FINER ANATOMY

OF THE

CENTRAL NERVOUS
SYSTEM

AN INTRODUCTION TO THE

FINER ANATOMY

OF THE

CENTRAL NERVOUS
SYSTEM

BASED UPON THAT OF THE

ALBINO RAT

WITH TWENTY-EIGHT PLATES AND TEN ILLUSTRATIONS
IN THE TEXT

BY

E. HORNE CRAIGIE, Ph.D.

ASSISTANT PROFESSOR OF COMPARATIVE ANATOMY AND NEUROLOGY
IN THE UNIVERSITY OF TORONTO

PHILADELPHLA

P. BLAKISTON'S SON & CO.

1925

COPYRIGHT, CANADA, 1925

BY

E. HORNE CRAIGIE, Ph.D.

PRINTED IN CANADA

PREFACE

SEA'ERAL guides for the study of the gross anatomy of
the brain have been published but up to the present
time there has been available no small handbook
devoted to the minute anatomy of the nerve centres and
pathways^which is at least as important as the gross anatomy
— in any laboratory animal. The present text is an attempt
to fill this gap— to provide a brief but comprehensive intro-
duction to the functional anatomy of the central nervous
system of a common laboratory type of size convenient for
the use of serial sections.

As the book is designed primarily for elementary students,
all important differences from the human structure (which
are not very numerous but are sometimes quite instructive)
are pointed out, and an introductory chapter upon the
general principles of nervous structure has been included,
as also an appendix outlining a simple form of the Weigert
method of preparing sections. From tAvo or three brains
enough short series of sections for a fair sized class may be
made.

Besides being a suitable form for didactic purposes, the
rat is so widely used in research that an atlas of the anatomy
of its central nervous organs and a summary of what has
been recorded regarding them should be valuable to many
investigators. With this end in view, an attempt has been
made to include all details of the neuro-anatomy of this type
which are to be found in the literature except those of the
histological localization in the cerebral cortex. Such details
are set in small type w^hen they are not of immediate import-
ance to the elementary student. A bibliography has been
added to bring up to date, so far as the central nervous system

6 Anatomy of the Nervous System

is concerned, that published by Donaldson in 1915. It is
hoped that it has been possible in this way to provide a
useful manual for research workers without lessening the value
of the book as an introductory text.

Special acknowledgements are due to Dr. Henry H.
Donaldson. The author is indebted also to Dr. B. A. Bensley,
to Dr. J. W. Papez, who kindly sent him manuscript notes of
his observations upon the brain of the rat, to Dr. C. Judson
Herrick for reading the manuscript, for the sketch upon
which Fig. 6 is based, and for permission to use Fig. 5, to
Dr. J. B. Johnston for Figs. 7 and 8, to Dr. /E. B. Droog-
leever Fortuyn for Fig. 10, and to the W. B. Saunders Co.
for Figs. 1 and 4.

F. HoRNE Craigie.
University of Toronto,
June, 1925.

CONTENTS

CHAPTER I. PAGE

Gross Anatomy of the Central Nervous System 9

CHAPTER H.

Fundamental Structure of Nervous Organs. Principle

of Nerve Components 17

CHAPTER HI.

Spinal Cord 22

CHAPTER IV.

Medulla Oblongata — General Somatic Afferent System . 29

CHAPTER V.
Special Somatic Afferent System 35

CHAPTER \T.
Visceral Afferent System 42

CHAPTER VH.
Motor System 45

CHAPTER Vni.

Correlation Centres and Tracts in the Medulla Ob-
longata and Pons 50

CHAPTER IX.

Cerebellum 56

CHAPTER X.
Midbrain 59

2

bX\3>

8 Anatomy of the Nervous System

CHAPTER XI. PAGE

DIencephalon 72

CHAPTER XH.

Telencephalon — Rhinencephalon 87

CHAPTER XHI.

Telencephalon — Non-olfactory Portions 105

Bibliography 116

Appendix. Preparation of Sections — Method of

Weigert 126

Plates 128

Index 185

CHAPTER I.
Gross Anatomy of the Central Nervous System.

WHILE the gross features of the brain may be studied
a little more conveniently if a larger type than that
of the rat is employed, the latter nevertheless
shows most of the important structures very satisfactorily.

The spinal cord extends through the vertebral canal from
the base of the skull to the sacral region. It is roughly
cylindrical, but presents two slightly swollen regions which
are somewhat depressed. These are the cervical and lumbar
enlargements, from which arise respectively the nerves for
the fore and hind limbs. Posteriorly, it tapers off to end in
the sacrum in a fine thread, the filum terminale, which runs
back for some distance in the tail. From the spinal cord
arises a series of paired spinal nerves, the first of these emerging
between the skull and the first vertebra, while the remainder
emerge between successive vertebrae. In the cervical region,
all the nerves except the last are named from the vertebrae
in front of which they emerge, but the nerve behind the last
cervical vertebra is also reckoned to this region, so that w^hile
there are seven cervical vertebrae, there are eight pairs of
cervical nerves. Each of the remaining nerves is named
from the vertebra behind which it emerges. Thus there are
thirteen pairs of thoracic nerves, six pairs of lumbar nerves,
four pairs of sacral nerves, and three pairs of caudal nerves.

Each spinal nerve is attached to the cord by dorsal and
ventral roots, and on the former may be observed a slight
enlargement, a spinal ganglion or dorsal root ganglion. Similar
masses occur near the bases of some of the cranial nerves, the

10 Anatomy of the Nervous System

cerebral ganglia, though the latter are not very evident in
gross dissection.

An almost imperceptible dorsal median sulcus (sulcus
medianus posterior) runs along the dorsal middle line and a
deep groove, the ventral median fissure (fissura mediana
anterior), occurs in the corresponding position ventrally. A
short distance to each side of the dorsal median sulcus is
another fairly distinct groove, in which the dorsal roots of
the spinal nerves enter the cord. This is the dorso-lateral
sulcus (sulcus lateralis posterior). The line of attachment
of the ventral roots is indicated by a much fainter depression,
the ventro-lateral sulcus (sulcus lateralis anterior).

A dorsal view of the entire brain (Plate I.) shows the
three primary subdivisions, the cerebral hemispheres not
being sufficiently developed in the rat to conceal entirely
the more posterior sections. Anteriorly, the forebrain or
prosencephalon is represented by the cerebral hemispheres
and the olfactory bulbs; posteriorly, the hindbrain or rhomb-
encephalon is represented by the cerebellum and the medulla
oblongata; while between the cerebral hemispheres and the
cerebellum, appear the posterior colliculi of the midbrain or
mesencephalon.

In the hindbrain, the cerebellum appears as a massive
thickening of the roof in the anterior part of the region,
which has attained so great a size as completely to conceal
the large cavity of the hindbrain, the fourth ventricle. Its
surface is thrown into numerous transverse folds, and shows
a division into different regions, of which the conspicuous
ones are the median vermis, the large paired cerebellar
hemispheres (lobuli ansiformes), at each side of it, and the
small, stalked floccular lobes projecting laterally. The
visible part of the floccular lobe is the paraflocculus, which
forms the greater part of this lobe in the rodent, the flocculus
being only an inconspicuous lobule at the base of the stalk.

Gross Anatomy 11

The cerebellar hemisphere represents only a part of the much
larger hemisphere of man.

The cerebellum consists morphologically of three lobes,
anterior, middle, and posterior, the fissures separating them
being the sulcus primarius and sulcus praepyramidalis re-
spectively (see Fig. 3). The anterior lobe consists of four
transverse lobules, but only the last of these can be seen
from the dorsal surface. The posterior lobe is composed of
a median part and the lateral floccular lobes, the median
part being made up of pyramis, uvula, and nodulus (named
dorso-ventrally), of which only the pyramis and the large
uvula behind it appear from above. The middle lobe like-
wise consists of median and lateral portions, the lateral ones
being the hemispheres, all of which are divided by transverse
grooves into narrow gyri.

The cerebellum may be removed by carefully cutting
through the peduncles which attach it to the medulla ob-
longata at each side. When this is done the cerebellum is
found to constitute only a very narrow transverse strip of the
actual roof of the ventricle below it. The greater part of
the ventricle is roofed by two membranes attached to the
cerebellum along this strip and extending forward (velum
medullare anterius or anterior medullary velum) and back-
ward (velum medullare posterius or posterior medullary
velum) to the extremities of the widely open fourth ventricle.
The posterior medullary velum contains greatly convoluted
groups of blood vessels which project into the ventricle,
pushing the velum before them — these vessels forming the
chorioid plexus of the fourth ventricle.

The trochlear nerve (I\') decussates in the anterior edge
of the anterior medullary velum and may be seen emerging
round the postero-lateral aspect of the midbrain.

The middle cerebellar peduncle, which has been cut
through, extends straight ventrally and forms a thick trans-
verse mass of fibres crossing the most anterior part of the

12 Anatomy of the Nervous System

ventral surface of the hindbrain — the pons (pons X^arolii).
Immediately behind this (PI. II.) is another, much less dis-
tinct transverse band, the trapezoid body. This shows at
each side of the middle line a marked elevation, which is the
anterior end of a distinct ridge, the pyramid, extending along
each side of the ventral median sulcus from the lower end
of the medulla oblongata up to this point, where the sulcus
itself is obliterated by the transverse fibres.

Coming back to the dorsal surface, the large fourth
ventricle or rhomboid fossa may now be studied (PI. III.).
This cavity narrows anteriorly into the aqueduct of Sylvius,
the cavity of the midbrain, and posteriorly into the central
canal of the spinal cord. A narrow deep posterior median
sulcus runs along the floor of the ventricle in the middle
line, and for about half the length of the ventricle a narrow,
low ridge, the funiculus teres, bounds this fissure at either
side. Just in front of this ridge the floor of the ventricle
forms a considerable eminence at each side, under which
lies the genu of the facial nerve (p. 48) and which is conse-
quently called the colliculus facialis. In front of this, a very
slight pit at each side of the ventricle is the fovea superior.
On a level with the funiculus teres appears a pair of large
rounded lateral elevations, which contain the vestibular
nuclei. The slight lateral pit at the posterior end of this
eminence is the fovea inferior. The triangular formation at
the posterior extremity of the ventricle is the calamus scrip-
torius.

The medial parts of the dorsal columns of the spinal cord
(funiculi graciles) end in a pair of enlargements, the clavae,
which are spread apart by the ventricle, and in front of each
of these the thick wall lateral to the vestibular nucleus is
formed by the restiform body or inferior cerebellar peduncle.
In the cut surface of the peduncles, this body, the middle
peduncle or brachium pontis, and the superior peduncle or
brachium conjunctivum can all be distinguished. The last

Gross Axatomy 13

of these forms the lateral wall of the anterior part of the
ventricle.

Immediately behind the cut part of the peduncles, crossing
the restiform body, is a transverse ridge, the tuberculum
acusticum, which appears to become continuous ventro-
laterally with the eighth or auditory nerve. Anterior and
ventral to this, the facial nerve (MI) emerges from the side
of the medulla, and just anterior to it again is the root of the
trigeminal nerve (\'), in which large, sensory and small,
motor parts may be distinguished.

In order that the midbrain may be observed the posterior
dorsal portions of the cerebral hemispheres must be raised
and pressed apart. This reveals two pairs of rounded emi-
nences, the corpora quadrigemina, which make up the dorsal
part of the midbrain. The ventral part is considerably
narrower antero-posteriorly than is the dorsal part, so that
the region is somewhat wedge-shaped. As seen from below,
it appears in a depression between the pons behind and the
cerebral hemispheres and mamillary body in front. A pair
of massive longitudinal fibre-tracts, the pedes pedunculi
cerebri, converge from the hemispheres and disappear under
the pons, covering the whole ventral aspect of the midbrain
except a median hollow between them, the interpeduncular
fossa. Through each peduncle emerges the oculomotor
nerve (III).

In the forebrain — indeed in the brain as a whole — ^the
most conspicuous structures are the large cerebral hemis-
pheres, which in the rat and other small animals have smooth
surfaces, but which in the larger mammals are greatly con-
voluted. The hemispheres of the rat are much smaller pro-
portionately than those of man and the higher mammals.
They are separated by a deep median cleft, the longitudinal
cerebral fissure.

At the front of each hemisphere lies the olfactory bulb, in
which end the numerous fine strands of the olfactorv nerve

14 Anatomy of the Nervous System

(I). Running back from this on the ventral surface is a
narrow, white band, the olfactory tract (PI. II.). The ventral
part of the hemisphere, along which the olfactory tract runs,
is marked off by a longitudinal groove, the rhinal (or limbic)
fissure, which separates this region (the olfactory lobe)
laterally from the more dorsal parts of the hemisphere. The
gray matter subjacent and lateral to the olfactory tract
forms the lateral olfactory gyrus, which enlarges posteriorly
into the pyriform lobe, while the rounded gray mass medial
to the tract is usually known as the tuberculum olfactorium,
though it represents also the anterior perforated area (see
p. 91). The superficial part of the brain dorsal to the rhinal
fissure is entirely non-olfactory in its relations, and is known
as the neopallium.

If the hemispheres be pressed apart so as to open up the
longitudinal cerebral fissure, a broad white band is seen con-
necting them. This is the corpus callosum.

Projecting between the dorsal posterior poles of the
hemispheres and lying in the furrow between the superior
corpora quadrigemina, is the pineal body, a small gland
attached by a stalk to the unpaired part of the forebrain,
the diencephalon. The latter may be exposed by raising, or
better by cutting away the back parts of the hemispheres.
The stalk of the pineal body is attached to the extreme
posterior portion of the membranous roof of the cavity of
the diencephalon, the third ventricle. This roof, like that
of the fourth ventricle, contains a chorioid plexus, and must be
removed to expose the ventricle. The latter is almost com-
pletely obliterated by the fusion of its lateral walls over the
greater part of their area, forming the soft commissure or
intermediate mass. The dorsal part of the ventricle is bounded
at each side by a conspicuous longitudinal ridge, the habe-
nula, the posterior ends of the two habenulae being connected
by a thin curved band, the habenular commissure. Lateral
to this, the wall is extremely massive, containing the thalamus

Gross Axatomy 15

and related parts (chapter XIII), Two eminences appear on
its lateral surface, the smaller, posterior one being the medial
geniculate body, the larger, anterior one the lateral geniculate
body, and in front of this the diencephalon is attached laterally
to the cerebral hemisphere (corpus striatum). A deep, oblique
groove, into which fits the fimbria of the hippocampus (vide
infra), marks roughly the boundary between thalamus and
corpus striatum on the dorsal surface.

The lateral geniculate body is covered with white fibres
of the optic tract, w^hich may be follow^ed dow^n and forward
to the optic chiasma, the decussation on the ventral surface of
the brain of the two optic tracts. From the chiasma the
tracts continue forwards as the optic nerves.

Behind the optic chiasma lies the pituitary body, or
hypophysis, which is attached to the base of the brain by
a thin, hollow stalk, the infundibulum. The hypophysis is
frequently left behind when the brain is removed from the
skull, the infundibulum being broken through, in w^hich case
the cavity of ihe infundibulum appears as a small median
slit opening into the third ventricle. This slit is surrounded
by an elevation of gray matter, the tuber cinereum, and this
is continuous posteriorly with the mamillary body, a con-
spicuous mass projecting postero-ventrally over the anterior
part of the interpeduncular fossa. There are paired mamil-
lary bodies in the human brain, but in the rat they are repre-
sented by a single median mass.

If one hemisphere be removed, there may be observed on
the part of its medial surface which lies in contact with the
diencephalon a distinct curved groove, the hippocampal
fissure, and parallel and a little anterior to it a conspicuous
ridge of white fibres, the fimbria hippocampi (PI. l\.). Be-
tween these is a fainter groove, which marks the div^ision
between the fascia dentata or dentate gyrus, lying between
it and the hippocampal fissure, and the hippocampus proper,
covered with a thin laver of white fibres which join the

16 Anatomy of the Nervous System

fimbria. The dorsal end of this (the postcommissural) part
of the hippocampal formation is connected posteriorly
through the gyrus subsplenialis with a narrow ridge, the
fasciola cinerea. This, in turn, curves round the splenium
of the corpus callosum and runs forward as the very thin
gyrus supracallosus or indusium griseum. All these struc-
tures are illustrated in PI. IV.

Scraping away the gray matter, or cerebral cortex, from
the dorsal part of the hemisphere reveals the white fibres of
the corpus callosum, and this also may be removed so as to
expose the cavity of the hemisphere, the lateral ventricle.
The postero-medial part of the floor of the ventricle is formed
by a curved ridge, the hippocampus, while the small antero-
lateral portion appears as a convex mass, part of the corpus
striatum. Between these projects the chorioid plexus of the
lateral ventricle. This is continuous with that of the third
ventricle through the foramen of Monro or interventricular
foramen, which connects these ventricles. In front of the
foramen, the medial wall is formed by the septum.^

Division of the brain in the median plane reveals another
instructive view, showing to particular advantage the larger
commissures and the relations of the ventricles, besides other
features. In addition to the commissures already mentioned,
there may be observed the hippocampal commissure, which
appears as a thin flat band directly ventral to the posterior
part of the corpus callosum with a thickened anterior portion;
the anterior commissure, a distinct cylindrical strand crossing
in the anterior wall of the third ventricle (the lamina termi-
nalis); and the posterior commissure, a less conspicuous
structure just below the habenular commissure (PI. I\'.,
XXVI.).

iSee p. 93.

CHAPTER II.

Fundamental Structure of Nervous Organs.
Principle of Nerve Components.

THE nervous system is made up of cells which fall into
two categories — those which perform the charac-
teristic nervous functions and those which do not.
The cells of the latter class, the neuroglia cells, are mainly
supportive in function and form a framework in which lie
the cells of the former group, the true nerve cells or neurons.
There are also strands of ordinary connective tissue extending
into the substance of the nervous organs from the membranes
which cover them, usually accompanying blood-vessels.

The neurons vary considerably in form and in structural
details, but the typical neuron consists essentially of a cell
body or perikaryon, a varying number of branched receptive
processes, the dendrites, and a single efferent process, the
axon, axis-cylinder, or neurite. The axon frequently has a
sheath of fatty material (myelin), the myelin (medullary)
sheath, and may give off small branches, usually perpendicular
to the main fibre, the collaterals. The nerve cells pass their
impulses from one to another through connections known
as synapses. The essential structure of the synapse is still
the subject of dispute, but it is usually held to be simply a
point of close contact, without actual continuity, between
the terminal of an axon and the next cell. The details of
structure of the nerve cell are described in practically all
text-books on the anatomy or physiology of the nervous
system and will not be considered here.

The perikarya of the afferent neurons are nearly always
situated outside the central nervous organs, where most of

18 Anatomy of the Nervous System

them are grouped in small masses, or ganglia. As noted in
the previous chapter, a ganglion occurs on the dorsal root of
each spinal nerve and near the bases of some of the cranial
nerves — i.e., near the point of entry into the central system
of every afferent nerve. These perikarya have certain
peculiarities of form, and it has been shown that in the rat
their volume varies during growth in direct proportion to
the area or skin-surface of the animal. This relation is con-
sidered to be an adaptation for maintaining the sensory
discrimination despite the extension during growth of the
area supplied by a single neuron. ^

The main functional unit in the nervous system is a chain
of such neurons consisting in the simplest possible condition
of two elements — a receptive, sensory, or afferent cell, which
receives a stimulus at the periphery and conducts the result-
ing impulse to the central organ, and a motor or efferent
cell, which is stimulated by the impulse in the afferent neuron
and transmits the resulting impulse to a muscle. The muscle
thus brought into action is termed the effector, while the
chain of neurons is called a reflex arc. It is the mechanism
which brings about that immediate, involuntary response
to a stimulus which is known as a reflex action.

Seldom, if ever, does the reflex arc consist of only two
neurons. There is practically always at least one internuncial
neuron between the afferent and efferent elements, and in
the great majority of cases there are many more. The whole
nervous system is made up of an infinitely complex system
of such arcs variously connected and superposed.

The internuncial neurons frequently are connected with
several other cells from which they receive stimuli, often of
different kinds, so that the impulse in such an element may
be the resultant of several stimuli instead of the effect pro-
duced by a single one. The internuncial cell thus becomes a
correlation neuron, and a group of such cells is a correlation

^Donaldson and Nagasaka, 1918.

Fundamental Structure 19

centre, if the various stimuli received are of different kinds.
A correlation neuron may send its impulses to several other
elements, which may include both efferent cells and other
correlation neurons. Thus one correlation centre may
influence another, and it itself may be controlled by yet
another, and so on. Hence we have lower and higher func-
tional levels in the central nervous system, the lowest being
the simple reflex level, the higher the correlation levels, of
which there are many, and the higher levels exerting a direct-
ing or controlling influence over the lower ones. To a certain
extent these correspond roughly with the structural levels,
the simple reflexes being confined to the lower parts of the
brain and to the spinal cord, w^hile only the lower correlation
centres are found near these, the higher ones occurring in
more or less regular succession towards the upper or anterior
part of the brain, and the highest being in the cerebral cortex.

Any part of the nervous system which is composed chiefly
of cell bodies and dendrites or unmyelinated axons appears
rather grayish in the fresh condition and hence is known as
gray matter, while the presence of a large number of axons
with myelin sheaths (myelinated fibres) gives a region a clear
white colour, so that it has received the name of white matter.

In all parts of the system, neurons of similar function
tend to be associated topographically. Hence we find more
or less definite groups of cells, or masses of gray matter,
which are concerned with particular functions and which are
given the name of nuclei. The axons proceeding from a
particular nucleus or group of nuclei also associate them-
selves into definite groups whicK are called fibre-tracts, when
all the fibres have similar connections. A bundle con-
taining fibres belonging to more than one tract is called a
fasciculus. It is the task of neuro-anatomy to identify the
various nuclei and tracts, showing how they are related, both
structurally and functionally, and to trace out in detail the

20 Anatomy of the Nervous System

pathways of the excitations which constitute or underlie all
nervous functions.

One of the most fruitful conceptions which have been
brought to bear upon the study of the structure of the central
nervous system is that of functional components, which has
made possible the analysis upon a functional basis of the
main patterns in the almost inconceivably complex system of
nerve centres and tracts which confronts the student of the
finer anatomy of the brain. According to this principle, all
peripheral nerve fibres may be divided into four groups
according, first, to whether they conduct impulses towards
(afferent) or away from (efferent) the central nervous system ;
and, second, to whether they connect at the periphery with
somatic or visceral structures. Moreover, while this suffices
for a complete classification of the fibres in the spinal nerves,
each of these groups as it occurs in the cranial nerves may be
subdivided into two components — general and special —
according to whether the structures innervated have changed
greatly from their primitive condition or not — e.g., the simple
nerve endings in the intestine belong to the general visceral
components while the gustatory fibres are special visceral
sensory. Thus the cranial nerve components are eight^ in
number: — general and special somatic afferent, general and
special visceral afferent, general and special somatic efferent,
and general and special visceral efferent.

The importance of this analysis from the standpoint of
neuro-anatomy is due to the fact that these different com-
ponents, having been identified at the periphery, can be
traced into the brain, where they are found to connect with
distinct centres of gray matter, or nuclei. The fibres arising
in these nuclei may thus be recognized as secondary fibres
of the particular functional type being studied, and these

^Actually only seven of these are found in the cranial nerves of
mammals, the general somatic efferent group being confined to the spinal
nerves.

Principle of Nerve Components 21

may be followed to their termination in secondary nuclei.
Thus the analysis may be continued until the various com-
ponents are found to end in centres common to two or more
of them — correlation centres, and from these, connections
may be followed to higher correlation centres, and the func-
tional pattern of the whole brain may be elucidated.

This principle is of value not only to the research worker,
but also to the beginning student, whom it provides with
what is probably the best available line of attack in approach-
ing a difficult subject. Experience has shown that the most
satisfactory way of making a first study of the finer anatomy
of the brain is not to study each section as a whole but to
follow out each functional system through the series of
available sections, completing its examination before looking
at the elements belonging to another system. This method
of presentation will accordingly be employed in the following
pages.

CHAPTER III.
The Spinal Cord.

IN transverse sections, the spinal cord shows a large mass of
gray matter surrounding the central canal, with white
matter disposed outside it. The shape of the section as
a whole and the shape of the mass of gray matter vary some-
what according to the level of the cord from which the section
is taken, the relative areas of gray and white matter also
varying (see PL V.). The gray matter, however, always
appears in transverse sections to have roughly the shape of
the letter H. The afferent nerve roots enter the cord opposite
the dorsal horn of the H, or dorsal column of gray matter
(cormc dorsalis), while the efferent fibres arise from cells in
the ventral horn, or ventral column of gray matter {cornu
ventralis), and leave the cord opposite it. The superficial
part of the dorsal column is distinct from the rest, having a
clearer appearance, and is known as the gelatinous substance
of Rolando {substantia gelatinosa Rolandi). This is very
massive in the rat, where it does not vary very greatly in
shape at different levels of the cord, as in man and many
other animals, but forms a broad, thick cap over the under-
lying parts.

The section is divided into lateral halves by the deep
fissure which runs along the ventro-median aspect of the cord
{ventral or anterior median fissure) and the shallow dorso-
median fissure, which is continued in towards the central
canal by a sheet of connective tissue, the dorsal or posterior
median septum. In each of the lateral halves thus formed,
the white matter is further divided by the gray columns and
the nerve roots into dorsal, lateral, and ventral funiculi. The

Spinal Cord 23

line of entry of the dorsal roots is marked by a superficial
groove, the dor so -lateral groove, and between this and the
dorsal median fissure is an intermedio-lateral groove, from
which also a connective tissue septum extends inward. The
last mentioned septum, which appears only in the upper levels
of the spinal cord, divides the dorsal funiculus into a lateral
fasciculus ctineattis {column of Burdach) and a medial fascicidus
gracilis {column of Goll) .

The various functional pathways, or fibre-tracts, within
these funiculi cannot be distinguished in normal adult material,
and have to be traced by experimental methods or by a study
of development. The only exceptions to this statement in
the case of the rat are the cortico-spinal tract and the tract
of Lissauer, which are described below. Such analyses have
not been made in the spinal cord of the rat, but as the general
pattern appears to be essentially similar throughout the
mammalia, a brief account of the pathways, based chiefly on
the human subject, is included here. A diagram of the
arrangement in the rabbit is also given in Fig. 2.

General Somatic Afferent System.

The peripheral receptive elements of this system fall into
two groups, exteroceptors and proprioceptors, of which the
first are stimulated by touch, pressure, heat, or cold, pain
due to excessive stimulation also being included usually in
this group, while the second transmit impulses arising in the
muscles, joints, etc. Impulses of this character from any
part of the animal except the head are carried to the spinal
cord, where they are passed on to secondary fibres which are
grouped according to the particular one of the above types
of sensation which they subserve.

Upon entering the spinal cord, each somatic sensory fibre
typically divides into two branches, which run up and down
the cord respectively, the ascending branch being the longer.
The fibres bearing pain and temperature impulses soon end

24 Anatomy of the Nervous System

in the gray matter of the dorsal column. Of those bearing
tactile impulses, some end very soon in the dorsal column,
but others extend for a longer or shorter distance up the
cord before terminating. Of the proprioceptive fibres, some
end after a short course in the cord while others run right
up to the lower end of the brain in company with some of
the exteroceptive fibres, which give the discriminative char-
acter to sensibility (Head). All these primary fibres running
along the spinal cord are grouped together in the dorsal
funiculus, which is thus made up largely of ascending pro-
prioceptive fibres mingled with some exteroceptive elements,
a number of shorter exteroceptive fibres, both ascending and
descending, and also certain axons which simply run from
one part of the cord to another — longitudinal association
fibres. At each level, the entering fibres take a position
lateral to those ascending from behind. Hence the fasciculus
gracilis contains the fibres which have come up from the more
posterior part of the cord, the fasciculus cuneatus those which
belong to the more anterior segments.

The cells of the dorsal column of the gray matter which
receive impulses from these primary sensory fibres give rise
to secondary axons, many of which pass across the middle
line and take up a position in the lateral or ventral column
of white matter on the opposite side, there ascending to the
brain. These fibres are grouped in separate bundles according
to the type of sensory impulse which they carry, and all the
bundles together are known as the spinal lemnisctis. Others
of the secondary fibres enter the deeper part of the white
matter in any of the three columns on the same side and pass
up and down the cord for varying distances, finally ending
in connection with the cells of the cord itself and thus serving
as association paths. Yet other secondary axons arising in
the dorsal column go into the ventral column to connect
there with the motor cells giving rise to the ventral nerve
roots. These fibres are thus part of short reflex paths pro-

Spinal Cord 25

ducing immediate responses at the same level as that where
the stimulus is received.

Those proprioceptive fibres which do not ascend to the
brain mostly end in connection with a group of cells near
the base of the dorsal column known as Clarke s column
{nucleus dorsalis), from which the secondary fibres pass to a
superficial position on the same side, where they ascend as
the dorsal and ventral spino-cerebellar tracts. This nucleus
is not clearly defined in the rat.

The dorsal nerve root contains unmyelinated afferent
fibres, of w^hich the function is not known, but which may be
somatic. These, upon entering the cord, take up a position
superficial to the gelatinous substance of Rolando (PI. \'.),
where, along with a number of endogenous fibres, they form
a thin layer which may be detected in normal sections because
the large preponderance of unmyelinated fibres makes it
stain very lightly by the Weigert method and very deeply
by silver methods. This is the tract of Lissaiier {fasciculus
dor so-later alls). The fibres run a short distance up or down
the cord and finally end in the gelatinous substance of Rolando.
The greater part of the tract is located in the lateral funiculus
in the rat, where'there is an area just lateral to the gelatinous
substance which has the same structure as the thin band
superficial to the latter and which is continuous w^ith it.

General Visceral Afferent System.

The peripheral fibres of this system convey from the
visceral organs to the central nervous system impulses which
do not ordinarily affect consciousness, but which make
possible the nervous control of the functioning of these organs.

They enter the spinal cord through the dorsal nerve root
along with the somatic afferent fibres and apparently end in
the ventro-lateral portion of the dorsal column. The im-
pulses may then be transmitted either to the visceral efferent

26

Anatomy of the Nervous System

neurons of the cord or up to the brain, but the exact course
of the fibres concerned in this has not yet been demonstrated.

Eferent Systems.

The efferent nerve fibres arise from cells situated in the
ventral column of the gray matter. These cells are arranged
in groups which correspond more or less to the areas of dis-
tribution of the fibres and which accordingly differ somewhat
in different regions of the spinal cord. The visceral efferent
cells form a group of small neurons situated laterally near the
base of the ventral column in the thoracic region, this group
being called the inter medio -lateral cell column. They lie
in a projection of the gray matter into the white matter,
which is named the lateral column and which is seen in the

Fasc. gracilis-

Fasc. cuneatufi

Pass, septo-marg.

Fasc. inter-fascic.

Tr.cortico^in. lat.

Tr. nibro-epin.

Nuc. doreo-lat.

Nuc. ventro-med.

Nuc. ventro-lat.

Tr. cortico-spm. ven .

Tr. olivo-spinalis

Tr. tecto-epinalis

Tr. vestibulo-spin.

Radix ventralis

Radix dorsalia
Fasc. dorso-lat.
Tr. spino-cereb. dor.
Columna don'alia
Fasc. proprius dora.
Fasc. proprius lat.
Tr. sp'mo-cereb. ven.
Tr. spino-thalam. lat.
Columna veotralis
Tr. spino-tectalis
— Tr. spino-thalam. ven.
Tr. spino-olivaris
Fasc. proprius ven.
Fasc. sulco-marg.

Fig. 1

Diagram of transverse section of fifth cervical segment of human
spinal cord, from Herrick, Introduction to Neurology.

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 are indicated. In the white matter, the out-
lines of some of the more important tracts are schematically indicated,
ascending fibres on the right side and descending fibres 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 fibres are mingled
in these regions.

Spinal Cord 27

thoracic and upper cervical regions in man. In the rat cord
examined by the writer, this projection was evident only in
the last two cervical and the first thoracic segments (PI. V.).

Fig 2.

Diagram of transverse section of the spinal cord of the rabbit f
in the region of the cervical enlargement, after Winkler. According to
V. Lenhossek and to Linowiecki, the cortico-spinal tract includes the
area here assigned to the rubro-spinal tract and extends dorso-laterally
to the surface of the cord. It will be observed that the location of the
crossed cortico-spinal tract in the rabbit, unlike that in the rat, cor-
responds with that in man.

The efferent cells receive their stimuli chiefly, if not
entirely, through correlation fibres which may come from
neighbouring or distant parts of the cord or may descend
from the brain. The most important descending tracts in
the white matter are shown in the diagram (Fig. 1). The
only one of these which can be observed in sections of the
normal spinal cord of the rat is the cortico-spinal or pyramidal
tract {fascicidiis c or tico- spinalis cruciatus sen lateralis),^
which stands out on account of the very poor myelination
of its fibres. It may be seen forming the ventral or deep
portion of the dorsal funiculus (PI. \^), a position which differs
entirely from that occupied by the same tract in man, where
it lies in the dorsal part of the lateral funiculus. In man,

^The direct or ventral cortico-spinal tract of man does not appear to
be represented in the rat (see p. 53).

28 Anatomy of the Nervous System

moreover, its fibres are as well myelinated as are those sur-
rounding it, so that it is not distinguishable in sections of
normal material. The course of the cortico-spinal tract in
the brain will be described later.

CHAPTER IV.

Medulla Oblongata — General Somatic Afferent
System.

THE transition from the spinal cord to the medulla oblon-
gata is gradual, the various parts changing their form
and relations by degrees. The boundary is usually con-
sidered arbitrarily to be at the decussation of the pyramids.
The greater part of the medulla oblongata contains the wide
fourth ventricle and is known as the open portion, so that the
section differs greatly in outline from sections of the spinal
cord, being relatively broad and low (dorso-ventrally) with a
hollow in the dorsal region representing the fourth ventricle.
This concavity is lined with gray matter which corresponds
to the central gray matter surrounding the canal of the
spinal cord, and in which various special groups of cells appear.
Extending right through the section in the median plane is a
partition formed largely by decussating fibres, the raphe.
Round the periphery of the section appear a number of more
or less sharply marked off tracts and nuclei, such as the
spinal V root and its nucleus, and between these and the
raphe is a large area of intermingled gray and white matter
known as the substantia reticularis or reticular formation. In
a zone extending dorso-ventrally along each side of the raphe,
the white fibres predominate, and hence this area is called
the substantia reticularis alba, while the remainder of the for-
mation, in which there is a larger proportion of gray matter,
is the substantia reticularis grisea (PI. VII.).

In the cranial nerves, three of the four primary functional
types are represented by both general and special subdivisions,
while the somatic motor group has special elements only,

30 Anatomy of the Nervous System

making up seven out of the complete set of eight components.
The general somatic afferent system is represented mainly in
the trigeminal nerve, and a small number of fibres of this
system occur in the facial/ glossopharyngeal, and vagus
nerves. It is also present, in the form of proprioceptive fibres
for muscle sense in the eye muscles, in the oculomotor, troch-
lear, and abducent nerves.

All the general somatic afferent fibres which carry extero-
ceptive impulses from the head region, no matter what their
peripheral course may be, end centrally in a single nucleus
of gray matter in the medulla oblongata. Since the great
majority of such fibres are found in the trigeminal nerve, this
centre has received the name of trigeminal nucleus. Upon
entering the brain, most of the afferent fibres bifurcate,
forming ascending and descending branches, of which the
latter are the longer. A considerable number of unmyelinated
and small myelinated fibres, however, have recently been
found in the rat and other mammals to descend without
bifurcating. They are believed to be pain fibres, as this sense
is absent from the chief nucleus. The fibres are grouped
together in a compact bundle running along the dorso-lateral
aspect of the medulla in a superficial position. This spinal V
root {radix spinalis sen descendens trigemini) extends down-
wards from the place of entry of the trigeminal nerve to the
beginning of the spinal cord, where it becomes continuous
with the tract of Lissauer (Pis. VL-XI.). Some of the ascend-
ing branches of the sensory fibres pass upwards beyond the
level of the entry of the fifth nerve before ending, forming
an ascending extension of the bundle for a short distance.
The number of fibres in this root and, consequently, the size
of the fasciculus as a whole vary directly with the degree of
development of general somatic sensibility in the head region.

^The presence of cutaneous sensory fibres in the facial nerve has been
demonstrated in the mouse, and in various lower vertebrates. They
probably occur in other mammals also.

General Somatic Afferent System 31

In man, where such sensibility is not specially highly de-
veloped, the root is relatively small as compared with its size
in the rat, which has great general sensibility in the snout
region, particularly in connection with the vibrissae, or
'' whiskers ".1 In transverse sections, it appears as a large,
crescentic area covering most of the lateral aspect of the
medulla oblongata, in which position it may be followed
from the beginning of the spinal cord up to the level of the
fifth nerve. Traced up in this way, it will be seen to increase
in size, being much smaller in the lower part of its course than
in the upper. The obvious explanation of this fact is that all
the fibres do not descend to the lower part of its course, a
series of them leaving it all the way along to end in the ad-
jacent gray matter.

The gray matter in which the afferent fibres of the general
somatic system end is made up of two parts which are, how-
ever, continuous with each other. The first of these is the
chief sensory nucleus of the trigeminal nerve (nucleus principalis
trigemini). It consists of a large mass of gray matter in the
region of the entry of the nerve, lying in the lateral part of
the medulla in close contact with the mass of sensory fibres
(PI. XL), and extending anteriorly a short distance along
with the ascending branches of these fibres, so as to receive
their terminations. The trigeminal elements are covered
superficially by other structures at this level, but a short
distance posteriorly they emerge. Continuous with the chief
nucleus is a column of gray matter situated immediately
medial to the descending root and partly surrounded by it.
This is the spinal V nucleus {nucleus spinalis trigemini),
which in the rat is as large as the chief nucleus in section, so
that no precise line of demarcation can be drawn between
them, though it is claimed that they differ in function (Pis.

^Meyersohn finds that, if cross sections at the level of the pyramidal
decussation be magnified so as to make their diameters equal, the size
of the spinal V root in the rat has the ratio to that in man of 1.21: 0.25.

32 Anatomy of the Nervous System

VI I. -X.). In man, the chief nucleus appears as an enlarge-
ment at the anterior end of the column formed by the spinal
nucleus. Many fibres run through the nuclei, the lower part
of the spinal nucleus enclosing conspicuous bundles.

About the level of the anterior end of the hypoglossal
nucleus (p. 45) a small area of rather clear gray matter,
similar to the gelatinous substance of Rolando of the spinal
cord, appears superficial to the dorsal part of the spinal V
nucleus, and about the lower extremity of the fourth ventricle
this spreads out and forms a layer over the outer surface of
the nucleus, exactly as the gelatinous substance covers the
dorsal column in the cord (PI. VII.). Traced downwards,
this is found to be continuous with the similar substance in
the cord, the nucleus as a whole being continuous with the
dorsal column. In connection with the cells of this nucleus,
the fibres of the spinal V root end, and from these cells arise
the secondary fibres which are to convey the impulse onwards.
Fasciculi concomitantes of the spinal V root lying in and
medial to the medial side of the spinal V nucleus, and largely
of heterolateral origin, are described for the rat by Fuse.

The secondary fibre-tracts arising from the chief sensory
and the spinal nuclei of the trigeminus cannot be followed in
normal material, coursing through the reticular substance
in a diffuse condition. They comprise both short reflex con-
nections, largely, if not entirely crossed, and ascending fibres
to the midbrain and to the thalamus [trigeminal lemniscus),
of which the majority are crossed. It would appear that the
fibres arising in the chief and spinal nuclei follow separate
routes, and that the trigeminal lemniscus arises only from
the chief nucleus. It comprises two portions, one in the
dorsal part of the reticular formation, the other, which
decussates slightly more posteriorly in the rat, running close
to the dorsal portion of the medial lemniscus and to the
raphe. Fuse states that in the rat the ventral trigeminal
lemniscus ends largely (he believes entirely) in the ventral

General Somatic Afferent System 33

reticular nucleus of the tegmentum, from which, apparently,
new fibres continue the pathway forward with the medial
lemniscus to the thalamus.

Dorso-medial to the chief sensory \^ nucleus, lies a con-
spicuous group of large unipolar cells mingled with several
small bundles of stout myelinated fibres. The cells give off
similar fibres, which join those in the small bundles and run
with them into the fifth nerve. Each fibre bifurcates, how-
ever, before passing out of the brain, one branch remaining
within and ending either in the motor \' nucleus or in a small
group of cells dorso-medial to the sensory \' nucleus. If the
small bundles be now followed up the brain, they are found
to be associated with a continually decreasing number of the
unipolar cells, each of w^hich gives rise to one of the fibres in
the bundles, scattered along the lateral aspect of the central
gray matter up to nearly the anterior end of the midbrain.
These fibres form the mesencephalic root of the trigeminus
{radix mesencephalica trigemini), the unipolar cells making
up its nucleus (Pis. XI. -XIV.). They are sensory in function,
probably concerned with muscle sensibility, the unipolar
cells being apparently equivalent to peripheral ganglion cells
which have developed within the brain instead of outside it.
In fact some of the fibres of the mesencephalic root do arise
from similar cells in the semilunar ganglion.

Degeneration experiments (Papez) reveal a group of descending
fibres (Tract of Probst) rising apparently in the mesencephalic V root and
running back in the rat dorsal to the chief sensory and the motor nuclei
of the trigeminus. It continues posteriorly in the dorsal part of the
reticular formation just ventro-medial to the solitary tract and its nuclei,
and probably ends in relation to the salivatory nuclei.

Besides those somatic afferent fibres which enter through
the cranial nerves, the medulla oblongata receives others
which come up from the spinal cord. The fibres of the spinal
lemniscus pass up from the white matter of the cord into the
reticular formation of the medulla oblongata, whence they

34 Anatomy of the Nervous System

continue upward through the midbrain to end finally in the
diencephalon. It is not possible to observe the course of
these fibres in the sections from normal material.

The dorsal funiculi of the cord are interrupted at the
lower end of the medulla oblongata. As one approaches
this region, passing up the spinal cord, the dorsal gray columns
are seen to spread apart somewhat, and a little below the decus-
sation of the pyramids, a small mass of gray matter appears
projecting dorsally from the central gray among the fibres of
the fasciculus cuneatus. This is the beginning of the nucleus
cuneatus. A little farther up than the lower end of the
pyramidal decussation, a broad median eminence also appears
on the dorsal surface of the central gray matter, the nucleus
gracilis. Still farther forward, another small part of the
nucleus gracilis is seen on each side among the fibres of its
fasciculus, and this becomes continuous with the projection
from the central gray matter. The latter, moreover, divides
into a small median nucleus and two lateral portions, con-
necting as just described with the groups of cells within the
white matter (PI. VI.). The cuneate and gracile nuclei
increase rapidly in size, and as the fibres of the fasciculi end
within them, the latter are correspondingly reduced and
finally disappear. The cells of these funicular nuclei give
rise to secondary fibres which take a course ventrally and
medially in small groups, appearing in sections as internal
arcuate fibres. They cross the middle line, forming the
decussation of the lemniscus, and take up a position close to
the raphe (PI. VII.). Here they form an ascending tract
which is known as the medial lemniscus {lemniscus medialis,
medial fillet), and which can be followed right through the
medulla oblongata in about the same position, though the
shape of its cross-section changes somewhat. It is a little
less conspicuous in the rat than in man. The details of form
of the gracile and cuneate nuclei in man also differ consider-
ably from those of the rat described above.

CHAPTER W
Special Somatic Afferent System.

THE special somatic afferent division comprises the
auditory and vestibular elements and also the visual
neurons. It really includes the olfactory neurons in
addition to these, though the olfactory functions combine
exteroceptive with interoceptive components, and it is usual
to consider the olfactory structures with the visceral afferent
group. Some authors object to this usage, however. Only
the auditory and vestibular systems, which occur in the
hindbrain, will be considered in the present chapter.

The fibres which convey auditory impulses to the brain
form the cochlear branch of the eighth nerve, which branch
ends in two large nuclei situated superficially on the dorso-
lateral aspect of the medulla oblongata in its more anterior
part. The dorsal cochlear nucleus {nucleus cochlearis dorsalis)
or tuherculum acusticum extends from the dorsal part of the
lateral surface round on to the dorsal surface, where it runs
medially as far as the edge of the ventricle. It is situated
immediately behind the attachment of the cerebellum to
the medulla and its ventral end is produced forward along
the lateral aspect of the cerebellar peduncles for some dis-
tance, where it is continuous with the flocculus. In the
angle between the ventral extremity of this nucleus and the
side of the oblongata lies another mass of gray matter of a
more rounded shape, the ventral cochlear nucleus {nucleus
cochlearis ventralis). This does not extend so far posteriorly
as the tuberculum acusticum, but reaches considerably farther

36 Anatomy of the Nervous System

forward.^ It is quite distinct from the latter nucleus in
sections owing to the arrangement of the nerve-fibres, and
the character of the cells in the two centres differs (Pis. III.,
IX.-XI.).

Some of the secondary fibres from the dorsal nucleus pass
ventrally along the medial aspect of the ventral nucleus,
where they join those arising in the latter. These fibres form
a large tract which passes ventrally and a little anteriorly
over the surface of the medulla until it gets near the median
line, where many of the fibres become deeper in position,
breaking up into smaller fascicles and leaving only a thin
layer superficial to the pyramids. They decussate and pass
towards the lateral part of the other side of the oblongata in
a somewhat more diffuse condition than in the earlier part
of their course. Up to this point the tract is known as the
trapezoid body {corpus trapezoides), but it now changes its
direction, running anteriorly, and is given a new name, the
lateral lemniscus {lemniscus lateralis, lateral fillet) (Pis. II.,
IX. -XIV.). In the human brain the trapezoid body is com-
pletely covered superficially by the greatly enlarged pons.

The other secondary fibres arising in the dorsal cochlear
nucleus pass medially {striae medullares acusticae), dipping
some distance below the floor of the fourth ventricle in the
form of small scattered groups of arcuate fibres. About two-
thirds of these decussate {decussation of v. Monakow) and,
running forward dorsal to the superior olive, join the lateral
lemniscus.

^There may be distinguished in the ventral cochlear nucleus: dorso-
medial, ventro-medial, dorso-lateral, central cell groups, and in the dorsal
cochlear nucleus: superficial, middle, and deep layers may be observed.

The difference in the level of the two nuclei is greater in man, where
they do not overlap, the dorsal nucleus having been pushed back by the
enlargement of the restiform body and the ventral nucleus having become
imbedded in the side of the brachium pontis. Also, the dorsal nucleus
is relatively smaller and the ventral nucleus is relatively larger in the
human brain.

Special So\l\tic Afferent System 37

The trapezoid body comes into relation with certain
masses of gray matter in the transverse part of its course, of
which the most conspicuous are the superior olivary nuclei.
These are more largely developed in the rat than in man — ^as
are also the cochlear nuclei — and have the form in this and
many other mammals of a lamina folded twice, so as to appear
S-shaped in cross section. Immediately medial to each is a
smaller lamina, the accessory superior olivary nucleus, and
some little distance medial to this again is a much less defi-
nitely circumscribed group of cells, the nucleus of the trapezoid
body. Some of the trapezoid fibres end in some one of these
nuclei, either before or after crossing the median plane, and
fibres arising from them pass into the lateral lemniscus. Many
of the fibres arising in the superior olivary nucleus, however,
run in a rather diffuse tract which may be seen passing dorso-
medially towards a small group of cells near the floor of the
ventricle, the nucleus of the sixth nerve^ (PI. X.). These
fibres mediate direct reflex movements of the eyes in response
to auditory stimuli. Other such fibres run through the reti-
cular substance from all the nuclei of this group to form short
reflex paths to other motor nuclei.

The superior olivary complex of the rat, which has been analyzed as
follows by Fuse, contains more different elements than are enumerated
above. Besides the principal superior olivary nucleus, there are (1) the
accessory superior olivary nucleus, consisting of a small, dorso-ventrally
elongated gray mass and a largely-developed dorso-medial accessory
group of cells; (2) the nucleus of the trapezoid body; (3) the nucleus
praeolivaris internus; (4) the dorsal accessory nucleus of the principal
superior olive; and (5) the nucleus praeolivaris externus (poorly developed
in the rat).

The accessory superior olivary nucleus receives trapezoid fibres
(mostly heterolateral) and gives off axons to the medial lemniscus and to
the homolateral nucleus of the trapezoid body. It also has connections
with its own medio-dorsal accessory cell-group, and with the nucleus
praeolivaris internus of the same side. The medio-dorsal cell-group is
larger than the rest of the nucleus, and is separated from the nucleus of

^Fuse doubts whether they reach the abducens nucleus in the rat.

38 Anatomy of the Nervous System

the trapezoid body by thick bundles of trapezoid fibres. Its connections
are similar to those of the other part of the nucleus.

The nucleus of the trapezoid body lies medio-ventral to the last-
mentioned group and is differentiated from the more ventro-laterally
placed nucleus praeolivaris internus by its larger cells. It is traversed by
many longitudinal fibre bundles. There are extensive fibre connections
with the lateral parts of the medial lemniscus and the pyramidal tract as
well as with the accessory superior olivary nucleus, but none with the
lateral lemniscus of the same side. There are considerable heterolateral
connections also.

The nucleus praeolivaris internus receives fibres of the corpus trape-
zoides, mostly of heterolateral origin. The lateral part of the nucleus
sends most of its axons across the raphe, while most of those from the
medial part enter the homolateral medial lemniscus and pyramidal tract.

The dorsal accessory nucleus of the principal superior olive lies over
the dorsal hilus of the latter in relation to the fibres of v. Monakow.

The trapezoid fibres ending in the principal superior olivary nucleus
are partly hetero- but chiefly homolateral in origin and only about one-
quarter to one-third of the fibres coming directly from the cochlear nuclei
decussate. These apparently originate chiefly in the ventral cochlear
nucleus. The crossed fibres of the trapezoid body originating in the nucleus
of the trapezoid body, superior accessory olivary nucleus, nucleus prae-
olivaris internus, ventral cochlear nucleus, dorsal cochlear nucleus and
chief superior olivary nucleus are of decreasing numerical importance in
the order named, and are accompanied by a few fibres from the spinal
V nucleus and from the reticular formation. Most of these fibres are
distributed more caudally after decussating, while most of those from
the cochlear nucleus end in the olivary complex. Apart from the fibres
of V. Monakow, the lateral lemniscus is very largely of homolateral origin.

The lateral lemniscus passes directly forwards into the
midbrain, where many of its fibres may be followed into the
posterior colliculus (p. 68). Others pass on with the fibres
arising in that region to the medial geniculate body (p. 79).
In the lower part of its course, the fibres of the lateral lem-
niscus enclose a group of cells, the ventral nucleus of the lateral
lemniscus {nucleus ventralis Umnisci lateralis), in connection
with which a number of them end. The lemniscus then
follows a somewhat antero-dorsal course and breaks up into
small fibre-bundles as it approaches the midbrain. Between

Special Somatic Afferent System 39

the bundles is a large amount of grey matter, constituting the
dorsal nucleus of the lateral lemniscus {nucleus dorsalis lemnisci
lateralis). Behind this point its fibres are not very easily
distinguished from those of the ventral spino-cerebellar tract
(p. 52), which have a similar arrangement and general direc-
tion, and which lie immediately lateral and dorsal to the
lemniscus.

The vestibular nerve, which conveys proprioceptive im-
pulses, chiefly of an equilibratory nature, ends in relation with
a somewhat extensive group of nuclei placed dorso-laterally.
These consist of a more or less continuous column of gray
matter which is rather clearly divisible into parts. At the
level of entry of the root is a large-celled mass in a lateral
position close to the ventricle, the lateral vestibular nucleus^
{nucleus lateralis nervi vestibuli) or nucleus ofDeiters (Pis. IX.
X.). Extending posteriorly from this, just as the spinal \^
nucleus extends posteriorly from the chief sensory \ nucleus,
is the descending vestibular nucleus {nucleus descendens nervi
vestibuli seu octavi), which is accompanied by bundles of the
root fibres, the descending vestibtdar root (PI. MIL), This
root can be traced down as far as the beginning of the spinal
cord, where it ends in connection with a small nucleus lying
between the cuneate nucleus and the gelatinous substance.

There are also ascending branches of the root fibres
forming an ascending root, as in the case of nerve \, each
fibre bifurcating on entering the brain. Many of these
ascending branches end in an anterior nucleus, the nucleus of
Bechterew or superior vestibular nucleus, which extends from
the nucleus of Deiters antero-dorsally, projecting into the
cerebellar peduncles (PI. X.).

Medial to the nucleus of Deiters and the descending

^Fuse distinguishes seven parts in this nucleus in the rat and other
rodents: dorso-lateral, dorsal, middle or central, dorso-medial, ventro-
medial, intravestibular, and triangularis portions. He also finds two or
three cell groups in the nucleus of Bechterew.

40 Anatomy of the Nervous System

nucleus, and immediately under the floor of the fourth
ventricle, lies a small-celled nucleus of roughly triangular form
in cross section, the chief vestibular nucleus {dorsal vestibular
nucleus, nucleus dorsalis seu medialis nervi vestibuli, nucleus
triangularis) .'^ This large nucleus extends all the way from
a point a little behind the entrance of the V nerve root to
the level of the anterior part of the hypoglossal nucleus
(Pis. VIII.-X.).

Some of the ascending vestibular fibres do not end in
any of these nuclei but run directly into the cerebellum,
where they end in the roof nuclei and in certain parts of the
cortex (PL X.).

Fortuyn also saw vestibular fibres entering the ventral
cochlear nucleus in the rat.

Secondary fibres from the nuclei of Deiters and Bechterew
join the direct fibres to the cerebellum and end with them in
that structure.

Other fibres arising in these nuclei pass medially to enter
an important longitudinal correlation tract on either the same
or the opposite side (PL X.). This tract is the medial longi-
tudinal bundle, which lies in the angle between the raphe
and the gray matter of the ventricular floor. Through this
tract, the vestibular impulses are conveyed to various motor
centres, particularly the eye-muscle nuclei, the ascending tract
of Deiters (tractus Deiters ascendens) forming a definite group
of fibres in the lateral part of the bundle in the midbrain.

Yet other fibres from Deiters' nucleus form a diffuse tract
in the reticular formation, which runs down to the spinal
cord, conveying impulses to the spinal motor neurons. This
is the vestibulospinal tract {fasciculus vestibulo-spinalis, tractus
Deiters descendens). Its fibres pass obliquely between the

^The term triangular nucleus has also been applied to the cuneate
nucleus.

The chief vestibular nucleus contains several centres and receives
vestibular fibres only in its lateral part (F"use).

Special Somatic Afferent System 41

more lateral fascicles of the ascending limb of the facial root,
and appear to be partly of heterolateral origin.

The fibres arising in the chief vestibular nucleus cross to
the opposite side, where they course through the reticular
formation to make various reflex connections. Fuse states
that many of these fibres are uncrossed in the rat.

The complexity of the cochlear and vestibular systems is
probably considerably greater than is indicated by the fore-
going account, as has been shown by Winkler to be the case
in the rabbit.

CHAPTER VI.
X'iscERAL Afferent System.

THE visceral afferent system, like the somatic, is repre-
sented in the cranial nerves by both general and
special subdivisions, of which the latter is concerned
with the sense of taste. Smell may also be included in this
category on physiological grounds, but its structural represen-
tatives are independent, whatever their phylogenetic origin
may have been. Both these components are represented in
each of the tenth, ninth and seventh nerves. Unlike the
somatic system, the two subdivisions of the visceral afferent
group end in the same column of gray matter according to
the account usually given.

The sensory fibres of the glossopharyngeal and vagus
nerves upon entering the substance of the brain run in small
fascicles to a position between the dorsal end of the spinal
V nucleus and the ventro-lateral angle of the chief vestibular
nucleus. Here they take up a longitudinal, descending
direction, forming the fasciculus solitaritis, which runs down
to the beginning of the spinal cord (Pis. VI. -IX.). The afferent
facial fibres, which enter the brain farther forward, also run
back and slightly mediad near the dorsal edge of the spinal V
root and join the descending fibres of nerve IX. The fasciculus
is accompanied by two columns of gray matter, of which the
large one medial to it is the nucleus of the fasciculus solitarius
{nucleus fasciculi solitarii), while the one which lies ventro-
lateral to it is the nucleus parasolitarius. The former of these
has a very clear appearance and is a conspicuous object in
sections. Its anterior end is covered dorsally by the chief
vestibular nucleus, but it soon emerges and occupies a position

Visceral Afferent System 43

medial to this and directly under the floor of the fourth
ventricle. Some of the fibres of the fasciculus solitarius end
in these nuclei/ but many of them descend to about the level
of the decussation of the pyramids, where they decussate in
a mass of gray matter dorsal to the central canal and con-
tinuous with the nuclei of the fasciculi solitarii of the two
sides. This mass is the commissural micleus of Cajal {nucleus
commissuralis, PI. \'I.), in which the fibres terminate after
decussating within it, forming the commissura infima. The
commissura infima is not ordinarily seen in Weigert prepara-
tions as it contains very few myelinated fibres. A few of the
fibres do not decussate, but run right into the upper part of
the spinal cord on the same side.

While the fasciculus solitarius and its gray matter are usually con-
sidered to contain both the general visceral and the gustatory connections,
there is some evidence in favour of the view that only the former com-
ponents end in this way. According to the supporters of this inter-
pretation, a considerable proportion of the visceral afferent fibres, at least
from nerves IX and X, do not end in the centres mentioned, and chiefly
on comparative grounds, these are believed to be the gustatory elements.
The nuclei with which they come into relation are two in number on each
side, and lie at the level of the entering nerve roots. The larger and
more distinct is a small-celled nucleus which is wedged in between the
anterior end of the hypoglossal nucleus and the dorsal motor vagus nucleus
(p. 47), and is named the nucleus intercalatus of Staderini (PI. VIII.).
This nucleus extends medially, forming a cap over the anterior extremity
of the hypoglossal nucleus, and becoming continuous with a small mass
next to the median line, the nucleus funiculi teretis. In front of the
hypoglossal nucleus, it enlarges and occupies the position of the latter for
a short distance. It also connects laterally, dorsal to the motor X nucleus,
with the second centre to which gustatory functions are attributed, viz.,
a small mass which forms the ventro-medial angle of the chief vestibular
nucleus, and which has usually been considered as part of that centre.
Allen, however, could trace no visceral afferent fibres to either of these
centres in the guinea pig, but found them all to enter the fasciculus soli-
tarius.

Wlen found no terminals from the tractus solitarius in the nucleus
para solitarius of the guinea pig (Jour. Comp. Neur., vol. 35).

44 Anatomy of the Nervous System

The secondary visceral pathways are not well known.
They include descending fibres to the cervical spinal cord,
concerned probably with respiratory reflexes and reflexes of
the digestive organs, such as vomiting, and an ascending path
probably to the hypothalamus. Allen, however, has demon-
strated in the guinea pig that the ascending secondary fibres,
unless they are unmyelated, must run with the medial lem-
niscus of the opposite side to the thalamus.

CHAPTER \ 11.
The Motor System.

Somatic Efferent Column.

ALL the somatic muscles (those derived from the myo-
tomes of the embryo) controlled by cranial nerves
are of a rather highly specialized character and their
nerve elements are consequently classified as special somatic
efferent, the general somatic efferent component being con-
fined to the spinal nerves. These special motor fibres are
found in the third, fourth, and sixth nerves, where they
control movements of the eyeball, and in the twelfth nerve,
where they regulate the movements of the musculature of the
tongue.

The nuclei of these nerves are arranged in a linear series,
being parts of a single somatic efferent column which has
become broken into separate centres corresponding to the
nerves through which its fibres run. The oculomotor and
trochlear nuclei lie in the midbrain but the abducent and
hypoglossal centres are in the medulla oblongata.

The position of the column corresponds primitively to
that of the ventral gray column in the spinal cord, of which
it is to be considered an anterior continuation. The location
of the cells, however, has undergone a change, there having
been a migration towards the most important source of
stimulation. In the case of the hypoglossal nucleus (XII),
the majority of stimuli are received, probably, from the
visceral afferent nuclei, and the nucleus has mov^ed antero-
dorsally and taken up a position near the floor of the ventricle,
close to these centres (Pis. VL-VIIL). Here it appears as a
slender, elongated mass of gray matter extending from the

46 Anatomy of the Nervous System

level of the decussation of the pyramids to a point some little
distance forward under the floor of the fourth ventricle.
Several different groups of cells, which are related to special
muscles or sets of muscles, are distinguishable in the nucleus.
The hypoglossal root fibres are arranged in small bundles
which pass directly ventrally and a little laterally to emerge
at the outer edge of the pyramid (pp. 12, 53).

The nucleus ahdttcens (VI) lies considerably farther
forward, having migrated from a position between the levels
of the nuclei VII and IX to one in front of the nucleus VII.
This change probably took place largely under the influence
of fibres from the anterior parts of the medial and lateral
vestibular nuclei running chiefly in the medial longitudinal
bundle. Having reached this more anterior position, it has
farther moved dorso-laterally in many mammals, so as to lie
still nearer these centres. This is the condition seen in man,
where the nucleus VI lies near the floor of the fourth ventricle
and lateral to the genu of the VII nerve root (p. 48), as it
does also in the rabbit. In the rat, however, the nucleus has
not undergone the lateral movement, but appears as a small,
rather scattered group of cells wedged in between the genu
of the VII root and the medial longitudinal fasciculus and
extending a short distance ventrally along the side of the
latter (PI. X.). Its root fibres pass out directly in a course
exactly parallel to that of the XII root fibres farther back.

Visceral Efferent Nuclei.

The visceral efferent centres, like the somatic, may be
considered to be an anterior continuation of the corresponding
column in the spinal cord. They have become divided into
separate general and special columns, however, and the
changes of location undergone by the individual nuclei are
more diverse than are those of the somatic centres.

General visceral efi^erent fibres occur in the roots of
nerves III, MI, IX, X, and XI, whence they run into

Motor System 47

sympathetic ganglia. Those of the X and XI nerves arise
in a single column of cells which has a dorsal situation near
the floor of the fourth ventricle (Pis. VI.-VHI.). This nucleus
— the dorsal motor vagus nucleus {nucleus motorius dorsalis X)
— ends anteriorly a short distance In front of the rostral
extremity of the nucleus XII, where it is separated from
the fourth ventricle by the large nucleus of Staderini and the
chief vestibular nucleus and lies just medial to the corre-
sponding sensory centre (nucleus fasciculi solitarii). Passing
backwards, the nucleus of Staderini rapidly decreases until
the vagus nucleus, which grows somewhat larger, comes to lie
lateral to It and immediately under the floor of the ventricle.
From here, It can be traced dow^n into the closed part of the
oblongata, where it ends near the posterior extremity of the
hypoglossal nucleus.

The general visceral components of the facial and the
glossopharyngeal nerves are connected with the salivary
glands, and their nuclei are the superior and the inferior
salivatory nuclei respectively. These are small groups of
scattered cells. They are practically continuous with each
other and occupy a more lateral and deeper position than the
dorsal motor X nucleus, a short distance in front of Its anterior
extremity.

The special visceral efferent cells, on the other hand,
which send fibres Into nerves V, \TI, IX, X, and XI, have
undergone a marked change In position In a ventral and
somewhat lateral direction, a change which has gone farthest
In the case of the motor facial nucleus {nucleus motorius VII).
This large group of cells Is to be found close to the ventral
surface of the brain, midway between the spinal V root and
the pyramid, and some distance behind the level where Its
root emerges (PI. IX.). It is composed of several separate
cell-masses, which Papez has found in both rat and cat to
give rise to the fibres of distinct branches of the nerve. The
root fibres do not run directly to the point of emergence, but

48 Anatomy of the Nervous System

pass in a dorso-medial direction towards the position whence
the cells have migrated. Close to the floor of the ventricle,
they turn forwards and group themselves into a compact
bundle, the genu or ascending portion of the motor VII root.
After ascending a short distance to a point just in front of
the nucleus VI, the root turns sharply at about a right angle
and runs ventro-laterally to emerge at the ventral edge of
the spinal V root (Pis. X., XL). In the human brain, the
facial nucleus is relatively rather smaller than in the rat, and
is not quite so far ventral in position, while its rostral end lies
dorso-lateral to the superior olive instead of practically
directly behind it, as in the rat. This is probably due partly
to the more dorsal position in man of the reduced spinal V
nucleus, from which the motor VII nucleus receives impulses,
and partly to the enormous development of the pons. The
latter factor has pushed back the emerging motor VII root
in man until it lies beside the front part of its nucleus, ventral
to which the posterior portion of the pons itself extends.

The special motor cells of the IX and X nerves form a
continuous column of very much smaller size than the motor
facial nucleus. This is the nucleus amhiguus, which is found in
a position similar to that of the motor VII nucleus, but not
so near the surface, being medial to the ventral edge of the
spinal V nucleus (PI. VIII.). Its anterior extremity is about
on a level with that of the dorsal motor X nucleus, but
posteriorly it gradually thins out until it is represented by
a few isolated cells so irregularly scattered that its posterior
end cannot be exactly determined.

The root fibres from this nucleus run dorso-medially and
join those from the dorsal nucleus, passing out with them in
small groups in a lateral direction.

The special motor fibres of the XI nerve arise from the
nucleus accessorius, which may be regarded also as part of
the visceral motor column which has changed its position,
migrating in a posterolateral direction and being joined by

Motor System 49

visceral motor cells of those segments of the spinal cord in
which it lies (PI. \l.). It extends from near the level of the
posterior end of the hypoglossal nucleus through the upper
cervical segments of the cord.

There remains to be considered one other special visceral
efferent centre, namely the motor trigeminal nucleus (nucleus
motoriusV, nucleus masticatorius) , which controls the move-
ments of the jaw muscles. This nucleus consists of a rather
conspicuous group of large cells which do not lie very close
together. It is situated medially to the chief sensory V
nucleus and gives rise to the motor fibres of the \^ nerve
(PI. XI.). These form large fascicles which may be observed
passing out in an antero-ventral and somewhat lateral direc-
tion.

The various changes in position which we have just seen
to have taken place in the development (both ontogenetic
and phylogenetic) of the motor nuclei are prominent ex-
. amples of a principle which seems to be active throughout
the nervous system, and which has been called by Kappers
neurohiotaxis. The most essential part of this principle may
be summed up by saying that two nerve cells which are
simultaneously or consecutively excited exert an attractive
influence on each other, as a result of which the cell body
tends to migrate during development towards any other
neuron from which it receives stimuli.

CHAPTER VIII.

Correlation Centres and Tracts in the Medulla
Oblongata and Pons.

THE nerve centres and pathways which have been con-
sidered so far do not by any means exhaust the list
of structures visible in sections of the medulla ob-
longata and pons. There are pathways leading to and from
both the cerebellum and the higher regions of the brain, and
there are structures which serve for correlation of impulses
within this region itself. The functional difference between
the correlation centres and the sensory nuclei is, however,
one of degree rather than of kind, for sensory nuclei usually
receive stimuli from several different sources, as do corre-
lation nuclei. The only real difference is that the sensory
centre receives a great majority of its stimuli from one source,
frequently outside the body.

One of the most conspicuous correlation centres in the
brain stem is a prominent mass of gray matter lying in a
ventro-medial situation and having a very characteristic
appearance in section (Pis. YH., YIII., XXVI.). This is the
inferior olivary complex, of which the functional relations
are still very imperfectly understood, but which is known to
be intimately related with the cerebellum. The mass is
divisible into three distinct nuclei, the medial olive {medial
accessory olivary nucletis), the dorsal olive {dorsal accessory
olivary nucleus), and the ventro-lateral or principal olive {in-
ferior olivary nucleus, oliva inferior), which are continuous with
each other at certain points. Each of these is an elongated
lamella of rather irregular outline, so that its exact form and
position vary at different levels. The medial nucleus extends

Correlation Centres and Tracts 51

considerably farther posteriorly than do the other two,
which are nearly co-extensive. The principal nucleus differs
from the others in being folded lengthwise so as to form a
sort of pocket with an opening, the hilns, directed medially.
Thus a section through the middle part of the nucleus is
U-shaped, but as the pocket is somewhat narrowed round
the opening, this form does not appear in sections near either
end of the nucleus. In higher mammals, including man, the
accessory nuclei are similar to those of the rat, but the principal
nucleus is very greatly enlarged and folded, though still
retaining the pocket-like form.

Streaming through the hilus and round and through the
nuclei are many fibres which are chiefly efferent axons from
the olives to the cerebellum. They cross directly to the
other side and curve up to the restiform body (vide infra),
with which they become incorporated.

Other correlation centres are present in the form of
large and medium-sized cells scattered through the reticular
formation. These are the reticular nuclei,^ which receive
impulses from various sources and transmit the resultant
impulses directly to motor centres (Pis. \TI., MIL).

The nucleus of Roller, which Hes just ventral to the hypoglossal
nucleus and has sometimes been supposed to be related to it, belongs to
this group. In another nucleus of the group, the ventral reticular nucleus
of the tegmentum {nucleus reticularis tegmenti ventralis), which extends
forward from the level of the oral end of the superior olive in a medial
position, Fuse believes that the trigeminal lemniscus and man\- of the
more dorsal fibres of the medial lemniscus are interrupted in the rat. He
finds that this group is composed of large cells, of which the axons are
largely uncrossed, and medium-sized and small cells of which the axons are
mostly crossed (especiall}- the more posterior ones), and that it receives
many arcuate fibres from the ventro-lateral part of the reticular formation
of the same side.

"^For a description of these nuclei in the rabbit, see Pekelsky, 1922.
This author examined also Mus rattus, where he found no important
difference in respect of these nuclei.

52 Anatomy of the Nervous System

Fuse has described in some detail for the rat and other types caudal
and oral ventro-medial cell groups of the. spinal V root lying between the
latter and the superior olive, and a few small cell groups ventral to the
motor V nucleus (Pis. IX., XL).

The medulla oblongata Is furthermore traversed by
numerous pathways connecting higher and lower regions of
the brain. Some of these — the secondary sensory tracts — ■
have already been considered. Among the ascending path-
ways, there are to be noted in addition to these the restiform
body (corpus resiiforme) or inferior cerebellar peduncle and the
ventral spino- cerebellar tract which forms a large part of the
composite tract of Gowers. The former is the direct con-
tinuation of the dorsal spino-cerebellar tract of the spinal
cord, to which are added various elements originating in the
medulla oblongata. It forms a conspicuous, compact bundle
in the dorso-lateral corner of the sections (oval area), where it
can be followed up into the cerebellum (Pis. IX. -XL). The
ventral spino-cerebellar tract lies superficially in the ventro-
lateral region, where it is not clearly marked off from the
surrounding tissue (PI. VHI.). In the upper part of the
oblongata, anterior to the trigeminal nuclei, it may be seen
to turn dorsally and pass to a position dorsal to the brachium
conjunctivum (vide infra), with which it then runs back into
the cerebellum (PI. XI.). As it passes dorsally, it appears in
the form of numerous small bundles which may easily be
confused with the lateral lemniscus, as this tract runs in a
similar direction immediately medial to it.

There may also be included in this group the brachium
conjunctivum, or superior cerebellar peduncle, which traverses
the uppermost part of the hindbrain in passing from the
cerebellum into the midbrain. It is a rather large bundle
situated laterally, at each side of the ventricle, and having a
dorso-ventrally flattened form. It passes in an antero-ventral
direction into the midbrain, becoming decreasingly compact
as it does so (Pis. XL, XII.).

Correlation Centres and Tracts 53

Containing both ascending and descending correlation
fibres, is the medial longitudinal bundle {fascicuhis longitudi-
nalis medialis, dorsalis, seu posterior), a rather distinct bundle
lying close to the central gray matter immediately on each
side of the raphe, and containing many different kinds of
fibres (Pis. MI.-X\\).

The most conspicuous descending tract is the pyramidal
tract, which runs directly posteriorly in a ventral position,
carrying motor impulses to the somatic muscles. As it enters
the region of the pons from the midbrain, this tract is accom-
panied by numerous cortico-pontine fibres which end in the
nucleus pontis (infra), thus decreasing considerably the size
of the bundle as a whole. It is also accompanied by cortico-
bulbar fibres, which pass to the motor nuclei of the medulla
oblongata. The remainder of the tract^ (cortico-spinal or
pyramidal tract), when it reaches the lower end of the ob-
longata, decussates in the form of numerous rather large
bundles — the pyramidal decussation (PI. VL) — and runs
dorsally to take up a position in the ventral part of the
dorsal funiculus of the spinal cord, where it has already been
observed.

In man and some other mammals, the decussation is
incomplete, a small proportion of the fibres continuing straight
down into the ventral funiculus of the same side and crossing
over singly just before ending in the ventral gray matter.
In the rat, however, the decussation appears to be complete
or practically so.

Related functionally with the pyramidal tract is the rubro-
spinal tract {fasciculus rubro-spinalis), which conveys impulses

^Bregmann finds that the area of cross section of the cortico-spinal
tract just before it decussates is 5.3% of the area of the section of the
spinal cord just behind the decussation in the rat. The corresponding
ratio for man is 30.0%. These figures do not represent the relative
number of fibres, however, since they do not consider the relative sizes
of the individual axons, a factor which is complicated by the great difference
in myelination.

54 Anatomy of the Nervous System

to Influence the tone of the body muscles. This bundle runs
through the reticular formation, where it is not distinct from
its surroundings. As it enters the region of the pons from
above, the rubro-spinal tract lies just medial to the lateral
lemniscus. Upon reaching the level of the trigeminal nuclei,
it takes up a more superficial position immediately ventral
to the spinal \' nucleus and root, separating the latter from
the ventral spino-cerebellar tract, and in this position it
descends directly into. the spinal cord (Pis. X., XIII.).

\^entral to the medial longitudinal fasciculus is situated a
bundle of fibres, somewhat diffuse in the rat, rising in the
anterior colliculi of the midbrain and descending to the spinal
cord, where degeneration experiments in the rat reveal it
as an extensive zone of finely scattered fibres external to the
fasciculus proprius in the ventral funiculus. This is the
tectospinal tract {fasciculus tecto-spinalis sen praedorsalis)

(Pis. vn.-xiii.).

The reticular formation, of course, contains many other
pathways, which, however, are mostly diffuse in arrangement
and cannot be observed in sections of normal material.

Finally, there may be mentioned here the pons {pons
Varolii) (Pis. II., XII., XIII., XXYL). This is a large mass
of transverse fibres running round the ventral surface of the
anterior part of the medulla oblongata and turning dorsally
at each side to pass up into the cerebellum as the brachium
pontis or middle cerebellar peduncle (Pis. III., XII.). In the
ventral part of their course, the fibres spread apart to allow
the cerebral peduncles (pyramidal tracts+cortico-bulbar
and -pontine tracts) to descend between them and also to
surrotind a considerable amount of gray matter, the pontine
nuclei. These nuclei receive impulses from various sources,
particularly from the cerebral cortex^ through the cortico-
pontine tracts, and give rise to the fibres of the pons itself.

Trontal and temporal lobes in man. The former, however, is
exceedingly rudimentary in the rat.

Correlation Centres axd Tracts 55

The latter usually decussate and convey their impulses up
to the cerebellar hemisphere on the opposite side. The pons
is to be regarded as a structure which has been added to the
primitive hindbrain coincident with the development of the
cerebellar hemispheres, for the purpose, primarily, of putting
these into communication with the cerebral cortex. Hence
it is very much smaller in the rat than in man, where both
the cerebral cortex and the cerebellar hemispheres are much
more largely developed.

CHAPTER IX.

The Cerebellum.

TRANSX^ERSE sections through the cerebellum show
the various lobules which cross the median plane, the
lohiis ansijormis or cerebellar hemisphere on each side,
and the floccular lobe projecting laterally below it. These
parts have already been described in the first chapter. Those
lobules which are medially situated are best seen in a sagittal
section (Fig. 3). Such a section also shows the branching
arrangement of the white matter which has received the
name of arbor vitae, in which each little branch is one of the
transversely disposed lamellae cut across.

Sulcus primariuj.
Lobus

epyrai

^^oriulU£

Fig. 3
Median sagittal section of the cerebellum of the rat.

In general, the sections of the cerebellum show it to
consist of a large mass of white matter, the medulla of the
cerebellum, over which is disposed a greatly folded layer of
gray matter, the cerebellar cortex. The cortex has a very
complex structure, being made up of many different kinds of

Cerebellum 57

neurons, but in ordinary sections it appears to be composed
of only two distinct layers. The outer, or molecular layer, is
of a homogeneous or very finely punctate appearance, with
a few small cell-nuclei scattered through it. The inner, or
granular layer, on the other hand, is composed of cell-bodies
closely packed together. The outermost row of cells in this
layer is made up of very characteristic large, pyriform cells,
the Purkinje cells, of which the axons pass straight down into
the white matter, while the large, branched dendrites extend
out through the molecular layer. The dendrites of each
Purkinje cell spread out in a single plane sagittally placed,
i.e., at right angles to the axis of the cerebellar lamella. Each
dendrite is thus in a position to receive a maximum number
of stimuli from the numerous, fine fibres in the molecular
layer which run lengthwise of the lamella.

The gray matter of the cerebellum, however, is not all
superficial in position. There are also certain masses im-
bedded in the deeper part of the white matter, near the roof
of the fourth ventricle (PI. IX.). Four pairs of these nuclei
are recognized, named in a medio-lateral direction respec-
tively nucleus fasti gii {nucleus tecti), nucleus globosus, nucleus
emboliformis, and nucleus dentatus {dentate nucleus or cere-
bellar olive). The dentate nucleus is a little larger than the
other nuclei and stretches out into the base of the stalk of
the floccular lobe. It is the ending-place of many of the
axons of the Purkinje cells, and the place of origin of most
of the fibres in the brachium conjunctivum. In the rat, this
nucleus is a compact mass slightly hollowed medially, so that
it appears somewhat curved in section. In man, the hollowing
has increased and the nucleus has grown as a whole until it
appears as a much-folded, pouch-shaped lamella, very similar
in form to the inferior olivary nucleus. This increase in
size of the dentate nucleus is apparently correlated with the
larger development of the cerebellar hemisphere. The three
remaining nuclei are proportionately larger in the rat than

58 Anatomy of the Nervous System

in man and are more or less fused with each other (particu-
larly the nuclei globosus and emboliformis, which are often
together called nucleus intermedins or interpositus in lower
mammals) in most sections. They receive fibres from the
Purkinje cells, and the nucleus fastigii, which is related
particularly to the vestibular apparatus, also receives nerve
endings from other parts of the nervous system. The axons
of the nuclei globosus and emboliformis join those of the
dentate nucleus in the brachium conjunctivum, while those of
the nucleus fastigii pass to the reticular formation of the hind
brain as the cerebello -bulbar or fastigio-bulbar tracts. Some of
these fibres to the hindbrain form a rather definite bundle,
the tr actus arcuatus Russell, or fasciculus cerebello-bulbaris
{fasciculus uncinatus) (PI. XL). This leaves the ventral
surfaces of the deep nuclei and curves anteriorly and laterally
round the dorsal aspect of the brachium conjunctivum,
between it and the ventral spino-cerebellar tract. It then
runs postero-ventro-laterally, medial to the restiform body,
and joins the descending vestibular root, with which it passes
backwards.

CHAPTER X.
The Midbrain.

THE general form of the midbrain, or mesencephalon, is
less modified from the early embryonic condition than
is that of either of the other primary subdivisions of
the brain. It retains the condition of a thick-walled tube
connecting the forebrain and the hindbrain with only a little
modification of the shape of the walls by a greater thickening
ventrally than dorsally, and by the moulding of the dorsal
wall, or tectum, into four eminences, the corpora quadrigemina
(Pis. HI., Xn.-XVL). The narrow ventricle is variously
known as the cerebral aqueduct, the aqueduct of Sylvius, or the
iter. The thick wall ventral to the aqueduct is the cerebral
peduncles, which are divided into a thick tegmentum, in the
deep position, and a pair of large flattened bands of longi-
tudinal fibres situated superficially, the basis peduncuU or
pes peduncuU. Tegmentum and basis are separated by a
layer of gray matter, the substantia nigra (Pis. XHI.-XM.).

Dorsal to the substantia nigra, the tegmentum consists of
reticular formation continuous with that of the medulla
oblongata, while dorsal to this again is a rather thick layer
of central gray matter. Into the tegmentum may be traced
many of the longitudinal tracts which have been observed
in the corresponding region of the hindbrain. In the posterior
part, the fibres of the lateral lemniscus are to be seen at each
side surrounding their dorsal nucleus and running antero-
dorsally into the tectum. From the enlarged anterior end of
the dorsal nucleus of the lateral lemniscus, arises a thin bundle
of transverse fibres, the commissure of the lateral lemniscus or
commissure of Probst (PI. XII.). After decussating, these

60 Anatomy of the Nervous System

fibres run into the corresponding nucleus of the other side
and curve dorsally in the lemniscus to end in the posterior
colliculus (Papez).

Ventrally, the medial lemniscus continues forward in the
same position which it occupied in the hindbrain, becoming
more flattened dorso-ventrally, however, and soon moving
aw^ay from the raphe to a somewhat more lateral position,
where it may be traced up into the thalamus.

In the dorsal part of the tegmentum, on the other hand,
the medial longitudinal bundle likewise continues forward
in the same position where it was observed farther back. It
also becomes more flattened dorso-ventrally, and it breaks
up into a number of distinct, compact, little bundles. At the
level of the trochlear nucleus, it forms a dorsal concavity
containing the nucleus, and some of its fascicles run right
through the latter (PI. XIII.). Then it swings round dorso-
medially so that at the level of the more anterior part of the
oculomotor nucleus the tracts of the two sides enclose be-
tween them a \^-shaped area of the central gray, in which
the nerve nuclei lie. In front of these nuclei, the ventral edges
of the two tracts separate a little before they continue into
the hypothalamus. Here the tract disappears in relation
with a small group of cells, the interstitial micleiis of Cajal,
situated at the anterior extremity of the red nucleus (vide
infra) and just lateral to the tract itself (PI. X\\). This
nucleus gives rise to the interstitio-spinal tract {Tract of Boyce),
which in the rat is a bundle of coarse fibres running back in
first the dorsal and then the medial part of the medial longi-
tudinal fasciculus. It gives off fibres to the eye-muscle
nuclei and passes on into the spinal cord, taking up a sulco-
marginal position there (Papez). In the lateral part of the
medial longitudinal bundle, for a considerable distance
through the midbrain, run secondary fibres from the nucleus
of Deiters to the oculomotor and trochlear nuclei (ascending
tract of Deiters).

Midbrain 61

Extending obliquely between the medial longitudinal
bundle and the lateral lemniscus, lies the cross section of the
superior cerebellar peduncle, which is thick and rounded
dorso-medially (caput brachii conjunctivi) and tapers out
almost to a point ventro-laterally (cauda brachii conjunctivi)
(PI. XII.). Passing upward, the cauda becomes less definite
and the whole tract becomes a little more medial in position
by the time it reaches the level of the trochlear nucleus, about
the front of which it turns medially and decussates immedi-
ately ventral to the medial longitudinal bundle (Pis. XIII.,
XXVI.). In the upper part of its course before decussation,
it is less sharply marked off from the surrounding reticular
formation than farther back. In man, the superior peduncle
is proportionally larger than in the rodent and the dense
decussation has a considerably greater dorso- ventral extent.

After decussating, the tract runs straight forward a short
distance to end largely in the red nucleus {nucleus ruber)
(Pis. XI\'., XXVI.). Some of the fit>res pass on into the
thalamus, while a few turn backward in the reticular forma-
tion {brachium conjunctivum descendens).

The red nucleus is a large group of cells which is very
conspicuous in sections prepared with cytoplasmic stains.
In the human brain, it is conspicuous also in Weigert pre-
parations, where it is marked off sharply from the surround-
ing reticular formation by a capsule made up largely of the
fibres of the brachium conjunctivum. In the rat, however,
there is no such definite capsule and a great many white
fibres run between the cells, so that the nucleus is less sharply
delimited in Weigert sections. A magno-cellular division
and dorso-lateral and ventro-Lateral divisions composed of
smaller cells may be distinguished in this nucleus, the magno-
cellular portion being relatively larger in the rat than in
man. Besides the cerebellar peduncles, fibres from the
cerebral hemispheres also end in the red nucleus.

From the magnocellular division of the red nucleus, arise

62 Anatomy of the Nervous System

descending fibres which convey to the motor columns of the
spinal cord the impulses received from the cerebellum-
impulses which are believed to be important factors in
muscular coordination and in the maintenance of muscle
tone. These fibres pass ventro-medially to cross the raphe
as the ventral tegmerUal decussation {fountain decussation of
Forel), after which they turn posteriorly, forming the rubro-
spinal tract {fascictdus ruhro-spinalis) in a position ventro-
medial to the red nucleus. Farther back than this nucleus,
the tract passes gradually in a lateral direction to a location
just ventral to the cauda of the brachium conjunctivum.
Papez describes its descending course in the rat as "medial
to the lateral lemniscus, ventral to the trigeminus, facial
nucleus, etc.".

From the small-celled portions of the red nucleus, arise
crossed and uncrossed fibres to other parts of the tegmentum
and to the thalamus. From the latter, the impulses are
transmitted to the corpus striatum and cerebral cortex. As
a result of these relations, the small-celled region is found to
correspond in size with the degree of development of the
cerebellar hemispheres and of the cerebral cortex (especially
the frontal cortex), both of which are small in the rat.

Dorsal to the ventral tegmental decussation and the
decussation of the superior cerebellar peduncles, is another
group of fibres crossing the raphe. These fibres arise in the
tectum, whence they curve round the central gray matter
to its ventral surface {fountain-like radiation of Meyn&rt)
and form the dorsal tegmental decussation {fountain decussation
of Meynert). They then turn directly posteriorly, just lateral
and ventral to the medial longitudinal bundle, as the tecto-
spinal tract, which has already been observed in the hind-
brain. Some of the fibres are said by certain authors not to
decussate but to pass posteriorly on the same side along with
the lateral lemniscus. After giving off fibres to the oculomotor
and trochlear nuclei, the tecto-spinal tract suddenly becomes

MlDBRAIX 63

more scattered near the posterior end of the decussation of
the brachia conjunctiva, some of its fibres as they continue
back being mingled with the medial longitudinal bundle and
with the medial lemniscus (Papez).

The central gray matter surrounding the aqueduct of
Sylvius contains several more or less distinct cell-masses.
Among the most important of these are the nuclei of the
fourth and third nerves, which lie in the ventral part of the
central gray, directly dorsal to the medial longitudinal bundle.
The trochlear nucleus is a small group of large cells in the
posterior part of the midbrain (PI. XIII.). In the rat, it lies
actually between the fascicles of the medial longitudinal
bundle, suggesting the intimacy of its relation to this tract,
from which it receives most of its afferent fibres. The axons
arising from this nucleus may be traced postero-laterally and
dorsally, running backwards at the boundary between the
central gray and the surrounding tissue (PI. XII.). This
course takes the root to the antero-lateral region of the
anterior medullary velum, in which the fibres decussate to
form the peripheral nerve roots (PI. XXVI.).

The oculomotor nucleus lies directly in front of the troch-
lear, being practically continuous with it but entirely dorsal
to the medial longitudinal bundle (PI. XIV.). It extends up
to near the anterior end of the midbrain. The root-fibres
pass ventrally in small fascicles which emerge along the
medial margin of the cerebral peduncles (Pis. XI\'., XXVI.),
a few of them first decussating between and dorsal to the
nuclei arid joining the root of the other side. In man, the
oculomotor nuclei consist of a median centre (nucleus of
Perlia) and paired lateral nuclei. The former, however, is
not present in rodents. Immediately dorsal and anterior to
the oculomotor nuclei proper, is a mass of small cells the
axons of which also emerge in the third nerve. This is the
nucleus of Edinger-Westphal, which is the origin of the general
visceral fibres in the oculomotor nerve. It is continuous

.64 Anatomy OF the Nervous System

across the median plane in the rodents, but in man is distinctly
paired.

The close association of the oculomotor and trochlear
nuclei with the medial longitudinal bundle is a significant
example of neurobiotaxis (p. 49), this tract containing the
majority of the fibres which carry stimuli to their cells.

Various correlation nuclei also occur in the tegmentum,
among which, two important centres are the dorsal and ventral
tegmental nuclei {nucleus tegmenti dorsalis or nucleus ventralis
grisea centralis, and nucleus tegmenti ventralis or nucleus of
V. Gudden). These occur at the extreme posterior end of the
midbrain and are better developed in the rodents than in
man (PI. XII.). The dorsal nucleus lies immediately dorsal
to the medial longitudinal bundle, a little behind the troch-
lear nucleus. Between the levels of Pis. XI and XII it
appears as a large rounded mass rather clearly outlined by a
diffuse capsule of fine fibres. It receives fibres from the
interpeduncular nucleus through the tegmental tract of that
centre and from the mamillary body through the mamillo-
tegmental tract {tractus mamillo-tegmentalis, tract of v. Gudden) .
Its axons descend in a system of scattered longitudinal fibres
close to the ventricular surface {dorsal longitudinal bundle
of Schtitz, periependymal longitudinal tract), which extends
back from the thalamic region but is most distinct at and
behind the level of this nucleus. The ventral nucleus is a
conspicuous mass ventral to the medial longitudinal bundle,
in which also many mamillo-tegmental fibres end. These
nuclei are apparently relay-stations on a reflex pathway
from the hypothalamus to the motor nuclei of the hindbrain.
The mamillo-tegmental tract is difficult to distinguish in
the lower part of its course in transverse sections prepared by
Weigert's method. It is a group of fine fibres which curve
backward in the vertical plane after leaving the mamillary
body, and run dorso-posteriorly through the tegmentum on
each side of the raphe, to end in the nuclei just described.

Midbrain 65

Other correlation centres are the dorsal nucleus of the
raphe, which lies in the median plane just dorsal to the
trochlear nuclei, some groups of cells in the raphe posterior
to the decussation of the brachia conjunctiva, and a number
of nuclei in various parts of the reticular formation which
are not usually distinct in Weigert sections. Among the last
of these, there must be mentioned particularly the nucleus
of Darkschewitsch (PI. X\'.). This mass of small cells is
situated at about the level of the anterior end of the medial
longitudinal bundle, to which it may contribute some fibres.
It lies just dorsal to the interstititial nucleus of Cajal and is
probably related particularly to the fibres of the posterior
commissure (p. 71).

Situated in a median position at the ventral surface of
the tegmentum, lies the interpeduncular nucleus (ganglion
inter peduncular e), a mass of gray matter occupying the
posterior perforated space of human anatomy, w^hich lies
at the bottom of the interpeduncular fossa (Pis. XIII., XIV.,
XX\T.). This centre receives a large fibre-bundle, the
fascicidus retroflexus of Meynert (Pis. XI\\-X\TIL, XXM.),
from each of the habenular nuclei of the diencephalon, and
gives off fibres, as already mentioned, which run to the
dorsal tegmental nucleus as the tegmental tract of the inter-
peduncidar nucleus (PI. XIII.). This is a group of very fine
fibres which may be observed to leave the dorsal surface of
the interpeduncular nucleus and to run dorso-posteriorly
on each side of the raphe. These structures, being concerned
with the correlation of olfactory with other, chiefly somatic,
impulses, are largest in animals having a highly developed
sense of smell. They are considerably larger in the rat than
in man, whose olfactory sense is rather poorly developed.

A bundle which is rather distinct in the ventral part of
its course, but of which the origin is still uncertain, becomes
visible just behind the mamillary body. This is the transverse
peduncular tract (PI. X\\). It probably contains some optic

66 Anato:my of the Nervous System

fibres, and Edinger believed it to be composed largely of
fibres belonging to the ciliary portion of the oculomotor. It
runs along the medial edge of the pes pedunculi and then
turns dorsally a short distance to end in a centre close to the
substantia nigra and posteroventral to the red nucleus
{nucleus tractits pedunculi transversus, ganglion ectomamillare
of Wallenberg). Kosaka and Hiraiwa found that removal
of the eye of the rat produced degeneration of this tract,
chiefly contra- but partly homo-laterally. Some of the fibres
enter the subthalamic nucleus. The secondary fibres from
the nucleus of the transverse peduncular tract are probably
short, running forward into the subthalamic nucleus for
reflexes of the pupil, eyelid, etc.

Just in front of the level of the anterior border of the pons,
there appears a mass of gray matter separating the teg-
mentum from the ventrally situated basis pedunculi. This
mass is the substantia nigra, which has the form of a broad,
thick band extending forward into the hypothalamus, and
becoming thicker and more rounded in cross section as it
does so (Pls.XI\^-XYI.). It receives fibres from the cerebral
hemispheres and gives rise to others which run through the
tegmentum, but its precise connections and function are
not clearly understood. It has received its name on account
of its dark colour in the human brain, where it stands out
conspicuously when examined in gross material. This colour
is due to the presence in many of the cell-bodies of large
numbers of brown pigment granules. There is less pigmenta-
tion in lower mammals than in higher ones, however, (Kap-
pers) and in the rat it is practically entirely absent.

The basis pedunculi is composed of a broad band of closely-
grouped, longitudinal fibres running along each side of the
ventral surface of the midbrain, and appearing roughly
crescentic in cross section (Pis. II., XIY.-XIX.). The fibres
originate in the cerebral cortex and pass through the internal
capsules to form these bands, which converge towards the

Midbrain 67

upper edge of the pons and penetrate into It. These large
bundles gradually decrease in size as they descend, giving off
fibres to various parts of the brain, and analysis shows that
they are composed of several distinct tracts. In man the
medial fifth of the bundle is composed of fibres running from
the frontal cortex to the pontine nuclei — the fronto-pontine
tract — but this portion may be presumed to be very small
in the rat if it is present at all, as the frontal cortex is developed
to a very slight extent. The lateral fifth in man is made up
of fibres from the temporal and occipital lobes to the same
nuclei — the temporo-pontine tract. These two together form
the cortico-pontine tract. Between these lies the large cortico-
spinal tract, with which are mingled many cortico-hulbar
fibres. Many of the latter, however, are grouped in the mid-
brain into two distinct bundles, w^hich pass to the nuclei of
the cranial nerves.

As might be expected, the basis pedunculi, consisting of
fibres of neocortical origin, varies in size in different animals
according to the development of the neocortex. It is largest
in man, where the neocortex is most highly developed, and
is considerably smaller in the rat.

The tectum is the dorsal portion or roof of the midbrain.
In the lower vertebrates, it is moulded into two rounded
optic lobes, where terminate most of the fibres of the optic
nerve, while the posterior part of its ventro-lateral portion
contains a more or less massive auditory correlation centre.
In the mammals, the latter centre has acquired a more dorsal
situation and appears as a second, usually smaller eminence
behind the reduced optic lobe. Hence the optic lobes have
come to be known in mamrnals as the anterior {superior)
corpora quadrigemina or anterior (superior) colliculi, while the
new eminences are the posterior (inferior) corpora qicadrigemina
or posterior (inferior) colliculi. In the rat and other lower
mammals, the latter are considerably smaller than the
former, while in man they are more nearly alike in size. This

68 Anatomy of the Nervous System

difference in proportion is due chiefly to a decrease in the
functional importance and consequently in the relative size
of the anterior coUiculi in man, a much larger percentage of
the optic fibres ending in the diencephalon, whence their
impulses are sent to the cerebral cortex.

As seen in sections, the posterior coUiculi appear simpler
in structure than the anterior ones. Each contains a large,
somewhat rounded mass of gray matter, the micleus of the
posterior coUiciilus, which is the end station of the greater
part of the lateral lemniscus (Pis. XII., XIII.). Most of the
fibres of this tract may be seen to plunge directly into the
nucleus. Some of them, however, pass round it, forming a
capsule, before entering its substance. A certain proportion
of these may decussate, to end in the nucleus of the other
side, but Papez finds no evidence of this in his studies of
degeneration in the brain of the rat.

Separated from the nucleus of the posterior coUiculus by
the deep portion of its capsule together with a mass of fibres
arising in the tectum (stratum profundum), lies a thick layer
of central gray matter. Dorsal to this, the space between
the two nuclei is filled by intermiclear cortex or laminated
gray matter of the tectum, which is penetrated by a great many
transverse white fibres.

The tectum being composed of important reflex corre-
lation centres, it receives many different kinds of fibres,
coming from the spinal cord, the medulla oblongata, and the
forebrain. These, however, are not easily traced in ordinary
sections. On the other hand, efferent fibres are said to leave
the tectum for all of these regions of the central nervous
system. Axons arising both in the nuclei of the posterior
coUiculi and in the laminated gray matter form a layer
between these and the central gray (stratum profundum) in
which they are said to run ventrally to the tegmentum to
join the tecto-bulbar and tecto-spinal tracts. Papez, however,
can find in Marchi preparations of rat brains no demonstrable

Midbrain 69

descending tract from the posterior colliculi, and concludes
that these cannot be regarded as reflex centres. Many of
these fibres decussate bet^\'een the two corpora quadrigemina
as the commissure of the posterior colliculi and Papez finds
these to end entirely in the opposite colliculus. Along with
the commissure of Probst and the incomplete decussation
of the trapezoid body, they provide for bilateral representa-
tion of auditory stimuli in the posterior colliculi and in the
medial geniculate bodies. Many fibres arising in each pos-
terior colliculus, moreover, enter its brachium (vide infra)
to ascend to the thalamus.

Not all the fibres of the lateral lemniscus end in the
posterior colliculi. Some divide before terminating there
and send a branch into the anterior colliculus, and it may be
that a few such fibres end in the latter region without pre-
viously dividing. A considerable number of the fibres termi-
nating in the colliculi are, moreover, collaterals from axons
which continue their course farther up the brain along with
some which have no connection in the midbrain. These
form a distinct tract on the lateral aspect of the tectum,
which is the direct continuation of the lateral lemniscus, but
which is now known as the brachium of the posterior collicuhis
or posterior (inferior) quadrigeminal brachium {F\s.Xll.-X.y.).
The fibres which end in the tectum form the chief reflex
auditory pathway, according to the usual view, while the
brachium is the cortical pathway, up which pass auditory
impulses destined to enter consciousness. The fibres of the
latter group end in the medial geniculate body, whence new
axpns pass to the auditory cortex. The brachium contains
also fibres arising in the posterior colliculi, and some of
cortical origin which terminate in these centres.

The nucleus and fibres of the mesencephalic root of the
trigeminal nerve, which lie in the lateral part of the tectum
at the outer border of the central gray matter, have already
been described.

70 Anatomy of the Nervous System

The anterior colliculi show a distinct lamination — a
character which is evidently very ancient and fundamental,
since it is found in the optic lobe of all vertebrates except
the cyclostomes and tailed amphibians. In the mammals,
the lamination is less complex than in lower forms, the reduc-
tion being chiefly in the outer layers; and in the lower mam-
mals, such as the rat, these layers are more largely developed
than in man, the purely reflex visual responses being of more
importance in the rat than in man. The cortex of the anterior
colliculi is considered to be made up of four layers, though
secondary divisions may be distinguished in these (Pis. XIV.-
X\T.). The surface is covered by a very thin layer of fibres,
the stratum zonale. Below this come in order the much
thicker stratum griseum, which contains very few myelinated
fibres, and the thick stratum opticum and stratum lemnisci,
in which end respectively fibres belonging to the systems
indicated by their names, along with many others. The two
latter laminae both contain large numbers of myelinated
fibres. The stratum profundum separates the stratum lem-
nisci, from which it is not sharply differentiated, from the
central gray matter and is made up chiefly of efferent fibres
from the superposed laminae, particularly from the stratum
griseum. The efferent fibres in the stratum profundum
mostly enter the dorsal tegmental decussation, after crossing
in which they form the tecto-spinal and -bulbar tract, though
some of these fibres are stated to descend without crossing.
Stratum profundum fibres crossing the middle line form the
commissure of the anterior colliculi.

Near the anterior end of the tectum, the fibres of the
optic tract (here called the brachium of the anterior corpora
quadrigemina) may be observed entering the stratum opticum,
and deeper than these are many fascicles which Cajal believes
to come from the cerebral cortex (cortico-tectal tract) . These
form a distinct layer in the deeper portion of the stratum
opticum.

Midbrain 71

Not only are the anterior colllculi much larger in the rat
than in man, but the structural differentiation within them,
as just described, is much more marked.

At the anterior boundary of the tectum, where it con-
tinues into the thalamus, lies the large posterior commissure
{commissura posterior), which appears as an anterior exten-
sion of the stratum profundum of the anterior corpora
quadrigemina (Pis. X\'I., XML, XXM.). This commissure
is apparently a rather fundamental structure as it is very
constant throughout the whole vertebrate series and develops
early in ontogeny. Nevertheless its relations and functional
significance are not yet clearly understood. At each side of
the commissure, its fibres spread out in a large mass of cells
which has been called the nucleus of the posterior commissure,
and which is continuous with an elongated cell mass in the
deep layers of the optic tract and with the deeper parts of
the anterior coUiculus. The nucleus of the posterior com-
missure receives optic tract fibres and gives rise to a large
part of the commissure itself in the rat. Papez is inclined to
believe that the commissural fibres rising in this centre
terminate exclusively in the region of the interstitial nucleus
or the nucleus of Darkschewitsch on the other side, thus
leading from the optic tract to the heterolateral interstitio-
spinal tract. Another important constituent of the com-
missure is probably made up of ascending vestibular fibres
from the lateral part of the medial longitudinal bundle, and
there may also be contributions from the posterior nucleus
of the thalamus (p. 82) and from the nucleus of Darksche-
witsch.

CHAPTER XI.
The Diencephalon.

AT the anterior extremity of the midbrain, the appear-
ance of sections changes considerably as one passes
forward into the next region, the 'tweenbrain, 'twixt-
brain, or diencephalon. At this point, the aqueduct of
Sylvius opens out into the third ventricle, which has the
form of a high, narrow slit with a rather thin floor, a mem-
branous roof, and an exceedingly massive wall at each side.

The diencephalon has three main subdivisions, the epi-
thalamiis, a small dorsal region; the hypothalamus, a small
ventral region; and between these the extensive thalamus,
including the geniculate bodies, which are sometimes dis-
tinguished as the metathalamus.

The epithalamus comprises the thin, membranous roof
of the third ventricle, the pineal body, and the habenulae.
The first of these is thrown into folds and richly supplied
with blood vessels forming the chorioid plexus of the third
ventricle. The pineal body or epiphysis is a small, unpaired
glandular body which projects back between the anterior
corpora quadrigemina, with the posterior portions of the
cerebral hemispheres largely covering it from the sides (PI. I.).
It is attached by a short stalk to the posterior portion of the
roof of the third ventricle, where a commissure connects the
habenulae.

The habenula is a paired centre which has the form of a
fusiform ridge along the dorsal edge of the wall of the third
ventricle where the membranous roof is attached (Pis. XMIL-
XX., XX\T.). In section, it is seen to consist of a column of
gray matter covered dorsally and ventrally by layers of

DiENCEPHALON 73

white fibres and composed of two distinct nuclei. These
nuclei are a larger, lateral mass of rather scattered cells with
many myelinated fibres and a smaller, medial group of many,
closely-packed, smaller cells with few white fibres. Anteriorly,
the medial nucleus di\'ides into dorsal and ventral portions,
in the rat, while posteriorly it is continuous with that of the
other side through the poorly-developed bed nucleus of the
habenular commissure. The dorsal layer of white matter is
a definite band composed of compact bundles of longitudinal
fibres, the stria medullaris thalami, of which the medial edge
projects slightly into the membranous roof as the taenia
thalami. The latter forms a small but sharp ridge marking
the dorsal extremity of the wall of the third ventricle. It is
composed of olfactory fibres of various kinds (see below), all
of which probably end in the nuclei habenulae, so that the
stria medullaris diminishes in size as it passes posteriorly.
Many of its fibres, nevertheless, are still to be seen just
behind the posterior end of the nucleus. These cross the
middle line directly dorsal to the posterior commissure to
terminate in the habenular nucleus of the other side, thereby
constituting the habenular commissure {commissura habenu-
larum, superior commissure) (Pis. XML, XXM., drawn but
not named in PI. III.), which is small in the rat. Traced
anteriorly, the stria medullaris is seen to turn ventrally at
the anterior end of the thalamus and to spread out in the
ventro-medial part of the hemisphere, where it meets the
longitudinal fibres of the diffuse medial forebrain bundle.

A summary of the composition of the stria medullaris of the albino
rat has been given by Herrick,^ who lists eight component tracts, as
follows: .

(1) Tractus olfacto-habenularis medialis — composed of fibres from the
praeoptic nucleus and surrounding gray matter. There are anterior and
posterior parts of this tract running respecti\'ely in front of and behind the

^"The conclusions as I published them should be accepted with some
reserve, though I believe them to be substantially correct". (Herrick^
personal communication.)

74 Anatomy of the Nervous System

column of the fornix, and there may be also a small lateral portion passing
up lateral to the fibres of the internal capsule.

(2) Tractus olfactohabenularis anterior — the largest component in
the rat. The fibres arise in the tuberculum olfactorium and pass back-
ward with those of the medial forebrain bundle (p. 97) to the level of the
optic chiasma, where they turn dorsally.

(3) Tractus septohabenularis — made up of fibres from the entire
septal region in the broad application of that term (p. 93), especially from
the portion near the anterior commissure.

(4) Tractus corticohabenularis medialis — composed of fibres arising in
the hippocampus and passing through the fimbria and column of the
fornix. They leave the latter for the stria medullaris when these structures
pass close to each other near the anterior commissure. They are drawn
on the lateral surface of the column of the fornix in Plate XXI, but are not
named.

(5) Tractus corticohabenularis lateralis — fibres coming from the
pyriform lobe, nucleus of the lateral olfactory tract, and tuberculum
olfactorium running dorsomedially and converging with component (2)
to enter the stria medullaris (PI. XX., sma).

(6) Tractus thalamohabenularis — consisting of fibres from the anterior
thalamic nucleus running through the stratum zonale thalami (the thin
superficial fibre layer) into the stria medullaris.

(7) Tractus striohabenularis — fibres between the stria medullaris and
the medial part of the globus pallidus (p. 106) — connections unknown.

(8) Imperfectly known connections between the habenula on the one
hand and the thalamus and the midbrain on the other.

Apparently not included in this classification is (9) the stria medullaris
bundle of the stria terminalis which arises in the pyriform lobe, the nucleus
of the lateral olfactory tract, and the tuberculum olfactorium, according
to Johnston, runs through the stria terminalis, and joins the stria medullaris
near the anterior commissure.

Gurdjian has recently confirmed for the rat the description of com-
ponents (1) — (5) and (9). He describes hypothalamic fibres in component
(1) and intimate synaptic relations between components (3), (4) and (9)
and the bed nuclei of the anterior commissure and the stria terminalis.

In the stria medullaris, components (3) and (4) lie dorsally, (4) medial
to (3), and components (1), (2) and (5) ventrally and medially, (1) being
most ventral.

Many of these connections are indicated in Fig. 4.

The efferent fibres of the habenula take up a position
ventral to the nucleus, where they descend to near its pos-

DiEXCEPHALOX 75

terior extremity, then, turning sharply ventrad, they run
straight to the interpeduncuhir nucleus as the fasciculus
retroflexus (Pis. XI\'.-X\'III.). Thus the habenular nuclei
are a relay station on the descending olfactory pathway and
probably serve for the correlation of the olfactory impulses
with somatic sensory ones received from neighbouring parts
of the thalamus (Herrick). This whole system, as already
pointed out, is considerably larger in macrosmatic animals,
such as the rat, than in microsmatic forms, such as man.

The hypothalamus forms the floor of the third ventricle,
and comprises the pars optica hypothalami, the tuber cinereum
and the infundibultcm , and the mamillary bodies. The pars
optica hypothalami is part of the telencephalon and will
therefore be considered later.

The mamillary bodies are a pair of rounded eminences
at the posterior extremity of the ventral surface of the dience-
phalon in the human subject. In the rat, however, as in
many other lower mammals, the two bodies are fused into
a single median mass (Pis. II., XV., XXVL). This fusion
is not only external but also internal, the bodies being separ-
ated in man by a ventral extension of the third ventricle
which does not occur in rodents.

Superficially, the bodies are covered by a very thin capsule
of white fibres. Each mamillary body consists of two main
nuclei, a larger medial one and a smaller lateral one, and
between these penetrates a great mass of descending olfactory
fibres of the fourth order — the column of the fornix {columna
fornicis) (Pis. X\\-XXII.). Many of these fibres (or at least
many collaterals from them) end in the mamillary nuclei,
dithers, however, pass dorso-medially, becoming more scat-
tered as they do so, and form a diffuse decussation {the siipra-
mamitlary decussation) (PI. X\\), after which they descend
through the tegmentum.

Another tract belonging to the mamillary body which is
well developed in the rat, and which is probably afferent, is

76 Anatomy of the Nervous System

the peduncle of the mamillary body {peduncidus mamillaris,
tractus mamillo-peduncidaris). This fasciculus ascends from
the hindbrain along the ventral surface, just medial to the
pes pedunculi (Pis. XIW, X\\, XXVL). It enters the mamil-
lary body dorso-lateral to the column of the fornix and
bifurcates there, sending many transverse fibres dorsal to
the fornix into the medial nucleus, while the rest of the tract
passes ventrally into the lateral nucleus.^ It can be traced
back down the brain to about the level of the most posterior
roots of the oculomotor nerve, where it joins the medial
lemniscus, with which at least some of its fibres have evidently
ascended. The mamillary body also receives afferent fibres
from the tuberculum olfactorium, etc., through the olfacto-
hypothalamic tract.

The efferent fibres of the mamillary body leave the
antero-dorsal part of the medial nucleus as a group of com-
pact bundles which form a conspicuous tract running an-
teriorly and dorsally to end in the anterior nucleus of the
thalamus (p. 82). This tract is the mamillo-thalamic tract or
bimdleof Vicqd'Azyr {fascictdtis mamUlo-tlialamicus) (Pls.X\\-
XX., XX\T.). Shortly before reaching its terminal nucleus,
it breaks up in the rat into many small scattered bundles,
so that it becomes much less evident than it is in the pre-
ceding part of its course. The early course of these fibres is
not straight, however, as is strikingly evident in sagittal
sections. They curve rapidly upwards until they are running
nearly xertically, when many of them bifurcate, the one
branch turning sharply forward in the mamillo-thalamic
tract, while the other continues its sweeping curve and takes
a posterior direction across the medial surface of the fasciculus
retroflexus and back to the dorsal nucleus of the tegmentum.
There, it has already been described as the mamillo-tegmental

K\ few fibres from this tract continue forward into the tuber cinereum
in the rat. A lesion in the peduncle in this animal produces no descending
degeneration (Papez).

DiEXCEPHALOX 77

tract (Pis. XII. -XM., XXM.). This fasciculus is considerably
smaller and less conspicuous than the mamillo-thalamic tract.
It probably conveys effector impulses which are perhaps
concerned with the act of sniffing.

Anterior to the mamillary bodies, is more gray matter,
which forms the tuber cinereum, and from which there pro-
jects ventrally the hollow stalk of the hypophysis, the in-
fundibulum (Pis. X\\-X\1II.). The tuber cinereum, like the
mamillary body, receives olfactory and other fibres, and these
two regions together constitute an important olfactory corre-
lation area, which is probably chiefly olfacto-visceral in func-
tion. The mamillo-tegmental tract leads the resultant nerve
impulses down towards the motor centres, while the mamillo-
thalamic tract conducts similar impulses to the anterior
thalamic nucleus, where they are further correlated with
others of somatic origin.

Thus we have in the epithalamus olfacto-somatic corre-
lation, in the hypothalamus olfacto-visceral correlation, and
in the anterior thalamic nucleus olfacto-viscero-somatic corre-
lation.

The thalamus is made up of dorsal and ventral parts, of
which the former is very much larger in mammals than is
the latter.

The ventral part is the subthalumus , which has sometimes
been confused with and sometimes included in the hypo-
thalamus. Its phylogenetic history, however, seems to show
that it should be regarded as a ventral part of the thalamus,
of which it is the motor coordination centre.

The subthalamus is the direct forward continuation of
the substantia nigra of the midbrain and the reticular for-
mation just dorsal to it. It is not so large in the lower mam-
mals as in man and the primates, though this fact does not
strike the observer on account of the still greater enlargement
in the latter forms of the dorsal part of the thalamus. The
ventral region is by no means simple, though not very large,

78 Anatomy of the Nervous System

and its structure and connections have not yet been thoroughly
elucidated. The outstanding structures in it which are usually
described are as follows.

The subthalamic nucleus {corpus subthalamic urn, body or
nucleus of Luys) is particularly large in primates. It appears
in the human brain as a rather conspicuous, lens-shaped
nucleus lying immediately dorsal to the pes pedunculi. In
the rat, however, it is not clearly marked off from surrounding
structures in Weigert preparations. It has the general form
of a flattened mass of numerous rather large cells, penetrated
by many medullated fibres and occupying about the same
position as in man (Pis. XVII., XVIII.). The centre is marked
in the figures according to the description of Cajal and a com-
parison of Weigert and Nissl sections of the rat brain with
Weigert sections of human material. This mass appears to
be the nucleus proprius pedunculi cerebri as identified by
Winkler and Potter,^ who indicate a more diffuse group of
cells dorso-lateral to this as corpus subthalamicum.

Dorsal to this region is the zona incerta (Pis. XVI I. -XIX.),
which in man lies between two distinct layers of white fibres
belonging, like the capsule of the subthalamic nucleus, to the
ansa lenticularis. In the rat, the fibres form a diffuse net
among the small cells of the region, which thus lacks, like
the subthalamic nucleus, the clearly marked boundaries in
Weigert sections which are seen in the human brain. The
ansa lenticularis probably contains both ascending and
descending fibres between the corpus striatum of the cerebral
hemisphere and the corpus subthalamicum and neighbouring
gray matter." Anteriorly this area continues beyond the

^An Anatomical (juide to Experimental Researches on the Rabbit's
Brain. Amsterdam, 1911.

^Morgan (Anat. Rec, vol. 29, p. 369) has recently called attention
to a tract in man running from the ansa lenticularis dorsally, medial to
the red nucleus, close to the mamillo-tegmental tract, and taking up a
position immediately lateral to the medial longitudinal bundle. While

DiEXCEPHALON 79

frontal extremity of the subthalamic nucleus as a similar
looking region known as the regio innomifiata, in which run
a large number of thalamic fibres to and from the cerebral
hemispheres.

Perhaps to be included with the subthalamus (Herrick '18),
is the nucleus reticularis thalami (formatio reticulata, lattice
nucleus, radiate nucleus), which occurs between the zona
incerta and the overlying white fibres (ventral medullary
lamina) in the more anterior part of the extent of the former.
It is continuous with a more lateral portion in the lateral part
of the thalamus and anteriorly extends between the nucleus
anterior and the fibres of the internal capsule (Pis. XIX., XX.).
Its names are derived from the many large bundles of cortico-
thalamic and thalamo-cortical fibres which run through it.

The dorsal portion of the thalamus is very large as com-
pared with the parts of the diencephalon hitherto considered,
this disproportion being even much greater in man than in
the lower mammals. The great increase in size is correlated
with the development of the cerebral cortex, the increase
being chiefly in those parts which serve as relay stations on
the sensory paths to the hemispheres — and every such path
is interrupted by a synapse in the thalamus.

One of the first parts of the thalamus to appear as one
passes forward in serial sections is the medial or internal
geniculate body (cor pics geniculatum mediate), which forms a
marked eminence on the lateral surface of the brain near the
frontal end of the anterior colliculi (Pis. III., XV. -XML).
The nucleus is surrounded and penetrated by many white
fibres, largely derived from the brachium of the inferior
coUiculus, which ends in this centre. Thus the body receives
auditory impulses, and these it transmits along its axons to
the auditory area of the cerebral cortex. Not all its axons

these fibres are not very distinct in the writer's preparations of the rat
brain, they appear to be present between the levels of Plates XIV and
XV.

80 Anatomy of the Nervous System

run to the cortex, however, some passing to the region of the
optic chiasma, in the posterior part of which they decussate
as the commissure of v. Gudden or postoplic commissure {com-
missura supraoptica ventralis), and then running back to the
medial geniculate body or posterior colliculus of the opposite
side. The fibres of this commissure are rather fine and lie in
the ventro-medial edge of the optic tract.

Covering the antero-dorsal aspect of the medial geniculate
body is the large, curved lateral or external geniculate body
{corpus geniculatum laterale), which is the terminal nucleus
for a large number of the fibres of the optic tract (Pis. III.,
XML, X\^III.). The latter forms a covering of white matter
over the outer surface of the nuclear mass, into which a
large proportion of its fibres are seen to plunge, while the
remainder pass on to enter the anterior quadrigeminal body.
The percentage of the optic fibres ending in the geniculate
body is, as we have seen, smaller in the rat than in man,
corresponding with the relative functional importance of the
cortical connections in the two cases. The difference is not
so great as it might have been, however, on account of the
fact that a considerable part of the rodent geniculate body
does not serve, apparently, as a relay station to the cortex.
In the rat and other lower mammals, the lateral geniculate
body consists of distinct dorsal and ventral nuclei (PI. X\TII.),
of which only the dorsal one sends its axons to the cerebral
cortex. The ventral nucleus perhaps transmits its impulses
to the tectum of the midbrain. As one examines forms
higher in the mammalian series, the dorsal nucleus is found to
increase in importance and the ventral one to decrease, until
in man the latter cannot be recognized. The dorsal nucleus
in man, on the other hand, is greatly developed and shows a
lamellated structure with crossed and uncrossed fibres ending
in alternate layers. The fibres arising in the dorsal nucleus
form the radiatio optica or optic radiation, which joins the

DiENCEPHALON 81

internal capsule. The radiation contains descending as well
as ascending fibres.

In man, some optic fibres end in the pulvinar, where they
meet fibres from the cerebellum, and whence impulses are
transmitted to the cerebral cortex. This centre, however,
which forms a large mass projecting back over the antero-
lateral aspect of the anterior quadrigeminal bodies in the
human brain, is not developed in the lower mammals. It is
a part of the lateral nucleus of the thalamus (vide infra
p. 85).

The optic chiasma itself belongs to the telencephalon.
The optic tracts and the nervous portions of the eye originate
developmentally from the wall of the diencephalon, however,
and the fibres end, as we have seen, either in the diencephalon
or in the midbrain, so that the chiasma also may reasonably
be considered at this point. It lies immediately in front of
the di-telencephalic boundary on the ventral surface of the
brain, and is composed essentially of the decussating fibres
of the optic nerves. These arise in the retina and most of
them cross over and pass to their terminal centres on the
other side. Some of the fibres, however, coming from the
lateral (in man) or posterior (in animals with the eyes in the
sides of the head) side of the retina, do not decussate, but run
through the homolateral optic tract to their termination.
The exact proportion of the fibres which cross over varies in
different animals according to the degree of overlapping of
the fields of vision of the two eyes. In man, where there is a
maximum of overlapping, about one quarter of the optic
fibres are uncrossed, but in the rat the proportion is much
smaller.^ The uncrossed fibres in the rat are scattered
among the crossed ones in the optic tract except near the
chiasma, where they are grouped chiefly in the dorsal part.

The optic fibres are accompanied in the chiasma by the
commissure of v. Gudden, which was discussed above, and

^But larger than in the rabbit.

82 Anatomy of the Nervous System

by the commissure of Meynert (commissura siipraoptica
dorsalis). The latter is better developed in the rat than in
the human species and forms a conspicuous decussation
postero-dorsal to the optic fibres. It is composed of fine
fibres, which may be observed running along the medial aspect
of the optic tract (PI. XMIL). These fibres are probably
telencephalic, but their exact relations are still uncertain.

A small, aberrant bundle of optic fibres is separated from
the rest by the commissures of v. Gudden and Meynert,
crossing in the most posterior part of the chiasma. They
gradually penetrate these commissures, however, and pass
through them to join the main optic tract, a few ending in
the subthalamic nucleus.

There may perhaps be included with the geniculate bodies
under the heading metathalamus^ the posterior nucleus of
the thalamus {nucleus posterior thalami, nucleus praebige-
minalis), which is related chiefly to the midbrain and has no
ascending cortical connections. It is a rather large group of
cells lying medial to the medial geniculate body and the
dorsal part of the lateral nucleus at the posterior end of the
thalamus (Pis. XVI., XYIL). It receives a group of collaterals
from the medial lemniscus.

The remainder of the thalamus falls into three chief
divisions, the anterior, medial, and ventro -lateral nuclei.

The anterior nucleus {nucleus anterior thalami, nucleus
dorsalis thalami) is wedged between the rostral ends of the
other two groups and extends farther anteriorly than either
of them (PI. XX.). The bundle of \'icq d'Azyr has already
been traced to this nucleus, which receives through it im-
pulses from olfactory centres. Thus it is not surprising to
find that the size of the anterior nucleus of the thalamus
varies in different animals with the degree of development of
the olfactory system, though not related to it alone. Hence

^According to Kappers. Other writers regard the posterior nucleus
as a part of the lateral nucleus.

DiENCEPHALON 83

this nucleus is considerably larger in the rat than in man.
It consists of two distinct cell masses — a smaller, dorsal,
large-celled group and a larger, ventral and lateral, small-
celled group. The anterior thalamic nucleus sends fibres to
the caudate nucleus (p. 105) and probably to the cerebral
cortex and also has connections with the medial thalamic
nucleus. It is probably of special importance in olfactory
exploration.

The anterior thalamic nucleus in the human brain is
separated from the adjacent gray matter by a layer of white
fibres formed by the bifurcation of the dorsal edge of a vertical
plate of such fibres which separates the medial and lateral
nuclei. This layer of w^hite matter is the internal medullary
lamina {lamina medullars s interna thalami), a very distinct
object in sections of the human thalamus, but very much
less so in that of the rat, through most of which there is a
more or less diffuse scattering of white fibres.

The medial nucleus {nucleus medialis thalami) (Pis. XVHI-
XX.) comprises several distinct cell groups and probably has
various functions in its different parts. It has both ascending
and descending connections with the corpus striatum and
probably with the cerebral cortex. This group and the
anterior nucleus are the older part of the thalamus phylo-
genetically and contain the intrinsic reflex mechanism of
that region of the brain. The medial nucleus probably
includes among its functions the more primitive and ele-
mentary forms of consciousness.

This mass is separated from the third ventricle in man
by a thin layer of central gray matter which contains scattered
cells constituting the nucleus paramedianus. The latter
is probably a visceral — particularly vasomotor — reflex centre.
The enlargement of the thalamus has reduced the ventricle
to a narrow slit and has brought its two walls into contact
over part of their area in a great many cases, forming a con-
tinuous mass of gray matter across the median plane — the

84 Anatomy of the Nervous System

intermediate mass or soft commissure {massa intermedia, corn-
mi ssura mollis, commissura media thalami) (Pis. XVIII.-XX.).

In the rat and other lower mammals the central gray
matter is more largely developed than in man, containing a
number of distinct nuclei and having the area of fusion in
the intermediate mass extended so as to obliterate a large
part of the third ventricle. The chief median mass of cells
is given the name of nucleus reuniens {nucleus rhomboidalis)
(Pis. XIX., XX.), though Winkler and Potter confine this
term to one of the cell-groups which can be distinguished in it.

Lateral and ventral to the foregoing portions of the
thalamus, we observe an extensive region which is a more
recent acquisition, from the phylogenetic standpoint. This
is the ventro-lateral group of nuclei, which is related in
development to the cerebral cortex, its general function
being that of an antechamber to the latter, particularly for
general somatic sensory impulses. It is divisible into two
main parts, ventral and lateral, in each of which a number
of nuclei are to be found. The term lateral nucleus is fre-
quently used to include both parts, i.e., the whole ventro-
lateral group, but will not be employed in that sense here.

In the human brain, the ventral nucleus {nucleus ventralis
thalami) contains, besides other gray matter, two centres
which are very clearly defined in Weigert sections. These
are a rather large, round mass, the nucleus centralis or medial
centre of Luys {centrum medianum, 7iucleus glohosus thaliami),^
and a curved body just ventral to this, the nucleus arcuatus
or nucleus semilunaris {corpus patellare). These, however,
do not stand out in the same distinct way in the brains of
lower mammals, and the homologies seem not to be certain
in many cases. Like the anterior and medial group, the
ventral nucleus is not most highly developed in the highest
mammals, and it is quite large in the rat (Pis. XVHI.-XX.)

The ventral nucleus receives most of the terminals of the

^This is included by Kappers in the medial nucleus.

DiENCEPHALON 85

medial, trigeminal, and spinal lemnisci; the former^ chiefly
medially, the latter laterally, and between them the trigeminal
lemniscus, which goes mainly to the nuclei centralis and arcu-
atus. While the ventral nucleus as a whole gives rise to an
extensive cortical radiation, some of its constituent centres
seem to have few or no such fibres, and the mass is apparently
rather intimately related with the medial nucleus.

The lateral nucleus, on the other hand, is overwhelmingly
cortical in its connections, and enlarges enormously as the
cerebral cortex becomes more extensive. It is thus much
smaller in the rat than in man, though the exact functional
significance of this difference is not clear. A considerable
part of the increased bulk in the higher form is represented
in the pulvinar,'- which is not distinguishable in the rodents.

The lateral and ventral surfaces of the ventro-lateral
nucleus are covered by the lattice nucleus (nucleus reticularis
thalami) (Pis. XIX., XX.), which is much more extensive in
the rat than in man. In the latter it is marked off from the
lateral nucleus by white fibres, the external medullary lamina
{lamina medullaris externa), but this lamina is not definite
in the rat — chiefly owing to its being more largely developed
and scattered through the nucleus.

A conspicuous object in the diencephalon is the crusta of
the cerebral peduncle (basis or pes pedunculi), which runs

'In the rat, "the principal terminals course along and in the ventral
surface of the ventral and lateral nucleus of the thalamus, forming a
narrow, crescentic, lemniscal stratum. The degeneration of these terminals
is limited almost exclusively to this nucleus", (Papez.)

2This is apparently a centre concerned largely with the correlation
of optic and cerebellar impulses, though it, no doubt, receives many others.
It has a largely developed connection with the cerebral cortex, particularly
with the area of the latter which seems to be largely concerned in spacial
recognition.

The suggestion has been made that the great enlargement of the
lateral nucleus is related to the emancipation of the fore limb, which has
reached such importance in the primates, and especially in man.

86 Anatomy of the Nervous System

back on each ventro-lateral aspect of this region to the mid-
brain. It is formed by fibres descending through the internal
capsule hiteral to the more anterior parts of the thalamus
and turning back to run to various lower parts of the central
nervous system as has already been described. These are
accompanied by many fibres which end in the thalamus
itself, so that even by the time it reaches the midbrain the
tract is already considerably smaller than when it started.

CHAPTER XII.
Telencephalon — Rhinencephalon.

THE telencephalon comprises all those parts of the brain
which have not yet been considered — all that lie
morphologically anterior to the diencephalon. It
consists of a very small median portion which continues the
diencephalon forward and encloses the anterior part of the
third ventricle, the telencephalon medinm {pars optica hypo-
thalami), and of paired lateral evaginations from this the
cerebral hemispheres.

Each hemisphere contains a cavity, the lateral ventricle,
which is connected with the anterior part of the third ventricle
by a small interventricular foramen {foramen of Monro). The
roof of the lateral ventricle with the dorsal parts of its walls
form the pallium, which in mammals has an external layer
of gray matter, the cerebral cortex, and is greatly expanded so
as largely to cover the outer surface of the rest of the hemi-
sphere. What were primitively the floor and more ventral
parts of the walls form the basal ganglia, which are very
largely hidden from view in the gross brain by the expansion
of the pallium. Of the structures visible externally, the
tuberculum olfactorium belongs to this division, as does also
the lower part of the medial wall just in front of the telence-
phalon medium. This part is known as the paraterminal
body {corpus paraterminale sen precommissurale) , or frequently
as the septum, though the latter term is confined in man to a
postero-dorsal extension of this region,^ as we shall see
presently (p. 93).

^According to Johnston, the septum pellucidum of higher mammals
is not genetically related to the paraterminal body, but is a separate

88 Anatomy of the Nervous System

At the anterior end of each hemisphere is the olfactory
bulb, which is connected by the olfactory tract with masses
of gray matter, both pallial and subpallial {i.e., belonging to
the basal ganglia). All of these, together with the olfactory
centres in the diencephalon, make up the rhinencephalon or
olfactory brain.

The olfactory portions of the pallium are the older phylo-
genetically and are given the name archipallium,^ while the
non-olfactory portions are called the neopallium. It is the
neopallium which has expanded in the mammals, as mentioned
above, and in the higher mammals it expands a great deal
more, crowding down the other parts until they are almost
completely concealed by it, and becoming very much con-
voluted.

A consideration of the rhinencephalon starts naturally
with the olfactory bulb (fig. 4, PI. XXV.), which receives the
axons of the olfactory nerve cells lying in the nasal mucous
membrane. Penetrating the anterior and ventral surfaces
of the bulb, these unmyelinated fibres terminate in little,
densely-matted knots called glomeruli, each of which contains
the terminal branches of several fibres and also one or more
dendrites from cells of the second order, the mitral cells. The
axons of the mitral cells form the olfactory tract, which
conducts the impulses back to the cerebral hemisphere proper
and end there in the secondary olfactory area. Just after
leaving the cell body, however, the axon gives ofT collaterals
which come into relation with the granule cells, small neurons
present in the olfactory bulb in great numbers. The axons
of the latter extend back to the mitral cells and discharge
among their dendrites. This arrangement and the glomeruli
are both to be regarded as mechanisms whereby strong central
impressions may be produced by weak peripheral stimuli.

structure of pallial origin dorsal to the latter, so that the name septum
should not be applied to the paraterminal body. (See Figs. 7 and 8.)
^See, however, footnote on p. 97.

Rhixexcephalox 89

All these elements in the olfactory bulb are arranged in
definite layers (PI. XXW). Superficially, we find, as would
be expected, a thin layer of unmyelinated fibres made up of
the entering primary olfactory axons {stratum fibrorum , lamina
fibronim ntrvi olfactorii). This passes into a layer in which
the glomeruli are seen lying side by side in a single series
{lamina glomeriilosa) . A thin band of granule cells {lamina
granulosa externa) lies just inside this, and a stratum made
up of dendrites and unmyelinated fibres {lamina gelatinosa)
separates the latter from the perikarya of the mitral cells,
which are disposed in a single row {lamina cellular um).
Alternating layers of granule cells and unmyelinated fibres
w4th a few fine myelinated fibres {lamina granulosa interna)
lie inside the layer of mitral cells, and between the former and
the centre of the bulb are two layers of myelinated fibres
{lami7ta fibrorum tractus olfactorii and lamina fibrorum medullae
lobi piriformis). The fibres in the outer of the two last
mentioned layers are grouped in little bundles which run
together posteriorly to form the olfactory tract. Those in
the innermost layer have thinner myelin sheaths, are scattered
in a rather diffuse reticulum, and run into the deep white
matter of the lateral olfactory gyrus (vide infra). They
include the fibres to and from the anterior commissure (p. 97).
Among them are many small cells, which are most numerous
near the centre, where the ventricle is situated.

Some of the olfactory nerve fibres (the vomeronasal nerve)
separate from the rest and run to a small body which is
imbedded in the posterior part of the dorsal surface of the
olfactory bulb. This is the accessory olfactory bulb or bulbus
parolfactorius (PL XXV.). Its structure is similar to that
of the olfactory bulb, except tbat the internal granular layer
is nearly absent and the fibres to the olfactory tract lie between
the mitral layer and the internal granular layer of the olfactory
bulb. These fibres run in the lateral olfactory stria to the
amygdaloid complex (p. 92).

90 Anatoimy of the Nervous System

If the observer work back through a series of sections of
the olfactory bulb, he will see a new mass of gray matter
appear just lateral to the layer of fibres of the medulla of the
lateral olfactory gyrus. This enlarges backwards and is
found to be the cortex of the lateral olfactory gyrus, on the
lateral aspect of which the olfactory tract is seen, while the
olfactory bulb extends some distance back along its ventro-
medial aspect.

According to the usual descriptions, the human olfactory
tract consists of three parts known respectively as the medial,
intermediate, and lateral olfactory striae. The last of these
runs into the lateral olfactory gyrus and gyrus hippocampi
(which is represented by the pyriform lobe of the rat) ; the
intermediate olfactory stria ends in the intermediate olfactory
nucleus, which occupies the anterior perforated area of man
and includes a vestige of the large tuberculum olfactorium
of lower mammals; while the medial olfactory stria is dis-
tributed to the subcallosal gyrus (paraterminal body) and
septum pellucidum of man, both of which are represented in
the septum of the rat.

The medial olfactory stria is small and diffuse and lies
below the surface in the rat, just dorso-medial to the inter-
mediate stria, running back over the anterior part of the
tuberculum olfactorium. The intermediate olfactory stria
is better developed, but runs along the medial aspect of the
lateral olfactory stria in the anterior part of the course of
this bundle, so that it does not appear as a distinct structure,
the two together forming a single conspicuous olfactory tract
on the latero-ventral aspect of the brain (PL XXIV.).

The olfactory tract (lateral olfactory stria only, in its
more posterior part) forms a conspicuous band extending
back along the ventral surface of the hemisphere for about
half the length of the latter, accompanied by the gradually
increasing gray matter of the lateral olfactory gyrus. The
depression in the surface of the cerebral cortex occupied by

Rhixexcephalox 91

the olfactory tract is the endorhinal groove {sulcus endo-
rhinalis). At each side of the optic chiasma, the lateral
olfactory gyrus expands rather suddenly in a ventro-medial
direction, forming the large pyrijorm lobe {lobus piriformis,
lobus sphenoidalis) with a prominent medial eminence, the
uncus of human anatomy, and the fibres of the olfactory tract
are dispersed in this enlarged lobe. The related gray matter
medial to the lateral olfactory gyrus forms the anterior
olfactory lobe {intermediate olfactory nucleus), while the pyriform
lobe and the lateral olfactory gyrus make up the posterior
olfactory lobe. There appears to be no real structural difference
between the lateral olfactory gyrus and the pyriform lobe,
so that the latter term is sometimes applied to the whole
posterior olfactory lobe, though the two parts are said to
have different origins (Herrick). This region is separated
laterally from the neocortex by a groove, the rhinal fissure
or limbic fissure {sulcus rhinalis, fissura rhinica, fovea limbica),
while a slight depression, the incisura rhinica, bounds the
anterior part of the lobe medially (Pis. II., XIV. -XXIV.).
The anterior olfactory lobe is probably concerned particularly
with the "oral sense", which is simply correlated smell,
touch, taste, and muscle sensibility of the snout region, and
which is important in feeding reflexes.

The cortex of the anterior olfactory lobe, or praepyriform cortex,
differs from the cortex of the pyriform lobe and the neocortex in being
thinner and in consisting of only three layers, the zonal or plexiform,
granular, and ganglionic layers {laminae zonalis, granularis, and ganglion-
aris). The granular layer of the praepyriform cortex is a dense band
of cells, which (except at its anterior and posterior extremities) forms a
series of longitudinal folds covering the ventral surface of the hemisphere
and extends at its medial edge to the ventral part of the septum. From
these folds of the granular layer, which serve to increase its area, small
groups of cells separate off as the islands of Calleja (ganglia olfactoria),
and certain special nuclei are distinguished in the medial part (nuclei of
the medial and lateral olfactory tracts). This large area with the cortical
folds and islands is usually called the tuherculum olfactorium, but includes
also the anterior perforated substance. Both of these are much reduced

92 Anatomy of the Nervous System

in the human brain, only the latter being recognized as a rule. The
tuberculum olfactorium has connections with the pyriform lobe and the
septum. Anteriorly, it becomes continuous with the anterior olfactory
nucleus (p. 96). Laterally and posteriorly, the praepyriform cortex
passes into that of the pyriform lobe, which is distinguished by greater
thickness and by a larger number of cell layers. The external granular
layer in it is particularly conspicuous, being composed of crowded, deeply-
staining cells, while the subjacent layers are more diffuse. It not only
receives secondary olfactory fibres, like the praepyriform cortex, but also
is the terminus of many tertiary fibres from the latter area as well as of
axons from other regions.

Closely related to the pyriform lobe in function, and
covered ventrally and laterally by it, is the amygdaloid
complex {amygdala, nucleus amygdalae). This is a group of
several distinct nuclei which may have various functions and
various origins, but at least some of which — and probably all,
according to Johnston — belong to the olfactory system in
company with the pyriform cortex. The amygdala certainly
has important somatic functions also, however (p. 107).

The nuclei of this complex are six in number according to Johnston,
who names them topographically. A rather large mass is the small-
celled central nucleus, directly medial to which is a superficial medial
nucleus (Pis. XVI 1 1. -XX.). These are derived from the posterior portion
of a ridge of gray matter (the bed of the stria terminalis — p. 96) belonging
to the primitive olfactory apparatus. A conspicuous, rounded group of
rather larger cells superficially situated anterior to the medial nucleus is
also a remnant of the latter apparatus. This is the nucleus of the lateral
olfactory tract {praesubicular area) (PI. XX.). Ventral to the central
nucleus, and pushing partly in between it and the medial nucleus, is the
basal nucleus (Pis. XVIII., XIX.), which is less well developed in the rat,
where it is composed of small cells, than in some other mammals, where
it has distinct large- and small-celled parts. Dorso-lateral to the basal,
and central nuclei is a distinct mass lying between the external capsule
and the ventricular surface, the lateral nucleus (Pis. XVIII. -XX.). The
basal and lateral nuclei are of more recent origin than those previously
mentioned and have been derived from the pyriform cortex by infolding
or immigration of cells along the line of the endorhinal groove. Finally,
there is a cortical nucleus in a superficial position between the medial

Rhinencephalon 93

nucleus and the endorhinal groove (Pis. XVIII., XIX.). This belongs with
the first group, but has been modified by the processes involved in the
formation of the newer centres.

Medially, the tuberculum olfactorium (except a small part
anteriorly) joins the ventral edge of the septum, which forms
the medial wall of the anterior part of the lateral ventricle.
This structure appears as a fairly thick plate of gray matter
penetrated by a large number of medullated fibres, most of
which are directed dorso-ventrally. Anteriorly, it rapidly
tapers off between the tuberculum olfactorium and the hippo-
campus (praecallosal portion — ^fig. 8) and it is most easily
recognized under the corpus callosum. The upper part of
the septum extends postero-dorsally between the corpus
callosum and the fornix and hippocampal commissure (Fig. 8)
and in the human subject this part becomes secondarily
separated from the remainder. To it the term septum
pellucidum is applied, while the remainder is known as the
subcallosal gyrus or paraterminal body (PL XXII.).

Thus the septum as a whole is composed of two parts, the area prae-
commissuralis septi or paraterminal body, and the pars supraforaminalis or
pars fimhrialis septi. According to Johnston, the latter is of pallial origin
and is alone entitled to the name of septum (Fig. 8), while the term par-
olfactory area may be applied to the remainder. This must not be con-
fused with Edinger's lobus parolfactorius, which is the tuberculum ol-
factorium.

The myelinated fibres in the septum fall into medial and
lateral groups, the former consisting of numerous relatively
thick fibres which form a distinct layer on the medial side
of the septum, the bundle of Zuckerkandl or fasciculus prae-
commissuralis {tractus lobo-hippocampalis, tractus cortico-
olfactorius septi), while the lattei" is made up of finer fibres
more diffusely arranged. This system comprises several
different kinds of fibres, including axlons both to and from the
septum as well as others which pass right through it. At
least some of the fibres arising in this region go to the hypo-

94

Anatomy of the Nervous System

thalamus and others go to the cerebral cortex, while many of
those ending in it are tertiary olfactory fibres from the various
secondary areas which have just been described. Many of
the latter, however, pass on to end in the hippocampus.
Ascending fibres from centres lower in the brain also reach
the septum. The medial olfactory stria sends many of its
fibres into this region.

Connecting the septum with the amygdala and the pyri-
form cortex is a thick strip of gray matter mingled with a
diffuse tract of white fibres, the whole being known as the

form bulb

Trolf.hypthS

Fig. 4

Diagram of some of the principal olfactory pathways in the rat,
from Herrick, Introduction to Neurology.

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. jorn., columna fornicis; com. ant., com-
missura anterior; com. hip., commissura hippocampi; com. post., com-
missura posterior; form, bulb., formatio bulbaris; /. retr., fasciculus retro-
flexus of Meynert; hab., habenula; h. pc, hippocampus praecommis-
suralis; h. sc, hippocampus supracommissuralis (indusium griseum);
n. ant., nucleus anterior thalami; n. olf. ant., nucleus olfactorius anterior;
n. pop., nucleus praeopticus (ganglion opticum basale); S., septum;
str. med., stria medullaris thalami; tr. mam. th., tractus mamillo-thala-
micus (Vicq d'Azyri); tr. olf. hypth., tractus olfacto-hypothalamicus or
basal olfactory tract; tr. olf. tegm., tractus olfacto-tegmentalis; tub. f.
dent., tuberculum fasciae dentatae (hippocampus postcommissuralis);
tub. olf., tuberculum olfactorium.

Rhixencephalox 95

diagonal band of Broca (Pis. XXL, XXII.). The posterior
extremity of this mass of gray matter becomes continuous
with the central and medial amygdaloid nuclei. It extends
antero-medially and turns up in the posterior part of the
septum, mingling with other fibres of the praecommissural
system. Most of the fibres of the diagonal band pass right
through the septum to end in the hippocampus. The diagonal
band also contains fibres connecting the amygdala with the
tuberculum olfactorium.

Another connection between the amygdala and the septum
is made by one of the bundles which form the stria terminalis
{stria sen taenia semicircidaris, stria cornea) . This is a massive
tract which accompanies the caudate nucleus (p. 105) — though
largely separated from it in the rat by the internal capsule — •
in its curving course from the septal region in front of the
anterior commissure (vide infra), where the tract appears
rather diffuse, round to the amygdaloid complex and is
distributed to the various nuclei of the latter and to the
neighbouring pyrlform cortex. These It connects with other
parts of the olfactory apparatus (Pis. XMII.-XXL).

Johnston finds the stria terminalis to be made up of five components
in the rabbit, man, and other mammals, and Gurdjian has largely con-
firmed his description for the rat.

(1) The commissural bundle is, in the rat, a distinct bundle of rather
fine fibres which can be followed from the nucleus of the lateral olfactory
tract through the stria terminalis, forming the most ventral part of its
dorsal loop and running into the anterior commissure. It crosses in the
most posterior part of the commissure and passes to the nucleus of the
lateral olfactory tract on the other side; or, according to Gurdjian, to the
cortical olfactory areas in the rat.

(2) The hypothalamic bundle (olfactory projection tract of Cajal) is
the largest bundle. It arises from the central and medial amygdaloid
nuclei, and anteriorly turns down behind the anterior commissure to run
back in the medial forebrain bundle (vide infra). In the rat it ends largely
in the praeoptic region.

(3) The infracommissural bundle passes forward under the anterior
commissure into the gray matter just lateral to the lateral ventricle, and
posteriorly connects with the basal nucleus of the amygdala.

96 Anatomy of the Nervous System

(4) The supracommissural bundle also connects with the basal
nucleus, while anteriorly it partly runs into the paraterminal body of the
septum and partly turns back under the anterior commissure to join the
medial forebrain bundle. The latter portion can be followed into the
praeoptic and hypothalamic regions in the rat.

(5) The stria medullaris bundle has already been described (p. 74).
The stria terminalis is accompanied in the reptiles and in mammalian

embryos by a ridge of gray matter, the hed of the stria termiyialis. In
adult mammals, this is reduced along the main part of the course of the
stria to the condition of a few scattered cells among the fibres of this tract.
At the anterior end of the stria, however, there remains a considerable
mass of gray matter near the anterior commissure, which is particularly
largely developed in the rat (PI. XXL). This is continuous with the gray
matter surrounding the commissure (the bed of the anterior commissure)
and extends forward and downward over the medial surface of the anterior
limb of the commissure (p. 97) to merge with the medial part of the head
of the caudate nucleus and with the nucleus accumbens (p. 106). Thus
these latter structures are seen to belong to the olfactory system, of which
the bed of the stria terminalis is a part. They are, moreover, both con-
tinuous anteriorly with a mass of secondary olfactory cells in the basal
wall near the olfactory bulb, which mass has received the name of anterior
olfactory nucleus (Fig. 4).^

Other fibres connecting the amygdala with several different regions
are contained in the rather diffuse longitudinal amygdalo-pyriform associa-
tion bundle (sagittales Ldngsbiindel , tractus cortico-amygdaloideus) shown in
Pis. XIX. and XX.

A crowded group of large cells situated at the ventral
surface immediately in front of the optic chiasma on each
side, and exitending posteriorly dorsal to the latter, is the
praeoptic nucleus {nucleus praeopticus, nucleus magnocellularis
praeopticus, ganglion basale opticum), which is related to the
paraterminal body both in its origin and in its connections,

^Authorities differ as to the definition of this nucleus. According
to Herrick (Jour. Comp. Neur., vol. 37, pp. 317-359) it includes the part
mentioned on p. 90 as anterior end of the lateral olfactory gyrus and
extends back from the olfactory bulb above the anterior end of the
tuberculum olfactorium. A distinct lamina superficial to the anterior end
of the lateral olfactory gyrus is the pars externa of the anterior olfactory
nucleus of Herrick. See particularly Gurdjian '25 and Obenchain '25.
The part marked noa in PI. XXIII belongs to the caudate nucleus.

Rhixen'cephalox 97

From the secondary olfactory areas, tertiary fibres run
to the habenula (stria meduUaris thalami) and to the hypo-
thalamus {iractus oljacto-hypothalamictis, basal olfactory tract,
basal olfactory projection tract (Pis. X\1I.-XX.), while others
descend to the lower parts of the brain. All these tracts
together form the large, more or less diffuse medial forebrain
bundle (Pis. XX., XXL). The farther pathways from the
lower centres have been considered in the previous chapter.
A great many of the tertiary olfactory fibres, however, belong
to the olfacto-cortical group, which ends in the hippocampus^
(vide infra).

The olfactory centres of the two sides are, moreover,
connected by the anterior commissure {commissura anterior
cerebri), a large structure crossing between the hemispheres
in the anterior wall of the telencephalon medium (lamina
terminalis). The bundle consists of two parts which are
quite distinct, the more anterior being composed of large
fibres deeply stained by the Weigert method, while the
remainder is made up of smaller, less heavily myelinated
fibres which take a paler stain. The anterior part is generally
held to be made up largely of secondary olfactory fibres con-
necting the two olfactory bulbs, while the posterior or temporal
portion is chiefly a commissure between the two pyriform
lobes.2 As might be expected, the anterior portion, which
is large in the rat and other macrosmatic animals, is very
much smaller in the human brain, where also the commissure

^Kappers differentiates the cortex of the olfactory lobe (cortex
piriformis and praepiriformis), which receives secondary olfactory fibres
as palaeocortex, and terms the hippocampus, which receives olfactory
fibres only of the third and higher orders, archicortex. The latter term is
frequently applied by other writers to the whole of the olfactory cortex.

The olfacto-cortical fibres from the tuberculum olfactorium (p. 94)
reach the hippocampus (and indusium) through the fimbria, the fornix
superior, and the striae Lancisii.

2A narrow zone of fibres on the lateral aspect of the bulbar portion
of the commissure connects the anterior parts of the lateral olfactory areas

98 Anatomy of the Nervous System

as a whole is relatively poorly developed. Between the two
parts there is an almost contiiluous series of fibres connecting
the olfactory areas all along the hemisphere (Gurdjian).

The arrangement of the tertiary olfactory cortex, hippo-
campus or Amnions horn {cornu Ammonis) is a little difficult
to understand at first, and as it is explained by the phylo-
genetic history of the hemisphere, a brief summary of the
latter is introduced here.

The hippocampus is derived from what was originally the dorsal half
of the medial wall of the cerebral hemisphere. Ventrally, this region met
the dorsal edge of what was to become the septum, while dorsally, it
came into contact with the part which was to become the pyriforrn lobe
(Fig. 5). Between it and the last-mentioned part, the neopallium later
developed in the dorsal wall of the hemisphere, separating these two
olfactory regions. As the neopallium grew in extent longitudinally as
well as transversely, it pushed the posterior parts of the hemisphere
downwards, so that the originally straight primordium of the hippocampus
was bent down posteriorly, curving round until its posterior end pointed
antero- ventrally in the temporal region of the hemisphere (Figs. 6, 7).
At the same time, the transverse growth of the neopallium forced the
primordium of the pyriform lobe down laterally until it reached the ventral
position where we have seen it in the rat and finally the ventro-medial
location which it occupies in man. The hippocampal area, however,
could not be thus displaced and consequently became folded inwards so
that most of it lay at the bottom of a deep groove, the hippocampal fissure.
It appears in this condition in the marsupials, but in the placental mammals
the increasing pressure of the neopallium and the development of the corpus
callosum have resulted in the degeneration of the anterior and dorsal part,
so that only the posterior part which curves down into the temporal lobe
remains well developed (Fig. 8). The remainder is represented only by

of the two hemispheres. There are also commissural and decussating
fibres from a pair of small bed nuclei of the anterior commissure and the
stria terminalis component which is mentioned on p. 95.

Kappers, in his recent text-book, calls the anterior part of this
commissure pars olfactoria and the posterior part pars neocorticalis, main-
taining that the latter is made up of neocortical commissural fibres like
those of the corpus callosum. He describes four different kinds of fibres
in the pars olfactoria (one of these being the commissural bundle of the
stria terminalis), but denies that any of these arise in the olfactory bulb.

Rhixexcephalox 99

a very thin band of gray matter, the indusium {indusiiim griseum, gyrus
siipracallosus) , accompanied by a few white fibres {striae longitudinales
Lancisii), which runs along the dorsal surface of the corpus callosum under
the edge of the outermost layer of the cerebral cortex. Johnston figures a
groove, the fimbrio-dentate fissure, between the indusium and the medial
stria of Lancisi in the rat, but the present writer has been unable to
recognize this in his specimens.

Fig. 5

Transverse section of the left cerebral hemisphere of the frog, from
Herrick,

C.S., corpus striatum; lob. p., lobus piriformis; nuc. ac, nucleus
accumbens septi; p. hip., primordium hippocampi; tr. ol. v. I., tractus
olfactorius ventro-lateralis.

The indusium, or supracallosal hippocampus, Is less
reduced in the rat than in man, but even here it appears in
cross sections as a very small group of cells, which require a
special cell stain to demonstrate them clearly (Fig. 9, Pis.
XVIII. -XX.). In front of the corpus callosum, there is an
unusually large remnant of the hippocampus to be found in
the rat, where the typical hippocampal structure Is to be
seen, though without any involution (Figs. 8, 9). At the
genu of the corpus callosum, part of the hippocampal forma-
tion extends back ventral to it for a short distance, while the
rest is seen to curve round it and become continuous with the
indusium. The dorsal band of cells is accompanied by a
somewhat dispersed group of fine myelinated fibres, mostly
belonging to the olfacto-cortical tract, which form the striae
Lancisii.

100 Anatomy of the Nervous System

The postcommissural hippocampus, i.e., the portion behind
and below the corpus callosum, is large and well-developed
(Fig. 8, Pis. IV., XV.-XX.).

The hippocampal formation is divided into two parts, the
hippocapipiis proper and the fascia dentata or gyrus dentatus

Fig. 6

Diagram of the medial surface of the cerebral hemisphere of the
frog, with the primordium hippocampi shaded. From a sketch by Herrick
made with sHght modification from a model by Johnston.

(Pis. IV., XV.-XX.), The hippocampus proper is the larger
part, which is folded in at the hippocampal fissure and forms
a prominent ridge in the ventricle. The ventricular surface
is covered with a layer of white fibres, the alveus, composed
of hippocampal projection fibres, and these join a large tract

Fig. 7

Diagram of the medial surface of the cerebral hemisphere of a
marsupial (opossum), from Johnston; hippocampus solid black; septum
shaded, with pallial and basal regions distinguished (see p. 93).

Rh I XEXCE PH ALOX

101

which runs along the side of the hippocampal ridge and is
known as the fimbria. The stria medialis Lancisii is the
reduced fimbria of the degenerated supracallosal part of the
hippocampus. The fornix, which has been seen ending in
the mamillary body, is the direct continuation of the fimbria.

Fig. 8

Diagram of the medial surface of the cerebral hemisphere of the
rat, from Johnston; shading as in Fig. 7.

Fig. 9

Middle part of a transverse -section of the brain of an albino rat
immediately behind the genu of the corpus callosum, stained with a cell
stain; c.c, corpus callosum; h., hippocampus (praecallosal portion extend-
ing back a short distance under the genu); i., indusium griseum; n.a.s.,
nucleus accumbens septi; s.c.c, sulcus corporis callosi; v. I., lateral
ventricle; /., lamina zonalis of the neocortex.

102 Anatomy of the Nervous System

The hippocampus is composed of three layers of cells, the
molecular, pyramidal, and polymorphic, of which the pyramidal
layer stands out clearly on account of the rather crowded
arrangement of its perikarya. The dentate gyrus is likewise
composed of three layers, in this case molecular, granular, and
polymorphic. It runs along the edge of the hippocampus and
is folded around it, the two portions overlapping considerably.
The cells of the granular layer are very closely crowded,
making it a conspicuous object in sections where they are
stained. The functions of the fascia dentata are chiefly
reception and local coordination. Most of the ax^ons of the
granule cells pass to the hippocampus proper, which gives rise
chiefly to commissural and descending projection fibres.

The afferent fibres to the hippocampal formation are
divisible into medial and lateral groups. The former arise
from the various parts of the secondary olfactory area and
the septum and make up a considerable part of the fibre
system which has already been noted in the latter structure.
They are distributed to the hippocampal cortex in various
ways, but chiefly through the fimbria and alveus. The
lateral group, or external hippocampal fibres (fibrae hippo-
campales externae)^ arise from both praepyriform and pyriform
areas and either join the alveus or run into the surface of the
molecular layer of the hippocampus. Non-olfactory fibres
enter the hippocampus directly from neighbouring parts of
the neopallium.

Efferent fibres from the hippocampus run anteriorly
through the fimbria into the fornix. The latter forms a
sweeping curve anteriorly {body of the fornix), which in the
rat is almost absent, as the dorsal end of the hippocampus
and the fimbria extend forward practically to the lamina
terminalis (Fig. 8) in the position otherwise occupied by the
body of the fornix (Pis. XIX. -XXI.). The fornix now turns
ventrally into the lamina terminalis, where it passes ventrally
and then posteriorly {column of the fornix), proceeding back

Rhinexcephalox 103

with a much more gradual curve In the medial part of the
thalamus. A large proportion of the fornix fibres {tractus
cortico-mamillaris) continue backwards in this way until they
reach the mamillary body, where, as already observed, many
end while a few decussate and pass farther down in the brain
stem. In the earlier part of their course, these fibres are
accompanied by hippocampal fibres for the habenula {tractus
cortico-hahenularis medialis), which separate off just after the
fornix turns backwards, postero-dorsal to the anterior com-
missure, and pass posteriorly and dorsally in the stria medul-
laris thalami. This bundle we have seen to accompany the
habenular nuclei and end in them on the same and the opposite
sides.

The hippocampus also gives rise to a large number of
commissural fibres which run to the same region of the other
hemisphere, forming the hippocampal commissure or psalterium
icommissura hippocampi, lyra). In man, this is a thin plate
of transverse fibres crossing between the bodies of the fornix
of the two hemispheres and merging at either side into the
alveus. In rodents, there is a very large commissure {ventral
hippocampal commissure, commissure of the fimbria, psalterium
ventrale) just behind the ventrally directed columns of the
fornix in the lamina terminalis, and this has a thin dorsal
extension {dorsal hippocampal commissure, psalterium dorsale)
which stretches back ventral to the corpus callosum until it
reaches the splenium, with which it seems to merge. This
extension is thickest in its posterior part, which is really a
distinct structure, and which Cajal believes not really to be
commissural in character, terming it the crossed temporo-
hippocampal tract. Some other authors seem to consider the
latter portion as part of the splenium, though it is clearly
different from the rest of that structure, taking a deeper stain
in Weigert preparations (PI. XXVL).

The fibres of the ventral hippocampal commissure partly run through
a small-celled mass of gray matter lying between it and the columns of the

104 Anatomy of the Nervous System

fornix. This bed of the commissure belongs morphologically to the
dorsal part of the septum. Continuous with it, a little mass of cells
just where the anterior end of the membranous roof of the third ventricle
meets the posterior surface of the commissure has been described in the
rat and other forms by Johnston as the noduliis marginalis.

Another longitudinal band of hippocampal projection fibres
may be seen tp separate from the alveus in its postero-dorsal
part and to take up a position near the middle line between
the corpus callosum and the dorsal hippocampal commissure.
There it is joined by small bundles which cross through
the corpus callosum from behind and above. This is thefornix
superior, a structure which is inconspicuous in man, but is
very prominent in the rat and in many other mammals
(Pis. XIX. -XXL). This band increases in size as one passes
forward to the region of the lamina terminalis, where many of
its fibres join the descending columns of the fornix, while
others mingle with the praecommissural system in the septum.
A much reduced remnant of the fornix superior may be
traced some distance farther forward, when it will be found
to come from the olfacto-cortical tract (Pis. XXII., XXIII.).
The perforating fibres which run through the corpus callosum
to the fornix superior are derived partly from the indusium,
but many come from the region of the cingulum (p. 109),
after originating in the interhemispheric cortex (chiefly from
its posterior and middle regions in the rat). These, with the
fornix superior itself, are frequently called fornix longus, a
term which seems to have been used in somewhat different
ways by different writers. A few fibres of the fornix system,
after traversing the septum, join the medial forebrain bundle
for the hypothalamus.

A diffuse tract running backwards in the molecular layer
of the cortex and mingling with the external hippocampal
fibres connects the deeper part of the corresponding layer of
the hippocampus with the subsplenial gyrus and the neo-
pallium caudal to it (Pis. XV.-XIX., XXVL).

CHAPTER XIII.
Telencephalon — Non-olfactory Portions.

THE non-olfactory portions of the telencephalon comprise
the basal ganglia and the neopallium. The basal
ganglia, according to the usual classification, consist of
four centres, the caudate and lentiform nuclei, the amygdala,
and the claustrum, though the claustrum is really derived from
the pallium. The amygdala, moreover, is not really non-
olfactory, as we have already seen. The caudate and lenti-
form nuclei, with the white fibres passing between or through
them, make up the corpus striatum, in which the amygdala is
sometimes included.

In the majority of mammals, including the human species,
the caudate and lentiform nuclei are separated by a thick mass
of myelinated fibres, the internal capsule, which is a direct
upward continuation of the pes pedunculi, and which is
composed chiefly of ascending and descending axpns between
the non-olfactory cerebral cortex and lower parts of the
brain (projection fibres). In the rat, however, as is found to
be the case in many other lower mammals, though usually in
a smaller degree, the internal capsule has a more primitive
form, being made up of small bundles of fibres scattered
through the gray matter, so that there is no sharp division
of the latter into two parts (Pis. XIX. -XXIII. ); As a matter
of fact, the internal capsule never extends far enough forward
to separate the two parts completely, and even in man, where
there is the greatest separation, the anterior end of the caudate
nucleus is continuous with that of the outer part (putamen)
of the lentiform nucleus.

The division into caudate and lentiform nuclei {nucleus

106 Anatomy of the Nervous System

caudatiis and nucleus lentijormis sen lenticularis) is not really
the fundamental one in the corpus striatum, however, the
lentiform nucleus consisting of two parts, the lateral putamen
and the medial globus pallidus, of which the putamen is much
more closely related to the caudate nucleus than to the
globus pallidus. The putamen and caudate nucleus together
form the neostriatum, the globus pallidus is the palaeo striatum,
being much older both phylogenetically and ontogenetically,
and the amygdala is called the archi striatum according to this
terminology.

The caudate nucleus, in animals where it is distinct, has
a large anterior portion or head {caput) and a greatly elongated
tail {cauda) which runs back from the caput dorso-medial to
the internal capsule and curves ventrally in the posterior part
of the hemisphere. Finally, it turns anteriorly again and
ends ventral to the globus pallidus in close topographical
relation with the amygdala. In the rat, the bundles of the
internal capsule come together posteriorly into a compact
mass separating the corpus striatum from the thalamus, but
even here there is no definite separation of a caudate nucleus.
Ordinary cell stains likewise show no distinction, so the
caudate nucleus may be considered to be represented by a
considerable anterior portion and a decreasing dorsal,
posterior, and postero-ventro-medial portion of the common
mass (neostriatum) formed by it and the putamen. At the
anterior end of this mass, a large and distinct nucleus projects
from it antero-medially under the lateral ventricle into the
region of the septum. This nucleus accumhens septi (collictdus
striatiy is found in nearly all mammals, as well as in lower
vertebrates, but has not yet been recognized in man. The
caudate nucleus is made up of two parts which are distinct
in the embryo but are indistinguishable in the adult. The
medio-ventral portion of the head of the caudate belongs to
the olfactory system along with the nucleus accumbens, as

^Nucleus parolfactorius lateralis of Johnston.

TeLEXCEPHALOX NOX-OLFACTORV PORTIOXS 107

previously noted (p. 96), while the rest is somatic in its
relations, like the remainder of the corpus striatum.

The globus pallidus is a much smaller mass than the
neostriatum.^ It lies directly medial to the putamen, from
which it is distinguished by the fact that it contains a large
number of diffuse myelinated fibres. In man and many
other animals, it is divided up by thin lamellae of such fibres.

The caudate nucleus and the putamen receive axons
through the internal capsule from the thalamus — particularly,
probably, from the centre of Luys and the anterior nucleus —
while some also come from the red nucleus. Perhaps col-
laterals from descending cortical fibres also end here. Associa-
tion fibres connect the striatal nuclei, but are stated to run
chiefly from the caudate nucleus to the putamen and from
this to the globus pallidus. There are probably descending
fibres accompanying the afi^erent ones mentioned, but most
of the descending fibres, at least in higher mammals, seem
to come from the globus pallidus through the ansa lenticularis,
whence they are distributed to the hypothalamus, the sub-
thalamic body, the substantia nigra, and the red nucleus.
There is, further, an extensive, difi^use fibre-connection between
the lentiform nucleus and the overlying pyriform cortex.

The functions of the corpus striatum are as yet very
uncertain. It seems to have an influence upon muscle tone
and also to act upon the visceral nervous system.

Related to the parts which we have been considering, is
the amygdala (archistriatum of Kappers and his school),
which difl^ers from them in receiving several orders of olfactory
fibres. Its function, however, is by no means entirely
olfactory, as is shown by its considerable development in
the dolphin, a totally anosmatic mammal. The amygdaloid
complex is to be regarded as a correlation mechanism for

^A closely related group of cells, the basal or central nucleus or nucleus
ansae pediincularis , is apparently incorporated in the globus pallidus itself
in the rat.

108 AXATOMY OF THE NeRVOUS SYSTEM

many dliYerent kinds of afferent impulses, both visceral and
somatic.

The remaining mass which is usually classified with the
basal nuclei is the claustrum, a thin plate of gray matter situ-
ated between the putamen and the overlying cortex (insular
area). It is separated from the putamen by a rather thin
layer of the white matter of the cerebral hemisphere, the
external capsule {capsula externa), and from the cerebral
cortex by a considerably thinner layer, the capsula extrema.
The latter is practically absent in the rat, so that the claustrum
cannot readily be distinguished in Weigert sections (Pis. XX.-
XXII.). It can be recognized fairly easily, how^ever, in
preparations which show the cell-structure, though even there
its limits are not very clear, tending to blend with the deepest
layer of the adjoining cortex. In the rat, as in some other
lower mammals, its ventral edge extends a short distance below
the rhinal fissure. The claustrum has been shown by Elliot
Smith to be an infolded portion of the cerebral cortex, a
source from which the putamen and the greater part of the
caudate nucleus have also been derived. Both the fibre-
connections and the function of the claustrum are still un-
known.

The neopallium, the non-olfactory cerebral cortex with
its subjacent white matter, makes up the dorso-medial, the
dorsal, and most of the lateral surfaces of the hemisphere
of the rat. The early steps of its evolutionary expansion
were briefly reviewed in the last chapter. The same process
of expansion has continued through the mammalian series
to reach its culmination in man. This part of the brain is
the physical substratum of all the finer forms of consciousness,
all sensory discrimination, and all those processes which we
know as mental, intellectual, or psychic.^ Experiments with

^The physical basis of the emotions, however, is in the thalamus
(Head).

TeLEXCEPHALOX — NOX-OLFACTORY PORTIOXS 109

rats have proved, however, that a large part of their ability
to learn by trial and error has a subcortical basis (Lashley).

The gray matter of the neopallium has all taken up a
superficial position, forming the cerebral cortex {cortex cerebri),
while between this and the lateral ventricle or the basal
ganglia, as the case may be, is a thick layer of white matter,
the medullary centre of the hemisphere, made up of intrinsic
association fibres, interhemispheric commissural fibres, and
projection fibres. The proportion of gray matter to white
matter is much greater in the rat and other small animals
than in man and larger forms generally.^

The intrinsic association fibres are of various lengths and
serve to connect the different parts of the cerebral cortex
of the same hemisphere. The longer ones are largely grouped
into rather definite tracts, though these do not show in
ordinary sections. The projection fibres include all the
ascending and descending fibres which run between the
cerebral cortex and lower parts of the brain. These all pass
through the internal capsule and spread out at its upper
extremity to run directly to the various parts of the cortex,
this spreading out of the fibres forming the corona radiata.

One prominent tract which is made up of both association
and projection fibres connecting with the cortex of the medial
surface of the hemisphere above the corpus callosum (inter-
hemispheric cortex) and with some related parts in front of
it is the cingtdum (Pis. X\TII. -XXIII. ). This is a longitudinal
band which curves round the genu of the corpus callosum,
runs straight back directly dorsal to the truncus, and extends
round the splenium (vide infra). Many of the axons from
the interhemispheric cortex bifurcate and send anteriorly-
and posteriorly-directed branches through this tract.

The interhemispheric commissural fibres form an immense
transverse mass running straight across betw^een the two
hemispheres and spreading out in each, the corpus callosum.

iSee de Vries, Anat. Anz., Bd. 37, p. 387.

110 Anatomy of the Nervous System

In a median sagittal section, this structure appears as a
rather flat arch, the main part of which is the body of the
corpus callosum {trunctis corporis callosi), while the posterior
edge shows a rounded thickening, the splenium, and the
anterior part, also thickened, turns sharply down as the
genu, tapering rapidly to a point, the rostrum, directed
postero-ventrally (PL XXYI.)- ^^ man, the rostrum becomes
secondarily connected with the lamina terminalis by a rostral
lamina, but in the rat, this is absent and the rostrum is not a
conspicuous part. The area of the corpus callosum in a
median sagittal section, where the corpus itself is cut exactly
transversely, gives the only exact indication of its real size.
In the adult rat, the area of such a section of the corpus
callosum is found to be 3.29% of the computed area of the
entire brain, while in man, similar measurements give a ratio
of 4.44% (Suitsu). The fibres of the corpus callosum connect
with all parts of the neocortex, though all parts are not equally
richly supplied with them. Callosal fibres arising from a
given area in one hemisphere are distributed to many different
parts of the opposite one.

The cortex of the neopallium, or neocortex, is greatly
convoluted in man and other large mammals (gyrencephalous
species), but in the rat, as in all mammals with small brains
the cortex is smooth (lissencephalous). The degree of con-
volution varies more or less with the gross size of the hemi-
sphere. This is due to the peculiar fact that increase in
volume of the neocortex takes place almost entirely by two-
dimensional growth — growth in area, while thickness increases
relatively little. Thus we find that the average thickness of
the cortex of the adult albino rat is fully 1.9 mm. and that
of the wild rat about 8% more (Sugita) while that of man
is about 3 mm., though the total weight of the human brain
is seven hundred times that of the rat brain. The actual
thickness of the cortex varies at different points. In a general
way, it diminishes from the frontal to the occipital pole, and

Telencephalon — Non-olfactory Portions 111

from the dorsal aspect to the ventral in the rat. This general
statement is probably correct for other mammals also,
including man.

The neocortex shows a very definite lamination in its
cell-structure, though the laminae are not all so precisely
delimited as to bring about complete agreement among the
authorities who have studied and enumerated them. Prob-
ably the most generally followed scheme is that of Brodmann,
who distinguishes six main layers, numbered from the surface
inwards as follows:

I. Lamina zonalis (Plexiform layer)

II. Lamina granularis interna. (Layer of small pyramidal

cells)

III. Lamina pyramidalis. (Layer of medium and large

pyramidal cells)

IV. Lamina granularis interna. (Granular layer)

y. Lamina ganglionaris. (Inner layer of large py-

ramidal cells)
VL Lamina multiformis. (Layer of spindle-shaped

cells)

These laminae are to be found throughout the mammalia,
but in the rat, as in other rodents, layer II. is practically
indistinguishable from layer III., so that only five layers
are to be recognized (PI. XXVIL). All the layers superficial
to the fifth {i.e., the lamina ganglionaris) probably have
receptive and intracortical associative functions, the granular
cells giving rise to shorter and the pyramidal cells to longer
(inter-regional) association fibres. The two deep layers (V.
and VL), on the other hand, contain the efferent neurons of
the cortex and send out projection and commissural fibres,
though also containing some short-fibred intra-regional
correlation neurons. The outer layers have been shown by
Bolton to be the only ones in which there are visible changes
corresponding with mental defects in man.

112 Anatomy of the Nervous System

Fig. 10

TeLEN'CEPHALOX — ^XON-OLFACTORY PORTIOXS 113

The appearance of the layers in a typical region of the
cortex of the rat may be summed up as follows. The super-
ficial layer, the lamina zonalis (I.), contains only a few
scattered glia cells. The lamina pyramidalis (III.) below
it is made up of typical, deeply-staining, pyramidal cells,
rather closely packed together. Under this layer, the lamina
granularis interna (IV.) appears as a band of crowded, deeply-
staining, small granules, somew^hat resembling glia cells.
Beneath this again Is the lamina gangllonarls (\'.), which
contains somewhat scattered, large-sized, deeply-staining py-
ramids. Finally, the deepest layer consists of a broad zone
of polymorphous cells, the lamina multiformis (VI.). These
are represented on the left side of PI. XXVII.

The laminae differ considerably in details of their structure
and relative development in different regions of the cerebral
cortex, one sometimes even appearing distinctly subdivided,
or again one disappearing entirely at certain points. Thus
It has been found possible to map out a considerable number
of areas In the neocortex, each with a distinct histological
structure and, presumably, with a more or less distinct
functional significance. Homologous areas can, moreover,
be recognized In the various groups of mammals. Further,
the principal areas have been shown to have definite differences

Fig. 10. Cortical localization in the wild Norway rat. Diagrams of
the lateral, medial and dorsal surfaces, from Fortuyn.

Fortuyns area. Brodmanns term.

f, f Regio praecentralis.

j, n Regio parietalis.

w Regio occipitalis.

k Regio insularis.

p, X -. Regio temporalis.

c Regio cingularis.

z, z", z'" Regio retrosplenialis.

r', s Regio hippocampica.

h, h" Regio olfactoria.

Regio postcentralis is represented in the anterior part of area j.

114 Anatomy of the Nervous System

in vascularity in the rat, as have also the different laminae in
a given area, which would seem to indicate differences in
functional activity^ (PI. XXVII.).

The main areas have been enumerated by Brodmann as
follows :-

1. Regio postcentralis

2. Regio praecentralis

3. Regio frontalis

4. Regio insularis

5. Regio parietalis

6. Regio temporalis

7. Regio occipitalis

8. Regio cingularis

9. Regio retrosplenialis.

Histological studies reveal a considerably larger number
of structurally different areas, however, as may be seen from
the accompanying plan of the localization in the cortex of
the rat^ (Fig- 10). In the legend for this illustration, the
areas indicated are correlated with the terms of Brodmann.
It will be observed that the regio frontalis is not mentioned,
this area, though exceedingly large in man, being very small
in the rat and being completely hidden at the bottom of the
rhinal fissure. It may perhaps be that the area marked d
also belongs to this region.

For what is known as to the functions of these areas, the
reader is referred to the standard text-books of neurology

^Similar differences have been shown between various centres in the
brain stem and the cerebellum, and the development of these differences
in the young rat has been found to be related temporally to the establish-
ment of functional activity.

-Brodmann lists also the olfactory cortex as:

10. Regio hippocampica.

11. Regio olfactoria.

^For details of the histological structure in each area shown in Fig. 10,
see F^ortuyn, Arch. Neur. and Psych., Path. Lab. London County Asylums,
vol. VL, 1914, or Sugita, Jour. Comp. Neur., vol. 28, 1917.

TeLEXCEPHALOX — XOX-OLFACTORV PORTIOXS 115

and to the special literature. It need only be mentioned
here that in a careful effort to show that his area p (temporal
area) contains the auditory cortex of the rodent, Fortuyn
was unable to obtain any conclusive evidence, though he
believes that this is the case. Lashley has plotted the
electrically ex^citable area in the rat, which included approxi-
mately areas f and f and the anterior edges of areas n(?),
j(?) and k, and has carried out a large number of experiments
which seem to indicate a high degree of transferability of
function in the cortex of this animal.

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118 Anatomy of the Nervous System

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1920. A study of brain repair in the rat by use of trypan

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macrophages. Arch. Neur., and Psychiat., vol. 3, pp.

353-394.
Meyersohn, F. 1913. Die Dicke der spinalen Trigeminus-

wurzel bei v^erschiedenen Saugern. Fol. Neurob., Bd. 7,

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Bibliography 123

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Nicholson, F. M. 1923. The changes in the amount and
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NiTTONO, K, 1923. On bilateral effects from the unilateral
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Obenchain, Jeannette B. 1925. The brains of the South
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Papez, J. W. 1923. The mamillary peduncle, Marchi
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Stewart, C. A. 1916. Growth of the body and of the
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1918a. Weights of various parts of the brain in normal
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124 Anatomy of the Nervous System

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APPENDIX.
Preparation of Sections — Method of Weigert.

WHILE the Weigert technique is a standard histological
method, there are many modifications of it, with
some of which the brain of the rat is sometimes
rather refractory. The writer has usually obtained satis-
factory results by the following simple form of the method.

The brain is removed from the skull as soon as possible
after death and is at once immersed in Miiller's fluid
(potassium bichromate, 2.5 gm.; sodium sulphate, 1 gm.;
distilled water, 100 c.c). This solution is renewed two or
three times during the first week, after which the material is
left undisturbed for two months or more, being kept in, the
dark during this time. The tissue is then washed for a few
hours in distilled water and passed into 50% alcohol for a
few hours more, after which it is transferred to 70% alcohol,
where it may remain for from twenty-four hours to several
weeks. It is next kept for twenty-four hours in each of 80%,
95%, and absolute alcohol successively, and passed through
cedar oil to parafftn.

Serial sections 15/i-20iu thick are mounted on slides by the
ordinary water-albumin method. The parafBn is now washed
out of the sections by immersing the slide in xylol, from which
it is transferred to absolute alcohol. The slide is next flooded
with thin celloidin (0.5%), which is drained off and allowed
to dry in the air for a few minutes, and after this the slide
is passed rapidly down through a graded series of alcohols to
distilled water.

The slides are placed upside down (supported on small
slips of glass) in a half-saturated solution of copper acetate

Preparation of Sections 127

which is kept at a temperature somewhere near 35° C, and
remain there over night. After rinsing in distilled water they
are transferred to Weigert's haematoxylin (10 c.c. 10%
haematoxylin in absolute alcohol +90 c.c. distilled water +
1 c.c. saturated solution of lithium carbonate), again upside
down, in which they are kept for 4-6 hours. Following
another rinsing, the sections, which are now deep blue or
blue-black, are subjtected to the action of Weigert's decolor-
izer, a fluid composed of equal parts of a 2% solution of
borax and a 2.5% solution of potassium ferricyanide. This
mixture is used freely until the desired differentiation is
obtained, which may be in a short time or may not be for
several hours.

The differentiating fluid of Pal (equal parts of 1% oxalic
acid and 1% potassium sulphite solutions used after a brief
immersion in 0.25% potassium permanganate) is very much
more rapid and may give more brilliant results, but is less
easily controlled in its action.

The sections are finally washed in running water for
twenty-four hours and mounted in Canada balsam.

Suitsu recommends as particularly suitable for the rat
brain a modification of the very similar Kultschitzky techni-
que, the description of which may be found in his paper in the
Journal of Comparative Neurology, vol. 32, pp. 36-37.

Plate I. Dorsal view of the brain of an adult albino rat four times natural
size.

bo, olfactory bulb; cer. cerebral hemisphere; ch, cerebellar hemi-
sphere; ci, posterior colliculus; e, pineal body; pf, paraflocculus;
py, pyramis; sip, dorso-lateral groove; smp, dorsal median groove;
si, first cervical spinal nerve, dorsal root; s2, second cervical spinal
nerve, dorsal root; u, uvula; vr, vermis.

Plate I

NT

pf

smp —

i- ^j

sip

--

"'

.;

'

Plate II. Ventral view of the brain of an adult albino rat four times
natural size. The representation of the nerve roots is somewhat
diagrammatic.

bo, olfactory bulb; cho, optic chiasma; cma, mamillary body; ct,
trapezoid body; frh, rhinal fissure; i, infundibulum; Ip, pyriform
lobe; nc, neopallium; p, pyramid; pf, paraflocculus; po, pons; pp,
basis (pes) pedunculi cerebri; si, first cervical spinal nerve, ventral
root; tc, tuber cinereum; to, tuberculum olfactorium; trol, olfactory
tract; III., oculomotor nerve; IV., trochlear nerve; V., trigeminal
nerve (large sensory and small motor portions); VI., abducent nerve;
VII., facial nerve; VIII., auditory nerve; IX. +X., glossopharyngeal
and vagus nerves; XL, spinal accessory nerve; XII., hypoglossal
nerve.

Platf 11

po

-m

-A

Plate III. Dorsal view of the brain of an adult albino rat with the cere-
bellum and much of the cerebral hemispheres removed, X 4.
be, brachium conjunctivum; bo, olfactory bulb; bpo, brachium
pontis; cc, corpus callosum; ci, posterior coUiculus; cl, clava; cr,
restiform body; est, corpus striatum; csu, anterior colliculus; dc,
tuberculum acusticum; ev, vestibular area (trigonum acustici); f,
facial colliculus; fc, fasciculus cuneatus; fi, fo\'ea inferior; fim,
fimbria; fsu, fovea superior; ft, funiculus teres; gl, lateral geniculate
body; gm, medial geniculate body; nc, cerebral cortex (neocortex);
o, obex; s, septum; sm, stria medullaris thalami running along the
habenula into the habenular commissure; st, stria terminalis; th,
thalamus; tv, taenia ventriculi (line of attachment of posterior
medullary velum); vc, ventral cochlear nucleus; V., trigeminal nerve;
MIL, auditory nerve.

Plate III

nc

cc

N

:%

V

^ est

V

1

"}' N

1 csu jSW

1

"^- , ^/^

/

C'l

^

r

- V AT'

/

Plate IV. Medial view of the right cerebral hemisphere of an albino rat
with most of the thalamus cut away so as to expose the hippocampal
formation, X 4.

bo, olfactory bulb; ca, anterior commissure in lamina terminalis;
cc, corpus callosum; chd, dorsal part of hippocampal commissure;
chv, large ventral part of hippocampal commissure or commissura
fimbriae; co, optic chiasma; fac, fasciola cinerea; fch, fissura chori-
oidea; fd, fascia dentata; fh, hippocampal fissure; fim, fimbria;
gs, gyrus subsplenialis; gsc, gyrus supracallosus; h, hippocampus
proper covered by white fibres connecting with the fimbria; s, septum;
tfd, tuberculum fasciae dentatae; th, cut surface of thalamus.

Plate IV

/

bo

clid

s

^■^-^ Ji^

£•§

X.

ith i^ fd

/

ca

fli

fcl

Plate V. Outline drawings of transverse sections through alternate
segments (except in the caudal region) of the spinal cord of an albino
rat, X 6.25.

df, dorsal funiculus; 1, tract of Lissauer; If, lateral funiculus; p,
pyramidal (cortico-spinal) tract; sr, gelatinous substance of Rolando;
vf, ventral funiculus.

Plate V

df

P ,sr

CI.

cm.

cv

c vn.

TV.

TXI.

LIV

TI.

T MI.

T XIII.

LVI.

SIV.

Till.

TIX.

LII.

SII.

Plate VI. c, central canal; dp, decussation of the pyramidal tract;
fc, fasciculus cuneatus; fr, reticular formation; fs, lower end of
fasciculus solitarius; nfl, nucleus funiculi lateralis; ng, nucleus
gracilis; nXI, nucleus of nerve XL; nXII, nucleus of nerve XII.;
p, pyramidal tract; sr, gelatinous substance of Rolando; XL, nerve
XL; XII., root of nerve XII.

Plates VI.-XXV. THE MAGNIFICATION IN PLATES VL-XXV.
INCLUSIVE IS 12.5. THESE ARE TRANSVERSE SECTIONS
THROUGH THE BRAIN OF AN ADULT ALBINO RAT
STAINED BY THE METHOD OF WEIGERT. THE LEVELS
OF THE SECTIONS ARE INDICATED IN PLATE XXVIII.

Plate VI

ns; c ^^0 ' '

Plate VII. ch, cerebellar hemisphere; dl, decussation of the lemniscus;
fai, internal arcuate fibres; flm, medial longitudinal bundle; fra,
formatio reticularis alba; frg, formatio reticularis grisea; fs, fasciculus
solitarius; ncu, cuneate nucleus; ni\, nucleus of lateral funiculus;
nfs, nucleus of fasciculus solitarius; ng, gracile nucleus; nsV, spinal V.
nucleus; nX., dorsal motor vagus nucleus; nXII., hypoglossal
nucleus; od, dorsal accessory olivary nucleus; oi, inferior oli\ary
nucleus; om, medial accessory olivary nucleus; p, pyramidal tract;
r, nucleus of Roller; rsV., spinal V, root; tt, tecto-spinal tract;
ug, granular layer of cortex of uvula of cerebellum; um, molecular
layer of cortex of uvula of cerebellum; XII., root of hypoglossal
nerve.

Plate

VII

.__ _

.

'

'J^

-

, \

"

c

h

1

'

... • "

^ -. -■''

• f,"

11

y

<^^.A,

(

1
1

^-

^ - ^

--"

^^ -riA

1

^ -

--T'-"'^

nJ^I

J

:i^-rsF

'>•

y

^

L >

'ft-

-^-nsF

far

fVadlt'toml)^

Plate VIII. fs, fasciculus solitarius; Im, medial lemniscus; n, nodulus of
cerebellum; na, nucleus ambiguus; ncu, cuneate nucleus; nfs, nucleus
of fasciculus solitarius; ng, gracile nucleus; nlr, lateral nucleus of
reticular formation; np, nucleus parasolitarius; ns, nucleus inter-
calatus of Staderini; nsV., spinal V. nucleus; nvr, ventral nucleus of
reticular formation; nX., dorsal motor vagus nucleus; nXII., hypo-
glossal nucleus; od, dorsal accessory olivary nucleus; oi, inferior
olivary nucleus; om, medial accessory nucleus; p. pyramidal tract;
py, pyramis of cerebellum; rsV., spinal V. root; tsd, dorsal spino-
cerebellar tract; tsv, ventral spino-cerebellar tract; u, uvula of
cerebellum; ve, fourth ventricle; VIII.c, chief vestibular nucleus;
VIII. d, descending ^'estibular root and nucleus; XII., root of the
hypoglossal nerve.

Plate VIII

P liii nni nv r

Plate IX. cr, restiform body; ct, trapezoid body; d, dentate nucleus:
dc, dorsal cochlear nucleus; dt, nucleus of Deiters; fl, flocculus;
flm, medial longitudinal bundle; fs, fasciculus solitarius and its
nucleus; Im, medial lemniscus; me, lobus medius; n, nodulus; ncr,
nucleus of restiform body; ncvm, caudal \'entro-medial nucleus of
spinal V. root; ne, nucleus emboliformis; nsV, spinal V. nucleus;
nVII., motor facial nucleus; nvr, ventral nucleus of reticular forma-
tion; p, pyramidal tract; pf, paraflocculus; rsV., spinal V. root;
sa, stria medullaris acustica; t, nucleus tecti; tt, tecto-spinal tract;
vc, ventral cochlear nucleus; VII., fascicles of motor root of facial
nerve; VIII.c, chief vestibular nucleus.

Plate IX

m'ti "'!, "\

-%!V^'"-

_-.-ncr

ct

Ti c\ rii

xxUI

Plate X. b, nucleus of Bechterew; cr, restiform body; ct, trapezoid
body; dct, decussation of trapezoid body; dt, nucleus of Deiters;
fl, flocculus; flm, medial longitudinal bundle; fo, fibres from superior
olivary nucleus to nucleus abducens, etc.; fv, vestibulo-cerebellar
fibres; g, nucleus globosus; gVI I., genu of motor root of facial nerve;
Im, medial lemniscus; npe, nucleus praeolivaris externus; npi,
nucleus praeolivaris internus; nsV., spinal V. nucleus; nt, nucleus of
trapezoid body; nVI., nucleus of abducent nerve; os, superior
olivary nucleus; osa, accessory superior olivary nucleus and its dorso-
medial cell group; p, pyramidal tract; pf, paraflocculus; rsV., spinal
W root; tr, rubro-spinal tract; tt, tecto-spinal tract; \c, ventral
cochlear nucleus; VI., root fibres of abducent nerve; VII., emerging
motor root of facial nerve; VIII. c, chief vestibular nucleus; \s,
secondary vestibular fibres.

Plate X

To

P J' 4-1 ' \ \ ^ V OS npe

tt m

iipi

Plate XI. be, brachium conjunctivum; cr, restiform body; ct, trapezoid
body; dct, decussation of trapezoid body; flm, medial longitudinal
bundle; fls, dorsal longitudinal bundle of Schiitz; la, lobulusansiformis;
Ic, locus coeruleus; II, lateral lemniscus; Im, medial lemniscus; Is,
lobulus simplex; 1 1-4, lobules of lobus anterior; ncV., chief sensory
nucleus of the \^ nerve; nmes, mesencephalic nucleus of the V.
nerve with fibres of the mesencephalic root running ventro-lateral
from it; nmV., motor nucleus of the V. nerve; novm, anterior cells
of oral ventro-medial nucleus of spinal V. root; npi, nucleus prae-
olivaris internus; nt, nuclei of the trapezoid body; os, superior olivary
nucleus; osa, accessory superior olivary nucleus; p, pyramidal tract;
rmV., motor root of V. nerve; rsV., beginning of spinal root of V.
nerve; tr, rubro-spinal tract; tsv, ventral spino-cerebellar tract;
tsvd, descending part of ventral spino-cerebellar tract accompanying
brachium conjunctivum into cerebellum, with fibres of cerebello-
bulbar tract (of Russel) under it; vc, ventral cochlear nucleus; vma,
anterior medullary velum; VU., anterior part of emerging motor
facial root.

Plate XI

Irridci

Plate XII. be, brachiuni conjunctivum; hi, brachium of posterior
colliculus; bpo, brachium pontis; cer, occipital pole of cerebral
hemisphere; ci, posterior colliculus; cp, secondary fibres from dorsal
nucleus of lateral lemniscus, many of which cross in the commissure
of Probst; 11, lateral lemniscus with cells of its dorsal nucleus between
its fascicles; Im, medial lemniscus; npo, nucleus pontis; nr, nucleus
of the raphe; ntd, dorsal tegmental nucleus; ntv, ventral tegmental
nucleus; p, pyramidal tract; po, pons; rmes, mesencephalic V. root;
rmV., motor V. root; rsV., sensory V. root; tg, mamillo-tegmental
tract; tpo, tecto-pontine tract; tt, tecto-spinal tract; IV., root of
trochlear nerve.

Plate XTI

CI

1

rmcs
nt<l ^

M

II

?--^^

/:-J^=^

i t>o

-/

11 uo IK) 11 r In

Plate XIII. as, aqueduct of Sylvius; bi, brachium of posterior colliculus;
cer, occipital portion of cerebral hemisphere; eg, central gray matter;
ci, nucleus of posterior colliculus; dbc, decussation of brachium con-
junctivum; ddt, dorsal tegmental decussation (Meynert); dvt,
ventral tegmental decussation (Forel); flm, medial longitudinal
bundle; Ig, laminated gray matter of tectum penetrated by com-
missure of posterior colliculi; 11, lateral lemniscus; Im, medial lemnis-
cus; ndr, dorsal nucleus of the raphe; ni, interpeduncular nucleus;
nIV., nucleus of the trochlear nerve; npo, nuclei of the pons; p,
pyramidal tract; po, pons; rmes, mesencephalic V. root; sp, stratum
profundum; tg, mamillo-tegmental tract; tr, rubro-spinal tract;
tt, tecto-spinal tract; tti, tegmental tract of interpeduncular nucleus.

Plate XIII

ci

^-^'

^t

cer

Li

Plate XIV. bi, brachium of posterior colliculus; bs, brachium of anterior
colliculus (optic tract); eg, central gray matter; dvt, ventral teg-
mental decussation with dorsal tegmental decussation dorsal to it;
ew, nucleus of Edinger-Westphal ; fm, fasciculus retrofiexus (of
Meynert); frh, rhinal fissure; 11, lateral lemniscus; Im, medial
lemniscus; nc, neocortex; ni, interpeduncular nucleus; nlll., nucleus
of oculomotor nerve; nrd, dorso-lateral portion of red nucleus; nrm,
magnocellular portion of red nucleus; nrv, ventro-Iateral portion of
red nucleus; pm, peduncle of the mamillary body; pp, basis pedunculi;
pyc, pyriform cortex; rm, fountain radiation of Meynert; rmes,
mesencephalic V. root; sg, stratum griseum of anterior colliculus;
si, stratum lemnisci of anterior colliculus; sn, substantia nigra; soa,
optic layer of stratum opticum of anterior colliculus; sob, non-optic
layer of stratum opticum of anterior colliculus; sp, stratum profun-
dum; sz, stratum zonale; ta, tapetum (a distinct layer of fibres from
the corpus callosum); tg, mamillo-tegmental tract; vl, lateral
ventricle; w, layer of white matter superficial to tapetum; III.,
oculomotor nerve root.

Plate XIV

'HI V .
■^ 1 ^ j)in

Plate XV. bi, brachium of posterior colHculus; bs, brachium of anterior
colliculus; cf, column of the fornix; ds, supramamillary decussation;
fd, fascia dentata; fh, hippocampal fissure; fhe, external hippocampal
fibres; fhs, fasciculus hippocampo-subsplenialis; flm, medial longi-
tudinal bundle; fm, fasciculus retroflexus; frh, rhinal fissure; gm,
medial geniculate body; i, posterior extension of cavity of infundi-
bulum; Im, medial lemniscus; ml, lateral nucleus of mamillary body;
mm, medial nucleus of mamillary body; nc, neocortex; nda, nucleus
of Darkschewitsch; nic, interstitial nucleus of Cajal; npt, nucleus
of transverse peduncular tract; pm, peduncle of mamillary body;
pp, basis pedunculi; pyc, pyriform cortex; sg, stratum griseum of
anterior colliculus; si, stratum lemnisci of anterior colliculus; sn,
substantia nigra; soa, optic layer of stratum opticum; sob, non-optic
layer of stratum opticum; sp, stratum profundum; sz, stratum
zonale; ta, tapetum; tg, mamillo-tegmental tract; tpt, transverse
peduncular tract; vd'a, mamillo-thalamic tract; vl, lateral ventricle;
w, layer of white matter superficial to tapetum.

Plate XV

nc

sz

ta-

sg--

:'■■ n'lC-

fhs

;■; ;;^ ^

: 111 ,,~

:H-'l^

>,l>s fhe

01

■ti :^m4 0 I

>^' ,-„

v^a \

mm

pyc

Plate XVI. as, aqueduct of Sylvius; bs, brachium of anterior colliculus;
cf, column of the fornix; eg, central gray matter; cpt, posterior
commissure; d\t, ventral tegmental decussation; fd, fascia dentata;
fh, hippocampal fissure; fhe, external hippocampal fibres; fhs,
fasciculus hippccampo-subsplenialis; fm, fasciculus retroflexus; frh,
rhinal fissure; gm, medial geniculate body; h, hippocampus; i,
infundibulum; Im, medial lemniscus; nc, neocortex; npr, posterior
nucleus of thalamus; pp, basis pedunculi; pyc, pyriform cortex;
sg, stratum griseum of anterior colliculus; sn, substantia nigra;
soa, optic layer of stratum opticum; sob, non-optic layer of stratum
opticum; sz, stratum zonale; tc, tuber cinereum; tg, mamillo-
tegmental tract; vd'a, mamillo-thalamic tract; vl, lateral ventricle;
vt, third ventricle.

Plate XVI

Plate XVII. as, aqueduct of Sylvius; cf, column of fornix; cha, habenu-
lar commissure; cpt, posterior commissure; cs, subthalamic nucleus;
fd, fascia dentata; fe, endorhinal groove; fh, hippocampal fissure;
fhe, external hippocampal fibres; fhs, fasciculus hippocampo-sub-
splenialis; fm, fasciculus retroflexus; frh, rhinal fissure; gl, lateral
geniculate body; gm, medial geniculate body; h, hippocampus; i,
infundibulum; Im, medial lemniscus; nc, neocortex; npr, posterior
nucleus of thalamus; pp, basis pedunculi; pyc, pyriform cortex;
tc, tuber cinereum; toh, olfacto-hypothalamic tract; tro, optic
tract; vd'a, mamillo-thalamic tract; vl, lateral ventricle; vt, third
ventricle; zi, zona incerta.

Plate XVII

nc

//■>(■[ Kf

frh:

tc

/

/ •
cf

I ;toh ;

izi
lln

pyc

fe

Plate XVIII. a, alveus; ab, basal nucleus of amygdala; al, lateral
nucleus of amygdala; am, medial nucleus of amygdala; cc, corpus
callosum; ce, external capsule; cf, column of fornix; chd, dorsal
hippocampal commissure; cm, commissure of Meynert; en, cingulum;
CO, cortical nucleus of amygdala; cs, subthalamic nucleus; fd, fascia
dentata; fe, endorhinal groove; fhe, external hippocampal fibres;
fhs, fasciculus hippocampo-subsplenialis; fim, fimbria; fm, fasciculus
retroflexus; frh, rhinal fissure; gld, dorsal nucleus of lateral geniculate
body; g\v, ventral nucleus of lateral geniculate body; h, hippocampus;
hi, lateral nucleus of habenula; hni, medial nucleus of habenula; i,
infundibulum; in, indusium griseum; 1, lateral nucleus of thalamus;
Ini, medial lemniscus; m, medial nucleus of thalamus; mi, inter-
mediate mass; nc, neocortex; npa, parependymal nucleus; pp, basis
pedunculi; pyc, pyriform cortex; sm, stria medullaris thalami; st,
stria terminalis; tc, tuber cinereum; toh, olfacto-hypothalamic tract;
V, ventral nucleus of thalamus; vd'a, mamillo-thalamic tract; vl,
lateral ventricle; zi, zona incerta.

Plate XVIII

vd'a-^"

to / Iff]

■ I lies

L \m
^f zi fe

Plate XIX. a, alveus; ab, basal nucleus of amygdala; ac, central nucleus
of amygdala; al, lateral nucleus of amygdala; am, medial nucleus of
amygdala; cc, corpus callosum; ce, external capsule; cf, column of
fornix; chd, dorsal hippocampal commissure: cho, optic chiasma ;
cin, internal capsule; en, cingulum; co, cortical nucleus of amygdala;
cp, caudate nucleus + putamen; fd, fascia dentata; fe, endorhinal
groove; fhe, external hippocampal fibres; fhs, fasciculus hippocampo-
subsplenialis; fim, fimbria; fos, fornix superior; frh, rhinal fissure;
gp, globus pallidus; h, hippocampus; hi, lateral nucleus of habenula,
hm, medial nucleus of habenula; in, indusium griseum; 1, lateral
nucleus of thalamus; lb, longitudinal amygdalo-pyriform association
bundle {sngittales Ldngshimdel); m, medial nucleus of thalamus;
mi, intermediate mass; nbo, praeoptic nucleus; nc, neocortex; nl,
nucleus reticularis thalami (lattice nucleus); npa, parependymal
nucleus; nre, nucleus reuniens; pp, basis pedunculi; pyc, pyriform
cortex; sm, stria medullaris thalami; st, stria terminalis; sti, com-
missural bundle of stria terminalis; toh, olfacto-hypothalamic tract;
V, ventral nucleus of thalamus; vd'a, mamillo-thalamic tract; vl,
lateral ventricle; vt, third ventricle; 2, 3, fibres of stria terminalis
connecting with amygdala; zi, zona incerta.

Plate XIX

r~'^hh

ml^:--S^:.

am

rcUo^^ r^^^i,

rnbo CO

lb

Plate XX. a, alveus; ac, central nucleus of amygdala; ad, dorso-medial
part of anterior nucleus of thalamus; al, lateral nucleus of amygdala;
av, ventro-lateral part of anterior nucleus of thalamus; ca, fibres
from anterior commissure; cc, corpus callosum; ce, external capsule;
cf, column of fornix; chd, dorsal hippocampal commissure; cho, optic
chiasmal cin, internal capsule; cla, claustrum; en, cingulum; era,
coron,a radiata; fd, fascia dentata; fe, endorhinal groove; fim,
fimbria; fos, fornix superior; frh, rhinal fissure; gp, globus pallidus;
h, hippocampus; hm, medial nucleus of habenula; in, indusium
griseum; lb, longitudinal amygdalo-pyriform association bundle;
m, medial nucleus of thalamus; mi, intermediate mass; nbo, praeoptic
nucleus; nl, nucleus reticularis thalami; npa, parependymal nucleus;
nre, nucleus reuniens; ntol, nucleus of lateral olfactory tract; pu, puta-
men; pyc, pyriform cortex; sm, stria medullaris thalami; sma, part of
antero- ventral continuation of stria medullaris thalami; st, stria
terminalis; sti, commissural bundle of stria terminalis about to end
in ntol; toh, olfacto-hypothalamic tract in medial forebrain bundle,
here mingled with diagonal band of Broca; tol, lateral olfactory tract;
V, ventral nucleus of thalamus; vd'a, mamillo-thalamic tract; vl,
lateral ventricle; 2, terminal fibres from stria terminalis.

Plate XX

7, mi V>,'

sma

/ / / ' \

ioh/niol H,

,tr

EX*^^

Plate XXI, caa, anterior division of anterior commissure; cat, temporal
division of anterior commissure; cau, caudate nucleus; cb, bed of
anterior commissure; cc, corpus callosum; ce, external capsule; cf,
column of fornix; chv, ventral or principal hippocampal commissure;
cin, internal capsule; cla, claustrum; en, cingulum; era, corona
radiata; db, diagonal band of Broca; fbl, lateral division of medial
forebrain bundle (olfacto-hypothalamic fibres, stria medullaris, etc.);
fbm, medial division of medial forebrain bundle (olfacto-hypothalamic
tract, etc.); fe, endorhinal groove; fim, fimbria; fos, fornix superior;
frh, rhinal fissure; gp, globus pallidus; in, indusium griseum; nc,
neocortex; op, optic nerve; pu, putamen; pyc, pyriform cortex
(lateral olfactory gyrus); sb, bed of stria terminalis; st, stria termin-
alis; to, tuberculum olfactorium; tol, lateral olfactory tract; vl,
lateral ventricle.

Plate XXI

iiin

-co

ihr

to

Plate XXII. caa, anterior limb of anterior commissure; cau, head of
caudate nucleus; cc, corpus callosum; ce, external capsule; cf,
column of fornix; cin, internal capsule; cla, claustrum; en, cingulum;
db, diagonal band of Broca mingled with other fibres of praecom-
missural system; fbl, lateral division of medial forebrain bundle;
fe, endorhinal groove; fos, fornix superior; frh, rhinal fissure; in,
indusium griseum; nac, nucleus accumbens septi; nc, neocortex; pc,
praecommissural system of fibres running through nucleus of diagonal
band of Broca; pyc, pyriform cortex (lateral olfactory gyrus); s,
septum (the letter is in the pallial part of Johnston — his primordium
hippocampi); to, tuberculum olfactorium; tol, lateral olfactory
tract; vl, lateral ventricle.

Plate XXII

,cin

ct-4i

^||::V:- '""■^ .:::V-v •,:;■■.

nc

/.';-a v

io

frli

fe

fW

Plate XXIII. caa, anterior limb of anterior commissure; cau, head of
caudate nucleus; cin, internal capsule; en, cingulum; fe, endorhinal
groove; frh, rhinal fissure; h, rostral extension of hippocampal
cortex; nc, neocortex; pyc, pyriform cortex (lateral olfactory gyrus);
to, tuberculum olfactorium; toe, olfacto-cortical tract; tol, lateral
olfactory tract; torn, medial olfactory tract; vl, lateral ventricle.

Plate XXIII

cau

nc

j,,^)^'* ^cin

ioc~

1 ''' no.a \

1

-caa

.-t'~.V

ionis
io

frh

Plate XXIV. frh, rhinal fissure; irh, incisura rhinica; mp, white matter
of the lateral olfactory gyrus containing the fibres of the anterior
limb of the anterior commissure; nc, neocortex; soi, fibres of inter-
mediate olfactory stria; tol, lateral olfactory tract; tom, medial
olfactory tract; vl, rhinocoele (anterior extension of lateral ventricle);
w, central white matter of cerebral hemisphere.

Plate XXIV

w\' '/

V" '^

SSM

M

irli frh

om y4

Vol;

\

nc

to]

Plate XXV. bp, parolfactory bulb; Ice, layer of mitral cells; If, layer
of olfactory nerve fibres; Ige, external granular layer; Igel, gelatinous
layer; Igi, internal granular layer; Igl, glomerular layer; Imp, layer
of fibres of the white matter of the lateral olfactory gyrus including
anterior commissure fibres; Ito, layer of fibres of the olfactory tract;
nc, neocortex on frontal pole of cerebral hemisphere; vrh, rhinocoele.

Platk XXV

li; 0.^ . Ho

Isi

\'A

Ilt

l-C'l

Plate XXV'I. Parasagittal section of the brain of an adult albino rat,
passing fully half a millimeter to one side of the median plane anteriorly
and a little farther from it posteriorly. X^A.

ca, anterior commissure; cc, corpus callosum; cf, column of
fornix; eg, central gray matter round aqueduct of Sylvius; chd,
dorsal hippccampal commissure; cho, optic chiasma; chs, thin part
of dorsal hippocampal commissure (section lies a little too far laterad
to cut fornix superior); chv, ventral hippccampal commissure; ci,
posterior colliculus; cma, mamillary body; cpt, posterior commissure;
csu, anterior colliculus; ct, trapezoid body; dbc, decussation of
brachia conjunctiva; dp, lateral fibres of pyramidal decussation;
fd, fascia dentata; fhs, fasciculus hippocampo-subsplenialis; fm,
fasciculus retroflexus; fs, fasciculus solitarius; ge, genu of corpus
callosum; gVII., genu of motor root of facial nerve; h, hippocampus;
hi, lateral nucleus of habenula; Im, medial lemniscus; nc, neocortex
of cerebral hemisphere; nfs, nucleus of fasciculus solitarius; ng,
nucleus gracilis; ni, interpeduncular nucleus; nrm, magno-cellular
portion of red nucleus; nt, nucleus of trapezoid body; oi, inferior
olivary nucleus; p, pyramidal tract; pc, praecommissural fibre
system; pm, peduncle of mamillary body; po, pons; ro, rostrum of
corpus callosum; s, septum; sm, stria medullaris thalami; sp,
splenium of corpus callosum; tc, tuber cinereum; tg, mamillo-
tegmental tract; th, thalamus; to, tuberculum olfactorium; tro, optic
tract; vd'a, mamillo-thalamic tract; ve, fourth ventricle; vma,
anterior medullary velum; vmp, posterior medullary velum; III.,
oculomotor nerve root; IV., decussating trochlear nerve roots; VI.,
abducent nerve root; VII., motor root fibres of facial nerve; VIII.c,
chief vestibular nucleus; XII., hypoglossal nerve root.

Plate XXVII. On the left, a diagram of the cell-lamination in a small
part of the occipital cortex of an adult albino rat. On the right, a
drawing of the blood vessels in the same piece of cortex. X 112,
I., lamina zonalis; III., lamina pyramidalis; IV., lamina granularis
interna; V., lamina ganglionaris; VI., lamina multiformis.

Plate XXVII

Gft 0^

-^ ^-.^i^Ur

In

'^'V^V. ^''^

[U 0

O Q

:iA^/*-«^^^

u

't'^

r^^z

"o-'^^'k

0^«J'

oV^S^

V

^

'-^IV

\

-S \

i\

)

*' \

m

ET

izr

Plate XXVIII. Diagrams of the dorsal and ventral surfaces of the brain
of the albino rat, showing the approximate levels of the transverse
sections represented in Plates VI. -XXV. The numbers on the
diagrams are those of the plates in which the corresponding sections
are illustrated.

INDEX

Arabic numerals refer to pages, Roman numerals to plates.

Ala cinerea — see trigonum vagi,
alveus, 100, 102, 103, 104, XVIII-XX.
amygdala — see nucleus amygdalae,
ansa lenticularis, 78, 107.
aqueduct, cerebral (aqueduct of Svl-

vius), 12, 72, XII-XVII.
arbor vatae, 56.
arc, reflex, IS.
archipallium, 88, 97.
archistriatum, 106.
area, anterior perforated, 14, 91.

of corpus callosum in median section,

110.
cortical — see region, of cerebral cor-
tex,
electrically excitable, 115.
of cortico-spinal tract in section, 53.
of V. Monakow, area occupied by
dorsal secondary cochlear fibres
after decussating and before
joining lateral lemniscus— dorsal
to superior olivary nucleus,
oval, 52.
parolfactory, 93.
praesubicular — see nucleus of lateral

olfactory tract,
of spinal V root in section, 31.
vestibular, 12, III.
association fibres, 24.
axon, 17.

Band, diagonal of Broca, 95, XX-

XII.
basis pedunculi cerebri, 13, 59, 66, 67,

76, 85, 86, 105, II, XIV-XIX.
bed, of anterior commissure, 74, 96,
98, XXI.
of habenular commissure, 73.
of stria terminalis, 74, 92, 96, XXI.
of ventral hippocampal commissure,
103, 104.
blood vessels in brain, 11, 17, 113, 114,
XXVII.

body, of corpus callosum, 110.

of fornix, 102.

geniculate, lateral (external), 15, 80,
III, XVII, XVIII.
medial (internal), 15, 38, 69, 79,
80, III, XV-XVII.

of Luvs — see nucleus, subthalamic.

mamillarv, 15, 64. 75, 76, 94, 101,
103,11, XV, XXVI.

paraterminal, 87, 90, 93, 96.

pineal, 14, 72, I.

pituitary, 15.

praecommissural — see body, para-
terminal.

quadrigeminal — see colliculus, an-
terior and posterior.

restiform, 12, 13, 36, 51, 52, 58, III,
IX-XI.

subthalamic — see nucleus, subthala-
mic.

trapezoid, 12, 36, 37, 38, II, IX-XI,
XXVI.
brachium, of anterior colliculus, 70,
XIV-XVI.

of posterior colliculus, 69, 79, XII-
XV.

conjunctivum, 12, 52, 57, 58, 61,
III, XI, XII, XXVI.

conjunctivum descendens, 61.

pontis, 12, 36, 54, III, XII.
Broca, diagonal band of — see band,
bulb, olfactorv, 10, 13, 88, 89, 90, 94,
96, 97, I- IV, XXV.

parolfactorv (accessorv olfactorv),
89, XXV.
bundle, basis — ^see fasciculus proprius.

commissural of stria terminalis, 95,
98, XIX.

dorsal longitudinal — • see bundle,
medial longitudinal,
of Schutz, 64, XI.

hypothalamic of stria terminalis, 95.

185

186

Index

infracommissural of stria terminalis,
95.

longitudinal am>gdalo-i:)yriform as-
sociation {so git Idles Ldngs-
binidel), 96, XIX, XX.

medial forebrain, 73, 74, 95, 96, 97,
104, XX-XXII.
lateral division, XXI, XXII.
medical di\ision, XXI.

medial longitudinal, 40, 46, 53, 60,
61, 63, 65, 71, VII-XV.

posterior longitudinal — see bundle,
medial longitudinal.

stria medullaris of stria terminalis,
74,96.

supracommissural of stria terminalis,
96.

of Vicq d'Azyr — see tract, mamillo-
thalamic.

of Zuckerkandl — see fasciculus prae-
commissuralis.

Calamus scriptorius, 12.
Calleja, islands of — see islands,
cat, 47.

canal, central, 12, 22, VI, VII.
capsula extrema, 108.
capsule, external, 92, 108, XVIII-
XXII.
internal, 66, 74, 79, 80, 86, 95, 105,
107, XIX-XXIII.
cells, granule, 88.
mitral, 88, XXV.
nerve, 17.
neuroglia, 17.
Purkinje, 57, 58.
cell-lamination of cerebral cortex — see

laminae,
centre, correlation, 18, 19, 21, 50-55, 65.
gustatory, 42, 43.
medial of Luys, 84, 107.
cerebellum, 10, 40, 52, 54, 55, 56-58,

62,81, II, VII-XII, XXVI.
chiasma, optic, 15, 80, 81, 82, II, IV,

XIX, XX, XXVI.
cingulum, 104, 109, XVIII-XXIII.
claustrum, 105, 108, XX-XXII.
clava, 12, III.
collaterals, 17.

colliculus, anterior, 13, 54, 67, 68, 69,
70, 71, 72, 79, 80, III, XIV-
XVI, XXVI.
facial, 12, III.

posterior, 10, 13, 38, 60, 67, 68, 69,
80, I, III, XII, XIII, XXVI.

striati — see nucleus accumbens septi.
column of Burdach — see fasciculus
cuneatus.

of Clarke, 25.

of the fornix, 74, 75, 94, 102, 103,
104, XV-XXII, XXVI.

of Goll — see fasciculus gracilis.

dorsal gray of spinal cord, 22, 24,

25, 26, 34, V.

ventral gray of spinal cord, 22, 24,

26, 45, 53, V.
intermedio-lateral, 26.
lateral of spinal cord, 26, 27.
white of spinal cord — see funiculus.

commissura fimbriae — see commissure,
hippocampal, ventral,
infima, 43.

media thalami — see mass, intermedi-
ate,
commissure, anterior, 16, 74, 89, 94,
95, 96, 97, 98, IV, XX-XXVI.
anterior division, 96, 97, 98, XXI-
XXV.
temporal division, 97, 98, XX,

XXI
bed of, 74, 96, 98, XXI.
of anterior colliculi, 70.
of V. Gudden — see commissure, post-
optic,
habenular, 14, 72, 73, III, XVII.
hippocampal, 16, 94, 103, IV.
dorsal, 103, XVIII-XX, XXVI.
ventral of principal, 103, IV, XXI,
XXVI.
of lateral lemniscus, 59, 60, 69, XII.
of Meynert, 82, XVIII.
posterior, 16, 65, 71, 73, 94, XVI,

XVII, xxyi.

of posterior colliculi, 69, XIII.

postoptic, 80, 81.

of Probst — see commissure of lateral
lemniscus.

soft — see mass, intermediate.

superior — see commissure, habenu-
lar.

dorsal supraoptic, 82.

ventral supraoptic — see commissure,
postoptic.
complex, amygdaloid — see nucleus

amygdalae,
components, principle of, 20.

Index

187

connective tissue, 17.
cord, spinal, 9, 10, 22-28, 34, 40, 43,
45, 46, 49, 53, 54, 60, 62, 68, V.
corona radiata, 109, XX, XXI.
cornu — see column, gray of spinal cord,
corpora quadrigemina, 59, see also
colliculi, anterior and posterior,
corpus callosum, 14, 16, 93, 98, 99, 101,
103, 104. 110, III, IV, XVIII-
XXII, XXVI.
geniculatum — see body, geniculate,
mamillare — see body, mamillary,
patellare — see nucleus semilunaris,
pineale — see bod\-, pineal,
restiforme — see bod\', restiform.
striatum, 15, 62, 78, 83, 99, 105,
106, 107, III, see also names of
component nuclei,
subthalamicum — see nucleus, sub-
thalamic,
trapezoides — see body, trapezoid,
cortex, cerebellar, 40, 56, 57, VII-XII.
cerebral, 54, 55, 62, 66, 67, 68, 69,
70, 79,80,81,83, 84, 85, 87,94,
105, 108, 109, 110-115, I-IV,
XII-XXVII.
blood vessels of, XXVII.
lamination of, XXVII.
hippocampal — see hippocampus,
internuclear, 68.
praepvriform, 91, 102.
pvriform, 91, 92. 94, 95, 102, 107,
II, XIV-XXIV.
crus cerebri — see peduncle, cerebral,
crusta cruris cerebri — see basis pedun-
culi cerebri.

Decussation, of brachia conjunctiva,
61, 62, 63, XIII, XXVI.

fountain of Forel — see decussation,
ventral tegmental.

of medial lemniscus, 34, VII.

fountain, of Me\nert — see decussa-
tion, dorsal tegmental.

of V. Monakow, 36.

of coulomotor root fibres, 63.

of pontine fibres, 55.

of pvramidal tracts, 29, 34, 46, 53,
VI, XXVI.

supramamillary, 75, XV.

of trochlear nerve root, 11, 63,
XXVI.

dorsal tegmental, 62, 70, XIII, XIV.

ventral tegmental, 62, XIII-XVI.

of trapezoid body, 36, 38, 69, X, XL
dendrite, 17.

diencephalon, 14, 68, 72-86.
dolphin, 107.

Enlargement, cervical, 9.

lumbar, 9.
epithalamus, 72, 77.
epiphysis — see body, pineal,
exteroceptor, 23.

Fascia dentata, 15, 100, 102, IV, XV-
XX, XXVI.

fasciculus, 19. (See also bundle and
tract.)

concomitans radicis trigemini spin-
alis, 32.

cuneatus, 23, 24, 26, 27, 34, III, VI.

dorso-lateralis — see tract of Lissauer.

gracilis, 23, 24, 26, 27, 34.

habenulo-peduncularis — see fascicu-
lus retroffexus.

hippocampo-subsplenialis, X\"-XIX,
XXVI.

interfascicularis, 26.

longitudinalis dorsalis, medialis, or
posterior — see bundle, medial
longitudinal.

of IMeynert— see fasciculus retro-
flexus.

praecommissuralis, 93, 95, 104,
XXII, XXVI.

praedorsalis — see tract, tecto-spinal.

proprius, 26, 54.

retroflexus, 65, 75, 76, 94, XIV-
XVIII. XXVI.

septo-marginalis, 26.

uncinatus, 58.
fasciola cinerea, 16, IW
fibres in cerebral hemisphere, 109.

cortico-bulbar and -pontine — see
tract.

cortico-thalamic, 79.

external hippocampal, 102, XV-XIX.

internal arcuate, 34, VII.

of V. Monakow (crossed dorsal
secondary cochlear), 38.

olivo-cerebellar, 51.

perforating, 104.

projection, 105, 109, 111.

thalamo-cortical, 79, 85.

188

Index

fillet — see lemniscus.

filum terminale, 9.

fimbria, 15, 74, 97, 101, 102, III. IV,

XVIII-XXI.
fissure, chorioid, IV'.

dorsal median — see sulcus.

fimbrio-dentate, 99.

hippocampal, 15, 98, 100, IV, XV-

XVII.
limbic — see fissure, rhinal.
longitudinal cerebral, 13, 14.
rhinal, 14, 91, 108, II, XIV-XXIV.
ventral (anterior) median, 10, 22,
II, V.
flocculus, 10, 35, IX, X.
foramen, interventricular, 16, 87.

of Monro — see foramen, inter-
ventricular,
formatio reticulata — see nucleus re-
ticularis thalami.
formation, reticular, 29, 33, 38, 40, 51,

54, 59, 61, 65, 77, VI-XIV.
fornix, 74. 75, 94, 101, 102, 103, XV-
XXII, XXVI.
longus, 104.

superior, 97, 104, XIX-XXII.
fossa, interpeduncular, 13, 15, 65.
fovea inferior, 12, III.

limbica — see fissure, rhinal.
superior, 12, III.
frog, 99, 100.

funiculus, dorsal, 22, 27, 34, 53, V.
lateral, 22, 25, 27, V.
teres, 12, III.
ventral, 22, 53, 54, V.

Ganglion, 9, 10, 18, see also nucleus,
basal, 87, 88, 105, 107.
basale opticum — see nucleus prae-

optic.
olfactory, 91.
sympathetic, 47.
genu of corpus callosum, 99, 109, 110,
XXVI.
of motor facial root, 46, 48, X, XXVI.
globus pallidus, 74, 106, 107, XIX-

XXI.
glomerulus, 88.

groove, dorsal of spinal cord, 10, I, V.
dorso-lateral of spinal cord, 10, 22,

I, V.
endorhinal, 91, 92, XVII-XXIII.
intermedio-lateral, 22.

guinea pig, 43, 44.
gyrencephalous, 110.
gyrus, dentate — see fascia dentata.
hippocampi, 90.
lateral olfactorv, 14, 89, 90, 91, 96,

XXI-XXIV.
subcallosal, 90, 93.
subsplenial, 16, 104, IV.
supracallosal, 16, IV, see also
indusium griseum.
Habenula, 14, 65, 72, 74, 75, 94, 97,

III, XVIII-XX, XXVI.
hemisphere, cerebellar, 10, 11, 55, 56,
57, 78, 79, I, VII-XI.
cerebral, 10, 13, 61, 66, 72, 73, 87-
115, I-IV, XII-XXVI.
hindbrain, 10, 55.

hippocampus, 15, 16, 74, 94, 95, 97,
98, 100, 102, IV, XVI-XX,
XXIII, XXVI.
praecallosal, 93, 94, 99, 101.
supracallosal — see indusium griseum.
hypophysis, 15, 77.

hypothalamus, 44, 60, 64, 66, 72, 75,
77, 94, 96, 97, 104, 107.
pars optica, 75, 87.

Incisura rhinica, 91, XXIV.

indusium griseum, 16, 94, 97, 99, 101,

104, XVIII-XXII.
infundibulum, 15, 75, 77, II, XV-

XVIII.
islands of Calleja, 91.
iter — see aqueduct, cerebral.

Lamina terminalis 16, 97, 102, 103,
110, IV.

external medullar}- of thalamus, 85.

internal medullary of thalamus, 83.

ventral medullary of thalamus, 79.

rostral, 110.
alminae of olfactorv bulb, 89.

of anterior colliculi, 70, XIV-XVI.

of cerebellar cortex, 57.

of cerebral cortex, 111, 113, XXVII.

of hippocampal cortex, 102.

of praepyriform cortex, 91.

of pyriform cortex, 92.
laminated gray matter of tectum — see

tectum,
layers of corte.x — see laminae,
lemniscus, lateral, 36, 37, 38, 39, 52,
54, 59, 62, 68, 69, XI-XIV.

Index

189

medial, 33, 34, 37, 3S, 44, 51, 60,
63, 76, 82, 85, VII-XVIII,
XXVI.
spinal, 24, 33, 85.
trigeminal, 32, 51, 85.
lissencephalous, 110.
lobe, floccular, 10, 11, 56, 57, I, II,
IX, X.
frontal, 54, 62, 67.
occipital, 67, 112.
olfactory, 14.
anterior, 91.
posterior, 91.
optic, 67.
pvriform, 14, 74, 90, 91, 92, 94, 97,

98,99, 102, II, XIV-XXIV.
temporal, 54, 67, 112.
see also lobus.
lobulus ansiformis, 10, 56, XI.

simplex, XI.
lobusanterior of cerebellum, 11, 56, XI.
medius of cerebellum, 11, 56, XI.
parolfactorius, 93.
posterior of cerebellum, 11, 56.
sphenoidalis — see lobe, pvriform.
localization in cerebral cortex. 111, 112,

113.
locus coeruleus, XI.
lyra — see commissure, hippocampal.

Mass, intermediate, 14, 84, XVIII-XX.
matter, gray, 19, 22.

white, 19, 22.
medulla oblongata, 10, 29, 68, II, III,
VI-XII, XXVI.

spinalis — see spinal cord,
mesencephalon — see midbrain,
metathalamus, 72, 82.
Meynert, decussation of — see decus-
sation, dorsal tegmental.

fountain radiation of — see radiation.

tract of — see fasciculus retroflexus.
midbrain, 10, 12, 13, 59-71, 74, 82,

II, III, XII-XVI, XXVI.
myelin, 17.

Neocortex — see cortex, cerebral and

neopallium,
neopallium, 14, 88, 98, 102, 104, 105,

107, 109, 110, see also cortex,

cerebral,
neostriatum, 106.

I nerve, abducent, 30, 45, 4(), II, X,
XXVI.

accessor — see nerve, spinal accessory.

auditory, 13, 35, 39, II, III.

cochlear, 35.

components, principle of, 20.

cranial (components in), 20, 29, 45.

eighth — see nerve, auditory.

eleventh — see nerve, spinal acces-
sory.

facial, 13, 30, 42, 46, 47, II, IX-XI,
XXVI.

fifth — see nerve, trigeminal.

first — see nerve, olfactory.

fourth — see nerve, trochlear.

glossopharvngeal, 30, 42, 43, 46, 47,
II.

hypoglossal, 45, 46, II, \1-VIII,
XXVI.

ninth — see nerve, glossopharvngeal.

oculomotor, 13, 30, 45, 46,' 63, 66,
II, XIV, XXVI.

olfactorv, 13, 88, 89.

optic, 15, 67, II, XXI.

second — see nerve, optic.

seventh — see ner\'e, facial.

sixth — see nerve, abducent.

spinal, 9, 20, 45, I, II.

spinal accessory, 46, 47, II, VI.

tenth — see nerve, vagus.

third — see nerve, oculomotor.

trigeminal, 13, 30, 47, 49, II, III.

trochlear, 11, 30, 45, 63, II, XII,
XXVI.

twelfth — see nerve, hvpoglossal.

vagus, 30, 42, 43, 46, 47, II.

vestibular, 39.

vomeronasal, 89.
neurite, 17.
neurobiotaxis, 49, 64.
neuroglia, 17.
neuron, 17, 18.

correlation, 18.

internuncial, 18.
nodulus of cerebellum, 11, 56, VIII, IX.

marginalis, 104.
nucleus, 19.

of abducent nerve, 37, 45, 46, 48, X.

accessorius, 48, VI.

accessory olivary — see nucleus, oli-
vary.

accumbens septi, 96, 99, 101, 106,
XXII.

190

Index

ambiguus, 48, VIII.

amygdalae, 82, 92, 94, 95, 96, 105,

' 107, XVIII-XX.
amygdalae, basal, 92, 95, 96, XVIII,
' XIX.

central, 92, 95. XIX, XX.
cortical, 92, XVIII, XIX.
lateral, 92, XVIII-XX.
medial, 92, 95, XVIII, XIX.
ansae peduncularis, 107.
anterior olfactory — see nucleus, ol-
factory-,
arcuatus, 84.
basal (of cerebral hemisphere), 87,

88, 105, 107.
basal (of corpus striatum), 107.
basal optic — see nucleus, praeoptic.
bed, of anterior commissure, 74, 96,
98, XXI.
of habenular commissure, 73.
of yentral hippocampal commis-
sure, 103, 104.
of stria terminalis, 74, 92, 96, XXI.
of Bechterew, 39, 40. X.
caudal yentro-medial of spinal \'

root, 52, IX.
caudate, 83, 95. 96, 105, 106, 107,

108, XIX-XXIII.
central (of corpus striatum), 107.
centralis (of thalamus), 84.
cochlear, dorsal, 13, 35, 36, 37, 38,
III, IX.
yentral, 35, 36, 37, 38, 40, III,
IX-XI.
of colliculus posterior, 68, XIII.
commissural (of Cajal), 43.
correlation — see centre, correlation,
cuneatus, 34, 39, 40, VII, VIII.
of Darkschewitsch, 65, 71, XV.
of Deiters, 39, 40, 46, 60, IX, X.
dentate, 57, 58, IX.
dorsalis — see column of Clarke,
dorso-lateral, 26.
ectomamillar>-, 66.
of Edinger-Westphal, 63, XIV.
emboliformis, 57, 58, IX.
facial, motor, 46, 47, 48, IX.
of fasciculus solitarius, 42, 43, 47,

VII. \III, XXVI.
fastigii, 40. 57, 58, IX.
funicular, 34.

funiculi lateralis — see nucleus of
lateral funiculus.

funiculi teretis, 43.
globosus, 57, 58, X.

t ha la mi, 84.
of glossopharyngeal nerye, 46, 47, 48.
gracilis, 34, VI-VIII, XXVI.
of y. Gudden — see nucleus, teg-
mental, yentral.
habenulae, 65, 73, 75.

lateral, 73, XVIII, XIX, XXVI.
medial, 73, XVIII-XX.
hypoglossal, 32, 43, 45, 47, VI-VIII.
intercalatus (Staderini), 43, 47, VIII.
intermedins, 58.
interpeduncular, 64, 65, 75, XIII,

XIV, XXVI.
interpositus, 58.

interstitial (Cajal), 60, 65, 71, XV.
of lateral funiculus, VI, VII.
of lateral geniculate body, dorsal, 80,
XVIII.
yentral, 80, XVIII.
of lateral lemniscus, dorsal, 39, 59,
XII.
yentral, 38.
of lateral olfactory tract, 74, 91, 92,

95, XX.
lattice — see nucleus reticularis

thalami.
lentiform, 105, 107.
of Luys — see nucleus, subthalamic,
of mamillary body, lateral, 75, 76,
XV.
medial, 75, 76, XV.
masticatorius — see nucleus, trigem-
inal, motor,
of medial olfactory tract, 91.
mesencephalic of V nerye, 33, XI.
oculomotor, 45, 60, 62, 63, 64, XIV.
olfactory, anterior, 92, 94, 96.

intermediate, 91.
olivary, accessory superior, 37, 38,
X, XI.
of the cerebellum — see nucleus,

dentate,
dorsal accessory, 50, VII, VIII.
of superior, 37, 38.
oliyary, inferior, 50, 51, 57, VII,
VIII, XXVI.
medial accessory, 50, VII, VIII.
principal — see nucleus, olivary,

inferior,
superior, 36, 37, 48, 52, X, XI.

IXDEX

191

ventro-lateral — see nucleus, oli-
vary, inferior,
oral ventro-medial of spinal V root,

52, XI.
paramedianus, S3,
parasolitarius, 42, 43, VIII.
parependymal, XVIII-XX.
parolfactorius lateralis — see nucleus

accumbens septi.
of Perlia, 63.
of the pons, 53, 54, 67, XII, XIII,

XXVI.
of the posterior commissure, 71.
praebigeminal — see nucleus of the

thalamus, posterior,
praceolivaris externus, 37, X.

internus, 37, 38, X, XI.
praeoptic, 94, 96, XIX, XX^^
proprius pedunculi cerebri, 78.
radiate — see nucleus reticularis thai-
ami,
of the raphe, 65, XII.

dorsal, 65, XIII.
red, 60, 61, 62, 107, XIV, XXVI.
of restiform body, IX.
of reticular formation, 51, 52, 65.
lateral, VIII.
ventral, 51, VIII, IX.
reticularis tegmenti — see nucleus of

reticular formation,
reticularis thalami, 79, 85, XIX, XX.
reuniens, 84, XIX, XX.
rhomboidalis — see nucleus reuniens,
of Roller, 51, VII.
roof, of cerebellum, 40, see also

nucleus fastigii.
salivatorv, superior and inferior, 33,

47. '
semilunaris, 84.

of spinal accessory nerve, 48, VI.
of Staderini — see nucleus intercala-

tus.
subthalamic, 66, 78, 79, 82, 107,

XVII, XVIII.
tecti — see nucleus fastigii.
tegmental, dorsal, 64, 65, 76, XII.
ventral, 64, XII.
ventral reticular, 33.
of thalamus, anterior, 74, 76, 77, 79,

82, 83, 94, 107, XX.
dorsal — see nucleus of thalamus,

anterior,
globose, 84.

lateral, 81, 84, 85, XVIII, XIX.
medial, 82, 83, 84, 85, XVIII-XX.
posterior, 71, 82, XVI, XVII.
reticular — see nucleus reticularis

thalami.
ventral, 84, 85, XVIII-XX.
ventro-lateral, 82, 84, 85.
of transverse peduncular tract, 66,

XV.
of trapezoid bod\-, 37, 38, X, XI,

XXVI.
triangular — see nucleus, vestibular,

chief; also footnote, p. 40.
trigeminal, chief sensor\', 31, 32, 33,
49, XI.
mesencephalic, 33, 69, XI.
motor, 33, 49, 52, XI.
spinal, 31, 32, 38,48, VII-X.
trochlear, 45, 60, 61, 62, 63, 64,

XIII.
vagus, dorsal motor, 43, 47, 48,
VII, VIII.
ventral motor — see nucleus am-
biguus.
ventralis grisea centralis — see nuc-
leus, tegmental, dorsal,
ventro-lateral, 26.
ventro-medial, 26.

vestibular, chief, 40. 41, 42, 43, 46,
47, VIII-X, XXVI.
descending, 39, Mil.
dorsal — ^see nucleus, vestibular,

chief,
lateral — see nucleus of Deiters.
medial — see nucleus, vestibular,

chief,
superior — see nucleus of Bech-
terew.

Obex, III.

olive — see nucleus, olivary.

opossum, 100.

oral sense, 91.

Pain, 23.

palaeocortex, 97.

paleostriatum, 106.

pallium, 87

paraflocculus, 10, I, II, IX, X.

peduncle, cerebellar, 11.

anterior or superior — see brachium
conjunctivum.

middle — see brachium pontis.

]92

Index

posterior or inferior — see body,

restiforni.
cerebral. 13, 54, 59, 63, 66, II, XIV-

XIX.
of mamillary body, 76, XIV, X\',

XXVI. '
perikaryon, 17.
pes pedunculi cerebri— see basis ped-

unculi cerebri,
pigment, 66

plexus, chorioid, 11, 14, 72.
pons, 12, 36, 48, 54, 67, II, XII, XIII,

XXVI.
pressure, sense of, 23.
preparation of sections, 126.
primordium hippocampi, 98, 99, XXII.
Probst, commissure of — see commis-
sure of lateral lemniscus,
tract of — see tract,
proprioceptive elements, 24, 30, 33, 39.
proprioceptor, 23.

prosencephalon, 10, chapters XI-XIII.
psalterium — see commissure, hippo-

campal.
pulvinar, 81, 85.
putamen, 105, 106, 107, 108, XIX-

XXI.
pyramid, 12, 36, II.
pyramis of cerebellum, 11, 56, I, VIII.

Rabbit, 27, 41, 46, 51, 78, 81, 95.
radiation, fountain, of Meynert, 62,
XIV.

optic, 80.
raphe, 29.
refiex action, 18.

arc, 18.
regio innominata, 79.
region, of cerebral cortex, 112, 113.

cingular, 112, 114.

frontal, 54, 62, 67, 114.

hipjwcampal, 112, 114, see also
hippocampus.

insular, 108, 112, 114.

occipital, 67, 112, 114.

olfactory, 14, 114, see also lobe,
pyriform.

parietal, 112, 114.

postcentral, 112, 114.

praecentral, 112, 114.

retrosplenial, 112, 114.

temporal, 54, 67, 112, 114, 115.
rhinencephalon, 88.

rhinocoele, XXIV, XXV.
rhombencephalon, 10, chapters IV-IX.
Rolando, gelatinous substance of — see

substance,
root of nerve III, 63, II, XIV, XXVI.
of nerve IV, 11, 63, XXVI.
of nerve V, ascending, 30.

mesencephalic, 33, 69, XI-XIV.
motor, 49, XI, XII.
sensory, 30, XII.

spinal (or descending), 30, 31, 32,
VII-XI.
of nerve VII, motor, 46, 47, IX-XI,

XXVI.
of nerve VIII, ascending vestibular,
39.
descending vestibular, 39, 58,
VIII.
of nerves IX and X, 48.
rostrum of corpus callosum, 110,
XXVI.

Septum, 16, 74, 87, 90, 92, 93, 94, 95,
96, 98, 100, 101, 102, 104, 106,
III, IV, XXII, XXVI.
dorsal (posterior) median, 22.
sheath, myelin (medullary), 17.
somatic nerve components, 20, 29, 30,

35, 45.
space, anterior perforated — see area,
anterior perforated,
posterior perforated, 65.
spinal cord, 9, 10, 22-28, 34, 40, 43,

45, 46, 49, 53, 54, 60, 62, V.
splenium of corpus callosum, 16, 103,

109, 110, XXVI.
staining, method of Weigert, 126.

method of Kultschitzky, 127.
stratum griseum of anterior colliculus,
70, XIV-XVI.
lemnisci of anterior colliculus, 70,

XIV, XV.
opticum of anterior colliculus, 70,

XIV-XVI.
profundum of coUiculi, 68, 70, 71,

XIII-XV.
zonale of anterior colliculus, 70,
XIV-XVI.
of thalamus, 74.
stria cornea — see stria terminalis.
Lancisii, 97, 99, 101.

Index

193

medullaris acustica, 36, IX.

thalami, 73, 74, 94, 97, 103, III,
XVIII-XXI, XXVI.
olfactory, intermediate, 90, XXIV.
lateral — see tract, olfactory,
medial — see tract, olfactory,
terminalis, 95. Ill, XVIII-XXI.
bed of, 74, 92, 96, XXI.
substance, anterior perforated — see
area,
gelatinous of Rolando, 22, 25, 32,
39, V, VI.
substantia nigra, 59, 66, 67, 107, XIV-
XVI.
reticularis — see formation, reticular,
subthalamus, 77, 78.
sulcus corporis callosi, 101.
dorsal median, 10, 22, I, V.
dorso-lateral, 10, I, V.
endorhinalis — see groove, endorhinal.
posterior median, 12 — see also sulcus,

dorsal median,
praepyramidalis, 11, 56.
primarius, 11, 56.
ventro-lateral, 10, II, V.
synapse, 17, 79.

system, praecommissural — see fasci-
culus praecommissuralis.

Taenia semicircularis— see stria termin-
alis.
thalami, 73.
ventriculi. III.
tapetum, XIV, XV.
taste — see centres, gustatory,
tectum, 59, 62, 67, 71, 80, III, XII-
XVI.
laminated gray matter of, 68, XIII.
tegmentum, .59-66, 75.
telencephalon, 75, 81, 87-115.

medium, 87.
temperature, sensations of, 23.
thalamus, 14, 44, 60, 61, 62, 64, 69, 72,
73, 75, 76, 77, 79, 82-86, 103,
106, 107, 108, III, IV, XVI-XX,
XXVI.
thickness of cerebral cortex, 110.
tissue, connective, 17.

nervous, 17.
touch, 23, 24.

tract, 19. (See also bundle and
fasciculus.)

arcuate, of Russell— see tract, cere-

bello-bulbar.
ascending, of Deiters, 40, 60, 71.
of Boyce — see tract, interstitio-

spinal.
cerebello-bulbar (of Russell), 58, XL
in cerebral hemisphere, 109.
cortico-bulbar, 53, 54, 67.
cortico-habenular, lateral, 74.

medial, 74, 103.
cortico- mamillary, 103.
cortico-olfactory of septum — see fas-
ciculus praecommissuralis.
cortico-pontine, 53, 54, 67.
cortico-spinal, 23, 26, 27, 36, 38, 53,

54, 67, V-XIII, XXVI.
cortico-tectal, 70.
descending, of Deiters^see tract,

vestibulo-spinal.
fastigio-bulbar, 58.
fronto-pontine, 67.
of Gowers, 52.
of V. Gudden — see tract, mamillo-

tegmental.
interstitio-spinal, 60, 71.
of Lissauer, 23, 25, 26, 27, V.
lobe-hippocampal — see fasciculus

praecommissuralis.
mamillo-peduncular, 76, XIV, XV,

XXVI.
mamillo-tegmental, 64, 76, 77, XII-

XVI, XXVI.
mamillo-thalamic, 76, 77, 82, 94,

XV-XX, XXVI.
olfacto-cortical, 97, 99, 104, XXIII.
olfacto-habenularis anterior, 74.
olfacto-habenularis medialis, 73.
olfacto-hypothalamic, 76, 94, 97,

XVII-XX.
olfacto-tegmental, 94.
olfactory, 14, 88, 89, 90, 91, II, XXV.
olfactory basal — see tract, olfacto-
hvpothalamic.

lateral, 89, 90, 94, 99, XX-XXIV.

medial, 90, 94, XXIII, XXIV.

projection, 95, 97.
olivo-cerebellar, 50.
6livo-spinal, 26.
optic, 15, 70, 71, 80, 81, 82, XIV-

XVII, XXVI.
uncrossed fibres, 81.

194

Index

periepenchmal longitudinal — see

bundle, dorsal longitudinal of

Schiitz.
of Probst, 33.

pyramidal — see tract, cortico-spinal.
rubro-spinal, 26, 27, 53, 54, 62, X-

XIII.
of Russell — see tract, cerebello-

bulbar.
septo-habenular, 74.
spino-cerebellar, dorsal, 25, 26, 27,

52, VIII.
ventral, 25, 26, 27, 39, 52, 58,

VIII, XI.
spino-olivary, 26.
spino-tectal, 26.
spino-thalamic, lateral, 26 — see also

lemniscus, spinal,
ventral, 26 — see also lemniscus,

spinal,
strio-habenular, 74.
tecto-bulbar, 68, 70.
tecto-pontine, XII.
tecto-spinal, 26, 27, 54, 62, 68, 70,

VII-XIII.
tegmental of interpeduncular nuc-
leus, 64, 65, XIII.
temporo-hippocampal, crossed, 103.
temporo-pontine, 67.
thalamo-habenular, 74.
transverse peduncular, 65, XV.
vestibular, secondary, 40, 41, 71, X.
vestibulo-cerebellar, 40, X.
vestibulo-spinal, 26, 27, 40.
visceral, secondary, 44.

trigonum acustici. superficial eminence
produced bv vestibular nuclei,
III.
vagi, eminence in floor of fourth
ventricle produced by dorsal
vagus nucleus,
truncus corporis callosi, 110.
tuber cinereum, 15, 75, 76, 77, II,

XVI-XVIII, XXVI.
tuberculum acusticum, 13, III — see
also nucleus, dorsal cochlear,
fasciae dentatae, 94, IV.
olfactorium, 14, 74, 76, 87, 90, 91,
92, 93, 94, 95, 96, 97, II, XXI-
XXIII, XXVI.

Uncus, 91.

uvula of cerebellum, 11, 56, I, VII,
VIII.

Vascularity of brain tissue, 113, 114.
velum, anterior medullarv, 11, 63, XI,
XXVI.
posterior medullarv, 11, XXVI.
ventricle, fourth, 10, 11, 12, 29, III,
VIII-XI, XXVI.
lateral, 16, 87, 92, 93, 100, 101,

XIV-XXV.
third, 14, 16, 72, 75, 83, 84, 87, III,
XVI-XIX.
vermis of cerebellum, 10, I.
vessels, blood — of brain, 11, 17, 113,

114, XXVII.
visceral nerve components, 20, 42, 46.

Zona incerta, 78, 79, XVII-XIX.