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AN INTRODUCTION

TO

NEUROLOGY

BY

C. JUDSON HERRICK

PROFESSOR OF NEUROLOGY IN THE UNIVERSITY OF CHICAGO

ILLUSTRATED

PHILADELPHIA AND LONDON

W. B. SAUNDERS COMPANY

1916

Copyright, 1915, by W. B. Saunders Company

PRINTED IN AMERICA

PRESS OF

W. B. SAUNDERS COMPANY
PHILADELPHIA

PREFACE

THERE are two groups of functions performed by the nervous
system which are of general interest; these are, first, the physio-
logical adjustment of the body as a whole to its environment
and the correlation of the activities of its organs among them-
selves, and, in the second place, the so-called higher functions
of the cerebral cortex related to the conscious life. The second
of these groups of functions cannot be studied apart from the
first, for the entire conscious experience depends for its materials
upon the content of sense, that is, upon the sensory data received
by the lower brain centers and transmitted through them to the
cerebral cortex. Since the organization of these lower centers is
extremely complex, and since even the simplest nervous proc-
esses involve the interaction and cooperation of several of these
mechanisms, it follows that an understanding of the workings
of any part of the nervous system requires the mastery of a large
amount of rather intricate anatomical detail.

Fortunately, the knowledge of the precautions which must be
observed in order to maintain the nervous system in healthy
working order is not difficult of acquisition (though surprisingly
few people seem to have gained it), just as any one can learn to
operate an automobile, even though quite ignorant of the engi-
neering problems involved in its design and construction. In-
formation regarding these matters of practical hygiene is readily
available,1 and it is not the primary purpose of this book to sup-

1 GULICK, LUTHER H. 1907. The Efficient Life, New York.

GULICK, LUTHER H. 1908. Mind and Work, New York.
^ JEWETT, FRANCIS GULICK, 1898. Control of Body and Mind, New York,
Ginn & Co. Adapted for use in the graded schools.

LUGARO, E. 1909. Modern Problems in Psychiatry, Manchester
University Press. A book written especially for physicians, but full of
stimulating ideas for every educated reader.

STILES, P. G. 1914. The Nervous System and Its Conservation, Phila-
delphia, W. B. Saunders Company.

11

12 PREFACE

ply it. But to understand the actual inner operation of the
nervous mechanisms is a much more difficult matter, and this
knowledge cannot be acquired without arduous and sustained
study of the peculiar form relations of the nervous organs and
their complex interconnections; and information of this sort is
indispensable for a grasp of the principles of nervous organiza-
tion, and especially for an intelligent treatment of nervous dis-
eases.

The study of neurology is, therefore, intrinsically difficult
if one is to advance beyond its most superficial phases; the more
so if the student is not well grounded in general biology and at
least the elements of the general anatomical structure of the ver-
tebrate body. To these inherent difficulties there is added a
purely artificial obstacle in the form of a cumbersome and con-
fused terminology which has grown up during several centuries of
anatomical study of the brain, in the early stages of which little
or no comprehension of the functional significance of the parts
discovered was possible, and fanciful or bizarre names were given
without reference to the mutual relationship of parts.

The problems which at present chiefly occupy the attention of
neurologists are of two sorts — first, to discover the regional
localization within the nervous system of the nerve-cells and
fibers which serve particular types of function or, briefly, archi-
tecture, and second, to discover the chemical or other changes
which take place during the process of nervous function, that is,
the metabolism of the nervous tissues. The first of these prob-
lems is at present further advanced than the second; the larger
part of this work is, therefore, devoted to a description of archi-
tectural relations. Without a knowledge of these relations,
moreover, the problems of metabolism are, in large measure,
meaningless.

It is impossible to understand clearly the form of the brain,
and especially the relations of its internal structures, from verbal
descriptions merely. Pictorial illustrations and the various brain
models which are on the market are of great assistance; but ac-
tual laboratory experience in dissecting the brain and, if possi-
ble, the study of microscopic preparations of selected parts of
it are indispensable for a thorough mastery of the subject.
The brains of the sheep, dog, and cat are easily obtained, and

PREFACE 13

are so similar to the human brain in all respects, save the
smaller relative size of the cerebral cortex, that they can readily
be used for such studies. Before dissection the brain should be
carefully removed from the skull and hardened by immersion for
a few days in a solution of formalin (to be obtained at any drug
store and diluted with water in the proportion of one part
formalin to nine parts water). Several neurological laboratory
guides have been published, and one of these should be followed
in the dissection.1

This work is designed as an introduction in a literal sense.
Several very excellent manuals and atlases of neurology are
available, and to these the reader is referred for the illustrations
and more detailed descriptions necessary to complete the rather
schematic outline here presented. The larger medical text-
books of anatomy and physiology are, however, often very diffi-
cult for the beginner, chiefly on account of the lack of correlation
of the structures described and their functions. This little
book has been prepared in the hope that it will help the student
to learn to organize his knowledge in definite functional pat-
terns earlier in his work than is often the case, and to appreciate
the significance of the nervous system as a working mechanism
from the beginning of his study.

The structure and functions of the nervous system are of
interest to students in several different fields — medicine, psy-
chology, sociology, education, general zoology, comparative
anatomy, and physiology, among others. The view-points and
special requirements of these various groups are, of course,
different; nevertheless the fundamental principles of nervous
structure and function are the same, no matter in what
field the principles are applied, and the aim here has been to

1BuRKHOLDER, J. F. 1912. The Anatomy of the Brain, Second Edition,
Chicago, G. P. Engelhard & Co. (Dissection of the brain of the sheep.)

FISKE, E. W. 1913. An Elementary Study of the Brain Based on the
Dissection of the Brain of the Sheep, New York, The Macmillan Company.

HARDESTY, 1. 1902. Neurological Technique, The University of Chicago
Press. (Dissection of the human brain by means of transverse gross
sections, methods of microscopic preparation, and lists of neurological
terms.)

HERRICK, C. JUDSON, and CROSBY, ELIZABETH. 1915. A Laboratory
Outline in Neurology, privately printed by the authors at the University of
Chicago. (Dissection of the dogfish, sheep, and human brains, and direc-
tions for study of prepared microscopic sections of the human brain.)

14 PREFACE

present these principles rather than any detailed application
of them. In the selection of subject matter and mode of
treatment the author has been fortunate in having the advice
of many experienced teachers in several of these fields, who
have read the manuscript of this work or of selected chapters
and whose suggestions have contributed greatly to its value.
Especial acknowlegement of generous assistance of this sort
should be made to Doctors G. W. Bartlemcz, R. R. Bensley,
Harvey A. Carr, C. M. Child, G. E. Coghill, Mabel R. Fcrnald,
Joseph W. Hayes, Mary Stevens Hayes, F. L. Landacre, John
T. McManis, and R. E. Sheldon.

The materials presented in this book are arranged in three
groups: (1) Chapters I to VII discuss the more general neurologi-
cal topics; (2) Chapters VIII to XVIII comprise a brief account of
the form of the nervous system and the functional significance of
its chief subdivisions in general, followed by a review of the archi-
tectural relations of the more important functional systems; (3)
Chapters XIX to XXI are devoted to the cerebral cortex and its
functions. Readers whose chief interest lies in the general neu-
rological questions may omit much of the detail comprised
within the second group of chapters or use these for reference
only. To facilitate ready reference the general index has been
prepared with especial care, and with it is combined a brief
glossary of some more commonly used technical terms. In the
text some of the more special topics, which may be omitted if
a briefer presentation is desired, are printed in smaller type.

C. JUDSON HERRICK.
CHICAGO, ILL.,
October, 1915.

CONTENTS

CHAPTER I PAGE-
BIOLOGICAL INTRODUCTION 17

CHAPTER II
THE NERVOUS FUNCTIONS 24

CHAPTER III

'THE NEURON 38

CHAPTER IV
THE REFLEX CIRCUITS 56

CHAPTER V

THE RECEPTORS AND EFFECTORS 69

CHAPTER VI

THE GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM 96

CHAPTER VII

THE GENERAL ANATOMY AND SUBDIVISION OF THE NERVOUS SYSTEM 106

CHAPTER VIII
THE SPINAL CORD AND ITS NERVES 125

CHAPTER IX
THE MEDULLA OBLONGATA AND CEREBELLUM 143

CHAPTER X

THE CEREBRUM 160

15

16 CONTENTS

CHAPTER XI

PAGE

THE GENERAL SOMATIC SYSTEMS OF CONDUCTION PATHS 172

CHAPTER XII
THE VESTIBULAR APPARATUS AND CEREBELLUM 183

CHAPTER XIII
THE AUDITORY APPARATUS 195

CHAPTER XIV
THE VISUAL APPARATUS 204

CHAPTER XV
THE OLFACTORY APPARATUS 215

CHAPTER XVI

THE SYMPATHETIC NERVOUS SYSTEM 224

CHAPTER XVII
THE VISCERAL AND GUSTATORY APPARATUS 234

CHAPTER XVIII
PAIN AND PLEASURE 249

CHAPTER XIX
THE STRUCTURE OF THE CEREBRAL CORTEX 263

CHAPTER XX
THE FUNCTIONS OF THE CERERRAI, CORTEX 279

CHAPTER XXI

THE EVOLUTION AND SIGNIFICANCE OF THE CEREBRAL CORTEX 301

INDEX AND GLOSSARY . 317

INTRODUCTION TO NEUROLOGY

CHAPTER I

BIOLOGICAL INTRODUCTION

THE living body is a little world set in the midst of a larger
world. It leads in no sense an independent life, but its con-
tinued welfare is conditioned upon a nicely balanced adjust-
ment between its own inner activities and those of surrounding
nature, some of which are beneficial and some harmful. The
great problem of neurology is the determination of the exact
part which the nervous system plays in this adjustment.

This problem is by no means simple. The search for its
solution will lead us, in the first place, back to an examination
of some of the fundamental properties of the simplest living
substance, of protoplasm itself; and in the last analysis it will
involve a consideration of the highest mental capacities of the
human race and of the physiological apparatus through which
these capacities come to expression. We shall first take up the
nature of this adjustment on the lower biological levels.

All of the infinitely diverse forms of living things have cer-
tain points in common, so that one rarely has any doubt
whether a given object is alive or dead. Nevertheless, the
precise definition of life itself proves very difficult. Herbert
Spencer, in his "Principles of Biology," after many pages
of close argument and rather formidable verbal gymnastics,
arrived at this formula: Life is "the definite combination of
heterogeneous changes, both simultaneous and successive, in
correspondence with external coexistences and sequences";
or, more briefly, "The continuous adjustment of internal re-
lations to external relations." A somewhat similar idea was

2 17

18 INTRODUCTION TO NEUROLOGY

subsequently more simply expressed by the late C. L. Herrick
in the proposition. "Life is the correlation of physical forces
for the conservation of the individual"; and this, in turn, may
be cast in the more general form, Life is a system of forces
maintained by a continuous interchange of energy between the
system and its environment, these forces being so correlated
as to conserve the identity of the system as an individual and
to propagate it. A certain measure of modifiability in the char-
acter of the system, without loss of its individuality, is not ex-
cluded.

No one of these definitions, or any other which has been sug-
gested, is fully satisfactory; but biologists generally agree that
the common characteristics of living beings can best be ex-
pressed in the present state of our knowledge in terms of their
actions, their behavior. The properties commonly ascribed to
any object are in last analysis names for its behavior, and the
so-called vital properties are very special forms of energy trans-
formation.

Now, the chief difference between a corpse and a living body
consists in the fact that the forces of surrounding nature tend
to the disintegration of the dead body, while in the living body
these forces are utilized for its upbuilding. If, then, the vital
process is essentially a special type of mutual interaction be-
tween the bodily mechanism and the forces of the surrounding
world, of the correspondence between the organism and the
environment, to use the Spencerian phrase, it follows that the
living body cannot be studied by itself alone. Quite the con-
trary, the analysis of the environmental forces upon which the
life of the body depends and of the parts of the body itself
in their relations to these external forces is the very kernel of
the problem of life.

The measure of the fulness of life in any organism is two-
fold. In the first place, the life is measured by the amount of
energy which the organism can assimilate from surrounding
nature and incorporate into its own organization. This enters
the body chiefly in the form of chemical potential energy in food
eaten, air breathed, and so on, and can be quantitatively de-
termined and stated in the form of standard units of energy,
such as calories or foot-pounds of work. This measures the

BIOLOGICAL INTRODUCTION 19

working capacity of the machine, but gives little insight into
the real value of the work done. In the second place, life may
be measured in terms of the extensity or number and diversity
of environmental relations. This takes account of the range
or working distance of the organization and, in general, of the
efficiency of the work done. For evidently the organism which
has few and simple relations with the environment, so that it
can adjust itself to only a small range of external conditions,
is less efficient than one which has many diverse relationships
and an extensive series of possible adjustments, even though the
actual amount of energy expended may be vastly greater in the
former than in the latter case. The first of these standards is a
tolerably satisfactory measure of the vegetative functions of
the body, sometimes less happily termed the "organic functions."
We have no word in common use which covers precisely the
group of activities embraced by our second standard of meas-
urement, though the terms "animal functions," "somatic or
exteroceptive activities" are sometimes used in about this
sense.

Let us now endeavor to illustrate the last topic a little more
concretely. We are standing on a hilltop overlooking a
meadow, through which runs a mountain brook, and beyond
the valley is another range of rugged hills. In the fence-corner
near us is a patch of daisies and clover with a honey-bee buzzing
from flower to flower. A plowboy is crossing the field, and at
our elbow an artistic friend is busy with sketching pad and
brushes. Here are four things which have this at least in
common, that they are alive — daisy, bee, plowboy, artist.
There can be no doubt about their vitality, but how differently
they respond to the sunshine, the rain, and the other forces of
nature.

The daisy expands in the vivifying light of the summer sun,
the energy of whose actinic rays is used to build up living proto-
plasm and vegetable fiber from the inert substances of air and
soil. Its vitality, measured in terms of energy transforma-
tion, is great; yet how limited its range of life, how helpless -in
the face of the storms of adversity which are sure to buffet it.
Rooted to its station, it can only assimilate what food is brought
to it and it cannot flee from scorching wind or blighting frost.

20 .INTRODUCTION TO NEUROLOGY

The honey-bee leads a more free and varied life. Instead of
passively and blindly waiting for such bane or blessing as fate
may bring, she hurries forth, strong of wing and with senses
alert, to gather the daily measure of honey and pollen. The
senses of touch, sight, and smell open realms of nature forever
closed to the plant, and enable her to seek food in new fields
when the local supply is exhausted, as well as to avoid enemies
and misfortunes. With the approach of the storm, she flies to
shelter in a home which she and her sisters have prepared with
consummate skill. Yet in this provision for the future in hive
and well-stocked honeycomb there is little evidence of intelli-
gent foresight or rational understanding of the purposes for
which they work. Though so much more highly organized than
the plant, the honey-bee is to a very large extent blindly follow-
ing out the inborn impulses of her hereditary organization and
she has no clear understanding of what she does, much less why
she does it. There is some evidence of intelligent adaptation in
her behavior, but the part played by this factor in her life as a
whole is probably very small compared with the blind inborn
impulses which dominate most of her activities. Like the plant,
the bee's reactions are determined chiefly by the past evo-
lutionary history of the species, which has shaped the innate
organization of the body and fixed its typical modes of re-
sponse to stimulation. But the bee lives much more in the
present than does the plant; that is, she can vary her behavior
much more widely in response to the needs of the moment. As
for the future, she knows naught of it.

The farmer's boy whistles as he goes about his work. He,
too, has a certain innate endowment, including the whole
range of his vegetative functions, together with an instinctive
love of sport and many other inborn aptitudes. This is his
inheritance from the past. By these instincts and appetites he
is, as Dewey says, "pushed from behind" through the per-
formance of many blindly impulsive acts. He is a creature of
the present, too, his whole nature overflowing with the joy of
living. But he also looks into the future and hastens through
the daily tasks that he may obtain the coveted hour of sunset
to fish in the brook. He flicks off the heads of the daisies with
his whip-stock and remarks in passing, "This meadow is

BIOLOGICAL INTRODUCTION 21

choking up with white-weed. The boss will have to plow it up
next year and replant it." The extraordinary natural beauty
of the place is, however, unnoticed amid the round of daily
work and simple pleasure.

The artist looks out upon the same scene, but through what
different eyes! The mass of white daisies and the rocky knoll
beyond ruddy with sheep sorrel suggest to him no waste of
valuable pasture land, but a harmony of color and grace of form
upon which he feasts his soul. The esthetic delights of the
forest, the sky, the brook, and the overhanging crag beyond are
for him unmixed with any utilitarian motive.

Each of these four organisms occupies, in one sense, the
same environment; but it is evident that the factors of this
environment with which each comes into active vital relations
are immeasurably different. They correspond with or are at-
tuned to quite different energy complexes, though the cor-
respondence or interaction is very real in each case. This has
been stated very simply by Dr. Jennings when he says that
every species of organism has its characteristic "action system,"
i. e., a habitual mode of reaction to its environment which is
determined wholly or in part by its inherited organization.

Every animal and every plant has, accordingly, • a definite
series of characteristic movements which it can make in re-
sponse to external stimulation. This is all that Jennings means
by the "action system." We humans are no exception to this
rule of life. We move along in a more or less stereotyped way,
through more or less familiar grooves, in our daily work.
Much of this work is routine, done about as mechanically as
the flower unfolds its petals to the morning sun or the honey-
bee gathers in her store of honey. This is our action system.
Of course, we have much else to do besides this routine, and our
actual value to the community is in large measure determined by
our ability to vary this routine in adaptation to new situations
as they arise. Even the daisy has a little of this capacity for
independently variable action; the insect has more; and man's
preeminence in the world is due primarily to his larger powers
of adapting his reactions not only to the needs of the moment,
but to probable future contingencies, i. e., of varying his
inborn action system by intelligently directed choices.

22 INTRODUCTION TO NEUROLOGY

This distinction between the blind working of a stereotyped
action system whose character is determined by the inherited
bodily structure, on the one hand, and individually acquired
variable adaptive actions (which may or may not be intelli-
gently performed), on the other hand, is very fundamental, and
we shall have occasion to return to it. Most animal activities
contain both of these factors, and it is often very difficult
to analyze a given example of behavior into its elements,
but the distinction is nevertheless important. Plant life is
characterized by the dominance of invariable types of reaction
which are determined by innate structure; these in their most
elementary forms give us, in fact, the so-called vegetative func-
tions. These same functions predominate in the lowest animals
also; but in the higher animals, as we shall see, there are two
rather distinct lines of evolutionary advance. In one line the
innate stereotyped functions are very highly specialized, leading
up to a complex instinctive mode of life; in the other line these
functions are subordinated to a higher development of the indi-
vidually acquired variable functions, leading up to the intelli-
gence and docility of the higher mammals, including the human
race.

The distinction between plants and animals is very difficult
to draw and, in fact, there are numerous groups of organisms
which at the present time occupy an ambiguous position, such
as the slime molds. The botanists claim them and call them
Myxomycetes; the zoologists also describe them under the name
Mycetozoa; still other naturalists frankly give up the problem
and assign them to an intermediate kingdom, neither vegetable
nor animal, which they call the Protista. As children we prob-
ably considered the chief distinction between plants and ani-
mals to be the ability of the latter to move freely about; but one
of the first lessons in our elementary biology was the correction
of this notion by the study of sedentary animals and motile
plants. Nevertheless, I fancy that in the broad view the child-
ish idea has the root of the matter in it. The plants and seden-
tary animals may have their vegetative functions of internal
adjustment never so highly specialized and yet remain rela-
tively low in the biological scale because their relations with
the environment are necessarily limited to the small circle within

BIOLOGICAL INTRODUCTION 23

which they first take root, whereas the power of locomotion
carries with it, at least potentially, the ability to choose between
many more environmental factors. It is only the free-moving
animals that have anything to gain by looking ahead in the
world, and here only do we find well-developed distance recep-
tors, i. e., sense organs adapted to respond to impressions from
objects remote from the body. And the distance receptors, as
we shall see, have dominated the evolution of the nervous sys-
tem in vertebrates and determined the lines it should follow.

The net result of this discussion can be briefly stated. The
differences between various kinds of organisms are, in the main,
incidental to the extent and character of their relations with the
forces of surrounding nature. A species which can adjust itself
to few elements of its environment we call low; one that can
adapt itself to a wide range of environmental conditions in a
great variety of ways we call higher. The supremacy of the
human race is directly due to our capacity for diversified living.
If man finds himself in an unfavorable climate, he may either
move to a more congenial locality or adapt his mode of life by
artificial aids, such as clothing, houses, and fire. And in these
adaptations he is not limited to a narrow range of inherited
instincts, like the hive of bees, but his greater powers of obser-
vation and reflection enable him to discover the general uni-
formities of natural process (he calls these laws of nature) and
thus to forecast future events and prepare himself for them in-
telligently. In other words, to return to our original point of
view, our advantage in the struggle for existence lies in our
ability to correlate our bodily activities with a wide range of
natural forces so as to make use of these forces for our good rather
than our hurt. (Of course, it should be borne in mind that this
formula makes no pretense of being an exhaustive account of
human faculty; but only that, in so far as biological evolutionary
factors have operated in the human realm, they act in accord-
ance with this principle.) The apparatus by which these exter-
nal adjustments are effected and by which the inner parts of the
body are kept in working order is the nervous system.

CHAPTER II

THE NERVOUS FUNCTIONS

THE body is composed of organs and tissues, the organs being
parts with particular functions to perform and the tissues being
the cellular fabric of which the organs are composed. The
tissues (which must be studied microscopically) are classified,
sometimes in accordance with the general functions which they
serve, such as the nervous and muscular tissues, and sometimes
with reference to the forms and arrangements of their compo-
nent cells. An illustration of the latter method of treatment is
furnished by the epithelial tissues, which are thin sheets of
cells, sometimes arranged in one layer (simple epithelia), some-
times in several layers (stratified epithelia). Epithelial tissues
may perform the most diverse functions.

All living substance (protoplasm) possesses in some measure
the distinctive nervous functions of sensitivity and conductivity,
that is, it responds in a characteristic fashion to certain exter-
nal forces (stimuli), and when thus stimulated at one point the
movement or other response may be effected by some remote
part. This last feature implies that some form of energy is
conducted from the site of the stimulus to the part moved.
Ordinary protoplasm also possesses the power of correlation,
that is, of combining a number of individual reactions to stimu-
lation in diverse special adjustments.

The one-celled animals and all plants lack the nervous sys-
tem entirely ; nevertheless they are able to make highly complex
adjustments. The leaves, roots, and stems of the higher plants
have individual functions which are, however, bound together
or integrated into a very perfect unity. In animals, as con-
trasted with plants, we see a further differentiation of parts of
the body for special functions, and at the same time a more per-
fect correlation of part with part and integration of the whole
for rapid and diversified reactions of the entire body. The

24

THE NERVOUS FUNCTIONS

25

nervous system is the apparatus of these more perfect adjust-
ments and its protoplasm is highly modified in different direc-
tions. Some parts may be especially sensitive to particular
forms of energy (such as light waves, sound waves, etc., this
being termed the adequate stimulus in each case) ; other parts,
the nerves, are highly modified so as to conduct nervous impulses
from part to part with a minimum expenditure of energy and loss
of efficiency; still other parts of the nervous system serve as
centers for receiving and redistributing nervous impulses some-
what after the fashion of the central exchange of an automatic

jKin

Fig. 1. — Diagram illustrating the simplest spinal reflex arc consisting
of two nervous elements or neurons (see Chapter III), a sensory neuron
connected with the skin and a motor neuron connected with a muscle.
Physiological connection between the two neurons is effected within the
spinal cord. (Modified from Van Gehuchten.)

telephone system. These are the correlation centers, and they
are larger and more complex in proportion to the range of diver-
sity in the possible reactions of the animal.

The simpler reactions to stimulation of the sort here under con-
sideration are called reflexes (Fig. 1 ; see also p. 56), and the essen-
tial mechanism is a reflex arc consisting of (1) a sensitive receiv-
ing organ (receptor or sense organ); (2) a conductor (afferent
or sensory nerve) transmitting the nervous impulse inward from
the receptor; (3) a correlation center or adjuster, generally
located within the central nervous system; (4) a second con-
ductor (efferent or motor nerve) transmitting the nervous im-

26 INTRODUCTION TO NEUROLOGY

pulse outward from the center to (5) the effector apparatus,
consisting of the organs of response (muscles, glands) and the
terminals of the efferent nerves upon them.

No part of the nervous system has any significance apart from
the peripheral receptor and effector apparatus with which it is
functionally related. This is true not only of the nervous mech-
anism of all physiological functions, but even of the centers con-
cerned with the highest manifestations of thought and feeling of
which we are capable, for the most abstract mental processes
use as their necessary instruments the data of sensory experience
directly or indirectly, and in many, if not all, cases are inti-
mately bound up with some form of peripheral expression.

The neurologist's problem is to disentangle the inconceivably
complex interrelations of the nerve-fibers which serve all the
manifold functions of adjustment of internal and external rela-
tions; to trace each functional system of fibers from its appro-
priate receptive apparatus (sense organ) to the centers of corre-
lation; to analyze the innumerable nervous pathways by which
these centers are connected with each other (correlation tracts) ;
and, finally, to trace the courses taken by all outgoing impulses
from these correlation centers to the peripheral organs of re-
sponse (muscles, glands, etc., or, collectively, the effectors).

This is no simple task. If it were possible to find an educated
man who knew nothing of electricity and had never heard of a
telegraph or telephone, and if this man were assigned the duty
of making an investigation of the telegraph and telephone sys-
tems of a great city without any outside assistance whatever,
and of preparing a report upon all the physical equipment with
detailed maps of all stations and circuits and with an explana-
tion of the method of operation of every part, his task would be
simple compared with the problem of the neurologists. The
human cerebral cortex alone contains some 9280 million nerve-
cells, most of which are provided with long nerve-fibers which
stretch away for great distances and branch in different direc-
tions, thus connecting each cell with many different nerve-
centers. The total number of possible nervous pathways is,
therefore, inconceivably great.

Fortunately for the neurologists, these interconnecting ner-
vous pathways do not run at random; but, just as the wires

THE NERVOUS FUNCTIONS 27

entering a telephone exchange are gathered together in great
cables and distributed to the switchboards in accordance with a
carefully elaborated system, so in the body nerve-fibers of like
function tend to run together in separate nerves or within the
brain in separate bundles called tracts. Notwithstanding the
complexity of organization of the nervous organs, the larger and
more important functional systems of nervous pathways have
been successfully analyzed, and the courses of nervous discharge
from the various receptors to the appropriate centers of adjust-
ment, and from these (after manifold correlations with other sys-
tems) to the organs of response, are fairly well known. The
acquisition of this knowledge has required several centuries of
painstaking anatomical and physiological study, and much
remains yet to be done.

The external forms of the brain and other parts of the nervous
system are dependent mainly upon the arrangements of the
nerve-cells of which they are composed (for the characteristics of
these cells see Chapter III), and these arrangements, in turn,
are correlated with the functions to be performed. The func-
tional connections of the nerve-cells can be investigated best by
the microscopical study of the tissues combined with physiolog-
ical experimentation. From this it follows that the study of the
gross anatomy, the microscopical anatomy (histology), and the
physiology of the nervous system should go hand in hand so far
as this is practicable.

A study of the comparative anatomy of the nervous system
shows that its form is always correlated with the behavior of the
animal possessing it. The simplest form of nervous system con-
sists of a diffuse network of nerve-cells and connecting fibers
distributed among the other tissues of the body. Such a ner-
vous system is found in some jelly-fishes and in parts of the
sympathetic nervous system of higher animals. Animals which
possess this diffuse type of nervous system can perform only
very simple acts, chiefly total movements of the whole body
or general movements of large parts of it, with relatively small
capacity for refined activities requiring the cooperation of
many different organs. But even the lowest animals which
possess nerves show a tendency for the nervous net to be con-
densed in some regions for the general control of the activities

28

INTRODUCTION TO NEUROLOGY

of the different parts of the body. Thus arose the central ner-
vous system. (Some works dealing with the evolution of the
nervous system are cited at the end of this chapter.)

The aggregations of nervous tissue to which reference has just
been made, containing the bodies of the nerve-cells, are called
ganglia,1 and in all invertebrate animals the central nervous
system is a series of such ganglia, variously arranged in the body
and connected by strands containing nerve-fibers only, that is,
by nerves.

Superior ganglia
Pharynx
Inferior ganglia

Ventral ganglia

Fig. 2. — The anterior end of an earthworm (Lumbricus) laid open from
above with all of the organs dissected away except the ventral body wall
and ventral ganglionic chain.

The central nervous systems of all but the lowest forms of
animals are developed in accordance with two chief structural
patterns, represented in typical form by the worms and insects
on the one hand, and by the back-boned animals or vertebrates
on the other hand.

In the segmented worms (such as the common earthworm,
Fig. 2) the central nervous system consists of a chain of ganglia
connected by a longitudinal cord along the lower or ventral wall

1 On the ganglia of the vertebrate nervous system, see page 108.

THE NERVOUS FUNCTIONS 29

of the body. Each of these ganglia is connected by means of
peripheral nerves with the skin and muscles of its own segment,
and each joint of the body with its contained ganglion (ventral
ganglion) has a certain measure of physiological independence so
that it can act as a unit. This is a typical segmented nervous
system. At the head end of the body the ventral ganglionic
chain divides around the pharynx and mouth, and there are
enlarged ganglia above and below the pharynx. The superior
ganglia (supra-esophageal ganglia) are sometimes called the
brain, and this organ dominates the local activities of the several
segments, enabling the animal to react as a whole to external
influences.

The nervous systems of crustaceans (crabs and their allies),
spiders, and insects have been derived from the type just
described. In these animals the segments of the body are more
or less united in three groups, constituting respectively the head,
thorax, and abdomen, and the ganglia of the central nervous
system are modified in a characteristic way in each of these
regions. Figure 3 illustrates the nervous systems of four species
of flies, showing different degrees of concentration of the ganglia.
In all cases the head part (brain) is greatly enlarged, and is
arranged, as in worms, in ganglia above and below the mouth
and esophagus. The other ganglia are diversely arranged, from
the simple condition (A} where there are three thoracic ganglia,
one for each pair of legs, and six abdominal ganglia, through in-
termediate stages (B and (7), to the highest form (D), where all
of the ganglia of both thorax and abdomen are united in a single
thoracic mass.

The type of nervous system just described is found throughout
the highest groups of invertebrate animals, as in insects and
spiders, and is constructed on a totally different plan from that
of all of the vertebrate or back-boned animals. In this latter
group we have, instead of a segmented chain of ventrally placed
solid ganglia, a hollow tube of nervous tissue which extends
along the back or dorsal wall of the body and constitutes the
spinal cord and brain. The cavity or lumen of this tube extends
throughout the entire length of the central nervous system,
forming the ventricles of the brain and the central canal of the
spinal cord. The details of the invertebrate nervous systems

30

INTRODUCTION TO NEUROLOGY

(whose structures are very diverse) will not be further consid-
ered in this work; the nervous systems of all vertebrates, how-
ever, are constructed on a common plan, and, though our prime
interest is the analysis of the human nervous system, we shall
find that many of the details sought can be seen much more
clearly in the lower vertebrates than in man.

Fig. 3. — The nervous systems of four species of flies, to illustrate the
various degrees of concentration of the ganglia: A, Chrionomus plumosus,
with three thoracic and six abdominal ganglia; B, Empis stercorea, with
two thoracic and five abdominal ganglia; C, Tabanus bovinus, with one
thoracic ganglion and the abdominal ganglia moved toward each other; D,
Sarcophaga carnaria, with all thoracic and abdominal ganglia united into
a single mass. (After Brand, from Lang's Text-book of Comparative
Anatomy.)

Correlated with these differences between the structure of
invertebrate and vertebrate nervous systems there are equally
fundamental differences in the behavior of these animals which
require a few words of further explanation. Living substance
exhibits as its most fundamental characteristic, as we saw at the
beginning, the capacity of adjusting its own activities to con-
stantly changing environmental conditions in such a way as to
promote its own welfare. This adjustment may be effected

THE NERVOUS FUNCTIONS 31

in two ways, both of which are universally present and which
throughout the remainder of this work we shall call the invariable
or innate behavior and the variable or individually modifiable
behavior.

Every animal reaction, then, contains these two factors, the
invariable and the variable or individually modifiable. The
first factor is a function of the relatively stable organization of
the particular living substance involved. The pattern of this
organization is inherited, and these characteristics of the be-
havior are, therefore, common, except for relatively slight devia-
tions, to all members of the race or species; they are rigidly
determined by innate bodily organization so arranged as to
facilitate the appropriate reactions, in an invariable mechanical
fashion, to every kind of stimulation to which the organism
is capable of responding at all. In the strictly vegetative func-
tions, in all true reflexes (as these are defined on page 56), and in
purely instinctive activities in general this factor of behavior is
dominant.

But in addition to this invariable innate behavior, all organ-
isms have some power to modify their characteristic action sys-
tems in adaptation to changed environmental relations. This
individual modifiability is known as biological regulation, a proc-
ess which has of late been very carefully studied. We cannot
here enter into the problems connected with form regulation,
that is, the power of an organism to restore its normal form after
mutilation or other injury. On regulation in behavior reference
should be made to the works of Jennings and Child. In lower
organisms Jennings recognizes three factors in the regulation of
behavior: First, the occurrence of definite internal processes;
these form part of the invariable hereditary action system re-
ferred to above. Second, interference with these processes
causes a change of behavior and varied movements, subjecting
the organism to many different conditions. Third, one of these
conditions may relieve the interference with the internal proc-
esses, so that the changes in behavior cease and the relieving
condition is thus retained. Lack of oxygen, for instance, would
interfere with an animal's internal processes; this leads it to move
about; if finally it enters a region plentifully supplied with oxy-
gen, the internal processes return to normal, the movement

32 INTRODUCTION TO NEUROLOGY

ceases, and the animal again settles down to rest. If this regu-
latory process is oft repeated another factor enters, viz., the
facilitation of a given adjustment by repetition. Thus arise
physiological habits or acquired automatisms.

The more highly complex forms of individual modifiability
are termed associative memory and intelligence, and the latter
of these is by definition consciously performed. Whether con-
sciousness is present in the simpler forms of "associative memory"
as these are demonstrated by students of animal behavior in
lower animals cannot be positively determined. In the behavior
of lower animals there are no criteria which enable us to tell
whether a given act is consciously performed or not, and, there-
fore, the lower limits of intelligence in the animal kingdom are
problematical. In other words, the manifestations of variable
behavior form a graded series from the simple regulatory phe-
nomena of unicellular organisms, as illustrated above, to the
highest human intelligence, so far as these express themselves
objectively.

In mankind, where intelligent behavior is dominant, the
stereotyping of the adjustments by repetition (true habit forma-
tion) may also take place, and in this case the acquired au-
tomatisms are sometimes said to arise by "lapsed intelligence,"
that is, an act which has been consciously learned may ulti-
mately come to be performed mechanically and nearly or quite
unconsciously. Much of the process of elementary education
is concerned with the establishment of such habitual reactions to
frequently recurring situations. How far "lapsed intelligence*'
is represented in the so-called instincts of other animals is still
a debated question (see p. 301).

Among the invertebrate animals, the insects and their allies
possess a bodily organization which favors the performance of
relatively few movements in a very perfect fashion, that is, the
action system is simple but highly perfected within its own range.
Their reflexes and instincts are very perfectly performed, but
the number of such reactions which the animal can make is rather
sharply limited and fixed by the inherited bodily structure.
Their behavior is dominated by the invariable and innate fac-
tors and they cannot readily adapt themselves to unusual condi-
tions. The vertebrates likewise have many elements of their

THE NERVOUS FUNCTIONS 33

behavior which are similarly fixed or stereotyped in their innate
organization; but, in addition to these stable reflexes and in-
stincts, the higher members of this group have also a consider-
able capacity for individual modifiability in behavior, and they
are characterized by greater individual plasticity and docility
(Yerkes). It appears that the tubular type of nervous system
found in vertebrates permits of the development of certain
kinds of correlation mechanisms which are impossible in the more
compact form of ganglia of the insects. These two branches of
the animal kingdom have, therefore, during all of the more
recent evolutionary epochs diverged farther from each other,
and now, in their highly differentiated conditions, neither type
could be derived from the other. The jointed animals (articu-
lates) developed from the lower worms, and this branch of the
animal kingdom, which may be called the articulate phylum,
culminates in the insects. The vertebrates were probably
developed from similar lowly worm-like forms along an inde-
pendent line of evolution, and this branch of the animal king-
dom, the vertebrate phylum, culminates in the human race.
Figure 4 illustrates in a rough diagrammatic way the relative de-
velopment of the variable and invariable factors of behavior in
the articulate and vertebrate phyla.

In unicellular organisms without nervous systems the general
protoplasm, of course, is the apparatus of both the invariable
and the variable factors of behavior, and the simpler forms of
nervous system likewise possess both of these capacities. But
in the more complex forms of nervous system among vertebrates
special correlation centers are set apart for the variable activi-
ties, particularly those which are intelligently performed, and
the most important of these centers are found in the cerebral
cortex. This is the part of the brain which is greatly enlarged in
mankind, as contrasted with all other animals, and the last three
chapters of this work are devoted to the structure and functions
of these cortical mechanisms with whose activity the progress of
human culture is so- intimately related.

It should be borne in mind that the higher correlation centers
which serve the individually variable or labile behavior in higher
vertebrates can act only through the agency of the lower reflex
centers. The point is, that all of the elements of behavior are

3

34

INTRODUCTION TO NEUROLOGY

represented in the innate neuro-muscular organization. Every
single act which the animal is capable of performing has its
mechanism provided in the inherited structure. But higher
animals may learn by experience to combine these simple ele-
ments in new patterns. The higher correlation centers serve this
function. The presence and general arrangement of these
centers is, of course, also determined in heredity; but the partic-

reptiies

Vertebrate Phylum

Fig. 4. — Two diagrams illustrating the relative development of the
invariable and variable factors in the behavior of the articulate phylum
and the vertebrate phylum of the animal kingdom. In the articulate
phylum the invariable factor (represented by the shaded area) predominates
throughout; in the vertebrate phylum the invariable factor predominates
in the lower members of the series, and the variable factor (represented by
the unshaded area) increases more rapidly in the higher members, attaining
its maximum in man, where intelligence assumes the dominant role.

ular associations which will be effected within them are deter-
mined by individual experience, and the building up of these
new associations is the chief business of education (see p. 312).
In the analysis of behavior and the related neurological mechan-
isms the distinction between the innate and the individually
acquired factors must always be kept clearly in mind. The
failure to do so, and also the failure to distinguish between these

THE NERVOUS FUNCTIONS 35

two factors and the acquired automatisms (p. 32), is responsible
for much confusion in the current discussions of instinct.

In the nomenclature of the correlation centers there is considerable
diversity of usage. In describing the adjustments made by these centers
neurologists frequently use the words coordination, correlation, and associ-
ation in about the same sense; but the adjustments made in those centers
which lie closer to the receptors or sense organs are physiologically of dif-
ferent type from those made in the centers related more closely to the
effector apparatus. In recognition of this fact the following usage has been
suggested to me by Dr. F. L. Landacre and will be adopted in this work:

The term correlation is applied to those combinations of the afferent
impulses within the sensory centers which provide for the integration of
these impulses into appropriate or adaptive responses; in other words, the
correlation centers determine what the reaction to a given combination of
stimuli will be. Nervous impulses from different receptors act upon the
correlation centers, and the reaction which follows will be the resultant of
the interaction of all of the afferent impulses (and physiological traces or
vestiges of previous similar responses) involved in the process. When this
resultant nervous discharge passes over into the motor centers and path-
ways, the final common paths (see p. 62) innervated will lead to a response
whose character is determined by the organization of the particular motor
centers and paths actuated.

To the term coordination we shall give a restricted significance, applying
it only to those processes employing anatomically fixed arrangements of the
motor apparatus which provide for the co-working of particular groups of
muscles (or other effectors) for the performance of definite adaptively useful
responses. Every reaction — even the simplest reflex — involves the com-
bined action of several different muscles, and these muscles are so inner-
vated as to facilitate their concerted action in this particular movement.
These are called synergic muscles. Coordination involves those adjust-
ments which are made on the effector side of the reflex arc (p. 56). This
is the sense in which the term is applied by Sherrington in the following
passage (Integrative Action of the Nervous System, p. 84) :

"Reflex coordination makes separate muscles whose contractions act
harmoniously, e. g., on a lever, contract together, although at separate
places, so that they assist toward the same end. In other words, it excites
synergic muscles. But it in many cases docs more than that. Where two
muscles would antagonize each other's action the reflex arc, instead of
activating merely one of the two, causes when it activates the one depression
of the activity (tonic or rhythmic contraction) of the other. The latter is
an inhibitory effect."

The motor paths and centers in general are more simply organized than
are the sensory paths and centers. The nervous discharges through these
motor systems are very direct and rapid. Complex nervous reactions
require more time than simple reflexes, and this delay or central pause is
chiefly in the correlation centers rather than in the efferent coordination
mechanisms (see pp. 98, 181).

The word association may be reserved for those higher correlations
where plasticity and modifiability are the dominant features of the response
and whose centers are separated from the peripheral sensory apparatus by
the lower correlation centers which are devoted to the stereotyped invari-
able reflex responses. Correlation may be mechanically determined by

36 INTRODUCTION TO NEUROLOGY

innate structure, or there may be some small measure of individual modifi-
ability, but when the modifiability comes to be the dominant characteristic,
so that the result of the stimulus cannot be readily predicted with mechan-
ical precision, the process may be called association. The intelligent types
of reaction and all higher rational processes belong here, and the cerebral
cortex is the chief apparatus employed.

The boundaries between the three types of centers just distinguished
are not always sharply drawn, especially in their simpler forms, though in
general they are easily distinguished. The mechanisms of coordination
are neurologically simpler than those of correlation and association, and in
general they are developed in the more ventral parts of the brain and
spinal cord, that is, below the limiting sulcus of the embryonic brain (p. 120).
The correlation and association centers are developed in the more dorsal
parts of the brain and cord, and the greater part of the thalamus and cere-
bral hemispheres is composed of tissues of this type. Nevertheless, the dis-
tinctions here drawn are fundamentally physiological rather than anatom-
ical, and coordination centers may be developed in the dorsal parts of the
brain, as in the case of the cerebellum and probably also the corpus striatum
of mammals (though not the striatum of lower vertebrates).

Summary. — The functions which characterize the nervous
system have been derived from those of ordinary protoplasm
by further development of three of the fundamental protoplas-
mic properties — viz., sensitivity, conductivity,, and correlation.
The most primitive form of nervous system known is diffuse and
local in its action, but in all the more highly developed forms the
chief nervous organs tend to be centralized for ease of general
correlation and control. Most of the types of nervous systems
found in the animal kingdom are represented in two distinct and
divergent lines of evolution, one adapted especially well for the
reflex and instinctive mode of life and found in the worms, in-
sects, and their allies, and the other found in the vertebrates and
culminating in the human brain with its remarkable capacity
for individually acquired and conscious functions.

LITERATURE

BARKER, L. F. 1901. The Nervous System and Its Constituent Neu-
rones, New York.

CHILD, C. M. 1911. The Regulatory Processes in Organisms, Journal of
Morphology, vol. xxii, pp. 171-222.

EDINGER, L. 1908. Th^ Relations of Comparative Anatomy to Compar-
ative Psychology, Jour. Comp. Neur., vol. xyiii, pp. 437-457.

HERRICK, C. JUDSON. 1910. The Evolution of Intelligence and Its
Organs, Science, N. S., vol. xxxi, pp. 7-18.

— . 1910. The Relations of the Central and Peripheral Nervous Systems
in Phylogeny, Anat. Record, vol. iv, pp. 59-69.

THE NERVOUS FUNCTIONS 37

JENNINGS, H. S. 1905. The Method of Regulation in Behavior and in
Other Fields, Jour. Exp. Zool., vol. ii, pp. 473-494.

— . 1906. Behavior of the Lower Organisms, New York.

LEWANDOWSKY, M. 1907. Die Funktionen des zentralen Nervensystems,
Jena.

LOEB, J. 1900. Comparative Physiology of the Brain and Comparative
Psychology, New York.

PARKER, G. H. 1909. The Origin of the Nervous System and Its Ap-
propriation of Effectors, Pop. Sci. Monthly, vol. Ixxv, pp. 56-64, 137-146,
253-263, 338-345.

— . 1914. The Origin and Evolution of the Nervous System, Pop. Sci.
Monthly, vol. Ixxxiv, pp. 118-127.

PARMELEE, M. 1913. The Science of Human Behavior, New York.

SHERRINGTON, C. S. 1906. The Integrative Action of the Nervous Sys-
tem, New York.

VERWORN, M. 1899. General Physiology, London.

WASHBURN, MARGARET F. 1908. The Animal Mind, New York.

WATSON, J. B. 1914. Behavior, An Introduction to Comparative Psy-
chology, New York.

YERKES, R. M. 1905. Concerning the Genetic Relations of Types of
Action, Jour. Comp. Neur., vol. xv, pp. 132-137.

CHAPTER III

THE NEURON

As we have seen in the last chapter, the functions of irrita-
bility, conduction, and correlation are the most distinctive fea-
tures of the nervous system. Like the rest of the body, the
nervous tissues are composed of cells, the irritability of whose
protoplasm is of diverse sorts in adaptation to different func-
tional requirements. Each sense organ, for instance, is irri-
table to its own adequate stimulus only (see pp. 25, 69). The
functions of correlation and integration of bodily actions cannot
be carried on by the nerve-cells as individuals, but they are
effected by various types of connections between the different
cells in the nerve-centers. The character of any particular cor-
relation, in other words, is a function of the pattern in accord-
ance with which the nerve-cells concerned are connected with
each other and with the end-organs of the reflex arcs involved.
The conducting function of nerve-cells is, perhaps, their most
striking peculiarity, and their very special forms are due largely
to the fact that their business is to connect remote parts of the
body so that these parts can cooperate in complicated move-
ments.

Not all of the cells which compose the central nervous system are nerve-
cells. The brain and spinal cord are surrounded by three connective-tissue
membranes (dura mater, arachnoid, and pia mater, in the aggregate
termed meninges) whose functions are chiefly protective and nutritive;
from the inner membrane, the pia mater, blood-vessels, and strands of
connective tissue extend into the true nervous substance. In addition to
these non-nervous elements which grow into the central nervous system
from without, the substance of the brain and spinal cord contains a sup-
porting framework composed of ependyma and neuroglia or glia cells which
develop from the primitive embryonic nervous system (the neural tube, see
pp. 106, 116), but are not known to perform nervous functions, though
nutritive and other functions have been ascribed to them (see p. 104).

The true nerve-cells are called neurom. There has been a
long controversy regarding the way in which the neurons of the

38

THE NEURON 39

adult body are developed from the cells of the embryonic nervous
system; but it is now generally accepted that each neuron is
developed from a single embryonic cell (known as a neuroblast),
and that in the adult body each neuron has a certain measure of
anatomical and physiological distinctness from all of the others.

The very young nerve-cell (neuroblast) is oval in form and is
composed of a nucleus and its surrounding protoplasm (cyto-
plasm) ; but in further development it rapidly elongates by the
outgrowth of one or more fibrous processes from the cell body,
so that the mature neuron may be regarded as a protoplasmic
fiber with a thickening somewhere in its course which is the cell
body of the original neuroblast and contains the cell nucleus
and a part only of its cytoplasm (this part being called the
perikaryon), the remainder of the cytoplasm composing the
fibrous processes, that is, the nerve-fibers. The cell body of
the mature neuron is sometimes loosely termed the nerve-cell,
though the latter term should strictly include the entire neuron.
The importance of the conducting function is reflected in the elon-
gated forms of the neurons and in the peculiar protoplasmic struc-
ture of the nerve-fibers. The function of the cell body is chiefly
nutritive; the entire neuron dies if the cell body is destroyed.

Each neuron may be regarded as essentially an elongated con-
ductor, and these units are arranged in chains in such a way that
a nervous impulse is passed from one to another in series. Since
the arrangement is such that the nervous impulse usually
passes through the series in only one direction (see the typical
reflex arc, Fig. 1, p. 25), each neuron has a receptive function
at one end and discharges its impulse at the other end. This is
what is meant by the polarity of the neuron (see pp. 52 and 97).

The simpler forms of neurons are bipolar, with one or more
processes known as dendrites conducting nervous impulses toward
the cell body, and (usually) only one process, the axon or neurite,
conducting away from the cell body. The dendrites are usually
short, and in this case their structure is similar to that of the cell
body. But where the dendrites are long, as in the neurons of the
spinal and cranial ganglia (Figs. 1, 10), they may have the
same structure as the axon. The axons are the axis-cylinders
of the longer nerve-fibers and ar.e structurally very different from
the protoplasm of the cell body, being composed chiefly of

40

INTRODUCTION TO NEUROLOGY

numerous very delicate longitudinally arranged neurofibrillae
embedded in a small amount of more fluid protoplasm.

Fig. 5. — Diagram of a motor neuron from the ventral column of gray
matter in the spinal cord. The cell body, dendrites, axon, collateral
branches, and terminal arborizations in muscle are all seen to be parts of a
single cell and together constitute the neuron: ah, Axon hillock free from
chromophilic bodies; ax, axon; c, cytoplasm of cell body containing chromo-
philic bodies, neurofibrils, and other constituents of protoplasm; d, den-
drites; m, myelin (medullary) sheath; m', striated muscle-fiber; n, nucleus;
n', nucleolus; nR, node of Ranvier where the axon divides; sf, collateral
branch; si, neurilemma (not apart of the neuron); tel, motor end-plate.
(After Barker, from Bailey's Histology.)

The forms of neurons are infinitely diverse and appear to have
been determined by two chief factors; these are (1) the nutrition

THE NEURON

41

of the cell and (2) the specific functions of conduction to be
served. The dendrites spread widely throughout the surround-
ing tissues, thus giving the cell a large surface for the rapid ab-
sorption of food materials from the surrounding lymph. This
was regarded as the only function of the dendrites by Golgi and
some of the other pioneers in the study of neurons, and led them
to apply the name "protoplasmic processes" to these structures.
We have already seen that the dendrites are more than this,

\\

Fig. 6. — Enlarged view of a cell body similar to that of Fig. 5, from the
spinal cord of an ox, showing the large chromophilic bodies: a, Pigment;
b, axon; c, axon hillock; d, dendrites. (After von Lenhossdk.)

however, being the usual avenues by which nervous impulses
enter the cell body. The size, length, and mode of branching
of the dendrites are, therefore, chiefly determined by their rela-
tions to other neurons from which they receive their nervous
impulses. The axon probably plays but little part in the gen-
eral nutrition of the cell, and its form is shaped almost entirely
by the distance to be traversed in order to reach the center or
centers into which it discharges.

42

INTRODUCTION TO NEUROLOGY

Neurons can function only when connected together in chains,
so that the nervous impulse can be passed from one to the other.
In any such chain the neuron first to be excited is called the
neuron of the first order, and the succeeding members of the
series neurons of the second, third, fourth order, and so forth.
All reflexes require an afferent neuron which conducts the ner-
vous impulse from the receptor to the center, one or more effer-
ent neurons conducting from the center to the organ of response,

W

Fig. 7. — The body of a pyramidal neuron from the cerebral cortex,
stained by Nissl's method, illustrating the arrangement of the chromophilic
substance and the form of the nucleus: a, Axon; 6, chromophilic bodies
surrounding the nucleus; c, a mass of chromophilic substance in the angle
formed by the branching of a dendrite; d, nucleus of a neuroglia cell (not a
part of the neuron). (After Ram6n y Cajal.)

and usually one or more neurons intercalated between these
within the center itself (see pp. 25, 56, 109). Figure 1, p. 25,
illustrates the simplest possible connection of neurons in a reflex
arc of the spinal cord, involving only two elements. The
afferent neuron sends its dendrite to the skin and its axon into
the spinal cord, where the nervous impulse is taken up by the
dendrites of the efferent neuron, which in turn transmits it to a
muscle. Figures 5 to 9 illustrate the forms of other neurons.

THE NEURON 43

The different dendrites of a neuron may be physiologically
all alike, or they may spread out in different directions to receive
nervous impulses of diverse sorts from different sources. Simi-
larly the axon may discharge its nervous impulse into a single
nerve center or peripheral end-organ, or it may branch, thus
connecting with and stimulating to activity two or more diverse
functional mechanisms. In other words, a given neuron may be
a link in a chain of some simple nervous circuit (Fig. 1), or it
may be adapted to collect nervous impulses from different
sources and discharge them into a single final common path, or
in the third place it may receive nervous impulses of one or more
functional sorts and then discharge its own nervous energy into
several remote parts of the nervous system. This, in brief, is the
mechanism of correlation, and illustrations of these different
types of connection will be found in the following chapters. If
animal reactions were simple responses so arranged that a given
stimulus could produce only one kind of movement, the only
nervous mechanism required would be a single neuron transmit-
ting the excitation from the point of stimulation to the organ of
response, as a call bell may be rung by pulling a bell cord. But
the actual reactions are always more complex than this, so that
several neurons must be connected in series with various di-
vergent pathways of nervous discharge which reach different
correlation centers, all of which must cooperate in the final
response. Illustrations of some of these complicated reflex
mechanisms will be found in Chapter IV.

Neurons with short dendrites and a single long axon are the
most common form and were termed Type I by Golgi (Fig. 8).
In some cases (Fig. 9) the axon also is very short, breaking up
in the immediate neighborhood of the cell body; these are the
Type II neurons of Golgi and appear to be adapted for the diffu-
sion and summation of stimuli within a nerve center. The
neurons of the spinal and cranial ganglia form a third type.
In embryonic development they begin as bipolar cells with a
dendritic process at one end and an axonal process at the
opposite end of the cell body; but in the course of further devel-
opment (Fig. 10) the two processes approach each other and
finally unite for a short distance into a single stem, which then
separates into an axon and a highly special form of dendrite

44

INTRODUCTION TO NEUROLOGY

Fig. 9. — Neuron of Type II from
the cerebral cortex of a cat. The
entire neuron is included in the
drawing: a, Axon which branches
freely and terminates close to the
cell body; d, dendrites. (After
Kolliker.)

Fig. 8. — Pyramidal neuron (Type
I of Golgi) from the cerebral cortex
of a rabbit. The axon gives off
numerous collateral branches close
to the cell body and then enters the
white substance, within which it ex-
tends for a long distance. Only a
small part of the axon is included in
the drawing: a, Axon; b, white sub-
stance; c, collateral branches of
axon; d, chief dendrite; p, its ter-
minal branches at the outer surface
of brain. (After Ram6n y Cajal.)

which has the same microscopic structure as the axon, but con-
ducts in the opposite direction with reference to the cell body.
This produces a T-form unipolar cell. The axon usually arises

THE NEURON

45

from the cell body; it may arise from the base of one of the den-
drites or, rarely, from the apex of the chief dendrite (Fig. 11).

Neurons differ in internal structure, as well as in form, from
the other cells of the body. The most important of these pecu-

Fig. 10. — A collection of cells from the ganglion of the trigeminus of the
embryonic guinea-pig, to illustrate various stages in the transformation of
bipolar neuroblasts into unipolar ganglion cells. (After Van Gehuchten.)

liarities are, first, the fibrillar structure of their cytoplasm,
and, second, the presence in the cytoplasm of a highly complex
protein substance chemically allied to the chromatin, which is

Fig. 11. — A neuron from the primary gustatory center in the medulla
oblongata of the carp. (Figure 136 (2), p. 303, illustrates the enormous
enlargement of the medulla oblongata of this fish which is produced by this
gustatory center.) The peripheral gustatory nerves end among the
dendrites, d. The axis of the main dendrite is directly prolonged to form
the axon, a. The heavy line at the right marks the external surface of the
brain. (From the Journal of Comparative Neurology, vol. xv, p. 395.)

the best known and probably the most important constituent of
the cell nucleus. This is the chromophilic substance, which in
nerve-cells as seen under the microscope is ordinarily arranged
in more or less definite flake-like masses scattered throughout

46 INTRODUCTION TO NEUROLOGY

the cytoplasm of the cell and extending out into the larger den-
drites (see Figs. 6, 7). These masses were first carefully investi-
gated by Nissl, who devised a special staining method for that
purpose; they are, accordingly, often called the Nissl bodies, and
sometimes tigroid bodies. They never occur in the axon nor
in a special conical protuberance of the cell body (the axon
hillock) from which the axon arises (see Fig. 5, ah, and Fig. 6, c).

The neurofibrils are very delicate strands of denser protoplasm
found in all parts of the neuron except the nucleus. They are by
many regarded as the specific conducting elements of the neuron,
though the evidence for this is not conclusive. They ramify
throughout the cytoplasm (Fig. 12), passing through the cell
body from one process to another.

The longer nerve-fibers are usually enveloped by a thick white
glistening sheath of myelin, a fat-like substance secreted by the
nerve-fibers themselves. This myelin sheath, or medullary
sheath, is a part of the neuron with which it is related and the
fibers which possess it are called myelinated or medullated fibers;
these fibers compose the white matter of the brain and a large
part of the peripheral nerves (see Fig. 5). There may be, in ad-
dition, in the case of the peripheral nerves an outer sheath, the
neurilemma (primitive sheath or sheath of Schwann). This is a
thinner nucleated membrane, not a part of the neuron to which
it is attached, but formed from surrounding cells.

The function of the myelin sheath has often been regarded as
simply that of an insulating substance to prevent the overflow
and loss of the nervous impulse conducted by the axon, but
there is some evidence that this sheath plays an important part
in the chemical processes involved in the act of nervous conduc-
tion. The neurilemma is likewise often spoken of as a protecting
membrane. Whether it has any other function in the normal
life of the nerve-fiber is unknown ; but if a nerve-fiber is by acci-
dent severed from its cell body, it is known that the nuclei of
the neurilemma play a very important part in the degeneration
and regeneration of the severed fiber and the restoration of its
normal function.

As has been suggested, nerve-fibers cut off from their cell
bodies immediately die and degenerate. But in the case of
peripheral nerves the neurilemma nuclei do not die; and, appa-

THE NEURON

rently with the aid of these nuclei, a new nerve-fiber may under
favorable conditions grow out from the central stump of the cut
nerve, and finally the entire nerve may regenerate. In the cen-

lii

Fig. 12. — Cell from the ventral gray column of the human spinal cord,
illustrating the arrangement of the neurofibrils: ax, Axon; Hi, interfibril-
lar spaces occupied by chromophilic substance; n, nucleus; x, neurofibrils
passing from one dendrite to another; y, similar neurofibrils passing through
the body of the cell. (After Bethe, from Heidenhain's Plasma und Zelle).

tral nervous system, where the neurilemma is absent or greatly
reduced, the regeneration of such injured nerves takes place
with great difficulty, if at all.

48

INTRODUCTION TO NEUROLOGY

It is possible by a special method of staining devised by Marchi
to differentiate myelinated fibers which are in process of degen-
eration from the normal fibers with which they may be mingled.
This method has often permitted a much more precise deter-
mination of the exact course of the fibers of a given peripheral

Fig. 13. — Two motor neurons from the ventral column of gray matter of
the spinal cord of a rabbit, taken fifteen days after cutting the sciatic nerve,
to illustrate the chromatolysis of the chromophilic substance: A, Cell in
which the chromophilic bodies are partially disintegrated (at 6) and the
nucleus eccentric; B, cell showing more advanced chromatolysis (c), the
chromophilic substance being present only in the dendrites and around the
nucleus in the form of a homogeneous mass; a, axon. Compare with these
appearances the normal cell of the ventral column shown in Fig. 6. (After
Ram6n y Cajal.)

nerve or central tract than would be possible by the examination
of normal material, especially after experimental operations on
the lower animals, where the particular collection of fibers under
investigation may be severed and then later the animal killed
and examined by Marchi's method (see p. 135).

It is also found that after cutting any group of nerve-fibers the

THE NEURON 49

cell bodies from which these fibers arise show structural changes.
The most important change is a solution of the chromophilic
substance or Nissl bodies so that they no longer appear in a
stained preparation (Fig. 13). This is termed chromatolysis,
and often enables the neurologist to determine exactly which
cells in the central nervous system give rise to a particular
bundle of fibers (for examples see pp. 136 and 284).

The neuron doctrine may be said to date from the publica-
tion of important papers by Golgi, of Pavia, in 1882 to 1885
(though his now famous method was published in 1873, and
many of Golgi's theoretical conclusions have been greatly
modified). The name Neuron (in English often spelled
"neurone") was first applied by Waldeyer in 1891 in connection
with a clear enunciation of the recently demonstrated facts upon
which the concept is based. The discovery of William His
that the nervous system is made up of cellular units which are
embryologically distinct, and the further demonstration by
others that these cellular elements retain some measure of ana-
tomical and physiological individuality (the exact degree of
anatomical separation is still in controversy — some say it is com-
plete) up to adult life revolutionized neurology, and this doctrine
has profoundly influenced all subsequent neurological work. The
history of this movement we cannot here go into (see the excel-
lent summaries in Barker's Nervous System and the article by
Adolf Meyer cited at the end of this chapter). The present
status of the neuron doctrine has been summarized by Heiden-
hain (1911, p. 711) in the following six propositions:

1. The neuron of the adult animal body is an anatomical unit;
it corresponds morphologically to one cell.

2. The neuron is, accordingly, also a genetic unit, for it is
differentiated from a single embryonic cell.

3. Nervous substance is composed of the contained neurons;
within the nervous system there are no elements other than
neurons which participate in nervous functions.

4. The neurons remain anatomically separate; they are merely
in contact with each other, that is, there are no connections
between them which are characterized as conditions of conti-
nuity or fusion of their substance.

5. The neuron is a trophic unit. This means that the injury

4

50

INTRODUCTION TO NEUROLOGY

of any part of the neuron affects the welfare of the whole, and
the destruction of the nucleus and cell body destroys the entire
neuron, but such injuries do not directly affect adjacent
neurons.

6. The neuron is a functional unit or, better, the functronal
unit of the nervous system.

Fig. 14. — Neurons from the trapezoid body of the medulla oblongata of a
cat, illustrating different forms of synapse: a, Delicate pericellular net
around the cell body of a neuron which is not shown; b, coarser endings; c,
still coarser net; d, calyx-like envelope. In b, c, and d, at the left of the
figure, the globular cell body of the neuron of the second order is shaded
with lighter stipple than the terminals of the axon of the neuron of the first
order. (After Veratti, from Edinger's Vorlesungen.) (It should be noted
that in this account we do not follow Veratti 's interpretation of these
structures, but that of Held, Ram6n y Cajal, and the majority of other
neurologists.)

These six propositions are accepted in their entirety by many
neurologists; but it should be clearly understood that all of
them are controverted by others. The fourth proposition, in
particular, has been the subject of violent attack (see the dis-
cussion of the synapse below). The neuron, moreover, is a
functional unit (proposition 6) in only a rather limited sense
(see p. 56). Without further discussion of the merits of these

THE NEURON

51

controversial questions, it may be regarded as generally accepted
that all of the preceding propositions have some measure of
factual basis, though different neurologists would give various
interpretations and modifications of some of them.

The place where the axon of one neuron comes into physio-
logical relation with another neuron is known as the synapse.

Fig. 15. — Synapse between an ascending fiber entering the cortex
of the cerebellum and the dendrites of a Pur kin je cell. (After Ram6n y
Cajal.)

Its precise nature is still obscure. Structurally it usually ex-
hibits a dense interlacing of the terminal arborization of an axon
of one neuron with the bushy dendrite of a second neuron.
In Fig. 1 (p. 25) such a synapse is seen between the dorsal root
neuron and the ventral root neuron. In other cases the ter-
minal arborization takes the form of a delicate network which

52 INTRODUCTION TO NEUROLOGY .

twines around the cell body of the second neuron or of a calyx-
like expansion or coarse-meshed reticulum closely enveloping
the cell body (Fig. 14). Another form of synapse is seen in Fig.
15 from the cortex of the cerebellum. The body and larger
dendrites of a single cortical neuron of the type known as
Purkinje cells (see p. 191) are shown in gray, and the terminal
branches of an afferent neuron are seen twining about the den-
dritic branches of the Purkinje cell, thus forming a very intimate
union. Similar synapses are found in the cerebral cortex (p.
272). Figure 16 illustrates a type of synapse also found in the

Fig. 16. — A "basket cell" from the cerebellar cortex of a rat, illustrating
the discharge of a single neuron, B, by synaptic connection with the cell
bodies of several Purkinje neurons, A, by basket-like endings of the axon:
A, cells of Purkinje; a, the basket-like synapse on the body of a Purkinje
cell; B, the basket cell; b, terminus of the axon; c, axon of basket cell.
(After Ram6n y Cajal; cf. Fig. 89, p. 190.)

cerebellar cortex. A single "basket cell," B, has a short axon
whose branches form synapses around the bodies of a large
number of Purkinje cells, thus diffusing and greatly strength-
ening the nervous discharge (see p. 192 and Fig. 89, 6). For
still other types of synapse see Figs. 61, 89, 98, 104, 109, 126.
The synapse has been a crucial point in recent discussions
regarding the general physiology of the nervous system, many
neurologists believing that it is the most important part of the
reflex circuits (see, for instance, on the theory of sleep, p. 103).
The doctrine of the polarization of the neuron (p. 39) implies

THE NEURON

53

that at the synapse there must be a reversal of the polarity with
reference to the cell body as the nervous impulse passes over
from an axon to a dendrite.

In the simple diffuse form of nervous system found in primi-
tive animals like the jelly-fishes and lowest worms (p. 27) the
nerve-cells are described as connected by protoplasmic strands
to form a continuous network. Here, of course, there are no

Fig. 17. — Plexus of sympathetic neurons in the villi of the small intes-
tine of a guinea-pig: a, b, c, d, Neurons of the subepithelial plexus; e, f,
neurons of the plexus within the villi; g, fibers of the submucous (Meissner's)
plexus. (After Ram6n y Cajal.)

synapses and the neurons are not polarized. Apparently the
nervous impulse may be transmitted equally well in all direc-
tions throughout this network. The physiological properties
of such an arrangement appear to be very different from those
of the synaptic nervous systems of higher animals. A non-
synaptic network similar to that mentioned above has. been des-
cribed as occurring in some of the diffuse ganglionic plexuses of
the human body (Fig. 1.7).

54 INTRODUCTION TO NEUROLOGY

In the synaptic systems, as found in all highly differentiated
nervous centers, the majority of neurologists teach that at the
synapse the two neurons involved are simply in contact and that
the nervous impulse passes from one to the other across a very
short gap in the conducting substance. Others believe that
they have demonstrated very delicate protoplasmic threads
which bridge this gap, thus establishing continuity of the con-
ducting substance across the synapse. Good histological prepara-
tions show, however, in some of the most intimate synapses
known where the axon ends directly on the cell body of the sec-
ond neuron that there is a distinct cellular membrane around
the terminals of the fibers of the first order and a second cellu-
lar membrane enveloping the body of the neuron of the second
order, so that continuity of the ordinary protoplasm of the
neurons here seems to be quite impossible, so far as our present
technic is adequate to decide the question.1

The following important points regarding the synapse seem
to be established:

1. Unimpeded protoplasmic continuity across the synapse has
not been clearly established, and in some cases there is clearly
a membranous barrier interposed between the two neurons.
But the exact nature of this barrier is unknown and it by no
means follows that the synaptic membrane is an inert substance.
It may be composed of living substance of a different nature
from that of the other protoplasm of the neurons.

2. The transmission of the nervous impulse across the synapse
involves a delay greater than that found in the nerve-fiber or
the cell body. This suggests that there is some sort of an ob-
struction here which does not occur elsewhere in the reflex arc
(see p. 98).

3. The synapse is more susceptible to certain toxic substances,
such as nicotin, than is any other part of the reflex arc.

4. Though a nerve-fiber seems to be capable of transmitting
an impulse in either direction, the nervous impulse can pass the
synapse only in one direction, viz., the direction of normal dis-
charge from the axon of one neuron to the dendrite of another.

1 For an illustration of such a synapse see BARTLEMEZ, G. W., Mauthner's
Cell and the Nucleus Motorius Tegmenti, Jour. Comp. Neur., vol. xxv,
1915, Figs. 11, 12, and 13, pp. 126-1 2S.

THE NEURON 55

The synapse, therefore, acts as a sort of valve, to use a crude
analogy, and appears to be one of the factors (not necessarily
the only one, see p. 97) in establishing the polarity of the neuron.

5. Observations upon injured -neurons show that the degenera-
tions caused by the severance of their fibrous processes (whether
these be manifested as degeneration of the fibers or as chroma-
tolysis, see p. 49) or by the destruction of the cell bodies from
which the fibers arise cannot cross the barriers interposed by the
synapses.

Summary. — In this chapter the form and internal structure of
neurons have been briefly reviewed and the present status of the
neuron doctrine is summarized on p. 49. The synapse is. the
place where the nervous impulse is transmitted from one neuron
to another, and it is regarded as of the utmost physiological
importance, its most important features being presented briefly
on p. 54. The doctrine of the polarization of the neuron teaches
that nervous impulses are received by the dendritic processes
and transmitted outward from the cell body through the axon.

LITERATURE

APATHY, S. 1898. Ueber Neurofibrillen, Proc. Internal.- Zoological
Congress, Cambridge, pp. 125-141.

BARKER, L. F. 1901. The Nervous System and Its Constituent Neurones,
New York.

BETHE, A. 1904. Der -heutige Stand der Neurontheorie, Deutsch.
med. Woch., No. 33.

. GOLGI, C. 1882-1885. Sulla fina anatomia degli organi centrali del
sistema nervoso, Riv. Sperim. di Freniatria, vols. viii, ix, and xi.

— . 1907. La dottrina del neurone, Teoria e fatti, Arch. Fisiol., vol. iv,
pp. 187-215.

HEIDENHAIN, M. 1911. Plasma und Zelle, 2 Lieferung (in Bardelcben's
Handbuch der Anatomic des Menschen, Bd. 8), Jena.

His, W. 1889. Die Neuroblasten und deren Entstehung im embry-
onalen Mark, Leipzig.

MEYER, ADOLF. 1898. Critical Review of the Data and General Meth-
ods and Deductions of Modern Neurology, Jour. Comp. Neur., vol. viii,
pp. 113-148 and 249-313.

NISSL, F. 1903. Die Neuronenlehre und ihre Anhanger, Jena.

RAM6N Y CAJAL, S. 1909. Histologie de Systeme Nerveux, Paris.

WALDEYER, W. 1891. Ueber einige neuere Forschungen im Gebiete
der Anatomic des Centralnervensystems, Deutsch. med. Woch., Bd. 17.

CHAPTER IV

THE REFLEX CIRCUITS

THE cellular unit of the nervous system, as we have seen, is
the neuron. Neurons, however, never function independently,
but only when joined together in chains whose connections are
correlated with the functions which they serve. Accordingly,
the most important unit of the nervous system, from the phys-
iological standpoint, is not the neuron, but ttae reflex circuit, a
chain of neurons consisting of a receptor or sensory organ, a cor-
relating center or adjuster, and an effector or organ of response,
together with afferent and efferent nerve-fibers which serve as
conductors between the center and the receptor and effector
respectively (see p. 25). In a reflex circuit the parts must be
so connected that upon stimulation of the receptive end-organ a
useful or adaptive response follows, such, for instance, as the
immediate jerking away of the hand upon accidentally touching
a hot stove.

A reflex act, as this term is usually defined by the physiologists,
is an invariable mechanically determined adaptive response to
the stimulation of a sense organ, involving the use of a center of
correlation and the conductors necessary to connect this center
with the appropriate receptor and effector apparatus. The act
is not voluntarily performed, though one may become aware of
the reaction during or after its performance.

The term "reflex" is often popularly very loosely applied, but
as generally used by physiologists it involves the rather complex
nervous function above described. If an electric shock is ap-
plied directly to a muscle or to the motor nerve which innervates
that muscle, the muscle will contract, but this direct contraction
is not a reflex act. Many acquired movements have become so
habitual as to be performed quite automatically, such as the
play of the fingers of an expert pianist or typist; but these

56

THE REFLEX CIRCUITS 57

acquired automatisms must be clearly distinguished from the
reflexes, which belong to the innate nervous organization with
which we are endowed at birth (see pp. 31, 301). The lowly
organisms which lack a differentiated nervous system exhibit
many kinds of behavior which closely resemble reflexes and, in
fact, are physiologically of the same type; but these non-nervous
responses are usually termed tropisms or taxes, though some
physiologists call them reflexes, and some reflexes, as above de-
fined, are often called tropisms.

The structure of the simple reflex circuit is diagrammatically
illustrated in Fig. 18, A. The receptor (R) may be a simple
terminal expansion of the sensory nerve-fiber or a very complex
sense organ. The effector (E) may be a muscle or a gland. The
cell body of the afferent neuron (1) may lie within the center (C)
or outside, as in the diagram. The latter condition is more usual,
as seen in the spinal and cranial ganglia (Fig. 1, p. 25). The
synapse and the cell body of the efferent neuron (2) lie in the
center.

A simple reflex act involving the use of so elementary a mech-
anism as has just been described is probably never performed
by any adult vertebrate. The nervous impulse somewhere in its
course always comes into relation with other reflex paths, and in
this way complications in the response are introduced. Some
illustrations of the simpler types of such complex reflex circuits
will next be considered.

Separate reflex circuits may be so compounded as to give the
so-called chain reflex (Fig. 18, B). Here the response of the
first reflex serves as the stimulus for the second, and so on in
series. The units of these chain reflexes are usually not simple
reflexes as diagrammed, but complex elements of the types next
to be described.

Figure 18, C illustrates another method of compounding re-
flexes so that the stimulation of a single sense organ may excite
either or both of two responses. If the two effectors, El and
E2, can cooperate in the performance of an adaptive response,
the case is similar to that of Fig. 18, A, with the possibility of a
more complex type of reaction. This is an allied reflex. If,
however, the two effectors produce antagonistic movements, so
that both cannot act at the same time, the result is a physiological

58

INTRODUCTION TO NEUROLOGY

dilemma. Either no reaction at all results, or there is a sort of
physiological resolution (sometimes called physiological choice),
one motor pathway being taken to the exclusion of the other.
Which path will be chosen in a given case may be determined by

Fig. 18. — Diagrams representing the relations of neurons in five types of
reflex arcs: A, Simple reflex arc; B, chain reflex; C, a complex system illus-
trating allied and antagonistic reflexes and physiological resolution; D, a
complex system illustrating allied and antagonistic reflexes with a final
common path; E, a complex system illustrating the mechanism of physio-
logical association. A, A, association neurons; C, C', C", Cl, and C2, centers
(adjusters); E, E', E", El, and E2, effectors; FCP, final common path;
R, R', R", Rl, and R2, receptors.

the physiological state of the organs. If, for instance, one motor
system, E2, is greatly fatigued and the other rested, the thresh-
old of E2 will be raised and the motor discharge will pass to El .
Figure 18, D illustrates the converse case, where two receptors

THE REFLEX CIRCUITS 59

discharge into a single center, which, in turn, by means of a final
common path (FCP) excites a single effector (E). If the two re-
ceptors upon stimulation normally call forth the same response,
they will reinforce each other if simultaneously stimulated,
the response will be strengthened, and we have another type of
allied reflex. But there are cases in which the stimulation of
Rl and R# (Fig. 18, D) would naturally call forth antagonistic
reflexes. Here, if they are simultaneously stimulated, a phys-
iological dilemma will again arise which can be resolved only by
one or the other afferent system getting control of the final com-
mon path.

Figure 18, E illustrates still another form of combination of
reflexes. Here there are connecting tracts (A, A) between the
two centers so arranged that stimulation of either of the two
receptors (Rl and R2] may call forth a response in either one of
two effectors (El and E2). These responses may be allied or
antagonistic, and much more complicated reflexes are here pos-
sible than in any of the preceding cases.

A few illustrations of the practical operation of these types
of reflex circuits will be given here and many other cases are
cited throughout the following discussions. A case of a simple
reflex has already been mentioned in the sudden twitch of the
hand in response to a painful stimulation of the skin. The
simplest possible mechanism of this reaction involving only two
neurons is shown in Fig. 1 (p. 25). In actual practice, however,
the arrangement figured is one element only of a more complex
reaction (see p. 61). Figure 19 illustrates a more usual form
of this type of reaction, where a series of three or more neurons
is involved and at least two cerebral centers. An auditory im-
pulse coming to the brain from the ear through the VIII cranial
nerve terminates in a primary acoustic center in the superior
olive (a deep nucleus of the medulla oblongata, see p. 201),
where it is taken up by an intercalary neuron of the second order
and transmitted to the nucleus of the VI nerve. The result is a
contraction of the external rectus muscle of the eyeball, turning
the eye toward the side from which the auditory stimulus was
received. So far as this reaction alone is concerned, it is a simple
reflex, but in practice the external rectus muscle of one eye is
never contracted apart from the other five muscles of that eye

60

INTRODUCTION TO NEUROLOGY

and all six muscles of the other eye. In this way alone can con-
jugate movements of the two eyes be effected for the accurate
fixation of the gaze upon any object. The entire system of con-
jugate .movements is also entirely reflex and it is effected by an
exceedingly complicated arrangement of nerve tracts and cen-
ters, of which the superior olive and the nucleus of the VI nerve
are integral parts.

The chain reflex (see Fig. 18, B) is a very common and a very
important type. Most of the ordinary acts in the routine of
daily life employ it in one form or another, the completion of one
stage of the process serving as the stimulus for the initiation
of the next.

nerve

Fig. 19. — Diagram of a simple auditory reflex. Upon stimulation of the
endings of the VIII nerve in the ear by sound waves, a nervous impulse may
pass to the superior olive, whence it is carried by an intercalary neuron of
the second order to the nucleus of the VI nerve. The fibers of this nerve
end on the external rectus muscle of the eyeball.

There are within the muscles elaborate sense organs (the mus-
cle spindles and their associated afferent nerves, see p. 87),
which are stimulated by the contraction of the muscle. These
afferent nerves of the muscle sense have their own centers of
adjustment within the central nervous system, from which in
turn efferent impulses go out which ultimately reach the same
muscles from which the sensory impulses came in. This, of
course, is a variety of chain reflex, and is the mechanism by
which refined movements of precision are executed, where differ-
ent sets of muscles must work against each other in constantly
varying relations without conscious control. In the case of a
sustained reflex series of this character this return flow of affer-

THE REFLEX CIRCUITS 61

ent impulses of the muscle sense, tendon sense, etc., exerts a
constant influence upon the center which receives the initial
stimulus, so that this center is constantly under the combined
influence of the external stimulus which sets the reflex in motion
and the internal stimuli arising from the muscles themselves
(proprioceptors, see p. 86) which control its course. In this
case there is a true physiological circuit rather than an arc or
segment of a circuit, as is commonly implied in the expression
"reflex arc." This case is typical of the complex reflexes of the
body in general, and for this and other considerations we follow
the usage of Dewey (1893) and term the mechanism of a com-
plete reflex a "reflex circuit" rather than an arc (see C. J.
Herrick, 1913, and p. 308).

It has been suggested by Loeb also that many instincts are
-^^xply complex chain reflexes. Even in animals whose behavior
^X^is so complex as .birds, a careful analysis of the cycle of nest
building and rearing of young reveals many clear illustrations of
this principle (see the works of F. H. Herrick, cited at the end of
this chapter). Each step in the cycle is a necessary antecedent
to the next, and if the series is interrupted it is often necessary
for the birds to go back to the beginning of the cycle. They
cannot make an intelligent adjustment midway of the series.

The complex circuit illustrated by Fig. 18, C presents two
possible types of reaction, either allied or antagonistic reflexes.
The former case is illustrated again by the sudden movement of
the hand in response to a painful stimulation of the skin. This
is brought about, as we saw in considering the simple reflex, by a
contraction of the arm muscles. But the muscles which move
the elbow-joint are not, when the arm is at rest, entirely flaccid.
Both flexors and extensors are always contracted to a certain
degree, one balanced against the other. Now at the same time
that the sensory stimulus from R (see Fig. 18, C) causes the con-
traction of the flexor muscle, El , it also causes the relaxation of
the antagonistic extensor, E2 the two efferent impulses coopera-
ting to effect the avoiding reaction as rapidly as possible. In
the antagonistic reflexes of our third type the physiological reso-
lution involved in the selection of one or the other possible
reaction always involves a delay in the response until one motor
pathway dominates the system to the exclusion of the other.

62

INTRODUCTION TO NEUROLOGY

In the fourth type of complex reflexes (see Fig. 18, D) two dif-
ferent sensory paths discharge into a single center, from which a
final common path goes out to the effector. This mechanism
also provides for both allied and antagonistic reflexes. A very
simple apparatus for this type of reflex is found in the roof of the
midbrain of the lowly amphibian, the common mud puppy,
Necturus. Here the upper part of the midbrain roof receives
optic fibers from the optic tracts, while the lower part receives
fibers from the primary acoustic and tactile centers (Fig. 20).

OPTIC

CENTER

/ ACOUSTiq
/and TACTILE
/ CENTER

m nerve

MOTOR
CENTER

Fig. 20. — Diagram of a cross-section through the midbrain of Necturus,
illustrating a single correlation neuron of the midbrain roof. One dendrite
spreads out in the optic center among terminals of the optic tracts; another
dendrite similarly spreads out in the acoustic and tactile center. The axon
descends to connect with the motor neurons of the III nerve.

A single neuron of the midbrain may send one dendrite down-
ward to receive acoustic or tactile stimuli (or both of these), and
another dendrite upward to receive optic stimuli. If the animal
receives visual and auditory stimuli simultaneously, the inter-
calary neuron of the midbrain may be excited by both sets of
stimuli. Its discharge through the axon to the motor organs of
response (say to the eye muscles by way of the III nerve, as in
Fig. 20) will be the physiological resultant of both sets of ex-
citations. If they reinforce each other, the discharge will be

THE REFLEX CIRCUITS

63

stronger and more rapid; if, on the other hand, they tend to pro-
duce antagonistic responses, there will be an inhibition of the
response or a delay until one or the other stimulus obtains the
mastery.

Yerkes has given a striking illustration of this method of re-
inforcement of stimuli in his experiments on the sense of hearing
in frogs. The reflex mechanism of touch, hearing, and vision in
the midbrain of the frog is similar to that of Necturus as des-
cribed above (Fig. 20). Yerkes found that frogs under labora-

midbrain

Fig. 21. — Diagram of some conduction paths in the brain of Necturus,
seen in longitudinal section. From the medulla oblongata an acoustic
impulse may be carried forward through the neuron A to the midbrain,
whose neurons, B, are of the type shown in Fig. 20, receiving both acoustic
and optic impulses. This neuron, B, may discharge downward through the
tract S to the motor nuclei of the III, V, VII, etc., nerves, or it may dis-
charge upward to a neuron of the thalamus, C, which also receives descend-
ing impulses from the cerebral hemisphere through the neuron, D, and, in
turn, discharges through the motor tract, S.

tory conditions do not ordinarily react at all to sounds alone,
but that they do react to tactual and visual -stimuli. When
these reactions are carefully measured, it is found that the sound
of an electric bell occurring simultaneously with a tactual or
visual stimulus markedly increases (reinforces) the strength of
the reaction.

The reflex centers of the midbrain are further complicated by
the fact that the efferent tract from the sensory centers above
the aqueduct of Sylvius is not simple as diagrammed in Fig.
20, but it divides into a descending and an ascending path, as

64 INTRODUCTION TO NEUROLOGY

shown by the neuron B of Fig. 21. The descending path
connects directly with motor centers, including the oculomotor,
bulbar, and spinal motor nuclei (Fig. 21, S), while the ascending
path enters the thalamus, where associations of a still higher
order are effected through the thalamic neuron, C. Here again is
introduced a physiological choice or dilemma; the response is not
a simple mechanical resultant of the interacting stimuli, but its
character may be influenced by variable physiological states.
The invariable type of action is replaced by a relatively variable
or labile type (see p. 31). In the thalamus the nervous impulse
is again subjected to modification under the influence of a still
greater variety of afferent impulses, for these centers receive all
sensory types found in the midbrain, and, in addition, important
descending tracts from the cerebral hemispheres (in lower ver-
tebrates the latter are chiefly olfactory).

The more complicated associations are effected by arrange-
ments of correlation tracts and centers illustrated in the simplest
possible form by Fig. 18, E. The mode of operation of such
a system may be illustrated by an example : A collie dog which I
once owned acquired the habit of rounding up my neighbor's
sheep at very unseasonable times. The sight of the flock in the
pasture (stimulus Rl, Fig. 18, E) led to the pleasurable reaction
(El) of chasing the sheep up to the barnyard. It became neces-
sary to break up the habit at once or lose a valuable dog at the
hands of an angry farmer with a shotgun. Accordingly, I
walked out to the pasture with the dog. She at once brought in
the sheep of her own accord and then ran up to me with every
expression of canine pride and self-satisfaction, whereupon I
immediately gave her a severe whipping (stimulus R2}. This
called forth the reaction (E2) of running home and hiding in her
kennel. The next day (the dog and I having meanwhile with
mutual forgiveness again arrived at friendly relations) we took a
walk in a different direction, in the course of which we unex-
pectedly met another flock of sheep. At sight of these the dog
immediately, with no word from me, put her tail between her
legs, ran home as fast as possible, and hid in her kennel. Here
the stimulus Rl led not to its own accustomed response, El,
but to E2, evidently under the influence of vestigeal traces of the
previous day's experience, wherein the activities of Cl and C2

THE REFLEX CIRCUITS 65

were related through the associational tract (A, A) passing be-
tween them.

In the case of the dog's experience just described the neural
mechanism was undoubtedly much more complex than our dia-
gram, though similar in principle, and the associative memory
process involved was probably vividly conscious (cf. p. 295).
But the simpler types of "associative memory" which have been
experimentally demonstrated in many of the lower organisms
may involve no more complex mechanism than this diagram, and
it is by no means certain that any conscious process is there
present.

It must be kept in mind that in higher vertebrates all parts of
the nervous system are bound together by connecting tracts
(internuncial pathways). Some of these tracts are long, well-
defined bundles of myelinated fibers whose connections are such
as to facilitate uniform and clear-cut responses to stimulation.
Others are very diffuse and poorly integrated. Permeating the
entire central nervous system is an entanglement of very deli-
cate short unmyelinated fibers. This nervous felt-work (neuro-
pil) is much more highly developed in some parts of the brain
than in others. It is not well adapted to conduct definite ner-
vous impulses for long distances, but it may serve to diffuse or
irradiate such impulses widely. Where tissue of this sort is
mingled with myelinated fibers it is termed the "reticular for-
mation" (see pp. 65, 127, 158, 304).

These manifold connections are so elaborate that every part
of the nervous system is in nervous connection with every other
part, directly or indirectly. This is illustrated by the way in
which the digestive functions (which normally are quite auton-
omous, the nervous control not going beyond the sympathetic
system, see p. 241) may be disturbed by mental processes whose
primary seat may be in the* association centers of the cerebra)
cortex; and also by the way in which strychnin-poisoning
seems to lower the neural resistance everywhere, so that a
very slight stimulus may serve to throw the whole body into
convulsions.

It follows that the localization of cerebral functions can be
only approximate. Every normal activity has what Sherring-
ton calls its reflex pattern, whose anatomical basis is a definite
5

66 INTRODUCTION TO NEUROLOGY

reflex path; but the stimulus is rarely simple and the nervous
discharge irradiates more or less widely, so that the activity is
by no means limited to the part which gives the act its reflex
pattern. Moreover, neither the stimulus complex nor the char-
acter of the irradiation will be repeated exactly in any higher
animal, so that the precise nature of the response cannot in any
case be infallibly predicted except under experimental conditions
(and not always then).

Our picture of the reflex act in a higher animal will, then,
include a view of the whole nervous system in a state of neural
tension. The stimulus disturbs the equilibrium at a definite
point (the receptor), and the wave of nervous discharge thus set
up irradiates through the complex lines determined by the neural
connections of the receptor. If the stimulus is weak and the
reflex path is simple and well insulated, a simple response may
follow immediately. Under other conditions the nervous dis-
charge may be inhibited before it reaches any effector, or it may
irradiate widely, producing a very complex reflex pattern. In
the former case the neural equilibrium will be only locally
disturbed; in the latter case almost the whole nervous system
may participate in the reaction, a part focal and sharply defined
and the rest marginal, diffuse, and exercising more or less of
inhibitory or reinforcing control on the final reaction.

The studies of Herrick and Coghill have shown that in the
development of the nervous system of Amphibia the first reflex
circuits to come to maturity are made up of rather complex
chains of neurons so arranged as to permit of only one type of
response, viz., a total reaction (the swimming movement), from
all possible forms of stimulation, and that in successive later
stages this generalized type is gradually replaced by a series of
special reflexes involving more diversified movements. Parallel
with this process the higher correlation centers are developed
for the integration of the several special reflexes into complex
action systems. The simple reflex arc, as illustrated in Fig.
1 (p. 25), which is adapted for the execution of a single movement
in response to a particular stimulus, is the final stage in this
developmental process, whose initial stages are much more
complex and diffuse arrangements of neurons adapted for total
reactions of a more general sort.

THE REFLEX CIRCUITS 67

We have just described the mechanisms of certain reflexes.
The question at once arises, In what sense do we know the
mechanism of a nervous reaction? Certainly not in the sense
that we understand all of the factors involved in nervous conduc-
tion and correlation. But we do have a practical knowledge of
the combinations of neurons necessary to effect certain definite
results, much as the practical electrician may be able to wind a
dynamo or build a telephone, even though his knowledge of the
theory of electricity be very small.

Summary. — The reflex arcs or reflex circuits rather than
the neurons of which these circuits are composed are, from
the physiological standpoint, the most important units of the
nervous system. Reflex acts are to be distinguished, on the
one hand, from the simpler non-nervous reactions known as
tropisms and taxes, and, on the other hand, from voluntary
acts and acquired automatisms. Many instincts are chain
reflexes of very complex sorts, the completion of one reaction
serving as the stimulus for the next, and so on in series. The
simplest true reflex requires a receptor, a center or adjuster,
an effector, and the afferent and efferent conductors which put
the center into physiological relation with the receptor and the
effector respectively. Five types of reflex circuits were distin-
guished (see Fig. 18) and illustrations of them given. All of the
reflex centers are interconnected by systems of fibers, either in
the form of definite tracts or else by more diffuse connections
in the neuropil. Localization of cerebral function is, therefore,
only approximate, with the possibility of all sorts of intercon-
nection of different reflex systems as occasion may require.
This is the neurological basis of the greater plasticity of
behavior of higher vertebratesas contrasted with invertebrates
and lower vertebrates.

LITERATURE

DEWEY, J. 1893. The Reflex Arc Concept in Psychology, Psychol.
Review, vol. iii, p. 357.

HERRICK, C. JUDSON. 1913. Some Reflections on the Origin and Sig-
nificance of the Cerebral Cortex, Jour, of Animal Behavior, vol. iii, pp. 222-
236.

HERRICK, C. JUDSON and COGHILL, G. E. 1915. The Development of
Reflex Mechanisms in Amblystoma, Jour. Comp. Neur., vol. xxv, pp. 65-85.

68 INTRODUCTION TO NEUROLOGY

HERRICK, F. H. 1905. The Home Life of Wild Birds. Revised edition,
New York.

— . 1907. Analysis of the Cyclical Instincts of Birds, Science, N. S., vol.
xxv, pp. 725, 726; and Jour. Comp. Neur., vol. xyii, pp. 194, 195.

— . 1907. The Blending and Overlap of Instincts, Science, N. S., vol.
xxv, pp. 781, 782; and Jour. Comp. Neur., vol. xyii, pp. 195-197.

— . 1908. The Relation of Instinct to Intelligence in Birds, Science,
N. S., vol. xxvii, pp. 847-850.

HOUGH, TH. 1915. The Classification of Nervous Reactions, Science,
N. S., vol. xli, pp. 407-418.

JENNINGS, H. S. 1905. The Basis for Taxis and Certain Other Terms in
the Behavior of Infusoria, Jour. Comp. Neur., vol. xv, pp. 138-143.

— . 1906. The Behavior of Lower Organisms, New York.

LOEB, J. 1900. Comparative Physiology of the Brain and Comparative
Psychology, New York.

— . 1912. The Mechanistic Conception of Life, Chicago.

SHERRINGTON, C. S. 1906. The Integrative Action of the Nervous Sys-
tem, New York.

YERKES, R. M. 1905. The Sense of Hearing in Frogs, Jour. Comp. Neur.,
vol. xv, pp. 279-304.

CHAPTER V
THE RECEPTORS AND EFFECTORS

IN the further study of the nervous system as the apparatus
of adjustment between the activities of the body and those of
environing nature, our first task is the analysis of the receptors
(that is, the sense organs) ; for these are the only places through
which the forces of the world outside can reach the nervous sys-
tem in order to excite its activity.

"The world is so full of a number of things
I'm sure we should all be as happy as kings."

But in order to attain this fortunate result it is necessary that we
should be able to discriminate the essential from the unimportant
elements of this environing complex, and to adjust our own be-
havior in relation thereto.

Protoplasm in its simplest form is sensitive to some sorts of
mechanical and chemical stimulation. In fact, as we have seen,
all of the so-called nervous functions are implicit in undifferen-
tiated protoplasm. But the bodies of all but a few of the lowest
organisms are protected by some sort of a shell or cuticle from
excessive stimulation from the outside, and individual parts of
the surface are then differentiated in such a way as to be sensi-
tive to only one group of excitations while remaining insensitive
to all other forms. Thus arose the sense organs, each of which
consists essentially of specialized protoplasm which is highly
sensitive to some particular form of energy manifestation, but
relatively insensitive to other forms of stimulation. Each sense
organ possesses, in addition, certain accessory parts, adapted to
concentrate the stimuli upon the essential sensitive protoplasm,
to intensify the force of the stimulus, or to so transform the

69

70 INTRODUCTION TO NEUROLOGY

energy of the stimulus as to enable it to act more efficiently upon
the essential end-organ.

Sherrington states the distinctive characteristic of the sense
organs in this form, "The main function of the receptor is, there-
fore, to lower the threshold of excitability of the arc for one kind
of stimulus and to heighten it for all others." The selective func-
tion of the receptors is illustrated by a consideration of the
different forms of vibratory energy which pervade the environ-
ment in which we live.

There are, first, rhythmically repeated mechanical impacts
perceived through the sense of touch. This series of tactile
sensations extends from a single isolated contact at one extreme
to rhythmically repeated contacts touching the skin as fre-
quently as 1552 vibrations per second.

A second series of vibratory phenomena is presented by the
mechanical vibrations of the surrounding medium perceived sub-
jectively as sound. Out of the entire series of such vibrations of
all possible frequencies the human ear is sensitive to a series of
approximately ten octaves from about 30 (in some cases 12) to
about 30,000 (in some cases 50,000) vibrations per second (wave
lengths from 1228 cm. or 40 ft. to 1.3 cm. or .5 inch in length).
To all other vibrations it is insensitive. Within this range the
average human ear can discriminate some 11,000 different pitch
qualities (Titchener).

Subjectively, the series of tone sensations is broken up into
a number of octaves, and it is found that a given tone of the
musical scale is excited by vibrations of exactly twice the fre-
quency which excites the corresponding tone of the next lower
octave. By analogy with this arrangement all series of physical
vibrations are sometimes spoken of as divisible into octaves,
the octave being defined as those vibration frequencies which
lie between a given rate and twice that rate or half that
rate.

A third type of vibratory phenomena is presented by the much
more rapid series of so-called ethereal vibrations, or waves in
immaterial media. The lower members of this series are the
Hertzian electric waves; the higher members are the re-rays.
Between these extremes lie waves perceived as radiant heat, the
light waves, and the ultra-violet rays of the spectrum. This

THE RECEPTORS AND EFFECTORS 71

series of ethereal vibrations may extend farther indefinitely both
downward and upward, but of its ultimate limits we have no
knowledge.

There is no human sense organ which can respond directly to
the electric waves, the ultra-violet rays, and the x-rays.
These have, accordingly, remained wholly unknown to us until
revealed indirectly by the researches of the physical laboratories.
Some ten octaves of this series are contained in the solar spec-
trum, from an infra-red wave length of about .1 mm. to an ultra-
violet wave length of .00035 mm. The light from metallic arcs
and from incandescent gases has, however, been found to contain
wave lengths as short as .00006 mm. The human eye is sensi-
tive to something over one octave of this series (waves from
.0008 to .0004 mm. in length, whose rates lie between 400,000
and 800,000 billions of vibrations per second), with six octaves
in the infra-red and three in the ultra-violet. The lower mem-
bers of this 'series of vibrations of the solar spectrum, and to a
less extent the higher also, are capable of stimulating the tem-
perature organs of the skin.

Thus it appears that of the complete series of ethereal vibra-
tions, we can sense directly only about one octave by the eye and
a number of others through the sense organs for temperature in
the skin, while to the lowest and highest members of the series
our sense organs are entirely insensitive. The sensitivity of the
skin to these vibrations is limited subjectively to a small range
of temperature sensations, while the retinal excitations give us
subjectively an extensive series of sensations of color and bright-
ness. The human eye can discriminate from 150 to 230 pure
spectral tints, besides various degrees of intensity and purity of
tone, making a total of between 500,000 and 600,000 possible
discriminations by the visual organs (von Kries). Some of the
preceding data are summarized in the table1 on page 73.

1 In the preparation of this table I have been assisted by Professor R. A.
Millikan, of the University of Chicago, whose kindness I gratefully ac-
knowledge. The figures given are based upon the formula —

velocity

— i — znr = rate
wave length

and the velocity of transmission is taken as 3 x 1010 cm. per second. The
actual velocity of light waves as worked out experimentally by Michelson
is 299,853 kilometers per second.

72

INTRODUCTION TO NEUROLOGY
TABLE OF PHYSICAL VIBRATIONS

Physical
process.

Wave
length.

Number of vibrations
per second.

Receptor.

Sensation.

Mechanical

From very slow
to

Skin.

Touch and

1552 per second.

pressure.

Below 12,280 mm.

Below 30 per second.

None.

None.

Waves in
material
media.

12,280 mm.
to
13 mm.

30 per second
to
30,000 per second.

Internal
ear.

Tone.

Above 13 mm.

Above 30,000 per second.

None.

None.

oo to .1 mm.
(electric waves).

0 to 3000 billion (3 X 10").

None.

None.

.1 mm.
to
.0004 mm.

3000 billion (3 X 10")
to
800,000 billion (8X10").

Skin.

Radiant
heat.

Ether waves.

.0008 mm.
to
.0004 mm.

400,000 billion (4 X 10")
to
800,000 billion (8X10").

Retina.

Light and
color.

.0004 mm.
to
.000059 mm.
(ultra- violet- rays) .

800,000 billion (8 X 10")
to
5,100,000 billion (5.1 X 10'6).

None.

None.

.0000008 mm.
to
.00000005 mm.
(x-raya).

400,000,000 billion (4 X 10")
to
6,000,000,000 billion (6X1018).

None.

None.

Similarly, the chemical senses, taste and smell, reveal to us only
a very small number out of the total series of actual excitations
to which our sense organs are exposed. Our organs of taste, in
fact, can respond to only four types of chemical substances, with
only four subjective sense qualities, viz., sour, salty, sweet, bitter.
The organs of smell respond to a larger range of chemical stimuli
and to far greater dilutions, i. e., the threshold of sensation is
far lower for smell than for taste.

Many of the lower animals have very different limits of sus-
ceptibility to the kinds of stimulation which we have just been
considering, and in some cases they have sense organs which are
attuned to respond to a quite different series of environmental
factors than are our own, as, for example, the lateral line sense
organs of fishes. We can form no idea how the world appears to
such organisms except in so far as their sensory equipment is
analogous with our own.

THE RECEPTORS AND EFFECTORS 73

From these illustrations it is plain that the sensory equipment
of the human body is adapted to respond directly to only a
limited part of the environing energy complex, the remainder
having little, if any, practical significance in the natural environ-
ment of primitive man. During the progress of the develop-
ment of human culture mankind has very considerably wid-
ened his contact with the environment by artificial aids to his
sense organs. The range of vision has been extended by the
microscope and the telescope, and of hearing by the micro-
phone and the telephone. The photographic plate enables him
to extend his knowledge of the solar spectrum beyond its visible
limits, and the Marconi wireless apparatus brings the Hertzian
electric waves under his control and thus enables him to
put a girdle round about the earth in less than Puck's forty
minutes.

We may conceive the body as immersed in a world full of
energy manifestations of diverse sorts, but more or less com-
pletely insulated from the play of these cosmic forces by an
impervious cuticle. The bodily surface, however, is permeable
in some places to these environing forces and in a differential
fashion, one part responding to a particular series of vibrations,
another part to a different series, much as the strings of a piano
when the dampers are lifted will vibrate sympathetically each to
its own tone when a musical production is played on a neighbor-
ing instrument. The sense organs, again, may be compared
with windows, each of which opens out into a particular field so
as to admit its own special series of environmental forces. In
each species of animals these windows are arranged in a charac-
teristic way, so as to admit only those forms of energy which are
of practical significance to that animal as it lives in its own natu-
ral environment. The sensory equipment of the human race was
thus established by the biological necessities of our immediate
animal ancestors, and there is no evidence of subsequent im-
provement in these physiological mechanisms or of any increase
in the number of our senses during the advancement of human
culture. What the progress of science has accomplished is to
supplement the limited sensory equipment of primitive man by
various indirect means. To recur to our analogy of a house with
many windows, we have not been able to increase the number of

74 INTRODUCTION TO NEUROLOGY

windows so as to look out directly into new fields; but we have
increased the range of vision through the old windows, much as a
telescope brings remote objects near and as a periscope enables
the observer to see around a corner. To the development of
the cerebral cortex we owe the acquisition of these new powers
which have opened to us the realms of electric vibrations,
ultra-violet rays, and many other natural phenomena to which
our unaided sense organs are quite insensitive.

Children in the kindergarten are taught that there are five
senses. In reality, there are more than twenty different senses.
Some of the sense organs are stimulated by external objects and
hence are termed exteroceptors ; others are stimulated by internal
excitations of the visceral organs and are termed inter oceptors.
Still further classifications have been suggested, to which refer-
ence will be made shortly. Here we must first consider the
criteria in accordance with which the various senses are dis-
tinguished.

The analysis and classification of the senses is by no means
so simple a task as one might at first suppose. It is true that
ordinarily we do not confuse a thing seen with a sound heard;
but, on the other hand, we do constantly confuse savors with
odors, and it often requires refined physiological experimentation
to determine whether the organ of taste or the organ of smell is
the source of the sensory excitation in question. Most of the
common "flavors" of food are, in reality, odors and are perceived
by the organ of smell only. A bad cold which closes the pos-
terior nasal passages makes "all food taste alike" for this reason.
In reality, as we have already seen, there are only four tastes
recognized by the physiologists, viz., sweet, sour, salty, and
bitter.

Confusion has arisen in the attempts to analyze these two
senses from the fact that different physiologists have used differ-
ent definitions of a "sense." One author, who defines these
senses in terms of the physical agents which excite them, says
that taste is stimulated by liquids and smell by vapors, and that,
accordingly, aquatic animals, whose nostrils are filled with water,
have by definition no sense of smell. Other authors separate
these senses according to the organ stimulated, the excitation of
the nose being smell, that of the taste-buds being taste, regard-

THE RECEPTORS AND EFFECTORS 75

less of the nature of the exciting substance or of the subjective
quality of the sensation.

There are, in reality, four different factors which must be
taken into account in defining a "sense." (1) Doubtless with us
human folk the most important criterion is direct introspective
experience, the psychological criterion. Ordinarily this is ade-
quate, but, as we have just seen, there are some cases where
it alone cannot be depended upon to distinguish between two
senses. (2) The adequate stimuli of the various senses exhibit
characteristic physical or chemical differences, the physical
criterion. This factor, too, must be carefully investigated or we
may be led astray. (3) The data of anatomy and experimental
physiology may differentiate structurally the receptive organs
and conduction paths of the several types of sensation, the ana-
tomical criterion. (4) Finally, the type of response varies in a
characteristic way for the different senses, the physiological
criterion.

The fourth criterion has been applied to solve the problem of
the reason for the development of two very different types of
sense organs and cerebral connections for the senses of smell and
taste, both of which are chemical senses with similar subjective
qualities. It has been pointed out by Sherrington that taste is
an interoceptive sense, calling forth visceral responses within the
body, while smell is, in part at least, an exteroceptive sense, being
excited by objects at a distance from the body and calling forth
movements of locomotion carrying the whole body toward or
away from the source of the odorous emanations. Thus the
form of the response is here the distinctive factor, and incidental
to this feature the organs of smell are sensitive to far smaller
quantities of the stimulating substance than are the taste-buds.
Parker and Stabler have shown that the human organ of smell
is sensitive to alcohol at a dilution 24,000 times greater than that
necessary to stimulate the organs of taste (see p. 218).

It is impossible in the present state of our knowledge to frame
adequate definitions of all the senses in terms of any one of
these four criteria alone, although it is a reasonable hope that
this may at some future time be attained. Even when all of
these criteria are taken into account, it is by no means easy to
determine how many separate senses the normal human being

76 INTRODUCTION TO NEUROLOGY

possesses. Not only is there a considerable number of sense
organs not represented at all in our traditional list of five senses,
but several of these five are complex. Thus, the internal ear
includes two quite distinct organs — the cochlea, which serves as
a receptor for sounds, and the labyrinth, whose semicircular
canals serve as the chief sense organs concerned in the regula-
tion of bodily position and the maintenance of equilibrium, func-
tions which are quite distinct from hearing. The skin, too,
serves not only as the chief organ of touch, but also the addi-
tional functions of response to warm, cold, and painful impres-
sions, besides some other more obscure sensory activities, such
as tickle.

An acceptable classification of the sense organs or receptors
of the body must take account of their anatomical relations, of
the nature of the physical or chemical forces which serve as the
adequate stimuli, of the subjective qualities which we experience
upon their excitation, and of the character of the physiological
responses which commonly follow their stimulation. The last
point has been too much neglected.

In fact, the most fundamental division of the nervous sys-
tem which we have, cutting down through the entire bodily
organization, is based upon this physiological criterion. From
this standpoint we divide the nervous organs into two great
groups: (1) a somatic group pertaining to the body in general
and its relations with the outer environment, and (2) a visceral,
splanchnic, or interoceptive group. The latter group comprises
the nerves and nerve-centers concerned chiefly with digestion,
respiration, circulation, excretion, and reproduction. These are
intimately related with the sympathetic nervous system and
those parts of the central nervous system directly connected
therewith, though the more highly specialized members of this
group are independent of the sympathetic system. The somatic
group comprises the greater part of the brain and spinal cord and
the cranial and spinal nerves, or, briefly, the cerebro-spinal ner-
vous system as distinguished from the sympathetic system (see
p. 225). This is the mechanism by which the body is able to ad-
just its own activities directly in relation to those of the outside
world — to procure food, avoid enemies, and engage in the
pursuit of happiness.

THE RECEPTORS AND EFFECTORS 77

The organs belonging to each of these two groups do much of
their work independently of the other group, i. e., visceral stimuli
call forth visceral responses and external or somatic stimuli
call forth somatic responses. Nevertheless, the two groups of
organs are by no means entirely independent, for external excita-
tions may produce strong visceral reactions, and conversely.
Thus, the sight of luscious fruit (exteroceptive stimulus) natu-
rally calls forth movements of the body (somatic responses) to go
to the desired object and seize it. But if one is hungry, the
mouth may water in anticipation, a purely visceral response.
On the other hand, the strictly visceral (interoceptive) sensation
of hunger is apt to set in motion the exteroceptive reactions
necessary to find a dinner.

Sherrington, whose analysis with some modifications is here
adopted, recognizes three types of sense organs or receptors:
(1) the interoceptors, or visceral receptive organs, which respond
only to stimulation arising within the body, chiefly in connection
with the processes of nutrition, excretion, etc.; (2) the extero-
ceptors, or somatic sense organs, which respond to stimulation
arising from objects outside the body; (3) the proprioceptors, a
system of sense organs found in the muscles, tendons, joints,
etc., to regulate the movements called forth by the stimulation
of the exteroceptors. This third group is really subsidiary
to the somatic group, or exteroceptors, and will be considered
more in detail below.

The proprioceptive sense organs are deeply embedded in the
tissues and are typically excited by those activities of the body
itself which arise in response to external stimulation. The
proprioceptors then excite to reaction the same organs of re-
sponse as the exteroceptors and regulate their action by reinforce-
ment or by compensation or by the maintenance of muscular
tone. All reactions concerned with motor coordination, with
maintenance of posture or attitude of the body, and with
equilibrium involve the proprioceptive system.

The important point to bear in mind here is that stimulation
of the visceral sense organs typically calls forth visceral responses,
i. e., adjustments wholly within the body, while stimulation of
the somatic (exteroceptive) sense organs typically calls forth
somatic responses, i. e., a readjustment of the body as a whole

78 INTRODUCTION TO NEUROLOGY

I

with reference to its environment. This is a very fundamental
distinction. These two functions are quite diverse and the
organization of these two parts of the nervous system shows cor-
responding structural differences.

The internal adjustments of the visceral systems are effected
by a nicely balanced mechanism of local and general reflexes so
arranged that most of their work is done quite mechanically and
unconsciously. The taking of food and its preliminary mastica-
tion are generally voluntary acts whose various processes are —
or may be — controlled at will. But once the food has passed into
the esophagus, the further work of swallowing, digestion, and
assimilation is no longer under direct control. The presence of
a morsel of food in the upper part of the esophagus excites the
muscular movements necessary for the completion of the act of
swallowing, which no act of will can prevent or modify. In fact,
any attempt at conscious interference or regulation is apt to
result in an incoordination of the movements involved, and
sputtering or gagging may result.

The mechanisms involved in these processes are inborn and
require no practice for their perfect performance. They are
innate, invariable, and essentially similar in all members of a
race or species. They are, moreover, nicely adapted to the
mode of life characteristic of the species. In a carnivorous ani-
mal the whole physiological machinery of nutrition is different
from that of a herbivorous animal. These physiological and
structural peculiarities by which each species of animal is
adapted to its mode of life have been brought about by natural
selection and other evolutionary factors. This is not absolutely
true of all visceral actions; some are acquired and modifiable.
But as a general rule this is their type.

Some of the somatic actions are likewise innate and relatively
fixed in character. This is true of most of the proprioceptive
reactions and of many of the exte receptive as well. Fish can
swim as soon as they are hatched ; chicks just out of the shell have
an instinctive tendency to peck at all small objects on the ground.
But in most of these cases (of which innumerable instances
might be cited) some practice is necessary before perfect re-
sponses are attained ; and a very large proportion of the extero-
ceptive acts are not innate, but acquired by long and often ardu-

THE RECEPTORS AND EFFECTORS 79

ous experience. In higher vertebrates, as a rule, all but the
simplest and most elementary exteroceptive activities are indi-
vidually acquired, variable, non-hereditary, plastic behavior
types. The elements of which these acts are made up are, of
course, necessarily present in the inherited reflex pattern; but
the pattern according to which these elements are combined is
not wholly predetermined in the hereditary organization of the
species (pp. 31, 301).

With these principles in mind, let us now undertake an anal-
ysis of the human receptors and of the nervous end-organs re-
lated to their effectors, or organs of response. The following list
is by no means complete and is in some parts merely provisional.

I. SOMATIC RECEPTORS

These are concerned with the adjustment of the body to external or
environmental relations.

A. THE EXTEROCEPTIVE GROUP

The sense organs of this group are stimulated by objects outside the
body and typically call forth reactions of the whole body, such as locomo-
tion, or of its parts, so as to change the relation of the body to its environ-
ment. This group includes a system of general cutaneous sense organs,
some organs of deep sensibility, and some of the higher sense organs. The
cutaneous exteroceptors comprise a very complex system whose analysis
has proved very difficult. The conclusions presented in the paragraphs
which follow are based chiefly upon the observations of von Frey, Henry
Head, and Trotter and Davies. The correlation of the data of physiological
experiment with the anatomical structure of the cutaneous end-organs is
still somewhat problematical and the assignment of end-organs here to the
various cutaneous senses should be regarded as provisional rather than as
demonstrated.

1. Organs of Touch and Pressure. — These fall into two groups, those
for deep sensibility (pressure) and those for cutaneous sensibility (touch).
The deep pressure sense is served by nerve-endings throughout the tissues
of the body and is preserved intact after the loss of all cutaneous nerves.
Most of the functions of the deep sensory nerves belong to the propriocep-
tive and interoceptive series (see below), but some exteroceptive functions
are here present also. The latter are probably related chiefly to the
Pacinian corpuscles and similar encapsulated end-organs. The Pacinian
corpuscle has a central nerve-fiber enclosed in a firm lamellated connective-
tissue sheath (Fig. 22). By these end-organs relatively coarse pressure
may be discriminated and localized (exteroceptive function), and movements
of muscles and joints can be recognized (proprioceptive function). The
sensory fibers concerned with the deep preesurexsense-are distributed through
the muscular branches of the spinal nerves in company with the motor
fibers. The point stimulated can be localized with a fair degree of accuracy,

80 INTRODUCTION TO NEUROLOGY

but there is no discrimination of two compass points applied simultaneously
to the overlying skin. The two points will appear as one stimulus, even
when widely separated.

The cutaneous organs of tactile sensibility are of several kinds, whose
precise functions are still obscure. There are two principal groups of these,
those arranged in the hair bulbs at the roots of the hairs and those on the
hairless parts, such as the lips, the palms of the hands, and the soles of the
feet. The latter are more highly differentiated endings and are organs of
the most refined active touch.

Most of the surface of the body is more or less hairy, though many of
these hairs may be so fine as to escape observation. The hairs are the most

Fig. 22. — Pacinian corpuscles from the peritoneum of a cat. (After Sala,
from Bohm-Davidoff-Huber's Histology.)

important sources of excitation of the first group of cutaneous sense organs,
and the sensitiveness of the hair-clad parts is greatly reduced after the hair
is shaved. The threshold of excitation to touch of the skin about the base
of a hair is from three to twelve times higher than that of a similar excita-
tion applied to the hair itself. The inneryation of the hair bulbs is very
complex and varies greatly for different animals and for the different kinds
of hairs on the same body, so that no general description is possible.

Miss Vincent has shown that the large vibrissse of the rat receive their
nerve-supply from two sources. A large nerve bundle pierces the deep

THE RECEPTORS AND EFFECTORS

81

layer of the skin (dermis) in the lower part of the hair bulb, spreads out
over the inner hair follicle in a heavy plexus, and terminates chiefly in a
mantle of touch cells, resembling Merkel's corpuscles (see Fig. 26), in the
outer root sheath all over the follicle. A second nerve supply comes from
the dermal plexus of the skin, from which branches run down and form a
nerve ring about the neck of the follicle. Experimental studies show that
these hairs are very important not only as general tactile organs, but

Fig. 23. — Nerve-endings about a large hair from the dog. The nerve-
fibers are shown in black surrounding the hair shaft, the straight fibers
at b and the circular fibers at c. (After Bonnet, from Barker's Nervous
System.)~

specifically as aids in locomotion and equilibration. The ordinary hairs of
man and other mammals have three forms of specific nerve-endings in addi-
tion to various forms of terminal arborizations in the surrounding tissues:
(1) straight and often forked endings running parallel with the base of the
hair; (2) circular fibers forming a plexiform ring around the root of the
hair external to the straight endings; and (3) leaf -like nerve-endings associ-
ated with special cells resembling Merkel's corpuscles. Figure 23 illustrates
the first and second types of these endings.

6

82

INTRODUCTION TO NEUROLOGY

Under the hairless parts of the skin there are special tactile bodies, such
as Meissner's corpuscles. These are generally found in the deep layer of
the skin (dermis) and in the underlying tissues, either as free skein-like
terminal arborizations of cutaneous nerves or as similar more elaborate
endings enclosed in connective-tissue capsules. Figures 24 and 25 illustrate
the most highly differentiated form of these endings, the Meissner cor-
puscles. Merkel's corpuscles (Fig. 26) are probably simpler organs of this
system.

Stratum
corneum

—Stratum lucidum

Stratum
_.- ?7 granulosum

Blood-vessels
and nerves

Fig. 24. — Section through the human skin, illustrating the four layers
of the epidermis and the papillae of the dermis or corium. A corpuscle of
Meissner is seen within one of the dermal papillae. (From Cunningham's
Anatomy.)

All forms of cutaneous sensibility (touch, temperature, and pain) when
studied physiologically are found to be localized in small areas or sensory
spots, each of which has a specific sensibility to one only of the cutaneous
sensory qualities. The intervening parts of the skin are insensitive. An
immense amount of physiological and clinical observation has been devoted
to the analysis of cutaneous sensibility, including the experimental division

THE RECEPTORS AND EFFECTORS

83

Fig. 25. — The details of the nerve-endings in a Meissner corpuscle from
the human skin. Only the outline of the corpuscle is shown, within which
the terminals of the nerve-fiber form a complex skein. (After Dogiel, from
Bohm-Davidoff-Huber's Histology.)

Fig. 26. — Merkel's corpuscles or tactile disks from the skin of the pig's
snout. The nerve-fiber, n, branches, and each division ends in an expanded
disk, ra, which is attached to a modified cell of the epidermis, a. The un-
modified cells of the epidermis are shown at c. (From Ranvier.)

84 INTRODUCTION TO NEUROLOGY

of cutaneous nerves in their own bodies by Head, Trotter, and Davies for
the purpose of studying more critically the distribution of the various
sensory functions in and around the anesthetic areas produced by the
injuries and the phenomena accompanying the restoration of these functions
during the regeneration of the nerves. But general agreement has not yet
been reached on all questions.

Head and his colleagues are of the opinion that all forms of cutaneous
sensibility (touch, temperature, and pain) are grouped in two series, each
served by different nerve-fibers and end-organs; these he terms "proto-
pathic" and "epicritic" sensibility. Protopathic sensibility is subjectively
general diffuse sensibility of a primitive form. Its sense organs are arranged
in definite spots, and yet these sensations have no clear local reference or
sign; that is, the spot stimulated cannot be accurately localized. There are
separate spots for touch, heat, cold, and pain; these spots being generally
grouped near the hair bulbs. In fact, the hairs are the most important
tactile organs of this system and the other sense qualities belonging here are
intimately associated with the roots of the hairs. Epicritic sensibility is

Fig. 27. — End-bulb of Krause from the conjunctiva of man. The
nerve-ending forms a globular skein within a delicate connective-tissue
cansulp,. (After Dncnpl.^

capsule. (After Dogiel.)

a more refined sort of discrimination, and is regarded as a later evolutionary
type. It includes light touch, on the hairless parts of the body particularly,
and the discrimination of the intermediate degrees of temperature. Cuta-
neous localization and the discrimination of the distance between two points
simultaneously stimulated (the "compass test") are functions of this sys-
tem; but pain sensibility is not included, this being wholly protopathic.

Trotter and Davies repeated some of Head's experiments and, while
confirming most of his observations, they were led to somewhat different
conclusions. They do not regard the protopathic and epicritic series as
served by distinct systems of nerves, but as different physiological phases of
the same systems of nerve-fibers and end-organs.

2. End-organs for Sensibility to Cold.

3. End-organs for Sensibility to Heat. — Physiological experiment
shows that warmth and cold are sensed by different parts of the skin (the
warm spots and the cold spots respectively), and Head is of the opinion that
each of these types of sensibility may be present in an epicritic and a proto-
pathic form. What end-organs are involved here is by no means certain.
The margin of the cornea was found by von Frey to be sensitive to pain and

THE RECEPTORS AND EFFECTORS

85

cold only. The free nerve-endings found here he assumes to be pain recep-
tors and the end-bulbs of Krause (Fig. 27) to be cold receptors. By an
analogous argument he assumes that the "genital corpuscles" of Dogiel and
some similar endings widely distributed in the skin are warmth receptors.
By some other physiologists these types of corpuscles are regarded as belong-
ing to the tactile system. Stimulation of the somatic nerves of deep sensi-
biEty causes no temperature sensations. (For temperature sensations in the
viscera see p. 242.)

4. End-organs for Pain. — Some physiolo-
gists believe that there are separate nerve-
endings for pain; others regard pain as a
quality which may be present in any sense,
and not as itself a true sensation (pp. 249
ff.). The free nerve-endings among the cells
of the epidermis are regarded by von Frey
as the pain receptors, because these endings
alone are present in some parts of the body
where susceptibility to pain is the only sense
quality present, such as the dentin and pulp
of the teeth (Fig. 28), the cornea, and the
tympanic membrane of the ear (J. G. Wilson).

Similar endings are found throughout the
epidermis (Fig. 29) and in many deep struc-
tures. The nerves of deep sensibility of the
somatic sensory type may also carry painful
impressions. (For visceral pain see pp. 243,
250.) According to Head, cutaneous pain is
wholly of protopathic type, and in case of in-
jury to the peripheral nerves it disappears
and reappears in regeneration simultaneously
with the protopathic type of tactile and tem-
perature sensation. This cutaneous pain is
not accurately localizable unless epicritic cu-
taneous sensibility is also present.

5. End-organs of General Chemical Sen-
sibility.— In man this type of sensibility is
found only on moist epithelial surfaces, such
as the mouth cavity; but in fishes it may be
present over the entire surface of the body.
The sense organ is probably the free nerve
terminals among the cells of the epithelium,
never special sense organs like taste-buds,
for these when present in the skin belong
to a quite different system. Coghill has

recently shown that the supposed sensitivity of the amphibian skin to acids
is really due to a destructive action of the reagents upon the epithelium, and
the entire question of diffuse chemical sensibility requires further study.

6. Organs of Hearing. — The stimulus is material vibrations whose
frequency ranges from 30 to 30,000 per second (see p. 70). The receptor is
the spiral organ (organ of Corti) in the cochlea of the ear (see p. 197), and
perhaps also the sensory spots in the saccule and utricle. There are two
forms of auditory sensations: (1) noise, stimulated by sound concussions or
irregular mixtures of aerial vibrations; (2) tone, stimulated by sound waves
or periodic aerial vibrations.

Fig. 28. — Longitudinal
section of a tooth of a fish,
Gobius, showing nerve ter-
minals: d, Dentin; n,
nerve-fibers entering the
cavity of the dentin and
ending free. (After Ret-
zius, from Barker's Ner-
vous System.)

SO INTRODUCTION TO NEUROLOGY

7. Organs of Vision. — The stimulus is ethereal vibrations ranging be-
tween 400,000 billions and 800,000 billions per second. Here also there
are two forms: (1) brightness, stimulated by mixed ethereal vibrations;
(2) color, stimulated by simpler ethereal vibrations. (On the structure of
the eye and its connections see p. 204.)

Fig. 29. — Transverse section through the skin of the ear of a white
mouse. The dotted line marks the lower border of the epidermis: a, Hori-
zontal nerve-fibers; b, bifurcation of nerve-fibers; fn, cutaneous nerve-
fibers. (After Van Gehuchten, from Barker's Nervous System.)

8. Organs of Smell. — This sense has both exteroceptive and intero-
ceptive qualities, the latter being apparently the more primitive. (See
pp. 75, 91, and 215.)

B.. THE PnopRiocEPTivK GROUP

These sense organs are contained within the skeletal muscles, joints, etc.,
and are stimulated by the normal functioning of these organs, thus report-
ing back to the central nervous system the exact state of contraction of the
muscle, flexion of the joint, and tension of the tendon. Cutaneous sensi-
bility may also participate in these reactions, which are generally uncon-
sciously performed.

9. End-organs of Muscular Sensibility. — The organ is a series of
nerve-endings among special groups of muscle-fibers known as muscle
spindles. These endings are usually spirally wound around their muscle-
fibers and are stimulated by the contraction of the muscle (Fig. 30).

THE RECEPTORS AND EFFECTORS

87

As we shall see below (p. 92), the muscles are classified for our purposes
into three groups: (1) somatic muscles (the striated skeletal muscles); (2)
general visceral muscles (generally unstriated and involuntary) ; and (3)
special visceral muscles of the head which are striated and voluntary. The
first and third of these groups receive their motor innervation from cere-
bro-spinal nerves; the second, from sympathetic nerves. The classification

Fig. 30. — Muscle spindle from the muscles of the foot of a dog. Three
muscle-fibers are shown, and three sensory nerve-fibers, which enter the
muscle spindle, branch, and wind spirally around the muscle-fibers (a, 6).
A sympathetic nerve-fiber (Sy.n.) also enters the muscle spindle. (After
Huber and DeWitt, from Barker's Nervous System.)

of the nerves of muscle sense related respectively to these three groups of
muscle offers some difficulties. The striated muscles of the first and third
groups are physiologically similar in that they act in general in response to
exteroceptive stimuli and they may be voluntarily excited, while the visceral
muscles of the second group are generally stimulated by interoceptive stim-

Fig. 31. — A teased preparation of a tendon of a small muscle from a
rabbit, showing the endings of the nerve-fibers of tendon sensibility, each
of which spreads out widely over the surface of the tendon. (After Huber
and DeWitt, from the Journal of Comparative Neurology.)

uli and their functions are usually involuntary. I have, accordingly, some-
what arbitrarily regarded the sensory nerves of the first and third groups
of muscles as proprioceptors and those of the second group as interoceptors.
10. End-organs of Tendon Sensibility. — Nerve-endings are spread
out over the surface of tendons and are stimulated by stretching the tendon
during muscular contraction (Fig. 31).

88

INTRODUCTION TO NEUROLOGY

11. End-organs of Joint Sensibility. — Nerve-endings found in the
joints and the surrounding tissues are stimulated by bending the joint, and

Fig. 32. — Diagram of the relations of a fiber of the vestibular branch
of the auditory nerve and its mode of termination in the semicircular
canal: co, The central nervous system; fz, non-nervous supporting cell of
the semicircular canal; hz, hair cell, one of the receptor cells of the sensory
surface; sn, axon of the vestibular neuron; sz, cell body of the vestibular
neuron. (After Retzius, from Barker's Nervous System.)

report back to the central nervous system the degree of flexion of the joint.
The chief end-organs are probably Pacinian corpuscles (see Fig. 22).

THE RECEPTORS AND EFFECTORS 89

12. Organs of static and equilibratory sensation arising from
stimulation of the semicircular canals of the internal ear (Fig. 32). This is
the most highly specialized member of the proprioceptive group and acts
in conjunction with all of the other somatic senses to maintain equilibrium,
posture, and the tone of the muscular system (see p. 189). The eyes and
most of the other exteroceptive sense organs, so far as they act in the way
just suggested, also serve as proprioceptors.

II. VISCERAL RECEPTORS

The visceral or interoceptive senses fall into two well-defined groups:
First, the general visceral systems are without highly specialized end-organs
and are innervated through the sympathetic nervous system. Their reac-
tions are chiefly unconsciously performed. Second, the special visceral
senses are provided with highly developed end-organs which are in-
nervated directly from the brain without any connection with the sympa-
thetic nervous system. The special visceral sense organs may in some
cases serve as exteroceptors as well as interoceptors. Their reactions
may be conscious and voluntary.

A. GENERAL VISCERAL GROUP

Many of the sensations of this group are obscure and a number of excito-
motor and excito-glandular reactions may be included here which never
come into clear consciousness, particularly those concerned with nutrition,
excretion, and vasomotpr adjustments. The number of these reactions
might be considerably increased; for further discussion of these reflexes
see p. 234.

13. Organs of Hunger. — The stimulus is strong periodic contractions
of the muscles of the stomach. Hunger is apparently a variety of muscle
sense, but other factors are also present (see p. 240) .

14. Organs of Thirst. — The specific stimulus here is probably a dry-
ing of the pharyngeal mucous membrane, together with more general
conditions.

15. Organs of Nausea. — The stimulus is probably an antiperistaltic
reflex in the digestive tract (see p. 243).

16. Organs giving rise to respiratory sensations, suffocation, etc. (see
p. 235).

17. Organs giving rise to circulatory sensations, flushing, heart panics,
etc. (see p. 234).

18. Organs giving rise to sexual sensations.

19. Organs of sensations of distention of cavi.ties, stomach, rectum,
bladder, etc. This is a variety of muscle sense.

20. Organs of visceral pain (see pp. 243, 250).

21. Organs of obscure abdominal sensations associated with strong
emotions of fright, anger, affection, etc., characterized (probably correctly)
by the ancients by such expressions as "yearning of the bowels," etc. The
stimulus is probably a tonic contraction, of the unstriped visceral muscula-
ture.

The nerve-endings of the general visceral receptors are generally either
simple terminals in the visceral muscles or free arborizations in or under the
various mucous surfaces, without the development of specialized accessory

90

INTRODUCTION TO NEUROLOGY

cells to form differentiated sense organs. Figure 33 illustrates a sensory end-
ing in the mucous membrane of the esophagus, and Fig. 34 types of nerve-

Fig. 33. — Free nerve-endings in the mucous membrane of the esoph-
agus of a cat. (After DeWitt, from Wood's Reference Handbook of the
Medical Sciences.)

Fig. 34. — Nerve-endings in the mouth epithelium of the frog: A, From
sensory papilla of the tongue; B, cylinder cells; C, isolated rod cell; D, upper
part of papilla; E, ciliate cells of palate. (After Bethe, from Wood's Refer-
ence Handbook of the Medical Sciences.)

endings upon epithelial cells. The nerve-endings in the visceral muscles
are very simple (see Figs. 37 and 38) and the separation of sensory from
motor endings here has not been effected.

THE RECEPTORS AND EFFECTORS

91

B. SPECIAL VISCERAL? GROUP.

22. Organs of Taste. — These are excited by chemical stimulation of
taste-buds on the tongue and pharynx by sweet, sour, salty, or bitter sub-
stances. In man this is a strictly interoceptive sense; but in some fishes
taste-buds are scattered over the outer body surface in addition to the mouth
cavity, and thus may serve as exteroceptors also. The organ is a flask-
shaped collection of specialized epithelial cells of two sorts, supporting and
specific sensory elements (Fig. 35). There is a double innervation, partly
by perigemmal fibers whose endings surround the bud, and partly by intra-
gemmal fibers which penetrate the bud and arborize in intimate relation
with the specific sensory cells.

23. Organs of Smell. — These are excited by chemical stimulation of the
specific olfactory mucous membrane of the nose. The number of substances

Fig. 35. — Taste-bud from the side wall of a circumvallate papilla of the
tongue: a, Taste-pore; b, nerve-fibers, some of which enter the taste-bud
(intragemmal fibers), while others end freely in the surrounding epithelium
(perigemmal fibers). (After Merkel-Henle.)

which may act as stimuli is greater than in the case of taste-buds, the num-
ber of subjective qualities is also greater, and the discrimination threshold
is much lower (see pp. 75 and 218). The peripheral organ of smell is a
specific sensory epithelium within the nose, whose sensory cells give rise
directly to the fibers of the olfactory nerve, this being the only peripheral
nerve of the human body whose fibers arise from superficially placed cell
bodies (Fig. 36).

That the olfactory system was originally an interoceptive sense seems
clear; but in all vertebrates living at the present time, the visceral responses
to smell are less important than the somatic reactions. The sense of smell
is the leading exteroceptor in most lower vertebrates, and this function has
been secondarily derived from the primary visceral function. We have seen
above that the sense of taste in some fishes has secondarily acquired extero-
ceptive functions; and in the case of smell this secondary change has been
carried still further until the exteroceptive function has come to dominate

INTRODUCTION TO NEUROLOGY

Olfactory
hairs

.Olfactory
hairs

Peripheral
"process

Body of

-cell with

nucleus

Central
process

C

Fig. 36. — Cells from the olfactory mucous membrane: A from the frog,
B and C from man. The supporting cells are non-nervous. The olfactory
hairs of the olfactory cells project out "into the mucus of the nose, and are
probably the specific receptors. The central process at the base of each
olfactory cell is prolonged mto a fiber of the olfactory nerve (not shown
in the figure), which extends inward to the brain (cf. Fig. 104, p. 217).
(After Schnlte and Brunn.)

the primitive interoceptive, though the latter has by no means been en-
tirely obliterated.

m. SOMATIC EFFECTORS

24. End-organs on Striated Skeletal Muscles. — This "motor end-
plate" is a complex terminal arborization of the motor nerve-fiber, associ-
ated with an elevated granular mass of protoplasm and a collection of
nuclei of the muscle-fiber (see Fig. 5, tel, p. 40).

The somatic muscles whose innervation is here under consideration are
derived embryologically from the somites, or primary mesodermal segments
of the embryo, while the visceral muscles have a different origin. They are
under the direct control of the will and are concerned chiefly with loco-
motion or other movements which change the relations of the body to its
environment. They are typically stimulated to action through the ex-
teroceptive sense organs. They make up the bulk of the musculature of
the trunk and limbs and are represented in the head only in the external
muscles of the eyeball and a part of the muscles of the tongue.

THE RECEPTORS AND EFFECTORS

93

IV. VISCERAL EFFECTORS

25. End-organs on the Involuntary Visceral Muscles. — These muscles
may be unstriated or striated (as in heart muscle). They are innervated

Fig. 37. — Two unstriated involuntary muscle-fibers, showing the nerve-
endings: a, Axon; b, its termination; n, nucleus of the smooth muscle
cell. (After Huber and DeWitt, from Barker's Nervous System.)

through the sympathetic nervous system and typically by a chain of two
neurons, the preganglionic and the postganglionic neurons (see p. 229).

Fig. 38. — Three striated cardiac muscle cells, with their nerve-endings.
(After Huber and DeWitt, from Barker's Nervous System.)

The body of the preganglionic neuron lies in the central nervous system and
its axon passes out into the sympathetic nervous system, where it ends in a
sympathetic ganglion. The efferent impulse is here taken up by a post-
ganglionic neuron, whose body lies in the sympathetic ganglion in question

94 INTRODUCTION TO NEUROLOGY

and whose axon passes onward through a sympathetic nerve to end in the
appropriate effector. The nerve-endings of this system are simple or
branched free terminals ending on the surface of the muscle-fiber (Fig. 37) ;
in the case of heart muscle the fibers usually have expanded tips (Fig. 38) .

26. End-organs on Glands. — The innervation of these organs is in most
respects similar to that of the involuntary muscles last described. A fine
plexus of unmyelinated fibers of sympathetic origin envelops the smaller
glands and pervades the larger ones; these are believed in some cases to be
the excito-glandular fibers.

27. Special Visceral Motor End-organs. — The nerves of these muscles
have no connection with the sympathetic nervous system. These effectors
are striated muscles which may act under the direct control of the will.
In their evolutionary origin they are derived from the muscles of the gills
of the lower vertebrates, and they are developed embryologically from the
ventral unsegmented mesoderm and not from the primitive mesodermal
segments which give rise to the somatic muscles. They are found only in
the head and neck and their nerve-endings are similar to those of the
striated muscles of the somatic series.

Summary. — We have seen that the chief function of the sense
organs is to lower the threshold of excitability of the body in
definite places to particular kinds of stimulation, and thus to
effect an analysis of the forces of nature so far as these concern
the welfare of the body. The nature of this analysis of the en-
vironing energy complex was illustrated by a review of the ways
in which the body may respond to different kinds of vibrations.
The senses, as this word is commonly used, were distinguished
by four criteria, termed briefly the psychological, physical, ana-
tomical, and physiological. Then followed a physiological classi-
fication of the receptors and effectors of the human body.

LITERATURE

BARKER, L. F. 1901. The Nervous System and Its Constituent Neurones,
New York.

COGHILL, G. E. 1914. Correlated Anatomical and Physiological Studies
of the Growth of the Nervous System of Amphibia. I. The Afferent Sys-
tem of the Trunk of Amblystoma, Jour. Comp. Neur., vol. xxiv, pp. 161-233.

VON FREY, M. 1897. Untersuchungen iiber die Sinnesfunctionen der
menschlichen Haut, Abhangl. kgl. sachs. Gesellsch., Bd. 40 (Math.-Phys.
Classe, Bd. 23).

HEAD, H., RIVERS, W. H. R., and SHERREN, J. 1905. The Afferent
Nervous System from a New Aspect, Brain, vol. xxviii, pp. 99-115.

HERRICK, C. JUDSON. 1903. On the Morphological and Physiological
Classification of the Cutaneous Sense Organs of Fishes, Amer. Naturalist,
vol. xxxvii, pp. 313-318.

— . 1908. On the Phylogenetic Differentiation of the Organs of Smell
and Taste, Jour. Comp. Neur., vol. xviii, pp. 157-166.

— . 1914. End-organs, Nervous, Wood's Reference Handbook of the
Medical Sciences, 3d ed., vol. iv, pp. 20-27, New York.

THE RECEPTORS AND EFFECTORS 95

HERTZ, A. F. 1911. The Sensibility of the Alimentary Canal, London.

HUBER, G. C. 1900. Observations on Sensory Nerve-fibers in Visceral
Nerves and on their Modes of Terminating, Jour. Comp. Neur., vol. x,
pp. 134-151.

HUBER, G. C., and DEWITT, LYDIA, M. A. 1897. A Contribution on
the Motor Nerve-endings in the Muscle-spindles, Jour. Comp. Neur., vol.
vii, pp. 169-230.

— . 1900. A Contribution on the Nerve Terminations in Neuro-tendi-
nous End-organs, Jour. Comp. Neur., vol. x, pp. 159-208.

PARKER, G. H. 1912. The Relation of Smell, Taste, and the Common
Chemical Sense in Vertebrates, Jour. Acad. Nat. Sci., Phila., 2 Ser., vol.
xv, pp. 221-234.

PARKER, G. H., and STABLER, ELEANOR M. 1913. On Certain Distinc-
tions Between Taste and Smell, Aincr. Jour. Physiol., vol. xxxii, pp. 230-
240.

RIVERS, W. H. R., and HEAD, H. 1908. A Human Experiment in Nerve
Division, Brain, vol. xxxi, p. 323.

SHELDON, R. E. 1909. The Reactions of the Dogfish to Chemical Stim-
uli, Jour. Comp. Neur., vol. xix, pp. 273-311.

SHERRINGTON, C. S. 1906. The Integrative Action of the Nervous Sys-
tem, New York.

TROTTER, W., and DAVIES, H. M. 1909. Experimental Studies in the
Innervation of the Skin, Jour, of Physiol., vol. xxxviii, pp. 134-246.

VINCENT, STELLA B. 1913. The Tactile Hair of the White Rat, Jour.
Comp. Neur., vol. xxiii, pp. 1-38.

— . 1913a. The Function of the Vibrissae in the Behavior of the White
Rat, Behavior Monographs, vol. i, No. 5, pp. 7-81.

WATSON, J. B. 1915. Behavior, An Introduction to Comparative Psy-
chology, Chapters XI-XIV, New York.

WILSON, J. G. 1911. The Nerves and Nerve-endings in the Membrana
Tympani of Man, Amer. Jour. Anat., vol. xi, pp. 101-112.

CHAPTER VI

THE GENERAL PHYSIOLOGY OF THE NERVOUS

SYSTEM

THE functions of the body are generally effected by chemical
changes within its protoplasm. These chemical changes in the
aggregate we term "metabolism" and they generally involve a
rather slow interchange of the chemical substances of food and
waste materials between the cytoplasm and the lymph which
surrounds the cells and between the cytoplasm and the proto-
plasm of the nucleus (karyoplasm). The rate of metabolism is
dependent upon many factors, one of which is the time required
for the passage of soluble substances through the cell membrane
and through the nuclear membrane which separates the cyto-
plasm from the karyoplasm.

In the nerve-cells both of these sorts of chemical interchange
are facilitated by the form and internal structure of the cell.
As we have already seen (p. 41), the widely branching dendrites
present a large surface for the absorption of food materials from
the surrounding lymph and the elimination of waste. The
specific nervous functions involve the consumption of living sub-
stance, both in the cell body and in the nerve-fibers. This is
in part an oxidation process, and this phase of the activity can
be roughly measured by the amount of carbon dioxid eliminated.
Until very recently it was not possible to secure any evidence of
CO2 production in nerve-fibers; in view of this and of the further
fact that nerve-fibers seem to be less susceptible to fatigue than
nerve-cells and synapses, many physiologists assumed that
nervous conduction is not a chemical process, but perhaps some
sort of molecular vibration. The conduction of a nervous im-
pulse through a living nerve-fiber is accompanied by an electric
change, the so-called negative variation, which by some physi-
ologists has been identified with the nervous impulse itself.
This and other complicated theories of nervous transmission

96

THE GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM 97

assume that the process is essentially a physical change (prob-
ably of an electric nature) which involves no chemical altera-
tions, no consumption of material, no metabolism.

But by means of recently devised apparatus of extreme deli-
cacy Tashiro has shown very clearly and quantitatively that
the resting nerve-fiber eliminates CO2 and that during functional
activity caused by stimulation the amount of CC>2 is increased
to about double that of the resting nerve. The same investiga-
tor subsequently showed that the amount of CC>2 given off by
nerve-fibers is quite as great per unit of weight as that given off
by the nerve-cell bodies of the ganglia. Tashiro has shown,
moreover, that the rate of C02 production is greater in that por-
tion of a nerve-fiber which lies nearer to the source of the stimu-
lus than in a similar portion of the same nerve-fiber farther from
the receptive end and nearer to the discharging end. This ap-
plies to both sensory and motor fibers. Child has confirmed this
by showing that different parts of the nerve-fiber show differ-
ences in susceptibility to certain poisons corresponding to the
differences in rate of oxidation of their substance. .There is,
accordingly, a physiological gradient in the nerve-fiber, the physi-
ological activity diminishing in the direction of the normal con-
duction of the nervous impulse. The neuron is thus seen to
have an intrinsic physiological polarity of its own quite apart
from that occasioned by the irreversible character of the
synapse (see p. 53).

It is, therefore, probable that the transmission of a nervous
impulse involves a wave of chemical change throughout the
length of the nerve-fiber, though a change of a quite different
character from that occurring in the cell body during its func-
tional activity. That the nervous conduction is not a simple
electric discharge through a free conductor, nor any other
sort of simple ethereal or molecular vibratory wave motion, is
evident from the fact that its velocity of propagation through
the nerve-fiber, which is easily measured, is slower than any
known wave movement of this character.

In the unmyelinated nerves of vertebrates the rate of pro-
gression of the nerve impulse varies from 0.2 to 8 meters per
second; in the myelinated sciatic nerve of the frog it varies
from 24 to 38 meters per second; and in human myelinated
7

98 INTRODUCTION TO NEUROLOGY

nerves it may be as rapid as 125 meters per second. This rate
of conduction of the nervous impulse in peripheral nerves varies
greatly with different animals, with different nerves in the same
animal, and in the same nerve under different physiological con-
ditions.

The reaction time required for the performance of various reflex acts
can be very accurately measured, and it is found that the time of even the
simplest reflex is considerably greater than is required for the transmission
of the nervous impulse through the conductors involved. The average rate
of conduction in human nerves is probably about 120 meters per second, and
the simplest reaction times which have been measured in psychological labor-
atories vary between 0.1 and 0.2 second (from 0.117 to 0.188 for reactions
to touch, and from 0.120 to 0.182 for reactions to sound). The total time
required for transmission of the nervous impulse through the nerve-fibers
involved in these reactions need not exceed 0.02 second, whence it appears
that the greater part of the reaction time is otherwise consumed. A part of
this excess time is required to overcome the inertia of the end-organs
(receptor and effector), and the remainder is used in the central nervous
system. This "central pause" is characteristic of all reflexes and, in fact,
has a profound significance in connection with the evolution of the higher
associational functions of the brain. The introduction of further complexity
in the reaction, of whatever sort, usually lengthens the time of the central
pause, though long training in making a discriminative reaction may reduce
this pause almost to the time of a simple reaction.

Many attempts have been made to determine the central time of reac-
tions of different degrees of complexity by substracting from the total time
in each case the probable time required for the peripheral processes and by
subtracting the total time required for the simpler reactions from the total
time taken in more complex discriminative reactions. But further analysis
(particularly more critical introspection) has shown that in these human
reactions the problem is too complex to be resolved by this method (see
Ladd and Woodworth, 1911, p. 497).

The simpler reflexes of lower vertebrates can be studied physiologically,
and these give data which are much more readily analyzed than the more
complex human reactions. In the case of the simplest reflex obtainable in
the spinal cord of the frog, the central pause was estimated by Wundt to be
only 0.008 second, i. c., all of the time required for the reaction except this
interval was used in the peripheral apparatus. But in a crossed reflex,
where the reaction occurs on the opposite side of the body from the stimu-
lus, the increased complexity of the central process consumed 0.004 second
additional.

Miss Buchanan (1908), with more accurate methods of study, finds in the
frog that the central time varies between .014 and .021 second. She also
measured the additional latent time required for a crossed reflex, and found
it to be of the same order of magnitude MS the latent time of the simple
reflex (instead of half as much as in Wundt's experiments), that is, the
crossed reflex required about twice the l.-itent time in the spinal cord as the
uncrossed reflex. It is assumed that this central pause in the uncrossed
reflex is consumed chiefly in the synapses between the peripheral sensory
and the peripheral motor neurons, and that only one such synapse is in-
volved in each simple reflex connection (a two-neuron circuit, see Fig. 1,

THE GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM 99

p. 25) ; but in the crossed reflex two such synapses are involved (a three-
neuron circuit such as the pathway from d.r.2 to v.r.l' through correlation
neuron 1 in Fig. 61, p. 134), and the introduction of the second synapse
doubles the time. It is, therefore, assumed that it requires in the frog
between .01 and .02 second for the nervous impulse to pass the synapse
between two neurons in a reflex circuit.

Turning now to the activities of the nerve-cell body, it will
be recalled (p. 45) that here the chroraophilic substance is gen-
erally scattered throughout the cytoplasm in the form of the
"Nissl bodies." This substance is very similar to that of the
chromatin of the nucleus, from which it is said to be derived
during the development and functional activity of the neuron.
During the resting state of the cell it and other reserve materials
accumulate in the cytoplasm; and now, when the cell is stimu-
lated to activity, the energy thus stored up may be liberated
almost instantly because the chemical substances necessary for
the reaction are widely diffused throughout the entire mass of
the cytoplasm.

The function of neurons, as compared with that of most other
cells of the body, may, therefore, be described as of the explo-
sive type. A word of explanation will render the analogy clear.
In ordinary combustion, oxygen is supplied to the surface of the
burning material, say a blazing log, and the chemical process
of burning goes on only as fast as the superficial parts can l»e
oxidized and removed. But explosive substances are chemic-
ally so constituted that as soon as combustion begins oxygen is
liberated in the interior of the material and the process of oxi-
dation takes place almost instantaneously throughout the
entire mass. Similarly in the nerve-cell, the processes of metab-
olksm are not dependent upon the slow interchange of substances
through the nuclear membrane between the cytoplasm and the
nuclear plasm; but the chromophilic substance distributed
through the cytoplasm permits of much more rapid responses.
The organization of the protoplasm of the nerve-cell is such
that a very small stimulus may liberate a large amount of energy
with explosive suddenness. The energy thus liberated does
not all leave the cell, but part of it is directed into the axon,
which is thereby excited to conduct a nervous impulse to the
appropriate end-organ or to the next synapse, and thence to a
second neuron.

100

INTRODUCTION TO NEUROLOGY

The conduction of nervous impulses within the central nervous
system in some cases takes place through well-defined and insu-
lated bundles of fibers, which are termed tracts; but in most
cases there is more or less complexity introduced by collateral
avenues of discharge to other specific centers, as in the complex
forms of reflex systems described in Chapter IV, or by a more
diffuse type of irradiation (p. 65). The organization of the
central nervous system is such that in general the excitation of
any peripheral sensory neuron may be transmitted to very di-
verse and remote parts of the brain, each of which may call forth
its own characteristic form of response.

The physiological effects of such a dispersal of an incoming nervous im-
pulse within the central nervous system may be very different, depending

sKin

Fig. 39. — Diagram of an arrangement of neurons adapted for the dis-
tribution of a single afferent nervous impulse to several different motor
organs.

on the connections of the pathways which are taken by the neurons of the
second order. If these pathways diverge so that the stimulus is distributed
among several different effector systems, this would tend to disperse the
energy of the afferent impulse and a relatively strong stimulus is necessary
to call forth a response. This is the situation in case a painful prick on the
skin of the face calls forth reflex movements of, say (1) twitching of the
facial muscles; (2) turning the head away, and (3) a movement of the hand
to remove the irritant. Here the stimulus arising at a single point in the
skin (Fig. 39) is distributed to three widely separated motor centers (M.I,

THE GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM 101

muscle

M.2, M.S). On the other hand, in case the stimulus received by the
neuron of the first order is distributed to several neurons, all of which dis-
charge into the same motor center, the stimulus may be reinforced because
each neuron of the second order may discharge its own reserve energy in
such a way as to send out a stronger impulse than the one received, so that
the total discharge into the motor
center is greatly strengthened (Fig.
40). Such an impulse may be said
to accumulate momentum as it ad-
vances like an avalanche on a moun-
tain slope, and hence this type of re-
action has been termed by Ramon
y Cajal "avalanche conduction."
In some parts of the brain there are
very special mechanisms for this sort
of cumulative discharge, as in the
cortex of the cerebellum (p. 192) and
the olfactory bulb (p. 218).

The intensity of nervous dis-
charge in all of its forms is very
dependent upon the general physio-
logical state of the body, some con-

Fig. 40. — Diagram of the mechan-
ism of reinforcement whereby a
single weak afferent nervous im-
pulse may be received by several
neurons of the second order which
discharge their greatly strengthened
nervous impulses into a single final
common path.

ditions, such as fatigue and various
intoxications, tending to depress the
activity, and other conditions tend-
ing to facilitate it. The main-
tenance of good nervous tone is,
therefore, essential to the highest
efficiency. Some of these physio-
logical agents may also act locally on particular parts of the nervous system
and thus determine the selection of one instead of another out of several
possible modes of response in the variable type of behavior.

Fatigue of nerve-cells may be brought about in two ways,
which have been clearly distinguished by Verworn: (1) by
the consumption of reserve material from which the energy of
the cell is derived more rapidly than this material can be re-
stored, and (2) by the accumulation of waste-products more
rapidly than they can be eliminated from the cell. These forms
of fatigue have recently been named by Dolley respectively
"fatigue of excitation" and "fatigue of depression."

In his interesting discussion of neuro-muscular fatigue, Stiles
(1914, p. 101) enumerates several particular ways (in addition
to the two general methods just mentioned) by which fatigue
may be brought about, among which are the following: (1)
fatigue of muscle-fibers, (2) fatigue of the junction of the motor
nerve with the muscle-fiber at- the motor end-plate (see Fig.
5, p. 40), (3) fatigue of the nerve-fibers, (4) fatigue of the motor

102 INTRODUCTION TO NEUROLOGY

nerve-cells, (5) fatigue of the synapses between the nerve-cells,
(0) fatigue of the sense organs and afferent apparatus, (7) fatigue
of the centers of voluntary control. The first, second, fourth,
and fifth types commonly play a part in ordinary fatigue, the
third is insignificant, and the sixth and seventh may be present.
The synapses and the motor end-plates are probably especially
susceptible to fatigue of depression by toxic substances, and the
muscle-fibers and nerve-cell bodies to fatigue of excitation by
consumption of their material.

A resting neuron when excited to activity at first increases in
size by reason of the stimulus given to general metabolic activ-
ity. The first signs of fatigue result from the exhaustion of the
oxygen supply of the cells; then follows the consumption of the
reserve food materials, chiefly those represented in the chromo-
philic substance, with consequent shrinkage of the Nissl bodies.
In extreme fatigue the ultimate dissolution and death of the cell
may be hastened by the accumulation of toxic products of cell
metabolism.

It appears to be well established by numerous experimental studies that
at the beginning of functional activity both the nucleus and the cytoplasm
of the resting neuron are enlarged, and that with the onset of fatigue
there is a shrinkage, especially of the nucleus, with vacuolation of the
cytoplasm and solution of the Nissl bodies due to the consumption of the
chromophilic substance during activity. The neurofibrils are also said to
be modified during functional activity. After excessive activity they be-
come more slender and apparently increase in number, while during rest and
after hibernation of those animals which have this habit the neurofibrils
become thicker and less numerous.

Cells whose chromophilic substance has been consumed by active
function may after rest return to the normal form; but if the excitation be
carried beyond the stage of normal fatigue, recovery of the neuron is im-
possible and it gradually disintegrates, resulting in the permanent enfeeble-
ment of the nervous system.

The observations of Dollcy have suggested to him that the volume of the
nucleus bears a constant relation to the volume of the cytoplasm in all
resting nerve-cells of the same type. In varying functional states of excita-
tion and depression this mass relation is disturbed in accordance with the
formula: Activity finally results in a disturbance of the normal nucleus-
cytoplasmic relation in favor of the cytoplasm (fatigue of excitation), while
depression resulting from accumulated toxins finally results in a disturbance
of this relation in favor of the nucleus. In short, the depression of the
neuron by any form of intoxication or otherwise gives the converse picture
of structural changes from that presented by fatigue of excitation.

Most of the physiological work which has been (lone upon fatigue has been
directed toward the isolation of special toxic substances such as in Dolley's
scheme would produce "fatigue of depression." It has been shown that

THE GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM 103

prolonged muscular exertion produces toxins (carbon dioxid, lactic acid, and
others) which are dissolved in the blood and exert a profound depressing
influence upon all of the tissues of the body. If the blood of a fatigued
animal be injected into or transfused with a perfectly fresh animal of the
same species, the latter immediately manifests all the signs of fatigue.

It is often taught that a change of work is physiologically equivalent to
complete rest. It is true that, so long as one is well within the limits of
extreme fatigue, a change of work will prolong efficiency far beyond that
which would be possible in continuous activity of a single nervous or mus-
cular mechanism. Nevertheless experiment shows that mental efficiency
is greatly impaired in extreme muscular fatigue, and, conversely, muscular
power is greatly weakened after long sustained mental work. Glandular
secretions are also apparently often reduced in extreme fatigue, thus, for
instance, reducing the efficiency of the digestive organs. These effects are
doubtless clue to the accumulation of toxic products in the blood, producing
a true "fatigue of depression" throughout the entire body.

It has been suggested that the local feelings of muscular fatigue are due
to excitations of the organs of the muscular sense in the muscle spindles (p.
87) ; but the evidence for this does not seem very convincing.

The experiments of Dolley suggest to him, further, that the more highly
differentiated nerve-centers are more susceptible to the structural altera-
tions of fatigue than are those of the lower reflex systems. It is a well-
known fact that sustained mental work produces the subjective evidences
of fatigue more promptly than does muscular work, and that during severe
mental training one is more apt to go "stale" than during physical training.
This principle has been widely recognized in the provision of short work-
ing hours and frequent holidays for pupils and teachers in our schools; it
should be still further extended, especially in commercial and professional
life. Its neglect is in large measure responsible for the prevalence of neu-
rasthenia and other forms of nervous breakdown.

The early fatigue of the higher voluntary centers is particularly evident
in young children, where continuous sustained attention is impossible except
for very short periods. By training, these periods can be greatly length-
ened, the nervous mechanism involved here probably being the acquisi-
tion of a wider range of associations related with the subject which occupies
the focus of attention, so that individual neurons or systems of neurons
which participate in the functional complex may be temporarily rested while
other related systems are brought into maximum activity, without thereby
interrupting the continuous progress of the train of thought.

The neurological basis of sleep is at present wholly unknown,
though the physiological phenomena seem to be in many respects
analogous with those of fatigue. Of the various theories which
have been suggested, the two which have excited greatest interest
are: (1) the belief that some soluble toxin is produced during
waking hours which induces sleep by a process similar to that of
the "fatigue of depression," and (2) the doctrine of the retraction
of the neuron, which teaches that during sleep (and according
to some authors in less measure during fatigue also) the dendrites
of the neurons retract toward their cell bodies and away from

104 INTRODUCTION TO NEUROLOGY

contact with the axons of other neurons with which they are in
synaptic union, thus increasing the resistance to nerve conduc-
tion at the synapse.

Many physiological experiments show that, though the predis-
position to sleep may be brought about by the accumulation of
toxins in the blood or by other general causes, the actual falling
asleep is accompanied by a fall in blood-pressure, which may be
the essential factor in sleep. Fatigue of the vasomotor center
has been suggested as the real physiological cause of sleep. No
adequate proof of any of these theories has been brought for-
ward.

The numerous theories regarding the neurological processes
taking place in the cerebral cortex during the progress of such
mental functions as attention, association of ideas, etc.,
are likewise as yet entirely unproved. It has been suggested
that during cerebral function the resistance of some pathways
may be diminished by the ameboid outgrowth of the dendrites
so as to effect more intimate synaptic union with the physiolog-
ically related neurons, while the resistance of other paths may
be increased by the retraction of dendrites from their synapses.
Others believe that the neuroglia may participate in the process
by thrusting out ameboid processes between the nervous ter-
minals in the synapses and thus increasing the resistance.
Lugaro has suggested a different interpretation, in accordance
with which during sleep there is a generally diffused extension
of all nervous processes, thus providing for the uniform diffusion
of incoming stimuli, while in the state of attention all of these
processes retract save those which are directed in some definite
direction, thus narrowing the stream of nervous discharge so as to
intensify it and direct it into the appropriate centers. There is
no direct evidence for any of these theories, and the scientific-
ally correct attitude toward them is frankly to admit that at
present we do not know what physiological processes are in-
volved in any of these functions.

Summary. — The forms assumed by neurons are shaped in
part by their nutritive requirements and in part by their func-
tional connections. The metabolism of nervous protoplasm, as
measured by its COz output, is found to be as active in nerve-
fibers as in the cell bodies. In a nerve-fiber the metabolic

THE GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM 105

activity is found to be greatly increased during the transmission
of a nervous impulse ; and nervous conduction evidently involves
a chemical change in the conducting fiber. The rate of trans-
mission of a nervous impulse depends on the structure and
physiological state of the nerve-fiber involved. The metabolic
activity of the nerve-cells is of a very different sort from that of
nerve-fibers, and may be characterized as of the explosive type.
There are at least two factors involved in the fatigue of the
nervous system: (1) fatigue of excitation, resulting from the
consumption of the materials of its protoplasm, and (2) fatigue
of depression, resulting from the accumulation of toxic products
of cellular activity. Each of these processes produces its own
very special series of morphological changes in the neurons.
The neurological functions involved in sleep and the higher
mental processes are as yet unknown.

LITERATURE

BUCHANAN, FLORENCE. 1908. On the Time Taken in Transmission of
Reflex Impulses in the Spinal Cord of the Frog, Quart. Jour. Exp. Physiol.,
vol. i, pp. 1-66.

CHILD, C. M. 1914. Susceptibility Gradients in Animals, Science, N. S.,
vol. xxxix, No. 993, pp. 73-76.

DOLLEY, D. H. 1911. Studies on the Recuperation of Nerve-cells After
Functional Activity from Youth to Senility, Jour. Med. Research, vol. xxiv,
pp. 309-343.

— . 1914. On a Law of Species Identity of the Nucleus-plasma Norm
for Nerve-cell Bodies of Corresponding Types, Jour. Comp. Neur., vol.
xxiv, pp. 445-501.

— . 1914. Fatigue of Excitation and Fatigue of Depression, Intern.
Monatsschrift f. Anat. u. Physiol., Bd. 31, pp. 35-62.

DONALDSON, H. H. 1899. The Growth of the Brain, New York, chapters
xiv to xvii.

HODGE, C. F. 1892. A Microscopical Study of Changes Due to Func-
tional Activity in Nerve-cells, Jour. Morphology, vol. vii, pp. 95-168.

LADD, G. T., and WOODWORTH, R. S. 1911. Elements of Physiological
Psychology, New York.

STILES, P. G. 1914. The Nervous System and Its Conservation, Phila-
delphia.

TASHIRO, S. 1913. Carbon Dioxide Production from Nerve-fibers when
Resting and when Stimulated; a Contribution to the Chemical Basis of
Irritability, Amer. Jour, of Physiol., vol. xxxii, pp. 107-136.

TASHIRO, S., and ADAMS, H. S. 1914. Carbon Dioxide Production from
the Nerve-fiber in a Hydrogen Atmosphere, Amer. Jour, of Physiol., vol.
xxxiv, pp. 405-413.

— . 1914. Comparison of the Carbon Dioxide Output of Nerve-
fibers and Ganglia in Limulus, Jour, of Biological Chemistry, vol. xviii,
pp. 329-334.

CHAPTER VII

THE GENERAL ANATOMY AND SUBDIVISION OF THE
NERVOUS SYSTEM

ON merely topographic grounds the nervous organs are divided
into the central nervous system, or axial nervous system, compris-
ing the brain and spinal cord, and the peripheral nervous system,
including the cranial and spinal nerves, their ganglia and periph-
eral end-organs, and the sympathetic nervous system. The
nerves are simply conductors, putting the end-organs into phys*-
iological connection with their respective centers. The general
form of the human central nervous system and its connections
with the peripheral nerves are seen in Fig. 41. The nerves
connected with the spinal cord are the spinal nerves, those con-
nected with the brain are the cranial or cerebral nerves, and
both of these systems of nerves together are called the cerebro-
spinal nerves, in contrast with the sympathetic nerves, which
latter may or may not be connected with the central nervous
system (see p. 225).

The central nervous system is the great organ of correlation
and integration of bodily processes. Its primitive form in verte-
brates is a simple tube, and this is the form shown in an early
human embryo (see Fig. 46, p. 116). The original tubular form
is but little modified in the trunk region of all vertebrates, where
the spinal cord (medulla spinalis) is formed by a tolerably uni-
form thickening of the lateral walls of the tube (see Figs. 41, 58).
But in the head region the brain (enccphalon) is formed by the
very unequal thickening of different parts of the walls of the tube
and by various foldings brought about thereby. The general
arrangement of the human central nervous system at successive
stages of development is seen in Figs. 47-51.

The external form of the brain has been shaped by the space
requirements of the nerve-cells and fibers which make up its
substance. A group of nerve-cells which performs a single
function is often spoken of as the "center" of that function; but

106

-/ - I CERVICAL KERVB

MIDDLE CERVICAL SYMPATHETIC
GANGLION

I THORACIC SERVE

OANOLIATED CORD

GAXGLIOX - -

WCOCCYGEAL SERVE

FILC.V TERMIXALE

Fig. 41. — The human central nervous system from the ventral side,
illustrating also its connections with the cerebro-spinal nerves and with
the sympathetic nervous system, the latter drawn in black. (After Allen
Thompson and Rauber, from Morris' Anatomy.)

107

108 INTRODUCTION TO NEUROLOGY

it should be borne in mind that this does not imply that this
function resides exclusively in that place. These functions are
all more or less complex and the "center" is usually the region
where various nervous impulses are received and redistributed;
it is, therefore, roughly analogous with the switchboard of an
electric plant.

The nerve-fibers which conduct nervous impulses toward a
given center are called afferent, and those which conduct away
from the center are called efferent with reference to that center.
Most of the peripheral nerves are mixed, in the sense that they
carry both afferent and efferent fibers with reference to the
central nervous system. The efferent fibers may excite move-
ment in muscles (motor fibers) or secretion in glands (excito-
glandular fibers); other efferent fibers which check the action
of the organ to which they are distributed are called inhibitory
fibers. The afferent fibers of the peripheral nerves are often
called sensory fibers, though it must be borne in mind that theif
excitation is not always followed by sensations or other conscious
processes.

The vertebrate nervous system when examined in the fresh
condition is found to be made up of white matter (substantia
alba) and gray matter (substantia grisea), the white matter
containing chiefly nerve-fibers with myelin sheaths (see p. 46)
and the gray matter nerve-cell bodies and unmyelinated fibers.
The centers are, therefore, generally gray in color and the inter-
vening parts of the central nervous system are white.

A group of nerve-cells constituting a center as above described is often
called a "nucleus," a term which has nothing to do with the nuclei of the
individual cells (see p. 39) of which the center is composed. Some critical
writers use the word "nidulus" (originally suggested by C. L. Herrick) or
"nidus" (Spitzka) for such a center, thus avoiding the ambiguity in the use
of the word nucleus. The term "ganglion" is also sometimes used for nuclei
or centers within the brain (ganglion habenulae, ganglion interpedunculare,
etc.), but this usage is objectionable, for the use of the word ganglion in
vertebrate neurology should be restricted to collections of neurons outside
the central nervous system, such as the ganglia of the cranial and spinal
nerves and the sympathetic ganglia.

A nucleus from which nerve-fibers arise for conduction to some remote
part of the nervous system is called the nucleus of origin of these fibers;
conversely, a nucleus into which nervous impulses are discharged by fibers
arising elsewhere is the terminal nucleus of those fibers. Any correlation
center is, therefore, a terminal nucleus for its afferent fibers and a nucleus
of origin for its efferent fibers.

ANATOMY AND SUBDIVISION OF NERVOUS SYSTEM 109

The centers or nuclei within the brain are of two general sorts :
(1) primary centers and (2) correlation centers. The primary
centers are directly connected with peripheral nerves, either as
terminal nuclei of afferent fibers or as nuclei of origin of efferent
fibers (see pp. 42, 108). The elements out of which most acts are
compounded are reflexes (see p. 56), and in the simplest of these
reflexes a sensory nervous impulse received from the periphery
by a terminal nucleus may be passed on to a nucleus of origin and
thence directly to the organ of response; but in more complex
reflexes the incoming nervous impulse is first transmitted from
the terminal nucleus to a correlation center, where it may meet
other types of sensory impulses and then be discharged into any
one of several possible motor pathways. For illustrations of
these types of connection see Chapter IV.

In general, ganglia or nerve-centers are interpolated in con-
duction pathways only where some complication of the reaction
is to be provided. The conduction path is usually here inter-
rupted by synapses and various forms of correlation or coordina-
tion mechanisms are present (see p. 35 and Chapter IV).
Many of the sympathetic ganglia provide the mechanism for
local reflexes in which the central nervous system does not par-
ticipate (p. 225). The spinal ganglia (see Fig. 1, p. 25) are
often regarded as merely trophic centers for the maintenance of
the fibers of the peripheral nerves; but they evidently have
functions of correlation in addition to this, for numerous syn-
apses between sympathetic and cerebro-spinal neurons occur
here (see p. 228 and Fig. 109) which play a part in the correla-
tion of visceral and somatic reactions.

The primary centers and the simpler correlation centers of the
brain can be studied much more readily in the brains of fishes,
which lack the cerebral cortex whose enormous development in
the human brain has obscured the relations and connections of
the more primitive reflex apparatus. Figures 42, 43, and 44 illus-
trate the relations of the principal sense organs to the brain in a
small shark, the common marine dogfish. Figures 42 and 43
(on the right side) illustrate the arrangement of the principal
roots and branches of the cranial nerves. On the left side of
Fig. 43 the relations of the nose, the eye, and the ear to the

110

INTRODUCTION TO NEUROLOGY

brain are indicated; and Fig. 44 shows an enlarged side view
of the brain and the sensory roots of the cranial nerves.

Fig. 42. — Dissection of the brain and cranial nerves of the dogfish,
Scylliurn catulus. The right eye has been removed. The cut surfaces
of the cartilaginous skull and spinal column are dotted, d.l-d.5, Bran-
chial (gill) clefts; ep., epiphysis; exl.rect., external rectus muscle of the
eyeball; gl.ph., glossopharyngeal nerve; hor.can., horizontal semicircular
canal; hy.mnd.VII, hyomandibular branch of the facial nerve; inf. obi.,
inferior oblique muscle; int.rect., internal rectus muscle; lal.vag., lateral
line branch of the vagus nerve; mnd.V, mandibular branch of the trigeminal
nerve; mx.V, maxillary branch of trigeminus; olf.cps., olfactory capsule;
G//.S., olfactory sac; oph.V.VII, superficial ophthalmic branches of the
trigeminal and facial nerves; pfilh., trochlear nerve (patheticus); fil.VII,
])alatine branch of facial nerve; s.obl., superior oblique muscle; .s/>.ro.,
spinal cord; .s'/nr., spiracle; s.rect., superior rectus muscle; vag., vagus nerve;
rr .<;/., vestibule. (After Marshall and Hurst, from Parker and Harwell's
Zoology.)

In fishes there is a system of small sensory canals widely dis-
tributed under the skin. These contain sense organs somewhat
similar to those in the semicircular canals of the internal ear, and

ANATOMY AND SUBDIVISION OF NERVOUS SYSTEM 111

their functions are probably intermediate between those of the
organs of touch in the skin and those of the internal ear, respond-

Olfactory bulb

Olfactory nerve

(n.I)
Somatic area

r. ophthal. superfic. V
r. ophthal. superfic. VTI

n. terminalis

r. ophthal. profundus V

Optic nerve (n. II)

r. mnxillaris V
r. mandib. V

Supra-orbital trunk

Infra-orbital trunk
Ganglion V
'. palatinus VII
-Gang. I'cnifiili VII
Gang, later. VII
r. prespirac. VII

Spiracle

r. hyomandib. VII

n. IX
n. X
r. lateralis X

r. branrhialis X
r. intestinalis X

Fig. 43. — Diagram of the brain and sensory nerves of the smooth dog-
fish, Mustelus canis, from above. Natural size. The Roman numerals
refer to the cranial nerves. The olfactory part of the brain is dotted, the
visual centers are shaded with oblique cross-hatching, theacoustico-lah-ral
centers with horizontal lines, the visceral sensory area with vertical lines,
and the general cutaneous area is left unshaded. On the right side the
lateral line nerves are drawn in black, the other nerves are unshaded.

112

INTRODUCTION TO NEUROLOGY

ing to water vibrations of slow frequency and probably assisting
in the orientation of the body in space. These are the lateral
line canals. They are innervated by special roots of the VII and
X pairs of cranial nerves (the lateralis roots of these nerves),
which are drawn in black in Figs. 43 and 44. The other nerves
are lightly shaded or white. The lateral line organs and their
nerves are entirely absent in higher vertebrates (see p. 199).

The lateral line nerves and the acoustic nerve (VIII pair) in
fishes terminate in a common center within the brain (the acous-
tico-lateral area), which is shaded with horizontal cross-hatch-
ing in Figs. 43 and 44. The nerves of general cutaneous sen-
sibility also terminate in a particular region which is unshaded

Optic lobe

Epithalamua

Thalam

Supra-orbital trunk

Acoustico-lateral area
Gang, lateralis VII

Hypothalam

Ganglion V
Infra-orbital trunk

hi

r. hyomandibularis VII

Spiracle

r. palatinus VII

Ganglion geniculi VII

Fig. 44. — The same brain as Fig. 43 seen from the side and slightly enlarged.

and marked "general cutaneous area." The visceral nerves
from the gills, stomach, etc., all enter a single "visceral area,"
which is shaded with vertical lines. The eye is also connected
with a special region in the midbrain, the "optic lobe," which is
shaded with oblique cross-hatching; and the nose is connected
with a part of the forebrain which is stippled.

We may, therefore, recognize in this fish a "nose brain," an
"eye brain," an "ear brain," a "visceral brain," and a "skin
brain," each of these peripheral organs having enlarged primary
terminal nuclei which make up definite parts of the brain sub-
stance. Remembering that the primitive brain was a simple
tubular structure, we observe that each one of the chief sense
organs and each group of similar sense organs sends sensory

ANATOMY AND SUBDIVISION OF NERVOUS SYSTEM 113

nerves inward to terminate in a special part of the wall of the
primitive neural tube, and that here a thickening of the wall of
the tube has taken place to provide space for the appropriate
terminal nucleus. It may be noticed, further, that all of these
structures (except a part of the olfactory centers) lie in the
dorsal part of the brain. An examination of the primary motor
centers would show that they are distributed in a somewhat
similar fashion along the ventral part of the brain.

The facts just recounted give a clear picture of the pattern of
functional localization of the primary reflex centers in a simple
type of brain, and they show that all of the more obvious parts
of this brain except the cerebellum are in simple direct relation
with particular peripheral organs. In other words, nearly the
whole of this brain is directly concerned with simple reflexes
and (aside from the cerebellum) no large centers for the higher
types of adjustments are present. The primary reflex centers
are found to be arranged in accordance with essentially the same
pattern in the human and all other higher brains, though in these
cases the pattern is slightly modified and obscured by the pres-
ence of greatly enlarged correlation centers, of which the cerebral
cortex is the chief. The structure and significance of the cere-
bral cortex form the theme of the last three chapters of this work.

The central nervous system of the earliest vertebrates was
probably a simple longitudinal tube of nervous tissue with which
the peripheral nerves were connected in a segmental fashion (see
p. 28). This is the permanent form of the spinal cord and ite
nerves in all vertebrates (see p. 125 and Fig. 41). In the brain
the enlargement of the primary reflex centers and of the corre-
lation centers directly related to them has changed the form of
the tube and disturbed the primitive segmental arrangement of
the cranial nerves, as is indicated in Figs. 43 and 44. Never-
theless, this more ancient part of the brain is sometimes called
the segmental apparatus, to distinguish it from two very large
coordination and correlation mechanisms which are of later
evolutionary origin, namely, the cerebellar cortex and the
cerebral cortex, which are termed suprasegmental structures.
The segmental apparatus is often called the brain stem. It
includes practically all of the fish brain (Figs. 43 and 44) except
the cerebellum, for in these animals there is no cerebral cortex.

8

114

INTRODUCTION TO NEUROLOGY

If in the human brain we dissect away the cerebral cortex and
the cerebellar cortex and the white matter immediately con-

Nucleus lentiformis

Capsula interna

(pars lenticulo-

caudata)

Tractus. __
olfactorius

Tractus ppticus '" \^-^f ! '"
Infundibulum-'''/^^ ' /

Hypophy- / anterior lobe ''' ^^B>' / /
sis ceiebri \ posterior lobe- ^i / / ,{

Tuber cinereum/// \

Corpus mamillare// /i

N. oculomotorius / /' }

Basis pedunculi' / ,'

Pons7 / X

Nervus trigeminus (portio major)xx "
Nervus trigeminus (portio minor)-' __--
N. facialis" "J^
N. intermedius^'-''^

N. abducens

N. glossopharyngeus^.-
Nervus vagus j ,,

Pytamis""

Oliva-

Fasciculus circumolivaris pyramidis "

Capsula interna (pars lenticulo-thalamica)
I Nucleus caudatus

, Nucleus amygdalae (cut)

Commissure anterior
terminalis

Capeula interna (pars
sublenticularis)
Nucleus caudatus

^Thalamus

_Corpus geniculatum

laterale

-Corpus pineale
-Cor. geniculatum mediate
-Colljculus superior
-Colliculus inferior

Lemniscus lateralis
Nervus trochlearis
Brachium conjunctivum

Brachium
N, pontis

^~_ Fossa flocculi
— Cni8 flocculi
| — Nucleus denta-
tus cerebelli

Corpus ponto-bulbare

Fasciculus spinocerebellaris

-__ Nervus spinalis

Fig. 45. — Left lateral aspect of a human brain from which the cerebral
hemisphere (with the exception of the corpus striatum, the olfactory bulb
and tract, and a small portion of the cortex adjacent to the latter) and the
cerebellum (excepting its nucleus dentatus) have been removed. The
brain stem (segmental apparatus, palaeencephalon) includes everything
here shown with the exception of the strip of cortex above the tractus
olfactorius and the nucleus dentatus. Within its substance, however, are
certain cortical dependencies (absent in the lowest vertebrates), which
have been developed to facilitate communication between the brain stem
and the cerebral cortex. The chief of these are found in the thalamus,
basis pedunculi, and pons. Compare this figure with the side view of the
intact brain, Fig. 54. (Modified from Cunningham's Anatomy.)

nected therewith we have the form shown in Fig. 45.
the human brain stem.

This is

ANATOMY AND SUBDIVISION OF NERVOUS SYSTEM 115

The cerebellum appears in the evolutionary history of the
vertebrate brain much earlier than the cerebral cortex; its
functions are wholly reflex and unconscious (see pp. 158, 186)
and are concerned chiefly with motor coordination, equilibra-
tion, and, in general, the orientation of the body and its members
in space. Its activities are of the invariable, innate, structurally
predetermined type (see pp. 22, 31, 78). The cerebral cortex,
on the other hand, is the organ of the highest and most plastic
correlations, which are in large measure individually acquired.
It attains its maximum size in the human brain.

In recognition of the late phylogenetic origin of the cerebral
cortex Edinger has called the entire brain stem and cerebellum
the old brain (palaencephalon), and the cerebral cortex and
parts of the brain developed in relation therewith the new brain
(neencephalon).

The terminology of the brain is in great confusion. Most of
the more obvious parts were named before their functions were
known, the same part often receiving many different names, and
sometimes the same name being applied to very different parts.
To remedy this situation the German Anatomical Society in
1895 published an official list of anatomical terms which is known
as the Basle Nomina Anatomica (commonly abbreviated as
B. N. A.). Each of these terms has a clearly denned significance
and they are now very widely used, though many anatomists
continue to use some older and unofficial names. The B. N. A.
terms or their English equivalents are used in this work, save in
a few cases which are specifically mentioned. The terminology
of the brain is based upon the embryological researches of Pro-
fessor His, and can best be outlined by reviewing the form of the
human brain at a few selected stages of development.

The B. N. A. terminology was developed with exclusive reference to the
human body. The names of many parts of the bodies of other animals than
man and of microscopic structures in general are not included. The names
of this list are all used and defined in W. Krause's Handbuch der Anatomic
des Menschen, Leipzig, 1905, and in most of the recent American and
English text-books of anatomy. At the end of Krause's book is a very com-
plete list of synonyms, including most of the anatomical terms in use and
their B. N. A. equivalents.

Following the example of many other recent anatomists, we shall in this
work replace the B. N. A. term "anterior" (on the front or belly side) by the
word "ventral," and the B. N. A. term "posterior" (on the back side) by

116

INTRODUCTION TO NEUROLOGY

the word "dorsal." The head end of the body will be referred to as the
"anterior" or "cephalic" end; the other end of the body as the "posterior"
or "caudal" end. The terms "upper" or "higher" and "lower" will refer
to the relations in the erect human body. In the nomenclature of the
medulla oblongata (see p. 122) and of the thalamus (p. 167) our usage
departs slightly from that of the B. N. A. Regarding the naming of fiber
tracts see page 128.

Figure 46 illustrates the form of the brain in a very early
human embryo. Its tubular form is very evident, and in the

-Optic ocsicle

Futvrt poritint
f/enure

Pallium
Anterior neuropore

Maencephalon
Isthmus

Mtsencephahn

Optic rtcai

Future panting
Rhombtnciptidon flexure

Fig. 46. — An enlarged model of the brain of a human embryo 3.2 mm.
long (about two weeks old). The outer surface is shown at the left, and
on the right the inner surface after division of the model in the median
plane. The Anterior neuropore marks a point where the neural tube
is still open to the surface of the body. The Pallium is the region from
which the cerebral cortex will develop. The Optic recess marks the
portion of the lateral wall of the Diencephalon from which the hollow
Optic vesicle has evaginated. (After His, from Prentiss' Embryology.)

brain the diameter of the tube is but little greater than that
of the spinal cord. The walls are thin and the cavity wide.
In a slightly older embryo the form is shown in Fig. 47, and
Fig. 48 illustrates diagrammatically the median section of an
embryo of about the same age as that shown in Fig. 47, upon
which the regions as denned by the B. N. A. are indicated. The

ANATOMY AND SUBDIVISION OF NERVOUS SYSTEM 117

A

a

I fblliunt .Thalama

r-^r-^K^J \ I

Mamtfhalai

Centillium

Myelencepholal —

Fig. 47. — Reconstruction of the brain of a 6.9 mm. human embryo
(about four weeks old): A, Lateral view; B, in median sagittal section;
Ceph.flex., cephalic flexure. (After His, from Prentiss' Embryology.)

Hypophysis (anterior lobe) - -

Ventro-lateral plate —
Dorso-lateral plate---

]

Fig. 48. — Diagram of the inner surface of the human brain, based on a
specimen of about the same age as shown in Fig. 47. The shaded area is
the ventro-lateral plate of the neural tube, giving rise to the motor centers.
Its upper boundary is marked by a groove on the ventricular surface, the
sulcus limitans, which separates the ventro-lateral plate from a dorso-
lateral plate (unshaded), which gives rise to the sensory centers and chief
correlation centers. (After His, from Morris' Anatomy.)

Cerebral ptduncln

Hypothalamta -•'-
Epithalamui
Tkalam
Diencephalon
(Inter-brain)

Cerebral aqueduct

/ Mrsencrphalon
' . (Uid-brain)

. (After-brain)

Corf,

Rhincnccfhahn, ''terminates slriat,
(Oljactory-braii,)

Fig. 49. — Vertical median section of a model of the brain of a human
embryo 13.6 mm. long: 1, Optic recess, marking the attachment of the
optic vesicle; 2, ridge formed by the optic chiasma; 3, optic chiasma;
4, infundibular recess. The limiting sulcus is visible in the model, though
not named, running upward from the optic recess between the thalamus and
the hypothalamus. (After His, from Sobotta's Atlas of Anatomy.)

Pallium

Corpus stratum. U

Epithalamus (Corpus plneale)
' Metathalamus
(Corpora geniculata)

Corpora quadrlgemina

• Pedunculus cerebri

Cerebellum
---Fossa rbomboidea

Medulla oblongata

Rhinencephalon / .-' ,-
Pars optica hypothalaml ,-' /
Chiasma opticum'' ,''
Hypophysis-'

Pars mamillaris bypothalami

Pons [Varoli] '

Fig. 50. — A vertical median section of a model of the brain of a human fetus
in the third month. (After His, from Spalteholz's Atlas.)
118

ANATOMY AND SUBDIVISION OF NERVOUS SYSTEM

119

Thalamus

Khinencephalon .

Rcccssus opticus

Chiasma opticum ./ /
ecesaus infundibuli ' /

Infundibnhi

Pedunculus cereb

Pons [Varoli]

< Velum medul-
lare anterius

irebcllum

Ventriculus quartus
edulla oblongata

Fig. 51. — Vertical median section of the adult human brain. (From Spalte-

holz's Atlas.)

SULCUS CINGUL1
(marginal portion)

SCBPARIETA1. SULCUS
PAKIETO-OCCin-
TAL

CENTRAL SULrrs (ROLAXHT)
i MASS A WTBJUIEDU

I SULCUS CIKGULt
I dub/ron tal par lion)

I

sui.crx conpoRis

CALLOSI

HESEXCEPIIA LOU

TVBBS CIXSBBVit

\ ' POSTERIOR PAROLFACTORT SPtCls
*SUH-CALLOSAL OYBUS (PBDVHCLE OF
\ CORPUS CALLOSOIf)

INFVUDIBULDM

Fig. 52. — Vertical median surface of the adult human brain. (After Toldt,
from Morris' Anatomy.)

120

INTRODUCTION TO NEUROLOGY

The brain as a whole is the encephalon, and its chief divisions
are indicated by prefixes having a topographic significance ap-
plied to this word. In Fig. 48 the ventral part of the neural tube
is shaded to indicate the region in which the motor centers of the
adult brain are found. The unshaded part of the figure indi-
cates the region devoted to the primary sensory centers and the

Optic chiasma

Infundibulum
Left corpus mamillarc

Substantia perforata
posterior

Pedunculus cerebri

Olfactory bulb

Olfactory tract

Optic nerve

Substantia perfora-
ta anterior

Optic tract

Tuber cinereum

Abducens nevre

Hypoglossal nerve

Trochlear nerve

medius
Glossopharyngeal nerve

Vagus nerve

Medulla oblongata
Medulla spinalis (cut)

Accessory nerve
Hypoglossal nerve

Fig. 53. — Ventral view of the adult human brain. Compare Fig. 41.
(From Cunningham's Anatomy.)

correlation centers related to them. The sensory and motor
regions are separated in early embryologic stages by a longi-
tudinal limiting sulcus (the sulcus limitans). Comparison with
the figures of later stages which follow shows that the supra-
segmental structures are developed wholly from the sensory
region. Figures 49 and 50 illustrate later stages of develop-

ANATOMY AND SUBDIVISION OF NERVOUS SYSTEM 121

ment and Fig. 51 the adult brain in median section. The exter-
nal form of the adult brain is illustrated also in Figs. 52, 53, 54.

TBAXS-
TEKSB
OCCIPI-
TAL SOL-

cvs

Fig. 54. — View of the left side of the adult human brain. • Some of the
principal sulci and gyri are named. The lateral cerebral fissure (sylvian
fissure) is not named; it lies immediately above the gyrus temporalis supe-
rior. (After Toldt, from Morris' Anatomy.)

The following table summarizes the relations of the subdivis-
ions of the brain (the ventricles of some of them being added in
parentheses), to which a few comments are here added:

Rhombencephalon, rhombic brain (fourth ventricle).
Myelencephalon, medulla oblongata.
Metencephalon.
Cerebellum.
Pons.

Isthmus rhombencephali.
Cerebrum.

Mesencephalon, midbrain or corpora quadrigemina and cerebral peduncles

(aqueduct of Sylvius).
Prosencephalon, forebrain.

Diencephalon, betweenbrain (third ventricle).
Hypothalamus.
Thalamus.
Metathalamus.
Epithalamus.
Telencephalon, endbrain.
Pars optica hypothalami.
Hemisphseria, cerebral hemispheres (lateral ventricles).

122 INTRODUCTION TO NEUROLOGY

The isthmus is a sharp constriction which separates the brain
into two major divisions, the rhombencephalon behind and the
cerebrum in front. In the B. N. A. 'table the isthmus is regarded
as a transverse segment or ring; it might better be regarded
simply as a plane of separation between the rhombencephalon
and cerebrum. In the table the medulla oblongata is regarded
as synonymous with myelencephalon, that is, the region between
the pons and the spinal cord. The older usage, which is still
widely current, regards the medulla oblongata as including
everything between the isthmus and the spinal cord except the
cerebellum dorsally and the fibers and nuclei of the pons and mid-
dle peduncle of the cerebellum ventrally. This is the old or seg-
mental part of the rhombencephalon, and the cerebellum and
pons fibers related to it are added to this primitive medulla
oblongata. The older usage is preferable to the B. N. A. division
and will be adopted here, for the medulla oblongata as here
defined is a structural and functional unit, whose form is not
modified in those animals which almost totally lack the cere-
bellum and its middle peduncle. The midbrain (mesencephalon)
is the least modified part of the neural tube in the adult brain.
The betweenbrain (diencephalon) has three principal divisions:
(1) below is the hypothalamus; (2) above is the epithalamus;
(3) between these is the thalamus which includes the thalamus
and metathalamus of the table (see p. 167). The hypothalamus
and epithalamus are highly developed in the lowest vertebrates
and are related to the olfactory apparatus; in these brains the
thalamus proper is very small, this part increasing in size in the
higher animals parallel with the evolution of the cerebral cortex.
The thalamus proper is really a sort of vestibule to the cere-
bral cortex; all nervous impulses which reach the cortex, except
those from the olfactory organs, enter it through the thalamus.
The endbrain (telencephalon) includes the cerebral hemispheres
and a very small part of the primitive unmodified neural tube
to which the hemispheres are attached, this being the pars
optica hypothalami of the table or, better, the telencephalon
medium.

If now we compare this subdivision of the human brain with
our rough functional analysis of the fish brain (p. 112), we notice
that the "ear brain" (acoustico-lateral area), "skin brain" or

ANATOMY AND SUBDIVISION OF NERVOUS SYSTEM 123

"face brain" (general cutaneous area), and "visceral brain" (vis-
ceral area) are all contained in the rhombencephalon, whose seg-
mental or stem portion is made up of these centers and the
corresponding motor centers. The same relations hold in the
human brain, and in both cases the cerebellum (and in man the
pons in the narrower sense in which I use that term) is added as
a suprasegmental part. In both cases the "eye brain" includes
the retina of the eye, the optic nerve, and a part of the roof of
the midbrain. In the fish a very small part of the thalamus
(not indicated on Figs. 43 and 44) also receives fibers from the
optic nerve. In man this optic part of the thalamus is greatly
enlarged, forming so large a part of that structure in fact that
the thalamus as a whole is often called the optic thalamus. It
should be remembered, however, that even in man the optic
centers comprise only a part of the thalamus. The "nose brain"
of the fish comprises most of the cerebral hemispheres (all except
the small "somatic area" of Fig. 44), and all of the epithalamus
and hypothalamus. In man these parts remain essentially un-
changed, but the "somatic area" of the hemisphere has greatly
enlarged to form the large corpus striatum and the enormous
cerebral cortex, the latter forming the suprasegmental apparatus
of the telencephalon, and greatly modifying the form relations
of all adjacent parts.

The details of the development of the brain lie outside the
scope of this work, as also do the anthropological questions grow-
ing out of the statistical study of brain weights1 and measure-
ments. These and many other topics of fundamental impor-
tance are presented in a very interesting way in Donaldson's
book on The Growth of the Brain.

Summary. — In all vertebrates the central nervous system is
fundamentally a hollow dorsal tube in which the primary seg-
mentation is subordinated to the development of important
longitudinal correlation tracts and centers. This tube is en-
larged at the front end to form the brain. The vertebrate brain
may be divided on physiological grounds into great divisions,

1 The weight of the brain is exceedingly variable, even in a homogeneous
population. The average weight of the normal adult European male brain
is commonly stated to be 1360 grams (48 oz.), and that of the female 1250
grams (44 oz.).

124 INTRODUCTION TO NEUROLOGY

first the brain stem, or primary segmental apparatus; and second
the cerebellum and cerebral cortex, or suprasegmental apparatus.
The brain stem and cerebellum are devoted chiefly to reflex and
instinctive activities and constitute the "old brain" of Edinger.
The cerebral cortex is devoted to the higher associations and
individually acquired activities and is called the "new brain"
by Edinger. No nervous impulses can enter the cortex without
first passing through the reflex centers of the brain stem.

In fishes the form of the brain is shaped almost wholly by the
development of the reflex centers, and here these mechanisms
can best be studied, each of the more obvious parts of the brain
being dominated by a single system of sensori-motor reflex cir-
cuits. The same pattern is preserved in the human brain, but
much distorted by the addition of the centers of higher correla-
tion.

The terminology of the brain now in most common use is
based on its embryological development, which is briefly re-
viewed.

LITERATURE

BARKER, L. F. 1907. Anatomical Terminology, Philadelphia.

DONALDSON, H. H. 1899. The Growth of the Brain, a Study of the Ner-
vous System in Relation to Education, New York.

EDINGER, L. 1908. The Relations of Comparative Anatomy to Com-
parative Psychology, Jour. Comp. Neur., vol. xviii, pp. 437-457.

HERRICK, C. JUDSON. 1910. The Morphology of the Forebrain in
Amphibia and Reptilia, Jour. Comp. Neur., vol. xx, pp. 413-547.

His, W. 1895. Die anatomische Nomenclatur: Nomina Anatomica,
Archiv f. Anat. und Physiol., Anat. Abt., Supplement-Band.

JOHNSTON, J. B. 1906. The Nervous System of Vertebrates, Philadel-
phia.

— . 1909. The Central Nervous System of Vertebrates, Ergebnisse und
Fortschritte der Zoologie, Bd. 2 Heft 2, pp. 1-170.

— . 1909. The Morphology of the Forebrain Vesicle in Vertebrates, Jour.
Comp. Neur., vol. xix, pp. 457-539; also important papers on the same sub-
ject in later volumes of The Journal of Comparative Neurology.

KEIBEL, F., and MALL, F. P. 1912. Manual of Human Embryology,
Philadelphia, vol. ii, pp. 1-156.

KRAUSE, W. 1905. Handbuch der Anatomie des Menschen, mit einem
Synonymenregister, auf Grundlage der neuen Baseler anatomischen Nomen-
clatur, Leipzig.

RETZIUS, G. 1896. Das Menschenhirn, 2 vols., Stockholm.

SHERRINGTON, C. S. 1906. The Integrative Action of the Nervous Sys-
tem, New York.

CHAPTER VIII
THE SPINAL CORD AND ITS NERVES

THE spinal cord (medulla spinalis) is the least modified part
of the embryonic neural tube, and the spinal nerves constitute
the only part of the nervous system in which the primitive seg-

Posterior divisio

Rami communicantes
Sympathetic ganglion

Anterior cutaneous n.

Fig. 55. — Diagram of a typical spinal nerve in the thoracic region.
The spinal column and the muscles are shown in gray, the nerves and their
ganglia in black. (Modified from Gray's Anatomy.)

mental pattern is clearly preserved in the adult body (see p.
113) The spinal nerves are connected with the spinal cord in
serial order, a pair of nerves for each vertebra of the spinal
column (see Fig. 41, p. 107).

125

126

INTRODUCTION TO NEUROLOGY

Each spinal nerve distributes efferent (motor) fibers to the
muscles and afferent (sensory) fibers to the skin and deep tissues
of its appropriate segment of the body, and through its connec-
tions with the sympathetic nervous system it may effect various
visceral connections (Figs. 55 and 56). The efferent fibers
leave the cord through the ventral roots of the spinal nerves,
these fibers arising from cells within the gray matter of the cord,
and the afferent fibers enter through the dorsal roots, these

Dorsal root

Lateral column

Ventral column . . , „ _ , . ,

Preganglionic fiber

Ramus communicans
Sympathetic ganglion
— Postganglionic fiber

Ventral root

Visceral muscle

Mucous membrane

Fig. 56. — Diagram illustrating the composition of a typical spinal nerve
in the thoracic region. The somatic sensory system is indicated by broken
lines, the visceral sensory by dotted lines, the somatic efferent by heavy
continuous lines, the visceral efferent by lighter continuous lines. (Compare
Figs. 1 and 55.)

fibers arising from cell bodies of the spinal ganglia (see Fig. 1,
p. 25, and Figs. 55, 56). The fibers of the spinal nerves are chssi-
fied in accordance with the same physiological criteria as their
end-organs (see pp. 79-94, and compare the cranial nerves, pp.
143-150) into somatic afferent (or sensory), visceral afferent
(or sensory), somatic efferent (or motor), and visceral efferent
(or motor) systems (Fig. 56).

In the spinal cord the originally wide cavity of the embryonic
neural tube (see p. 116) is reduced to a slender central canal and

THE SPINAL CORD AND ITS NERVES 127

the walls of the tube are thickened. The nerve-cells retain their
primary position bordering the central canal, thus forming a
mass of central gray matter which is roughly H -shaped in cross-
section. This gray matter on each side is accumulated in the
form of two massive longitudinal ridges, a dorsal column
(columna dorsalis, or posterior horn), whose neurons receive
terminals of the sensory fibers of the dorsal roots, and a ventral
column (columna ventralis, or anterior horn) whose neurons give
rise to the fibers of the ventral roots.

The white matter of the spinal cord is superficial to the gray
and is made up of sensory and motor root fibers of spinal nerves,
ascending and descending correlation fibers putting different
parts of the cord into functional connection, and longer ascend-
ing and descending tracts by which the spinal nerve-centers
are connected with the higher association centers of the brain.
In general, the shorter fibers lie near to the central gray and the
Ion7er tracts more superficially.

The white matter which borders the gray in the spinal cord
is more or less mingled with nerve-cells and fine unmyelinated
endings, and thus shows under low powers of the microscope a
reticulated appearance. This is the reticular formation (pro-
cessus reticularis) of the cord (see pp. 65, 158, and Fig. 58).
Immediately surrounding the reticular formation and partly
embedded within it are myelinated fibers belonging to neurons
intercalated between the sensory and the motor roots, which
run for relatively short distances in an ascending or descending
direction for the purpose of putting all levels of the cord into
functional connection in the performance of the more complex
spinal reflexes. These fibers form the deepest layer of the
white matter and are termed the fasciculi proprii (dorsalis,
lateralis, and ventralis, see Fig. 59). These fascicles are also
called ground bundles and fundamental columns.

In the narrow space between the ventral fissure and the cen-
tral canal (see Fig. 58) there is a bundle of nerve-fibers which
cross from one side of the spinal cord to the other. This is the
ventral commissure. A similar but smaller dorsal commissure
crosses immediately above the central canal.

There is considerable confusion in the terminology in use in the further
analysis of the spinal white matter, and the usage which follows differs

128

INTRODUCTION TO NEUROLOGY

in some respects from most of the classical descriptions, no two of which
agree among themselves. We shall limit the application of the term
funiculus to the three major divisions of the white matter of each half of the
spinal cord, viz., the dorsal funiculus bounded by the dorsal fissure and the
dorsal root, the lateral funiculus lying between the dorsal and ventral
roots, and the ventral funiculus between the ventral root and the ventral
fissure (Fig. 57).

Each funiculus may be divided in a purely topographic sense into
fasciculi, or collections of nerve-fibers which occupy the same general region
in the cross-section of the cord, such as the fasciculus gracilis of Goll and the
fasciculus cuneatus of Burdach (which together make up the greater part
of the funiculus dorsalis, see Figs. 57 and 59), and the superficial ventro-
lateral fasciculus of Gowers (including among other tracts the spino-
tectal tract and the ventral spino-cerebellar tract of Fig. 59) . These fasciculi
are usually mixed bundles containing tracts of diverse functional types.

Dorsal root

Dorsal funiculus

Dorsal column

Lateral funiculus

Lateral column

Ventral column

Ventral funiculus

Ventral root

Fig. 57. — Diagram of a cross-section through one-half of the spinal
cord to illustrate the arrangement of the funiculi of white matter and the
columns of gray matter.

The true physiological units of the spinal white matter are the tracts, i. e.,
collections of nerve-fibers of similar functional type and connections.
These tracts by some neurologists are termed fasciculi; and, like the other
tracts of the central nervous system, they are, in general, named in accord-
ance with the terminal relations of their fibers, the name of the location of
their cells of origin preceding that of their place of discharge in a hyphenated
compound word. Thus, the tractus cortico-spinalis arises from cells of the
cerebral cortex (p. 140), and terminates in the spinal cord, and the tractus
spino-cerebellaris arises in the spinal cord and terminates in the cerebellum
(p. 130). But, as already stated, there is no uniformity in the nomenclature
of these tracts and no two authorities agree exactly in the terminology
adopted. Moreover, few of the tracts have clearly defined anatomical limits,
in most cases the fibers of different systems being more or less mingled.

The appearance of a cross-section through the spinal cord in
the lower cervical (neck) region, after staining so as to reveal the
arrangement of both the nerve-cells and the nerve-fibers, is seen

THE SPINAL CORD AND ITS NERVES

129

in Fig. 58. Figure 59 illustrates diagrammatically the ar-
rangement of the chief fiber tracts in the same region.

The spinal cord has two main groups of functions, first, as a
system of reflex centers for all of the activities of the trunk and
limbs; second, as a path of conduction between these centers
and the higher correlation centers of the brain. The former
group is the more primitive, and there is evidence that in the

Dorsal median septum

Septum
Dorsal lateral groove

Dorsal nerve root
Substantia gelatinosa

Root-fibers entering gray

matter
Processus reticularia

Central canal

Nucleus from which \

motor fibers for

muscles of upper

limb arise

Ventral white commis-
sure

Ventral nerve root

Ventral median fissure

Fig. 58. — Cross-section through the human spinal cord at the level of
the fifth cervical nerve, stained by the method of Weigert-Pal, which colors
the white matter dark and leaves the gray matter uncolored. (From
Cunningham's Anatomy.)

course of vertebrate evolution the higher centers, especially the
cerebral hemispheres, exert an increasingly greater functional
control over these reflex centers (see p. 280). The long conduc-
tion paths between the spinal cord and the cerebral hemispheres
are, accordingly, much larger in man than in lower vertebrates.
It is impossible in the space at our disposal to summarize even
the most important of the internal connections of the spinal
nerves; we can only select a few typical illustrative examples.
9

130

INTRODUCTION TO NEUROLOGY

Faac.

Faac. cuneai

Fasc. scpto-marg.

Fasc. inter-fascie.

Tr. cortico-spin. lat

Tr. rubro-epin.

Nuc. dorso-lat.

Nuc. ventro-med.

Nuc. ventro-lat.

Tr. cortico-spin. ven .

Tr. olivo-spinalis

Tr. tecto-spinalis

Tr. vestibulo-spin.

Radix ventralis

Radix dorsalis
Fasc. dorso-lat.
Tr. spino-cereb. dor.
Columna dorsalis
Fasc. proprius dors.
Fasc. proprius lat.
Tr. spino-cereb. ven.
Tr. gpino-thalam. lat.
Columna ventralis
Tr. gpino-tectalis
Tr. spino-thalam. ven.
Tr. spino-olivaris
Fasc. proprius ven.
Fasc. sulco-marg.

Fig. 59. — Diagram of a cross-section through the human spinal cord at
the level of the fifth cervical nerve, to illustrate arrangement of the fiber
tracts in the white matter and of the nerve-cells in the gray matter of the
ventral column. On the right side the area occupied by the dorsal gray
column (posterior horn) is stippled; on the left side some of the groups of
cells of the ventral gray column (anterior horn) are indicated. In the white
matter the outlines of some of the more important tracts are schematically
indicated, ascending fibers on the right side and descending fibers on the
left. The same area of white matter is in some cases shaded on both sides
of the figure. This indicates that ascending and descending fibers are
mingled in these regions. A list of the tracts here illustrated follows. The
names here employed in some cases differ from those of the official German
Anatomical Society list (see p. 115), the B. N. A. terms here being italicized.

ASCENDING TRACTS

Fasciculus gracilis (column of Goll) and fasciculus cuneatus (column of
Burdach.) These are mixed bundles which in the aggregate make up the
greater part of the dorsal funiculus (old term, posterior columns). They are
made up chiefly of the ascending branches of dorsal root fibers (see Fig. 61 ),
those in the gracilis from the sacral, lumbar, and lower thoracic nerves (S,
L, T5-12), and those in the cuneatus from the upper thoracic and cervical
nerves (Tl-4, C), as indicated in the figure. These fasciculi terminate
respectively in the nuclei of the fasciculus gracilis (clava) and cuneatus
(tuberculum cuneatum) at the lower end of the medulla oblongata (cf. Fig.
83), and conduct chiefly impulses of the proprioceptive reflexes and those
concerned with sensations of posture, spatial discrimination, and the co-
ordination of movements of precision (see pp. 137, 175).

Fasciculus dorse-lateral is (tract of Lissauer, Lissauer's zone), made up
chiefly of unmyelinated fibers from the dorsal roots, together with myelin-
ated correlation fibers of the fasciculus proprius system.

Tractus spino-cerebellaris dorsalis (fasciculus cerebeUo-spinalis, direct
cerebellar tract, Flechsig's tract) . These fibers arise from the neurons of the
nucleus dorsalis (Clarke's column of gray matter between the dorsal and
ventral gray columns in the thoracic region, also called Stilling's nucleus)
of the same side and enter the cerebellum by way of its inferior peduncle
(corpus rcstiforme).

Tractus spino-cerebellaris ventralis (part of Cowers' tract, or the fascicu-
lus anlero-lateralis superficialis of the li. N . A .). These fibers also arise from
the nucleus dorsalis of the same side (A. N. Bruce) in the lower levels of the

THE SPINAL CORD AND ITS NERVES 131

spinal cord and enter the cerebellum by way of its superior peduncle (bra-
chium conjunctivum) .

The spinal lemniscus. Under this name are included several tracts to
the midbrain and thalamus. These fibers arise from neurons of the dorsal
gray column, cross in the ventral commissure, and ascend in the lateral and
ventral funiculi of the opposite side, partly superficially mingled with those
of the ventral spino-cerebellar tract and partly deeper in the fasciculus
proprius. This system of fibers includes a tractus spino-tectalis to the roof
(tectum) of the midbrain and a tractus spino-thalamicus to the ventral and
lateral nuclei of the thalamus. The deeper fibers of the latter tract are
arranged in two groups, the tractus spino-thalamicus lateralis for sensory
impulses of temperature and pain, and the tractus spino-thalamicus ven-
tralis for sensory impulses of touch and pressure (see p. 138, 173).

Tractus spino-olivaris, fibers arising from the entire length of the spinal
cord and terminating in the inferior olive (Goldstein).

DESCENDING TRACTS

Tractus cortico-spinalis (fasciculus cerebro-spinalis, pyramidal tract).
This system of fibers conducts voluntary motor impulses from the precentral
gyrus of the cerebral cortex to the motor centers of the spinal cord. It di-
vides at the upper end of the spinal cord into two tracts, the larger division
immediately crossing through the decussation of the pyramids to the oppo-
site side of the spinal cord, where it becomes the tractus cortico-spinalis
lateralis (fasciculus cerebro-spinalis laieralis, lateral or crossed pyramidal
tract). A smaller number of these fibers pass downward into the spinal
cord from the medulla oblongata without decussation to form the tractus
cortico-spinalis ventralis (fasciculus cerebro-spinalis anterior, direct pyra-
midal tract, column of Tiirck). These fibers cross in the ventral commis-
sure a few at a time throughout the upper levels of the cord, and finally ter-
minate in relation with the motor neurons of the opposite side. Both parts
of the pyramidal tract, therefore, decussate before their fibers terminate.

Tractus rubro-spinalis (tract of Monakow), from the nucleus ruber of the
midbrain to the spinal cord, for thalamic and cerebellar reflexes.

Tractus olivo-spinalis (Helwig's bundle, tractus triangularis), fibers de-
scending from the inferior olive of the medulla oblongata to the lower cer-
vical or upper thoracic segments of the spinal cord.

Tractus tecto-spinalis (predorsal bundle, tract of Lowenthal), from the
roof (tectum) of the midbrain to the spinal cord, chiefly for optic reflexes.

Tractus vestibulo-spinalis, from the primary centers of the vestibular
nerve in the medulla oblongata to the spinal cord, for equilibratory reflexes.

The two tracts last mentioned, together with several others, compose the
.fasciculus marginalis ventrahs.

THE FASCICULUS PROPRIUS

The fasciculus proprius system of fibers (also called ground bundles, basis
bundles, and fundamental bundles) comprises chiefly short ascending and
descending fibers arising from neurons of the spinal gray matter, for intrinsic
spinal reflexes. In general, these fibers border the gray pattern, but in the
dorsal funiculus some are aggregated in the tractus septo-marginalis and the
fasciculus interfascicularis (comma tract, tract of Schultze), these two tracts
also containing descending branches of the dorsal root fibers. Some fibers
of the fasciculus proprius ventralis lie adjacent to the ventral fissure and are
termed the fasciculus sulco-marginalis, these fibers forming the direct
continuation into the cord of the fasciculus longitudinalis medialis (posterior
longitudinal bundle) of the brain (see pp. 185, 211).

132 INTRODUCTION TO NEUROLOGY

The sensory nerves which enter the spinal cord come either
from the deep tissues or from the skin, and both of these types of
nerves carry fibers of very diverse functional sorts belonging to
the somatic sensory group, in addition to visceral fibers which
will not be considered here. It will be recalled (see pp. 77, 79)
that the general somatic sensory group includes: (1) propriocep-
tive systems, concerned with motor coordination and the orien-
tation of the body and its members in space (muscle sense, ten-
don sense, etc.), and (2) exteroceptive systems, concerned with
the relations of the body to its environment (touch, temperature,
and pain sensibility). The first of these systems is served
chiefly by the deep nerves, and the second chiefly by the cutane-
ous nerves, though this is not rigidly true. In particular it
should be noted that, even though the skin be completely anes-
thetic, the nerves of deep sensibility can still respond not only
to their proprioceptive functions, but also to the ordinary clinical
tests for the exteroceptive qualities of touch, temperature, and
pain, though with a higher threshold than in the case of the
cutaneous end-organs of these senses.

Henry Head and his colleagues have also separated the cuta-
neous fibers into a protopathic group (including cutaneous pain,
a diffuse non-localizable tactile sensibility, and the discrimina-
tion of extreme degrees of temperature) and an epicritic group
(light touch, cutaneous localization, discrimination of inter-
mediate degrees of temperature and some others) ; but there is
difference of opinion as to whether these groups represent two
distinct sets of nerve-fibers or different stages in regeneration
or different types of end-organs of the same fibers (see p. 84).

Upon entering the spinal cord all of these functional types
of fibers effect two sorts of connections: (1) for intrinsic spinal
reflexes, and (2) for the transmission of their impulses upward to
the higher centers of the brain. We shall first take up the in-
trinsic connections.

The simplest of these intrinsic connections is the direct motor
reflex illustrated by Fig. 1 (p. 25), but there are many more
complex forms of the connection between the dorsal and ven-
tral roots, some of which are indicated in Figs. 60 and 61. In
general, there is at least one neuron of the gray matter of the
spinal cord interpolated between the dorsal and the ventral root

THE SPINAL CORD AND ITS NERVES

133

neurons, and usually there is a complex chain of such neurons.
As may be observed in Fig. 61, the dorsal root fiber imme-

dorsal

root

ventral
root

Fig. 60. — Diagram of some of the types of connection between the sen-
sory fibers of the dorsal root and the motor fibers of the ventral root in the
spinal cord of the rabbit (chiefly after the researches of Philippson). The
visceral connections are not included.

1. Collateral branches of the dorsal root fibers effect synaptic relations
directly with dendrites of ventral column cells of the same or the opposite
side.

2. Dendrites of ventral column cells may cross to the opposite side and
here receive terminals of dorsal root fibers.

3. A correlation neuron may be intercalated between the two peripheral
neurons in either of the first two cases. These neurons may have short
axons for reflexes within a single segment (3a) or their axons may pass out
into the white matter (fasciculus proprius) and extend for longer or shorter
distances in either the ascending or the descending direction (or after branch-
ing in both directions) for connections with more remote motor centers of
the same or the opposite side (36, 3c).

4. The root-fibers arising from the cells of the ventral column themselves
may give off collateral branches which return to the gray matter and there
arborize about other cells of the ventral column belonging to different func-
tional groups or about correlation cells, thus facilitating the coordinated
contraction of several distinct muscles in the performance of some complex
reaction.

The neurons of the dorsal column apparently do not play an important
role as intercalary elements in the simpler spinal reflexes. The axons of
these cells are for the most part directed upward, after decussating in the
ventral commissure, and are chiefly concerned with the transmission of
nervous impulses from the spinal cord to the higher correlation centers of
the brain.

134

INTRODUCTION TO NEUROLOGY

diately upon entering the spinal cord divides into ascending and
descending branches, and secondary branchlets are given off in
large numbers from each of these, so that a single peripheral
sensory neuron may discharge its nervous impulses into very
many central neurons scattered throughout the entire length of
the spinal cord. When to these numerous endings we add the
countless ramifications of the correlation neurons, it is evident

-spinal lemniscus
correlation neuron 1
funiculus dorsalis

sp.g.l
correlation neuron, 2

sp.g.3

correlation neuron 3
( ) spg.4

Fig. 61. — Diagram of the spinal cord reflex apparatus. Some of the con-
nections of a single afferent neuron from the skin (d.r.2) are indicated : d.r.2,
Dorsal root from second spinal ganglion ;m, muscles; sp.g.l to sp.g.4, spinal
ganglia; v.r.l' to v.r.4, ventral roots.

that even in the spinal cord, which is the simplest part of the
central nervous system, there are reflex mechanisms of great
complexity. Some of these have been analyzed. Sherrington,
in his Integrative Action of the Nervous System, has presented
a very clear analysis of the scratch reflex of the dog and the
neural mechanisms involved. The mechanism of the locomotor
reflexes has been studied physiologically and histologically by

THE SPINAL CORD AND ITS NERVES 135

Steiner, Philippson, Polimanti, Herrick and Coghill, and very
many others.

Our most precise knowledge of the arrangement of the afferent
and efferent myelinated fibers in the spinal roots has been gained
by the application of Marchi's method (p. 48) to the study of
degenerations following accidental .and experimental injuries.
Nerve-fibers which have been cut off from their cells of origin
degenerate within about two weeks after the injury. It is,
therefore, possible by the microscopic study of a divided nerve
with Marchi's method (which stains only the degenerating mye-
linated fibers) to determine on which side of the injury are the
cells of origin from which these fibers arise.

Figure 62 illustrates the effects of section of the spinal roots
made at four different places. In the first case section of the
mixed trunk peripherally of the union of the dorsal and ventral
roots is followed by degeneration of all of the myelinated fibers
of the nerve-trunk, showing that the cell bodies of all of these
fibers lie centrally of the injury. In the second case, section of
the ventral root close to the spinal cord is followed by degenera-
tion of all the fibers of this root without disturbance of those
of the dorsal root, showing that the ventral root fibers arise as
axons of cells within the spinal cord. In the third case section
of the dorsal root fibers peripherally of the ganglion and before
their union with those of the ventral root results in the degenera-
tion of all of the fibers of the mixed nerve which arise in the
spinal ganglion (sensory fibers), without loss of any motor fibers
from the ventral root. In the fourth case section of the dorsal
root on the central side of the ganglion is followed by degenera-
tion of all myelinated fibers of the central stump of this root, but
not of the peripheral part of the root or the spinal ganglion.
This shows that the cells of origin of these fibers lie in the spinal
ganglion and not, like those of the ventral root, within the spinal
cord. The peripheral processes of these ganglion cells, there-
fore, are dendrites, and the centrally directed processes which
compose the dorsal roots are axons (cf. Fig. 1, p. 25, and Fig.
56, p. 126).

Another useful method for the solution of problems of this
character is the study of the fine structure of the cell bodies of
the neurons after such experimental lesions as those just des-

136

INTRODUCTION TO NEUROLOGY

cribed. Neurons whose peripheral fibers have been severed,
thus cutting the cell body off from its usual avenue of functional
discharge, within a few days thereafter undergo structural
changes, chief of which is chromatolysis, or the solution and dis-
appearance of the Nissl bodies (see p. 49). Thus, after cutting

m **

Fig. 62. — Four sketches to illustrate the degenerations of somatic sensory
and motor fibers which follow section of spinal nerve-roots indifferent places.
Fibers separated from their cells of origin will degenerate, as shown in black
(see the text, p. 135).

a ventral spinal root (Fig. 62, II), a microscopic examination of
the spinal cord will show the chromatolysis effect (see Fig. 13,
p. 48) in every neuron in the ventral gray column which gives
rise to a fiber of this root, while all of the other neurons will
remain normal.

Physiological experiments upon men and other animals where

THE SPINAL CORD AND ITS NERVES 137

such injuries have taken place give the necessary control to con-
firm the proof that efferent fibers leave the spinal cord through
the ventral roots and afferent fibers enter through the dorsal
roots, for the loss of ventral roots results in a motor paralysis of
the muscles supplied by them, while the destruction of dorsal
roots results in the loss of superficial and deep sensibility in the
regions innervated, with no loss of motor function save for the
imperfect coordination resulting from the loss of the sensory
control through the proprioceptive system (ataxia).

Turning now to the conduction paths between the spinal
cord and the brain, we notice first that the reactions involved
here may be performed either reflexly or consciously. In the
latter case a connection with the cerebral cortex is to be expected;
in the former case an infinite variety of reflex connections within
the brain stem is possible.

The sensory or ascending fibers which pass between the
spinal cord and the brain may be classified as follows:

I. Proprioceptive systems:

1. To the cerebellum (unconscious).

2. To the brain stem (unconscious).

3. To the thalamus and cerebral cortex (sensations of posture and

spatial adjustment).

II. Exteroceptive systems:

1. To the brain stem (unconscious).

2. To the thalamus and cerebral cortex (sensations of touch, tempera-

ture, and pain).

I. Proprioceptive Systems. — As soon as the afferent fibers of
the spinal nerves have entered the spinal cord they are im-
mediately segregated into proprioceptive and exteroceptive
groups, as suggested by the analysis above (see Figs. 63, 64, 81,
and 83). The proprioceptive fibers take quite different courses,
depending upon whether they are directed into the cerebellar
path or into the path to the brain stem and cerebral cortex.
Some terminals of this system end in the gray matter between the
dorsal and ventral columns (the nucleus dorsalis of Clarke, or
Clarke's column, and adjacent regions), whose neurons send their
axons into the dorsal and ventral spino-cerebellar tracts and
finally into the cerebellum. The cerebellum is the great center
of motor coordination, and these spino-cerebellar tracts are two

138 INTRODUCTION TO NEUROLOGY

only out of a larger number of paths by which afferent spinal
impulses may be discharged into it (see p. 188).

The remaining proprioceptive fibers of the spinal roots are
directed upward in the dorsal funiculus, of which they form the
larger part. At the point where the spinal cord passes over
into the medulla oblongata they terminate, and after a synapse
here the neurons of the second order carry the impulse across to
the opposite side of the brain and upward toward the thalamus
in a tract known as the medial lemniscus or fillet (Fig. 64).
After another synapse here, a final neuron may carry the nervous
impulse forward to the cerebral cortex. This medial lemniscus
system is largely concerned with unconscious motor adjustments
involving the muscles of the trunk and limbs. Disturbance of
its functions produces motor incoordination (ataxia), but not
necessarily any great loss of exteroceptive sensations. So far as
its functions come into consciousness, they are recognized as sen-
sations of position, spatial localization, and motor control.

II. Exteroceptive Systems. — The central course of the extero-
ceptive fibers of the spinal nerves is quite different from that
just described. Almost immediately after entering the spinal
cord these fibers terminate among the neurons of the dorsal
gray column. After a synapse here the fibers of the second order
cross to the opposite side of the spinal cord, and here turn and
ascend in the white matter of the lateral and ventral funiculi,
where they form the spinal lemniscus, or tractus spino-thalamicus.
Some fibers of the spinal lemniscus ascend throughout the entire
length of the spinal cord, medulla oblongata, and midbrain, to
end in the thalamus. In the upper part of their course these
fibers accompany those of the medial lemniscus already des-
cribed.

Collateral connections are effected between the ascending
fibers of the spinal lemniscus and the various motor nuclei of the
brain for different cranial reflexes, such as turning the eyes in
response to a cutaneous stimulation on the hand. But their
final terminus is in the thalamus, and after a synapse here the
nervous impulse may be carried forward to the cerebral cortex
by neurons of the third order. The spinal lemniscus system is
the chief ascending pathway for nervous impulses giving rise to
consciousness of touch, temperature, and pain from the trunk

THE SPINAL CORD AND ITS NERVES

139

and limbs. There is a similar but anatomically distinct path-
way to the thalamus for cutaneous sensibility from the head,
which is called the trigeminal lemniscus (see p. 180 and Figs.
64, 77, 81).

Within the spinal cord the nerve-fibers of sensibility to pres-
sure, pain, and temperature run in three distinct tracts of the
i

•Fasciculus graeilis "1

Vproprioceptive
'Fasciculus cuneatus j

Dorsal spino-cere-
bellar tract (pro-
prioceptive)

Nucleus dorsalis of
Clarke

Ventral spino-cere-
bellar tract (pro-
prioceptive)

Spinal lemniscus
(exteroceptive for
pain, heat, and
cold)

Spinal lemniscus
(exteroceptive for
touch and pres-

• sure)

Fig. 63. — Diagram to illustrate the terminations within the spinal cord
of some of the types of somatic sensory fibers and their secondary paths.
The central connections of root fibers 1, 2, and 5 provide for proprioceptive
responses; those of fibers 3 and 4, for exteroceptive responses. Root fiber
1 terminates in the nucleus of the fasciculus cuneatus of the same side at
the upper end of the spinal cord and conveys impulses of muscular sensi-
bility, sense of passive position and movement, and of spatial discrimina-
tion. Root fiber 2 terminates in the nucleus dorsalis of Clarke (Clarke's
column) and root fiber 5 in the same nucleus or adjacent parts of the gray
substance. These fibers call forth unconscious cerebellar activity underly-
ing the coordination and reflex tone of the muscles. Root fibers 3 and 4
terminate in the dorsal gray column and convey exteroceptive impulses.
Fiber 3 typifies all fibers which carry sensibility of pain, heat, and cold;
fiber 4, those which carry sensibility of touch and pressure.

spinal lemniscus (the pain and temperature tracts very close
together, see Figs. 59, 63, and 81), so that it occasionally hap-
pens that one may be destroyed by accident or disease without
affecting the other two. Thus, at the level of the fifth cervical
vertebra the destruction of the pathway for touch and pressure
(tractus spino-thalamicus ventralis of Fig. 59) would result in
the total loss of both cutaneous and deep sensibility to pressure

140 INTRODUCTION TO NEUROLOGY

over the whole of the opposite side of the body below the level
of the injury, but there would be no disturbance of either tem-
perature or pain sensibility. Similarly, by an injury of the trac-
tus spino-thalamicus lateralis, pain or temperature sensibility
might be lost with no disturbance of pressure sense. (For the
description of a case of this sort see p. 173.)

Such combinations of symptoms as just described could
not occur from any form of injury to the peripheral nerves, for
in these nerves the various kinds of fibers are all mingled in the
larger trunks, so that one functional component cannot be in-
jured without involvement of the others also. And at the first
division of these trunks into deep and superficial branches each
branch also carries all or nearly all of the functional systems
(see pp. 79-84, 132).

The return pathway for motor nervous impulses from the
cerebral cortex is the cortico-spinal tract or pyramidal tract
(Fig. 64), whose fibers descend without interruption from the
precentral gyrus of the cerebral cortex (see p. 283) to the spinal
cord, where they form the lateral and ventral cortico-spinal
tracts (Fig. 59). The various reflex centers of the brain stem
also send motor fibers downward into the cord for the excitation
of movements of the trunk and limbs. The tecto-spinal tract
(Fig. 59) is such a path, leading from the optic and acoustic
centers of the midbrain, as is also the vestibulo-spinal tract,
leading from the vestibular nuclei of the medulla oblongata
(p. 176, Fig. 83, neuron 16).

Summary. — The spinal nerves are segmentally arranged and
are named after the vertebrae adjacent to which they emerge
from the spinal canal of the vertebral column. Each nerve
arises by a series of dorsal rootlets afferent in function and a
series of ventral rootlets efferent in function. Most of the gray
matter of the spinal cord is massed in two longitudinal columns
on each side, for somatic sensory and somatic motor functions
respectively. These are separated by an intermediate region
containing the visceral sensory and motor centers and various
correlation neurons. The white matter of the cord is superficial
to the gray and contains myelinated fibers for various kinds of
correlation, besides root-fibers of the spinal nerves. The white
matter is divided topographically into funiculi and fasciculi and

THE SPINAL CORD AND ITS NERVES

141

physiologically into tracts. The latter are the really significant
units in the analysis of the cord. Peripherally, the spinal nerves
divide into deep and superficial branches, and the latter contain,

cerebral

cortex

trUjeminal lemniscus

sKin

medial lemniscus

nucleus of dorsal

funiculus

spinal lemniscus

ventral pyramidal
tract

dorsal funtcufus
— lateral pyramidal tract

spinal ganglion
sKin

muscle

Fig. 64. — Diagram of the chief connections between the spinal cord and
the cerebral cortex. The spinal lemniscus complex carries the ascending
exteroceptive systems (touch, temperature, and pain) ; the dorsal funiculus
and medial lemniscus complex carries chiefly ascending proprioceptive sys-
tems (a nerve of muscle sense is the only member of this group included
in the drawing) . The diagram also includes the sensory path from the skin
of the head to the cerebral cortex through the V cranial nerve (trigeminus)
and the trigeminal lemniscus (p. 157). The pyramidal tract (tractus cpr-
tico-spinalis) is the common descending motor path for both exteroceptive
and proprioceptive nervous impulses from the cerebral cortex.

142 INTRODUCTION TO NEUROLOGY

according to Henry Head, protopathic and epicritic functional
systems of fibers. As soon as the peripheral nerve-fibers have
entered into the spinal cord they are segregated into proprio-
ceptive and exteroceptive groups, and each of these again into
particular functional tracts. There are connections for local
spinal reflexes, reflexes of the brain stem and cerebellum, and
for the cerebral cortex. The spino-cerebellar tracts and the
dorsal funiculi are proprioceptive in function, and the spinal
lemniscus carries spino-thalamic tracts of the systems of touch,
temperature, and pain sensibility for the cerebral cortex.

LITERATURE

BARKER, L. F. 1901. The Nervous System and Its Constituent Neu-
rones, New York.

BROUWER, B. 1915. Die biologische Bedeutung der Dermatomerie.
Beitrag zur Kenntnis der Segmentalanatomie und der Sensibilitatsleitung
im Rlickenmark und in der Medulla Oblongata, Folia Neuro-biologica, Bd.
9, pp. 225-336.

BRUCE, A. 1901. A Topographic Atlas of the Spinal Cord, London.

BRUCE, A. N. 1910. The Tract of Cowers, Quart. Journ. Exp. Physiol.,
vol. iii, pp. 391-407.

HEAD, H., RIVERS, W. H. R., and SHERREN, J. 1905. The Afferent
Nervous System from a New Aspect, Brain, vol. xxviii, pp. 99-115.

HEAD, H., and THOMPSON, T. 1906. The Grouping of the Afferent
Impulses Within the Spinal Cord, Brain, vol. xxix, p. 537.

HERRICK, C. JUDSON, and COGHILL, G. E. 1915. The Development of
Reflex Mechanisms in Amblystoma, Jour. Comp. Neur., vol. xxv, pp. 65-85.

PHILIPPSON, M. 1905. L'autonomie et la centralisation dans le systeme
nerveux des animaux, Paris.

POLIMANTI, O. 1911. Contributi alia fisiologia del sistema nervoso cen-
trale e del movimento dei pesci, Zool. Jahrb., Abt. f. Zool. u. Physiol., Bd.
30, pp. 473-716.

RIVERS, W. H. R., and HEAD, H. 1908. A Human Experiment in Nerve
Division, Brain, vol. xxxi, p. 323.

SHERRINGTON, C. S. 1906. The Integrative Action of the Nervous Sys-
tem. New York.

STEINER, J. 1885. Die Functionen des Centralnervensystems und ihre
Phylogenese. I. Abteilung. Untersuchungen iiber die Physiologic des
Froschhirns, Braunschweig.

— . 1888. Idem. II. Abteilung, Die Fische.

— . 1900. Idem. IV. Abteilung, Reptilien-Ruckenmarksreflexe, Ver-
mischtes.

— . 1886. Ueber das Centralnervensystem der grunen Eidechse nebst
weiteren Untersuchungen iiber das des Haifisches, Sitzb. k. Akad. Wiss.,
Berlin, p. 541.

CHAPTER IX

THE MEDULLA OBLONGATA AND CEREBELLUM

THE brain contains a series of primary sensory and motor
centers related to the cranial nerves (see p. 109), the correlation
mechanism which serves these sensori-motor centers, and an
extensive system of conduction pathways between the brain and
spinal cord and between the various correlation centers of the
brain itself.

The brain is divided into two principal parts by a constriction
in front of the cerebellum and pons, the isthmus (see p. 122).
Above this level lies the cerebrum and below it the rhomben-
cephalon, comprising the medulla oblongata or bulb and the
cerebellum. The medulla oblongata contains the primary
centers concerned with most of the simpler cerebral reflexes,
especially those of the visceral, general cutaneous, auditory, and
proprioceptive systems (see pp. 112 and 123). The cerebellum
is a suprasegmental apparatus developed phylogenetically and
embryologically out of the more primitive bulbar nuclei of the
vestibular nerve, i. e., out of the acousti co-lateral area of fishes
(Figs. 43 and 44, pp. Ill, 112, and Fig. 68).

The olfactory nerve (I pair), the so-called optic nerve (II
pair), and the auditory nerve (VIII pair) are special sensory
nerves, whose central connections will be described more in
detail below. The remaining nine pairs of cranial nerves of the
human body may be briefly summarized as follows:

The oculomotor nerve (III pair), trochlear nerve (IV pair), and abducens
(VI pair) contain the somatic motor fibers and fibers of muscle sense related
to the six muscles which move the eyeball. The III pair also contains vis-
ceral motor fibers for the ciliary ganglion, from which are innervated
the muscles of the ciliary process and iris within the eyeball, i. e., the muscles
of accommodation. The trigeminal nerve (V pair) supplies general sensibil-
ity to the skin and deep tissues of the face and the motor innervation of the
muscles of mastication. The facial nerve (VII pair) innervates the taste-
buds of the anterior two-thirds of the tongue (special visceral sensory fibers),
the sublingual and submaxillary salivary glands (general visceral efferent

143

144 INTRODUCTION TO NEUROLOGY

fibers), and the muscles related with the hyoid bone and the superficial
facial muscles or muscles of facial expression, these two groups of muscles
belonging to the series of special visceral muscles (p. 94). The glosso-
pharyngeal nerve (IX pair) supplies fibers to the taste-buds on the posterior
third of the tongue (special visceral sensory), also general sensibility to this
region, motor fibers for the stylopharyngeus muscle (special visceral motor),
and excito-glandular fibers for the parotid salivary gland (general visceral
efferent). It also cooperates with the vagus nerve in innervating the skin
about the external auditory canal (by the auricular branch of the vagus).
The vagus nerve (X pair) is very complex. In addition to the general so-
matic sensory fibers of the auricular branch, which have just been men-
tioned, it contains general visceral sensory fibers from the pharynx, lungs,
stomach, and other abdominal viscera, and visceral efferent fibers of several
sorts to the pharynx, esophagus, stomach, intestines, lungs, heart, and
arteries. The peripheral and central courses of most of these functional
systems have been accurately determined, but are far too complex for sum-
mary here. The accessory nerve (XI pair) contains two parts: (1) the
bulbar part, which should be regarded as nothing other than detached
filaments of the vagus, for all of these fibers peripherally join vagus branches,
(2) the spinal part, which arises by numerous rootlets from the upper levels
of the spinal cord and participates in the innervation of two of the muscles
of the shoulder (the trapezius and sternocleidomastoid muscles). The hu-
man hypoglossus nerve (XII pair) is a modified derivative of the first spinal
nerve of lower vertebrates. It has lost its sensory fibers and innervates
a special part of the tongue musculature.

All of the nerves of the preceding list except the I, II, III, and
IV pairs connect with the medulla oblongata. In the dogfish
we have seen that this region of the brain presents special emi-
nences which form respectively the terminal nuclei of the
acoustic (and lateral line), cutaneous, and visceral (including
gustatory) sensory systems (see p. 112 and Figs. 42^14). The
primary motor centers lie ventrally of these sensory areas.

The cranial nerves are usually described in our text-books
as if they were segmental units like the spinal nerves (see p.
125). This was, in fact, the primitive condition; but in all
vertebrate animals this segmental pattern has been greatly
modified in such a way as to facilitate the discharge into the
brain of all sensory fibers of like physiological type into a single
center. These physiological systems are, accordingly, the most
useful units of structure in the cranial nerves. Each cranial
nerve may contain several of these functional systems, and no
two pairs of cranial nerves have the same composition. The
components of the cranial nerves, like those of the spinal nerves
(p. 126), are named in accordance with the same physiological
criteria as their end-organs (see pp. 79-94).

THE MEDULLA OBLONGATA AND CEREBELLUM 145

A functional system may be defined as the sum of all the
neurons in the body which possess certain physiological and
anatomical characters in common so that they may react in a
common mode. Morphologically, each system of peripheral
nerves is defined by the terminal relations of its fibers — by the
organs with which they are related peripherally and by the
centers in which the fibers arise or terminate. A single periph-
eral nerve may contain several of these systems. It becomes
necessary, therefore, to analyze the root complex of each pair of
spinal and cranial nerves into its components, and to trace not
only the central connections of these components within the
spinal cord and brain, but also their peripheral courses as well.
In other words, the description of any given nerve or ramus is not
complete when we have given its point of origin from the nerve-
trunk, root, or ganglion, the details of its devious courses, and
the exact points where the several ramuli terminate. In addi-
tion to this it is necessary to learn what functional systems are
represented in each ramus and the precise central and peripheral
relations of each system.

Each of the four primary divisions of the spinal nerves
(somatic sensory and motor, visceral sensory and motor, see
p. 126) is represented in the head region in the same primitive
unspecialized form as seen in the spinals, and also by specialized
systems found only in one or more cranial nerves. This gives
eight groups of functional systems represented in the cranial
nerves, as follows:

1. General somatic afferent nerves, supplying (1) general exteroceptive
sensibility to the skin and the underlying tissues, and (2) deep propriocep-
tive sensibility to the muscles, tendons, etc. Type 1 is represented in the
V, IX, and X nerves, and in some lower vertebrates in the VII nerve also
(there is some clinical evidence for its presence in the VII nerve of man) ;
type 2 is represented in the III, IV, V, VI nerves and probably in some of
the others also.

2. Special somatic afferent nerves, for the innervation of highly differ-
entiated sense organs. Here belong in the exteroceptive series the coch-
lear branch, and in the proprioceptive series the vestibular branch of the
VIII pair. The lateral line nerves of fishes belong here, and probably the
visual organ connected with the II pair in all vertebrates (though the so-
called optic nerve is not a true nerve, see p. 204).

3. General somatic efferent nerves, supplying the general skeletal muscu-
lature of the body. In fishes this system is represented in several cranial
nerves in addition to the spinalis, but in man it is lost in the cranial nerves,
unless, as some believe, a part of the fibers of the XI pair belong here.

10

146

INTRODUCTION TO NEUROLOGY

ONENTS

CHIEF BRANCHES.

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148 INTRODUCTION TO NEUROLOGY

4. Special somatic efferent nerves, supplying two groups of highly special-
ized somatic muscles, namely, the external eye muscles and a part of the
tongue muscles. They arise from a ventro-medial series of motor nuclei and
are represented in the III, IV, VI, and XII pairs.

5. General visceral afferent nerves, innervating visceral mucous surfaces
without highly differentiated sense organs. They distribute through the
sympathetic nervous system and are represented in the VII, IX, and X pairs
and perhaps in some others.

6. Special visceral afferent nerves, for the innervation of specialized sense
organs serving the senses of taste and smell. The gustatory fibers are
represented in the VII, IX, and X pairs. The olfactory nerve (I pair) is
probably a more highly differentiated member of this group (see pp. 91
and 215).

7. General visceral efferent nerves, for unstriped involuntary visceral
muscles, heart muscle, glands, etc., distributing through the sympathetic
nervous system. These fibers (preganglionic fibers of Langely, p. 229) are
present in the III, VII, IX, X, and XI pairs.

8. Special visceral efferent nerves, supplying highly specialized striated
muscles of a different origin (both embryologically and phylogenetically)
from the striated trunk muscles. These muscles are connected with the
visceral or facial skeleton of the head and are derived from the gill muscles
of fishes. These nerves in the adult body resemble those of the somatic
motor system, save that they arise from a different series of motor nuclei in
the brain (the ventro-lateral motor column) . They have no connection with
the sympathetic nervous system and are represented in the V, VII, IX, X,
and XI pairs.

In the preceding Table of Nerve Components (pages 146, 147) the sev-
eral cranial nerves are analyzed and compared with a typical spinal nerve.

The various functional systems of the head tend to be con-
centrated in one or a few cranial nerves for ease of central corre-
lation, and even in case a given system is represented in several
nerves, the fibers of this system may converge within the brain
to connect with a compact center. This is well illustrated by the
gustatory and acoustico-lateral systems of the cranial nerves of
the fish, Menidia, as shown in Fig. 65. Here the gustatory sys-
tem (indicated by cross-hatching) is present in the VII, IX, and
X cranial nerves, and all of these fibers, together with other
visceral fibers, converge within the brain to enter the visceral sen-
sory area in the vagal lobe (lob.X.). Similarly, the lateral line
components of the VII and X nerves and the VIII (printed in
solid black) converge to enter the acoustico-lateral area in the
tuberculum acusticum (t.a.}. The general cutaneous fibers
enter by the V and X nerves, and all of these fibers enter the
spinal V tract (sp.V.).

In the paragraphs which follow the chief central connections (terminal
nuclei of the sensory systems and nuclei of origin of the motor systems, see

THE MEDULLA OBLONGATA AND CEREBELLUM

149

p. 108) of some of the cranial nerve components are summarized (see Fig.
71). For the details of these connections the larger text-books of neurology
should be consulted.

1 . General Cutaneous System (part of the general somatic afferent, repre-
sented in the V, IX, and X nerves). — Chief sensory V nucleus and spinal
V nucleus, or gelatinous substance of Rolando of the medulla oblongata.

AcA

Fig. 65. — A diagram of the sensory components of the cranial nerves of a
fish, Menidia. The brain is outlined as seen from the right side with heavy
black lines. The general cutaneous nerves (somatic sensory) are outlined
with finer lines (unshaded), and all of these fibers are seen to enter a longi-
tudinal tract within the brain, the spinal trigeminal tract (sp.V.). The
special somatic sensory (acoustic and lateral line) nerves (black) converge
within the brain to a special center, theacoustico-lateral area, or tuberculum
acusticum (t.a.). The visceral sensory fibers (cross-hatched) likewise all
converge to a special center, the lobus vagi (lob.X.).

Reference letters: b.c.l to b.c.5, gill clefts; br.g.X., branchial ganglia of
X nerve; cil.g., ciliary ganglion ; d.l.g. VII., dorsal lateral line ganglion of VII
nerve; f.c., fasciculus solitarius; gen.g.VIL, geniculate ganglion of VII
nerve; IX., glossopharyngeal nerve; jug.g., jugular ganglion of X nerve;
lob.X., lobus vagi (visceral sensory area); n.I., olfactory nerve; n.IL, optic
nerve; n.IIL, oculomotor nerve; o.pr., ramus ophthalmicus profundus; pal.,
palatine branch of VII nerve; r. cut. dors. X., dorsal cutaneous branch of X
nerve; r.intest.X., intestinal branch of X nerve; r.lat.ac., ramus lateralis ac-
cessorius of VII nerve; r.lat.X., lateral line branch of X nerve; r.oph.sup.V
+ VIL, superficial ophthalmic branch of V and VII nerves; r.ol., ramus
oticus; r.st.X., supratemporal branch of X nerve; r.VII.p-t., pretrematic
branch of VII nerve; sp.V., spinal trigeminal tract; La.,- tuberculum acusti-
cum (acoustico-lateral area); t.hm., hyomandibular trunk; t.inf., infra-
orbital trunk; VIII., auditory nerve; v.l.g.VIL, ventral lateral line gan-
glion of VII nerve.

150 INTRODUCTION TO NEUROLOGY

2. Special Somatic Afferent Systems. — (1) Vestibular nuclei; (2) cochlear
nuclei; (3) optic tectum in the colliculus superior, optic part of the thala-
mus (lateral geniculate body and pulvinar).

3. General Somatic Efferent System. — Not represented in the human
cranial nerves.

4. Special Somatic Efferent Systems (III, IV, VI, and XII nerves). —
A series of ventral motor nuclei in the midbrain and medulla oblongata.

5 and 6. General and Special Visceral Afferent Systems (VII, IX, and X
nerves). — All of the fibers concerned with general visceral sensibility and
taste enter a single longitudinal tract, the fasciculus solitarius, and termin-
ate in the nucleus which accompanies this fasciculus. (The olfactory nerve
and its cerebral centers probably should also be included here.)

7. General Visceral Efferent Systems (III, VII, IX, X, and XI nerves). —
These are preganglionic fibers of the sympathetic system and arise from
laterally placed nuclei (except that of the III nerve, which is joined to the
ventral somatic motor nucleus) .

8. Special Visceral Efferent Systems (V, VII, IX, X, and XI nerves).
— A series of lateral motor nuclei of the medulla oblongata.

The spinal nerves, as we have seen, enter the spinal cord by a
series of segmentally arranged roots. Within the spinal cord,

Somatic sensory column

Dorsal funiculus
Visceral sensory column

/ f^>V"\5A^---^ V~~ Dorsal column

V menu motor column _ ^^, , „„ ^_,

' Lateral column
Somatic motor column .

' Ventral column

Fig. 66. — Diagrammatic transverse section through the spinal cord of a
fish (Menidia) to illustrate the relations of the functional columns of the
gray matter to the nerve roots. The relations of the visceral sensory
component are problematical, and fibers of the visceral motor component
probably emerge with the dorsal root, as well as with the ventral root,
though only the latter are included in the diagram.

however, their components are rearranged in longitudinal col-
umns which cut across and obscure the primary segmentation.
The sensory root-fibers and their terminal gray centers occupy
the dorsal part of the spinal cord and the motor roots and their
centers the ventral part (Figs. 66 and 67). In the brain the
same arrangement prevails, the sensory centers lying dorsal to
the motor. In the cranial nerves, moreover, the four primary
groups of functional systems of the peripheral nerves are more
clearly differentiated than in the spinal nerves, and from this

THE MEDULLA OBLONGATA AND CEREBELLUM

151

it follows that their primary centers are correspondingly highly
developed and distinct. The medulla oblongata, in fact, is
divided into four longitudinal columns related respectively to
the great primary groups of functional systems. In fishes,
where the amount of correlation tissue is less than in man, these
four primary columns appear as well-defined ridges in the wall
of the fourth ventricle.

An enlarged view of the medulla oblongata of the sturgeon,
which is very similar to that of the dogfish, is seen in Fig. 68,
which also illustrates the arrangements of the primary sensory
and motor centers in cross-section at several different levels.

Somatic sensory column

Visceral sensory column

Visceral motor column

Somatic motor column

Fig. 67. — Diagrammatic transverse section through the human spinal
cord. Compare Figs. 56 to 59 and note the relatively greater size of the
dorsal gray columns and dorsal funiculi in man than in the fish (Fig. 66).
This is correlated with the greater importance in man of the ascending
connections between the cord and the brain (see p. 129).

Figure 69 shows a cross-section through the medulla oblongata
in the region of the vagus nerve in another fish, the sea-robin.
In all of these cases the four principal functional systems (see
pp. 76 and 79-94) are arranged in longitudinal columns from
the dorsal to the ventral surface in the order: somatic sensory,
visceral sensory, visceral motor, and somatic motor centers, as
indicated diagrammatically on the left side of Fig. 69. The
arrangement of the peripheral nerve-fibers of these systems is
indicated on the right side. Figure 70 illustrates a cross-section
through the corresponding region of the medulla oblongata in an
early human embryo, where the same general arrangement of
the sensori-motor centers is evident.

152

INTRODUCTION TO NEUROLOGY

Lobus lines latcralis

Fasc. long, med
Lobus visceralis

-J

Fig. 68. — The medulla oblongata and cerebellum of the lake sturgeon
(Acipenser rubicundus) to show the longitudinal columns which have been
differentiated in correlation with the peripheral functional systems. Com-
pare Figs. 43 and 44 and note that the "Lobus lineae lateralis" and "Tu-
berculum acusticum" of this figure together correspond to the "acoustico-
lateral area" of the dogfish. A is a dorsal view with the membranous roof
of the fourth ventricle removed to show the longitudinal columns within
the ventricle. B, C, and D are sketches of cross-sections at the levels
indicated in which the four functional columns are diagrammatically shaded,
the somatic motor by white circles, the visceral motor by white rectangles,
the visceral sensory by oblique cross-hatching, and the somatic sensory by
vertical cross-hatching. The Roman numerals refer to the cranial nerves.
(From Johnston's Nervous System of Vertebrates.)

THE MEDULLA OBLONGATA AND CEREBELLUM

153

Figure 71 gives a view of the adult human medulla oblongata
and midbrain after the removal of the cerebellum and mem-

Somatic sensory column X?
Visceral sensory col

Visceral motor column
Somatic motor column

Acoustic area
pinal V tract
Vagal lobe

Nucleus ambiguus
Cutaneous root X
Visceral sensory root
Visceral motor root 2
Reticular formation
Ventral column

Spinal nerve (XII)

Fig. 69. — Diagrammatic cross-section through the medulla oblongata
at the level of the vagus nerve in a bony fish (the sea-robin, Prionotus caro-
linus), to illustrate the arrangement of the four principal functional columns.

branous roof of the fourth ventricle. (For the form of the ob-
longata, as seen from the side and from below, see Figs. 45 and
53.) In this figure the positions of the primary sensory and

Roof plate
Fourth ventricle

SZ /i^-X

Somatic sensory column
Visceral sensory column

Visceral motor column
Somatic motor column

Floor plate

Fig. 70. — Diagrammatic cross-section through the medulla oblongata
at the level of the vagus nerve of a human embryo of 10.2 mm. (fifth week),
to illustrate the arrangement of the four principal functional columns.
(Compare Fig. 69.)

motor nuclei are drawn as projected upon the dorsal surface,
the motor centers on the left and the sensory centers on the
right. The somatic motor nuclei are indicated by circles, the

154

INTRODUCTION TO NEUROLOGY

general visceral motor nuclei by small dots, the special visceral
motor nuclei by large dots, the visceral sensory nuclei by double

Trigonum
hypoglossi
Nuc. spinalia V

Nuc. com.
Cajal

Fig. 71. — Dorsal view of the human midbrain and medulla oblongata
after the removal of the cerebellum and the roof of the fourth ventricle,
with the positions of the cranial nerve nuclei projected upon the surface.
The motor nuclei are indicated on the left side and the sensory nuclei on the
right. The somatic motor nuclei are indicated by circles, the general vis-
ceral efferent nuclei by small dots, and the special visceral efferent nuclei by
large dots. The general somatic sensory area is indicated by horizontal lines,
the visceral sensory area by double cross-hatching, and the special somatic
sensory area by open stipple. (Compare Figs. 77, 86, and 114.)

n.IV, Nervus trochlearis; nuc.com.Cajal, the commissural nucleus of
Ram6n y Cajal ; nuc.III E-W ., the small-celled visceral motor nucleus of the
III nerve, or nucleus of Edingor-Westphal; nuc.III lat., lateral nucleus of
III nerve; nuc. Ill med., medial nucleus of III nerve; nuc. IV, nucleus of IV
nerve; nuc.mesenc. V, me-;encephalic nucleus of V nerve; nuc.mot.V, motor
nucleus of V nerve; nuc.mot.V II , chief motor nucleus of VII nerve; nuc.sal.
inf., nucleus salivatorius inferior; nuc.sal.sup., nucleus salivatorius superior;
nuc.sensor.V, chief sensory nucleus of V nerve; nuc.VJ, nucleus of VI nerve;
nuc. XII, nucleus of XII nerve; n.V, nervus trigeminus.

THE MEDULLA OBLONGATA AND CEREBELLUM

155

cross-hatching, the general somatic sensory nuclei by single
cross-hatching, and the cochlear and vestibular nuclei (special
somatic sensory) by open stipple bounded by heavy lines.

Figure 72 illustrates the appearance of a cross-section through
the adult human medulla oblongata at the level of the roots of
the IX nerve, and Fig. 73 presents an analysis of a section
slightly nearer the spinal cord at the level of the X nerve. Fig-
ure 74 is a diagrammatic representation of the relations of the

Vagoglossopharyngeal

roots Nucleus of the
Restifonn | fasciculus solitarius
botiy I Tsenia

Vagus nucleus

Fasciculus solitarius

Descending root of vestibu-
lar nerve (VIII)
Vago-glossophar-
yngeal roots

Fasc. long,
medialis
uc. spinal V.
jsr- \ tract

1 Spinal V. tr.
i. ambiguus
Olivo-cereb. tract
•orsal acces. olive
External arcuate fibers
Medial lemniscus

Medial acces. olive
Inferior olive

Pyramid
External arcuate fibers

Fig. 72. — Cross-section through the adult human medulla oblongata at the
level of the IX cranial nerve. (From Cunningham's Anatomy.)

four principal functional systems at the same level as shown by
Fig. 73 for comparison with Figs. 66, 67, 69, 70. It is obvious
that, while the general relations in the human embryo (Fig. 70)
resemble tolerably closely those of the adult fish (Fig. 69), in a
human adult (Fig. 74) this primary arrangement has been
greatly disturbed by the addition of many new tracts and cen-
ters in the ventral part of the cross-section.

156

INTRODUCTION TO NEUROLOGY

We cannot here undertake an analysis of the complex reflex
connections of the medulla oblongata. In general, each of the

Nuc. dorsalis vagi
Nuc. fasc. solitari
Fasc. solitarius

Nuc. fasciculus

cuneatus

Nuc. XII

Spinal V nuc.

Spinal V tr,

Nuc. sal. inf.

X root

Nuc. ambiguus

Reticular
formation

Inferior olive
XII root

Ala cinerea
Trigonum hypoglossi
Nuc. vestibularis spinalis

Fasc. long. med.

Lemniscus V

Fig. 73. — Diagrammatic cross-section through the human medulla
oblongata at the level of the vagus nerve, illustrating details of functional
localization in addition to those shown in Fig. 72.

Visc. mot. col.
Som. mot. col.

Area acustica
Nuc. fasc. sol.
Nuc. dors. X
Fasc. solitarius
Cor. restiforme
Spinal V tract

Cutan. root X
Vise. sens, root X
Vise. mot. root X
Nuc. ambiguus
Reticular form.
Inferior olive
XII root

Fig. 74. — Diagrammatic cross-section through the adult human medulla
oblongata at the same level as shown in Fig. 73, for comparison of the
arrangement of the principal functional columns with that of Figs. 69
and 70.

primary terminal nuclei of the sensory roots of the cranial nerves
effects four types of connections: (1) direct reflex connections

THE MEDULLA OBLONG ATA AND CEREBELLUM 157

with the motor nuclei of the medulla oblongata, these connec-
tions being effected through the reticular formation (Figs. 69,
73) ; (2) descending reflex connections with the motor centers of
the spinal cord, by way of the bulbo-spinal tracts (such as the
vestibulo-spinal tract, Fig. 59); (3) connections with the cere-
bellum (this applies only to such functional systems as have pro-
prioceptive value, of which the vestibular nerve from the semi-
circular canals of the ear is the most important) ; (4) connections
with the thalamus and (after a synapse here) with the cerebral
cortex.

The fibers of the type last mentioned comprise the bulbar
lemniscus (Figs. 64, 77) ; of this there are several distinct parts,
two of which require special mention, viz., the trigeminal lem-
niscus and the lateral lemniscus. The skin of the head is inner-
vated chiefly by the trigeminal nerve (V pair) and the fibers of
this type terminate in the general somatic sensory area (known as
the chief sensory V nucleus and the spinal V nucleus or gelatinous
substance of Rolando, Figs. 71-74). After a synapse in this
area the fibers of the trigeminal lemniscus cross to the opposite
side and ascend to the thalamus in a pathway distinct from all
other lemniscus fibers (see p. 180 and Figs. 64, 75, 77, 78, 81).

The lateral or acoustic lemniscus comprises by far the largest
component of the bulbar lemniscus complex. Its fibers arise
from the terminal nuclei of the cochlear nerve (VIII pair, Fig.
71), cross at once to the opposite side of the brain, and ascend
to the midbrain (Fig. 75). Some of these fibers continue di-
rectly to the thalamus, where they end in the medial geniculate
body (Fig. 77); others terminate in the roof of the inferior
colliculus of the midbrain. After a synapse here and various
reflex connections, the nervous impulse may be carried forward
to the medial geniculate body of the thalamus by way of the
brachium quadrigeminum inferius (Figs. 75, 86). (Regarding
this system see further on pp. 195-203.)

In fishes there is an ascending secondary visceral and gusta-
tory tract, or visceral lemniscus, from the visceral sensory area to
the midbrain (p. 246) ; this tract no doubt occurs in the human
brain also, though its exact course has never been demonstrated.

Having now reviewed cursorily the primary sensory and motor
centers of the medulla oblongata, we must next examine some of

158 INTRODUCTION TO NEUROLOGY

the centers of correlation. As has already been indicated, all of
these centers are interconnected by correlation neurons similar
to those of the spinal cord (Figs. 60, 61). These neurons are
loosely arranged in the spaces between the sensory and motor
groups of nuclei, this tissue being termed the reticular formation
(this region is also called the tegmentum, see pp. 65, 127 and
Figs. 69, 74). But the chief centers of correlation of the brain
stem are found in specially enlarged nuclei of the midbrain and
thalamus, some of which are mentioned in the next chapter.

In its more ventral parts the medulla oblongata contains a
number of large correlation centers and important conduction
pathways between remote parts of the brain. Of the former, the
largest are the inferior olives (Figs. 72, 73, 74), deeply buried
masses of gray matter arranged in the form of a hollow shell of
complex shape on each side of the median plane. The olives
receive fibers from the thalamus and spinal cord and discharge
into the cerebellum (olivo-cerebellar fibers of Fig. 72). Their
functions are unknown.

The cerebellum has already been referred to as a great supra-
segmental mechanism of unconscious motor coordination. It
is connected with the underlying brain stem by three pairs of
stalks or peduncles, two of which join the medulla oblongata and
one the midbrain. The inferior peduncle (restiform body)
connects with the dorsal margin of the medulla oblongata and
carries fibers into the cerebellum from the spinal cord and ob-
longata. The middle peduncle (brachium pontis) connects
with the pons and most of its fibers convey impulses from the
nuclei of the pons to the cerebellum. The superior peduncle
(brachium conjunctivum) connects with the cerebral peduncle
in the floor of the midbrain and contains chiefly fibers which
descend from the cerebellum, cross the midplane under the
aqueduct of Sylvius, and terminate in or near the red nucleus
(Fig. 75, nucleus ruber). The internal structure and connec-
tions of the cerebellum will be further considered on page 186.

Summary. — The rhombencephalon includes the medulla
oblongata and cerebellum, that is, all parts of the brain below
the isthmus. All of the cranial nerves except the first four pairs
connect with the medulla oblongata. An analysis of the func-
tional components of the cranial nerves shows that they can

THE MEDULLA OBLONGATA AND CEREBELLUM 159

best be understood by considering each functional system of
fibers as a unit and studying the connections of each component
separately. These connections are summarized in a table on pp.
146, 147. The medulla oblongata of lower vertebrates and of
the human embryo is seen to be composed chiefly of the primary
centers related to these functional components of the peripheral
nerves, arranged in longitudinal columns in the order from dorsal
to ventral surfaces on each side of somatic sensory, visceral
sensory, visceral motor, somatic motor centers. The same
arrangement appears in the adult human oblongata, though
somewhat distorted by the presence of large masses of correla-
tion tissue and of large conduction tracts which are not present
in the lower vertebrates. The sensory centers of the oblongata
are connected locally with the adjacent motor centers and also
by longer tracts with the spinal cord, cerebellum, and thalamus.
The latter fibers constitute the bulbar lemniscus, of which several
functional components can be distinguished, the most important
being the trigeminal lemniscus for general cutaneous sensibility
and the lateral or acoustic lemniscus for auditory sensibility.
The cerebellum is a proprioceptive center developed, out of the
vestibular area of the medulla oblongata.

LITERATURE

The details of the structure and functions of the parts mentioned in this
and the following chapters will be found fully presented in the standard text-
books of human anatomy and physiology and in the medical text-books of
neurology, and all of this literature up to the year 1899 is summarized in
Barker's Nervous System and Its Constituent Neurones. See also W.
von Bechterew, Die Funktionen der Nervencentra, Jena, 1908 to 1911,
3 vols. For discussions of comparative neurology and the evolution of the
nervous system, reference may be made to articles in the neurological
journals, especially the Journal of Comparative Neurology; see also the
Bibliographies on pp. 36, 124, 193, and 223, and the following works:

HERRICK, C. JUDSON. 1899. The Cranial and First Spinal Nerves of
Menidia: A Contribution Upon the Nerve Components of the Bony Fishes,
Jour. Comp. Neurology, vol. ix., pp. 153-455.

— . 1913. Brain Anatomy, Wood's Reference Handbook of the Medical
Sciences, 3d ed., vol. ii, pp. 274-342.

— . 1914. Cranial Nerves, ibid., vol. iii, pp. 321-339.

JOHNSTON, J. B. 1906. The Nervous System of Vertebrates, Philadel-
phia.

— . 1909. The Central Nervous System of Vertebrates, Spengel's
Ergebnisse und Fortschritte der Zoologie, Bd. 2. Heft 2, Jena.

EDINGER, L. 1908. Vorlesungen iiber den Ban der nervosen Zentral-
organe, 7th Auflage, Band 2, Vergleichende Anatomic des Gehirns, Leipzig.

— . 1911. Idem, 8th Auflage, Band 1.

CHAPTER X

THE CEREBRUM

THE cerebrum includes all of the brain lying in front of the
isthmus, that is, the midbrain (mesencephalon), betweenbrain
(diencephalon), and cerebral hemispheres (telencephalon), the
two last comprising the forebrain (prosencephalon). It con-
tains the primary sensory centers of the olfactory nerves (I
pair), the sensory correlation centers of smell and sight, the
primary motor and sensory centers of the oculomotor and troch-
lear nerves (III and IV pairs) for movements of the eyes, and all
of the most important higher correlation centers of the brain.
These higher correlation centers make up by far the larger part
of its substance in the human brain, though in fishes the converse
relation prevails, with the primary sensori-motor centers and the
simpler correlation mechanisms making up the larger part (see
Figs. 43, 44, pp. Ill, 112).

The mesencephalon (midbrain) is that part of the brain in
which the early embryonic neural tube (Figs. 46-51, pp. 116-
119) has been least modified in the adult. The ventral part
of the midbrain, i. e., the part lying ventrally of the ventricle,
which is here termed the aqueduct of Sylvius, is called the cere-
bral peduncle; the dorsal part is the corpora quadrigemina, the
upper pair of these four eminences being the superior colliculi,
and the lower pair the inferior colliculi (see Fig. 71, p. 154).

The corpora quadrigemina contain important correlation
centers, the superior colliculus chiefly visual (p. 209) and the
inferior colliculus chiefly auditory (p. 202). The cerebral
peduncle, as the name implies, contains the great ascending and
descending fiber tracts between the forebrain above and the
medulla oblongata, cerebellum, and spinal cord below. The
arrangement of some of these tracts can be seen in Fig. 75. The
cerebral peduncle also contains the nuclei of origin for the motor
fibers of the III and IV pairs of cranial nerves and several masses

160

THE CEREBRUM

161

of gray matter devoted to motor coordination, such as the black
substance (substantia nigra) and the red nucleus (nucleus ruber,
see p. 189).

The diencephalon (betweenbrain or thalamencephalon) in
early embryonic development is a transverse region of the simple
neural tube (Fig. 48, p. 117) surrounding the third ventricle. In

Tectum mesencephali
-"Commissure tecti
Nuc. and tr. mesen. V
Tr. spino-tectalia
. thalamo-olivaris
lemniscufl

Tractus optic
Brachium quad

Dora. tegm.
decuss.

Vent. tegm.
decuss.
Tr. rubro-
spinalia

Tr. cortico-
bulb. lat.

Tr. mam.-
pedunc.

Tr. cortico
bulb. med.

Fig. 75. — Diagrammatic cross-section through the midbrain at the level
of the superior colliculus (cf. Fig. 71), to illustrate the arrangement of the
chief conduction pathways: Aq., Aqueduct of Sylvius; m., medial part of
motor nucleus of oculomotor nerve; n.III, oculomotor nerve; nuc.III,
motor nucleus of oculomotor nerve; Tr.mam.-pedunc., tractus mamillo-
peduncularis. The fibers of the dorsal tegmental decussation (Dors.
tegm.decuss., also known as the fountain decussation of Meynert) arise
from the roof of the midbrain (tectum opticum) and immediately after
crossing the median plane descend toward the spinal cord, where they form
part of the tractus tecto-spinalis (Fig. 59, p. 130). The fibers of the ventral
tegmental decussation (Vent. tegm. decuss., also known as Forel's decussa-
tion) in a similar way arise from the nucleus ruber and enter the opposite
tractus rubro-spinalis.

the adult human brain, however, it is entirely concealed by other
parts. The posterior part of it is visible from the side in the
dissection shown in Fig. 45 (p. 114), its medial surface in Fig.
52 (p. 119), and its dorsal surface is exposed in the dissection,
Fig. 76 (see also Fig. 77). This part of the brain is devoted
11

162

INTRODUCTION TO NEUROLOGY

wholly to various types of correlation. It has three main divi-
sions, the thalamus, the epithalamus, and the hypothalamus, of
which the two last are dominated by the olfactory apparatus
(see p. 220).

The epithalamus consists of the membranous chorioid plexus
which forms the roof of the third ventricle (Fig. 79), the pineal
body or epiphysis (Fig. 76), the habenula (marked trigonum

Genu of corpus callosum
Corpus callosum (cut)

Cavum septi pellucidi
Septum pellucidum

Caudate nucleus

Fornix

Foramen interventriculare
Anterior commissure
Ant. tubercle of thalamus
Massa intermedia

Third ventricle
Stria terminals
Ta'nia thalami
Trigonum habenulse

Posterior commissure
talk of pineal body
Pulvinar
Pineal body

Non-ventricular part of
thalamus

Groove correspondin

to fornix
Quadrigeminal bodi

Trochlear nerv
Brachium ponti
Brachium conjunctivum
Lingula

Medulla oblongata

Fig. 76. — A dissection of the brain from above to expose the thalamus and
corpus striatum. (From Cunningham's Anatomy).

habenulae on Fig. 76), and the stria medullaris, a fiber tract which
connects the olfactory centers of the cerebral hemispheres with
the habenula (Figs. 78, 79). The habenula is a center for the cor-
relation of olfactory sensory impulses with the various somatic
sensory centers of the dorsal part of the thalamus. The pineal
body of some lower vertebrates is a sense organ, apparently
visual in function and known as the parietal eye (p. 212); in

THE CEREBRUM 163

man its primary sensory function is lost and it is said to pro-
duce an important internal secretion whose physiological value
is still obscure.

The hypothalamus includes the tuber cinereum and mammil-
lary bodies (see Figs. 53, 78, and 79), these structures being
olfactory centers, and the hypophysis or pituitary body (which
has been removed from the specimen shown in Fig. 53, its point
of attachment being the infundibulum). The hypophysis is a
glandular organ which produces an internal secretion of great
importance in maintaining the proper balance of the metabolic
activities of the body. The hypothalamus is an important cen-
ter for the correlation of olfactory impulses with various visceral
functions, including probably the sense of taste.

The thalamus is in the human brain chiefly a sort of vestibule
through which the systems of somatic sensory nervous impulses
reach the cerebral cortex. There are, however, two parts of the
thalamus which should be clearly distinguished. The ventral
part contains chiefly motor coordination centers. It is feebly
developed in the human brain, where it is termed the subthala-
mus (not to be confused, as is often done, with the hypothala-
mus, see Figs. 78, 79, and 81). The dorsal part of the thalamus,
in its turn, contains two distinct types of sensory correlation
centers: (1) primitive sensory reflex centers, chiefly in the medial
group of thalamic nuclei ; (2) the more lateral nuclei which form
the cortical vestibule to which reference was made above.
These lateral nuclei are sometimes called the new thalamus
(neothalamus) in distinction from all of the other thalamic
nuclei which form the old thalamus (palseothalamus).

The centers which comprise the new thalamus make up by far
the larger part of the thalamus in the human brain and include
the following nuclei: the lateral, ventral, and posterior nuclei
(for general cutaneous and deep sensibility) receiving the
spinal, trigeminal, and medial lemnisci; the lateral geniculate
body and pulvinar (visual sensibility) receiving the optic tracts;
the medial geniculate body (auditory sensibility) receiving the
lateral or acoustic lemniscus. The lateral and medial genicu-
late bodies comprise the metathalamus of the B. N. A. (see p.
121 and Fig. 50, p. 118), which in this work are described as
part of the thalamus.

^Jprnix

Superior olive

Ala cintTCa

Nuclei of funiculi gracilis
and cuneatus

n. V motor
n. V sensory
Chief sensory
nucleus V

Nuc. vestib. VIII
n. VII

Nuc. cochlearis VIII

Fasciculus solitarius

Nuc. commissuralis of Cajal

Nucleus spinalis V

Fig. 77. — A diagram of the human brain stem from above after the re-
moval of the cerebral hemisphere, to illustrate the nuclei of the thalamus
and some of the chief fiber tracts connected with them. Compare Figs. 71
and 45. The fibers of the sensory radiations between the thalamus and the
cerebral cortex fall into three groups : somesthetic (sow.) for touch, tempera-
ture, and spatial discrimination, auditory (era.), and optic (opt.). Descend-
ing cortico-thalamic fibers are shown in connection with the somesthetic
radiation only; but such fibers are present in the auditory and optic radia-
tions also, ant., Anterior nucleus of thalamus; ep., pineal body (epiphy-
sis); c.g.l., corpus geniculatum laterale; c.g.m., corpus geniculatum mediale;
col. inf., colliculus inferior; col. sup., colliculus superior; lat., lateral nucleus
of thalamus; med., medial nucleus of thalamus; post., posterior nucleus of
thalamus; pulv., pulvinar; ventr., ventral nucleus of thalamus.
164

THE CEREBRUM

165

All of the thalamic nuclei of the lateral group (the neothala-
mus) are connected by important systems of fibers with the
cerebral cortex, these fibers running both to and from the cortex
(Fig. 77). These are called sensory projection fibers and all
pass through or near the internal capsule of the corpus striatum
(p. 169). As we have just seen, the nuclei of the lateral group
receive special systems of somatic sensory fibers — optic, acous-
tic, and the general cutaneous and deep sensibility complex of

Corpus callosumx

Fimbria* X'

x
Nucleus anterior ^ *

Tr. mamillo-thalamicus^x

Nucleus lateral is s v
MetathalamuSv

Habenula — >
Fasc. ret.-
Nuc. post..
Nuc. ven— \ -

Subthal.
Nuc. ruber

Lemn. V

Br. conj.__ __

3. nigra — •

Lemn. med.

Tr. th.-pe<L

Fig. 78. — Diagram of the nuclei of the diencephalon and some of their
functional connections as seen in parasagittal section taken close to the
median plane. The epithalamus and the hypothalamus are stippled.
Br.conj., Brachium conjunctivum; Fasc.ret., fasciculus retroflexus; Lemn.
med., lemniscus median's; Lemn.V., lemniscus trigemini; Nuc.post., nucleus
posterior thalami; Nuc.ven., nucleus ventralis thalami; Subthal., subthala-
mus; S.nigra, substantia nigra; Tr.th.-ped., tractus thalamo-peduncularis.

the spinal, trigeminal, and medial lemnisci. The elements of
the latter complex (comprising touch, temperature, pain, general
proprioceptive sensibility, spatial localization, etc., termed as a
whole the somesthetic group) are no doubt separately represented
in the thalamus, but the analysis of their respective thalamic
centers has not yet been completely effected. Each of the chief
functional regions of the neothalamus which have just been
enumerated is connected by its own system of projection fibers

Stria medullaris
Columna fornicis
Commissure anterior
Lamina tcnninalis
Area olfactoria medialia
Corpus mamillare
Nervus opticus
Tractus olfactorius

166

INTRODUCTION TO NEUROLOGY

with a specific region in the cerebral cortex, viz., the optic,
auditory, and somesthetic projection centers (see p. 273).
These tracts are known as the optic, auditory, and somesthetic
radiations (see Fig. 80).

The old thalamus (palaeothalamus) comprises the more
medial thalamic centers, which were differentiated for the
primitive thalamic correlations which are present in fishes and
other lower vertebrates which lack the cerebral cortex. Clinical

Lateral (Sylvian)

fissure
Internal capsule

Island of Reil
Lentiform nucleus

External capsule

Corpus callosum
Fornix

Chorioid plexus

Stria medullaris

Nuc. ant. thai.

Nue. medial, thai.

Nuc. lateral, thai.

Nuc. ventral, thai.

Subthalamus

Mammillary body

Optic tract

Amygdala-

Lateral ventricle

Fig. 79. — Cross-section through the human cerebral hemisphere and
thalamus, including the mammillary body and the posterior end of the an-
terior nucleus of the thalamus (cf. Fig. 78). At this level the epithalamus
is represented only by the stria medullaris and the chorioid plexus of the
third ventricle, the hypothalamus by the mammillary body. The old
thalamus (pala;othalamus) is represented by the anterior and medial nuclei
and the subthalamus, the new thalamus (neothalamus) by the lateral and
ventral nuclei.

evidence (see especially Head and Holmes, 1911) seems to show
that many of these primitive functions are retained in the old
thalamus in man, and that some of the conscious activities are
served by these thalamic centers. In other words, the activity
of the cerebral cortex is not essential for all conscious processes,
though its participation is necessary for others, particularly
all intellectual and voluntary activities. The thalamus, on the
other hand, can act independently of the cortex in the case of

THE CEREBRUM 167

painful sensibility and the entire series of pleasurable and pain-
ful qualities; for the thalamic centers when isolated from their
cortical connections are found to be concerned mainly with
affective experience, and destructive lesions which involve the
cortex alone do not disturb the painful and affective qualities of
sensation (see p. 253).

The relations of the thalamic nuclei and of some of the tracts
connected with them are shown as seen from above in Fig. 77
and in a section parallel with the median plane in Fig. 78.

THE DIENCEPHALON
I. Epithalamus.

1 . Chorioid plexus of the third ventricle.

2. Pineal body (epiphysis).

3. Habenula (receives the stria medullaris from the olfactory centers

and sends fibers to the cerebral peduncle).

II. Thalamus.

1. Dorsal part.

(1) Medial group of nuclei.

(a) Medial nucleus (receives fibers from the olfactory area
and neothalamus and from the trigeminal lemniscus;
sends fibers to the olfactory area, corpus striatum,
subthalamus, and probably cerebral cortex).

(6) Anterior (or dorsal) nucleus (receives fibers from the
mammillary body and sends fibers to the corpus stri-
atum).

(2) Lateral group of nuclei (neothalamus).

(a) Lateral, ventral, and posterior nuclei (receive the
medial, spinal, and trigeminal lemnisci ; connect with
parietal and frontal cortex by ascending and descend-
ing somesthetic projection fibers).

(6) Pulvinar and lateral geniculate body (receive optic
tracts; connect with occipital cortex by ascending
and descending optic projection fibers).

(c) Medial geniculate body (receives the lateral or acoustic
lemniscus; connects with temporal cortex by ascend-
ing and descending auditory projection fibers).

[The two geniculate bodies = metathalamus, B. N. A.]

2. Ventral part, or subthalamus (a motor coordination center re-

ceiving fibers from the dorsal part of the thalamus, from the
corpus striatum and from the pyramidal tract; sends fibers to
the pedunculus cerebri; comprises the body of Luys, Forel's
field H2, and some adjacent gray matter; is continuous behind
with the substantia nigra of the cerebral peduncle).

III. Hypothalamus.

1. Tuber cinereum (olfacto-visceral correlation center).

2. Mammillary body (receives fibers from the olfactory centers; sends

fibers to the cerebral peduncle and nucleus anterior thalami).

3. Hypophysis.

168 INTRODUCTION TO NEUROLOGY

Some of these centers are seen in cross-section in Fig. 79. The
preceding analysis of the diencephalon, which differs in some
respects from that of the B. N. A. (p. 121), is summarized in the
accompanying table (p. 167), which includes also a few of the
more important fiber tracts connected with each nucleus.

In front of the thalamus lie the corpus striatum and olfactory
centers (see Fig. 45, p. 114), and above these last two is spread
the great expanse of the cerebral cortex or pallium. The corpus
striatum consists of masses of gray matter separated by sheets
of white matter, an arrangement which gives a striated appear-
ance in section.

In studying the comparative anatomy of the cerebral hemi-
spheres we find the corpus striatum well developed in some lower
vertebrates which lack the cerebral cortex, and very highly de-
veloped in others, like reptiles and birds, where the cortex is
present, though very small. In these animals the corpus stri-
atum appears to be a reflex center of great importance and of
higher order than the thalamus; and the dfferentiation of this
apparatus seems to have been a necessary precursor of the elabo-
ration of the cerebral cortex as we find it in the mammals.

The functions of the mammalian corpus striatum are very
obscure. It is connected by both ascending and descending
fibers with various nuclei of the thalamus and cerebral peduncle,
and also with the cerebral cortex. Ramon y Cajal is of the
opinion that the mammalian striatum functions chiefly to re-
inforce the descending motor impulses which leave the cerebral
cortex, these systems of fibers giving off collateral branches as
they traverse it, and the striatum itself sending important de-
scending tracts into the thalamus and cerebral peduncle.

The white matter of the corpus striatum consists partly of the
fibers already mentioned as passing between it and the thalamus
and cortex, but chiefly of fibers passing between the cortex and
deeper parts of the brain stem, having no functional connection
with the striatum itself. These are called projection fibers.
They are partly ascending and descending fibers passing between
the thalamus and the cortex (the optic, auditory, and somesthetic
projection systems, or radiations, which have already been
mentioned, p. 165), and partly descending motor projection
fibers of the cortico-spinal or pyramidal tract (p. 140 and Fig.

THE CEREBRUM 169

64, p. 141), cortico-bulbar tract, and cortico-pontile tracts (pp.
187 and 289).

The gray matter of the corpus striatum is gathered into two
principal masses, the caudate nucleus and the lentiform nucleus
(so-named from their shapes), and most of the projection fibers
pass between these nuclei in a wide band of white matter known
as the internal capsule. The broken ends of the internal capsule
fibers are seen in the dissection shown in Fig. 45 (p. 114). As
these fibers radiate from the internal capsule toward the cortex
they are called the corona radiata (Fig. 79). The external
capsule is a thinner sheet of fibers externally of the lentiform
nucleus (Figs. 79 and 80). Figure 79 illustrates a transverse
section through the cerebral hemisphere, showing the relations
of the thalamus and corpus striatum.

The exact arrangement of the functional systems of sensory
and motor projection fibers within the internal capsule is a
matter of great clinical importance; for a considerable propor-
tion of apoplexies and other cerebral diseases result from hemor-
rhage or other injury of the internal capsule causing destruction
of some of its fibers. A partial paralysis will result, whose symp-
toms will depend upon the particular functional systems of pro-
jection fibers affected. Figure 80 illustrates the arrangement
of some of the systems of fibers of the internal capsule as seen
in a horizontal section through the cerebral hemispheres.

The olfactory centers of the cerebral hemispheres and the
cerebral cortex will be considered in chapters which follow.

Summary. — The cerebrum contains the primary centers for
the I, II, III, and IV pairs of cranial nerves, but most of its
substance is concerned with the higher centers for the correla-
tion of sensory impressions, especially those involved in the
psychic activities. The midbrain contains in the corpora quad-
rigemina important reflex correlation centers of sight and hear-
ing, and in the cerebral peduncle centers for the coordination of
movements. The diencephalon is devoted chiefly to various
types of correlation. It is divided into three parts, the thalamus,
the epithalamus, and the hypothalamus, the two last being
dominated by the olfactory system. The thalamus contains a
medial group of nuclei concerned with thalamic reflexes and the
affective experience and a lateral group of nuclei which discharge

170

INTRODUCTION TO NEUROLOGY

Gyrus frontalis inf.
Gyrus cinguli
Corpus callosum
External capsule
Lateral ventricle
Caudate nucleus

Anterior limb of
internal capsule

Column of fornix
Lentiform nucleus

Posterior limb of
internal capsule

Medial nucleus of
thalamus
Third ventricle

Lateral nucleus

thalamus
Hippocampal com.

Corpus callosum

Caudate nucleus

Parieto-occip. fissure

Cuneus

Fig. 80. — Longitudinal section through the human cerebral hemisphere
passing through the internal capsule, some of the fiber systems of which are
numbered as listed below:

1. Frontal thalamic tracts between the medial nucleus of the thalamus
and the frontal lobe of the cerebral cortex.

2. Frontal pontile tract between the frontal lobe of the cortex and the
pons.

3. Cortico-oculomotor tract from the motor cortex to the nucleus of the
oculomotor nerve.

4. Cortico-bulbar tracts from the motor cortex to the motor nuclei of the
medulla oblongata.

THE CEREBRUM 171

the sensory projection systems of sight, hearing, and general
sensibility into the cerebral cortex. The subdivision of the
diencephalon is summarized in the table on p. 167. The cor-
pus striatum in lower vertebrates is an important reflex center;
in man its functions seem to be subsidiary to those of the cere-
bral cortex for the most part. It consists of two chief masses of
gray matter, the caudate and lentiform nuclei, with sheets of
white matter between and within these masses. The chief sys-
tems of fibers of the white matter are accumulated in the internal
capsule which lies between the lentiform nucleus laterally and the
caudate nucleus and thalamus medially. Through the internal
capsule run the projection fibers which connect the cerebral cor-
tex with the lower parts of the brain stem, including the sensory
radiations from the thalamus and the descending systems to the
pons and brain stem and the great pyramidal tract, which is the
voluntary motor path from the cortex to the spinal cord.

LITERATURE

HEAD, H., and HOLMES, G. 1911. Sensory Disturbances from Cerebral
Lesions, Brain, vol. xxxiv, pp. 109-254.

HERRICK, C. JUDSON. 1913. Article Brain Anatomy, in Wood's Refer-
ence Handbook of the Medical Sciences, 3d ed., vol. ii, pp. 274-342.

JOHNSTON, J. B. 1906. The Nervous System of Vertebrates, Philadel-
phia.

v. MONAKOW, C. 1895. Experimentelle und pathologische-anatomische
Untersuchungen liber die Haubenregion, den Sehhiigel und die Regio sub-
thalamica, Arch. f. Psychiat., Bd. 27.

SACHS, E. 1909. On the Structure and Functional Relations of the Optic
Thalamus, Brain, vol. xxxii, pp. 95-186.

5. Cortico-rubric tract from the motor cortex to the nucleus ruber.

6 to 10. Pyramidal tract (tractus cortico-spinalis) from the .motor cortex
to the spinal cord, with the following parts —

6. To the cervical spinal cord for the muscles of the shoulder.

7. To the cervjcal cord for the muscles of the arm.

8. To the cervical cord for the muscles of the hand.

9. To the lumbar cord for the muscles of the leg.

10. To the lumbar cord for the muscles of the foot.

11. Somesthetic radiations from the lateral and ventral nuclei of the
thalamus to the cerebral cortex.

12. Occipito-temporal pontile tract to the pons, and temporo-thalamic
tract to the thalamus.

13. Auditory radiation from the medial geniculate body to the superior
temporal gyrus.

14. Optic radiation from the pulvinar and lateral geniculate body to the
cuneus in the occipital lobe of the cortex.

CHAPTER XI

THE GENERAL SOMATIC SYSTEMS OF CONDUCTION

PATHS

IN this and the following chapters we shall review the con-
duction pathways followed by some of the chief sensori-motor
systems and add some further details to the general description
already given, beginning with the more generalized somatic
sensory functions.

Clinical neurologists have long been in the habit of grouping
together the different forms of deep and cutaneous sensibility
under the term "general sensibility." The more refined re-
searches of recent students (especially Sherrington, Head,
Trotter and Davies, Brouwer, see the bibliographies on pp.
94 and 142) have given us a much more precise analysis
of these systems, as already explained. The peripheral
nerves of deep sensibility (exclusive of those devoted to
strictly visceral functions) are anatomically distinct from
those of cutaneous sensibility. Physiologically, the nerves of
deep sensibility are devoted chiefly to proprioceptive functions
(muscle sensibility, joint sensibility, etc.), and the nerves of
cutaneous sensibility chiefly to exteroceptive functions (touch,
temperature, and pain); but this holds only approximately, for
nerves of deep sensibility may also serve the exteroceptive func-
tions of pressure and painful response to overstimulation,
though with a higher stimulus threshold than in the skin, and the
cutaneous nerves also participate to some extent in the proprio-
ceptive functions of spatial orientation of the body and its mem-
bers (see pp. 77 ff. and 132).

Exteroceptive Systems. — The nerves serving the functions
of touch, pressure, temperature, and pain of the body and
limbs, whether derived from the skin or the deep tissues, im-
mediately after their entrance into the spinal cord terminate in

172

GENERAL SOMATIC SYSTEMS OF CONDUCTION PATHS 173

the gray matter of the dorsal column of the same side. After
a synapse here the axons of the neurons of the second order cross
to the opposite side of the cord and ascend in the spinal lemniscus
to the thalamus. For further details of these connections see
pages 138-140, 163-169, and Figs. 59, 63, 64, 75, 77, 78, 80, 81;
on the pain path, see also p. 251. The pathway for cutaneous
sensibility from the head follows the trigeminal lemniscus (pp.
157, 180, and Figs. 64, 75, 77, 78, 81). The more important
exteroceptive pathways are assembled in Fig. 81.

It will be recalled that in the spinal lemniscus the pathways for
touch and pressure, for pain and for temperature are assembled
in three distinct tracts, those for pain and temperature being
close together (Fig. 63, p. 139). From this it follows that small
circumscribed injuries in the white substance of the spinal cord
may destroy all sensibility to pressure in a part of the body with-
out any disturbance whatever of pain or temperature sensibility,
or conversely, it may destroy pain or temperature sensibility
without any involvement of the other qualities of sensation.
And, in fact, in numerous clinical cases these conditions are
found, as will be clear from the following example..

Figure 82 illustrates such a case from Dr. Head's experience.
The patient suffered from an injury to the lower part of the
spinal cord caused by the overturning of a truck of concrete,
and when admitted to the London Hospital was paralyzed from
the hips downward. In the course of a year he partly recovered,
but showed a permanent loss of some sensation qualities over
the shaded area in the figure. The right leg below the knee was
insensitive to pain (prick) and to all degrees of temperature.
But over the whole of this area he could appreciate all tactile
stimuli and could localize accurately the spot touched or pressed
upon. Yet it was not possible to produce pain anywhere over
the right leg and foot by excessive pressure, although he fully
recognized its gradual increase. Referring to Fig. 63 (p. 139),
it is evident that to produce these symptoms the lesion must
have involved the conduction path for pain and temperature in
the lateral funiculus (fiber 8 of the figure) of the left side of the
spinal cord, and spared the path for touch and pressure in the
ventral funiculus (fiber 9). Both superficial pain (prick) and
deep pain caused by excessive pressure were abolished. This

174

INTRODUCTION TO NEUROLOGY

spinal V Tract
reficular formation

spinal Y Tract-
spinolfnudeus

-e. Touch and pressure
- 7. pain and tempraTure
- p} spinal lemniscus

Fig. 81. — Diagram of the exteroceptive conduction pathways contained
within the spinal cord and brain stem. The figure illustrates cross-sections
of the central nervous system in the lower cervical region of the spinal
cord, at the level where the cord passes over into the medulla oblongata, at
the level of the roots of the VIII cranial nerve, through the inferior collicu-
lus and through the thalamus.

1. Connections of peripheral neuron of touch, temperature, or pain for
intrinsic spinal reflexes.

GENERAL SOMATIC SYSTEMS OF CONDUCTION PATHS 175

combination of symptoms could not be produced by any injury
to the nerve-roots or peripheral branches.

Proprioceptive Systems. — Referring back to p. 137, we are
reminded that the ascending proprioceptive fibers of the spinal
cord effect three types of connections within the brain: (1) in

Fig. 82. — The sensory loss resulting from an injury to the lower part of
the spinal cord. The shaded area represents the parts insensitive to cuta-
neous painful stimuli and also to the pain of excessive pressure; yet over this
area light touch and the tactile element of pressure were appreciated.
(After Head and Thompson.)

the cerebellum; (2) in the brain stem; (3) in the cerebral cortex.
The connections of the second and third types are made through
the dorsal funiculus and medial lemniscus; they are shown in

2. Peripheral neuron of pain or temperature.

3. Peripheral neuron of touch and pressure.

4. Peripheral motor neurons of spinal nerve.

5. Peripheral cutaneous neuron of trigeminal nerve.

6. Secondary neuron of touch and pressure in spinal lemniscus.

7. Secondary neuron of pain or temperature in spinal lemniscus.

8. Secondary neuron from lower part of spinal V nucleus entering the
spinal lemniscus.

9. Secondary neuron from chief sensory V nucleus entering the trigemi-
nal lemniscus.

10. Intrinsic correlation neuron of thalamus for thalamic reflexes.

11, 12, 13. Thalamo-cortical radiations to the postcentral gyrus.

176

INTRODUCTION TO NEUROLOGY

— to cerebellum

(brochium conjuncTivuml

7, longitudinal medial
_i_^-^ fasciculus

^ r\\

dorsal spino-
cerebeliar tract, 7

nucleus of fast gracilis
nucleus of fosccuneotus

6,spino-olivary trod
5,ventral spino- cerebeltar Trocl

Fie. 83. — Diagram of the chief proprioceptive conduction pathways con-
tained within the spinal cord and brain stem. The mesencephalic root of
the trigeminal nerve (see p. 180 and Figs. 71 and 77) is omitted and not all
of the cerebellar connections are indicated. The connection to the cere-
bellum from the nuclei of the fasciculi gracilis and cuneatus (neuron 14) is
controverted, but it is well established that similar connections are effected
immediately below this level from the dorsal funiculus of the cord. The
figure illustrates cross-sections of the central nervous system in the lower
cervical region of the spinal cord, at the level where the cord passes over

GENERAL SOMATIC SYSTEMS OF CONDUCTION PATHS 177

Figs. 59, 63, 64, 75, 77, 78, and 80, and in a more comprehensive
way in Fig. 83.

The cortical proprioceptive pathway in its simplest form may
consist of a chain of only three neurons: (1) A peripheral neuron
whose cell body lies in some spinal ganglion, whose dendrite
reaches some organ of muscle sense, tendon sense, or similar
receptor, and whose axon terminates at the upper end of the cord
in the nucleus of the fasciculus gracilis or fasciculus cuneatus of
the same side; (2) the body of the second neuron lies in one of
the nuclei last mentioned (marked nucleus of dorsal funiculus in
Fig. 64), its axon ascends in the medial lemniscus, and termi-
nates in the lateral and ventral nuclei of the thalamus of the
opposite side (Figs. 77 and 83) ; (3) the neuron of the third order
lies in the thalamus and sends its axon through the internal cap-
sule to the somesthetic area of the cerebral cortex.

The dorsal funiculi of the spinal cord have until recently been
regarded as the chief ascending pathway for all forms of sensi-
bility, and much of the clinical practice now in vogue is based
upon this assumption. But evidently such an assumption is
untenable. The dorsal funiculi seem to be concerned chiefly

into the medulla oblongata, at the level of the roots of the VIII cranial
nerve, through the inferior colliculus, and through the thalamus.

1. Peripheral neuron entering the dorsal funiculus and also effecting
intrinsic spinal reflex connections.

2. Peripheral neuron entering the nucleus dorsalis of Clarke.

3. Peripheral neuron effecting connections with the intrinsic correla-
tion neurons of the spinal cord.

4. Peripheral motor neurons of spinal nerve.

5. Ventral spino-cerebellar tract.

6. Spino-olivary tract.

7. Dorsal spino-cerebellar tract.

8. 9. Medial lemniscus.

10. Vestibular root fiber passing directly into the cerebellum.

11. Vestibular root fiber entering the vestibular nucleus.

12. Vestibulo-cerebellar tract.

13. Olivo-cerebellar-tract.

14. Path from the dorsal funiculus (or its nuclei) to the cerebellum.

15. Path from the reticular formation to the cerebellum.

16. Vestibulo-spinal tract.

17. Path from the vestibular nucleus to the fasciculus longitudinalis
medialis.

18. Path from the vestibular nucleus to the reticular formation.

19. 20. Thalamic radiations to the cerebral cortex.
21. Tecto-cerebellar tract.

12

178 INTRODUCTION TO NEUROLOGY

with the proprioceptive group of reactions. These may be
unconscious reflexes of motor coordination and the maintenance
of equilibrium, or they may come into consciousness as sensa-
tions of position and orientation of the body and its parts and of
spatial discrimination. Purely exteroceptive stimuli, whether
transmitted by the deep nerves or by the cutaneous nerves, may
be carried for a few segments in the dorsal funiculi (Fig. 81,
neuron 1); but they are soon filtered off into the gray matter of
the dorsal column, and after a synapse here they are sorted into
functionally distinct tracts on the opposite side of the cord.
The tactile elements of the mixed peripheral root fibers entering
the dorsal funiculus are drawn off later than are the elements for
thermal and painful sensibility; and some of the components
commonly reckoned with cutaneous exteroceptive sensibility
remain in the dorsal funiculus for its entire length. These are
chiefly two-point discrimination, and discrimination of size,
shape, form, and texture of surfaces. These all involve a com-
parison and discrimination in consciousness of spatial factors
and are, therefore, bound up with those fibers which serve the
proprioceptive reflexes, which are unconscious spatial adjust-
ments.

Some peculiar combinations of symptoms arise from the fact
that, whereas the ascending proprioceptive impulses (so far as
these are consciously perceived) pass up in the dorsal funiculus
of the same side for the entire length of the cord, the impulses
of the exteroceptive impulses, within a few segments of their
point of entrance into the cord, are transferred to the opposite
side to ascend in the spinal lemniscus tracts. From this it fol-
lows that a localized central injury involving the dorsal gray
column and dorsal funiculus of one side only will cut off all
ascending proprioceptive impulses which pass through the dor-
sal funiculus from lower levels on the same side of the body as the
lesion, and at the same time will abolish both proprioceptive and
exteroceptive functions in a circumscribed region of the same
side of the body whose exteroceptive neurons of the first order
discharge into the injured part of the dorsal gray column.

Figure 84 illustrates the loss of sensibility to painful stimuli
resulting from a tumor in the cervical region of the spinal cord.
Tactile, temperature, and deep sensibility were also profoundly

179

disturbed over approximately the same region (the temperature
disturbance involving the right side also). These symptoms
resulted from the destruction of all dorsal root fibers in the
affected area at the point of their entrance into the cord or of the
gray substance containing the terminals of these fibers, a purely
local effect. That the dorsal funiculus of the same side was also
involved is shown by symptoms of remote effects of the injury
in the left foot. All forms of exteroceptive sensibility (touch,
temperature, pain) were perfectly preserved in both legs, but the
left leg was devoid of proprioceptive sensibility, as shown by the

Fig. 84. — The loss of sensibility to pain resulting from a tumor in the cervical
region of the spinal cord. (After Head and Thompson.)

loss of ability to appreciate the passive position or movement of
the leg and failure to discriminate two points with the compass
test.

The intrinsic connections within the cord for spinal reflexes are undoubt-
edly very primitive. These are both exteroceptive and proprioceptive in
type (p. 132). We have seen that the ascending tracts between the spinal
cord and the brain fall into two groups: (1) The exteroceptive systems in
the spinal lemniscus, and (2) the proprioceptive systems in the dorsal funicu-
lus and medial lemniscus. Comparative anatomy shows that the spinal
lemniscus system is much older phylogenetically than the medial lemniscus
system. The fishes possess well-defined spino-tectal and spino-thalamic
tracts, but their dorsal funiculus possesses only the fasciculus propriua

180 INTRODUCTION TO NEUROLOGY

fibers (cf. Figs. 66, 67, pp. 150, 151) and they lack the medial lemniscus
altogether. The spino-cerebellar tracts, on the other hand, are very ancient
and are present from the lowest to the highest vertebrates.

These considerations suggest that the first fibers to pass from the spinal
cord to the higher centers of the brain, and presumably the first sensory
impulses from the spinal nerves to be consciously perceived, were those of
touch and temperature transmitted through the spinal lemniscus. (Pain
is probably also very primitive as a conscious experience, but it is doubtful
whether it is represented in the spinal lemniscus of lower forms; see p. 251).
The proprioceptive impulses in lower vertebrates are coordinated quite un-
consciously in the brain stem and cerebellum, and it is only in the higher
forms that this system of nervous impulses reaches the thalamus (through
the medial lemniscus) and cerebral cortex for conscious control. Clinical
evidence shows that the medial lemniscus connections in man are concerned
with the conscious adjustments of the positions and orientation in space of
the body and its members and with spatial discriminations of various sorts,
rather than with the senses of touch and pressure as externally projected.

The innervation of the organs of muscular sensibility and tendon sensi-
bility in the head is not as fully known as in the case of those of the trunk
and limbs, as above described. Sherrington and Tozer have recently
shown that such organs are present in the muscles which move the eyeball
and that their nerves accompany the motor fibers of the III, IV, and VI
cranial nerves; but of the central connections of these sensory nerve-fibers
of the eye muscles nothing is known. It is suggested by the researches of
Johnston, Willems, and many others that the jaw muscles, which receive
their motor innervation from the motor V nucleus (nucleus masticatorius),
receive their sensory innervation from the mesencephalic nucleus of the V
nerve, whose position along the lateral border of the aqueduct of Sylvius
is seen in Figs. 71, 75, and 77. But recent studies of Edgeworth have
shown that these muscles also receive sensory fibers from the semilunar
or Gasserian ganglion of the V nerve, and the question requires further inves-
tigation. Possibly the sensory fibers from the Gasserian ganglion to the
muscular branches of the V nerve conduct impressions of deep sensibility
of pressure and pain of the exteroceptive type, while those from the mesen-
cephalic V nucleus innervate the muscle spindles for true proprioceptive
sensibility.

The fibers of the chief sensory root of the V nerve in part end in the chief
sensory V nucleus near the level of their entrance into the medulla oblongata
(Figs. 71, 77) and in part pass downward through the whole length of the
medulla oblongata and upper levels of the spinal cord as the spinal V tract
(Figs. 64, 71, 72, 81). It is suggested by clinical and comparative evidence
that the spinal V tract and its nucleus are connected with a phylogenetically
old type of reaction to touch, temperature, and pain, probably chiefly reflex,
while the chief nucleus is concerned with the more recently acquired dis-
criminations of these systems with more direct cortical connections. The
fibers of the trigeminal lemniscus (p. 157) follow two separate tracts arising
from these two parts of the sensory V nucleus, only the upper one of which
is shown in Fig. 77, though both are shown in Fig. 81 (neurons 8 and 9).

Motor Paths. — Throughout the length of the spinal cord and
brain stem the ascending fibers of both exteroceptive and pro-
prioceptive sensibility give off collateral branches into the reticu-

GENERAL SOMATIC SYSTEMS OF CONDUCTION PATHS 181

lar formation (p. 158) for reflex connections with the motor
nuclei at various levels. The arrangement of these motor nuclei
of the brain stem, from which peripheral motor fibers of the
cranial nerves arise, is shown on the left side of Fig. 71 (p. 154).
The details of these connections for local motor reflexes will not
be entered into here. From the ventral part of the thalamus
(p. 163) there are descending thalamo-bulbar and thalamo-
spinal tracts for local thalamic reflexes. The main descending
pathway for voluntary motor responses to general somatic
stimuli arises from the precentral gyrus of the cerebral cortex
(p. 283). This is the tractus cortico-bulbaris (Fig. 75) and trac-
tus cortico-spinalis or pyramidal tract (Figs. 64, 75, 137). The
reflex connections effected in the medulla oblongata are some-
what more complex than those of the spinal cord, that is, they
represent the integration of more different kinds of sensory im-
pulses and facilitate the performance of a greater variety of
movements by way of response. Similarly, the complexity of
the reflex adjustments increases as we pass forward into the mid-
brain, thalamus, and cerebral cortex (see p. 63).

Attention has already been called to the fact that the centers of adjust-
ment in the brain stem are of two physiologically different types which we
have termed centers of correlation and centers of coordination (p. 35). The
more labile and individually variable adjustments are effected in the corre-
lation centers which are developed from the more dorsal parts of the embry-
onic neural tube above the limiting sulcus (p. 120), while the more ventral
parts of the neural tube give rise to the motor centers and the centers of co-
ordination, whose adjustments are of a more fixed and invariable character.
In the embryonic development the coordination centers develop preco-
ciously, while the correlation centers mature more slowly; the higher asso-
ciation centers of the thalamus and cerebral cortex in particular are the last
to mature (p. 286).

In the phylogenetic development of the brain the same rule holds. In
the lowest vertebrates the coordination centers are much larger in pro-
portion to the size of the correlation centers than in higher vertebrates.
Bartlemez1 has analyzed these motor coordination mechanisms (which he
terms in the aggregate the nucleus motorius tegmenti) in fishes, and finds in
the motor tegmentum throughout the medulla oblongata a nucleus of a
primitive type whose neurons serve to connect the primary sensory nuclei
with the primary motor nuclei. Some of these connections are very short,
while others are very long, reaching remote parts of the brain and spinal
cord through the longitudinal medial fasciculus (pp. 185, 211). This nu-
cleus is the parent tissue out of which the more complex coordination centers
in the tegmentum of higher vertebrates have been differentiated.

1 BARTLEMEZ, G. W. 1915. Mauthner's Cell and the Nucleus Motorius
Tegmenti, Jour. Comp. Neur., vol. xxv, pp. 87-128.

182 INTRODUCTION TO NEUROLOGY

In very young amphibian embryos Coghill1 finds a still simpler condition
which is probably also more primitive. In the spinal cords of these larvae
the individual neurons of the motor tegmentum give rise both to fibers of the
longitudinal conduction tract of motor coordination (fasciculus proprius
ventralis) and to peripheral fibers of the ventral roots, the latter arising as
collaterals of the longitudinal axons. In older larvae separate neurons have
been differentiated for these two functions of peripheral conduction and
longitudinal conduction. The steps in the embryologic development and
probable evolution of the more complex centers of adjustment have been
briefly reviewed by Herrick and Coghill (see p. 66).

Summary. — The old clinical concept "general sensibility"
has recently been analyzed into a number of components, the
most fundamental division being the distinction between a
group of exteroceptive and a group of proprioceptive systems.
The exteroceptive systems are transmitted from the spinal
cord to the brain through a complex tract, the spinal lemniscus,
within which there are separate pathways for the three qualities
of sensation, touch, temperature, and pain. These sensation
qualities come into consciousness with a distinct peripheral or
external reference. The proprioceptive systems (muscle sense
and allied types) are transmitted to the brain through the dorsal
funiculus of the same side of the cord, the medial lemniscus of
the opposite side, the thalamus, and the somesthetic radiations
to the cerebral cortex; and also through the spino-cerebellar
tracts to the cerebellar cortex. Most of these reactions of
spatial adjustment do not come into consciousness at all, but
some appear subjectively as sensations of posture, bodily move-
ment, and spatial discrimination. The cerebellum is the great
clearing house for these and all other afferent systems which
are concerned in the proprioceptive functions, so far as these are
unconsciously performed.

1 COGHILL, G. E. 1913. The Primary Ventral Roots and Somatic Motor
Column of Amblystoma, Jour. Comp. Neur., vol. xxiii, pp. 121-144.

— . 1914. Correlated Anatomical and Physiological Studies of the
Growth of the Nervous System of Amphibia, Jour. Comp. Neur.. vol. xxiv.
pp. 161-233.

CHAPTER XII

THE VESTIBULAR APPARATUS AND CEREBELLUM

THE general somatic sensory systems considered in the last
chapter include some of the most primitive reflex mechanisms.
These fall into two groups — the exteroceptive systems and the
proprioceptive systems (pp. 77-89)— and each of these groups
comprises, in addition to its primitive generalized members,
certain so-called organs of special or higher sense. The special
exteroceptive sense organs are the organ of hearing (p. 195)
and the organ of vision (p. 204). The special proprioceptive
sense organs are the semicircular canals of the internal ear; and
those will next be described, together with their central mechan-
isms in the medulla oblongata and cerebellum.

The Vestibular Apparatus. — The internal ear contains two
quite distinct groups of sense organs, the organ of hearing in the
cochlea and the vestibular organs (utricle, saccule, and semicircu-
lar canals), both of which are supplied by the VIII cranial nerve,
which accordingly has two parts, the cochlear and the vestibu-
lar nerves. The semicircular canals are the most highly spe-
cialized end-organs of the proprioceptive series and are- con-
cerned chiefly with the maintenance of bodily equilibrium. The
general structure of the internal ear is described on p. 195; here
we need merely mention that the three semicircular canals
(ductus semicirculares) of each ear lie approximately at right
angles to each other, as shown diagrammatically in Fig. 85, and
each canal is dilated at one end to form the ampulla, within
which is a patch of sensory epithelium from which hairs project
into the contained fluid (see Figs. 32 and 91). A movement of
the head in any direction will cause a flow of the fluid in one or
more of these canals in each ear, which in turn will excite a
nervous impulse in the hair-cells of the corresponding ampullae.
These nervous impulses will be transmitted to the vestibular
centers of the brain, where they will be so analyzed as to call forth

183

184

INTRODUCTION TO NEUROLOGY

the appropriate reaction to the movement which has excited the
particular semicircular canals involved.

The fibers of the vestibular nerve enter the medulla oblongata
immediately behind the pons and terminate in a vestibular nu-

Fig. 85. — Diagram of the position of the semicircular canals in the head,
as seen from behind. On each side it will be seen that the three canals lie
in planes at right angles to one another. The external or horizontal canals
(E) of the two sides lie in the same plane. The anterior canal of one side
(A) lies in a plane parallel to that of the posterior canal (P) of the other
side. (After Ewald.)

cleus which forms an eminence on the floor of the fourth ventricle
in this region (Figs. 71, 96). This nucleus has four subdivisions,
as follows:

Nucleus nervi vestibuli medialis (of Schwalbe, also called nucleus dor-

salis and principal nucleus).
Nucleus nervi vestibuli lateralis (of Deiters).
Nucleus nervi vestibuli superior (of Bechterew).
Nucleus nervi vestibuli spinalis.

The arrangement of these nuclei and of some of their second-
ary connections is shown in Fig. 86. Some of these connections
are made with the motor nuclei and reticular formation of the

THE VESTIBULAR APPARATUS AND CEREBELLUM

185

medulla oblongata for local bulbar reflexes; there is a vestibulo-
spinal tract (tr.v.sp.) for movements of the trunk and limbs

brc.mf.

collie, sup
c.g.rn.

col lie inf.
f.Lm

— Tr.v.qereh

nuc amb.

Fig. 86. — Diagram of the nuclei of the vestibular nerve, together with
some of the associated fiber tracts. The secondary tracts associated with
the vestibular nuclei are drawn in full lines; a part of the secondary auditory
path from the cochlear nuclei is drawn in broken lines. Compare Figs.
71, 77, 96. br.c.inf., brachium quadrigeminum inferius; c.g.m., corpus genic-
ulatum mediale; collie, inf., colliculus inferior; collie, sup., colliculus superior;
f.l.m., fasciculus longitudinalis medialis; L, nucleus nervi vestibuli lateralis
(Deiters); Im. lat., lemniscus lateralis; M, nucleus nervi vestibuli medialis
(Schwalbe); n.lm. lot., nucleus of lemniscus lateralis; nuc. amb., nucleus
ambiguus; ol. sup., superior olive; S, nucleus nervi vestibuli superior (Bech-
terew); Sp., nucleus spinalis nervi vestibuli; tr. v. cereb., tractus vestibulo-
cerebellaris; tr.v.sp., tractus vestibulo-spinalis; VIII c., radix cochlearis of
VIII nerve; VIII v., radix vestibularis of VIII nerve.

in response to stimulation of the semicircular canals; and there
is also a strong connection with the longitudinal medial fascicu-

186 INTRODUCTION TO NEUROLOGY

lus (f.l.m.\ by which fibers descend to the spinal cord (chiefly
for turning movements of the head by the neck muscles) and
ascend to the midbrain. The last-mentioned fibers connect
chiefly with the nuclei of the motor nerves for the eye muscles
(III, IV, and VI pairs of cranial nerves), thus providing for
the conjugate movements of the eyes which accompany head
movements (in this way, for instance, enabling one to keep the
gaze fixed upon a stationary object while the head is moving,
cf. p. 211).

It will be noticed that there is no important pathway from
the vestibular nucleus to the thalamus and cerebral cortex, for
the equilibratory reactions excited from the semicircular canals
are normally unconsciously performed. This is in marked
contrast with the connections of the cochlear nerve, for the audi-
tory reactions are often consciously directed (p. 202). There
is, however, an important connection with the cerebellum,
partly directly by root fibers of the vestibular nerve and partly
by secondary fibers from the superior and lateral vestibular
nuclei (Fig. 86). The cerebellum is, accordingly, an important
center of adjustment for the proprioceptive reflexes, and to this
our attention will next be directed.

The Cerebellum. — This important organ is an overlord which
dominates the proprioceptive functions of the body in some-
what the same way that the cerebral cortex directs and controls
the exteroceptive reactions. Both of these organs are second-
arily added to the more primitive segmental structures of the
brain stem, that is, they are suprasegmental (p. 113).

The correlation centers of the brain stem, and particularly
those of the cerebral cortex, analyze the afferent impulses enter-
ing the brain and determine what particular reactions are ap-
propriate in each situation. After the character of the move-
ment has been determined in this way, the proprioceptive sys-
tems cooperate in its execution, and the cerebellum is the cen-
tral coordination station for the proprioceptive reactions.
None of its activities come into consciousness.

The cerebellum, therefore, is intimately connected with all
sensory centers which are concerned in the adjustment of the
body in space and motor control in general. The maintenance
of bodily equilibrium is the most important of these functions,

THE VESTIBULAR APPARATUS AND CEREBELLUM 187

and the semicircular canals of the internal ear (pp. 89, 196) are
the receptive organs, which are of chief importance in these reac-
tions. Comparative and embryological studies show that the
cerebellum was developed as a direct outgrowth from the pri-
mary centers for the semicircular canals in the medulla oblongata
(the acoustico-lateral area of fishes, Fig. 43), and even in the
human body root fibers from the vestibular branch of the VIII
cranial nerve enter the cerebellum directly. Neurons of the
second order also enter the cerebellum from the vestibular
nucleus, as well as from the spinal cord and from practically all
of the somatic sensory centers of the brain; there is also a very
important path from the cerebral cortex.

The human cerebellum consists of a median lobe, the worm
(vermis), and two larger cerebellar hemispheres. The vermis
receives fibers chiefly from the somatic sensory centers of the
brain stem and spinal cord, and it alone is well developed in
lower vertebrates (from fishes to birds, see Fig. 43). The cere-
bellar hemispheres vary in size in different mammals in propor-
tion to the size of the cerebral cortex, being, therefore, much
larger in man than in any other animal. Their appearance from
the ventral side is seen in Fig. 53. The cerebellum is attached
to the brain stem by three stalks or peduncles on each side, the
superior peduncle (brachium conjunctivum), the middle pe-
duncle (brachium pontis), and the inferior peduncle (corpus
restiforme).

Figure 87 illustrates diagrammatically the chief pathways
which enter the cerebellum, and Fig. 88 those by which nervous
impulses leave it. We cannot here describe these connections
in detail, but can mention a few only of their general features.

The cerebellum, as already stated, receives afferent impulses
from all of the important somatic sensory centers and also from
the cerebral cortex. The afferent fibers from the brain stem
enter by the superior and inferior peduncles. The pons is an
eminence under the upper part of the medulla oblongata (Fig.
53) which contains gray centers (the pontile nuclei). Fibers
pass into the pontile nuclei from the association centers of the
cerebral cortex by way of the cortico-pontile tracts, and from the
motor areas of the cerebral cortex by way of collateral branches
from the cortico-spinal tract as it passes through the pons.

188

INTRODUCTION TO NEUROLOGY

These nervous impulses enter the cerebellar hemispheres from
the pons by the middle cerebellar peduncles.

corpus
restiforme

nuc.fosc.
gracilis ef
cuneatus

tr. olivo<ereh

tr. spino-
cereb. dorg.

(Flechsi

TK spino -oil vans'
tr. cortico-spinalis'

cerebellum

brachium
conjuncfivum

tecto-cereb.
-cereb,

mesencephalon

central

tegmental

tract.

tr cortico-

xoliva inferior cerebellaris

tr.spino-cereb.ventr. (Cowers)

Fig. 87. — Diagram of the chief afferent tracts leading into the cerebellum.

nuc. dentatus
roof nuclei

ochium pontis

brachiurn

conjunctivum

tr. cereb.-Tegmentalis
hali

rnesencephalon

iro.thal.

corpus restiforme

-tr rubro-
spinalis

Tr cerebello-

tegmentalis

oliva inferior^ \ pontis

Jtr. cerebello-tegmentalis bulbi
Fig. 88. — Diagram of the chief efferent tracts leading out of the cerebellum.

Fibers leave the cerebellum by all three peduncles for the
motor centers of the brain stem (the cerebello-tegmental tracts,

THE VESTIBULAR APPARATUS AND CEREBELLUM 189

Fig. 88), and a much larger number leave by the superior pedun-
cle for the red nucleus (nucleus ruber, Fig. 75) and adjacent
parts of the brain stem, these fibers first crossing to the opposite
side of the brain in the cerebral peduncle under the aqueduct of
Sylvius. From the red nucleus fibers pass downward into the
spinal cord (rubro-spinal tract) and upward to the cerebral
cortex.

The connections just described illustrate some of the pathways
by which the cerebellum is able to reinforce, coordinate, or
otherwise modify the somatic motor mechanisms. There is an
immense amount of potential nervous energy always available
in the neurons of the cerebellar cortex, and the cerebellum ap-
pears to be constantly exerting a stimulating or tonic effect upon
the body muscles. An injury to the cerebellum (especially an
unsymmetric lesion) produces motor incoordination, and the
total removal of the cerebellum results in loss of muscular tone
and great weakness, though there is no abolition of any particular
motor functions. The cerebellar cortex and the cerebral cortex
are very intimately connected by large fiber tracts, and each
apparently exerts an important physiological effect upon the
other. But the exact nature of this reciprocal control is still
obscure.

The cerebellar cortex differs from the cerebral cortex in the
form and arrangement of its neurons and also, further, in that
it is structurally similar throughout its entire extent. The
cerebral cortex, on the other hand, shows differences in the forms
and arrangements of its neurons in different regions, and this
is correlated with a regional localization of diverse functions
(pp. 273, 281). There is some evidence that different parts
of the cerebellar cortex exert a dominant regulatory influence
over particular large groups of muscles; but this localization of
function is of a very general sort and is by no means so precise
as the localization of voluntary motor centers in the cerebral
cortex. Moreover, the physiological influence of the cerebellum
upon movement is of a very different sort from that of the cere-
bral cortex.

The surface of the cerebellum is divided by deep fissures or
sulci into narrow leaf-like subdivisions termed folia or gyri,
so that when it is cut open across the median plane the cut sur-

190

INTRODUCTION TO NEUROLOGY

face looks somewhat like a sprig of the common evergreen cedar
tree known as arbor vitae. Hence this cut surface by the an-
cients was termed the arbor vitse.

Fig. 89. — Semidiagrammatic section taken transversely through a
lamina of the cerebellar cortex (Golgi method): A, Molecular layer,
filled with axons of granule cells cut at right angles to their course; B,
granular layer; C, white matter; a, Purkinje cell, with the dendrite broadly
spread out in the transverse plane (compare Fig. 15) ; b, basket cell (com-
pare Fig. 16); d, terminal arborizations of the basket cells enveloping the
bodies of the Purkinje cells; e, superficial stellate cells;/, Golgi cell of type
II (see p. 43); g, granule cells with their axons ascending and bifurcating
in the molecular layer at i\ h, mossy fibers; j, neuroglia cell; m, neu-
roglia cell; n, climbing fibers. (After Ram6n y Cajal.)

The gray matter of the cerebellum is partly superficial (this
is the cortex to which reference has already been made) and
partly in the form of deep nuclei embedded within the white
matter. The largest of these deep gray centers are the dentate

THE VESTIBULAR APPARATUS AND CEREBELLUM 191

nuclei within the cerebellar hemispheres. Within the vermis
are other smaller centers, called the roof nuclei, because they lie
immediately above the fourth ventricle (nuclei emboliformis,
globosus, and fastigii, see Fig. 96). Some of the afferent fibers
which enter the cerebellum end in these nuclei, but most of them
end in the cortex. The efferent fibers, on the other hand, arise
from the deep nuclei, especially the dentate nuclei (Fig. 88).

The cerebellar cortex has three distinct layers. External to
the central white matter (Fig. 89, C) is a wide layer composed of
very minute granule cells (Fig. 89, B) densely crowded together,
with scanty cytoplasm, short, claw-like dendrites, and slender
unmyelinated axons which ascend to the superficial molecular
layer (Fig. 89, A), where they bifurcate (their branches running
lengthwise of the folium) and end among the dendrites of the
Purkinje cells, to be described immediately. The middle layer
of the cortex is composed of a single row of Purkinje cells (Fig.
89, a); these have large globose bodies with massive bushy
dendrites directed outward and slender axons directed inward.
These axons are myelinated and constitute the chief efferent
pathway from the cortex; they do not, however, leave the cere-
bellum, but end in the deep gray nuclei (chiefly the dentate
nuclei), from which other neurons carry the impulses out of the
cerebellum. The dendrites of the Purkinje cells are widely ex-
panded transversely to the length of the folium, but are very
narrow in the opposite direction; thus each cell comes into
contact with the largest possible number of axons of the
granule cells which run lengthwise of the folium. The outer-
most or molecular layer contains the dendrites of the Purkinje
cells, termini of the axons of the granule cells and of other
fibers, and a small number of neurons with short axons, among
which are the basket cells illustrated in Figs. 16 and 89, b.

Afferent fibers terminate in the cerebellar cortex in two ways.
They may pass directly out to the molecular layer as ascending
or climbing fibers, where they end in very intimate relation with
the dendrites of the Purkinje cells (Figs. 15 and 89, n), or they
may end as moss fibers (Fig. 89, h) among the cells of the gran-
ule layer. Here the granules take up the nervous impulses and
deliver them to the dendrites of the Purkinje cells. Ramon
y Cajal is of the opinion that the moss fibers are the terminals

192 INTRODUCTION TO NEUROLOGY

of the afferent fibers of the inferior cerebellar peduncle, and that
the ascending fibers are the terminals of the fibers from the
middle peduncle (brachium pontis).

Since each fiber from the inferior peduncle branches exten-
sively and reaches many granule cells in widely separated parts
of the cerebellum, and since the axon of each granule cell- reaches
the dendrites of a very large number of Purkinje cells, a single
incoming nervous impulse may excite a very large number of
Purkinje cells, and thus its physiological effect may be greatly
enhanced. The same result is also secured by the action of the
basket cells (Fig. 89, 6) and other forms of neurons with short
axons within the cortex (Fig. 89, e, /), each of which may
discharge powerful impulses directly upon several Purkinje
cells. The axons of the Purkinje cells themselves also give off
collateral fibers into the granular layer, whose neurons dis-
charge back into the Purkinje cells again. In all of these
ways provision is made for the diffusion, summation, and re-
inforcement of stimuli during the process of their transmission
through the cerebellar cortex, and also for prolongation of motor
reactions which would otherwise soon subside, and for the main-
tenance of muscular tone.

This type of reaction has been termed "avalanche conduction"
(see p. 101), and its mechanism here is similar to that found in
the olfactory bulb (p. 218), but much more complex. It is prob-
able that the reciprocal relation between the cerebellum and the
cerebral cortex is of a similar sort, all cortical activities exciting
also the cerebellum and drawing therefrom additional nervous
energy as needed to maintain the tone of the reacting mech-
anism; and voluntary movements excited by the cortico-spinal
or pyramidal tract from the cerebral cortex (see p. 283) are
under especially direct proprioceptive control from this source.

The relationships of the centers of the brain stem, the cerebral
cortex, and the cerebellum may be illustrated somewhat crudely
by the analogy of the three chief departments of the national
government. The reflex centers of the brain stem correspond
to the legislative branch of government, determining in advance
by virtue of their innate structure what actions may appropri-
ately be performed in each particular type of frequently recur-
ring situation. The cerebral cortex is a sort of glorified judicial

THE VESTIBULAE APPARATUS AND CEREBELLUM 193

branch of government, interpreting the decrees of the legislative
centers, integrating the behavior by combining its elements into
cooperating systems in view of all the factors of present and
past experience, ,and with extensive powers of veto over inap-
propriate reactions which may have been inaugurated by the
lower centers. The cerebellum is the great administrative
office which attends to the details of the proper execution of the
acts which have been previously determined upon and initiated
in the other departments of government.

Summary. — The vestibular apparatus and the cerebellum are
genetically and physiologically very closely related. The semi-
circular canals are the most highly differentiated proprioceptive
end-organs, serving chiefly the functions of equilibration and
the maintenance of muscular tone. These reactions are, for the
most part, unconsciously performed and there is no important
cortical path from the vestibular nuclei. These nuclei effect
reflex connections with the motor centers of the spinal cord and
medulla oblongata, especially the eye-muscle nuclei, and with the
cerebellum.

The cerebellum has been developed out of the primary vestibu-
lar area for the more perfect coordination and integration of the
somatic motor reactions and for strengthening these reactions.
It receives afferent fibers from all somatic sensory centers, and
in mammals it is also very intimately connected with the cere-
bral cortex, these two higher centers appearing always to act
conjointly. The cerebellum discharges into all of the somatic
motor centers and assists in preserving the proper balance of
muscular contraction and in the maintenance of muscular tone.

LITERATURE

For the original sources of the data presented in this and the preceding
chapters see the bibliographies appended to Chapters VII, VIII, IX, and X.
On the cerebellum see further:

ANDRE-THOMAS and DURUPT, A. 1914. Localisations cerebelleuses,
Paris.

BIANCHI, A. 1903. Sulle vie di connessione del cervelletto, Arch, di
Anat. e Embriol., vol. ii.

BOLK, L. 1906. Das Cerebellum der Saiigethiere, Jena.

BRUCE, A. N. 1910. The Tract of Cowers, Quart. Jour. Exp. Physiol.,
vol. iii, pp. 391-407.

13

194 INTRODUCTION TO NEUROLOGY

FERRIER, D., and TURNER, W. A. 1895. A Record of Experiments Illus-
trative of the Symptomatology and Degenerations Following Lesions of the
Cerebellum, Phil. Trans. Roy. Soc. London for 1894, vol. clxxxv B, pp.
755-761.

GEHUCHTEN, A. VAN. 1904. Le corps restiforme et les connexions
bulbo-cerebelleuses, Le Nevraxe, vol. vi.

— . 1905. Les pedoncules cerebelleuses superieurs, Le Neyraxe, vol. vii.

GOLDSTEIN, K. 1910. Ueber die aufsteigende Degeneration und Quer-
schnittsunterbrechung des Riickenmarks (Tractus spino-cerebellaris pos-
terior, Tractus spino-olivaris, Tractus spino-thalamicus), Neurol. Central-
blatt, No. 17.

HERRICK, C. JUDSON. 1914. The Cerebellum of Necturus and Other
Urodele Amphibia, Jour. Comp. Neur., vol. xxiv, pp. 1-29.

HERRICK, C. L. 1891. Illustrations of the Architectonic of the Cere-
bellum, Jour. Comp. Neur., vol. i, pp. 5-14.

LEWANDOWSKY, M. 1907. Die Funktionen des zentralen Nervensys-
tems, Jena.

LUCIANI, L. 1893. Das Kleinhirn, Leipzig.

RUSSELL, J. S. RIESEN. 1895. Experimental Researches into the Func-
tions of the Cerebellum, Phil. Trans. Roy. Soc. London, vol. clxxxv B,
pp. 819-861.

SCHAPER, A. 1894. Die morphologische und histologische Entwickelung
des Kleinhirns der Teleostier, Morph. Jahrb., Bd. xxi.

SHERRINGTON, C. S. 1909. On Plastic Tonus and Proprioceptive Re-
flexes, Quart. Jour. Exp. Physiol., vol. ii, p. 109.

WILSON, J. GORDON, and PIKE, F. H. 1912. The Effects of Stimulation
and Extirpation of the Labyrinth of the Ear, and Their Relation to the
Motor System, Phil. Trans. Roy. Soc. London, vol. cciii B, pp. 127-160.

CHAPTER XIII

THE AUDITORY APPARATUS

THE human organ of hearing consists of the external ear,
bounded within by the drum membrane (tympanic membrane,
membrana tympani); the middle ear, a cavity filled with air
which communicates with the pharynx through the auditory

Tympanic cavity, with chain of ossicles
Semicircular duct
Utricle j
Ductus endolymphaticus

Saccule
Ductus cochlearis

Auricula

Auditory tub«

Membrana tympani

Recessus epitympanicus

External acoustic meatus

Fig. 90. — Diagrammatic view of the parts of the human ear.
Cunningham's Anatomy.)

(From

or Eustachean tube and contains the auditory ossicles; and the
internal ear, a complex bony chamber, the bony labyrinth,
within which is the membranous labyrinth containing the specific
receptors or sensory surfaces of the internal ear (Fig. 90). The

195

196

INTRODUCTION TO NEUROLOGY

tympanic membrane receives the air waves which form the
physical stimuli of sound (pp. 70 and 85). These vibrations are
then transmitted (and at the same time intensified) by the au-
ditory ossicles of the middle ear to the liquid within the bony
labyrinth.

The membranous labyrinth is of approximately the same shape
as the bony labyrinth, but smaller, so that there is a space be-
tween the membranous labyrinth and the enclosing bony wall.
This space is filled with liquid, the perilymph, and the mem-
branous labyrinth is also filled with liquid, the endolymph. In
Fig. 90 the perilymphatic space is printed in black and the endo-
lymphatic space in white. The parts of the membranous laby-
rinth are shown diagrammatically in Fig. 91.

Recessus utriculi
Saccule |

Ductus cochlearis

Ductus reuniens

Ductus endolymphaticus

Ampulla of posterior duct
Saccus endolymphaticus

•Ampulla of superior semi-
circular duct
Ampulla of lateral duct

Crus commune

Ductus utriculosaccularis

Sinus inferior

Fig. 91. — Diagrammatic representation of the parts of the membranous
labyrinth. (From Cunningham's Anatomy.)

The membranous labyrinth is a closed sac which has four chief
parts: (1) the utricle (recessus utriculi), with a patch of sensory
epithelium, the macula utriculi; (2) the three semicircular canals
(ductus semicirculares), each of which communicates at both
ends with the utricle and has at one end a dilation (ampulla)
containing a patch of sensory epithelium, the crista; (3) the
saccule (sacculus) connected by a narrow ductus utriculosaccu-
laris with the utricle and containing a patch of sensory epithe-
lium, the macula sacculi; (4) the ductus cochlearis, which com-
municates by a narrow ductus reuniens with the saccule and is
spirally wound to fit the bony cochlea, which is shaped like a snail
shell. The ductus cochlearis (old name, scala media) is trian-

THE AUDITORY APPARATUS

197

gular in cross-section (Fig. 92) and contains the specific auditory
receptive epithelium in the spiral organ, or organ of Corti.

Membi
vestibularia

Inner hair cell

Membrana basilaris
Outer hair cells

Fig. 92. — Section across the ductus cochlearis (scala media) to illustrate
the relations of the spiral'organ (organ of Corti). (After Retzius.)

Membrana tectoria
Sulcus spiralis interims
Iambus lamina1 spiralis

Inner rod of Corti

Outer rod of Corti
Inner hair cell I 1 Outer hair cells
Hensen's stripe | | Cells of Hensen

Membrana basilaris

Cells of Claudius

Inner spiral fasciculus

Vas spirale
Tunnel of Corti

GJ<> Q.O- .«,- ** • ., '-Vcr

Cells of Deiters

Space of Nuel

Fig. 93. — Section across the spiral organ (organ of Corti) from the central
coil of the ductus cochlearis. (After Retzius.)

The generally accepted structure of the spiral organ, as presented in
the classical researches of Retzius, is shown in Fig. 93. Upon a basement
membrane (Fig. 92, membrana basilaris) is a very highly differentiated

198

INTRODUCTION TO NEUROLOGY

sensory epithelium, part of whose cells are supporting elements of diverse
sorts and part (the hair cells) are specific receptors. The termini of the
cochlear nerve arborize around the bodies of the hair cells in the same way
that fibers of the vestibular nerve are related to the hair cells of the cristaj
of the semicircular canals (Fig. 32, p. 88). The membrana tectoria is
a delicate gelatinous mass resting upon the spiral organ and intimately
connected with the hairs of the hair cells. Its shape has been very care-
fully studied by Hardesty.

Many details of the structure of the spiral organ, or organ of Corti, and
the whole question of the mode of its functioning are still controverted.
Our present knowledge of the histqlogical organization of the basilar mem-
brane shows that it is structurally incapable of serving the function of tone
analysis in the way postulated by Helmholtz's theory. Based upon im-
portant additions to our knowledge of the minute structure of the organ

Fig. 94. — Section through the apical turn of the cochlea of the pig at
about full term, showing outer auditory hairs embedded in the membrana
tectoria: ep.s.sp., Epithelium of spiral sulcus; i.h.c., inner hair cells; i.pil.,
inner pillar; m.bas., basement membrane; m.tect., membrana tectoria; lab.
vest., labium vestibulare; n.coch., cochlear nerve; o.h.c., outer hair cell; s.sp.,
sulcus spiralis. (After C. W. Prentiss.)

of Corti and clinical observations upon diseased conditions, several different
theories of the mechanism of tone analysis have recently been expressed.
Among the more important of these researches are those of Shambaugh.
This author has demonstrated that the hairs of the hair cells do not termi-
nate freely in the endolymph, as commonly figured, but that they are
firmly attached to the under surface of the tectorial membrane. This
membrane has a semigelatinous texture and is capable of taking up sym-
pathetically the vibrations of the endolymph within which it floats.

The development of the tectorial membrane has recently been restudied by
Prentiss and Hardesty. It first appears as a thin cuticular plate developed
over the free ends of the columnar cells which form the inner or axial part of
the epithelium of the basement membrane. In the adult ear it retains its
attachment to the labium vestibulare along the axial border of the ductus
cochlearis, but becomes free from the cells which form the lining of the

THE AUDITORY APPARATUS 199

sulcus spiralis (Fig. 94). Prentiss claims that it is in part formed from the
embryonic cells which develop into the spiral organ, and that its connection
with the spiral organ is retained in the adult (Fig. 94) ; but Hardesty (1915)
says that the cells of the embryonic spiral organ contribute little or nothing
to the formation of the tectorial membrane and that this membrane is free
from the spiral organ in the adult. Prentiss describes the membrane as
growing in thickness by the secretion of a cuticulum formed between the
ends of the epithelial cells, thus giving to the mature membrane a cham-
bered or honey-comb structure. Hardesty, however, regards it as produced
by fibrils growing out from the free ends of the epithelial cells lying be-
tween the embryonic spiral organ and the axis of the cochlea, these fibrils
being embedded in a gelatinous matrix.

Shambaugh concludes that the tectorial membrane takes the part of a
physical resonator by responding in its various parts to tones of different
pitch, depending on the size of the membrane, tones of higher pitch being
taken up by the hair cells located near the beginning of the basal coil, those
of lower pitch by the cells near the apex of the cochlea, where the tectorial
membrane attains its maximum size. The stimulation of the hair cells is
effected only through the medium of their projecting hairs, these being
excited by vibrations of the tectorial membrane to which they are attached.

In fishes the organs of the internal ear are intimately associated with an
extensive series of subcutaneous canals containing numerous sense organs
and with naked cutaneous sense organs of the same type, the entire complex
forming the system of lateral line sense organs (see p. 110 and Fig. 95).
The nerves which in fishes supply the lateral line sense organs (lateralis
roots of the VII and X cranial nerves) and the organs of the internal
ear (VIII nerve) are intimately associated and terminate together in the
acoustico-lateral area of the medulla oblongata (Figs. 43 and 44, pp. Ill,
112), and all of these end-organs have the same type of structure as those
of the human internal ear (Fig. 32, p. 88).

The internal ears of fishes are essentially similar to those of man save
that they lack the cochlea and the organ of Corti. They possess a small
sense organ in the saccule, the lagena, supplied by a special branch of the
VIII nerve (Fig. 95, RL), from which the cochlea of higher vertebrates has
been developed. The researches of Parker have shown that fishes hear,
though there is no evidence that they possess the power of tone analysis,
and the sense organs of the saccule are the essential receptors for sound
waves. The sense organs of the lateral line system are said by Parker to be
sensitive to water vibrations of slower frequency than the sound waves to
which the ear responds, while Hofer is of the opinion that these organs are
stimulated only by streaming movements of the water in which the animals
live. Probably the lateral line organs also participate in the equilibratory
reactions of the fish.

Though our knowledge of the functions of the various parts of the acous-
tico-lateral system of fishes is still very imperfect, it is evident that all of
these organs are both structurally and physiologically of common type, and
it is probable that they have had a common evolutionary origin from a more
generalized form of cutaneous tactile organ. This is the explanation of the
intimate association in the human ear of sense organs of so diverse functions
as the cochlea for hearing and the semicircular canals for equilibration, the
former being an exteroceptor whose reactions may be vividly conscious, and
the latter being a proprioceptor whose reactions are almost entirely uncon-
sciously performed. For further consideration of the semicircular canals
and their central connections see p. 183.

200

INTRODUCTION TO NEUROLOGY

In the human body the cochlear and vestibular nerves are
very intimately associated, but the embryological studies of

Fig. 95. — Diagram of the acoustico-lateral system of nerves with their
peripheral end-organs, as seen from the right side, in a fish, the common
silver-sides, Menidia (X 9). The relations here figured were recon-
structed from serial sections by projection upon the sagittal plane. For
the relations between the acoustico-lateral nerves and the other systems
of nerves in this fish, see the more detailed chart from which this was drawn
off, in the Journal of Comparative Neurology, vol. ix, 1899, plate 15; cf.
also Fig. 65, p. 149, of this book. The dotted outline represents the posi-
tion of the brain, the lateral line canals are shaded with cross-hatching, the
internal ear is stippled, and the nerves are drawn in black. The organs
of the lateral line system are drawn as black disks when naked on the
surface of the skin, and as black circles when lying in the canals. NAA,
Anterior nasal aperture; NAP, posterior nasal aperture; N OL, olfactory
nerve; N OPT, optic nerve; RAA, nerve of superior ampulla; RAE, nerve
of lateral ampulla; RAP, nerve of inferior ampulla; R BUG, ram us bucca-
lis of facial nerve; RL, nerve of the lagena (rudimentary spiral organ);
RLAT, ram us lateralis of the vagus; ROS, ramus ophthalmicus super-
ficialis of the facial nerve; R MAN EX, ramus mandibularis externus of
the facial nerve; R SAC, nerve of the sacculus; RU, nerve of the utriculus;
T, acoustico-lateral area. (After Herrick, from Wood's Reference Hand-
book of the Medical Sciences.)

Streeter and others have made it plain that these two nerves are
really more distinct than was formerly supposed. The periph-

THE AUDITORY APPARATUS

201

eral receptors of the cochlea and semicircular canals are obviously
as dissimilar as are their functions, but the functional significance

Medial geniculate body

^Inferior quadrigeminate body
^Nucleus of trochlear nerve

Nucleus fastigii

Nucleus emboliformis

• Dentate nucleus

Lateral nucleus of

vestibular nerve
_,Restiform body

Dorsal nucleus ol
•^ cochlear nerve
^Ventral nucleus of

cochlear nerve
,. Cochlear nerve

Nucleus of lateral lemniscus
Medial longitudinal fasciculus ---
Lateral lemniscus^.

Peduncle of superior olive

Superior olivary nucleus
Trapezoid body

Fig. 96. — Diagram of the auditory and vestibular connections. Com-
pare Figs. 71, 77, and 86. The fibers of the cochlear nerve enter the ven-
tral and dorsal cochlear nuclei (the latter being termed the tuberculum
acusticum) at the lateral border of the medulla oblongata. The auditory
path now divides, one tract, the trapezoid body, passing ventrally through
the pons to enter the lateral lemniscus of the opposite side, and the other
passing dorsally through the acoustic medullary striae (stria? medullares
acustici) across the floor of the fourth ventricle and also entering the lat-
eral lemniscus. These fibers may be interrupted by synapses in the supe-
rior olives, the nucleus of the lateral lemniscus or the inferior colliculus
(inferior quadrigeminate body) before they reach the medial geniculate
body of the thalamus, or they may pass by these nuclei without connecting
with them. The fibers shown in the diagram as passing from the inferior
quadrigeminate body to the temporal lobe of the cerebral cortex are prob-
ably interrupted by a synapse in the medial geniculate body. (From
Morris' Anatomy.)

of the sensory organs of the utricle and saccule is more uncertain.
The fact that fishes undoubtedly hear, notwithstanding their
lack of cochlea or any other receptors more complex than the

202 INTRODUCTION TO NEUROLOGY

sensory spots in the saccule, demonstrates the relatively late
phylogenetic origin of the cochlear system from the vestibular,
and has suggested to some physiologists that even in man these
two systems are not wholly distinct, and that the sense organs in
the saccule may also function as a sound receptor. It is clear,
however, that tone analysis is effected only in the cochlea.

The central connections of the cochlear and vestibular nerves
are fundamentally different. The vestibular nerve terminates
in reflex centers of the medulla oblongata and cerebellum (p.
185) with no important cortical connections, while the cochlear
nerve has, in addition to important reflex connections in the ob-
longata and midbrain, the much stronger ascending pathway of
the lateral lemniscus directly to the medial geniculate body of
the thalamus, and thence to the temporal lobe of the cerebral
cortex (see p. 157 and Figs. 75, 77, 80, 96). Some of the fibers
of the lateral lemniscus are interrupted in the inferior colliculus,
which is an important auditory reflex center.

Summary. — The human ear has three parts: (1) the external
ear, for receiving sound waves from the air; (2) the middle ear,
for intensifying the vibrations and transmitting them to (3) the
internal ear, which is filled with liquid and contains sense organs
of uncertain function in the utricle and saccule, sense organs for
equilibration in the semicircular canals, and the spiral organ
(organ of Corti) in the cochlea for tone analysis. The spiral
organ is a complicated epithelial structure resting on a basement
membrane and consisting of supporting cells of diverse kinds,
the hair cells (which are the specific receptors and receive the
endings of the fibers of the cochlear nerve), and the tectorial mem-
brane. Shambaugh is of the opinion that the tectorial membrane
is capable of responding in its various parts to different vibration
frequencies, and that the hair cells are stimulated through their
hairs which are attached to the tectorial membrane.

In fishes the organ of hearing is much simpler than in man, the
semicircular canals are, however, similar, and there is, in addi-
tion, an elaborate system of lateral line sense organs whose func-
tions seem to be intermediate between the tactile and auditory
organs. It is probable that these three systems of sense organs
were derived phylogenetically from some more generalized form
of cutaneous tactile organ. This accounts for the intimate as-

THE AUDITORY APPARATUS 203

sociation in the human ear of organs of so diverse functions as the
semicircular canals and the cochlea.

The central connections of the vestibular and cochlear nerves
are very different, the former effecting chiefly reflex connections
for equilibration in the medulla oblongata and cerebellum, and
the latter both reflex connections in the brain stem and cortical
connections through the lateral lemniscus, medial geniculate
body of the thalamus and auditory radiations, for conscious
sensations of hearing.

LITERATURE

EWALD, J. R. 1892. Physiologische Untersuchungen liber das End-
organ des Nervus octavus, Wiesbaden, J. F. Bergmann.

HARDESTY, I. 1908. On the Nature of the Tectorial Membrane and Its
Probable Role in the Anatomy of Hearing, Amer. Jour. Anat., vol. viii.

— . 1915. On the Proportions, Development, and Attachment of the
Tectorial Membrane, Amer. Jour. Anat., vol. xviii.

VON HELMHOLTZ, H. L. T. 1896. Die Lehre von den Tonempfindungen,
Ausgabe 5, Braunschweig.

HOFER, B. 1908. Studien iiber die Hautsinnesorgane der Fische,
Berichte kgl. Baverischen Biologischen Versuchsstation in Miinchen, Bd. 1,
p. 115.

KAPPERS, C. U. ARIENS. Kurze Skizze der Phylogenetischen Entwick-
lung der Oktavus und Lateralisbahnen mit Beriicksichtigung der neuesten
Ergebnisse, Zentralbl. f. Physiol., Bd. 23, 1909.

PARKER, G. H. 1903. Hearing and Allied Senses in Fishes, U. S. Fish
Commission Bulletin for 1902, Washington.

— . 1903. The Sense of Hearing in Fishes, Amer. Naturalist, vol. xxxvii.

— . 1905. The Function of the Lateral Line Organs in Fishes, Bui. of the
Bureau of Fisheries for 1904, Washington.

PRENTISS, C. W. 1913. On the Development of the Membrana Tectoria
with Reference to Its Structure and Attachments, Amer. Jour. Anat., vol.
xiv, No. 4.

RETZIUS, G. 1884. Das Gehororgan der Wirbeltiere, Stockholm.

ScHdNEMANN, A. 1904. Die Topographic des menschlichen Gehoror-
ganes, Wiesbaden.

SHAMBAUGH, G. E. 1907. A Restudy of the Minute Anatomy of Struc-
tures in the Cochlea with Conclusions Bearing on the Solution of the
Problem of Tone Perception, Am. Jour. Anat., vol. vii.

— . 1908. The Membrana Tectoria and the Theory of Tone Perception,
Arch. Otology, vol. xxxvii.

— . 1910. Das Verhiiltnis /wischen der Membrana Tectoria und dem
Cortischen Organ, Zeits. f. Ohrcnheilk., Bd. 62.

— . 1912. Ueber den Bau und die Funktion der Crista Ampullaris, Ibid.,
Bd. 65.

STREETER, G. L. 1907. On the Development of the Membranous Laby-
rinth and the Acoustic and Facial Nerves in the Human Embryo, Amer.
Jour. Anat., vol. vi.

WATSON, J. B. 1914. Behavior, An Introduction to Comparative Psy-
chology, New York, Chapter XII.

CHAPTER XIV

THE VISUAL APPARATUS

THE eye is the most highly specialized sense organ in the
human body, and in other respects it occupies a very unique
position. The essential receptive part of the eye is in the retina.
But the retina is much more than this; it is really a part of the
brain, and the so-called optic nerve is a true cerebral tract. This
is evident from a consideration of the embryologic development
of the retina.

In the early embryonic stages the neural tube expands laterally in the
position of the future thalamus, and from the upper part of this region a
"primary optic vesicle" is evaginated from the lateral wall on each side

Optic cup
Optic stalk | Lens rudiment

Cavity of forebrain

Ectoderm forming lens
rudiment

Optic vesicle becoming
cupped

Fig. 97. — Diagrammatic section through the head of a fetal rabbit to
illustrate the mode of formation of the primary and secondary optic vesicles
and of the lens of the eye. The right side of the figure is drawn from a more
advanced stage than the left side. (From Cunningham's Anatomy.)

(Figs. 46, 47, 49, 97). The optic vesicle grows outward toward the skin
and assumes the form of a hollow sphere, whose cavity remains in com-
munication with that of the third ventricle by a hollow stalk (Fig. 97).
While the formation of the primary optic vesicle is in progress the overlying
ectoderm (outer skin) is thickened and finally invaginated to form the lens
of the eye, the optic vesicle collapses so that its cavity is obliterated by the
apposition of its lateral and medial walls, and a secondary cavity (the sec-

204

THE VISUAL APPARATUS 205

ondary optic vesicle or optic cup) is formed whose walls are two-layered,
being composed of both the original lateral and medial parts of the primary
•optic vesicle (Fig. 97, on the right side). This secondary cavity contains
the vitreous humor in the adult eye; the layer of the secondary optic vesicle
which borders the vitreous humor forms the retina; the outer layer of the
vesicle forms the pigment layer of the retina; and the stalk forms the optic
nerve by the ingrowth of fibers throughout its length from the retina and
brain (Fig. 100).

The retina, then, is as truly a part of the brain as is the cerebral
hemisphere and its structure is, in general, similar to that of
other parts of the brain. There are supporting cells, the fibers
of Miiller (Fig. 98, M), and neuroglia elements (Fig. 98, d.s. and
s.s.), and lying among these are the neurons. The latter can
be classified in general in four groups: (1) the rods and cones
(Fig. 98, A); (2) the bipolar cells (Fig. 98, D); (3) the so-called
ganglion cells which give rise to fibers of the optic nerve (Fig.
98, F); (4) horizontally disposed correlation neurons (Fig. 98,
h) . All of these types except the third are intrinsic to the retina,
i. e., they send none of their fibrous processes beyond the limits
of the retina itself. The axons of the neurons of the third type
pass out of the retina and form the so-called optic nerve, termi-
nating in the thalamus or midbrain.

Immediately external to the nervous layer of the retina is the
pigment layer (Figs. 99, 100), which is formed from the outer
epithelial layer of the secondary optic vesicle (Fig. 97). Figure
99 illustrates the ten layers of the retina as figured by the older
histologists, and Fig. 98 illustrates the relations of some of the
nervous elements as revealed by the Golgi method. It is evi-
dent that the "nuclear" or "granular" layers are characterized
chiefly by the presence of the cell bodies of the neurons and their
nuclei, while the "molecular" layers are composed chiefly of the
fibrillar nerve-endings which form the synapses between the
various groups of neurons.

The rods and cones of the retina are the receptors and also
the neurons of the first order in the optic path. Their free ends
project through the external limiting membrane into the pigment
layer. Rays of light which pass through the dioptric apparatus
(lens, humors, etc.) of the eyeball must penetrate also the entire
thickness of the retina (which is very transparent) before they
reach these receptors (Fig. 100).

206

INTRODUCTION TO NEUROLOGY

i'E

Fig. 98. — Two transverse sections through the mammalian retina: A,
Layer of rods and cones; ar, internal arborizations of bipolar neurons related
to the cones; ar', internal arborizations of bipolar neurons related to
the rods; B, outer nuclear layer; C, outer molecular layer; c, cones; ex.,
contact of bipolar neurons with branches of the cone fibers; c.l., bi-
polar neurons related to cones; e.g., cone granules or nuclei of cones;
c.n., centrifugal nerve-fiber; c.r., contact of bipolar neurons with ends of
rod fibers; D, inner nuclear layer; d.s., diffuse neuroglia elements; E, inner
molecular layer; F, ganglionic layer; G, layer of nerve-fibers; g, neurons of
the ganglionic layer; h, horizontal cells; M, supporting fiber of Miiller;
r, rods; r.b., bipolar neurons related to rods; r.g., rod granules or nuclei of
rods; s.g., stratified ganglion cells; s.s., stratified neuroglia elements. (After
Ram6n y CajaL)

THE VISUAL APPARATUS

207

The peripheral ends of the rods contain a pigment, the visual
'purple or rhodopsin, which is chemically changed by light rays
and has been supposed to function as the exciting agent for ner-
vous impulses of sensibility to light in the rod cells. But recent
experiments go to show that the visual purple is concerned with

Istratum
f pigment!

Istratum
[opticum

Membrana limitans interns

Fig. 99. — Diagrammatic section through the human retina to illustrate
the ten layers as commonly enumerated. (After Schultze, from Cunning-
ham's Anatomy.)

the adaptation of the eye to different intensities of light rather
than with the specific receptor function itself. The brown pig-
ment of the pigment layer is probably also concerned with light
adaptation.

The exact mechanism through the agency of which the rods
and cones are excited to nervous activity by light is still obscure;

208

INTRODUCTION TO NEUROLOGY

but when the rods and cones are once actuated, they may trans-
mit their nervous impulses across synapses in the external
molecular layer to neurons of the second order whose cell bodies
lie in the internal granular layer. The neurons of the internal
granular layer are of diverse sorts, some of them spreading the
nervous impulse laterally (probably for summation effects in
weak illumination), but most of them conducting radially and
effecting synaptic connection with the dendrites of the "ganglion
cells of the retina." The latter are neurons of the third order
whose axons form the larger part of the fibers of the so-called

Fig. 100. — Diagram of the relations of the retina and the so-called optic
nerve to the other parts of the brain.

optic nerve, which is really not a peripheral nerve at all, but a
true cerebral tract.

The fibers of the "optic nerve," having reached the ventral
surface of the brain, enter the optic chiasma, where part of them
cross to the opposite side of the brain, while others enter the
"optic tract" of the same side. From the chiasma a big tract of
crossed and uncrossed optic fibers passes upward and backward
across the surface of the thalamus, where they divide into two
groups. Some terminate in the pulvinar and lateral geniculate
body which form the postero-dorsal part of the thalamus (Figs.
45, 76, 77) ; others pass these structures to end in the roof of the
superior colliculus of the midbrain, i. e., in the optic tectum.

THE VISUAL APPARATUS 209

The latter connection is for responses of purely reflex type,
chiefly those concerned with the movements of the eyeballs and
accommodation of the eyes; the thalamic connection is a station
in the cortical visual path.

From these relations it follows that there is nothing in the
visual organs which corresponds to a peripheral nerve. The
retina as a part of the brain is directly excited by the light waves
which penetrate its substance. The so-called optic nerve is a
tract within the brain, whose fibers for the most part come from
visual neurons of the third order in the retina, though there are
others also which come from the brain and pass outward to end
by free arborizations within the retina (Fig. 98, c.n.). The
function of these centrifugal fibers to the retina is unknown.
Identically the same nerve-fibers which make up the so-called
optic nerves peripherally of the optic chiasma are called the
optic tracts centrally of that point. It would be more logical
to name these fibers optic tracts for their entire length, these
tracts being very similar to those of the lemniscus system. Like
the lemniscus fibers, they decussate completely in the optic
chiasma in lower vertebrates before terminating in the thalamus
and midbrain. It is only in animals with an overlapping of the
fields of vision of the two eyes and stereoscopic vision that the
decussation of the optic tracts in the chiasma is incomplete.

The significance of the crossed and uncrossed fibers of the
optic tracts is seen in Fig. 101. In this diagram the shaded por-
tions of the retinae receive their light from the left side of the
median plane of the body; the unshaded portions, from the right
side. The nasal part of each retina recives visual images from
objects lying on the same side of the body exclusively, i. e.,
from the temporal portion of the visual field, while the temporal
part of the retina may receive images from objects on the oppo-
site side of the body. Accordingly, in order that the visual
images derived from all objects lying on one side of the body may
be represented by nervous excitations within the opposite half
of the brain, it is necessary that the nerve-fibers from the nasal
part of each retina cross in the chiasma, while those from the
temporal part pass through the chiasma without decussation.

The reflex optic centers in the roof of the midbrain occupy
most of the colliculus superior, which corresponds to the optic

14

210

INTRODUCTION TO NEUROLOGY

•

Fig. 101.

THE VISUAL APPARATUS 211

lobe of the fish brain (Figs. 43, 44). Here visual impressions
•are brought into physiological relations with those of the tactual
and auditory systems received by the lemnisci. The chief effer-
ent pathway from this center is by way of the underlying cere-
bral peduncle (Fig. 75). Here reflex connections are effected
directly with the nuclei of the III and IV cranial nerves for the
eye muscles, and through the fasciculus longitudinalis medialis
with the centers for all other cranial and spinal muscles. This
fasciculus is a strong bundle composed of both descending and
ascending fibers whose function is the general coordination of
reflex motor responses, and in particular those of the conjugate
movements of the two eyes (see p. 186).

The accommodation of the eye for distance is effected by
changes in the curvature of the lens, and the adaptation for
differences in illumination is effected in part by changes in the
diameter of the pupil (this is in addition to the changes in the
retinal pigment referred to on p. 207 and to changes in the rods
and cones and other neurons of the retina which may be excited
by the centrifugal fibers from the brain to the retina referred to
on p. 209). The nerves controlling the movements of the lens
and the pupillary reactions belong to the visceral motor system.
They leave the central nervous system in part through the ocu-
lomotor nerve and in part (for dilation of the iris) from the lower
cervical region of the spinal cord. The latter fibers pass by way
of roots of spinal nerves into the superior cervical sympathetic
ganglion (p. 234 and Fig. 41, p. 107) and then forward to the
eyeball. We cannot here enter into further details of the mech-
anism of accommodation or of the diopteric apparatus and the
accessory parts of the eye; see the larger text-books of anatomy
and physiology.

The thalamic connections of the optic tracts in the lowest
vertebrates are very insignificant, collaterals of these fibers

Fig. 101. — A diagram of the visual tract, illustrating the significance of
the partial decussation of nerve-fibers in the optic chiasma so as to ensure
the representation in the cerebral cortex of nervous impulses excited by ob-
jects on the opposite half of the body only. ///, Oculomotor nerve; L,
medial lemniscus; M, mammillary bodies; RN, red nucleus (nucleus ruber);
SN, black substance (substantia nigra) ; TG, optic tract to corpora quadri-
gemina (cf. Fig. 75). (From Starr's Nervous Diseases.)

212

INTRODUCTION TO NEUROLOGY

being given off to terminate in the unspecialized correlation
centers of the dorsal part of the thalamus. But in all forms with
a differentiated cerebral cortex these thalamic optic connections
assume greater importance, a special region in the dorsal part
of the thalamus being set apart for their use. Thus arose the
lateral geniculate body, and in higher mammals this is supple-
mented by the pulvinar. These centers are, in the strict sense
of the word, cortical dependencies, for they attain to only very

vfc I

Fig. 102. — Section through the parietal eye of a lizard (Anguis fragilis) :
ct, Connective-tissue cells around nerve; gc, ganglion cells; I, lens;-n, nerve-
fibers; pc, pigment cells; pn, parietal nerve from the parietal eye to the
brain; r, retinal cells; vb, vitreous body. (After Nowikoff.)

insignificant proportions in forms with rudimentary cerebral
cortex, but increase in proportion to the elaboration of the visual
cortex.

The early steps in the evolution of the eyes of vertebrates are imperfectly
understood. In structure and mode of function the vertebrate eyes are
unlike those of any of the invertebrate animals. The experiments of Parker
and others have shown that the skin of many aquatic vertebrates among the
fishes and amphibians is sensitive to light, and it has been supposed that the
vertebrate retina was differentiated from such cutaneous photoreceptors.
But it seems more probable (Parker, 1908) that the vertebrate organs of
vision were developed from the first within the central nervous system.

Some of the fishes and reptiles possess, in addition to lateral eyes of
typical form, a median eye, the parietal or pineal eye (Fig. 102), which is

THE VISUAL APPARATUS 213

developed from a tubular outgrowth from the roof of the diencephalon (the
- pineal organ or epiphysis, p. 162) ; this extends dorsalward from the brain
through a special foramen in the skull to reach the skin in the center of
the top of the head. The functions and evolutionary significance of this
eye are shrouded in mystery.

Summary. — The retina is developed as a lateral outgrowth
from the early neural tube and throughout life retains its char-
acter as a part of the brain, the "optic nerve" being really a cor-
relation tract comparable with the lemniscus systems. The rods
and cones of the retina are the photoreceptors and also the neu-
rons of the first order in the optic path. The "optic nerve" con-
tains neurons of the third order from the retina to the thalamus
and midbrain, and also centrifugal fibers from the midbrain to
the retina. In lower vertebrates the fibers of the optic path
decussate completely in the optic chiasma, but in those mammals
whose fields of vision overlap there is an incomplete decussation
so as to ensure the representation of the field of vision of one side
completely in the opposite cerebral hemisphere. Those fibers
of the optic tract which terminate in the midbrain effect various
kinds of reflex connections, while those which terminate in the
thalamus effect cortical connections. The parietal or pineal
eye of some fishes and reptiles is apparently functional as an
organ of vision which was developed quite independently of
the lateral eyes.

LITERATURE

In this chapter we have not attempted to present a systematic descrip-
tion of the structure of the eye or of the functions of the retina and theories
of vision. For the details of these questions reference must be made to the
larger text-books of anatomy, physiology, and physiological psychology.
A few general works are cited below, together with some special researches
to which reference has been made in the preceding text :

VON BECHTEREW, W. 1909. Die Funktionen der Nervencentra, Jena,
Bd. 2, pp. 996-1103. Idem, 1911, Bd. 3, pp. 1554-1583, 1883-1964.

COLE, L. J. 1907. An Experimental Study of the Image-forming Powers
of Various Types of Eyes, Proc. Amer. Acad. Arts and Sciences, vol. xlii,
No. 16.

HARRIS, W. 1904. Binocular and Stereoscopic Vision in Man and Other
Vertebrates, with Its Relation to the Decussation of the Optic Nerves, the
Ocular Movements, and the Pupil Light Reflex, Brain, vol. xxvii, pp. 106-
147.

LADD, G. T., and WOODWORTH, R. S. 1911. Elements of Physiological
Psychology, New York.

MAST, S. O. 1911. Light and the Behavior of Organisms, New York.

214 INTRODUCTION TO NEUROLOGY

NUEL, J. P. 1904. La Vision, Bibliotheque Internationale de Psychologic
Expe"rimentale Normal et Pathologique, Paris.

PARKER, G. H. 1908. The Origin of the Lateral Eyes of Vertebrates,
Amer. Nat., vol. xlii, pp. 601-609.

— . 1909. The Integumentary Nerves of Fishes as Photoreceptors and
Their Significance for the Origin of the Vertebrate Eyes, Amer. Jour, of
Physiol., vol. xxv, pp. 77-80.

RAMON Y CAJAL, S. 1894. Die Retina der Wirbeltiere, Wiesbaden.

SCHAFER, E. A. Text-book of Physiology, vol. ii, pp. 752-761, 1026-
1148.

VINCENT, S. B. 1912. The Mammalian Eye, Jour. Animal Behavior,
vol. ii, pp. 249-255.

WATSON, J. B. 1914. Behavior, an Introduction to Comparative Psy-
chology, New York, Chapter XL

CHAPTER XV

THE OLFACTORY APPARATUS

THE olfactory part of the brain as a whole is sometimes called
the rhinencephalon. In fishes (p. 112 and Figs. 43, 44) almost the
whole of the cerebral hemisphere is devoted to this function, and
as we pass up the scale of animal life more and more non-ol-
factory centers are added to the hemisphere in the corpus stria-
turn and cerebral cortex, until in man the non-olfactory part of
the hemisphere overshadows the rhinencephalon. The complex
form of the human cerebral hemisphere cannot be adequately
understood apart from a knowledge of this evolutionary history,
which has been studied with great care by comparative neurolo-
gists. The metamorphosis of the vertebrate cerebral hemisphere
from a simple olfactory reflex apparatus in the lower fishes to the
great organ of the higher mental processes upon which all human
culture depends is a very dramatic history, into which, unfortu-
nately, we cannot here enter.

Smell is evidently the dominant sense in many of the lower
vertebrates. That this is the case in the dogfish is shown by the
enormous development of the olfactory centers of the brain, to
which reference has just been made. And in most of the labora-
tory mammals, such as the rat and the dog, the sense of smell
still plays a very much more important part in the behavior
complex than in man and other primates, whose olfactory organs
are in a reduced condition.

The nervus terminalis is a slender ganglionated nerve found associated
with the olfactory nerve in most classes of vertebrates from fishes to man.
Its fibers, which are unmyelinated, reach the mucous membrane of the
nose, though the precise method of their ending is unknown. They pass
inward in company with those of the olfactory nerve as far as the olfactory
bulb. Here they separate from the olfactory fibers and enter the cerebral
hemisphere between the attachment of the olfactory bulb and the lamina
terminalis (Fig. 43, p. 111). Within the brain they have been followed
backward through the entire length of the olfactory area and hypothalamus,
but their cerebral connections have never been accurately determined.
The function of this nerve is likewise wholly unknown.

215

216

INTRODUCTION TO NEUROLOGY

The olfactory cerebral centers fall into two groups: (1) the
reflex centers of the brain stem and (2) the olfactory cerebral
cortex. The arrangements of the olfactory reflex centers and
their connecting tracts are essentially similar in plan in all ver-
tebrate brains (except in some aquatic mammals, like the dol-
phin, which lack olfactory organs altogether). The olfactory
cerebral cortex, on the other hand, is very diversely developed
in different groups of vertebrates. There is no true cerebral
cortex in fishes; in amphibians (particularly in the frog) the
olfactory cerebral cortex begins to emerge from the general

Fig. 103. — Dissection of the right olfactory bulb and nerve on the
lateral wall of the nasal cavity. (From Wood's Reference Handbook of the
Medical Sciences.)

olfactory reflex centers; in reptiles there is a well-formed olfac-
tory cortex of simple histologic pattern and the beginnings of the
non-olfactory cortex; in birds the olfactory apparatus is reduced
and the non-olfactory cortex is somewhat more extensive than
in reptiles; in mammals both the olfactory cerebral cortex and
the non-olfactory cortex attain their maximum dimensions, the
former in the lowest members of this group and the latter in the
highest.

The cerebral cortex as a whole is sometimes called the
pallium. That portion of the pallium which is related with the
olfactory apparatus was differentiated earlier in vertebrate evo-

THE OLFACTORY APPARATUS

217

lution than the non-olfactory pallium and has, therefore, been
called the archipallium. The non-olfactory cerebral cortex is
termed the neopallium (or somatic pallium, for it receives the
somatic projection fibers). The archipallium, as already indi-
cated, attains its maximum development in the lowest mammals,
particularly the marsupials, like the kangaroo and opossum,
consisting of the hippocampus and hippocampal gyrus (gyrus
hippocampi, or pyriform lobe). The neopallium attains its
maximum size in the human brain, and the indications are that
in civilized races it is now in process of further differentiation.
In the human brain practically all parts of the exposed cerebral
cortex are neopallium, the archipallium being of relatively small

Olfactory tract

Granule cell
Mitral cell
Glomerulus
Olfactory nerve
Ethmoid bone
~^ Olfactory epithelium

Fig. 104. — Diagram of the connections of the olfactory bulb.

size and mostly concealed by a process of infolding along the
posterior margin of the neopallium.

In the human body the specific olfactory receptors (see p. 92)
are limited to a small area of the mucous lining in the upper part
of the nasal cavity on both its lateral (Fig. 103) and its medial
walls. The cell bodies of the olfactory neurons of the first
order lie in this mucous membrane (Figs. 36 and 104). The
axons of these neurons form the fibers of the olfactory nerve,
which are unmyelinated; they pierce the ethmoid bone in nu-
merous small fascicles (fila olfactoria) and terminate by free
arborizations in the primary olfactory center within the olfac-
tory bulb (Figs. 53, 78, 103, 104). Several olfactory nerve-
fibers terminate together in a dense entanglement of fibers

218 INTRODUCTION TO NEUROLOGY

termed a glomerulus, which also receives one or more dendrites
from the olfactory neurons of the second order, or mitral cells.
The glomerulus, therefore, contains the first synapse in the ol-
factory pathway. The axons of the mitral cells form the ol-
factory tract and discharge into the olfactory area, or secondary
olfactory nucleus, at the base of the olfactory bulb. These axons
give off collateral branches which discharge among very small
neurons of the olfactory bulb, the granule cells, whose chief
processes are directed peripheralward, to end among dendrites
of the mitral cells.

Attention has already been called (pp. 75 and 91) to the fact
that, though smell and taste are both chemically excited senses,
the olfactory organs can be excited by much more dilute solu-
tions of the stimulating substances than can the gustatory or-
gans. The lowering of the threshold for olfactory stimuli has
been effected by several means, among which we may mention
the following: Whereas in the taste-buds there is a synapse
between the specific receptor cells and the peripheral nerve-fiber
(Fig. 35, p. 91), there is no such synapse in the olfactory organ,
the peripheral receptor cell giving rise directly to the olfactory
nerve-fiber (Fig. 104). In the second place, the peripheral
gustatory nerve-fiber discharges centrally into several neurons
of the primary gustatory center in the medulla oblongata; but
many peripheral olfactory fibers enter a single glomerulus, where
they are engaged by dendrites from only one or two mitral cells,
thus providing for the summation of stimuli in each mitral cell.
Again, the collateral discharge from the olfactory tract into the
granule cells (which are very numerous) carries the discharge
from the mitral cells back again into these cells and thus rein-
forces their discharge (see pp. 101, 192). By these and other
devices exceedingly feeble peripheral stimuli may give rise to
very strong excitations in the olfactory centers.

The fibers of the olfactory tract reach the olfactory area, or
secondary center, by three paths which spread out from the
base of the olfactory bulb and are known as the medial, inter-
mediate, and lateral olfactory striae (these are shown but not
named on Fig. 53, p. 120). The olfactory area has various sub-
divisions (Fig. 105), the most important of which are: (1) the
lateral olfactory nucleus (or gyrus) which receives the lateral

THE OLFACTORY APPARATUS

219

olfactory stria and extends backward directly into the tip of the
temporal lobe of the cerebral cortex (uncus), where the ventro-
lateral ends of the hippocampus and the hippocampal gyrus come
together; (2) the medial olfactory nucleus, including the sub-
callosal gyrus (Fig. 52, p. 119) and septum, which receive the
medial olfactory stria; (3) the intermediate olfactory nucleus,
which occupies the anterior perforated substance (Figs. 53, 105)
and receives the intermediate olfactory stria. These nuclei are
all important reflex centers, where olfactory stimuli are combined

Olfactory bulb

Lateral olfactory
(stria)

Posterior parolfactory
sulcus

Uncus (hippocampal
gyrus)

Medial olfactory gyrus (stria)
Olfactory tract

Limen insulae

Anterior perforated
substance

Hippocampal gyrus

Fig. 105. — Brain of a human fetus at the beginning of the fifth month
(22.5 cm. long), illustrating the olfactory centers visible on the ventral
surface. (After Retzius, from Morris' Anatomy.)

with other sensory impressions, each nucleus having its own par-
ticular reflex pattern. The intermediate nucleus (also called
tuberculum olfactorium and by Edinger lobus parolfactorius) is
better developed in many other mammals than in man, and is
probably especially concerned with the feeding reflexes of the
snout or muzzle, including smell, touch, taste, and muscular
sensibility, a physiological complex which Edinger has called
collectively the "oral sense." This complex of muzzle reflexes
has probably played a very important role in the earlier stages
of the evolutionary history of the correlation centers of the

220

INTRODUCTION TO NEUROLOGY

cerebral hemispheres (see the works by Edinger cited at the end
of this chapter).

From these nuclei of the olfactory area fiber tracts of the third
order pass to the mammillary bodies of the hypothalamus and
to the habenular bodies of the epithalamus, from both of which,
after another synapse, tracts lead downward into the motor

com. post,
hab.
f.reth
nanT.
str. med.
tr. mamTh.
Tr. olf. fegm

form bulb.

TfrolT.hyptKS

^com.ont.
^n. pop

Fig. 106. — Diagram of some of the olfactory tracts in the brain of the
rat. The chief connections of the medial and intermediate olfactory tracts
are indicated; those of the lateral olfactory tract are omitted: c.mam.,
corpus mamillare; col. forn., columna fornicis; com. ant., commissura an-
terior; com. hip., commissura hippocampi; com. post., commissura posterior;
form, bulb., formatio bulbaris; f.retr., fasciculus retroflexus of Meynert;
hob., habenula; h.pc., hippocampus precommissuralis; h.sc., hippocampus
supracommissuralis; n. ant., nucleus anterior thalami; n. olf. ant., nucleus
olfactorius anterior; n. pop., nucleus preopticus (ganglion opticum basale);
S, septum; str. med., stria medullaris thalami; tr. mam. th., tractus mamillo-
thalamicus (Vicq d'Azyri) ; tr. olf. hypth., tractus olfacto-hypothalamicus, or
basal olfactory tract; tr. olf. tegm., tractus olf acto-tegmentalis; tub. f. dent.,
tuberculum fasciae dentatse (hippocampus postcommissuralis) ; tub. olf.,
tuberculum olfacfcorium.

centers of the midbrain in the cerebral peduncle. The path from
the mammillary body is the tractus mamillo-peduncularis (Figs.
75, 78, 106). The path from the habenular body is the tractus
habenulo-peduncularis (fasciculus retroflexus B. N. A., or Mey-
nert's bundle — Fig. 106). The mammillary body also sends a
tract into the anterior nucleus of the thalamus, the tractus mam-
illo-thalamicus (fasciculus thalamo-mamillaris B. N. A., or tract

THE OLFACTORY APPARATUS 221

of Vicq d'Azyr, Figs. 78, 106), for the correlation of olfactory with
general somatic reactions. There is also a direct path between
the secondary olfactory area and the cerebral peduncle, without
connection with the diencephalon, by way of the tractus oLfacto-
tegmentalis (Fig. 106). In the epithalamus the olfactory ner-
vous impulses are correlated with those of the somatic sensory
centers of the thalamus, especially the optic and tactual sys-
tems (p. 162) ; in the hypothalamus they are correlated with gus-
tatory and various visceral sensory systems (p. 163).

The preceding account includes a description of a few of the
more important pathways involved in olfactory reflexes. Ol-
factory impulses which reach the cerebral cortex take a different
path. They are carried from all parts of the secondary olfactory
area at the base of the olfactory bulb into the hippocampus
(which composes the greater part of the archipallium in the
human brain) by several olfacto-cortical tracts, whose courses
in the human brain are so tortuous that we shall not attempt to
describe them here.

The hippocampus (formerly called the Ammon's horn or cornu
Ammonis, also the hippocampus major, Fig. 107) is a special
convolution which forms the postero-ventral border of the cere-
bral cortex; it is rolled into the posterior horn of the lateral
ventricle so that it does not appear on the surface of the brain.
It is connected with the remainder of the cortex (neopallium)
by cortex of transitional type, the hippocampal gyrus (gyrus
hippocampi), from which it is separated by a deep groove, the
fissura hippocampi. The free border of the hippocampus is
accompanied for its entire length by a strong band of fibers, the
fimbria, through which olfactory projection fibers enter it from
the secondary olfactory area. These fibers discharge into a
subsidiary part of the hippocampus, the dentate gyrus (gyrus
dentatus, also called fascia dentata), at a, Fig. 107.

The hippocampus is connected with all other parts of the cere-
bral cortex by an extensive system of association tracts forming
the alveus (Fig. 107), thus providing for those complex inter-
actions of diverse functional systems for which the cortex is
especially adapted. There is also an efferent pathway from the
hippocampus to the brain stem through the fimbria and the
column of the fornix (Figs. 78, 107), whose fibers terminate in

222

INTRODUCTION TO NEUROLOGY

both the hypothalamus and the epithalamus. This is the prob-
able pathway taken by voluntary motor impulses of cortical
origin, in which the olfactory element is dominant, such as
sniffing. Having reached the hypothalamus and epithalamus,
these motor impulses of cortical origin are conveyed to the motor
centers in the midbrain by the same pathways as are the reflex
impulses already described.

Fig. 107. — Section across the hippocampus and gyms hippocampi of the
human brain. (After Edinger.)

Summary. — The olfactory centers (rhinencephalon) make up
nearly the entire forebrain in fishes, and in higher vertebrates
progressively more non-olfactory centers are added to this part
of the brain. The non-olfactory parts of the cerebral hemi-
sphere comprise chiefly the corpus striatum and the neopallium;

THE OLFACTORY APPARATUS 223

the latter makes up by far the larger part of the human hemi-
sphere. The rhinencephalon consists of a reflex part in the brain
stem and a cortical part in the archipallium. Smell and taste are
both chemically excited senses, but the threshold of excitation is
much lower in the case of smell. This is brought about by the
suppression of a synapse in the peripheral receptor organ and by
a complex mechanism -for the summation and reinforcement of
stimuli in the primary olfactory center in the olfactory bulb.
The secondary olfactory center is the olfactory area, which has
three parts, each of which is a reflex center of distinctive type.
The reflex path from the secondary center passes backward to
the epithalamus and to the hypothalamus, from both of which a
descending path goes to the motor centers in the cerebral
peduncle. The secondary olfactory center also discharges into
the olfactory cerebral cortex, which is chiefly contained within
the hippocampus and from which manifold association pathways
connect with all other parts of the cerebral cortex.

LITERATURE

BARKER, L. F. 1901. The Nervous System, New York, pp. 747-781.

EDINGER, L. 1908. Vorlesungen liber den Bau der nervosen Zentral-
organe, Bd. 2, Vergleichende Anatomic des Gehirns, Leipzig.

— . 1908. The Relations of Comparative Anatomy to Comparative
Psychology, Jour. Comp. Neurology, vol. xviii, pp. 437-457.

— . 1908. Ueber die Oralsinne dienenden Apparate am Gehirn der
Sauger, Deutsch. Zeits. f! Nervenheilkunde, Bd. 36.

HERRICK, C. JUDSON. 1908. On the Phylogenetic Differentiation of
th.e Organs of Smell and Taste, Jour. Comp. Neurology, vol. xviii, pp.
157-166.

— . 1910. The Evolution of Intelligence and Its Organs, Science, N. S.,
vol. xxxi, pp. 7-18.

— . 1910. The Morphology of the Forebrain in Amphibia and Reptilia,
Jour. Comp. Neurology, vol. xx, pp. 413-547.

JOHNSTON, J. B. 1906. The Nervous System of Vertebrates, Phila-
delphia, pp. 176-189, 292-337.

— . 1909. The Morphology of the Forebrain Vesicle in Vertebrates,
Jour. Comp. Neurology, vol. xix, pp. 457-539.

— . 1913. The Morphology of the Septum, Hippocampus and Pallia!
Commissure in Reptiles and Mammals, Jour. Comp. Neurology, vol. xxiii,
pp. 371-478.

KAPPERS, C. U. A. 1908. Die Phylogenese des Rhinencephalons, des
Corpus Striatum und der Vorderhirnkommissuren, Folia Neurobiologica,
Bd. 1, pp. 173-288.

ZWAARDEMAKER, H. 1895. Die Physiologie des Geruchs, Leipzig.

— . 1900. Revue generate sur 1'olfaction, Annee Psychol., vol. vi.

— . 1902. Geruch. Ergebnisse der Physiologie, Bd. 1.

CHAPTER XVI

THE SYMPATHETIC NERVOUS SYSTEM

BEFORE we can extend our analysis of the conduction paths
into the realm of the visceral activities of the body we must con-
sider briefly the sympathetic nervous system through which the
regulatory control of these activities is effected. Most of the
visceral activities are performed either unconsciously or with
very imperfect awareness. The nervous mechanisms of many
of them are still obscure. Nevertheless the visceral functions
as a whole are of enormous importance, not only in the mainte-
nance of the physical welfare of the body, but also as the organic
background of the entire conscious life (see p. 259).

Many of the visceral functions can be performed quite apart
from any nervous control whatever by the intrinsic mechanisms
of the viscera themselves. The heart musculature, for instance,
beats automatically with a characteristic rhythm, and most of
the other visceral muscles have the power of automatic rhythmic
contraction. Some of the glands of the body may be excited
to secretion by chemical substances dissolved in the blood. For
instance, when food enters the small intestine from the stomach,
the intestinal glands are directly excited to activity by the pres-
ence of the food. Some of their secretions are poured out into
the intestine to act as digestive juices; others are absorbed di-
rectly by the blood (internal secretions). Among the latter is
secretin, a substance which is carried by the blood-stream to the
pancreas and there excites the secretory activity of this organ to
the formation of pancreatic juice, which is, in turn, poured into
the intestine. The very complex secretory activities involved in
the formation of the intestinal and pancreatic juices under the
stimulus offered by the presence of food in the intestine, there-
fore, are not directly excited by the nervous system, though they
may be brought under nervous control in a secondary way.

Most of the viscera are, however, under immediate nervous
control of two sorts. This control is partly derived from the

224

THE SYMPATHETIC NERVOUS SYSTEM 225

ganglia of the sympathetic nervous system which are distributed
widely throughout the body, and partly from the central nervous
system. The nervous impulses involved in the second type of
control are, moreover, always distributed to the viscera through
the sympathetic system.

A clear analytic description of the visceral nervous systems is
extremely difficult, and there is wide diversity of usage, not only
in the terminology employed in these descriptions, but also in
the fundamental concepts upon which they are based. The
brain and spinal cord and the cranial and spinal nerves and their
end-organs in the aggregate constitute the cerebro-spinal nervous
system. The cell bodies of the neurons of this system all lie
within the spinal cord and brain (including the retina) or in the
ganglia on the sensory roots of the cranial and spinal nerves.
There are, however, innumerable other ganglia distributed very
widely throughout the body, which are connected with each
other and with the central nervous system by intricate nervous
plexuses. These constitute the sympathetic ganglia and nerves,
or in the aggregate the sympathetic nervous system.

There is an especially important group of sympathetic ganglia
which are arranged in two longitudinal series extending one on
each side of the vertebral column. These ganglia constitute the
vertebral sympathetic trunks or chains, and throughout the
middle part of the body there is one ganglion of each trunk for
each spinal root (Fig. 41, p. 107). Communicating branches con-
nect the ganglia of the trunks with their respective spinal roots,
and from these ganglia sympathetic nerves extend out periph-
erally to ramify among the viscera and other tissues of the
body. Ganglion cells are scattered among these peripheral
sympathetic nerves, and in some places, especially among the
abdominal viscera, these cells are crowded together to form large
ganglionic plexuses (Fig. 108).

When further analyzed, the sympathetic nervous system is
found to consist of two imperfectly separable parts. The first
is a diffusely arranged peripheral plexus of nerve-cells and fibers
adapted for the local control of the organs with which it is con-
nected. This we shall call the peripheral autonomous part of the
sympathetic system (this is not the same as the autonomic ner-
vous system of Langley, see p. 229). The second part of the

15

226

INTRODUCTION TO NEUROLOGY

Maxillary nerve

Ciliary ganglion
Sphenopalatine ganglion \
Superior cervical ganglion of sympathetic \ \

Cervical plexus

Brachial plexus

Pharyngeal plexus

Middle cervical ganglion of

sympathetic

Inferior cervical g. of sympathetic
Recurrent nerve

Bronchial plexus

Cardiac plexus

Esophageal plexus
!oronary plexus

Greater splanchnic nerve
Lesser splanchnic nerve

Lumbar plexus
Sacral plexus

Left vagus nerve

ic plexus
eliac plexus

Superior mesenteric plexus

Aortic plexus

inferior mesenteric plexus

Hypogastric plexus

Fig. 108. — The sympathetic nervous system, illustrating the right sym-
pathetic trunk and its relation with the spinal nerves and with the per-
ipheral sympathetic ganglionated plexuses; cf. Fig. 41, p. 107. (After
Schwalbe.)

THE SYMPATHETIC NERVOUS SYSTEM 227

sympathetic system includes those neurons which put the periph-
eral autonomous system into functional connection with the
central nervous system, thus providing a central regulatory con-
trol over the autonomous system. This part of the sympa-
thetic nervous system includes the peripheral courses of the
neurons involved in the general cerebro-spinal visceral reflex sys-
tems (see pp. 76, 89, 93).

The peripheral autonomous nervous system appears to be a
direct survival of that diffuse type of nervous system which is
found in the lowest animals which possess nerves at all, such as
some jelly-fishes and worms. The central nervous system of
higher animals is supposed to have developed by a concentration
of ganglia in such a diffuse system (see p. 27), a portion of which
remains as the peripheral autonomous sympathetic system (Fig.
17, p. 53). But during evolution the central nervous system
increased in importance for integrating and regulating the
functions of the body, the central control of the viscera assumed
greater importance, and the general cerebro-spinal visceral sys-
tems were developed to serve this function.

Figure 56 (p. 126) illustrates the typical arrangement of the
visceral sensory and motor fibers in the spinal nerves, and their
relations to the sympathetic ganglia and nerves. These fibers,
of course, belong to the cerebro-spinal visceral systems; the
peripheral autonomous system is not included in the diagram.
The central control of the visceral apparatus is effected (1) by
afferent visceral nerve-fibers distributed peripherally through
the sympathetic nerves and entering the spinal cord through the
dorsal spinal roots, and (2) by efferent visceral nerves which
leave the spinal cord through the ventral roots and also enter the
sympathetic nerves. In lower vertebrates (and possibly also in
man) some of these fibers leave by the dorsal roots also.

The cell bodies of the afferent neurons lie in part in the
spinal ganglia and in part in the sympathetic ganglia. Figure
109 illustrates the connections of these two types of afferent
visceral neurons. Neuron 3 of this figure may transmit its
impulse either directly into the spinal cord through its centrally
directed process or by a collateral branch to some other cell
body of the spinal ganglion (neuron 1). The fiber marked 4
arises from a cell-body lying in some sympathetic ganglion and

228

INTRODUCTION TO NEUROLOGY

terminates in synaptic relation with some neuron whose cell
body lies in the spinal ganglion, which, in turn, may transmit this
visceral impulse into the spinal cord in addition to its own proper
function, say, of cutaneous sensibility.

spinal ganglion

peripheral
nerve

Fig. 109. — Diagram illustrating three ways in which afferent visceral
fibers may connect with the central nervous system through the spinal
ganglia (cf. Fig. 56, p. 126). Neurons 1 and 2 are typical somatic sensory
neurons, whose peripheral fibers reach the skin. Neuron 3 is a visceral
sensory neuron, whose peripheral fiber enters the sympathetic nervous sys-
tem through the communicating branch (this neuron is drawn in fine dotted
lines in Fig. 56). Neurons of the third type may bring in afferent impulses
from the viscera through their peripheral processes and transmit these im-
pulses directly to the spinal cord through their central processes. A col-
lateral branch from this neuron, moreover, may carry the visceral impulse
to the cell body of a neuron of type 1, which thus serves to convey both
somatic impulses from the skin and visceral impulses from some deep-seated
organ. The spinal ganglion also receives nerve-fibers of the type marked 4,
whose cell bodies lie in the sympathetic ganglia. These probably convey
visceral afferent impulses as far as the spinal ganglion, which are then trans-
mitted to the spinal cord through a somatic sensory neuron. These arrange-
ments are described in detail by Dogiel.

The relations just described probably provide the neurological
mechanism of some of the curious phenomena known as referred
pains. It is well known that disease of certain internal organs
may be accompanied by no pain at the site of the injury, but by
cutaneous pain and tenderness in remote parts of the body.
Fig. 110 illustrates some of these areas of referred pain and the

THE SYMPATHETIC NERVOUS SYSTEM 229

sources of the excitations. The mechanisms shown in Fig.
109 show how an inflammatory process or other injury of the
sympathetic nerves associated with these deep viscera may read-
ily be carried over to the related neurons of the somatic sensory
system. Many referred pains are undoubtedly due to similar
collocations of visceral and somatic sensory paths within the
spinal cord and brain. Since the functions of these visceral
nerves do not usually come into consciousness at all, the pain
will be referred to the peripheral area of distribution of the
associated somatic nerve, which has a distinct "local sign," or
habitual peripheral reference.

The efferent fibers of the cerebro-spinal visceral system arise
from several groups of cells in the intermediate zone between
the dorsal and ventral gray columns of the spinal cord, and in
particular from an intermedio-lateral column of cells at the mar-
gin of the lateral column of gray matter (Fig. 56, p. 126). These
efferent fibers never reach their peripheral terminations directly.
They always end in some sympathetic ganglion, either of the
vertebral ganglionic trunk or one of the peripheral sympathetic
ganglia. Here there is a synapse, and a second neuron of the
sympathetic ganglion in question takes up the nervous impulse
and transmits it to its termination in some unstriated visceral
muscle or gland. The efferent fiber arising from a cell body
within the spinal cord is termed the preganglionic fiber, and the
peripheral fiber arising from a neuron of the sympathetic gang-
lion is the postganglionic fiber. The former is usually a small
myelinated fiber; the latter is usually unmyelinated. The pre-
ceding description is applicable to the visceral nervous system
in the trunk region of the body. In the head the connections
of the nerves of this type are much more complex.

Langley and others have shown that what is here termed the
general cerebro-spinal visceral system is related to four distinct
regions of the central nervous system, as illustrated by Fig. 111.1

1 Langley calls the entire sympathetic system the autonomic system, and
limits the application of the term "sympathetic" to what is here called the
thoracic-lumbar sympathetic. There is no adequate ground for his belief
that the latter is genetically different from the other parts of the cerebro-
spinal visceral apparatus, though its physiological characteristics are very
distinctive. Many of the viscera have a double innervation through the sym-
pathetic, one set of fibers coming from the midbrain, bulbar, or sacral sym-
pathetic ganglia and an antagonistic set coming from the thoracic-lumbar
sympathetic ganglia.

230

INTRODUCTION TO NEUROLOGY

AMMfe

Kn,tf,mttrlt\t

Coiutipattm BlaJdtrf

Caritiofincifor

Fig. 110. — The locations of referred pains and their causes. (After Dana,
from Starr's Nervous Diseases.)

Area, cerebro-spinal
nerves.

I. Trigeminus, fa-
cial.
II. Upper cervical.

III. Lower four cer-

vical and first
thoracic.

IV. Upper six thor-

acic.

V. Lower six thor-
acic.

VI. Twelfth thoracic

and fourth lum-
bar.

VII. Fifth lumbar and

five sacral.

Associated ganglia

Distribution.

of sympathetic.

Distribution.

Face and anterior

Four cerebral.

Head.

scalp.

Occiput, neck.

First cervical.

Head, ear.

Upper extremity.

Second and third

Heart.

cervical, first

thoracic.

Thorax.

First to sixth

Lungs.

thoracic.

Abdomen, upper

Sixth to twelfth

Viscera of ab-

lumbar.

thoracic.

domen and

testes.

Lumbar, upper

First to fifth

Pelvic or-

gluteal, anterior

lumbar.

gans.

thigh, and knee.

Lower gluteal,

First to fifth

Pelvic or-

posterior thigh

sacral.

gans and

and leg.

legs.

THE SYMPATHETIC NERVOUS SYSTEM 231

The portions of the sympathetic system related to these respec-
tive regions are as follows: (1) The midbrain sympathetic, com-
prising chiefly the ciliary ganglion behind the eye and its nerves,
these being related to the brain through the III cranial nerve.
(2) The bulbar sympathetic, related to the brain chiefly through
the VII, IX, and X cranial nerves. (3) The thoracic-lumbar
sympathetic, related to the spinal cord through the I thoracic
to II or III lumbar nerves. (4) The sacral sympathetic,
related to the spinal cord through the II to IV sacral nerves.

Each of these four regions has its own distinctive physiological
characteristics, including in some cases a special type of reaction
to certain drugs. They all exhibit a common reaction to nico-
tin in physiological doses. The effect of this poison is to paralyze
the synapses between the preganglionic and the postganglionic
neurons and thus to isolate the peripheral sympathetic neurons
physiologically from efferent impulses arising within the central
nervous system. Adrenalin (extract of the suprarenal glands)
affects chiefly the thoracic-lumbar sympathetic system (see p.
255). On the other hand, poisons of a different group, including
atropin, muscarin, and pilocarpin, are said to act chiefly upon
the midbrain, bulbar and sacral sympathetic, but not upon the
thoracic-lumbar system. There are other cases of very specific
action of drugs upon special parts of the sympathetic nervous
system.

Summary. — From the preceding considerations it is evident
that the sympathetic nervous system cannot be sharply sepa-
rated anatomically or physiologically from the cerebro-spinal
system. The cell bodies of the neurons of the cerebro-spinal
visceral system lie partly within and partly without the central
nervous axis. A ganglionic sympathetic trunk extends on each
side of the body along the spinal column, and the ganglia of this
trunk are connected with most of the spinal nerves by com-
municating branches. The neurons of this trunk of vertebral
sympathetic ganglia belong chiefly to the cerebro-spinal visceral
system, since they are concerned with the central regulatory
mechanism of the viscera. All parts of the visceral nervous sys-
tem which lie peripherally of the communicating branches
between the sympathetic ganglionated trunks and the spinal
roots, and can be anatomically separated from the peripheral

232

INTRODUCTION TO NEUROLOGY

Sphincter of iris )
Ciliary muscle /

Heart, blood-vessels of mucous mem-")
branes of bead, salivary glands, walls of i
digestive tract from mouth to descending I
colon, including outgrowths of this re- f
gion — trachea and lungs, gastric glands, |
fiver, pancreas.

Dilator of iris, orbital muscles, arteries,
muscles and glands of the skin, blood-
vessels of lungs and abdominal viscera and
of digestive tract between mouth and rec-
tum, arteries of skeletal muscles, muscles
of spleen, ureter, and internal generative
organs.

Arteries of rectum, anus, and external |
generative organs, muscles of external gen- I
erative organs, walls of bladder and I
urethra, walls of descending colon to anus, j

Midbrain sympathetic

Bulbar sympathetic

/Thoracic-lumbar sympathetic
1 1 thoracic to II or III lumbar

/Sacral sympathetic
\II to IV sacral

Fig. 111. — Diagram of the central localization of the cerebro-spinal visceral
nervous connections. (Modified from Langley.)

THE SYMPATHETIC NERVOUS SYSTEM 233

branches of the cerebro-spinal nerves, are commonly described
as constituting the sympathetic nervous system. This system
includes the ganglionated trunks bordering the spinal column,
to which reference has just been made, the larger peripheral
ganglionated plexuses of the head, thorax, and abdomen, and a
very large number of minute sympathetic ganglia scattered
everywhere throughout the body. This sympathetic nervous
system we have regarded as composed of two imperfectly
separable parts: (1) a series of autonomous peripheral ganglia for
the local regulation of the organs within which they are found;
(2) the neurons of the cerebro-spinal visceral systems which en-
able the central nervous system to maintain a regulatory control
over the intrinsic autonomous systems.

LITERATURE

DOGIEL, A. S. 1908. Der Bau der Spinalganglien des Menschen und der
Saugetiere, Jena, G. Fischer, 151 pp., 14 plates.

HEAD, H. 1893. On Disturbances of Sensation with Especial Reference
to the Pain of Visceral Disease, Brain, vol. xvi, pp. 1-133.

— . 1901. The Gulstonian Lectures for 1901, Brain, vol. xxiv, p. 398.

HEAD and CAMPBELL. 1901. Pathology of Herpes Zoster, Brain, vol.
xxiii, p. 353.

HUBER, G. C. 1897. Lectures on the Sympathetic Nervous System,
Jour. Comp. Neur., vol. vii, pp. 73-145.

KUNTZ, A. 1911. The Evolution of the Sympathetic Nervous System in
Vertebrates, Jour. Comp. Neur., vol. xxi, pp. 215-236.

LANGLEY, J. N. 1900. The Sympathetic and Other Related Systems of
Nerves, in Schaefer's Text-book of Physiology, London, pp. 616-696.

— . 1900. On Axon-reflexes in the Preganglionic Fibers, Jour, of Physiol.,
vol. xxv, p. 364.

— . 1903. The Autonomic Nervous System, Brain, vol. xxvi, pp. 1-26.

ONUF, B., and COLLINS, J. 1900. Experimental Researches on the Central
Localization of the Sympathetic with a Critical Review of its Anatomy and
Physiology, Archives of Neurology and Psychopathology, vol. iii, p. 1-252.

CHAPTER XVII
THE VISCERAL AND GUSTATORY APPARATUS

OUR knowledge of the functional localization within the spinal
cord of the general visceral reflex centers related to the spinal
nerves is still rather indefinite. Most of the cerebro-spinal
control of the visceral reactions of the body is effected from the
bulbar sympathetic centers by way of the vagus nerve. The
afferent fibers of these systems all enter the fasciculus solitarius,
a longitudinal bundle of fibers in the lower part of the medulla
oblongata, and they terminate in the nucleus of visceral sensory
neurons which accompanies this fasciculus (Figs. 71-74, 77,
114). The special visceral fibers of the nerves of taste also ter-
minate in this nucleus. The efferent fibers of these systems arise
chiefly from the dorsal motor nucleus of the vagus, a cluster of
neurons which produces an eminence in the floor of the fourth
ventricle known as the ala cinerea or trigonum vagi (Figs. 71-
74, 114). From this nucleus arise preganglionic fibers for the
innervation of various systems of visceral muscles of blood-
vessels, esophagus, stomach, intestine, bronchi, and others.

Most viscera possess a double innervation — from the thoracic-
lumbar sympathetic system and from the midbrain, bulbar, or
sacral system (see p. 229). For instance, the heart-beat is
accelerated by the thoracic-lumbar system and inhibited by the
bulbar system through the vagus; and the iris is contracted
through the midbrain sympathetic, but dilated through the
thoracic by way of the superior cervical ganglion (p. 211).

Organs of Circulation. — The nervous control of the heart
and blood-vessels is far too complex for full description here. A
few general features only can be touched upon.

The rate of blood flow may be varied for the body as a whole
by changes in the rate and force of the pulsations of the heart,
and for particular parts of the body by changes in the caliber
of its blood-vessels. The heart beats automatically, but its

234

THE VISCERAL AND GUSTATORY APPARATUS 235

rate is regulated through the cardiac nerves. The caliber of
the smaller blood-vessels and hence the amount of blood which
can pass through them is regulated by vasomotor nerves. Both
the heart and the muscular walls of the vessels have a double
innervation. The heart has an accelerator nerve and an in-
hibitory nerve; the smaller arteries have vasodilator and vaso-
constrictor nerves. The amount of blood pumped by the heart
at any time will depend upon the equilibrium existing between
its accelerator and its inhibitory fibers and upon the resist-
ance offered by the peripheral vessels; that flowing through
any particular system of blood-vessels will be affected also by
the equilibrium between the vasodilator and the vasoconstrictor
nerves of these vessels.

There are sympathetic ganglia within the heart. Its extrinsic
nerve supply includes afferent fibers to the brain and efferent
fibers of two sorts, viz., the accelerator and inhibitory fibers
already mentioned. The afferent fibers are represented in a
small sympathetic nerve, the nerve of Cyon, which is also called
the depressor nerve. They arise from the walls of the ventricles
of the heart and join the vagus trunk, through which they enter
the medulla oblongata. Stimulation of this nerve produces a fall
of arterial pressure by dilating the vessels throughout the body,
especially in the viscera. It appears to act to reduce the labor
of the heart when intraventricular pressure becomes excessive.

The medulla oblongata contains a center whose stimulation
causes inhibition of the heart-beat. These efferent fibers go out
as preganglionic fibers of the vagus nerve and terminate in the
cardiac sympathetic plexus (Fig. 108), where their postganglionic
neurons are located. There is also a center in the medulla ob-
longata (which has not been precisely localized) whose stimula-
tion causes acceleration of the heart-beat. These accelerator
nerve-fibers do not leave the brain through the vagus, but appar-
ently they descend through the spinal cord to the lower cervical
region and pass out into the sympathetic nervous system at this
level. The centers of vasomotor control of various regions of
the body are indicated in Fig. 111.

Organs of Respiration. — Oxygen is supplied to the tissues of
the body in a great variety of ways in different animals. In
some of the simpler animals, as in plants generally, oxygen is

236 INTRODUCTION TO NEUROLOGY

simply absorbed from the surrounding medium by the exposed
surfaces. In all but the lowest animals there is a blood-vascular
system by means of which the oxygen absorbed at the surface is
transferred to the deeper tissues. In insects, however, this re-
sult is obtained chiefly by a different apparatus, namely, a sys-
tem of air tubes (tracheae) which ramify among the tissues and
supply oxygen directly to the functioning cells. In most water-
breathing animals a portion of the surface of the body is lamel-
lated and vascularized to form gills to facilitate the absorption of
oxygen by the blood-stream, and in air-breathing vertebrates
lungs are developed to accomplish the same result. The nervous
mechanisms of respiration will differ in all of the cases cited
above, and it is only in mammals that we shall here consider the
details of this mechanism.

In ordinary breathing, inspiration is effected by actively in-
creasing the volume of the thoracic cavity and thus creating a
suction through the trachea, while expiration is the result of the
passive return of the organs involved to their former positions
by reason of their own elasticity. The muscles involved in
inspiration belong to two groups: (1) the internal apparatus, i. e.,
the diaphragm, and (2) the external apparatus, the intercostal
and other muscles of the body wall. These are all somatic
muscles. In forced respiration various other muscles act in an
accessory way during both inspiration and expiration.

The diaphragm is innervated by the phrenic nerve, which
takes its origin from the fourth and fifth cervical spinal nerves;
and the intercostal muscles are innervated by ventral spinal
roots arising successively from all thoracic segments of the spinal
cord (Fig. 112). The accessory muscles are in part somatic
muscles of the abdomen and shoulder and in part special vis-
ceral muscles of the head, particularly those of the glottis
(innervated by the vagus) and of the nostrils (innervated by the
VII cranial nerve).

The anatomical relations just described imply that, although
respiration is a visceral function, in mammals the necessary
movements for ordinary breathing are performed by somatic
muscles. This is not true in fishes. Here the organs of respi-
ration (gills) are strictly visceral structures innervated by vis-
ceral components of the cranial nerves, whose cerebral center is

THE VISCERAL AND GUSTATORY APPARATUS

237

in the lower part of the medulla oblongata (the area visceralis of
Fig. 43, p. 111).

In the ordinary breathing of mammals the act of inspiration
is effected by an upward and outward movement of the ribs and
a downward movement of the diaphragm. Now, if the spinal
cord be cut through at the level of the seventh cervical nerve the
respiratory movement of the ribs is entirely abolished, though
the movements of the diaphragm go on as usual. The continuity

Dorsal motor X

nucleus
Nucleus of fascic. solitarius

Fasciculus solitarius
Vagus ganglion
Vagus nerv
Tr. solitario-spinal

Sympathetic ganglia

Lung
Intercostal nerve.

Intercostal muscle
Phrenic nerve

Diaphragm

Respiratory center

Fig. 112. — Diagram of the nervous mechanism of respiration,
from Ram6n y Cajal.)

(Modified

of the thoracic motor nerves which innervate the intercostal
muscles with their centers of origin in the spinal cord is undis-
turbed by this operation, yet they can no longer be coordinated
in the respiratory act. If in another animal the spinal cord be
divided at the level of the third cervical nerve, i. e., above the
level of origin of the phrenic nerve, the respiratory movement - < if
both the ribs and the diaphragm cease, even though the spinal
cord below the section is intact and its connection with the
peripheral respiratory apparatus is undisturbed. These experi-

238 INTRODUCTION TO NEUROLOGY

ments show that the spinal segments from which all of the motor
respiratory nerves arise cannot of themselves effect the coor-
dinations necessary in respiration. This is in marked contrast
with many other reactions (both visceral and somatic), whose
performance is still possible after the separation of the spinal
cord from the brain.

If now, in a third animal, the medulla oblongata is cut across
at any point above the middle of its length, say at the lower bor-
der of the pons, the respiratory processes are in no way disturbed.
This shows that there is a respiratory correlation center in the
lower half of the medulla oblongata, that is, somewhere in
the region corresponding to the "visceral area" of the fish
brain.

The air tubes of the lungs are provided with smooth muscle-
fibers by which their caliber may be contracted. These muscles
are innervated by the vagus, and the hyperexcitation of their
motor nerves may impede respiration, this being one of the fac-
tors which cause asthma. The cerebral center from which these
intrinsic muscles of the lungs are innervated has been shown to
lie in the middle part of the dorsal motor vagus nucleus (Fig. 73,
nuc dorsalis vagi). These are preganglionic neurons, the cor-
responding postganglionic neurons lying in sympathetic ganglia
distributed along the pulmonary branches of the vagus (Fig. 1 12).

The apparatus described in the preceding paragraph is, how-
ever, not responsible for the maintenance of the regular rhythm
of breathing. Physiological experiments show that there is some-
where in the lower part of the medulla oblongata a respiratory
center which performs this function. This center may appa-
rently be excited to activity directly by variations in the compo-
sition of the blood which reaches it, especially either by a de-
ficiency in oxygen or by an excess of carbon dioxid. Its activity
may also be modified by nervous influences reaching it through
the peripheral afferent nerves, the vagus being the only nerve
which appears to be able to act directly on the respiratory center,
though the strong excitation of almost any sensory nerve of the
body may under some circumstances indirectly affect the res-
piratory rhythm. Coughing and sneezing are special cases of
this sort. The reflex mechanism of the cough is illustrated in
Fig. 113.

THE VISCERAL AND GUSTATORY APPARATUS

239

Vagus
ganglion

Larynx

Sympathetic

lion
Postgangli-
onic neuron

Fig. 113. — Diagram of the nervous mechanisms of coughing and vomit-
ing. In the cough an irritation of the mucous membrane of the larynx is
transmitted to the nucleus of the fasciculus solitarius, from which the
tractus solitario-spinalis passes downward to the motor centers of the spinal
cord for the innervation of the muscles of the diaphragm, the abdominal
wall, and the ribs which cooperate in the production of the cough. In
vomiting, an irritation of the stomach is carried by sensory fibers of the
vagus to the nucleus of the fasciculus solitarius, from which the pathway is
as before to the spinal motor centers for the innervation of the diaphragm
and abdominal wall. In this case there is also an excitation of the dorsal
motor vagus nucleus, from which preganglionic fibers go out into the vagus
nerve for a sympathetic ganglion in the hypogastric plexus, from which,
in turn postganglionic fibers pass to the muscles of the stomach which par-
ticipate in the ejection of its contents. The diagram is suggested by one
in Ram6n y Cajal's text-book, though greatly modified.

Attempts to localize the respiratory center in the mammalian medulla
oblongata more accurately have led to contradictory results. The old

240 INTRODUCTION TO NEUROLOGY

conception of Flourens that there is a minute "vital node" under the lowest
point of the fourth ventricle which is the respiratory center must be aban-
doned. Later the fasciculus solitarius was identified as the "respiratory
tract," and the nucleus associated with this tract was regarded as the
respiratory center, but further experiment has shown that this is not an
exact statement of the case. Some physiological experiments have sug-
gested that the respiratory rhythm is maintained by a center in the
reticular formation of the vagus region ventrally of the fasciculus solitarius.

It has recently been shown, as stated above, that afferent visceral fibers
from the lungs whose cell bodies lie in the vagus ganglion enter the fascicu-
lus solitarius, and it is known that from the nucleus of this tract a "tractus
solitario-spinalis " (Fig. 112) descends into the motor centers of the upper
segments of the spinal cord. This descending visceral spinal tract probably
plays some part in the regulation of respiration, though not the chief role.
Rain6n y Cajal and Kappers believe that, while the upper part of the nucleus
of the fasciculus solitarius has nothing to do with respiration, the lower end
of this nucleus (commissural nucleus of Cajal, see Figs. 71, 112, and 114) is a
true respiratory center. Ram6n y Cajal, in fact, thinks that this nucleus
serves both for reflexes excited by the sensory pulmonary nerves and also for
the normal respiratory rhythm excited by carbon dioxid in the blood.
This hypothesis is not supported by direct physiological experiment, and for
the present we must content ourselves with the statement that the true
respiratory center has not been accurately located anatomically. Figure
112 may be regarded as a true picture of the essential relations of the
respiratory nerves, with the reservation that the position of the respiratory
center is not precisely known.

There is also a reflex center for the regulation of respiration in the medial
wall of the thalamus and others have been described in different parts of the
brain stem. The entire respiratory mechanism is also under partial volun-
tary control from the cerebral cortex.

While many features of the central respiratory mechanism
remain obscure, it seems evident that the location of the chief
respiratory center in the "visceral area" of the lower part of the
medulla oblongata instead of the portions of the spinal cord
directly connected with the respiratory muscles is a survival of
the ancestral condition found in fishes, where the entire respira-
tory function is carried on by a visceral apparatus (gills) inner-
vated from the vagus region.

Organs of Digestion. — Hunger seems to be a complex in which
at least three factors are present: (1) Specific hunger pangs due
to waves of muscular contraction in the stomach (Cannon,
Carlson) ; (2) appetite, or craving for food regardless of the state
of the stomach; (3) general malaise from starvation of the tissues
and weakness. Appetite may persist after section of the vagus
nerves and is probably a sensation distinct from the hunger
pangs.

THE VISCERAL AND GUSTATORY APPARATUS 241

The ordinary processes of digestion are carried on partly by
automatic activities of the organs without nervous control
(see p. 224), and partly by the intrinsic sympathetic nervous
system of the digestive organs. Throughout the length of the
digestive tract there are two sympathetic ganglionated plexuses.
One of these is located between the muscular coats of the stom-
ach and intestine, known as the myenteric or Auerbach's plexus;
the other lies immediately under the lining mucous membrane
and is known as the submucous or Meissner's plexus. It has
been shown physiologically that the local reflexes concerned in
the typical peristaltic contractions of the digestive tube are
effected chiefly by the myenteric plexus. Accordingly, this
reflex is called by Cannon the myenteric reflex.

The entire digestive mechanism (like most of the other visceral
systems) may also be influenced indirectly by nervous impulses
arising in the cerebral cortex, though these organs are not under
direct voluntary control. It is well known that the digestive
processes are especially sensitive to emotional states, pleasurable
experiences promoting digestion and painful or disagreeable emo-
tions inhibiting it. These facts can be studied on laboratory ani-
mals under experimental conditions (Cannon). A large amount
of information regarding the physiology of digestion has recently
been gathered by Carlson from the study of a man with an arti-
ficial opening into the stomach (gastric fistula), permitting
direct observation of the stomach at all times.

The salivary glands are excited to secretion from two nuclei
of the medulla oblongata, the superior salivatory nucleus (Figs.
71, 114), whose preganglionic fibers go out with the VII cranial
nerve for the sublingual and submaxillary salivary glands, and
the inferior salivatory nucleus (Figs. 71, 73, 114), whose fibers
go out with the IX nerve for the parotid gland. The secretion
of saliva may be produced either as a simple reflex from the
presence of food in the mouth through the gustatory nerves and
fasciculus solitarius, or as so-called psychic secretion excited by
the sight or thought of food. All of the digestive secretions
are susceptible to this sort of indirect excitation, as, indeed, are
most other processes which are under the control of the cerebro-
spinal visceral nervous system. These visceral reactions, in
their turn, are reported back to the central nervous system and

16

242 INTRODUCTION TO NEUROLOGY

no doubt play a very large part in shaping the organic back-
ground of the entire conscious life (see p. 259).

Students of animal behavior are in the habit of investigating
the ability of animals to make simple associations by training
them to perform particular acts under conditions such that the
normal stimulus to the act is always accompanied by a second
stimulus of a different type. After many repetitions the re-
sponse may be obtained by presenting the second or collateral
stimulus without the first. For the nervous mechanism of
"associative memory" of this sort see p. 64. Pawlow has
found that variations in the amount of saliva secreted form an
especially good index of associations of this type, and he has used
this method extensively in analyzing complex reactions, or con-
ditional reflexes, as he calls them. See the summary of his
researches in the paper by Morgulis cited in the appended bib-
liography.

Tactile sensibility is entirely absent throughout the entire
alimentary canal from the esophagus to the rectum, and the
same holds true for most of the other deep-seated viscera of the
body. Even the substance of the brain is insensitive to any
kind of mechanical irritation. Sensibility to changes in tem-
perature is feebly developed or absent in most of the viscera, the
esophagus and anal canal being very sensitive to heat and cold,
while the stomach and colon are feebly sensitive to these stimuli.
The entire alimentary canal is insensitive to hydrochloric and
organic acids in concentrations far in excess of what ordinarily QC-
curs in either normal or pathological conditions. The contact of
alcohol with all parts of the mucous membrane of the alimentary
canal gives rise to a sensation of warmth. This sensation is
different in character from that caused by hot fluids and is prob-
ably excited through the sympathetic nerves, while the sensa-
tion of warmth felt in consequence of the passage of hot fluid
through the esophagus is excited through the vagus.

The demonstrated absence of tactile sensibility throughout
the mucous membrane of the stomach and intestine is considered
by Hertz to indicate that the sensations of fulness arising from
the distention of different parts of the alimentary canal are due
to the stretching of the muscular coat, and that, therefore, these
are to be regarded as varieties of the muscle sense. The same

THE VISCERAL AND GUSTATORY APPARATUS 243

may also be true of the bladder. The free nerve-endings (see
Fig. 33, p. 90) known to be present in these mucous membranes,
particularly in the bladder, may, however, share in exciting these
sensations, for these membranes may well be sensitive to stretch-
ing, even though quite insensitive to simple pressure. The only
immediate cause of true visceral pain is tension, and it is stated
by Hertz that, so far as the alimentary canal is concerned, this
tension is exerted on the muscular coat, not on the mucous
lining. See the further discussion of visceral pain, p. 250.

The vomiting reflex may be caused by excitations of sensory
termini of the vagus nerve in the stomach, which are transmitted
to the nucleus of the fasciculus solitarius in the medulla oblon-
gata, whence the nervous impulses are distributed as shown in
Fig. 1 13 to the appropriate motor centers.

The Gustatory Apparatus. — Taste, like smell, is a chemical
sense (see pp. 75, 91, 218). Physiologically, it is classed by
Sherrington as an interoceptive or visceral sense, and its primary
cerebral center is intimately joined to the general visceral
sensory center in the-nucleus of the fasciculus solitarius. Unlike
the general visceral sensory system, however, its peripheral
fibers have no connection with the sympathetic nervous system
and the reactions may be vividly conscious. The end-organs,
or taste-buds (Fig. 35, p. 91), are present in the mucous mem-
brane of the tongue, soft palate, and pharynx and are innervated
by the VII and IX cranial nerves; there are a few taste-buds also
on the larynx and epiglottis which are probably supplied by the
vagus (J. G. Wilson). All of these peripheral gustatory fibers,
upon entering the medulla oblongata, terminate in the nucleus
of the fasciculus solitarius (Figs. 71, 72, 73, 114) along with
those of general visceral sensibility, those of the gustatory sys-
tem probably ending farther forward (toward the mouth) in this
nucleus than those of the general visceral systems.

There has been considerable controversy as to the exact course
taken by the peripheral nerves of taste on their way to the brain,
many clinical neurologists believing that all of these fibers enter
the medulla oblongata through the root of the V cranial nerve.
It has now been clearly shown by the studies of Gushing and
others that the V nerve takes no part in the innervation of
taste-buds. Figure 115 shows in continuous lines the true

244

INTRODUCTION TO NEUROLOGY

V motor

Nuc. sal. sup.

Nuc. sal. inf.

VII motor

VII pars. int.

VIII

IX motor

X motor

XI pars bulbaris

XI pars spinalis

Nuc. dorsalis X

Nuc. ambigu

V sensory
Nuc. sensory V

VII motor

VII pars. int.

VIII

IX sensory

X sensory

Ala cinerea
Fasciculus solitarius
Nuc. commissuralis Cajal

Nuc. spinalU V

Fig. 114. — Diagram of the visceral afferent and efferent connections in
the medulla oblongata, based on Fig. 71; compare also Figs. 77 and 86.
The afferent roots and centers are indicated on the right side ; the efferent,
on the left. Visceral sensory fibers enter by the VII nerve (pars intermedia
of Wrisberg, VII pars, int.) and by the IX and X nerves. These root-fibers
include both general visceral sensory and gustatory fibers, all of which enter
the fasciculus solitarius. (Fibers of the IX and X nerves also enter the
spinal V tract ; but since these are somatic sensory fibers from the auricular
branch they are not included in the diagram. For further details on the
composition of these cranial nerves see the table on pp. 146, 147.)

On the left side of the figure the general visceral efferent nuclei are indi-
cated by small dots and the special visceral nuclei by large dots. The
latter comprise the motor V nucleus for the jaw muscles, the motor VII
nucleus for the muscles related to the hyoid bone and the general facial
musculature, and the nucleus ambiguus supplying striated muscles of the
pharynx and larynx by way of the IX and X nerves. Three general visceral
efferent nuclei are indicated — the dorsal motor nucleus of the vagus under
the ala cinerea and the superior and inferior salivatory nuclei. The superior
nucleus (nuc. sal. sup.) supplies the sublingual and submaxillary salivary
glands by way of the VII nerve (pars intermedia of Wrisberg), and the in-
ferior nucleus (mic. sal. inf.) supplies the parotid salivary gland by way of
the IX nerve. All of the general visceral efferent fibers are preganglionic
sympathetic fibers (see p. 229) which end in sympathetic ganglia, whence
postganglionic fibers carry the nervous impulses onward to their respective
destinations.

THE VISCERAL AND GUSTATORY APPARATUS

245

courses of the nerve-fibers from the taste-buds of the tongue
through the VII and IX nerves, and in broken and dotted lines
some of the other courses which have been suggested.

In fishes the gustatory system is much more extensively developed than
in mammals, especially the vagal part which supplies taste-buds in the gill
region. In some species of fishes, moreover, taste-buds appear in great
numbers in the outer skin, and these are in all cases innervated from the VII

fac. rt:

Fig. 115. — Diagram showing some of the various courses which have been
advocated for the taste fibers in man. The courses advocated in this
work are shown by heavy black lines; other suggested courses are indicated
by broken or dotted lines: fac. rt., motor facial root ; G.G., Gasserian gang-
lion; G.g., geniculate ganglion; G. otic, otic ganglion; G. petr., ganglion
petrosum; G. sp., sphenopalatine ganglion ; g. s. p., great superficial petrosal
nerve; N. fac., facial trunk; N. Jac., Jacobson's or the tympanic nerve;
N. vid., vidian nerve; Rami anast., anastomotic rami between the geniculate
ganglion and tympanic plexus and the small and great superficial petrosal
nerves respectively; s. s. p., small superficial petrosal nerve; Tymp., tym-
panum. (After Gushing.)

cranial nerve. In the common horned-pouts or catfishes and in the carps
and suckers these cutaneous taste-buds are distributed over practically the
entire body surface, and especially on the barblets. The distribution of
these cutaneous gustatory branches of the facial nerve in the common bull-
pout, Ameiurus, is shown in Fig. 116. These sense-organs and their nerves
are entirely independent of those of the lateral line sensory system and of the
ordinary tactile system, though the gustatory and the tactile systems have
been shown experimentally to cooperate in the selection of food. The
primary terminal nuclei of these gustatory nerves make up by far the larger

246

INTRODUCTION TO NEUROLOGY

part of the visceral area (Fig. 43, p. Ill) of fish brains, and hi some species
these centers are enormously enlarged, as in the carp (Fig. 136 (2), p. 303).

The primary sensory center for the nerves of taste in the
nucleus of the fasciculus solitarius is very intimately connected
with all of the motor centers of the medulla oblongata for the
reactions of mastication and swallowing, and also with the motor
centers of the spinal cord. The ascending path from the prim-
ary gustatory nucleus to the thalamus and cerebral cortex is
wholly unknown in the human body. A gustatory center is

Fig. 116. — The cutaneous gustatory branches arising from the geniculate
ganglion of the facial nerve of the catfish (Ameiurus melas), projected upon
the right side of the body. Spinal cord and brain stippled. The geniculate
ganglion, its roots and cutaneous branches are drawn in black; the branches
of this nerve distributed to the mucous lining of the mouth cavity are
omitted. Taste-buds are found in all parts of the outer skin to which these
branches are distributed.

believed to exist in the cortex of the gyrus hippocampi near the
anterior end of the temporal lobe. In fishes, where this ascend-
ing gustatory path is much larger, it has been followed to the
roof of the midbrain and, after a synapse here, to the region of
the hypothalamus.

Visceral Efferent Centers. — The arrangement of the visceral
efferent nuclei and nerve-roots of the medulla oblongata is shown
in Fig. 114. There is also a general visceral efferent component
of the III cranial nerve (Fig. 71, p. 154, nuc. III. E-W.), whose
fibers pass out through this nerve to the ciliary ganglion in the
orbit, which in turn connects with the intrinsic muscles of the

THE VISCERAL AND GUSTATORY APPARATUS 247

eyeball in the ciliary process and iris. These fibers are involved
in the movements of accommodation of the eye for distance and
in the regulation of the diameter of the pupil. The nucleus of
the fasciculus solitarius is connected through the reticular forma-
tion with all of the motor centers of the medulla oblongata for
the reactions of mastication and swallowing and for many other
movements ; from this nucleus there is a descending tract to the
motor centers of the spinal cord, the tractus solitario-spinalis
(Figs. 112 and 113). There is also a connection with the supe-
rior and inferior salivatory nuclei of the VII and IX nerves.
The excitation of the gustatory fibers of these nerves by the
presence of food in the mouth is carried to the nucleus of the
fasciculus solitarius and thence through the reticular formation
to the salivatory nuclei, from which the flow of saliva is excited.
There are other connections with the motor centers of the spinal
cord through the descending fibers of the fasciculus solitarius,
some of these fibers crossing to the opposite side in the vicinity
of the commissural nucleus of Cajal (Fig. 114).

Summary. — The cerebro-spinal visceral systems fall into a
general group related peripherally to the sympathetic nerves and
a special group independent of the sympathetic. 'The second
group includes the apparatus for taste and probably for smell.
The central innervation of the viscera is partly from the spinal
and midbrain regions, but chiefly from the visceral area of the
medulla oblongata. The heart and blood-vessels have a double
innervation derived from both the spinal and the bulbar visceral
centers, and the nervous control of the organs of circulation is
very complex. Respiration in lower vertebrates is effected by
strictly visceral structures and is controlled from the visceral
area of the medulla oblongata. In mammals the muscles of
ordinary respiration are all of the somatic type, but the centers
of control are retained in the visceral area of the oblongata. The
sensations related to the digestive tract are served chiefly
(though not exclusively) by the vagus. There are special sali-
vatory nuclei related to the VII and IX cranial nerves. The
nerves of taste are the VII, IX, and to a very limited extent (in
man) the X pairs of cranial nerves. The primary cerebral gus-
tatory center is in the upper part of the nucleus of the fasciculus
solitarius, but the cortical path is unknown.

248 INTRODUCTION TO NEUROLOGY

LITERATURE

Any of the larger text-books of physiology will give further details of the
visceral reactions. For a very brief and simple account of the circulatory
apparatus see the book by Stiles (pp. 118-125) cited below. The experi-
ments of Molhant have given us the most detailed information regarding
the visceral functions of the vagus and their centers in the medulla oblon-
gata.

CANNON, W. B. 1898. The Movements of the Stomach Studied by
Means of the Rontgen Rays, Amer. Jour. Physiol., vol. i, pp. 359-382.

— . 1902. The Movements of the Intestines Studied by Means of the
Rontgen Rays, Amer. Jour. Physiol., vol. vi, p. 251.

— . 1912. Peristalsis, Segmentation, and the Myenteric Reflex, Amer.
Jour. Physiol., vol. xxx, pp. 114—128.

CANNON, W. B., and WASHBURN, A. L. 1912. An Explanation of Hunger,
Amer. Jour. Physiol., vol. xxix, pp. 441-450.

CARLSON, A. J. 1912-1915. Contributions to the Physiology of the
Stomach, Amer. Jour. Physiol., vols. xxxi-xxxv.

GUSHING, H. 1903. The Taste Fibers and Their Independence of the
N. Trigeminus, Johns Hopkins Hospital Bulletin, vol. xiv, pp. 71-78.

HERRICK, C. JUDSON. 1903. The Organ and Sense of Taste in Fishes,
Bui. U. S. Fish Commission for 1902, pp. 237-272.

— . 1905. The Central Gustatory Paths in the Brains of Bony Fishes,
Jour. Comp. Neurol., vol. xv, pp. 375-456.

— . 1908. On the Commissura Infima and Its Nuclei in the Brains of
Fishes, Jour. Comp. Neurol., vol. xviii, pp. 409-431.

HERTZ, A. F. 1911. The Sensibility of the Alimentary Canal, London,
Oxford University Press.

KAPPERS, C. U. A. 1914. Der Geschmack, perifer und central, zugleich
eine Skizze der phylogenetischen Veranderungen in der sensibelen VII, IX,
und X Wurzeln, Psychiat. en Neurol., Bladen, ,pp. 1-57.

MOLHANT, M. 1910-1913. Le nerf vague: Etude anatomique et experi-
mentale, Le Nevraxe, vols. xiii-xv.

MORGULIS, S. 1914. Pawlow's Theory of the Function of the Central
Nervous System and a Digest of Some of the More Recent Contributions
to this Subject from Pawlow's Laboratory, Jour. Animal Behavior, vol. iv,
pp. 362-379.

PAWLOW, I. 1913. The Investigation of the Higher Nervous Func-
tions, Brit. Med. Jour., vol. ii for 1913, pp. 973-978.

SHELDON, R. E. 1909. The Phylogeny of the Facial Nerve and Chorda
Tympani, Anat. Record, vol. iii, pp. 593-617.

— . 1909. The Reactions of the Dogfish to Chemical Stimuli, Jour.
Comp. Neurol., vol. xix, pp. 273-311.

STILES, P. G. 1915. The Nervous System and Its Conservation, Phila-
delphia.

WILSON, J. G. 1905. The Structure and Function of the Taste-buds of
the Larynx, Brain, vol. xxviii, pp. 339-351.

CHAPTER XVIII

PAIN AND PLEASURE

FEW problems in neurology are more difficult and involved
than those centering about the nerves of painful sensibility.
This question is intimately related with the disagreeable and
pleasurable feelings and with the affective and emotional life as a
whole. Nearly all sensations, whether of the somatic or visceral
series, appear to have an agreeable or disagreeable quality
(quale). There is difference of opinion as to whether any sensa-
tion is wholly indifferent in this respect. There are, however,
two factors in this situation which have not always been dis-
tinguished and whose introspective analysis is very difficult.
In the first place, many sensations are as such painful or pleasur-
able, and in the second place the related apperceptions, ideas,
etc., may have an agreeable or disagreeable feeling tone. The
intimate relation of these two factors in consciousness probably
grows out of a similarity in the type of physiological process
involved in their neurological mechanisms, and this, in turn,
may rest on the fact that the two mechanisms in question have
had a common evolutionary origin.

The stimulation of some of the sense organs results in the so-
called sensation of pain with no other quality recognizable;
this is true of the cornea, of the tooth pulps, of the tympanic
membrane, and of the "pain spots" of the outer skin. This fact
would suggest that there is a special system of neurons (or at
least of receptors, see p. 85) for pain as for the other senses.
But, on the other hand, the supernormal stimulation of most
other sense organs may result in a very similar type of pain,
though in this case the painful quality is accompanied by the
normal sensory quality of the organ in question unless the stimu-
lation is excessively strong. From this it would appear that
most sensory nerves may upon occasion function as pain nerves.
In other cases normal stimulation of a sense organ may result in

249

250 INTRODUCTION TO NEUROLOGY

a sensation of the quality typical for the organ in question, to
which there is added an agreeable or disagreeable quality which
may be very pronounced, the disagreeable quality not being
painful in the ordinary sense of that term. This mixed quality
of normal sensations is illustrated by certain odors and savors,
and on the agreeable side by certain sensations of tickle and
warmth. Finally, some ideational processes have an agreeable
or disagreeable quality, and these, in turn, are very intimately
related with the emotions and with esthetic and appreciative
functions of the most complex psychic sort, as well as with ques-
tions of habitual emotional attitude and temperament.

The superficial parts of the body which are more directly ex-
posed to traumatic injury are, in general, more sensitive to pain
than are the deeper parts, and painful stimuli here can be more
accurately localized. In some parts, like the conjunctiva of the
eyeball, where very slight irritation may seriously interfere
with the function, very gentle stimulation gives rise to acute
pain, and no other sensory quality may be present.

Surgeons find that the brain membranes are sensitive to
mechanical injury, especially to stretching or pulling. The brain
substance itself, however, is quite insensitive to pain from either
mechanical or chemical stimulation. The deeper viscera of the
thorax and abdomen are insensitive to pinching, cutting with a
sharp instrument, or other mechanical, chemical, or thermal
stimuli, though they are sensitive to pains arising from internal
disorders, as in colic (p. 243). The visceral portions of the
pleural and peritoneal membranes are insensitive to pain, but
their parietal portions, forming the innermost layer of the body
wall, are sensitive, and these pains can be accurately localized
(Capps).

From these considerations it appears that pain is an adaptive
function which is present only where it is of value to give warn-
ing of noxious influences liable to injure the body unless re-
moved. (See the excellent discussion by Sherrington in
Schafer's Physiology, vol. ii, pp. 965-1001.)

Pains of this sort are physiologically similar to other extero-
ceptive sensations, that is, they have a definite localization and
are externally projected like other somatic sensations. But
other pains and discomforts (especially those related to the

PAIN AND PLEASURE 251

visceral functions) and all pleasurable feelings are devoid of this
external projicience and are experienced merely as a non-local-
ized awareness of malaise or well-being (see p. 259). They are
also more variable in relation to habit, mental attitude, fatigue,
and general health. This latter group of affective processes is so
different from the ordinary sensations as to make it desirable to
consider them separately, and, as will appear beyond, they prob-
ably involve a quite different series of nervous processes.

There has been much controversy regarding the pathway
taken by painful impulses through the spinal cord and brain
stem, and it is probable that this pathway is very complex.
All painful impulses carried by the spinal nerves, no matter
what the peripheral source, are discharged immediately upon
entering the spinal cord into its gray matter, and after a synapse
here the nerve-fibers of the second order seem to take several
courses. The most recent experiments (Karplus and Kreidl,
1914) go to show that the ascending impulses of painful sensi-
bility in the spinal cord of cats follow a chain of short neurons,
some of whose axons immediately cross to the opposite side of
the cord and some ascend on the same side. These short fibers
belong to the fasciculus proprius system (p. 127), and the nervous
impulse is at frequent intervals returned to the gray matter to
pass from one neuron to another, and it may cross the midplane
repeatedly. This diffuse method of conduction appears to be
the primitive arrangement. In the human spinal cord it is
probably present to a limited extent, but has been largely sup-
planted by a more direct pathway in the spinal lemniscus, whose
precise localization has been determined by the clinical studies of
Henry Head and others (pp. 139, 173) . This direct path for fibers
of painful sensibility includes axons of neurons of the dorsal gray
column, which immediately cross to the opposite side of the cord
and ascend directly to the thalamus. Injury to this path in the
human body may cause complete insensitivity to both superficial
and deep pain on the opposite side of the body below the site
of the injury, without loss of general tactile sensibility. The
two methods of transmission of impulses of painful sensibility
are shown diagrammatically in Fig. 117.

It may be assumed that pain and an avoiding reaction and pleasure and a
seeking reaction have come to be instinctively associated by natural selec-

252

INTRODUCTION TO NEUROLOGY

tion or other biological agencies because this is an adaptation useful to the
organism. No separate neurons would be required for the transmission and
analysis of painful stimuli in their simpler forms. A peripheral neuron, say,
of the pressure sense, if excited by the optimum stimulus will transmit the
appropriate nervous impulse to the tactile centers of the thalamus and cere-
bral cortex. But the peripheral sensory neurons branch widely within the
spinal cord and there effect very diverse types of connection (see Fig. 61, p.
134) ; and supernormal or maximal stimulation of the end-organ may excite
so strong a nervous discharge as to overflow the tactile pathway in the spinal

To the thalamus

Fasciculus proprius

Spinal lemniscus

Spinal nerve

Fig. 117. — Diagram of the pathways of painful sensibility in the spinal
cord. The spinal lemniscus is the dominant path in the human body, and
the fasciculus proprius is the dominant path in other mammals.

cord by overcoming the synaptic resistance of certain other collateral path-
ways with a higher threshold than those of the tactile path, thus exciting to
function the pathway for painful sensibility with its own central connection
in the thalamus (Fig. 118, A).

In the course of the further differentiation of the cutaneous receptors,
the peripheral fiber of the sensory neuron may branch and effect connection
with two types of sense organs, one organ (a tactile spot) with a low thresh-
old for pressure stimuli whose nervous impulses are so attuned as to dis-

PAIN AND PLEASURE

253

charge centrally at the first synapse into the tactile tract, and another organ
differently constructed (a pain spot) which generates nervous impulses so
attuned as to discharge centrally into the pain tract (Fig. 118, B). In a
still more highly elaborated system two separate peripheral neurons may be
present to serve these functions, which are distinct throughout (Fig. 118, C).
All three of these methods of pain transmission and analysis may be present
in the spinal nerves; but by whatever pathway the pain impulses reach the
spinal cord, in the human body those which are destined to excite conscious-
ness of pain as a localizable sensation are immediately filtered off from the
other sensory qualities with which they may be associated and assembled in
a pathway of their own, which remains distinct from this time forth. With-
in the spinal cord and brain stem these pain impulses, especially those result-
ing from supernormal stimulation, also effect short reflex connections with
the adjacent motor centers for quick avoiding reflexes, and these may not
be associated with the spinal lemniscus, but with the more diffuse pain path
in the fasciculus proprius.

pain path
tactile path
sKin

pom path
tactile path

spinal lemniscus-^.

a, pain

b, touch

spinal cord

Fig. 118. — Three diagrams to illustrate various ways in which the
nerves of painful sensibility may be associated with those of other sensory
functions.

The terminus of the ascending pain tract is related within the
thalamus very differently from those of the pathways for tactile
and thermal sensitivity. The latter impulses are in part trans-
mitted to the motor centers of the thalamus for intrinsic thalamic
reflexes, but chiefly pass forward after a synapse in the thalamus
through the internal capsule to the somesthetic areas of the
cerebral cortex. Head is of the opinion that the painful im-
pulses do not reach the cortex at all in their simple elementary
form, but that the painful sensations are essentially thalamic.

Lesions of the lateral and ventral nuclei of the thalamus in-
volving the termini of the lemniscus, but leaving the geniculate
bodies and pulvinar and the medial and anterior nuclei intact,
result in the more or less complete loss of superficial sensation of
the opposite side of the body, with still more profound disturb-

254 INTRODUCTION TO NEUROLOGY

ance of deep sensibility and the postural sensations, together
with an exaggeration of painful sensibility. The modifications
of pain and affective sensibility are regarded by Head and
Holmes as the most constant and characteristic features of le-
sions of the lateral zone of the thalamus. Acute, persistent,
paroxysmal pains are always present, often intolerable and
yielding to no analgesic treatment. There is also a tendency to
react excessively to unpleasant stimuli. This is not necessarily
associated with a lowering of the threshold of stimulation.
Deep pressure is especially important here. The pain does not
develop gradually out of the general sensation, but appears
explosively. This pain has some factor to which the normal half
of the body is not particularly susceptible. Thermal, visceral,
and other sense qualities are similarly affected. Tickling is very
unpleasant on the affected side. The pleasurable aspect of
moderate heat is accentuated on the affected side, yet the
threshold for heat is never lowered. Not only does the side of
the body involved react more vigorously to an affective element
of a stimulus, but an overreaction can also be evoked by purely
mental states. The manifestations of this increased suscepti-
bility to states of pleasure and pain are strictly unilateral.
Associated with this overreaction to painful stimuli some loss
of general sensation will always be manifest on the affected side
of the body.

Pure cortical lesions cause no change in the threshold to pain,
nor is there the exaggerated affective quality characteristic of
thalamic lesions. Head and Holmes assume that both the
thalamus and the cortex are concerned in conscious activity.
They say:

"The most remarkable feature in that group of thalamic cases with
which we have dealt in this work is not the loss of sensation, but an excessive
response to affective stimuli. This positive effect, an actual overloading of
sensation with feeling tone, was present in all our 24 cases of this class."
This effect is interpreted as due to the release of the inhibitory or regulatory
influence of the cortex arising from the destruction of the ascending and
descending fibers between the thalamus and the cortex, thus isolating the
thalamus and allowing it to act to excess. These authors add, since "the
affective states can be increased when the thalamus is freed from cortical
control, we may conclude that the activity of the essential thalamic center
is mainly occupied with the affective side of sensation." "This conclusion is
strengthened by the fact that stationary cortical lesions, however extensive,

PAIN AND PLEASURE 255

which cause no convulsions or other signs of irritation and shock, produce
no effect on sensibility to pain. Destruction of the cortex alone does not
disturb the threshold for the painful or uncomfortable aspects of sensation."

Some recent experiments by Cannon have revealed a very
intimate relation between emotion and some of the ductless
glands. The suprarenal (or adrenal) glands, situated above the
kidneys, secrete and pour into the blood a remarkable substance
known as adrenalin or epinephrin. This substance exerts upon
structures which are innervated by sympathetic nerves the same
effects as are produced by impulses passing along those nerves.
The glands may themselves be excited to activity by nervous
impulses passing out through the sympathetic nerves. Cannon
has shown that the emotions of fear, rage, and pain excite these
glands to activity and cause the secretion of adrenalin. The
blood of a caged cat which has been tormented by the barking of
a dog will show an increased percentage of adrenalin. The
addition of adrenalin to the blood has the further effect of caus-
ing liberation of sugar from the liver into the blood to such an
extent that sugar may appear in the urine (glycosuria) ; and sugar
is known to be the most available form in which energy can be
quickly supplied to tissues which have been exhausted by exer-
cise. Adrenalin will in this and other ways act as an antidote to
muscular fatigue. It also renders more rapid the coagulation of
the blood.

If a muscle is fatigued, the threshold of irritability rises. It
may rise as much as 600 per cent., but the average increase is
approximately 200 per cent. If the fatigued muscle is allowed
to rest, the former irritability is gradually regained, though two
hours may pass before the recovery is complete. If a small dose
of adrenalin is administered intravenously, or the adrenal glands
are stimulated to secrete, Cannon has found that the former irri-
tability of the fatigued muscle may be recovered within three
minutes. In this way adrenal secretion may largely restore
efficiency after fatigue.

Fear and anger — as well as worry and distress — are attended
by cessation of the contractions of the stomach and intestines.
These mental states also reduce or temporarily abolish the secre-
tion of gastric juice. Adrenalin injected into the body has the
same effect. Besides checking the functions of the alimentary

256 INTRODUCTION TO NEUROLOGY

canal, adrenalin drives out the blood which, during digestive
activity, floods the abdominal viscera. This blood flows all the
more rapidly and abundantly through the heart, the lungs, the
central nervous system, and the limbs.

Cannon epitomizes the account from which the above has been
condensed in these words: "The emotional reactions above
described may each be interpreted, therefore, as making the
organism more efficient in the struggle which fear or rage or
pain may involve. And that organism which, with the aid of
adrenal secretion, best mobilizes its sugar, lessens its muscular
fatigue, sends its blood to the vitally important organs, and
provides against serious hemorrhage, will stand the best chance
of surviving in the struggle for existence."

The preceding account includes a summary of some of the
most securely established facts regarding the peripheral and
central nervous mechanisms of painful impressions and the
physiology of the emotions, together with a theoretical interpre-
tation of the apparently twofold nature of pain as a specific
sensation and as a component of the general affective state of the
body as a whole. The more general questions concerning the
physiological processes related with pleasurable and unpleasant
experience and the affective life in general are still more difficult
of analysis. It seems probable that pain, unpleasant and
pleasurable feelings, emotion, and, in short, the entire affective
life are very intimately related on the neurological side.

Many physiological theories of pleasure-pain have been elaborated, for
the most part on very slender observational grounds. It has been suggested
that the flexor movements of the body are associated with pain, the extensor
movements with pleasure; that constructive metabolism is pleasurable,
destructive metabolism disagreeable; that heightened nervous discharge is
pleasurable, and the reverse (some form of inhibition or of antagonistic
contraction) is unpleasant. Some hold that pain and unpleasantness or
disagreeableness are different in degree only, not in kind. Others regard
pain as a true sensation, but disagreeableness and pleasure (affective ex-
perience) as belonging to a different category which is non-sensory. In the
latter case the affective experience may be neurologically related in some
way with the various sensations (including pain) or the affective experience
and sensations may be independent variables with separate cerebral
mechanisms. None of these hypotheses, or many others which might be
mentioned, are competent to explain satisfactorily all of the known facts,
though strong arguments can be adduced in support of each of them.

Our own view is that pleasurable and unpleasant experiences are not true
sensations, that in the history of the psychogenesis of primitive animals a

PAIN AND PLEASURE 257

diffuse unlocalized affective experience of well-being or malaise probably
antedated anything so clearly analyzed as a sensation with specific external
reference, and that, parallel with the differentiation of true sensations of
touch, temperature, and so on in consciousness, pain sensations emerged out
of the diffuse affective experience and took their place among the other sense
qualities. An essential condition for the appearance in consciousness of a
definite sensation like touch or vision is the differentiation in the nervous
system of a system of localized tracts and centers related to this function,
and in the human body such localized tracts and centers seem to be present
for pain. Pain, therefore, considered psychologically and neurologically, is
a sensation, and a different neurological mechanism for unpleasantness and
pleasantness must be sought. To this problem we shall next turn our
attention.

We have seen above that it is possible to frame a neurological hypothesis
which allows a given peripheral sensory neuron to be conceived as trans-
mitting, say, a tactual impression from the skin and also a painful im-
pression from the same or a different end-organ. Upon reaching the spinal
cord the nervous impulses of the tactual series may pass through one type of
spinal synapse to the spinal lemniscus, and finally reach the tactual center
of the cerebral cortex, and the nervous impulses of the painful series may be
drawn off through a second system of synapses for transmission through a
distinct system of central pathways. Attention has also been called to the
fact that the specific pain nerves and central paths may have been developed
by a process of the further differentiation of separate neurons with different
peripheral and central connections for these two functions. But what of
the pleasurable qualities which seem similarly to be associated with some
sensory impulses?

The simplest view seems to the writer to be that the normal activity of
the body within physiological limits is intrinsically pleasurable, so far as it
comes into consciousness at all. There is a simple joy of living for its own
sake, and the more productive the life is, within well-defined physiological
limits of fatigue, good health, and diversified types of reaction, the greater
the happiness. The expenditure of energy within these physiological limits
is pleasurable per se except in so far as various psychological factors enter to
disturb the simple natural physiological expression of bodily activity. Such
disturbing factors are anxiety, want, rebellion against compulsory service,
and unrelieved routine. The expenditure of intelligently directed nervous
energy along lines of fruitful endeavor is probably the highest type of
pleasure known to mankind.

But it should be borne in mind that the normal activities of the body are
all combined into adaptive systems, that is, they are directed toward the
accomplishment of definite ends and not directed at random. Even in
instinctive activities of the invariable or innate type, though there may be no
consciousness of the end to be attained, the actions are not satisfying to the
animal unless they follow in the predetermined adaptive sequence (p. 61).
The play of both men and other animals is likewise always correlated around
some definite physiological motive. And it is even more conspicuously true
that the intelligently directed activities are unsatisfying unless they attain,
or at least approximate to, some particular end. Stated in other words, it is
not the activity which is pleasurable, so much as the accomplishment, or, in
the case of delayed reactions, the hope of accomplishment.

The normal discharge, then, of definitely elaborated nervous circuits
resulting in free unrestrained activity is pleasurable, in so far as the reaction

17

258 INTRODUCTION TO NEUROLOGY

comes into consciousness at all (of course, a large proportion of such reactions
are strictly reflex and have no conscious significance). Conversely, the
impediment to such discharge, no matter what the occasion, results in a
stasis in the nerve centers, the summation of stimuli and the development
of a situation of unrelieved nervous tension which is unpleasant until the
tension is relieved by the appropriate adaptive reaction. Such a stasis
may be brought about by a conflict of two sensory impulses for the same
final common path (see p. 59), by the dilemma occasioned by the necessity
for discrimination in an association center between two or more possible
final paths, by fatigue, auto-intoxication, or other physiological states which
lower the efficiency of the central mechanism, and by a variety of other
causes. The unrelieved summation of stimuli in the nerve centers, involving
stasis, tension, and interference with free discharge of nervous energy, gives
a feeling of unpleasantness which in turn (in the higher types of conscious
reaction at least) serves as a stimulus to other associated nerve centers to
participate in the reaction until finally the appropriate avenue for an
adaptive response is opened and the situation is relieved. With the release
of the tension and free discharge, the feeling tone changes to a distinctly
pleasurable quality (see C. L. Herrick, 1910).

The fact that the primitive pain path in the spinal cord seems to follow
a rather diffusely arranged system of fibers in the fasciculus proprius, fre-
quently interrupted by synapses in the gray matter (Fig. 117) with corres-
pondingly high resistance to nervous conduction, is perhaps correlated with
this general and diffuse quality of unpleasantness.

Now, pain as a distinct and localizable sensation has not been involved
in the situation described in the preceding paragraphs. Pain, considered as
a distinct sensation, was, however, born out of this situation or differentiated
from it. Certain sensational elements which have a high protective value
for the organism are naturally most often involved in such a situation.
These are warning calls, and usually necessitate an interruption of the
ordinary business of life which may be in process at the time the danger
threatens. The free flow of ordinary sensori-motor activity is abruptly
checked, and the organism suddenly stops and makes the necessary reaa-
justment as quickly as may be. In the interest of increasing the rapidity
of this avoiding reaction, which, of course, is frequently of vital importance,
the pathways of the exteroceptive pain reactions are well developed and
segregated from the more diffuse and poorly organized affective apparatus
which we have just been considering. Thus arose pain nerves (if such exist
separately) and the pain tract of the spinal cord (whose anatomical dis-
tinctness seems well established), and also perhaps a special mechanism
for painful reactions in the thalamus. Sherrington has given a graphic
statement of the probable history of this process in the following words
(Schafer's Physiology, vol. ii, p. 974):

"The facility of path of these motor reflexes colligated to pain hints at
their antiquity, or at their having been formed by some neural method par-
ticularly able to, as it were, make a good road. Each reaction that employs
a neural path seems to smooth it by sheer act of travel. This is true even
of slight impulses — light traffic — and more true of heavy. Pain reactions
are to be regarded as very heavy traffic. Their impressions summate with

Kculiar ease, take correspondingly long periods to subside, and, to judge
„• their inertia, move generally masses of neural material relatively great.
Such impressions might wear a road with quite especial speed. Many
spinal reflexes imply, so to say, well-worn habits based on ancient pain

PAIN AND PLEASURE 259

reactions. One is almost emboldened to figuratively imagine them as con-
nate memories of the spinal cord. The majority of them seem to be pro-
tective reactions that in organisms of high neural type are accompanied by
'pain.'"

But even in this case the apparatus for pain is incapable of acting as
rapidly as are those of some other sensations. If a sensitive corn on the foot
is struck a sharp blow, one will often feel a very distinct tactile sensation an
appreciable interval before the painful quality is perceived, the latter, how-
ever, soon welling up into consciousness and obscuring the tactile quality
entirely. This is an illustration of the fact that even the highly protective
exteroceptive painful stimuli pass through a mechanism of slower reaction
time than the primary exteroceptive sensations with which they may be
associated.

We cannot here enter into a full discussion of the larger questions center-
ing about the physiological correlates of the higher affective life, the emotions
and esthetics. It has often been pointed out that the conscious processes
resulting from exteroceptive stimulation tend to be directed outward, the
attention being focussed on the external objects giving rise to the stimuli
with a minimum of personal reference. The deep sensations, both of the
proprioceptive and the interoceptive group, on the other hand, have a less
clearly defined local sign and the mental attitude toward them is not one of
outwardly directed attention to the source of the stimulus, but rather a
change in the subjective state and an alteration, of the general feeling tone
of the body as a whole. Under ordinary circumstances the visceral afferent
and other deep nervous impulses do not come into clear consciousness sepa-
rately, but in the aggregate these complexes (often termed as a whole com-
mon sensation) profoundly modify the general mental attitude and equilib-
rium. The generalized feelings of both the pleasurable and the painful type
share this subjective reference with the common sensations. They are very
important factors in that sensory continuum which lies at the basis of the
maintenance of personal identity which the older psychologists sometimes
called the empirical ego. Only the pains associated with the sharply local-
ized cutaneous sensation qualities with a high adaptive value as warning
signs of external danger have a distinct peripheral reference, and even this is
less clearly defined than that of the accompanying sensations of pressure,
and so forth. The deep pains are imperfectly localized and have more of the
general subjective- reference which has just been mentioned, and all of the
pleasurable qualities are of this type.

The simpler affective types of experience, accordingly, seem to be most
intimately associated with the "common sensation" complex, especially
with the visceral sensation components of this complex. From this it has
been argued that the coarser emotions, as well as the elementary feelings, are
the direct expression in consciousness of these visceral activities, that the
well-known visceral changes associated with the emotions are not the results,
but the causes of the emotions (Lange and James). This hypothesis has
been attacked experimentally by Sherrington (see The Integrative Action
of the Nervous System, 1906, p. 260), who found that cutting the afferent
sympathetic fibers from the abdominal viscera in dogs made no apparent
difference in the emotional reactions of the animals ; but the experiments are
not very convincing, and the question is probably too complex for solution
by so simple means as those here employed.

The probability is that we have here a circular type of reaction. The
initial visceral afferent impulses, being heavily charged with affective quali-

260 INTRODUCTION TO NEUROLOGY

ties and with a minimum of objective reference, excite within the brain,
probably in the medial thalamic nuclei, a general non-localized pleasurable
or unpleasant feeling, a feeling of well-being or malaise, as the case may be.
These thalamic receptive centers are in very intimate relation with the
visceral efferent systems of the hypothalamus and a reflex response in the
viscera follows — a typical organic circuit. So long as this circuit involves
only the viscera and their thalamic centers the peripheral reference will be at
a minimum, and the feeling remains an unlocalized change in the affective
consciousness.

The higher emotional and esthetic activities are so charged with intellec-
tual content also as to require the ^participation of the association centers of
the cerebral cortex. But no pleasure-pain centers are known in the cortex
and the evidence at present available seems to negative the presence of such
centers. The agreeable or disagreeable components of the higher emotional
processes are very probably due to the colligation of thalamic activities
with cortical associational processes. In case these emotional or esthetic
processes are of cortical origin, that is, excited in the first instance by the
activity of cortical associational centers, their affective content may be due
to the involvement of the subcortical pleasure-pain apparatus in the asso-
ciational process, and this apparatus would, as above described, generate
efferent impulses from the related visceral centers, thus causing the charac-
teristic visceral movements, which in turn would reinforce the visceral activ-
ities of the brain centers, and thus by a "back-stroke" action strengthen
the emotional con ten t of the primary associational complex. Thus the com-

Eletion of the circular reaction may reinforce the affective consciousness so
>ng as it is operative.

That pleasure is correlated with free discharge of nervous energy is sug-
gested further by the fact that in most of the pleasurable emotions and senti-
ments there is present a large factor of recall of previous experiences. The
esthetic enjoyment of a given situation is in large measure proportional to
the wealth of associated memories incorporated within it, especially when
these are recombined into new patterns. The pleasure experienced in listen-
ing to a complicated musical production like a symphony may be enhanced
many fold after one has become thoroughly familiar with it, and still more
so if the listener has himself played it or parts of it.

In concluding this discussion of pleasure-pain we quote the following
paragraph from Sherrington's account of Cutaneous Sensations, already
referred to (Schafer's Physiology, 1900, vol. ii, p. 1000):

"Affective tone is an attribute of all sensation, and among the attribute
tones of skin sensation is skin-pain. Affective tone inheres more intensely
in senses which refer to the body than in those which refer to the environ-
ment, that is, it is strongest in the non-pro jicient senses. It is, therefore,
strong in the cutaneous senses, and in them is inversely as their projicience,
therefore least in touch spots, more in thermal spots, most in the so-called
'pain-spots.' . . . Stimuli evoking skin-pain are broadly such as injure
or threaten injury to the skin; the skin may be said to have gone far toward
developing a special sense of its own injuries. The central conducting
path concerned with these skin feelings seems a side-path into which the
impressions from the various skin spots embouch with various ease, those
from the 'pain spots' especially easily. The physiological reactions connected
with this side-path are characterized by tendency to 'summation, ' tendency
to 'collateral irradiation,' slow culmination, and slow subsidence. They
often involve with their own activity that of adjacent sensory channels (as-

PAIN AND PLEASURE 261

sociate pains, referred pains), and almost invariably of motor centers of
visceral, facial, and other muscles of expression (emotional discharge)."
Our own view is in harmony with that expressed in this paragraph except
that, while we recognize that sensations in general have an affective tone,
we do not consider that affective experience is to be regarded as essentially
an attribute or quale of sensation. These are independent variables which
are, however, usually intimately associated. Each has its own mechanism.
The mechanism of every sensation is a localizable system of tracts and
centers as expounded in the preceding chapters. The mechanism of the
affective experience is a more general neural attitude or physiological phase,
intimately bound up with the visceral reactions peripherally and inte-
grated centrally in the thalamus.

Summary. — In the human organism pain appears to be a true
sensation with its own receptors, probably with independent
peripheral neurons (in some cases at least), and certainly with
well localized conduction paths and cerebral centers, these cen-
ters being thalamic and not cortical. Pain appears to be closely
related neurologically with feelings of unpleasantness and pleas-
antness, and these, in turn, with the higher emotions and the
affective life in general. The intellectual elements in the higher
emotions and sentiments are, of course, cortical, and in nearly all
cases the affective experience probably involves a highly complex
interaction of cortical and subcortical activities. Pleasantness
and unpleasantness are not regarded simply as attributes of
specific sensory processes in any case, but rather as a mode of
reaction or physiological attitude of the whole nervous system
intimately bound up with certain visceral reactions of a protec-
tive sort whose central control is effected in the ventral and medial
parts of the thalamus. These parts of the thalamus form, ac-
cordingly, the chief integrating center of the nervous reactions
involved in purely affective experience. This mechanism is
phylogenetically very old, and in lower vertebrates which lack
the cerebral cortex it is adequate to direct avoiding reactions to
noxious stimuli and seeking reactions to beneficial stimuli.
With the appearance of the cortex in vertebrate evolution these
thalamic centers became intimately connected with the associ-
ation centers of the cerebral hemispheres, and an intelligent
analysis of the feelings of unpleasantness and pleasantness be-
came possible. As a final step in the development of the pro-
tective apparatus the peripheral nerves of painful sensibility,
with their own specific conduction paths and centers, were differ-

262 INTRODUCTION TO NEUROLOGY

entiated, and pain takes its place among the other exteroceptive
senses. But even in man the thalamic and visceral mechanisms
of affective experience are preserved and give a characteristic
organic background to the entire conscious life. In the normal
man these mechanisms may function with a minimum of cor-
tical control, giving the general feeling tone of well-being or
malaise, or they may be tied up with the most complex cortical
processes, thus entering into the fabric of the higher sentiments
and affections and becoming important factors in shaping human

conduct.

LITERATURE

CANNON, W. B. 1914. Recent Studies of Bodily Effects of Fear, Rage,
and Pain, Jour. Philos. Psych. Sci. Methods, vol. xi, pp. 162-165.

— . 1914. The Interrelations of Emotions as Suggested by Recent
Physiological Researches, Amer. Jour. Psychol., vol. xxv, pp. 256^-282.

— . 1914. The Emergency Function of the Adrenal Medulla in Pain and
the Major Emotions, Amer. Jour. Physiol., vol. xxxiii, pp. 356-372.

— . 1915. Bodily Changes in Pain, Hunger, Fear, and Rage, New York,
311 pages.

CAPPS, J. A. 1911. An Experimental Study of the Pain Sense in the
Pleural Membranes, Arch. Internal Medicine, vol. viii, pp. 717-733.

HEAD, H.; and HOLMES, G. 191 X. Sensory Disturbances from Cerebral
Lesions, Brain, vol. xxxiv, pp. 109-254.

HEAD, H., and THOMPSON, T. 1906. The Grouping of the Afferent Im-
pulses Within the Spinal Cord, Brain, vol. xxix, p. 537.

HERRICK, C. L. 1910. The Summation-irradiation Theory of Pleasure-
pain. In The Metaphysics of a Naturalist, Bui. Denison University
Scientific Laboratories, vol. xv.

HOLMES, S. J. 1910. Pleasure, Pain, and the Beginnings of Intelligence,
Jour. Comp. Neur., vol. xx, pp. 145-164.

JAMES, W. 1890. The Principles of Psychology, New York, vol. ii, pp.
442^85.

— . 1894. The Physical Basis of Emotions, Psych. Rev., vol. i, p. 516.

KARPLUS, J. P., and KREIDL, A. 1914. Ein Beitrag zur Kenntnis der
Schmerzleitung im Riickenmark, nach gleichzeitigen Durchschneidungen
beider Riickenmarkshalften in verschiedenen Hohen bei Katzen, Pfliiger's
Archiv, Bd. 158, pp. 275-287.

LANGE, C. 1887. Ueber Gemiithsbewegungen. Eine Psycho-physio-
logische Studie, Leipzig.

MEYER, MAX. 1908. The Nervous Correlate of Pleasantness and Un-
pleasantness, Psych. Rev., vol. xv, pp. 201-216. 292-322.

SHERRINGTON, C. S. 1900. Cutaneous Sensations, in Schafer's Physi-
ology, vol. ii, pp. 965-1001.

— . 1906. The Integrative Action of the Nervous System, New York.

WATSON, J. B. 1913. Image and Affection in Behavior, Jour. Philos.
Psych. Sci. Methods, vol. x. pp. 421-428.

CHAPTER XIX

THE STRUCTURE OF THE CEREBRAL CORTEX

THE preceding pages have included a brief chapter on some of
the general biological principles underlying the differentiation of
the structure and functions of the nervous system, some general
characteristics of the nervous tissues, a brief survey of the
structure of the various great divisions of the nervous system,
and finally an analysis of the more important sensori-motor
reflex circuits. Nearly all of the mechanisms hitherto consid-
ered are concerned with the innate invariable types of response
represented in the reflex and instinctive life of the organism
(p. 31). In the higher mammals, and especially in man, the
individually acquired relatively variable types of action, par-
ticularly those which are consciously performed, require the
cooperation of the cerebral cortex, and the following chapters
will be devoted to a consideration of the cortex, its structure,
functions, evolution, and biological significance.

We have already commented (pp. 109, 215) on the fact that the
cerebral cortex appeared later in vertebrate evolution than most
of the other parts of the brain, and that in general it serves the
individually acquired and intelligent functions, in contrast with
the brain stem and cerebellum, which contain the apparatus for
the innate activities of the reflex type. The primary reflex
centers of the brain stem and cerebellum, accordingly, are some-
times called the old brain (palaeencephalon, see Fig. 45, p. 114),
while the cerebral cortex and those parts of the brain stem which
develop as subsidiary to the cortex (such as the neothalamus, p.
163) are called the new brain (neencephalon).1

1 A review of the evolution of the brain and the phylogenetic origin of
the cerebral cortex would lie beyond the limits of this work, for the liter-
ature upon this subject is very extensive. The following papers may be
consulted in the present context. (See also the bibliographies on pp.
159, 223.)

HERRICK, C. JUDSON. 1910. The Evolution of Intelligence and Its
Organs, Science, N. S., vol. xxxi, pp. 7-18.

SMITH, G. ELLIOT. 1910. The Arris and Gale Lectures on Some Prob-

263

264 INTRODUCTION TO NEUROLOGY

In the embryologic development of the human brain the cere-
bral hemispheres grow out as lateral pouches from the anterior
end of the neural tube (Figs. 46-54, pp. 116-121). These pouches
are hollow and the cavities within them are the lateral ventricles
(also called the first and second ventricles), each of which com-
municates with the third ventricle of the thalamus by a narrow
opening, the interventricular foramen or foramen of Monro.

In a simply organized brain like that of the frog (Fig. 1 19) the
olfactory bulb forms the anterior end of each cerebral hemi-

Olfactory nerve

Ifactory bulb
Lateral ventricle
Corpus striatum
Lamina tenn'malis
Interventricular foramen
Third ventricle
.Optic lobe
.Cerebellum

'ourth ventricle

Fig. 119. — Diagrammatic representation of an amphibian brain from
which the roof of the thalamus and cerebral hemisphere has been dissected
off on the right side, exposing the third and the lateral ventricles and the
interventricular foramen (foramen of Monro). The membranous roof of
the fourth ventricle has also been removed.

sphere, behind which the massive wall contains ventrally the
basal olfactory centers (p. 218), laterally the corpus striatum
(p. 168), and dorsally the cerebral cortex or pallium (which has

lems Relating to the Evolution of the Brain, The Lancet for January 1, 15,
and 22, 1910.

SMITH, G. ELLIOT. 1912. The Evolution of Man, Report of the Anthro-
pological Section of the British Assoc. for the Advancement of Science,
Dundee Meeting. Printed also in Nature (London) for Sept. 26, 1912,
and in the Smithsonian Report (Washington) for 1912, pp. 553-572.

THE STRUCTURE OF THE CEREBRAL CORTEX 265

been removed on the right side of Fig. 119). In the human brain
the cerebral cortex is so greatly enlarged that it overlaps all other
structures of the hemisphere.

The anterior end of the early neural tube is an epithelial
plate, the terminal plate or lamina terminalis, which forms the
anterior wall of the third ventricle in the median plane. The
position of this plate is unchanged throughout all subsequent
stages of development (Figs. 46-51, pp. 116-119, and Fig. 119),
though the cerebral hemispheres grow forward on each side of it,
so that in the adult brain it lies deeply buried at the bottom
of the great longitudinal fissure which separates the hemispheres.

The reflex centers of the two sides of the spinal cord and brain
stem are connected by transverse bands of fibers known as
commissures, for the facilitation of bilateral adj ustments. There
is an extensive series of ventral commissures crossing below the
ventricle in the floor of the midbrain, medulla oblongata, and
spinal cord, and several smaller dorsal commissures are found
above the ventricle. In the diencephalon there is a large ventral
commissure associated with the optic chiasma, and a dorsal com-
missure, the superior or habenular commissure, connecting the
habenular bodies of the epithalamus. The basal parts of the
cerebral hemispheres are connected by the anterior commissure,
whose fibers cross in the lamina terminalis (Fig. 78, p. 165), and
there are two large commissures which connect the cerebral
cortex of the two hemispheres. One of these, the corpus callo-
sum (Figs. 52, p. 119, and 78, p. 165), connects the non-olfactory
cortex (neopallium, p. 217), the other one, the hippocampal
commissure, connects the olfactory cortex (hippocampus).
The fibers of the hippocampal commissure lie under the posterior
end of the corpus callosum in close relation with the fimbria
(Figs. 78, p. 165, and 80, p. 170).

In the smaller mammals the cerebral cortex is smooth, but in
the larger forms it is more or less wrinkled, so that the surface
is marked by gyri or convolutions separated by sulci or fissures.
A more highly convoluted cortical pattern is found in large
animals than in smaller ones of closely related species, and in
animals high in the zoological scale than in lower species; but
the factors which have determined this pattern in each individual
species are very complex (see Kappers, 1913 and 1914). The

266

INTRODUCTION TO NEUROLOGY

primary factor in the higher mammals has undoubtedly been
the great increase in the superficial area of cortical gray matter
without a corresponding enlargement of the skull.

The human cerebral cortex is somewhat arbitrarily divided
into frontal, temporal, parietal, and occipital lobes (Fig. 120).
These lobes have no special functional significance, but are dis-
tinguished merely for convenience of topographic description.
Some of the more important gyri and sulci are named on Figs.
52 and 54 (pp. 119 and 121). Between the temporal and frontal
lobes and under the lower end of the lateral or Sylvian fissure is
a buried convolution, the island of Reil (insula), which is seen in
section in Figs. 79 and 80 (pp. 166 and 170) . The cortical lobules
which cover the insula are called opercula (Fig. 54, p. 121).

OCCIPITAL
LOBE

Fig. 120. — The lateral aspect of the human brain, illustrating the boundaries
of the lobes of the cerebral cortex (cf. Fig. 54).

The walls of the cerebral hemispheres in the cortical region are
very thick, the greater part of this thickness being occupied by
white matter composed of nerve-fibers which effect various types
of connection with the neurons of the cerebral cortex. The
cortex itself is composed of gray matter and is relatively thin,
its inner border being marked by a broken line in Figs. 79 and 80.
The subcortical white matter contains three chief classes of
fibers: (1) Corona radiata fibers which connect the cortex with
the brain stem (Figs. 79, 80). Most of these fibers pass through
the internal capsule and comprise the sensory and motor pro-
jection fibers (pp. 165-169); (2) commissural fibers of the corpus
callosum and hippocampal commissure (Figs. 79, 80) ; (3) associ-
ation fibers, which connect different parts of the cerebral cortex

THE STRUCTURE OF THE CEREBRAL CORTEX

267

of each hemisphere. Some of these fibers are very short, passing
between adjacent gyri (arcuate fibers, or fibrae proprise, f.p.,
Fig. 121); others are very long fibers, forming compact fascicles
which can easily be dissected out and which connect the impor-
tant association centers of the cortex. All parts of the cerebral
cortex are directly or indirectly connected with all other parts
by these association fibers, so that no region can be regarded as
the exclusive seat of any particular cortical function.

str term
Us.

f. Tr. OOt

f.l.i.
f. occ.fr. inf.

Fig. 121. — Diagram illustrating some of the chief association tracts of
the cerebral hemisphere, seen as projected upon the median surface of the
right hemisphere: cin., cingulum; f.l.i., fasciculus longitudinalis inferior;
f.l.s., fasciculus longitudinalis superior; /. occ.fr. inf., fasciculus occipito-
frontalis inferior; f.p., arcuate fibers; f.tr.oc., fasciculus transversus occipi-
talis;/.imc., fasciculus uncinatus; str. term., stria terminalis.

The human cortex varies in thickness in different regions from
about 4 mm. in the motor area to less than half that thickness in
some other parts. When cut across and examined in the fresh
condition it shows alternate bands of light and dark gray, whose
arrangement varies in different parts of the hemisphere. The
light bands are composed of myelinated fibers which run parallel
with the surface. There are typically two of these light bands,
the outer and inner stripes of Baillarger (Fig. 122). In the
visual projection area (Figs. 130, 131, area 17) the outer stripe

268 INTRODUCTION TO NEUROLOGY

of Baillarger is greatly thickened by the optic projection fibers,
and here it is sometimes called the line of Gennari. The por-
tion of cortex exhibiting the line of Gennari is called the area
striata.

The most characteristic neurons of the cortex are pyramidal in
shape, with the apex directed toward the outer surface of the
brain and prolonged to form the principal dendrite. Smaller
dendrites arise from other parts of the cell body, and the axon
arising from the base of the cell body is directed inward into the
white matter (Figs. 7, 8, pp. 42, 44). The cortex contains, more-
over, many other types of neurons, some of irregular shape (poly-
morphic or multiform cells) and many whose axons are short and

Fig. 122. — Sections of the cerebral cortex, drawn nearly natural size and
showing the naked-eye appearance: 1 shows the layers as they appear in
many parts of the cortex, and 2 shows the appearance of a section from the
visual cortex (area striata) from the neighborhood of the calcarine fissure,
with the conspicuous line of Gennari. (After Baillarger.)

ramify close to the cell body without leaving the cortex itself
(Fig. 9, p. 44). These type II neurons probably assist in the
summation and irradiation of stimuli (see p. 101). Some other
types of neurons are shown in Fig. 123.

Figure 124 illustrates a typical arrangement of the neurons in
the postcentral gyrus (gyrus centralis posterior of Fig. 54, p. 121).
Most of the neurons here shown send their axons inward to
participate in the formation of the white matter and may dis-
charge their nervous impulses into remote parts of the brain.
The endings of the afferent nerve-fibers which effect synaptic
connection with the neurons here shown form a dense entangle-
ment of fine unmyelinated fibers between the dendrites of these
neurons. These afferent fibers are not included in Fig. 124; one

THE STRUCTURE OF THE CEREBRAL CORTEX

269

of them is shown in Fig. 123 and they are drawn separately in
Fig. 125 as they appear in the precentral gyms (gyrus centralis
anterior of Fig. 54). These afferent fibers may be either sensory
projection fibers or association fibers from other parts of the

Fig. 123. — Diagrammatic illustration of the arrangement of neurons in
the cerebral cortex as revealed by the Golgi method. The figure is copied
from Obersteiner and the layers are numbered differently than in Brod-
mann's scheme, Fig. 127. Obersteiner's layer III includes layers III, IV,
and V of Brodmann. The arrows indicate the direction of nervous conduc-
tion, and the axons of the neurons are marked by a cross, x ; gl., layer of su-
perficial neuroglia cells; m, beginning of the layer of white matter; 12, 13,
14, and 15 mark neuroglia (glia) cells; the other numbers designate different
types of neurons.

cortex. The synapses between these incoming fibers and the
neurons of the cortex among which they end are of various types.
Many of the afferent fibers end in the outermost layer of the
cortex (layer 1 of Figs. 123 and 124) among the dendrites of the

270

INTRODUCTION TO NEUROLOGY

Fig. 124. — Section from the cerebral cortex of a human infant from the
postcentral gyrus (gyrus centralis posterior), with the neurons impregnated
by the method of Golgi. The figure is taken from Ramon y Cajal's His-
tology of the Central Nervous System, and the layers are numbered accord-
ing to his system. Layer 1 corresponds to Brodmann's first layer (Fig.
127); layer 2, to his second layer; layers 3 and 4, to his third layer; layer 5,
to his fourth layer; layer 6, to his fifth layer; and layer 7, to his sixth layer.

THE STRUCTURE OF THE CEREBRAL CORTEX

271

pyramidal cells which are here widely expanded (see Fig. 8, p.
•44); others end in dense arborizations which closely envelop

Fig. 125. — Section of the human cerebral cortex from the precentral
gyrus (gyrus centralis anterior), illustrating the free endings of the incoming
fibers. This region contains a large number of cells similar to those shown
in Fig. 124; but none of the cells were stained in this preparation, which was
prepared by the method of Golgi. At a and 6 are seen the terminal arbori-
zation of two individual fibers. At B is a dense entanglement of such ter-
minal arborizations around the cell bodies of the pyramidal neurons of layer
3 (Fig. 124). C, D, and E illustrate horizontally directed nerve-fibers,
from which the terminal arborizations shown in the upper part of the
figure arise. (After Ram6n y Cajal.)

272

INTRODUCTION TO NEUROLOGY

the bodies of the pyramidal cells (Fig. 126). Still others twine
around the dendrites for their entire length. The dendrites of
the pyramidal cells are very rough and thorny, and these thorns
are supposed by some to be the points where the actual synaptic
connections are effected.

Besides the lamination caused by the bands of tangential
nerve-fibers already referred to, the cell bodies themselves are
arranged in layers whose pattern varies in different parts of the

Fig. 126. — Section of the human cerebral cortex from the precentral
gyrus, illustrating the details of the terminal arborizations of the incoming
fibers (a) in the form of a closely woven feltwork of fibers (b, c, d) around the
cell bodies of the large pyramidal cells of the cortex. The cells themselves
are not stained in the preparation, but their outlines are clearly indicated
by the pericellular basket-work by which they are enveloped. (After
Ilam6n y Cajal.)

cortex. Neurologists enumerate these layers differently. Brod-
mann, who has studied this question very exhaustively, enumer-
ates six primary layers which in most parts of the cortex are
arranged essentially as shown in the accompanying diagram
(Fig. 127). The six layers here recognized are present in most
but not in all parts of the cortex. In the different regions one or
more of these layers may be reduced, enlarged, or subdivided;
and on the basis of these differences the entire cortex has been

THE STRUCTURE OF THE CEREBRAL CORTEX 273

mapped out into areas, each of which is defined by the arrange-
ment of the layers of cortical cells and fibers.

Brodmann (Figs. 128, 129) divides the cerebral hemisphere
into eleven general regions, which he says are recognizable more
or less clearly throughout the entire group of mammals. These
are:

1. Regio postcentralis (tactile region).

2. Regio precentralis (motor region).

3. Regio frontalis (frontal association center).

4. Regio insularis (insula).

5. Regio parietalis (parietal association center).

6. Regio temporalis (auditory region).

7. Regio occipitalis (visual region).

8. Regio cingularis (supracallosal part of limbic lobe).

9. Regio retrosplenialis (postcallosal part of limbic lobe).

10. Regio hippocampica (gyrus hippocampi and hippocampus).

11. Regio olfactoria (uncus, amygdala, tuberculum olfactorium).

In the list as here given Brodmann's names of the regions are
given, and in parenthesis is added a brief description of each
region. Regions 8, 9, 10, and 11 are all concerned with the
olfactory reactions, though region 8 only to a small extent.
Region 11 is only in part cortical (the uncus); the other parts
of this region are subcortical olfactory centers. The specific
sensory and motor projection centers (see p. 165) lie within their
respective regions, as designated, but they do not occupy the
whole of their regions. On the basis of the arrangement of their
cells and fibers these regions are further subdivided by Brod-
mann into upward of 50 areas or fields, as shown in Figs. 130
and 131. The areas are less uniformly developed in different
animals than are the general regions, though many of them are
very constantly present.

Bolton, Campbell, Ramon y Cajal, Vogt, Elliot Smith, and
many others have investigated the lamination of the cerebral
cortex in man and other mammals, and many charts similar to
those here presented have been published. The conclusions
reached by these authors do not agree in all respects (particu-
larly in the number of areas separately recognized and the
nomenclature of the layers of cells and fibers in the various
regions) ; nevertheless there is a sufficiently close general agree-
ment to make it evident that there is a definite structural pattern

18

274

INTRODUCTION TO NEUROLOGY

>;,.'

(P

Fig. 127. — Diagram of the arrangement of the layers of cells and mye-
linated nerve-fibers in the cerebral cortex, according to Brodmann. At the
left of the figure is shown the arrangement of cells as shown by the Golgi
method, in the middle their arrangement as shown by Nissl's method, and at
the right the arrangement of nerve-fibers as shown by Weigert's method.
/. Lamina zonalis, or plexiform layer, containing tangential nerve-
fibers.

//. Lamina granularis externa, or layer of small pyramidal cells.
///. Lamina pyramidalis, or layer of medium and large pyramidal cells.

THE STRUCTURE OF THE CEREBRAL CORTEX 275

which is characteristic of the several cortical regions in each
' species of mammals, and that this pattern is broadly similar in
all of the higher members of this group of animals.

Data derived from physiological experiments made on dogs,
apes and other animals, and from the study of pathological hu-
man brains have shown also that the difference in structural
pattern of the cortical areas is correlated with differences in
the functions performed by them. To these functional ques-
tions our attention will next be directed.

Summary. — The cerebral cortex is the organ of the highest
individually modifiable functions, particularly those of the
intellectual life. It matures late in both phylogenetic and indi-
vidual development, and therefore has been called the neenceph-
alon. In early developmental stages it forms the roof of the
lateral ventricle of each cerebral hemisphere, but in the adult
human brain it is so enlarged as to envelop most other parts of
the hemisphere. The cortex of the two hemispheres is con-
nected by commissural fibers in the corpus callosum and the hip-
pocampal commissure. The various regions of each hemisphere
are connected by a complex web of association fibers, and some
parts of the cortex are connected with subcortical regions by
1 projection fibers. The sensory projection fibers discharge
among the neurons of the sensory projection centers, and the
motor projection fibers arise from neurons of the motor projec-
tion centers. The intervening association centers are connected
with the projection centers and with each other by very intricate
systems of association fibers. The cortex is laminated by bands
of horizontally arranged nerve-fibers and by an arrangement of
its cells in layers. The pattern of this lamination varies in
different regions, and charts of these structurally defined regions
are found to show a general correlation with the functionally
defined areas as physiologically and pathologically determined.

IV. Lamina granularis interna, or inner granular layer, containing the
medullated fibers of the external line of Baillarger (in the visual area called
the stripe of Gennari).

V. Lamina ganglionaris, or layer of large cells, containing in the motor
area the giant pyramidal cells or Betz cells, from which the fibers of the
pyramidal tract arise, and containing in most areas the medullated fibers of
the internal line of Baillarger.

VI. Lamina multifonnis, or layer of polymorphic cells.

276

INTRODUCTION TO NEUROLOGY

Fig. 128. — The chief regions of the human cerebral cortex as determined
by Brodmann from the study of the structural arrangements of the layers of
cells and fibers, seen from the left side.

Fig. 129. — The chief regions of the cortex, seen from the median side.

THE STRUCTURE OF THE CEREBRAL CORTEX 277
6

Fig. 130. — The detailed subdivisions of the cortical regions shown in Fig.
128 as determined by Brodmann, seen from the left side. Each area or
field which is here designated by a number and conventional symbols has a
distinctive lamination of its cells and fibers.

Fig. 131. — The same brain shown in Fig. 130, seen from the median side.

278 INTRODUCTION TO NEUROLOGY

LITERATURE

BOLTON, J. S. 1910. A Contribution to the Localization of Cerebral
Function, Based on the Clinico-pathological Study of Mental Disease,
Brain, vol. xxxiii, Part 129, pp. 26-148.

BOLTON, J. S., and MOYES, J. M. 1912. The Cytoarchitecture of the
Cerebral Cortex of a Human Fetus of Eighteen Weeks, Brain, vol. xxxv.

BRODMANN, K. 1907. Die Kortexgliederung des Menschen, Jour. f.
Psychol. u. Neurol., Bd. 10.

— . 1909. Vergleichende Lokalisationslehre der Grosshirnrinde, Leipzig.

— . 1910. Chapter entitled, Feinere Anatomic des Grosshirns, in Lewan-
dowsky's Handbuch der Neurologic, Bd. 1, pp. 206-307.

CAMPBELL, A. W. 1905. Histological Studies on the Localization of Cor-
tical Function, Cambridge.

KAPPERS, C. U. A. 1913. Cerebral Localization and the Significance of
Sulci, Proc. XVII Intern. Congress of Medicine, London.

— . 1914. Ueber das Rindenproblem und die Tendenz innerer Hirnteile
sich durch Oberflachen-Vermehrung statt Volumzunahme zu vergrosseren,
Folia Neuro-biologica, Bd. 8, pp. 507-531.

RAM6N Y CAJAL. 1900-1906. Studien uber die Hirnrinde des Menschen,
Leipzig.

SMITH, G. ELLIOT. 1907. A New Topographical Survey of the Human
Cerebral Cortex, Jour. Anat. and Physiol., vol. xli.

VOGT, O. 1903. Zur anatomischen Gliederung des Cortex Cerebri,
Jour. f. Psych, u. Neurol., Bd. 2.

— . 1904. Die Markreifung des Kindergehirns wahrend der ersten vier
Lebensmonate und ihre methodologische Bedeutung, Jena.

CHAPTER XX

THE FUNCTIONS OF THE CEREBRAL CORTEX

THE greatest diversity of view has prevailed and still prevails
regarding the method of cortical function. That the cerebral
cortex is concerned in some way with the higher conscious func-
tions is clearly shown by a large body of experimental and
clinical evidence.

The partial or complete removal of both cerebral hemispheres
has been accomplished in various species of animals, from fishes
to apes, and the changes in behavior carefully studied. In
fishes and frogs the behavior is but little modified, save for the
loss of the sense of smell, if the thalamus is left intact; but if the
thalamus also is destroyed, the animal loses all power of sponta-
neous movement, of feeding when hungry, etc., though it will still
react to some strong stimuli in an apparently normal manner.
The fundamental reflexes of the spinal cord and brain stem are
but little modified by this operation in frogs, save for the dis-
turbance of the olfactory and visual functions. The recent
experiments of Burnett have, moreover, shown that frogs in
which the cerebral hemispheres alone have been removed are
somewhat more excitable than normal frogs (probably due to the
loss of cortical inhibitions), and that simple associations easily
learned by normal frogs are in this case impossible.

In the dog the loss of the cerebral hemispheres alone leaves the
animal in a state of profound idiocy, though here also all of the
primary sensori-motor reflexes (except the olfactory) remain if
the thalamus is uninjured, and one such animal operated on by
Goltz lived for eighteen months. During this time, however, he
had to be artificially fed, for he had lost the ability to recognize
food when set before him, nor did he show any of his former signs
of intelligence. (These experiments are summarized in Schafer's
Physiology, vol. ii, pp. 698 ff., to which the reader is referred for

279

280 INTRODUCTION TO NEUROLOGY

references to the literature; see also the papers by Goltz, Edinger,
and Holmes, cited in the appended Bibliography.)

Edinger and Fischer report the case of a boy who lived three
years and nine months, whose brain when examined after death
showed total lack of the cerebral cortex with no other important
defects. In this boy there was practically no development in
sensory or motor power or in intelligence from birth to the time
of his death. The infant fed when put to the breast, but showed
no signs of hunger, thirst, or any other sensory process. It lay in
a profound stupor and during the first year of life made no
spontaneous movements of the limbs. Until the time of death
there was little change from this condition, save for continual
crying from the second year on. This case shows that the reflex
functions of the human brain stem are normally under cortical
control to a much greater extent than are those of any of the
lower animals, and that the absence of the cortex accordingly
involves a more profound disturbance of the subcortical ap-
paratus (see p. 129).

About a hundred years ago Gall and Spurzheim examined the
brain, form of skull, and physiognomy of many persons whose
mental characteristics were more or less fully known, and
reached very definite conclusions regarding the localization
within the brain of particular mental faculties, such as benevo-
lence, wit, and destructiveness; they claimed, further, that the
sizes of these specific parts of the brain (and hence their relative
physiological importance) can be determined by study of the
external configuration of the skull. Many valuable observations
were accumulated by these men and their followers, but the data
were so uncritically used and the psychological basis of their
generalizations was so faulty that the alleged science of phrenol-
ogy which they founded is now wholly discredited and is pro-
fessed today only by ignorant charlatans.

The great popularity of phrenology fifty years and more ago
grew out of the fact that it served to give a pseudoscientific
character to methods of reading character, and hence of forecast-
ing the future formerly claimed by astrologers and necromancers.
Modern psychology recognizes that the mind cannot be sub-
divided into any such distinct "faculties" as the phrenologists
used, and modern neurology finds no basis for the sharply

THE FUNCTIONS OF THE CEREBRAL CORTEX 281

defined localization of these or any other mental functions, in
the sense that a specific cortical area is the exclusive organ of a
particular mental element.

As a reaction against the crude theories of Gall and Spurzheim
it was commonly believed up to the year 1870 that there is no
definite localization of functions in the cerebral cortex, but that
the cortex functions as a whole, much like the cerebellar cortex,
with no clearly defined functional areas. This view and modi-
fications of it are still very prevalent. Goltz, who succeeded
in removing all of both cerebral hemispheres from several dogs,
holds that different psychic functions are not localizable in the
cortex, but that removal of cortical areas simply diminishes
general intelligence in proportion to the amount of cortex re-
moved. Even total removal of the cortex, in his opinion, does
not completely destroy consciousness. Many physiologists
have, on the other hand, taught that particular conscious func-
tions are localized in definite cortical areas, somewhat after the
fashion of a refined and modernized phrenology, and this view is
very prevalent among clinical neurologists.

The modern period of study of cortical functions was inaugu-
rated by a chance observation on the battlefield. During the
Franco-Prussian war an army surgeon, Fritsch, while operating
on a wounded soldier, applied the galvanic electric current to the
exposed surface of the brain and observed a twitching of some of
the muscles. This was followed immediately by experimental
researches upon the electric excitability of the cerebral cortex
of dogs, the first results of which were published by Fritsch and
Hitzig in 1870. They showed that there are regions in the
vicinity of the central sulcus (fissure of Rolando, cruciate sulcus)
whose excitation in the living animal is followed by movements
of definite groups of muscles on the opposite side of the body.

These observations have been followed by an immense number
of experimental researches on various animals (the animals
being anesthetized during the experiments) and clinico-patho-
logical studies of the human brain, whose correlation and integra-
tion have proved very difficult. The most careful studies have,
however, in general given concordant results. Without attempt-
ing a summary of these investigations here, we may mention the
recent investigations of Sherrington on the chimpanzee, whose

282

INTRODUCTION TO NEUROLOGY

results as summarized on Fig. 132 may be accepted as fully in
accord with the best previous experimental work, with the
anatomical investigations of the regional differentiation of the
cortex, and with the most recent clinical studies. The corre-
sponding areas of the human brain are seen in Fig. 133.

Anus and vagina

Hip

Shoulder
Elbow

Ear Eyelid ,

Noee Closure /
of jaw /
Opening of jaw

Vocal cords

I

Mastication

Sulcus centralia

Fig. 132. — Brain of a chimpanzee seen from the left side and from
above, upon which the cortical areas whose excitation causes bodily move-
ments are indicated by shading. The regions shaded by vertical lines and
marked " EYES " indicate the frontal and part of the occipital regions which
when electrically excited cause conjugate movements of the eyes. The
regions shaded with stipple comprise the motor projection centers from
which the fibers of the pyramidal tract arise. The names printed large
on the stippled area indicate the main regions of the motor area; the names
printed small outside the brain indicate broadly by their pointing lines
the relative topography of some of the chief subdivisions of the main
regions of the motor cortex. But there exists much overlapping of the
motor areas and of their subdivisions which the diagram does not attempt
to indicate. (After Griinbaum and Sherrington.)

The electric or mechanical stimulation of each one of the
shaded areas of Fig. 132 is followed by the contraction of a
particular group of muscles on the opposite side of the body, as

THE FUNCTIONS OF THE CEREBRAL CORTEX

283

designated on the figure. The electrically excitable motor cor-
tex is of two types, marked on the figure by stipple and vertical
cross-hatching respectively. Stimulation of the latter areas in
the frontal and occipital lobes calls forth conjugate movements
of the eyes, and the physiological characteristics of these areas
are very different from those of the areas in the precentral gyrus,
which are shaded with stipple. This gyrus is the true motor
projection center, and a comparison of Figs. 132 and 133 with
Fig. 130 shows that its limits coincide tolerable closely with

Fig. 133. — The human cerebral hemisphere seen from the left side, upon
which the functional areas of the cortex are indicated. The area marked
"motor speech" is Broca's convolution. (From Starr's Nervous Diseases.)

area 4 of Brodmann's chart of the anatomically distinct cortical
areas, including, however, a part of the cortex farther forward
in area 6.

The structure of the cortex in the precentral motor area
(Brodmann's area 4) is very characteristic. In this region the
fifth layer of the cortex (see Fig. 127) contains a type of large
pyramidal cells (giant pyramids or Betz cells) which are found
nowhere else in the brain. From these cells arise most of the
fibers of the pramidal tract (tractus cortico-spinalis). This

284 INTRODUCTION TO NEUROLOGY

connection has been proved in several ways in addition to the
direct physiological experiments by electric stimulation already
referred to. First, if this area of the cortex (and a portion of
area 6 in front of it) is destroyed, the entire pyramidal tract will
degenerate, a result which follows from the destruction of no
other part of the cortex. Conversely, if the pyramidal tract is
interrupted, the giant pyramidal cells of this area are the only
neurons of the cortex to give clear pictures of chromatolysis of
their chromophilic substance. In the third place, these giant
cells of the human cortex have been counted, and a count of the
number of fibers in the pyramidal tract shows that the numbers
are in tolerably close agreement (nearly 80,000 on each side of
the body). Finally, a case of sclerotic degeneration involving
almost the entire cortex has been described by Spielmeyer, in
which these giant cells and the fibers of the pyramidal tract alone
escaped injury.

The sensory projection centers of the cortex have also been
determined physiologically, though their limits are less precisely
known than are those of the motor cortex. The olfactory
receptive area has already been mentioned as comprised within
the archipallium (hippocampus and hippocampal gyms, see
p. 217), only a part of which is exposed on the surface of the
brain (the regio hippocampica of Fig. 129; areas 27, 28, 34, 35 of
Fig. 131). The visual projection center, which receives fibers
from the thalamic optic centers in the pulvinar and lateral genic-
ulate body (pp. 165, 212), is in the occipital region (Fig. 129).
Area 17 (Fig. 131) appears to be the chief center for the recep-
tion of these visual projection fibers, though the adjacent area 18
participates in this function, these areas together comprising
the area striata of the cortex (p. 268). The auditory projection
center is in the upper part of the temporal lobe (area 41, and
probably to some extent area 42 also, of Fig. 130). The tactual
projection center lies in the postcentral region (Fig. 128; areas 1,
2, and 3 of Fig. 130). The parts of the cerebral cortex which lie
between the sensory and motor projection centers which have
just been enumerated are the association centers (see pp. 287,
290).

Within each general sensory sphere there is a focal area which
is exclusively receptive in function, such as area 17 (Fig. 131) in

THE FUNCTIONS OF THE CEREBRAL CORTEX 285

the visual sphere. Each of these focal spheres is surrounded by
other areas which receive projection fibers, though in less
abundance, and also numerous association fibers from other
parts of the cortex. These marginal fields are, therefore, to be
regarded as association centers, each of which is under the
dominant physiological influence of the adjacent focal projection
center. These are sometimes called visual psychic, auditory
psychic fields, etc., after the adjacent projection centers; but
these terms are objectionable as implying the old phrenological
notion of localization of specific psychological faculties.

Each sensory projection center which receives afferent fibers
of course sends out association fibers to other parts of the cortex.
Some of these fibers may be very short, reaching only to the
adjacent marginal fields (these are arcuate fibers, see Fig. 121,
f.p.) ; other much longer association fibers may assist in forming
the great associational tracts of the subcortical white matter.
The association centers themselves are likewise connected by
fiber tracts of bewildering complexity, so that every part of the
cerebral cortex is in direct or indirect physiological connection
with every other part. All of these parts are, therefore, able to
influence the motor centers of the precentral gyms, from which
alone voluntary motor impulses can be discharged from the
cortex to the lower motor centers of the brain stem and spinal
cord.

The relations of the tactual and somesthetic sensory projection fibers to
the postcentral and precentral gyri have been variously described, and some
further consideration of the functional connections of these fibers may here
be appropriate. From a large body of anatomical, experimental, and clinical
evidence it was formerly assumed that the cortical motor centers are co-
extensive with those for the general somatic sensory projection systems of
cutaneous and muscular sensibility, the projection centers of both the
sensory and motor fibers related to each region of the body being located
on both the anterior and posterior sides of the central sulcus or fissure of
Rolando, that is, in both the precentral and postcentral gyri. Most of the
diagrams of cortical localization in all but the most recent manuals are
based upon this view of the case. But recent work has shown definitely
that the motor centers are confined to the region in front of this sulcus.
Here only are found the giant pyramidal cells of Betz which give rise to
most of the fibers of the pyramidal tract. It may, therefore, be regarded as
definitely established that motor projection fibers do not arise from the
postcentral gyrus, as formerly supposed.

Sensory projection fibers, however, are known to pass from the general
somatic sensory centers in the ventral and lateral nuclei of the thalamus to

286 INTRODUCTION TO NEUROLOGY

the postcentral gyrus, to the motor cortical centers of the precentral gyrus,
and to other widely separated parts of the cortex. The significance of this
fact is still obscure. That the postcentral gyrus is of different functional
type from the precentral gyrus is shown by the fact that motor projection
fibers arise from the latter and not from the former, by the differences in
anatomical structure of these regions, by a large amount of experimental
and clinical evidence which shows that tactile sensibility is not lost by the
destruction of the precentral motor areas, and finally by direct physiological
experiment upon human subjects.

Dr. Harvey Gushing (1909), in operating upon brain tumors in 2 cases in
which the use of an anesthetic was prohibited by the condition of the patient,
exposed the postcentral gyrus and, with the patient's consent, electrically
stimulated its surface. The patients, who were fully conscious during the
operation, reported distinct cutaneous sensations which were subjectively
localized as if coming from the skin of the hand. There were no motor
responses from this and adjacent parts of the cortex behind the central
sulcus, though in the same cases, upon stimulation of the precentral gyrus,
motor responses were obtained which were accompanied by no sensations
save those which came from the muscles during their contraction. In a
previous similar case Dr. Gushing (1908) obtained typical motor responses
from stimulation (with the patient's consent) of the precentral gyrus in an
operation without anesthesia, and these responses were unaccompanied by
painful sensations.

A very extensive series of experiments involving the stimulation and
extirpation of these cortical areas in apes, dogs, and other animals supports
the conclusion that the postcentral gyrus is the great receptive center for
cutaneous reactions of the general cutaneous system. What may be the
functions of those thalamic fibers which pass to the motor centers in front
of the central fissure is unsettled. Possibly these connections are concerned
in cortical reflexes of the proprioceptive system or acquired automatisms.

The myelinated fibers of the cerebral hemisphere mature,
that is, acquire their myelin sheaths, at various stages in the
development of the brain, some of these systems of fibers appear-
ing before birth and some after birth. Much investigation has
been directed to the determination of the exact facts regarding
the sequence of development of these fibers, and many interest-
ing theories have been developed regarding the significance of
these facts.

Flechsig in a long series of researches made the first thorough study of
this problem, and his conclusions have exerted a profound influence upon
all subsequent theories of the functions of the cerebral cortex. He proposed
a series of laws of developmental sequence (myelogeny) of the cortical fibers,
among which two may be mentioned : (1) The myelinated fiber tracts of the
brain do not all mature at the same time, and fiber systems which are of
like function, that is, which are so connected as to perform special move-
ments in response to excitation, tend to mature at the same time. This is
Flechsig's "fundamental myelogenetic law," which may be stated in this
form, The myelination of the nerve-fibers of the developing brain follows

THE FUNCTIONS OF THE CEREBRAL CORTEX 287

a definite sequence such that the fibers belonging to particular functional
systems mature at the same time. (2) A second law states that in the
'cerebral cortex there are two great functional groups of fibers which mature
at different times. One of these groups contains the projection fibers, which
mature early, chiefly before birth; the other group contains the association
fibers, which mature after birth. These groups are further subdivided into
subsidiary functional systems, each of which connects with a definite region
of the cerebral cortex, so that it is possible to map the cortical areas in ac-
cordance with the sequence of development of the related myelinated fibers.
There are, accordingly, two groups of cortical areas in this scheme: the
projection centers whose fibers mature early and the association centers
whose fibers mature late.

Figures 134 and 135 illustrate the arrangement of these areas, the prim-
ary areas (projection centers) being marked by double cross-hatching and
the association centers by single cross-hatching or unshaded areas. The
numbers printed on the charts indicate the approximate order in which
the corresponding parts acquire their myelinated fibers. It will be noticed
that Flechsig's projection areas do not correspond exactly with those deter-
mined by the physiological method and by the histological study of the adult
cortex (Figs. 130, 131, 132, 133).

On the basis of his studies, Flechsig elaborated a highly speculative
theory of the significance of the association centers, which has been criticized
as a return to the old attempt to localize particular mental functions in
definite cortical areas. These criticisms are not wholly justified; never-
theless it is even yet premature to attempt so detailed an analysis of the
cortical mechanisms of psychic processes as Flechsig has elaborated. His
observations on the facts of myelogeny, moreover, have not been confirmed
by more recent students of the question (Monakow, Vogt, Dejerine, and
others), though it seems to be established that the sensory and motor
projection centers in general acquire myelinated fibers earlier than other
parts of the cerebral cortex. (This entire question is critically reviewed
by Brodmann in Lewandowsky's Handbuch der Neurologic, Band 2, pp.
234-244.) The only conclusion at present possible is that the factors which
operate in determining the sequence of myelination of the nerve-fibers of
the brain are exceedingly complex, and it is impossible from the facts at
present known to formulate the laws of the myelogenetic development of
the brain.

Attention should be called here to the fact that there are many
different kinds of projection fibers, that is, fibers connecting the
cerebral cortex with the underlying structures of the brain stem
and spinal cord. Most of these projection fibers, except those
of the olfactory system, pass through the corona radiata and
internal capsule of the corpus striatum. The most important
of these projection systems are the great sensory radiations which
discharge their nervous impulses into the cortical centers of
vision, hearing, touch, and smell, as already described (the
exact course of the gustatory projection fibers has not been de-
termined), and the great motor system of the pyramidal tract

288

INTRODUCTION TO NEUROLOGY

wo

Fig. 135.

Figs. 134. 135. — Lateral and median views of the human cerebral hemi-
sphere, to illustrate the sequence of maturity of the myelinated fibers of
the cortex during the development of the brain, according to Flechsig's
observations. The numbers indicate approximately the order in which
different parts of the cortex acquire their mature fibers. Areas 1-12
(double cross-hatched) constitute the primordial region, made up chiefly
of the projection centers; these include the olfactory area (1, 3, 4, and 4a),

THE FUNCTIONS OF THE CEREBRAL CORTEX 289

arising from the precentral gyrus. Each of the thalamo-cortical
projection tracts of vision, hearing, and tactile sensibility is,
moreover, accompanied by cortico-thalamic fibers which conduct
in the reverse direction and whose functions are not well known,
and there are other cortico-thalamic and cortico-mesencephalic
systems. The cerebral cortex is in direct connection with the
red nucleus of the cerebral peduncle by a cortico-rubral tract,
arising in the frontal region of the cortex, and by ascending
fibers from the red nucleus to the same general part of the
cerebral hemisphere. From the frontal, parietal, temporal, and
occipital association centers there arise large descending fiber
tracts to the nuclei of the pons (cortico-pontile tracts). These
connections between the cerebral cortex and the red nucleus
and pons put the cerebral cortex and the cerebellum into very
intimate relations, but the exact way in which the cerebrum and
the cerebellum cooperate functionally is obscure (see p. 192).

From the preceding account it is plain that the cerebral cortex
is structurally differently organized in different parts, and that
each of these parts has its own characteristic fiber connections.
Physiological experiment and pathological studies have shown,
moreover, that some of these regions, the projection centers, are
functionally diverse, in that each one receives a particular type
of afferent fibers or discharges efferent impulses into a definite
subcortical motor center. Stated in other words, the cortex is
structurally a mosaic of diverse patterns ; and on the physiolog-
ical side there is a specific localization of function, at least in the
sense that the various systems of afferent and efferent projection
fibers connect each with its particular place in the structural
mosaic.

Several English neurologists, notably Bolton, from studies on the
development and adult structure of the cortex in normal and abnormal men
and in other mammals, have been led to the conclusion that, in addition to
the mosaic localization pattern of which we have been speaking, there is a
functional difference between the different layers of neurons of the cortex

the somesthetic area (2, 26, 2c, and 8), the visual area (7 and probably 76),
and the gustatory area (46 and 6). The remainder of the cortex is made up
of association centers, of which there are two groups, those which mature
soon after birth (lightly shaded areas 13-28), and the terminal areas (un-
shaded areas 28-36) which are the last to mature. (From Lewandowsky's
Handbuch der Neurologic.)

19

290 INTRODUCTION TO NEUROLOGY

in general. Bolton believes that the granular layer (layer IV of Fig. 127)
marks an important boundary between functionally different cortical
mechanisms. The infragranular portion of the cortex is thought to be
concerned especially with the performance of the simpler sensori-motor
reactions, particularly those of the instinctive type, while the supragranular
layers serve the higher associations manifested by the capacity to learn by
individual experience and to develop the intellectual life.

The infragranular layers mature earlier in the development of the brain,
and they are the last to suffer degeneration in the destruction of cortical
cells in the acute dementias or insanities. The supragranular layers
(notably the pyramidal neurons of Brodmann's third layer, Fig. 127) ma-
ture later than any other layers. They are thinner in lower animals and in
feeble-minded and imbecile men than in the normal man, and they are the
first to show degenerative changes in dementia.

This doctrine is controverted by some other neurologists, but the evi-
dence seems to show that the supragranular pyramidal neurons are physio-
logically the most important elements in the higher associative processes of
the cortex. In this connection it is significant that the granular and infra-
granular layers are thicker in the projection centers, while in the association
centers the supragranular layers of pyramidal cells are thicker. But all of
the layers in each region are very intimately related, the processes of
most of the cells of the deeper layers extending throughout the thickness of
the more superficial layers (see Figs. 123, 124, 125) to reach the most super-
ficial layer, and in the present state of our knowledge a functional differ-
ence between the layers cannot be said to have been, established, save in
very general terms.

It must be borne in mind that the most significant parts of
the human cerebral cortex are the association centers. These
alone are greatly enlarged in the human brain as compared with
those of the higher apes. In the latter animals the projection
centers are fully as large as those of man, the much smaller
brain weight being chiefly due to the relatively poor develop-
ment of the association centers.

The data which we have summarized in the preceding pages
have led to the most contradictory theories as to the exact
mode of functioning of the association centers. Neurologists
have been prone, even up to the present time, to fall into the
error of attempting to find specific centers for particular mental
functions or faculties. But the evidence at present available
gives small promise of success in the search for such centers.
It is, in fact, theoretically improbable that such discoveries will
ever be made, for psychology today recognizes no such mosaic
of discrete mental faculties as would be implied in such a doc-
trine.

The facts of cerebral localization as clinically and experi-

291

mentally demonstrated, in themselves and aside from any philo-
sophic theories based upon them, contribute no evidence what-
ever to a solution of the problem of a seat of consciousness or of
particular mental "faculties." That the proper functioning of
a given locus in the cortex is essential to the execution of a
given motion or the experience of a given sensation by no means
necessarily implies that the consciousness of the act is located
there. The latter is an entirely independent problem which
must be separately investigated. It is not, then, the facts of
cerebral localization which can be called in question so much as
the interpretation of these facts.

The search for a single seat of consciousness, such as psychol-
ogists and philosophers have so long sought, is vain. The higher
mental processes undoubtedly require the activity of associa-
tion centers of the cerebral cortex, and the integrity of the
associational mechanism as a whole is essential for their full
efficiency. The cerebral cortex differs from the reflex centers of
the brain stem chiefly in that all of its parts are interconnected
by inconceivably complex systems of associational connections,
many of which are probably acquired late in life under the influ-
ence of individual experience, and any combination of which
may, under appropriate conditions of external excitation and
internal physiological state, become involved in any cerebral
process whatever.

Nevertheless, some of these cortical association paths are
structurally more highly elaborated than others (Fig. 121, p.
267, illustrates the most distinct of these tracts), and certain
combinations of cortical functions are, therefore, more likely to
follow a given stimulus than others. This associational pattern
is doubtless partly innate and partly acquired. That there is a
fairly precise anatomical pattern of association tracts can be seen
in any good dissection of the cerebral hemisphere, and that the
elements of this pattern are related in definite functional systems
which are spatially separate is shown by numberless clinical ob-
servations in which sharply circumscribed mental defects are
found to be associated with definite cerebral lesions. The
phenomena of aphasia give the clearest illustrations of these
relations.

The term aphasia has commonly been applied to a variety of

292 INTRODUCTION TO NEUROLOGY

speech defects, but Hughlings Jackson extended the connotation
of the word to include "a loss or defect in symbolizing relations
of things in any way." The lesion which produces the defect
affects the association centers rather than the projection centers,
for there is no primary sensory defect- — no blindness or deafness
or loss of general sensation — nor is there any motor paralysis.
The problems connected with aphasia are very difficult and
confused, and there is by no means general agreement on either
the fundamental physiological mechanisms involved in speech or
on the nature of the lesions which produce the various types of
observed speech defects. The enormous literature relating to
this subject cannot be summarized here; see the text-books of
physiology, physiological psychology, and clinical neurology.

Lesions of the primary sensory or motor projection centers will not pro-
duce aphasia, for in these cases all sensations or all movements related to
the injured parts are lost, whereas in aphasia only the correlations involved
in speech or other associational processes are impaired and all other sensori-
motor correlations may be intact. Of course, the number of associational
pathways involved in the communicating of ideas by hearing, reading,
speaking, and writing words is very large; and the character of the speech
defect will depend in part upon the particular associational tracts affected
by the lesion and in part upon the effect of the lesion upon the general in-
telligence of the patient (diaschisis effect, see p. 293). The second factor
seems to be exceedingly variable and has given rise to much controversy.

Distinctive names have been given to the more important types of speech
defect as clinically observed ; such as agraphia or inability to write correctly,
aphemia or inability to utter words, word-blindness (alexia) or inability to
comprehend written words, word-deafness or inability to comprehend
spoken words, and many others. Evidently an aphasia may result from
injury to (1) a sensory association area contiguous to the primary visual or
auditory projection centers (sensory types of aphasia), or (2) to a motor
association center contiguous to the motor projection centers for the speech
muscles (motor types), or (3) to any of the associational tracts connecting
these association centers.

The second, or motor, type of aphasia usually, though not invariably,
results from injury to the posterior part of the inferior frontal gyrus (see
Fig. 54, p. 121) of the left hemisphere in right-handed persons and of the
right hemisphere in left-handed persons. This relation was first discovered
by Broca, and the area of motor speech correlations (marked "motor speech "
in Fig. 133, p. 283) has since been termed Broca's convolution.

It should be reiterated that Broca's convolution does not lie in the excit-
able motor zone of the cortex. Though the destruction of this area may be
followed by defects of speech, the muscles of the larynx, tongue, lips, etc.,
involved in vocalization are not paralyzed. This case is typical of many
other motor association centers of the cortex whose integrity is essential for
specific motor combinations, though separate motor centers are present for
all of the muscles involved in these movements.

THE FUNCTIONS OF THE CEREBRAL CORTEX 293

The correlations involved in the motor functions of speech appear to be
represented typically in only one hemisphere, though this is by no means
rigidly true. The corresponding structures in the other hemisphere may
cooperate in these functions normally, and after loss of speech from a uni-
lateral lesion speech may be reacquired by further education of the unin-
jured centers of the same or the opposite side. It has recently been shown
that Broca's convolution is often larger on the left side of the brain than on
the right side and that the average thickness of the cortex in this region is
greater on the left side.

Various attempts have been made to localize each of the various types of
aphasia mentioned above in a specific part of the cortex, but with no con-
cordant results. Each of these functions is, of course, very complex, and a
small circumscribed cortical injury may disturb or temporarily abolish the
entire complex by the destruction of one only of the component functional
connections. (See the summary by Dr. A. Meyer, 1910.)

The general conclusion to be drawn from the entire series of
physiological and pathological studies of the cortex is that spe-
cific mental entities are not resident in particular cortical areas,
but that cortical functions involve the discharge of nervous en-
ergy from one or more sensory centers to various near and remote
regions, each of which, in turn, may serve as a point of departure
for new nervous discharges, and so on until the complexity of
action and interaction of part upon part becomes too intricate
for the mind to conceive. The resultant effect of all of these
nervous activities which reverberate from one association center
to another will be the establishment by a process of which we are
still in ignorance of an equilibrium, usually by means of a motor
discharge of some precise form from the cortex through the
pyramidal tract.

This dynamic view of cortical function finds a further illustra-
tion in the realm of neuro-pathology in von Monakow's doctrine
of diaschisis. The onset of cerebral hemorrhage or any other
sudden injury to the cerebral cortex is usually marked by an
apoplectic "stroke," with profound shock and usually loss of
consciousness. The entire cortical equilibrium is disturbed and
this effect irradiates very widely throughout the nervous system.
If the injury is not too severe, there is soon a partial readjust-
ment of the nervous equilibrium and consciousness returns. But
the restoration is incomplete, for some of the normal factors
in the dynamic equilibrium complex are lacking by reason of the
destruction of the corresponding cortical areas or association
tracts. The intelligence is enfeebled and all voluntary control is

294 INTRODUCTION TO NEUROLOGY

impaired. In the course of a few weeks or months a new equi-
librium minus the lacking factors is established and the patient
very rapidly improves. Ultimately complete recovery may
occur, save for a permanent residual defect which results directly
from the loss of the tissue destroyed.

The immediate shock-like interference with the activity of
cerebral centers not directly affected by the lesion is what von
Monakow means by diaschisis. Upon the restoration of the
nervous equilibrium this transient diaschisis effect is wholly or
partially lost, and the residual symptoms of defect give a fairly
accurate picture of the intrinsic functions of the center directly
attacked by the lesion. It is commonly assumed that there
is also during the process of gradual recovery from such a corti-
cal injury a certain capacity for the compensatory development
of other centers of the same or the opposite cerebral hemisphere,
so that they learn to perform vicariously the functions of the
lost part.

All functions of the nervous system are facilitated by repeti-
tion, and many such repetitions lead to an enduring change in
the mode of response to stimulation which may be called physio-
logical habit. This implies that the performance of every reac-
tion leaves some sort of a residual change in the structure of the
neuron systems involved. These acquired modifications of
behavior are manifested in some degree by all organisms (see
pp. 22, 31), and this capacity lies at the basis of all associative
memory (whether consciously or unconsciously performed) and
the capacity of learning by experience. This modifiability
through individual experience is possessed by the cerebral cortex
in higher degree than by any other part of the nervous system ;
and the capacity for reacting to stimuli in terms of past experi-
ence as well as of the present situation lies at the basis of that
docility and intelligent adaptation of means to ends which are
characteristic of the higher mammals. It is a fact of common
observation that those animals which possess the capacity for in-
telligent adjustments of this sort have larger association centers
in the cerebral cortex than do other species whose behavior is
controlled by more simple reflex and instinctive factors, that is,
by inherited as contrasted with individually acquired organiza-
tion. This is brought out with especial distinctness by a com-

THE FUNCTIONS OF THE CEREBRAL CORTEX 295

parison of the brains of the higher apes with that of man (Figs.
132, 133), and of the lower races of men as contrasted with the
higher. In our own mental life we recognize the persistence of
traces of previous experience subjectively as memory, and mem-
ory lies at the basis of all human culture. From this it follows
that psychological memory is probably a function of the associa-
tion centers; but it must not be assumed that specific memories
reside in particular cortical areas, much less that they are pre-
served as structural traces left in individual cortical cells, as has
sometimes been done.1

The simplest concrete memory that can appear in conscious-
ness is a very complex process, and probably involves the activity
of an extensive system of association centers and tracts. That
which persists in the cerebral cortex between the initial experi-
ence and the recollection of it is, therefore, in all probability a
change in the interneuronic resistance such as to alter the
physiological equilibrium of the component neurons of some
particular associational system. What the nature of this change
may be is unknown, but it is conceivable that it might take the
form of a permanent modification of the synapses between the
neurons which were functionally active during the initial experi-
ence such as to facilitate the active participation of the same
neurons in the same physiological pattern during the reproduc-
tion.

That which we know subjectively as the association of ideas
may, in a somewhat similar way, be pictured as involving neuro-
logically the discharge of nervous energy in the cortex between
two systems of neurons which have in some previous experience
been physiologically united in some cortical reaction. If, for
instance, I heard a song of a mocking bird for the first time last
year while walking in a rose garden, upon revisiting the gar-
den I may recall the song of the bird. Here the sight of the
garden (a highly complex apperceptive process involving many
association tracts) actuates neuron system number one domi-
nated by present visual afferent impulses, and the association

1 These residua of past cerebral activities form the basis of those char-
acteristic "brain dispositions" which are important factors in each person-
ality. They have been termed "engrams" by Semonand "neurograms" by
Morton Prince (see Prince, The Unconscious, Chapter V, New York, 1914).

296 INTRODUCTION TO NEUROLOGY

tract leading to neuron system number two (the auditory com-
plex established last year when the song was heard) has a lowered
physiological resistance by virtue of the previous collocation
with system number one, and I remember the song (see p. 64) .
It should be emphasized that the mechanism of association
here suggested is purely theoretical; we have no scientific evi-
dence regarding the details of such physiological processes. But
it can be confidently asserted that even the simplest associational
processes are at least as complex as this, and may involve the
participation of thousands of neurons in widely separate parts of
the cortex; and the consciousness must be regarded as a function
of the entire process, not of any detached center (cf. p. 66).

In summarizing this dynamic conception of the nature of consciousness I
will quote a few sentences from my brother's writings (see C. L. Herrick,
1910, pp. 13, 14):

"The theory of consciousness which seems best to conform to the condi-
tions of brain structure and its observed unity is that each conscious state is
an expression of the total equilibrium of the conscious mechanism, and that
intercurrent stimuli are continually shifting the equilibrium from one to
another class of activities. In other words, the sensation accompanying
a given color presentation is not due to the vibrations in the visual center
in the occipital lobe, but to the state of cortical equilibrium or the equation
of cortical excitement when that color stimulus predominates. Previous
vestigeal excitements and coordinations [associations, c. J. H., see p. 35]
with the data from other cortical centers all enter into the conscious pres-
entation. As the wave of excitation passes from the visual center to other
parts, the proportional participation of other centers increases, producing a
composite containing more distantly related elements."

"Every specific sense-content with its escort of reflexly produced associ-
ated elements causes a more or less profound disturbance of the psychical
equilibrium, and the nature of this disturbance depends not only on the
intensity and state of concentration, but very largely on the kind of equi-
librium, already existing. . . . The character of the conscious act (and
the elements of consciousness are always acts) will, of course, depend upon
the extent to which the several factors in the associational system partici-
pate in the equilibrium. Each disturbance of the equilibrium spreads from
the point of impact in such a way that progressively more of the possible
reflex currents enter the complex, thus producing the extension from mere
sensation to the higher processes of apperceptive association. A conscious
act is always a fluctuation of equilibrium, so that all cognitive elements
are awakened in response to changes rather than invariable or monotonous
stimuli."

The dynamic view of consciousness here adopted makes such
expressions as "the unconscious mind" impossible contradic-
tions. Either the mental functions are in process or they are

THE FUNCTIONS OF THE CEREBRAL CORTEX 297

not, and unconscious cerebration is not consciousness. This is,
of course, not incompatible with a dissociation of consciousness
into multiple or co-conscious units, as Dr. Morton Prince so
forcibly illustrates (The Unconscious, p. 249), though how
far in normal men this dissociation may be carried is an open
question.

In my life as viewed by an outside observer there is continu-
ity of process, but not necessarily continuity of consciousness.
In my own experience consciousness appears to be continuous, of
course, because the periods of unconsciousness (as in coma, deep
sleep, etc.) do not appear in consciousness; that is, they do not
exist for me except as I learn of them by an indirection. In a
water mill the function of grinding corn may go on intermittently,
though the mechanism is there all the time and the energy is
there; but if the water passes from the mill race out over the dam
instead of through the water wheel the grinding function ceases.
While the mill is at rest changes may be made in the machinery
which will modify the character of the grinding when it is re-
sumed, but these changes are not grinding. So in the brain the
mechanism of consciousness and the structural memory vestiges
of past experience may be present continuously; indeed, these
vestigeal traces may be linked up in new ways by intercurrent
physiological processes. But these things do not constitute con-
sciousness. In fact, a large amount of unconscious cerebration
may go on, the end result of which alone becomes conscious.
The aim of physiological psychology is to clarify not only the
mechanism of consciousness, but also all of the antecedent and
subsequent physiological processes which are, from the stand-
point of an outside observer, demonstrably related to the con-
scious processes. It is possible, moreover, to develop a really
scientific introspective psychology in which abstraction is made
from all of these mechanisms and the individual experiences
alone are studied as given in consciousness. This makes up a
large part of general psychology.

Summary. — The functions of the cerebral cortex are still
largely wrapped in mystery, but the evidence thus far accumu-
lated suggests that these functions are, so far as physiologically
known, not different in kind from those of the other parts of the
brain. It is, however, manifest that these functions are con-

298 INTRODUCTION TO NEUROLOGY

cerned with the individually acquired and especially the intelli-
gently performed activities as distinguished from the fundamen-
tal reflex and instinctive processes whose mechanisms are innate.
There is a specific localization of function in the cerebral cortex,
in the sense that particular systems of sensory projection fibers
terminate in special regions (the sensory projection centers),
that from other special regions (the motor projection centers)
particular systems of efferent fibers arise for connection with the
lower motor centers related to groups of muscles concerned with
the bodily movements, and that between these projection centers
there are association centers, each of which has fibrous connec-
tions of a more or less definite pattern with all other parts of the
cortex. The destruction of any part of the cortex or of the
fiber tracts connected therewith involves, first, a permanent loss
of the particular functions served by the neurons affected, and,
in the second place, a transitory disturbance of the cortical
equilibrium as a whole (diaschisis effect). Specific mental acts
or faculties are not resident in particular cortical areas, but all
conscious processes probably require the discharge of nervous
energy throughout extensive regions of the cortex, and the char-
acter of the consciousness will depend in each case upon the
dynamic pattern of this discharge and the sequence of function of
its component systems. This pattern is inconceivably complex
and only the grosser features are at present open to observation
by experiment and pathological studies.

No cortical area can properly be described as the exclusive
center of a particular function. Such "centers" are merely
nodal points in an exceedingly complex system of neurons which
must act as a whole in order to perform any function whatsoever.
Their relation to cerebral functions is analogous to that of the
railway stations of a big city to traffic, each drawing from the
whole city its appropriate share of passengers and freight; and
their great clinical value grows out of just this segregation of
fibers of like functional systems in a narrow space, and not to any
mysterious power of generating psychic or any other special
forces of their own.

The essence of cortical function is correlation, and a cortical
center for the performance of a particular function is a physio-
logical absurdity, save in the restricted sense described above, as

THE FUNCTIONS OF THE CEREBRAL CORTEX 299

a nodal point in a very complex system of associated conduction
paths. Those reflexes whose simple functions can be localized
in a single center have their mechanisms abundantly provided for
in the brain stem. The resting brain is probably normally
during life in a state of neural tension in more or less stable
equilibrium. An effective stimulus disturbs this equilibrium
and the precise effect will depend upon variable synaptic resist-
ance or neuron thresholds which change with different functional
states of the organism as a whole and of the brain in particular.
If this activity involves the cerebral cortex of a human brain, it
may be a conscious activity, the kind of consciousness depending
on the kind of discharge. But the consciousness must not be
thought of as localized in any cortical area. The discharge in
question may reverberate to the extreme limits of the nervous
system and the peripheral activities may be as essential in deter-
mining the conscious content as the cortical.

LITERATURE

VON BECHTEREW, W. 1911. Die Funktionen der Nervencentra, vol.
iii, Jena.

BURNETT, T. C. 1912. Some Observations on Decerebrate Frogs, with
Special Reference to the Formation of Associations, Amer. Jour. Physiol.,
vol. xxx, pp. 80-87.

GUSHING, H. 1908. Removal of a Subcortical Cystic Tumor at a Second-
stage Operation Without Anesthesia, Jour. Amer. Med. Assoc., 1908, vol. i,
p. 847.

— . 1909. A Note upon the Faradic Stimulation of the Postcentral Gyrus
in Conscious Patients, Brain, vol. xxxii, pp. 44-54.

EDINGER, L. 1893. The Significance of the Cortex Considered in Con-
nection with a Report Upon a Dog from which the Whole Cerebrum had
been Removed by Professor Goltz, Jour. Comp. Neurol., vol. iii, pp. 69-77.

— . 1908. The Relations of Comparative Anatomy to Comparative
Psychology, Jour. Comp. Neurol., vol. xviii, pp. 437-457.

EDINGER. L., and FISCHER, B. 1913. Bin Mensch ohne Grosshirn, Arch,
f. ges. Physiol., Bd. 152, pp. 1-27.

FLECHSIG, P. 1896. Gehirn und Seele, Leipzig.

— . 1896. Die Lokalisation der geistigen Vorgange, Leipzig.

FRANZ, S. I. 1915. Variations in Distribution of the Motor Centers,
Psychological Monographs, Princeton, N. J., vol. xix, No. 1, pp. 80-162.

FRITSCH, G., and HITZIG, E. 1870. Ueber die elektrische Erregbarkeit
des Grosshirns, Arch. f. Anat., Physiol. u. Wissen. Med., p. 300.

GALL and SPURZHEIM. 1810-19. Anatomic et Physiologic du Systeme
Nerveux, Paris.

GOLTZ, F. 1869. Beitrage zur Lehre von den Functionen der Nerven-
centren des Frosches, Berlin.

300 INTRODUCTION TO NEUROLOGY

GOLTZ, F. 1892. Der Hund ohne Grosshirn, Arch. f. ges. Physiol., Bd.
51, p. 570.

GRUNBAUM, A. S. F., and SHERRINGTON, C. S. 1903. Observations on the
Physiology of the Cerebral Cortex of the Anthropoid Apes, Proc. Roy. Soc.,
vol. Ixxii, p. 152.

HEAD, H., and HOLMES, G. 1911. Sensory Disturbances from Cerebral
Lesions, Brain, vol. xxxiv, pp. 109-254.

HERRICK, C. L. 1910. The Equilibrium Theory of Consciousness, in
The Metaphysics of a Naturalist, Bui. Sci. Lab. Denison University, vol.
xv, pp. 12-22.

HITZIG, E. 1904. Physiologische und klinische Untersuchungen iiber
das Gehirn, Berlin.

HOLMES, G. W. 1901. The Nervous System of the Dog Without a
Forebrain, Jour. Physiol., vol. xxvii.

LEWANDOWSKY, M. 1907. Die Funktionen des zentralen Nervensys-
tems, Jena.

MARIE, P. 1906. Revision de la Question de 1'Aphasie, Semaine M6d-
icale, 23 May.

MEYER, A. 1910. The Present Status of Aphasia and Apraxia, The
Harvey Lectures for 1909-10, New York, pp. 228-250.

VON MONAKOW, C. 1909. Neue Gesichtspunkte in der Frage nach der
Lokalisation im Grosshirn, Zeits. f. Psychologic, Bd. 54, pp. 161-182.

— . 1910. Aufbau und Lokalisation der Bewegungen beim Menschen.
Arbeiten a. d. hirnanatom, Institut in Zurich, Bd. 5, pp. 1-37 ; also in Bericht
tiber den IV Kongress f. exp. Psychologic in Innsbruck, 1910.

— . 1913. Theoretische Betrachtungen liber die Lokalisation in Zentral-
nervensystem, insbesondere im Grosshirn, Ergebnisse der Physiol., Bd. 13,
pp. 206-278.

— . 1914. Die Lokalisation im Grosshirn. Gegenwilrtiger Stand der
Frage der Lokalisation in der Grosshirnrinde, Wiesbaden.

MUNK, H. 1890. Ueber die Funktionen der Grosshirnrinde. Gesam-
melte Abhandl., 2d ed., Berlin.

— . 1902. Zur Physiologic der Grosshirnrinde, Arch. f. Physiol., 1902.

PRINCE, M. 1914. The Unconscious, New York.

CHAPTER XXI

THE EVOLUTION AND SIGNIFICANCE OF THE CERE-
BRAL CORTEX

AT the conclusion of our analysis of the structure and func-
tions of the nervous system it will be of interest to review very
briefly a few topics of a more general sort related to our theme,
with special reference to the significance of the cerebral cortex
in the general scheme of human evolution and culture.

For the purpose of our analysis animal activities may be
classified under three heads (see p. 31): (1) Innate functions of
invariable or stereotyped character developed through natural
selection or other biological processes, whose mechanism is hered-
itary and common (with small differences only) to all members of
a race or species, typified by reflex action and purely instinctive
action; (2) variable and modifiable functions, whose pattern is
determined by individual experience through which the innate
action system is more or less permanently altered, intelligent
acts and the reasoning process representing the highest forms of
this type, though the lower members of this series are not neces-
sarily consciously performed; (3) acquired automatisms, or
individually acquired actions which have become so thoroughly
habitual as to be performed quite as mechanically as the heredi-
tary reflexes. Intelligently acquired actions which have finally
come to be automatically and even unconsciously performed are
sometimes designated "lapsed intelligence," but such lapsed
intelligence must be a purely individual acquisition. There
is no evidence that automatisms of this sort can be transmitted in
heredity, and, therefore, they can play no part directly in the
evolution of instincts, as some have taught.

The first and second of the types of action above distinguished
appear to be common to all organisms, though their relative im-
portance varies enormously from species to species. The first
type includes the reflexes and all of the pure instinct-actions,

301

302 INTRODUCTION TO NEUROLOGY

that is, the hereditary component of the commonly recognized
instincts (p. 61). There is no clear boundary between reflexes
and instinct-actions as just denned. These actions may be
exceedingly complex and their neuro-muscular mechanisms may
be complicated apparently without limit. The available evi-
dence suggests that they are always unconsciously performed.

Most of our common activities include all three of these types
of behavior in varying proportions, and accordingly they fre-
quently have not been distinguished. The first and third types
are especially liable to confusion, for both are manifested as
stereotyped, non-intelligent behavior. They can sometimes be
separated only by a study of their origins; nevertheless this dis-
tinction is of great importance, especially to educators.

The nervous organs of the invariable reactions are fairly well
known and are characterized in their more highly elaborated
forms by a closely knit system of nerve-centers and distinct con-
necting fiber tracts so organized that particular stimuli may call
forth a response or a combination of several responses selected
from a fixed number of possible actions. The range of possible
reactions of any given functional system of this type is limited
by the structural complexity of the nerve-centers involved.
This complexity may be very great, with a correspondingly great
number of movements necessary to complete the reaction, and it
may include the capacity for discriminating between two or more
structurally possible modes of response by means of variable
internal functional states of the nerve-centers. But in all of
these cases the response is finally determined within rather nar-
row limits by the nature of the stimuli and the innate structural
organization not only of the nervous organs, but of the body as a
whole.

In some cases an elaborate nervous reflex or instinctive act
may involve a more extensive nervous apparatus than is required
by an intelligent act. It is not a mere question of the size of the
nervous mechanisms involved. For instance, a comparison of
the brains of the two species of fishes shown in Fig. 136 shows
that in the medulla oblongata of these rather closely related
species there is an astonishing difference between the size of
certain reflex centers. The greater size of the medulla oblon-
gata of Carpiodes over that of Hyodon is due almost entirely to

EVOLUTION AND SIGNIFICANCE OF CEREBRAL CORTEX 303

the enlargement of the centers for taste,1 and these reflex centers
xare found to be very complex. The enormous increase in the
mass and complexity of arrangement of the gustatory neurons in
Carpiodes does not imply any higher organization from the
standpoint of range of behavior (see p. 19) than in Hyodon.
The apparatus is more efficient as a means of sorting out food
particles from mud, but we do not rank this form of activity
very high in our scale of behavior.

medulla
oblongata-'

Fig. 136. — Illustrations of the brains of two rather closely allied species
of fishes showing very different development of the reflex centers of the
medulla oblongata: (1) Hyodon tergisus, the moon-eye, (2) Carpiodes tumi-
dus, a carp-like species. (After C. L. Herrick.)

In general, in the execution of a complicated reflex many inter-
connected nerve-centers are so arranged that they discharge
into a common final path or an integrated series of such coordi-
nated paths. The movements involved in the act, if performed
at all, must follow in a definite sequence which is structurally

1 For an analysis of this gustatory apparatus in fishes, see HERRICK, C.
JTJDSON. The Central Gustatory Paths in the Brains of Bony Fishes,
Jour. Comp. Neurol., vol. xv, 1905, pp. 375-456.

304 INTRODUCTION TO NEUROLOGY

predetermined in the inborn organization of the nerve-centers
concerned. In the variable type of response, on the other hand,
the association centers involved are so arranged that many final
paths leading to different systems of coordinated motor centers
diverge from a single center of correlation. Which of these paths
will be taken in a given reaction, that is, which of several possible
different (or even antagonistic) movements will result, will be
determined by variable physiological factors of internal resist-
ance within the correlating system (fatigue, habit, the influence
of memory vestiges, etc.) ; accordingly, the response is not pre-
determined by the inborn organization of the apparatus.

Definite, well-established reflexes generally follow distinct
nervous pathways between sharply limited nerve-centers. Be-
tween these centers there is usually found, in addition to the
well insulated tracts just mentioned, a more diffuse and loosely
organized entanglement of nerve-cells and fibers, through which
nervous impulses may be more slowly transmitted in any direc-
tion. Tissue of this character is found throughout the entire
length of the central nervous system, and in some places it occu-
pies extensive regions (especially in the medulla oblongata and
upper part of the spinal cord) which are termed the reticular for-
mation (see pp. 65, 127, 158).

The reticular formation is the parent tissue out of which the
higher correlation centers have been differentiated. In the
spinal cord and medulla oblongata, where its character is most
clearly seen, it receives fibers from all of the sensory centers
and may discharge motor impulses into efferent centers of con-
tiguous or very remote regions. In the higher parts of the brain
the elaborate association centers of the thalamus and cerebral
hemispheres have been developed from such a primitive matrix,
and these centers are interconnected by similar undifferentiated
nervous tissue.

The details of the functional connections of the reflex centers
of the brain stem are much more precisely known than are those
of the higher correlation centers of the thalamus and cerebral
cortex. And, in fact, it is essential that these details be fairly
well understood before the functions of the higher centers can
be investigated; for all nervous impulses which reach these higher
centers must first pass through the lower centers and there be

EVOLUTION AND SIGNIFICANCE OF CEREBRAL CORTEX 305

combined into reflex systems or otherwise correlated. The
afferent stimuli which reach the cerebral cortex are not crude
sensory impressions, but purposeful reflex combinations, often
including sensory data from several different sense organs.

The nerve-centers of the spinal cord and brain stem in general
are of this more rigid type, the internal adjustments of the sys-
tem being, for the most part, as mechanically determined as are
those of an automatic telephone exchange. The cerebellum is
the highest member of this series, exerting a regulatory and re-
inforcing influence upon all of the other members. Nevertheless
the cerebellum adds no new types of reaction or combinations of
reactions to those of the brain stem; its cortex shows little de-
monstrable localization of different functions, and its efferent
tracts are physiologically related to a limited number of pre-
established systems of motor coordination in the brain stem and
spinal cord. In all of these respects the contrast between the
cerebellar cortex and the cerebral cortex is very striking.

The variable or individually modifiable type of reaction is
served chiefly by the cerebral cortex and its immediate depend-
encies, though some capacity of this sort is found in the brain
stem, as shown by the behavior of lower vertebrates which lack
the cerebral cortex. This type of reaction is genetically related
with that modifiability arising from variable internal physio-
logical states which we have mentioned as present in the reflex
centers. There is no proof that the simpler forms of this indi-
vidually modifiable behavior are conscious, though the higher
forms are certainly so.

The cerebral cortex can in no case act independently of the
reflex centers of the brain stem, but always through the agency
of these centers. It is superposed upon them much as the cere-
bellum is, though the control exerted is of a very different type.
Here there is a very elaborate regional differentiation of the
cortex with an infinite complexity of associational connections.
The efferent pathways, moreover, are not physiologically homo-
geneous; but they are so diversified that any possible combina-
tion of the organs of response may be effected by associations
within the cortex. The various afferent functional systems enter
sharply circumscribed cortical areas (the sensory projection
centers); and the efferent fibers likewise leave the cortex from
20

306 INTRODUCTION TO NEUROLOGY

functionally defined motor areas, each group of muscles which
cooperate in definite reaction complexes (termed synergic
muscles, see p. 35) being excited from a definite part of the motor
cortical iield, whose motor tract is anatomically distinct through-
out its entire further course from the cortex to the periphery.
Between the sensory projection centers and the motor areas are
interpolated the association centers, and these are so arranged
that all correlation, integration, and assimilation of present
sensory impulses with memory vestiges of past reactions are
completed, and the nature of the response to be made is deter-
mined before the resultant nervous impulses are discharged into
the motor centers. Only such of the motor areas will be excited
to function as are necessary for evoking the particular reaction
which is the appropriate (that is, adaptive) response to the total
situation in which the body finds itself. This arrangement of
association centers in relation to a series of distinct motor areas
provides the flexibility necessary for complex delayed reactions
whose character is not predetermined by the nature of the con-
genital pattern of the nervous connections.1

The thalamus, as we have seen (p. 163), has its own intrinsic system
of association centers which discharge downward into the cerebral pedun-
cles, and this is the primary reflex apparatus of this part of the brain The
thalamo-cortical connections arose to prominence later in the evolutionary
history, though feeble rudiments of these are present in lower brains.
Parallel with the enlargement of these cortical connections a special part of
the thalamus was set apart for them, and from the Amphibia upward in the
animal scale this dorsal part of the thalamus assumed increasingly greater
importance. This part is termed by Edinger the neothalamus, and makes
up by far the larger part of the thalamus in the human and all other mam-
malian brains. It occupies the dorso-lateral part of the thalamus proper and
comprises most of the great thalamic nuclei (lateral and ventral nuclei,
pulvinar and lateral and medial geniculate bodies). The primitive in-
trinsic reflex thalamic apparatus in man is a relatively unimportant area
of medial gray matter and the subthalamic region (corpus Luysii, lattice
nucleus, etc., not to be confused with the hypothalamus which lies farther
down in the tuber cinereum and mammillary bodies).

The neothalamus, accordingly, serves as a sort of vestibule to the cortex,
every afferent impulse from the sensory centers (except the olfactory sys-
tem) being here interrupted by a synapse and opportunity offered for a wide
range of subcortical associations. The olfactory cortex (hippocampal for-
mation) has a similar relation to subcortical correlation centers in the olfac-
tory area in the anterior perforated space, septum, etc.

l/The paragraphs which follow (pp. 306-311) are reproduced with slight
modification from The Journal of Animal Behavior, vol. iii, 1913, pp. 228-
236.

EVOLUTION AND SIGNIFICANCE OF CEREBRAL CORTEX 307

From these anatomical considerations it follows that no simple sensory
impulse can, under ordinary circumstances, reach the cerebral cortex
without first being influenced by subcortical correlation centers, within
which complex reflex combinations may be effected and various automatisms
set off in accordance with then; preformed structure. These subcortical
systems are to some extent modifiable by racial and individual experience,
but their reactions are chiefly of the invariable or stereotyped character,
with a relatively limited range of possible reaction types for any given
stimulus complex.

It is shown by the lower vertebrates which lack the cerebral cortex that
these subcortical mechanisms are adequate for all of the ordinary simple
processes of life, including some degree of associative memory. But here,
when emergencies arise which involve situations too complex to be resolved
by these mechanisms, the animal will pay the inevitable penalty of failure —
perhaps the loss of his dinner, or even of his life.

In the higher mammals with well-developed cortex the automatisms and
simple associations are likewise performed in the main by the subcortical
apparatus, but the inadequacy of this apparatus in any particular situation
presents not the certainty of failure, but rather a dilemma. The rapid
preformed reflex mechanisms fail to give relief, or perhaps the situation pre-
sents so many complex sensory excitations as to cause mutual interference
and inhibition of all reaction. There is a stasis in the subcortical centers.
Meanwhile the higher neural resistance of the cortical pathways has been
overcome by summation of stimuli and the cortex is excited to function.
Here is a mechanism adapted, not for a limited number of predetermined
and immediate responses, but for a much greater range of combination of
the afferent impressions with each other and with memory vestiges of pre-
vious reactions and a much larger range of possible modes of response to any
given set of afferent impressions. By a process of trial and error, perhaps,
the elements necessary to effect the adaptive response may be assembled
and the problem solved.

It is evident here that the physiological factors in the dilemma or problem
as this is presented to the cortex are by no means simple sensory impres-
sions, but definitely organized systems of neural discharge, each of which is a
physiological resultant of the reflexes, automatisms, impulses, and inhibi-
tions characteristic of its appropriate subcortical centers. The precise form
which these subcortical combinations will assume in response to any par-
ticular excitation is in large measure determined by the structural connec-
tions of these centers inter se. And the pattern of these connections is
tolerably uniform for all members of any animal race or species. This
implies that it is hereditary and innate. This is the underlying basis of
instinct.

The connections between the cortical centers, on the other hand, are
much less definitely laid down in the hereditary pattern. The details of
the definitive association pattern of any individual are to a greater degree
fixed by his particular experience. This is the basis of docility and the
individually modifiable or intelligent types of behavior. The typical cor-
tical activities, even when physiologically considered, are far removed indeed
from those of the brain stem.

It should be emphasized, however, that the differences between the cor-
tex and the lower centers of the brain stem, so far as these can be deduced
from a study of structure and from physiological experiment, are relative and
not absolute. Indeed, the general pattern of the regional localization of the

308 INTRODUCTION TO NEUROLOGY

cortex itself is innate, and in adult life the cortex has acquired many more
characteristics similar to those of the brain stem, with its own systems of
acquired automatisms and habitually fixed types of response. The larger
association centers retain their plasticity longest, but ultimately these also
cease to exhibit new types of correlation, and this marks the onset of
senility.

The relations of the cerebral cortex to the cerebellar cortex and the
brain stem have been compared (p. 192) to those of an enlarged judicial
branch of the central government charged with the duty of interpreting the
decrees of the lower legislative centers and dominating the administrative
machinery, and with the additional power of shaping the general policy of
the government.

Dewey's stimulating analysis1 of the reflex arc concept or, as he prefers
to say, the organic circuit concept implies that the synthesis of the elements
of a complex chain reflex into an organic unity is the essential prerequisite
of that apperceptive process which will make the total experience of value
for future discriminative responses — for learning by experience. This,
which is true in the individual learning process, is also true phylogenetically.
The correlation centers (and their capacity for the preservation of vestiges
of past reactions) are the organic mechanism for this synthesis. They make
it possible that a new stimulus may be reacted to, not as a detached element,
but as a component of a complex series of past and present adjustments, to
which it is assimilated in the association centers — apperception. This
assimilation or apperceptive process is an integral part of the receptor proc-
ess in the higher centers, giving the quale to the idea of the exciting object.
Cotemporaneously with this stimulus-apperception process we have an
apperception-response-activity giving the object- or purpose-idea, so that
the entire reaction is to be regarded as stimulus-apperception-response, as a
functional unity rather than as a sequence: stimulus j>apperception> re-
sponse.

Dewey's organic circuit concept is elaborated in terms of psychology.
Let us see how it may be applied to biological behavior.

The simple reflex is commonly regarded as a causal sequence: given the
gun (a physiologically adaptive structure), load the gun (the constructive
metabolic process), aim, pull the trigger (application of the stimulus), dis-
charge the projectile (physiological response), hit the mark (satisfaction of
the organic need). All of the factors may be related as members of a simple
mechanical causal sequence except the aim. For this in our illustration
a glance backward is necessary. An adaptive simple reflex is adaptive
because of a pre-established series of functional sequences which have been
biologically determined by natural selection or some other evolutionary
process. This gives the reaction a definite aim or objective purpose. In
short, the aim, like the gun, is provided by biological evolution, and the
whole process is implicit in the structure-function organization which is
characteristic of the species and whose nature and origin we need not here
further inquire into.

Now, passing to the more complex instinctive reactions, so far as these
are unconscious automatisms, they may be elaborations of chain reflexes
of the type discussed above (p. 61). But the aim (biological purpose) is

1 The Reflex Arc Concept in Psychology, Psych. Rev., vol. Hi, p. 357,
1893. See also Dewey's later statement in Jour. Philos., Psych., and Sci.
Methods, vol. ix, Nov., 1912, pp. 664-668, especially the.f ootnote on p. 667.

EVOLUTION AND SIGNIFICANCE OF CEREBRAL CORTEX 309

so inwrought into the course of the process that it cannot be dissociated.
Each step is an integral part of a unitary adaptive process to serve a definite
biological end, and the animal's motor acts are not satisfying to him unless
they follow this predetermined sequence, though he himself may have no
clear idea of the aim.

These reactions are typical organic circuits. The cycle in some of the
instincts of the deferred type comprises the whole life of the individual.
In other cases the cycle is annual (as in bird migrations, etc.), diurnal, or
linked up with definite physiological rhythms (e. g., the nidification of birds
as described by F. H. Herrick, see p. 61). In still other cases there is no
apparent simple rhythm. But always the process is not a simple sequence
of distinct elements, but rather a series of reactions, each of which is shaped
by the interactions of external stimuli and a preformed or innate structure
which has been adapted by biological factors to modify the response to the
stimuli in accordance with a purpose, which from the standpoint of an out-
side observer is teleological, i. e., adapted to conserve the welfare of the
species.

Every intelligently directed response to external stimulation involves a
large measure of highly complex unconscious cerebration of this type;
and it is possible to describe with considerable precision the mechanisms of
the subcortical activities involved in many of those organic circuits which
are commonly regarded as typically cortical.

Much of that which goes in psychological literature under such contra-
dictory terms as unconscious mind or subconscious mind is, in reality, the
subcortical elaboration of types of action system which ordinarily do not
involve the cortex at all, but which upon occasion may be linked up with
cortical associational processes and then come into consciousness in such a
form as to suggest to introspection that they are all of a. piece with the
conscious process with which they are related. In fact, within the cortex
itself there are doubtless many routine activities which do not ordinarily
come into consciousness, particularly of the sort known as acquired autom-
atisms or lapsed intelligence; and these, though of quite different origin
from the innate instinctive systems, cannot easily be distinguished from
them in the form in which they are experienced in the adult.

In the organic circuit as defined by Dewey the process is considered as a
whole, so that the response is conceived as logically implicit in the stimulus.
The motor reaction, he says, is not merely to the stimulus; it is into the
stimulus. "It occurs to change the sound, to get rid of it. What we
have is a circuit, not an arc, or broken segment of a circle. This circuit is
more truly termed organic than reflex, because the motor response deter-
mines the stimulus just as truly as sensory stimulus determines movement."
This notion, which is difficult for the practical scientific mind to understand,
is considerably clarified by some neurological considerations.

From the standpoint of the cerebral cortex considered as an essential
part of the mechanism of higher conscious acts, every afferent stimulus, as
we have seen, is to some extent affected by its passage through various sub-
cortical correlation centers (i. e., it carries a quale of central origin). But
this same afferent impulse in its passage through the spinal cord and brain
stem may, before reaching the cortex, discharge collateral impulses into the
lower centers of reflex coordination, from which incipient (or even actually
consummated) motor responses are discharged previous to the cortical reac-
tion. These motor discharges may, through the "back stroke" action, in
turn exert an influence upon the slower cortical reaction. Thus the lower

310

INTRODUCTION TO NEUROLOGY

reflex response may in a literal physiological sense act into the cortical stim-
ulus complex and become an integral part of it.

But there is another aspect of the problem which has recently been
brought to our notice by Kappers.1 It is a well-known fact, which is not
often taken account of in this connection, that the descending cortical
paths (pyramidal tracts) do not typically end directly upon the peripheral
motor neurons whose functions they excite, but rather upon intercalary
neurons which lie in the reticular formation or even in the adjacent sensory

muscle

Fig. 137. — Diagram of the relations of the pyramidal tract in a rabbit or
similar lower mammalian brain. Sensory stimuli enter the spinal cord from
the skin through the peripheral sensory neuron, S, and ascend to the cere-
bral cortex through the lemniscus, L. The descending pyramidal tract,
P, lies in the dorsal funiculus of the spinal cord. Its intercalary neuron, 7,
may be stimulated by both the peripheral neuron, S, and by the pyramidal
tract, P. It discharges upon the peripheral motor neuron, M .

centers. These intercalary neurons, in turn, oxcito the peripheral motor
neurons. The same intercalary neuron which receives the terminals of the

^ * KAPPERS, C. U. ARIENS. Ueber die Bildung von Faserverbindungen auf
Grund von simultanen und sukzessiven Reizen. Bericht iiber den III
Kongress fur experimentclle Psychologic in Frankfurt a. Main, 1908. Also
Weitere Mitteilungen iiber Neurobiotaxis. Folia Neuro-Biologica, Bd.
I, No. 4, April, 1908, pp. 507-532.

See also DEARBORN, G. V. N. Kinesthesia and the Intelligent Will,
Amer. Jour, of Psychol., vol. xxiv, 1913, pp. 204-255.

EVOLUTION AND SIGNIFICANCE OF CEREBRAL CORTEX 311

pyramidal tract also receives collaterals from the peripheral sensory neurons
of its own segment (Fig. 137) . This arrangement is the explanation of the
fact that the pyramidal tract fibers descend through the human spinal
cord for the most part in the dorso-lateral region, not in the ventral funic-
ulus like most other motor tracts. In most lower mammals the pyramidal
tract actually descends within the dorsal funiculus in the closest possible
association with the peripheral sensory fibers, and this arrangement is
clearly the primitive relation of the descending cortical pathway.

Accordingly, stimulation of the skin of the body excites a dorsal spinal
root fiber which ascends toward the cortex within the spinal cord and also
gives collateral branches to intercalary neurons of the spinal cord itself.
The latter neurons may excite motor elements of the spinal cord to an im-
mediate reflex response which is well under way before the cortical return
motor impulse gets back to the spinal cord and discharges into these same
intercalary neurons which are already under sensory stimulation directly
from the periphery. The effect of this arrangement is that the central
motor path during function is under the influence of sensory stimulation at
both ends, and is not, as commonly described, under simple sensory stimula-
tion at the cortical end and purely emissive in function at the spinal end.

Viewed from the standpoint of cerebral dynamics, the exact physiological
effect of the discharge of a central motor bundle such as the pyramidal tract
will be dependent upon the combined action of the sensory stimulation at
the cortical end and the state of sensory excitation at the spinal end, as well
as upon the resistance of the motor apparatus itself.

We saw in a previous paragraph how the simple reflexes of the spinal
cord may become factors in the stimulus complex of the cortex. Here we
find, conversely, that the efferent cortical discharge may become a factor in
the local reflex stimulation of a motor spinal neuron. Fropi both stand-
points Dewey's conception of the unitary nature of the organic circuit, as
contrasted with the classical reflex arc concept, receives strong support.

The thalamic correlation centers probably serve as the organs par excel-
lence where are elaborated those organic circuits which give to the higher
apperceptive processes of the cortex that quale to which Dewey refers.
The origin of this quale is to be sought partly in the subcortical assimilation
of a present stimulus complex to the pre-existing organic circuits structur-
ally laid down in the reflex mechanism, and partly in an affective quality
pertaining to the several organic circuits involved in the reaction. This
affective quality may be innate or it may have been acquired by experience
of the results of previous reactions of the sort in question.

Head and Holmes have brought forward some very interesting evidence
that not only the affective quale of sensations but also the emotional life
in general is functionally related to the primitive intrinsic nuclei of the
thalamus, rather than to cortical activity (see p. 253). And certainly
there is much evidence in the behavior of lower animals, especially birds,
that a high degree of emotional activity is possible where the basal centers
are highly elaborated but the cerebral cortex is small and very simply organ-
ized.

From all of these considerations it seems probable that the functions of
the higher association centers of the cerebral cortex do not consist of the
elaboration of crude sensory data or of any similar elements, but rather of
the assembling and integration of highly elaborated subcortical organic cir-
cuits which in the aggregate make up the greater part of the reflex and in-
stinctive life of the species.

312 INTRODUCTION TO NEUROLOGY

The normal newborn child brings into the world an inherited
form of body and brain and a complex web of nerve-cells and
nerve-fibers which provide a fixed mechanism, common except
for minor variations to all members of the race alike, for the
performance of the reflex and instinctive actions. The pat-
tern of this hereditary fabric can be changed only very slowly
by the agency of selective matings and other strictly biological
factors or by degenerations of a distinctly pathological sort. It is
thus manifest that the improvement of the racial stock of normal
individuals by the practice of eugenics must necessarily be very
slow, though the improvement of defective or pathological strains
by selective matings so as to breed out the objectionable charac-
teristics is fortunately in most cases more readily accomplished.

But in addition to this hereditary organization the newborn
child possesses the large association centers of the brain with
their vast and undetermined potencies, the exact form of whose
internal organization is not wholly laid down at birth, but is
in part shaped by each individual separately during the course of
the growth period by the processes of education to which he is
subjected, that is, by his experience. This capacity for indi-
viduality in development, this ability to profit by experience,
this docility, is man's most distinctive and valuable character-
istic. And since the form which this modifiable tissue will take
is determined by the environing influences to which the child
is subjected, and since these influences are largely under social
control, it follows that human culture can advance by leaps and
bounds wherever a high level of community life and educational
ideals is maintained.

So well have we learned the lesson that the child brings with
him into the world no mental endowments ready-made — no
knowledge, no ideas, no morals — but that these have to be
developed anew in each generation under the guiding hand of
education, that we devote one-third of the expected span of life
of our most promising youth to the educational training neces-
sary to ensure the highest possible development of the latent
cultural capacities of these association centers of the cerebral
cortex.

But we have often been blind to the other side of the picture.
We have seen above that the adult cortex cannot function save

EVOLUTION AND SIGNIFICANCE OF CEREBRAL CORTEX 313

through the reflex machinery of the brain stem, and it must not
be forgotten in our pedagogy that this relation holds in a much
more vital and significant sense in the formative years of the
child. It is true that the child is born with no mental endow-
ments; but how rich is his inheritance in other respects! He has
an immense capital of preformed and innate ability which takes
the form of physiological vigor and instinctive and impulsive
actions, performed for the most part automatically and uncon-
sciously. This so-called lower or animal nature is ever present
with us. In infancy it is dominant; childhood is a period of
storm and stress, seeking an equilibrium between the stereotyped
but powerful impulsive forces and the controls of the nascent
intellectual and moral nature; and in mature years one's value in
his social community life is measured by the resultant outcome of
this great struggle in childhood and adolescence. This struggle
is education.

The answer to the riddle of life, however, lies not in a success-
ful attack upon the native innate endowments of the child. No,
that would be unbiological and wasteful, for our world of ideas
and morals is no artificial world within the cosmos, but it is a
natural growth, which is as truly a part of the cosmic process as
are "ape and tiger methods" of evolution. No higher association
center of the human brain can function except upon materials of
experience furnished to it through the despised lower centers of
the reflex type. So also, no high intellectual, esthetic, or moral
culture can be reached save as it is built upon the foundation of
innate capacities and impulses.

We are gradually learning through the kindergarten that the
most economical way to lead a child into the realm of learning is
not to stamp out all of his natural interests and shut him up with
his face to the wall, while he learns by rote an a-b-c lesson which
is neither interesting nor useful. On the contrary, we accept as
given his native impulses and automatisms, his spontaneous
interests and his overproduction of useless movements, and we
use these as the capital with which we set the youngsters up in
the serious business of the acquisition of culture. But how does
it happen that we make so small use of the principles here learned
in the later years of the child's schooling?
- Not all of the instincts with which man is by nature endowed

314 INTRODUCTION TO NEUROLOGY

come into function in a sucking babe or a kindergarten pupil.
Childish curiosity is our strongest ally, if only we can use it
wisely, throughout the whole of the educational career from in-
fancy to the graduate school. Anger is a mighty passion in child-
hood. It is not wise to eradicate it altogether; rather keep it,
though under curb, for there are times when real abuses arise
which require that the man know how to hit and to hit hard.
And so with the instincts of self-preservation, of fear, of sex —
these all have their parts to play in the nobler works of life and
are by no means to be eradicated. The ascetic ideal of mortifica-
tion of the flesh as a means of grace is fundamentally wrong in
principle. Our case calls for no blind, indiscriminate attack
upon the world and the flesh, but rather the subjugation and dis-
cipline of these, so that we may use them effectively in our attack
upon the devil.

Conflict is inherent in the cosmic process, at least in the bio-
logical realm, from beginning to end. There is the struggle for
physical existence among the animals. And even in the lower
ranks of life there arises also the struggle within the individual
between stereotyped innate tendencies or instincts and individu-
ally acquired experience. This is clearly shown by experiments
on animals as low down as the Protozoa. And out of this inner
conflict or dilemma intelligence was born. With the gradual
emergence of self-consciousness in this process arises the eternal
struggle with self, that conflict which leads to the bitter cry,
"When I would do good evil is present with me." Conflict,
then, lies at the basis of all evolution, and the factors of social
and even of moral evolution can be traced downward throughout
the cosmic process.

The social and ethical standards, therefore, have not arisen
in opposition to the evolutionary process as seen in the brute
creation, but within that process. And our immediate educa-
tional problem is the elaboration of a practicable system of pub-
lic instruction which can use to the full the enormous dynamic
energy in the hereditary impulsive and instinctive endowment of
the child, and build upon this, in the form best suited to the re-
spective capacities of all the separate individuals, a properly
ordered sequence of studies which will develop the latent capac-
ities of each pupil and ensure a vital balance between the strong

EVOLUTION AND SIGNIFICANCE OF CEREBRAL CORTEX 315

blind impulse of the innate nature and the acquired intellectual,
esthetic, and moral control.

And herein lies the solution of the problem of human freedom,
so far as this rests within our own control. The limits of one's
powers and the range within which his freedom of action is cir-
cumscribed are in part determined by his hereditary endowments
and by environmental influences over which he has no control.
These are decreed to him by his fate, and the innate organization
of the nervous system is the chief instrument of this fate. But
man differs from the brute creation chiefly in that he can more
completely control his own environment and thereby to that
extent take his fate into his own hands; in other words, he can
enrich his own experience along lines of his own selection. To
some extent each individual can do this for himself through self-
culture; but to ensure the best results of such efforts there must
be a social control of the environment as a whole by concerted
community action. Individual freedom of action can, therefore,
attain its highest efficiency only through a certain amount of
voluntary renunciation of the selfish interests where these con-
flict with community welfare. Ethical ideals and altruism are
as truly evolutionary factors in human societies as are the ele-
mental laws of self-preservation and propagation of the species.1

To return now to the developing nervous system, we note that
the educational period is limited to the age during which the
association centers, whose form is not predetermined in heredity,
remain plastic and capable of modification under environmental
influence. Ultimately even the cerebral cortex matures and
loses its power of reacting except in fixed modes. Its unspecial-
ized tissue — originally a diffuse and equipotential nervous mesh-
work — becomes differentiated along definite lines and the funda-
mental pattern becomes more or less rigid. The docile period is
past, and though the man may continue to improve in the
technic of his performance, he can no longer do creative work.
He is apt to say, "The dog is too old to learn new tricks."

1 In this connection reference may be made to two very interesting ad-
dresses recently delivered before the American Society of Naturalists:

JENNINGS, H. S. 1911. Heredity and Personality, Science, N. S., vol.
xxxiv, pp. 902-910.

CONKLIN, EDWIN G. 1913. Heredity and Responsibility, Science, N. S.,
vol. xxxvii, pp. 46-54.

316 INTRODUCTION TO NEUROLOGY

Whether this process occurs at the age of twenty or eighty years,
it is the beginning of senility. And, alas, that this coagulation
of the mental powers often takes place so early! Many a boy's
brains are curdled and squeezed into traditional artificial molds
before he leaves the grades at school. His education is complete
and senile sclerosis of the mind has begun by the time he has
learned his trade. For how many such disasters our brick-yard
methods in the public schools are responsible is a question of
lively interest.

We who seek to enter into the kingdom of knowledge and to
continue to advance therein must not only become as little
children, but we must learn to continue so. The problem of
scientific pedagogy, then, is essentially this : to prolong the plas-
ticity of childhood, or otherwise expressed, to reduce the in-
terval between the first childhood and the second childhood to
as small dimensions as possible.

INDEX AND GLOSSARY

The references are, in all cases, to pages. Numbers referring to pages upon which the
item is figured are printed in black-faced type. Authors' names are printed in SMALL,
CAPITALS. Brief definitions of some of the more commonly used technical terms are in-
cluded in this Index; for fuller descriptions consult the pages cited. Terms which are
defined in this Glossary are printed in black-faced type. The names of fiber tracts, in
general, define their connections, the first part of the compound word indicating the
nucleus of origin and the last part the terminal nucleus (see page 128). To facilitate
cross-reference, the key-word of a polynomial term is capitalized wherever it occurs in this
Index and Glossary.

Accommodation of vision, 143, 207,
211, 232, 234, 247

Acids, sensitiveness to, 72, 91, 242

Acipenser rubicundus, nervous sys-
tem of, 151, 152

Acoustic apparatus. See Auditory
apparatus.

Acoustico-lateral apparatus, the ner-
vous mechanisms of the internal
ear and lateral line organs in fishes
and amphibians. See Nerves,
lateral, and Organs, lateral line.

Action. See Behavior and Reflex.

Action system, 21, 32, 66

Adrenalin (epinephrin), 231, 255, 256

Affection, affective experience, affec-
tive tone, pleasure-pain, emotions,
and allied phenomena; cf. Feeling
tone and Pain, 89, 167, 241, 249-
262, 311

Affective tone. See Feeling tone
and Affection.

Afferent, conducting toward a cen-
ter, 25, 42, 108, 126, 137, 145-150

Agraphia, loss of the power to write
correctly, 292

Agreeable and disagreeable. See
Affection.

Ala cinerea (vagal eminence, emi-
nentia vagi, trigonum vagi), an
eminence in the floor of the fourth
ventricle formed by the dorsal
Nucleus of the vagus, 154, 156,
164, 234, 244

Alcohol, sensitiveness to, 242

Alexia, loss of the power of reading
(word-blindness), 292

Altruism, 315 .

Alveus, association-fibers which con-
nect the Hippocampus with the
Gyrus hippocampi, 221, 222

Ameboid, resembling an ameba;
applied to the supposed outward
and inward movement of proc-
esses of cells during nervous func-
tion, 104

Ameiurus melas, gustatory nerves
of, 245, 246

Ammon's horn. See Hippocampus.

Amphibia, nervous system of, 182,
264

Ampulla, of semicircular canal, 183,
196

Amygdala. Seo Nucleus amygdalae.

Anatomy of nervous system, gen-
eral, 106

ANDRfi-TnoMAS, 193

Anger. See also Affection, 89, 255,
256, 314

Anguis fragilis, parietal eye of, 212

Animals, contrasted with plants,
22

Anterior, as used in this work,
means toward the head end of
the body; as used in the B. N. A.
tables it means toward the ventral
side, 115, 116

APATHY, S., 55

Apes, nervous system of, 282, 286,
290, 295

Aphasia, a speech defect due to a
cortical injury, or more broadly any
defect in symbolizing relations; cf.
Speech, apparatus of, 291-293

Aphemia, loss of the power to utter
words, 292

317

318

INDEX AND GLOSSARY

Apoplexy, 293
Apperception, 249, 296, 308
Appetite, 240

Aqueduct of Sylvius (iter, optoccele,
mesoccele), the ventricle of the
midbrain, 62, 121, 158, 160, 161
Arachnoid, the middle brain mem-
brane, 38

Arbor vitae, the tree-like appear-
ance of the white matter of the
cerebellum in section, 190
Archipallium, the olfactory cerebral
cortex, including the Hippocam-
pus and the Gyrus hippocampi
(in part), 217, 221, 222, 273, 284,
288, 306

Area, acoustic. See Area, acoustico-
lateral, Nucleus, cochlear, and
Nucleus, vestibular.
acoustico-lateral (in fishes = tuber-
culum acusticum), 111, 112,
123, 143, 149, 152, 187, 200
cortical, as used in this Index is
a part of the cerebral cortex
which can be differentiated from
its neighbors structurally by the
arrangement of its cells and
fibers (sometimes termed field);
cf. Center, cortical, 273, 277,
287, 288

cutaneous, 111, 112, 123, 157
general somatic sensory. See Area,

cutaneous.

olfactoria, the region containing
the secondary olfactory centers,
divided into anterior, medial,
intermediate and lateral olfac-
tory Nuclei, 165, 167, 215, 217,
218, 219, 221, 306
parolfactoria of Broca (gyrus ol-
factorius medialis of RETZITJS),
a portion of the medial Area ol-
factoria immediately in front of
the Gyrus subcallosus, 119
perforata. See Substantia per-

forata.

somatic, a small region in the fish
brain from which the Neopal-
lium and Corpus striatum were
developed, 111, 112, 123
striata, that part of the occipital
lobe of the cerebral cortex con-
taining the Line of Gennari; the
visual center, 268, 284

Area, visceral. See also Lobe, vis-
ceral, 111, 112, 123, 148, 149, 152,
153, 157, 237, 239, 240, 246, 303.

Arteries, nerves of. See Vasomotor
apparatus.

Articulates, behavior of, 33

Association, correlation involving a
high degree of modifiability and
also consciousness, 35, 64, 104,
242, 258, 279, 290, 292, 295, 296,
307

Association center. See Center,

association,
fibers. See Fiber, association, and

Tract, association,
of ideas, 295
pattern, 291
time of, 98

Asthma, 238

Ataxia, loss of the power of muscular
coordination, 137, 138

Atropin, 231

Attention, 103, 104

Auditory apparatus, 60, 62, 63, 70,
85, 145, 147, 150, 157, 160, 163-
167, 170, 195-203

Auditory reaction time, 98

AUERBACH, plexus of (myenteric
plexus), 241

Aula, the anterior end of the third
ventricle where it communicates
with the lateral ventricles by way
of the interventricular Foramina.

Auricle, of external ear, 195

Automatisms, acquired, 35, 57, 286,
301, 309

Avalanche conduction. See Conduc-
tion, avalanche.

Axis-cylinder, the central protoplas-
mic strand of a nerve-fiber; part
of the Axon, 39

Axon (axis-cylinder process, neurite,
neuraxon, neuraxis), a process of
a Neuron which conducts impulses
away from the cell body, 39, 40,
44, 45, 47

Axon hillock, the point of origin of an
axon from the cell body, 40, 41, 46

Axone. See Axon.

Back-stroke, the influence which a
peripheral organ of response exerts
back upon the center from which

INDEX AND GLOSSARY

319

the response was excited ; a form of
chain Reflex; cf. Reflex circuit,
260, 309
BAILLARGER, layer of, stripe of. See

Line of BAILLARGER.
BARKER, L. F., 36, 40, 49, 55, 94,

124, 142, 159, 223
BARTLEMEZ, G. W., 54, 181
Basis pedunculi (pes pedunculi,
crusta), the ventral part of the
cerebral Peduncle, composed of
descending fiber tracts, 114
Basle nomina anatomica (B. N. A.),

115, 116, 121, 122
BECHTEREW, W., 159, 213, 299
BECHTEREW, vestibular nucleus of,

184, 185

Behavior, invariable, activities whose
character is determined by in-
nate structure, typified by reflex
and instinctive actions, 31, 67,
78, 115, 181, 363, 294, 301-304,
312

range of, 19, 303

variable, activities which are modi-
fiable by individual experience,
with or without consciousness,
31, 64, 67, 78, 101, 115, 181, 263,
290, 294, 301-304, 312, 315, 316
BETHE, A., 47, 55, 90
Betweenbrain. See Diencephalon.
BETZ, cells of. See Cells of BETZ.
BIANCHI, A., 193
Birds, behavior of, 61, 309, 311

nervous system of, 187, 216
Bladder, innervation of, 226, 232,

243

Blood, coagulation of, 255
Blood-pressure, 104, 235
Blood-vessels, nerves of. See Cir-
culation of blood, apparatus of,
and Vasomotor apparatus.
B. N. A. See Basle nomina ana-
tomica.
Body of cell. See Cell body.

chromophilic. See Substance,

chromophilic.

of fornix. See Fornix body,
geniculate, lateral (corpus genicu-
latum laterale, external gen-
iculate body), a visual center
in the Thalamus, 114, 150,
163, 164, 167, 208, 210, 212,
284, 306

Body, geniculate, medial (corpus
geniculatum mediale, internal
geniculate body), an auditory
center in the Thalamus, 114,
118, 121, 154, 157, 163, 164,
167, 185, 201, 202, 306

habenular. See Habenula.

of LUYS, 167, 306

mammillary (corpus mamillare,
corpus candicans), one of a pair
of eminences at the posterior
end of the Tuber cinerium in
the Hypothalamus ; an olfac-
tory center, 114, 120, 163, 165,
166, 167, 210, 220, 306

of NISSL. See Substance, chromo-
philic.

pineal (corpus pineale, pineal
gland, epiphysis, conarium), a
glandular outgrowth from the
Epithalamus; in some lower
vertebrates it takes the form of
a median dorsal eye. See
Parietal eye, 110, 114, 118, 119,
162, 164, 167, 212

pituitary. See Hypophysis.

quadrigeminal. See Corpora quad-
rigemina.

restiform. See Corpus resti-
forme.

striate. See Corpus striatum.

tigroid. See Substance, chromo-
philic.

trapezoid (corpus trapezoideum),
transverse decussating fibers in
the ventral part of the medulla
oblongata which connect the
auditory nuclei of one side with
the lateral Lemniscus of the
other side, 50, 201
BOLK, L., 193

BOLTON, J. S., 273, 277, 289, 290
BONNET, R., 81

Brachium, of colliculus inferior. See
Brachium quadrigeminum in-
ferius.

conjunctivum (prepeduncle), the
superior or anterior peduncle of
the cerebellum; cf. Peduncle,
cerebellar, 114, 131, 158, 162,
165, 176, 1S7, 188

pontis (medipeduncle, processus
cerebelli ad pontem), the middle
peduncle of the cerebellum; cf.

320

INDEX AND GLOSSARY

Peduncle, cerebellar, 114, 122,
158, 162, 187, 188, 192
Brachium, quadrigeminum inferius
(brachium of colliculus inferior),
a ridge on the Corpora quadri-
gemina formed by fibers from the
Colliculus inferior to the medial
geniculate Body, 114, 161, 164,
185

Brain (encephalon), that portion of
the central nervous system con-
tained within the skull, 106
development of. See Nervous sys-
tem, development of.
measurements of, 123
new. See Neencephalon.
nomenclature of. See Nervous

system, nomenclature of.
old. See Palaeencephalon.
stem, all of the brain except the
cerebellum and the cerebral
cortex, i. e., the Segmental
apparatus, 113, 114, 115, 123,
164, 181, 185, 186, 192, 266,
280
reflexes of. See Reflexes of

brain stem.

terminology of. See Nervous sys-
tem, nomenclature of.
weight of, 123
Branch. See Ramus.
Branchial ganglia. See Ganglion,

branchial.

nerves. See Gills, innervation of.
Bridge. See Pons.
BROCA, P., 292
BUOCA'S area. See Area parolfac-

toria of Broca.

Broca's convolution, the posterior
part of the gyrus frontalis inferior,
supposed to function as a motor
correlation center of speech, 283,
292 293
BRODMANN, K., 269, 270, 272, 273-

278, 287, 290

Bronchial tubes, nerves of, 226, 238
BROUWER, B., 142, 172
BRUCE, A., 142
BRUCE, A. N., 130, 142, 193
BUCHANAN, FLORENCE, 98, 105
Bulb (bulbus), any bulb-like struc-
ture; specifically the Medulla ob-
longata, as in bulbar paralysis,
tractus bulbo-spinalis.

Bulb, olfactory, a protuberance from
the cerebral hemisphere contain-
ing the primary olfactory center,
110, 111, 112, 120, 165, 215, 216,
217, 218, 219, 264

Bulbar formation. See Formatio
bulbaris.

Bundle. See Tract and Fasciculus,
basis, fundamental, or ground.

See Fasciculus proprius.
longitudinal medial. See Fascicu-
lus longitudinalis medialis.
posterior longitudinal. See Fas-
ciculus longitudinalis medialis.
solitary. See Fasciculus solitarius.

BURDACH, column of. See Fascicu-
lus cuneatus.

BURNETT, T. C., 279, 299

CAJAL. See RAMON Y CAJAL.
CAJAL, commissural nucleus of. See

Nucleus, commissural, of CAJAL.
Calcar avis (hippocampus minor), a
projection into the posterior horn
of the lateral ventricle formed by
the calcarine fissure.
CAMPBELL, A. W., 273, 278
Canal, central (canalis centralis), the
ventricle of the spinal cord, 126,
129
lateral line. See Organs, lateral

line.

neural, the lumen of the embry-
onic Neural tube; also applied to
the spinal Canal of the vertebral
column.

semicircular (ductus semicircu-
Juris). See also Vestibular ap-
paratus, 111, 183, 184, 187,
195, 196, 201

spinal, the canal in the vertebral
column containing the Spinal
cord.
CANNON, W. B., 240, 241, 248, 255,

256, 262

CAPPS, J. A., 250, 252
Capsule, external (capsula cxterna),
a thin band of nerve-fibers form-
ing the outer border of the
Corpus striatum, 166, 169, 170
internal (capsula interna), a strong
band of nerve-fibers passing
through the Corpus striatum,

INDEX AND GLOSSARY

321

144, 165, 166, 1G9, 170, 174, 176,
210, 253, 266, 287
Capsule, nasal, 110, 111
Carbon dioxid, production of, in

neurons, 96, 97, 103
as respiratory stimulus, 238, 240
CARLSON, A. J., 240, 241, 247
Carp, nervous system of, 45, 245,

302, 303
Carpiodes tumidus, brain of, 302,

303

Cat, nervous system of, 90, 251
Catfish, nerves of, 245, 246
Cauda equina, a bundle of elongated
spinal nerve roots arising from the
lumbar and sacral segments of the
spinal cord.

Caudal, pertaining to the tail, or
directed toward the tail end of the
body, as opposed to cephalic, 116
Cavum septi pellucidi (fifth ven-
tricle, pseudocode), the space en-
closed between the Septum pellu-
cidum of the two cerebral hemi-
spheres; not a true ventricle, 162
Cell (or cells), auditory (hair cells of

organ of COKTI), 197, 198, 199
basket, of cerebellum, 52, 190, 191,

192

of BKTZ (giant pyramidal cells of
motor center of cerebral cortex),
274, 283, 284, 285
body, the nucleus and perykaryon

of M neuron, 39
of CLAUDIUS, 197
of CORTI (hair cells), 197, 198,

199

of DEITERS of organ of CORTI, 197
epondyrna. See Ependyma.
granule, of corcbollar cortex, 190,

191, 192

of cerebral cortex, 274, 290
of olfactory bulb, 217, 218
of retina, 206, 207
of HENSEN, 197
mitral, an olfactory neurone of the

second order, 217, 218
nerve. See Neuron,
neuroglia. See Neuroglia.
of PUUKINJE. See PURKINJE,

cells of.

Cellulifugal, conducting away from
the Cell body, applied to the proc-
esses of a neuron.

21

Cellulipetal, conducting toward the
Cell body, applied to the processes
of a neuron.

Center (centrum), a collection of
nerve cells concerned with a
particular function, 25, 106, 108,
109, 181, 302

association. See also Center, cor-
tical, association, 64, 65, 103,
104, 181, 258, 294, 302, 304
auditory. See Area, acoustic,
Auditory apparatus, and Center,
cortical, auditory,
correlation, 108, 109, 113, 120,

133, 158, 181, 186, 304
cortical, a part of the cerebral
cortex which can be differen-
tiated functionally from its
neighbors; cf. Area, cortical.
These centers are sometimes
called areas, fields, spheres, or
zones, 273, 282, 283
association, 104, 181, 273, 284,
285, 287, 288, 289, 290, 292,
306, 311, 312

auditory, 166, 202, 273, 283
gustatory, 246, 288
motor, 140, 141, '181, 187, 273,
281, 282, 283, 284, 285, 292,
306

olfactory. See Archipallium.
optic. See Center, cortical,

visual.

projection. Sec Projection cen-
ter.

of reading. See Speech, appara-
tus of.
somesthetic, 141, 166, 177, 253,

273, 282, 283, 284, 285, 288
of speech. See Speech, appara-
tus of.
tactile. See Center, cortical,

somesthetic.
of temperature. Sec Center,

cortical, sotnest lietic.
visual, 166, 210, 267, 268, 273,

283, 284, 288

of writing. See Speech, appara-
tus of.
motor. See Motor apparatus and

Center, cortical, motor,
optic. See Visual apparatus and

Center, cortical, visual.
oval. See Center, semioval.

322

INDEX AND GLOSSARY

Center for pain. See Thalamus,
pain center in.

primary, 109, 143, 151

projection, See Projection cen-
ters.

reflex. See also Reflex circuit,

109, 113, 129, 156
respiratory, 237, 238, 239, 240
semioval (centrum semiovale, cen-
trum ovale), the great mass of
white matter in the center of
each cerebral hemisphere.

sensory, 117, 120
tactile. See Area, cutaneous,
Touch, apparatus of, and Cen-
ter, cortical, somesthetic.
trophic, a nerve-center which
regulates the nutrition of an-
other part, 109

of trunk and limb reflexes, 129
vasomotor. See Vasomotor ap-
paratus.

visceral. See Area, visceral,
visual. See Visual apparatus and

Center, cortical, visual.
Central nervous system. See Ner-
vous system, central,
pause, 98

Centrifugal. See Efferent.
Centripetal. See Afferent.
Centrum. See Center.
Cephalic, pertaining to the head, or
directed toward the head end of
the body, as opposed to caudal,
116

Cerebellum, the massive coordina-
tion center dorsally of the upper
end of the Medulla oblongata,

110, 111, 112, 115, 117-120,
120-122, 143, 152, 158, 186-194,
264

cortex of. See Cortex, cerebellar.
development of, 117, 118, 119, 187
fiber tracts of, 130, 137, 158, 176,

187, 188

functions of, 186, 189, 192, 289, 305
lesions of, 189

Cerebration, unconscious, 297, 309,
311

Cerebrum, that portion of the brain
lying above the Isthmus; also used
as synonymous with Prosen-
cephalon and Cerebral hemi-
spheres, 121, 122, 143, 160

Chain, sympathetic. See Trunk,
sympathetic.

Chemical processes in nerve-cells,

96, 97, 99
sensibility, 72, 85

Chiasma, optic (chiasma opticum),
the partial decussation of the
optic Tracts on the ventral surface
of the brain, 118, 119, 120, 208,
209, 210

CHILD, C. M., 31, 36, 97, 105

Chimpanzee, cerebral cortex of, 282

Chorda tympani, 146, 245

Chorioid plexus (choroid plexus). See
Plexus, chorioid.

Chrionomus plumosos, nervous sys-
tem of, 30

Chromatin, a nucleo-protein sub-
stance found in the cell nucleus, 99

Chromatolysis, the solution and dis-
appearance of the chromophilic
Substance from a neuron, 48, 49,
136, 284

Chromophilic bodies, granules, or
substance. See Substance, chro-
mophilic.

Ciliary process. See Process, ciliary.

Cingulum, an association tract of the
cerebral hemisphere lying under
the Gyrus cinguli, 267

Circle of Willis, a potygonal circuit
of anastomosing arteries on the
ventral surface of the brain, from
which some of the arteries of the
brain arise.

Circuit, organic. See Reflex circuit.

Circulation of the blood, apparatus
of. See also Vasomotor apparatus,
89, 147, 234, 235.

CLARKE, column of, or dorsal nucleus
of. See Nucleus, dorsal, of CLARKE.

CLAUDIUS, cells of, 197

Claustrum, a thin band of gray mat-
ter between the external Capsule
and the cortex of the island of
REIL, or Insula.

Clava, an eminence on the dorsal
surface of the lower end of the
medulla oblongata produced by
the nucleus of the Fasciculus
gracilis, 130, 164, 176, 177, 188

Cochlea, the bony spirally wound
canal containing the auditory re-
ceptor, 85, 183, 196, 199, 201, 202

INDEX AND GLOSSARY

323

Co-consciousness, 297
Ccelenterates, nervous system of, 27,
227

COGHILL, G. E., 66, 67, 85, 94, 135,

142, 182

Cold, sensations of. See Tempera-
ture, apparatus of.
COLE, L. J., 213
Colic, 250
Collateral, a small side branch of an

Axon, 40, 44

Colliculus facialis (eminentia facialis,
eminentia abducentis, eminentia
teres, Eminentia medialis), an
eminence in the floor of the
fourth ventricle produced by
the VII nucleus and the Genu
of the facial nerve, 154

inferior, one of the lower pair of
Corpora quadrigemina, contain-
ing chiefly reflex auditory cen-
ters, 114, 154, 157, 160, 164,
174, 176, 185, 201, 202

superior (optic lobe, optic tectum,
mites), one of the upper pair of
Corpora quadrigemina, contain-
ing chiefly reflex optic centers,
62, 63, 111, 112, 114, 150, 154,
160, 161, 164, 185, 20S, 209, 210,
211, 264
COLLINS, J., 233
Colon, 242

Column, anterior. See Funiculus
ventralis.

of BURDACH. See Fasciculus
cuneatus.

of CLARKE. See Nucleus, dorsal,
of CLARKE.

dorsal (columna dorsalis grisea.
See Column, gray. This name
is also applied to the dorsal
Funiculus), 126, 127, 128-130,
i:«, 150, 151, ITS, 251

of Fornix. See Fornix column.

fundamental. See Fasciculus pro-
prius.

of GOLL. See Fasciculus gracilis.

gray (columna grisea), one of the
longitudinal columns of neurones
which make up the gray matter
of the spinal cord. There are
three columns: (1) dorsal (pos-
terior), (2) ventral (anterior),
and (3) lateral (middle or inter-

mediate). These columns were

formerly called horns (cornua);

cf. also Funiculus, 126, 127, 128,

130, 150, 151j

Column, intermedio-lateral, of spinal
cord, 229

lateral (columna lateralis grisea;
see Column, gray), 126, 128,
150, 151, 229

of medulla oblongata, 151-156

posterior. See Funiculus, dorsal.

somatic motor, 150-156
sensory, 150-156

of TURCK, the ventral cortico-
spinal Tract.

ventral (columna ventralis grisea;
see Column, gray. This term
is also applied to the ventral
Funiculus), 126, 127, 128, 129,
130, 133, 150, 151

vesicular. See Nucleus, dorsal, of
CLARKE.

visceral motor, 150-156

sensory, 150-156
Columna. See Column.
Coma, 297
Comma tract of SCHULTZE. See

Fasciculus mterfascicularis.
Commissure (commissura), a band
of fibers connecting correspond-
ing parts of the central nervous
system across the median plane;
many decussations are also
called commissures, 265

anterior (commissura anterior),
fibers passing transversely
through the Lamina terminalis
and connecting the basal por-
tions of the two cerebral hemi-
spheres, 114, 162, 165, 222, 265

dorsal, fibers which cross the mid-
plane of the spinal cord dorsally
of the ventricle, 127

of fornix. See Commissure of hip-
pocampus.

of GUDDEN. See Commissure,
postoptic.

habenular (superior commissure),
a band of fibers connecting the
two Habenulae immediately in
front of the pineal Body, 265

of hippocampus (commissura hip-
pocampi, commissura fornicis),
fibers connecting the Hippo-

324

INDEX AND GLOSSARY

campi of the two sides through

the Fornix body, 170, 220, 265,

266

Commissure, inferior. See Com-
missure, postoptic.

of MEYNEUT. See Commissure,
postoptic.

middle. See Massa intermedia.

mollis. See Massa intermedia.

posterior (commissura posterior),
fibers passing transversely
through the anterior end of the
roof of the midbrain, 162, 220

postoptic (inferior commissure),
fibers passing transversely across
the floor of the hypothalamus
associated with the optic chias-
ma ; contains the commissures of
GUDDEN, MEYNERT, and other
fibers, 265

soft. See Massa intermedia.

superior. See Commissure, habe-
nular.

of tectum (commissura tecti),
fibers passing transversely across
the roof of the midbrain, con-
tinuing backward the Com-
missure posterior, 161

ventral, fibers which cross the

midplane of the nervous system

ventrally of the ventricle, 127,

129, 131, 133, 265

Compensation of function in cortex,

294
Components of nerves. See System,

functional.

Conarium. See Body, pineal.
Conduct, neurological basis of, 262,

312-316

Conduction, avalanche, the summa-
tion of nervous impulses in :i
center so as to increase the
intensity of discharge, 101,
192

nervous, 38, 54, 90, 97, 98, 101
Cones of n-tin.-i, 20.'), 206, 207, 208,

211

Conflict in evolution, 314
Conjunctiva, 84, 85, 2~>()
Consciousness, dissociation of, 297

evolution of. Sec Psychogenesis.

of lower animals; cf. Psychogene-
sis, 32, 257, 305

multiple, 297

Consciousness, neurological mechan-
ism of, 104, 166, 224, 242, 249,
258, 259, 260, 261, 280, 281,
286, 287, 290-297, 305-316
seat of, 291
of self, 314

Continuity of consciousness, 297
Convolution. Set; Gyrus.
of BROCA. See Broca's convolu-
tion.

Coordination, the combination of
nervous impulses in motor cen-
ters to ensure the cooperation of
the appropriate muscles in a re-
action, 35, 130, 132, 133, 181
Cornea, 84, 85, 249
Cornu. See Horn.
Corona radiata, the Projection fibers
which radiate from the internal
Capsule into the cerebral hemi-
sphere, 164, 166, 169, 170, 266, 287
Corpora quadrigemina, the dorsal
part of the Mesencephalon, con-
taining the superior and inferior
Colliculi, 118, 119, 121, 160, 162,
201, 210

Corpus callosum, a large band of
commissural fibers connecting
the Neopallium of the two cere-
bral hemispheres, 119, 162, 165,
166, 170, 220, 265, 206, 267
candicans. See Body, mammil-

lary.

dentatum. See Nucleus, dentate,
fornicis. See Fornix body,
geniculatum. See Body, genicu-

late.
mamillare. See Body, mammil-

lary.

pineale. See Body, pineal,
ponto-bulbare, 114
quadrigeminum. Sec; Corpora

quadrigemina.

restiforme (restiform body), the
inferior peduncle of the cerebel-
lum; cf. Peduncle, cerebellar,
i::o. 155, 156, 158, 176, 187, 188,
192, 201

striatum (striate body), a subcor-
tical or ba.-al mass of gray and
white matter in each cerebral
hemisphere, 116, 117, 118, 12:5,
162, 105, 166, 107, 168, 169,
170, 174, 176, 215, 264, 287

INDEX AND GLOSSARY

325

Corpus trapezoideum. See Body,

trapezoid.

Correlation, the combination of ner-
vous impulses in sensory centers
resulting in adaptive reactions, 35,
38, 43, 106, 133, 181
Correlation neurone, 62, 133, 134
Cortex, cerebellar, the superficial
gray matter of the cerebellum,
51, 52, 189-192
compared with cerebral cortex,

186, 189, 192, 289, 305, 308
localization of function in, 189,

281, 305

neurones of, 51, 52, 190
cerebral (pallium, mantle), asso-
ciation tissue forming the
superficial gray matter of the
cerebral hemisphere, 26, "65,
74, 109, 116-119, 123, 141,
166, 266-316

areas of. See Area, cortical.
centers of. See Center, cortical.
dependencies of, 114, 115
development of, 116-119, 286,

287, 288, 289, 290
electric excitability of, 281
evolution of. See also Hemi-
spheres, cerebral, comparative
anatomy and evolution of,
115, 129, 263, 280, 301
functions of, 104, 115, 122, 186,

189, 192, 254, 279-316, 311
layers of. See Layers of cerebral

lesions of , 254, 275, 279, 280,

L'N:i, 284, 286, 290^294
localization of function in. See
Localization of function in
cerebral cortex, and Center,
cortical.
motor contors of. See Center,

cortical, motor.
neurones of, 42, 44, 268-274,

290

number of neurons in, 26
phylogeny of. See Cortex, cere-

bral, evolution of.
structure of, 21)0-277
somatic. Sec Neopallium.
CORTI, cells of (hair cells), 197, 198,

199

ganglion of. See Clanglion, spiral.
organ of. See Organ, spiral.

CORTI, rod of, 197, 198

tunnel of, 197

Cough, mechanism of, 238, 239
Crista ampullaris, 196, 198

basilaris of cochlea, 197
Cms, a stalk or peduncle, applied to
compact masses of fibers which
connect different parts of the
brain; cf. Peduncle.

commune, of internal ear, 196

flocculi, 114

fornicis. See Fornix, crus of.

olfactoria, the stalk or peduncle of

the Olfactory bulb.
Crusta. See Basis pedunculi.
Crustaceans, nervous system of, 29
Cuneus, a wedge-shaped gyrus on the

medial face of the posterior pole

of the cerebral hemisphere receiv-
ing visual projection fibers, 119,

170, 210
Cup, optic. See also Vesicle, optic,

204

Curiosity, 314

CUSHING, H., 243, 245, 248, 286, 299
CYON, nerve of, 235
Cytoplasm, all protoplasm of a cell

exclusive of that in the nucleus,

39, 96, 99, 102

DAVIES, H. M., 79, 84, 95, 172

DF.AKHORN, G. V. N., 310
Decussation (decussatio), a band of
fibers crossing the median plane
of the central nervous system
and connecting unlike centers of
the two sides; many docussations
are called commissures,
of FOREL. See Decussation, tog-
mental, ventral.

fountain. See Decussation, tog-
mental, dorsal,
of MEYNERT. Soo Decussation,

t elemental, dors.-il.
optic. See Chiasma, optic.
of pyramids, 131

tegmontal, dorsal (MXTKBBT's
decussation, fountain decus-
sation), 161
ventral (K<mi:i.'s decussation),

161

Degeneration of nervous tissues, 46,
48, 55, 135, 136, 283, 290

326

INDEX AND GLOSSARY

Degeneration, sclerotic, of cortex,
284

DEITERS, cells of, in spiral Organ, 197
vestibular nucleus of, 184, 185

DEJERINE, J. J., 287

Dementia, 290

Dendrite, a process of a Neuron which
conducts toward the cell body, 39,
40, 41, 44, 45, 47, 96, 103, 104

Dependency, cortical, a part of the
brain stem developed as a sub-
sidiary of the cerebral cortex, 212,
263, 305

Depression, 101, 102, 103

Development of the nervous system.
See Nervous system, development
of.

DEWEY, J., 20, 61, 67, 308, 309, 311

DEWITT, LYDIA M. A., 87, 90, 95

Diaphragm, innervation of, 236,
237-240

Diaschisis, a transitory defect of
function due to disturbance of
cortical equilibrium, 292, 293, 294

Diencephalon (betweenbrain, thala-
mencephalon), the brain region
lying between Mesencephalon
and Telencephalon ; sometimes
called Thalamus or Optic thala-
mus, but properly divided into
Thalamus, Epithalamus, and
Hypothalamus, 116-119, 121,
122, 160-167
development of, 116-119

Diffusion of nerve impulses, 104, 192

Digestion, apparatus of, 78, 224, 232,
240-243, 255, 256

Dilemma, 58, 307

Dioptric apparatus of eyeball, 205

Disagreeable and agreeable. See
Affection.

Discrimination. See Reaction, dis-
criminative.

I )i-~ociation of consciousness, 297

Detention, sensations of, s'i

Dog, functions of cortex of, 279, 281,

286
scratch reflex of, 134

Dogfish, nervous system of. See
Fishes, nervous system of.

DOGIEL, A. S., 83, 84, 85, 228, 233

Dou.i.v, I). II., 101, 102, 103, 105

Dolphin, absence of olfactory organs
of, 216

DONALDSON, H. H., 105, 123, 124
Dorsal, on the back side of the body,

termed posterior in the B. N. A.

lists, 116
Ductus cochlearis, 195, 196, 197, 198

endolymphaticus, 195, 196

reuniens, 196

semicircularis. See Canal, semi-
circular, and Vestibular appara-
tus.

utriculo-saccularis, 196
Dura mater, the outer brain mem-
brane, 38
DURUPT, A., 193
Dynamic theory of consciousness,

293, 296

Ear. See Auditory apparatus and

Vestibular apparatus,
brain, 112, 123
evolution of, 199, 201
Earthworm, nervous system of, 28
Ectoderm (epiblast), the outer germ
layer of the embryo, from which
the epidermis and the Neural tube
develop, 204
EDGEWORTH, F. H., 180
EDINGER, L., 36, 115. 124, 159, 219,

223, 280, 299, 306

EDINGER- WESTPHAL, nucleus of (the
visceral efferent nucleus of the III
nerve; cf. Nucleus of oculomotor
nerve).

Education, 32, 312-316
Effector, an organ of response, 26, 92
Efferent, conducting away from a
center, 25, 42, 108, 126, 137, 145-
160

Electric excitability of nervous tis-
sues, 281, 283," 286
phenomena in nervous tissue,

96
Embryology of nervous system, See

Nervous system, embryology of.
Eminent ia. abducent is. See Col-

liculus facialis.

facialis. See Colliculus facialis.
hypoglossi. See Trigonum hypo-

glossi.

medialis (eminent ia tens), a
medial longitudinal ridge in the
floor of the fourth ventricle; an
enlarged portion is the Collicu-
lus facialis.

INDEX AND GLOSSARY

327

Eminentia teres. See Eminentia

medialis.
vagi. Sec Ala cinerea.

Emotion. See Affection.

Empis stercorea, nervous system of,
30

Encephalon, the brain, 120

Endolymph, 196, 198

End-organ, the peripheral apparatus
related to a nerve; a Receptor or
Effector, 25, 40, 70, 79, 98

End-plate, motor, the terminal ar-
borization of a motor axon upon a
muscle-fiber, 40, 92, 101

Endyma. See Ependyma.

Engram, 295

Environment, 17, 18, 69, 312

Epencephalon, the cerebellum.

Ependyma (endyma), the lining
membrane of the ventricles of the
brain, derived from the original
epithelium of the Neural tube, 38

Epiblast. See Ectoderm.

Epicritic sensibility, a highly refined
type of cutaneous sensibility, espe-
cially on hairless parts, 84, 85, 132

Epiglottis, organs of taste upon, 147,
243

Epinephrin. See Adrenalin.

Epiphysis. See Body, pineal.

Epithalamus, the dorsal subdivision
of the Diencephalon, containing
the pineal Body and the Haben-
ula, an important olfactory correla-
tion center, 111, 112, 118, 119,
121, 122, 162, 165, 167, 220, 221,
222

Epithelium, a thin sheet of cells, 24
nerve endings in, 90
olfactory (Schneiderian mem-
brane), 217

Equilibrium, apparatus of. See also
Vestibuhir apparatus, 77, 88,
89, 147, 183, 186, 199, 200
nervous, 66, 293, 296
theory of consciousness, 296

Esophagus, 78, 90, 144, 147, 234, 242

Esthetic experience. See Affection.

Ethics. See Morals.

Eugenics, 312

Eustachean tube (auditory tube),
195

Evolution of mind. See Psycho-
genesis.

Evolution of Nervous system. See

Nervous system, evolution of.
KWALU, J. II., 184, 203
Excitability, electric. See Electric

excitability of nervous tissues.
Excitation, fatigue of, 101, 102, 103
Experience, learning by, 34, 294, 307,

308, 312, 315
Exteroceptor, a sense organ excited

by stimuli arising outside the body,

74, 77, 79
Exteroceptors, apparatus of, 132,

137, 138, 139, 141, 145, 163-167,

172-174, 250

Extirpation of cortical centers, 286
Eye. See Visual apparatus.

accommodation of. See Accom-
modation of vision.

brain (ophthalmencephalon), 112,
123

conjugate movements of, 186, 211,
283

development of, 116, 117, 204

evolution of, 212

muscles of, 92, 110, 143, 146, 148,
180, 232, 247

parietal. See Parietal eye.

pineal. See Parietal eye.

Face brain, 123

Faculties, mental, 280, 285, 290, 291
Falx cerebri, a longitudinal fold of
Dura mater which extends be-
tween the cerebral hemispheres in
the longitudinal fissure.
Fascia dentata. See Gyrus denta-

tus.

Fasciculus, a bundle of nerve-fibers
not necessarily of similar func-
tional connections. The term
is often used, however, as a
synonym for Tract, 128.
antero-lateralis superficialis (of
COWERS). See Fasciculus ven-
tro-latoralis superficialis.
cerebello-spinalis. See Tract,

spino-cerebellar.
corebro-spinalis. See Tract,

cortico-spinal.

circuniolivaris pyramidis, 114
communis, a name formerly ap-
plied to the Fasciculus solita-
rius in lower vertebrate hr.-iins.

328

INDEX AND GLOSSARY

Fasciculus cuneatus (column of Bi u-
DACH), the lateral portion of
the dorsal funiculus of the
spinal cord, 128, 130, 139, 176,
177

nucleus of. See Tuberculum
cuneatum.

dorso-lateralis (LISSAUEH'S zone,
LISSAUEK'S tract), 130

of GOWEKS. See Fasciculus
ventro-lateralis superficialis.

gracilis (column of GOLL), the
ini'dial portion of the dorsal
Funiculus of the spinal cord,
128, 130, 139, 176, 177
nucleus of. See Clava.

inner spiral, of Spiral organ, 197

interfascicularis (comma tract,
tract of SCHULTZE), 130, 131

longitudinalis inferior of cerebral
hemisphere, 222, 267

longitudinalis median's (medial
longitudinal bundle, posterior
longitudinal bundle, fasciculus
longitudinalis posterior or dorsa-
lis), a bundle of motor coordina-
tion fibers running tlirough the
brain stem, 131, 152, 155, 156,
161, 176, LSI, 185,201,211

longitudinalis superior of cerebral
hemisphere, 267

marginalis ventralis, 131

of MEYNEUT. See Tract, haben-
ulo-peduncular.

occipito-frontalis inferior of cere-
bral hemisphere, 267

proprius of cerebral hemisphere.
See Fibers, arcuate (1).

proprius of spinal cord (funda-
mental columns, basis bundles,
ground bundles), that portion of
the white matter of the spinal
cord \vhieh borders the »ray
matter ami contains correla-
tion filters; arranged in dorsal,
lateral, and ventral subdivi-
sions. 127, 130, 131, 133, 179,
lv_>, 251, 252,253, 258

retroflexus of MKYNI.KT. Sec-
Tract, habeniilo-pcduncular.

solitarius (trartus solitarius, soli-
tary bundle, in lower verte-
brates often called fasciculus
communis), a longitudinal

bundle of fibers in the medulla
oblongata containing the central
courses of the visceral sensory
root-fibers of the cranial nerves,
149, 150, 155, 156, 164, 234,
237, 239, 240, 241, 244, 247
Fasciculus sulco-marginalis, 130,

131
thalamo-mamillaris. Sec Tract,

mamillo-thalamic .
transvcrsus occipitalis of cerebral

hemisphere, 267
uncinatus of cerebral hemisphere,

267

ventro-lateralis superficialis (an-
tero-lateral fasciculus, ClowEK.s'
tract), 128, 130

Fatigue, 101-103, 255, 256, 258, 304
Fear. See also Affection, 89, 255,

256.

Feeble-mindedness. See Idiocy.
Feeding, reflexes of. See Reflexes of

feeding.

Feeling (affective). See Affection.
Feeling tone. See also Affection, 249,

254, 258-262
FEKKIER, D., 194

Fiber, or fibers, fibraj. See Nerve-
fiber.

arcuate, of the cerebral hemi-
sphere, short association
fibers connecting neighboring
gyri; also called fibne propria'
and fasciculi proprii, 267, 'Js5
of the medulla oblongata, decus-
sating fibers lying in a super-
ficial series (external arcuate
fibers) and a deep seri<-> ' in-
ternal arcuate fibers), 155
association; cf. Tract, association,
L'liti, 267,260, 2*5. '-'S7, 2<M 2'.i3
of MULLEH, 205, 206
postganglionic. See Neuron, post-

ganglionic.
preganglionic. See Neuron, pre-

ganglionic.

projection. See Projection fibers.
propria1 (arcuate fibers of the

cerebral hemisphere), 267, _^."J
Field, auditory psychic, 285

cortical, a term sometimes used as
a synonym of Center, cortical, or
of Area, cortical.
visual psychic, 285

INDEX AND GLOSSARY

329

Fila olfactoria, the filaments of

which the olfactory nerve is com-

posed, 146, 217
Fillet. See Lemniscus.
Filum terminate (terminal filament),

the slender caudal termination of

the spinal cord, 107
Fimbria, a band of fibers which

borders the Hippocampus and

joins the Fornix, 165, 220, 221,

222, 2(w
Final common path, 58, 59, 62, 101,

258

FISCHER, B., 280, 299
Fishes, nervous system of, 109, 110,

111, 112, 122, 123, 148-153, 166,

179-181, 187, 199, 200, 201, 212,

215, 237, 240, 245, 246, 279, 302,

303

Fissure (fissura), in the cerebral cor-
tex a deep fold which involves
the entire thickness of the brain
wall; cf. Sulcus. This is the
usage of the B. N. A., but fissure
and sulcus are often used as
synonyms and the B. N. A. is
not consistent in this matter.

calcarine, 110, 268

chorioid, the fold in the postero-
incilial wall of the cerebral
hemisphere through which the
lateral chorioid Plexus is in-
vagmated.

dorsal, of spinal cord (dorsal
median septum), 128

ectorhinal. See Fovea limbica.

hippocampal, 221, 222

lateral (lissura lateralis Svi.vn,
fissure of SYLVIUS), a deep fis-
sure on the lateral surface of the
cerebral hemisphere which sepa-
rates the temporal from the
frontal and parietal lobes. 121,
166, 2l)li

longitudinal, the great fissure be-
tween the two cerebral hemi-
spheres, 120, 21).")

parieto-occipital, 119, 121, 170

rhinal. See Fovea limbica.

of ROLANDO. See Sulcus cen-
tralis.

ventral, of spinal cord, 127, 128,

129
Fistula, gastric, 241

FLECHSIG, P., 286, 287, 288, 299
tract of. See Tract, spino-cere-

bellar, dorsal.

Flexure, a bending or crumpling of
the developing Neural tube caused
by unequal growth of its parts, aa
cervical, pontile, mesencephalic,
diencephalic, and telencephalic
flexures, 116-119
Flies, nervous system of, 30
Flocculus, the most lateral lobe of

the cerebellum.
FLOURENS, J. P. M., 240
Fluid, cerebro-spinal, a clear liquid
resembling lymph filling the ven-
tricles of the brain and spinal
cord.

Folium, one of the leaf-like subdivi-
sions of the cerebellar cortex;
these are termed Gyri in the B. N.
A., 189

Foramen interventriculare (foramen
of MOXHO, porta), the commu-
nication between the lateral
and the third ventricles, 162,
264

of Magendie, an aperature in the
membranous roof of- the fourth
Ventricle.

of MONRO. See Foramen inter-
ventriculare.

Forcbrain. See Prosencephalon.
FORKL, decussation of. See Decus-

•aiion, tegmental, ventral,
field of, 167

Formatio bulbaris (bulbar forma-
tion), the tissue comprising the
primary olfactory center in the
olfactory bulb, i. e., the Glome-
ruli, mitral Cells, and granule
Cells, 220

reticularis (reticular formation,
processiis reticularis in spinal
cord), a mixture of nerve-fibers
and cell bodies providing for
local reflexes. (}.->, 127, 129, 153,
156, 1.T7, lf,S, 174, 176, 181, 184,
•_'H>. -JIT. :;oi, 310

Fornix, a complex fiber system con-
necting the Hippocampus with
nt her parts of the brain, 162,
164, 165, 166, 222
body (corpus fornicis), the middle
part of the Fornix.

330

INDEX AND GLOSSARY

Fornix columns (columns fornicis,
anterior pillars of fornix), two
columnar masses of fibers diverg-
ing from the anterior end of the
Fornix body to descend into the
diencephalon, 165, 170, 220, 221
commissure. See Commissure of

hippocampus.

cms of (crus fornicis, posterior
pillar of fornix), a band of fibers
on each side of the brain con-
necting the posterior part of the
Fornix body with the Fimbria.
longus of FOREL, fibers which per-
forate the Corpus callosum and
pass through the Septum pelluci-
dum.
Fossa flocculi, 141

lateralis (fossa of SYLVIUS), a
deeper part of the Fissura
lateralis containing the Insula.
rhomboidal, the floor of the fourth

ventricle, 118

Fovea limbica (sulcus rhinalis, fis-
sura rhinica, fissura rhinalis, fis-
sura ectorhinalis), the sulcus
which marks the lateral border of
the lateral Area olfactoria and
Gyms hippocampi or pyriform
Lobe in the lower mammals.
FRANZ, S. I., 299
Freedom of action, 315
FREY, M. VON, 79, 84, 85, 94
FRITSCH, G., 281, 299
Frog, cerebral cortex of, 216, 264,
. 279

nerve endings in, 90, 92
olfactory receptors in, 92
reaction time of, 98
reactions of, 63
velocity of nervous transmission

in, 97

Funiculus, one of the three principal
divisions of white matter on
each side of the spinal cord;
these funiculi were formerly
called Columns, 128
dorsal (funirulus dorsalis or pos-
terior, posterior columns), the
white matter of the spinal cord
included between the dorsal
fissure and the dorsal root, 128,
130, 134, 138, 141, 150, 151,
175, 176, 177, 178, 179, 310

Funiculus, lateral (funiculus later-
alis, lateral columns), the white
matter of the spinal cord in-
cluded between the dorsal and
ventral roots, 128
ventral (funiculus ventralis or an-
terior, ventral, or anterior col-
umns), the white matter of the
spinal cord included between the
ventral fissure and the ventral
root, 128

GALL, F. G., 280, 281, 300
Ganglion, a collection of nerve-cells.

In vertebrates the term should be

applied only to peripheral cell

masses, though sometimes Nuclei

within the brain are so designated,

108, 109

Ganglion or ganglia, basal, a term
sometimes applied to the Cor-
pus striatum and other sub-
cortical parts of the cerebral
hemisphere.

branchial, of vagus, 149

cerebro-spinal, development of,
45, 225

cervical, inferior, 226
middle, 226
superior, 226, 234

ciliary, 143, 146, 149, 226, 231,
246

of CORTI. See Ganglion, spiral.

of facial nerve. See Ganglion,
geniculate.

GASSER'S. See Ganglion, semi-
lunar.

geniculate (ganglion geniculi, the
ganglion of the VII cranial or
facial nerve), 111, 112, 146,
149, 245, 256

habenula;. SOP Habenula.

of insects, 29, 30

interpedunculare. See Nucleus,
interpeduncular.

of invertebrates, 28, 29, 30, 227

jugular (ganglion jugulare), 147,
149

of lateral line nerves. 149

nodosum, 147, 237, 239, 240

opticum basale. See Nucleus,
preoptic.

otic, 147, 245

INDEX AND GLOSSARY

331

Ganglion, petrosal (ganglion petro-

sum), 147, 245
of SCARPA. See Ganglion vestibu-

lar.

semilunar (ganglion semilunare,
GASSER'S ganglion, the ganglion
of the V cranial or trigeminal
nerve), 45, 111, 112, 146, 180,
245

sphenopalatine, 226, 245
spinal, 25, 43, 109, 125, 126, 134,

135, 136, 141, 147, 227, 228
spiral (ganglion spirale, ganglion of

CORTI), 147
submaxillary, 146
superior (ganglion superius of IX

cranial nerve), 147
supra-esophageal, 29, 30
sympathetic, 53, 107, 109, 125, 126,
225, 226, 227, 230, 237, 238,
239

prevertebral, sympathetic gan-
glia of the thorax and abdo-
men other than those of the
sympathetic trunk,
vertebral, the ganglia of the

sympathetic Trunk,
of trigeminus. See Ganglion,

semilunar.
of vagus. See Ganglion, jugular,

and Ganglion nodosum.
of vertebrates, 108.
vestibular (ganglion of SCARPA),

147

GEHUCHTEN, A. VAN, 25, 45, 86, 194
Generative organs. See Sexual or-
gans.
Geniculate body. Sec Body, genicu-

late.
ganglion. See Ganglion, genicu-

late.
GENNARI, layer of stripe of. See

Line of Gennari.

Genu, a knee-shaped bond of an
organ, such as the genu of the
corpus callosum, of the facial
nerVo, etc.

of corpus callosum, 119
Gills, 236, 240

innervation of, 110, 111, 112, 149,

246

muscles of, 94, 148
Gland, adrenal. See Gland, supra-
renal.

Gland, intestinal, 224
nerve-endings on, 94
pineal. See Body, pineal,
pituitary. See Hypophysis,
salivary, innervation of, 143, 144,
146, 147, 154, 156, 232, 241,
244

suprarenal, 231, 255, 256
Glia. See Neuroglia.
Glomeruli, olfactory, small globular
masses of dense Neuropil in the
olfactory bulb containing the first
synapse in the olfactory pathway,
217, 218
Glycosuria, 255
GOLDSTEIN, K., 131, 194
GOLGI, C., 41, 43, 44, 49, 55, 190,

274
GOLL, column of. See Fasciculus

gracilis.

GOLTZ, F., 279, 280, 281, 300
GOWERS, fasciculus of. See Fascic-
ulus ventro-lateralis superficialis.
Gradient, physiological, in nerve-
fibers, 97

Granules. See Cells, granule,
chromophilic, tigroid, of NISSL.

See Substance, chromophilic.
Gray, central, relatively undifferen-
tiated gray Matter which retains
its primitive position near the
ventricles, 127.
Groove, medullary. See Neural

groove.

neural. See Neural groove.
GRUNBAUM, A. S. F., 282, 300
GUDDEN, commissure of. See Com-
missure, postoptic.
Gustatory apparatus, 72, 74, 91,
143, 144, 146, 147, 148, 149, 150,
157, 163, 218, 222, 234, 243-246,
303

Gyms, one of the convolutions or
folds of the cerebral cortex
bounded by Sulci or Fissures,
265, 266
angularis, 121

cent rails anterior (procentral gy-
rus), 121, 140, 181, 269, 271,
272, 282, 283, 285, 286, 288
posterior (postcentral gyms),
121, 268, 270, 282, 283', 285,
286, 288
cinguli, 119, 170

332

INDEX AND GLOSSARY

Gyrus dentatus (fascia dentata), a
subsidiary gyrus of the Hippo-
campus, 221, 222

fornicatus (limbic lobe), the mar-
ginal portion of the cerebral
cortex on the medial aspect
of the hemisphere, including
the Gyrus cinguli, Gyrus hippo-
campi, and others; there is a
variety of usage regarding its
limits, 273

frontalis inferior, 121, 170, 292
medius, 121
superior, 119, 121

hippocampi, that part of the cere-
bral cortex which borders the
Hippocampus. Part of it (the
Uncus) is Archipallium ; the re-
mainder is transitional to the
Neopallium. See Lobe, pyri-
form, 217, 219, 221, 222, 273,
284

lingualis, 119

occipitalis lateral is, 121

olfactorius lateralis. See Nucleus

olfactorius lateralis.
medialis. See Area parol-
f actoria of Broca

orbitalis, 121

postcentral. See Gyrus centralis
posterior.

precentral. See Gyrus centralis
anterior.

subcallosus (podunculus corporis
callosi), part of the Nucleus ol-
factorius medialis, 119, 219

supramarginalis, 121

temporalis inferior, 121
medius, 121
superior, 121, 170

uncinatus. See Uncus.

Habenula (nucleus habonulip, gang-
lion Imbonuhp), an important ol-
factory correlation center in the
Epithalamus, 162, 165, 167, 170,
220

Habit, physiological, 32, 294, 304

Hair colls (cells of CORTI), 197, 198,

199
innorvation of, SO, 81

II \iim:*-n. !.. I'.ts. 199,203

HARRIS, W., 213

HEAD, H., 79, 84, 85, 94, 95, 132,
142, 166, 171, 172, 173, 175, 179,
233, 251, 253, 254, 262, 300, 311
Hearing, organs of. See Auditory

apparatus.
Heart, innervation of, 144, 147, 232,

234

Heat, sensations of. See Tempera-
ture, apparatus of.
HEIDENHAIN, M., 49, 55
HELD, H., 50
HELMHOLTZ, H. L. T. VON, 198,

203
HELWIG, tract of. See Tract, olivo-

spinal.

Hemispheres, cerebellar, 120, 187
cerebral, 63, 64, 111, 112, 121, 122,
123, 129, 160, 215, 219, 264,
265, 279

comparative anatomy and evo-
lution of, 111, 112, 129, 215,
264, 265, 279, 280, 290, 294,
301-306

Hemorrhage, cerebral, 293
HK.VSKX, cells of , 197
stripe of, 197, 198

HERRICK, C. JUDSON, 36, 61, 66, 67,
95, 124, 135, 142, 159, 171, 182,
194, 200, 223, 248, 263, 303
HERRICK, C. L., 18, 108, 194, 258,

262, 296, 300, 303
HERRICK, F. H., 61, 68
HKRTS, A. F., 95, 242, 243, 248
Hibernation, nerve cells in, 102
Hindbrain, a term which has boon
variously applied to the cerebel-
lum, the cerebellum and puns, the
medulla oblongata, and the entire
rhombencephalon .

Hipporampal gyrus. See Gyrus hip-
pocampi.

Hippocampus (hippocampus major,
Ammon's horn, cornu Ammo-
nis), a submerged gyrus forming
the larger part of the Archipal-
lium, or olfactory cerebral cor-
tex, 217, 219, 220', 221, 222, 273,
284, 306
commissure of. See Commissure

of hippocampus,
minor. See Calcar avis.
His, WILLIAM, V.I :>;>, 115 US, 121
Histology, the study of Tissues,
27

INDEX AND GLOSSARY

333

HITZIG, E., 281, 299, 300

HODGE, C. F., 105
.HOFER, B., 199, 203

HOLMES, G., 166, 171, 254, 262, 280,
300, 311

HOLMES, S. J., 262

Horn (cornu), one of the three chief
parts of the lateral ventricle —
anterior, posterior, and inferior or
middle; also applied to the gray
Columns of the spinal cord.

HOUGH, TH., 68

HLBER, G. C., 87, 95, 233

Humor, vitreous, 205

Hunger, apparatus of, 89, 240

Hyodon tergissus, brain of, 302, 303

Hypophysis (pituitary body, pitui-
tary gland), a glandular appendage
to the ventral part of the hypo-
thalamus; its posterior lobe is an
outgrowth from the Neural tube,
its anterior lobe is an ingrowth
from the epithelium of the em-
bryonic mouth cavity, 114, 119,
163, 167

Hypothalamus, the ventral subdivi-
sion of the Diencephalon, contain-
in": the Hypophysis and the mam-
millary Body, an important olfac-
tory correlation center, 117, 118,
121, 122, 162, 163, 165, 166, 167,
174, 176, 215, 220, 221, 222, 246,
260

Idiocy, 279, 290

Imbecility. See Idiocy.

Impulse, nervous, nature of, 96, 97
velocity of, 97, 98.

Infundibulum, a funnel-shaped ex-
tension of the third ventricle pass-
ing through the Hypothalamus to
the end in the Hypophysis, 114,
119, 120, K.:i

Inhibition, the diminution or anv-t
of a function, 63, 66, 108, 25 J,
256, 279, 307

Insanity, 2','0

Insects, nervous system of, 29, 30
respiration of, 2: id

Instinct, a complex form of in-
variable Behavior, 32, 61, 257,
263, 290, 301, 307, 309, 311, 312,
313

Insula (island of REIL), a portion of
the cerebral cortex which is sub-
merged under the Fossa lateralis,
166, 170, 266, 273

Integration, the combination of dif-
ferent acts so that they cooperate
toward a common end, 106

Intelligence. See Consciousiu
lapsed, 32, 301, 309

Interbrain. See Diencephalon.

Interference of nervous impulses, 58,
61, 63, 307

Interoceptor, a sense organ excited by
stimuli arising within the viscera;
cf. Visceral apparatus and Visceral
organs, 74, 77, 89, 243

Intestines, nerves of, 144, 234, 241,
242

Intoxication, effects of, 97, 101, 102,
103, 104, 231, 258

Introspection, 98, 297, 309

Intumescentia cervicalis (cervical
enlargement), the enlargement
of the spinal cord from which
the nerves of the arm arise.
lumbalis (lumbar enlargement),
the enlargement of the spinal
cord from which thr nerves of
the leg arise.

Invariable behavior. See Behavior,
invariable.

Invertebrates, behavior of, 32
nervous system of,

Iris, 14:5. 211, I'::--'. 234, 217

Irradiation of n< rvous impulses, 65,
66, 100, 260, 20s

Island of KKII.. See Insula.

Isthmus, a narrow segment of the
brain forming the upper end of the
Rhombencephalon (H. X. A.); it
might better be regarded as
merely the plane of separation be-
tween Rhombencephalon and
Cerebrum, 116-119, 121, 122, 143,
160

Iter (itor a tertio ad quart um ven-
triculum). See Aqueduct of Syl-

JACKSON. Hr<;in.i\<.-. 2'.i2

JAC OHSO.X, nerve of. See Nerve,

tympanic.
JAMES, \\ .. 2.v.». 262

334

INDEX AND GLOSSARY

Jelly-fishes, nervous system of, 27,

227

JENNINGS, H. S., 21, 31, 37, 68
JOHNSTON, J. B., 124, 152, 159, 171,

180, 223
Joints, nerve-endings in, 88

Kangaroo, cerebral cortex of, 217
KAPPERS, C. U. AEIENS, 203, 223,

240, 248, 265, 278, 310
KARPLUS, J. P., 251, 262
Karyoplasm, the protoplasm of the

nucleus of a cell, 96
KEIBEL, F., 124
KOLLIKER, A., 44
KRAUSE, W., 115, 124

end-bulbs of, 84, 85
KREIDL, A., 251, 262
KRIES, J. VON, 71
KUNTZ, A., 233

Labium vestibulare, 198
Labyrinth of ear, 195, 196
Lactic acid, 103
LADD, G. T., 98, 105, 213
Lagena, 199, 200
Lamina. See also Layer.

affixa, a thin non-nervous part of

the medial wall of the cerebral

hemisphere attached to the

thalamus and bordered by the

lateral chorioid Plexus.

of neural tube. See Plate.

terminalis (terminal plate), the

anterior boundary of the third

ventricle, 118, 165, 215, 264, 265

LANCISI (LANCISICS), nerves of. See

Stria longitudinalis.
striae of. Sec Stria longitudinalis.
LANDACRE, F. L., 35
LANGE, C., 259, 262
LANGLEY, J. N., 148, 225, 229, 233
Laqueus. See Lemniscus.
Larynx, 239
Lateral line organs. See Organs,

lateral line.

Law, myelogenetic, of FLECHSIG, 287
Layer. See also Lamina.

of BAILLARGER. See Line of Bail-
larger.

of cerebellar cortex, 190
of cerebral cortex, 268-274, 290

Layer of GENNARI. See Line of

Gennari.

of retina, 205, 206, 207
Learning. See Experience, learning

by.

Lemniscus (fillet, laqueus), sensory
fibers of the second order ter-
minating in the thalamus.
acoustic. See Lemniscus, lateral,
bulbar, ascending sensory fibers of
the second order from the
medulla oblpngata to the thala-
mus, including several different
tracts, 157

gustatory. See Lemniscus, vis-
ceral.

lateral, the acoustic lemniscus,
fibers from the cochlear nuclei
to the colliculus inferior and
thalamus, 114, 157, 161, 163,
164, 167, 174, 176, 185, 201
medial, ascending fibers of the
proprioceptive system from the
spinal cord to the thalamus, 138,
141, 155, 156, 161, 163, 164, 165,
167, 174, 175, 176, 177, 179, 180,
210

optic, a term which might ap-
propriately replace optic Tract,
209

spinal, ascending fibers of touch,
temperature, and pain sensibility
from the spinal cord to the
thalamus. In the cord these
fibers form two tracts, the dor-
sal and ventral spino-thalamic
tracts, 130, 131, 134, 138, 139,
141, 156, 161, 163, 164, 167, 173,
174, 178, 179, 190, 252, 253
trigeminal, ascending sensory
fibers of the second order from
the sensory V nuclei to the
thalumus, 139, 141, 157, 161,
163, 164, 165, 167, 173, 174, 180
visceral, a name suggested for the
ascending secondary fibers from
the nucleus of the fasciculus soli-
tarius to the higher cerebral cen-
ters, 157, 246
LENHOSSEK, M. VON, 41
Lens, 204-208, 211, 212
LEWANDOWSKY, M., 37, 194, 300
Life, definition of, 17
Ligament, spiral, of Cochlea, 197

INDEX AND GLOSSARY

335

Limbus laminse spiralis, 197
Limen insulae. See Nucleus olfac-

torius lateral! s, 219
Line of Baillarger, a stripe of tan-
gential white fibers in the cere-
bral cortex; there is an outer and
an inner line, 267, 268, 274
of Gennari, a stripe of tangential
white fibers in the Area striata
of the cerebral cortex; it is the
outer Line of Baillarger in this
area, 268, 274

Lingula cerebelli, a small eminence
on the ventral surface of the cere-
bellum where the anterior medul-
lary Velum joins the Vermis,
162
LISSATJER, tract of, zone of. See

Fasciculus dorso-lateralis.
Lizard, parietal eye of, 212
Lobe, frontal, 120, 266, 283

of the lateral line (lobus lineae
lateralis), a highly differentiated
part of the acoustico-lateral Area
of fishes, 152

limbic. See Gyrus fornicatus.
occipital, 266, 283
olfactory (lobus olfactorius), the
olfactory Bulb, its Crus, and the
anterior part of the Area olfac-
toria; this is the B. N. A. usage;
the term is sometimes applied to
the olfactory Bulb alone and
sometimes to the Area olfac-
toria alone.

optic. See Colliculus superior.
parietal, 266

pyriform (lobus piriformis), the
lateral exposed portion of the
olfactory cerebral cortex in
lower mammals, bounded dor-
sally by the Fovea limbica; in
man it is represented by the
Uncus and part of the Gyrus
hippocampi, 217

temporal, 120, 201, 202, 219, 266
vagal. See Lobe, visceral,
visceral (lobus visceralis, vagal
lobe, lobus vagi), the visceral
sensory Area of fishes, 148, 149,
152, 153, 303

Lobulus paracen trails, 119
parietalis inferior, 121
superior, 121

Local sign; cf. Localization of sensa-
tion, 84, 229, 250, 259
Localization of functions in central
nervous system, 65, 113, 230-
232, 234, 280
in cerebellar cortex, 189, 281,

305

in cerebral cortex, 189, 273, 280,
281, 282, 283, 284-297, 305,
307
of sensation, 79, 84, 85, 228-230,

250, 259, 286

Locomotion, reflexes of, 134
LOEB, J., 37, 61, 68
LOWENTHAL, tract of. See Tract,

tecto-spinal.
LUCIANI, L., 194
LUGARO, E., 104

Lumbricus, nervous system of, 28
Lungs, innervation of. See Respira-
tory apparatus.

LUYS, body of. See body of LUYS.
Lyra. See Lyre of David.
Lyre of David (lyra Davidis, psal-
tcrium), the posterior part of the
Fornix body, including the Com-
missura hippocampi.

Macula sacculi, 196
utriculi, 196

MAGENDIE, foramen of. See Fora-
men of Magendie.

MALL, F. P., 124

Mammals, cortical regions of, 273

Mammillary body. See Body, mam-
millary.

Mantle. See Cortex, cerebral.

MAUCHI, method of, 48, 135

MAHIK, P., 300

Marsupial animals, cerebral cortex
of, 217

Massa intermedia (commissura mol-
lis, soft commissure, middle com-
missure), a band of gray matter
connecting the medial surfaces of
the two thalami across the third
ventricle; it is not a true commis-
sure, 119, 162

MAST, S.O., LM3

Mastication, apparatus of, 78, 143,
146, 180, 244, 247

Matter, central gray. See Gray,
central.

336

INDEX AND GLOSSARY

Matter, gray (substantia grisea),
gray nervous tissue compose/I
chiefly of nerve-cells and un-
myelinated nerve-fibers, 108, 128
white (substantia alba), white ner-
vous tissue composed chiefly of
myelinated nerve-fibers, 108,
127, 128, 130

Meatus, external auditory, 195
Medial (medialis), nearer the median

plane; opposed to lateral.
Median (medianus), lying in the axis
or middle plane of the body or one
of its members.
Medius, intermediate between two

other parts.

Medulla oblongata (bulb), the Myel-
encephalon B. N. A. ; the older
and better usage includes the
whole of the Rhomb encephal-
on except the Cerebellum and
Pons, 110, 111, 112, 116-120,
121, 122, 143, 152, 154, 162,
232, 244, 246, 302, 303
reflexes of, 143, 148, 181, 234,

235, 302, 303

spinalis. See Spinal cord.
Medullary sheath. See Myelin

sheath.

tube. See Neural tube.
MEISSNER, corpuscle of, 82, 83
plexus of (submucous plexus), 53,

241
Membrane, basilar, of spiral organ,

197, 198

of the brain. See Meninges.
limiting, of retina (membrana
limitans externa and internal.
207
mucous, nerves of, 90, 126, 146,

21.-,, 232
nuclear, 99

Sclmeiderian. See Epithelium, ol-
factory.

teetoriaC 197» 198, 199

tympanic (drum membrane), 85,

195, 190, 245, 249
vestibular (membrane of HKISS-

NKH), 197
Memory, 295, 297, 304, 307

associative, 32, 65, 242, 294, 295,

307

Menidia. nerves of. 148, 149, 200
spinal cord of, 150

Meninges, the membranes of the
brain and spinal- cord, 38, 146, 250
MERKEL, corpuscle of, 81, 82, 83
Mesencephalon (midbrain), the Cor-
pora quadrigemina and cerebral
Peduncles, 62, 63, 116-119, 121,
122, 160, 161, 232
development of, 116, 119, 160
Metabolism, chemical changes in

protoplasm, 96, 97, 99, 163
Metathalamus, the posterior part of
the Thalamus, comprising the me-
dial and lateral geniculate Bodies,
118, 121, 122, 163, 165, 167
Metencephalon (hindbrain), the an-
terior part of the Rhombenceph-
alon, including the Cerebellum,
Pons, and intervening part of the
Medulla oblongata, 117-119, 121
MI.YER, A., 49, 55, 293, 300
MEYER, MAX, 262
MEYNERT, commissure of. Sec

Commissure, postoptic.
decussation of (fountain decussa-
tion, dorsal tegmental decussa-
tion), 161
fasciculus retroflexiis of. See

Tract , habonulo-peduncular.
MICHKLSON, A. A., 71
Midbrain. See Mesencephalon.

MlLLIKAN, R. A., 71

Mind. See Consciousness.

evolution of. See Psychogenesis.

unconscious, 2%, 297
Molecular substance, Molecular

layers, a name applied to the

Neuropil.

MOLHANT, M., 248
MONAKOW, C. VON, 171, 2S7, 293,
294, 300

tract of. See Tract, rubro-spinal.
MONHO, foramen of. Sec, Foramen

interventriculare.
Moon-eye, brain of, 302, 303
Morab,313. 314, 315
Mouct-Lis. S., 242, 248
Motor apparatus, 62, 63, 117, 120,

154, 163, 180-182, 186, 192, 234,

244,24(5. LM7. L'r.i, 279, 309
MOYKS. .1. M., 278
Mucous membrane, nerve endings in.

See Membrane, mucous.
MiM.KH, fibers of, 205, 206
Multiple consciousness, 297

INDEX AND GLOSSARY

337

MUNK, H., 300
Muscarin, 231

Muscle, Muscles, of arm, motor
nuclei of, 129

cardiac, the muscle of the heart, a
visceral muscle whose fibers are
cross-striated, 93, 148, 224, 234

of eyeball. See Eye, muscles of.

of facial expression, innervation of,
144, 146, 244, 261

intercostal, innervation of, 236-
240

involuntary, muscles not under di-
rect control of the will; they are
of the general visceral type, 93

nerve endings in, 86, 87, 90, 92, 93

respiratory, 236-239

sense, 77, 87, 132, 141, 146, 172-
180, 242

skeletal. See Muscle, somatic.

smooth or unstriated, visceral
muscle whose fibers are not
cross-striated, 87, 93, 147, 148

somatic, striated muscles derived
from the Somites of the embryo,
skeletal muscles, 87, 92, 145,
147, 148

spindle, a bundle of muscle-fibers,
smaller than ordinary fibers,
which are supplied with special
nerve endings of the muscle
sense in addition to typical
motor End-plates, 87

sternocleidomastoid, 144, 147

striated, composed of fibers hav-
ing a cross-striped appearance;
may be somatic or visceral, 87,
92,93

synergic, muscles which act to-
gether for the performance of a
movement. :{.">, 3(X>

of tongue. See Tongue, muscles of.

trapexius, innervation of, 144, 147

visceral, unstriated or striated
muscles not derived from the
Somites of the embryo ; may be
involuntary or voluntary, 87,
93, 94, 126, 14X, _'_'!

voluntary, muscles under direct

control of the will; may be

either somatic or visceral, 92,

93,94

Mustelus, nervous system of, 111,

112

22

Mycetozoa, 22

Myel (myelon), the Spinal cord.

Myelencephalon (afterbrain), the
posterior part of the Rhomben-
cephalon, or that portion of the
Medulla oblongata lying behind
the Pons and Cerebellum, 117,
118, 119, 121, 122

Myelin, a f at-like substance formed as
a sheath around the myelinated
(medullated) nerve-fibers. 46
sheath, an envelope of Myelin
around the Axis-cylinder of some
nerve-fibers, 40, 46, 108, 286

Myelogeny, the sequence of matura-
tion of the Myelin sheaths of
nerve-fibers in the development of
the central nervous system, 286,
287, 288

Myelon (myel), the Spinal cord.

Myotom. See Somites.

Myxomycetes, 22

Nates. See Colliculus superior.
Nausea, apparatus of, 89
Necturus, nervous system of, 62, 63
Neencephalon, the new brain, i. e.,
the cerebral cortex and its depend-
encies, 115, 263

Negative variation in nerve-fibers, 96
Neopallium, the non-olfactory part
of the cerebral cortex, or somatic
cortex, 217, 220, 221
Neothalamus (new thalamus), the
phylogenetically new part of the
Thalamus, which is a cortical de-
pendency, 163-167, 263, 3<X>
Nerve (nervus), any bundle of nerve-
fibers outside the central ner-
vous system, 28, 106
abducens (VI cranial nerve), 114,
120, 143, 145, 146, 148, 150,
180, 186

accelerator, of heart, 234.
accessory (XI cranial nerve), 120,

144. U.->. 1 17, 14S, 244
acoustic (VIII cranial nerve, audi-
tory nerve, nervus acusticiist,
110, 111,112,114, 120. 1 i:i. 1 }.'..
147, 149, 1.83, 199, 200, 201
afferent. See Afferent.
anterior cutaneous, 125
auditory. See Nerve, acoustic.

338

INDEX AND GLOSSARY

Nerve, auricular, 144, 147

branchial, 110, 111, 112, 149, 245

buccal, 200

cardiac. See Heart, innervation

of.

cerebral. See Nerve, cranial,
cerebro-spinal, the peripheral

nerves connected with the brain

and spinal cord, 107
cervical, 107, 130, 237
chorda tympani, 140, 245
ciliary, 146
coccygeal, 107
cochlear, 145, 157, 183, 185, 197,

198, 200, 201

components, table of, 146, 147
cranial (cerebral nerve), a periph-
eral nerve connected with the
brain; these nerves are enu-
merated in 12 pairs, 10(5, 110,
111, 112, 143, 144-150, 152,
154, 164

of fishes, 110, 111, 112, 148, 149
cutaneous, 79-86, 125, 132, 134,

1 i:i, 1 15-150, 157, 172-180,

228, 245, 246, 252, 253
of CYON, 235 . .».
of deep sensibility; cf. Proprio-

ceptors, apparatus of, 79, 86,

132, 172-180
depressor, of heart, 235
efferent. See Efferent,
excito-glandular, I OS
facial (VII cranial nerve, facialis),

110, 111, 112, 114, 120, 143, 145,

146, 148, 150, 231, 236, 243,

244, 245, 246
gtosBopharyngeal (IX cranial

nerve), 110, 111, 112, 114, 120,

144, 145, 147, 148, 149, 150, 155,

231, 24:5, 244, 245
gustatory. 213-246
hyomandibular, 110 112, 149
hypogloBBUB XII cranial nerve),

120, 144, 147, 148, 150, 153,

156
inhibitory, a nerve which checks

or retards the action of the

<>rti;m in which it terminates.

UK •_>::!, •_>:•,.-,

intem.stal, 125, 2:57, 238, 239
intermediate i nerve of \VIMSHKUC,

p-irs intermedia facialis, portio
intermedia facialis, the smaller

of the two roots of the VII

cranial nerve), 114, 120, 146, 244,

245
Nerve, intestinal, 111, 149

of JACOBSON. See Nerve, tym-
panic.

of LANCISI. See Stria longitudi-
nal! s.

laryngeal, 147

lateral (nervus lateralis, lateral
line nerves), branches of the
VII, IX, and X cranial nerves
which supply the Lateral line
organs, 110, 111, 112, 145,
148, 149, 152, 199, 200
accessory (rannis lateralis ac-

cessorius facialis), 149, 246
cutaneous, 125

lingual, 147, 245

lumbar, 107, 130, 231, 232

mandibular, 110, 111, 146, 200, 245

maxillary, 110, 111, 146, 226, 245

motor, a peripheral nerve which
conducts efferent impulses to a
muscle, IDS, 120, 145-150

oculomotor (III cranial nerve),
114, 120, 143, 145, 146, 14S, 149,
150, 160, 161, ISO, lS(i, 210,231,
246

olfactory (nervus olfactorius, the
first cranial nerve), 91, 110, 111,
112, 143, 146, 148, 149, 150, 160,
200, 215, 216, 217, 264

ophthalmic, 110, 111, 140, 149,
200, 245

optic (nervus opticus, the second
cranial nerve); this is not a true
nerve, but, in reality, a cerebral
tract; cf. Tract, optic, 111, 120,
123, 1 I:1-. I C), 140, 149,165,200,
204, 20r,, 20S, 209

otic, 149

of pain, 249, 251, 252, 253, 254,
257, 2;.s

palatine, a nerve of fishes corre-
sponding to the human great
superficial petrosal nerve, 110,
111, 112, 149

parietal (nerve of the Parietal
eye), 212

phrenic, 231). 237, 238, 239

pneumogastric. See Nerve, vagus.

postganglionic. See Neuron, post-
ganglionic.

INDEX AND GLOSSARY

339

Nerve, prcganglionic. See Neuron,
preganglionic.

prespiracular (pretrcmatic branch
of the facial), 111, 149

pretrematic, of facial, 111, 149

recurrent, 226

sacral, 107, 130, 231, 232

sciatic, 97

sensory, a peripheral nerve which
conducts afferent impulses from
a sense organ to the spinal cord
or brain, 108, 126

somatic, 126, 139, 145, 172

spinal, a peripheral nerve con-
nected with the spinal cord,
106, 107, 114, 125, 126,252
central connections of, 129-140,
150, 151, 251, 252, 253, 254
components of, 145, 147, 150,
151

splanchnic, 226

superficial petrosal, 147, 245

supratrmporal, 149

sympathetic. See Nervous sys-
tem, sympathetic.

of taste. See Gustatory appara-
tus.

terminal, a slender nerve associ-
ated with the olfactory nerve,
111, 147, 215

thoracic, 107, 125, 126, 130, 231,
232, 236, 237

trigeminal (trifacial nerve, V
cranial nerve), 110, 111, 114,
120, 141, 113, 1-15, 116, 1 is.

149, 150, 152, 154, 157, 174,

150, 213, 244, 245

trochlear (pat liel icus, VI cranial
nerve), 110, 11 I 120, IK!, 1 15,
lit), 1 is, 150, 154, 160, 162, ISO,
186

tympanic (nerve of JACOBSON),
147, 245

vagus (pneumogastric nerve, X
cranial nerve), 110, 111, 112,
114, 120, 1 11, 1 15, 1 17, 1 IS, |50,
152, 153, 156,226,2:',!, 2:; I 215

vasoconstrictor, 235

vasodilator, 235

vasomotor. See Vasomotor ap-
paratus.

vestibular, 88, 110, 111, 145, 147,
176, is:j, 1S4, 185, 187, 188, 19S,
200, 201

Nerve, vidian, 245

visceral, 126, 144, 145-150, 259
of WRISBEUG. See Nerve, inter-
mediate.

Nerve-cell. See Neuron.
Nerve-fiber, a slender fibrous proc-
ess of a Neuron, 39
afferent, 108

carbon dioxid production in, 96, 97
conduction in, 96, 97
degeneration of, 46
efferent; cf. also Efferent, 108
electric changes in, 96
fatigue of, 96, 101
medullated. See Nerve-fiber, mye-

linated.
myelinated, a fiber provided with

a Myelin sheath, 97, 108, 286
postganglionic. See Neuron, post-

ganglionic.

preganglionic. See Neuron, pre-
ganglionic.

rate of transmission in, 97, 98
regeneration of, 46
unmyelinated or unmedullated, a
fiber devoid of a Myelin sheath,
108
Nervous impulse, nature of, 96, 97

velocity of, 97, 98
Nervous system, the aggregate of all

nervous tissues.
autonomie; cf. Nervous system,

sympathetic, 225, 229
central, 28, 106, 107
cercbro-spinal, 76, 225
development of, 106, 115, 116,
117, 118, 120, 123, 153, 181,
1S2, 204, 219, 264, 286, 290
diffuse. 27, 5:i, ti(i, 227, 251
embryonic, See Nervous sys-
tem, development of.
evolution of; sec also Cortex,
cerebral, evolution of. and
Hemisphere, cerebral, com-
parative anatomy and evolu-
tion of, 22, 24, 27, 33, 34, 113,
115, 1 29, ISO, isl, 182, 212,
215, 219. '2-27. 251, 252, 253,
2i»:;. 2so, 291, :;oi. :;or,

general anatomy of, 106

cerebro-spinal visceral, 227 •
invertebrate, 27, 53, 227
nomenclature of, 115, 121. 122,
1 1':;, 127, 128

340

INDEX AND GLOSSARY

Nervous system, peripheral, 106
phylogeny of. See Nervous sys-
tem, evolution of.
physiology of, 96
segmental. See Segmentation

and Segmental apparatus,
subdivision of, 106, 115-123
sympathetic, 53, 65, 76, 89, 93,
106, 107, 125, 126, 147, 148,
150,211,224-233,234,241,
242, 255, 259
peripheral autonomous part,

225, 227
synaptic, 53
vertebrate, 29, 106
Neural canal. See Canal, neural,
groove (medullary groove), the
trough-like form assumed by the
Neural plate during its in-
\ -agination to form the Neural
tube.

plate, a thickened plate of Ecto-
derm in early vertebrate em-
bryos from which the Neural
tube develops.

tube, the embryonic central ner-
vous system when in the form
of an epithelial tube, 106, 116,
126, 160, 181
Neurasthenia, 103
Neuraxis, the central nervous sys-
tem; and also applied to the
Axon.

Neuraxon. See Axon.
Neurenteric canal, in the embryo, a
communication between the
raudal end of the Neural tube and
tin' digestive tract.
Neurilemma, the outer sheath of a

peripheral nerve-fiber, 40, Hi
Neurite. See Axon.
Neuroblast, an immature nerve cell,

:;'.». 45

Neurocyte. See Neuron.
Neurofibrils, delicate protoplasmic
fibrils within the cytoplasm of the
Neuron, 40, 46, 47| 102
Neuroglia (glia), a supporting fabric
of cells and horny fibers pervad-
ing the central nervous system, 38,
101. 190, 20.-,, 206,269
Neurogram, 295

Xeuromasts. See Organs, lateral
line.

Neuromere, one of the segments of

the embryonic Neural tube.
Neuron (neurocyte), a nerve cell;
cf. Cell, 38, 40, 41, 42, 49, 56, 96,
190, 268, 269, 270, 274
afferent, 42
bipolar, 44, 45
correlation, 133, 134, 158
efferent, 42

fatigue of, 96, 101, 102
of first, second, etc., order, 42
multiform, See Neuron, poly-
morphic,
polarization of. See Polarity of

the Neuron.

polymorphic, 268, 269, 274
postganglionic, an efferent sym-
pathetic neuron which is ex-
cited by a preganglionic Neuron,
94, 126, 146, 147, 229, 231, 235,
238, 239. 244

preganglionic, an efferent sympa-
thetic neuron whose cell body
lies in the central nervous sys-
tem, 93, 126, 146-14S, i;,(), 229.
231, 234, 235, 23S, 239, 241, 244
pyramidal, of cerebral cortex, 42,

44, 269, 270, 274, 290
retraction of, 103
type I, 43, 44, 190
type II, 43, 44, 190, 268, 269
unipolar, 45
Neurone. See Neuron.
Neuropil (molecular substance, dot-
ted substance), an entanglement
of unmyelinated fibers containing
many synapses, 65
Neuropore, in the embryonic brain
an opening between the anterior
end of the neural Canal and the
exterior, 116
Nicotin, 231

N id ul us. See Nucleus (2).
Nidus, a depression on the ventral
surface of the cerebellum; also
used as a synonym for Nucleus
(2), ins
NlSSL, F., 42, 46, 55,274

bodies of, granules of, substance
of. See Substance, chromo-
philic.

Nociceptor, a sense organ or Recep-
tor which responds to injurious
influences.

INDEX AND GLOSSARY

341

Node of Ranvier, an interruption of
the Myelin sheath of a nerve-fiber,
40

Node, vital, 240

Nomenclature. See Nervous sys-
tem, nomenclature of.
Nose. See Olfactory apparatus.
Nose brain (Rhinencephalon), 112,

123

Nucleus (1), the differentiated cen-
tral protoplasm of a cell, 39, 40,
41, 42, 47, 96, 99, 102, 108
Nucleus (2), a group of nerve-cells
within the central nervous sys-
tem; also called Xidulus and
Nidus; cf. Ganglion, 108.
of abducens nerve, 60, 146, 150,

154, 185, 201

acoustic. See Nucleus, cochlear.
ambiguus, 147, 150, 153, 154, 155,

156, 185, 244

amygdalae (amygdala), a small
mass of subcortical gray matter
under the tip of the temporal
lobe which forms part of the
Nucleus olfactorius lateralis,
144, 166, 273
anterior thalami, 164-166, 167,

220

arcuate, 155

of auditory nerve. See Nucleus,
cochlear. and Nucleus, vestibu-
lar.
of BECHTEREW, vestibular, 181,

185

caudate (nucleus caudal u-0, one
of the two large gray in;
the Corpus striatum, 114, 162,
166, 169. 170, 222
of CLAHKK. See Nucleus, dorsal,

of CLAKKK.
cochlear. 60, 63, 150, 154, 157.

164, 185, 201
coniinissural, of CA.TAL, 154, 164,

210, 244, 247

of DKITKHS, vestibular, 184, 185
dentate, a large nucleus embedded
within the cerebellar hemisphere
from which the fibers of the
Brachium conjunctivum arise,
114, 188, 190. 191. 201
dorsal, of Clarke (nucleus dor-
salis of CI.AKKK or STII.I.IM:.
CLARKE'S column), a longitu-

dinal strand of neurones of the

spinal cord whose axones enter

the spino-cerebellar tracts, 130.

137, 139, 176

Nucleus of dorsal funiculus. See
Clava and Tuberculum cune-
atum.

dorsal, of vagus. See Nucleus of
vagus, dorsal.

dorsalis thalami. See Nucleus
anterior thalami.

dorso-lateral, of spinal cord, a col-
lection of neurones in the ven-
tral gray column which inner-
vate the muscles of the limbs,
129, 130

of Edinger-Westphal, the visceral
efferent nucleus of the oculo-
motor nerve, 146, 150, 154, 246

emboliformis, 191, 201

of facial nerve, 146, 150, 154, 244

of fasciculus cuneatus. Sec Tu-
berculum cuneatum.

of fasciculus graeilis. See Clava.

of fasciculus solitarius, the visceral
sensory nucleus of the VII, IX,
and X cranial nerves, 150. 154,
155, 156, 231, 237, 239, 2 In, 2 13.
246. 217

fastigii, 191, 201

globosus, 191

of glossopharyngeus nerve, 150

habenulie. See Habenula.

of hypoglossus nerve, 1 17, 150,
154, 156

interpeduncular, a nucleus lying

between the cerebral peduncles
which receives the habenulo-
peduncular tract.

of lateral lemniscus. 185, 201

lateralisthalami, 163, 164 166, HIT.
170, 174, 176, 177, 25:!. 2s:>. :',()6

lattice, of thalamus. See Xiicleus
reticularis thalami.

lentiform f nucleus lentiformis, len-
ticular nucleus), one of the two
lame uray masses of the Corpus
striatum,' 114, 166, 169. 170

magnocellularis tecti. See Xudeiis,
mesencephalic, of V nerve.

masticatory. See Xucleus of
trigeminus, motor.

medialis thal.-imi. Hi:!. 164, 166,
167, 170, 174, 176, :}(Mi

342

INDEX AND GLOSSARY

Nucleus, mesencephalic, of V nerve,

146, 154, 161, 164, 180
motorius tegmenti, 181
of oculomotor nerve, 62, 63, 64,
146, 150, 154, 160, 161, 185, 211,
246

olfactorius anterior, the anterior
undiffcrentiated portion of
the Area olfactoria, 220
intermedius. See Tuberculum

olfactorium.

lateralis, the lateral portion of
the Area olfactoria, lying be-
tween the olfactory Bulb and
the Uncus, 219

medialis, the medial portion of
the Area olfactoria, contain-
ing the Septum and Gyrus
subcallosus, 219
olivary. See Olive,
of origin, 108, 128
pontile (pontile nuclei, nuclei pon-

tis), 158, 1S7, 188, 289
posterior thalami, 163, 164, 165,

167
preoptic (ganglion opticum ba-

salc), 220
reticulartt thalami (lattice nucleus

(litterschicht), 306
roof, of cerebellum (nuclei fas-
tigii, globosus, and embolifor-
mis), 188, 1<)1
ruber (red nucleus), 158, 161, 166,

188, 1X9, 210, 289
salivatory, 1 111, 147, 154, 156,241,

244, 247

of SCHWALBE, vestibular, 184, 185
of STILLING. See Nucleus, dorsal.

of CLAUKK.
terminal. los. 112
of trigeininus, chief sensory, 1 I'.i.

154, 157. 164, 174, 180, 244
motor, 1H5. 150, 154, ISO, 244
spinal (nucleus of spinal V
t met ; old term, gelatinous sul >-
stance of KOI.\\I>O of medulla
oblongata). 1 I'.i. 154, 155,156,
157. 164, 174, ISO, 244
of trochlear nerve. 1 If), 150, 154,

160. 185,201,211

of vagus, dorsal, 147, 150, 154,
155, 156, 2:5 1, 237, IKS, 239, 244
of ventral gray column of spinal
cord, 129, 130

Nucleus, ventralis thalami, 163, 164,
165, 166, 167, 174, 176, 177, 253,
285, 306

ventro-lateral, of spinal cord, a
collection of neurones in the
ventral gray column which in-
nervate the muscles of the
limbs, 129, 130

ventro-medial, of the spinal cord,
a collection of neurones in the
ventral gray column which in-
nervate the muscles of the
trunk.

vestibular, 143, 150, 154, 155, 156,

164, 176, 1X4, 185, 186, 201
XI-EL, J. P., 214

space of, 197
Number of BETZ cells, 284

of fibers in human pyramidal tract,
284

of neurones in cerebral cortex, 26

( )ni:i(STi:i\KH, H., 269
Oblongata. See Medulla oblongata.
Olfactory apparatus. See also Rhi-
nencephalon, 72, 74, 86, 91, 92, 110,
111, 112, 122, 146, MX, KM), 162,
163, 215-223, 273, 279
Olive, accessory, 155

inferior (oliva, nucleus olivaris,
olivary body), a large gray cen-
ter in the medulla oblongata
which produces an eminence on
its lateral surface, 114, 131, 155,
156, 15X, 174, 188
superior, a nucleus in the second-
ary auditory path embedded
in the medulla oblongata dor-
sally of the pons, 60, 164, 1X5,
20f

peduncle of, 201
OXI-K, H., 233

Operculum, the lobules of the fron-
tal, parietal, and temporal cere-
bral cortex which cover the Insula,
121, 170, 'Jr.*;

Ophthalmencephalon, the retina, op-
tic nerve, and visual apparatus of
the brain.

Opossum, cerebral cortex of, L'17
Optic apparatus. Sec Visual ap-
paratus.
chiasma. See Chiasma, optic.

INDEX AND GLOSSARY

Optic tectum, an optic reflex center
in the roof of the midbrain. See
Colliculus, superior,
thalamus. See Diencephalon.
vesicle. See Vesicle, optic.
Oral, pertaining to the mouth, or
directed toward the mouth, as
opposed to Caudal.
sense of EDINGER, 219
Organ (organon), a part of the body

with a particular function, 24
of CORTI. See Organ, spiral,
generative. See sexual organs,
lateral line (neuromasts), sense
organs in or under the skin of
fishes and amphibians of in-
termediate type between tac-
tile and auditory organs, 110-
112, 145, 148, 149, 152, 199,
200

parietal. See Parietal eye.
pineal. See Body, pineal,
spiral (organon spirale), the organ
of CORTI or receptor for sound in
the Cochlea, 85, 197, 198, 199
Ossicles, auditory, 195, 196
Oxydation in neurones, '.Mi, 07. 99
Oxygen as respiratory stimulus, 238

PACINIAN corpuscle, 79, 80, 88
I'ain, apparatus of; cf. Affection, 85,
89, 131, 132, 137, 138, 139, 141,
163-167, 172-174, 178-180, 228-
230, 243, 249-262, 286
conduction paths for, 249, 251,

252, 253, 254. 257. 258
referred, 22S, 1>29, 230, 261
thalamic renter for. See Thala-
mus, pain center in.
Palaeencephalon, the old brain. /. >..
all of the brain except the cerebral
cortex and its dependencies, 115,
363

Palaeothalamus fold thalamus), the
phylogenetically old part of the
Thalamus, present in animals
which lack the cerebral cortex, It'.:;.
166

Palate, 21:1,

Pallium. See Cortex, cerebral, 216
Pancreas, 224

Paralysis from central lesion. 17"..
178, 292

Paraphysis, an evagination of the
membranous roof of the telen-
cephalon in front of the Velum
transversum in some vertebrate
brains.

Parietal eye (parietal organ, pineal
eye, epiphyseal eye), a modifica-
tion of the pineal Body in some
lower vertebrates to form a dorsal
median eye, 162, 212
PARKER, G. H., 37, 75, 95, 199, 203,

212, 214

PARMELEE, M., 37
Pars intermedia of WRISBERG. See

Nerve, intermediate.
Pars mamillaris hypothalmi, the
mammillary bodies and their
environs, 118

optica hypothalami, the optic
Chiasma and its environs, 118,
121, 122

Pause, central. 98
PAWLOW, I., 242, 248
Pedagogy. Sec Education.
Peduncle (peduncuhis), a peduncle

or stalk. See Cms.
cerebellar, one of the fibrous stalks
by which the cerebellum is at-
tached to the brain stem. Then-
are three peduncles on each side:
(1) the superior peduncle (Bra-
chium conjunctivum), (2) the
middle peduncle iBrachium pon-
tis), (3) the inferior peduncle
(Corpus restiforme), 158, 187,
188

cerebral (pedunculus cerebri), the

ventral part of the mesen-

cephalon. 118, 119, 120, 121.

L58> mo, ic.7, 210, 211. :;o»i

of corpus callosum. See (lyrus

subcallosus.
of superior olive, 201
Perikaryon, the protoplasm sur-
rounding the nucleus in the
Cell body of a Neuron.
functions of, 99
IVrilymph, 1911

Perineureum, the connective-tissue
sheath surrounding a peripheral
nerve .
Peristalsis, 241
Peritoneum, 80, '_'."><)
IV- pedimculi. See Basis pedunculi.

344

INDEX AND GLOSSARY

Pharynx, innervation of, 144, 147,

243

PniLippsoN, M., 133, 135, 142
Photoreceptors, nervous End-organs

sensitive to light, 212
Phrenology, 280, 281, 285
Phylogeny of nervous system. See

Nervous system, evolution of.
Physiognomy, 280
Pia mater, the inner brain membrane.
Pigment, retinal. See Retina, pig-
ment of.

PIKE, F. H., 194
Pillar of CORTI, 197, 198

of fornix. See Fornix column and

Fornix cms.
Pilocarpin, 231
Pineal body. See Body, pineal.

eye. See Parietal eye.
Pituitary body. See Hypophysis.
Plants contrasted with animals, 22
Plasticity in behavior. See Behavior,

variable.

Plate (lamina), a general term ap-
plied to any flat structure or
layer; specifically to the six
longitudinal bands or zones into
which the Neural tube is divided
as explained in the following
definitions, 117.

dorsal (roof plate, Deckplatte), the
unpaired dorsal longitudinal
epithelial zone of the Neural
tube; it is non-nervous and in
some parts of the adult brain is
enlarged to form a Tela, 153.
dorso-lateral (alar plate, wing
plate, epencephalic region, Flii-
gelplatte), one of a pair of
dorso-lateral longitudinal zones
of the Neural tube; it gives rise
to the dorsal gray column of the
spinal cord and to the sensory
centers of the brain, 117, 120,
153

floor. See Plate, ventral,
neural. See Neural plate,
roof. See Plate, dorsal,
ventral (floor plate, Bodenplatte),
the unpaired ventral longitu-
dinal zone of the Neural tube;
it is originally non-nervous, but
in the adult is invaded bv the
ventral Commissure, 153

Plate, ventro-lateral (basal plate,
hypencephalic region, Boden-
platte), one of a pair of ventro-
lateral longitudinal zones of the
Neural tube; it gives rise to the
ventral gray column of the cord
and to the motor centers of the
brain, 117, 120, 153
Play, 257

Pleasantness, Pleasure. See Affec-
tion.

Pleura, 125, 250

Plexus, chorioid (choroid plexus,
plexus chorioidcus), a thin
non-nervous portion of the
brain wall to which highly
vascular Pia mater is adherent
and which is crumpled and
thrust into the brain ven-
tricles.

lateral, the chorioid plexuses of
the lateral ventricles of the
cerebral hemispheres, 222
of fourth ventricle (plexus cho-
rioideusventriculi quart i), the
chorioid plexus which forms
the roof of the fourth ven-
tricle, 119, 121, 152, 264
of third ventricle (plexus chori-
oideus ventriculi tertii), the
chorioid plexus which forms
the roof of the third ven-
tricle, 162, 166, 167
ganglionic, of sympathetic nervous
system, an entanglement of
sympathetic nerves and ganglion
cells; most of the nervous plex-
uses enumerated in the follow-
ing list are ganglionic plexuses
of this type, 225, 226, 241
nervous, an interlacing of differ-
ent kinds of nerve-fibers, 53
aortic, 226
of AfKHBACH (myenteric plexus),

241

brachial. 226
bronchial, 226,238
cardiac, 226, 235
celiac, 226
cervical, 226
coronary, 226
e<ophageal, 226
gastric, 226
hypogastrie., 226, 239

INDEX AND GLOSSARY

345

Plexus, nervous, lumbar, 226

of MEISSNER (submucous plex-

us), 53, 241
mesenteric, 226
myenteric (plexus of AUER-

BACH), 241
pelvic, 226
pharyngeal, 226
sacral, 226

solar. See Plexus, celiac.
submucous (plexus of MEISS-
NER), 53, 241
vesical, 226
Poisons, susceptibility of neurones

to, 97, 101-104, 231, 258
Polarity of the neuron, 39, 52, 97
POLIMANTI, O., 135, 142
Pons (pons VAROLII), a projection
from the under surface of the
medulla oblongata below the
cerebellum, 114, 118-120, 121,
122, 143, 154, 158, 187, 188,
220

nuclei of. See Nucleus, pontile.
Portio dura facialis (motor facial

root), 146

intermedia. See Nerve, interme-
diate.
major trigemini (sensory root of

the trigeminus), 146
minor trigemini (motor root of the

trigeminus), 146

Posterior, as used in this work means
toward the tail end of the body; as
used in the B. N. A. tables it
means toward the dorsal side, 115,
116
Posture, apparatus of, 77, 89, 130,

137, 2.54
Procuneus, 119

PRBNTISS, ('. \V., 198, 199, 203
Pressure, apparatus of. See Touch,

apparatus of.

Primitive sheath. See Neurilemma.
PRINCE, MORTON, •_><>.->, 297, 300
Prionotus caroliuus, nervous system

of, 151, 153

Process, axis-cylinder. See Axon.
ciliary, of eyeball, 143, 146, 232,

247

protoplasmic. See Dendrite.
Processus reticularis, the Formatio
reticularis of the spinal cord, 127,
129

Projection centers, those parts of the
cerebral cortex which receive or
give rise to Projection fibers ; cf .
Center, cortical, 166, 273, 282,
283, 284-289, 292, 306
fibers, fibers which connect the
cerebral cortex with the brain
stem, 164, 165, 166, 167, 168,
221, 266, 268, 269, 271, 284-
287, 289

Proprioceptor, a sense organ lying
within the deep tissues of the
body for the coordination of
somatic reactions, 77, 86
apparatus of, 130, 132, 137, 138,
139, 141, 145, 163-167, 172, 175-
179, 2.54, 286

Prosencephalon (forebrain), the Di-
encephalon and Telencephalon;
sometimes applied to the Cerebral
hemispheres only, 117-119, 121,

160

Protista, 22

Protopathic sensibility, a primitive
type of diffuse cutaneous sensi-
bility, especially on hair-clad
parts, 84, 85, 132
Protoplasm, living substance, 24, 69,

96

nervous, 38, 69, 96
Protozoa, one-celled animals, 24, 314
Psalterium. See Lyre of David.
Pseudocode. See Cavum septi pel-

lucidi.

Psychogenesis, the development of
mind, 249, 256, 257, 294, :;(>:>.
312-316
Psychology, general, 297

physiological, 297

Pulvinar, a visual center in the
thalamus, 150. 162, 163, 164, 167,
170, 208, 212, 2S4. 3<X>
PrKKiN.n:, cells of, 51, 52, 190, 191,

192

Purple, visual, 207
Putamen, a part of the Nucleus lenti-

formis.

Pyramid (pyramis), an eminence on
the ventral surface of the medulla
oblongata produced by the pyra-
midal tract and from which the
latter receives its name, 114, 155
Pyramids, deciissation of, 131
Pyriform lobe. See Lobe, pyriform.

346

INDEX AND GLOSSARY

Quale, a quality pertaining to any-
thing; specifically a quality of sen-
sation or other conscious process,
249, 261, 308, 309, 311

Rabbit, cortico-spinal tract of, 310
development of eye of, 204
spinal cord of, 133

Radiations, sensory, the thalamo-
cortical tracts. See Tract,
thalamo-cortical, and Corona
radiata, 287, 289
auditory, 170, 287
gustatory, 287

olfactory, the olfacto-cortical
tracts; the term has also been
applied to various subcortical
olfactory tracts, 287
optic, 170, 210, 287
somesthetic (of tactile and general

sensation), 287
Radix. Sec Root.
Rage. See Anger.
RAM6N Y CAJAL, S., 42, 44, 48, 50-
53, 55, 190, 191, 214, 237, 239, 240,
270-273, 278

Ramus communicans, a communi-
cating branch between the ganglia
of the sympathetic Trunk and tin-
roots of t lie spinal nerves, 125, 126,
225, 228

Range of behavior, 19, 303
RANVIER, node of. See Node of

Ranvier.

Hat, nervous system of, 220
Rate of nervous conduction, 97, 98
Reaction, a change in bodily state
in response to stimulation; cf.
Reflex, (Hi

avoiding. See 1,'eflex. ;ivoidinii.
discriminative, 9S, 258, 302, 308
time, the time required for re-
sponse to stimulation, 98, 258,
259
Heading, apparatus of. See Speech.

apparatus of.

Receptor, a sense organ, 25, 38, 69
contact, a >ense organ adapted to
respond to impressions from ob-
jects in cont.-ict with the body;
opposed to distance Receptor,
distance, a sense organ adapted to
respond to impressions from ob-
ject.- remote from the body, 2'.',

Recess, epitympanic, 195

infundibular, 118, 119

lateral, the widest part of the
fourth Ventricle under the cere-
bellum.

optic, the depression in the lateral
wall of the diencephalon formed
by the evagination of the optic
Vesicle, 116-119

utricular. See Utricle.
Reflex act, a simple form of inva-
riable Behavior requiring a
nervous system, 25, 32, 56,
109, 363
time of. See Reaction time.

allied, 57, 58, 59, 61

antagonistic, 57, 58, 59, 61

arc. Sec Reflex circuit.

avoiding, 251, 253, 258

of brain stem, 181, 192, 279, 280,
304, 305

bulbar, 181, 279

chain, 57,58,60,61,308

circuit, a chain of neurons which
function in a Reflex act, 25, 42,
56, 58, 60, 62, 63, (ill, 109. 113.
132, 133, 134, 260, 80S, 309,
311

conditional, 242

cortical, 286, 290

cyclic, 61, 309

discriminative. Sec Reaction, dis-
criminative.

of feeding; cf. Oral sense, 219, 279

locomotor, 134

of medulla oblongata, 181, 27'.»

myenteric. 'J 1 1

pattern, (ft, 219, 305, 307, :',12

proprioeeptive, 130, 175-193

of spinal cord, 129, 131 13.1, 174,
176, 179- iv.'. 185,234,258,279,
301. 305

thalamic. 163. 166. 171. isi. L':,:i.

•2:>\. :;m. :;oii, :;n
Hencneration of nervous ti.~ lie-. It'..

132
Region, cortical, a group of related

cortical Areas, 273. 276
Regulation, the process of adapta-
tion of form or behavior of an or-
ganism to changed conditions, 31
HKII.. island of. See Insula.
neinforcement, 59, 02, 63, 101, 192,

218

INDEX AND GLOSSARY

347

REISSNER, membrane of. See Mem-
brane, vestibular.
s Reptiles, cerebral cortex of, 216
Resistance, nervous, 104, 252, 258,

295, 296, 304, 307
Resolution, physiological, 58, 293,

304, 306, 307
Respiratory apparatus, 89, 144, 147,

232, 234-240
Restiform body. See Corpus resti-

forme.
Reticular formation. See Formatio

reticularis.
Retina, 123, 146, 204, 208

pigment of, 205, 207, 208, 211
Retraction of the neuron, 103, 104
RETZIUS, G., 85, 88, 124, 197, 203,

219

Reverberation, cortical, 293, 296
Rhinencephalon (nose brain), the
olfactory part of the brain, 111,
112, 118, 119, 123,215,273
Rhodopsin, 207

Rhombencephalon, that part of the
bruin below the Isthmus, includ-
ing the Medulla oblongata and
Cerebellum, 116-119, 121, 122,
123, 143

development of, 116-119
limits. \V. II. H..<>1. 9.1. 142
Rod of COKTI (pillar of COHTI), 197,

198

of retina, 205, 206, 207, 208, 211
ROLAXDO, fissure of. See Sulcus

centralis.

gelatinous substance of. See
Substantia gelatinosa Rolandi.
Root (rudix), a nerve root, or the
part of a nerve adjacent to the
center with which it is con-
nected; in the case of spinal and
crunial nerves, the part lying
between the cells of origin or
termination and the ganglion,
anterior. See Root, ventral,
dorsal (radix dorsalis, posterior
root, rudix posterior), (lie dorsal
or sensory Root of a spinal or
cranial nerve. 126, 128, 129,
130, 133, 134, 139, 150, 151, 227,
228, 252

posterior. Sec Root, dorsal.
spinal, composition of. 126. ]'.}'•>.
136, 150, 151, 22.'), 227

Root, ventral (radix ventralis, radix
anterior), the ventral or motor root
of a spinal or cranial nerve, 126,
128, 129, 130, 133, 134, 150, 151,
182, 227

Rostral, pertaining to the beak or
snout, or directed toward the
front end of the body as opposed
to Caudal.

Host rum of corpus callosum, 119

RUSSELL, J. S. RIESEN, 194

Sac, dorsal (saccus dorsalis), a dorsal

evagination of the Tela chori-

oidea of the third ventricle in

some vertebrate bruins.

endolymphatic (saccus endo-

lymphaticus), 196
nasal, 110, 111

Saccule (sacculus), part of the mem-
branous labyrinth of the ear, 85,
1^1. 195, 196, 199,200,201

SACHS, E., 171

SALA, C. L., SO

Saliva, secretion of. See also Gland,
salivary 146, 147, 241, 242, 217

Sarcophaga earnuriu, nervou< >y>tem
of, 30

Srala media. See Ductus cochleark-
tympuni, 197
vestibuli, 197

SCARPA, ganglion of. Sec Ganglion,
vestibulur.

SCHAKFF.K, E. A., 214

Sell \PF.K. A.. I'.'l
SroHNF.MAVX, A., 203

S( in I.T/.K, tract of (comma tract).

See Fasciculus interfuseiculuris.
SCHWAI.BK, vestibular nucleus of,

1M. 185
SCIIWANV. sheath of. See Neuri-

lemma.

Seyllium, nervous system of, 110
Sea-robin, nervou< ^y>tem of, 151,

153

Secret in. 221

Secretions, effect of fatigue and emo-
tion on. 103, 2.-).")

internal. 1»>3. 221, 231. 2.V.. L'.'.r,

psychic. 211. 212
Segment, mesodennul, or primitive.

See Somites.

348

INDEX AND GLOSSARY

Segmental apparatus, the Brain

stem, 113, 114, 123
Segmentation of nervous system, 28,

29, 30, 113, 125, 144, 150
Self-consciousness, 314
Senility, 316
Semicircular canals, nerve endings in,

88,89

SEMON, R., 295

Sensation, a subjective experience
arising in response to stimula-
tion, 70, 108, 249, 250, 256, 257,
261

common, 259
in lower animals, 72
neurological mechanism of, 250,

257, 261
visceral, 77, 91, 148, 228, 234-246,

250, 259
Sense, criteria of, 74

organ. See Receptor.
Sentiments. See Affection.
Septum, the medial wall of the cere-
bral hemisphere between the
Lamina terminalis and the ol-
factory Bulb; in man its upper
part is thin and forms the Sep-
tum pellucidum, 219, 220, 306
dorsal median, of cord. See Fissure,

dorsal.

pellucidum, a thin sheet of nervous
tissue forming a portion of the
medial wall of each cerebral
hemisphere between the Corpus
callosum and the Fornix, 162
Sexual organs, innervation of, 232

sensations from, 89
SHAMBAUGH, G. E., 198, 199, 203
Shark, nervous system of. See

Fishes, nervous system of.
Sheath, medullary. See Myelin

sheath.

myelin. See Myelin sheath.
primitive. See Neurilemma.
of SCHWANN. See Neurilemma.
BHBLDON, H. s., 95, 248
SHKKKKV, .!., 94, 142
SIIKKIU\<;TO\, ('. S.. '.',:>. :!7. ti.1, C,S,
I:.. 77. '.).-,, r_H. i:U. 11_>. 17'_>. 180.
I'.H. •_'!:;. -J.-,i). •_>:,'.». LT.O, LV,L>. 281.
282, 300
Sight, organs of. See Vi-'Mal appara-

toa

Sinus, inferior, of labyrinth, 196

Skin brain, 112, 123
nerves of. Sec Nerves, cutaneous,
nerve-endings in, 79, 80, 81-83, 84,

86, 245, 253

sensibility of, 70, 72, 79, 80-86,
132, 172-180, 212, 228-230,
245, 249, 250, 252, 253, 260
Sleep, 103, 104, 297
Smell, organs of. See Olfactory ap-
paratus.

SMITH, G. ELLIOT, 263, 273, 278
Sneeze, mechanism of, 238
Social evolution, 314, 315
Somatic area. See Area, somatic,
cortex. See Neopallium.
nerves. See Nerve, somatic.
organs, those concerned with the
adjustment of the body to its
environment, 76, 79, 92, 109,
172

Somesthetic apparatus, the general
somatic sensory systems, includ-
ing cutaneous and deep sensibil-
ity, 164, 165, 172-180
Somites (myotoms, primitive seg-
ments, mesodermal segments),
-eirinented masses of mesoderm in
vertebrate embryos which give
rise to the somatic muscles, 92
Sound, reaction time to, 9S

receptors for. Sec Auditory ap-
paratus.
Space, discrimination of, 130, 132,

137, 172, 178-1 MI
of NUEL, 197
perforated. S<-e Substantia per-

forata.

Speecli, apparatus of (including read-
ing and writing); cf. Aphasia, 283,
291-293

SPENCER, HERHKHT, 17
Sphere, cortical. See Center, corti-
cal.

Spiders, nervous system of, 29
SPIELMKVI.U, W., L'M
Spinal cord (medulla spinalis), that
portion of the central nervous
system contained within the
spinal Canal of the spinal
column, 106, 107, 110-112,
117, 118, 120, 122, 125, 126,
127. 128-130, 150, 151, 182
cervical, 12S. 129, 130
development of, 182

INDEX AND GLOSSARY

349

Spinal cord, functions of, 129, 132,
182, 234, 237, 238, 251-253,
304, 311
lesions of, 173, 175, 178, 179,

237, 238, 251
tracts of, 130, 139, 141, 174,

176

Spiracle, a rudimentary gill cleft in
some fishes, represented in mam-
mals by the auditory or Eusta-
chean tube, 110, 111, 112
SPITZKA, E. C., 108
Splanchnic, visceral, 76
Spongioblast, one of the epithelial
cells of the embryonic Neural tube
which becomes transformed into
an Ependyma cell.
Si>t -RXHEIXI, J. K., 280, 281, 300
STABLER, ELEANOR M., 75
Stalk, optic, 204
STEINER, J., 135, 142
Stem. See Brain stem.
STILES, P. G., 101, 105, 248
STILLING, dorsal nucleus of. See

Nucleus, dorsal, of CLARK.
Stimulus, a force which excites ;m

organ to activity, 69
adequate, 25, 38, 70, 76
Stomach, 144, 147, 224, 234, 239,240-

243

STREETER, G. L., 200, 203
Stria acustica. Sec Stria medul-

laris acustica.

of BAILLARGER. See Line of Bail-
larger,
of GENNARI. See Line of Gen-

nari.

longitudinalis (stria of LANTISF,
nerve of LANCISI), slender bun-
dles of nerve-fibers running
along the dorsal surface of the
Corpus callosum in the floor of
the longitudinal fissure.
medullaris acustica, secondary
acoustic fibers arising in the
dorsal cochlear nucleus and
decussating across the floor
of the fourth ventricle to
reach the opposite lateral
Lemniscus, 201

thalami, a band of fibers accom-
panying the Taenia thalami
along the dorsal border of the
thalamus, ^untaining thetrac-

tus olfacto-habenularis, tractus
cortico-habenularis, and other
fibers, 162, 165, 166, 167, 220
Stria, olfactoria intermedia, a sec-
ondary olfactory Tract from
the olfactory Bulb to the Tu-
berculum olfactorium, most
of its fibers first crossing
in the anterior Commissure,
219

lateralis, a secondary olfactory
Tract from the olfactory Bulb
to the Nucleus olfactorius
lateralis, 219

median's, a secondary olfactory

Tract from the olfactory Bulb

to the Nucleus olfactorius

median's, 219

semicireularis. See Stria termina-

lis.

terminalis (stria semicireularis, old
term, tacnia semicireularis), a
correlation tract between the
Nucleus amygdalae of the lateral
olfactory Area and the medial
olfactory Area, 114, 162, 267
vascularis of cochlea, 197
Striate area. See Area striata.
body. See Corpus striatum.
Stripe of BAILLAKGER. See Line of

Baillarger.

of GENNARI. See Line of Gennari.
of HKXSKN-, 197, 198
Sturgeon, nervous system of, 151,

152
Subconscious mind. See Uncon-

.-cioiis cerebration.

Subiculum, that part of the Gyrus
hippocampi which borders the
fissura hippocampi; sometimes ap-
plied to the whole of this gyrus.
222
Substance, black. See Substantia

nigra.

chromophilic (Nis.-i, >ul»tance. ti-
groid substance, or bodies, or
granules), a proteid substance
typically present in the cyto-
plasm of nerve-cells. 40, 41, 42,
r>. Ki. 48, w. 102. i:;r,. 2M
gray. See Matter, gray.
perforated. See Substantia per-

fprata.
white. See Matter, white.

350

INDEX AND GLOSSARY

Substantia alba. Sec Matter, white,
gelatinosa Rolandi (gelatinous sub-
stance of ROLANDO), an area of
Neuropil bordering the dorsal
gray column of the spinal cord;
sometimes also applied to the
nucleus of the spinal V tract in
the medulla oblongata, 129
grisea. See Matter, gray,
nigra (black substance), an area of
gray matter immediately dor-
sal of the Basis pedunculi,
functionally related to the
cortico-pontile tracts, 161, 165,
107, 210

perforata, anterior (anterior per-
forated substance or space),
a region on the ventral sur-
face of the brain in front of
the optic Cniasma which is
pierced by many small arte-
ries, 120, 219, 300
posterior (posterior perforated
substance or space), a region
on the ventral surface of the
brain between the Bases pe-
dunculi which is pierced by
small arteries, 120
Subthalamus, the ventral part of
the Thalamus, 1(5:3, 165, 166, 1G7,
174, 176, 3<X>

Sulcus, in the cerebral cortex, a
superficial fold not involving
the entire thickness of the brain
wall; cf. Fissure, 266
anterior parolfactory, 119
central (fissure of ROLANDO, cru-
ciate sulcus), 119, 121, 281, 282
cinguli, 119
corporis callosi, 119
cruciate. See Sulcus, central,
frontalis, inferior, 121

superior, 121
interparietalis, 121
limiting (sulcus limitans), a longi-
tudinal groove on the ventricu-
lar surface of the embryonic
brain separating the dorao-
latcral sensory Plate from the
ventro-lateral motor Plate, 30,
117, 118, 120, 153, 181
occipitalis transversus, 121
posterior parolfactory, 119, 219
precentralis, 121

Sulcus rhinalis. See Fovea limbica.

spiralis, 197, 198, l«jy
Summation, central. See Conduc-
tion, avalanche, and Reinforce-
ment.

of stimuli, the enhancement of
effect by repeated stimulation,
59, 62, 03, 192, 208, 218, 258,
200, 208, 307

Suprasegmental apparatus, the cere-
bral cortex and cerebellum with
their immediate dependencies,
113, 120, 123, 143, 158, 186
Susceptibility of neurones to poisons,

97, 231

Swallowing, apparatus of, 78, 247
SYLVIUS, aqueduct of. See Aqueduct

of Sylvius.

fissure of. See Fissure, lateral.
fossa of. See Fossa lateralis.
Symbolizing, defects of, 292
Sympathetic nervous system.

Nervous system, sympathetic.
Synapse, the place where the nervous
impulse is transmitted from one
neuron to another, 50, 51, 52,
53, 54, 90, 97, 103, 109, 190,
218, 231, 252, 20s, 209. 272, 295
fatigue of, 102, 103
time of transmission through, 54,

98,99
Synergic muscles. See Muscles,

synergic.

System, functional, all neurons of
common physiological type. Most
peripheral nerves contain several
components belonging to different
systems, 145-150

Tabanus bovinus, nervous system of,

30

Taenia, the line of attachment of a
membranous part to a massive*
part of the brain wall; formerly
applied also to some fiber
tracts, as Ta-nia semicircularis
= Stria terminalis, and Ta-nia
thalami = Stria medullaris
thalami.

chorioidea, the line of attachment
of the lateral chorioid Plexus to
the medial wall of the cerebral

INDEX AND GLOSSARY

351

hemisphere. (This portion of the I
f medial wall is adherent to the thal-
amus, forming the Lamina affixa.)
Taenia fornicis, the line of attach-
ment of the lateral chorioid
Plexus to the Fimbria of the
Fornix.

thalami, the line of attachment of
the Tela chorioidea of the third
ventricle to the dorsal margin
of the thalamus. This name was
formerly applied to a band of
fibers, the Stria medullaris
thalami, which borders the
ta-nia. 162.

ventriculi quarti, the lino of at-
tachment of the membranous
roof of the fourth ventricle to
the medulla oblongata, 155.
TASIUKO, S., 97, 105
Taste, apparatus of. See Gustatory

apparatus.
bud, 91, 143, 144, 218, 243, 245,

246
peripheral nerves of. See Nerves,

gustatory.

Taxis. See Tropism.
Tectum mesencephali," the roof of
the mid!>rain, comprising the
Colliculus superior (teetum op-
ticinn) and the Colliculus infe-
rior, 161, 246
optic. See Colliculus, superior.

Teeth. 85, 141), 2M)

Tegmen ventriculi quarti, the root' of
the fourth ventricle, formed chiefly
by the Velum medullare anterius,
the Velum medullare posterius,
and the Plexus chorioideus ven-
triculi quarti.

Tegmentum, the dorsal part of the
cerebral Peduncle between tin-
Basis pedunculi and the Aqueduct
of Sylvius; often described as al<o
extending backward into the cor-
responding part of the medulla
oblongata, IX, 181, 182
Tela, any thin non-nervous part of

the brain wall.

chorioidea, that portion of the
Pia mater which covers any
thin non-nervous part of the
brain wall, including the chori-
oid Plexuses.

Telencephalon (endbrain), the ante-
rior end of the embryonic
Neural tube and its adult de-
rivatives, comprising chiefly the
cerebral hemispheres and Lam-
ina terminalis, 117-119, 121-
123, 160

medium, that portion of the em-
bryonic Telencephalon which is
not evaginated to form the cere-
bral hemispheres; it comprises
chiefly the Lamina terminalis
and Pars optica hypothalami,
122

Telodendron, the terminal branched
end of a Dendrite; sometimes ap-
plied also to that of an Axon; cf.
Terminal arborization.
Temperature, apparatus of, 71, 84,
131, 132, 137, 138, 139, 141, 163-
167, 172-174, 242, 254, 260
Tendon, nerve endings in, 87

sense, 77, 87, 132

Tentorium cerebeUi, a transverse
fold of Dura mater between the
cerebellum and the cerebral hemi-
spheres.

Terminal arborization, the- branched
end of an axon; sometimes ap-
plied also to that of a Dendrite,
40
Terminology. See Nervous system,

terminology of.

Te-tes. See Colliculus, inferior.
Thalamencephalon. See Dienceph-

alon.

Thalamus, the middle and larger
subdivision of the Dienceph-
alon, sometimes applied to the
entire diencephalon and called
Thalamus options, 63, 112, 114,
117, 121-123, 141, 162, 1(13,
164-166, 167, 174, 176, 204,
210, 279, 311
lesions of, 253, 254, 279
new. See Neothalamus.
old. See Palaeothalamus.
options. See Thalamus.
pain center in, 167, 253, 254, 258,

260, 261, 311
respiratory center in. I'll)
Thirst, apparatus of, s'.i
THOMPSON, T., 142, 17.'), 179, 262
Thorns of dendriles, '!,'!

352

INDEX AND GLOSSARY

Threshold, the minimal stimulus
which will excite an organ to ac-
tivity, 72, 80, 91, 132, 172, 218,
252, 254, 255
Tickle, 76, 254

Tigroid bodies, substance, or gran-
ules. See Substance, chromo-
philic.

Time, central. See Pause, central,
latent. See Pause, central,
reaction. See Reaction time.
Tissue, the cellular fabric of which

the body is composed, 24
TITCHKNEU, E. B., 70
Tone, affective. See Feeling tone

and Affection,
analysis, 198, 199, 202
feeling. See Feeling tone and

Affection.

muscular, 77, 89, 189, 192
nervous, 101, 192

Tongue, muscles of, 92, 144, 147, 148
nerves of, 143, 144, 146-148, 243,

245

Touch, apparatus of, 62, 70, 79, 80-
83, 131, 132, 137, 138, 139, 141,
103-167, 172-180, 242, 243, 245,
252, 253, 257, 260
reaction time of, 98
Toxines. See Poisons.
TOZEU. F. M., 180
Tract (tractus), a collection of nerve-
fibcrs of like origin, termination,
and function; cf. Fasciculus, 27,
100, 128, 304
association; cf. Fibers, association,

58, (54, 267, 285
bulbo-spinal, 157
central tcgmcntal. 188
cerebello-tegmental, 188
comma. See Fasciculus interfas-

cicularls.

cortico-bulbar, 161, 169, 170, 181
cortico-cerehellar. See Tract, cor-

tico-pontile.

cortico-mesencephalic, 289
cortico-oculomotor, 170
cortico-pontile, 161, 169, 170, 187,

188, 289

cortico-rubral, 170, 289

cortico-spinal (fasciculus ccrebro-

spinalis, B. X. A., pyramidal

tract), the voluntary motor

path from the precentral gy-

rus of the cerebral cortex to the

spinal cord, where it divides into

lateral and ventral parts, 128, 130,

131, 140, 141, 156, 161, 167, 168,

170, 181, 188, 192, 282-285, 289,

293, 310, 311

Tract, cortico-spinal, lateral (fas-
ciculus ccrebro-spinalis later-
alis, B. N. A., lateral or
crossed pyramidal tract), 130,
131, 141

ventral (fasciculus cerebro-
spinalis anterior, B. N. A.,
ventral or direct pyramidal
tract, column of TCRCK),
130, 131, 141

cortico-thalamic, 289

direct cerebellar. See Tract,
spino-cerebellar, dorsal.

of FLECHSIG. See Tract, spino-
cerebcllar, dorsal.

of GOWERS. See Fasciculus ven-
tro-lateralis superficialis.

habenulo-peduncular (fasciculus
retroflexus, MEYNERT'S bundle),
165, 220

of HELAVIG. See Tract, olivo-
spinal.

internuncial, a fiber tract connect-
ing two nuclei or centers, 65

of LISSAUER. See Fasciculus dor-
so-lateralis.

of LOWENTHAL. See Tract, tecto-
spinal.

mamillo-peduncular, 161, 220

mamillo-thalamic (fasciculus

thalamo-mamillaris B. N. A.,
tract of VICQ n'AzYit), 165, 220

inesencephalic, of V nerve, 161

of MEYNERT. See Tract, habcn-
ulo-peduncular.

of MONAKOW. See Tract, rubro-
spinal.

nomenclature of, 128

oUactory (tractus olfactorius), ol-
factory fibers of the second
order passing from the olfac-
tory Bulb to the nuclei of the
Area olfactoria. See Stria ol-
factoria, 114, 120, 165, 217, 218,
219, 220

olfacto-liypothalarnic, 220

olfacto-tegmental, 220, 221

olivo-cerebellar, 155, 176, 188

INDEX AND GLOSSARY

353

Tract, olivo-spinal (HELWIG'S bun-
dle, tractus triangularis), 130,
131

optic (tractus opticus), that por-
tion of the optic path which
passes between the optic Chi-
asma and the optic centers in
the thalamus and midbrain.
(The term might properly be
extended to include also the so-
called optic Nerve), 114, 120,
161, 166, 167, 208, 209, 210,
222

ponto-cerebellar, 188
predorsal. See Tract, tecto-spinal.
projection. See Projection fibers,
pyramidal. See Tract, cortico-

spinal.

respiratory, 240
rubro-spinal (MONAKOW'S tract),

130, 131, 156, 161, 188, 189
rubro-thalamic, 188
of SCHULTZE. See Fasciculus in-
ter fascicular is.
secondary gustatory. See Lem-

niscus, visceral.
visceral. See Lemniscus, vis-
ceral.

septo-marginal, 130, 131
solitary. See Fasciculus solitarius.
soli tar io-spinalis, 237, 239, 240,

247
of spinal cord. Sec Spinal cord,

tracts of.
spinal, of V nerve, 14X, 149, 153,

155, 156, 174, ISO, 244
of vestilmlar nerve, 155
spino-cerebellur, 114, 137, ISO
dorsal (fasciculus cerebello-spi-
nalis, B. N. A., direct cere-
bellar tract, FLECHSIG'S tract ),
130, 139, 156, 176, 188
ventral (part of (ioWKiss' fas-
ciculus, or Fasciculus antero-
lateralis superCicialis, B. N.
A.), 128, 130, 139, 156, 176,
188

spino-olivary, 130, 131, 176, 188
spino-tectal, 128, 130, 131, 161,

179
spino-thalamic, lateral, 130, 131,

139, 179

ventral, 130, 131, 139
tecto-cerebellar, 176, 188

23

Tract, tecto-spinal (predorsal bundle,
tract of LOWENTHAL), 130, 131,
140, 156

terminology of, 128

thalamo-bulbar, 181

thalamo-cortical; cf. Projection
fibers and Radiations, 170, 174,
176, 289, 306

thalarno-oh'vary, 161

thalamo-peduncular, 165

thalamo-spinal, 181

triangular. See Tract, olivo-spinal.

vestibulo-cerebellar, 157, 176, 187,
188

vestibulo-spinal, 130, 131, 140,
157, 176, 185

of VICQ D'AZYR. See Tract,

mamillo-thalamic.
Transmission of nervous impulse.

See Conduction, nervous.
Trapezoid body. See Body, trape-

zoid.

Trigonum habenulae, a triangular
area at the posterior end of the
Habenula, 162

hypoglossi (eminentia hypoglossi),
a ridge in the floor of the fourth
ventricle produced by the XII
nucleus, 154, 156

olfactorium, a triangular expan-
sion of the Cms olfactoria from
which the Striae olfactoriae arise.

vagi. .See Ala cinerea.
Tropism, a simple form of invariable

Behavior not requiring a nervous

system, 57

THOTTER, W., 79, 84, 95, 172
Trunk (truncus), the main stem of a
nerve from which the branches
(ranii) arc given off. See Nerve.

sympathetic (ganglionated sympa-
thetic cord, sympathetic chain,
vertebral sympathetic chain), a
strand of sympathetic nerves
and ganglia extending along
each side of the vertebral col-
umn, 107, 225, 226
Tube, auditory (Eustachean tube),
195

neural. See Neural tube.
Tuber cinereum, a gray eminence

forming the ventral part of the

Hypothalamus, 114, 119, 120,

163, 167, 306

354

INDEX AND GLOSSARY

Tubercle, anterior, of thalamus, an
eminence on the dorsal surface;
formed by the Nucleus anterior
thalami, 162

Tuberculum acusticum of fishes
(part of the Area acustico-
lateralis), 148, 149, 152, 200
of mammals (the dorsal cochlear

nucleus), 201

cinereum, an eminence on the
lateral aspect of the medulla
oblongata produced chiefly by
the spinal V tract and its nu-
cleus.

cuneatum, an eminence on the
dorsal surface of the lower end
of the medulla oblongata later-
ally of the Clava produced by
the nucleus of the Fasciculus
cuneatus, 130, 139, 141, 156,
164, 176, 177, 188
fascia- dentata?, 220
olfactorium (lobus parolfactorius
of EDINGER), the intermediate
olfactory Nucleus, lying in the
Substantia perf orata anterior ; cf .
Area olfactoria, 219, 220, 273
Tunnel of COHTI, 197
TI'KCK, column of, the ventral cor-

tico-spinal Tract.
Ti -UN-Kit, W. A., 194
Tympanic membrane, Tympanum.
See Membrane, tympanic.

cerebration, 297, 309,
311

mind, 29<i, 297, 309
Unconsciousness, 297
Uncus, the hook-shaped extremity of
the Gyrus hippocampi, part of the
Archipallium, 219, 273

rnpleasaiitness. See Affection.

Utricle (utriculus, recessus utriculi),
part of the membranous labyrinth
of the inner ear, 85, 183, 195, 196,
200, 201

Valve of ViKfssK.Ns. Set? Velum

medullare anterius.
Valvula eerehelli. See Velum medul-

lare anterius.
Variable behavior. See Behavior,

variable.

Variation, negative, in nerve-fibers,

VAROLI (VAROUUS). See Pons

Varolii.

Vas spirale, 197

Vasomotor apparatus, the neuro-
muscular mechanism which con-
trols the amount of blood sup-
plied to any part, 104, 114, 232,
234, 235

Veins, nerves of. See Vasomotor ap-
paratus.

Velocity of nervous conduction.
See Nervous impulse, velocity of.
Velum anticum. Bee Velum medul-
lare anterius.
interpositum, the Tela chorioidea

of the third ventricle,
medullare anterius, a thin portion
of the brain wall containing a
few myelinated fibers which
forms the roof of the fourth
ventricle in front of the cere-
bellum, 119, 154
posterius, a thin portion of the
brain wall containing a few
myelinated libers which forms
a small part of the roof of the
fourth ventricle immediately
behind the cerebellum.
superius. See Velum medullare

anterius.

transversum, a transverse fold of
the Tela chorioidea which marks
the boundary between the Di-
encephalon and the Telenceph-
alon in the embryonic brain.
Ventral, on the front or belly .side of
the body, termed Anterior in the
B. N. A. lists, 115

Ventricle, a cavity within the brain
and spinal cord derived from
the lumen of the embryonic
Neural tube.

fifth. See Cavum septi pellucidi.
first. Sec Ventricle, lateral,
fourth (metactt'le), the ventricle of
the medulla oblongata, 119, 121,
152, 264

lateral (paraccele), the ventricle
of each cerebral hemisphere;
these are also called first and
second ventricles, 121, 170, 210,
222, 264

INDEX AND GLOSSARY

355

Ventricle, second. See Ventricle,

lateral.

third (diaccele), the ventricle of
the dieneephalon, 121, 162, 170,
264, 265

VERATTI, E., 50

Vermis cerebelli (worm), the middle
lobe of the cerebellum, 119, 187,
191, 201

Vertebrates, behavior of, 33
nervous system of, 29

VERWORN, M., 37, 101

Vesicle, optic, an outgrowth from
the lateral wall of the dicnceph-
alon which forms the nervous part
of the eyeball. It first assumes
the form of a simple hollow sphere,
the primary optic vesicle, which
later collapses to form a two-lay-
ered optic cup, or secondary optic
vesicle, 116, 117, 204

Yestilmlar apparatus, 88, S9, 110,
111, 131, 145, 147, 150, 183-193,
11)'.), 200, 201

YeMige-:, memory, in cortex, 295-
2<)7, 304, 306-306

Vibrations, table of, 72

Vibri.ssu-, innervation of, 80

Vicarious function in cortex, 294

VICQ D'AZYR, tract of. See Tract,
mamillo-thalamie.

YINCKNT, STELLA B., 80, 95, 214

Viscera, the internal organs, espe-
cially those concerned with tin1
internal adjustments of the body,
70, 89, 93, 109, 144, 234-243, 250

Yisreral apparatus. 224-247, 250, 259
nerves. See Nerve, visceral,
brain. 111'. r_':j

Vision, stereoscopic, 209, 210

Visual apparatus, 62, 63, 71, 86, 110-
112, 123, 145, 146, 150, 160, 163-
167, 170, 186, 204-213, 268, 279

VOGT, O., 273, 278, 287

Voluntary movement, apparatus of,
78, 166, 181, 192, 222, 240, 241,
279, 285, 286

Vomiting, mechanism of, 239, 243

WALDEYER, W., 49, 55

Warmth, sensations of. See Tem-
perature, apparatus ol'.

WASHBURN, A. L., 248

WASHBI'KN, MARGARET F., 37

\\ M SON, J. B., 37, 95, 203, 214, 262

WEIGERT, method of, 129, 274

WILLEMS, E., 180

\\II.MS, circle of. See Circle of
Willis.

WILSON, J. G., 85, 95, 194, 243, 248

WOODWOKTH, R. S., 98, 105, 213

Word-blindness (Alexia), 292

Word-deafness, 292

Worm. See Vermis cerebelli.

Worms, nervous system of, 27, 28,
227

WKISHERG, nerve of. See Nerve, in-
termediate.

Writing, apparatus of. Sec Speech,
apparatus of.

WUNDT, W., 98

YKHKKS, R. M., 33, 37, 63, 68
Zone, cortical. See Center, cortical.

Of LlSSAUER. See I'a.-cicillus

dorso-lateralis.
of neural tube. Sec Plate.

ZWAARDEMAKER, H., 223

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