory nerve-endings in, 402 
 Tension, effect of, on contraction curve, 109 
 on irritability of nerves and muscles, 56 
 on latent period, 110 
 Tensor tympani muscle, 369 
 Tentacles of Actiniae, contractility of, 20 
 Terminal arborizations, definition of, 174 
 Tertiary positions of the eye, 299 
 Testis, 443 / 
 
 ducts of, 447 - 1 
 
 histology of, 446 
 Tetanic contractions, height of, 120 
 
 relative intensity of, 126 
 Tetanomotor, 56 
 Tetanus, 66 
 
 analysis of, 123 
 
 complete, 120 
 
 curves, introductory peaks of, 124 
 
 explanation of, 121 
 
 strychnin poisoning, 217 
 
 incomplete, 117 
 
 normal physiological, 132 
 
 of the muscles, 127 
 
 rate of stimulation required for, 125 
 
 Bitter's, 37, 61, 131 
 
 Tetanus, secondary, 150 
 
 voluntary, 133 
 
 Wundt's, 37 
 closing, 61 
 Thalamus, cortical connections of, 271 
 
 heat-centre of, 271 
 Thermal energy liberated in muscle, 141 
 
 stimulation of nerve, 25 
 Thermopile, 142 
 
 Thermotaxis, relation of cerebrum to, 270 
 Thirst, 404 
 
 Thyro-arytenoid muscles, 424, 426 
 Thyro-hyoid muscle, 425 
 Thyroid cartilage, 425 
 
 gland, relation of, to growth of the central 
 
 nervous system, 289 
 Tigroid of nerve-cells, 179 
 Timbre of musical tones, 383, 387 
 Time intervals, perception of, by the ear, 388 
 Tissue death, 492 
 Tissues, growth of, 486 
 Tobacco smoke, action of, on nerves, 60 
 Tones, combinational, 387 
 
 differential, 387 
 
 fundamental, 383 
 
 loudness of, 381 
 
 pitch of, 381 
 
 simple, 381 
 
 Tongue, distribution of taste-sensations on, 413 
 Tonus, muscular, in the insane, 220 
 reflex origin of, 220 
 
 of muscles, 143 
 Touch illusions, 396 
 
 sensations, 392 
 
 localization of, 394 
 Tractus solitarius, 236 
 Tremors, 132 
 
 Trigeminal nerves, central paths of, 238 
 Triplets, 483 
 Trophic impulses to muscles, 70 
 
 influence of neurones on one another, 197 
 
 nerves of the muscles, 70 
 Tubuli recti of the testis, 447 
 Turtle's striped muscle, time of contraction in, 
 
 108 
 
 Twins, 482 
 Tympanic membrane, 364 
 
 effect of destruction of, 370 
 pressure-sensations of, 382 
 vibrations of, 370 
 Tympanum, 363 
 
 mechanics of, 368 
 
 ULTIMUM moriens, 492 
 Umbilical arteries, 474 
 
 vein, 474 
 
 Umbo of the tympanic membrane, 365 
 Unconsciousness, 293 
 Unipolar excitation for localized excitation, 45 
 
 nerve-cells, development of, 178 
 
 stimulation, 30 
 
 principles of, 43 
 Urea in muscles, 167 
 Urethra, 443 
 
 structure of, 448 
 Uric acid in muscles, 167 
 Uterus, 443, 456 
 Utriculus of the internal ear, 373 
 
 VAGINA, 443, 462 
 
 Vagus, central path of the afferent fibres in, 
 
 236 
 nerve, fatigue of, 96 
 
 rate of conduction in, 90 
 
 Variation of the offspring in reproduction, 500 
 Variations, somatic, classification of, 497 
 Vas deferens, 447 
 
INDEX TO VOLUME II. 
 
 521 
 
 Vasa-deferentia, 443 
 
 efferentia of the testis, 447 
 Vaso-motor nerves of the cranial vessels, 286 
 Ventral nerve-roots, number of fibres of, 230 
 Ventricles of Morgagni, 4 V J^ 
 
 of the brain, capacity of, 274 
 Ventricular bauds, 422 
 
 Veratria, action of, on coagulation of muscle- 
 plasma, 164 
 
 on muscular contraction, 129, 137 
 on nerves and muscles, 60 
 effect of, on muscular contraction, 128 
 Vertigo in diseases of the ear labyrinth, 406 
 Vestibular root of the auditory nerve, central 
 
 path of, 237 
 
 Vestibule of the bony labyrinth, 371 
 Vibrations of the tympanic membrane, 370 
 
 transmission of, through the labyrinth, 376 
 Vision, binocular, 356 
 far-point of, 312 
 indirect, 341 
 near-point of, 312 
 pseudoscopic, 357 
 stereoscopic, 357 
 Visual area of the cortex, 253 
 impulses, place of origin of, in the retina, 
 
 327 
 
 judgments of distance, 348 
 of size, 350 
 
 and distance, 354 
 purple, 330 
 
 adaptation of the eye by, 326 
 sensation, intensity of, 339 
 Vitelliue membrane, absence of, in human ova, 
 
 450 
 Vitreous humor, opacities in, 321 
 
 refractive index of, 303 
 Vocal cords, false, 422 
 
 true, 423 
 Voice, 430 
 
 changes at puberty in, 489 
 effect of age on, 431 
 pitch of, 432 
 registers of, 432 
 Voice-production, 421 
 mechanism of, 431 
 Voices, classification of, 433 
 Volta, 28 
 Voltaic pile, 28 
 
 Voluntary muscular contractions, fatigue of. 1.11 
 
 tetanic character of. i:;:; 
 rein-lions, all'erent paths of. 
 anatomical mechanism of, 226 
 compared with reflex, 225 
 von Gudden's commissure, 238 
 Vorticella, movements of, 20 
 Vowel-sounds, 431 
 
 differences in quality of, 385 
 production of, 434 
 Vulva, 443, 462 
 
 WALKING, 420 
 
 Walleriau degeneration, changes of excitability 
 
 in, 69 
 
 of nerve-fibres, 197 
 of nerves, 69 
 
 Water, percentage of, in brain and cord, 274 
 pure, toxic action of, on nerves and muscles. 
 
 58 
 Weber's law, 340 
 
 applied to pressure-sensations, 393 
 Weight, increase of, in the embryo, 487 
 of the brain and spinal cord, 274 
 decrease of, in old age, 296 
 relation of, to social environment, 277 
 of the child at birth, 487 
 Weissmann's theory of heredity, 502 
 Whispering, 436 
 
 Whistling register of the voice, 433 
 White matter of the central nervous system, 
 
 water contents of, 274 
 Womb, 443, 456 
 Work done by contracting muscles, conditions 
 
 affecting, 139 
 
 by muscular contraction, curve of, 140 
 Worms, segmental nervous system of, 212 
 Wrisberg, cartilages of, 422, 425 
 Wundt's closing tetanus, 37, 61, 131 
 
 XANTHIN of muscles, 167 
 YOUNG-HELMHOLTZ theory of color vision, 335 
 
 ZOLLNER'S lines, 351 
 Zona pellucida, 454 
 
 of the ovum, 450 
 radiata of the ovum, 449, 450 
 
GENERAL INDEX. 
 
 ABDOMINAL muscles, action of, in vomiting, i. 
 
 387 
 n >piratory action of, i. 407 
 
 respiration, definition of, i. 398 
 Absolute muscular force, ii. 141 
 Absorbents, i. 318 
 Absorption, effect of alcohol on, i. 535 
 
 in the small intestine, i. 313 
 
 in the stomach, i. 312 
 
 mechanism of, i. 312 
 
 nature of process, i. 27 
 
 of fats, i. 317 
 
 of gases by liquids, i. 414 
 
 of proteids, i. 316 
 
 of sugars, i. 317 
 
 of water and salts, i. 318 
 
 part played by leucocytes in, i. 48 
 
 paths of, i. 311 
 
 spectrum of oxy haemoglobin, i. 41 
 Accelerator centre, cardiac, i. 177 
 respiratory, i. 457 
 
 nerves of the heart, i. 167, 169 
 
 urinae muscle, ii. 449 
 Accessory articles of the diet, i. 357 
 
 thyroids, i. 268 
 Accommodation, ii. 306 
 
 associated movements of, ii. 311 
 
 astigmatic, ii. 310 
 
 dissociation of, from convergence, ii. 312 
 
 influence of drugs on, ii. 311 
 
 in old age, ii. 314 
 
 mechanism of, ii. 309 
 
 nervous mechanism of, ii. 311 
 
 normal range of, ii. 312 
 
 range of, in hypermetropia, ii. 314 
 in myopia, ii. 314 
 
 relation of, to perception of distance, ii. 356 
 
 voluntary character of, ii. 311 
 Acetic acid, i. 536 
 
 Acetone, relation of, to fat metabolism, i. 539 
 Acetonitril, i. 542 
 Acetonuria, i. 537 
 Acetyl-acetic acid, i. 537 
 Acetyl-propionic acid, i. 538 
 Achromatic lenses, ii. 316 
 Achromatism of the eye, ii. 316 
 Achroodextrin, i. 2a5, 566 
 Acid, acetic, i. 536 
 
 acetyl-acetic, i. 537 
 
 acetyl-propionic, i. 538 
 
 amido-acetic, i. 537 
 
 amido-ethyl-sulphonic, i. 543 
 
 a-aiuido-a-thiopropionic, i. 546 
 
 aspartic, i. 557 
 
 benzoic, i. 569 
 
 butyric, i. 539 
 
 capric. i. 541 
 
 caproic, i. 540 
 
 caprylir, i. 541 
 
 carbamic, i. 548 
 
 carbolic, i. 569 
 
 carbonic, chemical structure of, i. 545 
 
 choleic, i. 543 
 
 cholic, i. 543 
 
 Acid, chondroitic. i. 578 
 
 cynurenic, i. 571 
 
 diamido-acetic, i. 551 
 
 o--diamido-caproic, i. 552 
 
 diamido-valeric, i. 552 
 
 dithio-diamido-ethideue lactic, i. 547 
 
 fellic, i. 543 
 
 formic, i. 534 
 
 glutamic, i. 558 
 
 glycerin phosphoric, i. 559 
 
 glycuronic, i. 567 
 
 hippuric, i. 339, 569 
 
 homogentisic, i. 570 
 
 hydriodic, i. 509 
 
 hydrobromic, i. 509 
 
 hydrochloric, i. 507 
 
 hydrocumaric, i. 570 
 
 hydrofluoric, i. 510 
 
 iso-butyl amido-acetic, i. 540 
 
 iso-valerianic, i. 539 
 
 lactic, i. 545 
 
 levulic, i. 538 
 
 malic, i. 558 
 
 mercapturic, i. 547 
 
 metaphosphoric, i. 514 
 
 methyl amido-acetic, i. 538 
 
 monobasic fatty, i. 532 
 
 nucleic, i. 579 
 
 oleic, i. 560 
 
 orthophosphoric, i. 514 
 
 oxalic, i. 557 
 
 oxaluric, i. 555 
 
 oxybutyric, i. 548 
 
 oxyphenyl-acetic, i. 570 
 
 oxyphenyl-amido-propionic, i. 570 
 
 palmitic, i. 541 
 
 parabanic, i. 555 
 
 phenaceturic, i. 569 
 
 phenyl-acetic, i. 569 
 
 propionic, i. 538 
 
 salts of muscle, ii. 168 
 
 sarco-lactic, i. 546 
 
 silicic, i. 519 
 
 stearic, i. 541 
 
 succinic, i. 557 
 
 sulphuric, i. 506 
 
 sulphurous, i. 506 
 
 thiolactic, i. 547 
 
 thymic, i. 579 
 
 uric, i. 322, 338, 554, 557 
 Acidity of worked muscles, ii. 168 
 Acids, action of, on nerves and muscles, ii. 60 
 
 effect of, on pancreas, i. 236 
 Acinus, definition of, i. 212 
 Acquired characters, inheritance of, ii. 496 
 
 variations, ii. 500 
 Acromegaly, i. 273 
 
 Actinic rays of the luminiferous ether, ii. 331 
 Action current, diphasic, ii. 152 
 in the heart, ii. l.Vj 
 in the muscles, ii. 150 
 in the nerves, ii. 153, 183 
 Adamkiewicz reaction for proteids, i. 576 
 Adam's apple, ii. }-'."> 
 
 523 
 
524 
 
 GENERAL INDEX. 
 
 Addison's disease, i. 271 
 Adenin, i. 339, 554 
 Adipocere, i. 541, 560 
 
 Adrenal bodies, internal secretion of, i. 272 
 removal of, i. 271 
 secretory nerves of, i. 272 
 
 extract, action of, on muscles, ii. }38 
 
 physiological action of, i. 271 
 Aerial perspective, ii. 355 
 
 Afferent impulses, effect of, on irritability of the 
 central nervous system, ii. 223 
 
 neurones of the spinal cord, ii. 203 
 
 paths in the cord traced electrically, ii. 230 
 traced histologically, ii. 229 
 traced physiologically, ii. 229 
 
 respiratory nerves, i. 460 
 After-birth, ii. 481 
 
 After-effect of retinal stimulation, ii. 345 
 After-images, ii. 346 
 After-loading of muscles, ii. 110 
 After-pressure, sensation of, ii. 394 
 Agamogenesis, ii. 439 
 
 Age, changes in organization of the central ner- 
 vous system with, ii. 284 
 
 influence of, on heat production, i. 482 
 on nerve-cells, ii. 490 
 on pulse-rate, i. 121 
 on respiration, i. 425 
 on visual accommodation, ii. 314 
 
 relation of body-temperature to, i. 469 
 of brain-weights to, ii. 276 
 of menstruation to, ii. 459 
 
 specific gravity of the nervous system with 
 
 changes in, ii. 284 
 Air, alveolar, composition of, i. 413 
 
 atmospheric, composition of, i. 410, 413 
 
 complemental, i. 427 
 
 expired, composition of, i. 410 
 
 inspired, composition of, i. 410 
 
 in the lungs, renewal of, i. 413 
 
 passages, obstruction of, i. 452 
 
 residual, i. 427 
 
 respiratory changes in, i. 410 
 
 stationary, i. 427 
 
 suction of, into veins, i. 97 
 
 supplemental, i. 427 
 
 tidal, volume of, i. 426 
 
 variations in the composition of, i. 435 
 Albinos, condition of the internal ear in, ii. 407 
 Albuminates, i. 577 
 Albuminoids, digestion of, in the stomach, i. 297 
 
 enumeration of, i. 577 
 
 nutritive value of, i. 277, 349 
 
 properties of, i. 579 
 
 protection of proteids by, i. 349 
 
 tryptic digestion of, i. 304 
 Albuminous glands, i. 216 
 Albumins, properties of, i. 577 
 Albumose injections, effect of, on blood-coagu- 
 lation, i. 62 
 Alcaptonuria, i. 570 
 Alcohol, absorption of, in the stomach, i. 313 
 
 action of, on conductivity of nerves, ii. 93 
 
 amyl, i. 539 
 
 cerotyl, i. 540 
 
 cetyl, i. 540 
 
 effect of, on nerve-currents, ii. 156 
 
 ethyl, i. 535 
 
 fumes, action of, on nerves, ii. 60 
 
 melicyl, i. 540 
 
 nutritive value of, i. 358 
 
 physiological action of, i. 357, 535 
 
 propyl, i. 536, 538 
 
 stimulating action of, ii. 75 
 
 toxic effects of, i. 359 
 Alcoholic fermentation, i. 535 
 Alcohols, monatomic, i. 531 
 
 Aldehydes, general properties of, i. 534 
 
 Aldoses, i. 561 
 
 Alimentary canal, movements of, i. 369 
 
 principles, i. 276 
 
 Alkalies, action of, on nerves and muscles, ii. 60 
 Allantoic arteries, ii. 474 
 
 vein, ii. 474 
 Allantoin, i. 555 
 Allantois, ii. 474 
 Allochiria, ii. 400 
 Alloxuric bases, i. 338, 339, 552 
 Altitude, effect of, on the number of red cor- 
 puscles, i. 46 
 Alveolar air, compostion of, i. 413 
 
 capacity, i. 427 
 
 tension of carbon-dioxide, i. 413 
 
 of oxygen, i. 413 
 
 Alveolus, glandular, definition of, i. 212 
 Amido-acetic acid, i. 537 
 Amido-acids, properties of, i. 538 
 Amines, definition of, i. 541 
 Ammonia, action of, on nerves, ii. 60 
 
 inhalation of, i. 440 
 
 occurrence of, i. 511 
 
 origin of, in the body, i. 511 
 
 properties of, i. 511 
 
 Ainmoniacal fermentation of urine, i. 512 
 Ammonium carbamate, i. 548 
 
 carbonate, i. 523 
 
 cyanate, i. 542 
 
 magnesium phosphate, i. 527 
 
 salts, action of, on muscles, ii. 138 
 Amnion, ii. 472 
 Amniotic cavity, ii. 472 
 
 fluid, inhibitory effect of, on respiration, i. 464 
 Amoeba, contractility of, ii. 19 
 Amoeboid movement, ii. 19 
 in neuroblasts, ii. 176 
 in ova, ii. 22 
 of leucocytes, i. 48 
 Amphiaster, ii. 469 
 Amphimixis as cause of congenital variation, 
 
 ii. 504 
 
 Amphioxus, reflexes in, ii. 212 
 Ampho-peptone, definition of, i. 293 
 Amplitude of sound-waves, ii. 381 
 Ampulla of Henle, ii. 447 
 Ampullae of the semicircular canals, ii. 372 
 Ampullary nerves, stimulation of, ii. 407 
 Amputation in man, effects of, on neurones, ii. 
 
 196 
 Amygdalin, fermentative decomposition of, i. 
 
 542 
 
 Amyl alcohol, i. 539 
 Amylodextrin, i. 566 
 Amyloid, i. 578 
 Amylolytic enzyme of gastric juice in the dog, 
 
 i. 296 
 
 of succus entericus, i. 308 
 of the liver, i. 330 
 
 enzymes, definition of, i. 280 
 
 action of, in the body, i. 285 
 Amylopsin, i. 232, 280 
 
 action of, on starch, i. 566 
 
 digestive action of, i. 305 
 
 occurrence of, i. 304 
 
 properties of, i. 305 
 Anabolism, definition of, i. 19 
 Anaemia of the brain during fatigue, ii. 288 
 Anaesthesia, contralateral, after hemisection of 
 
 the cord, ii. 233 
 Anaesthetics, action of, on nerve-currents, ii, 155 
 
 effect of, on body-temperature, i. 472 
 Analgesia, ii. 232 
 
 following removal of the cerebellum, ii. 272 
 Analysis of composite tones, ii. 384 
 Anatomy of the ear, ii. 362 
 
GENERAL INDEX. 
 
 525 
 
 Anelectrotonus, ii. 62 
 
 Angular movements of joints, ii. 416 
 
 Animal foods, composition of, i. 'JT 1 - 
 
 heat, i. 4(57 
 
 source of, i. 474 
 
 Anisotropic substance of muscle-fibres, ii. 104 
 Annulus Yicusscns, i. l.~>!i 
 Anodal contraction, ii. ."><i 
 Anode, physical, definition of, ii. 52 
 
 physiological, definition of, ii. 52 
 Anosmia, ii. 411 
 Antallmmid, i. X>93 
 Anterior association centre, ii. - > .')7 
 
 roots, recurrent sensibility of, ii. 204 
 Antilytic secretion, i. 230 
 Antimony poisoning, i. 514 
 Anti-peptone, definition of, i. 293 
 
 nature of. i. 302 
 Antiperistalsis, intestinal, i. 383 
 
 of the stomach, i. 379 
 Antrum pylori, i. 377 
 Apex beat, i. 117 
 
 preparation of the frog's heart, i. 188 
 
 ventricular, rhythmicity of, i. 151 
 Aphasia, ii. 257 
 Apnoea, definition of, i. 440 
 
 fatal, i. 464 
 
 phenomena of, i. 44 
 
 relation of vagi to, i. 442 
 Apoinorphia, action of, i. 389 
 Apraxia, ii. 259 
 
 Aqueous humor, index of refraction of, ii. 303 
 Arabinose, i. 562 
 Arginin, i. 552 
 Argon of the blood, i. 417 
 Aromatic compounds in urine, i. 572 
 
 metabolism of, i. 568, 569 
 Arsenic poisoning, i. 514 
 Arterial blood-pressure, explanation of, i. 92 
 
 pulse, cause of, i. 93 
 definition of, i. 139 
 extinction of, i. 94 
 Arteries, calcification of, in old age, ii. 491 
 
 coronary, i. 179 
 
 elongation of, i. 140 
 
 rate of flow in, i. 101 
 
 Arthropods, segmental nervous system of, ii. 212 
 Articular cartilages, ii. 415 
 Articulation, ii. 434 
 Articulations, varieties of, ii. 414 
 Artificial circulation through the heart, ii. 69 
 through the muscles, ii. 68 
 
 respiration, circulatory effects of, i. 453 
 methods of maintaining, i. 446 
 
 stimulation of muscle compared with normal, 
 
 ii. 134 
 
 Aryepiglottic fold, ii. -Vl'i. 
 Aryteno-epiglottidean muscle, ii. 426 
 Arytenoid cartilages, ii. 425 
 
 muscle, ii. 426 
 Asexual reproduction, ii. 439 
 
 theory of the origin of, ii. 441 
 Asparagin, i. 558 
 Aspartic acid, i. 557 
 Asphyxia, i. 441 
 
 circulatory changes in, i. 445 
 
 effects of, on the blood-vessels, i. 202 
 on the respiratory rhythm, i. 425 
 
 stages of, i. 4-1 ."> 
 Aspirates, ii. 437 
 Aspiration of the thorax, influence of, on the 
 
 circulation, i. 77. 95 
 on the lyinph-llmv, i. 147 
 on venous circulation, i. 77. !'"> 
 Aoimilation, general characteristics of, i. 19 
 Associated respiratory movements, i. 408 
 Association centre, anterior, ii. 257 
 
 Association centre, middle, ii. 257 
 posterior, ii. -J.TT 
 
 cerebral, variations in, ii. :_v,< 
 
 fibres and centres of the cortex, ii. 
 
 tracts, origin of, from central cells, n 
 Astasia after removal of the cerebellum, i, 
 Asthenia from removal of t lie cerebellum, n 
 Astigmatic accommodation, ii. 310 
 Astigmatism, ii. ::i7 
 
 detection of, ii. :;i! 
 
 irre-ular, ii. 319 
 
 Astral rays, contractility of, ii. 470 
 Asymmetrical carbon atom, definition of, i 
 Atavism, ii. 495 
 
 Ataxia after removal of the cerebellum, ii. 273 
 Atelectasis, i. 396 
 
 Atmospheric air, composition of, i. 410, 413 
 Atonia after removal of the cerebellum, ii. 273 
 Atrophy of the heart after section of the vagi, i. 
 167 
 
 of the nerve-cells from disuse, ii. 195 
 Atropin, action of, on accommodation, ii. 311 
 on salivary glands, i. 222, 229 
 on sweat glands, i. 260 
 on the eye, ii. 325 
 
 effect of, on body-temperature, i. 472 
 Attraction sphere of the ovum, ii. 449 
 Auditory area of the cortex, ii. 253 
 
 canal, ii. 363 
 
 epithelium of the utricle and saccule, ii. 373 
 
 judgments, ii. 389 
 
 meat us, external, ii. 362 
 
 nerves, central paths of, ii. 237 
 cochlear division of, ii. 376 
 subdivision of, ii. 373 
 
 ossicles, ii. 365 
 movements of, ii. 367 
 
 sensations, limits of, ii. 382 
 successive contrast in, ii. 388 
 theory of, ii. 380 
 Augmentor centre of the heart, i. 177 
 
 nerves of the heart, i. 161, 167 
 Auricle of the external ear, ii. 362 
 
 systolic changes in, i. 115 
 Auricles, connection of, i. 135 
 
 degree of emptying, in systole, i. 138 
 
 functions of, i. 135 
 
 influence of, on venous blood-flow, i. 136 
 
 negative pressure in, i. 137, 138 
 Auricular pressure, i. 135, 137 
 
 systole, duration of, i. 124, 136 
 effect of, on venous blood -flow, i. 138 
 on ventricular filling, i. 137 
 Auriculo-ventricular valves, i. 108 
 Auscultation, i. 118 
 Automatism, definition of, ii. 20 
 Axilla, temperature in, i. 468 
 Axones, definition of; ii. 21, 173 
 
 growth in diameter of, ii. 179 
 
 length of, ii. 174 
 
 BACTERIAL decomposition in the intestines, i. 
 
 309 
 
 Ball-and-socket joint, ii. 416 
 Banting diet, i. 353 
 
 Barium salts, action of, on muscles, ii. 138 
 Barometric pressures, effect of, on respiration, i. 
 
 434 
 Bartholin, duct of, i. 217 
 
 gland-* of. ii. UJ-J 
 Basilar membrane, structure of, ii. 377 
 
 width of, ii. 380 
 P,a-i,phili'<. i. 47 
 
 Baths, influence of, on body-temperature, i. 471 
 Mathyasthesia. ii. 233 
 
 Meats in musical tones, production of, ii. 386 
 Beckmann's apparatus, i. 68 
 
526 
 
 GENERAL INDEX. 
 
 Beef-tea, physiological action of, i. 359 
 
 Beer, i. 535 
 
 Beeswax, i. 540 
 
 Benham's spectrum top, ii. 344 
 
 Benzoic acid, i. 340, 569 
 
 Benzol, molecular constitution of, i. 568 
 
 Benzopyrol, i. 571 
 
 Bidder's ganglion, i. 148 
 
 Bile, amount secreted, i. 246, 321 
 
 antiseptic property of, i. 326 
 
 composition of, i. 245, 321 
 
 discharge of, from the gall-bladder, i. 248, 249 
 
 fatty acids of, i. 541 
 
 influence of, on emulsification of fats, i. 307 
 
 mineral constituents of, i. 530 
 
 physiological value of, i. 325 
 
 pigments of, i. 245, 322 
 
 relation of, to fat absorption, i. 325 
 
 secretion of, i. 246 
 
 sulphur of, i. 507 
 Bile-acids, i. 245 
 
 detection of, i. 324 
 
 Neukomm's test for, i. 545 
 
 occurrence of, i. 323 
 
 origin of, i. 324 
 
 Pettenkofer's test for, i. 324, 544 
 
 relation of, to fat absorption, i. 326 
 Bile-capillaries, i. 244 
 Bile-ducts, occlusion of, i. 249 
 Bile-pigments, i. 322 
 
 chemical properties of, i. 574 
 
 Gmelin's test for, i. 322, 574 
 
 origin of, i. 45, 530 
 Bile-salts, i. 245 
 
 chemistry of, i. 543 
 
 circulation of, i. 544 
 Bile-secretion, normal mechanism of, i. 248 
 
 relation of, to blood-flow in the liver, i. 247 
 Bile-vessels, motor nerves of, i. 248 
 Biliary fistulae, i. 321 
 Bilicyanin, i. 574 
 Bilirubin, i. 245, 574 
 Biliverdin, i. 245, 574 
 Bilixanthin, i. 574 
 Binocular combination of colors, ii. 358 
 
 vision, ii. 356 
 
 illusions in. ii. 359 
 
 rivalry of the fields of vision in, ii. 358 
 Biophors of the germ-plasm, ii. 503 
 Birds, removal of cerebral hemispheres in, ii. 267 
 Birth, size of the child at, ii. 487 
 Birth-rate of the two sexes, ii. 483 
 Births, multiple, ii. 482 
 
 ratio of male to female, ii. 483 
 Biuret, i. 549 
 Bladder, urinary, movements of, i. 369, 390 
 
 vaso-motor nerves of, i. 209 
 Blastomeres, ii. 470 
 Blind-spot, ii. 328 
 Blood, i. 33 
 
 amount of, in the central nervous system, ii. 
 288 
 
 changes in, during pregnancy, ii. 477 
 
 chemical composition of, i. 50 
 
 circulation of, i. 76 
 
 coagulation of, i. 54 
 
 defibrinated, i. 34 
 
 distribution of, in the body, i. 63 
 
 foreign, action of, on the heart, i. 192 
 
 gaseous exchanges of, i. 411 
 
 general function of, i. 33 
 
 histological structure of, i. 33 
 
 identification of, i. 573 
 
 oxidations in, i. 423 
 
 reaction of, i. 34, 290 
 
 regeneration of, after hemorrhage, i. 63 
 
 specific gravity of, i. 34 
 
 Blood, total quantity of, in the body, i. 63 
 
 transfusion of, i. 64 
 Blood-corpuscles, inorganic salts of, i. 50, 530 
 
 varieties of, i. 33 
 Blood-gases, analyses of, i. 411 
 
 extraction of, i. 420 
 
 tension of, i. 415 
 Blood-leucocytes, i. 47 
 Blood-plasma, color of, i. 33 
 
 composition of, i. 51 
 
 inorganic salts of, i. 50 
 Blood-plates, i. 49 
 Blood-pressure, aortic, i. 91 
 
 capillary, i. 84, 93 
 
 effect of the accelerator nerves on, i. 170 
 of the depressor nerves on, i. 173 
 on renal secretion, i. 253, 256 
 
 mean, definition of, i. 90 
 
 methods of measuring, i. 84, 85 
 
 origin of, i. 91, 92 
 
 pulmonary, i. 91 
 
 respiratory changes in, i. 447 
 
 venous, i. 91, 94 
 Blood-serum, composition of, i. 51 
 
 definition of, i. 34 
 
 mineral constituents of, i. 530 
 
 osmotic pressure of, i. 68 
 
 Blood-supply of the central nervous system, ii. 
 286 
 
 relation of, to irritability, ii. 67 
 Bodily metabolism, estimation of, i. 343 
 
 movements, effect of, on lymph-flow, i. 147 
 
 temperature, effect of, on respiratory ex- 
 changes, i. 432 
 
 Body-sense area of the cortex, ii. 252, 254 
 Body-temperature, rise of, from injury to the 
 
 optic thalami, ii. 271 
 
 from lesions in the corpora striata, ii. 271 
 Body-weight, influence of, on heat-production, 
 i. 482 
 
 loss of, from starvation, i. 362 
 Bolometer, ii. 142 
 
 Bones, action of muscles on, ii. 417 
 Border-cells of the gastric glands, i. 237, 238 
 Bottcher's crystals, ii. 445 
 Brain, circulation in, regulation of, ii. 287 
 
 curve of growth of, ii. 279 
 
 growth of, ii. 278 
 
 number of nerve-elements during, ii. 280 
 relation of, to growth of the body, ii. 280 
 size of neurones during, ii. 281 
 
 metabolic activity in, ii. 288 
 
 vaso-motor nerves of, i. 203 
 
 weight of, ii. 273 
 Brain-stem, ii. 274 
 Brain-ventricles, capacity of, ii. 274 
 Brain-weight, decrease of, in old age, ii. 296 
 
 relation of, to insanity, ii. 278 
 to sex, ii. 276 
 
 to social environment, ii. 277 
 Brain-weights of different races, ii. 278 
 Breaking contraction, point of origin of, ii. 35 
 " Breaking" shock, ii. 31 
 Broca's convolution, ii. 257 
 Bromelin, i. 280 
 Bromine, i. 508 
 Bronchial capacity, i. 427 
 
 constrictor nerves, i. 465 
 Broncho-dilator nerves, i. 465 
 Brown-Sequard's paralysis, ii. 233 
 Brucin, action of, on end-plates, ii. 27 
 Brunner's glands, i. 243 
 Buffy coat, i. 55 
 
 Bulbo-cavemosus muscle, ii. 449 
 action of, in erection, ii. 464 
 Bulimia, ii. 404 
 Butyric acid, i. 539 
 
GENERAL IXI>i:X. 
 
 527 
 
 CADAVERIN, i. 543 
 Caflein, i. .">;.:; 
 
 actiou of, on body-temperature, i. 472 
 on coagulation of muscle-plasma, ii. 164 
 oil kidneys, i. 254 
 Calcium, absortion of, i. >-"> 
 carbonates, i. 524 
 chloride, i. ")2i{ 
 excretion of, i. 526 
 fluoride, i. .-,10, 523 
 phosphates, i. .Vj:; 
 physiological value of, i. 524 
 relation of, to heart muscle, i. 151 
 salts, action of, on the heart, i. 190 
 
 on the muscles, ii. 138 
 amount of, in fibrin, i. 58 
 excretion of, i. .'!.~>i> 
 nutritive value of, i. 356 
 relation of, to blood-coagulation, i. 57, 524 
 
 to irritability, ii. 59 
 sulphate, i. -Vj:; 
 Calorie, definition of, i. 504 
 Calori metric equivalent, i. 478 
 Caloriuietry, direct and indirect, i. 365, 475, 
 
 478 
 Canalis cochlearis, ii. 373 
 
 reunions, ii. 373, 374 
 Cane sugar, injection of, i. 317 
 
 inversion of, i. 565 
 
 Capacity of the heart-ventricles, i. 105 
 Capillaries, biliary, i. 244 
 blood, length of, i. 79 
 permeability of, i. 70 
 pressure in, i.84 
 rate of flow in, i. 101 
 resistance in, i. 81 
 structure of, i. 80 
 time spent by the blood in, i. 103 
 secretion, of the fuudic glands, i. 238 
 Capillary circulation, microscopic characters of, 
 
 i. 80 
 
 electrometer, ii. 146 
 pressure, origin of, i. 93 
 
 relation of, to lymph-formation, i. 72, 75 
 Capric acid, i. 541 
 Caproic acid, i. 540 
 Caprylic acid, i. 541 
 
 Capsules, suprarenal, extirpation of, i. 271 
 Caput gallinagiuis, ii. 464 
 Carbamic acid, i. 548 
 
 relation of, to urea formation, i. 336 
 Carbamide, i. 548 
 
 Carbo-hsemoglobin, nature of, i. 39 
 Carbohydrates, absorption of, i. 317 
 affinity of cell-substance for, i. 568 
 chemistry of, i. 561 
 combustion equivalent of, i. 365 
 definition of, i. 561 
 digestion of, in the stomach, i. 296 
 dynamic value of, i. 475 
 fermentation of, in the intestines, i. 310 
 molecular constitution of, i. 561 
 nutritive value of, i. 277, 353 
 origin of fat from, i. ::.">-.' 
 proteid-protection by, i. 568 
 synthesis of, i. 26 
 Carbon dioxide, action of, on conductivity in 
 
 nerves, ii. 93 
 on the heart, i. 191 
 on the nerves, ii. 60 
 on warm spots, ii. 398 
 dysptuea. i. 11 1 
 
 effect of, on nerve-currents, ii. 156 
 elimination, conditions affecting, i. 429 
 cutaneous, i. 1'J-J 
 estimation of. i. 428 
 inhalation, effects of, i. 440 
 
 Carbon dioxide, occurrence of, i. r>i7 
 of imiM-le, ii. 168 
 of the blood, extraction of, i. 517 
 production ..f. in nerves, ii. !."> 
 properties of, i. 518 
 tension of, in the alveoli, i. 413 
 
 in the blood, i. 416 
 
 disulphide, action of, on nerves, ii. 60 
 equilibrium, definition of, i. 345 
 metabolism of, i. 518 
 monoxide haemoglobin, i. 517 
 
 absorption spectrum of, i. 44 
 composition of, i. 38 
 inhalation, i. 440 
 properties of, i. 517 
 occurrence of, i. 516 
 properties of, i. 516 
 
 Carbonic acid, chemical constitution of, i. 545 
 Carburetted hydrogen inhalation, i. 440 
 Cardiac centre, augmentor, i. 177 
 
 inhibitory, i. 17<> 
 cycle, analysis of, i. 122 
 definition of, i. 104 
 duration of, i. 123 
 dyspnoea, i. 444 
 
 excitation, propagation of, during vagus stim- 
 ulation, i. 163 
 impulse, i. 117 
 nerves, anatomy of, i. 159 
 classification of, i. 171 
 extrinsic, i. 159 
 of frogs, i. 160 
 of mammals, i. 160 
 palpitation at the climacteric, ii. 490 
 Cardio-inhibitory centre, respiratory variations 
 
 in, i. 451 
 
 Cardio-pneumatic movements, i. 412 
 Cardiogram, i. 117 
 Cardiometer, i. 106 
 Carnic acid of muscles, ii. 167 
 Carnin, i. 554 
 
 of muscles, ii. 167 
 Casein, i. 261 
 composition of, i. 579 
 curdling of, by acids, i. 296 
 
 by rennin, i. 295 
 Castration, effects of, ii. 463 
 
 on the voice, ii. 431 
 Cataleptic rigor, ii. 160 
 Catalysis, i. 282, 503 
 Caudate nucleus, heat-centre of, ii. 271 
 Cell, galvanic, ii. 29 
 Cell-differentiation, i. 22 ; ii. 22. 
 Cell-division, i. 20 
 Cell-granules of the glandular epithelium, i. 
 
 216 
 
 Cells, growth of, ii. 486 
 Central cells, importance of, in relation to in- 
 
 en-a-r of organization, ii. 285 
 nervous system, amount of blood in, ii. 288 
 arrangement of cell groups in, ii. 205 
 hlood-snpply of, ii. 286 
 change in specific gravity of, with age, ii. 
 
 284 
 
 condition of, in sleep, ii. 293 
 conscious phenomena of, ii. 172 
 daily rhythms of. ii. 289 
 development of. ii. I?'.' 
 fatigue of, ii. 289 
 general arrangement of, ii. 202 
 
 functions of, ii. 171 
 influence of the thyroid on growth of, ii. 
 
 >! 
 
 in old age, ii. _'!'.. 
 intensity of metabolism in. ii. 28* 
 medullation of nerves in, ii. 181 
 operation on, in fro-rs. ii 
 
528 
 
 GENERAL INDEX. 
 
 Central nervous system, organization of, at dif- 
 ferent ages, ii. 284 
 
 neurones of the spinal cord, ii. 203 
 
 stimulation of the nervous system, ii. 28 
 Centre, augmentor of the heart, i. 177 
 
 cardio-inhibitory, i. 176 
 
 defecation, i. 387 
 
 deglutition, i. 377 
 
 expiratory, i. 457 
 
 inspiratory, i. 457 
 
 micturition, i. 391, 393 
 
 of hearing, cortical, ii. 253 
 
 of rotation of the eye, ii. 298 
 
 of smell, cortical, ii. 253 
 
 of vision, cortical, ii. 253 
 
 peripheral reflex, i. 178 
 
 respiratory, i. 455 
 
 salivary, secretory, i. 230 
 
 spinal, of ejaculation, ii. 465 
 of erection, ii. 464 
 of parturition, ii. 481 
 
 sweat, i. 260 
 
 thermogenic, i. 491 
 
 vaso-motor, i. 198 
 
 vomiting, i. 389 
 Centres, association, ii. 256 
 Centripetal nerves of the heart, i. 171 
 Centrosome, i. 22 
 
 of human spermatozoa, ii. 444 
 
 of the fertilized egg, ii. 468 
 
 of the ovum, ii. 449 
 Cerebellum, anatomical connections of, ii. 273 
 
 effects of injury to, ii. 272 
 
 functions of, ii. 272 
 
 senile changes in, ii. 296 
 Cerebral circulation, i. 203 
 
 conditions affecting, ii. 288 
 crossed, i. 443 
 
 cortex, relation of, to the vaso-motor centre, 
 i. 202 
 
 hemispheres, effect of removal of, ii. 263 
 relative physiological values of, ii. 259 
 removal of, in birds, ii. 266 
 
 in dogs, ii. 262 
 Cerebrin, i. 559 
 
 of nerves, ii. 170 
 Cerebrum, heat regulating functions of, ii. 270 
 
 removal of, in dogs, ii. 267 
 Cerotyl alcohol, i. 540 
 Cerumen, i. 257 
 Cervical sympathetic, vaso-motor function of, i. 
 
 193 
 
 Cetyl alcohol, i. 540 
 
 Characters, acquired, inheritance of, ii. 496 
 Chemical reagents, action of, on irritability, ii. 
 58 
 
 stimulation of nerve, ii. 25 
 
 tonus, ii. 143 
 
 Chemistry of nerves, ii. 169 
 Chemotaxis, ii. 466 
 
 influence of, on neuroblasts, ii. 176 
 Chemo-tropism, ii. 466 
 Chest, effects of opening, i. 115 
 Chest-voice, ii. 432 
 Cheyne-Stokes respiration, i. 424 
 Chief cells of the gastric glands, i. 237 
 Chinese wax, i. 540 
 Chinolin, i. 571 
 
 Chloral, effect of, on the respiratory rhythm, i. 
 425 
 
 hydrate, i. 536 
 Chlorine, inhalation of, i. 440 
 
 occurrence of, i. 507 
 Chlorocruorin, i. 578 
 
 Chloroform, action of, on coagulation of muscle- 
 plasma, ii. 164 
 
 effect of, on nerve-currents, ii. 156 
 
 Chloroform, fate of, in the body, i. 533 
 
 vapor, action of, on nerves, ii. 60 
 Chocolate, nutritive value of, i. 357 
 Cholagogues, i. 246 
 Cholesterin, i. 575 
 amount of, in the blood, i. 51 
 distribution of, i. 325 
 excretion of, i. 325 
 in nerve, ii. 169 
 of bile, i. 245 
 of milk, i. 261 
 of sebaceous secretion, i. 257 
 Choletelin, i. 574 
 Cholin, i. 541, 543 
 Cholo-hsematin, i. 323 
 Chondroitic acid, i. 578 
 Chondro-mucoid, i. 578 
 Chorda tympani nerve, gustatory function of, ii. 
 
 410 
 
 vaso-dilator function of, i. 194 
 Chordae tendinese, i. 109 
 Chorion, ii. 473 
 frondosum, ii. 474 
 Iseve, ii. 474 
 Chorionic fluid, ii. 473 
 
 villi, ii. 473 
 
 Chromatic aberration, ii. 316 
 Chromatin, i. 22, 28 
 
 Chromatoblasts of pleuronectidae, ii. 20 
 Chromo-proteids, i. 576 
 Chromosomes, i. 22, 28 
 
 number of, in the segmentation nucleus, ii. 
 
 466 
 
 of germ-cells, hereditary function of, ii. 499 
 of human spermatozoa, ii. 443 
 of the sexual elements, reduced number of, ii. 
 
 454 
 ovarian, changes in, during maturation, ii. 
 
 451 
 
 number of, ii. 450 
 reduction of, in maturation of spermatozoa, ii. 
 
 445 
 
 Chronograph, description of, ii. 100 
 Chyme, i. 287, 381 
 Ciliary ganglion, ii. 323 
 
 muscles, action of, in accommodation, ii. 309 
 nerves, long, ii. 324 
 short, ii. 311, 323 
 
 Circulating proteid, definition of, i. 346 
 Circulation, artificial, through isolated organs, 
 
 ii. 68 
 
 capillary, velocity of, i. 83 
 cerebral, i. 203 
 of hydriodic acid, i. 509 
 of hydrofluoric acid, i. 510 
 of the bile, i. 323, 324 
 of the blood, causes of, i. 77 
 definition of, i. 76 
 discovery of, i. 76 
 in the retina, ii. 322 
 microscopic appearances of, i. 80 
 portal, i. 77 
 pulmonary, i. 78, 103 
 of the brain and cord, ii. 286 
 
 rate of, i. 79, 98 
 pulmonary, i. 103 
 renal, i. 255 
 Circulation-time, i. 79 
 Circumduction, movement of, ii. 416 
 Climacteric, ii. 459, 490 
 
 ovulation after, ii. 456 
 
 Climate, influence of, on age of puberty, ii. 489 
 on body-temperature, i. 469 
 on time of climacteric, ii. 490 
 Clitoris, ii. 462 
 
 hornology of, ii. 464 
 Clothing, influence of, on heat-loss, i. 486 
 
<>/:. YER A L IM>I:X. 
 
 529 
 
 Clotting of blood, i. 55 
 
 of milk. i. -JH5 
 Clupein, i. .'.HI 
 
 (.'<>_. cliinination, cutaneous, i. 'J.">8, 342 
 during muscular work, i. 3<>1 
 
 deep, i. :!;i 
 
 Coagulated proteids. properties of. i. 578 
 Coagulating t'li/.yincs, definition of. i. 280 
 Coagulation of tlie blood, accelerating agents of, 
 
 i. (il 
 
 conditions necessary for, i. 57 
 description of. i. "> I 
 intravascular, i. 60 
 nature of, i. HO 
 
 retarding influences affecting, i. 61, 62 
 theories of. i. 55, 56 
 rime taken by, i. 55 
 uses of, i. ">.") 
 of milk, i. -JIT. 
 
 of muscle-plasma, action of drugs on, ii. 164 
 of myosin, ii. 163 
 Cocaine, action of, on conductivity in nerves, ii. 
 
 93 
 
 on the tongue, ii. 413 
 effect of, on intestinal movements, i. 384 
 Cochlea, anatomy of, ii. 374 
 bony, ii. 372 
 
 membranous, structure of, ii. 376 
 Cochlear root of tbe auditory nerve, central 
 
 paths of/n. 237 
 Coefficient of absorption of liquids for gases, i. 
 
 414 
 Coffee, nutritive value of, i. 357 
 
 stimulating action of, ii. 75 
 Cold and warm points of the skin, ii. 398 
 
 effect of, on coagulation of the blood, i. 61 
 Collagen, i. 580 
 Collaterals of axones, ii. 173 
 Colloid, i. 578 
 
 substance of the thyroid, secretion of, 268 
 Color of objects, relation of, to intensity of il- 
 lumination, ii. 333 
 sensations in indirect vision, ii. 333 
 
 phenomena of, ii. 333 
 theories, ii. 335 
 triangle, ii. 334 
 vision, theories of, ii. 335 
 Color-blindness, ii. 338 
 
 hereditary transmission of, ii. 494 
 of the rods, ii. 342 
 Colored shadows from simultaneous contrast, ii. 
 
 347 
 
 Color-mixture, ii. 333 
 Colors, binocular combination of, ii. 358 
 complementary, ii. 334 
 physical basis 'for, ii. 332 
 relative luminosity of, ii. 340 
 saturation of, ii. 342 
 Colostrum corpuscles, origin of, i. 263 
 
 definition of, i. 264 
 Combinational tones, ii. 387 
 Combined proteids, i. 579 
 Combustion, i. 501 
 
 equivalent of foods, i. 365 
 Comedones, i. 257 
 Commissure, Meynert's, ii. 238 
 
 von Gudden's, ii. 238 
 
 Commissures, origin of, from central cells, ii. 205 
 Common sensation, definition of. ii. .">!'! 
 
 sensibility, afferent paths of the nerves of, 
 
 ii. 230 
 
 Commutators, method of using, ii. 36 
 Complemental air. i. 427 
 Complementary colors, ii. 334 
 Composite tones, analysis of, ii. 384 
 Compressed air, respiration of, i. 452 
 Conceptions, multiple, ii. 482 
 
 34 
 
 Concha of the external ear. ii. 3fi2 
 Condiments, nutritive value of, i. .",:>!i 
 Conduction by continuity, ii. M 
 
 directions ,f, ii. -[ 
 
 from neurone to neurone, ii. 84 
 
 in branching nerves, ii. -o 
 
 in ganglion-cells, ii. !7 
 
 in muscles, ii. 80 
 
 in nerve-trunks, ii. 
 
 in nerves, elleeK of, ii. !."> 
 
 in the heart of the contraction wave, i. 154 
 
 of nerve-impulses, dim-Hem of, ii. 184 
 from neurone to neurone, ii. 207 
 
 process, nature of, ii. 97 
 
 rate of, ii. 87 
 Conductivity, action of drugs on, ii. 93 
 
 definition of, ii. 20, 77 
 
 dependence of, on protoplasmic continuity, 
 ii. 77 
 
 effect of constant current on, ii. 94 
 
 influences affecting, ii. 91 
 
 of living matter, i. 21 
 
 of muscle, ii. 20 
 
 of nerves, ii. 21 
 
 of neurone, ii. 189 
 
 of ova, ii. 22 
 Condyloid joints, ii. 416 
 Cones, retinal, function of, ii. 341 
 
 movements of, ii. 331 
 Confluxion in space perception, ii. 353 
 Congenital resemblances, ii. 494 
 
 variations, ii. 500 
 
 Congo-red test for mineral acids, i. 289 
 Con in, action of, on end-plates, ii. 27 
 Conjugate foci in a dioptric system, ii. 302 
 Conjugated sulphates, nutritive, history of, i. 340 
 Conjugation, ii. 440 
 Consciousness, i. 29 
 
 cerebral origin of, ii. 172 
 Consonants, ii. 436 
 
 Constant current, contracture effect of, in mus- 
 cles, ii. 131 
 
 effect of, on conductivity, ii. 94 
 on muscles, ii. 61 
 on nerves, ii. 62 
 
 Constrictor nerves of the iris, ii. 323 
 Continuous contractions, ii. 127 
 Contractility, definition of, ii. 17, 98 
 
 in vorticella, ii. 20 
 
 occurrence of, ii. 20 
 
 of amcebfe, ii. 19 
 
 of astral rays, ii. 470 
 
 of living matter, i. 21 
 
 of muscle, ii. 17 
 
 adaptation of, to their normal functions, 
 ii. 108 
 
 of ova, ii. 22 
 
 of plain muscle, i. 370 
 
 Contraction curve of muscle, effect of frequent 
 excitations on, ii. 115 
 
 id io-rnu scalar, ii. !>J 
 
 of muscles, post-mortem, ii. 160 
 relation of, to structure, ii. 1C7 
 
 remainder, ii. 106 
 
 volume of the heart, i. 105 
 
 wave in muscle, rate of transmission of, ii. 87 
 length of, ii. 88 
 
 of the heart, rate of propagation of, i. 1":; 
 Contractions from repeated single stimuli, ii. 112 
 
 introductory, ii. 113 
 
 isometric, ii. 110 
 
 isotonic, ii. 110 
 
 normal, tetanic nature of, ii. 132 
 
 of rigor caloris. ii. !;." 
 Contract 11 re after frequent excitations, ii. 128 
 
 after single excitation, ii. 129 
 
 definition of, ii. 116 
 
530 
 
 GENERAL INDEX. 
 
 Contracture from fatigue, ii. 130 
 in dying muscles, ii. 132 
 in rigor mortis, ii. 128 
 in veratria poisoning, ii. 128 
 normal, ii. 129 
 
 of neck muscles after cerebellar injury, ii. 272 
 pathological, ii. 127, 132 
 relation of, to tetanus, ii. 117, 122, 124 
 Contractures, ii. 127 
 Contrast, ii. 346 
 
 in space perception, ii. 352 
 Convergence of the eyes in accommodation, ii. 
 
 311 
 
 muscular mechanism of, ii. 300 
 Co-ordination of the eiferent impulses in re- 
 flexes, ii. 214 
 Copulation, ii. 463 
 Core-conductors, ii. 158 
 Cornea, curvature of, ii. 303 
 Corniculum laryngis, ii. 425 
 Cornutine, action of, on muscles, ii. 137 
 Corona radiata, ii. 454 
 
 of the ovum, ii. 450 
 Coronary arteries, anatomy of, i. 179, 180 
 
 ligatiou of, i. 181, 183 
 circulation, effect of ventricular systole on, i. 
 
 185 
 
 volume of, i. 184 
 veins, closure of, i. 184 
 Corpora Arantii, i. 112 
 
 cavernosa of the penis, ii. 448 
 striata, functions of, ii. 271 
 Corpus callosum, functions of, ii. 270 
 luteum, ii. 455 
 
 spongiosum of the penis, ii. 448 
 Corpuscles, colostrum, i. 263 
 of the blood, i. 45 
 salivary, i. 283 
 
 Corresponding points of the retinas, ii. 359 
 Cortex cerebri, connection of, with the respira- 
 tory centre, i. 463 
 effects of localized electrical stimulation of, 
 
 ii. 241 
 
 electrical stimulation of, ii. 242 
 number of nerve-cells in, ii. 284 
 course of efferent impulses from, ii. 251 
 latent areas of, ii. 261 
 Corti, cells of, ii. 377 
 
 organ of, structure of, ii. 377 
 rods of, ii. 377 
 Cortical areas, ii. 243 
 
 motor, in man, ii. 250 
 size of, ii. 247 
 centres, ii. 243 
 motor control, crossed, ii. 251 
 
 multiple character of, ii. 250 
 regions, ii. 243 
 stimulation, inhibitory effects of, ii. 224 
 
 vascular effects of, i. 202 
 Costal respiration, definition of, i. 398 
 Coughing, i. 454 
 Coughs, sympathetic, i. 455 
 Cowper's gland, ii. 443 
 histology of, ii. 448 
 secretion of, ii. 446 
 
 Crab-extract, lymphagogic action of, i. 73 
 Crabs, regeneration of lost parts in, ii. 496 
 Cranial nerves, afferent, ii. 236 
 Creatin, chemical constitution of, i. 550 
 in muscle, ii. 166 
 nutritive history of, i. 339, 551 
 Creatinin, i. 551 
 
 nutritive history of, i. 339 
 of muscle, ii. 167 
 Cresol, i. 569 
 
 elimination, i. 340 
 Cretinism, sporadic, ii. 289 
 
 rico-arytenoid muscle, lateral, ii. 426 
 
 posterior, ii. 426 
 Cricoid cartilage, ii. 425 
 Crico-thyroid muscles, ii. 426 
 Criminals, weight of the brain in, ii. 277 
 Crista acustica of the semicircular canals, ii.373 
 Critical period of nerves, ii. 66 
 Crossed cerebral circulation, i. 443 
 Cross-suturing of nerve-trunks, ii. 201 
 Cruciate heat-centre, ii. 271 
 Crying, i. 454 
 
 Crystalloids, diffusion of, i. 69 
 Crystals of CO-hsemoglobin, i. 40 
 
 of hsemin, i. 44, 573 
 
 of haemoglobin, i. 39 
 Cupola of the cochlea, ii. 375 
 Curare, action of, ii. 26 
 Currents of action in muscle, ii. 150 
 in nerves, ii. 153 
 
 of rest, ii. 147 
 
 theories as to their cause, ii. 148 
 Curve of fatigue, with repeated single contrac- 
 tions, ii. 113 
 
 of intensity of sleep, ii. 294 
 
 of muscle contraction, effect of frequent ex- 
 citations on, ii. 115 
 
 of muscular contractions, ii. 100 
 
 of work for muscles, ii. 140 
 Cutaneous nerves, influence of, on respiration, 
 i. 463 
 
 respiration, i. 422 
 
 secretion, i. 257 
 
 sensations, cortical area for, ii. 253 
 disturbance of, in disease, ii. 403 
 varieties of, ii. 390 
 
 temperature points, ii, 398 
 Cyauamide, i. 542 
 Cyanogen gas, i. 541 
 
 inhalation, i. 440 
 Cynurenic acid, i. 571 
 Cystein, i. 546 
 Cystin, i. 547 
 
 Cytology, definition of, i. 31 
 Cytoplasmic changes in nerve-cells, ii. 182 
 Cytosin, i. 579 
 
 " DANGEROUS region," i. 97 
 Daniell cell, ii. 28 
 Darwin's theory of heredity, ii. 501 
 Death, definition of, ii. 491 
 
 of the tissues, ii. 492 
 
 somatic, ii. 491 
 
 theory of, ii. 492 
 Decidua graviditatis, ii. 461, 471 
 
 menstrualis, ii. 458, 461 
 
 reflexa, ii. 472 
 
 serotina, ii. 472 
 
 vera, ii. 472 
 Decidual cells, ii. 471 
 
 Decomposition, bacterial, in the intestines, i. 309 
 Defecation, i. 386 
 
 cerebral control of, ii. 270 
 
 reflex character of, ii. 213 
 Defibrinated blood, definition of, i. 34 
 
 preparation of, i. 55 
 
 Degeneration after hemisection of the spinal 
 cord, ii. 228 
 
 following removal of motor cortical areas, ii. 
 244 
 
 of cut nerves, ii. 78 
 
 of muscle after section of its nerve, ii. 70 
 
 of nerve-cells, ii. 199 
 
 of nerve-elements, ii. 197 
 
 of nerves after section, ii. 69 
 
 reaction of, ii. 47 
 Deglutition, i. 372 
 
 action of the epiglottis in, ii. 422 
 
GENERAL INDEX. 
 
 531 
 
 Deglutition, aualysis of, i. 376 
 
 apmra, i. 442 
 centre for, i. 377 
 explanat ii>ii of, i. '>7f> 
 
 nervous regulation of, i. 376 
 Deiters, cells of, in the organ of Corti, ii. 377 
 
 nucleus of, ii. 838 
 
 Demarcation currents of injured muscle, ii. 148 
 Demilunes, i. 21!> 
 Dendrites. definition of, ii. 174 
 Depressor nerve, i. 172, 2:; 
 Dermal sensations, cortical area for, ii. 253 
 
 path of conduction for. in the cord, ii. 235 
 sensibility, area of distribution of the nerves 
 
 of; 231 
 
 Descending impulses, course of, ii. 244 
 IVsiceation of nerve, ii. 59 
 Determinants of the germ-plasm, ii. 503 
 Deutero-proteose, definition of, i. 293 
 Deu topi asm of the ovum, ii. 450 
 
 composition of, ii. 451 
 Development of nerve-cells, ii. 176 
 Dextrose, action of, in delaying rigor mortis, ii. 
 
 164 
 
 on the heart, i. 191 
 amount of, in the blood, i. 51, 317 
 origin of, i. 563 
 
 oxidation of, in the tissues, i. 317 
 storage of, i. 563 
 Diabetes mellitus, dextrose excreted in, i. 354, 
 
 563 
 
 fatty acids in, i. 536 
 on proteid diet, i. 329 
 phosphorus excretion in, i. 515 
 relation of the pancreas to, i. 266 
 Dialysis, definition of, i. 65 
 
 of soluble substances, i. 69 
 Diapedesis of maternal leucocytes into the 
 
 foetus, ii. 476 
 
 Diaphoretics, effect of, on heat dissipation, i. 489 
 Diaphragm, movements of, i. 398 
 Diarthrosis, ii. 415 
 Diastase, i. 280 
 Diastatic enzymes, i. 280, 566 
 Diaxonic nerve-cells, ii. 178 
 Dicrotic pulse, i. 144 
 
 wave of the pulse-curve, i. 143 
 Diet, accessory articles of, i. 357 
 
 average, for man, i. 366 t 
 Dietetics, i. 366 
 Differential manometer, i. 131 
 
 tones, ii. 387 
 Diffusion, definition of, i. 65 
 
 of central nerve-impulses, ii. 208 
 
 of impulses in the cord, influences affecting, ii. 
 
 217 
 
 of nerve-impulse, peripheral, ii. 218 
 of proteids, i. 70 
 through membranes, i. 66 
 Digastric muscle, i. 372; ii. 426 
 
 ion, action of alcohol on, i. 535 
 ric, i. 287 
 
 influence of, on respiratory exchanges, i. 431 
 intestinal, i. 299 
 in the large intestine, i. 309 
 of fats, i. 305 
 of proteids. i. 292, 301 
 of starch, i. '284 
 pancreatic, i. 301, 308 
 purpose of, i. -2?.") 
 salivary, i. 283 
 Digitalin, action of, on coagulation of muscle- 
 
 plasma, ii. 164 
 on muscles, ii. 137 
 Digitalis, action of, on nerves and muscles, ii.60 
 
 effect of, on the respiratory rhythm, i 12.~> 
 Digits, supernumerary, ii. 494 
 
 Dioptric apparatus of the eye, defects of, ii. :;i I 
 Dioptrics of the eye, ii. ;{<H) 
 Dioptry, definition of, ii. :;o| 
 Dioxyacetone, i. 558 
 Dioxyplienyl-acetic acid, i. 570 
 Diphasic current of action, i. l.Yj 
 Direction, judgments of, by means of auditory 
 sensations, ii. 
 
 of the nerve-impulse, ii. 184 
 Disaccharides, i. 564 
 
 digestion of, i. 308 
 Disassimilatiou, definition of, i. 19 
 Discord, ii. 387 
 
 Discriminating sensibility of the skin for pres- 
 sure, ii. :;!_ 
 
 Discriminative sensibility for difference of tem- 
 perature, ii. :$7 
 Discus proligerus, ii. 450, 454 
 Diseases, inheritance of, ii. 498 
 Dispenny, ii. 471 
 Dispersion of light, ii. 316 
 Dissociation of electrolytes, i. 67 
 
 of the axial and focal adjustments of the eye, 
 
 ii. 312 
 
 Distance, judgments of sensation by means of 
 auditory, ii. 389 
 
 perception of, ii. 354 
 
 visual perception of, ii. 348 
 Disuse, effect of, on muscles, ii. 77 
 Diuretics, action of, i. 254 
 Dizziness, ii. 405 
 
 Dogs, removal of cerebrum in, ii. 267 
 Domestication, effect of, on menstruation in 
 
 animals, ii. 460, 462 
 Dorsal nerve-roots, efferent fibres in, ii. 203 
 
 roots, degeneration resulting from section of, 
 ii. 227 
 
 spinal nerve-roots, number of fibres of, ii. 230 
 Dreams, ii. 293 
 Drinking-water, i. 504 
 Dropsy, i. 147 
 Drowning, phenomena of, i. 445 
 
 resuscitation from, i. 445 
 Drugs, action of, on body-temperature, i. 472 
 on salivary glands, i. 222, 229 
 on sweat-glands, i. 260 
 on therinogenesis, i. 484 
 on thermolysis, i. 489 
 Du Bois-Reymond's key, ii. 30 
 
 law of stimulation, ii. 32 
 
 theory of currents of rest, ii. 148 
 Duct of Bartholin, i. 217 
 
 of Rivinus, i. 217 
 
 of Stenson, i. 217 
 
 of Wharton, i. 217 
 
 of Wirsung, i. 231 
 Ductus cochlearis, structure of, ii. 374 
 
 endolymphaticus, ii. 373 
 
 venosus of the embryo, ii. 476 
 Duration of electric currents, effect of, on their 
 
 irritating power, ii. 46 
 Dynamic equilibrium, organs of, ii. 407 
 Dyslysin, i. 544 
 
 Dyspepsia accompanying the climacteric, ii. 
 490 
 
 cause of, i. 309 
 Dyspnosa, definition of, i. 441 
 
 effect of, on the iris. ii. :;_'! 
 on intestinal movements, i. 386 
 
 phenomena of, i. 444 
 
 varieties of, i. 443, 444 
 
 EAR, analysis of composite tones by, ii. 384 
 anatomy of, ii. 362 
 
 discriminative sensibility of, for pitch, ii. 385 
 fatigue of, ii. 387 
 imperfections of, ii. 388 
 
532 
 
 GENERAL INDEX. 
 
 Ear, membranous labyrinth of, ii. 372 
 ossicles of, ii. 365 
 sensibility of, in perception of time intervals, 
 
 ii. 388 
 Earth-worms, regeneration of lost parts in, ii. 
 
 496 
 
 Eck fistula, i. 336 
 Edestine, i. 577 
 
 Efferent fibres of the optic nerves, ii. 240 
 impulse in reflexes, co-ordination of, ii. 214 
 neurones of the dorsal spinal nerve-roots, ii. 
 
 203 
 
 of the spinal cord, ii. 203 
 paths from the cortex, course of, ii. 244 
 respiratory nerves, i. 463 
 Egg albumin, absortion of, i. 315 
 Ejaculation, ii. 465 
 Ejaculatory duct, ii. 447 
 Elasticity of muscle, ii. 105 
 Elastin, i. 580 
 Electric currents, correlation of their duration 
 
 with histological structures, ii. 47 
 effect of, duration of, ii. 46 
 on muscles, ii. 61 
 on nerves, ii. 62 
 their density, ii. 41 
 galvanic, effect of, on normal human 
 
 nerves, ii. 51 
 influence of their direction in nerves, ii. 48 
 
 varying duration of, ii. 47 
 methods of detecting, ii. 145 
 spread of, in moist conductors, ii. 41 
 stimulating effect of, ii. 28 
 organs, ii. 145 
 
 Electrical changes in active glands, i. 231 
 in the beating heart, i. 152, 153 
 in the heart during vagus stimulation, i. 
 
 164 
 
 in the retina, ii. 331 
 phenomena of muscle and nerve, ii. 144 
 
 of nerves, interpretation of, ii. 158 
 stimulation of nerve, ii. 25 
 
 of nerves, law of, ii. 32 
 Electrodes, shielded, ii. 41 
 
 varieties of, ii. 29 
 Electrolytes, definition of, i. 67 
 Electrostatic changes, stimulating action of, ii. 42 
 Electrotonic changes of conductivity, ii. 50 
 of irritability, ii. 64 
 
 in human nerves, ii. 65 
 twitch, ii. 157 
 Electrotonus, ii. 62 
 Embryo, nutrition of, ii. 475 
 
 rate of growth of, ii. 487 
 Emigration of leucocytes, i. 83 
 Emmetropia, ii. 313 
 Emmetropic eye, ii. 312 
 Emphysema, influence of, on the respiratory 
 
 rhythm, i. 424 
 Emulsification of fats, i. 306 
 
 influence of the bile on, i. 307 
 Emulsions, preparation of, i. 307, 559 
 Encephala, classification of, according to weight, 
 
 ii. 275 
 Encephalon, specific gravity of, ii. 275 
 
 weight of, ii. 274, 275 
 End-bulbs, sensory, ii. 392 
 
 Endocardiac pressure. See Intracardiac pressure. 
 Endolymph, ii. 372 
 End-organs, importance of, in touch sensations, 
 
 ii. 396 
 
 transmission of excitation by means of, ii. 82 
 Enemata, nutritive, i. 315 
 Energy liberated in contracting muscles, ii. 138 
 
 potential, of foods, i. 364 
 
 Engelmann's theory of the nature of muscular 
 contraction, ii. 105 
 
 Environment, influence of, on organisms, ii. 493 
 Enzyme action, theories of, i. 282 
 
 glycolytic, i. 354 
 Enzymes, classification of, i. 280 
 
 composition of, i. 279 
 
 definition of, i. 279 
 
 effect of, on blood coagulation, i. 63 
 
 general properties of, i. 281 
 
 mode of action of, i. 282 
 
 of pancreatic juice, i. 232, 235, 301 
 
 solubility of, i. 281 
 Eosinophiles, i. 47 
 Epididymis, ii. 447 
 Epigenesis, theory of, ii. 500, 504 
 Epiglottis, ii. 421 
 Epiguanin, i. 554 
 Epinephriu, i. 272, 572 
 
 action of, on muscles, ii. 138 
 Episarcin, i. 554 
 Equilibrium of the body, definition of. ii. 404 
 
 relation of the cerebellum to, ii. 273 
 
 sense of, ii. 404 
 Erection, ii. 464 
 
 of the heart, i. 114 
 
 spinal centre for, ii. 464 
 Erector clitoridis muscle, ii. 464 
 
 penis, action of, in erection, ii. 464 
 
 muscles, ii. 449 
 Erectores spinse muscles, respiratory action of, 
 
 i. 405 
 
 Erythroblasts, i. 45 
 Erythrodextrin, i. 285, 566 
 Erythrose, i. 562 
 
 Escape of the heart from vagus inhibition, i. 163 
 Eserin, action of, on nerves and muscles, ii. 60 
 Ether, action of, on coagulation of muscle- 
 plasma, ii. 164 
 on conductivity of nerves, ii. 93 
 
 effect of, on nerve-currents, ii. 155 
 
 ethyl, i. 536 
 
 vapor, action of, on nerves, ii. 60 
 Ethereal sulphates, i. 506 
 
 of the urine, i. 572 
 Ethers, properties of, i. 536 
 Ethyl alcohol, i. 535 
 Ethylamine, i. 541 
 Eudiometer, i. 421 
 Eupncea, definition of, 440 
 Eustachian tube, ii. 363 
 function of, ii. 369 
 
 Excitability, changes in, during Wallerian de- 
 generation, ii. 69 
 
 Excitation, cardiac, electrical variation in, i. 153 
 propagation of, i. 153, 154 
 
 wave, cardiac, i. 152 
 Excretin, occurrence of, in feces, i. 320 
 Excretions, definition of, i. 213 
 Exercise, effect of, on growth-, ii. 489 
 on metabolism, i. 359 
 on muscular endurance, ii. 76 
 on pulse-rate, i. 121 
 Exhaustion of muscles, ii. 72 
 Expiration, forced, muscles of, i. 407 , 
 
 movements of, i. 406 
 Expiratory centre, i. 457 
 Explosive consonants, ii. 437 
 Extensibility of muscle, ii. 105 
 External auditory meatus, ii. 362 
 
 ear, anatomy of, ii. 362 
 
 rectus muscle, ii. 299 
 Extirpation of the liver, i, 336 
 
 of the pancreas, i. 266 
 
 of the thyroids, i. 268 
 Extractives, nitrogenous, of muscle, ii. 166 
 
 of the blood, i. 50, 51 
 Extracts, adrenal, i. 271 
 
 ovarian, i. 274 
 
GENERAL INDKX. 
 
 533 
 
 Extracts, testicular, i. 273 
 
 thyroid, i. '-'<>!) 
 Extrapolar region, ii. o'-j 
 Exudations, secretion of, i. Jl."> 
 Eye, abnormal positions of, after cerebellar in- 
 jury, ii. 272 
 
 adaptation of, to light, ii. ':>< 
 
 axes of rotation of, ii. -!! 
 
 chromatic aberration of. ii. ol<i 
 
 constants, changes in, during accommodation, 
 
 defects in the dioptric apparatus of, ii. 314 
 dioptric apparatus of, ii. 300 
 mechanical movements of. ii. -.'!- 
 movements, binocular co-ordination in, ii. 300 
 
 extent of, ii. 298 
 muscles of, ii. 2SW 
 
 innervation of, ii. 300 
 optical constants of, ii. 303 
 
 power of, ii. 304 
 positions of, ii. 299 
 refractive media of, ii. 302 
 
 surfaces of, ii. 303 
 spherical aberration of. ii. 315 
 
 FALLOPIAN tubes, ii. 443, 456 
 
 False amuion, ii. 473 
 
 Falsetto register of the voice, ii. 433 
 
 Far-point of vision, ii. 312 
 
 Fat, affinity of cell-substance for, i. 568 
 
 nutritive history of, ii. 559 
 
 origin of, from carbohydrates, i. 352 
 
 from proteid, i. 351, 560 
 Fat-absorption, influence of bile on, i. 325 
 
 mechanism of, i. 318 
 Fat-combustion, equivalent of, i. 365 
 Fat-formation in the body, i. 351, 560 
 Fatigue, cerebral anaemia from, ii. 288 
 
 curve with repeated single contraction, ii. 113 
 
 effect of, on height of contraction, ii. 113 
 on muscular contraction, ii. 130 
 on rigor caloris, ii. 165 
 mortis, ii. 160 
 
 from voluntary muscular contraction, ii. 134 
 
 in nerve-fibres, ii. 195 
 of central nervous system, ii. 289 * 
 
 of motor end-organs, ii. 83 
 
 of muscle, ii. 6h'. 70 
 
 recovery from, ii. 73^^ 
 - of nerve-cells, ii. 136T191 
 
 of nerves, ii. 75, 96 
 
 of retina, ii. 344 
 
 relation of, to sleep, ii. 291 
 
 theories of, ii. 72 
 
 to auditory sensations, ii. 387 
 Fat-metabolism, acetone formation in, i. 537 
 Fats, absorption of, in the stomach, i. 313 
 
 action of, on gastric secretion, i. 241 
 
 digestion of, i. 305 
 
 dynamic value of, i. 475 
 
 emulsification of, i. 306 
 
 gastric digestion of, i. 297 
 
 nutritive value of, i. 277, 350 
 
 of feces. i. 319 
 
 of muscle, ii. 167 
 
 origin of. in the body. i. 351, 560 
 
 relation of. to glycogen formation, i. 329 
 to muscular work. ii. 74 
 
 synthesis of, from fatty acids, i. 558 
 Fatty acids, monobasic, i. 532 
 
 degeneration in phosphorus-poisoning, i. 514 
 Feces, composition of, i. 319 
 Fellic acid, i. 543 
 Female births, relative number of, ii. 483 
 
 pronucleus, ii. !.">:; 
 Females, rate of growth in, ii. 488 
 Fenestra oval is, ii. 363 
 
 rotunda, ii. :;H.'{. 375 
 
 ii>e of, ii. :;TI; 
 
 Ferment, myo*iaofen-<x>culting, ii. 161 
 Fermentation, alcoholic, i. 535 
 
 lactic, i. .")!."> 
 
 Ferments, unorua nixed, i. 279 
 Ferralin, i 
 Ferric ph., 
 Ferrosulphide, i. 528 
 Fertilization, ii. 466 
 Fetal membranes, ii. 17_' 
 Fever, body-temperature in, i. l?j 
 cause of, i. 473 
 effect of, on blood coagulation, i. 55 
 
 on the respiratory centre, i. 458 
 heat dissipation in, i. 4*!) 
 Fibrillar contraction of the heart, i. 181, 183 
 Fibrin ferment, i. 56 
 
 absence of, in circulating blood, i. 61 
 nature of, i. 57 
 origin of, i. 59 
 preparation of, i. 59 
 mode of deposition of, i. 54, 55 
 Fibrin-globulin, i. 56 
 Fibrinogen, i. 53, 54 
 Fibrinoplastin, i. 56 
 Fictitious meal, effect of, on gastric secretion, i. 
 
 239 
 
 Field of vision, binocular rivalry of, ii. 358 
 Filtration processes in secretion, i. 213, 215 
 Fimbrise of the Fallopian tube, ii. 456 
 Fish, bony, removal of cerebral hemispheres in, 
 
 ii. 263 
 
 semicircular canals in, ii. 407 
 visual accommodation in, ii. 306 
 Flavors, nutritive value of, i. 359 
 Flicker photometry, ii. 345 
 Fluorine, occurrence of, i. 510 
 Focal illumination of the eye, ii. 320 
 Foci, conjugate, ii. 302 
 
 principal, ii. 302 
 
 Food, combustion equivalent of, i. 365 
 definition of, i. 275 
 dynamic value of, i. 364 
 effect of, on respiratory activity, i. 431 
 energy liberated by, i. 474 
 influence of, on thermogenesis, i. 484 
 rate of movement of, in the intestines, i. 314 
 Food-stuffs, classification of, i. 276 
 composition of, i. 278 
 Liebig's classification of, i. 346 
 Foramen ovale of the foetal heart, ii. 476 
 Force of ventricular systole during vagus stim- 
 ulation, i. i;:; 
 
 Forced movements after section of the semicir- 
 cular canals, ii. 405 
 in frogs, ii. 266 
 Formic acid, i. 534 
 aldehyde, i. 533 
 Formose, synthesis of, i. 533 
 Fovea centralis, ii. :;-J7 
 Franklin's theory of color vision, ii. 337 
 Frequency of respiration, conditions affect in<r. 
 
 i.' I'.T. 
 
 relation of, to the pulse-rate, i. 426 
 Frictionals, ii. l.'!7 
 Frogs, removal of cerebral hemispheres in, ii. 
 
 -.'.;} 
 striped muscle of, time of single contraction 
 
 in. ii. 108 
 
 Frontal lobes of the hemispheres, effect of re- 
 moval of. ii. 262 
 Fuhlspiire, cortical, ii. -'."_> 
 Fundamental tone, definition of, ii. 383 
 
 GALACTOSK, i. r,<;-2, 564 
 Gall-bladder, motor nerves of, i. 248 
 
534 
 
 GENERAL INDEX. 
 
 Galvani, Luigi, ii. 28 
 
 Galvanic current, action of, on conductivity, ii. 
 
 94 
 
 contracture effect of, on muscles, ii. 131 
 effect of, on heart apex, i. 150 
 on muscles, ii. 61 
 on nerves, ii. 62 
 
 of making and breaking, ii. 31 
 on normal human nerves, ii. 51 
 opening and closing contractions with, ii. 38 
 Galvanometers, ii. 145 
 Galvanotoiius, ii. 54, 131 
 Gamogenesis, ii. 440 
 Ganglion spirale of the ear, ii. 376 
 
 submaxillary, i. 219 
 Ganglion-cells, conduction in, ii. 97 
 
 of the heart, i. 148 
 Gas analysis, i. 421 
 
 Gaseous exchanges in the brain, ii. 288 
 interchanges in the lungs, i. 410, 417 
 
 in the tissues, i. 419 
 Gas-pump, description of, i. 420 
 Gases, absorption of, i. 414 
 
 in the blood, respiratory changes in, i. 411 
 in the large intestine, i. 320 
 law of partial pressure of, i. 413 
 of muscle, ii. 168 
 of the saliva, i. 221 
 poisonous, inhalation of, i. 440 
 solutions of, i. 415 
 Gastric digestion of proteids, i. 292 
 fistulse, i. 288 
 
 glands, histology of, i. 237 
 secretory changes in, i. 242 
 value of, i. 299 
 juice, acidity of, i. 289 
 
 action of, on carbohydrates, i. 296 
 
 on milk, i. 296 
 antiseptic property of, i. 288 
 artificial, preparation of, i. 291 
 composition of, i. 238, 288 
 methods of obtaining, i. 287 
 mineral constituents of, i. 530 
 secretion, inhibition of, i. 241 
 nervous regulation of, i. 239 
 normal mechanism of, i. 240 
 relation of, to the character of the diet, i. 
 
 241 
 
 stimulants for, i. 241 
 
 Gelatin, digestion of, in the stomach, i. 297 
 nutritive value of, i. 349 
 proteid, protecting power of, i. 567 
 Gelatoses, i. 297 
 
 Geminal fibres of the pyramidal tracts, ii. 245 
 Gemmules of the germ-plasma, ii. 499 
 Genio-hyoid muscle, ii. 426 
 
 function of, in mastication, i. 372 
 Gerhardt's reaction, i. 537 
 Germinal spot of the ovary, ii. 450 
 transmission of infectious diseases, ii. 498 
 vesicle, structure of, ii. 450 
 Germ-plasm as a basis of heredity, ii. 499 
 continuity of, ii. 502 
 definition of, ii. 496 
 morphological nature of, ii. 499 
 origin of, ii. 499 
 Gestation, duration of, ii. 478 
 Gland, adrenal, i. 271 
 mammary, i. 262 
 pancreatic, i. 231, 266 
 parathyroid, i. 268 
 parotid, i. 217 
 sublingual, i. 217 
 submaxillary, i. 217 
 thyroid, i. 267 
 
 Gland-cells, electric currents in, ii. 145 
 selective activity of, i. 27 
 
 | Glands, albuminous, histology of, i. 216 
 
 Brunner's, i. 243 
 
 cutaneous, i. 257 
 
 gastric, i. 237 
 
 intestinal, i. 243 
 
 Lieberkiihn's, i. 243 
 
 mucous, histology of, i. 216 
 
 of Bartholin, ii. 462 
 
 of Littre, ii. 448 
 
 salivary, i. 215 
 
 sebaceous, i. 257 
 
 serous, definition of, i. 216 
 
 structure of, i. 211 
 
 sweat, i. 259 
 Glans penis, ii. 449 
 Glauber's salt, i. 522 
 Gliding movements in joints, ii. 416 
 Globin, i. 37 
 
 Globulicidal action of serum, i. 36 
 Globulins, i. 577 
 
 Glomeruli, renal, secretory function of, i. 253 
 Glossopharyngeal nerve, gustatory function of, 
 ii. 410 
 
 nerves, influence of, on respiration, i. 462 
 Glossopharyngeus, central conduction paths for, 
 
 ii. 236 
 Glottis, ii. 423 
 
 oedema of, ii. 422 
 
 respiratory movements of, i. 408 ; ii. 429 
 Glucosamin, i. 564 
 Glucoses, i. 562 
 
 synthesis of, i. 563 
 Glutamic acid, i. 558 
 Glutamin, i. 558 
 Glutolin, i. 53 
 Glutoses, i. 297 
 Glycerin, i. 558 
 
 aldehyde, i. 558 
 
 phosphoric acid, i. 559. 
 Glycerose, i. 558 
 Glycocoll, i. 537, 543 
 
 in muscles, ii. 167 
 
 nutritive history of, i. 538 
 Glycogen, i. 566 
 
 amount of, in the liver, i. 327 
 
 demonstration of, in the liver, i. 327 
 
 distribution of, i. 330 
 
 effect of exercise on, i. 361 
 of starvation on, i. 362 
 of sugars on, i. 328 
 
 function of, i. 329 
 
 of muscles, i. 330 ; ii. 167 
 
 origin of, i. 326, 327 
 
 properties of, i. 327, 566 
 Glycogen-eliminatiou of the liver, i. 265 
 Glycogen-formation, effect of proteid diet on, i. 
 
 328 
 
 Glycogen-formers, i. 328 
 Glycogenic theory, i. 329 
 Glycolysis, i. 354 
 Glycolytic enzyme, i. 280, 354 
 
 origin of, i. 267 
 Glyco-proteids, i. 576, 578 
 Glycosazones, i. 562 
 Glyco-secretory nerves, i. 248 
 Glycoses, i. 562 
 Glycosuria after pancreas extirpation, i. 266, 
 
 563 
 
 Glycuronic acid, i. 567 
 Gmelin's test for bile-pigments, i. 322, 574 
 Goblet cells, i. 216 
 Goitre, i. 269 
 
 Golgi, organ of, in tendons, ii. 402 
 Gout, i. 557 
 
 Graafian follicles, ii. 454 
 Grammeter, i. 477 
 Gram-molecular solution, i. 67 
 
GENERAL INDEX. 
 
 536 
 
 Graphic method of studying muscular con 
 
 tractions, ii. 99 
 Gravity, influence of, on cerebral circulation, ii. 
 
 287 
 Gray matter of the cerebrum, water contents of, 
 
 ii. 274 
 
 Growth after birth, ii. 487 
 before birth, ii. l-< : 
 increase of fibres of the cortex during, ii. 282 
 
 of functional neurones during, ii. '-' 
 influence of sex on the rate of, ii. 488 
 influences which modify, ii. 489 
 of nerve-cells, ii. 176 
 Guanidin. i. 550 
 Guanin, i. :9 f 554 
 Gunzburg's reagent, i. 508 
 Gustatory nerves, ii. 410 
 
 sensations, ii. 411 
 Guttural consonants, ii. 437 
 Gymnemra silvestre, action of, on taste-nerves, 
 ii. 413 
 
 H.EMATIN, i. 37, 44, 573 
 Haematogen, i. 356 
 
 composition of, i. 579 
 
 nutritive value of, i. 528 
 Haeinatoidiu, i. 44, 323, 574 
 Haematopoiesis, definition of, i. 45 
 Haematopoietic tissues, embryonic, i. 46 
 Hsematoporphyrin, i. 44, 574 
 Haemerythriu, i. 578 
 Haemin, i. 44, 573 
 Haeniochromogen, i. 37, 44, 573 
 Haemocyanin, i. 578 
 Haemoglobin, i. 573 
 
 absorption spectra of, i. 43 
 
 action of. on carbonates, i. 517 
 
 affinity of, for COa, i. 417 
 
 amount of, i. 38 
 
 compounds of, with gases, i, 38 
 
 condition of. in the corpuscles, i. 35 
 
 crystals of, i. 39 
 
 decomposition products of, i. 37 
 
 derivatives of, i. 44 
 
 distribution of, in animals, i. 37 
 
 elementary composition of, i. 37 
 
 molecular formula of, i. 37, 38 
 
 nature of, i. 37 
 
 of muscle-serum, ii. 166 
 
 oxygen capacity of, i. 416 
 Hair-cells of the crista acustica, ii. 374 
 
 of the organ of Corti, ii. 377 
 Hamulus, ii. 376 
 Harmonic overtones, ii. 386 
 Harmony, ii. 387 
 Hawking, i. 454 
 Head register of the voice, ii. 433 
 
 vaso-motor nerves of, i. 204 
 Hearing, ii. 362 
 
 keenness of, ii. 371 
 
 relation of, to speech, ii. 431 
 Heart, anaemia of, i. 183 
 
 artificial stimulation of, i. 156 
 
 augmentor nerves of, i. 167 
 
 cause of rhythmic beat of, i. 148 
 
 centripetal nerves of, i. 171 
 
 changes in, due to pregnancy, ii. 477 
 in form of, i. 113 
 in position of. i. 114 
 in size of, i. 112 
 
 compensatory pause of, i. 156 
 
 diphasic action currents in, ii. 152 
 
 electrical currents of, i. 152 
 
 erection of, i. 114 
 
 fibrillar contraction of, i. 181 
 
 heat produced by. i. 108 
 
 human, output of, i. 106 
 
 Heart, intrinsic nerves of, i. 148 
 illation of, i. 148, 1>7 ; ii.iiii 
 lymphatics of, i. 186 
 muscle, atrophy of, after section of the vagi, 
 
 i. 167 
 
 conduction of the contraction wave by , i 1 ."4 
 normal stimulus of, i. ir.i 
 rate of conduction in, ii. 89 
 rhythinicity of, i. 151 
 rigor mortis of, ii. 162 
 nutrition of, i. 179 
 position of, i. 117 
 pumping action of, i. 78 
 refractory period of, i. 156 
 suction -pump action of, i. 134 
 tetanus of, i. 165 
 vaso-motor nerves of, i. 206 
 work done by, i. 107 
 Heart-beat, abnormal sequence of, i. 152 
 conduction of, from auricles to ventricles, i. 
 
 155 
 
 effect of blood-supply on, i. 186 
 genesis of, i. 149, 150 
 heat produced by, i. 108 
 rate of, i. 121 
 
 voluntary control of, ii. 214 
 Heart-pause, i. 122 
 Heart-sounds, i. 118 
 Heat, expenditure of, i. 476 
 income of, i. 475 
 source of, i. 474 
 Heat-centres, ii. 271 
 Heat-dissipation, conditions affecting, i. 485 
 
 estimation of, i. 480 
 Heat-dyspnosa, i. 441, 443 
 Heat-production, amount of, i. 364 
 by the heart, i. 108 
 conditions affecting, i. 482 
 estimation of, i. 481 
 in contracting muscles, ii. 138 
 in muscles, ii. 142 
 in nerves, ii. 96 
 in rigor mortis, ii. 160 
 relation of, to respiratory activity, i. 483 
 Heat-rays of ether, ii. 331 
 Heat-regulation, i. 495 
 Height of contraction, dependence of, on the 
 
 load, ii. Ill 
 
 effect of temperature on, ii. 136 
 Helico-proteid, composition of, i. 579 
 Helicotrema, ii. 376 
 Hemianopsia, anatomical basis for, ii. 240 
 
 from cortical lesions, ii. 255 
 Hemi-peptone, decomposition of, by trypsin, i. 
 
 303 
 
 definition of, i. 293 
 
 Hemisections of the cord alternating at differ- 
 ent levels, ii. 230 
 
 Brown-Se'quard's paralysis from, ii. 233 
 degeneration resulting from, ii. 228 
 effect of, in man, ii. 233 
 
 on sensation and motion, ii. 230 
 in animals, ii. 234 
 physiological effect of, ii. 234 
 Hemorrhage, effect of, on hematopoiesis, i. 46 
 fatal limits of, i. 63 
 regeneration of the blood after, i. 63 
 relation of, to blood -pressure, i. 91 
 saline injections after, i. 64 
 Hemorrhagic dyspno3a, i. 444 
 Hepatin. i. .vjs' 
 Heredity, definition of. ii. 493 
 physical basis ,,f. i. 28 
 theories of. ii. .|!- 
 
 Hering's theory of color vision, ii. 336 
 Hermann's theory of currents of rest, ii. 148 
 Heteromita, reproduction in. ii. 440 
 
536 
 
 GENERAL INDEX. 
 
 Hexon-bases, origin of, i. 580 
 Hexoses, i. 562 
 
 Hibernation, effect of, on the respiratory quo- 
 tient, i. 438 
 Hiccough, i. 455 
 
 Higher brain-centres for the heart, i. 178 
 Hinge-joints, ii. 416 
 
 Hippuric acid, nutritive history of, i. 339 
 Histidin, i. 552 
 Histohsematin, i. 44, 578 
 Histology of striped muscle, ii. 104 
 Histon, i. 580 
 
 effect of, on intra vascular clotting, i. 61 
 Hofacker-Sadler law, ii. 484 
 Holmgren method for testing color vision, ii. 
 
 339 
 
 Homogentisic acid, i. 570 
 Homothermous animals, i. 467 
 Hpropter, ii. 359 
 
 Hiifner's method of urea determination, i. 549 
 Human muscles, fatigue of, with artificial stim- 
 ulation, ii. 134 
 Hunger, ii. 404 
 
 Hunger-centre, clinical evidence for, ii. 404 
 Hydra, regeneration of lost parts in, ii. 496 
 Hydrsemia from saline injections, i. 69 
 Hydraemic plethora, effect of, on lymph secre- 
 tion, i. 74 
 
 Hydration, nature of the process of, i. 503 
 Hydriodic acid, i. 509 
 Hydrobilirubin, i. 320 
 Hydrobromic acid, i. 509 
 Hydrocarbons, saturated, i. 531 
 Hydrochloric acid, occurrence of, i. 507 
 of the gastric juice, i. 238 
 preparation of, i. 507 
 properties of, i. 508 
 secretion of, i. 289 
 tests for, i. 508 
 Hydrocumaric acid, i. 570 
 Hydrocyanic acid, i. 542 
 
 action of, on coagulation of muscle-plasma, 
 
 ii. 164 
 Hydrofluoric acid, circulation of, in the body, i. 
 
 510 
 Hydrogen, inhalation of, i. 440 
 
 occurrence of, i. 499 
 
 peroxide, i. 505 
 
 preparation of, i. 500 
 
 properties of, i. 500 
 Hydrolysis by enzyme action, i. 282 
 
 definition of, i. 504 
 
 of fats, i. 305 
 
 of proteids, i. 292 
 Hydroquinone, i. 569 
 Hymen, ii. 462 
 Hyo-glossus muscle, ii. 426 
 Hypersesthesia, homolateral, after hemisection 
 
 of the cord, ii. 233 
 Hypermetropia, ii. 313 
 
 range of accommodation in, ii. 314 
 Hyperpnoea, i. 440 
 
 from muscular activity, i. 442 
 Hypertonic solutions, physiological definition 
 
 of, i. 69 
 
 Hypertonicity, definition of, i. 37 
 Hypophysis cerebri, function of, i. 273 
 Hypotonicity, definition of, i. 37 
 Hypoxanthin, i. 553 
 
 of muscles, ii. 167 
 
 relation of, to uric-acid formation, i. 338 
 
 ICE calorimeter, principle of, i. 504 
 Icterus, i. 249, 544 
 Idants of the germ -plasm, ii. 503 
 Idiomuscular contraction, ii. 27, 92, 128 
 Idioplasm as a basis of heredity, ii. 499 
 
 Idio-ventricular rhythm, i. 152 
 
 Ids of the germ -plasm, ii. 503 
 
 Illusions, visual, in sizes of objects, ii. 354 
 
 of space perception, ii. 351 
 Imbibition of water, i. 504 
 Immunity, inherited, ii. 498 
 Impregnation, ii. 466 
 Incus, ii. 366 
 
 Independent irritability of muscle, ii. 25 
 Index of refraction of the aqueous humor, ii. 
 
 303 
 
 of the lens, ii. 303 
 of the vitreous humor, ii. 303 
 Indifferent point of polarized nerves, ii. 64 
 Indirect vision, color sensations in, ii. 333 
 Indol, i. 571 
 
 elimination of, i. 340 
 occurrence of, in feces, i. 320 
 Induced currents, making and breaking shocks 
 
 with, ii. 40 
 
 prevention of spread of, ii. 44 
 electric currents, stimulating effect of, ii. 33 
 Induction apparatus, schema of, ii. 33 
 Infections, intra-uterine, ii. 498 
 Infectious diseases, germinal transmission of, ii. 
 
 498 
 Inferior laryngeal nerve, respiratory function 
 
 of, i. 464 
 
 mesenteric ganglion, reflex activity of, i. 392 
 oblique muscle, ii. 299 
 rectus muscle, ii. 299 
 Inflammation, emigration of leucocytes in, i. 
 
 83 
 
 Infra-hyoidei muscles, i. 405 
 Infundibular body, function of, i. 272 
 Inharmonic overtones, ii. 386 
 Inheritance, facts of, ii. 494 
 of acquired characters, ii. 496 
 of diseases, ii. 498 
 theories of, ii. 498 
 
 Inhibition from cortica* stimulation, ii. 224 
 in the central nervous system, ii. 224 
 of the heart, reflex, i. 172 
 Inhibitory centre, cardiac, localization of, i. 176 
 
 ton us of, i. 176 
 centres, respiratory, i. 457 
 nerves of the heart, i. 161 
 of the intestines, i. 385 ,, 
 of the pancreas, i. 233 
 of the spleen, i. 333 
 of the stomach, i. 382 
 Innervation of the blood-vessels, i. 192 
 
 of the heart, i. 148 
 Inorganic salts of the blood, i. 50 
 of urine, i. 341 
 
 relation of, to blood coagulation, i. 56, 57 
 to irritability, ii. 59 
 to the heart beat, i. 151, 189 
 Inosit, i. 573 
 Insanity, relation of brain-weight to, ii. 278 
 
 variations of muscular tonus in, ii. 220 
 Insect muscle, time of contraction in, ii. 108 
 Inspiration, enlargement of the thorax in, i. 
 
 398 
 
 muscles of, i. 398, 404 
 Inspiratory centre, i. 457 
 Intensity of visual sensations, ii. 339 
 Intercostales muscles, respiratory action of, i. 
 
 402, 407 
 Intermedius nerve of Wrisberg, central path of, 
 
 ii. 236 
 
 Intermittent pulse, i. 141 
 Internal capsule, grouping of fibres in, ii. 248 
 ear, anatomy of, ii. 371 
 rectus muscle, ii. 299 
 secretion, definition of, i. 265 
 of the adrenal bodies, i. 272 
 
GENERAL AV/>AA. 
 
 537 
 
 Internal secretion of the kidneys, i. ^74 
 of the liver, i. 265 
 of the ovaries, i. 27-1 
 of the pancreas, i. :J(i( - > 
 of the pituitary body, i. -J73 
 of the testis, i. 273 
 of tlu thyroids, i. 27u 
 Intestinal eontent>, reaction of, i. 310 
 digestion, i. 2J>9 
 juice, i. 213 
 movements, i. 382-385 
 Intestines, iuuervatiou of, i. 364 
 
 intrinsic nervous mechanism of, i. 384 
 large, absorption in, i. 314 
 penduhir movements of, i. 384 
 peristalsis nf. i. 382 
 putrefactive changes in, i. 310 
 small, absorption in, i. 313 
 vaso-motor nerves of, i. '206 
 Intraeardiac pressure, i. 107, 125, 126 
 
 methods of measuring, i. 129, 130 
 Intraeranial pressure, relation of, to blood-pres- 
 sure, ii. 287 
 
 Intra-ocular images, ii. 320 
 Intrapolar region, ii. 62 
 Intrapulmonary pressure, i. 408 
 Intrathoracic pressure, i. 397, 409 
 Intravascular clotting, i. 60, 61 
 Intrinsic nerves of the heart, i. 148 
 Introductory contractions of a contraction series, 
 
 ii. 113 
 
 peak of tetanus curves, ii. 124 
 Inversion of retinal images, ii. 305 
 Invertase, occurrence of, i. 308 
 Invertebrates, conduction in the nerves of, ii. 91 
 Invertine, definition of, i. 280 
 Involuntary muscles, rigor mortis of, ii. 162 
 Iodine, i. 509 
 
 lodothyrin, properties of, i. 270 
 Ionic theory of solutions, i. 67 
 lon-proteid compounds of muscle, ii. 168 
 Iris, dilator nerves of, ii. 324 
 direct response to light by, ii. 324 
 innervation of, ii. '.'>'-'> 
 movements of, in accommodation, ii. 309 
 
 rate of, ii. 325 
 muscles of, ii. 323 
 
 relation of, to spherical aberration, ii. 315 
 Iron, amount of, in haemoglobin, i. 39 
 excretion of, i. 530 
 inorganic absorption of, i. 529 
 nutritive history of, i. 528 
 occurrence of, i. 528 
 salts, excretion of, i. 356 
 
 nutritive value of, i. 356 
 synthesis of, into haemoglobin, i. 529 
 Irradiation in the retina, ii. 349 
 of medullary centres, i. 201 
 of nerve-impulses in the central nervous sys- 
 tem, ii. 208 
 Irrigating fluids for the isolated heart, i. 189, 
 
 191 
 
 Irritability, definition of, ii. 20, 23 
 effect of blood-supply on, ii. 66 
 of constant current on, ii. 62 
 of repeated stimuli on, ii. 65 
 of living matter, i. 18 
 of muscle, ii. 'J."> 
 of nerve-fihn-s. ii. 21 
 of nerves, ii. 24 
 
 and muscles, conditions affecting, ii. 55 
 effect of section on, ii. 69 
 of ova, ii. 22 
 Irritants, classification of, ii. 23 
 
 conditions determining their efficiency, ii. 28 
 effect of, on irritability, ii. 55 
 
 of variations in strength of, ii. 39 
 
 Irritants, n-latinn of, to the IV^POIIM-. ii. Ml 
 
 Ischa-mia !' heart mnsele, i. 181 
 
 Ischio-cavernosi muscle.-,, ii. 1 )! 
 
 Isrhio caverno.Mis. action of. in erection, ii |>;i 
 
 Iso-lmtyl alcohol, i. .">:;; 
 
 Iso-butyrie acid, i. :>:;! 
 
 ISO-dynamic equivalence of foods, i, 365 
 
 Isolated apex of frog's heart. L 186 
 
 conduction in nerve-trunks, ii. T'.i 
 Isolation of tin- heart, i. 148, 191 
 Isomaltose, i. .';:> 
 
 Isometric contractions, definition of, ii. 110 
 Iso-pentyl alcohol, i. :>:;! 
 Isotonic contractions, definition of, ii. 110 
 
 solutions, i. 36, 69 
 Isotouicity, i. 36, 68 
 
 Isotropic substance of muscle-fibres, ii. 104 
 Iso-valeriaiiic acid, i. 539 
 
 JAUNDICE, i. 249, 544 
 Jecorin, i. 564 
 
 Joints, classification of, ii. 415 
 Jumping, ii. 420 
 
 KARYOKINESIS, i. 20 
 
 Karyokinetic figures in mature nerve-cells, ii. 
 
 202 
 
 Katabolism, definition of, i. 19 
 Katelectrotonus, ii. (>_' 
 Kathodal contraction, ii. 35 
 Kathode, physical definition of, ii. 52 
 
 physiological definition of, ii. 52 
 Keratin, i. 580 
 Ketoses, definition of, i. 561 
 Keys, electric, ii. 30 
 Kidneys, blood-flow through, i. 255 
 
 histology of, i. 249 
 
 internal secretion of, i. 274 
 
 nerve-endings in, i. 251 
 
 vaso-motor nerves of, i. 207, 256 
 " Klopf-versuch " of Goltz, i. 175 
 Knee-kick, reinforcement of, ii. 222 
 Krause's membrane, ii. 104 
 Kymograph, i. 89 
 
 LABIA majora, ii. 462 
 
 minora, ii. 462 
 Labial consonants, ii. 437 
 Labio-dental frictionals, ii. 438 
 Labium tympanicum of the internal ear, ii. 377 
 
 vestibulare of the limbus. ii. 377 
 Labor, nature of, ii. 481 
 
 stages of, ii. 47!> 
 Labor-pains, ii. 479 
 
 Labyrinth of the ear, anatomy of, ii. 371 
 Lactalbumin, i. :J'il 
 Lactation, ovtilation during, ii. 456 
 Lacteal vessels, i. 318 
 Lacteals, absorption through, i. 311 
 Lactic acid, i. 545 
 
 fermentation, i. 545 
 occurrence of, in the stomach, i. 289 
 of muscles, ii. 168 
 Lacto-globulin, i. 261 
 Lactose, i. 262, 565 
 Laky blood, i. 35 
 Lamina spiralis, ii. 372 
 
 Laminae of medullary tube in tin- fn-tus. ii. ?(>.") 
 Langerhans, bodies of, i. 232 
 Lanolin, i. 257. 575 
 Large intestine, digestion in, i. 3 
 Laryngeal muscles, specific action of. ii. 428 
 
 nerve, recurrent, ii. I'.'- 
 
 superior, ii. !_'- 
 Laryngoscope, ii. I'-!' 
 Larynx, cartilages of. ii. 425 
 
 closure of, during muscular effort, ii. 423 
 
538 
 
 GENERAL INDEX. 
 
 Larynx, muscles of, ii. 425 
 nerves of, ii. 42b 
 structure of, ii. 421 
 Latent areas of the cortex, ii. 261 
 
 characters, hereditary transmission of, ii, 495 
 
 heat, definition of, i. 504 
 
 period, effect of temperature on, ii. 136 
 
 of tension on, ii. 110 
 of cardiac accelerator nerves, i. 170 
 of heart muscle, i. 153 
 of motor end-plates, ii. 103 
 of muscle, ii. 103 
 of red muscle, ii. 109 
 of retinal stimulation, ii. 343 
 of simple muscular contraction, ii. 102 
 of vagus-stimulation, i. 162 
 Latham's hypothesis of the structure of pro- 
 toplasm, i. 24 
 Laughing, i. 454 
 
 "Law of contraction," Pfliiger's, ii. 50 
 Law of stimulation of human nerves by battery 
 
 currents, ii. 54 
 Lecithin, i. 559 
 amount of, in the blood, i. 51 
 occurrence of, i. 325 
 of bile, i. 45 
 of milk, i. 261 
 of nerves, ii. 169 
 Leech extract, effect of, on blood coagulation, i. 
 
 62 
 
 lymphagogic action of, i. 73 
 Lemniscus, medial, i. 226 
 
 sensory paths entering, ii. 235 
 Lens, changes in, during accommodation, ii. 307 
 crystalline, changes in, with old age, ii. 314 
 curvatures of, ii. 303 
 opacities in, ii. 321 
 refractive index of, ii. 303 
 thickness of, ii. 303 
 Lenticular ganglion, ii. 311 
 Leucin, action of, on end-plates, ii. 27 
 chemical properties of, i. 540 
 formation of, in tryptic digestion, i. 303 
 nutritive history of, i. 540 
 occurrence of, i. 540 
 Leucocytes, behavior of, in blood capillaries, i. 
 
 82 
 
 classification of, i. 47, 48 
 emigration of, i. 83 
 
 from the thymus gland, composition of, i. 51 
 functions of, i. 48 
 influence of, on blood-plasma, i. 49 
 movements of, ii. 19 
 origin of, i. 49 
 Leucocythsemia, fatty acids in, i. 530 
 
 purin bases excreted in, i. 557 
 Leuconuclein, effect of, on intravascular clot- 
 ting, i. 61 
 
 Leucophrys patula, reproduction of, ii. 442 
 Levatores ani muscles, expiratory action of, i. 
 
 407 
 
 costarum breves, inspiratory action of, i. 402 
 Levulic acid, i. 538 
 
 fate of, in pancreatic diabetes, i. 267 
 Levulose, i. 562 
 
 occurrence of, i. 564 
 oxidation of, in diabetes, i. 564 
 Lieberkiihn's crypts, histology of, i. 243 
 Liebig's method of urea determination, i. 549 
 Life, general hypothesis of, i. 25 
 
 of the individual, stages of, ii. 486 
 Ligaments of the incus, ii. 367 
 
 of the malleus, ii. 366 
 Ligatures of Stannius, i. 178 
 Light, changes in the retina produced by, ii. 330 
 definition of, ii. 298 
 dispersion of, ii. 316, 332 
 
 Light, monochromatic, ii. 316 
 
 physical theory of, ii. 331 
 
 rays of the luminiferous ether, ii. 331 
 
 sensations, intensity of, ii. 332, 339 
 
 mechanism for the production of, ii. 331 
 quality of, ii. 332 
 Light-waves, lengths of, ii. 332 
 Limbs, vaso-motor nerves of, i. 209 
 Limbus of the spiral lamina, ii. 377 
 Lingual frictiouals, ii. 438 
 
 nerve, gustatory function of, ii. 410 
 Linguo-palatal consonants, ii. 437 
 Lipase, i. 305 
 Lipochromes, i. 574 
 Liqueurs, i. 535 
 Liquids, ii. 436 
 Liquor amnii, ii. 472 
 
 folliculi, ii. 454 
 Listing's law, ii. 299 
 Littre, glands of, ii. 448 
 
 Liver, defensive action of, against intravascular 
 clotting, i. 61 
 
 extirpation of, i. 336 
 
 functions of, i. 320 
 
 histology of, i. 244, 321 
 
 internal secretion of, i. 265 
 
 lymph formation in, i. 73 
 
 nerve-endings in, i. 245 
 
 secretory function of, i. 244 
 nerves of, i. 247 
 
 urea formation in, i. 331 
 
 vaso-motor nerves of, i. 206 
 Living matter, elementary constituents of, i. 
 
 499 
 
 general properties of, i. 18 
 molecular structure of, i. 23 
 Load, effect of, on latent period of muscle, ii. 110 
 
 on the contraction curve, ii. Ill 
 Local signs of sensations, ii. 394 
 Localization, cutaneous variations of, ii. 395 
 
 in the skin, theory of, ii. 395 
 
 of cell-groups in the cerebral cortex, ii. 241 
 
 of cortical cell-groups for afferent impulses, ii. 
 252 
 
 of pain sensations, ii. 399 
 
 of touch sensations, ii. 394 
 
 power, relation of, to mobility, ii. 394 
 Locomotion, ii. 420 
 
 Locomotor ataxy, disturbance of equilibrium 
 in, ii. 405 
 
 mechanisms, action of, ii. 414 
 Loew's hypothesis of the structure of proto- 
 plasm, i. 23 
 
 Long-reed register of the voice, ii. 432 
 Long tracts of the cord, terminations of, ii. 235 
 Loop of Henle, i. 250 
 Loudness of the voice, factors determining, ii. 
 
 430 
 
 physical cause of, ii. 381 
 
 Luminiferous ether, rates of vibration of, ii.331 
 Luminosity, relative, of spectral colors, ii. 340 
 Luminous sensations, intensity of, ii. 339 
 Lungs, capacity of, i. 427 
 
 nerve-supply of, i. 465 
 
 structure of, i. 396 
 
 vaso-motor nerves of, i. 205 
 Lunulse of the semilunar valves, i. Ill 
 Lustre in visual sensations, explanation of, ii. 
 
 358 
 
 Lutein, i. 574 
 
 Luxus consumption, i. 348 
 Lymph, i. 33 
 
 amount of, i. 146 
 
 definition of, i. 70 
 
 formation of, i. 71 
 
 gases of, i. 419 
 
 glands, i. 146. 
 
GENERAL INDl-IX. 
 
 539 
 
 Lvmph, mechanical t hcory of t he origin of, 1.75 
 
 movement of. i. 71, 140 
 
 pressure of, i. 14G 
 
 -.cretiou of, i. ~1 I 
 Lymphagogues, action of, \.1'.\, 71 
 Lymphatic, system, nature of. i. 145 
 
 Lymphatics of tin- heart, i. 1><J 
 
 Lymphocytes, i. 48 
 
 Lysatin, i. .">.">! 
 
 Lysatinin, relation of, to urea formation, i. 337, 
 
 551 
 Lysin, i. 552 
 
 M \rKocKfiiAi.ic brains, weight of, ii. 275 
 Macula acustica, ii. 373 
 
 lutea. ii. :'-': 
 
 Macula- acus'ticse, relation of, to state of equi- 
 librium, ii. 407 
 Magnesium carbonate, i. 527 
 nutritive history of, i. 527 
 occurrence of, i. 527 
 phosphates,.!. 527 
 
 Making contraction, point of origin of, ii. 35 
 " Making" shock, ii. 31 
 Male births, relative number of, ii. 483 
 
 prouucleus, ii. 466 
 Mali-s, rate of growth in, ii. 488 
 Malic acid, i. 558 
 Malleus, ii. 365 
 
 ligaments of, ii. 366 
 Malpighian corpuscle of the kidney, structure 
 
 of, i. 249 
 Maltase, i. 280, 565 
 
 in starch digestion, i. 285 
 occurrence of, i. 308 
 Mammary glands, ii. 443, 462 
 histological changes in, i. 262 
 in pregnancy, ii. 477 
 normal secretion of, i. 264 
 secretory nerves of, i. 263 
 structure of, i. 261 
 Man nose, i. 562 
 
 Manometer, differential, i. 131 
 elastic, i. 127 
 maximum, i. 107 
 mercurial, i. 87 
 
 Manubrium of the malleus, ii. 365 
 Marsh gas, i. 532 
 Masseter muscle, i. 372 
 Mastication, i. 372 
 
 Masticatory movements, effect of, on taste-sen- 
 sations, ii. 411 
 Mastoid antrum, ii. 363 
 
 " Mastzellen," relation of, to colostrum corpus- 
 cles, i. 263 
 
 Maturation of germ-cells, significance of, ii. 454 
 of nerve-cells, ii. 177 
 of spermatozoa, ii. 445 
 of the ovum, ii. 451 
 
 Meat extracts, physiological action of, i. 359 
 Meats, composition of, i. 278 
 Meatus auditorius internus, ii. 373 
 Mechanical stimulation of nerves, ii. 25, 56 
 strain, influence of, on neuroblasts. ii. 176 
 work of muscular contraction, ii. 138 
 Meconium. biliary salts in, i. 544 
 Medial lemniscus, ii. 226 
 
 Medullary sheath, development of, in the cen- 
 tral nervous system, ii. 181 
 in the peripheral nerves, ii. 180 
 significance of, ii. 180 
 tube, fetal, ii. 204 
 
 lamina- of, in the foetus, ii. 205 
 Medullation. central, progressive character of, 
 
 ii. 1-1 
 
 of nerve-fibres, significance of, ii. 283 
 peripheral, ii. 180 
 
 Medusas, rate of conduct ion in, ii. 89 
 
 staircase contractions in. ii. 1 TJ 
 Melanins. i. 574 
 Melicyl alcohol, i. 540 
 Membrana hasilaris, ii. 374 
 
 flaccida, ii. '',>>'< 
 
 granulosa of the Graafian follicle, ii. -ir.1 
 
 reticulata, ii. 378 
 
 tectoria. ii. .'{77. :i7'.i 
 
 tympani, ii. :;ii| 
 
 Membrane of Reissue r, ii, :;7t, 379 
 Membranous labyrinth of the ear, ii. 372 
 Menopause, ii. 459, 490 
 Menstruation, ii. 457 
 
 age of onset of, ii. 459 
 
 cessation of, at the climacteric, ii. 490 
 
 general disturbances accompanying, ii. 459 
 
 in animals, ii. 460 
 
 relation of ovulation to, ii. 456 
 
 theory of, ii. 460 
 Mental activity, relation of cerebral circulation 
 
 to, ii. 288 
 
 Menthol, action of, on cold nerves, ii. 398 
 Mercapturic acids, i. 547 
 Mercury manometer, description of, i. 87 
 Metabolism, conditions influencing, i. 359 
 
 definition of, i. 20 
 
 during sleep, i, 361 
 starvation, i. 362 
 
 effect of temperature on, i. 362 
 
 influence of the cell-nucleus on, i. 22 
 
 intensity of. in the brain, ii. 288 
 
 methods of estimating, i. 343 
 Metaphosphoric acid, i. 514 
 Methsemoglobin, i. 44, 573 
 Methane, origin of, i. 532 
 Methods, physiological, i. 31 
 Methyl amido-acetic acid, i. 538 
 
 mercaptan, i. 534 
 
 selenide, i. 534 
 
 telluride, i. 534 
 
 violet, in testing for mineral acids, i. 289 
 Methylamine, i. 541 
 Meynert's commissure, ii. 238 
 Micella}, definition of, i. 25 
 Microcephalic brains, weight of, ii. 275 
 Microcephalies, ii. 268 
 Micturition, i. 389 
 
 centre for, i. 391, 393 
 
 cerebral control of, ii. 270 
 
 nervous mechanism of, i. 392 
 
 reflex character of, ii. 213 
 Middle ear, ii. 362 
 
 inflammatory disease of, ii. 364 
 mechanism of, ii. 368 
 muscles of, ii. 369 
 Migration of neuroblasts, ii. 176 
 Milk, composition of, i. 261 
 
 mineral constituents of, i. 530 
 
 normal secretion of, i. 264 
 Milk-sugar, i. 565 
 
 Millon's reaction for proteids, i. 576 
 nature of, i. 569 
 with phenol, i. 569 
 Mineral acids, tests for, i. 289 
 
 constituents, amount of, in the tissues, i. 530 
 Mitosis, i. 20 
 Modiolus, ii. 372 
 Molecular weight, relation of, to physiological 
 
 action, ii. Ho 
 
 Molecules, physical and physiological, i. 25 
 Monochromatic light, ii. 316 
 Mononuclear leucocytes, i. 48 
 Mons Veneris, ii. 462 
 Monstrosities, congenital, ii. 494 
 
 origin of, ii. 483 
 Morgagni, ventricles of, ii. 422 
 
540 
 
 GENERAL INDEX. 
 
 Morphin, effect of, on body-temperature, i. 472 
 
 Morula, ii. 470 
 
 Motor areas, cortical serial arrangement of, ii. 
 
 247 
 
 degeneration after removal of, ii. 244 
 paralysis following removal of, ii. 269 
 physiological characters of, ii. 243 
 subdivision of, into centres, ii. 247 
 
 centres, degree of separateness of, ii. 248 
 of the human cortex, ii. 250 
 
 disturbance from hemisection of the cord, ii. 
 230 
 
 end-plates, latent period of, ii. 103 
 
 transmission of excitation by means of, ii. 
 82 
 
 nerves, fatigue of, ii. 96 
 
 rate of conduction in, ii. 89 
 Mouth, temperature in, i. 469 
 Movements of joints, varieties of, ii. 416 
 
 of spermatozoa, ii. 444 
 
 of the eyeball, ii. 298 
 Mucin of bile, i. 325 
 
 of gastric juice, i. 288 
 
 of saliva, i. 283 
 
 physiological value of, i. 221 
 
 properties of, i. 578 
 
 secretion of, i. 217 
 Mucous glands, histology of, i. 216 
 Miiller's experiment, i. 452 
 Multiple conceptions, ii. 482 
 
 control from the cortex, ii. 250 
 Murexid, i. 555 
 Muscse volitantes, ii. 320 
 Muscarin, i. 543 
 
 action of, on the heart, i. 150 
 Muscle, accelerator urinse, ii. 449 
 
 aryteno-epiglottidean, ii. 426 
 
 arytenoid, ii. 426 
 
 bulbo-cavernosus, ii. 449 
 
 chemistry of, ii. 159 , 
 
 ciliary, in accommodation, ii. 309 
 
 crico-arytenoid, lateral, ii. 426 
 
 crico-thyroid, ii. 426 
 
 currents of action in, ii. 150 
 of rest in, ii. 147 
 
 digastric, i. 372; ii. 426 
 
 elasticity of, ii. 105 
 
 erector clitoridis, ii. 464 
 
 external rectus, ii. 299 
 
 fatigue of, ii. 70 
 
 frog's, rate of conduction in, ii. 89^. 
 
 gases of, ii. 168 
 
 general physiology of, ii. 17 
 
 genio-hyoid, i. 372; ii. 426 
 
 glycogenic function of, i. 330 
 
 human, rate of conduction in, ii. 89 
 
 hyo-glossus, ii. 426 
 
 independent irritability of, ii. 25 
 
 inferior oblique, ii. 299 
 rectus, ii. 299 
 
 inorganic constituents of, ii. 168 
 
 internal rectus, ii. 299 
 
 involuntary, properties of, i. 370 
 
 limitation of the rate of stimulation in, ii. 126 
 
 masseter, i. 327 
 
 mineral constituents of, i. 530 
 
 mylo-hyoid, i. 372 ; ii. 426 
 
 nitrogenous extractives of, ii. 166 
 
 non-nitrogenous constituents of, ii. 168 
 
 obliquus exteruus, i. 407 
 internus, i. 407 
 
 omo-hyoid, ii. 425 
 
 posterior crico-arytenoid, ii. 426 
 
 pterygoid, external, i. 372 
 internal, i. 372 
 
 pyramidalis, i. 407 
 
 reaction of, ii. 159 
 
 Muscle, red, capacity for tetanic contraction of, 
 
 ii. 127 
 
 retractor lentis, of fishes, ii. 306 
 skeletal, sensory nerve-endings in, ii. 402 
 specific gravity of, ii. 159 
 stapedius, ii. 370 
 sterno-hyoid, ii. 425 
 sterno-thyroid, ii. 425 
 striated, histology of, ii. 104 
 optical properties of, ii. 103 
 stylo-hyoid, ii. 426 
 superior oblique, ii. 299 
 
 rectus, ii. 299 
 temporalis, i. 372 
 thyro-arytenoid, ii. 426 
 external, ii. 424 
 internal, ii. 424 
 thyro-hyoid, ii. 425 
 transversalis abdominis, i. 407. 
 trapezius, i. 405 
 Muscle-contraction, Engelmann's theory of, ii. 
 
 105 
 
 Muscle-plasma, ii. 161, 163 
 Muscle-proteids, precipitation temperature of, ii. 
 
 166 
 Muscles, abdominales, action of, in vomiting, i. 
 
 387 
 
 respiratory function of, i. 407 - 
 absolute force of, ii. 141 
 action of, upon the bones, ii. 417 
 classification of, ii. 18 
 degeneration of, after section of motor nerves, 
 
 ii. 48, 54, 70 
 endurance of. ii. 76 
 erectores penis, ii. 449 
 
 spinse, i. 405 
 expiratory, i. 407 
 glycogeu of, i. 330 
 human, fatigue of, with artificial stimulation, 
 
 ii. 134 
 
 infrahyoidei, i. 405 
 inspiratory, i. 399, 405 
 intercostal, i. 402, 407 
 ischio-cavernosi, ii. 449 
 levatores ani, i. 407 
 
 costarum, i. 402 
 of mastication, i. 372 
 of the eye, ii. 299 
 of the iris, ii. 323 
 of the middle ear, ii. 369 
 pectorales, i. 405 
 quadrati lumborum, i. 399 
 rate of conduction in, ii. 89 
 rhomboidei, i. 405 
 scaleni, i. 401 
 serrati postici, i. 399, 402 
 sterno-cleido-mastoid, i. 404 
 tensor tympani, ii. 369 
 thermogenic function of, i. 490 
 triangulares sterni, i. 407 
 vaso-motor nerves of, i. 210 
 Muscle-serum, ii. 166 
 Muscle-sounds, ii. 132 
 Muscle-spindles, ii. 402 
 
 Muscle-structure, influence of, on its contrac- 
 tion, ii. 107 
 Muscle-tonus, ii. 143 
 
 Muscular contractions, effect of drugs on, ii. 137 
 of support on the height of, 122 
 of temperature on, ii. 136 
 graphic record of, ii. 99 
 influences affecting, ii. 107 
 post-mortem, ii. 160 
 source of energy in, ii. 74 
 effort, closure of larynx in, ii. 423 
 exercise, effect of, on metabolism, i. 359 
 on the pulse-rate, i. 121 
 
GENERAL INDEX. 
 
 541 
 
 Muscular exercise, effect of, on the rate of 
 
 respiration, i. Ui 
 on the respiratory exchanges, i. 433 
 
 quotient, i. \'.\^ 
 on the sweat glands, i. 260 
 on the venous circulation, i. 1)5 
 inhibit ion from cortical stimulation, ii. 224 
 movements, relations of antagonistic muscles 
 
 in, ii. 418 
 
 sensations, cortical area for, ii. 253 
 definition of, ii. 390 
 
 effect of hemisectiou of the cord on, ii. 235 
 in estimation of weights, ii. 403 
 nature of, ii. 401 
 
 path of conduction for, in the cord, ii. 235 
 psychological value of, ii. 391 
 work, effect of stimulants on, ii. 7"> 
 Musical sounds, characteristics of, ii. 387 
 tones, beats of, ii. 386 
 
 limits in the pitch of, ii. 282 
 production of, ii. 381 
 Mycoderma aceti, i. 537 
 Mydriatics, ii. 325 
 Mylo-hyoid muscle, i. 372 ; ii. 426 
 Myo-albuinin, ii. 166 
 Myo-albumose, ii. 166 
 Myogen-fibrin, ii. 164 
 Myoglobulin, ii. 166 
 
 Myogonic theory of the causation of the heart- 
 beat, i. 150 
 Myogram, ii. 34 
 definition of, ii. 100 
 
 effect of temperature on the form of, ii. 136 
 Myograph, ii. 35 
 
 description of, ii. 100 
 double, of Heriug, ii. 36 
 Myohsematin, i. 578 ; ii. 166 
 Myopia, ii. 313 
 
 range of accommodation in, ii. 314 
 Myosin, ii. 163 
 absorption of, i. 315 
 ferment, ii. 161, 163 
 Myosin-fibrin, ii. 164 
 Myosinogen, ii. 163 
 
 temperature of heat coagulation of, ii. 165 
 Myotics, ii. 325 
 Myxcedema, i. 269 
 
 NATIVE albumins, i. 577 
 
 Nausea from disturbance of equilibrium, ii. 405 
 Near-point of vision, ii. 312 
 Negative after-images, ii. 346 
 pressure in the auricles, i. 137 
 in the heart, i. 98 
 in the thorax, i. 95 
 in the veins, i. 94 
 variation in muscles, rate of propagation, ii. 
 
 152 
 
 relation of, to the contraction, ii. 153 
 of muscle-currents, ii. 150 
 of nerve-currents, ii. 154 
 of the beating heart, i. 153 
 Nerve, auriculo-temporal, i. 218 
 chorda tympani, i, 194, 219 
 
 gustatory function of, ii. 410 
 coronary, of the tortoise, i. 164 
 depressor, i. 172, 203 
 facial, secretory fibres of, i. 219 
 general physiology of, ii. 17 
 glossopharyngeal, gustatory function of, ii. 410 
 
 secretory fibres of. i. 218 
 Jacobson's, i. 218 
 lingual, secretory fibres of, i. 219 
 oculomotor, ii. '.\'l'-\ 
 recurrent laryngeal, ii. 428 
 small superficial petrosal, i. 218 
 superior laryngeal, ii. 428 
 
 
 Nerve, vakils, eanliae branches of, i. l.V.) 
 gastric branches of, i. :;-l 
 intestinal branches of, i. !{.<> 
 pulmonary branches of, i. Jiif, 
 repiratory functions of. i. !.">!! 
 secretory fibres of, i 
 
 trophic influence of, on the heart, i. 166 
 Nerve-cells, atrophy of. from disuse, ii. 195 
 
 changes in, with age, ii. mo 
 
 chemical changes in. ii. 1!M 
 
 classification of, ii. 177 
 
 connection between, ii. 206 
 
 degeneration of the cell-bodies of, ii. 199 
 
 diaxonic, ii. 178 
 
 effect of exercise on, ii. 76 
 
 fatigue of, ii. 191 
 
 generation of impulses in, ii. 188 
 
 growth of, ii. 176 
 
 histological change in, due to functional 
 activity, ii. I!'-.' 
 
 human, size of, ii. 174 
 
 internal structure of, ii. 179 
 
 maturation of, ii. 177 
 
 morphology of, ii. 173 
 
 number of, in the central nervous system, ii. 
 888 
 
 nutrition of, ii. 190 
 
 nutritive control of, over nerve-fibres, ii. 198 
 
 of animals, size of, ii. 175 
 
 of spinal ganglia, development of, ii. 178 
 
 peculiarities of, ii. 174 
 
 pyramidal, ii. 178 
 
 rate of discharge from, ii. 189 
 
 regeneration of, ii. 201 
 
 relation of size and function in, ii. 175 
 
 senescence of, ii. 182, 490 
 
 significance of the branches of, ii. 186 
 
 summation of stimuli in, ii. 190 
 
 volume relation of, ii. 175 
 
 Nerve-elements, primitive segmental arrange- 
 ment of, ii. 205 
 Nerve-endings in the liver, i. 245 
 
 in the salivary glands, i. 220 
 
 in the skin, ii. 392 
 Nerve-fibres, reaction of, ii. 170 
 
 classification of, ii. 21 
 
 cortical, increase in the number of, during 
 growth, ii. 282 
 
 fatigue in, ii. 95 
 
 functions of, ii. 21 
 Nerve-impulse, definition of, ii. 25 
 
 direction of the passage of, ii. 184 
 
 electrical variation accompanying, ii. 183 
 
 generation of, ii. 187 
 
 in peripheral nerves, ii. 183 
 
 peripheral diffusion of, ii. 218 
 
 reversed, in spinal ganglion-cells, ii. 185 
 
 theories of, ii. 97 
 
 transmission of, from neurone to neurone, ii. 
 
 -.'( )T 
 
 Nerve-muscle preparation of a frog, ii. 34 
 Nerves, action currents in, ii. 153 
 
 auditory, central path of, ii. 237 
 
 augmentor, of the heart, i. 167 
 
 cardiac, i. 148 
 
 cervical sympathetic, i. 193 
 
 chemistry of, ii. H>!> 
 
 cross-suturing of, ii. 200 
 
 current of rest in. ii. 149 
 
 degeneration of, after section, ii. 69, 78 
 
 depressor, of the heart, i. !?-' 
 
 fatigue of, ii. 7~> 
 
 glossopharyngeal, central conduction paths 
 for. li.238 
 
 in man, stimulation of. ii. .",1 
 
 law of stimulation of, with galvanic current, 
 ii. 50 
 
542 
 
 GENERAL INDEX. 
 
 Nerves, limitation of the rate of stimulation in, 
 
 ii, 126 
 
 lingual, gustatory function of, ii. 410 
 medullation of, ii. 180 
 
 non-medullated, rate of conduction in, ii. 90 
 of common sensation, central conduction 
 
 paths of, ii. 230 
 of cutaneous sensations, central conduction 
 
 paths of, ii. 233 
 of dermal sensation, area of distribution of, ii. 
 
 231 
 
 of invertebrates, rate of conduction in, ii. 91 
 of taste, nuclei of origin of, ii. 236 
 of temperature, ii. 397 
 of the bile vessels, i. 248 
 of Wrisberg (intermedius), central path of, 
 
 ii. 236 
 
 olfactory, central paths of, ii. 241 
 optic, central paths of, ii. 238 
 phrenic, i. 463 
 rate of conduction in, ii. 89 
 secondary degeneration of, ii. 197 
 septal, of the frog's heart, i. 166 
 specific energy of, ii. 232 
 splanchnic, i. 173 
 trigeminal. i. 463 
 
 central paths of, ii. 238 
 
 vagus, course of the afferent fibres in, ii. 236 
 Nerve-trunks, isolated conduction in, ii. 79 
 Nervi erigentes, ii. 464 
 
 intestinal branches of, i. 385 
 Neukomm's test for bile acids, i. 545 
 Neuridin, i. 543 
 Neurin, i. 543 
 
 Neuroblast, development of, ii. 176 
 Neurogenic theory of the causation of the 
 
 heart-beat, i. 149 
 Neuro-keratin, i. 580 
 
 of nerves, ii. 169 
 Neuromuscular spindle, ii. 390 
 Neurone, definition of, ii. 173 
 Neurones, ii. 21 
 
 afferent, to the spinal cord, ii. 203 
 changes in number and size of, ii. 280 
 conduction in, ii. 97 
 connection by, ii. 206 
 double conduction in, ii. 185 
 increase in number of, during growth, ii. 282 
 internal structure of, ii. 179 
 polarity of, ii. 184 
 total number of, ii. 283 
 Neutral salts, effects of, on blood coagulation, i. 
 
 62 
 
 Neutrophiles, i. 47 
 
 Nicotin, action of, on end-plates, ii. 27 
 on intestinal movements, i. 384 
 on secretory nerves, i. 229 
 on sympathetic ganglia, ii. 219 
 Nissl method for nerve-cells, ii. 195 
 substance, iron in, ii. 191 
 
 of nerve-cells, ii. 179 
 Nitric oxide, i. 512 
 
 haemoglobin, i. 39, 512 
 
 Nitrogen equilibrium, definition of, i. 344, 512 
 free, of muscles, ii. 168 
 history of, in the body, i. 512 
 inhalation, i. 440 
 occurrence of, i. 510 
 of the feces, i. 320 
 preparation of, i. 510 
 tension of the blood, i. 417 
 Nitrogenous equilibrium, definition of, i. 344, 
 
 512 
 excreta of milk, i. 262 
 
 of sweat, i. 259 
 extractives of muscle, ii. 166 
 of the spleen, i. 333 
 
 Nitrogenous metabolism, estimation of, i. 343 
 Nitrous oxide, inhalation of, i. 440 
 
 properties of, i. 512 
 Nodal point in the simplest dioptric system, ii. 
 
 301 
 
 Noaud vital, i. 456 ; ii. 236 
 Noises, definition of, ii. 388 
 Non-medullated nerves, rate of conduction in, 
 
 ii. 90 
 
 stimulation fatigue of, ii. 180 
 Non-polarizable electrodes, ii. 29 
 Nose, anatomy of, ii. 408 
 
 respiratory 'tract of, ii. 408 
 Nucleic acid, i. 579 
 Nuclein bases, i. 552 
 
 composition, i. 556, 579 
 Nucleo-histon of the blood-plates, i. 49 
 
 relation of, to intravascular clotting, i. 61 
 Nucleo-proteids, classification of, i. 577 
 
 properties of, i. 579 
 Nucleus, functions of, i. 22 
 
 relation of, to oxidation, i. 503 
 Nutrition of living matter, i. 18 
 of nerve-cells, ii. 190 
 of the embryo, ii. 475 
 Nutritive control of nerve-cell bodies over 
 
 nerve-fibres, ii. 198 
 value of albuminoids, i. 349 
 of carbohydrates, i, 353 
 of fats, i. 350 
 of proteids, i. 276, 345 
 of salts, i. 354 
 of water, i. 354 
 Nymphse, ii. 462 
 Nystagmus after cerebellar injury, ii. 272 
 
 OBLIQUUS externus, respiratory action of, i. 
 407 
 
 interims, respiratory action of, i. 407 
 Occlusion of the bile-duct, effect of, i. 249 
 Oculomotor nerve, ciliary fibres of, ii. 311 
 
 relation of, to the iris, ii. 323 
 Odors, ii. 410 
 CEdema, i. 148 
 
 of the glottis, ii. 422 
 (Esophagus, deglutition in the, i. 374 
 Oils, effect of, on gastric secretion, i. 241 
 
 on pancreatic secretion, i. 236 
 Old age of the central nervous system, ii. 295 
 Olefines, i. 542 
 Oleic acid, i. 541-560 
 Olfactory area of the cortex, ii. 253 
 
 cells, ii. 408 
 
 epithelium, ii. 408 
 
 nerves, central paths of, ii. 241 
 
 paths to the brain, ii. 409 
 
 stimuli, conditions affecting, ii. 409 
 
 tracts, section of, in sharks, ii. 264 
 Omo-hyoid muscle, ii. 425 
 Oncometer, i. 255 
 
 Ontogenetic development of nerve-cells, ii. 177 
 Onychodromus, reproduction of, ii. 442 
 Oocyte, ii. 451 
 
 Oophorin tablets, action of, i. 274 
 Opening of the chest, effect of, on heart, i. 115 
 Ophthalmometer, ii. 304 
 Ophthalmoscope, ii. 326 
 
 Opium, effect of, on respiratory rhythm, i. 425 
 Optic commissure, decussation of optic fibres in, 
 ii, 238 
 
 nerve, currents of action in, ii. 154 
 
 nerve-fibres, number of, ii. 330 
 
 nerves, central paths of. ii. 238 
 cortical centres of, ii. 240 
 efferent fibres of, ii. 240 
 
 thalami, functions of, ii. 271 
 Optical constants of the eye, ii. 303 
 
643 
 
 Optical illusion* in binocular vision, ii. T>! 
 of span- perceptions, ii. .Til 
 
 properties of striated mn>cle. ii. In;; 
 Optograms. ii. ."i^ 11 
 Organ of Corti, structure of, ii. 377 
 
 of Golgi in tendons, ii |n-j 
 11 < )ri:anej\veiss," i. '.\\*> 
 
 < )rirani/ation in the central nervous system, ii. 
 286 
 
 relation of educability to the establishment 
 
 of. ii. -.'Hi 
 
 Organs, growth of, ii. 486 
 Ornithin. i. 552 
 Orthophosphorie acid, i. 51-1 
 Os orbicnlare of the incus, ii. 366 
 Osa/.ones of glycoses. i. T.ti-J 
 Oscillatory activity of the retina, ii. 344 
 Osmosis, definition of, i. <>f> 
 
 relation of, to secretion, i. 213 
 Osmotic pressure, definition of, i. 65 
 method of determining, i. 67, 68 
 relation of, to concentration, i. 66 
 Osones, preparation of, i. 562 
 Ossicles, auditory, ii. 365 
 
 of the ear, movements of, ii. 367 
 Osteomalacia, i. 524, 525 
 
 ovariotomy in, i. 274 
 Osteoporosis, i. 525 
 Otitis media, ii. 364 
 Otoconia, ii. 374 
 Otoliths, ii. 374 
 Ova, ii. 440 
 
 number of, in human ovary, ii. 451 
 Ovaries, ii. 443 
 
 effect of removal of, on menstruation, ii. 459 
 internal secretion of, i. 274 
 Ovariotomy, effects of, i. '274 
 Ovary, structure of, ii. 454 
 Overtones, definition of, ii. 383 
 
 inharmonic, ii. 386 
 Oviducts, ii. 443. See Fallopian tubes. 
 Ovnlation, ii. -I.") 
 Ovum, chemistry of, ii. 450 
 
 fertilization of, ii. 466 
 
 human, structure of, ii. 449 
 
 maturation of, ii. 451 
 
 physiological properties of, ii. 22 
 
 segmentation of. ii. 407 
 
 stages in the maturation of, ii. 452 
 Oxalate solutions, effect of, on blood coagula- 
 tion, i. 63 
 Oxalic acid, i. 557 
 Oxaluric acid, i. r> ."."> 
 Oxidases, i. 281 
 Oxidation, i. 501 
 
 physiological, Hoppe-Seyler's theory of, i. 505 
 
 Traube's theory of, i. 502 
 Oxidizing enzymes, i. 280 
 Oxybutyric acid, i. 548 
 Oxycholin, i. 543 
 Oxygen, alveolar tension of, i. 413 
 
 occurrence of, i. 500 
 
 preparation of, i. 501 
 
 properties of, i. 501 
 
 storage of, in muscle, ii. 169 
 
 supply, relation of, to irritability, ii. 68 
 
 tension in the blood, i. -115 
 
 respiratory effects of varying, i. 440 
 Oxygen-absorption, coefficient of, i. 415 
 
 conditions affecting, i. 429 
 
 cutaneous, i. 422 
 
 estimation of, i. 428 
 Oxygen-dyspnrea, i. 444 
 Oxytaemoglobin, composition of, i. 38 
 
 dissociation of, i. 415, 501 
 Oxyntic cells of gastric glands, i. 237 
 Oxyphenyl-acetic acid, i. 570 
 
 Oxyphenyl-amido-propionic acid, i. .">?<) 
 
 Oxyphiles, i. 17 
 
 < )/.one inhalation, i. 1 |n 
 
 preparation of. i 
 
 properties ot, i. ;,n:j 
 
 PACINIAN body, ii. :;!! 
 of the pem>. ii. 449 
 Pain nerves, evidence for the existence of, ii. 232 
 
 points of the skin, ii. 400 
 
 sensations of. ii. :;!! 
 
 Pains, transferred or sympathetic, ii. 400 
 Pale striped muscle, physiological peculiarities 
 
 of. ii. 1U! 
 
 Palmitic acid, i. 541 . 
 Pancreas, anatomy of, i. 231 
 extirpation of, i. 266 
 grafting of, i. ~Ji>7 
 histology of, i. 231 
 innervation of, i. 232 
 internal secretion of, i. 266 
 mineral constituents of, i. 530 
 secretory changes in, i. 233 
 vaso-inotor nerves of, i. 207 
 Pancreatic diabetej^LJ267, 353, 563 
 fistulae, preparation ofpfc 300 
 juice, amyloly tic action of, i. 305 
 artificial, i. 30i- ... 
 collection of, i. 300 
 composition of, i. 232, 299 
 fat-splitting power of, i. 305 
 secretion, composition of, i. 232, 299 
 histological changes during, i. 233 
 nervous mechanism of, i. 232 
 normal mechanism of, i. 235 
 reflex character of, i. 236 
 relation of, to the character of the food, i. 
 
 237 
 
 Pangenesis, Darwin's theory of, ii. 501 
 Papain, i. 280 
 
 Papilla foliata of rabbits, ii. 410 
 Papillary muscles, i. 110 
 Parabamic acid, i. 555 
 Paracasein, i. 296 
 Paradoxical contraction, ii. 157 
 Paraffins, i. 531 
 Paraformic aldehyde, i. 533 
 Paraglobulin, amount of, in the blood, i. 53 
 composition of, i. 53 
 functions of, i. 53 
 origin of, i. 53 
 * properties of, i. 53 
 
 Parallax, use of, in estimation of distance, ii. 3."i 
 Paralysis after removal of motor areas, ii. 2<!!i 
 agitans, ii. 296 
 Brown-Sequard's, ii. 233 
 homolateral, after hemisection of the cord, 
 
 ii. 233 
 
 Paralytic secretion, i. 229 
 Paramcecium, reproduction in, ii. 440 
 PHramyosinogen. ii. 163 
 
 temperature of heat coagulation of, ii. 165 
 Parapeptone, definition of, i. 292 
 Parannclein. i. 579 
 Parathyroids, anatomy of, i. 268 
 
 function of, i. 269 
 Paresis following removal of the cerebellum, 
 
 ii. 272 
 
 from injury to motor areas, ii. 269 
 Parotid gland, anatomy of, i. 217 
 
 innervation of. i. 218 
 Partial tones, definition of, ii. 383 
 Parturition, ii. J7!> 
 
 spinal centre of, ii. 481 
 Pate de foie pr:<. i. ",t;n 
 
 Paths of conduction in the cord, clinical evi- 
 dence on, ii. 234 
 
544 
 
 GENERAL INDEX. 
 
 Pause, compensatory, of the heart, i. 156 
 Pauses, respiratory, i. 424 
 Pectoral muscles, respiratory action of, i. 405 
 Pendular movements of the intestines, i. 384 
 
 vibrations, ii. 381 
 Penis, ii. 443 
 
 structure of, ii. 448 
 Pentamethylene-diamin, i. 543 
 Pentoses, i. 562 
 Pepsin, i. 237, 238 
 
 effect of, on blood coagulation, i. 63 
 
 preparation of, i. 291 
 
 properties of, i. 290 
 
 Pepsin-hydrochloric acid, action of, i. 292 
 Pepsinogen granules, i. 242 
 Peptic digestion, i. 292, 294 
 Peptones, absorption of, in the stomach, i. 313 
 
 definition of, i. 292, 295 
 
 effect of, on blood coagulation, i. 62 
 
 properties of, i. 294, 577 
 
 Pepton-injection, effect of, on lymph formation, 
 i. 73 
 
 toxicity of, i. 316 
 Perfusion cannula, i. 187 
 Perilymph, ii. 372 
 Periodic reflexes, ii. 216 
 Peripheral nerves, medullation of, ii. 180 
 
 reference of special sensations, ii. 400 
 
 reflex centres, i. 178 
 Peristalsis, definition of, i. 372 
 
 intestinal, i. 382 
 
 of the stomach, i. 379 
 
 of the ureters, i. 389 
 Permeability of the capillary walls, i. 70 
 Peroxide of hydrogen, i. 505 
 Pettenkofer's reaction for bile acids, i. 324, 
 
 544 
 
 Pexinogen granules, i. 242 
 
 Pfliiger's hypothesis of the structure of proto- 
 plasm, i. 23 
 
 law of contraction, ii. 50 
 Phagocytosis, i. 48 
 Phakoscope, ii. 308 
 
 Phalangar process of the rods of Corti, ii. 378 
 Pharynx, deglutition in, i. 373 
 Phenaceturic acid, i. 569 
 Phenol, i. 569 
 
 elimination of, i. 340 
 Phenyl-acetic acid, i. 569 
 Phloridzin diabetes, i. 563 
 Phosphates, i, 514 
 Phosphene, pressure, ii. 305, 331 
 Phosphoric acid, salts of, i. 514 
 Phosphorus, nutritive history of, i. 515 
 
 occurrence of, i. 513 
 
 peroxide, i. 514 
 
 poisoning, i. 513 
 
 preparation of, i. 513 
 
 properties of, i. 513 
 Photometry, ii. 345 
 Phrenic nerve, current of action in, ii. 154 
 
 nerves, i. 463 
 
 Phylogenetic development of nerve-cells, ii. 177 
 Physical molecules, definition of, i. 25 
 Physiological anodes, definition of, ii. 52 
 
 division of labor, i. 22 
 
 molecules, i. 25 
 
 kathode, definition of, ii. 52 
 
 observations on afferent paths in the cord, ii. 
 229 
 
 rheoscope, ii. 148, 151 
 
 salt solution, ii. 59 
 
 in transfusions, i. 64 
 Physiology, definition of, i. 17 
 
 human, definition of. i. 30 
 
 methods employed in, i. 30 
 
 subdivisions of, i. 17, 29 
 
 Physostigmin, action of, on accommodation, ii. 
 
 311 
 
 on the eye, ii. 325 
 Pia mater, weight of, ii. 274 
 Pigment epithelium, retinal, movements of, ii. 
 
 330 
 retinal, relation of, to adaptation of the eye, 
 
 ii. 326 
 Pigments, biliary, i. 45, 245, 322, 530, 574 
 
 blood-, i. 37, 44, 573 
 Pilocarpin, action of, on salivary glands, i. 229 
 
 on sweat-glands, i. 260 
 
 Pilomotor mechanism, relation of, to thermo- 
 lysis, i. 494 
 
 Pi nee myographique, ii. 87 
 Pineal gland, calcification of, in old age, ii. 491 
 Pinna of the ear, ii. 362 
 Pitch, limits of perception of, ii. 382 
 of musical tones, ii. 381 
 of the voice, ii. 430, 432 
 Pituitary body, anatomy of, i. 272 
 functions of, i. 273 
 internal secretion of, i. 273 
 extracts, action of, i. 272 
 membrane, ii. 408 
 Pivot-joint, ii. 417 
 Placenta, ii. 474 
 Placental transmission of infectious diseases, ii. 
 
 498 
 
 villi, ii. 474 
 
 Plain muscle, histology of, i. 369 
 physiology of, i. 370 
 tone of, i. 371 
 
 Plant-cells, assimilation in, i. 18 
 Plants, regeneration of lost parts in, ii. 496 
 Plasma of blood, i. 33, 50 
 
 oxygen absorption-coefficient of, i. 416 
 Plastic food-stuffs, definition of, i. 346 
 Plethysmograph, i. 196 
 Pneumatic cabinet, i. 453 
 Pneumogastric nerve. See Vagus. 
 pulmonary branches of, i. 465 
 respiratory function of, i. 459, 460 
 Pneumograph, i. 423 
 Pohl's mercury commutator, ii. 36 
 Poikilothermous animals, i. 467 
 Polar amphiaster of the ripening egg, ii. 453 
 bodies, ii. 451 
 
 of the ovum, ii. 453 
 Polarity of neurones, ii. 184 
 Polarization, after-effects of, ii. 65 
 
 physiological, of neuroblasts, ii. 176 
 Polarizing current, effect of, on conductivity, 
 
 ii.50 
 
 on muscles, ii. 61 
 on nerves, ii. 62 
 Pole-changers, ii. 36 
 Polynucleated leucocytes, i. 48 
 Polypncea, i. 441 
 Polyspermy, ii, 471 
 Pomum Adami, ii. 425 
 Portal vein, vase-motor nerves of, i. 209 
 Positive after-images, ii. 346 
 variation of the heart during vagus stimula- 
 tion, i. 164 
 
 Posterior association centre, ii. 257 
 Post-ganglion ic fibres of the sympathetic sys- 
 tem, ii. 219 
 
 Post-mortem rise of temperature, i. 497 
 Posture sense, ii. 399 
 
 Potassium carbonates, nutritive history of, i. 520 
 chloride, nutritive history of, i. 519 
 cyanide, i. 542 
 occurrence of, i. 519 
 phosphates, nutritive history of, i. 520 
 relation of, to heart muscle, i. 151 
 salts, action of, on muscles, ii. 138 
 
GENERAL 1M>KX. 
 
 545 
 
 Potassium suits, relation of, to irritability, ii. .",! 
 toxicity of, i. iV.'o 
 
 sulphocyanide. detection of, i. 284 
 occurrence of, i. '2*'.\, ~>l'* 
 of tlir urine, i. r>()7 
 
 thiocyanide, i. "> |-J 
 Potential energy of food, i. ill! I 
 Preformat ion theory of heredity, ii. 500 
 Pre-ganul ionic fibres of the sympathetic system, 
 
 ii. -Jl!> 
 
 Pregnancy, effects of, on the mother, ii. 477 
 I'n-sbyopia, ii. .'!! I 
 Pressor nerves, i. 202 
 
 Pressure, eil'ert of, on irritability of nerves, ii. 
 66 
 
 influence of, on conductivity, ii. 92 
 
 intracanliac. i. 107 
 
 intrathoracic, i. 396, 409 
 
 intraventriciilar, i. 125 
 
 of the lymph, i. 146 
 Pressure-points of the skin, ii. 396 
 Pressure-sensations, fusion of, ii. 394 
 Pressure-sense, delicacy of, ii. 392 
 
 of the tympanic membrane, ii. 382 
 Primary position of the eye, ii. 299 
 
 taste-sensations, ii. 412 
 Principal foci in a dioptric system, ii. 302 
 
 point of the simplest dioptric system, ii. 301 
 
 ray in the simplest dioptric system, ii. 301 
 Processus brevis of the malleus, ii. 365 
 
 gracilis sive folianus of the malleus, ii. 365 
 Projection system of fibres, origin of, from 
 
 central cells, ii. 205 
 Pronucleus, female, ii. 453 
 
 male, ii. 466 
 
 Propeptones, definition of, i. 292 
 Propionic acid, i. 538 
 Proptosis after cerebellar injury, ii. 272 
 Propyl alcohol, i. 536, 538 
 Prostate glands, ii. 443 
 histology of, ii. 448 
 secretion of, ii. 446 
 Prostatic fluid, ii. 446, 448 
 Protagon, i. 559 
 
 of medullary substance, ii. 170 
 Protamin, nature and origin of, i. 24 
 Protamins, properties of, i. 580 
 Proteid, affinity of cell-substance for, i. 568 
 
 circulating, definition of, i. 346 
 
 metabolism during starvation, i. 363 
 effect of muscular work on, i. 360 
 end-products of, i. 337 
 Proteid-absorption, mechanism of, i. 316 
 Proteids, absorption of, i. 315 
 
 classification of, i. 576 
 
 color reactions of, i. 576 
 
 combined, classification of, i. 579 
 
 combustion equivalent of, i. 365 
 
 diffusion of, i. 70 
 
 dynamic value of. i. 475 
 
 effect of, on glycogen formation, i. 328 
 
 gastric digestion of, i. 292 
 
 general reactions of, i. ">?.") 
 significance of, i. 24 
 
 living, theoretical structure of, i. 23, 24 
 
 molecular structure of, i. 581 
 
 nutritive value of, i. :.'7i, :Jl.~> 
 
 of milk, i. ><)! 
 
 of muscle, precipitation temperatures of, ii. 
 166 
 
 of muscle-serum, ii. 166 
 
 of the blood, i 49, 50 
 
 origin of fat from, i. 351 
 
 osmotic pressure of, i. 69 
 
 putrefaction of. in the intestines, i. 310 
 
 rapidity of oxidation of, i. 347 
 
 relation of. to muscular work, ii. 74 
 
 IVoleids. simple, classification of, i. .~>7<; 
 substitutes for, in t he diet, i. 348 
 synthesis of, i. :>!-. 
 tryptic digestion of, i. 303 
 vegetable, i. .~>?7 
 Proteo lysis, i. -i:; 
 tryptic, i. 303 
 value of, i. :<!."> 
 
 Proteolytic enzymes, definition of, i. 280 
 Proteose injection, effects of, i. 316 
 Proteoses, definition of, i. 292 
 
 properties of, i. .777 
 Protbrombin. i. 58 
 Protoplasm, i. 17, 499 
 Pseudo-mucoid, i. 578 
 Pseudoscopic vision, ii. 318, .'!.">7 
 Psychical powers of the spinal cord, ii. 215 
 Psycho-physic law, ii. 340 
 
 of Fechner, ii. 393 
 Pterygoid muscles, i. 372 
 Ptomaines, chemical structure of, i. 542 
 Ptyalin, i. 221, 280 
 action of, i. 284, 286, 566 
 occurrence of, i. 284 
 Puberty, ii. 489 
 Pulmonary circulation, i. 78, 103 
 
 innervation of, i. 205 
 ventilation, forces concerned in, i. 413 
 Pulse, arterial, cause of, i. 93 
 celerity of, i. 142 
 definition of, i. 139 
 dicrotic wave of, i. 143 
 extinction of, i. 94 
 frequency of, i. 121, 141 
 regularity of, i. 141 
 
 respiratory variations in the rate of, i. 451 
 size of, i. 141 
 tension of, i. 141 
 transmission of, i. 140 
 relation of, to body-temperature, i. 471 
 respiratory, i. 96 
 Pulse-curve, i. 142 
 Pulse-rate, diurnal variations of, 121 
 Pulse-volume of the heart, definition of, i. 105 
 Pupil, changes during accommodation in, ii. 
 
 311 
 
 in size of, ii. 323 
 condition of, in sleep, ii. 325 
 dilator nerves, ii. 324 
 size of, in old age, ii. 314 
 Pupillary reflex to light, ii. 323 
 Purin, i.'553 
 bases, i. 552 
 
 in leucocythsemia, i. 557 
 Purkinje-Sanson's images, ii. 307 
 Purkinje's figure, ii. 321 
 phenomenon, ii. 340 
 
 explanation of, ii. 342 
 Purposeful reflexes, ii. 215 
 Putrefaction, intestinal, products of, i. 310 
 Putrescin, i. 543 
 Pyin, i. 579 
 
 Pyramidal fibres, number of, ii. 246 
 nerve-cells, development of, ii. 178 
 tracts in the cord, ii. 245 
 geminal fibres of, ii. 245 
 size of, ii. 252 
 Pyramidalis muscle, expiratory action of, i. 
 
 407 
 
 Pyridin. i. .",71 
 Pyrocatechin, i. 569 
 
 QUADRATI lumborum, respiratory action of, i. 
 399 
 
 (Quadruplets, ii. 483 
 Quality of musical tones, ii. 383 
 of the voice, ii. 430 
 
546 
 
 GENERAL INDEX. 
 
 Quinine, action of, on coagulation of muscle- 
 plasma, ii. 164 
 
 hydrochlorate, action of, on salivary glands, 
 i. 222 
 
 Quintuplets, ii. 483 
 
 RACE, relation of brain-weight to, ii. 278 
 Range of accommodation, normal, ii. 312 
 Rarefied air, respiration of, i. 452 
 Rate of conduction in heart muscle, i. 154 
 in muscles, ii. 87 
 in nerves, ii. 89 
 of excitation, effect of, on the contraction 
 
 curve, ii. Ill 
 
 of heart-beat, variations of, i. 121 
 of progress of the food in the intestines, i. 314 
 of respiratory movements, i. 425 
 of stimulation in voluntary contractions, ii. 
 
 133 
 
 limitations of, for muscle, ii. 126 
 required to tetanize, ii. 125 
 of transmission of the pulse, i. 140 
 Reaction, influence of, on action of ptyaliu, i. 
 
 286 
 
 of bile, i. 322 
 of blood, i. 34 
 of degeneration, ii. 48, 54 
 of gastric juice, i. 288 
 of intestinal contents, i. 310 
 of muscles, ii. 159 
 of nerve-cells, ii. 191 
 of nerve-fibres, ii. 170 
 of pancreatic juice, i. 232, 300 
 of succus entericus, i. 308 
 of sweat, i. 342 
 of urine, i. 250, 334 
 time, ii. 291 
 
 in old age, ii. 491 
 
 Rectus abdominis, expiratory action of, i. 407 
 Recurrent laryngeal nerve, ii. 428 
 
 sensibility of the anterior roots, ii. 204 
 Red corpuscles, behavior of, in the capillaries, i. 
 
 81 
 
 color of, i. 35 
 composition of, i. 51 
 disintegration of, i. 45 
 form of, i. 35 
 function of, i. 35 
 number of, i. 35 
 origin of, i. 45, 46, 333 
 size of, i. 35 
 structure of, i. 35 
 variations in the number of, i. 46 
 Red-striped muscles, physiological properties of, 
 
 ii. 109 
 
 Reduced eye, ii. 304 
 Reduction, i. 502 
 
 processes in the animal body, i. 536 
 Reflex acceleration of the heart, i. 177 
 actions, simple, ii. 208 
 arc, ii. 209 
 coughs, i. 455 
 discharge of bile, i. 248 
 frog, ii. 209 
 
 inhibition of the heart, i. 172 
 secretion of gastric juice, i. 239 
 of pancreatic juice, i. 236 
 of saliva, i. 230 
 segmental reaction, ii. 210 
 stimulation of the nervous system, ii. 208 
 tonus of muscular tissues, ii. 220 
 vaso-motor changes, i. 202 
 Reflexes, co-ordinated, ii. 211 
 
 co-ordination of the efferent impulses in, ii. 
 
 214 
 
 effect of location of stimulus on, ii. 209 
 of strength of stimulus on, ii. 210 
 
 Reflexes from the isolated cord in man, ii. 213 
 
 lumbar cord, ii. 213 
 in different vertebrates, ii. 212 
 in man, ii. 216 
 latent period of, ii. 211 
 of a purposeful character, ii. 215 
 periodic, ii. 216 
 simple, ii. 208 
 spinal, ii. 212 
 
 reinforcement of, ii. 222 
 summation of stimuli in, ii. 211 
 through sympathetic ganglia, vaso-motor, i. 
 
 200 
 
 voluntary control of, ii. 214 
 Refractive index of the aqueous humor, ii. 303 
 of the lens, ii. 303 
 of the vitreous humor, ii. 303 
 media of the eye, ii. 302 
 surfaces of the eye, ii. 303 
 "Refractory period " of nerves, ii. 57, 66 
 
 of the heart, i. 156, 158 
 
 Regeneration of blood after hemorrhage, i. 63 
 of lost parts, ii. 496 
 of nerves, ii. 78, 199 
 Registers of the voice, ii. 432 
 Regular astigmatism, ii. 317 
 Reinforcement of reflexes, ii. 222 
 
 of the knee-kick, ii. 222 
 Reissner, membrane of, ii. 374, 379 
 Rejuvenescence by sexual reproduction, ii. 442 
 Relaxation of muscle, nature of, ii. 99 
 Rennin, i. 238 
 
 action of, on milk, i. 296 
 occurrence of, in gastric juice, i. 295 
 of the kidneys, i. 274 
 preparation of, i. 295 
 Reproduction, asexual, ii. 439 
 of leucophrys patula, ii. 442 
 of living matter, i. 18, 20 
 of onychodromus, ii. 442 
 of stylonichia, ii. 442 
 sexual, ii. 440 
 
 elements of, ii. 440 
 theory of, ii. 441 
 
 Reproductive organs, female, ii. 449 
 internal secretions of, ii. 462 
 male, ii. 443 
 
 vaso-motor nerves of, i. 208 
 process, ii. 463 
 
 Residual air, definition of, i. 427 
 Resonance of the ear, ii. 388 
 Resonants, ii. 436 
 
 Resonators, analysis of sounds by, ii. 385 
 Respiration, artificial, i. 446 
 associated movements of, i. 408 
 cutaneous, i. 422 
 definition of, i. 395 
 heat dissipated in, i. 488 
 intensity of, i. 429 
 internal', i. 422 
 nervous mechanism of, i. 455 
 rhythm of, i. 423 
 
 Respiratory activity, conditions affecting, i. 429 
 centres, i. 455 
 
 afferent nerves to, i. 459 
 conditions influencing, i. 458 
 foetal, i. 464 
 rhythmicity of, i. 458 
 food-stuffs, definition of, i. 346 
 movements, circulatory effects of, i. 447 
 duration of, i. 424 
 effect of, on blood-pressure, i. 448 
 on venous circulation, i. 95, 96 
 frequency of, i. 425 
 special, i. 453 
 nerves, afferent, i. 460 
 efferent, i. 463 
 
547 
 
 R< -piratory pauses, i. I'.M 
 pressure, i. 408 
 quotient, i. 410 
 
 (luring hibernation, i. 434 
 relation of. to the diet, i. l^C, 
 variations of. i. 437 
 sounds, i. K'H 
 
 Resuscitation from drowning, i. 445 
 Kete inirabilc of the Ifalpighian corpuscles, i. 
 
 249 
 
 vasenlosiim of the testis, ii. 447 
 Retina, changes pioduced in, by light, ii. 330 
 circulation in. ii. .'!'-'.' 
 histology of. ii. ''>'"* 
 oscillatory activity of, ii. 344 
 pace-perceptions by, ii. 348 
 
 structure of, ii. 327 
 Retinal currents, ii. 331 
 
 images, inversion of, ii. 305 
 
 si/e of, ii. 305 
 
 stimulation, sifter-effect of, ii. 345 
 fati-ne in, ii. 344 
 latent period of, ii. 343 
 laws of, ii. 343 
 
 rise to maximum for different colors, ii. 343 
 vessels, demonstration of, ii. 321 
 Reversion to ancestral characters, ii. 495 
 Rhamnose, i. 562 
 Rheocord, ii. 41 
 Rheometer, i. 99 
 Rheonome, ii. 31 
 Rheoscope, physiological, ii. 148 
 Rheoscopic frog, ii. 148 
 Rheostat, ii. 40 
 Rhinencephalon, ii. 241 
 Rhomboideus muscles, respiratory action of, i. 
 
 405 
 
 Rhythm of the respiratory movements, i. 423 
 Rhythmic activity of the vaso-constrictor cen- 
 tre, i. 201 
 Rhythmicity of the heart, abnormal, i. 152 
 
 cause of, i. 148 
 
 Ribs, respiratory movements of. i. 400 
 Rickets, i. 356, 525 
 Right lymphatic duct, i. 145 
 Rigor caloris, ii. 57, 164 
 contracting in, ii. 128 
 effect of fatigue on, ii. 165 
 mortis, ii. 159 
 
 chemical changes accompanying, ii. 162 
 contracture of, ii. 128 
 disappearance of, ii. 162 
 influence of the nervous system on, ii. 220 
 nature of changes in, ii. 161 
 Rima glottidis. ii. 423 
 respiratoria, ii. 423 
 vocalis, ii. 423 
 
 Ringer's solution for the heart, i. 190 
 Ritter's opening tetanus, ii. 37, 61 
 
 tetanus, ii. 132 
 Rivinus, ducts of, i. 217 
 Rod-and-cone layer, function of, ii. 327 
 Rod-pigment, ii, 339. See Visual purple. 
 Rods and cones, function of, n. 341 
 
 number of, ii. 330 
 of Corti, ii. 377 
 retinal, function of, ii. 341 
 Rosel von Rosenhof on spontaneous changes in 
 
 form of living organisms, ii. 19 
 Rotation, movements of, ii. 416 
 Roy's tonometer, i. iss 
 Running, ii. 421 
 Rut of animals, ii. 460 
 
 \\ AROSE, i. 564 
 
 Sacculus of (ho internal ear, ii. 373 
 Saccus endolymphaticus, ii. 373 
 
 Saddle-joint, ii. 41! 
 Saliva, composition of, i. 220, 283 
 mineral constituents of. i. ~>:;o 
 properties of. i. j-jn. 888 
 uses of, i. '.'-Mi 
 Salivary corpuscles, i. 283 
 glands, i. :_ir 
 
 anatomy of, i. 217 
 histolou'ical changes in, i. 226 
 histology of, i. :.'!! 
 nerves of, i. 218, _'_> I 
 vaso-motor nerves of, i. 222 
 secretion, action of drugs on, i. 229 
 cerebral control of, ii. 'J70 
 normal mechanism of, i. '.'".<> 
 Salkowski's reaction for cholesterin, i. 575 
 Salmin, i. 580 
 
 Salt solution, physiological injection of, i. 64 
 Salt-licks, i. 355 
 Salts, absorption of, i. 318 
 inorganic, of muscle, ii. 168 
 
 relation of, to irritability, ii. 58 
 lymphagogic action of, i. 73 
 nutritive value of, i. 276, 354 
 of heavy metals, action of, on nerve and 
 
 muscle, ii. 60 
 
 Santorini, cartilage of, ii. 422, 425 
 Saponification of fats, i. 306, 558 
 Saprin, i. 543 
 Sarcin, i. 553 
 
 Sarcode of sponges, contractility of, ii. 20 
 Sarcolactic acid, i. 546 
 
 formation of, in rigor mortis, ii. 161, 162 
 in clotting of muscle-plasma, ii. 164 
 relation of, to fatigue contracture, ii. 131 
 Sarcoplasm, ii. 104 
 Sarcosin, i. 538 
 Saturation of colors, ii. 342 
 Scala media, ii. 375 
 tympani, ii. 372, 375 
 vestibuli, ii. 372, 375 
 
 Scaleni muscles, inspiratory action of, i. 401 
 Schneiderian membrane, ii. 408 
 Scombrin, i. 580 
 Scrotum, ii. 443 
 
 Sebaceous glands, structure of, i. 257 
 secretion, composition of, i. 342 
 function of, i. 258 
 physiological value of, i. 342 
 Sebum, composition of, i. 257 
 Secondary degeneration of nerves, ii. 197 
 position of the eye, ii. :1W 
 tetanus, ii. 150 
 Secreting glands, electrical changes in, i. 231 
 
 histological changes in, i. 226 
 Secretion, antilytic, i. 230 
 biliary, i. 248 
 
 capillaries of the gastric glands, i. 238 
 definition of, i. 211 
 gastric, 240 
 
 histological changes during, i. 226 
 internal, definition of, i. 211 
 intestinal, i. 243 
 mammary, i. 264 
 nierliaiiism of, i. 213 
 pancreatic, i. 235 
 paralytic, i. 229 
 
 psychical, of gastric juice, i. 239 
 relation of, to intensity of stimulus, i. 223 
 salivary, i. 230 
 
 cerebral control of, ii. 270 
 sebaceous, i. 257, 342 
 sweat, i. 259 
 urinary, i. 251 
 
 Secretions, general characteristics of. i. 213 
 Secret oiroL'iies for the gastric {.'lands, i. 359 
 Secretory centre, salivary, i. 230 
 
548 
 
 GENERAL INDEX. 
 
 Secretory fibres proper, definition of, i. 224 
 nerves, evidence for, i. 222 
 fatigue of, ii. 96 
 mode of action of, i. 225 
 of the adrenal bodies, i. 272 
 of the kidneys, i. 251 
 of the liver, i. 247 
 of the mammary glands, i. 263 
 of the pancreas, i. 232 
 of the stomach, i. 239 
 of the sweat glands, i. 259 
 salivary, endings of, i. 220 
 significance of, i. 214 
 stimulation of, i. 222 
 Segmental arrangement of nerve-elements, ii. 
 
 206 
 
 reactions, reflex, ii. 210 
 Segmentation, ii. 467 
 Segmeritation-centrosomes, ii. 469 
 Segmentation-nucleus, ii. 466 
 Semen, composition of, ii. 445 
 Semicircular canals, membranous, ii. 373 
 of the bony labyrinth, ii. 371 
 relation of, to equilibrium, ii. 405 
 section of, ii. 405 
 Semilunar valves, i. 110 
 Seminal vesicles, ii. 443 
 function of, ii. 448 
 secretion of, ii. 446 
 Semi-vowels, ii. 436 
 Senescence of nerve-cells, ii. 18%^~\ 
 of the central nervous system, ir! 295 
 phenomena of, ii. 486 
 Sensation, cutaneous, definition of, ii. 390 
 muscular, definition of, ii. 390 
 of after-pressure, ii. 394 \ 
 of light, mechanism for the production of, ii. 
 
 331 
 
 of temperature, ii. 397 
 Sense of equilibrium, ii. 404 
 
 of touch, ii. 392 
 Sensory areas of the cortex, determination of, ii. 
 
 253 
 conducting paths in the spinal cord, ii. 234 
 
 continuation of, in the brain, ii. 235 
 cortical areas in man, ii. 255 
 
 motor responses from, ii. 253 
 relative functional importance of, ii. 270 
 disturbance from hemisection of the cord, ii. 
 
 230 
 
 impulses, path of, in the central nervous sys- 
 tem, ii. 226 
 relation of, to the maintenance of the 
 
 erect posture, ii. 419 
 nerve-endings in skeletal muscle, ii. 402 
 in tendon, ii. 402 
 in the skin, ii. 391 
 
 nerves, influence of, on respiration, i. 463 
 of the heart, i. 172 
 rate of conduction in, ii. 91 
 relation of, to the respiratory centre, i. 459 
 reflex influence of, on the pulse-rate, i. 175 
 paths, degeneration of, after section of the 
 
 dorsal roots, ii. 227 
 in the central nervous system, ii. 226 
 regions of the cortex cerebri, ii. 252 
 stimulation, relation of, to sleep, ii. 291 
 Septal nerves of the frog's heart, i. 166 
 Serous cavities, i. 146 
 Serrati postici inferiores, respiratory function 
 
 of, i. 399 
 
 superiores, inspiratory action of, i. 402 
 Serum, bactericidal action of, i. 36 
 globulicidal action of, i. 36 
 osmotic pressure of, i. 68 
 toxicity of, i. 36 
 Serum-albumin, action of, on carbonates, i. 517 
 
 Serum-albumin, amount of, in the blood, i. 52 
 
 composition of, i. 52 
 
 functions of, i. 52 
 
 properties of, i. 52 
 Sex, characters of, ii. 442 
 
 influence of, on heat production, i. 482 
 on pulse-rate, i. 121 
 on respiration, i. 430 
 
 of offspring, determination of, ii. 483 
 
 origin of, ii. 441 
 
 relation of body-temperature to, i. 470 
 
 of brain-weight to, ii. 276 
 Sexual characters, ii. 442 
 
 glands, accessory, ii. 445 
 
 organs, ii. 443 
 
 reproduction, ii. 440 
 
 congenital variations resulting from, ii. 501 
 theory of origin of. ii. 441 
 Shark, reflexes in, ii. 2i2 
 
 removal of cerebral hemispheres in, ii. 263 
 Shivering, i. 362, 491 
 Shrapnell's membrane, ii. 365 
 Siamese twins, ii. 483 
 Silicic acid, properties of, i. 519 
 Silicon, i. 519 
 Simple muscular contraction, duration of, ii. 102, 
 
 108 
 explanation of, ii. 101 
 
 proteids, i. 576 
 
 Simultaneous contrast, ii. 347 
 Singing voice, ii. 434 
 Sinuses of Valsalva, i. Ill 
 Size, increase of the embryo in, ii. 487 
 
 influence of, on pulse-rate, i. 121 
 
 of nerve-cells, ii. 175 
 Skatol, i. 572 
 
 elimination of, i. 340 
 
 occurrence of, in feces, i. 320 
 Skiascopy, detection of astigmatism by means 
 
 of, ii. 319 
 Skin, functions of, i. 341 
 
 glands of, i. 257 
 
 tactile areas of, ii. 395 
 Sleep, ii. 291 
 
 cause of, ii. 292 
 
 condition of the pupils in, ii. 325 
 
 curve of intensity of, ii. 294 
 
 effect of loss of, ii. 295 
 on metabolism, i. 361 
 on respiration, i. 424 
 on the respiratory quotient, i. 438 
 
 responsiveness to stimuli in, ii. 293 
 Smegma prseputii, i. 257 
 Smell, ii. 408 
 
 comparative physiology of, ii. 409 
 
 subjective sensations of, ii. 410 
 Smooth muscle, rate of conduction in, ii. 89 
 Snails, regeneration of lost parts in, ii. 496 
 Sneezing, i. 454 
 Snoring, i. 455 
 Sobbing, i. 454 
 Sodium ammonium phosphate, i. 523 
 
 carbonates, i. 522, 523 
 
 chloride, nutritive history of, i. 521 
 
 phosphates, i. 522 
 
 sulphate, i. 522 
 Somatic death, ii. 491 
 Somatoplasm, definition of, ii. 496 
 Somatopleure, ii. 472 
 Sound, physical, ii. 381 
 
 quality of, ii. 383 
 Sound-waves, amplitude of, ii. 381 
 
 composite, ii. 384 
 
 limits of perceptions of, ii. 382 
 
 production of, ii. 381 
 Space illusions, ii. 354 
 Space-perception from visual sensations, ii. 347 
 
GENERAL INDI.X. 
 
 Special respiratory movements, i. .r>:; 
 Specialization of functions, i. 21 
 Specitic energies of nerves, doctrine of, ii. 
 er.eriry of I In- opiic nerve, ii. 331 
 gravity of blood, i. .'II 
 
 of blood-corpuscles, i. 34, 35 
 
 of muscle, ii. !.">!> 
 
 of tbe eiicephalon, ii. ~7."> 
 
 of the nervous system at different ages, ii. 
 
 284 
 
 of urine, i. _'.""> 1 
 beat, definition of, i. 477 
 
 of tbe human body, i. 504 
 nerve-energy, doctrine of, ii. 399 
 Spectral colors, incomplete saturation of, ii. 
 
 348 
 
 Spectroscope, i. 40 
 Spectrum, ii. 332 
 definition of, i. 40 
 
 luminous intensity of the colors of, ii. 340 
 of CO-htemoglobin, i. 44 
 of luernoglobin, i. 4'J 
 of oxyhsemoglobin, i. 41 
 solar, i. 41 
 
 top, Benham's, ii. 344 
 Speech, dependence of, on hearing, ii. 431 
 
 elements of, ii. 433 
 Speech-centre, ii. 257 
 Spermaceti, i. 540 
 Sperm-aster, ii. 467 
 Spermatids, ii. 445 
 Spennatocytes, ii. 445 
 Spermatozoa, ii. 440 
 contractility of, ii. 20 
 discovery of, ii. 443 
 entrance of, into the uterus, ii. 465 
 locomotion of, ii. 465 
 maturation of, ii. 445 
 movements of, ii. 444 
 structure of, ii. 443 
 Spermin, ii. 445 
 
 physiological action of, i. 273 
 Sperm-nucleus, ii. 466 
 Spherical aberration, ii. 315 
 Sphincter ani, cerebral control of, ii. 270 
 antri pylorici, i. 377 
 iridis, ii. 3-_:i 
 pylori, i. 377, 381 
 urethra, i. 390 
 
 contraction of, in erection, ii. 464 
 vesicse internus, i. 390 
 Sphincters ani, i. 386 
 Sphygruogram, i. 143 
 Sphygmograph, i. 142 
 Sphyginoraanometer, i. 141 
 Sphygmometer, i. 141 
 
 Spinal centres for vaso-motor nerves, i. 199 
 cord, afferent paths of, ii. 229 
 central neurones of, ii. 203 
 degeneration of, from hemlsection, ii. 228 
 efferent neurones of, ii. 203 
 motor tracts of, ii. 245 
 reflexes jjji man after section of, ii. 213 
 schematic? cross-section of, ii. 202 
 weight of, ii. 274 
 
 ganglion-cells, development of, ii. 178 
 nerve-roots, section of, ii. 198 
 Spiral ganglion of the ear, ii. 376 
 ligament of the cochlea, ii. 379 
 Spirometer, i. 427 
 
 Splanchnic nerves, gastric fibres of, i. 382 
 influence of, on blood-pressure, i. 173 
 
 on respiration, i. 463 
 intestinal fibres of, i. 385 
 stimulation of, i. 173 
 Spleen, composition of, i. 333 
 function of, i. 322 
 
 Spleen, innervation of, i. :;:;:; 
 
 movements of. i. :;-.".' 
 
 vaso-motor nerves of, i. -.'07 
 Staircase contraction-,, ii. Uii. 1 TJ 
 relation of. to tetanus, ii. U I 
 Standing, ii. 11* 
 Stannius's ligatures, i. 178 
 Stapedius muscle, ii. 370 
 Stapes, ii. ::i;7 
 Starch, i. r,i;<; 
 
 digestion of, i. 284, 305 
 
 hydrolysis of, by acids, i. 286 
 
 by amylolytic ferments, i. 285 
 Starvation, effect of, on metabolism, i. 362 
 on the nervous system, ii 
 
 glycogeu disappearance during, i. 331 
 
 nutrition during, i. .'!.'>( i 
 
 phosporus excretion in, i. 516 
 
 potassium excretion in, i. 520 
 Static equilibrium, organs of, ii. 407 
 Stature, relation of brain-weight to, ii. 276 
 Steapsin, i. 232, -J-o 
 
 demonstration of, i. 306 
 
 occurrence of, i. 305 
 Stearic acid, i. 541 
 Stenson's duct, i. 217 
 
 experiment, ii. 67 
 Stercorin, i. 575 
 Stereoscope, ii. 356 
 
 Sterno-cleido-mastoid muscles, respiratory ac- 
 tion of, i. 404 
 Sterno-hyoid muscle, ii. 425 
 Sterno-thyroid muscle, ii. 425 
 Sternum, respiratory movements of, i. 401 
 Stethograph, i. 423 
 Stimulants, effect of, on muscular work, ii. 75 
 
 of the sweat glands, i. 260 
 
 physiological action of, i. 357 
 Stimulation fatigue of non-medullated nerves, 
 ii. 180 
 
 of the cortex, ii. 241 
 
 Stimuli, artificial, effect of, on the heart, i. 156 
 classification of, ii. 23 
 
 chemical, of muscle, ii. 131 
 
 conditions determining efficiency of, ii. 28 
 
 effect of changing intensity of, ii. 32 
 of their repetition on irritability, ii. 65 
 of varying strength of, ii. 39 
 
 galvanic, contracture effect of, in muscles, ii. 
 131 
 
 variations in intensity of, ii. 31 
 Stokes' s reagent, composition of, i. 43 
 Stomach, absorption in, i. 312 
 
 extirpation of, i. 299 
 
 glands of, i. 237 
 
 immunity of, to its own secretion, i. 297 
 
 innervation of, i. 381 
 
 movements of, i. 377, 378 
 
 musculature of, i. 377 
 Strabismus, ii. 300 
 Strise acusticse, ii. 237 
 
 gravidarum, ii. 477 
 Stromuhr of Ludwig, i. 99 
 Strontium, i. 526 
 Strychnin, action of, on diffusion of impulses in 
 
 the cord, ii. 217 
 on end-plates, ii. 27 
 on sympathetic ganglia, ii. 219 
 
 effect of, on body-temperature, i. 472 
 
 tetanus, ii. 217 
 Sturin, i. 580 
 
 Stylo-hyoid muscle, ii. 426 
 Stylonychia, reproduction of, ii. 442 
 Sublingual gland, anatomy of, i. 217 
 Submaxillary gland, anatomy of, i. 217 
 Successive contrast, ii. 346 
 Succinic acid, i. 557 
 
550 
 
 GENERAL INDEX. 
 
 Succus entericus, i. 243 
 
 action of, ou carbohydrates, i. 309 
 collection of, i. 308 
 digestive action of, i. 308 
 ferments of, i. 308 
 Suction action of the heart, i. 134 
 Sudorific drugs, i. 260 
 Suffocation. See Asphyxia. 
 Sugar injections, lyruphagogic action of, i. 73 
 of muscles, ii. 167 . 
 use of, in muscular work, ii. 74 
 Sugars, absorption of, i. 313, 317 
 
 consumption of, by the tissues, i. 353 
 effect of, on glycogen formation, i. 328 
 synthesis of, i. 533 
 Sulphates of the urine, estimation of, i. 506 
 
 origin of, i. 506 
 Sulph-hsemoglobin, i. 506 
 Sulphur, elimination of, i. 340 
 metabolism of, i. 507 
 neutral, i. 506 
 occurrence of, i. 505 
 Sulphuretted hydrogen, inhalation of, i. 440 
 
 properties of, i. 506 
 Sulphuric acid, i. 506 
 Sulphurous acid, i. 506 
 
 Summation of contraction in muscle, ii. 121 
 of stimuli in nerve-cells, ii. 190 
 
 in reflex action, ii. 211 
 Superior laryngeal nerve, ii. 428 
 
 nerves, influence of, on respiration, i. 459, 
 
 462 
 
 oblique muscle, ii. 299 
 rectus muscle, ii. 299 
 Supernumerary digits, ii. 494 
 Supplemental air, definition of, i. 427 
 Suprarenal capsules, extirpation of, i. 271 
 Sustentacular cells of the crista acustica, ii. 374 
 Suture, ii. 414 
 Swallowing, i. 375 
 
 action of the epiglottis in, ii. 422 
 Sweat, amount of, i. 258, 342 
 composition of, i. 259, 342 
 nitrogenous constituents of, i. 512 
 Sweat-centres, spinal, i. 261 
 Sweat-glands, secretory nerves of, i. 259 
 stimulation of, i. 260 
 structure of, i. 258 
 Sweat-nerves, i. 259 
 
 Sweat-secretion, action of drugs on, i. 260 
 Sylvian heat-centre, ii. 271 
 Sympathetic ganglia, action of nicotin on, ii. 219 
 
 of strychnin on, ii. 219 
 nerves, cardiac, i. 168, 171 
 pulmonary, i. 466 
 
 reflex influence of, on the pulse-rate, i. 175 
 to the iris, ii. 324 
 pains, ii. 400 
 
 secretory fibres to the pancreas, i. 232 
 to the salivary glands, i. 218, 222 
 system, connection of, with the cerebro-spinal, 
 
 ii. 218 
 
 post-ganglionic fibres of, ii. 219 
 pre-ganglionic fibres of, ii. 218 
 vaso-motor centres, i. 200 
 vibration, ii. 385 
 Symphysis, ii. 414 
 Syndesmosis. ii. 414 
 Synovial fluid, ii. 415 
 Synthesis of proteids, i. 518, 582 
 
 of sugars, i. 563 
 
 Synthetic processes of plants, i. 518 
 Syntonin, absorption of, i. 315 
 
 occurrence of, in peptic digestion, i. 292 
 Syphilis, hereditary transmission of, ii. 498 
 Systole, auricular, i. 124. 136 
 ventricular, i. 123 
 
 TACTILE areas of the skin, ii, 395 
 
 corpuscle, ii. 390, 392 
 Tartar, i. 524 
 Taste, nerves of, ii. 410 
 
 organs of, ii. 430 
 Taste-buds, ii. 410 
 
 Taste-nerves, nuclei of origin of, ii. 236 
 Taste-perceptions, conditions affecting, ii. 411 
 Taste-sensations, conditions which influence, ii. 
 
 411 
 primary, ii. 412 
 
 distribution of. ii. 413 
 Tauriu, i. 507, 543 
 
 in muscles, ii. 167 
 Tea, nutritive value of, i. 357 
 stimulating action of, ii. 75 
 Tectorial membrane, ii. 379 
 Tegmen of the tympanum, ii. 364 
 Temperature, axillary, i. 468 
 
 body-, effect of, on respiratory activity, i. 432 
 influence of drugs on, i. 472 
 lowering of, i. 472 
 variations of, i. 469 
 effect of, on enzymes, i. 281 
 on heat dissipation, i. 487 
 on metabolism, i. 362 
 on muscular contraction, ii. 136 
 on sweat glands, i. 260 
 on the respiratory quotient, i. 438 
 on tryptic digestion, i. 301 
 external, effect of, on respiration, i. 426 
 on respiratory exchanges, i. 432 
 on thermotaxis, i. 496 
 influence of, on conductivity, ii. 92 
 on heat production, i. 483 
 on irritability, ii. 56 
 on ptyalin, i. 286 
 on rigor mortis, ii. 161 
 limits of muscular contraction, ii. 136 
 nerves, ii. 397 
 of animals, i. 467 
 of respired air, i. 410 
 of the blood from the brain, ii. 288 
 post-mortem rise of, i. 497 
 regulation of, i. 473 
 
 rise of, from lesions of corpora striata, ii. 271 
 sense, ii. 397 
 spots of the skin, ii. 398 
 topography of, i. 468 
 Temporal muscle, i. 372 
 Tendon reflexes after cerebellar injury, ii. 272 
 
 sensory nerve-endings in, ii. 402 
 Tension, effect of, on contraction curve, ii. 109 
 on irritability of nerves and muscles, ii. 56 
 on latent period, ii. 110 
 of the blood-gases, i. 415 
 Tensor tympani muscle, ii. 369 
 Tentacles of Actiniae, contractility of, ii. 20 
 Terminal arborizations, definition of, ii. 174 
 Tertiary positions of the eye, ii. 299 
 Testicular extracts, action of, i. 273 
 Testis, ii. 443 
 ducts of, ii. 447 
 histology of, ii. 446 
 internal secretion of, i. 273 
 Tetanic contractions, height of, ii. 120 
 
 relative intensity of, ii. 126 
 Tetanomotor, ii. 56 
 Tetanus, ii. 66 
 analysis of, ii. 123 
 complete, ii. 120 
 
 curves, introductory peaks of, ii. 124 
 explanation of, ii. 121 
 from strychnin poisoning, ii. 217 
 incomplete, ii. 117 
 normal physiological, ii. 132 
 of the heart, i. 165 
 
GENERAL /.V/>A'A'. 
 
 551 
 
 Tetanus of the muscles, ii. 127 
 
 rate of stimulation required for, ii. 125 
 
 Ritter's, ii. 37, 61 
 
 secondary, ii. 150 
 
 voluntary, ii. 133 
 
 Wundfs.'ii. 37. <il 
 Tetramethylene-diamin, i- 543 
 Thalamus, Vortical connections of, ii. 271 
 
 heat -cent re of. ii. 271 
 Theobromin, i. "),">:> 
 Theophyllin, i. 553 
 Thermal energy liberated in muscle, ii. 141 
 
 stimulation of nerve, ii. 25 
 Tln-i-i no-accelerator centres, i. 492 
 Thermogeneds, i. 477 
 
 mechanism of, i. 489 
 Thermogenio centres, i. 491 
 
 n.-rvcs. i. 490 
 
 tissues, i. 490 
 
 Thermo-inhibitory centres, i. 492 
 Thermolysis, i. 485 
 
 mechanism of, i. 494 
 Thermopile, ii. 1 I:.' 
 Thermotaxis, i. 489, 495, 496 
 
 relation of cerebrum to, ii. 270 
 Thiolactic acid, i. 547 
 Thirst, ii. 404 
 Thiry-Vella fistula, i. 308 
 Thoracic duct, i. 145 
 Thorax, effects of opening, i. 115 
 
 movements of, in respiration, i. 397 
 
 negative pressure in, i. 396 
 Thrombin, i. 58, 280 
 Thrombus, i. 60 
 Thymic acid, i. 579 
 Thyro-arytenoid muscles, ii. 424, 426 
 Thyroglobulin, i. 509 
 Thyro-hyoid muscle, ii. 425 
 Thyroid cartilage, ii. 425 
 
 extract, injection of, i. 269, 270 
 
 gland, relation of, to growth of the central 
 
 nervous system, ii. 289 
 Thyroidectomy, i. 269 
 Thyroids, anatomy of, i. 267 
 
 extirpation of, i. 268 
 
 functions of, i. 268 
 
 grafting of, i. 269 
 
 internal secretion of, i. 270 
 Thyroiodine, i. 509 
 Tidal air, volume of, i. 426 
 Tigroid of nerve-cells, ii. 179 
 Timbre of musical tones, ii. 383, 387 
 Time intervals, perception of, by the ear, ii. 388 
 
 of a complete circulation, i. 79 
 Tinctures, definition of, i. 535 
 Tissue death, ii. 492 
 Tissue-proteid, definition of, i. 346 
 Tissue-respiration, i. 422 
 Tissues, growth of, ii. 486 
 Tobacco smoke, action of, on nerves, ii. 60 
 Tones, combinational, ii.387 
 
 differential, ii. 387 
 
 fundamental, ii. 383 
 
 loud ness of, ii. 381 
 
 pitch of, ii. 381 
 
 simple, ii. 381 
 
 Tongue, distribution of taste-sensations on, ii. 
 413 
 
 vaso-motor nerves of, i. 204 
 Tonicity of involuntary muscle, i. 371 
 
 of vaso-con stricter centre, i. 199 
 Tonograph, definition of, i. 127 
 Tonometer, i. 188 
 
 Tonus, muscular, in the insane, ii. 220 
 reflex origin of, ii. 220 
 
 of muscles, ii. 143 
 
 ventricular, during vagus stimulation, i. 163 
 
 Touch illusions, ii. 3!Hi 
 sensations, ii. :i!i-j 
 
 locali/.at ion of. ii. 3!M 
 Tractus solitarius, ii. _':;; 
 Transfusion of blood, i. 64 
 Transversalis abdominis muscle, respiratory 
 
 action of, i. 407 
 
 Trapi'/.ius muscle, respiratory action of, i. 405 
 Traulie-Hering waves, i. 201 
 Tremors, ii. 132 
 Triangulares sterni muscles, expiratory action 
 
 of, i. 407 
 Trigeminal nerves, central paths of, ii. 238 
 
 influence of, on respiration, i. 463 
 Ti imethylamine, i. 541 
 Trioses, i. 559 
 Triplets, ii. 483 
 
 Trommer's test for carbohydrates, i. 562 
 Tropseolin 00 test for mineral acid, i. 289 
 Trophic impulses to muscles, ii. 70 
 
 influence of neurones on one another, ii. 197 
 
 of the vagi on the heart, i. 167 
 nerves of the muscles, ii. 70 
 of the salivary glands, i. 224 
 pulmonary, i. 466 
 Trypsin, i. 232 
 
 effect of, on blood coagulation, i. 63 
 extracts, preparation of, i. 301 
 properties of, i. 301 
 Trypsinogen, i. 235 
 
 granules, i. 235 
 Tryptic digestion, products of, i. 302 
 
 value of, i. 304 
 Tryptophan, i. 574 
 Tubules, uriniferous, i. 250 
 Tubuli recti of the testis, ii. 447 
 Tunicin, i. 566 
 Turtle's striped muscle, time of contraction in, 
 
 ii. 108 
 
 Twins, ii. 482 
 Tympanic membrane, ii. 364 
 
 effect of destruction of, ii. 370 
 pressure-sensations of, ii. 382 
 vibrations of, ii. 370 
 Tympanum, ii. 363 
 
 mechanics of, ii. 368 
 Tyrosin, i. 570 
 
 formation of, in tryptic digestion, i. 303 
 
 ULTIMUM moriens, ii. 492 
 Umbilical arteries, ii. 474 
 
 vein, ii. 474 
 
 Umbo of the tympanic membrane, ii. 365 
 Unconsciousness, ii. 293 
 Unipolar excitation for localized excitation, ii. 
 
 45 
 
 nerve-cells, development of, ii. 178 
 stimulation, ii. 30 
 
 principles of, ii. 43 
 Units, calorimetric, i. 477 
 Unorganized ferments, definition of, i. 279 
 Urea, amount of, in sweat, i. 335 
 in the blood, i. 51 
 in the urine, i. 335 
 antecedents of, i. 335 
 elimination of, i. 252 
 estimation of, i. 549 
 formation of, after removal of the liver, i. 337 
 
 in the liver, i. 331 
 in muscles, ii. lb? 
 origin of, in the body, i. 550 
 
 in the liver, i. 266 
 preparation of, i. 548 
 from proteid, i. 337 
 presence of, in sweat, i. 337 
 properties of, i. 549 
 Ureters, movements of, i. 371, 389 
 
552 
 
 GENERAL INDEX. 
 
 Urethra, ii. 443 
 
 structure of, ii. 448 
 Uric acid, formation of, i. 338 
 in the liver, i. 322 
 in the spleen, i. 333 
 in muscles, ii. 167 
 molecular structure of, i. 554 
 occurrence of, i. 338 
 origin of, in birds, i. 557 
 
 in mammals, i. 338, 556 
 preparation of, i. 555 
 properties of, i. 555 
 Urinary bladder, innervation of, i. 392 
 
 movements of, i. 390 
 
 pigments, origin of, from haemoglobin, i. 45 
 secretion, normal stimulus for, i. 255 
 relation of, to the blood-flow through the 
 
 kidney, i. 253 
 
 Urine, acidity of, after meals, i. 290 
 composition of, i. 250, 334 
 ethereal sulphates of, i. 572 
 secretion of, i. 251 
 Uriuiferous tubules, secretory function of. i. 
 
 252 
 
 structure of, i. 250 
 Urobilin, i. 574 
 Uterus, ii. 443, 456 
 Utriculus of the internal ear, ii. 373 
 
 VAGINA, ii. 443, 462 
 
 Vagus, anabolic action of, on the heart, i. 166 
 anatomy of, in the dog, i. 159 
 cardiac branches of, i. 159 
 central path of the afferent fibres in, ii. 236 
 effect on the heart, nature of, i. 166 
 gastric branches of, i. 381 
 inhibition, dependence of, on the character 
 
 of the stimulus, i. 165 
 intestinal branches of, i. 385 
 nerve, fatigue of, ii. 96 
 
 pulmonary branches of, i. 465 
 rate of conduction in, ii. 90 
 relation of, to apnoea, i. 442 
 respiratory function of, i. 459 
 pneumonia, i. 466 
 secretory fibres of, to the pancreas, i. 232 
 
 to the stomach, i. 239 
 stimulation, auricular effects of, i. 164 
 effect of, latent period of, i. 162 
 on the heart, i. 152, 163 
 on the ventricle, i. 162 
 Valsalva's experiment, i. 452 
 
 sinuses, i. Ill 
 
 Valves, auriculo- ventricular, i. 108 
 of lymphatic vessels, i. 146 
 semilunar, i. 110 
 Valvulse conniventes, value of, in absorption, i. 
 
 314 
 Variation of the offspring in reproduction, ii. 
 
 500 
 
 Variations, somatic, classification of, i. 497 
 Vas deferens, ii. 447 
 Vasa deferentia, ii. 443 
 
 efferentia of the testis, ii. 447 
 Vaseline, i. 531 
 Vaso-constrictor centre, rhythmical activity of, 
 
 i. 201, 451 
 
 nerves, discovery of, i. 193 
 Vaso-dilator nerves, discovery of, i. 194 
 Vaso-motor centre, medullary, i. 198 
 centres, spinal, i. 199 
 sympathetic, i. 200 
 nerves, anatomy of, i. 198 
 
 methods of investigating, i. 195 
 of the brain, i. 203 
 of the cranial vessels, ii. 286 
 of the generative organs, i. 208 
 
 Vaso-motor nerves of the head, i. 204 
 of the heart, i. 206 
 of the intestines, i. 206 
 of the kidneys, i. 207, 256 
 of the limbs, i. 209 
 of the liver, i. 206 
 of the lungs, i. 205, 466 
 of the muscles, i. 210 
 of the pancreas, i. 207 
 of the portal system, i. 209 
 of the salivary glands, i. 222 
 of the spleen, i. 207 
 of the tongue, i. 205 
 of the veins, i. 195 
 special properties of, i. 197 
 reflexes, i. 201 
 
 through the vagi, i. 172 
 Vegetable foods, composition of, i. 278 
 
 proteids, i. 577 
 
 Veins, effect of compression of, on lymph forma- 
 tion, i. 72 
 
 entrance of air into, i. 97 
 rate of flow in, i. 101 
 vaso-motor nerves of, i. 209 
 Velocity of blood-flow, i. 99-101 
 Venae Thebesii, i. 184 
 Veno-motor nerves of the limbs, i. 209 
 Venous blood-flow, effect of the auricles on, i. 
 
 137 
 
 circulation, i. 95, 96 
 pressure, i. 91, 94 
 pulse, respiratory, i. 96 
 Ventilation, principles of, i. 439 
 Ventral nerve-roots, number of fibres of, ii. 230 
 Ventricles, independent rhythm of, i. 152 
 of Morgagni, ii. 422 
 of the brain, capacity of, ii. 274 , 
 
 work done by, i. 106, 107 
 Ventricular bands, ii. 422 
 cycle, analysis of, i. 133 
 diastole, duration of, i. 123 
 pressure-curves, analysis of, i. 128 
 pressures, i. 125 
 systole, duration of, i. 123 
 
 Veratria, action of, on coagulation of muscle- 
 plasma, ii. 164 
 
 on muscular contraction, ii. 129, 137 
 on nerves and muscles, ii. 60 
 effect of, on muscular contraction, ii. 128 
 Vernix caseosa, i. 258 
 Vessels of Thebesius, i. 186 
 Vertigo in diseases of the ear labyrinth, ii. 406 
 Vestibular root of the auditory nerve, central 
 
 path of, ii. 237 
 
 Vestibule of the bony labyrinth, ii. 371 
 Vibrations of the tympanic membrane, ii. 370 
 transmission of, through the labyrinth, ii. 
 
 376 
 
 Villus, intestinal, structure of, i. 318 
 Viscero-motor nerves to the intestines, i. 385 
 Viscosity of irrigating media for the heart, i. 
 
 191 
 
 Vision, binocular, ii. 356 
 far-point of, ii. 312 
 indirect, ii. 341 
 near-point of, ii. 312 
 pseudoscopic, ii. 357 
 sterescopic, ii. 357 
 Visual area of the cortex, ii. 253 
 
 impulses, place of origin of, in the retina, ii. 
 
 327 
 
 judgments of distance, ii. 348 
 of size, ii. 350 
 
 and distance, ii. 354 
 purple, i. 575; ii. 330 
 
 adaptation of the eye by, ii. 326 
 sensation, intensity of, ii. 339 
 
GENERAL INDKX. 
 
 Vital capacity of the lungs, i. 427 
 
 force, definition of. i. 25 
 Vitrlliu, composition of, i. .~>?;i 
 Vitelline membrane, absence of, in human ova, 
 
 ii. 450 
 Vitreous humor, opacities in, ii. 321 
 
 refract ivf index of, ii. 303 
 Vocal cords, false, ii. 422 
 
 true, ii. -12.'$ 
 Voice, ii. 430 
 
 changes at puberty in, ii.489 
 etfect of am- on, ii. 431 
 pitch of, ii. \'.\-l 
 registers of. ii. 432 
 
 Voice-production, ii. 421 
 
 mechanism of, ii. 431 
 Voices, classification of, ii. 433 
 Volta, ii. > 
 Voltaic pile, ii. 28 
 Voluntary control of the heart, i. 178 
 
 muscular contractions, fatigue of, ii. 134 
 tetanic character of, ii. 133 
 
 reactions, atl'erent paths of, ii. 226 
 anatomical mechanism of, ii. 226 
 compared with reflex, ii. 225 
 Vomiting, i. 387 
 
 causes of, i. 388 
 
 centre for, i. 389 
 
 nervous mechanism of, i. 388 
 von Gudden's commissure, ii. 238 
 Vorticella, movements of, ii. 20 
 Vowel-sounds, ii. 431 
 
 differences in quality of, ii. 385 
 
 production of, ii. 434 
 Vulva, ii. 443, 462 
 
 WALKING, ii. 420 
 
 Wallerian degeneration, changes of excitability 
 
 in, ii. 69 
 
 of nerve-fibres, ii. 197 
 of nerves, ii. 69 
 
 Wandering cells, definition of, i. 48 
 Water, absorption of, i. 313, 318 
 amount lost through the lungs, i. 410 
 distribution of, i. 503 
 effect of, on pancreatic secretion, i. 236 
 elimination of, i. 340 
 imbibition of, i. 504 
 
 Water, latent heat of, i. 504 
 nutritive value of. i. -_'7i;. :;:, 
 percentage of, in brain and cord, ii. 274 
 properties of, i. 503 
 pure, toxic action of, on nerves and muscles, 
 
 ii. r,- 
 Weber's law, ii. 340 
 
 applied to pressure-sensations, ii. 393 
 Weight of embryo, increase of, ii. 487 
 of the brain and spinal cord, ii. 274 
 decrease of. in old age, ii. :.'Ji; 
 relation of, to social environment, ii. 277 
 of the child at birth, ii. 487 
 Weissmann's theory of heredity, ii. 502 
 Wharton's duct, i. 217 
 Whispering, ii. 436 
 
 Whistling register of the voice, ii. 433 
 White matter of the central nervous system, 
 
 water contents of, ii. 274 
 William's frog-heart apparatus, i. 188 
 
 valve, i. 187 
 Wines, i. :.:;:> 
 Wirsuug's duct, i. 231 
 Womb. See Uterus. 
 Work done by contracting muscles, conditions 
 
 affecting, ii. 139 
 
 by muscular contraction, curve of, ii. 140 
 by the heart ventricles, i. 106, 107 
 Worms, segmental nervous system of, ii. 212 
 Wrisberg, cartilages of, ii. 422, 425 
 Wundt's tetanus, ii. 131 
 closing, ii. 37, 61 
 
 XANTHIN, i. 553 
 
 of muscles, ii. 167 
 
 physiological significance of, i. 339 
 Xantho-proteid reaction, i. 576 
 Xylose, i. 562 
 
 YAWNING, i. 454 
 
 Young-Helmholtz theory of color vision, ii. 335 
 
 ZOLLNER'S lines, ii. 351 
 Zona pel lucida. ii. 454 
 
 of the ovum, ii. 450 
 radiata of the ovum, ii. 449, 450 
 Zymogen granules, definition of, i. 228 
 
 of the pancreas, i. 235 
 

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