For Reference

NOT TO BE TAKEN FROM THIS ROOM

JF&t^ fa &« -h J~^^

' 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.

WARREN P. LOMBARD, M.D.
GRAHAM LUSK, Ph.D.,F.R.S. (Edin.j
W. T. PORTER, M.D.
EDWARD T. REICHERT, M.D.
HENRY SEW ALL, Ph.D., M.D.

ORS. TASKER & TASKER
OSTEOPATHIC PHYSICIANS

526-529 Auditorium Bldg,
Le« Angeles, Cal.

WILLIAM H. HOWELL, Ph.D., M.D.

Professor of Physiology in the Johns Hopkins University, Baltimore, Md.

EDITED BY

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

1901

CM

Copyright, 1900,
By W. B. SAUNDERS & COMPANY

PRESS OF
W. B. SAUNDERS 4 COMPANY

CONTRIBUTORS TO VOLUME II.

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.

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 hook 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 textbook
that may be needed daily for consultation in the lecture-room or the labora-
torv. 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, 'flic 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 ami considerably amplified, a table
of contents has been added to each volume, and numerous new figures have
been introduced.

August, 1900.

PKEFACE.

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

PBEFA CE.

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 fad 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 lie 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
manv 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. II. Howell).

BLOOD (By W. H. Howell).

LYMPH (By W. H. Howell).

CIRCULATION (By John G. Curtis and W. T. Porter).

SECRETION (By W. II. 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 amcebje, 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 (Pfluger's law1, ID — 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 dh'eetions 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 Elate
of conduction in motor nerves, 89 — Rate of conduction in sensory nerves, 91 — The
effect of deatli 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. L02 -Optical properties of muscle daring rest
and contraction, 103 The elasticity of muscle. L05 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 fr repeated stimulation, 113 The

effect of repeated stimulations on the form of separate contractions, I 1 5 The pro-
duction of tetanus by repeated stimulations, 117 — The cause of summation in tetanic
contractions, 120 The effect of two excitations following rapidly, i".'i 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, 121 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-

11

12 CONTENTS.

PAGE

tractares and continuous contractions, 127— Contracture following frequent excita-
tions, L28 Contracture following single excitations. 129— The effect of fatigue ou
contracture, 130 Effect of tlie constanl 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 -Effecl of drugs and chemicals
upon muscular contractions, 137 — Liberation of energy by the contracting muscle,
13b — Conditions cou trolling the amount of work done by a muscle, L39 — The thermal
energy given oil' by a contracting muscle, 1 11 -Muscle-tonus and chemical tonus, 143.

E. Ele< rBiCAX 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, 117- Theories as
to the cause of the current of rest, 1 1> -The current of rest in nerves, 149 Currents
of action in muscle, 150- Secondary tetanus, loll- -The diphasic act ion currents, 152 —
Relation of the action current to the muscle contraction, 153 — Currents of action in
nerves, 153- Gelation of the electrical phenomena of nerves to the physiological pro-
cesses, L57.

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 — Rigor 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 cheni-
ist rv of nerves, 169.

CENTRAL NERVOUS SYSTEM (By Henry H. Donaldson) 171

Introduction 171

The unity of the, central nervous system. 171 — Phenomena in vol ving consciousness,
172 — Growth and organization, 172 — Plan of presentation of the subject, 172.

Part I.— Physiology of the Xkrve-cell 173

A. Anatomical Characteristics of the Nerve-celi 173

Form of nerve-cells, 173 — Peculiarities in structure of nerve-cells, 174 — The volume

relations of nerve-cells. 17." — Size of nerve-cells iu 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 ax one, 177 — Growth of
the branches of the nerve-cell, 179 — Internal structure of the neurones, 179 — Medulla-
tion of nerve-fihres, 179— Growth of the medullary sheath in peripheral nerves, 180 —
Medullation in the central system, 181 — Changes m the cytoplasm, 182 — Old age of
the nerve-cells, 182.

B. The Nebve 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, 1S7 — The rate of discharge of nerve-impulses, 189 — Points at which the
nerve-impulse can be aroused, L89— Irritability and conductivity, 189 — Summation of
stimuli in nerve-cells. 190.

C. Tin; Nutrition of the Nerve-cell 191

Chemical 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 1 >egenera1 ion of the cell-body, 199 -Regeneration of the axone, 199.

Part II. — The Physiology of Groi ps of Nerve-cells 202

a. Architects re \ni> Organization of the Central Nervoub 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 t he passage of the nerve- impulses, 207.

B. REFLEX Actions 207

The condition-; of stimulation controlling reflex actions, 297 — 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 Re Ilex reactions in other vertebrates, 212 Co-ordination of the.
liferent impulses iu reflex actions. 211 — 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

CONTEXTS. 13

PAGE

the sympathetic system, 219— Evidence for continuous outgoing impulses from the
central nervous system, 220— Rigor mortis as affected by the nervous system. 220
Modification of reflexes by simultaneous and successive afferent impulses, -2,21— Effects
of afferent impulses on reflexes, 223— Inhibition of reflexes, 223.

C. Reactions Involving the Encephalon 226

The path of afferent impulses in the central nervous system, 226— Degenerations in

the spinal cord after hemisection, 228— Physiological observations on afferenl pathways
in the central nervous system, 229— The nerves of common sensation. 230- I he nerves
of pain and their pathway in the cord, 231— The pathways ol impulses m the cord,
233— The nuclei and courses of the cranial nerves, 230— The pathway ot the libres ot
the optic nerve, 238— The pathway of the fibres of the olfactory nerve, 240.

D. Localization of Cell-gboups in the Cerebral Cortex 241

The discovery of localization of function in the cortex, 241— Effects of stimulation

of the i cortex, 241 Course of the descending impulses, 244— Mapping ol 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 ot
the pyramidal tracts in different mammals, 252.

E. Localization in the Cerebral Cortex of the Cell-groups Receiving the

Afferent Impulses • ~°2

Sensory regions of the cortex, 252— Delimitation of the sensory areas, 253— Hemi-
anopsia 255— Association-fibres and 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 2/4

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 2<8

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

( >rganization 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 ■«

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 Centbal 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 -!,s

A. Vision (By Hknky P. BOWDITCH) 298

The general physiology of vision. 298— The mechanical movements <>f the eyeballs
around various axes, 298- The muscles of the eye. 299 The dioptric apparatus of the
eye, 300 The refracting media of the eve. 302 The optical constants of the rye, 304
—The mechanism of accommodation, 306 The range of accommodation, 312 Myopia
and hypermetropia, 313— Presbyopia, 314— Spherical aberration, 315 Chromatic
aberration, 316 Astigmatism, :;i7 [ntraocular images, 320— Muscse volitantes, 320
—The retinal vessels, 32] 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. 328 Changes produced m
the retina bv 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 CO XT E NTS.

PAGE

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
latenl period <>f light-sensations, 343 -The rise to maximum of light-sensations, 3t3
— The fatigue of tin- retina, 344— The after-effect of stimulation, 315 — After-images,
346 — Color-contrasts, 346 -The perception of space. 347— Irradiation, 34!) — The false
judgments of snbdivided Bpace, 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 Eab and Hearing (By Hknkv Skwall) 362

The anatomy and histology of the ear, 362 — The external ear, 362 — The middle ear,

36:; 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, 3s3 —
Analysis of composite tones by the ear. 384 — Inharmonic overtones, 386 — The produc-
tion n( heats, 386 — Harmony and discord. 3S7 — 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. CUTANEOU8 AND MUSCULAB 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 Semicibculab 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, 115— 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, 121 — Functions of the epiglottis, 422— Ventricular hands and

ventricles of Morgagni, 122— The true vocal cords, 423— The cartilages of the larynx.
425— The muscles of the larynx. 125— Specific actions of the laryngeal muscles, 427—
The nerve-supply of the larynx, 428 — The laryngoscopy appearance of the larynx,
429— The production of voice,' 130— Loudness and pitch of voice. 430— Quality of voice,
130 Arrangements for changing the pitch of the voice, 432— The vocal registers, 132
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. Bepboductiom in Genebal 439

Asexual reproduction, 439 — Sexual reproduction, 440— Origin of sex and theory of

reproduction, 111 — Primary and secondary sexual characters, 442 — The sexual organs,
443.

B. The Male Repboductive Organs : • 413

Structure and properties of the spermatozoon, I 13- Maturation of the spermatozoon,
445— The composition of semen, 445 -The testis, 146 Tin' urethra. 1 18— The prostate
gland, II- -Cowper's glands, 448 — The penis, 41-".

CONTENTS. 15

. PAGE

C. The Female Eeproductive Organs 449

The ovum, 449 — Maturation of the ovum, 451 — The ovary and ovulation, 454 — The

Fallopian tubes, 456 — The uterus, 456 — The act of menstruation, 157 — 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, 462.

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 — Polyspermy, 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, 478 — 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 — Theoriesof heredity, 498- < Jcmi-plasm, 499 — Variation, 500 — Darwin's theory

of pangenesis, 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 aud 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 <>f m vasl Dumber 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 muscle-
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 of 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 bauds, 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

SL

Fig. 1.— Amoeba proteus, magnified 200 times: a, endosarc; b, simple pseudopodium ; C, ectosarc; d,
first stage in the growth of a pseudopodium : c, 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 matter 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

result from changes which have occurred within a,Ciiiaof ciliated disk; 6, ciliated disk;
its own substance, and apparently independently ''• peristome; </. vestibule; >. oesophagus;

n , • n r\ ii ,1 i i f, contractile vesicle; g, food-vacuoles ;

of any external influence. On the other hand, ft.endoplast; Undosarc; Mctosarc; I,

if its body be disturbed by being touched, by CQticle; m, axis of stem (after Brooks:

• i • i Handbook of Invertebrate Zon!ti<i)/).

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 littl" 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 Aikins2 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 Actinias, the surface sarcode of sponges,
the chromatoblasts of Pleuroneetida?,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 niu-cle-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 those movements, accompanied by illustrations, is given in
Quoin's Anatomy, vol. i., pt. 2. pp. 174-179.

a Hodge and Aikins: American Journal of Psychology, 1895, vol. vi., No. 4, p. 524.

3 Krukenberg: Vergleichend-physiologische Vbrtrdge, 1886, Bd. i. S. 271.

* A careful study of the different forms of movement exhibited by simple organisms has
been made by Engelmann: Hermann's Handbuch der Physiologie, 1879. r>d i., Th. 1, IS. 344.

GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 21

Irritability and conductivity are not confined to contractile mechan-
isms. They are possessed in a still higher degree by nerve-cells, neurones,
as they are called, which have not thus far been found to have the power of
movement, except that which is associated with the growth of a cell. Each
neurone is composed of a body and one or more branches. The bodies of the
nerve-cells are located chiefly in the spinal cord and brain, a smaller number
being found in the spinal ganglia and in the ganglia of the so-called sympa-
thetic system. The branches of a neurone are of two kinds, an axis-cylinder
process, or axone, which frequently carries at its extremity a specially formed
organ, through which it is able to excite to action the cells with which it
comes in contact, and protoplasmic processes, or dendrites, which have no
such exciting mechanism, and are destined to receive excitation and transmit
it to the body of the nerve-cell. Outside the central nervous system, at
least, the axone and the dendrite acquire a delicate membranous sheath,
the neurilemma, which invests it as the sarcolemma does the muscle-fibre.
The branches of nerve-cells together with their sheaths form the nerve-
fibres. There are two classes of nerve-fibres, medullated and non-medullated,
which are distinguished by the fact that the former has between the axis-
cyliuder 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
awaken 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 character of the activity which they excite : those which excite muscle-
fibres to contract are called motor nerves; those distributed to the museles
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 sensations, 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 these cases, the result depending wholly on the organ stimulated.

Nerve-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
nerves 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 these many strands are
bound together is of anatomical interest, but has little physiological significance.

The active substance of the nerve-fibre does not show contractility, but this

22 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 isdeveloped.
The fertilized ovum 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. During 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 repair
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 protopla-m 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 Tubularidae, which is
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-
in-, much a- 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.

GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 23

spermatozoon comes in contact with the mucilaginous covering of the ovum, "a
prominence pointing toward the nearest spermatozoon now rises from the super-
ficial layer of protoplasm of the egg and grows until it comes in contact with
the nearest spermatozoon." " At the moment of contact between the sperma-
tozoon and the egg, the outermost layer of protoplasm of the latter raises 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 likewise been observed to undergo amceboid movements as a result
of the physiological stimulus 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 these
properties are 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 the
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 raises
a given weight may be taken as an indication of its activity. As the nerve
gives no visible evidence of activity, the effecl of the irritant upoa it is usually
estimated 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
muscle is taken as an index.

To determine the exact relation of an irritant to its irritating effect we should

1 Korschelt: Zoologischea Jahrbueh, 1891, Aunt. Abtheil., Bd. iv., Heft 1, S. 1. Hertwig:
Morpkologisches Jahrbuch, 1876, Bd. 1. Herbst: Biologische Centralblatt, L891, xiii. S. 22.

2 Kor the physiology of the lower forms of :niim:il life, see (,'cmr<il I'!ii/siol<nji/} bv Yerworn;
translation by V. S. Lee, London and New York, 1899.

24 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY.

be able to accurately measure them. This we cannot do. We are unable to
state in irritation-units the relative value of different kinds of irritants. Even
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 ami 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
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 iniisele hold good, with but slight modification, for the
nerves and muscle- 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
it- groove, the branches going to the thigh-muscles being divided. The leg is
then cut through jusl above the knee. This gives a preparation consisting of
the uninjured lower leg and foot, and the carefully prepared nerve supplying
tiie muscles of these parts. The leg may be placed foot upward, and fastened

GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 25

in this position by a clamp which grasps the bones at the knee, the clamp
being supported by an upright (see Fig. 3). This preparation can then be
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 FlG" V -Pl-f '.T,' fofr determining

& ' the irritability of nerves.

flame of a match, be applied to the nerve, a

condition of activity will be developed in the rapidly heated nerve-fibres, and

be 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 change in
potential.

Pliysiological 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 muscles
was an evidence that the nerve had undergone a change at the point of stim-
ulation, and that the active state thus produced had been transmitted through the
length of the nerve, and had been sufficiently marked to stimulate the muscle
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 irritants
were applied to the nerve, and the muscle was indirectly stimulated. Muscle
protoplasm, like nerve protoplasm, may be directly excited to action by various
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 as
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 curare.

Curare, the South American arrow-poison, is used by the Indians in hunt-
ing. The bird shot by these poisoned arrows gradually becomes paralyzed,
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 same

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-
j id itcs?

Qurare 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
f< \v 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 («, Fig. 4)
causes no contraction of the muscles of that leg, while stimulation of the motor
nerve of the left leg (b), 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 Hud 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,
it is necessary to assume that the cause of the paralysis is something which pre-
vents the nerve-impulse from passing from the nerve to the muscle. Micro-
Bcopic examination shows that the nerve-fibre does not communicate directly
with the muscle-fibre, but end- in-i<le the sarcolemma in an organ which is
called the motor end-plate (see Fig. 31). It appears that the nerve acts on the
muscle through this organ, ami it> 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 dired excitation with all forms of irritants, viz.,

1 Ch. Bernard : " Analyse physiologiqne des Pmprietcs dee Systemes tnosculaires et nerveux
au moveri du Curare," Otnnptes-rendus, 1856, p. 825. Kolliker: " Physiologische T'ntersuch-
ungen iiber den Wirkunjjen einiger (lifte," Archiv fur pdthologische Anatomie, 1S56.

GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 27

electrical, mechanical, thermal, and chemical. Evidently the muscle-proto-
plasm is irritable and is capable of developing1 a contraction independently of
the nerves. There are a number of natural plant bases that have a " curare-
like" action — c. g.} brucin, strychnin, leucin, nicotin, conin, etc.1 If 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 Proofs llutt the 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- Fl«-4--curareexpenment:

& ... the shaded parts shovi there

tion and loses its irritability and conductivity in four gionofthebody to which the

or five days, and the excitation of such a nerve has S^^fSS*™

no effect upon the muscle although direct stimulation protected by the ligature

,. ri i •, Mf • r> ii ii ,• » from the 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 the sensory nerves which

... " , . . carry sensorv impulses from

nerve, but also the nerve end-plates, the contraction the skin to the central nerv-

must be attributed to the irritability of the muscle- ous system ; the broken lines

* m m 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, &om the central nervous sys-

* o 7 te]ll ,,,,( (I, (iR, muscles

instead of contracting as a whole it contracts at the Lauder Brunton: Pharmacol
i i •. • •■ ■ j ,i j • ,i t} otii/. Therapeutics, ami M

place where it was irritated, the drawing together ot „,,/„.<,)
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 mean.- 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 idiomuseiilar contraction.

1 Santesson : Archiv fur experimentelle Pathologic und Pharmakologie, 1 '.<•">, 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. (<1) The duration of
application, (e) The angle of application. (/) The direction of flow.

Irritating Effect of the Electric < 'urrent. — 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. Volta, 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 I><ii, i>ll 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 MUSCLE AND NERVE.

29

to enclose one of tlieni 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 cup is the sulphuric acid and tie
zinc plate, and inside the cup is the copper sulphate solution and the copper plate.
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 always 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 kathode. 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 the sudden flow of
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. 5.— Daniell cell.

^r

X)

: j

V

^>

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 : b, 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 day mixed with a 0.6 percent, solution
of sodium chloride, and a1 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 (day to the tissues; d /, electrodes for exciting human nerves and mus-
cles through the skin (after Erb): these may be of various forms and sizes, and arc 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.

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-lley-
niond, 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 S.

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 b, 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 passim; 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 PHYSIOLOGY OF MUSCLE AND NERVE. 31

nerve gives a single twitch at the moment that the current enters the
nerve, and then remains quiet; and thus we meet with the remarkable fact
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 ihc strength of which is increased or decreased
very gradually, no change occurs in the oerve 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 tin: circuit

32 AN AMERICAN TEXT- BOOK OF PHYSIOLOGY.

containing the nerve. The wires from the battery are connected with binding-posts, a, 6
(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 roils are connected with binding-posts on the movable bar, and
from these wins 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 < ml v 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-Revmond 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 irritant-. 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 ; b, 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, b, and a second coil, s, wholly
independent of the first, which is connected with
electrodes, e. At the instant that the key, h, 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 of fail of the Intensity of the primary cur-
zero. As long as the battery current continues to rcnt when u is broken; 4, curve of tin- rise

a ,i ., i .1 • -l n • and lull of intensity of the corresponding in-

flow constant ly tliroimli ( lie pnniarv coil, ( here is , , .

■ ' ' _ _• duced current.

no change in the electrical condition of the sec-
ondary coil, but at the instant (lie primary current is broken another induced current of short
duration is set up in the secondary coil, and again the nerve receives a shock. The rise and
V,,,.. ti._ 3

^

m*** —

^^>-^

4

Fig. 12.— Schema of the relative intensity
of induction currents (after Hermann, Hand
buck dir Physioloffie, Bd. ii. 8. 87): /', 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

34

AN AMKIUCAN TEXT-BOOK OF PHYSIOLOGY.

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 in the nature of the apparatus. At the
moment that the current begins to flow into the primary coil, it induces not only a current
in the secondary coil, but also currents in the coils of wire of the primary 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
is 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 MUSCLE AND NERVE. 35

ments 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 piG 14 _Effect of making
records of the breaking contractions being higher than and breaking induction
those of the making contractions (Fig. 14). Similar

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
Von Bezold,1 and has since been substantiated for nerves as well as muscles

Fig. 15.— Schema of Ilering's double myograph: C, clamp holding middle "I' muscle ; P.P. pulleys to
the axes of which the recording levers are attached ; p,p, pulleys for the light weights w hich keep the
muscle under slight tension; A, positive electrode; K, negative electrode ; r, commutator for reversing
the current ; k, key ; b, 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-

1TJntcrxiiclnui<j<n fiber die elektrische Erreguny run Muskeln und Xrnm, 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, hut 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

r rding 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. 10). 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, .4), 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 <i ami A (Fig. 16). The anchors can be rocked to one side or the
Other, BO that the ends of the curved arms shall dip into the cups c and d (in which case
cup '/ will he 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 ease 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, <; d, and in the reverse direction,
up the nerve and hack 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, B). 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 3IUSCLE AMD NERVE. 37

the electrodes on one part of the nerve, and the cups e, /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 OE 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
put 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, b). 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 ; b-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 iiber 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-
sistance in «, 6, c, d, e, /, and
in a, b, g, h, e, f. The bridge
c, (I can be slid along the fine
German -silver wires b, i and
e, j, and thus the resistance
a, 6, c, d, e, /, and the amount *1g. 2L— 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 Owrremt. — 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: Electrophysiologic, 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-Eeymond : Untersuchungen iiber 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
brok

cu

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., 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 witli 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: Handburh 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- BO OK OF PHYSIOLOGY.

to a ck-nsc How 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 maybe 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 (e*, 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 (A), 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 area battery (k) and a key {I); the needle (wi) 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 (b), which is on a plate (c), resting on a sheet of vulcanite (d) ; 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 OE PHYSIOLOGY.

Ill a like manner if a nerve-muscle preparation he isolated, as shown in
Fig. 22, and a needle, held in the hand or connected with a large metallic
conductor or a condenser, he 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. Xon-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 Physiologie, Bd. ii. Theil 1, S. 88.

2 Biedermann: Elektrophysiologie, 1S95, Bd. ii. S. 546.

GENERAL PHYSIOLOGY OF MUSCLE 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 Anodonta, 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 unstriated 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 curreuts
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 uerve 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 Engclm;mn : Pfluger'a Archiv, L870, 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-
lieymond (see p. 32) must be modi tied 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. Leicher1 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 Meyer2 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.

Fick3 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 Uhtersfuehimgen aus dem phytioloffischen Institut der Unircrsitat Halle, Heft i. S. 5.

'-' /'lliiyerJs Archiv, 1880, Bd. xxi. S. 462.

s Beitrage zur vergleichendc Phn/siologie du- irritublen Substanzcu, Braunschweig, 1863.

GENERAL PHYSIOLOGY OF MUSCLE ANU XEHVE.

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-

:■<

a

'

W>

10 SO

SO

4"

100 11"

'0>

b

10,

/

0

10

20

20'
10

0

d

\i

e

f

/ \

Fio. 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 tail 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 tin1 Direction in which the ( \wreni flows along the Neme. —
The result of the irritating change produced in a nerve by a battery current
has been found to depend upon whether the current Hows toward or away
from the organ stimulated by the nerve. This fact can be most readily ob-
served in the ease 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.— I

50

AN AMERICAN TEXT- BOOK OF PHYSIOLOGY.

Moreover, by a given rate of change of intensity, the stimulating effect varies
with the Btrength of the current employed. Pfliiger in his celebrated mono-
graphj Untermchungen uber die Physiologie des Eleldrotonus, 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. Eest.

Medium " Contr. Contr.

Strong " Eest. Contr.

Descending Current.
Closing. Opening.

Contr. Rest.

Contr. Contr.

Contr. Rest.

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).

b. 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

Strong current

Fig. 25.— Diagram illustrating Pfliiger's law.

The twelve cases i Deluded 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. 51

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, i. c, 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 tails 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 tor 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.

\? — \r

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
arc given in dotted lines (after Erb: Ziemsseu's Pathologie 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
inters 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, lor 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 effect 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
cases may arise, namely :

1. Anodic closing 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, o, 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. c. 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 In-
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.

K< i KCC KCC

ACC ACC

AOC AOC

KOC

The natural order, therefore, would be KCC, ACC, AOC, KOC. Some-
time-, 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. Bordier1 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 At ) ; and that it was least by Kl) —
i.e., 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, <',/).
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 renetion of the nerve can be readily tested.

When the currents employed are strong, it occasionally happens in the
case of meu thai 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 A( !< ! obtained with weaker currents than KOC, and KOC than
AOC (Babinski) 2. This is known as the reaction of degeneration. Under

1 Bordier: Archives de Physiologie normaleet Pathohgique, 1897, ]>i> 543-553.

2 I5;ii>in.ski : Comples rendus de la Sociele de Bioloyie, 1899, p. o4o.

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.

(«) The Influence of Irritant* 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 Tigerstedt1 to be 7000 to 8000 milligrammillimeters, which would corre-
spond roughly to a weight of 0.500 gram filling 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 shori intervals, without interfering with the activity

1 Studien iiber niechanisclie Nervenreizung," Acta Societatis Scientiarum Fennica, 1880,
Bd. xi. S. 82.

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 (<> from •'> to 4 millimeters in height. Rapidly following
light blows or twitches applied to a motor nerve, by the tetanomotorof Heiden-
hain or Tigerstedt, excite a -cries 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 lone a<n> 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
0 up to 20 grams the irritability of the nerve is continually increased, but
it lessens as soon as the weight is further increased."

Surgicallv 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 nerves 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. I'xhull: Zetockrifl, fur Biologic, 1894, Bd. xxxi. S. 118: 1895, Bd. xxxii. S. 438.

2 Op. cit., 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 ccdoris (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 Xot 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 <>/ Physiology, 1896, xx. p. 247.

2 Waller: Ibid., 1899, x.xiv. p.L

8 Gotch and Burch: Ibid., 1899, xxiii. |>. 'J-J ; Boycott : Ibid., 1899, xxiv. p. 144.

58 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY.

.-lows chemical processes and increases the endurance. It is noticeable that
aerves and muscles remain irritable much longer than ordinarily in ease the
body be cooled before their removal. In tie case of a mammal, the irritability
may last from six to eighl hours instead of two and a half, while in the ease
of frogs it may be preserved at 0° for ten days, although at summer heat it lasts
bnly 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

i stitution 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 <> 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
e<ll- 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 Nad, CaCl,
and KC1, even when there are no energy-giving substances present in the
fluid. Howell ' 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 Iv
are essential to the development of the rhythmic movements of the heart-
muscle. Loeb2 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 Xa, 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 woidd supply the Xa, 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 arc 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 Xa, 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. W 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. |» 17. 2 Ibid., 1900, i i i . p. 383.

-

■

58

IV 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 tRe case of a mammal, the irritability
may lasl from six to eighl hours instead of two and a half, while in the case
uC frogs it may be preserved at 0° lor ten days, although at summer heat it lasts
'onlv 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
t<> ordinary room-temperature.

Effect <</" ( 'hemicals "ml Drugs. — The irritability of nerve and muscle proto-
plasm is markedly influenced by even slight changes in its constitution. II'
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 take- 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 ;i 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 <i parts per 1000 of distilled water, has
keen 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 lot K) is approximately in osmotic equilibrium with the fluids of the
(•ell- 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 cell-. 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 salt- of tin' 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 tissue-, and ike need tk.it 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 OE 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 (Ritter'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, nol only doe- 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 curreni 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 (lie frog's heart,
and the kathode at some indifFerenl 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: Etektrophysiologie, 1895, S. 195.

62 AN AMERICA X 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 " an electrotonus ;" while the
change at the kathode was termed " katclectrotonic," 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 quicklv thrown into or removed from the nerve, and a commutator (O),
which allows the current to be reversed and to be sent through the nerve in

1 Waller: Lectures on Animal Electricity, London, 1897; Biedermann: Eleclrophysiology,
translated by F. A. Welby, 1898, rol. Li.

'' Pfliiger: Untermchwngcn iiber die Physiologic des Electrotonus, Berlin, 1859.

GENERAL PHYSIOLOGY OF MUSCLE AND NERVE.

63

either the ascending or descending direction. ( Jonnect 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 /.

Fig. 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 / 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 1 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 alter 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 Prliiger to illustrate the way in which the irritability is changed in
the anelectrotonic and katelectrotonic regions as the strength of the current is
increased :

Pig. 30.— Electrotonic alterations <>f irritability caused by weak, medium, and strong battery
currents: A and Bindicate the points of application of the electrodes to the nerve, A being the anode,
Bthe kathode. The horizontal line represents the nerve at normal irritability; the curved lines illus-
trate how the irritability is altered al differenl parts of the nerve with currents of different strengths.
curve v1 Bhows the effect ofs weak current, the part below the line indicating decreased, and that above
the line increased Irritability, at z* the curve crosses the line, this being the indifferent point at which
atelectrotonic effects are compensated for by anelectrotonic ctl'ects: //-gives t lie effect of a stronger
fa 8tm stronger current. As the strength of the current is increased the effect becomes
r and extend- farther Into the extrapolar regions. In the intrapolar region the indifferent point is
to advance <n ith 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. 05

of irritability immediately after the removal of the current, and vice versa.
When the current is withdrawn from the nerve, the irritability in the region
of the kathode is lowered, and in the region of the anode raised. It must be
added, however, that the decrease of irritability seen at the kathode gradually
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 the irritability rises from the condition of
decreased irritability up to something above the normal irritability. The change
from the normal to the anelectrotonic 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 :i result of this, induction shocks too feeble to
excite contractions may, if frequently repeated, after a little 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
cadi cause a larger contraction than its predecessor, until a certain height of
contraction has Ween 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 ton tractions, 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
aetivitv 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," as it
is called, responding very imperfectly to stimuli applied to it just before
and during its systole.- 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 <>f 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 1 <'.. with maximal stimuli, the "critical period"
may lie 0.007-0.008 second ; at higher temperatures it is shorter.

(I>) Influences which favor the maintenance of the Normal Physiological
Oondiiion <>f Nerve <m<l Muscle. — Effect of Blood-supply m, Nerve and Muscle.
— The vascular system is a path of communication between the several organs

1 Waller: Lectures on Physiology, first scries, 1897, p. C>8.

- Cashing: Journal of Physiology, 1897, vol. xxi. p. '_'] 1.

5 Boycott : Ibid., 1899, v<<\. sxiv. p. 144. * Gotch and Burch: Ibid., p. 410.

GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 67

and tissues, aud 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 iu 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 an1 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

68 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY.

not lost for four or five hours. In tin's 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 iu 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-
bodv (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 ;* 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 Frey2 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 kepi 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. X. Martin3 made a similar experiment on the heart

1 SUzungaberichte der math.-phys. Claisse der k. sacks. Gesellschaft der Wissenschaftm, vol. xx., 1868.

2 " Versuche iiber den StoflTweohsel des Muskels," Archiv fur Anatomic und Physiologic,
1885; physiologische AbtheilunK, S. 533.

3 Studies from the Biological Laboratory of Johns Hopkins University, 1882, vol. ii. p. 188.

GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 69

of a dog. The heart and lungs were isolated from the rest of the body, the
heart was fed with defibrinated blood from a Mariotte flask, and the Lungs
were supplied with air by an artificial respiration apparatus. The heart, which
was kept moist and at the normal temperature, continued to beat for four hours
and more. Porter1 has succeeded in keeping even small pieces of the ven-
tricle of the mammalian heart alive by maintaining a good circulation of
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 free 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 ease
of mammalian nerves loss of irritability may be complete at the end of three
or four days, but the nerves of cold-bl led 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.
2Sczelkow: Sitzungsbcr. d. k. A had, Wim, 1862, Bel. xlv. Abtli. 1.
3 Blix: Skandinavischcs Archil' 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
uerves 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 u-hicJi 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 NERVE. 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 indicis, 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 hv the flexion of
the second linger. 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 t<> prepare the chemical compounds from
which the energy is liberated as fast as they are used, and get rid of the

1 Archiv fiir Anatomie uittl Physiologie, 1890; physiologische Abtheilung, S. 191.

2 Mosso: Die Ermiidung, Leipzig, 181*2, S. '.hi ; 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 nut wholly clear. It is noticeable, however, that not only the
muscles employed in the work, hut 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 lung 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 dug 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. Lee2 would draw a sharp distinction between
fatigue and exhaustion. lb' 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 Arcliir fur Anatomic mul Physiologie, 1890; phvsiolmrische Abtheilung.

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 NERVE. 73

tability, may exert a protective influence and 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 tvpe
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. Waller1 has of late laid much stress upon the action
of C02 to stimulate protoplasm when present in small amounts and to anaes-
thetize it when in larger quantities. C02 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 recoverv.
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 completelv 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. It 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 OE 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 Kick 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 Pfluger and his pupils
have gone to the other extreme and conclude that proteid is the chief source
of energy.1

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 Vierteljahrexschrift der naturforsche Gesclkchafl in Zurich, 1865, Bd. X. S. 317.

2 Zeilschnfl fur Biologic, 1866, Bd. ii. 3 Ibid., 1876, Bd. vi. S. 305.
1 Pfluger' s Arclav, 1899, Bd. 77, S. 425.

s 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 follows
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 compounds
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: Archil) JUr Anatomic und Physiologie, 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
emploved 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-tissne, 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.

GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 77

Effect of Enforced Rest. — Not only is the strength of the muscles greatly
increased by exercise, but a lack of exercise soon results in a loss of strength.
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.

C. 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 fibrilhe 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 fibrillar
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 ncrvc-libre, most rapidly of all.

Protoplasmic Continuity is Essential to Conduction. — Effect of a
Break in Protoplasmic ( 'ontinmby.- — A break of protoplasmic continuity in any
1 Biedermann : Elektrophysiologic, 1895, 8. 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 davs 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 Huber1 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-cylinder, i> 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 power
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 tact
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 ' 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-tmt/nks. — 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, IS'.'T, vol. xxii. p. '2-A.

80 AN AMERICAN TEXT-BOOK OE 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-medullated 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).

(b) 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 toastill 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 fibrillse 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 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 15, 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. In 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
are little bridges of protoplasm binding the cells together. For example,
Engelmann describes the muscle of the intestine-- of the fly as composed of
striated cells, sheathed by sarcolemma, excepl where bound together by little
branches of sarcoplasma, which may ad 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 arc not known to have a sarcolemma, and which
are united not only by their broad ends, but by lateral branches. Engelmann
and lately Porter1 and others have concluded that conduction take- 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 medusae (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 resl by means of contact,
or through the mediation of cement-sul stance.

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 deli, ate 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. Kuhne's experiment with the sartorius muscle of the frog,

described on page 45, gives a g 1 proof that the activity of a striated

muscle-fibre is not normally transmitted to its neighbors ; nevertheless,
Kiihne3 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 ' reported that when a frog's muscle was partly dried a -light

1 Porter: Journal of Experimental Medicine, 1897, ii. p. 891 ; American Jon,,,,,! of Physi-
ology, 1899, ii. i». 127. ' Engelmann : Pfttger'i Arch,,; is:i. Bd iv.
' * Kiihne: Untersnchin>,en am der phy iohgUchen hutttvte in Heidelberg, 1880, Bd. 3, S. 1.
* Biedermann: Berichte <!>;• ]\'i,„a- Abulrmic, 1888, Bd. 97, Al.tli. 3, B. 1 !•">.
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 hem 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

cases, at least, the axone terminates in an

end-organ which is physiologically dif-

j ferent from the rest of the cell, and this

end-organ is the exciting agent. The

relation of the protoplasm of the end-

organ to the protoplasm of the cell which

it stimulates, whether one of continuity

or contiguity, is not certain, but most

histological and physiological observa-

Fig. 31. Nerve-termination in voluntary tions are distinctly in favor of the latter
muscle of the rabbit, stained in methylen-blue

vitam), fixed, sectioned, and counter- View,
stained in alum carmin. A, surface view; B, The physiology of the end-organs of

longitudinal section through nerve tormina- .. .. '

tion and muscle-fibre; C, cross-section; S, motor axones distributed to striated

sarcolemma; n. I., neurilemma. (From Text- muscles is best known
book oj Histology, Bohm and Davidoff, revised
by '■ < Huber W. B. Saunders, Philadel- Fig. 31 sllOWS a surface View and

pina, 1900). a longitU(ljnal an(] 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 sarcoplasroa is heaped together at this place,
making a little mound, and the axis-cylinder, after dividing into a number
of fine terminal twigs, vmh 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-
tain- it- 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 thai it is the sarco-
plasma rather than the fibrillary elements of the muscle that conies 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: Arehiv fur Anatomic una Physiologie, 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 fibrillae 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 eases 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 ; Aveak afferent impulses may. by summation, lead to a
strong efferent discharge, and continuous afferent stimulation may awaken
rhythmic efferent discharges.

In short, phvsiological 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 pari 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 i> 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 onlyal one extremity of the fibre.

Paul Bert made a well-known experiment, in which he tried t<> reverse a
sensorv 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, alter the tail was cut off at its base, it was still capa-
ble of giving sensations of pain. The experiment tailed 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 sensorv nerve caused movements in the pari supplied by tie'
motor nerve. Most of these experiments have given doubtful results, but
lately Budgett and Green ' have succeeded where others have failed, ami
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 wenl
up the old sensorv 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 tin; part connected with the peripheral -tump 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 tin; tongue, was excised. When the peripheral end of the pneumogastric
was excited the muscles of the tongue were seen t<> contract. Mechanical as
well as electrical stimuli were effective, and there would seem to be no escape
from the conclusion thai tin- sensory fibres of the pneumogastric had con-
ducted the impulse ecntripctally as far as the ganglion, and t hen centrifugally
down to the muscle of the tongue.

1 Budgett and (ircen : American Journal of Physiology, 1S99, iii. p. 1 15.

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-
tion-. 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 alter reaching it. Such branchings of fibres occur in muscle, and Kuehne1
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. 82.— Kuehne's ° f . . , .

preparation of sarto- as to supply every plate. In this case mechanical stimu-

fho* ,1"uble lation of the central end of one of the cut 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 oerve was excited, a negative variation current could be
detected in the anterior a- 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 Archir fur Anatomie und Physiologie, 1859, S. 595.

2 IbuL, is? 7, S 262.

3 Tkierische Electncitdt, 1849, Bd. ii. S. 587.

GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 87

Rate of Conduction. — The activity of the conduction process varies
greatly in differenl tissues. The nerves of* warm-blooded animals conducl more
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 ciliata."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 ]\'ar<- 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 stalls 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 docs not begin al
the further extremity of the mus-
cle until somewhal later. The re-
cord given in Figure 33 was ob-
tained in a similar experiment, but
one in which the contraction of
the muscle was registered by the

pince myographique and recording tambour of Many (see Fig. 34)
1 Biedermann: Eleklrophysiologie, 1895, Bd. i. S. 124.

Fig. 3:i. — Rate of conduction of the contraction pro-
cess along ■■' muscle, as Bhown by the difference In the
tlmi of thickening of the t\\<> extremities. The tuning*
f«.rk waves record ,\„ second (after Marey),

88 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY.

Bernstein1 measured the rate at which the irritating process is transmitted
along the muscle by recording the latent period, the time elapsing between the

Fin. 34.— 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 semimembranosus 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 pine— passes along the fibre, we can estimate the
length of the contraction wave, just as we could reckon the length of a train

1 Untersuchunyen iiber die elektrische Errerjung von Muskeln unci 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 al 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 (Engelmann).

Muscle of the heart-ventricle of the frog 0.1 I Waller).

Contractile substance of medusa; 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 (Rollet).

Sterno-mastoid of the dog 3. -6 (Bernstein and SteinerV

Semimembranosus (white muscle) of the rabbit . . . 5.4-11.4 (Rollet).

Human muscle 10. -13 (Hermann).

(b) 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, 'flic 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 Pouillefs 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 shork. 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 Anatomie und Physioloyu; 1850, S. 71-27U; 1852, S. 199.

90

AN AMERICAN TENT-BOOK OF PHYSIOLOGY.

excited in two succeeding experiments at two points, at a known distance apart,
and tin difference in the time records obtained was the time required for the
transmission of the nerve-impulse through this distance.

Fig. 35. — Method of estimating rate of conduction in motor nerve of frog, as vised by Helmholtz. The
horizontal bara?> is supported on an axis in such a manner that when the contact is made at a it is
broken at b, therefore at the same instant a current is made in the galvanometer circuit g and opened in
tin- primary circuit of the induction apparatus /). 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 1)4 meters per second, a
rate which is considerably quicker than that of our fastest express trains,
but ;i 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 -low 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 (esophagus, 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 rati' in non-medullated nerves of the
Cephalopodia is 3.5-5.5 meters per second | Boruttau).

Contrary to the view frequently expressed (Pfliiger1 and others), all parts
of the nerve have the same rate of conduction.2

Rate in Sensory Ni roes. — We have no definite knowledge of the rate of
conduction in sensory uerves. The attempt lias 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 ' 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 contracl at the same time ;
but there are certain conditions under which the transmission of the exciting
influence is very much slowed, <>r 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: Uhtersuchungen itber die Physiologic da ElectrotoTvua, Berlin, 1859, S. 465.
JR. du Rois-Reymon.l : CentralblcUtfur Physiologic, 1899, Bd. xiii. S. 513.
3()ehl : Archives italiennes de Biologic, 189"), xxi. .'>, p. 401.

*Dollc-y and ('attell : Psychological Review, New York and London, 1 894, i. p. 159.
'Schelske: Archiv fur Anatomic uml Plu/sinlniiir, lStil, S. 151 .

92 AN AMERICAN TENT-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. 1 fa 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 maybe 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
;i~ 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

1 Schenck : Pfiuger's Archiv, 1896, Bd. 64, S. 179.
'Oehl: Archives italiennes de Bioloyie, 1895, xiiv. 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 lias readied 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 0° C. The inhibitory fibres of the vagus nerve of the dog show dimin-
ished activity at 3° C, and become wholly inactive at 0° C. ; the inhibitory
fibres of the vagus of the rabbit become inactive at 15° ( '.

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 <>n 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 <ni<l 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 effecf upon the other. There are important
exceptions to the rule, however. The direct application of alcohol, ether,
('{c., to the nerve may destroy the conductivity without greatly lessening the
irritability, while carbon dioxide1 or cocain will destroy the irritability very
much sooner than the conductivity. Such observations suggest that con-

1 Stewart: Journal of Physiology, L891, vol. zii. No. '■<>. p. 22.

'Howell, Budgett, and Leonard i Journal <;/' Physiology, vol. xvi. Nob. •" ami t, L894.
3 Weir Mitchell : Injuries of Nerves and their Consequences, Philadelphia, 1S72, p. 59.
* Griinhagen : PflUger'a Archiv, 1872, vi. 8. 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. Yon Bezold1 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 the 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 -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 alter 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 Unterauehungen iiber die elektrische Erregung der Nerval und Muskeln, Leipzig, 1861.
'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 Btrong currents
explain the failure of the closing of the ascending current and opening of the
descending current t<> irritate the muscle (see Pfluger's law, p. 50). In the

former case the anode regit I" 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 <>J 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 lie 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 "cramps;" or the sending of
such a current through a sensory nerve may, during the (low 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.

(/) Efect of Conduction and Fatigue of Nerves. — Many experiments have
been made in the hope of detecting sonic 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 a- 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. -

1Funke: Archivfur AruUomieund l'liiisi'>l<><)i<\ 1 s ">'.>, S. 835. Ranker Cnitnilblutt fur medicin-
ische Wixseiisrlmfl, ISfiS and lNfif).

2 Heidenliain : Studien aus dem phyaiologuchen Tnalitui ra Bresiau, ix. S. 248 ; Centralblatt fur

Medicin, 1868, S. 833. Ti.ijersiedt : "Studien iilier iiieclianiselie Nervenreizuni;," Ada Societatia
Scientiamm 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 2Q100 of a
degree during excitation."

.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 '/ or the part of the nerve
near the muscle was temporarily deprived of its conducting power by an
anaesthetic, 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.* Other experimenters have examined the vagus nerve, to see if after
long-continued stimulation it was still capable of inhibiting the heart, 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 arc, like all protoplasmic structures, continually the seat of metabolic

1 Helmholtz: Archiv fiir Analomie und Physiologie, 1848, S. 158. Heidenhain : op. cit.
Rolleston: Journal of Physiology, 1890, vol. xi. p. 208. Stewart: ibid., 1891, vol. xii. p. 424.

2 Bernstein : Pfiiiger's Archiv, 1877, xv. S. 289. Wedenski : Centralblatt fur die medic inischen
Wwaenschqften, 1884.

•, Bowditch : Journal of Physiology, 1885, vi. p. 133.

* Wedenski : loc. cit. Maschek : Sittungsberichte der Wiener Academic, 1887, Bd. xcv. Abthl. 3.

5 Szana: Archiv fur Anatomic und Physiologie, 1891, 8. 315.

6 Howell, Budgett, and Leonard: Journal of Physiology, 1894, xvi. p. 312.

7 Lambert: Comples-rendus dc la Societe de Biologie, 1894, p. 511.

GENERAL PHYSIOLOGY OF MUSCLE AND NEHVE. 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
form of a wave; finally, that the molecules of the nerve-substance underwent
a form of physical vibration analogous to that assumed lor 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 t<. react to different
kinds of stimuli and to transmit the state of excitation to the dendrite. The
exciting organs at the extremities of the axonee of different .'lasses 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 tin-
body of a ganglion-cell of the posterior Bpinal 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, pupil lo-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-gangl ionic
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 affecl 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 ; it' 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 movement
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 the 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 udauce 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 !><• 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. Langendorfl': Physiologuehe Graphik, Kranz 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-
faee at nst; b, surface
moving slowly; c, sur-
face moving more rapidly ;
'/, surface moving even
faster.

GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 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 Interrup-
tions by means of an electro-magnet: a, battery; b, 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 fort vibrating; g, chronograph.
The current from the battery a, passes to the fork b, 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 Boft-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 (b, 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 ; t>, beginning of contraction ; <\ height of contraction ; '/, 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, J-,, second, or , fo second, as the case may be.

Myogram of Simple Muscle-contraction. — The rate of the movement of the
muscle during every pari of its contraction can be n'adilv determined by com-
paring the record it lias 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. Boon became very rapid, but toward its close was again
gradual.; the relaxation began almost immediately, and took a similar course,

102 AX 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 jfo second elapsed between the point b, 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 y^-g- 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 y^ second, that the rise of the curve occupies
about jfo, and the fall about j^-g- second, the total time occupying about -^
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. Tigerstedt2 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 s has found that by the aid of the latter instru-

1 Archiv fur Anatcmiie und Physiologie, 1850, S. 308.

7 Ibid., 1885, Suppl. Bd., S. 111.

6 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 Mould 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 cau 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 '
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.

Bernstein2 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 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 (if a very delicate elastic sheath, the sarcoleimna, 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 Untersuchungen iiber den Erregungsvorgang im Nen>en und Muxkelsystem, 1871.

104

AJV 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, Q, 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,
Mere 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
ittle compartments, each of which had
exactly the same construction. Kuehne

fig. 39.-Sehema of histological structure of chancecl 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

contracting fibre by ordinary light; 1>, contract- i -i ,1 , • , , 1 1 ,

big fibre by polarized light ' observed 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 bulk of the substance in the
light bands is singly refractive, isotropic (Z>, Fig. 39). The anisotropic sub-
stance is found to stain with hematoxylin, 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 striations are looked upon with great interest
as indicating that the particles of anisotropic material are arranged in long
chains as incomplete fibrillar 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 fibrillar of like structure,
and so placed as to give the effect of transverse disks (Z, n, (J, 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 osmic
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 (Q, Fig. 39, ( ') 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, D), 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 sareoplasm 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 Engelmann ' 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 a- a whole is the result of
a change in the minute elements of the fibrillffi, and that the various condi-
tions which influence the activity of the process of contraction musl 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 i- 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 he altered by
pressure, but the change is one of form and not of bulk ; since muscles arc

largely made up of fluid, their compressibility is i asiderable. The elasticity

of muscles is slight but quite perfect, by which is meant that a muscle yields
readily to a Btretching force, hut 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 fiber den Ursprung </<•/■ Muskdkrafi, 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 tlie weights.

conditions. Under these circumstances it is found that if a number of equal
weights be added t<> 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 eifect 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. Mosso3 suggests the following experiment on man :

1 Blix : Skandinavisches ArchivJ'ur Physiologie, 1893, iv. S. 392.

2 Philosophical Transactions, 1884, p. 197.

s Mosso : Archives ilaliennes de Biologie, 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 aeeompanying 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.— («) The Character of the Mused-. — 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 AX AMERICAN TEXT-BOOK OF PHYSIOLOGY.

kinds of muscle-tissue iu 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

Pectoralia major
Omuli yoid //\^\~~~~~'j racUis ^^~~-^^

J • • • • I I

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 & second, and
the longer marks seconds (after Cash).2

same animal exhibit different degrees of activity. Cash1 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 j 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 arc 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 w^ell as of con-

1 Arrhivfiir Anatomie und Physiologie, 1880, Suppl. Bd., S. 147. ' 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 parte, 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

mo

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 eases 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 ' 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 varving conditions. The isotropic substance, the Barcoplasma, 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 sonic
kind of recording mechanism. Every mechanical device for recording the
movements of the muscle has inertia, and, if given motion, acquires momen-
1 Bottazzi : Journalof Physiology, 1897, xxi. p, 1.

110

AN AMERICAN TEXT-BOOK OF PHYSIOLOGY.

turn. Roth 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) .1 Iteration in the shape of the ascend in;/ 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 bci 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
long 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 affair, 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 lust
illustration of this is the heart, which continues to beat at a regular rate
throughout the life of the individual. Tiege] 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 tor a considerable time the
ability to recover from the effects of excitation. It is evident that when a

1 Kaiser: Zeitechrift far Biologic, 1896, xxxiii. S. 157, 360.

2 Fick, v. K rifs, and others

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AN AMERICAN TENT-BOOK OE PHYSIOLOGY.

muscle is excited repeatedly, a certain interval of rest must be permitted
between the succeeding excitation- it* 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 eapaeity for work.

(1) Eti'cft of Frequent F.reitations on the Height of Separate Muscular
( 'ontracUons. — 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 lesseu 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 Bowditch,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 moviug surface, the summits of the succes-
sive contractions were seeu to rise oue above the other like a flight of steps.
This peculiar phenomenon received the name of the " staircase contractions "
(see Fig. 1 1).

A

■*m

1 1 1 j i

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 Bowditch). 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 Rossbach,3 on the muscles of warm-blooded
animals, and by Romanes4 on the contractile tissues of Medusae.

The following .-erics 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 w hich preceded it. A decline in the height of the first three or
tour contraction- is the rule when a normal resting muscle is called into action

1 BerichU der koniglichen sach&ischen Gesettsehqft der Wissenschaft, 1871.

9 Pfliiger>8' Archie, 1882, 1884, Bd. xiii.. xv.

* Romanes: Jelly-fish und Star-fish, International Science Series, p. 54.

2 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 uot
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 straight line. In the experiment, parts of the record of which are
reproduced in 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 grains ; the records of
the succeeding contractions were recorded on a slowly moving cylinder. The
experiment consisted of two parts — in the first (i(J contractions, in the second
over 1700 contractions were made; an interval of rest of five minutes was
permitted between the two series.

In the first part of the experiment there was a decline in the height of the
contractions tor tin; first five contractions, the "introductory contractions,"
then during the next sixty-one contractions a gradual rise in the height of the

Vol. II.— 8

114

AN AMERICAN TENT-BOOK OE 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

6fi contractions.

Rest. 1-30

400

500

G00

700

S00

900

1000

1100

1200

1300

1100

1500

1(300

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 hy an isolated gastrocnemius muscle of a frog. The con-
tractions were isotonic, the weight heing 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 rest 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 tirst twenty contractions of the 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 1700 contractions given 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

(1ENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 115

which has its normal blood-supply is rhythmically excited to a large Dumber
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 c lition

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

Fir,. 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; ;:, the 200th ; I, the 300th. The muscle was excited
automatically by an arrangement carried by the drum, and the excitation whs 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 Intent period, in the rate
at which the muscle shortens, and in the rate at which the muscle relaxes.
The etfect 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 100th 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
L50th 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 l 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 b, 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 1!'.
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 tin' time. In the experiment recorded in Figure 50 the
development of the condition of contracture was more marked than in the
1 Lee : American Journal of I'hytiology, 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-
in"- 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 (sec p. 130).

The record in Figure 51 shows many points of interest : a to b, a rapidly

Fig. 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 o and the beginning of section b, and 200 contractions between the end of section band tin
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 ; b 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 NERVE. 1 1 5>

a gradual fall of the curve of incomplete tetanus, due to the effect of fatigue
on the contracture; e, complete tetanus, but continued gradual declme 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 b 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 indirect ly
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 contractu re to raise the height

Fig. 51.— Developmenl anil fatigue of contracture, Experiment <>n a gastrocnemius muscle of a frog.
The weight was 10 grams. U 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

AX AMERICAN TEXT- BO OK 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. 121

tract much higher than it will as a result of a single excitation. As we have
seen, repeated excitations cause, in the case <»('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. — Helmholtz 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 h< aght
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 Helm-
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 i 'i' Hi. effect Of double excitations upon the gracilis muscle of n froc, by di Hi-r-
ent intervals of excitation. To obtain ibis 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 Jiir Anatomic und J'hysiologie, 1888. 2 Ibid., S. 213.

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AN AMERICAN TEXT-BOOK OE 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-

Fk;. 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 ; 6, 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 Kries1 and Von
Frey2 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. 115*.
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 Archir Jar Analmnie. und 1'hy.vnlor/ie, 188(5. - Ibid., 1887.

GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 123

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 be 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 56.

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 a
tuning-fork making 100 vibrations per second. The rate of excitation was gradually increased, and
then gradually decreased. The ascending curve, n-b, shows the effect of increasimr, 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 circuil of an induction apparatus and at t he same time short-circuiting the
making shocks. This interrupter was run by an electric motor Which was allowed to Speed lip 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 j this was followed by two eon-
tractions of le.-s height, " introductory contractions ;" then came three contrac-
tions each of which was higher than the preceding, "staircase contractions;"
these were followed by three contraction.-, which, in spite .,1" 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 ,"><! 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 still
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. 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
everv 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.

/;. 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 followiug 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

Fir,. 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 Blowly moving
kymograph drum. The time was recorded in seconds at the bottom of the record, and above this tin-
movement of the interrupter of the induction apparatus was written by an electric signal. The muscle
was excited four times by breaking induction shucks 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 shucks 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 F rev'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 <>f 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 OE PHYSIOLOGY.

muscle, for this divides 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.
( !omplete 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
docs 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 <>/ Tetanus and Single ( 'ontntctions. — The amount that
a muscle is capacle of shortening, when tetanized by maximal excitations, and

1 C. C. Stewart: American Journal of Physiology, 191)0, iii. p. 25.

1 Kronecker and Sterling: Archivfiir Anatomie und Physiologic, 1878, and Journal of Physi-
ology, 1880, vol. i. Von Frey nnd Wiedermann: Bcrichtc der sachsischen Gesellschaft der Wissen-
schuft, 1885. Roth: Pjluga's A rchiv, 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 nol show as great an
increase in the height and strength of contractions when tetanized as do th<
red; the latter, which are very rich in sarcoplasma, have likewise the greatei
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 ten-ion, by isometric
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 abdominalis 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 nin-t
find its explanation in the adjustment of protoplasmic activity in the nerve-
cells of the central nervous system.

10. Continuous Contractions <ni<l 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 : ElektrophysMogie, S. 109.

128 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY.

excitation by prolonged contractions. If a muscle be excited by frequent
induction shocks, oven 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 sonic 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
l>\ lessening the work of the nerve-cell. Richet3 writes that normal con-
tracture is not to be confused with the prolonged relaxation of fatigued and
dving 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 : PfHiget^a Archiv, 1876, xiii. S. 71-84.

J Mosso: Archives itcdiennea de Biolorjie, 1890, xiii. pp. 165-179.

s Richet: Dictionnairc de Physinlogie, 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 to
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 betweeu 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 maybe 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 thai 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 interims)
muscles of the rabbit, have two different rates of contraction.1 This expla-
nation is not very satisfactory, because it has been (bund that both the pale
and the red muscles of the rabbit can give typical veratria contracture curves.1
Moreover, both heart-muscle and non-striated muscles show independent tonus
and contraction movements though containing only one kind of muscle-fibre.3

1 Grutzncr: Pfluget>6 Archiv, 1887, Bd. 41, S. 256.

2 ( 'iirvallo and Weiss: Journal </< I'lii/siolonif ft I'atholoyie ijenerale, 1899, t. i. p. 1. Bu-
cannan : Journal of Physiology, 1899, vol. xxv. p. 145.

3 Jiottazzi : Journal of Physiology, 1897, vol. \xi. 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 sarcoplasina 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 contractu re.

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 tin' fir.-t experiment it was a true contraction process; in the second it

1 Kick and Boehtn : Verhand. der physikal.-med. Oesellschaft in Wiirzburg, 1872, Bd. iii., N.
]•'., S. 198.

2 Locke: Journal of Experimental Medicine, 1896, vol. i. p. 630.
BruntOE 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 and
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 a 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 Ritter'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 Frey3 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

2 Arrhirjiir . I iiatninif iiihI I'ln/siolotjie, 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 eases 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 tonus 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) Norma/ Physiological Contraction*. — 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 a> 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 Physiologie, 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 to 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. Loven1 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
Schafer2 excited the brain cortex and motor tracts in the corona radiata and the
spinal cord of mammals by induct ion 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 Cenlralblatt for die medicinischen fVissenschaften, 1881.

2 Journal of Physiology, 1886, vii. p. 96.

134 AN AMERICAN TEXT- HOOK 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. Tunstall1 found by the use
of tambours, in experiments on voluntary contract ions of men, a rate of 8 to 13
per second, with an average of 10. Griffiths2 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 witli 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's3 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 f>8.

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

1 Journal of Physiology, 1886, vii. p. 114. 2 Journal of Physiology, 1888, ix p. 39.

5 Pjluger\ Archiv, 1887, Bd. 41, S. 277.

GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 135

been ascertained by experiments upon cold-blooded animals, such as the
frog, can be accepted with but slight modifications for the muscles of 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 TEX1-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 he 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

c

FlG. f>0. — 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.* 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 0° C. the height increases, while

'Lombard: Archives italiennes de Biologie, xiii. p. 1 ; or American Journal of Psychology,
1890, p. 1 ; Journal of Physiology, 1892, p. 1 ; 1893, p. 97.
'Waller: Brain, 1891, p. 179.

SC. C. Stewart- American Journal of Physiology, 1900, iii. p. 25.
'•lad und Heymans: Arehivfur Anatomic wnd Physiologic, 1890, S. 73.

GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 137

below 0° 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 (sec p. 130).

There arc a number of drugs which have an action on muscle-tissue simi-
lar to that of veratria — e.g., cornutinc2 produces a similar effect on striated
muscles; digitaline increases the tonus of hearl muscle and of the smooth

'Bucannan: 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 etfect 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 elec-
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. Zuntz4
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

1 AU'l : Zeit8chrift fur phyxiologische Chemie, 1899, Bd. xxviii. S. 354.
'-'< Hivcr and Schafer: Journal of Physiology, 1895, xviii. pp. 230-276.

3 Fick : Pfliiger'a Archiv, 1878, xvi. S. 85.

4 Tbid., 1897. Bd. Ixviii., 8. 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 muscle. 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) T7ie method of contraction and the mechanical conditio)is under winch
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 conies 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 Physiologie, 1879, Bd. i. S. 64.

140 AN 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 iu the case of the heart muscle.
A l-ist' 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 contracting
muscle can just support at, but not raise above, its normal resting length
This weight will be a measure of the absolute muscular force.

Example.

Load Height of lift Work

(grams). (millimeters). (grammillimeters).

0 13 0

30 11 330

60 9 540

90 7 630

120 5 600

150 3 450

180 2 '. . 360

210 0 0

1 Fick : Mechanische A rbeit, etc., S. 132. Santesson : Skandinavisches Archiv fiir Physiologic,
1889, i. S. 56.

GENERAL PHYSIOLOGY OE 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 lias 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 Weiss2 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 2£ kilos is required to rupture it.
The increased resistance can be best explained on the idea that, as Prliiger
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 Pfluger's Archiv, 1898, Bd. 73, S. 429.

1 Carvallo and Weiss: Comptes rendus Societe de Biologic, 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 oreater 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-produceion 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.

TJie 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 : Physiology of Sport, translated from the German, second edition, London.
1892.

J Fick : Pfliiger'a Archiv, 1878, xvi. S. 89.

3 Michanische Leistung, Warmeentwicklung und Stoffumsatz bei der Muskdthatigkeit, Leipzig,
1864.

4 Mnothermische Untersuchungen aus den physiologischen Laboratorium zu Zurich and 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 efl'ect of this multitude of affer-
ent stimuli, in spite of their feebleness, is to cause the motor cells of the curd
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 may lie maintained
in the muscles by intermittent but frequent reflex excitations, a condition which
has been called chemical tonus.2 Thai the chemical activity of muscles is kept

1 Rrondgeest: Archiv fur Anatomie und Physiologie, 1860, S. 703; Hermann, Ibid., L861,
S. 350.

2 Koehrig und Zuntz: Pfliiger'a Archie, 1871, lid. iv ; Piliiger: Pjluger'a Archie, 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 Zimtz: Pjlii'in-'s Archiv, 1876, xii. 522; Colasanti, Ibid., 1878, xvi. S. 57.
3 Biedermann : Electrophysiologic, Jena, 1895, Bd. ii.; translation by F. A. Wei by, 1898;
Waller: Lectures on Animal Electricity, London, 1897.

Fig. 02.— Schema of galvanometer: n, .s, north
and south poles >>f 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 ;

, mirror ;/, fibre supporting the magnets; e,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, i , imn
polarizablc electrodes applied to the longitudinal
surface and cross section of a muscle.

GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 145

of a man may be interpreted from the movements of his shadow, but they
are very different phenomena; the activity of the 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. Got eh
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 Giilriiitu/iu fer — 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 he 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 earths 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 I lie effect of I lie earth's magnetism is partly compensated.
Still another magnet may he 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 hears a slightly concave mirror, from
which a beam of lighl can lie reflected on a scale. Or a scale may be placed so that iis
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.
and by earth currents which may he 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. lxv. p. I 12.
Vol. ir.— in

146

AN AMERICAN TEXT-BOOK OF PHYSIOLOGY.

because they may lead to uncontrollable movements of the magnet, and a consequent shift-
ing of the 0 point. There arc other forms of instruments, such as the Deprez-d' Arson val
mirror galvanometer,1 which arc not affected by Buch influences.

The galvanometer i- 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
tli*' 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 l<> 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 (<■). 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 he 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 come 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 (</). 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. , . , re <. i, „ •„ „ r„„„ *^„

1 ' 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 (d ), 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 he 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 he recorded photographically by letting the
beam of light reflected from the mirror fall through a horizontal slit on a photographic
plate. 1 1' the plate he 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 he 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 : Pjliirjer's Archiv, 1898, Bd. lxxiii. 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 Best. — 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 oif the two ends at right an-
gles to the long axis, the piece Mill
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.

Samjloff2 says that the electro-motive force of the currents ordinarily

1 Unterwchungen fiber thierische Etektricitat, Berlin, 1849.
- ^anijlotl': Pfluger'a Archiv, 189(J, 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 h, d), and two
artificial cross sections (a, 6 and c, d). The position
of the equator is shown by line e. The unbroken
lines connect points of different potential, and tin-
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 ^V AMERICAN TEXT-BOOK OF PHYSIOLOGY.

obtainable from muscle, 0.06-0.08 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 lie 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.

A It In ugh 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-Reymond : OesammdU Abhandlungen, 1877, Bd. ii. S. 319.
7 Hermann : Handbuch der Physiologie, 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 rest, although a current is found as soon as any part is hurt, the
part becoming immediately negative in relation to other uninjured parts. In
experiments 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.

HeringMias 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 (X025, 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.'

Hering found that an irritable nerve, like a muscle, could be excited by

1Hering: Lotos, 1888, Bd. ix.; translation in Brain, 1897.
» Philosophical Transactions, L891, B., vol. L82, pp. 267 526.
8 Knehne and Steincr : Heidelberger Untersuchungen, 1880, iii. S. 14U-169.

150

AN AMERICA X 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 hack 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 ueedle 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
A, when the nerve of A is stimulated,
not only the muscle of .1 but the
muscle of B will twitch (see Fig. 65).
If nerve A be excited by many
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 B, for the
tetanus of both muscles stops if nerve A lie 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 B could
only have been excited to tetanus by rhythmic excitations from A. Each of
1 Burdon-Sanderson : Journal of Physiology, 1898, xxiii. p. 323.

Fig. 65. — Secondary tetanus

GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 151

the rapidly following excitations applied to .1 was the cause of a separate eon-
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 Sanderson1 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 Q6 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 Du 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
diminutional effect." The continuous uegative change i- possibly attributable
to the presence of a continuous contraction process, perhaps the contracture
which avc observed in studying the tetanus curve (see Fig. 52). This " diminu-

Fig. 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 cul 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 tin- curve indicates an electrical
change of opposite direction t<> thai caused by the injury. The diminution of the current of injury,
which was less than in some oilier experiments, was onus volt. The time record at the bottom of the
curve was obtained from a tuning fork making 500 double vibrations perse id afb r 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 contract ing and the injured,
imperfectly contracting muscle-substance. When all parts of the muscle are
normal and contracting to an equal amount, the electrical force- would be
everywhere of the same nature, balance one another, and give no external
evidence. Although the diminutional 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 Owrr&rd of Actum. — 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 theru-
selves. If an electrode be applied near the place where the uninjured
muscle is stimulated. .1, and another at some distant point, B, 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.

lt^ 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 >ingle 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 start.-, is negative to
the apex, and in the second phase the apex is negative to the base. In ease
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 triphasic
change is observed, which need not be discussed here. Waller1 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 lie seen to give one, or, if the heart is uninjured,
sometimes two, contractions avevy time the heart beats.

Bernstein2 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 Anatomu und Physiologie, 1890; physiol. Abtheil., S. 187.

2 Untersuch a 1,,/,-n iiber den Erregungsvorgang im Nerven- mul Mu.<kel-system, 1871.

3 Bandbuch der Physiologie, 1879, i. 1, S. 224.

GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 153

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-Reymoud
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. c, 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,1 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
Lee3 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. 6ti), 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
isotonic.

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 fads 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 latin* distant parts showing the negative change. If the nerve

1 Jensen : Pfluger'a Archiv, L899, Bd. lx.wii. S. 107.
1 Journal of Physiology, ls7'.», vol. ii. p. 396.

8 Archir fur Anatomic mid Plti/.<it>l<>(fie, 1SS7, S. 'J04.

154 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY.

be injured, and the normal surface l>c compared with the dying or dead cross
section, the second phase is absent. If the nerve be frequently excited, each
excitation awakens a separate currenl of action.

Although nerves are excited most readily by electric currents, negative
variations of the demarcation current maybe called out by various chemicals
— e.g., salt or glycerin ' — 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 uerve 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 I Iorslev ; 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 Rwd ' 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. Stcinach5 observed currents of action to be
caused by mechanical pressure on the skin of the frog. If the pressure was
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 inedullated nerves of the
frog a length of IS 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 un<l Steiner : I rntersuchungen mix der physiologischen Laboratorium i* Heidelberg, 1880,
Bd. iii. S. 1 19.

2 Hering: Lotos, Neue Folge, 1888, Bd. 9, S. 3-",.

3 Physiological Transactions, 1891, vol. 182, pp. 267-526.

* Macdonaltl and Keed : Journal of Physiology, 1898, vol. xxiii. p. 100.
5Steinach: Pftuger,a Archiv, 1896, Bd. lxiii. 8. 495.

GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 155

and in the same nerve under different conditions. In the medullated nerves
of the frog it lasts 0.008 to 0.024 second ;l in the non-medullated nerves 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 same
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 phvsical 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. Waller3 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. Alter 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 : Eleetrophysiology, translation by F. A. Welby, 1898, vol. ii. p. 260.

2 Gartner: Pfliiger's Arehiv, 1899, Bd. lxxvii. S. 198.

3 Waller: Lectures on Animal Electricity, 1897, Lecture 11.

156 AN AMERICAN TEXT-BOOK OE 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 efl'eet, paralyzes
the nerve ; under favorable conditions the nerve recovers in a few hours.
The action of C02 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 0O2 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 C02 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. Waller2 and Green3 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 <>n .\>iim<il Electricity, 1897, p. 59.

2 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 Hering2 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. Raise 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
phvsiological 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 arc brought to the
part of the nerve from which the branches to the thigh muscles arc 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, rol. Ixv. p. I H.

* 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 anaesthetics, 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 chcmico-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 Pjliiger's Archiv, 1872, Bd. v.. vi., vii.; also Handbuch der Phyriologie, 1879, Bd. ii. S. 17.

2 Pflug&>s Archiv, L894-1897, Bd. Iviii., lix., lxiii., lxv., lxvi., Ixix.
• Lectures on Animal Electricity, London, 1897.

4 Boruttau: Pfliiger'a Archiv, 1894, lid. Iviii. S. 64.

5 Biedermann : I^lectrophysioloyy, translated by F. A. VVelby, 1898, vol. ii. p. 303.

GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 159

made. Purely physiological phenomena are such as can be 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. l When it is
fresh the reaction is slightly alkaline.

It contains about 75 parts of water and 25 parts of solids; nearly '_'<>
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 make- 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 bya loss of translucency, the muscle becoming opaque, and
by a gradual contraction, accompanied by a development of heal 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.
M'arvallo and Weiss: Journal dc I'lu/sinlixjir, ls'.i'.i, i. p. 1204.

160 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY.

Conditions which Influence the Development of Rigor. — Ordinarily on the
deatli 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 oil" 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 it 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 arc 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
Qol impossible. Muscle- 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. 101

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," if expressed from the frozen muscle, coagulates on
being wanned. The coagulation is the result of a chemical change, by which
two proteids of the plasma, paramyosinogen and my osinogen, 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.3
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 ami
the production of carbon dioxide, and the fact that the dying and presumably
coagulating muscle is, like normal contracting muscle, electrically negative

1 Brown-Sequard : Archives de Physwlogie, 1889, p. 675.
'Hermann: Handbuch der Phyaiologie, 1879, Bd. i. ■v;. 1 16.
Vol. II. ll

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 still!
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, lias 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 0° 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 : l'jlihjrr s Archir, 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 Kuehne1 in the study of the muscles of frogs,
and was later employed with slight modifications by Halliburton2 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 salt-, 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 fibt r das Protoplasma, Leipzig, 1864.

* Journal of Physiology, 1887, vol. viii. pp. 1 33 202.

164 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY.

as Fiirth says, to myogen-fibrin and niyosin-fibrin ' 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 different
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 aeid, 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's2 observation that the amount
of glycogen is not lessened in rigor ; besides, the muscles of starving animals
become aeid when entering into l'igor, although, as Bernard found, they con-
tain no glycogen. Bohni 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, eaffein, 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
Latimer3 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 different 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, then' will be a heat precipitation of the various proteids
of the muscle, which will lead to a still further shortening, the contraction

1 Faith: Archiv fur experimenteUe Paihologie umd Pharmakologie, 1895, xxvi. 231; and 1896,
.xxxvii. 389.

Pfluger*8 Archiv, 1880, Bd. xxiii. 8. 44.
8 Latimer: American Journal of Physiology, 1899, ii. p. 29.

GENERAL PHYSIOLOGY OF MUSCLE AND NEBVE. 165

of rigor caloris. The heated muscle may, therefore, be the .seat of three
different 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. c, 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. Latimer2
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.1 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 <>f
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.8 These
facts are brought out in experiments in which the temperature of the muscle

Stewart and Sollmann : L899, xxiv. p. 128.

'Latimer: American Journal of Physiology, 1899, ii. p. 20.

■> Philosophical Transactions <>/ the Royal Society <>/ London, 1899, Series B., v.. I. exci. p 363

4 Gotschlich : Journal of Physiology, 1^'.)7, vol. xxi. p. 353.

5 Ward, II. ( '. : Unpublished experiments at tin- University of Michigan.

166 AN AMERICAN TEXT- BO OK OF PHYSIOLOGY.

is more or less rapidly raised, at the same time that the changes in the length
of the muscle arc recorded on a slowly moving surface. Halliburton1 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 Myoglobin 63° C.

muscle-serum ... Myo-albumin 73° C.

I Mvo-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
.an be expressed from the clotted muscle, the muscle-serum, therefore lacks
at least two of the protcid constituents of the normal muscle. The proteids
of the muscle-serum are : myoglobulin, myo-albumin, and mvo-albumose.

The myoglobulin resembles serum-globulin, although precipitated at 63°
C. instead of 78° C. The myo-albumin is apparently identical with serum-
albumin.

To these proteids we must add the pigment haemoglobin. Another pig-
ment, myolncniatin, 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 nuelein, 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 <>r 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

'Halliburton: Physiological Chemistry, p. 414.

'Pfliiger: Pfluger's Archiv, 1899, lid. lxxvii. S. 425.

3 Dunlop, Paton, Stockman, Maccadam: Journal of Physiology, 1897, xxii. p. 67.

GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 167

addition to this we find small amounts of creatinin and of 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 matin
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 osrnic 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 in 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
C02 and H..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 ( 'hauveau 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.-' Although sugar
is considered a source of muscle-energy, the exact way in which it is employed
is doubtful.

1 Arehiv fur Anatomie vmd Physiologic, 1897, 8 1 l'.'.

2 Qmiptt'x rrnrfus rfc la Sorictr (If Biologic, 1886, civ.

168 AX 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 <>.<> 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 i.- 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 he in
pari caused by acid potassium phosphate produced from alkaline phosphates
as a result of the formation of phosphoric acid from lecithin, nuelein, etc.1
Rohmann2 attributes the acidity of worked muscles to monopotassium phos-
phate, and the alkaline reaction of the resting muscle to dipotassium phos-
phate ami sodium bicarbonate.

Inorganic Constituents of Muse/,. — 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
,,. 58).

Gases of Muscle. — Xo 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.
Tin' amount of carbonic acid developed varies greatly with the condition of
the muscle ; for instance, it i- 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.' Oxygen is not retained as free

1 Weyl imil Seither: Zeitschrift fur physiologische Cher/vie, vi. S. ~>~>7.

: Rohmann : Pfluger'a Archiv, 1892, 1. S. - 1, and 1893, lv. 589.

3 American Journal <■/' Physiology, 1900, vol. iii. p. 327.

*Ludwig mid Sczelkow: Stizungsberichte der I:. Akad. Wien, 1862, Bd, xlv. Abthl. 1; and
Ludwig mid Schmidt : Sitzungsberichte '/<•>• mnih.-phys. Clasxe d. k. Sachs. GeseUschafi <!<>■ Wissen-
schaft, 1868, Bd. xx.; Regnault and Reisel : Annates d>- Ghimie et de Physique, 1849, 3tne s6r.,
xxvi.; Pfliiger: Pfliiger's Archiv, 1>7"_\ vi. : and others.

GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 169

oxygen, but is stored in some combination more stable than oxyhemoglobin.
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 Tree 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 Riehct).

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.

I.". " 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 arc 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 nerve-
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 Bub-
stanee, and about a quarter complex fatty bodies, neiiro-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 constituent-: Proteids, 36.8; lecithin. 32.57; cholesterin
and fat, 12.22; cerebrins, 11.30; neurokeratin, .'1.07 ; other substances, 1.0.-
The nerve-fibre has a delicate sheath, the neurilemma, t he exact constitution

1 Zuntz: Pfluger'a Arehiv, L897, Bd. Ixx. 8. 346.

'J. chevalier: Zeitachrift fur phy&iologische Chemie, x. S. 97.

170 AN AMERICA X 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) neurokera-
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" 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 docs in shape to the frame-
work of the body, its branches extend to all parts. These branches form
pathwavs 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 fad of the
greatesl 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. < >wing
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 system.- 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 aervous system which can he studied without an appeal to con-
sciousness. A moment's consideration shows, however, that in the physiology
of the brain 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
i- 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 understanding 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
• •ells, ami (•"> i 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 pari is that of the pathway of any imj)ii/.<,
through tin- 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 resl and activity are most readily de-
scribed.

CENTRAL NERVOUS SYSTEM.

173

PART I.— PHYSIOLOGY OF THE NERVE-CELL.

A. Anatomical Characteristics of the Nerve-cell.

Form of Nerve-cells. — Morphologically the mature nerve-eel 1 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 short 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

'

,

/

^>

o

Fig. 67. — A group of human nerve-cells, all drawn to the same scale, from pioparations 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 ; c, small
nerve-cell from tip of dorsal horn, thoracic cord ; d, spinal ganglion-cell, cervical root ; > . three granules
from the granular layer of the cerebellum; /, l'urkinje's cell from the same preparation as « ; <i, small
pyramidal cell from the second layer of the cerebral cortex of the central gyri ; It, 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 t<> 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 axone, 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 <lni<lrites.

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 ft to 65 //,' 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 Jength 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 u = 0.001 of a millimeter.

Fig. 68.— A cell, with a short axone,
Rivinp 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 (Ramon y Cajal).
/' . di mlrites ; 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 axones are distributed to all intermediate points
both in the central 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
rarelv 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, Kaiser1 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 <.f 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 and Function. — 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 oi
these living substances, and hence the larger the amount of stored energy
represented by it. The larger cells are therefore those capable oi setting
free the greater amount of energy. The energy-producing changes are to be
associated with the cell-body rather than with any of the branches. More-
over, the nerve-cdls 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 Funktumen tin- Oangliemellen <l>:<i Halsmarkcs, Haag, 1891.

176

AN AMERICAN TEXT-BOOK OE PHYSIOLOGY.

w

G.

these brandies 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
f>?;~ ?f rr-^. -? ~~^~7 '/** 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;1 chemotaxis ; nu-
tritive attraction or electrical influences).3
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
x lioodiametera (His) .a, germinal ceil; cases to fa determined, and the cell is thus

A', neuroblasts.

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' to be 697 cubic fi in a human fetus (embryo It-
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 Kin- perform, 1874.

2 Davenport: Bulletin of the Museum of Comparative Zoology, Harvard College, Nov., 1895;
Hcrl.st: Biologiaekea Centralblatt, 1894, Bd., xiv.; H. Strasser: Ergebnme der Anatamie u. Ent-
wickclunr/.tr/esrfiirlite, Merkel and Bonnet, 1891, Bd. i. S. 731.

3 Mall: Journal of Morphology, 1893, vol. viii.
' Arrhir fur Anatomie mid Pkysiologie, 1889.

Fig. 69.— Portion of developing medul
lary tube (human) seen in frontal section

CENTRA L NER VO US S ) 'S TKM.

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 medullatiou 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 diiferent in different cases. Furthermore, many nerve-cells
never develop beyond the first stage of immaturity (Fig. 70).

Fig. 70.— A-D, showing the phylogenetic development "f mature nerve-cella in a series of vertebrates ;
a-e, the ontogenetic development of growing cells in a typical mammal; in both ra>.-v only pyramidal
cells from the cerebrum are shown : .1, frog ; /•', lizard ; ''. rut ; />, man ; o, neuroblast withoul dendrites ;
6, commencing dendrites ; r, dendrites further developed ; </. tirst appearance of collateral branches; >,
further development of collaterals ami dendrites (from S. Ram6n y Cajal).

Form of the Axone as a Means of Classification. — Of the various de-
vices used to 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 channel-
through which the impulses pass, and hence assume a special significance
Voi,. i r. — 12

178 AN AMERICAN TEXT- BOOK OF PHYSIOLOGY.

without indicating any fundamental change in the structure of the cell.
Where the cell lias well-developed branches we explain the arrangement by
assuming thai tli<' 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 maybe 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).

grows 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. 1'2 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: Anal. Am. Jeva, 1890, Bel. 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 twoaxones
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 fi for the smallest, up to 7-8 // for the
largest, with an average of 3-4 /x, while at maturity it is 10-15 [i 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 slivjitly
enlarged at the tip (Cajal), suggesting thai 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 no! l" 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 : Neurologisches Centralblalt, 1894, No. 2.
'Barker: The Nervous System, L 899, 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 giveD 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 vet 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 (B\ 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 growingaxone 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 intemodal 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 intemodal 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 x 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 Porter2 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 Flechsig4 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 thai 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 tin1 fibre. Tin1 proc-
ess, therefore, is not a progressive one, but is practically simultaneous.

From the observations of Ambronn and Held'' on rabbits a day or two old,
it appears that the efferent (motor) spinal ami cranial nerves acquire their

1 Langley : Journal of Physiology, 1897, xxii. p. 223.

- Quarterly Journal of Microscopical Science, 1890.

3Vulpius: Archiv fiir Psychiatric u/nd Nervenkrank., 1S92, lid. xxiii.

4 Archiv fur Anatomic unci Physiologic, 1889.

•''Ambronn and Held : Archiv fiir Anatomic and Physiologic, Anatom. A.btb.1., 1896, S. 208.

182

AN AMERICAN TEXT- BOOK OE PHYSIOLOGY.

sheaths before the corresponding afferent (sensory) nerves are medullated
(except in the ease 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 afferenl 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.

Ambrmin and Held1 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,' 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 factsare illustrated
by the observations of Hodge4 on the spinal ganglion-cells of an old man of
ninety-two year- ;is 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 hoc. cit., S. 222. * Vas: Archiv fur mikroskopische Anatomie, 1892.

3 Marinesco : Revue neurologique, October, 1899, No. 20.

1 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 extenf to
which the neurones may influence one another. Both of these conditions 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 18 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 toother 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 die
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 arc. of course, very lew
'(Ireene: American Journal of Plti/siotoi/i/, 1NJIS, vol. i. p. 1 15.

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 N6rve-irnpulse. — In the case of a given nerve-cell the
impulses which we usually consider, pa>> 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 different, 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 tin; spinal ganglion-cells, and thus, although the
1 Gotch and Ilorsley : 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 tho 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. — [f the view is correct, that in passing through the
spinal ganglion the normal impulse traverses the cell-body, then the nerve-

Pro. 75.— A longitudinal section
of the cord to show the branshing
of incoming root-fihres in dorsal
columns. At the left are three (DR)
root-fihres, each of which forms two
principal branches. These give off
at right angles other branches, col-
laterals, 0>l, which terminate in
brushes. O C, central cells, whose
axones give off similar collaterals
(Ramon y Cajal).

'Gad and Joseph : Archivf. Anatomie ». 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 Erregungsleitnng im Iterciche des Spinalganglions,"
Pfiiiger's Archiv, 1899, Bd. 78.

186 AN AMERICAN TEXT-BOOK OE 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 iibre.

The study of this modification brings with it the following suggestion :
It' 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 (V) 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
for 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 Iioyal Society, 1897, lxi. 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 ; b, 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 thai 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.

Birge1 found upon stabbing the spinal cord of a frog with a needle in
1 Birge: Arch./. Anat. u. Physiol., Physiol. Abthl., 1882, S. 471.

188

AN AMERICAN TEXT- BOOK OF PHYSIOLOGY.

the region where ftie 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-
-Mercury.

^Sulphuric acid 10%.

Microscope.

nj

Miriiiri/.

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-
plied directly to the
underlying fibres, the
discharge of the im-
pulses was found 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.

Fig. 78.— Schema illustrating the experiment fur 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,
one non-polarizable electrode is placed over the cut end of the pyramidal
fibres going to the lumbar enlargement ; the other, on the side of the cord.
These lead to the capillary electrometer, in which the column of mercury
moves each time an impulse passes.

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 sonic 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-

D < w*. I Excitation. |

I 1 Sec. |

Excitation.

I I i I I I I I I I Sec- I

Fig. 79. — From a photographic record of the movements of the column of mercury in a capillary
electrometer (Uotch 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 t<>
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-fibre can be distinctly separated from its conductivity,
the latter being present while the former is absent.

It is an old observation thai 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
'Schliferand Horsley: Journal of Physiology, 1885, vol. vii.; Schafer, Ibid.

190 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY.

nerve-fluid, and was held to demonstrate that after the nerve-tubes in the
portion ol* 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 that1 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. c, the discharge does
not follow until a scries 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.

C. 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 Ilnber: Journal of Physiology, 1892, vol. xiii.

1 Ward: Archiv f. Anatomie u. Physiologie, 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 active1 (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.
Macallum3 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, alter death, the sections
from the stimulated are compared with those of the corresponding, but mi-
stimulated, spinal ganglion, a picture like that represented by Fig. 81, is
obtained.4

1 Shimamura: Neurologisches CerUralblalt, 1894, Bd. xiii.

! Lilienfeld and Monti : Zeitschrift fur physiologische Chemie, L892, Bd. xvii.

3 Macallum : British Medical Journal, London, 1898, vol. ii. p. 778.

4 Hodge : Journal of Morphology, 1S92.

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. SI.— Two sections, A and /;, from the first thoracic spinal ganglion of a cat. B is from the gan-
glion which bad been electrically stimulated through its nerve for live hours. A, from the correspond-
ing resting ganglion, The shrinkage of the structures connected with the stimulated cells is the most
marked general change, n, 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. Tn 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 of Stimulated Spinal
Ganglion-cells of Cats. Stimulation for fifteen seconds alternating with
rest for forty -five 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

\ \

^,ms

« — V-

X

/

70

60

\

s

50

l 1

Y

1 1

1

1

\

Hours 1 2£

10 11'

17

23

29

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 was 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 thai 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 OE PHYSIOLOGY

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 nol stimulated re-
mained unchanged, while the stimulated cell went
through the series of changes exhibited in Fig. 83. l

It followed that if these changes were really
significant of normal processes they should he 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 antennary lobes
of hees. These observations therefore support the
conclusions drawn from the appearances following
direct stimulation.

( >ther observers2 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 ells of the rabbit, both Yas and Mann
found, after a short period of stimulation, a pre-
liminary swelling of the cell-body, anil the same
has been noted by Mann in the case of retinal cells

Fig. 83.— Showing the changes \\\ flic dog.
in the form <>f tin- nucleus re- rm .. . - , . . ,

Buitingfromthedirecteiectricai I he application of these observations to changes

Btimuiation of the living sym- jn t]1(l human nervous svstem has thus far been
pathetic nerve-cell oi a frog.

The hour ofobservation is given made only in a casual way, but enough has been

within each outline. The ex- already observed to make 'certain that the results

perimeul lasted six hours and

forty-nine minutes. \ control are applicable.

cell treated during this time in u .,, , lJj.1j.j_i i i 1 Ml

the same manner! except that U Will be noted that the changes above described

it was not stimulated, showed follow variations in the amount of stimulation, the
no changes (Hodgi . ... . . , ,.

nutrient conditions represented by the surrounding

plasma remaining nearly constant. This latter, however, may undergo

1 Hodge: Journal of Morphology, 1892, vol. vii.

- Vas: Archiv fur mikroskopische Anatomie, 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 Nissl2 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 ease
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.

^Schaffer: Ungarisches Arehivfur Medicin, 1893; Pandi: Ibid., 1894; Popoff: \'irrhow's
Archiv, 1S94; Tsehistowitsch : Petersburger medicinische Wochenschrift, 1895.

1 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, 1S99, pp. 105, 100.

196 AN AMERICAN TEXT-BOOK OE PHYSIOLOGY.

Amputation in Man. — When the oerves 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 Uehuchten) ; 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 some 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 Centralbl., 1892 (reviews the literature); Gregoriew : Zeilschrift /.
Heillcunde, 1894, T.d. xv.

'-' Weir Mitchell : Injuries of Nerves, 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 along 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 the 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 through
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 Waller1 that when sepa-
rated from the cell-body of which it wras 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 short time.

Concerning the progress of degenerative changes in the non-mcdiillatcd
fibres information is scanty. Bowditch and Warren3 observed that when
the sciatic nerve of the cat was sectioned, degeneration of the motor and

1 Nouvelle methode anatomi<]m pour ['investigation du Syateme nerveux, Bonn, 1851.
'Howell and Huber: Journal of Physiology, 1S92, vol. xii.
' Journal of Physiology, 1885, vol. vii.

198

AN AMERICAN TEXT-BOOK OF PHYSIOLOGY.

vaso-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 Tuckett1 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-bodv 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-

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 :;.

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 an' 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 spinal cord. Tn 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

'Tuckett: Journal of Physiology, 18%, vol. xix.

2 Kre£in:inn : Arbeiten aua <l<-m TnstUulJiir Anatomic and Physiologic des CentrcUneriiensystems
an der Wiener Universitat, 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 Gudden1
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. 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 Gehuehten). 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 arc brought
together under favorable conditions, the peripheral portion may be regener-
ated. This regeneration occurs only in axones possessing a nucleated (inedul-
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 : '

While the fragmentation and absorption of (he 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 Archivfur PsyckUUrie, 1S70, Bd. ii.

2 Forel : Festschrift zux run Nageli und run Kolliker, Zurich, 1891.

:t Raer, Dawson, and Marshall : Journal of Experimental Medicine, L899, 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 he 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 i]\w 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 don', and obtained regeneration of the nerve
with a return of function, although the initial interval between the two parts
of the nerve was more than three centimeters. The rate of growth from the
central end has been specially studied by Vanlair.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 ()."2"> 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 Iluber), from which we infer that the
afferent 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. Hut 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 doers. Reunion of the crossed

'Worcester: Journal of Experimental Medicine, 1S98, vol. iii. p. 597, describes a case of
apparent regeneration of a fibre-bundle in the mid-brain, and cites the literature.
1 Journal of Morphology, 1895, vol. xi.
s Archives de Physiologie normale et pathologique, 1894.

CENTRAL NERVOUS SYSTEM. 201

nerves occurred and sensation and motion returned to the affected parts of
the limbs.' It is plain thai 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 resull ; 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, Lam-lev ! 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, Langley4 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, L895, vol. xviii.

1 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. j>. 240.

202

AN AMERICAN TEXT-BOOK OE PHYSIOLOGY.

Karvokinetic figures in mature nerve-cells utter 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 lias 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.C

V.R

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 s i<U- only, efferent and central cells on the right side only (von
Lenhossek > : 1>. I'., dorsal ro"t ; V. /.'., ventral root ; 1>. P., direct pyramidal fibres; C. P., crossed pyramidal
films ; C., direct cerebellar tract; .1. L., antero-lateral tract ; D. C, dorsal columns. The various classes
nf 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
nerve- they arc again delivered :it 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 he 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 , : .1 natomi&ch-ejcperimentellen Beilrag aim Sludien der Regeneration des Gewebe dcs
Centralnervensystems. Bcitriige zur palhologischen Anatomie und zur allegemeinen Pathologie, Jena,
1897, xxi. 43-72, 3 pi.

CENTRAL NERVOUS SYSTEM.

203

Figs. 86, 87, the elements which compose it fall into three groups: (1)
The 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
severed dorsal root may cause
contraction of the skeletal mus-
cles.1 There is no good evi-
dence, however, that these fibres
are present in mammals.

(2) The ccui ml neurones;
those the axones of which never
leave the central system, and the function of which is 1<> 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. .1. Morton-Smith : Journ. Physiol, vol. xxi. p. L01.

Fig. s7.— Schema of the distribution of the efferent
fibres of the spinal roots: 4, afferent fibres in the dorsal
root only ; /•:, /■.', efferent fibres In both dorsal and ventral
roots. In the ventral root one group of efferent tiiires
goes to m, the striped muscles; another group to ganglion-
cells, S, forming a single sympathetic ganglion, or to n\
cells located In more than one sympathetic ganglion, but
all connected with one efferent fibre by means of Its col
laterals; P, peripheral plexuses into which the axonea
hi Bome sympathetic cells run.

204

AN AMERICAN TEXT-BOOK OE PHYSIOLOGY.

d.r.

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 ease 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 pari 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 Keen 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 hack 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 info 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. Sec Figs. 88, 89, which show the part of the medullary tube divided

1 Recent work on the spinal ganglia shows that in addition t<> the elements usually deserihed,
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. Anz. Jena, 1896, Bd. xii.

Fig. 88.— Cross-sectioD in the cer-
vical region of a fetal human spinal
cord at tlir sixth week ; < 50 diameters
(V. Kolliker): <\ central canal; o. a.,
groove separating the two laminae ;d. p.,
dorsal lamina ; v, p., ventral lamina, in
which alone arc Located nerve-cells
the axones of which leave the central
system; d. r., dorsal root; v. r., ventral
root.

( EN Til 1 1 /, NE RVOl rS 8 I rS TEM.

•_'< i.j

by His into the ventral and dorsal laminae during development. 'Flic ventral
horns of the gray substance form pari of the ventral laminae.

The cells of the sympathetic system which 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 neurone- (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 neurone- 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 Mi not) : C, cerebrum ; Cb, cerebellum ; F, foramen
of Monro; /, infundibulum ; .1/, bull.; 0, olfactory lube; P, pons; Q, quadrigemina ; Sp.c, spinal cord;
///, 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 ol
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 fibre- 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 ami the projection systems, while the
association tracts arise entirely from them.

Segmentation. — The grouping of the cell-bodies of the afferent fibres i-
originally segmental, one spinal ganglion corresponding to each segment of
the trunk. Tn 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 nerve- Is segmental, the area of skin involved forming a band
about the body. ( >n 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 eases 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 an' examples of concrescence. In the majority of eases, however,
a close approximation of the parts of two nerve-cells is alone to be seen
I Fig. 90). The termination of the discharging axone may be by fibrils or
expanded di>ks, and occur either close to or upon the body, dendrites, or
even collaterals' of the receiving neurone. If, as seems probable, the
dendrites form an important pathway by which the receiving neurone is
1 Held: Arehivf. Anat. a. Physiol., Anat. Abthl., Leipzig, 1897.

CENTRAL NERVOUS SYSTEM.

207

excited, then a cell with many dendrites should offer more receiving points
than one with few. It is perfectly evident, however, that in many cases the
dendrites are not the only pathway by 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 OF PHYSIOLOGY.

hence there is no iiMiim in at which the nervous system is not stimulated, and
no moment at which the effectiveness of this stimulus is not varied. The
response t<> this continuous and ever-varying stimulation i- 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 fad the variations in the physiological (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 to
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 clement 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 firsl place, the impulse
reaches a far greater number of cell- 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.

'flic simplest and most constant of the co-ordinated reactions of the nerv-

CENTRAL NERVOUS SYSTEM. 209

ous system are termed reflex. The term involves the idea thai the response
is not accompanied by consciousness, and is dependent on anatomical condi-
tions in the central system which are only in a slight degree subject to physi-
ological modifications. This view of reflex activities is in a large measure

justified by the facts, but at the same time it must he 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 reflex 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 for a 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 legs
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
cells, 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 the1 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 i> applied the hind Limbs have a tendency
to move at tlie same time that the muscles most directly concerned contract,
[f 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
i- extended some distance through the cord forming several terminations by
Vol. ir. -u

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 he remembered that the hind legs are.
•par excellence, the motile extremities of the frog, and therefore all general
movements involve their use. We inter from this, moreover, that the arrange-
ment in the spinal cord of the frog is not such that the sensory impulses com-
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
firs! appear in the muscles of the same side; and if the stimulus is slight they
may appear on that side only. The incoming impulses arc therefore first and
mosl 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 'hat 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 which
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 i> stimulated by pinching it or dipping it in dilute acid. In this case,
if the stimulus hi' slight, the leg is hut slightly raised, whereas, if the stimu-
lus he strong, it is drawn up high. In the same way by altering the stimulus
the muscles which enter into the contraction may he 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 and Mendelssohn : Neurologisches Centralblatt, 1897, Bd. xvi. S. 978.

CENTRAL NERVOUS SYSTEM. 211

The responses which are thus obtained arc not spasmodic, but are con-
tractions of muscles in regular series, giving the appearance of 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 ' 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 would 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 portion- of the pathway. Next to be considered is

1 Birge: Arehivfur Anatomic und Physiologie (Physiol. A.bthl>), 1882,8. 484.

2 Ward : Ibid,, 1880.

s Gad uud Goldscheider : " Ueber die Summation von Hautreizen," ZeU&chrift Jur klinische
Medicin, 1893, Bd. xx. Befte 4-ti.

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 thai 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, howrever,
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
hitter 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 cause- in both of them locomotor movements, such
as are made by the entire animal.

When a shark (Scyllium canicula) is beheaded, the torso swims in a co-or-
dinated manner when returned to the water. Separation of the cord from the

1 Loeb: Einleilung in dit vergleichen.de Gehirnphysiologie und vergleichende Psychologie, Leip-
zig, 1899.

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Fig. 92.— Diagrammatic representation of the lower portion of the human bulb and spinal cord.

The cord is divided into Lts four regions : 1, medulla cervicalis; _'. medulla dorsalis : 3, medulla lum-
balis; 1, medulla sacralis. Within each region the spinal segments bear Roman numbers. On the left
side of the diagram the locality supplied by the sensory (afferent) neurones is indicated by one or more
words and these latter are connected with the bulb or the segments of the cord at tin- levels at which
the nerves enter. The afferent character is indicated by the arrow-tip on the lines of referenci

On the right-hand side the names of muscles or groups of muscles are given, and to them arc drawn
reference lines which start from the segments of the cord in which the cell-bodies of origin have been
Located.

Within the cord itself, the designations for several reflex centres are inscribed in the segment where
the mechanism is localized. For example. Reflexus scapularis. Centrum eilio-spinale, Reflexus epigas-
tricus, Reflexus abdominalis. Reflexu> cremastericus, Reflexus patellaris, Reflexus tendo Achillis, Cen-
trum vesieale, Centrum analc (the last two on the left side of the diagram). (From Iccrnes yrtnoloyicx,
Striimpell and Jakob.)

Acusticus

Vestibularis
Gustus

Pharynx

Oesophagus
Larynx, Trachea

,f*tedulkj cervfcaDs

1 ' .',,

(Thorax. Abdomen)

Regio occipitalis

Regio colli

Regio nuchas

Regio humeri

Regio Nervi radialis

Regio N. mediani

Regio N. ufnaris

in

Muscuii faciei
Mm. pharyngis. palati
Mm. laryngis
Mm. linguae
Oesophagus
Sternocleidomastoideus
Muscuii colli et nuchae

Cucullaris
Rhomboidei
Diaphragma

DeltoirJeus, Supinator lonps

Biceps

Mil

dulla dorsalls

Abdomen

Thorax

[pigastriur

Umbilicus

Regio glutaealis
Regio inguinalis

IV

M

vn

Mil

Reflex epigastr

XI

WtexaWommafe

Ml

Pectoralis major (portio ciavicui.)

Teres minor ( "==

Pronatores. Brachialis internus \ ^

Triceps

Extensores carpi et digitor. longi j %

Pectoralis major (portio costalis] g_

Latissimus dorsi, teres major

Flexores carpi et digitor. longi)

Interossei lumbricales
Thenar Hypothenar

If lleopsoas

IS

Regio femoris

Regio cruris

Vesica

Rectum

Anus

Quadriceps femoris
Glulaei, tensor fasc. lat.
Adductores femoris
Abductores femoris
Tibialis anticus
Gastrocnemius, Soleus
Biceps Semitendi Semimembranosus

Flexores pedis

Extensores digitorum
Peronei
Flexores digitorum

Mm. vesicates
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 per cent, 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-sulphuric
acid bath is evident in the caudal part of the body alone. 1 n 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 Ave 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 Gerhardt4 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 Functional des Centralnervensystems mul ihre Phylogeneae, 2te Aluli., "Die
Fische," 1888.

2 Tarchanow : Pfl.ugei?8 Archiv, 1885, I'd xxxiii.

3 Goltz und Ewald : Ibid., 1896, Bd. lxiii.

4 Neurologisches Centrcdblatt, 1894, 8. 502. ' 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; tor 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 liar, 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 n 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 cell- 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

1 Kobinson : Nineteenth Century, 1891.

1 Archiv Jiir Anatomie und Fhysiologie, 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 arc not dis-
tributed in the centre in the same manner on each 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 x 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 tiny tail
to remove the stimulus, the prime fact remains that the stimulus continues /<>
ad 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 : Pfluger'a Archiv, Kd. lxvii.

1 Hering: Archiv fiir experimenleUe Paihologie und Pharmakologie, Bd. xxxviii.

216 AN AMERICAN TEXT- BO OK OF PHYSIOLOGY.

from precise ; and although the movements of the leg, when the stimulus i.-
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 fool 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 besl to explain the result by the diffusion of the impulses
within the cord, leaving quite to one side the psychical clement. Such reflex
actions are in a high degree predictable, but in reality this has little signifi-
cance, since there i> 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 mollification 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 the
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 reaction- 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

CENTRAL NERVOUS SYSTEM. 217

system of hibernating animals and all animals exhibiting well-marked periodic
variations in their habits of life.

Variations in Diffusibility. — The degree in which any set of incoming
impulses is diffused and modifies the responsiveness of the central system
depends, in the first instance, on the physiological connections of the fibres
by which they travel, and, in the second, on the particular condition in which
the central cells happen to be found. It is observed that by means of drugs
it is possible to alter the diffusibility of incoming stimuli to an enormous
extent. Strychnin and drugs with a similar physiological action have this as
one of their effects.

Influence of Strychnin. — The experimental study of strychnin-poisoning
shows the following relations : A frog poisoned by the injection of this drug
is easily thrown into tetanus whether the brain is intact or has been removed
previous to the injection. The drug is found to have accumulated in the
substance of the spinal cord.1 The peculiar change wrought in the nervous
system is such that a slight stimulus will cause an extended and prolonged
tetanic contraction of the skeletal muscles — i. e., the diffusion of impulses
within the cord is very wide and (efficient to an unusual degree. The direct
application of strychnin to the spinal cord has been carefully studied by
Houghton and Muirhead.2 AVhen the strychnin solution was applied locally
to the brachial enlargement of the spinal cord of a brainless frog, a subse-
quent stimulation of the skin of the arms produced tetanic contractions of
the arms, and later, after the poison had acted for a time, of the entire trunk
and legs. On the other hand, stimulation of the legs in such a case produced
a slight reflex or none at all. Since, in order to cause contraction Df the leg-
muscles, the efferent cells controlling the muscles of the leg must, be discharged
— and in the one case when the stimulus was applied to the arm region these
cells discharged so as to cause a tetanic spasm, while in the other, when the
stimulus was applied to the legs, they discharged only slightly — the alteration
in the cord produced by the drug must affect some other group than these
efferent cells. Since, moreover, a tetanus of the legs could be caused by the
stimulation of the skin of the arm, the application of the drug being to the
brachial enlargement only, it appears that the central cells, or those conduct-
ing the impulses entering by the dorsal root-fibres in the brachial region to
the nuclei of the lumbar enlargement, are probably affected ; and. further,
that it is on the bodies of these cells that the drug must act. since they
alone were in the locality at which the drug was applied. The application
of the drug to the dorsal root-ganglia and to the nerve-roots between the
ganglia and the cord proved to be without effect, SO that the two parts which
can possibly be influenced are the terminations of the sensory afferent nerves
within the cord ami some portion (the dendrites ?) of the central cells with
which these terminations are associated. lint whether the change is in both
these structures or only in one cannot now be determined.

1 Lovett : Journal <;/' Physiology, L888, vol. ix.
': Mrtliral News, June 1, 1S95.

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,
:i- 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; hut where the lateral root is
presenl 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 Haskell,- previously quoted, and
the physiological studies of Langleyj3 an outline of the relations of the sym-
pathetic cells, based on the arrangement found in the cat, is briefly as follows:

Pre-ganglionic fibre — i.e., those growing out of cell-bodies located in the

1 GaskeU : Journal of Physiology, 1885, vol. vii. ; von Kolliker, " Ueber die feinere Anatomie
and die physiologische Bedeutung des sympathischen Nervensystems," Verhandlunf/en Gesell-
8chaft deulscher Naturforscher und Aertze, 194, Allgemeiner Theil, 1894. 2 Loc. cit.

3 Langley : " A Short Account of the Sympathetic System," Physiological Congress, Berne,
1895.

Fig. 93.— Schema of the neurones forming the sympathetic nervous system (Huber: Journal of Com-
paraif . L897, vol. vii.i.

A solid black line designates the axone from an efferent neurone, with its cell-body in the ventral horn
of the cord, and the terminal brush ending in a striated muscle (m.n.).

A black line crossed by short dashes designates the axone from an afferent neurone, the cell-body of
which is in the spinal ganglion, and the peripheral axone of which terminates in the epidermis or some
special sense-organ (g.n.).

An interrupted black Km indicates an axone of similar origin to the one just described, but distributed
with the fibres of the sympathetic system (s.s/.). At the periphery it terminates in a free ending
(s.s.f.H)) or in a Pacinian corpuscle (s.s/.(2)).

A blue line shows a preganglionic fibre (of Langley), the cell-body of the neurone being located in the
lateral horn of the cord. The axone leaves the cord by the ventral root (as a fibre of very small calibre),
passes in the white ramus | W.R.), and terminates by a pericellular basket about the body of a sympa-
thetic neurone (drawn in red).

The various places where such an axone may terminate are indicated as follows: o. axone passing
through the chain-ganglion (I.C.6.) to terminate within the next higher chain-ganglion ; b, axone passing
as does (a), but terminating in the next lower chain-ganglion {II.C.O.) ; c, two axones ending in a gan-
glion of this same segment (I. CO.); d, axone passing through the chain-ganglion of the segment and
ending in a prevertebral ganglion (Pr.v.G.) : e, axone passing through both a chain-ganglion and a pre-
vertebral ganglion to end in a peripheral ganglion I Periph.6.) ; /, axone which gives off a collateral branch
to one ganglion {J.CO.) and passes on to terminate in a more distal ganglion (Pr.v.G.). Fibres arranged
like (/) probably account for some of the reflexes obtained from sympathetic ganglia ; g and h. axones
representing fibres which regularly pass to any given ganglion from the ganglia above and below it.
The sympathetic neurones are drawn in red, and about their cell-bodies terminal baskets of other axones
(always in blue) are shown. They enter the mixed nerve by the gray ramus (G.R.). m, the axones of the
sympathetic neurones, terminate: i, in the muscular coats of the blood-vessels (vaso-motor endings);
j, in the muscular coals of the viscera (viscero-motor endings), and in heart-muscle (not specially shown
in the figure) ; k, in glands (secretory fibres); I, in other sympathetic ganglia (a doubtful form of termi-
nation).

The figure further shows two " afferent " sympathetic neurones (Dogiel), in dotted red ; o, arising in a
peripheral ganglion [Periph.0.) and terminating in the prevertebral ganglion [Pr.v.O.) : p, arising in the
chain ganglion (I.C.G.) and passing to the spinal ganglion, to terminate about Dogiel's spinal ganglion-
cell of " type two," q (represented in solid back); g, spinal ganglion-cell (Dogiel's " type-two "), the ter-
minals of W Inch form baskets about the bodies of the ordinary spinal ganglion-cells.

CENTRAL NERVOUS SYSTEM. 219

cord — arise from the first thoracic to the fourth or fifth lumbar, and from these
segments only (Gaskell). The fibres are medullated. Langley'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 (Langlev).

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 eat 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 is
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 tiny 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

'Berkeley: Anatomixrhcr Anzeitjrr, IS',1'2.

2 Mall: Arch irf. Anatomie u. Physiologic, 1892

3 Proceedings of the ltoyul Socirli/, 1889, vol. xlvi.

220 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY.

contraction or tonus. When the nerves controlling any such sot of muscles
are cut, or their central connections injured, the muscles at first relax.

If a frog be hung up vertically alter 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 iu a state of slight contraction. Destruction of the entire cord
abolishes all outgoing impulses, and 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, arc
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. Admittedly, there is
in the case chosen but a fraction of the central system. It 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 under more than one impulse is
well illustrated by an experiment of Exner:1 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. ( )ne
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 loot overlying 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 the loot
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 firsl cortical stimulus in fig. 9 1
{A, a), put the efferenl cells in such a condition that the stimulus from the
skin (A, h, Fig. 1)4), 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

' Exner: Archivfur die gesammte Physiologie, Bd. xxvii.

■2-2-2

AN AMERICAN TEXT-BOOK OE PHYSIOLOGY.

the discharging cell for a short period of time more excitable. In the same
figure the record shows that ii'a longer interval — here more than three seconds —
be allowed to elapse, then the second stimulus from the skin remains ineffi-
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 curve B,
Fig. 94, shows the results when both stimuli are inefficient. In this the
stimuli [l> and a) produce no effect when given several seconds apart, but when
they occur within a short interval (/>' 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 ' that when a patient was being tested for the height of his knee-
kick, ;i voluntary muscular contraction, or an extra sensory stimulus, occur-

Moremi-iit of paw.

Stimtdatioti of cortex. ifl!

" b' " paw.

/.'.-.'.'(' .':■■• seconds.

rlHHrArlrHriru-\i

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 .1 the cortical stimulus at a causes a movement of the paw.
Dermal stimulus, within a second, at b causes a movement of the paw. Cortical stimulus at a' causes a
movement of the paw. Dermal stimulus several seconds later at V is ineffective. In curve li dermal
stimulus :it b is ineffective. The cortical stimulus at a several seconds later is also ineffective. The
dermal stimulus at 1/ is ineffective, but if followed within 0.13 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 Dentsches Archir fiir klinische Medizin, Bd. xxxiii.
1 Journal of Physiology, 1890, vol. xi.

CENTRAL NERVOUS SYSTEM.

223

Effects of Afferent Impulses. — Studies on inactivity show that a certain
amount of exercise in any given cell is necessary for its proper nutrition, and
if the excitation fall below the point which causes this, the responsiveness of
the cell is diminished.

For example, a strychnized reflex frog on being dipped into a solution of
cocaine loses in so large a measure its irritability that its responsiveness falls
far below that of a normal frog.1

In this case the central system is deprived by the action of the cocaine of
the impulses which even in the absence of any special form of irritation
normally arrive from the skin, and the abolition of these impulses causes a
diminution in central responsiveness. Effects which can thus be accom-
plished in a few seconds by cutting off the afferent impulses from the skin
may of course follow any slow diminution in these impulses, although all

40-
30-
20-
10-

0

10-

20-

30-

\

Normal.

O.I"0.2" 0.4"

0.7-

10"

1.7"

Fig. 95.=Showing in millimeters the amount by which the "reinforced" knee-kick varied from the
normal, the level of which is represented by the horizontal line at 0, "normal." The time intervals
elapsing hetween the clinching of the hand 'which constituted the reinforcement) and the tap on the
tendon are marked below. The reinforcement is greatest when the two events arc nearly simultaneous.
At an interval 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
when 1.7" is allowed to elapse the height of the kick ceases to be affected by the clinching of the hand
(Bowditch and Warren).

such slow changes arc much more likely to be accompanied by some sort of
compensation whereby oilier afferent impulses in a measure take the place of
those which have been suppressed, 'fhe loss of these impulses which rouse
the cells to activity is usually a more important condition than direct nutri-
tive change, and must for this reason always be kept in view.

Inhibition. — Evidence is accumulating to show that all the active tissues
of the body may be influenced through their nerves in two opposite ways.
That is, stimulation may increase or diminish their activity." Thus the
physiological processes in the glands, nerve-centres, or muscles can be so
varied. In most cases, nerves which cause inhibition are, except in the
central nervous system, distinct from those causing increased activity. The

1 Poulsson : Archiv fiir experimentelle Pathologie and Pharmakologie, L885, Bd. \xvi.
'Meltzer: "Inhibition," JVeto York Medical Journal, May, 1899.

224 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY.

chemical changes in the inhibited muscle of the heart arc 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 iu 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 lie 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 leaser 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.

AYithin the central nervous system very remarkable examples of inhibitory
phenomena have been investigated, chiefly by Sherrington.2 Boubnoff and
Heidenhain3 were the first to record the observation that under certain
conditions stimulation of the cerebral cortex might cause a relaxation of
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 flexor- of the arms also gave rise to impulses leaving the cortex and
causing a (inhibition) relaxation of the antagonistic extensors.

( )n stimulating the cortical ana 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. ruder these conditions the eye, when
;it 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 Graskell : Journal of Physiology, vol. vii.

'-' Sherrington : Ibid., vol. xvii.

3 Boubnotr und Heidenhain: Pfluger'a Arckiv, xxvi.

CENTRAL NERVOUS SYSTEM. 225

movement of the operated eye toward the median plane (the right), and i<>
the position in which it would be held by the clastic tissues alone. This
could be explained only through a relaxation or inhibition of the external
rectus 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, for the inhibition
follows when the fibre-bundles below the cortex arc 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 other
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 ccllu-
lipetal impulses causing inhibition.

C. 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 liomologues in
the brain stem, can carry the efferent impulses needed. Further, it musi 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 ami voluntary reactions.
The processes then which distinguish the two forms of reactions must take
place in the central cells. We turn, therefore, t<» the nervous connections <>f
the encephalon with the cord, since it is by means of these connections that

1 Hughlings-JacksoD : 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

fig. 9G.-schema showing the smaller pathway nerve-roots. The long paths in the

of the sensory impulses, on the lett side, s, S', dorsal funiculi of the cord are formed

represent afferent spinal nerve-fibres ;C, an afferent ... , _ . _

cranial nerve-fibre. This fibre in each case termi- by the ascending branches of the af-

nates near a central cell, the axone of which ferCnt axoneg an(J tl1(,S(, terminate, for

crosses the middle line and ends m the oppo- '

site hemisphere. The interruption of the larger the most part, about the cell-bodies

pathway in the thalamus is not indicated (van \ • \ r t,\ 1 • C j.1 1 1

, ,e] ,,„.,,',,.„, winch 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
al ->nce, and then the axones pass forward in the medial lemniscus to rind
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

CENTRAL NERVOUS SYSTEM. 227

collaterals and the ascending branches of the afferent axones, which end
before they reach the nuclei of the dorsal funiculi. 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 ' 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 b, and in
the thoracic region in c and d. The section c 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 t<> be kept
in view. Though a number of fibres among those degenerating alter 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 different
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-sect ion shows 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 funiculi, since the majority

1 Tschermak: Archivfur Anatomii v/nd Physiologie, Anat. Abthl., 1898, S. 291-400.

228

AN AMERICAN TEXT-BOOK OE PHYSIOLOGY.

terminate somewhere between this extreme limit and their point of en-
trance.

Since the fibres1 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 musl form pari 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 conned a- they terminate at the several levels of the cord.

Fig. '.'7.— The sections are from five levels of the spinal cord of a dog. The dorsal roots on one side
had heen 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 cam lail were examined in series
it would be found thai 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 t\\ enty-second and the twentieth nerves— that is, after passing the level at which the second
group of sectioned nerves joins the cord — there arc to be seen two bundles of degenerated tihres marked
by black spots in the sections ''*, 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 Beptum and approach one another; and. finally, that they completely fuse in the cervical
region

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, hut on the opposite side there are also degenerated fibres in
all these localities, although tiny are by no means s<> numerous. It is inferred
that all the fibres which thus degenerate form path.- 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 direci continuation- 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

crated 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 bull), 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 sonic 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, Wbroschiloff3 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.

' KohnatSLTam: Neurologischea Centralbla.lt, L900, S. 242.

'■ Archives iialiennes de Biologie, ls'.i">, t. sxiv.

3 Beriehteder math.-phys. Clawed. /.-. Qenellnch. >l. Wwsen. ~u Leipzig, 1874.

230 IV AMERICAN TEXT-BOOK OF PHYSIOLOGY.

The physiological observations of Gotch and Horsier1 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.

Osawa8 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. Tt 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 p.-iss 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 .-kin in a man of (>2 kilograms 1 136 pound-), and twenty-
six years of age, was found by Meeh to lie 1,900,000 square millimeters.8

From tin' study of the nerves going to the muscles of the dog, Sherring-

1 Croon ian Lectures: Philosophical Transaction* <</' the Iloyd Society, 1891.

2 Griinbaum : Journal of Physiology, 1894, vol. xvi.

; f'nii ,'.:iii-hiiii(j< n Hbt'r (lie, Leilungsbahnen im Riickenmark des JIundes, Strassburg, 1882.

1 It seems probable that both these estimates were too low.

■ \ Blight correction is called for hero, 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 genera] relations given below would not, however, be significantly modified by the altera-
tion of the data.

CENTRAL NERVOUS SYSTEM. 231

ton l reports that from one-third to one-half the number of these muscular
fibres arise from the dorsal root ganglion, and are therefore 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 300,000 sensory fibres
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. Woischwillo2 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 specially
marked in the leg. Assuming, however, one nerve-fibre to six square 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.

[f we include abnormal persons, it is found that while in a lew cases com-
plete absence of painful sensations has been noted — the other sensations
remaining normal — there arc at the other end of the scale those cases in
which pain is produced by many stimuli which would not have this effect <>n
persons in ordinary health. The capability of a given stimulus to produce
pain is therefore subject to wide variations according to the general condition
of the subject.3 The same stimulus ha- different effects in a given individual
according to several circumstances. Peripheral irritation, such as an inflam-
matory process in the skin, greatly increase- 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"T'cl)cr das Verhiiltniss des Calibers der Nerven zur Haut und den Muskeln des Men-
schen." [naug. Diss. (Russian), 1883; vide OerUrcHblail fur NervenheUkunde, 1883, Bd. vi.
"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 anaesthetics, such as cocaine, may reduce the sensibility to zero, and
the same follows the general anaesthesia 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 —
/'. <■., 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 he
without effect in causing any painful sensations whatever. This would
represent a condition of complete analgesia. Moreover, the capacity of the
>kin to cause abnormal painful sensations upon the adequate stimulation of
each of these groups of nerves may he 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. H 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 lie 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 Muller, 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 ma nner; 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 mill Goldscheider : Zeitschrift fiir klinische Me'dicin, 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 afferent
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 passing
along the optic nerve-fibres arc, for example, similar or dissimilar to those
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 bow
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 ease 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 hyperesthesia, accompanied by contra-lateral anaesthesia.

3. As to the several forms of sensation, the following may be stated :

(«) The muscle sensations (Bathyasthesia, Oppenheim): the defect is
never contra-lateral ; sometimes, however, it is bilateral, but in mosl cases is
homo-lateral.

(b) The contact sensations are very often not affected at all — sometimes

'Hering: "Zur Theorie der Nerventhatigkeit," Akademischer Vortrag, Leipzig, 1S99.
'Oppenheim: ArcMvfur Physiologic, Physiol. Al.tlil., Suppl. Bd., 1 licit. .Inly. 1899.

234 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY.

slightly, and in the latter case the hyperesthesia may appear on either or
both sides.

(c) The prominent and almost constant sensory symptom is the contra-
lateral loss of the sensation lor pain and temperature.

On the basis of a case1 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 case2 is recorded in which a stab-wound divided all of one-half
of the cord pins 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 Turner3 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/ 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 part- 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 oil'. 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.'1 I have seen no signs of paralysis cither motor or sensory on

'Gowers: Clinical Society's Transactions, 1878, vol. xi.

2 Muller : /<"' itri'n/i -in- jiniliiiliii/ischi Aii'iiniiiii imil I'/niswIogie dea Riickenmarkes, Leipzig, 1871.

3 Brain, 1891. ' 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. Trims. B. 1892), although my 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 or
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 afferent
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 Tschcrmak,
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
Starr2 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, 'fhe long tracts in the dorsal columns are
connected with the nuclei of those columns (nuclei of Goll and of Burdaeh)
on the same side. The cells of these nuclei ^-\u\ 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 I describe lose of sensibility in the opposite liind limb in
the monkey. Brown-Sequard, as is well known, obtained 1 1 1 i » result in the rahbit. (See also
Ferrier, Functions of the Brain, and Orooniam Lectures, 1890.)

'Starr: Transactions of the American Neurological Association, 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 briefly the relations of the sev-
eral atfcreiit cranial nerve.-, 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 (X. alse cinerse) falls within the area of
the mi ml vital. Concerning the axones of the neurones forming the nucleus
of termination, it can only he said that they are continued cephalad in the
medial lemniscus and the fasciculus longitudinalis medialis.

Nervus Glossopharyngeus [Ninth Nerve). — The ninth nerve (glossopharyn-
geus, Fig. 91) i> represented in the hull) by the tractus solitarius, the fibres
of which find their principal nucleus of termination in the cell-group lying
ju-t 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
ir 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 intermedins. The nuclei of termination for these three nerves
are very close to one another in the bulb, and hence the innervation of a
special -en-e-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
intermedins (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
nucleu- 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.'

Nervus Auditorius ( Eighth Nerve). — A. Cochlear Hoot. — The eighth nerve

1 Van Gehuchten : "Becherches sur la Terminaison centrale des Nerfs sensible peripheri-
ques — le Nerf intermediary de Wrisberg." Le N&uraxe, Mars, 1900, t. i. fasc. 1.

CENTRAL NERVOUS SYSTEM. 237

goes to the inner car. The cochlear portion of the inner car mediates sensa-
tions of sound and is connected with the bulb by means of the aervus coch-
leae; the cochlear branch of the eighth nerve. The cell-bodies of the aervus
cochlea? are located in the spiral ganglion of the cochleae, which is homologous
with the dorsal root ganglion of a spinal nerve. The ganglion cells are
bipolar or diaxonic, one axonc 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 axones enters in a
large measure the nucleus nervi cochlea? ventralis,1 and to a less extent the
nucleus nervi cochlea? dorsalis. According to Held." 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 trapezoid e um, the superior olive,
the lateral lemniscus, and the colliculus inferior; to all of which gray masses,
including the nucleus nervi cochlea? 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 cochlea? dorsalis send
their axones across the floor of the fourth ventricle, forming the stria? acustica?.
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 besf
evidence shows the nerve to convey those impulses from the macula acustica
utriculi and the crista' 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 rool than that
associated with the cochlea, join the bulb at the caudal (■d^c of the puns, 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 :i (1) the nucleus nervi vestibuli spinalis (the radix descendens); ('_') in

1 Barker: The Nervous System "nil its Constituent X* "nun*, 189'.*, pp. "ill 555.

2 Held : Arehivfur Physiologie, Aunt. Abth., Leipzig, L893.

3 Barker: '/'//< Nervous System mid Us Conxliiurnt \i-iirmim, 1899, p. <>_!7. >i .«</.

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 Trigemmus {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 eephalad 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
quadrigemina, 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 l 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
Derves, the nerve of the side opposite to the lesion being most affected, and a
smaller degeneration appearing in the nerve of the same side (sec Fig. 98).

In the fish, amphibia, reptiles, and birds — except the owls2 — 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 (bidden 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

Won Gudden : Qesammelte wnd hinterlassene Abhandlungen, Wiesbaden, 1889.
2 Fcrrier : The Croonian Lectures on Cerebral Localization, London, 1890, p. 70.
"' Singer and Munzer : IJenkschriJ'ten der math.-naturwiss. Classe der kais. Akademie der Wixscn-
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 decussation 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

Fig. 98.— Illustrating tin- relations or the afferent fibres in the optic nerve. The crossed fibres are
indicated by solid lines, the uncrossed fibres by broken lines: AT, nasal side of the right eye ; T, temporal
side of the same ; O. E, geniculatum externum ; /'. 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

' Cajal: Die Slruclur des Chiasma opticum nebst einer Allgemeinen Tkeorie der Kreuzung der

Nervenbahnen, Leipzig, 1899.

240 AN AMERICAN TEXT- HOOK OF PHYSIOLOGY.

view of complete decussation has been maintained by Michel.1 The central
ends of the afferenl optic fibres forming an optic tract arc for the most part
distributed between the anterior quadrigeminum, the geniculatum externum,
and the pulvinar of the same side. By central cells located in these latter
structures the pathway is continued to t In- 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 he remembered, however,
that between the cortex and the primary centre-, and again between these
centres and the retina, there are pathway- conducting from the cortex to the
primary centres, and also from the primary centres to the retina.2

As the result of partial decussation it will be seen that the relations of the
two retime 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 trad 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 hsikiI 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. Henschen3 indicates
the calcarine fissure and its immediate neighborhood as the most important
locality. Observations on the arrest in the development of the cortex duv to
early blindness following destruction of the retina in the ease 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 suffice of the hemisphere.4

First Nerve. — Comparative anatomy indicates that the part- of theenceph-
alon mediating the sen -e of smell are most closely connected with the cerebral
hemispheres, in the sense that phylogenetically the firsl 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 tract- are at the same time but poorly developed. The pathway

1 Kolliker's Festschrift, Wiirtsburg, 1887.

2 von Monakow: Archivfiir Psychiatric, 1890, Bd. xx. II. ■'!.

s Henschen: Klinische und anatomische Beitrtigezur Pathologie des Qehirns, Upsala, 1890-92.

4 Donaldson : American Journal of Psychology, 1892, vol. iv. No. 4.

5 Sir William Turner : Journal of Anatomy, 1890; Edinger: AnaUmmcher 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 rhinencephalon of the comparative anatomists. It has been shown,
nevertheless, by Hill1 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 jnterrupting it, to obtain movements of the limbs
and face when the electrodes arc 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 arc 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 eerehrum, the reader is referred t<> Barker's The Nervous System, 1899.

3 Archiv fur Anatomie und Physiologie, INTO.

Vol.. II.— 16

242

AN AMERICAN TENT-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, ami often immediately after the first exposure of the brain there is
a time during which no response can lie 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 Horsley1 (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-gronp 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 (Ilorsley).

cortex. The areas in turn are subdivided into centres, comprising the groups
of cells, which, for example, control the smaller masses of muscle1 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 muscle- — 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 etl'eet can lie produced by the

1 Beevor and Horsley: Philosophical Transactione of the Royal Society, 1SS8-90.

1244 AN AMERICA X TEXT- HOOK 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. AVe shall see later that this direct course
probably does not represent the sole pathway for these impulses.

Course of the Descending 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 Pitres1 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 Wcrtheimer and Lapage2 on

1 ProgreS medieale, Paris, 1882, x. 528.

1 Archives de Physiologic, 1897, No. 1, p. 168.

CENTRAL NERVOUS SYSTEM.

245

dogs, and by the studies of Melius1 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 fibres 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 ; />'. motor tract : I ', sen-
sory tract for touch (separated from B for the sake of clearness in the schema) ; />, visual tract ; E, audi-
tory tract; F, Q, II, superior, middle, and inferior cerebellar peduncles; J, fibres between the auditor;
nucleus and the inferior quadrigeminal body ; K, motor decussation in the bulb; 17, fourth ventricle.
The numerals refer to the cranial nerves. The sensory radiations kare 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 fco the
opposite side of the cord, while the other continues its lirst 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) aud

1 Proceedings of the Royal Society, London, 1891 and 1895.

246 IV AMERICAN TEXT- BOOK OF PHYSIOLOGY.

dogs, and probably the arrangement of these fibres in man is similar. The
observations are particularly significant as giving 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 tar 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, Schafer1 denies the existence of
such ;i din <t 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, Ave 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
— /*. 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, whereas, 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 arc for the arm,
trunk, ami leg, in man. Ti'.lll 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 x.xiv ; Proceedings of the Physiological Society.
» Blocq el ' >zanoff: Gaz. da Hdpil., 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 sonic 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 iro 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 anas 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 lingers, ami especially
the thumb. The centres from which these several reactions may lie 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.

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 coin-
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-

Fig. 102.— Horizontal section of the human cere- J

brum, showing the internal capsule on the left gethcr, SO that thev are not easily Sepa-
side: /', frontal region: G, knee of the capsule ; , , . '

NC.NC, muriate nucleus; NL, lenticular nucleus; rated by experiment.

0, occipital lobe; TO, thalamus; X,X, lateral ven- Jn the lower monkeys (Macacvs

trir-lc In the internal capsule the letters indicate

the probable position of the bundles of tii.r.-s whir h Milieux) these cell-groups are SCgre-

upon stimulation give rise to movements of the w^l g0 <),.,, those associated with the

l>urts named or which convey special sets of in- © > _ i i »

coming impulses: E, eyes ; ff.head; r.tongue; M, cervical portion Of the cord and lomi-

mouth : L, shoulder; B.elboy ; D, digits; .A.abdo- ,i __ __„_i, .„..„„,, *■„

, ' ,. , ... . . mtr the arm-area are nmen more to-

men ; P.hip; if, knee; U, toes; S, temporo-occip- a

itai tract ; oc, fibres jo the occipital lobe ; op, optic gether and quite separate from those

radiation (based on Ho rel . , ■, ..i ,1 1 i • ,1

associated with the lumbar region, the
leg-area. In the orang-utang,- and to a greater extent in man, a further sepa-

1 Mann : Journal of Anatomy and Physiology, 1895, vol. xxx.

2 Beevor and Horsley : Proceedings of the Royal Society, London, 1890-91, vol. xlviii.

CENTRAL NERVOUS SYSTEM.

249

Fir.. 103.— Rabbit's brain, dorsal view. The
areas indicated are those the stimulation of which
causes a movement of the parts named (Mann).

Fig. 10-4. — 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-utan^', showing the motor area about the cen-
tral fissure (Beevor and Horsley).

Kin. loo.— 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 buman brain the excitable localities are surrounded bj
rather extensive anas nol 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 direcl stimulation (see Fie;. 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

K 1 1 ; . 107.— Mesial view of a human hemisphere, showing motor areas. Formed in the same way as

Fig. 100.

the results are plotted they give a distrib
At the same time all such figures arc lari

FlG. 108.— Frontal section of human cerebrum on
the left side. The fibres forming the internal capsule

( ), the callosum ( ), and the anterior

COmmisMirr i . - . — . — ) have been i mlic:i t <■<! : T. cor-
tical area for the trunk ; /,. cortical area tor the leg: A,
cortical area for the arm; /•'. cortical area for the face;
A, anterior commissure ; C, callosum ; CO, optic chias-
ina : NC, caudate nucleus ; NL, lenticular nucleus; i:
fornix; TO, thalamus; .V, lateral ventricle.

to the cord. The relation of the ana- in
Multiple Control from the Cortex.

ution such as is shown in Fig. 106.
elv 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
anas 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

a frontal -eel ion is shown in Fig 10<S.

— It has been found that stimulation

1 Helen B. Thompson: Journal of ComjMtrative Neuroloyy, 1899, vol. ix. No. 2.

CENTRAL NERVOUS SYSTEM. 251

of the cortex in the region of the frontal lobes marked " eve " (Fig. 99) was
followed by movements of the eye. Schafer ' 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, lor 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 mammal-, 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 <>_( tin- Royal Society, 1888, vol. xliii.

2 Sherrington : Journal of Physiology, 1 897—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, :i 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 r>'\\\> 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
ana of the entire cord being taken as 100 per cent., are given by v. Len-
hossek3 for the following animals:

Mouse 1.14 per cent.

Guinea-pig 3.0 "

Rabbit 5.3 "

(at 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 of 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-
pulse.- t'roin the skin and muscles back to some organ controlled by the
efferent nerves. To Munk4 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 anas 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

: Journal of Gomparatiw Medicine and Surgery, 1886, vol. vii.

a von Lenhossek : Anatomi*cher Anzeiyer, 1889.

3 THefeiner Bait des Nervensystemi im l/ichte neuester Forschungen, Basel, 1893.

* Ueber die Functionen der Qro&hirnrinde, 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 Schafer 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 alter birth, Flechsig2
lias 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 trad (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 tin; fibres in question are mosi 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 project ion-

1 Ferrier : Functions of the Brain, 1876.
'Flechsig: Gehim 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 visual area as seen from the mesial surface. The portion comprising the deeply
colored tipof 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 ; S, anterior commissure ; 4, quadrigemina ;
5, callosum ; 6, fornix; 7, septum lucidum (from Flechsig).

Fig. no.— The colored portion at the tip of the occipital lobe represents the postero-lateral extension
ofthe visual area. 'I he colored portion about the central fissure and the neighboring parts of the frontal
lobe represents the lateral extension ofthe 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 theareas where the projec-
tion-fibres are most abundant. The uncolored portions of the cortes form the association centres of Flechsig
i 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 denned, 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 tin; calcarine
fissure. Conversely, the failure of the eyes to grow, arrests the development
of this portion of the hemisphere.

Hemianopsia. — Tt is found, moreover, that injury to the visual area in
one hemisphere usually produces a hemianopsia or partial defect of vision in
both retina'. 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 firsl sight appears, for the
point of most acute vision is often unaffected in such cases. This peculiarity
depends apparently on the (act that there is a binocular centre for macular
vision in the cortex lining the sides ami bottom of the posterior portion of
the calcarine fissure.2

1 Donaldson: American Journal <>/" Psychology, 1891- 2. vol. iv.

2 Laqueur and Schmidt : Virchotfs 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; Ii, 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 Flechsig1 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 parietotemporal 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 eon-
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 pari 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 : /.'»•. at.
Vol. II.— 17

L'.-.S

AN AMERICAN TEXT-ROOK OF PHYSIOLOGY.

from localities outside the third frontal gyrus. 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

fhe interesting observation was also made that in the normal right-handed

pei-oii the muscles of phonation could not he co-ordinated tor speech from
the right hemisphere alone. Thus the symmetrical portion of the right
hemisphere has not the same physiological value.

Besides this lesion, 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. 1T2).

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 ; hut because of the interruption
of the association fibres the object is not recognized, and the impulses reach-
ing this sensory area (licit 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,

Pig. 112. Lateral view of a human hemisphere; cor- ■ .

tical ana V, damage to which produces "mind-blind- '-^^i all the others. J Ills arrange-

ness"; cortical area //. damage to whirl, produces ment is to be regarded as modi-
" mind-deafness " ; cortical area S, damage to which

the loss of audible speech; cortical area If, dam- fied in several Ways,

age to which abolishes the power of writing. Jn t]|(. firgt plaC6j t])(, (.onnec_

tion between a given sensory and a given motor area differs widely accord-
ing to the area.- concerned. The connection, for example, between the visual
area and the motor area for the arm is probably represented by more nerve-
element-, and these better organized, than the connection between the gusta-
tory area and that for the movements of the leg.

When, therefore, it i- -aid that such connections exist, it must he added
always that the nexus i> different for the several regions concerned, and what
i- more, that in man, at least, it i- different for the two hemispheres.

Relative Importance of the Two Hemispheres. — The cerebral cortex

Semon ami Horsley : Philosophical Transactions of the Royal Society, 1890, vol. 181. Also
-. medicinische \Vo<-!i<-n-<rhri/t. N<>. .".]. lx'.Hi.

CENTRAL NERVOUS SYSTEM. 259

is always active daring our periods of consciousness, and it is to be thought
of as a region over which the local 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
Flechsig1 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 maybe 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. 1 1 .->>.

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 oi
aphasia. When an interruption of associative pathways occurs some one or
more of these tracts is broken, and hence the discharging cell doe- not receive
a stimulus adequate to cause a response.

1 Leitung8bahnen im Oehri/n und Riickenmark, 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 ureas 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. 118. — 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 on< — the response is made, though, acting
alone, any one -et 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, hut 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 he 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 arc by no means to be attained br-
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 arc 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 arc removed the change
is profound. < )n 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 hyperesthesia 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 ju>t described appear on the other side of the body, but still more
fundamental changes occur. A ceaseless wandering to and fro, such as
Goltz3 observed, in those dogs in which the anterior half of the brain had
lien) 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 lobe- 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 tin- less extent.

For the determination of the functions of the several parts of the enceph-
alon it i- 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
portion- 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 italiennea de Biohgie, 1895, t. xii.

2 Archiv fur Anatomic mid Physiologic, 1895.

3 Ueber dieVerichtungen des Grosshirns, Is- 1.

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 Goltz4 and Loci)/'
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, ami, 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 :i shark.'
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 Untersuehungen ueber daa Oehirn, Berlin, 1874.

2 Ueber die Funettonen der Orosshimrinde, Berlin, 1881.

3 The Functions i if tin- llrdhi. London, 1S7(>.

* Ueber die Veriehtungen dea Gfrosshirns, Bonn, 1881.

8 Archiv fur die gesnmmte Physiologic, 1884, Bde. 33 a. 34.

6 Steiner : Die Functional der Centralnervensystems, 1888. 'Steiner: Loe. cit.

264

AN AMERICAN TEXT- BOOK OF PHYSIOLOGY.

If the olfactory trad 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

Pig. 1U.— Schema of the encephalon of a bony fish— embryonic i Edinger). The vertical black Hue 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.

Passing next to the amphibia as represented by the (Vol;, there are several
series of observations on the physiological value <>f the divisions of the cen-
tral system. Schrader ' 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 Archir fin- die (jemmmte Physiologie, 1S87, 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 there-
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 inthedirec-

/V\

i \

UK

Fig. 116. — Frog's brain; the parts in dotted outline have been removed : J, brain intact ; /•'. cerebral
hemispheres removed ; C, cerebral hemispheres and thalami removed; IK cerebellum removed'; /-.'. two
sections through the optic lobes ; /•', two sections through the right half of the bulb (Stein

tion opposite to the motion of the support, and then to follow with move-
ments of the body. If the optic thalami arc removed (Fig. IK!, 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 depriv sd of the h im-
ispheres and thalami exhibits the lack of spontaneity which is usually d iscrib sd
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. 11<>, 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 theeye,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 lias been finally attained,
and vainly reach into space from this position. In the water they swim "dog-
fashion,1' 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 t 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 I, 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
arc the only defects noted (Fig. 116, />). 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 //), 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 with
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
hold- 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, dust 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 arc the most instructive.1 The removal of the hemispheres from
the bird involves taking away the mantle and the basal ganglia, the chiasm a
1 Archwfur die gesammte Physiologie, 1888, Bd. xliv.

CENTRAL NERVOUS SYSTEM. 267

and the optic nerves being left intact. For the Hist 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. Goltz1 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 lie 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 bid' 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 Archiv fur die gesammte Physiologie, 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 :i 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 eneephalon is less
than one-lburth 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 -till in view, it is possible to go into further
details. In tonus below the monkey the loss of portions of the cerebral
cortex from tin; motor area is accompanied by a greater or less paralysis of

CENTRAL NERVOUS SYSTEM. 269

the 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 difference are the following: The
efferent cells in the ventral horns are dominated principally by two set- of
impulses, those arriving directly over the dorsal roots of that segment in
which they are located, and those coining 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 series,
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 (/. c, 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 lias 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.} to detect objects in motion and read
to them normally. A pigeon can direct its movements in some measure, and
even select a special objeel 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 arc in a less and less degree reflected toward the cord.
\\ hen, 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 lie 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 .d 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 lie far less well determined. The dis-
turbances caused by the section and stimulation of the callosum have been
studied by Koi'anyi • 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
symmetrica] 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 Orosshirnrinde, Berlin. 1881.

2 Archiv fur Anatomic und Physiologic, lid. zlvii.
Brain, 1890.

CENTRAL NERVOUS SYSTEM. 271

by the uninjured hemisphere. These results seem to corroborate the conclu-
sion derived from histological work to the effecl that the Bystem of the eallo-
sum is composed only of commissural fibres, and that it sends no fibres directly

into the internal capsule of cither 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 ease 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 mid
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-Sylvian 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; (!. 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,
cither cortex or the thalamic nuclei, is followed by degeneration in the other
part, and that the location of the defeneration depends on that of the lesion.

Further, it has been observed by Melius'' that the axones passing from the

1 Aronsolm mid S:iolis: Archiv fur die gesammte Physiologie, 1885, Bd, xxxvii.; Richel :
Coviptrx nii/lux d,' VAcad. des Sciences, 1884; Ott: /.' ain, lSS'.t, vol. xi.

2 Kaiser: Neurobgisches Cenlralblalt, 1895, No. 10. 'Ott: Loc at.

4 Archiv fur Psychiatric mul Nervenkrankheiten, 1893, Bd. xxvii.

5 Proceedings of the Royal Society, London, 18U4 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 tract-.

Moreover, the studies of Tschermak ' ou 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 a> variable in its development as the
mantle of the cerebral hemispheres, and in many lish 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 Russell3 and by Ferrier.4

The cerebellum is not concerned with psychical functions. The removal
of it docs 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 anaesthesia 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 lo 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 "Xutiz betreffi des Rindenfeldes der Hinteretrangsbahnen," XeurologischesCrninilblatt, 1898,,
No. 4.

1 Archives italiennes df Biologic, 1891-92, xvi.

1 Philosophical Transactions of the Royal Society, 1894. * Brain, 1893, vol. xvi.

CENTRAL NERVOUS SYSTEM.

The existing symptoms arc 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 muscles ; an asthenia, or loss of
strength, which was measured by Luciani,and 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 contraction- of the
muscle-groups. The general expression of these symptom is a twisl 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-
sural 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 caudad 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 -how the cerebellum to be closely associated
with the proper contraction of the muscles, and this i> so directly connected
with the maintenance of equilibrium ' 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 TliE 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 possc-s potential energy. This energy varies
for any given element in accordance with :i 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," F.uu], anafomique, dinigue et physiologique, Paris, 1897.

2 Lee: Journal of Physiology, 1893, vol. sv. ; 1894, vol. wii.

Vol. II.— IS

■2:\

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 aervous 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

Til.. 117. — Showing tin- 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; 1, cerebellum, or hind-brain ; 5, bulb, or after-brain. Divisions 2, 3, and •'>. taken
r, 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, Si 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 <F ArUhropologie generate, 1885.

2 Halliburton : Journal of Physiology, 1894.

CENTRA L NER I T) I TS S ) ' V TEM.

275

The specific gravity of the entire encephalon is for the male, 1036.3, and
for the female, K).'5ii.

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 :

The Nomenclature of the Encephalon according to Weight.

(Topina rd.)

Classes. Males.

Microcephalic From 1925 to 1701

Large " 1700 to 1451

Medium " 1450 to 1251

Small " 1250 to 1001

Microcephalic " 1000 to 300

Weight in Grams.

Females.
From 1743 to 1501
" 1500 to 1351
" 1350 to 1151
" 1150 to 901
" 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 Sam-
Persons, the Records being arranged according to Sex, Age, and Stature
(from Marshall's tables based on Boyd's records).1

Males.

Ages.

a

o
%

A

«
a
W

a
P

u

£>
<a

O)

O

a

o
u
o
O

a
s

Stature 175 cm. and upward.

20-40

1409 1 1232 1 149

28

41-70

1363 1192 1 144

27

71-90

1330 1167 137
Stature 172-167 cm.

26

20-40

1360

1188 1 144

28

41-70

1335

11 6 1 144

27

71-90

1305

1135 142. s

28 as

Stature 164 cm. and under.

20-40

1331 1 1168

138

25

41-70

1297 1123

139 a

25

71-90

1251

1095

131

25

Females.

a

TO

Cerebellum.

i

2
2
3

H

O

"3

&

o

a
w

m

9

to

<

Stature 163 cm. and upward.

23

L34

1108 j 1265

20-40

23

131

1055 L209

11 To

24 a

130

1012 | L166

71-90

Mature 160-155 cm.

26 s

137 s I 1055 I 1218

20-40

26 a

131 L055 1 1212a

41-70

24

12S 969 a 1121
Stature 152 cm. and under.

71-90

24 s

L30 in 15 L199

20 to

25 a 8

129 1051.1 1205,,

41-70

25 <* 8

123

'.•7 1

1 1 22

71-90

1 a indicates that a record considered according to age is too large; s indicatesthata record
considered according to stature is too large.

276 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY.

The method of weighing the brain used by Dr. Boyd1 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. 1 18) 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 involutional1}' changes in the central system
arc 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 genera] relation of size according to sex among the mam-
malia, where the male as a rule has the greater body-weight and iargerskull,
it is to be anticipated that a similar difference in the weight of the brain will
be shown in other genera.

A i none; individuals of the same species, but of differenl 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 ti.-sues like muscle, and not with fat. As to the

1 Philosophical 'I'm tismi inns <>/ th< ll>\jnl S„rirhj, London, 1860; see also Marshall : Journal
of Anatomy and I'tnjxioloipj, ls'.t'i.

2 Keith: Journal of Anatomy and Physiology, 1895.

:; Imi Bois, in the Arch, fur Anthropol., Bd. xxw. maintains that among forms which may be
fairly compared, the formula E = S0'56, will give the weight of the encephalon, — E being the
encephalic weight and S the body-weight.

CENTRAL NERVOUS SYSTEM. -J 7,

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 leasts-
favored social classes, and hence it is not improbable that the figures id 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 li>t "t
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 exceptionally 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 indi-

1 Donaldson: The Orowth of the Brain, 1895.

278 AN AMERICAN TEXT- HOOK OF PHYSIOLOGY.

viduals usually examined in the Parisian hospitals. In the same manner, the
observations on the h rain-weight among the insane indicate, according to the
records of Boyd and others, that the insane as a class (the microcephalics
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 case- arc 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 : Mies3 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 i> 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,8 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, 1*69.

2 Donaldson: Growth of the Brain, 1895, p. 115.

3 Neurologischea Centralblatt, L893.

* Archivfur Anatomic »>"/ Physiologic, 1890.
Hirngewicht da Menschen, Bonn, 1^80.

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

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 iu both sexes this high percentage value of the central system is
mosi 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 bv 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 fetal 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 -hows 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.
I>irge ' 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 Phyxiologie, suppl., 1S82.

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

Man (based on Kaiser's records of the mean diameters).

The volume TOO,"3, in the fetus of four weeks, is taken from His, and the figures represent
multiples of that volume.

Subject.

Fetus

Child at birth . .
Boy at fifteen years
Man, adult . .

4 weeks
20 "
24 "
28 "
36 "

Proportional

volume of

the cell-bodies

1=70(V3.

n

17 I

31 \

67 |

81 j

124}

124 f

160

Time interval.

36 weeks.

15 years.
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 maybe 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. inns 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-' 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 lew observations on the increase in the number of

'Hardesty: Journal of Comparative Neurology, 1899, vol. ix.

2 Schiller: Comples rendus de VAcadtm.it des Sciences, I'uris. L889.

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, 10 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. IK". Diagram illustrating the extent of the cerebral cortex. The outer square i,* , shows a sur-
face approximately one-fiftieth of 2352 sq. cm. in extent ; the inner square (.1) has two-thirds of this area,
and is the proportion of the cortex sunken in the fissures. 2352 sq. cm. are approximately the areaof 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. 11!)) varies according to several conditions, but in general the more
voluminous the cerebral hemispheres the greater it- 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
Vulpius2 that the number of fibres in the different layers is greater in the

1 DU Functional der Oanglienzellen dea Haismarkes, Haag, 1891.
-' Vulpius: Archiv fur Psychiatric und Nervenkrankhcilen , 1892.

CENTRAL NERVOUS SYSTEM. 283

thirty-third year than at earlier periods, but 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 years.

In a similar way Kaes has shown 1 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. 17(i>.

Number of Cells. — Any attempt to determine the entire number of
nerve-cells, the bodies of which lie within the wall.- of the neural tube, must
be open to many sources of error.

LKaes: Wiener med. Woehenschrifl, 1895, NTos. 41,42; Kaes: Jahrbiichern der Hamburg,
Stoats JSsankenanstallen, J ahrgang, 1893-94, Bd. iv.
2 Passow: Archivfiir Psychiatrie, Bd. 31, 8. 859,860.
'Thompson: Journal of Comparative Anatomy, W'.i, vol, i\.: Donaldson: Ibid., No. '2.

284 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY.

A careful study of the cortex1 based on Hammarherg'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 Dumber in the entire central nervous system is probably
a conservative calculation.

From the foregoing tacts, 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.

C. Organization and Nutrition of the Central Nervous System.

What is here meant by organization maybe 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 powers of these tissues, which is mainly quantita-
tive, flie 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 bv 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.

\\ ith 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 clown, at first gradually, and later rapidly. It becomes important,
therefore, to examine the manner in which this organization 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-
lary 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 oilers. These cells are, lor 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 contribute-
to this complexity almost solely by the lbrniati< f dendrites, the collaterals

which come from the axones of these cells forming but an insignificanl 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 <>t' pathways both by
elaboration of the axones and the dendrites is alone possible.

Defective Development. — In view of these tacts, 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, 1 >ut in some measure a diminution in the strength
and range of those thai remain (Hammarberg *).

Laboratory Animals. — The 1 tearing of these facts on the conception
which we form of the nervous systems of those animals commonly employed
for laboratory experiment.- may be here mentioned. The frog, pigeon, rabbit,
cat, and dog form a series in which the total ma^s 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 IIuberJ 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-motor

1 Bammarberg: Studien ueber Klinik und Pathologie der f<liotie nebst Untersuchungen ueber
die normale Anatomie der Hirnrinde, CJpsala, 1895.

2 Journal of Compan ill ve Neurology, 1S99, 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 em. 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 lbntanelles are open
the brain normally pulsates, and we recognize in its variations in volume :ill
the different variations in blood-pressure with which we are familiar. The
pulsation of the brain is doubtless an important aid to the movements «>f the
fluids within, and hence tends to facilitate nutrition during the earlier Btages
of growth.

In pathological eases 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 Howell: American Journal of I'livsinh^n/, lS'JS, i. \(). 1.

1 Hill : Journal >>f Physiology, 1895, v<>l. xviii.

3 W. und Ed. Weber: Mechanikder memchlichen Qehwerkzeuge, 183G.

288 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY.

the >kull 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 effect, 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.

A.S 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 <>/ Oxygen and Carbonic Acid in virions Samples of Dogs' Blood

, c ... , . , . i <<>.,. . 37.64 percent

Average oi 52 arterial samples lO 18 25 "

a r aoi i i fC02. . 41.65 "

Average of 42 torcular samples lO 13 49 "

Average of 28 femoral vein samples - -' „'L.

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
activitv 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 <>f the cerebral circulation in the ease of those in whom the
skull-wall is at sonic point deficient shows a bulging of the skin over the

1 I>i> 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, occurto anything like such an
extent, and the space for the arterial blood must he 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 Kig. l'JO). (Sec ( Jeneral
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'-' found that if the
capacity for voluntary effort was measured by the amount of work which

1 Voit: Zeitechrift fiir Biologie, 1894, lid. xx.\.

2 Journal of Physiology, 1892, vol. xiii.
Vol.. tr. 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

- — ~\

lil. hill I I I I llllllll I I I

III I

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 ofthe flexor muscles by electricity. In the former then- are periods, in the latter
none. The arrow shows the direction in which the record is to be read (Lombard i.

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

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 ofthe forefinger before fatigue sets in. The curve is
highest at 10 to 11 a.m. and 10 to 11 p. m.; lowest, 8 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 i.

the immediate effect of taking food is to increase the work done by the sub-
ject. Alcohol lias 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.

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 arc decreased as
the result of the beginning fatigue. I low these conditions arc brought aboul
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 arc much less widely diffused in
early years than at maturity. Moreover, in childhood the amount of stored
material is small, large at maturity, ami small again in old age, and this holde

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 tissue- (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.

•J. Circulatory Influences. — The vaso-motor theories look upon the changes
in the blood-supply as :i prime cause of sleep; these changes to be referred
in the last instance to the fatigue of the vaso-motor centre in the bulb.

.'5. 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, anaesthetics, 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.

1

Mil.

.i"»i ,1'n1"1'

ii'

il,i"|lil.l|,|",'l,

"illininiii'lV"'
n

\
., "'

Fig. 123.— Plethysmography record taken from the arm of a person sleeping in the laboratory. A tail
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 a>leep
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 ease
it would not be possible to awaken the sleeper. The reactions most depressed
during sleep are those which require the lull 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 plethysmography 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 Experimental Medicine, 1897.

2 De Manaceine: "Sleep: Its Physiology, Pathology, Hygiene, and Psychology," Contem-
porary Science Series, London, 1897.

l".l 1

AN AMERICAN TEXT- BOOK OF PHYSIOLOGY.

withdrawal of blood from it, the blood supplied to the brain being probably
;ii the same time increased (see Fig. 123).

Tins 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
600
500
400

300

200

-A

<> i

100

Hours 0.5 1.0 1.5 20 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 63 7.0 7.5 7.8
Pig. 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 tie dropped increased during

the first hour, and then diminished, at first very rapidly, then slowly (Kolsehiitter).

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 fiir Biologic, 1893, Bd. xix.

CENTRAL NERVOUS SYSTEM. 295

thirds of the total time for rest and refreshment, vet 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 lias been found (Manace'ine) 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 tatty 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 l 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 virion (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 vita3 of
the human cerebellum decrease in size and number in old age.1

Changes in the Cerebellum. — Tn 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 youngesl 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
presenl 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 in some cases a granular degeneration ; the
fibres in part arc 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

' Engel: Wiener medicinisehe Woehensehrift, 18f>3.

2 Ketch. -r : Zeitechrififur Heilkunde, 1892; Redlich: Jahrbueh fur Psychiatric, 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 lnminiferons ether are enabled to aifeet 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 an Mind 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 shoulder-. The combined
eye and head movements are in mosl 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. Winn 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
lAchtxinn 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 tertia re-
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 advantage in reducing
the amount of muscular exertion required to put the eye into any given posi-
tion, and thus facilitating the recognition of slight differences of direction, for,
according to Fechner's psycho-physic law the smallest perceptible difference in
a sensation is proportionate to the total amount of the sensation. Hence if the
eye can be brought into a given position by a slight muscular effort, a change
from that position will be more easily perceived than if a powerful effort were
necessary.

Each of the eye-muscles, acting singly, tends to rotate the eye round an axis
which may be called the axis of rotation of that muscle. Now, none of the
muscles have axes of rotation King exactly in the equator of the eve — i. e.
in a plane passing through the centre of rotation perpendicular to the axis

300 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY.

of vision.1 Bui all movements of the eye from the primary position take place,
as we have seen, round an axis lying in this plane. Hence all such movements
must be produced by more than one muscle, and this circumstance also is prob-
ably of 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 each nerve-fibre supplies only two or three
muscle-fibres, while in the muscles of the limbs the ratio is as high as 1 to
40-125.2

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
anv 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,
through the nerves of muscular sense, the position into which the eye is nor-
mally brought in order to best receive the luminous rays, or, in other words,
tin; 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 effected it will be necessary to consider the refracting
media of the eve by means 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.

2 1'. Tergast : '' Leber das Verbiiltniss von Nerven unci Muskeln," Archiv fixr 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 AP B in Figure 125 represent in section such a spherical surface the

M'^ B

Fig. 125.— Diagram of simple optical system (after Foster).

centre of which is at N, 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
to the denser medium, is perpendicular to the spherical surface will be unchanged
in its direction — i. e. will undergo no refraction. Such rays are represented by
the lines OP, M I), and M' 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 ray8 have their
origin at a point infinitely distant on (he axis — i. e. if they are parallel to each
other — 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 coining from the opposite direction will be
brought to a focus. The position of these two principal foci may be deter-
mined bv the construction shown in Figure 126. Let CA C represent a sec-
tion of a spherical refracting surface with the axis A N, the nodal point N, and
the principal point .1. 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', A]/ proportionate to the rapidity of light in the two media
(e. ff. 2:3). The points where the lines p' o and po' prolonged will cut the
axis are the two principal foci Fand 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 1 27 let A N be the axis, and F and F' the principal foci of

f"\

Fig. 127. — Diagram to show method of finding conjugate foci.

the spherical refracting surface CA C, with a nodal poiut 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 rav will necessarily be refracted through the focus F, its course being
represented by the line C F and its prolongation. Similarly, the incident ray
passing through the focus F' and striking the spherical surface at C will, after
refraction, be parallel to the axis — i. e. it will have the direction C 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 b, 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 retracted 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;
lens, 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
of lens, 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.<> 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 betweeu 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 found 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 index 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 ;is a whole.

304

AX 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:

B : b — A : - : or r = — , in which 11 = the size of the object, b = the size of
2 jB

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 this
imaginary refracting surface is indicated by the dotted line p in Figure 128. The

Fig. 128— Diagram of the 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 tin.' 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 Bize and position of the retinal image of any external object —
e. g. the arrow in Figure 128 — it is only necessary to draw lines from various

1 Strictly speaking, there are in thia 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. Tin- distance is so small that the two points may, for all ordinary constructions, be
regarded as coincident.

aThe optical power of a lens is the reciprocal of its focal length. The dioptry or unit of
optical power is the power of a lens with a focal length of 1 meter.

THE SENSE OE VISION.

;;o:,

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
will 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 ;i 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 on 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 effected 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. The first 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 {Archivjvsr die gesammte Phygiologie, lviii. 523) that in fishes
the eyes when at rest are accommodated for neur objectB, and that accommodation for di.</<nit
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 conical
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
pei-son 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 tbe image reflected from
tbe posterior surface of tbe lens.

308

AX AMEIUCAX TEXT-HOOK 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' 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.

FlG. i:;i.— Diagram explaining the change in the position of the image reflected from the anterior surface
■ >f the crystalline lens (Williams, after Donders).

in the directions indicated by the dotted lines ending at a, b, 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 h to //, 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 b 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 lias
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 tin1 pressure of the capsule and the tension of the sus-
pensory ligaments (s, 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 (e, e, Fig. 133), most of whose fibres, having their origin at the

310

AN AMERICAN TEXT-BOOK OF PHYSIOLOGY.

Fig. 134— To illustrate

Scln ii-n's theory of ac-
commodation.

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 Tschcrning,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,2 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
offered 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
applied to the power said to be sometimes gradually
acquired by persons with astigmatic3 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 ' as the result of investigations into the hyperboloid
form of the lens in accommodation. On the other hand, it is maintained by
Priestley Smith5 that this form of the lens is not inconsistent with the Helm-
holtz theory. Moreover, it has been shown by Hess6 and Heine7 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 wastriped fibres of the ciliary
muscles. It is evident, therefore, that the term " involuntary " sometimes

1 Archives fie Physiologie, 1894, p. 40. 2 Archiv fur die gesammie Physiologie, lix. 427.

' flee |>. 317. * Archiv j'iir Ophthalmologic, xlii. (4) S. 36.

5 Ophthalmic Review, xvii. p. 341.

6 Archiv J'iir Ophthalmologic, 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 bv 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.

Radius 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 " 4.48 "

Distance from cornea to principal point 2.15 " 2.26 "

Distance from cornea to nodal point 7.16 " 6.74 "

Distance from cornea to anterior focus 12.918 " 11.241 "

Distance from cornea to posterior focus 22.231 " 20. 248 "

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 belter understood after the subject of

312 AN AMERICAN TEXT-BOOK OE 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 be
found that at the moment when the middle image of the letter becomes distinct
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 effort 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 then; 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
attected 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 i- important to discover this delict
where it exists and to apply the appropriate remedy — viz. convex lenses placed

Fig. 135.— Diagram showing the difference between

normal, myopic, aud 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. 'finis 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, C, 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: Archiv/ur Ophthalmologic, 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 role 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 maybe regarded :is 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. c 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. c. 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 an astronomical telescope) and sur-
rounded with borders of colored light. Now, the chromatic aberration of the
eve is so slight that it is not easily detected, 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 have 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 colored rays, and the achromatic
lens, thus constructed, became at once an essential part of every first-class opti-
cal instrument. Now, as there is not only no evidence that 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 tin- 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 V, while the red rays will be focussed behind the retina at R.

THE SENSE OE VISION.

317

On the retina itself will be formed not an accurate optical image of the point
A, but a small 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 ease the rays proceeding from a
single luminous point arc 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 minor 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 eves, 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 eve 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 diagrammatically 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 o 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.

Muscae 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 OE 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 m/uscce 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 l>\ the
absence of vessels from that portion of the macula lutea.

The retinal vessels may also be made visible in several other ways — e. </.,
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

v,„ TT — 21

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 held 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
offside 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

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. Kadiating 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
eye. The arrangement of the fibres

Course of constrictor nerve-fibres
" dilator "

Fig. 142. — Diagrammatic representation of the

nerves governing the pupil (after Foster) : U, optic

nerve: I. g, ciliary ganglion; r.b, i is Bbort rout from

III, motor-oculi nerve; sym, its sympathetic root; r.i,

in the optic commissure is in general its i.mg mot from r. o phthalmo-nasal branch of oph-

. , • i i • • n , thalmic division of fifth nerve; $.c, short ciliary

associated with the position of the m,n, f 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 arc usually

found to be distributed to both optic, tract-, while in animal- whose eves

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 com-

324 AN AMERICAN TENT-BOOK OE 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 pupil 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 far die gesammte Phys-
iologic, lviii. S. 155; also Sclienck : Ibid., Ixii. S. 19 L

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
mydriatics, 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 SJchultZ : Archiv fiir IVu/xiolorjie, 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
diagrammatical ly in Figure 143.

1 I.. 1 1:;.— 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'. 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 A, 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 Ophthalmologic, 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 aifected 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
cones.

Fig. 144.— Diagrammatic representation of the retina.

Figure 144 is intended to show, diagrammatically, 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 I. 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
luminiforous 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 images1 as
1 "Dimmer: Verb. d. phys. Clubs zu Wien, 24 April, 1894," Centralbl fiir Physiologic, 1894, 159.

32>

AX AMKIUCAN 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
A*Z Z TTTTZ ,,, , . ,. / the point of fixation of the eye, and

Fig. 146.— Form of the blind spot (Ilulmholtz). ' J '

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-
matical ly 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 C. 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)

i [i 147. — Diagrammatic representation of the
structure of the retina (Cajal): .i, layer of rods
and cones ; u, external nuclear layer ; ( . external
molecular (or plexiform) layer; /•.', Internal nu-
clear layer; F, Internal molecular (or plexiform)

layer: <!, layer of Kiinglion-eells ; //. layer or
nerve Qbres,

1 Die Retina der Wirbilthn srt , 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, vet 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. m.-optogram in eye Figure 148 shows the appearance of a rabbit's retina

of rabbit (Kiihne). . ° _ rr. , , , . ,

on which the optogram of a window lias been impressed.

Although the chemical changes in the visual purple under the influence of

light seem, at first sight, to afford 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 Sitzungaberichte, 1880, Bd. lxxxi. S. 3.

2 Kiihne : Untersuchungen a. d. phys. Inst. d. Universitat 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 stim-
ulus of 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 cone-, which, in their turn, act :i- 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 Kngelmann : Archiv fur die gesammte 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
anv 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
|P 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 illustrating the dispersion of light hjte gurface instead of forming a

by a prism. ' °

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 m icromilli meters a 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

1 < hie inicroniillinieter = 0.001 millimeter = one \u

'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 SEXSE 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," 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 cent re
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 :i prism, :i» 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 "//
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
King 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. Bv 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
trout 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,
ami are therefore united upon the retina.

3. Bv 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 /'

it has ceased to act — a phenomenon of /

great importance in physiological optics, .' \

and one which will be further discussed /

U \

/ \

in connection with the subject of " after- / \

/ \

images." A/ \B

The physiological mixing of Colors Fig. 151 — Diagram to illustrate color mixture by
,i |« v. j -i .1 • , reflected and transmitted light (Helmholtz).

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 appeal's
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 — <':iu
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 botli 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 modem theories of color-vision, see the address of Professor
Frank P. Whitman on "Color-vision," Science, Sept. 9, 1898.

336 AN AMERICAN TEXT- BOOK OE PHYSIOLOGY.

— 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- Hemholtz theory introduced by these sub-
sidiary 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 ether
vibrations, but each of these substances is supposed to be affected in two oppo-
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
sensation of blackness, which, on Hering's hypothesis, must correspond to active
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. L.
Franklin. In this theory it is supposed that, in its earlier periods of de-
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

1 The renewal of the rod pigment in ;i dim light may afford an explanation of tins 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 ?t 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.
Midler,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.

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-

1 7a itschriftfiir Psyehologie und Physiologie 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 way 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 luminiferons 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 Sinnesorganc, 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 TENT-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. 'Phis 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 SENSE 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

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degree of illumination (Konig).

our eyes in such a way that the image falls upon the fovea centralis of the
retina. The luminosity of a faint object, however, seems greatest when we
look not directly at it, but a little to one side 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 Kainon yCajal's1 recent observations, that the rods
are a comparatively rudimentary form of visual apparatus, taking cognizance

1 Zeitschriftfiir Psychologic und Physiologic dcr Sinnesorganr, xvi. S. 161.

342 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY.

of the existence of light with special reference to its varying intensity,
and that the cones arc organs specially modified for the localization of
stimuli ami 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. c. 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-ealled " 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 leas by all
the rays of light. Thus when rays of red light fall upon the retina they will

1 Zeiischrift Jur Psychologic wnd Physiologic der Siwiesorgane, ix. 81.

2 von Kries: CentralblaitfUr Physiologie, 1896, i.

THE SENSE OF VISION. 343

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 eifect 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 eifect
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 1) 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

154, A) is rotated before the eyes. The white hand as its image moves out-
ward or inward over the retinal surface appears bordered witli 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." x 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
ro 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 Physiofogie, 1892, pp. 541, ''>•_!'.) ; and 1886, p. 677.

THE SENSE OE VISION. 345

minated 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 Era. m-Msk to illustrate the persistence

. . ... . ... of retinal sensation (Helmholtz).

01 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 tin- present state of our knowledge, dis-
criminate between retinal fatigue and exhaustion of tin- 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 vellow, 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 eve 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 in/ens,' 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 l.y tl bjecl 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 he
thus stated : Every pari of the held 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 shadows 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- BO OK OF PHYSIOLOGY.

ous 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. 108.-T0 illustrate the over-esti- than will be demanded of the stronger muscles

mation of vertical lines. , in i

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 Scit nces, 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-

than its breadth. tion of verticai 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 eve can be accommo-
dated for the distance at which the objects arc 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

300

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

Pig. L62.— 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. 1 62) seems longer than that from
B to C, 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,

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 ^ j *

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, arc projected onto our retinas
as acute or obtuse angles. Knowing these angles ton.

. . . i i • ,i Fig. 164.— To illustrate illusion

be right angles, we are constantly applying mental of Bpace-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 fur Psychologie und Physiologic drr Sinncsorgane, 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.

Fig. 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-perceptiim (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 are 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 (Muller-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 figure's.

Fig. 169.— To illustrate so-called "confluxion" in space-perception (Muller-Lyer).

In some cases, however, an influence of the opposite sort1 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 16S), 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 (Muller-Lyer).

Lines meeting at an angle appear longer when the included angle is large
than when it is small, as is shown in Figure 17<>. 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 " lias been pro posed by Miiller Lyer, from whose
article in the Archivfiir Phygiologie, 1889, Sup. Bd., the above examples arc 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 Jhe horizontal line in the same way as the outside lines in Figure 168
influence our judgment of the length of the line between them.

Fig. 171— Illusion of space-perception.

Eintlioven1 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 h. 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 Ave 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 niie 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 thi' 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 log, are
judged to be larger, because we suppose them to be farther off than they

really are. The familiar fact that the n n seems to be larger when near the

horizon than when near the zenith is also an illustration of this form of illu-

1 Pfliiger's Archir, lxxi. S. 1.

Pig. 172.— Illustrating Einthoven'a explanation of
space illusions through indistinct vision.

THE SENSE OF VISION. 355

sion. When the moon is high above our heads we have do means 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 —
e. g. 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 eve,
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 (he eve, 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 he placed at a distance of about two meters and one
eye closed. liven 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 ai<l 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 object- 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 increased3 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 i- 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-

Fi'.. IT:1..— To illustrate stereoscopic vision.

lv 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 eve 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 related to each other as those shown in this experiment.

Stereoscopic slides often serve well to illustrate the superiority of binocular
over 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 cither
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.

Peeudoscopic Vision. — If the pictures of an ordinary stereoscopic slide be
reversed, so that the picture belonging in front of the right eve is presented to
the left eye, and viceversd, the stereoscopic gives place to what is called a pseudo-
scopic effect — i. 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 bv simply converging the visual axes so that the right eye looks at the
left and the lefl 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 objeel 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
dill, rent outlines of a geometrical solid, one in black upon white ground and
the other in white upon black ground, their combination in the stereoscope
wjH 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-
liant K' illuminated to one eve and dark to the other. Hence the stereoscopic
combination of black and white is interpreted as indicating a polished surface,
since it i< by means of a polished surface that this effect is usually produced.

Corresponding 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 ringer. In both eases 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 thefovexe centrales
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 foveoB 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 foveas
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 LS 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 17b' 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 tingle of about 20° with thai of the paper, the dis-
tance of the lines from the eyes being such thai cadi 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 eves.
and the combination of these images results in the perceptiou of a third line,
not lying in the plane of the paper, but apparently passing through ii 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 thai the illusion

360

AN AMERICAN TEXT-BOOK OF PHYSIOLOGY.

of a line placed vertically to the plane of the paper does not entirely dis-
appear when one eve 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
figure, the short lines which lie nearly in a plane with the
visual axis 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.

Fig. 176.— Binocu-
lar illusion of a ver-
tical line.

THE SENSE OE VISION.

361

Fig. 178.-

Illusion of lines always pointing
toward observer.

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
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.

:k;l'

AN AMERICAN TEXT-BOOK OF PHYSIOLOGY

B. The Ear and Hearing.

Anatomy and Histology of the Ear. — The organ of hearing may con-
venient Iv 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 ('(implex form which receives the peripheral terminations of the
auditory nerve. This sac, therefore, is t<> 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 lefl side (Quain, after Arnold): l. the pinna; 2. bottom of
concha ; 2-2', meatus externus; 3, tympanum ; above 8, the chain of ossicli - : •'•'. opening into the mastoid
cells; i, Eustachian tube; 5, meatus tnternus, containing the facial (uppermost) and auditory nerves;

ced "ii the vestibule of the labyrinth above the fenestra ovalis; a, apex of the petrous I e; b,

Internal carotid artery; c, Btyloid process; d, facial nerve, issuing from the stylo-mastoid foramen; e,
mastoid process ; /, squamous pari of the bone.

tiallv <it' yellow elastic cartilage covered with skin, and forming at the entrance
of the auditory meatus a cup-shaped depression called the "concha."

The <-<ni<-hay 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 ; lint in many of the lower animals the concha is relatively larger than
in man, and. their car- 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 HEARING.

363

%,

Yd

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
canal-wall is cartilaginous and
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
ceruminons 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

cells. 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

ovalis, 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. 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.

Fig. 181. Otoscopic view of left lmnihrana
tympani (Morris): 1, membrana flaccida; 2,2',
folds bounding the former ; 3, reflection from
processus brevis of malleus ; i. processus lon-
gus ni' incus (occasionally seen); i, mem-
brana tympani; 6, umbo and end of manu-
brium ; 7, pyramid <>f Light.

36-4

AN AMERICAN TEXT-BOOK OF I'HYSIOLOGY

membrane and the ring of hone 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, eoncave 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, i- 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
5 is longer than its roof. The
membrana tympani, though
8 so thin as to be semi-trans-
parent, is composed of three
layers of tissue. Externally
it is covered by a thin plate
12 of skin ; internally, by mu-
ll cons membrane ; a nd between
these lies the proper sub-

\P*~y£

Fig. 182.*— Tympanum of righl Bide with ossicles in place, viewed
in. in u itlun (after Morris) : I, body of incus ; 2, suspensory ligament
of malleus; :;, ligament of Incus; i, head of malleus; .".. epitym-
panic cavity; 6, chorda tympani nerve; 7, tendon ol tensor tympani

muscle; 8, fool pie< f stirrup; 9, ■ ■ ,• 10, manubrium;

U, tensor tympani muscle; 12, membrana tympani; i"., Eustachian
tuiir.

Fig. 188.— The chain of auditory
OBsicles, anterior view (after TeB-
tut) : l, head of malleus; 'J, long
process of incus; 3, stapes.

stance (membrana propia) 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 Burface of the tympanic membrane is not flat, but is
funnel-shaped, with the apex of the funnel pointing inward. Moreover lines
JMacewen: Pyogenic Diseases of the Brain ami Spinal ('mil, l.S'.t:;.
Figs. 180, L81,and 1 82 are taken by permission from Morris's Text-Book of Anatomy, Phila., 1893.

TEE 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 flaccida, 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. 1K0, 182, 183).
The malleus (Fig. 184) is 18 to 19 milli-
meters long; it presents a rounded head,

° ' . . Fig. 184.— Maueus of the right side : a, anterior

grooved on one side for articulation with face; B.internal face (after Testat): l, capita-
,i i i i i I ii him or head of malleus; 2, cervix or neck; 8.
the incus, a short neck, and a long handle procesem8 ,,n.vis: , pr !88tM gracilis; 5, manu'

or manubrium, which is inserted in the brium; 6, grooved articular surface for incus ;
. „ . . . .. 7, tendon of m. tensor tympani.

tissue of the tympanic membrane from

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 gracilis, or pro-

;;.;.;

AN AMERICAN TEXT-BOOK OF fUYSfOLOGY

cessus Folia nits, 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-
ti hi 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 gracilis ; 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.
The ligaments of the malleus serve to
keep its head in position. The exter-
nal ligament, being attached above the
axis of rotation of the hammer, pre-
vents the head of this bone from
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, umbos, 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. 185.— ligaments of the ossicles and their axis
of rotation (from Foster, after Bensen). The figure
represents a nearly horizontal section of tin- tym-
panum, carried through tin- heads of the malleus
and Incus: M, malleus; I, incus ;t, articular tooth
of incus: lii.n and lg.e, external ligament of mal-
leus : Ig.inc, ligament of the incus ; the line a-x rep-
resents the axis of rotation of the two ossicles.

Fig. 1S6.— The incus of the right side : \, anterior face; B,
Internal face (after Testut): 1. body of incus; 2, processus
brevis; :;, processus longus ; 1, articular rurface for the mal-
leus; 5, a convex tubercle, processus lenticularis, fur articu-
lation with stapes; i',, rough surface for attachment of the
ligament of the incus.

FlG. 187.— The stapes (after Testut): 1,
base; 2, anterior cms; S, posterior cms;
I, articulating surface of head of the
bone ; 5, cervix or neck.

each other; the former has a length of A\ millimeters, and the latter a length
of 3 to •'>.', millimeters. When in position the long process descends nearly
parallel with the manubrium, but it has less than three-fourths the length of
the latter. The i'veo end of the long process is turned sharply inward at right
angles, and terminates in a round projection, the 08 orbiculare, which is provided
with cartilage for articulation with the head of the stapes. The short process is

THE SENSE OF HEARING. 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 1| 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 Helraholtz1 says:
u 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 clastic
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 theshort 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 j 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 st;ipes 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 thai 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 arc directly
1 Sensations of Tone, trans, by Kllis, ISS'i, p. l.'W.

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 ,'s millimeter to y^ millimeter, while
that of the centre of the tympanic membrane is about -^ 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 perilymph 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 Helmholtz2 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-wave- 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 it- 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 edy;e 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 arc broughl
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 said
that the relaxed tympanic membrane, particularly alter 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 aerial impulses and
to (he periodic waves of musical notes. When the membrane is tense its
vibrations are damped, and it is particularly lilted 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 lie to decrease
the acuteness of hearing. According to Hensen,1 the tensor tympani muscle

1 Hermann's Handbuch der Physiologic, 1880.

Vol. 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 do 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 Archiv fur Anatomic a, a! 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 bv
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 pressed 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 (PI. 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 oue-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

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 Sdmmering): 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 trails (from
Quain, after Sdmmering): I, '-', ::, the superior, pos-
terior, and horizontal semicircular canals; i, fovea
hemi-elliptica ; 5, fovea hemispherical 6, common
opening of the superior and posterior semicircular
canals ; 7, opening of the aqueduct of the vestibule ;
8, opening of the aqueduct <>f the cochlea; '.'. the
scala vestihuli ; lo.seala 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'* Hindi, , ush ,/,,- Physiologie, 1880, Bd. iii. Tli. 'J, S. 26.

2 Seashore: "Studies in Psychology," Bulletin University of Iowa, 1899.

:;7i'

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
externa] 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-
putta. The anterior and posterior canals
unite near the ends not provided with am-
pulla1, and they enter the vestibule as a com-
mon tube. Anteriorly the cavitv of the

— 4 vestibule is continued as a tube of complex
internal structure which is coiled upon itself

l'i'.. 1'."'.- I tiiiLT.-iin lift he osseous cochlea _ _ l

laid open (after Quain) : I, scala vestibuli ; two and one-half times, and which, from its

LMamina spiralis. 3 scala tympani ; 4, cen- regemblance to the ghell of a snai] jg kncnyn
tral pillar or modiolus. '

as the cochlea (PI. 1, Fig. 3). The osseous
cochlea may be conceived as formed by a bony tube turned about a bony central
pillar, the modiolus, 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

—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-
lcus, with 7, 7', its twigs of origin : 8, Baccus endolymphaticus : 9, canalis cochlearis, with 9', its vestibular
cul-de-sac, and 9", its blind extremity ; 10, canalis reunions.

soon learn to know as the scala vestibuU, and the lower chamber as the scala
tympani ( Fig. 1 J»0; 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 cavitv 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 (PI. 1, Fig. 4 ; Fig. 11*1 ). 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

Explanation of Plate 1.

Fig. 1.— Schematic representation of displacement of the auditory ossicles due to contraction of the
tensor tympani muscle (Testut): a, external auditory meatus; 6, tympanic cavity; c, vestibule of the
bony labyrinth ; </, fenestra ovalis ; 1, membrana tympani : 2, handle of malleus; 3, head of malleus : 4,
insertion of tendon of tensor t\ mpani ; 5, Long or vertical process of incus ; 6, head oi incus; 7, stapes.
(The arrow indicates the direction of traction of the tensor tympani muscle; and the lines in red indi-
cate the change in the position of the parts produced by it.

Fig. 'J.— Schematic representation of the displacement of the stapes due to contraction of the stape-
dius muscle (Testut): A, the stapes in repose.; B, stapes during contraction of stapedius muscle; l. base
ipes : 2, anterior border of fenestra ovalis ; 3, the pyramid ; l, tendon of stapedius muscle ; a, anterior
portion of annular ligament of stapes, longer than b, posterior portion of same ligament; z,x, antero-
posterior diameter of fenestra ovalis, passing through the base of the resting stapes ; y, point of passage
of the vertical line w Inch represents the axis of rotation of the stapes.

Fig. 3.— The three parts making up the bony cochlea (schematic, from Testut) : A, the columella : B,
spiral tube containing the scala-; C, lamina spiralis; /', the three parts in their normal relations.

Fig. 4. — Schematic representation of the perilymphatic and endolymphatic spaces. The former
appear in black, and the latter are colored blue (Testut) : 1. utricle; 2, saccule; :'., semicircular canal : 4,
canalis cochlearis; .r>, ductus endolymphaticus with its two branches of origin ; 6, saceus endolymph-
aticus ; 7, canalis reuniens, or canal of Hensen ; 8, scala tympani . <», scala vestibuli; 10, their communi-
cation at the helicotrema ; 11, aqua-ductus vestibuli ; 12, aqua ductus cochlearis : 13, periosteum ; 14, dura
mater; 15, stapes in the fenestra ovalis ; Ifi, fenestra rotunda w itli its membrane.

THE SENSE OF HEARING.

Plate 1.

THE SENSE OF HEARING. 373

perilymph space, which is lined by lymphatic epithelium, is in communication,
along the sheath of the auditory nerve, with the subdural 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 oval is (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 endolymphaticus, a tube running inward through a
canal in the petrosal bone and ending blindly in a dilated flattened extremity,
the saccus endolymphaticus, this being supported between the layers of the
dura mater within the cavity of the skull (PI. 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 ampulla?, 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 cocJdearis. 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 auditorius internus, 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
are limited to a certain part of the ampulla of each canal. The tissue of the
w;ill of the ampulla is developed into a ridge projecting into the cavity in a
direction across its long axis. Tins 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 ;it 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

AX .\Mi:ni(AX TEXT-HOOK OF PHYSIOLOGY.

Pig. 192.— Diagram showing the epithelial cellsof
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-culls.

shape and reaching only pan way to the basement membrane, the hair-cells;
the other, aarrow and elongated, the supporting or mxientaeular cells. The
former are peculiar in the fact that from their free ends there project long,
-till*, hair-like processes. The filaments of the ampullary-nerve branches
puss through the crista and encircle the bodies of the hair-cells. The cells

/covering the maculae acustica have
/ essentially the same structure as those

just described, though in the maculae
the auditory hair- are shorter than in
the eristic. 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 crista'
in post-mortem section, a coagulum
due to the method of preparation.
Imbedded in the membrane over the
maculae of both saccule and utricle
are small crystals, otoliths or oto-
conia, 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. 188-191). 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
partlv 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 i- called the basilar membrane, iii<ni!>r<tii<t hasilaris.
The coiled tube forming the bony cochlea is thus divided by the lamina spiralis
and the <-<ni<ifis cochlearis into three tubes which wind spirally and parallel
round the modiolus. The canalis cochlearis contain- endolymph, and its cav-
itv 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 OE 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
cochlea, but, where this part bulges into the tympanum as the "promontory"
of its inner wall, it is perforated by the aperture known as the fenestra
rotunda, whose proper membrane alone prevents the perilymph from escaping

Pig. 198.— Diagram of a transverse section of a whorl of the cochlea (after Foster): Sc. V, scala vestib-
uli; Sc.T, scala tympani; C.Cld, canalis cochlearis; Lam.sp, Lamina spiralis; Og.sp, ganglion spirale;
it. aud, auditory nerve; m.R, membrane of Eteissner; Str.v, stria vascularis; Lg.sp, ligamentum Bpirale;
t.l, lymphatic epithelioid lining of basilar membrane on the tympanic side: m.b, basilar membrane;
Org.C, organ of Corti ; L.t, Labium tympanicum; lb, 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 scalamedia. 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 seel ion from base to
apex until near the top; hence the width of the basilar membrane and the

376 AN AMERICAS TEXT-BOOK OF PHYSIOLOGY.

length of its radial fibres increase from below upward. The seala vestibuli
and the scala tympani have qo communication excepl through a small aperture
under the cupola of the cochlea, known as the helicotrema; this is bounded
by the hook-like termination, the hamulus, of the bony lamina spiralis, which
forms the greater part of a ring completed by the pointed blind extremity of
the canalis cochlearis fastened above it t<> 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 seala vestibuli to its top, through the helicotrema,
and back by way of the seala 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. I) 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 peri lymph -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 saccnle in turn is
continuous with the seala media of the cochlea by way of the canalis reuniens.

The Membranous Cochlea and the Organ of Corti ( Figs. 193-li)o).—
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 <'<>r/i 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 gang/ion spirale. The

THE SENSE OF HEARING. 377

thin, free edge of the bony lamina spiralis is, in the recent state, thickened bv
a development of connective tissue forming a promontory known as the limbics.
The free edge of the limbics 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 vestibvlarei serves for the attachment of the tectorial membrane,
membrana tedoria, 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 limbns 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 Cord (Fig. 195, 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. 19 1), much like those described as seated on
the maculae and crista? of the labyriuth, to which hair-cells filaments of the
auditory nerve are distributed. Closely applied to the .-ingle 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 jusi 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 t<» a longitudinal or spiral one. These
fibres, and others having a more direci 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 OUtermosI 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 heighl externally into a layer of cuboidal epithelium covering the outer
part of the basilar membrane. The rods of < lorti arc 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-
la/ngar 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 (inembrana
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-

1,1 j

n.aud l.l i.spn t.xjiit o.spn c i>

lg.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.fte, inner hair-cell ; v.c, the group of nuclei beneath it; o.hc, outer hair-cells, or cells of
Corti; C.I), the twin cells of Deiters (four rows) ; n.aud, the auditory nerve perforating the tympanic lip,
l.l. and lost to view among the nuclei beneath the inner hair-cells: i.spn, the inner spiral strand <>f nerve-
fibrils; t.spn, the spiral strand of the tunnel: o.spn, the outer spiral strand belonging to the tirst 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^ //.<■, Etensen's cells; Cl.c, Claudius' cells;
t.i. lymphatic epithelioid lining on the side toward the Bcala tympani : lg.sp, Ugamentum 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 <>f 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 contacl of the cells of Corti with
those of Deiters is so intimat< — 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 Hat cells, and stretches from the
limbus of the spiral lamina outward and upward to the side wall of the

THE SENSE OF HEARING.

379

coclilea. It is known as the membrane of Reissner. The periosteal con-
nective tissne 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-

..o.r.h

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; pA, in each, is the phalangar pro-
cess; c, the twin outer hair-cell ; Cc, the cell of Corti; h, its auditory hairs; n, its nucleus; x, Ilensen's
body; D.c, cell of Deiters ; n' , its nucleus; ph.p, its phalangar process;/?/, 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-
ecu- <>f the inner rod ; o.r.li, the head of the outer rod, with ph.p, its phalangar process, covered to t!
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; pli, one of the phalangse of tin' 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 cuboidaJ epithelium lines the con-
nective tissue forming the outer wall of the canal is cochlearis. This epithe-
lium is peculiar in that the blood-vessels of the underlying connective tissue
penetrate between the epithelial cells themselves. The tectorial membrane
(membrana tectoria) is a sheet of radially-fibrillated tissue, thin at its poinl 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 tin- 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 ( 'orti.

380 AN A 31 ERICA N TEXT-BOOK OF PHYSIOLOGY.

The researches <»t' Howard Avers' have led him to conclusions concerning
the minute anatomy of the ear materially different from those just presented.
Thus, Avers 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
Avers, 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 times and probably of noises as well. But no trustworthy
conclusion can be maintained as to the precise mode of action of the auditory
apparatus, due fact that the rods of Corti are absent from the cochlea? 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, Helmholtz2 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
he 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. 0.3 1 milli-
meter, ami 0.36 millimeter. The waves of physical sound are thus supposed
to he 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.

'Avers: Journal of Morphology, May, 1892.

2 Helmholtz : Toiienipjiii(/iut(/i-ii, 1*77, S. 210.

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 torn.

Loudness and Musical Pitch. — The more vigorous the vibrations of the
oscillating body, the more forcibly arc 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 doe- this on
the amplitude of air-motion. Differed 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 thai the apparent pitch
of a tone is raised when its Intensity is lowered, and that such an elevation
of pitch may equal one-tilth 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 hotly. When
1 Broca : Jahresiberiehi der Physiologie, L897, 8. 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 iu 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'"") 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 sav, 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 indefinitelv
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-ncss," 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. Krics und Auerbach: D>i Bois-Rey-
moncTs Archiv fur Physiologie, 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 less 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' 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 overtones, 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 \\\>v together into
a single series of waves of peculiar form. Such a composite wave may be
1 W. .lames: 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. W 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. 19G). 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

B e

Fro. 196.— The curve B represents twice the vibration-rate of a. When the two curves are combined
by the algebraic addition of their ordinate*, 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
din 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 t lie 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 Ear. — 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 Belmholtz, op. ciL, pp. :!0-34.

THE SENSE OF HEARING. ■>■>

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 bodv 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 Veber die Grenzen der Tbnwahrnehmung, June, 1876.
Vol. II.— 25

386 s\X AMERKWX TEXT- HOOK OF PHYSIOLOGY.

Inharmonic Overtones. — It will ho 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 >tuck 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 eve. When sufficiently Dear 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 <■'" 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 4 5 6 8

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.

RisurnS. — 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 Dote 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-

'Tyndall: Sound. 'Waller: Human Physiology, 1891.

3 Foster : Text-book of FVn/siology, 5th etl., 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 mens 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 crista? 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 Lonfrworth) : a, ramification of nerve-
fibres in the mucous membrane, and their termination in end-bulbs, as Been 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 whirl)
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 ot

1 Seashore: hoc cit.

390

AX 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 tor 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 sensation- of touch, 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) : 71, n, two nerve-fibres pass-
ing to the corpuscle; a, a, ter-
minal varicose ramifications of
the axis-cylinder within the cor-
puscle.

Pig. 191 hematic figure of a neuromuscular spindle of the first type, namely, with complex

nerve-ending ; adult cat : '•.. capsule ; m. u.h.. motor nerve-bundle ; pi. < -. plate-ending; n. >>-.. nerve-trunk:
,,r. < .. primary ending; s. e., secondary ending ; b. to., axial muscle-fibres. 1 From Rufflni, Journal of l'i<<i*i-
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

CUT 1 XEO f TS A XI) M ( 'S< TLA II SKXS. I TIOXS.

391

the object to that of the whole field of vision ; but as objects of different
size may fill the same visual augle 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
surface in a body become more obscure as we recede from that body. But
more important data concerning distance come from the sense of muscular
innervation, 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. I n
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. Tn 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 thai 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 freelyamong 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): », stalk with nerve-
fibre enclosed in sheath of Henle,
passing to the corpuscle; »', its
continuation through the coil, m,
as a pale fibre ; a, termination of
the nerve in the distal end of the
core (the terminations are not
always arborescent); <l. 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 eve-
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 palm of the hand and the sole of the foot ; they are also numerous along
the nerves of the joints, and even among the sympathetic nerves supplying
the 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 Cf. Barker : The Nervous System, 1899, pp. 361-421.

2 Aubert und Kammler : Moleschotfs Untersvchungen, 1809, Bd. v. S. 145.

THE SENSE OF PRESSURE. :VX\

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: The 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 : The 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 ^ to ^ of the weight estimated, while the skin of the head, the fingers,
and the forearm requires an increase of ^V to -^ 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. Weber1 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 onlv 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 l»y
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 l>e 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 Gemeingefiihl," Wagners 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- HOOK OF PHYSIOLOGY.

but one warm ami 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 tins weight has been applied.
In fact, this after-sensation may produce a striking effect on consciousness,
a familiar example of which is the persistence of the sense of pressure of
the hat-band alter 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 -§\-$ 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.

Tlie 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 musl 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 -till 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.

- Funke, in Hermann's Handbuch der Physiologic, Bd. iii. 2, S. 404.

THE SENSE OF PRESSURE. 395

skin can thus be marked cut in areas (tactile areas), 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 figures1 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, bul to a more perfeel discrimination of sen-
sations in the nerve-centres. Dressier" found thai after practice tor four
weeks, the compass-points. which at die beginning had to be separated is
millimeters on the skin of the forearm to be distinguished, could, at the end of
the period, !»e recognized as two \\ hen only aboul I millimeters apart. A Imosl
as great an improvement of localizing power was gained by the unexercised

1 Foster's Physiology, ">tli ed., 1891.

2 Weber: " Tastsinn und Gemeingefuhl," Wagner's Hamdworterbueh (!>;■ Physiologic, 1846.

:1 Dressier : Loc. tit.

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-point*, 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 SENSE OF TEMPERATURE. 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 outer world. Such temperature differences prob-
ably make themselves manifest by raising or lowering the temperature of the
skin itself, and thus in someway 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, 1.20.1

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 : Deutsehes Archivfur klinische Medicin, L866, ii. S. 284.

398

AN AMERICAS TEXT-BOOK OF I'll Ysioijx ; V

Fig. 201.— Cutaneous " cold " spots
(vertical shading) and "hot" spots
(horizontal shading), anterior sur-
face uf the thigh (from Waller, after
Goldscheider).

Cold ami Warm Points. — The -kin 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
sin face. 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 surrouuding 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 cold, 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-
trnnk 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-ReymoncFa Archiv fur Physiologie, 1886, 1887 ; Blix: Zeitxchrift far

:ie, 1884; Donaldson: Mind, 1885, vol. xxxix.

2 Czermak : Sitzwngsbt richtt </. Wiener Akad., 1855,8.500; King: Arb. d. physiol. Anstcdi zu
Leipzig, 1876, S. 168.

THE 8EN8E OF PAIN. 399

the baud 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 thai position,
indicating the possession of a posture sense. The nerves of common sensation
must, then, be continuously active in carrying to the sensor ium 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 fell 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 TENT-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 arc 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 Jahresbemht ;;. Physiologie, 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 close 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 Meltzer2 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
naturallv 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 «>)' 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.

* S. J. Meltzer: Philadelphia Medical Journal, August 5, 1899, p. 12. 3 Brain, 1881.

Vol. II.— 2G

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
skeletal muscles. Ciaccio1 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 - 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
envelope.- 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,' " 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 thegenio-
glossus muscle. Sherrington ' 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 eve
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 striatum
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 bei es 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 otheris tendinous.

According to Rufrini/' sensory nerves may end upon the axial muscle-

1 Barker: The Nervous System, 1899, |>. 105 - Ibid.

3 Sherrington : Journal of Physiology, L895, vol. xvii. p. 229. 4 Ibid.

5 Ruffini : Ibid., 1898, sxiii. 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 nj> into a number of leaflets
applied to the muscle-fibre (secondary mode). :]. 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-spindle-
are numerous, the knee-jerk could no longer be excited through the muscle.

Our appreciation of the weight of bodies is determined by lilting 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."

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. Funke3 refers

'Seashore: Op. rit. 'Wolfe: Paycholoc/iml Rivim; ISDN, p. 25.

8 " Der Tastsinn," Hermann's Handbuch der Phyaiologie, Bd iii. 8. 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 Thirst. — 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." '

E. The Equilibrium of the Body; the Function of the
Semicircular Canals.

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 equUibrivm, 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 eves 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-
1 Ewald : Disease a 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 benl 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 eves, 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 I'll VSIOLOGY.

of a chicken whose head lias 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 l>c<n explained by the assumption that the hair-cells on
the crista <icnxtic<r. of the ampulla 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 i- 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 ainpullse 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
1 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 ».>• the semicircu- un(]er observation feels a sensation of motion

lar canals (after Waller): s, superior p

canal ; p, posterior canal ; h, horizontal in the opposite direction. Each of tliese re-
canaL suits 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 forced 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 Bewail: 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 eare, with sharp scis-
sors, so as to avoid traction on or crushing of the nerves, such forced move-
ments do not follow.

Lee1 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 ampulla? 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. Rawitz2 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 maoulce
acustlcce. 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. lint 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. :;il ; xvi. p. 192.

2 Rawitz : Zoologiseher Jahresbericht, 1896.

8 Journal of Physiology, xv. p. 31 1, xvi. p. 192.

408

AN AMERICA X TEXT-BOOK OE PHYSIOLOGY.

F. 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

Fn.i. 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-
lilpril; /. uucleua; c, knob-like clear termination
of peripheral process ; //, 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 SEWSE OF .SMELL.

409

appears to be not ciliated except near its juncture with the Sclineiderian
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

Fig. 205— Diagram of the connections of cells and fibres in the olfactory bulb (Schiifer, in Quoin's Avat-
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 glomeruli, containing
arborization of the olfactory nerve-fibres and of the dendrons of the mitral cells; ro.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; «', 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 common 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 thai 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 differenl 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 Sodete de Biologic, 1892, p. 84.

410 AN AMERICAN TEX1-BOOK 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 taster-buds (Fig. 206), seated in certain papillae 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 papillae, 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. Vintechgau : " Geruchsinn," Hermann's Handbnch der Physiologie, iii. 2, 1880.
2Cameror: Zeiischrifl fur Biologie, 1870, vi. 8. 440; Wilczynsky : Hofmann und Schwalbe's
Jahresbericht der Physiol., 1875.

THE SENSE OF TASTE.

411

WHSSmm

■%0mmmmtm

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 ; in,
leucocyte containing granules; e, superficial epi-
thelial cells ; a, nerve-fibres.

gustatory fibres arise from the brain as part of the glossopharyngeal nerve,
whatever may be their subsequent course to the tongue. On the contrary,
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 eves 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. Weber3 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.
'Shore: Journal of Physiology, 1892, vol. xiii. p. 191.
3 Archivfiir Anaiomie wnd Physiologie, is 17, S. 'M2.

U2

AN AMERICAN TEXT-BOOK OF I'll YSIOIJ Hi Y

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 11 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).

A uditory.

Gustatory.

Tactile.

PIG. 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 Mini 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. Vintschgau, 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 der Physiohgw, 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-benzoic 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 papillae.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 papillae 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 Mere 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 papillae 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. Miillcr). 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.

2 Skandinavisches Archil) fitr Physwlogie, 1890, vol. ii. S. 1.

3 Briicke: Vorlemngen iiber Physiologie, 1876.

IV. PHYSIOLOGY OF SPECIAL MUSCULAR
MECHANISMS.

A. The Action op Locomotor 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-
bra' and the sacro-iliac and pubic bones are closely bound together by disks
of fibro-cartilage. This material, which is very strong, but yielding and
elastic, acts as a butler 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 vertebras 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
-.iiiii' time that they bind the bones together, admit of a certain amount of

1 Kick : (.'oiiipfiiiliiim tier Phygiologie drs Menxchen, "Wien, 1891.
•Ill

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 aud 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 aud 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 tonus 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 bauds, 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 hones arc effected chiefly by muscular contractions,
but the direction and extent of the movements arc for the most part deter-
mined by the form of the joint-surfaces and the limitations to movement
1 Quoin's Anatomy, vol. ii. pt. 1 ; Grm/s Anatomy; Horrids Anatomy.

Ill) 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 ;i 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-
bra?, 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.
Singe 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 arc determined by the contact
of the coronoid and olecranon processes of the ulna with the bone in the cor-
responding fossa? 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 aboul 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
Abhancttungen dor math.-phys. Classe der konigl. Siichsischer Geselhchaft der Wissenschaften, Bd.
xvii. and others.

THE ACTION OF LOCOMOTOR MECHANISMS. 417

of the bone serving as an axis of rotation. The pivot-joint allow- of rotation
only; the atlanto-axial and radio-ulnar joint.- may be 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 are 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 I era- 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, upou which the rotation occurs, is the fulcrum. The
balancing of the head upou 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-joiut, aud the power is the pull of the
triceps on the olecranon. The raising of the body on the toes is another ex-
ample : the fulcrum is at the place where the toes are iu 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 bit ween the tendon of the muscle and the bone; for
example, when the arm is extended the flexor muscles work to great disad-
vantage, ibr a large part of the force is expended in pulling the ulnar and
radius against the humerus, and is Lost Ibr movement, but as the elbow is
flexed the force is directed more and more nearly at right angle- t.. 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 i^ the more important, since the power of the
muscle is greatest when the muscle has its normal length, ami 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: I'llii'/rr's Archiv, 189<J, Bd. lxiv. S. 53).
Vol. If.— 27

418 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY.

cases special processes projecting from the bones, the introduction of sesamoid
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 pari in the movement. It would appear from the observations of
certain writers2 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 tin; 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 ereel position is maintained with comparatively little muscular
exertion. It is an act of balancing in which the centre of gravity of the
body is kepi 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 oe-
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 'In- Pkysiologie, 1871, Bd. i. pt. 2. S. I'll.
- Sherrington: Proceedings of the Royal Society, Feb., 1893, vol. liii.

THE ACTION OF LOCOMOTOR 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 tall forward is
resisted by the strong muscles of the calf of the leg. The creel 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 round 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 OE 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.' — 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 i- 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-
mit- the propelling limit 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 Beauni< : Physiol ogu hwmame, 1888, vol. ii. p. 269, gives many references to the litera-
ture of this subject.

: W. and E. Weber: Media ink ilrr mrnxchlidien (Idinrerkzruiii , 183(>.

3 ha Methode graphique, 1885.

' Marey: M&hode 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 each 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 are slight. The tendency
for the centre of gravity to move from side to side as the Legs 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 voice 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

'Fischer: Abhandl, </. math.-physik. ('I. </. Sachs. QeseUsch. <l. Wissensch., xxv. Nr. i.
-Quoted by Griitzner: Hermann's Handi. der Physiologic, L879, Bd. 11. 'I'll. 2, S. 14.

422

AX AM /:/>/< AX TEXT-BOOK OF PHYSIOLOGY.

the epiglottis. The epiglottis, which is extremely movable in a median plane,
mav lie lilted 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 1< iss 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 larynx (after Testut) : 1. posterior face of i ,1 • / ,,• r u

epiglottis, with r, us cushion; 2, aryteno- mucous membrane, the ary-epigloUic fold,

epiglottic fold; 3. ventricular band, or false which forms the lateral rim of the superior

vocal cord; 4, true vocal cord; 5, central . . i i • i i

of Merkei; 6, ventricle of larynx, with aperture of the larynx and which ends in,

6', its ascending pouch ; 7, anterior portion an(J covers posteriori V, the arytenoid carti-

of cricoid; 8, section of cricoid; 1), thyroid, mi i i • l

cut surface; 10, thyrohyoid membrane; 11, lages. Ihe rounded prominence on the pos-

thyrohyoid muscle; L2, aryteno-epiglottic terior cornei- of this fold is made l)V the car-

muscle; 13, thyro-arytenoid muscle, with _ t J

13', its inner division, contained in the vocal tilagc of Saiit< >i*i hi, and a sce< >nd, less marked,

cord;n,cnc,,thyroidnn1sc,e K, Md.lottic m „,,,,.„.,] tO it, bv the eartilaqeof

portion ol Larynx; l6,cavityoi tin' trachea. '^ » ' & •>

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 Morg-agrii. — The ventricular bands
or false mral cords arise from the thyroid cartilage near the median line, a
shorl 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 "(edema 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. 123

Tlie 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). — Xo 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 extensive 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 nol
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,1
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 sides 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 base-, the space between
the cords themselves is sometimes called the rima uocalis and thai 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 freeedges 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 (ash : Journ. Anal, and Phx/8., 1883, vol. xvii.

424

AN AMERICAN TEXT-BOOK OE 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 clastic tissue and the internal and part
of the external thyroarytenoid 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; '_', petiolus; 3, median
in itch (if thyroid; 1, superior cornu of thyroid;
5, attachment of stylo-pharyngeus muscle; 6,
origin of thyroepiglottic ligament; 7, origin of
the thyro-arytenoid muscle; 8, origin of true
vocal conl ; 9, interior cornu of thyroid ; 10, car-
tilage of Wrisherg : 11, cartilage of Santorini ; 12,
12', arytenoid cartilages, showing attachments of
the transverse arytenoid muscle ; 13, 13', pro-
. . -u- muscularis, showing attachments of the
posterior and lateral cricoarytenoid muscles;
II. base Of the arytenoid cartilage; 15, vocal pro-
cesses of the arytenoids ; III, articular surface for
the base Of the arytenoid cartilage ; 17, posterior
view of cricoid cartilage, with outline of attach-
iiii ill of the posterior cricoarytenoid 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
Wrisherg; 4, ary-epiglottic ligament; 5, 8, mucous
membrane ; 6, cartilage of Santorini; 7, arytenoid
cartilage; i», its processus muscularis; 10, crico-
arytenoid ligament; 11, cricoid cartilage; 12, in-
ferior cornu of thyroid cartilage; 1"', posterior
superior cerato-cricoid ligament; 13', posterior
inferior cerato-cricoid ligament; 14, cartilages
of the trachea; 1">, 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 witli 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 iu 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, cornicutum laryngis (Figs. 209. 210). Outside
and in front of the latter is the minute cuneiform cartilage or cartilage of
Wrisberg, 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 proces.su* 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 rocalis.

The thyroid and cricoid cartilages and the body of the arytenoids 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 %terno-hyoidi the stemo-ihy-
roid, and the omo-hi/oid muscles, which depress the larynx or hyoid bone; the
thyro-hyoid muscle, which depresses the hyoid bone or elevates the thyroid

426

AN AMERKA.X TEXT-ROOK 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 loud 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
larynx, and the constrictors of the larynx contract, completely closing the
entrance into that organ.

Tin- intrinsic muscles of the larynx an; the crico-thyroids, the lateral crico-
arytenoids, the postc -lor crieo-arytenoids, the arytenoid, the aryteno-epiglot-
tideans, and the tkyro-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-
tilage- 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
/minimi Adami while the ascending scale is sung
in the middle chest register, both descent of the
fronl 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 ., , . , , , , ,

cartilages of larynx with the crico- cartilage, and passes upward and backward to be

thyroid muscle (Quain'* Anatomy, iDserted into the outer edue of the arytenoid car-
after Willis): I, crico-thyroid mus-

crico-thyroid membrane ; 3, tilage, on and in front of the lateral prominence

cricoid cartilage; 4, thyroid carti- /pj gjgy Its main action is to wheel the
lage ; 5, upper rings of the trachea. v

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 colds. The posterior crico-arytenoid occupies an important position in the
group of respiratory muscles; during vigorous inspiration it is brought into action

VOICE AND SPEECH.

427

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 arytenoid muscle, the single unpaired

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 ; 6, oblique arytenoid
muscles; 7, transverse arytenoid muscle; 8,
posterior crico-arytenoid muscle; 9, interior
cornu of thyroid cartilage; 10, cricoid car-
tilage; 11, posterior inferior cerato-ericoid 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, thyroepiglottic mus-
cle; 3, cartilage of Wrisberg; 4, ary-epiglottic mus-
cle; 5, cartilage of Santorini ; 6, oblique arytenoid
muscles; 7, thyroarytenoid 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,
membrane ms 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 aryteno-epiglottidean, sometimes culled 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 middle 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 t<> close the glottis. The tivyro-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

\-2s

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 musoularis (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
thvroid 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 Laryngeal 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,
m.thy.ar.i. anc] tne lateral crico-arytenoids ; the
m.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
sphincter group and especially by the
external thyro-arytenoids.

It will easily be seen that in the
larynx, as in the skeleton at large, the
efficiency of any single muscle involves
the action of accessory muscles ; thus,
contraction of the crico-thyroid could
have little effect in tightening the vocal
cords were not the arytenoid cartilages
fixed !>\ 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
i- 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

Pig. 214.— Diagram to illustrate the thyro-aryte-
noid muscles ; the figure represents a transverse
section of the larynx through the bases of the
arytenoid cartilages (redrawn from Foster): Ary,
arytenoid cartilage; p.m., processus muscularis;
p.v, processus vocalis; Th, thyroid cartilage; c.v,
vocal cords; CE is placed in the oesophagus;
m.thy.ar.i, internal thyro- arytenoid muscle;
m.thy.ar.e, external thyro-arytenoid muscle;
m.thiuir.rp, part of the thyro-ary-epiglottic mus-
cle, cut more or less transversely; m.ar.t, trans-
verse arytenoid muscle.

VOICE AND SPEECH.

429

larynx receive their motor impulses from the inferior laryngeal 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 two rounded nodules, the cartilages of AVris-
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

17

Pig. 215. — The laryngoscopic image in easy breathing (Stoerk): 1, base of the tongne; 2, median
glosso-epiglottic ligament ; 3, vallecula; -l, 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 ; it, sinus
pyriformis; 12, cartilage of Wrisberg ; L8, aryteno epiglottic fold; L4, 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 ex-en its bifurcation may he 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

130 AN AMERTCAN 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, pitcli, 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
tip- 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. 883).
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.

J Helmholtz: Sensations of Time, trans, by Ellis, LS85, p. 98.

3 Op. cit., 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 h 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 they 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 nods.

The voice and the larynx of the child foil 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. 133). In hoys
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 n
peculiar quality, much valued in some countries in the rendition of vocal
music. The practice of castration for eesthetic 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 Changing the Pitch of the Voice. — As has frequently
been mentioned, the vocal cords are stretched, and the pilch 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 it.- 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-
vnx 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-
able added. The most common designation of the lowesl 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. Seiler3) 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. Iu -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

ABC

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,l\ lal.se vocal cords;
n.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 lias the remark-
able power of emitting from the larynx nuics which arc indistinguishable in quality from

an ordinary whistle. He writes, " The whistle cannot he made to '.slide' into vocal tones
of any sort, nor can any other tones he produced simultaneously with it. Its range is
about one and a half octaves, or hall' 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 t he 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 li|x lirinU closed as
in the ordinary position. Any other movement id' 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 id' whistling. No notes were written at the lime, hut the picture reineni

bered is that of vocal cords closely approximated, except lor 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, vowel* or sonants, which arc produced purely by vibration of the vocal

1 Mackenzie : Hygiene <</' the Vocal Organs, 1891, p. 55.

2 Garcia; Lond., Edin., and Dull. Mag., vol. x. 1855, p. 218. (Quoted bj 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 .-peed), usually effected by action
of the lips, the tongue, the palate, or the jaws, and the place of articulation
depends, in any definite ease, 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, ?*, o, u (pronounced ah, a, e, o, oo), it is found that the

FlG. J17.— Section of the parts concerned in phonation, and the changes in their relations in sound-
ing the vowels A (<■*), I ("), [/(">) (after Landoisand 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 vocal
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. ciL, 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 Hipping
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 with 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-
holtz5 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 lor believ-
ing that the form of the chest cavity and the tension of its walls arc factors in
determining the pitch of its fundamental tone; so that through the varied

1 Helmholtz: op. cit., p. 108. 'l Op. cil., p. 114. 3 Op. cit., p. 118.

4 Quoted by Griitzner: op. cit., p. 179. 5 Op. cil., 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 month 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 arc 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 (Jriitzner,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 /•. 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,
Inn is the abbreviated labial expression, as in pack or jticrv when all the letters
are eliminated after the first. Of the liquids the v. 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. cit., p. 93. -.wall and Pollard: Journal of Physiology, 1890. vol. xi., p. 159.

"Griitzner: op. cit., \>. 196. 4 Op. cit., 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 to
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. 11 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 prat/.

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, 6, 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 voire
(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 voice 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?

Oral.

Place of Articulation.

Momentary.

Continuous.

Surds Sonants Surds sonants

(without voice), (with voice), (without voice), (with voice),

Nasal.

Continuous.

Sonants

iwitii \ oice]

Lips

Lips and teeth

Tongue and teeth . . .
Tongue and hard palate

( forward )

Tongue and hard palate

(hack)

Tongue, hard palate, and

soft palate

Tongue and soft palate .
Various places

t
ch

f
th(in)

sh

<1| 7
z. r
zh, r

y. 1

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 t" tin' sound,
they are known as continuous as distinguished from the momentary sounds of

1 Griitzner, op. cit., pp. 211, 213.

* Webster's International Dictionary, 1891, p. lxvi.

438 AN AMERICAN TEXT- BOOK OF PHYSIOLOGY.

group 2. They may be divided into labio-dental JricfionaU, f (without voice) ;
r, w (with voice) ; the lingual friciionah 8, fh (as in them); sh, ch soft (with-
out voice); ~, / (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
//, as hark, 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, i> the
chief method of reproduction among unicellular plants and animals, and
throughout the plants and in the lower multicellular animals it is important.
Among various species it takes various forms, known as fission or division,
gemmation or budding, endogenous cell-formation 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 Haeckel 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,
Heteromita, 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, Paramaeeium, the fusion of the two similar individ-
uals is a partial and temporary one, during which a partial exchange of
nuclear material takes place • tin- is followed by separation, after which each
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 and
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, muscular, and nervous
function- of daily life, the latter subserving reproduction. The germ-cells of
the male, called spermatozoa, are relatively small and active, those of the
female, called ova, are relatively large and passive; the reproductive process

consists of a fusi >f a male and a female germ-cell, the essential part being

a fusion of their nuclei ; and this is followed by continued asexual cell-division
and growth into a new individual. Among both plants and animals it is not

B EPR OB UCTJON. 44 1

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 an1
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 vigor, restores the power of fission, and giants a new lease of life to

442 AN AMERICAN TENT-BOOK OF PHYSIOLOGY.

the species. From his observations upon the [nfusoria, Maupas1 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,
Onyehudromus 140 to 230, and Leucophrys patida 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 (incoming 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 subsequenl 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 arc 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 exp&rirnentale et genSrale, 2e se>ie, vii , 1889.

2 O. uiid II. Hertwig: Experimentelle Studien am thieri&chen Ei for, wahrend uml nnch der
Befruchtung, i., 1890.

REPR OD UCTION. 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 Hainm, a
student at Ley den, in 1677. Ilamm's teacher, Leeuwenhoek, 6rs1 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 (^j-g- 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 al'xc and often slightly depressed sides. It terminates anteriorly in a
slender, projecting, and sharply pointed thread <»r spear. It consists of a
nucleus composed of a dense mass of chromatin and covered l>v an excessively
thin layer of cytoplasm, von Bardeleben ' claims the number of chromo-
somes in the chromatin after maturation to be eight.

1 K. v. Bardeleben: Verhawtttungen der anatomischen Oeselhchaft; Analomischer An
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 (/) is a delicate filiform, apparently cytoplasmic structure.
and analogous to a single ciliuni of a ciliated cell. The tail is tipped by an
excessively fine, short filament, the end-piect (e). The most
. abundant of the .-olid 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 clement in fertilization ; it seeks the ovum, and
it i- 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
movement is a complex one, and is effected by the lashing
of the tail from side to side, accompanied by a rotary move-
ment about the longitudinal axis. The rate of movement has
fig. 218.— HamaD been variously estimated at from 1.2 to 3.6 millimeters in the
spermatozoa (after minute. Spermatozoa taken directly from the testis are

Retziust : .1, seeD en . •'

/ace; h, head!; m, quiescent ; normally they begin to move when mixed with

renSpTece65 jT^ the secre1 ions of the access017 sexual organs.1 Toward heat,
seen from the si.ie. cold, and chemical agents spermatozoa behave like ciliated
cell-.

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. Tt is not
certain how long they may remain alive within the genital passages of the
human female. Diihrssen- claims to have found motile spermatozoa in the
oviduct at leasl 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 interesl to know that
spermatozoa capable of fertilizing have been known to live within the recep-
taculum ■•« minis 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 thai serves a a compensation for the

lCf. Walker: Archivfur AnatomU und Physiologic, Anatomischer Abtheilung, 1899, S. 313.
2 Diihrssen: CmtraJblatt fur Gynakologie, 1893, xvii. S. 592.
'A. Lode: Pjiiiger's Archivfur die gesammte Physvdogie, 1891, 1.

BEPR 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 interest
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. But 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 spermatids, 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,
Asearis, 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 be 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).

Semen. — 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 vasa 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 (5 cubic centimeters. Its
chemical composition has not been examined exhaustively, lie-ides water, it
contains approximately 18 per cent, of solid substances, which comprise nuclein,
protamine, proteids, xanthin, lecithin, cholesterin, and other extractives. Cat, and
Sodium and potassium chlorides, sulphates, and phosphates. I Jnder 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 s/),rm-
ine. Interest in the semen centres in its histological rather than its chemical

features. The fluid portion serves as a vehicle for the transportation of and pos-
sibly also for the nutrition of the ripe spermatozoa. Colorless particles, called

1 v. Bardeleben : loc eit.

44G AN AMERICAN TEXT- HOOK OF 1'JI YSIOLOGY.

seminal granules, exist in Bemen. They are possibly parts of nuclei of disin-
tegrated cells. < Jomparatively 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 L.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, Buch as the rodents, seems to contain fibrinogen. This enables the fluid
to clol after it- reception in the female passages, and thus to prevent loss of
spermatozoa. Camus and Gley1 find that this coagulation is caused by a
specific fermenl present in the prostatic fluid. Cowper's glands secrete a
mucous fluid. By careful experiments upon white rats Steinaeh 2 has shown
thai removal of the seminal vesicles and the prostate gland, while not dimin-
ishing sexual pa— ion 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 -emeu, ruder normal circumstances the secretions of these
accessory glands arc essential to the motility of the spermatozoa,3 and they
may have other important functions. Ivanoh",' however, has been able to
impregnate dogs, rabbits, and guinea-pigs artificially by injecting into the
vagina spermatozoa taken directly from the epididymis and mixed with a 0.5
per cent, solution of .-odium carbonate.

The Testis. — The testes ( Fig. 219, /) are compound tubular glands with
a unique structure. Formed early in embryonic life as solid structures, with
the seminiferous tubules (to) represented by solid cords of cells, they remain
in the embryonic condition until the time of puberty. Some of the cells,
the mother-cells of the -pern 1a t ozoa, t hen 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 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 year- of age. The spermatozoa accumulate within the seminal

L Camas and Gley: Oomptes rendus de la SocieHS de Biologic, 1896, p. 7*7, and 1897, p. 787.
I. Steinaeh: Pfluger'a Archie fiir die gesammte Physiologic, 1894, lvi. ( f. also Rehfisch :
V medicinischt Woch L896, xxii. S. 245; and Lode: Sitsungsber. <l. Kais. Akad.d.

W W ■■■■. Math, naturw. QL, 1895, civ., Abth.iii

'Cf. Walker: Archivfur Anatomic und Physiologic, Anatomisclier Abtheilung, 1899, S. 313.
4 Ivanoff: Journal de Physiologic ei de Pathologic generate, 1900, ii. p. 95.

BE PR OD UCTION.

447

tubules, and by the constant formation of others behind them are gradually
pushed outward along the ducts.

The Ducts of the Testis. — The ducts of the testis (Fig. 219) comprise a
succession of tubes of different morphological and physiological values.
They are approximately twenty-five feet in length, and are named, in
order, tubuli recti, rete vaseulosum, vasa efferentia, canal of the epididymis,
vas deferens, and ejaculatory duct. The tubuli recti (tr) and rete vaseulosum
(rv), being mere channels for the
passage of spermatozoa, present no
special physiological features. The
vasa efferentia (ve) and the canal of
the epididymis (e) contain smooth mus-
cular tissue in their walls, and, more-
over, are lined by ciliated epithelium,
the cilia causing a movement out-
ward ; both of these features doubt-
less aid in the outward passage of
the spermatozoa. The excretory duct
of the testis, or vas deferens (vd),
with its offshoot, the seminal vesicle,
is more important physiologically.
It is nearly two feet in length, with
a diameter throughout the greater
part of its course of one-tenth of an
inch. Near its termination, however,
it is larger and sacculated, and re-
sembles the seminal vesicle ; it is
known here as the ampulla of Henle.
Its epithelium is not ciliated, but its
walls contain a very thick, plain mus-
cular layer consisting of outer longi-
tudinal and inner circular fibres. In
the walls of the ampulla of Henle
exist small tubular glands. The vas
deferens is an important storehouse
for the spermatozoa. The glands
near its termination supply a part of the liquid of the semen. The muscles
in its walls, by contracting, aid in the seminal discharges. The seminal
vesicle (vs) is a branched diverticulum from the vas deferens. In structure
it is not radically unlike the ampulla of Henle, its walls containing muscular
layers and glands. An important function is to contribute liquid to the
semen. Of all the organs the seminal vesicles contribute probably the
greatest share of liquid. Microscopic examination has somewhat weakened
the old belief that the vesicles are storehouses for spermatozoa, but Rehfisch '
'Rehlisrli : I)< utxrhe medicinische Wochcmchrift , 1896, xxii. 8. 245.

Fig. 219.— Diagram of the male productive
organs; t, testis; ts, seminiferous tubules; tr,
tubuli recti; rv, rete vaseulosum; ve, vasa effer-
entia; e, canal of the epididymis; bo, vas aberrans;
vd, vd, vas deferens; va, Beminal vesicle: de, ejac-
ulatory duct j pr, prostate gland; b, urinary blad-
der; en. cuwper's gland; u, urethra; pn, penis.

148 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY.

finds the anatomical relation of the vesicle to the vas deferens in the human
being to be such that liquids injected into the testicular end of the vas
deferens pass Brs1 into the vesicle before going out the urethra; and lie
believes strongly thai the vesicles exert the double function of serving as
storehouses for spermatozoa and finding liquid for the semen. The ejacuki-
tory duct (de) on each side is a -hurt, thin-walled muscular tube, passing
partly through the substance of the prostate -land and serving to convey the
semen to the urethra.

The Urethra. — The urethra (Fig. 219, u), the common excretory duct
for the urine and the semen, is commonly described as consisting of three parts,
named, respectively, the prostatic, the membranous, and the spongy portions.
The first is characterized by the presence of the prostate gland, the second by
the absence of special features, and the third by the presence of Cowper's glands
and the penis. Throughout its length the wall of the urethra contains plain
muscular tissue arranged longitudinally within and circularly without ; and,
except at the external opening, the small racemose mucous glands of Littre.
It- wall is hence contractile and its lumen is kept moist. Beyond these its
special physiological feature- are given it by the organs above mentioned.

The Prostate Gland. — The prostate gland (Fig. 21!», pr) is a compound
tubular gland whose alveoli are mingled with a large quantity of plain mus-
cular tissue. Jt completely surrounds the urethra at the base of the bladder,
and open- into it by numerous small ducts situated about the openings of the
vasa defereniia. Its function is to contribute prostatic fluid to the semen.
The composition of this fluid and its specific use so far as it is known have
been already mentioned (p. 446).

Cowper's Glands. — Courper's glands | Fig. 219, Og), two in number, are
tubulo-racemose glauds, the duets of which open into the spongy portion of
the urethra by two orifices situated some two inches below the openings of the
vasa defereniia. Their viscid secretion is thought to be one of the components
of the seminal fluid, but its specific function is unknown. It has been sug-
• I thai Cowper's fluid cleanses the urethra of urine and of semen, instead
of contributing actually to the seminal fluid.

The Penis. — The penis (Fig. 219, pn) has as its constant function merely
the conveying of the urine to the outside world, and for this purpose it has no
Bpecial features beyond those belonging to the urethra, which runs throughout
it- whole length. Specifically, however, it is the intromittent organ, and
serves to convey the -emeu into the genital passages of the female. This
(unction is based upon it- power of erection, and this power is dependent
upon the presence of the erectile tissue which constitutes the bulk of the
organ. The erectile tissue is arranged in the form of three long cylindrical
masses imperfectly separated from, but parallel to. one another and extending
lengthwise. < >f these, the two corpora cavernosa lie at the sides, and meet each
other in the middle line along the upper side of the penis ; the corpus spongi-
osum lies in the middle line below, and is pierced throughout its length by the
urethra. At its proximal end each corpus is enlarged into a bulbous part,

REPR OD UCTION. 449

and is covered by a layer of muscular fibres constituting a distinct muscle — the
bulbs of the corpora cavernosa by the ischio-cavernosi (erectores penis), that of
the corpus spongiosum (called bulbus urethne\ by the bulbo-cavcrnosu.s (accel-
erator urinai). 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.

C. 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, /
that everything living is derived from
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 (jyt inch) in diameter. As in
other cells, the cell-body may be distin-
guished from the nucleus, the proto- Fig. 220,-HTuaan ovum (modified from Na-
o ' tr gel): », nucleus (germinal vesicle) containing

plasm of the former being called cyto- the amoeboid nucleolus (germinal spot) ; d,deu-
7 t •, n , ,i , toplasmic zone; />. protoplasmic zone; c, zona

plasm. In its finer structure the cyto- riuiiatu; v I,(,,vit(.lliu,sl,a(.(,

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, .4). For some distance inward from the border the cytoplasm is
pure and transparent, and this portion is often called the protoplasmic zone
Vol. IT.— 29

150 AiX 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
/one ((/). 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 embrvonic 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 nudevs (/<), 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 lor 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 likewise 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
(rsViJ mcn) m 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 Cor the passage of nutriment to the egg. Between the zona radiata
and the ovum a narrow space, the perivitelline space (x), exists. Attached to
the outside of the zona r<i<li<it<i 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 c<>i-<>,i<i 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

BE PR OD UCTION. 45 1

among its solid constituents. Deutoplasm 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" (/>). 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

462 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; ». its nucleus, containing fbur chromosomes ; c, its double centrosome, surrounded
by the attraction Bphere; in /.' much of the chromatin has begun to degenerate; the rest has become
arranged Into two quadrupli ■ chromosomes or tetrads; the formation of the spindle and the

asters has begun; in Cthe flrsl polar amphlaater, bearing the chromosomes, is completed ; in Dtheam-
phiaster has become rotated and has travelled toward the Burface 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 J" the first polar body, containing two dyads, has
been extruded ; the formation ofthe Becond polar amphlaster has begun ; In G the firsi polar body is pre-
paring to divide . polar amphlaster is fully funned ; in // the division ofthe dyads into single

chromosomes In both the Brsl and the egg has begun, and the Becond polar body, p. 6», is in-

dicated : in / the formation of the [Hilar bodies is completed ; j . the egg-nucleus, containing two small
chromosomes, one-half the original Dumber. In fertilization the spermatozoon will bring in two addi-
tional chromosomes, thus restoring the total number Oi lour.

REPR OD UCTIOX. 453

substance lying between them — either the cytoplasmic 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, D). The groups of chromosomes become attached to
the spindle threads midway between the poles. At each pole there may lie a
centrosome, and about it the cytoplasm may become arranged in the form of
a star, the aster, though these structures are not universal among species. The
spindle with the two asters is known as the polar amphiastei', 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 hod;/ ( /•'. p. 61).
The diminished nucleus within the ovum proceeds at once to undergo a second
mitotic division (G, H, I); each of the remaining dyads divides into two
single chromosomes, which are separated from each other ; and a second polar
body (p. b2), 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. Tin1 nucleus
develops a membrane and again enters the resting stage. It is known hence-
forth as the egg-nucleus, or female pronucleus, and it await- 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 spermatocyte
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 c>j;<j;*) and one functional ovum. It is entirely
probable, however, that the essence of the process is exactly the same in the
two cases, and lies in the reduction of the number of the chromosomes. The
three important facts have now been demonstrated in a large number of
species, viz. — that in the 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-halt' 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 every 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
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 membrane granulosa, and is filled with a clear, serous, viscid liquid, the
liquor foUicvM. 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 proligervs. 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
by 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 peUueida 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 Inheril-
1900, 2d ed., Now York.

2 Heyse ArcMv fiir Gynakologie, 1897, liii. 8. 321.
3Henle: Handbuch dcr Anatomie, 1873.

REP ROD UL'TION.

455

ovary to be taken up by the Fallopian tube. The empty follicle undergoes
changes and becomes the corpus luteum (cl). 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 ovulation 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./, Graafian follicle containing an ovum; cl, 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 nymphiu; /•»(, 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 presence in the ovary of freshly-
discharged Graafian follicles at the menstrual period afford evidence of the
frequent coincidence of the two phenomena. But ovulation at the time of
menstruation, though probably usual, is not exclusive of ovulation at other

456 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY.

times, for intermenstrual observations of fresh ovarian sears 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 eases, 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 Falloj>ia)i tubes (Fig. 222, F. t), 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,
the fimbrke (/), 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
fimbria? ; and the cilia with which the tube is lined throughout, and which are
present also upon the inner side of the fimbria?. 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 foUkuU. 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 fimbria?. Once within the tube, the
ciliarv action, assisted perhaps by contraction of the muscular fibres in the
walls, carries the ovum slowly along toward and finally into the uterus. In
-nine mammals the passage occupies three to five days; the time in woman is
not definitely known, but i- 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
other hand, it receives from the vagina spermatozoa and transmits them to the
Fallopian tubes ; it is the seat of the function of menstruation ; when impreg-
nation has taken place, it retains and nourishes the growing embryo, and ulti-
mately expels the child from the body. Its structure accords with these func-

BEPBOD ( '( 'T/ON. 457

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 muscularis mucosae, 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 bl< >o< 1-
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 arc
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 : Citilralhlnlf j'iir ihjuiikolixjic, lS'.t.'i, xvii. S. 7<>l.

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 nut 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 hyperemia 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 deddua
nwnstrualis is applied to the lost coat. The flow continues upon an average
four days or more. From observations upon 2080 American women Emmet2
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 100 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 lias 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. Minot3 suggests the following approximate times :

Tumefaction of the mucosa, with accompanying structural changes 5 days.

Menstruation proper 4

Restoration of the resting mucosa ... 7

Resting period 1-

Total 28 days-

The menstrua] changes in the uterus are accompanied by characteristic
phenomena in other parts of the body. The Fallopian tubes are congested,
lSee Westphalen : Archivjur Qynakologie, 1896, lii. S. 35 ; and Mandl: Ibid., 8. 557.
2T. A. Emmet- The Principle* and Practice of Gyrwecology, 1880, 2d ed.
SC. S. Minot: Human Embryology, L892.

BE PR OD UCTION. 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, conges-
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.

( 'omparative Physiology of Menstrua/ion. — The comparative physiology of
menstruation, although it has been studied only incompletely in a few domesti-

1 Of. Mary Putnam Jacobi: "The Question of lust tor Women during Menstruation,"
Boylston Prize Essay, 1870; C. ReinI : Smnmlung kliniache Vortrdge, 1884, No. 243; 0. Ott:
NouveUea archives a" obsUtrique et de gynecologic, 1890, v.; and A. E. Giles: Transactions of (he
Obstetrical Society of London, 1897, xxxix. p. 115.

460 AN AMERICAN TEXT-HOOK 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
arid 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 awhile with great frequency and regularity (monkey,
mare, buffalo, zebra, hippopotamus, al intervals of four weeks; cow, three
week-; sow, fifteen to eighteen days; sheep, two weeks; bitch, twelve to
sixteen weeks). They arc 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-
fid whether the epithelium is actually shed. It is generally believed that
" heat " in the lower mammals is accompanied by ovulation. It is not neces-
sarilv 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 ohildbearing. The How 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 tli'- 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 i> 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; I-'.. Retterer: Comptea rendm <I<'s

memoires '/* /<< soeiete • /• biologie, 1892; W. Heape: Philosophical Transactions of the
Royal Society, 1894, (B), vol. 185. pt. i. ; and Proceedings of the Royal Society, 1897, lx. p. 202.

2 E. F. W. Pfliiger: Untersuchungen aus dem physiologischen Laboratorium zu Bonn, 1865.

BEPR OD UCTION. 4 (i 1

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 decidua menstrualis is the fore-
runner of the decidua graviditatis (p. 471) ; if an ovum, whenever it is discharged,
be fertilized, it attaches itself to the thickened uterine wall, the tissues become
the decidua graviditatis, pregnancy follows, and the decidua 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 decidua men-
strualis, and the flow occurs. The suggestion of Jacobi * 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 Arrhenius2 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 : (his sig-

1 Mary Putnam Jacobi; American Journal of Obstetrics, L885, xviii.
'■'Arrhenius: Skandinavisches Archivfiir Physiologie, L898, viii. S. M07.
s Strassmann : Archivfiir Oynakologie, 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 nympha> (n), the clitoris, which is the diminutive homologue
of the penis of the male, and the hymen (//), 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
Bartholin'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 of the two
germ-cells. Investigation is making it more and more probable that the
spermatozoon and the ovum, although so different in appearance and general
behavior, are fundamentally and in origin both morphologically and 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 about 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
cavernosa and the corpus spongiosum. Simultaneous relaxation of the smooth

1 Compare Th. Boveri : " Befrnchtunp;," Merkel und Bonnet's Ergebnwe der Atiatomie und

Entwickelungsgeschichte, 1892, i.

1 1 i I AN AM EB ICA X TEXT-B O OK OF PHYSIOL O G ) '.

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 aud 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 cresla of the urethra or capvi galfinaginis, 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 urethrce, 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
coming from 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
passing along the sensory nerves of the organs to the spinal centre ; or they
may originate in the brain and pass downward through the cord, the impulses
in this case corresponding to sexual emotions. The centrifugal paths for the
arteries are along the nervi erigentcs, which are true vaso-dilator nerves, and
in the mammals, where experiment has proved their existence, pass from the
spinal cord along the posterior lumbar (monkey) or anterior sacral (monkey,
dog, cat) nerves to their arterial distribution. The ischio- and bulbo-caverno-
sus muscles are under the control of their motor nerve supply, consisting of
branches of the perineal nerve.

In the female, anatomists recognize the homologues of the male erectile
parts as follows : the clitoris with its corpora cavernosa and glans as the horno-
logue of the penis, the two bulbi vestibuli as that of the bulb of the corpus
spongiosum, the pars intermedia perhaps as that of the corpus spongiosum
itself, and the erector clitoridis muscle as the homologue of the erector ])enis
(iscfiio-cavernosus). 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 sexual orgasm, when the emis-
sion of semen from the penis into the vagina occurs. It 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
and the canal of the epididymis, and sweeps along the powerful muscular
walls of the vasa deferentia in the form of a series of peristaltic waves. The
Beminal vesicles also contract, and the mixed liquid and spermatozoa are poured
through the ejaculatory ducts into the prostatic portion of the urethra. The
muscles of the prostate expel the prostatic fluid and help to pass the semen

RE PR OD UCTION. 465

onward. The glands of Cowper possibly add their contribution. But the
final urethral discharge is effected especially by powerful rhythmic contractions
of the already partially contracted striped muscles, viz. the ischio- and bulbo-
cavernosi, the constrictor urethrce, 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 uteri. Ejaculation is a reflex act.
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. Bat 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. Kraft2 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, i. 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 th<>
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 arc 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 •<■ approach and

surround it in great numbers, being apparently attracted t<> it in some myste-

1 Schuworski : Abstract in Monatsschrift fwr Oeburlshulfe nu<l Oynakologie, L896, iv. S. 275.
-II. Kraft: Pfiuger'i Archiv fur die gesammte Phyeiologie, f890, slvii.

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 chemotaxis, 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 fertiUzation
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 ouly 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, i>).

This body has the conventional nuclear structurt — namely, an achr atic

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 nber presenl in either germ-cell before its

maturation, hence in the human being perhaps sixteen, one-half of them

1 \V. Pfeffer: Untersuchungen au&dem Botanischen Tn&titutzu Tubingen, 1884, i.
-J. Riickert: Archiv fiir mikroskopische Anatomic, 1895, xlv.
K. Zoja: Anatomischer Anzeiger, 1896, xi.

BE PR OD UCTION. 467

bavins: come, however, from the mule cell and one-half from the female cell.
On the commonly accepted theory that they constitute the hereditary .-ub-
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
centrosame, 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 e, the entrance-cone for the spermatozoon ; the
head (ft) 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, ami then growth begins.

Segmentation. — The process of growth is a complex process of repealed
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 6rs1 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

}»;>

AN AMERICAN TENT-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. AVe have
spoken of the difference of opinion regarding the origin of the original cen-
fcrosome of fertilization. The origin of the two centrosonies present in seg-
mentation has likewise been disputed. The question is of considerable the-
oreticaJ interest in connection with the problem of the physical basis of

Pig 224.— Stages in the fertilization oJ the egg (continued from Fig. 223).

minuti - after entrance of the spermatozoon. The male and the female pronuclei have met
in a r the centre of the egg and the fusion 1ms 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 Bperm-nucleus.

d. "-till later after entrance of the spermatozoon. The two pronuclei have united to form the firsl
segmentation-nucleus. The sperm-aster bas become divided into two asters, which have moved to
opposite poles of the nucleus. The i gg is nov 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. Oilier
observers, following Boveri, maintain that the centrosome of the sperma-

BE PROD UCTION.

469

/

m<

tozoon divides into the two segmentation-centrosomes, the latter hence being
exclusively of male origin. Still
others believe that the sperm-
centrosome disappears, its place
being taken by two new ccntro-
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-eentro-
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
ccntrosomes (Fig. 225, a). The
spindle, ccntrosomes, and asters
form the body known as the
amphiaster. The chromatic sub-
stance becomes changed into the
definite rod-like; chromosomes,

which are collected in the equa- tag as delicate filaments extending in the f<

. , « , • n l from poll' tn polr.

tonal zone ot the spindle, ana B, Each chromosome has become split into two. and the

constitute the equatorial plate, latter are being pulled toward the poles.

* * c. The divergence oi the chromosomes has ceased and

Each chromosome proceeds to the Latter are becoming converted Into vesicular masses

split lengthwise and the two ,"'si'''' the centrosomes. The spindle is becoming resolved

'' & * Into ordinary cytoplasm,
halves move toward the t\v<>

ccntrosomes (Fig. 22o, b). The cause of this movement is not known. The

'For a critical review of iliis and oilier problems in fertilization and segmentation Bee E.
B. Wilson: The Cell in Development and Inheritance, 1900, 2d ed., New York.

Fig 225.— Stages in the segmentation of the egg (after
Wilson). The drawings were made from sections of eggs
of the sea-urchin, Toxopneuxtes variegatus, kg.

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 defined chromosomes aggregated in the centre to
form the equatorial plate, the achromatic substance exist

nn of a spindle

470

AN AMERICA* TEXT- BOOK OF PHYSIOLOGY.

original idea of Van Beneden,1 that the astral raws are contractile and
mechanically pull apart the half-chromosomes, is supported by considerable
lait unconvincing evidence. The idea appears to be growing that by reason
of chemical changes taking place in the centrosomes the halt-chromosomes
are attracted t<> the two poles of the spindle.2 Strasburger3 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 ecpial parts, the separation taking place along the plane of nuclear

/

/

I Jrar

''<7\~*

\

/

\

i

i
i

/

Fir,. 226.— Stages in the segmentation of the egg (continued from Fig. 226).

d. The vesicular chromatic masses have become converted into two typical resting nuclei, each with
n chromatic network. The single aster, formerly connected with each nuclear mass, bas 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, i>). Each part contains one of the new nuclei, and the
result of the firsl 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 resull being four in all, and by subsequent divisions, eight, sixteen,
and more, the division- not proceeding, however, with mathematical rcgu-
laritv. By such repeated mitotic processes the original fertilized ovum
becomes a mas- 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.

'Van Beneden: Arclt'n-rs ck Iliolat/k, iss.'j, iv.

1 - v . Biitechli: Verh. NalurhwL med. Ver. Heidelberg, 1891 ; and E. B. "Wilson, op. cit.

'Strasburger: Analomi&cher Anzeiger, 1893, viii.

REPRODUCTION. All

Polyspermy. — It happens occasionally that two or more spermatozoa enter
the ovum ; such a phenomenon is known as dispermy or 'polyspermy, according
to the number of entering sperms. Each sperm with its nucleus and centro-
somc becomes a male pronucleus and proceeds to conjugate with the female
pronucleus. In the case of dispermy the one female and the two male pro-
nuclei i'u^c together; each centrosome gives place as usual to two, making
four in all, which take up a quadrilateral position about the firsl segmenta-
tion-nucleus; the chromatic figure consists of two crossed spindles; and the
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 cells. 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 arc accompanied In-
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 docs not break away as 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-HOOK OF PHYSIOLOGY.

membrane that forms its bed is henceforth known as the decM.ua serotina; as
the seat of the future placenta, it is physiologically the most interesting and
important portion of the uterine mucosa. 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 rejiexa. 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 Fetal 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 amnion 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 (If 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 fital kidneys. As growth proceeds, the amnion expands and becomes
loosely attached to the outer fetal membrane, the chorion.

REPRODUCTIOX.

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 serotina.
Chorion frondosum.

—Mucous plug within cervical canal.

Fig. 227.— Diagram of the human uterus nt the seventh or eighth week of pregnancy modified from
Allen Thompson). The fetal villi are shown growing into the Binuses of the decidua serotina and the
decidua reflexa; in the latter they are becoming atrophied. They are marked by the black fetal vessels,
whiehcanbe traced backward along the umbilical conl 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 vUU, extending outward in all directions and joining by their
tips the decidua serotina and the decidva 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

AX AMIUUCAX TEXT-HOOK OF PHYSIOLOGY.

Amnion.

i

i 'horion.

i

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 deeidua reflexa, and, as the latter disappears,
with the deeidua vera; this portion of it is called chorion Iceve. In the region
of the deeidua serotina it enters into the formation of the placenta, and is
here called chorion frondosum.

The Placenta. — The placenta (Fig. --7), or organ of attachment of mother
and fetus, is a disk-shaped body, approximately 20 centimeters (7—8 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 ictus. It consists of
a maternal part, the modified
deeidua 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 an- 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 greai development of
their contained blood-vessels, and
a linn attachment of their tips to
the uneven surface of the serotina,

so that their branches come to float
Pig. 228. Diagram ol the placenta (Schafer) : s, pla-
centa) sinuses, into which project the fetal villi, con- freely within the uterine sinuses
taining tin- nil fetal > otina; m. 1 . i 1 .1 1 ■ • 11 1

spongy layer, and m, muscular layer, of the uterus; a, :iml to De bathed in uterine blood
uterine artery, and v, uterine vein, opening into tin- (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 by the fetal villi ; this is in con-
tact upon the fetal side with the thin unmodified chorion covered within by
the amnion, and upon the maternal side with the thin relatively unmodified
serotina covered without by the uterine muscle. The pure maternal blood
brought by the uterine arteries moves slowly through the sinuses and retiree
by the uterine veins; the fetal blood is propelled by the fetal hear! along the
umbilical cord within the allantoic arteries and through the villous capillaries,
and returns by the allantoic vein. The two kinds of blood never mix, but
are always separated by the thin capillary walls 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 e<r<r 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 Graafian follicle
obtains its food from its surrounding stroma, so within the highly vascular
decidua it absorbs directly from the decidual tissue. Bui thai this source
soon proves insufficient for the rapid growth is indicated by the early develop-
ment of the chorion with its villi and the embryonic vascular system. In
the youngest known human embryo,' believed to be scarcely seven days old,
the villi are already well marked. From this time onward throughout
tation the chorion takes an important pari in the embryonic nutrition, becom-
ing, as we have seen, an integral pari of the placenta. The placenta is par
excellence 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-iinportani functions of grow th and prepa-
ration for the future independent existence. The organism requires, therefore,
1 Peters: Verhandlungen der deutschen Qesellschafl fur Gynakologie, 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 he 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 (Indus venoms, 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 leucocyte- into the ictus may take place.

REPRODUCTION. 477

The uterine glands arc thought by some to afford a nutritive secretion to the
sinuses, and to the amniotic liquid has been ascribed a nutritive function.
Theoretically, these various means are not impossible, but true placental diffu-
sion must be regarded as the chief principle at work. The result is that the
mother relieves the child of all the labor of nutrition except that connected
directly with the hitter's own cellular and protoplasmic metabolism. The
fetal energies are, therefore, free to be expended in the process of growth,
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 i*egard 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 hyperemia. 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 haemoglobin than at other times. This is perhaps true for
the earlier months, but Schroeder2 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 i- increased during
gestation. It is maintained by some that the heart beat- 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 sphygmographic
tracings, that the left ventricle is hypertrophied during pregnancy. Post-
mortem examination confirms this inference. Pregnancv necessarily throws

'(). Spiegelberg und R. Gscheidelen: Archivfur Gynakologie, 1872, iv.
2E. Schroeder: Ibid., L890 91, xxxix.; Wild: Ibid., 1897, liii. S. 363.
3F. A. Kelirer: Uebei- die Verdnderungen der Ptdmirve im Paerperium, 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 davs 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
decidual, the thrombosis of the placental vessels, the venosity of the fetal
blood due to the growing functional importance of the fetal right ventricle
acting asa 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 no!
constant phenomena, and the others are not definitely proved to be efficient
causes. It i> probable that, with the uterus undoubtedly irritable, in different

1 J. Veit : Midler's Handbuch der QeburtohiUfe, 1888, 1.

REPRODUCTION. 479

cases different stimuli act to inaugurate the process of birth, and a priori
several of the above causes seem 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 pf 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 oh 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, arc passive. The contractions
are probably peristaltic in character, as in lower animals. Schatz1 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
contraction- 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 tonieally contracted state.
Each contraction is accompanied by a pain, localized in the early pari 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 firsl 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.

1 V. Schatz : Archiv fur Uyuil koloyir, INN") S(i, xwii. < ompare O. Schaeffer : Experimentelle
Untersuchungen iiier die Wehentkatigkeii den menschlichen Uterus, auagefiihrt miitelst einer neuen
Pelolte iokI eines neuen Kymoyraphionf Berlin, 1896; abstract in Centralblatt fur Gynalcologie,
1896, xx. 8. 85.

480 AN AMERICAN TEXT-BOOK OE 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 eervix. 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 eease 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

REPRODUCTION. 4*1

membrane of the uterus from the placenta outward and for a considerable
depth is gradually torn free from the deeper parts through the spongy layer,
and with the attached chorion and amnion follows the placenta. As a rule,
this after-birth appears at the vulva within fifteen minutes after the expulsion
of the child; it consists of the placenta, the amnion, the chorion, the deddua
reflexa, and a considerable portion of the <le<-i<ln<t r<r<t.

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 placental vessels the maternal blood flows freely and continue- 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. Rein1 found that in the rabbit after section of all uterine nerves
normal conception, pregnancy, and birth may occur, [n some animal- 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. Goltz2 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 tor uterine contraction must
hence be supposed to exist in the lumbar cord. Centripetal and centrifugal
fibres exist in both sympathetic and spinal uerves, and reflex uterine contrac-
tions are readily obtained by stimulation of the central end- of the divided
nerve-trunks. According to Langley and Anderson,3 in the cat :nid the rab-

bit both the longitudinal and the circular muscular coats and the arteries of
the uterus are supplied with motor nerve-fibres mainly by the third, fourth,
and fifth lumbar nerves; the fibres pas- to the sympathetic system, and

'<;. Rein: PfiUger'B Archiv fur die gesammte Physiologic, 1880, \.\iii.
2Fr. Goltz: Ibid., 1874, ix.

8 Langley and Anderson: Journal of Physiology, 1895 96, six. p. 122.
Vol. II.— 31

482 AN AMERICAN TEXT-HOOK 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 ami 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 quadrigemina, 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 In-
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
effort s.

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 ' 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 -ingle 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 amnion.-. 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 Geburtekunde und FrauenkrankheUen, L855, vi.

REPR OD UCTION. 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. Xo 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 AVilson 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 decidua 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 witli 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:5

Boys bom to 1000 Girls born (1887-91).

Italy 1058 England 1036

Ireland 1055

German Empire 1052

Erance 1046

< lonnecticut 1072

Rhode Island L049

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 Cf. Er. Ahlfeld : Arehiv fur Qynakologie, i\., 1876.

2 II. Driesch : Zriixrhrifi fiir nissenschaftliche Zoologie, liii , 1892; lv., 1898; MiUheilungen

mis dir Zonlni/isrltrn Station zn Xi/ip,/, xi., 1893.

? E. R. Wilson: Journal of Morphology, viii., L893.

4 R. Zoja : Ar<-liir fiir Entwickelungsmechanik der Organismen, ii., 1895.

5 Bulletin de I'institut vnternational </< statislique, vii.

184 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY.

more hypotheses, bearing upon the determination of sex, have been brought
forward. The Bofacker-Sadler law (Hofacker, 1828; Sadler, 1830) is well
known, as follows: [f the lather 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 us truth. Thury in L863 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. Schenk1 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. Diising2 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
nutritions flesh Yung ; increased the percentage of females from 56 to 92.
Mrs. Treat4 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 I.. Schenk : Einflusz auf das Oeschlechtverhdltniss, Magdeburg, 1898. Authorized transla-
tion: The Determination of Sex, London, L898.

'-' K. Diising: Jenaische Zeitschrift fur Natunrisscnschnft, 1 SS:i, xvi., and 18S4, xvii.; also
published separately, Die Regulierung des GeschlectverhaUnisses bei der Vermehruna der Menschen,
Tiere und Pflanzen, Jena, 1884.

E. Yung: Comptes rendus de P academic des science*, Paris, INN], xcii.

1 Mrs. Mary Treat: The American Naturalist, 1S7I!, vii.

B i: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. Maupas1 found that sex in the rotifer.
Uydatina senta, 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 resl
to females; at 14°-15° C. only 5-24 per cent, were male.-, the much larger
majority females. Nussbaum2 has brought Ma upas'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
Ripley3 of the two races inhabiting Belgium, the Walloons, of the same
origin as the Kelts in France, and the Flemish of German stock. Where
these races are purest, the number of boys born to 1000 girls is 106 1 : 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" 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. I low largely
and in what manner this may be true of the human species is wholly unknown.
Diising6 urges that the various factors determining sex have arisen through
natural selection ; they are conducive to the continuance of the species, and

1 K. Maupas: Complex nnrfus </<■ i'unitlt'mie des sciences, Paris, 1891, cziii.
'Nussbaum: Archivfiir mikroskopische Anatomic, 1897, \li\- 8. 227.

3 W. X. Ripley : Quarterly Publications of the Imerican Statistical Association, March, L896, v,

4 For a critical review of the various theories Bee I. ( ohn: Die willkiirliche Bestimtnung des
Geschlechts, 2d ed., Wiirzburg, 1898.

5S. Watase: Journal of Morphology, 1892, vi.
6 Dusing : Loc. cit.

486 AN AMERICAN TEXT-BOOK OE 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 Mar 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-
fication-, 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 topicsof 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 organ-, is growth or increase in size of the
body. Growth of the body continue- actively from the beginning of the seg-
mentation of the ovum up to about the age of twenty-five years, and results in

REPR OD UCTION. 487

an increase in all dimensions and in weight. In determining the extenl of
growth, the two most convenient and mosl commonly used measurements are

those of length, or height, and weight. For the embryo the following table
has been compiled by Hecker : !

Table shouting flic Average Length and Wt ight of the Human Embryo at

Different Ages.

Month. Length of embryo in centimeters. Weight of embryo in grams.

Third 4 to 9 11

Fourth 10 to 17 57

Fifth 18 to 27 284

Sixth 28 to 34 634

Seventh 35 to 38 1218

Eighth 39 to 41 1569

Ninth 42 to 44 1971

Tenth 45 to 47 2334

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. Minot2 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 i-
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; ;t 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 live 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 live to ten years thereafter slight increase in

1 C. Heokor : Monatsschrift fur Oeburtskunde mi, I Frauenkrankheiien, 1866, >. wli.
• ( '. S. Minot : Human Embryology, Is'.1'-!.

488

AN AMERICA X TEXT-BOOK OE 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 Bowditch1 from observa-

Fig. 229.— Diagram showing increase of stature and weight of both sexes, as determined by the Anthropo-
metric ( lommittee of the British Association.2

tions on several thousand Boston school children, has been abundantly con-
tinued 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 II. P. Bowditch : Eighth Annual Report of the State Board of Health of Massachusetts, 1877.

2 Roberts: Manual of Anthropometry, 1878.

REPRODUCTION. 489

others in both weight and stature. Beyer1 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 less upon
external conditions than upon race and sex. As an interesting accessory
fact it was found by Porter2 that well-developed children take a higher rank
in school than less-developed children of the same age. Tf the percentage
annual increase of the total weight he 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 change- i-
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 a«;e of fourteen to seventeen, lint other characteristic anatoin-
ical and physiological changes in the body occur. The uterus, the external
reproductive organs, and the breasts become larger, while the pelvis widen-.
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 II. G. Beyer: "The Influence of Kxercise on Growth," Journal of Experimental Medicine,
189(i, 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 arc frequent; vertigo, neuralgia,
rheumatism, and gout are not rare; a tendency to obesity occurs, though
sometimes the reverse ; irritability, fear, hysteria, ami melancholia may be
present ; the disposition may be temporarily altered ; all of which changes
indicate that the female organism at this time sutlers 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
[lowers. 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" in the nerve-cells of the first
cervical spinal ganglion :

Volume of
nucleus.

Fetus (at birth) 100 per cent.

Old man (at ninety-two years) 64.2 "

1 <'/. ( '. s. Minut : Journal of Physiology, 1891, xii.

2 C. F. Hodge: Anatomischer Anzeiger, 1894, ix.: Journal of Physiology, 1894, xvii.

Nucleoli observ-

Pigment

Pigment

able in nuclei.

much.

little.

in •"))') per cent.

in 5 "

67 per cent.

33 per cent.

REPR 01) UCTION. 491

Thus with the progress of age the nuclei become small and irregular in out-
line, and the cytoplasm pigmented, while the nucleoli are often wanting. The
nuclear differences are even more marked in the cerebral ganglia of lues, where
moreover, aged individuals possess a smaller number of nerve-cells than the
young. The nuclear differences accord with the common belief thai 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-live 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 ' 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 thai 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 "I' 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 ^vxvv<- 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 kidney- 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 da- Berliner Anthropologischen QezeUschaft, L895.

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 maybe 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 living, 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." According to Weis-
mann, primitive protoplasm was not endowed with the property of death.
As found in the simplest individuals, like the Anxrha, even at the present

' Lee, Adler, and Balkley : American Journal of Physiology, 1900, iii. p. xxix.
2 A. Weismann: Essays upon Heredity, 1889, i.

RE PR OB UCTION. 493

day, with a continuance of the proper nutritive conditions 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 tl ther bodily functions. Germ-
cells are passed on from parent to offspring; they never die, they arc 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
every 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 thai 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 tacts and to evolve narrow
and erroneous views as to the causes of vital phenomena. Biologists are
-ccking 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-
le-- 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.

Pacts 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 Weismann. 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 eves 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. 495

voice, haeniatophilia or tendency to profuse hemorrhage from .slight wounds,
gout, epilepsy, and asthma. Psychological inheritances comprise 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 ; Reversion. — Characters that never appear in the parent
may yet be 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 1 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 hcrbivora, 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 mav
be due to reversion to a primitive savage condition which, usually latent in

1 Charles Darwin : The Variation of Animals <uul Plants under Domestication, 1892, vol. ii.,
•2d ed.

496 AN AMERICAN TEXT- HOOK OF PHYSIOLOGY.

both civilized ami 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.

Regt in ration. — The farts 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 -lied. I'atho/or/ical
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 bl 1-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 follow - the same course as the
original embryonic growth. It is properly a phenomenon of hereditv.

The Inheritance of Acquired Characters. — No topic in heredity has been
more debated during the past twenty years than that of the possibilitv 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 t<> 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 i- transmitted is transmitted through germinal cells,
or germ-plasm, a- the hereditary substance contained in the germ-cells is now

commonly called. The problem then resolves itself into that of the relation
of tin' germ-plasm to the protoplasm of the rest of the body, the so-called
somatoplasm; and the question to he answered is this: Are variations in the

REPRODUCTION. 497

parental somatoplasm capable of inducing such changes in the germ-plasm thai
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 ' 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. Bos2
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-
Sequard,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 <»(* mutila-
tions, yet in these eases 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 Inward epileptiform convulsions,
the cause of which is still to be explained. Evidence from palaeontology
regarding the apparent gradual accumulation of the effects of use ami disuse
throughout a long-continued animal series seems to require the assumption of

1 A. Weismann : Essays upon Heredity, vol. L, L889, p. 432.

* J. R. Bos: Biologisches Centmlblatt, xi., 1891, S. 7:11.

3 E. Brown S('i|ii:u(l : Researches on Epilepsy, ''<•.. Boston, 1857; also various later papers.

4 H. Obersteiner : Medizmische Jahrbiicher, Wien L875, S. 17'.'.

Vol. 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 a Tier uterine development has begun, and chiefly through the
circulation. Of germinal infection- 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 .-aid to date from 1864, when Herbert
Spencer published his Principles <>f 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

REPR OD UCTION. 499

majority of scientific workers, and the excuse for introducing them into a
text-book of physiology is the hope that a brief discussion of them may
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 sperma-
tozoon and the ovum respectively, and within these two bits of protoplasm
there must be contained potentially the qualities of the two parents. There
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-grand pa rent, 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 eg<r 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 Pflfiger, 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 larva? ; 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 Epir/enesis. 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.

I rdriation. — It is a commonplace in observation that, however close hereditary
resemblance may be, it is never absolute; the child is never the exacl image
of the parent either physically or mentally. Variations from the parental type
may be either <i<-<inirc<l 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
hereditv 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 role 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 Tlieory of Pangenesis. — Darwin's " Provisional Hypothesis of
Pangenesis" was published in 1868 as chapter xxvii. of his work on Tfie 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 arc 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 may be 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 generation-, 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 tin-own oil' by every unit,

not only during the adult stale, 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
oif 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
Gal ton,1 Brooks,2 and de Vries.3 But neither the original theory nor its
modifications have been generally accepted.

Wei&mann's Theory. — Since 1880, Professor Weismann*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 Francie I ralton : "A Theory of Heredity," Journal of the Anthropological Institute, 1875.

2 \V. K. Brooks: The. Laws of Heredity, 1883.

8 II. de Vries: Die Intra cellular e Pangenesis, 1889.

* August Weismann : /■,'.- <(y upon Heredity and Kindred Biological Problems, authorized
translation, vol. i., 1889; vol. ii., L892; The Germ-plasm, authorized translation, 1893; The
Effect of External Injlncnccs upon Dccclopuinit, the Romanes Lecture, lS'.M.

REPR OD UCTION. 503

origin of sex — germ-plasm is continuous from individual to individual along
any one line of descent. Weisniaun draws a sharp line between germ-plasm,
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 gemmnles
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 th 'ganisrn. 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 organism.
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 somatoplasm
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 Weismann'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
re-eiublance 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
ot' 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

he pr on UCTION. 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 urinae 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, •'ill

in myopia, 314
relation of, to perception of distance, 356
voluntary character of, .'ill
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
cut ral nervous system, 2:23
neurones of i 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, IH»
After-pressure, sensation of, 394
Agamogenesis, 139

Age, changes in organization of the central
nervous system wit 1 1 , 28 I
influence of. on nerve-cells, 190
on visual accommodation, :!l 1
relation of brain-weights to. .'Tii

of menstrual ion to, 159
specific gravity of the nervous system with
changes in, 2*4
Albinos, condition of the internal ear in, l<>7
Alcohol, action of, on conductivity of nerves.
93

effect of, on nerve-currents, 156
fumes, action of, on nerves, 60
st imulating acl ion of, 7.")

Alkalies, action of, On nerves and muscles, till

Allantoic arteries, 17 1

vein, 474
Allantois, 171
Allochiria, 400

Ammonia, action of, on nerves, 60
Ammonium salts, action of, on muscles, 138
Amnion, 472

Amniotic cavity, 472
Amceba, contractility of, 19
Amoeboid movement, 19
in neuroblasts, 176
in ova, 22
Amphiaster, 169
Amphimixis as cause of congenital variation.

504
Amphioxus, reflexes in, 212
Amplitude of sound-waves, 381
Ampulla of 1 lenle, 1 17
Ampullae of the semicircular canals, 372
Ampullary nerves, stimulation of, 107
Amputation in man, effects of, on neurones, 196
Anaemia of the brain during fatigue, 288
Anaesthesia, contralateral, after hemisection of

the cord, 233
Anaesthetics, action of. on nerve-currents, 155
Analgesia, 232

following removal of the cerebellum, 272
Analysis of composite tones, 364
Anatomy of the ear, 362
Anelectrotonus, 62
Angular movements of joints, lit!
Anisotropic substance of muscle-fibres, 104
A nodal cont racl ion, .';•>
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 refract ion of. 303

Arteries, calcificati f. in old age, 191

Arthropods, segmental nervous system of, 212
\ it icular cart ilages, 415
Articulation. i:;i
Articulations, varieties of, 414
Artificial circulation through the heart. 69
I In ough t he muscles. 68
stimulation of muscle compared with normal,
134
Aryepiglottic fold, I ! !
Aryteno-epiglottidean muscle. 126
Arytenoid cartilages, 125

muscle. 426
Asexual reproduction. 139

I heory of t lie origin of, 1 1 1

Aspirates. |.;7

Association centre, anterior, 257

middle, 257

posterior, 257

cerebral, varial ions in. 260

fibres and centres of the cortex. 256

tracts, origin Of, from central cells. 205
Astasia after removal of the cerehel In in. 273

Asthenia from removal of the cerebellum, 27:'.
\-i igmat ic accommodal ion, •"•i|»
Astigmal ism. ::i7

detect ion of, 319

irregular, 319
Astral rays, contractility of, 470

\tavisin. !!».">

507

01 IS

INDEX TO VOLUME II.

Ataxia after removal of the cerebellum, 273
Atonia after removal of the cerebellum, 273
Atrophy of the oerve-cellB from disuse, 195
Atropia, action of, on accommodation, 311
Atropin, action of, <m the eye, 325
Attraction sphere of th«' ovum. 149
Auditory area of the cortex. 253
canal. 363

epithelium of the utricle and saccule, 37.'!
judgments, 389
meatus, external, 362
nerves, central paths of. 237
cochlear division of, 376
subdivisions of, '■>'•'•
ossicles. 365

movements of, 3<>7
sensations, limits of, 382
successive contrast in, 3S.s
theory of, 380
Auritde of the external ear, 362
Automatism, definition of, 20
Axones, definition of, 21, 173
growth in diameter of, 179
length of, 174

Ball-and-socket joint, 416

Barium salts, action of, on muscles, 138

Bartholin i, glands of, 162

Basilar membrane, structure of, '■'•77

width of, 380
Bathyasthesia, 233

Beats in musical tones, production of, 386
Benham's spectrum top, oil
Binocular combination of colors, 358
vision, 356

illusions in. .T>!i

rivalry of the fields of vision in, 358
Biophors of the germ-plasm, 503
Birds, removal of cerebral hemispheres in, ~<>7
Birth, size of the child at, 487
Birth-rate of the two sexes, 483
Births, multiple, 182

ratio of male to female. 483
Blastomeres, 17"
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, (17
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. 1 12

Bones, action of muscles on, 417
Bottcher's crystals, 1 15
Brain, curve of growth of, 279
growth of, 278

number of nerve-elements during, 2*0
relation of, to growth of the body, 280
size of neurone- during, 281
metabolic activity in, 288
regulation of circulation in. '.'-7
weight of, '-'7o
Brain-stem, .'71

Brain-ventricles, capacity of, '-71
Brain-weight, decrease of, in old age, 296
relation of, to insanity, 278
to sex, 276

to social environment, 277
Brain-weights of different races, 278
Breaking contraction, point of origin of, '■>'<
" I'.m aking " -hock. :;i
Broca'e convolution, 257
Brown-Seqnard's paralysis. 233
Brucin, action of, <>n end-plates. -.'7

Bulbo-cavernosus muscle, 449
act ion of, in erection, 4til
Bulimia, 104

Caffkin. action of. on coagulation of muscle-
plasma, 164
Calcium salts, action of, on the muscles. L38

relation of, to irritability, 59
Canalis cochlearis, 373

reuniens, 373, 374
Capillary electrometer, 14b'
( 'aput gallinagims, v\ I

Carbon dioxide, action of, on conductivity in
nerves. 'X',
on the nerves, 60
on warm spots, 398
effect of, on nerve-currents, 156
of t he muscle, 168
production of, in nerves, 95
disnlphide, action of, on nerves, 60
t lardiac palpitation at the climacteric, 490
Carnic acid of muscles, 167
( larnin of muscles 167
I a-t ration, 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
bh.od-supply of. 2N>
change in specific gravity of, with age,

284
condition of, in sleep, 293
conscious phenomena of, 172
daily rhyi Inn- 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, 29o

intensity of metabolism in, 288
mednllation 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 erect ion, 4<i4
of parturit ion. 1-1
Cent ns. associal ion, 256
Centrosome of human spermatozoa, 444
of the fertilized egg, 168
of t he on inn. 1 1!"
Cerebellum, anatomical connections of. 273
effects of injury to. 272
fund ions of, 272
senile changes in, 296
Cerebral circulation, conditions affecting, 288
hemisphere-, etl'ect of removal of, 263

relative physiological values of, 259

removal of, in birds. 266
in dogs, 262, 267
( 'ere brin of nerves. 170
Cerebrum, heat -regulat ing functions of, 270

removal of, in dogs, 267
Characters, acquired, inheritance of, 496
< lo mica! reagents, action of, on irritability, 58

INDEX TO VOLUME 11.

50(J

Chemical stimulation of nerve, 25

tonus, 143
Chemistry of nerves, 169

Chernotaxis, 406

influence of, on neuroblasts, 17»i
Chemo-tropism, 466
Chest-voice, 43:2

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 tvmpani nerve, gustatorv function of,

410
Chorion, 473

frondosum. 474

lseve, 474
Chorionic fluid, 473

villi, 473
Chromatic aberration, 316
Chromatoblasts of pleuronectidae, 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 axoms, 173

Color of objects, relation of, to intensity of illu-
mination, '■'>■'<■'>

sensations in indirect vision, '■)'■'<'■'<
phenomena of, 333

theories, 335

triangle, 334

vision, theories of, 335
Color blindness, 338

hereditary transmission of, l* 1 1

of the rods, 342
Colored shadows from simultaneous contrast, 347
Color-mixture, '■'.'■','■'.
Colors, binocular combination of, 358

complementary, •'!■'> I

physical basis of, 332

relative luminosity of, 340

saturation of, 342

i lombinational tones, 387
Commissure. Meynert's, 238

von Gidden's, 238
Commissures, origin of, from central cells. 205
Common sensation, definition of, 399

sensibility, afferent paths of the nerves of, 230
Commutators, method of using, 36
Complementary colors. :;:;i
Composite tones, analysis of, 384
Conceptions, multiple, 182
Concha of the external ear, 362
Conduction by contiguity, 81

directions of, >1

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. '.'<i7

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 voi't icella, 20

occurrence of, 20

of amoebae, 1!>

of muscle, 17

of muscles, adaptation of, to their normal
functions, 108

of ova, 22

of the astral rays, 17u
Contraction curve of muscle, effect of frequeul
excitat ion on, 115

idio-muscular, 92

of muscles, post-mortem, 160
relation of, to structure, 107

remainder, L06

wave in muscle, rate of transmission, >7
length of, B8
Contractions from repeated single stimuli, 112

introductory, 1 13

isomet ric, 110

isotonic, 1 lo

normal, tetanic nature of, 132

of rigor calorie, 165
Contracture after frequenl excitation. 128
after single excital ion, 129

ilelinit ion of, 1 16
from fat [gue, 130

510

INDEX TO VOLUME II.

Contracture in dying muscles, 132

in rigor mortis, L28

in veratria poisoning, 128

normal, 129

of the nick muscles after cerebellar injury,
272

pathological, 127, L32

relation of, to tetanus, 117. 122, 124
( 'out ractures, 127
Contrast, visual, 3 16

in space perception, 352
Convergence of the eyes in accommodation, 311

muscular mechanism of, 300
Co-ordination of the efferenl impulses in re-
flexes, -.'l 1

< lopulal ion, i<>3

( tore-conductors, 158
Cornea, curvature of, 303
Cornicnlum laryngis, 125
Cornutine, action of, on muscles, 137
Corona radiata, 454

of the ovum, 150
Corpora cavernosa of the penis, 148

striata, functions of, •-'71
Corpus callosum, functions of, 27<»
luteum, 455

spongiosum 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 efferenl impulses from, 251
latent areas of, 261
Corti, cells of, 377

organ of, structure of, 377
rorls of, 377
Cortical areas, 213

motor, in man, 250

size of, '.'17

centres, 243

motor control, crossed, 251

multiple character of, 250
regions. 243

stimulation, inhibitory effects of, 224
Cowper's gland, 443
histology of, 448
secret ion of, 1 16
Crabs, regeneration of lost parts in, 4!»6
Cranial nerves, afferent, 236
Creatin in muscle, 166
( Ireal inin of muscle, 167
Cretinism, sporadic, 289
t i ic arytenoid muscle, lateral, 426

posterior, 126
Cricoid cartilage, 425
Crico-thyroid muscles, 426
Criminals, weigh 1 of the brain in, 277

< 'rista acusl ica of the semicircular canals. 373
Critical period of nerve.-,, 66
Cross-suturing of nerve-trunks, 201

( Iruciate heat cent re, 271
Cupola of the cochlea. 375
( 'urare, act ion of, 26
Current - of acl ion in muscle, L50
in nerves, 153
of rest. 1 17

theories as to their cause, 148
Curve of fatigue with repeated single contrac-
tions, 113
of intensity of sleep, 294
of musch contraction, effect of frequent exci-
tation on, 1 15
of mnscular contractions, l1"1
of work for muscles, 1 l"
Cutaneous sensations, cortical area for, 253
disturbance of, in disease, 103

Cutaneous sensations, varieties of, 390

temperature points, 398
Cytoplasmic changes in nerve-cells, 182

Daniell cell, 28

Darwin's tin cry of heredity, 501

Death, definition of, 491

of the tissues, 192

somatic, 191

theory of, 192
Decidua graviditatis, 461,471

menstrualis, 458, 461

reflex a, 172

serotina, 172

vera, 172

I decidual cells, 171

Defecation, cerebral control of, 270

reflex character of, 213
Degeneration after hemisection of the spinal
co nl, 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, 17, 54, 7(1
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, 176
DiaTthrosis, 115

I >ia SOniC nerve-cells, 178

Differential tones, 3-7

Diffusion of central nerve-impulses, 208
of impulses in the cord, influences affecting,

217
of nerve-impulses, peripheral, 21s
I ligasl lie muscle. 126

Digitalin, action of, on coagulation of niuscle-
plasma, 164
on muscles, 137
Digitalis, action of, on nerve-, and muscles, 60
Digits, supernumerary, 194
Diopl lie apparatus of the eye, defects of, 314
Dioptric- of the eye, 300

1 >iopt i y. definition of, 30 1

Diphasic current of action, 152

Direction, judgments of, by means of auditory
sensal ions, 389
of the nerve-impulse, 184

Discord, 3-7

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, 171

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

perception of, 354

visual perception of, 348
Disuse, effect of, on muscles, 77
Dizziness, 405

Dogs, removal of cerebrum in, 267
Domestication, effect of, on menstruation in

animals, 460, 462
Dorsal nerve-roots, efferent fibres in, 203

roots, degeneration resulting from injury to,
227

spinal nerve-roots, number of fibres of, 230
Dreams, 293
Du Bois-Reymond's key, 30

law of stimulation, 32

theory of currents of rest, 148
Ductus cochlearis, structure of, 374

endolympliaticus, 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
witli 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 iii I lie retina, 331
phenomena of muscle and aerve, 144

of nerves, interpretation of, 158
st imulation of nerve, 25
of nerves, law of, :;■_'

Electrodes, shielded, 1 1

varieties of, 29
Electrostatic changes, stimulating action of, 42
Electrotonic changes of conductivity, 50
of irritability, til

in human nerves, 65
twitch, 157
Electrotonus, 62
Embryo, nutrition of, 175

rate of growth of. 1-7
Emmctropia, 313
Emmetropic eye. 312

Encephala, classification of, according to
weigbt, 275

Encephalon, specific gravity of, 275

weight of, -.'7 1. ■-,7."i
End-bulbs, sensory, 392
Endolymph, 372

End-organs, importance of, in touch sensations,
396

transmission of excitation by means of, 82
Energy liberated in contracting muscles, 138
Engelmann's theory of the nature of muscular

contraction, 105
Environment, influence of, on organisms, 493
Epididymis, 1 17
Epigenesis, theory of, 500, 504
Epiglottis, 121

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. <>!>
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

innervation of, 300
optical constants of, 303

power ill', ::n|
positions of, 299
refractive media of, 302

surfaces of, 303
spherical aherral ion of, 315

V M.i.nri w tubes, 1 13, 156

False amnion, 173

Falsetto register of the voice, 133

Far-point of vision, 312

Fatigue, cerebral amemia from, 288

curve with repealed single contractions. 111!

effect of. mi In i<_;lit of coui racl ion. 113

on muscular conl raction, 130

on rigor caloris, 165
morlis. Kin
from voluntary muscular contraction, 134
in nerve-fibres, 195

of central nervous system, 289

512

INDEX TO VOLUME II.

Fatigue of motor end-organs, 83
of muscle, <iti, TO

reCO\ <-ry from. ?;;
of nerve-cells, 136, 191

Of nerves, 75, 96

of retina. 3 I I

relation of, to sleep, 291

I heories of, 72

to auditor^ sensations, 387
Fats of muscle, 1<>7

relation of, to muscular work, 74
Female births, relative number of, 483

pronucleus, 453
Females, rate of growth in, 488
Fenestra ovalis, '■'<*>'■'>

rotunda, 363, 375

Use Of, 37b'

Ferment, myosinogen-coagulating, L61

Fertilization, Ititi

Fetal membranes, 472

Field of vision, binocular rivalry of, 358

Fimbriae of the Fallopian tube, 156

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

Frietionals, 137

Frogs, removal of cerebral hemisphere in, 264
striped muscle, time of single contraction in,
108

Frontal lobes of the hemispheres, effecl <>f re-
removal of, 262

Fuhlsphare, cortical, 252

Fundamental tone, definition of, 383

Galvani, Luigi, 28

Galvanic current, action of, on conductivity, it)

contracture effecl 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
Galvaiiotonus, 54, 131
Gamogenesis, 1 1<»
( langliou spirale of the ear, 376
Ganglion-cells, conduction in, 97
Gaseous exchanges in the brain, 288
( ia-.es of muscle, 168
Geminal fibres of the pyramidal tracts. 245

< temmules of t he germ-plasma, 499

< tenio-hyoid muscle, 426
Germinal spot of the ovary, 450

transmission of infectious diseases. Ill*
vesicle, si ructnre of, 150
( term -plasm as a basis of heredity, 499
continuity of, 502

detinit iOD Of, l!"l

morphological nature of, 199

origin of, 199
I testation, duration of, 478
Gland-cells, electric currents in, 145
Glands of Bartholini, 162

of Fittre, 1 18
Glans penis, 1 1!'
Gliding movements in joints, 41t;

Glossopharyngeal nerve, gustatory function of,

410
Glossopharyngeus, central conducting paths of,

236
Glottis. 423
oedema of, 122

respiratory movements of, 429
( HyCOCOl] in muscles, ll>7

< Hycogen of muscles, l<>7
Golgi, organ of, in tendons, 402
Graafian follicle, 15 1

Graphic method of studying muscular contrac-
tions, 99
I ira\ ity, influence of, on cerebral circulation, 287
(irav matter of the cerebrum, water contents of,

274
Growth after birth. 187

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, I7<i
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, 1 12
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
Bro\vn-Sei)iiard's paralysis from, 233
degeneration resulting from, 228
effecl of, on man, •.';'».'!

on sensation and motion, 230
in animals, 234
physiological effects of, 234
Heredity, definition of, 493

I heories of, \'.i>

Bering theory of color vision, 336
Hermann's theory of currents of rest. 1 I-
Ileteromita. reproduction in, 440
Hinge-joints, 116
Histology of striped muscle, 104
Hofacker Sadler law. 1-1

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 of lost parts in, 496
Hydrocyanic acid, action of, on coagulation of

muscle-plasma, Ki4
Hymen. 462
Hyo-glossus muscle, 426
Hyperesthesia, homolateral, after hemisection

of the cord, •!'■'•'■',
Hypermetropia, 313

range of accommodation in, 314
Hypoxanthin of muscles, 167

Idants of the germ-plasm, 503
Idiomuscular contraction, 27, !'•_>, 128
Idioplasm as a basis of heredity, 499
Ids of the germ-plasm, 503
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
Intermedins 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
series, 113

peak of tetanus curves, 124
Inversion of retinal images, 305

Invertebrates, conduction in the nerves of, 91
Involuntary muscles, rigor mortis of, 162

Ion-proteid compounds of muscle, 168
Iris, dilator nerves of, 32 t

direct response to lifjlit by, •">'-' I

innervation of, 323

movements of, in accommodation, 309
rate of, 325

muscles of, 323

relation of, to spherical alienation. 315
Irradiation in the retina, ''. 19

of nerve-impulses in the central nervous sys-
tem, 208
Irritability, definition of, 20, 23

33

Irritability, effect of blood-supply on, 66
of constant currents on. i>2
of repeated stimuli on, 65
of muscle, 25
of nerve-fibres, 21
of nerves. 24
and muscles, conditions affecting, 55
effect of section on, 69
of ova, 22
Irritants, classification of, 23
conditions determining their efficiency, 28
effect of, on irritability, 55

of variations in strength of, 39
relation of, to the response, :.' 1
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

Kakyokinetic figures in mature nerve-cells,

21 12
Katelectrotonus, 62
Kathodal contraction, 115
Kathode, physical, definition of, 52

physiological, definition of, •">■.!
Keys, electric, 30
Knee-kick, reinforcement id', 222
Krause's membrane, 104

Labia majora, 462

minora, 462
Labial consonants, 137
Labio-dental frietionals, 438
Labium tympanicum of the internal ear, 377

vest ibul are of the limbus, 377
Labor, nature of, 4*1

stages of, 179
Labor-pains, 479

Labyrinth of the ear, anatomy of, 371
Lactation, ovulation (luring, 15(1
Lactic acid of muscles, L68

Lamina spiralis, 372

Lamina- of medullary tube in the foetus, 205
Laryngeal muscles, specific actions of, 428
nerve, recurrent, 428
superior. 128
Laryngoscope, 129

Larynx, earl ilages of, 425

closure of, during muscular effort, 423

muscles of, 125
nerves of, 128
-,t met ore id', 421
Latent areas of the cortex, 261

characters, hereditary transmission of. 195
period, effed of temperature on. 136
of tension on. l lit

of inoi in- end plates, 103

of muscle, 103

of red muscle. L09

of re1 inal stimulation, 343
of simple muscular cont raction, in-.'
" Law of contraction,*' Pfluger'8, 50
Law of stimulation of human nerves by bat ter\
currents. 54

Lecit hin of nerves, L6fl
Lemniscus, medial. 226

sensory paths entering, 235
Lens, changes in. during accommodation, 307

crystalline, changes in. -with old age, 31 I
curvatures of, 303

opacities in. 321

refractive index of. 303

5] 1

INDEX TO VOLUME 11.

Lens, thickness of, 303

Lenticular ganglion, 311

Leucin, action of, on end-plates, 27

Leucocytes, movements of, 19

Leucophrys patula. reproduction of, 142

Life of the individual, stages of, 4 — ■ * >

Ligaments of tin- malleus, 366

of the incus, :!<>?
Light, changes in the retina produced by, 330

definition of, 298

dispersion of, 316, :;:;•_'

monochromatic, 316

physical theory of, :;.;i

rays of the luininiferous ether, 331

sensations, intensity of, 332, 339,

mechanism for the production of, 331
quality of, 332
Light-waves, lengths of, :;:;■.'
Limbus of the spiral lamina, 37?
Lingual frictionals, 138

uerve, gustatory function of, 410
Linguo-palatal consonants, 437
Liquids, 136
LiquoT amnii, 472

folliculi, 454
Listing's law, 299
Littre, glands of, 448
Load, effect of, mi 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 sensat ions. III! I
power, relation of, to mobility, 394
Locomotion. 120

Locomotor ataxy, disturbance of equilibrium
in. 405
mechanisms, action of, 11 1
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. :;~i
Luminiferous ether, rates of vibration of, 331
Luminosity, relative, of spectral colors, 340
Luminous sensations, intensity of, 339
Lustre in visual sensations, explanation of, 3r>s

MICROCEPHALIC brains, weight of. 275
Macula acustica, 3?::

lutea, 3-.'?
.Macula- acusticse, relation of. to static equilib-
rium. 407
Making contraction, point of origin, 35
" Making " shock. 31
Mai'- births, relative number of, 483

pronucleus. Pit;
Mali-, rate of growth in, 1-*
Malleus. 365

ligaments of. 366
Mammary glands. II.'!. 462

in pregnancy, 17?
Manubrium of the malleus, 365
Masticatory movements, effect of, on taste sen-
sations, 11 1
Mastoid antrum, 363
Maturation of germ-cells, significance of, 451

of nerve-cells. 177

of spermatozoa, 1 l">

of tin- ovum, 151
Meatus auditorius interims, 373
Mechanical stimulation of nerve. •.'.">, 56

strain, influence of, on neuroblasts, 1 Tf >

work of muscular contract ion. 13*

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

lamina- of, in the foetus, 205
Medullation, central, progressive character of,
181
of nerve-fibres, significance of, 283
peripheral. 180
Medusse, rate of conduction in, 89

staircase contractions in, 112
Membrana basilaris, 374
Saccida, 365

granulosa of the Graafian follicle, 454
reticulata, :;7-
tectoria, 377. 379
tympani, 364

Membrai f Beissner, 374, 379

Membranous labyrinth of the ear, 372
Menopause, 459, 490
Meiisi mat ion, 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, iii the brain, 288
Meynert's commissure, 238
Microcephalic brains, weight of, 275
Microcephalics, 268
Micturition, cerebral control of, 270

reflex character of, 213
Middle ear, 3112

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
Mmis Veneris, 162
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. 213
subdivision of, into centres. 217
centres, degree of separateness of. 248

of the human cortex, 250
disturbance from bemisection of the cord, 230
end-plates, latent period of. 103

i ransmission of excitation by means of, 82
nerves, fatigue of, 96

rate of conduction in, 89
Movements of joints, varieties of, 416
of spermatozoa. 1 1 1
of the eyeball. 298

Multiple conceptions, 182

conl rol from t he cortex. 250

Muscse volitantes, 320
Muscle, accelerator urimi\ 449

aryteno-epiglottidean, 42<i

arytenoid, 126

bulbo-cavernosus, 1 19

chemistry of. 150

ciliary, in accommodation. 309

crico-arytenoid, lateral, 126

INDEX TO VOLUME II.

51;

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
genio-hyoid, 426
hyo-glossus, 426
independent irritability of, 25
inferior oblique, ""»!)

rectus, 299
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,
l.;i

rate of conduction in, 89
ischio-cavemosi, I in

Of tint eve, 299

of the iris, :;•.':;
of the middle ear, i!<ii)
rate of conduction in, 89
stapedius, 370
tensor tympani, 369
Muscle-serum, 166
Muscle-sounds, L32
Muscle-spindles, 402
Muscle-structure, relation of, to its contractility,

107
Muscle-tonus, 143

Muscular < tractions, effect of drugs on, 137

of support on t lie heighl of, L22
of temperal ore on. L36
graphic record of, 99
influences affecting, in?
post-mortem, L60
-.on rce of energy in, 71
effort, closure of larynx in. 123
inhibition from cortical stimulation, 224
movements, relations of antagonistic muscles
in, 418

Muscular sensations, cortical area for, 253
definition of, 390

effect of heinisection of the cord on, 235
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, 75
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
Myohsematin, 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, 105
Near-pointof vision, 312
Negative after-images, 346

variation in muscles, rate of propagation. L52
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, •'!■-'•'!

recurrent laryngeal, 428

superior laryngeal. 428
Nerve-cells, atrophy of, from disuse, 195

changes with age in, 190

chemical changes in, 191

classification of, 177

connections between, 206

degeneral ion of the cell-bodies of, 199

diaxonic, 17*

elfect Of exercise on. To'

fatigue of, 191

general ion of impulses in, 188

growth of, 17<>

histological changes due to functional activity

in, I!'-.'
human, size of, 17 I
internal si rucl lire of, 1 T! '
maturation of, 177
morphology of, 17:'.

number of, in the central nervous system, 283
ii ii t ri 1 ion of, 190
nutritive com vol of, over nerve-fibres, ins

Of animal-, size of, 175

of spinal ganglia, development of, 178

peculiarities of. 1 7 I

pyramidal, 17*

rate of discharge from. L89

regeneration of, 201

relation of si/.e and function in. 175

senescence of. 182, 490

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 tin- skin, 392
Nerve-fibres, classification of, '-'l

cortical, increase in the aumber of, during
-i-owili. 282

fat igue in, L95

functions of, 21

reaction of, 170
Nerve-impulse, definition of, 25

direction of tin- passage of. 1-1

electrical variation accompanying, 1-:;

generation of, 1-7

in peripheral nerves. i-:;

peripheral diffusion of. 218

reversed, in spinal ganglion-cells, L85

theories of, !'?

transmission of, from neurone to neurone, 207
Nerve-muscle preparation, :;i
Nerve-trunks, isolated conduction in, 7!)
Nerves, action currents in, 153

auditory, central path of, 237

chemistry of, 169
cross-suturing of, 200

current of rest in, 1 1!'

degeneration id', after section, til', 78

fatigue of, 75

glossopharyngeal, central conduction paths

for, 236
in man, stimulation of, "1
law of stimulation of. with galvanic currenl . 50
limitation of the rate id' stimulation in, 126
lingual, gustatory function of, 410

medulla! ion 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 intermedins), 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 Mores in, 236
Nervi erigentes, 164

Neuroblast, development of. IT*;

Neuro-kerat in 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. !'7

connection between, 206

double conduction in. 185

increase in number of, during growth, 282

internal structure of. 179

polarity of. 1-1

total number of. 283
Nicotin, action of, on end-plates, 27

on sympathetic ganglia, 219
Nissl method for study id' nerve-cells, 195

substance, iron in. 191
of nerve-cell-, 170
Nitrogen, free, of muscles, 168
Nitrogenous extractives of muscle, 166

Nodal point in the simplest dioptric system, 301

Nceud vital. 236

Noises, definition of, 388

Non-medullated nerves, rate of conduction in, 90

stimulation fatigue of, L80
Non-polarizable electrodes, 29
Nose, anatomy of. 108

respiratory t ract of, 408
Nutrition of nerve-cell.-. L90

of the embryo, 475
Nutritive control of mrve-cell bodies over

nerve-fibres, 198
Nymphse, 462
Nystagmus after cerebellar injury, 27'.'

< it l LOMOTOB nerve, ciliary fibres of, 311

relation of, to the iris, 323
Odors. 110
(Edema of the .-lot t is. 422

< ild age of the central nervous system, 295

Olfactory area id' the cortex, 253

cells, 108

epithelium. 10-

nerves, central paths of, 241

paths to the brain, 409

stimuli, conditions affecting, 109

tracts, section id', in sharks, 264
( hno-hyoid muscle. 425

Ontogenetic development of nerve-cells, 177
Onychodromus, reproduction of, 442
Oocyte, 4r.l

< Ophthalmometer, 304
Ophthalmoscope, 326

Optic commissure, decussation 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, :J>51
properties of striated muscle, 103

< Optograms, 330

Organ of Corti, structure of, :;77

of Golgi in tendons, 402
I Organization in the central nervous system, 285

relation of educahilitv to the establishment
of, 286
Organs, growth of, 486
Oscillatory activity of the retina, 344
( Os orbiculare of the incus, 366
I Ossicles, auditory, 365

of the ear. movements of, 367
Otitis media. 364
( Otoconia, 37 l
otolith-. :;7l
Ova, 140

number of. in human ovary, 451
( Ovaries, 1 13

effect of removal of. on menstruation, 459

st rucl me id'. 154

Overtone-, definit ion of. :!-::

inharmonic, 386
Oviduct-. 1 13, 150
Ovulation, 155
( Ovum, chemist ry of, 450

fertilization of. 166

human, structure of. 1 I''

maturation of, 151

physiological properties of. 22

segmental ion of, 167

Stages in t he maturation of. 152
Oxygen, storage of, in muscle, L69
Supply, relation of, to irritability, lis

IXDEX TO VOLUME II.

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 muscle, phvsiological peculiarities

of, 109
Pangenesis, Darwin's theory of, 501
Papilla foliata of rabbits, 410
Paradoxical contraction. 157
Parallax, use of, in estimation of distance, 356
Paralysis after removal of motor areas, 269

agitans, 296

Brown-Sequard's, 233

homolateral, after hemisection of the cord,
233
Paramcecium, reproduction in, 440
Paramyosinogen, 163

temperature of heat coagulation of, 165
Paresis following removal of the cerebellum, 272

from injury 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 tours, 381

of the voice, 130, 432
Pituitary membrane, 408
Pivot-joint, 417
Placenta, IT I

Placental transmission of infectious diseases,
498

villi, 17 1
Plants, regeneration of lost parts in. !!><;
Pohl'a mercury commutator, '■>*>
Polar amphiaster of the ripening egg, 153

bodies, 151

of the ovum, 453
Polarity of neurones, 184
Polarization, after-effects of, (;.">

physiological, of neuroblasts, 1 T« ;
Polarizing current, effect of. on conductivity, 50
on muscles, o'!
on nerves. 62
Pole-changers, ::<i

Polyspermy, 471
Poinum Adami, 42~>
Positive after-images, 346
Posterior association-centre, 257
Post-ganglionic 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,

•_'l!l
Pregnancy, effects of, on the mother, 17?
Presbyopia, 314
Pressure, effect of, on irritability of nerves, 56

infiuence of, on conductivity, 92
Pressure-points of the skin. '■'>'.)<>
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 -ysteru, 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, 166
Proptosis after cerebellar injury. 272
Prostate glands, 1 13
histology of, 4 18
secret ion of. 1 Hi
Prostatic lluid. 14(5. IIS
Protagon of medullary substance. l?(i
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

l'urkinje's figure, 321
phenomenon, .".In
explanation of. :: 12
Purposeful reflexes, 215
Pyramidal fibres. Dumber of, 246
t racts in t lie cord, 2 15
geminal fibres of, 215
si/.e of. 252
nerve-cells, development of. it-'

<VM M)i;[ P] BT8, I-:'.

Quality of musical tones. 383

of t he voice. 130

Quinine, action of. on coagulation of muscle-
plasma, I'ii
Quintuplets, !-•'•

Race, relation of brain-weight to. 278
Range of accommodation, normal, 312
Kate of conduction in muscles, -T

in nerves, 89
of excitation, effect of. on the contraction
curve. 1 1 1

of stimulation in voluntary contractions, 133

518

INDEX TO VOLUME II.

Bate of stimulation, limitations of, for muscle,
L26
required to tetanize, L25

lii'Mi'timi ot' degeneration, 48, 54, 70
of muscles, L59

of nerve-cells, 191

of nerve-fibres, lTn

time, 291
in old age, 491
Recurrent laryngeal nerves, 428

sensibility of the anterior roots, 204
Bed striped muscles, physiological properties of,

109
Beduced eye, 304
k( !l< \ actions, simple, 208

air, -.'(in

frog, 209

segmental reaction, 210

stimulation of the nervous system, 208

tonus of muscular tissues, 220
Reflexes, co-ordinated, 211

en 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
Refractive index of the aqueous humor, 303
of the lens, 303
of the vitreous humor, 303

media of the eye. .'!()•.'

surfaces of the eye. 303
" Refractory period " of nerves, 57, 66
Regeneration of lost parts, 496

of nerves, 7-. 199
Registers of the voice, 132
Begular astigmatism, :il7
Reinforcement of reflexes, 222

of the knee-kick, 222
Reissner, membrane of, 371
Rejuvenescence by sexual reproduction, 442
Relaxation of muscle, nature of, 99
Beproduction, asexual. 439

of Leucophrys patula, 442

of onychodromus, l 12

of stj lonychia, 1 12

sexual, 140

element-- of, 440

theory of, 1 II
Reproductive organs, female, 449
internal secretions of, 162
male, 443

process, 463
l;> 3onance of the ear, 388
Resonants, 136

Resonators, analysis of sounds by, 385
Bete vasculosum of the testis, l'7
Betina, changes produced by light in, 330

circulation in. 322
histology of, 329
oscillatory activity of, '■'■ 1 1

Space percept ions by, 3 18

structure of, 327

Ret inal currents, 331
images, inversion of, 305
size of, 305

stimulation, after-effect of, 345
fatigue in. 34 l

Ret inal stimulation, latent period of, 343
laws of, 343

rise to maximum for different colors, 343
vessels, demonstration of, 321
Ri version to ancestral characters, 495
Bheocord, 11
Bheonome, .'il

Rheoscope, physiological, 148
Rbeoscopic frog, 1 18
Rheostat, 40
Bhinencephalon, 241
Rigor caloris, 57, 164
contracture in, 128
effect of fatigue ou, 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
Rima glottidis, 423
respiratoria, 423
vocalis, 423
Ritter's opening tetanus, 37, 61

tetanus, 131
Rod-and-cone layer, function of, 327
Rod-pigment, 339. See also Visual purple.
Rods and cones, function of, 341
number of, 330
of Corti, 377
retinal, function of, 341
Rota i ion, movements of, 416
Running, 421
Rut of animals, 460

Sacculus of the internal ear, 373
Saccus endolymphaticus, 373
Saddle-joint, 41(i

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, (ill
Santorini, cartilage of, 122, 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, 37.">
vest ibuli, 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

react ions, reflex, 210
Segmental ion, h;7
Segmentation-centrosomes, 469
Segmentation-nucleus. 466
Semen, composition of, 445
Semicircular canals, membranous, 373
of the bony labyrinth, :'.?1
relation of, to equilibrium, 405
sect ion of, 105
Seminal vesicles. 1 13
function of, I 18
secretion of, I l'>
Semivowels, 436
Senescence of nerve-cells, 182

of the central nervous system, 295

INDEX TO VOLUME II.

519

Senescence, phenomena of, 186
Sensation, cutaneous, definition of, 390

muscular, definition of, 390

of after-pressure, 394

of light, mechanism for the production of,

331
of temperature, 3!»7
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, 253
relative functional importance of, 270
disturbance from hemisection of the cord, 230
impulses, path of, in the central nervous sys-
tem, 226
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 iu, 263
Shrapn ell's membrane, 365
Siamese twins, 483

Simple muscular contraction, duration of, 102,
108
explanation of, 101
Simultaneous contrast, 347
Singing voice, 13 1
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, 108

comparative physiology of, 409

subjective sensations of, 410
Smooth muscle, rate of conduction in, 89
Snails, regeneration of lost parts in. i!tt>

Somatic death, 191
Somatoplasm, definition of, 496
Somatopleure, 172
Sound, physical, 381
Sounds, quality of, 383
Sound-waves, amplitude of, 381

composite, 384

limits of perception of, -'I-0-'

production of, 381
Space illusions, 354

Space-perception from visual sensations, :;it
Specific energies of nerves, doctrine of, 232, 399

Specific energy of the optic nerve. 331
gravity of muscle, 159
of the encephalon, 275
of the nervous system at different ages, 28 I

nerve-energy, doctrine of, 232, 399
Spectral colors, incomplete saturation of, 342
Spectrum, 332

luminous intensity of the colors of, 340
top, Benham's, 31 1
Speech, dependence of, on hearing, 431

elements of, 133
Speech-centre. 257
Spermatids, 445
Spermatocytes, 445
Spermatozoa, 1 lo
contractility of, 20
discovery of, 1 13, 498
entrance of, into the uterus, 465
locomotion of, 4<!5
maturation of, 445
movements of, 1 14
structure of, 113
Sperm-aster, 4U7
Sperm-nucleus, 466
Spermine, 4 15
Spherical aberration. 315
Sphincter ani, cerebral control of, 270
iridis, 325

urethra-, 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, 2 15
reflexes in man, after section of, 213
schematic cross-sectiou of, 202
weight of, 271
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, :;<;7

Starvation, effect of, on the nervous system, 289
static equilibrium, organs of, 107
Stature, relation of brain-weight to, 276
Stenson's experiment, 67
Stereoscope, 356
Sterno-hyoid muscle, 125
Sterno-thyroid muscle, 425
Stimulants, etfect of. on DQUSCUlar work, 75
Stimulation fatigue of noii-iucdullaled nerves,
l-o

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 b1 rengl h of, 39
galvanic, contracture effect of, in muscles, 131

variat ions iu intensity Of, 31

Strabismus, 300

Stria' aCUSl ide, 237

g] :i\ idaruin, 177
St r\ clmin. act ion of. on diffusion of impulses in
the cord. 217
on end-plates. 27

on sympathetic ganglia, 219

tetanus, 217
Stylo-hyoid muscle. 126
Stylonychia, reproduction of, 142

Successive colli l\l-t. 346

Sugar of muscles, L67

520

INDEX TO VOLUME II.

Sugar, use of, iu muscular work, 74
Summation of contractions in muscle, 121

of stimuli in nerve-cells, 190
in reflex action, 211
Superior laryngeal nerve, 428

oblique muscle, 299

rectos muscle, 299
Supernumerary digits, 494
Sustentacula! cells of the crista acostica, 374
Suture "f skull-bones, 414
Swallowing, action of the epiglottis in, 422
Sylvian heat-centre, 271

Sympathetic ganglia, action of nicotiu on, 219
of st rychnin on, 219

nerves to the iris, 324

pains, 400

system, connection of, with the cerebro-spinal,
2 I B
post-ganglion ie fibres of, 219
pre-ganglionic fibres of, 218

vibration, 385
Symphysis, 414
Syndesmosis, 414
Syuovial 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-sensat ions, conditions which influence, 411

primary, 412
distribution of, 413
Taurin in muscles, 167
Tea, stimulating action of, 7.">
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 ill' muscular contraction, 136

nerves, 397

of the blood from the brain, 288

rise of, from lesions of corpora striata. 271

sense, 397

spots of tin- skin. 39S
Tendon rethxes 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. Ill)
Tensor fcympani muscle, 369
Tentacles of Actinia-, contractility of, 20
Terminal arborizations, definition of, 174
Tertiary positions of the eve, 299
Testis, 443

ducts of, 1 17

histology of, 1 16
Tetanic contractions, height of, 120

relative intensity of, 126
Tetanomotor, 56

Tetanus. 66

anal} sis of, 123

complete, 120

curves, introductory peaks of. 124

explanation of, 121

from strychnin poisoning, '.'17

iiii iplete, 1 17

normal physiological, L32

of the muscles, 127

rati- of stimulation required for. 125

Bitter's, 37, 61. 131

Tetanus, secondary, 150

voluntary, 133

W'undt's' :;7
closing, 61
Thalamus, cortical connections of, 271

heat -cell tie of. 271
Thermal energy liberated in muscle. 141

stiiuulat ion of nerve, 25
Thermopile, 142

Thermotaxis, relation of cerebrum to, 270
Thirst. 10 1

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. :\-\
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, 1-:;
Trophic impulses to muscles, 70

influence of neurones on one another, 197

nerves of the muscles. 70
Tubuli recti of the testis. 1 17
Turtle's striped muscle, time of conti'action 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, 174

vein. 17 1
Umbo of the tympanic membrane, 365
Unconsciousness, 293
Unipolar excitation for localized excitation, 45

nerve-cells, development of, 178

stimulation, •';"
principles of, 43
Urea in muscles, 167
Urethra, 143

structure of, 1 18
Uric acid iii muscles, 167
Uterus, 1 13, 156
Utriculus of the internal ear. 373

Vagina, 143, 162

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
Yas deferens. 1 17

INDEX TO VOLUME II

521

Vasa-deferentia, 443

efferentia of the testis, 447
Vaso-niotor nerves of the cranial vessels, 286
Ventral nerve-roots, number of fibres of, 230
Ventricles of Morgagni, 422

of the brain, capacity of, 274
Ventricular bands, 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
Vitelline 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, 134
tetanic character of, 133
reactions, afferent paths of, 226
anatomical mechanism of, 226
compared with reflex, 225
von Gudden's commissure, 238
Vorticella. movements of, 20
Vowel-sounds, 131
differences in quality of, 385
production of. 434
Vulva, 443, 462

Walking, 420

Wallerian 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, condition^
affecting, L39
by muscular contraction, curve of, 140
Worms, segmental nervous system of, 212
Wrisberg, cartilages of, 422, 425
Wundt's closing tetanus, 37, 61, 131

Xantiiix of muscles, 167

Young-Helmholtz theory of color vision, :;:;"

Zollner's lines. :;r.i
Zona pellucida, 4~>4
of the ovum, 450
radiata of the ovum, 449, 450

GENERAL INDEX.

Abdominal muscles, action of, in vomiting, i.
387
respiratory action of, i. 407

respiration, definition of, i. 398
Absolute muscular force, ii. Ill
Absorbents, i. 318
Absorption, effect of alcohol on, i. 535

in the small intestine, i. 313

in the stomach, i. 312

mechanism of, i. .">12

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 oxyhemoglobin, i. 41
Accelerator centre, cardiac, i. 177
respiratory, i. 457

nerves of the heart, i. 167, 169

uriuse 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. 5 12
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. 285, 566
Acid, acetic, i. 536

acetyl-acetic, i. 537

acetyl-propionic, i. 538

amido-acetic, i. 5:;?

amido-ethyl-sulphonic, i. 543

o-amido-a-thiopropionic, i. 546

aspartic, i. 557

benzoic, i- 569

butyric, i. 539

capric, i. 5 1 1

caproic, i. 540

caprylic, i. 511

carbamic, i. 5 is

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

a-e-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. 55s

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-acel ic, i. 569

propionic, L. 538

salts of muscle, ii. 168

sarco-lactic, i. 546

silicic, i. 519

stearic, i. 511

succinic, i. 557

sulphuric, i. 506

sulphurous, i. 506

thiolactic, i. 517

thymic, i. 57!'

uric. i. 322, 338, 551, 557
Acidity of worked muscles, ii. 168
Acids, action of, on nerves and muscles, ii. 60

effect of, on pancreas, i. 236
AcinUS, delinit ion of, i. 212

Acquired characters, inheritance of, ii. 496

variat ions, ii. 500

Acromegaly, i. 278

Ad [nic rays of the luminiferous ether, ii. 331
Action current, diphasic, ii. 152
in t he heart, ii. 152

in the muscles, ii. 150

in t he nerves, ii. 15.",. IS.",
\damkie\vic/. reaction for proteids. i. 576

Adam's apple, ii- 125

523

524

GENERAL INDEX.

Addison's disease, i. 271
Adenin, i. o.i'.K 55 1
Adipocere, i. 5 1 1 . •">•>!•

Adrenal bodies, internal secretion of, i. 272
removal of, i 271
secretory nerves of, i. 272

extract, action of, on muscles, ii. 138
physiological action of, i. 271
Aerial perspective, ii. 355

Afferent impulses, effect of, on irritability of the
central nervous system, ii. "J23

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. 28 1
Air, alveolar, composition of, i. 413

atmospheric, composition of, i. 410, 413

complemental, i. 4:27

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. ii'.'
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

melicvl, i. 540

nutritive value of, i. 358

physiological action of, i :'..">7, 535

propyl, i- 536, 538

stimulating action of. ii. 7">

toxic effects of. i. 359
Alcoholic fermentation, i. 535
Alcohols, mon atomic, i. 531

Aldehydes, general properties of, i. 534

Aldoses, i. 561

Alimentary canal, movements of, i. 369

principles, i. 276
Alkalies, action of, oh nerves and muscles, ii.60
Allantoic arteries, ii. 474

vein. ii. 171
Allan toin, i. 555
Allautois, 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
Ammoniacal 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. Ill
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
Ampulla? of the semicircular canals, ii. 372
Ampullary nerves, stimulation of, ii. 407
Amputation in man, effects of, on neurones, ii.

196
Amvgdalin, 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 bodv, 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
AnSBmia of the brain during fatigue, ii. 288
Anicsthesia, contralateral, after hemisection of

the cord, ii. •.':;:;
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.

:>-i:>

Aiielectrotonus, ii. 62
Angular movements of joints, ii. 416
Animal foods, composition of, i. 278
heat, i. 467

source of, i. 474
Anisotropic substance of muscle-fibres, ii. 104
Annulus Vieussens, i. 159
Anodal contraction, ii. '■',*'<
Anode, physical, definition of, ii. 52
physiological, definition of, ii. 52
Anosmia, ii. 411
Antalbumid, i. 293
Anterior association centre, ii. 257

roots, recurrent sensibility of, ii. 204
Antilytic secretion, i. 230
Antimony poisoning, i. 514
Anti-peptone, definition of, i. 293

nature of, i. 302
An ti peristalsis, 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
Apncea, definition of, i. 440
foetal, i. 464
phenomena of, i. 44
relation of vagi to, i. 442
Apomorphia, 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. I hi
stimulation of muscle compared with normal,
ii. 131
Aryepiglottic fold, ii. 422
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. 411
circulatory changes in, i. 445
effects of, on the blood-vessels, i. 202

on the respiratory rhythm, i. 125
stages of, i. 145
Aspirates, ii. 437

Aspiration of the thorax, influence of, on the
circulation, i. 77. 95
on the lymph-flow, i. 1 17
on venous circulation, i. 77. 95

Assimilation, general characteristics of, i. 19

Associated respiratory movements, i. 408
Association centre, anterior, ii. 257

Association centre, middle, ii. 257
posterior, ii. 257
cerebral, variations in, ii. 260
fibres and centres of the cortex, ii. 256
tracts, origin of, from central cells, ii. 205
Astasia alter removal of the cerebellum, ii. 273
Asthenia from removal of the cerebellum, ii. 273
Astigmatic accommodation, ii. 310
Astigmatism, ii. 317
detection of, ii. 319
irregular, ii. 319
Astral rays, contractility of, ii. 470
Asymmetrical carbon atom, definition of, i. 515
Atavism, ii. 195

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.
Iti7
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
meatus, 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. L15
Auricles, connection of. i. 135
degree of emptying, in systole, i. 138
functions of, i. L35

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 tilling, i. 137
Auriculo-ventricular valves, i. 108
Auscultation, i. L18
Automal ism, deflnil ion of. ii. 20
Axilla, temperature in. i. 468
Axones, definition of. ii. 21. 173
growth in diameter of, ii. 179
length of, ii. 171

|: \. ii kiai. decomposition in the intestines, i.

309
Ball-and-sockel joint, ii. 1 16

Banl ing diet, i. 353

Barium salts, action of, on muscles, ii. 138

Barometric pressures, effecl of, on respiration, i.

134
Bartholin, duct of. i. 217

Lilands of. ii. 162
Basilar membrane, structure of. ii. 377

width of. ii. 380
Basophiles, i. 17

Baths, influence of, on body-temperature, i. 471
Bathyasthesia, ii. 233

Beats 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 arid, i. 340, -— >• ii •

Benzol, molecular constitution of, i. 56?!

BenzopjTol, i. 571

Bidder's ganglion, i. 1 1-

Bile, amount secreted, i. 246, 321

antiseptic property of, i. 326

composition of, i. 245, 32J

discharge of, from the gall-liladder, 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

secret ion 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

Pettenkoi'cr- 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 fistula, i. 321
Bilicyanin, i. 571
Bilirubin, i. 245, 574
Biliverdin, i. 245. 574
Bilixanthin, i. 571
Binocular combination of colors, ii. 358

vision, ii. 356

illusions in. ii. 359

rivalry of the fields of vision in. ii. 358
Biopbors of the germ-plasm, ii. 503
Birds, removal of cerebral hemispheres in, ii. 267
Birth, size of the child at. ii. 187
Birth-rate of the two sexes, ii. 483
Births, multiple, ii. 482

ratio of male to female, ii. 483
Biuret, i. 5 1!'
Bladder, urinary, movements of, i. 369, 390

vaso-motor nerves of, i. 209
Blastomeres, ii. 17o
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. 51

defibrinated, i. 34

distribution of, in the body, i. 63

foreign, action of, on the heart, i. 192
■ n- exchanges of, i . Ill

general function of, i. •'!:;

histological structure of, i. 33

identification of, i. 57:;

oxidations in, i. 123

reaction of, i. 34, 290

regeneration of, after hemorrhage, i. 63

specific gravity of, i. ::i

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. 17

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 svstem, 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. 132
Body-sense area of the cortex, ii. 252, 254
Body-temperature, rise of, from injury to the
opt ic thalami. ii. 271
from lesions in the corpora striata, ii. 271
Body-weight, influence of, on heat-productiou,
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 (luring, 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. 27<i

to social environment, ii. 277
Brain-weights of different races, ii. 278
Breaking contraction, point of origin of, ii. 35
•' Breaking " shock, ii. :;i
Broca's convolution, ii. 257
Bromelin, i. 280
Bromine, i. 508
Bronchia] capacity, i. 427

const rictor nerves, i. p;~,
Broncho-dilator nerves, i. p;.~>
Brown-Sequard's paralysis, ii. 233
Brucin, action of, on end-plates, ii. 27
Brunner's glands, i. 243
Buffy coat, i. .">5

Bulbo-cavernosus muscle, ii. 149
action of, in erection, ii. 464
Bulimia, ii. 404
Butvric acid, i. 539

GENERAL INDEX.

527

Cadaverin, i. 543
Caffein, i. 553

action of, on body-temperature, i. 472
on coagulation of muscle-plasma, ii. 164
on kidneys, i. 254
Calcium, absortion of, i. 525
carbonates, i. 524
chloride, i. 523
excretion of, i. 526
fluoride, i. 510, 523
phosphates, i. 523
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. 356
nutritive value of, i. 356
relation of, to blood-coagulation, i. 57, 524
to irritability, ii. 59
sulphate, i. 523
Calorie, definition of. i. 504
Calorimetric equivalent, i. 478
Calorimetrv, direct and indirect, i. 365, 475,

478
Canalis cochlearis, ii. 373

reuniens, 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. 5 HI
Caprylic acid, i. 541

Capsules, suprarenal, extirpation of, i. 271
Caput gallinaginis, ii. 464
Carbaniic acid, i. 548

relation of, to urea formation, i. 336
Carbamide, i. 548

Carbo-hsemoglobin, nature of, i. 39
Carbohydrates, absorption of, i. .':I7
affinity of cell-substance for, i. 568
chemistry of, i. 56]
combustion equivalent of, i. 365
definition of, i. 561
digestion of, in the stomach, i. 296
dynamic value of, i. 175
fermentation of, in the intestines, i. 310
molecular constitution of, i. 561
nutritive value of, i. 277, 353

origin of fat from, i. 352
proteid-protection by, i. 56H
synthesis of, i. •_'<>
Carbon dioxide, action of, on conductivity in
nerves, ii. '*'■>

on the heart, i. I'll

on the nerves, ii. fill

on warm spots, ii. 398
dyspnoea, i. 444

etfeet of, on nerve-en rrents. ii. 156
elimination, conditions affecting, i. 429

cutaneous, i. 122

estimation of, i. 428
inhalation, effects of, i. 1 10

Carbou dioxide, occurrence of, i. 517
of muscle, ii. 168
of the blood, extraction of, i. 517
production of, in nerves, ii. 95
properties of, i. 518
tension of, in the alveoli, i. 413
in the blood, i. 416
disulpbide, anion 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. 176
cycle, analysis of, i. 122
definition of, i. 101
duration of, i. 123
dyspnoea, i. 11 1

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
C'arnic acid of muscles, ii. 167
Carnin, i. 551

of muscles, ii. 167
Casein, i. -jtil

composition of, i. 579
curdling of, by acids, i. 296
by rennin, i. 295
Castration, effects of, ii. 463

on t he voice, ii. 131
Cataleptic riijor, ii. 1(50
Catalysis, i. •>•_', 503
< Saudate 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
('ells, growth of, ii. 186

Central cells, importance of, in relation to in-
crease of organization, ii. 285
nervous system, amount of blood in, ii. \>,ss
arrangement of cell groups in. ii. 205
blood-supply of, ii. 286

change in specific gravity of, with age, ii.
284

( lit ion of, in sleep, ii. 293

conscious phenomena of, ii. 17'.'
daily rhyt Inns of, ii. 289

development of, ii. L72

fatigue of, ii. 289

general arrangement of, ii. 202

fund ions of. ii. 171

influence of the thyroid <>n growth of, ii.

289
in old age. ii. 295
intensity of metabolism in, ii. 288

medullation of nerves in, ii. ls|
operation on, in frogs, ii. 265

528

GENERAL INDEX.

Central nervous system, organization of, at dif-
ferent ages, ii. 28 l

neurones of tin- spinal cord, ii. 203

stimulation "t' the oervous system, ii. 28
Centre, augmentor of the heart, i. 17?
cardio-inhibitory, i. 176
defecation, i. 387
deglutition, i. :s?7
expiratory, i. 457
inspiratory, i. l.~>7
micturition, i. 391, 393
of bearing, 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. 46 1
of parturition, ii. 1-1
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. Ill
of the fertilized egg, ii. 168
of the ovum, ii. 449
f erebelluin. 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-niotor 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
Cerebri n, i. .v><i

of nerves, ii. 170
Cerebrum, heat regulating functions of, ii. 270
removal of, in dogs. ii. 267

< terotyl alcohol, i. .", pi
I e] nmen, i. 257

Cervical sympathetic, vaso motor function of, i.
193

< > t\ 1 alcohol, i. 540

Characters, acquired, inheritance of, ii. 496
Chemical reagents, action of, on irritability, ii.
58

stimulation of nerve, ii. 25

tonus, ii. 1 13
Chemistry of nerves, ii. 169
( Ihemotaxis, ii. 166

influence of, on neuroblasts, ii. 176

• b.emo-1 ropism, ii. 166

Chest, effects of opening, i. 115

• Ihest-voice, ii. 132
Cheyne-Stokes respiration, i. 124
chief cells of the gastric glands, i. '.'.''.7

( hinese wax. i. 540

< Ihinolin, i 571

Chloral, effect of, on the respiratory rhythm, i.
125

hydrate, i. 536
Chlorine, inhalation of. i. 1 111

occurrence of, i. 507
i Shlorocruorin, i. 578

< Ihloroform, action of, on coagulation of muscle

plasma, ii 164
effect of, on nerve-currents, ii. L56

Chloroform, fate of, in the body, i. 533

vapor, action of, on nerves, ii. 60
Chocolate, nutritive value of, i. 357
Cholagogues, i. 2 16
Cholesterin, i. 575

amount of, in the blood, i. 51

disl ribution of, i. 325

e.xcret ion of, i. 325

in nerve, ii. L69

of bile, i. 245

of milk, i. •.'ill

Of sebaceous secretion, i. 257

< boletelin, i. 57 1
Cholin, i. 511, r,4:5
Cholo-lnematin, i. 323
Chondroitic acid, i. 578

< 'homl ro-mucoid, i. 578

Chorda t\ in pa 11 i nerve, gustatory function of, ii.

llll

vaso-dilator function of, i. 194

Chorda- tcndineie, i. 109
Chorion, ii. IT",
frondosum, ii. 474
laeve, ii. 17 I
Chorionic fluid, ii. 473

villi, ii. 4?:;
Chromatic aberration, ii. 316
Chromatin, i. 22, 28

Chromatoblasts of pleuronectidae, ii. 20
( 'hromo-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.

4. ".4
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 bydriodic acid. i. 509
of hydrofluoric acid. i. 510
of the bile, i. :;•.':;. 324
of the blood, causes of, i. 77
definition of, i. 76
discovery of, i. 7t>
in the retina, ii. 322
microscopic appearances of, i. 80
portal, i. 77
pulmonary, i. 78, 103
of the brain ami cord, ii. 286

rate of. i. 79, 98
pulmonary, i. 103
renal, i. 255
Circulal ion-time. i. 79
Circumduction, movement of. ii. 416
Climacteric, ii. 159, 190

ovulation after, ii. loll
Climate, influence of, on age of puberty, ii. 489
on bodj temperal a re, i. 169
on time of climacteric, ii. 490

< litoris. ii. 462

homology of, ii. in 1
Clothing, influence of. on heat-loss, i. 486

CKXFJiAL f.XDF.X.

529

Clotting of blood, i. 55

of milk, i. 295
Clupein, i. 580

CO; elimination, cutaneous, i. 258, 342
during muscular wmk, i. 361
sleep, i. 361
Coagulated proteids, properties of, i. 578
Coagulating enzymes, definition of, i. 280
Coagulation of the blood, accelerating agents of,
i. 61
conditions necessary for, i. 57
description of, i. 54
intravascular, i. 60
nature of, i. 60

retarding influences affecting, i. 61, 62
theories of, i. 55, 56
time taken by, i. 55
uses of, i. 55
of milk, i. 295

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 the auditory nerve, central

paths of, ii. 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. 26 1
Combinational tunes, ii. 3*7
( 'onibined proteids. i. 579
Combustion, i. 501

equivalent of foods, i. 365
Comedones, i. 257
Commissure, Meynert's. ii. 23*

von Gudden's, ii. 238
Commissures, origin of, from central cells, ii.205
Common sensation, definition of, ii. 399
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. 1*2

::i

Concha of the external ear. ii. 362
Condiments, nutritive value of, i. 359
( londucl ion by contiguity, ii. 81

directions of, ii. 84

from neurone to neurone, ii. 84

in branching nerves, ii. bO

in ganglion-cells, ii. 97

in muscles, ii. 80

in nerve-trunks, ii. 79

in ner\ es, effects of, ii. 95

in the heart of the contraction wave. i. 154

of nerve-impulses, direction 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
Contluxion in space perception, ii. 353
Congenita] resemblances, ii. 494

varial ions, ii. ."><>()
Congo-red test for mineral acids, i. 289
Conin, action of, on end-plates, ii. 27
Conjugate foci in a dioptric system, ii. 302
Conjugated sulphates, nutritive, history of, i. 340
( lonjugal ion, 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 amoebae, ii. L9

of as1 ral lays. ii. 470

of living mat ler. i. 21

of muscle, ii. 17
adaptation of, to their normal functions,
ii. Id*

of ova. ii. 22

of plain muscle, i 370
Contraction curve <>f muscle, effect of frequent
excital ions on, ii. 1 1">

idio muscular, ii. 92

of muscles, post-mortem, ii. 160
relation of, to structure, ii. IC7

remainder, ii. 106

volume of t lie heart, i 105

wave in muscle, rate of transmission of, ii. B7
length of, ii 88
of the heart, rate of propagation of, i. 153

Contractions from repeated single stimuli, ii. 1 12
inl roductory, ii. 113
isomet lie, ii 110

isotonic, ii. 110

normal, tetanic nature of, ii 132
of rigor caloris, ii 1 65
Contracture after frequenl excitations, ii. 128
after single excitation, ii. 129
definition of, ii- 1 1<>

530

( } EX ERA L INDEX.

Contracture from fatigne, ii. 130
in dying muscles, ii. 132
in rigor mortis, ii. 128
in veratria poisoning, ii. 128
normal, ii. L29

hi aeck muscles after cerebellar injury, ii. 272
pathological, ii. 127, 132
relation of, to tetanus, ii. 117, 122, 124
Contractures, ii. 12?
( 'mit rast, ii. 346

in spare perception, ii. 352
Convergence of the eyes in accommodation, ii.
311
muscular mechanism of, ii. 300
Co-ordination of the efferenl impulses in re-
flexes, ii. 214
Copulation, ii. 463
( lore-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
ligation of, i. 181, 183
circulation, effect of ventricular systole on, i.
185
volume of, i. 1- I
veins, closure of, i. 184
Corpora Arautii. i. 112

cavernosa of 1 he penis, ii. 448
striata, fund ions of, ii. 271
Corpus callosum, ('unctions of, ii. 270
luteum, ii. 455

spongiosum of the penis, ii. 448
Corpuscles, colostrum, i. 263
ot' t lie blood, i. 45
salivary, i. 283
Corresponding points of the retinas, ii. :;.">!(
Cortex cerebri, connection of, with the respira-
tory centre, i. 463
effects of localized electrical stimulation of,

ii. 211
electrical stimulation of, ii. 242
number of nerve-cells in. ii. 2*1
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. ; :77
Cortical anas. ii. 243

motor, in man. ii. 250
size of, ii. 247
centre-, ii. 243
motor control, crossed, ii. 251

multiple character of, ii. 250
regions, ii. 2 13

stimulation, inhibitory effects of, ii 221
vascular effects of, i. 202
Costal respiration, definition of, i. 398
( Soughing, i. 154
Coughs, sympathetic, i. 155

< o\\ per'- gland, ii. 1 13

histology of, ii. 148
secret ion ol'. ii. t 16

Crab-extract, Lymphagogic action of, i. 73
<'rab-, regeneration of lost parts in, ii. 196

< Irania I 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. 1C7
Cre.-ol. i. 569

elimination, i. 340
Cretinism, sporadic, ii. 280

Crico-arytenoid muscle, lateral, ii. 426

posterior, ii. 126
Cricoid cartilage, ii. I2">
( 'rico-t liyroid muscles, ii. 426
Criminals, weight of the brain in, ii. 277
Crista acustiea 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

( i\ stalloids, diffusion of, i. 69
Crystal- of CO-hsemoglobin, i. 40

of ha-niiii, i. 44, 573

of haemoglobin, i. :i;»
( tupola of the cochlea, ii. 375
( 'ura re, action of, ii. 26
Currents of action in muscle, ii. 150
in nerves, ii. 153

of rest. ii. 1 17

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-
cital ions on, ii. 1 15

of muscular contractions, ii. 100

of work for muscles, ii. 140
Cutaneous nerves, influence of, on respiration,
i. 163

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
Cyanamide, i. 5 12
( lyanogen gas, i. 541

inhalation, i. 440
( 'ynurenic acid, i. 571
( lystein, i. 546
Cystin, i. 517

Cytology, definition of, i. 31
Cytoplasmic changes in nerve-cells, ii. 182
Cytosin, i. 579

" Dangerous region," i. 97
Daniel] cell, ii. 28

Darwin's theory of heredity, ii. 501
Death, definition of, ii. 491

of the tissues, ii. 192

somatic, ii. 491

theory of. ii. 192
Decidua graviditatis, ii. 461, 471

menstrualis, ii. 15S, 4<il

reflexa, ii. 472

serotina, ii. 172

vera. ii. 172
Decidual cells, ii. 171

Di composition, bacterial, in the intestines, i.309
Defecation, i. 386

cerebral cont rol of, ii. 270

reflex character of, ii. 213
Defibrinated blood, definition of, i. 34

preparal ion of, i. 55
Degeneration after hemisection of the spinal
cord, ii. 228

following removal of motor cortical areas, ii.
211

of cut nerves, ii. 78

of muscle after section of its nerve, ii. 70

of nerve-cells, ii. 199

of lierve-eleiiients, ii. 197
of nerves after section, ii. 69
read ion of. ii. 47
Deglutition, i. 372
action of the epiglottis ill, ii. 422

GENERAL IXDEX.

531

Deglutition, analysis of, i. 376
apncea, i. 1 12
centre for, i. .'!??
explanation of, i. 375
nervous regulation of, i. 376
Deiters, cells of, in the organ of L'orti, ii. 377

nucleus of, ii. 238
Demarcation currents of injured muscle, ii. 148
Demilunes, i. 219
Dendrites, definition of, ii. 174
1 Repressor nerve, i. 172, 203
Dermal sensations, cortical area for, ii. 253

path of conduction for, in the cord, ii. 235
sensibility, ana of distribution of the nerves
of, 231
Descending impulses, course of, ii. 241
Desiccation of nerve, ii. 59
Determinants of the germ-plasm, ii. 503
Deutero-proteose, definition of, i. 293
Deutoplasm of the ovum, ii. 450

composition of, ii. 451
Development of nerve-cells, ii. 176'
Dextrose, action of, in delaving 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. <>5

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. 1 1 1

wave of the pulse-curve, i. 143
Diet, accessory articles of, i.357

average, for man, i. 366
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
Digestion, action of alcohol on, i. 535
gastric, 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. :;<il. 3(»s
purpose of, i. 275
salivary, i 283
Digitalin, action of, on coagulation of muscle
plasma, ii. 16 1
on muscles, ii. HIT
Digitalis, action of, on nerves and muscles, ii. 60

effect of, on the respiratory rhythm, i 125
Digits, supernumerary, ii. 494

1 dioptric apparatus of t he eye. defects of, ii. 314
Dioptrics of the eye. ii. 300
I >ioptry, definition of, ii. 304
Dioxyacetone, i. 558
Dioxyphenyl-acetic acid, i. 570
1 diphasic current of action, i. L52
Direction, judgments of, by means of auditory
sensations, ii. 389

of the nerve-impulse, ii. 1-1
Disaccharides, i. 56 1

digestion of, i. 308
Disassimilation, definition of, i. l!t
Discord, ii. :>1

Discriminating sensibility of the skin for pres-
sure, ii. :;!»-.'
Discriminative sensibility for difference of tem-
perature, ii. 397
Discus proligerus, ii. 450, 454
Diseases, inheritance of, ii. 498
Dispermy, 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
i Sureties, action of, i. 254
Dizziness, ii. 105

Dogs, removal of cerebrum in, ii. 267
Domestication, effect of, on menstruation in

animals, ii. 160. 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

resuscital ion from, i. 1 15
Drugs, action of, on body-temperature, i. 472
on salivary glands, i. 222, 229
on sweat-glands, i. -jilil
on thermogenesis, i. 18 1
on thermolysis, i. 189
Du Bois-Eeymond's key, ii. 30

law of st iinulat ion. ii. .".-.'

theory of currents of rest, ii. 148
Duct of Bartholin, i. 217

of Rivinus, i. 217

of Stenson, i. 217

of Wharton, i. '.'17

of Wirsung, i. 231
Ductus cochlearis, structure of. ii. :;7l

endol vmphal icus. ii. ,",7-'!
venosus of t he embryo, ii. I7n'
Duration of electric currents, effeel of. mi their

irritating power, ii. 16

Dynamic equilibrium, organs of, ii. L07

Dyslysin. i. 511

Dyspepsia accompanying the climacteric, ii.

I'M)

cause of, i. 309
I >\ spncea, definition of. i. l li
effeel of, on t he iris, ii. 32 I

on intest inal \ ements, i. 386

phenomena of, i. 1 1 1

varieties of. i. 1 13, I 1 I

Ea.b, analysis of composite tones by, ii. 384
anatomy of. ii. 362

discriminative sensibility of, for pitch, ii. :;<,
lai Lgue of, ii. :;s7
imperfeel ion- of, ii. 3SS

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 iu, ii.

4! Mi
Ecfc fistula, i. 336
Edestine, i. 577

Efferenl 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. 163
Egg albumin, absortion of, i. 315
Ejaculation, ii. 165
Ejaculatory duct. ii. I IT
Elasticity of muscle, ii. 105
Elastin, i. 580

Electric currents, correlation of their duration
with histological structures, ii. 47
effect of, duntt ion 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 iii 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. 15S
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. til

in human nerves, ii. 65
twitch, ii. 157
Electrotonus, ii. 62
Embryo, nutrition of, ii 175

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 Eats, i. 306

influence of the bile on, i. 307
Emulsions, preparation of, i. - 1< >T . 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

glycolyl ic, i. 354
Enzymes, classification of, i. 280

composition of, i. 279

definition of, i. 27!i

effect of, on blood coagulation, i. 63

genera] properties of, i. 281

nn.de 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
Epinephrin, 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. 1<>4
Erect ion, ii. 464

of t he heart, i. 114

spinal centre for, ii. 464
lire, tor clitoridis muscle, ii. 464

penis, action of, in erection, ii. 464
muscles, ii. 1 19
Hreetores spime 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

El li\ la in i tie, i. 541

Eudiometer, i. 421
Eupnoea, definition of, 440
Eustachian tube, ii. :;<;:;

function of, ii. 369
Excitability, changes in, during Walleriau de-
generation, ii. 69
Excitation, cardiac, electrical variation in, i. 153

propagat ion 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 id', i. 406
Expiratory centre, i. 157
Explosive consonants, ii 137
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 t in- pancreas, i. 266

of the thyroids, i. 268
Extractives, nitrogenous, of muscle, ii. 166

of t he blood, i. 50, 51
Ext tacts, adrenal, i. 271

ovarian, i. 271

GENERAL J XI > EX.

533

Extracts, testicular, i. 273

thyroid, i. 269
Extrapolar region, ii. 62
Exudations, secretion of, i- 215
Eye, abnormal positions of, alter cerebellar in-
jury, ii. 272
adaptation of, to light, ii. 326
axes of rotation of, ii. 299
chromatic aberration of, ii. 316
constants, chauges in, during accommodatiou,

ii. 311
defects in the dioptric apparatus of, ii. 314
dioptric apparatus of, ii. 300
mechanical movements of, ii. 298
movements, binocular co-ordination in, ii. 300

extent of, ii. 298
muscles of, ii. 299

innervation of, ii. 300
optical constants of, ii. 303

power of, ii 3<»1
positions of, ii. 299
refractive media of, ii. 302

surfaces of, ii. 303
spherical aberration of. ii. 315

Fallopian tubes, ii. 443, 456

False amnion, ii. 473

Falsetto register of the voice, ii. 433

Far-point of vision, ii. 312

Fat, affinity of cell-substance for, i. 5(58

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. 66, 70
recovery from, ii. 73

of nerve-cells, ii. 136, 191

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 '.'ll

digestion of, i. 305

dynamic value of. i. 175

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 format ion. i. 329
to muscular work, ii. 71

synthesis of, from fatty acids, i. 558
Fatty acids, monobasic, i. 532

degeneration in phosphorus-poisoning, i. 51 l
Feces, composil ion of. i. 319
Fellic acid, i. 543
Female births, relative number of, ii. \-''>

pronucleus, ii. 453
Females, rate of growth in, ii. 4*8
Fenestra ovalis, ii. 363

Fenestra rotunda, ii. 363. 375

use of, ii. 376
Ferment, myosinogen-coagulating, ii. 161

Fermentation, alcoholic, i. 535

lactic, i. 545
Ferment--, unorganized, i. 279
Ferratin, i. 528, 529
Ferric phosphates, i 528
Ferrosulphide, i. 528
Fertilization, ii. 466
Fetal membranes, ii. 172
Fever, body-temperature in, i. 472

cause of, i. 473

effect of, on blood coagulation, i. 55
on the respiratory centre, i. 458

heat dissipation in, i. 489
Fibrillar contraction of the heart, i. 181. l-'J
Fibrin fermeut, i. 56

absence of, iu 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
Fimbria? of the Fallopian tube, ii. 456
Fish, bonv, removal of cerebral hemispheres in,
ii. 263

semicircular canals in, ii. 4(>7

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, combust ion equivalent of, i. 365

definif ion of, i. 275

dynamic value of. i. 364

effect of, on respiratory activity, i. 431

energy liberated by, i. 171

influence of, on thermogenesis, i. 484

rate of movement of. in the intestines, i. 31 I
Food-stuffs, classification of, i. 276

composition of, i. 278

Liebig's classification of, i. •"> 16
Foramen ovale of the foetal heart, ii. 476
Force of ventricular systole during vagus stim-
ulation, i. 163
Forced movements after section of the semicir-
cular canal-, ii. 405
in frogs, ii. 266
Formic acid, i. 534

aldehyde, i. 533
Formose, synthesis of, i. 533
Fovea cent ralis, ii. 327

Franklin's theory of Color vision, ii. .'!.".7

Frequency of respiration, conditions affecting,
i. 125
relation of, to the pulse-rate, i. 126

Frict ionals. ii. 137

Frogs, removal of cerebral hemispheres in, ii.
264
striked muscle of, time of single contraction
in. ii. 108

Frontal lobes of the hemispheres, effect of re-
moval of, ii. 262

Fuhlspare, cortical, ii. 252

Fundamental tone, definition of, ii. 383

Gal ictose, L 562. 564
Gall-bladder, motor nerves of, i.

:i-

534

< } i:\KIi. 1 /. INDEX.

Galvani, Luigi, ii. 28

Galvanic current, action of, ou conductivity, ii.
94

contracture effect of, on muscles, ii. 131
effect of, on heart apex, i. 150
on muscles, ii. * : I
mi nerves, ii. 62

of making and breaking, ii. 31
on normal human nerves, ii. 51
opening and closing contractions with, ii. 38
Galvanometers, ii. l l">
GalvanOtonus, ii. 54, 131
I kunogenesis, ii. 140
Ganglion spirale of the ear, ii. 376

submaxillary, i. 219
Ganglion-cells, conduction in, ii. 97

of the heart, i. 1 18
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. 42U
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
< lelatoses, i. 297

Geminal fibres of the pyramidal tracts, ii. 245
Gemmules of the germ-plasma, ii. 499
Genio-hyoid muscle, ii. 126

function of, in mastication, i. 372
Gerhardt's reaction, i ."37
Germinal spot of the ovary, ii. 450
transmission of infectious diseases, ii. 498
vesicle, al ructure of, ii. 450
Germ-plasm as a basis of heredity, ii. 499
coin inuity of, ii. 502
definition of, ii- 196
morphological nature of, ii. 499
origin of, ii. I!'!'
Gestation, duration of, ii. 478
i Hand, 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

gasi ric, i. 237

intesl tnal, i. 243

Lieberkiihn's, i. 243

mucous, histology of, i. 216

of Bartholin, ii. 462

of Littrc. ii. 448

salivary, i. 215

sebaceous, i. 257

serous, definition of, i. 216

structure of, i. 211

sweat, i. 259
I Hans penis, ii. 449
Glauber's salt, i. 522
Gliding movements in joints, ii. 416
Gb.bin. i. 37

Globulicidal action of serum, i. 36
< rlobulins, i. 577

Glomeruli, renal, secretory function of, i. 253
Glossopharyngeal nerve, gustatory function of,
ii. 410

nerves, influence of, on respiration, i. 462
Glossopharvngeus, central conduction paths for.

ii. *236
Glottis, ii. 423

tedenia of, ii. 422

respiratory movements of, i. 408 ; ii. 429
Glucosamin. i. 564
Glucoses, i. 562

S3 nthesis 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
Glyeocoll. i. 537. 543

in muscles, ii. 107

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-elimination of the liver, i. 265
Glycogen-formation, effect of proteid diet on. i.

328
Gl\ cogeii-formers, i. 32*
Glycogenic theory, i. 329
Glycolysis, i. 354
Glycolytic enzyme, i. 280, 354

origin of, i. 2(17
Glyco-proteids, i. 576, 578
( Hycosazones, i. 562
i Hyco secretory nerves, i. 248
Gl> coses, i. 562
Glycosuria after pancreas extirpation, i. 266,

563
Glycuronic acid, i. 567
Gmelin's test fur bile-pigments, i. 322, 574
Goblet ■•ells. i. -.'Hi
Goitre, i. 269

Golgi, organ of. in tendons, ii. 402
Gout, i. 557

( Graafian follicles, ii. 154
Grammeter, i. 477
Gram-molecular solution, i. 67

CKXERAL I XD EX.

535

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. 271

Growth alter birth, ii. 487

before birth, ii. 186

increase of fibres of the cortex during, ii. 282
of functional neurones during, ii. 282

influence of sex on I lie rate of, ii. 4S8

influences which modify, ii. 4s>L>

of nerve-cells, ii. L76
Guanidin, i. 550
Guanin, i. 339, 554
Gunzburg's reagent, i. 508
Gustatory nerves, ii. 410

sensations, ii. 411
Guttural consonants, ii. 437
Gyniueinra silvestre, action of, on taste-nerves,
ii. 413

H.emati.v, i. 37, 44, 573
Hsematogen, i. 356

composition of, i. 579

nutritive value of, i. 528
Haeinatoidin, i. 44, 323, 574
Hsematopoiesis, definition of, i. 45
Hematopoietic tissues, embryonic, i. 46
Hsematoporphyrin, i. 44, 574
Hsnierythrin, i. 578
Hsemin, i. 44, 573
Hpemochromogen, i. 37, 44, 573
Hreniocyanin, i. 578
Haemoglobin, i. 573

absorption spectra of, i. 43

action of, on carbonates, i. 517

affinity of, for CO2, 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-eel Is of the crista acustica, ii. 374

of the organ of < lorti, ii- :!77
Hamulus, ii. -iTii
Harmonic overtones, ii. 386
Harmony, ii. 387
Hawking, i. 151
Head register of the voice, ii. 433

vaso-motor nerves of, i. 204
Hearing, ii. 362

keenness of, ii. .'!71

relation of, to speech, ii. 431
Heart, anaemia of, i. 1 33

artificial stimulation of, i. 156

augmentor nerves of, i. 167

cause of rhythmic heat of, i. 148
cent ripetal nerves of. i. 171
changes in, due to pregnancy, ii. 477

in form of. i. 111!

in position of, i. 114

in si/.e of, i. 1 12
compensatory pause of, i. 156
diphasic act ion iai rrente in. ii. L52
electrical currents of, i. 152
erection of. i. 1 1 I
fibrillar emit rait ion of, i. 181
heat produced by, i. L08
human, output of, i. Kit!

Heart, intrinsic nerves of, i. 148

isolation of, i. 1 I-. 1 -? ; ii 69

lymphatics of, i. 186

muscle, atrophy of, after section of the vagi,
i. L67
conduction of the contraction wave l>y, i l" 1
normal stimulus of, i. 151
rate of conduction in, ii. 89
rhythmicity 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. L65

vaso-motor nerves of, i. 206

work done by, i. 1U7
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. 175

source of, i. 474
Heat-centres, ii. 271
Heat-dissipation, conditions affecting, i. 485

estimation of, i. 480
Heat-dyspnoea, i. 1 11, 443
Heat-production, amount of, i. 364

by the heart, i. 108

conditions affecting, i. 482

estimation of, i. 4M

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. .'Ilil
Heat -regulation, i. 195

Height of contraction, dependence of, on the
load, ii. Ill
effect of temperature on, ii. 136

Helico-proteid, composition of, i. .".?ii

Helicotrema, ii. ::7<>

Hemianopsia, anatomical hasis 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-Sequard's paralysis from, ii. •.':::;
degeneration resulting from, ii. 228

effect of, in man, ii. '.'.;:;

on vriivat ion and mot ion, ii. }30
in animals, ii. 234
physiological effeel of. ii. -.':;i
Hemorrhage, effeel of. on hematopoiesis, i. It!

fatal limits of, i. liii

regeneration of the hi 1 after, i. 63

reiat ion of, to blood-pressure, i. 91
saline injections after, i. ill

Hemorrhagic dyspnoea, i. ill
Hepatin, 1. 528

Heredity, definition of. ii. pi;;
physical hasis of, i. 28

theories of. ii. 198
Bering's theory of color vision, ii. 336
Hermann's theory of currents of rest, ii. 1 18

Heteromita, reproducl ion in. ii. 1 10

536

GENEBAL INDEX.

Hexou-bases, origin of, i. 580

Hexoses, i. 562

Hibernation, effect of, ou the respiratory quo-

tient, i. 438
Biccough, i 155

Higher brain-centres for the beart, i. 17b
Hinge-joints, ii. 416

Hippuric acid, nutritive history of, i. 339
Histidin, i. 552
1 1 istohtematin, i. 44, 578
Histology of striped muscle, ii. 104
Histon, i. 580

t-ll't'ci of, cm intravascular clotting, i. 61
Hofacker-Sadhr law, ii. 484
Holmgren method for testing color visiou, ii.

339
Homogciitisic acid, i. 570
Homotbermous animals, i. 407
Horopter, ii. '.'>'<'.>

Hiifner's method of urea determination, i. 549
Human muscles, fatigue of, with artificial stim-
ulation, ii. 134
Hunger, ii. 101

Hunger-centre, clinical evidence for, ii. 404
Hydra, regeneration of lost parts in, ii. 496
Hydremia from saline injections, i. 69
Hydremic plethora, effect of, on lymph secre-
tion, i. 74
Hydration, nature of the process of, i. 503
Hydriodic acid. i. 509
Hydrobilirabin, 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. 54'.'

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
Hydroqninone, i. 569
ll.\ men. ii. 162
11\ o-glossus muscle, ii. 126
Hyperesthesia, homolateral, after hemisection

of t he cord, ii. 233
Hypennel ropia, ii. 313

range of accommodation in. ii. 314
Hyperpnoea, i. l hi

from muscular activity, i. 1 12
Hypertonic solutions, physiological definition

of. i. 69

Hypertonicity, definition of. i. ::7
Hypophysis cerebri, function of. i. 273
Hypotonicity, definition of. i. ,';7
Hypozanthin, i. 553

of muscles, ii. 107

relation of, to uric-acid formation, i. 338

Ii i. calorimeter, principle of, i. 504
Icterus, i. -.'l!i, .Ml
[dants of the germ-plasm, ii. 5n.",
Idiomuscular contraction, ii. ".'7. 92, 128
Idioplasm as a l»:i~ i~ of heredity, ii. 199

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 [ens, 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

eliminat ion 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-uterhie, ii. 498
Infections diseases, germinal transmission of, ii.

498
Inferior laryngeal nerve, respiratory function
of, i. 464
mesenteric ganglion, reflex activity of, i. 392
oblique muscle, ii. •.'!»!!
rectus muscle, ii. -.'!»!•
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
tonus of. i. 176
centres, respiratory, i. 457
nerves of the heart, i. 101
of the intestines, i. 385
of the pancreas, i. 233

of the spleen, i. '.','.','■',

of the stomach, i. 382
Innervation of the blood-vessels, i. 192

of the heart, i. 148
Inorganic salts of the hlood, i. 50
of urine, i 34 l

relation of. to blood coagulation, i. 56, 57
to irritability, ii. 59

to the heart heat. i. 151, 189
Inosil . i 57:'.
Insanity, relation of brain-weight to, ii. 2/78

variations of muscular tonus in. ii. 220
Insecl muscle, time of contraction in. ii. 108
Inspiration, enlargement of the thorax in, i.
398
muscles of. i :;<•>, 101

Inspiratory centre, i 157

Intensity of visual sensations, ii. 339

Intercostales muscles, respiratory action of, i.

102, 107
Intermedins nerve of Wrisberg, central path of.

ii 236
Intermittent pulse, i. Ml
Internal capsule, grouping of fibres in. ii. 248
ear, anatomy of. ii. '.\'\
rectus muscle, ii. 299
secret ion. definition of. i. -.'05
of the adrenal bodies, i. 272

GENERAL INDEX.

537

Internal secretion of the kidneys, i. 274

of the liver, i. 265
of the ovaries, i. -'71
of the pancreas, i. 266
of the pituitary body, i. 273
of tlie testis, i. 273
of the thyroids, i. 270
Intestinal contents, reaction of, i. 310

digestion, i. 299

juice, i. 243

movements, i. 382-385
Intestines, innervation of, i. 3a4

intrinsic nervous mechanism of, i. 384

large, absorption in, i. 314

pendular movements of, i. 384

peristalsis of, i. 382

putrefactive changes in, i. 310

small, absorpl ion in, i. 313

vaso-motor nerves of, i. 206'
Intracardiac pressure, i. lo7, 1 25, 126

methods of measuring, i. 129, 130
Intracranial pressure, relation of, to blood-pres-
sure, ii. 287
Intra-ocular images, ii. 320
Intrapolar region, ii. 62
Intrapnlmonary pressure, i. 408
intrathoracic pressure, i. 397, 409
Intravascular clotting, i. 60, ' > 1
Intrinsic nerves of the heart, i. 148
Introductory contractions of a contraction series,
ii. 1*13

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

Iodothyrin, properties of. i. 270
Ionic theory of solutions, i. 67
Ion-proteid compounds of muscle, ii. 168
Iris, dilator nerves of, ii. 324

direct response to light by, ii. 324

innervation of, ii. 323

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 neivous sys-
tem, ii. 208
Irrigating fluids for the isolated heart, i. L89,

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. •>.">
of living matter, i. 18
of muscle, ii. 25
of nerve-fibres, ii. 21
of nerves, ii. 2 1

ami 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, relation of, to the response, ii. 24

[schsemia of heart muscle, i. l-i

[schio-cavernosi muscles, ii. 449

[schio cavernosus, action of, in erection, ii. 464

Iso-butyl alcohol, i. 539

Iso-butyric acid, i. 7>:;'.i

Iso-dynamic equivalence of foods, i, 365

Isolated apex of frog's heart, i. 1 —

conduction in nerve-trunks, ii. 79
Isolation of the heart, i. 148, 191
Isomaltose, i. 565

isometric contractions, definition of. ii. 110
l-ii pent; 1 alcohol, i. 539
isotonic contractions, definition of. ii. llo

solul ions, i. 36, on
Isotonicity, i. 36, 68

Isotropic substance of muscle-fibres, ii. 104
Iso- valerianic acid. i. .">39

.1 \i n dick. i. 249, "'1 1
Jecorin, i. 564

Joints, classification of, ii. 415
.lumping, ii. 420

K.vkvokinksis, i. 20

Karyokinetic figures in mature nerve-cells, ii.

202
Katabolism, definition of, i. 19
Katelectrotonus, ii. 62
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. 2 19

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. -137
Labio-dental frictionals, ii. l">-
I. allium tympanicum of tin- internal ear. ii. 377

vesl ibulare of (he limbus, ii. 377
Labor, nature of, ii l-l

stages of. ii. i t; •
Labor-pains, ii. 179

Labyrinth of the ear, anatomy of. ii. 371
Lactalbumin, i. 261
Lactation, ovulation during, ii. 156
Lacteal vessels, i. ."I -
I .acteals, absorpl ion t hrough, i. 31 1
Lactic acid, i 545

fermental ion, i. ."> 15

occurrence of, in the stomach, i. 289

of muscles, ii. 10-

Lacto globulin, i. 261

Lac; ose, i 202. 565

Laky blood, i. 35

Lamina spiralis, ii. 372

Laminae of medullar; tube in the foetus, ii. 205

Langerhans, bodies of, i. 232

I ianol in. i. '.'."'7. .">7.">

Larue intestine, digestion in. i. 309
Laryngeal muscles, specific action of. \\. ij-
oerve, recurrent, ii. 128

superior, ii. 128
Laryngoscope, ii. 129
I. :n vii \. carl ilages of. ii. 12.".
closure of. during muscular effort, ii. 123

538

GENERAL INDEX.

Larynx, muscles of, Li. 125
in i \ ea hi', 11. I'J-
31 nictate of, ii. i.l
Latent ana- of the cortex, ii. 2<il
characters, hereditary transmission of, ii, 495
beat, definit inn hi', i. 504
period, effect of u mperature on, ii. L36
..I' tension mi, Li. l in
hi' cardiac accelerator nerves, i L70
oi' In-art muscle, i. L53
ni' motor end-plates, ii. 103
..I" muscle, ii. L03
• ■I' nil muscle, ii. 109
i.f retinal si Lmulation, ii. 343
of simple muscular contraction, ii. 102
of \ agus-stimulal Lon, i. L62
Latham's hypothesis of tin- structure of pro-
toplasm, i. •-' l
Laughing, i. 154

"Law ui' contraction," Pfliiger's, ii. 50
I, aw uf -i Lmulation of human nerves by battery

currents, ii. 51
Lecil liin. i. 559
amount of. in tin- blood, i. 51
occurrence uf, i. 325
of hilc, i. 45
of milk, i. •.'HI
of nerves, ii. 169
l.i . eh extract, effect of, mi blood coagulation, i.
• ;-.'
lymphagogic action of, i. 73
Lemniscus, medial, i. 226

-i nsoiy paths entering, ii. 235
Lens, changes in. during accommodation, ii. :;n7
crystalline, changes in, with old age, ii. 314
curvatures of, ii. 303
opacities in. ii. :;■-'!
refract Lve index of, ii. 303
thickness of, ii. 303
Lent Lcular ganglion, ii -';i l
Leucin, action of, on end-plates, ii. '-'7
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.

classifical L< f, L. 17, 48

emigration of, i. 83

from the thymus gland, composition of, i. 51

functions of, i. lv

influence of, on blood-plasma, i. L9

movements of, ii. L9

origin of, i. Ii'
Leucocythsemia, fatty acids in, i. 530

purin bases excreted in, i. 557
Leuconuclein, effecl of, on intravascular clot-
ting, i. 61
Leucophrys patula, reproduction of, ii. U2
Levatores ani muscles, expiratory action of, i.
107

costarum breves, inspiratory action of, i. 402
l.i vu lie acid, i. 538

fate of, m pancreatic diabetes, i. 267
Levulose, i. 562

occurrence of, i. 56 1

oxidation of, in diabett -. i. 564
Lieberkuhn'e crypts, histology of, i. 243
Liebig's method of area determination, i. 549
Life, general hypothesis of, i. 25

of i he individual, stages of, ii. 186
Ligaments of the incus, ii. 367

of the malleus, ii. 366
Ligatures of Stannins, i. 178
Light, changes in the retina produced by, ii. •"■•';|i

definition of, ii 298

dispersion of, ii- 316, 332

Light, monochromatic, ii. .">1<J
physical theory of, ii. 33]
rays of the Luminiferous ether, ii. 381
i lions, intensity of, ii. 332, 339
mechanism for the production of, ii. 331
quality of, ii. 332
Light-waves, lengths of, ii. 332
Limbs, vaso-inotor nerves of, i. 209
Liinhiis of the spiral lamina, ii. 377
Lingual frict ionals, ii 138

nerve, gustatory function of, ii. 410
Liuguo-palatal consonants, ii. 437
Lipase, i. 305
Lipochroines, i. 574
Liqueurs, i. 535
Liquids, ii. 43ti
Liquor amnii, ii. 472

folliculi, ii. 454
List ing's law, ii. 299
I. ill iv. 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. 217
urea formation in, i :'.:il
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

<>n the contraction curve, ii 111
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 sensat ion-, ii. 394
power, relation of. to mobility, ii. 394
Locomotion, ii. 420

Locomotor ataxy, disturbance of equilibrium
in, ii. 105
mechanisms, action of. ii. Ill
I. new- hypothesis of the structure of proto-
plasm, i. •_':;
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, Li. 331
Luminosity, relative, of spectral colors, ii. 340
Luminous sensations, intensity of, ii. 339
Lungs, capacity of, i. 4'.'7
ini'Ve-supply of. i. 165

structure of. i. 396

vaso-motor nerves of, i. 205
Lunulas of the semilunar valves, i. Ill
Lustre in \isual sensations, explanation of, ii.

Lutein, i. 571

Luxus consumption, i. 348

Lymph, i. 33

amount of. i. 1 hi

definition of. i. to

formation of. i. 71
gases of, i. mi
glands, i. 140'.

GENERAL IXDEX.

539

Lymph, mechanical theory of the origin of, i. 75

movement of, i. 71, 146

pressure of, i. 146

secretion of, i. 21 1
Lymphagogues, action of, i ?•!. 7i
Lymphatic system, nature of, i. 143
Lymphatics of the heart, i. 186
Lymphocytes, i. 48
Lysatin, i. 55i
Lysatinin, relation of, to una formation, i. 337,

551
Lysin, i. 552

Macroceph \i.i< brains, weight of, ii. 275

.Macula acustica, ii. 373

lutea, ii. 327
Macula? acustica. relation of, to state of equi-
librium, ii. 1<>7
Magnesium carbonate, i. 527

nutritive history of, i. 527

occurrence of, i. 527

phosphates, i. 527
Making contraction, point of origin of, ii. 35
" Slaking "' shock, ii. 31
Male births, relative number of, ii. 483

pronucleus, ii. 466
Males, 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
Manuose, i. •"><>.'
Manometer, differential, i. 131

elastic, i. 127

maximum, i. 107

mercurial, i. -7
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. Ill
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 tiie ovum. ii. I'll
Meat extracts, physiological action of. i. 359
Meats, composii ion of, i. 278
Meatus auditorius interims, ii. 373
Mechanical stimulation of uerves, ii. 25, 56

strain, influence of, on neuroblasts, ii. i?i>

work (if muscular contraction, ii. 138
Meconium, biliary salt- in. i. •" I 1
Medial lemniscus, ii. 226
Medullary -heath, development of, in the C6E

tral nervous system, ii. 181
in the peripheral nerves, ii. 180
significance of, ii. L80

tube, fetal, ii. 204

lamina' of, in t lie foetus, ii. 205
Medullation, central, progressive character of,

ii. 1-1
of nerve-fibres, significance of. ii. 283
peripheral, ii. 180

Medusae, rate of conduction in. ii. 89

staircase contractions in. ii. 112
Melauius, i. 57 I
Melicyl alcohol, i. 540
Membrana basilaris, ii. 374

flaccida, ii. 365

granulosa of the Graafian follicle, ii. 454

reticulata, ii. .17-

tectoria. ii. 377. :!7i»

tympani, ii. '■'>*> I
Membrane of Reissner, ii. 371. :;7!»
Membranous labyrinth of the ear, ii. 372
Menopause, ii. 159, 190
Menstrual ion. ii. 457

age of onset of. ii. 459

cessation of, at the climacteric, ii. 490

genera] 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. 517
Mercury manometer, description of. i. -7
Metabolism, conditions influencing, i. 359

definition of, i. 20

during sleep, i, 361
starvation, i. 3(52

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. 1 1. 573
Methane, origin of, i. 532
Methods, physiological, i. 31
Methyl amido-acetic acid, i. 538

mercaptan, i. 534

seleuide, i. 534

telluride, i. 5:>4

violet, in testing for mineral acid-, i. 289
Methylamine, i. 541
Meynert's commissure, ii. 238
M icellsa, definition of, i. 25

Microcephalic brains, weight of, ii. '.'75
Microcephalics, ii. 268
Micturition, i. :;-:i

en i,-e for. i. 391, 393

cerebral control of. ii. 070

nervous mechanism of. i. 392

reflex character of, ii. 213
Mi. Idle ear, ii. 3d-.'

inflammatory disease of, ii. 364
mechanism of, ii. 368

muscles of. ii. .".ii!"
Migration of neuroblasts, ii. 17ii
Milk, composition of, i. 261

mineral constituents of. i. 530

normal secret ion of. i. 26 1
Milk-sugar, i. 565
Millon's reaction for proteids, i. 576

nature of, i. .".flit

wit h phenol, i. 569

M ineral acids, tests for. i. 289

constituents, amount of, in the tissues, i. 630
Mitosis, i. 20
Modiolus, ii. 372
M..i,cular weight, relation of. to physiological

act ion. ii. 60

Molecules, physical ami physiological, i. 25
Monochromal ic light, ii. 316

M .nuclear leucocytes, i. |s

M.ms Veneris, ii. 162
Monstrosities, congenital, ii. 1**1

origin of, ii. 183
Morgagni, v.m ricles of. ii. 122

540

« / i:XERA L INDEX.

Morphin, effect of, on body-temperature, i. 472
Morula, ii. 47o

Motor areas, cortical serial arrangement of, ii.
247
degeneration after removal of, ii. '-44
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 bv 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
Muscae volitantes, ii. 320
Muscarin, i. 543

action of, on the heart, i. 150
Muscle, accelerator urinae, ii. 449

aryteno-epiglott ideal), ii. 426

arytenoid, ii. 426

bulbo-cavernosus, ii. 4 19

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. -".72: 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. 126

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

in voluntary, properties of, i. 370

limitation of the rate of st i initiation in, ii. 126

masseter, i. 327

mineral constituents of, i. 530

inylodiyoid, i. 372 : ii. 126

nitrogenous extractives of, ii. 166

DOn-nitrogenous constituents of, ii. 168

obliquus externus, i. 407
interims i. |u7

omo-hyoid, ii. 125

posterior crico-arytenoid, ii. 426

pterygoid, external, i. 372
internal, i. 372

pyramidalis, i. H>7

reaction of, ii. L59

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
Btylo-hyoid, ii. 426
superior oblique, ii. 299

rectus, ii. 299
temporalis, i. 372
thy ro-arytenoid. ii. 426
external, ii. 121
internal, ii. 424
thy lo-hyoid, ii. 425
transversalis abdominis, i. 407
trapezius, i. 405
Muscle-contraction, Engelmaun's theory of, ii.

105
Muscle-plasma, ii. 161, 163
Muscle-proteids, precipitation temperature of, ii.

166
Muscles, abdomiuales. 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
glycogen of. i. 33i i
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. 102
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 conduct ion 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. H»7
vaso-motor aerves of, i. 210
Muscle-serum, ii. L66
Muscle-sounds, ii. 132
Muscle-spindles, ii. 102

Muscle-structure, influence of, on its contrac-
tion, ii. 1H7
Muscle-tonus, ii. 1 13

Muscular contracl ions, effect of drugs on, ii. 137
Of Support on the height of, L22
of temporal u re on, ii. 136
graphic record of, ii. 99
influences affecting, ii. 107
post-mortem, ii. 160
BOB rce of energy in, ii. 74
effort, closure of larynx in, ii. 423
exercise, effecl of, on metabolism, i. 359
on the pulse-rate, i. 121

a EXE HAL IXDEX.

541

Muscular exercise, effect of, on the rate of
respiration, i. 426
on the respiratory exchanges, i. 433

quotient, i. 138
on the sweat glands, i. 260
on the venous circulation, i. 95
inhibition from cortical stimulation, ii. 224
movements, relations of antagonistic muscles

in, ii. 418
sensatious, cortical area for, ii. 253
definition of, ii. 390

effect of hemisection 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. '■'>'. il
work, effect of stimulants on, ii. 75
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-albumin, 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 Hering, 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. 05
in the veins, i. !H
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, aurieulo-teniporal, i.'.'I-
chorda tympani. i. 101, 210

gustatory function of, ii. 1 10
coronary, of t be 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. 323
recurrent laryngeal, ii. 128
small superficial petrosal, i. 218
superior laryngeal, ii. 428

Nerve, vagus, cardiac branches of, i. 159

gastric branches of. i. 381

intestinal branches of, i. 385

pulmonary branches of, i. 465

repiratory functions of. i. i59

secretory fibres of. i. 232, 239

trophic influence of, on the heart, i. 166
Nerve-cells, atrophy of, from disuse, ii. 195
changes in, with age, ii. 400
chemical changes in. ii. 191
classification of, ii. 177
connection between, ii. 206
degeneration <>f the cell-bodies of, ii. 199
diaxonic, ii. ITS
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. 192
human, size of, ii. 171
internal structure of, ii. 179
maturation of, ii. 177
morphology of. ii. 17^1
number of, in the central nervous system, ii.

283
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
genera! ion of, ii. 187
in peripheral nerves, ii. 1>.'I
peripheral diffusion of. ii. '.'is
reversed, in spinal ganglion-cells, ii. 185

theories of. ii. 07

transmission of. from neurone to neurone, ii.
207
Nerve-muscle preparation of a frog, ii. 34
Nerves, action cu rrents in, ii. 153

auditory, central path of. ii. 237

augmentor, of i lie heart, i. ltiT

cardiac, i. 1 1*

cervical sympai hel ic, i. 193

chemist ry of. ii. 169

cross sin uring of, ii. 200

current of resl in. ii. 1 10

degeneration of, after section, ii, 69, 78

depressor, of the heart, i. 172

fatigue of, ii. 75

glossopharyngeal, central conduction paths

for. ii. 236
in man. stimulation of. ii. 51
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-mcdullated, 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, una of distribution of, ii.

23]
of invertebrates, rate of conduction in, ii. 91
of taste, nuclei of origin of, ii. 236
of temperature, ii. -'197

of tile bile \e-scls. i. 248

of Wrisberg intermedins), central path of,

ii. 236
olfactory, central paths of, ii. 241
o[itic, central paths of, ii. 238
phrenic, i. 463
rate of conduction in. ii. B9
secondary degeneration of. ii. 197
septal, of the frog's heart, i. 166
specific energy of, ii. 232
splanchnic, i. 1?:;
trigeminal, i. 463

centra] paths of, ii. 238
vagus, course of the afferenl fibres in, ii. 236
Nerve-trunk-, isolated conduction in, ii. 79
Nervi erigentes, ii. 464

intestinal branches of, i. 385
Neukomm's test for bile acids, i. 545
Neuridin, i. 5 13
Neurin, i. 5 13

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. 17;;
Neurones, ii. 21
afferent, to the spinal cord, ii. 203
changes in number and size of, ii. 280
conduct ion 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. 2S3
Neutral salts, effects of, on blood coagulation, i.

62
Neutrophils, 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

hemoglobin, 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. ::■-'"
preparation of. i. 510

tension of the hi I. i. 117

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
Nceud 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. L90
of the embryo, ii. 475
Nutritive control of nerve-cell bodies over
nerve-fibres, ii. \\\>
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
CEderna, i. 148

of the glottis, ii. 422
Oesophagus, 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
(defines, 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 lot he brain, ii. 109

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
( >ocyte. ii. 451

Oophorin tablets, action of, i. 274
Opening of the chest, effect of, on heart, i. 115
( > | >l 1 1 halmometer, ii. 304
I ipht halmoscope, 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

i-ort ical centres of, ii. 210

efferent fibres of, ii. 240
t halami, functions of, ii. 271
Optical constants of the eye, ii. 303

(iEXEBAL IX I) EX.

543

Optical illusions in binocular vision, ii. 359
of space perceptions, ii. 351

properties of striated muscle, ii. 103
Optograms, ii. 330
Organ of Corti, structure of, ii. 377

of Golgi in tendons, ii. 402
" Organeiweiss," i. 346

Organization in the central nervous system, ii.
285

relation of educability to the establishment
of, ii. 286
Organs, growth of, ii. 486
Ornithin. i. 552
Orthophosphoric acid, i. 514
<)s orbiculare of the incus, ii. 366
Osazones of glycoses, i. 562
Oscillatory activity of the retina, ii. 344
Osmosis, definition of, i. 65

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. 3S6
Oviducts, ii. 443. See Fallopian tubes.
Ovulation, ii. 455
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. 467

Stages in the maturation of, ii. 452
Oxalate solutions, effect of, on blood coagula-
tion, i. 63
Oxalic acid, i. 557
Oxalnrie acid, i. 555
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

prepara' ion of, i. 501

properties of, i. 501

storage of, in muscle, ii. L69

supply, relation of, to irritability, ii. 68

tension in the blood, i. 415

respiratory effects of varying, i. I In
Oxygen-absorption, coefficient of, i. 115

conditions affect ing, i. 129

cutaneous, i. 122

estimation of, i. 428
( Oxygen-dyspnoea, i. Ill
Oxyhemoglobin, 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. 570
Oxyphiles, i. 17
Ozone inhalation, i. 140

preparation of, i. 502

properties of, i. 502

Pacinian body, ii. 391
of the penis, ii. 1 1!'
Pain nerves, evidence for t he existence of, ii. 232
points of the skin, ii. 400
sensations of, ii. 399
Pains, transferred or sympathetic, ii. 400
Pale striped muscle, physiological peculiarities

of, ii. 109
Palmitic acid, i. 541
Pancreas, anatomy of, i. 231
extirpation of. i. 266
grafting of, i. 267
histology of, i. 231
innervation of, i. 232
internal secretion of, i. 266
mineral constituents of, i. 530
secretory changes in. i. 233
vaso-motor nerves of, i. 207
Pancreatic diabetes, i. 267, 353, 563
fistulse, preparation of, i. 300
juice, amylolytic action of, i. 305
artificial, i. 301
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. 356
Paralysis after removal of motor anas, ii. 26!'
agitans, ii. 296
Brown-Sequard's, ii. 233

homolateral, after heinisection of the cord,
ii. 233
Paralytic secretion, i. 229
Paramcecium, reproduction in, ii. 440
Paramyosinogen, ii. 163

temperature of heal coagulation of, ii. L65
Parapeptone, definition of, i. 292

Paraniichin. i. 579

Parathyroids, anatomy of, i. 268
fund ion of, i. 269

Paresis following removal of the cerebellum,
ii. 272
from injury to motor areas, ii. '.'('.'.i
Parotid gland, anatomy of, i 2Vi

in nerval ion of, i. 218
Partial tones, definition of, ii. 383
Parturition, ii. 17!»

Spinal cent re of, ii 1~ '

Pate* de foie gras, i. 560

Paths of conduction in the cord, clinical evi-
dence on, ii. 23 1

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. '-'>-\
Penis, ii. 443

structure of, ii. 148
Pentamethylene-diamin, i. 543
Pentoses, i. 562
Pepsin, i. 237, 238

effect of, on blood coagulation, i. 63

preparal ion 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. t>2

propert ies of, i. 294, 577
Pepton-injection, effect of, on lymph format inn
i. 73

toxicity of, i. 316
Perfusion cannula, i. 1.^7
Perilymph, ii. :;72
Periodic reflexes, ii. 2ln'
Peripheral nerves, medullation of, ii. 180

reference of special sensations, ii. 400

reflex cenl res, i. L78
Peristalsis, definition of, i. 372

intestinal, i. 382

of the stomach, i. 37!»

of the ureters, i. 389
Permeability of the capillary walls, i. 70
Peroxide of bydrogen, i. 505
Pettenkofer's reaction for bile acids, i. 324,

514
Pexinogen granules, i. 242
Pfliiger's hypothesis of the structure of proto-
plasm, i. 23

law of contract ion, ii. 50
Phagocytosis, i. 48
Phakoscope, ii. 308

Phalangar process of the rods of Corti, ii. 378
Pharynx, deglutition in, i. 373
I'lieiiaceturic acid, i. 569
Phenol, i. 569

elimination of. i. 340
Phenyl-acetic acid. i. 569
Phloridzin diabetes, i. 563
Phosphates, i. -"1 I
Phosphene, pressure, ii. 305, 331
Phosphoric acid, -alts of. i. 511
Phosphorus, nutritive history of. i. 515

occurrence of, i. 513

peroxide, i. 51 1

poisoning, i. 513

preparal ion of, i. 513

properties of. i. 513
Photomet ry, ii. '■'> 15
Phrenic' nerve, current of action in. ii. 151

mrves. i. 463
Phylogenetic development of nerve-cells, ii. 177

Physical molecules, di Sniti f. i. 25

Physiological anodes, definition of, ii. 52

division of labor, i. 22

molecules, i. •.'."I

kat bode, definil ion of, ii. 52
abservations on afferent paths in the cord, ii.
229

rheoscope, ii. 1 K 151
-alt solut ion, ii. 59

in t ransfusions, i. 64
Physiology, definition of. i. 17

human, definil ion of. i. .'Ill
met hod- employed in. i. 30
subdivisions of, i. 17, 29

Physostigmin, action of, on accommodation, ii.
:;il
on the eye, ii. 325
l'ia mater, weight of, ii. 274

Pigment epithelium, retinal, movements of, ii.
:;:;n
retinal, relation of, to adaptation of the eye,
ii. 326
Pigments, biliary, i. 45, 215, 322, 530, 574

blood-, i. 37, 4-1, 573
Pilocarpin, action of, on salivary glands, i. 229

on sweat-glands, i. 260
Pilomotor mechanism, relation of, to thermo-
lysis, i. 494
Pince myographique, ii. ~7
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. 130, 432
Pituitary body, anatomy of, i. 272
functions of, i. '.'7;;
internal secretion of, i. 273
extracts, action of. i. 272
membrane, ii. 408
Pivot-joint, ii. 417
Placenta, ii. 17 I
Placental transmission of infectious diseases, ii.

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. !!)(>
Pneumatic cabinet, i. 453
Pneumogastric nerve. See I'/tyus.
pulmonary branches of, i. 465
respiratory function of, i. 459, 460
Pneumograph, i. 423
Pohl's mercury commutator, ii. 36
Poikilotherinoiis animals, i. 4(i7
Polar amphiaster of the ripening egg, ii. 453
bodies, ii. -151

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. til
on nerves, ii. t>2
Pole-changers, ii. 3*i
Polynucleated leucocytes, i. 48
Polypncea, i. 4-11
Polyspermy, ii. 17 1
Pomum Adami, ii. 125
Portal vein, vaso-motor nerves of, i. 209
Positive after-images, ii. •'!!<>
variation of the heart during vagus stimula-
tion, i. 164
Posterior association centre, ii. '.'57
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. 15]
salts, action of, on muscles, ii. 138

GENERAL INDEX.

545

Potassium salts, relation of, to irritability, ii. 59
toxicity of, i. 520

sulphocyanide, detection of, i. 284
occurrence of, i. 283, 542
of the urine, i. 507

thiocyanide, i. 542
Potential energy of food, i. 364
Preformation theory of heredity, ii. 500
Pre-ganglionic fibres of the sympathetic system,

ii. 219
Pregnancy, effects of, on the mother, ii. 477
Presbyopia, ii. 314
Pressor nerves, i. 202

Pressure, ett'ect of, on irritability of nerves, ii.
56

influence of, on conductivity, ii. 92

intracardiac, i. 107

intrathoracic, i. 396, 409

intraventricular, 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. 175

effect of, on glycogen formation, i. 328

gastric digestion of, i. 292

general reactions of, i. 575
significance of, i. 2 1

living, theoretical structure of, i. 23, 24

molecular structure of, i. 581

nutritive- value of, i. 276, 345

of milk, i. 261

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

Proteids, simple, classification of, i. 576
substitutes for, in the diet, i. 348
synthesis of, i. 518, 582
tryptic digestion of, i. 303
vegetable, i. 577
Proteolysis, i. 293
tryptic, i. 303
value of, i. 315
Proteolytic enzymes, definition of, i. 280
Proteose injection, effects of, i. 316
Proteoses, definition of, i. 292

properties of, i. 577
Prothrombin, i. 58
Protoplasm, i. 17, 199
Pseudo-mucoid, i. 578
Pseudoscopic vision, ii. 318, 357
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. 1 12
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. 1 11
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
I'lirin, i. 553
bases, i. 552
in leucocythsemia, i. 557
Purkinje-Sanson's images, ii. 307
Purkinje's figure, ii. 321
phenomenon, ii. 340
explanat ion of. ii. .'! 12
Purposeful reflexes, ii. 215

Putrefaction, intestinal, products of, i. 310
Putrescin, i. 543

Pyin, i. 579

Pyramidal fibres, number of, ii. 216

nerve-cells, development of, ii. 178
tracts in t he cold. ii. 215
geminal fibres of, ii. 215
size of, ii. 252
Pyramidal is muscle, expiratory action of, i.

107
Pyridin, L 571
Pyrocatechin, i. 569

Qi ldbat] lumborum, respiratory action of, i.

399
Quadruplets, ii. 183

Qualitv of musical tones, ii. 383
of the voice, ii. 130

546

<; i;m:l'.\l index.

Quinine, action of, on coagulation of muscle-
plasma, ii. 164

hydrochlorate, actii f, on salivary glands,

i. 222

Quintuplets, ii. 483

I; \i IB, relation of brain-weight to, ii. 278
Range of accommodation, normal, ii. 312
Rarefied an-, respiration of, i. 152
Kali of conduction in heart muscle, i. 154
in muscles, ii. -7
in nerves, ii. 89
of excitation, effect of. <m 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 jitvalin, 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. lit]
of nerve-fibres, ii. 170
of pancreatic juice, i. -232, 300
of succiis 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. 4.">, 4fi, 333
size of. i. :;.">
structure of, i. 35
variations in the number of, i. 46
Red-striped muscles, physiological properties of,

ii. 10!)
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. 155
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.

21 1
effect of location of stimulus on, ii. 209
of strength of stimulus on. ii. 210

Relieves 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.
2()() '

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
Eennin, 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 onychodronius, 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

cent res. i. 155

afferenl nerves to, i. 459

conditions influencing, i. 458

total, i. Mil

rhythmicity of. i. 15-
food-Stuffs, definition of, i. 346
movements, circulatory effects of, i. 447

duration of, i. I'.'l

effect of, on blood-pressure, i. 448
on venous circulation, i. 95, 96

frequency of, i. 125

Bpecial, i. 453
nerves, afferent, i. 460

efferent, i. 463

GENERAL INDEX.

547

Respiratory pauses, i. 424
pressure, i. 408
quotient, i. 410

during hibernation, i. 134
relation of, to the diet, i. 353
variations of, i. 437
sounds, i. 4U!)
Resuscitation from drowning, i. 445
Rete mirabile of the Malpighian corpuscles, i.
249
vasculosum of the testis, ii. 447
Retina, changes produced in, by light, ii. 330
circulation in, ii. 322
histology of, ii. 329
oscillatory activity of, ii. 344
space-perceptions by, ii. 348
structure of, ii. 327
Retinal currents, ii. 331
images, inversion of, ii. 305

size of, ii. 305
stimulation, after-effect of, ii. 345
fatigue in, ii. 31 1
latent period of, ii. 343
laws of, ii. 343

rise to maximum for different colors, ii. 313
vessels, demonstration of, ii. 321
Reversion to ancestral characters, ii. 495
Rhamnose, i. 562
Rheocord, ii. 41
Rheometer, i. !)!)
Rheonome, ii. 31
Rheoscope, physiological, ii. 148
Rheoseopie frog, ii. 148
Rheostat, ii. 40
Rhineneephalon, 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. 1 15
Rigor caloris, ii. 57, 161
contracture in, ii. 128
elicit 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. 123
respiratoria, ii. 423
vocalis, ii. 423
Ringer's solution for the heart, i. 190
Hitter's opening tetanus, ii. 37, til

tetanus, ii. 132
Rivinus. duds (if, i. -.'17
Rod and-cone layer, function of, ii. 327
Rod-pigment, ii, 339. See Visual purple.
Rods and cones, function of, ii. 341
number of, ii. 330
of Corti, ii. 377
ret inal, function of, ii. 311
Rosel von Rosenhofon spontaneous changes in

form of living organisms, ii. 19
Rotal ion, movements of, ii. 116
Roy's tonometer, i. L88
Running, ii. 421
Rut of animals, ii. 460

SA.CCH IBOSE, i. 561

Sacculus of the internal ear, ii. 373

Saccus endolymphaticus, ii. 373

Saddle-joint, ii. 116

Saliva, composition of, i. 220, 283
mineral constituents of. i. 53d
properties of. i. 220, 283
uses of, i. 286
Salivary corpuscles, i. 283
glands, i. 215

anatomy of, i. 217
histological changes in, i. 226
histology of, i. 219
nerves of, i. 218, '-"-'I

vaso-motor nerves of. i. 222
secretion, action of drugs on, i. :.".'!»
cerebral control of, ii. 270
normal mechanism of. i. 230
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, 125
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 mollis, ii. 161, 162
in clotting of muscle-plasma, ii. 161
relation of, to fatigue contracture, ii. 131
Sarcoplasm, ii. 104
Sarcosin, i. 538
Saturation of colors, ii. 342
Seal a media, ii. 375
tvmpani, ii. 372, 375
vestibuli, ii. 37:.', 375
Scaleni muscles, inspiratory action of, i. 401
Schneiderian membrane, ii. 408
Scombrin, i. 580
Scrotum, ii. 1 13

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 defeneration of nerves, ii. 1!»7
position of the eye, ii. •_':•!•
tetanus, ii. 150
Secreting glands, electrical changes in, i. 231

histological changes in, i. \.'\.'6
Secretion, antilytie, i. '.'.".n
biliary, i. 248

capillaries of tin- gastric glands, i. 238
definition of. i. 21 1

gastric. •.Mil

histological changes during, i. 226

internal, deflnil ion of, i. ".M 1

intest inal. i. 243

mammary, i. 264

mechanism of. i. •.'13

pancreal ic, i. 235

paralytic, i. 229

psychical, of gaBl ric j u ice. i. 239
relation of, to intensity of stimulus, i. 223
salivary, i. 230
cerebral control of, ii. 270

sebaceous, i. '.'57. 342
sweat, i. 259
u rinary. i. 251

Secretions, general characteristics of, i. 213
Secretogogues for the gastric glands, i. 359

Secretory centre, salivary, i. '.'30

548

GENERAL IXDEX.

Secretory fibres proper, definition of, i. 224
nerves, evidence for, i. 222

fatigue of, ii. !••>

mode of action of, i. 225

of i he adrenal bodies, i. 272

of the kidneys, i. 251

of the liver, i. 247

of the mammary glands, i. 263

of the pancreas, i. 2:52

of tin- stomach, i. 239

of the sweat •.'lands, 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
Segmental ion, ii. 167
Segmentation-centrosomes, ii. 469
Segmentation-nucleus, ii. 466
Semen, composition of, ii. I !•">
Semicircular canals, membranous, ii. 373

of t he bony labyrinth, ii. 371

relation of, fco equilibrium, ii. 405

section of, ii. 405
Semilunar valves, i. 1 10
Seminal vesicles, ii. 1 13

fond ion of, ii. 1 18

secretion of, ii. 1 16

Semi-vowels, ii. 136

Senescence of tiel'Ve-cel Is, ii. 182

of tin' central nervous system, ii. 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. .",<i7
Sense of equilibrium, ii. 404

of touch, ii. 392
Sensory arc-as of t he 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
i;i tendon, ii. 102
in the skin, ii. 391
nerves, influence of, on respiration, i. 463
of the heart, i. 172
rate of conduct ion 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. 29]
Septal nerves of the frog's heart, i. 1 * >*»
Serous cavities, i. 1 16

Senat i postici inferiores, respiratory function
of, i. 399
superiores, inspiratory action of, i. 102
Scrum, bactericidal action of, i. 36
globulicidal acl ion of, i. ■'!*>
osmol ic 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

composit ion 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. 130

of offspring, determination of, ii. 483

origin of, ii. 441

relation of body-temperature to, i. 470
of brain-weigh! to, ii. 276
Sexual characters, ii. 142

glands, accessory, ii. 445

organs, ii. 443

reproduction, ii. 440
congenital variations resulting from, ii. 501
t heory of origin of, ii. 441
Shark, reflexes in, ii. 212

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. Jul

proteids, i. 57o'
Simultaneous contrast, ii. 347
Singing voice, ii. 434
Sinuses of Valsalva, i. Ill
Size, increase of the embryo in, ii. 4*7

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 iu, 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, phvsieal, ii. 381

quality of, ii. 383
Sound-waves, amplitude of, ii. 381

composite, ii. 384

limits of perceptions of, ii. 382

production of. ii. :\-\
Space illusion-., ii. 35 1
Space-perception from visual sensations, ii. 347

GENERAL INDEX.

549

Special respiratory movements, i. 453
Specialization of functions, i. 21
Specific energies of nerves, doctrine of, ii. 232
energy of the optic nerve, ii. 331
gravity of blood, i. 34

of blood-corpuscles, i. 34, 35

of muscle, ii. 159

of the encephalon, ii. 275

of the nervous system at different ages, ii.

284
of urine, i. 251
heat, definition of, i. 47?

of the human body, i. 504
nerve-energy, doctrine of, ii. 399
Spectral colors, incomplete saturation of, ii.

342
Spectroscope, i. 40
Spectrum, ii. 332
definition of, i. 40

luminous intensity of the colors of, ii. 340
of CO-hremoglobin, i. 44
of haemoglobin, i. 42
of oxyhemoglobin, 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
Spermatocytes, 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
Spermiu, 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. 325
pylori, i. 377, 381
urethrse, i. 390

contraction of, in erection, ii. Hit
vesica? interims, i. 390
Sphincters ani, i. 3*6
Sphygmogram, i. 143
Sphygmograph, i. 1 12
Sphygmomanometer, i. 141
Sphygmometer, i. 141

.Spinal (('ntrcs for vaso-inotor nerves, i. 199
cord, afferent paths of, ii. 229
central neurones of, ii. 203
degeneration of, from heinisect ion, ii. 228
efferent neurones of, ii. 203
motor tracts of, ii. 245
reflexes in 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. 19s
Spiral ganglion of the. car, ii. 376
ligament of the cochlea, ii. 379
Spirometer, i. 427
Splanchnic nerves, gastric fibres of, 1.382

influence of, on blood-pressure, i. 173
on respiral ion, i. 463

intestinal fibres of. i. 385

stimulation of, i. 173
Spleen, composition of, i. 333
function of, i. 322

Spleen, innervation of, i. 333

movements of, i. 322

vaso-motor nerves of, i. 207
Staircase contractions, ii. 66, 112
relation of, to tetanus, ii. 124
Standing, ii. 418
Stanniiis's Ligatures, i. L78
Stapedius muscle, ii. 370
Stapes, ii. 367
Starch, i. 566

digestion of, i. 284, 305

hydrolysis of. by acids, i. 286
by amylolytic ferments, i. 2-5
Starvation, effect of. on metabolism, i. 362
on tin- nervous system, ii. 289

glycogen disappearance during, i. 331

nutrition during, i. 350

phosporus excretion in, i. 516

potassium excretion in, i. 520
Stal ic equilibrium, organs of, ii. 407
Stature, relation of brain-weight to, ii. 276
Steapsin, i. 232, 280

demonstration of. i. 306

occurrence of. i. 305
Stearic acid, i. ."11
Stenson's duct, i. 217

experiment, ii. 67
Stercorin, i. 575
Stereoscope, ii. 356

Sterno-cleido-mastoid muscles, respiratorv 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. 211
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

ex1 irpation of. i. 299

Clauds of, i. 237

immunity of, to its own secretion, i. 297

innervation of, i. ::-l

movements of, i. 377, 378

musculature of. i. 377
Strabismus, ii. .".(in
Stria' acust icie, ii. 237

gravidarum, ii. 477
Stromuhr of Ludwig, i. 99
St rout ium, i. 526

Strychnin, action of. on diffusion of impulses in
Hie cord. ii. 217
on end-plates, ii. '.'7
on sympathetic ganglia, ii. 219

effect of, on body-temperature, i. 172

tetanus, ii. 217
Sturin, i. 580

Stylo-byoid muscle, ii. 126
St ylonychia. reproduction of. ii. 442
Sublingual gland, anatomy of, i. 217
Submaxillary gland, anatomy of. i. 217
Successive contrast, ii. 3 Hi
Succinic acid. i. 557

550

GENERAL INDEX.

Saccns 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 drags, i. 260
Suffocation. See Asphyxia.
Sugar injections, lymphagogic action of, i. 73
hi muscles, ii. L67
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. 32ti
synthesis of, i. 533
Sulphates of the urine, estimation of, i. 506

origin of, i. .">« >t ;
Sulph-haemoglobin, 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. 1*21
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. '.Tilt

Sweat -secretion, action of drugs on, i. 260
Sylvian heat-cent re. ii. 271
Sympathetic ganglia, action of nicotin on, ii. 219
of strychnin on, ii. 219
nerves, cardial', i. 168, 171
pulmonary, i. 466

reflex influence of, on the pulse-rate, i. 175
to tlu- iris, ii. 324
pains, ii. 400

secretory fibres to tin- pancreas, i. 232
to die salivary glands, i. 21-. 222
system, connect ion of. with the cerehro-spinal,
ii. 21-
post-ganglionic fibres of. ii. 219
pre ganglionic fibres of, ii. 218
vaso-motor centres, i. 200
vibration, ii. 3-5
Symphysis, ii. 11 1
Syndesmosis, ii. Ill

Synovial fluid, ii 115
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. 121. L36

ventricular, i. 123

Tactile anas of the skin, ii, 395

corpuscle, ii. 390, 392
Tartar, i. 524

Taste, nerves of, ii. lln

organs of, ii. flu
Taste-buds, ii. 410

Taste-nerves, nuclei of origin of, ii. 23d
Taste perceptions, conditions affecting, ii. 411

Taste-sensations, conditions which influenc . ii.
Ill
primary, ii. 412
distribution of. ii. 413
Taurin, i. 507, 543

in muscles, ii. li>7
'Tea, nutritive value of. i. 357
stimulating action of. ii. 75
'Tectorial membrane, ii. 379
Tegmen of the tympanum, ii. 3G4
Temperature, axillary, i. 4d-

body-, efl'ect <d'. ou 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. 4s7
on metabolism, i. 362
on muscular contraction, ii. 136
on sweat glands, i. 260
on the respiratory quotient, i. 438
on tryptic digest ion. i. :;<il
external, effect of. on respiration, i. 426
on respiratory exchanges, i. 432
on thermotaxis, i. 196
influence of. on conductivity, ii. 92
on heat product ion. i. 4-3
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
ot' the blood from the brain, ii. 288
post-mortem rise of, i. 497
regulation of, i. 173

rise of. from lesions of corpora striata, ii. 271
sense, ii. 397
spots of the skin, ii. 398
topography of, i. 46s
Temporal muscle, i. 372
Tendon reflexes after cerebellar injury, ii. 272

sensory nerve-endings in. ii. 402
Tension, efl'ect of. on contraction curve, ii. 109
on irritability of nerves and muscles, ii. 56
on latent period, ii. Ill)
of the blood-gases, i. 415
Tensor tympani muscle, ii. 369
Tentacles ot' Actinia-, 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. 113

ducts of. ii. 117

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 INDEX.

551

Tetanus of the muscles, ii. 127
rate of stimulation required for, ii. 125
Bitter's, ii. 37, 61
secondary, ii. 150
voluntary, ii. 133
Wundt's, ii. 37. <il
Tetramethylene-diamin, i. 513
Thalamus, cortical connections of, ii. 271

heat-centre of, ii. 271
Theobromin, i. 553
Theophyllin, i. 553
Thermal energy liberated in muscle, ii. Ill

stimulation of nerve, ii. 25
Thermo-accelerator centres, i. 492
Thermogenesis, i. 477

mechanism of, i. 489
Thermogenic centres, i. 491
nerves, i. 490
tissues, i. 490
Thermo-inhibitory centres, i. 492
Thermolysis, i. 485

mechanism of, i. 494
Thermopile, ii. 142
Thermotaxis, i. 489, 495, 496

relation of cerebrum to, ii. 270
Thinlactic 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. 122
Tissues, growth of, ii. 186
Tobacco smoke, action of, on nerves, ii. 60
Tones, combinal ional, ii. 387
differential, ii. 387
fundamental, ii. 383
loudness of, ii. 381
pitch of. ii. 381
simple, ii. 381
Tongue, distribution of taste-sensations on, ii.
413
vaso-inotor nerves of, i. 204
Tonicity of involuntary muscle, i. 371

of vaso-cou strict or centre, i. 199
Tonography definition of, i. 127
Tonometer, i. 188

Tonus, muscular, in the insane, ii. 220
rellex origin of, ii. 220
id' muscles, ii. 143
ventricular, during vagus stimulation, i. 163

Touch illusions, ii. 396
sensations, ii. '■>'*'!

localization of, ii. 394
Tractus solitarius, ii. 236
Transfusion of blood, i. 64
Transversal is abdominis muscle, respiratory

action of. i. 407
Trapezius muscle, respiratory action of, i. 405
Traube-Hering waves, i. 201
Tremors, ii. 132

Triangulares sterni muscles, expiratory action

of, i. 107
Trigeminal nerves, central paths of, ii. 238

influence of, on respiration, i. 463
Trimethylamine, i. 511
Trioses, i. 559
Triplets, ii. 483

Trommel'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. 4iii!
Trypsin, i. 232

effect of, on blood coagulation, i. 63
extracts, preparation of, i. 301
properties of, i. 301
Tripsinogen, i. 235

granules, i. 235
Tryptie 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. 1*2
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 tryptie digestion, i. 303

Ultimum moriens, ii. 492

Umbilical arteries, ii. 474

vein, ii. 17 I
Umbo of the tympanic membrane, ii. 365
Unconsciousness, ii. 293

Unipolar excitation for localized excitation, ii.
15
nerve-cells, development of, ii. 17-
stimulation, ii. 30
principles of, ii. 13
Unite, calorimel ric, i. 177
Unorganized ferments, definition of. i. 271'
Urea, amount of, in sweat, i. .'!:;.">

in the bl 1, i. 51

in the mine, i. :;:',.">

antecedents of', i. ::::.">
eliminal ion of. i. 252
est imaf ion of, i. ."> lit

formation of. after removal of the liver, i. 3:57
in f he liver, i. :;::i

in muscles, ii. L67

origin of, in the body. i. 550

in the liver, i. 266
preparation of, i. ."> i^

from proteid, i. 337

presence of, in sweat, i. .'!.".7
proper! ies of, i. .". |<>
Ureters, movements of, j. 371, 389

552

G ENEMA L IX J) EX.

Urethra, ii. 443

structure of, ii. 448
Qric acid, formation of, i. 338
in the liver, i. 322
in the spleen, i. 333
in muscles, ii. lt>7
molecular structure of, i. 554
occurrence "t', i. 338
origin of, in birds, i. 557

in mammals, i. 338, 556
preparal ion of, i. 555
proper! ies of, i. 555
Urinary bladder, innervation of, i. 392
movements of, i. 390
pigments, origin of, from haemoglobin, i. 45
se< retion. 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
et In real sulphates of, i. 572
secretion of, i. 251
Uriniferous tubules, secretory function of, i.
252
-t met ure of, i. 250
Urobilin, i. 57 1
1 'terns, ii. 1 13, 15<>
Utri cuius of the internal ear, ii. 373

Vagina, ii. 443, 162

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
etl'eet on the heart, nature of, i. 166
gastric branches of, i. 381
inhibition, dependence of, on the character

of the stimulus, i. L65
intestinal branches of, i. 385
nerve, fatigue of. ii. 96

pulmonary branches of, i. 465
rate of conduction in, ii. 90
relation of, to apncea, i 442
respiratory function of, i. 459
pneumonia, i. 166
secretory fibres ,.f. 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. L52, 163
on the ventricle, i. L62
Valsalva's experiment, i. 152

sinuses, i. Ill
Valves, auriculo ventricular, i. 108
of lymphatic vessels, i. 146
.semilunar, i. 110
Valvuhe conniventes, value of. in absorption, i.

:;n
Variation of the offspring in reproduction, ii.

500
Variations, somatic, classification of, i. 197
Vas deferens, ii. 1 17
Yasa deferentia, ii. I !•">

efferentia of the testis, ii. 1 17
Vaseline, i. 531

Vaso-constrictor centre, rhythmical activity of,
i. -.'(il. 151
nerves, discovery of. i. 19.'!
Vaso-dilatoT nerves, discovery of. i. mi
Vaso-motor centre, medullary, i. 198
centres, spinal, i. lii!»
sympal be tic, i. '.'on
nerves, anatomy of. i. 198
methods of investigating, i. 195
of the brain, i. 203
of the cranial vessels, ii. 2*6
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 tin- kidneys, i. 207, 256
of the limbs, i. 209
of the liver, i. 206
of the luugs, i. 205, 466
of the muscles, i. 2 in
of the pancreas, i. 207
of the portal system, i. 209
of the salivary glands, i. 222
of tin- spleen, i. 207
of the tongue, i. 205
of t he veins, i. 195
special properties of, i. 197
reflexes, i. 201
through the vagi, i. 172
Vegetable food.-, composition of, i. 278

proteids, i. 577
Veins, ethct of compression of, on lymph forma-
tion, i . 72
entrance of air into. i. 97
rate of flow in. i. in]
vaso-motor nerves of, i. 209
Velocity of blood-flow, i. 99-101
Vense Thebesii, i. 184
Veno-motor nerves of the limbs, i. 209
Venous blood-flow, effect of the auricles on, i.
L37
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 hrain. capacity of, ii. 271
work done by, i. 106, 107
Ventricular hands, 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.
:;7<i
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
mar-point of. ii. 312
pseudoscopic, ii. 357
sterescopic, ii. 357
Visual ana of the cortex, ii. 253

impulses, place id' 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. 359

GENERAL INDEX.

553

Vital capacity of the lungs, i. 427

force, definition of, i. 25
Vitellin, composition of, i. 579
Vitelline membrane, absence of, in liuman ova,

ii. 450
Vitreous humor, opacities in, ii. 321

refractive index of, ii. 303
Vocal cords, false, ii. 422

true, ii. 423
Voice, ii. 430

changes at puberty in, ii. 489

effect of age on, ii. 431

pitch of. ii. 432

registers of, ii. 432
Voice-production, ii. 421

mechanism of, ii. 431
Voices, classification of, ii. 433
Volta, ii. 28
Voltaic pile, ii. 28
Voluntary control of the heart, i. 178

muscular contractions, fatigue of, ii. 134
tetanic character of, ii. 133

reactions, afferent 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. 27b', 354
percentage of, in brain and cord, ii. 274
properties of, i. 503

pure, toxic action of, on nerves and muscles,
ii. 58
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. 296
relation of, to social environment, ii. 277
of the child at birth, ii. 487
Weissinann'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. 535
Wirsung'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 pellucida, 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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Second edition, thoroughly revised.

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revised and enlarged.

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Atlas and Epitome of Syphilis and the Venereal
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By Dr. O. Haab, of Zurich. From the Third Revised and Enlarged
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Including a Text-Book of Special Bacteriologic Diagnosis. By Prof.
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'7

NOTHNAGEL'S ENCYCLOPEDIA

OF

PRACTICAL MEDICINE

Edited by ALFRED STENGEL, M. D.

Professor of Clinical Medicine in the University of Pennsylvania; Visiting
Physician to the Pennsylvania Hospital

IT is universally acknowledged that the Germans lead the world in Internal
.Medicine ; and of all the German works on this subject, Nothnagel's " Ency-
clopedia of Special Pathology and Therapeutics" is conceded by scholars to
be without question the best System of Medicine in existence. So necessary
is this book in the study of Internal Medicine that it comes largely to this country
in the original German. In view of these facts, Messrs. W. B. Saunders & Com-
pany have arranged with the publishers to issue at once an authorized edition
of this great encyclopedia of medicine in English.

For the present a set of some ten or twelve volumes, representing the most
practical part of this encyclopedia, and selected with especial thought of the needs
of the practical physician, will be published. The volumes will contain the real
essence of the entire work, and the purchaser will therefore obtain at less than
half the cost the cream of the original. Later the special and more strictly
scientific volumes will be offered from time to time.

The work will be translated by men possessing thorough knowledge of both
English and German, and each volume will be edited by a prominent specialist
on the subject to which it is devoted. It will thus be brought thoroughly up to
date, and the American edition will be more than a mere translation of the Ger-
man ; for, in addition to the matter contained in the original, it will represent the
very latest views of the leading American specialists in the various departments
of Internal Medicine. The whole System will be under the editorial super-
vision of Dr. Alfred Stengel, who will select the subjects for the American edition,
and will choose the editors of the different volumes.

Unlike most encyclopedias, the publication of this work will not be extended
over a number of years, but five or six volumes will be issued during the coming
year, and the remainder of the series at the same rate. Moreover, each volume
will be revised to the date of its publicatfon by the American editor. This will
obviate the objection that has heretofore existed to systems published in a number
of volumes, since the subscriber will receive the completed work while the earlier
volumes are still fresh.

The usual method of publishers, when issuing a work of this kind, has been
to compel physicians to take the entire System. This seems to us in many cases
to be undesirable. Therefore, in purchasing this encyclopedia, physicians will be
given the opportunity of subscribing for the entire System at one time ; but any
single volume or any number of volumes may be obtained by those who do not
desire the complete series. This latter method, while not so profitable to the pub-
lisher, offers to the purchaser many advantages which will be appreciated by those
who do not care to subscribe for the entire work at one time.

This American edition of Nothnagel's Encyclopedia will, without question,
form the greatest System of Medicine ever produced, and the publishers feel con-
fident that it will meet with general favor in the medical profession.

18

NOTHNAGEL'S ENCYCLOPEDIA

VOLUMES JUST ISSUED AND IN PRESS

VOLUME I
Editor, William Osier, M. D„ F. R. C. P.

Professor of Medicine in Johns Hopkins
University

CONTENTS

Typhoid Fever. By Dr. H. Ckrschmann,
of Leipsic. Typhus Fever. By Dr. II.
Curschmann, of Leipsic.

Handsome octavo volume of about 600 pages.
Just Issued

VOLUME vn
Editor, John H. Musser, M. D.

Professor 0/ Clinical Medicine, University of
Pennsylvania

CONTENTS •

Diseases of the Bronchi. By Dr. F. A. HOFF-
MANN, of Leipsic. Diseases of the Pleura.
By Dr. Rosenbach, of Berlin. Pneumonia.
By Dr. E. Aufrecht, of Magdeburg.

VOLUME II

Editor, Sir J. W. Moore, B. A., M.D.,
F.R.C.P.I., of Dublin

Professor of Practice of Medicine, Royal College
of Surgeons in Ireland

CONTENTS

Erysipelas and Erysipeloid. By Dr. H. Len-
uartz, of Hamburg. Cholera Asiatica and
Cholera Nostras. By Dr. K. von Lieber-
meister, of Tiibingen. "Whoooing Cough
and Hay Fever. By Dr. G. Sticker, of
Giessen. Varicella. By Dr. Tir. von Jur-
gensen, of Tiibingen. Variola (including
Vaccination). By Dr. H. Immermann, of
Basle.

Handsome octavo volume of over 700 pages.
Just Issued

volume vm

Editor, Charles G. Stockton, M. D.

Professor of Medicine, University of Buffalo

CONTENTS

Diseasesof the Stomach. By Dr. F. Kiegel,

of Giessen.

VOLUME IX
Editor, Frederick A. Packard, M. D.

Physician to the Pennsylvania Hospital and to the
Children's Hospital, Philadelphia

CONTENTS

Diseases of the Liver. By Drs. H.QuiNi ke
and G. Hoppe-Seyler, of Kiel.

VOLUME in
Editor, William P. Northrup, M. D.

Professor of Pediatrics, University and Bellevue
Medical College

CONTENTS

Measles, By Dr. Tii. von JCrgensen, of
Tubingen. Scarlet Fever. By the same
author. Rbtheln. By the same author.

VOLUME X
Editor, Reginald H. Fitz, A.M., M. D.

Hersey Professor of the Theory and Prat :.' •■
of Physic , Ha* vard I University

CONTENTS

Diseases of the Pancreas. By Dk. L. < 'mi:.
of Vienna. Diseases of the Suprarenals.
By 1 >R. E. Nl 1 SSER, "i \ ienna.

VOLUME VI
Editor, Alfred Stengel, M. D.

Professor of Clinical Medi, inc. University 0/

Pennsylvania
CONTENTS

Anemia. By Dr. P. EHRLICH, of Frankfort-
on-the-Main, and 1 >r. A. Lazarus, of Char
lottenburg. Chlorosis. By Dk. K. von
Noorden, of Frankfort-on-the-Main. Dis-
eases of the Spleen and Hemorrhagic
Diathesis. By Dk. M. I.itti n, of Berlin.

VOLUMES IV, V, and XI
Editors announced later

Vol. IV. — Influenza and Dengue. By Dk. i >.

I. Kirn 1 1 \sti rn,oI I lologne. MalarialDis-

eases. By Dk. J. Mannaberg, ol Vienna.
Vol. \. Tuberculosis and Acute General

Miliary Tuberculosis. By Dr.G.< < >km i ,

of Berlip.

Vol. XI Diseases of the Intestines and
Peritoneum. By Dr. II. NOTHNAGKL,
of Vienna.

CLASSIFIED LIST

OF THE

MEDICAL PUBLICATIONS

OF

W. B. SAUNDERS Ct COMPANY

ANATOMY, EMBRYOLOGY,
HISTOLOGY.
Bbhm, Davidoff, and Huber— Histology,
Clarkson A Text-Book of Histology
Haynes - A Manual of Anatomj
Heisler —A Text-Book of Embryology
Leroy— Essentials of Histology,. .

McClellan — Art Anatomy

McClellan — Regional Anatomy, . .
Nancrede — Essentials of Anat< >m) , .
Nancrede— Essentials of Anatomy
Manual of Practical Dissection. .

and

BACTERIOLOGY.

Ball — Essentials of Bacteriology

Frothingham — Laboratory Guide, . . . .

Gorham — Laboratory Bacteriology, . . .

Lehmann and Neumann — Atlas of Bacte-
riology,

Levy and Klemperer's Clinical Bacteri-
ology

Mallory and Wright— Pathological Tech-
nique

McFarland — Pathogenic Bacteria

CHARTS, DIET-LISTS, ETC.

Griffith— Infant's Weight Chart

Hart — Diet in Sickness and in Health, . .

Keen — Operation Blank,

Laine — Temperature Chart

Meigs — Feeding in Early Infancy

Starr — Diets for Infants and Children, . .
Thomas— Diet-Lists

CHEMISTRY AND PHYSICS.
Brockway Essentials of Medical Physics,
Jelliffe and Diekman— Chemistry, '. . .

Wolf — Urine Examination

Wolff— Essentials of Medical Chemistry, .

CHILDREN.
American Text-Book Dis.of Children,

Griffith— Care of the Baby

Griffith —Diseases of Children, . . .
Griffith— Infant's Weight Chart, . . .
Meigs — Feeding in Early Infancy, . .
Powell —Essentials of Diseases of ( 'hildren
Starr— Diets for Infants and Children, .

DIAGNOSIS.
Cohen and Eshner — Essentials of Diag-

n sis

Corwin — Physical Diagnosis

Vierordt — Medical Diagnosis

DICTIONARIES.
The American Illustrated Medical Dic-
tionary

The American Pocket Medical Dictionary,
Morten — Nurses' Dictionary

I 4

EYE, EAR, NOSE, AND THROAT.

An American Text-Book of Diseases of

the Eye, Ear, Nose, and Throat, .... i
De Schweinitz— Diseases of the Eye, . . 6
Friedrich and Curtis— Rhinology, Laryn-
gology and Otology, .• 6

Gleason — Essentials of Diseases of the Ear, 15

Gleason— Ess. of Dis. of Nose and Throat, 15

Gradle — Ear, Nose, and Throat 22

Griinwald and Grayson— Atlas of Dis-
eases of the Larynx 16

Haab and De Schweinitz — Atlas of Exter-
nal Diseases of the Eye 16

Haab and De Schweinitz— Atlas of Oph-
thalmoscopy, 17

Jackson — Manual of Diseases of the Eye, 8

Jackson — Essentials of Diseases of Eye, 15

Kyle — Diseases of the Nose and Throat, . 9

GENITO-URINARY.

An American Text-Book of Genito-Uri-

nary and Skin Diseases 2

Hyde and Montgomery— Syphilis and the

Venereal Diseases 8

Martin — Essentials of Minor Surgery,

Bandaging, and Venereal Diseases, . . . 15
Mracek and Bangs— Atlas of Syphilis and

the Venereal Diseases 16

Saundby — Renal and Urinary Diseases, . . n
Senn — Genito-Urinary Tuberculosis, ... 12
Vecki — Sexual Impotence 14

GYNECOLOGY.

American Text-Book of Gynecology,
Cragin — Essentials of Gynecology, .... 15

Garrigues — Diseases of Women 6

Long — Syllabus of Gynecology,
Penrose — Diseases of Women, .
Pryor — Pelvic Inflammations, .
Schaeffer & Norris — Atlas of Gynecology, 17

HYGIENE.
Abbott — Hygiene of Transmissible Diseases 3

Bergey — Principles of Hygiene 4

Pyle — Personal Hygiene 11

MATERIA MEDICA, PHARMACOL-
OGY, AND THERAPEUTICS.

American Text-Book of Therapeutics, . . 1
Butler — Text-Book of Materia Medica,

Therapeutics, and Pharmacology, ... 4

Morris — Ess. of M. M. and Therapeutics, 15

Saunders' Pocket Medical Formulary, . . 12

Sayre — Essentials of Pharmacy 15

Sollmann— Text- Book of Pharmacology, . 12

Stevens — Manual of Therapeutics, ... 13

Stoney — Materia Medica for Nurses, . . 13

Thornton — Prescription-Writing 14

MEDICAL PUBLICATIONS OF IV. B. SAUNDERS & CO. 21

MEDICAL JURISPRUDENCE AND
TOXICOLOGY.

Chapman — Medical Jurisprudence and
Toxicology 5

Golebiewski and Bailey— Atlas of Dis-
eases Caused by Accidents 17

Hofmann and Peterson— Atlas of Legal
Medicine 16

NERVOUS AND MENTAL
DISEASES, ETC.

Brower — Manual of Insanity 22

Chapin — Compendium of Insanity, ... 5
Church and Peterson — Nervous and Men-
tal Diseases 5

Jakob & Fisher — Atlas of NervousSystem, 17
Shaw — Essentials of Nervous Diseases and
Insanity 15

NURSING.

Davis — Obstetric and Gynecologic Nursing, 5

Griffith— The Care of the Baby 7

Hart — Diet in Sickness and in Health, . . 7

Meigs — Feeding in Early Infancy 10

Morten — Nurses' Dictionary 10

Stoney — Materia Medica for Nurses, . . 13

Stoney — Practical Points in Nursing, ... 13

Stoney — Surgical Technic for Nurses, . . 13

Watson — Handbook for Nurses 14

OBSTETRICS.

An American Text-Book of Obstetrics, . 2

Ashton — Essentials of Obstetrics 15

Boisliniere— Obstetric Accidents, .... 4

Dorland — Modern Obstetrics 6

Hirst — Text-Book of Obstetrics, .... 7

Norris — Syllabus of Obstetrics 10

Schaeffer and Edgar— Atlas of Obstetri-
cal Diagnosis and Treatment, 17

PATHOLOGY.

An American Text-Book of Pathology, . 2
Diirck and Hektoen— Atlas of Pathologic

Histology 16

Kalteyer— Essentials of Pathology, ... 22
Malloryand Wright— Pathological Tech-
nique 9

Senn — Pathology and Surgical Treatment

of Tumors, 12

Stengel— Text-Book of Pathology, ... 13
Warren— Surgical Pathology and Thera-
peutics x4

PHYSIOLOGY.

An American Text-Book of Physiology, 2

Budgett— Essentials of Physiology, ... 22

Raymond — Human Physiology 11

Stewart— Manual of Physiology, .... 13

PRACTICE OF MEDICINE.
An American Year-Book of Medicine and

Sun

Anders — Practice of Medicine 4

Eichhorst— Practice of Medicine 6

Lockwood— Manual of the Practice of

Medicine 9

Morris— Ess. of Practice of Medicine, . . 15
Salinger and Kalteyer — Modern Medi-
cine Il

Stevens— Manual of Practice of Medicine, 13

SKIN AND VENEREAL.

An American Text-Book of Genito-
urinary and Skin Diseases 2

Hyde and Montgomery — Syphilis and the
Venereal Diseases 8

Martin — Essentials of Minor Surgery,
Bandaging, and Venereal Diseases, . . 15

Mracek and Stelwagon — Atlas of Diseases
of the Skin, 16

Stelwagon — Essentials of Diseases of the
Skin 15

SURGERY.

An American Text-Book of Surgery, . . 2
An American Year-Book of Medicine and

Surgery 3

Beck — Fractures 4

Beck — Manual of Surgical Asepsis, ... 4

Da Costa — Manual of Surgery 5

International Text-Book of Surgery, . . 8

Keen — Operation Blank 8

Keen — The Surgical Complications and

Sequels of Typhoid Fever 8

Macdonald — Surgical Diagnosis and Treat-
ment 9

Martin— Essentials of Minor Surgery,

Bandaging, and Venereal Diseases, . . 15

Martin— Essentials of Surgery 15

Moore — Orthopedic Surgery 10

Nancrede — Principles of Surgery 10

Pye — Bandaging and Surgical Dressing, . 11

Scudder — Treatment of Fractures, ... 12

Senn — Genito-Urinary Tuberculosis, ... 12

Senn — Practical Surgery, 12

Senn — Syllabus of Surgery 12

Senn — Pathology and Surgical Treatment

of Tumors 12

Warren — Surgical Pathology and Thera-
peutics *4

Zuckerkandl and Da Costa— Atlas of

Operative Surgery 16

URINE AND URINARY DISEASES.

Ogden — Clinical Examination of the Urine, 11
Saundby -Renal and Urinary Diseases, . n
Wolf - Handbook of Urine- Examina-
tion 14

Wolff — Essentials of Examination of
Urine *5

MISCELLANEOUS.

Bastin — Laboratory Exercises in Botany, . 4
Golebiewski and Bailey- Atlas of Dis-

e tses Caused by Accidents 17

Gould and Pyle — Anomalies and Curiosi-
ties of Medicine 7

Grafstrom— Massage 7

Keating— 1 low to Examine for Life Insur-
ance . .

Saunders' Medical Hand-Atlases, . . 16,17
Saunders' Pocket Medical Formulary, . . 12
Saunderb' Question-Compends, . . .14,15
Stewart and Lawrance— Essentials of

Medical Electricity 15

Thornton — Dose-Book and Manual of

Prescription-Writing 13

Warwick and Tunstall First Aid to the

Injured and Sick 14

Books in Preparation.

Jelliffe arid Diekman's Chemistry.

A Text-Book of Chemistry. By Smith Ely Jelliffe, M. D., Ph.D.,
Professor of Pharmacology, College of Pharmacy, New York ; and
George C. Diekman, Ph. (J., M. 1)., Professor of Theoretical and
Applied Pharmacy, College of Pharmacy, New York. Octavo, 550
pages, illustrated.

Brower's Manual of Insanity.

A Practical Manual of Insanity. By Daniel R. Brower, M. D., Pro-
fessor of Nervous and Mental Diseases, Rush Medical College, Chicago.
121110 volume of 425 pages, illustrated.

Kalteyer's Pathology.

Essentials of Pathology. By F. J. Kalteyer, M. D., Assistant Demon-
strator of Clinical Medicine, Jefferson Medical College ; Pathologist to
the Lying-in Charity Hospital ; Assistant Pathologist to the Philadel-
phia Hospital. A New Volume in Saunders' Question- Compend Series.

Gradle on the Nose, Throat, arid Ear.

Diseases of the Nose, Throat, and Ear. By Henry Gradle, M. D.,
Professor of Ophthalmology and Otology, Northwestern University
Medical School, Chicago. Octavo, 800 pages, illustrated.

Budgett's Physiology.

Essentials of Physiology. By Sidney P. Budgett, M. D., Professor of
Physiology, Washington University, St. Louis, Mo. A New Volume
in Saunders' Question- Compend Series.

Griffith's Diseases of Children.

A Text-Book of the Diseases of Children. By J. P. Crozer Griffith,
Clinical Professor of Diseases of Children, University of Pennsylvania.

Galbraith on the Four Epochs of Woman's Life.

The Four Epochs of Woman's Life: A Study in Hygiene. By Anna
M. Galbraith, M. I)., Fellow New York Academy of Medicine; At-
tending Physician Neurologic Department New York Orthopedic Hos-
pital and Dispensary, etc. With an Introduction by John H. Musser,
M. 1)., Professor of Clinical Medicine, University of Pennsylvania.
1 21110 volume of about 200 pages.

Date Due

.HINTED IN U.S. A CAT NO. 24 161

D 000 224 608 o

CO

== a?

k

11859a

1900

v. 2
Hove 11, William II

An American text-book of physic-lory

MEDICAL SCIENCES LIBRARY

UNIVERSITY OF CALIFORNIA, IRVINE

IRVINE, CALIFORNIA 92664

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